Fundamentals and applications of pervaporation through zeolite membranes

Size: px
Start display at page:

Download "Fundamentals and applications of pervaporation through zeolite membranes"

Transcription

1 Journal of Membrane Science 245 (2004) 1 33 Review Fundamentals and applications of pervaporation through zeolite membranes Travis C. Bowen 1, Richard D. Noble, John L. Falconer Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO , USA Received 15 January 2004; received in revised form 24 June 2004; accepted 25 June 2004 Available online 21 September 2004 Abstract Zeolite membranes have uniform, molecular-sized pores, and they separate molecules based on differences in the molecules adsorption and diffusion properties. Zeolite membranes are thus well suited for separating liquid-phase mixtures by pervaporation, and the first commercial application of zeolite membranes has been for dehydrating organic compounds. Because of the large number of zeolites that can be prepared, zeolite membranes have also been used to remove organic compounds from water, separate organic mixtures, and remove water from acid solutions on the laboratory scale. The fundamental aspects of separations by pervaporation through zeolite membranes are reviewed, and examples of the selectivities and fluxes obtained are presented. Some aspects of these separations are similar to gas-phase separations using zeolite membranes, but feed-side coverages are close to saturation during pervaporation, making competitive adsorption and molecule molecule interactions more important during multicomponent diffusion. Some of the topics that are discussed include: (1) the use of feed fugacities to predict separation selectivities; (2) the effects of coverage, competitive adsorption, heats of adsorption, molecular sizes, temperature, membrane structure, non-zeolite pores, concentration polarization, and support resistance on transport and separations; (3) the ability of one molecule to slow down or speed up another molecule in the zeolite pores, and (4) the techniques used to measure adsorption and diffusion properties. Several possibilities for improving understanding and effectiveness of pervaporation through zeolite membranes are also suggested Published by Elsevier B.V. Keywords: Pervaporation; Inorganic membranes; Ceramic membranes; Zeolite membranes; Adsorption Contents 1. Introduction Characteristics of zeolite membranes Membrane synthesis Membrane characterization Pervaporation measurement techniques Modules Analysis Membrane maintenance... 9 Corresponding author. Tel.: ; fax: address: john.falconer@colorado.edu (J.L. Falconer). 1 Present address: National Risk Management Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, OH 45268, USA /$ see front matter 2004 Published by Elsevier B.V. doi: /j.memsci

2 2 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Adsorption Hydrophobicity/hydrophilicity Adsorption measurement Driving force Diffusion Pore diameter Kinetic diameters of polar molecules Diffusivities Coverage dependence Multicomponent diffusion Non-zeolite pores External transport resistance Concentration polarization Support resistance Pure component pervaporation Separations Organic dehydration Removal of organic compounds from water Organic/organic separations Acid separations Multicomponent separations Comparison to vapor permeation Commercial pervaporation Conclusions and recommendations Acknowledgments Nomenclature References Introduction Pervaporation allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation has advantages in separating azeotropes, close-boiling mixtures, and thermally sensitive compounds, and removing species present in low concentrations. Only a fraction of a mixture is vaporized during pervaporation, and lower temperatures than those required in distillation are usually used. In addition, membranes operate continuously without requiring sorbent regeneration, and they are modular, which allows design flexibility. These advantages make membrane processes or hybrid processes involving membranes economically attractive in many industrial applications. Many types of membranes can remove salts or large impurities and particles from a stream with high efficiency. These separations are the most common large-scale uses of membranes, but molecular separations of both gas and liquid feeds are also possible with dense polymeric membranes or membranes that have molecular-sized pores. Hydrogen removal from other gases involved in fuel cell applications, N 2 /O 2 separation, removal of CO 2 and H 2 S from natural gas, and various gas and vapor separations for chemical processing are some of the desired gas-phase separations. Liquid separations, such as organic dehydration and ethanol removal from fermentation broths are also desired. Pervaporation is the most commonly used membrane process for these separations, but filtration and perstraction, which use liquids on both sides of the membrane, can also be used. The driving force for pervaporation, however, is usually higher because it uses a vacuum on the permeate side. Furthermore, perstraction requires additional separation steps to remove the permeated component from a permeate solvent. Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side as shown by the diagram in Fig. 1. The vapor is enriched in the preferentially permeating component and is condensed for future

3 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Fig. 1. Diagram of pervaporation across a membrane. processing. Meanwhile, the retentate is enriched in the nonpreferentially permeating component and can either be used in another process or recycled for further separation. Several reviews on pervaporation have been published [1 6],but these only discuss separations with polymeric membranes. Difficulties that reduce the effectiveness of pervaporation through polymeric membranes include concentration polarization (depletion of the fast permeating component near the membrane surface, see Section 9.1) on the feed side and membrane swelling. These and other problems have been overcome for some separations, and polymeric membranes have been successfully commercialized for pervaporation. As of 1996, 63 pervaporation systems were used industrially in the world [2]. Only one of these removed volatile organic compounds (VOCs) from water; the rest dehydrated organic solvents. By 2001, the number of large-scale pervaporation systems had risen to about 100 [7]. Recently, W.R. Grace & Co. developed a pervaporation process that uses a polymeric membrane to remove sulfur compounds from naptha streams, and this system has been tested on a demonstration plant [8]. Other separations, including organic/organic separations, have been tested in the laboratory and on the pilot scale, but we are not aware that they have been commercialized. Zeolite membranes have also been used for pervaporation both industrially and in laboratory studies. These membranes are polycrystalline zeolite layers deposited on porous inorganic supports, and they offer several advantages over polymeric membranes: Zeolite membranes do not swell, whereas polymeric membranes do. Zeolites have uniform, molecular-sized pores that cause significant differences in transport rates for some molecules, and allow molecular sieving in some cases. Most zeolite structures are more chemically stable than polymeric membranes, allowing separations of strong solvents or low ph mixtures. Zeolites are stable at high temperatures (as high as 1270 K for some zeolites [9]). In contrast, zeolite membranes in general cost significantly more to produce than polymer membranes, and zeolite layers are more brittle than polymers. The advantages make zeolite membranes attractive alternatives for separating mixtures whose components have adsorption or size differences, but are difficult to perform using polymeric membranes and other conventional separation methods. Zeolite crystals have been deposited in a polymeric layer to make zeolite-filled polymeric (mixed-matrix) membranes, which are more flexible and easier to work with than zeolite membranes. The zeolite crystals, however, are not formed in a continuous layer. Instead, isolated zeolite particles are surrounded by polymeric membrane material. Ideally, zeolite particles increase the mobility of the component that is more permeable in the polymer, while decreasing the mobility of the component that is less permeable. Zeolite incorporation often increases pervaporation flux with little or no decrease in separation factor [10] or increases separation factor with little or no decrease in flux [11 13]. Zeolite incorporation has increased both the separation factor and flux for some separations, such as ethanol removal from water using silicalite-1 [14,15] and Y-type zeolite [16,17] in polydimethylsiloxane (PDMS) membranes. Other polymeric systems that exhibited improved separation factor and flux upon zeolite incorporation include alcohol dehydration with polyamidesulfonamide and polyvinylalcohol membranes filled with NaA zeolite [18,19], removal of methanol from toluene with a viton membrane filled with NaX zeolite [20], and removal of chlorinated hydrocarbons from water with a PDMS membrane filled with silicalite-1 [21]. Zeolite membranes usually have higher separation factors than zeolite-filled membranes because the zeolite layer is continuous. The flux through zeolite pores in a zeolite membrane can be described by J = ρd t (q) dq (1) dz where q is the coverage or occupancy, which is the amount adsorbed in the zeolite pores, z the position along the transmembrane direction, D t (q) the coverage-dependent transport diffusivity, and ρ the zeolite density. Transport through zeolite pores takes place by surface diffusion at low temperatures and activated gaseous diffusion at high temperatures where adsorption is insignificant [22,23]. Surface diffusion follows an adsorption diffusion mechanism [24,25]. That is, molecules first diffuse from the bulk feed to the zeolite surface. Next, they adsorb to the sites on the zeolite surface and in the zeolite pores. After entering the zeolite pores, the molecules diffuse along the surface of the pores by jumping from site to site, driven by the chemical potential gradient within the pore. At the permeate side of the membrane, molecules desorb from the zeolite and diffuse into the bulk permeate through the support pores. For many mixtures, zeolite membranes separate because of adsorption differences, and this is especially true for water/organic separations because of the hydrophobic/hydrophilic nature of zeolites. Diffusion differences are also important in some separations. Most zeolite membrane separations occur in the adsorption and diffusion steps, but concentration polarization and support resistance also affect selectivity. Pervaporation has many similarities to vapor permeation, which uses gaseous components on the feed side of the mem-

4 4 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 brane. Studies that have compared vapor permeation and pervaporation show that vapor permeation with the feed at the saturated vapor pressure gives almost identical fluxes and selectivities as pervaporation at the same temperature [26 28]. Despite this similarity, pervaporation and vapor permeation have several differences: Vapor permeation fluxes depend strongly on feed pressure. Higher pressures, and thus higher fugacities than in pervaporation are typically used because vapor permeation temperatures are usually higher. Pervaporation fluxes, however, are independent of feed pressure and the pressure need only be high enough to maintain a liquid feed. Heat transfer within the membrane is more of a consideration for pervaporation because of the energy required to vaporize the fraction of the feed that permeates through the membrane. When vapor permeation is performed with feed pressures well below the saturated vapor pressure, the feed-side adsorption coverage is probably lower than during pervaporation. Concentration polarization in the feed is more likely during pervaporation because diffusion is slower in liquids than in vapors. Several reviews of zeolite membranes [9,23,29 37] have focused mainly on membrane synthesis and gas separations. Significant progress has been made in developing new membranes and synthesis techniques, and understanding transport and separation fundamentals over the last decade. This progress suggests that many applications of zeolite membranes in separations, chemical sensors, and catalytic membrane reactors are promising. A few of the reviews discuss pervaporation [23,30,33,34,36], but only briefly. In contrast, zeolite membranes have been used extensively for pervaporation in laboratory studies, and the only current large-scale commercial application of zeolite membranes is dehydrating alcohols and other organic compounds by pervaporation for solvent recovery. Mitsui Engineering & Shipbuilding Co. of Japan commercialized this process using NaA zeolite membranes [38]. Furthermore, pervaporation through zeolite membranes has potential applications in removing organic compounds from water for water treatment and purification, in concentrating ethanol from fermentation broths for alternative fuel production, and in separating isomers, such as xylenes or linear and branched alkanes if the fraction to be removed is small. Recent advances in preparing thin zeolite membranes have dramatically increased gas permeation fluxes while maintaining good selectivities. Hedlund et al. [39] prepared 0.5 m thick silicalite-1 membranes that had light gas fluxes that are one to two orders of magnitude higher than other silicalite-1 membranes reported in the literature, and had n-/i-c 4 H 10 gas-phase selectivities as high as 9 at 298 K for a 50/50 feed. In addition, Lai et al. [40] prepared 1 m thick oriented silicalite-1 membranes that performed significantly better for xylene isomer gas-phase separations than previously reported membranes. They obtained p-/o-xylene separation factors as high as 500 with a permeance of mol/m 2 s Pa at 473 K. Similarly, reducing the thickness of zeolite membranes may also dramatically increase their potential for pervaporation applications. Current and potential applications of pervaporation through zeolite membranes are discussed in this review. Physical properties, synthesis, and characterization of zeolite membranes are only briefly described because they have already been discussed in the previously mentioned reviews. Experimental considerations and the influence of adsorption, diffusion, concentration polarization, support resistance, and non-zeolite pores on pervaporation separations are also discussed. Moreover, common problems encountered and several recommendations for improvement are mentioned. 2. Characteristics of zeolite membranes Zeolites are crystalline structures that have uniform, molecular-sized pores. These inorganic structures have been used extensively as catalysts and adsorbents. More recently, continuous polycrystalline zeolite layers have been deposited on porous supports and used as zeolite membranes. Suzuki reported the first zeolite membranes in 1987 [41]. Since then, significant progress has been made to improve the quality of zeolite membranes and widen their range of applications. Today, more than 14 zeolite structures, including MFI [22,39,42 49], LTA[50 52], MOR [53 55], FAU[56 58], CHA [59,60], MEL [61], AFI [62], FER [63], BEA [64], GIS [65], ANA[66], DON [67], OFF [68], and ATN [69] have been prepared as membranes. The MFI structure (Fig. 2b) is most commonly used in zeolite membranes because of its pore size and ease of preparation, and this structure includes silicalite-1 and ZSM-5. Silicalite-1 is made up of pure silica, and ZSM-5 has Al substituted for some of the Si atoms. Zeolite structures are made up of TO 2, where T represents tetrahedral framework atoms, such as Si, Al, B, Ge, Fe, and P. The ETS-4 molecular sieve has also been prepared as a membrane and is a titanosilicate that contains octahedral framework atoms in addition to tetrahedral atoms [70,71]. Most often, zeolites contain Si with other metals substituted into the framework. The zeolite pores are made from rings in the framework and are designated by the number of oxygen atoms making up the ring. Small-pore zeolites include those structures made up of eight-member oxygen rings, medium-pore zeolites have 10-member rings, and large-pore zeolites have 12-member rings [32]. Examples of each of these are shown in the ball and stick representations of zeolite frameworks in Fig. 2 and are also described in Table 1. Zeolites with 6-, 9-, and larger than 12-member rings have been prepared, but, of these, the DON structure [67], which has 14-member rings, is the only one prepared as a membrane that we are aware of. Some zeolites have a three-dimensional pore system with pores along all crystal axes. Other zeolites, however, have only one- or two-dimensional pore systems. Some of these

5 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Fig. 2. Atomic stick representations for the frameworks of CHA (a), MFI (b), and MOR (c) zeolite structures. The nodes represent tetrahedral framework atoms and the sticks represent oxygen bridges. Table 1 Examples of small-, medium-, and large-pore zeolites and their structures [236] Structure (pore size) Zeolites with this structure Pore dimensions XRD pore diameters (nm) Similar sized zeolites (pore diameter in nm) a CHA (small) SSZ-13, SAPO LTA (0.41), GIS (0.48), ANA (0.42), ATN (0.40) MFI (medium) ZSM-5, Silicalite-1 2 (sinusoidal in , MEL (0.54), FER (0.54) one dimension) (elliptical) MOR (large) Mordenite (elliptical) BEA (0.77), FAU (0.74), AFI (0.73), OFF (0.68) a MOR and OFF also have 8-MR pores. structures have windows that allow limited movement along the axes with no pores, but others do not. The orientation of these types of zeolite membranes is therefore important. Pure silica zeolites have neutral frameworks because Si is tetravalent. When trivalent atoms, such as Al, are substituted in place of Si, charge-balancing cations are located in the structure near each trivalent atom. Different cations [72] and tetrahedral framework atoms [73] have different sizes and bond energies, causing changes in zeolite pore sizes and adsorption properties. Currently, however, quantitative effects of metal substitution and ion exchange on pore size and adsorption properties are not well understood for most zeolites. Membrane properties have also been controlled by posttreatment. Post-treatment methods include silylation to decrease pore size [74] and to increase hydrophobicity [75,76], and chemical vapor deposition (CVD) [77], atomic layer deposition (ALD) [78], or coking [79] to fill non-zeolite pores. 3. Membrane synthesis Zeolite membranes are most often prepared by hydrothermal synthesis, although dry gel methods [54,80,81] have also been used. Hydrothermal synthesis involves crystallization of a zeolite layer onto a porous support from a gel that is usually composed of water, amorphous silica, a source for tetrahedral framework atoms other than Si, a structure directing organic template, and sometimes a mineralizing agent, such as NaOH. This gel is placed in contact with the support in an autoclave, and the time, temperature, and gel com-

6 6 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 position for crystallization depend on the zeolite. Supports are generally alumina or stainless steel tubes or discs, although other ceramics [42,48,82], and other materials have been used. Alumina supports typically have pore diameters between 5 nm ( -Al 2 O 3 ) and 200 nm ( -Al 2 O 3 ), and stainless steel support pore diameters are typically between 0.5 and 4 m. During in situ crystallization, zeolite crystals nucleate and grow on the support surface. Crystals sometimes nucleate in the bulk solution, but this is not preferred. Nucleation in the bulk is less likely for dilute gels [23]. Techniques have been developed to prepare membranes without organic template molecules [58,83], and A-type membranes are usually prepared without a template, but if a template is used, the zeolite structure forms around the organic template molecules, making the pores. A polycrystalline zeolite layer forms on the support and this acts as the separating layer. Fig. 3a shows a cross-sectional SEM photograph of a zeolite layer on top of a porous support. From this view, only the zeolite crystals on top of the support can be seen, but zeolite crystals also penetrate into the pores of the support in many zeolite membranes. Seed crystals are sometimes added to the support prior to the crystallization step to provide sites for zeolite growth and improve control of crystal growth. Using seed crystals is referred to as two-step crystallization. This technique has been used to prepare oriented zeolite membranes (Fig. 3b), which have increased fluxes by aligning pores in a desired direction [40], and to prepare the thin membranes [39,40] that were mentioned previously. Cationic polymers on the support surface [39], dip coating [84], and covalent linking [85] have increased seed crystal adherence and improved membrane quality. Zeolite membrane preparation has been described in detail in several review articles [23,29,31,34,36]. If template molecules are used during crystallization, they occupy the zeolite pores and small non-zeolite pores in the intercrystalline boundaries. One method to check membranes for large defects is to measure N 2 permeation while the template is in place; if no large defects are present, the membranes are impermeable to N 2. Successive layers are often crystallized to eliminate defects significantly larger than the zeolite pores. The template is then removed from the membrane pores by calcining the membrane at temperatures around Fig. 3. Cross-sectional SEM photographs of a non-oriented B-ZSM-5 zeolite membrane on an -Al 2 O 3 coated SiC porous support (a), and an oriented silicalite-1 membrane on silica coated -Al 2 O 3 porous support [40] (b). (b) is reprinted with permission from Lai et al. [40]. Copyright (2002), AAAS.

7 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) K to desorb or combust the organic molecules. The temperature required for this process depends on the zeolite and the template used in synthesis. Even though zeolites are stable at high temperature, care must be taken when calcining the membranes because the thermal expansion coefficients of zeolites are usually different from that of the support. Zeolite membranes that do not use a template are calcined at lower temperatures or not at all. Ideally, membranes have a low defect concentration to obtain high selectivities, are as thin as possible to obtain high fluxes, and are reliably reproducible. Because they are polycrystalline, however, eliminating every defect or non-zeolite pore is not possible. Proper synthesis techniques reduce the number of non-zeolite pores. Although many studies have reported that membranes prepared by the same procedure had different properties, recent studies have shown that zeolite membrane preparation can be reproducible. The thin silicalite-1 membranes prepared by Hedlund et al. [39] had He and SF 6 permeances with relative standard deviations that were only 8 and 12% of the average permeances, respectively. Likewise, the thin silicalite-1 membranes prepared by Lai et al. [40] had p-xylene permeances with a 3% relative standard deviation during p-/o-xylene separations [86]. Lin et al. [87] also prepared silicalite-1 membranes with high reproducibility. Their membranes had ethanol/water separation factors as high as 106, and the relative standard deviations of the fluxes and separation factors were 10 and 5% of the average values, respectively. 4. Membrane characterization Zeolite membranes used in pervaporation measurements have been characterized using some of the methods described below. Zeolite crystal size and shape, and membrane thickness on top of the support are measured with scanning electron microscopy (SEM). Membranes ranging from 0.5 m to approximately 500 m thickness have been reported. In addition, SEM gives a qualitative idea of layer uniformity and continuity. Zeolite framework structure and crystallinity are typically determined using X-ray diffraction (XRD) [88]. Zeolite composition is measured with inductively coupled plasma (ICP) or electron probe microanalysis (EPMA), which is also known as energy dispersive X-ray spectroscopy (EDX). Several studies have also used EPMA to determine the distance the zeolite layer penetrates into the pores of the support [89 91]. The surfaces of flat membranes are sometimes analyzed without damaging the membranes, but tubular and monolithic membranes must be broken to carry out these measurements. Zeolite powders that form during membrane synthesis can be analyzed with XRD and ICP, and though these crystals have been shown to be similar to those in the membranes, this is not a direct measure of the membrane. Gas permeation is also a common method of membrane characterization. Ideal selectivity is defined as the ratio of single-gas permeances, and is often used as an indication Fig. 4. Single-gas permeances of H 2,CO 2, n-c 4 H 10, and i-c 4 H 10 through SAPO-34 [247], Ge-ZSM-5 [73], mordenite [142], and X-type [58] zeolite membranes at 473 K vs. maximum XRD pore diameters. of membrane quality. Permeation depends on both adsorption and diffusion, but molecules close to or larger than the zeolite pore size have difficulty entering zeolite channels. Fig. 4 shows that n-c 4 H 10 (0.43 nm kinetic diameter [92]) and i-c 4 H 10 (0.50 nm) permeances through zeolite membranes at 473 K decrease dramatically as the zeolite pore diameter decreases. In contrast, H 2 (0.29 nm) and CO 2 (0.33 nm) permeances at the same temperature decrease less because H 2 and CO 2 are smaller than the pores of all the membranes. Molecules larger than the zeolite XRD pore diameters sometimes fit into the zeolite pores, and this is possibly because some zeolite frameworks are flexible [93,94]. Moreover, XRD pore diameters may not be accurate representations of zeolite pore size [95], and kinetic diameters may not accurately represent molecular sizes because the diameters assume molecules are hard spheres. Appreciable permeation of molecules significantly larger than zeolite pores, however, indicates flow through non-zeolite pores. Gas mixture permeation is an additional characterization method. Some mixtures permeate differently from single components because of competitive adsorption, which is discussed further in Section 6. Also, some molecules are inhibited and some are sped up by the presence of other molecules [96 98], and multicomponent diffusion is discussed in more detail in Section 7. These effects are generally larger when adsorption coverages are high. Several studies have characterized 10-member oxygen ring (MR) zeolite membranes by measuring n-c 4 H 10 /i-c 4 H 10 separation selectivities [25,80,99 101]. Because these molecules fit into the zeolite pores, mixtures that are separated by molecular sieving are probably better for characterizations. Lai et al. [40] observed that 1 m thick silicalite-1 membranes with significant grain boundary defects separated n-c 4 H 10 /i-c 4 H 10 and N 2 /SF 6, but did not separate vaporphase p-/o-xylene mixtures. Only membranes with few grain boundary defects separated p-xylene (0.58-nm kinetic diameter) and o-xylene (0.68 nm) mixtures. In another study, Lai and Tsapatsis [86] showed that these high-quality

8 8 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 membranes exhibited separation factors of 15 and 25 for benzene (0.59 nm)/cyclohexane (0.60 nm) and benzene/2,2- dimethylbutane (DMB; 0.62 nm) vapor mixtures, respectively, at 473 K, even though their n-c 4 H 10 /i-c 4 H 10 and N 2 (0.36 nm)/sf 6 (0.55 nm) separation factors were only 5 and 2.5, respectively, at this temperature. The feeds for these separations were approximately 50/50 mixtures except for benzene/cyclohexane, which was approximately 25/75. For the benzene/cyclohexane and benzene/2,2-dmb binary separations, the benzene, cyclohexane, and 2,2-DMB feed partial pressures were 6.25, 21.2, and 6.45 kpa, respectively. For the n-c 4 H 10 /i-c 4 H 10 and N 2 /SF 6 binary separations, the feed partial pressure of each component was 50 kpa. Mixtures of n-hexane (0.43 nm) and 2,2-DMB have also been used to characterize MFI membranes [102]. Furthermore, separations at high temperatures ( 470 K and above) are better for characterizations because adsorption is low, and high temperatures are a harsher test of the membranes [103]. Gas permeation characterizations, however, are not always a good indication of membrane performance for pervaporation. For example, A-type membranes have effectively dehydrated organic compounds even though their gas selectivities are near the Knudsen values [28]. For characterization of smaller pore, 8 MR zeolite membranes, light gas separations, such as H 2 /CH 4 [59],CO 2 /CH 4 [104], and H 2 /N 2 [50] have been used in several studies. Similarly, larger pore zeolite membranes require larger test molecules. Pure component pervaporation, discussed in Section 10, may be the best characterization method for these membranes [105]. For laboratory-scale studies, permeate vapor is usually condensed in a liquid N 2 trap and permeate pressures below 300 Pa are maintained. Some studies have used a sweep gas on the permeate side and obtained similar permeation results as when using vacuum [106]. Vacuum, however, is used in industrial pervaporation because it does not require secondary separation from the sweep gas, and vapors are often liquefied in a pre-condenser and a liquid-ring pump Modules During pervaporation, membranes are sealed into a membrane module. Disc-shaped membranes are usually sealed between two plates in a manner similar to that shown in Fig. 6a. The feed is typically on the zeolite side of the support so that concentration polarization is minimized. Concentration polarization is discussed in greater detail in Section 9.1. Tubular membranes have non-porous ends on each side of the porous support to allow O-rings to seal the membrane, as shown in Fig. 6b. Non-porous tubes are welded to the ends of porous stainless steel supports, and the ends of ceramic and SiC porous supports are glazed to create the non-porous ends. Tubular membrane modules have been operated with the feed on either the inside or outside of the tube, but as with the flat membranes, the feed is typically on the zeolite side of the membrane. To maximize space efficiency, commercial 5. Pervaporation measurement techniques A typical experimental set-up for pervaporation studies is shown in Fig. 5. Adsorption from the feed depends on feed component fugacities, which depend on their molar concentrations. Because liquids are nearly incompressible, their fugacities are essentially independent of pressure, and thus the feed pressure has a negligible effect on pervaporation. Fig. 5. Schematic representation of a typical experimental set-up for pervaporation. Fig. 6. Cross-sectional sketches of modules for disc-shaped (a) and tubular (b) zeolite membranes, and a sketch of a multi-channel monolith membrane (c).

9 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) membrane modules house banks of tubular membranes [38]. A more efficient multi-channel monolith support (Fig. 6c) has been used on the lab scale, and it has the potential to be more efficient in commercial applications because its surface area to volume ratio is higher than that of tubes [42,107]. Monolith membranes are sealed in a module similar to the tubular membrane module. Zeolite membranes have also been prepared on a ceramic hollow fiber [108], which also has a high surface area to volume ratio, but this configuration has not been used for pervaporation Analysis Pervaporation fluxes in lab-scale studies are usually measured by weighing the amount of permeate collected in a liquid N 2 trap during a given time period. Fluxes are typically reported in units of kg/m 2 h because these are the units used in industrial separations. Molar fluxes in mol/m 2 h, however, are more useful for comparing permeation of components with different molecular weights, and fluxes are reported in these units in this review. For comparison, a 1 kg/m 2 h water flux is the same as 55 mol/m 2 h. Gas or liquid chromatography is usually used off-line to measure permeate concentrations, but on-line mass spectroscopy [98, ] and titration [59] have also been used. Separation performance in pervaporation is usually reported by a separation factor, α a/b = y a/y b (2) x a /x b where y and x are permeate and feed compositions, respectively. This definition is analogous to the definition of relative volatility in vapor liquid equilibrium. Separation factors are used instead of flux ratios because pervaporation feeds usually contain low concentrations of one component in a mixture. Note that the separation factor is a ratio of ratios, and small composition changes can lead to large changes in separation factors, especially at low feed concentrations and high permeate concentrations. molecules adsorbed in the zeolite pores. Another method for removing impurities is purging the membrane with a weakly adsorbing gas like N 2 or He. Funke et al. [112] recommended that laboratory membranes be stored in a vacuum oven or similar inert environment when not in use to reduce adsorption of impurities. These methods of removing undesired adsorbates are less feasible for large-scale separations. Few studies have investigated the effects of zeolite membrane fouling, how to prevent it, or better ways to regenerate a fouled membrane. 6. Adsorption Adsorption in zeolites under pervaporation conditions is physical adsorption, and is therefore a non-activated, exothermic process that is reversible [92]. Molecules adsorb into zeolite pores because of intermolecular attractive forces between the adsorbent and adsorbate. All other properties being equal, heats of adsorption are higher for molecules with larger dipole moments [115]. Furthermore, dispersion forces increase with size or molecular weight of a molecule, but decrease as the amount of branching increases because branching decreases the contact surface area. For this reason, heats of adsorption increase with increasing molecular weight if the molecules are otherwise similar [116]. For alcohol vapor adsorption on silicalite-1, coverages near saturation are typically reached at pressures well below the saturated vapor pressures, p sat (Fig. 7) [117]. This adsorption behavior is also true for A-type [28] and mordenite [55] zeolites. Adsorption isotherms for benzene and p-xylene [118], 2- and 3-methylpentane [119], and C 1 C 10 linear alkanes [120,121] adsorbed on silicalite-1, and n-hexane and p- xylene adsorbed on ZSM-5 zeolites [122] also indicate that the coverages are near saturation at pressures below P sat.in contrast, coverages of H 2 O on A-type [28] and mordenite [55] zeolites increase with pressure up to the saturated pressure, 5.3. Membrane maintenance Zeolite membranes can adsorb impurities during pervaporation and also from the atmosphere during storage, and these impurities can significantly affect permeation and separation performance [112]. Even if no impurities adsorb, some components used during pervaporation are strongly adsorbed in the zeolite pores and then affect permeation of other components if the feed is changed. Nomura et al. [113] and Li et al. [114] reported that fermentation broth fluxes through zeolite membranes decreased after h of pervaporation because minor components in the broths adsorbed in the membranes (see Section 11.5). The fluxes in both studies returned to the original values after the membranes were calcined at high temperature. Calcination between 550 and 650 K is usually the easiest way in the laboratory to remove unwanted Fig. 7. Adsorption isotherms of methanol (1), ethanol (2), 1-propanol (3), 1-butanol (4), 2-methyl-2-butanol (5), and 1-hexanol (6) on silicalite-1 zeolite (Si/Al = 990) at 293 K. The alcohol saturated vapor pressures at the adsorption temperature are designated by p sat. Reprinted in part with permission from Nayak and Moffat [117]. Copyright (1988), American Chemical Society.

10 10 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 and Flanigen et al. [123] showed that the silicalite pore volume occupied by H 2 O adsorbed at P sat was only about 25% of the theoretical pore volume. Saturation coverage, which is the maximum number of molecules adsorbed into the pore volume, is not expected to depend on temperature if the zeolite structure, and thus the total pore volume, does not change with temperature. Instead, saturation coverages are only expected to depend on molecular size. For feed mixtures, the total coverage at the feed side is high, but the species competitively adsorb. The coverage decreases through the membrane and is low on the permeate side during steady-state permeation. At low coverages (b i ˆf i 1inEq. (4)), the amount adsorbed is related to temperature by the following proportionality: ( ) Hads q exp (3) RT where H ads is the heat of adsorption [124]. Desorption is endothermic, and thus increasing temperature increases the desorption rate. Adsorption on zeolites is usually modeled with a Langmuir adsorption isotherm. Nomura et al. [125] used a singlesite Langmuir adsorption isotherm q i = q sat,ib i ˆf i (4) 1 + b i ˆf i to calculate the amount of component i adsorbed on a zeolite in contact with a liquid. Here q sat,i is the saturation coverage, b i is an adsorption equilibrium constant, and ˆf i is the component fugacity. In gas-phase adsorption at low pressures where gases are ideal, fugacity is equal to the partial pressure. Larger molecules and molecules at high coverages have shown preferences for specific adsorption sites in the zeolite pores, and for these cases, adsorption is usually better represented by a dual-site Langmuir adsorption isotherm [126]: q tot = q sat,ab A ˆf 1 + b A ˆf + q sat,bb B ˆf (5) 1 + b B ˆf where q tot is the total coverage, and subscripts A and B indicate independent adsorption sites, which are usually the channels and intersections within a zeolite structure. Modeling adsorption of hexane [126,127] and butane [128] isomers required a dual-site Langmuir adsorption isotherm because the linear molecules preferentially adsorbed in the channels of silicalite, and the branched molecules preferred the intersections. Both molecules adsorbed in both sites at high coverages. Moreover, a multicomponent isotherm is required when modeling adsorption of mixtures [126]. Dual-site, multicomponent isotherms that account for different saturation capacities for each component are expected to better represent adsorption of most mixtures, but ideal adsorbed solution (IAS) [ ] and real adsorbed solution (RAS) [126] theories and simpler Langmuir isotherms are currently used as approximations in most studies. The IAS and RAS theories allow predictions of mixture adsorption isotherms based on the pure component isotherms Hydrophobicity/hydrophilicity Perhaps the greatest attribute of zeolite membranes for pervaporation to date has been their hydrophobic/hydrophilic nature. This allows efficient separations of water/organic mixtures. Hydrophilic zeolite membranes, such as NaA, have effectively dehydrated alcohols with high separation factors [24,51], and hydrophobic zeolite membranes, such as silicalite-1, have removed organic compounds from water [26,48,132,133]. The latter separations have been successful despite water being smaller than the organic molecules in the mixtures. Table 2 shows that NaA zeolite has a greater average affinity for water than for methanol, but the reverse is true for silicalite-1. Giaya et al. [134] discussed several definitions of hydrophobicity for zeolites: the ratio of water desorbed at 423 and 673 K from zeolite initially saturated with water, the fraction of the available pore volume in a zeolite not occupied by water at a specific temperature and water partial pressure, the amount of organic compound adsorbed divided by the amount of water adsorbed when a zeolite is placed in contact with a vapor-phase organic/water mixture to give a hydrophobicity index. The hydrophobicity index is computed by HI = q organic q water (6) where q organic and q water are the amounts of organic compound and water adsorbed from a vapor mixture. This definition requires that the same organic compound, pressure, temperature, and composition be used when comparing different zeolites. Because competitive adsorption between water and organic molecules is important in organic/water pervaporation, the hydrophobicity index is the most meaningful definition for this application [135]. Amounts adsorbed from mixtures of pervaporation components, however, are not known in general, and studies have used single component organic/water adsorption ratios to qualitatively estimate hydrophobicity [136]. For this review, hydrophobicity refers to preferential adsorption of organic compounds over water. Likewise, hydrophilicity implies that water preferentially adsorbs over organic compounds. These terms, however, do not indicate individual amounts of each component adsorbed. For instance, a zeolite that adsorbs more water than another zeolite would still be considered the more hydrophobic of the two if its organic/water adsorption ratio is higher. Giaya et al. [134] showed that the hydrophobicity of dealuminated Y-type (DAY) zeolite increased as the aluminum content in the zeolite framework decreased. This dependence on Si/Al ratio has also been shown for other zeolite struc-

11 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Table 2 Heats of adsorption of water and alcohols on zeolites Adsorbate H ads (kj/mol) NaA a (calorimetric) Silicalite-1 a (calorimetric) Silicalite-1 b (gravimetric) [117] ZSM-5 a (calorimetric) Water 100 ± 25[92] 30 ± 15 [151] 90 ± 10 [151] Methanol 85 ± 20 [237] 65 ± 10 [151] ± 5 [151] Ethanol 70 ± 10 [151] ± 5 [151] 1-Propanol 90 ± 10 [151] ± 5 [151] 1-Butanol 60 2-Methyl-2-butanol 55 1-Hexanol 64 Acetone 67 ± 3 [238] 130 ± 5 [238] Diethyl ether 70 ± 5 [151] 135 ± 5 [151] a Averages and ranges of H ads are shown. b Si/Al = 990 and fractional coverage = 0.6. tures [ ]. The aluminum content of A-type zeolites is high (Si/Al = 1), making them hydrophilic [139]. Other zeolite structures with high aluminum contents, such as X-type (Si/Al = 1 1.5) [114,140], Y-type (1.5 3) [100,140,141], and mordenite (Si/Al = 5) [142] are also hydrophilic and membranes composed of these zeolites have separated water from organic compounds (see Section 11.1). Aluminum is trivalent, and thus requires a chargebalancing cation when it is in the zeolite framework in place of Si. The localized electrostatic poles between the negatively charged framework and the positively charged cations strongly attract highly polar molecules, resulting in a hydrophilic structure. Because of this, zeolite hydrophobicity likely also increases as the Si/Me ratio (Me = metal) increases when other trivalent metals, such as B and Fe, are substituted into the framework. Ion exchange changes the local polarity in the pores and therefore the adsorption [57], and may also affect hydrophobicity. Zeolite hydrophobicity also appears to depend on the structure. All-silica zeolite beta (0.55 nm 0.55 nm and 0.76 nm 0.64 nm XRD pore sizes) has been shown to be more hydrophobic than silicalite-1 zeolite ( nm), even though both contain almost no aluminum [143]. Also, germanium substituted ZSM-5 zeolite has smaller pores than silicalite-1 because the Ge atoms are larger than the Si atoms, and the Si O Ge bridging oxygen atoms may shift into the channels [144]. Germanium is tetravalent and chemically similar to Si, but Ge-ZSM-5 zeolite has been shown to adsorb more 2-propanol and less H 2 O than silicalite-1 powder prepared by the same method [145]. Silicalite-1 membranes [87], however, have produced significantly higher ethanol/water separation factors than those for Ge-ZSM-5 [146] zeolite membranes, though the membranes in these studies were not prepared by the same procedure. Thus, the effects of Ge substitution on hydrophobicity are not clear. In addition, intercrystalline boundaries and defects in the zeolite framework contain silanol (OH) groups that terminate the zeolite structure, and these silanol groups increase the local hydrophilicity of the zeolite [138, ]. Reducing the defects in hydrophobic zeolite membranes may therefore increase hydrophobicity Adsorption measurement Microcalorimetry [150,151], gravimetric [120] and volumetric uptake [145,152], temperature programmed desorption [153], tapered element oscillating microbalance measurements [127], chromatography [154], and transient permeation [49] have been used to measure adsorption on zeolite powders. These techniques work well for pure components, but are difficult to use for mixture adsorption. For liquid mixtures, differentiating between adsorbed molecules and wetting on the outer surface of zeolite crystals is also difficult. Liquid-phase adsorption from organic/water mixtures is most often measured with a batch concentration depletion method [155,156]. This method, however, only allows estimation of organic uptake and gives no information on water uptake because calculating adsorbed organic amount from a concentration change assumes the total amount of water in the liquid remains unchanged. The best technique for measuring liquid-phase mixture adsorption appears to be liquid chromatography using an adsorbent column packed with zeolite [93,157]. Liquid chromatography has the additional advantage of measuring diffusivities. Another method for estimating liquid-phase adsorption from mixtures is to use a technique for vapor mixture adsorption with a pressure near the saturated vapor pressure. Nomura et al. [26] measured ethanol/water mixture adsorption by first adsorbing both species onto silicalite-1 zeolite from a vapor mixture. Next, the silicalite-1 zeolite was heated, and desorbed molecules were condensed in a cold trap. A GC determined the amounts of each species. This method works better than liquid-phase adsorption if the vapor is at a pressure that is low enough to prevent condensation because the outer surfaces of the zeolite do not wet when contacted with vapor. Theoretically, adsorption from the saturated vapor of a liquid mixture should be the same as adsorption from the liquid mixture because the component fugacities are the same for both phases. A few studies have measured adsorption isotherms of components commonly used in pervaporation, with alcohols receiving the most attention. Nayak and Moffat [117] measured pure alcohol vapor adsorption isotherms gravimetrically on silicalite-1 (Si/Al = 990) zeolite. Their results are shown in

12 12 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 Fig. 7 as examples of alcohol adsorption isotherms. Methanol reaches saturation at a higher relative pressure, p/p sat, than the other alcohols because it has the lowest H ads (Table 2). Its molar coverage at saturation, however, is larger than the other alcohols because it is the smallest molecule. Note that heats of adsorption measured with different techniques have different values in Table 2. Lin and Ma [157] also measured isotherms for several alcohols on silicalite-1, using aqueous liquids with the batch concentration depletion method and high performance liquid chromatography (HPLC). They found that the two methods gave comparable alcohol adsorption amounts. Yamazaki and Tsutsumi [55] determined isotherms for pure water, methanol, t-butanol, and n-hexane vapors on mordenite using a gravimetric method, and Giaya et al. [158] reported isotherms of several chlorinated compounds on various zeolites using tapered element oscillating microbalance. The maximum adsorption capacities they measured are shown in Table 3 along with data from other adsorption studies. In addition, Cruz et al. [159] measured acetic acid vapor isotherms on NaY and NaX zeolites Driving force Adsorption at the feed-membrane and membrane support interfaces is usually assumed to be at equilibrium during steady-state permeation through a zeolite membrane because adsorption is fast compared to permeation. At equilibrium, the chemical potential of a given component is the same in the adsorbed phase and in the phase contacting the zeolite. Adsorption isotherms are usually determined as a function of gas partial pressures. Adsorption from liquids, however, is nearly independent of feed pressure. Instead, fugacities of each component in the feed are the driving forces for adsorption from liquids during pervaporation [160]. Fugacity is a form of the chemical potential, and it is the driving force for mass transfer from one phase to another. The feed fugacity for component i is determined from ˆf f i = x i γ i p sat i (7) where x i is the feed mole fraction, γ i the activity coefficient, and p sat i the saturated vapor pressure. At constant concentration, fugacity increases with temperature. In mixtures, adsorption selectivity appears to increase as the ratio of feed fugacities increases [160]. For example, at 5% organic concentration, the acetone/water adsorption selectivity is higher than the acetic acid/water adsorption selectivity for silicalite-1. Kinetic diameters and heats of adsorption are roughly similar for acetone and acetic acid, but acetone s fugacity in the aqueous mixture is about 60 times higher than that of acetic acid. 7. Diffusion A chemical potential gradient in the membrane pores drives surface diffusion. Molecules diffuse along pore surfaces when the molecules continuously interact with the potential field of the zeolite walls because of the zeolite s high affinity for the molecules [161]. Molecules with molecular diameters larger than about 60% of the pore diameter transport by configurational diffusion [162]. Configurational diffusion is essentially activated surface diffusion, and is the most common method of transport through zeolite pores. Because of this, some molecules whose diameters differ by a few hundredths of a nanometer diffuse at significantly different rates in zeolite membranes, allowing separations. If a mixture contains some molecules that fit into a membrane s pores and other molecules that cannot, the membrane separates the mixture by molecular sieving Pore diameter Zeolite pore diameters are usually determined using XRD along with assumed atomic radii for Si and O. These diameters are shown for several zeolite structures in Table 1, but their accuracy has been disputed because molecules that are slightly larger than a reported XRD zeolite pore diameter can adsorb and diffuse through the pores [95]. Reported zeolite pore diameters are only an estimate of the pore size because the zeolite surfaces do not behave as well-defined walls. In addition, some zeolite frameworks are flexible at high temperatures [163] and when coverages are high [93,94]. Xylenes, even at lower coverages, also appear to distort ZSM-5 zeolite pores, making them more elliptical [ ]. Pore diameters for different zeolite structures are typically measured for the Al forms of the corresponding structures, but other metals substituted into the framework affect the pore size if the Si/metal ratio is high enough [144]. These changes in pore size have not been quantified, but the differences may significantly affect diffusion. Moreover, chargebalancing cations in the zeolite channels reduce the local pore size, and the extent of this reduction depends on the size and number of cations present. Guan et al. [72] observed that light-gas permeation through ion-exchanged A-type zeolite membranes was highest for CaA, lower for NaA, and lowest for KA membranes, and this is the same order that their pore sizes decrease Kinetic diameters of polar molecules Reported molecular diameters should be used with caution because they assume diffusing molecules are hard spheres. Despite this, kinetic diameters give an approximation of molecular size and allow greater understanding of diffusion trends in zeolites. Most zeolite membrane studies with nonpolar permeating molecules use the characteristic length parameter of the Lennard Jones (6 12) potential as the kinetic diameter for spherical molecules and the diameter of the smallest cylinder that encompasses the van der Waals radii of the atoms as the kinetic diameter for non-spherical molecules [92]. The Lennard Jones potential only accounts for non-polar forces, but dipoles cause additional interac-

13 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Table 3 Maximum adsorption capacities of components used in pervaporation Component Zeolite Amount adsorbed (mmol/g zeolite) Method Water Silicalite Volumetric (vapor) [123] Methanol 4.8 Water Silicalite Gravimetric (liquid) [187] Ethanol 2.8 Water Silicalite Volumetric (vapor) [145] 2-Propanol 2.8 Methanol Silicalite Gravimetric (vapor) [117] Ethanol Propanol Butanol 1.8 Ethanol Silicalite HPLC (aqueous liquid) [157] 1-Propanol Propanol Butanol Butanol Propanol Silicalite Gravimetric (aqueous liquid) [239] 1-Butanol Pentanol 1.1 TCE Silicalite TEOM (vapor) [158] Chloroform 1.4 TCE Dealuminated Y 2.0 Chloroform 2.1 Water NaY 6.2 Chromatographic (vapor) [154] Methanol 1.2 MEK 2.2 Water Ge-ZSM Volumetric (vapor) [145] 2-Propanol 3.0 Water NaZSM Chromatographic (vapor) [154] Methanol 0.6 MEK 1.5 Water NaA 15 Volumetric (vapor) [28] Methanol 6.2 Ethanol 4.0 Water Mordenite 7.8 Gravimetric (vapor) [55] Methanol 4.7 tert-butanol 0.2 Water Mordenite 5.8 Chromatographic (vapor) [154] Methanol 1.6 MEK 1.4 TCE: trichloroethylene; MEK: methyl ethyl ketone; TEOM: tapered element oscillating microbalance; HPLC: high performance liquid chromatography. tions between molecules. For a polar molecule, the characteristic length parameter of the Stockmayer potential function, which accounts for polar interactions, is smaller than that of the Lennard Jones potential. Breck [92] used Stockmayer length parameters for the kinetic diameters of H 2 O and NH 3, which are polar and approximately spherical. Stockmayer length parameters based on phase-coexistence data [167] appear to best represent kinetic diameters for spherical, polar molecules [42]. The kinetic diameters for some molecules commonly used in pervaporation are shown in Table 4, but molecules that are more cylindrical are not shown because the Stockmayer length parameter does not represent the minimum cross-sectional diameter. A good estimate of kinetic diameters for non-spherical polar molecules is lacking Diffusivities Two types of diffusivities self-diffusivities, D s, and transport diffusivities, D t have been measured in zeolites [168]. Techniques such as pulsed-field-gradient nuclear magnetic resonance (PFG NMR) [169], quasi-elastic neutron scattering [170], chromatography [157], transient uptake [117], frequency response [171], and transient permeation [49,98,111] have measured these diffusivities. The first two techniques are equilibrium microscopic methods that mea-

14 14 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 Table 4 Kinetic diameters of planar and nearly spherical polar molecules used in pervaporation Molecule Formula Kinetic diameter (nm) Water H 2 O Methanol CH 4 O Ethanol C 2 H 6 O Acetic acid C 2 H 4 O Acetone C 3 H 6 O Propanol C 3 H 8 O Methyl acetate C 3 H 6 O Trichloromethane CHCl THF C 4 H 8 O Pyridine C 5 H 5 N Butanol C 4 H 10 O MEK C 4 H 8 O Diameters are Stockmayer length parameters based on phase-coexistence data [167]. sure the self-diffusivity, and the others are non-equilibrium macroscopic methods that measure the transport diffusivities. Self-diffusivities have also been calculated using equilibrium molecular dynamics (EMD) simulations [172], and non-equilibrium molecular dynamics (NEMD) simulations have been used to predict transport diffusivities [173]. In general, self-diffusivities from EMD simulations tend to agree satisfactorily with those determined from microscopic methods. Diffusivities measured with macroscopic methods are often two to three orders of magnitude lower [124,174,175]. Diffusion in some systems (e.g., benzene in ZSM-5 [176]), however, exhibits good agreement between macroscopic and microscopic diffusivity measurements. Diffusion in zeolite membranes is an activated process that is modeled by D = D 0 e E D/RT (8) where D is the diffusivity, D 0 a pre-exponential factor, and E D the activation energy of diffusion. In order to diffuse, the molecules must have enough energy to overcome the potential wells at each adsorption site. Activation energy tends to increase as molecular size approaches the pore diameter because molecule wall interactions increase [162]. Choudhary et al. [177] reported activation energies for C 4 C 8 alcohols in ZSM-5 zeolite (H + form, Si/Al = 39.7) that ranged between 16 and 41 kj/mol and they increased with increasing number of carbons. As with adsorption, most diffusion studies have been performed with permanent gases. Thus, diffusivities have not been measured for many molecules used in pervaporation. Diffusivities have been reported for water and alcohols at low loading, and these are shown in Table 5. Note that the different techniques give different diffusivities. The three diffusivities determined for ethanol, for example, have almost a fourfold range. Linear alcohol diffusivities measured using transient uptake of pure vapor decrease as the number of carbons increase [117], and this is the expected trend because the alcohol diameters and heats of adsorption increase in the same order. Methanol, ethanol, and 2-propanol dif- Table 5 Self- and transport diffusivities of water and alcohols at low loading in silicalite-1 zeolite near 298 K Component Self-diffusivity, D s 10 9 (m 2 /s) Method Water 1.7 PFG NMR [169] 3.3 Molecular dynamics [169] 4 NMR [240] MeOH 4 NMR [240] Component Transport diffusivity, Method D t (m 2 /s) MeOH 57.5 Pure vapor uptake [117] EtOH 13.2 Pure vapor uptake [117] 50 Pure liquid uptake [187] 16 HPLC [157] 1-PrOH 5.9 Pure vapor uptake [117] 79 HPLC [157] 2-PrOH 7.9 HPLC [157] 1-BuOH 4.5 Pure vapor uptake [117] 152 HPLC [157] 2-BuOH 4.1 HPLC [157] 2-Me-2-BuOH 1.5 Pure vapor uptake [117] 1-HxOH 0.8 Pure vapor uptake [117] Ref. [117] used ZSM-5 with Si/Al = 990. HPLC: high performance liquid chromatography with aqueous liquids; PFG NMR: pulsed field gradient nuclear magnetic resonance. fusion rates measured with isotopic-transient pervaporation also decreased with increasing carbon number [111]. These trends are consistent with diffusion of linear alkanes; diffusivities in silicalite-1 decrease with increasing alkane chain length, in general [ ]. In contrast, the diffusivities measured using HPLC with alcohol/water liquids increase as the number of carbons increases [157], but the reason for this discrepancy is not clear Coverage dependence Diffusivities of some species in zeolites depend on the coverage, so when these species are present in a coverage gradient across a membrane, transport diffusivities can vary across the thickness of the membrane [181]. Transport diffusivities of many molecules diffusing through zeolites increase with coverage [162,168,182], but transport diffusivities of some molecules that are near the zeolite pore size exhibit a maximum with increasing coverage or are nearly independent of coverage [182,183]. The Darken factor ( ln ˆf / ln q) is commonly used to account for the transport diffusivity coverage dependence, and the corrected diffusivity (D C ) is the proportionality constant relating the transport diffusivity to the Darken factor [49]: ( ) ln ˆf D t (q) = D C (9) ln q T The Darken factor accounts for the fact that the driving force for surface diffusion through zeolites is the chemical po-

15 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) tential gradient rather than the concentration gradient. The Maxwell Stefan (M S) diffusivity (Ð MS ) is equivalent to D C for single-component diffusion. These corrected diffusivities are assumed to be independent of coverage, although several studies indicate that Ð MS and D C depend on coverage. For example, experimental corrected diffusivities of C 2 H 4 and C 2 H 6 in silicalite-1 [184] and n-c 4 H 10 and i-c 4 H 10 in ZSM- 5 [124] increased, and those of D 2 in NaX zeolite [168] exhibited a maximum with increasing coverage. Likewise, simulated corrected diffusivities of CF 4 in silicalite-1 decreased with increasing coverage [183]. The M S approach has been successfully applied to multicomponent diffusion for many mixtures containing molecules with similar saturation capacities [182,183,185]. This approach, however, uses multicomponent Langmuir adsorption isotherms that are not thermodynamically consistent when molecules in a mixture have different saturation capacities [130,185]. For C 2 H 6 /CH 4 and C 3 H 8 /CH 4 mixtures, which have different saturation capacities for each molecule in silicalite-1, Kapteijn et al. [130] showed that the ideal adsorbed solution (IAS) theory with the M S model provided better predictions of permeation through a silicalite-1 membrane than the M S model with multicomponent Langmuir adsorption did. Similarly, Krishna [126] obtained reasonable predictions of n-hexane/2,2-dimethylbutane mixture permeation through a silicalite-1 membrane using the real adsorbed solution (RAS) theory combined with the M S model. Boulicaut et al. [93] reported diffusivities for hexane isomers at saturated loading in ZSM-5 zeolite that were several orders of magnitude larger than those for the same components at low loading. They suggested that the high loading may distort the ZSM-5 zeolite framework and increase the pore diameters slightly. In contrast, diffusivities at high and low coverages in X-type zeolites were approximately the same [186]. Moreover, Farhadpour and Bono [187] reported that water diffusion rates in silicalite crystals decreased with increased loading. Clearly, diffusion at high coverages is less understood than at low coverages Multicomponent diffusion Molecular simulations [188] and Maxwell Stefan (M S) modeling [189] of multicomponent diffusion through zeolite pores indicate that in some mixtures, slower molecules inhibit diffusion of faster molecules. Molecules have difficulty passing each other in the membrane and the slower molecules slow down faster molecules. In contrast, the slower moving molecule is also sped up in some mixtures [96,97]. For a pure species, a molecule that moves forward in a zeolite pore might move forward again or back to its original position if the site remains unoccupied (Fig. 8a). In a mixture, when a slower molecule jumps forward to a new adsorption site, a faster molecule can follow it and prevent it from jumping back to its original location [97], as shown in Fig. 8b. These effects are expected to increase as coverage increases because molecules at high coverages are more constrained Fig. 8. Representation of surface diffusion of a pure component (a) and the same component coadsorbed with a faster diffusing molecule (b) on a zeolite surface. and likely have a greater influence on one another. Inhibition of one molecule in a mixture has been observed using the desorption under reduced pressure method [190] and a tracer-exchange positron emission profiling technique [191]. Moreover, one molecule diffusing slower and the other diffusing faster in a binary mixture than in the corresponding pure components has been measured with PFG NMR [96], which is a microscopic method. We recently reported macroscopic measurements of the times required for species to transport through a Ge-ZSM-5 zeolite membrane during steady-state pervaporation [111]. Diffusion times for each component were measured by introducing isotopically labeled species to the feed as a step-change in concentration and monitoring their permeate concentrations as a function of time with a mass spectrometer. This technique has potential to separate the contributions of diffusion and adsorption during pervaporation, and to directly observe diffusivity changes for one species when other components are present in a mixture. Fig. 9 shows normalized isotope responses in the permeate during methanol, ethanol, and ethanol/methanol mixture pervaporation through the Ge-ZSM-5 membrane at 313 K. For a 5 wt.% methanol/ethanol mixture feed, ethanol diffused slightly faster than pure ethanol, but methanol diffused about 1/4 as fast as pure methanol (Fig. 9a). In addition, methanol in a 5 wt.% ethanol/methanol mixture feed diffused slightly slower than pure methanol, but ethanol in the mixture diffused about 1.6 times faster than pure ethanol (Fig. 9b). Methanol inhibition and ethanol speeding up is consistent with observations and simulations of other mixtures discussed above. In contrast, for acetone/methanol mixture feeds, both components in the mixtures diffused slower than either pure component, but not enough is known about the mixture adsorption isotherms and the diffusion coverage dependencies of these species to explain this behavior [98]. 8. Non-zeolite pores The above discussions on adsorption and diffusion represent the ideal case, where only zeolite pores are present in a zeolite membrane. Polycrystalline zeolite membranes, however, contain transport pathways in intercrystalline regions, or non-zeolite pores (Fig. 10). The synthesis procedure, the type of zeolite, and the calcination conditions affect the number and size of non-zeolite pores. For instance, Hedlund et

16 16 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 Fig. 9. Normalized isotope permeate responses for steady-state pervaporation of (a) 5 wt.% methanol/ethanol and (b) 5 wt.% ethanol/methanol mixtures and each pure component through a Ge-ZSM-5 zeolite membrane at 313 K [111]. The methanol response for the 5 wt.% methanol/ethanol mixture and the methanol and ethanol responses for the 5 wt.% ethanol/methanol mixture were smoothed, and the noise in the original signals was about twice that shown here. al. [39] prepared 0.5 m thick silicalite-1 membranes in a low-dust environment to reduce defects in the zeolite layers. Defects on support surfaces could also cause some of the nonzeolite pores present in zeolite membranes. Several methods have been used to estimate contributions of non-zeolite pores to fluxes through zeolite membranes [39, ]. Jareman et al. [195] used a mass transfer model with porosimetry data to determine non-zeolite pore distributions in MFI zeolite membranes. Molecules in non-zeolite pores have different adsorption and diffusion properties from those in zeolite pores. The differences, however, are difficult to quantify because of irreg- ularities in the shape and size of non-zeolite pores. Usually, only non-zeolite pores that are larger than the zeolite pores are considered, but non-zeolite pores have a size distribution, and pores smaller than the zeolite pores may also affect flux and selectivity [42]. Transport through non-zeolite pores that are larger than zeolite pores has contributions from both surface diffusion and Knudsen diffusion, and might also have viscous flow contributions. Knudsen diffusion requires that the pores are smaller than the mean free path of the diffusing molecules [196], whereas viscous flow requires a pressure gradient across the membrane and sufficient interactions between diffusing molecules that their motions are driven by the pressure gradient [197]. Non-zeolite pores that are larger than zeolite pores are usually detrimental to the membrane selectivity. Some molecules have difficulty passing each other in zeolite pores, and a larger, slower molecule, such as ethanol, can inhibit diffusion of a smaller, faster molecule, such as H 2 O(Fig. 11a). A larger, non-zeolite pore, however, allows the faster molecule to pass the slower molecule (Fig. 11b). In addition, silanol groups in non-zeolite pores are hydrophilic. Both of these non-zeolite pore properties should reduce the organic/h 2 O selectivity of a hydrophobic zeolite membrane. Some studies have shown, however, that non-zeolite pores are selective for some mixtures and pervaporation separations are hardly affected or even positively affected by non-zeolite pores. Okamoto et al. [28] prepared hydrophilic NaA zeolite membranes with H 2 /SF 6 selectivities almost the same as the Knudsen selectivity, indicating significant non-zeolite pores were present. In spite of this, their membranes dehydrated alcohols and other organic compounds with separation factors as high as 18,000 and 16,000 for aqueous 1-propanol and ethanol, respectively, because water preferentially adsorbed in both zeolite and non-zeolite pores. These results suggest that the non-zeolite pores in these membranes were selective for water over organic molecules, and this is shown qualitatively by the sketch in Fig. 11c. Furthermore, Nomura et al. [125] reported that a silicalite-1 membrane had a higher ethanol/water separation factor (17.5) than the separation factor (16.0) of a similar membrane they determined had fewer non-zeolite pores by using an intercrystalline intracrystalline model. This suggests that the non-zeolite pores are selective for ethanol, although the reason for the apparent conflict with data showing that non-zeolite pores contain hydrophilic silanol groups [138, ] is not clear. 9. External transport resistance 9.1. Concentration polarization Fig. 10. Representation of non-zeolite pores in the intercrystalline boundaries of a zeolite membrane layer. Drawing is not to scale, and the intercrystalline gaps are exaggerated. Selective permeation from a feed mixture sometimes causes depletion of the preferentially permeating component near the feed-zeolite interface, creating a feed concentration gradient (Fig. 12), or concentration polarization. This happens more easily during pervaporation than gas separations

17 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Fig. 12. Concentration profiles during pervaporation through a zeolite membrane when concentration polarization and support resistance are negligible (a) and when both are significant (b). The concentrations in the feed, support, and permeate are normalized by the mixture saturated vapor pressure, and the concentrations in the membrane are normalized by the mixture saturated coverage. nent driving force. Concentration polarization therefore reduces selectivity that can be obtained. Zeolite membranes are usually oriented with the zeolite layer toward the feed because concentration polarization is greater if the support is on the feed side. van de Graaf et al. [204] demonstrated that membrane selectivity for ethane/methane was reduced when the membrane orientation was reversed and the feed was on the support side for gas-phase species. Although boundary layers near the membrane surface in flowing feeds exhibit concentration gradients, sufficient feed mixing or circulation minimizes concentration polarization during pervaporation through zeolite membranes [205] Support resistance Fig. 11. Representation of transport of an organic/water mixture through: (a) a hydrophobic zeolite membrane with only zeolite pores; (b) a hydrophobic membrane containing a larger, hydrophilic non-zeolite pore, and (c) a hydrophilic zeolite membrane containing a larger, hydrophilic non-zeolite pore. Solid and open circles represent water and organic molecules, respectively. Drawing is not to scale. because diffusion is slower in liquid feeds, and the depleted species is not replenished easily by diffusion from the bulk. Several studies have shown that concentration polarization limits the flux in some cases and reduces separation factors to as low as 10% of their values with negligible concentration polarization during pervaporation through polymeric [ ] and zeolite-filled polymeric membranes [202,203]. Concentration polarization shifts the adsorption equilibrium at the feed membrane interface and reduces the fractional coverage, and thus the driving force of the preferentially permeating component, as shown in Fig. 12b. Likewise, it increases the non-preferentially permeating compo- Appreciable transport resistance in the support layer leads to concentration gradients on the permeate side, and this is also shown in Fig. 12. Low coverage, corresponding to the bulk permeate pressure, or zero coverage on the membrane support interface is often assumed in modeling permeation through zeolite membranes. In contrast, studies of light hydrocarbons permeating through silicalite-1 membranes on stainless steel supports with a He sweep gas showed that the support resistance was significant [204,206]. Gas permeances with the support facing the feed side were as much as 50% higher than permeances with the support facing the permeate side. Modeling studies that account for support resistance in gas permeation typically assume molecules transport in the support by molecular diffusion [124,207]. This is a reasonable assumption for gas permeation because permeate molecules diffuse through a stagnant gas layer in the support. Diffusion at low pressures, however, is significantly faster because molecules travel farther between collisions. The mean free path length of a molecule is calculated from λ = k BT 2πσ 2 ii p (10)

18 18 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 where k B is the Boltzmann s constant, σ ii the collision diameter of the diffusing species in the gas phase, and p the pressure. Knudsen diffusion occurs when the mean free path of a molecule is larger than the diameter of the pore it is diffusing through and the molecule collides with the pore walls more often than it collides with other molecules. As an example, the mean free path of MEK (0.50 nm kinetic diameter, Table 2) at 500 Pa and 298 K would be approximately 7.4 m. This is larger than the pore diameters of stainless steel (0.5 4 m) and Al 2 O 3 (0.2 1 m) supports usually used for zeolite membranes even though the pressure and kinetic diameter are on the high end for pervaporation. Mean free path increases with decreasing kinetic diameter and pressure, and increasing temperature, and for H 2 O at 200 Pa and 298 K, it is around 53 m. The Knudsen diffusivity is given by D Kn = d 2kB T (11) 3 M where d is the pore diameter and M the molecular mass (molecular weight divided by Avogadro s number) [196]. Pressure drop for Knudsen diffusion through the support is then estimated using J = εd Kn L S RT (P m P p ) (12) where J is the flux through the membrane, ε the support porosity, P m the pressure at the membrane support interface, P p the permeate pressure, and L S the effective thickness of the support. Viscous flow also takes place in pores with diameters large enough to allow appreciable intermolecular collisions when a pressure gradient is present [197]. Pressure drop for viscous flow in the support is estimated using Darcy s law of flow through porous media: J = εd2 (Pm 2 P2 p ) (13) 32RTµL S where µ is the gas viscosity in the permeate. The membrane support interface pressures shown in Fig. 13 were calculated using Eqs. (12) and (13) for wa- ter transport through a membrane at 333 K with a 0.5 kpa permeate pressure, a 0.27 support porosity, and a 3 mm effective support thickness. For the fluxes shown, more than 90% of the flow through the 4 m pore diameter support was Knudsen diffusion. The Knudsen diffusion contribution increased to 97% of the flow through the 0.2 m pore diameter support. For a water flux of 55 mol/m 2 h (1.0 kg/m 2 h) through a support with 4 m diameter pores, the pressure at the membrane support interface is approximately 1 kpa. This pressure is not expected to significantly reduce the coverage gradient in the membrane for most pervaporation mixtures, and support resistance during pervaporation can probably be neglected when a large-pore (>4 m diameter) support is used and the flux is around 50 mol/m 2 h and below. Fig. 13 shows that support resistance increases significantly if supports with smaller pore diameters are used. The higher membrane support interface pressures correspond to increased permeate side coverages and a lower pervaporation driving force. de Bruijn et al. [208] showed that when the support pore diameters were around 0.5 m and smaller, and the fluxes were higher than 80 mol/m 2 h, the support resistance was significant and sometimes limiting for pervaporation separations reported in the literature. 10. Pure component pervaporation Pure component pervaporation mainly serves as a characterization of a membrane s quality. Molecules that are smaller than the zeolite pores have pervaporation fluxes that differ by orders-of-magnitude, as shown in Fig. 14. Single-component pervaporation fluxes decrease with increasing kinetic diameter, in general, and for a membrane with few non-zeolite pores, fluxes decrease dramatically when molecular size gets larger than the zeolite pores. Appreciable fluxes of molecules that are significantly larger than zeolite pores indicate flow Fig. 13. Pressure at the membrane support interface vs. water flux through porous supports with 4, 1, 0.5, and 0.2 m diameter pores. Fig. 14. Single component pervaporation fluxes vs. kinetic diameter for A-type [24], -type [64], and Ge-ZSM-5 [146] zeolite membranes at K. Temperature was constant for each membrane and the largest XRD pore dimension for each zeolite is given in parentheses. The permeating components are designated in the figure, where DMB is 2,2-dimethylbutane and TIPB is triisopropyl benzene.

19 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) through non-zeolite pores. The 2-propanol flux through a Mitsui A-type zeolite membrane [24] was higher than the ethanol flux through the same membrane (Fig. 14) even though 2- propanol is larger than ethanol and is not expected to fit in the zeolite pores. Other studies [28,38] have reported that these membranes have approximately Knudsen selectivities for light gases, indicating a large portion of the flow was through non-zeolite pores. The single-component fluxes through the other membranes shown in Fig. 14 exemplify the behavior of membranes with relatively few non-zeolite pores. Pure component pervaporation at elevated temperatures is probably better for membrane characterization than low temperatures because high temperatures are a harsher test of membrane performance [103]. For large-pore zeolite membranes, including those with beta-type zeolites, single-component pervaporation of larger molecules than those used in vapor-phase separations may be the best permeation method for characterization [105]. The coverage at the feed side of a membrane is near saturation during pervaporation of most components (see Section 6), but whether the feed interface is saturated is not as clear for molecules with low fugacities. Li et al. [146] measured pure acetone, 2,2-dimethylbutane (DMB), m-xylene, and triisopropylbenzene (TIPB) pervaporation fluxes through Ge- ZSM-5 membranes. If feed-side coverages of low-fugacity molecules (DMB, m-xylene, and TIPB) are below saturation, temperature might be expected to have a greater influence on their fluxes than on fluxes of a molecule with higher fugacity (acetone) because fugacities increase with temperature. In contrast, the fluxes for all the components increased by about the same factor (1.5 2 times) when the temperature increased from 303 to 323 K, even though the fugacities of DMB, m- xylene, and TIPB were much lower than that of acetone. In addition, Yang and Rees [209] reported maximum coverages for decane, undecane, dodecane, and hexadecane adsorbed on silicalite-1 that were approximately the same as the theoretical pore volume of silicalite-1, indicating that the coverages were near saturation. The molecules were adsorbed from the vapor phase, but dodecane and hexadecane were heated to 383 K because of their low fugacities. These results, however, do not conclusively show that the feed-side coverages of low-fugacity compounds are near saturation during purecomponent pervaporation. If a low fugacity causes only partial coverage in the zeolite and non-zeolite pores at the feed side of the membrane, the driving force for pervaporation would be lower than if the feed side were saturated. This possibility leads to some doubt in the validity of comparing pure component fluxes directly if the coverage gradients are not known. Measuring adsorption isotherms of low fugacity molecules that fit into zeolite pores would lead to a better understanding of how large the coverage gradients are for low fugacity molecules during pervaporation. Fig. 15 shows that fluxes increased with temperature for pure alcohol and water pervaporation through a ZSM-11 zeolite membrane [210]. This temperature dependence is typical for pervaporation through most zeolite membranes, but dif- Fig. 15. Alcohol and water fluxes through a ZSM-11 zeolite membrane vs. inverse temperature [210]. ferent from the temperature dependence of most gas permeation. Bakker et al. [22] showed that gas permeance exhibits a maximum as temperature increases. Initially, the diffusivity increases with temperature faster than coverage decreases and permeance increases (region 1). Eventually, the coverage decreases faster than the diffusivity increases and permeance decreases (region 2). Pervaporation fluxes are in region 1 because fugacities increase with temperature, and coverages remain high at increased temperature. Thus, diffusion rates increase more than adsorption decreases with increasing temperature, and on a log scale, the fluxes are nearly linear with inverse temperature because diffusion is activated. 11. Separations Organic dehydration Type-A zeolite membranes are nearly ideally suited for organic dehydration because they are highly hydrophilic and their XRD pore diameter (0.4 nm) is smaller than almost all organic molecules but larger than water. The A- type membranes that have been prepared also have nonzeolite pores [28,38] that contain hydrophilic silanol groups [138, ]. These properties allow preferential permeation of water over organic compounds with separation factors that are often over 1000 and sometimes higher than 10,000. These high separation factors are sensitive to permeate concentration because the water concentrations are often higher than 98% and the separation factor is a ratio of ratios. Table 6 shows that several studies have achieved these high separation factors with high fluxes for ethanol dehydration using NaA zeolite membranes. Okamoto and co-workers [28,38,91,211] and van den Berg et al. [212] have reported the most successful zeolite membranes for water removal from organic compounds. Fluxes during ethanol dehydration tend to increase as the water concentration in the feed increases. In contrast, permeate concentrations are relatively independent of feed concentration except near 0% H 2 O. Jafar et al.

20 20 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 Table 6 Ethanol dehydration using NaA zeolite membranes Membrane thickness ( m) Support Water in feed (wt.%) Temperature (K) Flux (mol/m 2 h) α Reference 7 Al 2 O [70] 5 SS [109] NA Zirconia/carbon [213] NA Mullite >10000 [241] 10 Mullite/Al 2 O [91] NA Mullite [211] 30 Al 2 O [28] TiO [212] SS: stainless steel; NA: not available. [213] reported that H 2 O/EtOH separation factors increased as the water concentration approached zero using a NaA zeolite membrane on a ZrO 2 TiO 2 coated porous carbon tube. On the other hand, Kondo et al. [91] reported that H 2 O/EtOH separation factors dramatically decreased as the H 2 O feed concentration neared zero for a NaA zeolite membrane on a mullite/alumina/cristobalite porous support. These studies also both found that increasing the temperature increased the flux with little effect on the separation factor. Water has also been removed from ethanol with ZSM-5, mordenite, and X-, Y-, and T-type zeolite membranes, but fluxes and separation factors (Table 7) are generally lower than when using A-type zeolite membranes. This is because the pore diameters are larger for these membranes than A- type membranes so they are less size selective for water, and because their Si/Al ratios are higher so they are less hydrophilic than A-type membranes. Zeolite T membranes (OFF structure, 0.68 nm XRD pore diameter, Si/Al = 3.6) prepared by Tanaka et al. [68] and the mordenite membrane prepared by Zhang et al. [214] had higher separation factors than the ZSM-5 and X- and Y-type zeolite membranes did, but their fluxes and separation factors were, respectively, about 1/3 and 1/9 of those using the best A-type membranes [91]. The X- and Y-type zeolite structures have a three-dimensional pore system of 12-member rings (MR), whereas the T-type and mordenite zeolite structures have 12-MR pores in only one direction, but have smaller 8-member ring pores in parallel with and perpendicular to their 12-MR pores. Because the membranes were randomly oriented, the 8-MR pores in the T-type and mordenite membranes might have provided Table 7 Ethanol dehydration using 10 wt.% H 2 O/ethanol mixtures Membrane Thickness ( m) Support Temperature (K) Flux (mol/m 2 h) α Reference ZSM-5 NA Al 2 O [242] NaX 20 NA NaY 20 NA X-type NA NA [68] Y-type NA NA T-type NA -Al 2 O Mordenite 30 -Al 2 O [214] NA: not available.

21 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Table 8 Organic dehydration using mixtures near 10 wt.% water Membrane Thickness ( m) Support Organic Water in feed (wt.%) Temperature (K) Flux (mol/m 2 h) α Reference NaA 5 10 SS t-buoh [234] NaA NA Zirconia 2-PrOH [82] ZSM Al 2 O 3 2-PrOH [243] Mordenite 3 -Al 2 O 3 2-PrOH Y-type NA Al 2 O 3 THF [100] Mordenite NA Al 2 O ZSM-5 NA Al 2 O A-type NA Al 2 O Mordenite 35 -Al 2 O 3 1-PrOH [244] Mordenite 35 -Al 2 O 3 2-PrOH ZSM-5 60 Al 2 O 3 2-PrOH [46] NaA 30 Al 2 O 3 MeOH [28] EtOH PrOH PrOH Acetone Dioxane DMF NaA 5 10 Ceramic THF [235] Mordenite 30 -Al 2 O 3 MeOH [214] SS: stainless steel; DMF: dimethyl formamide; THF: tetrahydrofuran; NA: not available. alternate pathways for H 2 O that ethanol could not diffuse through. Thus, these 8-MR pores might explain why the T- type and mordenite membrane H 2 O/ethanol separation factors were higher than those of the X- and Y-type membranes. Several other organic compounds have been dehydrated by pervaporation through zeolite membranes with similar success (Table 8). Okamoto et al. [28] measured high separation factors and fluxes using the same NaA zeolite membrane to dehydrate methanol, ethanol, 1-propanol, 2-propanol, acetone, dioxane, and dimethyl formamide. In general, zeolite membranes with lower Si/Al ratios have higher separation factors for organic dehydration. Organic dehydration has also been investigated in a zeolite membrane pervaporation reactor, which combines liquidphase chemical reaction and pervaporation in a single process. The removal of one or more product species from a reversible reaction system shifts the equilibrium towards the products, and thus increases the conversion. The concept of using pervaporation membrane reactors was proposed in an early patent [215]. Tanaka et al. [68] removed water generated by the liquid-phase ethanol/acetic acid esterification reaction with pervaporation through the T-type zeolite membrane mentioned above. The membrane (38 cm 2 surface area) was in a batch reactor containing ethanol and acetic acid (molar ratio = 2) and a slurry of cation exchange resin used as catalyst. The esterification was carried out at 343 K, and water and ethyl acetate were produced. Without pervaporation, the reaction reached equilibrium conversion (80%) after about 2 h. Pervaporation removed water and shifted the reaction equilibrium, and the conversion reached 98% after 7 h. Similarly, Aiouache and Goto [216] used a Mitsui NaA zeolite membrane to remove water during etherification of tert-amyl alcohol with ethanol. The tert-amyl ethyl ether yield increased from 31 to 49% when the membrane was used. Etherification was carried out in a reactive distillation column that contained the membrane (980 cm 2 surface area) and a cation exchange resin used as catalyst Removal of organic compounds from water Highly hydrophobic zeolite membranes, such as silicalite- 1 [132], Ge-ZSM-5 [145], and -type [105] have separated various organic compounds from water. The highest fluxes for separating organic compounds from water, however, still remain lower than the highest fluxes for dehydrations with hydrophilic membranes, and the separation factors are one to three orders of magnitude lower than those for dehydrations. These differences are seen when comparing Table 9 to Tables 6 and 7. Diffusion favors water permeation because organic molecules used in pervaporation are larger than water molecules. Hydrophobic zeolites overcome this drawback by preferentially adsorbing organic compounds, but nonzeolite pores and structural defects in the membranes may be a more significant disadvantage to hydrophobic membranes than hydrophilic ones because of silanol groups on the zeolite surface. Takaba et al. [147] used duel-ensemble Monte Carlo simulations performed with Lennard Jones interatomic potentials to model separation of ethanol from water by pervaporation through a silicalite-1 single crystal. For a binary

22 22 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 Table 9 Ethanol removal from water Membrane Thickness ( m) Support EtOH in feed (wt.%) Temperature (K) Flux (mol/m 2 h) α Reference B-ZSM Al 2 O 3 /SiC monolith [42] Ge-ZSM-5 30 SS [160] Silicalite SS [229] ZSM-5 NA -Al 2 O [242] ZSM Al 2 O [210] B-ZSM SS [210] Silicalite-1 10 Mullite [48] Silicalite-1 NA -Al 2 O [218] NA -Al 2 O NA -Al 2 O NA -Al 2 O Silicalite Mullite [87] Silicalite-1 NA -Al 2 O [133] Silicalite-1 (silicone coated) NA SS [217] Silicalite-1 (before coating) NA SS [217] Silicalite-1 60 SS [245] Silicalite SS [26] Silicalite-1 (silane modified) NA SS [76] Silicalite-1 (before silane) NA SS [76] Silicalite SS [132] Silicalite SS [43] B-ZSM-5 NA SS [205] SS: stainless steel; NA: not available. system, ethanol permeated through the zeolite pores, but no water penetrated into the zeolite pores. Water, however, adsorbed on silanol groups along the external surface of the zeolite crystal. These results suggest that structural defects and non-zeolite pores in hydrophobic polycrystalline zeolite membranes increase water transport, and further improvements in organic separations from water may be possible. Table 9 shows results for ethanol removal from ethanol/water mixtures with various hydrophobic membranes. A silicalite-1 membrane reported by Lin et al. [87] is the best membrane reported so far for this separation. Their membrane had a 14 mol/m 2 h flux and an ethanol/h 2 O separation factor of 106 for a 5 wt.% ethanol/h 2 O feed at 333 K. This membrane was prepared by in situ crystallization on both the inside and outside surfaces of a mullite tube. The zeolite layer was approximately 20 m thick, and the separations were highly reproducible. They concluded that better membranes were produced by in situ crystallization than by seeding followed by crystallization, and randomly oriented membranes had higher separation factors than oriented membranes. Matsuda et al. [217] used a silicone-coated silicalite-1 membrane on a stainless steel support to obtain a higher separation factor (125), but a significantly lower flux (3.7 mol/m 2 h) for a 4 wt.% ethanol/h 2 O feed at 303 K. The silicone was added to fill the non-zeolite pores and increase the membrane hydrophobicity. Sano et al. [76] also

23 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) Fig. 16. Organic/water separation factors vs. organic feed fugacity for 5 wt.% organic/water binary mixture pervaporation through a Ge-ZSM-5 zeolite membrane at 303 K. The number of carbons for the organic compound in the feed is indicated and the line is a least squares fit. Reprinted from Bowen et al. [160]. Copyright (2003), with permission from Elsevier. increased the hydrophobicity of silicalite-1 zeolite using a silane coupling reagent. The amount of water adsorbed on silicalite-1 powder decreased after modifying it with octyltrichlorosilane. Modifying a silicalite-1 membrane with the same silane improved ethanol/h 2 O separation factors by up to fourfold, and the flux and separation factor after modification were 22 mol/m 2 h and 44, respectively, at 323 K. This modification may have also filled non-zeolite pores because the ethanol flux before modification was approximately twice that after modification. Separation factors increase with decreasing ethanol concentrations for most membranes. Li et al. [210] reported a 34 mol/m 2 h flux and a separation factor of 96 for a 1 wt.% ethanol/h 2 O feed using a B-ZSM-11 membrane at 333 K, even though the flux and separation factor were only 30 mol/m 2 h and 42, respectively, for a 5 wt.% ethanol/h 2 O feed at the same temperature. This trend with ethanol concentration agrees with the trends seen by Nomura et al. [26] and Sano et al. [132]. In general, fluxes for ethanol separations from water increase significantly with increasing temperature, but with the sacrifice of slightly decreasing separation factors. The dependence on temperature is most evident in the data of Lin et al. [218] shown in Table 9. Many other organic components have also been separated from water. Organic/H 2 O separation factors of organic compounds with different functional groups differ by more than two orders of magnitude because their fugacities are different, and this affects adsorption as discussed in Section 6.3. For 5 wt.% organic/ H 2 O mixtures permeating through a Ge- ZSM-5 zeolite membrane [160], the organic/h 2 O separation factor for diethyl ether was higher than that for ethanol, which was higher than that for acetic acid, and this is the same order that their organic fugacities increase (Fig. 16). Fugacities of organic compounds with the same functional group are similar. Organic/H 2 O separation factors for alcohols tend to increase as carbon number increases for methanol, ethanol, and 1-propanol, but then decrease for 2-propanol and 1-butanol, Fig. 17. Fluxes and alcohol/h 2 O separation factors for 1 5% alcohol/water pervaporation through silicalite-1 [132], Ge-ZSM-5 [146], and B-ZSM-5 [42] zeolite membranes at 303 K. and this is shown in Fig. 17. Adsorption selectivity for linear alcohols/h 2 O increases as the alcohol carbon number increases because the alcohol adsorption strengths increase in this order [160]. The alcohol diffusion rates, however, decrease with increasing carbon number because of increasing size and adsorption strength. Branching also increases size and decreases diffusion rate compared to linear alcohols. Water diffuses faster than alcohols with two or more carbons. As alcohol size increases, the water/alcohol diffusion selectivity eventually becomes larger than the alcohol/water adsorption selectivity, and the separation factor decreases. For the MFI zeolite structure, this change appears to take place approximately at 1-propanol because 2-propanol and 1-butanol have lower separation factors. Multi-channel monolith membranes, which have larger surface area to volume ratios than tubular membranes have also been used for pervaporation [42]. The B-ZSM-5 zeolite membranes on monolith supports effectively removed alcohols and acetone from 5 wt.% organic/water binary feeds over a temperature range of K. The organic/water separation factors for methanol, ethanol, 2-propanol, 1-propanol, and acetone at 303 K were 8.5, 25, 43, 85, and 330, respectively. With the exception of methanol, these separation factors were significantly higher than those reported for a B-ZSM-5 tubular membrane [205], and the methanol and ethanol fluxes were comparable to those through the tubular membrane. The separation factors and fluxes were lower than those of the best silicalite-1 membranes [87], but these B-ZSM-5 membranes were the first reported pervaporation separations using a zeolite membrane on a multi-channel monolith support, and improvements are expected Organic/organic separations Some non-aqueous organic mixtures also exhibit adsorption and diffusion differences, allowing separations. Table 10 shows that methanol/mtbe and methanol/benzene separations have been the most successful organic/organic separa-

24 24 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 tions with zeolite membranes. Methanol (1.7-debye dipole moment) is more polar than MTBE (1.4 debye) and benzene, which does not have a permanent dipole. Thus, NaY and NaX zeolite membranes that have high aluminum contents separate these mixtures better than a silicalite-1 membrane because of their localized electrostatic poles. Kita et al. [141] showed that methanol/mtbe total fluxes through a NaY zeolite membrane at 323 K were nearly independent of feed concentration, whereas the separation factor decreased from 10,000 to 800 as the methanol feed concentration increased from 5 to 75%. Hexane isomers and benzene/p-xylene mixtures have also been separated by pervaporation through zeolite membranes. Matsufuji et al. [219] observed separation factors as high as 270 for n-hexane/2,3-dmb separations using a ZSM-5 membrane, but the fluxes were about two orders of magnitude lower than those obtained by Flanders et al. [220] for the n-hexane/2,2-dmb separations shown in Table 10. The membrane prepared by Matsufuji et al. was on an - Al 2 O 3 support, and the continuous zeolite layer was in the support pores instead of on the surface of the support. In addition, the TIPB flux was less than the detection limit ( mol/m 2 h). Thus, the low n-hexane/2,3-dmb flux was attributed to flow only through zeolite pores and reduced effective permeable area because of the porosity of the support (0.4). The membrane prepared by Flanders et al. was a ZSM-5 zeolite layer on a stainless steel support. Sommer et al. [221] obtained good fits for the hexane isomer singlecomponent and mixture fluxes and selectivities through this membrane. The data were fit using adsorption isotherms and diffusivities from the literature, a Maxwell Stefan diffusion model for flow through zeolite pores, and a Knudsen diffusion model for flow through non-zeolite pores. They determined that 2,2-DMB transported almost exclusively by activated Knudsen diffusion through non-zeolite pores. Nishiyama et al. [66,222] used a method similar to that of Matsufuji et al. to prepare mordenite and ferrierite zeolite membranes that had separation factors for benzene/p-xylene mixtures higher than 100 (Table 10) [66,222]. The fluxes, however, were less than 1 mol/m 2 h. These membranes also had their continuous zeolite layers inside the support pores and TIPB fluxes below the detection limit, and this may explain their low fluxes. Furthermore, the FER membrane had a lower flux than the MOR membrane, and this was probably because it has smaller zeolite pores. Although zeolite membranes have been used to separate vapor mixtures of xylene isomers [39,40,101,164] with p- /o-xylene separation factors as high as 500 in a silicalite-1 membrane, [40] few pervaporation studies of xylene isomer separations have been reported for zeolite membranes. For p- /m-xylene and p-/o-xylene mixtures, Matsufuji et al. [80,223] and Nishiyama et al. [63] observed that p-xylene preferentially permeated with pervaporation separation factors between 2 and 16, but the fluxes were below 0.01 mol/m 2 h. Wegner et al. [47] obtained separation factors that were approximately 1.0. Recently, Yuan et al. [224] used a templatefree synthesis method to prepare a silicalite-1 membrane and obtained a p-/o-xylene separation factor of 40 at 323 K; the total flux was 1.3 mol/m 2 h Acid separations A few studies have investigated pervaporation with acidic feeds, which are industrially interesting because finding a suitable stable material for these separations is often difficult, and some aqueous acids have azeotropes. Hydrophilic zeolites, in general, are not stable in low ph environments because acid leaches Al from the framework. In addition to steaming [225], acid leaching is commonly used to dealuminate zeolite structures [226]. Zeolite membranes used for low ph pervaporation, therefore, need to have relatively high Si/Al ratios so that the framework is not destroyed when Al is removed. Stainless steel supports are usually used for these applications because Al 2 O 3 supports are susceptible to degradation by acids. A Ge-ZSM-5 membrane removed acetic acid from a 5 wt.% acetic acid/h 2 O mixture at 363 K with α =14 Table 10 Organic/organic mixture separations Membrane Thickness ( m) Support Feed (A/B) a wt.% A Temperature (K) Flux (mol/m 2 h) α Reference NaY NA MeOH/MTBE [140] MeOH/benzene MeOH/DMC EtOH/benzene EtOH/cyclohexane EtOH/ETBE NaX NA MeOH/MTBE [140] Silicalite SS MeOH/MTBE [246] ZSM-5 NA SS n-hexane/2,2-dmb [220] Mordenite 10 Al 2 O 3 Benzene/p-xylene [66] Ferrierite 10 Al 2 O 3 Benzene/p-xylene [222] Silicalite Al 2 O 3 p-xylene/o-xylene [224] SS: stainless steel; MTBE: methyl-tert-butylether; DMC: dimethylcyclohexane; ETBE: ethyl-tert-butylether; DMB: dimethylbutane; NA: not available. a Component A is selectively removed.

25 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) and a 16.8 mol/m 2 h flux [145]. The Ge-ZSM-5 membrane had higher separation factors than silicalite-1 membranes in the same study. Sano et al. [227] reported acetic acid/h 2 O separation factors as high as 2.7, but the fluxes were approximately five times higher than those through the Ge-ZSM-5 membrane. Sano et al. also showed that the permeate concentration during pervaporation of 15 vol.% acetic acid through a silicalite-1 membrane remained nearly unchanged after almost 1 month of operation. Li et al. [228] used a more hydrophilic ZSM-5 zeolite membrane (Si/Al = 50) to dehydrate 50 wt.% H 2 O/acetic acid mixtures. To further increase hydrophilicity, the membrane was treated with an alkali solution to extract Si from the zeolite framework to create structure defects containing silanol groups. After this treatment, the flux and separation factor were 44 mol/m 2 h and 380, respectively. We recently reported SSZ-13 zeolite membranes (CHA structure, Si/Al = 14) that dehydrated HNO 3, which is a significantly stronger acid than acetic acid, and exhibits an azeotrope at approximately 69 wt.% HNO 3 [59]. The H 2 O/HNO 3 separation factors were approximately 3.3 using a 69 wt.% HNO 3 feed at 298 K, and the fluxes were about 2.8 mol/m 2 h. About 5% of the initial Al was removed from SSZ-13 powder during 3 days of exposure to 69 wt.% HNO 3, but a membrane continued to separate with a flux of 8.5 mol/m 2 h and α = 2.6, even after 13 days of pervaporation with approximately 69 wt.% HNO 3 feed Multicomponent separations Most laboratory pervaporation studies have investigated unary and binary pervaporation. Industrial conditions, however, most often require separations of multicomponent feeds, and these are sometimes more difficult to separate than binary mixtures. Even at low concentrations, other components in the feed can significantly affect pervaporation by blocking membrane pores, or they may distort the zeolite pores [93,94, ], as discussed in Section 7.1. A few studies have reported successful separations of ethanol [113,227, ] and acetic acid [227] from fermentation broths using silicalite-1 membranes with separation factors as high as or higher than the separations of the corresponding binary mixtures. Ethanol produced in fermentation broths must be concentrated before it is used as a commodity chemical or an alternative fuel. Moreover, removing ethanol during fermentation leads to a higher conversion of sugar to ethanol. Nomura et al. [113] showed that salts present in the fermentation broths were responsible for higher ethanol/water separation factors for the broths than for ethanol/water binary mixtures. They speculated that accumulation of salt in the membrane increased the ethanol/water fugacity ratio. Broth fluxes were about 0.5 kg/m 2 h initially, but then decreased to less than 0.1 kg/m 2 h after 48 h. These fluxes were comparable to the fluxes in the binary separation studies. Likewise, Ikegami et al. [230] used a silicalite-1 membrane to show that ethanol/water separation factors increased, but fluxes decreased when glucose or lactose were present in the ethanol/water feed. Sano et al. [227] also used a silicalite- 1 membrane to remove acetic acid from vinegar and reported fluxes around 0.02 kg/m 2 h. Mass fluxes are given for multicomponent mixtures instead of molar fluxes because the permeate concentrations of all components are often not given, and thus the molar fluxes could not be calculated. Li and co-workers [58,114,142,232] used silicalite-1, ZSM-5, ZSM-11, mordenite, and X-, Y-, and -type zeolite membranes, to remove 1,3-propanediol from glycerol and glucose in aqueous mixtures. Binary, ternary, and quaternary mixtures, and a cell-free fermentation broth were investigated. The 1,3-propanediol/glycerol separations were mainly controlled by preferential adsorption of 1,3- propanediol, whereas the 1,3-propanediol/glucose separations were mainly controlled by differences in diffusion rates. Although larger pore membranes had larger fluxes for these mixtures, zeolite pore size had a small effect on selectivity. An X-type zeolite membrane was stable during pervaporation of a model quaternary mixture for at least 1 week, and the total flux was 2.1 kg/m 2 h at 308 K [114]. Inaddition, their membranes separated 1,3-propanediol from a fermentation broth that also contained water, glycerol, glucose, and at least 12 other species [114]. They measured 1,3-propanediol/glycerol separation factors up to approximately 100 and 1,3-propanediol/glucose separation factors up to approximately 2000 at 328 K. Separation factors for the broth were comparable to those for the quaternary solution at 308 K, but the fluxes were reduced to about 60% of the quaternary solution flux by the presence of other components in the broth. The total flux of the fermentation broth at 308 K was 1.2 kg/m 2 h through the X-type zeolite membrane. Components in the broth began to deposit on the membrane surface and foul the membrane after about 60 h, and the 1,3-propanediol permeate concentration decreased to 38% of the original concentration in the next 15 h. The total flux also decreased by about 35% because of the fouling, but calcination regenerated the membrane Comparison to vapor permeation Some studies have shown that, for a given zeolite membrane, fluxes and selectivities are the same for pervaporation and vapor permeation if the feed fugacities are the same [26,27]. Vapor permeation, however, is usually not carried out with the feed at the saturated vapor pressure. Pervaporation fluxes are usually significantly higher than vapor permeation fluxes at the same temperature because of higher feed-side coverages during pervaporation. Fluxes are divided by feed fugacities to obtain permeances. Permeances are sometimes lower during pervaporation than during vapor permeation because fugacities may be significantly higher during pervaporation, but the feed-side coverage is limited by the saturation

26 26 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) 1 33 perature dependence during pervaporation was different from that during vapor permeation. The membrane was more selective for n-hexane as temperature increased during pervaporation, perhaps because n-hexane s fugacity increases more than 2,2-DMB s fugacity with increasing temperature, resulting in higher competitive adsorption for n-hexane. In contrast, over the same temperature range, the vapor permeation selectivities were higher and relatively constant, but the membrane was less selective for n-hexane at higher temperatures, due to increasing 2,2-DMB permeation. Kita et al. [140] reported similar temperature dependencies for methanol/mtbe separation factors during pervaporation and vapor permeation through a NaY zeolite membrane. Fig. 18. Vapor permeation and pervaporation permeances of n-hexane and 2,2-dimethylbutane (50/50 feed) through a ZSM-5 zeolite membrane vs. temperature. Reprinted from Flanders et al. [220]. Copyright (2000), with permission from Elsevier. capacity, and thus the coverage does not increase linearly with fugacity. Flanders et al. [220] measured vapor permeation and pervaporation permeances of 50/50 mixtures of n-hexane and 2,2-dimethylbutane (DMB) through a ZSM-5 membrane (Fig. 18). During vapor permeation, the hexane isomer feed partial pressures were approximately 4 kpa each, and the remainder of the flow was He. Helium was also used as a sweep gas on the permeate side, and the feed and permeate total pressures were 85 kpa. At the same temperatures, the n-hexane and 2,2-DMB pervaporation permeances were about 1/10 and 10 times those of the same components during vapor permeation, respectively, even though the pervaporation fluxes of n- hexane and 2,2-DMB were 10 and 100 times higher than those during vapor permeation, respectively. The increase in 2,2- DMB vapor permeance above 400 K was explained by either zeolite pores expanding or non-zeolite pores opening at elevated temperatures. Fig. 19 indicates that the separation tem Commercial pervaporation Although progress in improving separations suggests that zeolites may have further uses in large-scale pervaporation, the only current large-scale commercial use of zeolite membranes we are aware of is in organic dehydration. Mitsui Engineering & Shipbuilding Co. in Japan has implemented A-type zeolite membranes for this application. These zeolite membranes have higher separation factors and significantly higher fluxes than polymeric membranes used for dehydration in other commercial pervaporation applications. The Mitsui Engineering & Shipbuilding Co. zeolite membrane pervaporation plant uses m thick NaA zeolite membranes on porous, tubular ceramic supports, and processes alcohols up to 530 L/h with separation factors as high as 10,000, bringing them from 10 wt.% water to 0.2 wt.% water [38]. Fig. 20 shows banks of membranes in a shell and tube geometry used in this plant. Mitsui Engineering & Shipbuilding Co. has also supplied laboratory-scale zeolite membranes for some studies [24]. Smart Chemical Co., Ltd. in the UK commercialized zeolite membranes for large-scale dehydration by pervaporation [233] and supplied laboratory-scale membranes [234,235]. Artisan Industries Inc., USA [106] and Christison Scientific, UK [30] have also been mentioned as zeolite membrane suppliers. A few other companies are currently working to commercialize zeolite membranes for small-scale applications. 12. Conclusions and recommendations Fig. 19. Permeate composition vs. temperature during pervaporation and vapor permeation of n-hexane and 2,2-dimethylbutane (50/50 feed) through a ZSM-5 zeolite membrane. Reprinted from Flanders et al. [220]. Copyright (2000), with permission from Elsevier. Pervaporation has advantages for separating azeotropes, close-boiling mixtures, and thermally sensitive compounds, but only for removing the species present in low concentration because heat transfer becomes important if large quantities are removed. Zeolite membranes have additional advantages in separating mixtures with molecular size differences and/or adsorption differences, and high or low ph mixtures, many of which also form azeotropes. Separating mixtures by pervaporation through zeolite membranes depends strongly on competitive adsorption, and also on

27 T.C. Bowen et al. / Journal of Membrane Science 245 (2004) water. Simulations of ethanol/water pervaporation suggest that perfect, single-crystal silicalite-1 membranes could have significantly higher separation factors. Furthermore, fluxes during removal of organic compounds from water and organic/organic separations are currently too low to be industrially practical. Recent advances have allowed researchers to produce zeolite membranes as thin as 0.5 m with high selectivities [39,40]. These thin membranes may dramatically increase pervaporation fluxes, although their use in pervaporation has not yet been reported. Advances in the following areas have potential to improve understanding and effectiveness of pervaporation through zeolite membranes: Fig. 20. Zeolite membrane pervaporation module layout for a Mitsui Engineering & Shipbuilding Co. large-scale solvent dehydration plant. Reprinted from Ref. [38]. Copyright (2001), with permission from Elsevier. diffusion differences. Separation factors during pervaporation correlate with feed fugacity ratios [160], suggesting that competitive adsorption depends on these ratios. Several methods have been used to measure adsorption and diffusion properties of molecules in zeolite crystals, but adsorption and diffusion effects are indistinguishable during steady-state permeation through zeolite membranes. Isotopic-transient permeation, however, has potential to separate the adsorption and diffusion steps and improve understanding of pervaporation transport [111]. Molecule molecule and molecule zeolite interactions can lead to dramatic diffusion differences between high- and low-coverage conditions. Multicomponent adsorption and diffusion are also often significantly different from single component adsorption and diffusion due to molecule molecule interactions. Zeolite membranes have been more successful in dehydrating organic compounds than in removing organic compounds from water, and this is mainly due to the high hydrophilicity of A-type zeolite membranes. Silanol groups present in structural defects and intercrystalline boundaries decrease the hydrophobicity of zeolite membranes currently used for removing organic compounds from Investigate pervaporation performance of thin membranes. Increase zeolite membrane hydrophobicity for removal of organic compounds from water. Either eliminating or masking the silanol groups in intercrystalline boundaries, or developing new zeolite membranes may accomplish this. Continue to improve membrane production techniques and reproducibility. Oriented zeolite membranes have shown promising improvements to fluxes and membrane quality. Also, less complex and less expensive synthesis routes are industrially attractive. Measure mixture isotherms and multicomponent diffusivities of mixtures used in pervaporation. These data would be useful in understanding pervaporation transport, and would also contribute to better predictions. Use thinner supports for high flux applications so that the support resistance is minimized. Improve modeling and simulations of transport through zeolites at high coverages, and focus studies on polar molecules and other components used in pervaporation. More modeling of transport in supports would also be useful. Because of the high adsorption strengths of many molecules, zeolite membranes are vulnerable to fouling, which should be investigated during pervaporation. Most mixtures separated in industrial applications contain impurities or are multicomponent mixtures, but few studies on pervaporation using these mixtures have been performed. Acknowledgments We gratefully acknowledge funding from the National Science Foundation (Grant No. CTS ) and the Department of Education Graduate Assistantships in Areas of National Need (GAANN) Program (TCB). We also appreciate useful discussions with Dr. Shiguang Li, Dr. Manuel Arruebo, Dr. Tracy Q. Gardner of the Colorado School of Mines, Dr. Leland M. Vane of the U.S. Environmental Protection Agency, National Risk Management Research Laboratory, Dr. Paul F. Bryan of ChevronTexaco Energy Research and Technology Co., and Jeffrey C. Wyss.

Adsorption Processes. Ali Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad

Adsorption Processes. Ali Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad Adsorption Processes Ali Ahmadpour Chemical Eng. Dept. Ferdowsi University of Mashhad Contents Introduction Principles of adsorption Types of adsorption Definitions Brief history Adsorption isotherms Mechanism

More information

Synthesis and Properties of Zeolitic Membranes

Synthesis and Properties of Zeolitic Membranes 17 Synthesis and Properties of Zeolitic Membranes Sankar Nair* and Michael Tsapatsis University of Massachusetts Amherst, Amherst, Massachusetts, U.S.A. I. INTRODUCTION Zeolite-based separations involve

More information

Adsorption (Ch 12) - mass transfer to an interface

Adsorption (Ch 12) - mass transfer to an interface Adsorption (Ch 12) - mass transfer to an interface (Absorption - mass transfer to another phase) Gas or liquid adsorption (molecular) onto solid surface Porous solids provide high surface area per weight

More information

SEPARATION BY BARRIER

SEPARATION BY BARRIER SEPARATION BY BARRIER SEPARATION BY BARRIER Phase 1 Feed Barrier Phase 2 Separation by barrier uses a barrier which restricts and/or enhances the movement of certain chemical species with respect to other

More information

Technologies and Approaches of CO 2 Capture

Technologies and Approaches of CO 2 Capture Southwest Regional Partnership Project Technologies and Approaches of CO 2 Capture Liangxiong Li, Brian McPherson, Robert Lee Petroleum Recovery Research Center New Mexico Institute of Mining and Technology,

More information

Pervaporation: An Overview

Pervaporation: An Overview Pervaporation: An Overview Pervaporation, in its simplest form, is an energy efficient combination of membrane permeation and evaporation. It's considered an attractive alternative to other separation

More information

LATEST TECHNOLOGY IN Safe handling & Recovery OF Solvents in Pharma Industry

LATEST TECHNOLOGY IN Safe handling & Recovery OF Solvents in Pharma Industry LATEST TECHNOLOGY IN Safe handling & Recovery OF Solvents in Pharma Industry TYPICAL SOLVENT USE IN Pharma Industry Usage of solvents in an API process development is for: Diluent to carry out reaction

More information

General Separation Techniques

General Separation Techniques ecture 2. Basic Separation Concepts (1) [Ch. 1] General Separation Techniques - Separation by phase creation - Separation by phase addition - Separation by barrier - Separation by solid agent - Separation

More information

Chromatography. Gas Chromatography

Chromatography. Gas Chromatography Chromatography Chromatography is essentially the separation of a mixture into its component parts for qualitative and quantitative analysis. The basis of separation is the partitioning of the analyte mixture

More information

ZEOLITES AS ALCOHOL ADSORBENTS FROM AQUEOUS SOLUTIONS

ZEOLITES AS ALCOHOL ADSORBENTS FROM AQUEOUS SOLUTIONS UDC 661.183.6:66.021.3.081.3:547.260.2 APTEFF, 37, 1-192 (2006) BIBLID: 1450 7188 (2006) 37, 83-87 Original scientific paper ZEOLITES AS ALCOHOL ADSORBENTS FROM AQUEOUS SOLUTIONS Blagica Cekova, Dragi

More information

Intermolecular forces Liquids and Solids

Intermolecular forces Liquids and Solids Intermolecular forces Liquids and Solids Chapter objectives Understand the three intermolecular forces in pure liquid in relation to molecular structure/polarity Understand the physical properties of liquids

More information

Synthesis of Zeolite Composite Membranes for CO2 Separation

Synthesis of Zeolite Composite Membranes for CO2 Separation Synthesis of Zeolite Composite Membranes for CO2 Separation April. 10. 2003 Sang Hoon Hyun, Dong Wook Shin, Young Eun Lee, Moon Hee Han*, and Churl Hee Cho* School of Materials Science & Engineering Yonsei

More information

Multi-stage synthesis of nanopore NaA zeolite membranes for separation of water/ethanol mixtures

Multi-stage synthesis of nanopore NaA zeolite membranes for separation of water/ethanol mixtures International Journal of Research in Engineering and Innovation Vol-1, Issue-6 (2017), 42-46 International Journal of Research in Engineering and Innovation (IJREI) journal home page: http://www.ijrei.com

More information

Permeation of Hexane Isomers across ZSM-5 Zeolite Membranes

Permeation of Hexane Isomers across ZSM-5 Zeolite Membranes 2618 Ind. Eng. Chem. Res. 2000, 39, 2618-2622 Permeation of Hexane Isomers across ZSM-5 Zeolite Membranes Rajamani Krishna* and Dietmar Paschek Department of Chemical Engineering, University of Amsterdam,

More information

Review Topic 8: Phases of Matter and Mixtures

Review Topic 8: Phases of Matter and Mixtures Name: Score: 24 / 24 points (100%) Review Topic 8: Phases of Matter and Mixtures Multiple Choice Identify the choice that best completes the statement or answers the question. C 1. Soda water is a solution

More information

- intermolecular forces forces that exist between molecules

- intermolecular forces forces that exist between molecules Chapter 11: Intermolecular Forces, Liquids, and Solids - intermolecular forces forces that exist between molecules 11.1 A Molecular Comparison of Liquids and Solids - gases - average kinetic energy of

More information

Part I.

Part I. Part I bblee@unimp . Introduction to Mass Transfer and Diffusion 2. Molecular Diffusion in Gasses 3. Molecular Diffusion in Liquids Part I 4. Molecular Diffusion in Biological Solutions and Gels 5. Molecular

More information

Chemistry Instrumental Analysis Lecture 28. Chem 4631

Chemistry Instrumental Analysis Lecture 28. Chem 4631 Chemistry 4631 Instrumental Analysis Lecture 28 Two types in general use: -packed (stationary phase) -open tubular or capillary determine selectivity and efficiency of the sample. Column Materials Column

More information

A- Determination Of Boiling point B- Distillation

A- Determination Of Boiling point B- Distillation EXP. NO. 2 A- Determination Of Boiling point B- Distillation The boiling point of a liquid is the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure. The normal boiling

More information

Ceramic Membranes in Process Technology

Ceramic Membranes in Process Technology BASF SE Ludwigshafen Hartwig Voß, Jacek Malisz, Patrick Schmidt, Jörg Therre Ceramic Membranes in Process Technology Status, future Trends, Challenges Strategie WS Hochleistungskeramiken, Bonn 20.01.2015

More information

m WILEY- ADSORBENTS: FUNDAMENTALS AND APPLICATIONS Ralph T. Yang Dwight F. Benton Professor of Chemical Engineering University of Michigan

m WILEY- ADSORBENTS: FUNDAMENTALS AND APPLICATIONS Ralph T. Yang Dwight F. Benton Professor of Chemical Engineering University of Michigan ADSORBENTS: FUNDAMENTALS AND APPLICATIONS Ralph T. Yang Dwight F. Benton Professor of Chemical Engineering University of Michigan m WILEY- INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION Preface xi

More information

Materials development for inorganic membrane layers at ECN

Materials development for inorganic membrane layers at ECN Materials development for inorganic membrane layers at ECN B.C. Bonekamp Presented at XXV EMS Summerschool, Leuven, Belgium, September ECN-M--09-062 May Materials Development for Inorganic Membrane Layers

More information

RECENT DEVELOPMENT IN THE ZEOLITE MEMBRANE SYNTHESIS

RECENT DEVELOPMENT IN THE ZEOLITE MEMBRANE SYNTHESIS RECENT DEVELOPMENT IN THE ZEOLITE MEMBRANE SYNTHESIS L. GÓRA, J.C. JANSEN, A.W.C. van den BERG and Th. MASCHMEYER Laboratory for Applied Organic Chemistry and Catalysis, DelftChemTech, Delft University

More information

Solids, liquids and gases

Solids, liquids and gases Solids, liquids and gases Solids, liquids, and gases are held together by intermolecular forces. Intermolecular forces occur between molecules, not within molecules (as in bonding). When a molecule changes

More information

Chapter 10: Liquids and Solids

Chapter 10: Liquids and Solids Chapter 10: Liquids and Solids Chapter 10: Liquids and Solids *Liquids and solids show many similarities and are strikingly different from their gaseous state. 10.1 Intermolecular Forces Intermolecular

More information

Molecular Sieves Principles of Synthesis and Identification

Molecular Sieves Principles of Synthesis and Identification Molecular Sieves Principles of Synthesis and Identification Second Edition R. SZOSTAK Clark Atlanta University Atlanta, GA USA V D BLACKIE ACADEMIC & PROFESSIONAL An Imprint of Chapman & Hail London Weinheim

More information

London Dispersion Forces (LDFs) Intermolecular Forces Attractions BETWEEN molecules. London Dispersion Forces (LDFs) London Dispersion Forces (LDFs)

London Dispersion Forces (LDFs) Intermolecular Forces Attractions BETWEEN molecules. London Dispersion Forces (LDFs) London Dispersion Forces (LDFs) LIQUIDS / SOLIDS / IMFs Intermolecular Forces (IMFs) Attractions BETWEEN molecules NOT within molecules NOT true bonds weaker attractions Represented by dashed lines Physical properties (melting points,

More information

Lecture 10. Membrane Separation Materials and Modules

Lecture 10. Membrane Separation Materials and Modules ecture 10. Membrane Separation Materials and Modules Membrane Separation Types of Membrane Membrane Separation Operations - Microporous membrane - Dense membrane Membrane Materials Asymmetric Polymer Membrane

More information

Liquids & Solids. Mr. Hollister Holliday Legacy High School Regular & Honors Chemistry

Liquids & Solids. Mr. Hollister Holliday Legacy High School Regular & Honors Chemistry Liquids & Solids Mr. Hollister Holliday Legacy High School Regular & Honors Chemistry 1 Liquids 2 Properties of the States of Matter: Liquids High densities compared to gases. Fluid. The material exhibits

More information

Recap: Introduction 12/1/2015. EVE 402 Air Pollution Generation and Control. Adsorption

Recap: Introduction 12/1/2015. EVE 402 Air Pollution Generation and Control. Adsorption EVE 402 Air Pollution Generation and Control Chapter #6 Lectures Adsorption Recap: Solubility: the extent of absorption into the bulk liquid after the gas has diffused through the interface An internal

More information

Speakers. Moderator. John V Hinshaw GC Dept. Dean CHROMacademy. Tony Taylor Technical Director CHROMacademy. Dave Walsh Editor In Chief LCGC Magazine

Speakers. Moderator. John V Hinshaw GC Dept. Dean CHROMacademy. Tony Taylor Technical Director CHROMacademy. Dave Walsh Editor In Chief LCGC Magazine Webcast Notes Type your questions in the Submit Question box, located below the slide window You can enlarge the slide window at any time by clicking on the Enlarge Slides button, located below the presentation

More information

What type of samples are common? Time spent on different operations during LC analyses. Number of samples? Aims. Sources of error. Sample preparation

What type of samples are common? Time spent on different operations during LC analyses. Number of samples? Aims. Sources of error. Sample preparation What type of samples are common? Sample preparation 1 2 Number of samples? Time spent on different operations during LC analyses 3 4 Sources of error Aims Sample has to be representative Sample has to

More information

CHAPTER 13. States of Matter. Kinetic = motion. Polar vs. Nonpolar. Gases. Hon Chem 13.notebook

CHAPTER 13. States of Matter. Kinetic = motion. Polar vs. Nonpolar. Gases. Hon Chem 13.notebook CHAPTER 13 States of Matter States that the tiny particles in all forms of matter are in constant motion. Kinetic = motion A gas is composed of particles, usually molecules or atoms, with negligible volume

More information

Carbon dioxide removal processes by alkanolamines in aqueous organic solvents Hamborg, Espen Steinseth

Carbon dioxide removal processes by alkanolamines in aqueous organic solvents Hamborg, Espen Steinseth University of Groningen Carbon dioxide removal processes by alkanolamines in aqueous organic solvents Hamborg, Espen Steinseth IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's

More information

Physicochemical Processes

Physicochemical Processes Lecture 3 Physicochemical Processes Physicochemical Processes Air stripping Carbon adsorption Steam stripping Chemical oxidation Supercritical fluids Membrane processes 1 1. Air Stripping A mass transfer

More information

Modeling and Analysis on Pervaporation Separation of. Composite Zeolite Membranes. Stewart Mann

Modeling and Analysis on Pervaporation Separation of. Composite Zeolite Membranes. Stewart Mann Modeling and Analysis on Pervaporation Separation of Composite Zeolite Membranes by Stewart Mann A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science Approved

More information

Complex Compounds Background of Complex Compound Technology

Complex Compounds Background of Complex Compound Technology Complex Compounds For more than 20 years, Rocky Research has been a pioneer in the field of sorption refrigeration utilizing complex compounds. Our technology earned special recognition from NASA in 1999.

More information

Edexcel Chemistry Checklist

Edexcel Chemistry Checklist Topic 1. Key concepts in chemistry Video: Developing the atomic model Describe how and why the atomic model has changed over time. Describe the difference between the plum-pudding model of the atom and

More information

Chapter 27: Gas Chromatography

Chapter 27: Gas Chromatography Chapter 27: Gas Chromatography Gas Chromatography Mobile phase (carrier gas): gas (He, N 2, H 2 ) - do not interact with analytes - only transport the analyte through the column Analyte: volatile liquid

More information

Characterization of zeolites by advanced SEM/STEM techniques

Characterization of zeolites by advanced SEM/STEM techniques SCIENTIFIC INSTRUMENT NEWS 2016 Vol. 7 SEPTEMBER Technical magazine of Electron Microscope and Analytical Instruments. Article Characterization of zeolites by advanced SEM/STEM techniques Toshiyuki Yokoi

More information

The Liquid and Solid States

The Liquid and Solid States : The Liquid and Solid States 10-1 10.1 Changes of State How do solids, liquids and gases differ? Figure 10.4 10-2 1 10.1 Changes of State : transitions between physical states Vaporization/Condensation

More information

Chromatography. What is Chromatography?

Chromatography. What is Chromatography? Chromatography What is Chromatography? Chromatography is a technique for separating mixtures into their components in order to analyze, identify, purify, and/or quantify the mixture or components. Mixture

More information

Chapter 10. Lesson Starter. Why did you not smell the odor of the vapor immediately? Explain this event in terms of the motion of molecules.

Chapter 10. Lesson Starter. Why did you not smell the odor of the vapor immediately? Explain this event in terms of the motion of molecules. Preview Lesson Starter Objectives The Kinetic-Molecular Theory of Gases The Kinetic-Molecular Theory and the Nature of Gases Deviations of Real Gases from Ideal Behavior Section 1 The Kinetic-Molecular

More information

Gas Chromatography. Introduction

Gas Chromatography. Introduction Gas Chromatography Introduction 1.) Gas Chromatography Mobile phase (carrier gas) is a gas - Usually N 2, He, Ar and maybe H 2 - Mobile phase in liquid chromatography is a liquid Requires analyte to be

More information

Experiment 1: Thin Layer Chromatography

Experiment 1: Thin Layer Chromatography Experiment 1: Thin Layer Chromatography Part A: understanding R f values Part B: R f values & solvent polarity Part C: R f values & compound functionality Part D: identification of commercial food dye

More information

What is Chromatography?

What is Chromatography? What is Chromatography? Chromatography is a physico-chemical process that belongs to fractionation methods same as distillation, crystallization or fractionated extraction. It is believed that the separation

More information

The Vacuum Sorption Solution

The Vacuum Sorption Solution The Total Sorption Solution The Vacuum Sorption Solution The Vacuum Sorption Solution www.thesorptionsolution.com About the Technique DVS Vacuum - the only gravimetric system that supports static and dynamic

More information

Open Column Chromatography, GC, TLC, and HPLC

Open Column Chromatography, GC, TLC, and HPLC Open Column Chromatography, GC, TLC, and HPLC Murphy, B. (2017). Introduction to Chromatography: Lecture 1. Lecture presented at PHAR 423 Lecture in UIC College of Pharmacy, Chicago. USES OF CHROMATOGRAPHY

More information

GCSE CHEMISTRY REVISION LIST

GCSE CHEMISTRY REVISION LIST GCSE CHEMISTRY REVISION LIST OCR Gateway Chemistry (J248) from 2016 Topic C1: Particles C1.1 Describe the main features of the particle model in terms of states of matter and change of state Explain, in

More information

Aviation Fuel Production from Lipids by a Single-Step Route using

Aviation Fuel Production from Lipids by a Single-Step Route using Aviation Fuel Production from Lipids by a Single-Step Route using Hierarchical Mesoporous Zeolites Deepak Verma, Rohit Kumar, Bharat S. Rana, Anil K. Sinha* CSIR-Indian Institute of Petroleum, Dehradun-2485,

More information

Methods of pollution control and waste management - laboratory. Adsorptive removal of volatile organic compounds from gases streams

Methods of pollution control and waste management - laboratory. Adsorptive removal of volatile organic compounds from gases streams Methods of pollution control and waste management - laboratory Adsorptive removal of volatile organic compounds from gases streams Manual for experiment 17 dr Hanna Wilczura-Wachnik and dr inż. Jadwiga

More information

Chapter 10: Liquids, Solids, and Phase Changes

Chapter 10: Liquids, Solids, and Phase Changes Chapter 10: Liquids, Solids, and Phase Changes In-chapter exercises: 10.1 10.6, 10.11; End-of-chapter Problems: 10.26, 10.31, 10.32, 10.33, 10.34, 10.35, 10.36, 10.39, 10.40, 10.42, 10.44, 10.45, 10.66,

More information

2. a) R N and L N so R L or L R 2.

2. a) R N and L N so R L or L R 2. 1. Use the formulae on the Some Key Equations and Definitions for Chromatography sheet. a) 0.74 (remember that w b = 1.70 x w ½ ) b) 5 c) 0.893 (α always refers to two adjacent peaks) d) 1.0x10 3 e) 0.1

More information

Ch. 11: Liquids and Intermolecular Forces

Ch. 11: Liquids and Intermolecular Forces Ch. 11: Liquids and Intermolecular Forces Learning goals and key skills: Identify the intermolecular attractive interactions (dispersion, dipole-dipole, hydrogen bonding, ion-dipole) that exist between

More information

Supplementary Information. Experimental Methods

Supplementary Information. Experimental Methods Extremely thin Pd-silica mixed-matrix membranes with nano-dispersion for improved hydrogen permeability Masakoto Kanezashi, Mitsunori Sano, Tomohisa Yoshioka, and Toshinori Tsuru Department of Chemical

More information

Chapter 12 Intermolecular Forces and Liquids

Chapter 12 Intermolecular Forces and Liquids Chapter 12 Intermolecular Forces and Liquids Jeffrey Mack California State University, Sacramento Why? Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature

More information

Copyright SOIL STRUCTURE and CLAY MINERALS

Copyright SOIL STRUCTURE and CLAY MINERALS SOIL STRUCTURE and CLAY MINERALS Soil Structure Structure of a soil may be defined as the mode of arrangement of soil grains relative to each other and the forces acting between them to hold them in their

More information

Pressure Swing Adsorption: A Gas Separation & Purification Process

Pressure Swing Adsorption: A Gas Separation & Purification Process Pressure Swing Adsorption: A Gas Separation & Purification Process Pressure swing adsorption is an adsorption-based process that has been used for various gas separation and purification purposes. Separation

More information

AQA Chemistry (Combined Science) Specification Checklists. Name: Teacher:

AQA Chemistry (Combined Science) Specification Checklists. Name: Teacher: AQA Chemistry (Combined Science) Specification Checklists Name: Teacher: Paper 1-4.1 Atomic structure and the periodic table 4.1.1 A simple model of the atom, symbols, relative atomic mass, electronic

More information

High-Pressure Volumetric Analyzer

High-Pressure Volumetric Analyzer High-Pressure Volumetric Analyzer High-Pressure Volumetric Analysis HPVA II Benefits Dual free-space measurement for accurate isotherm data Free space can be measured or entered Correction for non-ideality

More information

Abstract: An minimalist overview of chromatography for the person who would conduct chromatographic experiments, but not design experiments.

Abstract: An minimalist overview of chromatography for the person who would conduct chromatographic experiments, but not design experiments. Chromatography Primer Abstract: An minimalist overview of chromatography for the person who would conduct chromatographic experiments, but not design experiments. At its heart, chromatography is a technique

More information

Processes and Process Variables

Processes and Process Variables FACULTY OF PETROLEUM & RENEWABLE ENERGY ENGINEERING Course Learning Outcomes Chapter 2 Processes and Process Variables At the end of this course students will be able to Calculate the composition in term

More information

AP Chemistry: Liquids and Solids Practice Problems

AP Chemistry: Liquids and Solids Practice Problems AP Chemistry: Liquids and Solids Practice Problems Directions: Write your answers to the following questions in the space provided. or problem solving, show all of your work. Make sure that your answers

More information

BAE 820 Physical Principles of Environmental Systems

BAE 820 Physical Principles of Environmental Systems BAE 820 Physical Principles of Environmental Systems Catalysis of environmental reactions Dr. Zifei Liu Catalysis and catalysts Catalysis is the increase in the rate of a chemical reaction due to the participation

More information

AQA Chemistry Checklist

AQA Chemistry Checklist Topic 1. Atomic structure Video: Atoms, elements, compounds, mixtures Use the names and symbols of the first 20 elements in the periodic table, the elements in Groups 1 and 7, and other elements in this

More information

High Performance Liquid Chromatography

High Performance Liquid Chromatography Updated: 3 November 2014 Print version High Performance Liquid Chromatography David Reckhow CEE 772 #18 1 HPLC System David Reckhow CEE 772 #18 2 Instrument Basics PUMP INJECTION POINT DETECTOR COLUMN

More information

High Performance Liquid Chromatography

High Performance Liquid Chromatography Updated: 3 November 2014 Print version High Performance Liquid Chromatography David Reckhow CEE 772 #18 1 HPLC System David Reckhow CEE 772 #18 2 1 Instrument Basics PUMP INJECTION POINT DETECTOR COLUMN

More information

CHAPTER CHROMATOGRAPHIC METHODS OF SEPARATIONS

CHAPTER CHROMATOGRAPHIC METHODS OF SEPARATIONS Islamic University in Madinah Department of Chemistry CHAPTER - ----- CHROMATOGRAPHIC METHODS OF SEPARATIONS Prepared By Dr. Khalid Ahmad Shadid Chemistry Department Islamic University in Madinah TRADITIONAL

More information

OCR Chemistry Checklist

OCR Chemistry Checklist Topic 1. Particles Video: The Particle Model Describe the main features of the particle model in terms of states of matter. Explain in terms of the particle model the distinction between physical changes

More information

************************************************************************************** NSS1 ( )

************************************************************************************** NSS1 ( ) Yan Oi Tong Tin Ka Ping Secondary School NSS1 Chemistry Teaching Schedule (2018 2021) NSS1 Chemistry (2018 2021) / Teaching Schedule / P.1 **************************************************************************************

More information

HPLC Background Chem 250 F 2008 Page 1 of 24

HPLC Background Chem 250 F 2008 Page 1 of 24 HPLC Background Chem 250 F 2008 Page 1 of 24 Outline: General and descriptive aspects of chromatographic retention and separation: phenomenological k, efficiency, selectivity. Quantitative description

More information

Name: Class: Date: SHORT ANSWER Answer the following questions in the space provided.

Name: Class: Date: SHORT ANSWER Answer the following questions in the space provided. CHAPTER 10 REVIEW States of Matter SECTION 1 SHORT ANSWER Answer the following questions in the space provided. 1. Identify whether the descriptions below describe an ideal gas or a real gas. a. The gas

More information

GAS CHROMATOGRAPHY. Mobile phase is a gas! Stationary phase could be anything but a gas

GAS CHROMATOGRAPHY. Mobile phase is a gas! Stationary phase could be anything but a gas GAS CHROMATOGRAPHY Mobile phase is a gas! Stationary phase could be anything but a gas Gas Chromatography (GC) GC is currently one of the most popular methods for separating and analyzing compounds. This

More information

Basic Principles of Membrane Technolog

Basic Principles of Membrane Technolog Basic Principles of Membrane Technolog by Marcel Mulder Center for Membrane Science and Technology, University oftwente, Enschede, The Netherlands ff KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON / LONDON

More information

PHYSICAL CONSTANTS: MELTING POINTS, BOILING POINTS, DENSITY

PHYSICAL CONSTANTS: MELTING POINTS, BOILING POINTS, DENSITY CRYSTALLIZATION: PURIFICATION OF SOLIDS ANSWERS TO PROBLEMS: 1. (a) (b) (c) (d) A plot similar to line A in Figure 5.1 on page 559 will be obtained. The line will be slightly curved. All of the substance

More information

Mass Transfer Operations

Mass Transfer Operations College of Engineering Tutorial # 1 Chemical Engineering Dept. 14/9/1428 1. Methane and helium gas mixture is contained in a tube at 101.32 k Pa pressure and 298 K. At one point the partial pressure methane

More information

Nanowires and nanorods

Nanowires and nanorods Nanowires and nanorods One-dimensional structures have been called in different ways: nanowires, nanorod, fibers of fibrils, whiskers, etc. These structures have a nanometer size in one of the dimensions,

More information

Template-Free Synthesis of Beta Zeolite Membranes on Porous α-al 2 O 3 Supports

Template-Free Synthesis of Beta Zeolite Membranes on Porous α-al 2 O 3 Supports Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2014 Electronic Supplementary Information for Template-Free Synthesis of Beta Zeolite Membranes on Porous

More information

Spanish Fork High School Unit Topics and I Can Statements Honors Chemistry

Spanish Fork High School Unit Topics and I Can Statements Honors Chemistry Spanish Fork High School 2014-15 Unit Topics and I Can Statements Honors Chemistry Module 1 I Can: Module 2 I Can: Distinguish between elements, compounds, and mixtures Summarize the major experimental

More information

The Liquid and Solid States

The Liquid and Solid States : The Liquid and Solid States 10-1 10.1 Changes of State How do solids, liquids and gases differ? Figure 10.4 10-2 10.1 Changes of State : transitions between physical states Vaporization/Condensation

More information

Membrane processes selective hydromechanical diffusion-based porous nonporous

Membrane processes selective hydromechanical diffusion-based porous nonporous Membrane processes Separation of liquid or gaseous mixtures by mass transport through membrane (= permeation). Membrane is selective, i.e. it has different permeability for different components. Conditions

More information

Lecture 25: Manufacture of Maleic Anhydride and DDT

Lecture 25: Manufacture of Maleic Anhydride and DDT Lecture 25: Manufacture of Maleic Anhydride and DDT 25.1 Introduction - In this last lecture for the petrochemicals module, we demonstrate the process technology for Maleic anhydride and DDT. - Maleic

More information

The first three categories are considered a bottom-up approach while lithography is a topdown

The first three categories are considered a bottom-up approach while lithography is a topdown Nanowires and Nanorods One-dimensional structures have been called in different ways: nanowires, nanorod, fibers of fibrils, whiskers, etc. The common characteristic of these structures is that all they

More information

Pre-seeding -assisted synthesis of high performance polyamide-zeolite nanocomposie membrane for water purification

Pre-seeding -assisted synthesis of high performance polyamide-zeolite nanocomposie membrane for water purification Electronic Supporting Information: Pre-seeding -assisted synthesis of high performance polyamide-zeolite nanocomposie membrane for water purification Chunlong Kong, a Takuji Shintani b and Toshinori Tsuru*

More information

Properties of Liquids and Solids

Properties of Liquids and Solids Properties of Liquids and Solids World of Chemistry Chapter 14 14.1 Intermolecular Forces Most substances made of small molecules are gases at normal temperature and pressure. ex: oxygen gas, O 2 ; nitrogen

More information

GCSE Chemistry. Module C7 Further Chemistry: What you should know. Name: Science Group: Teacher:

GCSE Chemistry. Module C7 Further Chemistry: What you should know. Name: Science Group: Teacher: GCSE Chemistry Module C7 Further Chemistry: What you should know Name: Science Group: Teacher: R.A.G. each of the statements to help focus your revision: R = Red: I don t know this A = Amber: I partly

More information

A User-Defined Pervaporation Unit Operation in AspenPlus on the Basis of Experimental Results from Three Different Organophilic Membranes

A User-Defined Pervaporation Unit Operation in AspenPlus on the Basis of Experimental Results from Three Different Organophilic Membranes 127 A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 39, 2014 Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong Copyright 2014, AIDIC Servizi S.r.l., ISBN 978-88-95608-30-3;

More information

Aalborg Universitet. Transport phenomena in gas-selective silica membranes Boffa, Vittorio. Creative Commons License Unspecified

Aalborg Universitet. Transport phenomena in gas-selective silica membranes Boffa, Vittorio. Creative Commons License Unspecified Aalborg Universitet Transport phenomena in gas-selective silica membranes Boffa, Vittorio Creative Commons License Unspecified Publication date: 2016 Link to publication from Aalborg University Citation

More information

Describe how the inter-conversion of solids, liquids and gases are achieved and recall names used for these inter-conversions

Describe how the inter-conversion of solids, liquids and gases are achieved and recall names used for these inter-conversions Understand the arrangements, movements and energy of the particle in each of the 3 states of matter : solid, liquid and gas Describe how the inter-conversion of solids, liquids and gases are achieved and

More information

Chapter 11. Liquids and Intermolecular Forces

Chapter 11. Liquids and Intermolecular Forces Chapter 11 Liquids and Intermolecular Forces States of Matter The three states of matter are 1) Solid Definite shape Definite volume 2) Liquid Indefinite shape Definite volume 3) Gas Indefinite shape Indefinite

More information

Chapter 11. Intermolecular Forces and Liquids & Solids

Chapter 11. Intermolecular Forces and Liquids & Solids Chapter 11 Intermolecular Forces and Liquids & Solids The Kinetic Molecular Theory of Liquids & Solids Gases vs. Liquids & Solids difference is distance between molecules Liquids Molecules close together;

More information

Instrumental Analysis II Course Code: CH3109. Chromatographic &Thermal Methods of Analysis Part 1: General Introduction. Prof. Tarek A.

Instrumental Analysis II Course Code: CH3109. Chromatographic &Thermal Methods of Analysis Part 1: General Introduction. Prof. Tarek A. Instrumental Analysis II Course Code: CH3109 Chromatographic &Thermal Methods of Analysis Part 1: General Introduction Prof. Tarek A. Fayed What is chemical analysis? Qualitative analysis (1) Chemical

More information

Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro

Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro hemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay ommunity ollege Wellesley ills, MA 2008, Prentice all omparisons of the States of Matter the solid and liquid states have

More information

2. As gas P increases and/or T is lowered, intermolecular forces become significant, and deviations from ideal gas laws occur (van der Waal equation).

2. As gas P increases and/or T is lowered, intermolecular forces become significant, and deviations from ideal gas laws occur (van der Waal equation). A. Introduction. (Section 11.1) CHAPTER 11: STATES OF MATTER, LIQUIDS AND SOLIDS 1. Gases are easily treated mathematically because molecules behave independently. 2. As gas P increases and/or T is lowered,

More information

DETERMINATION OF OPTIMAL ENERGY EFFICIENT SEPARATION SCHEMES BASED ON DRIVING FORCES

DETERMINATION OF OPTIMAL ENERGY EFFICIENT SEPARATION SCHEMES BASED ON DRIVING FORCES DETERMINATION OF OPTIMAL ENERGY EFFICIENT SEPARATION SCHEMES BASED ON DRIVING FORCES Abstract Erik Bek-Pedersen, Rafiqul Gani CAPEC, Department of Chemical Engineering, Technical University of Denmark,

More information

INDUSTRIAL EXPERIENCE WITH HYBRID DISTILLATION PERVAPORATION OR VAPOR PERMEATION APPLICATIONS

INDUSTRIAL EXPERIENCE WITH HYBRID DISTILLATION PERVAPORATION OR VAPOR PERMEATION APPLICATIONS INDUSTRIAL EXPERIENCE WITH HYBRID DISTILLATION PERVAPORATION OR VAPOR PERMEATION APPLICATIONS Mario Roza, Eva Maus Sulzer Chemtech AG, Winterthur, Switzerland; E-mails: mario.roza@sulzer.com, eva.maus@sulzer.com

More information

Chapter 11. Intermolecular Forces, Liquids, and Solids

Chapter 11. Intermolecular Forces, Liquids, and Solids Chapter 11. Intermolecular Forces, Liquids, and Solids A Molecular Comparison of Gases, Liquids, and Solids Physical properties of substances are understood in terms of kinetic-molecular theory: Gases

More information

Liquids, Solids, and Intermolecular Forces or. Why your Water Evaporates and your Cheerios Don t. Why are molecules attracted to each other?

Liquids, Solids, and Intermolecular Forces or. Why your Water Evaporates and your Cheerios Don t. Why are molecules attracted to each other? Liquids, Solids, and Intermolecular Forces or Why your Water Evaporates and your heerios Don t Why are molecules attracted to each other? 1 Intermolecular attractions determine how tightly liquids and

More information

VOCABULARY. Set #2. Set #1

VOCABULARY. Set #2. Set #1 VOCABULARY Set #1 1. Absolute zero 2. Accepted value 3. Accuracy 4. Celsius scale 5. Conversion factor 6. Density 7. Dimensional analysis 8. Experimental value 9. Gram 10. International system of units

More information

Facilitated transport of thiophenes through Ag 2 O-filled PDMS membranes

Facilitated transport of thiophenes through Ag 2 O-filled PDMS membranes Facilitated transport of thiophenes through PDMS membranes Rongbin Qi, Yujun Wang, Jiding Li *, Shenlin Zhu State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University.

More information