Commercial PTFE membranes for membrane distillation application: effect of microstructure and support material

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1 NOTICE: this is the author s version of a work that was accepted for publication in Desalination. Changes resulting fro the publishing process, such as peer review, editing, corrections, structural foratting, and other quality control echaniss ay not be reflected in this docuent. Changes ay have been ade to this work since it was subitted for publication. A definitive version was subsequently published in Desalination, [Vol. 284, (2012)] doi: /j.desal Coercial PTFE ebranes for ebrane distillation application: effect of icrostructure and support aterial Shariza Adnan 1, Manh Hoang 2, Huanting Wang 1, Zongli Xie 2 1 Cheical Engineering Departent, Monash University, Clayton, Vic. 3800, Australia 2 CSIRO Material Science and Engineering, Clayton Vic. 3168, Australia ABSTRACT Mebrane distillation (MD), a therally-driven ebrane separation process has been widely studied in recent years and the ebrane properties are known to have significant effect on the perforance of the MD process. This paper studied the icrostructure of PTFE ebranes fro three different sources for MD application. The paraeters investigated include pore size, thickness, porosity and tortuosity. Non-supported ebranes were also tested for coparison. The experiental results for different pore size, thickness, porosity, tortuosity and support aterials in direct contact ebrane distillation (DCMD) were copared with the theoretical flux siulated fro the Schofield odel using Polyath NLE solver. The teperature polarization coefficient was also deterined to establish the relationship between the ebrane icrostructure and perforance. The results indicated that the structure and porosity of support aterials play iportant roles in deterining the perforance of DCMD. Higher fluxes and lower teperature polarizations were observed over ebranes with large pore size, low thickness, high porosity and low tortuosity. Mebrane porosity, pore size and the presence of support aterials were found to have significant effects on flux and teperature polarization. Keywords Mebrane distillation, ebrane icrostructure 1. Introduction In desalination, ebrane distillation (MD) involves the transport of water vapour through a hydrophobic ebrane. Condensation takes place on the cooler side of the ebrane with the teperature difference being the driving force, resulting in the pressure difference between the war and cool surfaces [1]. Aongst the advantages of MD are the production of high purity distillate, the possibility to operate by low energy sources such as waste heats, solar and geotheral. MD can also 1

2 be integrated to conventional desalination processes such as ulti-effect distillation (MED) or reverse ososis (RO) to increase water recovery and/or iprove the energetic efficiency of the syste [2]. Copared to conventional distillation and reverse ososis processes, the benefits of MD are low operating teperature and pressure, high ions and non-volatile rejections, less stringent ebrane property requireents and low cheical interaction between the ebrane and feed solution. The low operating teperature (between 30 and 80 C) of MD process akes it suitable for separation of therally sensitive aterials such as food and agricultural produce and waste or low grade energy fro the production process can be used [1, 3]. One of ajor challenges in MD process is the absence of affordable high perforance ebranes. Low pereate fluxes copared to other separation process such as RO, flux decay due to teperature polarization, fouling and ebrane wetting have been reported in the literature [4-7]. Lei et al. [8] listed soe of the challenges in MD process include (i) the absence of affordable coercial ebranes with high porosity and low thickness, (ii) lack of available ebrane odules with good fluid dynaic to enhance ass and heat transfer, (iii) the need for extensive studies on ebrane fouling, (iv) iproveent of theral efficiency and (v) difficulty in getting pereates with high purity. Khayet [9] outlined ten following ebrane requireents for an effective MD application: i) A single or ulti-layers with at least one being hydrophobic ii) The pore size ranging fro several nanoeters to a few icroeters with a narrow distribution desired and high liquid entry pressure (LEP) iii) Sall tortuosity factor, defined as the straightness of the pores iv) High porosity of the hydrophobic layer v) An optiu thickness so as axiise ass transport and iniise heat loss through the ebrane aterial. vi) Low theral conductivity vii) The surface in contact with the feed solution should be highly resistant to fouling viii) Good theral stability for long ter application for ixtures of up to 100 C ix) Excellent cheical resistance to various feed solutions as well as acidic and basic cleaning solutions x) Ability to provide stable MD perforance and long life It is known that the ebrane properties including the ebrane aterial and icrostructure are critical in deterining the vapour transport and consequently the overall perforance of MD. Polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE) are the ost coon ebrane aterials for MD applications due to their hydrophobic nature and high theral stability. The PVDF has a elting point of 170 C whereas the PTFE of about 327 C. A recent study indicated that PFTE 2

3 ebranes showed better MD perforance copared to PVDF due to better operability and higher ass transfer coefficient [10]. The ebrane icrostructure and its influence on the perforance of MD have been studied by several researchers. Al-Rub et al. had carried out a study on effects of different paraeters on the ass flux [11]. The ebrane porosity and thickness were found to contribute to flux, teperature polarization and theral efficiency. High porosity was favoured for high flux, low teperature polarization coefficient (θ) and high theral efficiency. On the other hand, even though low ebrane thickness results in high flux and low θ, Al-Rub concluded that it does not affect the theral efficiency. Despite their coprehensive study on the two ebrane paraeters, no result was presented on the relationship between ebrane pore size or tortuosity and flux, teperature polarization coefficient or theral efficiency. In addition, the noinal ebrane thickness of 1.5 and 3.0 used are uch thicker than ost of the coercial available ebranes. The iportance of ebrane pore size on DCMD perforance has not been studied widely copared to the porosity and thickness suggesting that it is less iportant for flux iproveent in the range of µ [12]. This paper is based on a study using coercial PTFE ebranes with thickness < 0.16 fro three different sources with varied icrostructure. The paraeters investigated include pore size, thickness, porosity and tortuosity. Two non-supported PTFE ebranes were also tested for coparison. The results were copared with the theoretical flux using Schofield Model siulated by the Polyath software with incorporated vapour transport equations. The teperature polarization coefficient was deterined to establish its relationship with the ebrane icrostructure. 3

4 2. Theory 2.1. Mass transfer The ass and heat transport for MD can be divided into five regions (Fig. 1): the bulk feed, feed boundary layer, across ebrane, pereate boundary layer and pereate bulk regions. The ass diffusing rate across the ebrane results in a decrease in the total feed flow and an increase in the total pereate flow. The ratio of this rate to the ebrane area is defined as the ass flux, J (kg/ 2 h). T bf T bf,out M f,o T f P f Hydrophobic Mebrane pore Coolant flow M p,i T bp,i J War feed flow M f,,i T bf,i Feed boundary layer T p P p Pereate boundary layer T bp,o M p,oi T bp Q f Q Q p Figure 1. Teperature and pressure profiles across MD There are several diffusion echaniss for the vapour transport across porous edia such as ebranes used in MD. The ost iportant ones are the Maxwell-Stefan (olecular), the Hagen- Poiseuille (viscous) and the Knudsen diffusions [13-14]. These three diffusion echaniss are used in cobination to explain the total ass transfer in MD. The general for for ass flux can be represented below: M J = Ri p RT (1) 4

5 where M is the olecular ass of the volatile coponent, R is the gas constant, T is the average teperature, p is the saturated pressure gradient and R i is the resistance to ass transport i. To siplify the equation, the pressure gradient is only evaluated across the thickness of the ebrane, hence p becoes the pressure difference across the ebrane. The resistance to ass transport in MD can norally be odelled using the electrical circuit analogies. Two ost acceptable odels in MD are those proposed by Schofield et al. [15] and the Dusty Gas Model (DGM). Both odels incorporate the Knudsen, olecular and viscous diffusions. The difference in between the odels are in ter of resistance to transport which can be represented by the electrical circuit analogies in Figure 2 [16]. The resistance for both viscous and Knudsen diffusions are in parallel for both odels, but olecular diffusion s resistance in the DGM is in series with the Knudsen diffusion. In cases where the viscous or olecular flows are negligible, these two odels will be equivalent. (a) Schofield s odel (b) Dusty Gas Model Figure 2. Electrical circuit analogies of Schofield and Dusty Gas odels The ordinary or olecular diffusion is a ulti-coponent diffusion in gases at low density. The transfer rate of the olecules depends on the concentration difference and the resistance. In MD, the presence of air in the ebrane pores can be considered as a stagnant fil which allows the diffusion of a volatile coponent. When there is no pressure difference across the ebrane, the two liquid phases are kept at the pore entrance via the capillary forces. It is also assued that vapour-liquid equilibria (VLE) exist at these ends. If a ebrane is assued as having a unifor sized pores and equal velocities relative to position in the pores, the ebrane tortuosity can influence the vapour velocities. The Maxwell-Stefan equation for ulti-coponent gas diffusion has been used as a basis to estiate the ass flux of vapour olecules in olecular diffusion [17]. The volatile coponent and air inside the pores are considered as a binary ixture. The resistance ter in olecular diffusion, R M, depends on diffusivity of volatile coponent in air, the partial pressures of air and volatile coponents and the porosity as well as tortuosity of the ebrane. The olecular diffusion resistance can be estiated by equations such as the one used by Martinez [18], incorporating the pressure in the ebrane pores, and the diffusivity of the volatile coponent 5

6 (water vapour) in air. The total pressure of volatile coponent and air inside the pores is assued as constant and roughly equals to the atospheric pressure. The viscous or Hagen-Poiseuille flow occurs in the lainar region for flowing incopressible fluid through a capillary of constant circular cross-section. The flow of a Newtonian fluid through a channel such as the ebrane pore is influenced by the pressure drop, the fluid viscosity, the radius and the length of the pipe. This type of diffusion is doinated by inter olecular collisions, assuing the absence of walls. A single pore in ebranes can be visualised as a tube or capillary of radius r and length δ. If the ean free path of the diffusing vapour through a ebrane is shorter than the pore size, the likelihood of olecule-wall collision is low copared to inter-olecule collisions, thus Hagen-Poiseuille is the doinant flow. The transport resistance in viscous flow is influenced by the pore size, ebrane porosity, the pressure inside the pore, ebrane tortuosity and the gas viscosity inside the pores with equations siilar as entioned by Fernandez-Pineda [16]. The Knudsen diffusion, on the other hand, is defined as a free olecule transport of gases in which the olecular diaeter is shorter than the ean free path relative to the pore size, i.e. interaction between olecules is negligible [17]. The resistance to Knudsen diffusion depends on the ebrane properties (pore size, porosity, tortuosity, and teperature) and the olecular ass of the diffusing coponent. The equations for resistance to these vapour transport echaniss used in this study are shown in Appendix B. The saturated vapour pressure of a coponent at a specified teperature can be deterined using the Antoine equation [19]. For water as the ore volatile coponent, this value can be obtained fro handbooks or the literature. The presence of a solute in water will reduce the vapour pressure of the water according to Raoult s law. To apply Raoult s law to non-ideal solutions, i.e., the vapour pressure of the solution not only depends on the vapour pressure of individual coponents and their ole fractions, but also on the interaction between olecules of different substances, a correction factor called the activity coefficient (γ w ) is applied to account for the interaction between olecules in the liquid phase. Hence, the corrected vapour pressure is a product of saturated vapour pressure, the olar fraction of the volatile coponent and the activity coefficient (Appendix B). In ost MD studies where salt solutions are used as feeds, γ w correlation can be deterined as a function of salt ole fraction, x NaCl [20]: γ w NaCl 2 = 1 0.5x 10x (2) NaCl 2.2. Heat transfer As the feed approaches the ebrane surface, its bulk teperature drops fro T bf to T f. In this region, the aount of heat transferred (Q f ) equals to the product of fil heat transfer coefficient and the teperature difference across the feed side boundary layer is calculated as below: 6

7 Q f = h T T ) (3) f ( bf f The heat transferred to the pereate, Q p is deterined in the sae anner as shown below: Q p = h (T T ) (4) p p bp The heat supplied by the feed provides enough energy for volatile olecules to evaporate and enter the ebrane pores. Across the ebrane, there are two ajor heat transfers occurring: the heat transfer fro the vapour flux and the conduction through the ebrane. The heat transfer across the ebrane, Q, can be represented as: Q k = Jλ v+ (Tf Tp ) (5) δ where λ v is the latent heat of vaporisation, δ is the ebrane thickness and k is the ebrane theral conductivity calculated by: k = (1 ε)k + ε k (6) where ε is the ebrane porosity, k pl is the theral conductivity of the ebrane polyer aterial and k g is the theral conductivity of the air/vapour ixture in the pores. Equation (6) can be applied directly provided that the ebrane is non-supported. However, if the ebrane is supported, the theral conductivity and porosity of the support aterial will be taken into consideration. The heat transfer coefficients, h f and h p can be estiated using the Nusselt nuber (Nu) which represents the ratio of convective to conductive heat transfer across the boundary layers. In this study, the feed and pereate streas are in the lainar region (Reynolds nuber less than 2100) and there are nuerous correlations for Nusselt nuber which can be used for this regie [21]. These Nu correlate Reynolds and Prandtl nubers, Re and Pr, and the configuration of the flow channels. To iniise the errors in the estiation of flux, the contribution of net spacers used in this study was included in the Reynolds and Nusselt nuber calculations according to Phattaranawik et al. [22] with the spacer factor α s = The use of net spacers in this study increased the Reynolds nuber by 11%. Based on the available Nusselt nuber correlations outlined by Gryta et al. [21], equations B.13a and B.13b (in Appendix B) were chosen due to the low standard deviation observed in their study and the channel height relatively close to the one used in this study. This correlation indicated good agreeent of ore than 98% with all six ebranes used in this study. pl g 7

8 2.3. Schofield s odel The odel by Schofield has been used widely to explain the ass transport across hydrophobic ebranes in MD. The following equation for theoretical ass flux was used in this study: J = M RTδ R M 1 + R V 1 + R K 1 p (7) where T is the average ebrane teperature, δ is the ebrane thickness and p is the pressure difference across the ebrane. The expressions for R M, R K and R V are listed in Appendix B. Whilst the bulk teperatures can be estiated fro the inlet and outlet teperatures of the feed and pereate, the teperatures on the ebrane surfaces, T f and T p are not norally easurable experientally. By assuing that there is negligible heat loss to the surrounding and the syste is well insulated, the heat transferred, Q f, Q and Q p are equal. Hence, T f and T p can be calculated by solving equations (3), (4) and (5) to for the following expressions which are then used to calculate the average ebrane teperature T : T f = k δ hp + k δ h f k + 1 δ h p T bf k + δ h f T bp Jλ h f (8) T p = 1 + k δ h 1 p + k δ h f k + 1 δ h f T bp k + δ h p T bf Jλ + h p (9) Equations 8 and 9, however, do not take into account the heat absorbed by any ebrane support aterials. Since ost flat sheet ebranes used on MD studies have very low thickness, they have low echanical strength and require stronger support aterials such as polypropylene (PP) or poly ethylene terephthalate (PET) to ease handling. Taking this effect into account, Gryta et al. proposed heat transfer coefficient correction ters to account for the presence of nets as ebrane supports [21]. Since the supported ebranes used in this study were only supported on one side, the effective fil heat transfer coefficient of the pereate side, h p is now represented by the following equation: 1 ' 1 δ s h p = + (10) h p k s in which δ s represents the thickness of the support aterial and k s represents its effective theral conductivity. k s can be deterined as: s s ' s k = (1 ε )k + ε k (11) s w 8

9 9 with k s is the support aterial theral conductivity, k w is the theral conductivity of water on pereate side and ε s is its porosity. Taking into consideration the heat loss through the support aterial, the corrected equations to estiate the ebrane surface teperatures becoe: = f bp f bf ' p h δ k h δ k f h λ J T h δ k T h δ k ' T f ' p (12) = ' p bf ' p bp f h δ k h δ k p h λ J T h δ k T h δ k ' T f ' p (13)

10 2.4. Teperature polarization The teperature polarization has been identified as one of the shortcoings in MD [23]. Depending on the operating paraeters and the ebrane properties, the teperature difference between the ebrane surfaces (T f and T p ) is saller than that of the bulk teperatures. The teperature polarization coefficient (θ) is generally used as a benchark for the energy efficiency. The θ value ranges fro zero to one (0-1) and can be calculated as below [23]: Tf ' Tp ' θ = (14) T T bf bp where T bf and T bp are the bulk teperatures of the feed and pereate streas, respectively. As the teperature on the ebrane surfaces becoe closer to the bulk teperatures, θ approaches unity. Flux and teperature polarization are interrelated with each other. High flux will increase the aount of heat transferred to the ebrane, hence increasing the pereate teperature on the ebrane surface (refer to equation 13) whilst at the sae tie decreasing the teperature at the ebranefeed interface (equation 12). This will result in lower θ value. High θ indicates that the difference between the ebrane surface teperatures is close to the difference in bulk teperatures, thus the effect of teperature polarization is negligible. This usually happens at low feed teperatures where the flux is also low Siulation for theoretical values deterination This study used the Polyath 6.10 version with NLE via the Newton-Raphson ethod to calculate theoretical fluxes as described by the Schofield odel. This ethod is based on a truncated Taylor series approxiation of the function values to obtain better estiated values of the unknown variables. The function values and the atrix of partial derivatives were calculated using an iterative ethod. The iteration stops if the su of the agnitudes of the functions is less than the tolerance value of 1.0x10-7. The paraeters used for the estiates are listed in Appendix A. To solve for the NLEs, the feed and cooling water bulk teperatures based on experiental data were used as initial estiated values for the ebrane surface teperatures. 10

11 3. Experiental 3.1. Materials Hydrophobic Polytetrafluoroethylene (PTFE) ebranes supplied by Mebrane Solutions (MS series), GE Osonics and Sartorius Stedi Biotech were used with specification listed in Table 1. The GE, MS 3000 and 4000 series ebranes have polyethylene tetraphthalate (PET) and polypropylene (PP) as support aterials. Two non-supported ebranes (MS-2000 and Sartorius 11807) were also used for coparison. Table 1. Properties of PTFE ebranes used Mebrane Pore size a (µ) Porosity (%) Overall thickness a (µ) Support GE a 150 PET scri MS a 160 PP non-woven MS a 160 PP non-woven MS a 140 PET scri MS a 30 Non-supported Sartorius b 65 Non-supported a Manufacturer s specification b Reported by Hong et al. [24] 3.2. Mebrane properties A Philips XL30 FEG scanning electron icroscopy (SEM) was used for studying the orphology of the surface and cross section of ebranes. It was also used to estiate the thickness of different layers of ebranes (Fig. 4). A digital icroeter with 0.1 µ accuracy was used to verify the overall thickness. The hydrophobicity of ebrane can be indicated by the contact angle which was easured by a DataPhysics OCAH 230 instruent equipped with a high speed caera and the SCA 20 software Mebrane distillation A ebrane odule ade fro plexiglass with 50 c 2 contact area was used with a direct contact configuration (Fig. 3). The feed solution was heated to the desired teperature by passing it through a teperature-controlled water bath. The feed flow rate was controlled using a variable speed peristaltic pup (Masterflex ). A circulated pereate strea with teperature controlled by a HetoFrig chiller with ethylene glycol as the refrigerant and the flow rate controlled by a single-speed peristaltic pup. The pereate strea was kept at a teperature of 20 C±1 and flow rate of 0.5 L/in. The teperatures at the inlet and outlet of both the feed and pereate were onitored using 11

12 K-type therocouples. The operating conditions of the experient are suarised in Table 2. A 0.5 wt% NaCl solution was prepared as the feed by dissolving analytical grade NaCl salt in distilled water. The weight loss fro the feed reservoir was onitored continuously using a Kern electronic balance Model A (with ±0.1g accuracy) which was interfaced with a coputer. Readings at 30 second intervals were recorded. Saples were collected fro the pereate reservoir at 10 inutes intervals and the conductivity analysed using Oakton CON 110 conductivity eter was used to deterine the salt content. The conductivity less than 10µS indicates pure water obtained on the pereate side. The concentration of salt in the feed reservoir was deterined at the end of each run. The flux is calculated by the ratio of the weight gain over a certain tie period to the area of ebrane (50 c 2 ). Table 2. Operating paraeters used Paraeter Range Feed teperature ( C) Feed flow rate (L/in) 0.6 Cooling water flow rate (L/in) 0.5 Feed concentration (wt/wt) 0.5% (x NaCl.=0.0015) Figure 3. Apparatus for DCMD 4. Results and discussion 4.1. SEM iages and ebrane properties 12

13 The SEM iages (Fig. 4) were used to estiate the thickness of the active ebrane layers. The active layers of the ebranes used showed the thickness values ranging fro 21 to 36 µ (Fig. 4c, d). The support aterials used consisted of ore than 80% of the overall ebrane thickness. The SEM iages also indicate that the PTFE ebranes have a loose packed sphere structure; hence equation B.4 in Appendix B can be used to predict the tortuosity values. (a) (b) (c) (d) Figure 4. SEM iages of active surface and cross-section of MS-3010 (a and b) and MS-4010 (c and d) ebranes Table 3 lists the contact angle and easured thickness of the ebrane. The contact angles of the MS-series and GE ebranes are higher than 130, with the GE ebrane showing the highest value copared to the other ebranes. The Sartorius ebrane showed a lower hydrophobicity with a contact angle of 123. The pore size of the scri and non-woven supports (Fig. 5) differs significantly. The scri support has large oval shape pores 900µ x 275µ in size whereas the nonwoven support has highly varied pore size between 40µ and 190µ. Table 3. Contact angle and easured thickness of ebranes. Mebrane Contact angle( ) Thickness of support layer (µ) Thickness of active layer (µ) GE 165±

14 133±4 153±3 157±5 161±9 123±1 MS-3010 MS-3020 MS-4010 MS-2000 Sartorius n/a n/a (a) (b) Figure 5. SEM iages of (a) scri-backing (MS-4010) and (b) non-woven supports of ebranes (MS- 3010) 4.2. Effect of pore size The ebrane pore radius influences both the vapour transport via viscous and Knudsen diffusions. A larger pore size eans that the viscous diffusion increases to the second power whilst the Knudsen diffusion increases proportionally. Thus, it is expected that ebranes with larger pore size will yield higher MD flux. Studies by several researchers supported this for DCMD using coercial and laboratory fabricated PTFE or PVDF ebranes in either flat sheet or hollow fibre fors [25-28]. 14

15 (a) (b) Figure 6. (a) Theoretical and experiental flux and (b) teperature polarization coefficient for ebranes with different pore sizes (porosity = 0.82, thickness = 0.24 µ/ 0.36 µ) Figures 6(a) and (b) show the flux and teperature polarization coefficient for ebranes with 0.45 and 0.22 µ pore size. Generally, with all other properties constant, ebranes with bigger pores show higher flux. This becoes ore pronounced at higher feed teperatures. This is because 15

16 the vapour pressure of water in the feed solution increased with increasing feed teperature, consequently the driving force increased The teperature polarization coefficient was found to be higher for MS-3020 ebrane (pore size of 0.22 µ) copared to MS-3010 (0.45 µ pore size). This could be due to the fact that less heat can be transferred by vaporization due to the restricted pore size for transport when copared to a ebrane with larger pores. This will lessen the teperature drop at the feed side ebrane interface and decrease teperature polarization (higher θ), hence lower flux was observed. The higher feed teperature also resulted in lower teperature polarization coefficient, and consequently higher teperature polarisation Effect of thickness The thicker ebrane extends the distance travelled by the diffusing olecules thus reducing the flux. It is therefore desirable to iniise the ebrane thickness for MD to achieve higher fluxes. On the other hand, it is iportant to have reasonable ebrane thickness to prevent wetting when higher feed flow rates are used. The optiu ebrane thickness depends on the type of ebrane used (supported or non-supported) and the type of feed solutions (salt contents). In a study by Martinez and Rodriguez-Maroto, the flux for DCMD using supported PVDF and PTFE ebranes was found to decrease with increasing thickness for both water and 2M NaCl solution [18]. The sae study showed that this trend differed slightly when higher concentration of NaCl (4M) was used. Initial increents in flux and energy efficiency were observed before a gradual decrease becae apparent with increasing ebrane thickness. The effect of thickness on energy efficiency for nonsupported ebranes is less evident with only increased efficiencies observed up to 20 µ of thickness before levelling off [18]. In this study, MS-4010 and GE ebranes were used to copare the effect of thickness on flux and teperature polarization. The overall and active layer thicknesses are 140 µ and 21 µ (MS- 4010) and 150 µ and 29 µ (GE), respectively. Figure 7 shows the flux and teperature polarisation coefficient for ebranes with different thickness. As expected, the thinner ebrane generally shows higher flux than the thicker ebrane. In coparison to a thick ebrane, a thinner ebrane not only provides a shorter route for diffusion, but it allows ore heat to be transferred across the ebrane via conduction (second ter in equation 5). The flux obtained by the MS-4010 ebrane was uch higher than that of GE ebrane at 50 C and 70 C feed inlet teperatures (Fig. 7a). At 30 C feed inlet teperature, both ebranes showed alost siilar fluxes. This trend is consistent with other studies [11, 18]. The teperature increase will affect the vapour pressure exponentially, and the ipact was obvious at higher teperatures. 16

17 (a) (b) Figure 7. (a) Theoretical and experiental flux and (b) teperature polarization coefficient for ebranes with different thickness (pore size=0.45 µ) The difference of about 32% in the thickness of the ebrane active layers contributes to 32 to 44% difference in θ, indicating the influence of ebrane thickness on the teperature polarization, particularly at low teperatures. Increasing the ebrane thickness will increase the resistance (δ /k ) to heat transfer for the sae aount of heat being transferred, resulting in a lower driving force for heat transfer. The difference between T f and T p will increase and thus θ increases. 17

18 4.4. Effect of porosity and tortuosity In a previous study, Al-Rub et al. found that the MD ass flux is linearly related to the ebrane porosity whereas the teperature polarization increases slightly with an increase in ebrane porosity [11]. This is due to the fact that higher porosity eans ore pore channels are opened for diffusion, hence higher flux. Both the voluetric and surface ebrane porosities affect MD perforance. The hollow fibre ebrane consisting of an additional low porosity layer at the internal surface was found to reduce the flux up to 15% [29]. The porosity of available coercial icrofiltration ebranes varies fro 60% to 90%, depending on the type of aterial, ebrane for (flat sheet or hollow fibre) and the anufacturing ethod. Several researchers have also reported successful attepts to fabricate PVDF hollow fibre ebranes with high porosity for MD applications [28, 30-31]. MS-2000 and Sartorius non-supported ebranes were tested to copare the effect of porosity and tortuosity on flux and teperature polarization. The porosity of MS ebrane was specified by the anufacturer, whereas the reported porosity for the Sartorius ebrane was used in this study [24]. The tortuosity for both ebranes was estiated using equation B.4 (Appendix B). The porosity of MS-2000 and Sartorius ebranes are 82% and 62%, respectively. The estiated values of tortuosity are 1.22 and 1.61, respectively. Figure 8 shows flux and teperature polarization coefficient for ebranes with different porosity. As expected, the ebrane with higher porosity (MS-2000) has higher flux. Using the known porosity and estiated values for tortuosity, the Schofield odel predicted that higher porosity and lower tortuosity will lead to lower teperature polarization coefficient, hence higher flux. This is because higher porosity reduces the effect of heat loss via conduction [2], hence increasing the flux and causing T f to decrease and T p to increase. As a result, the overall θ decreases. The θ values for lower porosity Sartorius ebrane (Fig. 8b) are 50% higher than the MS [32]. The results fro this study are in good agreeent with a previous study by Abu Al-Rub that proved porosity to be the ost influence on DCMD perforance in coparison with other ebrane properties such as thickness [11]. 18

19 (a) (b) Figure 8. (a) Theoretical and experiental flux and (b) teperature polarization coefficient for ebranes with different porosity (pore size 0.2 µ) 4.5. Effect of ebrane support Mebranes are usually consisted of a thin active layer supported on another aterial to assist in handling and iprove durability. Polypropylene (PP), polysulfone (PS), and polyethylene 19

20 terephthalate (PET) are the ost coon supporting aterials. The effect of ebrane support was studied by coparing the perforance of a non-supported ebrane (MS-2000) and a supported ebrane, (MS-3020) with siilar ebrane properties. Figure 9 shows that the supported MS-3020 gives lower flux copared to the non-supported ebrane. Support aterials not only block soe of the pores of ebrane, but also reduce the porosity. A reduction in porosity of 50% in a scri backing ebrane has been estiated using SEM iages. In addition, the 20% difference in thickness could be another factor that contributes to this observation. Moreover, support aterials absorb soe heat supplied by the feed, thus increasing the teperature polarization. The influence of additional heat absorption by the support aterial has been incorporated into the theoretical flux calculations for MS-3010, MS-3020, MS-4010 and GE ebranes. The presence of ebrane support aterial on the pereate side reduces the flux by up to 56%. Figure 10 shows the coparison between the non-woven and scri-backing supported ebranes. The experiental fluxes at 30 C for both types of support did show significant difference. However, as the feed teperature was increased, the fluxes obtained by the ebrane with non-woven support (MS-3010) were higher. The SEM iage (Fig. 5b) indicates that the porosity of non-woven support aterial is higher copared to the scri-backing, but it was not possible to verify this experientally since it is not possible to copletely separate the two layers. The estiated porosity of the non-woven support aterials for the MS-3010 and MS-3020 is around 68% based on the literature value reported for a siilar ebrane by the sae anufacturer [12]. The non-woven support fibres were fored in four to five layers on the active ebrane surface, thus the contact points between the support aterial and ebrane are iniised. However, this structure tends to trap soe stationary pereate (water) olecules. At higher teperatures, when flux across the ebrane is expected to be higher, the effect of non-woven ebrane support is ore evident. This observation differs fro that reported by Zhang et al. [12]. However, the scri-backing ebranes used in their study have a higher porosity (70%). Thus, it is an evidence that the porosity of ebrane support aterial has soe influence on the flux of DCMD. 20

21 (a) (b) Figure 9. (a) Theoretical and experiental flux and (b) teperature polarization coefficient for supported (MS-3020) and non-supported (MS-2000) ebranes (pore size = 0.22 µ, porosity = 82%, thickness = 36 µ for MS-3020 and 30 µ for MS-2000) 21

22 (a) (b) Figure 10. (a) Theoretical and experiental flux and (b) teperature polarization coefficient for scri-backing (MS-4010) and non-woven supported (MS-3010) ebranes (pore size 0.45µ, porosity 82%) Figure 11 shows the relationship between the flux and teperature polarization coefficient for the ebranes used in this study. As expected, high teperature polarization (as indicated by low θ) is 22

23 observed when the flux is high and vice-versa. The unsupported ebranes showed higher θ values up to 0.64) which indicates lower teperature polarization copared to supported ebranes with θ only up to 0.5. Figure 11 also indicated that the teperature polarization effect on flux is ore significant in the following order of support aterials: scri-backing > non-woven support>nonsupported. In ters of ebrane properties, increasing the ebrane thickness will shift the relationship on Figure 11 to the right in x-axis (increases θ). The flux (y-axis) is ore affected by the pore size and porosity. Figure 11. The relationship between flux and teperature polarization coefficient for various ebranes 5. Conclusions The MD perforance of several coercial icroporous hydrophobic PTFE ebranes with various paraeters and different support aterials were investigated in this study. The experiental results fro different pore size, thickness, porosity, tortuosity and support aterials were copared to the theoretical Schofield odel fluxes which were obtained using Polyath NLE solver. It was found that: Mebrane porosity has the ost influence on DCMD perforance copared to pore size and thickness. For very thin ebranes, the inversed relationship between thickness and flux becoes saller and negligible as δ<<k /U. 23

24 Non-supported ebranes show better perforance in DCMD than supported ebranes due to the absence of flux blockage at the pereate side and additional teperature polarization by the ebrane support aterial. The use of ebranes with non-woven support aterials results in higher fluxes in coparison with the scri-backing aterial if it has higher porosity due to its likelihood to have less contact with the active ebrane surface. The presence of ebrane support aterial on the pereate side reduces the flux by up to 56%. Although the fil heat transfer coefficient on the feed side is not affected, the overall heat balance is affected thus increases the teperature polarization (indicated by reduced θ). Acknowledgeents The authors would like to thank CSIRO and Monash University for providing the research facilities and the Ministry of Natural Resources and Environent Malaysia for financial supports. Noenclature C p Specific heat capacity (J kg -1 K -1 ) D Diffusion coefficient ( 2 s -1 ) D H Hydraulic diaeter () D Molecule diaeter () H Channel height () h Fil heat transfer coefficient (W -2 K -1 ) J Mass flux (kg -2 s -1 ) K Theral conductivity (W -1 K -1 ) M Molecular weight (kg ol -1 ) Nu Nusselt nuber P Pressure (Pa) p Average pressure (Pa) p * Saturated pressure (Pa) Pr Prandtl nuber Q i Heat transfer (W -2 ) R Gas constant (8.314 J ol -1 K -1 ) Re Reynolds nuber R i Resistance to transport ( 2 s -1 ) R Mebrane pore radius () T Teperature (K) W Channel width () x Mole fraction Greek letters δ ebrane thickness () ε ebrane porosity τ ebrane tortuosity λ latent heat of vaporization (J kg -1 ) ρ fluid density (kg -3 ) 24

25 χ ean free path () γ activity coefficient υ linear velocity ( s -1 ) µ dynaic viscosity (kg -1 s -1 ) θ Teperature polarization coefficient Subscripts A Air B Bulk F Feed G Gas I Inlet K Knudsen M Molecular Mebrane NaCl Sodiu chloride o Outlet p Pereate pl Polyer s Solid v Viscous w Water References [1] B. Bolto, T. Tran, M. Hoang, Mebrane distillation - A low energy desalting technique?, Water, 34 (4) (2007) [2] E. Curcio, E. Drioli, Mebrane distillation and related operations - A review, Separation and Purification Reviews, 34 (1) (2005) [3] G.W. Meindersa, C.M. Guijt, A.B. De Haan, Water recycling and desalination by air gap ebrane distillation, Environental Progress, 24 (4) (2005) [4] S. Srisurichan, R. Jiraratananon, A.G. Fane, Mass transfer echaniss and transport resistances in direct contact ebrane distillation process, J. Mebr. Sc., 277 (1-2) (2006) [5] F.A. Banat, J. Siandl, Theoretical and experiental study in ebrane distillation, Desalination, 95 (1) (1994) [6] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A fraework for better understanding ebrane distillation separation process, J. Mebr. Sc., 285 (1-2) (2006) [7] M. Gryta, Fouling in direct contact ebrane distillation process, J. Mebr. Sc., 325 (2008) [8] Z. Lei, B. Chen, Z. Ding, Mebrane distillation, in Special Distillation Processes. 2005, Elsevier Science: Asterda. p [9] M. Khayet, Mebranes and theoretical odeling of ebrane distillation: A review, Advances in Colloid and Interface Science, 164 (2011) [10] J. Zhang, M. Duke, E. Ostarcevic, N. Dow, S. Gray, J.-d. Li, Perforance of new generation ebrane distillation ebranes, in Singapore Water Week. 2008: Singapore. [11] F.A. Abu Al-Rub, F. Banat, K. Beni-Melhi, Paraetric sensitivity analysis of direct contact ebrane distillation, Sep. Sci. Technol., 37 (14) (2002) [12] J. Zhang, N. Dow, M. Duke, E. Ostarcevic, J.D. Li, S. Gray, Identification of aterial and physical features of ebrane distillation ebranes for high perforance desalination, Journal of Mebrane Science, (2009). [13] K.Y. Wang, S.W. Foo, T.S. Chung, Mixed atrix PVDF hollow fiber ebranes with nanoscale pores for desalination through direct contact ebrane distillation, Industrial and Engineering Cheistry Research, 48 (9) (2009)

26 [14] R.W. Schofield, A.G. Fane, C.J.D. Fell, Gas and vapour transport through icroporous ebranes. II. Mebrane distillation, Journal of Mebrane Science, 53 (1-2) (1990) [15] R.W. Schofield, A.G. Fane, C.J.D. Fell, Heat and ass transfer in ebrane distillation, Journal of Mebrane Science, 33 (3) (1987) [16] C. Fernández-Pineda, M.A. Izquierdo-Gil, M.C. García-Payo, Gas pereation and direct contact ebrane distillation experients and their analysis using different odels, J. Mebr. Sc., 198 (1) (2002) [17] B.R. Bird, E. Stewart, E.N. Lightfoot, Transport Phenoena, John Wiley & Sons.New York, [18] L. Martínez, J.M. Rodríguez-Maroto, Mebrane thickness reduction effects on direct contact ebrane distillation perforance, J. Mebr. Sc., 312 (1-2) (2008) [19] R.M. Felder, R.W. Rousseau, Eleentary principles of cheical processes, John Wiley & Sons.New York, [20] K.W. Lawson, D.R. Lloyd, Mebrane distillation, J. Mebr. Sc., 124 (1) (1997) [21] M. Gryta, M. Toaszewska, A.W. Morawski, Mebrane distillation with lainar flow, Separation and Purification Technology, 11 (2) (1997) [22] J. Phattaranawik, R. Jiraratananon, A.G. Fane, Effects of net-type spacers on heat and ass transfer in direct contact ebrane distillation and coparison with ultrafiltration studies, J. Mebr. Sc., 217 (1-2) (2003) [23] M. Qtaishat, T. Matsuura, B. Kruczek, M. Khayet, Heat and ass transfer analysis in direct contact ebrane distillation, Desalination, 219 (1-3) (2008) [24] A. Hong, A.G. Fane, R. Burford, Factors affecting ebrane coalescence of stable oil-inwater eulsions, J. Mebr. Sc., 222 (1-2) (2003) [25] M. Khayet, J.I. Mengual, T. Matsuura, Porous hydrophobic/hydrophilic coposite ebranes: Application in desalination using direct contact ebrane distillation, J. Mebr. Sc., 252 (1-2) (2005) [26] M.C. Garcia-Payo, M. Essalhi, M. Khayet, Effects of PVDF-HFP concentration on ebrane distillation perforance and structural orphology of hollow fiber ebranes, Journal of Mebrane Science, 347 (1-2) (2009) [27] M.M. Teoh, T.-S. Chung, Mebrane distillation with hydrophobic acrovoid-free PVDF- PTFE hollow fiber ebranes, Separation and Purification Technology, 66 (2) (2009) [28] D. Hou, J. Wang, D. Qu, Z. Luan, X. Ren, Fabrication and characterization of hydrophobic PVDF hollow fiber ebranes for desalination through direct contact ebrane distillation, Separation and Purification Technology, 69 (1) (2009) [29] M. Gryta, Influence of polypropylene ebrane surface porosity on the perforance of ebrane distillation process, J. Mebr. Sc., 287 (1) (2007) [30] M.C. García-Payo, M. Essalhi, M. Khayet, Effects of PVDF-HFP concentration on ebrane distillation perforance and structural orphology of hollow fiber ebranes, Journal of Mebrane Science, 347 (1-2) [31] S. Bonyadi, T.S. Chung, R. Rajagopalan, A novel approach to fabricate acrovoid-free and highly pereable PVDF hollow fiber ebranes for ebrane distillation, AIChE Journal, 55 (3) (2009) [32] F.A. Abu Al-Rub, F. Banat, K. Bani-Melhe, Sensitivity analysis of air gap ebrane distillation, Sep. Sci. Technol., 38 (15) (2003) [33] B.E. Poling, G.H. Thoson, D.G. Friend, R.L. Rowley, W.V. Wilding, Physical and Cheical Data, in Perry's Cheical Engineers' Handbook (8th Edition), D.W. Green, Editor. 2008, McGraw-Hill: New York. [34] J. Blu, A. Lindeann, M. Meyer, C. Strasser, Characterization of PTFE Using Advanced Theral Analysis Techniques, International Journal of Therophysics, (2008) 1-9. [35] A. Boudenne, L. Ibos, E. Gehin, Y. Candau, A siultaneous characterization of theral conductivity and diffusivity of polyer aterials by a periodic ethod, Journal of Physics D: Applied Physics, 37 (1) (2004)

27 [36] M. Hu, D. Yu, J. Wei, Theral conductivity deterination of sall polyer saples by differential scanning calorietry, Polyer Testing, 26 (3) (2007) [37] D.M. Price, M. Jarratt, Theral conductivity of PTFE and PTFE coposites, Therochiica Acta, (2002) [38] C.M.A. Lopes, M.I. Felisberti, Theral conductivity of PET/(LDPE/AI) coposites deterined by MDSC, Polyer Testing, 23 (6) (2004) [39] N. Sobatsopop, A.K. Wood, Measureent of theral conductivity of polyers using an iproved Lee's Disc apparatus, Polyer Testing, 16 (3) (1997) [40] B. Weidenfeller, M. Höfer, F.R. Schilling, Theral conductivity, theral diffusivity, and specific heat capacity of particle filled polypropylene, Coposites Part A: Applied Science and Manufacturing, 35 (4) (2004) [41] E.W. Leon, Fluid Properties, in CRC Handbook of Cheistry and Physics, D.R. Lide and W.M. Haynes, Editors [42] R.M. Felder, R.W. Rousseau, Eleentary principles of cheical processes, John Wiley & Sons.New York, [43] S.B. Iversen, V.K. Bhatia, K. Da-Johansen, G. Jonsson, Characterization of icroporous ebranes for use in ebrane contactors, J. Mebr. Sc., 130 (1-2) (1997)

28 Appendix A Paraeters used for theoretical calculations Paraeter Sybol Unit Correlation Ref. Latent heat of λ V -1 J kg water a vaporization of Theral conductivity of water Theral conductivity of PTFE b Theral conductivity of PET b Theral conductivity of PP b Theral conductivity of air Heat capacity of water Viscosity of liquid water T T [33] T x k w W -1 K x10-9 T T T [33] k s W -1 K [34-37] k s W -1 K [38] k s W -1 K [39-40] k g W -1 K x10-9 T x10-6 T x10-4 [41] C p J kg -1 K [42] µ kg -1 s x 10 T 140 Viscosity of air a µ g kg -1 s x10-11 T x10-8 T x10-6 [41] Density of liquid ρ kg -3 water a [33] T T [41] a Interpolated fro handbook b Median value fro references 28

29 Appendix B Definition Resistance to olecular diffusion Total pressure inside ebrane pores Relationship between average pressure inside pores, diffusivity and teperature Tortuosity relation to porosity for loosed packed ebrane structure [43] Resistance to viscous flow Resistance to Knudsen diffusion Vapour pressure of water Equation R M ε pd = (B.1) τ p air p = p air + p v (B.2) T pd = 4.46x10 (B.3) 1 τ = (B.4) ε R v = 2 εr p 8τµ g (B.5) 2r ε 8RT R K = (B.6) 3τ πm * pw = x T (B.7) Corrected vapour pressure * pw = γ w x w pw (B.8) Nusselt nuber Reynolds nuber Prandtl nuber Hydraulic diaeter hd H Nu = = k B B = 11.5 for cooling (feed side) D H 0.5 ( Re) ( Pr) ( ) B = 15 for heating (pereate side) L (B.9) υρd H Re = (B.10) ε µ s µc Pr = k p (B.11) D 2WH = (B.12 ) H W + H Heat transfer coefficient * 11.5 k DH h f = Re Pr D L H (B.13a) h p = 1.524* 15 k D H Re 0.23 Pr 0.23 D L H 0.5 (B.13b) Feed side bulk teperature Pereate side bulk teperature Average ebrane teperature Tbf,i + T Tbf = 2 Tbp,i + T Tbp = 2 Tf ' + Tp' T = 2 bf,o bp,o (B.14) (B.15) (B.16) 29