The esterification of acetic acid with ethanol in a pervaporation membrane reactor

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1 Desalination 245 (2009) The esterification of acetic acid with ethanol in a pervaporation membrane reactor Ayça Hasanoğlu*, Yavuz Salt, Sevinç Keleşer, Salih Dinçer Chemical Engineering Department, Yıldız Technical University, Davutpaşa Campus, Esenler-Istanbul, Turkey aycameric@yahoo.com Received 21 July 2008; revised 01 December 2008; accepted 09 February 2009 Abstract Pervaporation membrane reactors are the systems in which the separation and reaction are carried out simultaneously in order to increase conversions by removing one or more of the products formed during equilibrium reactions. In this study the esterification reaction of acetic acid and ethanol to produce ethyl acetate and water was investigated using a batch pervaporation membrane reactor. The experiments were carried out in the temperature range of C. The ratios of ethanol concentration to acetic acid concentration were chosen as 1 and 1.5. Amberlyst 15 and sulfuric acid were used as the catalysts. Polydimethylsiloxane (PDMS) prepared in our labs was used as the membrane material, permselective to ethyl acetate formed by reaction. In this way, conversions were increased by continuous removal of ethyl acetate from the reaction media. Conversions are found to increase with an increase in both molar ratios of reactants and temperature. Temperature has a strong influence on the performance of the pervaporation membrane reactor because it acts on both the esterification kinetics and pervaporation. Keywords: Pervaporation membrane reactor; Esterification; Ethyl acetate; Catalyst 1. Introduction The use of membranes in chemical reaction processes has attracted much attention. Since separation membranes permit selective permeation of a component from a mixture, they help to *Corresponding author. enhance the conversions of thermodynamically or kinetically limited reactions through controlled removal of one or more product species from the reaction mixture. Using membranes to separate products in a reversible reaction is an effective method for producing esters. By applying a hybrid process, such as esterification pervaporation, it is possible to shift the equilibrium towards Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling From the Nano to the Macro Scale (EWM 2008), May 25 28, 2008, Vale do Lobo, Algarve, Portugal /09/$ See front matter 2009 Elsevier B.V. All rights reserved. doi: /j.desal

2 A. Hasanoğlu et al. / Desalination 245 (2009) higher reaction yields. Considerable savings can also be made in the amount of reactants required (as there is hardly any need for excess amount of one of the starting components) and the reaction time. Pervaporation, itself, has become in recent years a promising technology, potentially useful in applications such as the dehydration and removal/recovery of organic compounds from aqueous solutions, as well as the separation of organic mixtures. Pervaporation is often applied in combination with another technology in a hybrid process; among these, pervaporation-distillation systems and PMR (pervaporation membrane reactor) are already finding industrial applications. In PMR, the membrane either removes the desired product or the undesired product [1 2]. Ethyl acetate (EAc) is an important raw material for many applications in chemical industry including coatings, adhesives, perfumes, and plasticizers [3]. With the consciousness of the importance of the mixture behaviour and separation of ternary or quaternary mixtures consisting of ethyl acetate, water, ethanol (EOH), and acetic acid (AAc) in industrial applications; pervaporation assisted esterification reaction of acetic acid and ethyl alcohol, yielding ethyl acetate and water was investigated in this study. In the literature several studies of pervaporation coupled esterification based on water removal have been reported, while ester removal was not much investigated. In this study, the focus is on the removal of ethyl acetate formed by the reaction between ethanol and acetic acid using PMR in the temperature range of C. The catalysts used were sulfuric acid and Amberlyst 15. The influence of parameters such as temperature and ratios of the reactants on the reactor performance were analyzed [4]. 2. Materials and methods Acetic acid, ethanol and sulfuric acid used were analytical grade and purchased from J.T. Baker. Amberlyst 15 was purchased from Acros. The membrane used in this work was a crosslinked hydrophobic membrane, polydimethylsiloxane (PDMS) prepared in our labs [5]. PDMS (RTV 615 A) and its crosslinking agent (RTV 615 B) were purchased from GE Silicone representative in Turkey. A solution of PDMS and its crosslinking agent (10 wt.%) was degassed under vacuum, then cast on membrane plates using a film applicator, and crosslinked for 1 h at 100 C in an oven by heat treatment. The thickness of the resulting membranes was ~ 200 µm. Thinner membranes were not used because they deformed during PMR applications. Reactions were carried out in batch reactors both with and without membrane under the same conditions. Thus, the effect of membrane on the conversions was determined. Initial molar ratios of ethanol/acetic acid were selected as 1 and 1.5. Batch reactor without membrane and batch pervaporation membrane reactor used are illustrated in Fig. 1. The membrane cell was maintained at constant temperature by a heating jacket. Reaction mixture was stirred by a mechanical stirrer during the pervaporation. The membrane was supported on a perforated stainless steel disk with a hole diameter of 5 mm. Two pairs of teflon o-rings between flanges provided the vacuum seal. The pressure at the downstream side was kept at approximately 1 kpa. During the pervaporation runs product samples were taken from the collection tubes, and analyzed every 50 min. Permeate compositions were determined by using a Shimadzu GC 9A model gas chromatograph equipped with a TCD detector, using Poropak T80/100 column with dimensions of 6 1/8. The oven temperature was kept at 180 C. Helium was used as the carrier gas. Effective membrane area was cm 2, while the feed mixture was 100 ml giving a membrane area/volume ratio of cm Results and discussions The esterification reaction was performed both with and without membrane at two different ethanol/acetic acid ratios (M = 1 and 1.5) and three

3 664 A. Hasanoğlu et al. / Desalination 245 (2009) Fig. 1. (a) Batch reactor without membrane; 1 2: cold water outlet and inlet of the condenser, 3: condenser, 4: temperature sensor, 5: sample inlet, 6: circulator, 7: water bath, 8: magnet, 9: magnetic stirrer (b) Batch pervaporation membrane reactor; 1 2: cold water outlet and inlet of the condenser, 3: condenser, 4: stirrer, 5: temperature sensor, 6: sample valve, 7-8: hot water inlet and outlet of the membrane reactor jacket, 9: membrane, 10: vacuum gauge,11: vacuum pump, D1,D2,D3: Dewar containers [4]. different temperatures (50, 60 and 70 C) in the presence of Amberlyst 15 catalyst, where 5 g Amberlyst 15 /100 g acetic acid was used. Fig. 2 compares the variation of conversions with time for different temperatures at equimolar condition (M = 1) in the presence of Amberlyst 15 catalyst. It can be seen that the conversions determined using the batch pervaporation membrane reactor are slightly higher than the conversions obtained using the batch reactor without membrane. Fig. 2 shows that conversions increase with temperature. Temperature has a strong influence on the performance of PMR because it acts on both the kinetics of esterification and pervaporation [6]. An increase in

4 A. Hasanoğlu et al. / Desalination 245 (2009) x t (min) 50 C M = 1 without membrane 50 C M = 1 membrane reactor 60 C M = 1 without mebrane 60 C M = 1 membrane reactor 70 C M = 1 without membrane 70 C M = 1 membrane reactor Fig. 2. The variation of conversion with time at different temperatures in the presence of Amberlyst 15 (M = 1). temperature induced accelerations of both esterification reaction and pervaporation separation. Furthermore, the ester content increased much faster at the higher temperature, however the ethyl acetate content in the reaction mixture decreased abruptly because of membrane permeation flux increase, thus accelerating the esterification. Fig. 3 compares the conversions at different temperatures when the ratio of ethanol/acetic acid (M) was increased to 1.5 in the presence of Amberlyst 15. The conversions obtained at M = 1.5 were higher than the conversions obtained at M = 1. It is well known that an excess amount of one reactant leads to increased conversions. Thus increasing M to a reasonable value in PMR improves the reactor performance. Fig. 4 shows the fluxes of each component through the membrane for different M values at x t (min) 50 C M = 1.5 without membrane 50 C M = 1.5 membrane reactor 60 C M = 1.5 without membrane 60 C M = 1.5 membrane reactor 70 C M = 1.5 without membrane 70 C M = 1.5 membrane reactor Fig. 3. The variation of conversion with time at different temperatures in the presence of Amberlyst 15 (M = 1.5).

5 666 A. Hasanoğlu et al. / Desalination 245 (2009) Fig. 4. Partial fluxes of each component through the membrane at 60 C in the presence of Amberlyst 15: (a) M = 1, (b) M = C in the presence of Amberlyst 15. As can be seen, the fluxes of ethyl acetate through PDMS membrane are much higher than those of other components. This is not unexpected because the solubility parameter of PDMS (δ PDMS = 8.1 (cal/cm 3 ) 0.5 ) is closer to ethyl acetate (δ EAc = 9.1 (cal/cm 3 ) 0.5 ) than the other components [7]. Thus, PDMS is more selective to ethyl acetate than other components. The solubility parameter is a measure of the affinity between polymer and penetrant, and can give at least a qualitative information about interaction between polymer and penetrant. As the affinity between permeant and polymer increases, the amount of liquid inside the polymer increases, and consequently the flux through the membrane increases [8]. As can be seen in Fig. 4, ethyl acetate fluxes at M = 1 are higher than at M = 1.5. Since the fluxes increase with feed concentration in a pervaporation process, the EAc fluxes at equimolar condition are higher because ethyl acetate concentration at M = 1 is much higher than at M = 1.5 at a given time. Experiments were also carried out in the presence of sulfuric acid at 60 C, where 1 g sulfuric acid/100 g AAc was used. Fig. 5 shows the variation of conversions with time at two different molar ratios in the presence of sulfuric acid catalyst at 60 C. As expected, conversions increase with increasing molar ratios. The conversions obtained with sulfuric acid are higher than the conversions obtained with Amberlyst 15 at a given time, as shown previously in Figs. 2 and 3, indicating that the reaction is more rapid with sulfuric acid. It is well known that homogeneous catalysts are usually more efficient than heterogeneous catalysts. Despite a strong catalytic effect, the use of homogeneous catalyst, e.g., sulfuric acid, suffers from drawbacks, such as the existence of possible side reactions, equipment corrosion, and having to Fig. 5. The variation of conversion with time at 60 C in the presence of sulfuric acid.

6 A. Hasanoğlu et al. / Desalination 245 (2009) Fig. 6. Partial fluxes of each component through the membrane at 60 C in the presence of sulfuric acid: (a) M =1, (b) M = 1.5. deal with acid-containing waste. The use of ion exchange catalyst such as Amberlyst 15 holds following distinct advantages over homogeneous catalysts in catalysis the purity of the products is higher, as the side reactions can be completely eliminated or are significantly less; the catalyst can be easily removed from the reaction mixture by filtration, and the corrosive environment caused by the discharge of acidcontaining waste is eliminated [9]. Fig. 6 represents the partial fluxes of components through the membrane at 60 C using sulfuric acid catalyst. The ethyl acetate fluxes are much higher than those obtained in the presence of Amberlyst 15. Since the EAc production is faster with sulfuric acid catalyst, EAc concentrations in the feed mixture are higher than those obtained with Amberlyst 15 catalyst. Therefore an increase in the production rate causes an increase in the fluxes. As for the esterification pervaporation coupling process, the ratio (F) of rates of EAc removal to EAc production is found to be the key factor defined as [10]: Here, F is a dimensionless parameter that stands for the interaction between EAc removal and production during the coupling process, and when F > 1, the rate of EAc removal is larger than the production rate, indicating that the conversion could attain 100%, and limited by the EAc production rate. Fig. 7 represents the variations of F with time for the reaction in the presence of Amberlyst 15 at different temperatures for M = 1.5. Fig. 7 shows that F increases with increasing temperature indicating that the F J S V EAc / = dc / dt EAc (1) Fig. 7. The variations of F with time at different temperatures in the presence of Amberlyst 15 (M = 1.5).

7 668 A. Hasanoğlu et al. / Desalination 245 (2009) to remove selectively the ethyl acetate formed in the esterification reaction to obtain relatively higher conversions by using PMR. Temperature has a strong influence on the membrane reactor performance because it acts on both the kinetics of esterification and pervaporation. Although use of sulfuric acid instead of a hetereogenous catalyst such as Amberlyst 15 enhances the conversions, its corrosive action on the system and removal from the reaction media present challenging problems. Fig. 8. The comparison of the variations of F with time in the presence of two different catalysts at 60 C (M = 1.5). rate of EAc permeation is faster than the rate of EAc production. Although both the EAc production rate and permeability coefficient increase with temperature, EAc production rate changes lesser as temperature increases, while the rate of EAc removal changes more as temperature increases. Fig. 8 represents the variation of F with time in the presence of two different catalysts at the same conditions (60 C and M = 1.5). The F values obtained with sulfuric acid are higher than those obtained with Amberlyst 15. EAc production rate is higher with sulfuric acid than with Amberlyst 15 indicating that sulfuric acid is a more efficient catalyst than Amberlyst 15. EAc production rate increases with the efficiency of the catalyst, thus resulting in the increased EAc permeation flux. When sulfuric acid was used as the catalyst, the maximum EAc content in the mixture was achieved in a shorter time. The permeation of EAc was proportional to EAc content in the mixture, and increased with the increasing concentrations of EAc. 4. Conclusions Based on the experimental results it can be concluded that the PDMS membrane can be used Acknowledgement The financial support of YTUAF ( ) is appreciated. The scholarship of TUBITAK- BİDEB for A.Hasanoğlu is also acknowledged. Nomenclature AAc acetic acid c EAc concentration of EAc (mol m 3 ) EAc ethyl acetate EOH ethanol F ratio of the rate of EAc removal to EAc production J EAc permeation flux of EAc (mol m 2 h 1 ) M ratio of EOH concentration to AAc concentration S membrane area (m 2 ) t time (minute) V volume of reaction mixture (m 3 ) x conversion (AAc is the limiting reactant) Greek δ solubility parameter (cal/cm 3 ) 0.5 References [1] S.Y. Lim, B. Park, F. Hung, M. Sahimi and T.T. Tsotsis, Chem. Eng. Sci., 57 (2002) [2] R. Krupiczka and Z. Koszorz, Separ. Purif. Tech., 16 (1999) [3] K.C. Wu and Y.W. Chen, Appl. Catal. Gen., 257 (2004)

8 A. Hasanoğlu et al. / Desalination 245 (2009) [4] A. Hasanoglu, Investigation of the esterification reaction for the ethyl acetate production in the presence of various catalysts using pervaporation membrane reactor, PhD Thesis, Yıldız Technical University, Istanbul, Turkey, [5] A. Hasanoğlu, Y. Salt, S. Keleşer, S. Özkan and S. Dinçer, Chem. Eng. Process., 44 (2005) [6] M.O. David, T.Q. Nyugen and J. Neel, Trans. IChemE. 69, Part A (1991) [7] A. Hasanoğlu, Y. Salt, S. Keleşer, S. Özkan and S. Dinçer, Chem. Eng. Process., 46 (2007) [8] M.H.V. Mulder, Thermodynamic Principles of Pervaporation, in Pervaporation Membrane Separation Processes, R.Y.M. Huang (ed), Elsevier, Amsterdam, 1991, pp [9] H.T.R. Teo and B. Saha, J. Catal., 228 (2004) [10] Q. Liu, Z. Zhang and H. Chen, J. Membr. Sci., 182 (2001)