Journal of Membrane Science Pervaporation Dehydration of Ethyl

Journal of Membrane Science 327 (2009) 274–280

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Journal of Membrane Science

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Pervaporation dehydration of ethyl acetate/ethanol/water azeotrope using chitosan/poly (vinyl pyrrolidone) blend membranes

Xiu Hua Zhang, Qing Lin Liu ∗, Ying Xiong, Ai Mei Zhu, Yu Chen, Qiu Gen Zhang

National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Department of Chemical and Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China article info abstract

Article history: In this paper, chitosan (CS)/poly (vinyl pyrrolidone) (PVP) blend membranes crosslinked by glutaralde- Received 18 July 2008 hyde (GA) were prepared for the separation of ethyl acetate/ethanol/water azeotrope by pervaporation Received in revised form (PV). Their chemical and physical characteristics were studied by Fourier transform infrared (FTIR), 19 November 2008 environmental scanning electron microscopy (ESEM), X-ray diffraction (XRD), differential scanning Accepted 19 November 2008 calorimeter (DSC), thermogravimetric analysis (TGA), and contact angle measurement. The PV properties Available online 27 November 2008 of the membranes were investigated through dehydration of the azeotrope. Permeation flux increased with increasing feed temperature and PVP content, while separation factor decreased. However, the sep- Keywords: Ethyl acetate/ethanol/water azeotrope aration factor increased with increasing GA content, whereas the flux decreased. The results showed that −2 −1 Chitosan/PVP blend the membranes with PVP content of 10 wt% exhibited excellent PV properties with a flux of 953 g m h ◦ Pervaporation and separation factor of 746 at 35 C. Membranes © 2008 Elsevier B.V. All rights reserved.

1. Introduction boiling point mixtures [13–15]. Pervaporation coupled distillation is considered as a promising method for separation of homogeneous Ethyl acetate has been widely used in industrial processes, such azeotropic mixtures since it can reduce the energy consumption as organic intermediate of pharmacy, solvent of essence, rayon, and avoid the use of entrainers [16–18]. paint, printing ink, etc. [1,2]. The application of ethyl acetate is Due to the extensive applications of ethyl acetate, its purifica- attracting increasing attention and the demand is also increasing tion was widely studied. Tian and Jiang [19] and Zhu et al. [8] used rapidly with the development of industry due to its low toxicity. separately the poly (vinylidene fluoride-co-hexafluoropropene) The production of ethyl acetate is commonly based on a classical membrane to separate ethyl acetate from its aqueous solutions. Fischer esterification process of acetic acid with excessive ethanol Salt et al. [20] tried to remove the water from ethyl acetate–water in conventional industry [3,4]. Therefore the resultant ethyl acetate mixtures through a poly (vinyl alcohol) membrane crosslinked contains produced water and the residual ethanol, which can form with tartaric acid. A good separation factor was obtained though binary and ternary azeotropes; it is hence difficult to purify the ethyl the permeation flux was not so good. Hasanoglu et al. [15] also acetate just by conventional distillation. In recent years, azeotropic investigated the separation of ethyl acetate–ethanol binary mix- distillation and extractive distillation have been thoroughly inves- tures using polydimethylsiloxane membranes, the permeation tigated and used for the purification of ethyl acetate [5]. However, was good, however, the separation factor was poor. Pervapora- both of the two processes are suffering from high capital and oper- tion of toluene/acetone/ethyl acetate aqueous mixtures through ating costs because entrainers are required. Comparing with them, dense composite polydimethylsiloxane membranes had been done pervaporation (PV) demonstrates a great advantage because of its by Panek and Konieczny, aiming at removing organic solvents mild operating condition and no entrainers required [6]. from aqueous solutions, although the result was not so sat- Pervaporation is an energy efficient process for separation of isfactory [21]. Pervaporation separation of organics from ethyl organic mixtures [7,8]. The separation mechanism is based on dif- acetate/ethanol/water mixtures with the water content above ference in the sorption and diffusion of the liquid pairs through 90 wt% using a hydrophobic PDMS membrane was also reported membranes instead of the relative volatility [6,7,9–12]. Therefore, in the literature [22]. To our knowledge, there was no report on it is especially attractive for separation of azeotropic and close- dehydration of ethyl acetate/ethanol/water azeotrope using membranes, whereas pervaporation dehydration of ethanol solution was thoroughly studied [23]. The membranes used showed either high ∗ Corresponding author. Tel.: +86 592 2183751; fax: +86 592 2184822. permeation flux with low separation factor, or high separation E-mail addresses: [email protected], [email protected] (Q.L. Liu). factor with low permeation flux. In order to improve separation

0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.11.034 X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280 275 efficiency, it is of great significance to design membranes with both CS and PVP were mixed uniformly in the membrane. The accu- high permeation flux and separation factor. rately weighed samples (5 mg) were placed into aluminum cups The affinity between membranes and feed components plays and then were heated from 30 to 320 ◦C at a constant heating rate an important role in pervaporation efficiency. Chitosan (CS), of 10 ◦Cmin−1 under constant nitrogen purging at 10 ml min−1. The with many reactive amino and hydroxyl groups, is a promis- thermal stability of the membranes were analyzed using a TG209F1 ing hydrophilic membrane material and has been widely used (Netzsch, Germany) system. It was conducted by heating from 30 in pervaporation [8,24]. Poly (vinyl pyrrolidone) (PVP) is also an to 990 ◦C at a constant heating rate of 10 ◦C min−1 under a nitrogen idea hydrophilic membrane material and can easily blend with atmosphere. other organic or inorganic compounds. Currently, the relationship between the structures of polymer membranes and their perms- 2.4. Pervaporation experiments electivity has been intensively studied. Some groups reported that polymer blending, crosslinking and annealing can all strongly Pervaporation experiments were carried out on a laboratory influence permeability or selectivity [9,23]. In the present work, chi- scale apparatus. The azeotrope of 82.6 wt% ethyl acetate, 8.4 wt% tosan/poly (vinyl pyrrolidone) blend membranes crosslinked with ethanol and 9 wt% water was used as feed solution. A vacuum pump GA were prepared, characterized and used for pervaporation dehy- was used to maintain the permeate side pressure at 650 Pa. The dration of ethyl acetate/ethanol/water azeotrope. CS/PVP blend permeate was collected using a liquid nitrogen cold trap and was membranes had ever been prepared for direct methanol fuel cell analyzed by a gas chromatography (GC-950) for its concentration. applications, and were found to be effective methanol barriers and The total permeation flux (J) including all components in the per- to possess sufficient thermal stability [25]. meate and the separation factor (˛) are calculated from Eqs. (1) and (2), respectively 2. Experimental G J = (1) A × t 2.1. Materials yi/(1 − yi) ␣ = (2) xi/(1 − xi) Chitosan, with 95% deacetylation and an average molecular − weight of 2 × 105 gmol 1, was provided by Chekiang Golden-shell where G refers to the amount of the permeate (g) including water, 2 Biochemical Co., Ltd. Poly (vinyl pyrrolidone) K-30, glutaralde- ethanol and ethyl acetate, A refers to the effective area (m )ofthe hyde (GA) 25 wt%, ethanol and ethyl acetate were all purchased membrane used in pervaporation and t is the time (h) used to col- from Sinopharm Chemical Reagent Co., Ltd. Both ethanol and ethyl lect the permeate. In Eq. (2), yi is the mass fraction of component i acetate used were of analytical grade. in the permeate, xi is the mass fraction of component i in the feed solution. The subscript i represents the component in the solution such as 1 referring to water, 2 ethanol and 3 ethyl acetate. In the 2.2. Membrane preparation present work, water was named one component, both ethanol and ethyl acetate another pseudo-component, in the calculation of sep- 1.5 g CS and PVP in range of 0–0.35 g was dissolved in 100 ml aration factor, sorption selectivity and diffusion selectivity. In this 0.8 wt% acetic acid solution under stirring at 25 ◦C for 2 h, where- paper all the separation factor, sorption selectivity and diffusion after a certain amount of 1 wt% GA was added into the solution and selectivity discussed are referred to water. the mixture was then stirred for 0.5 h. The insoluble impurities were removed from the solution by pumping filtration. After deaeration 2.5. Sorption and desorption for 24 h, the resulting solution was cast onto a clean glass plate, dried in an oven at 35 ◦C for 8 h and then the obtained membranes Before each swelling test, the CS/PVP blend membranes were were annealed in vacuum at 100 ◦C for 4 h. completely dried in vacuum at 80 ◦C to evacuate the moisture and weighed immediately. Whereafter, they were immersed into 2.3. Membrane characterization ethyl acetate/ethanol/water azeotrope until an equilibrium was achieved. The membranes were taken out, wiped quickly with fil- Water static contact angles of the membranes were measured ter paper and weighed. This process was repeated for several times by the pendant drop method using a contact angle meter (SL200B, until the weight of the membrane was maintained a constant value. ◦ SOLON TECH, Shanghai, China) at 26 ± 1 C with 67 ± 2% relative The degree of swelling (DS) is calculated from Eq. (3). humidity. The density of the membranes was measured to inves- Desorption was performed to estimate the concentration of the tigate the variation of the free volume of the membranes with sorbed solution. The sorbed solution was completely desorbed from different composition [26] using a digital microbalance (Mettler the membrane at 90 ◦C under vacuum and collected in a liquid nitro- Toledo, AB204-S) with density kit. The density was determined by gen cold trap. The concentration of the collected solution was also measuring the weights of the membranes in air and cyclohexane determined by gas chromatography (GC-950). The sorption selec- ± ◦ ˛ ˛ at 26 0.5 C. The IR spectra of the membranes were recorded by tivity s and the diffusion selectivity d are calculated from Eqs. (4) Fourier transform infrared (FTIR) 740SX (Nicolet, USA) to analyze and (5), respectively the composition of the membranes. Crystal structure characteriza- Ww − Wd tion was carried out by using X-ray diffraction (XRD) (Panalytical DS = (3) Wd X’pert Philip, Holland) with Cu K␣ radiation. The diffraction was ◦ C / − C operated at 40 kV and 30 mA at a 2 range of 5–35 , using a step ˛ = i (1 i) ◦ s (4) size of 0.0167 and a counting time of 10 s per step. Environmen- xi/(1 − xi) tal scanning electron microscopy (ESEM) (XL30ESEM-TMP, Philips, ˛d = ˛/˛s (5) Holland) was used to characterize the surface and cross-section of W the membranes. The sample used to characterize the cross-section where w is the weight of the wet membrane(g), Wd is the weight was quenched by immersing the membrane in liquid nitrogen. of the dry membrane(g), Ci is the mass fraction of component i in The membranes were also characterized by a differential scan- the sorbed solution, and xi is the mass fraction of component i in ning calorimeter (Netzsch DSC 204 Phoenix) to analyze whether the azeotrope. The subscript i is the same as that in Eq. (2). 276 X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280

Fig. 1. IR spectra of the membranes of CS (a), CS/GA (b), CS/PVP (c) and CS/PVP/GA Fig. 3. XRD spectra of the CS membranes crosslinked by GA with various amounts. (d).

3.1.3. XRD patterns of the membranes 3. Results and discussion Fig. 3 shows the effect of GA and PVP content on the crystallinity of the membranes. Sharp diffraction peaks were observed at about 3.1. Membrane characterization 15◦ and 21◦ for the membranes. The CS membranes crosslinked by GA has the highest crystallinity. The intensity of the peak around 21◦ 3.1.1. FTIR spectra decreased with introduction of PVP into CS indicating that the poly- FTIR spectra allow for a qualitative and quantitative determina- mer chains flexibility increased, possibly resulting in an increase in tion of functional groups of polymers. Fig. 1 shows the spectra of permeation flux of the CS/PVP blend membranes over the CS mem- the membranes of CS, the CS crosslinked by GA, the CS blended with branes. The crystallinity decreased with increasing the GA content PVP and the CS/PVP blend crosslinked by GA, respectively. The peak for the CS/PVP blend membranes. The GA was distributed almost −1 around 1660 cm is a characteristic of carboxyl groups of PVP. The homogeneously into the CS/PVP blend during the preparation of the −1 characteristic peak of C–N bond at about 1300 cm was intensi- CS/PVP/GA solution. And networks formed between the polymer fied after crosslinking with the GA possibly because the C–N bond chains through chemical reactions and disordered the structure of formed. Comparing the CS with the CS/PVP blend membranes, one the CS/PVP blend, resulting in a decrease in the crystallinity of the −1 finds that the characteristic of C–O bond at about 1280 cm was blend membranes. weakened with introduction of PVP. 3.1.4. DSC and DTG analysis 3.1.2. Morphology of the membranes The DSC curves (Fig. 4a) of the membranes of the CS, the PVP There were no flaws in the surface and the cross-section of the and the CS/PVP blend reveal that the CS and PVP in the membrane CS/PVP blend membranes with the PVP content of 10 wt% as shown mixed rather uniformly and their compatibility was good. With the by the ESEM images in Fig. 2. This indicates that the compatibil- thermal analysis, one can find that the peak top temperature of ity between CS and PVP was quite good possibly because chemical DTG (Fig. 4b) of the CS membrane was 290.6 ◦C. It was decreased bonds formed between the CS and the PVP after crosslinked by GA. to 275.1 ◦C with introduction of 10 wt% PVP. After crosslinking with There is no phase separation observed in the CS/PVP blend mem- GA, the peak top temperature of the DTG of the CS/PVP10 membranes. And the thickness of the membranes for pervaporation was brane raised to 284.9 ◦C. This suggests that introduction of PVP about 14–17 ± 0.5 ␮m. can weaken the thermal stability of the CS membranes whereas

Fig. 2. The SEM image of the GA crosslinked CS/PVP blend membranes: the surface (a) and cross-section view (b). X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280 277

Fig. 4. DSC (a) and DTG (b) curves of CS membrane, PVP membrane, and CS/PVP10 blend membrane. crosslinking by GA can enhance the thermal stability of the CS/PVP meation flux. PVP, an amorphous hydrophilic polymer with amide blend membranes. groups and carbonyl groups, was introduced into CS to prepare CS/PVP blend membranes to solve the inherent decrease in per- 3.2. Effect of composition on pervaporation performance meation flux arising from crosslinking. It is found that the membranes demonstrate quite good PV per- Chitosan is a strong hydrophilic membrane material since it formance. The permeation flux was high. In the meanwhile, the contains many reactive amino and hydroxyl groups. However, mass fraction of ethanol and ethyl acetate in the permeate was the pristine chitosan membranes swell excessively in the ethyl below 3.5 and 0.5%, respectively, due in part to their bigger molec- acetate/ethanol/water azeotrope, resulting in a low permselectiv- ular size. ity, or even worse, the destruction of structure of the membranes. Therefore it is necessary to control the swelling of the pris- 3.2.1. Effect of PVP content on pervaporation performance tine chitosan membranes. Crosslinking could remarkably depress Polymer blending is an efficient method for membrane mod- the mobility of the polymer chains. However, it was found that ification. In this paper, PVP was introduced to modify the CS crosslinking could not effectively control the swelling of the mem- membranes. Fig. 5 reflects the effect of PVP content on the per- branes just dried at 35 ◦C, as the membranes might still broken after meation flux and selectivity. It was obvious that the permeation a short time when applying in separation of the azeotrope. Anneal- flux increased with increasing the PVP content, whereas the sepa- ing is reported to be an efficient method for controlling the swelling ration factor decreased. On the one hand, introduction of PVP into of a membrane since it not only can enhance the crosslinking [27] CS can disorder the arrangement of the CS chains, resulting in a but also lead to condensation of the polymers which will form net- decrease in the compactness of the membranes and an increase in works and reduce the polymer chain flexibility. In this work, the the swelling degree of the membranes, reflected by the variation membranes were annealed at 100 ◦C. of the density of the membranes with introducing the PVP (Fig. 6). Although crosslinking and annealing can depress the swelling As a result, the degree of the crystallinity of the CS decreased, and of membranes, it will also simultaneously lead to a decrease in per- the free volume of the membranes increased with increasing the PVP loading. All these lead to a decrease in the density of the membranes, as shown in Fig. 6. On the other hand, introduction of PVP could significantly enhance the hydrophilicity of the membranes because it has strong polarity and hydrophilic groups. The forma- tion of hydrogen bonding between the membranes and water was hence promoted. Therefore the affinity between water and membranes increased accordingly, which was confirmed by the contact angle measurement, as shown in Table 1. All these will cause an increase in the sorption of water. The decrease of sorption selectivity is unusual when the hydrophilicity of the membranes increased with increasing the PVP content (Fig. 7). Examination of the sorption selectivity and diffusion selectivity for the liquid pairs reveals

Table 1 Water contact angle of the membranes.

Membranes Contact angles (◦)

CS 69.91 ± 0.94

CS/GA0.33 72.75 ± 0.76 CS/PVP10 65.60 ± 0.82 CS/PVP10 /GA0.33 70.29 ± 0.59 Fig. 5. Effect of PVP content on the PV performance of the CS/PVP blend membranes at 25 ◦C. Subscript 0.33 is the weight fraction of GA, and 10 is that of PVP. 278 X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280

Fig. 6. Density of CS/PVP10 /GA membranes with different GA content (a) and CS/PVP/GA0.33 membranes with different PVP content (b). that the diffusion selectivity contributes significantly to the separa- 3.2.2. Effect of GA content on pervaporation performance tion factor. This can be explained by that increasing swelling degree The performance of membranes is closely related to their struc- leads to an increase in the free volume of the membranes, result- ture. Crosslinking is an efficient way to modify the structure of ing in an increase in the diffusion of all components, especially in membranes. Fig. 8 shows the effect of GA content on pervapo- favor of ethanol and ethyl acetate owing to their high feed con- ration properties. Permeation flux decreased with increasing GA centration. And the free volume played a dominant role over the content in the range of 0.25–0.50 wt%, whereas separation factor affinity possibly because of the plasticization (or synergy) effects increased. Networks formed in the blend of CS/PVP crosslinked for such kind of polymers. The effect of the free volume was even by GA can restrain the mobility of the polymer chains, and hence more remarkable with increasing PVP content. The diffusion selec- make the structure of the membranes more compact, and result tivity decreased slowly in the range of PVP content below 15%, and in a decrease in the free volume of the membranes. Therefore the then decreased remarkably with increasing the PVP content. The density of the membranes increases (Fig. 6). In addition, the crys- sorption selectivity always decreased possibly because the blend tallinity decreased due to the disorder of the polymer chains arising membranes were readily plasticized by ethanol and ethyl acetate from the crosslinking (Fig. 3). The depression of the mobility of and that their “plasticization” might be a significant contributor the polymer chains led to a decrease in the swelling degree. And to their enhancement in liquid sorption with increasing PVP con- the free volume of the membranes crosslinked by GA was also tent. As a result, separation factor decreased and permeation flux reduced. Furthermore, the hydrophilic groups of the membranes increased with increasing PVP content arising from both the affinity reduced after crosslinking with GA. As a result, the affinity between and high swelling degree. the membranes and water decreased, which was confirmed by the

Fig. 7. Effect of PVP content on the DS, ˛s and ˛d of the CS/PVP blend membranes at Fig. 8. Effect of the amount of GA on the PV performance of the CS/PVP blend 25 ◦C. membranes at 25 ◦C. X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280 279 contact angle measurement as shown in Table 1. The sorption of water would decrease after the membranes crosslinked by GA, and hence the sorption selectivity should decrease. However, the sorption selectivity increased practically. The reason may be that the

Fig. 12. Effect of temperature on the ˛s and ˛d of the CS/PVP blend membranes.

reduction of free volume restrained the sorption and diffusion of ethanol and ethyl acetate into the membranes due to their bigger molecular size. And the diffusion selectivity also increased for the same reason. As a result, the permselectivity increased as well. Whereas the permeation ﬂux decreased since both the afﬁnity for

Fig. 9. Effect of the amount of GA on the DS, ˛s and ˛d of the CS/PVP blend mem- the permeant and the swelling degree decreased with increas- ◦ branes at 25 C. ing the GA content. Fig. 9 shows the effect of GA content on the swelling degree, sorption and diffusion selectivity of the membranes, respectively. As expected, the degree of swelling of the membranes decreased with increasing the GA content; whereas both the sorption and the diffusion selectivity increased.

3.3. Effect of temperature on pervaporation performance

Temperature is an important operating parameter in pervaporation because it affects both the sorption and diffusion rates; hence it can affect significantly the performance of membranes. Fig. 10 shows the effect of temperature on pervaporation performance in the range of 25–45 ◦C. Permeation flux increased with increasing temperature, whereas separation factor decreased. This is because the frequency and amplitude of the polymer chain vibra- tion increased with increasing temperature, resulting in an increase in the swelling degree, as shown in Fig. 11. The free volume of the membranes increased after swelling. In addition, the vapor pressure of all components in the feed mixture increased with increasing the Fig. 10. Effect of temperature on the permeation flux and the separation factor of the CS/PVP blend membranes. feed temperature, and the vapor pressure at the permeate side was not affected. As a result, the diving force increased with increasing feed temperature. Hence the permeation flux increased, whereas both the sorption selectivity and the diffusion selectivity decreased (Fig. 12) as explained previously. All these lead to a decrease in the separation factor. One can find that the swelling degree first increased with increasing the temperature, and then decreased (Fig. 11). The solvation effects might be responsible for such phe- nomenon.

4. Conclusion

The membranes were prepared by blending chitosan with poly (vinyl pyrrolidone), and then crosslinked with glutaraldehyde. They have good performance in pervaporation dehydration of ethyl acetate/ethanol/water azeotrope. Crosslinking by GA and annealing can efficiently control the swelling of the membranes to ensure a high separation factor. And the addition of a certain amount of PVP can enhance the hydrophilicity of the membranes and is in favor of increasing permeation flux. In addition, the permeation flux also increased with increasing feed temperature, whereas the separa- Fig. 11. Effect of temperature on the DS of the CS/PVP blend membranes. tion factor decreased accordingly. Of all the membranes prepared, 280 X.H. Zhang et al. / Journal of Membrane Science 327 (2009) 274–280

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