Pervaporation separation of organics from multicomponent aqueous mixtures

Pervaporation separation of organics from multicomponent aqueous mixtures

Chemical Engineering and Processing 46 (2007) 300–306 Pervaporation separation of organics from multicomponent aqueous mixtures ¨ Ayc¸a Hasano˘glu, Y...

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Chemical Engineering and Processing 46 (2007) 300–306

Pervaporation separation of organics from multicomponent aqueous mixtures ¨ Ayc¸a Hasano˘glu, Yavuz Salt, Sevinc¸ Keles¸er, Semra Ozkan, Salih Dinc¸er ∗ Yıldız Technical University, Chemical Engineering Department, Davutpa¸sa Cad., No. 127, 34210 Istanbul, Turkey Received 23 February 2006; received in revised form 28 June 2006; accepted 28 June 2006 Available online 3 July 2006

Abstract Pervaporation (PV) separation of ternary and quaternary mixtures of ethyl acetate (EAc), water, ethanol (EOH) and acetic acid (AsAc), which are present in the hydrolysis of ethyl acetate encountered in pharmaceutical industries was investigated. Water concentrations in the feed mixtures are in the range of 90–98 wt.%, while the concentrations of ethyl acetate, ethanol and acetic acid are much lower. Polydimethylsiloxane (PDMS) was used as membrane to separate organic compounds from aqueous streams. Pervaporation experiments were conducted at 30 ◦ C. The effect of feed concentration on flux and selectivity was discussed. Experiments show that PDMS membrane is much more selective to ethyl acetate than other organic components. Increase in the ethyl acetate concentration in feed mixture yields higher total fluxes but lower selectivities of ethyl acetate. © 2006 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Multicomponent; PDMS; Ethyl acetate

1. Introduction Pervaporation, a promising separation method, is an energyefficient way of separating liquid mixtures, such as azeotropic mixtures and mixtures in which relative volatilities of the components are low, that are difficult to separate by conventional methods [1]. Pervaporation offers advantages over conventional techniques like distillation, such as, less energy requirement, separation of azeotropes and modular design [2]. However, as pervaporation is a diffusion-controlled process, the flux through the membrane is generally low. For this reason, this process becomes economically more attractive when the preferentially permeable component is present in the feed at low concentration [2,3]. Important applications of pervaporation technology are removal of water from organics, removal of organics from water and organic/organic separations [4]. Removal of organics from aqueous solutions is of particular interest for recycling process water and for the treatment of wastewater. In environmental applications, removal of organic compounds by pervaporation is becoming more important and

Corresponding author. Tel.: +90 212 4491925; fax: +90 212 4491895. E-mail address: [email protected] (S. Dinc¸er).

0255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2006.06.010

can be more attractive economically than classical methods. To optimize the overall separation process, pervaporation is usually combined with other separation methods to be used in hybrid systems [5]. Lipnizki et al. made a detailed study of hybrid processes [6]. The integration of pervaporation–distillation hybrid processes can play an important role in waste minimisation and clean technology. In most cases, the hybrid processes may offer potential savings in energy because of reduced thermal and pressure requirements. Also savings due to the elimination of chemical entrainers will occur and upgrading of the product quality can be expected. The first integration of a distillation–pervaporation hybrid process into the isopropanol (IPA) production was suggested by Binning and James [7]. In their process layout, the top product of an IPA distillation column, a ternary mixture of isopropanol–ethanol–water mixture, was dehydrated by a PV unit using hydrophilic membranes to produce a saleable alcohol mixture with less than 0.5 wt.% water. An economic comparison revealed that the investment costs of the hybrid process were 31% lower than for a two-column azeotropic distillation process with hexane as entrainer. The combination of pervaporation with a chemical reactor has been also found to be an interesting alternative to conventional processes. The integration of pervaporation into the conventional esterification process, therefore, offers the opportunity to shift the chemical equilib-

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rium by removing water. This hybrid process overcomes the inhibition of the chemical equilibrium of the process and therefore leads an increased productivity. This process also allows one to use heat of the chemical reaction to increase the efficiency of the pervaporation process and leads consequently to potential savings in energy costs. The economic comparison discovered that these potential savings in operating costs combined with an increased productivity means that the hybrid process is often more favourable than conventional alternatives [6]. Pervaporation with hydrophobic membranes is considered to be an interesting alternative process for the separation of organic components from aqueous media. Organics to be separated could be pollutants or high value products like aroma compounds. One of the most used polymeric materials for organic separation is PDMS. PDMS exhibits high selectivity and permeability towards organic substances because of the flexible structure, and therefore is preferred for the removal of organic compounds from water [8]. EAc is an important raw material for many applications in chemical industry including coatings, adhesives, perfumes, plasticizers and pharmacy [9,10]. In this study, ethyl acetate, ethanol, water and acetic acid mixtures, which are encountered in the hydrolysis reaction of ethyl acetate were separated by pervaporation using PDMS membrane. Pervaporation experiments were carried out for selected concentrations at 30 ◦ C. In the presence of ethyl acetate and water, the hydrolysis reaction of ethyl acetate occurs forming acetic acid and ethanol. In addition, the reaction rate increases with increasing temperature. Thus, at higher temperatures concentrations of compounds change very sharply while concentration change is negligible at lower temperatures. In the studied mixtures, water is present in excess; therefore, practically the concentration of water does not change much but the acetic acid, ethyl acetate and ethanol concentrations change considerably with rising temperature. Thus, the experiments were conducted at a relatively low temperature of 30 ◦ C. However, in our previous work, pervaporation study of ethanol and ethyl acetate binary system was performed at higher temperatures resulting in increase of flux with rising temperature [11]. 2. Experimental 2.1. Material Ethyl acetate, ethanol and acetic acid used were analytical grade, and purchased from J.T. Baker. Distilled water is used in feed mixtures. PDMS (RTV 615 A) and its crosslinking agent (RTV 615 B) were purchased from GE Silicone representative in Turkey. 2.2. Membrane preparation PDMS (General Electric, RTV 615) is a two-component system, consisting of a vinyl terminate prepolymer with high molecular weight and a crosslinker containing several hydride groups on shorter PDMS chains [12]. 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 thicknesses of the resulting membranes were 250 and 300 ␮m. We could not obtain uniform membranes and had problems with smaller thicknesses than the ones we used in this study. However, by using modified partial flux equation we normalized the effect of membrane thickness in ternary mixtures. 2.3. Pervaporation set-up and procedure The apparatus used for pervaporation is illustrated in Fig. 1a [11]. Feed tank and membrane cell were maintained at constant temperature by a water bath. The pressure at the downstream side was kept at approximately ≤1 mbar. The effective membrane area was 23 cm2 . During the pervaporation runs product samples were taken from the collection tubes and analyzed every 1 h. The selectivities (α) are calculated from the equation below, where wi and wj are the weight percentages of components i and j, respectively: selectivity α(i/j) =

(wi /wj )permeate (wi /wj )feed


The reproducibilities of permeate compositions and fluxes were within ±1%. Permeate compositions were determined by using a Schimadzu GC 9A model gas chromatograph equipped with a TCD detector, using Porapak T 80/100 column with dimensions of 6 × 1/8 . The oven temperature was kept at 200 ◦ C. Helium was used as the carrier gas. The membrane cell is illustrated in Fig. 1b. The membrane was supported on a perforated steel disk with a hole diameter of 1.5 mm. The perforated disk diameter is 9 cm, while the feed channel height is 4 cm. Two pairs of o-rings between flanges provided the vacuum seal. The resistance-in-series model and semi-empirical Sherwood correlations given by Liang et al. [13] were employed to calculate mass transfer coefficients of bulk components to membrane surface and boundary layer thickness from a knowledge of Reynolds and Schmidt numbers based on a feed flow rate of 50 ml/min. The calculated boundary layer thickness was 47.6 ␮m. The membrane surface concentrations were calculated from the concentration polarisation equation given by She and Hwang [14]. The ethyl acetate concentrations on membrane surface were about 5% lesser than the corresponding bulk concentrations while the other organic constituent concentrations on the membrane surface were almost the same as their bulk concentrations. These results indicate that although some concentration polarisation takes place the concentration change from the bulk to the membrane surface is low enough indicating negligible decrease in membrane performance. The feed mixture concentrations and the thicknesses of membranes used for quaternary and ternary mixtures are shown in Table 1. The feed concentrations were selected in view of the solubility limits of ethyl acetate in water. Table 2 presents selected physicochemical properties of water and the organic components investigated.


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Fig. 1. (a) Pervaporation apparatus: 1, pervaporation module (PVM); 2, feed tank; 3, digital circulator; 4, water bath; 5, feed pump (peristaltic); 6, collection tubes; 7, dewar flask; 8, digital thermometer; 9, digital gauge; 10, vacuum pump. (b) Schematic representation of membrane cell. Table 1 Feed concentrations investigated (wt.%) Quaternary mixtures (300 ␮m) %Water 97.5 %EOH 1 %EAc 1 %AsAc 0.5 Ternary mixtures (300 ␮m) %Water 95 %EOH 4 %EAc 1 Ternary mixtures (250 ␮m) %Water %EOH %EAc

95 3 1 1

92.5 4 1 2.5

97.5 1.5 1

95 2.5 1.5 1

92 6 2

98 1 1

95 2 2 1

90 6 3 1

90 7 3

92 5 3

92 5 3

92.5 3.5 3 1

95 1 3 1

95 2 3

90 6 4

90 5 5

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Table 2 Physicochemical data for components [15–17] Component

Molar volume (cm3 /mol)

Solubility in water (g/100 g H2 O)

Solubility parameter, δ [(cal/cm3 )0.5 ]

Water EAc EOH AsAc

18 98 58 57

– 7.9 Miscible Miscible

23.4 9.1 12.7 12.4

3. Results and discussion Fig. 2a and b represents partial fluxes of each component in the quaternary and ternary mixtures for 300 ␮m PDMS membrane. As seen in Fig. 2, PDMS membrane permeates

Fig. 2. (a) Partial fluxes of water, EAc, EOH and AsAc in the quaternary mixtures and (b) partial fluxes of water, EAc and EOH in the ternary mixtures.

EAc much more than the other components. This is not unexpected because the solubility parameter of PDMS is closer to ethyl acetate than the other components as shown in Table 2 (δPDMS = 8.1(cal/cm3 )0.5 ) [17]. 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 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 [18]. With the increase in the ethyl acetate concentration in the feed, the ethyl acetate fluxes increase. While the ethyl acetate concentrations increase from 1 to 3 wt.%, total fluxes (summation of partial fluxes) change in the range of 79–307 g/m2 h for the quaternary mixtures. Acetic acid and ethanol permeate through the PDMS membrane in very small quantities. For the mixtures, which have the same EAc concentrations, the increase in ethanol concentration in feed yields higher partial fluxes of ethanol while the acetic acid flux is not much affected by feed concentration of acetic acid. As seen in Fig. 2a, the three different feed mixtures having 3% EAc concentrations have different partial fluxes of EAc indicating the effects of mutual interactions of each component on the fluxes. Permeation of components is not only affected by the interaction of the individual permeants with the membrane but also affected by their mutual interactions. This phenomenon can be explained in terms of the plasticizing effect of the organic molecules. As the EAc concentration in the feed increases, membrane swells more and the polymer chains become more flexible, thus decreasing the energy required for the diffusive transport through the membrane. Many possible pairs of interactions such as water–water, ethanol–ethanol, water–ethanol, ethyl acetate–water, etc., exist. Thus, the mobility of any given penetrant molecule may widely differ from that of another, since mobility depends on the relative strength of short-range interactions existing between these molecules [19,20]. The transport of multicomponent mixtures through a polymeric membrane is generally very complex. Fig. 3 shows the dependence of EAc/water ratio on the total flux of the quaternary systems. The total fluxes increase with increasing EAc/water ratio. This is a result of hydrophobicity of the membrane and the affinity of the membrane to the EAc molecules. Figs. 4 and 5 show the effects of EAc/organic ratio and organic% in feed mixture on the EAc/organic and EAc/water selectivities, respectively. The increase of ethyl acetate amount in the feed mixture, which PDMS is more permeable causes the membrane to swell. Therefore, the amount of other soluble com-


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Fig. 3. Effect of EAc/water ratio on the total flux of the quaternary mixtures of water, EAc, EOH and AsAc.

Fig. 5. Effect of organic% in the feed mixture on the selectivity (EAc/water) of the quaternary mixtures of water, EAc, EOH and AsAc.

ponents dissolved in the membrane increases due to presence of ethyl acetate. Consequently, an increase in EAc/organic ratio in feed yields a decrease in EAc/organic selectivity. As the organic concentration increases in feed, the selectivity of EAc to water decreases as seen in Fig. 5. With rising organic concentration, the membrane swells more and the penetration of smaller solvent molecules through the membrane becomes much easier. Thus, selectivities decrease with rising organic concentration. For ternary mixtures pervaporation experiments were performed with 250 and 300 ␮m-thick PDMS membranes. Thus, the effect of the membrane thickness was determined. According to the Fick’s equation and the solution-diffusion model, permeability of a permeant through a membrane should be independent of membrane thickness, but the flux is inversely proportional to membrane thickness [21]. In this study although selectivities

are not affected, fluxes change with thickness. In Fig. 6, both the variation of fluxes through 250 and 300 ␮m membranes with the EAc/water ratios in feed mixtures are presented and it is seen that the fluxes through 250 ␮m membranes are higher than those through the 300 ␮m membranes, which is in agreement with the solution diffusion model. To eliminate the effect of the membrane thickness on fluxes a modified flux equation was used:

Fig. 4. Effect of EAc/organics ratio in the feed mixture on the selectivity (EAc/organic) of quaternary mixtures of water, EAc, EOH and AsAc.

J =

m At


where J is the modified flux, m the permeate weight (g), A the membrane area (m2 ),  the membrane thickness (␮m) and t is the time (h). The calculated modified partial fluxes through 250 and 300 ␮m-thick membranes in ternary mixtures are shown in Fig. 7. As can be seen, the EAc fluxes are much higher than

Fig. 6. Effect of EAc/water ratio in the feed on the total fluxes through 250 and 300 ␮m PDMS membranes for the ternary mixtures of water, EAc and EOH.

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Fig. 9. Effect of EAc/organic ratio on the selectivity (αEAc/organic ) of the ternary mixtures of water, EAc and EOH. Fig. 7. Modified partial fluxes of the ternary mixtures of water, EAc and EOH.

4. Conclusion the other component fluxes as also encountered in quaternary mixtures (see Fig. 2). Figs. 8 and 9 show the effects of EAc/water and EAc/organics ratios on the respective selectivities. Increase in these ratios yields lower selectivities as in quaternary systems. As the composition of the more selective component ethyl acetate increases in the feed, the degree of swelling increases, and the ethyl acetate selectivity of the membrane decreases, resulting in more ethanol and water transport by increase of diffusion of the smaller molecules. The selectivities of EAc and EOH (αi/water ) to water change as, EAc: 866–5327, EOH: 11–82, while the selectivities of each component to total organics (αi/organic ) change as, EAc: 1.65–6.95, EOH: 0.03–0.12, water: 0.0006–0.0046.

In this study, it was shown experimentally that PDMS membrane could be used to separate ethyl acetate–ethanol–water– acetic acid quaternary and ethyl acetate–ethanol–water ternary mixtures with acceptable flux and selectivities. Pervaporation results show that PDMS membrane is much more selective to ethyl acetate than to the other components. As the concentration of ethyl acetate increases, the total flux increases but selectivity decreases slightly. Membrane thickness affects the fluxes but the selectivity is unaffected. In such a case, a modified flux equation can be used to eliminate the thickness effect. PDMS membrane strongly favors the permeation of EAc over that of the other organic compounds present in the aqueous organic mixture. This may be attributed to the fact that the effective diffusivity of EAc in the PDMS membrane is higher than that of the other organic compounds, which is a result of the affinity of PDMS to EAc in the mixture. It can be concluded the PDMS membrane could be used to separate ethyl acetate from aqueous streams of ethanol, acetic acid and ethyl acetate with appropriate fluxes and selectivities. Acknowledgement ¨ The financial supports of YTU-BAPK-(23-07-01-02) and ¨ YTU-BAPK-(25-07-01-06) are appreciated. The support of ¨ ˙ITAK-BAYG is gratefully acknowledged. TUB Appendix A. Nomenclature

Fig. 8. Effect of EAc/water ratio on the selectivity (αEAc/water ) of the ternary mixtures of water, EAc and EOH.


membrane area acetic acid ethyl acetate ethanol isopropanol


J J  m PDMS PV PVM t TCD wi , wj W

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flux modified flux membrane thickness mass of permeate polydimethylsiloxane pervaporation pervaporation module time thermal conductivity detector weight percentages of components i and j, respectively water

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