ceramic composite pervaporation membrane

Separation and Purification Technology 77 (2011) 53–59

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Dehydration of ethyl acetate–water mixtures using PVA/ceramic composite pervaporation membrane Shanshan Xia, Xueliang Dong, Yuexin Zhu, Wang Wei, Fenjuan Xiangli, Wanqin Jin ∗ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, PR China

a r t i c l e

i n f o

Article history: Received 3 September 2010 Received in revised form 12 November 2010 Accepted 16 November 2010 Keywords: PVA/ceramic composite membrane Pervaporation Ethyl acetate dehydration Interaction

a b s t r a c t Dehydration of ethyl acetate (EAC)–water mixtures by pervaporation (PV) was studied using a ceramicsupported polyvinyl alcohol (PVA) composite membrane. The effects of feed temperature, feed water content and feed flow rate on the PV performance of the membrane were systematically investigated. In addition, swelling experiments were performed to evaluate the sorption characteristic of the membrane. Flory-Huggins and NRTL (non-random two liquid) theories were applied to analyze the interactions of the membrane and penetrants and the mutual interaction between the penetrants. The membrane exhibited high PV performance, the water permeance and selectivity of water to EAC were 1.45 × 10−5 kg m−2 s−1 kPa−1 and 129, respectively, at 60 ◦ C with the feed water content of 5.1 wt% and the feed flow rate of 252 mL min−1 . © 2010 Elsevier B.V. All rights reserved.

1. Introduction Ethyl acetate (EAC) has been widely used in the manufacture of varnishes, thinners, nitrocellulose lacquers and various drugs because of its low toxicity, good volatility (heat of vaporization is 31.9 kJ mol−1 ) and favorable solubility (miscible with almost all common organic liquids) [1–3]. In 2001, the international market demand of EAC was 1.0 million tons, whereas it reached 2.5 million tons in 2008. As world’s chemical and pharmaceutical industry grows rapidly, the demand of EAC will increase at a fast pace in the future [3]. EAC is mainly synthesized industrially via reactive distillation which combines reaction and distillation in one tower. As an efficient process, reactive distillation has attracted increasing attention in recent years [4,5]. For further purification of EAC, a decanter and two distillation columns are necessary [6]. At the top of the reactive column, a decanter separates the organic phase from the aqueous phase. One part of the organic phase of the reactive distillation column is used as reflux, the other part of the organic distillate is further purified in a second unit. The aqueous distillate is purified in a third unit. Since EAC, ethanol (EA) and water in the organic distillate can form several binary and ternary azotropes such as EAC–water, EA–water, and EAC–EA–water, ordinary distillation is not suitable to separate the azeotrope. Therefore,

∗ Corresponding author. Tel.: +86 25 83172266; fax: +86 25 83172292. E-mail address: [email protected] (W. Jin). 1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2010.11.019

azeotropic distillation or extractive distillation is needed in the further purification of EAC [7]. However, both of the two processes suffered from high-energy consumption and large capital cost. In addition, the product is easily to be contaminated due to the addition of entrainers or extractants. Consequently, the development of an alternative process with high-energy efficiency and no entrainer required is a major challenge. As a low energy consumption, moderate cost, compact and modular design and no entrainer required process, pervaporation (PV) has attracted intensive attentions [8–10] and been widely used in dehydration of organic mixtures [11–15], recovery of organic compounds from diluted mixtures [16,17] and separation of organic–organic mixtures [18,19]. Among these applications, the dehydration of the azeotropes or the close-boiling mixtures has important commercial benefits [10]. Although PV has high potential in dehydration of various mixtures, most of the researches focused on the PV dehydration of alcohol–water mixtures, and few literatures reported the PV dehydration of EAC–water mixtures [20–23]. In this work, PV was applied to replace the azeotropic distillation or extractive distillation in the further purification of EAC. A selfmade ceramic-supported PVA composite membrane was employed for dehydration of EAC–water mixtures. The PV performances of the prepared composite membrane were systematically investigated. Meantime, in order to analyze the transport mechanism in the PV process, the swelling of the composite membrane and the interactions between the molecules of penetrant and the membrane, and the mutual interactions among molecules of penetrant were studied.

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Nomenclature List of symbols Ji permeation flux of component i Wi weight of component i in the permeate A effective membrane area t measurement time xi fraction of component i in feed yi fraction of component i in permeate Pisat saturated vapor pressure of component i permeance of component i Qi Pp permeate pressure fˆ fugacity of the penetrant in equilibrium with the membrane fsat fugacity both for saturated liquid and for saturated vapor Vi partial molar volume of component i in the membrane Ds degree of swelling weight of wet membrane Ww Wd weight of dry membrane Ssorb selectivity of sorption ywater fraction of water in collected liquid xwater fraction of water in feed T absolute temperature excess Gibb’s free energy GE Gij parameter of the NRTL model Greek letters ˛i separation factor activity coefficient of component i i ˇ selectivity i3 interaction parameter of composite i with the membrane volume fraction of component i in membrane i 12 interaction parameter of water and ethyl acetate ui relative volume fraction of component i in membrane Subscript ij component i, j 1 water 2 ethyl acetate 3 membrane

2. Experimental 2.1. Membrane preparation and characterization Tubular ZrO2 /Al2 O3 asymmetrical ceramic supports (The average pore size, length, outer diameter and inner diameter of the ceramic membrane were 0.2 ␮m, 70 mm, 12 mm and 8 mm, respectively) were polished with metallographic sandpaper (991A, Starcke GmbH & Co. KG, Germany). 7 wt% PVA solution was prepared and the undissolved particles were removed by filtration subsequently. Maleic acid and sulfuric acid were used as crosslinking agent and catalyst, respectively. The resultant solution was coated onto the pretreated ceramic supports by dip-coating method. The membranes were air dried and then annealed at 120 ◦ C for 3 h. In order to measure the surface hydrophilicity, a planar membrane was prepared on disk ceramic support using the same solution. The static contact angles of the PVA/ceramic composite membranes were measured by the sessile water drop method using

contact angle measurement system (DSA100, Krüss, Germany) at room temperature. Water droplets (about 3 ␮L) were dropped carefully on the membrane surface. The contact angle of the PVA/ceramic composite membrane was 77.6◦ , which indicated that the membrane was hydrophilic. 2.2. Swelling experiment Samples of the active layer of the composite membrane were weighted and immersed in various EAC solutions at 60 ◦ C. The weight of each sample was monitoring periodically until a constant weight was achieved. The swollen membranes were then removed from the solutions, rapidly blotted with tissue paper and then weighted immediately. The degree of swelling (Ds ) was defined as: Ds =

Ww − Wd × 100% Wd

(1)

where Ww and Wd are the weight of wet membrane and dry membrane, respectively. The adsorbed liquid in the swollen membrane was removed by vacuum. The pressure of the vacuum was maintained at 500 Pa. Heating was used to maintain desorption temperature at 60 ◦ C, which was in accordance with the pervaporation temperature. The liquid was collected in a cold trap filled with liquid nitrogen. The composition of the collected liquid was analyzed by gas chromatography. The mass balance and the composition of the mass collected in the cold trap were listed in the Table A.1 (Appendix A). The selectivity of sorption (Ssorp ) was calculated by: S sorb =

ywater /(1 − ywater ) xwater /(1 − xwater )

(2)

where ywater and xwater represent the fraction of water in collected liquid and feed respectively. 2.3. Pervaporation experiment The PV performance of the membrane was measured on a pervaporation apparatus, which was reported in our previous work [24]. In this work, every pervaporation experiment was repeated at least three times. The composite membrane was sealed with O-rings (material of the O-rings was rubber) and installed in the tubular membrane module. The feed was heated by the water-bath heater and the temperature of the feed in the membrane module was controlled with a thermocouple and feedback loop. The feed was circulated from the feed tank to the membrane module at the desired flow rate. A vacuum pump was applied to maintain the permeate pressure at less than 1000 Pa. The vacuum pressure was monitored by a vacuum regulator. The penetrant was collected in a cold trap filled with liquid nitrogen. Composition of permeate was determined by gas chromatography (GC-2014, SHIMADZU, Japan) equipped with a thermal conductivity detector (TCD) using a Porapak Q packed column and helium (He) as the carrier gas. The temperature of the injector, detector and oven were set at 180, 180 and 160 ◦ C, respectively. The permeation flux of component i (Ji ) and the separation factor was calculated according to the following equations: Ji =

Wi At

(3)

where Wi represents the weight of component i in the permeate (kg), A is the effective membrane area (m2 ), t is the measurement time (h). ˛i =

yi /(1 − yi ) xi /(1 − xi )

(4)

where yi and xi represent the weight fraction of component i in permeate and feed, respectively.

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55

On the basis of solution-diffusion model, the transport equation for PV process can be expressed as [25]: Ji (xi i Pisat − yi P p )

Qi =

(5)

where Qi and xi are the permeance and mole fraction of component i in feed side,  i is the activity coefficient calculated by NRTL theory [26]. The NRTL parameters for EAC–water mixture [27] were listed in Table B.1 (Appendix B). Saturated vapor pressure Pisat can be obtained by Antoine equation and the Antoine parameters [28] were listed in Table C.1 (Appendix C). yi is the permeate mole fraction and Pp is the permeate pressure. The selectivity of the membrane ˇ is defined as: ˇ=

Qi Qj

(6)

where Qi and Qj are the permeance of component i and j, respectively.

Fig. 1. Effect of water content on the Ds and selectivity of sorption at 60 ◦ C.

3. Theory In PV process the separation performance of the membrane is determined by the swelling of the membrane and the interactions of the penetrant molecules with the membrane and the mutual interactions among the penetrant molecules. In order to analyze the transport mechanism of the PV process, Flory-Huggins [29,30] and NRTL theories were used to calculate the interaction of the penetrant molecules with the membrane and the mutual interaction among the penetrant molecules in PV process. For a ternary system consist of the membrane and EAC–water mixture, the thermodynamic swelling equilibrium can be expressed as [29]: ln

fˆ1 f1sat

ln

V1 2 3 V2

fˆi fi

sat

=

1 RT



V2 V2 + 23 3 )(1 + 3 ) − 13 1 3 V1 V1

P

Pi sat

Vi dP =

Vi (P − Pi sat ) RT

1 x1 x2 GE (xs ln + x2s ln + ) u1 u2 RT x1s u2 1

u2 =

2 1 + 2

(12)

4.1. Swelling study

(8)

(9)

where fˆ is the fugacity of the penetrant in equilibrium with the membrane and fsat is the fugacity both for saturated liquid and for saturated vapor. P is the total pressure. i is the volume fraction of component i in membrane which can be calculated by swelling experiment, the detailed calculation of i was given in Appendix D; Superscripts 1, 2 and 3 denote water, EAC and the membrane, respectively. Vi is the partial molar volume of component i in the membrane.i3 is the interaction parameter of the component i and the membrane. The lowest value of i3 indicates the highest sorption level and, thus, the best affinity between the component i and the membrane. i2 is the mutual interaction parameter of the water and EAC molecule. i2 is usually calculated via fitting VLE measurements [32] and related to excess Gibb’s free energy (GE ). Flory-Huggins thermodynamics is applied to calculate i2 : 12 =

(11)

4. Results and discussion

From thermodynamics, we have [31]: ln

1 1 + 2

(7)

fˆ2 V2 V2 = ln 2 + (1 − 2 ) − 1 − 3 V1 V3 f2sat + (12 1

u1 =

13 and 23 can be calculated by Eqs. (7)–(10).

V1 V1 = ln 1 + (1 − 1 ) − 2 − 3 V2 V3 + (12 2 + 13 3 )(2 + 3 ) − 23

where x1 and x2 are the mole fraction of water and EAC; GE is Excess Gibb’s free energy which can be calculated by NRTL model; u1 and u2 are the relative volume fraction of water and EAC in membrane.

(10)

The effect of water content on the Ds and the sorption selectivity of the membrane were firstly investigated. As shown in Fig. 1, the Ds increased significantly with increasing the water content. This phenomenon indicated that the membrane had good affinity for water, which was consistent with the contact angle analysis. The sorption selectivity of the membrane decreased with increasing the water content. When the water content increased, more water molecules interacted with the hydrophilic PVA chains, resulting in an enlarged swollen structure. This structure could generate the increase of free volume and consequently more EAC molecules could be adsorbed into the membrane, resulting in decreased sorption selectivity. As shown in Fig. 2, 13 and 23 decreased with increasing the water content, When water content increased, there were more water absorbed in to the membrane, the amorphous regions of the membrane were more swollen and the polymer chains became more flexible, the structure of the membrane became more relaxed [33,34]. Thus, the interaction between EAC and the membrane (EAC–M) and the interaction between water and the membrane (Water–M) both increased [35]. From Fig. 2, we can also find that Water–M was much more intensive than EAC–M, indicating that Water–M played more important role in the PV process. In PV process, the transport of a component is also affected by the mutual interaction among the penetrants. The influence of water content on 12 is illustrated in Fig. 3. The mutual interaction between EAC and water varied slightly with increasing the water content, which attributed to the weak polarity of the EAC molecules.

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Fig. 2. Effect of water content on the interaction parameter of 13 and 23 at 60 ◦ C.

4.2. Pervaporation characterization 4.2.1. Effect of operation temperature Operation temperature has a significant effect on the PV performance of the composite membrane. Fig. 4a and b displays the operation temperature effect on the permeance and flux of water and EAC, respectively. It could be observed that water flux and EAC flux increased significantly with increasing the temperature. When operation temperature increased, the partial pressure of the water

Fig. 3. Effect of water content on the interaction parameter 12 at 60 ◦ C.

and EAC in the feed side increased. As a consequence, the driving force for mass transfer across the membrane increased [36], resulting in the increase of flux. In addition, high temperature accelerated the diffusion of the water and EAC molecules, which favored the mass transfer of the process, thus the flux increased. However, permeance of both water and EAC decreased with increasing the temperature, which was opposite to the flux verse temperature relationship observed in most PV process. This may attribute to the definition of permeance [37]. As shown in Eq. (5),

Fig. 4. Effect of operation temperature on (a) permeance and flux of water (b) permeance and flux of EAC (c) selectivity and separation factor of the composite membrane (feed condition: feed water content 5.1 wt% and the feed flow rate 252 mL min−1 ).

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Table 1 Comparison of activity coefficients and saturated vapor pressure of water and EAC at different temperature. Temperature (◦ C)

Water activity coefficient

EAC activity coefficient

Water saturated vapor pressure (Pa)

EAC saturated vapor pressure (Pa)

30 40 50 60 70

6.73 5.99 5.35 4.80 4.32

1.77 1.75 1.73 1.70 1.68

4204 7362 12392 20127 31650

24205 36617 53333 75119 102690

Water content: 5.1 wt% EAC: Ethyl acetate.

the permeance is defined as flux divided by driving force. The driving force includes two temperature dependent factors:  i and Pisat . The variation of  i and Pisat with temperature was listed in Table 1. As shown in Table 1, Pisat increased with increasing the temperature while  i decreased. However, the variation of  i was much smaller than Pisat . Thus Pisat played more important role than  i in driving force. Pisat increased with increasing the temperature, resulting in the increase of the denominator in Eq. (5), and consequently the decrease of the permeance. Generally, the increase of the flux is at the expense of the separation factor. However, in this work (as shown in Fig. 4c) when the temperature increased from 50 ◦ C to 70 ◦ C, the separation factor increased remarkably. This phenomenon was similar to Peters’ work [38,39]. When the temperature varied from 30 ◦ C to 50 ◦ C, the rotating frequency and amplitude of the polymer chain increased, leading to the increase of free volume. Hence, the transport of water and EAC were both accelerated, which caused the decrease of separation factor. However, when the temperature increased from 50 ◦ C to 70 ◦ C, the excessive swelling of the active polymer layer was prevented by the rigid ceramic support [24,40]. So, the density of the amorphous region in the active polymer layer did not further expand [38], and the transport of EAC (with larger molecule diameter) was still impeded. As a result, the separation factor of the composite membrane increased from 50 ◦ C to 70 ◦ C. In contrast, the effect of temperature on the selectivity is not so obvious. A possible explanation is that the selectivity is mainly dominated by the membrane transport properties, while separation factor depends on both membrane transport properties and thermophysical properties of the feed mixture. When the operation temperature varied, the activity coefficient and saturation vapor pressure of water and EAC both changed, thus the separation factor varied remarkably. However, the selectivity varied slightly with the operation temperature for it decoupled the impact of operation conditions on the membrane properties.

Fig. 5. Effect of feed water content on the water permeance and selectivity of the composite membrane (feed condition: feed flow rate 252 mL min−1 and the operation temperature 60 ◦ C).

molecules dissolved in the membrane and the swelling degree of the membrane enlarged, which resulted in the increase of the water permeance. The slightly variation of selectivity may attribute to that selectivity decoupled the impact of operation conditions on the membrane properties. When the operation temperature was 60 ◦ C, the feed water content was 5.1 wt%, and the feed flow rate was 252 mL min−1 the composite membrane exhibited high PV performance (the water permeance was 1.45 × 10−5 kg m−2 s−1 kPa−1 and the selectivity reached 129). Comparison of the PV performance of the composite membrane with other published works is illustrated in Table 2. The PVA/ceramic composite membrane exhibited high PV performance in the PV dehydration of EAC–water mixtures. The large porosity of the ceramic support favored the diffusion of the penetrants, resulting in the high permeance. The high performance of

4.2.2. Effect of feed water content The effects of feed water content on the water permeance and selectivity are shown in Fig. 5. Water permeance increased remarkably while the selectivity decreased with increasing the feed water concentration. As displayed in Fig. 1, the Ds increased significantly with increasing the water content. Furthermore, when water content increased the interaction of both water and EAC with the membrane became more intensive (Fig. 2), which could accelerate the transport of the permeation molecules, resulting in the increment of the water permeance. The decrease of the sorption selectivity with increasing water content (Fig. 1) could account for the decrease of the selectivity. 4.2.3. Effect of feed flow rate Fig. 6 illustrated the effect of the feed flow rate on the PV performance of the composite membrane. The water permeance increased with increasing the feed flow rate. When the feed flow rate increased, the concentration polarization and the temperature polarization extent decreased. Hence, there were more water

Fig. 6. Effect of feed flow rate on the water permeance and selectivity of the composite membrane (feed condition: feed water content 5.1 wt% and the operation temperature 60 ◦ C).

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Table 2 Comparison of PV performance of the composite membrane with other published works. Membrane

Feed composition (wt%)

Temperature (◦ C)

Water flux (kg m−2 s−1 )

Water permeance (kg m−2 s−1 kPa−1 )

Separation factor

Selectivity

Ref

PVA(crosslinked by TAC) PFSA-TEOS/PAN PU PVA/ceramic PVA/ceramic

EAC/W = 97.5/2.5 EAC/W = 98/2 EAC/W = 92/8 EAC/W = 94.9/5.1 EAC/W = 97.4/2.6

50 40 30 60 60

1.51 × 10−5 5.18 × 10−5 4.10 × 10−5 2.83 × 10−4 5.32 × 10−5

1.02 × 10−6 5.63 × 10−6 9.97 × 10−6 1.45 × 10−5 4.18 × 10−6

4000 496 42 633 1265

216 23 24 129 138

[20] [22] [23] This work This work

EAC: Ethyl acetate; W: Water; TAC: Tartaric acid. PFSA: Perfluorosulfonic acid; TEOS: Tetraethoxysilane; PU: Polyurethane.

the PVA/ceramic composite membrane in our work greatly meets the requirement of commercial production. 5. Conclusions A PVA/ceramic composite pervaporation membrane was used in dehydration of EAC–water mixtures. The influence of operation temperature, feed water content and feed flow rate on the PV performance was thoroughly investigated. A simultaneous increase in both flux and separation factor with increasing the temperature (from 50 ◦ C to 70 ◦ C) was observed in PV process. This particular property of the membrane was benefit to PV process. When the operation temperature was 60 ◦ C, the feed was 5.1 wt% and the feed flow rate was 252 mL min−1 , the membrane exhibited excellent PV performance with the water permeance of 1.45 × 10−5 kg m−2 s−1 kPa−1 and selectivity (water to EAC) of 129. This work provided a promising process for the further purification of EAC. Acknowledgements This work was supported by the National Basic Research Program of China (No. 2009CB623400); National Natural Science Foundation of China (No. 20990222); the “Six kinds of important talents” program of Jiang Su (2007007); the Nature Science Foundation of Jiangsu Province (SBK200930313).

Table B.1 Parameters of NTRL model. Component

Water(1)–EAC(2)

(g12 − g22 )/R (g21 − g11 )/R ˛12

1059.54 523.16 0.37

Table C.1 Parameters of antoine equation. Component

A

B

C

Water EAC

11.68 9.52

3816.44 2790.5

−46.13 −57.15

Table D.1 Density of the penetrants and membrane (active layer). Component

Density (g/cm3 )

EAC Water Membrane

0.90 1.00 1.58

Parameters of NTRL model are listed in Table B.1. Appendix C. Antoine equation See Table C.1.

Appendix A. LnP sat = A −

See Table A.1.

Psat , bar; T, K.

Table A.1 Experimental results of the swelling experiment. Water content in the solution (wt%) 1.5 3.0 4.0 5.1 6.0 7.2

W2 (g)

W1 (g)

0.249 0.286 0.341 0.234 0.352 0.376

0.270 0.322 0.393 0.297 0.462 0.495

B T +C

Water content in the cooling trap (wt%) 96.7 95.5 94.5 94.1 92.6 91.3

W1 : The mass of the membrane before desorption; W2 : The mass of the membrane after desorption.

Appendix D. The calculation of i i =

((Ds

(Ds ∗ xwi )/i ∗ xwj )/j ) + (100/m )

Ds : The mass of adsorbed liquid in the 100 g dry membrane. xwi : The weight fraction of component i in the adsorbed liquid. m : The density of the dry membrane. The density of the penetrants and membrane (active layer) in this study is listed in Table D.1. References

Appendix B. NRTL model



21 G21 GE 12 G12 x1 x2 + RT x1 + x2 G21 x2 + x1 G12



where G12 = exp(−˛12 12 ),

G21 = exp(−˛12 21 ),

12 = (g12 − g22 )/RT, 21 = (g21 − g11 )/RT

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