water solution through response surface methodology

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

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Poly(vinyl alcohol)/ZSM-5 zeolite mixed matrix membranes for pervaporation dehydration of isopropanol/water solution through response surface methodology Zhen Huang ∗ , Xiao-fei Ru, Ya-Tong Zhu, Yu-hua Guo, Li-jun Teng Department of Packaging Engineering, Institute of Materials Science & Chemical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Mixed matrix membranes (MMMs) of poly(vinyl alcohol) (PVA) containing certain amounts

Received 15 April 2018

of ZSM-5 zeolite were evaluated for pervaporation dehydration of highly concentrated

Received in revised form 7 January

isopropanol aqueous solution. The effects of zeolite content, feed composition and feed

2019

temperature on the membrane separation performance are in detail examined by using a

Accepted 25 January 2019

preliminarily one-factor-each time method and a systematically response surface method-

Available online 4 February 2019

ology (RSM). Preliminarily results show the dehydration separation factor/selectivity have been greatly boosted but without the cost of pervaporation flux/permeance after adding

Keywords:

zeolite ZSM-5, and it is consistent very well with the Arrhenius activation energy estima-

Isopropanol dehydration

tions where water molecules require much less energy whereas ethanol molecules need

Pervaporation

much more energy to transport through the membrane, probably because of the favorable

ZSM-5 zeolite

hydrophilic and porous features of zeolite ZSM-5 as revealed by swelling and water con-

PVA

tact angle results. High feed isopropanol concentration and low feed temperature are both

Mixed matrix membrane

observed to lead to very low pervaporation flux/permeance but very relatively high separation factor/selectivity. The RSM results suggest that zeolite content, feed composition and feed temperature all have highly significant impacts on total pervaporation flux and separation factor. The interaction effect of zeolite content and feed temperature on separation factor is for the first time found to be also significant. The polynomial models established according to the RSM analysis can fit very well against the experimental data with a very high coefficient of determination and the predictions given for optimized conditions have been experimentally confirmed by the validation results with a deviation of less than 2.0%. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

may be described with three successive steps: sorption of the permeating species from the feed upstream into the membrane surface

At present, membrane-based pervaporation process has been widely accepted as an energy efficient separation technique and successfully commissioned to dehydrate highly concentrated organic solvents or remove dilute organics from aqueous solutions (Bolto et al., 2011; Cheng et al., 2017; Jiang et al., 2009; Liu et al., 2014; Ong et al., 2016). According to the solution-diffusion theory, the whole pervaporation process

∗

layer, diffusion of the permeating species from the surface layer across the membrane and desorption of the permeating species to the vapor phase at the downstream surface of the membrane. Among various pervaporation membranes, polymeric membranes, such as poly(vinyl alcohol) (PVA) (Amirilargani and Sadatnia, 2014; Deng et al., 2016; Huang et al., 2006a; Shirazi et al., 2011), polydimethylsiloxane (PDMS) (Fan

Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Huang). https://doi.org/10.1016/j.cherd.2019.01.026 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

et al., 2014; Naik et al., 2016; Zhou et al., 2017; Zhuang et al., 2015), chitosan (Fazlifard et al., 2017; Kang et al., 2013; Sun et al., 2008a), cellulose (Dogan and Hilmioglu, 2010; Zafar et al., 2012), alginate (Adoor et al., 2008; Choudhari et al., 2016), and polyimide (Hua et al., 2014; Mosleh et al., 2012), are widely used and competitively advantageous in their flexibility, relatively low cost and easy processability. However, these membranes have serious drawbacks in terms of separation performance since there is a compromise between the flexibility of macromolecular chains and the ability to discriminate between permeating species, which means that membranes if more permeable are generally less selective or vice versa. To overcome such drawback, many researchers have attempted different porous inorganic materials as a modifier such as zeolite (Adoor et al., 2008; Dogan and Hilmioglu, 2010; Huang et al., 2006a; Mosleh et al., 2012; Naik et al., 2016; Sun et al., 2008a; Zhou et al., 2017; Zhuang et al., 2015), carbon nanotube (Qiu et al., 2010; Shirazi et al., 2011), silica (Beltran et al., 2013; Choudhari et al., 2016; Peng et al., 2011), and metal organic framework (Amirilargani and Sadatnia, 2014; Deng et al., 2016; Fan et al., 2014; Fazlifard et al., 2017; Kang et al., 2013; Hua et al., 2014) to improve the pervaporation performances of polymer membranes. For the purpose of dehydration of various organics, hydrophilic membranes are demanded and, from this aspect, PVA seems to be the most attractive polymer material in view of its excellent film-forming ability, appreciated hydrophilic nature, easy modification, and good chemical stability. In our earlier works (Huang et al., 2006a; Huang et al., 2006b), a series of zeolite filled PVA MMMs have been fabricated for ethanol dehydration, and the separation performances of these membranes are found to be strongly influenced by the pore size and hydrophilic nature of the zeolites used. To continue our work on pervaporation dehydration, the present work deals with the pervaporation performance of PVA/ZSM-5 zeolite MMMs for isopropanol dehydration. It may be noted that ZSM-5 zeolite has the same MFI framework structure as silicalite-1 zeolite with three-dimensional channel system: straight channels with an aperture of 0.54 × 0.56 nm intersecting with sinusoidal channels with a pore size of 0.51 × 0.55 nm (Breck, 1964). They differ with each other regarding the Si/Al molar ratio, which is usually in the 20–1000 range for ZSM-5 but is infinite for silicalite-1. The presence of the Al atoms in the zeolite framework has rendered ZSM-5 more hydrophilic than its organophilic counterpart silicalite-1, which enables ZSM-5 to remove water through pervaporation from its mixture with organic compounds for the dehydration purpose. In present work, a number of new PVA-based MMMs were fabricated with the addition of ZSM-5 zeolite and preliminarily evaluated by using conventional one time-one factor method for pervaporation separation. To our knowledge, there is no report on isopropanol dehydration of PVA/ZSM-5 zeolite MMMs. The effects of zeolite content, feed composition and feed temperature on pervaporation results were systematically examined by using response surface methodology (RSM) coupled with Box–Behnken design (BBD). Many studies (Chew et al., 2017; Mesli and Belkhouche, 2018; Rahma et al., 2017; Shirvani et al., 2016) have demonstrated that the statistical RSM analysis is the most effective mathematical tool to investigate the effects of multiple process factors, alone or in combination on response parameters and consequently, the number of experimental runs can be greatly reduced with high efficiency. Based on the analysis results, mathematical models for the permeate flux and separation factor have thus been developed along with the optimized process conditions.

2.

Experimental

2.1.

Membrane preparations

The membranes considered in our present work included pure PVA membrane and zeolite-filled PVA counterparts prepared by means of a solution casting method. The preparation procedure could be described as follows. After adding 264 mL of double-distilled water (with a electric conductivity of 485 ␮S/m) into a 500 mL round-bottom flask, 36.0 g of PVA (a

degree of polymerization of 1750 ± 50 and a degree of hydrolysis of 99%, Tianjin Guangfu Fine Chemical Co., LTD., China) and 3.0 g of fumaric acid (FA, 99.0%, Tianjin Guangfu Fine Chemical Co., LTD., China) were fed into the flask and heated in a water bath at temperature of 95 ◦ C. The PVA dissolving was conducted under agitation for 12 h, finally resulting in a 12 wt.% PVA aqueous solution. For making zeolite-filled dopes, the batch addition of polymer was adopted to make the polymer macromolecular chains better attached to the zeolite surface and yield good zeolite-polymer interactions, and here 12.0 g of PVA was thrice fed into the system for this purpose. The zeolite used as the filler was ZSM-5 with a SiO2 /Al2 O3 molar ratio of 50 and a particle size of approximately 1 ␮m and purchased from Tianjin Nankai Catalyst Co., LTD., China. A desired amount of zeolite powder at tested membrane contents was first dispersed into 88.0 g of distilled water under moderate agitation for 1 h, followed by adding 4 g of polymer and 1.0 g of fumaric acid into the zeolite-liquid mixture. The obtained mixture was heated at 95 ◦ C and stirred at moderate speed for another 4 h. The left PVA was added twice into the resultant mixture and continuously stirred for 12 h to produce a homogeneous suspension. The resultant PVA solution and zeolite-added polymeric suspensions before casting were placed in a fumehood for 12 h for the purpose of bubble removal. The resultant dopes was spread onto a 30 cm × 40 cm glass wafer and flattened with a casting knife. After dried in the fumehood under room temperature for 24 h, the nascent film was placed in an oven at 160 ◦ C for a period of 2 h for cross-linking to occur, with fumaric acid at 0.03 mole per mole monomeric unit of PVA as the cross-linking agent according to our earlier work (Huang et al., 2006a; Huang et al., 2006b). All the PVA membranes were put in an air-tight desiccator before any subsequent characterizations.

2.2.

Membrane characterizations

The cross-sectional and surface morphologies of the PVAbased MMMs were investigated using a scanning electron microscope (SEM, SS-550, Shimadzu). The membrane crosssections were obtained by chilling the sample strips in liquid nitrogen for a couple of minutes and then cracking with forceps. The wide-angle X-ray diffraction (XRD, D/max 2500v/PC, Rigaku) analysis was performed on the MMM samples with a diffraction angle  range of 2 = 3–50 ◦ at a speed of 8◦ /min. The contact angles for water on the surface of the pure PVA membrane and its MMMs were determined using a contact angle tester (Powereach, Shanghai Zhongchen Digital Tech Apparatus Co., Ltd.) at room temperature. The membrane swelling measurements were conducted at 30 ◦ C gravimetrically in pure water and binary isopropanol (IPA, analytical grade, >99.7%, Tianjin Kaitong Chemical Reagent Co., Ltd., China) aqueous solution with a water content of 90 wt.%. The dried membrane samples, after weighed on a digital microbalance (FA2104S, Shanghai Jingke Industrial Co., Ltd., China) with an accuracy of ± 0.01 mg, were immersed in pure water and water-IPA mixture in sealed vessels at 30 ◦ C for 48 h. The swollen membranes were weighed immediately after carefully removing the water adhered to the sample surface. The measurements conducted in triplicate were averaged for each membrane. The percent degree of swelling (DS) was calculated as: DS(%) =

mS − mD × 100 mD

(1)

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

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Fig. 1 – Surface and cross-sectional SEM images of pure PVA membrane (a–c) and its MMM with a ZSM-5 zeolite content of 20 wt.% (d–f). where mS and mD are the mass of the swollen and dry membranes, respectively.

2.3.

Pervaporation experiments

The experimental pervaporation setup used in this work is shown in Fig. 1 for dehydration separation of isopropanol aqueous solutions, similar to the laboratory-scale Sulzer Chemtech pervaporation system used for ethanol dehydration (Huang et al., 2006a; Huang et al., 2006b). Mainly, the setup consists of an upstream unit (a 5.0 L feed tank, a liquid flow meter, a circulating pump and an oil-heating system), a permeating unit (a pervaporation permeation cell of stainless steel, a testing film sheet and a digital thermometer), a downstream unit (a penetrant collector, liquid N2 cold trap and a vacuum pump), as well as different type of tubings, valves and fittings. The experimental operating procedure has been described in detail elsewhere (Huang et al., 2006a,b) and may be briefed as follows. A circular test film sheet properly cut with a working area of 13.8 cm2 was mounted into the permeation cell, and then an IPA aqueous solution of about 4.0 L, prepared with

pure IPA and double-distilled water, was fed into the feed tank and circulated at a flow rate of 40 L/h. The feed solution was heated to test temperatures with the circulating oil-heating system while the permeate side was applied with a vacuum pump to keep the downstream pressure at around 100 Pa. The penetrant vapor was condensed using liquid N2 and collected in a round-bottom flask cold trap. The sample mass was weighed within ± 0.01 mg using a digital balance and the compositions of IPA-water mixtures were determined with a gas chromatography analyzer (GC 1100 Beijing Purkinje General Instrument Co., Ltd.). Triplicate independent measurements were performed for each pervaporation condition, leading to reproducible flux and separation factor values with standard errors of less than 3%. The membrane pervaporation performances can be easily examined in terms of permeation flux (J) and separation factor (␣). The total pervaporation flux may be calculated using the following equation:

Jtotal = W/At

(2)

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

Table 1 – Independent factors and levels of experimental BBD design. Code

Factor

Coded factor level

Feed temperature (◦ C) Feed IPA concentration (wt.%) Zeolite content (wt.%)

A B C

High 1

Center 0

Low −1

70 95 25

60 90 20

50 85 15

where W (g) is the total amount of the permeate collected during the experimental time interval t (h) at a steady state, A is the effective working area of 13.8 cm2 . The water over IPA separation factor (␣) for tested membranes can be calculated as follows: ˛=

ywater /yIPA xwater /xIPA

(3)

where y and x are the weight fraction of either water or IPA in the permeate and feed, respectively. Apart from pervaporation flux and separation factor, the membrane performances can also be evaluated in terms of the permeate permeances and selectivity since these parameters can reflect the intrinsic properties of tested membranes. Considering that the permeate pressure is very low and might be neglected, the permeance of IPA or water can be written as: PIPA =

JIPA feed pIPA

Pwater =

permeate − pIPA

≈

Jwater feed

permeate

pwater − pwater

JIPA feed pIPA

≈

=

JIPA xIPA IPA psat IPA

Jwater feed

pwater

=

Jwater xwater water psat water

(4)

(5)

where P is the membrane permeance, p is the partial vapor pressure and psat is saturated vapor pressure of the pure species of IPA or water at given temperature.  is the activity coefficient of water and IPA that can be calculated by using the Wilson’s equation as described in our preceding work (Huang et al., 2006b). The membrane dehydration selectivity (ˇ) is defined as the ratio of the water permeance over the IPA permeance. ˇ=

2.4.

Pwater PIPA

(6)

Factorial design of pervaporation experiments

In this work, a factorial experimental design was adopted to evaluate the pervaporation performances of PVA-based MMM membranes since such design usually allows researchers to investigate the influences of those factors involved by conducting a relatively few experiments (Chew et al., 2017; Mesli and Belkhouche, 2018; Rahma et al., 2017; Shirvani et al., 2016). For better choosing the levels of process factors, preliminary tests via conventional one-factor-each time-method were carried out in order to evaluate the effects of various factors on pervaporation flux and separation factor. All of the experiments were carried out in triplicate and mean values were used for the regression analyses. Based on these experimental results presented later in Section 3.1, the factors A (zeolite content, wt. %), B (feed temperature, ◦ C) and C (feed IPA concentration, wt.%) were selected for the RSM design and Table 1 lists the three independent factors and its significant levels considered in RSM. Each factor was changed over three levels

between −1 and +1 over the determined ranges and the ranges for each factor were determined according to the preliminary experiments. For the BBD experiment, total 17 runs were conducted to determine the optimum values of selected factors (zeolite content, feed temperature and feed IPA concentration) for maximum permeation flux and maximum separation factor. Because each factor has just three levels, a polynomial quadratic model has thus been considered:

Y = a0 +

k  i=1

ai Xi +

k 

aii Xi2 +

i=1

k k  

aij Xi Xj

(7)

i=1 j≥2

where Y is the predicted response, Xi and Xj are the actual levels of the independent factors considered, a0 , ai , aii , and aij are the regression coefficients of intercept, linear, quadratic and interaction terms, respectively; and k is the number of the factors studied. The Design-Expert software version 7.1.3 was used to analyze experimental data against the regressed model so as to predict the optimal conditions. Analysis of variance (ANOVA) of the data was performed and the quality of the fitted polynomial model was examined by using the coefficient of determination (R2 ), the adjusted coefficient of determination and the coefficient of variation (C.V.). The significance of the independent parameters and their interactions, the adequacy of the developed model and statistical significance of the regression coefficients were evaluated with F-test and they could be considered significant once the resultant p-value is less than 0.05.

3.

Results and discussion

3.1.

Characterizations of PVA-based MMMs

3.1.1.

SEM analysis result

The morphology investigation by SEM examination of zeoliteembedded PVA membrane can be used to obtain some useful information about the compatibility achieved between the inorganic zeolite and organic PVA matrix. Fig. 1 presents surface and cross-sectional SEM images of crosslinked PVA membrane and those of PVA-based MMM at a zeolite content of 20 wt.%. As can be seen from Fig. 1a, b and c, the unfilled flat PVA membrane is apparently dense and defect-free. For the filled MMM, its surface is homogeneously distributed with zeolite particles and no observable pores or cavities can be found, as shown in Fig. 1d. The cross-sectional view at large magnification, given in Fig. 1f, shows that the zeolite particles seem to embed with PVA matrix tightly and no obvious defects could be seen between zeolite ZSM-5 and PVA matrix. These morphological results tend to suggest very good inorganic–organic contact between the filler and matrix, possibly leading to appreciated separation performances.

3.1.2.

XRD analysis results

Fig. 2 presents XRD patterns of ZSM-5 powder, unfilled PVA membrane and 20 wt.% zeolite filled MMM samples. The unfilled PVA membrane shows a characteristic peak that appears at 2 = 20◦ , which is rather broad and reflects relatively imperfect crystallization feature of a semi-crystalline polymeric material. This peak still holds very well after crosslinked with a small amount of formic acid. Furthermore, XRD pattern of the 20 wt.% zeolite-loaded MMM sample show that the peak intensity at 2 = 20◦ decreases considerably as compared

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

Table 2 – Hansen solubility parameters and distance parameters considered in present work. Various HSPs(MPa1/2 )

Substance

a

Water 2-Propanola PVAa ZSM-5b a b c

Distance parameters (MPa1/2 )c

ıd

ıp

ıh

ıt

ıp–s

ız–s

18.1 15.8 17.0 18.3

12.9 6.1 9.0 9.4

15.5 16.4 18.0 13.6

27.1 23.5 28.9 24.7

4.8 3.5

4.0 5.0

ıw-IPA

ıp–z

All HSP data are collected from Ref. Hansen (2007). HSP data of ZSM-5 are averaged on those of some other metal oxides (Hansen, 2007). Subscripts p, z and w, respectively, stand for PVA, ZSM-5 zeolite and water, and subscript s represents solvent of either water or IPA.

Fig. 2 – XRD results of ZSM-5 zeolite powder, unfilled PVA and filled MMM samples. to that of the unfilled PVA membrane, reflecting that the crosslinked PVA membrane has still retained its crystal structure but its relative crystallinity has decreased a lot after the addition of zeolite ZSM-5. For the MMM sample, some additional peaks are observed at 2 = 8◦ , 9◦ and 23◦ and they are due to the incorporation of zeolite ZSM-5 in the MMM if compared to typical XRD patterns of ZSM-5 powder shown in Fig. 2.

3.1.3.

not be achieved by adding the same amount of zeolite. 2) The free volume of PVA membrane is greatly reduced as a result of the presence of zeolite particles in polymer matrix. The zeolite particles keep rigid due to the crystalline structure and may act as the physical crosslink in the PVA matrix, and then hinder the movement of PVA chains around the zeolite (Amnuaypanich et al., 2009; Khoonsap and Amnuaypanich, 2011). Also presented in Fig. 3a is the swelling behavior of PVA-based MMMs in IPA aqueous solution with a water concentration of 90 wt.%. It can be seen, the swelling result of the MMMs looks alike that in pure water, and the degree of swelling is the highest for the unfilled PVA membrane and decreases as zeolite content in the MMMs increases. The molecular affinity between two substances may be assessed using Hansen solubility parameters (HSP). In the Hansen approach (Hansen, 2007), the solubility parameter ı, i.e., the cohesive energy parameter, can be divided into three components according to the nature of molecular interactions: ıh – hydrogen bonding interactions, ıp – polar interactions and ıd – dispersion interactions. Substances with similar HSP may have high affinity for each other, and usually this is done by examining a distance parameter, ıi−j , between two substances of interest. The distance parameter can be estimated by using the partial components of the solubility parameter as follows (Hansen, 2007):

Swelling and water contact angle measurements

Fig.3a shows the swelling results of PVA-based MMMs of different zeolite contents in pure water and 90 wt.% water-containing IPA mixture. For a polymeric pervaporation membrane, its swelling property usually reflects its capability to form specific interactions with the absorbed molecules and thereafter controls the diffusion of dissolved molecules across the membrane. As shown in Fig. 3a, the unfilled PVA membrane can swell considerably in pure water even after crosslinked with fumaric acid, which is mainly attributed to hydrogen-bonding interactions between water molecules and the –OH groups of PVA macromolecules. The PVA membrane shows higher degree of swelling in pure water than that in IPA/water mixture, suggesting it possesses higher affinity to water than to IPA. This is because that water is more polar than IPA, and consequently forms relatively stronger interaction with the –OH groups of PVA chains, enhancing its swelling in the membrane. Upon introducing zeolite ZSM-5 to the PVA membrane, it is clearly observed that the DS value for the PVA membranes in pure water decreases with increasing zeolite content. It is likely because that 1) the net weight of PVA in MMMs decreases with the increase in the zeolite content and hydrophilic zeolite incorporated is capable to adsorb water but exhibits a rather lower amount of water absorption as compared with PVA membrane. In other word, a make-up to the less water adsorbed due to the reduction in PVA mass could

 ıi-j =

2

2

(ıd,i − ıd,j ) + (ıp,i − ıp,j ) + (ıh,i − ıh,j )

2

(8)

where ıi−j is the distance parameter (MPa1/2 ) between substance i and j. ıd , ıp and ıh are Hansen solubility parameters determining dispersion (d), polar (p) and hydrogen bonding (h) interactions, respectively. Accordingly, the lower the ıi−j value, the stronger the interactions between substance i and substance j. In present work, the HSP data for water, IPA and PVA are all collected from the book given by Hansen (2007) and shown in Table 2. The HSP data for zeolites including ZSM-5 zeolite are not available in the literature but available for some metal oxide fillers like TiO2 , Al2 O3 , Fe2 O3 and ZnO (Hansen, 2007). Considering the metal oxide nature of ZSM-5 zeolite, its HSP data are then taken as the averaged values of these metal oxides. The calculated distance parameters between different substances considered here are also listed in Table 2, and it can be seen that the ıp-IPA between PVA and IPA is lower than the ıp–w between PVA and water while the ız–w between ZSM-5 zeolite and water is lower than the ız-IPA between PVA and IPA. Therefore, the PVA based MMMs seem to become more hydrophilic after filling more ZSM-5 zeolite, which may in turns explain the smaller DS values resulting from a water/IPA mixture with 90 wt.% of water than from pure water. Never-

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

Fig. 3 – Swelling and water contact angle results of PVA-based MMMs of different zeolite contents. theless, using the HSP approach only cannot account for the decrease of DS value in the membrane swelling tests with the increase of added ZSM-5 zeolite, possibly related to the relatively low zeolite adsorption capacity. Likewise, Kujawska et al. (2016) have also stated that the POMS membrane exhibits the highest affinity to organic solvents according to the HSP approach whereas its pervaporation performance results cannot be understood if strictly following the HSP theory only. The hydrophilic property of pure PVA membrane may also be evaluated by the contact angle measurements and the contact angle measurements for water on the membrane surface is shown in Fig. 3b. It can be seen that the water contact angles (49–61 ± 3◦ ) for all PVA based MMMs investigated are lower than 90◦ , confirming the hydrophilic feature of these membranes. Further, the value of water contact angle is observed to decrease with the zeolite loading in the MMMs. This is possibly because the increment in hydrophilicity, due to the presence of ZSM-5 zeolite particles, has increased the affinity for water to some extent and hence led to the decrease of the water contact angle, which seems to be in good consistence with the HSP approach discussed above.

3.2.

Preliminary pervaporation results

In preliminary tests, the effects of three key independent factors, including zeolite content, feed composition and feed temperature, on pervaporation performances of PVA-based MMM membranes have been examined so that the factor levels with most significant influence on the pervaporation results can be determined for RSM experiments. However, it may be noted that the pressure at the permeate side is also very important factor to influence the pervaporation results (Kujawska et al., 2016; Mosleh et al., 2012; Smitha et al., 2004). Kujawska et al. (2016) have recently detailed the effect of the downstream pressure of 1–25 mbar on the hydrophobic PDMS/POMS membranes and found that the pervaporation performances are strongly influenced for removal of acetone, 1-butanol, ethanol and ethyl acetate from its aqueous binary mixture of 4 mass% organic concentration. It can be readily deduced that once the permeate pressure varies, the pressure difference across the membrane will change, causing the change of the driving force for pervaporation process and subsequently affecting the pervaporation performances. Mosleh et al. (2012) have reported earlier that when permeate pressure decreases higher fluxes are resulted while the IPA dehydration separation factor increases for pure polyimide Matrimid 5218 membrane and its zeolite 4 A and ZSM-5 zeolite filled MMMs. Thus, in this work, the pressure at the permeate side was kept

constantly at around 100 Pa. In the meantime, there may be a concentration polarization phenomenon to influence pervaporation of organic–water mixtures to certain extent when the feed is kept in a static state (Smitha et al., 2004), and then the feed mixture in present work was dynamically circulated at a flow rate of 40 L/min.

3.2.1.

Effect of zeolite contents

Fig. 4 shows the effects of zeolite ZSM-5 content on the mixed matrix membrane pervaporation performance in terms of separation factor, fluxes, selectivity, permeance and permeate water concentration. The zeolite contents considered here is in the range of 0–30 wt.% and the pervaporation test was conducted at 60 ◦ C for a feed of 80 wt.% IPA aqueous solution. The porous zeolite added is known to be able to change the intrinsic properties of filled membranes (Shirazi et al., 2011) and then the pervaporation results could be subsequently affected after the incorporation of zeolite ZSM-5. As shown in Fig. 4a, total pervaporation flux is seen to substantially increase when the zeolite content increases, which may be due to the hydrophilic nature and the large pore aperture of zeolite ZSM-5. As described earlier, the PVA/ZSM-5 zeolite MMMs show decreased water contact angles with the increase of the zeolite content, that is to say, these membranes become more hydrophilic at higher zeolite content. As such, the hydrophilic zeolite might have led to the formation of strong interactions between zeolite surface and water molecules, consequently rendering water species readily pass through the mixed matrix membranes. ZSM-5 zeolite is known to have a pore size larger than the kinetic diameter of water molecule (0.296 nm) and that of IPA molecule (0.470 nm) (Wang et al., 2009). For these reasons, the more zeolite added into the polymer, the higher the pervaporation flux. In terms of separation factor, Fig. 4a shows that the zeolite-loaded PVA membranes have performed outstandingly better than the neat polymer counterpart. For example, the neat PVA results in a separation factor value of only 328 but the MMM with a ZSM-5 content of 20 wt.% has yielded the highest value of 1254, increasing by more than 3.82 times as compared to the pure PVA membrane. Further increasing the zeolite content to over 20 wt.% has led to a substantial decrease in separation factor. The reason for it is possibly related to the agglomeration of the zeolite particles and subsequent poor zeolite-polymer interfacial properties, and more detail can be referred to our earlier work (Huang et al., 2006b). Fig. 4b shows that the water content in the permeate is greatly higher than the value of 20 wt.% in the feed solution, spanning from 98.8 to 99.7 wt.%.

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

25

Fig. 4 – Effect of zeolite content on separation factor/selectivity, flux/permeance and permeate water concentration of the zeolite ZSM-5 filled PVA membrane for the pervaporation of 80 wt.% IPA aqueous solution at 60 ◦ C. This is also reflected by the considerable difference between the water flux and IPA flux as shown in Fig. 4b and d. The pervaporation performances of the MMMs can be more appropriately represented in terms of selectivity and permeance, as shown in Fig. 4c and d. Promisingly, the MMMs have exhibited appreciable separation performance when the zeolite content ranges within 15–25 wt.%, showing considerably higher separation selectivity than the unfilled PVA counterpart. It can be seen that water flux and water permeance follow the same trend and both increases monotonously with the zeolite content. Likewise, IPA flux and permeance, separation factor and selectivity, respectively, demonstrated similar zeolite content dependence. This finding is understandable since operational conditions like feed IPA concentration and temperatures are holding constant when considering the zeolite content, and thus the membrane permeation flux and separation factor are entirely dependent on the MMM intrinsic permeation properties as the permeance and selectivity are. As discussed above, the zeolite content added has solely changed the membrane structural properties and then intrinsic permeation properties as well. Similar observation has also applied to ethanol dehydration with zeolite 4 A filled PVA MMMs (Huang et al., 2006b), IPA dehydration with zeolite 13X or 5 A embedded P84 co-polyimide membranes (Qiao et al., 2006) and with cyclodextin filled Matrimid 5218 polyimide (Jiang and Chung, 2010).

3.2.2.

Effect of feed composition

Fig. 5 shows the influences of feed IPA composition on pervaporation results for the 20 wt.% zeolite ZSM-5 filled MMM membrane. Over the whole IPA content range of 75–95 wt.% investigated here, total pervaporation flux decreases drasti-

cally whereas water/IPA separation factor increases substantially when the feed IPA concentration increases as shown in Fig. 5a. This finding may be due to the hydrophilic nature of both PVA polymer and zeolite ZSM-5, and the coupling effect between fast water and slow IPA molecules. The former may make the membrane materials form relatively stronger interactions with polar water molecules rather than less polar IPA molecules, resulting in preferred water transport across the membrane. It is always true that pervaporation water flux for hydrophilic membranes can increase significantly as the feed water concentration increases or the feed IPA content decreases. The latter suggests that there are mutual dragging forces between two penetrates, which could make the fast water species move forward become slowly or the slow IPA molecules move forward become rapidly due to the presence of the other species. In our case, the coupling effect appears to be more pronounced for slow IPA species at high feed water content. Because of such compromise, i.e., promoted IPA permeation, the water/IPA separation factor becomes too much lower at high feed water concentration than at low feed water concentration. The same dependence of total flux and separation factor on feed water concentration has also been reported earlier for ethanol pervaporation dehydration with the PVA-based MMMs filled different zeolites (Huang et al., 2006a, 2006b) and IPA dehydration with zeolite-4A filled polyimide MMMs (Mosleh et al., 2012). Shown in Fig. 5b are feed composition dependence results of water flux and permeate water composition for the PVA/ ZSM-5 zeolite MMMs. As can be seen, the water content in the permeate stream is very higher and it spans in the range of 99.1–99.8 wt.%, tending to suggest that the present method may be efficient in dehydrating IPA. As also can be shown, the

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

Fig. 5 – Effect of feed IPA content on separation factor/selectivity, flux/permeance and permeate water concentration of the 20 wt.% zeolite ZSM-5 filled PVA membrane for the pervaporation of IPA aqueous solution at 60 ◦ C. water flux is extremely higher than the IPA flux (see Fig. 5d) under the pervaporation condition, and the former approximately equals to the total pervaporation flux. Fig. 5c and d reflect the effect of feed IPA content on selectivity, water permeance, IPA permeance and flux results. It can be seen that the permeance of water or IPA generally decreases a lot as the feed IPA content increases from 75 to 90 wt.% but increase a little as the IPA content further goes up to 95 wt.%, unlike the monotonous decreasing trend for water or IPA flux with high IPA concentration in the feed. As for the membrane selectivity, it, unlike separation factor again, shows different dependence on the feed IPA concentration. Clearly as shown in Fig. 5c, the selectivity considerably increases from 254 to 542 as the IPA content varies from 75 to 80 wt.%, then reaches the maximum value of 587 at the IPA content of 90 wt.%, and finally drops to 458 as the IPA content goes up to 95 wt.%. The differences observed for selectivity and separation factor, and component flux and permeance as a function of feed IPA content, may be taken as a comparison result between the intrinsic membrane properties only, and the combined results of both operation conditions and intrinsic membrane properties. As detailed elsewhere (Jiang and Chung, 2010), the permeance can be derived by removing the feed component partial vapor pressure from the pervaporation flux and then thought to be pertinent parameters to describe the membrane intrinsic separation property. That is to say, the component permeances and water/IPA selectivity belong to the intrinsic properties of the PVA/ZSM-5 zeolite membrane. On the other hand, the water/PA flux and separation factor are not only influenced by the membrane intrinsic property but also affected by the partial vapor pressures of operation condition dependence (Baker et al., 2010). It can be deduced from Eqs. (4) and (5) that both

IPA and water partial vapor pressures are varying with feed IPA concentration, and after removing such effect both water and IPA permeances are seen to increase a little when feed IPA composition goes up from 90 to 95 mass%, leading to a decrease other than an increase for the water/IPA selectivity.

3.2.3.

Effect of feed temperature

Fig. 6 shows the pervaporation results achieved in the temperature range of 50–90 ◦ C for the MMM membrane. According to the results shown in Fig. 6, higher pervaporation fluxes including total flux, water flux and IPA flux have been resulted from higher feed temperature whereas higher separation factor is obtained at lower feed temperature. For instances, the total flux and separation factor values obtained at 50 ◦ C are 745 g/m2 h and 1218 but at 90 ◦ C they are 2383 g/m2 h and 126, respectively. The dependence of pervaporation flux and water/IPA separation factor on feed temperature may be explained as a combined result of effect of temperature on membrane structural properties and effect of temperature on penetrate properties (Huang et al., 2006a,b). As feed temperature increases, the polymer free volume tends to expand and polymer matrix becomes loose and swollen, rendering the permeating molecules of water and IPA diffuse or move more readily. In the meantime, the penetrate molecules may possess higher molecular mobility at higher temperature. As such, higher flux can be resulted as feed temperature becomes higher. On the other hand, IPA molecules seem to become more volatile than water molecules at higher feed temperature. Furthermore, the change in the polymer free volume due to the influence of increased feed temperature may probably promote the IPA transport more remarkably than the water transport. Such two aspects tend to result in more pronounced

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

27

Fig. 6 – Effect of feed temperature on separation factor/selectivity, flux/permeance and permeate water concentration of the 20 wt.% zeolite ZSM-5 filled PVA membrane for the pervaporation of 80 wt.% IPA aqueous solution. increase in IPA flux than that in water flux, subsequently leading to lower water/IPA separation factor at higher operation temperature. Likewise, similar observations about the temperature effect on IPA dehydration separation properties have also been reported for the other MMMs in the literatures (Adoor et al., 2007; Qiao et al., 2006; Mosleh et al., 2012). Fig. 6b shows that at temperature higher than 80 ◦ C, the permeate water composition value has a substantial drop, further suggesting high temperature is not favorable for maintaining very satisfactory separation performance. As shown in Fig. 5c, the water permeance shows a rather different dependence on feed temperature and it is observed to decrease substantially when feed temperature increases, contrary to the pervaporation flux increase with temperature. It is known that the membrane diffusion for the permeating molecules is almost always increasing with the increase in feed temperature while the membrane sorption is generally decreasing (Baker et al., 2010; Luis and Van der Bruggen, 2015). This turns to suggest that for the MMM considered, the variation in water sorption is more significant than the variation in water diffusivity. It may be noted that if considering the intrinsic permeances, the deteriorated water sorption with feed temperature is not properly reflected by the membrane water flux. As for the IPA permeance and water/IPA selectivity, the former increases with feed temperature whereas the latter decreases with feed temperature. In spite of the decreased water permeance, the newly-developed membrane is observed to still have a water/IPA selectivity of 50 at 90 ◦ C, in large part possibly due to the high preferential diffusion of smaller water molecule compared to a little larger isopropanol molecule.

For better understanding the temperature effect on pervaporation performance, a quantitative Arrhenius equation may be used to express the temperature dependence of pervaporation flux. Ji = Ji0 exp(−EJi /RT)

(9)

where EJi is the apparent activation energy of penetrant i (i.e., the energy barrier for the species i to transport through the membrane), R and Ji0 are universal gas constant (J/mol K) and the pre-exponential factor, respectively. Based on the respective water and IPA fluxes obtained at pervaporation temperature of 50, 60, 70, 80 and 90 ◦ C, the EJ values of water and IPA for the filled PVA membranes can be estimated from the slope of ln Ji versus 1/T plot. Shown in Fig. 7 are the linear Arrhenius plots for penetrate fluxes of water and IPA, respectively. The activation energies estimated for water and IPA are also included in Fig. 7 for the zeolite-filled PVA membranes. As expected, IPA has much higher activation energy than water since both polymer and zeolite used here are highly hydrophilic, favorable for water permeation. As such, IPA molecules tend to require much higher energy than water molecules to go through the membrane at the same conditions. In the meantime, it may be found that the IPA flux is more dependent on the feed temperature than the water flux because the apparent activation energy for IPA (82.15 kJ/mol) is around 3.0 times as much as that for water (27.44 kJ/mol). As a comparison, the activation energy values for IPA and water in the work of Qiao et al. (2006) are reported to be 60.6 and 34.6 kJ/mol for the P84/zeolite 5A MMM and 51.5 and

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

Fig. 7 – Effect of feed temperature on pervaporation water and IPA fluxes of the 20 wt.% zeolite ZSM-5 filled PVA membranes for the dehydration of 80 wt.% IPA aqueous solution. Table 3 – Box–Behnken design of independent factors and observed response values. Run no.

A

B

C

Total flux (g/m2 h)

Separation factor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1 0 0 1 −1 0 −1 −1 0 0 0 0 0 −1 1 1 0

0 0 0 1 0 1 1 −1 1 0 −1 −1 0 0 −1 0 0

−1 0 0 0 −1 −1 0 0 1 0 1 −1 0 1 0 1 0

689.12 ± 11.34 632.49 ± 10.09 602.22 ± 13.22 731.56 ± 12.45 568.42 ± 12.02 581.88 ± 9.15 577.11 ± 10.34 628.33 ± 13.06 698.22 ± 14.34 656.78 ± 11.04 863.93 ± 13.41 678.91 ± 10.32 642.89 ± 8.07 772.73 ± 8.23 876.38 ± 7.71 892.78 ± 9.63 612.73 ± 11.75

1524 ± 5 2013 ± 7 2037 ± 7 1957 ± 9 1609 ± 6 1723 ± 8 2205 ± 3 1953 ± 6 2023 ± 8 2045 ± 6 1823 ± 9 1587 ± 7 1995 ± 4 2016 ± 6 1689 ± 5 1678 ± 4 2024 ± 7

30.8 kJ/mol for the P84/zeolite13X MMM, respectively. Similarly, the values for ethanol and water are reported in our earlier work (2006a) to be 49.2 and 74.8 kJ/mol for the PVA MMM containing 20 wt.% silicalite-1.

3.3.

Response surface analysis of experimental results

3.3.1.

Fitting the model against the experimental data

Data from single-factor experiments suggested that the abovementioned three different factors had important influences on two responses of total pervaporation flux and separation factor. The levels of each factor were selected according to the membrane pervaporation performances. In terms of water/IPA separation factor along with the membrane selectivity, feed temperature of 50–70 ◦ C, feed IPA concentration of 85–95 wt.% and zeolite content of 15–25 wt.% have led appreciated dehydration separation results, and then the levels for three factors are determined and given in Table 1 for the RSM design. The experimental data obtained from the seventeen-run experiments are given in Table 3. It can be seen that total pervaporation flux and water/IPA separation factor obtained with the filled PVA membranes are in the ranges of 568–893 g/m2 h and 1524–2205, respectively, with dif-

ferent combination of three independent factors. It may be noted that the two responses were equivalently considered in this work. Thus, the experimental data for both responses were statistically analyzed in parallel by means of the variance analysis of the factors investigated for the response surface method. The statistical significances and regression coefficients were determined by absolute F-test and p-value. The p-value is evaluated as the significance of each coefficient and can be used to suggest the strength of interaction between each independent factor. The mathematical model was generated from experimental data using Design-Expert 7.1.3 software. Table 4 presents the analysis of variance results for response surface quadratic model of total pervaporation flux. A polynomial model based on the experimental data can be resulted for total pervaporation flux and represented as follows. Y1 = 629.42 + 80.41A − 57.35B + 88.67C − 23.41AB − 0.16AC −17.17BC + 49.48A2 + 24.45B2 + 51.87C2

(10)

where Y1 is the total pervaporation flux (g/m2 h), A is the feed temperature (◦ C), B is the feed IPA concentration (wt.%) and C is the zeolite content (wt.%). It can be seen from Table 4 that the F-value and p-value for the polynomial model are 18.45 and 0.0004 (<0.001), respectively, indicating that the regressed model is of statistical significance. The coefficient of determination (R2 ) in statistics is representing the proportion of the total variation in the response that can be predicted from independent factors via the regression model and higher R2 means that the response surface model is workable. R2 of 0.9596 indicates that the model could satisfactorily describe the experimental data and 95.96 of the variations can be explained by the regression model. In the meantime, the adjusted coefficient of determination (Ra 2 ) may be used to analyze the model accuracy and a high Ra 2 value of 0.9228 tends to suggest that a very good correlation between the experimental data and the predicted ones has been resulted. The coefficient of variation (C.V.) defined as the ratio of standard deviation to the averaged response value is used to be a measure of reproducibility of the fitted model and the low C.V. of 5.16% suggests the regression model has very good reproducibility. A non-significant lack of fit further validates the model with a p-value of 0.125 (>0.05). As also can be seen from Table 4, the influences of three independent factors (feed temperature, feed IPA concentration and zeolite content) on the total pervaporation flux are all statistically significant as reflected by the p-values (p < 0.001) of the regressed coefficients of the model linear terms. Feed temperature and zeolite content are seen to have positive effects and their positive changes can cause an increase in the response value. This seems to be reasonable as the increase of either feed temperature or zeolite content tends to result in higher total flux. On the contrary, feed IPA concentration has a negative effect on the pervaporation flux. These findings are consistent with preliminary results discussed above. In the meantime, the quadratic terms of feed temperature and zeolite content are both found to have a p-value lower than 0.05, indicating that the second-order quadratic effect of feed temperature and zeolite content may have a significant influence on the total pervaporation flux. On the other hand, the quadratic effect of feed IPA concentration and all three interaction terms are seen to be insignificant as reflected by the

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Table 4 – Analysis of variance results for response surface quadratic model of total pervaporation flux. Source of variation

Degree of freedom

Sum of squares

Mean squares

F-value

p-Value

Significance*

Model A B C AB AC BC A2 B2 C2 Residual Lack of fit Pure error Corrected total C.V. = 5.16%

9 1 1 1 1 1 1 1 1 1 7 3 4 16

1.71 × 105 5.17 × 104 2.63 × 104 6.29 × 104 2.19 × 103 0.11 1.18 × 103 1.03 × 104 2.52 × 103 1.13 × 104 7.20 × 103 5.25 × 103 1.96 × 103 1.78 × 105 R2 = 0.9596

1.90 × 104 5.17 × 104 2.63 × 104 6.29 × 104 2.19 × 103 0.11 1.18 × 103 1.03 × 104 2.52 × 103 1.13 × 104 1.03 × 103 1.75 × 103 4.89 × 102

18.5 50.3 25.6 61.1 2.13 1.03 × 10-4 1.15 10.0 2.44 11

0.0004 0.0002 0.0015 0.0001 0.188 0.992 0.32 0.0158 0.162 0.0128

Significant Significant Significant Significant Insignificant Insignificant Insignificant Significant Insignificant Significant

3.57

0.125

insignificant

∗

Ra 2 = 0.9228

Significant at p < 0.05.

Table 5 – Analysis of variance results for response surface quadratic model of separation factor. Source of variation

Degree of freedom

Sum of squares

Mean squares

F-value

p-Value

Significance*

Model A B C AB AC BC A2 B2 C2 Residual Lack of fit Pure error Corrected total C.V. = 2.12 %

9 1 1 1 1 1 1 1 1 1 7 3 4 16

6.44 × 105 1.09 × 105 9.16 × 104 1.50 × 105 64.0 1.60 × 104 1.02 × 103 2.50 × 104 1.15 × 102 2.41 × 105 6.87 × 103 5.30 × 103 1.56 × 103 6.50 × 105 R2 = 0.9894

7.15 × 104 1.09 × 105 916 × 104 1.50 × 105 64.0 1.60 × 104 1.02 × 103 2.50 × 104 1.15 × 102 2.41 × 105 9.81 × 102 1.77 × 103 3.91 × 102

72.7 1.11 × 102 93.4 1.53 × 102 0.065 16.3 1.04 25.5 0.12 2.45 × 102

<0.0001 <0.0001 <0.0001 <0.0001 0.807 0.0049 0.342 0.0015 0.742 <0.0001

Significant Significant Significant Significant Insignificant Significant Insignificant Significant Insignificant Significant

4.53

0.0897

Insignificant

∗

Ra 2 = 0.9536

Significant at p < 0.05.

much higher p-values (p > 0.05). The predicted model can be further revised once the non-significant factors and interactions are removed, resulting in a very simple model as given below: Y1 = 629.42 + 80.41A − 57.35B + 88.67C + 49.48A2 + 51.87C2 (10a) In the case of water/IPA separation factor, a second-order polynomial model can be generated by a nonlinear regression analysis of the experimental data, and the analysis results of the regressed quadratic model are given in Table 5. The polynomial model regressed for water/IPA separation factor, Y2 , is obtained as the following expression. Y2 = 2022.81 − 116.87A + 107.01B + 137.13C + 4.01AB −63.25AC + 16.02BC − 77.03A2 + 5.22B2 − 239.02C2

(11)

Table 5 shows that the F-value and p-value for the regressed model are 72.69 and <0.0001, respectively, suggesting that the obtained quadratic model is statistically significant. The model thus regressed seems to perform very well and lead to a satisfactory fit against the experimental data, as indicated by sufficiently high determination coefficient R2 of 0.9894 and

very high adjusted determination coefficient Ra 2 of 0.9536. Further, the satisfactory C.V. value of 2.12% demonstrates that the model has led to sufficient repeatability. A non-significant lack of fit, with a p-value of 0.0897, further suggests the model is excellent for predicting water/IPA separation factor. Similar to the case of total pervaporation flux, the influences of feed temperature, feed IPA concentration and zeolite content on separation factor are all statistically significant as reflected by the p-values (p < 0.001) for three model linear terms. As can be seen, feed IPA concentration and zeolite content tend to have positive effects while feed temperature seems to have negative effect on water/IPA separation factor. That is to say, the increase of either feed IPA concentration or zeolite content will yield higher separation factor whereas the increase of feed temperature leads to decreased separation factor. Seemingly, these findings are agreeing very well with previous discussion. Table 5 also shows that the quadratic terms of feed temperature and zeolite content and their interaction term are all significant with a p-value lower than 0.001 but their effects on separation factor are negative. The statistical significance of the model terms of feed temperature and zeolite content indicate that the influence of feed temperature or zeolite content on water/IPA separation factor is not a simple linear relationship but a rather complex one. That is to say, feed temperature and zeolite content both have signif-

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Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

icant first order linear effect and significant second-order quadratic effect on water/IPA separation factor. Like the revised model for total pervaporation flux, the regressed model for water/IPA separation factor can be simplified after removing the non-significant factors and interactions:

Y2 = 2022.81 − 116.87A + 107.01B + 137.13C − 63.25AC −77.03A2 − 239.02C2

3.3.2.

(11a)

RSM optimization of pervaporation conditions

The response surface models obtained as above were applied to investigate the effects of independent factors and their interactions on dependent responses. In order to visualize the influences of three independent parameters on the response Y1 (total pervaporation flux) and response Y2 (separation factor), the 3D response surface graphs were plotted by sketching the response (z-axis) versus two parameters (x and y coordinates) changed in the experimental ranges and holding the rest parameter constant at zero (zero-level). The zero values considered here were 60 ◦ C, 90 wt.% and 20 wt.% for feed temperature, feed IPA concentration and ZSM-5 zeolite content, respectively. Fig. 8 shows the simultaneous effects of feed IPA concentration and ZSM-5 zeolite content, feed temperature and ZSM-5 zeolite content, and feed temperature and feed IPA concentration on total pervaporation flux, respectively. According to the 3D total flux plot, it can be seen that there does exist a desirable location in the design space where the response (Y1 ) tends to have the highest value. It can be seen from Fig. 8a that when the feed temperature is kept at constant, the total flux gradually decreases with the increase of the feed IPA concentration under different zeolite contents but it increases considerably with the increase of zeolite content under different feed compositions. Appreciated larger response values could be resulted at about 25 wt.% of zeolite content and a feed of 85 wt.% IPA aqueous solutions. Higher IPA concentration or lower zeolite content leads to the decrease of the overall pervaporation flux, but their interaction effect on the flux is not significant as the p-value is larger than 0.05. The increment of total flux with the zeolite content can be also observed from Fig. 8b regardless of the feed temperature considered. As well, the overall flux increases with the rise of feed temperature irrespective of the zeolite content used. Higher feed temperature or larger zeolite content tends to get larger response value, namely the larger flux, although their interaction effect on the flux is insignificant (p > 0.05). Such dependence of total flux on feed temperature can be also clearly referred from Fig. 8c. On the other hand, the flux is seen to decrease under different feed temperatures as the feed IPA concentration increases but their interaction effect is not significant (p > 0.05). Relatively larger flux values can be reached in the case of 85 wt.% IPA aqueous solution and feed temperature of 70 ◦ C. The 3D plots given in Fig. 9 show that the response two Y2 (separation factor) seems to reach the highest value in a desirable location, similar to the case of the response one Y1 (total flux). It can be seen from Fig. 9a and b that water/IPA separation factor increases first and then decreases with the increase of zeolite content under different feed temperatures or different feed IPA concentrations. Relatively larger response value, namely the higher separation factor, can be obtained

Fig. 8 – 3D response surface plots for total pervaporation flux as a function of (a) zeolite content and feed temperature, (b) zeolite content and feed IPA concentration, (c) feed temperature and feed IPA concentration.

Chemical Engineering Research and Design 1 4 4 ( 2 0 1 9 ) 19–34

31

IPA concentration their interaction effect is not significant (p > 0.05). Fig. 9c shows the case of keeping the zeolite content at constant and water/IPA separation factor is seen to increase with the increase of the feed IPA concentration under different feed temperatures whereas it substantially decreases with the increase of the feed temperature irrespective of the feed IPA concentration used. Lower feed temperature and higher feed IPA concentration, as expected, can result in larger response value, i.e., higher separation factor, or vice versa. However, their interaction effect on separation factor is noted to be insignificant (p > 0.05). Overall, relatively higher separation factor can be achieved as feed temperature is 50 ◦ C, IPA concentration in the feed is 95 wt.%, and the zeolite content in PVA matrix is 20 wt.%.

3.3.3.

Experimental validation

According to the p-values shown in Tables 4 and 5, the influences of feed temperature, feed IPA concentration and zeolite content on both total pervaporation flux and water/IPA separation factor are all highly significant. Using the RMS design model, the optimum combination of three factors can be predicted to render two response values both reach at a very desirable level. One set of optimized conditions thus obtained are 50 ◦ C of feed temperature, 85 wt.% of feed IPA concentration and 25 wt.% of zeolite content. Under such combined conditions, the total pervaporation flux and water/IPA separation factor are estimated to be 814 g/m2 h and 1910, respectively. In order to examine the accuracy of the optimization procedure, the model was assessed by conducting the pervaporation tests under optimum conditions. Three parallel experimental runs were carried out and the averaged total flux and separation factor determined from the validation experiment were 831 ± 18 g/m2 h and 1930 ± 38, respectively, in very good agreement with the predicted values. The differences of experimental data from the predicted results are less than ±2.0%. Hence, the optimum conditions determined by RSM have thus been validated, confirming that RSM can be used to optimize the dehydration separation of highly concentrated IPA aqueous solution via pervaporation for PVA/ZSM-5 zeolite MMM membranes.

3.4.

Fig. 9 – 3D response surface plots for water/IPA separation factor as a function of (a) zeolite content and feed temperature, (b) zeolite content and feed IPA concentration, (c) feed temperature and feed IPA concentration. at around 20 wt.% zeolite content. Regardless of the zeolite content investigated the water/IPA separation factor is seen to increase considerably with the increase of feed IPA concentration (Fig. 9a) or decrease with the increase of feed temperature (Fig. 9b). Very interestingly, the interaction effect of zeolite content and feed temperature on separation factor is significant (p < 0.05) and such finding is possibly reported for the first time, indicating the influence of either zeolite content or feed temperature on separation factor is not independent but depends on each other. As for zeolite content and feed

Comparison with literature contributions

A comparison of the PVA/ZSM-5 zeolite pervaporation MMM with other MFI zeolite incorporated MMMs published in the literatures (Adoor et al., 2007; Huang et al., 2006a; Mosleh et al., 2012; Sun et al., 2008a,b) for alcohol dehydration from its highly concentrated aqueous solution is shown in Table 6. It can be seen from separation factor and flux values included that the MMM with 20 wt.% ZSM5 zeolite developed in this work shows a separation factor of 2013 for dehydrating 90 wt.% IPA aqueous solution and 1254 for 80 wt.% IPA aqueous solution at 60 ◦ C, comparable to of higher than those MMM performances under similar feed temperatures (Huang et al., 2006a; Mosleh et al., 2012; Sun et al., 2008a,b). Furthermore, the 20 wt.% ZSM5 filled PVA MMM in the meantime has also possessed very promising total fluxes of 0.632 or 0.944 kg/m2 h for 90 and 80 wt.% IPA aqueous solutions at 60 ◦ C and such pervaporation fluxes are very higher than those of the other MFI zeolite filled MMMs. As can be seen from Table 6, the newly-developed MMMs exhibit much better dehydration performance than the others if compared in terms of

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229.29 35.11 52.48 75.89 438.36 485.73 108.70 154.56 1271.58 1182.83 694.82

This work

Huang et al. (2006a,b) Sun et al. (2008a) Mosleh et al. (2012); Sun et al. (2008b) Adoor et al. (2007)

Refs. PSI (kg/m2 h)

0.219 0.231 0.025 0.278 0.039 0.027 0.084 0.069 0.632 0.944 1.768

Flux (kg/m2 h)

60 80 50 80 30 30 30 30 60 60 80 80 90 90 90 90 90 90 90 90 80 80 Ethanol Ethanol IPA Ethanol IPA IPA IPA IPA IPA IPA IPA

4.

Conclusion

In this communication, we have investigated the effect of incorporated ZSM-5 zeolite on the pervaporation performances of the PVA/ZSM-5 zeolite MMMs for water removal from highly concentrated isopropanol aqueous solution. These mixed matrix membranes were prepared by means of a solution casting method and characterized by SEM, XRD, swelling and water contact angle tests. The pervaporation performances of the zeolite-filled and unfilled membranes have been preliminarily evaluated in terms of pervaporation flux/permeance and separation factor/selectivity. Preliminary results show that the zeolite addition has considerably increased separation factor/selectivity and permeation flux/permeance, indicating that incorporated ZSM-5 zeolite can promote water permeation and in the meantime constrain the IPA transport across the membrane. The separation factor/selectivity and permeation flux/permeance are found for the MMMs to increase with the increase of the zeolite content, and this is possibly because the PVA/ZSM-5 zeolite MMMs become more hydrophilic after the zeolite addition, as reflected by the decreased water contact angle with the addition of more ZSM-5 zeolite and more considerably suppressed swelling in water/IPA mixture than in pure water. The satisfactory dehydration results are in good agreement with calculated Arrhenius activation energies where the energy is much lower required for water component but greatly higher for IPA component to go through these hydrophilic MMMs. Increasing feed isopropanol content and decreasing feed temperature are seen to make water/IPA separation factor/selectivity increase substantially but pervaporation flux/permeance decrease a lot. Based on these preliminary results, the RSM analysis coupled with a three-factor-threelevel BBD has been successfully used to account for the effects of three independent factors, i.e. zeolite content, feed composition and feed temperature, on total pervaporation flux and separation factor. The analysis results demonstrate that three factors all have significant effects on both total flux and separation factor, and zeolite content and feed temperature are found for the first time to have significant interaction effect on water/IPA separation factor. The RMS quadratic models are predictable and perform very well to describe the experimental data with very high coefficients of determination and very low coefficient of variation. Within the range scanned, a set of optimized conditions are determined to be 50 ◦ C of feed temperature, 85 wt.% of feed IPA concentration and 25 wt.% of zeolite content. Under these optimum conditions, the predicted total pervaporation flux and water/IPA separation factor are 814 g/m2 h and 1910, respectively, while the experimental values, from the validation experiment, agree very well against these predictions with the deviation less than 2.0%.

Acknowledgement PVA

Sodium alginate

Silicalite-1 (Si/Al = 196, 20 wt.%) ZSM-5 zeolite (Si/Al = 50, 8 wt.%) ZSM-5 (Si/Al = 40, 10 wt.%) MPTMS-H-ZSM-5 (Si/Al = 25, 8 wt.%) Silicalite-1 (5 wt.%) Silicalite-1 (10 wt.%) Silicalite-1 (5 wt.%) Silicalite-1 (10 wt.%) ZSM-5 zeolite (Si/Al = 25, 20 wt.%) PVA Chitosan Matrimid 5218 Chitosan PVA

Alcohol

Content (wt.%)

Temp. (◦ C)

1048 153 2100 274 11241 17991 1295 2241 2013 1254 394

Separation factor(␣) Feed Filler Polymer

Table 6 – Comparison of alcohol dehydration performances of some MFI zeolite filled pervaporation MMMs.

Pervaporation Separation Index (PSI, i.e., PSI = Jtotal · (˛ − 1)). Deduced from these pervaporation results, we may consider the PVA/ZSM-5 zeolite MMMs for practical IPA dehydration applications.

The authors would like to thank Tianjin University of Commerce for partially funding this project (TJCU-2018-002).

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References Adoor, S.G., Manjeshwar, L.S., Bhat, S.D., Aminabhavi, T.M., 2008. Aluminum-rich zeolite beta incorporated sodium alginate mixed matrix membranes for pervaporation dehydration and esterification of ethanol and acetic acid. J. Membr. Sci. 318, 233–246. Adoor, S.G., Prathab, B., Manjeshwar, L.S., Aminabhavi, T.A., 2007. Mixed matrix membranes of sodium alginate and poly(vinyl alcohol) for pervaporation dehydration of isopropanol at different temperatures. Polymer 48 (18), 5417–5430. Amirilargani, M., Sadatnia, B., 2014. Poly(vinyl alcohol)/zeolitic imidazolate frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of isopropanol. J. Membr. Sci. 469, 1–10. Amnuaypanich, S., Patthana, J., Phinyocheep, P., 2009. Mixed matrix membranes prepared from natural rubber/poly(vinyl alcohol) semi-interpenetrating polymer network (NR/PVA semi-IPN) incorporating with zeolite 4A for the pervaporation dehydration of water–ethanol mixtures. Chem. Eng. Sci. 64, 4908–4918. Beltran, A.B., Nisola, G.M., Choi, S.S., Kim, Y., Chung, W.J., 2013. Surface-functionalized silica nanoparticles as fillers in polydimethylsiloxane membrane for the pervaporative recovery of 1-butanol from aqueous solution. J. Chem. Technol. Biotechnol. 88, 2216–2226. Baker, R.W., Wijmans, J.G., Huang, Y., 2010. Permeability, permeance and selectivity: a preferred way of reporting pervaporation performance data. J. Membr. Sci. 348, 346–352. Breck, D.W., 1964. Zeolite Molecule Sieves. John Wiley, New York. Bolto, B., Hoang, M., Xie, Z.L., 2011. A review of membrane selection for the dehydration of aqueous ethanol by pervaporation. Chem. Eng. Process. 50, 227–235. Cheng, X.X., Pan, F.S., Wang, M.R., Li, W.D., Song, Y.M., Liu, G.H., Yang, H., Gao, B.X., Wu, H., Jiang, Z.Y., 2017. Hybrid membranes for pervaporation separations. J. Membr. Sci. 541, 329–346. Chew, S.C., Tan, C.P., Nyam, K.L., 2017. Optimization of degumming parameters in chemical refining process to reduce phosphorus contents in kenaf seed oil. Sep. Purif. Technol. 188, 379–385. Choudhari, S.K., Premakshi, H.G., Kariduraganavar, M.Y., 2016. Development of novel alginate-silica hybrid membranes for pervaporation dehydration of isopropanol. Polym. Bull. 73, 743–762. Deng, Y.H., Chen, J.T., Chang, C.H., Liao, K.S., Tung, K.L., Price, W.E., Yamauchi, Y., Wu, K.C.W., 2016. A drying-free, water-based process for fabricating mixed-matrix membranes with outstanding pervaporation performance. Angew. Chem. Int. Ed. 55, 12793–12796. Dogan, H., Hilmioglu, N.D., 2010. Zeolite-filled regenerated cellulose membranes for pervaporative dehydration of glycerol. Vacuum 84, 1123–1132. Fan, H.W., Shi, Q., Yan, H., Ji, S.L., Dong, J.X., Zhang, G.J., 2014. Simultaneous spray self-assembly of highly loaded ZIF-8-PDMS nanohybrid membranes exhibiting exceptionally high biobutanol permselective pervaporation. Angew. Chem. Int. Ed. 53, 5578–5582. Fazlifard, S., Mohammadi, T., Bakhtiari, O., 2017. Chitosan/ZIF-8 mixed-matrix membranes for pervaporation dehydration of isopropanol. Chem. Eng. Technol. 40, 648–655. Hansen, C.M., 2007. Hansen Solubility Parameters: A User’s Handbook, 2nd ed. CRC Press, New York. Hua, D., Ong, Y.K., Wang, Y., Yang, T., Chung, T.S., 2014. ZIF-90/P84 mixed matrix membranes for pervaporation dehydration of isopropanol. J. Membr. Sci. 453, 155–167. Huang, Z., Guan, H.M., Tan, W.L., Qiao, X.Y., Kulprathipanja, S., 2006a. Pervaporation study of aqueous ethanol solution through zeolite-incorporated multilayer poly(vinyl alcohol) membranes: effect of zeolites. J. Membr. Sci. 276, 260–271. Huang, Z., Shi, Y., Wen, R., Guo, Y.H., Su, J.F., Matsuura, T., 2006b. Multilayer poly(vinyl alcohol)-zeolite 4A composite

33

membranes for ethanol dehydration by means of pervaporation. Sep. Purif. Technol. 51, 126–136. Jiang, L.Y., Chung, T.S., 2010. Homogeneous polyimide/cyclodextrin composite membranes for pervaporation of isopropanol. J. Membr. Sci. 346, 45–58. Jiang, L.Y., Wang, Y., Chung, T.S., Qiao, X.Y., Lai, J.Y., 2009. Polyimides membranes for pervaporation and biofuels separation. Prog. Polym. Sci. 34, 1135–1160. Kang, C.H., Lin, Y.F., Huang, Y.S., Tung, K.L., Chang, K.S., Chen, J.T., Hung, W.S., Lee, K.R., Lai, J.Y., 2013. Synthesis of ZIF-7/chitosan mixed-matrix membranes with improved separation performance of water/ethanol mixtures. J. Membr. Sci. 438, 105–111. Khoonsap, S., Amnuaypanich, S., 2011. Mixed matrix membranes prepared from poly(vinyl alcohol) (PVA) incorporated with zeolite 4A-graft-poly(2-hydroxyethyl methacrylate) (zeolite-g-PHEMA) for the pervaporation dehydration of water–acetone mixtures. J. Membr. Sci. 367, 182–189. Kujawska, A., Knozowska, K., Kujawa, J., Kujawski, W., 2016. Influence of downstream pressure on pervaporation properties of PDMS and POMS based membranes. Sep. Purif. Technol. 159, 68–80. Liu, G.P., Wei, W., Jin, W.Q., 2014. Pervaporation membranes for biobutanol production. ACS Sustain. Chem. Eng. 2, 546–560. Luis, P., Van der Bruggen, B., 2015. The driving force as key element to evaluate the pervaporation performance of multicomponent mixtures. Sep. Purif. Technol. 148, 94–102. Mesli, M., Belkhouche, N.E., 2018. Emulsion ionic liquid membrane for recovery process of lead. Comparative study of experimental and response surface design. Chem. Eng. Res. Des. 129, 160–169. Mosleh, S., Khosravi, T., Bakhtiari, O., Mohammadi, T., 2012. Zeolite filled polyimide membranes for dehydration of isopropanol through pervaporation process. Chem. Eng. Res. Des. 90, 433–441. Naik, P.V., Kerkhofs, S., Martens, J.A., Vankelecom, I.F.J., 2016. PDMS mixed matrix membranes containing hollow silicalite sphere for ethanol/water separation by pervaporation. J. Membr. Sci. 502, 48–56. Ong, Y.K., Shi, G.M., Le, N.L., Tang, Y.P., Zuo, J., Nunes, S.P., Chung, T.S., 2016. Recent membrane development for pervaporation processes. Prog. Polym. Sci. 57, 1–31. Peng, P., Shi, B.L., Lan, Y.Q., 2011. Preparation of PDMS-silica nanocomposite membranes with silane coupling for recovering ethanol by pervaporation. Sep. Sci. Technol. 46, 420–427. Qiao, X.Y., Chung, T.S., Rajagopalan, R., 2006. Zeolite filled P84 co-polyimide membranes for dehydration of isopropanol through pervaporation process. Chem. Eng. Sci. 61, 6816–6825. Qiu, S., Wu, L.G., Shi, G.Z., Zhang, L., Chen, H.L., Gao, C.J., 2010. Preparation and pervaporation property of chitosan membrane with functionalized multiwalled carbon nanotubes. Ind. Eng. Chem. Res. 49, 11667–11675. Rahma, W.S.A., Mjalli, F.S., Al-Wahaibi, T., Al-Hashmi, A.A., 2017. Polymeric-based deep eutectic solvents for effective extractive desulfurization of liquid fuel at ambient conditions. Chem. Eng. Res. Des. 120, 271–283. Shirazi, Y., Tofighy, M.A., Mohammadi, T., 2011. Synthesis and characterization of carbon nanotubes/poly vinyl alcohol nanocomposite membranes for dehydration of isopropanol. J. Membr. Sci. 378, 551–561. Shirvani, S., Mallah, M.H., Moosavian, M.A., Safdari, J., 2016. Magnetic ionic liquid in magmolecular process for Uranium removal. Chem. Eng. Res. Des. 109, 108–115. Smitha, B., Suhanya, D., Sridhar, S., Ramakrishna, M., 2004. Separation of organic–organic mixtures by pervaporation—a review. J. Membr. Sci. 241, 1–21. Sun, H.L., Lu, L.Y., Chen, X., Jiang, Z.Y., 2008a. Pervaporation dehydration of aqueous ethanol solution using H-ZSM-5 filled chitosan membranes. Sep. Purif. Technol. 58, 429–436. Sun, H.L., Lu, L.Y., Chen, X., Jiang, Z.Y., 2008b. Surface-modified zeolite-filled chitosan membranes for pervaporation dehydration of ethanol. Appl. Surf. Sci. 254, 5367–5374.

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Wang, Y., Goh, S.H., Chung, T.S., Na, P., 2009. Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols. J. Membr. Sci. 326, 222–233. Zafar, M., Ali, M., Khan, S.M., Jamil, T., Butt, M.T.Z., 2012. Effect of additives on the properties and performance of cellulose acetate derivative membranes in the separation of isopropanol/water mixtures. Desalination 285, 359–365.

Zhou, H.L., Zhang, J.Q., Wan, Y.H., Jin, W.Q., 2017. Fabrication of high silicalite-1 content filled PDMS thin composite pervaporation membrane for the separation of ethanol from aqueous solutions. J. Membr. Sci. 524, 1–11. Zhuang, X.J., Chen, X.R., Su, Y., Luo, J.Q., Cao, W.F., Wan, Y.H., 2015. Improved performance of PDMS/silicalite-1 pervaporation membranes via designing new silicalite-1 particles. J. Membr. Sci. 493, 37–45.