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Chemical Engineering Journal 175 (2011) 306– 315

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

he potential of pervaporation for separation of acetic acid and water mixturessing polyphenylsulfone membranes

ora Julloka,b,∗, Siavash Darvishmanesha, Patricia Luisa, Bart Van der Bruggena

Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, Katholieke Universiteit Leuven, W. de Croylaan 46,-3001 Heverlee, BelgiumSchool of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3,02600 Arau, Perlis, Malaysia

r t i c l e i n f o

rticle history:eceived 26 March 2011eceived in revised form5 September 2011ccepted 27 September 2011

eywords:ervaporationcetic acid dehydrationolyphenylsulfone (PPSU)embrane swellingembrane stability

a b s t r a c t

Conventional pervaporation (PV) membranes usually have limited resistance to acetic acid (HAc), par-ticularly in high pressure and temperature conditions, resulting in a cumbersome water-acetic acidseparation. When acetic acid is to be recycled in process conditions in a hybrid pervaporation approach,the PV membrane may experience these conditions of high temperatures and pressures. This studyexplores the potential of dehydrating acetic acid using pervaporation with novel polyphenylsulfone(PPSU) membranes. These membranes were tested for PV dehydration of mixtures of acetic acid–waterwith 80 and 90 wt.% acetic acid in the temperature range between 30 and 80 ◦C. In addition to that,an experimental study of membrane stability was performed at high concentration of HAc and hightemperatures.

It was found that a higher polymer concentration does not necessary yield a better separation factor:PPSU-based membranes with 27.5 wt.% of the polymer (PPSU-27.5) were similar to 30 wt.% (PPSU-30) interms of overall performance, considering both flux and separation factor. Although the total flux of PPSU-27.5 (∼0.12–0.83 kg/m2 h) is lower than PPSU-25 (∼0.24–1.48 kg/m2 h) the average separation factor canbe higher than for the PPSU-30 membrane. For example, in 90 wt.% HAc, the separation factor is 8.4 for

PPSU-27.5 and 5.7 for PPSU-30. The swelling degree (DS) was found to decrease with feed temperature,while an increase of the selectivity and flux was observed. The activation energy of permeability (Ep)shows that PPSU membranes have negative Ep values. This indicates that the membrane partial perme-abilities decrease with increasing temperature. With the enrichment of acetic acid on the feed side of themembrane, the degree of swelling, flux and separation factor all increase. Regarding on the membrane

emb

stability tests, the PPSU m

. Introduction

Separation processes are required in the chemical industry inrder to obtain high purity of raw materials, intermediates ornd products. Separation of compounds from a mixture requiresnergy and in many cases, the separation is challenging to per-orm due to energy consumption and cost. The chemical or physicalroperties (e.g., molecular size, vapour pressure, freezing point,ffinity, charge, density and the chemical nature) of the target com-

ounds are important factors in determining the process viability1,2]. The mixture of HAc with water has been the main focusf several previous investigations [3–7], which consider the close

∗ Corresponding author at: Department of Chemical Engineering, Laboratory forpplied Physical Chemistry and Environmental Technology, Katholieke Universiteiteuven, W. de Croylaan 46, B-3001 Heverlee, Belgium. Tel.: +32 16 32 23 48,ax: +32 16 32 29 91.

E-mail address: Nora.Jullok@cit.kuleuven.be (N. Jullok).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.09.109

ranes showed promising results at tested conditions.© 2011 Elsevier B.V. All rights reserved.

boiling point of both compounds. The main challenges of suchseparation process are (1) cost-minimization and (2) increase ofseparation efficiency. The conventional HAc production processby methanol carbonylation operates at both elevated tempera-ture (∼190 ◦C) and pressure (∼28 bar) [8]. To some extent, theprocess may entail higher pressure states, reaching up to 80 bar.One example of such a case is the ethylene direct oxidation pro-cess. In addition, several studies reported yields as low as 63–86%selectivity to HAc [9]. PV could be a beneficial option for furtherpurification and/or product/by-product recycling. Mixtures con-taining acetic acid and water do not form an azeotropic mixture[3–7,10]. However, separation of HAc from water using a normaldistillation process is far from being the best process to considerdue to the large number of trays necessary in the distillation col-umn to perform the separation, which increases the associated

costs [10]. Many alternative processes have been proposed in orderto improve the efficiency of the separation of HAc and watersolution in order to reduce the energy consumption, which maylead to lower operational costs. Pervaporation is considered as a

eering

pmabispap[dtpaidatatmPaoetiOsits3tpb[tcebplamstHabtifcdiorouchtiPo

N. Jullok et al. / Chemical Engin

romising process to separate azeotropic and close boiling pointixtures. However, the choice of the membrane is a key consider-

tion that defines the type of application. The specific component toe separated from the mixture determines the membrane type that

s needed (hydrophilic, organophilic or hydrophobic). Normally, themallest weight fraction of component in the mixture is to be trans-orted across the membrane: hydrophilic polymeric membranesre used for the dehydration of organic liquids and hydrophobicolymeric membranes for removal of organics from water streams11]. Wee et al. [12] found that polymeric membranes had the bestehydration performance under azeotropic conditions comparedo inorganic membranes. Nevertheless, polymeric membranes alsoossess disadvantages such as limited solvent [12] and temper-ture stability. Poly(vinyl alcohol) (PVA) membrane, for instance,s a highly hydrophilic membrane that is studied for the dehy-ration of organic solutions, including the dehydration of aceticcid but PVA membranes have a poor stability in aqueous mix-ures, requiring an intensive pretreatment, modification and/orlteration of the membrane to assure a good performance. Fur-hermore, polymeric membranes also suffer from swelling, which

ight change the membrane structure and properties significantly.olyphenylsulfone (PPSU), as a new membrane material, may beble to provide a new option and/or serve as an alternative inrganic dehydration, especially in solutions at low pH, as found in.g., pervaporation dehydration of acetic acid. In recent years, inves-igations related to PPSU membranes have been conducted to studyts potential use as proton-conducting fuel cell membrane [13–16].thers have studied PPSU/PBNPI blend membrane for hydrogen

eparation [17] and as a support to polymeric liquid membranesn recovery of aromatic compounds from wastewater [18]. In addi-ion, PPSU membranes have been reported to have a high thermaltability with a decomposition temperature between 300 ◦C and50 ◦C, and a good mechanical stability [13]. The robustness ofhe material in terms of physical durability and chemical stabilityoints out that PPSU may be a good polymer candidate, which coulde potentially developed for further use in HAc–water separation19]. Furthermore, in industrial applications, excessive tempera-ures and pressures frequently occur. Since PPSU is easily sourced,ommercially available and is relatively low cost, it will effectivelynable a time- and cost-effective operation [13]. No studies haveeen made yet on the use of PPSU for HAc–water separation inervaporation. PPSU is generally considered hydrophobic, but pre-

iminary experiments (not shown) proved that the water contactngle for the manufactured PPSU membranes was below 90◦, whichakes them sufficiently hydrophilic for this application. Thus, this

tudy is focused on achieving the separation of HAc–water mix-ures in harsh environments using self-made PPSU membrane.Ac–water mixtures containing up to 90 wt.% HAc will be studiednd a deep discussion about the swelling phenomena in the mem-rane is performed. In dense membranes used for pervaporation,he flux through the membrane is proportional to the driving force,.e. the temperature, pressure, concentration and electromotiveorce gradient. These factors, concluded as an overall driving force,reate mobility of the permeant through the membrane. Besides theriving force, the membrane itself is the prime factor in determin-

ng the selectivity and flux. The nature of the membrane – in termsf structure and material used – determines the type of application,anging from separation of microscale particles to the separationf molecules of identical size or shape. Colman and Naylor [20]sed pervaporation in the dehydration of isopropanol showing thatoncentration polarization still exists even when the feed flow isighly turbulent (with Reynolds numbers above 104). However, in

his study, the effects of concentration polarization were not takennto account; the focus is on the analysis of applicability of novelPSU membrane in HAc dehydration and data estimation insteadf precise calculations.

Journal 175 (2011) 306– 315 307

2. Material and methods

2.1. Materials

Radel® R-5000, a transparent polyphenylsulfone (PPSU), waspurchased from Solvay Advanced Polymer Belgium and dissolvedat specific ratio with N-methyl-2-pyrrolidinone (NMP) which actsas the solvent, purchased from Aldrich. Acetic acid (HAc) was sup-plied by Chem Lab Belgium. Demineralised water is produced usinga reverse osmosis system having a conductivity of 5.6 �S/cm.

2.2. Membrane synthesis

PPSU membranes were prepared using a phaseinversion–immersion precipitation method. Three PPSU-basedmembranes were prepared at higher polymer concentration range(25, 27.5 and 30 wt.% PPSU) in order to achieve the formation of adense top layer. The casting solution was prepared by dissolvingPPSU in NMP at ambient temperature using a Stovall Low ProfileRoller. After the PPSU pellets have completely dissolved, thesolution was placed inside a vacuum chamber with the bottlecap partially opened to release present bubbles in the castingsolution. The bubble free solution was then cast on a glass plateinside a controlled humidity (<40% RH) chamber using an auto-matic driven casting blade of 250 �m thickness. The humidityinside the chamber was controlled by feeding water-bubblednitrogen. Immediately after the casting process was completed,the glass plate was taken off from the platform and immersed in acoagulation bath containing demineralised water at 20 ◦C.

2.3. Membrane characterization

2.3.1. Measurement of degree of swelling (DS)The degree of swelling (DS) was measured gravimetrically in 80

and 90 wt.% HAc containing water. The initial mass of circularly cutPPSU membrane (dia. = 3.5 cm) was weighed on a single-pan digitalmicrobalance (Model AB204-S, Mettler Toledo) with sensitivity of±0.0001 mg, recorded and labelled as dry membrane (mdry). Thedry membrane was then immersed in the 80 wt.% HAc and 90 wt.%HAc mixture at 30–70 ◦C for 24 h [4]; the membrane was taken outfrom the immersion solution and wiped using a cleansing tissue andimmediately weighed, recorded and labelled as swollen membrane(mswollen). Sorption on the membranes in pure water and pure HAcwas also measured. The swelling degree was calculated as follows:

DS (%) = mswollen − mdry × 100mdry

(1)

2.3.2. Scanning electron microscopy (SEM)The cross-section of the studied membranes was obtained using

SEM to study the morphology of the PPSU membranes as a func-tion of the polymer composition and to measure the thickness ofthe skin top layers. Each sample of PPSU membrane was brokenduring the immersion in liquid nitrogen. The broken membraneswere glued on an agar before coated with a conductive layer ofgold. The cross-sections were then investigated using Philip XL 30ESEM FEG (The Netherlands).

2.3.3. Pervaporation experimentsPervaporation experiments were carried out using a Lab Test

Cell Unit, (Sulzer Chemtech). The feed solution of acetic acid–waterwas kept in a 3 L stainless steel tank with controlled tempera-

ture using heater and temperature controller loop. First, the flatsheet PPSU membrane was cut and then immersed in the feedsolution overnight. Subsequently, the membrane was placed intothe membrane test cell. The effective membrane diameter was

3 eering Journal 175 (2011) 306– 315

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08 N. Jullok et al. / Chemical Engin

bout 5 cm, corresponding to an effective surface area of around.963 × 10−3 m2. The feed solution was circulated over the mem-rane using a centrifugal pump.

In pervaporation, the feed is normally heated because it isxpected that with every 10 ◦C change in the temperature, perme-bility will increase by 20–40% with a minimal loss in selectivity.he optimal operating pressure above the active layer of the mem-rane is atmospheric [21,22]. This condition will avoid partialaporization of the feed; thus, a pressure of 1 atm was used in thisork. The feed temperatures ranged from 30 ◦C to 80 ◦C. Before

tarting the PV experiments, test membranes were equilibrated for h with the feed mixture. In order to collect permeate samples, a-glass was used. One side of the U-tube was connected to the per-eate line while the other end was screwed to the vacuum line.

he permeate pressure was maintained below 2.5 mbar. The per-eate was collected for every 30–60 min in a cold trap consisting

f U-glass which was immersed in a Dewar flask containing liq-id nitrogen; at least three permeate samples of each feed mixtureere collected.

The membrane flux (J) was determined gravimetrically using aeighing scale with an accuracy of 10−4 g. The weight difference

etween the initial empty U-glass and post permeate collection isivided by the duration of permeate collection and the effectiveembrane surface area.

= w

At(2)

here w is the weight of permeate collected (g), t is the durationf the experiment (h), and A is the effective area of the membranem2). The calculated flux value can be then used to estimate theater and/or acetic acid permeability defined in Eq. (3) [23]:

p

l= J

�xpsat − ypp(3)

here P/l is the membrane permeability (P) to membrane thick-ess (l) ratio, � is the activity coefficient calculated using the Vanaar equation, x is the mol fraction in the feed, Psat is the saturatedapour pressure determined using the Antoine equation, y is theolar fraction in the permeate and Pp is the permeate pressure

btained during the PV experiment.The permeate compositions were analyzed by measuring its

efractive index with an accuracy of ±0.0005 units using a refrac-ometer. The obtained refractive index was then compared to atandard curve for acetic acid and water mixtures. Each measure-ent was performed at least three times and the average value was

aken as final reading. The separation factor was determined as:

w/HAc =(

Yw/YHAc

Xw/XHAc

)(4)

here X and Y are weight fractions in the feed and permeate,espectively.

.4. Membrane stability test in pervaporation for acetic acidehydration

In the membrane stability test, the impact of HAc–water mix-ure on PPSU membranes was assessed at 0, 2 and 4 barg. Threeieces of 30 wt.% PPSU membranes were tested, and denoted asP-0, MP-2 and MP-4, respectively, where each membrane was

ept in a cell containing water while applying pressure at desiredonditions. After 2 weeks of immersion in the designated storageondition, each membrane was mounted onto the membrane test

ell to undergo PV experiment with 80 wt.% HAc at 80 ◦C for 12 h.he test was terminated when the PV has undergone 12 h opera-ion. The PPSU membrane stability towards elevated temperaturerom 30 ◦C to 80 ◦C was also conducted. The experiment was ended

Fig. 1. SEM images for PPSU membranes top layers (A) PPSU-25, (B) PPSU-27.5 and(C) PPSU-30.

once each membrane was probed to all temperatures in both HAcconcentrations, 80 wt.% and 90 wt.% HAc.

3. Results and discussion

3.1. Membrane characterization

Membrane characterization allows studying the membranemorphology. Fig. 1(A–C) presents the PPSU membranes top lay-ers structures taken at 20,000× (A), 50,000× (B) and 35,000× (C).In Fig. 1A, it can be seen that the PPSU membranes exhibited densepolymer network structures with finger-like pores at the lower partof the membrane, having a top layer thickness of 0.549 �m. Fig. 1Bshows a more dense top layer thickness of 0.227 �m in average,which is lower than in Fig. 1 A. Fig. 1C shows a very dense toplayer, thicker than the one observed in Fig. 1B; the thickness was0.308 �m.

These images clearly explain that small changes in polymer

concentration result in different membrane characteristics (densetop layer membrane obtained using high polymer concentration),which will be further studied in pervaporation experiments.

N. Jullok et al. / Chemical Engineering Journal 175 (2011) 306– 315 309

0

20

40

60

80

DS,

%30°C 50° C 70° C

Ft

3

b3imtpbtltaodTtitdt

iTittmPtmtbDiitbpDscHHco

m

Pervaporation dehydration of acetic acid–water as a function

ig. 2. Total DS for PPSU membranes in water and pure HAc at different immersionemperatures.

.2. Effect of temperature on membrane swelling

Pervaporation performance is influenced by the extent of mem-rane swelling (DS). DS results for the studied PPSU membranes at0, 50 and 70 ◦C (±5 ◦C) in pure water and pure HAc are shown

n Fig. 2. In general, it was observed that DS values for PPSUembranes were higher in pure HAc solution than in water at all

emperatures, for all the membranes. During the DS process, it isossible that liquid sorption occurred at both sides of the mem-ranes, where sorption of the membranes at the top layers is lowerhan that at the sub-layers. This is due to the membranes sub-ayers, which appear to have higher porosity than at the membranesop layers, enabled the HAc and/or water molecules to sorb fasternd cluster inside the membranes’ free volume. The SEM imagesf PPSU membranes in Fig. 3 shows that the sub-layer porosityecreases in the following order: PPSU-25 > PPSU-27.5 > PPSU-30.he temperature effect on membrane swelling is initially expectedo result in significant and observable degree of swelling. However,t is not seen to produce any dramatic changes as expected. Onhe contrary, the DS analysis shows a stable membrane throughoutifferent immersion conditions; different feed concentrations andemperatures.

The free volume of PPSU membranes was observed; it decreasedn the order PPSU-25 > PPSU-27.5 > PPSU-30, as could be expected.his shows that when the free volume increases (decreases), the DSncreases (decreases). Moreover, the results in Fig. 2 also indicatehat the DS values in pure HAc were more likely to be influenced byhe membranes’ sub-layer porosity rather than the sorption of the

embranes’ dense top layers. These outcomes were expected sincePSU membranes are no organo-selective membranes. Thus sorp-ion of HAc on the dense top layers was probably lower than water

olecules. Since the molar mass of HAc (60.05 g/mol) is higherhan that of water (18.01 g/mol), the total DS values of PPSU mem-ranes were higher in HAc compared to water. The fluctuation ofS observed in PPSU-25 may have resulted from the membrane

nstability when exposed to high HAc concentrations. As observedn Fig. 2, the total DS somewhat decreased with the increase ofemperature. This is a remarkable observation since the contraryehaviour is normally reported [19]. This is because at higher tem-eratures, the density of HAc is lower, resulting in a total decrease ofS. Furthermore, the mobility of HAc molecules in the solution was

uspected to be more random and active. This resulted in a lowerontact time with the PPSU membranes, thus causing a decrease inAc sorption. As a consequence, the total DS decreased when theAc sorption decreases. This analysis, indicates that a higher PPSUontent produces lower DS, which is an indication of the stability

f the membrane.

In pervaporation, swelling is often observed when using a poly-eric membrane. Hence, the concentration gradient is non-linear

Fig. 3. SEM images of PPSU membranes sub-layers construction (a) PPSU-25, (b)PPSU-27.5 and (c) PPSU-30.

due to the swelling in the polymeric membrane. The fully swollenpolymer may be 10–100 times the volume, weight or surface area ofthe dense, unswollen polymer [7]. Therefore, analysis of DS for PPSUmembranes in feed solutions is a vital step to study the chemistrybetween the membrane and the feed solutions, and at the sametime, obtaining the DS. The DS analyses are presented in terms ofpartial sorption at 50 ◦C in all three different PPSU membranes andshown in Fig. 4. All PPSU membranes indicate a decrease of watersorption. This is parallel with the decreasing ‘weight fraction ofwater in feed solution’, labelled on the X-axis, and vice versa. Thisbehaviour is expected and well understood.

3.3. Effect of temperature and feed concentration onpervaporation

of temperature for 80 wt.% and 90 wt.% HAc are shown in Fig. 5(a)and (b), respectively. In Fig. 5(a), PPSU-25 shows the highest flux,∼0.24–1.48 kg/m2 h while PPSU-30 gives the lowest flux in the

310 N. Jullok et al. / Chemical Engineering Journal 175 (2011) 306– 315

0

10

20

30

40

50

60

70

100 20 10 0

Sorp

�on

at 5

0°C,

%

25-Wate r27.5-Wate r30-Wate r25-HA c27.5-HAc30-HA c

F

rtioasTtl

igcPsttt

Fu(

y = -30 .53 x + 9.84 2R² = 0.96 7

y = -33 .19 x + 10 .27R² = 0.99 4

y = -27 .18 x + 7.48 6R² = 0.96 0

-4

-3

-2

-1

0

0.32 0 0.34 0 0.36 0 0.38 0 0.40 0

ln(J

), kg

/m².

h

1000/RT

PPSU-25 PPSU-27 .5 PPSU-30

Weight frac�on of water, wt %

ig. 4. The partial sorption of PPSU membranes against weight fraction of water.

ange of ∼0.1–0.5 kg/m2 h. This was observed for both HAc concen-rations. In this study, it was found that as the PPSU concentrationncreases, the flux decreases. This is explained by the dependencyf the permeance as a function of the membrane top layer char-cteristics. Increasing the initial PPSU concentration in the castingolution leads to a much higher PPSU concentration at the interface.hus, the transport of the permeance in a more dense, top layer ofhe membrane becomes slower, (PPSU-27.5 and 30), resulting in aower flux.

As can be seen in Fig. 5(a) and (b), fluxes increase when increas-ng temperature. In addition, remarkably, the PPSU-27.5 membraneenerated the highest value in overall separation factor. This resultan be explained by referring to Fig. 1. In Fig. 1, it can be seen thatPSU-27.5 and PPSU-30 consist of a dense membrane top layer

tructure, which is not seen in PPSU-25. PPSU-27.5 has a thinnerop layer skin than PPSU-30, as explained in section 3.1. Thus,he figures indicate that PPSU-27.5 may be a good compromiseo obtain the optimal performance. This indicates that no further

-8

-3

2

7

12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

30 40 50 60 70 80

Separation factorFlux

, J (k

g/m

²h)

Feed te mperature ( C )

a

b

-8

-3

2

7

12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

30 40 50 60 70 80

Separation factorFlux

, J (k

g/m

²h)

Feed temperature ( C )

ig. 5. Effect of temperature on pervaporation dehydration of HAc–water solutionsing three PV membranes in (a) 80 wt.% HAc and (b) 90 wt.% HAc; (�)PPSU-25,�)PPSU-27.5 and (�)PPSU-30.

Fig. 6. The EJ data obtained from the slope of plot Ln(J) against inverse temperature(1000/RT) in 10 wt.% water.

improvement in the separation is to be expected by increasingthe membrane thickness. The increase of separation factor may besmall, as in the case for PPSU-25 for both feed concentrations, butit is significant for the PPSU-30 membrane. The results also showthat when a PPSU membrane is introduced to a HAc/water solu-tion, HAc tends to become more reactive to PPSU, in comparison towater. As the HAc molecule associates with the PPSU membrane,the development of hydrogen bonds with PPSU is facilitated. Inaddition, as the temperature increases, the vapor pressure differ-ence increases, which enhances the driving force [21]. As a result,more HAc and water penetrate through the PPSU membrane lead-ing to a higher flux. Because the water molecule is smaller thanHAc and the PPSU membrane is hydrophilic type, the permeationrate was faster than for HAc, resulting in increase of the separa-tion factor. This is applicable for all feed conditions and membranecompositions. The increase of flux is larger at higher PPSU concen-trations, which can be seen by a comparison of the slopes at 25, 27.5and 30 wt.% PPSU, respectively. It is also obvious that the activationenergy may have a larger influence for the more dense membranesthan for PPSU-25 membrane (Ref in table: [24]).

The temperature dependency of the flux was analyzed using anArrhenius equation (Eq. (5)):

X = X0 exp(

− Ex

RT

)(5)

where X can be the flux (J) or permeability (P), X0 is the pre-exponential factor (permeation rate constant), R is the gas constant(J/mol K). T is the temperature (K) and Ex is the activation energy

(kJ/mol). If the activation energy is positive, the permeation fluxincreases with an increase in temperature, which is also observedin most PV experiments in the literature [25]. In Figs. 6 and 7,

y = 12 .17 x - 3.00 1R² = 0.83 5

y = 9.869 x - 2.69 9R² = 0.93 6

y = 16 .11 x - 5.57 3R² = 0.90 4

-0.5

0

0.5

1

1.5

2

0.32 0.34 0.36 0.38 0.4 0.42

Ln(P

), kg

/m².

h.ba

r

1000/R T

PPSU-25 PP SU-27 .5 PPSU-30

Fig. 7. The EP data obtained from the slope of plot Ln(P) against inverse temperature(1000/RT) in 10 wt.% water.

N. Jullok et al. / Chemical Engineering Journal 175 (2011) 306– 315 311

Table 1Arrhenius activation energy of flux and permeability for water.

Membrane Feed mixture EJ (kJ/mol) EP (kJ/mol) (Fitted, Fig. 7) EP (kJ/mol) Ref.(HAc/water) (Fitted, Fig. 6) (Calculated, Eq. (9))

PPSU-25 90/10 30.53 −12.7 −10.13 This workPPSU-27.5 90/10 33.19 −9.87 −7.47 This workPPSU-30 90/10 27.18 −16.11 −13.48 This workPVA 10–90 29.6–33.5 NA −11.06 to −7.16a [23]PVA 10-90 28.6–42.9 NA −12.06 to 2.24a [24]bPPSU-25 80/20 28.48 −14.83 −12.18 This workbPPSU-27.5 80/20 33.86 −9.51 −6.8 This workbPPSU-30 80/20 28.59 −14.64 −12.07 This work

N

lictuef

i

wb1p(vafa

J

Tw

E

wt

oc

E

TA

N

A, not available from the reference cited.a Calculated by authors in this work.b Fitted graphs were not shown.

ogarithmic plots of flux and permeability as a function of thenverse temperature are shown. The straight line correlation indi-ates the validity of the adsorption–diffusion model to describeransport through the PPSU membrane and the slope of these fig-res give the activation energy of flux EJ (Fig. 6) and the activationnergy of permeability Ep (Fig. 7). Values are indicated in Table 1or water and Table 2 for acetic acid.

According to Bettens et al. [26] the transport can be describedn terms of the permeability coefficient (P), expressed in Eq. (6):

p

L= J

�F= SD

L=

(S0D0

L

)exp

[−�HS − ED

RT

](6)

here J is the permeant flux (kg/m2 h), �F is the transmem-rane partial pressure difference (bar), S is the solubility (g per00 g water), D is the surface diffusivity (m2/s), S0 and D0 arere-exponential factors, L is the membrane thickness (�m), �Hs

kJ/mol) is the heat of adsorption and ED (kJ/mol) is the acti-ation energy of diffusion. Burggraaf [27] has related both thectivated microscopic models based on configuration and on sur-ace diffusion with the temperature dependence in the classicaldsorption–diffusion model as shown in Eq. (7):

= S0 exp(

−�HS

RT

)D0 exp

(− ED

RT

)A exp

(−�H�ap

RT

)(7)

hus, combining Eqs. (5)–(7), the activation energy of flux can beritten as shown in Eq. (8).

J = �HS + ED + �H�ap (8)

here �Hvap is the heat of vaporization of the permeant (water)hrough the membrane (40.66 kJ/mol).

In Eqs. (6) and (8), the term (�HS + ED) is the activation energy

f permeability, Ep (kJ/mol), when the permeate pressure is suffi-iently low; thus, Eq. (8) can be written as follows:

p = EJ − �Hvap (9)

able 2rrhenius activation energy of flux and permeability for HAc.

Membrane Feed mixture EJ (kJ/mol)

(HAc/Water) (Fitted)

PPSU-25 90/10 24.67

PPSU-27.5 90/10 31.23

PPSU-30 90/10 13.8

PVA 10–90 29.6–34.2

PVA 10–90 42.5–62.8

PPSU-25 80/20 19.78

PPSU-27.5 80/20 17.54

PPSU-30 80/20 7.36

ote: The EJ and EP values were obtained the same way shown in Table 1, however, the fia Calculated by authors in this work.

Values of Ep calculated with Eq. (9) are indicated in Tables 1 and 2.The comparison of these values with those obtained from exper-imental data (slope of Fig. 7) shows that there are deviations:experimental values which are the true values were higher thancalculated, reaching deviations between 16% and 28% for water(Table 1) and 2–5% for acetic acid (Table 2). This different behaviouris worth of further investigation since the high deviation for watermay indicate that Eq. (9) cannot be applied for calculating its acti-vation energy of permeability.

Theoretically, the vapour pressure (Psat) of the feed componentincreases when the feed temperature is increased, while the vapourpressure on the permeate side will not be affected. In this study,negative values of Ep were expected because values of EJ were inthe range of 14–34 kJ/mol, below the heat of vaporization of water,indicating that membrane permeability coefficient decreases (withan increase in flux) as the temperature increases. This is due to themore significant effect of temperature on saturated vapour pres-sure [28].

3.4. Membrane stability

The long term-stability of the membranes is a vital parameterto determine their durability in industrial applications. However,this is rarely reported. In this work, a membrane stability testfor PPSU was set up and evaluated. The PPSU membrane wasassessed by studying the effect of pressure and HAc exposure onthe membrane performance. The membrane stability test was alsoconducted along with the PV analysis in the temperature range of30–80 ◦C, and in high acetic acid concentrations, 80–90 wt.% HAc.

3.4.1. Pressure effect on PPSU membraneThe results indicate that the application of pressure on the PPSU

membrane caused compaction without further alteration of themembrane performance. In Fig. 8, it can be seen that MP-2 andMP-4 membranes showed a similar trend as for MP-0. In fact, themembranes were stable following that trend. The pressure effect

EP (kJ/mol) EP (kJ/mol) Ref.(Fitted) (Calculated, Eq.(9))

−15.48 −15.99 This work−9.9 −9.43 This work−27.46 −26.86 This workNA −11.06 to −6.46a [23]NA 1.84–22.14a [24]−21.63 −20.88 This work−23.75 −23.12 This work−33.99 −33.3 This work

tted graphs for HAc were not shown. NA, not available from the reference cited.

312 N. Jullok et al. / Chemical Engineering Journal 175 (2011) 306– 315

Stabilization tim e

0.000.050.100.150.200.250.300.350.40

0 1 2 3 4 5 6 7 8 9 10 11 12

Part

ial f

luxe

s, kg

/m²h

Pervapor ation time (hour)

a

b

Water-0 Water-2 Water-4 Hac- 0 Hac- 2 Hac- 4

6789

10111213

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Sepa

ratio

n fa

ctor

, (α w

/a)

Pervaporat ion time (hour)MP-0 MP- 2 MP- 4

Fig. 8. Effect of pressure on PPSU-30 membrane in PV: (a) flux vs. time and (b)s

rftaIatstflrl(MSTbtfsrpcittabu

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

22.00

0

0.05

0.1

0.15

0.2

0.25

80 85 90 95

Separa�on factor

Flux

, kg/

m²h

Weight fr ac�o n of HAc in feed sol u�on , wt %

HAc

Water

Sep.facto r

branes. Table 4 summarizes each type of pervaporation membrane

eparation factor vs. time.

esulted in membrane thickness contraction, which was observedor MP-2 and MP-4 where the fluxes were higher than MP-0. Onhe other hand, the contraction is suspected to be significant onlyt the sub-layer (porous structure) rather than the dense top layer.n this analysis, water transport through the PPSU-30 membrane isgain seen to be the major cause of total flux fluctuation amongsthe tested membranes. During the initial hours (considered as thetabilization time), variations due to the membrane adaptationo the system was observed. MP-4 shows a great decrease in itsux during stabilization time. This is because of the membraneeaction towards different environment from high pressure (sub-ayer compaction) to high temperature and high HAc concentrationmembrane swelling). At high HAc concentration (80 wt.%), the

P-4 membrane swelled, resulting in an increased flux after 5 h.omehow, MP-4 still has a relatively lower flux compared to MP-2.he existing pressure (4 barg) which was applied to the MP-4 mem-rane has caused the porous sub-layer to compact, with a reducedhickness. This combined with the already dense top layer thenormed a thicker, denser membrane layer. Thus, the effect of pres-ure has caused the existence of a thinner, but denser membrane,esulting in a lower flux compared to MP-2. On the other hand, theressure applied onto membrane MP-2 did not cause significantompaction, even though thickness reduction and flux increase arenevitable. This again reiterates the initial finding in Section 3.3hat the thickening of the dense top layer will not further improvehe membrane’s performance. Apart from obtaining membrane flux

nd selectivity, this study showed that PPSU membrane is safe toe applied when pressure above 1 atmospheric in the system isnavoidable.

Fig. 9. Partial fluxes for HAc and water in higher HAc concentration range usingPPSU-27.5.

In Fig. 8(b), it shows that, the separation factors for MP-4 havea stepwise trend beginning from the fourth hour until the end ofthe experiment. It can be seen that the separation factors increasedevery 3 h. This analysis indicates that the MP-4 membrane requiredmore time compared to MP-2 to achieve a steady condition. Inthe end, it is believed that MP-4 will be able to regain its initialbehaviour and perform as well as MP-0.

3.4.2. Stability in high acetic acid feed concentrationIn Section 3.3, it was explained that PPSU-27.5 is the best choice

for HAc–water mixture separation in this study. Therefore, thismembrane was used as reference to study the membrane stabilityin high HAc feed concentration. The partial fluxes and separationfactor through PPSU-27.5 membranes at 50 ◦C as a function of HAcconcentration are shown in Fig. 9. Before hand, it was foreseen thatthe separation factor may decrease with increase of HAc composi-tion in the feed solution, due to domination of HAc molecules in thesolution which can possibly hinder the interaction between watermolecules and membrane surface. In contrast, it can be seen that thewater flux decreases when the HAc concentration in the feed solu-tion increases. This is rational because the water molecules reduceswhen HAc molecules increases. On the other hand, the separationfactor increases when the concentration of HAc in the feed getshigher. The explanation for this phenomenon is twofold. Firstly,the water solubility and diffusivity increases when the water sat-uration in the feed decreases. The decrease in water saturation inthe feed solution may contribute to the decrease of intermolecularfriction during transport through the membrane. As a consequence,water permeates at higher proportion when the HAc concentrationis high. Secondly, the plasticizing effect may facilitate the watertransport through the membrane. As a result, the separation factorincreases.

A complete analysis of PPSU membranes in higher range ofHAc concentration at 50 ◦C is summarized in Table 3. The partialfluxes and separation factors were presented in a systematic way.Moreover, this investigation proved that PPSU membranes werestable for high HAc concentrations. Generally, it can be seen thatPPSU-27.5 performed better than PPSU-25 and PPSU-30 and simul-taneously reassured the findings in Section 3.3.

3.5. Comparison of present membranes with literature data

Several studies have been undertaken on dehydration ofHAc–water mixtures using organic, inorganic and composite mem-

applied in acetic acid and water separation. It has been shownthat inorganic membranes, in most cases, are capable to providea very high water selective barrier, but with a very low permeate

N. Jullok et al. / Chemical Engineering

Table 3The partial fluxes and separation factors for PPSU membranes in high HAc concen-tration at 50 ◦C.

PPSU-conc. (wt.% HAc) Partial flux (kg/m2 h) at 50 ◦C Sep. factor’Water HAc

PPSU-25–80 0.34 0.37 3.7PPSU-25–85 0.26 0.43 3.3PPSU-25–90 0.26 0.54 4.4PPSU-25–95 0.38 0.80 8.9

PPSU-27.5–80 0.23 0.14 6.6PPSU-27.5–85 0.15 0.13 6.9PPSU-27.5–90 0.12 0.13 8.2PPSU-27.5–95 0.14 0.21 12.7

PPSU-30–80 0.18 0.13 5.3

flwtmlb

vmehtp

TC

Ap

PPSU-30–85 0.11 0.09 6.7PPSU-30–90 0.08 0.13 5.7PPSU-30–95 0.12 0.16 14.9

ux, e.g., using zeolites. In an acid-proof silicalite-1-zeolite studyhich was carried out by Masuda et al. [29], it was found that

he separation factor achieved near infinite to water. However, theembrane was not able to be commercialized due to the extremely

ow flux (0.00045 kg/m2 h). Moreover, zeolites are normally unsta-le in highly acidic solution.

A study using a free-radical graft polymerization of 1-inylimidazole (VI) onto the as-synthesized mordernite zeoliteembrane in acetic acid dehydration has been conducted by Chen

t al. [30]. It was found that by applying this membrane in the PV atigher temperature, it produces higher flux and also higher selec-ivity. PV of 83 wt.%HAc at 80 ◦C gives infinite selectivity due toure water at permeate and flux of 0.258 kg/m2 h was obtained.

able 4omparison of PV membranes used in HAc–water separation and their performance.

Membrane Binary mixt

Inorganic membraneSilicalite-1-zeolite 98/2

Poly(1-vinylimidazolo)/modernite 83/17

STA-NaAlg-5 90/10

STA-NaAlg-1 90/10

Silica 50/50

Organic membrane25 wt.%PPSU 80/20–90/127.5 wt.%PPSU 80/20–90/130 wt.%PPSU 80/20–90/1

Modified PVA membraneModified PVA membrane with poly(acrylic acid) 10–90

Modified PVA membrane with malic acid 20–90

Modified PVA membrane with amic acid 10–90

Modified PVA with glutaraldehyde 10–90

Modified PVA with formaldehyde 10–90

Charged membraneNafion(C8H17)4N+ 90/10

PSF(SO3−)–H+ 90/10

AMV-CH3COO− 80/20

CMV-H+ 80/20

PVC membranePoly(vinyl chloride) Polyacrylonitrile 80/20

Poly(vinyl chloride) 50/50

Composite membranePoly(4-methyl-1pentane)/ethylene-vinyl acetate copolymer TPX/P4-VP 84/16

NaAlg and PAN crosslinked with PVA 85/15

NaAlg + 5%PVA + 10%PEG 90/10

NaAlg and PAN crosslinked with HDM (ion: Na+) 90/10

90/10

90/10

90/10

MV, CMV, crosslinked styrene−co−butadiene base, mechanically stabilized with pooly(sulfone).

Journal 175 (2011) 306– 315 313

Conversely, when HAc concentration increases, the selectivity andthe flux dropped. Although the explanation was more onto themolecule movement, the worsening in quality may also have beencontributed by the collapsed and/or damaged membrane due tohigh HAc concentration. However, the stability of the membranewas not reported.

Another interesting study was the investigation by Shivanandet al. [25] in applying a silicotungstic acid incorporated sodiumalginate (STA-NaAlg-5). This study gave an excellent separation fac-tor and flux, ranging from 0.194 to 0.489 kg/m2 h. However, themembrane performance decreased when applied at high tempera-ture and in a more dilute feed solution. As temperature and watercontent in feed solution increases, flux increase and selectivitydecreased. There were also no stability-related details provided.

PV using organic membranes such as poly(4-methyl-1-pentene)/Co (III) (acetylacetonate), grafted PVA membrane withpolyacrylamide and PVA membrane modified with PAA whichwere reported by Chapman et al. [6] were less attractive due itspoor durability in high temperature and mixture concentration.Improvement of the organic membrane performance through mod-ification and new membranes’ construction has to be carried outto enable better compatibility with the inorganic and compositemembranes. This is to ensure feasibility for practical industrialapplications. In this study, PPSU membranes were found to be veryconvincing in terms of stability. Even though the selectivity of mostof the modified PVA membrane was better than PPSU membranes,

the fluxes produced were lower than PPSU membrane. Another dis-advantage in applying PVA membranes is that it has poor stabilityin aqueous solutions. The proposed PPSU membranes also pro-vided better performance than some of the charged membranes,

ure (HAc/water) ˛w/HAc Flux (kg/m2 h) T (◦C) Year Ref.

∞ 0.00045 80 2003 [29]∞ 0.258 80 2009 [30]124–22,491 0.194–0.489 30–70 2007 [25]817 0.15–0.35 30–70 2007 [25]125 5400 90 1990 [31]

0 2.5–6.1 ∼0.24–1.48 30–80 This work0 5.0–11.4 ∼0.12–0.83 30–80 This work0 2.8–12.0 ∼0.09–0.48 30–80 This work

34–3548 0.03–0.6 30–55 2003 [32]121–670 0.05–0.29 40 2005 [33]13–42 0.08–2.28 30–75 1991 [34]4.0–9.0 ∼0.0001–0.0003 30–60 2001 [35]1.0–5.5 ∼0.0001–0.0004 30–60 2001 [35]

243 0.18 25 1999 [36]9.4 0.02 – 1997 [37]4.3 0.83 80 1997 [37]4 0.33 80 1997 [37]

182–274 0.56–0.74 80 1993 [38]45 6 40 1993 [39]

606 0.215 25 1994 [2]162 0.262 70 2000 [3]40 0.0239 30 2004 [40]38 0.037 40 2000 [3]60 0.054 50 200098 0.092 60 2000134 0.138 70 2000

ly (vinyl chloride) backing and containing, respectively, PSF(SO3−), sulfonated

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14 N. Jullok et al. / Chemical Engin

specially in term of flux and selectivity. Details of this comparisonan be seen in Table 4.

The investigation of PV dehydration of acetic acid using aomposite membrane of poly(4-methyl-1pentane)/ethylene–vinylcetate copolymer TPX/P4-VP by Lee and Lai [2] in 90 wt.% HAct 25 ◦C showed an impressive selectivity of 606 and the flux was.215 kg/m2 h. On the other hand, at high HAc concentration, theembrane compacted resulted in flux decrement. There was no

urther information about the change of the separation factor.Comparing the present work with other organic, composite

nd inorganic membranes listed in Table 4, PPSU membranesave shown an impressive performance and stability at high HAcqueous solution and high temperature. A stable polymeric baseembrane promoting a high flux can serve as an excellent basis

or further attempts on membrane selectivity improvements, asentioned by Chapman et al. [6]. Thus, this work will be a good

latform for further PPSU membrane improvement in separationsy pervaporation.

The PV study on the dehydration of 80 and 90 wt.%HAc showedmpressive performance. Even though the lab-prepared membrane

as not further improved by thermal treatment, cross-linkinggent or other potential method, it is still functioning as a goodater separation membrane at high acetic acid concentration andigh temperatures. When the acetic acid concentration increases

n the feed solution, the DS, flux and separation factor increased.his phenomenon occurs possibly because acetic acid and the PPSUembrane have a good interaction with each other and are always

ttracted to each other. Thus, as the wt.% HAc increases, the PPSUembrane reacts by creating more free volume to accommodateore acetic acid molecules. This results in an increase of DS. Whenany of the acetic acid molecules form a network within the

PSU membrane, the water molecules are able to penetrate acrosshe membrane, resulting in an increased separation factor. As theeed temperature increases, DS decreases, but the flux and separa-ion factor increase. At a relatively high temperature, the physicalxpansion of the polymer network becomes restricted, ensuing in

lower DS and free volume. The increase of flux and separationactor at high temperature occurred possibly due to the interac-ion of acetic acid with PPSU blend. This consequently increaseshe hydrogen bond potential between the feed solution and PPSU

embrane. The hydrogen bond facilitates the transport of waterolecules through the membrane. This PPSU membrane is also

table at high wt.% HAc content and reasonably high operating tem-eratures, as long as the applied system pressure does not exceed

atm.

. Conclusions

In this study, PPSU membranes were successfully developedsing the phase inversion method for pervaporation dehydrationf HAc. The novel membranes show an impressive performance,hich offers a feasible method to separate HAc and water, withigh flux, good selectivity and stability. DS values show that HAcas a larger influence on membrane swelling than water. At higherAc concentrations, DS increases, which results in an increase of

he total flux. This condition somehow does not weaken the mem-ranes since the separation factors improved with the increasef the HAc concentration. The plasticizing effect was postulatedo result in a special sieving property, which assisted the water

olecules to be transported across the membrane. Furthermore,he binary mixture’s density at different temperatures also influ-

nced the DS of each membrane. The density of the mixture is lowerhen the solution has higher temperature. Overall, the PPSU-27.5embrane had the optimum performance; following further devel-

pment and improvement on the membrane morphology, PPSU

[

g Journal 175 (2011) 306– 315

membranes are thought to be promising for purification of highlyacidic organic solutions using pervaporation.

Acknowledgement

The financial support from Ministry of Higher Education ofMalaysia is gratefully acknowledged.

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