The potential of pervaporation for separation of acetic acid and water mixtures using polyphenylsulfone membranes

The potential of pervaporation for separation of acetic acid and water mixtures using polyphenylsulfone membranes

Chemical Engineering Journal 175 (2011) 306–315 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: ww...

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Chemical Engineering Journal 175 (2011) 306–315

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

The potential of pervaporation for separation of acetic acid and water mixtures using polyphenylsulfone membranes Nora Jullok a,b,∗ , Siavash Darvishmanesh a , Patricia Luis a , Bart Van der Bruggen a a Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium b School of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3,02600 Arau, Perlis, Malaysia

a r t i c l e

i n f o

Article history: Received 26 March 2011 Received in revised form 25 September 2011 Accepted 27 September 2011 Keywords: Pervaporation Acetic acid dehydration Polyphenylsulfone (PPSU) Membrane swelling Membrane stability

a b s t r a c t Conventional pervaporation (PV) membranes usually have limited resistance to acetic acid (HAc), particularly in high pressure and temperature conditions, resulting in a cumbersome water-acetic acid separation. 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 study explores 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–water with 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 high temperatures. 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) in terms of overall performance, considering both flux and separation factor. Although the total flux of PPSU27.5 (∼0.12–0.83 kg/m2 h) is lower than PPSU-25 (∼0.24–1.48 kg/m2 h) the average separation factor can be 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 permeabilities decrease with increasing temperature. With the enrichment of acetic acid on the feed side of the membrane, the degree of swelling, flux and separation factor all increase. Regarding on the membrane stability tests, the PPSU membranes showed promising results at tested conditions. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Separation processes are required in the chemical industry in order to obtain high purity of raw materials, intermediates or end products. Separation of compounds from a mixture requires energy and in many cases, the separation is challenging to perform due to energy consumption and cost. The chemical or physical properties (e.g., molecular size, vapour pressure, freezing point, affinity, charge, density and the chemical nature) of the target compounds are important factors in determining the process viability [1,2]. The mixture of HAc with water has been the main focus of several previous investigations [3–7], which consider the close

∗ Corresponding author at: Department of Chemical Engineering, Laboratory for Applied Physical Chemistry and Environmental Technology, Katholieke Universiteit Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium. Tel.: +32 16 32 23 48, fax: +32 16 32 29 91. E-mail address: [email protected] (N. Jullok). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.09.109

boiling point of both compounds. The main challenges of such separation process are (1) cost-minimization and (2) increase of separation efficiency. The conventional HAc production process by methanol carbonylation operates at both elevated temperature (∼190 ◦ C) and pressure (∼28 bar) [8]. To some extent, the process may entail higher pressure states, reaching up to 80 bar. One example of such a case is the ethylene direct oxidation process. In addition, several studies reported yields as low as 63–86% selectivity to HAc [9]. PV could be a beneficial option for further purification and/or product/by-product recycling. Mixtures containing acetic acid and water do not form an azeotropic mixture [3–7,10]. However, separation of HAc from water using a normal distillation process is far from being the best process to consider due to the large number of trays necessary in the distillation column to perform the separation, which increases the associated costs [10]. Many alternative processes have been proposed in order to improve the efficiency of the separation of HAc and water solution in order to reduce the energy consumption, which may lead to lower operational costs. Pervaporation is considered as a

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

promising process to separate azeotropic and close boiling point mixtures. However, the choice of the membrane is a key consideration that defines the type of application. The specific component to be separated from the mixture determines the membrane type that is needed (hydrophilic, organophilic or hydrophobic). Normally, the smallest weight fraction of component in the mixture is to be transported across the membrane: hydrophilic polymeric membranes are used for the dehydration of organic liquids and hydrophobic polymeric membranes for removal of organics from water streams [11]. Wee et al. [12] found that polymeric membranes had the best dehydration performance under azeotropic conditions compared to inorganic membranes. Nevertheless, polymeric membranes also possess disadvantages such as limited solvent [12] and temperature stability. Poly(vinyl alcohol) (PVA) membrane, for instance, is a highly hydrophilic membrane that is studied for the dehydration of organic solutions, including the dehydration of acetic acid but PVA membranes have a poor stability in aqueous mixtures, requiring an intensive pretreatment, modification and/or alteration of the membrane to assure a good performance. Furthermore, polymeric membranes also suffer from swelling, which might change the membrane structure and properties significantly. Polyphenylsulfone (PPSU), as a new membrane material, may be able to provide a new option and/or serve as an alternative in organic dehydration, especially in solutions at low pH, as found in e.g., pervaporation dehydration of acetic acid. In recent years, investigations related to PPSU membranes have been conducted to study its potential use as proton-conducting fuel cell membrane [13–16]. Others have studied PPSU/PBNPI blend membrane for hydrogen separation [17] and as a support to polymeric liquid membranes in recovery of aromatic compounds from wastewater [18]. In addition, PPSU membranes have been reported to have a high thermal stability with a decomposition temperature between 300 ◦ C and 350 ◦ C, and a good mechanical stability [13]. The robustness of the material in terms of physical durability and chemical stability points out that PPSU may be a good polymer candidate, which could be potentially developed for further use in HAc–water separation [19]. Furthermore, in industrial applications, excessive temperatures and pressures frequently occur. Since PPSU is easily sourced, commercially available and is relatively low cost, it will effectively enable a time- and cost-effective operation [13]. No studies have been made yet on the use of PPSU for HAc–water separation in pervaporation. PPSU is generally considered hydrophobic, but preliminary experiments (not shown) proved that the water contact angle for the manufactured PPSU membranes was below 90◦ , which makes them sufficiently hydrophilic for this application. Thus, this study is focused on achieving the separation of HAc–water mixtures in harsh environments using self-made PPSU membrane. HAc–water mixtures containing up to 90 wt.% HAc will be studied and a deep discussion about the swelling phenomena in the membrane is performed. In dense membranes used for pervaporation, the flux through the membrane is proportional to the driving force, i.e. the temperature, pressure, concentration and electromotive force gradient. These factors, concluded as an overall driving force, create mobility of the permeant through the membrane. Besides the driving force, the membrane itself is the prime factor in determining the selectivity and flux. The nature of the membrane – in terms of structure and material used – determines the type of application, ranging from separation of microscale particles to the separation of molecules of identical size or shape. Colman and Naylor [20] used pervaporation in the dehydration of isopropanol showing that concentration polarization still exists even when the feed flow is highly turbulent (with Reynolds numbers above 104 ). However, in this study, the effects of concentration polarization were not taken into account; the focus is on the analysis of applicability of novel PPSU membrane in HAc dehydration and data estimation instead of precise calculations.

307

2. Material and methods 2.1. Materials Radel® R-5000, a transparent polyphenylsulfone (PPSU), was purchased from Solvay Advanced Polymer Belgium and dissolved at specific ratio with N-methyl-2-pyrrolidinone (NMP) which acts as the solvent, purchased from Aldrich. Acetic acid (HAc) was supplied by Chem Lab Belgium. Demineralised water is produced using a reverse osmosis system having a conductivity of 5.6 ␮S/cm. 2.2. Membrane synthesis PPSU membranes were prepared using a phase inversion–immersion precipitation method. Three PPSU-based membranes were prepared at higher polymer concentration range (25, 27.5 and 30 wt.% PPSU) in order to achieve the formation of a dense top layer. The casting solution was prepared by dissolving PPSU in NMP at ambient temperature using a Stovall Low Profile Roller. After the PPSU pellets have completely dissolved, the solution was placed inside a vacuum chamber with the bottle cap partially opened to release present bubbles in the casting solution. The bubble free solution was then cast on a glass plate inside a controlled humidity (<40% RH) chamber using an automatic driven casting blade of 250 ␮m thickness. The humidity inside the chamber was controlled by feeding water-bubbled nitrogen. Immediately after the casting process was completed, the glass plate was taken off from the platform and immersed in a coagulation 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 cut PPSU membrane (dia. = 3.5 cm) was weighed on a single-pan digital microbalance (Model AB204-S, Mettler Toledo) with sensitivity of ±0.0001 mg, recorded and labelled as dry membrane (mdry ). The dry 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 out from the immersion solution and wiped using a cleansing tissue and immediately weighed, recorded and labelled as swollen membrane (mswollen ). Sorption on the membranes in pure water and pure HAc was also measured. The swelling degree was calculated as follows: DS (%) =

mswollen − mdry × 100 mdry

(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 function of the polymer composition and to measure the thickness of the skin top layers. Each sample of PPSU membrane was broken during the immersion in liquid nitrogen. The broken membranes were glued on an agar before coated with a conductive layer of gold. The cross-sections were then investigated using Philip XL 30 ESEM FEG (The Netherlands). 2.3.3. Pervaporation experiments Pervaporation experiments were carried out using a Lab Test Cell Unit, (Sulzer Chemtech). The feed solution of acetic acid–water was kept in a 3 L stainless steel tank with controlled temperature using heater and temperature controller loop. First, the flat sheet PPSU membrane was cut and then immersed in the feed solution overnight. Subsequently, the membrane was placed into the membrane test cell. The effective membrane diameter was

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about 5 cm, corresponding to an effective surface area of around 1.963 × 10−3 m2 . The feed solution was circulated over the membrane using a centrifugal pump. In pervaporation, the feed is normally heated because it is expected that with every 10 ◦ C change in the temperature, permeability will increase by 20–40% with a minimal loss in selectivity. The optimal operating pressure above the active layer of the membrane is atmospheric [21,22]. This condition will avoid partial vaporization of the feed; thus, a pressure of 1 atm was used in this work. The feed temperatures ranged from 30 ◦ C to 80 ◦ C. Before starting the PV experiments, test membranes were equilibrated for 1 h with the feed mixture. In order to collect permeate samples, a U-glass was used. One side of the U-tube was connected to the permeate line while the other end was screwed to the vacuum line. The permeate pressure was maintained below 2.5 mbar. The permeate was collected for every 30–60 min in a cold trap consisting of U-glass which was immersed in a Dewar flask containing liquid nitrogen; at least three permeate samples of each feed mixture were collected. The membrane flux (J) was determined gravimetrically using a weighing scale with an accuracy of 10−4 g. The weight difference between the initial empty U-glass and post permeate collection is divided by the duration of permeate collection and the effective membrane surface area. w J= (2) At where w is the weight of permeate collected (g), t is the duration of the experiment (h), and A is the effective area of the membrane (m2 ). The calculated flux value can be then used to estimate the water and/or acetic acid permeability defined in Eq. (3) [23]: J p = xpsat − ypp l

(3)

where P/l is the membrane permeability (P) to membrane thickness (l) ratio,  is the activity coefficient calculated using the Van Laar equation, x is the mol fraction in the feed, Psat is the saturated vapour pressure determined using the Antoine equation, y is the molar fraction in the permeate and Pp is the permeate pressure obtained during the PV experiment. The permeate compositions were analyzed by measuring its refractive index with an accuracy of ±0.0005 units using a refractometer. The obtained refractive index was then compared to a standard curve for acetic acid and water mixtures. Each measurement was performed at least three times and the average value was taken as final reading. The separation factor was determined as:



˛w/HAc =

Yw /YHAc Xw /XHAc



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 HAc concentrations, 80 wt.% and 90 wt.% HAc.

(4)

where X and Y are weight fractions in the feed and permeate, respectively. 2.4. Membrane stability test in pervaporation for acetic acid dehydration In the membrane stability test, the impact of HAc–water mixture on PPSU membranes was assessed at 0, 2 and 4 barg. Three pieces of 30 wt.% PPSU membranes were tested, and denoted as MP-0, MP-2 and MP-4, respectively, where each membrane was kept in a cell containing water while applying pressure at desired conditions. After 2 weeks of immersion in the designated storage condition, each membrane was mounted onto the membrane test cell to undergo PV experiment with 80 wt.% HAc at 80 ◦ C for 12 h. The test was terminated when the PV has undergone 12 h operation. The PPSU membrane stability towards elevated temperature from 30 ◦ C to 80 ◦ C was also conducted. The experiment was ended

3. Results and discussion 3.1. Membrane characterization Membrane characterization allows studying the membrane morphology. Fig. 1(A–C) presents the PPSU membranes top layers 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 dense polymer network structures with finger-like pores at the lower part of the membrane, having a top layer thickness of 0.549 ␮m. Fig. 1B shows 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 top layer, thicker than the one observed in Fig. 1B; the thickness was 0.308 ␮m. These images clearly explain that small changes in polymer concentration result in different membrane characteristics (dense top 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

30°C

50°C

309

70°C

DS, %

80 60 40 20 0

Fig. 2. Total DS for PPSU membranes in water and pure HAc at different immersion temperatures.

3.2. Effect of temperature on membrane swelling Pervaporation performance is influenced by the extent of membrane swelling (DS). DS results for the studied PPSU membranes at 30, 50 and 70 ◦ C (±5 ◦ C) in pure water and pure HAc are shown in Fig. 2. In general, it was observed that DS values for PPSU membranes were higher in pure HAc solution than in water at all temperatures, for all the membranes. During the DS process, it is possible that liquid sorption occurred at both sides of the membranes, where sorption of the membranes at the top layers is lower than that at the sub-layers. This is due to the membranes sublayers, which appear to have higher porosity than at the membranes top layers, enabled the HAc and/or water molecules to sorb faster and cluster inside the membranes’ free volume. The SEM images of PPSU membranes in Fig. 3 shows that the sub-layer porosity decreases in the following order: PPSU-25 > PPSU-27.5 > PPSU-30. The temperature effect on membrane swelling is initially expected to result in significant and observable degree of swelling. However, it is not seen to produce any dramatic changes as expected. On the contrary, the DS analysis shows a stable membrane throughout different immersion conditions; different feed concentrations and temperatures. The free volume of PPSU membranes was observed; it decreased in the order PPSU-25 > PPSU-27.5 > PPSU-30, as could be expected. This shows that when the free volume increases (decreases), the DS increases (decreases). Moreover, the results in Fig. 2 also indicate that the DS values in pure HAc were more likely to be influenced by the membranes’ sub-layer porosity rather than the sorption of the membranes’ dense top layers. These outcomes were expected since PPSU membranes are no organo-selective membranes. Thus sorption of HAc on the dense top layers was probably lower than water molecules. Since the molar mass of HAc (60.05 g/mol) is higher than that of water (18.01 g/mol), the total DS values of PPSU membranes were higher in HAc compared to water. The fluctuation of DS observed in PPSU-25 may have resulted from the membrane instability when exposed to high HAc concentrations. As observed in Fig. 2, the total DS somewhat decreased with the increase of temperature. This is a remarkable observation since the contrary behaviour is normally reported [19]. This is because at higher temperatures, the density of HAc is lower, resulting in a total decrease of DS. Furthermore, the mobility of HAc molecules in the solution was suspected to be more random and active. This resulted in a lower contact time with the PPSU membranes, thus causing a decrease in HAc sorption. As a consequence, the total DS decreased when the HAc sorption decreases. This analysis, indicates that a higher PPSU content produces lower DS, which is an indication of the stability of the membrane. In pervaporation, swelling is often observed when using a polymeric 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 swollen polymer may be 10–100 times the volume, weight or surface area of the dense, unswollen polymer [7]. Therefore, analysis of DS for PPSU membranes in feed solutions is a vital step to study the chemistry between the membrane and the feed solutions, and at the same time, obtaining the DS. The DS analyses are presented in terms of partial sorption at 50 ◦ C in all three different PPSU membranes and shown in Fig. 4. All PPSU membranes indicate a decrease of water sorption. This is parallel with the decreasing ‘weight fraction of water in feed solution’, labelled on the X-axis, and vice versa. This behaviour is expected and well understood. 3.3. Effect of temperature and feed concentration on pervaporation Pervaporation dehydration of acetic acid–water as a function 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

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0

60 50

ln(J), kg/m².h

Sorpon at 50°C, %

70

25-Water 27.5-Water 30-Water 25-HAc 27.5-HAc 30-HAc

40 30 20 10

-1

y = -30.53x + 9.842 R² = 0.967 y = -33.19x + 10.27 R² = 0.994

-2 y = -27.18x + 7.486 R² = 0.960

-3 -4 0.320

0 20

100

10

0.340 PPSU-25

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

range of ∼0.1–0.5 kg/m2 h. This was observed for both HAc concentrations. In this study, it was found that as the PPSU concentration increases, the flux decreases. This is explained by the dependency of the permeance as a function of the membrane top layer characteristics. Increasing the initial PPSU concentration in the casting solution leads to a much higher PPSU concentration at the interface. Thus, the transport of the permeance in a more dense, top layer of the membrane becomes slower, (PPSU-27.5 and 30), resulting in a lower flux. As can be seen in Fig. 5(a) and (b), fluxes increase when increasing temperature. In addition, remarkably, the PPSU-27.5 membrane generated the highest value in overall separation factor. This result can be explained by referring to Fig. 1. In Fig. 1, it can be seen that PPSU-27.5 and PPSU-30 consist of a dense membrane top layer structure, which is not seen in PPSU-25. PPSU-27.5 has a thinner top layer skin than PPSU-30, as explained in section 3.1. Thus, the figures indicate that PPSU-27.5 may be a good compromise to obtain the optimal performance. This indicates that no further

12

2.5

7

2.0 1.5

2

1.0

-3

0.5 0.0

Separation factor

-8 40

50

60

70

80

Feed temperature ( C ) 3.0

7

2.0 1.5

2

1.0

-3

0.5 0.0 40

50

60

70

PPSU-30

 E  x

X = X0 exp −

(5)

RT

where X can be the flux (J) or permeability (P), X0 is the preexponential 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 flux increases with an increase in temperature, which is also observed in most PV experiments in the literature [25]. In Figs. 6 and 7,

2

-8 30

PPSU-27.5

improvement in the separation is to be expected by increasing the membrane thickness. The increase of separation factor may be small, as in the case for PPSU-25 for both feed concentrations, but it is significant for the PPSU-30 membrane. The results also show that when a PPSU membrane is introduced to a HAc/water solution, HAc tends to become more reactive to PPSU, in comparison to water. As the HAc molecule associates with the PPSU membrane, the development of hydrogen bonds with PPSU is facilitated. In addition, as the temperature increases, the vapor pressure difference increases, which enhances the driving force [21]. As a result, more HAc and water penetrate through the PPSU membrane leading to a higher flux. Because the water molecule is smaller than HAc and the PPSU membrane is hydrophilic type, the permeation rate was faster than for HAc, resulting in increase of the separation factor. This is applicable for all feed conditions and membrane compositions. The increase of flux is larger at higher PPSU concentrations, which can be seen by a comparison of the slopes at 25, 27.5 and 30 wt.% PPSU, respectively. It is also obvious that the activation energy may have a larger influence for the more dense membranes than for PPSU-25 membrane (Ref in table: [24]). The temperature dependency of the flux was analyzed using an Arrhenius equation (Eq. (5)):

12

2.5

Separation factor

Flux, J (kg/m²h)

b

0.400

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

80

Feed temperature ( C ) Fig. 5. Effect of temperature on pervaporation dehydration of HAc–water solution using three PV membranes in (a) 80 wt.% HAc and (b) 90 wt.% HAc; ()PPSU-25, ()PPSU-27.5 and ()PPSU-30.

Ln(P), kg/m².h.bar

Flux, J (kg/m²h)

a 3.0

0.380

1000/RT

Weight fracon of water, wt%

30

0.360

0

y = 12.17x - 3.001 R² = 0.835

1.5

y = 9.869x - 2.699 R² = 0.936

1

y = 16.11x - 5.573 R² = 0.904

0.5 0 -0.5 0.32

0.34

0.36

0.38

0.4

0.42

1000/RT PPSU-25

PPSU-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 1 Arrhenius activation energy of flux and permeability for water. Membrane

Feed mixture (HAc/water)

EJ (kJ/mol) (Fitted, Fig. 6)

EP (kJ/mol) (Fitted, Fig. 7)

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

Ref.

PPSU-25 PPSU-27.5 PPSU-30 PVA PVA b PPSU-25 b PPSU-27.5 b PPSU-30

90/10 90/10 90/10 10–90 10-90 80/20 80/20 80/20

30.53 33.19 27.18 29.6–33.5 28.6–42.9 28.48 33.86 28.59

−12.7 −9.87 −16.11 NA NA −14.83 −9.51 −14.64

−10.13 −7.47 −13.48 −11.06 to −7.16a −12.06 to 2.24a −12.18 −6.8 −12.07

This work This work This work [23] [24] This work This work This work

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

logarithmic plots of flux and permeability as a function of the inverse temperature are shown. The straight line correlation indicates the validity of the adsorption–diffusion model to describe transport through the PPSU membrane and the slope of these figures give the activation energy of flux EJ (Fig. 6) and the activation energy of permeability Ep (Fig. 7). Values are indicated in Table 1 for water and Table 2 for acetic acid. According to Bettens et al. [26] the transport can be described in terms of the permeability coefficient (P), expressed in Eq. (6): p J SD = = = L L F

S D  0 0 L

exp

 −H − E  D S

(6)

RT

where J is the permeant flux (kg/m2 h), F is the transmembrane partial pressure difference (bar), S is the solubility (g per 100 g water), D is the surface diffusivity (m2 /s), S0 and D0 are pre-exponential factors, L is the membrane thickness (␮m), Hs (kJ/mol) is the heat of adsorption and ED (kJ/mol) is the activation energy of diffusion. Burggraaf [27] has related both the activated microscopic models based on configuration and on surface diffusion with the temperature dependence in the classical adsorption–diffusion model as shown in Eq. (7):

 H  S

J = S0 exp −

RT

 E  D

D0 exp −

RT

 A exp

H␯ap − RT

 (7)

Thus, combining Eqs. (5)–(7), the activation energy of flux can be written as shown in Eq. (8). EJ = HS + ED + H␯ap

(8)

where Hvap is the heat of vaporization of the permeant (water) through the membrane (40.66 kJ/mol). In Eqs. (6) and (8), the term (HS + ED ) is the activation energy of permeability, Ep (kJ/mol), when the permeate pressure is sufficiently low; thus, Eq. (8) can be written as follows: Ep = EJ − Hvap

(9)

Values of Ep calculated with Eq. (9) are indicated in Tables 1 and 2. The comparison of these values with those obtained from experimental data (slope of Fig. 7) shows that there are deviations: experimental values which are the true values were higher than calculated, reaching deviations between 16% and 28% for water (Table 1) and 2–5% for acetic acid (Table 2). This different behaviour is worth of further investigation since the high deviation for water may indicate that Eq. (9) cannot be applied for calculating its activation energy of permeability. Theoretically, the vapour pressure (Psat ) of the feed component increases when the feed temperature is increased, while the vapour pressure on the permeate side will not be affected. In this study, negative values of Ep were expected because values of EJ were in the range of 14–34 kJ/mol, below the heat of vaporization of water, indicating that membrane permeability coefficient decreases (with an increase in flux) as the temperature increases. This is due to the more significant effect of temperature on saturated vapour pressure [28]. 3.4. Membrane stability The long term-stability of the membranes is a vital parameter to determine their durability in industrial applications. However, this is rarely reported. In this work, a membrane stability test for PPSU was set up and evaluated. The PPSU membrane was assessed by studying the effect of pressure and HAc exposure on the membrane performance. The membrane stability test was also conducted along with the PV analysis in the temperature range of 30–80 ◦ C, and in high acetic acid concentrations, 80–90 wt.% HAc. 3.4.1. Pressure effect on PPSU membrane The results indicate that the application of pressure on the PPSU membrane caused compaction without further alteration of the membrane performance. In Fig. 8, it can be seen that MP-2 and MP-4 membranes showed a similar trend as for MP-0. In fact, the membranes were stable following that trend. The pressure effect

Table 2 Arrhenius activation energy of flux and permeability for HAc. Membrane

Feed mixture (HAc/Water)

EJ (kJ/mol) (Fitted)

EP (kJ/mol) (Fitted)

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

Ref.

PPSU-25 PPSU-27.5 PPSU-30 PVA PVA PPSU-25 PPSU-27.5 PPSU-30

90/10 90/10 90/10 10–90 10–90 80/20 80/20 80/20

24.67 31.23 13.8 29.6–34.2 42.5–62.8 19.78 17.54 7.36

−15.48 −9.9 −27.46 NA NA −21.63 −23.75 −33.99

−15.99 −9.43 −26.86 −11.06 to −6.46a 1.84–22.14a −20.88 −23.12 −33.3

This work This work This work [23] [24] This work This work This work

Note: The EJ and EP values were obtained the same way shown in Table 1, however, the fitted graphs for HAc were not shown. NA, not available from the reference cited. a Calculated by authors in this work.

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22.00

0.25

Stabilization time

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

20.00

Flux, kg/m²h

0.2

18.00 16.00

0.15

14.00 12.00

0.1

10.00 HAc

0.05

0

1

2

3

4

6

5

7

8

9

10 11 12

Water Sep.factor

0 80

Pervaporation time (hour)

85

90

8.00

Separaon factor

Partial fluxes, kg/m²h

a

6.00 4.00

95

Weight fracon of HAc in feed soluon, wt%

Water-0

Water-2

Water-4

Hac-0

Hac-2

Hac-4

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

b Separation factor, (αw/a)

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

13 12 11 10 9 8 7 6 0

1

2

3

4

5

6

7

8

9 10 11 12 13

Pervaporation 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) separation factor vs. time.

resulted in membrane thickness contraction, which was observed for MP-2 and MP-4 where the fluxes were higher than MP-0. On the other hand, the contraction is suspected to be significant only at the sub-layer (porous structure) rather than the dense top layer. In this analysis, water transport through the PPSU-30 membrane is again seen to be the major cause of total flux fluctuation amongst the tested membranes. During the initial hours (considered as the stabilization time), variations due to the membrane adaptation to the system was observed. MP-4 shows a great decrease in its flux during stabilization time. This is because of the membrane reaction towards different environment from high pressure (sublayer compaction) to high temperature and high HAc concentration (membrane swelling). At high HAc concentration (80 wt.%), the MP-4 membrane swelled, resulting in an increased flux after 5 h. Somehow, MP-4 still has a relatively lower flux compared to MP-2. The existing pressure (4 barg) which was applied to the MP-4 membrane has caused the porous sub-layer to compact, with a reduced thickness. This combined with the already dense top layer then formed a thicker, denser membrane layer. Thus, the effect of pressure has caused the existence of a thinner, but denser membrane, resulting in a lower flux compared to MP-2. On the other hand, the pressure applied onto membrane MP-2 did not cause significant compaction, even though thickness reduction and flux increase are inevitable. This again reiterates the initial finding in Section 3.3 that the thickening of the dense top layer will not further improve the membrane’s performance. Apart from obtaining membrane flux and selectivity, this study showed that PPSU membrane is safe to be applied when pressure above 1 atmospheric in the system is unavoidable.

3.4.2. Stability in high acetic acid feed concentration In Section 3.3, it was explained that PPSU-27.5 is the best choice for HAc–water mixture separation in this study. Therefore, this membrane was used as reference to study the membrane stability in high HAc feed concentration. The partial fluxes and separation factor through PPSU-27.5 membranes at 50 ◦ C as a function of HAc concentration are shown in Fig. 9. Before hand, it was foreseen that the separation factor may decrease with increase of HAc composition in the feed solution, due to domination of HAc molecules in the solution which can possibly hinder the interaction between water molecules and membrane surface. In contrast, it can be seen that the water flux decreases when the HAc concentration in the feed solution increases. This is rational because the water molecules reduces when HAc molecules increases. On the other hand, the separation factor increases when the concentration of HAc in the feed gets higher. The explanation for this phenomenon is twofold. Firstly, the water solubility and diffusivity increases when the water saturation in the feed decreases. The decrease in water saturation in the feed solution may contribute to the decrease of intermolecular friction during transport through the membrane. As a consequence, water permeates at higher proportion when the HAc concentration is high. Secondly, the plasticizing effect may facilitate the water transport through the membrane. As a result, the separation factor increases. A complete analysis of PPSU membranes in higher range of HAc concentration at 50 ◦ C is summarized in Table 3. The partial fluxes and separation factors were presented in a systematic way. Moreover, this investigation proved that PPSU membranes were stable for high HAc concentrations. Generally, it can be seen that PPSU-27.5 performed better than PPSU-25 and PPSU-30 and simultaneously reassured the findings in Section 3.3. 3.5. Comparison of present membranes with literature data Several studies have been undertaken on dehydration of HAc–water mixtures using organic, inorganic and composite membranes. Table 4 summarizes each type of pervaporation membrane applied in acetic acid and water separation. It has been shown that inorganic membranes, in most cases, are capable to provide a very high water selective barrier, but with a very low permeate

N. Jullok et al. / Chemical Engineering Journal 175 (2011) 306–315 Table 3 The partial fluxes and separation factors for PPSU membranes in high HAc concentration at 50 ◦ C. PPSU-conc. (wt.% HAc)

Partial flux (kg/m2 h) at 50 ◦ C Water HAc

PPSU-25–80 PPSU-25–85 PPSU-25–90 PPSU-25–95

0.34 0.26 0.26 0.38

0.37 0.43 0.54 0.80

3.7 3.3 4.4 8.9

PPSU-27.5–80 PPSU-27.5–85 PPSU-27.5–90 PPSU-27.5–95

0.23 0.15 0.12 0.14

0.14 0.13 0.13 0.21

6.6 6.9 8.2 12.7

PPSU-30–80 PPSU-30–85 PPSU-30–90 PPSU-30–95

0.18 0.11 0.08 0.12

0.13 0.09 0.13 0.16

5.3 6.7 5.7 14.9

Sep. factor’

flux, e.g., using zeolites. In an acid-proof silicalite-1-zeolite study which was carried out by Masuda et al. [29], it was found that the separation factor achieved near infinite to water. However, the membrane was not able to be commercialized due to the extremely low flux (0.00045 kg/m2 h). Moreover, zeolites are normally unstable in highly acidic solution. A study using a free-radical graft polymerization of 1vinylimidazole (VI) onto the as-synthesized mordernite zeolite membrane in acetic acid dehydration has been conducted by Chen et al. [30]. It was found that by applying this membrane in the PV at higher temperature, it produces higher flux and also higher selectivity. PV of 83 wt.%HAc at 80 ◦ C gives infinite selectivity due to pure water at permeate and flux of 0.258 kg/m2 h was obtained.

313

Conversely, when HAc concentration increases, the selectivity and the flux dropped. Although the explanation was more onto the molecule movement, the worsening in quality may also have been contributed by the collapsed and/or damaged membrane due to high HAc concentration. However, the stability of the membrane was not reported. Another interesting study was the investigation by Shivanand et al. [25] in applying a silicotungstic acid incorporated sodium alginate (STA-NaAlg-5). This study gave an excellent separation factor and flux, ranging from 0.194 to 0.489 kg/m2 h. However, the membrane performance decreased when applied at high temperature and in a more dilute feed solution. As temperature and water content in feed solution increases, flux increase and selectivity decreased. There were also no stability-related details provided. PV using organic membranes such as poly(4-methyl-1pentene)/Co (III) (acetylacetonate), grafted PVA membrane with polyacrylamide and PVA membrane modified with PAA which were reported by Chapman et al. [6] were less attractive due its poor durability in high temperature and mixture concentration. Improvement of the organic membrane performance through modification and new membranes’ construction has to be carried out to enable better compatibility with the inorganic and composite membranes. This is to ensure feasibility for practical industrial applications. In this study, PPSU membranes were found to be very convincing in terms of stability. Even though the selectivity of most of the modified PVA membrane was better than PPSU membranes, the fluxes produced were lower than PPSU membrane. Another disadvantage in applying PVA membranes is that it has poor stability in aqueous solutions. The proposed PPSU membranes also provided better performance than some of the charged membranes,

Table 4 Comparison of PV membranes used in HAc–water separation and their performance. Membrane

Binary mixture (HAc/water)

˛w/HAc

Flux (kg/m2 h)

T (◦ C)

Year

Ref.

Inorganic membrane Silicalite-1-zeolite Poly(1-vinylimidazolo)/modernite STA-NaAlg-5 STA-NaAlg-1 Silica

98/2 83/17 90/10 90/10 50/50

∞ ∞ 124–22,491 817 125

0.00045 0.258 0.194–0.489 0.15–0.35 5400

80 80 30–70 30–70 90

2003 2009 2007 2007 1990

[29] [30] [25] [25] [31]

Organic membrane 25 wt.%PPSU 27.5 wt.%PPSU 30 wt.%PPSU

80/20–90/10 80/20–90/10 80/20–90/10

2.5–6.1 5.0–11.4 2.8–12.0

∼0.24–1.48 ∼0.12–0.83 ∼0.09–0.48

30–80 30–80 30–80

This work This work This work

Modified PVA membrane Modified PVA membrane with poly(acrylic acid) Modified PVA membrane with malic acid Modified PVA membrane with amic acid Modified PVA with glutaraldehyde Modified PVA with formaldehyde

10–90 20–90 10–90 10–90 10–90

34–3548 121–670 13–42 4.0–9.0 1.0–5.5

0.03–0.6 0.05–0.29 0.08–2.28 ∼0.0001–0.0003 ∼0.0001–0.0004

30–55 40 30–75 30–60 30–60

2003 2005 1991 2001 2001

[32] [33] [34] [35] [35]

Charged membrane Nafion(C8 H17 )4 N+ PSF(SO3 − )–H+ AMV-CH3 COO− CMV-H+

90/10 90/10 80/20 80/20

243 9.4 4.3 4

0.18 0.02 0.83 0.33

25 – 80 80

1999 1997 1997 1997

[36] [37] [37] [37]

PVC membrane Poly(vinyl chloride) Polyacrylonitrile Poly(vinyl chloride)

80/20 50/50

182–274 45

0.56–0.74 6

80 40

1993 1993

[38] [39]

84/16 85/15 90/10 90/10 90/10 90/10 90/10

606 162 40 38 60 98 134

0.215 0.262 0.0239 0.037 0.054 0.092 0.138

25 70 30 40 50 60 70

1994 2000 2004 2000 2000 2000 2000

[2] [3] [40] [3]

Composite membrane Poly(4-methyl-1pentane)/ethylene-vinyl acetate copolymer TPX/P4-VP NaAlg and PAN crosslinked with PVA NaAlg + 5%PVA + 10%PEG NaAlg and PAN crosslinked with HDM (ion: Na+ )

AMV, CMV, crosslinked styrene−co−butadiene base, mechanically stabilized with poly (vinyl chloride) backing and containing, respectively, PSF(SO3 − ), sulfonated poly(sulfone).

314

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

especially in term of flux and selectivity. Details of this comparison can be seen in Table 4. The investigation of PV dehydration of acetic acid using a composite membrane of poly(4-methyl-1pentane)/ethylene–vinyl acetate copolymer TPX/P4-VP by Lee and Lai [2] in 90 wt.% HAc at 25 ◦ C showed an impressive selectivity of 606 and the flux was 0.215 kg/m2 h. On the other hand, at high HAc concentration, the membrane compacted resulted in flux decrement. There was no further information about the change of the separation factor. Comparing the present work with other organic, composite and inorganic membranes listed in Table 4, PPSU membranes have shown an impressive performance and stability at high HAc aqueous solution and high temperature. A stable polymeric base membrane promoting a high flux can serve as an excellent basis for further attempts on membrane selectivity improvements, as mentioned by Chapman et al. [6]. Thus, this work will be a good platform for further PPSU membrane improvement in separations by pervaporation. The PV study on the dehydration of 80 and 90 wt.%HAc showed impressive performance. Even though the lab-prepared membrane was not further improved by thermal treatment, cross-linking agent or other potential method, it is still functioning as a good water separation membrane at high acetic acid concentration and high temperatures. When the acetic acid concentration increases in the feed solution, the DS, flux and separation factor increased. This phenomenon occurs possibly because acetic acid and the PPSU membrane have a good interaction with each other and are always attracted to each other. Thus, as the wt.% HAc increases, the PPSU membrane reacts by creating more free volume to accommodate more acetic acid molecules. This results in an increase of DS. When many of the acetic acid molecules form a network within the PPSU membrane, the water molecules are able to penetrate across the membrane, resulting in an increased separation factor. As the feed temperature increases, DS decreases, but the flux and separation factor increase. At a relatively high temperature, the physical expansion of the polymer network becomes restricted, ensuing in a lower DS and free volume. The increase of flux and separation factor at high temperature occurred possibly due to the interaction of acetic acid with PPSU blend. This consequently increases the hydrogen bond potential between the feed solution and PPSU membrane. The hydrogen bond facilitates the transport of water molecules through the membrane. This PPSU membrane is also stable at high wt.% HAc content and reasonably high operating temperatures, as long as the applied system pressure does not exceed 1 atm.

4. Conclusions In this study, PPSU membranes were successfully developed using the phase inversion method for pervaporation dehydration of HAc. The novel membranes show an impressive performance, which offers a feasible method to separate HAc and water, with high flux, good selectivity and stability. DS values show that HAc has a larger influence on membrane swelling than water. At higher HAc concentrations, DS increases, which results in an increase of the total flux. This condition somehow does not weaken the membranes since the separation factors improved with the increase of the HAc concentration. The plasticizing effect was postulated to result in a special sieving property, which assisted the water molecules to be transported across the membrane. Furthermore, the binary mixture’s density at different temperatures also influenced the DS of each membrane. The density of the mixture is lower when the solution has higher temperature. Overall, the PPSU-27.5 membrane had the optimum performance; following further development and improvement on the membrane morphology, PPSU

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