water azeotrope using a novel chitosan-impregnated bacterial cellulose membrane and chitosan–poly(vinyl alcohol) blends

water azeotrope using a novel chitosan-impregnated bacterial cellulose membrane and chitosan–poly(vinyl alcohol) blends

Journal of Membrane Science 251 (2005) 131–136 Pervaporative separation of ethanol/water azeotrope using a novel chitosan-impregnated bacterial cellu...

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Journal of Membrane Science 251 (2005) 131–136

Pervaporative separation of ethanol/water azeotrope using a novel chitosan-impregnated bacterial cellulose membrane and chitosan–poly(vinyl alcohol) blends Vinita Dubey∗ , Lokesh Kumar Pandey, Chhaya Saxena Synthetic Chemistry Division, Defence Research and Development Establishment, Jhansi Road, Gwalior 474002, Madhya Pradesh, India Received 30 July 2004; received in revised form 10 November 2004; accepted 10 November 2004 Available online 15 December 2004

Abstract The pervaporative (PV) performance with respect to the separation of ethanol/water (EtOH/H2 O) azeotrope was assessed for a bacterial cellulose membrane (BCM) impregnated with chitosan (CTSN), designated as CTSN–BCM. The PV potential of CTSN–BCM was compared with that of parent polymers and also with the blends of CTSN with poly(vinyl alcohol) (PVA). The blends and CTSN–BCM were characterized using spectroscopic and thermoanalytical techniques, and also tested for their mechanical strength and sorption of EtOH/H2 O mixtures. While PVA and the blends tend to dissolve at higher concentration of water, CTSN–BCM was found to be intact over the entire composition range. When evaluated against pervaporation of 95:5 w/w EtOH/H2 O azeotrope at 24 ± 1 ◦ C; CTSN–PVA (1:1) blend showed comparatively higher selectivity (22.0) but a poorer flux (1.7 kg ␮m m−2 h−1 ). The flux was significantly higher (42.8 kg ␮m m−2 h−1 ) in CTSN–BCM and selectivity (∼10) was at par with PVA. The temperature dependence of selectivity and flux through CTSN–BCM was also investigated. Substantially high pervaporative separation index (PSI) of the order of 350 kg ␮m m−2 h−1 and low energy of activation of 10 kJ/mol is indicative of the potential of CTSN–BCM in the pervaporative separation of EtOH/H2 O azeotrope. © 2004 Published by Elsevier B.V. Keywords: Pervaporation; Bacterial cellulose; Membrane; Chitosan; Poly(vinyl alcohol)

1. Introduction The separation of azeotropes by distillation poses a challenging problem as the vapour distils over without change in the composition irrespective of the change in temperature or pressure. Complex processing steps involving addition of entrainers followed by extractive distillation are generally adopted to achieve high purity. This leads to high energy consumption and uneconomical operating costs. Moreover, selectivity of these processes is limited by the vapour–liquid equilibrium of the constituents. In the recent years, pervaporation (PV) is rapidly emerging as an economical and simple alternative to conventional ∗ Corresponding author. Tel.: +91 751 2233482 85; fax: +91 751 2341148. E-mail address: india [email protected] (V. Dubey).

0376-7388/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.memsci.2004.11.009

energy-intensive technologies for separating azeotropic, close-boiling, isomeric or temperature-sensitive liquid mixtures [1–11]. Unlike distillation process, the separation mechanism in PV is not based on the relative volatility of the components, but on the difference in sorption and diffusion properties of the feed components as well as permselectivity of the membrane. Hydrophilic polymers such as PVA, polysaccharides (cellulose and CTSN) show a stronger affinity to water, hence their copolymers, blends or composites have been widely investigated for pervaporative separation of EtOH/H2 O mixtures [1–11]. CTSN is generally preferred due to its high abundance, natural occurrence, hydrophilicity, chemical resistance, adequate mechanical strength, good membrane forming properties and ease of processing. PV performance of EtOH/H2 O mixtures through the surface cross-linked CTSN composite membranes exhibited a high selectivity

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value but a low permeation flux [2]. The PV membranes of derivatives of CTSN obtained by its chemical modification have also been widely studied [3,4]. Blending and/or formation of composites is one of the attractive means of tuning the performance of a membrane to achieve the desired flux and/or selectivity. Moon et al. [5] achieved a high selectivity (1 1 1 0) but a very low flux (0.07 kg m−2 h−1 ) for the separation of EtOH/H2 O azeotrope through CTSN-sodium alginate two-ply composite membrane. The composite of CTSN with polysulphone was also investigated for pervaporative dehydration [6]. Blends of CTSN with many polymers such as Nylon-6 [7], polyethylene oxide [8], hydroxyethyl cellulose [9], polyacrylic acid [10] have also been evaluated for the same purpose. In spite of prolific literature on PV behaviour of CTSN and its blends/composites, very few studies report specifically the flux and selectivity values for the separation of azeotropic composition at ambient temperature. In the present work, we primarily focussed on the aspect of enhancing the pervaporative flux and water selectivity for the separation of EtOH/H2 O azeotrope. In our previous work [11] on EtOH/H2 O separation, we observed remarkable water flux with reasonable selectivity especially in the water-lean (<50 wt.% water) regions for the pervaporation of EtOH/H2 O binary mixtures through BCM. The bacterial cellulose is a microbial extracellulose polymer comprising of ␤-1,4-linked glucopyranose units. It originates as a white gelatinous pellicle on the surface of the liquid medium at about 30 ◦ C, in a static culture containing Acetobacter xylinum, a rod shaped, aerobic gram negative bacterium which occurs as a contaminant in vinegar production [12]. By controlling the physiological conditions of bacterial growth such as the composition of the culture media, its pH, temperature and oxygen tension, morphologically reproducible pellicle is obtained which can be easily flattened, pressed and processed into membranes of desired thickness [11,12]. The membrane also possess superior mechanical properties, high resistance to corrosive chemicals, biodegradability, ease of tailorability and economical processing [12]. The unique features of BCM and CTSN prompted us to combine the potentials of both these naturally occurring polysaccharides for the “breaking of EtOH/H2 O azeotrope”, in the present work. The pervaporative separation of EtOH/H2 O azeotrope through morphologically modified BCM prepared by impregnation of CTSN and PVA-blended CTSN membranes was evaluated vis-`a-vis parent polymers (BCM, CTSN and PVA) in the present study.

(molecular weight 1–3 lakhs) and EtOH (99.9%) were received from Across Organics, USA. Distilled water was used to prepare aqueous solutions. 2.1.1. Preparation of BCM BCM grown from A. xylinum, as per the earlier reported [11] method was obtained from Biotechnology Division of DRDE Gwalior. The membranes were dipped for 48 h in a flat-bottomed large Petri dish containing saturated solution of NaOH for deproteination and were then rinsed several times with distilled water, until a neutral pH was attained in the drained liquid. The specimens were dried for a week under vacuum at 60 ◦ C prior to use. The average thickness of the dried membranes was 80 ␮m. 2.1.2. Preparation of CTSN membranes The 1.5% solution of CTSN in 1% aqueous acetic acid, having a viscosity of 240 cP at 20 ◦ C was cast on a flat glass mould at 35 ± 2 ◦ C. After 24 h, the membrane was peeled off gently. It was further dried at 60 ◦ C for 15 h under a vacuum of 10 mmHg. The average thickness of the dried membrane was 40 ␮m. 2.1.3. Preparation of PVA membranes Ten percent aqueous solution of PVA was poured on a flat glass mould and allowed to gel gradually at 38 ± 2 ◦ C under a draft of air. After 48 h, the membrane was peeled off gently by dipping the mould in cold water. It was further dried at 60 ◦ C for 75 h under a vacuum of 10 mmHg. The average thickness of dried membrane was 220 ␮m. 2.1.4. Preparation of CTSN–PVA blends The individual sols of CTSN and PVA prepared as described above were blended in different volume ratios (1:3, 1:1, 3:1) by continuous stirring for 30 min. After degassing, the resultant solutions were poured on flat glass moulds and allowed to gel gradually at 35 ± 2 ◦ C under a draft of air. After 48 h, the membranes were peeled off gently and further dried at 60 ◦ C for 75 h, under a vacuum of 10 mmHg. The average thickness of the dried membranes was found to be 80 ␮m for 3:1 blend and 60 ␮m for 1:3 and 1:1 blends.

2. Experimental

2.1.5. Preparation of CTSN-impregnated BCM The deproteinated BCM of pre-determined weight, prepared as described above, was dipped into 1% acetic acid solution of CTSN in a large Petri dish for about 48 h. The specimen was removed, washed several times with distilled water and allowed to dry. After 24 h, the membranes were further dried at 60 ◦ C under a vacuum of 10 mmHg for about 30 h and weighed. From the mass balance, the membrane was found to contain 8 ± 2 wt.% of CTSN. The average thickness of the impregnated specimens was 200 ␮m.

2.1. Materials

2.2. Characterization of membranes

Hydrolysed PVA (86–89% S.D. Fine Chemicals, India) of molecular weight 125,000 was used as received. Chitosan

The surface morphology of non-stained membranes was studied using scanning electron microscope (JEOL JSM 840,

V. Dubey et al. / Journal of Membrane Science 251 (2005) 131–136

magnification 20 000× at 5 kV and 6 × 10−10 A) after coating with gold using an ion sputter (JFC-1100). Thickness was measured using a dial caliper (Mitutoyo, Japan) with an accuracy of ±0.01 mm. Viscosity of sols was determined at 20 ◦ C using a viscometer (Brookfield, DV-E) equipped with s62 spindle at 100 rpm. The infra-red absorption spectra of the membranes were recorded on Perkin-Elmer FTIR. The mechanical properties of rectangular membrane specimens (8 mm × 5 mm × 0.14 mm) were determined using a minitensile tester (Rheometrics, USA) at a cross-head speed of 0.5 mm/min. The thermal stability of the membranes was assessed using a thermogravimetric analyzer (TA Instruments, US) in nitrogen atmosphere at a heating rate of 20 ◦ C/min. The chemical resistance was determined by immersing the membrane specimens in various solvents at 30 ◦ C for 1 week. 2.3. Sorption studies Dry membranes of pre-determined weight (Wd ) were immersed in various compositions of EtOH/H2 O mixtures at 24 ± 1 ◦ C for 72 h to allow them to attain equilibrium sorption. The specimens were removed from the mixture and quickly wiped with tissue paper to remove the superfluous liquid. The weight of swelled membrane, Ws was recorded on Shimadzu balance with an accuracy of 1 × 10−4 g. The extent of sorption, also called swelling index (Q) was calculated using the formula [11,13]: Q=

Ws − Wd × 100 Wd

(1)

The mean of three readings was reported; the standard deviation ranged from 9.5 to 18.7. 2.3.1. Sorption isotherms The sorption isotherms for water vapour were determined using an automated sorption analyzer (IGA, Hiden Analytical, UK) based on the principle of mass relaxation in the polymer upon uptake of the solvent vapours. The relative pressure (P/P0 ∼ activity or concentration) of the vapours was increased in 10 equal steps corresponding to a pressure change of 4.2 mbar at a constant temperature at 25 ◦ C. The change in weight of the polymer with time was recorded and the isotherm was approximated from the kinetic data by autofitting into the appropriate sorption models [14]. 2.4. Pervaporation studies A stirred batch cell reactor reported by Natke et al. [15] was used for evaluating the pervaporative performance of the membranes. The effective membrane area was 29.40 cm2 . The membrane was supported on a filter paper over a porous sintered steel disk 6.4 cm in a diameter. Prior to PV, the specimens were equilibrated with the feed solution for 12 h at 30 ◦ C. After a steady state was attained, the permeate was condensed at −75 ◦ C in liquid nitrogen–alcohol mixture. The feed chamber was at 24 ± 1 ◦ C and downstream pressure was

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maintained at 2.0 mmHg in all the experiments. The flux (J) was determined by measuring the weight (W) of the permeate and using the following equation [11,13]: J=

W Ah

(2)

where A is the effective membrane area and h the time for pervaporation. The normalized flux J was obtained from the product of flux and thickness (␮m) of the membrane [13]. The composition of the permeate was determined by measuring the water content with the help of Karl Fischer Coulometer (model 831, Metrohm devices, Switzerland) having a sensitivity of 1 ppm at 30 ◦ C. The permeation selectivity (αp ) was calculated using the standard equation [11,13]: αp =

Ywater /Yethanol Xwater /Xethanol

(3)

where X andY are the weight fractions of species in the feed and permeate, respectively. The pervaporation separation index (PSI) was calculated using the following equation [1,16]: PSI = J  (αp − 1)

(4)

3. Results and discussion 3.1. Characterization of membranes The FTIR spectra of CTSN–PVA blends and CTSNimpregnated BCM showed only a marginal shift in absorption frequencies; indicating the absence of any chemical bonding of PVA or cellulose moieties with CTSN. The porous non-woven mesh structure of BCM (Fig. 1a) with interwoven microfibrils was masked upon impregnation by CTSN (Fig. 1b) revealing an overlaid mat-like morphology devoid of any surface details. As observed from Table 1, the impregnation of BCM lowers its tensile strength from 74 to 54 MPa, which is at par with CTSN. Comparatively the blends have a lower tensile strength ranging from 12 to 38 MPa but a higher elongation at break (12–72%). The thermal stability of the blends is also relatively lower than that of CTSN; the temperature for 50% weight loss (T50 ) is 340 ◦ C for 1:3 blend as compared to 350 ◦ C for CTSN and 390 ◦ C for CTSN–BCM. Additionally, a residual weight of ≤2% is obtained in the blends at 450 ◦ C, Table 1 Mechanical properties of membranes Membrane

Tensile strength (MPa)

Elongation at break (%)

PVA CTSN–PVA (1:3) CTSN–PVA (1:1) CTSN–PVA (3:1) CTSN BCM CTSN–BCM

28 38 15 12 54 74 54

8.0 72.0 26.0 12.0 11.0 6.8 7.4

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Fig. 1. SEM micrographs (magnification 20 000×) of the gold-coated surface of: (a) BCM; (b) CTSN–BCM.

while much higher values viz. 34 and 24% are obtained for CTSN and CTSN–BCM, respectively. A representative thermogram for CTSN–BCM is depicted in Fig. 2. The polymer is fairly stable showing <10% degradation up to 325 ◦ C and 75% degradation at 700 ◦ C. 3.2. Sorption studies Unlike PVA and its blends with CTSN which suffer dissolution or excessive swelling at higher concentration of water, CTSN–BCM maintains its dimensional stability over the entire composition of EtOH/H2 O binary mixtures. From Table 2 it is observed that as the amount of CTSN in the blends increases, the degree of overall sorption Q, decreases for the mixtures containing ≤50% water. For instance, in contrast to 126% sorption in 1:3 blend (25% CTSN content), the 3:1 blend with 75% CTSN content shows only 95% sorption in aqueous EtOH containing 50% water. Comparatively, CTSN–BCM has lowest sorption; Q values range from 2% for pure alcohol to 83% for pure water. In general the degree of sorption follows the trend: PVA > CTSN–PVA > CTSN > CTSN–BCM. It is also observed that invariably the sorption increases with the increas-

ing concentration of water in the binary mixture. Consequently, in the water-lean and azeotropic region (<5% water), the blend membranes can retain their integrity; while in the water-rich feed compositions, the high sorption hampers the practical utility. A disparity in the general sorption trends is observed for the blends at higher concentration (≥50%) of water which may be attributed to the partial dissolution and leaching/extraction of the constituent PVA into the binary mixture, thereby altering the true composition of the blends. By weighing the vacuum-desorbed and dried blend specimens after swelling in 50:50 v/v EtOH/H2 O mixture, the blends were found to have suffered more than one-fourth weight loss over their initial weight. The mass balance indicated that the initial weight reduced by 34, 31 and 27% in 1:3, 1:1 and 3:1 blends, respectively. A higher CTSN content in these cases may retard the leaching of PVA thereby showing a higher value of Q. In other words, greater the amount of PVA (as in 1:3 blend), higher is the leaching/extraction, causing a loss in the polymer content of the blends and consequently lowering the overall sorption. The sorption trends are further corroborated by the water sorption isotherms depicted in Fig. 3 for CTSN, BCM and CTSN–BCM. At low concentration of water (P/P0 ≤ 0.4)

Fig. 2. Thermogram for CTSN–BCM.

V. Dubey et al. / Journal of Membrane Science 251 (2005) 131–136 Table 2 Swelling index Q (%) for the membranes in EtOH/H2 O mixtures at 24 ± 1 ◦ C Water (%, v/v)

100 90 70 50 30 10 0 a b

CTSN–PVA blends 1:3

1:1

3:1

Da 178 –b 126 86 35 17

D 234 – 105 – 21 04

D 241 175 95 83 18 01

PVA

D D D D 386 42 18

CTSN

205 118 106 83 36 07 06

CTSN–BCM

83 61 52 51 30 – 02

Dissolution. Reading not taken.

the isotherms are nearer to the abcissa indicating a low interaction with the polymers. The water sorption increases significantly at P/P0 ≥ 0.5. In the region corresponding to P/P0 = 0.3–0.9, the water-uptake by CTSN is higher than BCM or CTSN–BCM. The isotherm for CTSN–BCM lies below that of the parent polymers, viz. CTSN and BCM, implying the reduced water sorption in the latter. 3.3. Pervaporation characteristics The pervaporation potential of a membrane quantified by PSI is assessed in terms of two important parameters namely, flux (J), i.e. the mass crossing the membrane per unit area in a unit time and selectivity (αp ) towards the preferentially permeated component. The flux values are generally normalized with respect to thickness of the membrane. The comparison of PV performance of the modified membranes vis-`a-vis the parent polymers is shown in Table 3 for azeotropic EtOH/H2 O composition. The PSI values are of the order of 4–380 kg ␮m m−2 h−1 . The highest PSI ∼3 × 102 kg ␮m m−2 h−1 was obtained for PVA and CTSN–BCM. The flux and selectivity values in these two membranes were also comparable. On the

Fig. 3. . Water vapour isotherms for CTSN, BCM and CTSN–BCM at 25 ◦ C (P is the pressure; P0 the saturation vapour pressure; P/P0 the relative pressure ≈ activity or concentration; m the mass-uptake of vapours).

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Table 3 PV performance of membranes for separation of EtOH/H2 O azeotrope at 24 ± 1 ◦ C Membrane

Normalized flux, J (kg ␮m m−2 h−1 )

Selectivity, PSI α (kg ␮m m−2 h−1 )

PVA CTSN–PVA (1:3) CTSN–PVA (1:1) CTSN–PVA (3:1) CTSN BCM CTSN–BCM

41.8 4.6 1.7 2.3 4.8 60.3 42.8

10.1 19.3 22.0 2.8 2.4 1.6 9.2

380 83 36 04 06 36 350

other hand addition of CTSN to PVA reduces the flux but increases selectivity; high selectivities corresponding to 19.3 and 22.0 were achieved in 1:3 and 1:1 blends, respectively; nevertheless the PSI was not significant due to the poor flux. The decrease in flux may probably be due to reduction of free volume by CTSN moieties in the blends. Both the flux and selectivity were poor in CTSN thereby lowering the PSI to 6 kg ␮m m−2 h−1 . Despite the highest flux (60 kg ␮m m−2 h−1 ) obtained for BCM, attributed to its reticulated microfibrillar structure as described earlier, the PSI was comparatively low ∼36 kg ␮m m−2 h−1 , owing to a very low selectivity. 3.4. Temperature dependence of flux and selectivity The PV process is known to be temperature dependent [13,16] as both flux and selectivity are influenced by the change in temperature. Due to a high PSI and excellent dimensional stability, the CTSN–BCM was chosen for studying the temperature dependence in PV of EtOH/H2 O azeotrope. From Fig. 4, it is observed that the total flux increases from 69 kg ␮m m−2 h−1 at 30 ◦ C to 124 kg ␮m m−2 h−1 at 50 ◦ C with the concomitant decrease in selectivity from 6.3 to 2.1. The increase in flux at higher temperature may be attributed to the increased free volume of the polymers and enhanced kinetic energy of the permeants which assists their diffusivity through the matrix of the membrane. The latter effect also

Fig. 4. Temperature dependence of total flux (J ) and selectivity (αp ) in the pervaporation of EtOH/H2 O azeotrope through CTSN–BCM.

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causes the decrease in selectivity as the diffusivity of both the permeants is facilitated [17]. In order to determine the energy of activation (Ej ) for the PV process, ln J was plotted against the inverse of temperature (K) in accordance with the extensively used Arrhenius type of relationship [17,18]:   Ej J = J0 exp − (5) RT Ej as determined from the slope of the plot was 10 kJ/mol. Ej thus calculated is a compounded parameter characterizing the overall temperature dependence of permeation flux [17]. The Ej is also dependent upon the nature of the membrane material and feed composition; the reported values [17,18] range from 13 to 53 kJ/mol for various other polymers. A comparatively lower Ej obtained in the present study implies that the pervaporation of EtOH/H2 O azeotrope may be more facilitated in CTSN–BCM as compared to the other membranes.

4. Conclusion A novel membrane CTSN–BCM was prepared by the impregnation of bacterial cellulose membrane with chitosan. The potential of the membrane for the pervaporative separation of EtOH/H2 O azeotrope is manifest by substantially high PSI of the order of 350 kg ␮m m−2 h−1 and a lower energy of activation (Ej ) of 10 kJ/mol. The normalized flux and selectivity values are at par with PVA, but as compared to PVA the membrane has an excellent dimensional stability, better mechanical strength and improved thermal stability.

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

Acknowledgements We are grateful to Dr. Lokendra Singh, Head, Biotechnology, DRDE, for providing the bacterial cellulose membranes. Dr. P. Pandey, Scientist, DRDE, is sincerely acknowledged for providing SEM facility. We also thank Dr. R.C. Malhotra, Joint Director, and Er.K. Sekhar, Director, DRDE, for unstinted support and encouragement.

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