Pervaporation of binary water–ethanol mixtures through bacterial cellulose membrane

Separation and Purification Technology 27 (2002) 163– 171 www.elsevier.com/locate/seppur

Pervaporation of binary water–ethanol mixtures through bacterial cellulose membrane Vinita Dubey a,*,1, Chhaya Saxena a, Lokendra Singh a, K.V. Ramana b, R.S. Chauhan a a

Defence Research & De6elopment Establishment, Jhansi Road, Gwalior, MP 474002, India b Defence Food Research Laboratory, Mysore, India

Received 2 August 2001; received in revised form 15 November 2001; accepted 27 November 2001

Abstract Cellulose membrane produced by the bacterium Acetobacter xylinum was deproteinated and investigated for pervaporation (PV) of binary water–ethanol mixtures. The membrane was characterised using elemental analysis, infra-red spectroscopy, scanning electron microscopy as well as for mechanical strength and sorption characteristics. A batch stirred cell was used to study the PV behaviour of water– ethanol mixtures through the membrane. The permeate flux, selectivity, PV separation index (PSI), solubility and degree of sorption were studied as a function of increasing ethanol concentration in the feed. The membrane was found highly selective to water. Even when the feed was rich in ethanol ( \70% (w/w)) the permeate contained higher than 95% (w/w) of water. For feed compositions containing less than 30% water, the selectivity towards water was in the range of 125– 287, the flux was greater than 100 g/m2 h and PSI was of the order of 104 g/m2 h. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bacterial cellulose membrane; Pervaporation; Water– ethanol mixtures; Flux; Selectivity; Acetobacter xylinum

1. Introduction Acetobacter xylinum, a rod shaped, aerobic gram negative bacterium which occurs as a contaminant in vinegar production produces a white gelatinous material (pellicle) on the surface of the liquid medium in a static culture system at 30 – 40 °C. In 1886, Brown first reported [1] that the pellicle was composed of pure cellulose called * Corresponding author. Fax: + 91-751-341148. E-mail address: [email protected] (V. Dubey). 1 india – [email protected].

‘bacterial cellulose’. The cellulose of bacterial origin is distinguished by a high degree of crystallinity and superior mechanical properties. Its unique structural features and properties facilitate diverse applications, ranging from wound-dressing, carrier for mammalian cell culture, immobilization of enzymes and other biomolecules, to diaphragms in speakers for audio-communication [1–3]. The pellicle which is flat can be easily processed into a porous membrane possessing good mechanical strength, unlike the cellulose from plant biomass. Plant cellulose is often interspersed with

1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 1 ) 0 0 2 1 0 - 6

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lignin, hemicellulose and pectin leading to nonuniformity in porosity and unpredicatable permeability. In contrast the porosity of membranes from bacterial cellulose can be suitably tailored by varying the physiological conditions of bacterial growth such as composition of the culture media, its pH, temperature, oxygen tension as well as by chemical modifications [3–6]. In contrast to polymer membranes, the bacterial cellulose membrane (BCM) can be economically produced and processed, it also has high resistance to corrosive chemicals and is biodegradable too, and hence ecofriendly. These unique features and the nonavailability of any reports on use of BCM for chemical separations prompted us to investigate its pervaporation (PV) characteristics. PV is a membrane based separation process in which the membrane functions as a selective barrier for the mixture to be separated. Low energy consumption and mild working conditions make the process attractive for separating azeotropic and close-boiling mixtures or dehydrating temperature-sensitive products. Pervaporative dehydration of ethanol has been widely studied using membranes based on poly(vinyl alcohol), polyamides, polysulfonamides, poly(ethyleneimine) polysiloxanes [7– 10], etc. Chitosan and its derivatives as well as sodium alginate have also been used for water– ethanol separations [11,12]. The potential of BCM in the dehydration of alcohol, especially in the azeotropic mixtures, is shown in the present study. The effect of varying ethanol concentrations in the feed, on the permeate composition and flux, selectivity, sorption and PV separation index (PSI) has been investigated.

2.2. Preparation of inoculum and membranes Acetobacter culture was maintained on sucrose medium. The composition of the medium is given in Table 1. The pH of the medium was adjusted at 5.0 using 1.0 M HCl. The inoculum was kept for 2 weeks at 35 °C in a BOD incubator. The pellicle was gently removed from the flask, thoroughly washed with distilled water several times and pressed to form a membrane. The membrane was then allowed to dry at ambient temperature, and was referred to as nascent or untreated membrane. These membranes are largely composed of proteins and cellulose. Some of these membranes were dipped for 48 h, in a flat-bottomed large petridish containing saturated solution of NaOH. Since NaOH solubilises the proteinaceous matter from the membrane, only a cellulosic matrix remains in the membrane. NaOH treated membranes are, therefore, also termed as deproteinated membranes. These membranes were rinsed several times with distilled water, until a neutral pH was attained in the drained liquid. The membranes were dried for a week at ambient conditions, prior to use.

2.3. Characterisation of membranes The elemental analysis and the infra-red spectra (Perkin–Elmer FTIR) of the membranes were recorded before and after the alkali treatment. Scanning electron microscope (JEOL) was used to obtain the micrographs of both (untreated and treated) membranes. The mechanical properties of deproteinated membrane specimen (rectangular, 50×7.5 mm) were evaluated using Goodbrand Jeffrey Micro 5000 tensile testing machine at a

2. Experimental

2.1. Materials

Table 1 Medium for cellulose production from Acetobacter xylinum

BCM from A. xylinum was grown in Biotechnology Division of DRDE, Gwalior. The chemicals used in the preparation of culture medium were reagent grade obtained from M/s Fluka, E. Merck or Aldrich. Absolute ethanol (E. Merck, HPLC grade) and deionized distilled water were used to prepare the binary mixtures.

Chemical

Concentration (g/l)

Sucrose Yeast extract Ammonium sulphate Potassium hydrogen phosphate Magnesium sulphate, hydrated

50.0 5.0 5.0 3.0 0.05

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cross-head speed of 10 mm/min. An average of five readings was reported.

2.4. Determination of sorption The degree of sorption (Q) was determined by immersing 1.0 g of the membrane in 50 ml of water –ethanol mixtures at 3091 °C until equilibration. The membranes were then removed and dried between the folds of the filter paper and weighed using analytical balance (205ACS Precisa, Switzerland) with an accuracy of 0.1 mg. From the weight of the dry membrane (wd), the degree (%) of sorption Q was calculated using the equation given below: Q=

  n

ws −1 × 100 wd

(1)

where ws is the weight of the swollen membrane. The increase in weight of the membranes was due to sorption of water and alcohol.

2.5. E6aluation of solubility selecti6ity In order to determine the amount of water, in the membrane phase, the above membrane specimens swollen to equilibrium in binary water– alcohol mixtures were desorbed using the cold-finger technique. The specimen was placed in a reservoir at 100 °C under a vacuum of 1.0 mmHg for 2 h. The desorbed vapours were collected in a trap cooled by ice– salt mixture at a temperature of −10 °C. The composition of the condensate was determined by the measurement of refractive indices using an Abbe Refractometer (Model RCR-1, from M/s Rajdhani Scientific Instruments, New Delhi, India) having an accuracy of 1×10 − 3 and lower detection limit of 1% for water –alcohol mixtures. The solubility selectivity (hs) was calculated using the following equation: hs =





Ym/Xm Yf/Xf

(2)

where Y and X are the weight fractions of water and alcohol, respectively, in the membrane (Ym, Xm) and feed (Yf, Xf).

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2.6. Per6aporation The batch-stirred PV cell used for the PV experiments was the same as that of Netke et al. [13]. The effective membrane area was 32.15 cm2. The membrane was supported by a filter paper over a porous sintered steel disc 6.4 cm in diameter. Prior to PV, the specimen was allowed to equilibrate with the feed solution for 12 h at 30 °C. After a steady state was attained, the permeate was condensed in a cold-trap by ice–water mixture at −10 °C. The feed chamber was at 329 1 °C and downstream pressure was maintained at 1.0 mmHg in all the experiments. The flux (J) was determined by measuring the weight of the permeate. The composition of the feed solution and the permeate were determined by Abbe Refractometer. The permeation selectivity was calculated using the following equation: hp(water/ethanol) =



(Ywater/Yethanol) ( Xwater/Xethanol)

n

(3)

where X and Y are the weight fractions of species in the feed and permeate, respectively. The PSI was calculated from Eq. (4) defined by Feng and Huang [14]. PSI = J(hp − 1)

(4)

3. Results and discussion A previous study [4] revealed that the production of cellulose by Acetobacter is maximised in sucrose as compared with glucose and fructose. Also the cellulose mass was higher for all pH values (4.0–7.0) at 30 °C than that at 20 °C. The pH variation caused a small alteration (B 5%) in the amount of cellulose produced. The final pH of the medium was in the range 2.20–3.24 due to the evolution of acetic acid [3,4]. For the present study, the membranes were prepared as per the conditions stated in Table 1. The thickness of the membranes was maintained at 100 mm after deproteination by NaOH. The membrane was characterised before and after the alkali treatment. All the membranes used in this study were prepared in the same batch to ensure identical growth and thermal history.

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Fig. 1. FTIR spectra of nascent and NaOH treated BCM.

3.1. Characterisation of membrane The elemental analysis revealed the composition of carbon and hydrogen conforming to the structure of cellulose. The amide bond at 1538 cm − 1 in the IR spectra of the untreated membrane disappeared upon exposure to the alkali (Fig. 1). Low wave number of carbonyl amide group in untreated specimen is rationalised by the formation of intermolecular hydrogen bond between the amide group and an adjacent NH group. Also the 1650 cm − 1 stretch (CONH) shifts to 1659 cm − 1 upon alkali treatment as the CO linkage is freed from NH. The SDS PAGE electrophoresis reported earlier [4] showed that the molecular weight of the proteins in the untreated membrane was in the range 116– 20 kDa. The SEM micrograph (20 000× 10 kV) of the untreated membrane (Fig. 2a) revealed a mat-like structure with interwoven microfibrils.

Some intact bacteria were also seen. The fibril diameter was in the range 0.03–0.05 mm. On treatment with the alkali, the membrane was deproteinated, and a fibrous network resembling a porous non-woven fabric structure having no preferential direction of fibres (Fig. 2b) was revealed. These fibres were devoid of any surface details and associated intact bacteria or debris. The fibres appeared swollen after the NaOH treatment, their diameter was in the range of 0.044–0.63 mm. The wide-angle X-ray diffraction reported by Takai et al. [15] also showed sharp peaks indicating the crystalline structure of bacterial cellulose. The mesh-like interwoven structure of the membrane imparts high tensile properties akin to bonded non-woven fabric. From Table 2, it observed that the tensile strength is of the order of 0.1 N mm − 2 and the Youngs modulus is 102 N mm − 2. The membrane also possesses excellent

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Fig. 3. (a) Degree of sorption Q of deproteinated BCM for water– alcohol binary mixtures; (b) solubility of water in the membrane phase for water – alcohol binary mixtures.

3.2. Sorption and solubility in the membrane phase

Fig. 2. SEM micrographs of (a) untreated BCM, (b) NaOH treated BCM.

chemical resistance. It is insoluble in alcohols, ketones, aldehydes, hydrocarbons, ether as well as aprotic solvents (DMF, DMSO, THF). It is also resistant to dilute acids and bases. It swells in water without dissolution. Table 2 Mechanical properties of NaOH treated BCM Property

Strain @ break (%) Elongation @ break (mm) Stress @ break (N mm−2) Young’s Modulus (N mm−2)

Mean value

Standard deviation (S.D.)

10.50 3.33

1.58 0.54

0.1

0.03

101.6

0.96

According to the prevalent solution-diffusion model, widely used in explaining separation characteristics of membrane materials, the flux and the separation factor of a component in a binary system through a membrane are functions of the solubility and diffusivity of the component. The solubility of the components in the membrane was determined using the sorption–desorption method. The preferential sorption properties of the deproteinated membrane were investigated in ethanol –water mixtures of different compositions and the overall degree of sorption was calculated using Eq. (1). The results shown in Fig. 3 indicate that as the water content in the feed mixture increased, the degree of sorption Q, also increased from 20 to 135%. The value of Q in pure alcohol was only 18%, it increased to 110% in equi-weight (1:1) mixture of water and alcohol and to 135% for pure water. It indicates that the membrane is hydrophilic in nature and sorbs almost seven times more water than alcohol. In order to determine the relative amounts of water and alcohol in the membrane phase sorbed from the binary mixtures, the swollen membranes were desorbed at 100 °C under a vacuum of 1 mmHg and the condensate was analysed using refractive index measurement. From the results plotted in Fig. 3, it is seen that as the amount of

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Table 3 Sorption selectivity (hs) in NaOH treated BCM for water–ethanol binary mixtures Feed composition (v/v)

hs for water

Composition (v/v) in the membrane

Water

Alcohol

Water

Alcohol

90 70 50 30 10

10 30 50 70 90

97.5 93.0 91.25 85.0 30.0

2.5 7.0 8.75 15.0 70.0

water in the feed increased from 10 to 100%, the water sorbed in the membrane also increased from 0.2 to 1.4 g/g. Correlating the above data in Fig. 3, it can be said that solubility of water in the membrane phase increases as the overall sorption increases. This implies that water is sorbed preferentially. The sorption selectivity for water (hs), calculated using Eq. (2), was in the range of 4– 10. From Table 3, it is observed that the hs values were favoured when the feed was richer in ethanol and contained 30–50% of water by volume. Reduced sorption selectivity at higher contents of water may be attributed to excessive swelling of macromolecular chains and flow coupling. Due to interaction with one component viz. water, the membrane becomes accessible to the other component (alcohol, in this case). Hence, the useful range of sorption is considered to be 5– 25% (refer Fig. 3); selectivity reduces at higher sorption values.

4.3 5.7 10.4 13.5 3.9

was used at a temperature of 309 1 °C. Eqs. (3) and (4) were used to calculate the permeation selectivity and PSI values.

3.3.1. Effect of alkali treatment The cellulose membrane initially used was in the nascent form without any chemical treatment. The flux obtained was low (32.3 g m2 h − 1) and so was the selectivity (4.88) for 1:1 compositions of alcohol–water mixtures. For the deproteinated membrane the flux increased considerably to 55.1 g m − 2 h and selectivity was 10.1. The membrane thickness was about 100 mm. The PV characteristics of the deproteinated membrane were studied over the entire range of compositions of binary mixtures. 3.3.2. Effect of concentration Figs. 4 and 5 depict total flux (J) and selectivity (hp) as a function of varying feed composition. A ‘trade-off’ relationship between selectivity for wa-

3.3. Per6aporation properties The PV process combines the evaporation of volatile components of a mixture with their permeation through a polymeric membrane under reduced pressure conditions. It, therefore, involves a sorption step at the membrane upstream face, followed by a diffusion through the dense film and a desorption into the vacuum. Thus PV performance of a membrane, termed PSI is described in terms of two important parameters namely, flux (J) i.e. the mass crossing the membrane per unit area in a unit time and the selectivity towards the preferentially permeated component. For the membranes under study, a batch stirred PV cell

Fig. 4. Water flux of binary water – alcohol mixtures through deproteinated BCM.

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Fig. 5. Permeation selectivity for binary water – alcohol mixtures through deproteinated BCM.

Fig. 6. Pervaporation separation index for binary water – alcohol mixtures through deproteinated BCM.

ter and its flux is observed from these figures. With an increase in feed water concentration from 10 to 50%, the flux increases from 112 to 153 g m − 2 h and water selectivity decreases from 285 to 40 for all water– alcohol systems. This phenomenon may be due to plasticizing effect of water, and probable flow coupling between water and ethanol. As the water concentration in the feed increases, the amorphous regions of the membrane swell and the polymer chains become more flexible, allowing alcohol molecules also to pass through, thus lowering the ability of the membrane for selective transport. When the concentration of water exceeds 60 wt.%, a reversal in the phenomenon occurs, viz., the flux decreased drastically with slight increase in the selectivity. This may be attributed to the clustering of water-molecules [7– 11]. The intermolecular hydrogen-bonding forms large clusters of water thereby reducing their diffusivity through the membrane. Since the swelling of the membrane is appreciable (Q \ 100%, refer Fig. 3) when the feed is richer in water, the macromolecular chains may become flexible due to plasticisation. This facilitates the solubility of the associated water molecules in the membrane phase but retards their diffusivity, and hence the overall flux reduces. This is supported by the interesting observation that the flux for pure water through the membrane was only 31 g m2 h − 1, while the solubility in the membrane phase was high viz. 1.34 g g − 1. With a small amount (  10 wt.%) of alcohol in the binary feed mixture, the overall flux increased to 111 g m2 h − 1. Even though the membrane is hydrophilic,

the flux for pure alcohol is substantial (41 g m2 h − 1); while the solubility in the membrane is as low as 0.19 g g − 1. The overall contribution of flux and selectivity to PV is defined by the PSI. The PSI values are plotted in Fig. 6 as a function of feed composition. Two distinct regions can be recognised corresponding to water-lean mixtures (B30 wt.%) and waterrich mixtures. In the former region, PSI is of the order of 104 g m − 2 h ranging from 13 600 g m − 2 h for 30% water to 32 700 for 10% water in the feed. For the water-rich region, PSI is lowered and is in the range 4535–13000 g m − 2 h. It, therefore, appears that the membrane may be effective in dehydration of azeotropes of ethanol. This is further substantiated by the vapour–liquid equilibrium (VLE) diagram shown in Fig. 7. The PV curve for ethanol lies below the VLE, indicating

Fig. 7. Vapour – liquid equilibrium diagram and pervaporation curve for binary water – alcohol mixtures.

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Table 4 Flux and selectivity of ethanol–water (70:30 wt.%) through homogeneous membranes (50 mm) at 70 °C (ref. [16]) Polymer

Flux (kg m2 h−1)

hp

Poly(acrylonitrile) Poly(acrylamide) Poly(vinyl alcohol) 98% Poly(ether sulphone) Polyhydrazide Bacterial cellulose membrane, (100 mm, 30 °C)

0.007 0.011 0.08 0.072 0.132 0.112

12 500 4080 350 52 19 287

that PV is favoured. The permeate is richer in water over the entire feed composition and contains at least 95% water by weight. It is interesting to compare the flux and selectivity values for azeotrope of ethanol (90 wt.% alcohol) through the BCM and the various synthetic polymer membranes [16] in Table 4. Polyacrylonitrile, polyacrylamide and polyvinylalcohol have a very high selectivity compared with BCM but their flux is very low. Polyhydrazide has a flux value comparable to BCM but its selectivity is poor. BCM has a high selectivity and reasonable flux. With an increase in temperature from 30 to 70 °C and thinning of the membrane from 100 to 50 mm; flux is expected to improve further. These aspects are under investigation. Efforts are also being made to design suitable membrane module and integrate it in a hybrid distillation–PV process.

4. Conclusion The PV characteristics of deproteinated BCM were investigated over the wide range of water– ethanol feed composition. The membrane is promising for dehydration of azeotropes of ethanol. It has a high selectivity towards water and a reasonable flux.

Acknowledgements We are grateful to Professor V.G. Pangarkar of University Department of Chemical Technology,

Mumbai, for fruitful discussions. We thank Dr R.V. Swamy, Chief Controller (R&D), DRDO, New Delhi for keen interest and invaluable suggestions. Dr D.K. Jaiswal, Director, DRDE, Gwalior is thanked for providing the necessary facilities in the laboratory. Our thanks to T.K. Das for secretarial assistance.

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