Chemical Engineering and Processing 40 (2001) 263– 267 www.elsevier.com/locate/cep
Acetic acid dehydration by pervaporation N. Durmaz-Hilmioglu a,*, A.E. Yildirim a, A.S. Sakaoglu b, S. Tulbentci a a
Department of Chemical Engineering, Chemical and Metallurgical Faculty, Istanbul Technical Uni6ersity, 80626, Maslak, Istanbul, Turkey b Eczacibasi –Beiersdorf, 80640, Le6ent, Istanbul, Turkey Received 5 January 2000; received in revised form 4 July 2000; accepted 6 July 2000
Abstract In this study polyvinyl alcohol (PVA) was modified with two different agents — glutaraldehyde and formaldehyde — by a method called crosslinking and the membranes produced were used to separate mixtures of acetic acid and water. The effect of type of crosslinking agent, feed composition and feed temperature on the pervaporation characteristics were investigated. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Acetic acid; PVA; Formaldehyde; Glutaraldehyde
1. Introduction Pervaporation has gained acceptance by the chemical industries for the past 10 years. Pervaporation can offer favorable economics, efficiency and simplicity and it can be integrated into distillation and extraction processes . Among the other membrane processes, the pervaporation process is well known and has been used for separating low molecular weight close-boiling organic liquids, aqueous –organic mixtures . In pervaporation, the liquid mixture to be separated is placed in contact with one side of a membrane and the permeate is removed as a low pressure vapor from the other side. According to solution – diffusion model, pervaporation consists of three steps: (1) sorption of the permeant from the liquid mixture to the membrane; (2) diffusion of the permeant through the membrane; and (3) desorption of the permeant to the vapor phase . With a production capacity of :4 million tpa worldwide, acetic acid is an important product in the chemical industry. Most of the acetic acid produced is mainly used for the production of vinyl plastics, hot-melt adhesives, textile finishes, latex paints and acetic anhydride. The chemical processes for the production of these * Corresponding author. Tel.: +90-212-2852925; fax: + 90-2122853425. E-mail address: [email protected]
end-products and intermediates are always accompanied by waste and/or recycle streams containing acetic acid/water mixtures. Furthermore, the synthesis of acetic acid itself results in the production of water as a by-product. In order to obtain pure acetic acid, water must be removed. Because of all these facts, separation of acetic acid/water mixtures plays a very important role in the chemical industry . The aim of this study was to separate acetic acid/water azeotropic mixtures by using polyvinyl alcohol membranes that were prepared. Polyvinyl alcohol was modified with two different agents, such as formaldehyde and glutaraldehyde, by a method called cross-linking. Pervaporation experiments were performed at different temperatures and different liquid feed mixture concentrations for two types of membranes. The studies in the literature investigating acetic acid dehydration by pervaporation can be given as followings. Until recently, research in pervaporation was mainly focussed on the ethanol dehydration. Over the past few years, other systems have been studied. The first publication on acetic acid dehydration is patent from Kirkland et al. published in 1965 . They used modified and crosslinked copolymers of vinyl chloride and vinyl acetate. The selectivity is not high at high temperatures. Yoshikawa et al. studied the separation properties for acetic acid dehydration through a membrane made
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Table 1 Membranes based PVA for pervaporation applications Polymer
Membrane preparation method
PVA PVA PVA, AN PVA, AA, MA PVA, MA, MMA PVA, latex PVA, poly(N-vinyl-2-pyrolidone) PVA, CS PVA, poly(AMcoAANa)
Solution-casting Photocrosslinking Grafting Grafting Grafting Grafting Blending
      
Blending Annealing, blending Annealing Heat treatment Heat treatment Freezing–thawing Freezing–thawing Composite Composite Composite Composite Composite Irradiation Ion exchange
             
PVA PVA PVA, PAN, PSSA PVA PVA PVA PVA, PAN PVA, PSSA PVA, PAN PVA, PVAc PVA PVA, poly(isobutylene-co-maleic anhyride PVA, chitosan
from polyacrylic acid-co-acrylonitrile. The membrane permeates water from aqueous acetic acid mixtures . Nguyen et al. investigated polyvinyl alcohol membranes containing covalently bonded carboxylic groups for acetic acid dehydration . The loss in selectivity is largely compensated by the gain in permeability at higher carboxylate contents. Huang et al. studied acetic acid dehydration of membranes prepared from crosslinked PAA and nylon-6 blends and PVA crosslinked with amic acid [8 – 11]. The crosslinked PVA membrane showed a higher selectivity than the PVA membrane measured by Nguyen et al. Koops et al. investigated poly (vinyl chloride) poly
acrylonitrile composite membranes for the acetic acid dehydration. The fluxes are low in the study . Okuno and Nishimoto developed the PVC membrane preferentially incorporated acetic acid . In this study, the material science aspects for developing acetic acid resistant membranes suitable for the dehydration of acetic acid. PVA is the object of pervaporation researchers because of its chemical and physical nature and its excellent permselective characteristics, even though the PVA composite membrane material, known as GFT membrane, has been commercialized . PVA can be well adapted for dehydration, but it has poor stability in aqueous mixtures. Its stability can be improved by modification of chemical structure in the polymer through crosslinking reaction . Studies on PVA membranes are concerned with the separation characteristics and their applications. There is less study on the effect of crosslinking of PVA membrane. Crosslinking may play an important role in designing a permselective PVA membrane . A complete survey of the literature for the preparation of PVA membranes are represented in Tables 1 and 2. In the literature, PVA membranes are prepared by chemical crosslinking with glutaraldehyde or by heat treatment for pervaporation applications. Formaldehyde is used for reverse osmosis applications. In this study, membrane was prepared by chemical crosslinking with formaldehyde and glutaraldehyde and thermal crosslinking. Since glutaraldehyde is a dialdehyde, chemical crosslinking reaction would be more effective and membrane would be more resistant by thermal crosslinking.
2. Experimental procedure
2.1. Material Acetic acid, formaldehyde and glutaraldehyde were of analytical grade. Membranes were prepared by cross-linking technique at a laboratory.
Table 2 PVA crosslinking methods Polymer
Membrane preparation method
PVA PVA, b CD PVA PVA
Crosslinking with metal salts, dicarboxylic acids, ketones Crosslinking with glutaraldehyde+sodium sulfate+sulfuric acid at room temperature Crosslinking with glutaraldehyde+acetic acid+sulfuric acid+methanol at room temperature Crosslinking with glutaraldehyde+sodium sulfate+sulfuric acid and precipitation in sodium sulfate+sodium hydroxide solution at room temperature Crosslinking with glutaraldehyde+hydrogen chloride+sodium hydroxide at room temperature Crosslinking with glutaraldehyde
   
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tion of the collected permeate was determined by a refractometer. Pervaporation flux was determined by weighing of the permeate. During the runs, permeate samples were taken and analyzed every hour for 8 h. Steady state was obtained after 3 h throughout the experiments. The pervaporation selectivity was calculated with the concentrations in the liquid feed mixture and in the vapor permeate. The reproducibility of permeate compositions and fluxes were within 95%. Fig. 1. Experimental set-up.
Fig. 2. Influence of the feed concentration on the pervaporation flux.
2.3. Membrane preparation PVA was dissolved in water by refluxing and stirring for 8 h at 100°C. Homogeneous solution of 10 wt% polymer in water was obtained. To this solution, 0.1 wt% crosslinking agent was added and the reaction was started with 10 wt% 1N hydrogen chloride solution. The mixture was stirred for 20 h at room temperature and the reaction was stopped by neutralization with 10 wt% 1N sodium hydroxide solution. Formaldehyde and glutaraldehyde were used as crosslinking agents. For degassing, the solution was stored at room temperature for 1 week. The cooled and degassed solution was cast on a glass plate by a blade. Then, water was allowed to evaporate slowly at room temperature and ambient pressure. After 1 week, drying was completed and the polymer was treated in an air-circulating oven at temperature 150°C for 1 h for thermal crosslinking [14,17,18,21,25,26,39 –41,43].
3. Results and discussion The pervaporation properties are characterized by the flux J and the selectivity h. Fluxes were determined by measuring the weight of liquid collected in the cooling traps at steady state conditions. The pervaporation selectivity (h) is defined by Fig. 3. Influence of the feed concentration on the selectivity.
2.2. Per6aporation experiments Pervaporation apparatus is shown in Fig. 1. The pervaporation experiments were performed with a continuous steel set-up at different temperatures and different liquid feed mixture concentrations for two types of membranes. The membrane with an effective membrane diameter of 6 cm was installed in the pervaporation cell. The feed temperature was kept constant. The liquid feed was circulated through the pervaporation cell from a feed tank by a pump with a rate of 2 l/h. The pressure at the downstream side was kept : 10 mbar within 9 1 mbar by a vacuum pump. Permeate was condensed in liquid nitrogen traps. The composi-
h1/2 = (y1/y2)/(x1/x2)
Where x and y represent the molar fractions in the feed and permeate, respectively. Indices 1 and 2 refer to the more permeable component and the less permeable component, respectively.
3.1. Effect of feed composition The relationships between liquid feed mixture composition and pervaporation data were investigated over a composition range from 10 to 90 vol% of acetic acid/water binary system at 30°C for the two membranes were given in Figs. 2 and 3. The pervaporation flux increased with increasing liquid feed concentration of water composition for both
N. Durmaz-Hilmioglu et al. / Chemical Engineering and Processing 40 (2001) 263–267
membranes, but selectivity decreased. The pervaporation fluxes were less in the membrane that crosslinked with glutaraldehyde than the membrane that crosslinked with formaldehyde. The membrane that crosslinked with glutaraldehyde provided the highest selectivity. As the water concentration in the feed mixture increases because of a strong interaction between water and membrane, the membranes may become more swollen and as a result, polymer chains may become more flexible and transport may become easier. The permeabilities decreased in the membrane that crosslinked with glutaraldehyde. It showed that the crosslinking portions in the membranes increase with increasing aldehyde group. This can be evidence that the crosslinking reactions were well progressed. The selectivities increased in the membrane that crosslinked with glutaraldehyde. Since water has more polarity, the membrane can prefentially interact with water.
3.2. Effect of feed temperature The temperature dependence of pervaporation data is investigated over a temperature range from 30 to 60°C with the membranes that crosslinked formaldehyde and glutaraldehyde for the feed mixture containing 30 vol% water. The relationships between pervaporation characteristics and temperature are given in Figs. 4 and 5.
For the two membranes, the flux increased with the feed temperature, while selectivity decreased. As the feed temperature increased the vapor pressure at the feed side increased, while the vapor pressure at the permeate side was not affected. Therefore, the driving force for transport of the penetrants through the membranes increased with temperature.
4. Conclusions There is a disproportionality between the flux and selectivity for all liquid feed mixture compositions and for all operating temperatures. The membrane that crosslinked with glutaraldehyde has the higher selectivity and the lower flux. In these membranes, crosslinking reaction efficiency may be higher and cross-links may prevent the permeability and act selective. Swelling of the membrane decreases while crosslinking degree increases. Therefore, the flux decreases. The membrane acts selective while swelling decreases and so selectivity increases. The fluxes increase with increasing water concentration in the feed mixture and selectivities decrease for the operating conditions in both membranes. Sorption may increase with higher water concentration and this causes higher permeability, but because of the decreasing of the preferential sorption, selectivity decreases also.
Fig. 4. Temperature dependence of flux.
Fig. 5. Temperature dependence of selectivity.
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