Study of alcohol permselective separation membrane with a high flux

Abstract On the basis of preparation of CS/PAN flat composite membrane, the membrane performance toward alcohol/water system is studied. The results show that the membrane possesses the character of alcohol-permselectivity at lower feed concentration and water-permselectivity at higher feed concentration. Feed temperature and pore size of matrix remarkably affect the flux of the composite membrane.
Key words chitosan/polyacrylonitrile(CS/PAN), pervaporation, composite membrane, alcohol-permselectivity
摘要 在研制出壳聚糖/聚丙烯腈(CS/PAN)平板复合膜的基础上，考察了其对乙醇/水体系的渗透蒸发分离特性，实验结果表明：使用该膜可以实现料液低浓区优先透醇，高浓区优先透水，料液温度及基膜孔径对复合膜的通量均有较大的影响。
关键词 壳聚糖/聚丙烯腈 渗透蒸发 复合膜 优先透醇

崔永芳 尹燕 葛继均
(天津纺织工学院材料科学系 天津 300160)

The research on the pervaporation (PV) process and the membrane materials has been developing very fast since the pervaporation phenomenon was discovered. Although lots of water-permselective membranes have been widely used^[1-3], only a few alcohol-permselective membranes were studied such as silicon rubber membrane^[4], which shows low flux and selectivity toward alcohol. In this paper, we deal mainly with the study of alcohol-permselective CS/PAN flat composite membrane. The result shows that this kind of composite membrane has a good performance of alcohol-permselectivity.

1 Experimental
1.1 Materials
The chitosan(CS) was purchased from Japan and its deacetylation degree is 75%. The molecular weight of polyacrylonitrile(PAN) is 3.0×10⁴. The separating system is alcohol/water mixture. Liquid nitrogen is applied during evaluation.

1.2 Membrane Preparation
The PAN matrix is prepared first. PAN is put in DMF at 25°C for dissolution for half an hour and then is further dissolved at 60-70°C. The concentration is controlled at 18wt%. When PAN is completely dissolved and is defoamed with the decrease of pressure, the solution is casted on glass. Then the film is put into the coagulation bath and the matrix is formed. The second procedure is the fabrication of CS/PAN composite membrane. The deacetylated CS solution(with a deacetylation degree of 84%) is casted onto the matrix to form the composite membrane. After heat treatment and chemical cross-linking with sulfuric acid, the formation of composite membrane is eventually finished.

1.3 Instrumentation
The instruments of evaluation are self-made. The schematic diagram is shown in Fig.1. The downstream pressure is kept below 5mmHg during evaluation.

	Fig.1 Schematic diagram of instruments of membrane evaluation of pervaporation 1.thermostatic bath 2.feed tank 3.recycle pump 4.permeation chamber 5.plugcock 6.safety device 7.vacuum instrument 8.cold trap 9.three-way valve 10.buffer tank 11.drying chamber 12.vacuum pump

2 Results and discussion
2.1 Influence of feed concentration on composite membrane performance

	Fig.2 Influence of feed concentration on membrane performance

The relationship between feed concentration and membrane behavior is presented in Fig.2. The results show that the membrane has a unique separation performance toward water/alcohol mixture. With the separation factor α < 1 at lower feed concentration (13%~58%), the contact angle observed between alcohol and membrane is almost 0^º, which means alcohol-permselectivity^[5]. And α > 1 at higher feed concentration(90%~95%) means water-permselectivity. To the system of lower feed concentration, the average separation factorαis about 1/12.1 and the flux amounts to 5527g/m²h. This shows a better alcohol-permselectivity than that of silicon rubber membrane. The application of this special property will economize energy greatly and make it possible to recover alcohol from waste liquid mixture and at the same time reduce the pollution of discharging water. This kind of performance of the membrane has not been reported in the literature.

2.2 Influence of pore size of matrix on flux of composite membrane
From Fig.3 we can see that with the increase of draw ratio of PAN matrix the flux tends to increase gradually. When the draw ratio comes to1.5, the flux of composite membrane can amount to 1.0×10⁴g/m²h. Compared with the membrane undrawn, the flux is raised by almost 9 times. This is mainly because that, after drawing, the pore size of matrix becomes larger. And then the resistance against mass transfer becomes weaker, which means the flux increases respectively. So we can improve the working efficiency by post-drawing on the matrix.

	Fig.3 Relationship between draw ratio and flux

2.3 Influence of feed temperature on membrane performance

	Fig.4 Relationship between feed temperature and the membrane performance

The alcohol-permselectivity of the composite membrane can be seen in Fig.4. The separation rate goes through the highest point (the lowest value) and then decreases with the increase of the feed temperature. The flux gradually increases at higher temperatures. When the feed temperature is low, the molecular segments of chitosan layer are in freezing status and the free volume is small, so there exist only smaller moving units than segments, which results in “the densification” of the membrane. Therefore the flux through membrane is low and the separation rate is low too. When the feed temperature increases, the free volume, together with the moving units in chitosan layer, becomes larger. Thus, “the membrane densification” decreases and the flux increases. At the same time, water keeps diffusing into chitosan layer at higher temperature and destroys the crystal structure inside. The permeation of water impedes the permeation of alcohol, which results in the increase of the water-permselectivity, so the separation rate increases respectively. Generally speaking, when the temperature is below 333K(60°C), the comprehensive consequence is better than others above 333K. We can achieve the optimum result when the temperature approaches 328K(55°C). The separation rate is 0.068 and the flux amounts to 4500g/m²h.

3 Conclusion
Using this kind of CS/PAN composite membrane, we are able to achieve effective separation toward alcohol/water mixture. When the feed concentration is low (13%~58%), the membrane possesses a performance of alcohol-permselectivity, which will expand the selectivity of membrane materials toward alcohol-permselective. After a proper post-draw by 1.5 times or so, we can achieve an excellent composite membrane with higher flux. This will improve working efficiency greatly. When the mixture system is separated at 55°C the net effect may reach optimum.

4 References
[1] Lee Y M, Nam S Y and Kim J H, Polymer Bulletin, 1992, 29: 423.
[2] Feng Xianshe, Huang R Y M, J. Membr. Sci., 1996, 116: 67-76.
[3] Shieh J J and Huang Y M Robert, J. Membrane Sci., 1997, 127:185-202.
[4] Xu Jiping, China-Japan Symposium on Membrane and Membrane Processes, April 2-3,1997, p3-4.
[5] Rong Hueichen, Horng-Dar Hwa, Carbohydrate Polymers. 1996, 29:353-358