Development of the system for membrane separation of fatty aicd from supereritical carbon dioxide

采用溶胶-凝胶法制备了平均孔径为3.3 nm的多孔二氧化硅薄膜，并将其用于分离超临界二氧化碳中的溶质。用分子模拟技术研究了溶质的吸附特性，测定了溶质从膜上的解吸速率，计算了溶质在二氧化硅表面附近的势能。研究发现，咖啡因被强烈吸附到表面，然后缓慢解吸，形成吸附层，使孔变窄，产生分子筛分效应。因此，拒绝值是正的。而吸附良好、解吸速度快的正辛酸的截留值为负值。据推测，由于它们的势能，这些分子填充了毛孔，然后在跨膜压力的作用下被迫流过毛孔。用分子筛膜从超临界二氧化碳中分离咖啡因，并进行了渗透汽化实验。Dubinin-Astakhov分析结果表明，NaY沸石膜的孔径为亚纳米级。对月桂酸的截留率为0.88。结果表明，分子筛膜可用于SCCO_2膜分离。

A thin porous silica membrane (average pore size of 3.3 nm) was prepared by the sol-gel method and used to separate the solute from supercritical carbon dioxide. The characteristics of solute permeation were investigated in respect of the adsorption properties of the solute, the desorption rate of the solute from the membrane being measured and the potential energy of soulte near the silica surface being calculated by the molecular modeling technique. It was found that caffeine was strongly adsorbed to the surface and then slowly desorbed to form an adsorption layr, making the pores narrower and causing a molecular-sieving effect. Therefore, the rejection value was positive. On the other hand, the rejection value of n-octanoic acid, which was well adsorbed and rapidly desorbed, was negative. It is presumed that the molecules filled the pores due to their potential energy and were then forced to flow through the pores by the transmembrane pressure. Separation of caffeine from supercritical carbon dioxide was carried out with a zeolite membrane that has been tested in pervaporation. The pore size of a NaY zeolite membrane was evaluated to be subnanometer scale from the result of Dubinin-Astakhov analysis. The rejection of lauric acid was 0.88 for the zeolite membrane. A zeolite membrane was shown to be useful in SCCO_2 membrane separation.