Industrial application of membrane separation processes

Abstract

During the past two decades membrane separation processes have been developed and optimized for even large scale industrial applications. The most important of theae proceases include: (i) microfiltration and ultrafiltration for purification of aqueous streams, concentration and recovery of valuable products; (ii) reverse osmosis for the production of demineralized or potable water; (iii) electrodialysis for the concentration or removal of dissolved ions; (iv) gas separation for splitting gas streams, removal or recovery of specific gases; (v) pervaporation for separation and concentration of liquid mixtures, especially of aqueous-organic azeotropes.Whereas the first three of these processes are well established and have reached a high degree of maturity, the last two ones are still in developing stage, although development is fairly fast. Both procam, gas separation and pervaporation are very closely related with respect to their physicochemical fundamentals. In this paper principles of the separation processes are outlined. Examples of membrane performance and applications, with the emphasis on Pervaporation processas are presented.In general all processes used for the separation of fluid mixtures can be split into two categories: separation by equilibrium distribution separation by differences in transport ratesThe most common separation processes used on large scales in the industry are based on equilibrium distribution. Evaporation, distillation, extraction, adsorption, absorption are all going back to the same principle shown in fig. 1. A first phase comprising a mixture to be separated is brought into contact with a second phase. After a certain time thermodynamic equilibrium is established between the two phases. That means both phases show the same temperature and all components have the same chemical potential in both phases. The analytical concentrations of a component in the two phases, however, may differ e.g. a component can be highly enriched in one phase and be depleted in the other one. If now the two phases are separated by appropriate means, the enriched component can be recovered usually by establishing a new equilibrium at a different temperature or pressure. Repetition of this procedure at the end will lead to a phase in which one of the components is present at the wanted purity.If separation by differences in transport rates is to be achieved, an additional means is required. In fig. 2 such a means is a membrane, separating to phases from each other. A driving force, a gradient in pressure, concentration, temperature, electrical field is applied and has to be maintained over the membrane. Under the influence of this driving force components from the mixture to be separated, held at a higher chemical potential, migrate through the membrane to the side of the lower chemical potential. Separation between different components is effected by the difference in transport rates. The gradient in the chemical potential has to be maintained by continuous removal 993 Unauthenticated Download Date | 6/...

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