Liquid processes with nonporous membrane

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

The membrane extraction process mostly makes use of nonporous membranes. Such a membrane can be a liquid or a solid phase (a polymer impregnated with a liquid), which is placed between two... 

Pervaporation is a process in which organic solvent water mixture or organic solvent mixture can be separated by partial vaporization through a nonporous permeate selective membrane. In this process liquid feed mixture circulates in contact with the active nonporous side of the membrane while a vacuum is applied on the other side of the membrane. A phase change of membrane-selective permeate takes place in the membrane. The membrane-selective permeate diffuses through the membrane and desorbs on the posterior side of the membrane. Later, it evaporates with the help of a vacuum from the posterior side of the active nonporous membrane. The transport of the permeate through a nonporous permeate-selective membrane is quite complex. This could be explained in three steps, which are as follows ... 

Synthetic membranes for molecular liquid separation can be classified according to their selective barrier, their structure and morphology and the membrane material. The selective barrier- porous, nonporous, charged or with special chemical affinity -dictates the mechanism of permeation and separation. In combination with the applied driving force for transport through the membrane, different types of membrane processes can be distinguished (Table 2.1). 

Membrane extraction encompasses a class of liquid-phase separations where the primary driving force for transport stems from the concentration difference between the feed and extractant liquids rather than a pressure gradient, as is the case with most of the other processes discussed above. A microporous membrane placed between the feed and the extractant liquids functions primarily as a phase separator. The degree of separation achievable is determined by the relative partition coefficients among individual solutes. This operationx is known as membrane solvent extraction. If a nonporous, permselective membrane is used instead, however, the selectivity of the membrane would be superimposed on the partitioning selectivity in this case the process may be referred to as perstraction. These process concepts are illustrated in Fig. 34. 

These developments follow naturally from the basic configuration of a liquid membrane contained between porous polymeric membranes. Any pure liquid or liquid solution or liquid suspension can be used as the liquid membrane regardless of whether it wets the porous hollow fiber substrate (required in SLM otherwise a complex exchange process is needed (25-26)) or not. Further, a variety of hollow fibers may be used to expand the capabilities of the technique. In addition, the CLM can be used as a reaction medium as well as a separating membrane. Further, the CLM may be used to separate a mixture to provide the right feed to a reaction system. All such concepts will be briefly illustrated here for a basic configuration of two microporous/porous (or nonporous) fibers next to each other with a layer of liquid in between. 

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