Membrane separations, multistage separation requirements

With the proper choice of membrane material, a membrane separator can achieve significant separations in a single stage. Indeed, as we shall see, it usually turns out that multistage operations with membranes are not economical (compared to cryogenic distillation). Thus most industrial applications of membranes do not require very high purity products. 

It is theoretically possible to accomplish a complete separation of a pair of gases by membrane separation, either by very highly selective membranes combined with very low partial pressure of the permeating component on the permeate side of the membrane, or by multistage cascades. However, it usually is not economical to accomplish high purity and high recovery of both components of a binaiy by membrane separation alone. Therefore, it usually b necessary to couple membrane gas separation widi other processing steps when neatly complete separation te required. 

Membrane gas separation processes can be improved by using multistage separation as depicted in Figure 3.3.68 for natural gas treatment with a two-stage membrane for the separation of CO2. Traditionally, amine absorption is used to separate CO2 from natural gas. Membrane plants require less operator attention and smaller units may even operate unattended. Hence, membrane separations are favored in remote locations like offshore platforms. 

The book may be used for a methodical study of the subject or as a reference for solving day-to-day problems. It follows a logical flow of ideas within each chapter and from one chapter to the next yet each chapter is quite self-contained for quick reference. The discussion starts with fundamental principles, prediction of thermodynamic properties, the equilibrium stage, and moves on to the different types of multistage and complex multistage and multicolumn processes, batch distillation, and membrane separation operations. Although computer simulation is a central theme of this book, no previous experience in the use of simulation software is required. 

Caustic Concentration. The elaborate multistage evaporators required for the concentration of the diaphragm-cell caustic and the separation of NaCl and Na2S04 must be nickel plated because of the corrosiveness of the cell liquor containing NaCl and NaC10 j. These evaporators cost 20 - 35 % of the total. The evaporators for the membrane process may be constructed of stainless steel and are much smaller because the essentially salt-free cell liquor is more concentrated, costing 3 - 4 % of the total. The mercury process produces 50 % caustic directly, evaporation is not required. 

In general, high selectivities can be obtained in liquid membrane systems. However, one disadvantage of this technique is that the enantiomer ratio in the permeate decreases rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed would be an approach to tackle this problem or, alternatively, using a system containing the two opposite selectors, so that the feed stream remains virtually racemic [21]. Another potential drawback of supported enantioselective liquid membranes is the application on an industrial scale. Often a complex multistage process is required in order to achieve the desired purity of the product. This leads to a relatively complicated flow scheme and expensive process equipment for large-scale separations. 

An example of a multistage gas separation membrane process is given in Figure 7.1 for natural gas treating [Spillman, 1989]. It is generally accepted that for a gas separation membrane to be cost effective, a separation factor of at least 5-10 (sometimes even higher) will be required. 

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