Separation trains azeotropic

Residue curve maps (RCMs) have been long used as a tool for analyzing a given ternary system s phase equilibrium behavior. These maps, originally pioneered by Schreinemakers in 1902 [1], enable design engineers to quickly scan possible separation trains or sequences, and also to identify areas of difficult separation due to azeotropes. 

Figure 7-6. The PPG Industries Inc. Chloroethylene process for producing perchloro- and trichloroethylene (1) reactor, (2) graphite exchanger, (3) refrigerated condenser, (4) scrubber, (5) phase separation of perchlor from trichlor, (6, 7) azeotropic distillation, (8) distillation train, (9-11) crude trichlor separation—purification, (10-16) crude perchlor separation—purification.

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

A separation process is sought that can satisfy both our present economic and enviromental constraints. It would also provide an alternative to present practice that relies on expensive azeotropic or extractive distillation processes used in the recovery of products from low relative volatility streams. As an example, virtually all industrial butadiene recovery processes now rely on extractive distillation using acetonitrile or other equivalent agent to enhance the relative volatility of the C4 components. The use of supercritical or near critical separation of these streams may satisfy these requirements provided certain pressure, temperature and recompression criteria can be met. Such a process would also reduce the need for a complex train of distillation towers. 

Unless there is an azeotrope formed between the components to be separated, it is possible to separate completely a binary mixture into its components in a single distillation column. In some cases it is eveo possible, with the use of sidestreams, to obtain more than two pure componoms from a single distillation colume. In general, a multicomponent mixture in which there are no azeotropes and no sidestreams withdrawn can be separated completely in a distillation train using one oolumn less than the number of product streams required. 

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