Column distillation countercurrent cascade

Here we understand by complex colunms a countercurrent cascade without branching of flows, without recycles and bypasses, which, in contrast to simple columns, contains more than two sections. The complex colunm is a column with several inputs and/or outputs of flows. The column of extractive distillation with two inputs of flows - feed input and entrainer input - is an example of a complex column. 

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Continuous Binary Distillation Column 496 Controller Tuning Problem 427 Three-Stage Reactor Cascade with Countercurrent Cooling 287... 

The countercurrent (recycle) cascade (Fig. 8.3) is much more useful because by reprocessing the waste stream a larger fraction of the desired isotope is recovered. In a recycle cascade the i th stage is fed by a mixture of the product, Y( i) from the (i — l) th, and the waste, X(i+ X) from the (i + l) th, stage. A distillation column... 

As an alternative method, Qi et al. [14] proposed to use a reactive condenser (see Fig. 4.1) to predict possible top products of a countercurrent reactive distillation column. The feasibility analyses of the reactive condenser and the reactive reboiler are analogous to the flash-cascade approach. The latter authors used transformed... 

A flow diagram for countercurrent extraction with reflux is shown in Fig. 20.14. To emphasize the analogy between this method and fractionation, it is assumed that the cascade is a plate column. Any other kind of cascade, however, may be used. The method requires that sufficient solvent be removed from the extract leaving the cascade to form a raffinate, part of which is returned to the cascade as reflux, the remainder being withdrawn from the plant as a product. Raffinate is withdrawn from the cascade as bottoms product, and fresh solvent is admitted directly to the bottom of the cascade. None of the bottom raffinate needs to be returned as reflux, for the number of stages required is the same whether or not any of the raffinate is recycled to the bottom of the cascade. The situation is not the same as in continuous distillation, in which part of the bottoms must be vaporized to supply heat to the column. 

Whereas in ordinary countercurrent operation the richest possible extract product leaving the plant is at best only in equilibrium with the feed solution, the use of reflux at the extract end of the cascade can provide a product even richer, as in the rectifying section of a distillation column. Reflux is not needed at the raffinate end of the cascade since, unlike distillation, where heat must be carried in from the reboiler by a vapor reflux, in extraction the solvent (the analog of heat) can enter without a carrier stream. 

Gas extraction extends the possibilities of separation processes like distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. Furthermore, it enables separation of components with very similar properties if used in the countercurrent mode. Process temperatures in gas extraction are determined by the critical temperature of the solvent and not, as is the case of distillation of any kind, by the liquid-vapor transition of the feed mixture. As compared to liquid-liquid extraction, gas extraction makes easily possible to operate a two cascade separation column, applying a stripping and an enriching section. Combined, these possibilites allow gas extraction to be operated at very moderate temperatures and as a separation process for difficult separations. 

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