Distillation complexes with side strippings

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. 

Three kinds of distillation complexes with thermal coupling flows (with branching of liquid and/or vapor flows) - columns with side stripping, columns with side rectifiers and complexes with full thermal coupling flows, called Petlyuk column -are used in industry at present. 

Figure 6.12. Some complex columns for ternary mixtures (a) with side rectifying (b) with side stripping (c) Petlyuk column (d) with prefractionator (e) more operable Petlyuk column (f) with divided wall and (g) with divided wall for extractive distillation.

The comparison of various distillation complexes and of ordinary flowsheets is given in Tedder and Rudd (1978). Columns with side strippings and side rectifiers, Petl50ik columns, flowsheet with prefracionator, and also some other feasible configurations of two columns were examined. It was shown, in particular, that Petlyuk columns are preferable at big content of average volatile component. 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

Among multisection distillation complexes, only columns with side strippings bring practically the whole heat into the feeding and bring hve steam into the bottom. Application of pumparounds decreases energy expenditures and recuperates withdrawn heat for heating of petroleum before separation. 

Therefore, column with side strippings, with live steam into the bottom, and with pumparounds is the best distillation complex for petroleum refining. An optimum way of designing such column is discussed in Section 7.5.2. 

For petroleum mixtures, their peculiarities predetermine the choice of the main distillation complex (column with side strippings) and of main sequence (atmospheric and vacuum columns), but different modifications are possible for different sets of products and compositions of crude are possible. These modifications may significantly improve the economy of the equipment. 

Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

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