Reversible distillation column sequence

When multicomponent mixtures are to be separated into three or more products, sequences of simple distillation columns of the type shown in Fig. 13-1 are commonly used. For example, if aternaiy mixture is to be separated into three relatively pure products, either of the two sequences in Fig. 13-4 can be used. In the direct sequence, shown in Fig. 13-4, all products but the heaviest are removed one by one as distillates. The reverse is true for the indirect sequence, shown in Fig. 13-4 7. The number of possible sequences of simple distillation columns increases rapidly with the number of products. Thus, although only the 2 sequences shown in Fig. 13-4 are possible for a mixture separated into 3 products, 14 different sequences, one of which is shown in Fig. 13-5, can be synthesized when 5 products are to be obtained. 

The novel approach finally taken was to conduct the reaction and purification steps in a reactor-distillation column in which methyl acetate could be made with no additional purification steps and with no unconverted reactant streams. Since the reaction is reversible and equilibrium-limited, high conversion of one reactant can be achieved only with a large excess of the other. However, if the reacting mixture is allowed to flash, the conversion is increased by removal of the methyl acetate from the liquid phase. With the reactants flowing countercurrently in a sequence of... 

It may be worthwhile to check the outcome if the column sequence is reversed. Sending the feed initially to the low-pressure column, the bottoms will be essentially pure benzene and the distillate would be close to the 36 mole% ethanol azeotrope. If this stream is sent to the high-pressure column, with its ethanol concentration lower than the high-pressure azeotrope (44.8 mole% ethanol), the bottoms would again be essentially pure benzene and the distillate would be the azeotrope. Therefore this setup would not produce the required separation. 

First, the role of reaction kinetics is analyzed considering RD processes for the simple reversible reaction Aj o Aj in an ideal binary mixture. The educt Aj is assumed to be the reaction component with the higher boiling point, so the product A2 is obtained in the distillate. The reaction can be carried out in an RD column sequence with an external recycling loop (Fig. 5.1), a non-RD column on top of a reactive reboiler (Fig. 5.2), or a full RD column (Fig. 5.3). More possible configurations are analyzed elsewhere [1]. 

Column Sequence of Ideal Mixtures Reversible Distillation... 

Figure 4.4. A sequence of three-component reversible distillation. C-1, C-2, C-3, columns arrows, heat input and output.

Later, these columns were independently rediscovered (Petlyuk, Platonov, Slavinskii, 1965 Platonov, Petlyuk, Zhvanetskiy, 1970) on the basis of theoretical analysis of thermodynamically reversible distillation because this distillation complex by its configuration coincides with the sequence of thermodynamically reversible distillation of three-component mixture (see Chapter 4), but in contrast to this sequence it contains regular adiabatic columns. The peculiarities of Petlyuk columns for multicomponent mixtures are (1) total number of sections is n(n - 1) instead of 2(n - 1) in regular separation sequences (2) it is sufficient to have one reboiler and one condenser (3) the lightest and the heaviest components are the key components in each two-section constituent of the complex and (4) n components of a set purity are products. 

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