Direct sequence separations

Two-column case. If the relative volatility of the product C is intermediate between the two reactants, a two-column distillation system is typically used. Either the light-out-first (LOFj, direct separation sequence, or the heavy-out-first (HOF), indirect separation sequence, can be used. The former is more common because the lightest component only has to be taken overhead once (in the first column) and not twice (as would be the case in the HOF configuration). However there are processes in which the HOF is preferred because it sometimes has the advantage of reducing the exposure of temperature-sensitive components to high base temperatures. 

Thus, for the separation of a feed mixture into three fractions there are two column circuits with two columns. Figure 2.3.2-18 shows the two possibilities. In the direct separation sequence the low-boiling components are separated one after the other overhead, and in the inverse sequence the high-boiling components are removed at the bottoms of the columns. 

Energy reductions of up to 30% have been reported in some systems for Petlyuk and divided-wall column configurations compared with the direct-separation sequence. Figure 12.1 gives the flowsheet of a divided-wall column for the numerical benzene/toluene/xylene separation example considered later in this chapter. The material presented in this chapter is based on the paper that studied the control of divided-wall columns. ... 

Table 12.1 gives detailed information of the equipment sizing and economics for the RadFrac divided-wall column and the conventional direct-separation sequence. The economic gain is about 30%. 

The optimum economic steady-state design of a two-column direct-separation sequence is shown in Figure 12.18. The number of trays, feed tray locations, and reflux ratios were varied in each column to find the configuration giving the minimum TAC. An additional... 

The control objectives for the conventional direct-separation sequence are quite similar to those of the divided wall. Each column has two manipulated variable reflux and reboiler heat input. Therefore, two variables can be controlled in each column. Figure 12.26 gives the proposed control structure for this conventional two-column process. 

Figure 17.20 shows that the heavy reactant excess design is favored over the hght reactant excess design by a factor of almost 30%. As for the column sequencing, the direct separation sequence is preferred over the indirect sequence. For the separation sequence, the LLK is boiled up twice for the indirect sequence as compared to that of the direct sequence where the LLK is only boiled up once. Therefore, we expect a lower separation cost using the direct sequence. 

Figure 2.2 compares these two possible configurations for a simple plant. A fresh feed stream containing a mixture of chemical components A, B, and C is fed into a two-column distillation train. The relative volatilities are > aB > separation sequence A is taken out the top of the first column and B out the top of the second column. 

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The above sequencing methods valid for zeotropic systems cannot be applied in the case of mixture with strong non-ideal character and displaying distillation boundaries, as those in the case of breaking azeotropes. Fortunately, the sequencing problem in this case has a different character. Most of the separations of multi-component non-ideal mixtures can be reduced by appropriate splits to the treatment of ternary mixtures, for which two or three columns are normally sufficient. The separation sequence follows direct or indirect sequence. The energetic consumption due to the recycle of entrainer dominates the economics. From this viewpoint preferred is that sequence in which the entrainer is recycled as bottoms. Hence, in azeotropic distillation the main problem is the solvent selection and not columns sequencing. 

Saddle packing, 57 Safety, in design, 34 Savannah River plant, 23, 73 Scatchard-Hamer equation, 198 Scheibel column, 82 Schmidt number, 664 Secondary reflux and boilup, 698-704 Selection of separation operations, 32, 534 factors in selection, 34-35 Selectivity, relative, 145 Separation operations, 8-11, 20 Separation process, 1-47 synthesis, 527-547 Separator sequences, 530 Sequence of separators, 527-528 direct, 529 indirect, 529 Shell Chemical Co., 513 Shortcut design methods, 427-493 Shower trays, 77 Side streams, 392 Sieve trays, 68-69... 

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