Distillation indirect split

Indirect. split, where C is removed first (AB/C) and then mixtnre AB is distilled... 

A bifurcation study to predict the distillate and bottoms products for the entire range of reaction rates from the limit of no reaction to the limit of chemical equilibrium. This provides a global view of the direct and indirect split products from a continuous RD at all rates of reaction. 

Figure 5.25 shows the evolution of top section trajectory bundle at separation of four-component ideal mixture, when the product is ternary mixture 1,23 (i.e., at indirect split) K >K2>Kj,> K, xdi -I-xd2+xd3 = 1). At small values of the parameter L/V, the stable node Ai+ that at the increase of the parameter L/F moves away from the product point along reversible distillation trajectory appears at face 1-2-3 (Fig. 5.25a). After this node reaches reversible distillation trajectory tear-off point from face 1-2-3 inside concentration tetrahedron, it turns into the saddle...

However, if we want to achieve full satisfaction of the distillation equation system and to obtain precise product compositions xb and x, it is necessary to execute iterations by these compositions (i.e., to take into consideration the fact that at the direct split not only the second component is an impurity one in the top product). These iterations become more necessary the larger the set value of (1 - t d) at the direct split or the set value of (1 - rja) at the indirect split. 

However, sharp extractive distillation is not expedient because bottom product Xb is a binary mixture with azeotrope 13. Split of extractive semisharp distillation 2 1,2,3 (xg ) is preferable. Indirect split 2,3 1, at which the top product point... 

Figure 8.27. Phase equilibria map and sequences for distillation of a binary azeotropic mixture (1,2) with an intermediate boiling en-trainer (3) (a) indirect split 2,3 1 in the first column, (b) direct split 2 1,3 in the first column, and (c) preferred split 2,3 13 in the first column.

The six sequencing heuristics are formulated to reduce the separation load on downstream columns, favoring easier separations early and difficult separations in the absence of nonkey components. If only two products are to be derived from a mixture and all of the components in one product are more volatile than all of the components in the other product, then the next split should divide the mixture into the two products. The presence of hazardous or corrosive materials can gready increase costs, and such components should be removed as early as possible. The most plentiful product in a mixture should be removed (if it can be) with one separation and if the relative volatility is favorable. Direct sequences, ie, removing a light product as distillate, generally are favored over indirect sequences, ie, removing a heavy product as bottoms. If no product dominates the feed composition, then separations that yield approximately equimolar splits are favored. Only if no other heuristic applies should the easiest separation be performed next. 

For example, the separations d,h, and d2h2 are sloppy splits with different amounts of A in the distillate, and accordingly with different recoveries. The separation of pure A at the top is represented by the split d h, which corresponds to a direct sequence . Accordingly, the separation of B/C in a second column is represented by the edge BC, on which h is the feed. Similarly, the first split in an indirect sequence , in which C is separated in bottoms and A/B at the top, is shown by the segment d"b". The locus of all splits between the above limit cases allows the regions of attainable products to be defined. 

The separation section receives liquid streams from both reactors. For assessment the residue curve map in Figure 5.7 is of help. The first separation step is the removal of lights. This operation can take place in a distillation column operated under vacuum (200mmHg) with a partial condenser. Next, the separation of the ternary mixture cyclohexanone/cyclohexanol/phenol follows. Two columns are necessary. In a direct sequence (Figure 5.15) both cyclohexanone and cyclohexanol are separated as top products. The azeotrope phenol/cyclohexanol to be recycled is the bottoms from the second split In an indirect sequence (Figure 5.16) the azeotropic phenol mixture is a bottom product already from the first split. Then, in the second split cyclohexanone is obtained as the top distillate, while cyclohexanol is taken off as the bottom product The final column separates the phenol from the heavies. 

The above separation sequence, although the most used in industry, is not unique. Another possibility would consist of adopting the indirect sequence . In this case, EDC separates in the first split as bottoms, followed by VCM/HC1 distillation, as pictured in Figure 7.7 (left-hand). This alternative is penalized by excessive bottom temperature in the first split at pressures above 5 bar. In addition, in the second step an intermediate compression would be necessary for an efficient separation HC1/VCM. 

The transition split divides direct-type sphts from indirect-type splits as discussed by Doherty and Malone (Conceptual Desisn of Distillation Systems, 2001, chaps. 4 andS) also see Fidkowski, Doherty, and Malone [AlChE J., 39,1301(1993)]. The upper line in Fig. 13-70 is the minimum vapor flow leaving the reboiler of the main column, which also corresponds to the minimum vapor flow for the entire system since all the vapor for the total wstem is generated by this reboiler. For P = 0 the minimum vapor flow for the entire thermally coupled system (i.e., main column) becomes equal to the minimum vapor flow for the side rectifier system (i.e., main column of the side-rectifier system see Fig. 13-65b or c) (Vsr) for P = 1 it is equal to the minimum vapor flow of the entire side stripper system (Vss) (which is the sum of the vapor flows from both the reboilers in this system see Fig. 13-66h or c). Coincidentally, the values of these two minimum vapor flows are always the same (Vsr), = (Vss)mm- For P = Pr the main column is pinched at both feed locations i.e., the minimum vapor flows for separations A/B and B/C are equal. 

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. 

A second alternative, called indirect sequence , is depicted in the Figure 9.7-right. In the first split the pure high-boiler is recovered completely as bottoms, followed by the separation of the light and medium components. Note that the two alternatives make use of simple distillation columns, with one feed and two products. A suitable design can fiilfil any type of feed composition and purity specifications. 

Let s consider the separation of a binary minimum azeotrope AB with a medium boiling entrainer C (Fig. 9.12). Note that the AB azeotrope and the component B are nodes, while both A and C are saddles. The separation regions for the first split are delimited by direct and indirect sequences, respectively. If the boiling points of A and AB azeotrope are not too close, A can be obtained as distillate, even if it is a saddle Rooks et al. (1998) has given recently a consistent explanation the split is feasible when the concentration profiles of both rectification and stripping zones points to the same common saddle, in this case the component C. 

Split sequencing follows the representation sketched in Figure 9.12. Direct sequence gets acetone as distillate in the first split, and heptane as bottom product in the second split. In indirect sequence heptane can be recovered as bottoms from the first split, while acetone is obtained as top distillate from the second split. 

In this chapter, we describe an algorithm for predicting feasible splits for continuous single-feed RD that is not limited by the number of reactions or components. The method described here uses minimal information to determine the feasibility of reactive columns phase equilibrium between the components in the mixture, a reaction rate model, and feed state specification. This is based on a bifurcation analysis of the fixed points for a co-current flash cascade model. Unstable nodes ( light species ) and stable nodes ( heavy species ) in the flash cascade model are candidate distillate and bottom products, respectively, from a RD column. Therefore, we focus our attention on those splits that are equivalent to the direct and indirect sharp splits in non-RD. One of the products in these sharp splits will be a pure component, an azeotrope, or a kinetic pinch point the other product will be in material balance with the first. 

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