Distillation sequencing superstructure

Consider now ways in which the best arrangement of a distillation sequence can be determined more systematically. Given the possibilities for changing the sequence of simple columns or the introduction of prefractionators, side-strippers, side-rectifiers and fully thermally coupled arrangements, the problem is complex with many structural options. The problem can be addressed using the optimization of a superstructure. As discussed in Chapter 1, this approach starts by setting up a grand flowsheet in which all structural features for an optimal solution are embedded. 

Figure 11.22 Superstructure for distillation sequence for a four-product mixture based on task representation. 

HEAT-INTEGRATED DISTILLATION SEQUENCES BASED ON THE OPTIMIZATION OF A SUPERSTRUCTURE... 

In postulating a superstructure of alternatives for the heat-integrated distillation sequencing problem, we simultaneously embed ... 

The superstructure for the illustrative example is shown in Figure 9.2. Note that we have two sequences of distillation columns (i.e., column 1 followed by column 2, and column 3 followed by column 4) since the feed stream has three components. In Figure 9.2, the potential process-process matches and the process-utility matches are indicated. The variables shown in Figure 9.2 are the unknown flow rates for the column sequences (i.e., F, F2,F3, F4) and existence variables for the columns (i.e., yi,y2, y3, Vi) and the process-process matches (i.e., yci,R2, yc2,Ri,yc3,R4, yc, R3)-The temperatures of the condensers and reboilers of each column, as well as the existence variables of the process-utility matches are not shown in Figure 9.2. 

To determine a least total annual cost heat-integrated sequence of distillation columns from the postulated superstructure, we define variables for... 

Another class of separation problem attacked has been that of designing the most effective thermally coupled distillation column arrangement to separate a multicomponent mixture. Sargent and Gaxninibandara (1975) present a general column superstructure which they optimize. Imbedded in the superstructure are all the alternative thermally coupled and ordinary column sequences to be considered. The optimization eliminates those portions of the superstructure which are not economic leaving, hopefully, the optimal substructure. 

A case-based program has developed, which in the case of a new separation problem can help to generate a superstructure and an MINLP model for the design of distillation column or sequences. During retrieval the important design and operational parameters are compared. The method is tested by heptane-toluene example, which has been solved from the problem statement through the retrieval. 

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