Sequencing distillation columns

This chapter discusses mixed-integer nonlinear optimization applications in the area of distillation-based separation system synthesis. Section 9.1 introduces the reader to the generic definition of separation system synthesis and provides a classification of the different types of separations based on distillation column sequencing. For further reading on separation system synthesis the reader is referred to the reviews by Westerberg (1985) and Nishida et al. (1981)... 

Freshwater and Zigou23 examined several four- and five-component systems with and wilhont heal integration. They noted that without heat integration a direct sequence was usually best. However, if heat integration were considered, the optimal sequence was often distinctly different. Their work suggests the importance of heat integration in distillation column sequencing. 

Heat integration, multiple feed and withdrawal, partial splits, and medium recoveiy fractions in distillation column sequencing. 

Eirico, M., Tola, G., Rong, B.G. et al. (2009) Energy saving and capital cost evaluation in distillation column sequences with a divided wall column. Chemical Engineering Research and Design, 87, 1649-1657. 

Proios P. and Pistikopoulos E. (2005). Generalized modular framework for the representation and synthesis of complex distillation column sequences. Industrial and Engineering Chemistry Research 44 (13), 4656-4675. 6.1.3, 9.5... 

As this book serves as an introduction, we do not go into more complex examples whose formulation and solution procedure are out of the scope of this work. For those interested in the topic, we recommend specialized literature on systematic process design and optimization [14,15]. Furthermore, the literature is rich in papers on the design of different facilities of industrial interest such as hydrocarbon separation [16], hydrodealkylation (HDA) production process optimization [4], distillation columns sequence design, heat exchanger networks, power plants and IGCC modeling... 

Probably the most common method used for sequence selection for simple distillation columns is heuristic. Many heuristics have been proposed, but they can be summarized by the following four ... 

Heat Integration of Sequences of Simple Distillation Columns... 

This remixing which occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast. 

Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegrated sequences and (2) study... 

Porter, K. E., and Momoh, S. O., Finding the Optimum Sequence of Distillation Columns—An Equation to Replace the Rules of Thumb (Heuristics), Chem. Engg. J., 46 97, 1991. 

Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

Figure 5 illustrates a typical distillation train in a styrene plant. Benzene and toluene by-products are recovered in the overhead of the benzene—toluene column. The bottoms from the benzene—toluene column are distilled in the ethylbenzene recycle column, where the separation of ethylbenzene and styrene is effected. The ethylbenzene, containing up to 3% styrene, is taken overhead and recycled to the dehydrogenation section. The bottoms, which contain styrene, by-products heavier than styrene, polymers, inhibitor, and up to 1000 ppm ethylbenzene, are pumped to the styrene finishing column. The overhead product from this column is purified styrene. The bottoms are further processed in a residue-finishing system to recover additional styrene from the residue, which consists of heavy by-products, polymers, and inhibitor. The residue is used as fuel. The residue-finishing system can be a flash evaporator or a small distillation column. This distillation sequence is used in the Fina-Badger process and the Dow process. 

Minimum Boiling Azeotropes. AH extractive distillations of binary minimum boiling azeotropic mixtures are represented by the residue curve map and column sequence shown in Figure 6b. Typical tray-by-tray composition profiles are shown in Figure 7. 

Historically azeotropic distillation processes were developed on an individual basis using experimentation to guide the design. The use of residue curve maps as a vehicle to explain the behavior of entire sequences of heterogeneous azeotropic distillation columns as weU as the individual columns that make up the sequence provides a unifying framework for design. This process can be appHed rapidly, and produces an exceUent starting point for detailed simulations and experiments. 

Fig. 19. Separation of ethanol and water from an ethanol—water—benzene mixture. Bottoms and are water, B is ethanol, (a) Kubierschky three-column sequence where columns 1, 2, and 3 represent the preconcentration, azeotropic, and entrainer recovery columns, respectively, (b) Material balance lines from the azeotropic and the entrainer recovery columns, A and E, respectively, where represents the overall vapor composition from the azeo-column, 2 1SP Hquid in equiUbrium with overhead vapor composition from the azeo-column, Xj, distillate composition from entrainer...

Fig. 20. Three sets of material balance lines for the Kubierschky three-column sequence where design 1 corresponds to the upper tie-line having Tmin = 8.78 design 2, to the subcooled upper tie-line having = 12.23 and design 3, to the lower tie-line having = 17.31 represents overall decanter composition , the overall feed composition to the azeo-column , the distillate composition from the entrainer recovery column and O, the...

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. 

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