Process synthesis distillation sequencing

Samanta A (2001) Modelling and Optimisation for Synthesis of Heat-Integrated Distillation Sequences in the Context of Overall Processes, PhD Thesis, UMIST, UK. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Early work in process synthesis focused on the solution of specific problems, such as the best sequence of distillation columns to perform separation of components in feedstreams into product streams. Another early problem was the synthesis of heat-exchanger networks. 

The first chapter, by Siirola, reviews the impact of process synthesis in industry and shows how process synthesis fits into the innovation process within industrial manufacturing and research. It also highlights a number of industrial successes leading to substantial energy savings and overall cost reductions. Most of these savings are in the areas of distillation sequences, and examples include heat-integrated separation sequences and separation of azeotropic systems. 

Process synthesis heuristics fall into several categories. Many of these were proposed origiually for multicomponent distillation but have since been applied 10 other types of separation. Some heuristics have bean verified through calculation and experimentation while others are described best as common senes, in any case, they serve as effective guidelines in the selection and sequencing of separation processes. 

Heuristics far Multicomponent Distillation Sequences. The origin of meny heuristics can be traced to multicomponent distillation problems and, with the possible exception of heat exchenge network synthesis, most process synthesis study has been in this area. This is not surprising since distillation is the backbone of the chemical industry. Heuristics for multicomponent distillation are summarized in Table 4.2-2 and discussed below. 

Very little process synthesis literature has dealt successfully with the problem of selection between alternative methods of separation. Indeed, most process synthesis literature has dealt with the selection of the flowsheet sequences Tor a single sepamtion method with only simple, sharp separations between components of ndjacent selectivity, without recycle. Furthermore, most of this literature has used only distillation in illustrative epplications. In practice, much use is made of recycle, nonsharp eplits. and complex columns (in distillation) and, of coorse. distillation is not always the separation merited of choice. 

In Chapter 7, the focus is on separation processes, in which the criteria for the selection of separation processes and the choices of equipment are reviewed before systematic methods of process synthesis are covered. The latter begin with sequences of ordinary distillation columns, then general vapor-liquid separation processes, and subsequently sequences that include azeotropic distillation columns. Also covered are considerations in selecting separation systems for gas mixtures and for solid-fluid systems. 

Distillation is the most widespread separation method in chemical process industry. Since the equipment and utility costs are very high the synthesis of distillation columns and distillation sequences is important task. Whereas it is very difficult due to the complexity of structures and equilibrium models. 

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