Synthesis of general distillation sequences—nonsharp separations

Abstract

The synthesis problem of distillation sequences that separate a given multicomponent feed stream into several desired multicomponent product streams, while allowing nonsharp separations in the columns, is addressed. A superstructure is proposed that contains options for distribution of the light and heavy key components, all possible sequences and all alternatives for stream splitting, bypassing and mixing. Light and heavy key component recoveries define the nonsharpness of separation and are treated explicitly as optimization variables. This superstructure is modeled as a mixed-integer nonlinear programming (MINLP) formulation whose objective is to minimize the total annual cost. Shortcut simulations and regression analysis are used to develop cost functions to be used in this formulation. The solution of the MINLP formulation results in an optimal separation sequence that may involve sharp and/or nonsharp distillation columns. The nature of the proposed mathematical formulation is investigated and the search for a global optimum solution is performed using a new decomposition approach. The proposed approach, which is implemented automatically in the procedure NOUS (NOt Universally Sharp separation), is illustrated with six example problems and the results show that using nonsharp separations can result in savings of 10–30% and even 70% in some cases (see Example 2) as compared to sharp sequences. It is also illustrated that columns with nonadjacent key components can be included easily by extending the proposed approach.