The retrofit design of distillation-based separation systems.

摘要

Changing economic conditions often force the redesign of existing production facilities. In terms of distillation based separation systems, the need for retrofit designs results from the revision of production goals, such as increased demand for throughput capacity and/or tighter purity specifications, or the desire to reduce utility consumption.; Under new production goals, the fixed attributes of a column, such as the cross-sectional area and the number of trays, place constraints on the performance of a separation task. Columns often have attributes that make them appropriate to handle a portion of a separation task either in a series or parallel. A solution strategy for such production goal retrofit problems requires a two phase procedure. In order to be comprehensive, the search must include all possible distillation sequences. The first phase characterizes each column's ability towards performing separation tasks in series and parallel. The second phase uses this information in the formulation of a mixed integer nonlinear program (MINLP). Solution of the MINLP provides the optimal distillation sequence under retrofit conditions. In an alternate approach, individual coverage scenarios are generated and optimized. Solution of an integer linear program (ILP) determines the optimal combination of scenarios.; Heat integration between condensers and reboilers is an effective means to reduce utility consumption. To explore heat integration possibilities with existing columns requires a two step procedure. Once again, to be comprehensive, the search must include all possible sequences. In the first step, a screening procedure identifies all feasible heat matches of interest. In the second step, the solution of a MINLP indicates the optimal heat integrated sequence.; Complex column configurations also have the potential to reduce utility consumption. Insight into complex column heat flow facilitates extension of Underwood's method to columns with side strippers and side enrichers. The same insights show that, even though these configurations are more energy efficient, they require a larger temperature range for operation than analogous simple column sequences. The set of Underwood equations developed for the side stripper and side enricher easily extend to the Petlyuk configuration. Close scrutiny of these equations shows a family of middle component split fractions that give rise to minimum reflux. This analysis is valid for any multicomponent feed mixture with any number of middle components.