New Synthesis Procedure To Find the Optimal Distillation Sequence with Internal and External Heat Integrations

Abstract

Recent developments in energy conservation have sought internal heat integration through side heat exchangers as an attractive energy-efficient alternative to reduce the energy consumption in conventional distillation. However, it is difficult to estimate the effect on the reduction of reboiler duty through internal heat integration without executing rigorous simulations. Thus, the stages used for heat exchange have been selected by trial and error or some heuristics. For a fixed design where heat integration between a rectifying section and a stripping section takes place, current research has derived the optimal structure by trial-and-error or systematic approaches. In this work, the combination of superstructure representation, rigorous simulations, and mathematical programming is effectively used to develop a systematic synthesis procedure, which can derive the optimal distillation process structure including the internal and external heat integration. In the proposed procedure, piece-wise linear functions between the amount of internal heat exchange and the reduction of reboiler and condenser heat duties are derived in advance by executing rigorous simulations. Then, for every pair of rectifying and stripping sections of two columns, the optimal structure with internal heat integration is derived by solving an MILP problem. The cost of side heat exchangers, and the condenser and reboiler duties and areas resulted from internal heat integration are used as parameters in the formulation of a synthesis problem which comprises all the possible distillation sequences. By adopting such a procedure, the overall synthesis problem can also be formulated as an MILP problem, and the optimal process structure can be derived by solving it. The result of a case study to separate a ternary mixture shows the effectiveness of the proposed synthesis method.