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Synthesis of Heat-Integrated Separation Sequences Involving Heat-Pumping

Robin Smith, Megan Jobson, and Sonia Farrokhpanah. Centre for Process Integration, The University of Manchester, Sackville Street, PO Box 88, M60 1QD, Manchester, United Kingdom

The objective of separation sequence synthesis is to design a sequence of separations that recover desired products with minimum operating and capital cost. Determining the optimal sequence of separations, types of separations and associated operating conditions, to separate a feed into the desired products, is a challenging task. Vapour-liquid separations are very energy intensive. Heat integration of the units within the sequence and with other associated process streams can lead to significant savings in operating costs. Simultaneous optimisation of separation configuration, operating conditions and heat integration, should be adopted to capture the trade-offs.

In this work, heat pumping (e.g. heat transfer to the reboiler from the compressed overhead vapour of a distillation column) has been considered in the design of separation sequences. It is illustrated how heat pumping exploits the trade-offs between separation operating conditions and heat integration opportunities. The work builds on previous research on distillation sequence synthesis and design, with and without heat integration. The approach includes simple and complex distillation configurations, various heat pumping configurations, as well as direct heat integration options.

The sequence synthesis problem is formulated as an optimisation problem that is solved using a genetic algorithm (a stochastic optimisation technique). Case studies consider the separation of ethylene cracker products and of other mixtures of light hydrocarbons. These case studies illustrate that the separation sequences generated by this approach can achieve significant reductions in costs, compared to conventional sequence and column configurations.