Electrifying distillation − Optimization-based evaluation of internally heat-integrated distillation columns

Improving the energy efficiency of distillation processes is essential for reducing the chemical industry’s substantial energy demand and environmental footprint. The use of mechanical heat pumps with compressors is an important asset in this transformation process, as it not only enables the recovery of heat rejected at low temperature, reducing external energy requirements, but also facilitates the electrification of chemical processes and distillation in specific. The necessary temperature lift dictates the required compression rate for the compressor and is therefore of considerable importance for the applicability of mechanical heat pumps. By operating the rectifying and stripping sections of a column at different pressures and enabling heat exchange between the respective sections, temperature lift and compression ratio can be reduced for the so-called Internally Heat-Integrated Distillation Columns compared to mechanical vapor recompression. In order to enable a quick problem specific evaluation of the possible benefits of this concept we propose two novel superstructure models for optimal design, that allow for heat exchange between stages at the same height or arbitrary stages in the rectifying and stripping section, provided a minimum temperature difference is maintained. The respective optimization problems are solved as a series of successively relaxed mixed-integer nonlinear programming problems in GAMS. An automatic stepwise initialization and optimization strategy provides a computationally efficient approach for the determination of optimized designs.