Thermally coupled distillation columns without vapor transfer – Current state and further needs

Abstract

Reducing the energy requirements is one essential pillar in the effort to reduce the carbon footprint of the energy intensive chemical industry. Thermal coupling of distillation columns and the equipment-integrated implementation in dividing wall columns can be considered a mature and established technology to reduce the energy demand of distillation sequences. Despite the widespread investigations and applications, the vaporous transfer stream between thermally linked columns or vapor split in the dividing wall column is associated with certain limitations. Some of these can effectively be overcome by adding an additional column section with reboiler or condenser to the thermally linked column, converting the bidirectional transfer of liquid and vapor to a unidirectional liquid transfer, which mandates additional investment, but provides an additional degree of freedom and retains the energy saving potential of the thermally coupled sequence. Although these so-called Liquid-Only-Transfer (LOT) sequences have been introduced almost 25 years ago, the process concept is receiving an increasing interest in recent years, with specific focus on conceptual design and control studies. The current review provides a methodological overview on synthesis, design and control studies for LOT-sequences, considering zeotropic and azeotropic distillation processes. It analyzes the advantages and challenges of this interesting process concept and highlights research areas and questions that still require additional attention.