Recuperative vapor recompression heat pumps in cryogenic air separation processes

Vapor recompression heat pumps are used in distillation processes to reduce energy consumption. For sub-ambient distillation, one of the largest difficulties is to produce liquid reflux since the condensation of highly volatile components requires expensive refrigeration energy. Thermally coupled distillation columns have been commonly applied to produce reflux in cryogenic air separation processes. In order to condense the nitrogen vapor for reflux by the oxygen liquid, all the air feed has been compressed to a pressure considerably above ambient pressure. In cases where the oxygen product does not have to be at elevated pressure, this causes unnecessary compression of the oxygen in the air feed. One such large scale application is oxy-combustion in coal based power plants. A scheme of recuperative vapor recompression heat pumps is developed to substitute the thermally coupled distillation columns, and this scheme is applied in cryogenic air separation processes in this paper. A portion of the vapor nitrogen is compressed and condensed for reflux, thus the mass flow through the compression stages is reduced. The power consumption has been significantly reduced compared to conventional double-column distillation cycles, and further reduced when the principle of distributed reboiling is applied to reduce irreversibilities in distillation columns.