Investigation of novel integrated air separation processes, cold energy recovery of liquefied natural gas and carbon dioxide power cycle

Abstract

Cryogenic air separation unit (ASU) consume high amount of energy to produce oxygen and nitrogen with high purity. Liquefied natural gas (LNG) regasification process provides low temperature cryogenic refrigeration source which can be used in ASU with high efficiency. In this study two cryogenic air separation processes for production of high purity nitrogen and oxygen with low energy consumption are proposed and analyzed. The first proposed process is a two-column configuration which power consumption per oxygen production is 16.6% lower, compared to a convectional cryogenic air separation process. In the second one, liquefied natural gas cold energy is used for pre-cooling the feed air. This greatly reduces the energy consumption of the compressors located before the columns by more than 55.6% compared to the first proposed process, without much change in purity of the products. In the second process, energy saving in the air separation unit and power generation cycle is 2715 kW and 17,810 kW respectively. Both processes use an integrated heat exchanger which is both condenser of the high pressure column and reboiler of the low pressure column. With this integration latent heat of the pure nitrogen and pure oxygen can be exchanged in a two-column process configuration. The second proposed process is a hybrid with a carbon dioxide trans-critical power cycle. The liquefied natural gas cold energy is used for cooling the power cycle condenser. Energy and exergy analysis are carried out on the air separation unit and power generation cycle.