Efficient cryogenic nitrogen generators : An exergy analysis

Abstract Through exergy analysis, inefficiencies were identified in the distillation system for an efficient cryogenic air separation plant producing large-tonnage quantities of nitrogen. Both exergy losses and carefully chosen exergy efficiencies were used for analysing this process. An equation for the calculation of the overall efficiency of a distillation column has been extended to calculate section efficiencies of the column. Its use along with an overall efficiency of a distillation column can help in quantifying the efficiency of various sections within a distillation column. Two solutions using two vaporizer/condensers in the bottom section of the low pressure (LP) column were suggested, which reduce the exergy loss of the cryogenic part of the plant by 8–9.5%. It was found that, when a limited number of vaporizer/condensers are used in a stripper, depending on the product need, it can be more desirable to condense streams of the same composition but at different pressures in more than one vaporizer/condenser. A process in which nitrogen is condensed in both the vaporizer/condensers located in the bottom section of the LP column yielded the lowest exergy losses for the distillation system.