Pinch point analysis of heat exchange for liquid nature gas (LNG) cryogenic energy using in air separation unit

Abstract

Due to strong nonlinear variation of supercritical liquid nature gas (LNG) and nitrogen (N2) isobaric heat capacity with temperature and pressure, pinch point analysis is conducted for heat exchange process between LNG and N2 and its effect on the air separation unit cooled by LNG cryogenic energy (CEASU) is studied in the present study. The effect of pinch point value, temperature difference at the cold end and pressure of N2 on mass flow ratio (N2 to LNG), approach temperature difference, energy consumption of CEASU and cryogenic energy utilization rate are analyzed in detail. For the heat exchanger process between LNG and N2, the different value of pinch point have little effect on its location, and increasing the temperature difference at the cold end effectively increases the mass flow ratio and decreases the approach temperature difference, while rising the pressure of N2 leads to a valley value of the mass flow ratio, in contrast to a peak value in the approach temperature difference. For the energy consumption of CEASU, choosing the lower temperature difference at the cold end is good for the energy consumption and equivalent energy consumption per kilogram liquid production, but a large amount of LNG is needed and cryogenic energy could not be fully used; while the influence of pressure of N2 on the energy consumption presents different variation trend. The result shows the energy consumption per kilogram liquid production of proposed CEASU is roughly 5–12% lower than that of convectional processes which the cryogenic energy is only used for cooling the nitrogen or the feed air.