Performance analyses on four configurations of natural gas liquefaction process operating with mixed refrigerants and a rectifying column

Abstract

Natural gas (NG) fields of low pressure and small reserve are usually located remotely and dispersedly in China, for which pipeline transport in gaseous state is not economic. As a result, liquefying natural gas with small-scale liquefiers is a more preferable choice. In this paper, four configurations of natural gas liquefaction process (NGLP) operating with mixed refrigerants (MRs) and a rectifying column were proposed. The mixture was composed of R728, R50, R1150, R290 and R600a. In the rectifying column, the volatile and less volatile components of the MR were better separated, and the entrained lubricating oil of the compressor was trapped, thus avoiding blockage in the low temperature section. Simulations were conducted for these configurations using modified PT equation and Virial mixing rule in vapor-liquid equilibrium (VLE) calculation of MR. The influences of operating pressures, refrigerant composition and process configuration on system performance indicators, such as specific power consumption (SPC) and discharge temperature, were detailly investigated. It was found by simulation that SPC first decreases and then increases with the decrease of suction pressure in all configurations, while the minimum is achieved at around 250 kPa. The simulation results also suggested keeping the molar fraction of R600a the highest among the components, and increasing the molar fractions of R50 and R728 as much as possible provided that the compressor discharge temperature does not exceed the limit. Preliminary experiments have been conducted to compare the performances of four configurations under specified MR composition and NG flow rate.