Performance improvement of nitrogen expansion liquefaction process for small-scale LNG plant

• R410a and propane cycles are adopted to improve the nitrogen expansion LNG process.
• Key parameters are optimized to approach the minimum unit energy consumption.
• The unit energy consumption of NEC-R410a is 0.3607 kW h/N m3.
• The unit energy consumption of NEC-Propane is 0.3734 kW h/N m3.
• Thermodynamic analyses are applied to two precooling cycles.

Liquefaction of natural gas is usually a kind of high energy consumption process. Therefore, any performance improvement of the liquefaction process will definitely reduce the energy consumption. Nitrogen expansion liquefaction process is regarded as a suitable process for small-scale LNG plant due to its simplicity, quick startup and convenient maintenance. However, the disadvantage of the process is high-energy consumption. An efficient way to lower its energy consumption is to add a precooling cycle. In this paper, two different precooling cycles including propane precooling cycle and R410a precooling cycle are proposed to the nitrogen expansion liquefaction process to improve the liquefaction process performance. Unit energy consumption as an objective function is optimized in terms of several key operating parameters. Based on the optimization results, the effects of the liquefaction rate and methane recovery rate on the process performance are investigated. The thermodynamic analyses are adopted to the processes as well as the two precooling cycles. Furthermore, the exergy analyses of the main equipment are also presented and discussed. The results show that the unit energy consumption for the nitrogen expansion process with R410a precooling and with propane precooling reduce by 22.74% and 20.02% respectively, compared with nitrogen expansion process without precooling.