Energy optimization of cascade refrigeration systems for Liquefied Natural Gas (LNG) plant

Liquefying natural gas consumes high amounts of energy as its need to be cooled from ambient temperature down to ‐161degreesC. One of the key energy consumers in the liquefaction process is the refrigeration section. In this study, LNG cascade refrigeration process was modelled using Aspen Hysys for a five million tonne per annum production plant. Cascade refrigeration systems consists of three refrigeration cycles viz. propane, ethylene and methane. Appropriate load is desirable for each cycle to obtain an efficient process operation. In this paper, different evaporator pressure levels applied for propane refrigeration were studied, optimized and evaluated to obtain an optimum power balance among the other refrigeration cycles. The simulation results shows that case 1 with 5.50 bar, 3.00 bar and 1.32 bar are the optimized pressure level for high pressure, medium pressure and low pressure propane evaporator respectively. The case study indicated that the highest coefficient of performance (COP) of propane refrigerant cycle achieved is 1.66. The results indicate that the higher the COP, the more efficient is the refrigeration cycle.