Design and Optimization of Pressurized Liquefaction Processes for Offshore Natural Gas Using Two-Stage Cascade Refrigeration Cycles

Because of increasing interests in offshore natural gas and due to limited deck space, simpler liquefied natural gas processes are necessary. The aim of this paper is to develop three pressurized liquefaction processes using two-stage cascade refrigeration cycles instead of the conventional three-stage refrigeration cycles. The proposed processes are CH4C2H6, CH4C2H4, and C2H4C3H8 processes. Taking the specific energy consumption as the objective function, simulation based optimization is conducted for the three novel processes as well as two conventional cascade processes by a sequential search method. Optimization results show that the C2H4C3H8 process is the most efficient one with a specific energy consumption of 0.2089 kWh/Nm(3), 22% less than that of the conventional CH4C2H4-C3H8 process. Moreover, a detailed thermodynamic analysis is Carried out for the five processes. The thermodynamic analysis results confirm that the C2H4C3H8 process presents the best composite curve match and the highest coefficient of performance. The CH4C2H6 process requires the smallest heat transfer area, 70% less than that of the conventional CH4C2H6C3H8 process. The CH4C2H4 process uses the least amount of key equipment, 35% less than that of the conventional CH4C2H4C3H8 process. In addition, an exergy analysis is performed for all processes and the results indicate that further improvements are requested the most in valves and heat exchangers, respectively, for the conventional and novel processes.