Design and optimization of natural gas liquefaction process by utilizing gas pipeline pressure energy

Transmission pipelines carrying natural gas across long distances usually work at high pressures (up to 10 MPa). At city gate stations the gas pressure is regulated through a set of pressure reducing processes, which are normally realized by a throttling device and waste much pressure energy. In this paper, a novel natural gas expansion liquefaction process is designed and simulated by Aspen HYSYS to utilize the pressure energy. The unit energy consumption and liquefaction rate are selected as the objective functions and are optimized with several key variables of the process. The available specific work of the gas pipelines at different pressures is also calculated and discussed. Furthermore, the exergy losses of different equipment in the process are evaluated and analyzed in details. The results show that although the liquefaction rate of this process is much lower than that of normal liquefaction processes, the novel natural gas expansion liquefaction process is suitable to utilize the gas pipeline pressure due to its low unit energy consumption, simplicity and flexibility.