In the liquefied natural gas (LNG) industry, CO2 needs to be removed to 50 ppm to avoid CO2 freeze-out under cryogenic conditions. However, in LNG produced from unconventional natural gas that is rich in ethane, the allowance of CO2 may be much higher due to the much higher CO2 solubility than that in conventional LNG. Considering that CO2 solubility data in liquid CH4 + C2H6 mixtures are still quite lacking, a static solid–liquid equilibrium experimental setup is built to measure the CO2 solubility data. The experiments are carried out at the temperature range of 148–203 K, covering an ethane content of 0–100%. The experimental results are compared with theoretical calculated results based on equations of state and Gibbs–Helmholtz relation. CO2 solubility in the CH4 + C2H6 mixture increases exponentially with increasing temperature. The addition of ethane significantly increases the CO2 solubility in the entire temperature zone, indicating that the purification specification of CO2 in natural gas with high ethane content can be significantly enlarged, and it is easier to achieve carbon removal by cryogenic distillation. Furthermore, the increase in CO2 solubility in the CH4 + C2H6 mixture approximates a logarithmic relationship to the C2H6 content.
Ethane recovery is an important way to increase the economic benefits of unconventional natural gas, such as shale gas. However, current researches did not focus on obtaining high purity and high recovery rate of ethane. In this study, two pure refrigerant cascade natural gas liquefaction processes without (Process 1) or with (Process 2) flash gas recovery are proposed to coproduce high-purity liquefied ethane gas (LEG) and liquefied natural gas (LNG). The proposed processes are combined with advanced distillation methods to reduce the energy consumption through process and energy integration. Aspen HYSYS V11 and genetic algorithm (GA) are used to simulate proposed processes and optimize the refrigeration cycles, respectively. To search the optimal conditions of distillation column, power consumption under different distillation pressure is comparatively analyzed, and the optimal distillation pressure is about 2.7 MPa. According to the optimization results, the specific power consumption of Process 1 is 0.4384 ~0.4584 kWh/Nm 3 (NG), and that of Process 2 is 0.4336 ~0.4503 kWh/Nm 3 (NG) when ethane content is 10% ~40%. Process 2 is preferred for lower power consumption and no material waste.Further study is conducted to research the effect of throttling temperature and ethane content on the system performance of Process 2. In addition, the feasibility exploration of cryogenic compression shows that re-compression of natural gas after distillation is not energy saving for the proposed process.
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