Process Research On Low Concentration Gas System LNG

Coal mine gas is a clean unconventional natural gas resource with huge reserves in China. However, most of the gas is low concentration gas (methane content less than 30%). so it is difficult to be effectively utilized. Low concentration gas is often directly discharged into the air, resulting in a waste of energy and increasing the greenhouse effect. Therefore, it is necessary to realize the clean and efficient utilization of low concentration gas.According to the relevant technology of gas for LNG and the characteristics of the low concentration gas from a coal mining in Sichuan, producing LNG with low concentration gas was determined. In gas enrichment and deoxidation pretreatment part, the merits and drawbacks of the membrane separation, adsorption, cryogenic separation method, gas hydrate separation and solvent absorption method of five kinds of gas enrichment technology were discussed and cryogenic separation, catalytic combustion deoxidizing, coke combustion deoxidization, adsorption and deoxidization of four gas deoxidation process were investigated, too. Adsorption method was chosen as the low concentration gas enrichment and deoxidation pretreatment process for its adaptability. In gas liquefaction for LNG part, the advantages and disadvantages of three kinds of refrigeration liquefaction process, which are the cascade refrigeration technology, mixed refrigerant refrigeration technology and nitrogen expansion refrigeration process, were studied. Considering the gas volume, the cost of equipment and process requirements, the nitrogen expansion refrigeration was selected as liquefied gas for refrigeration process.The results of gas enrichment and oxygen extraction (CH4 62.9%, O2 7.26%) were estimated using the equilibrium adsorption model. Based on the pretreatment results, the LNG process of gas liquefaction was simulated by ASPEN HYSYS with the settings--outlet pressure of the compressor was 300kPa, the cold box outlet temperature was-150 degrees Celsius, the distillation column pressure was 190kPa, the content of methane on the top of distillation column was 26%.The simulation results showed that LNG containing 99% methane can be obtained by the process. The methane recovery rate was 79.6% and the liquefaction rate was 81.2%. The low concentration gas for LNG process was realized safety.After analyzing the affecting factors on the rate of methane recovery, the process operating parameters were optimized by reducing pressure and controlling methane concentration of tower top. These methods can guarantee the distillation tower away from explosion range. Meanwhile, methane recovery rate increased to 90.5%, liquefaction rate up to 92.2% and the LNG product with methane content of 99% can be achieved.