Optimization And Analysis Of Floating LNG’s Pretreatment And Liquefaction Process

With the deepening research of offshore natural gas’ development and utilization, floating LNG energy is becoming a new hotspot of future energy field, and the pretreatment and the liquefaction processes are the key links of the floating LNG. Offshore natural gas’ development and utilization are different from onshore because it’s floating production platform has a high requirement on the device of its compactness and security, so it needs to choose natural gas pretreatment and liquefaction processes meeting floating LNG.For this purpose,the paper has done the following researches as follows:(1) Introduced the methods of removing various impurities in raw natural gas, then chose the pressure swing adsorption method as the main method of the natural gas pretreatment, and summarized the present research status of pressure swing adsorption methods.(2) Through comparing the liquefaction cycle methods, chose the expansion liquefaction process as the main method of nature gas liquefaction, and make an introduction to it.(3) Using the Aspen Adsorption software to simulate the pressure swing adsorption process, the adsorption bed of the pressure swing adsorption was divided into two layers. The first layer uses activated carbon as the adsorbent of CO2 impurities, while the second layer uses molecular sieve as the adsorbent of N2 impurities. Finally the results show the concentration of CH4 product gas was 98.9%, and the recovery rate can reach 91%, as well as 99.77% volume fraction of CO2 and 98.8% of N2 can be gotten.(4) Designed a new expansion type of natural gas liquefaction process with the CO2 as the precoolant and air as the refrigerant, and using the HYSYS software to simulate the process, the liquefaction rate of the process can reach 91%,and the energy consumption per unit product is 0.85 k Wh·m-3. Also these processes are suitable for offshore operations.(5) Simulated the traditional two levels of nitrogen expander cycle liquefaction process and the propane precooling nitrogen expander liquefaction process.Comparing them with the new expansion type circulating liquefaction process for studying,the results shows that the new liquefaction process is more suitable for the process of offshore natural gas liquefaction for its device compactness, security, and efficiency which are more superior to the other two processes.(6) Optimized the parameters of the liquefaction process with the conclusions are as follows: the energy consumption of the device was low when the pressure of refrigerant before expansion is of 3~3.3Mpa; the lower of the raw gas initial temperature, the lower of the energy consumption in the process of circulation; seawater for the raw gas cooling can be used; when the temperature of CO2 after throttling is at-43~-48℃, the dry ice will never form; Taking the whole process into consideration, when the LNG storage pressure remains 0.2 Mpa, it will be more appropriate.