Investigation And Analysis On Purification And Liquefaction Processes For Offshore Associated Natural Gases

With the gradual reduction of large oil and gas fields, the development ofoffshore associated gas has received increasing attention. The floatingproduction, storage and offloading device for LNG （LNG-FPSO） is a newtechnology for the development of offshore associated gases. However, themarine environment is harsh and LNG-FPSO is often operated in the state ofthe irregular shaking caused by the waves, typhoons, and other factors.Furthermore, the space on LNG-FPSO is very small. Therefore, the selectednatural gas processes on LNG-FPSO should be able to adapt to the harshoperating environment, compact and take up less deck area. The traditionalabsorption method encountered a great challenge used on LNG-FPSO for thereason that it employes liquid as absorbent in natural gas purification. Theresearch of adsorption method applied to remove sour gas contained in naturalgas is lack of systemic.As for the lquefication of natural gas, nitrogen gasexpansion refrigeration cycle （N2expansion refrigeration cycle） and mixedrefrigerant cycle are two of classic processes, but as to what kind of process ismore applicable to the floating platform, the experts and designers did notgive comprehensive comparison and analysis. For these reasons, adsorptionpurification set-up is designed and constructed, and detailed experiments arecarried out. The influence of the acid gas content and adsorption pressure onthe adsorbent performance is researched. The adsorption purification processis designed according to the measured dynamic adsorption data and comparedto the traditional absorption method. The conditions of the adsorption methodapplied to the floating platform are determined. As for the liquefaction of natural gas, N2expansion refrigeration cycle and mixed refrigerant cycleincluding propane pre-cooling cycle C3/MRC and no pre-cooling cycle MRCare designed and analyzed according to LNG-FPSO characteristics. Theliquefaction process best suited to the floating platform is determined andthen the process is improved.The main contents of this paper are as follows.（1） With the adsorption test rig, the typical X-type zeolites of13X-PG,13X-HP and APG-II are applied to test the the adsorbent performance indifferent adsorption pressure and CO₂concentrations. The test results revealthat the adsorption performance of adsorbents improves with the increase ofadsorption pressure in a certain extent, but it decreases sharply with theincrease of CO₂content in the gas mixture. The difference of adsorptionperformance of13X-PG,13X-HP and APG-II is large. In contrast, theadsorption performances of13X-HP and APG-II are better than13X-PG andthe adsorption performance of APG-II is a little better than13X-HP.（2） The desorption experiments of13X-PG,13X-HP and APG-II arecarried out also. The results show that desorption time is shortened with theincrease of N2flow under the same heating temperature and desorption time isshortened with the heating temperature increasing at the same N₂flowconditions. The desorption process is more difficult for the molecular sievewith better adsorption performance. Therefore, desorption time is longer andthe N2amount consumed is more.（3） As for the processing capacity of20×10⁴Nm³/d, the adsorptionpurification process is designed according to the dynamic adsorption data of13X-HP and compared to the designed absorption purification process. Theresults show that although the adsorption purification process is simpleroperated and reliable, the renewable heat is much larger and it closes to2000kW when the CO₂content in gas mixture is2%. Therefore, it is notrecommend using adsorption method for the purification of natural gas on theoffshore floating platform when the CO₂content in gas mixture is over2%.（4） The designed N₂expansion refrigeration cycle, C₃/MRC and MRC arecalculated by phase equilibrium equation and then conducted comprehensive comparison and analysis. The results reveal that the energy consumption ofN2expansion refrigeration cycle is relatively higher and the process runningeconomy is slightly worse compared to C₃/MRC and MRC. However, N2expansion refrigeration cycle is the more appropriate liquefaction processapplied to the floating platform for its advantages of better applicability fordifferent offshore gas sources and the small onshore coalbed methane sources,less insensitive to the floating platform, employing refrigerant N₂withoutflammable and no existing the ratio of refrigerant and more ease to start/stopquickly.（5） Compared with C₃/MRC and MRC, the N2expansion refrigerationcycle has a significant competitive applied to the floating platform, but one ofthe most obvious shortcomings of the process is the energy consumption istoo higher. The process is analysed from the point of exergy and the energyutilization efficiency of it is analysed with the concept of exergy efficiency.According to the analysis results, R410A is used to pre-cool the liquefactionprocess. Calculation results show that the energy consumption of the processis reduced significantly with pre-cooling measures and the exergy efficiencyis increased also. Thus further enhance the competitiveness of N₂expansionrefrigeration cycle to be applied for floating platform natural gas liquefaction.