Optimization In Membrane-Expender Hybrid Condensation Process To Recover Light Hydrocarbons

As an important process in the comprehensive utilization of natural gas, the recovery of light hydrocarbons helps to make the components of the gas used efficiently and meanwhile to recover the pressure energy from the gas, hence is meaningful against the energy crisis at present.However, there is still a problem:shallow cooling is widely used in most traditional recovery processes of light hydrocarbons, and the residual gas, pressure-relived, is pumped into the pipes to be burned as fuel. However, actually the vent gas still has a high concentration of light hydrocarbons and a high pressure; directly burning the vent gas is a waste of resources and energy. So a further recovery of light hydrocarbons from vent gas is necessary. We proposed a Membrane -Expender Hybrid condensation technology based on shallow cooling to resolve this problem. The process was simulated and optimized with UniSim Design software to achieve the purposes of improving the utilization of resources and the energy, and getting a higher profit.In this work we studied the process of a large oilfield, and the flow rate of the associated gas was 40000 Nm3·d-1 in winter and 66000Nm3·d-1 in summer. Based on the existing shallow cooling hydrocarbons recovery process, we proposed the process of Membrane-Expender Hybrid single stage condensation technology and Membrane-Expender Hybrid secondary condensation technology to further recover the light hydrocarbons and energy from the vent gas. In Membrane-Expender Hybrid single stage condensation process,-30℃was chosen as condensation temperature and the membrane area was 80m2 in winter and 140m2 in summer, the expander had an efficiency of 75.66%. With the operating conditions above, a profit of 3115.5Wanyuan/a was obtained, and the light hydrocarbons in vent gas were reduced from 6.77v% to 2.87v% in winter and 6.41v% to 2.80v% in summer respectively, and the energy recovered was 66.6kW/h. In Membrane-Expender Hybrid secondary condensation process-12℃and -30℃were chosen as condensation temperature respectively, and the membrane area was 80m2 in winter and 140m2 in summer, the expander had an efficiency of 75.66%. With the operating conditions above, a profit of 3131.2Wanyuan/a was gained, and the light hydrocarbons in vent gas were reduced from 6.77v% to 2.85v% in winter and 6.41v% to 2.83v% in summer respectively, and the energy recovered was 65.4kW/h. As for the safety aspect, the explosion limits of the three processes were caltulated and compared. The results showed that reformation methods had changed the explosion limits slightly, thus generally had no effect on safety production. Comprehensively considering the profit, safety and the difficulty degree of reformation, Membrane-Expender Hybrid single stage condensation technology is the best.Considering the reduction of the associated gas as the exploitation goes on and the seasonal fluctuations of the flow rate and the composition of the gas, we took a simulation under these conditions. The results showed that: on those'conditions the new method still had a higher profit than the original process. Additionally, during the seasonal variation process, we should also adjust the membrane area to maximize the profit.