| The conversion of natural gas to liquid fuels(GTL) is an important and effective way to utilize natural gas,which is helpful to reduce the cost of natural gas utilization in the past.However,the accumulation of the inert gases such as nitrogen in the process always decreases the conversion efficiency of syngas and reduces the utilization of energy,and the GTL vent gas is released.The vent gas contains much synthesized light hydrocarbons,if it is flared or used as low grade fuel,its value is depreciated.Therefore,it is imperative to design an efficient process to recover light hydrocarbons from the vent gas.However,no economical method of separating these gases is available in the past.Based on the simulation and optimization of the UniSim Design software,this paper designed a hybrid process combining with the membrane separation technology and the compression-condensation unit to recover light hydrocarbons from GTL vent gas efficiently.To recover light hydrocarbons from 133000Nm3·h-1 GTL vent gas at low cost, three processes,including refrigeration process,HM-VOCM-compression condensation hybrid process and VOCM-HM-compression condensation hybrid process,were proposed,and the optimal parameters were achieved.The condenation temperature of refrigeration process was -70℃.The condensation temperature,total membrane area and the area ratio of the first and second stage membranes of HM-VOCM-compression condensation hybrid process and VOCM-HM-compression condensation hybrid process were -30℃,-30℃;7000m2,11000m2 and 2.0,2.0, respectively.In comparison with other processes,HM-VOCM-compression condensation hybrid process was the optimal process,in which the annual economic profit and output-input ratio were 2.96 hundred million yuan and 1.283.On this basis,the distillation system was built and simulated.The plate numbers of the demethanizer,dethanzier and depentanizer were 36,37 and 34,respectively. The feed stages of them were 18,16 and 14.The reflux ratios of them were 0.55,10 and 1,and the bottom flow rates of them were 194kmol·h-1,170kmol·h-1 and 46kmol·h-1,respectively.Finally,the results of light hydrocarbons recovery were obtained.The outputs of ethane,LPG and light oil were 0.74t·h-1,6.18t·h-1 and 3.66t·h-1,respectively.The recovery ratios of C2H6,C3H8 and C4H10 in LPG,C5+ in light oil were 48.9%,85.3%,88.0%,respectively,which met the requirements of recovering the light hydrocarbons at low cost and high efficiency. Considering the fluctuation of factors in practice,the effects of vent gas change. prices of membrane module and products on the light hydrocarbons recovery were investigated.Firstly,with the increase of pressure,flow rate and C2+ concentration of vent gas,the annual economic profit increased.However,the recovery ratios of light hydrocarbons changed slightly,which meant that the HM-VOCM-compression condensation hybrid process was flexible with the change of the vent gas.Secondly, the annual economic profit increased when the membrane module price decreased and the products price increased.With the development of the membrane science technology and shortage of petroleum oil,HM-VOCM-compression condensation hybrid process would be more and more economical.According to the analysis above, the basis law of the change of the light hydrocarbons recovery ratios and the annual economic profit with these factors were achieved,which could be used to provide certain guidance for the practical production.
|
PDF Full Text Request