Study On Separation Process For GTL Purge Gas

GTL technology is an effective way to solve the world's energy problems and product clean fuels of the 21st century, however, it is still in its infancy and far from perfect: Fischer-Tropsch synthesis in GTL process produces a great quantity of tail gas and CH4, N2 and light hydrocarbon(C2～C5) in tail gas have negative impact on reaction equilibrium and conversion rate of Fischer-Tropsch synthesis, hence most tail gas is delivered into fuel gas pipeline as GTL purge gas; H2 and CO in purge gas account for about 15vol% of syngas and C2+ in it is valuable, the tail gas is converted into CO2 and energy after combustion, which both bring down the GTL technology efficiency and cause a loss to feedstork. Therefore, taking full advantage of tail gas and recoverying GTL purge gas reasonably have become key technical problem for GTL technology.To improve the accuracy of process simulation, the experiment of separating GTL model purge gas using polymide hollow fiber membrane module was carried out firstly. The actual membrane separation factor was obtained based on the experimental results, and the effect of nonideal phenomenon on separation process was studied, which provided HM process simulation with basic data and certain guidance for the practical production.With regard to the GTL technical problem, a new process for comprehensively recoveying GTL purge gas was proposed. Based on the project of comprehensively recoveying 133000Nm3/h GTL purge gas, according to the two separation sequences initially formulated, the corresponding processes, including HM+PSA+VM/CC process and HM+VM/CC+PSA process, were proposed to comprehensively recovery CH4, N2 and C2+ and simulated using UniSim Design to optimize the processes:the membrane area, the H2 concentration of product, the H2 recovery ratio, and the annual economic profit of HM in two processes were both 5.82×104m2,91.2%,85.6% and 1.22 hundred million yuan respectively; in HM+PSA+VM/CC process, the condensation temperature, the total membrane area and the area ratio of the first and the second stage membranes were -17℃,4500 m2 and 1:2 respectively; in HM+VM/CC+PSA process, the condensation temperature and the total membrane area of the single stage membrane process were -40℃and 6000m2 respectively. Considering the separation performance and annual economic benefit, HM+VM/CC+PSA process was the better one. Based on the above optimum process, for the purpose of energy saving, the VM+demethanizer process and two-stage HM process were proposed to reducing demethanizer utilities and recovering H2 in adsorption tail gas of PSA respectively,which were to intensify the separation process for GTL purge gas. In the VM+demethanizer process, the theoretical plate number of demethanizer was 11, the recovery ratio of C2 was 95%, the recovery ratio of C3+ was more than 99% and the utilities decreased as the membrane area of VM increasing, which showed a significant energy saving performance; In the two-stage HM process, the membrane area of two stages were 40000m2 and 5200m2, the H2 concentration of product and the H2 recovery ratio were 90.00% and 70.44% respectively, which showed a good H2 recovery effect, by which the recovery ratio and the annual economic benefit of the whole GTL separation process raised to 95.88% and 1.54 hundred million yuan respectively.