Study On The Recovery Of Hydrogen From Vent Gas Of Ammonia Synthesis And Regulation Of Hydrogen-nitrogen Ratio By Membrane Technology

Synthesis ammonia industry is an important basic chemical industry and one of pillar industry in china. Two streams of vent gas with about 50mol%-60mol%hydrogen are released in the production, namely purge gas and tank gas. In order to recover hydrogen from purge gas, many recovery technologies, for example, the cryogenic and membrane separation technology are used in the ammonia plant, and better economic profit has been obtained.Because of the low flowrate and pressure, tank gas usually is discharged into the fuel gas network as fuel after ammonia recovery in most of plants. In the cryogenic hydrogen recovery process, the concentration of hydrogen depends on condensation temperature and insulation effect of cold-box, while the purge gas can only be used as fuel when operation condition can not meet the requirement. To resolve the problems metioned above, a membrane-cryogenic hybrid process based on the cryogenic process was proposed and simulated by the Unisim Design.Based on current data of the hydrogen-rich vent gas in a Kellogg ammonia plant, the Unisim Design model of membrane-cryogenic process was established in this study. First of all, effects of membrane area on composition of feed of cold-box, concentration and recovery ratio of high-pressure hydrogen product were analyzed. Secondly, influences of composition of feed of cold-box and condensation pressure on throttling temperature change, heat exchange of exchanger and low-pressure hydrogen were analyzed. Finally, for the practical device, checking calculation of heat exchanger was carried out to determine whether it can meet the requirement; effect of membrane area and condensation temperature on total hydrogen recovery ratio was analyzed too. For the membrane-cryogenic process, the optimum membrane area, condensation temperature, total hydrogen recovery ratio and annual economic profit were 650m2,-189℃,97.64%and 3.133 million Yuan, respectively.When the hydrogen product is recoveryed, the hydrogen-nitrogen ratio of fresh synthesis gas increased. To eliminate this influence, the hydrogen-nitrogen ratio can be set to 2.85 by adjust flowrate of process air. However, the simulation result showed that:the yield of hydrogen in the synthesis gas decreased when the hydrogen-nitrogen ratio was set to 2.85. In order to increase the yield of hydrogen, a membrane process for nitrogen-rich process air was designed. In the designed process,20%compressed air can be used as feed of the membrane to produce the nitrogen-rich air in the residual side. Accordingly, the nitrogen-rich air mixed with the remaining compressed air is used as new nitrogen-rich process air for the secondary reformer. Compared to the process with the pure air, the economic profit of nitrogen-rich air process increased 0.806 million Yuan per year.