| Ethane, an important industrial chemical, is regarded as the most appropriate source to ethylene production, mainly extracted from natural gas and refinery vent gas. But it is hard to produce high purity ethane from natural gas containing low concentration of ethane with the traditional technologies. Therefore, a majority of ethane is burned as fuel along with methane, except for a small portion separated. Gas membrane technology can recover ethane and enrich ethane in permeated flow. Pipeline natural gas has high pressure which will reduce the compressor power consumption of membrane separation process. Then a new process, which combines membrane separation and traditional separation technologies, is designed for ethane recovery from natural gas within high pressure pipeline and built, simulated, and optimized in UniSim Design.The calculation accuracy of membrane separation unit strongly depends on the accuracy of the adopted membrane separation model. Our research group integrated a discrete model of membrane separation into UniSim Design, yielding the results of membrane separation more exactly and rapidly. Average models are commonly applied in the simulation of gas membrane separation process. With the development of membrane technology, both the performance of gas separation membranes and operational parameters substantially vary. Therefore, the accuracy and applicable scope of average models need to again be assessed. The simulation of three typical gas separation systems including H2/N2, CO2/N2, O2/N2 is performed with discrete model and two average models. Moreover, the accuracy of two average models has been investigated at varying conditions, i.e., membrane permeabilities, operating pressures, and concentrations. The simulation results show that the arithmetic average model exhibits a better accuracy in low pressure drop systems, whereas the logarithm average model is more suitable for high pressure drop systems.Three membrane separation processes which have different membrane stages were constructed and simulated applying different pressure ratios and membrane areas. The simulation results show that the membrane area mainly influences the permeation flow, but slightly affect the ethane concentration, and the pressure ratio can act on the concentration of permeated ethane. As the stages of membrane are increased, the effect of ethane concentration of the membrane area becomes weakened, whereas the pressure ratio’s impact gets more evident. Three stages process are distinctly different from one stage process, for example, the ethane concentration of three stages process is three times as much as that for one stage process, besides the significantly dropped permeation flow.Based on characteristics of the permeate, an absorber unit and a distillation unit are chosen for CO2 and CH4 separation, subsequently the VOC membrane-absorber-distillation process is constructed. In the process, DEA solution is used to absorb CO2 in absorber unit and CH4/C2H6 separation is achieved in a cryogenic rectification column. Processes with different membrane stages are calculated in UniSim Design. Moreover the profits and costs of each operating process are contrasted. It is shown that the cost of membrane separation unit rises and the cost of distillation unit declines when the membrane stages increases. The total profit initially climbs up and then declines. In conclusion, the profit of the two stages membrane combined process is maximum when ethane recovery is same. The operating parameters in the two-stage membrane-absorber-distillation combined process are optimized, this process can recover 44.17% ethane from pipeline natural gas. |
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