非平衡分子动力学在炭膜气体分离CH_4/CO_2中的模拟研究

Nonequilibrium molecular dynamics simulation on gas separation of CH_4/CO_2 in carbon membrane

通过建立单孔狭缝模型,采用非平衡分子动力学方法研究CH_4/CO_2二元气体混合物通过管状炭膜的传递和分离特性,考察了系统温度、气体组成和膜孔径对通量的影响。模拟过程中将膜低势,区压力取为更符合实际情况的非零值(大于零)。研究结果表明,在温度为20℃～160℃范围内,随温度升高,CH_4的通量增加,CO_2的通量则先降低后升高(80℃出现最低值);随混合气中CH_4组成的增加,CO_2通量下降、CH_4通量升高;随膜孔径增大,CH_4通量先增后减(9.77 (?)时出现最大值)、CO_2通量则呈下降趋势。以上模拟结果与实验数据相比较,吻合良好。在此基础上,本文还考察了跨膜压差对过程的影响,发现CH_4和CO_2的通量均随跨膜压差的增大而增大,膜的分离性能则随之降低。本研究结果充分表明,所建模型能够正确地描述CH_4/CO_2气体混合物的炭膜分离过程。

A nonequilibrium molecular dynamics simulation （NEMD） was carried out to investigate transport and separation of a binary mixture of CH4 and CO2 in a tubular carbon membrane using a slit pore model determined in the presence of an external chemical potential. A non-zero pressure on the side of lower chemical potential control volume was adopted during simulation, which is more practical than the zero pressure used in the previous studies. The effects of system temperature, gas composition and membrane pore size on the flux were examined. Results show that with increase in the system temperature, the flux of CH4 increases monotonically while the flux of CO2 first decreases and then increases; with increase in the CH4 mole fraction, the flux of CH4 increases while the flux of CO2 decreases; with increase in the pore size, the flux of CH4 first increases and then decreases gradually while the flux of CO2 decreases. Comparisons were made between simulation results and existed experimental data, and good agreements were achieved. Based on the results obtained, the effect of pressure on separation process was also examined, which was found that the fluxes of both CH4 and CO2 increase all with increase in the trans-membrane pressure difference, but the separation factor declines. The present study indicates that the model determined can describe gas separation of CH4 and CO2 accurately in carbon membrane.