活化扩散和支撑层阻力对MFI沸石膜分离正丁烷和异丁烷的影响

采用Maxwell-Stefan方程描述MFI沸石膜及支撑层内的表面扩散、气相活化MFI扩散、分子扩散和努森扩散,预测正丁烷和异丁烷在MFI沸石膜中单、双组分的扩散特性。预测结果表明,考虑支撑层阻力时,在MFI沸石膜层中,正丁烷和异丁烷活化扩散的活化能分别为(12.7±0.1)kJ/mol和(20.8±0.4)kJ/mol,活化扩散通量均随温度的升高而增大,异丁烷扩散主要受活化扩散控制;支撑层阻力对正丁烷和异丁烷混合物的扩散通量有显著影响;400K以下时,混合物中异丁烷活化扩散通量较小,正丁烷选择性高,有利于分离;400K以上时,异丁烷活化扩散通量增长显著,正丁烷选择性减小。

Activated Gaseous Diffusion and Support Layer Resistance of MFI Zeolite Membrane in Separation of n-Butane and i-Butane

A mass transfer model for predicting diffusion fluxes of single gas——n-butane,i-butane and their binary mixture in a modular MFI zeolite membrane with support layer was developed.Surface diffusion,activated gaseous diffusion and bulk diffusion and Knudsen diffusion of the gases in membrane were described in the light of Maxwell-Stefan theory.Considering the resistance of support layer,activation energies of activated gaseous diffusion of n-butane and i-butane in the membrane were estimated to be(12.7±0.1) kJ/mol and(20.8±0.4) kJ/mol,respectively.The activated gaseous fluxes of both n-butane and i-butane increased with rising of temperature.For i-butane,activated gaseous flux exceeded the corresponding surface diffusion flux,that is,the former is the controlling factor.The support layer resistance was found to inhibit the diffusion flux of the two gases and the mixture,and depended upon its pore structures.For the mixture of the two gases,at temperature below 400 K,due to this competitive adsorption of the two,the activated diffusion flux of i-butane was lower,so selectivity to n-butane became higher which was favorable to separation of two gases.But above 400 K,the activated diffusion flux of i-butane obviously increased,therefore the selectivity to n-butane dropped.