Natural gas purification with a DDR zeolite membrane; permeation modelling with maxwell-stefan equations

The permeation of CO2, N2 and CH4 and their mixtures through a DDR membrane has been investigated over a wide range of temperatures and pressures. The synthesized DDR membrane exhibits a very high selectivity for CO2 and a moderate selectivity for N2 over CH4 with good permeances. At a total pressure of 101 kPa and temperature of 225 K, the selectivity for CO2 was found to be over 3000 and 40 for N2 in a 50/50 feed mixture with CH4, both decrease with temperature. The N2/CH4 selectivity remains constant with pressure, while that for CO2/CH4 decreases. An engineering model, based on the generalized Maxwell-Stefan equations, has been used to interpret the transport phenomena in the membrane. The diffusivity of these permanent gases is strongly dependent on the loading in the membrane, severely complicating modelling work. A model developed by Reed and Ehrlich [1] could describe this phenomenon well for both the unary as the binary permeances. The feasibility of DDR membranes as applied to CO2 and N2 removal from natural gas is anticipated.