On the performance of customized MFI membranes for the separation of n-butane from methane

The separation of higher alkanes from methane is a key aspect for conditioning natural gases and accompanying gases. Against this background, a systematic study on related separation performances of several MFI membranes in dependence on their synthetic origin is presented with this contribution. Basically, MFI slurry was used for the activation of inert Al2O3 supports for heterogeneous crystallization. The resulting MFI seed layer acts both as heterogeneous nucleation side and as flexible distance holder between support and MFI membrane suppressing defect formations during thermal template removal. For the purpose of further optimization, synthesis parameters like temperature, reaction time, pore size of the support and reactant ratios were varied. Characterization of the membranes via permporometry and single gas permeation using methane and n-butane gave first results on the characteristic properties of the membranes obtained. Also, the non isobar performed separations of C4/C1 binary 1:1 mixtures (pfeed = 1 bar, ppermeate = 2 bar) are discussed with respect to membrane synthesis. It was found that either selectivity (M10, α = 11.3; J = 128 l m−2 h−1 bar−1 at 75 °C) or permeation flux (M16, α = 3.6; J = 477 l m−2 h−1 bar−1) optimized membranes can be generated via different synthetic routes. Both archetypes were tested in a simulated cascade like separation experiment for the isolation of n-butane from C4/C1 = 95:5 mixtures. Under elevated feed pressures (pfeed = 2–11 bar, ppermeate = 1 bar) only separation optimized membranes showed a promising applicability due to an increased separation performance also at a lower n-butane content of 6.8%. Increasing feed pressures induced an increase in separation factors from 2.0 to 13.9 at nearly constant permeances of 93–136 l m−2 h−1 bar−1 making customized MFI membranes attractive for technological applications.