Carbon molecular sieve membranes supported on non-modified ceramic tubes for hydrogen separation in membrane reactors

Abstract

Supported carbon membranes have been regarded as more competitive than traditional gas separation materials due to the versatile combination of different pyrolyzable polymers and supports which in turn leads to high separation factors and mechanical stability. In order to determine the extent to which supported carbon membranes are more competitive, the transport mechanism of supported carbon membranes was investigated in the range 32–150 °C and 1–2.5 bar. Polyimide (Matrimid 5218) material was coated and pyrolyzed under N2 atmosphere on TiO2-ZrO2 macroporous tubes (Tami) that had not been structurally modified in any way. The supported carbon membrane was studied to determine its permeation for low molecular weight gases such as H2, CH4, CO, N2 and CO2. For these gases, the permeance of the composite supported carbon membranes obtained after pyrolysis at 550 °C increased with inverse square root of molecular weight. The temperature dependence of the permeance was described using an Arrhenius law with the negative activation energies for hydrogen, carbon dioxide and nitrogen providing evidence of a non-activated process. The ideal separation selectivity computed from single gas measurements leads to values slightly lower than the Knudsen because of the influence of viscous flow. The coexistence of more than one transport mechanism in the composite membrane was confirmed. After plugging the possible defects with Polydimethylsiloxane (PDMS), the supported carbon membranes obtained at a pyrolysis temperature of 650 °C showed evidence of a molecular sieving mechanism. This paper shows the separation properties of a crack-free supported carbon membrane obtained using a simple fabrication method that does not require modification of the mesoporous support. The permeance and selectivity values were compared with those of other hydrogen selective materials. Finally, the membranes were applied to methanol steam reforming (MSR).