Sulfur inhibition of PdCu membranes in the presence of external mass flow resistance

Coal gasification products typically contain less than 40% H-2 and less than 40 ppm H2S after desulfurization with ZnO beds at operation temperatures around 773 K. Hence, both sulfur poisoning and external mass flow resistance must play a major role in separation of such mixtures with Pd-type membranes. We have investigated the separation of 1:1 and 91 H-2/N-2 mixtures contaminated with 7-35 ppm H2S to elucidate the relative importance of these transport resistances using a ca. 5 mu m thick PdCu membrane supported on a ceramic substrate. Sulfur inhibition depended strongly on H-2 recovery (10-80%) and temperature (673-773 K) in the investigated range. Sulfur poisoning of the membrane dominated H-2 permeation rates at lower temperatures and in the 9:1 H-2/N-2 mixture especially. However, its impact declined rapidly with increasing H-2 recovery in the 1:1 H-2/N-2 mixture. As a consequence H-2 recovery was only slightly reduced from 75% to 70% at 773 K even after adding 35 ppm H2S to that mixture. This demonstrates that concentration polarization is a stronger limitation to H-2 permeation than sulfur inhibition in practical separation situations where very high H-2 recovery will be an economic necessity. Exposure to H2S for altogether 75 h had no lasting effect on H-2 permeability of the membrane but the N-2 leak rate doubled presumably due to sulfide formation at defect sites at lower temperatures. (C) 2015 Elsevier B.V. All rights reserved