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

• Separation of 1:1 and 9:1 H2/N2 mixtures charged with 7–35 ppm H2S.
• Separation efficiency depends strongly on temperature and H2 recovery.
• External mass flow resistance more important than H2S inhibition of membrane.
• Marginal H2S inhibition at 773 K and industrially relevant H2 recovery levels.

Coal gasification products typically contain less than 40% H2 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 9:1 H2/N2 mixtures contaminated with 7–35 ppm H2S to elucidate the relative importance of these transport resistances using a ca. 5 µm thick PdCu membrane supported on a ceramic substrate. Sulfur inhibition depended strongly on H2 recovery (10–80%) and temperature (673–773 K) in the investigated range. Sulfur poisoning of the membrane dominated H2 permeation rates at lower temperatures and in the 9:1 H2/N2 mixture especially. However, its impact declined rapidly with increasing H2 recovery in the 1:1 H2/N2 mixture. As a consequence H2 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 H2 permeation than sulfur inhibition in practical separation situations where very high H2 recovery will be an economic necessity. Exposure to H2S for altogether 75 h had no lasting effect on H2 permeability of the membrane but the N2 leak rate doubled presumably due to sulfide formation at defect sites at lower temperatures.