Hydrogen Production From Syngas Containing H2S with PSS Supported Pd-Membranes

The IGCC process incorporating a palladium-based catalytic membrane reactor (CMR) or membrane separator represents an alternative method of fossil fuel use which lends itself well to carbon capture and sequestration. For Pd-membranes to be utilized in the IGCC process, at least some degree of tolerance to H₂S is necessary. It is known that pure Pd-membranes cannot tolerate more than 2.8 ppm of H₂S at 400ºC, but that Pd/Au and Pd/Cu membranes can. The objective of this study was thus to develop a better understanding of the H₂ permeation behavior of Pd and Pd/Au membranes in the presence syngas and under WGS reaction conditions, with up to 20 ppm of H₂S present.

Thin (3 – 5 μm) Pd-membranes were prepared by electroless deposition techniques onto air oxidized and alumina graded PSS-supports. Some membranes were plated with an Au-layer so that the surface would contain 10 – 30wt% Au. A pure Pd membrane and a Pd/Au (10 wt% Au) membrane have been tested under shifted syngas (50% H₂, 30% CO₂, 19% H₂O, and 1% CO, with 0 – 20 ppm H₂S) at 400°C and 14.0 atm. A different Pd membrane was loaded into the WGS CMR and the system was fed with syngas (18% H₂, 8% CO₂, 55% H₂O, and 19% CO, with 0 - 2 ppm H₂S) at 400°C, 14.0 atm, and 2700 h^-1 feed rate for 330 hours.

The Pd/Au membrane tested under shifted syngas achieved a stable H₂ flux of 7.3 m³/m²-h with 20 ppm H₂S in the feed, down from 13.9 m³/m²-h when no H₂S was present, indicating considerable tolerance to H₂S. The majority of the loss in flux occurred due to the addition of 1 – 5 ppm H₂S, showing saturation behavior. For the WGS CMR incorporating a Pd membrane, the CO conversion and H₂ recovery of 98 and 86% respectively were only moderately reduced to 96 and 78% when 2 ppm H₂S was added to the feed, highlighting the overall system stability. With 2 ppm H₂S present in the syngas feed, the system remained stable for over 230 hours. The Pd₄S phase was not detected on the Pd membrane surface (by XRD analysis) which was surprising considering that the H₂S/H₂ ratio of 0.000023 in the retentate was in significant excess of the Pd₄S formation threshold (0.0000025 mol_H2S/mol_H2 at 400°C).