Prediction of the early breakthrough of a diluted H₂S and dry gas mixture when treated by Sulfatreat commercial sorbent

Among the available technologies, gas clean up by adsorption on commercial sorbents is largely employed, although the overall process efficiency is often lowered by operation at fuel compositions and process conditions far from the advised specifications. This work reports on the removal mechanism of one of the most technically harmful trace compounds of biogas, hydrogen sulfide (H₂S), when treated at dilute concentrations in a dry gas mixture by the commercial Sulfatreat iron oxide sorbent. A kinetic study is presented, which aims to predict the early breakthrough of H₂S during fixed bed adsorption and, therefore, to provide the fundamental design tools for its integration into a fuel cell power system. On the basis of a simple schematization of the Sulfatreat pellet, the initial rate of adsorption of H₂S was studied by employing a differential reactor set up; the contribution of both the external mass transport and the intrinsic chemical reaction has been investigated by carrying out specific, low conversion, pseudo steady state tests by means of a dedicated adsorption rig. Results from complete saturation of the sorbent material show that the initial breakthrough is controlled by occurrence of a fast reaction (order 0.53) between H₂S and the external surface of the active iron oxide granules, which evolves towards a slower, diffusive mass transport controlled, first order reaction. Modelling work here reported shows that the initial breakthrough of H₂S can be predicted beyond fuel cells typical tolerance levels by a non linear kinetic expression, which takes into account of both reaction mechanisms. - Highlights: • Early breakthrough of H₂S at dilute levels is controlled by a fast reaction. • Depending on sorbent saturation level, a change of rate is caused by diffusive mechanisms. • Numerical prediction of the initial breakthrough was obtained within fuel cells tolerances