Experimental Study On Hydrogen Sulfide Removal By Reactive Adsorption On Sulfatreat Sorbent

EXPERIMENTAL STUDY ON HYDROGEN SULFIDE REMOVAL BY REACTIVE ADSORPTION ON SULFATREAT SORBENT

P. Pagliai and R. Di Felice

University of Genova, DICCA, via Opera Pia 15, 16145 Genoa, Italy;

tel. +39 0103532594, e-mail: paolo.pagliai@unige.it;

Passive adsorption on a porous bed of selected sorbents (e.g. activated carbons, zeolites) proved to be a cost effective technology in a wide range of industrial applications, which require the removal or separation of a few substances from a gas stream. However, in the energy production, the use of biogas derived from anaerobic digestion (AD) or collected from landfill (LFG) requires the sorbent materials to operate at conditions far from the ideal saturation capacity, thus raising the overall cost of the clean up process.

 

In fact, LFG and ADG contain a variety of trace compounds: more than 140 substances have been identified so far, which may account for about 0.15 % vol. (2000 mg/m³) of the total amount. Such trace compounds may damage or hamper the normal operation of the power unit and therefore need to be removed down to tolerance levels.

Among these, Sulphur compounds, and particularly H₂S, can reach concentrations as high as 1000 ppm_v, which are several order of magnitude higher than the limit values (i.e. 0.1 ppm_v H₂S for Solid Oxide Fuel Cells) required to avoid catalyst poisoning or equipment failure.  Because of the substantial differences among biogas contaminants and their concentrations, it's very difficult to establish a standard method for biogas clean-up. Moreover, in sulphur removal by passive adsorption, the efficiency of the process is often lowered by the mechanism of competitive adsorption, which takes place between H₂S and other pollutants, water and carbon dioxide.

A commercially available porous sorbent, namely Sulfatreat HP 410, is claimed to be a particularly efficient (about 20% wt.) material for sulphur removal: sulphur compounds (H₂S, Mercaptans) are removed by reaction with a fine mixture of solids composed by iron oxides and noble metals and supported by a coarse size inert material.

However, as reported in a few experimental studies, the investigation on the removal efficiency by Sulfatreat at different operative conditions has been only partially fulfilled: particularly, the reproducibility of breakthrough experimental curves and the relevant modelling of the reactive process is not trivial because of the wide particle size distribution and packing of the tested material.

The objective of the experimental study here presented is to ascertain whether the reactive adsorption of H₂S can be described by considering two mechanisms, namely: the external mass transport of sulphur towards the particle surface and the irreversible reaction with the active superficial layer of the sorbent. Different flow conditions have been tested over a differential porous bed of sieved Sulfatreat particles in order to determine the relevant contribution of the external mass transfer. Experimental data are compared with theoretical data obtained by coupling mass balance and mass transport equations; the dependence of the reaction rate on H₂S concentration and the presence of humidity in the gas stream is also reported.