The removal of hydrogen sulfide from gas streams using an aqueous metal sulfate absorbent

The desulfurization of gas streams using aqueous iron(II)sulfate (Fe(II)SO4), zinc sulfate (ZnSO4) and copper sulfate (CuSO4) solutions as washing liquor is studied theoretically and experimentally. The desulfurization is accomplished by a precipitation reaction that occurs when sulfide ions and metal ions are brought into contact with each other. A thermodynamic study has been used to determine a theoretical operating window, with respect to the pH of the scrubbing solution, in which the metal sulfate solution can react with hydrogen sulfide (H2S), but not with carbon dioxide (CO2) from the gas or hydroxide ions from the scrubbing solution. When the absorption is carried out in this window the proposed process should be capable of removing H2S from the gas stream without uptake of CO2 or the formation of metal hydroxides. The pH operating window increases in the order of iron, zinc to copper. Experimental verification showed that the proposed process indeed efficiently removes H2S when an aqueous Fe(II)SO4, ZnSO4 or CuSO4 solution is used as absorbent. However, for an efficient desulfurization the lower pH of the experimental pH operating window using the Fe(II)SO4 or ZnSO4 solution was higher than indicated by thermodynamics. The reason for this must probably be attributed to a reduced precipitation rate at decreasing pH. When a CuSO4 solution is used as washing liquor the solution can efficiently remove H2S over the entire pH range studied (as low as pH = 1.4). In this case only the upper pH boundary of the operating window (that indicates the possible formation of copper hydroxide or copper carbonates) seems to be a relevant limit in practice. The laboratory experiments indicate that the absorption of H2S in a CuSO4 solution, at the experimental conditions tested, is a gas phase mass transfer limited process. This allows a high degree of H2S removal in a relatively compact contactor. In addition to the lab scale experiments the potential of the new desulfurization process has also been successfully demonstrated for an industrial biogas using a pilot scale packed bed reactor operated with a fresh and regenerated CuSO4 solution. This study indicates that the precipitation reaction of metal sulfates with H2S can be used successfully in a (selective) desulfurization process, and that it can be an attractive alternative to the desulfurization methods currently used.