Influence of pH on gas phase controlled mass transfer in a membrane contactor for hydrogen sulphide absorption

Abstract

The influence of pH on mass transfer in a hollow fibre membrane contactor applied to a dilute hydrogen sulphide (H2S) gas stream has been studied. When using sodium hydroxide to control pH, a threshold was established at pH 10 where the reaction between sodium hydroxide and H2S could be described as pseudo-first order with respect to H2S, indicating an excess of reactive hydroxide. For solvent pH above 10, the reactivity counteracted the low H2S solubility constant and negated liquid side resistance thus the process was predominantly gas phase controlled. Whilst the gas side hydrodynamics were firmly within the laminar regime, the experimentally determined overall mass transfer coefficient recorded in this study compared well to previous investigations at higher solvent concentrations. Based on the experimental data, a solvent pH of 11 was deemed most economically attractive. This sodium hydroxide concentration facilitates efficient H2S removal at solvent concentrations several orders of magnitude below those proposed in previous contactor studies. The data therefore illustrates that less concentrated chemical solvents can be used without demanding further membrane area to counterbalance the lower reactivity since [OH−] is already in excess at these solvent concentrations.