Mass transfer modelling of H2S in odour treatment technologies

Abstract

The development of more efficient methods of membrane-based gas absorption has recently received considerable interest in two areas: the biological treatment of contaminated gas streams and the abiotic gas absorption of waste gases. The present work refers to a hollow fibre membrane gas absorber operating in non-wetted mode applied for the scrubbing of hydrogen sulphide from air, with pure water as the contact solvent. Two case studies were investigated: (1) H2S absorption within single passage through the membrane and (2) absorption of H2S using recycle. In both cases, countercurrent flow configuration was studied. In all cases under investigation, the test contaminant was passing through the shell side of the membrane module while the liquid absorbent was flowing inside the fibre lumens. The systems studied were pure H2S absorption in water and absorption of H2S from its mixtures in air by water. Measurements were conducted at liquid pH values in the range between 7 and 13. A numerical model describing the absorption process for each case either as physical transport or reactive absorption of species was developed. The chemistry and the kinetics of H2S in alkaline environment have been studied and the experimentally determined kinetic data were used to determine the values of rate parameters. Laminar parabolic velocity profile was assumed to describe the flow in the tube side while Happel’s free surface model was applied to characterise the shell side flow. The model predictions were in good agreement with the experimental observations. The absorption characteristics of the hollow fibre membrane were compared with those of the conventional processes. The system’s performance, based on data available from both pilot plant and full scale applications, was reviewed and the dependence of the overall mass transfer coefficient on the system’s process parameters was investigated. Overall, the performance of the unit was strongly dependent on the gas phase velocity and the initial water pH. Correlation of the mass transfer data in terms of classical dimensionless numbers revealed the hydrodynamic parameter (Gz) to increase as a function of pH indicating a greater importance on the gas phase velocity as the pH increases.