The Evolution, Chemistry and Applications of Homogeneous Liquid Redox Sulfur Recovery Techniques

Hydrogen sulfide is a highly undesirable component of natural gas and other gaseous streams. Its removal from sour natural gas streams is necessary for health and safety reasons and to prevent corrosion of equipment during its transmission and distribution. Herein an overview of desulfurization processes mainly concerning liquid redox sulfur recovery (LRSR) techniques is given. Three successful commercial applications of LRSR processes are the Stretford process, the LO-CAT process and the SulFerox process. The first using V(V)/V(IV) redox couple has dominated the 1980's, but now stopped spreading because of the chemical degradation and environmental problems with vanadium. The iron based processes employing Fe(III)/Fe(II) chelate systems, particularly the Fe(III)/Fe(II)-NTA system, is gradually replacing V(V)/V(IV) systems. However, the degradation of the chelating ligand in Fe(III)/Fe(II)-NTA system results in high chemical costs, which obviously obstructs the use of this system in large installations for the removal of H2S by homogeneous catalytic oxidation. Although the rate of degradation can be slowed considerably by addition of Na2S2O3, which is known as an effective scavenger for the hydroxyl radical, it seems that the problem involving the degradation of NTA in-process is not resolved completely yet. The serious deficiency calls for research or selection of robust ligands. Therefore, Processes using of non-degrading ligands or regeneration facilitated by microorganism, either with or without organic ligand hake appeared, some of them have been applied on a pilot scale. Other processes, Such as the biological treatment without an auxiliary redox couple and the combined sulfur/hydrogen recovery, are now being developed.