Catalytic Purification of Natural Gas from Sulfur

Hydrogen sulfide is highly toxic—at concentrations of 0.01% it may cause unconsciousness, and exposure to H₂S concentrations of 0.1% can quickly lead to death. Therefore, special precautions should be taken during the hydrogen sulfide production process. For a long time, deposits of hydrogen sulfide-containing gases were closed down for this reason. Many of them have now been reopened because it was recognized that for sulfur production such gas is a technologically more convenient and cheaper raw material than sulfur ores.

Natural gas can be purified from hydrogen sulfide in two ways:
1) by interaction of H₂S with a reagent or sorbent followed by regeneration of the sorbents and utilization of the separated sulfur compounds
2) by conversion of H₂S to elemental sulfur (S²⁻→ Sᵒ) with catalysts. It can also be purified by high temperature or radiation.

The most common method to recover sulfur from hydrogen sulfide is the Claus process (using an aluminum oxide catalyst for sulfur recovery) which was first implemented in 1980. Up to 20 million tonnes of sulfur are produced worldwide every year by the Claus process. Natural bauxite (ore containing a mixture of aluminum hydroxides) was widely used in catalysts due to its relatively high activity, availability, and low cost. Their shortcomings are fast deactivation and inconsistent content. Spherical aluminum oxide granules or extrudates are now used as catalysts. Al₂O₃ is deactivated by reducing the specific surface area, carburization, and sulfation. The catalyst is regenerated by treatment in Н₂S interacting with sulfate.

It would be more convenient if, during desulfurization, hydrogen sulfide were oxidized to sulfur directly and not to SO₂. Many porous solids could be used as catalysts for hydrogen sulfide oxidation into sulfur. Their activity may decrease over time, pass through a maximum, or even increase. This is due to the formation and deposition of elemental sulfur in the pores which often manifests a higher catalytic activity than the original solid. Catalysts active in the Claus reaction usually are not very useful for selective oxidation of hydrogen sulfide to sulfur. Bauxite and Al₂O₃ have relatively small degrees of conversion of H₂S to sulfur. Upon studying several catalysts—transition metal oxides—Fe₂O₃ promoted by other oxide additives turned out to be the best among them. It is active, selective, and has high mechanical strength.

At temperatures above 1000ᵒС, thermal dissociation of H₂S molecules may occur. With the use of catalysts, the temperature can be reduced to 600-800ᵒС. At lower temperatures, a reverse reaction occurs. To avoid a reverse reaction of H₂S formation, it is necessary to develop an effective rapid method for separating hydrogen from sulfur vapor at high temperatures. Transition metal sulfides can be used as effective catalysts. At high temperatures, mercaptans are also decomposed, forming hydrocarbons and sulfur or hydrocarbons and H₂S. The rate of mercaptan decomposition can be increased in the presence of catalysts, such as Al₂O₃ and zeolites. Sulfur and hydrogen sulfide are found in the reaction products.