Crude oil and natural gases that are naturally available from underground invariably contain sulphur in the form of poly-sulphides. The sulphur content in crude oils varies from less than 0.05 to 5.00 % by weight. Oils containing less than 1% by weight of sulphur are generally categorised as sweet and with that of higher than 1% by weight are generally considered as sour. Natural Gas ,also,may contain up to 28% by weight of sulphur compounds. Natural gas may also contain carbon-di-oxide. Both sulphur and carbon di oxide is required to be removed to levels of 4- 10 ppm to make the natural gas sweet. Crude Oils by itself has limited applications. It needs to be processed in oil refineries value added products like motor spirit (MS), diesel, fuel oils and gas etc. MS, diesel and fuel gas find maximum applications in the transportation sector. Sulphur, if not removed from crude, crude products, fuel oils and gases releases So_x   to the atmosphere which significantly contributes to pollution. It is a greenhouse gas and contributes towards to global warming. It causes acid rain which destroys plants, agriculture and foliage. It also adversely human health. Since the advent of and gas processing technologies towards the early and middle parts of the last century, the emphasis was more on production of crude derivatives and low emphasis was given to reduction of pollutants like NO_x and SO_X CO₂ CO etc. Towards the end of the last century, environment emission standards came into force and refineries were forced to adopt technologies for reduction of SO_X, NO_X and other pollutants. This created an opportunity to extract sulphur from crude and gas derivates. It was a WIN-WIN situation as refiners got an additional product to supply through sulphur extraction process and at the same time it helped reduce pollution. International Maritime Organization (IMO), on 1st Jan 2020, introduced new regulations mandating the use of max 0.50% sulphur in fuel oils to be used for the marine industry. This has sent ripples around the world.
More such regulations are under way in other areas as well.
Sulphur is an extremely useful element. Its largest application is for the manufacturing of fertilizers with other principal uses including rubber industries, sulphuric acid manufacturing industries, pharmaceutical industries, cosmetic industries etc. Sulphur is present in many raw industrial gases and in natural gas in the form of H₂S. The toxic H₂S fumes that characterize many gas processing, refinery operations and petroleum production sites represent a genuine threat to our environment. Concentration level of H₂S varies significantly depending upon their source. H₂S produced from absorption processes such as amine treating of natural gas or refinery off gases can contain 50-75% H₂S by volume or higher. Many other processes can produce H₂S with only ppm concentration but in quantities which preclude the gases from being vented without treatment.
The large variation in concentration and flow requires different methods of H₂S removal. For relatively small quantities of H₂S, scavenger processes are used. For Sulphur production up to approximately five tons per day, liquid reduction– oxidation (REDOX) processes are common. The Claus process is the most widely used process for conversion of H₂S to elementary Sulphur.