Additionally, the iron sulfide can cause upsets in treating systems and plugging in disposal wells. Aside from its corrosive nature, H2S is also a very toxic and very flammable gas. At low levels, H2S has a "rotten egg" smell. At levels of 100 ppm, H2S will paralyze the olfactory system, making it appear odorless. At levels above 700 ppm, H2S can kill instantly. To add to the threat this creates, H2S is heavier than air, allowing it to creep along the surface where it becomes a potentially life threatening, explosive hazard. Therefore it is extremely important to extract this compound from refineries for safety of humans and expensive machinery.
However, Hydrogen Sulphide also has some significant uses. The most important industrial use of hydrogen sulfide is as a source of about 25% of the world production of elemental sulphur. The manufacturing process is based on burning about 1/3 of hydrogen sulfide to sulphur dioxide, then letting the resulting SO2 react with H2S. Other uses are in metallurgy for the preparation of metallic sulfides. It also finds use in preparation of phosphors and oil additives, in separation of metals, removal of metallic impurities, and in organic chemical synthesis. Hydrogen sulfide is also used in nuclear engineering, in the Girdler Sulfide process of manufacturing heavy water.
The primary source of H2S is the Desulfovibrio sulfide reducing bacteria (SRB). SRBs reduce naturally occurring sulfate found in oilfield waters to hydrogen sulfide, which in turn reacts with iron to form iron sulfide. They are especially efficient in low-oxygen environments, such as in swamps and standing waters. Some other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids. Hydrogen Sulphide can be created anywhere where sulphur comes in contact with organic material at high temperatures.
Processes for Hydrogen Sulphide recovery
The most obvious method to avoid creation of Hydrogen Sulphide is to eliminate the sulphate from water prior to its injection. This can be done by using a nanofiltration membrane which removes all particles greater than one one-thousands of a micron resulting in high quality injection water free of silica and bacterial materials thereby insuring continued injection rates reflective of initial reservoir conditions.This process is very cost effective because it reduces the cost related to sour gas and oil treatment or dedicated "sour safe" pipelines and allows use of less costly metallurgy for the operation due to reduced stress cracking and corrosion. It also reduces the potential for necessary addition equipment on a platform with limited space and weight capacities.
Another method for removing Hydrogen Sulphide is by caustic washing. Caustic treating (sweetening) removes all the hydrogen sulphide and convert most of the mercaptans to disulphides. Chlorine is also used to control odor and for H2S control. Continuous chlorination is a widely used and effective method for oxidizing hydrogen sulfide, especially if the water pH is 6.0-8.0. Chlorine is usually administered as sodium hypochlorite, which reacts with sulfide, hydrogen sulfide, and bisulfates to form compounds that do not cause foul taste or odors. Other oxidizing agents besides chlorine can be added to the water to oxidize H2S. These include hydrogen peroxide, potassium