Hydrodesulfurization (HDS) removes sulfur from crude oil and petroleum feedstocks to reduce the emission of sulfur dioxide, a pollutant that causes respiratory problems and acid rain. HDS also prevents sulfur from reducing the effectiveness of catalysts and poisoning catalytic converters. The process involves heating a mixture of feedstock and hydrogen gas with a catalyst to break sulfur-carbon bonds and form hydrogen sulfide, which is then converted to elemental sulfur through the Claus process. Most sulfur production today comes from petroleum through the HDS and Claus processes.
Hydrodesulfurization, also known as HDS, is a process in which sulfur is removed from crude oil and petroleum-derived feedstocks for the petrochemical industry by combining it with hydrogen to form hydrogen sulfide (H2S). Crude oil is thought to have formed over millions of years from layers of dead plankton on the sea floor. Since sulfur is present in all living things, there is a residual sulfur in the oil, in the form of sulfides and organic sulfur compounds such as thiols and thiophenes. These compounds are considered undesirable.
When oil or petroleum products containing sulfur compounds are burned, the sulfur is oxidized to sulfur dioxide (SO2), a pollutant that can cause respiratory problems and is a major cause of acid rain. The burning of fossil fuels is the main source of SO2 pollution. Hydrodesulfurization has become increasingly important following the approval, in many countries, of new regulations on the levels of sulfur in petroleum products in order to reduce SO2 emissions. Consequently, oil refineries usually have an HDS unit.
Another problem with sulfur in oil is that it can reduce the effectiveness of the catalysts used to convert the petroleum feedstock into other products. The sulfur in gasoline also affects catalytic converters in automobiles. It requires only very low levels to produce this effect, known as “poisoning” the catalyst.
In the hydrodesulfurization process, a mixture of petroleum-based feedstock and hydrogen gas is heated to 300-400°C and pumped at a pressure of up to 130 atmospheres into a hydrodesulfurization reactor. Here, the mixture passes over a catalyst that breaks the sulfur-carbon bonds, allowing the sulfur to react with hydrogen to form hydrogen sulfide. There are a number of hydrodesulfurization catalysts, but the most commonly used is molybdenum sulfide, which contains cobalt on an aluminum oxide base. The H2S escapes from the reactor, along with the excess hydrogen, and into a treatment unit where it is separated, allowing the hydrogen to be recycled through the process. It may take several cycles to reduce the sulfur content to the required level.
The hydrogen sulfide produced from HDS is converted to elemental sulfur by a procedure known as the Claus process – refineries generally have a Claus unit for this purpose. Much of the sulfur thus recovered is used in the production of sulfuric acid. Although sulfur deposits are still mined, most sulfur production today comes from petroleum through the HDS and Claus processes.
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