Hydrodesulfurization, also known as HDS, is a process in which sulfur is removed from crude oil, and oil-derived feedstock 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 seabed. Since sulfur is present in all living things, there is a residue of sulfur in oil, in the form of sulfides and organic sulfur compounds such as thiols and thiophenes. These compounds are regarded as undesirable.
When oil or oil 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 contributor to acid rain. The burning of fossil fuels is the main source of SO2 pollution. Hydrodesulfurization became increasingly important following the passing, in many countries, of new regulations regarding sulfur levels in oil products in order to cut down SO2 emissions. As a result, 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 petroleum feedstock into other products. Sulfur in gasoline also affects catalytic converters in cars. It only requires very low levels to produce this effect, which is known as “poisoning” the catalyst.
In the hydrodesulfurization process, a mixture of the oil-based raw material and hydrogen gas is heated to 300-400°C and pumped under a pressure of up to 130 atmospheres into a hydrodesulfurization reactor. Here, the mixture passes over a catalyst which breaks the sulfur-carbon bonds, allowing the sulfur to react with the hydrogen to form hydrogen sulfide. There are a number of hydrodesulfurization catalysts, but the one most commonly used consists of molybdenum sulfide, which contains cobalt on an aluminum oxide base. The H2S flows out of the reactor, along with excess hydrogen, and into a treatment unit where it is separated out, allowing the hydrogen to be recycled through the process. Several cycles may be required to reduce the sulfur content to the required level.
The hydrogen sulfide produced by 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 recovered in this way is used in the production of sulfuric acid. Although sulfur deposits are still mined, most sulfur production today is from petroleum via the HDS and Claus processes.