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Hydrotreating, formally known as hydrodesulfurization (HDS), is a chemical process used on natural gas and refined oil. The objective of this process is to decrease the amount of sulfur in the oil, increasing the amount of hydrogen in the product. This is done for several reasons, such as lessening the environmental impact when using these petroleum products and to prevent the review units that process the petroleum from becoming poisoned. Most of the sulfur produced annually comes from this process.
When oil or natural gas is first extracted from the Earth, it contains a lot of sulfur. This sulfur, which was not removed until the end of World War II, is harmful to the environment when the petroleum derivative is used. While the environment is the main reason for sulfur reduction, another reason is that less sulfur provides better quality fuel. The smell and color of the fuel is also better when the sulfur is reduced.
Another reason is to help the longevity of the parts involved in fuel processing. When the fuel is processed, it passes through a catalytic reforming unit capable of raising the octane rating of the fuel. Catalytic reforming units are made of noble metals, mainly platinum and rhenium. While both metals are resistant to corrosion, trace amounts of sulfur poison these metals and render them useless for boosting octane levels.
In hydrotreating, hydrogen is introduced into a mixture to increase the total hydrogen count. The oil undergoes what is specifically called hydrogenolysis. This is when the carbon and sulfur bond contained in the oil splits and a hydrogen atom bonds to both the carbon and sulfur atom. Through this process, sulfur can be removed from the oil until it reaches an acceptable level.
After hydrotreating, the remaining sulfur is reduced to sulfuric acid or sulfur by-product, which is similar to its natural elemental state. Most of the sulfur produced and purchased comes from this process, because there is a large amount of sulfur left in oil refineries after hydrotreating is complete. In addition to generating more money for the oil refinery, this means that less sulfur needs to be extracted to adequately supply the market. Sulfur is the main element removed from fuel with hydrotreating, but other elements that are considered undesirable in the oil are also removed. Unsaturated hydrocarbons and nitrogen are taken from the oil supply. This further purifies the fuel and improves its value.