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Hydrotreating, formally known as hydrodesulfurization (HDS), is a chemical process used on natural gas and refined petroleum. The aim of this process is to decrease the amount of sulfur in the petroleum by increasing the amount of hydrogen in the product. This is done for a variety of reasons, such as decreasing the environment impact when these petroleum products are used, and to keep the reforming units that process the petroleum from being poisoned. Most of the sulfur produced annually comes from this process.
When petroleum or natural gas is first mined from the Earth, it contains a high amount of sulfur. This sulfur, which until the end of World War II was not removed, is harmful to the environment when the petroleum product is used. While the environment is the main reason for reducing the sulfur, another reason is because a lower sulfur amount provides a higher-quality fuel. The smell and color of the fuel also is better when the sulfur is reduced.
Another reason is to help the longevity of the parts involved in processing the fuel. When fuel is processed, it goes through a catalytic reforming unit that is able to enhance the octane rating of the fuel. Catalytic reforming units are made of noble metals, primarily platinum and rhenium. While both metals are resistant to corrosion, slight amounts of sulfur poison these metals and make them useless in octane enhancement.
In hydrotreating, hydrogen is introduced into a mixture to increase the overall hydrogen count. Petroleum undergoes what is specifically called hydrogenolysis. This is when the carbon and sulfur bond contained within petroleum is split, and a hydrogen atom is linked to both the carbon and the sulfur atom. Through this process, the sulfur can be removed from the petroleum until it reaches an acceptable level.
After hydrotreating, the remaining sulfur is reduced to either sulfuric acid or byproduct sulfur, which is similar to its natural elemental state. Most of the sulfur made and purchased comes from this process, because there is a large supply of sulfur leftover from the petroleum refineries after hydrotreating is finished. Aside from making the petroleum refinery more money, this means less sulfur has to be mined to adequately supply the market. Sulfur is the primary element removed from the fuel with hydrotreating, but there are other elements removed as well that are thought of as unwanted in petroleum. Both unsaturated hydrocarbons and nitrogen are taken out of the petroleum supply. This further purifies the fuel and improves its value.