Hydrodesulphurization (HDS) is a catalytic process typically used in the Oil & Gas and Refining industries to remove sulfur from natural gas and refined products, including gasoline, jet fuel, diesel fuel, fuel oil, and naphtha. Sulfur removal is critical in order to reduce the sulfur dioxide emissions from the combustion of those fuels in automobiles, aircraft, locomotives, ships, power plants, residential or industrial furnaces, etc.
Sulphur removal from naphtha is also important in a refinery as it can poison, or render ineffective, the noble metal catalysts, such as platinum and rhenium, used in the catalytic reforming units necessary to upgrade the naphtha octane ratings, even in very minute concentrations.
Hydrogenation is a class of chemical reactions which result in the addition of hydrogen. Hydrogenolysis is a type of hydrogenation resulting in the splitting of a C – X bond (C is carbon and X is sulfur, nitrogen or oxygen). This results in the formation of C – H and H - X bonds.
In a refinery HDS unit, the reaction occurs in a fixed-bed reactor at temperature ranging from 300 to 400 °C and at elevated pressures. This reaction takes place in the presence of an alumina based catalyst, often impregnated with one of the following:
- Molybdenum and cobalt (CoMo catalyst)
- Molybdenum and nickel (NiMo catalyst – added to CoMo when feedstock contains high levels of chemically bound nitrogen)
The feed (olefin free) is pumped to the required elevated operating pressure and is combined with a stream of hydrogen rich recycle gas. The combined feedstock is preheated via heat exchanger, and then further heated in a fired heater until the feed mixture is vaporized and heated to the operating temperature of the reactor. The feed mixture then flows through the fixed-bed catalyst in the reactor where the hydrodesulphurization reaction occurs.
The products are partially cooled by passing through the feed preheater, and further cooled in a water-cooled heat exchanger. The products then passes through a pressure controller to reduce the pressure prior to entering a gas separator at approximately 35 °C and 3 to 5 atmospheres of absolute pressure.
Most of the gas from the separator is hydrogen and hydrogen sulfide. This recycle gas goes to an amine contactor to remove the hydrogen sulfide and is then sent back for reuse in the reactor. All excess gas from the separator with join with the sour gas from the overhead of the product stripper.
Liquids from the gas separator go to the stripper distillation column, where the bottoms yield a final desulfurized liquid product from the HDS unit.
The stripper sour gas contains many components, including methane, ethane, propane, hydrogen sulfide, and possibly some butane and heavier hydrocarbons. This sour gas is routed to the central gas processing plant to remove the hydrogen sulfide in the main amine gas treating unit. It will probably also be routed through a series of distillation columns to recover propane, butane, etc. The remaining methane, ethane, propane, and hydrogen is utilized in the refinery fuel gas system.
The removed hydrogen sulfide is usually converted to elemental Sulfur or sulfuric acid in other processing units.
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