Diesel HDS
Hydrotreating is a catalytic process to stabilize products and remove objectionable elements like sulfur, nitrogen and aromatics by reacting them with hydrogen. Cobalt-molybdenum catalysts are used for desulphurization. When nitrogen removal is required in addition to sulfur, nickel-molybdenum catalysts are used. In some instances, aromatics saturation is pursued during the hydrotreating process in order to improve diesel fuel performance.
Most hydrotreating reactions take place between 600-800˚F (315-425˚C) and at moderately high pressures 500-1500 psi (35-100 bar). As coke deposits on the catalyst, reactor temperature must be raised. Once the reactor temperature reaches ~750˚F (400˚C), the unit is scheduled for shutdown and catalyst replacement.
Hydrogen is combined with feed either before or after it has been heated to reaction temperature. The combined feed enters the top of a fixed bed reactor, or series of reactors depending on the level of contaminant removal required, where it flows downward over a bed of metal-oxide catalyst
Hydrogen reacts with the oil to produce hydrogen sulfide from sulfur, ammonia from nitrogen, saturated hydrocarbons and free metals. Metals remain on the catalyst and other products leave with the oil-hydrogen steam. Hydrogen is separated from oil in a product separator.
Hydrogen sulfide and light ends are stripped from the desulfurized product. Hydrogen sulfide is sent to sour gas processing and water removed from the process is sent to sour water stripping prior to use as desalter water or discharge.
