The purpose of this unit is to reduce the sulphur, nitrogen, and aromatic hydrocarbons to acceptable levels to allow subsequent processing of the gas oil in a conventional refinery catalytic cracker or hydrocracker.
The Gas Oil Hydrotreater processes straight-run atmospheric and vacuum gas oils from the Crude Unit and cracked atmospheric and vacuum gas oils from the Hydrocracker Fractionator and Delayed Coking Units. As a result of hydrotreating, the sulphur compounds are converted to hydrogen sulphide, nitrogen compounds changed to ammonia, and aromatics are saturated. Some chemical conversion of gas oil to jet kerosene and lighter components also occurs. The hot reactor effluent is cooled to separate the unrecalled hydrogen and produced gases from the liquids.
The individual gas oil streams entering the unit are combined into a single stream, pre-heated and mixed with a hot hydrogen-rich recycle gas stream. This mixture is split into two equal streams and passed through two multi-catalyst bed parallel reaction vessels where hydrogenation of the contaminants occurs.
The high-pressure hydrogen-rich gas contacts an aqueous amine solution which absorbs hydrogen sulphide. After purging a portion to maintain hydrogen purity, the remainder of the hydrogen-rich gas, along with make-up hydrogen, is recycled to the reaction vessels for reuse. The ammonia produced in the reaction vessels will be dissolved in the process water which is removed as sour water.
The liquid hydrocarbon products from the separation stage are routed to a fractionation section for removal of any dissolved gases and fractionation into naphtha/jet and gas oil. The sour gases are sent to the Hydrogen Recovery Unit for further nitrogen and aromatics reduction. The gas oil is sent to storage for eventual blending into the synthetic crude oil.
Diagram used with permission (© SAIT Polytechnic, MacPhail School of Energy, 2009)