Distillate Desulphurization Unit (DDS)

The DDS unit, Unit 17, processes light cycle stock from the FCCU's, No. 1 distillate and virgin light gas oil from the crude units. The unit catalytically removes sulphur, oxygen, and nitrogen compounds and partially saturates olefinic hydrocarbon feed components. The unit has a nominal design capacity of 6,800 BSPD. The DDS consists of a charge heater, reactor, high and low pressure separators, and a stripper.

The charge is preheated in the feed/effluent exchangers and is combined with hydrogen from the Reformer before entering a charge heater furnace, where the mixture is raised to the reaction temperature. After exiting the heater, the stream enters the reactor where the sulphur compounds are converted to hydrogen sulphide. Oxygen compounds are converted to water and nitrogen compounds are converted to ammonia. The reactor effluents product stream passes back through the feed/effluent exchangers for heat recovery and then enters the high pressure separator. The HP separator off-gas is recycled back to the reactor via the recycle gas compressors and the hydrocarbon liquid is routed into the low pressure separator. The waste gas from the LP separator is compressed in the off-gas compressor, and after scrubbing with amine to remove the hydrogen sulfides is routed into the fuel gas system.

The liquid is fed to the stripper where light ends and dissolved H₂S gases are removed in the stripper overhead. The stripper off gas is also compressed in the off-gas compressor, and goes to fuel gas after amine scrubbing. The overhead liquid goes to the #1 FCCU Gas Plant for recovery of light ends. The stripper bottoms is the final treated gas oil product and its is routed to diesel blending and storage.

The unit has been recently revamped with a new larger reactor, and additional pumps to provide

for increased gas oils desulphurization capacity.