Virgin heavy naphtha from the Crude Unit and cracked naphtha from the Visbreaker Unit are processed in the Naphtha Hydrotreater Unit. The virgin heavy naphtha stream is produced as a bottoms stream from the Naphtha Splitter Unit installed in 1993-94.
Because of its high olefin content, the Visbreaker naphtha is first combined with all of the hydrogen-rich Reformer Separator Off-Gas, which is compressed by the HDS Compressor “A” cylinders, and passed downward through a fixed bed of cobalt-molybdena catalyst in the Visbreaker Naphtha Hydrotreater Reactor. The visbreaker Naphtha Reactor effluent is then combined with Virgin Naphtha, and the total mixture, containing the hydrogen-rich gas not consumed in the Visbreaker Naphtha Reactor, is passed downward through the virgin Naphtha Hydrotreater Reactor.
A small portion of the fresh Virgin Naphtha feed can be sent to the Visbreaker Naphtha Reactor as a quench stream to help control the reactor temperature.
The Virgin Naphtha Reactor effluent is cooled and separated. Separated hydrogen-rich gas from the Naphtha HDS Separator is compressed and fed to the Distillate HDS. In addition, some of the gas is recycled through the Naphtha HDS.
The treated naphtha from the Naphtha HDS Separator is first dewatered in a coalescer, and then countercurrently contacted with Reformer Separator Off-Gas in the Stripper-Absorber. The purpose of this is to strip traces of hydrogen sulfide, ammonia, and hydrogen chloride from the treated naphtha to make this treated naphtha suitable as Reformer charge. The stripped hydrogen sulfide, ammonia and hydrogen chloride remain in the hydrogen-rich gas stream through the Naphtha and Distillate HDS reaction stages, and are finally separated with the gas from the Distillate HDS Separators.
The Stripper-Absorber serves a two-fold purpose: it is used to strip the undesirable contaminants from the naphtha, and in addition, the naphtha absorbs light hydrocarbons from the Reformer Separator Off-Gas, thereby permitting return of these light hydrocarbons to the Reformer. The quantity of Reformer Charge (Stripper-Absorber bottoms) is thus always greater than the total quantity of naphthas charged to the HDS Unit.
The original design of this unit is based on the feed to the Diesel HDS consisting of combined Kerosene and Diesel Oil fractions from the crude Unit. However, as a portion of the 1967 refinery revamp, the diesel and kerosene fractions have separate Hydrotreater units to process the streams separately.
A new reactor and new charge heater were installed as a portion of this revamp.
The feed to the Diesel HDS is combined with the net hydrogen-rich off-gas from the Naphtha HDS Separator and recycle hydrogen-rich gas from the Diesel HDS (Secondary HP) Separator. These combined hydrogen-rich gas streams are compressed in the “B” cylinder of the HDS Compressor before joining the Distillate feed.
The mixture of hydrogen-rich gas and Diesel feed is heated to reaction temperature and passed downward through a fixed bed of cobalt-molybdena catalyst in the HDS Reactor, where the reactions occur. The reactor effluent is partially cooled and separated at high pressure.
The liquid from the second separation is flashed at low pressure, and the resulting gas is vented to the Fuel Gas system. The vapor from the second separation is hydrogen-rich gas, of which a constant quantity is recycled back to the “B” Cylinder of the HDS Compressor while the balance is unreacted hydrogen-rich gas which is vented to the Fuel Gas System.
Liquid from this separation is sent back to the Crude Unit side strippers to remove the hydrogen sulfide.
The Kerosene Hydrotreater is charged with the Kerosene Hydrotreater Charge Pumps (MP-301/A). The feed is combined with a hydrogen rich gas stream from the Naphtha Hydrotreater. The feed stream is then preheated by cross exchanging with the reactor effluent stream prior to entering the Kerosene Charge Heater. The charger heater boosts the temperature to approximately 650o F and then enters the Kerosene Hydrotreater Reactor vessel filled with cobalt-moly hydrotreating catalyst.
The effluent of the reaction is cross exchanged with the feed which cools the effluent stream prior to entering the HP Kerosene Separator and then the LP Separator. Hydrogen is separated in the two stage separation process. A third stage separation drum removes any remaining hydrogen which is then sent to the Diesel Hydrotreating Unit.
The resulting desulfurized kerosene is then sent back to the Kerosene Stripper, W-102A to remove the H2S and offgas, then is routed to storage.