accompanied by a significant temperature rise due to the exothermic saturation
reactions of olefins and aromatics. Finally, the presence of fragments of the
Si-based antifoam agent used in cokers will deactivate the hydrotreating catalysts
processing the coker streams.
Light cycle oils are characterized by low gravities (API gravities in the range of
12–22) and high levels of aromatics. Aromatics levels in LCOs may be as high as
70 wt% with the di-aromatics being the most prevalent. A typical aromatics
distribution of an LCO containing 70 wt% aromatics may be 10 wt% monoaromatics, 45 wt% di-aromatics, and 15 wt% tri-aromatics. The concentration of
sulfur in an LCO may range from ~500 wppm (for an LCO derived from an FCC
with an FCC feed pretreater) to ~2 wt% for FCC units processing high-sulfur feeds
without a FCC feed pretreater. Nitrogen levels are not usually very high, reaching
into several hundred wppm. Processing LCO in a hydrotreater is accompanied by
high hydrogen consumption due to the saturation of the aromatic compounds and
high heat release and temperature rise due to the exothermic nature of the saturation
reactions. Temperature control is important due to the rapid saturation rate of the
multi-ring aromatics.
Process Configuration
A simplified process flow diagram of a distillate hydrotreater is shown in Fig. 9.
The feed enters the feed surge drum from which it is pumped through a bank of
feed/effluent exchangers, is mixed with recycle gas, is heated to reaction temperature, and enters the reactor. Reactor effluent is passed through a bank of feed/
effluent exchangers and a fin fan cooler and then enters the separator. The gas is
recovered and sent to an amine scrubber to remove hydrogen sulfide and ammonia
and then routed to a recycle gas compressor and recycled to the reactor. The liquid
product is then directed to a flash drum, where any light gases are recovered
overhead while the hydrocarbon is routed to a stripper to remove any dissolved
hydrogen sulfide and ammonia in the product as well as any wild naphtha formed.
The stabilized product is then sent to blending or storage. Wash water is added to
the reactor effluent just upstream of the air fin cooler to absorb the ammonia
produced in order to avoid precipitation of ammonium hydrosulfide salts in the
colder sections for the plant.
The process flow diagram shows one quench location between two catalyst beds.
The number of beds and quenches will depend on the type of feed being processed
with SR distillates requiring only one point while LCGOs and LCOs requiring three
or more beds with interbed quenches to control temperature rise.
Process Conditions
Process conditions applied in distillate hydrotreating today are much more severe
than they were in the past. This is due primarily to the requirement to produce
Hydrotreating in Petroleum Processing
389
reactions of olefins and aromatics. Finally, the presence of fragments of the
Si-based antifoam agent used in cokers will deactivate the hydrotreating catalysts
processing the coker streams.
Light cycle oils are characterized by low gravities (API gravities in the range of
12–22) and high levels of aromatics. Aromatics levels in LCOs may be as high as
70 wt% with the di-aromatics being the most prevalent. A typical aromatics
distribution of an LCO containing 70 wt% aromatics may be 10 wt% monoaromatics, 45 wt% di-aromatics, and 15 wt% tri-aromatics. The concentration of
sulfur in an LCO may range from ~500 wppm (for an LCO derived from an FCC
with an FCC feed pretreater) to ~2 wt% for FCC units processing high-sulfur feeds
without a FCC feed pretreater. Nitrogen levels are not usually very high, reaching
into several hundred wppm. Processing LCO in a hydrotreater is accompanied by
high hydrogen consumption due to the saturation of the aromatic compounds and
high heat release and temperature rise due to the exothermic nature of the saturation
reactions. Temperature control is important due to the rapid saturation rate of the
multi-ring aromatics.
Process Configuration
A simplified process flow diagram of a distillate hydrotreater is shown in Fig. 9.
The feed enters the feed surge drum from which it is pumped through a bank of
feed/effluent exchangers, is mixed with recycle gas, is heated to reaction temperature, and enters the reactor. Reactor effluent is passed through a bank of feed/
effluent exchangers and a fin fan cooler and then enters the separator. The gas is
recovered and sent to an amine scrubber to remove hydrogen sulfide and ammonia
and then routed to a recycle gas compressor and recycled to the reactor. The liquid
product is then directed to a flash drum, where any light gases are recovered
overhead while the hydrocarbon is routed to a stripper to remove any dissolved
hydrogen sulfide and ammonia in the product as well as any wild naphtha formed.
The stabilized product is then sent to blending or storage. Wash water is added to
the reactor effluent just upstream of the air fin cooler to absorb the ammonia
produced in order to avoid precipitation of ammonium hydrosulfide salts in the
colder sections for the plant.
The process flow diagram shows one quench location between two catalyst beds.
The number of beds and quenches will depend on the type of feed being processed
with SR distillates requiring only one point while LCGOs and LCOs requiring three
or more beds with interbed quenches to control temperature rise.
Process Conditions
Process conditions applied in distillate hydrotreating today are much more severe
than they were in the past. This is due primarily to the requirement to produce
Hydrotreating in Petroleum Processing
389