as the reactor effluent is cooled. Likewise, ammonium chloride may be formed if
there is any chloride in the system. The purpose of the water is to keep the salts of
hydrogen sulfide and ammonia in solution and not allow them to precipitate.
Various companies have slightly different guidelines for the quality of the water
injection; in general, boiler feedwater is preferred.
Vapor/Liquid Separation
The exact method of separating vapor and liquid will vary depending on the
optimum heat integration scheme. Up to four separate vessels may be used to
disengage and individually remove vapor, water, and hydrocarbon liquid. A hot
high-pressure separator is sometimes installed after the feed/effluent exchangers to
collect the heavier hydrocarbon material from the reactor effluent and send it to
fractionation via a hot low-pressure flash drum. The overhead vapor from the hot
high-pressure separator continues through an air cooler into a cold high-pressure
separator. The two-separator system is depicted in Fig. 2.
Hydrogen Purification
Increasing the recycle gas hydrogen partial pressure will decrease the catalyst
deactivation rate. Depending upon the feedstock and type of unit, additional
measures may be taken to increase the hydrogen purity. These measures may
include hydrogen enrichment and/or membrane separation.
Fractionation Section
A schematic flow diagram of a typical fractionation section is shown in Fig. 3.
The function of the fractionation section is to separate the net reactor effluent
into the desired products. This can be accomplished with either a one- or a
two-column fractionation scheme, depending on the type of hydrotreating unit.
In the two-column scheme, the flash drum liquids combine and flow to a stripper
column. Steam and/or a fired heater reboiler is used to strip naphtha (if desired) and
lighter material overhead. The stripper bottoms stream flows to a fractionator where it
is further separated into naphtha (if desired) and heavier products. The fractionator feed
is typically preheated with fractionator bottoms and a fired heater before entering the
column. Stripping steam is used to drive lighter material up the column, and various
product strippers are used to pull side-cut products to the desired specifications.
Chemistry
The following chemical steps and/or reactions occur during the hydrotreating
process (depending on the impurities present):
• Sulfur removal, also referred to as desulfurization or hydrodesulfurization
(HDS) in which the organic sulfur compounds are converted to hydrogen sulfide
Hydrotreating in Petroleum Processing
369
there is any chloride in the system. The purpose of the water is to keep the salts of
hydrogen sulfide and ammonia in solution and not allow them to precipitate.
Various companies have slightly different guidelines for the quality of the water
injection; in general, boiler feedwater is preferred.
Vapor/Liquid Separation
The exact method of separating vapor and liquid will vary depending on the
optimum heat integration scheme. Up to four separate vessels may be used to
disengage and individually remove vapor, water, and hydrocarbon liquid. A hot
high-pressure separator is sometimes installed after the feed/effluent exchangers to
collect the heavier hydrocarbon material from the reactor effluent and send it to
fractionation via a hot low-pressure flash drum. The overhead vapor from the hot
high-pressure separator continues through an air cooler into a cold high-pressure
separator. The two-separator system is depicted in Fig. 2.
Hydrogen Purification
Increasing the recycle gas hydrogen partial pressure will decrease the catalyst
deactivation rate. Depending upon the feedstock and type of unit, additional
measures may be taken to increase the hydrogen purity. These measures may
include hydrogen enrichment and/or membrane separation.
Fractionation Section
A schematic flow diagram of a typical fractionation section is shown in Fig. 3.
The function of the fractionation section is to separate the net reactor effluent
into the desired products. This can be accomplished with either a one- or a
two-column fractionation scheme, depending on the type of hydrotreating unit.
In the two-column scheme, the flash drum liquids combine and flow to a stripper
column. Steam and/or a fired heater reboiler is used to strip naphtha (if desired) and
lighter material overhead. The stripper bottoms stream flows to a fractionator where it
is further separated into naphtha (if desired) and heavier products. The fractionator feed
is typically preheated with fractionator bottoms and a fired heater before entering the
column. Stripping steam is used to drive lighter material up the column, and various
product strippers are used to pull side-cut products to the desired specifications.
Chemistry
The following chemical steps and/or reactions occur during the hydrotreating
process (depending on the impurities present):
• Sulfur removal, also referred to as desulfurization or hydrodesulfurization
(HDS) in which the organic sulfur compounds are converted to hydrogen sulfide
Hydrotreating in Petroleum Processing
369