Hydrocarbon Processing - July 2022 - 33

Special Focus Valves, Pumps and Turbomachinery
S. ZARDYNEZHAD, TurboTech Consulting Corporation,
Calgary, Alberta, Canada
Fouling in hydrogen recycle gas compressors
Hydrogen (H2
) recycle gas compressors
are one of the most significant rotating
equipment in oil refineries, petrochemical
plants and upgraders. The compressor is
usually a centrifugal gas compressor between
bearings. A single-shaft centrifugal
compressor design is based on the API
Standard 617.1
Without the compressor,
recycle gas compressor
ina
plant's production will be stopped. FIG. 1
shows a typical H2
in a hydrotreating unit.
H2
oil
recycle compressors are used with
refinery processing equipment,
cluding hydrocrackers, hydrotreaters,
platformers and isomerization units. The
hydrotreating process is similar in oil refineries
and petrochemical plants like aromatic
plants and upgraders. Depending
on the licensor, the number and configuration
of the equipment may be different;
however, the process is very similar.
The risk of fouling is high in recycle
gas compressors due to chemical injection
and reactions in the reactor. Stress corrosion
cracking, possibly combined with
high cycle fatigue, causes impeller and rotor
failures.
This article will explain the hydrotreating
process and address compressor fouling,
which impacts the recycle gas compressor's
performance and reliability,
based on the author's experience working
in large oil refineries, petrochemical plants
and upgrader plants.
The hydrotreating process. The hydrotreating
process reduces mercaptan,
sulfur, metal content, nitrogen, oxygen,
organic halides (chlorides and bromides),
olefins and aromatic components of the
feedstock while enhancing the cetane/
octane number in oil refining and the density
and smoke point of the product. Some
hydrocracking and thermal cracking reactions
may take place in the hydrotreating
process. FIG. 1 shows a typical sketch of the
hydrotreating process in oil refineries, upgraders
and petrochemical plants. The primary
duty of the recycle gas compressor is
to move the feedstock (Point A) through
a fired heater, reactor, reactor feed effluent
exchanger, air and trim cooler, separator
and finally, the compressor's suction.
Make-up gas (Point C) supplied by a separate
reciprocating gas compressor replaces
the H2
H2 gas recycled from the separator to the
reactor has the following purposes:
* Absorbing some of the heat
of reaction, minimizing the
catalyst bed temperatures.
* Lowering the heater and feed
effluent exchanger tube wall
temperatures by increasing the
flow through the equipment.
* Preventing coke formation
as the feedstock is heated to
the reaction temperature.
* Ensuring the reactions are
carried out completely.
FIG. 1 shows a liquid feedstock sent to
consumed. The large quantity of
the heater by a charge pump, normally
a centrifugal pump. Consider the pump
shutoff pressure when designing the compressor
to prevent surging. Typically, two
check valves are placed close to the injection
point to prevent reverse liquid flow towards
the pump. Another check valve is required
downstream of the anti-surge valve.
H2 gas. H2 gas is colorless, odorless and
highly flammable. H2
gas forms explosive
mixtures with air at concentrations from
4%-74% and with chlorine at 5%-95%. A
spark, heat or sunlight can trigger explosive
reactions. The H2
auto-ignition temperature-the
temperature of spontaneous
ignition in the air-is 500°C (932°F).
Pure H2
-oxygen flames emit ultraviolet
light, and high-oxygen mixes are nearly inFIG.
1. Simplified hydrotreating process.
Hydrocarbon Processing | JULY 2022 33
Hydrocarbon Processing - July 2022

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