Hydrocarbon Processing - July 2022 - 63
Process
Optimization
A. SHARMA, Process Engineer, Jaipur, India
Caustic tower design guidelines
and recommendations
A caustic tower is a critical piece of
equipment in an ethylene plant that supplies
feed to the downstream section after
removing impurities. It mainly absorbs
contaminants such as carbon dioxide
(CO2
), hydrogen sulfide (H2
gases that are harmful to the downstream
process. If left untreated, CO2
at lower (negative) temperatures and H2
S
can deactivate the activity of catalyst for
the acetylene reactor and hydrogenation
reactor, which in turn leads to offspec
product. Additionally, any upset in
the caustic tower may lead to reduced
throughput and can force a complete
plant shutdown, which directly affects a
company's gross refinery margin.
FIG. 1 depicts a plant's caustic system
where cracked gas is compressed with
the help of a compressor and treated in
the caustic tower to eliminate contaminants.
The compressor may have 2-5
stages based on the pressure requirements
along with a provision for the
caustic tower between the stages.
A caustic tower is a combination of
packing and trays that splits into different
stages, such as weak, medium and strong
caustic sections, based on concentration.
Cracked gas from a compressor's firststage
discharge enters the tower bottom
section and comes in contact with weak
caustic. In the weak caustic section, most
acid gases are absorbed (compared to
the remaining stages) and the remaining
unabsorbed acid gases are later routed
to the medium and strong caustic sections.
A wash (water) section facility is
also provided to wash over the entrained
caustic from the cracked gas. All sections
are provided with individual pumps
for circulation of the caustic solution,
shown in FIG. 2.
Surface
cond
S) and acid
can freeze
HS
ST
Compressor
1st stage
2nd stage
This article addresses major concerns
associated with caustic towers during normal
operation and startup, as well as troubleshooting
and resolving these issues.
Small errors can quickly turn into
larger issues if not corrected. These errors
can be eliminated during design or
resolved later to avoid abnormal condiFrom
furnace
KOD
KOD
KOD
KOD
Caustic
Tower
FIG.
1. Overview of a cracked gas compressor and caustic tower system with two stages
and four knock-out drums. The flow direction of hydrocarbon is from the furnace to the
compressor and the compressor to downstream through the caustic tower.
Hydrocarbon Processing | JULY 2022 63
To downstream
Hydrocarbon Processing - July 2022
Table of Contents for the Digital Edition of Hydrocarbon Processing - July 2022
Hydrocarbon Processing - July 2022 - Cover1
Hydrocarbon Processing - July 2022 - Cover2
Hydrocarbon Processing - July 2022 - 3
Hydrocarbon Processing - July 2022 - 4
Hydrocarbon Processing - July 2022 - 5
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Hydrocarbon Processing - July 2022 - 12A
Hydrocarbon Processing - July 2022 - 12B
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Hydrocarbon Processing - July 2022 - Cover3
Hydrocarbon Processing - July 2022 - Cover4
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