Hydrocarbon Processing - April 2022 - 29

Process
Optimization
N. GEZER and K. KAHRAMAN,
Tüpraş, Kırıkkale, Turkey
Combating naphtha reflux section corrosion
mechanism and production losses in a CDU
Corrosion is one of the most important
challenges that refineries face
(FIG. 1).1
Sometimes, corrosion can cause
the loss of production. Managing this issue
in an optimal manner can mitigate or
eliminate production loss due to maintenance
shutdowns.
In refining process units, especially in
the crude distillation unit (CDU), there
are different kinds of corrosion. The primary
corrosion problem facing the CDU
is ammonium chloride and hydrochloric
(HCl) acid corrosion, which is discussed
in this article. Opportunities are always
available to decrease or prevent these
types of corrosion, either with additional
investment or with operational changes.
The aim of the CDU is to remove
salt and water in crude oil from the system,
and to separate fuel gas, LPG, light
naphtha, heavy naphtha, kerosene, light
diesel, heavy diesel and atmospheric bottom
fractions by taking advantage of the
differences in boiling points. The separated
fractions are sent to other process
units in the refinery.
Crude oil fed to the unit is processed
through four basic processes: preheating
and desalting, atmospheric distillation
and the reflux system, the naphtha splitter
and debutanizer columns.
Charge pumps send the crude oil to
the unit, where it is heated with the boiler
feedwater to provide the desalter temperature.
Then, crude oil passes through
the first group of preheating exchangers
and is heated with naphtha reflux, kerosene,
light diesel, heavy diesel and atmospheric
bottoms, respectively, to provide
the optimum desalting temperature.
The crude oil mixed with washwater
that is supplied to the desalter is then
FIG. 1. Corrosion is one of the most important challenges that refineries face.1
sent to the second group of heat exchangers
after the desalting operation.
The crude oil is sent from the desalter
outlet and enters the second group of
heat exchangers, where it is heated with
kerosene reflux and atmospheric bottoms.
Continuing from two branches,
the crude oil enters the furnaces. The
furnace outlet is charged to the atmospheric
distillation column. Some heat
is withdrawn from the column by reflux
circulation to balance the heat loading
along the column. The naphtha reflux
stream heats the crude oil in the first
group of heat exchangers and then cools
and returns it to the column as reflux.
The main corrosion mechanisms
through the naphtha reflux section are
ammonium chloride and HCl corrosion
(FIG. 2). HCl corrosion occurs with dewpoint
corrosion if there is water vapor
and HCl in the environment. When the
water vapor condenses, it forms liquid
drops with HCl, causing the material to
be exposed to a solution with a very high
HCl concentration and low pH. High
existing acidity means high corrosion
rates. Liquid HCl may form in the heat
exchanger under the salts of ammonium
chloride or amine hydrochloride.
Ammonium chloride is an acidic salt
formed because of the combination of
Hydrocarbon Processing | APRIL 2022 29
Hydrocarbon Processing - April 2022

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