Hydrocarbon Processing - April 2022 - 30

Process Optimization
gaseous HCl and ammonia. In the absence
of water in the environment, it does
not show corrosive properties; however,
excessive accumulation may cause fouling
problems. When the dewpoint temperature
is lowered, it may form an aqueAtmospheric
distillation column
Crude oil/naphtha
reflux exchangers
ous acidic solution and cause corrosion
at high rates, especially since the pH of
the first water drop formed will be very
low. The average ambient temperature is
kept 20°C-30°C (68°F-86°F) above the
condensation temperature.
To minimize the corrosion mechanisms,
a corrosion inhibitor is injected
into the reflux section. In terms of protection,
pH values in the stream should
be in the range of 6-8. If pH values are
not in that range, then there can be exchanger
tube leakage in the preheat train,
naphtha reflux/crude oil exchanger.
If there is any tube leakage in the
First group preheater exchangers
Crude oil
charge to unit
BFW
Naphtha reflux kerosene
Light diesel
Heavy diesel
Atmospheric residue
FIG. 2. The main corrosion mechanisms through the naphtha reflux section are ammonium
chloride and HCl corrosion.
Crude oil inlet
Crude oil inlet
naphtha reflux exchanger, the kerosene
product will be off-spec due to crude oil
that penetrates the reflux stream. The
primary cause of tube leakage is low pH
(e.g., 2-3). If there are droplets of water
in the naphtha reflux section, this will
cause a dramatic decrease in pH and result
in leakage. If liquid water droplets can
be eliminated from the stream, the pH
values will increase, and leakage will not
occur or the leakage period will be longer
than the previous operational period.
At a Tüpraş refinery, the leakage period
was 6 mos-12 mos. The leakage
was detected by the color of the kerosene
product, with the root cause being a
blockage in the exchanger group. This led
to increased temperatures in the column's
overhead section, leading to heavier light
ends-especially in heavy naphtha-and
an increase in offgas flow from the overhead
drum.
The most effective way to overcome
FIG. 3. Naphtha reflux flow rate, m3/hr.
this challenge is upgrading material. However,
this option is costly. A second option
is to increase the pH value of the naphtha
reflux stream by increasing the column return
temperature of the reflux stream.
As shown in FIGS. 3-5, naphtha reflux
FIG. 4. Naphtha reflux return, °C.
30 APRIL 2022 | HydrocarbonProcessing.com
pH values increased with the increase in
naphtha reflux return temperature. To
increase the temperature of the naphtha
reflux stream, the reflux flowrate was increased
from 700 m3/hr to 1,000 m3/hr.
After this operation, the trend of pH tends
to increase from 4-5 to 7-8. As shown in
FIGS. 3-5, temperature is changing pH in
an effective way. From the last leakage issue,
the authors' company's refinery heat
exchangers were in operation for nearly
3-4 times longer than the previous period.
This means that there were no operational
upsets and no maintenance issues.
This also means that refining personnel
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