Journal of Applied and Industrial Sciences, 2014, 2 (5): 238-243, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)
Research Article 238
The Effects of High HCl and Changes in pH Levels in CDU
Overhead Corrosion
Elnour M.M*1, Gasmelseed G.A*2, Karama B.A*3
(1-3)
University, Of Karari, Sudan, Department of Chemical Engineering.
2
. Email: [email protected]
3
Email: [email protected]
(Received: May 31, 2014; Accepted: September 28, 2014)

Abstract— many corrosion management methods for Alfulla
treatment had been investigated .Corrosion inhibitors and
neutralizer in addition to high efficient desalting have been
proposed as the superior corrosion treatment technique for
Alfulla crude processing equipment. Generally the overhead of a
crude unit can be subjected to a multitude of corrosives species .
Hydrochloric acid, formed from the hydrolysis of calcium and
magnesium chlorides, is the principal strong acid responsible for
corrosion in crude unit overhead . Mitigation of this type of
corrosion is performed by process changes, materials upgrading,
design changes, and injection of chemicals such as neutralizers
and corrosion inhibitors. Process changes include any action to
remove or at least reduce the amount of acid gas present and to
prevent accumulation of water on the tower trays. Material
upgrading includes lining of distillation tower tops with alloys
resistant to hydrochloric acid. Design changes are used to prevent
the accumulation of water. They include coalesces and water
draws. The applications of chemicals include the injection of a
neutralizer to increase the pH and a corrosion inhibitor. This
investigation proved that maintaining a low chloride level and
stable pH levels were the most effective ways to control equipment
damage from corrosion. Also, the study found that several
inspection techniques were particularly useful in estimating
service life for pipes and other crude-unit equipment. Applying
better pH control and improved monitoring and inspection
programs can reduce equipment damage from corrosion. In this
study, some field results are presented, including chemical
analysis, pH and corrosion rate for a CDU tower overhead in
Viscosity Breaking Unit.
Index Terms — Corrosion rate, pH, Neutralizer, HCl.
I. INTRODUCTION
significant corrosion problems have been reported in crude
unit overhead operations, including those due to chloride
and sulfur species in the crude [1]. Most of these problems,
such as those due to NH4Cl or HCl have been dealt with after
the damage has occurred, and continue to be a source of
significant material degradation in CDU overhead operations
[2].Chloride salts are found in most production wells, either
dissolved in water emulsified in the crude oil or as suspended
solids. Salts also originate from salt water injected for
secondary recovery or from seawater ballast in marine tankers
[3]. The amount of salt contained in the emulsified water may
S
1*
corresponding author Email: [email protected]
range from 10 pounds to 250 pounds per thousand barrels of
crude oil. The salt typically contains 75% sodium chloride,
15%
magnesium
chloride,
and
10%
calcium
chloride3.Hydrogen chloride corrosion is caused by the
presence of hydrogen chloride. Hydrogen chloride evolves
from heating magnesium chloride and calcium chloride to
above 300 0F (149 0C). Sodium chloride is essentially stable up
to about 8000F (426 0C) [4]. Hydrogen chloride evolution
occurs primarily in the crude preheat furnace. Dry hydrogen
chloride is not corrosive to carbon or low alloy steel, especially
when large amounts of hydrocarbon vapor or liquid are present.
However, when steam is added to the bottom of the crude tower
to facilitate the distillation process, dilute hydrochloric acid is
produced [3]. The hydrochloric acid can cause severe corrosion
in carbon steel equipment at temperatures below the initial
water dew point [5]. The corrosion rate increases with a
decrease in water pH [6].
Alfulla Crude Oil properties
Alfulla crude from South Kurdufan area wells has such
properties: High calcium content, high acid value, high water
content, high density and viscosity [7]. These properties need to
be reduced to a minimum; to improve the product quality [8]
.Alfulla has quite different characteristics, depending of the
time of sampling and analytical lab performing [7]. It is
especially related to TAN, viscosity, Fe, Ca and specific
gravity. High quality corrosion monitoring and control must be
celebrated and adjusted to give optimum units performance
with the ever- changing properties of crude [8]. For crude-unit
overhead systems, pH is the main process parameter that
impacts corrosion rates [9]. To control corrosion conditions,
many operators use various neutralizers at optimum ranges
determined by site-specific conditions [10]. A three-year study
(2011–2014) was conducted at a VBU using amine-blend
solutions to control pH. Over this period, corrosion rates were
measured through ultrasonic inspections and weight-loss
coupons [7].
II. MATERIALS AND METHODS
L ABORATORY M ETHODS
The laboratory test methods should be carried out under
conditions that simulate operational conditions in the field,
including composition of material and environment,
temperature, flow, pressure, and the method by which the
inhibitor is added, continuous or batch. The simplest, and
longest-established, method of estimating corrosion losses in
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Journal of Applied and Industrial Sciences, 2014, 2 (5): 238-243, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)
plant and equipment is weight loss analysis [11]. A weighed
introduced into the process, and later removed after a
reasonable time interval. The coupon is then cleaned of all
corrosion products and is reweighed. The weight loss is
converted to a corrosion rate (CR) or a metal loss [8].
Field data
First year, the observed corrosion rates in pipes, were obtained
from thickness measuring via ultrasonic testing. Corrosion
rates reached values of 0.25 mm/yr. Some thickness loss and
stress-corrosion cracking were also reported on the safety vents
of the CDU, which can be attributed to H2S levels (2,553 ppm)
in the neutralizing solution used. Second year, high chloride
levels, caused by inefficiencies in the crude preparation and
desalting processes, generated high corrosion rates, the average
corrosion rate observed by the coupon weight loss over two
years was 0.29 mm/yr. The pipes connecting the top of the
atmospheric tower and condensers have flaws from previous
campaigns. Localized under-deposit corrosion in the lower
blank of the condenser shell was observed. Third year,
monitoring results for coupons installed in the overhead, had
average corrosion losses of 0.16 mm/yr. This reflects a uniform
thickness loss expected in equipment and pipes. But there were
failures in reflux pipes caused by localized under-deposit
corrosion. The average service life was five years, and failures
were reported immediately after the flow entrance, where
condensation begins.
Thickness measuring
For this study, two regions of the overhead pipe were selected
at train A to conduct thickness measurement via ultrasonic
testing [11]. Initially, the testing was separated into two areas:
•Overhead atmospheric tower and condenser
•Between the condenser and accumulator drum.
Different behaviors are expected from the pipes carrying the
fluid before the condenser in the vapor phase (by design), then
after, where the water is already in liquid phase. Thus, the
observed corrosion rates were different, as shown in Fig. 1.
sample (coupon) of the metal or alloy under consideration is
•Regions of encounter between two pipes, in the form of “T,”
are also preferred regions for erosion.
•In the straight sections, fewer points are selected, which are
expected to be representative of the system.
In 2011, measurements were made on about 10 points before
the condensers. Another 10 points were inspected with the
same technique in pipes after the condensers. The same 20
points were inspected again in 2012. The results allowed
defining several average corrosion rates:
•Before the condensers: 0.15 mm/yr
•After the condensers: 0.18 mm/yr.
Also, there were high standard deviations in cases, 0.16 mm/yr
before and 0.15 mm/yr after the condensers. The highest rate
observed in the first case was 0.63 mm/yr, and the lowest 0.02
mm/yr. After the condensers, the highest rate was equal to 0.61
mm/yr and the lowest 0.02 mm/yr.Before the condenser, 25
points were measured again in 2014; the average corrosion rate
(2011–2014) was equal to 0.17 mm/yr. The data from the
corrosion coupons indicated an average rate of 0.19 mm/yr in
the same period.
In December 2013 and March 2014, five points were measured
in random areas of the pipes before the condensers, resulting in
an average corrosion rate equal to 0.21 mm/yr. In the same
period, some corrosion coupons were analyzed monthly,
positioned on the inlet connections of overhead condensers;
these coupons had an average corrosion rate equal to 0.28
mm/yr.
III. RESULTS AND DISCUSSION
In the overhead atmospheric system, pH, chloride, iron and
corrosion rates were monitored by weight-loss coupons. Fig. 2
shows data of pH values measured in the accumulator drum
since 2008. The figure shows the mean values and standard
deviations for measurements over each month, except May and
September, when there were no analysis reports.
7.5
7
6.5
22-Nov02-Mar10-Jun 18-Sep27-Dec06-Apr
Figure 2. Data of pH values in the overhead accumulator drum
of the tower for train A
Figure 1. CDU showing locations identified for thickness
monitoring by ultrasonic testing
The locations for thickness measurement are always chosen
based on the experience of the inspection team supervising the
unit and the measurements usually apply these aspects:
•In the curves, the corrosion rates may be higher due to an
increased propensity for the occurrence of corrosion associated
with erosion.
It is observed that the average pH over the years has always
been very close to or within the recommended range. But the
high standard deviations showed a lack of control during some
periods. There were some incidents in January in which a pH
reaching 1.5 was observed and adjusted to 3 on the same day
and recovered to a pH = 6.6 on the next day. On two days, the
pH reached 4. The higher standard deviation observed in this
month contributed significantly to increased corrosion rates.
Fig. 3 shows the measured chlorine values in the same drum.
The target is 40 ppm as the maximum, which can only be
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Journal of Applied and Industrial Sciences, 2014, 2 (5): 238-243, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)
guaranteed with efficient control in crude preparation at the
storage tanks and desalter.
5
80
60
0
22-Nov 02-Mar 10-Jun 18-Sep 27-Dec
40
20
0
22-Nov02-Mar 10-Jun 18-Sep 27-Dec 06-Apr
Figure 3. Data of chloride values in theoverhead accumulator
drum of the tower for Train A.
It is observed that the values remained above the recommended
targets throughout the year, showing deficiency in the early
stages of crude processing. As an immediate consequence, we
can expect greater usage of neutralizers and corrosion
inhibitors. What is not always sufficient to maintain is the
appropriate pH and low corrosion rates over slack periods, as
observed in January (average of 70 ppm chloride).
150
100
50
0
22-Nov02-Mar 10-Jun 18-Sep 27-Dec 06-Apr
Figure 5. Data of neutralizer amine Flowrates at overhead pipe in the
atmospheric tower for Train A
Figure 4. Data of iron values in the overhead accumulator
drum of the tower for Train A.
The iron level in the water was also monitored. Iron can be
another indicator of corrosion in the overhead system. In Fig. 4,
the measurements from 2013 are shown; conditions exceeded
the maximum value of 1 ppm over the year. Also, we can
observe that the iron content was below the recommended limit
4 of the 10 months evaluated. These results vary greatly over
the month, with standard deviations above the mean values; the
data is not included in Fig. 4. Intakes of neutralizing solutions
and corrosion inhibitors also represent relevant data on
analyzing control parameters in the overhead system. Figs. 5
and 6 show the injection rates for neutralizers and inhibitors for
Train A in 2013.
The mass balance at the tower overhead is shown in Fig. 7. It is
known that the chloride content measured in the top
accumulator is directly linked to the presence of HC1 formed
from the hydrolysis of salts present in the feed. Thus, it is
possible to set base values for neutralizing agent flowrates.
From the condensate analysis in the overhead drum, several
periods were selected in which the chloride content was close to
80 ppm, or 50% of this, 40 ppm. On the same dates, the average
flowrates of the neutralizing solution and pH were recorded, as
listed in Table 3.
Hydrocarbons 85%
Aqueous Solution
60
40
20
0
22-Nov 02-Mar 10-Jun 18-Sep 27-Dec 06-Apr
Figure 6.. Data of inhibitor flowrates atoverhead pipe in the
atmospheric tower ofTrain A
Figure 7. Mass balance of the atmospheric tower overhead system Train A
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150
Cl,
mean
100
Flowrat
e
N2,l/b
50
0
0
5
10
15
Figure 8. Relationship between levels of Chloride Neutralizer flow
(N2) and pH analyzed in the condensate from the overhead drum –
Train A
With the pH near the equivalence point, if we consider only the
presence of HCl, neutralizer and water, the result is salt
formation, N2Cl, which dissociates. We can determine the
resulting pH; the reactions are
N2Cl = N2+ + Cl- (1)
N2+ + H2O = N2OH + H+ (2)
From the salt concentration, it is possible to determine the
expected pH:
As Ka is very low, the salt concentration Cs = [N2+]
0.4
0.2
0
01-Jan
01-Apr
01-Jul
01-Oct
Fig.10.A Weight loss monitored by corrosion coupons, installed at the
inlet connection ofthe overhead condenser—Train AfromNovember
2008 to November 2009
0.3
0.2
0.1
0
Fig.10.B Weight loss monitored by corrosion coupons, installed at the
inlet connection oft he overhead condenser—Train A fromNovember
2009 to November 2010
0.6
0.4
0.2
There are many other contaminants in the overhead system,
such as H2S, ammonia (NH4), sulfur oxides (SOx) and others
that can alter conditions and force changes on the predicted pH
values. We cannot establish a direct relationship between the
chlorides (Cl-), flowrate and pH neutralizer from field results.
However, we can determine the salt concentration (N2Cl) from
the N2 solutions, as described in Table 4, and compare it with
the expected resulting pH. Table 4 lists the results; observing
that, in a few cases the values coincide, as in D1, D3, D4 and
D5, and the neutralizing added on top is extremely diluted into
the total water solution (264,000 l)
200
100
0
-100 D1 D3 D5 D7 D9
Flow
rate,
l/b
Time
Figure 9. Relationship between levels of Chloride Neutralizer flow
(N2) and pH analyzed in the condensate from the overhead drum –
Train A
Fig. 10 shows the data of the weight-loss coupons, installed on
the inlet connection of the overhead condenser. There were
many lack periods, in which the corrosion rate is at greater than
the established limit (0.125 mm), such as in November 2008
(0.60 mm/yr), January 2011 (0.53 mm/yr) and November 2012
(0.55 mm/yr).
0
Fig.10.C Weight loss monitored by corrosion coupons, installed at the
inlet connection ofthe overhead condenser—Train A fromNovember
2008 to November 2011
At the VB unit presented in this study, various problems caused
by corrosion are sourced to low operating efficiencies in the
crude desalting unit, which is initiated at the storage tanks.
Checking field data and literature to find benchmark values for
evaluating the effectiveness of existing desalters can help
maximize salt-removal efforts. Also, leakages observed in
pipelines in Train A were mainly caused by deficiencies in pH
control. This is the main control parameter in the tower
overhead, and it must be kept within the range with the
minimum possible deviation. We could not associate a
neutralizer type to observed failures.The results showed that
even with at stable pH behavior over the study period, corrosion
increased. The standard deviation observed during 2012 was
0.54, with daily routine measurements. This value is consistent
with observed deviation cited in the literature, equal to 0.78,
when gaseous ammonia was used as a neutralizer in the same
unit.The literature shows that low pH values lead to high
corrosion rates on mild steel, even though the presence of
inhibitors may be insufficient to alleviate this problem.
Conversely, a pH too high can also bring negative
consequences:
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•Using excess neutralizing solutions, based on amine or
ammonia, favors the occurrence of deposits, leading to
localized corrosion with extremely fast kinetics.
•In stream containing H2S, such as the CDU, stability of the
protective iron-sulfide film is compromised while increasing its
solubility, thus accelerating corrosion.
We can analyze a phase diagram for H2O-HCl and correlate it
to the overhead corrosion process. It is possible to observe a
temperature range of approximately 100°C to 102°C, in which
an average concentration observed in the field (0.7% HCl), and
in which two phases are present in equilibrium conditions:
vapor (rich in water) and liquid (rich in HC1). At the
temperature where condensation begins, the HCl concentration
in the liquid is 10 times higher than vapor phase. Only below
100°C, in equilibrium condition, the steam is fully condensed,
and the final concentration of the liquid is reached.
To increase the process data analysis, the measured
consumption of neutralizing amine in the overhead during 2012
were compared with values originally estimated by the supplier
data presented in Figure 5. The comparison was done in a
period in which the main process variables, such as pH and
chloride content in the overhead drum, did not suffer
interference from typical discontinuities, such as high levels of
base sediment and water in oil. The selected period was the
months of January 2012 to March 2012, in which the corrosion
rate was below the recommended target of 0.09 mm/yr, as
shown in Figure 6.
Application of neutralizing amine can be varied for many
reasons, such as incorrect pH measurement, which interferes
directly in injection flowrate. If the quality of the crude is kept
almost constant, the product amount injected into the overhead
stabilizes. This is the condition studied in the chosen period
optimum injection to compare the predicted with the far field.
From Figure 5, the relationship between the measured and
predicted consumption of neutralizer is doable.
For a maximum chloride content of 50 ppm at the overhead
using data of steam injection background and specific
consumption provided by the manufacturer, the amount of
amine provided at the top would be 60 l/d, while in practice,
keeping variables under control, the measured consumption
was 120 l/d. We can conclude that the predicted flowrate for the
neutralizing solution can be a guide for the process, but only
constant pH monitoring (preferably online) can promote
adequate control for amine injection.
Corrosion rates are directly proportional to pH. Accordingly,
field monitoring uses weight loss coupons to validate the
quality of process parameters control. Measurements were
made from 2008 until early 2013, when only 35% of cases were
below the limit 0.13 mm/yr. throughout 2008, the weight loss
was framed in only 30% of the months monitored. Comparing
these results with inspections by thickness measurement, we
realized that the difference between the rates obtained with both
techniques was short only at the second decimal number.
120
100
80
60
40
20
0
01-Jan
01-Feb
01-Mar
Chloride
content
Kg
MCl/Kg
solution
Predicte
d
consum
ption,
l/d
Figure 11. Relationship between predicted consumption and
measured one
IV. CONCLUSIONS
Among the available neutralizing solutions, we should use the
one that provides the best efficiency, coupled with the cost
benefit for each unit, while considering environmental aspects
from waste generation and final treatment. There are pros and
cons associated with each neutralizer. The results showed that
the type of neutralizer used on the CDU atmospheric tower
overhead was not the determining factor in minimizing
corrosion. Only a good control of process parameters,
especially the desalting efficiency (low chloride level at the
overhead accumulator drum), can increase equipment service
life. We can also establish a direct relationship between the
historic data of the process parameters (chloride level, pH,
temperature and pressure) and the expected thickness loss of
the equipment and pipes.Monitoring weight-loss coupons is
essential to validate the quality of the process parameters’
control. The rates obtained with the coupons were compared to
results from inspections by ultrasonic thickness measurement,
where only a small difference in the second decimal number
(0.02 mm to 0.07 mm) was observed. With these low rates and
constant monitoring, the likelihood of failure is minimized, and
it becomes possible to predict damage to equipment and avoid
unplanned shutdowns due to equipment failures by
corrosion.Plant results and literature data indicate that there is
an optimal pH control range for the CDU overhead system. The
main process parameter, defined in terms of two main corrosion
mechanisms are:
•At low pH (pH below 5.5) the HCl causes severe corrosion in
the mild steel
•At high pH (pH above 6.8), due to the presence of H2S, there is
an increase in the uniform corrosion rate due to the breakdown
of the iron sulfide layer, and localized corrosion under deposit
is also more likely to occur because of the salts formed.For each
system, an optimal range should be specified. It will depend on
the chemical composition of the final solution obtained in the
accumulator drum. It is important to note that pH stability is
dependent on system automation. More reliable online
information enables low deviations if there is an instrumented
injection control fed by online pH measurement .From this
study, it was observed that the average chloride concentration
was 10 ppm to 30 ppm in accumulator overhead drums.1
Chlorides are generated from some salts contained in the crude
oil that is processed in the CDU; thus HCl is formed.Regardless
of the neutralization technique applied, the pH is lower than the
dew point of water. This adds more challenges in measuring pH
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when condensation occurs; this is the preferred region for the
corrosion process to begin. One concern for neutralization is
the difficulty of controlling the ammonia or amine flowrates,
which depend on the varying HCl levels in the CDU.
ACKNOWLEDGEMENTS
The authors wish to thank the College of graduate studies and
scientific research, of Karrary University, for their support.
This research is made in fulfillment of the requirements for the
degree of Ph.D. in Chemical Engineering at Karrary University.
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