benefits of novel cooling water chemistry on refinery ... aiche spring meeting, refinery economics...
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2017 AIChE Spring Meeting, Refinery Economics Session, San Antonio, TX
March 27, 2017
Benefits of Novel Cooling Water Chemistry on Refinery
Economics
Prasad Kalakodimi, Ph.D.
Raymond M. Post, P.E.
ChemTreat Inc.
Some 35 years ago, phosphate and zinc based programs emerged as the cooling water treatment
technology of choice when the industry was strongly encouraged to eliminate chromates. At that time,
the many troublesome issues associated with phosphorus based programs were recognized: the precise
control required to prevent phosphate deposits on hot bundles, especially in refineries; inadequate
admiralty brass corrosion using only azoles; escalating dispersant demand due to phosphate
precipitation with well water iron and aluminum carryover; and excessive algae growth on the towers
and the associated chlorine demand. Although the industry was aware of impending phosphorus
regulations, efforts to perfect phosphorus based cooling water programs continued because there was
no reasonable alternative. This paper describes the development of a promising phosphorus-free
corrosion and deposit control program, including laboratory and field application, economics, and
performance data from several challenging applications. This paper also discusses the ability of these
new corrosion inhibitors to passivate rusted surfaces, mitigate pitting and stress cracking on stainless
steels, form persistent passive films, and function in waters with little or no calcium hardness. The
results of various electrochemical will be presented.
Introduction
Cooling water treatment from the 1930s through the early 1980s relied primarily upon 50-500 ppm
hexavalent chromium to inhibit corrosion of steel and copper alloys in conjunction with acid to maintain
the pH of the system in the 6.0-7.0 range to control scale formation. In the last decades of the chromate
era, zinc and polyphosphate supplements were added to support lower levels of chromate. Chromate
proved to be an excellent steel and copper corrosion inhibitor, but its greatest attribute was its forgiving
nature. As chromate was phased out due to human health concerns and zinc has been mostly phased
out due to aquatic toxicity, the cooling water treatment industry in the United States and Western
Europe focused primarily on phosphate-based chemistries for both corrosion and scale control.
Progressive advances have led to polymers that are more efficient in maintaining higher levels of
orthophosphate in solution. Organic phosphate components provide both scale inhibition and cathodic
corrosion inhibition for steel. Aromatic azole supplements are used to overcome phosphate’s deficiency
in protecting copper alloys.
Today’s phosphate chemistries perform adequately in most circumstances but demand precise control.
The concentration of phosphate must be balanced carefully with calcium, polymeric dispersant, pH, and
temperature. If all five factors are not perfectly balanced at all times and at all points in the system,
either corrosion or fouling will occur. This is particularly problematic in the chemical industry due to the
prevalence of high temperature, low flow bundles together with steel piping operating at much lower
temperature. Apart from unforgiving control requirements, phosphate has several additional
weaknesses. Phosphate by itself is an effective inhibitor only for steel and a marginal inhibitor for
galvanized surfaces. It has little or no beneficial effect on copper or aluminum corrosion. Phosphate
programs often perform poorly in soft or low hardness waters, requiring much higher levels of
phosphate to form an effective calcium phosphate film. Phosphate will also precipitate with well-water
iron and aluminum clarifier carryover, forming deposits and causing excessive polymeric dispersant
demand.
Emerging Phosphorus Regulations
Recently, US regulations have begun to restrict the industrial discharge of phosphorus as an undesirable
aquatic nutrient that promotes the growth of cyanobacteria and algae in the environment. According to
USEPA, phosphorus and nitrogen nutrients are the cause of degradation in half of impaired water bodies
and are associated with fish kills, sediment accumulation, toxic trihalomethanes (THMs) in chlorinated
drinking water, and a 6,000 square mile low dissolved oxygen “dead zone” in the Mississippi River delta
drainage area. In cooling towers, algae and cyanobacteria convert inorganic bicarbonate into organic
carbon which supports the growth of bacteria. The dense algae mats that form on cooling tower decks
and exposed areas support higher life forms such as protozoans and amoeba which can harbor and
amplify Legionella bacteria.
Objective
Due to emerging environmental restrictions on phosphorus discharge and the many shortcomings of
phosphorus based cooling water treatment technologies, a multi-year research effort was undertaken to
develop a versatile and totally phosphorus and zinc free cooling water treatment technology. The
requirements for the program were to have no orthophosphate, polyphosphate, or organic
phosphonates or phosphinates, yet be cost competitive with traditional phosphorus programs. The
program also had to be non-toxic to aquatic life at 10x the nominal use concentration, with an overall
Environmental Health and Safety (EH&S) profile similar to or better than current phosphorus-based
programs. Non-phosphorus and non-zinc corrosion inhibitor that meets the criteria is a reactive
polyhydroxy starch inhibitor (RPSI).
The development and application of this corrosion inhibitor for use in cooling towers under normal
flowing conditions has been described in earlier publications (Post, Kalakodimi, & Tribble, 2014). As
shown in the cyclic polarization curves in Figure 1, RPSI is very effective in inhibiting both anodic and
cathodic corrosion reactions. The anodic corrosion potential of the metal is shifted to more positive
values. Also, the corrosion current values decrease in both the anodic and cathodic sweeps in the
presence of the RPSI corrosion inhibitor.
Several field applications of this RPSI chemistry were also discussed in that publication. Since the
publication, several products containing the RPSI chemistry have been formulated and applied in several
industries including power, refinery, chemical, steel, power, and light industrial.
The RPSI chemistry was also shown to be effective in field applications in forming a passive film under
stagnant conditions and during preoperational cleanings conducted at pH 3 (Post, et al., 2015).
Figure 1. Electrochemical cyclic polarization graphs of mild steel coupon in Richmond tap water
Figure 2. OCP measurement over time on carbon steel coupon passivated with RPSI during chemical cleaning compared to a new carbon steel coupon.
Figure 3. OCP test baths after 3 days showing the rust-colored solution containing the new coupon on the left and the clear solution containing the RPSI-passivated solution on the right.
In a later publication (Post, Kalakodimi, & Tribble, 2016), the ability of the RPSI chemistry to passivate
during preoperational cleaning was reported. Efficacy of the RPSI chemistry was compared to the
traditional passivation chemistries such as polyphosphates and organic phosphates. Electrochemical
methods, including open circuit potential (OCP), cyclic polarization (CP) and electrochemical impedance
spectroscopy (EIS), were used to demonstrate the film persistency of the RPSI chemistry in comparison
with the traditional phosphate based chemistries. It was demonstrated that the non-P chemistry forms
a passive film which persists for several days in untreated blank water.
More recently, the RPSI chemistry was shown to be capable of passivating lightly rusted surfaces to a
degree comparable to that obtained on a fresh steel surface (Kalakodimi, Tribble, Post, 2017). The data
indicated that RPSI chemistry enables passivation of new equipment on-line without a separate
precleaning step for lightly corroded metal surfaces. It also allows water systems that have experienced
moderate corrosion resulting from upset conditions to be effectively re-passivated and treated without
removing them from service. The RPSI chemistry was found to be superior to traditional
polyphosphate, organic phosphate, ortho phosphate, molybdate, and nitrite based treatments in terms
of forming a passive film on the rusted mild steel surface.
Figure 4. Effectiveness of RPSI in passivating pre-rusted surfaces compared to phosphate
RPSI
Figure 4 shows the open circuit potential (OCP) measurement of the mild steel electrode during the
corrosion process and after adding various treatment programs to the corroded surface. The RPSI
chemistry produced a large anodic shift in OCP values on the pre-corroded surface. The water used in
the study contained 150 mg/L Ca as CaCO3, 100 mg/L Mg as CaCO3, 100 mg/L M-Alkalinity as CaCO3, 50
mg/L chloride as Cl-, 10 mg/L silica as SiO2=, at pH 8.0 and the temperature of the study was 50 oC.
Pitting and Stress Corrosion Cracking (SCC) Inhibition:
Austenitic stainless steels such as Type 304 (UNS 30400) and Type 316 (UNS 31600) are commonly used
as the metallurgy of choice for heat exchangers in cooling waters. These steels are characterized as
having excellent corrosion resistance and good mechanical and physical properties for long service life.
However, austenitic stainless steels are subject to pitting and crevice corrosion in warm chloride
environments and to stress corrosion cracking above about 140°F. Other mechanisms such as
deposition, microbial activity, and low flow have also been reported to promote pitting corrosion.
Cooling water cycles of concentration are often limited by the chloride levels in order to reduce pitting
and stress cracking tendencies, which increases the water consumption and operating cost.
The presence of chromium is mainly responsible for the resistance of stainless steels to corrosion. The
presence of chromium promotes a protective oxide film on stainless steels, which is also called passive
layer. Passivity, the mechanism by which the stainless steels derive their corrosion resistance, has been
a subject of electrochemical research for many years. The passive film provides a protective barrier
between the stainless steel surface and the surrounding environment. Some aggressive ions such as
chlorides and sulfates are capable of causing localized breakdown to the passive film. When the
breakdown of the passivation occurs under the conditions where repassivation is not possible, pitting
attack can occur on stainless steels. The austenitic stainless steels may also be subjected to stress
corrosion cracking (SCC) in chloride environments at high temperatures (at above 130-140 °F), if tensile
stresses are present.
Austenitic stainless steels are extensively used as heat exchangers (shell & tube and plate & frame) in
petroleum refining applications due to corrosion resistance against sulfur compounds and various acid
contaminants which may be present in the refining process of the crude oil. Other applications of
stainless steels are condensers, reactors, and piping.
In many refineries and petrochemical applications, the material selection is governed by the cooling
water and the chloride content in the cooling water, which can lead to pitting and SCC.
Pitting is a form of localized corrosion which is known to initiate due to the breakdown of passive form.
The most common cause of SS pitting is contact with water containing high chlorides. It is very common
for refineries to maintain high residual chlorine in their cooling towers. Hypochlorite ions in bleach
solutions are highly aggressive pitting corrosion agent. Localized corrosion in the form of pit and
crevices in corrosion resistant alloys is one of the biggest challenges for material selection for
applications in the oil and gas industry. Pitting resistance equivalent number (PREN) is often used to
predict pitting behavior and select the appropriate grade of stainless steel.
None of the traditional corrosion inhibitors used in the cooling waters, such as inorganic phosphates,
organic phosphates, zinc, molybdate, and nitrite, provide any significant inhibition towards pitting and
SCC of SS. Consequently, the cycles of concentration (COC) are often limited in cooling towers to avoid
exceeding the acceptable chloride limit for the alloy. The present work was conducted to evaluate the
performance of the RPSI chemistry in inhibiting pitting and stress corrosion cracking (SCC) of stainless
steels.
Electrochemical techniques such as cyclic polarization have been extensively used in the laboratory to
evaluate susceptibility to the localized corrosion. Critical parameters such as corrosion potential, pitting
potential, corrosion current and repassivation potentials can be determined from the cyclic polarization
experiment.
ELECTROCHEMICAL EXPERIMENTAL – The schematic of the electrochemical setup and the
representative cyclic polarization graphs are shown in Figures 4 and 5.
From the cyclic polarization graphs, important parameters can be calculated such as:
Pitting potential (Epit) = Eb - Ecorr
Repassivation Potential (ERP) = Epass - Ecorr
It is generally accepted that an Epit value of >350-400 mV coupled with an ERP of >150 mV indicates that
there is minimal to no possibility of localized corrosion, and the alloy is suitable for long-term
applicability in that environment.
Figure 6 shows the cyclic polarization curves for Type 304 stainless steel in Richmond, VA tap water
containing 750 ppm chloride at a temperature of 150 °F. Various critical corrosion parameters are
derived from the graphs and are shown in Table 2. It is clear from Figure 6 and Table 2 that the pitting
and repassivation potentials increase with increase in the dosage of RPSI. At 100 ppm dosage of RPSI,
Figure 4. Schematic of the electrochemical setup.
WE: Working Electrode; RE: Reference Electrode,
CE: Counter Electrode
Figure 5. Schematic of the cyclic polarization curve.
Ecorr: Corrosion Potential; Eb: Breakdown potential;
Epass: Repassivation potential; Icorr: Corrosion Current
the Epit and ERP satisfy the requirement of Epit>400 mV and ERP>150 mV and confirm that Type 304
stainless steel does not undergo localized corrosion under these conditions.
Figure 6. Cyclic voltammetry graphs of Type 304SS in Richmond tap water with 750 ppm chlorides at 150 °F
Table 2. Various corrosion parameters calculated from the cyclic polarizations curves shown in Figure 6.
Figure 7 shows the cyclic polarization curves of SS304 in Richmond tap water with 1000 ppm chlorides at
a temperature of 150 °F. Table 3 shows the critical corrosion parameters derived from the cyclic
polarization curves shown in Figure 7.
Figure 7. Cyclic voltammetry graphs of Type 304SS in Richmond tap water with 750ppm chlorides at 150 °F
Treatment Ecorr mV Epit mV Erp mV Icorr μA/cm2
Blank 62 180 -5 0.68 50ppm RPSI 42 225 122 0.32
100ppm RPSI 47 420 168 0.14
150ppm RPSI 39 518 219 0.072
Table 3. Corrosion parameters calculated from the cyclic polarizations curves shown in Figure 7
If it clear from Figure 7 and Table 3 that the RPSI treatment program was superior to other treatment
program in terms of higher pitting and repassivation potentials. The observed pitting potential for the
RPSI treatment is 553 mV, well above the generally accepted criteria of 400 mV for pitting resistance.
Similarly, the observed repassivation potential for RPSI of 195 mV is well above the accepted criteria of
150 mV for pitting resistance. The data is conclusive that Type 304 stainless steel will not undergo
localized corrosion under these conditions when treated with 25 ppm RPSI.
Chlorides are the essential contributor to SCC of stainless steels. High chloride concentrations, resulting
from elevated chloride levels in the makeup water, high cycles of concentration, and chlorination, will
increase susceptibility to SCC. SCC in stainless steels mainly occurs at temperatures above 130-140 °F.
Laboratory studies were always conducted at temperatures greater than 200 °F to accelerate the
cracking process. The most likely areas for SCC to be initiated are crevices or areas where the flow of
water is restricted. Hence, stopping crevice corrosion is critical to mitigating SCC in stainless steels.
High temperature autoclaves made of Hastelloy material were used to carrying out immersion studies
with U-bent Type 304SS specimens. The autoclave apparatus is shown in Figure 8.
Figure 8. Autoclave apparatus used to conduct SCC evaluations
Treatment Ecorr mV Epit mV Erp mV Icorr μA/cm2
Blank 97 107
15 ppm Zn -27 128 52 1.42
250 ppm Mo 55 300 120 0.68
25 ppm RPSI 79 553 195 0.12
These Type 304SS coupons were immersed in Richmond tap water with 1000 ppm added chlorides at
220 °F under compressed air pressure. After 15 days of immersion, U-bent coupons were taken out of
the autoclaves, photographed, and examined under microscope for possible localized corrosion and SCC.
Figures 9(a) and 9(b) show the U-bent Type 304SS coupons with no treatment and with 25 ppm of RPSI
treatment.
Figure 9: (a) Type 304SS U-bent coupon immersed in Richmond tap water + 1000ppm chloride at 220 °F under air
pressure for 15 days with no treatment program, (b) with 25 ppm RPSI
It is clear from Figure 9(a) and 9(b) that the untreated coupon exhibited large pits over the entire
surface area, with slightly larger pits at the U-bent. However, the coupon with 25 ppm RPSI looked
clean, with no localized corrosion. General discoloration of the untreated coupon indicates that the
alloy underwent general corrosion, whereas the coupon with 25 ppm RPSI is shiny and clean.
Figure 10(a) and 10(b) show the same coupons at the crevice washers.
Figure 10: (a) picture of the crevice area of SS304 U-bent coupon immersed in Richmond tap water + 1000ppm
chloride at 220 °F under air pressure for 15 days with no treatment program, (b) with 25 ppm RPSI
As seen in Figure 10(a) and 10(b), blank Type 304SS undergoes severe crevice corrosion under these
conditions, whereas 25 ppm of RPSI provides sufficient corrosion inhibition to mitigate the crevice
a b
attack. As mentioned above, crevice corrosion is one of the main reasons for SCC in stainless steels.
This is due to the buildup of corrosion products and reduced or restricted water flow. It can be
concluded that there is high probability for untreated Type 304SS to undergo SCC under these
conditions, wheras 25 ppm RPSI provides localized corrosion inhibition sufficient to mitigate SCC.
Areas around the U-bent were observed under optical microscope for possible SCC. Figures 11(a) and
11(b) show representative images of the exposed specimens in untreated water and in the presence of
25 ppm RPSI.
Figure 11(a): Microscopic images of the untreated Type 304SS coupon at the stressed area (U-bend) and (b) the
Type 304 SS specimen in the same solution chemistry with the addition of 25 ppm RPSI
It can be clearly seen from Figure 11(a) that there is an initiation of stress crack at the U-bend, whereas
there were no cracks with 25 ppm RPSI. From the visual evidence of crack development at the U-bend
as well as smaller cracks observed in the crevice washer area, it can be concluded that Type 304 SS
undergoes SCC under these conditions in the absence of an effective corrosion inhibitor. It is also
evident that the RPSI chemistry effectively inhibits localized corrosion and SCC on Type 304SS.
Gulf Coast Fractionation Plant Case Study:
Natural gas liquids (NGL) fractionation plant separate a Y-grade NGL feed into liquid products – ethane,
propane, butane, isobutane, and natural gasoline. NGL fractionation plants in the Gulf coast frequently
use wet surface air coolers (WSAC) for cooling in addition to conventional induced draft cooling towers
with external heat exchangers. A gulf coast fractionation plant suffered severe scaling of their WSAC
and other heat exchangers due to the inability of the deposit control program to maintain the
phosphate-based corrosion inhibitors and calcium carbonate in solution at the high process fluid
temperatures characteristic of these plants. Scale build up on the WSAC spray nozzles and heat
exchangers reduced the water flow rate, resulting in further loss of heat transfer and increased the
process side exit temperatures. The combined effect was a significant loss of fractionation capacity.
To restore and maintain performance, an online cleaning program was proposed to remove the deposits
and regain its performance, followed by the implementation of a non-phosphorous low fouling
corrosion control program. Commonly used cleaning methods, both acidic and neutral pH, are
aggressive to the base metal, particularly the galvanized WSAC tubes, and can result in excessive
corrosion during the cleaning process. A proprietary cleaning agent was used to enhance the scale
removal on-line under mild conditions. To mitigate the corrosive effects, RPSI chemistry was added to
the systems during the on-line cleaning process. The WSAC system was cleaned slowly over a 1 month
period at a pH of 6.5-7.5 to avoid damaging the galvanized tubes. The induced draft tower with
conventional heat exchangers was cleaned more aggressively over a period of 20 days, with the pH
reaching a low of 4.0.
Figures 12a and 12b show the condition of the WSAC tube bundle before and after the cleaning process.
Figures 13a and 13b show the corresponding thermal imaging photos, clearly indicating the
effectiveness of the program.
Figures 12a and 12b. Photos of the WSAC before cleaning (a) and (b) after cleaning
Figures 13a and 13b. Photos of the WSAC before cleaning (a) and (b) after cleaning
12a 12b
13a 13b
Figures 14a and 14b show the thermal images photos of the depropanizer approach temperatures
before and after cleaning on the induced draft tower.
Figures 14a and 14b. Thermal imaging photos of the depropanizer approach temperatures before cleaning (a)
and (b) after cleaning
The depropanizer approach temperatures during and shortly after the cleaning process on the RPSI
program are shown in Figure 15. Approximately one month after the cleaning process, the system was
returned to the original phosphate based corrosion inhibitor program on 1/19/2016. Approach
temperatures immediately began to increase. The phosphate based cooling program was then
permanently replaced with the non-fouling RPSI chemistry.
Figure 15. Depropanizer approach temperature decreased during the cleaning process and using the RPSI
program. Approach temps again increased on 1/19 as the program was replaced with phosphate. The system
was returned to the non-fouling RPSI program.
14a 14b
The cleaning and RPSI program was estimated to have resulted in a production increase of 2000 to 3000
BPD at an overall annual saving of $8 million.
Conclusions:
The inability of common cooling tower corrosion inhibitors to inhibit to pitting and stress cracking on
stainless steels have limited water utilization efficiency by restricting cycles of concentration in order to
maintain low chloride concentration. Additionally, chloride induced corrosion and SCC have led to the
use of more expensive stainless steel alloys. Apart from providing superior general corrosion rates on
mild steel, a unique non-fouling RPSI chemistry has demonstrated the ability to provide inhibition to
localized corrosion and SCC. The degree of stainless steel protection is significantly better than
phosphate or zinc, and even superior to high levels of molybdate. The RPSI chemistry enables the
industries to push the COCs higher and thereby saving water, providing superior asset protection, and
reducing the overall cost of plant operation.
References:
1. Post, R.M., Kalakodimi, R.P., Tribble, R.H., Development and Application of Phosphorus Free
Cooling Water Treatment, Cooling Technology Institute, Houston, February, 2014.
2. Post, R.M, Kalakodimi, R.P., Tribble, R.H., Lamm, J, Nelson, J.L., Advances in Pretreatment,
Passivation, and Layup of Cooling Systems, International Water Conference, IWC 15-75, Orlando,
November 2015.
3. Post, R.M., Kalakodimi, R.P., Tribble, R.H., Advancements in Cleaning and Passivation of Cooling
Systems, Cooling Technology Institute, Houston, February, 2016.
4. Kalakodimi, R.P., Tribble, R.H., and Post, R.M., Advancements in Cleaning and Passivation of
Cooling Water Systems, Cooling Technology Institute, New Orleans, February, 2017.