Benefits of Novel Cooling Water Chemistry on Refinery Economics

Transcription

1 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.

2 Introduction Cooling water treatment from the 1930s through the early 1980s relied primarily upon ppm hexavalent chromium to inhibit corrosion of steel and copper alloys in conjunction with acid to maintain the ph of the system in the 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

3 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). Figure 1. Electrochemical cyclic polarization graphs of mild steel coupon in Richmond tap water 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).

4 RPSI 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

5 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 CaCO 3, 100 mg/l Mg as CaCO 3, 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 o C. 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 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.

6 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. Figure 4. Schematic of the electrochemical setup. WE: Working Electrode; RE: Reference Electrode, CE: Counter Electrode Figure 5. Schematic of the cyclic polarization curve. E corr : Corrosion Potential; E b : Breakdown potential; E pass : Repassivation potential; I corr : Corrosion Current From the cyclic polarization graphs, important parameters can be calculated such as: Pitting potential (E pit ) = E b - E corr Repassivation Potential (E RP ) = E pass - E corr It is generally accepted that an E pit value of > mv coupled with an E RP 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,

7 the E pit and E RP satisfy the requirement of E pit >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 Treatment E corr mv E pit mv E rp mv I corr μa/cm 2 Blank ppm RPSI ppm RPSI ppm RPSI 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

8 Treatment E corr mv E pit mv E rp mv I corr μa/cm 2 Blank ppm Zn ppm Mo ppm RPSI 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 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

9 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. a b Figure 9: (a) Type 304SS U-bent coupon immersed in Richmond tap water ppm 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 ppm 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

10 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

11 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 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. 12a 12b Figures 12a and 12b. Photos of the WSAC before cleaning (a) and (b) after cleaning 13a 13b Figures 13a and 13b. Photos of the WSAC before cleaning (a) and (b) after cleaning

12 Figures 14a and 14b show the thermal images photos of the depropanizer approach temperatures before and after cleaning on the induced draft tower. 14a 14b 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.

13 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, 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 Post, R.M., Kalakodimi, R.P., Tribble, R.H., Advancements in Cleaning and Passivation of Cooling Systems, Cooling Technology Institute, Houston, February, 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.