June 2020

Water Management

Advanced cooling tower water treatment—Part 1

Refineries, petrochemical plants and similar facilities rely on a significant number of heat exchangers for process control and product formulation throughout the plant.

Refineries, petrochemical plants and similar facilities rely on a significant number of heat exchangers for process control and product formulation throughout the plant. Distillation, cracking and reforming, polymerization, steam generation, etc., require heat addition and heat removal.

While many heat exchangers may supply closed-loop circuits, their cooling water supply comes from cooling towers. Others, such as steam surface condensers, are directly supplied by raw cooling water. Thus, it is not unusual to see many cooling towers dotted over the vast landscape of a refinery or chemical plant. Because the towers often sit in somewhat isolated locations, it is easy to overlook cooling water treatment until a problem occurs that forces a shutdown. For example, microbiological fouling of cooling tower film fill has, in some cases, caused partial or full tower collapse. Thus, establishing and maintaining the proper chemistry in these cooling systems is of paramount importance.

In Part 1 of this series, the authors examine the evolution of scale and corrosion control chemistry for cooling tower systems. Part 2, which will be published in the July issue, will focus on control of microbiological fouling. Continuing improvements to treatment programs allow for enhanced performance of cooling systems and better protection of the environment.

The good old days

To understand the evolution of cooling water chemistry, a brief review of the most common cooling water corrosion mechanisms is appropriate. All corrosion mechanisms are electrochemical in nature, although some—such as erosion corrosion—are also influenced by mechanical factors. FIG. 1 provides a schematic of the primary corrosion mechanism of carbon steel—the most common cooling system piping material—in aerated water.

Iron (Fe) is oxidized at the anode and enters the solution as ferrous ion (Fe⁺²). The process releases electrons that flow through the metal to the cathode, where the electrons reduce dissolved oxygen to hydroxyl ions (OH^–). Hydroxyl ions then react with the Fe ions to complete the electrical circuit and form an initial product of Fe(OH)₂, which continues to oxidize to form rust—with a basic formula of Fe₂O₃.xH₂O. Uncontrolled oxygen attack can cause severe damage in piping networks, and can also generate deposits that may partially or completely restrict flow (FIG. 2).

Corrosion inhibitors function by slowing down reactions at either the anode, the cathode or sometimes both. For a long time during the middle of the last century, chromate (CrO₄^–2) was very popular for corrosion control in many cooling systems—both open-recirculating and closed. While CrO₄^–2 is considered an anodic inhibitor, with enough dosage, it will form a complete surface layer of Fe chromate (pseudo-stainless steel), which can be quite protective. CrO₄^–2 programs were often coupled with acid feed to react with bicarbonate ion (HCO₃^–) and convert it to carbon dioxide (CO₂) that escapes. Removal of alkalinity greatly reduces the potential for calcium carbonate (CaCO₃) scaling, which is typically the first mineral deposit that would otherwise precipitate without treatment. CrO₄^–2/acid chemistry is very straightforward and effective; however, environmental issues regarding chromium discharge, particularly in relation to the toxicity of hexavalent chromium (Cr⁺⁶), essentially led to the abandonment of this treatment method.

Phosphorus chemistry to the rescue?

With the phase-out of CrO₄^2–, alternative treatment methods became a priority. For 40 yr, the most common treatment programs for large industrial cooling tower-based systems have relied on a combination of inorganic and organic phosphate (i.e., phosphonate) chemistry for both scale and corrosion control. These programs typically function at a mildly alkaline pH, which minimizes general corrosion, but at the cost, without proper protection, of increased scaling potential (FIG. 3).

The chemistry also provides more specific corrosion protection, as phosphate will react with Fe⁺² produced at anodic sites to form a reaction-limiting deposit, while calcium phosphate [Ca₃(PO₄)₂] precipitates in the local alkaline environment at cathodic sites to inhibit electron transfer. However, even small upsets in phosphate programs can cause severe Ca₃(PO₄)₂ fouling, and, at one time, excess Ca₃(PO₄)₂ deposition became almost as great a problem as calcium carbonate scaling had been before. Accordingly, treatment methods evolved to more forgiving methodologies, where in many cases, the backbone of these programs are organic phosphates (phosphonates), with a supplemental polymer to control Ca₃(PO₄)₂ deposition. Phosphonates attach to crystal nuclei in solution and limit deposition by disrupting crystal growth and lattice strength (FIG. 4).

A common phosphate/phosphonate treatment program might include one or two of the phosphonate compounds in low mg/l dosages for primary scale control, 5 mg/l–15 mg/l of orthophosphate and some polyphosphate for additional scale control and corrosion protection. Phosphate programs are sometimes supplemented with 0.5 mg/l–2.5 mg/l of zinc (Zn) for improved corrosion control. Zn reacts with the hydroxyl ions generated at cathodes to form a precipitate [Zn(OH)₂], which provides additional cathodic protection. However, Zn solubility is also a strong function of temperature and pH, further increasing deposition on hotter heat exchangers. Moreover, Zn discharge is also tightly regulated due to its effects on aquatic life. It is included on the U.S. Environmental Protection Agency (EPA) list of 129 priority pollutants and 65 toxic pollutants. Typically included in these formulations is 5 mg/l–10 mg/l of dispersant polymer to control Ca₃(PO₄)₂ and zinc hydroxide deposition.

Phosphate/zinc programs are far from simple, and under- or over-feed can result in either corrosion or scale formation. Even with seemingly proper chemistry, the corrosion-inhibiting deposits are porous, and may wash away. Beyond those issues, two important factors are driving an evolution away from phosphate-based chemistry towards polymer treatment methods. One is the increasingly problematic issue of phosphorus discharge and its effects on the generation of toxic algae blooms in receiving bodies of water. The second is the growing evidence that well-formulated polymer programs are more effective—from both a performance and economic standpoint—than phosphate/phosphonate chemistry for scale prevention and corrosion protection.

Influence of phosphate in the natural environment

Phosphorus, along with nitrogen and carbon, is a macronutrient that is essential for all life forms. Algae derive their carbon requirements from inorganic bicarbonate and carbonate, utilizing energy from sunlight to convert the inorganic carbon into organic carbon for cellular tissue growth. Some species of algae are also capable of “fixing” atmospheric nitrogen gas, using the nitrogenase enzyme to convert N₂ into ammonia and other compounds required for the biosynthesis of nucleic acids and proteins. Common among the photosynthetic nitrogen fixing species are cyanobacteria, commonly referred to as “blue-green algae.” Phosphorus is often the limiting nutrient for growth in aquatic systems because it is present in very low concentrations relative to that required by plants and microorganisms.

Cyanobacteria are known for their extensive and highly visible green blooms. FIG. 5 shows an aerial photograph of a cyanobacteria bloom in the shallow western basin of Lake Erie in 2011. The unpleasant and unsightly algae growth resulted in fouled beaches, sharply reduced tourism and a decline in fish populations. Apart from their noxious sensory impact, cyanobacteria also produce microcystins and other cyanotoxins that are toxic to fish, birds and mammals. Many lakes closed in 2019 to recreational activities due to concern over the health effects of harmful algal blooms (HAB).

The presence of phosphorus in aquatic systems is also problematic because it ultimately leads to a reduction in dissolved oxygen, which is required by fish and other aquatic life forms. Dissolved oxygen is consumed rapidly by bacteria associated with the decay of algae, resulting in hypoxic conditions (< 2 mg/l dissolved oxygen) that do not support aquatic life. A notable example is the hypoxic or “dead” zone in the Gulf of Mexico, influenced by nutrient loading from the Mississippi River and other nutrient-laden streams that enter the Gulf. In the summer of 2017, the hypoxic area reached 22,730 km² in size, and stretched from the Louisiana-Alabama coast westward to the Texas border.

The emergence of polymer and other non-phosphorous chemistry

Polymer formulations containing the carboxylate group (FIG. 6) have been successfully utilized for decades to control CaCO₃ scale in cooling water.

However, many other scaling compounds are possible, including calcium and magnesium silicates, calcium sulfate, calcium fluoride and manganese dioxide. The need to combat these and other scale-formers has generated development of co- and ter-polymers, containing more than one functional group. These compounds act as crystal modifiers and sequestering agents, and, when tailored properly, each application can be quite effective at low concentrations.

Corrosion protection with non-phosphorous chemistry

The chemistry outlined has proven very effective for scale inhibition, but what about corrosion control? Significant development has occurred in that area, as well. Particularly effective is a chemical formulation that can be described generically as a reactive polyhydroxy starch inhibitor (RPSI).^a In this chemistry, the compounds, by virtue of many active sites on the molecules, attach to the base metal and form a protective layer.

Initial laboratory tests of RPSI illustrated the effectiveness of the chemistry in providing a synergistic combination of anodic and cathodic corrosion inhibition. FIG. 7 is a cyclic polarization evaluation of the compound as compared to the cathodic inhibitor, Zn and the anodic inhibitor (ortho-phosphate).

As indicated in FIG. 7, 12.5 mg/l of the nonphosphorus RPSI is shown to inhibit the cathodic corrosion reaction as effectively as 5 mg/l Zn and inhibit the anodic reaction as effectively as 15 mg/l of ortho-phosphate. The figure clearly illustrates the dual cathodic and anodic inhibition, indicative of RPSI’s film-forming nature.

Full-scale application of the chemistry has proven to be more effective than phosphate chemistry. During late 2015, a phosphate-based program at a large U.S. Gulf Coast chemical plant was replaced by a non-phosphate and non-Zn technology to mitigate the corrosion and scaling issues on the high-temperature (71°C/160°F) heat exchangers. After a year of using the improved chemistry, the equipment inspection during the turnaround cycle showed much cleaner heat exchangers. The Fe and copper (Cu) in the tower also decreased, consistent with the improved corrosion performance. Further improvements to the non-phosphorous, non-Zn chemistry were implemented after the 2017 turnaround, including the addition of a halogen stable triazole for Cu inhibition. This resulted in a dramatic improvement in both mild steel and Cu corrosion rates and heat transfer efficiency, as well as maintaining Fe and Cu levels in the water at historically low levels.

Corrosion is an electrochemical reaction, and the corrosion rate will roughly double with every 10°C increase in temperature, similarly to most chemical reactions. Placing the corrosion coupons at the outlet of the hottest heat exchangers provides a severe, but realistic, indication of the corrosion at the exchanger outlet. FIG. 8 shows the mild-steel corrosion coupons that were placed at the exit of the hottest heat exchangers.

The heat exchanger results on the phosphate program from the 2015 turnaround (FIG. 9) can be contrasted with the results of the non-phosphorous, non-Zn program in FIG. 10. Conditions under the phosphate program exhibited significant corrosion in the lower part of the bundle where there is insufficient phosphate to form a barrier film. The upper part of the bundle is hotter and induced substantial phosphate deposition. With the non-phosphorous, non-Zn program (FIG. 10), the heat exchanger shows minimal deposition in the hot upper part of the bundle and little to no corrosion in the colder zone at the bottom.

Tower Fe and Cu levels

When corrosion occurs in a recirculating cooling system, corrosion products, primarily Fe and Cu, are released to the cooling water. The concentration of corrosion products in the cooling water is often a more accurate indicator of actual equipment corrosion than corrosion coupon results, especially in the chemical and hydrocarbon processing applications where the heat exchanger surfaces are frequently much hotter than the cooling water in a coupon rack. Cooling tower Fe and Cu concentrations declined steadily, to vanishingly low levels, after transition to the non-phosphorous, non-Zn program in late 2015, confirming that cooling system assets are effectively protected (FIGS. 11 and 12).

Since implementing the non-phosphorous technology, the large chemical plant has been able to extend its turnaround schedule from 1 yr to 2 yr., resulting in significant production increases and reduced cleaning costs associated with removing phosphate deposits in high-temperature bundles.

Potential application for stainless-steel (SS) protection

SS is the material of choice for heat exchanger tubes in many applications. However, austenitic stainless steels, such as common grades 304 and 316, are susceptible to stress corrosion cracking (SCC) at seemingly low-chloride concentrations with increasing temperature.²

The ability of RPSI to potentially protect 304 SS from SCC has been demonstrated in laboratory tests, in which U-bend stressed specimens were placed for 15 d in a bath of 105°C tap water spiked with 1,000 mg/l of chloride. FIG. 13 shows the condition of the samples following the test.

The untreated specimen is clearly tarnished and pitted. The specimen exposed to the same solution containing the RPSI corrosion inhibitor shows no evidence of tarnish or pitting. Under magnification, cracking is evident at the bend of the untreated specimen, while the treated specimen shows no indication of cracking.

Environmental sustainability

The original goal behind the development of non-phosphorus and non-Zn chemistry was to provide a more environmentally sustainable alternative to phosphates in cooling systems. The RPSI corrosion inhibitor chemistry is formulated into several finished products, typically applied at a dosage of 100 mg/l, which have an LC₅₀ per common U.S. EPA aquatic marker organisms in the range of 1,000 mg/l–10,000 mg/l. In most cases, the products can be formulated at mild pH ranges, making them less hazardous to handle than many products that must be prepared at extreme pH values.

Takeaway

Scale and corrosion control in refinery and petrochemical cooling water systems are of primary importance in maintaining reliability. Corrosion and fouling can directly affect the bottom line, and, most importantly, sometimes present safety issues. Technologies that have emerged and continue to be enhanced to improve cooling water chemistry have been outlined here. This article offers only a general overview. Any facility that decides to adopt a treatment program should conduct due diligence and consult with water treatment experts. These enhanced chemistries, like their predecessors, may be quite ineffective without proper microbiological control of the cooling water. Formation of bacterial colonies and accompanying protective slime layers, or accumulation of other microorganisms, can wreak havoc in cooling systems. Part 2 will focus on improved methods to control microbiological fouling. HP

NOTES

       ^a Refers to ChemTreat’s FlexPro technology

ACKNOWLEDGEMENT

      The authors wish to acknowledge Bryan Shipman, ChemTreat Area Manager, for providing data on the case history outlined in this article.

LITERATURE CITED

1. Post, R., B. Buecker and S. Shulder, “Power Plant Cooling Water Fundamentals,” pre-conference seminar to the 37th Annual Electric Utility Chemistry Workshop, Champaign, Illinois, June 2017.
2. D. Janikowski, “Factors for Selecting Reliable Heat Exchanger Materials,” presented at the 33rd Annual Electric Utility Chemistry Workshop, Champaign, Illinois, June 2013.

The Authors

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