Advanced cooling water treatment concepts (Part 1)

Editor’s note: This is the first of a multi-part series by Brad Buecker, President of Buecker & Associates, LLC.

Power and co-generation facilities have multiple cooling systems for various plant processes such as steam condensation, chemical reactor temperature control, rotating equipment bearing cooling, lubricating oil cooling and many others.

Cooling systems need protection from corrosion, scaling, and microbiological fouling to maximize performance and prevent upsets that can partially or completely shut down plant operations. A representation of these issues and their interdependence is shown below.

A fourth, increasingly important factor is the potential environmental impact of water treatment chemistry, especially regarding chemicals that might appear in the plant discharge. Treatment programs that were once commonplace may no longer be allowed, or may be severely restricted due to discharge regulations.  

In this series, we will examine treatment concepts to minimize these issues in cooling networks, including once-through systems, open-recirculating systems with cooling towers, and closed systems. Treatment programs have significantly evolved over the last several decades, with additional research underway.

An overview of open-recirculating systems

The most complex water treatment issues typically occur in open-recirculating systems, where a cooling tower or hybrid version thereof is the heart of the system. Figure 2 outlines the basic flow path of a cooling tower-based system.

Several aspects of these systems enhance, or perhaps the better word is exacerbate, scaling, fouling and corrosion potential. Foremost is that the bulk of heat transfer in a tower occurs from the evaporation of a small amount, perhaps 2-3%, of the warm return water.

(For those interested in learning more about cooling tower design and fundamental heat transfer, references 1-3 provide additional information. A review of these references will be helpful for understanding this series. Also, a wonderful resource for detailed cooling water and cooling system information is the Cooling Technology Institute at www.cti.org.) 

As water evaporates, dissolved and suspended solids concentrate, which increases the scaling/corrosion potential of the cooling water. Open-recirculating systems are typically equipped with blowdown controls to bleed off concentrated water and replace it with fresh makeup, but blowdown usually only offers a partial solution to scale/corrosion control. 

Cooling towers are also highly efficient air scrubbers. Thus, they remove airborne dust and particulates and microbes, the latter of which also enter with makeup water. Without proper treatment, the warm and wet environment of cooling systems allows rapid microbiological growth that will foul cooling system components including heat exchangers and cooling tower fill.

To a large extent, scale and corrosion control treatment methods have evolved together; and in the first two parts of this series, we will examine important aspects of this history and the emergence of more effective technologies. Later installments of this series will examine microbiological fouling and control, which are often the primary concerns of cooling water treatment.

Scale and corrosion control methods

In the middle of the last century, a very reliable scale/corrosion treatment method emerged that we will now quickly explore. First, consider that many systems at that time had a fresh water source such as lake, reservoir, or possibly treated municipal water. Even these pristine sources still contain dissolved ions, which, if concentrated in a cooling tower system, can form scale. Two of the most common ions are calcium (Ca²⁺) and bicarbonate alkalinity (HCO₃^–), whose scale-forming potential increases with increasing concentration and also temperature.

            Ca²⁺ + 2HCO₃^– + heat –> CaCO₃ + CO₂­ + H₂O                            Eq. 1

In untreated waters, CaCO₃ is usually the predominant scaling compound. This is the same deposit that forms in home hot water piping and showerheads, and is somewhat incorrectly referred to as “lime scale.”  

A very effective solution to reduce CaCO₃ scaling potential is addition of sulfuric acid to convert bicarbonate alkalinity into carbon dioxide that escapes from solution.

            HCO₃^–_(aq) + H₂SO₄ –> HSO₄^2-_(aq) + H₂CO_{3 (aq)}                        Eq. 2

            H₂CO_{3 (aq)} ⇌ CO₂­ + H₂O                                                                  Eq. 3

Acid feed to the makeup or cooling water to reduce bicarbonate alkalinity proved straightforward for scale control, although instances are well-known where a system malfunction greatly lowered the cooling water pH and caused problems.

From a corrosion standpoint, the primary material for many cooling water systems is mild carbon steel, with perhaps stainless steel or copper alloys as the material for heat exchanger tubes. Accordingly, in the middle of the last century a hugely popular treatment program for open-recirculating systems consisted of sulfuric acid feed for scale control (to establish a pH range of 6.5-7.0), and use of disodium chromate (Na₂Cr₂O₇) for corrosion control. This latter compound provides chromate ions (CrO₄^2-) that react with carbon steel to form a pseudo stainless-steel layer that passivates the metal surface. Protecting metal surfaces is the key aspect for corrosion control, and we will return to that thought later.

In the 1970s and 1980s, a growing recognition of hexavalent chromium (Cr⁶⁺) toxicity led to a ban on chromium discharge to the environment. This essentially eliminated chromate treatment for open cooling water systems. The general replacement program was quite different, with a key factor being operation at a mildly basic pH (typically around 8.0 or perhaps a bit higher) to assist with corrosion control. Figure 3 illustrates the fundamental concept of such chemistry.

Sodium phosphates, e.g., tri-sodium ortho-phosphate (Na₃PO₄) and sodium hexametaphosphate (NaPO₃)₆), which partially reverts to ortho-phosphate in water) were the first choices for pH control. However, problems from calcium phosphate (Ca₃(PO₄)₂) deposition became nearly as severe as those of calcium carbonate previously. So, programs evolved to include the use of organic phosphates, more commonly known as phosphonates. The structures of several of the most common are shown below.

These compounds helped to reduce scale formation by several methods including ion sequestration and crystal modification. ATMP was introduced in the early 1970s for calcium carbonate scale control, and served as a replacement for polyphosphates. The compound exhibited fair to good corrosion inhibitor properties at the alkaline pH ranges of the (then new) phosphate-phosphonate programs, however, it has a low tolerance for oxidizers like chlorine, and it can also produce calcium-phosphonate precipitates.

The other phosphonates shown in Figure 4 were developed, in part, to resist oxidizing biocide decomposition. However, note the carboxylic acid groups (COOH) on PBTC and HPA. Carboxylate (COO^–) is a key functional group for many deposit-control dispersants, as will be further outlined in Part 2.

Researchers found that phosphate/phosphonate chemistry could also assist with corrosion control. Depending on the reaction product, phosphate and polyphosphate precipitates will settle at anodic and cathodic sites of corrosion cells and reduce the corrosion current. 

The addition of a small amount of zinc became common, as it forms a zinc hydroxide (Zn(OH)₂) deposit at cathodes that further inhibits electron transfer. Additional refinements included development of polymers to control calcium phosphate precipitation and other deposition. We will examine some of these polymers in the next installment.

Cracks in the armor

While phosphate/phosphonate programs have been successful in many applications, the methodology can have significant flaws. As was previously mentioned, the primary purpose of corrosion control chemistry is to protect metal surfaces. The corrosion-inhibiting precipitates formed by the chemistry are not tightly adherent and can wash away due to flow imbalances or other issues. (4) This may expose some surfaces to localized corrosion. Conversely, excess chemical feed can generate heavy deposits of calcium phosphate or perhaps even calcium phosphonate. Proper control may be a “walking on the razor’s edge” proposition.

A growing influence on treatment chemistry is that some facilities are required to take makeup from other sources besides fresh water supplies. An increasingly common, and in some arid locations, mandated, makeup source is discharge from a municipal wastewater treatment plant, aka, Publicly Owned Treatment Works (POTW). Often, POTW effluent contains a significant and variable phosphate concentration, which can have a strong effect on cooling and service water chemistry at the industrial plants taking that makeup.

Finally, of growing concern is phosphorus discharge to natural bodies of water, and the effects such discharge has on proliferation of toxic algae blooms.

Many people think of these algae blooms as being confined to warm locations such as Florida or the Gulf of Mexico, but as Figure 4 indicates, almost any water body may be susceptible. At many locations now, phosphorus discharge is limited if not entirely banned. Also being restricted is metals discharge, including zinc and copper. Such restrictions may completely eliminate the option of phosphate/phosphonate treatment chemistry.

The issues outlined above are leading to substantial movement away from traditional cooling water scale/corrosion treatment programs. Part 2 will provide an overview of combined film-forming and polymer technologies that offer better corrosion and deposit control, while at the same time greatly reducing the potential environmental impact of discharge chemistry.

This discussion represents general concepts for consideration and does not offer direct opinions from any water treatment chemical company.  Every project must be evaluated individually, with selection of treatment programs based on careful evaluation of system design and makeup water chemistry.

References

1. B. Buecker and R. Aull, “Cooling Tower Heat Transfer 101”; Power Engineering, July 2010.
2. B. Buecker and R. Aull, “Cooling Tower Heat Transfer 201”; Power Engineering, November 2010.
3. J.C. Hensley, ed., Cooling Tower Fundamentals, Second Edition; SPX Cooling Technologies, Overland Park, Kansas, 2009.
4. R. Post, B. Buecker and S. Shulder, “Power Plant Cooling Water Fundamentals”; pre-workshop seminar to the 37^th Annual Electric Utility Chemistry Workshop, June 6-8, 2017, Champaign, Illinois.

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He may be reached at beakertoo@aol.com.

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