Cooling towers serve a crucial role at many chemical plants but often receive only passing attention. However, cooling system fouling, scaling and corrosion can incur enormous costs related to materials replacement and from efficiency degradation and perhaps even lost production. Fortunately, cooling tower water-treatment technologies and programs have evolved significantly over the last decade, in large part due to more-stringent regulations regarding discharge and choice of makeup water supplies.
Today, plants generally can take steps to effectively combat fouling, scaling and corrosion. Let’s look at how to address these issues.
An obvious but challenging aspect of cooling tower operation is keeping the tower, condenser and other system components free of microbiological fouling, scaling and solids deposition. Even with fresh water from a lake or river as makeup, cooling system chemistry control requires diligence and good planning.
Cooling systems provide an ideal environment — warm and wet — for microbial colony formation. Bacteria will grow in condensers and on cooling tower fill (Figure 1), fungi on and in cooling tower wood, and algae on wetted cooling tower components exposed to sunlight. Biocide treatment is absolutely essential to maintain cooling system performance and integrity.
Bacteria fall into the following three categories:
• aerobic, which utilize oxygen in the metabolic process;
• anaerobic, which live in oxygen-free environments and use other sources, i.e., sulfates, nitrates or other donors for their energy supply; and
• facultative, which can thrive in aerobic or anaerobic environments.
A problem with microbes, particularly bacteria, is that once they settle on a surface the organisms secrete a slimy polysaccharide film for protection. This film then will collect silt from the water, thus growing even thicker. Even though the bacteria at the surface may be aerobic, the slime layer allows anaerobic bacteria underneath to flourish. These organisms in turn can generate acids and other harmful compounds that directly attack the metal. Microbial deposits also establish concentration cells, where the lack of oxygen underneath the deposit causes the locations to become anodic to other areas of exposed metal. This often results in pitting, which can cause tube failure well before the expected lifetime of the material.
Fungi will attack cooling tower wood in an irreversible manner, which can eventually lead to structural failure. Algae will foul cooling tower spray decks, potentially reducing performance and creating unsafe working locations. (See: “Enter a Tower with Caution”).
Algae are also a food source for amoeba and other protozoans, which in turn are important in the growth and amplification of airborne pathogens, such as Legionella pneumophila, the bacteria that cause Legionnaires disease .
The core of most microbiological treatment programs is an oxidizing biocide to kill organisms before they can settle on heat exchanger tube surfaces, cooling tower fill and other locations. Chlorine was the workhorse for many years; adding gaseous chlorine to water causes the following reaction:
Cl2 + H2O ⇔ HOCl + HCl
Hypochlorous acid (HOCl) is the killing agent, as it oxidizes internal cell components and disrupts the organism’s metabolic processes. Due to safety concerns with gaseous chlorine, many plants switched to bleach (sodium hypochlorite, NaOCl) as the oxidizing biocide. And most recently, reliable systems have been developed that generate chlorine on-site via electrolysis of salt solutions. These use ordinary salt, softened water and electricity to produce a dilute solution of hypochlorite, which is safer than storing and handling large quantities of sodium hypochlorite .
Regardless of the source of chlorine, pH greatly affects the functionality and killing power of the chemicals due to the equilibrium nature of HOCl in water:
HOCl ⇔ H+ + OCl-
OCl- is a much weaker biocide than HOCl, probably because the charge on the OCl- ion doesn’t allow it to penetrate cell walls. The dissociation of HOCl rapidly increases as the pH rises above neutral (Figure 2). Thus, for common alkaline-scale/corrosion-treatment programs, chlorine chemistry may not work well.
Ammonia or amines in the water further compromise performance by reacting irreversibly with chlorine to form much less potent chloramines. This is becoming more of a factor because both existing and especially new plants — by choice or mandate — increasingly are moving away from fresh water for makeup and relying upon alternative sources. A prime example is use of secondary wastewater from a publicly owned treatment works. This water often contains significant concentrations of ammonia, phosphorus and organics. If fed directly to the cooling tower, water containing these microbiological nutrients and food can totally disrupt any chemical treatment. Technologies such as membrane bioreactors, moving bed bioreactors and others are becoming more common as pretreatment methods for makeup water . (For details on some specific developments, see: “Wastewater Treatment Gets a New Spin.”)