Chemical useful in cooling tower applications.
by: Scott Marckini
Oxidation chemistry has long been an accepted and effective part of many water treatment programs. Oxidizing chemicals used in today’s water treatment programs include: chlorine, chlorine dioxide, bromine, bromine/chlorine releasing compounds, ozone and hydrogen peroxide.
Oxidizing microbiocides are often found at the forefront of many cooling water treatment programs. In large volume or once-through cooling systems they are usually the primary biocide and often are the most cost-effective programs available to a plant.
When selecting these economical and versatile chemicals, several factors should be considered before a technically sound program is implemented. Environmental and regulatory impact, system pH, process contamination, and equipment capital and maintenance expense all play a role in the decision-making process.
The primary killing mechanism these types of microbiocides use is oxidizing protein groups within a microorganism. Proteins are the basic components of essential cellular enzymes that are necessary for life-sustaining cellular processes such as respiration. The destruction of these proteins deprives the cell of its ability to carry out fundamental life functions and quickly kills it.
One oxidant is chlorine dioxide, which appears to provide an additional killing mechanism. Chlorine dioxide is able to diffuse readily through hydrophobic lipid layers of an organism, allowing it to react with cellular amino acids, which directly inhibits protein synthesis. Since amino acids are the basic building blocks of all cellular proteins, destruction of these molecules has a devastating effect on the microorganism.
Chlorine gas is likely the most widely used oxidizing microbiocide. It has traditionally been the biocide of choice in many cooling water treatment systems. It is a strong oxidizer that is relatively easy to feed and is quite inexpensive. Upon introduction into the water stream, chlorine hydrolyzes into hypochlorous acid (HOCl) and hydrochloric acid (HCl).
This hydrolization provides the active toxicant, HOCl, which is pH-dependent. In alkaline cooling systems, it readily dissociates to form the hypochlorite ion (OCl-). This dissociation phenomenon is important to remember when working with systems that will operate at a higher pH. In alkaline conditions, OCl- becomes the predominant species and lacks the biocidal efficacy of the non- dissociated form. Considerably more HOCl is present at a pH of 7.0 than at pH 8.5.
It is also widely known that chlorine is non-selective, making it very sensitive to contamination from either cooling water makeup or from in-plant process leaks. Ammonia, organic acids and organic compounds, sulfides, iron and manganese all easily react with HOCl. The amount of chlorine needed to react with these contamination species is referred to as chlorine demand and it must be satisfied before active HOCl is available to provide a free chlorine residual.
The combination of high chlorine demand in process-contaminated systems and the dissociation process in alkaline systems creates the need for greater chlorine feed to obtain the same microbial efficacy. This results in a higher concentration of HCl in the cooling system. Since HCl removes alkalinity, pH depression and system corrosion could occur. In low pH water the passive, metal oxide layers protecting the metal may resolubulize, exposing the surface to corrosion. At free mineral acidity (pH <4.3), many passivating inhibitors become ineffective, and corrosion will proceed rapidly. Increased chloride may also have a negative impact on system corrosion. The chloride ion (Cl-) can damage or penetrate the passive oxide layer, leading to localized damage of the metal surface.
High chlorine concentrations have also been shown to directly attack traditional organic-based corrosion inhibitors. When these inhibitors are “deactivated,” the metal surface would then be susceptible to corrosion. Process Safety Management (PSM) guidelines dictated by the U.S. Occupational Safety and Health Adminstration (OSHA), discharge problems related to chlorinated organic compounds such as trihalomethane (THM), dezincafication of admiralty brass and delignification of cooling tower wood are other significant concerns associated with the use of chlorine.
Sodium Hypochlorite Widely Used
The hypochlorite form of chlorine has been used since 1850. The most widely used form of hypochlorite is the liquid, sodium hypochlorite (NaOCl), with more than 150 tons per day consumed in the United States.
Sodium hypochlorite application in cooling water is essentially the same as with gas chlorine; HOCl is produced as the active toxicant. The HOCl is equally susceptible to process contamination, has the same chlorine demand as gas chlorine and displays the same tendency to dissociate.
Sodium hypochlorite differs from chlorine gas in two respects: method of feed and hydrolization properties. Sodium hypochlorite can either be gravity-fed or applied with a metering pump. The latter is generally recognized as a consistently more accurate method. The second difference, in hydrolysis, lies in the end products. The NaOCl reaction with water liberates sodium hydroxide (NaOH).
The addition of NaOH differs in that it tends to add alkalinity to the water. In large concentrations it may artificially elevate pH, leading to precipitation of calcium carbonate.
While NaOCl eliminates low pH corrosion as a concern, the use of large quantities in contaminated systems still introduces a high concentration of the chloride ion, which can be very aggressive to cooling system metals. Many of the other problems associated with chlorine remain present with sodium hypochlorite.
Chlorine Dioxide Effective Alternative
Chlorine dioxide (ClO2), long used in the paper industry, has been an acceptable and effective alternative to chlorination in cooling systems. Chlorine dioxide is a yellow-green gas with an irritating odor not unlike chlorine. It cannot be compressed and shipped in a container, so it must be generated on site.
There are three proven methods of efficiently generating chlorine dioxide. The most common is the chlorine/chlorite or “one pump” method. ClO2 generation uses sodium chlorite (NaClO2) and chlorine gas.
Chlorine gas is educted into a motive water stream in a ClO2 generator forming HOCl and HCl. Sodium chlorite is pumped into the stream and allowed to react in a generating column to produce ClO2.
A second, common method of generation uses NaOCl and HCl in place of chlorine gas. Also referred to as the “three pump” method of generation, this method is valuable to a facility that wants to eliminate gaseous chlorine.
A third, more recent method of generation uses sodium chlorate (NaClO3) and sulfuric acid. This differs from the other two methods in that ClO2 is generated in a vacuum and is then educted into the motive water stream.
Chlorine dioxide holds many advantages over chlorine in cooling water systems. ClO2 is considerably more selective than chlorine in the presence of various compounds, which allows it to be more effective in contaminated systems. Table 2 lists a series of compounds for which chlorine would show a greater affinity than ClO2. Under certain conditions ClO2 may, in fact, be two-and-one-half times more reactive than chlorine. Under efficient ClO2 generation, THMs are not formed and THM precursors are reduced. In one application, THM formation was reduced from 34 µ g/l to 1 µ g/l using ClO2.
Chlorine dioxide does not hydrolyze in water as does chlorine and there is no dissociation of ClO2. It remains fully active in a pH range far broader than chlorine or sodium hypochlorite. Since ClO2 remains a gas in water, it does not have the corrosive tendencies of chlorine gas. Its selectivity generally allows for lower dosages compared to chlorine, limiting the amount of aggressive Cl- available to attack passivated metal surfaces. Finally, ClO2 is much less aggressive to traditional corrosion inhibitors.
Hypochlorous acid, whether formed from the dissolution of chlorine gas or sodium hypochlorite in water, has satisfactorily controlled microorganisms in cooling water systems. However, dissolution does yield a mineral acid or caustic soda which may adversely affect system pH, inhibitor passivation layers or metal surfaces.
Hypochlorous acid is heavily pH-dependent, because as system pH increases, there is a correspondingly rapid decrease in the concentration of the biocidally active species. It is also a non-specific oxidant which readily reacts with various organic and inorganic compounds that may be present in a cooling water system. Some of these reactions tend to yield undesirable by- products which are regulated or may be regulated in the future.
The effects of pH on hypochlorous acid and its reactivity with a variety of compounds both combine to vastly diminish its effectiveness in contaminated, high-pH cooling water systems. Conversely, chlorine dioxide remains completely pH-independent in the range where recirculating and once-through cooling systems are typically operated. Since ClO2 is a dissolved gas in water, there is no mineral acid or caustic soda formation as happens when using HOCl. Chlorine dioxide tends to be much less, if not totally non-reactive, with many organic and inorganic compounds.
Scott A. Marckini is employed by Ashland Chemical Co., Drew Industrial Division, Boonton, NJ. This article was adapted from a presentation at the International Water Conference, October 1996.