Chemical Treatment For Cooling Water

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Process Engineering Guide: GBHE-PEG-UTL-901

Chemical Treatment For Cooling Water Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE will accept no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Chemical Treatment For Cooling Water

CONTENTS 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 PARTICULATE FOULING 2

5 CRYSTALLIZATION SCALING 2 6 CORROSION 4

6.1 Anodic Inhibitors 4 6.2 Cathodic Inhibitors 5 6.3 Combined Inhibitors 5 6.4 Yellow Metal Inhibitors 5

7 CHOICE OF COOLING WATER TREATMENT 5 8 MICROBIOLOGICAL FOULING 8

8.1 Oxidizing Biocides 8 8.2 Non oxidizing Biocides 8

9 ENVIRONMENTAL CONSIDERATIONS 9



FIGURES

1 SCHEMATIC DIAGRAM FOR CHOICE OF COOLING WATER TREATMENT 7

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 10



0 INTRODUCTION/PURPOSE Chemical treatment is an essential component in the avoidance of fouling in cooling water systems. However, chemicals alone cannot correct design or construction faults in cooling systems, nor will they work if the cooling system is not controlled correctly or if the dosing system is incapable of delivering the correct chemical dosage at all times. Both overdosing and underdosing of chemicals can exacerbate fouling problems in cooling water. 1 SCOPE This Guide covers the chemical treatments available for use in cooling systems and the problems associated with their use. It is not a selection guide for every cooling system since there are many factors to be taken into account. 2 FIELD OF APPLICATION This Guide applies to process engineers and water technologists in GBH Enterprises world-wide. 3 DEFINITIONS

No special definitions apply to this Guide. 4 PARTICULATE FOULING

For a chemical dispersant to be effective, there needs to be some turbulence in the system. Ideally cooling water velocities of 1 to 2 m/s will be achieved, but this may not be possible in the design of certain heat exchangers particularly on the shell side of the exchanger. Dispersants will only work on fine particles; particles larger than 20 om should have been removed by filtration.

The basic mechanism of dispersion is that of charge reinforcement of the particles to stop them coagulating in suspension and then settling out in low flow areas. The use of lignins or sulfonated lignins and tannins has been common for many years for the dispersion of hydrated oxides. More recently polyacrylates have been used to disperse silt, sand and iron oxides. Polymaleic acid and polyacrylates have also been shown to prevent the settling of calcium carbonate. Proprietary polymers are now available for the dispersion of calcium and iron phosphates.



The choice of dispersant is determined by an examination of the solids present and then by trial on line.

5 CRYSTALLIZATION SCALING

The simplest and cheapest method to prevent crystallization of calcium carbonate is by reducing the pH with acid addition. This method also serves to reduce calcium phosphate scaling, but can increase the likelihood of calcium sulfate scaling if sulfuric acid is used and the 'makeup' water is high in alkalinity, since calcium sulfate scaling is not very pH dependent.

This can be resolved by the use of hydrochloric acid. Reducing the scaling tendency by acid addition can increase the corrosiveness of the recirculating water, particularly if the pH is <7.0 and hence good pH control and the use of a good corrosion inhibitor are essential if this is not to lead to corrosion fouling. In the past the use of synergized chromate treatments has allowed operation under these conditions, but increasing environmental and toxicological concerns are likely to lead to a ban on the use of chromate for cooling duties in the near future. Scale control treatments are available which modify or inhibit the crystal growth of the scale forming materials by adsorption on specific sites in the crystal lattice. In this way, supersaturation of the salt is relieved by the nucleation of very fine particles which can be easily dispersed. Such materials frequently work at concentrations far below those expected if considering the chemical reactions occurring, since they act on!y on specific sites on the crystal lattice. For this reason they are known as "threshold agents". Examples of such materials are:

(a) Polyphosphates

Polyphosphate is a scale inhibitor at threshold levels as low as 1-5 mg/l. However it hydrolyses naturally in the recirculating water to orthophosphate, which may then react with calcium and/or iron ions in solution to form phosphate deposits. Thus it is only used in systems with a relatively short half-life (<48 h) so that the degree of reversion is small. They are now most commonly used in conjunction with other scale inhibitors.



(b) Phosphonates

These have the group C-PO3H2. The most commonly used materials are hydroxyethylidene diphosphonic acid, CH3COH(PO3H2)2, known as HEDP and aminomethylene phosphonic acid, or AMP, (H2O3P)CH2N(CH2PO3H2)2. These materials will not hydrolyze as easily as the inorganic polyphosphates and can complex metal ions by sequestration. Most phosphonates are, however, subject to some degree, to degradation by the presence of strong oxidizing biocides, such as chlorine. At the same time, these materials also provide some nutrients for microbiological growth, so well controlled biocide application is essential.

(c) Polyolesters

These contain the group C-O-PO3H2 and can be a range of different compounds depending on the nature of the polyol used and how they are made. They are threshold agents.

(d) Acrylates

These compounds contain carboxylic groups on an aliphatic carbon chain. The most common materials in use are polyacrylates, polymethacrylates and the polymaleics. They retard the precipitation of calcium carbonate by distorting the crystal habit so that the crystals do not stick together.

(e) Phosphonocarboxylic acids

These compounds contain both C-PO3H2 and COOH groups and act in a similar way to the phosphonates.

The selection of a scale inhibitor depends on the water quality in the system and which species is likely to form.



6 CORROSION

An atomic study of the metal surface would reveal that it is far from the uniform surface expected. Manufacturing defects and interfaces between the crystalline phases of the alloy result in polarization of the atoms, with some sites being positively charged (anodes) compared with others (cathodes). The presence of a conducting solution results in the electrical connection of these sites allowing corrosion to occur, with loss of metal into solution from the anode and deposition of metallic oxides and hydroxides at the cathodes. The rate and severity of the attack will depend on the nature of the metal alloy, the conductivity and the pH of the solution, the temperature, the presence of oxygen and several other factors. Corrosion can also be prevented by the use of specific inhibitors which work by several distinct mechanisms such as oxidation, deposition and filming. Corrosion inhibitors can be divided into two basic types, anodic and cathodic. In some circumstances the two types are used in combination to produce a synergistic effect.

6.1 Anodic Inhibitors

These work by oxidation of the anodic sites on the metal surface, thus preventing interaction through the water and dissolution of iron ions. In extreme conditions, anodic attack can result in severe pitting of the metal surface, in particular if there is poor control of addition of the anodic corrosion inhibitor. Typical examples of anodic inhibitors are: (a) Chromate

Used at 300 to 500 mg/l as CrO4. Generally regarded as the best performers, but now being phased out for environmental and toxicological reasons.

(b) Phosphates Used at 6 to 30 mg/l as PO4. Good performers if well controlled, but with the risk of heavy scale formation if not. Basis of the new generation of inhibitors to replace chromates, but even now under environmental scrutiny.



(c) Molybdate

Used at 8 to 12 mg/l as Mo. Not very effective by themselves, but enhance performance of other inhibitors.

(d) Nitrite

Used at 800 to 1000 mg/l as NO2, higher in closed loop systems, at pH values >8.5. Excellent performance, even at temperatures up to 100°C, but subject to microbiological degradation, so rarely used in open systems.

(d) Silicate

Rarely used alone. Regarded as outdated, but still the basis of many formulations.

(e) Phosphonate

Used at 10 to 15 mg/l as active ingredient. A moderate corrosion inhibitor, in addition to its scale inhibiting properties. The basis for the new "all organic" treatments.

6.2 Cathodic Inhibitors

Cathodic inhibitors work by coating the cathodic sites on the metal surface with salts which then prevent interaction with the anodic sites. Deposition of the salts occurs naturally at the high pH conditions at the cathodes which are the result of the formation of hydroxide ions during the first stages of corrosion. Cathodic inhibitors are not as good as anodic inhibitors for carbon steel and are generally used synergistically. The best examples of cathodic inhibitors are: (a) Zinc

Used at 1 to 5 mg/l as Zn, but above pH 8 can only be used in conjunction with solubilizing polymer. At pH values <7.5, the high levels of zinc required make the treatment expensive and environmentally unacceptable.



(b) Polyphosphates

Used at 10 to 20 mg/l as PO4. Rarely used alone due to degradation to orthophosphates which can result in fouling of the metal surface.

(c) Calcium carbonate

Naturally occurs as scale in hard waters. This is the basis of the Langelier Index control (see GBHE-PEG-UTL-900).

6.3 Combined Inhibitors

Generally used to produce a better performance than either of the component parts but, depending on the synergy, may bring further constraints of pH control, etc. For example, zinc/chromate inhibitors permit operation at chromate levels of 15 mg/l (c.f. 300 to 500 mg/l if used alone) and zinc levels of 2 to 4 mg/l, but require pH values of 6.5 to 7.0. Various combinations are available, for example zinc/phosphate, zinc/phosphonate, molybdate/ phosphonate, molybdate/nitrite, orthophosphate/polyphosphate etc., but most are proprietary blends with closely guarded formulations. Corrosion inhibitors can also be added in combination with scale inhibitors when the cooling water is on the border line of scale forming and corrosive. Examples are zinc/carboxylic acids, zinc/polyolesters and phosphonate/phosphonate.

6.4 Yellow Metal Inhibitors

The above treatments are all for carbon steel. In most cases stainless steels will not be susceptible to corrosion except where deposits have already formed and there are high metal temperatures in the presence of, for example, high concentrations of chloride ions. With many treatments, however, it is common to find a copper corrosion inhibitor which acts by forming a film on the surface of the metal in copper, brass or cupronickel forms, thus preventing attack, particularly in low pH conditions. Examples of such inhibitors are mercaptobenzothiazole, benzotriazole and tolyltriazole.



7 CHOICE OF COOLING WATER TREATMENT

Figure 1 illustrates the steps involved in deciding the correct cooling water treatment for a given application. In most cases there is no choice of water available for cooling. It is, therefore necessary to be able to use the water given without any further external treatment and modify the internal treatment to give acceptable cooling performance. The choice of treatment is dictated by:

(a) The raw water quality

Whether it is potable or non-potable, whether it is hard or soft, etc. (b) The size of the system

Whether it is possible to justify expenditure on control equipment for acid addition or whether the system half life is short or long.

(c) The cooling duties

For example, water flow, metal skin temperatures and water temperature.

There are two basic strategies, as indicated in Figure 1:

(1) Adjust the pH to make the recirculating water corrosive and non-scaling in nature and then treat it with a strong corrosion inhibitor (see Example 1). This is used for large systems where pH control can be afforded and where high water or skin temperatures are involved, since these can lead to local scaling problems. It is also used where the incoming raw water can vary in quality.

(2) Allow the water to concentrate to make it scaling, but non-corrosive, and then treat it with a good scale inhibitor (see Example 2). This is used for small systems where there are no high temperatures, but pH control is considered to be too expensive, and for harder make up waters.

A third strategy is to make the recirculating water mildly corrosive and non-scaling in nature by a combination of allowing it to concentrate and/or adjusting the pH and then treating it with a mild corrosion inhibitor plus a low concentration of scale inhibitor. This is used for softer make-up waters where scaling is not a major risk and for systems where the materials of construction present no major risks of corrosion (e.g. stainless steel ).



It is essential that each case is viewed individually to ensure that the correct balance between dosing and control costs, treatment costs and water costs have been achieved. FIGURE 1 SCHEMATIC DIAGRAM FOR CHOICE OF COOLING WATER TREATMENT



8 MICROBIOLOGICAL FOULING

Cooling systems are excellent incubators for a wide variety of microbiological organisms. Many of these are innocuous and cause few problems directly, but can act as nutrients for other organisms. Some form slimes which will foul the metal surface and restrict the activity of the corrosion inhibitor. Others will cause corrosion themselves, or will produce corrosive acids as part of their life cycle. Yet more can be harmful to humans if allowed to thrive, although they appear not to cause fouling problems. In short, the discovery of any significant microbiological activity in a cooling system is an indication of poor control, although it is impossible to operate open systems under aseptic conditions. Control can only be achieved by the correct application of a biocide program. Biocides used in cooling systems can be divided into two classes, oxidizing and non-oxidizing, although non intrusive biocide treatments such as UV light are also used.

8.1 Oxidizing Biocides

In general, oxidizing biocides are rapid acting at low concentrations, but are not persistent. They will react with organic material in the system indiscriminately, including some of the organic polymers added as corrosion and/or scale inhibitors, and can be corrosive at high concentrations. One major failing is their lack of dispersancy and their inability to penetrate existing microbiological deposits very rapidly. They may also be more effective in particular pH ranges. Addition can be carried out continuously or intermittently, depending on the choice of treatment, but the body of opinion is moving towards low level, continuous dosing. Examples of oxidizing biocides are: (a) Chlorine.

(b) Bromine. (c) Sodium hypochlorite. (d) Chlorine dioxide. (e) Peracetic acid.



(f) Ozone. (g) Hydrogen peroxide. (h) Hydantoin + chlorine or bromine. But it is the application of biocide (frequency, duration, concentration, pH) which is often more important than the choice of biocide.

8.2 Non-oxidizing Biocides

There is a wide range of biocides for cooling systems, each of which has its own application requirements of pH, concentration, and time if it is to be effective. Such biocides are frequently specific and will therefore not provide a general kill of all microorganisms (c.f. an oxidizing biocide). They may be rapid acting or persistent, depending on type. Because of their nature, they are frequently used alternately, in pairs, with additions once or twice per week, depending on system half-life, etc.. Dosing requirements are typically between 50 and 200 mg/l, which makes treatment much more expensive than most oxidizing biocides. Their ability to penetrate existing deposits, however, increases their value as supplementary biocides. When nitrite is used as the corrosion inhibitor, the use of non-oxidizing biocides is essential as it reacts with oxidizing agents such as chlorine. Examples of non oxidizing biocides are: (a) Quaternary ammonium compounds. (b) Chlorinated phenols. (c) Methylene bis-thiocyanate. (d) Isothiazalones. (e) Dibromonitrilopropionamide. (f) Biguanides.



9 ENVIRONMENTAL CONSIDERATIONS A critical assessment of the environmental impact of the chemical treatment of any cooling system is needed before a decision can be made about the treatment program. Pressure has already resulted in the ban on the use of chromate in Europe and (shortly) in the USA. Organotins and chlorinated phenols are now actively discouraged as biocides. Attention is also being drawn to the discharge of zinc and phosphates, and many biocides are, by their nature, toxic to humans and to the environment. The inevitable result of this concern will be a move towards less toxic, but, by their nature, less effective treatment chemicals which will require much tighter control of dosing if they are to give comparable performance. This will place even greater emphasis on the dosing and control system installed and the need to keep it properly maintained. There will also be pressure to move towards "zero discharge" of water, bringing with it the requirement for water 're-use' and the likelihood of worse fouling problems. At some stage, sooner rather than later, an assessment of the "life time" cost of fouling of cooling systems will reveal that, in many cases and for all but the largest duties, the installation of closed, recirculating cooling systems with indirect air or once through water cooling will be an economic proposition.



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