Chap. 25: Treatment and conditioning of industrial water

6. COOLING WATER The operator of a cooling system generally encounters three main problems: - fouling and biological growth; - scale deposits;

- corrosion. Figure 918 shows an open recirculating cooling system with the different devices used to treat the water or to inject conditioning products.

6.1. PROTECTION AGAINST FOULING AND BIOLOGICAL GROWTH (See Chapter 2, Page 60.) Prevention is more important than cure. The ultimate objective is to implement an inexpensive solution that is both efficient and ecological. Colloidal matter is particularly harmful in that it coagulates on hot surfaces and creates an insulating film, which supports and feeds biological growth.

6.1.1. Treatment of make-up water Depending on its degree of pollution, make-up water will be treated as follows: -clarification with coagulation and filtration; - direct filtration; - oxidation where necessary (NaClO, etc.). Lime softening makes it possible to coagulate the water and reduce salinity in

6. Cooling water the same settler simultaneously. Further, this process has a direct effect on the biological quality of the water in the system. 6.1.2. Treatment of the system Several methods may be implemented. 6.1.2.1. Sidestream filtration with optional in-line coagulation

It is advisable to filter a certain fraction of the circulated flow rate in order to limit the quantity of suspended solids in the system. The sidestream filtration flow rate normally totals between 5 and 10% of the recycled flow rate. This percentage can naturally be increased if the system needs to meet particularly stringent requirements or if it is vulnerable to certain types of pollution (sand storms, dust produced by certain processes, etc.).

The use of organic coagulants is recommended for water liable to clogging.

6.1.2.2. Use of organic dispersants The role of these products is to maintain particles in suspension, thereby preventing the formation of deposits in areas of low circulation and on exchange walls. They can also regulate the development of the protective films formed by the corrosion inhibitors with which they are often associated (see Page 155). 6.1.2.3. Use of surface-active compounds

Lowering the surface tension makes it easier for biocides to access and destroy fouling. The surface-active agents implemented can be cationic, anionic, amphoteric or non-ionic. The length of the carbon chain influences the detergent properties of the agents used.

Use of surface-active compounds is particularly recommended in the following cases:

Figure 919. Nuclear power station in Leibstadt (Switzerland). Flow rare: 3,600 m3.h-1. Carbonate removal of water from the Rhne river.

Chap. 25: Treatment and conditioning of industrial water

- system preparation (see Page 1377); - disinfection in association with biocides; - cleaning and pollution control (oil, grease, hydrocarbons, etc.). 6.1.2.4. Use of chlorine and bromine derivatives

The most commonly used reagent is NaClO. The chlorine acts as an oxidant and biocide and also has coagulant properties.

Cl2 requirements may be high owing to the fact that the release of organic matter trapped on the different surfaces can multiply initial chlorine demand by a factor of between ten and fifteen.

Chlorination treatments are generally performed on a fairly irregular basis, i.e., between three times a day and four times a year. To prevent the acceleration of corrosion phenomena, the quantity of residual free Cl2 in the systems should not exceed 1 mg.l-1.

Bromine derivatives offer an advantage

over chlorine in that they are more active in a basic pH and in an ammoniacal medium. However, they can be difficult to implement. 6.1.2.5. Use of biocides

If preventive treatment is performed using the products described above, it is generally possible to limit the use of biocides such as algicides and bactericides. These products will then be reserved primarily for major offensives in periods where biological growth is particularly likely to occur (Spring, Autumn) or when the water is accidentally polluted. (a) Effect of biocides

The chemical compounds used to control the biological content of the water in cooling systems act in two main ways:

. Compounds acting on the membrane of the cell wall - quaternary ammonium (cationic products) with surface-active effects; - certain amine derivatives;

Figure 920. Uerdingen facility (Germany) operated by Bayer. Flow rate: 350 m3.h-1. Sidestream filtration on a valueless filter, dia. 6.7 m.

6. Cooling water - phenols and chlorophenols: these compounds produce hazardous discharges and are prohibited in systems with blowdown devices; - certain aldehyde compounds. . Compounds acting on cell metabolism (enzymatic inhibitors) - organosulphur compounds; -certain amine derivatives. (b) Implementation . The following factors must be taken into account in the choice of a biocide: - the pH of the water (important for optimum conservation and efficiency); - compatibility with treatment aids (ionicity, chemical affinity, etc.); - contact time; - habit-forming phenomena (particularly in the case of frequent injections). . The association of biocides and sidestream filtration makes it possible to optimize two processes:

- protection of the filtration media by frequent cleaning of sand; - preliminary cleaning of the systems prior to start-up. A number of steps can be taken to prevent the development of microorganisms: - ascertain the direct or indirect causes of formation (contamination, nutrients, system design and operation); - define possible methods of prevention; - closely monitor the development of microorganisms from an objective standpoint on the basis of reliable analyses. The development of microorganisms should be controlled through a specific program, which takes into account the objectives of the treatment, the effects in the long term and the effects on the medium concerned. The success of the program will hinge upon the use of suitable conditioning products and the expertise of the operators involved, who must possess an in-depth knowledge of treatment processes.

6.2. PROTECTION AGAINST SCALE DEPOSITS AND CORROSION Three types of process are commonly used: 6.2.1. The natural balance process Based on the Ryznar index (see Page 425), this process consists in adjusting the pH and the M alk. of the water circulating through the system to achieve a balance. This is done by introducing alkaline or acid reagents in limited concentrations. Attractive by its simplicity, the above process nevertheless involves a numb er of constraints:

- The optimum water balance is defined for a specific temperature whereas, in reality, the temperature of the water in a cooling system varies constantly. - The permissible concentration of dissolved salts in the water circulating through the system is limited. Frequent blowdowns and significant amounts of make-up water are therefore required. - The water is in a state of unstable balance: the pH can drop considerably when the water enters the cooling tower (excessive absorption of CO2, etc.).

Figure 921 shows the average pH on the basis of the M alk. of the water flow-

Chap. 25: Treatment and conditioning of industrial water ing through an open recirculating system on a cooling tower.

This process is still applied to systems in, power stations operating on a virtually once-through basis (with very low concentrations). In this type of system, temperature deviations are slight and cleaning is performed using a ball-based system. 6.2.2. Processes with scale inhibitors 6.2.2.1. Principle

Especially suitable for water that has a tendency to scale, this process consists in adding chemicals intended to retard the precipitation of calcium carbonate - particularly in hot spots -into the system.

The pH, M alk. and total hardness of the water are then calculated to establish a balance - even where such a balance is scale-forming - for cold temperatures.

The retarders used are generally a mixture of polyphosphates, phosphonates and

above all organic polymers with increased dispersive properties.

These scale inhibitors increase the range of temperatures at which the water can be considered to be at equilibrium. It is thereby possible to apply Ryznar indices, which can drop to 4 and sometimes lower. 6.2.2.2. Advantages and disadvantages of precipitation retarders

Stabilization processes offer a considerable advantage in that they make it possible to operate in "free pH" conditions. However, a direct link exists between the pH and the M alk. (Figure 921).

In reality, a "free pH" limits the M alk. of the water between 15 and 30 Fr. deg., i.e., a pH of between 8.5 and 9.3. The risk of corrosion increases towards M alk. 10 Fr. deg. and the risk of scale forming towards M alk. 40 Fr. deg. The pH concentration ratio is therefore also limited.

The use of sulphuric or hydrochloric acid can lower the M alk. but the pH is therefore no longer free and the SSA is increased.

The use of scale inhibitors in stabilization procedures therefore controls the formation of a protective film of calcium carbonate. This film can, however, be highly sensitive to variations in the system's parameters (acidity, SSA, insufficient reagents, etc.).

The scale-forming characteristics and the pH of the water limit the risks of corrosion. The role of the SSA is nevertheless essential in that chlorides and sulphates accelerate the disintegration of passivating protective films. Generally speaking, in controlled scale-forming operating conditions, the expected corrosion rates for steel are:

6. Cooling water - lower than 100 gm per year if SSA < 50 Fr. deg. - lower than 150 gm per year if SSA < 75 Fr. deg. 6.2.2.3. Improvements obtained through the injection of additional inhibitors In order to reduce corrosion rates, other ingredients can be added to the anti-scale compounds used. Zinc. Zinc is a cathodic corrosion inhibitor. At the usual doses of anti-scale compounds and at a pH generally higher than 8.5, the zinc content in the system remains below mg.l-1. The presence of a dispersant has a favourable effect on the zinc content. In these conditions, the corrosion rate can be reduced by between 20 and 50%, particularly where the water has a low SSA. Zinc can also intensify the effects of certain biocides. Copper corrosion inhibitors: (azole derivatives, see Page 428). present in the water because they can accelerate the corrosion of cuprous metals. Chromates: Chromates are efficient in concentrations of a few mg.l-1. However, Cr(VI) is toxic and must be transformed

into Cr(III) during the blowdown process prior to discharge. 6.2.3. Processes with corrosion inhibitors 6.2.3.1. Principle The risk of scale forming is eliminated by lowering the pH of the water to around 7 (controlled pH) or by making it softer (through a softening or demineralization process). At the same time, a corrosion inhibitor is introduced into the system and forms a protective, adhesive, homogeneous and non-porous film, which has no effect on heat exchange. Most of the corrosion inhibitors used in open recirculating systems are composite substances ensuring both anodic and catholic protection (see Page 427 and table This type of compound is particularly 98). The requisite dose is usually a few recommended when phosphonates are dozen mg.l-1.

Table 98. Comparative table of the properties of the four main families of corrosion inhibitors for use in open recirculating systems (with a cooling tower). Polyphosphate- Phosphonate- Phosphates Chromate-zinc zinc zinc + dispersants + dispersant + dispersant + dispersant + organic inhibitors pH range 6.4-6.8 6.4-7.5 6.5-8 6.8-7.5 Contact time (h)

> 100 approx. 50 approx. 70 > 100

Efficiency Excellent Very good Good Excellent Corrosion rate, pm < 50 < 60 < 100 < 50 per year

Chap. 25: Treatment and conditioning of industrial water

Zinc is one of the most frequently encountered binary elements. According to the formulations used, the pH of the water in the system should be close to neutral or slightly acidic in order to ensure that zinc remains ionized close to the wall. 6.2.3.2. Optimizing the pH

The M alk. of the make-up water must be low in order to ensure that the pH of the water in the system remains within the desired range. This requirement can be satisfied using one of three possible methods: - injection of acid: this method has the disadvantage of increasing salinity; - removal of carbonates on carboxylic resin: this reduces both the M allc. and the hardness of the water and hence decreases salinity; - lime softening: if the water needs to be clarified. 6.2.3.3. Finding the optimum concen-tration ratio

The concentration ratio must be as high as possible in order to minimize the quantities of water and conditioning products required. The optimum level will depend on: - system operating conditions (search for spurious leaks, etc.); - risk of salt precipitation; - the recommended contact time for the inhibitor selected.

If sulphuric acid is used to maintain the pH at the correct level, the consequent

increase in the quantity of SO42- ions makes

it necessary to limit the concentration ratio in order not to exceed the solubility product of calcium sulphate. It may be useful to perform preliminary carbonate removal, which will remove alkalinity and calcium and thereby make it possible to increase the concentration.

Silica rarely poses a problem except in the Far East where fresh make-up water can contain several dozen mg.l-1.

The desired concentration rates generally range between 3 and 6 but can rise to 8 or higher with certain types of purification (demineralized make-up water, sidestream purification), or conditioning treatments, thereby simplifying the problem of subsequent discharges. 6.2.4. Discharges

The vast majority of cooling systems require blowdown and this raises the problem of discharges.

It is forbidden to discharge water containing chromates where the concentration of Cr(VI) exceeds 0.1 g.m-3. However, a number of industrial processes have been developed to destroy both chromates (see Page 260) and zinc.

The concentrations of P2O5 and zinc generally required in cooling systems ( < 20 mg.l-1 and < 5 mg.l-1 respectively) are usually tolerated in discharges although standards are currently changing.

6.3. CHOICE OF PROCESSES: HOW TO DESIGN A COOLING SYSTEM The chemical composition of water is

not the only factor to be taken into consideration when designing a cooling system, but it nevertheless affects a number of aspects of system design. Consulting a specialist in water treatment and conditioning at the outset of the project offers a number of advantages:

6. Cooling water - It would be unfortunate if the new installation did not make use of any new processes that may have been developed and tested. - It is sometimes possible to choose between water from two or more sources. - The treatment of make-up water can easily be integrated into the installation's general water treatment system. - The quality of the water available on the site could make it possible to use a system that is relatively easy to protect. If the water in contact with the surfaces to be protected is renewed at an excessively slow rate, it is practically impossible to protect the system by adding chemicals to the water. - The type of protection chosen influences the choice of construction materials: cooling apparatus, pumps, pipes, etc. 6.3.1. Basic data

The basic data involved are as follows: -analyses of water and system design; - system characteristics (flow velocities, materials, temperature gradients, "skin" temperatures, etc.); - desired efficiency; - operating constraints:

. possible concentration,

. use of acid reagents (H2SO4 or HCl); - discharge constraints; - operating costs. 6.3.2. Operating reports

It may be necessary to consider several alternatives and to draw up operating reports in order to compare different protection processes. These reports should include all items related to operating costs, such as: - product consumption; - cost of water, which is often the most expensive item (several times more expensive than products, particularly if drinking water is required); - labour costs for the monitoring and maintenance of equipment; - amortization expenses; - reduction in cooling system maintenance costs and improved production capacity.

Operating reports make it possible to better assess the consequences of increasing the concentration ratios. Such an increase would accelerate amortization of purification facilities (carbonate removal, filtration, etc.) by reducing operating costs.

6.4. SAVING WATER 6 4.1. Re-using industrial or domestic wastewater

Purified wastewater is sometimes recycled to improve operating results. Where water resources are inadequate, however, as in the case of oil refineries, it

may be a necessity. (See chapter 26, par. 4.3, page 1413.) 6.4.2. Sidestream purification

Sidestream chemical purification can be a useful option or a necessity: - to maintain high levels of concentration (8 to 12); - to remove most of the silica: quantities of water far smaller than the amount of make-up can be softened using lime and

Chap. 25: Treatment and conditioning of industrial water caustic soda owing to the substantial precipitations of carbonate and silica. Sidestream treatment is now imple- mented fairly widely in the United States in order to

achieve zero blowdown. The treatment can be implemented in association with a zinc chromate corrosion inhibitor (see Figure 922).

Fig. 923. Refinery in Tosco (USA). Flow race: 37 m3.h-1. Sidestream purification for cooling system.

6. Cooling water

6.5. SPECIAL CASES: ONCE-THROUGH SYSTEMS

In these systems, the concentration factor cannot be used to limit reagent consumption. Contact time is usually short, however. Table 99 lists the different kinds of treatment that can be applied.

Table 99. Protection of once -through cooling systems. Problem Solution

- Fouling and - Straining and/or Clarification- - Biological growth Filtration. Removal of iron (for certain types of drilling water) Dispersants Chlorination Biocides (on a periodic basis) - Scale deposit - Scale inhibiting compounds-Dis- persants - corrosion - Corrosion inhibitors (e.g., small quantities of polyphosphate - zinc) pH adjustment

6.6. SPECIAL CASES: CLOSED SYSTEMS

The system is filled with softened or demineralized "noble" water.

The main methods of protection are listed in table 100.

Corrosion inhibitors must ensure complete anodic protection and the level of concentration should be maintained at about one gramme per litre. Formulations must also be adapted to the different metals in the system, particularly when copper is used.

Oxygen scavenging is carried out on hot systems or where particular resistivity requirements need to be met. The oxygen input is controlled by the introduction of nitrogen.

There now exist an increasingly large

number of systems, which, although referred to as "closed", undergo considerable losses in reality and therefore require a certain amount of make-up water and effective pollutant control measures. Table 100. Protection of closed cooling systems.

Problem Solution - Foaling - Sidestrearn filtration Dispersants - Biological growth - Biocides (shock treatment) - Scale deposit - Softening Demineralization - Corrosion - Corrosion inhibitors in large quantities (chromates, nitrites, phosphates, molybdates, organ- ic) + buffering salt (borax) or Oxygen scavengers (sulphite, tannate, hydrazine, etc.) + increased pH (amine, phos- phate)

6.7. SYSTEM PREPARATION The start-up phase is of capital importance: an irreversible process of corrosion can

begin as soon as the water comes into contact with the steel. Conditioning must therefore be started as soon as the water enters the system. The preparatory process is as follows:

Chap. 25: Treatment and conditioning of industrial water - clean the system with dispersants and detergents - add large doses of inhibitors to form the protective layer.

This layer must subsequently be maintained by means of a metering unit associated with the make-up water flow rate.

6.8. TESTS

It is essential to conduct tests to make sure that the conditioning treatment has been properly implemented and is effective. Tests focus on: - water and reagent consumption; - functioning of water purification and reagent injection stations; - analyses of the water (make-up and system, in particular: pH, M alk., Ca, Cl, inhibitor content); - corrosion measurements: corrosion meter, coupons, test nipples;

- the quality of heat exchange: if possible, the heat transfer coefficient of one of the exchangers in the system should be measured to provide a reference.

Thorough conditioning of the cooling

water is essential for the efficiency of the production unit and the performance of the overall system, particularly during the first few weeks of operation.

The user and the water treatment specialist must work in close association.

Degrmont has developed a range of products and processes ("Kemazur" and Complexes) to meet all conditioning requirements.

6.9. COOLING SYSTEMS USING SEA WATER

These systems are used to cool power station condensers and arrays of heat exchangers.

The water intakes comprise a screen and a 4 mm mechanical strainer. The strainer provides essential mechanical protection and also plays a role in protecting the system from corrosion (less deposits). Construction materials must be selected with caution (see Page 447): - concrete or enamelled iron for pump

volutes and bronze or 18.8.3 stainless steel for impellers; - plasticized steel water box; - concrete collectors and steel auxiliary systems protected by a sacrificial anode; - screens with fixed-voltage cathodic protection. 6.9.1. Once-through systems

Once-through systems are generally used for condensers operating in coastal power stations or on board ships.

Corrosion protection: Titanium exchangers, which are used in the most recently built nuclear power stations, need no special treatment. However, copper alloys, particularly admiralty brass,

6. Cooling water require a protective film of iron hydroxide, which is formed by adding 1 mg.l-1 of Fe2+ in the form of sulphate heptahydrate to the water for a period of one hour daily on approximately 300 days of the year. Protection against deposits (to reduce corrosion). The requisite biocide treatment is aimed primarily at mussel control. Doses of chlorine (obtained by the in-situ electrolysis of sea water) are added into the system. The residual dose at the condenser outlets should be 0.2 mg.l-1. Chlorination will be performed on a continuous basis if the temperature of the sea water is 30C or higher and on a discontinuous basis for lower temperatures, e.g., 15 minutes every 6 hours. Chlorination may be omitted altogether at temperatures of less than 12C.

The on-line mechanical cleaning of condensers is also becoming increasingly common in nuclear power stations. The process relies on zircon-spiked balls, which also remove scale deposits. 6.9.2. Open recirculating systems Open recirculating systems are rare but may be necessary in certain cases. The concentration ratio for this type of system cannot exceed 1.2 -1.3. In addition to the precautions described for open systems, make-up water must undergo an injection of acid to reduce the M alk. It is also advisable to condition the water using an inhibiting-dispersing agent.