General Water Treatment For Cooling Water

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

GBH Enterprises, Ltd.

Process Engineering Guide: GBHE-PEG-UTL-900

General Water 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: General Water Treatment

for Cooling Water CONTENTS 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 CHOICE OF COOLING SYSTEM 2

4.1 ‘Once through' Cooling Systems 2 4.2 Open Evaporative Recirculating Systems 3 4.3 Closed Recirculating Systems 3 4.4 Comparison of Cooling Systems 3

5 MAKE-UP WATER QUALITY 4 6 FOULING PROCESSES 4

6.1 Deposition 4 6.2 Scaling 4 6.3 Corrosion 4 6.4 Biological Growth 5

7 CONTROL OF THE COOLING SYSTEM 9

7.1 ‘Once through' Cooling Systems 9 7.2 Closed Recirculating Systems 9 7.3 Open Evaporative Cooling Systems 9



TABLES

1 RELATIVE IMPORTANCE OF FOULING PROCESSES AND INSTALLED COSTS 3

2 WATER QUALITY PARAMETERS 4

FIGURES 1 PREDICTION OF CALCIUM CARBONATE SCALING 6 2 CALCIUM SULFATE SOLUBILITY 7

3 CALCIUM PHOSPHATE SCALING INDEX 8 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 11



0 INTRODUCTION/PURPOSE Fouling in cooling systems occurs by four potential processes, viz: (a) Crystallization scaling. (b) Deposition of particulate matter. (c) Corrosion and subsequent transfer of corrosion deposits. (d) Microbiological growth. These processes do not occur in isolation and it is frequently the interaction between them which results in the worst fouling problems. It is, therefore, essential to identify the potential sources of each and then choose the best cooling technology and apply the appropriate control, both mechanical and chemical, in order to minimize the effects of fouling. The water treatment required for cooling is determined by three factors: (a) The cooling process in use. (b) The make-up water quality. (c) The control of the cooling tower and chemical dosing system. It is important to understand how these interact if fouling of cooling systems is to be avoided. Clauses 4 to 6 describe the important parameters before chemical treatment is discussed. 1 SCOPE This Process Engineering Guide discusses the factors influencing the choice of cooling water system, defines the parameters influencing make-up water quality, explains the four basic processes of fouling and gives general advice on the cooling water selected. This Guide does not cover the design of a treatment system nor does it make specific quantitative treatment recommendations.



2 FIELD OF APPLICATION This Guide applies to process engineers and water technologists in GBH Enterprises world-wide. 3 DEFINITIONS The following definition applies to this Guide. Concentration Ratio is the ratio of the concentration (e.g. of Mg++) in the

circulating water to that in the make-up water. 4 CHOICE OF COOLING SYSTEM 4.1 'Once through' Cooling Systems

Water is abstracted, used for cooling and then discharged, generally with a low temperature rise. The high water usage means that a cheap surface or groundwater has to be used. Pretreatment may be considered when serious fouling problems due to silt or river muds occur.

Chemical treatment is difficult because of the quantities involved and the risk to the environment, but some biocidal treatment may be applied to reduce fouling due to microbiological or animal life. For this reason, heat exchangers on such systems are generally made of corrosion resistant materials, at extra cost, and mechanical methods of controlling fouling are employed.

4.2 Open Evaporative Recirculating Systems

Water is circulated through heat exchangers and then returned to a cooling tower in which heat is transferred to the atmosphere by evaporation. The evaporation loss is generally 1 to 2% of the total recirculating rate.

The cooled water is reused and the water lost by evaporation is replaced by make-up water, which will inevitably result in increasing mineral concentration of the water. This can be kept constant by purging a small amount of water from the system, generally equivalent to less then half of the evaporative loss. Chemical treatment of the water is economic when the sum of the purge losses and involuntary losses from leaks, etc., amounts to less than the evaporative loss.



While the capital cost of the Open Evaporative system is higher than for 'Once through' systems, the reduced use of water, the reduced environmental impact, the ability to chemically treat and the removal of the need to use corrosion resistant materials make this the system of choice for most applications.

Recent developments in new packing materials have made Open Evaporative towers even cheaper to purchase, but have brought a new fouling problem. The plastic packings come in various spacings, from as little as 12 mm to 38 mm and higher. At the smaller end, there is a tendency for the fouling deposits to bridge the gap between packing sheets, which reduces the cooling efficiency of the system significantly. At this point the packing has to be replaced as, because of the spacing, there is no way of cleaning it successfully by water jetting and chemicals cannot penetrate sufficiently to remove the deposits. Good control of the chemical regime in the recirculating water can ease the use of such packing.

4.3 Closed Recirculating Systems

Water is circulated through heat exchangers and then passed through an additional set of heat exchangers where the heat is transferred to air or water. No deliberate loss of water occurs from the closed loop and hence little or no make-up is required. Chemical treatment can readily be applied and control of the water chemistry is easy, particularly if condensate or deionized water is used. This system is used for critical heat transfer duties or where elevated temperatures are used. Capital costs are high, however, and some loss in cooling efficiency has to be borne from using an indirect cooling method. Treatment of the indirect cooling water has also to be considered, whether from a 'Once through' system or an Open Recirculating system.

4.4 Comparison of Cooling Systems Table 1 shows which fouling processes occur in each of the cooling systems described. The



TABLE 1 RELATIVE IMPORTANCE OF FOULING PROCESSES AND INSTALLED COSTS

5 MAKE-UP WATER QUALITY The "quality" of natural waters can be defined by measurement of the parameters given in Table 2.

TABLE 2 WATER QUALITY PARAMETERS

6 FOULING PROCESSES There are four basic fouling processes which can occur in cooling systems, all of which can be affected by the quality of the make-up water used. These four processes are now described. 6.1 Deposition If the make-up water contains a high turbidity or suspended solids, a high concentration of organic matter and/or a high alkaline hardness, it may be cost effective to pretreat. In any case, the worst effects of deposition fouling can be offset by maintaining water velocities in excess of at least 1 and preferably 2 m/s



6.2 Scaling The solubility of "hardness" salts, i.e. calcium and magnesium salts, decreases with increasing temperature, thus making any cooling duty a potential cause of scaling for heat transfer surface temperature constraints). For 'Once through' systems the temperature rise is generally too small for major problems, and closed systems are limited to the scale potential of the contained water if there is no make-up. However, Open Evaporative systems are prone to scaling with the increased mineral concentration due to evaporation. This is exacerbated if the make-up water is "hard". The calcium carbonate scale potential of any water can be estimated using either the Langelier Index or the Ryznar Index (see Figure 1). Calcium sulfate scaling may be a possibility where sulfuric acid is added to reduce the alkalinity (see Figure 2). Calcium phosphate scaling may be a problem, particularly with certain chemical treatments where control is poor or directly due to poor make-up waters (see Figure 3). Calcium silicate is not usually a problem unless the silica content of the recirculating water exceeds 150 ppm. 6.3 Corrosion For hard waters, the high alkalinity and mineral content make the water less aggressive towards, for example, carbon steel. However, soft waters are generally low in both alkalinity and mineral content and tend to be far more corrosive. Some correlation can be gained by using the Langelier or Ryznar Indices to predict the aggressiveness of the water, and this is the basis of some control methods, to hold the water at a neutral Index value to minimize both scaling and corrosion. In general, increasing the mineral content decreases the corrosiveness; the addition of acid to reduce scale potential increases corrosiveness. 6.4 Biological Growth With the exception of closed loop systems, where a one-off shot of biocide may be sufficient to control biological activity, no cooling system should ever be considered to be biologically stable, whatever the make-up water quality. Waterborne or airborne organisms find the warm, aerobic or anaerobic conditions almost ideal for growth and can very quickly become established, causing major fouling problems if not controlled.



FIGURE 1 PREDICTION OF CALCIUM CARBONATE SCALING

Calcium carbonate scaling can be predicted qualitatively by the Langelier Saturation Index or the Ryznor Stability Index. The indices are determined as follows: (LSI) Lagelier Saturation Index = pH (actual) – pHs (1) (RSI)) Ryznor Stability Index = 2 (pHs) - pH (actual) (2) The value pHs (pH of saturation) is a function of total dissolved solids, temperature, calcium and alkalinity; pH actual is the measured pH of the water. A positive LSI indicates a tendency for calcium carbonate to deposit. The RSI shows the same tendency when a value of 6 or less is calculated. The pHs value may be calculated from the nomogram in Figure 1.



FIGURE 2 CALCIUM SULFATE SOLUBILITY



FIGURE 3 CALCIUM PHOSPHATE SCALING INDEX

Calcium Phosphate scaling can be predicted by calculation of the Scaling Index This is a function of Calcium, pH, temperature and also Ortho-phosphate and is calculated as: Scaling Index = pH (actual) – pHs (3) where pH actual is the actual pH value of the solution as measured and pHs is the pH of saturation of calcium phosphate and is determined from the nomogram in Figure 3. When the scaling index is positive, precipitation is likely.



7 CONTROL OF THE COOLING SYSTEM

Given that the cooling process and make-up water quality are determined, much can be done to prevent the interaction of the four processes for fouling by good control of the operation of the cooling system and of the chemical dosing applied.

7.1 'Once through' Cooling Systems

For 'Once through' cooling systems, care should be taken to avoid a high temperature rise through the exchangers as this can increase the risk of scaling and can change the nature of particulate fouling from a sludge to a baked deposit. Dosing with biocides, if used, should be frequent enough to prevent deposit build-up. The use of corrosion resistant materials and regular mechanical removal of deposits should then prevent major problems.

7.2 Closed Recirculating Systems

At the other extreme, closed loop systems should only use a high quality make-up water, ideally condensate or deionized water. A one-off shot dose of corrosion inhibitor and biocide plus the adjustment to pH > 8.5 using alkali should then be sufficient to prevent fouling as long as there is no significant loss of water. At elevated temperatures (>100 °C), it may be better to employ a boiler water type of chemical treatment to minimize corrosion. With either method, the water should be analyzed on a weekly basis to guard against loss of treatment.

7.3 Open Evaporative Cooling Systems 7.3.1 Concentration Ratio

There is a natural tendency to concentrate minerals in the recirculating water by evaporation. This can lead to severe scaling problems if the make-up water is already hard, if the system is allowed to over concentrate or if the bulk water temperatures are high.

Control is on the basis of Concentration Ratio, which is defined as the ratio of concentration of the recirculating water to the make-up water, usually based on the magnesium concentration in each. In practice, the Concentration Ratio is determined by the rate of loss of water from the system, either by deliberate purging or through leaks and other involuntary losses.



In general, the aim is to control the cooling system at a minimum ratio of 3 to 5, but in some systems ratios as high as 8 or 10 can be achieved. This ratio determines how much water is used and how much chemical treatment has to be added. The control range is largely determined by the quality of the make-up water and the flows and water temperatures in the various plant heat exchangers. Operation at elevated concentrations can be achieved even with hard make-up waters by using acid addition to depress the pH, but failure of the dosing system can have serious consequences. Small systems with low purge rates may be very difficult to control manually.

7.3.2 Half-life

The rate of change of chemical composition of the water also has an impact on the ability to control fouling. This is increasingly important with the introduction of high efficiency PVC packings into cooling towers, which has allowed manufacturers to reduce the physical size of the towers and thus the volume of water stored in the sump.

It is best represented by the half-life (or Holding Time Index) of the system, which is proportional to the ratio of the volume of water in the system to the purge rate. For older towers, the half-life is typically 40-50 h, giving a relatively slow rate of change which can be easily managed given a daily analysis. Modern towers have the advantage of being considerably cheaper to build, but operate with half-lives of typically 4-10 h, making manual control very difficult. Variations in make-up water quality or cyclical variations in heat load, such as are experienced in batch processes, make these problems even worse.

7.3.3 Filtration

It may be possible to prevent suspended solids entering the cooling system by filtering all the make-up water, but this will not remove solids (several kilograms/day in industrial areas) sucked into the cooling tower and scrubbed from the air by the falling water droplets. Chemical dispersants may be able to prevent the finest of these particles from settling; maintaining high water velocities will avoid the worst of the fouling problems. Side-stream filtration of 3 to 10% of the recirculating water will greatly assist the removal of the finer suspended solids (10 – 100 om) which will otherwise cause major fouling problems.



7.3.4 Chemical Addition

However good the chemical treatment, it will be of little value if it is not added consistently and at the correct rate. Such treatments will rarely remove deposits once formed, so the intention is to prevent any deposition. In view of the interaction between the various types of fouling, it is essential that the treatment is regarded as a complete program in which all parts shall be added if it is to work. It is, for example, of little value to add a corrosion inhibitor when the metal surfaces are covered with deposits, since it will not be able to penetrate to the metal surface to prevent corrosion from occurring and adding to the fouling problems.

The dosing system required will depend on the treatment being applied (corrosion inhibitor, dispersant, biocides, acid, etc.), the make-up water quality, heat exchanger duty, half-life, etc.. There is no standard system applicable for all conditions. At one extreme it may be sufficient to use a manually adjusted purge and add the chemicals as "shot" doses once per day. For more demanding systems it may be necessary to pump in treatment chemicals and purge water in proportion to the make-up water flow while adding two biocides, one continuously and the other on a weekly basis, and acid on pH control. In this case the control system is likely to be microprocessor controlled with a full alarm system.

General Water Treatment For Cooling Water

Technology

Transcript of General Water Treatment For Cooling Water