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Developing Holistic Approach towards Cooling Water Treatment
Tushar Jhaveri,CEO, Vasu Chemicals and Dr S Kedar, Technical Advisor, Vasu Chemicals Water Cooling is a method of heat removal from components and industrial equipment, where water is used as the heat conductor. Corrosion, scaling, fouling and microbial growth are the major problems associated with any cooling water system. The authors discuss the importance of water audits and various methodologies to optimise consumption of cooling water in industrial processes.

The main criteria of a good cooling water treatment programme is - inhibition of corrosion, to extend equipment life, control scale and deposits to maintain heat transfer across the exchangers or condensers and control the growth of micro-organisms, which can lead to corrosion and fouling in the system. The objective is also to permit extended operation of the plant without necessitating any shutdown due to water-related problems. In todayís scenario, it is also necessary to continuously provide value addition and reduce the total cost of operations while fulfilling the above criteria, and it is this area that many treatment programmes are found with deficiencies.

On many occasions in public sector industries, the selection of water treatment vendor is based on the lowest cost. As is the case with most of the industries, the competition is getting stronger in the field of water treatment as well. The fine balance between low cost and genuine performance limits the flexibility of the treatment programme and operation, which is essential for value-added programmes and services. Correct technical water treatment recommendation at the right price creates a partnership situation for both the client and the water treatment vendor.

Value addition and reduction in the cost of operations can be realised only when both work as true partners. After all, the plant personnel knows his plant better and the water treatment vendor knows his chemicals and applications better. Both have to join hands together to pool their resources and work together to get the best out of any treatment programme. This approach brings in an atmosphere of trust, and transparency.

Maximising Returns on Investment through Water Audits
The foremost step in providing value-added services is to conduct a water audit. The audit exercise should cover all angles of operations to arrive at the right conclusions and implementation of best practices within the operational constraints. The aim should be to maximise the returns on the investment that the customer makes by selecting a water treatment company as their partner in water management.

Intent of any cooling water treatment audit should be three fold and comprise of measuring existing setup, improving the ongoing procedures, and institute and standardise improvements.

Suggested Procedure for Water Audits A. Review the Current Conditions and Service Standards - This is the first area and forms the baseline data. The following information needs to be collected:

• Details of the ongoing treatment programme
• Overall treatment performance currently achieved and past history of failures if any
• History of critical equipment and parameters
• Critical process data relevant to water treatment
• Specific contaminants, if present

B. Collect In-depth System Information - The operational system details are then collected along with the water flow chart. Storage and pretreatment data, stagnant zones, cooling tower information, seasonal changes are some areas that can often provide valuable inputs for overall improvement.

C. Data Generation - The data for screening should include:

• Daily log sheets
• Daily water analysis
• The water flow diagrams
• Flow mapping data
• Microbial analysis
• Corrosion rates analysis
• Exchanger monitoring reports
• Chemicals consumptions pattern • Specific observations
• Seasonal variations

D. Evaluating the Current Treatment Programme - The programme performance evaluation is generally based on:

• Corrosion control
• Scaling control
• Microbiological control
• Critical heat exchanger profiles
• Heat exchanger shutdown inspection reports

E. Conduct On-site Interview of Plant Personnel - The very important and integral part of any audit is - site interviews with key plant personnel at all levels.

While providing important feedback on the present condition and performance, these interviews go a long way in providing critical information on the concerns and needs of the plant and also highlight operational limitations, if any.

F. Cost Analysis: Actual Vs Capability - This information will allow optimisation of the feed rates of various chemicals and fine tune their frequency of addition, to bring in direct benefits to the plant. Variations in the regular maintenance level can be optimised and the chemicals can be maintained in a specific band of operation to avoid wide fluctuations and overfeed of chemicals.

G. Overall Review and Presentation - Once all the information is collected, a detailed study of the information gathered, is necessary to suggest any improvements with regard to the system, programme and services, to bring in value addition and cost-benefit, which is the sole objective of such an audit.

The findings and recommendations should then be logically presented to the decision-makers who can then assist in the implementation of the suggested improvements.

Output
The main purpose of an audit is the 'Payoff'. Some of the payoffs would be:

• Optimisation of the treatment programme
• Improvement of the treatment performance
• Operation at higher cycles to conserve water
• Use discharge of other areas that would normally drain to the effluent as make up
• Effluent recycle
• Removal of bottlenecks to improve plant load (productivity)

Futuristic Approaches
A. Chemical Treatment Programmes
- As indicated earlier, the main problems associated with any cooling water system are corrosion, scaling, fouling and microbial growth. The most commonly recommended programme today includes zinc and orthophosphate as the main corrosion inhibitors along with organophosphonates and polymers for scale and deposit control. Microbial control is achieved with the continuous use of an oxidising biocide-like chlorine, along with a bio-dispersant, which are specific surfactants, for the removal of slime and organic contaminants. Once these biofilms are prevented from adhering to the exchanger tubes and kept in suspension and circulation in the cooling water, the oxidising biocide can attack and kill these organisms, thereby preventing the further growth of slime in the system. Periodically, non-oxidising biocides are used to supplement the action of chlorine and bio-dispersants.

B. Corrosion Inhibition - It is a well-established fact that a synergistic combination of a cathodic and an anodic corrosion inhibitor provides the adequate corrosion protection. However, there are specific conditions or instances where these common inhibitors need to be further supplemented or replaced for better performance. India has a wide spectrum of water quality ranging from low hardness and TDS water from one region to hard water with high TDS in other regions. Low hardness waters are more corrosive while higher hardness have potential towards deposition. High TDS water with high concentrations of aggressive ions like chloride and sulfate can lead to higher corrosion propensity. In this complex scenario, supplementing common corrosion inhibitors like zinc and orthophosphate with newer molecules often becomes necessary.

Azoles have been used, since long, as a corrosion inhibitor for exchangers with metallurgy based on copper and copper alloys.

Molybdates are another group of molecules used to enhance the corrosion protection particularly in situations where one encounters high temperatures, lean low hardness water and systems with deposits already present. Molybdate, besides functioning as an anodic corrosion inhibitor, can also form plugs on existing deposits, making it impervious to aggressive ions like chloride and sulphate. Once these ions are prevented from reaching the metal under the deposits, their concentration cannot build up and further localised corrosion is arrested. The disadvantage of molybdate is its prohibitive price and that is the reason for its very selective use.

Today, by using various combinations of corrosion inhibitors, it is possible to obtain a corrosion rate of less than 2 mpy (mils per year) on a regular basis.

C. Scale and Deposit Control - Operation of cooling water systems at high cycles of concentration is becoming the trend everywhere. This is essential considering the fact that water is increasingly becoming a very valuable and expensive resource. Under such operating conditions, the deposition tendency of the water increases manifold and good deposit control can determine the success or failure of a treatment programme.

Organophosphonates and polymers used in combination have provided excellent deposit control. But, there is a shift towards polymers in present water treatment practices. It has particularly assumed significance after the advent of non-chromate programmes, where scale and fouling control is extremely critical, while corrosion still has to be restricted to minimum limits. There are various polymers available, each with its strengths and weaknesses and it is vital that the right polymers are selected in a treatment programme, taking into account the needs and demands of a specific system.

The most widely used polymers are low in molecular weight(2000 to 20,000) and usually use acrylic acid as one of the monomers. Polymers can be homo-polymers (using a single monomer), co-polymers (two monomers), ter-polymer (three monomers), tetra-polymer (four monomers) and so on. It is very important to know what the polymers specifically function as, in the selected programme. Besides being cost-effective, the polymers must provide improved performance and must be function-specific.

Most polymers exhibit multifunctional properties such as dispersion. A polymer may exhibit excellent control of suspended matter, while another may excel as an inhibitor of phosphate salts while providing multi-functional benefits of other control properties to varying degrees. Specific polymers are available for control of individual scale forming salts, phosphate, metal ion foulants and suspended matter.

A well-designed cooling water treatment programme takes into account all these aspects and selects components depending on the actual system needs, established after a thorough survey. This provides the basis of selection along with other scale and corrosion inhibitors. Coupled with the total microbial control programme, it forms the basis for the total cooling water treatment package.

New generation polymers have been designed for specific functions like inhibition of phosphates, calcium carbonate, calcium sulfate, strontium sulfate and silica. These are also being used for metal ion and foulant control as well as suspended matter dispersion.

Microbiological Control The primary mode of microbiological control includes use of an oxidising biocide along with a bio-dispersant. Bio-dispersants are essentially specific surfactants that target microbiological slime and biofilms and dislodge them from heat exchanger surfaces. Once brought into circulation, regular biocides (oxidising or non-oxidising) can then control these organisms by killing them. It is important to control the formation of biofilms very efficiently because they can cause the following problems:
• Reduction of plant performance due to the growth of biofilms.
• Reduction of plant integrity due to MIC

Bio-dispersants preferably should be non-foaming or low foaming; their latest varieties that have been developed also incorporate ingredients, which impart biostatic property to the product.

Chlorine is one the oldest and cheapest oxidising biocide used even today. The effort is to increase its effectiveness further by supplementing it with bromides or chlorine dioxide. This enhances the microbial control even in the presence of contaminants that otherwise use up chlorine and convert it to micrbiologically useless chlorides. Also, it extends the effectiveness of control to an alkaline pH where chlorineís activity as a biocide is reduced. Increasingly, chlorine dioxide is being extensively used in refineries, petrochemical, power and fertiliser plants.

Chlorine dioxide is one of the fastest growing chlorine alternatives in water treatment application. This technology offers the benefit of effectiveness at high pH, being unaffected by ammonia, and demonstrating significant advantages over chlorine, in systems susceptible to high levels of organic contaminants. Because chlorine dioxide is a selective oxidiser, its ability to control micro-organisms in water at very low dosages makes it an especially cost-effective solution.

Disadvantages of Chlorine
• Reacts with organics and hence exerts a chlorine demand and hence, additional chlorine has to be dosed to overcome the demand, before a free residual is available to act as a biocide
• Efficacy of chlorine is pH dependent. Hypochlorous acid, which is a desired component, will dissociate to H+ and OClions with increase in pH HOCl content will decrease from 80 per cent at a pH of 7 to 20 per cent at a pH of 8
• Is highly corrosive and exposure to it can lead to rapid corrosion of unprotected metal
• A toxic gas and hence storing and handling chlorine poses severe risk to environment, health and safety
• Reaction of chlorine with organics produces Trihalomethanes or THMís which are known to be carcinogenic and hence many European countries and America are bringing use of chlorine under increased regulation
• Does not have the ability to penetra teandre move biofilms formed

Major Advantages of Chlorine dioxide
• Very powerful oxidising biocide
• Effective over a broad pH range
• Has 2.5 times the oxidising capability of chlorine
• Selective oxidant
• Unlike chlorine, chlorine dioxide remains a true gas dissolved in solution. Since chlorine dioxide does not react with water, it retains its biocidal effectiveness even over a wide pH range
• In waste treatment applications, where various levels of contaminants are present, consumption of chlorine is significantly higher as that compared to chlorine dioxide to maintain similar residual levels
• Required contact time for chlorine dioxide is lower compared to chlorine
• Destroys phenols
• Has the ability to penetrate and remove biofilm formed and kills bacteria, spores and viruses
• Less corrosive than chlorine and does not hydrolyse to form acid
• Does not form chloramines
• No formation of trihalomethanes upon reaction with organic matter

Water treatment vendors are trying many new molecules and various, as effective, non-oxidising biocides. However the main concerns that exist include safety, biodegradability and effectiveness against a broad spectrum of micro-organisms.

The effectiveness in alkaline pH is another important concern since there has been a shift towards operating cooling water systems in the alkaline pH range and most earlier generation non-oxidising biocides lose their effectiveness in alkaline pH.

Monitoring & Dosing
Significant increase has been observed in focus on developing monitoring tools and feed systems. There has been much increase in concentration on developing equipment and tools that simulate the operating conditions in the cooling water systems to provide meaningful data to ensure proactive management of the cooling water system and treatment programme.

Continuous monitoring and recording of corrosion rates, pH, ORP (oxidation reduction potential) and conductivity has almost become a standard practice. This is supplemented with data from various kinds of deposit and biofouling monitors to give a comprehensive status of the treatment progress. All these data can be made available on the DCS in the control room, making monitoring even more stringent. Critical water parameters like phosphate, zinc and other inhibitors can also be continuously monitored. These can also be provided by the water treatment companies as a part of their value added services.

Once such data are available, automated control systems can be devised to automatically run the feed pumps continuously as per the parameters. Another interesting feed mode based on all these available data is to control the feed pumps with the help of a computer and a modem which can facilitate monitoring and operation of the feed pumps even from a remote location.

Reuse of ETP Water
This is an area where a good audit can be very handy. Of the various operational value additions like flow measurements, re-engineering etc, the recycle of ETP water has become most important and critical for future operations. As mentioned earlier, water is increasingly becoming a very valuable and expensive resource. Its conservation and reuse will therefore be of prime importance in future operations of any plant. The reuse of ETP water is being considered as an alternative option by many plants all over India.

The treatment philosophy will need to undergo a major change depending on the nature of contaminants and biological degradation process will need to be supplemented or replaced with physico-chemical treatment systems. ETP water, properly treated, is being used as partial make up in cooling water systems in plants, where water shortage is acute and cost-intensive. There are also cases where the ETP water undergoes elaborate treatment with a final RO step and final treated water is used as boiler feed.

Finally, the client and their selected water treatment vendor have to work as partners and pool in their combined knowledge to create innovative solutions to suit the specific needs of individual installations. This, we believe is the need of the future.