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INTRODUCTION

Most people see a large cooling tower as a sizable struc-ture polluting the air with a considerable plume of white"smoke." The white column is actually really very cleanwater vapor which has come into contact with cooler air.Cooling towers clean vast quantities of air by acting as airscrubbers. Dust, pollen, and trash in the air are capturedby small water droplets to be collected at the bottom ofthe cooling tower basin in the bulk water. Dissolved miner-als from the make-up water such as hardness ions are alsoconcentrated in the cooling tower as the water evaporates.Evaporated or "clean" water vapor leaves the coolingtower along with the cleaned air as exhaust.

Wind and water borne "additives" collected in cooledwater present problems for equipment using that water.Chemicals and acid are usually added to the water to pre-vent solids from precipitating and fouling heat transfer sur-faces in machinery being cooled. At some level of concen-tration a portion of the high-solids water must be drainedaway to prevent equipment fouling. This quantity of wateris referred to as bleed or blowdown. The water removed asblowdown and lost to evaporation is made-up with a fresh,less contaminated supply. This cycle is repeated on a con-tinuous basis to produce a high-solids waste stream. Theexcessive solids wastewater often presents a disposalproblem for wastewater treatment facilities and regulatoryauthorities.

A method to eliminate or greatly reduce the disposal prob-lem in a user-friendly system has been patented [1]. Usingthis new water chemistry, hardness ions and most silicaare concentrated to an easy-to-handle fluid. With the addi-tion of a flocculant, the solids are easily concentrated to amoist substance in a bag filter.

Water conservation and environmentally friendly technolo-gies are becoming more important, especially in areaswhere water is expensive, restricted or heavily regulated.The impact of water wasted by cooling towers is signifi-cant.

For example, a cooling water system operating at a con-centration ratio of 5 wastes 20 % of the total water used inthe system. A cooling tower operating at 11 360 m3 · h–1

(50 000 gpm), 10 °C temperature difference and a concen-tration ratio of 5 will blowdown 20 400 m3 (5.4 million gal-lons) of cooling water per month.

A typical ethanol plant uses 3.63 m3 of water for each m3

of ethanol produced [2]. Approximately 90 % of the totalutility water is specifically cooling water [3]. The dischargeeffluent comes primarily from cooling tower blowdown [4].A revolutionary new technology can reduce the amount ofwater required from 3.63 m3 to 2.9 m3 for each m3 ofethanol produced. Saving 0.73 m3 of water per m3 ofethanol is a 20 % reduction.

A NOVEL WATER CONSERVATION PROCESS

There is an economical process that can reuse the blow-down of cooling systems. HiCycler® is a registered trade-mark of CHEMICO International, Inc. This unique waterconservation process is designed to reduce and reusecooling water blowdown by up to 95 % by recycling thewater. This is accomplished by concentrating hardnessand silica into a viscous solution. Zero blowdown can beachieved by using one or more flocculants and a filter onthe blowdown from the reactor. A bag type filter has beenused successfully to remove water from the reactor solids.The water is returned to the cooling system bulk water.

This patented process works well in high hardness opencirculation water systems. Hardness in the cooling watersystem is controlled. Hardness and silica are removedfrom the water system as a viscous fluid, which is calledfluid bed.

The advantages of this process include its being a signifi-cant water conservation process. The heat transfer sur-

Achieving Zero Blowdown for Cooling Towers

© 2008 by PowerPlantChemistry GmbH. All rights reserved.

Achieving Zero Blowdown for Cooling Towers

Sam R. Owens and Rick H. Maxey

ABSTRACT

A new chemistry approach provides water savings in hard, alkaline cooling waters. Hardness and silica are removed asa semi-viscous fluid. This economical treatment program reclaims over 95 % of open circulation cooling tower waterblowdown. Significantly reduced make-up and wastewater treatment costs often make this the preferred treatmentprogram.

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faces are cleaner than with conventional treatment. Thecleaner heat transfer surfaces are due to the fact thatorganic salts and chelates of hardness are more soluble atelevated temperatures. This process reduces water sys-tem corrosivity along with any scaling potential of calcium,magnesium and silica. Reduced corrosivity is due to theoperating pH of 8.5 to 9.0. There is a multimetal corrosioninhibitor in one of the chemical blends which may helpreduce corrosivity. Dissolved solids are also rather high inthis process. Total hardness is usually above 650 mg · L–1

as calcium carbonate. Photos of corrosion coupons(Figures 1 and 2) show the marginal corrosion over a six-month interval in a heating, ventilation, and air conditioning(HVAC) system.

The drift from a tower using this process is more solublethan drift from conventional treatments, which is beneficialwhen the equipment is located in high traffic areas. Usingthis process, organic deposits from drift are easy to cleanoff.

This process uses a reactor and two solutions, C-875, abuffer containing organic acids, and C-845 (a conditioningagent). C-875 contains the chemicals required for the initialreaction. C-845 contains the products to drive a chain ofreactions which are necessary for hardness removal. Thereactions and clarification occur inside a vessel or reactor.The solids collect in the bottom of the reactor, where theyare blown down in a concentrated form as fluid bed.

C-875 has the chemicals necessary to form organic com-pounds with calcium, magnesium and some forms ofphosphorus. It reacts with existing scale deposits in thesystem and hardness entering in the make-up water.C-875 is an excellent cleaning agent, but is too slow andexpensive to use by itself as a system cleaner. This solu-tion also contains corrosion inhibitors for multimetal sys-tems. A small portion of the C-875 is lost in the blowdown.Most of it is released in the reactor and returns to the bulkwater in the bottom of the cooling tower.

C-875 is usually added near the reactor outlet. This loca-tion provides ample time for the products to be thoroughlymixed with water prior to entering the cooling water sys-tem.

C-845 contains a different blend of chemicals necessary toprecipitate the hardness. It initiates cross-linking, makingthe material heavy enough to settle to the bottom of thereaction chamber. It is always added in the reactor, caus-ing hardness to form a relatively thick solution. Contacttime inside the reactor determines the density and viscos-ity of the material to be released as blowdown. Typically,blowdown from the reactor resembles a thin milkshake.

This process is quite tolerant. If one or both products arenot added for 2 to 3 days, the fluid bed will continue toremove some of the hardness. This depends on theamount of fluid bed in the reactor at the time the chemicalsare no longer being added.

It is necessary to maintain a quantity of fluid bed in thebottom of the reactor. The reactor blowdown is locatedabove the mixing and contact area in the reactor. The ele-vated position of the blowdown valve reduces the chanceof losing all of the fluid bed during blowdown. The depth orquantity of fluid bed determines the weight of the productmade. 3.8 L (1 gal) of fluid bed blowdown contains0.9–2.3 kg (2 to 5 lb) of hardness as calcium carbonate.

In a field application of this process, the chloride concen-tration in the make-up water was 122 mg · L–1. Chlorides inthe tower water were 6 360 mg · L–1. The highest cycles ofconcentration obtained have been 52. Drift loss account-edfor a significant amount of the system water loss,depending on the condition of the mist eliminators.Maximum cycles of concentration were limited by thecooling tower drift loss. Persistent high winds in CorpusChristi, Texas, U.S.A., where the process was applied,usually limited a system to 22 cycles of concentration.

Figure 1:

Photo of the front of a mild steel (top) and a copper (bottom)corrosion coupon.

Figure 2:

Photo of the back of a mild steel (top) and a copper (bottom)corrosion coupon.

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The reactor solids have a pH of 9.5 to 11.0. Water exitingthe reactor has a pH of 9.4 to 10.2. System water enteringthe reactor is 8.5 to 9.0. C-875 is added to water leavingthe reactor, which lowers the pH by 0.1 to 0.4. This addsthe sufficient reactants required to maintain the system.The system pH will normally buffer to 8.5 to 9.4.

EQUIPMENT

The flow diagram in Figure 3 illustrates the simplicity of theprocess and shows the redirected flow. The side stream isbetween 0.15 and 0.5 % of the recirculation rate; 0.3 % isthe rate used most often.

An interesting event results from this chemistry – the waterusually changes to a golden color.

The system equipment is usually set up on a stainless steelpanel. In some cases a cabinet is used. At a minimum thecabinet or panel contains a controller, manifolds with sen-sors and pumps. The piping and any other equipmentsuch as flow indicators and corrosion racks are alsomounted on the panel.

The preferred system control method is by pH and specificion electrode for calcium or total hardness. If conductivityis measured, it is for information only, not for control. Theflow rate through the reactor is important information toknow. Flow meters are included which read in liters or gal-lons per minute. New controllers were developed for thisprocess because of the dual pH and hardness require-ments. Hardness is measured in the water to and from thereactor. The new controllers include monitoringsoftware. All units are monitored from a centrallocation where controller changes are made asrequired. Several customers also monitor thesystems from their locations. The control equip-ment is located near the reactor.

Reactors are sized based on the recirculationflow rates and on the quantity of hardness enter-ing the system in the make-up water. The hard-ness removal through the reactor is from 150 to950 mg · L–1. The maximum hardness for thereactor supply is maintained at less than1 400 mg · L–1 in the cooling tower bulk water.

There are several factors that affect the reactor'sperformance. They include:

• Flow velocityIf flow velocity is too high, fluid bed is carriedover with the water returned to the coolingtower.

• Chemical feed rateUniform chemical feed results in an efficientreactor. The pump is always set at maximumspeed, with a low stroke rate.

• Fluid bed ageAs the material ages, the fluid can form a thick paste,which slows the reaction process.

• Fluid bed heightWhen excess fluid bed forms in the reactor, flow rate isusually reduced because of the weight of the sludge.The specific gravity of the fluid bed is typically 1.14.With aging and mixing in the reactor some fluid bed hashad a specific gravity of 1.42. At a specific gravity of1.42, the material resembles pancake batter. Thickermaterial will probably plug some reactor drain lines.Where zero blowdown is required, the thicker the reac-tor product, the easier and more economical it is toremove the excess water with filters.

OPERATING COSTS

The cost to operate a system using this process is usuallymore than for chemicals used for conventional treatment.However, a savings is realized through the reduction ofmake-up water and wastewater. The control equipmentpackage without the reactor accounts for most of the initialcosts. The reactor size and cost depend on the system cir-culation and the amount of hardness in the make-up water.

The cost of cooling tower blowdown, make-up water plusdisposal or treatment fees is often more than the dollarvalue of conventional chemical treatment. It is not unusualfor the water savings to be two or more times the salesvalue of the conventional treatment chemicals.

Figure 3:

Flow diagram.

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In 2004, the building of a major bank saved over $27 000 inmake-up and wastewater costs, based on the prior years'expenses.

WATER CONSERVATION EXAMPLE

For a 4 540 m3 · h–1 (20 000 gpm) circulation HVAC systemwith a 6.7 °C (12 °F) temperature difference, operating witha concentration ratio of 5 and using make-up water with atotal hardness of 180 mg · kg–1, the water supply cost is$2.00 per thousand gallons (about 3.8 m3) and the waste-water or disposal cost is $3.00 per thousand gallons.

Evaporation

54.5 m3 · h–1 or 1 308 m3 per day (240 gpm or 36 000 gpd)

Blowdown (at 5 cycles of concentration)

326 m3 per day (86 000 gpd).

Blowdown cost$5.00

86 0000 gal per day · ––––––––1 000 gal

= $430 per day ($12900 per month)

Reminder: it is only the reduction of blowdown and make-up that will produce a savings. Evaporation and drift lossremain the same.

As with conventional treatment, bacteria were an antici-pated problem. While it appears that adding chlorine for aweekly shock treatment is effective, bromine is recom-mended because of the high pH. There has only been onecase of bacteria in this type of system. It appeared that theproblem was caused by fluid bed carry-over mixed withflour from an adjacent bakery. The reactor water pH of 9.2to 10.2 may account for the limited bacteriological prob-lems. The momentary high pH in the mixing chamber mayalso help reduce bacteria. Algae are as much of a problemas with conventional cooling water treatment systems.

SUMMARY

This new two-part water conservation chemistry offers aunique method to treat open circulation cooling water byproviding a major reduction in water consumption.Hardness and silica are removed with this extraordinaryprocess. Water savings is achieved through blowdownreduction of 95 % or more. Concentration of water hard-ness enhances the use of filters to achieve zero blowdown.Zero blowdown can now be achieved at a profit. True zeroblowdown is possible by using a filter on the reactor blow-down.

The economics of this process are justified by the signifi-cant reduction in make-up water and wastewater costs.This green technology provides an excellent alternative toconventional cooling water treatment.

REFERENCES

[1] Owens, S. R, Apparatus and Process for WaterConditioning, 2007. United States Patent andTrademark Office, Alexandria, VA, U.S.A., U. S.Patent 7,157,008.

[2] Jessen, H., Ethanol Producer Magazine 2007 (Feb-ruary), 115.

[3] Stanich, T., Ethanol Producer Magazine 2007 (Feb-ruary), 154.

[4] Stanich, T., Ethanol Producer Magazine 2007 (Feb-ruary), 155.

This paper was presented as part of the 68th AnnualInternational Water Conference®, which took place inOrlando, FL (U.S.A.), October 21–25, 2007.

THE AUTHORS

Sam R. Owens (B.S., Mechanical Engineering, PacificInternational University, California, U.S.A.) has been a reg-istered professional engineer in corrosion since 1976. Hiswork includes research and development focusing on safeand effective products for the user and the environment.He enjoys creating new products and improving existingproducts. He currently has two patents. He has given anumber of conference talks on reducing cooling towerblowdown. He has forty-five years' experience working incorrosion, metallurgy and consulting and fifteen years'experience in petrochemical refinery failure analysis. He isthe founder and serves as president of CHEMICOInternational, Inc.

Rick H. Maxey (B.S., Biology, University of Texas,Arlington, Texas, U.S.A.) has worked in various capacitiesin industrial water treatment. He has given talks on reduc-ing cooling tower blowdown at many conferences. RickMaxey has 25 years of experience working in corrosionand is currently the sales manager of CHEMICOInternational, Inc.

CONTACT

Sam R. Owens4801 Baldwin Blvd. #200Corpus Christi, TX 78408U.S.A.

E-mail: sam@chemico.com