Tower Water

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FULL-SCALE PILOT INVESTIGATION INTOOZONE TREATMENT FOR COOLINGTOWER WATER

Brendan van WykAir Products South Africa (Pty) Ltd.

Abstract

Air Products South Africa (Pty) Ltd. andOzonic, the local Wedeco representatives,have successfully piloted a full-scale ozonetreatment system (OxyZONE) for treatingcooling tower water at an Air Productsproduction facility in Vanderbijlpark.

The system has improved the efficiencies ofthe two heat exchangers and the cooling

tower. The tower is operating on an almostZero Blowdown basis; with only water used forback washing the sand filters being sent tosewer. Concentration cycles have beenimproved from two cycles with conventionalwater treatment to about seven cycles utilizingthe OxyZONE system.Savings realized are; overall waterconsumption of the system due to increasedcycles of concentration and power savings onthe compressors, due to improved heattransfer efficiencies.

Introduction

Air Products had been experiencing problemswith the current chemical based cooling towerwater treatment system, including:

- High bacterial plate counts (21,100 cfu/ml)- Loss of efficiency on the heat exchangers

due to fouling, scaling and corrosion.- Towers operating at less than two times

cycles of concentration.

- High water and chemical consumptions, aswell as high operating costs.

- Increased compressor power consumption.- Loss of overall production efficiencies- Large volumes of low quality blow down

water being sent to sewer.

After extensive research, Air Products andOzonic, the local Wedeco representatives,designed and built a containerized ozonetreatment system. This containerized systemwas installed on a small cooling tower within

the Air Products facility in Vanderbijlpark toinvestigate this technology.

Being containerized, this unit is fully portableand can be relocated for other pilot trials.

Equipment description

OxyZONE:

The OxyZONE system consists of:

- A 1.5kg/hr Wedeco Ozone Generator(oxygen fed).

- Two process water pumps (combinedflow of 60m3 /hr) with two venturis forintroducing the ozone gas into theprocess water

- A separate cooling water pump forcirculating cooling water to the ozonegenerator.

All that is required when the container isplaced on site is an electrical supply andplumbing into the cooling tower watersystem.

Figure 1: OxyZone Container

Maintaining a set Oxidation/reduction Potential(ORP value) in the process water controls theozone dosage.

Cooling Tower:

The tower chosen for this pilot test is used to

cool the oxygen from a 4.5MW two-stagecompressor. The tower has a basin volume of102m3 and a recycle rate of 325m3 /hr. Thecooling water is pumped through two shell-and-tube heat exchangers running in parallel,one after each stage of compression.The gas temperatures entering the heatexchangers are on average 1250C and thetarget exit gas temperature is below 300C.The lower the exit gas temperature achievedthrough the heat exchangers, the moreefficiently the compressor operates.


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Figure 2: Cooling tower used for pilot study

Trial Objectives

The objectives of this pilot trial were to provethe efficiency of ozone as a treatment methodby:

1) Improving the heat transfer efficienciesof the cooling tower and heatexchangers

2) Stopping all chemical treatment of thecooling tower water

3) Increasing the cycles of concentrationof the system in order to reduce waterconsumption and minimize blow down

4) Ultimately save money on operatingand power costs

Methodology

The water quality in the tower, and the heatexchanger efficiencies were monitored for a

few weeks prior to starting the trial in order toobtain a baseline for comparison with theozone treated system. Once the OxyZONEcontainer was installed on site, the existingwater treatment system, consisting of pHcontrol, biocides, anti-corrosion chemicals,anti-scaling and anti-foaming agents wasdisconnected, and the ozone treatmentstarted.

The trial was started by running the ozonegenerator at maximum output (1.5kg/hr, 13%

ozone in an oxygen stream) to overcome theinitial algae and bacterial load. After 5 hours ofoperation, the ORP had increased from 250mVto 750mV. The generator power was reducedto 50% and an ORP reading between 700mVand 750mV was maintained for the next 4days. After this period, generator power wasfurther reduced, maintaining an ORP of below650mV, correlating to about 200g/hr of ozonebeing injected into the cooling tower basin. Themake-up and basin water was then sampledregularly and sent to ERWAT Laboratories for

analysis.

The following method was used to calculatethe efficiency of the heat exchangers:The difference between the exiting cooled gasand the cold incoming cooling watertemperatures were monitored. The greater thetemperature difference (Delta T) is for the heatexchanger system, the more inefficient that

system is running.

Figure 3: Heat Exchanger Efficiency

The pH, conductivity, ORP, residual ozoneconcentration, temperatures and flows weremonitored hourly.

Results and Discussion

Water Control and Chemistry

Water is lost from the cooling tower in threeways: evaporation, drift, and blowdown.

The rate of evaporation from the tower isdetermined by the heat load on the tower, andarises from air passing through the tower andabsorbing heat and mass. The evaporation ofwater concentrates up the salts in the basin.

Drift occurs when water droplets becomeentrained in the discharge air stream and areblown out of the tower. This acts as a purge ofthe basin as this water contains salts.

Blowdown is the intentional bleed-off of waterfrom the basin in order to reduce theconcentration of salts and contaminants.

All the above water losses need to beconstantly replaced with fresh make-up water.Water savings can be realized by reducingwater blown down to sewer, however, there isa limit to the amount that blowdown can bereduced. This limit is set by the systemspotential to scale the heat transfer surfaces.

Hotgas

Cooledgas

ColdWater

HeatedWater

MonitorDifference


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Table 1: Water quality before ozonation

Table 1 shows the initial analysis of theincoming make-up water and the basin waterprior to ozonation. The table shows that theexisting water treatment method was noteffective in treating the bacterial contaminationof the cooling tower and that the water was notbeing cycled up effectively.

Due to 3rd party water supply issues, thesupply of make-up water was switched fromrecycled industrial water to primary treatedriver water just prior to the start-up of theozone generator. The make-up water waschanged back to recycled process water two

weeks into the trial of the ozone system. Ascan be seen from Table 2, this water had highTDS (1822mg/l) and high conductivity(309uS/cm). This change in water quality hadvery little effect on the operation of the coolingtower as can be seen in Graph 1.

Prior to start-up of the ozone system, thevisual quality of the water in the basin waspoor, as can be seen in Figure 4. After 4 daysof operation, the water clarity increased to apoint where the bottom of the basin was clearly

visible.

Table 2: Water quality after ozonation

Figure 4: Water quality before and afterozonation

The cycles of concentration were increased inthe tower by closing the blowdown valvecompletely. The tower is configured in such away that basin water is used to backwash thesand filters. This means that the basin ispurged every time the filters are backwashed.This is not the most effective system forbackwashing sand filters as high TDS and TSS

water from the basin can scale and plug thefilters. It would be more effective to backwashfilters with make-up water.

Make Up Basin Make Up Basin

31-Mar 31-Mar 10-Jul 10-Jul

Conductivity

(mS/m @25'c) 240 86 16 44

pH 7.8 8.1 7.8 8.2

TSS

(mg/l @105'C)


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The limit on cycles of concentration that maybe obtained in a cooling tower water systemusing ozone for treatment is far higher than forconventional treatment methods.

Using a conventional water treatment system,

and taking into account the standardSaturation Index, as below;

Saturation Index:1). P1 = log 10 (calcium * 2.5 * total alkalinity)2). P2 = (max. system water temp, F* 0.00912) + P13). P3 = (log 10 (conductivity * 0.8) * 0.1) + 12.274). pH saturation = P3 P25). SI = pH of cycled water - pH saturation

The tower would be limited to two cycles ofconcentration before scaling would become aproblem with this make-up water.

However, using the Practical Ozone ScalingIndex (POSI number) developed by Alan Pryorand Terill Buffum as shown in the formulabelow, a minimum of 3.74 cycles ofconcentration is obtained for the same make-up water.

Maximum conductivity:= 10 (1/(log(CaxMg)/(Na+Cl)]xlog(Alk/10)x cond(mu)

Where:Ca = Calcium Hardness in makeup water (AsCaCO3)Mg = Magnesium Hardness in makeup water(As CaCO3)Alk = Total Alkalinity makeup water (CaCO3)Cl = Chlorine in makeup water (Cl-)Na = Sodium in makeup water (Na+)Cond(Tower)= Tower water ConductivityCond(MU)= Makeup water Conductivity

This formula gives the maximum conductivity

that the water in the basin should obtain beforeblowdown becomes necessary to avoidscaling. The cycles of concentration that canbe obtained is then calculated by dividing thePOSI number by the conductivity of themakeup water. The authors state that theresulting figure is conservative, and the towermay operate at far higher cycles thanpredicted. This was found to be correct as thewater in the basin is currently running at closeto 7 cycles and there has been no evidence ofscale, or loss of heat transfer efficiency.

Although there have been numerousdocumented cases of ozonation systems

operating for extended periods with zerointentional blowdown, the underlying physical-chemical mechanisms are not well understood.

One mechanism that has been proposed forthe ability of ozonated systems to run at highcycles of concentration is that under the

influence of ozone, calcite tends to precipitateout into the bulk of the water body. Because ofthe very high surface area of the precipitatingcrystals compared to the metal surface in thesystem, continuing precipitation leads tocrystal growth in the bulk water rather thanscale formation on the heat transfer surfaces.

This theory seems to hold true, as it was foundthat the concentration of calcium did notincrease in the basin water to the same extentas the more soluble metals like sodium. Thegraph below shows the cycles of concentrationfor conductivity, calcium, sodium and chlorine.As the cycles of concentration increase for theconductivity, so the soluble elements follow,increasing in concentration, however, calciumseems to maintain a steady state at about 4cycles of concentration. This means thatcalcium is being removed from the water, but itdoes not do so as scale.

Graph 1: Cycles of concentration of elements

as Conductivity increases

After the analyses were received, the POSInumber was calculated, and the basin waterwas controlled on the maximum conductivity of14,000 uS/cm that was calculated.This corresponds to about 7 cycles ofconcentration, as can be seen from the plot ofconductivity and pH of the makeup water andthe basin water in graph 2.

0.00

1.00

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31-Mar 10-Jul 19-Jul 25-Jul 8-Aug 14-Aug 15-Aug 22-AugCl Na Ca Con


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Graph 2: pH and Conductivity of Make-up water

and Tower Basin Water

Heat Exchanger and Tower Efficiency:

The baseline results show that in the week priorto the start of ozonation, the Delta Ts had risenby over 30C, indicating that the heat transfersurfaces were becoming fouled, decreasing thecompressor efficiencies. Once the system wasconverted to ozone, there was an immediatereduction in the Delta Ts, with more than 30Cbeing recovered within 2 weeks of operation.

This efficiency was obtained, even while thecycles of concentration were being increased.

Graph 3: DeltaTs for heat exchanges beforeand after ozonation

Due to improved efficiencies of the heatexchangers, power consumption of the oxygen

compressor is reduced for the same volumethroughput. For every 3oC that the gas iscooled, there is a 1% saving in the powerconsumption of the compressor.

The lowest temperature that cooling tower

basin water can obtain is dependant on the wetbulb atmospheric temperature.

In order to monitor the efficiency of the coolingtower the water temperature in the basin, andwet bulb temperatures were monitored and thedifference plotted in the graph below.This plot shows that the cooling tower started tooperate more efficiently after ozone treatmentwas initiated.

Graph 4: Delta Ts for cooling tower

The tower efficiency increased as a result of thealmost total removal of algae and biologicalfouling present on the tower wetted surfaces.As shown in the tables above, the biologicalactivity in the tower was reduced from platecounts of as high as 14,200cfu/ml to as low as42cfu/ml after ozonation. This result wasachievable, even when the incoming make-upwater had plate counts as high as9,300,000cfu/ml.

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Corrosion:

The corrosion rates of the metals in the heatexchangers and pipe work are currently underinvestigation; however, chemical analysis of thebasin water does not show any corrosionproducts being present.

Various papers written on the subject ofcorrosion in ozonated systems show thatcorrosion should be no worse than conventionalsystems.R.J. Strittmatter et al, state the rate of corrosionis dominated by the saturation level of the water,

and not by the presence or absence of ozone in the

system. They also found that the corrosion ratesdropped as the cycles of concentrationincreased, with low-cycle tests having thehighest corrosion rates, and high-cycle testshaving extremely low corrosion rates.

Conclusions

The full-scale pilot investigation has proved tobe a success. All chemical treatment to thetower has been stopped; only ozone is used totreat the water. Heat transfer efficiencies haveimproved in both the heat exchangers and thetower packing. Almost zero blowdownconditions have been maintained in the towerfor 4 months with the only bleed coming from

the backwashing of the sand filters. The cyclesof concentration have been improved from 2cycles to 7 cycles, saving water. The overallsystem has proved to be simple to operate andeasy to maintain, while still being aneconomical alternative to conventionaltreatment methods.

Acknowledgements

The author would like to thank Cobus Coetzeeand Leon de Goede from Ozonic, and Phillip

Fivaz from CP Projects for their design andconstruction of the Pilot Plant, as well as theirhelp in the operation of the system. We wouldnot have been able to get the results we didwithout the able assistance of Noluvuyo Godlo,our operating student, and all the operationsstaff at our Production Facility in Vanderbijlpark.

Reference:

1. A.E. Pryor and M. Fisher, PracticalGuidelines for Safe Operation of CoolingTower Water Ozonation Systems,Ozone Science & Engineering Vol.16April 1994

2. D.J. Tierney, Ozone for Cooling TowerSystems An Update and LessonsLearned at the Kennedy Space Centerwww.zentox.com/CleanStreams/Ozone_Cooling_Tower_Systems_KSC.pdf

3. R.G. Rice and J.F. Wilkes,Fundamental Aspects of OzoneChemistry in Recirculating CoolingWater Systems Data EvaluationNeeds Ozone Science & EngineeringVol. 14 January 1992

4. R.J. Strittmatter, B Yang and D.A.Johnson, Application of Ozone inCooling Water Systems, NationalAssociation of Corrosion EngineersCorrosion92 Meeting, Tennessee, April27- May 1 1992.

5. Advanced Cooling Tower WaterTreatment,http://www.nelsonenviromentaltechnologies.com/ACTWT.html

6. U.S. Department Of Energy, FederalTechnology Alert Ozone TreatmentFor Cooling Towers,http://www.pnl.gov/fta/6_ozone.htm

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