1.0 Use chlorine as biocide
ESEC cooling tower using the city reclaim water which has high organically source, hence there is high biological activity in the cooling tower, city bylaw request cooling daily average cooling tower free chlorine should be 0.5 ppm, ESEC use sodium hypochlorite as biocide to control cooling tower biological activity and also keep service water free chlorine level as pretreatment of the RO feedwater source
Figure 1 cooling tower
2 System
Two bleach tanks installed on site, one take for cooling tower with two bleach pumps another for the service tank which provide two pumps for service tank to keep normal concentration, and two pumps for the normal injection after ultrafiltration system(ZV), two high capacity pumps for the ZV cleaning.
Due to large amount of the cooling water using per day, and highly chlorine consumption rate over 4400kg/day usage, every 5 days it will have 1 truck of bleach offloading, yearly 80 trucks delivery to site. it causes a challenge from the free chlorine offloading, accuracy test free chlorine and maintain chlorine level to keep plant long term reliability and economical operation is challenge for the plant staff.
Figure 2 bleach pump skid
3 Free chlorine test
3.1 method to test the free chlorine
DPD Colorimetric Method used for the free chlorine test, online chlorine meter is HACH CL17 and hands test is DR 3900,
The chemical basis for the DPD chlorine reaction is depicted in Figure 1.1. The DPD amine is oxidized by chlorine to two oxidation products. At a near neutral pH, the primary oxidation product is a semi-quinoid cationic compound known as a Würster dye. This relatively stable free radical species accounts for the magenta color in the DPD colorimetric test. DPD can be further oxidized to a relatively unstable, colorless imine compound. When DPD reacts with small amounts of chlorine at a near neutral pH, the Würster dye is the principal oxidation product. At higher oxidant levels, the formation of the unstable colorless imine is favored — resulting in apparent “fading” of the colored solution.
The DPD Würster dye color has been measured photometrically at wavelengths ranging from 490 to 555 nanometers (nm). The absorption spectrum indicates a doublet peak with maxima at 512 and 553 nm. For maximum sensitivity, absorption measurements can be made between 510 and 515 nm. Hach Company has selected 530 nm as the measuring wavelength for most of its DPD systems. This “saddle”between the peaks minimizes any variation in wavelength accuracy between instruments and extends the working range of the test on some instruments.
Figure 3 DPD free chlorine test method
3.2 free chlorine and total chlorine grip sample test
grip sample use HACH DR 3900 to test free chlorine and total chlorine, test free chlorine within 1 minutes which due to the reaction is very fast of the free chlorine reagent, when the time past, the total chlorine will react with reagent then the reading will increase, and provide the false reading
Figure 3 free and total chlorine grip sample test
Plant operator each shift will grip sample test the cooling tower and service tank free chlorine level and to confirm the online meter reading accuracy.
Free chlorine reagent reacts immediately and directly with the free chlorine in the sample. If given enough time some combined chlorine forms will start to react with the indicator as well. This is why the procedure for free chlorine states to proceed to the next step immediately after the mixing step and that the reacted sample cell should be read within 60 seconds of reagent addition. If given enough time the free chlorine results will more closely resemble the total chlorine results as more and more combined chlorine forms react with the indicator, but no matter how much time is given not all combined forms will react with the indicator. So even if the sample is read hours after reagent addition, the results will always be lower than a true total chlorine result. Therefore it is not possible to get total chlorine results with free chlorine reagent.
Total Chlorine reagent is the free chlorine reagent with additional components. These additional components convert all combined chlorine forms to free chlorine, which then react with the indicator. The conversion of some combined forms to free chlorine take longer than others, which is why there is a 3 minute reaction time for the total chlorine test. But some combined forms will be converted to free chlorine immediately and then immediately after react with the indicator. So even if the sample is read immediately after reagent addition, the results will always be higher than a true free chlorine result. Therefore it is not possible to get a free chlorine result with total chlorine reagent
4.0 Operation challenge and solution
4.1 Increase the chlorine concentration from 12 to 16%
Due to large amount of chlorine truck offload yearly, the design use 12% bleach, Engineer Reviewed feasibility for using 16% Sodium Hypochlorite solution in cooling tower tank and service water treatment tank based on Manufacturer and Tank Liner material manufacturer both approved.
Higher concentration means less truck loads and better control of free chlorine residual. The concentration currently being used is 12%. Utilizing a higher concentration will reduce the number of chemical offloads required. There will also be a cost savings as the chemical supply vendor charges a fee for diluting the chemical down to 12% from the standard 16%
4.2 Install second online free chlorine meter
Challenge:
Designed one HACH CL free chlorine online meter to monitory the cooling tower free chlorine, Redundancy for basin 0.5 ppm chlorine residual all the time is environmental compliance requirements, so each time when the online meter to do calibration (monthly), request manual grip sample every 5 minutes to test the chlorine free chlorine level and logged for future audit, when the chlorine meter in maintenance times, also requests 5 minutes interval test, this is create large amount test task for the plant staff, so install second chlorine analyzer is necessary
Solution:
Install 2nd CWS chlorine analyzer at circ water pump house. Same type as one already installed. feed come from test ports on CWS pump discharges. Plumb a feed each pumps test port to a common line, not use analyzer one suction piping, needs to be independent to the analyzer. changing analyzer, one’s suction feed to this configuration as well to get feed from both pumps. Tee off the test port then have strainer for each analyzer at each discharge. Install cross over valve on the suction piping which for further options if we have issues down the road.
On the DCS set up the alarm if the difference of between two chlorine meters to request to calibration
Figure 4 second online chlorine meter installed
4.3 Increase service tank free chlorine level
Service water low free chlorine will cause rapid bio growth which will migrate into the Reverse Osmosis Units and cause fouling. Service water low free chlorine will cause rapid bio growth which will migrate into the Reverse Osmosis Units and cause fouling. This means loss of water quality, more downtime and frequent offline cleaning. The danger is that the Service tank will become populated with bio growth to a point where it will need to be drained and cleaned which means a total plant outage. There is high organic loading in the Service tank due to the reclaimed water supply coming into the plant.
Solution is increasing the free chlorine level from 1.0 ppm to 2.0 ppm
4.4 Install cartridge filter for each chlorine meters
There is high organic loading in the Service tank due to the reclaimed water supply coming into the plant. Which will cause the online chlorine meter measure chamber fouling very quickly then provide the false reading and injection higher than request chlorine to cooling tower
Solution: install cartridge filter to each online chlorine meter and keep change each shift to minimize the chlorine feeding and also meet environmental compliance
