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National Starch & Chemical Co.’s (NSC) specialty organic chemicals Plant 98 requires temperature control between 5 to 165 degrees Celsius, so the reactor jackets are supplied a propylene glycol/water mixture to operate from 5 to 20 degrees Celsius (mode 1); cooling tower water to operate from 15 to 25 degrees Celsius (mode 2); steam injection into cooling tower water to operate from 25 to 110 degrees Celsius (mode 3); and steam to operate from 110 to 165 degrees Celsius (mode 4).
Each jacket is blown out with air before switching to the next jacket mode. Even with extended jacket blowout times, some residual glycol remains in the reactor jacket piping, which contaminates the cooling tower water.
A wide variety of chemical treatment programs for the cooling tower were tried over the past 10 years, with little success.
The primary mechanism of corrosion in the system was bacteria related. It was theorized that residual glycol, together with the expected natural organic loading from the air, provided an environment for excessive bacteria growth, resulting in severe pipe corrosion.
Faced with increasing chemical costs, a deteriorating cooling tower piping system and high maintenance repair costs for piping leaks, NSC decided to try ozone treatment.Principles of operation
Ozone was chosen due to its superior oxidizing power and its capability of being generated on-site. Ozone generation is accomplished by passing a high-voltage alternating current (8-20kV) across a dielectric discharge gap through which oxygen-enriched air is injected. As oxygen is exposed to the current, some molecules disassociate to mono-atomic oxygen and combine with diatomic oxygen molecules.
The ozone-rich gas stream is then introduced into the cooling tower water through a side stream using a venturi as an injection device.Technology application
Ozone treatment has found success in cooling towers associated with HVAC or light industrial processes with minimal (mostly airborne organic materials) demands. It has experienced only limited success in heavy industrial processes or cooling towers operating with high organic loading, water temperatures greater than 40 degrees Celsius, poor quality make-up water; and long piping systems.
When sizing a typical ozone system, the rule of thumb is to apply 0.1 gram/hour/ton of cooling tower capacity (based on 85 degrees Fahrenheit water and typical COD loading). Plant 98 has a 209-ton tower recirculating at 600 gpm. Therefore, an ozone system producing 21 grams ozone/hour (0.1 x 209) was required.
The initial ozone unit was a Marley Ozone Model #MOL140 with three electrodes, each producing about 10 grams/hour/electrode, for a total of 26.5 gram of ozone per hour.
After oxygen enrichment of the air supply the resultant feed gas stream was approximately 90 percent oxygen. Electrodes in the ozone generator transform the oxygen so that the gas stream leaving the electrodes contains about 3 to 5 percent ozone (about 15 scfh total air flow).
The ozonated gas stream was introduced by venturi injection into a cooling tower sidestream supplied by a 0.75 HP pump producing 25 gpm at 25 psi. A venturi injector with 5 to 7 psi differential pressure provided adequate vacuum to pull the gas stream into the water.
The water sidestream to the ozone injection was supplied from the bottom of the cooling tower basin, and the ozonated liquid return stream was directed into the ends of the cooling tower basin through distribution pipes containing drilled holes to provide even distribution.Initial operation
The target oxidation-reduction potential (ORP) setpoint for the unit was set at 600 mV, with ozone supplied at maximum capacity until the setpoint is reached. The initial startup of the system resulted in an immediate bacteria kill in the tower basin, followed by the subsequent release of a bio-film/rust particulate layer from the surface of the cooling tower supply and return piping.
Ferrous iron was oxidized to ferric iron, coloring the water red. This confirmed the seriousness of the corrosion problem and effectiveness of the ozone system but necessitated several complete flushes of the piping system to remove the initial suspended solids load and avoid cleaning out pick heater and compressor heat exchanger tube bundles.
As the ozone residual penetrated further along the piping system, additional suspended solids were generated and a regular clean/flush of the tower was scheduled every two months.
Make-up water quality was good, with TDS less than 250 mg/L as CaCO3, turbidity at 7-10, pH at 7.15 and low hardness, so the system recirculating water quality continued to improve with blowdown.
After the initial startup and cleanout period of three months during the second to third quarters of 2000, the system was able to maintain at or near its ORP setpoint during the fall and winter months of 2000-2001.
However, during the summer of 2001, the ozone system was not able to maintain the ORP setpoint the majority of the time due to the higher water temperatures and a higher than expected COD load from residual glycol.System improvements
To achieve desired system performance (steady ORP of 600 mV) during future summer months, additional electrodes were installed during December of 2001 to increase ozone system capacity. These extra electrodes provided an additional 8.9 grams ozone/hour based on a recommendation that excess ozone be provided in a 0.5:1 ratio of applied ozone to organic load in excess of 8 mg/L of TOC in the makeup water.
This increased the total ozone system capacity by 30 percent, or 34 grams ozone/hour, almost double the typical ozone system capacity for Plant 98’s size of tower.
Additional improvements to the system during 2002 to 2003 included upgrading the cooling tower water sidestream pump from 25 gpm to 75 gpm to improve contact/turnover time, directing the sidestream return flow against the bottom corner of each end of the cooling tower basin to provide a sweeping flow action to prevent solids accumulation behind the return flow distribution piping, and replacing an existing screen filter with a bag filtration system for more effective removal of small suspended solids.Corrosion monitoring
Despite the significant release of corroded piping particles during biofilm destruction upon start-up of the ozone system, the number of piping leaks and associated maintenance repair costs immediately showed a dramatic decrease.
An examination of reactor jacket piping removed after six months of ozone system operation showed the pits left over from classic bacteria corrosion, but a clean, smooth internal piping surface everywhere else.
Before the ozone system, corrosion rates were characterized as poor or less than poor based on maintenance repair data, with problems including pitting and repeated failures at elbows and fittings. Table 1 (see below) shows corrosion rates measured over a two-year period after ozone system startup using a standard metals coupon rack.
The stainless steel and copper corrosion rates were considered excellent and the steel corrosion rate was considered good. Interference of the passivation layer by glycol slime was theorized as the reason for the variable steel corrosion rates.
However, if tower water pH fell below 7 and conductivity below 300 uS/cm, increased corrosion rates for mild steel were seen.References
Jeffrey Mueller, PE is the engineering supervisor at National Starch and Chemical Company’s electronics and engineering materials specialty organics plant, Salisbury, NC.
William Ketchie is a PhD candidate at the University of Virginia, Charlottesville, VA. He completed three semesters of co-op work as a chemical engineer at NSC Salisbury’s specialty organics plant.