March 1, 2010

Strategies to Reduce Sulfuric Acid Usage in Evaporative Cooling Water Systems

Daniel C. Sampson, WorleyParsons

Automate Your Cooling Tower

Minimizing acid usage requires higher pH and increases risk. Manual control of cooling tower chemistry increases this risk. Using online analyzers and monitoring tools can significantly mitigate the risk of scaling up your equipment. Specifically, the following online analyzers should be part of the cooling system monitoring program:

• pH

• Specific conductivity

• Turbidity

• Oxidation/reduction potential

• Microbiological activity

• Corrosion rates (specific to cooling system metallurgy)

• Total dispersant feed

• Dispersant residual

• Dispersant consumption

• Scale formation (using a heated element)

These instruments provide information on those key performance indicators that can quickly determine cooling system scaling, deposition, and corrosion potential. Information on dispersant feed and consumption is especially important. An increase in dispersant consumption, for example, provides a leading indicator for scale formation or deposition. It should be used to automatically adjust chemical feeds as required to maintain acceptable deposit and scale control. Likewise, online corrosion rate analysis allows rapid adjustment of the treatment program before damage occurs. Corrosion coupons should also be used, but they can only quantify corrosion after it has occurred.

A Case History

A nuclear plant with a pressurized water reactor wished to minimize sulfate concentration in the steam cycle. The premise was that minimizing sulfate in the cooling tower would result in less sulfate ingress to the secondary system. The expected payback included shortened chemistry holds during start-up due to lower sulfate levels, steam generator life extension due to fewer impurities, fewer acid trucks to unload, and a safety improvement associated with handling less acid.

Sulfate in cooling towers averaged 700 to 800 ppm and came from two sources. Some sulfate entered the cooling system with the makeup water (approximately 40 ppm) and cycled approximately 3.5 times to provide a cooling water sulfate level of approximately 140 ppm. The majority of sulfate came from acid addition (approximately 500 to 600 ppm).

The study evaluated several solutions, including the use of other acids, minimizing cycles of concentration (to reduce the sulfate contribution from the makeup water), and increasing pH (to minimize sulfate contribution from the acid feed). The use of other acids wasn’t practical, so the focus shifted to the other two water treatment alternatives.

Modeling and Mineral Solubility Analysis. The pH/alkalinity relationship was validated using existing plant data. The pH/alkalinity pairs were plotted and curve fitting was used to establish the relationship. Modeling indicated that pH could be safely increased from 8.4 to 8.8 and cooling tower cycles of concentration could be reduced to 2.45. Sulfate concentration in the cooling tower would lower to approximately 300 ppm (at least a 50% reduction). Modeling also predicted a substantial decrease in acid handling — about a 48% reduction in the number of acid trucks and subsequent deliveries.

The modeling included several scenarios. The preferred solution minimized acid usage and sulfate contribution. Specialty chemical costs increased, but H₂SO₄ costs decreased, and this solution was essentially cost-neutral.

A pilot cooling tower study provided confirmation of the modeling results. Cycles of concentration were set at 2.5 with a pH control setpoint of 8.8. The scale inhibitor was changed from HEDP to PBTC to provide additional calcium carbonate scale inhibition. The zinc-phosphate corrosion inhibitor was eliminated.

The pilot cooling tower study indicated no scale present on heat exchanger tubes. Corrosion rates were low (<2 mpy mild steel and <0.1 mpy copper nickel and stainless steel). Acid consumption and sulfate both dropped by approximately 50%, and there were no microbiological control issues.

The plant achieved other performance improvements: reduced sulfate in the cooling tower blowdown, less accumulation of mud and silt (because of operating at lower cycles of concentration), and significant reductions in both zinc and phosphate in the cooling tower blowdown. The reduction of sulfate, zinc, and phosphate in the blowdown were especially advantageous because the plant discharges directly to a river.

Identifying and Mitigating Risks. The acid reduction program increased the risk of scale formation (by operating at a higher pH) and also increased the risk of a National Pollutant Discharge Elimination System violation under the Clean Water Act, as the proposed operating pH (8.8) was closer to the plant’s discharge limit of 9.0.

The risk of scale formation was mitigated in several ways. Changes to the cooling tower chemical treatment program included automatic feed of a more powerful calcium carbonate scale inhibitor. Automatic feed was based on consumption; the controller can sense the onset of scaling and react immediately with increased dispersant feed. Alarm functions and an online deposit monitor were also added to the instrumentation suite.

The risk of a pH violation necessitated a thorough design review of the acid feed system. The review concluded that the existing acid feed system was sufficiently reliable. It was gravity-based (no pumps), included two separate systems, and had operated for 20 years with an availability of >99%. Additional pH alarm functions were added, and the calibration frequency of the five existing pH analyzers was increased. Finally, operating at lower cycles of concentration provided a slower rise in both alkalinity and pH in the unlikely event of a loss of acid feed.

With mitigation measures in place, the plant initiated a trial. Data after six months of operation confirmed findings from both the modeling and pilot cooling tower testing. Cooling water pH averaged 8.70 to 8.75 at 2.2 to 2.7 cycles of concentration. Makeup water alkalinity averaged 250 to 270 ppm with cooling water alkalinity of 400 to 450 ppm. The reduced cycles of concentration led to higher concentrations of active polymer (from about 1.5 ppm to an average over 3 ppm). Zinc corrosion inhibitor has not been needed to achieve acceptable corrosion rates (between 1.0 and 2.0 mpy, down from 2.5 to 4.5 mpy). Reduced acid usage also reduced the acid truck deliveries from a high of 13 trucks per week to an average of five trucks per week.

—Daniel C. Sampson (dan.sampson@worleyparsons.com) is water/wastewater engineer in the WorleyParsons Sacramento Office.