Reducing Cooling Tower Acid Usage -- Key Concepts and Concerns
Dan Sampson, Industry Technical Consultant, Nalco Company
While the recent economic downturn has mitigated the issue somewhat, price volatility and supply issues are expected to continue. That being the case, there's a strong driver to minimize or eliminate sulfuric acid use in cooling towers. Cooling tower pH control represents the single largest acid use at most power plants. While it may be possible to minimize sulfuric acid usage, there are several technical and operational challenges that must be addressed.
Acid -- The Alkalinity Destroyer
It's important to understand the role that acid plays in controlling pH and that requires a basic understanding of the relationship between pH and alkalinity. Alkalinity occurs naturally and enters the cooling water with the makeup. Alkalinity remains in the water and its concentration increases as evaporation occurs. Increasing alkalinity increases pH. Carbonate and other scales can form more readily as pH and alkalinity increase. Figure 1 shows the relationship between M-Alkalinity and pH. The curve in Figure 1 is commonly used in modeling, but it's not consistent across all cooling systems. Any sulfuric acid reduction program must begin with an accurate model of the pH/alkalinity relationship in the specific cooling system under study.
The M-Alkalinity measurement is simply an acid titration -- acid is added to sample until the pH lowers to approximately 4.3 (the M-Alkalinity endpoint). As such, the M-Alkalinity measurement tells us the total alkalinity in the sample. This total alkalinity may be present in several different forms. Figure 2 shows a curve similar to Figure 1, but describes the relationship between the different alkalinity species as pH changes. At higher pH we see carbonate and bicarbonate alkalinity. We convert these forms into carbon dioxide as pH lowers through acid addition. The free carbon dioxide formed is scrubbed into the atmosphere as cooling water recirculates through the tower. Sulfuric acid destroys alkalinity by liberating CO2 to the atmosphere through equations (1) and (2).
(1) H2SO4 + Na2CO3 --> H2O + CO2 ^ + Na2SO4 (Destruction of Carbonate Alkalinity)
(2) H2SO4 + 2NaHCO3 --> 2H2O + 2CO2 ^ + Na2SO4 (Destruction of Bicarbonate Alkalinity)
The amount of acid we need is directly proportional to the amount of alkalinity we want to neutralize and that depends on the pH we want in the recirculating cooling water. It would be nice to simply shut off the acid feed and run the cooling tower at higher pH (and higher alkalinity). Unfortunately scaling tendency increases with pH, so it's not quite that easy.
Figure 2: pH and Alkalinity Species in Cooling Water
How High Can I Run My Cooling Tower PH And How Much Can Save?
The savings potential is necessarily plant specific. It depends on a host of factors including the concentration of scale forming minerals (primarily calcium and alkalinity) in the makeup water, the current pH target, and cycles of concentration. Table 1 analyzes potential acid savings by comparing normal and high pH cooling tower operation.
Table 1: Acid Reduction Example (Makeup versus Cycles versus M-Alkalinity
The red area in Table 1 indicates the "no go" zone -- the point above which scale formation will occur. A separate mineral solubility analysis (discussed later) performed for this example indicates that scale formation will occur if M-alkalinity exceeds 400 ppm.
In this example the maximum allowable M-Alkalinity was set at 375 ppm to provide some protection from transients based on the mineral solubility analysis. The "Normal pH Operation" cases assume tower operation at a pH setpoint of 7.65 and an M-Alkalinity of approximately 80 ppm. The "High pH Operation" cases limit M-Alkalinity to 375 ppm with a corresponding pH of approximately 8.8. At very low cycles of concentration (3 cycles or less), no sulfuric acid feed is required and M-Alkalinity is naturally less than the 375 ppm limit. The need for sulfuric acid feed increases as cycles increase and potential savings decrease as cycles increase due to the concentration of makeup alkalinity in the tower circulating water.
Figure 3 shows the change in operation. The increase in pH allows an increase in alkalinity and a reduction in sulfuric acid feed equal to the increase in alkalinity.
Figure 3: Operating Window -- Increasing pH from 7.65 to 8.81
Table 2 shows the impact on sulfuric acid usage in pounds and dollars for one scenario. This particular scenario assumes operation at 4 cycles of concentration and an M-Alkalinity increase in the cooling water from 80 to 375 ppm. Acid cost decreases by approximately $210,00 per year and acid usage lowers by approximately 92 percent in this scenario.
Table 2: Cooling Tower Sulfuric Acid Calculations
The Need For Mineral Solubility Analysis
Detailed mineral solubility analysis is critical before plants attempt to minimize sulfuric acid use. Simply increasing pH is an invitation to disaster -- or at least a badly scaled condenser. There are many mineral solubility programs available in the marketplace and most specialty chemical suppliers have been trained in their use. Plants must either educate themselves or get expert advice before attempting to minimize acid usage. Table 3 shows the typical output from a computer program that evaluates the risk associated with corrosion, scale, and microbial growth based on makeup water chemistry, cycles of concentration, pH, and product dosage.
The willingness to accept additional risk is an important component of an acid reduction program. Any increase in pH or alkalinity carries an increased risk of mineral scale formation. Plants can mitigate risk through automation. Redundant pH analyzers, for example, can minimize the potential for high pH transients. Similarly, changes in acid feed systems to include variable speed and redundant pumps can improve pH control and minimize risk further.
Risk still increases, however, and mineral solubility analysis has an important role to play. The analysis should be performed initially and periodically thereafter to ensure that the treatment program and operating limits continue to provide adequate protection. It should become part of the plant's monitoring program. Condenser performance monitoring should also be used, if it's not already, as part of the normal monitoring program.
Table 3: Mineral Solubility Program Output Example
Chemical Treatment Programs
A chemical treatment program designed for use at lower pH probably won't work as pH increases. Mineral solubility analysis again can help. Specialty chemical suppliers should know what scales their products can inhibit and, more importantly, the point at which those same scale inhibitors will fail. These specific product capabilities must be included in the mineral solubility analysis.
In general, operation at higher pH requires significant changes in the treatment program. Bleach, for example, may work well at lower pH but it loses its ability to control microbial growth at higher pH and bromine compounds may be required. General corrosion rates tend to lower as pH increases, but traditional corrosion control chemistries (like phosphate) may increase the risk of scale formation. Finally, the function of scale and deposit inhibitors is extremely pH dependent. A product that works well at lower pH may work poorly or not at all as pH increases. In addition, additional scale inhibitors may be required to stabilize scales that don't form at lower pH.
Why Not CO2?
Many plants attempt to replace sulfuric acid with carbon dioxide. That can work, but it's important to understand the limitations of carbon dioxide when it's used for pH adjustment. The addition of carbon dioxide to cooling water temporarily shifts carbonate alkalinity to bicarbonate as pH is lowered by formation of carbonic acid. This does work to lower pH in the circulating water, but the CO2 that's liberated is stripped in the cooling tower. So cooling water pH returns to equilibrium as the water drops through the cooling tower fill and the carbon dioxide is stripped. Tower M-Alkalinity stays the same. Figure 4 shows what happens.
This stripping action limits the usefulness of carbon dioxide for pH control. Large amounts of carbon dioxide may be required because it's all lost across the cooling tower. The use of carbon dioxide may have future environmental impacts in terms of greenhouse gas emissions. Injection points are critical. The gas must be injected into the circulating water after the water leaves the tower. Carbon dioxide can be injected into the discharge side of the cooling tower circulating water pumps or into the cooling water supply header just before the condenser. In either case the carbon dioxide can lower the pH in the pipes and condenser, but pH will again increase when the water passes through the tower. Carbon dioxide can be used to protect the condenser, but pH of the cooling water passing through the tower fill and in the tower basin cannot be adjusted and scale formation may result.
Carbon dioxide can be used, but plants need to have a clear understanding of its limitations.
Figure 4: CO2 Stripping in a Recirculating Cooling Tower
Final Thoughts
Lowering acid usage offers several benefits in addition to cost reduction. Safety improves as acid truck deliveries decrease (see Figure 5). Operators save time from fewer acid trucks. Corrosion rates tend to decrease at higher pH. There's an environmental benefit --increasing pH actually lowers the amount of carbon dioxide stripped into the atmosphere and so lowers the plant's greenhouse gas emissions. Sulfate and phosphate concentration in the cooling tower blowdown are also lower -- another important environmental concern for many plants. Zinc, if used, can often be discontinued or fed at a significantly lower dosage as pH increases.
Figure 5: Acid Truck Deliveries Before, During, and After Acid Reduction Project
While these benefits are considerable, acid reduction requires careful study, monitoring, and control. Mitigate risks with new chemistries, new equipment, new alarms, and increased monitoring.
Plants can operate safely and use less acid. Attention to the details described here can help ensure success.
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