CO2A Cost Effective Alternative to Sulfuric Acid in Cooling Systems

Captured CO2 from the fermentation process can be used for pH control in cooling water, reducing scale formation.
By Mike Mowbray | June 10, 2010
Sulfuric acid has been used to reduce alkalinity in cooling towers for decades. By neutralizing carbonate alkalinity with acid, scaling potential is reduced on heat exchangers, and often systems can be run with less water. However, handling large quantities of acid can be hazardous and requires special permitting. More importantly, the cost and availability of sulfuric acid varies from year to year. In 2009, acid prices were more than double current costs. World market sulfur prices are increasing, so it's likely that the price of sulfuric acid will be volatile again in 2010.

Using carbon dioxide gas for pH control as an alternative to sulfuric acid can be economically practical in many applications. This is particularly true at fuel ethanol plants which generate CO2 as part of the fermentation process. This gas is vented through a scrubber, where a portion of it can be recaptured for use as a replacement for sulfuric acid. This recovered CO2 can be injected into the cooling water, where it dissolves in the water to create carbonic acid. The acid formed can replace 60-90 percent of sulfuric acid used in the tower. For a typical 50 MMgy ethanol plant, this can save $15,000 to $35,000 annually at today's acid prices.

Benefits to CO2 Use
Carbon dioxide influences water pH differently than sulfuric acid. When sulfuric acid is added to water containing carbonate alkalinity, the alkalinity is neutralized, as shown in the following equation:
When carbon dioxide is added to water containing carbonate alkalinity, the alkalinity is converted to bicarbonate alkalinity, as shown:

Carbon dioxide doesn't eliminate alkalinity like sulfuric acid does. Instead, by converting carbonate to bicarbonate alkalinity, the potential for scale formation is significantly reduced. This distinction is important, and can be useful beyond just pH control in many applications.

There are other benefits to using CO2 for cooling water pH control. First, it is less likely to cause corrosion than sulfuric acid, since CO2 treated water will typically buffer out at a pH of 8.3. It is possible to drive the pH lower with excessive overfeed, but in practical applications, the pH won't ever go below 6.0, so the chance of an accidental acidification of the cooling water is greatly reduced. Also, reducing sulfuric acid feed reduces the amount of sulfate in your discharge water. Again, using a hypothetical 50 MMgy ethanol plant, sulfate concentration in the cooling tower bleed would decrease by up to 200 parts per million, eliminating between 180,000-270,000 pounds of sulfate discharge annually.

One more benefit to CO2 injection is seen in plants trying to reduce water discharge through recycling where the introduction of additional contaminants reduces its usefulness. Replacing sulfuric acid with carbon dioxide can provide more flexibility for water reuse within a plant.

Planning Considerations
U.S. Water Services has successfully converted several plants to pH control using carbon dioxide. This experience has yielded several important lessons, the most important one being that CO2 is not a complete replacement for sulfuric acid. A sulfuric acid backup system must be available for two reasonsas a failsafe in case your gas feed breaks down for some reason and, because a supplemental acid feed may be required in the summer, due to the reduced solubility of carbon dioxide in warm water. In the winter months, we have found that CO2 is usually sufficient on its own.

Carbon dioxide can be transferred into cooling water by means of either a blower or a compressor. At the cooling tower, a specially designed injector is used to introduce the carbon dioxide gas into the system. The location and design of the injector are crucial to effective carbonic acid formulation. If multiple towers are treated from the same system, careful balancing of the piping pressure and header elevation is required, because the gas will always go to the point of least resistance. This can be challenging, as the back pressure will change based on water level in the tower basin, pipe length and even temperature. It's often easier to have a separate feed system for each tower.

Choosing the correct transfer system is also important. Compressor systems are generally the less expensive option, and they can control gas injection over the broadest range of conditions. Downsides for compressors are high noise levels while operating, shorter life time, and higher maintenance costs. In particular, it is critical that all moisture be removed from the CO2 prior to compressing it in the storage tank. Carbon dioxide can dissolve into the water, and form carbonic acid in the bottom of the tank, resulting in tank leaks.

Positive displacement blowers have the benefit of being less maintenance intensive and have a longer life expectancy. They can be fitted with silencers, which make them quiet enough to have in the main plant area. The downside is the up front cost, which is higher than a compressor system. In addition, the operating range is not as broad, so the blower system must be carefully designed for the application in which it will be used.

Microbiological Controls
One of the concerns of adding CO2 to a cooling tower is the quality of that gas. In an ethanol production process, CO2 is recovered from a fermentation scrubber. A well-run scrubber will remove the majority of ethanol, ethyl acetate and acetaldehyde vapors. However, a small percentage will still get into the CO2 and then into the tower, where it could potentially act as a food source for bacteria growth. It is important to maintain good microbiological control in your cooling system when you switch to CO2 for pH control. A well-thought-out biocide program, coupled with regular monitoring for microbiological growth, makes this managable.

Adding carbon dioxide to your cooling water system can be an effective method of controlling cooling water pH. It has the potential to provide several benefits to a plant, beyond just the economic payback. The basic concept is to remove CO2 from the scrubber vent, and transfer it to the cooling tower basin. Some careful design is required to deal with the temperature and moisture content of the gas, and the distance that it needs to be transferred. In addition, there are several ways to control the amount of CO2 added to the system. Even the injection header must be designed specific to the plant layout. Depending on the amount of piping required, and the spot market price of acid, the payback can be 12 months or less. As with any project, the ultimate success will be based on careful design up front, and careful monitoring during operation. EP

Mike Mowbray is the marketing and technology manager for U.S. Water Services. Reach him at or (866) 663-7632.