Oxidation chemistry has many uses in heavy industrial water treatment. The major ones are microbiological control, emulsion breaking, H2S and phenol destruction.
There are several different oxidation chemistry’s that can perform these various jobs when properly applied. However, there are downsides to most of them; either safety, shelf-life, environmental or cost. While there is no universal panacea, there is one oxidizing chemistry that has been shown to be the best overall solution to many of these issues – chlorine dioxide (ClO2).
One thing to clearly note in the beginning is that chlorine dioxide is a relatively weak oxidant (compared to chlorine, bleach and bromine). As such, it does not react with hydrocarbons (oil and gas) or most other organics that may be present. This is why is has wide applicability in the oil and gas processing industry. Even if, as is frequently the case, the water being treated has organics present the chlorine dioxide will not react with them. This means it is safe to use. In addition, it will not form halogenated organic compounds with any organics present in the water – which is a marked advantage when compared to stronger oxidizing chemicals.
My goal is to be an independent source of information focusing on the application of chlorine dioxide chemistry. I do not sell chemicals or equipment. I help you make an informed decision on the best generation process, equipment and applications method for your unique needs. I also provide trial and startup support and training for your personnel to insure you obtain optimum performance and a safe operation.
HEAVY INDUSTRY MICROBIOLOGICAL CONTROL
By “heavy industry” we are referring here to the hydrocarbon processing industries – refining, petrochemical and ammonia. These industries use large amounts of process cooling water. There are almost always at least small process leaks into the cooling water systems from the large number of heat exchangers in the process. The hydrocarbons (and ammonia) provide an excellent food source for bacteria. In addition, the water is warm and saturated with oxygen – an ideal environment for bacterial growth.
It is not an exaggeration to say that at least 80% of all cooling water problems relate to poor microbiological control. Microbial growth results in multiple serious problems for these industries. The most important are:
- Loss of heat transfer in
- Process heat exchangers. A few microns of biofilm cause a major loss of heat transfer
- Cooling tower fill. Film, or high efficiency, cooling tower fill is an ideal environment for bacteria. Bacterial growth can proceed so far as to collapse cooling tower fill due to the weight of the microorganisms.
- Equipment corrosion from several sources
- Underdeposit corrosion
- Differential oxygen cell corrosion
- Inability of corrosion inhibitors to contact and passivate metal surfaces under the biofilm
In the past it was like this …
Industrial cooling water treatment evolved around the use of chromate-based corrosion control programs, and used this primarily until the late 1970’s, and into the mid-1980’s. This approach had several advantages from the performance perspective.
- Chromate is an excellent corrosion inhibitor and is soluble at high concentrations and across a wide pH range.
- Chromate is toxic to most organisms and itself aided in inhibiting microbiological growth in cooling water systems.
- Chromate treated cooling water systems provided excellent corrosion inhibition at a pH as low as 6.0, which allowed the use of chlorine gas at its most effective pH range.
- If microbiological growth did get out of control when a process leak occurred, various non-oxidizing biocides were available to supplement the treatment.
These programs worked well, and costs were economical. While chlorine dioxide was seen occasionally in the market, it was only viable where an unusually bad microbiological problem existed that couldn’t be controlled by these means.
Then a couple of things happened …
Elimination of Chromate
The toxicity of chromate resulted in new governmental regulations that mandated it be discontinued. The best alternative technology was the use of phosphates for corrosion control. Phosphates were not as good a corrosion inhibitor as chromate, so the pH of the cooling water systems was increased to a slightly alkaline environment (7.0 to 8.0) to lessen corrosivity. This resulted in decreased effectiveness of chlorine as a large portion of it converted to hypochlorite by the time pH reached the mid-7’s. Thus, higher dosages were required to maintain microbiological control.
Phosphates (typically in the form of orthophosphate) were decent corrosion inhibitors, but still couldn’t provide the performance of chromate treated systems. In addition, their solubility in the range of 7.5+ pH was very limited. This resulted in always walking a thin line between maintaining corrosion control and precipitating calcium phosphate and severely scaling the cooling water system.
Thus, further work was done to increase the performance of scale inhibitors to maintain the solubility of dissolved mineral species in the cooling water. By the late 1980’s water treatment companies were using “alkaline” cooling water programs. These operated at pH’s of 8.0 to 9.0+, with very low levels of phosphate or phosphonate. Corrosion was controlled primarily by operating in an alkaline environment.
Of course, this also meant that chlorine was largely ineffective at these high pH ranges. The industry then began using bromine as a biocide. This was either in the form of solid bromine tablets (BCDMH) or liquid sodium bromide reacted with bleach and fed in combination. Bromine has a disassociation curve shifted about 1 pH unit higher than chlorine, so offers some performance advantages over chlorine at these elevated pH’s. However, the performance of bromine as a biocide was always a poor second to chlorine, as well as being much, much more expensive. Bromine treatment costs were typically seen to be 10X to 20X that of chlorine, or even higher.
Elimination of Chlorine Gas
Concurrent with the elimination of chromate as a corrosion inhibitor, safety concerns were forcing many industries to eliminate the use of chlorine gas as a biocide, particularly in urban areas. Evaluation of the consequences of failure of a one-ton chlorine cylinder showed that a lethal concentration would cover quite a few square miles and could results in thousands of deaths. While a few industrial locations are still to be found using chlorine gas, they are almost universally in remote areas with low population density. Almost everyone else is using an alternative.
The first alternative was sodium hypochlorite (bleach). While it eliminates the possibility of mass deaths, it suffers the same performance problems as chlorine gas with the new phosphate and alkaline phosphate treatment programs – poor microbial kill at pH’s above about 7.5. In addition, the cost is significantly higher than using chlorine gas – 2X to 3X more.
As mentioned above, bromine is another alternative, but suffers from extremely high costs and mediocre (at best) performance.
And let’s not forget that both chlorine and bromine are strong oxidizers that will react with both hydrocarbons and ammonia. Thus, where process leaks are present, their demand will increase, resulting in higher dosage requirements and further increased cost. In cases of severe process leaks, it may not be possible to feed enough of either to obtain a residual at all. This is where you can lose total control of the cooling system microbial growth and literally shut down a process unit.
Both chlorine and bromine will react with organics and hydrocarbons to form chlorinated organics (THM’s or HAA’s). These are coming under increasing environmental regulation, putting further pressure on the use of these chemistries.
So…now there exists a situation where:
- Industry has been forced to transition to a less effective corrosion inhibitor
- Cooling system pH must be run higher to control corrosion
- An economical and reasonably effective microbial control chemistry (chlorine) no longer works due to the higher required system pH
- Chlorine gas itself has been eliminated due to safety issues, forcing the change to more expensive alternatives
- Concerns with DBP’s (THM’s and HAA’s) are putting pressure on the use of chlorine and bromine
So the solution is?
Chlorine dioxide is an ideal solution to the problems in these cooling systems.
- Performance is unaffected within the pH operating range of the system
- It is a weaker oxidant that will not react with process leaks, and whose performance is largely unaffected by these leaks.
- Has a lower demand than the stronger oxidants.
- Is less corrosive than other oxidants
- Maintains better microbial control while being fed intermittently (2 to 4 hours per day) rather than continuously. (see Recycle below)
- Provides lower system corrosion rates than any other chemistry, typically half or less, by:
- Eliminating biofilms and associated corrosion processes due to a recycle effect
- Providing a clean surface for corrosion inhibitors to contact and passivate.
- Intermittent instead of continuous feed
- Price competitive with all chlorine gas alternative and less expensive than many.
- No environmentally objectionable byproducts (THM’s, etc.)
- No supplemental nonoxidizing biocides are required.
Chlorine Dioxide “Recycle”
ClO2 is so effective on biofilm because of a “recycle” feature – when ClO2 reacts with the bacteria/biomass the majority reverts to chlorite ion. Acid from acid producing anaerobic bacteria species reacts with the chlorite and forms additional ClO2. Thus, a high level of ClO2 is regenerated inside the biomass and results in a rapid and complete kill and dissolution of the biomass.