Ozone Generator Feed gas preparation:
Ozone is produced from oxygen. This requires that some form of oxygen is fed into the ozone generator. Most all ozone generation for industrial and commercial uses is from corona discharge of some type. Due to this fact, the following information on Feed Gas Preparation will cover feed gas preparation for corona discharge ozone generators.
On-Site Oxygen Concentration:
- Pressure Swing Absorption (PSA)
- Vacuum swing adsorption (VSA)
Pressure swing adsorption (PSA)
- Desiccant-based air dryer
- High pressure
Liquid Oxygen (LOX)
- Cryogenic on-site storage
Compressed Oxygen Tank
- Small volume tanks, on-site storage
Dry air for Ozone Generation:
Air within the atmosphere is made up of ~21% Oxygen, 78% Nitrogen, and 1% Other inert gasses. The oxygen within the air we breathe every day can be used to generate ozone with great success. However, contamination within this air can produce impurities during the production of ozone and with the ozone gas. Therefore, any air fed into an ozone generator should be clean and filtered. This air must also be very DRY.
Most early ozone systems were air fed as oxygen separation devices were fairly expensive. Air-fed ozone generation systems are still popular when ozone is used for uses other than water treatment. When ozone mass transfer into water is not a primary concern air fed ozone generation systems are very cost-effective.
Advantages of Dry Air Ozone Generation:
- Low power consumption, no extra compressed air
- Can be lower capital costs – no oxygen concentrator
- Simple design, installation and operation
Disadvantages of Dry air Ozone Generation:
- Low concentration ozone (3% or less), causes poor mass transfer of ozone into water
- More undesirable by-product production
- More frequent maintenance intervals
Importance of Dry Air:
Moisture in air in the form of humidity will decrease ozone production efficiencies due to undesirable reactions and the loss of efficiency of the dielectric barrier to H2O in the air stream. The chart below reflects this efficiency decrease.
Moisture within the air carries hydrogen atoms in the form of H2O. The hydrogen atom in combination with nitrogen creates the potential to create nitric oxides that will eventually become nitric acid (HNO3). Nitric acid is the most common by-product found within poorly designed or maintained ozone systems. Over time, Nitric Acid will build up inside the ozone generator and the tubing directly after the ozone generator and cause reliability issues within the system.
When air is used for ozone generation that air should be as dry as possible. The level of moisture of air is measured by Dew-Point, in degrees. Dew-point levels in the air for ozone generation should be at least -40 deg C and ideally should be -70 deg C. For reference, -70 deg C contains 2.54 ppm H2O while -40 deg C contains 128 ppm H2O. Each H2O molecule contains two hydrogen atoms and has the potential to create two HNO3 (nitric acid) molecules. Ozone systems operated with dry air will always have some undesirable nitrogen by-products and will require cleaning for reliable long-term operation.
Dry air is always produced on-site ambient air that is dried with an on-site air dryer. Dry air can be produced from either heat-regenerative or pressure swing adsorption (PSA) air dryers. Both have the potential of providing sufficiently dry air for ozone generation and each has unique advantages and disadvantages.
Vacuum Driven, Heat Regenerative Air Dryer Systems:
Producing ozone from dry air can be simple by using a vacuum-driven,
heat-regenerative air dryer to provide dry air. A venturi injector can be used to provide the suction and vacuum to pull air through the air dryer and ozone generator. If a venturi injector will be used to mix ozone gas with water a vacuum-driven system will offer simplicity and potential energy savings. In addition, the ozone generator will be operated under a vacuum, eliminating the potential of ozone leaks into the environment.
Common applications for these systems would be swimming pools, spas, bottled water, or general clean-water disinfection.
In this system, a vacuum-driven heat regenerative air dryer provides moisture removal from the air, while a venturi injector provides the motive flow of gas into the water. This is an extremely simple system as the only moving parts are a solenoid valve inside the air dryer, and the water pump (which may be part of the existing water system). The potential inefficiency of ozone mass transfer is offset by the overall system simplicity and cost savings that can be used to purchase a larger ozone generator for this application. This is a great application of an air-fed ozone generator.
This system will be limited by water flow and the potential for the venturi injector to draw sufficient ozone gas flow into the water. This type of system will only be applicable when small ozone production rates and dosages in water are required.
Pressure Swing Adsorption (PSA) Air Dryer Systems:
For larger ozone applications using air-fed ozone generators a setup similar to the drawing on the right may be used. This uses a PSA (pressure swing absorption) air dryer. Using this technology the air can be dried to lower dew point than other methods, with more consistency. This also allows ozone to be generated under pressure allowing for greater versatility in downstream applications. Systems like this are used for anything from odor treatment applications to medium municipal water treatment systems. When using a venturi injector air and ozone gas can be pushed into the venturi, increasing potential ozone dosage rates into water.
A PSA air dryer will require a compressed air source and will purge some air from the air-dryer. Therefore energy consumption will be higher using this system than the vacuum-driven system. On the contrary, air quality will be improved. In high temperature and high humidity environments, this system has the ability to provide the same quality of air and dew-point as any other environmental conditions due to the higher pressures.
Oxygen for Ozone Generation:
Oxygen concentrations can be increased by removing nitrogen and other gasses from the air stream. Oxygen used industrially may range in purity from 90 – 99%. This oxygen may be produced using a variety of methods that are outlined below.
Oxygen feed gas for ozone generation allows greater production of ozone with the same ozone generator. Typical ozone generators will, at a minimum, double the output of ozone when operated on 90% oxygen feed gas versus dry air feed. Typical oxygen-fed ozone generators will achieve ozone concentrations of 5 – 10% for commercial and industrial applications and can achieve ozone concentrations greater than 20% by weight.
Factors to consider when looking to use oxygen:
Ozone generation from air is simple, effective, and low-cost. However, the move to oxygen many be the right move for your application. In fact, most ozone generators use oxygen as a feed gas in industrial applications. Before making the switch to concentrated oxygen make sure this is the right move. Consider some of these factors
How much more ozone will your ozone generator produce using oxygen? While most ozone generators more than double the output of an ozone generator versus dry air, some ozone generators will see an even greater improvement in efficiency.
What oxygen supply will be used? The concentrated oxygen must be generated. There are various methods listed below. Ensure that the downside of the extra equipment necessary does not outweigh the benefits of increased ozone production.
What will the added costs be? The oxygen equipment and potential extra energy costs may increase the capitol cost, or operating cost of your ozone system. Conversely, using oxygen for ozone generation may allow for a much small (less expensive) ozone generator that consumes much less energy.
In an oxygen stream with 90% purity the nitrogen content has been reduced from 78% to about 8%. There are still inert gasses and potentially a trace amount of H2O in this gas-stream. However, with much less nitrogen, undesirable nitrogen by-products and most importantly, HNO3 will be dramatically reduced. This will lower overall maintenance costs of the ozone generator and potential down-stream piping.
Advantages of oxygen fed ozone generation:
- Higher concentration ozone generation
- Increased solubility of ozone into water
- Decreased ozone generator maintenance
- Less undesirable by-product formation inside the ozone generator
Disadvantages of oxygen fed ozone generation:
- Higher compressed air energy costs
- Can be higher capital cost
- Higher complexity of overall system vs dry-air
Ozone can be produced on-site with an oxygen concentrator, or be provided by an off-site source that delivers oxygen gas via LOX or compressed gas. Examples below illustrate options and provide examples.
Bottled Compressed Oxygen:
Bottled compressed oxygen gas is a common method of providing concentrated oxygen in industrial applications. Using bottled oxygen for ozone generation may be the perfect solution for lab applications, or short term pilot tests. However, long term use of bottled oxygen will get very expensive and labor intensive with oxygen bottle changes required. For example, and H-series oxygen tank holds 7,080 liters of oxygen when pressurized to 2,200 PSI. At an oxygen flow-rate of 10 LPM this will supply 11.8 hours of oxygen flow.
Liquid Oxygen (LOX):
LOX is oxygen stored in a tank in liquid form. This oxygen must then be converted to a gaseous state by a Dewar to return the oxygen to a usable state. When oxygen is stored in this form 600 times more oxygen can be stored in the same size tank. Depending upon the availability of LOX this may be a great source of oxygen for long-term applications, or LOX could be used as a backup for other on-site oxygen generation.
Most large industrial applications using LOX add a small amount of dry air to supply a 1% nitrogen moisture into the feed gas. It has been found that on most ozone systems this 1% nitrogen addition increases the efficiency of the ozone generation process while reducing the necessary maintenance on the ozone generator dielectrics.
Many ozone systems generating large amounts of ozone for drinking water at municipalities use LOX due to the reliability and relatively low costs. Depending upon availability discounted rates for LOX can be negotiated with suppliers making LOX a very attractive option for oxygen supply.
On-Site Oxygen Concentrators:
An oxygen concentrator is a mechanical means of removing nitrogen from compressed air and capturing a concentrated form of oxygen after the nitrogen has been purged from the gas stream. This method uses a zeolite material that absorbs nitrogen under pressure and releases that nitrogen when un-pressurized, allowing the oxygen to be captured by the system of varying pressures. PSA oxygen concentrators require a compressed air supply to provide the necessary PSA action for oxygen concentration. This compressor should include a method of removing bulk moisture from the air along with buffer tanks for the compressed air and oxygen. These components will carry a capital cost and consume a sizable footprint.
This method is practical and cost-effective for small ozone systems and remains practical for very large ozone systems. The air compressors used, and styles of oxygen concentrator vary greatly as these systems scale, however on-site oxygen generation with a PSA oxygen concentrator is typically the most cost-effective method of providing oxygen for an ozone system
Note: Vacuum Swing Absorption (VSA) is also an option for on-site oxygen production. Similar zeolite material is used to absorb nitrogen along with air compressors to create a vacuum rather than pressure. Overall, the process has similar advantages and disadvantages as PSA.
Ozone in aqueous applications:
Ozone is commonly dissolved into water in a wide variety of applications. When the purpose of generating ozone is to dissolve it into water oxygen-fed systems have a few great advantages. First, if you recall in the Ozone Properties section the solubility of ozone is greater when ozone is generated at a higher concentration. Since ozone generated from oxygen will typically be much higher concentration than the air fed counterparts the mass transfer efficiency of the ozone system will be much higher, resulting in more ozone in the water.
Another advantage of oxygen-fed ozone system is the oxygen itself. Oxygen is more soluble into water than air. This means the entire volume of oxygen and ozone will be more soluble in the water, again, increasing the mass transfer of ozone into water. These factors, along with the lack of impurities gives oxygen-fed systems a clear advantage when the purpose of generating ozone is to dissolve that ozone into water.
If the purpose of ozone generation is to dissolve ozone into water, carefully review all options, information above, and ozone solubility into water before using an air-fed ozone generator. While air-fed ozone generators can be used with good success in aqueous ozone application, there are limitations that need consideration before implementation.