Ozone Mass Transfer – What is so Important?

Having the proper ozone equipment is only part of the solution. Ozone is a gas and is generated in situ. Making sure that this ozone gas is transferred into a liquid and then mixed properly is very important. This process is known as ozone mass transfer.

Making sure that the ozone is properly dissolved and allowed a complete chemical reaction of the gas is how to ensure a reliable ozone mass transfer. Most failures of ozone applications are the results of faulty ozone mass transfers.

The Solubility of Ozone and Importance of Choosing Correct Mass Transfer Devices

Temperature has an effect on the different solubility of ozone. Ozone is able to dissolve more and easier in cooler water. Because of this, ozone doses for the for the same applications will differ from country to country. When a proper ozone mass transfer is conducted, it will enhance the solubility levels of ozone, thereby ensuring required actions of ozone. A list of simple mass transfer devices are:

1) Hydro injector/ venturi/ eductors

2) Diffusers

3) Static mixers

4) Mass multipliers

5) Combination of the above

Venturis: These devices use high water pressure to suck the ozone into the water. They achieve at least 90-95% efficiency because of their design. The pump required has to be sized properly, or there will be ozone leakages.

Diffusers: These are devices that bubble ozone across water in order to diffuse it into the water. They work under normal water pressure. The number of diffusers needed will be determined by the volume of air, and surface area of the contact tank. Diffusions requires a minimum bubbling height of 5-6 meters for maximum efficacy. Some problems that arise from diffusing is the channeling of bubbles, and inadequate gas liquid contact. Because the diffusers is always in contact with high concentration ozone, it is important that the diffuser is made of the right material. SS or ceramic diffusers are the most preferred materials. For waters with high turbidity, diffusers are not recommended. Ozone destructors are also needed for the destruction of any unused ozone.

Static Mixers: These are devices placed in a pipe that are always used in conjunction with venturis. If used after venturi injections, they increase the ozone mass transfer. The material of construction is also very important here as well. SS 316 are normally used.

Mass Multipliers: These are devices that are placed in pips that discharge water mixed with ozone under high pressure into the water. They are normally used for large applications and in conjunction with venturis. Mass Multipliers are normally made of Kynar plastics.

Combination of Devices: A combination of mass transfer devices are often used. Combinations work efficiently and will guarantee higher percentage of mass transfer. However, in larger applications, it can also increase the cost it takes for mass transfer. This is something that is very important to keep in mind when planning your method of mass transfer.

Other factors that determine the efficiency of the ozone mass transfer are:

1) pH of the water: needs to be between 7-8 pH.

2) Temperature of the water: the higher the temperature the less the ozone will dissolve.

3) The concentration at which the ozone is used. The higher the concentration, the more ozone will dissolve.

4) The pressure: ozone mass transfer under pressure is recommended and this can reduce ozone dose, since efficiency is very high.

5) The design of the contact tank: ensures that the diffused ozone is efficiently mixed in the water.

Other Forms of Ozone Mass Transfer: There are also some unconventional types of ozone mass transfer that can be used with equal efficacy. U Tube devices are one of these methods. U Tubes have water that is forced into the tube under pressure and the water mixes within the pipelines when the direction of the water if forcibly changed. Essentially, it is like using a diffuser system under pressure. A U Tube system that is designed properly can have an efficiency of over 95%.

Ozone Generator Performance Defined

Every ozone generator is rated using certain metrics.  These are illustrated in a performance chart that should be available with every ozone generator.  Here is some important information about ozone generator performance, and how interpret this information.

Example of ozone generator performance chart
Example of ozone generator performance chart

An ozone generator performance chart may show various data in relationship to ozone production or concentration. Below are some definitions of this data and what you might look for.

Ozone Production:

Ozone production is commonly shown in g/hr, but also may be shown in mg/hr, lb/day or another unit of measure. The ozone production value must be shown in a mass over time value that indicates mass of ozone produced in a given period of time. This value is a calculated value based on ozone concentration and feed-gas flow rate.

Online ozone output calculator HERE

Ozone Concentration:

Ozone concentration is a measurement that shows the ratio of ozone to feed-gas produced from the ozone generator. This is real-time measurement that is obtained via a high concentration UV Ozone Analyzer. Ozone concentration may be shown in % by weight, g/m3, or another unit of measure that illustrates a mass of ozone at a given moment.

Feed-gas:

The ozone generator feed-gas may be oxygen, or air. This information must be present to determine what gas was used, and what gas must be used to achieve the same results. Oxygen purity, or air dew-point values would be helpful information.

Flow-rate:

The flow-rate of gas must be given to show the required flow-rate to achieve the reported ozone concentration and calculated production. Ensure that measured flow is compensated for pressure as manual float type flowmeters readings need to be calculated for pressure. We have found some performance charts that were created using direct readings from manual flow meters without calculating for pressure, this gives a false low flow-rate, false high ozone concentration, and false high ozone production rate.

Feed-gas Pressure:

Feed-gas pressure would indicate the pressure of fed-gas on the ozone generator corona cell. Some ozone generators ozone production will rise or fall as gas pressure on the corona cell changes. Therefore you might see a performance chart show the relationship between ozone production and pressure.

Power Setting:

The power setting of the ozone generator could be shown to illustrate how ozone production varies with changes in the adjustable ozone output dial or remote signal. This could be shown in a % or specific indication of the dial.

Power Consumption:

Power consumption could be shown in relationship to ozone production to show the net result or change of power consumption based on the power setting value on the ozone production. This would illustrate energy consumed by the ozone generator for a specific ozone production rate. This information could be used to determine electrical efficiency of the ozone generator.

Additional information links:

White paper: Validation of Gas Flow Measurement During Ozone Generator Performance Testing

White Paper: Guideline for Measurement of Ozone Concentration in the Process Gas From an Ozone Generator