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%.

What Happened at the IOA Today?

On Monday, August 20 the International Ozone Association (IOA) hosted an ozone process workshop giving an overview of ozone and Advanced Oxidation Processes (AOPs).   Seven presenters representing various research organizations and water treatment operations around the country reviewed the basics of AOP fundamentals and considerations for safe and successful process operation and maintenance.  Four companies set up materials for practical, hands-on instruction in feed gas, ozone generator and destruct repair and maintenance, ozone mass transfer, and ozone safety and monitoring.  The discussion and problem solving generated by these presentations served everyone from the large treatment facility to specialized applications such as medicine, food production, or ground water remediation.  I am very excited about this growing industry and will share some of the ideas and information that I am learning as someone new to the industry.

Ozone Laundry Facts vs. Fallacy

Over the last twenty years I have witnessed Ozone laundry companies come and go.

I have witnessed outlandish claims from some of these companies, and quite frankly am tired of listening to the outright lies and fallacies that some of these companies are stating.

I would like to look at some of the facts and hope to help our readers make an educated decision about ozone laundry.

Click here for more info on ozone and luanrdy

This month, let’s look at the two major design differences.

Some ozone laundry companies have taken the path of the “fine bubble diffuser” to inject ozone into the wash water, while other have chosen the Direct Venturi Injection as their means of transferring the ozone gas into the water.  There are claims by manufacturers of advantages to both, so we should look at the design criteria of each and pick what is best for your application.

Ozone gas is dissolved into water by utilizing pressure.   It does not take a lot of pressure to start dissolving ozone into water.  In the industry we call the ability to dissolve gas into water “Mass Transfer.”  The efficiency of Mass Transfer of either style of injection device is highly dependant upon the design criteria of the device.

Ozone bubble diffuser

 The idea of the fine bubble diffusion injection method used on washing machines is borrowed from the bottled water industry.  In bottled water plants, ozone in introduced into a large column (or tank) of water.  These are usually in excess of 15 feet tall. Water flows in to the top of the tank and exits the tank through the bottom.  The fine bubble diffuser is placed in the bottom of the tank.  Ozone is introduced thru that diffuser in the bottom of the tank. Water is introduced in to the top of the tank and flows downward, where it exits out the bottom of the tank.  As the ozone bubbles rise, they are violently thrashed around due to the counter flow if the water flowing through the tank.  This turbulence lends for a pretty decent mass transfer of the ozone gas into the water.  The tanks used in the Las Vegas water treatment plant are 32 feet tall.  Remember that pressure is needed to transfer the ozone into the water solution.  Every inch of water column used adds more pressure to the diffuser stone, located in the bottom of the tank.  Therefore the taller the tank, the more pressure created at the bottom of the tank from the weight of the column of water, and the better the mass transfer of the ozone into the water.

ozone contact basin

 Once the un-dissolved ozone reached the top of the tank, it is removed thru an air vent and destroyed by passing thru an ozone destruct system.

All in all, this is a very safe and effective way to dissolve ozone in to water.

However, when the designers of the fine bubble diffusion systems implemented this type of system on a washer extractor,  they left out a couple key ingredients.

Clean mazzei ozone venturi in action
Ozone venturi in action

 First, the column of water in a washer/extractor is nowhere near that of a 32 foot tall water tank.  It is more like 12 inches.  This allows for a much lower Mass Transfer efficiency of the ozone gas.  Several ozone engineers that have studied this design have figured that the efficiency is somewhere in the 25-40% range at best.  That means in the best case scenario, 60% of the ozone gas does not get dissolved.  This is important in that 1% of un-dissolved ozone gas can easily make a room unsafe for the laundry workers.   In the bottled water design, you have the ability to capture this un-dissolved gas and destroy it.  The main problem here is that the designers somehow forgot to include this important piece of the puzzle.  Common sense tells you that if you force air(or Ozone) into an unsealed vessel, air (or Ozone) will escape out the vents of the machine on into the laundry room where it comes into contact with the laundry workers.  Some companies have tried to cover up this major design error by installing an ambient ozone room monitor that in theory shuts off the ozone generators when this level is reached.  In reality, the ozone generator is only on a few seconds before it is shut down due to over saturation of ambient ozone in the room.  Then many times it takes several minutes for the room sensor to reset and let the system turn on the ozone generator again.  I have witnessed the ozone generator being in operation for all of 45 seconds during a 7 minute wash step.  That’s not going to kill many “Super Bugs”.  I recently attended an ozone conference where the guest speaker said that a fine bubble diffusion system on a washer/extractor was “So easy to Build, that even a Caveman could do it”

It is not only that it is easy to build, it is also cheap to build this type of system.  That is why there are so many on the market, and the low price keeps the attention of the potential uninformed buyer.

The Venturi Injection System uses the pressure of the water stream in the piping to create suction.  When applied to this suction port, 85-90% of the ozone gas is instantly dissolved in to the water.  Since we still have this water and un-dissolved gas contained inside the water piping, it can easily be run thru an Ozone Degassing chamber that removes the un-dissolved ozone gas where it can be destroyed.   The end results are 100% dissolved ozonated water going to the washing machine.  This is an over-simplification of the process.  But it should get the point across without going into all the hydraulic engineering facts that make this a much more reliable and stable way to dissolves ozone into water. The disadvantage of the Venturi Injection System is that it costs more to build this type of system.

Another reason not to consider the fine bubble diffusion type of technology is the time it takes to build up to concentration.  Those of you operating commercial laundries know that you never want to fill up a washer with cold water then steam up the temperature to 160 degrees, because it takes too long.  I know there are some cases where this is unavoidable.  But normally you would fill your washer up with your standard hot water usually 130-140 degrees, then add steam injection to raise the temperature in those wash steps where you need a little higher temperature.  The same is true with the fine bubble diffuser.  A Venturi Injection system instantly fills the washer with highly concentrated ozonated water that instantly starts the cleaning and sanitizing process.  It can take as much as 20 minutes for a fine bubble diffusion system to reach the ozone levels where the Venturi Injection system starts.  None of us have that much extra time available in our wash cycle.

Ozone Laundry system
Ozone laundry system showing bubble diffusers and direct venturi injection.

Finally, some Venturi Injection Systems on the market allow for larger than normal fill valves and water lines to be utilized.  This will allow for much faster fill times.  I have witnessed a 60 lb washer/extractor being filled in under 25 seconds, whereas the standard fill time was in excess of 2 minutes.  On a standard hospitality wash cycle with five fills, this would shorten your wash cycle by more than 7 minutes of total operating time.  Washing an average of ten loads per day, this type of ozone laundry system would easily eliminate over one hour of processing per day.

In the graph below, you can see two identical tests that show the concentration of ozone throughout a complete wash cycle.  Notice in the Venturi Injection System, the levels start off high and slowly dissipate before the water is drained in each bath.  You should also notice the effect of PH on each bath in the wash cycle.  During the detergent and bleach steps, the ozone is reduced as it is counteracted by the higher PH from the wash chemistry.  As the detergent and bleach are rinsed out, you can see that the ozone hangs around much longer.

In the fine bubble diffuser, you will notice that the ozone levels never reach above the 0.2 PPM of ozone in the water.  It is also evident that the higher PH makes it hard for the ozone to overcome when using a Fine Bubble Diffuser.

Chart compares bubble diffuser vs venturi in ozone laundry

 

I am not stating that the fine bubble diffusion system does not work, or does not clean laundry, but rather, it is no where as effective as a properly designed Venturi Injection System.  Results will be more consistent with the Venturi Injection System than a comparable Fine Bubble Diffusion System.

 By: Mark E. Moore