Fundementals of Ozone Solubility
CL = CG x S x P
- CL = dissolved concentration in liquid (mg/l)
- CG = gas concentration (g/m3)
- S = bunsen coefficient (solubility ratio), temperature dependent
- P = gas pressure (in atmospheres)
Ozone Solubility Ratio
The table below illustrates the solubility ratio of ozone, or S as shown in the calculation above.
For an example of this calculation at work:
|50 mg/l||=||100 g/m3||*||0.5||*||1|
|33.4 mg/l||=||85.8 g/m3||*||0.39||*||1|
This calculates for ozone solubility in water at atmospheric pressure (1 atm= 14.7 psia). Actual solubility is a theoretical maximum based on saturation of ozone into water. Many other factors will play a role in actual dissolved ozone levels in water. Organic loading, PH, ozone half life, etc will all lower actual dissolved ozone levels in water. These calculations are helpful in determining what is and is not possible with your current ozone equipment, along with understanding the factors that play a role on ozone dissolved into water.
Effect of Temperature and Ozone Concentration on Ozone Solubility
The table below shows ozone solubility in water based on water temperature and ozone concentration (shown in % by weight). These are the values most commonly used in the industry and give an easy reference tool.
This data is also illustrated in the chart below for easy visual reference.
This chart shows the dramatic difference temperature and ozone concentration plays on ozone solubility into water. This shows that a small change in water temperature may create a large difference in potential ozone dissolved into water. Also, changes in ozone concentration (dry air to oxygen) will change the ozone solubility dramatically.
While temperature is a variable we cannot control on many systems, ozone concentration is a variable we can control. Increasing the ozone concentration from 3 to 9% can overcome a dramatic temperature difference.
Effect of Pressure on Ozone Solubility
Table below shows the difference pressure plays on Ozone Solubility. Ozone concentration of 6% by weight, and water temp of 15-deg C. Calculated for differences in ozone solubility based on water pressure.
Water pressure is entered into the solubility calculation as P. Water pressure is calculated as atmospheric pressure. 0 psig = 1 atmosphere = 14.7 psia. As pressure on the water increases the solubility of ozone increases. The table above, and chart below is an example of this calculation based on ozone concentration of 6% by weight (85.8 g/m3) and 15-deg C water temperature. These are very average values for many ozone applications.
The Chart below shows the table data above illustrated in a chart.
It is evident that pressure makes a dramatic difference on ozone solubility in water. Higher water pressure = higher ozone solubuility = higher mass transfer of ozone into water. However, also be aware what differences water pressure may make on gas transfer for sheer effect, and more rapid decomposition of ozone gas.
- Higher ozone concentrations = higher dissolved ozone levels in water
- Lower water temperatures = higher dissolved ozone levels in water
- Higher water pressure = higher dissolved ozone levels in water
Apply this information to your application
Ozone is transferred into water in 2 main methods. Bubble dissufers and Venturi Injectors. Both methods goals are the same, dissolve ozone into water. With the information we reviewed we can put it to use to achieve the best mass transfer of ozone into water as possible. We will discuss each along with ozone generator differences.
At the heart of every ozone system is an ozone generator. It is evident from the information provided that ozone concentration plays a dramatic role in ozone solubility, and therefore, mass transfer of ozone into water. The type of ozone generator you choose will play a major role on mass transfer of ozone in water. Only corona discharge ozone generators will be discussed as UV ozone generators have no merit in dissolving ozone in water, and electrolytic ozone generators already did this job.
Ozone can be produced from dry air or oxygen. As air fed ozone generators are using only the oxygen in the ambient air (~20%) the resulting ozone concentration is much lower. Therefore, an oxygen fed ozone generator will ALWAYS have a better mass transfer of ozone into water than dry air. Most dry air ozone generators produce ozone at 1.5 - 3% by weight. While most oxygen fed ozone generators produce ozone at 4.5 - 10% by weight.
Also, oxygen is more soluble into water than air. Therefore the carrier of ozone itself will also dissolve into water more efficiently.
Ozone Generators are air cooled or water cooled to remove the heat produced from ozone production. Water cooled ozone generators will typically cool the cell more efficiently and produce ozone at higher concentrations. Also, cooling water (or ambient air temp) will affect ozone concentrations. Cooler temperatures will create higher concentrations of ozone, and higher ozone solubiliy!
Ozone output (g/hr) is not the only number to base your ozone generator choice on. If your goal is to create 10 mg/l of ozone in water but your ozone generator only produces ozone at 1% by weight, it can produce as much ozone in g/hr as you desire, but it will never achieve your goal. Ozone concentration is just as important, and in some applications, more important than overall ozone output in g/hr.
In most applications, we have no control over water temperature. This is simply a value we must be aware of, and plan accordingly. In higher temperature, applications use ozone generators with the highest concentration of ozone available to overcome this issue. Be careful when increasing water pressure as increased pressures can also increase temperature.
In small applications, it may be acceptable to place laboratory glassware in ice, or a cooler environment. Whenever possible use the coldest water temperatures possible. In process applications when choosing where to dissolve ozone into water, look for the process step where water temperature is the coolest.
Ozone may be bubbled into water, or another liquid in systems as small as a beacon in a lab, to a municipal water plant contact basin. Bubble diffusers can, and do provide sufficient mass transfer of ozone into liquid if implemented properly.
Smaller bubbles have greater surface area per volume and therefore increase mass transfer into water. Use the smallest pore diffuser available.
When using venturi systems a pressure differential across the venturi creates a vacuum that pulls ozone into water and mixes ozone with water efficiently. This creates opportunities to increase water pressures on the discharge of the venturi. Higher pressures on the ozone contact tank will create higher mass transfer of ozone into water (within reason).