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Ozone Solubility in water

Ozone Solubility in Liquid can be expressed as the saturation point of ozone in water.  Solubility of ozone will be the greatest limiting factor in getting ozone to dissolve into any liquid.  As water is the most common liquid ozone is dissolved into, for the purpose of this article we will use only water as the liquid we are calculating ozone saturation.

Ozone Solubility is dependent upon the temperature of water, concentration of ozone gas, and pressure of water.  This article will illustrate how these factors alter ozone solubility.

When ozone is dissolved in a liquid Henry's law is obeyed closely.  Therefore a saturation ratio must be determined (volume of gas dissolved / volume of liquid).  Based on the saturation ratio of ozone the solubility of ozone can be expressed as the following:

CL = CG x S x P

where:

• 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.

The ozone solubility ratio is the ratio of ozone gas volume / liquid volume that can dissolve into water.   These are derived from Henry's law constants based on water temperature.  This ratio is used to determine the potential solubility of ozone gas into water using Henry's Law equation listed above.  Below this calculation is used in a few examples for reference.

For an example of this calculation at work:

 CL = CG * S * P 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.

% by weight assumes ozone from oxygen.  14.3 g/m3 = 1%.

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.

Summary:

• 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.

Ozone Generators:

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.

Water Temperature:

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.

Water Pressure:

The data above shows that an increase in water pressure from 5 PSIG to 25 PSIG doubles ozone solubility.  In some applications, a small increase in water pressure may make a dramatic difference in ozone mass transfer into water.

It is also important to remember that as water pressure decrease ozone solubility decreases.  Therefore, when high levels of ozone are dissolved into water and that water is released to atmosphere with a spray wand, or the end of a hose, ozone off-gassing may occur.  This could be a safety concern that can be alleviated by adjusting other variables, or eliminating a sharp decrease in water pressure.

Bubble Diffusers:

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.

Vessel height will provide a 2 fold advantage.  Taller vessels create a higher water pressure at the bottom of the vessel where the diffuser is normally placed.  Higher water pressure equal higher ozone solubility.  Also, taller vessels will increase the time the bubble is in the water and increase the time available for ozone to transfer into the water.  With sufficient height and other necessary variables, bubble diffusers can achieve mass transfer of ozone into water that equals any other system.

Venturi Systems:

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).

Remember to call our staff with any questions you may have.  We are passionate about ozone and enjoy taking the time to ensure every ozone project is a success.

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