In the last few postings we discussed in general the subsystems of a ozone water treatment system. In this posting, we will focus in a little more detail about the factors that influence the dissolution of ozone gas into water.
There are a number of factors that create a driving force for ozone to enter solution. One of the most important factors is the concentration of ozone in the gas phase. As we mentioned in earlier postings, ozone concentration in the gas phase typically varies from 2-12% depending on the way it is produced. Henry’s Law states: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Essentially, the higher the concentration in the gas phase the higher the concentration can be in the liquid phase.
Another factor that will influence the transfer of ozone from the liquid to gas phase is the size of the gas bubbles. The smaller the bubbles, the easier the transfer from the gas phase into the liquid phase due to the ratio of surface area to volume. The bubble size can be affected by the mechanical method of mixing. A venturi can create small bubbles because of this mechanical action. Fine bubble diffusers that are placed at sufficient water depth can create small bubbles. As the depth of the water increases, the pressure of the water column compresses the gas in the bubble making it smaller.
The ratio of the gas volume to the liquid volume (G/L) also will influence the transfer of ozone from the gas to liquid phase. The smaller the ratio the higher the ozone transfer efficiency. In practice this ratio is typically in the range of 0.1 to 1.0.
The amount of time the bubbles are in contact with the water will also increase the transfer of ozone into the liquid phase. In practice this is affected by the positioning of the degas stage. As we discussed in one of the preceding posts, most of the gas fed to the water is either oxygen or nitrogen and will not dissolve into the water. The gas has thus to be removed. In some applications, this is done immediately after injection which means that the ozone has a short time to dissolve into the water. In other applications the dissolving is allowed to occur in a contact vessel where the ozone has substantially more time to dissolve. As a consequence the G/L ratio can be larger while achieving the same ozone transfer efficiency.
In subsequent postings we will further discuss factors associated with ozone transfer. If you have questions regarding this posting or about an application for ozone please contact Spartan Environmental Technologies.