Lenntech Water treatment & purification Lenntech Water treatment & purification

Ozone generation

Henry coefficient Bunsen coefficient Solubility ratio coefficient Influencing the solubility

Mass transfer Ozone injection techniques

To disinfect (waste)water with ozone, ozone must be dissolved in water. Ozone gas is produced on-site by an ozone generator. It can be dissolved in water in various ways.
To bring about a proper disinfection and oxidation, the ozone concentration must be as high as possible. The prediction of ozone solubility is more complicated than for other gasses, because ozone solubility is influenced by several factors, such as temperature, pH and dissolved matter. This is a consequence of the instability of ozone in water. The solubility of a gas in water is usually defined by Henry's law. For the solubility of ozone the Bunsen (β) and particularly the solubility ratio (S) factor is used.

Henry coefficient

Henry's law states that the solubility of a gas in a liquid is proportional to the pressure of the gas over the liquid. Principally, Henry's law can only be applied on gasses that do not chemically change in water, during transfer.

Y = HX

Y = pressure of substance over fluid [atm]
X = molar fraction ozone (gaseous) [-]
H = Henry [atm/l/mol]

An ozone generator produces ozone in a mixed form, so the term partial pressure is used. The partial pressure of a gas in a mixture (here: ozone) is its contribute to the total pressure of the mixture. Figure 1 describes the solubility of ozone at different pressures. In practice this will be less, because above a certain feed gas concentration stabilisation will occur [10]. Nowadays, much higher ozone concentrations are used to dissolve ozone in water. Lenntech delivers generators that produce ozone concentrations of 240 g/Nm3.

Figure 1: effect of pressure and feed gas concentration on ozone solubility

Henry coefficient (Hc)

Y = Concentration of gas above the liquid in equilibrium with the gas dissolved in the liquid, mg/l
X = Concentration of gas in the liquid in equilibrium with the gas above the liquid, mg/l
Hc = (mg gas/liter of gas)/ (mg gas/liter of liquid)

This form of Henry is more practical because the terms are expressed in mg/l.

Bunsen coefficient (β)

The solubility of ozone can also be calculated with the Bunsen adsorption coefficient [5]. The formula that can be used to calculate the solubility is:

Cs = β * M * P

Cs = Concentration dissolved gas (kg/m3)
Β = Bunsen adsorption coefficient (-)
M = Density of the gas (kg/m3)
P = Partial pressure (Pa)

The Bunsen adsorption coefficient is expressed as a volume of gas at NTP (normal pressure and temperature), which is dissolved at equilibrium by a unit volume of liquid at a given temperature, when the partial pressure of the gas is the unit atmosphere. The Bunsen coefficient has no dimension.

β = Vg / Vl

Solubility ratio coefficient

In general the mentioned formulas are not very practical, although they are used alternately (with the exception of the derived Hc coefficient). Another more practical method to calculate the solubility is by using the solubility ratio coefficient. This solubility ratio is given as mg/l per liter in water to mg/l in gas. This factor is dimensionless and relates to the Henry coefficient [6]. The relation between these constants is defined as follows:



H = Henry (atm/l/mol)
pO3 = partitial pressure ozone (Pa)
Cg = concentration zozone gas (kg/m3)
S = solubility factor (-)
Hc = Henry's constant (-)

Figure 2 illustrates the solubility ratio (S) at increasing temperature, carried out by different researchers. The solubility is studied at different ion concentrations (μ). This figure illustrates that the solubility of ozone is influenced by several factors in the water.

Figure 2: ozone solubility (S) as a function of the temperature (T = 5-35 ˚C)

The following formula can be used to calculate of the solubility ratio (S) at different temperatures:

log10s = -0,25 –0,013T [˚C]

This formula is more of a guideline because the real ozone concentration is influenced by a number of factors in the water. On the other hand, that the temperature is one of the most important factors influencing the solubility.

An computation of this coefficient at 20 ˚C:

log10s = -0,25 –0,013T
log10s = -0,25 –0,013 * 20
log10s = -0,51
s = 10-0,51
s = 0,31

At 20 ˚C the solubility ratio coefficient s = 0,31 mg/l water per mg/l carrying gas (see also figure 2). For example at 20 mg/l ozone in air, 20 * 0,31 = 6,2 mg/l will dissolve in water at 20˚C.

Influencing the solubility

The degree of solubility of ozone gas is dependant on the concentration in gas and thus dependant on the partial pressure. Another important factor influencing the solubility is the temperature. Besides temperature, pH and ion concentration in the solution are the main factors influencing the solubility. Summarized, the solubility can be increased by:

- Increasing the ozone concentration in the air (oxygen);
- Increasing air pressure (oxygen);
- Decreasing the water temperature;
- Decreasing the amount of solutes;
- Decreasing the pH;
- Excess of UV-light.

Mass-transfer

When matter is transferred from one phase to another across a gas-liquid interface, a concentration gradient will occur in each phase because of a resistance. This transfer into another phase is called mass-transfer and is represented by figure 3 (double-layer model). During the transfer of ozone from gas to liquid, the following stages are processed: diffusion of ozone across the gas/liquid phase, dissolving into the liquid, diffusion into the liquid.

Figure 3: model for ozone transfer

The transfer rate is dependant on [10]:

- The physical properties of gas and liquid
- The difference in concentration across the surface
- Turbulence

Ozone injection techniques

Ozone can be injected in water in several ways. The most commonly used techniques are diffuser and venturi [33,52]. A diffuser often exists of a disc or bar. A diffuser works under pressure and creates a bubble column. Advantages of a diffuser system are high yield, simple construction and advantageous for high water volumes (i.e. drinking water systems). Disadvantages are that it is not a very compact system and the efficiency depends on contact column dept and bubble size. Moreover, a diffuser can be stuffed-up, causing the efficiency to decrease. Here, you can see an application of a diffuser system (counter- and equal-flow contact column):

Figure 4: counter- and equal-flow contact column

In case of a venturi system, the ozone gas is dissolved in water by pressure. Pressure creates a constriction in the fluid flow, causing sucktion of ozone (figure 5). A venturi system has various benefits, namely: compact installation, no moving particles, high yield.

Figure 5: venturi system

About Lenntech

Lenntech BV
Distributieweg 3
2645 EG Delfgauw

tel: +31 152 610 900
fax: +31 152 616 289
e-mail: info@lenntech.com


Copyright © 1998-2018 Lenntech B.V. All rights reserved