Mass transfer of ozone in purified water

This work presents the study of ozone mass transfer in water in bubble column with 0.5cm inside diameter and 2 m height. The volumetric mass transfer, rate constant of ozone decomposition and mass transfer efficiency were studied with different gas flow rates (50,100, 150, 200, 250) L/min. The volumetric mass transfer as well as rate of decomposition of ozone was found to increase with increasing gas flow rate. They were ranged between 0.21-0.41min -1 and 0.018-0.036min -1 respectively. Mass transfer efficiency was increased from 77 to 98% with increasing gas flow rate. A mathematical model was numerically solved to describe the liquid ozone concentration with time. The model and experimental results agreed well.


Introduction
Ozone is one of the strongest oxidants technically applied. It utilized in a variety of chemical processes, as well as in the treatment of both drinking and wastewater.
Aims of ozonation in water treatment include the removal of color and smell and the disinfecting of biologically polluted water in order to destroy or inactivate microorganisms and event viruses n ( Janknecht, et.al, 2001).
In addition, ozone can be effective degradation of organic compounds. Thus, ozonation can be used to increase the biodegradability of biologically refractory compounds. Even small doses of ozone can be sufficient to change the structure of the compounds and make them more amenable to biological breakdown (Chang and Chian, 1981). The kinetic of reaction with organic compounds depends on the nature of them (Hoigne and Bader, 1983), (Beltran, et.al, 2001), and(Von Gunten, 2003).
When ozone is to be applied to water it needs to be transferred from the gas phase mixture into the liquid phase. The necessary condition for that is the surface between the two  (Hsu,etal., 2002) , (Murayama, etal., 2005).
In this work a bubble column ozone contactor was used, which is very popular for ozone application and more traditional contactor. An orifice diffuser was used to introduce the gas at the bottom of the column. The size of the bubbles depends on the size of the hole used and type of liquid.

Theory
The transfer of ozone to water without chemical reaction is currently accepted as occurring according to the double-film model (Sotelo, et al., 1989), in which the driving force is the difference of ozone concentration between liquid and gas phase. The general form of mass balance is given as: In organic free-purified water, ozone self decomposition reaction is the only reaction could be undergoing since no organic compounds exist for ozonation reaction. It's assumed that, for very clean water, the ozone decomposition can be approximated as first order process (Zhou, 1995;Roustan, 1996). The rate law is formulated as: The volumetric mass transfer constant K La is a key parameter for the description of gas/liquid contactor. It's a function of the physicochemical properties of the liquid, the bubble and the hydrodynamic conditions (Wu et al, 2000). At steady state, after the ozonation started, the dissolved ozone concentration constant with time. According to this equation (1) and (2) will be: and it can be calculated from Henrys law ( 6) in which: Once steady state achieved, the ozone production stopped then the gas introduced contains no ozone i.e. C * =0, so equation (1) and (2) will be: Plotting equation (6), the term -K D (C ss * / C ss * -C ss ) is the slope of [lnC=f(t)], so the values of K La and K D can be calculated. In order to compare the values of K La and K D , all of these values have been corrected to 20 ○ C, because some of the physical properties depend on temperature, such as viscosity, density, surface tension. K La and K D at 20 ○ C are (Ting, et al., 2001): The schematic diagram of the process is shown in figure (1), it consists of; a glass bubble column with 5 cm inside diameter and 2m height. Ozone generator device made by Fisher Company generating about 10 mg/L of ozone as maximum using pure oxygen.

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The flow rate of oxygen controlled and measured by manual control valve and calibrated flow meter. The electrical consumed by the device also had been watched as an indication for the discrepancy of any fluctuation in amount of ozone generated in the device may occur.
First the column filled with 1.5 liter of water (its quality given in table (1)) then the generated gas dispersed at the bottom of the column through an orifice plate with fine holes to insure forming small bubbles.
Different gas flow rates of 50, 100, 150, 200,250L/min were used in this work with 10 mg/L of ozone concentration.
To estimate the amount of ozone concentration in the inlet and outlet gas stream, the gas stream first lead to two traps connecting in series. Each trap contained Potassium Iodide (KI) with few drops of acetic acid to react with ozone. The solution in two traps then collected and titrated with Sodium Thiosulphite Na2S2O3 using starch as an indicator. A sample of water with drew each 5 minute to determine ozone concentration.

Results and Discussion Volumetric Mass Transfer
The effect of gas flow rate on volumetric mass transfer is shown in table (2). The values of KLa were found to be in the range of 0.2141min-1 to 0.41min-1 at 30○C and from 0.16 to 0.3min-1 at 20○C. These values are lower than those calculated by Chedeville,et.al (2007) ranged between 0.7-1.2min-1, while they are close to those obtained by Ting, et.al (2001). It could be seen from table (2) that the volumetric mass transfer increases with increasing gas flow rate due to the increase in the velocity of bubble and number of bubbles that causes greater specific surface area. This phenomenon is similar to other workers (Zhou andSmith, 2000 andTing, et al., 2001). While this effect is different on mass transfer coefficient since the last is mainly controlled by the diffusion of ozone in liquid film. The liquid film is not accelerated by the velocity of gas bubble, therefore the rate of diffusion will not be effected by the increase in the gas velocity. It was assumed that the relationship between volumetric mass transfer and superficial gas velocity (Ug) takes the form of a power law as follows: The relationship is determined from a linear regression of the logarithms of KLa and Ug as below with correlation coefficient equals to 0.94: KLa=0.0404Ug0.414 … (10) The rate of ozone decomposition shows similar behavior i.e. these values increase with increasing gas flow rate. They rise from 0.018 to 0.036min-1 at 30○C and from 0.0081 to 0.016 min-1 at 20○C. These values are lower than those obtained by Ting, et.al (2001) and Chedeville,et.al (2007). This can be explained by the mix use; purified water used in this study which was more basic in pH=7 than process water used by these workers. Moreover, the process water contains more competing lead to ozone decomposition.

Mass Transfer Efficiency
Mass transfer efficiency can be calculated from the amount of ozone in feed gas and in the off gas as follows: The effect of detention time on mass transfer efficiency is shown in figure  (2) . This figure shows that the PDF created with pdfFactory Pro trial version www.pdffactory.com 779 transfer efficiency increases with increasing detention time for all gas flow rates and 90% transfer efficiency achieved with detention time only 5 minutes and this value increases to 98% with 20 minutes. The effect of gas flow rate on overall mass transfer efficiency can be shown in figure (3)

Conclusions
Mass transfer rates were measured in purified water at different gas velocities. The mass transfer rates were roughly proportional gas velocity. It was found that the volumetric mass transfer as well as rate of ozone decomposition increase with increasing gas flow rate from 50 mg/L to 250 mg/L. Also, mass transfer efficiency increases with gas flow rate.