Ozone decomposition reaction

In the schemes considered to this point, even the complex ones, the products form by a limited succession of steps. In these ordinary reaction sequences the overall process is completed when the products appear from the given quantity of reactants in accord with the stoichiometry of the net reaction. The only exception encountered to this point has been the ozone decomposition reaction presented in Chapter 5, which is a chain reaction. In this chapter we shall consider the special characteristics of elementary reactions that occur in a chain sequence. 

Note that since the atomic chlorine consumed in reaction 8.5 is regenerated in 8.7, it acts as a catalyst for ozone decomposition. Reaction 8.6 represents the decomposition of ozone by ultraviolet light, so that reactions 8.6 and 8.2 form a dynamic system that accounts for the absorption of ultraviolet light by the ozone layer. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

We have recently and critically evaluated the available high temperature experimental data for the ozone decomposition reaction (8). The expression used here and shown in Table I is consistent with all the direct experimental data known to us and is valid over a decade range in temperature. 

The rate of ozone decomposition reaction and instantaneous ozone demand also depend on the content of dissolved organic matter. An example of different treated waters with varying content of total organic carbon (TOC) is shown in Figure 6. 

Kagawa et al. [109] utilized the two-zone (core-annulus) model of Brereton et al. [96] to interpret their results of gas-solid contacting using the ozone decomposition reaction. A cross-flow coefficient of 0.001 m/s was used to fit their data, and r R was assumed equal to 0.85. 

Figure 4.5 Simple single-phase model predictions for first-order irreversible catalytic ozone decomposition reaction in comparison with experimental fluidized-bed reactor data of Sun and Grace [44],...

The particle size distribution (PSD) significantly impacts the reactant conversion in a fluidized bed reactor. Sun and Grace (1990) examined the three different particle size distributions, wide, narrow, and bimodal, on the performance of a catalytic fluidized bed reactor using the ozone decomposition reaction. They found that a fluidized bed with particles of wide size distribution yields the highest reactant conversion. Of further interest, the property of particle entrainment and elu-triation is a function of particle size, density, and shape. Both entrainment and elutriation rates increase... 

Van Swaaij and Zuiderwag (1972) used the ozone decomposition reaction to study the eonversion characteristics in a bubbling bed. Studies were made with beds of 5, 10, 23, 30, and 60 cm diameter, respectively and up to 300 cm bed heights. The results were com-... 

Frye CG, Lake WC, Eckstrom HC. Gas-sohd contacting with ozone decomposition reaction. AIChE J 4(4) 403, 1958. 

Relatively low gas-solids contact efficiency has also been obtained in CFB risers based on the ozone decomposition reaction (Jiang et al., 1991 Kagawa et ah, 1991 Sun and Grace, 1992 Ouyang et al., 1993). The low contact efficiency must be mainly attributed to the formation of particle clusters and the core-annulus flow structure in the riser. 

Table 9 lists reactions/processes featured in CFB reactor studies or in the development of reactor models, together with relevant references. The ozone decomposition reaction is of no commercial interest, but it is convenient for tests since it is essentially first order, irreversible, and able to proceed at room temperature, and since the concentration of ozone can be readily analyzed at low partial pressures. The other reactions are all of commercial interest. 

The gas-phase ozone decomposition reaction, 2O3 —> 3O2, can be catalyzed by chlorine at low temperature [13]. The experimental rate equation was reported to be ... 

The net reaction for this two-step mechanism is the conversion of an O3 molecule and an oxygen atom into two O2 molecules. In this mechanism, chlorine atoms catalyze ozone decomposition. They participate in the mechanism, but they do not appear in the overall stoichiometry. Although chlorine atoms are consumed in the first step, they are regenerated in the second. The cyclical nature of this process means that each chlorine atom can catalyze the destruction of many O3 molecules. It has been estimated that each chlorine atom produced by a CFC molecule in the upper stratosphere destroys about 100,000 molecules of ozone before it is removed by other reactions such as recombination CF2 Cl -b Cl CF2 CI2... 

The cataiysis of ozone decomposition by chiorine atoms occurs entirety in the gas phase. Chlorine is classified as a homogeneous catalyst because the cataiyst and the reactants are present in the same phase, in this case the gas phase. A heterogeneous catalyst, on the other hand, is in a different phase than the one where the reaction occurs. A heterogeneous cataiyst is usuaiiy a soiid, and the reactants are gases or are dissolved in a liquid solvent. 

The quantum yield of ozone decomposition at 334 nm (L2) is 4, indicating that one of the products must be an excited species capable of decomposing 0 further. The primary process of the 0 photolysis at 334 nm occurs according to the reactions ... 

Van Swaaij, W. P. M., andZuiderweg, F. J., Investigation of Ozone Decomposition in Fluidized Beds on the Basis of a Two-phase Model, Chemical Reaction Eng., Proc. 5th European/2ndInt. Symp. Chem. Reaction Eng., Elsevier, Amsterdam/London/New York (1972)... 

The enthalpy of ozone decomposition AH=Do2—o = 107kJ mol-1. The most probable reaction of initiation by ozone in solution is the abstraction reaction [133] ... 

Due to high activity in reactions with free radicals, ozone undergoes the chain decomposition in solutions also. The chain reaction of ozone decomposition was evidenced in 1973 in the kinetic study of cyclohexane and butanone-2 oxidation by a mixture of 02 and 03 [146-151], It was observed that the rate of ozone consumption obeys the equation [112] ... 

So, three different chain reactions of ozone decomposition were observed in solutions ... 

Regarding ozonization, it is only applied in a limited number of WWTPs after secondary treatment [61]. Several investigations have proven that it is a very effective technique to eliminate pharmaceutical [25, 62, 63]. Oxidation reactions take place due to direct reaction with ozone (03), which are very selective or with free OH radicals, which are generated by ozone decomposition and are very powerful and not selective oxidants. In advanced oxidation processes, 03 is completely transformed onto OH radicals and they are recommended when compounds are ozone resistant. 

Ozone undergoes decomposition when irradiated with a-particles352, 353 or with y-rays354. The decomposition may proceed through a number of distinct reaction sequences354, such as negative ion chain decomposition reactions in allglass vessels and involvement of impurities in vessels fitted with fluorocarbon-lubricated stopcocks. 

TOC and phenol concentration decreased same trend and the forming of by product was determined by mass spectrum. It was shown that formed by product was not stabile, whereas it immediately decomposes to different products. It was also determined that the amounts of ozone were sufficient for phenol decomposing. As a result of kinetic investigation of phenol, decomposition reaction was found first order because of obtaining almost constant k values (Fig. 24.2a-f). As expected... 

A mechanism has been proposed recently by O Neal and Blumstein for the gas-phase ozone-olefin reaction. This mechanism postulates that molozonide-biradical equilibrium is reached fast and postulates a competition between a-, 8-, and y-hydrogen abstraction reactions and the classical mechanism proposed by Criegee for the liquid-phase reaction. The main features of the Criegee mechanism (Figure 3-9) are the formation, from the initial molozonide, of the major carbonyl products and a second biradical intermediate, the zwitterion. The decomposition pathways of the zwitterion comprise unimolecular re-... 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

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