Modeling of ozonation processes

Kinetic models can be used in the design of treatment plants. With their help it is possible to predict the influence of important parameters on the oxidation process. Knowing the kinetic parameters quantitively allows the size of the reactor system to be calculated. Models are also important research tools, which help us to understand the system being investigated. 

Models in general are a mathematical representation of a conceptual picture. Rate equations and mass balances for the oxidants and their reactants are the basic tools for the mathematical description. As Levenspiel (1972, p.359) pointed out the requirement for a good engineering model is that it be the closest representation of reality which can be treated without too many mathematical complexities. It is of little use to select a model which closely mirrors reality but is so complicated that we cannot do anything with it. In cases where the complete theoretical description of the system is not desirable or achievable, experiments are used to calculate coefficients to adjust the theory to the observations this procedure is called semi-empirical modeling. 

Every ozonation process where gaseous ozone is transferred into the liquid phase and where it subsequently reacts, involves physical and chemical processes which need to be considered in modeling. Physical processes include mass transfer and hydrodynamic properties of the reaction system, e. g. gas- and liquid-phase mixing. Chemical processes include, ideally, all direct and/or indirect reactions of ozone with water constituents. Of course these processes cannot be seen independently. For example, fast reactions can enhance mass transfer. 

Lab-scale systems used for experiments are usually completely mixed, especially with respect to the liquid phase. This has been assumed for the following discussion, It simplifies the description of the system hydrodynamics in the models immensely, but often leads to neglect of the system hydrodynamics in the scale-up of lab-scale results for pilot or full- 

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Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

Many wastewater flows in industry can not be treated by standard aerobic or anaerobic treatment methods due to the presence of relatively low concentration of toxic pollutants. Ozone can be used as a pretreatment step for the selective oxidation of these toxic pollutants. Due to the high costs of ozone it is important to minimise the loss of ozone due to reaction of ozone with non-toxic easily biodegradable compounds, ozone decay and discharge of ozone with the effluent from the ozone reactor. By means of a mathematical model, set up for a plug flow reactor and a continuos flow stirred tank reactor, it is possible to calculate more quantitatively the efficiency of the ozone use, independent of reaction kinetics, mass transfer rates of ozone and reactor type. The model predicts that the oxidation process is most efficiently realised by application of a plug flow reactor instead of a continuous flow stirred tank reactor. 

This comparison is only theoretical. In reality a high production of OH° can lead to a low reaction rate because the radicals recombine and are not useful for the oxidation process. Also not considerd are the effects of different inorganic and/or organic compounds in the water. Various models to calculate the actual OH-radical concentration can be found in the literature, some are described in Chapter B 5, Further information concerning the parameters which influence the concentration of hydroxyl radicals is given in Section B 4.4, as well as a short overview about the application of ozone in AOPs in Section B 6.2. 

The two following studies are noteworthy as successful examples for the modeling of waste water ozonation, where a close match between the measured and the calculated concentration profiles was achieved. In each case only one organic model compound was initially present. In both studies it was found that the kta value of the completely-mixed semi-batch reactors was very dependent on the concentration of the original compound, thus exerting considerable influence on the oxidation process. In order to assess and model the changing kLa, two different approaches were made. 

Due to the complexity of most waste waters and unknown oxidation products, differences in lumped parameters such as COD or preferably DOC are used to quantify treatment success. A model to describe the oxidation process, including physical and chemical processes, based on a lumped parameter has been tried (Beltran et al., 1995). COD was used as a global parameter for all reactions of ozone with organic compounds in the chemical model. The physical model included the Henry s law constant, the kLa, mass transfer enhancement (i. e. the determination of the kinetic regime of ozone absorption) as well as the... 

Model calculations performed during PAUR I included three-dimensional regional Chemistry-Transport Models to study chemical processes and transport at the regional and the urban level and radiation transfer models. These models were applied for a number of ozone depletion scenaria. 

Hauchecorne,A,Marchand,M.,Godin,S.and Souprayen,C. (1998) A High resolution advection model for the interpretation of ozone filaments observed in lower stratospheric ozone lidar profiles at mid-latitudes,Proceeding of the Europe Workshop on Mesoscale Processes in the stratosphere,Bad Tolz,Germany,in Press... 

At pH less than 3, the reaction will occur slowly enough for kinetic models to be true. Thus, a pH 2 or 7 can be utilized for the slow kinetics of atrazine oxidation by UV /ozone processes, while rapid reactions will take place at pH 12. All of the three reaction mechanisms will be affected by other variables such as temperature, pH, and bicarbonate ion concentrations. 

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