Computational Fluid Dynamics (CFD) Modeling of Photochemical Reactors

Advanced oxidation processes (AOPs) play an important role in the degradation or the production of a wide range of organic materials. Many organic compounds such as pharmaceuticals, dyes, herbicides, and pesticides have been subjected to degradation and remediation purposes in water and wastewater treatment systems using AOPs. Some of the organic compounds such as drugs, vitamins, or fragrances could be also produced by oxidation processes. As the standard of living increases, many chemicals such as pharmaceuticals, pesticides, herbicides, and dyes are extensively consumed. Each of these products may cause health issues by their accumulation in aquatic environment. Pharmaceuticals such as antibiotics are partially metabolized and excreted by humans and animals. Improper disposal, dumping, and accidental discharge of drugs lead to the increase of the concentration of compounds such as analgesics, antibiotics, steroids, and hormones in aquatic environment, which cause environmental and health problems. Residual pesticides and herbicides originate from the direct pollutant in production plant, disposal of empty containers, equipment washing, and surface runoff. High levels of these compounds are toxic, mutagenic, carcinogenic, and tumorigenic. Some other wastes such as landfill leachate are subjected to advanced treatment methods. Old landfill leachates (>10 years) are nonbiodegradable in nature due to the existence of organic compounds with high molecular weights. Although the composition of landfill leachates varies widely with respect to the age of the landfill, type of wastes, and climate conditions, they can be categorized into four groups of dissolved organic matter, inorganic macro components, heavy materials, and xenobiotic organic substances. Another type of toxic chemicals which cannot be removed using conventional treatment methods is endocrine disrupting compounds (EDCs). EDCs, especially the steroidal hormones, are well recognized exogenous agents that interfere with the synthesis, action, and/or elimination of natural hormones in the body. Conventional processes are not effective in destruction of these types of organic compounds; therefore, powerful advanced treatment processes are required in order to mineralize them. There are several options for choosing an oxidation process: wet air oxidation, supercritical water oxidation, incineration, and AOPs. AOPs have been promising in the treatment of contaminated soils and waters. The AOPs could be employed to fully or partially oxidize organic pollutants usually using a combination of different oxidants. In contrary to the conventional physical and chemical treatment processes, AOPs do not transfer pollutants from one phase to another, i.e., organic www.intechopen.com Applied Computational Fluid Dynamics 156 pollutants are completely destroyed. Most AOPs are able to generate hydroxyl radicals. These hydroxyl radicals are active and powerful, capable of reacting with almost all types of organics, including non-biodegradable and recalcitrant compounds. These oxidizing hydroxyl radicals are initiated by photolysis, photocatalysis, sonolysis, radiolysis, and other AOPs alone or in combination in the presence of some reagents such as hydrogen peroxide (H 2 O 2 ), ozone, and homogeneous or heterogeneous catalysts. Hydroxyl radicals are extremely powerful, short-lived oxidizing agent, and non-selective in nature and could react with a wide range of organic chemicals with reaction constant of several orders of magnitude higher than the reaction with molecular ozone under the same conditions. AOPs are used to convert toxic, non-biodegradable, inhibitory, and recalcitrant pollutants into simpler and less harmful intermediates, so they could be treated subsequently in biological processes. Due to the high operating and capital costs of AOPs, the complete mineralization of organic compounds using AOPs is practically impossible; therefore they are mainly combined with other processes such as biological systems to be cost-effective. With sufficient contact time and proper operating conditions and design, the mineralization efficiency of the AOPs is maximized by optimization of the processes in such a way that the total costs of the processes are minimized while the removal rates of organic pollutants are maximized. Since conducting experiments is time-consuming and expensive or sometimes impossible to do, the modeling of the photoreactors is necessary. Computational fluid dynamics (CFD) is one of the numerical methods to solve and analyze the transport equations. CFD has been used recently in order to find the velocity, radiation intensity, pollutant concentration distribution inside the photoreacotrs (

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