Design and Analysis of Homogeneous and Heterogeneous Photochemical Reactors

Abstract

A short chapter to make a complete discussion of photoreactoreactor analysis and design is an impossible task unless we decide that the readers of this work have a previous background on the subject and chemical engineering fundamentals. Moreover, we can increase its feasibility if coverage is restricted to only a fraction, albeit significant, of some homogeneous and heterogeneous photoreactions. On these premises, it is possible to concentrate our effort in those aspects that are distinctive of homogeneous photochemical and heterogeneous photocatalytic processes. For the missing details, the reader is referred to the original publications. The distinct aspect of these reactions is the unavoidable existence of a radiation field inside the reactor, which only in very special and unusual cases can be considered uniform in space and frequently is not even constant in time. This is so because inside the reaction space, besides geometrical effects produced by the characteristics of the reactor geometry, there must be absorption of radiation to produce the reaction activation. This absorption means attenuation of the incoming intensities; i.e. without attenuation, there is no photochemical reaction. In some heterogeneous systems, scattering is another source of variation in the incoming rays. Hence, spatial variations are unavoidable. These intrinsic non-uniformities, unfortunately often neglected or not properly accounted for, are responsible for the majority of the difficulties associated with photoreactor analysis and design. Many different shapes and configurations are possible for either single-phase or multiphase reactors (Braun et al., 1993; Cassano et al., 1995; Puma and Yue, 1998; Ray, 1998; Cassano and Alfano, 2000; Alfano et al., 2000). Again, we will restrict ourselves to describe in more details only a few of them. A systematic approach to the design of a reactor should start by discussing the field of velocity distributions. Much progress has been achieved in this area and the hydrodynamic characterization of a great variety of reactors is already known. For the sake of brevity in our work we will concentrate on two types of systems: a perfectly mixed reaction space and a fully developed unidirectional flow in a tubular reactor. In practical terms this is not a serious limitation; computational fluid mechanics commercially available calculating codes can be used to solve almost any other form of reactor configuration.