F. Santarelli

Scattering effects in photosensitized reactions

Abstract Scattering phenomena in photosensitized reactions are considered for continuous processes. A Monte Carlo approach is used to solve the radiant energy balance equation and to get the distribution of the local rate of energy absorption as a function of the most significant parameters. Scattering is found to reduce the achievable conversion in all the situations except that of the practically relevant case where completely segregated flow, coupled to high optical thickness, occurs.

Photosensitized reactions in an annular continuous photoreactor

The process of energy emission by a lamp of finite spatial dimensions is considered, and a general expression for the local volumetric rate of energy absorption (L.V.R.E.A.) is given under the assumptions of no reflection and of negligible scattering and emission phenomena in the absorbing medium. When a reaction is occurring in the absorbing medium, L.V.R.E.A. can be easily computed if the reaction is a photosensitized one. In this case profiles of L.V.R.E.A., computed in an annular photoreactor solving the equations which govern rigorously the phenomenon, can differ in a relevant way from those computed assuming a radial model for the emission by the same lamp, the difference being mainly dependent, for a given radial geometry, on the reactor length and on the value of the optical thickness of the absorbing medium. Photosensitized reactions of order 0 and 1 with respect to the reactant concentration were studied for different values of the optical thickness, of Damkoheler and Peclet numbers. In particular the effects of variations of Pe number on the conversion were taken into consideration and the results were explained on the basis of physical and mathematical arguments.

A rigorous approach to photochemicals reactors

Abstract The modelling of photochemical reactors has been rigorously treated describing the radiation field within the realm of radiant energy transfer. This kind of approach causes a heavy integro-differential problem to arise any time the reacting species is the absorbing one while the differential nature of the model equations is maintained in the case of photosensitized reactions in a purely absorbing medium. An annular photoreactor has been considered for this last situation and the influence on the absorption process of the most parameters has been investigated for both the cases of a reactor shielded and unshielded by a reflector.

Radiative transfer in an absorbing and anisotropically scattering slab with a reflecting boundary

Abstract Radiative transfer has been considered within a participating plane slab assuming the externallyapplied radiation to impinge normally on one boundary of the slab while the other boundary is assumed to reflect in a diffuse way. The linearly anisotropic and the Rayleigh modes of scattering have been both considered. A rigorous solution is developed following a constructive procedure based on projectional methods: the resulting computational formulae have been numerically processed to obtain the distribution of the physically relevant variables for some significant situations.

Rigorous and simplified approach to the modelling of continuous photoreactors

Abstract Photochemical reactions where the absorption of radiant energy occurs by the reacting species are considered. In this case the radiant energy balance equation can not be decoupled from the mass balance of the reacting species and a highly non linear integro-differential problem arises. Mathematical difficulties have been usually bypassed by modelling the radiation field on the basis of simplified models of questionable physical reliability. A rigorous modelling of the radiation field has not yet been considered for such a class of photochemical processes. An approach based on a rigorous modelling is therefore presented here, the solution being obtained through a transient approach. Results are compared with those obtained on the basis of simplified models in order to investigate the range of the significant parameters where such simple models, which are less computer-time demanding, can be conveniently used.

Heat transfer in mixed laminar convection in a reacting fluid

Abstract The influences of natural convection on a reaction occurring in a fluid flowing laminarly in a vertical tube and on heat transfer parameters have been examined. A numerical solution of the dimensionless continuity, momentum, heat and reactant mass balance equations was carried out using a finite difference technique. The principal assumptions under which solution was obtained were: “in compressible natural convection”, a first order reaction and a constant wall temperature equal to the inlet temperature of the fluid. The results bring out the fact that natural convection may significantly affect conversion, the final effect depending essentially on the heat of reaction. In the case of the local and average Nusselt numbers, natural convection always causes appreciable variations. The values of bulk properties and Nu number, both local and average, the axial distribution of the velocity and radial profiles of temperature and reactant concentration are given for some typical situations.

Effects of scattering and reflection of radiation on batch photochemical reaction in a slab geometry

Abstract Effects of scattering of radiation within the medium, boundary surface reflectivity and initial optical thickness of the medium on the conversion occurring in a batch photochemical reaction in plane-slab geometry are investigated analytically. Results are presented for the local and average conversion within the medium, reflectivity and transmissivity of the medium, and normalized optical thickness of the medium as a function of time. The scattering is found to reduce the conversion and tends to increase the uniformity of the distribution of the reactant within the medium.

Effective design of photocatalytic reactors: influence of radiative transfer on their performance

Abstract The role of radiative transfer on a photocatalytic reaction occurring in an annular reactor has been investigated by modeling the degradation of chloroform contaminating the processed aqueous stream. From the analysis of the resulting effects it is apparent that the crucial factor in assessing the performance of the reactor is the distribution of radiant energy absorption rate which, in turn, depends on the mass of catalyst and on the way it is dispersed within the reactor. An optimization of the reactor is conceivable through a proper selection of the catalyst dispersion.

The role of a reflecting boundary in improving the operation of an annular photoreactor

Abstract The influence of a cylindrical reflector on-a photochemical reaction occurring in an annular photoreactor has been investigated for both batch and continuous operations of the reactor. Absorption of radiant energy has been assumed by the reacting species itself and for each of the operating modes both the extreme conditions of mixing have been considered. Results show that a reflecting wall can be effective in enhancing the conversion only when the optical thickness m (based on the initial conditions and on the inlet) is neither too small nor too large, since in these two cases absorption and reflection, respectively, are negligible anyway. In any case conversion has been found to increase when mixing within the reactor is improved.

Radiative transfer in an absorbing-scattering slab bounded by emitting and reflecting surfaces☆

Abstract An exact integral theory of the planar radiative transfer with isotropic scattering and general boundary conditions is presented in the paper. The analytical solution to the problem is numerically processed for two different specializations of the emissivity and reflectivity properties of the bounding surfaces. Results are given for the total and angular radiation intensities as well as for the net radiative flux.