J.C. André

Industrial photochemistry II: influence of the stirring of unstable radical species on the kinetics of long chain photochemical reactions

Abstract The experimental rate of a photochemically initiated long chain reaction seems to be a function of the absorption profile of the light, the mean lifetime of the free radical chain carriers and finally of all the processes of homogenization of the medium. The influence of these different factors is defined theoretically and illustrated with the aid of gas—liquid-type reactions, i.e. photooxidation of heptanal and benzaldehyde and photochlorination of benzyl chloride.

Industrial photochemistry I: measurement of the absorption profile of the light in a photochemical reactor

Abstract Knowledge of the local distribution of the light energy in a photochemical reactor in the presence of an absorbing substance, leading to a reaction of which the rate is of an order different from first order with respect to the absorbed light intensity, is important to calculate the mean rate of reaction and the design of the reactor. We report a simple method for the determination of this profile as a function of the concentration of an absorbing substance and we compare the results obtained with the models of emission which have been published. A semiempirical model accounting for our experimental results is presented.

Problems arising during the determination of the lifetime of electronically excited molecules by fluorescence inhibition techniques under continuous excitation

Abstract Under the conditions at which the measurement of the lifetime τ of emissive electronic excited states becomes critical, a good estimation of τ can be determined from the inhibition of fluorescence under continuous excitation, the kinetics of which are diffusion controlled. However, this procedure requires that the inner quenching mechanism, the mutual diffusion coefficient D and the collision distance σ be known. These constants are sometimes difficult to evaluate, and several assumptions are then required. This method was used to determine the excited singlet lifetimes of a series of substituted anthraquinones which were compared with the values obtained by laser flash photolysis and single-photon counting measurements.

Industrial photochemistry III: influence of the stirring of reactants on the kinetics and selectivity of consecutive long-chain photochemical reactions

Abstract Under industrial conditions it is chlorine which, after absorption of light, gives free atoms which generate chlorination chains. In the particular case of the photochemical chlorination of toluene where several successive chlorination reactions are involved we show that the presence of an intense exciting light flux can be responsible for a reduction in the selectivity of the reaction; indeed, under such conditions the chlorination reaction does not occur in a homogeneous manner in the liquid phase (kinetic regime) but rather in the vicinity of the chlorine bubbles entering the reactor (diffusion regime). Thus successive chlorination reactions occur in a region in which chlorine can diffuse; each bubble then constitutes a kind of plug flow reactor which is independent of the buffer medium. This phenomenon is responsible for a modification of the selectivity of the reaction.

Computer analysis of mixing problems arising in consecutive photochemical reactions

Abstract The importance of agitation processes, particularly molecular diffusion, in the kinetics of formation of products of consecutive photoreactions is demonstrated and some types of photoreactors adapted to the simplified study of such reactions are proposed.

Industrial photochemistry VII: Light distribution in a diffusing medium of titanium dioxide in water

Abstract The experimental determination of the total light distribution in an aqueous solution of titanium dioxide for an almost non-absorbing wavelength shows that the extinction coefficient exhibits a non-linear dependence on the concentration at high solute concentrations. The axial and radial light distributions, measured relative to an incident light beam of small cross section, are monoexponential and bi-exponential decay functions respectively. The proportion of scattered light relative to the total amount of light incident on an axial cross section is a function which increases exponentially with increasing distance. A computer-run Monte Carlo simulation of multiple diffusion shows the same behaviour.

Industrial Photochemistry IV: influence of additives on the selectivity of successive photochemical reactions

The selectivity of consecutive photochlorination reactions does not appear to be increased appreciably when the macroscopic factors which govern the working of a photochemical reactor are modified. In the case of toluene and its chlorinated derivatives, we show that in theory it is possible to use additives to orient the photochlorination steps selectively towards the formation of benzyl chloride. However, at present the formation of secondary products perturbing the kinetics limits the potential interest of the use of additives such as phenol or di-tert-butylhydroxytoluene.

Industrial photochemistry V: modelling and trial runs of a reactor with separate photochemical and thermal steps

Abstract Annular reactors adapted to successive toluene chlorination have been described in the preceding papers in this series. In this paper a theoretical working model of a reactor in which the purely photochemical stage (initiation) is separated from the subsequent thermal stage (propagation) is reported. The model is in satisfactory agreement with experimental data obtained using the photochlorination of toluene as a reaction model. The advantages and disadvantages of this type of reactor are described.

Initiation of aldehyde photo-oxidation in the liquid phase

Abstract The oxidation of aldehydes in liquid phase proceeds by a chain radical mechanism which can be initiated by a photochemical process. In this study we report the influence of different macroscopic factors on the photochemical initiation rate (concentration of aldehydes, pressure of oxygen, temperature). The initiation consists in a self-deactivation process of the excited states of the aldehyde, and oxygen only intervenes in a physical process favouring the intersystem transition between the singlet and triplet states of aldehydes. Different products have been studied, the mechanism is always the same, but there is a change in the relative importance of the several processes.

Industrial photochemistry VIII: Light repartition in heterogeneous cylindrical photoreactors

Abstract Several models of light repartition in homogeneous absorbing media have already been proposed in the literature. They are based on the general assumption that the light path is a straight line. However, for systems which both scatter and partially absorb light, this principle of optics is no longer realistic. As the scattering by a particle cannot be expressed by an analytical relationship, we have treated the modelling of light repartition in a scattering and absorbing medium using a Monte Carlo method. It is shown that the model is in satisfactory agreement with experimental. results.