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The effects of microwave irradiation of an alumina-supported 0.2 wt.% platinum catalyst are studied. In the case of the isomerization of hexanes, the selectivity in isomers is increased (by a factor 1.3 to 2) when the thermal treatment of the catalyst and/or the reaction are performed under the electromagnetic field. The isomer selectivity remains high even when such catalysts are subsequently tested under classical heating conditions. These permanent and profitable effects of the microwaves could be interpreted in terms of modifications of the geometric and/or electronic properties of the platinum particles. It is pointed out that the microwave irradiation could be a new and promising way for activating catalysts.

The authors give an expression of the distribution function of relaxation times associated with the universal dielectric response. The calculations are based on the derivative of the susceptibility with respect to the frequency, and leads to an integral expression which is easy to compute.

The spatial-density function which characterizes how metal particles are embedded in a dielectric host medium, and which is used in calculating the effective permittivity of the mixture according to the theory of Bergman, can be simply related to the relaxation-time distribution function which describes the frequency dependence of the permittivity of the mixture. The direct and inverse formulae, for computing one from the other are given explicitly. New basic functions can consequently be proposed to expand spatial-density functions.

The quasi-stationary state of microwave evaporation of a polar liquid is studied experimentally. When the energy supplied by the field to the interface is larger than the heat flux exchanged with the surroundings, the rate of evaporation is then proportional to the microwave power absorbed and can be calculated simply by dividing the power by the standard value of the latent heat of evaporation. When the field is low, the energy balance does not govern the process; the irreversibility must be taken into account.

Heat transfer between microwave heated granular material and a surrounding moving gas is both theoretically and experimentally studied. When the microwave field is homogenous, the temperature of the material can be well estimated from the gas temperature at the output and controlled through its variations. Chemical reactors heated by an inhomogeneous microwave field are also studied. Two elementary models are considered. The behavior of a long reactor, for which the travelling wave and the fluid flow have the same direction, may not be temperature stable.

Donnees sur la deshydratation d'un zeolite 13X dans un champ electromagnetique, qui entraine une desorption directe des molecules d'eau

The temperature dependence of density d, electric permittivity ϵs and nonlinear high field dielectric variation Δϵ are reported for various benzonitrile-alkane solutions. For the benzonitrile-isooctane mixtures which exhibit a phase separation in the considered temperature range, the phase diagram has been obtained. In the “homogeneous” one phase region, significant deviations from the normal temperature dependence of d, ϵ0, Δϵ have been observed and characterize a precritical region of several degrees C above the consolute temperature. These results support the hypothesis of strong precritical orientational interactions or local ordering phenomena. The precritical dielectric behaviour of the benzonitrile-isooctane system is shown to be very different from those of nitrobenzene-alkane or aniline-alkane mixtures.