Apparatus for Subjecting a Material to Electromagnetic Waves

The present invention relates to apparatus for subjecting a material to electromagnetic waves. Such apparatus, hereinafter referred to as an "applicator", is typically used to treat a confined material, one at least of whose constituents is a dielectric in liquid, solid or even gaseous phase, with electromagnetic waves having a frequency above 1 MHz and preferably above 400 MHz, with a view to supplying it with energy.

Applicators of this type are used in the most varied fields, for example, the treatment of food products, the heating of insulating materials, plastics materials, concrete, rubber and ores, the effecting of polymerizations, desorptions, drying, separations or other physical and chemical actions or reactions which involve one of the constituents of the material or take place between the constituents of the latter.

Applicators using an antenna to radiate electromagnetic energy are known, for example, in high frequency electric ovens. In such an applicator, the antenna is fixed to one of the two extreme right sections of the applicator and radiates towards the corresponding right section of the material to be treated. Such an apparatus is satisfactory in the field of application for which it has been conceived, namely an oven concerned with heating a confined material, within the applicator, in a region whose transverse and longitudinal dimensions are appreciably of the same order.

On the other hand, it presents grave drawbacks in the case of elaborate industrial applicators, where one is concerned with inducing the aforementioned reactions by carefully proportioned contribution of electromagnetic energy, such as applicators used in the chemical industry. In this case, the dimensions of the applicator are imposed by the conditions of use, that of chemical engineering in the example mentioned. These conditions often involve an important relation between the longitudinal and transverse dimensions of the applicator: its section being most often circular, the latter looks like a true column whose height is equal to several times the diameter, in particular at least twice and generally at least four times.

In these conditions, the supplying of electromagnetic energy through the means of an antenna placed as indicated above leads to the major drawback of a preferential absorption of this energy in those areas of material to be treated which are nearest the antenna, and consequently to a rapid attenuation of the residual energy available for the treatment of the next areas. In order to supply the necessary minimal energy in the area furthest from the antenna, it is therefore necessary to provide an excess of energy in the nearest areas. This situation leads to overheating in these areas, if heating is the effect sought, or to the racings of reactions, with all the consequences that that implies, in the case of more elaborate industrial applicators.

It is an object of the present invention to reduce this grave drawback by ensuring for each right section of the applicator an identical contribution of electrical energy, whatever its longitudinal position within the applicator.

The present invention provides an applicator for subjecting a material to microwaves, comprising a microwave generator, a cylindrical receptacle for the material and an antenna placed in the receptacle and connected to the generator, characterised in that the antenna extends in a notional cylinder having the same axis as the receptacle.

The diagram of radiation of the antenna is then the same in every plane perpendicular to the axis of the applicator. The man skilled in the art will understand that, in such a right section, the path travelled by the waves is clearly shorter than in the case of the prior art apparatus mentioned above. The attenuation of the available residual energy is therefore considerably reduced.

Moreover, the invention allows the efficiency of "energetic recovery" in each right section to be increased by effecting the shape of the diagram of the antenna, the arrangement of the latter in the receptacle and the systematic and controlled use of the receptacle itself as reflector, which use, in the particular case of the invention, offers a certain interest since the available residual energy is far from being negligible.

The notional cylinder in which the antenna extends can be reduced to the axis of the receptacle. The antenna then extends on the axis of the receptacle. The antenna can also have the shape of a helix, especially in the case of a cylindrical receptacle of circular right section, which helix intersects the generatrices of the notional cylinder at a constant angle, homothetic of the receptacle in relation to the axis of the latter. It is advantageous that the antenna and the receptacle exhibit a common symmetry, of the type supplied by a helix.

According to a favourable variant, the antenna extends along a generatrix of the receptacle and, better still, the theoretical diagram of radiation of the antenna is a surface, therefore a curve in a right section, homothetic of the receptacle in relation to the antenna. Thus the quantity of radiated energy is proportioned to the path of the wave in the material.

To obtain the required homogeneity in the treatment, several transmitter antennae can be distributed angularly around the axis, preferably in an irregular way so that two antennae are not opposite each other. In the case of a column of circular right section, k antennae will be distributed angularly in a regular manner, k being an odd whole number greater than 1 or rather any number of antennae irregularly distributed. It is advantageous that the tangential planes in the diagram of radiation of an antenna do not include another antenna, that is to say that the latter is outside the acute dihedral angle formed by these planes so that the radiation of one antenna does not reach the other.

The efficiency of the antenna can be improved by placing it in the focal plane of the reflector formed by the interior wall of the receptacle and/or by actuating it with a movement, especially of oscillation, around its axis.

In the attached drawings, given solely by way of example:

FIG. 1 is a partial diagram of an applicator according to the invention in axial section;

FIG. 2 is a diagram, similar to that of FIG. 1, of a variant;

FIGS. 3 to 6 are views representing the right sections of other variants; and

FIG. 7 is a view in transverse section, on a larger scale, of an antenna used in the applicator of FIG. 6.

In FIG. 1, a molecular filter T to be desorbed is placed between two grids 2, 3 of dielectric material in a cylindrical receptacle 1 of circular right section and having a vertical axis XX'. The diameter of the column thus formed is less than the quarter of the height. An antenna 4 stretches along the axis XX' and radiates perpendicularly to the axis XX'. The antenna 4 is connected at one of its ends to a magnetron 5 serving as microwave generator.

In FIG. 2, another antenna 4 is shown which extends along a helix inside the receptacle 1.

In FIG. 3, four antennae 4a, 4b, 4c and 4d represented schematically by circles extend along generatrices of the cylindrical receptacle 1, which has vertical axis, against the interior wall of the receptacle. The theoretical diagram of radiation of the antenna 4a is, in the cross section of th applicator, a circle 7 homothetic of the receptacle 1 in relation to the antenna 4. The antennae 4b to 4c are outside the angle a delimited by the boundary radii 8 and 9 of the antenna 4.

In FIG. 4, five antennae 4.sub.1 to 4.sub.5 symbolized by dots are distributed according to a regular pentagon on the generatrices of the receptacle against the interior wall of the latter.

In FIG. 5, six antennae 4.sub.1 to 4.sub.6 are distributed according to a regular hexagon on the generatrices of the receptacle 1, against the interior wall of the latter.

In FIG. 6, the whole of the receptacle 1 is filled with material to be treated. Three antennae 4a, 4b, 4c with slots have their axes parallel to that of the receptacle 1. The interior wall of the receptacle 1 serves as reflector. To ensure a controllable reflection in a simple way, the antennae are distributed on a cylindrical surface of radius half that of the reflecting envelope of the reflector (and thus at the focal surface of the cylindrical reflector). In the example chosen, the three antennae are at the three apices of an equilateral triangle inscribed in the right section of this focal surface. The efficiency and homegeneity of the "energetic recovery" of each right section of the applicator can, in addition, be increased by causing each antenna to oscillate in an identical and calculated angle around its axis 10 by an apparatus 11 giving a to-and-fro rotation movement to the axis.

The arrangement of the slots of the antennae can, in all cases, be such as to radiate the electromagnetic energy only in one or several, well-determined, areas of the applicator, so as to increase the selectivity of the action of this energy.