Background Art Drying plastic materials is generally necessary before these materials are used in subsequent operations, for example in extrusion or injection molding processes. The presence of humidity in the materials may in fact impair the quality and/or appearance of subsequent production. In some cases, drying may also be performed prior to a storage phase, although always in view of the subsequent need for material with a low degree of humidity.

In some particular cases, as well as drying, it may be necessary to heat the material to a predetermined temperature to obtain the desired molecular structural characteristics prior to use in subsequent processes.

For example, polyethylene terephtalate (PET) may also be subjected to pre-crystallization procedures before use.

In general, the most widely used heating/drying systems in the prior art are essentially based on storage of the plastic materials in containers of considerable size. In fact, containers are required with a volume ranging from 3 to 20 times the volume of the plastic material to be dried, generally processed in the form of flakes or granules, as a function of the apparent specific weight of the material, of the processing time and of the productivity required. The heating or drying process is performed by maintaining the materials inside these containers for a time depending on the diffusion speed of the water molecules through the polymer up to the surface of the granule or flake, and on the degree of humidity of the

material subjected to processing.

In discontinuous processes, or in batch processes, the material is heated by contact with the heated walls of the container. In some cases, vacuum pressure is produced inside the container to accelerate the drying process and evacuate the gases produced. The material is mixed continuously by stirrers to even out the temperature of the mass.

With specific regard to continuous processes, heating takes place by blowing heated air while, if necessary, the material can be maintained in constant movement by stirrers to prevent possible compacting. The volatile substances released from the material are thus removed from the container by means of the hot air current thus produced and then dispersed into the environment. To obtain particularly high degrees of dehumidification, that is up to a few tens of ppm of residual humidity in the processed material, the process air is previously subjected to a drying phase using specific resins, until reaching condensation temperatures ranging from-40°C to-60°C, then heated and fed into the container.

The resins utilized for forced drying must in turn be regenerated, that is freed from the humidity absorbed. This is produced by air heated to high temperatures. The water released by regeneration of the resins is dispersed into the environment, with consequent loss of energy.

In more recent times it has been suggested to use microwave energy in heating and/or drying processes of plastic materials, for example, hygroscopic plastic materials, in which the humidity found in the same materials constitutes the means capable of absorbing this energy, or plastic materials that are in any case sensitive to this form of energy notwithstanding their capacity to absorb humidity.

A first example of a process of the discontinuous type with the use of microwave energy is described in USA patent n. US-A-5105555.

According to this document, the material is processed in a container

inside which a microwave generator produces an electromagnetic field.

The container is also supplied with a current of heated and dehumidified air destined to remove the humidity released by the processed materials.

The degree of humidity of the processed material is determined by measuring the degree of humidity in the air delivered from the container.

The solution proposed in this document, besides concerning a batch process, does not appear to be particularly efficient as regards the utilization of microwave energy. In fact, the electromagnetic field appears to be directed prevalently towards a zone of the container above the top level of the material and not directed towards the same material.

As it is known that the electromagnetic field produced by a microwave generator is highly directional, the greater part of the microwave energy is used to heat the air circulating inside the container rather than the material. In this case, by taking account of the reduced heat exchange efficiency of the air above in respect of the humidity contained in the material, the energy contribution of the microwaves in the heating/drying process of the materials may even be negligible in respect of the contribution provided by heating the air upstream of the container.

Another example of the use of microwave energy in heating/drying processes of polymers is disclosed in USA patent n. US-A-4055001. This document describes a continuous process which is carried out with a series of resonating cavities disposed along a pneumatic conveyor duct inside which the material is conveyed. The conveying air is pre-heated, to reduce the amount of humidity.

However, a solution of this type to produce a continuous process has several drawbacks. Firstly, the conveyor plant must be dimensioned as a function of the exposure time of the materials to the various

electromagnetic fields. In this case, as the material travels through each resonating cavity for extremely limited times (approximately from 1 to 3 seconds), it is unlikely that sufficiently high level of dehumidification can be obtained with systems having particularly small overall dimensions.

Moreover, besides the typical drawbacks of the pneumatic conveyors used for humid materials (for example, agglomeration of flakes or their deposit on the inner walls of the ducts), it must be considered that the material processed and the humidity removed from it travel together in the same duct for considerable distances.

Therefore, any cooling in some zones of the pneumatic conveyor ducts can cause the humidity present in the conveying air to condensate, with possible re-absorption of humidity by the materials or humidity depositing on the inside walls of the ducts. For these reasons, the process described in this prior art document may be somewhat inefficient, if not actually unacceptable, to obtain particularly high degrees of drying and the results attainable are not easily or directly controllable.

Summary of the invention This being stated, the task of the present invention is to propose a method and a device for heating and/or drying plastic materials in a continuous way which make it possible to overcome the drawbacks of prior art.

Within the scope of this task, an object of the present invention is to propose a method and a device of the aforesaid type which make it possible to produce heating and/or drying of plastic materials in a particularly rapid and efficient way.

Another object of the present invention is to propose a method and a device of the aforesaid type which make it possible to obtain a particularly high degree of dehumidification.

A further object of the present invention is to propose a device and a

method of the aforesaid type which make it possible to control the heating/drying process of plastic materials in a particularly accurate way.

Yet another object of the present invention is to propose a device of the aforesaid type which has particularly limited overall dimensions with the same level of productivity compared to the prior art systems proposed.

These objects are attained by the present invention, which relates to a device for heating and/or drying plastic materials in a continuous way, including at least a process container, feeding means to continuously convey the materials inside the container, handling means to stir the materials and/or make them advance inside the container, drawing means to remove the materials continuously from the container and heating means to heat the materials conveyed inside the container, characterized in that the heating means of the materials include one or more microwave generators to generate inside the container an electromagnetic field directed prevalently towards the materials being conveyed, and that the handling means are mechanical handling means.

The use of microwave generators with electromagnetic fields directed towards the materials being conveyed, together with handling performed at least in part by mechanical means, make it possible to control the heating of the conveyed materials optimally. This also makes it possible to obtain a particularly high level of dehumidification with considerable decreases in the dimensions of the device compared to prior art systems. The use of microwave generators also makes it possible to reduce specific energy consumptions (kW/kg) and makes the process particularly rapid both during the start up and the steady phases.

As well as contributing towards making the materials advance, the handling means also exert a stirring and mixing action on them. The

handling means preferably include one or more rotating elements made to rotate inside the container.

According to a possible embodiment, at least two rotating elements may be provided, positioned side by side and made to rotate inside the container in respect of mutually parallel axes. In this case, the microwave generators may be disposed so as to generate an electromagnetic field directed prevalently towards a direction not incident with the axis of rotation of the rotating element (s), for example, a direction shifted laterally towards the outside in respect of a perpendicular incident to the axes of rotation, or towards the space between the axes of rotation of the rotating elements if there are two of them.

The rotating elements may, for example, be constituted by augers, or may be constituted by rotating shafts on which a plurality of radial blades are fixed. In any case, in addition to the aforesaid functions of stirring the materials and/or making them advance, the rotating elements are produced in materials that reflect the waves of the electromagnetic field, so as to promote diffusion of the electromagnetic field in the mass of materials being conveyed in the container.

Control of the degree of dehumidification of the materials subjected to processing is preferably performed by detecting the temperature of the materials in one or more points along their path inside the container. The power emitted by the various microwave generators disposed along the path along which the materials are conveyed inside the container is regulated as a function of the detected temperature. One or more temperature sensors are thus provided, as well as means suitable to control the emission power of the microwave generators as a function of the detected temperatures.

The invention also relates to a method for heating and/or drying plastic

materials in a continuous way, wherein the materials are fed continuously into the container and drawn continuously from the container, and wherein the materials are heated along their path inside the container, characterized in that the materials are heated by a microwave electromagnetic field produced inside the container and directed prevalently towards the materials being conveyed, and that the materials are made to advance at least in part by mechanical handling means.

The materials in the container may also be made to advance at least in part by gravity, for example by positioning the container with the axis inclined in respect of the ground or, at the very most, with the axis substantially perpendicular to the ground. In this extreme case, in which the container is disposed vertically, the mechanical handling means could have the sole function of stirring the materials, or also a function of regulating (or even slowing) the speed at which the materials advance in the container.

Removal of humidity from the container may be produced by circulation of a conveying fluid made to circulate inside the container, for example a gas previously conditioned as regards characteristics of temperature and humidity.

Alternatively, the humidity released by the processed materials may be removed by maintaining a vacuum inside the container, or in any case by maintaining the pressure inside the container below the external atmospheric pressure.

Brief description of the drawings Further characteristics and advantages of the present invention shall become more apparent from the description hereunder, provided purely as a non-limiting example with reference to the accompanying schematic drawings, in which:

- Figure 1 is a partly sectional side elevation view of a heating/cooling device according to a possible embodiment of the present invention; - Figure 2 is a simplified top plan view of the device shown in Figure 1; and - Figure 3 is a sectional view of another possible embodiment of the device according to the present invention.

Modes for carrying out the invention Figures 1 and 2 schematically show a possible embodiment of a device according to the present invention.

The device is essentially constituted by a process container 10 made preferably in a reflecting material for the microwaves at the wavelength utilized. The cross section of the process container 10 may, for example, be circular or polygonal. Even if not expressly shown in the views of Figures 1 and 2, the process container 10 preferably includes an external coating of heat-insulating material.

The container 10 is mounted on supporting structures 100 and 200 positioned at its respective ends. Structure 100 includes in particular a base 101 connected to which are a pair of rods 102 equipped with a plurality of holes 103 suitably spaced apart. Repositionable pins 104 make it possible to clamp the container 10 and hold it in position with a preset angle of inclination in respect of the ground. Structure 200 in turn includes a base 201 on which a pair of rods 202 are rigidly mounted. The process container 10 is hinged to allow it to rotate in 203 at the upper ends of the rods 202. In the embodiment shown, inclination of the container 10 may be regulated according to angles such as to oppose (or rather slow down) through gravity, advance of materials in the container 10. Even if not expressly shown, the necessary modifications may be made to allow regulation of the inclination of the container 10 also with angles that can to promote advance of the materials through

gravity.

The materials to be processed are fed (arrow M) into the container 10 through a hopper 20 and a duct 21 connected to a feed manifold 11.

The materials are fed continuously to the hopper 20 through known devices (not shown), for example augers, band or bucket elevators or the like, and reach the process container 10 through gravity.

One or more suppressor elements 22 are preferably disposed along the duct 21 and are provided with passages of a suitable dimension to allow the material to pass through while simultaneously blocking propagation of the microwaves towards the outside environment. Some examples of suppressor elements that can be utilized in this case can be found in the USA patent n. US-A-4055001 mentioned above.

Inside the process container 10 are handling means, the function of which is essentially to stir the materials and/or make them advance. To perform these functions, rotating elements, such as augers or blades mounted on rotating shafts, both preferably made of materials capable of reflecting the microwaves, are preferably utilized.

In the embodiment shown herein, the handling means are constituted by a rotating shaft 30, on which a plurality of blades 31 are mounted extending radially almost to the inside wall of the container 10.

Orientation of the blades 31 and the direction of rotation of the shaft 30 are chosen so as to make the material advance and keep it moving towards a discharge outlet 12 in the container 10.

Rotation of the shaft 30 is imparted by a motor 32 connected to the shaft 30 through a reduction unit 33. The motor 32 is preferably controlled to allow speed to be varied as a function of the conditions and characteristics of the processed materials.

An advantageous feature of the device according to the present invention is that the shaft 30 can be easily removed to facilitate cleaning

operations, especially when materials with different characteristics must be processed in succession, and also maintenance operations on the blades and the shaft. For this reason, a removable covering element 13 is provided mounted at one end of the process container 10 and provided with an end support, for example a bearing or the like, for rotation of the shaft 30. By removing the covering element 13 the shaft 30 can be removed from its driving connection with the reduction unit 33 and withdrawn completely from the container 10.

The materials that reach the discharge outlet 12 fall by gravity towards drawing means, made in this case by an auger 40 that brings the processed materials, indicated by the arrow N, towards a discharge outlet 45. Similarly to the feed section, if necessary, a suppressor element 42 of the type mentioned previously may be disposed between the discharge port 12 and the auger 40.

The auger 40 is drawn in movement by a motor 43 connected to it through a reduction unit 44. In this case too, the motor 43 may be subject to a control unit that determines the rotation speed as a function of the output rate to be obtained.

Alternatively, the discharge port 12 may be connected directly to the feed section of an extruder positioned immediately downstream of a device according to the present invention.

The materials traveling along the container 10 are heated by microwave electromagnetic field generators 50 (Figure 1) disposed along the path of the materials in the container 10 and positioned so as to generate inside the container 10 an electromagnetic field directed prevalently towards the materials being conveyed. For simplicity, Figure 2 only shows the assembly flanges 51 of the generators 50.

In the embodiment of Figures 1 and 2, the microwave generators 50 are installed on the container in a position to produce an electromagnetic

field directed prevalently towards a shifted direction in respect of the axis of rotation of the shaft 30.

The device according to the present invention may utilize generators 50 commonly available on the market. In an experimental single-shaft prototype substantially conforming to the embodiment described herein, three generators were used, each with a power of at least about 2 kW, designed to emit microwaves at a frequency of around 2450 MHz.

The number, layout and power of the generators 50 may obviously vary as a function of the length of the container 10, and as a function of the conditions and characteristics of the materials that are prevalently processed in the device and the flow rates of material that the device must be capable of sustaining.

It must nonetheless be born in mind that the powers of the most widely used generators on the market for industrial applications in general do not exceed a power of 20 kW. Too many generators with limited power could be somewhat disadvantageous from the viewpoint of cost, while too few generators with high power could be somewhat ineffective or, even worse, entirely unusable.

A control unit (not shown) is preferably provided to allow the emission power of each of the generators 50 to be varied as a function of the characteristics, the conditions of humidity of the processed materials, and the degree of humidity to be obtained in the materials delivered.

Control is preferably produced as a function of the temperature of the materials detected in different points of the container. Temperature sensors 60 are thereby provided to produce a signal representing the temperature in each zone of the container 10 in which they are installed.

These signals are sent to the control unit to control the power emitted by the generators 50 to vary the emission power of each generator almost instantly.

Removal of the humidity released from the materials may be obtained, for example, by circulation of a fluid inside the container 10. Therefore, feed and delivery manifolds 70 may be provided to allow circulation of gaseous fluids with conditioned humidity and temperature. Alternatively, humidity may be removed by generating a vacuum inside the process container 10. It is in any case possible also to provide a combination of these two techniques, that is circulation of a certain amount of gaseous fluid simultaneously maintaining a vacuum inside the container 10.

Figure 3 represents another possible embodiment of the device according to the invention, according to which the process container includes two chambers 80 with a circular section and housing two respective parallel rotating shafts 90 equipped with radial blades 31.

Similarly to the previous embodiment, the container 10 includes an external coating 15 in heat-insulating material.

In this embodiment, the microwave generators 50 are disposed to act in both the chambers 80 and directed prevalently towards the external space in respect of the axes of rotation of the respective rotating shafts 90. Alternatively, or in combination, one or more microwave generators 50 (not shown) may be provided, directed prevalently towards the space between the axes of rotation of the rotating elements 90, that is towards the intersection space between the two chambers 80.

During steady operation, the materials to be processed are fed continuously to the hopper 20 and expelled through the discharge port 45. The speed of advance of the materials inside the process container 10 may be adjusted, for example, by controlling the rotation speed of the motor 32 that operates the shaft 30, and/or the rotation speed of the motor 43 that operates the removal auger 40. The variation in the inclination of the axis of the container 10 provides a further factor that may influence the speed at which the materials advance inside the

container.

In the start-up phase of the heating/drying process, in which the temperatures of materials conveyed in the various zones of the container have not yet reached the pre-established values, materials can be recirculated from the discharge port 12, or the discharge port 45, towards the feed hopper 20. In this case, known conveying means may be provided, such as conveyor belts, augers or the like (not shown) that make it possible to obtain a similar function for the time required to bring the temperatures of the materials to the pre-established values.

In any case, the time required to reach the steady operating temperatures is particularly fast and in any case much less than the times required to reach the steady phases in common drying systems using heated air currents. The device is thus particularly suitable and inexpensive even in the case of small quantities of materials to be processed.

Various modification may be made without however departing from the scope of the present invention. For example, the container 10 may also be disposed vertically, that is with its axis substantially perpendicular to the ground. In this case, advance of the materials in the container 10 takes place through gravity, while the handling means, as well as the actions to remix and stir the materials, may act to slow down the flow of materials towards the bottom part.