EP2394122A1

EP2394122A1 - Dehumidifier for plastics materials

Info

Publication number: EP2394122A1
Application number: EP10706383A
Authority: EP
Inventors: Christian Schiavolin
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-02-09
Filing date: 2010-02-08
Publication date: 2011-12-14
Also published as: IT1393150B1; ITPD20090024A1; WO2010089721A1

Abstract

A dehumidifying device (1) for plastics materials comprises a container body (2) which is suitable for receiving a desired amount of plastics material and which is externally delimited by a wall (3), a sheathing wall (4) which is suitable for at least partially surrounding said wall (3) and which is positioned in such a manner that a space (5) is defined between said sheathing wall (4) and said wall (3), wherein said space (5) is hermetically closed and is operatively connected to vacuum-producing means (7) suitable for producing a desired reduced pressure inside said space (5) in order to insulate said container body (2) thermally from the outside.

Description

DEHUMIDIFIER FOR PLASTICS MATERIALS

Description

Technical field

The present invention relates to a dehumidifier for plastics materials, having the features listed in the preamble of the main claim.

The invention also relates to a dehumidifying system for plastics materials and to a plant for treating plastics materials, which are provided with a dehumidifier according to the invention.

Technological background In order to produce manufactured or semi-manufactured plastics goods, it is possible to use various production processes, including, for example, injection-moulding and extrusion.

The plastics material to be worked, for example in the form of pellets, is stored in suitable containers and then transformed into finished or semi- finished objects in suitable transforming machines, for example injection presses, extruders, blowing machines, etc.

However, plastics materials, above all those classified as "hygroscopic", absorb moisture which comes principally from the atmosphere. The moisture absorbed depends, apart from on the type of plastics material, also, inter alia, on the exposure to air, the time and the conditions of storage of the plastics material before the subsequent transformation, etc.

Moisture has negative effects on the .transformation process, in addition to impairing the aesthetic and, above all, mechanical characteristics (tensile, flexural and impact strength) of the end product. It is therefore necessary to dehumidify the plastics material before feeding it to the transforming machines.

To that end, in the processes for transforming plasties materials, a dehumidifying phase is provided upstream of the transformation phase in order to extract the moisture present in the plasties material. The dehumidifying phase is particularly important for hygroscopic polymers, such as, for example, PA, ABS, PET, TPU, PC, which tend to readily absorb a substantial amount of water.

In order to extract the moisture from the plastics material, provision is currently made for treating the plastics material with various drying agents, such as, for example, hot, dry air, infrared rays, microwaves, or also a vacuum.

In the dehumidifying systems currently used, a specific amount of plastics material to be dehumidified is introduced into a dehumidifying container in which it is subjected to the action of the selected drying agent which removes the moisture therefrom.

The removal of the moisture from the plastics material depends on the temperature at which the dehumidification is carried out. By increasing the temperature, the breaking of the bonds between the water and the plastics material is facilitated and the mobility of the water is increased, facilitating its evaporation and, therefore, its removal from the plastics material.

The temperature to which the plastics material is heated before it is sent to the dehumidifier is generally from approximately 4O⁰C to approximately 200⁰C, depending on the type of plastics material, in order to facilitate the dehumidification thereof. The dehumidifiers generally used are provided with layers of insulating material having a thickness of approximately from 2 to 6 cm in order to limit heat losses to the outside. Rock wool or composite materials thereof is generally used.

However, the known dehumidifiers are subject to considerable heat losses to the outside and it is therefore necessary to apply an additional amount of energy in order to limit the effects of the decrease in temperature which are caused by the thermal dispersion and the heat absorbed by the insulating material.

A decrease in temperature slows the dehumidifying process, reduces the energy efficiency of the process substantially and increases energy consumption in the transforming machines and also prolongs the time necessary for dehumidification. The energy consumption of the known dehumidifying processes is therefore very high.

In addition, it is not possible with such insulating systems to regulate the degree of thermal dispersion associated therewith, the heat loss of the dehumidifiers being constant and dependent on the type and/or thickness and/or other characteristics of the particular insulating material used.

The duration of dehumidification depends on the temperature of the dehumidifying process, as well as on the dehumidifying agent used to remove the moisture and also on the type of plastics treated.

Description of the invention

An object of the invention is to provide a dehumidifier having increased thermal efficiency.

A further object is to provide a dehumidifier having increased dehumidifying efficiency in respect of plastics material. A further object is to provide a dehumidifier having a low operating energy consumption and enabling the plastics material to be treated effectively in considerably reduced times.

A further object is to provide an effective dehumidifying system which is efficient in terms of energy and which has a low energy consumption.

Yet another object is to provide a dehumidifier in which it is possible to regulate in a simple manner the level of thermal dispersion to the outside, and/or the temperature inside the dehumidifier, and/or the degree of dehumidification of the plastics material inside the dehumidifier. A further object is, finally, to provide a dehumidifier which is also capable of carrying out treatments of crystallising, regrading, increasing the intrinsic viscosity of and super-cleaning amorphous or recycled PET. Those objects are achieved by the present invention by means of a dehumidifier and a dehumidifying system for plastics materials, which are produced in accordance with the claims, which follow. Brief description of the drawings

The features and advantages of the invention will emerge more clearly from the detailed description of a preferred embodiment thereof illustrated by way of non-limiting example with reference to the appended drawings in which:

- Figure 1 is a diagram of a process for treating plastics materials;

- Figure 2 is a diagrammatic view of a dehumidifying system according to the invention;

- Figure 3 is a diagrammatic view of a dehumidifying device according to the invention; - Figure 3a is a diagrammatic view of a variant of the dehumidifying device according to the invention of Figure 3;

- Figure 4 is a diagrammatic view of a second variant of a dehumidifying device according to the invention; - Figure 5 is a diagrammatic view of a third variant of a dehumidifying device according to the invention;

- Figure 6 is a diagrammatic view of a fourth variant of a dehumidifying device according to the invention.

Preferred embodiment of the invention Figure 1 shows diagrammatically a process for transforming plastics materials 100 which can be carried out in a conventional plant for transforming plastics materials.

The plastics material may be, for example, in the form of pellets, flakes, reground material, that is to say, material obtained by grinding production waste and/or from post-consumption material, or it may also be in powder form.

The plastics material is first of all made, for example, into pellets, which are then stored in suitable storage containers 101 where they are left to await subjection to subsequent transformation processes in suitable transforming machines 102, by means of which the pellets are transformed into finished objects or semi-manufactured goods.

Although the following description refers particularly to plastics material in pellet form, the invention may be applied to plastics material in any other form, such as flakes or even powder. Before being sent to the transforming machines 102, the pellets are sent to a dehumidifying system 103 in order to remove any moisture absorbed by the pellets and to prevent the occurrence in the transforming machines 102 of any problems, such as air bubbles, defective edges and scoring, depressions, degradation of the polymer, burrs, low viscosity, roughness, etc. ... caused by the presence of moisture.

Inside the dehumidifying system 103, the pellets of plastics material coming from the storage container 101 are subjected to dehumidification by means of at least one drying agent, for example a current of hot, dry air, a vacuum, infrared radiation, microwaves, or a combination of those drying agents, in order to extract the water present.

Inside, or upstream of, the dehumidifying system 103, the pellets of plastics material are also subjected to the action of at least one heating agent in order to be heated to a temperature suitable for facilitating the dehumidification thereof. Figure 2 shows diagrammatically a dehumidifying system 103 in which hot, dry air is used to heat the plastics material and a vacuum is used as the drying agent and in order to maintain the heated plastics material at an almost constant temperature. The dehumidifying system 103 comprises a dehumidifying device 1, shown in greater detail in Figure 3, into which the pellets are fed, as indicated by the arrow Fa, in order to be subjected to dehumidification and from which they are subsequently fed, as indicated by the arrow Fb, to the transforming machines 102. The pellets are kept inside the dehumidifying device 1 and subjected to the action of the selected drying agents, in this case the air and the vacuum, for a specific period of time which depends, for example, on the type and on the degree of hygroscopicity of the plastics material, on the optimum treatment temperature, etc. ... The pellets of plastics material are preferably fed to the dehumidifying device 1 and then heated with a heating agent, for example hot, dry air, or infrared rays, or microwaves, up to a desired and optimum temperature, this also triggering the dehumidifying process, and subsequently, or simultaneously, as will be explained in more detail hereinafter, the pellets are subjected to the action of the vacuum in order to effect the dehumidification thereof.

In one version, the pellets of plastics material may be heated before being fed to the dehumidifying device 1.

The temperature to which the pellets are heated depends on the temperature at which it is desired to carry out the dehumidification, on the type of plastics material from which the pellets are made and on the temperature at which it is desired to feed the pellets to the subsequent transforming machines 102. Usually, the pellets are heated and dehumidified in the dehumidifying device 1 at a temperature of from approximately 4O⁰C to approximately 200⁰C. For example, in the case of PET intended for the manufacture of bottles, the material is treated at a temperature of from 16O⁰C to 18O⁰C for approximately 4-6 hours, depending on the initial degree of humidity. Given the high temperatures, therefore, in this process the energy consumption is a very critical and important parameter. The dehumidifying system 103 comprises an air-treatment system 104 placed upstream of the dehumidifying device 1 and arranged to treat the air before sending it to the dehumidifying device 1, an air-filtration system 105 placed downstream of the dehumidifying device 1 and arranged to treat the air leaving the dehumidifying device 1, and a blower 106 arranged to produce a flow of air inside the dehumidifying system 103.

In versions which are not shown, it is possible to use other devices suitable for moving the air inside the dehumidifying system 103, for example fans of suitable dimensions. The dehumidifying device 1 comprises a container body 2 in which the plastics material coming from the storage container 101 and to be subjected to dehumidification is collected, and which includes an opening 31 through which the plastics material is introduced, and an outlet mouth 32 which is positioned in the container body 2 on the opposite side to the opening 31 and which is arranged to permit the discharge of the plastics material from the container body 2. The opening 31 is reclosable, preferably in a sealed manner, with a cover 20.

The cover 20 is kept closed during the treatment of the plastics material, while it can be opened and optionally removed for operations of maintenance, cleaning, etc. An inlet valve 50 for the material is provided in the cover 20, while an outlet valve 51 for the material is provided at the location of the outlet mouth 32. The inlet valve 50 and the outlet valve 51 are configured in such a manner as to close the container body 2 in a sealed manner, so that, when they are both closed, with the cover 20 closed, the container body 2 is hermetically closed with respect to the outside. The dehumidifying device 1 can operate continuously, the residence time of the plastics material being varied in accordance with the consumption of plastics material in the transforming machines 102. The dehumidifying device 1 is also suitable for operating batchwise and that enables the dehumidifying device 1 to be used efficiently, for example in a case where it is necessary to store the plastics material after dehumidification, optionally in the container body 2 itself, or also in another container, before the transforming machines. That occurs, for example, when the dehumidified plastics material is mixed, before being fed to the transforming machines, with additives which cannot be subjected to the dehumidifying treatment.

The container body 2 is delimited by a wall 3, outside which is provided a sheathing wall 4 positioned in such a manner that a space 5 is defined between it and the wall 3, which space 5 is connected by means of a duct 6 and a vacuum-breaking valve 8 to a vacuum pump 7 suitable for producing a desired reduced pressure inside the space 5.

The vacuum pump 7 is caused to operate in such a manner as to generate inside the space 5 a relative pressure of from approximately -900 mbar to approximately -980 mbar, and in some applications the relative pressure may reach -1000 mbar.

The container 2 is produced from stainless steel or another material which is suitable for being in contact with the plastics material and supporting the structure of the dehumidifying device 1 and which is suitable for sealing the vacuum. The wall 3 and the sheathing wall 4 are configured in such a manner that the space 5 is hermetically closed with respect to the outside. In addition, in one version it is provided that the space 5 be hermetically closed also with respect to the container body 2.

Thus, any desired pressure can be maintained inside the container body 2, regardless of the degree of vacuum generated in the space 5. The presence of the space 5 and the vacuum inside it enable the dehumidifying device 1 to be insulated thermally from the outside, limiting the heat losses thereof and thus reducing the energy consumption compared with the known dehumidifiers.

The energy efficiency of the dehumidifying device 1 according to the invention will be compared hereinafter with a dehumidifier in which the thermal insulation is produced using an insulating material, such as rock wool.

Rock wool, having a density of from 8 to 100 kg/m³, has a thermal conductivity of approximately from 0.032 to 0.045 W/(m*K). By producing a reduced pressure of from -900 mbar to -980 mbar inside the space 5, the amount of air in the space 5 is reduced to values of approximately from 2% to 10%, and a thermal conductivity of approximately from 0.013 to 0.00104

W/(m*K) is obtained.

Those conductivity values are considerably lower than the conductivity values obtained with a sheathing of insulating material; for example, an improvement in the thermal insulation of approximately from 94 to 98% is obtained compared with rock wool.

That substantially increases the energy efficiency of a dehumidifying process carried out with the dehumidifying device 1, drastically reducing the heat losses to the outside and substantially limiting the dehumidifying energy consumption compared with the known dehumidifiers.

The presence of the space 5 and the degree of vacuum produced therein enable the energy efficiency of the dehumidifying device 1 to be maximised, reducing the heat exchange with the outside to a minimum. In addition, the necessity to supply heat to the dehumidifying device 1 during dehumidification in order to make up for the cooling of the plastics material is limited or even removed.

Furthermore, it is possible to reduce the dehumidifying times considerably, and a more uniform temperature of the plastics material is produced inside the container body 2 during dehumidification.

Therefore, dehumidification is carried out with greater efficiency.

The vacuum-breaking valve 8 of the dehumidifying device 1 according to the invention enables the degree of vacuum inside the space 5 to be regulated.

By acting in a suitable manner on the vacuum pump 7 and/or on the vacuum-breaking valve 8, it is possible to regulate and maintain a desired level of vacuum in the space 5, thus regulating the heat losses of the dehumidifying device 1 and, therefore, the temperature of the plastics material inside the dehumidifying device 1.

By reducing the degree of vacuum in the space 5, it is possible to increase the heat losses of the dehumidifying device 1, effecting rapid cooling of the plastics material inside the body 2, for example in the case of over-heating, and bringing the plastics material to the desired temperature.

Therefore, the degree of insulation produced by means of the space 5 can be varied in a controlled and rapid manner. It is thus possible, in addition, to regulate rapidly and efficiently the temperature inside the container body 2; by producing a high degree of vacuum in the space 5, thermal dispersion is minimised, whereas by increasing the pressure in the space 5, the heat losses are increased, bringing about a rapid reduction in the temperature inside the container body 2.

The possibility of varying the thermal insulation of the container body 2 as desired has numerous advantages. Reducing the degree of vacuum by means of the valve 8 is useful, in particular in some processes in which the material may have to be cooled and stored inside the dehumidifier 1 after the actual dehumidification, but also in some emergency cases in which it is necessary to reduce the temperature in order to prevent the triggering of dangerous processes, and varying the degree of vacuum enables the process to be adapted in a simple and rapid manner to the type of plastics material being treated and/or to the specific end purposes required for such plastics material.

In contrast, in the known dehumidifiers, it is not possible to vary the thermal dispersion, the latter being constant in accordance with the characteristics of the thickness and the type of insulating material used. In the known dehumidifiers, in order to vary the temperature, it is necessary to vary the supply of heat from outside by acting on the heating devices connected to the dehumidifiers, with very high energy consumption. Prolonged times are also necessary to reach the desired temperature conditions in the dehumidifier, owing to the inertia of the known systems. Because the space 5 is hermetic, the vacuum pump 7 can operate discontinuously. The pump 7 is operated until, in the space 5, the desired level of vacuum is reached in accordance with the specific characteristics of the process and/or of the material being treated, and it is turned off and turned on again in order, if necessary, to restore any vacuum losses caused by the incomplete hermeticity of the space 5, or after any opening of the valve 8 caused, for example, by emergency conditions resulting from the excessive temperature of the plastics material.

The container body 2 of the dehumidifying device 1 is connected, by means of a second duct 60 and a second valve 80, or another vacuum-regulating means, to a second vacuum pump 70 arranged to generate a specific degree of vacuum inside the container body 2.

The second valve 80 is connected to a source of fluid (not shown), for example air, dry air, nitrogen, etc. ..., which is drawn into the container body 2 by the opening of the second valve 80. That source of fluid may optionally be provided with heating means in order to pre-heat the fluid before it enters the container body 2; that enables the heat of the material in the container body 2 to be preserved. The vacuum inside the container body 2 enables the dehumidification of the plastics material to be carried out. The second vacuum pump 70 is caused to operate in such a manner as to generate a degree of vacuum inside the container body 2 of approximately from -900 to -1000 mbar.

By acting on the second valve 80, it is possible to regulate rapidly and effectively the temperature inside the container body 2, thus regulating the degree of dehumidification and/or the temperature of the plastics material being treated. The vacuum in the container body 2 enables the plastics material to be dehumidified owing to the stripping action of the second vacuum pump 70.

The second vacuum pump 70 generates a reduced pressure in the container body 2 such as to bring about the discharge of water from the pellets of plastics material, the bonds of which water with the plastics material have been broken beforehand by the action of the heat in the preceding heating phase. In addition, the vacuum reduces the boiling point of the water, further promoting the discharge thereof.

The action of the second pump 70 is preferably discontinuous and a desired level of reduced pressure is produced which is within a range of values up to a maximum of -1000 mbar.

After operating the second pump 70 and reaching the desired level of vacuum in the container body 2, which depends on the type of material being treated, the second vacuum pump 70 is turned off and turned on again to restore any losses of vacuum caused by any incomplete hermeticity of the container body 2.

By operating the second vacuum-breaking valve 80 in a suitable manner, it is possible to reduce the degree of vacuum in the container body 2 and subsequently to operate the second vacuum pump 70 again to return the degree of vacuum to the desired levels.

That further increases the dehumidification efficiency obtainable with the device 1.

The variation in the vacuum, or rather in the level of reduced pressure inside the container body 2, promotes dehumidification. As a matter of fact, at the level of vacuum set, the moisture is stripped from the plasties material but, after leaving that material, it tends to remain immobile in the container body 2, or else to move only slowly towards the colder points of the container.

The air, or the fluid, introduced into the container body 2 by the action of the second vacuum-breaking valve 80, acts as a means of transport for the moisture, promoting easier and faster removal thereof from the container body 2.

In the container body 2, phases of increasing the vacuum to up to -1000 mbar by the action of the second vacuum pump 70 alternate with phases of decreasing the vacuum by the action of the second vacuum-breaking valve

80 which allows fluid to pass into the container body 2 by way of the second duct 60.

The treatment process therefore provides that the increase in reduced pressure brought about by the second vacuum pump 70 alternates with a reduction in reduced pressure by the action of the second vacuum-breaking valve 80, according to a sequence and with effective pressure values which depend on the type of plastics material being treated.

The succession of phases of reducing/increasing the pressure enables the dehumidification efficiency to be maximised. The degree of vacuum generated in the container body 2 and/or in the space 5 depends on the phase of the process which has been reached, as will be explained in more detail hereinafter.

Optionally, the second duct 60, by which the moisture, or rather the air, is drawn in from the container body 2, can be cooled, this facilitating the movement of the extracted moisture which tends to condense towards the coldest point.

The container body 2 and the space 5 are produced with materials and thicknesses capable of withstanding the reduced pressure, humidity and heat, such as, for example, stainless steel. The space 5 and the container body 2 can be hermetically closed with respect to each other and the respective levels of vacuum can thus be regulated completely independently.

The presence of dedicated vacuum-regulating means, that is to say, the valve 8 and the second valve 80, enables the degree of vacuum in the space 5 and in the container body 2 to be regulated independently.

In the versions of Figures 3 and 3a, there is provided between the space 5 and the container body 2, a connecting means, for example a connecting valve 52, which is arranged to produce fluid communication between the space 5 and the container body 2. The connecting means 52 can be alternately opened/closed in order alternately to bring into communication/separate the container body and the space 5; by opening the connecting valve 52, it is possible to produce the same pressure inside the container body 2 and inside the space 5, whereas, by closing the connecting valve 52, it is possible to produce different and independent pressure levels in the container body 2 and in the space 5. In the version shown in Figure 3a, a single pump 7 is provided and is arranged to produce the desired degree of vacuum both in the space 5 and in the container body 2. In that case, the vacuum pump 7 and the vacuum- breaking valve 8 are connected alternately, by a by-pass valve 53, to the container body 2 and to the space 5. The presence of the by-pass valve 53 and the connecting valve 52 enables a particularly high degree of operating flexibility to be obtained: when the connecting valve 52 is open, in any position of the by-pass valve 53, it is possible to produce almost the same degree of vacuum in the container body 2 and in the space, whereas, by closing the connecting valve 52 and acting on the by-pass valve 53, it is possible to produce independent levels of pressure in the container body 2 and in the space 5. The container body 2 and the space 5 are hermetically closed with respect to the outside and are configured in such a manner as to impede re- balancing between the internal pressure and the external pressure when the respective vacuum pumps 7, 70 are turned off.

In the version of Figure 2, the dehumidifying device 1 is connected to a delivery tube 9 by means of which hot, dry air coming from the air- treatment system 104 is fed into the container body 2, as indicated by the arrow Fl, The delivery tube 9 is terminated by a diffuser 10 which leads into a first portion 2a of the container body 2 located towards the outlet mouth 32.

The diffuser 10 is configured in such a manner as to promote the diffusion of the air inside the plastics material present in the container body 2. In a vertical configuration of the dehumidifying device 1, in which the plastics material is caused to leave the body 2 by gravity, the first portion 2a is positioned in a lower portion of the body 2. The positioning of the diffuser 10 in the first portion 2a promotes the substantially uniform diffusion, from the top to the bottom, of the air in the plastics material. Contact between the hot air and the plastics material is increased, promoting dehumidification.

The humid air, that is to say, the air which has absorbed the moisture from the plastics material, is removed from a second, upper, portion 2b of the body 2 by way of a return tube 11 and is sent to the air-filtration system 105, as indicated by the arrow F3 and as explained in more detail hereinafter. The positioning of the return tube 11 in the upper portion 2b of the body 2 reduces the number of particles of plastics material being carried along with the air leaving the body 2. The blower 106 of the dehumidifying system 103 comprises a delivery portion 12 which sends out a stream of air fed to the air-treatment system 104, as indicated by the arrow F2, and an intake 13 which is reached by air coming from the air-filtration system 105, as indicated by the arrow F3, which air is forced towards the delivery portion 12 in order to be re-used in the air-treatment system 104. Therefore, the dehumidifying system 103 shown operates in a closed cycle. In other versions, which are not shown, it is possible to provide a dehumidifying system having an open cycle, in which the humid air extracted by the dehumidifying device is discharged into the atmosphere, optionally after suitable treatments. The air-treatment system 104 enables the air to be treated in such a manner that the air reaches the dehumidifying device 1 in the dry state, that is to say, substantially free from moisture, and in the hot state, in order to promote and accelerate the process of dehumidification inside the dehumidifying device. Preferably, the air-treatment system 104 treats the air in such a manner as to feed it to the dehumidifying device 1 at a temperature of from approximately 4O⁰C to approximately 200⁰C and with a dew-point value of from -1O⁰C to -7O⁰C, so that the air can heat and dehumidify the plastics material in the dehumidifying device 1. The air emitted by the blower 106 is sent by means of a distributor member 17 having two distinct outlet portions 19, 21 to a first and a second molecular sieve, 15, 150, respectively, to a heat generator 16, for example a resistor, and subsequently to the dehumidifying device 1. The first and second molecular sieves 15, 150 are preferably based on silicates, for example aluminium silicates, or alkali metal or alkaline-earth metal silicates, or other materials having excellent adsorbent properties for adsorbing air moisture.

The molecular sieves can absorb moisture below a specific temperature, approximately 10O⁰C, and they release the moisture absorbed if they are heated to a temperature of usually from 15O⁰C to 25O⁰C. The molecular sieves have a limited capacity to absorb water, beyond which they become saturated, and it is therefore necessary to subject them periodically to regeneration in order to eliminate the previously absorbed moisture therefrom. In order to permit the regeneration of the molecular sieves and, at the same time, to avoid stopping the dehumidifying system 103, two molecular sieves 15, 150 are provided which are used alternately, that is to say, one sieve is traversed by a current of air to be dried and to be sent subsequently to the dehumidifying device 1, and the other is regenerated, that is to say, traversed by a current of hot air which absorbs and carries away with it the moisture previously accumulated in the sieves. Provided upstream of each sieve 15, 150 is a respective heating member 14, 140 for heating the air before it enters the first and the second sieve 15, 150, respectively. When a sieve is in the use phase, the corresponding heating member is turned off and the air is not heated between the blower 106 and the sieve in use. In contrast, when a sieve is in the regeneration phase, the corresponding heating member is turned on and the air is heated before being fed to the sieve which is being regenerated, in order to promote the regeneration thereof. At predetermined time intervals, or in accordance with the flow of air, and/or the humidity thereof, and/or the humidity of the pellets, the sieve to be used in the process and the sieve to be subjected to regeneration, respectively, are changed round. The air-treatment system 104 further comprises a diverting member 30 which is connected by means of respective inlet portions to the first molecular sieve 15 and to the second sieve 150, and by way of an outlet 24 to the heat generator 16.

The diverter 30 further comprises a discharge portion 25 for discharging the air used for the regeneration of one of the sieves, as indicated by the arrow F4.

Provided on the delivery tube 9 and on the return tube 11 are respective closure valves 55, 54 which are arranged to be closed, after heating with hot, dry air has been carried out, in such a manner as to render the container body 2 hermetic. The operation of the air-treatment system 104 is the following. When the first sieve 15 is used to dehumidify the air to be sent to the dehumidifying device 1, and the second sieve 150 is in the regeneration phase, the air coming out of the first outlet portion 19 of the distributor member 17 flows in sequence through the first heating member 14, which is turned off, the first sieve 15, which absorbs the moisture thereof, the diverting member 30 and the heat generator 16 and thus to the dehumidifying device 1.

The air coming out of the second outlet portion 21 flows in sequence through the second heating member 140, which is turned on and heats it to a temperature suitable for regeneration, the second sieve 150, which absorbs the moisture thereof, and the diverting member 30, which discharges it into the surroundings, or into collection tanks provided, through the discharge portion 25. On leaving the dehumidifying device 1, the air is sent to the air-filtration system 105 comprising a first filter 26, a heat exchanger 27 and a second filter 28 arranged in sequence- Each filter 26, 28 is provided with a control system 29 comprising a system for cleaning the filter which periodically cleanses the filter of dust, and with a control system for controlling any blockages of the filter. The first and second filters 26, 28 enable any dust or other residues released into the air by the pellets to be removed in order to avoid damage to the blower 106.

Interposed between the first and second filters 26, 28 is the heat exchanger 27 which absorbs some of the heat of the air, reducing the air temperature and promoting the subsequent deposition of dust in the second filter 28 in order to protect the blower 106.

In a version which is not shown, it is possible to provide alternative methods for generating air under conditions suitable for dehumidification, such as, for example, methods which exploit the spontaneous dehumidification of air following rapid expansion after compression.

In that case, instead of the air-treatment system 104, there are provided a compressor, which generates compressed air, an expansion chamber downstream of the compressor, in which the compressed air expands, losing the moisture present, and at least one heating member for heating the air leaving the expansion chamber to the desired temperature for feeding it to the dehumidification device 1.

It is optionally possible to use mixed systems, that is to say, systems in which the air is treated both by means of molecular sieves and by means of compression/expansion. In alternative versions of the dehumidifying system 103, it is possible to provide different systems for heating the plastics material, which are positioned upstream of the dehumidifying device 1, that is to say, in order to heat the plastics material before it is introduced into the container body 2, or inside the container body 2 itself. The dehumidifying system 103 further comprises control members which are not shown in the drawings and which are arranged to control some desired process variables, for example the temperature of the pellets leaving the dehumidifying device 1, the temperature of the suction air 13 of the blower 106, the temperature of the air entering the dehumidifying device 1, entering the diverter 30, and entering and leaving the sieves 15, 150, the degree of vacuum in the space 5 and/or in the container body 2, the opening/closing of the inlet valve 50, the outlet valve 51, the connecting valve 52 and the by-pass valve 53, if provided. The control members regulate, for example, the opening/closing of the valve 8 and of the second valve 80, of the connecting valve 52, of the bypass valve 53, the inlet valve 50 and the outlet valve 51, the closure valves 54 and 55, the operation of the pump 7 and/or the second pump 70, in order consequently to regulate the degree of vacuum in the space 5 and in the container body 2, respectively. By means of suitable control members it is, in addition, possible to control the operation of the means for heating the plastics material. Each control member can also be associated with an alarm device suitable for generating an alarm signal if the value of a quantity controlled by a control member strays from a desired value. The control members are operatively connected to a control unit which controls the entire operation of the dehumidifying system 103. In particular, the control unit, in accordance with the information detected by the control members, operates respective heating members in order to vary the heating intensity and/or to turn on/turn off one or more of the heating members provided in the dehumidifying system 103, in order to bring to or maintain at the desired values the temperature of the air and/or of the plastics material in the dehumidifying system 103.

In operation, a specific amount of plastics material is first charged into the container body 2 by means of the inlet valve 50. The plastics material may already be hot, or may be heated inside the container body 2. In the latter case, hot, dry air is fed in to heat the plastics material to a desired temperature of preferably from 4O⁰C to 200⁰C, depending on the type of plastics material being treated. At the same time, the vacuum pump 7 is operated in order to generate

5 inside the space 5 a specific reduced pressure for limiting the thermal dispersion of the container body. Preferably, the reduced pressure in the space 5 is forced, during this phase, to higher levels.

Subsequently, the blowing of air into the container body is interrupted and the second vacuum pump 70 is operated to generate a specific degree ofo vacuum also inside the container body 2 and to carry out the dehumidifying process.

The inlet valve 50 and the outlet valve 51 are closed in order to close hermetically with respect to the outside the container body 2 and the space 5. s If the heating of the plastics material is carried out with the blowing-in of hot, dry air, the closure valves 55, 54 provided in the delivery tube 9 and the outlet tube 11 for air, respectively, are also closed in order to close hermetically with respect to the outside the container body 2 and the space 5. o The action of the vacuum, as described above, is discontinuous, that is to say, in the container body 2, phases of increasing the vacuum to up to - 1000 mbar alternate with phases of decreasing the vacuum by opening the second vacuum-breaking valve 80, in order to allow fluid to pass into the container body 2. 5 The increase/decrease phases of the vacuum in the container body 2 continue for a period of time which depends on the type of plastics material being treated and on the level of dehumidification required. When the plastics material has reached the desired degree of humidity, that is to say, when it has been treated for a specific time interval, it is expelled from the container body 2 by means of the outlet valve 51 and another treatment cycle can be started. If the plastics material in the container body 2 is heated with hot air, the phase of generating the vacuum in the container body 2 is not contemporaneous with the heating phase. It is optionally possible to provide a plurality of alternating heating and vacuum phases, for example when the plastics material is sensitive to excessively rapid temperature increases.

In order to improve the process, it is optionally possible to blow into the container body hot, dry air (that is to say, having a dew-point of from -10⁰C to -7O⁰C) which also has a dehumidifying action. If heating agents other than hot air, such as IR radiation or microwaves, are used, it is possible to effect the heating of the plastics material inside the container body 2 and also simultaneously to heat the plastics material and generate the vacuum in the container body 2, as will be explained in more detail hereinafter. The heating phase and the real dehumidifying phase can therefore be carried out simultaneously.

In a preferred version, as will be explained in more detail hereinafter, a plurality of heating phases alternating with phases in which the vacuum is generated, are provided for. As mentioned, the vacuum is generated in a discontinuous manner, that is to say, phases of increasing the degree of vacuum alternate with phases of reducing the degree of vacuum with the entry into the container body 2 of fluid which transports the moisture previously stripped from the plastics material owing to the effect of the vacuum. In an alternative version of the process, it is possible to provide for the charging into the container body 2 of already-hot plastics material which has been heated beforehand by suitable heating devices capable of bringing it to an optimum temperature for triggering the dehumidifying process, generally from 40^OC to 200⁰C. The plastics material can be heated by conduction, for example by means of resistors, and/or irradiation, for example infrared rays, and/or friction, for example by rubbing with a metal, and/or dielectric vibration, by means of microwaves. The dehumidifying device 1 can be used for the treatment of various types of plastics material, in particular hygroscopic plastics materials, such as, for example, ABS, PC, PET, PA, TPU, etc. ...

The dehumidifying device 1 may, in addition, be used in dehumidifying systems which use drying agents other than dry air, or in addition to dry air, for example in dehumidifying systems under vacuum, with infrared rays, with microwaves, etc. ... or also in mixed dehumidifying systems, that is to say, systems which use any desired combination of dehumidifying agents. The dehumidifying device 1 can be used, in addition, in the crystallisation of PET. The crystallisation process, which consists in modifying the structure of the material from amorphous to crystalline, is carried out by bringing the materia), while agitating, to a temperature of approximately 120-140⁰C for a period of time which can be varied in accordance with the type of heating used, generally approximately 60 minutes with hot air and approximately 5- 10 minutes with infrared rays. In that application, the dehumidifying device of the invention can therefore act as a container for the material being treated, in which the presence of the space and the vacuum inside it insulate the container from the outside, conferring the advantages of energy-saving explained above. The high temperatures (180-230⁰C) and the stripping action of the vacuum can produce other effects in the PET in addition to dehumidification, that is to say, regrading, increase in intrinsic viscosity and mechanical super-cleaning. Regrading and increase in viscosity are aimed at rendering the PET (irrespective of whether this is virgin, amorphous or post-consumption) more workable by the transforming machines, optimising the relationship between the amount of material used and the mechanical performance of the finished product.

PET having a low viscosity (0.5-0.6 dl/gr - so-called "fibre grade") tends to resemble "honey" and therefore to have a limited degree of workability. On the other hand, PET having a high viscosity (0.80-0.85 dl/gr - so-called "bottle grade") tends to resemble chewing gum and therefore to have a high degree of workability.

A clear effect of the improvement in the workability of PET has been noticed in recent years in bottles for mineral water which, for the same performance, have a lighter weight. Since the dehumidifying device according to the invention is able to keep PET both in a vacuum and at high temperatures, it is able to bring about the SSP (Solid State Polycondensation) reaction, in order words, it permits the regrading of PET. As regards mechanical super-cleaning, on the other hand, this consists in a process of treating PET which, starting from washed and ground post- consumption PET, makes it possible to obtain raw material again compatible with food, so-called "food grade" PET.

Specifically, mechanical super-cleaning treatment brings about a decontamination of recycled PET, using high temperatures (200⁰C) and vacuum, for a period of time ranging from 2 to 4 hours; at the end of the treatment, poisonous substances, such as, for example, chloroform, toluene, benzophenone, lindane, etc. ... are reduced to a sufficiently low level no longer to constitute a danger to humans. Since the dehumidifying device according to the invention can keep PET both in a vacuum and at high temperatures, it figures among the apparatuses that are able to effect mechanical super-cleaning of post- consumption PET.

In other words, the residue of any poisons is reduced to the point at which it is no longer able to migrate into the food with which the plastics material would come into contact. The effectiveness of the mechanical super- cleaning of an apparatus is demonstrated by so-called challenge tests: a sample of raw material which is suitably contaminated is treated with high temperatures and vacuum and then examined again to check the reduction in the contaminants and to check possible food compatibility. The compatibility criteria vary from country to country but currently have as their point of reference some control parameters outlined by the FDA (Food and Drug Administration).

Since the dehumidifying device 1 according to the invention can keep PET both in a vacuum, relative pressure up to -1000 mbar, and at high temperatures, 180°C-230°C, it figures among the apparatuses that are able to effect mechanical super-cleaning of post-consumption PET. Therefore, with the dehumidifying device 1 according to the invention, it is possible, in a simple and economical manner, to treat recycled PET in such a manner as to obtain PET which can be re-used for food containers. In any case, the dehumidifying device according to the invention permits an improvement in the performance and energy efficiency and a reduction in the costs of the dehumidifying system in which it is inserted. The dehumidifying device 1 according to the invention also makes it possible to reduce the dehumidifying times to a considerable extent and to maintain the dehumidifying temperature at a substantially constant level, with reduced supplies of energy, and therefore with greater efficiency. Figure 4 shows a variant of a dehumidifying device 1' according to the invention in which parts corresponding to the version described previously are indicated with similar numerical references. The container body 2' of the device 1' is delimited by a wall 3' provided with holes which have dimensions based on the dimensions of the pellets to be dehumidified and which are variously spaced over the wall 3'. The wall 3' may optionally be in the form of a net having meshes of a size of from approximately 0.1 mm to approximately 3 mm. In that case, the inside of the container body 2' is in communication with the space 5' and, therefore, almost the same degree of vacuum as that generated in the space 5' is produced there.

In that configuration, it is preferable to use the actual vacuum generated by the pump 7 as the dehumidifying agent; the container body 2' and the space 5' are almost at the same degree of vacuum since they are in fluid communication.

Also in that configuration, the vacuum is used to dry the plasties material as well as to generate the vacuum in the space 5' in order to insulate the container body 2' from the outside. The device 1' is provided with vacuum-regulating means, that is to say, the vacuum-breaking valve 8 associated with the vacuum pump 7 and arranged to regulate the degree of vacuum inside the space 5' and, therefore, inside the container body 2'. Thus, the operating temperature and the heat losses of the device 1' are regulated in a simple manner. In that version too, provision is made to alternate phases of increasing the vacuum (lowering the pressure) with phases of decreasing the vacuum (increasing the pressure) in order to maximise dehumidification efficiency. Optionally, as a further drying agent in addition to the vacuum, it is possible to provide for the irradiation or heating of the plastics material contained in the container body 2' with an irradiating or heating device 33 provided upstream of the dehumidifying device 1', for example using infrared radiation or microwaves, in order further to improve the dehumidifying process. Optionally, the heating/irradiation can be carried out during dehumidification, optionally alternating heating phases with phases of increasing the vacuum.

Figure 5 shows a further variant of a dehumidifying device 1" according to the invention, in which parts corresponding to the version previously described are indicated with similar numerical references. Provided inside the container body 2" is an irradiation device 34 for irradiating the plastics material contained in the container body 2" with infrared radiation, in order to bring about rapid heating of the plastics material and the elimination of the moisture present. In a preferred version, the irradiation device 34 comprises an irradiation device which emits infrared radiation, such as an infrared lamp.

The presence of the vacuum in the container body 2" also increases the operating efficiency of the irradiation device 34, which has a greater efficiency operating under vacuum. The infrared radiation increases efficiency under conditions of vacuum by up to 40% compared with transmission in air because there are substantially no obstacles to its propagation.

In addition, infrared lamps lend themselves to operation in discontinuous mode, that is to say, with phases of being turned on alternating with phases of being turned off. That enables a considerable energy saving to be achieved in the heating operation. Experiments carried out have shown that, by performing the dehumidification of plastics material with an alternation of phases of irradiation with the infrared lamp and, therefore of heating the plastics material, with phases of the infrared lamp being turned off, the same heating is obtained per unit of time as would be obtained by operating with continuous emission, but clearly using a lesser amount of energy. As a matter of fact plastics is an insulating material which tends to resist heating and therefore an increase in energy supplied by the infrared lamp, or similar irradiation device, does not correspond to a proportional increase in temperature per unit of time.

In the space 5" and in the container body 2" of the dehumidifying device 1", the pump 7 generates a desired degree of vacuum for thermally insulating the dehumidifying device 1" from the outside and for carrying out the dehumidifying process. The by-pass valve 53 and the connecting valve 52 enable the degree of vacuum in the container body 2" and in the space 5" to be regulated. In an alternative version of the dehumidifying device (not shown), with infrared heating, it is possible to provide two separate vacuum pumps connected by means of respective valves to the container body 2" and to the space 5". Also in that version, it is provided that phases of increasing the vacuum (reducing the pressure) alternate with phases of decreasing the vacuum (increasing the pressure) in order to maximise dehumidification efficiency. In another variant (not shown), the wall 3" may be provided with holes so that dehumidification takes place as a result of the combined action of the radiation and the vacuum.

In a further version, it is possible to provide an irradiation device which is arranged to generate microwaves for irradiating the plastics material contained in the container body. In that case too, a desired degree of vacuum is produced in a space positioned outside a wall of the container body in order to thermally insulate the container body from the outside, it being possible to regulate the degree of vacuum by means of suitable regulating means acting on the vacuum pump. The wall of the container body may be provided with holes in order to bring the container body into communication with the space, or it may be in a form such as to seal the space with respect to the inside of the container body.

If the heating is carried out with IR radiation and/or microwaves, the heating may be contemporaneous with the phase of stripping with a vacuum; optionally, phases of heating and phases of increasing the vacuum, which alternate with each other, are provided for. Figure 6 shows a further variant of a dehumidifying device 200 according to the invention, in which parts corresponding to the version described previously are indicated with similar numerical references and are not explained in detail. Provided inside the dehumidifying device 200 is a heating device 201 which is arranged to heat the plastics material in the container body 2 by friction. The heating device 201 comprises a shaft 202 which is operated by means of a motor 203 and on which a plurality of discs 204 driven in rotation by the motor 203 are positioned. The motor is caused to operate at a suitable rate, for example 200 rpm (revolutions per minute). The discs 204 rub the plastics material inside the container body 2 and heat it by friction. In that variant too, the heating phase may take place at the same time as the phase in which the desired degree of vacuum is generated in the container body 2. It is also possible to produce dehumidifying devices which are provided with an external space, in which the vacuum for insulating the devices from the outside is generated, and in which devices dehumidification takes place by means of drying agents other than those mentioned above, or by means of a combination of known drying agents. A dehumidifying device 1 according to the invention may be used as a container for storing plastics materials, for example in cases where it is desired to maintain the plastics materials at a specific temperature, before the transformation process.

Because heat losses to the outside are negligible, it is possible to dehumidify or, in general, to pre-treat the plastics materials, and then to store them in the device 1 to await a transformation process which can be carried out even after a considerable time interval without having to re-heat the plastics materials.

Therefore, with the dehumidifying device 1 according to the invention, it is not necessary to work the plastics material in the transforming machines immediately after dehumidification.

That enables the energy consumption to be reduced and pre-treatments and transformation processes to be managed independently, it not being necessary for the transformation process to be carried out immediately after the pre-treatments in order to avoid wasting energy. In addition, the invention can be extended to other application sectors in which it is necessary to dehumidify material, in particular hygroscopic materials, and/or in cases where moisture may produce negative effects in the subsequent treatment processes, for example foodstuffs, building materials, etc.

Claims

1. Dehumidifying device (1; 1'; 1") for plastics materials, comprising a container body (2; 2'; 2") which is suitable for receiving a desired amount of plastics material and which is externally delimited by a wall (3; 3'; 3"), a sheathing wall (4; 4'; 4") which is suitable for at least partially surrounding said wall (3; 3'; 3") and which is positioned in such a manner that a space (5; 5'; 5") is defined between said sheathing wall (4; 4'; 4") and said wall (3; 3'; 3"), wherein said space (5; 5'; 5") is hermetically closed, said device comprising vacuum- regulating means (8; 8, 80) interposed between said vacuum- producing means (7) and said container body (2; 2'; 2") and said space (5; 5'; 5"), respectively, in order to regulate said degree of vacuum in said container body (2; 2'; 2") and in said space (5; 5'; 5") in order consequently to vary the dehumidification of said plastics material and the thermal insulation of said container body (2; 2'; 2").

2. Device according to the preceding claim, wherein said vacuum- producing means (7; 7, 70) is arranged for producing a reduced pressure of approximately from -900 to -980 mbar, preferably up to approximately -1000 mbar, in said space (5; 5'; 5").

3. Device according to any one of the preceding claims, wherein said vacuum-producing means (7; 7, 70) comprises a vacuum pump (7) suitable for generating the vacuum both in said space (5; 5'; 5") and in said container body (2; 2'; 2").

4. Device according to any one of the preceding claims, wherein said vacuum-producing means comprises a first vacuum pump (7) suitable for generating the vacuum in said space (5; 5'; 5") and a second vacuum pump (70) suitable for generating the vacuum in said container body (2; 2'; 2"), said first (7) and said second vacuum pump (70) being operable independently of each other.

5. Device according to any one of the preceding claims, and further comprising control members for controlling the operation of said vacuum-regulating means (8; 8, 80) in a mutually independent manner in order to generate in said space (5; 5'; 5") and in said container body (2; 2'; 2") a mutually independent degree of vacuum.

6. Device according to any one of the preceding claims, and further comprising control members for jointly controlling the operation of said vacuum-regulating means (8; 8, 80).

7. Device according to any one of the preceding claims, and further comprising connecting means (52) for bringing said container body (2; 2'; 2") and said space (5; 5'; 5") into fluid communication.

8. Device according to any one of the preceding claims, and further comprising operating means for alternately operating said vacuum- producing means (7; 7, 70) and said vacuum-regulating means (8, 80) in such a manner as to alternate in said container body (2; 2'; 2") and in said space (5; 5'; 5") phases of increasing the degree of vacuum and phases of decreasing the degree of vacuum.

9. Device according to any one of the preceding claims, and further comprising heating means (9, 10, 106; 7; 34) operatively associated with said container body (2; 2'; 2") in order to heat said plastics material in said container body (2; 2'; 2").

10. Device according to the preceding claim, wherein said heating means (9, 10, 106; 7; 34) comprises a device for generating infrared radiation or microwaves, or a mechanical heating device (201, 202, 203, 204).

11. Device according to the preceding claim, and further comprising regulating means for alternately operating said vacuum-producing means (7; 7, 70) and/or said vacuum-regulating means (8, 80) and said heating means (9, 10, 106; 7; 34) in such a manner as to alternate in said container body (2; 2'; 2") phases of heating said plastics material with phases of increasing/decreasing the degree of vacuum.

12. Device according to any one of the preceding claims, and further comprising drying means (9, 10, 106; 7; 34) which is operatively associated with said container body (2; 2'; 2") in order to subject said plastics material in said container body (2; 2'; 2") to at least one drying agent for drying said plastics material.

13. Device according to the preceding claim, wherein said drying means comprises a feed device (9, 10, 106) for feeding a drying fluid to the inside of said container body (2).

14. Device according to the preceding claim, wherein said feed device comprises a blower (106) for feeding said fluid into said container body

(2) by way of a duct (9) leading into said container body (2) and including a diffusing member (10) configured in such a manner as to promote mixing of said air with said plastics material inside said container body (2).

15. Device according to claim 13, or 14, wherein said fluid is air which is fed into said container body (2) at a temperature of from approximately 4O⁰C to approximately 200⁰C with a dew-point value of approximately from -10⁰C to -7O⁰C, in order to heat and dehumidify said plastics material.

16. Device according to any one of claims 12 to 15, wherein said drying means comprises radiation-generating means (34) for supplying radiation having a desired wavelength to said container body (2") in order to dry said plastics material.

17. Device according to the preceding claim, wherein said generating means (34) comprises at least one generator of infrared radiation.

18. Device according to claim 16, or 17, wherein said generating means comprises at least one microwave generator.

19. Device according to any one of claims 16 to 18, and further comprising operating means for operating said radiation-generating means (34) which is suitable for operating said radiation-generating means (34) in a discontinuous manner.

20. Device according to any one of the preceding claims, wherein said wall (3; 3") is configured in such a manner as to isolate said container body (2; 2") in a sealed manner with respect to said space (5; 5").

21. Device according to any one of claims 1 to 20, wherein said wall (3') is provided with holes in order to connect said space (5') to said container body (2') in such a manner as to produce in said container body (2') almost the same degree of vacuum as in said space (5').

22. Device according to the preceding claim, wherein said wall (3') is in the form of a net having meshes of a size of from approximately 0.1 mm to approximately 3 mm.

23. Device according to any one of the preceding claims, wherein said plastics material is PET (amorphous or recycled).

24. Method for treating PET, comprising treating the PET in a dehumidifying device according to any one of claims 1 to 22 in such a manner as to produce PET which is more workable at the transforming machines.

25. Method for treating recycled PET, comprising charging the recycled PET into a dehumidifying device according to any one of claims 1 to 22, and operating said device for a period of time sufficient to produce PET in so-called food-grade, that is to say, which can be re-used to produce containers for food.

26. Dehumidifying system (103) for dehumidifying plastics materials, comprising a dehumidifying device (1; 1'; 1") according to any one of the preceding claims.

27. Plant for treating plastics materials, comprising a device according to any one of claims 1 to 22.

28. Plant for treating plastics materials, comprising a dehumidifying system for plastics materials according to claim 26.