DESCRIPTION

The present invention relates to a drying plant for granular polymer material and a relevant drying process.

The present invention is used particularly, though not exclusively, in industrial processes for transforming granular plastic materials by means of extrusion or moulding.

These operations are generally carried out in a transformation machine, in which the polymer material is brought to the molten or semi-molten state before being introduced into a mould or being extruded.

In order to ensure an adequate level of quality of the final product, however, it is necessary for the polymer material which is supplied to the transformation machine to have to be free from humidity to the greatest possible extent.

Furthermore, this requirement is difficult to reconcile with the high levels of hygroscopic properties of some plastic materials commonly used, such as, for example, the ones based on polyethylene terephthalate (PET) or polyamide (PA) or polycarbonate (PC) or some copolymers, such as ABS (acrylonitrile butadiene styrene).

Mainly for this purpose, but also in order to reduce the quantity of energy necessary to bring the polymer material to the molten or semi-molten state inside the transformation machine, it is known to process the granules beforehand in suitable drying plants, where the water content of the granules is reduced to the minimum quantities required by the transformation process.

An example of a known drying plant provides for the granular polymer material to be processed inside a drying hopper which is positioned immediately upstream of the transformation machine, in which there is introduced a continuous flow of a hot process gas, where applicable with a low content of humidity, which provides for desorbing the water from the polymer material.

The process gas, which is typically air, is introduced into the drying hopper through a suitable supply circuit where the process gas is brought to the most suitable conditions for efficiently drying the polymer material.

In particular, the supply circuit may comprise a heating unit for bringing the process gas to a desired temperature and, where applicable, a dehumidification unit for reducing the humidity content of the gas to a predefined value.

The process gas is introduced into drying hoppers at a predefined flow rate, urged along the supply circuit by a movement unit and controlled by a flow rate adjustment unit.

Furthermore, in quite a common embodiment, the process gas introduced into the hopper is recovered at the discharge and recirculated after being filtered.

The Applicant has observed that the known drying processes may be poorly suitable if the granular polymer material to be dried, when it is brought to the temperatures necessary for obtaining the required drying level, tends to become agglomerated, forming polymer blocks with great dimensions and also forming, in the worst cases, a single polymer block inside the hopper.

This event is highly undesirable because it involves different and serious disadvantages in the drying process, including an incorrect discharge of the material (or even the block thereof) and an insufficient and non-homogeneous drying of the material.

In fact, this brings about an extended blockage of production which is caused by the need to intervene, generally manually, in order to empty the hopper which contains the polymer material which is consolidated into blocks.

In particular, the Applicant has observed that this phenomenon can also occur in the case of polyethylene terephthalate (PET) when an adequate degree of crystallinity, which is generally variable from polymer to polymer and which in the case of PET has to be equal to or greater than 50%, is not present.

The Applicant has further observed that sometimes the PET to be dried has a degree of crystallinity less than this value, particularly when the PET originates, completely or at a relevant fraction, from recycled material.

The Applicant has verified that in these cases there is generally provision for subjecting the granular material with a low degree of crystallinity to a crystallization process (otherwise referred to as re-gradation) before being dried. The crystallization process provides for bringing the granular material to a suitable temperature (in the case of PET, for example, to approximately from 135°C to 140°C) and maintaining it at this temperature for a suitable time in a state of constant agitation, which is obtained by means of fluidized bed techniques, or by using mixers with blades so as to prevent the granules of polymer material from becoming sticking to each other.

The statements set out above indicate that, in order to dry a granular polymer material with a low degree of crystallinity, it is necessary to subject this material beforehand to a crystallization treatment which involves, however, the acquisition and installation of a dedicated plant and the connection thereof to the downstream drying plant, with possible problems of operational integration between the two plants and with an increase in processing costs and times.

Furthermore, the Applicant has observed that the composition of the granular polymer material fed to the drying process (in the case of PET, for example, the fraction of recycled material) may often vary and sometimes in an unexpected manner so that it is not always possible to understand in good time whether a particular batch of material does or does not have to be subjected to crystallization. Furthermore, the Applicant has observed that sometimes the degree of crystallinity of the granular polymer material is near the minimum threshold provided so that, in the case of unforeseen extensions of the drying processing operation (for example, as a result of the slowing down or stopping of the transformation machine which is positioned downstream), it is possible that the undesirable packing phenomena of the above-mentioned granules occur.

Therefore, the Applicant has perceived the need to provide a drying plant and process which are capable of processing granular also polymer material with a not always sufficiently high degree of crystallinity and in case a polymer material with a variable composition, but without using a crystallization unit.

In the present description and the appended claims, the term "granular material" is intended to be understood to be a plurality of solid elements which are different and separate from each other and which have suitable dimensions and configurations in accordance with the processing to be carried out and the polymer material used, including polymer material in powdered form or in flakes.

Furthermore, the term "drying" is intended to be understood to be the process, as a result of which the humidity content of the granular polymer material is reduced to the values required by the subsequent trasnformation process (moulding or extrusion). In accordance with the type of polymer material being processed, the water to be removed may be mainly the water present on the external surface of the granules or may also be the water present, where applicable, in the internal regions of the granules, for example, in the case of PET. By way of reference, the maximum residual humidity value present in the granular material being discharged from the drying process, as required by the transformation machine, may be approximately from 50 to 100 ppm (parts per million).

The term "fresh" granular polymer material is intended to be understood to be granular polymer material which is not dried, for example, material under conditions of temperature and humidity in balance with the environment.

The problem addressed by the present invention is to provide a drying plant and a drying process for granular polymer material which is structurally and functionally configured to comply with the requirement set out above by at least partially overcoming the disadvantages set out above with reference to the cited prior art.

In particular, an object of the present invention is to provide a drying plant and a drying process which allow granular polymer material to be readily processed both with a sufficiently high degree of crystallinity and with a relatively low degree of crystallinity without introducing costly and bulky processing units.

Furthermore, another object of the invention is to provide a drying plant and a drying process which can also be readily carried out in existing drying plants by means of simple modifications to plants.

The problem and the objects indicated above are solved and achieved by the present invention by means of a drying plant and a drying process comprising one or more of the features set out in the appended claims.

In a first aspect thereof, the present invention is directed towards a drying plant for granular polymer material, comprising a drying hopper, in which the granular polymer material is dried. Preferably, this plant comprises a feeding device which is provided to charge granular polymer material into the drying hopper.

Preferably, the granular polymer material which is charged in the drying hopper is fresh granular polymer material.

Preferably, this plant comprises a heating circuit which is provided to introduce a gas flow into the drying hopper.

Preferably, this gas flow has a predefined temperature suitable for heating the granular polymer material.

Preferably, this gas has a predefined humidity level in order to dry the granular polymer material.

Preferably, this plant comprises a recirculation line for the granular polymer material, which is provided to take from the drying hopper a portion of the granular polymer material and to re-introduce it into the drying hopper.

In a second aspect thereof, the present invention is directed towards a drying process for drying granular polymer material.

Preferably, this process comprises the step of charging granular polymer material inside a drying hopper.

Preferably, this process comprises the step of introducing a gas flow inside the drying hopper.

Preferably, the gas has a predefined temperature so as to heat the granular polymer material.

Preferably, the gas has a predefined humidity level so as to dry the granular polymer material.

Preferably, this process comprises the step of keeping the granular polymer material inside the drying hopper in contact with the gas flow for a predetermined mean residence time.

Preferably, this process comprises the step of discharging a portion of the granular polymer material which is contained inside the drying hopper and re-introducing it inside the drying hopper so as to recirculate the granular polymer material in the drying hopper.

As a result of the features set out above, with the drying plant and process of the present invention, the polymer material is continuously moved inside the drying hopper, causing it to flow continuously between the inlet and the outlet of the drying hopper.

It is thereby possible to ensure the movement of the granular polymer material inside the drying hopper in a manner that is not dependent on the material required by the transformation machine that is positioned downstream of the drying hopper.

In particular, the granular polymer material can be continuously discharged and recharged inside the drying hopper even when the downstream transformation machine does not require material or requires little material, for example, because it is stopped for maintenance.

It is thereby possible to keep the granular polymer material moving inside the drying hopper without providing any crystallization units which are positioned upstream of the drying hopper and without providing into the drying hopper a mechanical agitation device, such as, for example, a bladed agitator.

Not only this, it may be noted that the same recirculation velocity of the granular polymer material can advantageously be modified so as to conform to the characteristics of the polymer material being processed.

Furthermore, the recirculation mechanism of the granular polymer material can readily be stopped when it is not necessary, for example, when the material to be processed in the hopper has a degree of crystallinity which is sufficiently high for the temperatures and the mean residence times inside the drying hopper.

All these advantageous features allow the plant of the invention to adequately dry a granular polymer material inside the drying hopper even when the degree of crystallinity of the granular polymer material is relatively low and would therefore tend to become sticked, forming blocks and masses and/or when the material has to be kept in the drying hopper for a much longer time than the intended time.

In a third aspect thereof, the present invention is directed towards a transformation plant for granular polymer material.

Preferably, the transformation plant comprises a drying plant for the granular polymer material which is constructed according to the first aspect.

Preferably, the transformation plant comprises a moulding or extrusion unit which is positioned downstream of the drying plant.

Preferably, the moulding or extrusion unit is provided to receive the dried granular material which has been dried in the drying plant.

In one or more of the aspects described above, the present invention may further have at least one of the preferred features described below.

In some embodiments, the recirculation line extends from a discharge pipe of the drying hopper to the feeding device.

Preferably, the discharge pipe is connected to the recirculation line by means of a shut-off valve.

In one embodiment, the opening of the shut-off valve is adjustable.

It is thereby possible to vary the flow of granular polymer material which has been re-introduced into the drying hopper by means of the recirculation line. In alternative embodiments, the flow rate of the granular polymer material is varied by means of suitable adjustment devices which are inserted in the recirculation line.

In some embodiments, the feeding device comprises a feeding hopper. Preferably, the feeding hopper is positioned immediately above the drying hopper so that the granular polymer material is supplied to the drying hopper by falling.

Preferably, the feeding device comprises a pneumatic transport line which brings the granular polymer material up to the feeding hopper.

Preferably, the granular polymer material which is transferred to the feeding hopper by the pneumatic transport line is taken from a storage container of the granular polymer material.

More preferably, the granular polymer material which is taken from the storage container and which is brought to the feeding hopper is fresh polymer material.

Preferably, the granular polymer material polyethylene terephthalate (PET) based material.

In some embodiments, the PET has a degree of crystallinity less than 50%.

In some embodiments, the PET contains a fraction of recycled material greater than 30%.

In some embodiments, the recirculation line is connected to the feeding hopper.

In this manner, the recirculated granular polymer material is re-introduced into the drying hopper by means of the same devices which are used for charging the fresh granular polymer material without any need to provide additional containers, filters or other devices.

In some embodiments, the pneumatic transport line comprises a fan which is positioned downstream of the feeding hopper. In this manner, the pneumatic transport is brought about by the granular polymer material being drawn in.

Furthermore, the same fan can be used for transporting the fresh granular polymer material and also for recirculating the granular polymer material.

In some embodiments, upstream of the feeding hopper there is provided a diverter valve which is provided to place the feeding hopper and the fan alternatively in communication with the storage container or the recirculation line.

The features and advantages of the invention will be better appreciated from the detailed description of a preferred embodiment thereof, which is illustrated by way of non-limiting example with reference to the appended drawing, in which the single Figure 1 is a schematic view of a transformation plant for granular polymer material, comprising a drying plant for granular polymer material which is constructed according to the present invention.

In Figure 1, there is generally designated 200 a transformation plant for a granular polymer material 2 comprising a drying plant 1 constructed according to the present invention.

The transformation plant 200 is provided to transform any polymer material into granules, for example, polyamide, polycarbonate or ABS copolymer even if, in the specific example described here, the material processed is formed by PET granules (polyethylene terephthalate).

In particular, the PET may comprise a high fraction of recycled material and may therefore have a relatively low degree of crystallinity, for example, less than 50%, no crystallization processing operation being previously provided.

The transformation n plant 200 further comprises a transformation machine 100 for the granular polymer material which is provided immediately downstream of the drying plant 1 and which is provided to receive the granular material coming from the drying plant 1.

The transformation machine 100 for the granular polymer material comprises, in the specific example, an extruder 101, in which the dried granular material is brought to the molten state and moved forward by means of a screw 104, an injection chamber 105 which is positioned downstream of the extruder 101 and in which there is collected the molten polymer material which is discharged from the extruder 101 and a mould 102, in which the molten polymer material which is contained in the injection chamber 105 is urged by means of a piston 106.

In the present embodiment, therefore, the transformation machine 100 is a moulding unit but it is similarly preferred that it is an extrusion unit, providing an extrusion head in place of the mould 102.

The drying plant 1 comprises a drying hopper 10, at the top of which there is formed an inlet opening 11, through which the granular polymer material 2 to be dried is introduced into the drying hopper 10 and on the base of which there is provided a discharge pipe 12, through which the dried granular polymer material

2 is discharged from the drying hopper 10 in order to supply the transformation machine 100.

In greater detail, the passage of the dried granular polymer material 2 towards the transformation machine 100 is adjusted by a discharge valve 18 which is mounted in the discharge pipe 12.

The discharge valve 18 may be a shutter type valve or a star-like valve or any other type of valve suitable and known in the field.

The drying plant 1 further comprises a feeding device which is generally designated

3 and which is provided to charge the granular polymer material 2 in the drying hopper 10.

The feeding device 3 comprises a feeding hopper 13 which is mounted immediately upstream of the drying hopper 10 in the region of the inlet opening 11, a pneumatic transport line 4 which is provided to transport a suitable quantity of fresh granular polymer material 2a from a storage container 5 up to the feeding hopper 13.

In particular, the pneumatic transport line 4 comprises a line 8 which extends from the storage container 5 to the feeding hopper 13 and a fan 6 which is positioned downstream of the feeding hopper 13 and which is connected thereto by a line 7 which can be intercepted by a valve 7a. In this manner, the pneumatic transport of the granular polymer material is brought about by suction under reduced pressure conditions inside the lines 7 and 8.

A feeding valve 17 is provided between the feeding hopper 13 and the drying hopper 10 in order to allow the introduction of the fresh granular material 2a into the drying hopper 10 and to prevent the outflow of material or gas from the drying hopper 10 to the feeding hopper 13.

The drying plant 1 further comprises a recirculation line 14 for the granular polymer material 2 which is provided to take a portion of granular polymer material which is present in the discharge pipe 12 and to re-introduce it into the drying hopper 10 through the pneumatic transport line 4.

In particular the recirculation line 14 extends from the discharge pipe 12, in a position upstream of the discharge valve 18, as far as a diverter valve 15 which is mounted on the pneumatic transport line 4 upstream of the feeding hopper 13.

In particular, the diverter valve 15 is connected at the inlet to the recirculation line 14 coming from the discharge pipe 12 of the drying hopper 10 and to the line 8 coming from the storage container 5 and is connected at the outlet to a section 16 of the pneumatic transport line 4 which opens in the feeding hopper 13.

The diverter valve 15 is therefore movable between a first operating configuration, in which the section 16 is in communication with the line 8 and a second operating configuration, in which the section 16 is in communication with the recirculation line 14.

As a result of the action of the diverter valve 15, therefore, the fan 6 can be placed in communication with the line 8 or, alternatively, with the recirculation line 14, so that it is possible to charge in the feeding hopper 13 fresh granular polymer material 2a which comes from the storage container 5 or, in the second operating configuration, granular polymer material coming from the discharge pipe of the drying hopper 10.

In an embodiment which is not illustrated, there can be provided on the recirculation line 4 a shut-off valve or an adjustment valve which is capable of preventing or adjusting the passage of the granular polymer material.

The plant 1 further comprises a heating circuit 20, by means of which there is introduced into the drying hopper 10 a process gas having a predetermined temperature, humidity level and flow rate in order to heat and dry the granular material 2 up to the desired temperature and humidity content values.

The process gas is preferably air.

The heating circuit 20 comprises a supply line 21 which brings the process gas inside the drying hopper 10 and a recovery line 22, through which the process gas is extracted from the drying hopper 10.

The supply line 21 opens near the bottom of the drying hopper in the region of a diffusor 23 while the recovery line 22 is open near the top of the drying hopper 10 so that the process gas travels through the hopper in an upward direction in counter-current with respect to the granular material 2.

There is provided on the supply line 21 a heater 24 which is arranged for heating the process gas to a predefined heating temperature and a flow rate adjustment device 25 which is provided to adjust the flow rate of the process gas which is supplied to the drying hopper 10.

The flow rate adjustment device 25, in the embodiment described here, acts by varying the opening of one or more adjustment valves 25a, verifying the correctness of the adjustment action by means of a flow rate measuring unit 25b. Alternatively, the adjustment device 25 could act on an inverter which adjusts the rotation speed of the blades of a fan which urges the process gas along the supply line 21.

The process gas which is introduced into the drying hopper 10 by means of the supply line 21 has a particularly low humidity value which is obtained as a result of a dehumidification processing operation which is known per se and which is not illustrated in the appended Figure.

The plant 1 further comprises a control unit 30 which is provided to control and adjust the operating parameters of the drying plant 1.

The control unit 30 is also connected to the diverter valve 15 in addition to being connected to the flow rate adjustment device 25 of the process gas, the heater 24 and the discharge valve 18.

The plant 1, which is controlled by the control unit 30, operates in the manners described below.

In the drying hopper 10, the granular polymer material 2 is heated and dried by the process gas which is dispersed in the mass of material through the diffusor 23. The process gas, which is suitably dehumidified, is supplied at a predefined heating temperature which is obtained and controlled in the heater 24, for example, approximately 180°C, corresponding to the maximum temperature at which PET can be maintained under air without involving relevant oxidation phenomena. The flow rate of the process gas which is introduced into the drying hopper is adjusted by the adjustment device 25.

After a sufficient residence time, the granular polymer material 2 reaches the discharge pipe 12 at a temperature of approximately 180°C and with a reduced residual humidity content, for example, of approximately 40 ppm.

At the request of the transformation machine 100, a portion of the granular material present in the discharge pipe 12 is supplied, by suitably opening the discharge valve 18, to the extruder 101, where it is molten and, after being collected in the injection chamber 105, injected into the mould 102 by means of the piston 106.

When the level of granular material 2 inside the drying hopper 10 drops until reaching a predetermined level, a new quantity of fresh granular material 2a is introduced into the drying hopper 10.

The fresh granular material 2a is introduced into the drying hopper 10 through the feeding hopper 13 which, in turn, is charged with the fresh granular material 2a as a result of the pneumatic transport line 4.

In particular, this is brought about by opening the interception valve 7a so as to place the pneumatic transport line 4 in communication with the fan 6 and moving the diverter valve 15 into the first operating configuration so as to place the feeding hopper 13 in communication with the storage container 5.

In the first operating configuration of the diverter valve 15, the recirculation line 14 is therefore closed. Once the loading operation of the drying hopper 10 is completed, it is possible to move the diverter valve 15 into the second operating configuration so as to place the pneumatic transport line 4 in communication with the recirculation line 14.

In this manner, if the interception valve 7a is open, a portion of the granular polymer material 2 present in the discharge pipe 12 is taken by suction of the fan 6 and brought into the feeding hopper 13, from where it is re-introduced into the drying hopper 10 through the valve 17.

It may be noted that the step of recirculating the granular polymer material from the drying hopper 10 can be activated and deactivated substantially at any time of the process (with the exception of the step of charging the fresh material) simply by moving the diverter valve 15 into the second operating configuration.

In particular, the recirculation of the granular polymer material will be carried out whenever it is necessary to keep the granular material moving inside the drying hopper 10. For example, this may be necessary in the case of a stoppage of the transformation machine 100 without completely emptying the drying hopper.

Alternatively, this may be necessary in the case of fresh granular polymer material 2a with a low degree of crystallinity which, at the predefined drying temperatures, could tend to become sticked and to form undesirable blocks of granular material inside the drying hopper.

The invention thereby solves the problem proposed, further allowing additional advantages to be achieved, including the advantage of being able to readily manage granular polymer materials with a degree of crystallinity which is particularly variable, such as, for example, polymer materials with a high percentage originating from recycled material.

It will further be appreciated that the solution proposed by the present invention does not require particularly expensive installations and may readily also be able to be implemented in already existing drying plants.