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Title:
MULTI-HOPPER DRYING PLANT FOR GRANULAR POLYMERIC MATERIAL
Document Type and Number:
WIPO Patent Application WO/2021/074845
Kind Code:
A1
Abstract:
A plant for drying granular polymeric material comprises: - at least two drying hoppers (10) for drying the granular polymeric material; - a circuit (2) for feeding a process gas, which circuit is connected to the drying hoppers, for introducing a flow of gas suitable for drying the granular polymeric material into each of the drying hoppers; - a plurality of units (20) for dehumidifying the process gas, which units are inserted in the feed circuit, wherein each dehumidification unit (20) is individually connected to a corresponding drying hopper (10); and - a movement unit (5) for moving the process gas along the feed circuit towards each of the drying hoppers.

Inventors:
CATTAPAN GIANFRANCO (IT)
Application Number:
PCT/IB2020/059706
Publication Date:
April 22, 2021
Filing Date:
October 15, 2020
Export Citation:
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Assignee:
PEGASO IND S P A (IT)
International Classes:
F26B3/06; F26B17/14; F26B21/04; F26B21/08
Foreign References:
US20190030774A12019-01-31
US20190105806A12019-04-11
US20100217445A12010-08-26
EP1923649A22008-05-21
US20100205821A12010-08-19
Attorney, Agent or Firm:
SUSANETTO, Carlo et al. (IT)
Download PDF:
Claims:
CLAIMS

1. Plant for drying granular polymeric material, comprising:

- at least two drying hoppers (10) for drying said granular polymeric material, - a circuit (2) for feeding a process gas, which circuit is connected to said drying hoppers, for introducing a flow of gas suitable for drying said granular polymeric material into each of said drying hoppers,

- a plurality of units (20) for dehumidifying said process gas, which units are inserted in said feed circuit, wherein each dehumidification unit (20) is individually connected to a corresponding drying hopper

(10),

- a movement unit (5) for moving said process gas along said feed circuit towards each of said drying hoppers.

2. Drying plant according to claim 1, wherein said feed circuit (2) is a closed circuit.

3. Drying plant according to claim 1 or claim 2, wherein each of said dehumidification units (20) comprises at least one tower (21) containing desiccant material capable of reducing the humidity content present in said process gas. 4. Drying plant according to any one of the preceding claims, wherein said dehumidification unit (20) is a rotary tower dehumidification unit, and comprises:

- a process manifold (24) for connecting a first portion (24a) of said tower (21) to said feed circuit and to the inlet of said drying hopper (10) so as to dehumidify said process gas prior to entering said drying hopper,

- a regeneration manifold (25) for connecting a second portion (25a) of said tower (21) to a circuit (27) for regenerating said desiccant material, in order to regenerate said desiccant material after having dehumidified said process gas, and

- a cooling manifold (26) for connecting a third portion (26a) of said tower (21) to a cooling duct (28) so as to cool said desiccant material prior to dehumidifying said process gas,

- a device (22) for rotating said tower in order to successively put said first, second and third portions (24a, 25a, 26a) of said tower into communication with said process manifold (24), said regeneration manifold (25) and said cooling manifold (26).

5. Drying plant according to claim 4, wherein said cooling duct (28) extends between said feed circuit (2), upstream of said dehumidification unit (20), and an outlet branch (9) exiting from said drying hopper (10).

6. Drying plant according to either claim 4 or claim 5, wherein said dehumidification units (20) are served by a single regeneration circuit (27).

7. Drying plant according to claim 6, wherein a heat exchanger (34) is provided between said regeneration circuit (27) and said feed circuit (2) in order to heat the regeneration air by means of said process gas.

8. Drying plant according to claim 7, wherein said heat exchanger (34) is positioned on the feed circuit downstream of said movement unit (5).

9. Drying plant according to any one of the preceding claims, wherein a heater (30) arranged between said dehumidification unit (20) and said drying hopper (10) is associated with each of said drying hoppers (10).

Description:
Multi-hopper drying plant for granular polymeric material

DESCRIPTION

Technical field

The present invention relates to a drying plant for granular polymeric material, in particular a plant of the type comprising a plurality of drying hoppers. Technological background

It is known that, in order to guarantee an adequate quality level of the moulded product, the transformation of granular plastics materials by means of extrusion or moulding requires a very low level of humidity of the granular material.

However, this requirement is incompatible with the high hygroscopic properties of some plastics materials which are widely used in the sector, such as, for example, those which are based on polyethylene terephthalate (PET), or polyamide (PA), or polycarbonate (PC) or some copolymers such as ABS (acrylonitrile butadiene styrene).

Before being subjected to the extrusion or moulding process, these plastics materials must therefore be adequately dried in suitable drying plants, where the water content of the granules is reduced to the minimum quantities required by the transformation process. In a commonly used process, the drying of the granular polymeric material is carried out inside a hopper which contains the material to be dried, and into which a continuous flow of hot dry air is introduced.

Drying plants are also known which comprise a plurality of hoppers, each connected to one or more machines which transform the granular polymeric material dried by the hoppers, for example by means of a moulding or extrusion technique.

This solution is appreciated by the market since, inter alia, it makes it possible to control the transformation machine directly, thus being able to regulate the process of drying in the hopper according to the specific operative requirements of the transformation machine.

The Applicant has also noted that this type of multi-hopper plant makes it possible to increase the operative flexibility of the plant, by being able also to dry different polymeric materials in the different hoppers.

The Applicant has observed that in the known multi-hopper plants, all the hoppers are served by a single circuit for feeding process gas, generally air, into which circuit process gas is introduced that has been duly dehumidified by an appropriate dehumidification unit. In this plant configuration, a corresponding duct extends from the feed circuit in order to convey the quantity of process gas required to each hopper However, the Applicant has observed that, in order to dry a given polymeric material correctly, the process gas used must also have a degree of residual humidity (which in general is quantified by means of the value of its dew point) within a given range.

However, this requirement cannot be fulfilled by means of the above-described plant configuration, provided with a single dehumidification unit, which is generally regulated so as to introduce into the feed circuit a process gas which has the lowest dew point value from amongst those required by the different materials being dried.

In the present description and in the appended claims, "granular material" means a plurality of solid elements which are distinct and separate from one another, with appropriate dimensions and forms, according to the processing to be carried out and the polymeric material used, including polymeric material in powder or flake form.

In addition, the term "drying" means the process by means of which the humidity content of the granular polymeric material is reduced to the values required by the subsequent transformation process (moulding or extrusion) by means of substantial elimination of the water present in the inner regions of the granules.

By way of reference, the maximum residual humidity value required by the transformation unit can be approximately 20 - 200 ppm (parts per million).

"Desiccant material", with reference to a process gas dehumidification unit, means a material which can substantially reduce the humidity content of the process gas, as a result of contact between the desiccant material and process gas. The chemical-physical process which gives rise to this reduction of humidity can be any suitable process, such as, for example, a process of absorption or adsorption.

Disclosure of the invention

The problem addressed by the present invention is that of providing a drying plant of the multi-hopper type for granular polymeric material, which plant is structurally and functionally designed to eliminate, at least in part, one or more of the disadvantages described above with reference to the cited prior art.

This problem is solved by the present invention by means of a drying plant produced in accordance with the following claims. According to a first aspect thereof, the invention relates to a plant for drying granular polymeric material comprising at least two drying hoppers which are designed to dry the granular polymeric material.

Preferably, the plant also comprises a circuit for feeding a process gas, which circuit is connected to all the drying hoppers, in order to introduce into each of them a flow of gas which is suitable for drying the granular polymeric material. Preferably, the plant also comprises a plurality of units for dehumidification of the process gas, which units are inserted in the feed circuit, wherein each dehumidification unit is individually connected to a corresponding drying hopper. Preferably, the plant also comprises a movement unit which is designed to move the process gas along the feed circuit towards each drying hopper. Thanks to the above-described features, in the drying plant according to the present invention, each drying hopper is associated with its own unit for dehumidification of the process gas, which makes it possible to set for each drying hopper the correct dew point value required according to the type of material to be dried in the hopper, independently from the other hoppers.

Also, at the same time, the plant has a single unit for moving the process gas along the feed circuit, so as not to increase the cost of the plant and the maintenance work. In the aforementioned aspect, the present invention can have one or more of the preferred features expressed hereinafter.

According to one embodiment, the feed circuit is a closed circuit.

Preferably, all the drying hoppers are connected in parallel to the process gas feed circuit. In particular, the feed circuit comprises an delivery branch and a return branch, and each drying hopper is connected to the delivery branch and to the return branch by means of an inlet branch and an outlet branch, respectively. By this means, from time to time each drying hopper can be excluded from the feed circuit or included in the feed circuit independently from the other drying hoppers.

For this purpose, corresponding shut-off valves are provided at the inlet branches and the outlet branches.

Preferably, the movement unit comprises a blower, designed to supply a flow of process gas which is sufficient to serve all the drying hoppers of the plant According to one embodiment, the blower is controlled by a motor with a variator for the number of revolutions. By this means it is possible to regulate the flow of the process gas easily.

Preferably, the process gas is air.

According to one embodiment, each dehumidification unit comprises at least one tower containing desiccant material which can reduce the content of humidity present in the process gas. This reduction can be obtained by means of phenomena of absorption or adsorption.

By this means, the process gas, pushed by the movement unit along the feed circuit, is dehumidified before entering a drying hopper, thus taking the level of residual humidity to the desired dew point values, which are generally between -10°C and -45°C.

According to an embodiment of the invention, the dehumidification unit is of the rotor-type (hereinafter also identified as a "rotary tower").

This type of dehumidification unit generally comprises a cylindrical container (the tower) which is rotated about its own axis between a pair of fixed heads. These heads are connected to a plurality of manifolds, which are separate from one another and define respective operative portions of the tower, which portions, by rotating, move the desiccant material contained therein from one portion to another in continuous succession. Preferably, the tower comprises a support which is configured to form a plurality of channels which extend parallel to the axis of the tower and do not communicate with one another, which channels permit the passage of the process gas from one head to the other.

According to one embodiment, the support is formed by multi-layer corrugated cardboard.

Preferably, the desiccant material contained in the tower is impregnated in the support material.

Preferably, the desiccant material comprises a resin based on silica gel, glass fibre and molecular sieves. According to one embodiment, the dehumidification unit comprises a first, a second and a third manifold, each open on the heads of the rotary tower. Preferably, the dehumidification unit comprises a device for rotation of the tower, which device can put a first, a second and third portion of the tower successively into communication respectively with the first, the second and the third manifold.

Preferably, the first manifold is a process manifold, which is designed to connect a first portion of the tower to the feed circuit and to the inlet of the hopper. By this means, the dehumidified process gas to be introduced into the drying hopper in order to dry the granular polymeric material is obtained. Preferably, the second manifold is a regeneration manifold which is designed to connect a second portion of the tower to a circuit for regeneration of the desiccant material.

By this means, the desiccant material, full of the humidity extracted from the process gas at the first portion, is dehumidified in order to be able to be reused once more in the first portion.

Preferably, the third manifold is a cooling manifold which is designed to connect a third portion of the tower to a cooling duct. By this means, the regenerated desiccant material obtained from the second portion is taken to a lower temperature, so as to promote the subsequent process of reducing the humidity content of the process gas.

Preferably, the cooling duct connects the third portion of the tower both to the inlet branch of the drying hopper, upstream of the dehumidification unit, thus collecting a flow of process gas which is not yet dehumidified, and to the outlet branch of the drying hopper, into which it introduces the process gas which has passed through the third portion of the tower.

According to a preferred embodiment, the dehumidification units which are associated with the respective drying hoppers are served by a single regeneration circuit.

By this means, each rotary tower is connected individually to a single regeneration circuit which can be advantageously treated individually, since the process of regeneration of the desiccant material is not dependent on the type of granular polymeric material treated in the drying hopper.

According to one embodiment, the regeneration circuit comprises a heater which is individually associated with each dehumidification unit. By this means, each unit can be regenerated at a specific temperature, which is different from that of the other dehumidification units.

Preferably, the dew point of the process gas downstream of the dehumidification unit, or in other words, the dehumidification capacity of this unit, is regulated by varying the regeneration temperature of the desiccant material in the second portion of the tower.

In addition or as an alternative to this system, it is possible to regulate the dew point value of the process gas by providing a line for bypassing the dehumidification unit, wherein a fraction of non-dehumidified process gas is collected upstream of the dehumidification unit and is reintroduced into the dehumidified process gas downstream of the dehumidification unit, so as to obtain a dew point which is intermediate between that of the non-dehumidified process gas and that which is dehumidified.

Preferably, between the regeneration circuit and the feed circuit, a heat exchanger is provided which is designed to heat the air present in the regeneration circuit by means of the process gas, which is hotter than the regeneration air.

This produces the first advantage of preheating the regeneration air, which must be taken to high temperatures in an appropriate heater before being introduced into the second portion of the rotary tower. Preferably, the heat exchanger is positioned on the feed circuit downstream of the movement unit.

This produces the second advantage of lowering the temperature of the process gas output from the movement unit, thus promoting both the process of reducing the humidity in the first portion of the rotary tower and the process of cooling the desiccant material in the third portion of the rotary tower.

According to an alternative embodiment, the dehumidification unit comprises a pair of fixed towers filled with desiccant material, for example molecular sieves, which towers are connected operatively, so as to be connected alternately, to the drying hopper in order to dehumidify the process gas before introducing it into the hopper, and to a regeneration circuit in order to regenerate the desiccant material.

Preferably, a heater is provided between the dehumidification unit and the drying hopper. By this means, the dehumidified process gas is taken to the temperature planned for drying the granular polymeric material contained in the drying hopper, such that each drying hopper can be served by process gas at the most appropriate temperature for the polymeric material it is treating.

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

- Fig. 1 is a schematic view of a plant for drying granular polymeric material produced according to the present invention;

- Fig. 2 is an enlarged schematic view of a component of the drying plant in Fig. 1, indicated as II.

Preferred mode to carry out the invention

With reference to the appended figures, 1 indicates as a whole a plant for drying a granular polymeric material, produced in accordance with the present invention.

The plant 1 comprises a plurality of drying hoppers, all indicated as 10, which are all connected in parallel to a single feed circuit 2 designed to introduce into each drying hopper 10 a process gas, preferably air, which is appropriately heated and dehumidified in order to dry the granular polymeric material present in the drying hopper 10.

In particular, the feed circuit 2 is a closed circuit and comprises an delivery branch 3, which conveys the process gas to the drying hoppers 10, and a return branch 4, which collects the process gas output by the drying hoppers 10 and conveys it to a movement unit 5 which, after passing the gas through a filter 5a, returns it along the delivery branch 3.

In turn, the movement unit 5 includes a blower 6 rotated by a motor 6a, the number of revolutions of which is variable on account of the provision of an inverter. Each drying hopper 10 is connected to the delivery branch 3 by means of an inlet branch 7 which conveys the process gas to an open diffuser 8 inside the drying hopper. Access to the inlet branch 7 and the flow rate of the process gas along this inlet branch are regulated by a control valve 7a, which can be controlled by a flow rate sensor 7b. Similarly, each drying hopper 10 is connected to the return branch 4 by means of an outlet branch 9 which collects the process gas from the top of the drying hopper 10 and conveys it again to the return branch 4 through a valve 9a.

The valves 7a and 9a also act as shut-off valves which, when closed, can exclude the drying hopper 10 from the delivery and return branches 3, 4 of the feed circuit 2. Each drying hopper 10 comprises an inlet opening 11, through which the granular polymeric material to be dried is introduced, and an outlet opening 12, through which the dried granular polymeric material is discharged in order to feed a machine for transformation of the granular polymeric material, such as, for example, a mould.

The inlet 11 and outlet 12 openings are provided respectively at the top and bottom of the drying hopper 10.

Above each drying hopper 10, at the relevant inlet opening 11, a loading hopper 13 is fitted into which, by means of a loading line 14, a quantity of fresh granular polymeric material is introduced ready to be introduced into the drying hopper 10.

Between the loading hopper 13 and the drying hopper 10, a loading valve 13a is provided, in order, when necessary, to permit the intake of the fresh granular polymeric material into the drying hopper 10. Each drying hopper 10 is designed to dry any polymeric material in granule form, for example polyamide, polycarbonate or ABS copolymer (acrylonitrile butadiene styrene) or PET (polyethylene terephthalate).

Each loading hopper 13 is connected by means of its own loading line 14 to a relevant container for the granular polymeric material to be dried, for example a tank or bag (not represented in the appended figures), such that, in different drying hoppers 10, different granular polymeric materials can be treated simultaneously. Each loading hopper 13 is also connected, by means of a valve 14a, to a vacuum line 14b, which vacuum line is common to all the loading hoppers 13. Preferably, each loading hopper 13 is fitted on a weighing sensor 15, comprising for example load cells which are designed to detect the weight of the loading hopper 13 and the granular polymeric material the hopper contains.

On the top of the drying hopper 10, facing towards the interior thereof, a level sensor 16 is also fitted which is designed to detect the level of granular polymeric material present in the drying hopper, or, in other words, the filling level of the drying hopper 10.

On the bottom of the drying hopper 10, at the outlet opening 12, a humidity sensor 17 is also fitted which is designed to measure the humidity of the granular polymeric material discharged from the drying hopper 10.

A first temperature sensor 18 and a second temperature sensor 19 are also fitted on the inlet branch 7 and on the outlet branch 9, in order to measure, respectively, the temperature of the process gas input into and output from the drying hopper 10. On the inlet branch 7 of each drying hopper 10, downstream of the control valve 7a, advantageously a dehumidification unit 20 is provided, designed to dehumidify the process gas, and a heater 13 is provided, designed to heat the process gas which is output from the dehumidification unit 20 at a predefined temperature, before entering the drying hopper 10. By this means, each drying hopper 10 is individually connected to a corresponding dehumidification unit 20.

The dehumidification unit 20 is of the rotary tower type (or "rotor" type), and comprises a cylindrical tower 21, which is rotated about its own axis by a motor 22, between a pair of fixed heads 23 provided at the axially opposite ends of the tower 21. The tower 21 contains desiccant material which can collect a large part of the humidity present in the process gas.

In particular, the desiccant material can comprise a resin based on silica gel (approximately 40%-50%), glass fibre (10%-20%) and molecular sieves (30%-40%), which resin is impregnated on a support advantageously configured to form a plurality of channels which do not communicate with one another and extend parallel to the axis of the tower between the heads 23.

For example, the support can be formed by multi-layer corrugated cardboard. Each head 23 is connected to a first manifold 24, to a second manifold 25 and to a third manifold 26, which do not communicate with one another and are disposed adjacent to one another. The three manifolds are formed so as to cover the entire surface area of the base of the tower 21; for example, in a preferred form, they subdivide the surface area of the base of the tower 21 into three adjacent circular sectors which are not equal to one another The first, the second and the third manifolds of one head are axially aligned with the first, the second and the third manifolds of the other head, by this means defining respectively a first, a second and a third portion of the tower 21, indicated respectively by 24a, 25a and 26a, wherein each portion is formed by the cylindrical sector which is subtended to the respective pair of manifolds 24, 25 and 26.

In particular, the first manifold 24 is a process manifold, which is designed to connect the first portion 24a of the tower 21 to the inlet branch 7, between the control valve 7a and the heater 30, such that the process gas coming from the feed circuit 2 is suitably dehumidified before being heated, and then being introduced into the drying hopper 10 through the diffuser 8. The second manifold 25 is a regeneration manifold, which is designed to connect the second portion 25a of the tower 21 to a circuit 27 for regeneration of the desiccant material.

The third manifold 26 is a cooling manifold, which is designed to connect the third portion 26a of the tower 21 to a cooling duct 28, which extends between the inlet branch 7, upstream of the dehumidification unit 20, and the outlet branch 9 of the drying hopper 10. A valve 28a controls the flow rate of the process gas which is collected from the inlet branch 7 and is conveyed to the outlet branch 9, passing via the third portion 26a without entering the drying hopper 10.

The regeneration circuit 27 is advantageously fed by a single blower 29, which collects air from the environment, and, after passing said air through a filter 29a, pushes the air towards each dehumidification unit 20 to which it is connected by means of respective ducts 31, which are selectively opened by respective valves 32.

On each duct 31, downstream of the valve 32 and upstream of the dehumidification unit 20, a regeneration heater 33 is also provided which heats the air to a predefined temperature, for example between approximately 120°C and 170°C, suitable for regenerating the desiccant material contained in the tower 21.

The temperature of the regeneration air also determines the level of regeneration of the desiccant material, and thus, indirectly, its capacity for absorbing the humidity from the process gas in the first portion 24a.

This feature makes it possible to use the temperature of the regeneration air as a tool for regulating the dew point of the process gas at the outlet of the dehumidification unit 20.

For example, if a dew point of the process gas downstream of the dehumidification unit 20 of between approximately -25°C and -30°C corresponds to a temperature of the regeneration air of approximately 150°C, then a dew point of the process gas downstream of the dehumidification unit 20 of between approximately -35°C and -40°C can correspond to a temperature of the regeneration air of approximately 170°C.

The dependency of the level of dehumidification of the process gas on the regeneration temperature also depends on the flow rate of the process gas, and can be determined and refined by means of successive tests.

In addition or as an alternative to this system, it is possible to regulate the value of the process gas dew point by providing a line for bypassing the dehumidification unit 20.

Between the regeneration circuit 27, upstream of the blower 29, and the feed circuit 2, downstream of the blower 6, a heat exchanger 34 is also advantageously interposed, such that the air in the regeneration circuit 27 is preheated by means of the process gas.

Downstream of the dehumidification unit 20, and before the heater 30, a humidity sensor 20a is also advantageously fitted, which sensor is designed to measure the dew point of the process gas output from the dehumidification unit 20.

The plant 1 also comprises a control unit 50, which is designed to control and regulate the operating parameters of the entire plant 1 and of each individual drying hopper 10, such that the plant 1 operates in the ways described hereinafter. Each drying hopper 10 is loaded, by means of the loading hopper 13, with a suitable quantity of granular polymeric material to be dried, which material, as previously stated, can be different for each drying hopper 10.

On the basis of the type of granular polymeric material to be dried and the processing specifications defined by the transformation machine positioned downstream of the drying hopper, the main parameters of the drying process are set, including the hourly flow rate of the granular polymeric material to be dried, the time during which the granular polymeric material remains in the hopper (thus determining the filling level of the drying hopper), the flow rate of the process gas (determined for example from the hourly flow rate of dried polymeric material by means of a proportionality factor), as well as the temperature and the dew point of the process gas to be introduced into the drying hopper (which are dependent in the first instance on the type of granular polymeric material to be dried). The process gas is pushed along the delivery branch 3 of the feed circuit 2 by the blower 6, and, by means of the control valve 7a, part of the gas is introduced into the inlet branch 7. In particular, the flow rate of the process gas introduced into the inlet branch 7 is regulated by varying the opening of the control valve 7a, on the basis of the flow rate value provided by the control unit 50 (set value).

The process gas which is introduced into the inlet branch 7 (except for the small fraction taken from the cooling duct 28) is conveyed to the first manifold 24 of the dehumidification unit 20, where it enters the first portion 24a of the tower 21 and is dehumidified by the desiccant material contained therein at the dew point level predefined for the type of granular polymeric material to be dried, for example by correspondingly regulating the temperature of the regeneration air.

After leaving the dehumidification unit 20, the process gas is heated by the heater 30 to the temperature predefined for the type of granular polymeric material to be dried (for example between approximately 60°C and approximately 180°C) and is introduced into the drying hopper 10 through the diffuser 8.

After having passed through the granular polymeric material present in the drying hopper 10, the process gas exits from the hopper through the outlet branch 9 and, after having passed the valve 9a, reconnects with the return branch 4 of the feed circuit 2, which takes it to the blower 6 after it has passed through the filter 5a.

Downstream of the blower 6, the process gas yields part of its heat to the air of the regeneration circuit 27, due to the provision of the heat exchanger 34. As previously stated, the tower 21 is rotated slowly and continuously about its own axis by the motor 22, such that the desiccant material contained therein passes from the first portion 24a to the second portion 25a, where it is regenerated by the hot air coming from the regeneration circuit 27, and finally to the third portion 26a, where it is partly cooled by the small fraction of process gas taken from the cooling duct 28.

At the request of the transformation machine, the granular polymeric material is gradually discharged from the drying hopper 10, until the filling level of the drying hopper 10, detected by the level sensor 16, decreases by a predefined quantity relative to the predetermined filling level. At this point, the control unit 50 commands topping up with fresh granular polymeric material from the loading hopper 13.

The quantity of fresh granular polymeric material introduced into the drying hopper 10 from the loading hopper 13 is also weighed by the weight sensor 15. The plant and the process according to the present invention can be produced in variants which differ from the above-described preferred example.

The plant according to the present invention permits extensive operative flexibility, and in particular makes it possible to treat, in the different drying hoppers, a granular polymeric material which needs to be dried, by means of process gases which have different characteristics from one another, in particular process gases with different dew point values, at the same time optimising the energy efficiency of the process.