WO2021074847A1

WO2021074847A1 - Plant for drying granular polymeric material and related drying process

Info

Publication number: WO2021074847A1
Application number: PCT/IB2020/059708
Authority: WO
Inventors: Gianfranco Cattapan
Original assignee: Pegaso Industries SpA
Current assignee: Pegaso Industries SpA
Priority date: 2019-10-15
Filing date: 2020-10-15
Publication date: 2021-04-22
Anticipated expiration: 2022-04-15
Also published as: DE112020005023T5; IT201900018851A1

Abstract

A plant for drying granular polymeric material comprises: - a drying hopper (10) provided for containing the granular polymeric material during drying, - a loading hopper (13) provided for introducing the granular polymeric material to be dried into the drying hopper (10), - a drying system arranged for drying the granular polymeric material contained in the drying hopper, and - a weight sensor (15) provided for detecting the weight of the loading hopper (13) and the granular polymeric material contained therein. A control unit (50) determines, on the basis of the weight values detected by the weight sensor (15), the quantity of granular polymeric material introduced into the drying hopper (10) by the loading hopper (13) in a period of time, and controls at least one parameter of the drying system depending on the quantity of granular polymeric material introduced into the drying hopper (10) by the loading hopper (13) in such period of time.

Description

Plant for drying granular polymeric material and related drying process

DESCRIPTION

Technical field The present invention relates to a plant for drying granular polymeric material having the features set out in the preamble of the main claim. It also relates to a drying process carried out by such a plant.

Technological background

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

This need, however, does not reconcile with the high hygroscopic properties of some plastics widely used in the industry, such as those based on polyethylene terephthalate (PET), or polyamide (PA), or polycarbonate (PC) or some copolymers such as ABS (acrylonitrile- butadiene-styrene).

Therefore, these plastics materials, before being subjected to the extrusion or moulding process, must be adequately dried in special drying plants, where the water content of the granules is reduced to the minimum quantities required by the transformation process.

In general terms, these plants comprise a drying hopper, into which the granular polymeric material is introduced, as well as a drying system which dries the granular polymeric material contained in the drying hopper. In this field, drying plants are known in which the granular polymeric material is introduced into the drying hopper through a loading hopper, which is typically mounted above the drying hopper, so that the introduction of the granular polymeric material occurs by gravity and can be easily controlled by the opening of a special loading valve arranged on the connection between the two hoppers.

The granular polymeric material dried by the drying hopper is discharged, in successive steps, to feed a transformation machine, such as for example a mould or an extruder, which forms the granular polymeric material into a desired article.

The quantity of dried polymeric material discharged from the drying hopper depends on the request from the transformation machine which, generally, may not be constant over time.

This, however, involves lowering the energy efficiency of the process and, at times, can also penalise the quality of the dried granular polymeric material, for example due to over-drying of the polymer.

One of the greatest needs in the reference technical field is therefore that of adapting the parameters of the drying process to the variability of the flow rate of dried polymeric material required by the transformation machine.

Examples of drying plants which aim to meet this need are described in EP 2186613 and EP 2447027.

In such plants, the drying hopper rests on load cells capable of measuring the weight of the drying hopper and the granular polymeric material contained therein. On account of successive measurements carried out by the load cells, the quantity of granular polymeric material discharged from the drying hopper per unit of time is calculated in order to appropriately adjust some parameters of the drying process. In particular, in the first case, the flow rate of the process gas introduced into the drying hopper for drying the granular polymeric material is regulated and, in the second case, the filling level of the drying hopper is regulated.

However, the Applicant has noted that such plants are rather onerous in terms of cost and in terms of maintenance Furthermore, this solution is difficult to implement on plants with traditional drying hoppers, which were not originally designed to be positioned on load cells.

In the present description and in the appended claims, "granular material" is understood to mean a plurality of distinct and separate solid elements, having suitable dimensions and shape, depending on the process to be carried out and the polymeric material used, including polymeric material in powder or flake form.

Furthermore, the term "drying" means the process by which the moisture content of the granular polymeric material is reduced to the desired values by the subsequent transformation process (moulding or extrusion), by means of substantial elimination of the water present in the internal regions of the granules.

As a reference, the maximum residual humidity value required by the transformation unit may be approximately 20 - 200 ppm (parts per million). The "flow rate" of a fluid or solid is defined as the quantity of fluid or solid passing through a reference surface in a predefined unit of time, which for example may be one hour.

"Effective flow rate", referring to the process gas, is understood to mean the gas flow rate which, maintaining equal other parameters, such as temperature and dew point, allows a specific flow rate of granular polymeric material to be dried to the desired degree of drying. The effective flow rate of the process gas may be substantially proportional to the flow rate of the dried granular polymeric material in the drying hopper and its proportionality constant may be obtained from specific tables depending on the type of material, on the inlet temperatures of the process gas and of the material, and possibly on other parameters. "Effective filling level", referred to the drying hopper, is understood to mean the filling level which, for a certain flow rate of dried granular polymeric material, ensures a predefined residence time inside the drying hopper. The predefined residence time may depend on the type of material, the type of process gas and the temperature inside the drying hopper.

Description of the invention The problem addressed by the present invention is that of providing a plant for drying granular polymeric material as well as a corresponding drying process, which are structurally and functionally designed to overcome, at least in part, one or more of the drawbacks mentioned above with reference to the cited prior art. This problem is solved by the present invention by a drying plant and a drying process implemented according to the following claims.

In a first aspect thereof, the invention relates to a plant for drying granular polymeric material comprising:

- a drying hopper, provided for containing the granular polymeric material during a drying phase of the granular polymeric material,

- a loading hopper, provided for introducing the granular polymeric material to be dried into the drying hopper,

- a drying system associated with the drying hopper and arranged for drying the granular polymeric material contained in the drying hopper, and

- a control unit, provided for controlling at least the drying system. Preferably, the plant further comprises a weight sensor provided for detecting the weight of the loading hopper and the granular polymeric material contained therein. Preferably, the control unit is intended to determine, on the basis of the weight values detected by the weight sensor, the quantity of granular polymeric material introduced into the drying hopper by the loading hopper in a period of time, and to control at least one parameter of the drying system depending on the quantity of granular polymeric material introduced into the drying hopper by the loading hopper in the period of time.

In a second aspect thereof, the present invention relates to a process for drying granular polymeric material comprising the steps of:

- providing a drying hopper suitable for containing the granular polymeric material during the drying thereof, and a loading hopper provided for introducing the granular polymeric material to be dried into the drying hopper,

- loading granular polymeric material to be dried that is present in the loading hopper into the drying hopper, - drying the granular polymeric material in the drying hopper,

- unloading part of the dried granular polymeric material from the drying hopper, and

- loading further granular polymeric material to be dried that is present in the loading hopper into the drying hopper in order to replenish, at least in part, the unloaded granular polymeric material.

Preferably, the process includes measuring the quantity of granular polymeric material introduced into the drying hopper from the loading hopper in a period of time and controlling at least one parameter of the drying phase depending on the quantity of granular polymeric material introduced into the drying hopper from the loading hopper in the period of time.

On account of these features, it is possible to adjust one or more parameters of the process for drying the granular polymeric material without placing the entire drying hopper on load cells, thereby being limited to weighing only the loading hopper which is known to be much smaller in terms of dimensions and weight. In particular, the volumetric ratio between a loading hopper and the relevant drying hopper is preferably between approximately 1:10 and approximately 1:50, for example between approximately 1:30 and approximately 1:40. In this way, the design, installation and management of the drying plant is simplified.

The load cells themselves can in fact be sized for measuring lower weights. Furthermore, their maintenance, as well as their replacement, requires much simpler equipment than if the load cells were positioned below the drying hopper.

Moreover, on account of these features, it is possible to transform existing plants, which are already installed, into a plant according to the present invention. The measurement of the weight of the granular polymeric material that is introduced from the loading hopper to the drying hopper to be dried takes place mainly during the feeding phase of the drying hopper. In particular, the weight of the loading hopper with its content of granular polymeric material is detected at the beginning and at the end of the feeding phase, so as to obtain, by subtraction, the quantity of polymeric material introduced into the drying hopper.

When necessary, the loading hopper is filled again with granular polymeric material, but, suitably, this filling does not occur during the feeding phase of the drying hopper. For example, the filling of the loading hopper may be controlled by a level sensor placed inside the loading hopper, by means of a pneumatic conveying line of the granular polymeric material.

By correctly taking into account the time in which the feeding phases are carried out, it is therefore possible to calculate the quantity of granular polymeric material introduced into the drying hopper in a suitably defined period of time, thus obtaining an average hourly flow rate.

The period of time over which the average hourly flow rate is calculated may be selected in the way deemed most appropriate, such as 20 minutes, 30 minutes, or an hour. In any case, it is preferred that this period of time be greater than the time that normally elapses between two feeding phases of the drying hopper.

Generally, the feeding phases of the drying hopper follow each other at a rate of a few minutes from each other, for example every 1- 15 minutes, more preferably every 2 - 10 minutes. The average hourly flow rate of the material loaded into the drying hopper may be considered, in the first instance, equal to the average hourly flow rate of the granular polymeric material discharged therefrom, because in sufficiently long periods of time, the granular polymeric material inside the drying hopper remains substantially the same, without significant accumulations and without significant losses.

In a preferred embodiment, the filling level of the drying hopper is advantageously kept constant, at least in each period of time in which the weight measurements are made to calculate the average hourly flow rate. The parameter of the drying phase that is regulated depends on the method by which the drying is carried out and therefore on the drying system adopted by the plant.

In any case, this parameter is preferably dependent on the average hourly flow rate of granular polymeric material dried in the drying hopper. Another advantage obtained by the present invention is that of the possibility of accurately determining the quantity of granular polymeric material that is sent to dry and therefore of defining in advance the quantity of material subject to drying and subsequent processing. In this way, it becomes very easy to precisely respect the production demands of a certain article, which are generally defined in quantitative terms.

In fact, when the total weight of granular polymeric material introduced into the drying hopper, as calculated by adding all the introductions of material detected by the weight sensor, reaches the quantity required for that specific process, then the control unit can block the opening of the loading valve, thus allowing the drying hopper to progressively come to emptying.

This advantageously allows the residual quantities of granular polymeric material to be eliminated, or at least substantially minimized, between one process and another.

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

In one embodiment, the drying system comprises a circuit for feeding a process gas, which circuit is connected to the drying hopper, for introducing a flow of process gas suitable for drying the granular polymeric material into said drying hopper.

In this first type of drying, the contact of the granular polymeric material with a flow of a hot and dry process gas which absorbs the humidity contained in the granules of polymeric material is exploited. The process gas is, for example, air.

In this case, a first parameter of the drying phase controlled by the control unit depending on the average hourly flow rate of granular polymeric material fed into the drying hopper from the loading hopper is the effective flow rate of the process gas.

The latter, in fact, (other parameters being equal such as the inlet temperature of the process gas) depends in a substantially proportional manner on the flow rate of the granular polymeric material dried in the drying hopper. Therefore, if the average hourly flow rate calculated in the last predefined period of time differs at least by a minimum quantity from the previous set value, then the control unit determines a new effective flow rate value of the process gas calculated on the basis of the new average hourly flow rate value of the granular polymeric material and transmits this new value to the process gas flow rate regulation device.

In other embodiments, the drying system may exploit different phenomena to dry the material contained in the drying hopper, for example a vacuum. In this case, the parameter controlled by the process unit is different. A second parameter of the drying phase controlled depending on the average hourly flow rate of granular polymeric material introduced into the drying hopper from the loading hopper is the effective filling level of the drying hopper.

The filling level of the drying hopper is preferably detected by a level sensor, provided for measuring the level of granular polymeric material contained in the drying hopper.

The level sensor is preferably mounted in the drying hopper.

Preferably, the introduction of the granular polymeric material into the drying hopper from the loading hopper is determined by the opening of a loading valve, interposed between the loading hopper and the drying hopper.

Preferably, the loading hopper is positioned above the drying hopper, so that the granular polymeric material passes from the loading hopper to the drying hopper by gravity, with the loading valve open Both the effective flow rate of the process gas and the effective filling level of the drying hopper are determined by the control unit on the basis of the data relating to the type of polymeric material to be treated, to the inlet temperature of the granular polymeric material and the process gas in the drying hopper, to the flow rate of dried granular polymeric material and possibly other parameters. Some of these values, for example the temperatures, may be provided to the control unit by direct measurements (for example by temperature sensors placed on the circuit for feeding the process gas).

The initial value of the flow rate of dried granular polymeric material may be initially provided by the operator, for example based on the processing capacity of the transformation machine located downstream of the drying hopper, while, as previously mentioned, this value is then determined by the control unit on the basis of the measurements of the weight of the granular polymeric material introduced by the loading hopper that are detected by the weight sensor. In one embodiment, the weight sensor comprises at least one load cell on which the loading hopper is mounted.

Preferably, the load cell is interposed between the loading hopper and the drying hopper. In one embodiment, the plant is of the multi-hopper type and comprises at least two drying hoppers connected in parallel to a single circuit for feeding process gas, each of said hoppers being fed by a relevant loading hopper.

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

- Figure 1 is a schematic view of a plant for drying granular polymeric material implemented according to the present invention;

- Figure 2 is an enlarged schematic view of a component of the drying plant of Figure 1, indicated by II.

Preferred mode to carry out the invention With reference to the accompanying figures, reference numeral 1 indicates as a whole a plant for drying a granular polymeric material, implemented according to the present invention.

The plant 1 comprises a plurality of drying hoppers, all indicated by reference numeral 10, and a drying system provided for drying the granular polymeric material contained therein up to a degree of residual humidity defined by the subsequent processing specifications.

In particular, the drying system comprises a feeding circuit 2, connected in parallel to all the drying hoppers 10, and arranged to introduce into each drying hopper 10 a process gas, preferably air, that is suitably heated and dehumidified for drying the granular polymeric material present in the drying hopper 10.

In particular, the feeding circuit 2 is a closed circuit and comprises a delivery branch 3, which carries the process gas to the drying hoppers 10, and a return branch 4, which collects the process gas exiting the drying hoppers 10 and returns it to a handling unit 5 which, after passing said gas through a filter 5a, returns it along the delivery branch 3.

The handling unit 5, in turn, includes a blower 6 driven in rotation by a motor 6a of which the number of revolutions is variable, due to the provision of an inverter. Each drying hopper 10 is connected to the delivery branch 3 by means of an inlet branch 7 which brings the process gas up to an open diffuser 8 inside the drying hopper. Access to the inlet branch 7 and the flow rate of the process gas along said inlet branch are regulated by a control valve 7a, which can be controlled by a flow sensor 7b. Similarly, each drying hopper 10 is connected to the return branch 4 by means of an outlet branch 9 which takes the process gas from the top of the drying hopper 10 and returns it to the return branch 4 through a valve 9a.

The valves 7a and 9a also act as shut-off valves which are capable, when closed, of excluding the drying hopper 10 from the delivery and return branches 3, 4 of the feeding circuit 2.

Hereinafter, the description is limited to a single drying hopper 10, it being understood that the same features are similarly present in the other drying hoppers of the plant 1. In the preferred example described herein, the plant 1 is a multi-hopper plant, but the present invention similarly relates to a plant comprising a single hopper or multi-hopper plants in which the hoppers are connected together in a different way.

The 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 transformation machine of the granular polymeric material, such as a mould.

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

A loading hopper 13 is mounted above the drying hopper 10 at the inlet opening 11, into which loading hopper, through a loading line 14, a quantity of fresh granular polymeric material is introduced, ready to be fed into the drying hopper 10. The loading hopper 13 has dimensions much smaller than the drying hopper 10, with a volumetric ratio between the two hoppers of approximately 1:30 - 1:40.

A loading valve 13a is provided between the loading hopper 13 and the drying hopper 10 to allow, when necessary, the fresh granular polymeric material to enter the drying hopper 10. The loading valve 13a may be any valve suitable for selectively opening or closing the passage between the loading hopper 13 and the drying hopper 10. In a preferred embodiment, the loading valve 13a is a tilting flap valve, or a throttle valve, or a knife gate valve. The drying hopper 10 is provided for drying any polymeric material in granules, for example polyamide, polycarbonate or ABS copolymer (acrylonitrile-butadiene-styrene) or PET (polyethylene terephthalate).

The loading hopper 13 is connected via a loading line 14 thereof to a container of the granular polymeric material to be dried, for example a tank or a bag (not shown in the appended figures), as well as, via a valve 14a, to a vacuum line 14b, the latter being common to all loading hoppers 13.

The loading hopper 13 is mounted on a weight sensor 15 provided for detecting the weight of the loading hopper 13 and of the granular polymeric material contained therein.

In particular, the weight sensor 15 is preferably formed by a load cell comprising a metal ring interposed between the top of the drying hopper 10 and the support base of the loading hopper 13. The load cell comprises some strain gauges (preferably 3) which are integral with the metal ring and capable of accurately detecting the deformation of the ring due to the weight of the loading hopper 13 resting thereon.

On the top of the drying hopper 10, facing inwards, a level sensor 16 is also mounted, provided for detecting 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 mounted, provided for measuring 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 mounted on the inlet branch 7 and on the outlet branch 9 for measuring, respectively, the temperature of the process gas entering and leaving the drying hopper 10.

On the inlet branch 7 of the drying hopper 10, downstream of the control valve 7a, advantageously a dehumidification unit 20 is provided, for dehumidifying the process gas, and a heater 30 is provided, for heating the process gas outgoing from the dehumidification unit 20 at a predefined temperature, before entering the drying hopper 10.

The dehumidification unit 20 is of the rotating tower (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 capable of withdrawing a large part of the humidity present in the process gas.

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 each other and are arranged side by side and shaped in such a way as to cover the entire base surface of the tower 21, for example dividing the base area of the tower 21 into three adjacent circular sectors, which are not equal to each other. The first, second and third manifolds of one head are axially aligned with the first, second and third manifolds of the other head, thus defining, respectively, a first, second and third section of the tower 21, respectively indicated by 24a, 25a and 26a, wherein each section is formed by the cylindrical sector underlying the relevant pair of manifolds 24, 25 and 26.

In particular, the first manifold 24 is a process manifold, arranged to connect the first section 24a of the tower 21 to the inlet branch 7, between the control valve 7a and the heater 30, so that the process gas coming from the feeding circuit 2 is suitably dehumidified before being heated and then fed into the drying hopper 10 through the diffuser 8.

The second manifold 25 is a regeneration manifold, arranged to connect the second section 25a of the tower 21 to a regeneration circuit 27 of the desiccant material.

The third manifold 26 is a cooling manifold, arranged to connect the third section 26a of the tower 21 to a cooling duct 28, extending 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 taken from the inlet branch 7 and conducted to the outlet branch 9 passing through the third section 26a without entering the drying hopper 10.

The regeneration circuit 27 is advantageously fed by a single blower 29 which draws air from the environment and, after passing said air through a filter 29a, pushes it towards each dehumidification unit 20 to which it is connected by means of respective ducts 31, 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 of approximately 120°C - 170°C, suitable for regenerating the desiccant material contained in the tower 21.

Between the regeneration circuit 27, upstream of the blower 29, and the feeding circuit 2, downstream of the blower 6, advantageously a heat exchanger 34 is also arranged, so that the air in the regeneration circuit 27 is pre-heated at the expense of the process gas. A humidity sensor 20a is suitably mounted downstream of the dehumidification unit 20 and before the heater 30, which humidity sensor is provided for measuring the dew point of the process gas leaving the dehumidification unit 20.

The plant 1 further comprises a control unit 50 provided for controlling and adjusting the operating parameters of the entire plant 1 and of each individual drying hopper 10 in such a way that, with reference to any drying hopper 10, the plant 1 operates in the manner described below. Once the type and total quantity of granular polymeric material to be dried have been established, the initial operating parameters of the plant 1, in particular of the drying system, are set.

In particular, based on the type of granular polymeric material to be dried and the process specifications defined by the transformation machine located downstream of the drying hopper, the main parameters of the drying process are set, including: the initial hourly flow rate of the granular polymeric material to be dried, generally defined by the transformation machine,

- the residence time of the granular polymeric material in the hopper and consequently the initial effective filling level of the drying hopper, - the initial effective flow rate of the process gas (determined, for example, starting from the hourly flow rate of dried polymeric material by means of an appropriate proportionality factor), as well as

- the temperature and dew point of the process gas to be introduced into the drying hopper (depending in the first instance on the type of granular polymeric material to be dried).

The initial values set in the control unit 50 may be entered manually or may be obtained from suitable internal tables.

A quantity of granular polymeric material to be dried is then introduced into the drying hopper 10 by means of the loading hopper 13 until the initial effective filling level is reached as detected by the level sensor 16. At the end of the initial filling phase, the loading valve 13a is closed and the drying phase is started by opening the control valve 7a, so as to allow the passage of process gas from the feeding circuit 2, through the inlet branch 7, up to being introduced into the drying hopper 10.

In particular, the process gas is introduced into the inlet branch 7 by varying the opening of the control valve 7a. 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 effective flow rate value supplied by the control unit 50 (set value). The process gas introduced into the inlet branch 7 (except for the small fraction taken from the cooling duct 28) is conducted to the first manifold 24 of the dehumidification unit 20 where it is dehumidified by the desiccant material contained therein at the predefined humidity level for the type of granular polymeric material to be dried.

After leaving the dehumidification unit 20, the process gas is heated by the heater 30 to the predefined temperature for the type of granular polymeric material to be dried (for example between 60°C and 180°C) and is introduced into the drying hopper 10 through the diffuser 8. After passing through the granular polymeric material present in the drying hopper 10, the process gas exits from the latter through the outlet branch 9, reconnecting, after passing through the valve 9a, to the return branch 4 of the feeding circuit 2 which returns it to the blower 6 after being passed through the filter 5a. When the granular polymeric material present in the drying hopper 10 reaches the desired degree of drying, a part of it is discharged to feed the transformation machine downstream of the plant 1.

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 with respect to the effective filling level. At this point, the control unit 50 controls the start of the feeding phase of the drying hopper 10, opening the loading valve 13a to allow the entry of fresh granular polymeric material. The filling valve 13a is closed again when the level sensor 16 detects that the effective filling level set by the control unit 50 has been reached.

The weight sensor 15 detects the weight of the loading hopper 13 and its contents before and after the feeding phase, allowing the quantity of fresh granular polymeric material moved from the loading hopper 13 to the drying hopper 10 to be obtained.

This operation is carried out in successive cycles, and, once the predefined time period for calculating the average hourly flow rate has been reached, for example 30 minutes, the control unit 50 adds up all the quantities of material fed into the drying hopper 10 in this period of time, divides them by the predefined period of time (in this case 0.5 hours), thus obtaining the average hourly flow rate of the granular polymeric material treated in the hopper in the period of time considered.

If the new average hourly flow rate thus calculated differs significantly from the average hourly flow rate previously set in the control unit 50, the new average hourly flow rate value is used by the control unit 50 to define the new effective flow rate values of the process gas and of the effective filling level of the drying hopper 10.

These new values are then used as set values for regulating the opening of the control valve 7a and for the opening and closing of the loading valve 13a.

In this way, the drying process maintains its maximum energy efficiency even when the production speed of the transformation machine downstream of the plant varies, thus adapting to its productivity.

The plant and the process of the present invention may be implemented in different variants with respect to the preferred example described above.

In particular, the plant may comprise a single drying hopper.

Furthermore, the circuit for feeding process gas may be an open circuit and the dehumidification unit may be of a different type, for example comprising a pair of molecular sieve towers, or alternatively one operating tower and one regeneration tower.

The plant of the present invention allows a wide operational flexibility, an excellent adaptability to variations in the production capacity of the transformation machine downstream of the plant, while maintaining high efficiency of the drying process, with installation and maintenance costs substantially reduced compared to the known plants mentioned. Furthermore, the plant of the present invention allows any processing residues and process times to be minimised, particularly in the event of production changes.

Claims

1. Plant for drying granular polymeric material, comprising:

- a drying hopper (10) provided for containing said granular polymeric material during a drying phase of said granular polymeric material,

- a loading hopper (13) provided for introducing the granular polymeric material to be dried into said drying hopper (10),

- a drying system associated with said drying hopper (10) and arranged for drying said granular polymeric material when contained in said drying hopper,

- a control unit (50) for controlling at least said drying system, characterised in that:

- a weight sensor (15) is provided for detecting the weight of said loading hopper (13) and the granular polymeric material contained therein,

- said control unit (50) is intended to determine, on the basis of the weight values detected by said weight sensor (15), the quantity of granular polymeric material introduced into said drying hopper (10) by said loading hopper (13) in a period of time, and - said control unit (50) is intended to control at least one parameter of said drying system depending on said quantity of granular polymeric material introduced into said drying hopper (10) by said loading hopper (13) in said period of time.

2. Drying plant according to claim 1, wherein said drying system comprises a circuit (2) for feeding a process gas, which circuit is connected to said drying hopper (10), for introducing a flow of process gas suitable for drying said granular polymeric material into said drying hopper.

3. Drying plant according to claim 2, wherein said control unit (50) is intended to determine an effective flow rate value of said process gas so as to guarantee an appropriate degree of drying for said granular polymeric material, and said at least one parameter controlled by said control unit comprises said effective flow rate of said process gas.

4. Drying plant according to any one of the preceding claims, wherein said drying system comprises a level sensor (16) for measuring the level of said granular polymeric material contained in said drying hopper (10) and a loading valve (13a) which allows said granular polymeric material to be introduced into said drying hopper from said loading hopper (13) and said control unit is arranged to: - determine an effective filling level of said drying hopper so as to guarantee an appropriate residence time for said granular polymeric material inside said drying hopper, and - control the opening of said loading valve (13a) in order to allow said granular polymeric material to be introduced into said drying hopper (10) from said loading hopper (13) until said effective filling level is reached, on the basis of the measurement carried out by said level sensor (16).

5. Drying plant according to claim 4, wherein said at least one parameter controlled by said control unit (50) comprises said effective filling level.

6. Drying plant according to any one of the preceding claims, wherein said weight sensor (15) comprises at least one load cell and said loading hopper (13) is mounted on said at least one load cell.

7. Drying plant according to claim 6, wherein said at least one load cell is arranged between said loading hopper (13) and said drying hopper (10).

8. Process for drying granular polymeric material, comprising:

- providing a drying hopper (10) provided for containing said granular polymeric material during a drying phase of said granular polymeric material, and a loading hopper (13) for introducing the granular polymeric material to be dried into said drying hopper,

- moving granular polymeric material to be dried from said loading hopper to said drying hopper,

- drying said granular polymeric material in said drying hopper, - unloading part of said dried granular polymeric material from said drying hopper,

- moving further granular polymeric material to be dried from said loading hopper to said drying hopper in order to replenish, at least in part, the unloaded granular polymeric material, characterised by:

- measuring the quantity of granular polymeric material introduced into said drying hopper from said loading hopper in a period of time and

- controlling at least one parameter of said drying phase depending on said quantity of granular polymeric material introduced into said drying hopper from said loading hopper in said period of time.

9. Drying process according to claim 8, wherein said granular polymeric material is dried by means of introducing a process gas into said drying hopper (10) and said at least one parameter comprises the flow rate of said process gas.

10. Drying process according to claim 8 or claim 9, wherein said at least one parameter comprises an effective filling level at which said drying hopper (10) is loaded from said loading hopper (13).