IT201900018860A1 - Drying process of polymeric granular material and drying plant operating according to this process - Google Patents

Description

Drying process of polymeric granular material and drying plant operating according to this process

DESCRIPTION

Technical field

The present invention relates to a drying process of polymeric granular material having the characteristics set out in the preamble of the main claim. It is also directed to a drying plant operating according to this process.

Technological background

It is known that the transformation of plastic materials into granules by extrusion or molding requires, in order to guarantee an adequate quality level of the molded product, a very low level of humidity of the granular material.

This need, however, does not reconcile with the high hygroscopic properties of some plastics widely used in the sector, 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, before being subjected to the extrusion or molding 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 polymeric granular material is introduced, as well as a drying system which dries the polymeric granular material contained in the drying hopper.

For example, drying plants are known in which the polymeric granular material is dried by placing a hot and dry process gas into the drying hopper which absorbs the moisture of the granules.

As previously mentioned, the dried polymeric granular material must have a degree of humidity lower than a predefined nominal value, required for example by the type of processing that the material undergoes downstream of the drying plant.

However, the Applicant has observed that if the polymeric granular material is dried at a humidity degree that is too lower than this nominal value, some drawbacks may be encountered.

A first drawback is given by the fact that some materials, if too dry, can give rise to quality problems in the final product obtained from the subsequent processing.

For example, in some applications, a polymeric material dried at a humidity level that is too low can give rise to post-shrinkage phenomena after molding.

Furthermore, a drying phase that is too intense can cause the loss of any volatile additives or, in some materials, it can cause the formation of unwanted by-products, such as the formation of acetaldehyde in the case of PET.

Furthermore, a polymeric material dried at a humidity level that is too low can cause problems in the subsequent extrusion or molding steps, since the polymeric material that is too dry can be scarcely fluid when melted.

A further drawback is given by the fact that the energy supply supplied to the drying plant is obviously greater than that necessary to dry the polymeric granular material at the required degree of humidity, so that, in fact, a part of the energy supplied is wasted. ,

In the present description and in the attached claims, "granular material" means a plurality of distinct and separate solid elements, having suitable dimensions and conformation, depending on the processing to be carried out and the polymeric material used, including the polymeric material in powder form or flakes.

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

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

The "flow rate" of a fluid or solid is defined as the amount of fluid or solid that passes through a reference surface in a predefined unit of time, which, for example, can be an hour.

By “effective flow rate” referring to the process gas we mean the gas flow rate which, with the same parameters as temperature and dew point, allows to dry a certain flow rate of polymeric granular material to the desired degree of drying. The effective flow rate of the process gas can be substantially proportional to the flow rate of the dried polymeric granular material in the drying hopper and its proportionality constant can be obtained from specific tables depending on the type of material, the inlet temperatures of the process gas and of the material, and possibly other parameters.

Description of the invention

The problem underlying the present invention is that of providing a drying process for polymeric granular material as well as a drying plant operating according to this process, which are structurally and functionally designed to overcome, at least in part, one or more of the drawbacks complained above with reference to the prior art cited.

This problem is solved by the present invention by means of a drying process and a drying plant made in accordance with the following claims.

In its first aspect, the invention is directed to the drying process of polymeric granular material comprising the steps of:

- prepare a drying hopper, suitable for containing the polymer granular material,

- subject the polymeric granular material contained in the drying hopper to a drying phase,

- measure the degree of humidity of the dried polymer granular material,

- compare the measured humidity degree with a desired nominal humidity degree, e

- if the humidity degree measured is lower than the nominal humidity degree, adjust a parameter of the drying phase in order to bring the humidity degree of the dried polymeric granular material back to the nominal humidity degree.

In a second aspect thereof, the present invention is directed to a drying plant for polymeric granular material comprising:

- a drying hopper, designed to contain the polymer granular material,

- a drying system, designed to dry the polymer granular material when contained in the drying hopper,

- a humidity meter, designed to measure the degree of humidity of the polymeric granular material when discharged from the drying hopper,

- a control unit designed to control at least the drying system,

in which the control unit is designed to compare a degree of humidity measured by the humidity meter, with a preset nominal humidity degree and, when the degree of humidity measured is lower than the nominal degree of humidity, to adjust at least one parameter of the drying system.

Thanks to these characteristics, it is possible to adjust the final humidity level of the dried polymer granular material so that it is not lower than the nominal humidity level required, thus avoiding the occurrence of the problems described above and, at the same time, saving energy.

In at least one of the aforementioned aspects, the present invention may also exhibit one or more of the preferred characteristics expressed below.

In a first embodiment, the parameter of the drying step is adjusted when the measured humidity degree is lower than the nominal degree of humidity by at least 5%, more preferably when it is lower by at least 10%.

This defines a tolerance range with respect to the moisture content of the polymer granular material as measured, in which no adjustment is made. This makes it possible not to place an excessive burden on the control system of the drying system in a range of values which, in terms of quality and energy savings, would not be a source of particularly significant benefits.

In a preferred embodiment, the control over the degree of humidity of the dried polymeric granular material also takes place when the degree of humidity measured is greater than the nominal degree of humidity, so as to ensure compliance with this requirement.

In one embodiment, the drying step of the polymeric granular material comprises the introduction into said drying hopper of a flow rate of a dehumidified and heated process gas.

In another embodiment, the drying step of the polymeric granular material can comprise a step in which the polymeric granular material is placed in depression, in order to favor the desorption of moisture, and / or a heating step of the material. polymer granular by infrared or microwave radiation,

In one embodiment, the regulated parameter is the flow rate of the process gas fed into the drying hopper.

Preferably, the flow rate of the process gas is regulated by means of a control valve located on a process gas supply circuit.

In one embodiment, the adjusted parameter is the degree of dehumidification of the process gas fed into the drying hopper. The degree of dehumidification of the process gas is defined by its dew point.

Preferably, the dew point of the process gas is regulated by means of a dehumidification unit located on the process gas supply circuit.

In one embodiment, the adjusted parameter is the residence time of the polymer granular material in the drying hopper.

Preferably, the residence time is regulated by varying the filling level of the drying hopper.

In another embodiment, the regulated parameter is the inlet temperature of the polymer granular material into the drying hopper.

Preferably, the inlet temperature of the polymeric granular material is regulated by means of a heater located on the process gas supply circuit.

Preferably, the moisture meter is of the capacitive type.

In particular, the humidity meter determines the degree of humidity of the dried polymeric granular material by measuring its dielectric constant, which, for each material, depends on the degree of humidity.

Preferably, the moisture meter is positioned at the discharge of the drying hopper.

In this way, the process gas flow is continuously adapted to the actual productivity of the drying hopper.

Preferably, the process gas is air.

In one embodiment, the plant is of the multi-hopper type and comprises at least two drying hoppers connected in parallel to a single process gas supply circuit.

Brief description of the drawings

The characteristics and advantages of the invention will become clearer from the detailed description of a preferred embodiment thereof, illustrated by way of non-limiting example with reference to the attached drawing, in which Figure 1 is a schematic view of a drying plant for granular material. polymer made according to the present invention.

Preferred way of carrying out the invention

With reference to the accompanying figures, 1 generally indicates a drying plant for a polymeric granular material, made in accordance with the present invention.

The plant 1 comprises a drying hopper 10 and a feeding circuit 2 arranged to introduce into the drying hopper 10 a process gas, preferably air, suitably heated and dehumidified to dry the polymeric granular material contained therein up to a degree of desired nominal humidity.

In particular, the supply circuit 2 is a closed circuit and comprises a forward branch 3, which carries the process gas to an open diffuser 5 inside the drying hopper 10 and a return branch 4 which collects the gas. output from the drying hopper 10 and returns it to a handling unit, which, after suitable filtration, returns it along the forward branch 3. The process gas handling unit, for example, can comprise a 2nd blower driven in rotation by a motor whose number of revolutions is variable thanks to the provision of an inverter.

The flow rate of the process gas along the delivery branch 3 is regulated by a control valve 6.

On the outgoing branch 3, downstream of the control valve 6, there are a dehumidification unit 20 and a heater 30, arranged respectively to dehumidify the process gas and to heat the process gas before entering the drying hopper 10.

Preferably, the dehumidification unit 20 is placed upstream of the heater 30.

A first temperature sensor 7 is provided between the dehumidification unit 20 and the heater 30, designed to measure the temperature of the process gas before it enters the heater 30, while a second temperature sensor 8 is mounted downstream of the heater 30 to measure the process gas temperature at the outlet of the heater 30.

A third temperature sensor 18 is also mounted on the return branch 4 to measure the temperature of the process gas leaving the drying hopper 10.

The drying hopper 10 comprises an inlet opening 11, through which the polymeric granular material to be dried is introduced, and an outlet opening 12, through which the dried polymeric granular material is discharged to feed a machine for processing the polymer granular material, such as a mold or extruder.

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

A loading hopper 13 is mounted above the drying hopper 10, in correspondence with the inlet opening 11, into which, through a loading line 14, a quantity of fresh polymeric granular material is introduced, ready to be introduced into the drying hopper 10.

Between the loading hopper 13 and the drying hopper 10 there is a loading valve 13a to allow, when necessary, the entry of fresh polymeric granular material into the drying hopper 10.

The loading valve 13a is any valve suitable for selectively opening or closing the passage between the loading hopper 13 and the drying hopper 10, for example a tilting flap valve, or a butterfly valve, or a guillotine valve.

The drying hopper 10 is designed to dry any polymeric material in granules, for example polyamide, polycarbonate or copolymer ABS (acrylonitrile-butadiene-styrene) or PET (polyethylene terephthalate).

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

The loading hopper 13 is mounted on a weight sensor 15, for example a load cell, arranged to detect the weight of the loading hopper 13 and of the polymeric granular material contained therein.

On the top of the drying hopper 10, facing inwards, there is also mounted a level sensor 16, designed to detect the level of polymeric granular material present in the drying hopper, or, in other words, the filling level of the hopper. drying 10.

On the bottom of the drying hopper 10, at the outlet opening 12, there is also mounted a capacitive moisture meter 17, designed to measure the humidity of the polymer granular material discharged from the drying hopper 10.

The dehumidification unit 20 can be of any type suitable for the purpose and, in the preferred example described here, it is of the rotating tower (or "rotor") type.

It therefore comprises a cylindrical tower 21, which is rotated around its own axis by a motor 22, between a pair of fixed heads 23, provided at the axially opposite ends of the tower 21.

Tower 21 contains desiccant material capable of withdrawing a large part of the moisture 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, not communicating with each other, arranged side by side and shaped in such a way as to cover the entire base surface of the tower 21 , for example by dividing the base area of the tower 21 into three adjacent circular sectors, 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 tower 21, respectively indicated with 24a, 25a and 26a, in which each section is formed by the cylindrical sector underlying the respective 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 with the delivery branch 3, between the control valve 6 and the heater 30, so that the process gas coming from the supply circuit 2 is suitably dehumidified before being heated and then introduced into the drying hopper 10 through the diffuser 5. The second manifold 25 is a regeneration manifold, designed to connect the second section 25a of the tower 21 with a 27 regeneration of the desiccant.

The third manifold 26 is a cooling manifold, arranged to connect the third section 26a of the tower 21 with a cooling duct 28, extending between the forward branch 3, upstream of the dehumidification unit 20 and the return branch 4 of the drying hopper 10. A valve 28a controls the flow rate of the process gas taken from the delivery branch 3 and conducted to the return branch 4 passing through the third section 26a without entering the drying hopper 10.

In the regeneration circuit 27 is introduced air taken from the environment by a blower and heated before entering the dehumidification unit 20 by a regeneration heater 33.

In particular, the air is heated by the heater 33 to a predefined temperature, for example between about 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 degree of regeneration of the desiccant material and therefore, indirectly, its ability to absorb moisture from the process gas in the first section 24a.

This feature allows the regeneration air temperature to be used as a tool for adjusting the dew point of the process gas at the outlet of the dehumidification unit 20.

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

Downstream of the dehumidification unit 20 and before the heater 30, a humidity sensor 20a is also appropriately mounted, designed to measure the dew point of the process gas leaving the dehumidification unit 20.

The heater 30 is preferably of the electric type, comprising one or more resistances, controlled by an on / off switch 31.

Plant 1 also includes a control unit 50 designed to control and adjust the operating parameters of plant 1.

In particular, the control unit 50 is designed to regulate one or more parameters of the drying process to ensure that the dried polymeric granular material unloading from the drying hopper 10 has a degree of humidity not higher than a desired nominal value. and not less than 90% of said nominal value.

The nominal value of the degree of humidity of the dried polymeric granular material is preset in the control unit 50 and is generally determined by the type of material and the type of processing to which the dried material is intended downstream of the drying plant.

For example, if the nominal value is equal to 100 ppm (parts per million), the control unit is expected to act on the drying process by changing one or more parameters when the degree of humidity measured by the humidity meter 17 is higher than 100 ppm or less than 90 ppm, while when the humidity degree measured is between 90 and 100 ppm the process is not modified.

The operation of the plant 1 provides for an initial phase in which the polymeric granular material is loaded into the drying hopper 10 through the loading hopper 13 and a subsequent drying phase of the material contained in the drying hopper 10 by introducing the process.

The initial parameters of the process can be preset in the control unit and include:

- the initial filling level of the hopper, detected by the level sensor 16 and determined in such a way as to ensure an appropriate residence time of the material inside the drying hopper,

- the initial flow rate of the process gas, determined based on the type of material and the hourly flow rate of dried granular material, and controlled by appropriately adjusting the opening of the control valve 6 and / or the flow rate of the blower 2a,

- the temperature of the process gas entering the drying hopper, controlled by suitably varying the power of the heater 30, and

- the dew point of the process gas entering the drying hopper, controlled by acting on the dehumidification unit 20 (for example by appropriately varying the regeneration temperature).

When the granular polymeric material is dried, it is partly discharged from the drying hopper 10, based on the request of the transformation machine downstream of plant 1.

When the filling level of the drying hopper 10 drops by a predefined quantity with respect to the initial filling level, the control unit 50 starts a feeding phase which involves the introduction into the hopper of new polymeric granular material coming from the loading hopper. 13 until the filling level is brought back to the initial value.

The filling level value can remain constant or can vary during the process, depending on the average hourly flow rate of polymer granular material dried and discharged from the drying hopper 10. This flow rate value can be obtained from the quantity of material actually loaded into the hopper. of drying suitably measured by the weight sensor 15. Alternatively, it can be obtained from the working parameters of the processing machine downstream of the drying plant 1.

The dried polymeric granular material, discharged from the drying hopper 10, passes through the outlet 12 and through the humidity meter 17 which measures the degree of humidity, sending the measured value to the control unit 50. The latter compares it with the value set nominal value and if the measured value is greater than the nominal value or less than 90% of the nominal value, then it commands the variation of one or more process parameters that affect the drying level of the polymer granular material in the hopper.

In particular, the control unit 50 can adjust the flow rate of the process gas, acting on the control valve 6, on the dew point of the process gas, acting on the dehumidification unit 20, or, on the temperature of the process gas. at the entrance to the drying hopper, by acting on the heater 30, or again on the time spent in the hopper, by acting on the filling level set in the material loading phase.

The plant and the process of the present invention can be made in different variants with respect to the preferred example described above. The system of the present invention allows you to maintain excellent control of the overall energy supplied to the drying system and to ensure the correct degree of drying of the dried polymer granular material, in order to prevent any quality problems in subsequent processing of the material.

Claims (8)

CLAIMS 1. Drying process of polymeric granular material comprising: - providing a drying hopper (10), suitable for containing said polymeric granular material, - subjecting said polymeric granular material contained in said drying hopper (10) to a drying step, - measuring the degree of humidity of said dried polymeric granular material, - comparing said humidity degree measured with a desired nominal humidity degree, e - if said measured humidity degree is lower than said nominal humidity degree, adjust a parameter of said drying step in order to bring the humidity degree of said dried polymeric granular material to said nominal humidity degree. Process according to claim 1, wherein said parameter of said drying step is regulated when said measured humidity degree is lower than said nominal humidity degree by at least 5%, preferably at least 10%. 3. Process according to claim 1 or 2, wherein said drying step comprises the introduction into said drying hopper (10) of a flow rate of a dehumidified and heated process gas. 4. Process according to claim 3, wherein said regulated parameter is the flow rate of said process gas introduced into said drying hopper (10). Process according to claim 3 or 4, wherein said regulated parameter is the dew point of said process gas fed into said drying hopper (10). Process according to any one of the preceding claims, wherein said regulated parameter is the residence time of said polymeric granular material in said drying hopper (10). 7. Drying plant for polymeric granular material comprising: - a drying hopper (10), arranged to contain said polymeric granular material, - a drying system, arranged to dry said polymeric granular material when contained in said drying hopper (10), - a humidity meter (17), arranged to measure the degree of humidity of said polymeric granular material when discharged from said drying hopper (10), - a control unit (50) designed to control at least said drying system, characterized by the fact that: - said control unit (50) is arranged to compare a humidity degree measured by said humidity meter (17), with a preset nominal humidity degree and to regulate at least one parameter of said drying system when said humidity degree measured is lower than said nominal humidity degree. 8. Drying plant according to claim 7, wherein said drying system comprises: - a process gas supply circuit (2) connected to said drying hopper for introducing into said drying hopper a process gas flow rate suitable for drying said polymeric granular material, - a control valve (6) arranged to vary the flow rate of said process gas introduced into said drying hopper, and - a dehumidification unit (20) designed to dehumidify said process gas before being placed in said drying hopper (10), and said control unit (50), when said measured humidity degree is lower than said nominal humidity degree, is arranged to regulate said process gas flow rate by means of said control valve (6) or to regulate the degree of dehumidification of said process gas by means of said dehumidification unit (20).