Description

The present invention relates to a process for drying granular polymeric material and a drying plant which operates according to this process.

The present invention is used particularly in industrial processes for converting plastics materials into granules by means of extrusion or moulding.

It is known that these operations require, in order to ensure an adequate level of quality of the moulded product, that the converted plastics material be free from humidity to the greatest possible extent.

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

Therefore, these plastics materials, before being subjected to the extrusion or moulding process, advantageously have to be processed in suitable drying plants, where the water content of the granules is reduced to the minimum quantities required by the conversion process.

In a commonly used process, the drying of the granular polymeric material is carried out inside a drying hopper in which the material to be dried is introduced and in which there is introduced a continuous flow of a hot and dry process gas which provides for desorbing the water present in the granules by coming into contact with the granular material ("stripping").

The process gas, which is typically air, before being introduced into the hopper in order to come into contact with the granular material to be dried, is typically dehumidified in a suitable dehumidification device and subsequently heated to the desired temperature.

In a common embodiment, the process gas which provides for the drying of the polymer material is always the same (except for any reintegration fractions) so that the drying plant comprises a supply and recirculation circuit which recovers the process gas being discharged from the hopper and, after it has been dehumidified and heated, provides for reintroducing the process gas into the hopper.

In other words, the process gas is recirculated in the hopper after being suitably processed.

The Applicant has observed that, during the drying process of a number of polymeric materials, and in particular PET, there can be formed or released a number of undesirable contaminating substances, the presence of which inside the final product can prejudice the correct use thereof or even make it unsuitable for particular applications.

This disadvantage can become particularly serious in the case of final products which are intended for food contact, such as, for example, PET, where the potential substances released from the plastics material to the food product must not modify in any manner the organoleptic characteristics of the food product (particularly the flavour or the odour thereof) and not compromise the safety thereof in terms of toxicity.

Therefore, there is provision for the concentration of the contaminating substances in the plastic materials for food use to be maintained below predefined thresholds which, in a number of cases, are specifically determined by the standards applicable in these matters.

A significant example of an undesirable contaminating substance is constituted by benzene (or the derivatives thereof), the presence of which inside a plastic material which is intended for contact with food products, for example, for containing drinks, is standardized in a particularly strict manner.

A second example of an undesirable contaminating substance is constituted by acetaldehyde which has an odour and a flavour which are very pungent as a result of which the release thereof could affect the flavour of the drink contained in the bottle.

The Applicant has further observed that the presence of these contaminating substances is particularly significant when the polymer material to be dried comprises a relevant percentage of recycled material.

On the other hand, the Applicant has found that the use of recycled polymer material is also increasingly required by the market, in the light of the evident positive savings from the environmental point of view.

From what has been set out above, therefore, it is evident that it is necessary to develop solutions which allow a substantial reduction in the content of contaminating substances inside polymeric material during the processing process thereof.

Devices which can remove these undesirable contaminating substances are known.

An example of these devices is a condenser device (oil condenser) which comprises a heat exchanger which can condense the process gas so as to remove contaminating substances from the gas itself. One disadvantage of this device is connected with the poor thermal efficiency thereof because it is necessary to cool the process gas to such temperatures as to allow the condensation thereof; the process gas will subsequently have to be heated before being introduced into the hopper.

Another example of such devices is a catalyst group which is configured to promote a decomposition reaction of contaminating substances which are present in the process gas. A plant comprising such a catalyst group is described in the Italian patent application no. 102021000006737. This catalyst group has the disadvantage of needing high temperatures of the process gas in order to be able to operate efficiently. These temperatures may cause damage (for example, oxidation) to the granular polymeric material in the drying hoppers.

The problem addressed by the present invention is to provide a drying process for granular polymeric material and a corresponding drying plant which are structurally and functionally configured to at least partially overcome one or more of the disadvantages set out above with reference to the cited prior art.

In particular, an object of the present invention is to provide a drying plant and process for granular polymeric material which reduces the concentration of contaminating substances of polymeric material which is intended to produce, for example, containers for food products, in particular drinks bottles.

Another object of the present invention is to provide a drying plant and process for granular polymeric material which is particularly versatile and adaptable with respect to various working situations.

Another object of the present invention is to provide a drying plant and process for granular polymeric material which may be able to be configured in various configurations.

Another object of the present invention is to provide a drying plant and process for granular polymeric material which is particularly efficient from the point of view of energy.

The problem mentioned above is at least partially solved and one or more of the objects indicated above are at least partially achieved by the present invention by means of a drying plant and process for granular polymeric material comprising one or more of the characteristics set out in the appended claims.

In a first aspect thereof, the present invention is directed to a plant for drying granular polymeric material, comprising a hopper, in which the granular polymeric material is dried, and a supply and recirculation circuit configured to supply a process gas to the hopper in order to dry the granular polymeric material and to at least partially recover the process gas coming out of the hopper in order to supply it again to the hopper.

Below, for simplicity, the plant for drying granular polymeric material may be called the "plant", the supply and recirculation circuit may be called the "circuit", the granular polymeric material may be called the "material" and the process gas may be called the "gas".

The supply and recirculation circuit comprises a catalyst group configured to promote a decomposition reaction of contaminating substances which are present in the process gas and released from the granular polymeric material in the hopper and a first bypass pipe suitable for bypassing the catalyst group.

The circuit preferably comprises a first bypass valve suitable for allowing the passage of a first percentage of the process gas into the first bypass pipe.

The first bypass pipe is preferably a first branch of the supply and recirculation circuit which allows the passage of the gas to be prevented through the catalyst group. The circuit preferably comprises a first heating unit configured to heat the process gas.

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

- selecting the granular polymeric material to be introduced into a hopper of a drying plant,

- introducing the granular polymeric material inside the hopper,

- supplying a process gas to the hopper in order to dry the granular polymeric material by means of a supply and recirculation circuit of the plant;

- at least partially recovering the process gas coming out of the hopper and supplying it to the supply and recirculation circuit,

- setting the supply and recirculation circuit so as to introduce a first percentage of the process gas into a first bypass pipe which is suitable for bypassing a catalyst group placed in the circuit, this catalyst group being configured to promote a decomposition reaction of contaminating substances which are present in the process gas and released in the hopper from the granular polymeric material;

- re-supplying the process gas to the hopper.

Below, for simplicity, the drying process for granular polymeric material may be called the "process".

As a result of the features set out above, the process and the plant for drying granular polymeric material of the present invention are found to be particularly efficient and versatile. In particular, by setting the circuit of the plant in predetermined configurations, it is possible to comply with specific requirements. These predetermined configurations of the circuit are preferably established on the basis of parameters which will be described below.

The setting of the circuit can preferably be carried out after the granular polymeric material to be introduced into the hopper has been selected. In particular, the setting of the circuit can preferably be carried out after a desired admixture of granular polymeric material is selected. Alternatively, this setting can be carried out after the introduction of the granular polymeric material inside the hopper.

The term "bypass pipe of a circuit" is intended to be understood to be a pipe which can generate a deviation flow, in this case for deviating the process gas. Preferably, the first bypass pipe is positioned so as to straddle the catalyst group. Preferably, the first bypass pipe provides a deviation path for the first percentage of process gas in such a manner that the first percentage of process gas does not pass through the catalyst group. In other words, by means of the first bypass pipe, it is also possible to control the percentage of gas flow which passes through the catalyst group by subtraction. The first percentage of the gas is established on the basis of the parameters which will be described below. As a result of the first bypass pipe, there is generated in the circuit a first path for a first percentage of the process gas which does not pass through the catalyst group and a second path for a further percentage of the process gas which passes through the catalyst group. The further percentage of process gas is equal to the difference between the total flow of the process gas and the first percentage thereof.

Preferably, the first bypass pipe comprises an inlet, in the region of which the first path and the second path branch apart, and an outlet in which the first path and the second path rejoin each other. Preferably, the first bypass pipe comprises an inlet, at which the first percentage of process gas which passes into the first bypass pipe and the further percentage of process gas which passes through the catalyst pipe are separated.

Preferably, the first bypass pipe comprises an outlet, at which the first percentage and the further percentage of process gas rejoin each other.

Preferably, the first heating unit is configured to heat the process gas before it reaches the catalyst group.

On the basis of setting the circuit, there may also be provided the possibility that the first percentage of the gas may be equal to 100%, that is to say that all the process gas flows into the bypass pipe, completely bypassing the catalyst group.

In one embodiment, the first heating unit is preferably placed upstream of the bypass circuit and the catalyst group: in this case, the first heating unit is configured to heat all the process gas.

Preferably, the further percentage of process gas then flows into the catalyst group, that is to say, the flow of process gas from which the first percentage of the gas, which will instead flow into the bypass pipe, has been subtracted.

In another embodiment, the first heating unit is positioned downstream of the inlet of the bypass circuit and upstream of the catalyst group: in this case, the first heating unit is configured to heat only the process gas which passes through the catalyst group, that is to say, the further percentage of the process gas. Preferably, the supply and recirculation circuit comprises a valve system.

Preferably, by controlling the valve system, it is possible to set the supply and recirculation circuit. By controlling the valve system, it is possible to vary the first percentage and the further percentage.

This valve system preferably comprises the first bypass valve. Preferably, by controlling the first bypass valve, it is possible to set the circuit so as to introduce the first percentage of the process gas in the first bypass pipe.

In one embodiment, the first bypass valve may be an "on/off" valve, or a valve of the type which allows a value of the first percentage of the gas equal to 0% or equal to 100% to be imposed. This type of valve has the advantage of being very inexpensive and allows particularly simplified control of the circuit because the bypass pipe can be only either completely open or completely closed.

In another embodiment, the first bypass valve may be a "modulating" valve, or a valve of the type with "gradual" opening which allows a value of the first percentage of the gas between 0% and 100% to be imposed. This type of valve allows a more precise setting of the circuit to be obtained by being able to select between a higher range of percentages and therefore not only the complete opening or closing of the bypass pipe.

In one embodiment, the catalyst group comprises a support frame which is at least partially covered by a catalyst element.

In one embodiment, the support frame comprises at least one grille which is defined by a plurality of mesh (also called "cells"), through which the process gas flows. The catalyst element is preferably deposited on the mesh. In a preferred embodiment, the catalyst element is based on platinum. The catalyst group is configured to promote a decomposition reaction of contaminating substances which are present in the process gas. The contaminating substances present in the process gas may, for example, be benzene, acetaldehyde, etc.

Preferably, the plant is operationally connected to a transformation machine for transforming the granular polymeric material (not depicted) which is able to receive from the hopper the granular polymeric material which is dried, to process the material and to obtain a final product, for example, a container or a bottle. This machine may, for example, comprise an extruder, a press or both. Preferably, once the granular polymeric material has been dried by means of the supply and recirculation circuit, it can be discharged from the hopper by means of a pipeline which connects the hopper to the transforming machine for the granular material.

Preferably, the circuit comprises a movement device which is configured to supply the flow of the process gas in the circuit.

Preferably, this movement device comprises at least one fan. Preferably, the movement device is positioned upstream of the first heating unit.

Preferably, the movement device is positioned upstream of the inlet of the first bypass pipe.

Preferably, the circuit comprises a dehumidification device which is configured to dehumidify the process gas.

Preferably, the plant comprises a second heating unit which is positioned downstream of the first bypass pipe.

The second heating unit is preferably positioned downstream of the outlet of the first bypass pipe.

Preferably, the catalyst group is received in the first heating unit.

Preferably, the first bypass pipe is able to bypass both the catalyst group and the first heating unit.

In this manner, the first percentage of process gas passes neither from the catalyst group, nor from the first heating unit, but is instead directed directly from the dehumidification device to the second heating unit, if present. By positioning the catalyst group in the first heating unit, it is possible to obtain a particularly effective operation of this catalyst group. The catalyst group operates in a particularly effective manner when the process gas is heated to a predetermined temperature. By positioning the catalyst group in the heating unit, it is possible to obtain particularly precise control of the temperature because the catalyst group is in close contact with the heating elements of the first heating unit and therefore receives the process gas which has just been heated by the heating unit. By positioning the catalyst group in the first heating unit, it is possible to dedicate this heating unit to the catalyst, causing it to operate at high temperatures and therefore optimizing the operation thereof.

It is particularly possible to heat only a portion of the process gas to a particularly high temperature, for example, approximately 200°C, so as to obtain particularly effective operation of the catalyst and therefore a high decomposition of contaminating substances which are present in the gas portion. This portion of process gas is the one which does not pass into the first bypass pipe, that is to say, the further percentage. Downstream of the first bypass circuit, this portion of process gas will rejoin the first percentage of the process gas which is at a lower temperature because it passed into the first bypass pipe and avoided the first heating unit. In this manner, it is possible to adjust an overall temperature of the process gas so that it is less than the temperature of the portion of gas which passes into the catalyst. It is thereby possible to make this overall temperature of the gas suitable for coming into contact with the granular polymeric material which is present in the hopper, at the same time increasing the activity of the catalyst and therefore the removal efficiency obtained with the catalyst.

This suitable temperature may be, for example, between 160°C and 180°C. If necessary, in order to reach this suitable temperature it may potentially be possible also to use the second heating unit, heating the gas downstream of the first bypass pipe.

If the circuit is set in such a manner that the first percentage of the gas is 100%, the gas completely bypasses the first heating unit. In order to obtain the suitable temperature for being introduced into the hopper, in this case only the second heating unit is used.

Preferably, the second heating unit is positioned in the supply and recirculation circuit of the plant. Preferably, the second heating unit is configured to heat the process gas.

Preferably, the plant comprises a condenser device configured to eliminate contaminating substances present in said process gas.

Preferably, the plant comprises a second bypass pipe suitable for bypassing the condenser device. The second bypass pipe is preferably a second branch of the supply and recirculation circuit which allows the passage of the gas through the condenser device to be avoided.

Preferably, the plant comprises a second bypass valve suitable for allowing the passage of a second percentage of the process gas into the second bypass pipe. As a result of the second bypass pipe, there is generated in the circuit a third path for a third percentage of the process gas which passes through the condenser device and a fourth path for a second percentage of the process gas which does not pass through the condenser device.

Preferably, the second bypass pipe comprises a second inlet at which the third path and the fourth path branch apart and a second outlet at which the third and fourth paths rejoin each other.

The second bypass pipe allows division of the process gas into a second percentage which passes into the second bypass pipe and therefore does not pass through the condenser device and a third percentage which, conversely, does pass through the condenser device.

In particular, the second and third percentages of process gas rejoin each other at the second outlet of the second bypass pipe and are directed together towards the devices which are positioned downstream of the second bypass pipe.

Preferably, the process comprises setting the circuit so as to introduce the second percentage of the process gas in the second bypass pipe which is suitable for bypassing the condenser device which is positioned in the circuit.

Preferably, the condenser device comprises a heat exchanger which is configured to reduce the temperature of the gas.

Preferably, the condenser device is positioned in the supply and recirculation circuit of the plant.

Although the condenser device is particularly effective in terms of removing the contaminating substances, it does not have a particularly high thermal efficiency because the process gas passing from the exchanger is cooled and the temperature thereof when discharged from the condenser device is particularly low, for example, approximately 20°C. Therefore, the gas necessarily has to be heated downstream of the condenser device so as to reach the temperature suitable for coming into contact with the granular polymeric material which is present in the hopper. As a result of this poor thermal efficiency, it is preferable to use the condenser device only in some particular circumstances which will be described below. Preferably, the second bypass pipe is included in the supply and recirculation circuit.

Preferably, the second bypass pipe is positioned so as to straddle the condenser. Preferably, the second bypass pipe provides a deviation path for the second percentage of the process gas so that the second percentage of the process gas does not pass through the condenser device.

In other words, by means of the second bypass pipe, it is also possible to control by subtraction the percentage of flow which passes into the condenser device. The second percentage of the gas is established on the basis of the parameters which will be described below.

Preferably, the valve system comprises the second bypass valve. Preferably, by controlling the second bypass valve, it is possible to set the circuit so as to introduce the second percentage of the process gas into the second bypass pipe. In one embodiment, the second bypass valve may be an "on/off" valve. In another embodiment, the second bypass valve may be a "modulating" valve. The advantages connected with the type of valve are the same as those described with reference to the first bypass valve.

The circuit is preferably set by means of controlling the valve system. Setting the circuit preferably comprises imposing the first percentage of the gas and where applicable also the second percentage of the gas. This is preferably carried out by controlling the first and where applicable also the second valve.

As described above, the circuit can be set on the basis of predetermined parameters. These parameters preferably comprise first parameters relating to the granular polymeric material to be dried. These parameters preferably comprise first parameters relating to the type of granular polymeric material to be dried.

Preferably, the process comprises setting the circuit in a first configuration on the basis of the first parameters.

Preferably, the granular polymeric material is a granular polymeric material based on polyethylene terephthalate (PET).

Preferably, the granular polymeric material to be dried may be an admixture composed of different types of granular polymeric material.

Preferably, this admixture may comprise three types of granular polymeric material, that is to say, virgin, recycled and re-ground polymeric material.

The virgin material is a new material, that is to say, it does not result from being re-used. Generally, it contains a low concentration of contaminating substances and has not been subjected to thermal stresses.

The recycled material comprises material which has already been used and which it is desirable to re-use. In general, it contains a high concentration of contaminating substances and has been subjected to thermal stresses.

The re-ground material originates from industrial waste, for example, it may originate from swarf which is obtained during moulding and which is re-ground and put aside for re-use. In general, the re-ground material has a medium/low concentration of contaminating substances and has been subjected to thermal stresses.

In this admixture, these types of granular polymeric material can be present in respective percentages which vary from 0% to 100% in such a manner that the total sum of the percentages is 100%. For example, one type of admixture may comprise 70% virgin material, 25% recycled material and 5% re-ground material. There may also be used admixtures in which there is present only virgin material and therefore the percentages of recycled and re-ground materials are both 0%, or admixtures in which there is present only recycled material and therefore the percentages of virgin material and re-ground material are both 0%. On the basis of requirements, therefore, it is possible to select the correct admixture of granular polymeric material. The selection of the desired mixture may depend on, for example, the type of finished pieces which it is desirable to obtain or the type of polymer material which is available.

Preferably, the plant comprises a gravimetric device by means of which the admixture is obtained. In particular, the gravimetric device produces the correct admixture of granular plastic material.

Preferably, the plant comprises a supply system, by means of which the granular polymeric material is supplied to the hopper.

Preferably, the gravimetric device forms part of the supply system.

The first configuration allows an initial setup to be obtained for the circuit so as to adapt the circuit to the admixture of granular polymeric material to be dried. Setting the first configuration preferably comprises setting the circuit by imposing the first percentage and where applicable the second percentage of the gas and consequently the further percentage and where applicable the third percentage.

On the basis of the admixture of granular polymeric material, it is possible to imagine the concentrations of contaminating substances. Preferably, the concentration of contaminating substances is the one which will be present in the granular polymeric material and/or the process gas. On the basis of this concentration, the circuit is set to the first configuration.

If, for example, the admixture of granular polymeric material is composed of granular polymeric material which is 100% virgin material, it is possible expect that the concentrations of contaminating substances is initially low. Therefore, it is advantageous to set the circuit so that both the first and the second percentages of the gas are equal to 100%, or so that the process gas completely bypasses both the catalyst group and the condenser device. In other words, with this setting the process gas passes into the first and second bypass pipes and does not pass either from the catalyst group or from the condenser device. Conversely, if the granular polymeric material admixture has a high percentage of recycled material, it is possible to expect that the concentrations of contaminating substances could be also initially high. Therefore, it is advantageous to set the circuit so that both the first and the second percentages of the gas are equal to 0%, or so that all the process gas passes both through the catalyst group and through the condenser device. This may in fact be one of the circumstances in which it is necessary to use the condenser device despite the poor thermal efficiency thereof, because for high percentages of recycled polymer material, the action of the catalyst could be insufficient to remove the necessary quantity of contaminating substances.

Therefore, the setting to the first configuration may depend on the percentage of recycled material in the admixture.

Furthermore, the setting to the first configuration may depend on the type of recycled polymeric material which is present in the admixture. This is because, on the basis of the type of recycled material, the concentration of the provided contaminating substances, for example benzene, may vary.

Therefore, the first parameters may relate both to the percentage of recycled material in the admixture and to the type of recycled material present in the admixture of granular polymeric material. In a preferred embodiment, the setting of the circuit may be carried out by means of a control system.

Preferably, the control system is able to automatically set the circuit in a predetermined configuration on the basis of predetermined parameters. Preferably, the control system sets the circuit by controlling the valve system.

Preferably, the control system is able to set the circuit to the first configuration on the basis of the first parameters. Preferably, the control system sets the circuit to the first configuration by controlling the first bypass valve and where applicable the second bypass valve.

Preferably, the first parameters are supplied to the control system in an automated manner. Alternatively, it is possible to manually supply the first parameters to the control system. On the basis of these first parameters, the control system may therefore automatically set the circuit to the first configuration. In some embodiments, the data can be supplied by the operator to the gravimetric device and the control system is configured in such a manner that these data are sent automatically to the dehumidification device.

In an alternative embodiment, the setting of the circuit can be carried out manually. For example, it is possible to obtain the first parameters by knowing the composition of the admixture of the polymeric material and manually setting the circuit to the first configuration, for example, by acting on the valve system, by regulating the first bypass valve and where applicable the second bypass valve.

The automated setting of the circuit by means of a control system is preferable because it improves the efficiency of the production process and speeds up the response times with respect to the manual setting. As described above, the circuit can be set on the basis of predetermined parameters. These parameters preferably comprise second parameters relating to the concentration of contaminating substances. The concentration of contaminating substances is preferably the one present in the granular polymeric material and/or in the process gas.

Preferably, the process comprises measuring this concentration of contaminating substances.

Preferably, the process comprises obtaining second parameters relating to this measured concentration.

Preferably, the process comprises setting the circuit to the second configuration on the basis of these second parameters.

Setting the second configuration preferably comprises setting the circuit by imposing the first percentage of the gas which passes into the first bypass pipe and where applicable the second percentage of the gas which passes into the second bypass pipe. Consequently, the further percentage and where applicable the third percentage are therefore defined.

The second configuration allows a subsequent setup of the circuit which is adapted to the measured concentration of contaminating substances to be obtained. Therefore, this second configuration constitutes a type of verification in order to evaluate whether the first configuration has been adequate or not. In this manner, the plant becomes particularly reliable because by means of this verification it is possible to avoid the concentration of contaminating substances from going beyond predefined thresholds.

The second configuration may, for example, be equal to the first configuration if the measured concentrations of contaminated substances are below the predefined thresholds. These thresholds may in some cases be established by standards. In this case, the first percentage of the gas which passes into the first bypass pipe and the second percentage of the gas which passes into the second bypass pipe may remain identical.

Conversely, if the measured concentrations are above the predefined thresholds, the second configuration may be different from the first one. In this last case, the first percentage of the gas which passes into the first pipe may be reduced and where applicable the second percentage of the gas which passes into the second bypass pipe may also be reduced. The elimination of the contaminating substances from the gas is thereby promoted.

If it is necessary to increase the capacity for removing the contaminating substances, it is preferable to initially reduce the first percentage of the gas by therefore increasing the portion of gas which passes into the catalyst group. If the increase is not sufficient to reduce the concentration of the contaminating substances below the predefined thresholds, it is preferably possible to also reduce the second percentage of the gas, therefore also using the condenser device to remove these contaminating substances from the gas.

This capacity of the plant to adapt to the different requirements for removing the contaminating substances makes it particularly versatile.

Preferably, the plant comprises a measuring system suitable for measuring the concentration of contaminating substances.

Preferably, the control system is suitable for setting the circuit to the second configuration on the basis of the second parameters.

Preferably, the control system sets the circuit to the second configuration by controlling the first valve and where applicable the second valve. By means of the interaction between the measuring system and the control system, it is possible to configure the circuit in a particularly rapid and efficient manner. It is possible to impose the control system with feedback so as to maintain the concentration of contaminants within a predetermined range.

By means of the measuring system, it is possible to carry out monitoring of the efficiency of the devices present in the circuit, for example, by measuring the concentration of contaminating substances downstream of the catalyst group and therefore by monitoring the capacity for absorption of contaminating substances of the catalyst.

In an alternative embodiment, it is possible to manually set the circuit to the second configuration. For example, it is possible to obtain the second parameters by reading the concentration of contaminating substances from the measuring system and providing for manual setting of the circuit to the second configuration, for example, by acting on the valve system, regulating the first valve and where applicable the second valve.

Preferably, the measuring system comprises a first measuring device which is positioned downstream of the hopper.

Preferably, the first measuring device is suitable for measuring the concentration of contaminating substances of the granular polymeric material.

This measurement is particularly accurate because it is carried out directly on the granular polymeric material being discharged from the hopper, or on the granular polymeric material which is intended to be moulded.

Preferably, the first measuring device is suitable for measuring the concentration of contaminating substances in the granular polymeric material being discharged from the hopper. Preferably, the measuring system comprises a second measuring device. Preferably, the second measuring device is suitable for measuring a concentration of contaminating substances of the process gas.

This measurement is found to be effective because it is possible to obtain the measurement of the concentration within particularly rapid times. Given that the measurement is on the process gas, there is preferably not present a cryogenic grinding of the granular material.

In one embodiment, the measuring system may comprise both the first and the second measuring devices. In this case, the second measuring device may be used as an additional verification of the measurement of the contaminating substances. This may be necessary in particular situations in which a particular precision is required in measuring the contaminating substances.

In an alternative embodiment, the measuring system may comprise either only the first or only the second measuring device. Both the first and second measuring devices in fact provide a sufficiently accurate measurement of the contaminating substances.

Preferably, the second measuring device comprises a chromatography gas.

This device allows a particularly reliable measurement to be obtained of the contaminating substances.

Preferably, the second measuring device is positioned near an inlet of the hopper. Even more preferably, the second measuring device is positioned at the inlet of the hopper.

It is thereby possible to obtain a measurement of the concentration of contaminating substances which is particularly accurate because it is carried out on the process gas just before it is introduced into the hopper and therefore just before it comes into contact with the granular polymeric material present therein. Preferably, there is provided an inlet pipe for the gas in the hopper. Preferably, the inlet pipe extends inside the hopper and terminates at an end thereof positioned inside the hopper with a diffuser which is intended to promote the diffusion of the gas inside the hopper and between the granular material.

In another embodiment, the second measuring device may be positioned in this inlet pipe before a diffuser. This diffuser may be mounted at one end of an inlet pipe by means of which the circuit introduces the process gas into the hopper. This positioning also allows a particularly accurate measurement of the concentration of contaminating substances to be obtained for the same reasons as those set out for the positioning near or in the region of the inlet of the hopper.

In other embodiments, the second measuring device may be positioned elsewhere in the circuit, for example, downstream of the catalyst group so as to indicate whether this catalyst group is removing the contaminating substances in an adequate manner or whether a possible malfunction is present.

The features and advantages of the invention will be better appreciated from the detailed description of a preferred embodiment thereof, which is illustrated by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is a schematic view of a plant for drying granular polymeric material which is constructed according to the present invention.

In the Figures, there is generally designated with reference 1 a drying plant for drying granular polymeric material which is constructed according to the present invention. The plant 1 may be configured to supply a transformation machine for transforming a granular polymeric material (not illustrated). This machine may, for example, comprise an extruder, a press or both.

The plant comprises a drying hopper 2, in which an admixture of granular polymeric material to be dried is supplied and dried.

In the example described herein, there is provided a single drying hopper 2 but there may also be provided two or more hoppers which are arranged in series or in parallel.

The granular polymeric material to be dried may be an admixture composed of different types of granular polymeric material. This admixture may comprise three types of granular polymeric material, that is to say, virgin, recycled and re-ground granular polymeric material. The plant comprises a supply system, by means of which the granular polymeric material is supplied to the hopper 2. The supply system comprises a gravimetric device 30, by means of which the admixture is obtained. This gravimetric device 30 may preferably comprise a first tank 31 which is suitable for receiving the virgin material, a second tank 32 suitable for receiving the re-ground material and a third tank 33 suitable for receiving the recycled material. The gravimetric device 30 is suitable for mixing the different types of material, therefore preparing the admixture. Preferably, this admixture is obtained in a container of the gravimetric device 30 in which the quantities of granular polymeric material of the respective types are introduced. The gravimetric device 30 can comprise a valve, preferably a star- like valve, by means of which the admixture of granular polymeric material is gradually caused to be discharged from the gravimetric device 30 in order to be able to be conveyed into the hopper 2. The supply system is preferably of the pneumatic type. The supply system may comprise a charging line 4 and a compressor. The gravimetric device 30 may be connected to the hopper 2 by means of the charging line 4 through which the granular polymeric material to be dried in the hopper 2 is urged by the action of the compressor.

The plant 1 comprises a supply and recirculation circuit 10 which is associated with the hopper 2 in order to introduce therein a hot and dry process gas which, by passing through the granular material which is contained in the hopper 2, is capable of reducing the degree of humidity thereof to desired and appropriate levels for the subsequent processing steps. The process gas is typically air but may also be an inert gas without any oxygen.

Once the granular polymeric material has been dried by means of the supply and recirculation circuit 10, it can be discharged from the hopper 2 by means of a pipeline 88 which connects the hopper 2 to the transformation machine for transforming the granular polymeric material, in which the granular polymeric material is transformed in a transforming machine, for example, moulded, in order to obtain a final product, for example, a container or a bottle. When the level of the granular polymeric material present in the hopper 2 sinks below a specific level, the granular polymeric material may be reintegrated in the hopper 2 by means of the supply system.

In particular, the supply and recirculation circuit 10 introduces the process gas into the hopper 2 through an inlet pipe 11, at the internal end of which with respect to the hopper a diffuser 12 is mounted.

After passing through the granular polymeric material which is contained in the hopper 2, the process gas is recovered at the outlet from the top of the hopper 2 by an outlet pipe 13 of the supply and recirculation circuit 10. There may be mounted on the outlet pipe 13 a filtration device 6, for example, a separation cyclone, which is configured to separate the process gas from any powder which is conveyed out of the interior of the hopper 2. The outlet pipe 13 is therefore connected to the inlet pipe 11 in order to re-introduce into circulation the process gas which is discharged from the hopper 2. A reintegration line which is not shown in the Figure may be further provided in order to re-integrate where necessary the process gas present in the supply and recirculation circuit 10 with fresh process gas.

The supply and recirculation circuit 10 may comprise a condenser device 50 which is preferably positioned downstream of the filtration device 6. The condenser device 50 may comprise a heat exchanger. Preferably, the exchanger is passed through by a heat exchange liquid, for example, water, which is introduced into the exchanger at low temperature, for example, at approximately 15°C, so as to condense the gas. The gas may be introduced into the condenser device 50 at approximately from 25 to 30°C. It will be appreciated that the contaminants present in the supply and recirculation circuit 10, such as, for example, benzene and acetaldehyde, condense at these temperatures, allowing the removal of a significant portion thereof from the gas. The gas may be introduced into the condenser device 50 at approximately 80°C and is particularly humid because it has absorbed humidity from the granular polymeric material present in the hopper 2. Preferably, when the gas comes into contact with the cold wall of the exchanger, the humidity present in the gas condenses. During condensation, there is advantageously removed from the gas a given portion of contaminating substances, such as, for example, benzene and acetaldehyde. The condenser device 50 preferably comprises a bowl which is suitable for collecting the condensate. Preferably, the bowl is connected to a drain so as to be periodically emptied.

The circuit 10 may comprise a second bypass pipe 51 which is suitable for bypassing the condenser device 50. Preferably, the second bypass pipe 51 comprises a second inlet 52, through which a second percentage of the process gas can be introduced into the second bypass pipe 51, and a second outlet 53 through which the second percentage of the process gas can be discharged from the second bypass pipe 51.

At the second inlet 52, a second percentage of the process gas which is introduced into the second bypass pipe 51 and a third percentage of the process gas which is introduced into the condenser device 50 are separated. In this manner, the third percentage of the process gas is processed in the condenser device 50 while the second percentage flows into the second bypass pipe 51. At the second outlet 53, the second percentage and the third percentage of the process gas rejoin each other. The circuit 10 may comprise a second bypass valve 54, by controlling which it is possible to set the circuit 10 so as to introduce the second percentage of the process gas into the second bypass pipe 51.

The supply and recirculation circuit 10 may comprise a dehumidification device 18 which is positioned downstream of the second bypass pipe 51 and which is configured to dehumidify the process gas up to absolute humidity values which are predefined (for example, corresponding to a dew point of the process gas between -30°C and -50°C) and which are suitable for drying the granular polymeric material inside the hopper 2.

The dehumidification device 18 may be of any known type in the sector and, for example, may comprise a pair of towers, each one containing a suitable quantity of drying compound, for example, molecular sieves, which are connected to each other in parallel so as to be selectively and alternately connected to the supply and recirculation circuit 10.

The degree of humidification of the process gas may be measured downstream of the dehumidification device 18 and is preferably adjustable by acting on the operating conditions of the towers.

The supply and recirculation circuit 10 may further comprise a movement device 16, comprising one or more fans which are suitable for moving the process gas along the supply and recirculation circuit 10. This movement device 16 may preferably be positioned downstream of the dehumidification device 18.

The circuit 10 comprises a first heating unit 40 comprising a plurality of heating elements 41 which are formed, for example, by groups of electrical resistors. In the embodiment depicted, a catalyst group 42 is received in the first heating unit 40, preferably downstream of the heating elements 41. This catalyst group 42 is configured to promote a decomposition reaction of contaminating substances present in the process gas. In particular, the catalyst group 42 is configured to promote an oxidation reaction of the hydrocarbons present in the process gas and, preferably, to promote the oxidation reaction of benzene and acetaldehyde.

In a preferred embodiment, the catalyst group 42 may comprise a support frame which is fixed to the casing of the first heating unit 40. The frame may comprise at least one grille which is positioned transversely relative to the passage direction of the process gas. The grille may preferably have mesh which are formed in a honeycomb-like manner with a density of approximately 600 mesh per square inch. The mesh may preferably be covered with platinum which acts as a catalyst element for the oxidation reaction of the hydrocarbons.

The circuit 10 may comprise a first bypass pipe 43 which is suitable for bypassing the first heating unit 40 and therefore also the catalyst group 42. Preferably, the first bypass pipe 43 comprises an inlet 45, through which a first percentage of the process gas is introduced into the first bypass pipe 43, and an outlet 46, through which the first percentage of the process gas can be discharged from the bypass pipe 43.

At the inlet 45, a first percentage of the process gas which is introduced into the first bypass pipe 43 and a further percentage of the process gas which is introduced into the first heating unit 40 are separated. In this manner, the further percentage of the process gas is processed in the first heating unit 40 while the first percentage flows in the first bypass pipe 43 and does not pass through the first heating unit 40.

The supply and recirculation circuit 10 may comprise a first bypass valve 44, by controlling which it is possible to set the circuit 10 so as to introduce the first percentage of the process gas in the first bypass pipe 43.

The supply and recirculation circuit 10 may comprise a second heating unit 90 which is positioned downstream of the first bypass pipe 43 and which is provided to heat the process gas to a predefined temperature near an inlet 73 in the hopper 2, for example, to a temperature between approximately 160°C and 180°C. The second heating unit may comprise a plurality of heating elements 91 which are formed, for example, by groups of electrical resistors.

The second heating unit 90 is positioned downstream of the outlet 46. At the outlet 46, the first percentage and the further percentage of the process gas rejoin each other and are directed together towards the second heating unit 90, when provided.

The plant 1 may comprise a control system which is suitable for setting the circuit 10 to a predetermined configuration on the basis of predetermined parameters. Preferably, the control system sets the supply and recirculation circuit 10 to the configuration by controlling the first valve 44 and where applicable the second valve 54.

The control system is suitable for setting the circuit 10 to a first configuration on the basis of first parameters relating to the granular polymeric material to be dried.

Preferably, the control system sets the circuit 10 to the first configuration by controlling the first valve 44 and where applicable the second valve 54. The processed polymer material is formed by granules, for example, of polyethylene terephthalate (PET). The first parameters may be different if a different material is used.

Preferably, the plant 1 comprises a measuring system which is suitable for measuring the concentration of contaminating substances. Preferably, the control system is suitable for setting the circuit 10 to a second configuration on the basis of second parameters relating to the measured concentration of the contaminating substances. Preferably, the control system sets the circuit 10 to the second configuration by controlling the first valve 44 and where applicable the second valve 54.

In the embodiment shown in the Figure, the measuring system may comprise both a first measuring device 71 and a second measuring device 72.

The first measuring device 71 may be positioned downstream of the hopper 2 and is suitable for measuring the concentration of contaminating substances of the granular polymeric material being discharged from the hopper 2. This material being discharged may flow through the pipeline 88 which connects the hopper 2 to the transformation machine for transforming the granular polymeric material. The granular polymeric material is preferably taken from the pipeline 88 in order to be able to carry out the measurement of the concentration of contaminating substances. Before the measurement, the material may be subjected to cryogenic grinding. The measurement can last approximately 15 minutes. By measuring with the first measuring device 71, the concentration of benzene and/or acetaldehyde is preferably measured.

The second measuring device 72 is suitable for measuring a concentration of contaminating substances of the process gas. The second measuring device 72 is positioned near the inlet 73 of the hopper 2. The second measuring device 72 may comprise a chromatography gas. The chromatography gas may include a covered tube, downstream of which the substances contained in the air flow are separated, leaving at different time intervals. This occurs because the covering of the tube retains for longer time a number of substances with respect to other substances. By selecting the adequate covering for the tube, on the basis of which substance it is desirable to identify, it is possible to define a time period in which to verify the concentration of the substance. Preferably, the substances to be identified are benzene and/or acetaldehyde. In order to measure the quantity of the substances being discharged from the tube, it is possible to use a detector. Preferably, the detector may be an FID or flame ionization detector (which is more economical) or a gas mass.

The setting of the supply and recirculation circuit 10 could be carried out, by way of example, in the manners described below. In a first example, the granular polymeric material to be dried is an admixture composed of a single type of granular polymeric material, or virgin polymer material. The control system receives first parameters relating to the granular polymeric material to be dried, in this case 100% virgin material, and sets the circuit 10 to a first configuration. In this case, the first configuration comprises setting the first and second percentages of the gas both to be equal to 100%. In this manner, it is possible to impose in the circuit 10 a situation so that the process gas completely bypasses both the catalyst group 42 and the condenser device 50 because there is provision for the concentration of contaminating substances for this admixture to be zero, in view of the absence of recycled material. This setting is preferably carried out by controlling the first and second bypass valves 54 by means of the control system. The first configuration can be an initial setup. It may be the case that the concentration of contaminating substances also increases over time if the admixture is 100% virgin polymer material. Therefore, the measuring system can measure the concentration of contaminating substances and can communicate it to the control system. This system can set the circuit 10 to a second configuration on the basis of the second parameters relating to this measured concentration. In this second configuration, if the concentration of contaminating substances is above predefined thresholds, the first percentage of the gas is reduced in the first bypass pipe 43, for example, by setting it between 50% and 60%, in order to cause a portion of the gas to pass into the catalyst group 42. It is further possible to control with feedback this concentration of contaminating substances so as to vary periodically this second configuration by adapting it to the measurements carried out.

In a second example, the granular polymeric material to be dried is an admixture composed of three types of granular polymeric material at the following percentages: 20% virgin material, 5% re-ground material and 75% recycled material. The control system receives first parameters relating to the granular polymeric material to be dried, in this case 20% virgin material, 5% re-ground material and 75% recycled material, and sets the circuit 10 to the first configuration. In this case, the first configuration comprises setting the first and second percentages of the gas both to be equal to 0%. In this manner, it is possible to impose a situation in the circuit 10 so that the process gas completely passes both through the catalyst group 42 and through the condenser device 50 because there is provision for the concentration of contaminating substances for this admixture to be particularly high, given the high presence of recycled material. This setting is preferably carried out by controlling the first bypass valve 44 and the second bypass valve 54 by means of the control system. In this example, it is possible for the second configuration to be identical to the first one and therefore for the first and second percentages of the gas to remain unchanged, given the high percentage of recycled material in the admixture. The measuring system can be used to verify that the concentrations of contaminating substances remain below the predefined thresholds. By means of the measuring system, it is also possible to verify that the catalyst group and the condenser device are operating as they should, or that there are no malfunctions or repairs to be carried out.

In a third example, the granular polymeric material to be dried is an admixture which is composed of three types of granular polymeric material, at the following percentages: 60% virgin material, 5% re-ground material and 35% recycled material. The control system receives first parameters relating to the granular polymeric material to be dried, in this case 60% virgin material, 5% re-ground material and 35% recycled material, and sets the circuit 10 to the first configuration. In this case, in the first configuration the first percentage of the gas is set to be equal to 0% and the second percentage of the gas to be equal to 100%. It is thereby possible to impose a situation on the circuit 10 so that all the process gas passes through the catalyst group 42 and bypasses the condenser device 50 because there is provision, for this admixture, for the single catalyst group to be at least initially capable of reducing the concentration of contaminating substances below the predefined thresholds. This setting is preferably carried out by controlling the first bypass valve 44 and the second bypass valve 54 by means of the control system. It may be the case that the concentration of contaminating substances increases over time. Therefore, the measuring system can measure the concentration of contaminating substances and can communicate it to the control system. This system can set the circuit 10 to the second configuration on the basis of second parameters relating to this measured concentration. In this second configuration, if the concentration of contaminating substances measured is above the predefined thresholds, the second percentage of the gas is reduced in the second bypass pipe 51, for example, between 50% and 60%, in order to make a portion of the gas pass into the condenser device 50. It is further possible to control with feedback this concentration of contaminating substances so as to vary periodically this second configuration. For example, by means of the measuring system, there can be measured a concentration of contaminating substances which is still above the predefined thresholds, and in this case it serves to vary the second percentage of the gas until reaching 0%, so that all the gas passes into the condenser device 50.

The control system can be used to set the power of the first heating unit 40 and the second heating unit 90 so as to obtain a pre-established temperature of the gas, for example, a pre-established temperature of the gas near the inlet 73 of the hopper 2. This pre-established temperature may depend on the first parameters, for example, for an admixture comprising 100% virgin material that temperature may be approximately 180°C while for an admixture comprising a high percentage of recycled material that temperature may be more limited, reaching a maximum of approximately 160°C for an admixture comprising 100% recycled material.

The control system may further be used to set the plant 1 so as to obtain a desired residence time for the granular polymeric material in the hopper 2, or the time for which material has to remain in the hopper 2 so that the process gas, at the pre-established temperature, can absorb from it the humidity desired before being able to be discharged from the hopper by means of the pipework 88. This desired residence time may depend on the first parameters, for example, for an admixture comprising 100% virgin material the desired dwell time may be approximately 6 hours while for an admixture comprising a high percentage of recycled material this desired residence time may be approximately 8 hours. One method for setting the plant 1 in order to obtain the desired residence time may be the method of controlling a filling level of the hopper.

The control system can further be used to set the power of the movement device 16 so as to obtain a pre-established flow rate of the gas in the circuit 10. This pre-established flow rate has to be such as to ensure a sufficient removal of humidity per unit of time from the granular polymeric material in the hopper 2. The higher the value of the flow rate, the lower may be the desired residence time of the material, for the same temperature of the gas. The flow rate value of the gas is preferably adjustable by acting on the power of the fan.

The power of the movement device 16 can be varied by means of the control system on the basis of the first percentage of the gas which is introduced into the first bypass pipe 43. If, for example, the first percentage of the gas which is introduced into the first bypass pipe 43 is set to 0%, all the gas will have to pass through the catalyst group. In this case, it is possible to verify pressure losses as a result of the passage of the gas into the catalyst group 42 and, to compensate for these losses, it may be necessary to increase the power of the movement device 16 by means of the control system. Similarly, the power of the movement device 16 may be varied on the basis of the second percentage of the gas which is introduced into the second bypass pipe 51.

The plant 1 can operate in the manners described below.

The hopper 2 is supplied with granular polymeric material to be dried. The control system sets the circuit 10 to the first configuration on the basis of the first parameters, as described above. The process gas is moved along the supply and recirculation circuit 10 by means of the action of the movement device 16. The process gas being discharged from the hopper 2 passes into the filtration device 6 and then, on the basis of the configuration of the supply and recirculation circuit 10, it can pass through the second bypass pipe 51 or through the condenser device 50. Downstream of the second bypass pipe 51, the process gas is dehumidified in the dehumidification device 18. Subsequently, on the basis of the configuration of the circuit 10, the process gas can pass through the first bypass pipe 43 or through the first heating unit 40 and therefore also through the catalyst group 42. Downstream of the first bypass pipe 43, the gas passes through the second heating unit 90 which is controlled so that the temperature of the gas before the introduction into the hopper 2 has the desired value. Near the inlet of the hopper 2, there may be present the second measuring device which is suitable for measuring the concentration of contaminating substances in the gas. The process gas is therefore introduced into the hopper 2 through the inlet pipe 11 and the diffuser 12 and therefore brought back to the supply and recirculation circuit 10 by means of the outlet pipe 13.