Description

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

The present invention is used particularly in industrial processes for transforming plastics materials in 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 transformed plastics material be free from humidity to the greatest possible extent.

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

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

The process gas, which is typically air, before being introduced into the hopper 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 it into the hopper.

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

The Applicant has observed the necessity for calibrating the drying process on the basis of the granular polymer material which will be dried. On the basis of the properties of the material, the plant has to be set in order to operate under conditions which allow to dry the polymer material without deteriorating it.

The Applicant has observed that the correct setting of the plant has to be defined not only in accordance with the specific polymer or the specific polymer composition which forms the granular material to be dried, but also in accordance with the origin of this granular material. In particular, the Applicant has observed that the correct setting of the plant may vary substantially on the basis of the percentage of recycled granular polymer material present in the material which it is desirable to dry. In general, the greater the percentage of recycled material, the greater is the possibility that the material can be degraded during the drying operation. In these cases, therefore, it may be necessary to operate under milder conditions by extending the times for the drying. 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.

This setting is generally carried out manually and this involves a poor efficiency of the production process because it is necessary for an operator to manually set the different operating parameters of the plant, often on the basis of individual experience. This generally involves a poor reproducibility of the process with a high risk of error during the initial setting of the drying parameters which can reduce the efficiency of the plant. Furthermore, these settings have to be repeated when it is desirable to use another granular polymer material which comprises, for example, a different percentage of recycled material.

The problem addressed by the present invention is to provide a drying process for granular polymer 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 a process for granular polymer material which are particularly versatile and adaptable in order to rapidly and efficiently process polymer material having different percentages of virgin material and recycled material.

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

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

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

Preferably, the plant comprises a control system, configured for receiving input parameters related to the granular polymer material to be dried.

These input parameters preferably comprise at least the percentage of virgin material and the percentage of recycled material.

Preferably, the control system is configured to determine, on the basis of the input parameters, a desired temperature of the process gas in the hopper.

Preferably, the control system is configured to determine, on the basis of the input parameters, a desired residence time of the granular polymer material subjected to drying in the hopper.

Preferably, the control system is configured to automatically set the plant to a first configuration by controlling:

• the heating system to obtain the desired temperature and

• a filling level of the hopper to obtain the desired residence time.

Preferably, the control system is configured to automatically set the plant to the first configuration on the basis of the input parameters.

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

Preferably, the process comprises the step of selecting the granular polymer material to be introduced into a hopper of a drying plant.

Preferably, the process comprises the step of introducing the granular polymer material inside the hopper.

Preferably, the process comprises the step of supplying a process gas to the hopper in order to dry the granular polymer material by means of a supply and recirculation circuit of the plant.

Preferably, the process comprises the step of at least partially recovering the process gas coming out of the hopper and supplying it to the supply and recirculation circuit.

Preferably, the process comprises the step of setting input parameters relating to the granular polymer material to be dried. More preferably, these input parameters comprise at least the percentage of virgin material and the percentage of recycled material.

Preferably, the process comprises the step of determining on the basis of these input parameters a desired temperature of the process gas in the hopper.

Preferably, the process comprises the step of determining on the basis of these input parameters a desired residence time of the granular polymer material which is subjected to drying in the hopper.

Preferably, the process comprises the step of automatically setting the plant to a first configuration on the basis of the input parameters.

Preferably, the automatic setting of the plant to the first configuration comprises the step of controlling a heating system of the plant in order to obtain the desired temperature. Preferably, the automatic setting of the plant to the first configuration comprises the step of controlling a filling level of the hopper in order to obtain the desired residence time.

Preferably, the process comprises the step of recirculating the process gas in the hopper.

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

As a result of the features set out above, the process and plant for drying granular polymer material of the present invention are found to be particularly efficient and versatile. In particular, it is possible to dry, in addition to different polymer materials, also admixtures of materials based on the same polymer but having different origins (virgin or recycled material) in any percentage combination, ensuring under any conditions the correct drying of the material.

The control system of the plant is in fact configured to calculate on the basis of the material and the percentage of recycled material a maximum temperature to which the material can be subjected without degrading and the residence time necessary for being adequately dried at the temperature calculated in this manner. Therefore, the control system prepares the plant to operate in a first configuration which complies with these specifications, controlling the heating system of the gas and the filling level of the hopper (directly obtainable if the geometry of the hopper is known and the hourly production rate of the plant is known).

This process and this plant, once the type of material to be dried and the percentage of recycled material and virgin material are known, can therefore operate completely automatically without requiring the manual intervention of an operator. This allows an increase in the efficiency and the reproducibility of the process, containing the production costs and reducing setup times as a result of any changes in configuration.

Preferably, the plant is connected downstream to a transforming machine for the granular polymer material suitable for receiving the dried granular polymer material from the hopper in order to transform it into a semifinished product or a final product, for example, a container or a bottle. This transforming machine may, for example, comprise an extruder or a mould. Preferably, once the granular polymer material has been dried, it can be discharged from the hopper by means of a pipeline which connects the hopper to the transforming machine for the granular material.

In one embodiment, the granular polymer material is based on polyethylene terephthalate (PET). The input parameters may be different if a different polymer material is used.

In one embodiment, the granular polymer material to be dried may be an admixture composed of different types of granular polymer material, for example, it may comprise three types of granular polymer material, that is to say, re-ground polymer material in addition to virgin and recycled material.

The term "virgin" is intended to be understood to be 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 term "recycled" material is intended to be understood to be a material which originates from a finished product and 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 term "re-ground" material is intended to be understood to be a material which originates from industrial waste, for example, from moulding swarf which is re-ground and put aside for re-use. In general, the material has a medium/low concentration of contaminating substances and has been subjected to thermal stresses.

In this admixture, these types of granular polymer material can be present in respective percentages which vary from 0% to 100% in such a manner that the total sum 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%. Naturally, given that the total of the different percentages has to be equal to 100%, one of these percentages may be omitted and obtained directly by the control system.

On the basis of the specific requirements and demands, the admixture of granular plastics material is selected, for example, the type of finished products which it is intended to obtain or the material which is available.

Preferably, the plant comprises a gravimetric device by means of which the admixture of polymer material to be dried is obtained. In particular, the gravimetric device produces the admixture of granular plastics material having a percentage composition which is precise and predetermined, as defined by the input parameters, from which there are then calculated the desired drying temperature and the desired residence time.

In one embodiment, the gravimetric device is controlled by the control system of the plant in which the percentages of virgin, recycled and re-ground material to be dried are input.

The term "residence time" is intended to be understood to be the mean time period which the polymer material spends in the hopper in a state subjected to the drying action of the process gas before being discharged into the transforming machine.

Therefore, the desired residence time is a minimum time, below which there is provision for the material not to be able to be sufficiently dehumidified with the gas at the desired temperature in order to be correctly processed in the downstream transforming machine.

The desired temperature is the maximum temperature at which it is possible to dry the material without involving degradation phenomena of the material and depends on the characteristics of the material, in particular the type of polymer and the origin thereof, as already set out above. On the basis of these calculated parameters, the plant is set to the first configuration.

If, for example, the admixture is composed of 100% virgin PET material, the desired temperature may be approximately 180°C and the desired residence time may be approximately 6 hours. If, for example, the admixture is composed of 100% recycled PET material, the desired temperature may be approximately 160°C and the desired residence time may be approximately 8 hours.

The desired temperature and the desired residence time are preferably calculated by the control system of the plant automatically, for example, being based on tables or curves which are stored in the internal memory thereof which correlate these parameters with the input parameters relating to the polymer admixture to be dried.

In one embodiment, it is possible to manually modify these calculated parameters.

The plant will consequently be automatically set to the first configuration on the basis of these parameters which are calculated so as to efficiently dry the material without the risk of deterioration.

The setting of the plant to the first configuration may therefore depend on the percentage of recycled, re-ground and virgin material in the admixture. Furthermore, the setting to the first configuration may depend on the type of polymer present in the admixture. Therefore, the input parameters can relate both to the percentage of recycled material in the admixture and to the specific type of polymer present in the admixture.

The first configuration allows an initial setup of the plant to be obtained so as to adapt the plant to the admixture of granular polymer material to be dried. Setting the plant to the first configuration advantageously comprises controlling the heating system in order to obtain the desired temperature. Setting the plant to the first configuration advantageously comprises controlling the filling level of the hopper in order to obtain the desired residence time.

Preferably, the circuit comprises a movement device which is configured to supply the flow of the process gas in the circuit. Advantageously, this movement device comprises at least one blower. Preferably, the movement device is positioned upstream of the heating system.

Preferably, setting the plant to the first configuration comprises setting the movement device in order to obtain a desired flow rate for the gas in the circuit. This setting is advantageously carried out automatically by means of the control system.

The desired flow rate may be an additional parameter which is calculated by the control system on the basis of the input parameters which define the type of admixture to be dried, where applicable taking account of the temperature of the gas and the residence time in the hopper. Preferably, the desired flow rate is such as to ensure a sufficient removal of humidity per unit of time from the granular polymer material in the hopper.

The flow rate value of the gas is preferably adjustable by acting on the power of the blower or the opening of a control valve.

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

Preferably, setting the plant to the first configuration comprises setting the dehumidification device in order to dehumidify the process gas up to a desired humidity value, preferably a desired absolute humidity value. This setting is advantageously carried out automatically by the control system.

The desired humidity value may be an additional parameter which is calculated by the control system on the basis of the input parameters which define the type of admixture to be dried. Preferably, the desired humidity of the gas is such as to ensure a sufficient removal of humidity per unit of time from the granular polymer material in the hopper. Advantageously, the lower is the value of the humidity of the gas, the shorter may be the desired residence time of the material for the same gas temperature. The dehumidification device 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, the towers being connected to each other in parallel so as to be selectively and alternately connected to the supply and recirculation circuit. In this manner, while one tower operates in the circuit in order to dehumidify the process gas, the other tower may be adequately regenerated. The humidity value of the gas is preferably adjustable by acting on the operating conditions of the towers.

Preferably, the heating system of the plant comprises a first heating unit, in which a catalyst group is received. This catalyst group is configured to promote a decomposition reaction of contaminating substances which may be present in the process gas, such as, for example, benzene and acetaldehyde compounds. Preferably, the plant comprises a second heating unit.

Preferably, the second heating unit is positioned downstream of the first heating unit.

Preferably, the plant comprises a first bypass pipe suitable for bypassing the first heating unit. Advantageously, the plant comprises a first bypass valve, with adjustable opening, suitable for allowing the passage of a first percentage of the process gas into the first bypass pipe.

Preferably, the control system is suitable for automatically setting the plant to the first configuration so as to obtain the desired temperature by means of controlling the heating system.

Advantageously, this control of the heating system can be brought about by controlling the first and/or second heating unit and/or by controlling the first bypass valve in order to control the passage of the first percentage of the gas. Preferably, the process comprises controlling a power which is supplied by the first heating unit. Preferably, the process comprises controlling a power which is supplied by the second heating unit.

Preferably, the process comprises controlling the first percentage of the gas in the first bypass pipe.

Preferably, the first heating unit is controlled at a different temperature from the second heating unit, more preferably higher than the second heating unit.

Preferably, the first heating unit is controlled at a temperature higher than the desired temperature.

The control system is advantageously configured to automatically set the plant to the first configuration in accordance with the input parameters (that is to say, the type of polymer admixture to be dried) so as to maintain the concentration of the contaminating substances below predefined thresholds, for example, by controlling this first percentage of gas which bypasses the first heating unit by acting on the first bypass valve.

In particular, this first percentage of gas may depend on the percentage of recycled material of the admixture and where applicable the type of polymer present in the admixture. These input parameters can advantageously supply a prediction with respect to the concentration of contaminating substance which can develop in the process, in particular benzene and acetaldehyde. In this manner, the control system allows the plant to be set to the first configuration so that the concentration of contaminating substances is kept below predefined thresholds which, in some cases, are specifically determined by the standards. There may be provided embodiments, in which the heating system comprises a single heating unit or in which the catalyst group is not received in a heating unit, for example, in which it is positioned downstream of the heating unit and in which the first bypass pipe bypasses only the catalyst group and not the heating unit. In these alternative embodiments, the control system may control the heating system so as to obtain the desired temperature of the gas.

In some embodiments, the plant comprises a condenser device which is configured to condense contaminating substances present in the process gas so as to be able to separate them from the process gas. Advantageously, the plant comprises a second bypass pipe suitable for bypassing the condenser device. 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. Preferably, the setting of the plant to the first configuration comprises setting the condenser device and the second bypass valve so as to introduce the second percentage of the process gas into the second bypass pipe suitable for bypassing the condenser device.

The control system can advantageously automatically control this second percentage of the gas, which can be determined by the input parameters, in particular may depend on the percentage of recycled material of the polymer admixture to be dried and where applicable the type of polymer present in the admixture. As previously described, these input parameters can advantageously supply a prediction with respect to the type and the concentration of contaminating substances which can develop in the process. The condenser device can be particularly advantageous if there is provision for using admixtures with high percentages of recycled material, that is to say, if there is provision for the catalyst group not to be capable alone of reducing the concentration of contaminating substances below the predefined thresholds, or when the contaminating substances have condensation temperatures which are relatively high. In these cases, in fact, the control system may advantageously reduce the second percentage of the gas so as to pass more gas into the condenser device.

Preferably, the plant comprises a level sensor suitable for monitoring the filling level of the hopper. Preferably, the plant comprises a supply system, by means of which the granular polymer material is supplied to the hopper. Preferably, the control system, when the level sensor measures a desired filling level, controls the supply system in order to supply material to the hopper.

Preferably, the process comprises monitoring the filling level of the hopper and supplying granular polymer material to the hopper so as to maintain a desired filling level in the hopper. Preferably, the gravimetric device forms part of the supply system.

The filling level of the hopper may depend on various factors in addition to the desired residence time, for example, the quantity of material to be dried in the unit of time which generally corresponds to the capacity for hourly production of the transforming machine.

The filling level of the hopper can advantageously also be measured, alternatively or additionally to the level sensor, by means of load cells. Preferably, once the material reaches the desired filling level, the control system automatically controls the supply system so as to supply in the hopper a specific quantity of additional material. Advantageously, this additional material which still has to be dried is deposited above the material already present in the hopper, raising the filling level above the desired level.

Preferably, the process comprises obtaining parameters of the gas during the drying operation of the polymer material and, if necessary, carrying out an automatic setting of the plant to a second configuration on the basis of the obtained parameters of the gas.

In particular, the parameters of the gas are obtained from respective measurements of properties of the gas. Preferably, the measured properties of the gas may comprise the temperature, the flow rate and the humidity thereof. The second configuration allows a subsequent setup of the plant which is adapted to the measurement carried out 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 for obtaining the values desired. In this manner, the plant becomes particularly reliable because by means of this verification it is possible to carry out in the plant corrective actions directed towards obtaining the desired values. These corrective actions are advantageously controlled automatically by means of the control system which sets the plant to the second configuration.

The second configuration may, for example, be identical to the first configuration if the measured values are identical to, or very similar to, the desired ones. Conversely, if the measured values are remote from the desired ones, the second configuration can be different from the first.

Preferably, the plant comprises a measuring system suitable for measuring properties of the gas and the control system is suitable for automatically setting the plant to a second configuration on the basis of the measurement.

Preferably, the measuring system comprises at least one sensor suitable for measuring a property of the gas. Advantageously, the control system receives parameters of the gas relating to the measurement and on the basis of these parameters of the gas it can set the plant automatically to the second configuration.

The greater is the number of properties of the gas measured and relevant systems or devices of the plant controlled automatically by the control system, the greater is the reliability of the plant because a more accurate verification of the correct operation of the plant is made. By means of the interaction between the measuring system and the control system, it is possible to configure the plant in a particularly rapid and efficient manner. It is advantageously possible to set the control system with feedback so as to maintain the desired values of the properties of the gas within a predetermined range.

Preferably, the process comprises measuring the temperature of the gas and automatically setting the plant to the second configuration by controlling the heating system in order to obtain the desired temperature of the gas on the basis of the measured temperature.

Advantageously, the measuring system comprises a temperature sensor suitable for measuring the temperature of the gas. Preferably, the control system is suitable for automatically setting the plant to the second configuration by controlling the heating system in order to obtain the desired temperature of the gas on the basis of the temperature measured.

Preferably, the temperature of the gas can be measured downstream of the heating system and even more preferably between the heating system and an inlet of the gas into the hopper. It is thereby possible to obtain a measurement of the temperature of the gas particularly similar to that of the gas in the hopper. Advantageously, there are present a pair of temperature sensors downstream of the first and second heating units. In this manner, it is possible to obtain particularly precise control of the power supplied by the heating unit. The arrangement of sensors in pairs allows an increase in the reliability of the plant in the case of failure of one of the two sensors.

Preferably, the process comprises measuring the flow rate of the gas and automatically setting the plant to the second configuration by controlling the movement device of the plant in order to obtain the desired flow rate of the gas on the basis of the measured flow rate.

Advantageously, the measuring system comprises a flow rate sensor which is suitable for measuring the flow rate of the gas. Preferably, the control system is suitable for automatically setting the plant to the second configuration by controlling the movement device in order to obtain the desired flow rate of the gas on the basis of the measured flow rate.

Preferably, the flow rate sensor may be positioned downstream of the movement device.

In some embodiments, the flow rate sensor may not be present and in this case the flow rate of the air may be estimated by interpolating the working curve of the blower.

Preferably, the process comprises measuring the humidity of the gas and automatically setting the plant to the second configuration by controlling the dehumidification device of the plant in order to obtain the humidity value desired of the gas on the basis of the humidity measured.

Advantageously, the measurement system comprises a humidity sensor which is suitable for measuring the humidity of the gas. Preferably, the control system is suitable for automatically setting the plant to the second configuration by controlling the dehumidification device in order to obtain the humidity value desired of the gas on the basis of the humidity measured. Advantageously, the humidity sensor can be positioned downstream of the dehumidification device. Preferably, the humidity sensor is a sensor for the dew point of the gas.

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 polymer material realized according to the present invention;

- Figure 2 is an exemplary graph of a relationship between the composition of a polymer admixture to be dried, expressed as a percentage of recycled material and virgin material, and the desired temperature of the gas and the desired residence time in the hopper in order to correctly dry the polymer admixture.

In the Figures, there is generally designated 1 a plant for drying granular polymer material which is realized according to the present invention.

The plant 1 may be provided to supply a machine for transforming granular polymer material (not illustrated). This transforming machine may, for example, comprise an extruder, a press or both.

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 polymer material to be dried may be an admixture composed of polymer material having different origins. For example, this admixture may comprise three types of granular polymer material, that is to say, virgin, recycled and re-ground material.

The plant comprises a supply system, by means of which the granular polymer 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 may comprise load cells suitable for metering the quantity of granular polymer material of the respective types. 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 polymer 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 polymer 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, comprising a charging line 4 supplied by a compressor. The gravimetric device 30 may be connected to the hopper 2 by means of the charging line 4 through which the granular polymer material to be dried is urged by the action of the compressor into a supply hopper 5 which is positioned at the top of the hopper 2 and which is provided to discharge therein the granular material by gravitational force.

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 polymer material has been dried by means of the circuit 10, it can be discharged from the hopper 2 by means of a pipeline 88 which connects the bottom of the hopper 2 to the transforming machine for the granular polymer material, in which it is transformed into the final desired product. This pipeline 88 is advantageously connected to the bottom of the hopper 2 by means of a valve 74.

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 2 a diffusor 12 is mounted.

After passing through the granular polymer 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 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.

Upstream of the inlet pipe 11, the circuit 10 comprises a heating system which includes a first heating unit 40 and a second heating unit 90. The first heating unit 40 comprises 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 advantageously 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 comprises 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 is discharged from the bypass pipe 43.

The circuit 10 further comprises a first bypass valve 44, with adjustable opening, by controlling which it is possible to set the circuit 10 so as to vary the first percentage of the process gas which passes through the first bypass pipe 43.

The second heating unit 90 is positioned downstream of the first bypass pipe 43 and is provided to heat the process gas to a desired 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. Downstream of the filtration device 6, the supply and recirculation circuit 10 may comprise a condenser device 50. The condenser device 50 comprises a heat exchanger which 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 any high-boiling contaminants present in the gas. The gas is cooled in the condenser device 50 up to approximately from 25 to 30°C. It will be appreciated that at this temperature any contaminants potentially present in the circuit, such as, for example, benzene and acetaldehyde (in a smaller measure) and other organic components, such as oils, condense so as to allow the removal thereof from the gas, at least at a significant portion. The condenser device 50 may advantageously comprise a tray which is suitable for collecting the condensate. Preferably, the tray is connected to a drain so as to be periodically emptied.

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

The circuit 10 comprises 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 comprises a dehumidification device 18 which is positioned downstream of the condenser device 50 and the second bypass pipe 51 and which is configured to dehumidify the process gas up to absolute humidity values which are desired (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 polymer 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 supply and recirculation circuit 10 further comprises a movement device 16, comprising one or more blowers 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 plant 1 further comprises a control system (not illustrated) which is provided to control the plant 1 and which is suitable for setting the circuit 10 to a predetermined configuration on the basis of predetermined parameters.

Preferably, the control system is suitable for setting the plant 1 to a first configuration on the basis of input parameters relating to the granular polymer material to be dried comprising both the type of polymer which forms the material to be dried and the origin of the material, which is defined by the percentages of virgin material, recycled material and re-ground material which form the granular material to be dried. In the example described herein, the polymer material processed is formed by granules of polyethylene terephthalate (PET) from virgin material and recycled material at various percentages.

On the basis of the input parameters, the control system is capable of automatically setting the plant 1 to a first configuration, illustrating the initial operating configuration of the plant, which is suitable for drying the granular material adequately and efficiently, avoiding phenomena of degradation of the polymer and adequately containing the presence of contaminating substances.

In particular, from the input parameters, the control system is capable of obtaining a desired temperature of the process gas to be introduced into the hopper 2 and a desired residence time of the granular polymer material in the hopper 2.

An example of how the desired temperature and the desired residence time are obtained by the control system in accordance with the input parameters is depicted in the graph of Figure 2, where the desired temperature and the desired residence time are set out as a function of the percentage of recycled material and virgin material. Similar curves can be present in the control system in order to determine desired temperatures and desired residence times when the material to be dried also contains re-ground material.

Therefore, the control system is programmed to set the plant 1 to the first configuration by controlling the heating system in order to obtain the desired temperature and a filling level of the hopper 2 in order to obtain the desired residence time.

In particular, the control system is configured to obtain the desired temperature by controlling the power supplied by the first heating unit 40 and the second heating unit 90 and by controlling the first bypass valve 44 in order to control the passage of the first percentage of the gas into the first bypass pipe 43.

Preferably, the first heating unit 40 is adjusted to a higher temperature than the second heating unit 90 (and therefore greater than the desired temperature) so as to make the catalyst 42 operate under the best operating conditions. In this case, a portion of the process gas is passed through the first bypass pipe 43 by acting suitably on the first bypass valve 44. All the process gas is therefore directed into the second heating unit 90 which, if necessary, brings the temperature of the gas to the desired temperature.

The degree of use of the first heating unit and therefore of the catalyst 42 is determined by the control system on the basis of the input parameters. For example, if the percentage of recycled material in the granular polymer material to be dried is high, then the percentage of process gas directed through the first heating unit 40 will be greater with respect to an admixture formed only by virgin material.

Therefore, the control system in the first configuration of the plant 1 also determines the degree of opening of the first bypass valve 44 so as to control the first percentage of process gas which bypasses the first heating unit 40. The filling level of the hopper 2 is controlled by the control system by means of a level sensor 61 which is provided to measure the level of the granular polymer material inside the hopper 2.

Initially, the hopper 2 is filled up to a specific filling level which is determined on the basis of the hourly production of the press and the desired residence time of the material.

The material, once dried, is caused to gradually be discharged from the hopper

2 in accordance with the requirement of the transforming machine which is positioned downstream of the plant 1 which will preferably be adjusted in order to operate at the maximum capacity thereof.

When the level sensor 61 measures the desired filling level, the control system controls the supply system in order to introduce into the hopper 2 a specific quantity of additional material which is deposited above the material already present in the hopper, increasing the filling level above the desired level.

The control system, when defining the first configuration of the plant 1 on the basis of the input parameters, also determines the degree of opening of the second bypass valve 54 so as to control the second percentage of process gas which bypasses the condenser device 50.

In particular, if the percentage of recycled material in the granular polymer material to be dried is high, then it might be advantageous to pass a portion or all of the process gas through the condenser device 50.

In general, if possible, it is preferable to prioritize the use of the catalyst 42 with respect to the condenser device 50 because it can have a negative impact on the energy efficiency of the plant.

The control system, when defining the first configuration of the plant 1 on the basis of the input parameters (or other parameters, such as, for example, the production capacity of the transforming machine), also determines the desired flow rate of process gas to be introduced into the hopper 2. The desired flow rate is preferably adjustable by acting on the power of the blowers.

The control system, when defining the first configuration of the plant 1 on the basis of the input parameters, also determines the absolute humidity value desired of the process gas to be introduced into the hopper 2 so as to suitably control the dehumidification device 18. The plant 1 further comprises a measuring system for the properties of the gas so that the control system can verify that the plant 1 configured in order to operate in the first configuration is capable of producing the results expected and, if necessary, can automatically modify this first configuration to a second configuration.

The measured properties of the gas can comprise the temperature, flow rate, humidity and concentration of one or more contaminating substances thereof. The measuring system may therefore comprise at least one temperature sensor 62 which is suitable for measuring the temperature of the gas and for allowing the control of the heating system in order to obtain the desired temperature of the gas.

There are preferably present two pairs of temperature sensors 62, a first pair downstream of the first heating unit 40 and a second pair downstream of the second heating unit 90.

In some embodiments, there may be present a flow rate sensor 63 which is suitable for measuring the gas flow rate and for allowing the control of the movement device 16 in order to obtain the desired flow rate of the gas. Preferably, the flow rate sensor 63 is positioned between the movement device 16 and the first bypass pipe 43.

The measuring system may further comprise a humidity sensor 64 which is suitable for measuring the absolute humidity of the gas and for allowing the control of the dehumidification device 18 in order to obtain the desired humidity value of the gas.

The humidity sensor 64 can be positioned between the dehumidification device 18 and the movement device 16. The humidity sensor 64 can be a sensor for the dew point.

On the basis of the humidity value of the gas measured, it is where applicable possible to vary the operating conditions of the towers by means of the control system. Furthermore, on the basis of the measured value, it is possible to invert the towers or to convey a message for maintenance or replacement of the towers.

The measuring system may further comprise a measuring device 72 which is suitable for measuring the concentration of one or more contaminating substances which are present in the process gas and allowing the control of the opening of the first bypass valve 44 and second bypass valve 54. In this manner, it is possible to vary the quantity of process gas which is processed by the catalyst 42 and the condenser device 50.

The measuring device 72 is preferably arranged near the inlet of the hopper 2 downstream of the second heating unit 90. The plant 1 can operate in the manners described below.

The hopper 2 is supplied with granular polymer material to be dried. The control system sets the plant 1 to the first configuration on the basis of the input 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 and the flow rate thereof is measured by the flow rate sensor 63. 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 circuit 10, it can pass through the second bypass pipe 51 or through the condenser device 50. Downstream of the second bypass pipe 51, all the gas is dehumidified in the dehumidification device 18 and the humidity thereof is measured by the humidity sensor 64. Subsequently, on the basis of the configuration of the circuit 10, the 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, all 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. The temperature of the gas is therefore measured by the temperature sensor 62. Near the inlet of the hopper 2, there may be present a measuring device 72 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 diffusor 12 and therefore brought back to the supply and recirculation circuit 10 by means of the outlet pipe 13.