Process for drying polymeric granular material and system operating according to said process

Abstract

A process for drying polymeric granular material (2) comprises the steps of: —introducing into said drying hopper (10) a process gas having a predefined flow rate so as to heat and dry the polymeric granular material, —discharging a portion of the heated polymeric granular material into a transformation unit (100) for the polymeric material; —loading an amount of fresh polymeric granular material (2a) into the drying hopper. The process gas flow rate is regulated by measuring the inlet temperature of the fresh polymeric granular material (2a) and comparing it with a predefined inlet temperature of the fresh polymeric granular material, on the basis of which the predefined process gas flow rate has been calculated. If the measured inlet temperature is different from the predefined inlet temperature, the flow rate of the process gas is regulated on the basis of the measured inlet temperature.

Claims

1. Process for drying polymeric granular material (2), comprising: providing said polymeric granular material in a drying hopper (10), introducing into said drying hopper a process gas having a predefined flow rate and temperature so as to heat said polymeric granular material to a predefined discharging temperature and to dry said polymeric granular material to a predefined residual moisture value, discharging a portion of said polymeric granular material heated to said discharging temperature into a transformation unit (100) for said polymeric material; loading an amount of fresh polymeric granular material (2a) having an inlet temperature into said drying hopper, measuring said inlet temperature and comparing it with a predefined inlet temperature of said polymeric granular material, on the basis of which said predefined process gas flow rate has been calculated, and if said measured inlet temperature is different from said predefined inlet temperature, regulating the flow rate of said process gas on the basis of said measured inlet temperature.

2. The process according to claim 1, wherein said inlet temperature of said polymeric granular material is measured in a loading hopper (13) that is positioned upstream of said drying hopper.

3. The process according to claim 1 further comprising: measuring the discharging temperature of said polymeric granular material at predefined time intervals, determining a progression over time of said discharging temperature on the basis of said measurements, and regulating said flow rate of said process gas on the basis of said progression over time of said discharging temperature.

4. The process according to claim 3, wherein, if said progression over time of said discharging temperature is decreasing, the flow rate of said process gas is increased.

5. The process according to claim 3, wherein, if said progression over time of said discharging temperature is increasing, the flow rate of said process gas is decreased.

6. The process according to claim 1, wherein said transformation unit (100) comprises: a heating device (101) in which said polymeric granular material discharged from said hopper is brought into a molten or semi-molten state, and a mold (102) or an extrusion head, into which said polymeric granular material is introduced in the molten or semi-molten state, and the method further comprises: measuring the pressure at which said molten or semi-molten polymeric granular material is injected into said mold or said extrusion head at predefined time intervals, so as to stabilize a progression over time of said injection pressure, and regulating said flow rate of said process gas on the basis of said progression over time of said injection pressure.

7. The process according to claim 6, wherein, if said progression over time of said injection pressure is decreasing, the flow rate of said process gas is increased.

8. The process according to claim 6, wherein, if said progression over time of said injection pressure is increasing, the flow rate of said process gas is decreased.

9. The process according to claim 1, further comprising: measuring said discharging temperature of said polymeric granular material at predefined time intervals so as to define a progression over time of said discharging temperature, and regulating the flow rate of said process gas on the basis of said progression over time of said discharging temperature.

10. Process for drying polymeric granular material (2), comprising: providing said polymeric granular material in a drying hopper (10), introducing into said drying hopper a process gas having a predefined flow rate and temperature so as to heat said polymeric granular material to a discharging temperature and to dry said polymeric granular material to a predefined residual moisture value, discharging a portion of said polymeric granular material heated to said discharging temperature into a transformation unit (100) for said polymeric material, said transformation unit comprises a heating device (101) in which said polymeric granular material discharged from said hopper is brought into a molten or semi-molten state, and a mold (102) or an extrusion head, into which said polymeric granular material is introduced in the molten or semi-molten state; measuring the pressure at which said molten or semi-molten polymeric granular material is injected into said mold or said extrusion head at predefined time intervals, so as to define a progression over time of said injection pressure, and regulating the flow rate of said process gas on the basis of said progression over time of said injection pressure.

11. System (1) for drying polymeric granular material (2), comprising: at least one drying hopper (10), in which an entrance (11) for said polymeric granular material and an exit (12) for said polymeric granular material are formed, said exit being connected to a transformation unit (100) for said polymeric granular material, a heating circuit (20), via which a process gas, for heating said polymeric granular material to a discharging temperature and for drying said polymeric granular material to a predefined residual moisture value, is introduced into said drying hopper, a device (25) for regulating the flow rate of said process gas, a first temperature sensor (14) placed at said entrance for measuring an inlet temperature of said granular material, and a control unit (30) connected at least to said first temperature sensor and to said device for regulating the flow rate of said process gas and provided to control said device (25) for regulating the process gas flow rate depending on the difference between said inlet temperature, measured by said first sensor, and a predefined inlet temperature of said fresh polymeric granular material, on the basis of which a predefined process gas flow rate has been calculated.

12. The system according to claim 11, wherein a second temperature sensor (15) connected to said control unit (30) and positioned at said exit (12) is provided for measuring said discharging temperature of said granular material, and wherein said control unit is provided to control said device (25) for regulating the process gas flow rate depending on said discharging temperature.

13. The system according to claim 12, wherein a metering device (19) is provided for adding additives to said polymeric granular material discharged from said drying hopper (10) before the polymeric granular material enters said transformation unit (100), and said system comprises a third temperature sensor (15a) connected to said control unit (30) and positioned upstream of said transformation unit and downstream of a mixing point between said metering device and said drying hopper.

14. The system according claim 11, wherein said transformation unit comprises: a heating device (101) in which said polymeric granular material discharged from said hopper is brought into a molten or semi-molten state, and a mold (102) or an extrusion head, into which said polymeric granular material is introduced in the molten or semi-molten state, and wherein said control unit (30) is connected to a gauge (103) for measuring the pressure at which said molten or semi-molten polymeric granular material is injected into said mold or extrusion head, and is provided for controlling said device (25) for regulating the process gas flow rate depending on said injection pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and advantages of the invention will become clearer from the detailed description of a preferred embodiment thereof, shown by way of nonrestrictive example and with reference to the single accompanying drawing, in which FIG. 1 is a schematic view of a system for drying polymeric granular material, which system is produced in order to operate according to the process of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

(2) With reference to FIG. 1, the reference numeral 1 indicates, as a whole, a system for drying a polymeric granular material 2, which system operates according to the process of the present invention.

(3) The system 1 is designed to dry any granulated polymeric material, for example polyamide, polycarbonate or ABS copolymer, even if, in the specific example described herein, the treated material is formed of PET (polyethylene terephthalate) granules.

(4) PET has a melting temperature of approximately 260° C., and the maximum temperature at which the polymer can be maintained in air without incurring considerable oxidation is approximately 180° C.

(5) The system 1 is designed to supply the polymeric granular material to a transformation unit 100 which, in the specific example, comprises an extruder 101, in which the dried granular material is brought into the molten state inside a heating chamber in which the material is made to advance by means of at least one screw 104, an injection chamber 105 positioned downstream of the extruder 101, in which chamber the molten polymeric material discharged from the extruder 101 is collected, and a mould 102 into which the molten polymeric material is introduced from the injection chamber 105 by means of the action of a piston 106.

(6) A pressure gauge 103 is mounted in the injection chamber 105, which gauge is designed to measure the pressure inside the injection chamber and, in particular, the pressure at which the molten polymeric material is introduced into the mould 102.

(7) In the present embodiment, the transformation unit comprises a mould, or similarly it can preferably comprise an extrusion head.

(8) The system 1 comprises a drying hopper 10, in which an inlet opening 11, through which the granular material 2 to be dried is introduced into the drying hopper 10, and an outlet opening 12, through which the dried granular material 2 is discharged from the drying hopper 10, are made.

(9) The inlet opening 11 and outlet opening 12 are made in the top and bottom, respectively, of the drying hopper 10.

(10) The system 1 also comprises a loading hopper 13, which is mounted immediately upstream of the drying hopper 10 at the inlet opening 11 and in which, by means of a load line 13a, an amount of fresh polymeric granular material 2a is prepared ready to be introduced into the drying hopper 10.

(11) A first temperature sensor 14 is provided on the bottom of the loading hopper 13, which sensor measures the inlet temperature of the fresh granular material 2a. The fresh granular material 2a is generally the same as the granular material 2 that is already present in the drying hopper 10, but has a different temperature and moisture level and can differ from this material in the typology of PET used, for example it can have a different percentage of recycled material.

(12) A supply valve 17 is provided between the loading hopper 13 and the drying hopper 10 to allow, when necessary, the fresh granular material to enter the drying hopper 10.

(13) At the bottom of the drying hopper 10, at the outlet opening 12, a second temperature sensor 15 is also provided, which is designed to measure a discharging temperature of the granular material 2 that is ready to be discharged from the drying hopper 10.

(14) The second temperature sensor 15 can be mounted in the discharging duct that connects the drying hopper 10 to the extruder 101, upstream of a discharge valve 18 which allows the dried granular material to be discharged towards the transformation unit 100.

(15) The system 1 also comprises a metering device 19, which is connected immediately downstream of the outlet 12, on the discharging duct of the drying hopper 10, which device defines a point for mixing the additives and the granular material 2 exiting the drying hopper. This mixing point is preferably upstream of the discharge valve 18.

(16) A third temperature sensor 15a is also preferably mounted in the discharging duct of the drying hopper 10, downstream of the discharge valve 18 and therefore also downstream of the aforementioned mixing point.

(17) The system 1 also comprises a heating circuit 20, by means of which a process gas having a predefined temperature and flow rate is introduced into the drying hopper 10, in order to heat and dry the granular material 2 to desired temperature and moisture content values.

(18) The process gas is preferably air.

(19) The heating circuit 20 comprises a supply line 21, which brings the process gas inside the drying hopper 10, and a recovery line 22 through which the process gas is extracted from the drying hopper 10.

(20) The supply line widens close to the bottom of the drying hopper at a diffuser 23, whereas the recovery line 22 is open close to the top of the drying hopper 10, such that the process gas flows through the hopper from the bottom towards the top, counter-currently with respect to the granular material 2.

(21) A heater 24, which heats the process gas to a predefined heating temperature, and a device 25 for regulating the flow rate, which device regulates the flow rate of the process gas supplied to the drying hopper 10, are provided on the supply line 21.

(22) The regulation device 25, in the embodiment described herein, operates by varying the opening of one or more regulating valves 25a, thereby confirming the correctness of the regulating action using a flow rate gauge 25b. Alternatively, the regulation device 25 could act on an inverter which regulates the speed of rotation of the blades of a blower which propels the process gas along the supply line 21.

(23) The process gas introduced into the drying hopper 10 via the supply line 21 has a moisture value that is particularly low, which value has been obtained on account of a dehumidification treatment that is known per se and is not shown in the accompanying drawing.

(24) The system 1 also comprises a control unit 30 for controlling and regulating the operating parameters of the system 1.

(25) The control unit 30 is connected to the first temperature sensor 14, to the second temperature sensor 15, to the device 25 for regulating the flow rate of the process gas, to the heater 24 and to the injection pressure gauge 103. Furthermore, the control unit 30 is preferably also connected to the transformation unit 100 to receive information on the amount of granular material supplied to said transformation unit from the drying hopper 10, to a fourth temperature sensor 26 which measures the outlet temperature of the process gas from the drying hopper 10, to a level sensor 16 which measures the level of granular material 2 inside the drying hopper 10, to the third temperature sensor 15a and to the supply and discharge valves 17, 18.

(26) In one embodiment which is not shown, the control unit 30 is also connected to load cells which detect the weight of the drying hopper 10 and the granular material 2 contained therein.

(27) The system 1, controlled by the control unit 30, operates in the mode described in the following.

(28) In the drying hopper 10, the polymeric granular material 2 is heated and dried by the process gas, which is dispersed in the mass of material by the diffuser 23.

(29) The process gas, suitably dehumidified, is supplied at a predefined heating temperature, which is obtained and controlled in the heater 24, of approximately 180° C. for example, corresponding to the maximum temperature at which the PET can be maintained in air without incurring considerable oxidation.

(30) The flow rate of the process gas introduced into the drying hopper is regulated by the regulation device 25, which operates by varying the opening of the regulating valves 25a on the basis of the flow rate value provided by the control unit 30 (set value) and the flow rate value measured by the flow rate gauge 25b (real value).

(31) Alternatively, the flow rate of the process gas can be regulated by varying the speed of rotation of the blower by means of an inverter, and the process gas flow rate can be measured indirectly.

(32) As described above, the value at which the flow rate of the process gas has to be set (set value) is defined by the control unit 30.

(33) Initially, this flow rate value is a predefined value calculated, for example, on the basis of the heating temperature of the process gas (generally predefined), a predefined inlet temperature of the granular material, and the hourly flow rate of the granular material discharged from the drying hopper.

(34) In particular, the predefined inlet temperature of the material can be equal to a predefined standard value which is typical of the system 1, or a value set manually by the operator, or also a first temperature value measured by the first temperature sensor 14.

(35) The hourly flow rate of the granular material discharged from the drying hopper can also be a predefined value set by the operator or typical of the system 1.

(36) On account of the action of the process gas, the granular material 2 reaches, at the outlet opening 12, a discharging temperature of approximately 180° C. and a reduced residual moisture content, for example of approximately 40 ppm.

(37) At the request of the transformation unit 100, a part of the granular material present at the bottom of the drying hopper 10 is discharged through the discharge opening 12 and supplied to the extruder 101 where it is melted and, after having been collected in the injection chamber 105, injected into the mould 102 by means of the piston 106.

(38) When the level of granular material inside the drying hopper 10 decreases until it has reached a certain level, a predefined amount of fresh granular material 2a is introduced into the drying hopper 10. This fresh granular material is introduced through the inlet opening 11 from the loading hopper 13.

(39) The flow rate of the process gas can be regulated by the control unit 30, by means of the regulation device 25, on the basis of measuring, directly and indirectly, various process parameters.

(40) In a first operating mode, the control unit regulates the flow rate of the process gas on the basis of the measurement of the inlet temperature of the fresh polymeric granular material 2a as detected by the first temperature sensor 14.

(41) In this case, the control unit 30 detects the inlet temperature and compares it to the predefined inlet temperature value on the basis of which the predefined process gas flow rate has been calculated, and, if the deviation between the measured inlet temperature and the predefined inlet temperature exceeds a certain tolerance threshold, it acts on the regulation device 25.

(42) For example, if the measured inlet temperature is less than the predefined inlet temperature by more than 3° C., the process gas flow rate is subsequently increased in order to anticipate the greater thermal power demand necessary to heat the freshest material introduced into the drying hopper 10.

(43) On the contrary, if the measured inlet temperature is greater than the predefined inlet temperature by more than 3° C., the process gas flow rate is subsequently reduced in order to not overheat the granular material and to not waste energy.

(44) In a second operating mode, the control unit 30 regulates the flow rate of the process gas on the basis of the measurement of the discharging temperature of the dried polymeric granular material 2 as detected by the second temperature sensor 15.

(45) In this case, the control unit 30 detects the discharging temperature at regular time intervals, for example every 2 minutes or every mould 102 cycle time (that is to say every time the piston 106 introduces the molten material into the mould, typically corresponding to approximately 10 to 20 seconds), and from this determines the progression over time.

(46) The pressure considered to be the injection pressure is preferably the maximum pressure measured in the injection chamber 105 in the cycle time. Then, if this progression reveals that the discharging temperature is not sufficiently constant, but tends to decrease or increase, the control unit 30 acts on the regulation device 25 in order to bring the discharging temperature to predefined optimal values.

(47) In particular, if the progression over time of the discharging temperature is decreasing, the flow rate of the process gas is increased, whereas if the progression over time of the discharging temperature is increasing, the flow rate of the process gas is decreased.

(48) The control unit preferably also measures the progression of the discharging temperature measured by the third temperature sensor 15a. This provides verification of the measurements from the second sensor 15 and also provides an indication of the thermal input of any addition of additives into the dried polymeric granular material, which have been added by the metering device 19.

(49) In a third operating mode, the control unit 30 regulates the flow rate of the process gas on the basis of the measurement, detected by the pressure gauge 103, of the injection pressure, which corresponds to the pressure of the polymeric granular material in the molten state when introduced into the mould 102. The injection pressure preferably corresponds to the maximum pressure detected by the gauge 103 inside the injection chamber 105 during the cycle time for injection into the mould 102.

(50) In this case, the control unit 30 detects the injection pressure at regular time intervals, for example every 2 minutes or every mould 102 cycle time (that is to say every time the piston 106 introduces the molten material into the mould, typically corresponding to approximately 10 to 20 seconds), and from this determines the progression over time.

(51) Then, if this progression reveals that the injection pressure is not sufficiently constant, but tends to decrease or increase, the control unit 30 acts on the regulation device 25 in order to bring the injection pressure to predefined optimal values.

(52) In particular, if the progression over time of the injection pressure is decreasing, the flow rate of the process gas is increased, whereas if the progression over time of the injection pressure is increasing, the flow rate of the process gas is decreased.

(53) The control unit 30 can be designed to control the process gas flow rate according to any one of the operating modes described above, as well as to control the process gas flow rate according to any combination of these operating modes.

(54) It is particularly preferred for the control unit 30 to control the process gas flow rate on the basis of all of these operating modes at the same time. In fact, it is noted that these are not mutually exclusive and that the flow rate can therefore be controlled by taking into account the inlet temperature of the polymeric material, the progression over time of the discharging temperature, and the progression over time of the injection pressure.

(55) Furthermore, the control unit 30 can also regulate the flow rate of the process gas on the basis of the hourly amount of granular material dried in the drying hopper.

(56) This value can be derived from measurements over time of the total weight of the hopper, which in this case is preferably supported on load cells, or can be derived indirectly by counting the moulding cycles of the transformation unit 100.

(57) The system and the process of the present invention can be produced in variants which differ from the preferred example described above.

(58) On account of the process and the system of the present invention, it is possible to obtain excellent results in terms of stability of the process for drying polymeric granular material, thereby optimising the energy efficiency and reliability of the process.

(59) Another advantage is that the system is flexible in its operation and simple for the operators to control.

(60) A further important advantage is that the process of the invention allows the performance in terms of qualitative yield to be improved and the production of processing discards to be reduced.