Method for producing a plastic granulate

Abstract

The invention relates to a method for producing a plastic granulate (16), in which a process fluid (12) is contained in a process chamber (10) where an underwater granulation takes place and the process fluid in the process chamber has a temperature greater than 120° C. A process pressure of at least 2.0 bar is obtained in the process chamber, at which a granulation of the plastic strands (14) into plastic granulate occurs. From the process chamber, a mixture (18) of process fluid and plastic granulate is diverted into a first cooling zone (25) during cooling of the plastic granulate, while maintaining the process pressure. In a first separating device (22), the plastic granulate is separated from the process fluid under process pressure. In the process chamber, the process fluid has a temperature in the range from 120° C. to 160° C., and the process pressure obtained there is greater than the pressure of the vapour pressure curve of the process fluid. After separation from the process fluid in the first separating device, the plastic granulate is fed continuously in a line to a dealdehydization container (46).

Claims

1. Method for producing a plastic granulate, in which a process fluid is contained in a process chamber in which chamber an underwater granulation takes place and the process fluid in the process chamber has a temperature which is in a temperature range between 120° C. and 160° C., a process pressure of at least 2.0 bar prevails in the process chamber and at which process pressure a granulation of the plastic strands into plastic granulate is performed, a mixture of process fluid and plastic granulate is diverted from the process chamber into a first cooling zone while optimizing the cooling temperature to cause nucleation forming crystallization seeds on the surface of the plastic granulate during cooling of the plastic granulate, in a first separating device, the plastic granulate is separated from the process fluid under process pressure, wherein the process pressure is maintained in said first cooling zone, the process pressure prevails in the process chamber is higher than the pressure of the vapor pressure curve of the process fluid, and after separation from the process fluid in the first separating device, the plastic granulate is mixed with dry gas in a mixing zone and is fed continuously in a line to a dealdehydization container in which the plastic granulate is treated with a purge gas in order to reduce the acetaldehyde content of the plastic granulate, wherein the plastic granulate is fed through a crystallization zone forming a second cooling stage of the plastic granulates, to the dealdehydization container by means of the dry gas and wherein part of the dry gas is diverted downstream of the mixing zone but upstream of the crystallization zone and is fed as purge gas to a lower part of the dealdehydization container and remainder of the dry gas is fed into the dealdehydization container together with the plastic granulate, and a valve is used to regulate the amount of dry gas to be diverted.

2. Method according to claim 1, wherein the process chamber is partially delimited by a perforated plate for generating molten plastic strands, a cutting device is arranged in the process chamber and cooperates with the perforated plate for cutting up the plastic strands discharged from the perforated plate, and that said underwater granulation is performed in the process fluid by the cutting device acting on the perforated plate in the process chamber.

3. Method according to claim 1, wherein the dry gas comprises dry air.

4. Method according to claim 1, wherein the dry gas is introduced into the dealdehydization container together with fresh gas.

5. Method according to claim 1, wherein the plastic granulate, when it is introduced into the dealdehydization container, has a surface temperature of between 165° C. and 185° C.

6. Method according to claim 5, wherein the dry gas has a dew point of between −25° C. and −40° C. as it entrains the plastic granules downstream of the first separating device.

7. Method according to claim 6, wherein the dry gas has a temperature of between 180° C. and 210° C. as it entrains the plastic granules downstream of the first separating device.

8. Method according to claim 1, wherein the underwater granulation produces plastic granules which, having passed through the dealdehydization container, have a weight of between 8 mg and 36 mg.

9. Method according to claim 1, wherein the dry gas forms a closed cycle by returning from the dealdehydization container to a gas processing unit.

10. Method according to claim 1, wherein a process gas consisting of the dry gas introduced into the dealdehydization container together with the plastic granules, and of the purge gas introduced into the dealdehydization container, is supplied together with a fresh gas added to yield the required amount of dry gas—through an outlet provided in the dealdehydization container—to a gas treatment unit which processes said gas to dry gas and supplies said gas to the plastic granules again downstream of the first separating device.

11. Method according to claim 1, wherein downstream of said first separating device and upstream of said crystallization zone, pressure is reduced appropriately to ensure a predetermined dew point of the dry gas.

12. Method according to claim 1, wherein starting from the first separating device and up to the dealdehydization container, a pressure higher than 2 bar is maintained in the dry gas and that pressure is then reduced downstream of the dealdehydization container, preferably to atmospheric pressure.

13. Method according to claim 12, wherein the pressure prevailing in the dealdehydization container is lower than 10 bar.

14. Method according to claim 1, wherein the pressure reduction downstream of the dealdehydization container is performed using a cell wheel lock or a lock or by means of an intermediate chamber having a slide valve at its inlet and a slide valve at its outlet, which valves are opened alternately.

15. Method according to claim 14, wherein during dealdehydization under pressure, the pressure prevailing in the dealdehydization container is maintained constant in the dew point range at atmospheric pressure of less than 0° C.

16. Method according to claim 1, wherein following dealdehydization, the plastic granulate is fed to an additional cooling zone having a cooling fluid where the plastic granulate is cooled to below 65° C., at a cooling fluid temperature of below 40° C.

17. Method according to claim 16, wherein the cooling fluid has a loading density of more than 30% by weight of plastic granules in the additional cooling zone.

18. Method according to claim 1, wherein the plastic granulate consists of a semi-crystalline thermoplastic polyester or copolyester, for example poly-ethylene terephthalate.

19. Method according to claim 9, wherein the dry gas is mixed with fresh gas in the gas processing unit.

Description

(1) Throughout the description, the claims and the drawings, those terms and associated reference characters are used as are listed in the List of Reference Characters below. In the drawings:

(2) FIG. 1 is a schematic view of a method for producing plastic granulate according to a first embodiment;

(3) FIG. 2 is a schematic view of a method for producing plastic granulate according to a second embodiment, and

(4) FIG. 3 is a schematic view of a method for producing plastic granulate according to a third embodiment.

(5) FIG. 1 is a schematic view of a method according to the invention. In a process chamber 10, a blade rotor is arranged which is associated with a perforated plate 8. The blade rotor and the perforated plate 8 together form a conventional underwater granulator. Successively, hot process fluid 12, usually hot process water, is supplied to the process chamber 10. Moreover, a molten plastic flow 14 is introduced into the process chamber 10 via the perforated plate 8, which plastic is then discharged from the perforated plate 8 in the form of plastic strands and cut into plastic granules 16 by the blade rotor. This yields a mixture 18 of plastic granules 16 and process water 12.

(6) The molten plastic flow consists of a plastic, for example thermoplastic polyester or copolyester, for example polyethylene terephthalate. Moreover, the molten plastic flow 14 is pressed through the perforated plate 8 as a function of the speed of the blade rotor. The cool-down areas can be determined by appropriately adapting the speed at which the molten plastic passes through the perforated plate 8 in relation to the rotational speed of the blade rotor as well as to the temperature of the process water 12. The preferred option is cooling simply through convection since this will yield the best surface quality and thus also the best quality of the plastic granulate 16.

(7) In the process chamber 10, the process water 12 has a temperature ranging between 120° C. and 160° C. Moreover, the process pressure prevailing in the process chamber and in the sections of the process described hereinafter is higher than the pressure of the vapor pressure curve of the process water 12, but is at least 2.0 bar.

(8) The plastic granulate 16 is fed to a mixing zone 24 via a line 20 and a separating device 22 which latter separates the components plastic granulate 16 and process water 12 of the mixture 18 from one another. The zone extending from the process chamber 10 via the line 20 and the separating device 22 constitutes the first cooling zone 25.

(9) The process water 12 separated from the mixture 18 in the separating device 22 is fed to a process water tank 26. From there, the process water 12 is fed to a heat exchanger 32 via a filter 28, a pump 30, which heat exchanger 32 reheats the process water 12 to the required process temperature necessary in the process chamber 10. For this purpose, heating medium 34 is supplied to the heat exchanger 32. Filter residues 36 are discharged from the filter 28. The process water 12 heated in the heat exchanger 32 is then returned to the process chamber 10.

(10) In the mixing area 24, the plastic granulate 16 is mixed with dry air 38 to form a mixture 40. The dry air 38 entrains the plastic granules 16 in the mixing zone 24 downstream of the first separating device 22 with a dew point of between −25° C. and −40° C. The temperature of the dry air 38 is between 180° C. and 210° C. The mixture 40 is fed to a pressure lock 42, which may take the form of an impeller lock for example, where the pressure level is reduced to ambient pressure. The dry air 38 continues to have a predefined dew point. The mixture 40 is fed to a dealdehydization container 46 via a valve 48a and a crystallization zone 44. The valve 48a is used to control the speed at which the mixture passes through the crystallization zone 44.

(11) Downstream of the pressure lock 42, dry air 38 is diverted and fed as purge air 38a to the lower part of the dealdehydization container 46. A valve 48a is used to regulate/control the amount to be diverted.

(12) The mixture 40 of plastic granulate 16 and dry air 38 passes through the crystallization zone 44. The length of the crystallization zone 44 but also the flow rate of the dry air 38 are used to determine the dwell time in the crystallization zone 44. Before entering the dealdehydization container 46, the plastic granules 16 have a surface temperature of between 165° C. and 185° C. The mixture 40 of plastic granules 16 and dry air 38 enters the dealdehydization container 46. Here, this portion of the dry air 38 is used as additional purge air 38b.

(13) In its upper part, the dealdehydization container 46 has an outlet 50 which is used to re-supply the purge air 38a and the additional purge air 38b to a dry air treatment device 52. The dry air treatment device 52 has a fresh air supply 54, a fresh air filter 56, a control/regulating valve 58 for setting the required amount of fresh air, a pump 60 as well as a processing unit 62 having temperature control and dehumidification. This is used to remove acetaldehyde and excess water through absorption on a molecular sieve or by using similar substances and processes known to the person of skill in the art. In this step, acetaldehyde largely decomposes into water and carbon dioxide. The purge air 38c exiting the dealdehydization container 46 via outlet 50 and consisting of purge air 38a and 38b is supplied to the fresh air 54 downstream of the control/regulating valve 58. The purge air 38c and the fresh air 54 form the new dry air 38, thus closing the cycle.

(14) The plastic granules 16 exit the dealdehydization container 46 through a lock 64, at a temperature of more than 200° C. The plastic granulate 16 has a granule weight of between 8 mg and 36 mg, in particular of between 12 mg and 24 mg. Underwater granulation in the process chamber 10 is appropriately adapted thereto. The weight of the plastic granules 16 deviates by 10% at the most from a weight average of the plastic granules. The term weight average refers to a statistically significant proportion of plastic granules 16 during a small time slot downstream of the dealdehydization container 46, i.e. the process section.

(15) Downstream of the dealdehydization container 46, the plastic granulate 16 is supplied to an additional cooling zone 66 with a cooling fluid, in which the plastic granulate 16 is cooled down to a temperature of less than 65° C., with the cooling fluid temperature being less than 40° C. In the additional cooling zone 66, the cooling fluid has a loading density of more than 30% by weight of plastic granules 16.

(16) The plastic granulate 16 consists of a semi-crystalline thermoplastic polyester or copolyester, for example polyethylene terephthalate.

(17) FIGS. 2 and 3 each illustrate another embodiment. In each case, the process pressure is maintained until the plastic granules 16 exit the dealdehydization container 46.

(18) For this reason, the pressure reduction will only occur downstream of the dealdehydization container 46. The purge air 38c will therefore only be supplied to the fresh air 54 downstream of the high-pressure fan 60 since the purge air 38c is still pressurized. Starting from the first separating device 22 and up to the dealdehydization container 46, a pressure of more than 2 bar is maintained in the dry air 38. Pressure will only be reduced downstream of the dealdehydization container 46, in particular to atmospheric pressure. The pressure in the dealdehydization container 46 is lower than 10 bar. During dealdehydization in the dealdehydization container 46 under pressure, the dew point at atmospheric pressure of less than 0° C. is maintained constant.

(19) As seen in FIG. 2, pressure reduction is performed by means of a cell wheel lock 64a. As an alternative, it can also be performed using a lock, or an intermediate chamber having a slide valve at its inlet and a slide valve at its outlet and alternately opening said slide valves.

(20) As seen in FIG. 3, a pressure lock 70 is provided downstream of the lock 64. Apart from this, the sequence of the processes is the same and the same devices are provided in a line in the manner de-scribed with reference to FIG. 1. One advantage of the method also is the production of the plastic granules in a line, i.e. continuously and not sequentially.

(21) Moreover, the direct crystallization described in the method saves energy because firstly, energy contained in the molten plastic is used for crystallization, and secondly, this eliminates the need for inter-mediate storage.

(22) Furthermore, its reduced aldehyde content makes the plastic granulate better compatible with products it comes into contact with.

(23) The method according to the invention considerably improves the quality of the plastic granules. It ensures a constantly high viscosity and/or high molecular weight of the plastic. This thus makes the method far better suited for producing higher-quality products.

LIST OF REFERENCE CHARACTERS

(24) 8 perforated plate

(25) 10 process chamber

(26) 12 process fluid, process water

(27) 14 molten plastic

(28) 16 plastic granulate

(29) 18 mixture of process fluid 12 and plastic granulate 16

(30) 20 line

(31) 22 separating device

(32) 24 mixing chamber

(33) 25 first cooling zone

(34) 26 process water tank

(35) 28 first filter

(36) 30 first pump

(37) 32 first heat exchanger

(38) 34 heating medium

(39) 36 filter residues

(40) 38 dry air

(41) 38a purge air

(42) 38b purge air

(43) 38c purge air

(44) 40 mixture of plastic granulate 16 and dry air 38

(45) 42 pressure lock

(46) 44 crystallization zone

(47) 46 dealdehydization container

(48) 48a control/regulating valve

(49) 48b control/regulating valve

(50) 50 outlet of dealdehydization container 46

(51) 52 dry air treatment device

(52) 56 fresh air filter

(53) 58 fresh air control/regulating valve

(54) 60 high-pressure fan

(55) 62 processing unit consisting of temperature control and dehumidification units

(56) 64 lock of the dealdehydization container 46

(57) 64a cell wheel lock

(58) 66 additional cooling zone

(59) 68 additional cooling fluid

(60) 70 pressure lock