WO2007065184A1

WO2007065184A1 - System for the thermal treatment of plastic material

Info

Publication number: WO2007065184A1
Application number: PCT/AT2006/000500
Authority: WO
Inventors: Markus Fellinger; Christian Lovranich
Original assignee: Starlinger & Co Gesellschaft M.B.H.
Priority date: 2005-12-05
Filing date: 2006-12-05
Publication date: 2007-06-14
Also published as: AT502958B1; AT502958A1

Abstract

Disclosed is a system for thermally treating plastic material, especially in order to increase the intrinsic viscosity of polyester material by means of solid state polycondensation (SSP). Said system comprises a heatable reaction vessel (2) inside which the plastic material can dwell for a predefined dwell time at a given heat treatment temperature. An optional preheating vessel (17, 17') which is configured so as to preheat and transfer the preheated plastic material to the reaction vessel is mounted upstream of the reaction vessel (2'). A cooling vessel (6, 6') into which the plastic material can be transferred from the reaction vessel (2) in order to cool the same to a cooling temperature lying below the heat treatment temperature is disposed downstream of the reaction vessel (2). The inventive cooling vessel (6, 6') is mounted inside a cooling circuit conducting a cooling medium (K). The cooling circuit is directly or indirectly connected to a heating circuit via heat exchangers (4) downstream of the cooling vessel, said heating circuit being used for heating the preheating vessel (17, 17') and/or the reaction vessel (2)

Description

Plant for the heat treatment of plastic material

The invention relates to a plant for the heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by means of solid-phase polycondensation with a heatable reaction container, in which the plastic material can be left at a predetermined heat treatment temperature for a predetermined residence time, the reaction container optionally being preceded by a preheating container, which is designed for preheating and dispensing the preheated plastic material to the reaction container, and with a cooling container arranged downstream of the reaction container, into which the plastic material can be dispensed from the reaction container for cooling to a cooling temperature below the heat treatment temperature.

When processing or reprocessing plastics, heat treatment of the plastic is very often necessary. So you make yourself at the

Manufacture high molecular weight polyesters, such as PET and PEN, take advantage of the fact that when

Polyester at high temperatures Polycondensation of the polyester molecules occurs and thus the viscosity of the polyester increases. The polyester is left under vacuum or inert gas to prevent oxidative degradation. This recovery of high molecular weight polyester from low molecular weight polyester starting material usually takes place by means of melt polycondensation (MPPC) or solid phase polycondensation

(SSP) or a combination of both methods.

In melt polycondensation, polyester melt is processed at temperatures of about 27O ⁰ C to 300 ⁰ C for about 30 minutes to 5 hours under a strong vacuum of about 1 mbar.

In solid-phase polycondensation, the polyester melt is usually extruded through several nozzles, and the resulting plastic strands are then cooled in a water bath. After the plastic strands have hardened, they are granulated, ie cut into pellets. Due to the rapid cooling, the polyester is in the amorphous state. This is important because originally transparent polyester materials remain transparent in the amorphous state, whereas slowly cooling polyester takes on a crystalline state in which the originally transparent material turns white. For further processing, the polyester granules must be heated again, which is in the range of the crystallization temperature - is (80 120 ⁰ C) to a bonding of the granules body. Therefore, the granulate is first a so-called Crystallizer supplied, in which it is brought to a temperature above the crystallization temperature with vigorous stirring, in order to regain the pourability of the granules for further treatment, which is of great importance for transport and drying in a container without a stirrer. In crystalline form, the granulate also absorbs less moisture and therefore allows shorter dwell times during drying. The granulate is then called for increasing the intrinsic viscosity of a Festphasenpolykondensations container, also SSP (Solid State Polycondensation) reactor or heat treatment vessel is supplied, is heated to about 190 to 250 ⁰ C and then for about 1-40 hours under leave these conditions in the SSP reactor until the desired intrinsic viscosity is reached.

It is also conceivable to produce the granules by means of underwater granulation with a very short dwell time using cooling water which is as hot as possible and to feed the hot granules to the SSP reactor directly or via a heatable intermediate container after the separation from the cooling water.

Basically, the procedures for SSP can be divided into two groups, namely continuous SSP, such as described in DE 100 54 240, in which nitrogen is normally used as the inert gas, which simultaneously serves as a heat transfer medium and transport medium for cleavage products separated from the polycondensation, and discontinuous SSP, as described in DE 197 10 098, in which the plastic material is filled into a tumble dryer , heated under vacuum to above the reaction temperature, allowed to remain at this temperature until the desired intrinsic viscosity is reached, and then again cooled below the reaction temperature in the same drum. A semi-continuous SSP process is further disclosed in WO 2004/029130 A1. It should be mentioned that the continuous or semi-continuous SSP processes described in the publications mentioned are only suitable for increasing the intrinsic viscosity of plastic granules in such a way that the plastic granules, after reaching the desired intrinsic viscosity, remain in the SSP reactor at temperatures above the reaction temperature is cooled in the SSP reactor and stored therein in the cooled state. As an alternative to this, the granulate is fed to a silo in the cooled state, the SSP reactor being used both as a heating container for carrying out the heat treatment and as a cooling container after the heat treatment has taken place.

High-molecular polyesters, such as PET, PEN, PA or PC, are used after their treatment to increase the intrinsic viscosity in numerous fields of application, for example in so-called preforms for the production of plastic bottles, further for strapping tapes, thermoforming foils and sheets etc. The treatment of the polyester material to increase the intrinsic viscosity is carried out according to one of the processes described above in refineries, then the treated plastic granulate is cooled to ambient temperature and filled into transport containers and delivered to a preform factory, for example. Since the polyester granulate has bound water due to its hygroscopic properties during storage and transport, the high-molecular polyester granulate must first be heated in a dryer from the cold state to approx. 160 - 180 ° C in the preform factory remove the bound water before it can be processed into preforms in injection molding machines. To dry the granules, residence times in the dryer of six hours or more are required, otherwise there would be great losses in the viscosity of the polyester. This required drying step limits the processing speed of the plastic granulate and requires a high energy input, the cooling of the polyester granulate in the refinery and its subsequent reheating for drying in the processing factory being extremely disadvantageous in view of the overall energy balance.

In order to improve the overall energy balance, it is expedient if the reaction vessel of the SSP reactor is connected directly upstream of a plastic processing machine, such as an injection molding or extrusion machine, but fluctuating throughput of the plastic material or downtime of the plastic processing machine must not lead to a fluctuating quality of the plastic material supplied , or by stopping and restarting the SSP reactor, long start-up times and / or high material losses have to be accepted. In order to meet these requirements, the not yet published Austrian patent application A 1000/2005 of the present inventor proposes to arrange a cooling container downstream of the reaction container, which cooling container is designed to cool the polyester material discharged from the reaction container to a cooling temperature below the reaction temperature.

In times of rising energy prices and the generally prevailing desire for a careful consumption of raw materials and possible protection of the environment, the present invention is based on the task of a plant for the heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by means of solid-phase polycondensation (SSP). to create, which brings a further reduction in energy consumption. The present invention achieves the stated object by developing a plant for heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by means of solid-phase polycondensation (SSP), in that the cooling container is connected to a cooling circuit carrying a cooling medium, the cooling circuit downstream of the cooling container directly or is indirectly connected via a heat exchanger to a heating circuit for heating the preheating container and / or the reaction container. As a result of this measure, the heat released during the cooling of the plastic material in the cooling container is used further within the system and thus makes a significant contribution to reducing the energy consumption when operating the system according to the invention.

Excellent heat recovery for operating the system according to the invention is achieved if the cooling medium flows through the cooling container. As a result of this measure, the cooling medium flows through the entire plastic material in the cooling container and there is direct heat transfer from the plastic material to the cooling medium.

In order to prevent the cooling medium from undesirably influencing the plastic material in the cooling container, it proves to be advantageous if the cooling medium is dry air or an inert gas.

In one embodiment of the invention, the cooling medium “charged” with thermal energy is used directly for heating the preheating container or the reaction container. This is achieved by the cooling circuit and the heating circuit being in fluid communication, so that the cooling medium heated when flowing through the cooling container passes through the heating circuit Excellent heat transfer is achieved if the preheating container and / or the reaction container are connected to the heating circuit so that he / she is removed from the heated cooling medium that flows through the heating circuit The prerequisite for this is, of course, that the cooling medium is compatible with the plastic material in the reaction container or preheating container. Compatible cooling media include dry air and inert gas design effort, inert gas, such as nitrogen, is mainly used in large systems. Since the plastic material in the cooling container is already dry after its heat treatment in the reaction container, it proves most favorable if dry air is used as the cooling medium which flows through the cooling container. Because of the pre-dried plastic material in the cooling container does not cause undesired loading of the dry air with moisture.

In an alternative embodiment of the invention, the cooling circuit comprises a heat exchanger built into the cooling container. This avoids direct contact between the cooling medium and the plastic material in the cooling container. With this configuration, for example, a liquid cooling medium (water or oil-based) can also be used, which has a higher heat storage capacity than gaseous cooling media.

In a further embodiment of the invention, the cooling medium and the heating medium are not in direct contact with one another, but the cooling circuit is connected to the heating circuit via a heat exchanger in a heat-transferring manner. With this configuration, different media can be used as the cooling medium and heating medium, which, depending on the heat treatment process of the plastic material, entails greater flexibility. Due to the higher heat storage capacity, it can prove to be useful if a liquid heat transfer medium flows through the heating circuit. On the other hand, in order to avoid impairment of the plastic material in the preheating container or in the reaction container, direct contact between the liquid heat transfer medium and plastic material is to be avoided by the heat transfer medium flowing around a wall of the preheating container and / or reaction container, but not allowing it to flow through the preheating container and / or reaction container itself becomes. This can be done, for example, by a double-walled configuration of the said containers or by forming channels for guiding the heat transfer medium in the container wall.

Since the heat given off by the plastic material in the cooling container to the cooling medium and the subsequent heat transfer from the cooling medium to the heat transfer medium (if not identical) are generally not sufficient to heat the preheating container or reaction container, and to achieve better control of the temperatures in the system according to the invention, it is furthermore expedient to switch a heating register in the heating circuit upstream of the preheating container and / or reaction container.

If the cooling container is not filled with plastic material or the process flow does not allow the cooling of the plastic material in the cooling container, then on the one hand a bypass line can be switched on and off in the cooling circuit to bypass the cooling container, and on the other hand a shut-off valve is provided in the cooling circuit in front of the cooling container. The invention will now be explained in more detail with reference to the drawings using two exemplary embodiments, to which the invention is, however, not restricted.

In the drawings, FIG. 1 shows a schematic circuit diagram of a first embodiment of an installation according to the invention for the heat treatment of plastic material, and FIG. 2 shows a schematic diagram of a second embodiment of an installation according to the invention for the heat treatment of plastic material.

First, referring to FIG. 1, a first embodiment of the plant for heat treatment of plastic material according to the invention is explained, which is designed as a plant for increasing the intrinsic viscosity of polyester material with a reaction vessel 2 that can be heated by dry air, alternatively by inert gas. Polyester material P, which comprises PET, PA, PC etc. and is present as granules or regrind from bottles, preforms etc., is introduced in crystalline form, preferably preheated, in batches by means of vacuum conveying into an evacuable preheating container 17, which at the same time acts as a vacuum lock for the SSP Reaction vessel 2 is used. For this purpose, the preheating container 17 has vacuum-tight shut-off devices 20, 21 both at its inlet and at its outlet. The temperature in the preheating container is detected by a temperature sensor T4. After the polyester material P has been introduced, it is heated in the preheating container 17 to a heat treatment temperature which corresponds at least to the reaction temperature, but in any case is below its melting temperature. For faster heating of the polyester P to the reaction temperature, the preheating container 17 is provided with an agitator 18. The reaction temperature is the temperature at which the increasing the intrinsic viscosity of the polyester material starts to measure, the set heat treatment temperature is dependent on the supplied polyester material, but is generally at least 180 ⁰ C, preferably about 220 °. After the polyester material has reached the desired temperature, it is released to the SSP reaction container 2 by opening the shut-off member 21 and kept at the reaction temperature in the SSP reaction container 2 for a defined residence time. A solid-phase polycondensation of the polyester material occurs over the residence time of the polyester material in the SSP reaction container 2, which leads to an increase in its intrinsic viscosity. In the lower part of the Reaction container 2 is provided with an opening 11 for sampling in order to determine the degree of polymerisation of the plastic.

In the present exemplary embodiment, the SSP reaction container 2 is operated under vacuum, but the invention is not restricted to this. Numeral 13 denotes one

Level sensor for the reaction container 2. After the desired dwell time in the

Reaction tank 2 (i.e. after the desired IV has set in) will

Polyester material is delivered in batches to the cooling container 6 via a shut-off device 3, which is designed here as a slide, in which the polyester material is cooled to a suitable cooling temperature. To accelerate the cooling process, the cooling container 6 has an agitator 19.

At the outlet of the cooling container 6 there is a further shut-off device 15, which is followed by a polyester material switch 7 which has two outlets 7A, 7B, outlet 7A being connected via a pipe to a polyester material processing machine 8, here designed as a screw extruder, and Output 7B is connected via a pipeline to an intermediate storage container 10. Depending on the position, the polyester material switch 7 conducts the polyester material flow passing through the shut-off element 15 as material flow P1 to the processing machine 8 or as material flow P2 to the intermediate storage container 10, intermediate positions of the switch 7 and thus the formation of partial polyester material flows being possible.

In the cooling tank 6, the polyester material is cooled to a temperature below the reaction temperature, cooling temperature, when the downstream processing machine requires 8 material, which is measured by a level sensor 14 to a feed hopper 16 of the processing machine 8, for example, is set to a maximum of 180 ⁰ C for PET In order to stop the process of increasing the intrinsic viscosity on the one hand, but on the other hand to keep the cooling as low as possible for reasons of energy saving, since an increase in the temperature of the plastic material to the melting temperature is required again in the processing machine 8 with the supply of energy. If the processing machine 8 does not require any material, a significantly lower cooling temperature can be selected, for example below 16O ⁰ C for PET, in order to prevent the polyester material from yellowing. It goes without saying that the cooling temperature can also be selected to be significantly lower, for example between 70 and 120 ^° C. The cooling of the polyester material to this lower cooling temperature is carried out when the polyester material is to be delivered to the intermediate store 10. The temperature in the cooling container 6 can be regulated either as a function of the signals from the level sensor 14, or in Dependence on the position of the polyester material switch 7. If the fill level sensor 14 reports that the feed hopper 16 is full, the polyester material switch 7 is switched over in such a way that the polyester material stream P2 flows into the intermediate storage container 10, the polyester material in the material stream P2 having previously been said at the lower cooling temperature has been cooled. The material flow P2 can be conveyed by a conveying element, such as a vacuum conveying 9. Thus, even if the processing machine 8 cannot accommodate polyester material, be it due to malfunction or due to machine downtime due to a retrofit, the heat treatment process in the reaction container 2 can continue to run unchanged by dispensing the cooled polyester material to the intermediate storage container 10. If the processing machine 8 needs polyester material again, the polyester material switch 7 is switched over in order to direct the material flow P1 to the processing machine 8, the cooling of the material in the cooling container being simultaneously set to the higher cooling temperature. The preheating container 17, the reaction container 2 and the cooling container 6 are connected via vacuum lines to a vacuum source, not shown, so that a vacuum between 0.1 and 10 mbar can be generated in the said containers. The control ^'of the evacuation of each container takes place via valves Vl, V2, V3 in the vacuum lines.

According to the invention, the cooling of the plastic material in the cooling container 6 takes place by the cooling container 6 being connected to a cooling circuit in which a cooling medium K is guided. In the present exemplary embodiment, the cooling circuit comprises a dry air source 1 for generating dry air as the cooling medium K. From the dry air source, a pipeline 22 leads into the cooling container 6, through which the cooling medium K is introduced into the cooling container 6. The cooling medium K flows through the cooling container 6, thereby extracting the heat from the plastic material therein, where it heats itself up and leaves the cooling container 6 in the heated state through a pipeline 23. Since the plastic material in the cooling container 6 is absolutely dry due to the previous heat treatment process in the reaction container 2 is, the cooling medium K can not be loaded with moisture, which would be absolutely undesirable. Dry air as a cooling medium does not affect the plastic goods and in particular does not trigger any undesirable reactions of the plastic goods. It should be mentioned that instead of dry air, an inert gas, such as nitrogen, can also be used as the cooling medium. In this case, however, care must be taken to ensure that the inert gas is cleaned after use and returned to the cooling circuit. The pipeline 23 opens into a further pipeline 24, which supplies the heated cooling medium K to a heating register 5, where it is further heated to a predetermined heating temperature and is then passed through a heating circuit which comprises the pipeline 25 from which two heating circuits branch off, namely one Heating circuit for heating the preheating container 17 and a heating circuit for heating the reaction container 2. The heating circuit for heating the preheating container 17 comprises a pipe 26 which can be shut off by a valve V5 and which leads into the preheating container 17 so that the heated cooling medium flows through the preheating container 17 and thereby Releases thermal energy to the plastic material in the preheating container. The cooled cooling medium leaves the preheating container 17 via a pipe 27. The heating circuit for heating the reaction container 2 comprises a pipe 28 which can be shut off by a valve V6 and which leads the heated cooling medium into the space between the peripheral walls of the double-walled reaction container 2, where it leads to the inner wall and thus the plastic material located inside the reaction container 2 is heated or kept at the reaction temperature. Since in the present exemplary embodiment the reaction container 2 is operated under vacuum, it is important that the cooling medium does not get into the interior of the container, but rather only flows through the space between the peripheral walls and - after the stored thermal energy has indirectly given off to the plastic material in the reaction container 2 - is withdrawn via a pipeline 29.

In the event that the cooling container 6 should not be filled with plastic material or the process flow temporarily does not allow the cooling of the plastic material in the cooling container 6, a bypass line 30 which can be shut off by a valve V4 is connected between the pipes 22 and 24 in order to do this Guide cooling medium K past cooling container 6. Furthermore, the pipeline 22 between the branching of the bypass line 30 and the cooling container 6 can be blocked by a valve V7.

Referring now to FIG. 2, a second embodiment of the plant for heat treating plastic material according to the invention will be explained. Components of the second embodiment which correspond to those of the first embodiment are denoted by the same reference numerals and, in the case of structural differences but have the same function, are additionally provided with an apostrophe, and reference is made to the above explanations in this regard.

The second embodiment of the invention differs from the first by a changed type of management of the cooling circuit and heating circuit. The cooling circuit includes a cooling medium source 31, which provides a cooling medium K, for example in liquid form based on water or oil. The cooling medium K is guided through a pipe 32 into the cavity between the walls of the double-walled cooling container 6 'and thereby absorbs the heat from the inner wall, which in turn is in a heat-conducting relationship with the plastic material located inside the cooling container 6'. In this way, the cooling medium K indirectly extracts the heat from the plastic material and heats itself. As an alternative to the double-walled design of the cooling container, this could also be provided inside the container with a heat exchanger through which the cooling medium flows and thereby removes the heat from the plastic material. It is essential that the liquid cooling medium K does not come into contact with the plastic material. After the cooling medium K has been indirectly heated by the plastic material, it leaves the intermediate space of the double-walled cooling container 6 through a pipe 33, flows through a heat exchanger 4 and then returns to the cooling medium source 31. In the heat exchanger 4, the cooling medium K is used to heat a heat transfer medium W of a heating circuit without coming into direct contact with it. The heating circuit comprises a heat transfer medium source 34, which provides a heat transfer medium W, for example based on water or oil. The heat transfer medium W is passed through a pipe 35 to the heat exchanger 4, in which, as mentioned, indirectly heated by the heated cooling medium K and, after leaving the heat exchanger, it is led further to a heating register 5, where it is additionally heated and thereby brought to a predetermined heating temperature becomes. From the heating register 5, the heat transfer medium W passes through a pipe 36 into the space between the double-walled preheating container 17 ', where it is used to heat the inner wall of the preheating container 17 and thereby heat up the plastic material located inside the preheating container. After the heat transfer material has released its stored thermal energy to the inner wall, it is returned to the heat transfer medium source 34 via a pipeline 37.

Claims

Claims:

1. Plant for the heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by means of solid-phase polycondensation (SSP), with a heatable reaction container (2), in which the plastic material can be left at a predetermined heat treatment temperature for a predetermined residence time, the reaction container (2 ') is optionally preceded by a preheating container (17, 17') which is designed to preheat and deliver the preheated plastic material to the reaction container (2), and with a cooling container (6, 6 ') arranged downstream of the reaction container (2). , into which the plastic material can be discharged from the reaction container for cooling to a cooling temperature below the heat treatment temperature, characterized in that the cooling container (6, 6 ') is connected to a cooling circuit carrying a cooling medium (K), the cooling circuit downstream of the cooling container lter directly or indirectly via heat exchanger (4) to a heating circuit for heating of the preheating (17, 17 ') and / or the reaction vessel (T) is connected.

2. Plant for the heat treatment of plastic material according to claim 1, characterized in that the cooling container (6) is flowed through by the cooling medium (K).

3. Plant for the heat treatment of plastic material according to claim 1 or 2, characterized in that the cooling medium (K) is dry air or an inert gas.

4. Plant for the heat treatment of plastic material according to one of the preceding claims, characterized in that the cooling circuit and the heating circuit in

Fluid communication stand.

5. Plant for the heat treatment of plastic material according to claim 4, characterized in that the preheating container (17) and / or the reaction container (2) are connected to the heating circuit so that he / she from the heated cooling medium (K) through the heating circuit flows, is / will be flowed through.

6. Plant for the heat treatment of plastic material according to claim 1, characterized in that the cooling circuit comprises a heat exchanger built into the cooling container (6 '), through which the cooling medium (K) flows.

7. Plant for the heat treatment of plastic material according to claim 6, characterized in that the cooling medium (K) is a cooling liquid, in particular water or oil-based.

8. Plant for the heat treatment of plastic material according to one of the preceding claims, characterized in that the cooling circuit with the heating circuit via a heat exchanger (4) are connected to transfer heat.

9. Plant for the heat treatment of plastic material according to claim 8, characterized in that the heating circuit is flowed through by a liquid heat transfer medium (W).

10. Plant for the heat treatment of plastic material according to claim 9, characterized in that the heat transfer medium (W) flows around a wall of the preheating container (17 ') and / or reaction container (2).

11. System for heat treatment according to one of the preceding claims, characterized in that a heating register (5) is connected in the heating circuit upstream of the preheating container (17, 17 ') and / or reaction container (2).

12. System for heat treatment according to one of the preceding claims, characterized in that the cooling circuit has a bypass line (30) which can be switched on and off to bypass the cooling container (6).

13. Plant for heat treatment according to one of the preceding claims, characterized in that the cooling circuit in front of the cooling container (6) has a shut-off valve (V7).