AT502958B1 - PLANT FOR HEAT TREATMENT OF PLASTIC MATERIAL - Google Patents

Abstract

A plant for the heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by solid state polycondensation (SSP) comprises a heatable reaction vessel (2), in which the plastic material can be left at a predetermined heat treatment temperature for a given residence time, wherein the reaction vessel (2 '; ) is optionally preceded by a Vorwärmbehälter (17, 17 '), which is designed for preheating and delivery of the preheated plastic material to the reaction vessel (2). Downstream of the reaction vessel (2), a cooling vessel (6, 6 ') is arranged, in which the plastic material from the reaction vessel (2) for cooling to a below the heat treatment temperature cooling temperature can be delivered. According to the invention, the cooling container (6, 6 ') is connected in a cooling circuit leading to a cooling medium (K), wherein the cooling circuit downstream of the cooling container directly or indirectly via heat exchanger (4) to a heating circuit for heating the Vorwärmbehälters (17, 17') and / or the reaction vessel (2) is connected.

Description

Austrian Patent Office AT 502 958 B1 2011-07-15

description

PLANT FOR HEAT TREATMENT OF PLASTIC MATERIAL

The invention relates to a plant for heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by solid phase polycondensation with a heated reaction vessel in which the plastic material can be left at a predetermined heat treatment temperature for a given residence time, the reaction vessel optionally a Vorwärmbehälter upstream, which is designed for preheating and delivery of the preheated plastic material to the reaction vessel, and with a cooling vessel arranged downstream of the reaction vessel, in which the plastic material from the reaction vessel for cooling to a below the heat treatment temperature cooling temperature can be delivered.

In the processing or recycling of plastics very often a heat treatment of the plastic is required. Thus, for example, in the production of high molecular weight polyesters, such as PET and PEN, use is made of the fact that when polyester is allowed to remain at high temperatures, polycondensation of the polyester molecules occurs, thereby increasing the viscosity of the polyester. The polyester is allowed to sit under vacuum or inert gas to prevent oxidative degradation. This recovery of high molecular weight polyesters from low molecular weight polyester starting material is usually accomplished by melt polycondensation (MPPC) or solid phase polycondensation (SSP) or a combination of both.

In the melt polycondensation polyester melt is processed at temperatures of about 270 ° C to 300 ° C for about 30 minutes to 5 hours under a strong vacuum of about 1 mbar.

In the solid phase polycondensation, the polyester melt is usually extruded through a plurality of nozzles, and the resulting plastic strands are subsequently cooled in a water bath. After the plastic strands have hardened, they are granulated, i. 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 with slow cooling polyester assumes a crystalline state in which originally transparent material turns white. For further processing, the polyester granules must be reheated, whereby it comes in the range of crystallization temperature (80-120 ° C) to a sticking of the granules. Therefore, the granules are first fed to a so-called. Crystallizer in which it is brought to a temperature above the crystallization temperature with vigorous stirring to regain the flowability of the granules for further treatment, which is for transport and drying in a vessel without agitator great importance. In crystalline form, the granules also absorb less moisture and thus allows shorter residence times during drying. The granules are then added to increase the intrinsic viscosity of a solid state polycondensation container, also called SSP (solid state polycondensation) reactor or heat treatment vessel, heated therein to about 190 to 250 ° 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 residence time using as hot as possible cooling water and to feed the hot granules after separation from the cooling water directly or via a heatable intermediate tank to the SSP reactor.

In principle, the procedures for SSP can be divided into two groups, namely continuous SSP, e.g. described in DE 100 54 240, in which normally nitrogen is used as the inert gas, which serves as a heat transfer medium and transport medium for separated from the polycondensation cleavage products, and discontinuous SSP, as described in DE 197 10 098, in which the plastic material in a tumble filled, heated under vacuum to above the reaction temperature, allowed to stand at this temperature until the desired intrinsic viscosity is reached, and then in turn cooled in the same drum below the reaction temperature - AT Austrian Patent Office AT 502 958 B1 2011-07-15. 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 abovementioned publications are only suitable for increasing the intrinsic viscosity of plastic granulate 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. Alternatively, the granules are fed to a silo in the cooled state, wherein the SSP reactor is used both as a heating vessel for carrying out the heat treatment and after the heat treatment as a cooling vessel.

High molecular weight polyesters, such as PET, PEN, PA or PC, are used after their treatment to increase the intrinsic viscosity in numerous applications, for example in so-called preforms for the production of plastic bottles, further for strapping, thermoforming films and plates etc .. Die Treatment of the polyester material to increase the intrinsic viscosity is carried out according to one of the methods described above in refineries, then the treated plastic granules are cooled to ambient temperature, filled into transport containers and eg delivered to a preform factory. Since the polyester granules have bound water during storage and transport due to its hygroscopic properties, the high-molecular polyester granules in the preform factory must first be heated in a dryer from the cold state to about 160-180 ° C 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 large viscosity losses of the polyester. This required drying step restricts the processing speed of the plastic granules and requires high energy input, the cooling of the polyester granules in the refinery and its subsequent rewarming for drying in the processing plant are extremely disadvantageous in consideration of the overall energy balance.

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

In times of rising energy prices and the generally prevailing desire for a careful consumption of raw materials and the greatest possible environmental protection, the present invention has for its object to provide a plant for heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by solid phase polycondensation (SSP), which brings about a further reduction in energy consumption.

The present invention solves the problem by further developing an initially described plant for heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by solid phase polycondensation (SSP) by the cooling tank is connected in a cooling medium leading cooling circuit, the cooling circuit downstream from the cooling tank is connected directly or indirectly via heat exchangers to a heating circuit for heating the Vorwärmbehälters and / or the reaction vessel. As a result of this measure, the heat released during cooling of the plastic product in the refrigerator is reused within the system and thus makes a significant contribution to reducing the energy consumption during operation of the system according to the invention.

Excellent recovery of heat to operate the system according to the invention is achieved when the cooling tank is flowed through by the cooling medium. By this measure, the entire plastic material in the cooling tank flows through the cooling medium and there is a direct heat transfer from the plastic material to the cooling medium instead.

In order to avoid undesired influence of the plastic material in the cooling tank by the cooling medium, it proves to be advantageous if the cooling medium is dry air or an inert gas.

In one embodiment of the invention, the thermal energy "charged " Cooling medium used directly to heat the Vorwärmbehälters or the reaction vessel. This is achieved by the cooling circuit and the heating circuit are in fluid communication, so that the heated as it flows through the cooling tank cooling medium is passed through the heating circuit, where it delivers the stored heat to the preheating tank and / or the reaction vessel. Excellent heat transfer is achieved when the preheat tank and / or the reaction tank are connected in the heating circuit, so that he / she is flowed through by the heated cooling medium flowing through the heating circuit. The prerequisite for this is, of course, that the cooling medium is compatible with the plastic material in the reaction vessel or Vorwärmbehälter. Compatible cooling media include dry air and inert gas, for reasons of cost and because of increased design complexity, inert gas, such as nitrogen, is mainly used in large-scale plants for use. Since the plastic material located in the cooling tank is already dry after its heat treatment in the reaction vessel, it proves to be most advantageous if drying air is used as the cooling medium, which flows through the cooling tank. Due to the pre-dried plastic material in the cooling container no unwanted loading of the dry air with moisture takes place.

In an alternative embodiment of the invention, the cooling circuit comprises a built-in cooling tank heat exchanger. This avoids the direct contact between the cooling medium and the plastic material located in the cooling container. By this embodiment, for example, a liquid cooling medium (based on water or oil) can 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 each other, but the cooling circuit with the heating circuit via a heat exchanger connected heat transfer. As a result of this configuration, different media can be used as the cooling medium and the heating medium, which results in greater flexibility as a function of the heat treatment process of the plastic material. Due to higher heat storage capacity, it may prove to be expedient if in this case the heating circuit is flowed through by a liquid heat transfer medium. On the other hand, to avoid deterioration of the plastic material in the preheating container or in the reaction vessel to avoid direct contact between the liquid heat transfer medium and plastic material by the heat transfer medium a wall of the preheating tank and / or reaction vessel flows around, but not by the Vorwärmbehälter and / or reaction vessel itself is allowed to flow. This can be done for example by a double-walled design of said container or by forming channels for guiding the heat transfer medium in the container wall.

Since the heat dissipation of the plastic material in the cooling tank on the cooling medium and the subsequent heat transfer from the cooling medium to the heat transfer medium (if not identical) generally not sufficient for heating of preheating tank or reaction vessel, and to better control the temperatures in the invention It is furthermore appropriate to switch a heating register in the heating circuit upstream of the preheating container and / or reaction container.

If the cooling tank should not be filled with plastic material or the procedural sequence do not allow the cooling of the plastic material in the cooling tank, so on the one hand in the cooling circuit an on and wegschaltbare bypass line to bypass the cooling tank provided, and on the other hand in the cooling circuit in front of the cooling tank a shut-off valve intended.

The invention will now be explained in more detail with reference to the drawings with reference to two embodiments, to which the invention is not limited.

1 shows in a schematic circuit diagram a first embodiment of a system according to the invention for the heat treatment of plastic material, and [0021] FIG. 2 shows a schematic diagram of a second embodiment of an installation according to the invention for the heat treatment of FIG plastic material.

Referring first to Fig. 1, a first embodiment of the plant according to the invention for the heat treatment of plastic material is explained, which is designed as a plant to increase the intrinsic viscosity of polyester material with a by dry air, alternatively by inert gas, heatable reaction vessel 2. Polyester material P, which comprises PET, PA, PC, etc. and is present as granules or ground material from bottles, preforms, etc., is introduced in a crystalline form, preferably preheated, batchwise by means of vacuum conveyance into an evacuatable preheating tank 17, which at the same time serves as a vacuum lock for the SSP Reaction vessel 2 is used. For this purpose, the Vorwärmbehälter 17 has both at its entrance and at its output via vacuum-tight shut-off valves 20, 21. The temperature in the preheating container is detected by a temperature sensor T4. After introduction of the polyester material P, it is heated in the preheating tank 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 at the reaction temperature of the preheating tank 17 is provided with a stirrer 18. The reaction temperature is the temperature at which the increase of the intrinsic viscosity of the polyester material begins measurably. The set heat treatment temperature is dependent on the supplied polyester material, but is generally at least 180 ° C, preferably about 220 °. After the polyester material has reached the desired temperature, it is released by opening the obturator 21 to the SSP reaction vessel 2 and kept in the SSP reaction vessel 2 for a defined residence time at the reaction temperature. Over the residence time of the polyester material in the SSP reaction vessel 2, a solid-phase polycondensation of the polyester material occurs, which leads to an increase in its intrinsic viscosity. In the lower region of the reaction vessel 2, an opening 11 is provided for sampling in order to determine the degree of polymerisation of the plastic.

In the present embodiment, the SSP reaction vessel 2 is operated under vacuum, but the invention is not limited thereto. Reference numeral 13 denotes a level sensor for the reaction vessel 2. After the desired residence time in the reaction vessel 2 (ie, after the desired IV has been set), the polyester material is discharged via a shut-off device 3, which is designed here as a slide, batchwise to the cooling tank 6, in the polyester material is cooled to a suitable cooling temperature. To accelerate the cooling process, the cooling tank 6 has a stirrer 19.

At the outlet of the cooling tank 6, a further obturator 15 is arranged, which follows a polyester material switch 7, which has two outputs 7A, 7B, wherein output 7A via a pipeline with a polyester material processing machine 8, designed here as a screw extruder, is connected, and output 7B is connected via a pipeline to a buffer tank 10. Depending on the position, the polyester material diverter 7 directs the flow of polyester material passing through the obturator 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 diverter 7 also being present and thus the formation of polyester material substreams are possible.

In the cooling vessel 6, the polyester material is cooled to a temperature below the reaction temperature cooling temperature, which, when the downstream processing machine 8 requires material, which is measured by a level sensor 14 to a feed hopper 16 of the processing machine 8, e.g. for PET is set to a maximum of 180 ° C, on the one hand to stop the process of increasing the intrinsic viscosity, on the other hand for energy saving reasons but to keep the cooling as low as possible in the processing machine 8 under energy supply an increase in the temperature of the plastic material to the melting temperature is. If the processing machine 8 does not require material, a substantially lower cooling temperature may be selected, e.g. below 160 ° C for PET to prevent yellowing of the polyester material. It is understood that the cooling temperature can also be much lower, for example between 70 and 120 ° C, can be selected. The cooling of the polyester material to this lower cooling temperature is then carried out when the polyester material is to be delivered to the buffer 10. The regulation of the temperature in the cooling container 6 can be carried out either as a function of the signals of the level sensor 14, or depending on the position of the polyester material switch 7. If the level sensor 14 reports that the feed hopper 16 is full, the polyester material switch 7 is switched so that the polyester material stream P2 flows into the intermediate storage container 10, wherein the polyester material in the material stream P2 has previously been cooled to said lower cooling temperature. The material stream P2 may be conveyed by a conveyor such as e.g. a vacuum conveyor 9, be promoted. Thus, even if the processing machine 8 can not absorb polyester material, either due to malfunction or because of machine downtime in the course of retrofitting, the heat treatment process in the reaction vessel 2 can proceed unchanged as the cooled polyester material is delivered to the intermediate storage vessel 10. If the processing machine 8 again requires polyester material, the polyester material diverter 7 is switched over in order to guide the material flow P1 to the processing machine 8, at the same time setting the cooling of the material in the cooling container to the higher cooling temperature.

The Vorwärmbehälter 17, the reaction vessel 2 and the cooling tank 6 are connected via vacuum lines to a vacuum source, not shown, so that in the containers mentioned a vacuum between 0.1 and 10 mbar can be generated. The evacuation of the individual containers is controlled via valves V1, V2, V3 in the vacuum lines.

According to the invention, the cooling of the plastic material in the cooling tank 6 by the cooling tank 6 is connected in a cooling circuit in which a cooling medium K is performed. In the present 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 pipe 22 leads into the cooling tank 6, through which the cooling medium K is introduced into the cooling tank 6. The cooling medium K flows through the cooling container 6, while the plastic material therein removes the heat, it heats itself and leaves in the heated state, the cooling tank 6 through a pipe 23. Since the plastic material in the cooling tank 6 due to the previous heat treatment process in the reaction vessel 2 absolutely dry 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 material and in particular triggers no unwanted reactions of the plastic material. It should be noted that instead of dry air, an inert gas, e.g. Nitrogen, can be used as a cooling medium. In this case, however, care must be taken to clean the inert gas after it has been used and to return it to the cooling circuit.

The pipe 23 opens into a further pipe 24 which supplies the heated cooling medium K to a heating register 5, where it is further heated to a predetermined heating temperature and then passed through a heating circuit comprising the pipe 25, branch off from the two heating circuits , namely a heating circuit for heating the preheating tank 17 and a heating circuit for heating the reaction tank 2. The heating circuit for heating the preheating tank 17 comprises a pipe 26, which can be shut off by a valve V5 leads into the Vorwärmbehälter 17, so that the heated cooling medium flows through the Vorwärmbehälter 17 and thereby emits heat energy to the container located in the preheating Kunststoffgut. The cooled cooling medium leaves the preheating tank 17 via a pipe 27. The heating circuit for heating the reaction tank 2 comprises a valve 28 which can be shut off by a valve V6, which leads the heated cooling medium into the space between the peripheral walls of the double-walled reaction tank 2, where it Inner wall and thus the inside of the reaction container 2 befindliches plastic material heated or maintained at the reaction temperature. Since in the present embodiment, the reaction vessel 2 is operated under vacuum, it is important that the cooling medium does not enter the container interior, but only flows through the gap between the peripheral walls and -after which the stored heat energy has discharged indirectly to the plastic material in the reaction vessel 2 - is withdrawn via a pipe 29.

In the event that the cooling tank 6 should not be filled with plastic material once or the procedural process temporarily does not allow the cooling of the plastic material in the cooling tank 6, a bypass line 30 which can be shut off by a valve V4 is connected between the pipes 22 and 24 to pass the cooling medium K to the cooling container 6. Furthermore, the pipe 22 between the junction of the bypass line 30 and the cooling tank 6 by a valve V7 can be locked.

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

The second embodiment of the invention differs from the first by a modified type of leadership of the cooling circuit and heating circuit. The refrigeration cycle includes a cooling medium source 31 which provides a cooling medium K, e.g. 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 tank 6 'and thereby absorbs the heat from the inner wall, which in turn is in heat-conducting relationship with the inside of the cooling tank 6' plastic material. In this way, the cooling medium K indirectly withdraws the heat from the plastic material and heats up itself. As an alternative to the double-walled design of the cooling container, it could also be provided inside the container with a heat exchanger through which the cooling medium flows, thereby extracting 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 tank 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 contact with it directly. The heating circuit comprises a heat transfer medium source 34, which contains a heat transfer medium W, e.g. water or oil based. The heat transfer medium W is passed through a pipe 35 to the heat exchanger 4, therein, as mentioned, indirectly heated by the heated cooling medium K and is passed after leaving the heat exchanger to a heating coil 5, where it is additionally heated and thereby brought to a predetermined heating temperature becomes. From the heater 5, the heat transfer medium W passes through a pipe 36 in the space of the double-walled preheating container 17 ', where it is used to heat the inner wall of the Vorwärmbehälters 17 and thereby heat the located inside the Vorwärmbehälters plastic material. After the heat transfer material has delivered its stored heat energy to the inner wall, it is returned via a pipe 37 to the heat transfer medium source 34. 6.9

Claims (12)

Austrian Patent Office AT 502 958 B1 2011-07-15 Claims 1. Plant for heat treatment of plastic material, in particular for increasing the intrinsic viscosity of polyester material by solid phase polycondensation (SSP), with a heatable reaction vessel (2) in which the plastic material at a predetermined heat treatment temperature for a predetermined residence time can be allowed to linger, wherein the reaction vessel (2 ') is optionally preceded by a Vorwärmbehälter (17, 17'), which is for preheating and delivery of the preheated plastic material to the reaction vessel (2) is formed, and with a downstream of the reaction vessel (2) arranged cooling container (6, 6 '), in which the plastic material from the reaction vessel for cooling to below the heat treatment temperature lying cooling temperature can be delivered, wherein the cooling tank (6, 6') connected in a cooling medium (K) leading cooling circuit is, wob ei the cooling circuit downstream of the cooling tank directly or indirectly via heat exchangers (4) to a heating circuit for heating the Vorwärmbehälters (17, 17 ') and / or the reaction vessel (2) is connected, characterized in that the cooling circuit is an on and wegschaltbare bypass line (30) for bypassing the cooling container (6). 2. Plant for heat treatment of plastic material according to claim 1, characterized in that the cooling tank (6) from the cooling medium (K) is flowed through. 3. Plant for 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 heat treatment of plastic material according to one of the preceding claims, characterized in that the cooling circuit and the heating circuit are in fluid communication. 5. Plant for heat treatment of plastic material according to claim 4, characterized in that the Vorwärmbehälter (17) and / or the reaction vessel (2) are connected in the heating circuit, so that he / she from the heated cooling medium (K) by the Heating circuit flows, is flowed through / be. 6. Plant for heat treatment of plastic material according to claim 1, characterized in that the cooling circuit comprises in the cooling container (6 ') built-in heat exchanger, which is flowed through by the cooling medium (K). 7. Plant for 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 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 heat-transmitting connected. 9. Plant for 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 heat treatment of plastic material according to claim 9, characterized in that the heat transfer medium (W) flows around a wall of Vorwärmbehälters (17 ') and / or reaction vessel (2). 11. Plant for heat treatment according to one of the preceding claims, characterized in that in the heating circuit upstream of Vorwärmbehälter (17, 17 ') and / or reaction vessel (2), a heating coil (5) is connected. 12. Plant for heat treatment according to one of the preceding claims, characterized in that the cooling circuit in front of the cooling tank (6) has a shut-off valve (V7). For this 2 sheets drawings 7/9