PROCESS AND SYSTEM FOR PRODUCING PET GRANULES

Abstract

A process and a system for producing polyethylene terephthalate (PET) granules by transesterification of dimethyl terephthalate with ethylene glycol or by esterification of (fiber) purified terephthalic acid with ethylene glycol suitable for further processing to form packaging films and bottles, comprising the steps of polycondensation, granulation and latent heat crystallization, aftertreatment of the crude granules to adjust the polymer quality values required for the further processing, in particular the intrinsic viscosity, the acetaldehyde content and the moisture content, wherein the aftertreatment is carried out in multiple moving bed tubular reactors operated in parallel.

Claims

1. A process for producing polyethylene terephthalate (PET) granules suitable for further processing to form packaging film and bottles, comprising the steps: a) Producing a PET melt or PET copolymer melt in a continuous single-line polymerization system, either by transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) or by esterification of terephthalate acid (PTA) with ethylene glycol, b) Transport of the melt preferably through a pipeline for further treatment in step c), c) Granulation of the melt with cooling to form crude granules, wherein partial crystallization of the polymer takes place with the release of a portion of the heat of crystallization of the polymer (latent heat crystallization), d) Aftertreatment of the crude granules to adjust the polymer quality values required for the further processing, in particular the intrinsic viscosity, the acetaldehyde and moisture content, characterized in that the aftertreatment takes place in multiple moving bed tubular reactors operated in parallel, wherein the dwell time of the granules in the reactor, the type and chemical composition of the process gas and its temperature and dew point at the entrance in to the reactor are adjusted individually for each one of the reactors.

2. The process according to claim 1, wherein the polycondensation of the PET takes place in step a) until establishing an intrinsic viscosity in the range of 0.70 to 0.80 dl/g.

3. The process according to claim 1, wherein step c) takes place in multiple lines, wherein an additive to influence the polymer quality or a stream of recycled material is fed into the respective line.

4. The process according to claim 1, wherein the additives added in step b) are introduced into the PET stream either directly or embedded in a polymer matrix.

5. The process according to claim 1, wherein the additives added in step b) are dye additives such as blue toners, phosphorus stabilizers to prevent yellowing of the PET, comonomers and/or acetaldehyde scavengers.

6. The process according to claim 1, wherein in step d) of claim 1 the aftertreatment of the crude granules takes place in at least one of the moving bed tubular reactors with the goal of reducing the acetaldehyde content in the granules, wherein air at an inlet temperature between 160° C. and 200° C., preferably 180° C. and 190° C. is introduced into the reactor as the process gas.

7. The process according to claim 1, wherein in step d) of claim 1, in at least one of the moving bed tubular reactors, the aftertreatment of the crude granules is carried out with the goal of increasing the intrinsic viscosity by up to 0.1 dl/g wherein air at an inlet temperature into the reactor between 160° C. and 190° C. and a dew point of less than −15° C. is used as the process gas.

8. The process according to claim 1, wherein in step d) of claim 1, in at least one of the moving bed tubular reactors, the aftertreatment of the crude granules is carried out with the goal of increasing the intrinsic viscosity by up to 0.1 dl/g wherein inert gas preferably nitrogen at an inlet temperature into the reactor between 150° C. and 230° C. and a dew point of less than −15° C. is used as the process gas.

9. A system for carrying out the process according to claim 1 comprising the following system parts: a system part for producing a PET melt from the raw materials DMT/EG or PTA/EG, a conveyor device for transporting the PET melt thereby produced into the system to produce partially crystalline PET granules, a system part for producing partially crystalline PET granules, a conveyor device for the partially crystalline polymer granules, at least two system parts for aftertreatment of the granules, each comprising a moving bed tubular reactor and devices for conveying, introducing and discharging the process gas.

Description

[0037] The invention will now be explained in greater detail on the basis of the drawings, in which:

[0038] FIG. 1 shows a block diagram of a process and/or a system for producing aftertreated PET granules according to the prior art,

[0039] FIGS. 2 and 3 each show a block diagram of the process according to the invention and/or a system according to the invention.

[0040] FIG. 1 according to the prior art shows how starting materials 5, PTA/EG or DMT/EG are fed into the single-line continuously producing polycondensation system 1 and converted to a PET melt 6. The production capacity of system 1 is 600 tons of PET melt/day. It is not shown that the additives, for example, isophthalic acid and catalysts such as antimony, for example, that are needed to adjust the basic polymer quality in the melt 6 are used to make the polymer suitable for the production of beverage containers and packaging films. A suitable IV value (intrinsic viscosity) of the melt 6 for this purpose is 0.75 dl/g.

[0041] The term “quality” as used here and in the following is not to be understood in the sense of “good” or “bad” but rather in the sense of “type” or “grade.”

[0042] The melt 6 is transferred by means of a pump through a pipeline into a system for granulation and latent heat crystallization 2. The drawings do not show that this system includes two parallel production units for granulation and latent heat crystallization. In this system the PET melt is converted into partially crystalline PET granules. The granules are cooled to a temperature in the range of 160° C. to 180° C. The granules produced in the two units are transferred to a common container (not shown) and conveyed from there by a pneumatic conveyor 7 to the system 3 for the aftertreatment.

[0043] To avoid a loss of temperature by the granules, the pneumatic conveyance is accomplished using tempered gas accordingly.

[0044] The aftertreatment 3 is carried out according to the prior art in a continuous single-line system in campaigns, in which the system comprises a moving bed tubular reactor.

[0045] One aftertreatment process that is frequently used is dealdehydization, which serves to remove most aldehydes from the polymer. This is necessary in many cases when the polymer is to be processed to beverage bottles because aldehydes have a negative influence on the taste of the beverages. In this aftertreatment process, the dwell time of the granules in the moving bed tubular reactor is between 8 hours and 15 hours, or in many cases 12 hours. The process gas, whose temperature is adjusted so that the temperature of the granules is between 160° C. and 190° C., flows through the moving bed. In this temperature range, air is often used as the process gas.

[0046] Another aftertreatment process that is often used is to adjust a certain intrinsic viscosity. The change in intrinsic viscosity of the polymer in such a treatment is influenced by the moisture of the process gas. Table 1 shows this relationship as an example for a temperature and dwell time. In dealdehydization, the goal is to achieve a constant intrinsic viscosity and the dew point of the process gas is adjusted accordingly.

[0047] If the goal of the aftertreatment is a slight increase in the intrinsic viscosity, i.e., by at most 0.1 dl/g, then the procedure followed is the same. The dew point of the process gas is set lower accordingly.

[0048] The goal of the aftertreatment is often to raise the intrinsic viscosity of the polymer up to 0.95 dl/g as a result of the aftertreatment. In these cases it is favorable to keep the dwell time of the granules in the reactor within certain limits to adjust the temperature of the granules in the range of 200° C. to 230° C. It has proven suitable to use an inert gas such as nitrogen as the process gas in this temperature range. The dew point of the process gas is kept low accordingly as shown in Table 1.

TABLE-US-00001 TABLE 1 Change in the intrinsic viscosity Δ IV of the PET as a function of the dew point of the process gas with a dwell time of 12 hours and a temperature of the granules of 180° C. Δ IV (dl/g) Dew point (° C.) 0.08 −40 0.05 −30 0.03 −20 0.00 −15 −0.05 0 −0.09 10 −0.1 20

[0049] The aftertreated PET granules are conveyed 8 into the product storage bin 4. The different polymer qualities produced are stored separately 4a, 4b, 4c. Since large quantities of granules, so-called off-spec quality, i.e., product that does not conform to specifications, are produced when switching an aftertreatment process to a different polymer quality, such changes are to be avoided as much as possible. Off-spec quality granules have only a very low market value and thus ultimately result in an increase in production costs. Off-spec granules 13 are usually bagged in shipping containers such as big bags directly from the aftertreatment reactor and then shipped off for further treatment.

[0050] To keep the frequency of changes in the aftertreatment process as low as possible, the production campaigns must be as long as possible so that a product storage site with a large holding capacity is required. This also increases the cost of production.

[0051] FIG. 2 illustrates how the production of off-spec granules can largely be prevented according to the invention and how such a process and/or such a system can be operated with a smaller product storage capacity.

[0052] FIG. 2 shows that the aftertreatment is carried out in several systems that are operated in parallel in this example for aftertreatment 3a, 3b, 3c. The systems 3a to 3c can be operated independently of one another so that three different polymer qualities 8a, 8b, 8c can be created at the same time from one basic quality 7a, 7b, 7c. The polymer qualities 8a through 8c are stored separately 4a, 4b, 4c and, as shown by the streams 9a, 9b, 9c, conveyed out of the respective storage for further processing. The product storage area may be designed to be smaller than that in the process according to FIG. 1 because now it is not the length of the production campaigns that is the relevant factor in designing the storage capacity but instead only the logistics of the further transport of the granules for further processing determine the size of the storage area.

[0053] FIG. 3 shows an example of one variant of the invention, in which the product quality can be altered in addition to being altered by the aftertreatment by feeding an additive into the polymer line 6a, 6b leading from the polycondensation system into the granulation system and thereby incorporating it into the polymer melt. This incorporation can take place by merely introducing the additive stream 12a, 12b into the polymer stream 6a, 6b or the mixing may be supported by mixers installed in the polymer line 6a, 6b.

[0054] For example, dye additives, such as so-called blue toners, stabilizers such as phosphorus compounds, diethylene glycol, IPA or other comonomers, aldehyde scavengers and recycled materials, i.e., shredded polymer material produced from used bottles, for example, may be considered as additives to be by from the dosing systems 11a, 11b into the polymer lines and/or polymer streams 6a, 6b by way of the lines 12a, 12b.

LIST OF REFERENCE NUMERALS

[0055] 1 Polymerization system

[0056] 2a and b System for granulation and latent heat crystallization

[0057] 3a to d System for aftertreatment

[0058] 4a to d Storage for aftertreated granules

[0059] 5 PTA/EG or DMT/EG

[0060] 6a and b Transfer line for polymer melt

[0061] 7a to d Pneumatic conveyance of the granules

[0062] 8a to d Conveyance of the granules into the storage

[0063] 9a to d Conveyance of the granules for further processing

[0064] 10a and b Additives

[0065] 11a and b System for dosing additives

[0066] 12a and b Pipeline for feeding the additives into the transfer line

[0067] 13 Off-spec granules