Method for the production of polyester granulates from highly viscous polyester melts and also device for the production of the polyester granulates
The invention relates to a method and device for the direct production of polyester granulate from a highly viscous polyester melt with a polymerization degree of 132 to 165, as well as the granulates formed thereform. In the method, the highly viscous polyester melt is subjected to a pre-drying and drying/degassing after a hot cutting method. Hot cutting is implemented at water temperatures of 70° C. to 95° C. and with a liquid to solid ratio of 8 to 12:1.
1. A method for the direct production of polyester granulate from a highly viscous polyester melt (hiV) with a polymerization degree (PG) of 132 to 165, the method comprising:
hot cutting the hiV melt into granulate in water having a temperature of 70 to 95° C. and maintaining a water to granulate ratio of 8 to 12:1;
pre-drying the granulate in a pre-dryer;
classifying the granulate with a classifying screen;
purging the classified granulate with temperature-controlled, dry air in a collecting container for a maximum dwell time of 8 minutes;
drying/degassing the granulate in a temperature-control/cooling silo; and
wherein a moisture of the pre-dryer is controlled by controlling a quantity of the temperature-controlled dry air.
2. The method according to claim 1 , wherein the hiV melt has a dwell time in the water of <1 second.
3. The method according to claim 1 , the water and the granulate comprises a liquor that is maintained in its entirety until entry into the pre-dryer.
4. The method according to claim 1 , wherein 99% of circulating water is separated from the granulate within <10 seconds in a lower fifth of the pre-dryer.
5. The method according to claim 1 , a crystallization degree of at least 5% is achieved during pre-drying so that the agglomeration of granulates is prevented.
6. The method according to claim 1 , wherein an outlet moisture of the granulate from the pre-drying of <200 ppm is achieved.
7. The method according to claim 1 , wherein a dew point during pre-drying in the pre-dryer is controlled to 8 to 12° C. by means of a quantity of purging air from a subsequently connected collecting container.
8. The method according to claim 7 , wherein a moisture of the granulate at the outlet of the collecting container of <200 ppm to >100 ppm is achieved.
9. The method according to claim 7 , wherein a crystallization degree at the outlet of the collecting container of <25% is reached.
10. The method according to claim 1 , wherein a dwell time on the classifying screen is at most 30 seconds.
11. The method according to claim 1 , wherein the granulate is brought from the collecting container to the drying in the temperature-control/cooling silo by means of hot air conveyance.
12. The method according to claim 1 , wherein the temperature-control/cooling silo is actively insulated such that combined drying and cooling is effected.
13. The method according to claim 12 , wherein the drying is implemented over 6 to 12 hours at 150 to 180° C., and the cooling over 0.5 to 1.5 hours reducing to 50° C.
14. The method according to claim 1 , wherein vapors from the pre-drying are condensed in a mixing condenser, the cooling medium of which is taken from a partial flow of a main water circulation and the mixing condensate of which is used for temperature control of the main water circulation.
15. The method according to claim 14 , having a mixing ratio of partial flow to main flow of a ratio of 1:4 to 1:6.
16. The method according to claim 11 , wherein processing of dried air is effected at dew points of −10° C. to −40° C.
17. The method according to claim 11 , wherein air introduced in the temperature-control/cooling silo at a maximum temperature of 40° C. in a ratio of 1:5 to 1:10 experiences a quasi laminar distribution through a cooler.
18. The method according to claim 1 , wherein polyethylene terephthalate is produced.
19. The method according to claim 1 , wherein a reduction in a polycondensation degree starting from the hiV melt to the granulate after drying/degassing is less than 2%.
20. The method according to claim 1 , wherein a reduction in a polycondensation degree starting from the hiV melt to the granulate after drying/degassing is less than 1.5%.
21. The method according to claim 1 , wherein the cutting phase is effected at water temperatures of 70° C. to 90° C.
22. The method of claim 1 wherein the temperature-controlled, dry air is has a temperature of 140 to 180° C.
23. The method of claim 1 wherein the steps are performed in the recited order.