FIELD OF THE INVENTION

The present invention concerns a drying device for plastic granulates produced by means of under-water or water-ring granulation from all types of plastics known to be suitable for this purpose, but in particular including composite materials. Composite granulates in accordance with the present invention are all conventional plastics present in a mixture with other organic or inorganic substances, e.g., wood fiber, glass fiber or other additives, or mixtures of additives as well.

BACKGROUND OF THE INVENTION

In under-water or water-ring granulation, molten plastic is forced through the nozzles of an orifice plate and cut off at the nozzle outlet by a rotating blade. Due to the effect of the water, the cut granules automatically assume an approximately spherical shape. The plastic granulate is transported to a water separation and drying device by means of processing water.

Conventional drying devices may feature as their main component a centrifuge dryer wherein the plastic granulate is separated from the processing water by centrifugal forces. The centrifugal effects can be adjusted such that the granulate exiting the centrifugal dryer exhibits residual surface moisture of up to 0.1%.

However, the plastic granulate is exposed to high mechanical loads due to the centrifugal forces in a centrifugal dryer, resulting in surface roughness of the plastic pellets. Especially in composite granulates these mechanical loads can also result in damage of the plastic granulates since they are generally more brittle, which results in higher dust content.

Another problem is the abrasive effect on the centrifuge dryer baffles which must be therefore repaired or replaced regularly. This abrasive wear is particularly great with composite granulates, and among them, especially those containing glass fiber.

Another disadvantage is the great cleaning expense when the granulate load is changed, for example when the color is altered. Then the centrifugal dryer has to be completely cleaned of all adhering granulate remnants before a load can be processed.

It is known from DE 103 49 016, which describes a process for producing PET granulate, to subject the granulate, after under-water granulation and after water separation, to shaking or vibration during a subsequent heat treatment. During this phase a fluid stream can pass through the pellets and it is imaginable that the pellets are swirled about by the fluid. In another further development of this idea, DE 10 2004 021 595 describes that the fluid is warmer than the ambient room temperature. This document also mentions that the granulation and drying device is followed by a transport device in form of an oscillating conveyor or a conveyor trough.

Other known drying methods utilize a swirl or flow bed where the plastic granulate is separated from adhering processing water by a fluid such as for example an inert gas or air.

SUMMARY OF THE INVENTION

The subject of the present invention is to create a process and a device which in particular serves to implement this process making it possible to dry plastic granulates made of any material, especially composite material, such that a drying completely free of wear is achieved. Another object is optimization of the energy requirements for the drying. A third object is to further reduce the adhering residual moisture to prevent problems during long-term storage and further processing, especially where composite granulates are concerned.

The object is achieved by a process having the technical features of Claim 1, as well as by a device with the technical features of Claim 7. Advantageous further developments are subjects of the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a device for the implementation of the process according to the invention.

FIG. 2 is an elevational view of an alternate embodiment of a device for the implementation of the process according to the invention.

DETAILED DESCRIPTION

One possible embodiment of the invention is explained by way of drawings, where FIG. 1 shows a device for the implementation of the process according to the invention. An advantageous further development of the device in FIG. 1 is shown in FIG. 2.

The granulate exits the granulation apparatus (not shown) and is transported by a process water supply 16 to a perforated vibrating chute 3. The drained process water is collected in the water tank 17 located below. The vibrating chute 3 is functionally connected to at least one conventional vibrator 4. After passing through an oversize grain separator 5, the granulate with adhering residual moisture is transferred to a second vibrating chute 6 which in turn is also functionally linked with at least one vibrator 7. The vibrating chute 6 is disposed in a housing 8 which is provided with an exhaust hood 9 comprising a fan 10. An air inlet with an air filter 11 is disposed in the floor area of the housing 8. At least one heat exchanger 12 is arranged between the air filter 11 and the vibrating chute 6. The processing water 1 collected in the water tank 3 is returned by a pump 13 to the granulating unit by way of a drain line 14.

The processing water is heated by the introduction of the granulate. The amount of the added heat primarily depends on the composition of the granulate. However, the temperature of the processing water may not exceed a predetermined value. In conventional granulating units, processing water temperature is kept below said value by means of a water-to-water heat exchanger. The heat content of the discharged processing water is lost.

In the process according to the invention however, the heat content of the processing water is used to heat the drying air in the heat exchanger 12. The colder water regained from the heat exchanger 12 via the cooling water pump 15 is either returned to the water tank 17, and from there via pump 13 to the granulating unit or it is fed directly back to the granulating unit.

By this heating of the drying air which is aspirated by the fan 10 through the granulate disposed on the vibrating chute 6, the residual surface moisture adhering to the granulate product can be reduced to 0.01% or even further, resulting in a substantial increase of long-term storage stability of the granulate, and especially of its properties related to further processing.

In FIG. 2, the space underneath the vibrating chute 6 is completely or partially divided into smaller spaces that are separated from one another by a is sheet metal divider 20. Two heat exchangers 12 and 18 are arranged between the vibrating chute 6 and the air filter 11. The drying air aspirated by the fan 10 is partially heated by the first heat exchanger 12 in the manner described above and is passed through the granulate disposed on the perforated vibrating sheet 6. The heat content of this first heat exchanger 12 is extracted from the processing water in tank 17, in the above-described manner. The heat content of the second heat exchanger 18 is extracted from the moist exhaust air behind the fan 10 by a further heat exchanger 21, and is supplied by a pump 22 to the heat exchanger 18.

This allows on the one hand for the moisture contained in the exhaust air to condense on the surface of the heat exchanger 21, and is thus easily collected which substantially reduces the moisture content of the exhaust exiting the apparatus into the ambient air, while on the other hand the residual surface moisture adhering to the granulate can be further reduced in an advantageous manner.

A further advantage of the invention is the gentle form of drying as the granulate is subjected to substantially less mechanical stress in the vibrating chutes than in a centrifugal dryer. As concerns composite granulates, this provides considerably more degrees of freedom in selecting the types of compositions, and the mixture ratios of the individual components.

Additional advantages are that the drying can be accomplished practically without wear of the apparatus or its parts and that far less servicing is required which advantageously reduces the downtime of the device. Furthermore, a substantial reduction of dust content in the granulate product is achievable.