DE8624172U1 - Screw extruder for separating gaseous components from product melts - Google Patents

Description

BASF Aktiengesellschaft O.Z. 0050/38677

Screw extruder for separating gaseous components from product melts

Description

The present invention relates to a screw extruder for separating gaseous components from product melts, consisting of a screw shaft surrounded by a cylinder with a feed opening for the product melts, with a shaft, a screw drive and a degassing device arranged between the feed opening and the screw drive. consisting of a stuffing box housing, a stuffing box, an annular chamber, two openings and an annular gap.

Plastics must be processed so that they can be marketed more easily. To do this, the melted masses produced during polymerization are fed from the reactor to an extruder via a downpipe at temperatures above 200 ^° C.

The extruder then conveys the incoming, gas-laden product melt to a granulating device, where it is formed into an easily handled shape. The majority of the product melt takes the path determined by the conveying direction of the screw extruder.

However, a small part of the product melt escapes together with significant amounts of gas through the annular gap formed between the screw shaft and the housing or through the sealing elements attached there and exits in the direction of the screw drive. Due to the temperature gradient that exists between the product melt and the screw drive, which is only slightly above room temperature during operation, the product melt solidifies. This results in firmly adhering coatings on the shaft of the screw shaft. This firmly adhering coating spreads further as the product melt flows in and builds up further, particularly due to the material build-up on the seals. The polymer coating presses against the sealing elements, destroys them or pushes itself under them, thereby increasingly reducing their sealing effect.

For example, stuffing box seals that are intended to seal the molten polymer from the extruder’s intake opening to the drive side of the shaft* can be milled open, so to speak, and polymer and, in particular, gas can thus get into the screw drive. To prevent this, it is known to connect extruders to the gearbox at the end of their intake section.

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The extruder screw is provided with a return thread at the end of the extruder housing, which is intended to prevent too large a quantity of product melt from escaping from the lantern. However, the gas arriving with the product melt passes unhindered through the openings of the lantern and is usually released into the atmosphere.

Experience has shown that this well-known mechanical measure does not reliably prevent the product melt from escaping in the direction of the screw drive. In addition, the gas emissions represent a significant environmental burden.

In order to eliminate this source of interference, a degassing device was developed which consists of a stuffing box housing which covers and seals the screw shaft, which is positively and force-fittedly connected to the cylinder and which has on its inner wall on the side facing the cylinder a spiral groove which runs in the opposite direction to the screw cut and an eccentric annular chamber which has one or more openings extending from it.

However, the known degassing device has the major disadvantage that it cannot be used for all products.

As described, one works with extruder screws that have a return thread at the inlet to the degassing device.

The product melt coming into the area of the return thread crosslinks due to the long residence time in contact with the

metal surface and the relatively high temperatures prevailing there.

Brittle particles form which cannot flow towards the screw drive or the ring chamber if the degassing device is blocked.

On the contrary, the brittle particles are from time to time - especially when there are fluctuations in the product flow from the reactor - rewound into the extruder intake and thus into the main product flow. This leads to contamination in thin film products. Depending on the type of product extruded, the above-mentioned faults occur within a few hours. This is particularly the case with thermally unstable products and copolymers of ethylene, which

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have the property of sticking to metals, the known devices cannot prevent the described disturbances*

The EXTRUDATE is continually contaminated by brittle particles, i.e. there is a loss of quality,

Ultimately, the production of products with good film properties is no longer possible.

IQ For the above-mentioned products, an extruder screw without a return thread must therefore be used. However, this has the disadvantage that, depending on the product type/melt index of the product, very large quantities of melt can pass into the degassing device and clog it.

This means that degassing is no longer possible and disruptions occur in the product discharge due to foaming of the granulate and,

Product agglomeration.

The result is expensive downtime due to shutdowns of the polymerization system, costly repairs and environmental pollution due to increased hydrocarbon emissions.

It was therefore the object of the invention to make the degassing of product melts in screw extruders more effective, so that only a small, defined amount of product is drawn from the extruder inlet in the direction of the screw drive into the degassing device and perfectly functioning degassing is guaranteed at all times.

This object is achieved in a screw extruder of the type mentioned above according to the invention in that the degassing device is provided with a gap adjustment device that can be operated from the outside.

Advantageously, the gap adjustment device is arranged between the feed opening of the extruder and the annular chamber of the degassing device.

With the help of the invention, it is possible to limit the amount of product entering the degassing device and thus to protect the degassing device from clogging with product melt.

With the screw extruder according to the invention it is possible to

product types without compromising quality, whereby the degassing of the product melt works smoothly at all times, which

BASF Aktiengesellschaft 4 0.Z. 0050/38677

This results in an overall increase in economic efficiency. The gas escaping from the screw extruder is also fed back into the reaction process. The screw extruder according to the invention is suitable for all polymers which are gas-laden and which arise from the Sf reactors.

It has proven particularly useful in ethylene polymerization.

An embodiment of the invention is shown schematically in the drawings and explained in more detail below:

They show:

Figure 1: a side view of the screw extruder with degassing device in section.

Figure 2: a representation of the gap adjustment device (section perpendicular to the axis).

The screw extruder has a screw shaft 2 and a

surrounding cylinder 1. The worm shaft 2 is connected to the worm drive λ via a shaft.

A feed opening 5 leads into the cylinder 1 for the product melt coming from a reactor not shown in the drawing.

The actual degassing device, which is arranged coaxially to the screw extruder, consists of a stuffing box housing 6 and is positively and non-positively connected to the cylinder 1.

The stuffing box housing 6 has an eccentric ring chamber 8 in front of the stuffing box 7. Two openings 9, 10 extend from the ring chamber 8. The separated gaseous components of the product melt are drained through the upper opening 9 and returned to the process. The degassed polymer that entered the ring chamber 8 via an annular gap 11 is discharged through the lower opening 10.

The feature of the present invention is that an adjustment device for the annular gap 11 is installed between the extruder feed opening 5 and the annular chamber 8, with which the width of the annular gap 11 can be changed.

This gap adjustment device consists of adjustment blades 12, adjustment spindles 13, which are inserted into a threaded bushing H. Sealing and guide bushings 15 and end cover plates

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BASF Aktiengesellschaft 5 0.2. 0050/38677

This gap adjustment device consists in detail of adjustment blades 12, adjustment spindles 13, which are embedded in a threaded bushing H, sealing and guide bushings 15 and end covers 16.

All parts 12 to 16 of the gap adjustment device according to the invention are available in duplicate.

In the following, an embodiment of a gap adjustment device with two adjustment blades 12, which are arranged opposite each other, is described.

However, an asymmetrical arrangement of the adjustment blades 12 is also conceivable.

For extruders with screw diameters up to approx. 300 mm,

Gap adjustment devices with two adjustment blades 12 are used, while for extruders with larger screw diameters the use of gap adjustment devices with three or four adjustment blades is recommended.

20

The two adjustment blades 12 of the gap adjustment device 12 to 16 are »■each adapted to the shaft 3 in such a way that their turning diameter is 1.5 mm larger than the outer diameter of the shaft 3.

The cross-section of the adjusting blades 12 is preferably rectangular.

The adjusting blades 12 are fitted in the stuffing box housing 6 so that they slide easily.

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The adjustment blades 12 can be adjusted radially from the outside using auxiliary units via the valve spindles 13 to a distance of 0.75 to a maximum of 10 mm from the shaft 3. The end positions of the adjustment blades 12 are limited by a positive fit. This prevents the adjustment blades 12 from coming into metallic contact with the shaft 3 when setting the minimum gap 11.

To prevent gas and product from escaping from the adjusting blade chamber 17 due to the sliding surface clearances between the adjusting blades 12 and the stuffing box housing 6, a sealing cover 16 is mounted on the stuffing box housing 6.

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The passage of the adjustment spindle 13 with the sealing and guide bushing 15 is located in the shut-off valve cover 1B. The latter has the task of guiding the adjustment spindle 13 and preventing gas and product from escaping from the adjustment blade chamber 17. The threaded bushing H 5 of the adjustment spindle 13 is screwed to the extruder lantern in a force-locking manner. To facilitate the assembly of the

"Gap adjustment device 12 to 16, the adjustment spindle 13 is designed in two parts, the two parts being connected by a coupling 19. The respective setting of the gap 11 between the adjustment blades 10 and the shaft 3 can be read with the aid of a display device 20 which is mounted on the holder of the threaded bush 14.

Claims (2)

Protection2claims 1. Screw extruder for separating gaseous components from product melts, consisting of a screw shaft (2) surrounded by a cylinder (1) with an addition opening (5) for the product melts, with a shaft (3), a screw drive (A) and a degassing device arranged between the addition opening (5) and the screw drive (4), consisting of a stuffing box housing (6), a stuffing box (7), an annular chamber (8), two openings (9, 10) and an annular gap (11), characterized in that the degassing device (6 to 11) is provided with a gap adjustment device (12 to 16) that can be operated from the outside. 2. Screw extruder according to claim 1, characterized in that the gap adjustment device (12 to 16) is arranged between the feed opening (5) and the annular chamber (8) of the degassing device (6 to 11). 274/86 spr/ro 09.09.1986 Drawing. 060274