DE7540389U - SCREW EXTRUDER FOR SEPARATING GAS-COMPONENTS FROM PLASTIC BULK - Google Patents

Description

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BASF Aktiengesellschaft U

Our sign O ₈ Z. 51 729 Wr / MK 6700 Ludwigshafen, 12.1? .1975

Screw extruder for separating gaseous components from plastic masses

! The invention relates to a screw extruder for disconnecting \ separating gaseous components from plastic materials, consisting of one of a cylinder with a feed opening for the masses to be treated surrounded worm shaft, a drive unit and, arranged between the feed opening and the drive unit degassing "

Plastics have to be packaged so that they can be brought onto the market more easily. For this purpose, the masses obtained as melt during the polymerization are ^{fed from the reactor to an extruder at temperatures above 200 °} C. via a downpipe. The extruder then conveys the incoming, gas-laden polymer melt to a granulating device, where it is brought into an easy-to-use shape. »Most of the melt admittedly takes the path given by the screw extruder conveyor system» A small part of the melt escapes with it considerable gas shares through the gap that is formed between the worm shaft and the housing or through the sealing elements attached there and emerges in the direction of the drive. Due to the temperature gradient that prevails between the melt and the drive part of the extruder, which is only slightly above room temperature in the operating state, the melt solidifies. There are adherent coatings on the drive socket of the worm shaft. This firmly adhering coating spreads further as the melt flows in and builds up further, especially due to the accumulation of material on the seals. The polymer coating presses against the sealing elements, destroys or pushes itself under them and thus reduces their sealing effect more and more "

7540389 2 ^ 05.76 ■ ² -

O.Z, 31 729

For example, stuffing packs, which are intended to seal the molten polymer from the feed opening of the extruder to the drive side of the shaft, can be milled open. Polymer and in particular gas thus passes into the transmission. In order to prevent this, it is known at the end of its collection portion towards the gearbox with a so-called lantern, d »h _o a housing having a plurality of circumferentially spaced openings to provide Extruder» The extruder screw is provided with a return thread at the end of the extruder housing, which is intended to prevent excessively large amounts of polymer from escaping at the lantern. However, the gas arriving with the polymer passes through the openings in the lantern unhindered,

This known mechanical engineering measure includes an exit experience has shown that the melt in the direction of the drive is not reliable Environmental pollution.

It was therefore an object of the invention to make the degassing of plastic masses in screw extruders more effective, with at the same time an escape of the melt and in particular its gaseous constituents into the atmosphere is prevented.

In the case of a screw extruder, this task is described at the outset Art according to the invention in that the degassing device consists of a screw shaft covering and sealing gland housing is formed, which is positively and non-positively connected to the cylinder and on his Inner wall on the side facing the cylinder a spiral groove running in the opposite direction to the worm section and adjoining it has a concentric annular chamber which is provided with a partition wall and from which one or more openings extend.

In this way, the separation of the melt from its gaseous components is achieved in a simple manner, and beyond that can be withdrawn from the annular chamber. The melt no longer comes into contact with the shaft seals, so that the described Consequential damage is avoided «Expensive downtimes and costly repairs are significantly reduced, what

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Ultimately, the extruder has a longer service life and more reliable operation with consistent quality of the Product and thus overall in an increased profitability. The gas exiting the extruder can can also be returned to the reaction process »The device according to the invention is suitable for all polymers the gas-laden accumulate at the reactors. It has proven particularly useful in ethylene polymerization.

In the drawings, exemplary embodiments of the invention are shown schematically and explained in more detail below;

They show;

Figure 1 is a side view of the screw extruder with degassing device on average,

FIG. 2 shows a plan view of the degassing device and FIG. 3 shows a section through the degassing device according to FIG Figure 2 according to the line A-B.

According to FIG. 1, the worm shaft is denoted by 2 and the cylinder surrounding it is denoted by 1. A downpipe connects the feed opening 3 with the reactor (not shown in the drawing). The actual degassing device arranged coaxially to the extruder consists of a stuffing box housing 4 and is positively and non-positively connected to the cylinder 1.

Appropriately, the stuffing box housing 4 is on almost his Provided the entire length with a heating jacket 5 and has on his Inner wall 6 on the side facing the cylinder 1

opposite to the worm cut spiral groove 7 and then an annular chamber 8 on.

The concentric annular chamber 8 contains a partition 9, by means of which the gaseous components are separated from the plastic masses are »The separation of the relatively small amounts of plastic product, which over the strong shear field of the degassing ring reaches and adheres to the cylindrical part of the worm shaft, is a purely mechanical process of stripping ^.

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The partition is in the direction of rotation in front of the gas outlet opening appropriate.

According to a further feature of the invention, the annular chamber is designed to be eccentric. In this case a partition is not necessary. The plastic components are forcibly fed to the lower outlet opening.

Two openings 10 and 11 extend from the annular chamber 8. The separated gaseous constituents are returned to the process via the opening 10 and a pipe 12. The polymer passes through the opening 11 into a separator 13, in which further degassing of the masses can be carried out if necessary. On the side facing the drive unit Ik , the stuffing box housing k is sealed gas-tight, for example by means of packing rings 15 and a stuffing box gland l6 "

It is of decisive importance for the entire facility that the product which has passed through the return thread is before it is reached the annular chamber in the stuffing box housing passes through a shear field, whereby it is very strongly degraded at relatively high temperatures, i.e. is converted into low molecular weight components. Depending on the intensity of the shear surface, depending on the shear surface, the shear gradient and depending on the viscosity of the product melt entering this shear field, the degradation can occur take place so far that only liquid components emerge at the lower outlet opening of the annular chamber.

The spiral groove of the stuffing box housing must be designed in such a way be that

1. the gap between the spiral groove web in the cylindrical part of the worm shaft is so large that a perfect Unconstrained degassing from the extruder intake in the direction of the annular chamber within the stuffing box housing is guaranteed is,

2. The spiral groove web width and the spiral groove length form such a large shear surface that at a given relative speed

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- between the cylindrical, rotating worm _{shaft and the stationary stuffing box housing - and the gap resulting from I 0} , the specific shear capacity guarantees thermal degradation of the plastic material,

3. the pitch angle of the spiral groove is chosen so that a return of the thermally degraded product in the The feed area of the extruder does not take place »

A key figure that represents a measure of the trouble-free operation of the facility results from the product of the shear gradient χ Residence time of the material in the stuffing box housing,

This dimensionless number is called Z in the following: Z = f ' t

Shear rate

C "_ IT. Pcm" Ί "V = relative speed between ⁵ ~ a Ls cmJ see neighboring work

surfaces (shear surfaces)

a = distance (gap) between adjacent work surfaces

Residence time of the material in the screw

V = mass volume in the screw G = output of the screw

t = g

For the required thermal degradation of the product melt flowing through the stuffing box housing, the code Z must be 10- ^.

In a practical example, the following design data was used:

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Determination of the designation number "Z"

ζ = ¹ ^
a d * DD • do Rome"] d = 22.5 [cm] [cm] or 60 <xv - 120 Γ — i— I
LminJ = 0.075 = 80 cm a = 250 cn £ V 250
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(J i5. · DD 7 - Vl
^Z "a 60 Ö, 22nd 18, 10 . 120. 80. 3600 = 2, 075. 250 6th

Claims (4)

I «» (III V I ί »<I I 1 I * - 7 - O.Z, 51 7S9 Protection claims 1. Son corner extruder for separating gaseous components from plastic masses, consisting of a screw shaft surrounded by a cylinder with a feed opening for the masses to be treated, a drive unit and a degassing device arranged between feed opening and drive unit, characterized in that the degassing device consists of a screw shaft (2 ) covering and sealing stuffing box housing (4) is formed, which is positively and non-positively connected to the cylinder (1) and which on its inner wall (6) on the side facing the cylinder has a spiral groove (7) running in the opposite direction to the worm section and adjoining it has a concentric annular chamber (8) which is provided with a partition (9) and from which one or more openings (10, 11) extend. 2. Screw extruder according to claim 1, characterized in that »that the annular chamber (8) is arranged eccentrically to the stuffing box housing (4). 3. Screw extruder according to claims 1 and 2, characterized » that the annular chamber (8) on the side facing the drive unit (14) is sealed gas-tight by means of packing rings (15) and a gland (16). 4. Screw extruder according to claims 1 to 3, characterized in that the annular chamber (8) contains a single opening (11) which is related to a separator (13). 5 _β screw extruder according to claims 1 to 4, characterized in that the stuffing box housing (4) has a heating jacket (5). Sign. BASF Aktiengesellschaft