DE1579106A1 - Screw press for pre-dewatering and drying synthetic rubber - Google Patents

Description

Screw press for pre-draining and drying synthetic rubber It is known to dry synthetic rubber in belt dryers or in extruders. Screw machines or extruders are particularly suitable for drying Synthetic rubbers, which due to their physical properties have an impact on the Do not let the usual hot air tape dry with difficulty or not at all.

The drying of the synthetic rubber comprises two process stages, namely on the one hand the mechanical pre-drainage and on the other hand the thermal drying. For pre-drainage, for example, Vibrating sieves, squeegee rollers, rotary cell filters, Centrifuges and screw machines. Since the mechanical pre-drainage is essential Cheaper than thermal drying, the highest possible degree of dewatering strived for. Let the highest pressures and therefore the best drainage effects reach each other with screw machines. In the case of the drainage screws, the Conveyor screw the product to be dewatered through a damming element, which acts as a return thread, as a perforated disk or as a conical part, which consists of two mutually extending, if necessary consists of grooved cones, can be formed.

The screw conveyor is one with narrow slots Basket, the so-called colander. The pinched-off material passes through these slits Water out. A special 12/63 3 drawings Shaping the slots largely prevents product leakage, although the strainer does not stick cannot be avoided in the long run by small amounts of product pressed through. This leads to a decrease in the degree of dewatering that can neither be fixed nor controlled over time. For this reason, the machine must be cleaned at appropriate intervals.

There are two ways of drying in the extruder. Runs through once which is pre-dewatered, which is continuously heated by friction and convection in the extruder Rubber has several relaxation zones in which the water vapor escapes. This However, the method has the disadvantage that product escapes from the evaporation openings, that sticks to the hot degassing elements, is there due to the effect of temperature decomposes and thus creates a risk of contamination for the rubber; simultaneously As a result, the exhaust openings gradually clog.

Various attempts have been made, but mostly in vain, this product leakage through the use of clearance screws, by increasing the Prevent the number of snails and a suitable choice of the direction of rotation of the snails. These methods are advantageously designed with the use of devices Evaporation openings by measuring the steam conditions of the escaping Vapors can follow the drying process very precisely for a short time, which is a regulation or control of the drying process is made possible. One also already has pre-drainage and drying is carried out in a device which has a plurality of exhaust openings. However, an energetic disadvantage of this drying process is that the heat energy the hot, dry rubber that escapes is not used for the drying process will. m others can take advantage of the disadvantages of this mode of operation the fact that water heated under pressure8 explodes when it is released evaporates.

In this procedure, which has become known as expander drying the pre-drained rubber is heated under pressure until the absorbed Energy is sufficient to evaporate the water down to a content of less than 0.5 ffi. Care must be taken to ensure that the pressure exerted on the rubber is always kept above the boiling pressure of the water, otherwise it will already be in the machine Evaporation is coming.

It will be appreciated that in this procedure those in rubber The amount of water contained plays an important role in tearing it open and drying it plays. Since the "expander process" requires a radially closed housing, for technical reasons it is not possible to measure the moisture for a short time as to be determined in the case of machines with degassing openings. One must therefore follow the procedure Carry out in such a way that you first pre-drained in a machine and with introduces controllable humidity into the expander machine.

Knowing the moisture content is essential for setting the temperature especially important because the rubber is both sufficiently dry and without damage should leave the drying device due to excessive thermal stress.

It has now been found that synthetic rubber can be avoided while avoiding it can pre-drain and dry the aforementioned disadvantages in an advantageous manner, if you use a screw press with one or more screws for mechanical purposes Pre-aeration of synthetic rubber in - connected in series - one pressureless and one pressurized stage and for thermal drying by means of expansion drying, in which the screw housing on the hopper (5) equipped with a bottom sieve (6) which half encloses the worm shaft (1) is that the housing section adjoining in the conveying direction is a round cord (7) and the following housing section (8) closed, but internally with axially extending Liquid backflow channels (16) are provided and can be heated from the outside, while the now adjoining housing section is known as a closed one also heatable from the outside, with a narrowed product outlet cross-section provided screw cylinder is formed and that the screw is known per se Way is equipped with different threaded sections, being from the funnel (5) up to the 3rd end of the housing section (8) the screw with a conveyor thread (2) is equipped, which is a short section of a return thread as a throttle device (3) and then a conveyor thread (4) again up to the end of the screw.

With this screw press synthetic rubbers can be processed, obtained by polymerization of dienes, e.g. B. 3utadiene and its derivatives, such as isoprene or 2-chlorobutadiene, or by copolymerization of dienes and olefinically unsaturated ones Monomers, e.g. butadiene / styrene, butadiene / acrylonitrile, can be obtained. Even Kautachuk-like copolymers based on olefinic unsaturated monomers, such as ethylene / propylene rubbers, which also contain small amounts of Serve, z03. Biscyclopentadiene, polymerized in, can then be worked up will.

The mechanical separation of the larger part of the liquid the synthetic rubber-water mixture takes place in the radial not closed, so-called strainer zone, through the openings of which the to be removed Water can escape. During the transport of the rubber through the screw flights if there are shear forces that put the rubber under local pressure, see above that the surface moisture adhering to the rubber and part of the eapillar moisture Will get removed. It is essential to the process that there is no pressure in this zone builds up, for which the following conditions must be met: 1.) The worm threads may only be partially filled, d. H. the effectively possible delivery rate - accordingly multiplied by the product of the volume that can be displaced per revolution per pitch unit with the number of revolutions per time - must be greater than the amount of product to be conveyed be.

2.) The screw flights are allowed to hit the area to be freed from the liquid Mixture only exert forces in the circumferential direction, but not in the radial direction. This is achieved by the screw flights shown in Figure 2, in which the so-called pushing edge, i.e. the rear edge - seen in the direction of rotation - perpendicular to the direction of rotation.

3.) Between the outer diameter of the screw flights and the strainer there must be a clearance of at least 1 mm.

In this way it is possible to mix synthetic rubber and water through the strainer zone and at the same time squeeze out the water, without rubber parts whose geometric dimensions are larger than the strainer dimensions are to be pressed through the opening of the colander and thus for gluing and clogging of the colander. Rubber parts, their geometric dimensions smaller than the openings of the colander are flushed through them.

The way in which the strainer openings are formed depends greatly on the properties of the product. So they will z. B. at the Drainage of synthetic rubber as long, narrow slots with gap widths from 0.15 to 2 mm, while more or less in the case of powdery material selects fine sieves with mesh sizes of 80 to 1000 p, which are mechanically suitably selected be supported.

The further removal of the water takes place in a radially closed Zone of increasing pressure immediately following the strainer zone. Elevated the pressure acting on the synthetic rubber-water mixture increases the amount of water separated from the mixture, since the equilibrium water-synthetic rubber is shifted towards the synthetic rubber side. The separated water migrates in areas of low pressure, in which the rubber can absorb more water. In the screw press designed in this way, one of the medium transport speeds occurs of the rubber in the opposite direction.

The maximum possible degree of squeezing at a certain pressure is reached when the backflow velocity of the water in the direction of the partial pressure gradient, which at the same time corresponds to the absolute pressure gradient, equal to the mean transport speed of rubber is. The pressure corresponding to a certain degree of squeezing is once a function of the chemical composition, e.g. in the case of polymers, physical due to the amount and type of monomers, emulsifiers, fillers Properties of the rubber, e.g. B. the cohesive force between rubber and water, the shape and size of the capillaries, the resistance of the rubber to deformation, thus changes in the capillary dimensions and, on the other hand, in the geometric data and the performance of the machine.

The one required to achieve a certain degree of squeezing Pressure is achieved by a dam or throttle device, d. H. by Organs that oppose the flow of rubber and thus resistance Require a certain pressure to overcome this resistance.

As throttle organs, two types of devices are preferably shown in FIG equally suitable. Once the housing surrounding the worm shafts can on Discharge end closed by a so-called perforated plate or a perforated ring be on which a certain number of holes of certain diameters is arranged.

The most important parameters of this perforated plate or this tochring are the free cross-section of all holes, the hole diameter and the hole depth. While the resistance decreases as the hole area increases, it increases as the hole area increases Hole depth and, with decreasing hole diameter, the resistance per unit area. Whether perforated rings or perforated plates are used depends on the purely mechanical engineering Circumstances. So z. B. in machines with bearings on one side, the shaft not to protrude through the stowage organ; therefore a perforated plate is suitable. Against it In the case of machines with two-sided bearings, the shaft must protrude through the dam, see above that only a perforated ring comes into question as a stowage element.

In addition, return threads are suitable as throttling devices, including Threads are understood to be the mixture to be conveyed in one of the conveying threads try to move the induced flow in the opposite direction.

In this way, there is resistance to the mixture being pressed through opposite. The extent of this resistance can both be reduced by a decrease the return thread as well as by reducing the pitch can be increased.

What these dams have in common is that when the mixture hits the highest pressure on the inner storage area prevails while the pressure when leaving the dam on the external pressure, generally on normal pressure, falls off.

In terms of process technology, the types of throttle devices described are equivalent to; So they have no different influence on the course of the Drying process.

The one for pressing through the synthetic rubber, which has largely been freed from the water The pressure required by the throttle elements must be built up by the delivery threads will. The effective delivery rate of a worm thread is made up of the theoretical delivery rate multiplied by the amount displaced per original rotation with the number of revolutions per unit of time, and the counter pressure resulting return flow together. The Rtokstrom increases with increasing back pressure and increasing gap size, while with increasing screw length at constant Counterpressure decreases because the pressure gradient then becomes smaller. That means once that to achieve a certain back pressure with a given screw training a certain screw length is required.

On the other hand, with a given screw length, the counter pressure or increase the pressure gradient if the return flow, z. B. by enlarging the Gap dimensions, increased.

Finally, by increasing the return flow, the efletively returning flow can be reduced Increase the amount of water and thus significantly improve the squeezing effect.

In Figures 1 to 3 is an exemplary embodiment the screw press according to the invention shown.

The rubber to be drained enters the funnel (5) a housing provided with the bottom sieve (6) is grasped by the conveying threads (2) and ends up in the round sieve (7). Due to the occurring during transport Shear forces some of the water is already removed, ohllc that is in the strainer zone, the z. B. is formed by the round screen (7), a more effective for the pre-drainage Pressure builds up. Most of the separated water flows through the strainer Part with no significant pressure gradient. So there is a risk of clogging removed the colander.

The rubber that has been pre-drained in this way is transported by the screw conveyors (2) into the radially closed heatable housing (8) transported. As the product through the stowage organs, e.g. B. the return thread (3), must be pressed, have the feed thread (2) a corresponding pressure built up. This pressure build-up increases with Press the solid / liquid equilibrium more and more to the side of the solid postponed. The squeezed off liquid flows in the direction of the pressure gradient, which at the same time corresponds to the partial pressure gradient of the liquid, comes back into the colander (7) and exits there through the openings. Pressureless around the colander To keep the radially closed housing the screw conveyor (2) at least until the point where pressure begins to build up. The length of the closed The housing part in the pressure zone is primarily determined by the screw speed, the thread pitch of the conveying thread and the effectiveness of the retaining thread used (3). This can require housing lengths that are 10 times or more the Screw pitch. In order to keep the machine as short as possible, it is advantageous to the backflow of the rubber in the through the arrangement of longitudinal channels (16) To increase the pressure zone and thereby strengthen the pressure gradient (see section A - B in Figure 2). The number and design of the longitudinal channels (16) is both physical Properties of the rubber to be dewatered as well as its initial and final moisture addicted.

The pre-drained rubber leaving the damming zone is in, the directly adjoining radially closed housing (9) (heating zone) by convection and frictional heat is heated to the temperature required to keep it still vor- @ ene water after emerging from the perforated plate (10) with relaxation to normal pressure to evaporate. At no point in the heating zone (9) should the pressure fall below the The vapor pressure of the water will drop, otherwise it will be caused by evaporation in the machine Irregularities in operation can occur. For this reason one leaves the The pressure in the machine increases as the temperature rises. The outlet openings the perforated plate (10) must therefore be dimensioned in such a way that the for pressing through the pressure required of the rubber is greater than that prevailing in front of the perforated plate Is vapor pressure.

As soon as the pressure drops below the steam pressure in the holes in the perforated plate falls, the water starts to evaporate. The rubber is torn open in the process and leaves the perforated plate (10) as a torn strand, from which the residual moisture outwashed by the amount of heat still present in it. This has the advantage that the Rubber is cooled quickly. The emerging strands can through the rotating Knives (11) are cut into pieces of a defined length and fall onto a transport device, with which they are, for example, transported to the packaging system and on which the last ones Moisture residues evaporate. This process can be enhanced by blowing cold or warm air. A conveyor belt, for example, can be used as a transport device a vibrating machine or a pneumatic transport device can be used.

In the extrusion of plastics it is common practice to use built-up Destroy the pent-up plastic with impellers. The assurance constant product intake is given with plastics because their Deilchengrä- * e uorr,; nlerwcise is considerably below the dimensions of the snail and so that it does not affect the indentation.

With the device especially suitable for water-damp rubber types it is an impeller (13), the wing (15) (see FIG. 2) under a certain angle to the impeller axis (14) are arranged. The geometric Dimensions of the impeller (13) are functions of the rubber to be dewatered and the dimensions of the screw press. The product falling into the funnel (5) (See FIG. 3) is employed by the with the angle M to the impeller axis (14) Wings (15) detected and pressed against the screw conveyor (2). By the result caused by the relative movement of the blades (15) against the screw conveyors (2) Shear forces are already exerted locally on the rubber, which can lead to an initial pre-drainage. The water squeezed out in the process can drain through the bottom siphon (7). Through this feed device both the throughput increases and the intake amplitude is smoothed, i.e. the retracted Product quantity per unit of time kept constant.

The smooth implementation of the drying process continues uniform residual moisture after the pre-drainage stage. This means, that the strainer, from which the squeezed water flows off, does not stick over time allowed. A constant price maintenance of the colander is only possible if there is none in it There is pressure through which the product is forced through. Exactly this condition is met with the claimed devices.

The mode of operation of the claimed screw press should be based on the example below. the Execution conditions even rubber can of course be dried according to the type @es vary in many ways. So it is necessary when rooking for example cis-1,4-polybutadiene or the butadiene / styrene recycled rubber known as "SBR" to choose other drying conditions and arrangements in the devices than when drying e.g. a terpolymer based on ethylene / propylene. The changes in the devices can, for example, extend to the total length of the Machine, the length of the pre-drainage zone used in each zone Screw arrangements, the execution of the perforated plate, the number of holes, hole diameter and total exit area can be designed differently, as well as the strainer, where the length, gap width and design of the gap are variable.

The achievable throughput values are reduced by these changes in the devices, as well as influenced by the type of elastomer.

Example In a twin screw extruder of the type "ZSK" (trade name from Werner and Pfleiderer, Stuttgart) are arranged one below the other Screws with an outer diameter of 83 mm and a center distance of 76 mm a housing length of 2410 mm. The length of the with a bottom sieve (6) with 2 mm Gap width and one on the side (cf.

Figure 1) arranged impeller (13) (diameter: 167 mm) equipped The draw-in part is 280 mm. The pre-drainage zone consists of a 120 mm long Round cord with 2 mm gap width and a 360 mm long one that can be heated with 20 atmospheres of steam Housing part with 10 radially arranged channels (16) of 10 mm diameter. Thereon follows the part used to heat the rubber, consisting of closed, with 20 atm. steam heatable housing parts with a overall length of 1650 mm. The screw conveyors (2) are right-handed, cut smoothly and have with a slope of 120 mm a length of 720 mm. Have the following snails (3) with a length of 20 mm each, pitches of 60 and 30 mm and are 3-start with left-hand thread cut. The adjoining screw conveyors (4) are 3-threaded and right-handed and have the following dimensions in the direction of the product flow: 320 mm length with 120 mm pitch, 880 mm length with 80 mm pitch, 120 mm length with 60 mm pitch, 100 mm length with 50 mm pitch and finally 40 mm length with 40 mm pitch.

The interior of this screw extruder is closed by a Perforated plate with 25 round holes of 5 mm diameter, in front of which a double-leaf rotating knife is arranged.

A is processed by ionic polymerisation in hexane Terpolymer made from ethylene / propylene / biscyclopentadiene. The terpolymer, the mooneyplasticity of which is ML-4 = 45, falls in the course of abortion of the hexane carried out in hot water in the form of water-moist crumbs that contain about 50 percent by weight of water, based on moist material, and one Have a diameter of 5 to 10 mm. The moist rubber crumbs that have a temperature of about 20 ° C, are separated from the aqueous phase via a sieve in which the work-up was carried out, separated and placed in the intake section (5) of the extruder introduced. There they become from the under 45 ° to the impeller axis employed blades (15) of the impeller (13) rotating at 52 rpm and detected the screw conveyor (2) supplied. The screws rotate at 175 rpm on one Drive power of 51 KW. The humidity of the terpolymer after passing through the Selher zone, operated without pressure, falls to about 30%, based on on moist material, from. After going through the immediately following, under In the pressure-operated dewatering zone, the moisture content is still around -9.5 %, based on moist material.

The rubber pre-drained in this way, which has a temperature of 85 ° C, is further heated in the radially closed housing part until its temperature at a pressure of 43 atm in front of the perforated plate is 205 ° C. The rubber kicks in then out of the perforated plate, whereby the superheated water relaxes to normal pressure and the temperature of the rubber drops to 155 ° C. The moisture content of the granulated rubber now falls by 0.8%, based on the moisture used Rubber crumbs, in the course of the cooling of the rubber caused by tuft less than 0.5. The throughput is 390 kg / hour.

With the claimed screw presses, it is possible with reduced Energy consumption and lower investment costs than with the previously usual processing methods to carry out the drying of synthetic rubber, with the implementation of pre-drainage and drying in a machine guarantee a contamination-free end product offers.

Claims (3)

Claims 1. Screw press with one or more screws for mechanical pre-drainage of synthetic rubber in - connected in series - a pressureless and a pressurized stage and for thermal drying by means of expansion drying, it is not indicated that the The worm housing on the hopper (5) with a half-enclosing the worm shaft (1) Bottom sieve (6) is equipped so that the housing section adjoining in the conveying direction as a round screen (7) and the following housing section (8) closed, but inside provided with axially extending liquid return flow channels (16) and from the outside is designed to be heated, while the now adjoining housing section as well-known, closed one, also heatable from the outside, with a narrowing Product outlet throat provided screw cylinder is formed and that the screw is equipped with various thread sections in a manner known per se is, the screw from the funnel (5) to the end of the housing section (8) is equipped with a delivery thread (2), which is a short throttle body Section of a return thread 3) and then back to the end of the screw a conveyor thread (4) is connected. 2. Screw press according to claim 1, d a d u r c h g e k e n u z e i c h n e t that the narrowed product outlet cross-section as a per se known perforated plate is executed. 3. Screw press according to claim 1 for better feeding in of the coagulate, d u r c h e k e nnn s e i c h n e t that below the funnel opening (5) in the side wall next to the worm shaft (1) a known impeller is arranged, the axis of which is horizontal and at a right angle to the screw axis runs and whose wings have an angle of attack between 0 and 90 °.