Degassing extruder

Figure 10.11 Twin-screw degassing extruder located vertically above the ZSK (DSE) and horizontally to the side of the ZSK (ZS-EG).

An in-depth analysis is made of the wear behaviour of Krauss-Maffei s twin-screw extruders for manufacturing PVC pipes. Wear mechanisms and factors for minimising wear and influencing wear behaviour are discussed and the development of a new series of 36D double degassing extruders, which are less prone to wear, is reported. The energy balance, output rates and configuration of these extruders are briefly described and a policy for dealing with wear problems in extruders is outlined. 

In the second flash vessel (5), further concentrating to over 95% polymer is done and the product is fed into a degassing extruder (6) to remove the last traces of solvent and octene (when used as comonomer). Vapors from the second flash vessel and degassing extruder are purified in the purification section (7) and then recycled to the reactor section. In the degassing extruder, specified additives are added, and the polymer is pelletized. After drying, the pelletized resins are transported to the silos. 

The resin was fed to a degassing extruder where it was mixed with PO, fillers, and/or additives. The process was used for the preparation of blends in a simpler and more eost-effective manner than that of prior art... 

Solvent is recovered in two steps. 90% of the solvent is recovered in a scraped film evaporator (FI). The remaining 10% of the volatiles are recovered by a degassing extruder (El) equipped with a pelletizer. The pellets are cooled with water. The cooling water is removed by filtration and the pellets are dried in two centrifugal dryers. 

According to reaction kinetics measured by Maeder (3) and experiments of Troelstra (1) a conversion of over 95% can be achieved within a residence time of 4 min in the extruder. This corresponds with a flow rate of 80 kg/h for a Buss kneader with a diameter of 100 mm and a flow rate of 640 kg/h for a Buss kneader with a diameter of 200 mm. The process is designed for a conversion of 95%. The remaining 5% of monomer is removed with a degassing extruder (E2). The monomer vapor is condensed in a condenser (H7) and stored (VI). The monomers are subsequently purified by means of a batchwise distillation. 

The degassing extruder equipped with a pelletizer is equal to the extruder used to remove the solvent in the solution polymerization. 

Compared to a normal extruder with a length of 12 D (D = diameter of screw) a degassing extruder needs an additional 8 D length to house the degassing section. The additional foot print required makes it difficult to retrofit existing production lines due to space limitations. 

Safe degassing of compounds, independent of the conveying behaviour of the compound, by avoiding problems caused by flooding of the vacuum zone of an extruder. 

In this process, a vacuum extrusion step (melt treatment) is followed by a continuous SSP (solid treatment). The process starts with PET flakes which are introduced without pre-drying into a ring extruder. The flakes are dried, melted and degassed inside the extruder. A gear pump builds the necessary pressure for melt filtration, where solid particles are removed. After granulation the pellets are fed to a continuous three-step SSP unit [10]. 

A powder coating is the result of the application of a powder binder system, which generally consists of a resin, a crosslinker, flow additives, degassing agents, pigments or dyes, charge control agents, etc. Such a binder system is ap-pHed to the substrate as a fine, extruded powder (Fig. 20, step A), then melted at... 

The polymer collected in the large-volume high-pressure separator is fed to a low-pressure separator for further degassing and is then pelletized in a melt-extruder by a die-face cutter. The pellets are dried and conveyed to silos. 

The extruder is used since it mixes, melts, and degases the polymer. However, as said in the previous paragraph, from an exergetic point of view, it is not an efficient apparatus, since it dissipates mechanical work into heat by frictional forces. A possible alternative to the extruder could therefore be a separate degasser, a static mixer, or a gear pump to push the polymer melt through the mixer and perforated plate (Figure 11.6). 

T. Sakai and N. Hashimoto, Application of Novel Counter-rotating Intermeshed Twin Extruder for Degassing Operation, SPE ANTEC Tech. Papers, 32, 860-863 (1986) also T. Sakai, N. Hashimoto, and N. Kobayashi, Experimental Comparison between Counterrotation and Co-rotation on the Twin Screw Extrusion Performance, SPE ANTEC Tech. Papers, 33, 146-151 (1987). 

Degassing Polymer Melts with Co-Rotating Twin Screw Extruders... 

In addition to the technical reasons mentioned earlier and quality requirements, such as odor improvements, improved visual and mechanical characteristics, and nonporous pellets, the maximum legal limits for volatile components, e. g., monomers or solvents, also require degassing of polymers. Depending on the processing tasks, degassing in a co-rotating twin screw extruder can be divided in ... 

Figure 10.1 Schematic process designs for degassing polymer melts with co-rotating twin screw extruders...

Compared to conventional processing tasks, the extruder drive motor is usually quite small for degassing processes, because the intake of polymer solutions or polymer melts does not require a high torque consumption. The use of a back vent depends on the amount of... 

Coperion Werner Pfleiderer have a wealth of test and production experience related to degassing polymer melts with twin-screw extruders. Based on the process features described, the ZSK offers optimum solutions and can also be easily adapted for new tasks. The ZSK degassing machines also meet increasing quality requirements for polymers regarding residual degassing of solvents and monomers and the drive to save energy and raw materials in continuous manufacturing processes. 

Examples of products and a twin-screw extruder line for the manufacture of hard metal mixtures are shown in Figs. 14.19 and 14.20. The raw materials are supplied to the extruder as a pre-mix, plasticized and homogenized. If necessary, excess air can be removed by vacuum degassing. 

Screw extruders are also used, however, to produce pelleted materials, either as a final product or for further processing, in the food, ceramics, chemical and other industries. Many orifices in a die plate form a multiplicity of extruded rods which are cut into pellets by rotary cutter blades. The extrusion operation is frequently the final step in a sequence of processes which may include binder/lubricant addition, mixing, heating, cooling or vacuum degassing. 

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