Devolatilizing extruder screws

The final portion of the screw has a deep channel section following a decompression section. The channel depth is constant over the last screw section. Again, the deeper channel in the final screw section will reduce the pressure generating capability of the screw. A more effective power reduction can be obtained by not only changing the channel depth, but the channel depth, helix angle, flight width, and radial clearance in an optimum fashion as discussed in Section 8.3. 

Devolatilizing extruder screws are used to extract volatiles from the polymer in a continuous fashion. Such extruders have one or more vent ports along the length of the extruder through which volatiles escape. Some of the applications of vented extruders are  

Removal of water from hygroscopic polymers is a common use of vented extruders. Most polymers require less than 0.2% moisture in order to properly extrude. In some polymers, this percentage is considerably lower, e.g., PMMA 0.1%, ABS 0.1%, CA 0.05%, PBTB 0.05%, and PC 0.02%. Many polymers have an equilibrium moisture content at room temperature and 50% R. H. (relative humidity) that is considerably higher than the maximum allowable percentage moisture content for extrusion. Some values of the equilibrium moisture content of hygroscopic polymers [10] are ABS 1.5% PMMA 0.8% PBTP 0.2% PC 0.2% and PA 3%. Such poly- 

Conventional two-stage extruders can generally reduce the level of volatiles only a fraction of one percent. For example, for PP/xylene with an initial solvent concentration of 0.3 to 1.0%, the amount of solvent removed by single vent extrusion is about 50%. 

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Figure 8.37 shows a typical two-stage devolatilizing extruder screw. 

There are many different designs of devolatilizing single screw extruders with widely differing devolatilization capacity. Some of the more common ones will be described and discussed next. 

Twin screw extruders are finding increasing use in speciaity operations such as reactive processing of polymers and devolatilization. Twin screw extruders are used as continuous chemical reactors for polymerization and polymer modifications, e.g., grafting of side groups. 

Extruder screw profile design is critical to HME devolatilization. For optimal devolatilization performance, the screw must completely melt the material fed into the extruder in a mixing zone of the screw, prior to the first vent zone, and this mixing zone must be filled with molten material. This melt seal upstream of the first vent zone will ensure that any vacuum applied to the first vent zone will not leak past the first mixing zone and pull powders from the feed zone up into the vent. The melt seal downstream of the first vent zone can be formed by adding another mixing... 

A twin-screw devolatilization extruder processes 1.136 kg/s of polymer. The original percent volatiles are 4.5%, which are reduced to 0.45%. What will the final volatile percent be if the amount processed increases to 1.705 kg/s ... 

Single-screw and double-screw extruders are normally used for polymer melts to accomplish the deaeration or devolatilization of residual volatiles. Devolatilization in an extruder is effected through formation of the venting zone inside the chamber by carefully designed upstream and downstream screw sections. 

Fig. 1. Process flow sheet for the continuous conversion of latex in a counterrotating, tangential twin-screw extruder as it might be arranged for the production of acrylonitrile-butadiene-styrene polymer (Nichols and Kheradi, 1982). Polystyrene (or styrene-acrylonitrile) melt is fed upstream of the reactor zone where the coagulation reaction takes place. Washing (countercurrent liquid-liquid extraction) and solids separation are conducted in zones immediately downstream of the reactor zone. The remainii zones are reserved for devolatilization and pumping.

When devolatilization processes are conducted in screw extruders, the screw channels are only partially filled with the polymeric solution to be stripped of the volatile component (see Fig. 5) while the unoccupied portion of the screw channel serves to carry away the evaporated liquid. Because the barrel has a component of motion Vbz in the down channel direction, the solution is caused to flow from the extruder inlet to the outlet, which, in this case, is out of the plane of the paper. The crosschannel component of the barrel motion, Vtx, has two effects. First, it causes a circulation of the fluid in the nip and because of the continual... 

A specific expression for the mass transfer rate in Eq. (11) was first developed by Latinen (1962) in a classic paper that showed how penetration theory can be applied to the analysis of devolatilization processes in single-screw extruders. The derivation presented here parallels that by Latinen but differs in some respects for reasons of clarity. 

Roberts (1970) has identified the exact mathematical form for axial dispersion in a screw extruder by noting that the devolatilized film is remixed with material in the bulk at an upstream position which can be determined exactly once the geometry of the system is specified. According to Roberts, the Peclet number is given by... 

Published experimental studies on devolatilization rates in twin-screw extruders fall into two categories studies conducted at pressures in excess of the equilibrium partial pressure of the volatile component in the feedstream using an inert gas as the stripping agent and those conducted at pressures lower than the equilibrium partial pressure of the volatile component. 

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

Werner (1980) has studied devolatilization in corotating twin-screw extruders when the volatile component was stripped from the polymeric solution by applying a vacuum to the system. Rough estimates of the equilibrium partial pressure of the volatile component in the feedstream for each of the systems studied by Werner indicate that this pressure was less than the applied pressure, which means that bubbles could have been formed. Figure 17 shows the influence of the externally applied pressure on the exit concentration for a methyl methacrylate-poly(methyl methacrylate) system of fixed concentration. Note that the exit concentration decreases as the pressure is decreased, but seems to approach an asymptotic value at the lowest pressures studied. Werner also reported that at a fixed flow rate and feed concentration the exit concentration did not vary with screw speed (over the range 150-300 min" ), which also suggests that ky alay, is independent of screw speed. Figure 18 is a plot of data obtained by Werner on an ethylene-low-density poly(ethylene) system and also shows that decreases in the applied pressure result in decreases in the exit concentration, but here a lower asymptote is not observed. 

Collins et al. (1 3) have suggested that this same concept can be applied to polymer devolatilization processes, except that the HTU might be more appropriately termed the LTU when screw extruders are used since these need not be vertical. By analogy, the following expressions can be written ... 

This TSE consists of two parallel counterrotating screws, as shown in Fig. 6.53. The distance between the screw centers is L < Db, where Db is the barrel diameter thus, there is an open axial slit along the barrel. This type of extruder has advantages for the feeding of particulate solids (e.g., powder), venting, and devolatilization of the molten polymer. 

Single Screw Extruder Devolatilization Using Latinen s Model Review the paper by Biesenberger and Kessidis and discuss (a) the experimental method used... 

J.A. Biesenberger and G. Kessidis, Devolatilization of Polymer Melts in Single Screw Extruders, Polym. Eng. Sci., 22, 832 (1982). 

Devolatilizing Screw Extruder A 150-mm-diameter, square-pitched, single-flighted screw extruder, with screw channel depth of 25 mm and 20-mm flight width is used to devolatilize a 1000-kg/h stream with 0.78-g/cm3 density at 200° C and 125 torr. (a) At what frequency of screw rotation will the channel be 30% or less full (b) With water injection, if density is halved by formation of 1-mm bubbles, how much surface area (per meter length) is created (c) How does... 

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