Extruders, devolatilizing

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

Robust equipment has been developed for the various processing steps, including stirred-tank and tubular reactors, flash devolatilizers, extruder reactors, and extruder devolatilizers. Equipment costs are high based on working volume, but the volumetric efficiency of bulk polymerization is also high. If a polymer can be made in bulk, manufacturing economics will most likely favor this approach. 

The major advantage of the cascade devolatilizing system is that the control of the output of the first stage to the pressure generating capability of the second stage is much better than in a single extruder devolatilization system. Obviously, the cost will be higher, and the decision for one system or the other must be based on the importance of improved flexibility and controllability. 

Many types of equipment have been developed to improve the evaporation of solvent in order to provide energy savings. The most widely used techniques for devolatilization are the falling strand devolatilizer (FSD), the thin-film evaporator and the vented extruder [113]. 

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. 

The continuous mass process is divided into 4 steps rubber solution in styrene monomer, polymerization, devolatilization and compounding. In 1970 N. Platzer (40) drew up a survey of the state of the art. Polymerization is divided into prepolymerization and main polymerization for both steps reactor designs other than the tower reactors shown in Figure 2 have been proposed. Main polymerization is taken to a conversion of 75 to 85% residual monomer and any solvent are separated under vacuum. The copolymer then passes to granulating equipment, frequently through one or more intermediate extruders in which colorant and other auxiliaries are added. 

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. 

Consider a situation in which a concentrated polymeric solution enters the extraction zone of, say, an extruder in circumstances when the pressure in the extraction zone. Pa, is less than the equilibrium partial pressure of the volatile component in the feed solution. Under these conditions the solution will be supersaturated at the extraction pressure, flashing of the volatile component will occur, gas bubbles of radius Rq will be formed, and the concentration will immediately fall from Wi to wq. If bubble formation occurs by homogeneous nucleation, the rate at which these bubbles will be formed per unit volume of solution should depend on the difference between the equilibrium partial pressure of the volatile component and the devolatilization pressure. Since this pressure difference is greatest when the solution first enters the extraction zone, the rate of formation of bubbles will at first be high but as devolatilization pro-... 

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 ... 

Large diameter, melt-fed extruders are commonly used for the final devolatilization and pelletization of LDPE and PE copolymers in resin manufacturing plants. A full description of this type of extruder and process is provided in Section 15.3. Simulation of these processes is complicated by the multiple flights used in the design and the high H/W aspect ratios of the channels. The processes can be simulated from the feed hopper to discharge, however, since they are not required to convey solids and melt resin. This section will show the requirements and difficulties for simulating these processes. 

Two-stage and multiple-stage vented extruders are commonly used to remove volatile components from molten resin streams prior to downstream processing. The vent ports can be open to the atmosphere, or they can be attached to elaborate vacuum systems. For very specialized systems, stripping agents such as water, nitrogen, and carbon dioxide can be added upstream of the vent, mixed into the resin stream, and then devolatilized in the vent area. This technique can be employed to remove difficult components or components at a higher rate from the resin. 

Figure 15.4 Schematic for a melt-fed devolatilizing extruder for an LDPE resin production...

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. 

Fig. 8.10 Residual styrene concentration in PS extruded at 225°C. The open symbols refer to experiments without ultrasound, while the filled ones refer to experiments where ultrasound radiation was applied. The parameter is the absolute pressure in the chamber. Triangles 150 mmHg squares 50 mmHg, and circles 12 mmHg. [Reprinted by permission from A. Tukachinsky, Z. Tadmor, and Y. Talmon, Ultrasound-enhanced Devolatilization in Polymer Melt, AIChE J., 39, 359 (1993).]...

Fig. 8.14 PS-styrene sample extruded at 180°C into atmospheric pressure. The micrograph shows the smooth lateral surface and part of the cross section there is no evidence of huhhles. [Reprinted by permission from R. J. Albalak, Z. Tadmor, and Y. Talmon, Scanning Electron Microscopy Studies of Polymer Melt Devolatilization, AIChE J., 33, 808-818 (1987).]...

A. Tukachinsky, Y. Talmon, and Z. Tadmor Foam-enhanced Devolatilization of Polystyrene Melt in a Vented Extruder, AIChE J., 40, 670-675 (1994). 

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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