Twin-screw extruders devolatilization

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.

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. 

A twin-screw extruder is used to reduce residual monomers from ca 50 to 0.6%, at 170 °C and 3 kPa widi a residence time of 2 ruin (94). In another design, a heated casing encloses die vented devolatilization chamber, which encloses a rotating shaft widi specially designed blades (99,100). These continuously regenerate a large surface area to facilitate die efficient vaporization of monomers. The devolatilization equipment used for die production of polystyrene and ABS is generally suitable for SAN production. 

If a linear rubber is used as a feedstock for the mass process (85), the rubber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete rubber particles are formed. This is referred to as phase inversion since the continuous phase shifts from rubber to SAN. Grafting of some of the SAN onto the rubber particles occurs as in the emulsion process. Typically, the mass-produced rubber particles are larger (0.5 to 5 JJ.ni) than those of emulsion-based ABS (0.1 to 1 Jim) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to facilitate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extruders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Polymer devolatilization can also be accomplished by the use of single and twin screw extruders [61]. Extruders are well suited for high viscosity materials and, when equipped with appropriate vents, allow the discharge of volatiles. 

Tangential (Non-intermeshing) Counter-rotating Twin-Screw Extruder This extruder is used more for latex coagulation, recovery, devolatilization of polymer solutions, and for reactive extmsion. 

The modular co-rotating twin-screw extruder is the most widely used of all twin-screw extruders. There is probably no application for twin-screw extruders to which it has not been applied. Applications include all aspects of compounding and blending of thermoplastics with particulates, oils, and other polymers. The machine is also widely used for removal of liquids, that is, devolatilization and for reactive extrusion. The modular co-rotating twin-screw extruder has also been used for both polymerization and for grafting reactions. 

Ganzeveld (4) studied the grafting of maleic anhydride on high-density polyethylene in a counterrotating 40 mm twin-screw extruder. The polymer (Stamylan 7359, DSM) was tumble mixed with maleic anhydride (Nourymix MA-901 and 903, AKZO) and fed with a controlled feeder to the extruder. The initiator (di-tert butyl peroxide) was fed separately. The wall temperature ranged from 120 to 210°C. No devolatilization was used in this study, but the samples obtained were dried in a vacuum oven for 2 h to remove the unreacted maleic anhydride. The amount of maleic anhydride grafted was determined by titration. 

Applications for the diskpack are specialty polymer processing operations, such as polymerization, post-reactor processing (devolatilization), continuous compounding, etc. As such, the diskpack competes mostly with twin screw extruders. Presently, twin screw extruders are usually the first choice when it comes to specialty polymer processing operations. 

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. 

Todd [37] proposed an equation to describe devolatilization in co-rotating twin screw extruders based on the penetration theory discussed in Section 5.4 and Section 7.6. The equation contains the Peclet number (see Eq. 7.371), which represents the effect of longitudinal backmixing. The Peclet number must be measured or estimated to predict the devolatilizing performance of an extruder. Todd selected a Peclet number of 40 to correlate predictions to experimental results. A similar approach was followed by Werner [38], A visualization study was made by Han and Han [39], particularly to study foam devolatilization, They found substantial entrainment of the bubbles in a circulatory flow region in a partially filled screw devolatilizer. Collins, Denson, and Astarita [40] published an experimental and theoretical study of devolatilization in a co-rotating twin screw extruder. The experimentally determined mass transfer coefficients were about one-third those predicted by the mathematical model. They concluded, therefore, that the effective surface area for mass transfer is substantially less than the sum of the areas of the screws and barrel. 

Secor [41] presented an intermeshing model for devolatilization in co-rotating twin screw extruders that incorporates the major characteristics of fluid motion. These characteristics were experimentally observed in a twin screw extruder with a transparent barrel. The observed flow pattern consisted of alternating rotation in the tangential direction with the screw and axial forward motion at the entrance to the intermeshing region see Fig. 10.61. 

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