Devolatilization in Twin Screw Extruders

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. 

These fluid flow patterns are fully consistent with the flow patterns described in Section 10.3. The model is based on the following assumptions  

All the fluid is exposed for a series of intervals, each of duration X, in which devolatilization occurs. 

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