Foam-based devolatilization

Several models for foam-based devolatilization are available. The fundamentals are reviewed by Lee [22]. Bubble nucleation during devolatilization of polymer melts has been explained by homogeneous nucleation [23, 24], heterogeneous nucleation [25-27], and a mixed-mode nucleation [28]. Yarin et al. [29] considered a secondary nucleation. 

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. 

The model developed here is based on diffusion. However, in many cases the length required to reduce the concentration of a volatile below some specified value is overpredicted. Hence, it is believed that the formation of bubbles or foam accelerates the devolatilization process. There is no model available at the time of writing this book which deals with foam DV. 

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