Determination of mass transfer coefficients for bubble-free devolatilization of polymeric solutions in twin-screw extruders



Theoretical and experimental studies have been conducted on the mass transfer rates which occur during the continuous devolatilization of bubble-free polymeric solutions in a corotating twin-screw extruder. Experimental measurements of the mass transfer rates, when reported in terms of the product of the individual liquid phase mass transfer coefficient and the surface area for mass transfer per unit volume of empty extruder, were found to correlate well with screw speed in accordance with the predictions of a model based on penetration theory. It is shown, however, that significant differences exist between the measured and predicted values for the rates of mass transfer, which leads to the conclusion that the surface area for mass transfer is substantially smaller than the value computed on the basis that continuous films are formed on the surfaces of the screws and the barrel wall.