Mass transfer during bubble-free polymer devolatilization: a systematic study of surface renewal and mixing effects

Abstract

This paper deals with a systematic study of bubble-free polymer devolatilization. Experimental degassing tests are combined with a surface renewal analysis. For this purpose, a simplified extruder model is used as representative rotating degassing machine. It is a partially filled agitator vessel with a blade stirrer. A model substance system consisting of highly viscous polydimethylsiloxane as polymer and 1,1,2-trichloro-1,2,2-trifluoroethane as volatile allows the experiments to be carried out at ambient temperature. The driving force for the mass transfer is achieved by reducing the partial pressure in the gas phase via an inert gas flow of nitrogen. This allows the specific investigation of the mechanism of film degassing. The temporal decrease in the concentration of volatiles in the polymer solution is measured thermogravimetrically at three different filling levels and three different rotational speeds in a total of 18 test series. At the same time, the free surfaces and renewal times are determined by video recordings and numerical simulations. For this, the free CFD software OpenFOAM is used with a VOF-based free surface solver. The numerical results are validated by the experiments. It is shown that the deviations of the measured mass transfer data from the theoretical predictions of the common surface renewal models depend on the degassing degree. Mixing is the decisive factor which can be taken into account in the form of a corrected model for the design of degassing machines. Furthermore, it is shown that the gas-side mass transfer resistance is not negligibly small in all cases.