Novel Gas Dispersion in Highly Viscous Fluids Using a Plasticating Extruder

Abstract This paper describes a novel gas dispersion system for use in highly viscous/polymeric fluids using a plasticating extruder. The unique but not yet widespread method employs a rotating submerged orifice constructed on an extruder mixing head. The gas is injected directly into the mixing zone of the extruder radially through the rotating orifice. This generates large deformation of the bubbles formed at the orifice by virtue of the shear stress in the surrounding fluid. As a result, very fine bubbles (0.1–3 mm) may be generated in continuous-phase viscosities as high as 500 Pa·s. Results on the isothermal bubble formation in a model polymeric system show that the bubble size produced is a strong function of the screw speed and is also affected by the gas injection rate, melt pressure, continuous-phase viscosity, and to some extent by the size of the orifice used. A progressive diminution in the bubble size was recorded with increasing screw speed irrespective of the conditions employed. Depending upon the conditions employed (screw speeds, fluid pressures, continuous-phase viscosity, etc.), the dispersions generated may be controlled to the desired degree of fineness within certain limits. Only low gas injection rates (0.5–3.5 cc/s) using an isolated orifice were employed in this study. It is expected, however, that an understanding of the processes of formation at a single orifice will serve as a useful preliminary to investigations employing multiple orifices. The new process is discussed in the light of existing technologies, and its salient design features are described and illustrated. An outstanding feature of the process is its ability to produce the desired dispersion at relatively much lower but constant system pressures. This feature is especially attractive as it holds the potential for a practical gas subdivision device which offers the minimum of energy outlay. Linear low density polyethylene (LLDPE) was used as a model dispersion resin in this study although other resin systems may be employed without the loss of generality.