On the modeling of continuous mixers. Part II: The cokneader

Abstract

The Buss cokneader is a single-screw extruder with interrupted flights. Pins from the barrel are inserted into the screw channel. The screw is both rotating and oscillating. Due to this action, screw flights are continuously wiped by the pins. During one passage of the pin, the material is not only subjected to high shear stress, but it is reoriented as well, thus promoting the distributive mixing process by the local weaving action of the pins and screw flights. Attempts to model the cokneader tend to focus on a single pin passing through the hole in a screw flight (1,2). However, a more comprehensive model can start with the same equations that apply to the corotating twin-screw extruder (3). Because the effect of leakage flows on the local pressure gradient has to be considered along with the effect of the local dragging action of the pins (neglecting the oscillatory action), experiments with model liquids have been performed to evaluate the comprehensive model. Additional experiments with a Plexiglas-walled cokneader support the calculations concerning filled lengths in various screw geometries. These results, and those of model calculations, which are extended to the nonisothermal, nonNewtonian situation, are presented.

Dynamics of liquid‐liquid mixing: A 2‐zone model

“…The contact force F is the Stokes drag force acting on the drops: its order of magnitude can be estimated from the external simple shear flow considered: (16) although in reality it is not constant during the collision. The initial film thickness h, corresponds to the separation at which the constant approach velocity at large separations ( y R ) equals the decaying approach velocity from the drainage models.…”

Analysis of flow and mixing in screw elements of corotating twin‐screw extruders via SPH

“…This was confirmed experimentally by Pawlowsk 28 for a Newtonian creeping flow in a single-screw extruder, and is also well known from onedimensional screw models. 39,40 The obtained values for the parameters A 1 and A 2 (the axis intercepts of the pressure characteristic of actively conveying elements) are provided in Table 4 (i.e., A 1 characterizes the inherent conveying capacity). For the nonconveying element K90 in which both A 1 and A 2 are zero, the parameter A 0 (i.e., the slope of the pressure characteristic function) is shown.…”

Residence time distributions in a co‐kneader: A chemical engineering approach

“…However, though the presentation of the RTD E ( t ) is a good mean to view the influence of the processing conditions and profiles on the mean residence time, it is not the best way to emphasize the difference in the shape of the distribution. As shown by Elemans and Meijer , the representation of the normalized RTD with cumulative response plotted versus dimensionless time is more interesting for this goal since it removes the influence of the mean residence time. On Figure , the cumulative RTD F ( t ) was plotted versus dimensionless time ( t / τ ) for various elements in similar processing conditions.…”

“…Booy and Kafka have described theoretically the flow the co‐kneader by finite elements method with a movement of a pin passing between two flights and considering isothermal Newtonian flow. Elemans and Meijer have used a model that neglected the axial oscillation, starting from previous analysis of corotating twin screw extruder. They have also used an original experimental system which consisted of a plexiglas barrel to visualize the filled length and small glass tubes as open manometers to measure local pressure.…”