Modelling and optimization of the Cavity Transfer Mixer

The blending of different materials is an important process in polymer industry, where a good mixing is essential to guarantee adequate performances of the finished product. In the 80s a new device called the Cavity Transfer Mixer (CTM) was invented and patented by Gale at Rapra Technology Limited, as an add-on to be mounted downstream of existing extruders, in order to improve distributive mixing. The CTM consists of two concentric cylinders, the rotor and the stator, both provided with staggered rows of hemispherical cavities. The inner cylinder (rotor) rotates, while the outer (stator) remains still. At the same time, the pressure load imposed upstream, pushes the fluid through the mixer. The result of the interaction between the moving geometry, the imposed pressure load and the rheology of the fluid is the complex flow field driving the mixing mechanisms inside the device. Because of the variety of the phenomena involved, a clear understanding of the CTM mixing processes is still missing and the system development and optimization encounter noticeable difficulties. In this context, the present work proposes a full three dimensional model of the CTM, able to accurately simulate the device operations. A finite element solver provides the transient velocity field, which is used in the mapping method implementation in order to compute the concentration field evolution. A broad range of simulations is run assessing the impact on mixing of several geometrical and functioning parameters, such as the number of cavities per row, the number of rows, the size of the mixer, the rheology of the fluid and the ratio between the rotation speed and the fluid throughput. Results are used to develop some design and operation guidelines for the CTM.