A study of distributive mixing in counterrotating twin screw extruders

Counterrotating twin screw extruders are widely used for compounding, devolatilization, blending, and reactive extrusion. A fluid dynamics analysis package-FIDAP, using the finite element method, was implemented to simulate the 3-D, isothermal flow patterns in intermeshing and tangential counterrotating twin screw extruders. The rheology of the fluid was described by a power-law model. We developed a framework to evaluate the distributive mixing efficiency. The dynamics of distributive mixing were studied numerically by means of tracking the evolution of particles originally gathered as clusters. The extent or goodness of mixing was characterized in terms of length stretch, pairwise correlation function and volume fraction of islands. The length stretch reflects the capability of the mixer to spread particles away from their neighbors initially present in the same cluster. The pairwise correlation function characterizes the global distribution of particles in the mixing region. The volume fraction action of islands quantifies the regions void of minor component and provides the local analysis of mixing. This framework can then be used to differentiate among various operating conditions or various designs of the same type of equipment.