Analysis of mixing efficiency in continuous polymer processing equipment

Continuous mixing equipment is widely used in polymer processing. Typical continuous mixers include single screw and twin screw extruders. The flow field in these mixing devices is difficult to analyze due to the complex geometry. In this work, a fluid dynamics analysis package-FIDAP, using the finite element method, was implemented to simulate the three dimensional, isothermal flow patterns in the region of conveying elements and shearing discs of a Leistritz intermeshing counterrotating twin screw extruder model LSM30.34, a 2{dollar}sp{lcub}primeprime{rcub}{dollar} Welding Engineers tangential counterrotating twin screw extruder and the MCT section of a Multi-Cut Transfermix extrusion system. The rheology of the fluid was described by a power-law model. The problem of time dependent flow boundaries was solved by selecting a number of sequential geometries to represent a complete mixing cycle.; The dispersive mixing in the region of conveying elements and shearing discs of the intermeshing counterrotating twin screw extruder was characterized in terms of shear stresses generated in the flow field and a parameter {dollar}beta{dollar} quantifying the elongational flow components. The influence of screw rotational speed and axial pressure difference on these flow characteristics was analyzed. Comparisons of the flow characteristics in the region of conveying elements and in the shearing discs, and between corotating and counterrotating modes in the shearing discs were also presented.; A framework was developed to evaluate distributive mixing efficiency in the intermeshing and tangential counterrotating twin screw extruders and in the Multi-Cut Transfermix. The dynamics of distributive mixing was 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 of islands quantifies the regions void of minor component and provides a more local mixing analysis. This framework can then be used to differentiate among various operating conditions or various designs of the same type of equipment. Moreover, this framework can also be applied to other batch or continuous mixing equipment with some minor modifications.