Studies of Mixing Efficiency in Batch and Continuous Mixers

Abstract

Abstract In this paper we have reviewed some of the results of 3-D, isothermal flow simulations for batch and continuous mixers. The flow patterns were obtained by using FIDAP, a fluid dynamics analysis package based on the finite element method. The problem of time dependent flow boundaries due to the rotations of the rotors in the Banbury mixer, was solved by selecting a number of sequential geometries to represent a complete cycle. Non-slip boundary conditions for the stationary and moving surfaces were employed. For different regions of continuous mixers we used a nominal value for the normal stress difference in the axial direction as an additional boundary condition. The results obtained in terms of velocity and pressure profiles were further processed to analyze mixing efficiency. Dispersive mixing efficiency was studied in terms of the shear stresses generated and the elongational characteristics of the flow field. Distribution plots for shear stresses and a parameter λ, quantifying the elongational flow components, were obtained in various regions of the mixing equipment. Volumetric distributions for the same parameters as well as average values can be also used to analyze dispersive mixing efficiency. Distributive mixing was studied numerically by means of tracking particles in the mixing equipment. In batch mixers or batch elements of continuous mixers distributive mixing can be well characterized in terms of the pairwise correlation function. In continuous mixing equipment, distributive mixing can be analyzed in terms of concentration functions and the spatial evolution of pairwise correlation functions. The framework developed to analyze dispersive and distributive mixing efficiency opens the way for an unambiguous evaluation of the performance of any mixing equipment and, furthermore, for their design and process optimization.