(368b) Mixing in Double Screw Mixers

The mixing of granular materials has a significant influence on the yield and/or quality of the desired products in numerous industrial processes including energy generation, concrete manufacturing, food processing, and pharmaceutical production. The operating conditions of the mixing equipment require careful selection because of their impact on the homogeneity of the mixture, especially when handling materials which vary in size, shape, and/or density. Mixer scale can also have a significant impact on the desired level of mixing. In this study, the mixing effectiveness of three double screw mixers is compared for several operating conditions. The mixers have screw diameters of D = 2.54 cm, 3.81 cm, and 5.08 cm, characteristic mixing lengths of L/D = 10, and are identified as 1x, 1.5x, and 2x scale mixers. Two different size ranges of glass beads (300-500 μm and 800-1000 μm) are mixed using screws with one of three dimensionless screw pitches (p/D = 0.75, 1.25, and 1.75) and one of three rotation speeds (Ï? = 20, 40, and 60 rpm). Three different screw orientations are also investigated and include co-rotating (CoR), counter-rotating up pumping (CrtR UP), and counter-rotating down pumping (CrtR DP). The mixers were designed such that four equal area exit ports separate the exit stream; these exit streams were captured to determine the mass fraction of each material exiting the port. The coefficient of variation of the mixture composition across the exit ports was then determined for various operating conditions. Preliminary results indicate that counter-rotating down pumping has better mixing than the counter-rotating up pumping or the co-rotating configurations, as the coefficients of variation are low and consistent. The counter-rotating up pumping configuration has the highest coefficients of variation, particularly at the highest mixing speeds used in this study.