(431g) Optimizing Screw Mixer Effectiveness Using Composition and Statistical Analysis Techniques

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. In this study, the mixing effectiveness of a double screw mixer blending red oak chips and glass beads was analyzed under numerous operating conditions. The mixer exit stream was sectioned into four equal regions and samples were collected across the exit stream to determine the mixture composition variance, which indicates the spatial homogeneity (or lack thereof) of the granular mixture. The effect of four factors on the composition variance was investigated: (i) screw rotation speed at levels of ω = 20, 40, and 60 rpm, (ii) dimensionless screw pitch at levels of p/D = 0.75, 1.25, and 1.75, (iii) screw rotation orientation at levels of co-rotating, countering-rotating up-pumping, and counter-rotating down-pumping, and (iv) material injection configuration at levels of the red oak and glass beads being injected into port one and two and port two and one, respectively. A full factorial design of experiments was used taking on all possible combinations of the prescribed factors, and repeatability was incorporated by performing three tests at each of the 54 operating conditions. A model equation was applied using an analysis of variance (ANOVA) statistical procedure. Of the 15 main-effect and interaction terms, eight were found to be statistically significant, and a reduced model equation was applied using the significant terms. Most notably, two three-way interaction terms consisting of the interaction between the screw rotation speed, dimensionless screw pitch, and screw rotation orientation, and between the screw rotation speed, screw rotation orientation, and material injection configuration were significant. The mixture composition variance was minimized (i.e., highest mixing effectiveness) at a 60 rpm screw rotation speed, dimensionless screw pitch of 1.75, counter-rotating down-pumping screw rotation orientation, and the red oak chips and glass beads injected into port one and two, respectively.