Methods to Improve Backmixing

A major concern in backmixing is the fluid in the center region of the screw channel where the axial shear strain is zero or close to zero. In a simple conveying screw the fluid in the inner recirculation region will stay within this region until it reaches the end of the screw. When this happens, the material flowing into the die will be poorly mixed. 

Mixing pins and slots in the screw flights will improve axial mixing because they achieve a short-term splitting and reorientation of the fluid. The effect of mixing pins on backmixing is illustrated in Fig. 7.160. 

Particle tracking results in a mixing section with elongaticnal mixing pins 

These results were obtained using a three-dimensional BEM flow analysis. It is clear that one row of mixing pins has limited effect on axial mixing. Backmixing can be improved by varying the spacing between the pins, thus intentionally creating streams of different axial velocities. 

The challenge in improving axial mixing is to efficiently transfer fluid from the inner re-circulation region to the outer region and vice versa. A simple but effective method of doing this is the inside-out mixer shown in Fig. 7.161. 

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