Distributive Mixing Elements

The pins cause disturbances in the velocity profile and thus cause mixing. Many different patterns have been used to place the pins however, there is little agreement as to what pattern is most effective. 

Another well-known mixing element is the Dulmage mixing section shown in Fig. 8.96. 

The difference between the Saxton and the Dulmage mixers is that the slots in the Saxton mixer are machined in the helical rather than the circumferential direction. The advantage of this geometry over the Dulmage is that the Saxton mixer completely wipes the barrel surface. Therefore, there is less chance of stagnation and 

A simple screw geometry with slots machined into the flights is shown in Fig. 8.99. 

A Swedish company, Axon, has patented such a design in many European countries [43]. Many other slotted mixing sections have been developed over the years. 

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As an extension to the above study, several types of dispersive and distributive mixing elements (26) were evaluated using a single-screw extruder (27) compared with the Maddock mixing element used in the previous study. Elements evaluated included dispersive elements such as straight and tapered Maddock, pineapple and Twente mixers, and distributive elements such as pin and gear... 

The barrier screw geometry is shown in Fig. 8.108. The Maddock screw has a fluted mixing element similar to the one shown in the bottom of Fig. 8.108. The CRD screw is a screw with a distributive mixing element that is based on elongational flow rather than shear flow [128-131]. 

Good distributive mixing with effective distributive mixing elements... 

Motionless mixers continuously interchange fluid elements between the walls and the center of the conduit, thereby providing enhanced heat transfer and relatively uniform residence times. Distributive mixing is usually excellent however, dispersive mixing may be poor, especially when viscosity ratios are high,... 

Statie mixers, as reviewed in Chapter 7, eontain mixing elements enelosed in a tubular housing through whieh radial mixing is aehieved. They redistribute fluid aeross the flow ehannel and eonsequently rearrange temperature and eomposition distributions. They are often used to promote mixing in laminar flow systems thus having a pro-nouneed effeet on the RTD. 

M 31] [P 28] The time evolution of the flow patterns in the cross-shaped micro mixer with two static mixing elements was monitored by simulation at time intervals of 50,150, 500 ps and 1 ms after application of pressure [71]. In addition to seeing the evolution of the swirling patterns, it was concluded from this analysis that at 500 ps a nearly homogeneous distribution of the mass fractions is given and at 1 ms this is indeed completed. Hence the theoretical mixing time of the mixer may be below 1 ms. 

Figure 1.65 CFD simulations giving 3-D mass contour plots in the cross-channel structure for a design without and one with two static mixing elements. The completeness of mixing can be judged from the cross-sectional mass distribution at the outlet [71] (by courtesy of Elsevier Ltd.).

Fig. 7.29 Schematic representation of (a) the Ross ISG mixing element, and (b) its ordered distributive mixing action.

J. P. Christiano and M. Lindenfelzer, Investigation of Mixing Patterns in Co-rotating Fully Intermeshing Twin Screw Extruders Mixing Elements Using Dynamic Pressure Distributions, SPEANTEC Tech. Papers, 43, 78-83 (1997). 

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