Dispersive Mixing Elements

Maddock from Union Carbide published results of experiments with this mixing section [38] since then the mixing section is often referred to as the Maddock mixing section. 

Tangential pushing barrier flight flank tangential with inlet channel radius R 

A similar mixing device was later patented by Gregory [48]. The difference between this mixing device and the Egan mixing section is a constant depth in inlet and outlet channel and a concave channel geometry. Another similar mixing section was patented by Dray [49] see Fig. 8.68. 

The main difference in this mixing section is that the outlet channel is open at the start of the mixing section. Thus, not all material is forced over the barrier clearance. Therefore, this mixing device will not result in a uniform shear history of the material. An extreme form of a fluted mixing section is the annular blister ring see Fig. 8.69. 

The annular blister ring is simply a smooth cylindrical screw section with a small radial clearance. All the material has to pass through this clearance to exit from the extruder. Since no positive drag transport takes place over the barrier clearance, the 

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Taylor-Flow Microreactors Taylor-flow microreactors contain a dispersing mixing element for gas and liquid streams, typically of Tand Y shapes, followed by a reaction channel for the segmented gas-liquid flow, often of quite extended length, as the Taylor flow is dominant in typical flow-pattern maps [230,248-250]. In general, all Taylor-flow microreactors can induce other flow patterns as well as the ones mentioned above. 

As discussed in Section 7.7.3, dispersive mixing in shear flow is substantially less efficient than in elongational flow. Important requirements for dispersive mixing elements were formulated by Rauwendaal [66] they are ... 

C. Rauwendaal, P. Gramann, B. Davis, and T. Osswald, U.S. Patent 6,136,246, Screw Extruder with Improved Dispersive Mixing Elements, October 24 (2000)... 

Good dispersive mixing with effective dispersive 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,... 

Complete dispersion fluid elements interact and mix completely at the microscopic level. 

Two-roll mills have been analyzed in terms of the pressure distribution and velocity profiles created between the rolls [95], the shear imposed on fluid elements exposed to these conditions in the nip region [129] and their resulting efficiency as dispersive mixing devices [130,131]. An earlier mathematical model was proposed to describe the dispersive mixing process of carbon black in rubber on roll mills, through consideration of agglomerate size distribution and... 

For a flow rate of 1 pi min-1, CCD-camera monitoring of a dilution-type experiment showed a homogeneous texture of the images, when the flow had passed five mixing elements [163]. Albeit this does not necessarily indicate completeness of mixing, it illustrates that a certain degree of dispersion was achieved. 

Also noteworthy is the appreciable coalescence caused by the shear flows in the single screws, of the rheology section of the TSMEE following the mixing element section. Flow of dispersed immiscible blends involves continuous breakdown and coalescence of the dispersed domains (122). Shear flows, where droplet-to-droplet collisions are frequent—in contrast to extensional flows—favor coalescence over dispersion. The presence of compatibilizers shifts the balance toward reduced coalescence rate. Macosko et al. (123) attribute this to the entropic repulsion of the compatibilizer molecules located at the interface as they balance the van der Waals forces and reduce coalescence, as shown on Fig. 11.36. 

Fig. 11.41 Evolution of the dispersed morphology along the TSMEE mixing element at three operating conditions of the weak matrix Blend 1 (a) 140°C and 60rpm (b) 140°C and 120 rpm (c) 180°C and 120 rpm. [Reprinted by permission from the Proceedings of the Thirteenth Semi-annual Meeting of the Polymer Mixing Study, Polymer Processing Institute, Hoboken, NJ (1996).]...

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