Dispersive/Distributive Static Mixer

A relatively recent development in static mixers is the Dispersive/Distributive Static Mixer (DDSM) [308]. This mixer is specifically designed to generate strong elonga-tional flow for improved dispersive mixing. The DDSM is an extension of the CRD... 

Two repeating units of the Dispersive/Distributive Static Mixer... 

Figure 12.55 Computed particle tracking in one module of the Dispersive/ Distributive Static Mixer using BEM...

Gramann, P.J., Davls, B.A., Osswald, T.A., Rauwendaal, C.J., A New Dispersive and Distributive Static Mixer for the Compounding of Highly Viscous Materials, SPEAN-TEC (1999)... 

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

Kataoka T and Nishiki T. Dispersed mean drop sizes of (W/0)/W emulsions in a stirred tank. J Chem Eng Jpn 1986 19 408-412. Nishikawa M, Mori F, and Fujieda S. Average drop size in a liquid-liquid phase mixing vessel. J Chem Eng Jpn 1987 20 82-88. Nishikawa M, Mori F, Fujieda S, and Kayama T. Scale-up of liquid-liquid phase mixing vessel. J Chem Eng Jpn 1987 20 454—459. Berkman PD and Calabrese RV. Dispersion of viscous liquids by turbulent flow in a static mixer. AIChE J 1988 34 602-609. Chatzi EG, Gavrielides AD, and Kiparissides C. Generalized model for prediction of the steady-state drop size distributions in batch stirred vessels. Ind Eng Chem Res 1989 28 1704—1711. 

Tubular reactors are also used to carry out some multiphase reactions. Wamecke et al. (1999) reported use of a computational flow model to simulate an industrial tubular reactor carrying out a gas-liquid reaction (propylene oxide manufacturing process). In this process, liquid is a dispersed phase and gas is a continuous phase. The two-fluid model discussed earlier may be used to carry out simulations of gas-liquid flow through a tubular reactor. Warnecke et al. (1999) applied such a model to evaluate the influence of bends etc. on flow distribution and reactor performance. The model may be used to evolve better reactor configurations. In many tubular reactors, static mixers are employed to enhance mixing and other transport processes. Computational flow models can also make significant contributions to understanding the role of static mixers and for their optimization. Visser et al. (1999) reported CFD... 

The hydrodynamic enviroiunent in a static mixer is characterized by a more uniform distribution of energy and shear rates than in a conventional agitated tank. Thus, for dispersion processes, a static mixer can produce a narrower drop or bubble size distribution. Also, the narrower residencetime distribution in a static mixer makes it the preferred mixing device for processes involving fast reactions or polymerizations requiring no back-mixing. 

B. Weinstein. "Drop Size and Distribution of Water Dispersions Generated by Static Mixer Ele-... 

Common means of enhancing the mixing efficiency of single-screw extruders include use of modified screw forms containing special mixing devices, or static mixers, which are located between the extruder and die. The various forms of mixing screw design available may function in either a dispersive and/or distributive manner. 

In addition, Chandavimol et al. (1991a,b) have estimated the kinetic rate at which the bubbles go from initial size to the maximum equilibrium size as a function of energy dissipation. The rate of dispersion was found to be approximately proportional to energy dissipation rate. [See Figure 7-24 for a comparison of bubble breakup rate between vortex (HEV) and spiral (KMS type) static mixers.] In general, the equilibrium drop size is reached in a few pipe diameters. However, the drop size distribution is narrowed as the simultaneous processes of drop breakup and coalescence are continued, depending on the mixer design and fluid properties. See also Hesketh et al. (1987, 1991). 

If designed and operated at design pressure drop, these static mixers can produce a narrow drop size distribution. For example, 70% of dispersed volume can be within 20% of the mean drop diameter. However, these mixers work poorly at low velocities, because the pressure drop can fall below the necessary level. Also, at extremely high velocities, there is a danger of stable emulsion formation. 

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