Mixing of miscible liquids

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

In the following the state of the art of microstructured mixing devices is presented. Only the mixing of miscible liquids (and gases) is considered the same micro mixers, however, can usually be used for making liquid/liquid and gas/liquid dispersions, which is outside the scope of this chapter, but certainly is worth consideration in the future. If not otherwise mentioned, liquid mixing is involved. The few examples on gas mixing are explicitly dealt with. 

For suspension of free-settling particles, circulation of pseudoplastic slurries, and heat transfer or mixing of miscible liquids to obtain uniformity, a speed of 350 or 420 r/min should be stipulated. For dispersion of dry particles in liquids or for rapid initial mixing of liquid reactants in a vessel, an 1150- or 1750- r/min propeller should be used at a distance Df/4 above the vessel bottom. A second propeller can be added to the shaft at a depth Da below the liquid surface if the submergence of floating liquids or particulate solids is otherwise inadequate. Such propeller mixers are readily available up to 2.2 kW (3 hp) for off-center sloped-shaft mounting. 

Examples of axial impellers are marine propeller and pitched blade turbine, shown in Fig 7.2. A three bladed marine t3rpe propeller is similar to the propeller blade used in driving boats. The propeller can be a side entering type in a tank or be clamped on the side of an open vessel in an off-center position. Axial flow impellers are used in blending and mixing of miscible liquids. 

H. Dogan, S. Nas, and M. Muradoglu, Mixing of Miscible Liquids in Gas-Segmented Serpentine Channels, Int. J. Multiphase Flow (in press) (2009). 

Gogate, P.R., and Pandit, A.B. (1999a), Mixing of miscible liquids with density differences Effect of volume and density of the tracer fluid, Canadian Journal of Chemical Engineering, 77(5) 988-996. 

A large number of micromixers have been used, most often their design was not oriented on a special use for gas-liquid reactions, but rather on mixing of miscible liquids [78]. Interdigital micromixers comprise respective feed charmel arrays that... 

Raimondi, P., Gardner, G. H. F. and Petrick, C. B. Effect of Pore Structure and Molecular Diffusion on the Mixing of Miscible Liquids Flowing in Porous Media , Preprint 43 presented at AIChE-SPE Joint Symposium on Fundamental Concepts of Miscible Fluid Displacement, San Francisco, Calif. (Dec. 6-9, 1959). 

The fundamentals and numerical treatment ot mixing ot two or more liquid components are described in this section. The simplest fundamental explanation is for two miscible liquid components. For example, the spontaneous mixing of two liquid materials is fundamentally controlled by the free energy of mixing ... 

The volume-related mixing power PIV presents an adequate scale-up criterion only in liquid/liquid dispersion processes and can be deduced from the pertinent process characteristics dpid We ° (dp is the particle or droplet diameter We is the Weber number). In the most common mixing operation, the homogenization of miscible liquids, where a macro- and back-mixing is required, this criterion fails completely [10] ... 

The mechanisms of mixing of miscible and immiscible liquids and soft agglomerates are depicted in Fig. 7.5, and that of hard solid agglomerates in Fig. 7.6. In the former, elongational and shear stretching is the dominant mechanism, whereas, the latter is shear stress dominated. 

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