Bubbling suspensions

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

Sankaranarayanan, K., Shan, X., Kevrekidis, I. G. Sundaresan, S. 2002 Analysis of drag and virtual mass forces in bubbly suspensions using an implicit formulation of the lattice Boltzmann method. Journal ofEluid Mechanics 452, 61-96. 

Sankaranarayanan, K. Sundaresan, S. 2002 Lift force in bubbly suspensions. Chemical Engineering Science 57, 3521-3542. 

Foams are dispersions of bubbles where neighboring bubbles touch each other and form a jammed solid-like closed packing [62]. They are characterized by polyhedral bubbles and a high gas-phase fraction. When the gas fraction is relatively low, the bubbles retain their spherical shape (unless they are severely confined) and bubble suspensions are obtained. Monodisperse foams are advantageous, since coalescence, driven by the difference of Laplace pressure between neighboring bubbles, is reduced. Due to the high interfacial tension between gases and liquids, surfactants are usually introduced in the liquid phase to facilitate bubble formation and reduce coalescence. 

When a bubble suspension generated in the mixer is fed to a large diameter "delay tube, hexagonal close packing ofbubbles is obtained (Figure 8.10). Pulses of tracer to these systems for residence time distribution determination showed that dispersion in the delay tube is negligible [80]. Dispersion increased with total flow rate and... 

The generation ofliquid-liquid dispersions, commonly used as emulsions, has many commonalities with the generation of bubble suspensions, but differences also exist, due to the different ranges of interfacial tensions, viscosities and densities between the two systems. Surfactants are often introduced to facilitate dispersion formation and reduce coalescence. 

Sankaranarayanan K, Shan X, Kevrekidis IG, Sundaresan S. Analysis of drag and virtual mass forces in bubble suspensions using an implicit formulation of the Lattice Boltzmann method, J. Fluid Mech., in press, 2002. Saxena SC. Bubble column reactors and Fischer-Tropsch synthesis. Catal Rev Sci Eng 37 227-309, 1995. 

In summary, then, free bubble suspensions are thermodynamically unstable, and phase separation will occur, but the time scale for this depends on the detailed circumstances. 

Industrial flotation cells use a variety of strategies to create bubble surface area and contact it with the suspension. These include mixing the gas (typically 10 to 20 vol %) with the suspension in some manner. Injector nozzles, turbines, or impellers can be used for this purpose. No well-defined criteria are published for designing gas/suspension mixing in flotation systems. However, all the mix-ing/contacting strategies attempt to create small gas bubbles and ensure efficient bubble/suspension contact in a well-mixed zone. This is followed by a quiescent zone in which the gas (and the attached ink) can readily separate from the suspension (McKinney, 1998). 

Prior to about 1920, flotation procedures were rather crude and rested primarily on the observation that copper and lead-zinc ore pulps (crushed ore mixed with water) could be benefacted (improved in mineral content) by treatment with large amounts of fatty and oily materials. The mineral particles collected in the oily layer and thus could be separated from the gangue and the water. Since then, oil flotation has been largely replaced by froth or foam flotation. Here, only minor amounts of oil or surfactant are used and a froth is formed by agitating or bubbling air through the suspension. The oily froth or foam is concentrated in mineral particles and can be skimmed off as shown schematically in Fig. XIII-4. 

To a solution of indoline (20 mM in MeOH) was added Co(salen) (0.10 equiv.) and O2 was bubbled through the suspension at 25°C. After 1 h the suspension became homogeneous and the solvent was removed in vacuo and the product purified by chromatography on silica gel. 

Submerged-Culture Generators. Adaptation of the surface-film growth procedure for producing antibiotics to an aerated submerged-culture process has been successful in making vinegar. A mechanical system keeps the bacteria in suspension in the Hquid in the tank, in intimate contact with fine bubbles of air. The excess heat must be removed and the foam, which accumulates at the top of the tank, must be destroyed. 

Fluidized-Bed Vinegar Reactors. Intimate contact of air A.cetohacter is achieved in fluidized-bed or tower-type systems. Air introduced through perforations in the bottom of each unit suspends the mixture of Hquid and microorganisms within the unit. Air bubbles penetrating the bottom plate keep Jicetobacter m. suspension and active for the ethanol oxidation in the Hquid phase. Addition of a carrier for the bacterial ceUs to the Hquid suspension is reported to improve the performance (58—60). 

Eroth flotation (qv) is the most important method for cleaning fine coal because very smak particles caimot be separated by settling methods. Air is passed through a suspension of coal in water to which conditioning reagents, usuaky special oils, have been added. The oils are selected so that the coal particles preferentiaky attach themselves to the bubbles and separate from the refuse that remains in suspension. 

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