Bubbles coalescence bubble column reactors

External and internal loop air-lifts and bubble column reactors containing a range of coalescing and non-Newtonian fluids, have been studied (52,53). It was shown that there are distinct differences in the characteristics of external and internal loop reactors (54). Overall, in this type of equipment... 

Example 12-2 An aqueous solution contains 10 ppm by weight of an organic contaminant af molecular weight 120, which must be removed by air oxidation in a lo-cm-diameter bubble column reactor at 25°C. The liquid flows downward in the tube at an average velocity af 1 cm/sec. The air at 1 atm is admitted at 0.1 liter/sec and is injected as bubbles 1 mm diameter, which rise at 2 cm/sec. Assume no coalescence or breakup and that both gas and liquid are in plug flow. The reaction in the Hquid phase has the stoichiometry A + 2O2 products with a rate C. ... 

Luo H (1993) Coalescence, break-up and hquid circulation in bubble column reactors. Dr. ing. thesis. Department of Chemical Engineering, the Norwegian Institute of Technology, Trondheim, Norway... 

Olmos E, Centric C, Vial C, Wild G, Midoux N (2001) Numerical simulation of multiphase flow in bubble column reactors. Influence of bubble coalescence and breakup. Chem Eng Sci 56(21-22) 6359-6365. 

Olmos E, Centric C, Midoux N (2003) Numerical description of flow regime transitions in bubble column reactors by multiple gas phase model. Chem Eng Sci 58(10) 2113-2121 Oolman TO, Blanch HW (1986) Bubble coalescence in stagnant liquid. Chem Eng Commun 43(4-6) 237-261... 

Figure 12-12 Sketches of possible flow patterns of bubbles rising through a liquid phase in a bubble column. Stirring of the continuous phase will cause the residence time distribution to be broadened, and coalescence and breakup of drops will cause mixing between bubbles. Both of these effects cause the residence time distribution in the bubble phase to approach that of a CSTR. For falling drops in a spray tower, the situation is similar but now the drops fall instead of rising in the reactor.

The bubble column and spray tower depend on nozzles to disperse the drop or bubble phase and thus provide the high area and small particle size necessary for a high rate. Drop and bubble coalescence are therefore problems except in dilute systems because coalescence reduces the surface area. An option is to use an impeller, which continuously redisperses the drop or bubble phase. For gases this is called a sparger reactor, which might look as shown in Figure 12-16. 

An interesting class of exact self-similar solutions (H2) can be deduced for the case where the newly formed phase density is a function of temperature only. The method involves a transformation to Lagrangian coordinates, based upon the principle of conservation of mass within the new phase. A similarity variable akin to that employed by Zener (Z2) is then introduced which immobilizes the moving boundary in the transformed space. A particular case which has been studied in detail is that of a column of liquid, initially at the saturation temperature T , in contact with a flat, horizontal plate whose temperature is suddenly increased to a large value, Tw T . Suppose that the density of nucleation sites is so great that individual bubbles coalesce immediately upon formation into a continuous vapor film of uniform thickness, which increases with time. Eventually the liquid-vapor interface becomes severely distorted, in part due to Taylor instability but the vapor film growth, before such effects become important, can be treated as a one-dimensional problem. This problem is closely related to reactor safety problems associated with fast power transients. The assumptions made are ... 

Zahradnik J. et al., The effect of electrolytes on bubble coalescence and gas holdup in bubble column reactors, Trans IChemE 73 (1995), Part A,... 

Laari A, Turunen I (2003) Experimental Determination of Bubble Coalescence and Break-up Rates in a Bubble Column Reactor. The Canadian Journal of Chemical Engineering 81(3-4) 395-401... 

In general, the gas holdups and kLa for suspensions in bubbling gas-liquid reactors decrease substantially with increasing concentrations of solid particles, possibly because the coalescence of bubbles is promoted by presence of particles, which in turn results in a larger bubble size and hence a smaller gas-liquid interfacial area. Various empirical correlations have been proposed for the kLa and gas holdup in slurry bubble columns. Equation 7.46 [24], which is dimensionless and based on data for suspensions with four bubble columns, 10-30 cm in diameter, over a range of particle concentrations from 0 to 200 kg m 3 and particle diameter of 50-200 pm, can be used to predict the ratio r of the ordinary kLo values in bubble columns. This can, in turn, be predicted for example by Equation 7.41, to the kLa values with suspensions. 

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