Gas phase, in bubble columns

In small lab-scale STRs almost complete mixing of the gas-phase can also be assumed whereas plug-flow or mixed behavior can be generally assumed for the gas phase in bubble columns (Marinas et. al., 1993 Stockinger, 1995 Huang et al., 1998) and packed towers (Lin and Peng, 1997). 

Figure 12-10 Sketches of reactant concentration Ca around a spherical bubble or drop that reacts after migrating from ftie gas phase into the liquid phase in bubble column and spray tower reactors.

The superficial gas velocity, U is sometimes not the most pertinent process parameter, and the liquid velocity reflects better and more directly the complex liquid phase mixing when the liquids are non-Newtonian. The liquid circulation velocity increases with the increasing superficial gas velocity however, Kawase and Moo-Young developed a hydrodynamic model for the liquid phase in bubble columns with non-Newtonian fluids on the basis of an energy balance and the... 

In bubble-column slurry reactors, momentum is transferred to the liquid phase by the movement of gas bubbles. The liquid medium is stationary in most cases. Finely divided solids with particle diameters of the order of 0.01 mm are used. The operation is usually carried out in columns with high height-to-diameter ratios. The operation may be employed for batchwise conversion of a liquid reactant, or for continuous reaction between gaseous reactants. 

Zahradnik, J. and M. Fialova, The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors. Chemical Engineering Science, 1996. 51(10) p. 2491-2500. 

The term three-phase fluidization, in this chapter, is taken as a system consisting of a gas, liquid, and solid phase, wherein the solid phase is in a non-stationary state, and includes three-phase slurry bubble columns, three-phase fluidized beds, and three-phase flotation columns, but excludes three-phase fixed bed systems. The individual phases in three-phase fluidization systems can be reactants, products, catalysts, or inert. For example, in the hydrotreating of light gas oils, the solid phase is catalyst, and the liquid and gas phases are either reactants or products in the bleaching of paper pulp, the solid phase is both reactant and product, and the gas phase is a reactant while the liquid phase is inert in anaerobic fermentation, the gas phase results from the biological activity, the liquid phase is product, and the solid is either a biological carrier or the microorganism itself. 

Shetty et al. (1992) studied gas-phase backmixing for the air-water system in bubble-column reactors by measuring RTDs of pulse-injected helium tracer. 

To transfer ozone to a water phase very often fine bubble columns or stirred or agitated tank reactors are applied. The bubbles in these reactors are small. The value of m for ozone is relatively high. Furthermore the diffusion coefficient of ozone in the gas phase is very high compared with the diffusion coefficient of ozone in the water phase. These three reasons lead to the conclusion that in bubble columns and stirred tank reactors the resistance to mass transfer in the gas phase can be neglected. Equation (20) can than be simplified to ... 

In bubble columns the static head of the fluid is the dominant component of the pressure drop and consequendy it is important to determine the void fraction of the dispersion. All quanuties will be measured as posidve in the upward direction, this being the direction of flow of the dispersed phase. Assuming that the gas bubbles are of uniform size and are uniformly distributed over any cross section of the column, the gas and liquid velocities relative to the column are... 

Chlorination processes in bubble column reactors<9> are unusual in showing a significant gas-phase resistance to mass transfer. It will be seen from the low value of the Henry law constant 3 in the list of data for the example below, that the solubility of chlorine in toluene is much greater than the solubility of either the carbon dioxide or oxygen considered in the previous examples. This means that when the gas-phase mass transfer resistance is taken in combination with the liquid-phase resistance according to equation 4.19 which is derived in Volume 2, Chapter 12, then the gas side contribution to the resistance is much greater if 3 is small. 

Although the mixing patterns in bubble columns do not obviously correspond to simple axial dispersion, the dispersed plug flow model has been found to hold reasonably well in practice. For a two-phase gas-liquid system, the equation for gas-phase convection and dispersion (Chapter 2, equation 2.14) becomes ... 

Lapin A, Lubbert A. Numerical simulation of the dynamics of two-phase gas-liquid flows in bubble columns. Chem Eng Sci 1994 49 3661-3674. 

Albeit originally proposed for gas-solid fluidization, the concepts of structure resolution and compromise between dominant mechanisms embodied in the EMMS model can be generalized into the so-called variational multi-scale methodology (Li and Kwauk, 2003) and extended to other complex systems (Ge et al., 2007). One typical example out of these extensions is the Dual-Bubble-Size (DBS) model for gas-liquid two-phase flow in bubble columns (Yang et al., 2007, 2010). 

Other work has been mainly concerned with the scale-up to pilot plant or full-scale installations. For example, Beltran et al. [225] studied the scale-up of the ozonation of industrial wastewaters from alcohol distilleries and tomato-processing plants. They used kinetic data obtained in small laboratory bubble columns to predict the COD reduction that could be reached during ozonation in a geometrically similar pilot bubble column. In the kinetic model, assumptions were made about the flow characteristics of the gas phase through the column. From the solution of mass balance equations of the main species in the process (ozone in gas and water and pollution characterized by COD) calculated results of COD and ozone concentrations were determined and compared to the corresponding experimental values. 

Bubble columns are used very widely for reaction absorption applications. In bubble columns, the gas phase flows in the form of bubbles, either countercurrently or co-currently. Bubble columns provide significant liquid hold-up and sufficient liquid residence time. The column diameter sometimes exceeds 5 m, and its height reaches 10 m or more. 

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