Needed Bubble Column Research

A major study should be attempted to compare the different bioreactor performance characteristics. Comparisons between BCs and airlift reactors are available, but a wider array is lacking, but needed. Research towards this end, such as Bouaifi et al. (2001) who compared stirred-tank reactors and BCs, is sparse. Different bioreactor designs have quite unique scaling abilities and associated costs, and the economic benefits and decisions would be better understood if such studies would be more common. 

Bubble interactions are tightly connected to hydrodynamics so that gas holdup is usually capable of representing gas-liquid mass transfer trends fairly accurately and, more importantly, predicting reactor hydrodynamics. Thus, any BC experiment (or series of experiments) starts with a gas holdup study. The literature provides a wide array of gas holdup information however, the more detailed experiments, especially those that investigate gas-liquid mass transfer, are much fewer in number and smaller in scope. 

Most studies, for example, are based on air-water interaction in an isothermal setting, even though most industrial bioprocesses have thermal interactions and complex fluid properties. It is also common for industrial processes to be performed at relatively high temperatures and/or pressure. These settings still lack experimental coverage (Lau et al., 2004). Furthermore, gas holdup and gas-liquid mass transfer are decoupled under thermal operation, and pressure variances are rarely accounted for in currently available correlations. 

Gas-liquid mass transfer correlations and their design applications have to be handled very carefully. Bubble-bubble interactions are very complicated processes, which still have not been mastered and are not easily represented with the current set of tools. Although a change in gas holdup can predict the direction of the change in gas-liquid mass transfer, it cannot predict the amount 

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Correlations are needed to predict whether two-phase flow will occur after vapor venting is initiated by rupture disc failure or relief valve opening. Research is needed in this area, but for the present we recommend the following correlations to predict batch swell. For systems with low viscosity (less than 500 cp) an equation based on bubble column hold-up is used to obtain a swell ratio ... 

In the section on bubble column reactors, the hydrodynamic parameters needed for scale-up are presented along with models for reaction and heat transfer. The mixing characteristics of colunms are described as are the directions for future research work on bubble column reactors. 

The most important parameter governing stability is the dispersion coefficient of the dispersed phase such as bubbles, drops, and particles. The published information is not sufficient. A comprehensive research program is needed for the measurement of dispersion in all multiphase reactors over a wide range of terminal velocities, column diameters, column heights, sparger designs, phase velocities, and continuous-phase physical properties. 

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