Performance of Bubble Columns

The correlations detailed in Sections 7.6.2.1 to 7.6.2.5 [17, 18] are based on data for the turbulent regime with four bubble columns, up to 60 cm in diameter, and for 

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As Eg is usually small the detrimental effect of gas phase dispersion on the performance of bubble columns can be neglected in columns less than 20 cm in diameter (61). For illustrating the influence of gas phase dispersion some computed conversions are presented in Fig. 10 (J ). The simulations refer to CO2 absorption in carbonate buffer in a column 5 m in length. Eq was calculated from eqn. (15). The liquid phase dispersion does not affect the conversion in the present case as the process takes place in the diffusional regime of mass transfer theory. As shown in Fig. 10, the decrease in conversion due to gas phase dispersion increases with increasing diameter and gas velocity. However, in the favorable bubbly flow regime and in small diameter columns the effect is less pronounced. 

Although absorption plus chemical reaction sounds more complex than physical absorption or desorption, the absorption of oxygen in sulfite solutions is often used to characterize the performance of bubble columns or stirred reactors. With a 0.2-1.0 N solution of sodium sulfite and a small amount of copper sulfate as catalyst (10 M), the rate of oxidation is independent of sulfite concentration, and the oxygen absorption rate is constant until nearly all the sulfite has reacted ... 

Mouza, A. A., Dalakoglou, G.K., and Paras, S.V. (2005), Effect of liquid properties on the performance of bubble column reactors with fine pore spargers, Chemical Engineering... 

Mhaskar.R.D. "Effects of backmixing on the performance of bubble column reactors". Chem.Engng.Sci. 29 (1974) 897. 

Non-Newtonian fluids are frequently encountered in the process industries, especially in biochemical and biotechnological applications. It has been known that the rheological properties of the liquids have profound effects on the performance of bubble columns. The non-Newtonian anomalies create uncertainties in the design, scale-up, and operation of bubble columns. Most analyses and correlations are based on the following two premises, which are not necessarily always applicable ... 

Since gas holdup is one of the important design parameters for the performance of bubble columns, study of gas holdup has been extensively carried out. Gas holdup depends mainly on gas velocity and the physical properties of the liquid. Although a number of correlations have been proposed, no single generalized correlation is available at present. Only a few attempts have been made to predict gas holdup theoretically. 

This type of reactor operates well with low-viscosity mixtures, such as yeast and nonfilamentous bacteria. Citric acid and bakers yeast fermentations are well known in this respect. Also, beer fermentation can be considered as a special case, because the gas is process-generated CO2 and not air. At low viscosities, mixing can be much better than a stirred tank, while the mass transfer is similar or even better [4]. However, at higher viscosities, the performance of bubble columns will sharply fall, and so this type will lose functionality against the stirred tank. 

So far, we have considered only mass transfer within a single phase - that is, mass transfer between fluids and solid surfaces. For gas absorption and desorption, in which mass transfer takes place between a gas and a liquid, packed columns are extensively used, while bubble columns and sparged stirred vessels are used mainly for gas-liquid reactions or aerobic fermentation. As the latter types of equipment are discussed fully in Chapter 7, we shall, at this point, describe only the performance of packed columns. 

Experiments [25] performed in bubble columns of different sizes gave the following expression for the mixing time, 0 ... 

If the values of local mean bubble diameter and local gas flux are available, a fluid dynamic model can estimate the required influence of mass transfer and reactions on the fluid dynamics of bubble columns. Fortunately, for most reactions, conversion and selectivity do not depend on details of the inherently unsteady fluid dynamics of bubble column reactors. Despite the complex, unsteady fluid dynamics, conversion and selectivity attain sufficiently constant steady state values in most industrial operations of bubble column reactors. Accurate knowledge of fluid dynamics, which controls the local as well as global mixing, is however, essential to predict reactor performance with a sufficient degree of accuracy. Based on this, Bauer and Eigenberger (1999) proposed a multiscale approach, which is shown schematically in Fig. 9.13. 

After completing reaction-engineering work, it is first necessary to evolve a reactor configuration before one can start evaluating whether such hardware can perform the expected duties. In the case of bubble columns, evolving reactor hardware involves at least the following (also see Fig. 11.2) ... 

Unfortunately, the present models are still on a level aiming at reasonable solutions with several model parameters tuned to known flow fields. For predictive purposes, these models are hardly able to predict unknown flow fields with reasonable degree of accuracy. It appears that the CFD evaluations of bubble columns by use of multi-dimensional multi-fluid models still have very limited inherent capabilities to fully replace the empirical based analysis (i.e., in the framework of axial dispersion models) in use today [63]. After two decades performing fluid dynamic modeling of bubble columns, it has been realized that there is a limit for how accurate one will be able to formulate closure laws adopting the Eulerian framework. In the subsequent sections a survay of the present status on bubble column modeling is given. 

Lehr and Mewes [67] included a model for a var3dng local bubble size in their 3D dynamic two-fluid calculations of bubble column flows performed by use of a commercial CFD code. A transport equation for the interfacial area density in bubbly flow was adopted from Millies and Mewes [82]. In deriving the simplified population balance equation it was assumed that a dynamic equilibrium between coalescence and breakage was reached, so that the relative volume fraction of large and small bubbles remain constant. The population balance was then integrated analytically in an approximate manner. 

The oxidation of cumene with oxygen from air is nowadays performed in a series of bubble columns. The off-gas is treated by a combination of cooling and adsorption. The recovered cumene from the off-gas treatment is recycled to the oxidation unit. In the concentration unit, the product from oxidation with around 20 -40 wt% CHP is distilled under vacuum to increase the CHP concentration to 65-90wt%. The separated cumene is again recycled to the oxidation unit. In the cleavage, the CHP is converted into phenol and acetone using sulfuric acid as the catalyst. In parallel, the DMBA from oxidation is nearly quantitatively converted... 

Most problems in the design and performance prediction of bubble column reactors appear, because it is - up to now - not possible to control the fluid dynamics in such reactors. Especially the parcuneters of the turbulent flow in these reactors are of major importance. 

Bubble columns with vibrating internals and columns with low-amplitude pulsations are currently under investigation in an attempt to improve the column performance. Vibrating internals consist of helical springs leading to 140% gas hold, which is much higher than the ones without internals. Sauter mean bubble diameter is estimated around 0.3 cm for the new type of bubble columns. 

CARPT experiments were performed with bubble columns of three different diameters (11.4 cm 19 cm 29.2 cm) using air-water. Four superficial velocities, spanning the range from 2 cm/s to 18.4 cm/s were used in each column. This covers all flow regimes from bubble to churn turbulent flow. All runs were done with batch liquid (i.e. zero liquid superficial velocity). A porous plate distributor was employed. 

A small amount of collector (surfactant) or other appropriate additive in the liquid may greatly increase adsorption (Shah and Lemlich, op. cit.). Column performance can also be improved by skimming the surface of the liquid pool or, when possible, by removing adsorbed solute in even a tenuous foam overflow. Alternatively, an immiscible liquid can be floated on top. Then the concentration gradient in the tall pool of main hquid, plus the trapping action of the immiscible layer above it, will yield a combination of bubble fractionation and solvent sublation. 

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