Bubble columns dispersion effect

To illustrate, consider the hmiting case in which the feed stream and the two liquid takeoff streams of Fig. 22-45 are each zero, thus resulting in batch operation. At steady state the rate of adsorbed carty-up will equal the rate of downward dispersion, or afV = DAdC/dh. Here a is the surface area of a bubble,/is the frequency of bubble formation. D is the dispersion (effective diffusion) coefficient based on the column cross-sectional area A, and C is the concentration at height h within the column. 

Bubble Reactors In bubble columns the gas is dispersed by nozzles or spargers without mechanical agitation. In order to improve the operation, redispersion at intei vals may be effected by static mixers, such as perforated plates. The liquid may be clear or be a slurry. 

One of the purposes of giving Example 4.4 (on the chlorination of toluene) is to demonstrate the effect of different gas flowrates on the performance of a bubble column. The higher the gas flowrate, the larger the interfacial area a per unit volume of dispersion gas-liquid mass transfer will take place more readily and the concentration of the dissolved gas in the liquid will rise. Although the rate of reaction will increase, this is offset, as will be seen, by the disadvantage of a lower... 

Toluene is to be chlorinated in a batch-operated bubble column such that, for any particular gas rate chosen, the height of the dispersion in the column is 2.2 m. The reactor will operate at a temperature of 20eC and a pressure of 1 bar (any effect of hydrostatic pressure differences in the column may be neglected). The catalyst will be stannic chloride at a concentration of 5 x 10 4 kmol/m3. 

Effect of Gas-Phase Dispersion on the Gas Outlet Concentration from a Bubble Column The Absorption of C02 from an Air Stream... 

The rapid development of biotechnology during the 1980s provided new opportunities for the application of reaction engineering principles. In biochemical systems, reactions are catalyzed by enzymes. These biocatalysts may be dispersed in an aqueous phase or in a reverse micelle, supported on a polymeric carrier, or contained within whole cells. The reactors used are most often stirred tanks, bubble columns, or hollow fibers. If the kinetics for the enzymatic process is known, then the effects of reaction conditions and mass transfer phenomena can be analyzed quite successfully using classical reactor models. Where living cells are present, the growth of the cell mass as well as the kinetics of the desired reaction must be modeled [16, 17]. 

Some studies13,141 on the liquid-phase axial dispersion in horizontally-sectionalized bubble-columns have also been reported. In these studies, the bubble-column was sectionalized by a series of sieve plates with bubble caps. The data indicated that the axial dispersion in this type of column was considerably less than in an open bubble-column. There was no effect of length-to-diameter ratio up to a ratio of 24 on the axial dispersion. The axial dispersion increased with... 

The effects of suspended solid particles on liquid-phase axial dispersion in a cocurrent-upflow system have been studied by Schiigerl123 and Michelsen and Ostergaard.82 They showed that, in a three-phase column, the axial dispersion increases with gas rate. Unlike in a gas-liquid bubble-column, the liquid-phase axial dispersion coefficient in a three-phase column depends upon the liquid velocity. The nature of the effect is, however, dependent upon the gas rate and solids particle size. Similarly, the nature of the effect of solid size on the axial dispersion depends on the gas and liquid flow rates. 

A useful description of mixing in bubble columns is provided by the dispersion model. The global mixing effects are generally characterized by the dispersion coefficients El and Eq of the two phases which are defined in analogy to Fick s law for diffusive transport. Dispersion in bubble columns has been the subject of many investigations which have recently been reviewed by Shah et al. (45). Particularly, plenty of data are available for liquid-phase dispersion. 

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. 

Fig. 26. Effect of proportionality constant for dispersion on transition gas hold-up bubble column [Cy = 1.0, m = 1.9, dB-v oo Clift et al relation].

In fluidization, a suspension of fine solid particles behaves like a liquid during the upflow of a supportive gas or liquid phase. Thus the bed of fluidized solid itself may be analyzed similarly to liquid systems. The gas-lift effect produces internal recirculation, by providing a descending flow of high particle concentration and an ascending flow of low particle concentration. This effect resembles the circulation in bubble columns. Whereas bubble columns contain dispersed gas and a continuous liquid phase, the fluidized bed comprises the bubble phase and the emulsion phase in which particles have gained fluid-like properties by the interstitial gas flow. 

In the bubble column the velocity profile of recirculating liquid is shown in Fig. 27, where the momentum of the mixed gas and liquid phases diffuses radially, controlled by the turbulent kinematic viscosity Pf When I/l = 0 (essentially no liquid feed), there is still an intense recirculation flow inside the column. If a tracer solution is introduced at a given cross section of the column, the solution diffuses radially with the radial diffusion coefficient Er and axially with the axial diffusion coefficient E. At the same time the tracer solution is transported axially Iby the recirculating liquid flow. Thus, the tracer material disperses axially by virtue of both the axial diffusivity and the combined effect of radial diffusion and the radial velocity profile. 

To simulate the effects of reaction kinetics, mass transfer, and flow pattern on homogeneously catalyzed gas-liquid reactions, a bubble column model is described [29, 30], Numerical solutions for the description of mass transfer accompanied by single or parallel reversible chemical reactions are known [31]. Engineering aspects of dispersion, mass transfer, and chemical reaction in multiphase contactors [32], and detailed analyses of the reaction kinetics of some new homogeneously catalyzed reactions have been recently presented, for instance, for polybutadiene functionalization by hydroformylation in the liquid phase [33], car-bonylation of 1,4-butanediol diacetate [34] and hydrogenation of cw-1,4-polybutadiene and acrylonitrile-butadiene copolymers, respectively [10], which can be used to develop design equations for different reactors. 

Finally, we studied the effect of liquid dispersion on catalyst performance by comparing the performance of the powder catalyst in a bubble column reactor with the HyperCat-FT system. As shown in Figure 6, the CO conversion is much lower for a low Peclet number (bubble colunm reactor with a back-mixed liquid phase) as opposed to a higher Peclet number for the HyperCat system. The tests were conducted under the same process conditions and Damkohler number. The change in Peclet number did not change the liquid product... 

In bubble columns, since the gas bubbles are dispersed in the continuous liquid phase, fractional gas holdup (Eg) is an important design parameter, affecting column performance. The most direct and obvious effect is on the column volume, since a significant fraction of the volume is occupied by the gas. The indirect influences are also important. For instance, the possible spatial variation of Eg gives rise to pressure variation, which results in intense liquid phase motion. These secondary motions govern the rates of mixing plus heat and mass transfer. 

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