Columns bubble

Bubble columns in which gas is bubbled through suspensions of solid particles in liquids are known as slurry bubble columns . These are widely used as reactors for a variety of chemical reactions, and also as bioreactors with suspensions of microbial cells or particles of immobilized enzymes. 

Bubble columns find frequent application in the process industries due to their relatively simple construction and advantageous properties such as excellent heat transfer characteristics to immersed surfaces. Despite their frequent use in a variety of industrial processes, many important fluid dynamical aspects of the prevailing gas-liquid two-phase flow in bubble columns are unfortunately poorly un- 

Computed and measured (Bader et ai, 1988) radial profiles of (a) solids concentration and (b) axial solids velocity in a CFB riser for a superficial gas velocity U of 3.7 m/s and a mass flux Gs of 98 kg/(m s). Riser diameter D = 0.304 m, physical properties of the particles diameter, 76 /rm density, 1714 kg/m.  

Delnoij etal. (1997b) developed a computer code based on the volume of fluid (VOF) method (Hirt and Nichols, 1981 Nichols et al, 1980) to describe the dynamics of single gas bubbles rising in a quiescent Newtonian liquid. They were able to show that the predicted bubble shape and associated flow patterns induced in the liquid phase could be predicted very well with the VOF method over a wide range of Eotvbs (Eo) and Morton (M) numbers (see Sec. 1V,B,1 for the definitions of Eo and M). In Fig. 26 the formation and rise of single gas bubbles emanating from a central orifice is shown for various values of the Eotvos and Morton num- 

Values of the Eotvos (Eo) and Morton (M) Numbers Used IN THE Simulations Depicted in Fig. 6.11 

The mass transfer process which takes place in the gasAiquid reactor is determined by the volumetric mass transfer coefficient, that is the product of the mass transfer coefficient and the specific interfacial area. In most processes in gas in liquid dispersions, the gas phase resistance to mass transfer is negligible. There are two reasons for this the diffusivities in gases are several orders of magnitude higher than in liquids, and most gaseous compounds have a relatively low solubility in liquids (notable exceptions are, e.g., ammonia,sulfur trioxide). 

The liquid side volumetric mass transfer coefficient is defined as k a (s ). 

The index l indicates the liquid side. We shall add the index for the transferred component (a, b etc.) only where such distinction is necessary. The liquid side mass transfer coefficient depends on the bubble diameter, the hydrodynamic conditions and the diffusivity of the transferred component in the liquid. 

The specific interfacial area a mim- m ) follows from the next equation the bubbles are assumed to be spherical (compare eq. (4.35)  

The hydrodynamic conditions and the properties of the system determine both the gas fraction and the average bubble diameter d. 

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Eig. 5. Examples of air driven bioreactors (a) bubble column, (b) draught tube, and (c) external loop. 

Eigure 6 enables a comparison to be made of kj a values in stirred bioreactors and bubble columns (51). It can be seen that bubble columns are at least as energy-efficient as stirred bioreactors in coalescing systems and considerably more so when coalescence is repressed at low specific power inputs (gas velocities). 

Fig. 6. A comparison of k a values (51). Represented are 1, stirred bioreactor using water, = 0.02 m/s, kj a (eq. 16) 2, stirred bioreactor using water, t 3 = 0.04 m/s, kj a (eq. 16) 3, bubble column using water, kj a (eq. 18) 4, stirred bioreactor using salt water, = 0.02 m/s, kj a (eq. 17) 5, stirred bioreactor using salt water, = 0.04 m/s, kj a (eq. 17) and 6, bubble column using salt water (noncoalescing).

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... 

A similar process to SMDS using an improved catalyst is under development by Norway s state oil company, den norske state oHjeselskap AS (Statod) (46). High synthesis gas conversion per pass and high selectivity to wax are claimed. The process has been studied in bubble columns and a demonstration plant is planned. 

Fig. 4. Multiphase fluid and fluid—solids reactors (a) bubble column, (b) spray column, (c) slurry reactor and auxiUaries, (d) fluidization unit, (e) gas—bquid—sobd fluidized reactor, (f) rotary kiln, and (g) traveling grate or belt drier.

Fig. 13. Bubble column flow characteristics (a) data processing system for split-film probe used to determine flow characteristics, where ADC = automated data center (b) schematic representation of primary flow patterns.

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

K. H. Reichert and R. Michael, "Polymerization iu Bubble Columns, Problems of Mass and Heat Transfer at High SoHd Contents," Inst. Chem. 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

The reaction is carried out ia a bubble column at 120—130°C and 0.3 MPa (3 bar). Palladium chloride is reduced to palladium duriag the reaction, and then is reoxidized by cupric chloride. Oxygen converts the reduced cuprous chloride to cupric chloride. 

The difference between the curves for pure water and seawater again illustrates the significance of small concentrations of solute with respecl to bubble behavior. In commercial bubble columns and agitated vessels coalescence and breakup are so rapid and violent that the rise velocity of a single bubble is meaningless. The average rise velocity can, however, be readily calculated from holdup correlations that will be given later. 

The choice of a bubble column or an agitated vessel depends primarily on the solubihty of the gas in the liquid, the corrosiveness of the liquid (often a gas compressor can be made of inexpensive material, whereas a mechanical agitator may have to be made of exotic, expensive materials), and the rate of chemical reac tion as compared with the mass-transfer rate. Bubble columns and agitated vessels are seldom used for gas absorption except in chemical reac tors. As a general rule. 

Mass Transfer Mass transfer in plate and packed gas-liquid contactors has been covered earHer in this subsection. Attention nere will be limited to deep-bed contactors (bubble columns and agitated vessels). Theory underlying mass transfer between phases is discussed in Sec. 5 of this handbook. 

Gas Holdup (e) in Bubble Columns With coalescing systems, holdup may be estimated from a correlation by Hughmark [Ind. Eng... 

Axial Dispersion Backmixing in bubble columns has been extensively studied. An excellent review article by Shah et al. [AIChE... 

Entrained Solids Bubble Columns with the Solid Fluidized... 

Region II, 0.02 < P < 2. Most of the reaction occurs in the bulk of the liquid. Both interfacial area and holdup of liquid should be high. Stirred tanks or bubble columns will be suitable. 

Region III, P < 0.02. Reaction is slow and occurs in the bulk hquid. Interfacial area and liquid holdup should be high, especially the latter. Bubble columns will be suitable. 

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