Bubble columns column diameter effect

Yoshida and Akita (Yl) determined volumetric mass-transfer coefficients for the absorption of oxygen by aqueous sodium sulfite solutions in counter-current-ffow bubble-columns. Columns of various diameters (from 7.7 to 60.0 cm) and liquid heights (from 90 to 350 cm) were used in order to examine the effects of equipment size. The volumetric absorption coefficient reportedly increases with increasing gas velocity over the entire range investigated (up to approximately 30 cm/sec nominal velocity), and with increasing column diameter, but is independent of liquid height. These observations are somewhat at variance with those of other workers. 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

It has been reported that for diameters less than 7.62 cm, the gas holdup depends on the column diameter, whereas it is independent of it for diameters greater than 10.2 cm (Hughmark, 1967 Saxena, 1991). The same has been found in studies of the Fisher-Tropsch synthesis in slurry bubble columns, where it has been reported that the effect of the column diameter is negligible when foam is not present in the system (Fox and Degen, 1990). 

An important aspect of the design of three phase bubble columns is the variation of catalyst distribution along the reactor height, and its effect on reactor performance. Many factors influence the degree of catalyst distribution, including gas velocity, liquid velocity, solid particle size, phase densities, slurry viscosity, and, to a lesser extent, column diameter, solid shape and chemical affinity between the solid and liquid phases. 

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 heat-transfer coefficient was constant above nominal liquid velocities of 10 cm s l. No effect of Prandtl number (varied from 5 through 1,200) was obtained. The heat-transfer coefficient decreased with an increase in viscosity and a decrease in surface tension at low gas velocities, but was unaffected by changes in the column diameter (as long as the column diameter-bubble diameter ratio > 20) and the height above the gas distributor. 

It is interesting to note two orders of magnitude difference in the predictions of Ezl by the two methods described above. Method 1 does not include the effect of column diameter on EZL, whereas Method 2 does not include the effects of fluid properties and the particle diameter on EZL. It is well known that in gas-liquid (no solids) bubble-columns, the diameter of the column plays an important role in the determination of EZL. The fluid properties affect EZL only mildly and the solid particles affect ZL significantly only when their size is large. For the small particle size examined in this problem, Method 2 should therefore be more appropriate. 

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. 

In bubble columns, the estimation of parameters is more difficult than in the case of either gas-solid or solid-liquid fluidized beds. Major uncertainties in the case of bubble columns are due to the essential differences between solid particles and gas bubbles. The solid particles are rigid, and hence the solid-hquid (or gas-solid) interface is nondeformable, whereas the bubbles cannot be considered as rigid and the gas-liquid interface is deformable. Further, the effect of surface active agents is much more pronounced in the case of gas-liquid interfaces. This leads to uncertainties in the prediction of all the major parameters such as terminal bubble rise velocity, the relation between bubble diameter and terminal bubble rise velocity, and the relation between hindered rise velocity and terminal rise velocity. The estimation procedure for these parameters is reviewed next. 

Fig. 24. Effect of bubble diameter-terminal rise velocity on transitions bubble columns [oi = 0.5, Cv = 1.0, m = 1.9].

Nevertheless, as a first approximation for design, use of Eq. (Ill) is recommended with the coefficient doubled for a gas-liquid reaction in pulse or spray flow over inert packing. For the bubble flow regime, with G > 0.01 kg/m sec, it is conservative to assume Al = 0.15 sec for any gas-liquid reaction. For the dependence of effective interfacial area, despite the lack of a general correlation, it is judicious to consider that this area will vary with the 0.5 power of superficial gas velocity regardless of packing size and type, column diameter, and liquid superficial velocity. 

When a perforated plate is used as a gas sparger in a bubble column, the mean gas holdup b is influenced by the size and arrangement of holes as well as column diameter as a result of change in both of bubble size and the flow pattern of the bubble swarm. These effects have been reviewed and correlated by Kato and Nishiwaki (K6) for air-water systems where a sparger is perforated uniformly. When the hole diameter 8 is smaller than 1.0-1.4 mm and the gas velocity f/c is low, 5 for a given diameter column... 

Equation (3-23a) gives Ug from experimental knowledge of Mq + wl and b- In the case of Fig. 28, Mq + mwI = 71 cm/sec and b = 0.123, so that Ms = 28.8 cm/sec at Uq = 5.2 cm/sec withZJx = 25 cm. Figure 34 (from U5) summarizes Ms so obtained as a frinction of Uq. New data are added for a commercial-scale column (K15, D-y = 550 cm). The mean slip velocity Ms remains essentially unchanged at 50 cm/sec for C/g -20 cm/sec, but drops gradually as Uq decreases below this value. Also the column diameter has-essentially no effect, for Dx 10 cm. The intense turbulent field induced in the column seems to keep a steady bubble size, by dynamic balance between colaescence and redispersion of bubbles. 

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