Interfacial area and gas holdup

The terminal velocity enters into Eqs. (9.45) and (9.46), but this velocity does not change much with diameter for bubbles larger than 1 mm (Fig. 7.8), and a value of 0.2 m/s can be used for gases in water or similar pure liquids. For air bubbles in electrolyte solutions, coalescence is greatly retarded, and the average bubble size can be much less than in pure water, with corresponding increases in interfacial area and gas holdup. 

Stegeman D, Knop PA, Wijnands AJG, Westerterp KR. Interfacial area and gas holdup in a bubble column reactor at elevated pressures. Ind Eng Chem Res 35 3842-3847, 1996. 

Interfacial area and gas holdup (bubble coalescence, generation of large bubbles and bubble slugs) Important, but both k. a and r. determine R a, search for Ug, wnich gives maximum value of Ry a Very important R.a is proportional to a ... 

TABLE 23-9 Mass-Transfer Coefficients/ Interfacial Areas and Liquid Holdup in Gas/Liquid Reactions... 

Some of this theoretical thinking may be utilized in reactor analysis and design. Illustrations of gas-liquid reactors are shown in Fig. 19-26. Unfortunately, some of the parameter values required to undertake a rigorous analysis often are not available. As discussed in Sec. 7, the intrinsic rate constant kc for a liquid-phase reaction without the complications of diffusional resistances may be estimated from properly designed laboratory experiments. Gas- and liquid-phase holdups may be estimated from correlations or measured. The interfacial area per unit reactor volume a may be estimated from correlations or measurements that utilize techniques of transmission or reflection of light, though these are limited to small diameters. The combined volumetric mass-transfer coefficient kLa, can be also directly measured in reactive or nonreactive systems (see, e.g., Char-pentier, Advances in Chemical Engineering, vol. 11, Academic Press, 1981, pp. 2-135). Mass-transfer coefficients, interfacial areas, and liquid holdup typical for various gas-liquid reactors are provided in Tables 19-10 and 19-11. 

The physical and thermal properties of the gas and liquid, interfacial area and liquid holdup, physical mass transfer coefficients, diffusion coefficients, and volumetric flow rate of the liquid are independent of temperature and conversion. 

Static mixing of gas—Hquid systems can provide good interphase contacting for mass transfer and heat transfer. Specific interfacial area for the SMV (Koch/Sulzer) mixer is related to gas velocity and gas holdup ( ) by the following ... 

The correlations detailed in Sections 7.6.2.1-7.6.2.5 [17,18] are based on data for the turbulent regime with 4 bubble columns, up to 60 cm in diameter, and for 11 liquid-gas systems with varying physical properties. Unless otherwise stated, the gas holdup, interfacial area, and volumetric mass transfer coefficients in the correlations are defined per unit volume of aerated liquid, that is, for the liquid-gas mixture. 

Mass transfer is essential in EL-ALRs. Smaller bubbles and a uniform gas holdup radial distribution increase the interfacial area and improve mass transfer. Intensified turbulence increases the surface renewal frequency and decreases bubble size. A novel internal to improve mass transfer and the hydrodynamic behavior in a gas-liquid system is reported. Experiments were carried out to study the effect of the internal on the bubble behavior and liquid velocity in an EL-ALR. 

A. Schumpe, W.D. Deckwer, Gas holdups, specific interfacial areas, and mass transfer coefficients of aerated carboxymethyl cellulose solutions in a bubble column, I EC Process Des. Develop. 21 (1982) 706-711. 

Interfacial area per unit volume of liquid and gas holdup, ft3/ft3. Interfacial area, ft3. 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

Gas holdup is an important hydrodynamic parameter in stirred reactors, because it determines the gas-liquid interfacial area and hence the mass transfer rate. Several studies on gas holdup in agitated gas-liquid systems have been reported, and a number of correlations have been proposed. These are summarized in Table VIII. For a slurry system, only a few studies have been reported (Kurten and Zehner, 1979 Wiedmann et al, 1980). In general, the gas holdup depends on superficial gas velocity, power consumption, surface tension and viscosity of liquids, and the solid concentration. The dependence of gas holdup on gas velocity, power consumption, and surface tension of the liquid can be described as... 

Gas absorption is a function of the gas and liquid mass transfer coefficients, the interfacial area, and the enhancement due to chemical reaction. The gas-liquid interfacial area is related to the Sauter mean bubble diameter and the gas holdup fraction. The gas holdup fraction has been reported to vary with radial position (7-11) for column internal diameters up to 0.6 m. Koide et al" Tl2), however, found that the radial distribution of gas holdup was nearly constant for a column diameter of 5.5 m. Axial distribution of average gas holdup has been reported by Ueyama et al. (10). The average gas holdup... 

Some qualitative observations can be made. Increase of the superficial gas velocity increases the holdup of gas, the interfacial area, and the overall mass-transfer coefficient. The ratio of height to diameter is not important in the range of 4 to 10. Increase of viscosity and decrease of surface tension increase the interfacial area. Electrolyte solutions have smaller bubbles, higher gas holdup, and higher interfacial area. Sparger design is unimportant for superficial gas velocities > 5 to 10 cm/s (0.16 to 0.32 ft/s). Gas conversion falls off at higher superficial velocities, so values under 10 cm/s (0.32 ft/s) are advisable. 

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

The choice of a suitable reactor for gas-liquid reaction or absorption is very often a question of matching the reaction kinetics with the capabilities of the proposed reactor. The specific interfacial area a, liquid holdup /3, and mass-transfer coefficients ki and k (or kifl and k fl) are the most significant characteristics of a reactor. A synthesis of published values of the mass-transfer parameters will be given in this section. 

For the absorption of a dilute gas solute, this technique has been used for absorption of CO2 into NaOH and Na2C03 solutions (D7, L2). Thus the path has been prepared for simulation to be used, whenever desired, for slow, complex, or simultaneous reactions whenever these are met in practice. For such systems, more elaborate laboratory equipment is required that makes it possible to vary the ratios of interfacial area to liquid holdup (Lll). 

Equation (10-51) indicates that the important parameters for this case are ki and the gas-liquid interfacial area Cg. The latter depends on the size and concentration of bubbles in the slurry. Alternately, it may be expressed in terms of the bubble diameter and gas holdup. The volume of the gas bubbles is the difference between the volume of the bubbling slurry n. and that of the bubble-free slurry Vq. Both Vf, and Vq can be measured directly. Then for spherical particles of diameter dj,... 

The interfacial mass transfer area, a, is related to the superficial gas velocity, Vg, and gas holdup, Og, for a Sulzer SMV [73], as shown in Figure 9.46. The plot shows that... 

In general, the gas holdups and kLa for suspensions in bubbling gas-liquid reactors decrease substantially with increasing concentrations of solid particles, possibly because the coalescence of bubbles is promoted by presence of particles, which in turn results in a larger bubble size and hence a smaller gas-liquid interfacial area. Various empirical correlations have been proposed for the kLa and gas holdup in slurry bubble columns. Equation 7.46 [24], which is dimensionless and based on data for suspensions with four bubble columns, 10-30 cm in diameter, over a range of particle concentrations from 0 to 200 kg m 3 and particle diameter of 50-200 pm, can be used to predict the ratio r of the ordinary kLo values in bubble columns. This can, in turn, be predicted for example by Equation 7.41, to the kLa values with suspensions. 

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