Gas Holdup in Stirred Tank Reactors

Yawalkar AA, Beenackers AACM, Pangarkar VG. (2002a) Gas holdup in stirred tank reactors. Can. J. Chem. Eng., 80 158-166. 

A.5 GAS HOLDUP IN STIRRED TANK REACTORS 7A.5.1 Some Basic Considerations... 

TABLE 7A.1 Correlations for Gas Holdup in Stirred Tank Reactors Available in the Literature... 

There is absolutely no reported study on k a in stirred tank reactors containing internal coils. The work of Nikhade (2006) indicates that the gas holdup in such stirred tank reactors increases due to the presence of the coils. This observation implies that the effective gas-liquid interfacial area is also correspondingly higher. However, because of the dampening of the turbulence caused by the coil, the true... 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Figure 4.2 Gas holdup in a 21 -cm-diameter stirred-tank reactor obtained using X-ray computed tomography imaging.

X-rays, also work on larger tanks because the y-rays are strong enough to pass through substantial thicknesses of metals, overcoming the reactor wall thickness. For example, Veera et al. (2001) used a Cs source to measure gas holdup in a three-phase, 4.9-m-diameter stirred-tank reactor equipped with two impellers. 

Nikhade (2006) has determined the gas holdup in 0.57 m diameter stirred tank reactors equipped with different impellers and internal coils. The investigation covered three impellers (PTD, DT, and PTU) and three internal coils with different surface areas. He could successfully obtain a unique correlation for the gas holdup data for three different impellers and coils using the approach proposed by Yawalkar et al. (2002a). The correlation obtained was... 

When carrying out a gas-liquid reaction, the gas may be dispersed in the liquid, as in bubble-column reactors or stirred tanks, or the gas phase may be continuous, as in spray contactors or trickle-bed reactors. The fundamental kinetics are independent of the reactor type, but the reaction rate per unit volume and the selectivity may differ because of differences in surface area, mass transfer coefficient, and extent of mixing. In the following sections, gas holdup and mass transfer correlations and other performance data for gas liquid reactors are reviewed and some problems of scaleup are discussed. 

This type of reactor consists of two parts, that are essentially different. In the venturi tube, a very effective gas/liquid dispersion is obtained, with unusually Itigh values of bubble holdup, specific surface area and volumetric mass transfer coefficients. However, its volume is small. In the tank itself, considerable coalescence will occur, but the volume is of course much larger. Both zones are effective with respect to mass transfer. In recent papers by Dirix and Van der Wiele (1990) and Cramers et al. (1992), the feasibility of this reactor type, which is still relatively new, is demonstrated convincingly. It follows from these studies, that for large gas/liquid reactors, that require a large surface area despite a relatively low gas flow rate, the venturi-loop reactor compares favourably with a stirred tank. However, for a reliable scale-up of this type of reactor a lot of experimental work has to be done, preferably under realistic conditions. 

---