Hydrodynamic Regimes in Two-Phase Gas-Liquid Stirred Tank Reactors

3 HYDRODYNAMIC REGIMES IN TWO-PHASE (GAS-LIQUID) STIRRED TANK REACTORS 

Chapman et al. (1983b) have discussed the dispersion of a gas in a stirred vessel. In general, when a gas is introduced into a stirred vessel, it is sucked into the low pressure region at the rear face of the impeller blade. The picture can be viewed from two different perspectives (1) constant impeller speed and (2) constant gas velocity. Considering the second view, at relatively low gas rates, a large cavity covers the entire rear face of the impeller blade (Fig. 7A.5a). As the speed of the impeller 

FIGURE 7A.5 Changes in cavity shape with increasing speed of agitation. (Reprodnced from Nienow and Wisdom 1974 with permission from Elsevier Ltd. 1974 Pnblished by Elsevier Ltd.) 

FIGURE 7A.6 Plot of versus (a) Constant impeller speed and (b) constant gas flow rate. (Reproduced from Chapman et al. 1983b with permission from The Institution of Chemical Engineers. 1983, The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.) 

The power drawn by the impeller varies with the progress of the vortices as indicated earlier, and it is generally expressed in terms of a power number, Ap, (PJpJ D ) in the presence of the gas as a function of the flow number, (QJND ). 

---

FIGURE 7A.8 Hydrodynamic regimes in two-phase (gas-liquid) stirred tank reactor. 1, flooding of the impeller 2, gas dispersion above the impeller 3, gas circulation above the impeller with marginal dispersion helow the impeller 4, gas circulation both above and below the impeller 5, recirculation of the gas resulting in the formation of secondary loops besides main discharge streams from the impeller. (Reproduced from Middleton 2000 with permission from Elsevier. 1997, Elsevier.)... 

---