Critical Speed for Gas Dispersion

The following logical conclusions can be drawn for the case when PTU is used as the bottom impeller (data of Saravanan and Joshi 1996) (i) The strength of the flow generated by the lower impeller is oiD. Therefore, should decrease with increase in diameter of the lower impeller, (ii) increases with an increase in the interimpeller distance, Cj and (iii) does not have a significant effect on for 0.16 (C JT) 0.5 (for CJT= 0.17 and T= 1 m). 

These conclusions must be seen in the light of higher and optimum configuration recommended in Section 9.4.2. 

---

Fractional holdup of the gas phase at the critical speed for gas dispersion (—) Power inpnt per unit volnme (kW/m )... 

Henry s constant for component x (kmol/m )/(N/m ) reaction rate constant for CO2 hydrolysis (m /kmol/s) gas-liquid mass transfer coefficient (1/s) solid-liquid mass transfer coefficient (m/s) term defined by Equation (CS10.13) (-) molecular weight of component i (kg/kmol) speed of rotation of impeller (rps) critical speed for complete dispersion (rps)... 

There is absolutely no information in the literature on the critical speed for complete dispersion of the gas phase, N, in stirred tank reactors fitted with helical coils. The only work reported so far is that of Nikhade (2006) in the 0.57 m diameter stirred tank reactor referred to earlier. The correlations obtained for the inCTease in and AN were (Nikhade and Pangarkar 2006)... 

Critical speed for complete dispersion of the gas phase (rev/s) Critical speed of agitation for gas dispersion in the presence of helical coil (rev/s)... 

The critical speed for gas induction is solely decided by the ability of the rotor to generate a suction higher than the sum of static head and other pressure losses. The rotor design obviously plays an important role. Even when a second impeller is employed for gas dispersion/solid suspension, its action is limited to the role assigned to it in a region substantially away from the gas-inducing device. Therefore, the presence of the second impeller should not have any effect on the critical speed for gas induction in a multiple-impeller system. This obvious fact was experimentally proved by Saravanan and Joshi (1995). Consequently, Equations 9.23 or 9.28, which yield similar predictions, can be used to predict The effect of liquid viscosity can be accounted through Equation 9.24. 

For the upflow impeller, the Pq/Po vs. impeller speed curve is shown in Figure 11.44b. The flow generated by this impeller and the gas flow are complementary to each other, and thus formation of large cavities is prevented (Bujalski et al., 1988). Mhetras et al. (1994) have correlated the critical impeller speed for complete dispersion with the gas flow number as... 

Chapman et al. (1983c) found that in the case of the Rushton turbine, the value of critical impeller speed for solid suspension (N ) is higher than the impeller speed for complete dispersion of the gas phase except for some low density particles such... 

Apart from the critical impeller speed for solid suspension and efficient gas dispersion, flooding is also a very important phenomenon in three-phase systems. Flooding may take place at low impeller speed or high gassing rate. Under these conditions, the gas is dispersed just around the central shaft of the tank, whereas the solids are settled at the bottom. Flooding characteristics are not affected by particles. Furthermore, high-viscosity liquids are able to handle more gas before flooding than low-viscosity liquids. 

In order to achieve simultaneous suspension of solid particles and dispersion of gas, it is necessary to define the state when the gas phase is well dispersed. Nienow (1975) defined this to be coincident with the minimum in Power number, Ne, against the aeration number, 1VA, relationship (see Fig. 12 [Sicardi et al., 1981]). While Chapman et al. (1981) accept this definition, their study also showed that there is some critical particle density (relative to the liquid density) above which particle suspension governs the power necessary to achieve a well-mixed system and below which gas dispersion governs the power requirements. Thus, aeration at the critical stirrer speed for complete suspension of solid particles in nonaerated systems causes partial sedimentation of relatively heavy particles and aids suspension of relatively light particles. Furthermore, there may be a similar (but weaker) effect with particle size. Wiedmann et al. (1980), on the other hand, define the complete state of suspension to be the one where the maximum in the Ne-Ren diagram occurs for a constant gas Reynolds number. 

Note In the above table it has been confirmed in all the cases that operating speed is more than critical impeller speed for solid suspension and gas dispersion. 

The critical impeller speed for surface aeration (/Vcsa) can be identified using indirect sparging. A simple ki a versus N graph produces a sharp increase in ki a at iVcsA- Direct sparging makes this identification more difficult. Although gas may be entrained, additional gas dispersion does not occur until the impeller speed is increased by about 20% above the initial entrainment speed. Other factors... 

While a number of good correlations for the gas holdup in mechanically agitated reactors are available (Joshi et ai, 1982), the best correlations are those by Hughmark (1980) and Sridhar and Potter (1980), and they are recommended. The critical impeller speed, N0, required for effective gas-liquid dispersion, and the impeller speed at which gas above liquid is first entrapped, Nc, can be reasonably well calculated using Eqs. (2.4) and (2.5), respectively. 

---