Critical Speed for Solid Suspension

Similar to there is absolutely no information in the literature on the critical speed for suspension of solids, in stirred tank reactors fitted with cooling coils. Nikhade et al. (2005) have provided Equations 7A.32,7A.33, and 7A.34 for AA, for the three impellers referred to earlier. is defined as the difference between 

The dependence of AN on the various operating parameters is different for the three types of impellers studied, which has been rationally explained by Nikhade et al. (2005). It has been shown in section 7A.8.2 that the presence of an internal coil leads to relatively very high power requirement for gas dispersion. This argument is equally valid for the case of solid suspension. Overall, an internal coil is counter productive particularly when the coil area required is high due to a substantial heat load. 

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 

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The critical speed for solid suspension in the presence of a coil was obtained by an equation similar to Equation 7A.31 AN was... 

Both these correlations require a knowledge of the minimum speed for just suspension of the solids. Details of estimation of the critical speed for solid suspension are discussed earlier in Section 7B.11 as well as Chapter 7A. 

Sections 7A.7.1.3 and 7B.11.2. In the present case, has been found to be higher than However, the difference in the critical speeds for solid suspension in two- and three-phase systems, is practically zero. This behavior has been explained in Sections 7A.7.1.3 and 7B.11.2. The difference is further lower for two reasons (i) the low density difference (Ap 30-50 kg/m ) between the microcarrier support and the broth and (ii) relatively very low vvm of 0.008. As per conventional design practice, the higher among and (0.43 rev/s) is selected. 

Critical speed for solid suspension in solid-liquid system (rev/s) Speed of rotation of spin filter (rev/s)... 

Critical speed for solid suspension in a three-phase stirred reactor... 

It is well known that the critical impeller speed for solid suspensions is higher in the presence of a gas, depending mainly on the superficial gas velocity (Rewatkar et al., 1991). This is because of a decrease in the impeller power draw due to the formation of ventilated cavities behind the impeller blades on gassing. For example, for Rushton turbines, /)T//)a - 2-3.3 ... 

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. 

The critical stirrer speed for solid suspension increases slightly with increasing aeration rate, sohd loading, and non-Newtonian flow behavior [14]. 

The introduction of the gas phase leads to the formation of cavities behind the impeller blades. As a result, the power number and the impeller pumping capacity are reduced. Hence, the impeller speed has to be increased to compensate for the loss of pumping capacity. Consequently, the critical impeller speed for solid suspension was always higher in the presence of the gas phase (N g)... 

Critical speeds of agitator Critical impeller speed for solid suspension The speed shonld be adequate to keep the particles suspended. The critical speed for snspension in the presence of gas, Nj q, is calcnlated from the following correlation (Rewatkar and Joshi 1991d) ... 

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. 

Murthy, B.N., Ghadge, R.S., and Joshi, J.B. (2007), CFD simulations of gas-liquid-solid stirred reactor Prediction of critical impeller speed for solid suspension, Chemical Engineering Science, 62(24) 7184-7195. 

The solid phase (catalyst/reactant) shows different behavior in different mnlti-phase reactors. In sparged reactors, there is an exponential decay of the solid concentration along the vertical axis. For stirred multiphase reactors operated at the critical speed for just suspension of the solid, N, there is a substantial variation in axial solid concentration. The speed required for achieving uniform solid concentration and the corresponding power input are relatively very high (Nienow 1969, 2000 Shaw 1992). Hence, most stirred multiphase reactors operate at rather than For venturi loop reactors, Bhutada and Pangarkar (1989) have shown that above a certain power input at which the three-phase jet reaches the reactor bottom, the solid concentration is uniform both axially and radially. In this respect, venturi loop reactor is a definitely better option (Chapter 8). 

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... 

Critical speed for just suspension solid (2 phase), Ns 0.338 0.193... 

Difference in the critical speed for just suspension of solid in three-phase (gas-liquid-solid) and two-phase (solid-liquid) system (rev/s) Difference in the minimum speed for just suspension of the solid in three-phase system in the presence and absence of helical coil (rev/s) Power input (W, kW)... 

Aravinth S, Gangadhar Rao P, Murugesan T. (1996) Critical impeller speed for solid suspension in turbine agitated contactors. Bioprocess Eng., 14 97-99. 

Dutta NN, Pangarkar VG. (1995a) Critical impeller speed for solid suspension in multi-impeUer agitated contactors. Solid-liquid system. Chem. Eng. Commun., 137 135-146. 

Rao KSMS, Rewatkar VB, Joshi JB. (1988) Critical impeller speed for solid suspension in mechanically agitated contactors. AlCHEJ, 34 1332-1340. 

Lima OA, Deglon DA, Leal FUho LS. (2009) A comparison of the critical impeller speed for solids suspension in a hench-scale and a pilot-scale mechanical flotation cell. Miner. Eng., 22 1147-1153. 

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