Rate of Gas Induction

Joshi (1980) modified the pipe impeller design. Holes were drilled near the tip of the impeller and on the hub (Fig. 9b). In this design, the increase in the liquid flow increases the rate of gas induction by 30-60% at practically the same power consumption. Martin (1972) used a flattened cylindrical tube so as to simulate the aerofil design (Fig. 9c). In flotation applications, the Denver and Wamco type of self-inducing impellers has been widely used for several decades. Zundelevich (1979) has modified the Denver design so as to increase the rate of gas induction by about 100% (Fig. 9d). 

For an air-water system with a liquid volume of 0.05 m3 and a height of 0.4 m, Joshi et al. (1982) have compared the gas induction rate for a variety of impellers. Their analysis indicates that for a power consumption per unit liquid volume greater than approximately 2 KW/m3, the turbo-aerator gives the maximum rate of gas induction for a given power consumption. The procedures for the calculations of gas induction rate for pipe, Denver, and turbo impellers are given by Zlokarnik (1966), Zundelevich (1979), and Sawant and Joshi (1979), respectively. 

In a gas-inducing reactor, both gas and liquid phases are generally considered to be completely backmixed. The use of Eqs. (2.39) and (2.40) for the calculations of the critical speed for gas induction is recommended. The rate of gas induction can be expressed by a dimensionless relation NA = /(FrdJH, Ga, dT/du HJdj). The most important parameters are Fr dJH and dT/di. For a given power input per unit volume, the turbo aerator appears... 

Multiorifice nozzle yields higher value of rate of gas induction as compared to a single-orifice nozzle having the same area as the combined hole area for a given multiorifice nozzle. Pitch of 2xDjj found to yield the highest values of gas induction rate. 

Ejfect of a Swirl Device in the Nozzle As discussed earlier in Section 8.6.2.1, the presence of a swirl device in the primary nozzle creates a disturbance in the jet. This results in roughness on the nozzle surface and consequently higher rates of gas induction. It would therefore be expected that there may be concomitant changes in mass transfer parameters. Cramers et al. (2001) and Baier (2001) have shown that a swirl piece acm-ally causes a decrease in k a. For an upflow system, Havelka et al. (2(X)0) have also made similar observations. Cramers et al. (2001) found that for nozzles with a swirl piece, k a oc (D / ) . On the other hand, for conventional nozzles (without... 

MPa yields smooth jets for properly designed primary nozzles (Bhutada 1989 Dirix and van der Wiele 1990). Lower nozzle velocity results in lower rate of gas induction. The combined operating conditions should be such that the rate of gas induction is sufficiently in excess of (i) the stoichiometric requirement and (ii) minimum flow rate required to keep the catalyst in suspension (Eq. 8.25). 

Prediction of the Rate of Gas Induction The approach used is a typical phenomenological approach involving the driving force and a resistance. The driving force for gas induction is the pressure differential between the orifice pressure and that in the headspace (Rielly et al. 1992) ... 

Knowing APp and AP, Equations 9.11-9.15 can be used to calculate the rate of gas induction. RieUy et al. (1992) found that the predicted rates were higher than the experimental rates. This was attributed to Equation 9.7 that was employed to calculate Cpg. The pressure driving force thus calculated is applicable only at the orifice. In practice, and AP deaease as the evolving bubble expands and departs from the orifice. Information on the variation of bubble radius with time along with the fluid velocity in the vicinity of the orifice is required to predict the variation in the pressure driving force. 

The rate of gas induction is decided by the type of impeller used, the design of the stator, the speed of the impeller, and the static head above the gas ports. Analogous to the venturi loop reactor (Sections 8.6 and 8.7), the rate of gas induction cannot be varied independent of the aforementioned design and operating features. However, in the present case of gas-inducing impeller, in the event that the induced... 

A Prediction of the Rate of Gas Induction In this case also. White and de Villiers (1977) used an empirical approach based on dimensional analysis. The flow number, was given as... 

Total pressure on the orifice, sum of hydrostatic and pressure head (m) Time-averaged rate of gas induction (mVs)... 

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