Gas inducing impellers

The size of the bubbles produced in the reactor and the gas volume fraction will depend on the agitation conditions, and the rate at which fresh hydrogen is fed to the impeller, as shown in Fig. 4.20. (Some hydrogenation reactors use gas-inducing impellers to recirculate gas from the head-space above the liquid while others use an external compressor.) For the purposes of the present example, the typical values, db = 0.8 mm and eg m 0.20, will be taken. Also dependent to some extent on the agitation conditions are the values of the transfer coefficients kL and k, although these will depend mainly on the physical properties of the system such as the viscosity of the liquid and the diffusivity of the dissolved gas. The values taken here will be kL = 1.23 x 10 5 m/s and k, = 0.54x 10"3 m/s. 

When the gas-inducing impeller rotates in a fluid, the sudden acceleration of the fluid elements causes reduction in the static pressure. In all the designs, the reduced pressure area is connected to the gas space, and the gas is induced if the reduction in pressure is sufficient to overcome all the resistance in the path of the gas. 

Because the gas is aspirated through a hollow stirrer in a gas-inducing impeller, the gas must overcome the hydrostatic pressure of the liquid above the stirrer, H pg. The liquid head over the stirrer, H = (Hh — HJ, therefore has a significant influence on the gas flowrate q. A uniform aeration of... 

R. Gas-inducing impeller with dense solids 1c nr 2 ShGL = = (1.26 x 10- ) N XX1 NRe = pmil/p,Nsc = W(pD), NWe = pbPdl/c [E] Hydrogenation with Raney-type nickel catalyst in stirred autoclave. Used varying T,p, solvents. dst = stirrer diameter. [78]... 

FIGURE 10.1 Types of stirred tank reactor, (a) Multiphase stirred reactor. I impeller, 2 baffles, 3 cooling coils, 4 gas sparger, (b) Stirred reactor with gas-inducing impeller (dead-end type), (c) Stirred reactor with helical ribbon impeller (used with or without a draft tube). 

Joshi J.B., Modifications to the design of gas inducing impeller, Chem. Engng. Commun. 5 (1980), p. 109-114... 

Figure CS11.2a shows a comparison of fractional gas holdup under sparging conditions, with and without the use of a gas-inducing impeller. In the absence of a gas-inducing impeller, such a system behaves like a conventional mechanically agitated contactor (MAC). The comparison is made in terms of gas holdup as a function of power consumption per unit volume for different superficial velocities of sparged gas (uq = 0, 6,18, and 29 mm/s). It can be seen that the fractional...

For this value of superficial gas velocity, the gas induction rate (Qg), fractional gas holdup (Sg), and power consumption (Pg) are calcnlated, both in the presence of a gas-inducing impeller and in its absence. Hydrodynamic characteristics and the mass transfer coefficient are estimated using the correlations of Patwardhan and Joshi (1998). 

Forrester SE, Rielly CD, Carpenter KJ. (1998) Gas inducing impeller design and performance characteristics. Chem. Eng. Sci., 53 603-615. 

Gas induction rate is sensitive to the liquid level (submergence) above the main gas-inducing impeller (Conway et al. 2002). 

In view of (2), the gas-inducing impeller has to be necessarily located closer to the top liquid level (Section 3.4.2.4.3 and Figure 3.3 Praxair liquid oxidation reactor). For three-phase reactions involving a solid catalyst, a multi-impeller system is required. This is because a single impeller used to induce the gas is not capable of suspending the solid settled at the bottom. Further, because of the poor gas dispersion by the top gas-inducing impeller, the second lower impeller has to also perform this duty of gas dispersion in the lower part of the reactor. 

FIGURE 9.2 Schematic of 1-1 type gas-inducing impeller. (Reproduced from Forrester and Rielly, 1994, with permission from Elsevier Copyright 1995 Published by Elsevier Ltd. http //www.joumals.elsevier.com/chemical-engineering-science/)... 

Before discussing the hydrodynamics and mass transfer aspects of gas-inducing impellers in this category, it is desirable to briefly describe these two variants. 

Impellers The important components of 1-2 type gas-inducing impeller are (i) hollow shaft, (ii) impeller, (iii) orifices for gas entry, and (iv) a stator/ diffuser (Fig. 9.3). The impeller is mounted on the hollow shaft. 

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... 

FIGURE 9.3 Schematic of 1-2 type gas-inducing impeller. (Adapted from Koen and Pingaud (1977) with permission from BHRA, The Fluid Engineering Centre, Cranefield, England, Copyright BHRA 1977 and reproduced liom Patwardhan and Joshi (1999) with permission from American Chemical Society, Copyright 1999, American Chemical Society.)... 

FIGURE 9.4 Schematic of 2-2 type gas-inducing impeller with impeller details. (Reproduced from Sawant et al. (1981) with permission from Elsevier. Copyright 1981. Published by Elsevier B.V. http //www.journals.elsevier.com/chemical-engineering-joumal/.)... 

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