Radial disperser disk

To disperse gas, the gas is usually injected into the liquid from the bottom of the tank or near the impeller to enhance dispersion. Disk style turbines are found to be most convenient for gas dispersion because the disk disturbs the freely rising gas bubbles. The turbines with flat blades give radial flow and are very useful for gas dispersion where the gas is introduced just below the impeller at its axis and drawn up to the blades and chopped into fine bubbles. 

Another typical radial flow impeller is the disperser disk, also known as the toothed disk or dissolver disk. The EKATO MIZER disk belongs to this category, and consists... 

Several modifications of the design have appeared. Modifications of the rotors include perforation of the disk [Krishnara et al., Br Chem. Eng., 12, 719 (1967)] and radially supported arc plates [Nakamura and Hiratsuka, Kagaku Kogaku, 30, 1003 (1966)]. An asymmetric modification, with off-center rotors and arrangement of settling spaces for the liquids between dispersions (Misek, loc. cit.) is available in Europe. 

This dispersion of the gas passes through several stages depending on the gas feed rate to the underside of the impeller and the horsepower to the impeller, varying from inadequate dispersion at low flow to total gas bubble dispersion throughout the vessel. The open, without disk, radial flow type impeller is the preferred dispersing unit because it requires lower horsepower than the axial flow impeller. The impeller determines the bubble size and interfacial area. 

Accretion. Viscous stresses and gravitational torques within the disk transport angular momentum to the outer regions allowing disk matter to flow inward and accrete onto the star. Because the source of viscosity is still not well understood (see also Chapter 4), it is common to describe the viscosity via a dimensionless parameter a (Shakura Syunyaev 1973). Using this simplification, the viscous dispersal timescale, i.e. the time for the disk to disperse via accretion, becomes inversely proportional to a and increases linearly with the radial distance from the star (Hartmann et al. 1998). While the inner-disk material accretes onto the star, material further out moves in and replenishes the inner disk. Thus, the disk-dispersal timescale from accretion alone is set by the timescale to disperse the mass at the outer disk. 

For gas-dispersion applications, a radial discharge impeller, such as the straight-blade and disk style impellers in Fig. 12.1, should be used. A straight-blade impeller with a power number NP of 3.86 will be used for this design. Assume that the operating speed will be 100 r/min. 

The rotating disk electrode, described in Section 11.6, has the advantage that the fluid flow is well defined emd that the system is compact and simple to use. The rotation of the disk imposes a centrifugal flow that in turn causes a radially uniform flow toward the disk. If the reaction on the disk is mass-transfer controlled, the associated current density is imiform, which greatly simplifies the mathematical description. As discussed in sections 5.6.1 and 8.1.3, the current distribution below the mass-transfer-limited current is not uniform. The distribution of current and potential associated with the disk geometry has been demonstrated to cause a frequency dispersion in impedance results. The rotating disk is therefore ideally suited for experiments in which the disk rotation speed is modulated while im-der the mass-transfer limited condition. Such experiments yield another t)q)e of impedance known as the electrohydrodynamic impedance, discussed in Chapter 15. 

If the application requires high interfacial area (i.e., small drop diameters), a high-shear impeller, such as the Rushton or radial disk turbine (RDT), is a good choice (Fig. 1). Acceptable substitutes include the Scaba and Chemineer s BT-6 and CD-6 impellers, commonly used for gas-liquid dispersion. If moderate, yet gentle shear is required, such as for emulsion polymerization, the retreat-curve impeller is commonly chosen. When larger drops of a narrow size distribution are required, the loop impeller is a reasonable choice. Broad-blade paddles are also used. 

Recommended impeller diameter-to-tank diameter D/T) ratios, for liquid-liquid operations vary from 0.25 to 0.40 for radial disk turbines, from 0.4 to 0.6 for hydrofoils and propellers, and from 0.5 to 0.8 for retreat-curve, glassed-steel impellers. Vertical placement of the impeller depends on the vessel shape and the application. For dispersion by continuous addition of a dense phase fluid into a less dense fluid, the impeller should have a relatively small impeller clearance off the reactor bottom, C, with respect to the final height of the dispersion, H, i.e., the impeller should be placed low in the vessel (C H/A to H/S). For dispersion of light liquids, it is good practice to place a single impeller between 0.20 < C/H < 0.50. 

Multiple impellers are recommended it H/T 1.2 or if Ap > 0.15 kg/m. Assuming a less dense dispersed phase, the second or top impeller often is a hydrofoil impeller placed midway between the radial disk turbine and the surface of the liquid. This impeller produces high flow at low power, provides effective circulation, and complements the flow pattern produced... 

As previously discussed, placing a radial disk or Rush-ton turbine in the aqueous or lower phase, close to the interface, can be effective when making oil-in-water dispersions. A central interfacial vortex forms with the commencement of impeller motion. This directs a stream of the lighter oil phase to the impeller where it disperses. The volume of the oil layer decreases with continued dispersion until it is exhausted. Placing the turbine in the oil or upper phase, close to the interface, can result in water-in-oil dispersions, because a water-containing vortex forms, allowing water to be dispersed into the lighter oil phase. 

Of the stirrer types which set the liquid in a radial motion - or into a tangential flow in the case of high viscosities - only the turbine stirrer ) (so-called Rushton turbine , a disk 2d/3 in diameter supporting 6 blades each d/5 high and d/4 wide [474]) belongs to the high speed stirrers. It can be sensibly utilized only with low viscosity liquids and in baffled tanks. Its diameter ratio D/d is 3-5. The turbine stirrer causes high levels of shear and hence is well suited for dispersion processes. 

If the stirrer consists of a flat toothed disk, as e.g. the ZAR design [526] (Fig. 1.8), the liquid is accelerated radially in a small ring away from the disk and then rapidly decelerated. This produces high shear forces even in the absence of a stator ring and baffles. These two stirrer types are particularly suitable for emulsification and dispersion in a wide range of viscosities (e.g. in the production of pigment paints). 

Problem 3-37. Taylor Dispersion with Streamwise Variations of Mean Velocity. We consider steady, pressure-driven axisymmetric flow in the radial direction between two parallel disks that are separated by a distance 2h. We assume that the volumetric flow rate in the radial direction is fixed at a value Q and that the Reynolds number is small enough that the Navier-Stokes equations are dominated by the viscous and pressure-gradient terms. Finally, the flow is ID in the sense that u = [nr(r, z), 0, 0]. In this problem, we consider flow-induced dispersion of a dilute solute. We follow the precedent set by the classical analysis of Taylor for axial dispersion of a solute in flow through a tube by considering only the concentration profile averaged across the gap, ( ) = f h dz. 

The induced surface flow also gives rise to secondary bulk fluid motion, in the same way that bulk meridional vortices are generated in a fluid trapped between rotating and stationary disks in Batchelor flows [19], as depicted in Fig. 12. In this flow recirculation mode, particles dispersed in the flow are convected to the bottom by the bulk meridional recirculation. However, due to the inward radial velocity in the Ekman boundary layer (see Fig. 13), the particles begin to swirl in a helical-like manner toward the center of the base [19]. Although the flow recirculates back up a central spinal coluirm, the gravitational... 

Newman found that at disk electrodes current distribution is nonuniform in the radial direction (known as the primary [355] and secondary [356] current distributions), which leads to impedance dispersion [357]. Recently, Huang et al. [310,358, 359] continued these studies in more detail using a local impedance approach. Global admittance corresponds to the integration of the local admittances over the total disk area. Impedance can also be defined (and experimentally measured) locally as a function of the position on the electrode surface. In the case of the disk geometry, it changes radially from the disk center, r = 0, to the disk radius, r = ro- The authors distinguished two types of distribution of time constants ... 

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