Radial mixer

The instrument uses a radial mixer/observation cell shown in Figure 2. The reagents are introduced through alternating slits arranged around the observation tube and then travel through the tube at a rate of 2 m/s to 9 m/s. 

Figure 2. Top and end views of a radial mixer/observation cell used in pulse-accelerated-flow spectrometer. W, windows, A and B, reactants. Reproduced with permission from S. A. Jacobs, M. T. Nemeth, G.W. Kramer, T. Y. Ridley, and D. W. Margerum, Anal. Chem. 1984, 56, 1058. Copyright 1984 American Chemical Society.

Radial mixers flat-blade impellers and turbines... 

The closer the impeller is to the bottom of the tank, the more the induced velocity is suppressed. A proximity factor hJD is defined as the ratio of the gap of the impeller to the diameter. This principle is similar to those in the world of aeronautics. The ground effect is essentially the pressure exerted by a hehcopter or airplane. The closer it is to the ground, the more pronounced is the effect and, eventually, the induced drag is reduced when the machine flies very close to the ground. In the case of the mixers, the flow is restricted as the impeller gets closer to the bottom of the tank. This affects power consumption. A radial mixer tends to induce flow tangentially, whereas an axial machine tends to induce it vertically. Propeller and radial mixers tend to create different patterns of recirculation around their blades. 

For heat transfer, solid suspension, blending, or solid dissolving, bulk pumping is critical. A hydrofoil mixer would be the preferred machine. However, for gas-hquid contacting, molecular mixing, solid dispersion (reduction of agglomerates), an axihl flow, flat-blade machine or radial mixer are preferred options. 

Where columns with tower packings or sieve trays are used, the difference in densities between the liquid phases is the force causing flow, as well as dispersion of one phase into the other. However, mechanically agitated columns are equipped with a series of radial mixers driven by a vertical shaft, which produce the dispersion. Also, there is a hybrid device that uses radial mixers separated by wire mesh packing. 

Air is compressed to modest pressures, typically 100 to 200 kPa ( 15-30 psig) with either a centrifugal or radial compressor, and mixed with superheated vaporized butane. Static mixers are normally employed to ensure good mixing. Butane concentrations are often limited to less than 1.7 mol 1 to stay below the lower flammable limit of butane (144). Operation of the reactor at butane concentrations below the flammable limit does not eliminate the requirement for combustion venting, and consequendy most processes use mpture disks on both the inlet and exit reactor heads. A dow diagram of the Huntsman fixed-bed maleic anhydride process is shown in Figure 1. 

Axial-Flow Fluidfoil Impellers For vessel volumes of 4 to 200 m (1000 to 50,000 gal), a turbine mixer mounted coaxiaUy within the vessel with four or more baffles should be the initial choice. Here also the vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5. Four vertical baffles should be fastened perpendicularly to the vessel wall with a gap between baffle and wall equal to Df/24 and a radial baffle width equal to Df/12. 

Data are not currently available on the dispersion with the newer fluidfoil impellers, but they are often used in industrial mixer-settler systems to maintain dispersion when additional resonance time holdup is required, after an initial dispersion is made by a radial- or axial-flow turbine. 

Top entering mixers are heavy duty equipment. They are usually fixed to a rigid structure or tank mounting. Either radial flow or axial flow turbines may be used. Speeds vary from 50 to 100 rpm and usually require a double set of helical gearing or a single set of worm gears to achieve these low speeds. Therefore, they are more expensive than single reduction mixers. 

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

Vortex formation leads to a considerable drop in mixing efficiency and should be suppressed as much as possible in practical applications to increase the homogenizing effects of mixers. The preferable method of vortex suppression is to install vertical baffles at the walls of the mixing tank. These impede rotational flow without interfering with the radial or longitudinal flow. Figure 11 illustrates such a system. 

In either laminar or turbulent flow, rotational eireulation of a proeessed material around its own hydraulie eenter in eaeh ehannel of the mixer eauses radial mixing of the material. All proeessed material is eontinuously and eompletely intermixed, virmally eliminating radial gradients in temperature, veloeity, and material eomposition. 

Statie mixers, as reviewed in Chapter 7, eontain mixing elements enelosed in a tubular housing through whieh radial mixing is aehieved. They redistribute fluid aeross the flow ehannel and eonsequently rearrange temperature and eomposition distributions. They are often used to promote mixing in laminar flow systems thus having a pro-nouneed effeet on the RTD. 

The Kenics mixer has been shown to provide thorough radial mixing. This results in a reduction in radial gradients in velocity, composition, and temperature. Because the unit possesses nearly plug flow characteristics, both temperature and product quality controls are achieved. 

Self-aligning ball bearings supporting radial and thrust load where shaft and housing are subject to misalignment excellent rubber mixers, oil field drilling rigs, vertical pumps... 

The top entering mixer units are better for producing flow at constant power than the side entering units [29]. The radial flow impellers require more horsepower when compared to the marine impellers. 

The Kenics STATIC MIXER unit is a series of fixed, helical elements enclosed within a tubular housing. The fixed geometric design of the unit produces the following unique patterns of flow division and radial mixing simultaneously. 

Figure 7.27. Twisted-blade type of static mixer operating in the laminar flow regime (a) Distributive mixing mechanism showing, in principle, the reduction in striation thickness produced (f>) Radial mixing contribution...

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