Stirring, impeller

The dependence of the limiting current density on the rate of stirring was first established in 1904 by Nernst (N2) and Brunner (Blla). They interpreted this dependence using the stagnant layer concept first proposed by Noyes and Whitney. The thickness of this layer ( Nernst diffusion layer thickness ) was correlated simply with the speed of the stirring impeller or rotated electrode tip. 

Experiments which were essentially batch experiments were performed in reactors of three different sizes, which could be equipped with different kinds of stirring impellers. Special attention was devoted to the injection of the dye also, the dye itself was carefully selected in relation to the requirement of nonsurface activity. Some of their results are shown in Fig. 26. 

For a closed reactor (e.g., a batch reactor), the potential and kinetic energy terms in Equation (9.2.1) are not relevant. Additionally, = 0 for most cases (including the work input from the stirring impellers). Since most reactions carried out in closed reactors involve condensed phases, A (RE) is small relative to AU, and for this case is ... 

Stainless steel is the material of choice for reactors used in solution polymerization. Nickel and glass can also be employed, provided that the reactor is constructed to withstand pressures of 446 kPa (65 psi). Because of the wide range of viscosities encountered in solution polymerization, a variety of stirring impellers are employed. For lower viscosities (<1000 cP) an anchor-type agitator is used higher viscosities necessitate the use of a ribbon-type impeller, which sweeps virtually the entire volume of the reactor and, therefore, prevents polymer from remaining near the edges of the reactor. 

Digester employed by Shen et al. and impellers for simulation (a) Diagram of the anaerobic digester (1, digestion tank 2, blade 3, motor, 4. jacket for water bath 5, inlet 6, outlet 7, gas flow meter) (b) Types of stirring impellers for anaerobic digestion (c) Number of impellers [65],... 

FIG. 6-39 Typical stirred tank configurations, showing time-averaged flow patterns for axial flow and radial flow impellers. From Oldshue, Fluid Mixing Technology, McGraw-Hill, New Yo7 k, 1983.)... 

Blend time tb, the time required to achieve a specified maximum standard deviation of concentration after injection of a tracer into a stirred tank, is made dimensionless by multipfying by the impeller rotational speed ... 

Impeller Reynolds Number The presence or absence of turbulence in an impeller-stirred vessel can be correlated with an impeller Reynolds number defined... 

Suspensions of fine sohds may have pseudoplastic or plastic-flow properties. When they are in laminar flow in a stirred vessel, motion in remote parts of the vessel where shear rates are low may become negligible or cease completely. To compensate for this behavior of slurries, large-diameter impellers or paddles are used, with (D /Df) > 0.6, where Df is the tank diameter. In some cases, for example, with some anchors, > 0.95 Df. Two or more paddles may be used in deep tanks to avoid stagnant regions in slurries. 

Top-Entering Impellers For vessels less than 1.8 m (6 ft) in diameter, a clamp- or flange-mounted, angular, off-center fluidfoil impeller with no baffles should be the initial choice for meeting a wide range of process requirements (Fig. 18-14). The vessel straight-side-height-to-diameter ratio should be 0.75 to 1.5, and the volume of stirred liqiiid should not exceed 4 m (about 1000 gal). 

For circulation and mixingto obtain uniformity, the impeller should be located at one-third of the liquid depth above the vessel bottom unless rapidly settling material or a need to stir a nearly empty vessel requires a lower impeller location. 

Batch Stirred Tanks Tanks agitated by coaxial impellers (turbines, paddles, or propellers) are commonly used for batch dissolution of solids in liquids and may be used for leaching fine solids. Insofar as the controlhng rate in the mass transfer is the rate of transfer of mate-... 

Impeller-agitated tanks These can be operated as continuous leaching tanks, singly or in a series. If the sohds feed is a mixture of particles of different setthng velocities and if it is desirable that all particles reside in the leaching tank the same lengths of time, design of a continuous stirred leach tank is difficult and uncertain. 

The general charac teristics of the main types of reac tors—batch and continuous—are clear. Batch processes are suited to small production rates, to long reaction times, or to reactions where they may have superior selectivity, as in some polymerizations. They are conducted in tanks with stirring of the contents bv internal impellers, gas bubbles, or pumparound. Temperature controf is with internal surfaces or jackets, reflux condensers, or pumparound through an exchanger. 

Real reactors deviate more or less from these ideal behaviors. Deviations may be detected with re.sidence time distributions (RTD) obtained with the aid of tracer tests. In other cases a mechanism may be postulated and its parameters checked against test data. The commonest models are combinations of CSTRs and PFRs in series and/or parallel. Thus, a stirred tank may be assumed completely mixed in the vicinity of the impeller and in plug flow near the outlet. 

A basic stirred tank design is shown in Fig. 23-30. Height to diameter ratio is H/D = 2 to 3. Heat transfer may be provided through a jacket or internal coils. Baffles prevent movement of the mass as a whole. A draft tube enhances vertical circulation. The vapor space is about 20 percent of the total volume. A hollow shaft and impeller increase gas circulation (as in Fig. 23-31). A splasher can be attached to the shaft at the hquid surface to improve entrainment of gas. A variety of impellers is in use. The pitched propeller moves the liquid axially, the flat blade moves it radially, and inclined blades move it both axially and radially. The anchor and some other designs are suited to viscous hquids. 

FIG. 23-30a A basic stirred tank design, not to scale, showing a lower radial impeller and an upper axial impeller boused in a draft tube. Four equally spaced baffles are standard. H = beigbt of liquid level, Dj = tank diameter, d = impeller diameter. For radial impellers, 0.3 < d/Dt < 0.6. 

FIG. 23-30 Basic stirred tank design and selected lands of impellers, (h) Propeller, (c) Turbine, (d) Hollow, (e) Anchor,... 

Consider a stirred tank vessel having a Newtonian liquid of density p and viseosity p, is agitated by an impeller of diameter D, rotating at a rotational speed N. Let the tank diameter be D, the impeller width W, and the liquid depth H. The power P required for agitation of a single-phase liquid ean be expressed as ... 

Concluding, it is essential to represent complex, real-life flow situations by computationally tractable models that retain adequate details. As an example, a computational snapshot approach that simulates the flow in stirred reactors or other vessels for any arbitrary impeller has been developed [5]. This approach lets the engineer simulate the detailed fluid dynamics around the impeller blades with much less computations that would otherwise be required. Improvements in CFD technique are likely to encourage further work along these lines. 

Collisional break-up of erystals suspended in stirred vessels may oeeur as a result of eollision between erystal-crystal, erystal-impeller or erystal-vessel, and has been deseribed by many authors e.g. Ottens and de Jong (1973), Kuboi etal. (1984), Mazzarotta (1992). 

This medium was incubated in a 100 gallon stainless steel fermentor, at 24°C with sparged air being introduced at the rate of 50 C/min and with agitation by an impeller. After 66 hours of fermentation the beer was harvested. To 100 gal Ions of harvested beer was added 17 pounds of diatomite, and 35 pounds of activated carbon. The mixture was stirred well and then filtered, the cake was water-washed with 10 gal Ions of tap water, and then washed with 25 gallons of acetone followed by 30 gallons of 1 1 aqueous acetone. The acetone solutions of strepto-zotocin were pooled and dried in vacuo to 3.88 pounds. 

Johnson et al. (J4) investigated the hydrogenation of a-methylstyrene catalyzed by a palladium-alumina catalyst suspended in a stirred reactor. The experimental data have recently been reinterpreted in a paper by Polejes and Hougen (P4), in which the original treatment is extended to take account of variations in catalyst loading, variations in impeller type, and variations of gas-phase composition. Empirical correlations for liquid-side resistance to gas-liquid and liquid-solid mass transfer are presented. 

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