Mechanically stirred tanks

Stirred (agitated) tanks, which are widely used as bioreactors (especially as fer-mentors), are vertical cylindrical vessels equipped with a mechanical stirrer (agitator) or stirrers that rotate around the axis of the tank. 

The objectives of liquid mixing in stirred tanks are to (i) make the liquid concentration as uniform as possible (ii) suspend the particles or cells in the liquid (iii) disperse the liquid droplets in another immiscible liquid, as in the case of a liquid-liquid extractor (iv) disperse gas as bubbles in a liquid in the case of aerated (gassed) stirred tanks and (v) transfer heat from or to a liquid in the tank, through the tank wall, or to the wall of coiled tube installed in the tank. 

When this type of impeller is used, typically four vertical baffle plates, each one-tenth of the tank diameter in width and the total liquid depth in length, are fixed perpendicular to the tank wall so as to prevent any circular flow of liquid and the formation of a concave vortex at the free liquid surface. 

With this type of impeller in operation, liquid is sucked to the impeller center from below and above, and then driven radially towards the tank wall, along which it is deflected upwards and downwards. It then returns to the central region of the impeller. Consequently, this type of impeller is referred to as a radial flow impeller. If the ratio of the liquid depth to the tank diameter is 2 or more, then multiple impellers fixed to a common rotating shaft are often used. 

Details of heat transfer in stirred tanks are provided in Sections 5.4.3 and 12.3. 

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The present research was focused on the study of acetaldehyde oxidation rising air with aqueous mangan acetate catalyst in mechanically stirred tank reactor. 

A continuous, mechanically stirred tank reactor with a sparger located below the agitator or... 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

Many correlations allow estimation of the gas-liquid volumetric mass transfer coefficient kLa in mechanically stirred tank reactors. The following intends not to provide a comprehensive review but rather a critical evaluation of selected correlations adapted to hydrogenations [Eqs. (40) to (43)] [25, 51-53]. 

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

Chevalier J.L., Effective Viscosity of non-Newtonian fluids in a mechanically stirred tank, Chem. Eng. Commun. 21 (1983), p. 29-36... 

As discussed above, in the production of fine chemicals the mechanically stirred batch reactor is most popular. This is not surprising. In the laboratory reactions are usually conducted in mechanically stirred tank reactors, a natural (but not always correct) choice is to use a larger mechanically stirred tank reactor on the industrial scale. This illustrates the traditional method of scale-up. Furthermore, a tendency to duplicate known equipment usually wins when considering the choice of the reactor type for a particular process. 

The mechanically stirred tank reactor is most commonly used in batch pi o-cesses. It is the workhorse of the fine-chemicals industry. The catalyst particles are suspended in the liquid, which is almost perfectly mixed by a mechanical agitator. It is also possible to apply a hollow stirrer that encompasses two functions,... 

Each adsorption stage consists of a mechanically stirred tank 11 ft (3.3 m) in diameter by 10 ft (3.0 m) deep, from which the mixture of resin and pulp flows to an airlift, which in turn discharges to a set of screens SI. These separate the coarser resin particles from the finer slimes. The resin drops by gravity into the next lower numbered tank, toward the feed end of the cascade, and the slurry flows to the next higher numbered tank, toward the tailings end. This counterflow is made possible by the absence of particles coarser than 325 mesh in the pulp and the absence of particles finer than 50 mesh in the resin. Residence time in each adsorbing stage is 18 to 20 min. 

Scargiali, R, D Orazio, A., Grisafi, R, and Brucato, A. (2007), Modelling and simulation of gas-liquid hydrodynamics in mechanically stirred tanks, Chemical Engineering Research Design, 85(5) 637-646. 

Lane GL, Schwarz MP, Evans GM. (2002) Predicting gas-liquid flow in a mechanically stirred tank. Appl. Math. Model., 26 223-235. 

Yung CN, Wong CW, Chang CL. (1979) Gas hold-up and aerated power consumption in mechanically stirred tanks. Can. J. Chem. Eng., 57(6) 672-676. 

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