Stirring operations

If the liquid component predominates in the mixture of substances to be mixed, the mixing operation is named stirring and a stirrer (an impeller) is used as the mixing device. The following five stirring operations can be distinguished [611]  

The term homogenization is used, if a uniform liquid phase has to be realized, e.g. a molecularly homogeneous mixture of several miscible liquids or equalization of concentration and temperature differences during a chemical reaction in the liquid phase. (The same term is used in the food industry for a completely different operation, namely for L/L (liquid/liquid) dispersion under extreme shear conditions e.g. the homogenization of milk). 

Intensification of heat transfer in a stirred tank can represent, especially in case of viscous liquids, an important stirring operation, particularly if a strongly exothermic reaction takes place (e.g. block polymerization). In such cases the stirring operation consists of reducing the thickness of the liquid boundary layer on the tank wall and realizing liquid transport to and from the heat exchanger surface. 

If particulate matter has to be dissolved in a liquid or if a chemical reaction catalyzed by a solid is involved, the particles must be suspended from the vessel bottom, so that the total surface can participate in the process. In continuous processes a stochastically homogeneous distribution of the solid in the bulk of the liquid is required, so that the solid particles can be transported with the liquid from stage to stage (for example in a cascade crystallization process). In this intensive suspension process, the solid is, as a rule, subjected to high mechanical stress, which can result in its attrition. 

In the case of dispersion in a L/L or L/G (liquid/gas) systems, one fluid phase is distributed in the other in the form of fine droplets or gas bubbles to accelerate 

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This stirrer is not dependable for stirring operations lasting several hours the rubber tubing may stick to the shaft sind may also be attsu ked by the orgsinic vapours causing it to swell and allow the escape of vapours. 

Stirring vessels. Upon the examination of different stirring operations it was indeed found that the intensively formulated process parameter P/V represented the pertinent scale-up criterion only if the stirring power has to be dissipated in the volume as evenly as possible (micro-mixing, isotropic turbulence). Examples of this are the dispersion of a gas in a liquid or the dispersion of immiscible liquids s. [22]. 

In the most important stirring operation - the homogenization of liquid mixtures - the convective transport of liquid balls (macro-mixing) is of predominant importance. Thus, this process depends to a large degree on space geometry and type of stirrer. It is influenced by the extensive parameters such as stirrer speed, n, and stirrer diameter, d. Here, the similarity with respect to fluid dynamics is given by Re = n d2 p/p= idem. 

Micro-mixing is important in such stirring operations where the result depends upon the size of the smallest eddies, for example dispersion processes in liquid/liq-uid systems and shear damage to microorganisms. Therefore, it is not surprising that in such tasks it is the volume related power, P/V, that counts it is an intensively formulated process quantity. 

At this point in time, leverage flour, baking soda and salt into a bowl and aggregate. Equalize with prior mixture and develop intense and continuous liaison among inputs until well-coordinated. Associate key chocolate and nut subsystems and execute stirring operations. 

Often different stirring operations must be carried out simultaneosly, an example being solids-catalyzed hydrogenation, in which the stirrer disperses the gas (hydrogen) in the liquid phase, swirls up the catalyst particles (e.g. Raney nickel) from the bottom of the reactor and intensifies the removal of reaction heat. In such cases the stirring conditions are determined by that stirring operation which is the bottleneck in the process. 

The stirring operations discussed in the introduction can obviously not be performed with a single type of stirrer. There are many types of stirrers appropriate for particular stirring operations and particular material systems. In this section only those stirrer types will be discussed which are widely used in the chemical industry and for which reliable design guidelines exist. The dimensions of stirrer types have also been standardized to a large extent [161]. 

In Fig. 1.4 the stirrer types are arranged according to the predominant flow pattern they produce, as well as to the range of viscosities over which they can be effectively used. 90% of all stirring operations can be carried out with these standard stirrer types. The flow patterns obtained with typical radially and axially conveying stirrers are shown in Fig. 1,5. 

The first of the five stirring operations listed in Section 1.1 is homogenization. The term homogenization concerns the equalization of concentration and temperature differences, which is by far the most important and the most frequently carried out stirring operation. Its objective is the production of a uniform molecularly homogeneous mixture (solution). 

The target quantity of this stirring operation, the mixing time 6 or 0m - in the following for the sake of simplicity shortened to 0 - depends, for a material system without density and viscosity differences and for a particular stirrer type under given geometric conditions, only upon the characteristic measurement of length (stirrer diameter d), both material parameters (density p and kinematic viscosity v) and the stirrer speed n ... 

There are stirring operations in chemical engineering in which a large gas throughput is involved (e.g, blowing out of volatile compounds from a liquid), for which a minimization of stirrer power input is desirable. The condition for which P/q = min can be obtained from the process characteristics Q = /j(Fr) and Ne = f2(Fr) (Fig. 4.31 and 4.32), if the Ne, Q, Fr numbers are combined in a way, that a dimensionless expression for P/q arises ... 

This stirring operation is exceedingly frequent according to a well known German stirrer manufacturer it was in the period 1976-80, at 65%, the most frequent of all stirring operations, closely followed by the stirring operation homogenization [277]. 

Final Discussion from the Viewpoint of the Dimensional Analysis No industrial stirring operation in Germany in connection with the scale-up criterion has caused more stir and none was more controversially discussed than the suspension of solid particles. It is noteworthy that the authors without exception ascribed to dimensional analysis the blame for the inappropriate or deficient, in the sense of scale-up criteria, at any rate inadequate pi space. 

The enhancement of mass transfer in the solid/liquid system is a frequent stirring operation. It should be remembered, that many salts must be dissolved in the liquid, to prepare a salt solution or to initiate a chemical reaction. In order that the dissolution process proceeds rapidly, the whole surface of the solid particles must be wetted as completely as possible by the liquid and the liquid flow should be turbulent, so that the boundary layer on the liquid side is small and the transfer of the dissolved material to the bulk of the liquid proceeds rapidly. 

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