Mixing time characteristics -♦ Homogenization

To gain information on minimum mixing work (P6 = min) necessary for homogenization, the mixing time characteristics, as well as the power... 

The impact, which the introduction of intermediate quantities can have on the relevance list, will be demonstrated in the following by one elegant example. Example 3 Mixing-Time Characteristics for Liquid Mixtures with Differences in Density and Viscosity. The mixing time 0 necessary to achieve a molecular homogeneity of a liquid mixture—normally measured by decolorizatiorr methods—depends, in material systems without differences in density and viscosity, on only four parameters stirrer diameter d, density p, kinematic viscosity v, rotational speed ti ... 

Example 5.2 Mixing-Time Characteristics of a Stirrer. Mixing time 0 is the time necessary to completely homogenize an admixture with the liquid contents of the vessel. It can easily be determined visually by a decolorization reaction (neutralization, redox reaction in the presence of a color indicator). The relevance list of this task consists of the target quantity (mixing time 0) and of the same parameters as in the case of mixing power—on condition that (contrary to Example 3) both liquids have similar physical properties) ... 

In order to design and dimension stirrers for the homogenization of liquid mixtures - and this is by far the most common task when it comes to stirring - it is vital to know the power characteristic and the mixing time characteristic of the type of stirrer in question. If this information is available for various types of stirrers, it is possible to determine both the best type of stirrer for the given mixing task and the optimum operating conditions for this particular type. 

It should be emphasized that in this diagram only those stirrers are exemplary represented, whose geometric conditions are given in Fig. 2.1 and whose power and mixing time characteristics are presented in Fig. 2.2 and Fig. 3.5. To ascertain which stirrer type is actually the most suitable for a particular homogenization operation, a whole range of stirrer types must be evaluated and documented while changing their parameters (number of stirrer elements, pitch of the blades or stirrer vanes) and particularly their installation conditions (H/D, D/d, h/d etc.), which vastly exceeds the scope of this book. [0.19] is recommended literature in this connection. 

Opara stated, that it was not to be expected that a correlation between the mixing time and and hence with Rseg, could be realized, because the concept of Metzner and Otto was derived from work on the power characteristic. There was determined from the shear rates, which occured at the largest rate differences, whereas the regions close to the walls, in which the smallest rate differences were to be found, were determining for homogenization times. 

The decision about which stirrer type is the most suitable under particular material and geometric conditions for the homogenization of a liquid mixture without density and viscosity differences, can only be answered by coupling the mixing time and power characteristics. Optimum conditions are then realized, under which this process can be carried out with the minimum mixing work PO [Ws = J]. 

In liquids with high viscosities, on the other hand, the gas bubbles are larger and furthermore the liquid circulation is reduced with the result that the mixing time is longer. For the homogenization characteristics which apply under such conditions, see [613], Other papers over this subject are [324, 483]. 

The parameters which determine the characteristic mixing time are the linear flow rate V, device diameter D, diffuser opening angle y, as well as the kinematic viscosity V for micromixing processes. Practically, a single and available way of influencing the reaction mixture homogeneity, in a diffuser-confusor reactor, is the variation of its diameter and linear flow rate of reactants. 

In both cases the first reaction (Equations 4.33a and 4.34a) is instantaneous (e.g., a neutrahzation) while the second reaction has characteristic reaction time comparable to the characteristic mixing time. The reactant A2 is added in an overall stoichiometric defect to the stream of Aj in consecutive reactions or to the stream of solution containing A and A in parallel reactions. If the aggregates of A2 are rapidly mixed forming homogenous solution with a rate much faster than the rate of the second reaction t. ), species A2 will be almost totally consumed by... 

---