Agitation mixing time

Figure 7-16. Mixing times in agitated vesseis. Dashed iines represent unbaffied tanks soiid iines represents a baffied tank. (Source McCabe, N. L., et a ., Unit Operations of Chemicai Engineering, 4th ed., McGraw-Hill Book Company, New York, 1985.)...

Carreav. P, J.. Patterson, I., and Yap, C. Y. Can J. Chem. Eng. 54 (1976) 135. Mixing of viscoelastic fluids with helical ribbon agitators 1. Mixing time and flow pattern. 

Mixing times in mechanically agitated vessels typically range from a few seconds in laboratory glassware to a few minutes in large industrial reactors. The classic correlation by Norwood and Metzner for turbine impellers in baffled vessels can be used for order of magnitude estimates of... 

The net flow though the reactor will be small compared with the circulating flow caused by the agitator. The existence of the throughput has little influence on the mixing time so that mixing time correlations for batch vessels can be used for CSTRs as well. 

A real continuous-flow stirred tank will approximate a perfectly mixed CSTR provided that tmix h/i and tmix i. Mixing time correlations are developed using batch vessels, but they can be applied to flow vessels provided the ratio of throughput to circulatory flow is small. This idea is explored in Section 4.5.3 where a recycle loop reactor is used as a model of an internally agitated vessel. 

A 100-gal pilot-plant reactor is agitated with a six-blade pitched turbine of 6 in diameter that consumes 0.35 kW at 300 rpm. Experiments with acid-base titrations showed that the mixing time in the vessel is 2 min. Scale up to a 1000-gal vessel with the same mixing time is desired. 

Gray (1963) investigated the mixing times of helical ribbon agitators and found the following equation to hold ... 

Figure 1 Various dimensionless parameters [dimensionless velocity, v = v/ND pumping number, Nq = Q/ND power number, Np=[Pgc/pN D ) and dimensionless mixing time, f = as a function of the Reynolds number for the analysis of turbine-agitator systems. Source Adapted from Ref. 22.

Quinn and Paton (16) observed that the intensity and duration of mixing influenced the water uptake of the materials investigated, and that the measurement was not reproducible unless the time of agitation was constant (2 min was chosen). One product, sodium caseinate, required longer than a 2-min mixing time. These investigators state that the proposed technique measures water absorbed and retained under specific conditions, which may or may not apply to particular manufacturing applications. They do, however, state that this method with the application of limited... 

Mixing between mutual-soluble liquids is usually carried out by the action of certain external forces, e.g. agitation. Mixing occurs at various scales and the segregation scale varies with time. An idealized model of the mixing between a black liquid and a white liquid is illustrated in Fig. It). I. 

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