MIXING. CHARACTERISTICS OF MIXTURES (1). Mixing is the dispersing of components, one throughout the other. Ideally, a mixing process begins with the components, grouped together in some container, but still separate as pure components.
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where (M) is the mixing index and K is a constant, and on integrating from t = 0 to t = t during which (M) goes from 0 to (M),
[(1 - (M))] = e-Ktor (M) = 1 - e-Kt
(Po) = K(Re)n(Fr)m
(Re) = (D2N/µ)
(Po) = (P/D5N3)
the Power number
(relating drag forces to inertial forces)
(Fr) = (DN2/g)
the Froude number
(relating inertial forces to those of gravity)
D the diameter of the propeller,
N the rotational frequency of the propeller (rev/sec),
r the density of the liquid,
m the viscosity of the liquid and
P the power consumed by the propeller.
(Po) = K(Re)n
Figure 2.1 Performance of propeller mixersAdapted from Rushton (1950)
EXAMPLE Blending vitamin concentrate into molassesVitamin concentrate is being blended into molasses and it has been found that satisfactory mixing rates can be obtained in a small tank 0.67 m diameter, height 0.75 m,with a propeller 0.33 m diameter rotating at 450 rev min-1. If a large-scale plant is to be designed which will require a tank 2 m diameter, what will be suitable values to choose for tank depth, propeller diameter and rotational speed, if it is desired to preserve the same mixing conditions as in the smaller plant? What would be the power requirement for the motor driving the propeller? Assume that the viscosity of molasses is 6.6 N s m-2 and its density is 1520 kg m-3.