The Mixing of Liquids

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Chapter: Pharmaceutical Engineering: Mixing

In the mixing of miscible liquids, any practical scale of scrutiny embraces a very large number of particles.


In the mixing of miscible liquids, any practical scale of scrutiny embraces a very large number of particles. If, therefore, a mixture of liquids is randomized by agitation, for all practical purposes, it can be regarded as uniform. Miscible liquids are classified as positive mixtures and would, given time, mix com-pletely without external help. The time required for mixing is reduced by agi-tation during which the scale of segregation is reduced, allowing a fast decay in the intensity of segregation by natural diffusion. In general, no great problems are encountered unless the scale of the operation is very large.

Miscible liquids are most commonly mixed by impellers rotating in tanks. These are classified as

1. paddles,

2. propellers, and

3. turbines.

In conjunction with the design of the containing vessel, these provide

1. a. a region of intense shear in the vicinity of the impeller with the induction of high-velocity gradients and turbulence within the liquid and

2. the projection of the disturbance as a flow pattern extending throughout the volume of the container. This is dictated by the type and position of the impeller, the design of the tank, and the flow properties of the material.

All the material should pass through the impeller zone at frequent intervals of time, the design of the mixer preventing the formation of “dead” zones. The turbulent, high-velocity flow of liquid from the impeller causes mixing by projecting eddies into and entraining liquid from the neighboring zones. The thin ribbons of one component in another rapidly become diffuse and finally disappear through molecular diffusion.

The flow pattern may be analyzed in terms of its three components of motion; radial flow, in a direction perpendicular to the impeller shaft; longitu-dinal or axial flow, in a direction parallel to the shaft; and tangential flow, in which the liquid follows a circular path around the shaft. A satisfactory flow pattern depends on the correct balance of these components. In a cylindrical tank, radial flow will give rise to axial flow by reaction at the wall of the tank. Tangential flow receives no such modification. Its predominance as laminar flow circulation supports stratification at various levels. Furthermore, a vortex is created at the surface of the liquid, which may penetrate to the impeller, causing air to be dispersed in the liquid. In general, tangential flow should be minimized by moving the impeller to an off-center position, thus destroying the symmetry of the mixer, or by modification of the flow pattern by means of baffles. Tanks with vertical agitators may be baffled by one, two, or more strips mounted vertically on or just away from the vessel wall. These reduce but do not elimi-nate tangential flow, whereas little modification of radial and axial flow occurs. Baffles produce additional turbulence.

Additional factors must be applied to the mixing of two immiscible liq-uids. This operation, which is encountered, for example, in liquid-liquid extraction, involves the production and maintenance of a large interfacial con-tact area. In addition, separation of phases because of differences in density must be opposed by an adequate axial flow pattern. The high rates of shear induced by the rotation of a propeller or turbine cause globules of the disperse phase to be drawn into an unstable filament of liquid, which breaks and reforms into smaller globules. Unless stabilized by surface-active agents, the reverse process, coalescence, occurs in zones where velocity gradients are small.

Paddle Mixers

Four types of paddle mixer are illustrated in Figure 13.7. The mixing element is large in relation to the vessel and rotates at low speeds (10–100 rpm). A simple paddle, with upper and lower blades, suitable for mixing miscible liquids of low viscosity, is shown in Figure 13.7A. A tangential flow pattern predominates with zones of turbulence to the rear of the blades. The gate paddle (Fig. 13.7B) is suitable for mixing liquids of higher viscosity, and the anchor paddle (Fig. 13.7D), with low clearance between pan and blade, is useful for working across a heat transfer surface. Stationary paddles intermeshing with the moving element sup-press swirling in the mixer, illustrated in Figure 13.7C. In the other examples, baffles are necessary. Unless paddle blades are pitched, poor axial turnover of the liquid occurs. Paddles are not, therefore, suitable for mixtures that separate.

Propeller Mixers

Propellers are commonly used for mixing miscible and immiscible liquids of low viscosity. The marine propeller is typical of the group. High-speed rotation (400– 1500 rpm) of the relatively small element provides high shear rates in the vicinity of the impeller and a flow pattern with mainly axial and tangential components. They may be used in unbaffled tanks when mounted in an off-center position or are inclined from the vertical. Horizontal mounting in the side of the vessel is frequently used when the scale of the operation is large.

FIGURE 13.7 Paddle mixers.


Turbine designs are intermediate between paddles and propellers. Turbines are effective mixers over a wide viscosity range and provide a very versatile mixing tool. The ratio of radial flow to tangential flow, which are the predominating components with this impeller, increases as the operating speed increases. Pitched-blade turbines are sometimes used to increase axial flow. Baffles must be used to limit swirling unless a shrouded turbine is used. With this impeller, a discharge with no tangential component is produced.

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