Mixing usually implies the taking of two or more separate phases, such as a fluid and a powdered solid or two fluids, and causing them to be randomly distributed through one another.

Agitation refers to forcing a fluid by mechanical means to flow in a circulatory or other pattern inside a vessel

Blending: Agitation of two miscible liquid phase

Suspension: Suspending solid particles into liquid.

Dispersion: Dispersing a gas through the liquid in the form of small bubbles.

Purposes for Agitating Fluids,

1. Blending of two miscible liquids, such as ethyl alcohol and water.

2. Dissolving solids in liquids, such as salt in water.

3. Dispersing a gas in a liquid as fine bubbles, such as oxygen from air in a suspension of microorganisms for fermentation or for the activated sludge process in waste treatment.

4. Suspending of fine solid particles in a liquid, as in the catalytic hydrogenation of a liquid, where solid catalyst particles and hydrogen bubbles are dispersed in the liquid.

5. Agitation of the fluid to increase heat transfer between the fluid and a coil or jacket in the vessel wall.

Paddle agitators

Various types of paddle agitators are often used at low speeds, between about 20 and 200 rpm. Two-bladed and four-bladed flat paddles are often used. The total length of the paddle impeller is usually 60–80% of the tank diameter and the width of the blade to of its length. At low speeds mild agitation is obtained in an unbaffled vessel. At higher speeds baffles are used, since, without baffles, the liquid is simply swirled around with little actual mixing. The paddle agitator is ineffective for suspending solids, since good radial flow is present but little vertical or axial flow. An anchor or gate paddle, shown in below Fig. is often used. It sweeps or scrapes the tank walls and sometimes the tank bottom. It is used with viscous liquids where deposits on walls can occur and to improve heat transfer to the walls. However, it is a poor mixer. Paddle agitators are often used to process starch pastes, paints, adhesives, and cosmetics

Equipment for Agitation

1. Three-blade propeller agitator

These propellers turn at high speeds of 400 to 1750 rpm (revolutions per minute) and are used for liquids of low viscosity. The flow pattern in a baffled tank with a propeller positioned on the center of the tank is shown in Fig.  This type of flow pattern is called axial flow since the fluid flows axially down the center axis or propeller shaft and up on the sides of the tank as shown.

Turbine agitators

Turbines that resemble multibladed paddle agitators with shorter blades are used at high speeds for liquids with a very wide range of viscosities. The diameter of a turbine is normally between 30 and 50% of the tank diameter. The turbines usually have four or six blades. They are also useful for good gas dispersion; the gas is introduced just below the impeller at its axis and is drawn up to the blades and chopped into fine bubbles. This type is useful in suspending solids since the currents flow downward and then sweep up the solids.

Helical-ribbon agitators

This type of agitator is used in highly viscous solutions and operates at a low RPM in the laminar region. The ribbon is formed in a helical path and is attached to a central shaft. The liquid moves in a tortuous flow path down the center and up along the sides in a twisting motion

a) high-efficiency, three-blade impeller,

b) double- helical-ribbon,

c) helical-screw.

Agitator selection and viscosity ranges

The viscosity of the fluid is one of several factors affecting the selection of the type of agitator. Indications of the viscosity ranges of these agitators are as follows.

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