Equipment Batch Mixing: When the material to be mixed is limited in volume to that which may be conveniently contained in a suitable mixer, batch mixing is usually most feasible. It is commonly consists of two primary components: 1- A tank or other container suitable to hold the material being mixed. 2- A means of supplying energy to the system so as to bring about reasonably rapid mixing. Power may be supplied to the fluid mass by means of an impeller, air stream, or liquid jet. Besides supplying power, these also serve to direct the flow of material within the vessel. Baffles, vanes, or ducts also are used to direct the bulk movement of material in such mixers, thereby increasing their efficiency.

1. Impellers: The distinction between impeller types is often made on the basis of type of flow pattern they produce, or on the basis of the shape and pitch of the blades. Three basic types of flow may be produced: radial, axial and tangential. These may occur singly or in various combinations. The figure below shows the axial flow impeller (left) and radial flow impeller (right).

Three basic types of flow may be produced: radial, axial and tangential. The figure below shows the axial flow impeller (left) and radial flow impeller (right). Tangential flow axial radial tangential

Batch mixing is the most commonly used mixing technology, providing precise control of mix quality, flexibility of production and often times reduce installation and operation costs. With a batch mixing system, all ingredients are loaded into the mixer together, mixed homogeneously and then ejected in a single lot, or batch.

Radial: acts in the direction vertical to the impeller shaft. Excessive radial flow takes the material to the container wall then material falls to the bottom and rotates as the mass beneath the impeller. Axial or longitudinal or vertical:acts in the direction parallel to the impeller shaft. Inadequate longitudinal component causes the liquid and solid to rotate in layers without mixing. Adequate longitudinal pattern is best used to generate strong vertical currents particularly when suspending solids are present in a liquid. Tangentialor circular: acts in the direction tangent to the circle of rotation around the impeller shaft. If shaft is placed vertically and centrally, tangential flow follows a circular path around the shaft and creates a vortex in the liquid.

1- Propeller consists of number of blades that may be right or left handed depending upon the slant of their blades. Used when high mixing capacity is required. Three main types of impeller are used namely: propeller, turbine and paddles. 2- A turbine consists of a circular disc to which a number of short blades are attached. Blades may be straight or curved.

A paddle consists of a central hub with long flat blades attached to it vertically. Paddles are also employed as impellers; their blades have a large surface area in relation to the tank in which they are employed, a feature that permits them to pass close to the tank walls and effectively mix viscous liquids or semisolids.

Propellers characteristically produce flow parallel to their axes of rotation and this is more efficient when they can be run at high speed in liquids of relatively low viscosity, whereas turbine may produce either axial or tangential flow or a combination of these, so they are used for more viscous liquids.

2. Air jets: Subsurface jets of air, or less commonly of some other gas, are effective mixing devices for certain liquids. Of necessity and for obvious reasons, the liquids must be of low viscosity, non-foaming, unreactive with the gas employed, and reasonably nonvolatile. The jets are usually arranged so that the buoyancy of the bubbles lifts liquid from the bottom to the top of the mixing vessel. This is often accomplished with the aid of draft tubes. These serve to confine the expanding bubbles and entrained liquid, resulting in a more efficient lifting action by the bubbles.

The overall circulation in the mixing vessel brings fluid from all parts of the tank to the region of the jet itself. Here, the intense turbulence generated by the jet produces intimate mixing.

3. Fluid jets: When liquids are to be pumped into a tank for mixing, the power required for pumping often can be used to accomplish the mixing operation, either partially or completely. In such a case, the fluids are pumped through nozzles arranged to permit good circulation of material throughout the tank. In operation, fluid jets behave somewhat like propellers in that they generate turbulent flow in the direction of their axes. They do not in themselves, however, generate tangential flow as do propellers.

4. Baffles: It is an auxiliary device for directing the flow of the fluid, usually baffle plates. Baffle placement depends largely on the type of agitator used. The movement of fluids is maintained vertically in the cylindrical tank and the overall mixing process in turbulent flow around the axis.

Vortex formation Astrong circulatory flow pattern sometimes manifests into formation of a vortex near the impeller shaft & they can be formed when: Shaft is placed symmetrically in the tank. Blades in the turbines are arranged perpendicular to the central shaft. At high impeller speeds. In unbaffled tanks.

Prevention of vortex formation 1- Impeller should in in any one of the position that can avoid symmetry such as off central, inclined, side entering, etc., and should be deep in the liquid. 2- Baffled containers should be used. In such case impeller can be mounted vertically at the center.

Applications of Batch Mixing Batch mixing is preferred for applications where: 1-Many formulations are produced on the same production line. 2-Ingredient properties change over time and compensation must be on a batch-by-batch basis. 3-Production quantities are small. 4-Strict control of mix composition is required. 5-It is required to identify a batch for further follow up, example - pharmaceutical formulations, food products.

Advantages of Batch Mixing The advantages of the batch mixing operation are as follows: Precise control of mix quality. Batch traceability. Lower installed and operating costs for small to medium capacities compared to continuous mixing Flexibility of production. Control of mixing time. Pre-mixing of minor ingredients is easily accomplished. Easy cleaning, lower cleaning costs when product changes are frequent. Disadvantages and Limitations of Batch Mixing Batch mixing is uneconomical when large quantities of material are to be mixed. Batch mixing is more labor intensive compared to continuous mixing.