Theory of Fluidized Bed Mixing
The mechanically fluidized plough mixer provides quick, intense mixing without the application of high shear. This is done using particle fluidization, which by definition is the separation and agitation of solid particles using a rising stream of process gas.
Mechanical fluidization is the process of separation and agitation of the particles using impingement of the material on mixing elements mounted on a rotating shaft located within a cylindrical horizontal vessel.
The plough shaped mixing elements are designed to lift and separate the product within the vessel.
The design of the ploughs, their number and spacing, and the speed of rotation all contribute to fluidization. With the right design of the mixing elements and appropriate selection speed of rotation, the plough share mixer provides intense mixing in a gentle manner, with very little shear.
Design, Construction, and Operation
The plough share mixer consists of a cylindrical drum containing plough shaped mixing elements that are mounted on a horizontal shaft. The agitator’s shaft is positioned in the center of the drum and has welded / bolted arms on which the “Plough” shaped mixing elements are mounted. The plough share mixer gets is name because of these plough shaped mixing elements. Independently driven high-speed choppers may be installed to disperse agglomerations and control granulation during the mixing process.
The plough shaft is powered by a drive system comprising of a motor, gearbox, and couplings. The shaft exits the mixer drum at either end through the end plates bolted or welded to the cylindrical shell. The area where the shaft exits the container is provided with a sealing arrangement to ensure that material does not travel from the container to the outside and vice-versa. The mixer assembly along with the drive system components viz. motor, gearbox, couplings, and bearing supports is mounted on a supporting frame.
In operation, material is loaded through the top inlet. The working volume (volume up to which material should be loaded) of the plough share mixer ranges from 30% to 70% of the total drum volume. The shaft rotates at higher speed, producing a plough tip speed of more than 200 meters per minute to effect a fluidizing action. The ploughs throw the material upward, and the material settles in a random pattern.
The size, shape, positioning, and peripheral speed of the ploughs are coordinated in such a way that they produce a whirling, three-dimensional product movement. The turbulence and material movement within the mixer container prevents formation of dead-spots or low-movement zones and ensures fast and homogeneous mixing. The ploughs are designed to lift the material from the inside wall of the container without squashing the particles against the wall
The profile and arrangement of the ploughs ensure that after mixing, the material is directed towards the centrally located discharge valve. For larger mixers, more than one discharge valve can be provided depending on the requirement of the downstream process.
Flush bottom discharge valves are preferred to avoid any dead pockets within the mixer. These valves may be manually, pneumatically of hydraulically operated. Large side doors may be provided for cleaning access. Figure 1 shows 600 liter plough share mixer.