Stirred Tank Systems

The tank should be stirred so as to produce an upwelling flow of liquid at the wall and a vortex at the agitator that does not draw air under the surface at the agitator axis. The residence time for surface elements should be optimized to balance the desire for a high rate of powder addition with the need for adequate time for gas release from the clumps as they become wetted.

Powder should be added near the edge of the tank so as to maximize the time available for wetin before the surface element is drawn into the vortex. The addition area should be large enough to spread the powder our and avoid formation of a bed of powder on the surface.

Baffles in the tank will force liquid into upflow rather than circumferential flow. Baffles should not extend to the tank bottom nor to the tank walls so as to avoid dead flow areas and buildup of wall cake. Speical coatings may reduce problems with caking on the walls, baffles, or agitator shaft.

At high RPM a vortex may gulp air, which is then broken into tiny bubbles which may be quite difficult to remove from a dispersion which is highly loaded with solids. To avoid this some manufacturers operate the tank under vacuum, using a lock-hopper to keep the solids under vacuum during addition.

A turbine impellor placed near the bottom of a tank can produce strong liquid circulation for low-viscosity dispersions. A disk impellor placed near the center of the tank can generate high shear, which may be needed to break clumps as the viscosity of the batch increases with solids loading. However, interchange of material between the top and bottom of the tank may be slow, and the top may become overloaded with solids and develop bad rheological properties. To avoid such problems some manufacturers sell multiple-agitator systems, with large slow-moving blades to circulate the material throughout the system and high-shear blades or disks to provide the high shear needed for breaking up small clumps.

Surface residence time for a stirred tank is proportional to the height of liquid in the tank (H), the square of the tank diameter (D), and the inverse of RPM

t_surfres D² H / RPM

Wetted Wall Systems

Although it is more difficult to manufacture and to operate, the wetted wall device provides a more uniform surface residence time than a stirred tank and can be used for once-through makeup of slurries for materials that wet in quickly. In the device shown, a rapidly-spinning ridged disk breaks up dry clumps and throws them into a sheet of liquid flowing down the tank walls. The force of impact can cause even low-density powders to submerge, but this is most effective for clumps larger than 10 micrometers in diameter and near-vacuum conditions. In other implementations the liquid may flow down a flat wall or a funnel shape, and the powder may be simply dropped onto the flowing liquid without being pre-dispersed.

Surface residence time for a wetted wall system is proportional to the diameter of the pipe (D), the length of the flow region (H), the viscosity () and the inverse of volumetric flow rate (G)

t_surfres D H / G

Wet-Mill Systems