Stirred Tank Problems Of Suspend Solids in Liquid Phase

A stirred tank contains agitator fitted to an electric motor by gearbox. Liquids and solids charged into the tank opening, filling up to 60 percent volume of complete tank. Agitator mechanically induces the motion to the liquid and solid compounds. The two phases mix and form a homogeneous phase. This phenomenon works for liquid and liquid components also. Stirred tanks meant mainly to do this type of operations. Inside the tank, baffles fitted to the surface of the wall. Agitating parts like impellers, propellers, turbines and agitators are used.

Factors like impeller diameter, number of wings, shape of paddles, impeller clearance height from bottom of tank, speed of rotation, baffle spacing etc determine the extent of equipment running time for solids to suspend in liquid phase.

Power consumption for minimum speed of agitation is the design factor for any stirred tank. One of the main issues in suspending solids in liquid phase is that the solids settled at the bottom of the tank will not have intimate contact with the liquid. Agitators also do not influence much at the bottom of the tank. The reason would be that agitator fitted to distance away from bottom of the stirred tank. When it rotates, it creates a dead zone below it. The liquid is less influenced and do not participate actively in turbulent motion. Solid trapped in this dead zone settle down and do not suspend. This will alter the determined composition of final solution. One of the main practical problems will be the line chocking during the discharge of suspension because of left over solid particles at the bottom.

Commonly impellers like flat blade, pitched blade, six blades are used to overcome this problem. The selection of impellers should be axial- flow or mixed flow rather than radial flow to deal with the condition like above.  Axial flow impellers influence the liquid motion forcing towards the bottom of tank and then flow up at the tank wall and again circulate back. The pattern of fluid flow disturbers the stagnant solids and fluidize them finally, thus fully suspended solution formed.