The mixing of liquids is achieved in an agitated tank.
A large number of different types of impellers are in use; different impellers impart different flow patterns to the liquid and they must be matched to the rheology of the liquid and to the desired shear rate.
Mixing vessels usually have rounded bottoms, rather than flat ones, to prevent the formation of dead spaces.
The liquid depth is normally equal to the tank diameter.
When an impeller rotates in a liquid the liquid is likely to swirl in a mass and a vortex will form (Figure).
This is undesirable; the vortex may well draw air from the surface down to the impeller with the possibility either of unwanted dissolution of air or a waste of energy as the impeller rotates partly in air at the expense of agitating the liquid.
Consequently baffles are fitted to the tank which consists of vertical strips of metal running the full depth of the inside surface of the tank.
Normally four baffles are used, their width being about 10 % of the tank diameter.
Baffles minimize vortex formation, prevent swirling of the liquid, and result in more rapid mixing.