Low Power, Baffle-Free Mixing With Contra-Rotating Impellers

A novel method to achieve good mixing without baffles in tank reactors and other mixing vessels will be presented.  Baffle free mixing can be achieved with contra-rotating impellers on coaxial shafts, driven by a gearbox connected to a single motor.  Contra-rotating impellers can be run in a variety of configurations, such as the top impeller pushing down, bottom pushing up, or top pushing up and bottom down.

Mixing experiments were conducted in a 5 gallon cylindrical tank using a Caframo BDC 3030 Stirrer and standard pitched blade turbines.  Torque was measured as a function of impeller speed and the degree of mixing was imaged with suspended colored particles.  A single impeller, with and without baffles, was compared to dual impellers on the same shaft, with and without baffles, and then to baffle-free contra-rotating impellers in a number of configurations. 

Good mixing was never achieved without baffles using single or dual impellers on a single shaft.  The measured torque was proportional to the turbine blade area; dual impellers required about twice as much torque as a single impellers.  Baffles approximately double the required torque for both of these cases at the conditions tested.  Contra-rotating impellers have higher torque than single impellers because of the added blade area, but incur no torque penalty of baffles.  Good mixing was achieved in the optimal contra-rotating configuration mixing using about 20% less torque, and at lower rotational speeds than the single or dual impellers with baffles on a single shaft.  Thus, the power required for good mixing with contra-rotating impellers with no baffles is significantly less than single shaft mixing with baffles.  A number of applications of this mixing apparatus will be discussed.