(73b) Design of Small Scale Impellers for High Throughput Mixing Challenges

With the increasing use of automated high throughput research labs to perform rapid screening experiments, we are often faced with unique mixing challenges on a fairly small scale.  In this talk we will present some examples of small scale impellers that have been developed to meet specific goals in high throughput equipment where the mixing vessels are small unbaffled tubes or vials.  We have employed computational fluid dynamics and rapid prototyping capabilities to enhance our ability to analyze and optimize the impeller geometry.

One example will focus on a parallel reactor system where one of the reactants is contained in the head space gas.  The mixing impeller has to effectively draw down the gaseous reactant into the liquid and provide adequate fluid and thermal mixing in the tube.  Due to the manner in which the drive mechanisms is set-up, the impeller needs to perform in the same manner whether it is turning clockwise or counterclockwise.   It also needs to be inexpensive to manufacture since it is disposed of at the end of an experimental run.  This led to a 2D impeller design that provided an efficient compromise between draw down and axial flow development.

In another example involving a multi-step robotic system, it is imperative that the impeller does not draw down any air bubbles into the fairly viscous fluid during the blending step.  The impeller torque also needs to serve as a viscosity indicator that can be calibrated against a lab bench viscosity analyzer.  In this case, since the impeller is re-used, the design needs to allow for easy and effective cleaning in a separate robotic step.   For chemical and mechanical integrity the impeller needs to be constructed of metal, but once again a low impeller cost is a primary goal.

In a third example, a unique impeller based on a twisted tape geometry was developed for rapid blending of high viscosity fluids in the range of 5000 to 30,000 cp.  A number of geometric parameters were evaluated and optimized.  The impeller is able to generate good axial mixing and is a significant improvement over the performance of the small scale impellers offered by venders.