Scale-Up (and Down) of Viscous Mixing Processes

Understanding the role of mixing in scale-up is one of the keys to successful process development. Generally, scale-up is achieved by maintaining a constant ratio between the rate limiting step of the process, usually a reaction, and the rate of the mixing. Essentially scale-up is achieved by keeping Damkohler number constant.

 

In the turbulent regime, when scaling at constant power input by the agitator per mass of fluid, the blend time increases and, if the reaction is fast and competitive, steps must be taken to ensure that the reactants come into contact in a region of high energy dissipation and mixing rate by, for example, using dip pipes to feed at the tip of the agitator’s blades.

 

When processing viscous fluids in the transitional and laminar regimes the blend time will decrease on scale-up. The reason for this is that the impeller Reynolds number increases with increase in scale and a different scaling problem arises.

 

The mixing at small scale may be slower in the lab and pilot reactors than achievable in the plant. If a mixing sensitive reaction is being studied at small scale, it is possible that the chemistry will “fail” because the mixing is too slow, compared to the rate of reaction, compared to the rate achievable when performing the same reaction at a larger scale.

 

It is possible to get a “false negative” solely due to slow mixing at small scale that would not be observed if the process was carried out a larger scale and Reynolds number. This presentation will discuss how to identify these issues and what steps can be taken to reduce their effects resulting in successful scale-up.