Evaluating Mixing Techniques for Scale-up Processes

The issues associated with scale-up of mixing processes have been recognized for many years (for example, Bowen, Chemical Engineering, March 1985; Leng, Chemical Engineering Progress, June 1991). As processes are scaled-up, the timescales of mixing generally increase while the timescales of the process do not change. For example, if the concentration of reactants and the reactor temperature and pressure are constant on scale-up, the timescale of the reaction will also be constant. However, if a stirred tank is scaled using the common rule of geometric similarity and constant power per mass, a process at the 20m3 full scale will take almost 5 times longer to blend than a 20 liter pilot plant. For some processes this longer blend time may not matter, but for many processes this difference in blend time will be critical for success or failure.<br />Successful scale-up for mixing sensitive systems can be achieved by maintaining a constant ratio between the process rate and the rate of mixing – essentially constant Damkohler number. The key to this approach is identifying the rate limiting step, which may change on scale-up.<br />Speakers in this session will discuss their experiences developing and scaling-up mixing sensitive processes.