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Agitator/Impeller selection for stirred reactors
A large fraction of lab reactors are stirred reactors whether pressurized or not. A decision most users face is about the selection of the right size and type of impeller for the reactor they order. This decision can be made in consultation with your vendor and it usually helps if the vendor has process engineering experience in addition to just fabrication knowledge. Good agitator selection is both an art and a science and a complex undertaking but, in this blog, post we try to outline certain tips for common applications. (This is part 1 of a two blog post series on Agitator choices for lab reactors)
Firstly, the customer must be clear what their ultimate goal is for the reactor they are buying and the constraints of their particular application. For example we see a variety of use cases and the optimal choice of agitator will vary. E.g.:
- Blending of miscible fluids (e.g. alcohols)
- Single phase liquid reaction
- Gas liquid reactions (e.g. hydrogenations)
- Gas liquid solid reactions (some of the most challenging systems)
- Reactions in liquid liquid immiscible systems. These can also be quite challenging systems and often are seen where one of the reactants is aquous e.g. H2O2 or an acid and the others are organic liquids
- Dissolving a solid into solution
This is just a sampling of systems and real life systems can be much more varied but this should give the customer a flavor of how to characterize their own application.
Typically the advice at lab scale is to go for a agitator that can be generously sized since minimizing power consumption is usually not a crucial goal at lab or pilot scale unlike in actual production. A good starting point is to ask your lab reactor vendor for a sketch that summarizes all the variety of impeller designs that they are capable of providing.
One of the most common impeller types is a pitched blade turbine which should work for most applications at lab scale. This impeller will work well for most blending and reaction applications in miscible systems. It is important to specify multiple levels of impellers for reactors which are tall e.g. H / D >1.5 . This ensures that there are no dead zones at the upper levels of a reactor.
Refer Fig. A for a reactor with three impellers.
For more demanding applications where higher shear rates may be desirable a Rushton turbine may be a better choice. Although this may draw higher power mixing and interfacial mass transfer rates would be better. Often for multiphase systems this is an important goal.
In systems where gas – liquid mixing is involved it is crucial to consider some mechanism for redispersion of the gas from the headspace into the liquid. Hollow shaft impellers are a good option for such situations. Hydrogenation reactions often work best with such agitators where the rotation allows a suction to be created that continuously pulls hydrogen from the headspace into the liquid and redisperses it as tiny bubbles near the impeller blades
Refer Fig.C for a hollow shaft impeller.
Also, Refer Fig. D for an image of the plexiglass.
On the other extreme if your system involves fermentations or specialized polymers such systems may be sensitive to shear rate and could degrade at high shear situations. This can lead to constraints on the tip speeds for impellers. Finally for viscous fluids or non-Newtonian situations or systems involving slurries an anchor type agitator is often the choice. One constraint to recognize is that anchors cannot be run at the high rpms of other impeller types.
Refer Fig.B for an anchor type impeller inside a reactor
Overall during lab reactions and scale up all liquid parameters may not be accurately known and hence a recommendation is to ask for a VFD (Variable Frequency Drive) so that the rpm can be varied during the actual lab runs. Often in order to investigate whether mixing or mass transfer is a limiting parameter the rpm would be varied and some critical parameter tracked by measurement (e.g. reaction yield). For such goals the slightly added cost of a VFD pays itself off very quickly.
Stay tuned for Part 2 of this blog post for additional tips on agitator selection