March 25, 2010

Selection of Right Impeller

Its a long time since I wrote my last post in Aug'2009. Now I am retrying to find out some time out of my schedule to the benefit of all of you again. So I thought to start with this info on agitators design.



Generally in small chemical industries & particularly by different agitator designers pitch blade turbines are suggested for most of the mixing applications. But, before you select any agitator just based on the suggestion from your vendor or from some knowledge from books, re-think in terms of your process.

First & most important thing is to understand the process requirement. The process means following three major categories.

1. Gas Liquid applications.
2. Liquid Liquid applications.
3. Solid Liquid applications.

Now in most of the specilaity chemical companies the second category i.e. liquid liquid mixing is the most used application. This is the area of problem during selection of impellers, because most of the literature suggest pitch blade turbines in these cases without understanding or emphasizing on the basic chemistry & mixing need.

In my opinion, based on my experience, it is important to understand the requirement of process e.g. in case of mixing need for two liquids, it is not necessary that only homogeneous mixing is sufficient but you need to understand how the reaction rate is changing with respect to fine mixing. Means, if you use more shattering or shear during mixing does it imapct on reaction rate?

I am sure in case of mass transfer controlled reactions it is going to help a lot significantly. If you do not believe, then try changing one of your impellers & see the impact.

In such cases, the shear forces provided by flat blade turbine (FBT, Rushton) or by CD-6 turbine is much higher & very useful for increasing the reaction rate by providing fine liquid dispersion. PBT will not be my choice in this case as it does not provide any radial or shear component in mixing.






This is one example of going against well written suggestions.

Similarly in case of solid liquid mixing requirement, I am again not in favor of using PBT every time. In such cases, it depends on process behavior. if you need to break lumps or if you need to avoid formation of lumps you should have some radial mixing component along with axial component.

For example, if liquid is viscous or lump formation is to be avoided, use shear component to your advantage.

Conclusion
In my opinion mostly the shear or dispersion is helpful in case of chemical reactions, whether it is liquid liquid mixing or liquid solid mixing.

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