Wednesday, October 27, 2010

Process Wednesday: Reaction optimization tips

From Neal Anderson's "Practical Process Research and Development", page 170:
"Running a reaction at or near room temperature offers many potential advantages for scale-up operations. First, operations on scale are extended if large masses of chemicals must be heated or cooled. Second, running under extreme levels of heating or cooling requires increases energy expenditures. Third, reactions run at an excessively high temperature may generate unacceptably high amounts of by-products, including colored impurities that may be difficult to remove from the product."

"The rule of thumb is that increasing a reaction by 10°C will double the reaction rate. In practice, the reaction rate may decrease or even quadruple."
You mean you can't just lift the reactor out of a silicone oil bath, spray it off with hexanes and drop it into an ice water bath? Darn.

3 comments:

  1. I'm hoping that there are more reasons given than just that. The volume of a reactor and thereby the heat generation for an exopthermic reaction scales with L^3 while the surface area scales with L^2. Can you say "unconstrained exothermic reaction"?

    (Sorry, this is junior level Chem Engineering.)

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  2. There are definitely more reasons. But in the book (which is basically "Process Chemistry for Dummies"), there's a little "tip" box that states the above.

    But I think a typical bench-scale chemist may think "Why not just heat the jebeebers out of it?", and Anderson explains a few reasons why not.

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  3. I don't think cooling and heating is such a big deal on scale up if the temperature range is reasonable (say between -30 and +140C). Sometimes you may even want to run the reaction little higher on scale so as to deal with a prompt exothermic process - instead of delayed exotherm. From what I have heard its the problems with too slow loading of reagents (that can take too long to add with a large reactor - while the needed time exceeds the stability of reactive intermediates or products), the issues with stirring and heat transfer is where you usually get screwed up on plant scale. (Of course it would be nice to be able to run all reactions at room temp in denatured alcohol as a solvent but sometimes one has to use Grignard...)

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