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Author Topic: Minimize side-product formation  (Read 535 times)

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curiouscat

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Minimize side-product formation
« on: November 26, 2012, 01:39:44 AM »

Given the scheme of reactions below; what strategies could one apply to maximize desired product P and minimize undesired Q. [A,B,P,Q are all miscible liquids]

A + B  :rarrow: P ....desired product..(Rxn. 1)

A + A   :rarrow: Q  ....undesired product....(Rxn. 2)

The ways I could think of:

1. Semi batch with A added slowly to B in a tank
2. Massive dilution with some inert I also miscible
3. If Rxn.1 and Rxn.2 are of varying orders a PFR vs. CSTR choice might also help? PFR favoring the reaction of higher order?

Any other tips? No really very sure if #2 and #3 are relevant or not.
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Babcock_Hall

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Re: Minimize side-product formation
« Reply #1 on: November 26, 2012, 03:14:04 AM »

Can we assume that the first reaction is first order in A and the second reaction is second order in A?
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curiouscat

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Re: Minimize side-product formation
« Reply #2 on: November 26, 2012, 03:15:56 AM »

Can we assume that the first reaction is first order in A and the second reaction is second order in A?

Sure. Actually I don't yet know the orders but yes, that might be one quite likely possibility.
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Babcock_Hall

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Re: Minimize side-product formation
« Reply #3 on: November 26, 2012, 03:50:15 AM »

Then raising the concentration of B should help, all else held equal.  If the two rate constants responded differently to temperature, then one might also get some modest benefit from raising or lowering temperature, assuming kinetic control.
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curiouscat

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Re: Minimize side-product formation
« Reply #4 on: November 26, 2012, 07:46:35 PM »

Then raising the concentration of B should help, all else held equal.  If the two rate constants responded differently to temperature, then one might also get some modest benefit from raising or lowering temperature, assuming kinetic control.

Thanks. Adding inert solvent I, reduces conc. of both A and B. Not clear to me which effect dominates. Is having c_B less or more important than having low c_A?
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Babcock_Hall

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Re: Minimize side-product formation
« Reply #5 on: November 27, 2012, 02:40:14 AM »

I don't think that the inert solvent helps in this case.  Suppose that the concentrations of A and B both go down by half, by doubling the volume of the reaction.  Then both reactions slow down by fourfold, because both are second-order, overall.  It would help if the desirable reaction were first-order in A and zero-order in B.
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curiouscat

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Re: Minimize side-product formation
« Reply #6 on: November 27, 2012, 02:55:09 AM »

In which case would going solventless be ideal? And having a large excess of B? That should max. c_B.

Somehow seems counter-intuitive to me..
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Babcock_Hall

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Re: Minimize side-product formation
« Reply #7 on: November 27, 2012, 03:02:50 AM »

It is always best to write out the rate equations and look at the math.  However, I am inclined to say that having a high concentration of B relative to A is helpful.  One way to think about this is that it becomes more likely for a molecule of A to collide with B than with another molecule of A.
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