andre178
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Scaling up reaction times
Is there a general trend that when scaling up or down reactions, you have to scale the time, but maintain molar proportions, catalysts, temperatures,
etc.
This has been a hurdle in reactions.
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andre178
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really no one can answer this?
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Random
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Is this what you want? http://en.wikipedia.org/wiki/Chemical_kinetics
But, you can't change the speed if you don't change anything because you need to change something for a change to occur. That's how the things work I
think, there must be some cause.
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smuv
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Short answer: for homogenous reactions, if the procedure is scaled up with the same ratio of reagents, the reaction rate should stay the same. Of
course, there are probably tons of cases where this doesn't apply. For homogenous reactions, the rate is highly dependent upon mass transport between
phases, so the big factor determining rate is mixing, so when scaling up rates can drop quite a lot, because it is a lot easier to efficiently stir a
small quantity of solution than a very large quantity.
But really there are so many factors, it is hard to make a blanket statement.
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andre178
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thank you, any bit helps! yeah I understand the chemical kinetics etc, I don't know why people even put references to wikipedia, I think when people
ask on these forums they are asking people who have a lot of practice under their belt. I'm a beginner chemist, who has more than his share of theory
and not much real practice, thank you college experience.
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Magpie
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Once upon a time at work I was asked to determine why a corn starch cooker of about 5000 gallons capacity was so much more efficient than a laboratory
cooker of 1 gallon capacity. Both were operated at atmospheric pressure and the same time-temperature schedule was used. Concentrations of the
starch in water, and of the alpha-amylase enzyme, were the same. The only thing that was radically different was the mixing. Because of the large
radius of the plant-scale mixer the tip velocity of the agitator blades was much greater. We did increase the rpm of the 1-gallon mixer but still
could not come close to the plant conversion rate. We had to increase the enzyme concentration by at least 2X to match the rate of the 5000 gallon
converter.
Now that I think about it the hydrostatic pressure in the plant mixer would, on average, have been greater also. The lab converter was heated with a
steam jacket, the plant converter by live steam sparging. That could have been a major factor, I suppose.
But, I agree with smuv that if all microscopic parameters are the same the rates should be the same, regardless of the bulk size of the reactor.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Nicodem
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I think the proper question is more like why Random posted such an irrelevant reference to Wikipedia.
Anyway, if you are interested in knowing more on the topic try searching the literature for "scale-up effect" as keyword. One such scale-up effect is
that described by Magpie above. This topic is one of major importance for the chemical engineers, but it does not hurt if chemists learn about it as
well (often huge differences can be observed already going from 10 mmol scale to 100 mmol and it is good to have an understand about the possible
causes). Like smuv said, it is quite difficult to scale-up heterogeneous reactions. It is also difficult to scale-up exothermic reactions. And it is
extremely difficult to scale up exothermic heterogeneous reactions. The reaction conditions used on small scale simply can not remain the same in a
large reactor, thus the reaction time changes as a consequence.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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andre178
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thank you for the info, it helps a lot
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