Sciencemadness Discussion Board

Scaling up reaction times

andre178 - 17-2-2011 at 19:26

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.

andre178 - 18-2-2011 at 12:54

really no one can answer this?

Random - 18-2-2011 at 13:26

Is this what you want?

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.

smuv - 18-2-2011 at 13:38

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.

andre178 - 18-2-2011 at 13:50

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.

Magpie - 18-2-2011 at 18:47

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.

Nicodem - 19-2-2011 at 03:19

Quote: Originally posted by andre178  
yeah I understand the chemical kinetics etc, I don't know why people even put references to wikipedia

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.

andre178 - 22-2-2011 at 09:52

thank you for the info, it helps a lot