Sciencemadness Discussion Board

Is there a general rule about stirring rate & reaction speed like there is for temperature?

FireLion3 - 8-7-2014 at 22:47

I thought I read somewhere that every 10 degree increase usually doubles reaction speed. Is there any such rule like this for stirring rate? I'm my practices I've only been limited to 1500 RPM (stir plate limitation), but a friend is allowing me to use her overhead stirrer that can reach RPM of 5000-7000+... Very curious to see how much reaction enhancement I could get with these rates, any idea on what might be a good estimation?

Töilet Plünger - 8-7-2014 at 22:57

I'm not sure how that would affect the kinetics of the reaction. Part of what you need is an increase in the occurence of molecular collisions. I'm guessing a turbulent flow would do this, but make the rates somewhat unpredictable. It would be cool to try this with clock reactions and figure out the results.

Is your friend an amateur chemist too or a professional/student?

FireLion3 - 8-7-2014 at 23:49

Student/Amateur, but from what I knew her father works professionally in the field and has a bunch of equipment around his house that was retired from the lab he works at. He's got a bunch of other toys there I would love to try out sometime but as far as buying them, they're way above my price range.

I wonder if it's possible for too fast stirring rate to inhibit the reaction.

[Edited on 9-7-2014 by FireLion3]

forgottenpassword - 9-7-2014 at 00:39

Obviously stirring is adding energy to a reaction, and stirring at 200 rpm is adding twice the amount of energy as you would be adding at 100 rpm. Whether or not that energy is significant compared to heat energy I do not know, but I would imagine that it is not. If a solid reactant is present, stirring can increase the rate of reaction simply by exposing more of the reactant's surface area to the solution; and dissolution always takes place more quickly with faster stirring.

papaya - 9-7-2014 at 01:41

https://en.wikipedia.org/wiki/Diffusion_control

blogfast25 - 9-7-2014 at 06:25

Quote: Originally posted by forgottenpassword  
Obviously stirring is adding energy to a reaction, and stirring at 200 rpm is adding twice the amount of energy as you would be adding at 100 rpm. Whether or not that energy is significant compared to heat energy I do not know, but I would imagine that it is not. If a solid reactant is present, stirring can increase the rate of reaction simply by exposing more of the reactant's surface area to the solution; and dissolution always takes place more quickly with faster stirring.


Actually it's roughly four times the amount of kinetic energy when you double RPM, because kinetic energy is proportional to the square of velocity/angular velocity.

But kinetic energy of the bulk mass doesn't really contribute anything here: it'd be like saying that reactions would proceed faster if you conducted them in a fast space ship; they don't.

Only heat (or rather temperature) affects the kinetic energy of the atoms/ions/molecules in the reaction mix and thus reaction rate, all other variables being equal (see collision theory).

High speed stirring would very, very slightly heat the stirred liquid but in an almost negligible way.

Stirring helps reactions because it keeps the reacting mixture homogeneous. But once above a certain speed threshold, I doubt that increasing RPM really has any significant additional effect.


[Edited on 9-7-2014 by blogfast25]

aga - 9-7-2014 at 06:56

Stirring at 7000rpm might not alter the Rate of the reaction, but it may well change Where the reaction takes place.

At that speed the liquid will be all over the walls !

Praxichys - 9-7-2014 at 07:33

Agitation speed can be a factor in several cases:

If the reaction occurs between two immiscible phases, higher agitation can increase the surface area of the boundary layer. Think about a liquid/liquid extraction, or a hydrogenation.

There could also be instances where the reaction between solids and fluids could increase in speed with agitation by preventing localized depletion of fluid reactants in the solid layer. Examples include zinc reductions, coal power plants, fluidized bed reactors, etc..

Collisions of solid particles with each other can help physically abrade them into smaller pieces which have a higher surface area. Think about dissolving anything with stirring compared to without stirring. The homogenization of the liquid solution also plays a big role here.

Note that all of these examples rely on macroscopic physical processes to decrease the duration of a reaction. Actual solution reaction kinetics are, as blogfast mentioned, not affected by stirring unless the stirring is so fast that it begins to heat the mixture. Essentially, this is a testament to the low efficiency of imparting physical force on a liquid to heat it.

blogfast25 - 9-7-2014 at 07:58

Yes, I was of course specifically referring to reactions in homogenous phase (one phase). In multi-phase systems high shearing forces associated with high speed mixing can make a tremendous difference but I wouldn't really class that as 'stirring' anymore. I doubt that that was the spirit of the OP, though...

FireLion3 - 9-7-2014 at 10:53

The reaction would be kept more homogenous, but wouldn't doubling the stirring speed also double the rate at which new molecules are colliding? Thus potentially doubling the reaction speed? Assuming the mixture does not fly over the walls and foam up.

PHILOU Zrealone - 9-7-2014 at 12:11

At the University we had interdepartemental demo's and presentation.
I was in the Physico-chemical building and in the Photo-chemistry departement.
I had the chance to assist to a presentation from the corona-chemistry and sono-chemistry departement.

They studied the effect of sonochemistry with a Nickel horn on various reaction...they had the idea to test for cavitation bubbles from ulta-speed-agitation (ultra-turax with rpm between 10k and 60k...) sonochemical reaction occurs.
You could speed up Grignard's type reaction even without ether (only magnesium dust); there was also a reaction with a minor endothermic product vs a major product whose % passes from traces to nearly 50%.

So yes speed has its role to play but it is sonochemical in nature!

FireLion3 - 9-7-2014 at 13:01

Quote: Originally posted by PHILOU Zrealone  
At the University we had interdepartemental demo's and presentation.
I was in the Physico-chemical building and in the Photo-chemistry departement.
I had the chance to assist to a presentation from the corona-chemistry and sono-chemistry departement.

They studied the effect of sonochemistry with a Nickel horn on various reaction...they had the idea to test for cavitation bubbles from ulta-speed-agitation (ultra-turax with rpm between 10k and 60k...) sonochemical reaction occurs.
You could speed up Grignard's type reaction even without ether (only magnesium dust); there was also a reaction with a minor endothermic product vs a major product whose % passes from traces to nearly 50%.

So yes speed has its role to play but it is sonochemical in nature!


I had thought there was a connection between the sonochemical mechanism and the kinetic energy being generated by stirring, but blogfast25 above seems to imply that this kinetic energy would barely effect the reaction rate.

If kinetic energy of the stirring is not effecting the reaction, then I was assuming that the reaction rate would almost exclusively increase by the nature of more molecules colliding at a more frequent rate. Higher degrees and rapidity of homogenization thus increasing the likely hood that the correct reacants will intereact with eachother on a molecular level.

blogfast25 - 9-7-2014 at 13:15

FireLion3:

Stirring, even in a highly turbulent regime (very high Reynolds Number), is a macroscopic phenomenon: even the smallest vortex contains billions of reacting species and solvent molecules that don't even 'notice' the turbulence. The smallest vortex is billions of times larger than the atoms/molecules/ions that are bumping into each other.

But the turbulence due to stirring does affect the macroscopic homogeneity of your reacting mass.

To draw an analogy, say you have people bumping into each other on a busy train: do you think at linear (and uniform) speed they bump into each other more at higher speed?

Sonochemistry affects non-homogeneous systems (like Mg + alkyl halide + solvent) and other non-homogeneous systems through shearing. But a grain of Mg powder is billions of times larger than an Mg atom!
Quote: Originally posted by FireLion3  
[Higher degrees and rapidity of homogenization thus increasing the likely hood that the correct reacants will intereact with eachother on a molecular level.


Yes, but the phenomenon isn't due to kinetic energy of the reactants but simply due to constant redistribution of the reactant mix. From a certain point of speed onwards increasing speed will have little effect. By contrast increasing the overall temperature of the mix has almost no limits in how much it can affect reaction rate, temperature obviously being limited by BPs and other practical considerations.

So stirring/agitating can affect reaction speed but not by increasing kinetic energy of the participating species.


[Edited on 9-7-2014 by blogfast25]

FireLion3 - 9-7-2014 at 15:12

blogfast25, I was agreeing with you. I quoting PHILOU Zrealone because what he said came into conflict with what you said. Sonochemical excitation is much different from simple stirring.

When I was searching I came upon a thread on chemicalforums about a process engineer discussing how scaling up his reaction from 1kg to 100kg caused it to take 5x as long, but yields were not effected. He also mentioned that he was only able to stir at 150rpm in the bigger batch versus at 500rpm in the 1kg batch.

I agree with you that there is a limit to how much the reaction can increase. If the stirring is so fast that it inhibits mixing, like if an air vortex is created in the middle, that could become problematic. Though I'll bet that most reacation can benefit from much higher stirring rates than the typical 1500-2000 rpm limit of most stir plates. This is without a doubt true for immiscible reactions.

ziqquratu - 9-7-2014 at 15:48

Quote: Originally posted by FireLion3  

When I was searching I came upon a thread on chemicalforums about a process engineer discussing how scaling up his reaction from 1kg to 100kg caused it to take 5x as long, but yields were not effected. He also mentioned that he was only able to stir at 150rpm in the bigger batch versus at 500rpm in the 1kg batch.


But when you go from 1kg to 100kg, there are a lot of other factors to consider, too. Probably the biggest is heat transfer - which of course has a dramatic effect on reaction rate. This, of course, is limited by mixing, too - but the mixing itself doesn't affect the rate directly. On the other hand, in a large scale reactor, you may get local inhomogeneity (e.g. when you add the bag of reagent via the little opening in the top, or if the solution near the walls is warmer than that in the middle), which may be overcome by better mixing. But speed of mixing isn't really important here - rather it's the efficiency of the mixing.

As far as I can think, I've never come across a homogeneous reaction where the speed of mixing had a dramatic effect. And for small scale reactions, I'd probably argue that it doesn't matter at all - heat transfer should be good in most cases, and any tiny regions of local inhomogeneity would be disrupted by simple convection.

Of course, for non-homogeneous reactions, mixing is critical, but I think that mixing faster would, at a certain point, become counter-productive - once you get into big vortex territory, the mixing is less turbulent, and so less efficient (and this would be true of large-scale homogeneous reactions that have local inhomogeneity, too). I would think a lower speed but using a paddle which creates more turbulence would be significantly better than simply cranking the dial to 11!

FireLion3 - 9-7-2014 at 18:35

Quote: Originally posted by ziqquratu  
Quote: Originally posted by FireLion3  

When I was searching I came upon a thread on chemicalforums about a process engineer discussing how scaling up his reaction from 1kg to 100kg caused it to take 5x as long, but yields were not effected. He also mentioned that he was only able to stir at 150rpm in the bigger batch versus at 500rpm in the 1kg batch.


But when you go from 1kg to 100kg, there are a lot of other factors to consider, too. Probably the biggest is heat transfer - which of course has a dramatic effect on reaction rate. This, of course, is limited by mixing, too - but the mixing itself doesn't affect the rate directly. On the other hand, in a large scale reactor, you may get local inhomogeneity (e.g. when you add the bag of reagent via the little opening in the top, or if the solution near the walls is warmer than that in the middle), which may be overcome by better mixing. But speed of mixing isn't really important here - rather it's the efficiency of the mixing.

As far as I can think, I've never come across a homogeneous reaction where the speed of mixing had a dramatic effect. And for small scale reactions, I'd probably argue that it doesn't matter at all - heat transfer should be good in most cases, and any tiny regions of local inhomogeneity would be disrupted by simple convection.

Of course, for non-homogeneous reactions, mixing is critical, but I think that mixing faster would, at a certain point, become counter-productive - once you get into big vortex territory, the mixing is less turbulent, and so less efficient (and this would be true of large-scale homogeneous reactions that have local inhomogeneity, too). I would think a lower speed but using a paddle which creates more turbulence would be significantly better than simply cranking the dial to 11!


Most of the reactions I run aren't technically homogenous. I am a huge fan of green chemistry. I.E. wasting 10x excess in methanol to keep a reaction homogenous when I can just use some water and 1% of a phase transfer catalyst. Not only is green chemistry more appealing, it is easier to do, very clean to start, very clean to clean up, and in addition very cheap. Though, this only applies to some reactions. Though reactions with phase transfer catalysts would obviously get a tremendous increase in rate via mixing speed.

I've done heterogenous reactions with the reactive intermediate was soluble in both water and the reactant. The reaction goes to completion easily, but works far better in other solvents. Mixing rates would play a huge role here.

These are just a few examples. I think even minor mixing would be suitable for a homogenous reacton, but this is not always easy to come by, and definitely not always cheap.

I will be interested to see how high speeds I can reach with this thing without hitting any vortexes.

ziqquratu - 9-7-2014 at 22:06

You're definitely correct that a biphasic reaction using a phase transfer catalyst would likely benefit from better mixing - but I would again argue that efficiency of mixing, not simply speed, is the more important factor to consider.


As an aside, water is not necessarily as green as some proponents would argue. Operationally, you have to deal with a solvent which requires significantly more energy to heat up than do most organic solvents - and if you want to remove it, then that's even more energetically painful! It can also be harder on your equipment (more corrosion, leading to shorter lifespan, which can be exacerbated depending on the reagents you're using).

On top of the energy costs and capital risks, you also have the issue of waste - you can't incinerate water, so you have to make sure you clean it up before disposal. And have you ever tried to get your average phase transfer catalyst out of water? Ultimately, for many processes cleaning up the water prior to disposal (or, worse, simply dumping the contaminated water!) can actually be less green than simply incinerating 10 times the amount of methanol would have been.

Of course, these are more significant issues for large scale work - but it never hurts to bear them in mind.

AJKOER - 11-7-2014 at 18:21

One should not forget the improvement stirring can offer in electrochemical settings. For example, in a cell where Cl2 is formed, stirring can improve hypochlorite (and chlorate) formation with formed hydroxide at another electrode. Without stirring not only is the product formation slowed down, but as Cl2 can escape, total yield could also fall.

Also, stirring may keep electrodes free from inhibiting coatings that reduce output.

Note, the benefits of stirring in such cells cannot generally be duplicated by heating alone (as the latter, for example, negatively impacts gas solubilities which may be a general advantage that stirring has over heating).

[Edited on 12-7-2014 by AJKOER]