Tymir
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DIY Rotovap Vacuum Controller
I am currently in the process of assembling a rotovap setup with the intent of using it for culinary purposes. I'm wondering if distillation will run
too long (and thus I will lose my precious volatile aromatics) if I just rig up a stepper controller to a diaphragm pump, combined with a bleed valve
as an emergency option incase bumping starts bad. Buying a real vacuum controller seems like it could be overkill for my application. I will probably
be using ethanol as the solvent and running around 20 mbar, from what I've heard I need to get down to 8-10 mbar to do water based extractions or to
make things like deliciously unreacted port reduction syrup.
Some input from the more experienced would help. Thanks.
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peach
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Ethanol and water will be boiling around 35 to 25C if it's a 100mBar pump, which means you could catch both with cold enough tap water or using a
bucket filled with ice.
I'd be interested to see how a stepper motor works but, given the speed AC motors usually turn, unless you use a gear box, it may be better to use a
servo with an encoder on it. If only so it runs quieter.
Easier than both would be to run the motor that's already in it from a variac or some other form of speed controller and simply turn it down. Or bleed
some gas into it. The controllers are probably more useful for people in science labs, who are constantly putting different things through and need it
removed as fast and cold as possible. You've only got two different solvents of interest there, water and alcohol. Unless you're using anhydrous
ethanol, and the thing being washed with it is also dry, that's going to end up boiling around 80C in an azeotrope, which is fairly close to water, so
you shouldn't need to do too much adjusting of the pump once you've found a pressure that works. The diaphragm pumps they use can also have a
condenser after the pump, and the pump needs to clear it's self of condensing solvent so it periodically blows the chamber clear. Someone has a patent
on that. Can't remember who, Pfeiffer?
I wonder if one of those pressure transducers from a car's intake manifold would suffice for reading the pressure?
Here's a post a made a while ago in which I calculated the pressures needed for various solvents based on using tap water. They're not far off the
one's Buchi recommend, which I hadn't checked beforehand, so they seem about right.
As someone who does a fair bit of cooking himself, I am somewhat bemused by the sudden uptake of rotovaps in the kitchen. As I can't really see any
use for such an expensive bit of gear in cooking dinner. Walk into pretty much any three star restaurant and they'll get rid of the alcohol by just
igniting the pan. Or leaving it to simmer for a few hours. Never underestimate the power of the simmer Luke!
I hope you're using brand new glassware on the entire thing. Christ knows what's been through it if it's used. Also be wary of using laboratory grade
solvents in it, as they're not made to a consuming standard so there's a chance there could be something nasty lurking in some of them.
[Edited on 6-3-2011 by peach]
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Tymir
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Thanks for the advice. Stepper control does sounds a bit too involved and costly.
I guess I better start thinking more about a pressure gauge as well, I'm sure it won't be too long before I'm tired of farting around with guesswork,
even if it is only for culinary experimentation.
I was taking a look at the KNF Neuberger 843.3 as a potential pump and I believe the specs say the A/C voltage range is 198 to 241 V. Will a variac
give enough variance in motor speed over that range? Or is that just one of those useless manufacturers' advisories that can be disregarded,
especially on the lower end?  I'm still not too sure which form of motor speed
control is ideal since I have very little experience with vacuum motors of this sort.
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peach
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The voltage range they quote will be the range for it to meet it's specifications, but it'll likely still turn even if the voltage browns out.
You can build and buy various setups to control AC motors by solid state, but it can be kind of tricky since the motor has capacitance and inductance,
making it more than a simple resistive load; then you end up with feedback and power factors causing problems. For simplicity, and given that the pump
probably doesn't need much power, a small variac would do.
You don't need a pressure gauge if you're just trying to remove a solvent, you can do that by eye and get the same result. You'd lower the flask into
the bath at 40C or so, then start turning the speed of the pump up until the solvent begins to boil, and then leave it. You know what you're trying to
remove, and that is by far the major component in the flask. You only really need to know the pressure when you're trying to identify a distillate by
temperature, under vacuum.
Starman made a post just recently asking about the pressures and gauges here.
If you're not aware of it yet, you need to get to know and love the nomograph.
You can estimate pressures with one, without a gauge. If you have mixture that is largely water, you know the water's rough boiling point at 1ATM is
100C, so you can calculate what the pressure inside the glass must be based on the new temperature it's boiling at. Then use that number to predict
where other fractions in it will boil. Although, since this is all about removing solvent from things for dinner, that's not going to much of an
issue. You can just watch the thermometer. It'll sit still at a specific temperature or in a small band as most of the solvent comes off. When the
flask is looking substantially reduced and the temperature changes, it's time to stop.
What are you planning to do first, reduce port?
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Tymir
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Thanks for the help. Probably gonna make a yuzu syrup first actually.
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watson.fawkes
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Quote: Originally posted by peach  | | As I can't really see any use for such an expensive bit of gear in cooking dinner. Walk into pretty much any three star restaurant and they'll get rid
of the alcohol by just igniting the pan. Or leaving it to simmer for a few hours. |
You get different flavors
if you extract without heat; call it a "raw" concentration, one done without cooking.
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arsphenamine
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[mode="redneck"
style:= "JimVarney" ]
Vern, I got just one wird fur yew: molecular gastronomy.
[/mode]
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