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Amy Winehouse
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Rotary vane pump during distillation question
First off, sorry if this is the wrong place to have this, but I know many organic distillation procedures call for a vacuum much stronger than an
aspirator can pull. I have a rotary vane 3CFM pump that can supposedly pull 6 pascals and this is the first time im using something other than an
aspirator. Now, to my question, and this sounds like such a basic question but:
Lets say I was distilling something using a rotary vane instead. Normally, with the water aspirator, I just leave it on the entire time. But the
rotary vane is very powerful and heats up after like 10 minutes of use. Couldn't I just wait 10 minutes until the vacuum has pulled it's full
potential then close a gate valve between the system and the vacuum? The system would remain at it's pressure and as long as the rate of vapor
production rate was close enough to the condensation rate while something was boiling(assuming the temperature is monitored correctly, and the
pressure would remain relatively constant sans vapor pressure).
Put simply:
A) Keep the rotary vane vacuum pump running for hours at a time during the distillation
B) Let the rotary vane run for 10 minutes until it displaces all of the air molecules out of the system into the air. Then, once the vacuum has pulled
out as much as it can, you close a valve between the system and the vacuum, basically sealing it shut.
Which is it? I think B would work fine, but I feel like i am missing something really obvious. Thanks for your help in advance, I've never used one of
these fancy vacuums before and i don't want to break it.
"I'm too drunk to taste this chicken"
-Colonel Sanders
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Magpie
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Quote: Originally posted by Amy Winehouse  |
B) Let the rotary vane run for 10 minutes until it displaces all of the air molecules out of the system into the air. Then, once the vacuum has pulled
out as much as it can, you close a valve between the system and the vacuum, basically sealing it shut.
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Amy,
I've never tried something like that as my $100 Harbor Freight 2 CFM rotary vane pump will pump happily away against a closed system for as long as
needed, ie, hours if need be.
I don't see why your pump would not do likewise. Isn't it rated for continuous duty? Is there free air circulation so it can cool properly? Running
hot won't hurt it a bit if the max temp is within its design limitation.
Your idea might work but your system will likely have small leaks (as do most all systems) and so you will then slowly lose vacuum and have to restart
the pump, resulting in a very non-steady state condition. It could be a real pain.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Amy Winehouse
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Okay, thats awesome man thanks because I pretty much have the same kind as you, just the 3 cfm. From the same store. I guess I assumed it was rated
for continuous duty, and yeah its circulating, just getting hot and I was concerned so I thought I would ask. I just read in the manual that there is
a temperature that it will auto-shut off. Is there an OTC oil I can buy in bulk? I feel kind of >_> about the whole "specified vacuum oil"
thing, seems kind of retail shammish
"I'm too drunk to taste this chicken"
-Colonel Sanders
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leu
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The vacuum has to be applied continuously since vapors are produced during distillation  Please download and read:
http://ia700209.us.archive.org/6/items/vacuumpractice001008m...
http://ia600806.us.archive.org/24/items/FundamentalsOfVacuum...
and
http://ia700305.us.archive.org/22/items/HighVacuumPumpingEqu...
and then study these web pages:
http://www.belljar.net/index.htm
http://www.repairfaq.org/sam/lasercva.htm#cvaoilc
http://www.tau.ac.il/~phchlab/experiments_new/SemB02_Vacuum/...
to understand what's going on and where various supplies may be obtained 
Chemistry is our Covalent Bond
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Magpie
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IIRC the oil specified was Chinese and wasn't readily available. I bought some replacement oil at my local Grainger's. I just made sure it met the
specifications of the Chinese oil. I think the only specification was a viscosity range.
The single most important condition for a successful synthesis is good mixing - Nicodem
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CaliusOptimus
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Rotary vane pumps are designed to be run continuously. In fact they won't pull a very good vacuum until fully warmed up. The average vane pump can
pull around 1x10-2mBar, so don't run it full bore on lower boiling stuff. Keep water vapor out of your pump, or the oil will cloud up and
pressure will rise significantly. A few mL of water per hour shouldn't be an issue though. Take glycerol distillation for example, the first bit of
boiling is water, but it won't condense due to the vacuum level. Instead it runs out the pump, which isn't really a big issue if the pump is nice and
hot. These pumps are ideal for high BP stuff like H2SO4, but can also be used with success for non-polar solvents. Keep in mind that pump oil vapors
are able to make it back into your rig under low flow conditions.
Pump oil should be available OTC in most cities, through an HVAC supply house. Grade or brand isn't critical. If it says "Vacuum Pump Oil" on the
bottle you won't have any problems 
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unionised
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And they are continuously condensed by the condenser so they shouldn't reach the pump.
It's normal practice to leave vac pumps running continuously- but mainly because it saves the cost of having a valve.
Also if the material unexpectedly decomposes and gives off a non-condensible gas like CO" or N2 that gas can make its way out through the vac pump. If
there's a valve in the system you can overpressure the glassware which is never a good thing.
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Dr.Bob
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I have never seen a vacuum system that does not have some leakage, and in practical terms it is nearly impossible to maintain a consistent vacuum for
any distillation or chemical process without continuing to run the vacuum. That does not mean that there won't be some residual vacuum, but not one
stable enough to do good science. But unless you have some great hardware, it will leak more vacuum that you might realize. The same holds for a
vacuum desiccator, if you have anything volatile in it, it will slowly lose vacuum. That's why I learned to pull a vacuum on it overnight first
before closing it off, as that allows them to bled away first. The same will hold for any tubing, glassware, etc, as they all hold water vapor,
organics, and plasticizers which slowly leach into the vacuum to fill it.
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watson.fawkes
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| Quote: Originally posted by Dr.Bob | | The same holds for a vacuum desiccator, if you have anything volatile in it, it will slowly lose vacuum. That's why I learned to pull a vacuum on it
overnight first before closing it off, as that allows them to bled away first. The same will hold for any tubing, glassware, etc, as they all hold
water vapor, organics, and plasticizers which slowly leach into the vacuum to fill it. |
It sounds from this
description like you're seeing desorption. In vacuum systems used for physics, which are typically not constructed of jointed glassware, one standard
procedure is "bake-out", which is just a cycle of heat-and-hold with a pump or trap active. The heat speeds the rate of desorption of individual
molecules or atoms that are adsorbed onto the inner surfaces of the apparatus. Small apparatus go in an oven. Anything large gets heaters built into
it or temporarily wrapped around it. With jointed glassware, a heat gun would speed desorption rather a lot without threatening breakage. Just to be
sure, heat up both sides of each glass joint evenly, to avoid causing leaks on one hand or socket breakage on the other.
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Organikum
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Running an aspirator as pre-stage-pump - a small one with low water consumption suffices - will prevent your rotary vane from running too hot and
contamination of the oil will be greatly diminished, in special if it has no bleed-valve this is a necessity. In addition you can install a
needle-type valve between setup and pump and open this as far as you like your vacuum, this also helps a lot.
Actually even an electric airpump as sold cheap for in-and-deflating boats and mattresses and stuff should work just fine, it is just to get some
suction going.
A rotary vane pump which is able to reach 5 PA will easily get to 1 PA with a aspirator-stage not to talk about exhaust fumes etc. As told if you
don't need this strong vacuum install a bleed-valve. Sadly most 110/220V motors are not to regulate on rpm as this would be the preferred of choice.
/ORG
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Salmo
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Organikum could you pleas explain me how to connect the aspirator to the vacuum pump to use it as a pre-stage-pump?
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watson.fawkes
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Connect them in the
following order: atmosphere <- aspirator (forepump) <- rotary vane pump <- vacuum chamber. The arrows show the direction of mass flow.
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Salmo
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Thank you but I don't understand what kind of aspirator I should use, do you mean an electric water aspirator? Because a non electric one has the air
out exit that is the water exit too.. (sorry for my english, I do my best)
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watson.fawkes
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| Quote: Originally posted by Salmo | | Thank you but I don't understand what kind of aspirator I should use, do you mean an electric water aspirator? Because a non electric one has the air
out exit that is the water exit too.. |
Any aspirator will do. The mass flow arrows above are for the gas
being evacuated. Any water that goes along for the ride doesn't affect the gas flow diagram.
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Salmo
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Where should I connect the tube that would go to the vacuum pump?
Oh maybe I understand what you mean, Should I connect the tube to the air out of the vacuum pump? my vacuum pump hasn't got an air out metal tube..
[Edited on 19-10-2012 by Salmo]
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watson.fawkes
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| Quote: Originally posted by Salmo | | Should I connect the tube to the air out of the vacuum pump? my vacuum pump hasn't got an air out metal tube. |
There's almost always a pipe-thread fitting for the gas-out port of a rotary vane pump. Sometimes there's an output filter screwed
into it. You asked about plumbing; please be prepared to do some plumbing.
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Salmo
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Thank you man I will check and I will let you know, or I will take a photo and I will ask you
This pump is similar to mine what do you think about it?
[Edited on 19-10-2012 by Salmo]
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starman
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| Quote: Originally posted by watson.fawkes | | Connect them in the
following order: atmosphere <- aspirator (forepump) <- rotary vane pump <- vacuum chamber. The arrows show the direction of mass flow.
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Interesting. Additional to the increased vacuum it deals with the oil mist in the lab with no hood.
Is an additonal suck back trap requred to guard the pump,or does the positive pressure at the rotary vane outlet negate the necessity?
Is there increased 'out misting' causing accelerated oil consumption when utilising a forepump ?
@Salmo the black knob at the top of the pump in your pic is the outlet filter.You remove this and screw in your spigot.
Chemistry- The journey from the end of physics to the beginning of life.(starman)
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watson.fawkes
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| Quote: Originally posted by starman | Is an additonal suck back trap requred to guard the pump,or does the positive pressure at the rotary vane outlet negate the necessity?
Is there increased 'out misting' causing accelerated oil consumption when utilising a forepump ? |
In ordinary
operation, suck back of the pump oil shouldn't be a problem. On the other hand, if you take down the vacuum in the wrong order, you could have exactly
such a failure. If you're doing a fixed installation, it wouldn't be a bad idea to buy some insurance there.
Pump mist is most easily dealt with by a column at the exhaust loosely packed with steel wool or gauze. Mist droplets hit the obstruction, coalesce,
and drip down back to the reservoir. Gas proceeds as normal.
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Salmo
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Watson do you think that with this setup it would be possible to remove solvents too like a rotavap or is it still too dangerous for the vacuum pump?
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watson.fawkes
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| Quote: Originally posted by Salmo | | Watson do you think that with this setup it would be possible to remove solvents too like a rotavap or is it still too dangerous for the vacuum pump?
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Vacuum pumps aren't made of magic. This configuration has a specific purpose, and it's not to make the pump
chemically safe nor to make it magically applicable to anything that needs vacuum.
The purpose of using a pump train is to increase ultimate vacuum and the pumping rate. For solvent removal, ultimate vacuum is definitely not the
important figure of merit, as too much vacuum can cause problems. Plenty has been written on this here and elsewhere.
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ElectroWin
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you are describing a perpetual motion machine. it wont work in the real world. you must keep the pump running, at least intermittently.
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watson.fawkes
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| Quote: Originally posted by ElectroWin | | you are describing a perpetual motion machine. it wont work in the real world. you must keep the pump running, at least intermittently.
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Nope. @unionised was describing a heat engine driven by phase changes. Evaporate at the boiler, vapor
transport, condense at the condenser. In the kinds of high-vacuum systems used in materials processing, it's quite common to use a trap or a getter
rather than a pump to remove residual pressure. A well known example is the silver top of old vacuum tubes. Many of those mirrors are metallic
calcium, flash evaporated inductively after sealing, under pump, typically. The calcium reacts with many of the common contaminants, so much so that
the pressure in such tubes actually drops during the period of initial operation. Whether you trap particles with a chemical reaction or with a phase
change, it's still a heat engine.
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Salmo
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Thank you watson do you think that DCM is dangerous to strip for a "cheap" vacuum pump like mine?
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watson.fawkes
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| Quote: Originally posted by Salmo | | Thank you watson do you think that DCM is dangerous to strip for a "cheap" vacuum pump like mine? |
Why is it
that you think you need vacuum to strip DCM?
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