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Author: Subject: Rotary vane pump during distillation question
Amy Winehouse
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[*] posted on 11-9-2012 at 14:29
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.




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Magpie
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[*] posted on 11-9-2012 at 14:43


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.


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.




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Amy Winehouse
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[*] posted on 11-9-2012 at 15:22


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



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leu
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[*] posted on 11-9-2012 at 17:23


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 :cool:





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Magpie
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[*] posted on 11-9-2012 at 17:38


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.



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CaliusOptimus
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[*] posted on 15-9-2012 at 16:04


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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[*] posted on 16-9-2012 at 01:31


Quote: Originally posted by leu  
The vacuum has to be applied continuously since vapors are produced during distillation ;)




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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[*] posted on 16-9-2012 at 18:23


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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[*] posted on 17-9-2012 at 03:13


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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[*] posted on 11-10-2012 at 12:08


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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[*] posted on 18-10-2012 at 11:31


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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[*] posted on 18-10-2012 at 14:44


Quote: Originally posted by Salmo  
how to connect the aspirator to the vacuum pump to use it as a pre-stage-pump?
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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[*] posted on 18-10-2012 at 23:33


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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[*] posted on 19-10-2012 at 06:06


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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[*] posted on 19-10-2012 at 09:03


Where should I connect the tube that would go to the vacuum pump?

g007631.jpg - 61kB
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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[*] posted on 19-10-2012 at 10:45


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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[*] posted on 19-10-2012 at 11:12


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]

Screenshot_2012-10-19-22-06-54.png - 140kB
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[*] posted on 19-10-2012 at 18:31


Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Salmo  
how to connect the aspirator to the vacuum pump to use it as a pre-stage-pump?
Connect them in the following order: atmosphere <- aspirator (forepump) <- rotary vane pump <- vacuum chamber. The arrows show the direction of mass flow.


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.




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watson.fawkes
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[*] posted on 19-10-2012 at 19:46


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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[*] posted on 20-10-2012 at 02:24


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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[*] posted on 20-10-2012 at 03:43


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?
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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[*] posted on 20-10-2012 at 09:44


Quote: Originally posted by unionised  
And they are continuously condensed by the condenser so they shouldn't reach the pump.


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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[*] posted on 20-10-2012 at 13:09


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.
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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[*] posted on 21-10-2012 at 02:47


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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[*] posted on 21-10-2012 at 06:15


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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