andre178
Hazard to Self
Posts: 61
Registered: 11-12-2009
Location: San Diego, CA
Member Is Offline
Mood: pacified
|
|
new rotovap or improvise
I'm curious, after a bit of searching on google, and some major manufacturers, I found that new rotovaps are hard to purchase, if not impossible. Is
it a technology that can be improvised or gone around using a condenser and stirring rod?
The old one I'm using right now is slowly giving out, it leaks everywhere, and I'm not sure if I should spend time replacing it or just making a
tilted system with a condenser upright improvisation for distilling solvents.
|
|
|
Magic Muzzlet
Hazard to Others
Posts: 146
Registered: 22-7-2010
Member Is Offline
Mood: No Mood
|
|
Well you could get one off eBay or labx, or you could just use a rapidly spinning stir bar and remove your solvent in a distillation setup. This is
what I do, it takes just a bit longer but it's all the same. No way in hell I'm paying thousands or even hundreds for rotovap when I can do the same
with equipment I already have. 
|
|
|
not_important
International Hazard
Posts: 3873
Registered: 21-7-2006
Member Is Offline
Mood: No Mood
|
|
Early 20th century tech was a pair of flasks, connected using a pair of stoppers and a wide glass tube so as to resemble a cartoon barbell. Laid on
their sides, one flask in a hot water bath and the other in cold/ice water, possibly with a stream of cold water running over the cold flask. The
flasks were rotated around the axis formed by the tube, in some cases using a simple water turbine on the tube near the condensing flask and turned
using a 2nd stream of water. The fancy setups had a small tube offset in one the the stoppers, a rubber tube attached to that and through which the
flasks were evacuated and then the rubber tube clamped closed.
So, early versions of rotovaps, not as effective with the really low boiling solvents.
|
|
|
Lambda-Eyde
National Hazard
Posts: 860
Registered: 20-11-2008
Location: Norway
Member Is Offline
Mood: Cleaved
|
|
Having a rotavap in the lab makes life easier for you. Not having to deal with clips, clamps and tubes every time you want to remove a solvent is
preferable - that's why it's so nice to have a rotavap ready for work at all times! If I had one with a few problems, I'd try to find replacement
parts for it. They can be quite expensive, but MUCH less expensive than buying a new rotavap.
|
|
|
peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
|
|
There is a PDF floating around google somewhere written by a guy at university (a lecturer I think) who was complaining about the cost of rotovaps.
He'd made one using a ball and socket joint he'd turned on a lathe from PTFE, sealed with vacuum grease.
If you've got a lathe to hand and are good with your hands, you could try that. If not, I wouldn't try randomly connecting bits of glass and motors
together.
One possibility to produce the vacuum tight, but rotating, socket may be to use an overhead stir gland, as these are designed to maintain a vacuum
whilst also spinning - with one section staying still.
A side effect of using a stir gland is that they are usually glass. Balancing a heavy flask full of solvent on one end and spinning it, you would need
to be sure of what you were doing or I can envisage accidents fast approaching.
As some of the others have already pointed out, you can remove solvent very easily using standard glassware and a vacuum. Even without the vacuum,
most of the volatiles will come off quickly.
The benefit of the rotovap is simplicity. Meaning, if you have to be incredibly careful with your DIY setup, you may be better just using a normal
still setup.
I am also a little upset by rotovaps, as they seem to be promoting people to just ignore what is going on. I have seen numerous complaints from people
about solvent getting through the traps. This is always a side effect of them running them from high vacuum sources.
I suspect the older generations, more used to having to set up a still and put a bit more care into it, will appreciate why the solvent may be getting
through the trap.
Worse, the solutions I see to the problem involving spending more money as opposed to less.
Curly arrow has a post on his blog where he's using a double oblique stopper to control the vacuum, with one of the ports in the stopper being a fine
capillary. The reasoning being, a finer capillary will create a poorer connection with the vacuum pump.
That demonstrates that the vacuum pump is pulling an excessively hard vacuum in the first place. And, if you need a continuous flow from the pump,
that also indicates that the trap is not actually trapping the solvent with a high efficiency.
If the trap is functioning, there should be zero flow through the vacuum pump - bar the leakage from the joints.
Also on the point of convenience, rotovaps tend to reduce the boiling point to the lowest possible, to effect a very rapid boil off rate at low bath
temperatures.
That means dry ice or liquid nitrogen really needs to be involved.
At home, it may be less of an issue. You could use a weaker vacuum and a decent, none dewar, condenser. In fact, quite a few rotovaps use coil
condensers.
Going back to curly arrow and the others, I can understand that the most convenient source of a decent vacuum in universities is usually coming from a
rotary or a rotary with a diffusion pump on it.
However, if you connect most volatile solvents to the vacuum from a fridge compressor, the boiling point will drop to ten to a few tens of degrees C.
This is an important point because, without resorting to dry ice and liquid nitrogen, it is also the temperature range where water cooled condensers
begin to fail in terms of liquefying.
It's usually recommended you have around 20C of temperature different between your condenser and the BP of the thing going through it. This changes
based on the efficiency of the condenser and the flow rate of the thing you are putting through it. You can use liebigs with very little temperature
difference, provided you spend hours doing it slowly.
In a rotovap, the goal is usually speed - as much solvent through as quickly as possible.
In general then, you want your new BP under vacuum to not be below 10C, and preferably more like 20C. A coil condenser is needed for any form of
decent flow rate. You will struggle to keep a water condenser close to zero without resorting to buckets of ice or putting the entire thing in the
freezer.
[Edited on 8-2-2011 by peach]
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
When using a rotovap it is actually advised to have the vacuum set so that the boiling point of the solvent is 40*C. This then allows for your 20*C
difference with the condenser cooling water, and the bath itself should be set to 60*C - this is what Buchi teaches, according to a past member. And
it seems to work fine. The issue most people will have is controlling the vacuum. If you can get a gauge on there somewhere, the air leak at the top
of the condenser can be adjusted to give some crude control.
In reality, vacuum is still pulled with solvents like diethyl ether and dichloromethane, and as such these usually end up going through the pumps.
There are commercial vacuumstations which utilise a PTFE diaphragm pump and a condenser assembly on the OUTLET, allowing for near 100% solvent
recovery.
[Edited on 8-2-2011 by DJF90]
|
|
|
peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
|
|
Using buckets of ice, I've seen the temperature of the water in my condenser a few degrees above zero. Buckets of ice are a lot of hassle, so I'll
assume you're working straight off the tap. It's winter here in the UK and, having just stuck a thermometer under the tap, the water is coming out at
7-8C. Assuming a 20C difference for condensation, that's a BP of around 30. And, given that the tap water is going to warm up in summer, that's 40C.
Atmospheric pressure (1 ATM) is 760 Torr (mmHg), 1013.5 mBar and 101,350 Pascals.
Running the more common solvents through the pressure calculation for a BP of 40C gives;
Acetone
424 torr, 560 mBar
Ethyl Acetate
208 torr, 277 mBar
Toluene
51 torr, 67 mBar
Hexane
297 torr, 396 mBar
Cyclohexane
183 torr, 244 mBar
Chloroform
357 torr, 472 mBar
Diethyl Ether - Not even going to bother working that out, it boils at 35C under 1 ATM. If you're running ether through a tap water condenser, under
vacuum, you should be immediately disqualified from the course.
DCM - Same, boils at 40C under 1 ATM
Isopropyl Alcohol
103 torr, 137 mBar
Ethanol
135 torr, 180 mBar
Methanol
258 torr, 344 mBar
Water
62 torr, 82 mBar
Here are some examples of the levels of vacuum generated by different methods;
A vacuum cleaner
800 mBar
Sucking with my mouth 
200 mBar
Basic diaphragm pump (I wonder how aquarium pumps do? Anyone got one they want to post me to check?)
100 mBar
Water aspirator
32 mBar at 25C
The fridge pump I've left on since last week, after putting all kinds of stuff through it
29 mBar
Dual stage HVAC rotary vane
0.05 mBar
Dual stage laboratory rotary vane (the vacuum source attached to Schlenk lines)
0.001 mBar
Diffusion pump (also found on Schlenk lines)
0.000000001 mBar
As you can see.... both the rotary and diffusion pump are WAAAAAAAAY out the other end.
If I put the rotary pump pressure through for say, acetone, we get a BP of -124C.
Dry ice melts (sublimes) at -78C. No where near close enough, it's on the wrong side of the BP for a start.
If a diffusion pump is used, the BP of the acetone will drop to -176C. The only way to trap it is going to be liquid nitrogen, at -196C. Once you're
using liquid nitrogen, you then have the potential of liquefying oxygen (-183C) in the condenser as well. Liquid oxygen + volatile solvent --->
Fire
Let's now take a look at the prices!
Vacuum cleaner
Free, steal it and pretend you're doing the tidying up. Downside, it's not strong enough and vacuum cleaner motors SPEW sparks out in the gas path
My mouth
Depends on the mood and how broke I am at the time. Downside, not a particularly controllable source of vacuum and I'll get a headache
Diaphragm pump
£880 for the BOC Edwards D-Lab (their most basic)
Aspirator
£20 for a new Chapman from eGimp
Fridge pump
Free. Or, if you want to be classy like MossyDie, £2.30 new from eBay.
Dual stage HVAC rotary vane
£73
Dual stage rotary vane
£1376.4 Edwards E2M0.7 (cheapest they do)
Diffusion pump
£692 Chemglass with mantle
SO!!!!!!!
You can see that for a 40C condensation point, you are looking at a diaphragm, fridge or aspirator pump for the majority of volatile solvents. A HVAC
if you want it all covered.
Taking acetone as a model example, a dual stage rotary vane lab pump is five orders of magnitude too strong in terms of pressure and a diffusion is 11
orders of magnitude out.
Looking at price, and comparing it to MossyDies new fridge pump, at £2.30, the laboratory rotary is three orders of magnitude more expensive, and the
diffusion is the same.
You also have to factor in for other issues. For example, if you use the laboratory rotary and diffusion, that means buying dry ice and liquid
nitrogen, storing them, and having the equipment to handle them.
Putting things under vacuum dose not change their enthalpy of vaporisation. It doesn't make more boil off per unit of heat going in. The only thing
that changes is the temperature gradient. Going back to acetone, you're looking at a gradient of about 250C (DIFFERENCE) if the flask is in a hot
water bath. Borosilicate is rated to +400C. So you can see there is potential to put the flask under vacuum using one of the other methods, then heat
it in an oil bath to establish the same gradient or more.
The gas in a system at 100 mBar is at a density of 10^17 molecules per cubic centimetre. Under a lab rotary it's 10^12, and under a diffusion it's
10^7. The density of the material going through the condenser has dropped many zeros.
The bad method
One Buchi R-210 rotovap
£4246
+ A BOC Edward E2M0.7 + Chemglass Diffusion + Chemglass Schlenk line
£8705
I mean FUCK! To boil a solvent off? And not condense it?
Conclusion
If condensing with tap water or a tank of water and a pump is the goal, it should be very apparent which are the smarter choices from the above
numbers.
Tanks of water with a pump in them have a horrible tendency to warm up whilst you go for that quick cup of coffee. And if you're using ice slush, the
pressures won't need to go much lower to meet up with those new BPs.
This, £8705 way of improving education, is why tuition fees have jumped from £3k to £9k this year. And it's improved it so much
people, with PhD's, now think that is a good idea.
[Edited on 8-2-2011 by peach]
|
|
|
Regolith
Hazard to Self
Posts: 73
Registered: 4-2-2011
Location: Mining the moon.
Member Is Offline
Mood: Glacial
|
|
I DO like Peach doing all the googling for me, I like to have actual boiling points not just memory to go by when composing my thoughts. I'm confused
by the difference between the 2 devices I know what each does and it's intent but if you could only own ONE. The answer is an obvious full
distillation setup with stir bar. The downside is it's gonna take a little longer, you can't go above the curie temperature of the magnetic stir bar
and your wallet is gonna be way WAY heavier, like its gonna take some effort to haul around the extra money you didn't spend. Wait, did I say bad
thing ?
|
|
|
turd
National Hazard
Posts: 800
Registered: 5-3-2006
Member Is Offline
Mood: No Mood
|
|
No way you'll get 10^-9 mbar on a Schlenk line. 
| Quote: |
The bad method
One Buchi R-210 rotovap
£4246
+ A BOC Edward E2M0.7 + Chemglass Diffusion + Chemglass Schlenk line
£8705
I mean FUCK! To boil a solvent off? And not condense it?
|
All roties I have seen were connected to controlled(!) diaphragm pumps like this one:
They're too pricey for the amateur, but a research institution will buy them by the truck load (literally!), making them much less expensive. Also, in
the situation you describe the rotary pump is needed for the Schlenk anyway.
Concerning tuition fees, complain to your politicians. 
|
|
|
madcedar
Hazard to Others
Posts: 116
Registered: 10-9-2009
Member Is Offline
Mood: No Mood
|
|
Another article on rotary evaporators here: http://www.cookingissues.com/primers/rotovap/
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
