posted by Dave Arnold
As readers of this blog might know, I’m building my own rotovap. I think I can improve on the laboratory rotovap (I know I can). For the reasons to make a new one, see here. If you don’t know rotovaps, see here.
Aluminum condenser for my rotovap
Condenser for my rotovap top view
Here is the condenser section of the new rotovap. I made it from 3/16 in soft aluminum tubing (purchased from McMaster Carr). I chose aluminum because it is super-easy to bend, is a great heat conductor, and doesn’t impart any taste to the distillate. I have had all of my distillate running through an aluminum pump adapter for years and haven’t had any corrosion or off tastes. This condenser has a larger surface area than the one on my Buchi, but is of similar design. Unlike regular rotovaps, however, this condenser can be taken apart to clean easily (finally!). This condenser also won’t break. The cover for the condenser is a super-custom piece of polycarbonate (a Click-Clack pasta storage container from the Container Store).
If you’re like me and don’t have a real shop or a bending jig, this type of tubing can be bent fairly well by first bending the coil into a large cylinder (a large bain marie) and then tightening the coil around progressively smaller cylinders (I ended up on a rolling pin). When the coil is large you can make pretty large reductions in diameter between steps. When the coil is smaller you have to make smaller reductions in size. I used maybe 7 different sized cylinders (wine bottles, pipes, etc). The only tough part is doubling the coil back on itself. It just takes some practice.
I’m just sorry I won’t be able to work on the project more till July 13th cause I’ll be traveling.
9 responses so far ↓
Fabulous // July 2, 2009 at 12:38 am |
Coming to Mexico?… This new rotovap looks pretty pretty pretty good Dave, no more being afraid of habanero.
H. Alexander Talbot // July 2, 2009 at 9:43 am |
wonderful to see the process and desire to create a functional kitchen version of the rotovap
A
Rochelle // July 2, 2009 at 11:24 am |
Ian // July 7, 2009 at 10:41 pm |
I can’t wait to see the finished plans. I would love to build one of these.
There is a better, faster, foolproof way to bend the tubing. The big danger when bending tubing like this is that it kinks, which is why you have to start large and carefully get smaller. To prevent kinking, pinch off one end of the tubing with a vise or a good pair of pliers. Then straighten out the tubing and fill it with a fine sand. Any clean sand will do. Once the tubing is completely full of sand (tap the end on a concrete floor a couple times to pack it down real good), pinch off the other end. Now when you bend it, the sand is incompressible so the tubing will keep a constant volume and it won’t kink. You can start out with the rolling pin, bend the remaining ends up the center, or put any type of curvature you want into it and not worry about kinking it.
When you’ve got the shape you want, use a tube cutter to cut off the pinched ends and dump the sand out. Finish off by running water through it to remove the residual sand. I’ve made several heat exchangers in my lab this way and it works flawlessly every time.
I can’t wait to see the finished rotovap!
Dave A // July 9, 2009 at 2:45 am |
I actually purchased some cerro-bend alloy to do the tube bending but didn’t end up using it because I couldn’t immerse the entire tube in hot water. I’ve always wondered if the sand trick works.
Ian // July 28, 2009 at 1:37 am
The sand trick works great. I use it on thin walled copper refrigerator tubing all the time. We get a very fine sand in the lab, Washed Sea Sand from Fisher Scientific. It’s cheap, it’s reusable, and because it’s a relatively uniform size and well washed, it flows very well, so it’s not a problem to fill as small as 1/8″ tubing.
milkshake // July 27, 2009 at 4:06 pm |
Why do you need to build a rotovap? In synthetic chemistry lab, rotovaps are great if the non-volatile evaporation residue are what you are after. But they literally suck at recovering the distillates; the losses are too high.
What you really want is vacuum distillation still. The rotovaps prevent “bumping” of the distilling liquid by spinning the flask. But there is much simplier way: stirring. Buy some second-hand magnetic stirplate on e-bay, preferably a large Corning model because it is more powerful. You can get one for less than $150. Add a large teflon-coated egg-shaped stirbar into your distillation flask placed on a heating bath, connect it to some common condenzer, with a receiving flask. For a bigger oomph cool also the receiving flask on crushed ice/salt slush bath. The advantage: no motor and spinning parts (except for the stirplate), no problems with rotating seals, very little losses of the distillate if you adjust the pressure and temperature correctly.
You don’t have to worry about continuously removing the distillate. Breaking the vacuum is non-issue when your total allowed volume in the distillation flask is 3 liters. At 15% original alcohol concentration and 90% final concentration in distillates you will produce at most only half liter of distillates – so get a bigger receiving flask instead!
Dave A // July 27, 2009 at 5:54 pm |
Hi Milkshake,
I’ve used stir bar agitation for my vacuum distillation tests in the past but they never generated the surface area that I get out of my rotovap. I agree that the rotary seal is a huge weakness of the rotovap which is why my new one won’t have one. I am spinning an inclined cylinder inside the vacuum to agitate and provide surface area without having to use a crappy vacuum seal (using a magnet). You’d be surprised how much flavor loss you get from your distillation flask even when it’s kept on a crushed ice/salt mixture. That was the system I used to use prior to getting the peristaltic pump. Remember, I’m dealing with a distillate that has dozens, sometimes hundreds of different compounds in it in trace amounts. It isn’t that the flask isn’t big enough, it’s that it steals flavor! That is the main difference between kitchen rotovapping and lab rotovapping. I also need to be able to taste to see when the distillation is over, which is the other advantage of the pump. Theoretically I shouldn’t be getting a lot of loss off the flask because it is so much colder than the water bath, but empirically, I do.
syuzo fujimoto // December 21, 2009 at 1:20 am |
I wish to product your imaging rotovap with you .
Your idea is as same as I.