Operation of rotary evaporator

Operation of rotary evaporator,here’s the information you’ve been waiting for: a long, nitty-gritty section on flying the rotovap.

Correct Order to Set Up a Rotovap

Plug water bath, rotovap, peristaltic pump, and vacuum pump into one 20 amp circuit. Plug chiller into separate 15-20 amp circuit. Do not attempt to run entire system off of one circuit. Turn on chiller and allow it to reach temperature. Fill water bath (usually 2/3 full) with warm to hot water, set the temperature and turn it on. Put the thermometer from the rotovap unit into the water bath. Clean the vapor duct area and install the vacuum seal. Install a DRY evaporation flask. Close off (seal) the receiving flask/pump takeoff area. Close the product in/air vent valve.

If some numbskull turned the bath on without water, you’ll need to reset the bath. There is usually a hole big enough for a paper clip in the front panel of the bath. Stick a paper clip in there. The heater indicator should come on.

Perform Vacuum Test

With the evaporation flask stationary, turn on the vacuum pump and let the system get to full vacuum. You are looking for ultimate vacuum and for how fast you reach full vacuum. If you don’t get any vacuum at all, you have a major leak. Most likely you left something open. You can try listening for where the leak is, but the sound never appears to be coming from where the leak actually is. If it takes a long time to reach vacuum, and the vacuum isn’t very good, you might have a slow leak. Inspect all your valves and closures. A prime candidate for leaks is the rotary vacuum seal. Make sure it is clean and installed properly; make sure the O rings are good on the secondary seal; make sure there are no nicks on the seal; make sure the vapor duct is running true. If you can’t find a leak but your ultimate vacuum isn’t very good, your pump may be contaminated with vapor. Run it for a while in free air (no vacuum) to clear it out.

Once this test is complete, start the flask rotating and make sure you still have a good vacuum. A leaky rotary seal will make a ssssht ssssht ssssht sound as the flask rotates. If this happens you need to take off the flask and readjust the seal, the vapor duct, add grease (sorry, manufacturers), etc. You want to do this only after you have done the stationary test because the rotary seal works better once vacuum is attained anyway, and testing rotation second makes diagnosing problems easier. Next install the receiving flask or peristaltic pump take-off and ensure that the vacuum is still good. If everything checks out set the vacuum level to your typical starting value. We use 100 millibar. Vent slightly to raise the pressure.

Setting Up the Run

If your product has been strained and is not too thick, you are ready to go. Suck the product into the evaporation flask using the product in/air vent valve. If your product contains particles you have to open the product in/air vent valve to break vacuum, stop the flask rotation, remove the flask, pour the product into the flask, and reinstall. If possible, it is better to suck the product into the flask rather than to break the vacuum. Firstly, every time you break vacuum there is a chance the vacuum seal won’t re-seal properly, and that is a pain in the butt when you are running product. Secondly, it is technically better from a solvent recovery perspective to start from a vacuum (we don’t think this really matters but the scientists do).

Thirdly, sucking the product into the system de-aerates it as it sprays into the evaporation flask, lessening the chance of boil-over.

Starting the Run

Do not let an evaporation flask full of product sit in a warm water bath while you are fooling around with the equipment. The flask should only be in the water bath when you are actually distilling. During distillation, the product never reaches the temperature of the water bath. It could be as much as 10 degrees cooler than the water bath (because of evaporative cooling). A flask just sitting in bath will reach bath temperature and will boil over when you start distilling because it will be hotter than you expect.

Once your product is in and the peristaltic pump is turned on, lower the flask into the water bath using the lift-jack so that the product level in the flask matches the water line in the bath. If the water line is way below the product line, the flask can wobble quite a bit as it rotates. When you are lowering the flask make sure you don’t smash the flask into the water bath (it is very easy to do).

Now comes the most critical point of the distillation. Turn off your cell phone and make sure your buddies aren’t going to ask you a lot of questions. Concentrate on what you are doing. Make sure you can vent the system quickly. If the vacuum is stable and your product is not boiling, reduce the pressure until it does boil. As the product first starts to boil, you are stabilizing the distillation and getting rid of entrained air. This is where boil-over is most likely to happen. Suddenly, the product will begin to boil violently and shoot through the vapor duct, contaminating the distillation side and ruining your run. This occurrence is common and really, really, sucks. Some products are easy to clean out of the system “in place,” but some require you to break down the whole system and clean it ultrasonically. There is something called a bump-flask, which fits between the evaporation flask and the vapor duct, which can prevent small boil-overs from contaminating your rig, but they don’t fit on our flask. Keep adjusting the vacuum pressure, periodically venting the system if boil-over starts to happen (short vents—less than a second long) until a stable distillation is obtained.

The Main Run

The hard part is now over. The trick is to constantly adjust the level of vacuum so that the condenser is always running at full tilt without saturating. The faster it distills, the better the flavor. Period. This, along with not letting your product hang around in a receiving flask, is the most important rule of flying the rotovap. Side by side, you can always taste which product was made by the person who kept the distillation going fast. For our explanation on this, see Section IV.

If you see distillation only in the bottom part of the condenser that means you need to decrease the system pressure to make it boil more. If the distillation goes all the way to the top you have saturated the condenser. This means that vapor is actually entering the pump system and that you are losing flavor and contaminating your pump. Double whammy. If you saturate, raise the pressure and momentarily vent the system.

If you have a good vacuum controller and a dry vacuum pump, your pump will turn itself off when it is not actually pumping. Your pump should not need to run all the time (unless you are at the limit of what it can pump). If does run constantly, you are probably saturated (or you have a leak, or both).

You should always keep an eye on the evaporation flask to make sure you aren’t boiling over. If you have a peristaltic pump you should always keep an eye on the adapter to make sure the pump isn’t backing up with distillate. If it is, your tubing is collapsing, and you should replace it for your next run. For now, vent the system till the tubing un-collapses and the pump starts pumping again, and resume the run.

For more techniques (like “burping”) see the recipes in the next section.

Ending the Run

Part of the fun of flying the rotovap is tasting the distillate as you go and seeing how the flavor changes from moment to moment. Part of the art of rotovapping is knowing when to cut the distillation off. Invariably, the product is best when the distillation is cut off while there is still flavor in the distillate. Experience will tell you when to quit.

When the distillation is done you have two choices: 1) raise system pressure, suck in new product, and start distilling some more (this only works if you have enough room in your flask for more product); or 2) break the vacuum. To break the vacuum, turn the product in valve to vent, stop the flask rotation, turn off the peristaltic pump, turn of the vacuum (if desired), then remove the flask.