Rotary evaporator, or rotovap, is an instrument used to distill a solvent. (Everyone calls it a “rotovap” even though rotovap is a brand name). The purpose of distillation is to separate a given mixture into its components based on their respective volatilities, through the process of evaporation and condensation (liquid-gas-liquid).
Now we’ll revisit some key components of the rotovap, providing more details on our own setup and tips to help you build your own.
Evaporation Flask: The mixture to be evaporated starts in this flask, which rotates in a heated water bath. The size of your flask determines how much liquid you can distill at one time. Bigger is better, so long as your condenser can handle it. Ours is 3 liters.
Ideally, you want to attach the flask to the rotovap first, and then pump the solvent directly in (more on why in Section VII). Don’t fill the flask more than half full or you’ll have problems with boil-over. And you’ll decrease surface area and slow down distillation. Our flask has a special wide neck, which makes it easy to get product in and out. If your machine can’t accommodate a 3-liter flask, invest in an evaporation beaker, a wide mouth beaker that can be hooked up to a rotovap.
Rotary Drive: Controls the rpm of the flask. We keep ours at around 120.
Water Bath: Filled just up to the point where the mixture in the flask is submerged, the water bath transfers heat to the mixture quickly and efficiently. Water is the optimal medium to use since it’s safe and cheap, distributes heat well, and has a high specific thermal capacity (though on certain occasions, it may be necessary to heat the mixture using an oil bath).
Vapor Duct: This is where the vapor is transmitted from the evaporation flask to the condenser. It is also where your system is most likely to leak. Make sure your vapor duct is super-duper clean and properly centered. On our EL model, the vapor duct is stationary, and the seal rotates around it, as opposed to most other models, where the seal is stationary and the vapor duct rotates. This difference won’t make matter to most people, but since the EL series aren’t made anymore, they are often cheaper. They also allow reflux distillation (for the hard core rotovap users out there), but the seal goes bad on them more often.
Rotary Vacuum Seal: These cost 50 bucks a pop for a tiny ring of rubber so take good care of them! Remove the seal from the rotovap when you’re not running it. The manufacturer says to never grease them because grit can get into the grease and ruin the vapor duct. We do anyway because is the only way we can get the seal to work. Make sure you install them the right way (check your manual for details).
Condenser: As solvent surrounds the condensing coil with vapor, this part of the instrument turns the vapor back into a liquid (using the same amount of energy to cool that was originally needed to heat the liquid into a vapor). The vapors condense into liquid droplets that, by the power of gravity, make their way down the coil into a receiving flask.
You need enough cooling power in the condenser to condense everything that has vaporized in the evaporation flask, or the condenser will get saturated. This will contaminate your pump and make the mixture lose flavor. The more cooling power your condenser has, the more liters per hour you can distill without saturating and without losing temperature. There is no point, however, in having more cooling capacity than you have heating capacity in the water bath.
Another type of condenser is the “cold finger,” which consists of a sleeve filled with extremely cold liquid, like liquid nitrogen or dry ice and acetone, instead of a coil. Cold finger condensers completely freeze the distillate. They get extremely good capture of the volatile aromas, but I don’t like them because you have to thaw them out between batches. And there is no way to tell when you are done because you can’t taste the product as you go (more on that later).
Vacuum Pump/Controller: The rotovap would be just a souped-up still if it weren’t for the vacuum pump. The vacuum is what lowers the boiling point of product and allows us to do distillations all the way down to room temperature or below.
Choosing a good pump is important. First, your pump needs to be powerful enough to obtain a reasonable vacuum in a reasonable amount of time. Second, your pump needs to be able to reach an ultimate vacuum (the highest level your machine has the ability to achieve) good enough for the work you are doing. Atmospheric pressure is about 1000 millibar. To do straight liquor work at 60°C, your pump needs to be able to go down to 40 millibar without breaking a sweat. To do all liquor work down to 40°C your pump better be able to pull an ultimate vacuum of 20 millibar. To do syrups or for water-based work, your pump better get down to 8-10 millibar. Most people fail by getting a pump with an inadequate ultimate vacuum.
Water aspirators will not work for low temperature distillation; they also have low flow rates. I have tried using them with ice water, in big banks, etc. I don’t like them. Refrigeration vacuum pumps work well and are cheap, but are difficult to control, noisy, and fill the air with an oil-haze . They need oil to operate and don’t come with a filter, like the pumps in a vacuum sealer. If you do decide to use one, leave a rag over the outlet to catch oil. Also never, ever turn off the pump when under vacuum or you risk sucking oil back into your system. First vent the system, and then turn off the pump. To control a refrigeration pump, you need to install a bleeder needle valve between the pump and the rotovap. Let the pump run and adjust the vacuum level using the valve. Also install a second air/vent valve, preferably push-button. This is your emergency button that quickly lets air into the system to stop a boil-over. Other than noise, oil-haze, and control difficulties, the other problem with oil-based pumps is that the oil gets contaminated whenever you saturate the condenser.
Much better than an oil based pump is a dry vacuum pump and a vacuum controller. They are quieter, cleaner, easier to control, and don’t get contaminated. A dry pump with a good vacuum control is such a pleasure; you’ll never want to go back to the needle valve/oil-pump routine! The problem is the control units cost thousands of dollars and the dry pumps alone are many hundreds. Most controllers have an “automatic” distillation function that finds the correct vacuum level for your current product. I don’t like these. I’ve used several companies’ controllers and find that having a human being control the distillation level consistently delivers a higher quality product. It is a huge timesaver, however, to let the machine do the thinking for you. I think it would be possible to design a controller for maximum flavor; but it hasn’t been done yet.