Using a rotary evaporator is an inescapable step in many processes because it allows you to gently and efficiently remove solvents from your samples. In some cases, evaporation rate is not an issue, for example, when you are evaporating very little solvent or when time is not critical. However, in most cases you want the process to be as fast as possible, especially if you are running the application many times in a row or dealing with a large number of applications.
Thankfully, there are a few ways to make the rotavap process faster:
increase ship size
Boost
Raise the bath temperature
increase vacuum
While these methods all involve increasing certain parameters, there are some caveats in each case. In this post, we discuss each strategy in detail and point out their limitations.
1. Increase the speed
Another important factor in a rotary evaporator setup is the speed at which the evaporating flask rotates. The rotation of the flask has two main functions. First, it agitates the Water Bath to improve heat transfer to the flask and solvent. Additionally, rotation increases the surface area of the liquid within the flask, thereby increasing the rate of evaporation.
While faster is usually better, at some point faster speeds can actually make evaporation less efficient. It has been shown that, at a certain velocity, the liquid is turbulent as the sample becomes depressurized against the sides of the flask.
In addition to potentially reducing efficiency, using too much speed can cause premature wear on equipment.
Typically, a speed of 250-280 rpm is recommended for maximum turbulence in a benchtop rotary evaporator, but factors such as solvent and sample concentration, flask size and fill volume, etc. may change this.
2. Increase container size
It seems logical to use a small container to collect a small sample, since you might think it is more practical to extract a small amount of product from a small flask. However, when it comes to creating more efficient rotavaps, usually bigger is better.
The increased surface area of the flask means more of the flask is in contact with the water in the Water Bath. This results in faster and more uniform heating of the sample.
Also, in larger flasks, the sample surface area exposed to the air inside the vessel is greater. This makes evaporation more efficient.
Beyond efficiency, there are practical implications of using containers that are too small. Due to the tilting of the flask during the rotavap setup, there is more chance of overflow if the flask is too full.
Ideally, the evaporating flask should not be more than half full, both to prevent spillage and to allow sufficient liquid surface area for efficient evaporation.
3. Raise the bath temperature
A Water Bath is one of the ways a rotary evaporator can speed up evaporation. The Water Bath heats the solvent, increasing the rate of evaporation.
However, it's important not to be overzealous when raising the temperature of your bath. Too high a temperature has several potential problems:
Bumping: Bumping occurs when the sample boils too quickly and forms bubbles that splash out of the flask. Bubbles can contain both solvent and the sample you are retrieving, resulting in lower yields. Collisions can usually be avoided by slowly increasing the temperature. However, an excessively high vacuum could also be the culprit. If you are concerned about impact, you can collect boiled samples with an impact trap.
Detrimental reactions: While your solvent may be able to handle the temperature increase without collision, the sample may not be very soluble. Make sure that no matter what temperature you raise it to, it won't cause an adverse reaction inside the container.
Excessive solvent vapor: Depending on the cooler used, using too high a temperature may increase the rate of evaporation so that the cooler cannot keep up. In this case, the coolant cannot condense all the vapors that flow into the condenser. Unevaporated solvent vapor will continue to flow to the Vacuum Pump instead of condensing into the condensation bottle. Vapors in the solvent may damage the pump and shorten its life. Potentially toxic vapors may also pass through the pump and be released into the atmosphere. To avoid this, use the rule of 20 and make sure the coolant is always at least 20°C cooler than the vapor temperature. Also, check the cooling capacity of the cooler against the heating capacity of the sink to ensure it will keep functioning properly.
4. Increase the vacuum
One of the main reasons for using a rotary evaporator is to lower the boiling temperature of the solvent by using a vacuum to lower the pressure within the system. Therefore, increasing the vacuum and reducing the pressure further is helpful. Indeed it is.
If the vacuum is increased too quickly or too quickly, you may bump (as above) or bubble. Foaming occurs when a sample contains surfactants and small air bubbles (foam) develop on the surface. This can usually be mitigated by carefully monitoring the sample and slowly increasing the vacuum.
Reducing the pressure in the system can also cause the evaporation rate to be too fast. If the solvent does not have time to evaporate in the condenser, uncondensed vapors may enter the Vacuum Pump.
Finally, too high a vacuum increases the risk of glassware imploding.
