The Rule of 20: setting the right temperature for your rotary evaporator

You just bought your rotary evaporator, you set it up on your benchtop after making sure to follow the manual instructions word for word. Everything’s all set and you’re ready to get some work done with your evaporator. And then it hits you: What temperature should you set your rotary evaporator to? Enter the “Rule of 20”, AKA the “20/40/60 Rule” AKA the “Delta 20 Rule”. 

 

The Theoretical How

 

In a nutshell, the Rule of 20 states that the temperature of your coolant should be at least 20 lower than the vapor’s temperature, and that your bath’s temperature should be 20 higher than the vapor temperature or boiling point your are trying to reach. Let’s say that the boiling temperature of your substance is 30. You’ll need a coolant temperature of 10 and a bath temperature of 50 to be operating under optimal conditions. 

 

The Why

 

By following the Rule of 20, you’re trying to find the perfect balance between energy usage and process efficiency: Higher bath temperatures merged with lower coolant temperatures increase the distillation’s efficiency. But there’s a catch: the higher the temperature of your bath and the lower your coolant temperature, the more energy you end up spending. This means that at a certain point, investing more energy into your process won’t be impactful on the overall efficiency of the process anymore. 

 

The Rule of 20 bypasses this issue by ensuring an efficient distillation. 

 

You should keep in mind that in certain scenarios, like working with heat-sensitive compounds, you might be forced to keep your bath’s temperature at levels lower than the one advise by the Rule of 20.

 

The Practical How

 

Now that you’re all set, it’s time to evaporate some solvents! In certain scenarios, and with a bit of luck, you might be able to apply the 20/40/60 rule literally: Simply set your bath temperature to 60, your coolant to 20, and your vapor temperature to 40 using your vacuum controller. By applying vacuum and reducing the boiling point, you can use the same temperature settings for most common solvents. We thought we’d save you some time: you can find the vacuum pressures of some common solvents below. 

 

SolventBoiling Point (°C)Solvent
Acetic Acid118.0Ethyl Acetate
Acetic Acid Anhydride139.0Ethyl Ether
Acetone56.3Ethylene Dichloride
Acetonitrile81.6Ethylene Glycol
Benzene80.1Heptane
iso-Butanol107.7n-Hexane
n-Butanol117.7Hydrochloric Acid
tert-Butanol82.5Methanol
Carbon Tetrachloride76.5Methylene Chloride
Chlorobenzene131.7MTBE
Chloroform61.2Pentane
Cyclohexane80.7Petroleum Ether
Cyclopentane49.3iso-Propanol
Dichloromethane39.8n-Propanol
Diethyl Ether34.6Pyridine
Dimethyl Acetamide166.1Tetrahydrofuran
Dimethyl Formamide153.0Toluene
Dimethyl Sulfoxide189.0Trifluoroacetic Acid
Dioxane101.0Water
Ethanol78.3Xylene

 

While the rule of 20 is a good rule of thumb for most evaporative procedures, there is one aspect where it doesn’t hurt to overshoot: cooling. There is no downside to having the coolant even colder than 20 below the vapor temperature, and in fact it can help improve evaporation rates to do so. This increases solvent recovery, making your process as efficient as possible.  This is where chillers come in, but there’s a trick: chillers lose cooling power as they operate at colder temperatures due to less efficient heat transfer. Enter EcoChyll: by pumping refrigerant directly through the condenser coils from the compressor, the EcoChyll gets colder much faster, and can maintain very low temperatures better than what would be considered a suitably sized chiller hooked up to a rotary evaporator.

 

References:

 

https://brandtech.com/solventboilingpointschart/