How the delta 20 rule can make all the difference in your rotary evaporation

We don’t just invent rotary evaporators, we also invent the best ways to use them. The most cherished of these tips would be our golden delta 20 rule. This rule of thumb is a great way to ensure you are using the rotary evaporator under optimal conditions. Read the post, follow the rule and see if you yourself can achieve a perfect balance between evaporation output and energy consumption.

I missed my nephew during the quarantine before they started loosening up the measures. He is 11 now and was often bored at home. I had a video call with him a few weeks back and we decided to make some DIY slime together!

When we started, I told him that in my slime recipe, as a “rule of thumb”, I add 1 glass of white glue to 1 cap of laundry detergent. He didn’t really understand what “rule of thumb” meant. I explained that I am referring to an approximation based on practical experience rather than theory. Okay, he didn’t really understand this explanation either, but I managed to get my point across at the end.

Meanwhile, I had to laugh at my own reliance and use of the phrase “rule of thumb”. After all, science is full of rules of thumb.

So I decided to write a post about one of my favourites: the delta 20 rule in evaporation.

Here is the backstory. There is a direct relationship between the heating bath temperature and the evaporation rate. The more energy you apply to the evaporation side and at the same time remove from the condensation side, the more efficient your distillation becomes. Bear in mind, you also need to pay attention to sufficient cooling and stable under pressure.

You can achieve a fine balance of energy addition and removal by adjusting the heating bath temperature, vacuum pump and cooling temperature to the condenser’s capacity. For example, you can select the energy supplied for cooling the condenser with a “recirculating chiller” to produce temperatures as low as -5 to 10°C. And you can use the vacuum pump to select a precise and stable pressure value.

How can you tell if the condenser is working at optimal conditions?

A condenser is working at its optimal capacity if two-thirds of its height is covered with condensate. In this situation, the top third acts as a safety barrier for “entrained” low-boiling solvent plus pressure fluctuations. A condenser is overloaded if condensate forms downstream from the condenser or if the vacuum pump sucks continuously in order to maintain a specific pressure. Condenser loading is an interesting topic, so I think I will write more about this in future posts.

What I said here is all fine and dandy, but some samples are thermo-sensitive and you want to reduce energy consumption.

The delta 20 rule offers a great compromise between high evaporation output and energy usage.

For a sufficient condensation of the vapor, you should set the cooling temperature at about 20 ºC lower than the vapor temperature. And here comes the golden “Delta 20 Rule” in practice now. You can use this rule of thumb as follows:

Set the bath temperature at 50 °C to yield a solvent vapor temperature of 30 °C, which is subsequently condensed at 10 ºC.

These 10/30/50 parameters are suitable for the evaporation process in order to bring and carry off accumulated energy efficiently.

Here is an easy way to visualize the delta 20 rule in relation to the different components of your rotary evaporation system:

delta 20 rule, rotary evaporation, laboratory evaporation, heating bath temperature, vapor temperature, cooling temperature

You can also use the “Delta 20 Rule” with lower heating bath temperatures for solvents with a low boiling point or for thermo-sensitive products. For example, you can set up as follows: cooling media: 0 ºC; vapor: 20 ºC; heating bath: 40 °C and the pressure lowered in order to lower the solvent’s boiling temperature. Importantly, you should not go below the ambient temperature or you will get reboiling.

As you might have noticed, these conditions could never work if your condenser is connected to tap water. This is because tap water is never at 0°C or we might be facing more serious problems. But this roadblock highlights the importance of having a chiller, so all kinds of possibilities for you to implement the delta 20 rule are available.

So I think that’s all I have to say about the delta 20 rule at this time. But if you are still in learning mode, check out our evaporation poster for more time-saving tips! Or if the image of the rotary evaporator has inspired you to think about all the parts of the system, then check out my blog post on “Is your rotary evaporator well configured?” As a rule of thumb, I think two additional resources to keep your curiosity satisfied should be enough!

Ah, and my nephew managed to follow my recipe with all its rules of thumb for making slime and now has two jars full of fantastic yellow slime!

Till next time,

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