Condenser loading should not be at 100% and here is why.

You want to achieve the fastest distillation rate possible, so you might be tempted to blast all the vapor you can into the condenser. I am here to tell you that saturating your condenser renders your performance less efficient and leads to solvent loss. So how can you make sure your condenser is running at optimal conditions? Read on and find a few tips on condenser loading and how to avoid under- and overloading.

While many of you might have been enjoying the last summer rays last weekend, my wife and I were helping some old family friends move. They were certain they had so few items to relocate that they do not need a moving company. They were wrong. I cannot remember carrying so many moving boxes in my entire life. My wife had the strategy of picking out the smaller items and running around between the car and the apartment. I had the strategy of carrying as much as I could but making less trips back and forth. Soon, she was tired of running and my back was aching.

Clearly, to achieve optimal moving performance, you need to pack both brain and muscle. If we had just optimized our carry load to about 75% of our capacity, we would have been more efficient, less tired and with less aches.

Knowing limits is something I’ve touched upon before, especially in freeze-drying and freeze-drying endpoint determination. Now we can add rotary evaporation to the list, particularly when it comes to condenser loading.

The two factors that govern overall distillation rate and performance are evaporation and condensation. The distillation needs to be kept in balance, meaning the energy input for evaporation and the energy output through the condenser need to be kept in equilibrium. In other words, you need enough condenser loading and a high evaporation rate to reach optimal distillation performance.

The condenser’s ability to condense solvent vapor greatly affects the distillation speed. To ensure the condenser is running at optimal conditions, you should monitor the height of the condensed solvent on the coils of the condenser.

To avoid over or underloading, ideally two-thirds of the condenser height should be consistently covered with condensate.

Here is how optimal condenser loading looks like:

If your condenser is underloaded, you are not working at maximal efficiency levels. To improve the condenser loading, you could try to decrease the pressure values to 340 to 300 mbar. You could also try to increase the heating bath temperature of your rotary evaporation system.

If the condenser is overloaded, there will be loss of valuable solvent through the vacuum pump. To avoid saturation, you could try increasing the pressure to 300 to 430 mbar. Alternatively, you could see if lowering the cooling temperature produces the desired effect.

For optimal condenser loading, you should respect the ∆20 rule. This rule is so important in rotary evaporation, that I’ve already dedicated an entire post to the ∆20 rule. In brief, you should maintain a 20°C difference between cooling, vapor and heating bath temperature. For example, if you set the bath temperature to 50°C, you will get solvent vapor temperature of 30°C, which is then condensed at 10°C. If you are working with low-boiling-point solvents or thermo-sensitive products, you could also set your values to cooling media at 0 ºC, vapor at 20 ºC and heating bath temperature at 40 °C.

If you feel limited by the condenser capacity, especially when trying to evaporate low boiling point solvents, you could try to get a larger condenser. Condensers with a larger cooling surface area can help improve distillation efficiency, but they are not suitable for every application. To see if a high-performance condenser can help you, check out one of my previous blog posts on the topic.

I think I’ve condensed enough knowledge about condenser loading in this post to leave you room for more rotary evaporation tips. If this is true, check out more suggestions on how to speed up your distillations with a free poster. But be careful not to overload or underload yourself with information or your head will need a cool pack, just like my back.

Till next time,

The Signature of Bart Denoulet at Bart's Blog