Three parameters you need to boost your distillation rate

Whenever we try to boost the distillation rate, our first instinct tells us to increase the heating bath temperature. This is problematic for several reasons, including sample damage and process efficiency. If you want to achieve high distillation rates in the most effective way possible, then consider looking at pressure, condenser loading and evaporation flask size in addition to heating bath temperature. In this post, see how these parameters affect the speed of your distillation, in theory and in practice.

After years of cooking side by side with my wife, we have still not resolved the battle of who the true iron chef is. She always adds a bit more salt to my stew, I always turn down her boiling rice. And speaking of her rice, I do not understand why she insists on letting it boil on high, when medium would have just the same effect, but cost us less money, reduce the risk of the rice burning and sticking to the bottom of the pot and do less damage to the pot!

It’s actually a battle to be fought in the lab as well. Naturally, scientists performing distillations on their rotary evaporator want to achieve the highest distillation rates. Just as a reminder, the distillation rate is a measure of the amount of clean solvent that can be obtained in a specific time period. Understandably, we often try to achieve this by increasing the heating bath temperature. But wait a minute. Is using 90°C heating temperature the most efficient and safest solution for achieving a high distillation rate?

I am here to tell you, no. In fact, there are three key points you should consider whenever you are trying to increase your distillation rate.

1. The heating bath temperature

Of course, going as high as possible seems to intuitively make sense. More energy, faster distillation. However, thermo-sensitive samples should not be heated at high temperatures to protect some of their properties, for example flavours. Additionally, it is dangerous to set the heating bath beyond a certain level, not to mention that it sucks up energy and money from the lab. There are ways to avoid working at temperatures exceeding 60°C, which I will discuss below.

2. Condenser loading and pressure

One approach to reducing the heating bath temperature without affecting the distillation rate is to adjust the pressure. However, if you set the pressure too low, you can trigger re-evaporation of the solvent at room temperature, which loads the condenser with unnecessary vapor from the receiving flask. Take the capacity of the condenser into account if you want to maximize the distillation rate while avoiding flooding of the pump at the same time.

In fact, you should make sure that the heating bath temperature, the pressure and the cooling temperature are adjusted to condenser’s capacity. Optimal capacity occurs when two thirds of the condenser height is covered with condensate. In this case, the top third acts as a safety barrier for “entrained” low boiling solvents and for pressure fluctuations. A condenser is overloaded if condensate is observed downstream from the condenser or if the vacuum pump sucks continually to maintain a set pressure. It’s not the first time I discuss this topic, check out a previous blog post on condenser loading for more information.

3. Flask size

Another way to tackle loss of distillation rate when operating at lower heating bath temperatures is to increase size of your evaporation flask. In fact, my colleagues have performed several experiments to demonstrate this and I’ll share the results with you now.

We set up the experiments with the following parameters:

Heating bath temperature40 °C; 60 °C; 90 °C
Cooling temperature0 °C; 20 °C; 20 °C
Evaporating flask size0.5 L; 1.0 L; 5.0 L
Sample volume1/3 of flask volume
Rotation speed280 rpm

And we tested three different solvents: water, ethanol and acetone using the Rotavapor® R-300 .

As expected, we observe a net increase in the distillation rate when working at higher bath temperatures.

distillation rate; rotary evaporation; laboratory evaporation; water
distillation rate; ethanol; rotary evaporation; laboratory evaporation
distillation rate; evaporation rate; rotary evaporation; laboratory evaporation

Our results confirmed that when the bath temperature is raised, the evaporation output increases significantly. However, as I previously mentioned, not every sample can withstand heat of more than 60° C. For these cases, the distillation rates obtained at 90°C can often be achieved or sometimes even exceeded when working at 60°C with a larger flask. The same is true when comparing rates at bath temperatures of 60°C and 40°C.

A compromise can be found between the temperature of the heating bath and the speed of evaporation by adapting the size of the evaporating flask.

Working at lower bath temperature is also better for sample preservation, safety and energy costs . For fans of automation, I would highly recommend an interface that can automatically adapt the pressure to achieve evaporation. You can still always adjust the values manually to improve your distillation rate.

So indeed, turning down the temperature on rice boiling in a large pot with a lid on it can help you achieve a nice result at lower costs and lower risks. Similarly, playing with pressure and flask size can help you achieve a high distillation rate with less damage to your sample and to your wallet. Or get a rice cooker/ an automated rotary evaporator, then you really have nothing to worry about. I hope I convinced you in this post, at least I would have had more success than in convincing my wife to change her cooking techniques. Not that we are always competing mind you, read about the time we made strawberry jam together and what it had to do with evaporation flask sizes in a previous blog post.

Till next time,

The Signature of Bart Denoulet at Bart's Blog