When using a rotary evaporator system, you’ll likely want to set it up such that you have the most efficient distillation process possible. One of the factors to consider here is rotation speed. It’s tempting to think that faster is better, but this is not always the case.
There are a couple of factors at play, including the fact that the rate of evaporation can actually decrease at speeds above a certain point. You also have to think about the potential damaging effect that high speeds can have on the equipment.
Taking the above factors into account, an experiment conducted by Buchi determined that a speed between 250-280 rpm is optimal. This experiment used water in a half-filled 1 L flask. Optimal speeds may vary depending on solvent and sample consistency, the size of the flask, and its fill level.
In this post we explain the contributing factors in more detail to help you decide on the best speed for you rotary evaporator. We use the findings of the above experiment as a guide.
The Relationship Between Rotation Speed and Evaporation
Using a rotary evaporator provides more efficient evaporation than static distillation equipment for two main reasons:
- Rotation of the flask results in agitation of the liquid in the water bath, improving the heat transfer to the flask and subsequently the solvent.
- The surface area of the liquid inside the flask is higher, resulting in an increased evaporation rate.
The figure below shows a representation of the two setups with the arrows indicating heat transfer.
Since these factors depend on the rotation of the flask, it makes sense that more rotation will result in faster evaporation, but this is only true up to a point. The experiment found that once the speed reached 400 rpm (using a modified piece of equipment), the rate of evaporation actually decreased. At this point the contents of the flask are pressed up against the wall, lowering the turbulence of the liquid and subsequently the evaporation rate.
Basically, the speed at which you have maximum turbulence will give you the highest evaporation rate. As mentioned earlier this can depend on solvent and sample consistency, flask size, and fill level.
In terms of fill level, you want to make sure you minimize the risk of any bumping or foaming contaminating the flask. What’s more, too high a liquid level reduces the effective surface area, thereby slowing evaporation.
You should also always ensure the vapor tube isn’t sealed off by liquid at the start of operation. At any speed and turbulence, this could cause a bubble that pushes the liquid up the vapor tube and contaminates the collection flask immediately.
Potential Equipment Damage at High Speeds
Another factor to consider when choosing your rotary evaporator speed is potential damage to the equipment itself. High speeds can have two main consequences:
- Mechanical problems with the rotary evaporator
- Vibrations deteriorating the equipment
The Buchi experiment found that the amount of time to mechanical failure decreases fairly linearly up to a point but drops more quickly when you reach higher speeds.
One more thing to bear in mind is that high turbulence could result in spillage from the water bath, which could pose a safety risk. This is another reason to err on the side of caution when increasing speeds.
By intercepting the curves for evaporation performance and time to mechanical failure, an optimal speed can be deduced.
The Optimal Rotation Speed for Your Rotary Evaporator
When the curves were intercepted, this resulted in an optimal speed of 250 - 280 rpm. A speed within this range should result in near-optimal evaporation performance while still mitigating the risk of damage to your equipment.
Some evaporators, such as the Ai SolventVap SE53, max out well below that speed range, while others go up to 280 rpm and beyond. For example, the Hei-VAP Precision has a range of 10 - 280 rpm, so you shouldn't have to worry about going too high when using similar parameters to those used in the experiment.
From left to right: The Ai SolventVap SE53, the Hei-VAP Precision, and an RV 8.
Evaporators in the RV 8 range go up to 300 rpm. In this case, you should be aware that as you go up to the highest speeds, the trade-off between efficiency and possible equipment wear may not be worth it.