When operating a rotary evaporator in your lab, you want to make sure that you’re being as efficient as possible. One of the main factors to be considered is the rotational speed of your rotary evaporator, and while it might seem like maxing it out is your best option, it might not always be the case. You wouldn’t want things to spin out of control now, would you? Some of the factors that you need to keep in mind when operating your rotary evaporator at its upper rotational speed is mechanical damage to your equipment caused by high speeds, and the decrease in evaporation rates beyond said speeds. And while certain studies postulate that the optimal speed lies somewhere between 250 to 280 rpm, it isn’t truly a definite rule of thumb. In this post, we’ll discuss the factors you need to take into consideration when setting the rotational speed of your rotary evaporator to make the best out of your processes.
High Speeds and Potential Equipment Failure
High speeds are synonymous with an increased risk of equipment damage, mainly in two forms:
- Vibratory forces which increase the wear and tear of the evaporator
- Mechanical problems with the evaporator
Studies have shown that there is a linear relationship between mechanical failure and higher rotational speeds. Another thing worth noting is the increase in spillage risk due to the higher turbulence in water baths when operating at higher speeds.
High Speeds and Evaporation
The rotation of the flask in a rotary evaporator leads to an increase in the surface area of the liquid inside of the flask, which increases the evaporation rate. The rotation also leads to an increase in the agitation of the liquid in the water bath. This improves the heat transfer to the flask, and the solvent. Both of the aforementioned factors depend on the rotation of the flask. Intuitively, one would think that an increase in the rpm of the evaporator would result in quicker evaporation, but this applies only to a certain degree. A study by Buchi showed that at rates above 400 rpm, the rate of evaporation started decreasing. The sharp increase in centrifugal forces eventually leads to the particles inside of the flask pressing up against the walls, decreasing the turbulence and eventually the evaporation rate.
Simply put, you want to maximize the turbulence to achieve the highest evaporation rate. This will require you to factor in the flask size, the fill level (make sure to minimize the risk of both foaming and bumping to avoid contaminating your sample), the solvent, and sample consistency.
One last thing you want to avoid is any liquid sealing off your vapor tube. This could lead to the formation of a bubble that pushes the liquid up the vapor tube, contaminating the collection flask almost instantaneously.
Optimal Rotation Speed
While 250 to 280 rpm works best for most rotary evaporators, some of them operate better at other speeds. This is particularly true when considering different sizes of rotary evaporators; a larger rotovap will almost always have a lower ideal speed (and lower maximum speed) than a smaller rotary evaporator of the same brand. Similarly, flask size will play a role in the ideal rotation speed as well, with smaller flasks meriting a higher rotation speed. The takeaway: read your user manual, and find the sweet-spot between speed and equipment wear.