Applying vacuum using a pump is either helpful or necessary in most rotovap applications as it reduces the pressure within the system. This lowers the boiling temperature of the solvent resulting in faster evaporation and safer rotary evaporator operation.
When composing a rotary evaporator setup, not just any pump will be up to the task. You may even come across rotovap packages that come with a pump included, but we’ve discussed in detail why these bundles are generally not a good idea (mainly because you end up paying more than you need to).
Within rotovap applications, there’s often the goal to make production as fast as possible. In terms of pumps, a “bigger is better” mindset is commonly adopted, where it’s assumed that the higher the flow rate, the faster the production. However, this can lead to problems such as pump damage and an inefficient process. Plus, there are other factors to consider than flow rate alone.
Evaporation is one of the most common tasks performed in a chemistry laboratory. But, how do you go about selecting the correct vacuum pump for your rotary evaporator?
The key to correctly selecting and sizing a vacuum pump for rotary evaporation can be found in your answers to the following four questions:
What solvents are you working with?
This is important due to the different boiling points of solvents. Solvents with a low boiling point, like acetone, methylene chloride and pentane, do not require as deep of an end vacuum to evaporate as solvents such as acetonitrile, benzene and chloroform. The removal of solvents with high boiling points, such as water, DMSO, DMF and toluene requires a relatively deep end vacuum. You must ensure that the pump is capable of reaching an end vacuum deep enough to remove the target solvents efficiently. For reference, see the Boiling Points Table.
This topic then naturally progresses to the need for chemical compatibility. Based on the solvent vapors going through the pump, one can determine the best materials for a long, trouble-free service life. Dry, oil-free pumps offer significant advantages in this area in comparison to rotary vane and water aspirators. In addition, they can feature premium, corrosion-resistant wetted parts, are easily maintained on site, and do not create hazardous waste.
What is the size of your evaporation flask?
This question allows you to select the best sized pump for your application. Bigger isn’t always better. A pump that is too large or has too high of a flow capacity for a given application is difficult to control, terribly inefficient and more expensive to own and operate. On the other hand, a pump that’s sized too small or doesn’t have enough flow capacity will slow the evaporation and lead to longer processing times. A pump that is used for a 250 mL flask is probably not the best pump for use with a 20 L evaporation flask, and vice versa.
What is the temperature of your heating bath?
The goal is to determine whether or not the pump is able to provide the necessary vacuum level and handle the vapor temperatures. The closer the bath temperature is to the boiling point of the solvent, the faster the rate of evaporation. If the mixture is not sensitive to elevated temperatures, increasing the bath temperature will speed the process and reduce the need for a deep vacuum pump. This may have an effect on which pump is best for the job.
How do you want to control the vacuum?
Manual, two-point, or adaptive control? Vacuum control allows for optimization of the vacuum level in the system. Optimum vacuum level shortens evaporation times and provides superior product yields. Control of the vacuum also reduces bumping and enables greater solvent recovery and repeatability.
- Manual vacuum control involves the use of a stop cock or manual valve to adjust the vacuum level based on visual cues. Using this method, you will need to monitor throughout the process as the vacuum requirements may change throughout the evaporation.
- Two point or on/off vacuum control allows the vacuum in the system to cycle between a high and low set point by turning the pump on and off. This method allows you to leave the process unattended once the min/max settings are determined.
- Adaptive vacuum control provides the best results in terms of separation, speed and solvent recovery, and allows you to leave the process unattended once set. This type of control modifies the speed of the pump to precisely match the requirements of the process. It is the most accurate method; allowing for the system to operate at a specified vacuum level while responding to changing conditions without the need to monitor and adjust settings.
Few Tips About Vacuum pumps for rotary evaporation
Rotary evaporators are among the most common applications for vacuum in chemistry labs. Yet how many people give a lot of thought to the pump used to provide the vacuum? Here are a few tips.
Decide first how much vacuum you need.
Any solvent but DMSO can be brought to boiling at room temperature with a diaphragm pump, so the first step is to avoid using oil-sealed pumps whenever possible. The excess vacuum makes your evaporations too difficult to control, and you incur needless maintenance demands.
Next, avoid oversizing the pump.
Remember, when the condenser on your evaporator is 20 degrees Celsius cooler than your sample, it will condense most of the vapors leaving the evaporator. That means that a small pump with modest pumping capacity – 1 cfm or less – will provide enough pumping to manage the residual vapors that reach the pump from most benchtop evaporators.
Then, consider vacuum control.
Too much vacuum can cause foaming and bumping, and too little vacuum will slow your evaporations. A pump with electronic vacuum control will ensure that you optimize your evaporations for both speed and sample protection.
Finally, think about vapor capture.
The vapors coming off the evaporation can condense in the vacuum lines as they cool. An inlet separator protects the pump from condensed liquids. A catchpot on the outlet will collect exhausted vapors that condense as the vacuum is released. Add an exhaust condenser to your pump, and you can bring vapor capture to 98 percent or more – and avoid discharging those vapors through your fume hood to the atmosphere.