Using a pump to apply a vacuum is both helpful and necessary in most rotary evaporator applications because 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 unit, not just any pump will do the job. You might even come across rotovap packages that come with a pump, but we've discussed at length why these bundles are generally not a good idea (mainly because you end up paying more than you need).
In rotary evaporator applications, there is often a goal to get production as fast as possible. When it comes to pumps, the "bigger is better" mentality is often used, where the assumption is that higher flow rates lead to faster production rates. However, this can lead to problems such as pump damage and inefficiency. Also, there are other factors to consider, not just flow rate.
Choosing the right Vacuum Pump for your rotary evaporator
Evaporation is one of the common tasks performed in chemistry laboratories. But how do you choose the right Vacuum Pump for your rotary evaporator?
The key to the correct selection and sizing of a Vacuum Pump for rotary evaporation can be found in the answers to the following four questions:
1. What solvent are you using?
This is important due to the different boiling points of solvents. Low-boiling solvents such as acetone, methylene chloride, and pentane do not require as deep a final vacuum as solvents such as acetonitrile, benzene, and chloroform to evaporate. Removal of high boiling point solvents, such as water, DMSO, DMF, and toluene, requires a relatively deep end vacuum. You need to ensure that the pump is capable of achieving a high enough final vacuum to effectively remove the solvent of interest. For reference, see boiling point table.
The topic then developed naturally into the need for chemical compatibility. Based on the solvent vapor flowing through the pump, the best material can be identified for a long trouble-free life. Dry oil-free pumps offer clear advantages in this field compared to rotary vane and water aspirators. Plus, they have good, corrosion-resistant wetted parts, are easy to maintain in the field, and produce no hazardous waste.
2. What is the size of your evaporating flask?
This question allows you to choose the best sized pump for your application. Bigger isn't always better. Pumps that are too large or have too much capacity for a given application are difficult to control, are extremely inefficient, and more expensive to own and operate. On the other hand, a pump that is undersized or has insufficient flow will slow down evaporation and result in longer process times. A pump for a 250 mL flask may not be a very good pump for use with a 20 L evaporating flask, and vice versa.
3. What is the temperature of your heating bath?
The goal is to determine if the pump can provide the necessary vacuum and handle the steam temperature. The closer the bath temperature is to the boiling point of the solvent, the faster the evaporation rate will be. If the mixture is not sensitive to high temperatures, increasing the bath temperature will speed up the process and reduce the need for a deep Vacuum Pump. This can affect which pump is right for the job.
4. How do you control the vacuum degree?
Manual, two-point or adaptive control? Vacuum control optimizes the vacuum level in the system. Very good vacuum reduces evaporation time and provides excellent product yield. Controlled vacuum also reduces collisions and improves solvent recovery and reproducibility.
Manual vacuum control involves using a stopcock or manual valve to adjust the vacuum level based on visual cues. With this method, you will need to monitor the entire process, as vacuum requirements may vary throughout the evaporation process.
Two-point or on/off vacuum control cycles the vacuum in the system between high-low set points by turning the pump on and off. This method allows you to unattend the process once the min/max settings are established.
Adaptive vacuum control provides great results in separation, speed and solvent recovery, and leaves you with nothing to do after setup. This type of control modifies the speed of the pump to precisely match the requirements of the process. This is the exact method; allowing the system to operate at a specified vacuum while responding to changing conditions without the need to monitor and adjust settings.
Some Tips on Rotary Evaporation Vacuum Pumps
Rotary evaporators are a common vacuum application in chemistry laboratories. But how many people think a lot about the pumps they use to provide vacuum? Here are some tips.
1) First determine how much vacuum you need.
With the exception of DMSO, any other solvent will boil at room temperature with a Diaphragm Pump, so the first step is to avoid using an oil-sealed pump. Too much vacuum can make your evaporation unmanageable and create unnecessary maintenance needs.
2) Next, avoid oversized pumps.
Remember that when the condenser on the evaporator is 20°C below the sample, it will condense most of the vapor leaving the evaporator. This means that a small pump with a modest pumping capacity (1 cfm or less) will provide enough pumping capacity to manage the residual vapor reaching the pump from most benchtop evaporators.
3) Then, consider vacuum control.
Too high a vacuum will cause foaming and collisions, too low a vacuum will slow down evaporation. A pump with electronic vacuum control will ensure you optimize evaporation for speed and sample protection.
4) Finally, consider vapor capture.
The evaporated vapor condenses in the vacuum line as it cools. An inlet separator protects the pump from condensate. A catch bucket on the outlet will collect exhaust that condenses as the vacuum is released. Add an exhaust condenser to your pump and you can increase vapor capture to 98% or more – and avoid venting these vapors into the atmosphere through a fume hood.
