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.
In this post, we look at the potential implications of using the wrong pump for a given application and the factors you should consider when choosing a pump.
Potential Consequences of Using the Wrong Pump
There are a few main negative impacts of an unsuitable pump:
- Damage to the pump
- Safety risks
- Solvent loss
- An inefficient process
Let’s look at those in more detail.
1. Damage to the Pump
Allowing liquid to go through the pump is a big no-no. This can happen if the flow rate is too high or the chiller doesn’t have enough cooling capacity. In either case, if the solvent doesn’t have chance to condense in the condenser, it will enter the vacuum pump. This will, at the very least, cause excessive wear and tear on the pump, drastically shortening its lifespan. It could even flood the pump causing irreparable damage.
2. Safety Risks
In some cases, flooding of the pump can result in solvent squirting out of the back of the pump. This could create a fire safety hazard (for example, if your solvent is ethanol), or health risks (such as when using corrosive or carcinogenic solvents).
3. Solvent Loss
In a rotary evaporator process, you're often able to recover fairly high-purity solvent that can be reused. If your solvent doesn’t have time to condense into the collection flask, your solvent recovery rate is going to decrease. There’s no way to recover solvent that has made its way to the pump.
4. An Inefficient Process
So far, we’ve discussed the impact of using a pump with too high a flow rate for your application. However, there is also a major downside to using a pump that is not powerful enough: you end up with a slow, inefficient process.
Factors to Consider When Buying a Pump
We’ve discussed the potential negative consequences of using the wrong pump, so what factors should play into your decision? Answering the following questions will help you decide on the right pump:
How Much Vacuum Pressure Do You Need?
The solvents you’re working with and their corresponding boiling points will be the main determining factors in deciding how much vacuum pressure you need.
Depending on the boiling point of your solvent and the temperature at which you want to set your bath, you may need more or less vacuum.
For example, for a given solvent, you’ll need more vacuum when your bath is at 30°C than when it’s at 40°C. At a given bath temperature, you’ll need more vacuum for a solvent with a boiling point of 60°C than a solvent with a boiling point of 50°C.
The VP Series Vacuum Pump comes with several options for speed and pressure combinations: 18 L/min & 20 mbar, 50 L/min & 150 mbar or 30 L/min & 250 mbar.
What Pump Speed Do You Need?
The pump speed – which affects the flow rate (also referred to as evacuation rate or clearance) – is also a major concern. As mentioned, too high a speed for a given application could result in damage to the pump and other issues such as safety risks and product carryover. Aside from the practical implications, pumps with higher flow rates tend to be more expensive, and you’ll no doubt want to avoid paying more than you need to.
Flask size is an important determining factor for required pump speed. The larger the flask, the greater the air displacement required and the higher the flow rate needed. For example, a flow rate of 50 L/min may be suitable for a 20L flask whereas a rate of 70 L/min may be more suitable for a 50L flask. Also keep in mind that not all pumps are created equal. Due to the pump curves and design factors, a pump speed that is sufficient for a certain pump may not be for another pump.
If you already have a pump and are wondering if it can be used for your application, there are a couple of things to consider in terms of pump speed. Setting a low pumping speed – and as a result, low flow rate – will allow the solvent vapor more residence time in the condenser. However, many pumps are fixed-speed, so this might not be an option.
If you’re stuck with a pump that’s oversized for your application, there is an alternative option. You can reduce the flow rate of the pump by adding an additional length of vacuum hose between the condenser and the pump. The inner diameter of the hose should be smaller than that of the existing hose. The additional resistance to airflow will reduce the speed at which air is evacuated from the system. Granted, there will be some trial and error in finding the right length of hose that avoids flooding the pump but that doesn’t slow down the process too much.
When considering flow rate, you also need to consider what pressure you’ll be using your pump at. When pumps are operated at or close to their minimum pressure, the flow rate will diminish. For example, take the VP18R vacuum pump model with a flow rate of 18 L/min and a minimum pressure of 20 mbar. As that minimum pressure is approached, the pump won’t be able to clear 18 L/min.
Pump curves like the one shown above can help you determine what your flow rate will be at a given pressure. Note that not all pump curves will have the same shape.
Is Your Solvent Compatible With the Pump Materials?
With solvent vapors flowing through a pump, it’s important that the pump materials are compatible with that solvent. Ideally, a pump used in a rotovap setup should be oilless and chemical-resistant to make sure it lasts as long as possible. The most commonly used pumps for this purpose are diaphragm (or membrane displacement) pumps.
What are Your Vacuum Control Preferences?
For many rotovap applications, control over the level of vacuum is desirable to create a safer, more efficient setup. By controlling the vacuum, you can precisely adjust the pressure in the system to achieve the desired evaporation rate. This level of control can also help you avoid bumping and foaming and give you better solvent-product separation.
Some rotovaps come with a built-in vacuum controller, in which case you might not need to have a pump that has its own controller. If you do need a pump with a controller, there are analog and digital controllers to choose from, with the latter generally being pricier.
Heidolph’s RPM-regulated Rotavac Vario Pumping Unit (above left) comes with a digital controller. If you already have a pump but want to add a controller, you can buy one separately. Options include digital controllers like the LabTech VC1000 Vacuum Controller (above middle) or manual ones such as the Heidolph Manual Vacuum Controller (above right).
Note that a vacuum controller can greatly increase the cost of a pump, so if you’re on a small budget, you might want to try juryrigging one instead.
Choosing the right pump for your rotary evaporator setup can save you money, both in upfront equipment costs and in avoiding damaging equipment during use. It will also ensure you have an efficient process that strikes a balance between optimal evaporation rate and minimal product carryover. By considering the factors above, you will have a much better chance of selecting a suitable pump for your application.