Evaporation is commonplace in laboratories, typically for solvent removal and recovery. When done right evaporation is a safe and efficient process, but there are many factors to consider when optimizing your evaporation. Depending on your level of evaporation system, there are basic levels to increase the efficiency of distillation, automated methods of multiple solvent removal, and even ways to fully automate your distillation process.
Increase The Efficiency of Distillation
On a basic level, there are steps you can take the optimize your distillation process.
The size of your Condenser matters, the larger your condenser is, the larger surface area you have for condensing your vapor back into a liquid. This is your hard limit on how much you can evaporate at any time, as exceeding your condenser surface area means your evaporated solvent is being pulled into your vacuum pump.
The size of your evaporation flask matters. The size of your evaporation flask affects you in two ways. The first is in the overall product it can hold. The more you can fit in your evaporation flask, the less often you will be spending time replacing it. The second way it affects your process is by increasing the surface area for evaporation. The larger the evaporation flask, the more material that will be on the surface. Evaporation occurs on the top layer of liquid, so this can increase your evaporation rates without changing any evaporation parameters.
The first and most important rule you should observe in evaporation is the ¾ condenser rule. When optimizing your evaporation process, you do not want condensing to happen past the ¾ point of your condenser coils. You are looking for the highest point on your condenser where you start seeing liquid dripping as your point of reference. If you go higher than this point, you will find that solvent will begin to be pulled through your vacuum pump, exposing yourself as well as losing potentially valuable material.
The Second rule you should observe is the Delta 20 Rule. This means that you should see a delta of 20 degrees between your heating bath temperature – vapor temperature – cooling coil temperature. To condense your evaporated solvent efficiently, your chiller should be strong enough to bring your cooling to 40 degrees lower than your heating bath.
The speed of evaporation can be increased without a change in temperature or pressure by rotating your evaporation flask. Rotation of your evaporation flask helps in two ways. One, it makes the heating in both the bath and inside the flask more even by inducing turbulence, increasing the heat transfer speed. Two, it increases the surface area of your product against the evaporation flask, meaning more can evaporate at a time. The faster you rotate, the more efficiently you will evaporate.
Once you have decided on a maximum safe bath temperature, followed the second rule to determine your chiller temperature, and turned up your rotation speed, you can begin using pressure to dial in your evaporation rate. You will want to pull your vacuum down until you begin to see evaporation occurring in your system. Once that begins, you can slowly reduce your vacuum until you meet the first rule and your condensing hits the ¾ point of your condenser. The third rule to observe in evaporation is to avoid boiling/bumping/foaming inside your evaporation flask. When you evaporate too strongly and your material bumps or foams, you run the risk of your product being ejected along with the solvent into your condenser and receiving flask. There are two common methods of dealing with this. The first is to reduce your vacuum so that the bumping and foaming no longer occurs. The second method you can take is to install a bump/foam trap. This is another piece of glass that provides a buffer preventing material from making it to your condenser.
Programming and Automation
When you begin to reach the higher levels of rotary evaporation systems, there is a degree of preprogramming and process automation that can be applied, particularly with multiple solvent evaporation.
- Most higher-level rotary evaporators have a gradient function. This is a method to create preset conditions that will occur at different points in time. When you have a strong understanding of your evaporation process, you can preset your conditions for evaporation of your first solvent (bath temperature, pressure, rotation speed), and after a predetermined amount of time, the program will move onto the next set of conditions for the second solvent. This can be used anywhere from single solvent evaporation to multiple solvent evaporation. Beyond just basic solvent evaporation, this can be used for advanced level drying of products such as polymers. After removing the bulk of the solvent from a polymer, it has been found that the last bit can take significantly longer. A gradient can be set so that after the bulk the of solvent is removed from the polymer, the vacuum can be reduced gradually to speed up the drying speed.
- Higher-level evaporators may also come with an automatic evaporation function. In Heidolph’s case, this function is called AutoAccurate. This provides the user with a “fire and forget” method of evaporation. The process starts by raising the bath temperature to a preset, and automatically reducing the vacuum pressure. AutoAccurate uses a vapor temperature sensor to detect when a change in vapor temperature occurs. After this change in vapor temperature occurs, the system knows that evaporation is happening and holds the vacuum until the evaporation ends and the vapor temperature drops back down. If the user was evaporating one solvent, the process will end here; if the user was evaporating multiple solvents then the process will continue until the end pressure is reached.
Heidolph North America's goal is to give our customers the ability to focus on their research. Accomplishing this is achieved by providing the best in class service and support for solution-oriented equipment. Heidolph specializes in both benchtop and industrial rotary evaporation, making distillation more efficient, and in turn, allowing you, the researcher, to focus on the research. To learn more about Heidolph’s line of Rotary Evaporators, read our previous blog article, Redefining Evaporation, or contact your Thomas Scientific Sales Representative for a virtual demonstration.