Frequently asked questions

When choosing components or using the rotary evaporator for the first time, your head might start to spin. In today's blog post, we have therefore compiled and answered the most frequently asked questions about the rotary evaporator.


Questions about the setup:


Do I really need a recirculation chiller? Isn't a water supply also sufficient?

It is true: There is also possible to simply connect the condenser of a rotary evaporator to a water line with inlet and outlet. In respect of performance, this only has a negative effect if the tap water is not cold enough or the pressure is too low. In order to work properly, it must not be warmer than 15 °C. Nevertheless, when connecting to the water pipe, there are several aspects to consider: First of all, the water is being wasted. Approx. 8 liters of water per minute flow through a condenser - that is, 480 liters for one hour of operation. This is why, for reasons of sustainability, a recirculation chiller setup that pumps a specific volume of coolant in a closed circuit is to be preferred over connecting it to a water line. Furthermore, the safety aspect also has to be taken into consideration: In the event of a hose coming off or bursting, the line water will keep flowing without restraint if the device is not supervised, whereas in the case of the recirculation chiller, only a limited amount may be leaking.

What is the difference between standard vacuum pumps with valve regulation and vacuum pumps with speed controlling?

The biggest differences between the two are on the one hand in the power consumption and on the other hand in the noise level.
Standard vacuum pumps are very simple. They are either on or off; meaning one switches them on and they run all the way through the process.  Control can only be achieved indirectly by connecting a vacuum valve upstream of them, which opens when the negative pressure needs to be generated or maintained. Recognition of this need is provided by a vacuum sensor, which monitors the pressure in the rotary evaporator and releases it if necessary. The fact that its motor is constantly running during operation causes a standard pump to also consume electricity all the time - and be relatively noisy. That is the reason you should carefully consider where the device will be located and whether it might interfere with guests or colleagues.
Speed-controlled vacuum pumps, on the other hand, have a motor control. Similarly monitored by a vacuum sensor, the motor only switches on when negative pressure is created or has to be adjusted - apart from that, the pump is off and thus silent. It also uses up to 90% less energy than a standard vacuum pump.

I have concerns about dealing with such a sophisticated device. Are there functionalities which allow an easier start?

Technology is getting smarter. This also applies to rotary evaporators. Nowadays, there are devices where everything works at the push of a button and proceeds on its own - all you have to do is filling the evaporation flask with your own ingredients and leave the rest to the device. With the Flavorizer Expert package, you get the Hei-VAP Expert Control, a model with such a function. Additionally, it has a library where you can save your recipes and upload them at the touch of a button. The package also includes a one-day workshop with one of our experts on-site at your facility, so that you also have a smooth introduction in using the device.


Questions about the process:


What role does the heating bath temperature play? What are the effects obtained with higher or lower temperatures of the heating bath?

A higher heating bath temperature ensures that more steam is generated. More volatile components (which include some of the flavoring compounds) are thus also released from the mixture presented, which depends on the initial product. Excessive temperature, on the other hand, can also cause valuable flavors to be destroyed and thus lost. Additionally, the separation performance deteriorates. For example, if 40% alcohol (e.g. grain) is distilled at a high temperature, the water fraction will eventually be higher than if you choose gentler parameters, as more water will be distilled over as well.
As one of the parameters, temperature can be considered as an inert variable, since changes here have a slow effect (the heating bath must first warm up or cool down completely and transfer these changes). For quick and effective control of the process, vacuum is the parameter of choice. It is recommended to work with 45 °C for most applications.

To what extent does the rotation of the flask play a role? Can you make general statements?

The rotation of the flask has several effects: An even temperature distribution is ensured, both in the flask itself and in the heating bath. The ingredients whose flavors are to be extracted are optimally and evenly flushed by the alcohol/water throughout the flask. As a result, it is more efficient than, for example, infusion, where herbs, etc. are easily added and later filtered. Likewise, the rotation applies a thin coating of alcohol/water to the flask wall, allowing evaporation to occur much more easily compared to the static system. Due to turbulence in the flask caused by rotation, the surface area available for evaporation increases further. The rotation also prevents splashes and foaming.
But the faster, the better is a misconception: Rotating the flask too vigorously can cause parts of the flask contents to splash over into the receiving flask and condenser, and it also always depends on which size of flask is used and how full the flask is.
For a good starting point for the rotation, you can set 100 to 120 rpm and then gradually work your way up. Moreover, a flask should never be filled to more than half its design nominal volume, i.e. a maximum of 1 L of mixture into a 2 L flask.  An overfill will otherwise have a negative effect on the process runtime.

Which effects does the cooling temperature have? The warmer or colder the chiller, the ...?

One common misconception is: "The colder the chiller, the better". The recirculation chiller has a particular performance spectrum: The colder the setting, the more energy it spends on cooling itself down and maintaining its temperature.  This is frequently combined with condensation and even ice formation on the cooling hoses, particularly in high humidity conditions. A cooling temperature that is not too cold is ideal, so that the available cooling surface in the condenser of the rotary evaporator is optimally used. If not, the distillation process will last unnecessarily longer. If the chiller is set too warm, the result is that the steam rising into the condenser is not condensed quickly enough and is drawn into the vacuum pump. This is indicated by the fact that distillate is collected in the condensate cooler (the small glass flask at the back of the pump). If so, you must either set the chiller colder or turn the vacuum down a bit so as not to overload the condenser. We have learned from experience that the vast majority of processes run optimally at a setting of 5 °C cooling temperature.

How do the parameters measures differ between hydrosoles and distillates?

Water requires more energy (heat) to evaporate and more cold (cooler) to condense than alcohol. Therefore, you will realize that the process is always slower with a hydrosoles than with alcohol. It is possible to counteract this slightly by increasing the temperature of the heating bath (50 to 60 °C, provided that the aroma does not suffer as a result) and by selecting a lower vacuum value.

By the way: You can find a more detailed description of the processes occurring in the rotary evaporator in this blog article.

Didn't find your question? No problem! Just write them in the comments. We are looking forward to receiving your feedback!

Your Heidolph Team

Heidolph Team
Heidolph Instruments is a premium manufacturer of high-quality laboratory equipment.

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