Why is rotary evaporation more efficient than standard distillation?

Rotary evaporation is often preferred over standard distillation, and one of the main reasons is that you can achieve faster evaporation rates with the rotary evaporator setup. You might be wondering why this is the case.

At first glance, factors that contribute to faster evaporation are:

pressure drop in the system

Solvent temperature rise

flask rotation

While both rotary evaporation and standard distillation can modify the first two factors, only rotary evaporation has the added benefit of rotating the sample. In this article, we'll take a closer look at the similarities and differences between these two methods and why rotary evaporation is more effective.

pressure drop in the system

The presence of vacuum reduces the pressure within the system. This lowers the vapor pressure required for boiling to occur, thereby lowering the solvent's boiling point. This means less thermal energy is required for evaporation.

In addition to lowering the boiling point, lower pressure also speeds up evaporation. That is, when a solvent is close to its boiling point, it evaporates more quickly. The downward pressure on the solvent makes it difficult for the molecules to escape as a vapor. Lower pressure means these molecules can enter the atmosphere at a faster rate.

Many standard distillation setups include the use of vacuum so that pressure can be controlled in these systems. Thus, rotary evaporation and standard distillation can be considered comparable in this way.

Solvent temperature rise

An increase in temperature will increase the vapor pressure, thereby causing evaporation. However, heating the bath takes time and effort, whether in a rotary evaporator or at higher temperatures in a standard distillation setup. This reduces the efficiency of evaporating solvents using elevated temperatures.

Water requires a lot of energy to heat, so it takes longer to heat a bath from room temperature (typically 20-25°C) to, say, 80°C than to heat it from room temperature to 30°C (a common goal for rotary evaporation steam temperature). Oil baths may take longer to heat up as they can be as high as 180°C.

Also, as the temperature difference between the bath fluid and the internal heating element decreases, at higher temperatures more heat is lost to the surrounding environment and less heat is added to the bath. This makes operation at higher temperatures slower and less energy efficient.

Both standard distillation and rotary evaporation setups use elevated temperatures, so this factor does not necessarily make rotary evaporation more efficient than standard distillation. The key is in the rotation.

flask rotation

The rotation of the flask plays an important role in a rotary evaporator for two main reasons: increasing the surface area of ​​the solvent and stirring the Water Bath.

In a static setup, the surface area of ​​the solvent is limited as seen in standard distillation. In a spinner flask, the solvent forms a thin film on the sides of the flask, greatly increasing the surface area and speeding up evaporation.

Also, as the flask rotates, it stirs the liquid around the flask in the Water Bath. This helps transfer heat to the flask and solvent.