The Difference Between Rotary Evaporator And Molecular Distillation

The difference between a rotary evaporator and molecular distillation lies in their separation principles and applicable scenarios: Rotary evaporation is based on conventional vacuum distillation and is used for the rapid removal of common solvents; molecular distillation, on the other hand, is based on differences in molecular free paths and is used for the precise separation and purification of high-boiling-point and heat-sensitive substances.

The specific differences between the two can be distinguished from each other along the following four dimensions:

1. Separation Principle

A rotary evaporator uses reduced pressure to lower the boiling point of the solvent. Simultaneously, the rotation of the flask increases the evaporation area, accelerating solvent vaporization and condensation recovery. Essentially, it is an intensification of conventional distillation.

Molecular distillation: Under extremely high vacuum (10⁻² to 10⁻⁵ Pa), molecular distillation exploits the differences in the mean free paths of different molecules to cause the substances to vaporize on the heated surface and then fly directly to the condensation surface (collision-free), achieving efficient separation. This is a form of "molecular-level" distillation.

2. Operating Temperature

A rotary evaporator operates at a higher temperature, near or at the boiling point of the substance under the corresponding vacuum. It is typically used to separate low-boiling-point solvents (such as ethanol and acetone). Molecular distillation: Operating temperatures are significantly lower than the normal boiling point of a substance (up to 50-100°C lower), preventing heat-sensitive substances (such as vitamins and essential oils) from decomposing at high temperatures.

3. Separation Efficiency and Precision

Rotary evaporators: Low separation efficiency makes them primarily used for "rough separations" (e.g., concentrating solutions and removing solvents). They are unable to separate mixtures with similar boiling points.

Molecular distillation: High separation precision allows separation of mixtures with very small boiling point differences (even just a few degrees) or those that readily form azeotropes. It is often used for the preparation of high-purity substances (e.g., purification of pharmaceutical intermediates).

4. Application Scenarios

Rotary evaporators: Routine laboratory operations such as sample concentration and solvent recovery, as well as low-demand solvent removal in industry.

Molecular distillation: High-value-added applications include fish oil purification (isolation of EPA/DHA) in the food industry, high-purity drug preparation in the pharmaceutical industry, and separation of heat-sensitive polymers in the chemical industry.