Molecular distillation, also known as short-path distillation, is a highly efficient and important separation method in modern separation science. Its key distinctions from conventional distillation lie in the operating principles and process conditions.
This technique separates substances by utilizing differences in the mean free path of molecules under high vacuum. This leads to low operating temperatures, short heating durations, and high separation efficiency, making it particularly suitable for purifying high-boiling-point, heat-sensitive, and easily oxidized materials.
In conventional distillation, separation typically occurs near the boiling point of the substance. Evaporation and condensation are reversible processes, and the vapor and liquid phases eventually reach equilibrium. Molecular distillation operates at temperatures significantly below the boiling point at atmospheric pressure. Evaporation takes place at the surface of the liquid without boiling. By maintaining a high vacuum in the system, evaporated molecules travel from the liquid surface to the condensation surface with minimal collisions. This process is characterized as a non-equilibrium process, with molecules moving in a single direction.
In terms of equipment design, the core of a molecular distillation unit must ensure that the distance between the evaporation and condensation surfaces is less than the mean free path of the vapor molecules. Under these conditions, molecules can travel unimpeded to the condenser and condense rapidly, preventing re-mixing and re-evaporation, thereby achieving efficient separation. Common types of equipment include falling film, wiped film, and centrifugal molecular stills. The wiped film design employs mechanical components to form a thin, uniform liquid film, effectively enhancing mass and heat transfer, and is widely used in industrial applications.
This technology is now extensively applied across various industrial sectors. In the fine chemicals industry, molecular distillation is used to purify synthetic oils, extract monoglycerides, and process other polymer additives. In the food and oils industry, this technique can concentrate DHA and EPA from fish oils, as well as remove free fatty acids and pesticide residues from vegetable oils. In natural product extraction, it is employed to refine essential oils and isolate heat-sensitive components such as carotenoids. In the petrochemical industry, molecular distillation is also utilized in processing high-boiling-point lubricating oil fractions, effectively removing residues and impurities.
As related industries continue to demand higher product purity, quality, and process efficiency, molecular distillation technology is constantly being optimized and innovated. Its future development will focus more on refining equipment design, precise control of process parameters, and integration with other separation and reaction processes, so as to better meet the needs of high-precision manufacturing in sectors such as high-end chemicals, pharmaceuticals, and functional materials.