How much do you know about molecular distillation?

process

Short path stills are also suitable for molecular distillation. Molecular flow is directed from the heating surface to the condenser surface. We can make the molecular distillation process in the following four steps:

Diffusion of molecules from the bulk of the liquid phase towards the evaporating surface

Usually, the diffusion rate in the liquid phase is the main factor controlling the molecular distillation rate, so we should reduce the thickness of the liquid layer as much as possible and strengthen the flow of the liquid layer.

Free evaporation of molecules on the surface of the liquid layer

The evaporation rate increases with the increase of temperature, but the separation factor sometimes decreases with the increase of temperature. Therefore, it should select an economical and reasonable distillation temperature based on the processed material's thermal stability.

Molecules fly from the evaporating surface to the condensing surface.

As vapor molecules fly from the evaporating surface to the condensing surface, they may collide with each other or with the remaining air molecules between the two surfaces; this is because evaporating molecules are much heavier than air molecules, and most of them have the same movement direction, so the remaining gas molecules are in disordered thermal motion between the two sides, so the number of remaining gas molecules is the main factor affecting the flying direction and evaporation speed.

Molecules condense on the condensing surface.

As long as there is enough temperature difference between the hot and cold surfaces (usually 70~100℃), the condensation surface's form is reasonable and smooth. It is considered that the condensation process is completed instantaneously, so it is very important to choose the correct form of a condenser.

condition

1. The partial pressure of the residual gas must be so low that the mean free path length of the residual gas is several times the distance between the surface of the distiller and the front of the condenser.
2. At saturation pressure, the mean free path length of the vapor molecules must be of the same magnitude as the distance between the evaporator and condenser surfaces. Under these ideal conditions, evaporation occurs without encountering other molecules and returning to the liquid without hindrance from the remaining gas molecules. All vapor molecules reach the condenser surface without encountering other molecules and return to the liquid. The evaporation rate reaches the highest possible value at the set temperature. The evaporation rate is proportional to the pressure. Therefore, the amount of distillate from molecular distillation is relatively small.
3. In large and medium-sized short-path distillation, when the distance between the condenser and the heating surface is 20-50mm, and the residual gas pressure is 10-3bar, the mean free path length of the residual gas molecules is about twice as long. Short path distillation fully meets all the necessary conditions for molecular distillation.

Features

1. Conventional distillation is carried out at the boiling temperature, and molecular distillation can be carried out at any temperature, as long as there is a temperature difference between the hot and cold sides. It can also achieve the purpose of separation.
2. Ordinary distillation is a reversible process of evaporation and condensation, and a phase equilibrium state can occur between the liquid phase and the gas phase; In contrast, in the molecular distillation process, the molecules overflowing from the evaporation surface fly directly to the condensation surface, and there will be no other molecules in the middle collision. In theory, there is no possibility of returning to the evaporation surface, so the molecular distillation process is irreversible.
3. Ordinary distillation has the phenomenon of foaming and boiling. The molecular distillation process is the free evaporation of the surface of the liquid layer without foaming.
4. The separation factor, which represents the separation ability of ordinary distillation, is related to the ratio of the vapor pressure of each component. The separation factor, which represents the separation ability of molecular distillation, is related to the ratio of the vapor pressure and molecular weight of the component, which can be calculated from the relative evaporation rate.

Advantages

1. The distillation temperature is low. Molecular distillation works at a temperature far below the boiling point. As long as there is a temperature difference, it can achieve the purpose of separation. This is the essential difference between molecular distillation and conventional distillation.

2. Distillation vacuum degree is high, molecular distillation equipment can achieve a high internal vacuum degree, molecular distillation is usually carried out under extremely low pressure, so the material is not easy to be oxidized and damaged.

3. The distillate film is thin, and the heat transfer efficiency is high.

4. The heating time of the material is short, and the distance between the heated liquid surface and the condensing surface is less than the mean free path of light molecules. Therefore, the light molecules escaping from the liquid surface to the condensation surface rarely collide. Therefore, the heating time of the distillate is short, and the interruption time at the distillation temperature is generally between several seconds and tens of seconds, which reduces the possibility of thermal degradation of the material.

5. Higher degree of separation, molecular distillation can separate conventional substances that are difficult to separate.

6. No boiling, bubble phenomenon, molecular distillation is the free evaporation of the surface of the liquid layer, carried out under low pressure; there is no dissolved air in the liquid, so the whole liquid cannot be boiled during the distillation process, and there is no bubble phenomenon.

7. Non-toxic, harmless, non-polluting, no residue, pure and safe product, simple operation procedure, and less equipment. Molecular distillation technology can separate substances that are not easily separated by conventional distillation.

8. Molecular distillation equipment is expensive. Molecular distillation equipment must provide high vacuum system pressure and has high requirements on sealing materials. The distance between the evaporation surface and the condensation surface should be moderate, the equipment is difficult to maintain, and the process is expensive.

9. The product consumes less energy. Due to the low loss of overheating in the entire separation in all molecular distillations and the unique structure of the molecular distillation unit, the internal pressure is very low, and the internal resistance is much smaller than traditional distillation so it can save a lot of energy.

Advantage:

Based on the above characteristics of molecular distillation technology, it seems that  can see that compared with traditional distillation technology in practical industrial applications, it has the following obvious advantages:

1. Molecular distillation separates high-boiling, heat-sensitive and oxidizable materials. Because molecular distillation operates at temperatures well below the boiling point of the material, and the material residence time is short.

2. Molecular distillation can effectively remove organic solvents, odors, and other substances in liquids. It is a very efficient method of liquid precipitation after solvent extraction.

3. Molecular distillation can selectively distill out the target product, remove other impurities, and separate more than two substances simultaneously through multi-stage separation.

4. The fractional distillation process of molecular distillation is a physical process, so it can effectively prevent the separated substances from contamination and damage.