Conditions for molecular distillation:
1. The partial pressure of the residual gas should be very low so that the average free path length of the residual gas is a multiple of the distance between the surface of the distiller and the condenser.
2. At saturation pressure, the average free path length of the vapor molecules should be of the same order of magnitude as the distance between the surface of the evaporator and the condenser.
1. The partial pressure of the residual gas should be very low so that the average free path length of the residual gas is a multiple of the distance between the surface of the distiller and the condenser.
2. At saturation pressure, the average free path length of the vapor molecules should be of the same order of magnitude as the distance between the surface of the evaporator and the condenser.
Under these ideal conditions, evaporation occurs from residual gas molecules without any obstacles. The vapor molecules reach the condenser surface without encountering other molecules and returning to the liquid. The evaporation rate reaches a possible value at the temperature. The evaporation rate is proportional to the pressure, so the amount of distillate from molecular distillation is relatively small.
In large and medium-sized short-path distillation, the distance between the condenser and the heating surface is about 20 ~ 50mm, and the residual gas molecules have an average free path length of about 2 times longer when the residual gas pressure is 10-3mbar. The short path distiller can reach the conditions of molecular distillation.
The basic steps of the molecular distillation process
Short-path distiller is also suitable for molecular distillation. The molecular flow goes directly from the heating surface to the condenser surface. The molecular distillation process can be made in the following four steps:
1. Molecules diffuse from the liquid phase to the evaporation surface
Generally, the diffusion rate in the liquid phase is the main factor that controls the molecular distillation rate, so the thickness of the liquid layer should be reduced.
2. Free evaporation of molecules on the surface of the liquid layer
The evaporation rate increases with increasing temperature, but the separation factor sometimes decreases with increasing temperature. Therefore, the economic and reasonable distillation temperature should be selected on the premise of the thermal stability of the processed material.
3. The molecules fly from the evaporation surface to the condensation surface
Vapor molecules flying from the evaporation surface to the condensation surface may collide with each other or may collide with air molecules remaining between the two surfaces. Since evaporating molecules are much heavier than air molecules, and most of them have the same direction of motion, their collisions have little effect on the direction of flying and the rate of evaporation. The residual gas molecules are in chaotic thermal motion between the two sides, so the number of residual gas molecules is the main factor that affects the direction of flight and the rate of evaporation.
4. Molecules condense on the condensation surface
As long as there is a temperature difference between the cold and hot sides (generally 70 ~ 100 ℃), the form of the condensation surface is reasonable and smooth, it is considered that the condensation step can be completed, so it is very important to choose a reasonable condenser form.
The basic steps of the molecular distillation process
Short-path distiller is also suitable for molecular distillation. The molecular flow goes directly from the heating surface to the condenser surface. The molecular distillation process can be made in the following four steps:
1. Molecules diffuse from the liquid phase to the evaporation surface
Generally, the diffusion rate in the liquid phase is the main factor that controls the molecular distillation rate, so the thickness of the liquid layer should be reduced.
2. Free evaporation of molecules on the surface of the liquid layer
The evaporation rate increases with increasing temperature, but the separation factor sometimes decreases with increasing temperature. Therefore, the economic and reasonable distillation temperature should be selected on the premise of the thermal stability of the processed material.
3. The molecules fly from the evaporation surface to the condensation surface
Vapor molecules flying from the evaporation surface to the condensation surface may collide with each other or may collide with air molecules remaining between the two surfaces. Since evaporating molecules are much heavier than air molecules, and most of them have the same direction of motion, their collisions have little effect on the direction of flying and the rate of evaporation. The residual gas molecules are in chaotic thermal motion between the two sides, so the number of residual gas molecules is the main factor that affects the direction of flight and the rate of evaporation.
4. Molecules condense on the condensation surface
As long as there is a temperature difference between the cold and hot sides (generally 70 ~ 100 ℃), the form of the condensation surface is reasonable and smooth, it is considered that the condensation step can be completed, so it is very important to choose a reasonable condenser form.
Application of molecular distillation technology in purifying lubricating oil
Silicone compound is good lubricating oil, which can improve the smoothness of the disc and the stability of the disc under different humidity and high temperature.
Since siloxane compounds are heat-sensitive substances and have boiling points above 200 ℃, conventional distillation separation methods are easy to denature, and molecular distillation can not only greatly reduce the content of color-forming substances in lubricating oil, but also make the same amount of distillation The time of siloxane is reduced by 40%. At the same time, the molecular distillation technology is also used to purify erucamide to make its mass fraction reach more than 98%.
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