At a certain temperature, the lower the pressure is, the larger the average free path of gas molecules is. When the pressure of evaporation space is very low (10-2-10-4 mmHg), and the condensation surface is close to the evaporation surface, and the vertical distance between them is less than the average free path of gas molecules, the vapor molecules vaporized from the evaporation surface can reach the condensation surface directly without colliding with other molecules.

working principle:

Molecular distillation is a special liquid-liquid separation technology, which is different from the traditional distillation that relies on the principle of boiling point difference separation, but on the difference of the average free path of the movement of different substances. When the liquid mixture flows along the heating plate and is heated, the light and heavy molecules will escape from the liquid surface and enter the gas phase. Because the free path of the light and heavy molecules is different, the molecules of different substances will move at different distances after escaping from the liquid surface. If a condensation plate can be properly set, the light molecules will reach the condensation plate and be discharged, while the heavy molecules will not reach the condensation plate and be discharged along the mixture. In this way, the purpose of material separation is achieved.

Molecular distillation equipment:

The pressure difference between the boiling film and the condensing surface is the driving force of the steam flow direction. For a small pressure drop, the steam flow will be caused. Running at 1mbar requires a very short distance between the boiling surface and the condensing surface. The distiller based on this principle is called a short range distiller. The short range distiller (molecular distillation) has a built-in condenser on the opposite side of the heating surface and reduces the operating pressure to 0.001mbar.

The short-range distiller is a process of heat separation technology under the pressure of 1 ~ 0.001 mbar. It has a low boiling temperature and is very suitable for heat sensitive and high boiling point substances. Its basic structure: cylindrical cylinder with heating jacket, rotor and built-in condenser; film scraper and anti splashing device are precisely installed on the fixed frame of rotor. The built-in condenser is located in the center of the evaporator, and the rotor rotates between the cylindrical cylinder and the condenser.

The short-range distiller consists of a vertical cylinder heated externally, a central condenser and a membrane scraper rotating between the distiller and the condenser.

The distillation process is: the material is added from the top of the evaporator, distributed on the heating surface continuously and evenly through the material liquid distributor on the rotor, and then the film scraper scrapes the material liquid into a very thin, turbulent liquid film, and advances downward in a spiral shape. In this process, the light molecules escaping from the heating surface condense into liquid on the built-in condenser after a short route and almost without collision, and flow down the condenser tube, and discharge through the discharge tube at the bottom of the evaporator; the residual liquid is the heavy molecules collected in the circular channel under the heating area, and then flow out through the side discharge tube.

The short range 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 divided into four steps as follows:

In general, the diffusion speed in the liquid phase is the main factor to control the molecular distillation speed, so we should try to reduce the thickness of the liquid layer and strengthen the flow of the liquid layer. The free evaporation rate of molecules on the surface of liquid layer increases with the increase of temperature, but the separation factor sometimes decreases with the increase of temperature. Therefore, the economic and reasonable distillation temperature should be selected on the premise of the thermal stability of the processed substances. In the process of the molecules flying from the evaporation surface to the condensation surface, they may collide with each other or with the air molecules remaining between the two sides. Because the evaporation molecules are much heavier than the air molecules, and most of them have the same direction of motion, so their collision has little effect on the direction of flight and evaporation speed. The number of residual molecules is the main factor affecting the direction of flight and evaporation speed. As long as there is enough temperature difference between the cold and hot sides (generally 70 ~ 100 ℃) for molecular condensation on the condensation surface, and the form of condensation surface is reasonable and smooth, it is considered that the condensation step can be completed in an instant, so it is very important to choose a reasonable form of condenser.

1. The partial pressure of residual gas must be very low, so that the average free path length of residual gas is multiple of the distance between the surface of distiller and condenser.

2. At saturation pressure, the average free path length of steam molecules must have the same order of magnitude as the distance between the surface of evaporator and condenser.

molecular distillation

Under this ideal condition, evaporation occurs from the residual gas molecules without any obstacles. All steam molecules reach the condenser surface without encountering other molecules and returning to the liquid. The evaporation rate reaches the maximum possible value at the temperature. The evaporation rate is directly proportional to the pressure, so the distillate volume of molecular distillation is relatively small.

In large and medium-sized short-range distillation, when the distance between the condenser and the heating surface is about 20-50 mm and the pressure of the residual gas is 10-3 mbar, the average free path length of the residual gas molecule is about twice as long. The short distance distiller can completely meet all the necessary conditions of molecular distillation.