Many materials, including petroleum, heavy residue, chemical medicine, medicine and natural food, health care products, fatty acid and so on, are generally heat sensitive and viscous. Moreover, they also possess high boiling points. Separating these materials from their components and maintaining the quality of such products that are only able to distillate at low boiling points or even lower than the boiling temperature. Besides, it is also capable of reducing the thermal decomposition or polymerization to the minimum in a short period, so as to eliminate the damage to products.
When the operating vacuum is about 500 Pa, it is capable of conducting in the scratch film evaporator or falling film evaporator. However, if the distillation is operate under the high vacuum status (for instance, the pressure ranges from 0.001 to 100Pa), problems are bound to cause. Therefore, it is necessary to choose the evaporator which the vapor pressure is no less than pressure difference caused by heating and cooling surface, and the evaporator equipped with outside condenser is excluded from the distillation process under the high vacuum range.
In contrast, the short-path evaporator is quite adaptable to this occasion, and the condenser should be placed within evaporator and opposite to heat distillation. Therefore, as a relatively new equipment which is not widely applied in industrial production, the short-path distillation is able to solve many tough issues that normal distillation technical cannot conquered.
2. Working Principle
The pressure difference between the boiling film and condensing surface is the driven force of stem flow, and the minimum pressure will cause the flowing of steam. Under the 1mbar, the operation requires the boiling surface and condensing surface have the shortest distance. The evaporator manufactured based on the principle is called short-path distillation. The short-path distillation (molecular distillation) is equipped with an inside condenser which is opposite to the heating surface, and make the operation pressure reduce to under 0.001mbar. Its relative low boiling temperature is very suitable to the material with heat sensitive and high boiling point.
Basic constitution: The column cylinder is equipped with outside heating mantle, rotor and inside condenser. The fixing frame of rotor is precisely equipped with scratch film and anti-splash device. The inside condenser is placed in the center of evaporator, and the rotor should rotate between column cylinder and condenser.
Distillation process: The material should add from the top of evaporator and continuously spread to the heating surface through the material covered on rotor. Then, the scratch film is capable of scratching the material into a thin and flowing liquid film. During this process, the light molecular exceeded the heating surface forms the condense liquid on inside condenser without any collision. Then, the molecular flows along the condenser tube, and discharges through the discharge tube on the bottom of evaporator. The residual liquid（heavy molecule）is collected on the circular channel under the heating zone, and flows through the discharge tube on one side.
The molecular distillation serves as a special distillation process which is conducted under the high vacuum status within the range of 1Pa-0.01Pa. The molecular flows directly from heating surface to condensing surface. The molecular distillation can be concluded as the following four steps.
▲ Molecular spreads from liquid body to evaporation surface:
Generally speaking, the spread speed of liquid is the main factor in controlling the speed of molecular distillation, so it is adoptable to reduce the liquid thickness as much as possible to strength the flowing of liquid.
▲ Molecular is capable of free evaporating on the liquid surface:
The evaporation speed lifts with the increase of temperature, but the separation factors sometimes reduce with the increase of temperature. Therefore, it is wise to choose the economical evaporation temperature with the premise of heat stability of manufactured materials.
▲ Molecular spreads from evaporation surface to condensation surface:
During the process of spreading from evaporation surface to condensation surface, they are capable of conducting collision with each other, or with the air molecular between two surfaces. Because the evaporation surface is much heavier than air molecular, and both of them have the same operating direction.
▲ The condensation of molecular on condensing surface:
As long as to keep the heating and cooling surface have abundant temperature difference( in general 70-100℃), and the adoptable and smooth condensing surface guarantee the condensing steps can be completed in a moment. Therefore, it is of great importance to choose the suitable condenser.
From the above-mentioned text, we can concluded the conditions of molecular distillation as following:
▲. The partial pressure of residual gas must be very low, making the average length of residual gas is a multiple of distance between distiller and condenser surface.
▲. Under the saturation pressure, the average free length of steam molecular should have the same number with the distance between evaporator and condensing surface.
Under the ideal condition, the evaporation is capable of conducting from residual gas without any obstruction. If all vapor molecular do not encounter any other molecular, they will reach to the condenser surface during process of returning to liquid. Under the certain temperature, the evaporation speed is bound to reach the maximum. Because the distillation speed is proportional to vacuum degree, the amount of distillate during molecular distillation is relatively small.
During the SPE short-path distillation, the distance between condenser and heating surface is about 20-60mm. When the pressure of residual gas is about 10-3mba, the average free length of residual gas molecular is double than its original length. Therefore, the short-path evaporator could meet all necessary requirements of molecular distillation.
3. The Differences Between the Short-Path Distillation and Molecular Distillation
Short-path distillation: “short” refers to a short distance. In a broad sense, the evaporator adopts a design scheme with a distance of less than 300MM from the evaporation surface to the condensation surface, which can be called a short-path stiller.
The distillation process is: the material is added from the top of the evaporator, and it is continuously and evenly distributed on the heating surface through the material liquid distributor on the rotor, and then the material liquid is scraped into a very thin, turbulent liquid Membrane by the film scraper and advance downward in a spiral. The light components that overflowing from the heating surface are condensed into liquid on the built-in condenser after a short route and almost without collision, and flow down the condenser tube through the discharge tube at the bottom of the evaporator for discharge. The residual liquid, the heavy component, is collected in the circular channel under the heating zone, and then flows out through the side discharge pipe.
Molecular distillation, which is different from the separation principle of traditional distillation according to the difference of boiling point, it relies on the difference of the mean free path of the molecular motion of different substances to achieve separation.
The distillation process is: when the liquid mixture flows along the heating plate and is heated under a certain pressure condition, light and heavy molecules will overflow the liquid surface and enter the gas phase. Because the free paths of light molecules and heavy molecules are different, the molecules of different substances move at different distances after overflowing from the liquid surface. If a condensing plate can be properly set up (the distance between the evaporation area and the condensing surface is generally less than 100MM), the light molecules will first reach the condensing plate and be condensed and discharged, while the heavy molecules will not reach the condensing plate and be discharged along the mixed liquid. So achieve the purpose of material separation.
From the description of the distillation process, attention should be paid to “specific pressure conditions” and “evaporation area and condensation distance”. That is to say, not all raw materials can achieve molecular distillation; the molecular distillation process can only be achieved in a scraped short-path distiller or a centrifugal short-path distiller; Molecular distillation is not simply distilled in a short-path still, it must be a molecular motion at a specific pressure and temperature to achieve separation process.
Therefore, short-path distillation and molecular distillation are essentially different, and the two should not be confused. The distillation equipment should be reasonably selected according to the raw materials.
4. Device Characteristics
▲ Material with high boiling point
Using the specialized metal components is capable of treating the material whose boiling points reaches 450℃ under the vacuum condition of 0.001mbar.
▲ Viscous solid material
Scratch immersing into the liquid film produce the strong shear force, which is greatly reducing the material with the viscosity of 50Pas.
▲ Solid content surplus
Scratch immersing into the liquid film produce a strong stirring force, and it is also capable of preventing the solid scale on the heating surface.
▲ High distillation ratio
Using the scratch with precise manufacture means the liquid film is capable of smearing on the heating surface and making it heated and damped, so as to satisfy the requirement of high distillation ratio. Thus, more than 90% products are capable of being distillated, remaining a certain amount of residual.
5. Structural Characteristics
▲ Precise condenser
The condenser generally adopts coil, U-tube or bundle, which are fixed at the bottom of distiller. The shape of condenser and its distance with heating surface are directly connected with the separate purification, yield ratio and vacuum degree of distiller.
The inside condenser in tube bundle type, which is located in the center of distiller. Moreover, it also boasts such advantages as smooth surface, average temperature and large space of gas transfer. It is also set the distance between heating surface and condensing surface appropriately.
▲ Mist separator
Mist separator is necessary for some products, and it is capable of setting between the rotor and scratch as well as behind the scratch. These splash prevention equipment is capable of discarding any liquid added by steam stream.
As a complete component, the splash prevention equipment is easy to stretch. According to the technical characteristics, it can be divided into blind shape, herringbone shape and groove shape.