Short-path distillation (molecular distillation)

  1.Outline

  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 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 id no less than pressure difference caused by heating and cooling surface, and the evaporator quipped with outside condenser is excluded from the distillation process under the high vacuum range.

Short-path distillation

  In contrast, the short-path evaporator is quietly 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 a 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.

molecular distillation

  Basic constitution: the column cylinder 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 rotor 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 form the condense liquid on inside condenser without any collision. Then, the molecular flows along the condenser tube, and discharge through the discharge tube on the bottom of evaporator. The residue 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.

  1.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.

  2.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.

  3. 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. Due to the evaporation surface is far weight than air molecular, and both two of them have the same operating direction

  4.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:

  1. 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.

  2. 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. Due to the distillation speed is proportional to vacuum degree, so the 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. 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.

  4.Structure characteristics

  1. Precise condenser

  The condenser is generally adopt 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.

  2.Mist separator

  Mist separator is necessary to 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.

Short-path distillation

Molecular Distillation

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