molecular distillation(分子蒸馏)

Molecular distillation ProfileMolecular distillation is a distillation method operated in high vacuum, At this time, the mean free path of steam molecules is greater than the distance between evaporation surface and condensation surface, liquid mixtures can be separated because of the difference of evaporation rate of each component in liquid.Under certain temperature, the lower the pressure, the gas molecule mean free path greater. When the pressure of evaporation space (10-2 ~ 10-4 mmHg) is very low, and evaporation surface is near condensation surface, the vertical distance between them is less than the mean free path of steam molecules , gas molecules can reach condensation surface and condensate without collisions.The working principle of molecular distillationMolecular distillation is a special kind of liquid - liquid separation technology, which is different from the traditional distillation separation principle. Separation can be achieved because of the difference of the mean free path of the different molecules. When the liquid mixture flows along the heating plate and is heated, light and heavy liquid molecules escape into the gas phase because of the difference of the mean free path of the light and heavy molecules, the moving distance of the different molecules after escaping from liquid surface. If a condensation plate can be set up properly, the light molecules can reach condensation surface and condensate and be exhausted, the heavy molecules can not reach condensation surface and be exhausted. In this way, the purpose of the separation material is achieved. The difference of pressure between the boiling film and the condensation surface is the driving force whose the steam flows, the small pressure drop will cause the flow of steam. It is required that the distance between boiling surface and condensation surface is very short under the pressure of 1mbar, the stills based on the principle is namedshort-path distillation device. Still short-range (molecular distillation)has a built-in condenser in the opposite surface of heating surface which makes operating pressure reduce to 0.001mba Short-path distillation is a process of thermal separation technology under the pressure of 1 ~ 0.001mbar, which has the lower boiling temperature, is ideal for heat-sensitive, high boiling point materials. Its basic components: the cylinder cylindrical with a heating jacket, the rotor and the built-in condenser, the scraping membrane device, anti-splash installment. The built-in condenser is located in the center of the evaporator, the rotor between the cylinder and condenser.Distillation process: the materials are added from the top of the evaporator and are distributed uniformly, continuously on heating surface by the liquid distributor. the scraping membrane device make the liquid materials into a single very thin liquid film like turbulence, and advance downward spirally. In the process, the light molecules escaping from the heating sarurface reach the built-in condenser and condense into fluid through a short line and almost without a collision . It flow along the condenser tube, is exhausted through the exhaust pipe at the bottom of the evaporator; the residues(the heavy molecules ) are collected in a circular path under the heating zone, flow out through the side of the pipe.The process of molecular distillationShort-path distillation is also suitable for molecular distillation. The flow is from the surface of the heating zone directly to the condenser’s surface. Molecular distillation process can be ma de up ofthe following four steps:1, The molecules proliferate from the main body of liquid phase to the evaporating surface. Typically, in the liquid phase, the proliferation rate is the primary factor of controlling the molecular distillation speed, therefore, the thickness of the thin liquid layer should be reduced and the flow of liquid should be enhanced.2, the free evaporation of the molecules on the liquid’s surfaceWith temperature rises, the evaporation rate enhances, but in some instances, the separation factor decreases. Therefore, based on the thermal stability of the material the economic rational temperature of distillation is chose, which based on the thermal stability of the material.3, the molecules fly from the evaporation surface to the condensation surfaceIn the process, the molecules may collide one another and may collide with the residual air molecules between the two sides in the air. The evaporated molecules are far more heavier than air molecules, so the collision of their own has little effect on evaporation rates and the flying direction. The hot campaign of the residual gas molecules between two sides is chaotic state. so the number of residual gas molecules is the main factor influencing evaporation rates and the flying direction.4, the molecules condensate on the surface of condensationIf there is sufficient temperature difference (typically 70 ~ 100 ℃) between the hot side and cold side , and the form of condensation surface is smooth and reasonable, it is considered that the condensation steps can be completed in an instant, so it is very important to choose the form of the condenser.The conditions of molecular distillation1, the residual gas pressure must be very low, so that the length of mean free path of the residual gas is several times as the distance between the condenser’s surface and the distiller.2, under the saturation pressure, the length of mean free path of the vapor molecules must have the same magnitude with the distance between the condenser’s surface and the distiller.In this the ideal conditions, the evaporation is produced from the residual gas molecules without any obstacles. Without encountering any molecules, all the vapor molecules return to the liquid and reach on the surface of condenser. Evaporation rate achieve the maximum possible at the current temperature. Evaporation rate is proportional to the pressure, thus, the dis tillated liquid’s volume by molecular distillation is relatively small.The characteristics of molecular distillation1, the ordinary distillation may be separated at boiling point, and molecular distillation can be carried out at any temperature, as long as there is a temperature difference between two hot sides, the purpose of separation can be achieved.2, ordinary distillation evaporation and condensation is a reversible process, the equilibrium state of liquid and gas phase can be formed; in the process of the molecular distillation, the molecules escaping from the evaporation surface fly to the condensing surface directly, but there is not a collision with other molecules, in theory, there is not the possibility to return the evaporation surface, Therefore, molecular distillation process is not reversible.3, ordinary distillation has bubbling, boiling phenomena; there is no bubbling phenomenon because the liquid molecules on the surface of liquid evaporate freely.4 The separation factors which represent the separation capacity of ordinary distillation is relative to the ratio of the vapor pressure of the component , The separation factors which represent the separation capacity of molecular distillation is relative to the ratio of the vapor pressure and molecular weight of the component, them can be obtained from the relative speed of evaporation.The advantages of molecular distillation1 The temperature of molecular distillation is low. It is operated at the temperature much lower than the boiling point, as long as there is a temperature difference between two hot sides, the purpose of separation can be achieved, which is the essence of the distinction between conventional distillation and molecular distillation.2 Distillation has high vacuity .High vacuity can be obtained in the internal unit of molecular distillation, usually, molecular distillation can be operated at a very low pressure, so material is not easy to be oxidized and damaged.3 The membrane of distillation liquid is thin. The efficiency of heat transfer is high.4 Materials are heated in a short time. The distance between the surface of the liquid heated and condensing surface is less than the mean free path of light molecules, so the materials distillated are heated in a short time, residence time is from a few seconds to dozens of seconds, the chance of thermal decomposition of material should reduce.5 A higher degree of separation. The material that is not separated can be separated by molecular distillation.6 There is no the phenomena bubbling of boiling. Molecular distillation is a process of free evaporation on the surface of the liquid. Under the low pressure, there is no air dissolving in the liquid. So it is impossible to make all the liquid boil, and there is no bubbling phenomenon.7 Non-toxic, harmless and pollution-free, non-residual, the product of safe and pure can be obtained. the operation of molecular distillation is simple and the equipment is less.8 the equipment of molecular distillation is expensive, and it must ensure that the pressure of the system can achieve the high vacuity. A higher sealing material is required and the distance between evaporation surface and condensation surface is proper. The equipment is difficult to be processed and the cost is high.9 Energy-consuming of products is low. There is few loss of heat in the separation process of molecular distillation: the equipment of molecular distillation has special structure, the internal pressure is extremely low, the internal resistance is lower than the conventional distillation, and therefore power consumption can be reduced substantially.From the molecular distillation of the characteristics of the above, compared to conventi onal distillation technology, we can see that it has the following obvious advantages in the actual application of industrialization:1 molecular distillation provide the best separation method for materials of high boiling point, heat sensitive materials, materials that are easy to be oxidized, because It is operated at the temperature much lower than the boiling point and the retention time is short.2 the substances such as organic solvents, odor and so on can be removed effectively by molecular distillation. It is an effective method of desolution for the liquid using the solvent extraction.3 molecular distillation can make the goal product to be distilled off selectively and removes other impurities, it can separate 2 above materials through the multistage separations.4 the fractional process of molecular distillation is the physical process, thus may protect well the separated material not to polluted and violated.Molecular distillation equipmentA complete set of molecular distillation equipment include: molecular evaporator, degassing systems, feed systems, heating systems, cooling systems and vacuum control system. Molecular distillation unit is the core of the evaporator elements, of the types are mainly three kinds:(1) Falling film molecular distillation device: it is the early form of simple structure, but the liquid film is thickness, the efficiency is poor, countries of all over the world seldom used today. The device take up the following working way: the gravity make the material into the down liquid film. The materials are heated and the product of evaporation can be condensed on the surface of the across direction.(2) Wiped-Film molecular distillation deviceIn the end of 80's the installations and applied research of Wiped-Film molecular distillation s was carried out in our country. The forming liquid membrane is thin, the efficiency of separation is high, but the structure is more complex than falling-film. The device take up the following working way: the gravity make the material into the down liquid film, but in order to enable the thickness of the liquid film on the evaporation surface to be small and distributed evenly, a rotating scraper made of hard carbon or PTFE was set in the distiller. The scraper can not only make the down liquid be stirred fully, but also speed up the liquid surface layer to renew, thus the process of heat transfer and mass transfer was strengthened. Its advantages are: the thickness of liquid membrane is thin, and liquid flow along the evaporation surface; the remain time of distilled materials is short at operation temperature, the risk of thermal decomposition is small, the distillation can be operated continuously, the production capacity is large. The disadvantage is that: it is difficult to improve the liquid distribution device, it is difficult to ensure that all the liquid evaporation surface to be covered evenly; when liquid flows, there is frequent rolling phenomenon happening, foam and mist generated often spilled onto the condensation surface.. However, due to the structure of the device is relatively simple, relatively inexpensive, in the most cases the device is still used in the laboratories and the process of industrial production.(3) Centrifugal molecular distillation device: centrifugal film, thin film, evaporation efficiency, but t he structure is complex and is difficult to manufacture and operate, vacuum sealing is more difficult, the equipment is expensive. To enhance the efficiency of separation, multi-level series are often used to achieve multi-stage separation of the different substances. The materials are carried onto the centre of high-speed rotating turntable though the device and expend to form film on the rotation surface , at the same time they are heated and evaporate.Application of molecular distillation(A)the food industry1, the production of Monoglyceride Profits.2, the refinement of fish oil3, oil deacidification4, the refinement higher alcohol(B) in the Fine Chemical Industry1, aromatic oil purification2, the purification of polymer intermediates3, the extraction of lanolin(C) the pharmaceutical industryIn conclusion, as a special kind of new separation technology, molecular distillation technology mainly used in separation and purification of high boiling point,heat-sensitive materials. Practice has proved that this technology is not only a high technology, but also the range of applications is wide. It has a very broad prospect for industrial application of high-tech.。