Short-path distillation is a distillation technique where the distillate travels a short distance.
Scraped film short path molecular distillation equipment
The wiped film short-distance molecular distillation equipment is a distillation method operating under high vacuum. At this time, the average free path of vapor molecules is greater than the distance between the evaporation surface and the condensation surface, so that the evaporation rate of each component in the feed liquid can be used. Difference, the liquid mixture is separated.
At a certain temperature, the lower the pressure, the greater the average free path of the gas molecules. When the pressure in the evaporation space is very low (10-2 ~ 10-4 mmHg), and the condensation surface is close to the evaporation surface, the vertical distance between them is less than the average free path of the gas molecules, the vapor molecules vaporized from the evaporation surface can Other molecules collide and directly reach the condensation surface to condense.
Working principle of wiped film short path molecular distillation
Molecular distillation is a liquid-liquid separation technology, which is different from the traditional distillation principle of relying on the difference in boiling point separation, but the difference is achieved by the difference in the average free path of the molecular movement of different substances.
When the liquid mixture flows along the heating plate and is heated, light and heavy molecules will escape from the liquid surface and enter the gas phase. Due to the different free paths of light and heavy molecules, molecules of different substances will move different distances after escaping from the liquid surface If a condensing plate can be set properly, light molecules reach the condensing plate and are condensed and discharged, while heavy molecules cannot reach the condensing plate and are discharged along the mixed liquid. In this way, the purpose of material separation is achieved.
The pressure difference between the boiling film and the condensing surface is the driving force for the flow of steam, and a small pressure drop will cause the flow of steam. Operation at 1 mbar requires a very short distance between the boiling surface and the condensation surface. A distiller made based on this principle is called a short-path distiller. The short path distiller (molecular distillation) has a built-in condenser opposite the heating surface and reduces the operating pressure to 0.001 mbar.
The short-path distiller is a process of thermal separation technology working at a pressure of 1 to 0.001 mbar. Its lower boiling temperature is very suitable for heat-sensitive and high-boiling substances. Its basic structure: a cylindrical barrel with a heating jacket, a rotor and a built-in condenser; a membrane scraper and an anti-spatter device are accurately installed on the rotor’s fixed frame. The built-in condenser is located in the center of the evaporator, and the rotor rotates between the cylindrical barrel and the condenser.
The short-path distiller is composed of an externally heated vertical cylinder, a central condenser located in it, and a wiper film rotating between the distiller and the condenser.
Process of wiped film short-range molecular distillation
Short-path distillers are 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:
- The diffusion of molecules from the liquid phase body to the evaporation surface:
Generally, the diffusion rate in the liquid phase is the main factor controlling the molecular distillation rate, so the thickness of the liquid layer should be reduced as much as possible and the flow of the liquid layer should be strengthened.
- 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.
- The molecules fly from the evaporation surface to the condensation surface:
As the vapor molecules fly from the evaporation surface to the condensation surface, they may collide with each other, or they 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.
- The molecules condense on the condensation surface:
As long as there is sufficient 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 in an instant, so it is very important to choose a reasonable condenser form.
Conditions of wiped film short-range molecular distillation
- The partial pressure of the residual gas must 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.
- At saturation pressure, the average free path length of the steam molecules must 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 the residual gas molecules without any obstacles. All vapor molecules reach the condenser surface without encountering other molecules and returning to the liquid. The evaporation rate reaches the maximum value possible 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 fully meet all the necessary conditions for molecular distillation.
Characteristics of wiped film short-range molecular distillation
- Ordinary distillation is separated at the boiling temperature. Molecular distillation can be performed at any temperature. As long as there is a temperature difference between the cold and hot sides, the separation point can be reached.
- Ordinary distillation is a reversible process of evaporation and condensation, and a phase equilibrium state can be formed between the liquid phase and the gas phase; while in the molecular distillation process, the molecules escaping from the evaporation surface fly directly onto the condensation surface without colliding with other molecules in the middle In theory, there is no possibility of returning to the evaporation surface, so the molecular distillation process is irreversible.
- The ordinary distillation has bubbling and boiling phenomena; the molecular distillation process is free evaporation on the surface of the liquid layer, and there is no bubbling phenomenon.
- The separation factor representing the ordinary distillation separation ability is related to the ratio of the component’s vapor pressure, and the separation factor representing the molecular distillation separation ability is related to the ratio of the component’s vapor pressure and molecular weight, and can be obtained from the relative evaporation rate.
Advantages and advantages of wiped film short path molecular distillation
- The distillation temperature is low. Molecular distillation is operated at a temperature far below the boiling point. As long as there is a temperature difference, separation can be achieved. This is the essential difference between molecular distillation and conventional distillation.
- The distillation vacuum is high, and a high vacuum degree can be obtained inside the molecular distillation device. Generally, molecular distillation is operated under a very low pressure, so the material is not easily oxidized and damaged.
- Thin distillate film and high heat transfer efficiency.
- The heating time of the material is short, and the distance between the heated liquid surface and the condensation surface is less than the average free path of the light molecules, so the light molecules that escape from the liquid surface reach the condensation surface almost without collision. Therefore, the heating time of the distillation material is short, and the residence time at the distillation temperature is generally between a few seconds and tens of seconds, which reduces the chance of thermal decomposition of the material.
- Higher degree of separation, molecular distillation can separate conventional materials that are not easy to separate
- There is no boiling bubbling phenomenon. Molecular distillation is the free evaporation on the surface of the liquid layer. It is carried out under low pressure and there is no dissolved air in the liquid. Therefore, the whole liquid cannot be boiled during the distillation process, and there is no bubbling phenomenon.
- Non-toxic, harmless, pollution-free and residue-free, it can obtain pure and safe products, and the operation process is simple and the equipment is few. Molecular distillation technology can separate materials that are difficult to separate by conventional distillation.
- The molecular distillation equipment is expensive, and the molecular distillation device must ensure the high vacuum degree of the system pressure, the requirements for material sealing are high, and the distance between the evaporation surface and the condensation surface should be moderate, the equipment processing is difficult, and the cost is high.
- The energy consumption of the product is small. Due to the low overheat loss of the entire separation of molecular distillation, and due to the special structure of the molecular distillation device, the internal pressure is extremely low, and the internal resistance is smaller than that of conventional distillation, which can greatly save energy consumption.
Application fields of wiped film short-range molecular distillation
- Petrochemical industry: Ammonia, amine, quinoline derivatives, epoxy resin, resin acid, isocyanate, isobutyl ketone, peroxide, liquid crystal, polyethylene glycol, silicone oil, softener, etc. .;
- Pharmaceutical industry: amino acid esters, polymer intermediate products, vitamin E, vitamins, etc. .;
- Oil industry: diglycerides, dimerized fatty acids, fatty acids, fatty acid derivatives, polyglycerides, acids, lanolin alcohols, etc. .;
- Food industry: monoglyceride, lauric acid, propylene glycol grease, L-lactic acid, cod liver oil, rice bran oil, triticale germ oil, cinnamon oil, rose oil, etc. .;
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Can be customized depends on requirements.