What Is The Difference Between Distillation And Molecular Distillation

Distillation and molecular distillation are obviously different in principle, equipment and application.

Principle: Distillation is a traditional liquid separation technology, which is based on the difference of boiling points of different substances. Specifically, distillation is a method of separating different components by heating a liquid mixture and vaporizing it, and then condensing the vapor into liquid. Distillation uses the difference of boiling points to separate substances, so the distillation effect is better for mixtures with large boiling points.

Molecular distillation technology is a more advanced liquid separation technology, which is based on the difference of average free path of molecular motion of different substances. Molecular distillation can be operated at very low pressure, so the material is not easy to be oxidized and damaged. In addition, the distillation membrane of molecular distillation is very thin, which has high heat transfer efficiency and can complete the separation of substances in a short time. Because molecular distillation is based on the difference of free paths of molecular motion, it can also achieve effective separation for mixtures with small boiling points.

Equipment components: The distillation equipment is relatively simple in structure and mainly consists of a heating chamber and an evaporation chamber. The equipment structure of molecular distillation system is complex, which consists of heating plate, evaporator, condenser, vacuum pump and so on.

Application: Distillation is mainly used to separate mixtures with large boiling points, such as fractionation of petroleum. Molecular distillation machine is especially suitable for the separation of substances with high boiling point, heat sensitivity and easy oxidation, such as some polymer compounds, amino acids and antibiotics.

Boiling point of common reactants

• Water (H2O), boiling point 100°C: Water is an essential reactant in many chemical reactions. For example, acid-base neutralization reaction, redox reaction and hydrolysis reaction all need water to participate.
• Ethanol (C2H5OH, boiling point 78.5°C): Ethanol is an organic solvent widely used in pharmaceutical, cosmetic and food industries. It is also a reactant of some important reactions, such as esterification, etherification and acid catalysis.
• Ammonia (NH3), boiling point-33.3 C: Ammonia is a colorless gas with strong odor, which has important applications in manufacturing fertilizers, refrigerants and detergents. It is also an important raw material for synthesizing other compounds, such as nitration and preparation of ammonium salts.
• Oxygen (O2), boiling point-183 C: Oxygen is a highly active molecular gas, which plays an important role in organic synthesis and biological processes. For example, both oxidation and reduction reactions require the participation of oxygen.
• Sodium azide (NaN3), boiling point about 250°C: Sodium azide is an important inorganic compound, which can be used to prepare other compounds, such as azide and amino compounds. It is also the main chemical explosive in air passive airbag.
• Carbon dioxide (CO2), boiling point-78.5 C: CO2 is a gas widely existing in nature and plays an important role in biological processes and environment. For example, it participates in respiration, photosynthesis and acid-base reaction.

The average free path of molecular motion of matter refers to the average distance that molecules can travel freely between collisions in gas or liquid. It is an important parameter to describe the interaction and energy transfer between molecules.

Factors affecting the average free path of molecular motion of matter

1. Molecular diameter: The larger the molecular diameter, the more chances of collision and the smaller the free path. On the contrary, the molecular diameter is small and the free path is relatively large.

2. Molecular concentration: with the increase of molecular concentration, the collision frequency between molecules increases and the free path is relatively small.

3. Temperature: with the increase of temperature, the average kinetic energy of molecules increases, the speed of molecular motion increases, the collision frequency of molecules increases, and the free path is relatively small.

4. Medium properties: the interaction between molecules in the medium has an influence on the average free path of molecular motion. For example, in a liquid with strong interaction, the intermolecular attraction is large and the free path is small.

In the process of molecular distillation, the average free path of molecular motion of a substance will affect its separation effect from the mixture. Generally speaking, substances with smaller average free path of molecular motion are easier to be separated, because their intermolecular interaction is weak and the average free path of molecular motion is large, so they are easier to "escape" from the liquid surface and enter the vapor phase, and at the same time they are easier to be re-condensed in the condenser. Therefore, in molecular distillation, generally speaking, substances with low molecular weight and low boiling point are easier to be separated.

Molecules suitable for efficient separation by molecular distillation method

• Alcohol (ethanol): The molecular weight of alcohol is small, the intermolecular interaction is weak, and it is easy to evaporate from the mixture. Therefore, in the process of brewing and alcohol production, alcohol can be separated from fermentation broth or mixture by molecular distillation.
• Water and organic solvents: Water and many organic solvents (such as ether, toluene, etc.) often need to be separated. Because the intermolecular interaction of water is large, the average free path of molecular motion is small, while the intermolecular interaction of organic solvents is weak and the average free path of molecular motion is large. Therefore, in the process of molecular distillation, organic solvents are more likely to evaporate to the upper part of the condenser and thus be separated.
• Hydrocarbons in petroleum: Petroleum is a complex mixture, which contains many hydrocarbon compounds with different carbon chain lengths, such as methane, ethane and propane. Because the molecular weight and intermolecular interaction force of different hydrocarbons are quite different, they can be separated by molecular distillation.
• Flavor components in essential oil: Essential oil is a complex mixture extracted from plants, which contains many fragrant compounds, such as menthol and eucalyptus oil. These perfume components usually have small molecular weight and weak intermolecular interaction, which are suitable for separation and purification by molecular distillation.

Molecular distillation technology is widely used to extract natural products from animals, such as refined fish oil. Fish oil is a kind of oil extracted from fatty fish. Fish oil is rich in cis highly unsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). It has the effects of inhibiting platelet aggregation, reducing blood viscosity, resisting inflammation, cancer and enhancing immunity. It is considered as a potential natural medicine and functional food. The traditional separation methods include urea inclusion precipitation and freezing, and the recovery rate is low.

Using urea inclusion precipitation method can effectively remove saturated and low unsaturated fatty acids from the product and increase the content of DHA and EPA in the product, but it is difficult to separate other highly unsaturated fatty acids from DHA and EPA. Can make w(DHA+EPA)<80%. In addition, the product has heavy color, strong fishy smell and high peroxide value. The product needs further decoloration and deodorization, and the recovery rate is only 16%. Because the average free path of impurity fatty acids in the material is similar to EPA and DHA ethyl ester, molecular distillation can only make w(EPA+DHA)=72.5%, but the recovery rate can reach over 70%. The product has good color, pure smell and low peroxide value, and the mixture can be divided into products with different contents of DHA and EPA. Therefore, molecular distillation tech is an effective method to separate and purify EPA and DHA.