Molecular distillation, also known as short pathdistillation, is a kind of non-equilibrium distillation, which separates substances under high vacuum (generally less than 5 Pa) based on the difference in the average free range of molecular motion of different substances. It has the characteristics of high vacuum, low distillation temperature, short heating time and high separation degree, so it can greatly reduce the separation cost of high boiling point materials, and can protect the quality of heat-sensitive materials, especially for the separation and purification of high boiling point, heat-sensitive and easily oxidized substances. This technology has been widely used in the separation and purification of high-purity substances, especially for the separation of natural substances. At present, molecular distillation technology has been successfully applied to petrochemical, fine chemical, food, plastic, pharmaceutical and other industries. In recent years, molecular distillation technology has gradually become one of the key technologies for the modernization and production of traditional Chinese medicine.
1. The principle and characteristics of molecular distillation
1.1 The basic principle
Conventional distillation is based on the equilibrium of the gas-liquid phase, according to the relative volatility of the components and the separation, the separation operation is carried out at the boiling point temperature of the mixture. In contrast, molecular distillation is based on the difference in the average free range of molecular motion of different substances, and the separation of substances is achieved under high vacuum (pressure generally less than 5 Pa). Molecular distillation is a high vacuum distillation, where the separation operation is carried out below the boiling point of the substance, which is different from both simple distillation and evaporation, and is a completely non-equilibrium distillation process. It is a completely non-equilibrium distillation process. The separation operation can be performed when there is a temperature difference between the condensing surface and the evaporating surface. In the process of molecular distillation, the average free range of light component molecules is large, and the average free range of recombinant molecules is small. If a condensation surface is set at the point where the average free range of light component molecules is smaller than the evaporation surface but larger than the average free range of recombinant molecules, the light component molecules fall on the condensation surface and are condensed, thus destroying the dynamic equilibrium of their molecular motion and causing them to continuously escape, while the recombinant molecules do not reach the condensation surface and return to the original liquid surface. The molecules of the recombination do not reach the condensation surface and return to the original liquid surface, and soon tend to dynamic equilibrium, so that the different components of the mixture will be separated.
Molecular distillation process can be generally divided into five steps: (1) the formation of liquid film on the heating surface of the material; (2) the free evaporation of component molecules on the surface of the liquid film; (3) the movement of component molecules from the heating surface to the condensing surface; (4) the condensation of component molecules on the condensing surface; (5) the collection of distillates and residues.
1.2 Advantages and limitations of molecular distillation technology
From the principle of molecular distillation, it is known that the operation of molecular distillation must meet three necessary conditions: (1) the average free range of molecular motion of light and heavy components must be different; (2) the distance between the evaporation surface and the condensation surface should be less than the average free range of molecular motion of light components; (3) there must be a very high vacuum. Molecular distillation has many outstanding advantages compared with conventional distillation, but at the same time there are certain limitations, because molecular distillation requires separation under high vacuum, the equipment required is too high cost, complex structure, high design technology requirements, the corresponding supporting equipment is also too much, the investment is too large, the domestic has not seen large-scale use; molecular distillation is limited by the structure of the equipment and heating area, the volume of equipment than conventional distillation The volume of equipment is larger than that of conventional distillation equipment, and there are many difficulties in large-scale production applications.
2 The application of molecular distillation technology in the separation and purification of natural drugs
2.1 Characteristics of natural drugs
Natural drugs are various kinds of material components that exist in plants, animals, marine organisms and microorganisms, and are mainly used in the application of substances that are biologically active from the pharmacological and biological points of view, which are called active ingredients. The active ingredients in natural drugs often need to be extracted from the raw materials of natural drugs with complex compositions, and then purified and refined by separating and removing various impurities before they can be applied as medicines. On the other hand, the active ingredients in many natural drugs are heat-sensitive, easy to oxidize and decompose, which requires the extraction, separation and refining process to avoid the oxidation and decomposition of the active ingredients to maintain their biological activity. For example, the main components of medicinal volatile oils are aldehydes, ketones and alcohols, which have a high boiling point and are heat-sensitive substances, so when the general distillation method is used for refining, it is very easy to cause the oxidation of the components, decomposition and polymerization and damage. Molecular distillation method of refining, because of the high vacuum, low temperature, can effectively avoid the oxidation, decomposition, polymerization of the active ingredients, in the purification of the same time can also deodorization and decolorization, greatly improving the quality of the product. For example, through molecular distillation refining, the content of patchouli alcohol and patchouli ketone in patchouli oil is increased from 30% to 60%-90%, the content of citral in sorrel oil is increased from 40% to more than 90%, the content of allicin in garlic oil is increased from 0.5% to 8.0%-10%, the content of pine oil in Australian tea tree oil is increased from 30% to 60%-80%, and the content of eucalyptus oil is reduced from 15% to less than 1%. from 15% to less than 1%. Therefore, this shows that molecular distillation is a very suitable method for refining natural drugs.
2.2 The application of molecular distillation technology in the separation and purification of natural drugs mainly includes the following aspects.
(1) Purification of volatile oils in natural drugs.
(2) Refinement of natural drug extracts.
(3) Preparation of natural drug standards.
(4) New methods for the development of new drugs of natural drugs.
(5) Removal of residual pesticides and harmful heavy metals from natural drugs.
To sum up, molecular distillation technology has unique advantages when applied to the separation and purification of natural drugs. With the development of the trend of returning to nature and the pursuit of natural, as well as the progress of modernization of Chinese medicine in China, molecular distillation has been more and more widely used in the separation and purification of natural drugs; in particular, the combination of molecular distillation technology with traditional extraction and separation technology and supercritical extraction technology for the Chinese medicine industry has attractive application prospects, and molecular distillation technology will be more and more widely used in the separation and purification of active ingredients of Chinese medicine.
