Analysis of the principle and application of molecular distillation technology

Analysis of the principle and application of molecular distillation technology

Molecular distillation is a relatively new separation technology that has not been widely used in industrial production, and can solve a problem that cannot be solved by a large number of conventional distillation techniques. Molecular distillation is a special liquid-liquid separation technology that can be operated under extremely high vacuum. It can separate liquids at temperatures far below their boiling points according to the difference in the free path of molecular motion. It is especially suitable for High boiling point, heat sensitive and easy oxide separation. Because of its characteristics that the distillation temperature is lower than the boiling point of the material, the distillation pressure is low, the heating time is short, and the degree of separation is high, the separation cost of the high-boiling material can be greatly reduced, and the characteristic quality of the heat-sensitive substance is well protected. The extraction of pure natural health products can get rid of the shackles of chemical treatment methods, truly maintain the pure natural characteristics, and make the quality of health care products take a new step.

Molecular distillation technology, as an effective means of separating high-boiling and heat-sensitive materials, has gained the attention of all countries in the world since its appearance in the 1930s. In the 1960s, in order to meet the needs of vitamin A in concentrated cod liver oil, molecular distillation technology has been used in large-scale industrial applications. In Japan, the United States, Britain, Germany and the Soviet Union, several sets of molecular distillation devices were designed and manufactured for the concentration of vitamin A. However, for various reasons, the application surface was too narrow and the development speed was very slow. However, in the past 30 years, people have been paying attention to the development of this new liquid-liquid separation technology. The separation device has been refined and perfected, and the application field has been continuously explored and expanded. Patents and new applications have emerged. Especially since the end of the 1980s, with the popularization of natural materials and the rise of the natural trend, molecular distillation technology has developed rapidly.

For the equipment of molecular distillation, various forms have been developed by various countries. Most of them have been eliminated so far, and the current widely used methods are centrifugal film type and rotor wiped film type. These two forms of separation devices have also been constantly improving and perfecting, especially for different products, and their device structure and supporting equipment have different characteristics. Therefore, the content of the research and development of the molecular distillation device itself is still Very rich.

In terms of application fields, foreign countries have industrialized production in several products. In particular, in recent years, the application of natural substances has been prominently applied, such as extracting EPA and DHA from fish oil and extracting natural vitamin E from vegetable oil. In addition, there are more and more varieties in the extraction and separation of fine chemical intermediates.

China's research on molecular distillation technology started late. In the late 1980s, several sets of molecular distillation production lines were introduced in China for the production of stearic acid monoglyceride. Domestic research staff have also done some research, but there is no report on industrial application.

The molecular distillation complete industrialization device has novel design, unique structure and advanced technology, which can significantly improve the separation efficiency. From the small test to the industrialized production and the repeated trials and experiments of the small test, the outstanding problems that are easy to appear in industrialized production are especially solved. If the material backmixing problem is effectively solved, the product quality is obviously improved, and the message sealing method with compensation function is creatively designed; the long-term stable operation under the high vacuum of the industrial device is realized. This technology is domestically advanced and internationally advanced.

More than 20 kinds of products have been developed so far, such as: monoglyceride stearate, propylene glycol ester, rose oil, wheat germ oil, rice bran oil, oryzanol. The relevant process conditions for the application of molecular distillation technology have been determined, laying a foundation for industrial production.

The principle of molecular distillation and the structure of the device determine its characteristics:

1. The operating temperature of molecular distillation is much lower than the boiling point of the material:

According to the principle of molecular distillation, the separation of the mixture is realized by the different free radicals of different kinds of molecules after overflowing the liquid surface, and does not need boiling, so the molecular distillation is operated at a temperature far below the boiling point. This is fundamentally different from conventional distillation.

2. Low distillation pressure:

Due to the unique structural form of the molecular distillation unit, the internal pressure is extremely small, and a high vacuum can be obtained. Therefore, molecular distillation is operated at a very low pressure, generally on the order of ×10-1 Pa (×10-3 is supported). Number of orders).

3. Short heating time:

According to the principle of molecular distillation, the interval between the heated liquid surface and the condensation surface is required to be smaller than the uniform free path of the light molecules, and the light molecules overflowing from the liquid surface reach the condensation surface almost without collision, so the heating time is short. Further, the mixed liquid is in the form of a film so that the area of ​​the liquid surface and the heating surface are almost equal, so that the heating time of the material becomes shorter during the distillation. For vacuum distillation, the heating time is one hour, while the molecular distillation is only a dozen seconds.

4. Molecular distillation is more highly separated than conventional distillation, and can separate substances that are difficult to separate by conventional distillation:

The relative volatility of molecular distillation is ατ = (P1/P2) • (M2/M1) where: M1-light component molecular weight M2-recombination molecular weight and conventional distillation relative volatilization α = P1/P2. In the case where P1/P2 are the same, the molecular weight M2 of the recombinant component is larger than the molecular weight M1 of the light component, so ατ>α. This indicates that molecular distillation of the same mixture is more easily decomposed than conventional distillation.

The characteristics of molecular distillation determine that it has obvious advantages over traditional technologies in practical applications:

1. Due to the high vacuum degree of molecular distillation, low operating temperature and short heating time, for the separation of high boiling point and heat sensitive and easy oxide materials, there are incomparable characteristics of conventional methods, which can ensure the natural quality of materials. Can be widely used in the extraction of natural materials.

2. Molecular distillation can not only effectively remove low molecular substances in liquids, such as organic solvents, odors, etc., but also selectively distill the desired product to remove other impurities, so it is regarded as a protector of natural quality. And returnees.

3. Molecular distillation can realize the physical process that cannot be realized by traditional separation methods. Therefore, it is widely used as a means of deodorization, decolorization and purification in the separation of some high-value materials.

Introduction to Molecular Distillation Technology ▲ Principle of Molecular Distillation One-by-one molecular distillation is a special liquid-liquid separation technique. It differs from traditional distillation in that it relies on the principle of separation of boiling points, but it is separated by the uniform free path of different molecular motions. Here, the molecular motion free path (indicated by λ) refers to the distance traveled between two collisions of one molecule.

When the liquid mixture moves along the heating plate one by one and is heated, light and heavy molecules will escape from the liquid surface and enter the air intake phase. Because the free paths of light and heavy molecules are different, the gas phase movement intervals of different substances are different. When a condensing plate can be properly set, the light molecules reach the condensing plate and are condensed and discharged, and the heavy molecules do not reach the condensing plate and are discharged along the mixed liquid. In this way, the purpose of material separation is achieved.

â–²Characteristics of Molecular Distillation Technology As a high-tech separation technology that is synchronized with the international, molecular distillation technology has the advantages that other separation technologies cannot compare:

1. Low operating temperature (far below boiling point), high vacuum (no load ≤ 1Pa), short heating time (in seconds), high separation efficiency, etc., especially suitable for separation of high boiling point, heat sensitive and oxidizable substances;

2, can effectively remove low molecular substances (deodorization), heavy molecular substances (decolorization) and remove impurities in the mixture;

3. The separation process is a physical separation process, which can well protect the separated substances from being contaminated, in particular, can maintain the original quality of the natural extract;

4. High degree of separation, higher than traditional distillation.

â–²Molecular distillation technology industrialization application range chemical industry:

High carbon alcohol, hydrocarbon, erucamide, oleic acid amide, tal oil, silicone oil, lubricating oil, vacuum pump oil, brake fluid, asphalt dewaxing, crude paraffin, microcrystalline wax, tar, waste oil recovery, etc.

Food industry:

Refined fish oil, cod liver oil, fatty acids and their derivatives, dimer acid, nutrient phenol, monoglyceride, fatty acid ester, tallow and lard cholesterol, wheat germ oil, lactic acid, diglyceride, chili oleoresin, vegetable wax Wait.

Pharmaceutical industry:

Acid chloride, amino acid ester, glucose derivative, solanesol, terpene, natural and synthetic vitamins.

Cosmetics industry:

Lanolin, lanolin, alkyl polyglycosides, rose oil, ginger oil, capsanthin and the like.

Plastics and coatings industry:

Isocyanate, epoxy resin, acrylate, plasticizer, and the like.

Agriculture: permethrin, piperonyl ether, omethoate, herbicides, insecticides, etc.

The basic principle of molecular distillation technology (1) Molecular motion uniform free path:

Any molecule is constantly changing its free path during exercise. The uniformity of the free path over a time interval is a uniform free path.

Let Vm = uniform velocity of a certain molecule f = collision frequency λm = uniform free path then λm = Vm / f ∴ f = Vm / λm

Πd睵 is known by thermodynamics principle, f=(2)絍m•────

KT

Where: d-molecular effective diameter P-molecule in the space where the pressure T-molecule is in the environment K-Boltzmann constant KT

Then: λm=────•────

(2) Splashing small cockroaches (2) Distribution law of uniform free path of molecular motion:

The distribution law of molecular motion free path is normal distribution, and its probability formula is: F=1-e-λ/λm

Among them: probability of F-degree of freedom ≤ λm λm-uniform free path of molecular motion λ-molecular motion free path can be derived from the formula, for a group of moving molecules in the same state, the free path is greater than or equal to the mean free path λm The probability is:

1-F=e-λ/λm=e-1=36.8%

(3) The basic principle of molecular distillation:

It can be seen from the formula of the unitary free path of the molecule that different types of molecules have different uniform free paths due to different molecular effective diameters. In other words, flight intervals in which different kinds of molecules do not collide with other molecules after overflowing the liquid surface It is different.

Molecular distillation technology is realized by using different kinds of molecules to overflow the liquid surface and then the uniform free path is different. The uniform free path of the light molecule is large, and the uniform free path of the heavy molecule is small. If the liquid surface is smaller than the uniform free path of the light molecule and larger than the uniform free path of the heavy molecule, a condensation surface is set, so that the light molecule falls on the condensation surface. Condensation, and the heavy molecules return to the original liquid level because they do not reach the condensation surface, so the mixture separates.

(3) Relevant models in molecular distillation technology:

For many materials, there is no mathematical model for practical application to correctly describe the variable parameters in molecular distillation. The actual application is still based on experience. However, the evaporation conditions obtained from experience in various specifications of the evaporator can be safely extended to the design of the production unit. Related models are:

1. Membrane formation For the film thickness on the vertical wall with falling film and no mechanical motion, the Nasselt formula is:

Σm=(3v2Re/g)1/3

Where: σm - nominal film thickness [m]

V-material dynamic viscosity [m 2 • sec -1]

G-gravity acceleration [m•second-2]

The Re-Reynolds number, when the dimensionless Re>400, the equation is established.

Here: Re=Ï…/v

Υ-surface load [m3•sec-1•m-1]

For the mechanical scraping film, the above formula is not applicable, and it is generally determined by experiments, and the film thickness is approximately between 0.05 and 0.5 mm.

However, it can be seen from the above formula that the influence parameters of the film thickness in the mechanical wiper film mainly include the surface load, the viscosity of the material, and the force acting on the film by the blade member.

2. Thermal decomposition Hickman and Embree give the following formula for the probability of decomposition:

Z=p•t

Where: Z-decomposition probability p-working pressure (proportional to working temperature T)

T-residence time [seconds]

The residence time depends on the length of the heating surface, the viscosity of the material, the surface load and the flow rate of the material. The thermal damage of the material can be seen by the probability of decomposition.

The table below compares the thermal damage of the same material in different distillation processes. It can be seen that the decomposition rate and residence time of the material in molecular distillation are several orders of magnitude lower than other types of distillers. Therefore, molecular distillation can ensure that the material is less damaged, thus ensuring the quality of the material.

System type dwell time [sec] working pressure [mTorr] decomposition probability [Z=p•t] stability index [Z1=lgz]

Batch distillation column 4000760×1033×1099.48

Batch distillation 300020×1036×1077.78

Rotary evaporator 30002×1036×1066.78

Vacuum circulation evaporator 10020×1032×1066.30

Thin film evaporator 252×1035×1044.70

Molecular evaporator 101101.00

3. Evaporation speed With regard to the calculation of evaporation rate, the existing mathematical formula is applicable to the theoretical model study of molecular distillation, and the actual application should be determined experimentally.

The generalized LangMuir-Knudsen equation is:

G=k•p•(M/T) Jianjie Street School G-evaporation speed [Kg/m2•h]

M-molecular weight p-vapor pressure [mbar]

T-distillation temperature [k]

Application of k-constant molecular distillation technology (I) Application range Molecular distillation can be widely applied to all aspects of the national economy, especially for the separation of high boiling point and heat sensitive and easy oxide materials. At present, hundreds of kinds of products can be produced by using molecular distillation technology. In the future, with the rise of modern people's advocating nature and returning to nature, the products produced by molecular distillation technology will have a broader and longer-term vision. The application areas are as follows:

Petrochemicals:

Separation of hydrocarbons, separation of crude oil residues and the like, purification of surfactants, and purification of chemical intermediates, such as purification of higher alcohols and alkylpolyhydrazines, vinylpyrrolidone, etc. Preparation of lanolin esters, etc.

Plastic industry:

Purification of a plasticizer, deodorization of a polymer substance, purification of a resin substance, and the like.

Food industry:

Separating mixed fats and oils, obtaining monoglycerides with a purity of more than 90%, such as monoglyceride stearate, monoglyceride laurate, propylene glycol ester, etc.; extracting fatty acids and derivatives thereof, producing dimerized fatty acids, etc.; Natural products such as fish oil, rice bran oil, wheat germ oil and the like are extracted.

Pharmaceutical industry:

Extraction of synthetic and natural vitamins A and E; preparation of amino acids and glucose derivatives.

Spice industry:

It handles natural essential oils, deodorizes, decolorizes, and improves purity, making the taste of natural spices greatly improved. Such as cinnamon oil, rose oil, fragrant root oil, citronella oil, Litsea cubeba oil and so on.

(II) Application examples Some typical molecular distillation production processes are listed below:

1. Separation of low-boiling and heat-sensitive materials • Such as perfume substances, such substances have strong volatility and high heat sensitivity. The common process requirements are deodorization, decolorization and purification. Generally, three-stage molecular distillation and *-stage degassing can be used. The second stage is deodorized or purified, and the third stage is decolorized or purified. Such as:

┎→Product 1

Rose oil→* grade (degassing)→second grade→┃┎→product 2

┖→Steaming residue→Level 3→┃

┖→ slag through the above treatment, can solve the problem of poor fragrance, deep color and high wax content, so that the added value of the product is greatly improved.

• 2. Separation of high-boiling, heat-sensitive materials • The key to the separation of such substances is the control of temperature and heating time. If the temperature and heating time are not well controlled, it will not only affect the separation effect, but also cause the material to dissipate. Such as:

┎→Pay product fish oil ethyl ester→* grade (dislocation)→second grade (purification)→┃┎→main product┖→steam residue→third grade (decolorization)→┃

┖→ slag Through the above treatment, the obtained product not only has excellent odor, color, but also an improved content of active ingredients, and protects the natural structure.

• 3, the main product is non-volatile, distillate is a small amount of low-boiling components • such substances such as capsicum red desolvent, fish oil triglyceride deacidification. This can be done using two-stage molecular distillation. Such as: capsanthin desolvation ┎ → solvent capsanthin → * grade (desolvation) → ┃┎ → solvent ┖ → steaming residue → second level (desolvent) → ┃

┖→Qualified product Capsanthin contains 1%-2% solvent. After molecular distillation, the solvent residue in the product is <20ppm, which meets the quality requirements.

• 4. Separation of substances that are solid or highly viscous at room temperature • Such substances as monoglyceride stearate, propylene glycol ester, etc. Taking the production of monoglyceride stearate as an example, the raw material usually contains about 40% monoglyceride after esterification, and the other components are diglyceride, triglyceride, glycerin and stearic acid, if not When the ester is refined, the emulsification effect is not good and the value is not high, and after purification by tertiary molecular distillation, monoglyceride having a purity of 90% or more can be obtained, thereby improving the use effect and added value.

┎→glycerin (return to utilization)┎→glycerol+free fatty acid raw material→* grade→┃┃(return to utilization)┎→monoglyceride (degassing)┖→steam residue→second level→┃┃

┖→ steaming residue → third level → ┃ (return to use)

┖→Di- and tri-ester •5. Separation of the product from its catalyst • This type of material is characterized by easy decomposition of the product and the catalyst, and Zui is able to recycle expensive catalysts.

In the process of carbonylation of an olefin to a higher fatty acid alcohol catalyzed by a catalyst cobalt phosphorus compound, the catalyst and the product alcohol are separated. This can be done by secondary molecular distillation.

┎→solvent mixture→* level (degassing)→second level→┃

┖→ steaming residue (catalyst, return utilization)

Through the above treatment, not only high-quality product alcohol is obtained, but also decomposition of the catalyst is greatly reduced.

The performance specifications of the molecular distillation unit are summarized. The molecular distillation unit mainly has the following characteristics:

1. It adopts a fabric structure that can adapt to different viscosity materials, so that the liquid distribution is uniform, effectively avoiding back mixing, and obviously improving product quality.

2. The centrifugal force strengthening film forming device is designed in an original way, which effectively reduces the thickness of the concentrated film and reduces the mass transfer resistance of the liquid film, thereby greatly improving the separation efficiency and the production capacity, and saving energy consumption.

3. Successfully solved the problem of liquid splashing, saved the traditional liquid baffle, reduced the stroke of molecular motion, and improved the separation efficiency of the device.

4. Designed a unique and novel dynamic and static sealing structure, which solves the compensation problem of sealing deformation under high temperature and high vacuum, and ensures the long-term stable operation performance under high vacuum.

5. Developed a heating method that can adapt to the temperature requirements of many different materials, and improved the adjustment performance and adaptability of the equipment.

6. Completely solve the problem of vacuum leakage between the inter-stage material conveying and input and output under the operation of the device, and ensure the continuous operation of the device.

7. Optimized the vacuum acquisition method, improved the handling flexibility of the equipment, and avoided the interference of the normal handling performance of the equipment due to pressure fluctuations.

8. The equipment runs reliably and the product quality is stable.

9, adapt to a variety of industrial fields, can be produced in a variety of products, especially for the separation of high boiling point, heat sensitive and easy oxide materials have the advantages of traditional distillation methods.

At present, the complete set of equipment has been serialized, and its main specifications and performance indicators are:

Project indicator unit processing capacity 1-1000L/h

Operating temperature ≤300 °C

No-load vacuum ≤0.7-1Pa

Equipment level ≤ 1-6 At present, a series of devices from laboratory to industrial production scale have been developed. The completed specifications are: the processing capacity (in L/h) is:

1 5 10 15 25 50 75 100 200 300 400 500 1000, etc., fully meet the needs of existing industrial applications of molecular distillation technology.

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