Home > Blog about supercritical fluid > Wiki Library > Supercritical fluid extraction > What is supercritical fluid extraction (SFE)

What is supercritical fluid extraction (SFE)

Overview

Among several supercritical fluid (abbreviated SCF) technologies, the earliest developed, most researched and industrialized product technology is undoubtedly the supercritical fluid extraction (abbreviated SFE) technology.

Principle

Supercritical extraction uses a supercritical fluid as the separation medium (extractant), and uses the fluid to have a highly enhanced dissolving ability in a supercritical state to achieve effective extraction of certain components (solutes) in the raw material, and then pass the temperature and The continuous adjustment of the pressure can reduce the density of the extractant, that is, reduce its solubility to the solute, so as to achieve high-efficiency separation of specific components in the raw material.

Advantage

Because supercritical fluid has excellent mass transfer characteristics of gas and solvation ability equivalent to liquid solvents, supercritical extraction using it as a separation medium is considered to integrate the two unit operations of distillation and liquid-liquid extraction to a certain extent. The advantages of this form a unique separation technology.

The basis of the theory is the phase equilibrium relationship of the fluid mixture in the supercritical state, which operates in the mass transfer process.

Extraction process

The most commonly used supercritical fluid in the supercritical extraction process is SC-CO2, and the products obtained by supercritical CO2 are mostly a mixture of volatile oils, greases, alcohols, ethers, esters, resins and other lipophilic chemical components;

Cosolvent

For substances with greater polarity (such as flavonoids, saponins, alkaloids, etc.), a small amount of polar entrainer (or co-solvent, modifier, co-solvent) is often added to increase the CO2 polarity. The ability to dissolve sexual substances.

The addition of a small amount of co-solvents (such as water, methanol, ethanol, acetone, ethyl acetate, etc.) can not only increase the density of the supercritical fluid, but more importantly, it can form a new strong intermolecular force with some solute molecules , Thereby improving the selectivity of the process.

Expanding technology

In addition, in order to improve the extraction capacity and selectivity of CO2 for target solutes, researchers have also developed various ways to expand supercritical extraction technology, such as the use of physical fields (ultrasound, high-voltage pulsed electric field, etc.) The combination of SFE technology and other separation and purification technologies (such as rectification, molecular distillation, adsorption, membrane separation, crystallization, etc.), supercritical carbon dioxide microemulsion system for the extraction of hydrophilic compounds with high polarity and high molecular weight, and The supercritical carbon dioxide extraction of complexing agents, etc., the above-mentioned multiple approaches have expanded the application range of SFE technology.

Application

The application of SFE technology mainly includes: extraction of high value-added useful components (natural pigments, flavors and fragrances, edible or medicinal ingredients, etc.) or removal of harmful components from natural products; extraction and deashing of coal liquefied oil; hydrocarbons Selective extraction of linear alkanes or aromatic hydrocarbons; regeneration of catalysts (such as activated carbon); separation of monomers or residual solvents from polymers, separation of azeotropes; seawater desalination; separation of isomers; environmental pollution control ( Such as extraction of metal ions or organic matter from wastewater solutions, removal of soil pollutants, nuclear waste treatment
and many more.

In recent years, in the process of modernization of traditional Chinese medicine in China, supercritical extraction technology has received more attention and has been listed as the key technology of traditional Chinese medicine modernization.

In addition, analytical supercritical carbon dioxide extraction is used for the pretreatment of biological samples for trace drug test samples, and it has shown attractive application prospects in terms of gradually replacing some traditional methods and developing into a conventional method, showing faster and safer , Economic and environmentally harmless superiority.

For example, the study on the detection and removal of pesticide residues in ginseng with SC-CO2 extraction, the results show that the SFE method has high extraction rate, less solvents and fewer separation steps, which is significantly better than traditional methods.

In addition, in recent years, critical water has been successfully applied in the extraction of organic pollutants in environmental samples, the extraction of active ingredients from natural products, and the pre-analysis process, which is regarded as a green and environmentally friendly and promising revolutionary technology.

Principles of Supercritical Fluid Extraction Technology

Supercritical fluid extraction is a chemical separation technology with the earliest development, the most research, and the first industrialization in supercritical fluid technology. It is used in chemical engineering, energy, fuel, medicine, food, fragrance, environmental protection, marine chemical industry, biochemical industry, Many fields such as analytical chemistry have broad application prospects and are regarded as environmentally friendly, high-efficiency and energy-saving green high-tech.

The basic principle of supercritical extraction technology

The design of the supercritical extraction process is to use the density of the supercritical fluid to be sensitive to pressure and temperature changes, and use the supercritical fluid (such as SC-CO2) as the separation medium (extractant), and the fluid has a high degree of enhancement in the supercritical state. The ability to dissolve the material can effectively extract certain components (solutes) in the raw material at a higher density, and then the density of the extractant is reduced by continuous adjustment of temperature and pressure, thereby reducing its solubility to the solute and making the solvent It is effectively separated from the solute (extract).

Supercritical extraction process

Schematic diagram of supercritical fluid extraction process
1-Pressure booster; 2,6-heat exchanger; 3-extractor; 4-pressure-reducing valve; 5-separator.
E: Extraction state S: Separation state

The figure below is a schematic diagram of the most basic supercritical extraction process. First, the solvent (such as CO2) is passed through the booster 1 (high-pressure pump or compressor) to reach the supercritical state;

Then the supercritical fluid enters the extractor 3 to contact the pre-loaded raw materials (e.g., solid raw materials crushed into a certain particle size) and extract the target solute therein under the conditions of supercritical temperature T1 and pressure p1;

The extract (solute) dissolved in the supercritical fluid is throttled and expanded by the pressure reducing valve 4 as the supercritical fluid leaves from the top of the extractor, so that the density of the supercritical fluid is significantly reduced, thereby reducing the extract in the fluid. The solubility of the extract and the solvent can be separated in the separator 5.

Then make the solvent into a gaseous or supercritical state (the fluid state depends on T2, p2) and leave the separator 5, and then pressurize it to the supercritical state (T1, p1) by the booster 1, and repeat the above-mentioned extraction-separation step, After multiple cycles of the fluid, the extraction effect of the supercritical fluid on the solute reaches the expected value.

The extract deposited in the separator 5 generally enters the collection container through a discharge valve at the bottom of the separator.

Supercritical fluid extraction system

At present, the large-scale equipment of supercritical extraction process mostly uses high-pressure pumps, and the supercritical extraction of solid materials mostly adopts batch operation; in order to realize a semi-continuous process, several extractors can be operated in parallel, such as three extractors in parallel, when one operation During operation, the other two can be loaded and unloaded separately, and for liquid materials, a continuous co-current (or counter-current) extraction process can be used.

The material system involved in the supercritical extraction process includes the raw material mixture), the benzene extract (the solute) and the extraction medium (single supercritical fluid or mixed supercritical fluid), which constitute a multi-element system.

Solute extracted by supercritical fluid

The vapor pressure, polarity and molecular weight of the solute are important factors that affect the solubility of the solute in the supercritical fluid.

Therefore, the solubility of the solute in the supercritical fluid is not only related to the density of the fluid, but also directly related to the affinity and volatility between the supercritical fluid molecules and the solute molecules, and the difference in volatility between the separated substances during the extraction process These two factors work together with the difference in their intermolecular affinity; for example, in the supercritical extraction process, substances with low boiling points are extracted first than substances with high boiling points. Non-polar supercritical carbon dioxide only affects non-polar and weak substances. Polar substances have a high extraction capacity.

In the chemical unit operation, rectification uses the difference in the volatility of each component to achieve the purpose of separation, and liquid-liquid extraction uses the difference in solubility between the extractant and the molecule to be extracted to separate the extracted components from the mixture.

Because supercritical fluid has excellent mass transfer characteristics of gas and solvation ability equivalent to liquid solvents, supercritical extraction with it as a separation medium is considered to be a combination of distillation and liquid-liquid extraction to a certain extent. The advantages of this form a unique separation technology.

Because this separation process requires auxiliary agents (extractants), most scholars believe that SFE is closer to liquid benzene extraction and solid leaching, which is an extension and expansion of the classic extraction process. Its theoretical basis is that the fluid mixture is in a supercritical state. The phase balance relationship, and its operation belongs to the mass transfer process.

Basic types of supercritical fluid extraction process

Three separation conditions for supercritical fluid extraction

According to different separation conditions, the supercritical fluid extraction process is generally divided into three basic types: pressure reduction method, variable temperature method and constant temperature and constant pressure adsorption method.

They are further divided into more modes according to the difference of the phase state of the extracted and separated fluids. The following table shows the 7 common operating modes.

Mode FeatureModePressure pTemperature TExtraction stateSeparated state
Buck1p1>pc>p2T1>Tc>T2SCFVapor-liquid mixture
Buck2p1>pc>p2T1<Tc>T2Subcritical fluidVapor-liquid mixture
Buck3p1>pc>p2T1≥T2>TcSCFGas
Buck4p1>p2>pcT1≥T2>TcSCFSCF
Change t5p1=p2>pcT1>T2>TcSCFSCF
Change t6p1=p2>pcT<T2>TcSCFSCF
Constant
T & p
adsorption
7p1=p2>pcT1=T2>TcSCFSCF
Basic types of supercritical fluid extraction process

Depressurization separation process

Use the temperature-entropy diagram (a) and the schematic diagram of the supercritical extraction process (b) to describe the depressurization method (mode 1)

The above picture shows the process of mode 1 in the depressurization method described by the temperature-entropy diagram and the schematic diagram of the supercritical extraction process.

Separation process

Initial separation

First, the fluid (such as CO2) is compressed to the extraction pressure (1→2) by the high-pressure pump, and then reaches the required supercritical temperature and supercritical state (3) through the heat exchanger (W1), and then enters the extractor.

The solid or liquid raw materials in the supercritical fluid in the extractor are contacted under the conditions of p1 and T1 to make the dissolved solute enter the SCF (3→3°), and the SCF that has dissolved the solute leaves the extractor and is throttled and expanded by the pressure reducing valve. (Isenthalpic process), so that the fluid state changes from 3° to the state in the gas-liquid two-phase coexistence zone (4) and enters the separation tank.

At this time, part of the solvent that becomes gaseous will inevitably separate from the solute because the density is much lower than the SCF state, and leave from the separation tank outlet (5); while part of the solvent that becomes liquid will be in the supercritical state (3→3°) The extracted solutes remain at the bottom of the separation tank together.

In order to control the stability of the liquid solvent level in the separation tank, the separation tank is equipped with a heating jacket. The gas solvent needs to pass through the heat exchanger (W2) before entering the high-pressure pump P to condense (5→1) to the initial liquid state (1), which can avoid cavitation in the pressure pump.

Cycle

Then the liquid solvent (1) enters the high-pressure pump again and pressurizes to the extraction pressure (1→2), and re-contacts the solid or liquid mixture to be separated, and repeats the above extraction-separation step (1→2→ 3→3°→4→-5→1) to reach the predetermined extraction rate.

Compressor instead of high-pressure pump

If the process of mode 1 uses a compressor instead of a high-pressure pump, the gas solvent does not need to be condensed in the heat exchanger (W2) before entering the compressor, and the role of the heat exchanger (W1) becomes cooling.

CO2 high pressure pump for supercritical fluid extraction

It should be (1’→2’→3→3°→4→-5→1′) on the temperature-entropy graph.

Intercooling system for supercritical fluid extraction

The advantage of using a compressor is that the separated and recovered extractant can be recycled without being condensed into a liquid, but an intermediate cooling system must be configured to reduce the large temperature rise produced by the compression process. In order to save energy, heat exchangers are generally combined together appropriately.

Analysis of four modes of depressurization separation method

The difference between mode 2 in the depressurization separation method and the above mode 1 is only that its extraction condition is a subcritical high-pressure liquid, so it can only be transported by a high-pressure pump. The difference between mode 3 and mode 1 is its separation conditions SCF becomes a gas; the difference between Mode 4 and Mode 1 is that its separation condition is that SCF is still in a supercritical state, but its separation pressure or separation temperature is lower than the extraction pressure or temperature.

Summary

From the perspective of energy conservation, mode 1 will consume a lot of heat energy due to the evaporation of liquid solvent in the separation tank, so it is generally not necessary to set the separation pressure too low. However, mode 1 is easier to stabilize the operation by controlling the separation conditions by liquid level, so mode 1 is more often selected.

Isobaric variable temperature separation process

Mode 5 and Mode 6 belong to equal pressure variable temperature operation.

The choice of the separation temperature depends on the relationship between the solubility of the solute in the supercritical fluid and the temperature.

The effect of temperature on solute solubility

The influence of temperature on the solubility of a solute is restricted by two competing factors: increasing the temperature will increase the vapor pressure of the liquid solute or increase the sublimation pressure of the solid solute, thereby increasing the solubility of the solute in the supercritical fluid; but increasing the temperature will also increase the solubility of the solute in the supercritical fluid. The density of the supercritical fluid decreases and therefore decreases
The solubility of the solute in the supercritical fluid.

Solubility of naphthalene in SC-CO2

Combine the solubility of naphthalene in SC-CO2 and the temperature-entropy diagram to describe the variable temperature separation method (mode 5 and mode 6)
From 1 to 2 is mode 5; from 3 to 4 is mode 6

The right side b of the above figure shows the solubility of naphthalene in SC-CO2 at different temperatures. When the pressure is above 15MPa, the density of CO2 is not sensitive to temperature, and the vapor pressure of the solute plays a leading role in the solubility of the solute in the supercritical fluid.

Therefore, the solubility of naphthalene in supercritical CO2 increases almost linearly with the increase of temperature.

When the pressure is below 12 MPa, especially near the critical pressure (7-8 MPa), a small change in temperature leads to a large decrease in density. At this time, the density plays a leading role in the solubility of the solute in the supercritical fluid, and the solubility of the solute is increased. The so-called “reverse condensation zone” where the temperature increases and decreases.

The difference between the two modes

Therefore, the two modes of isobaric variable temperature operation are mainly based on the difference of the extraction pressure, that is, the extraction and separation pressures are both 30MPa, and the extraction temperature of 55°C and the separation temperature of 32°C (from 1 to 2) Supercritical extraction and separation of naphthalene (mode 5); it can also be extracted at a pressure of 8 MPa and a temperature of 32 °C, and then the pressure is increased to 42 °C (from 3 to 4) for separation (mode 6) ), as shown in the figure.

From the perspective of energy utilization, the variable temperature method is superior to the depressurization method. However, heating up will sometimes cause degradation and loss of heat-sensitive extraction components.

In addition, if the extract is a solid extraction component, the extract will be directly deposited on the wall of the heat exchanger due to the temperature change, making the collection of the extract in the separator more difficult than the pressure reduction method.

Constant temperature and constant pressure adsorption separation process

Mode 7 belongs to isothermal and isostatic operation, so the fluid does not need to repeatedly increase and decrease pressure or change temperature operation, from
To make this mode easy to operate is also the most energy-saving operation in theory.

However, the separation of this mode depends on the adsorption performance and selectivity of the adsorbent under high pressure, and the experimental data and theoretical research on the thermodynamics and kinetics of high pressure adsorption are relatively in-depth.

This method is mainly used in the removal process of a small amount of impurities in the product.If the adsorbed extract is the target product, the desorption process of the extract must be studied.

The above three types of modes have their own advantages and disadvantages. Usually, supercritical extraction adopts the depressurization method. In practical applications, the above 7 modes are often used individually or in combination according to the needs of the specific material system.

Features of supercritical extraction technology

The main characteristics of supercritical extraction technology can be summarized as follows

High efficiency and energy saving separation

Supercritical extraction is a high-efficiency and energy-saving separation technology. Because supercritical fluid has a lower viscosity and larger diffusion coefficient than pure liquids, and the surface tension is zero, supercritical fluids are easier to diffuse and penetrate into the porous matrix than organic solvents to achieve efficient extraction of solutes.

The process in which the solute in the extractor is dissolved in the supercritical fluid is a spontaneous process; the throttling expansion of the pressure reducing valve is an isenthalpic process; the main energy-consuming equipment in the process is the pressurization device (compressor or high-pressure pump) and The heat exchanger necessary for the temperature when the fluid is extracted and separated.

Therefore, supercritical extraction is considered to be an advanced process with fast extraction speed, high efficiency and low energy consumption.

Integration of extraction and separation processes

Supercritical extraction has the characteristics of both rectification and liquid-liquid extraction, and realizes the integration of extraction and separation processes.

The supercritical extraction process is mainly composed of two parts: an extractor and a separator. It is simpler than the traditional process. Unlike the conventional extraction method, the subsequent complex and energy-intensive sample concentration or separation process is usually carried out after extraction, resulting in solvent damage. Increase in usage and energy consumption.

Since the supercritical fluid becomes liquid or the gas has a phase interface, the latent heat of vaporization of the fluid near the critical point is very small, and the volatility difference between the general solvent and the solute is very large.Therefore, the separation of the solute and the solvent in the separator is very convenient.

Easy adjustment of operating parameters

The operating parameters of supercritical extraction are easy to adjust. Since the benzene extraction capacity of the supercritical fluid is related to the density of the fluid, the density of the supercritical fluid can be easily controlled by continuously adjusting the temperature and pressure, so the fractional extraction and fractionation of multiple components in the extract can be realized.

Fluid circulation

Supercritical fluid can be recycled in the extraction process, which can reduce costs and reduce pollution emissions.
Solvent separation and recovery are superior to general liquid-liquid extraction and distillation technologies

No solvent residue

It is especially suitable for separating heat-sensitive substances, and can achieve no solvent residue in the product.

For example, the operating temperature of supercritical CO2 extraction is close to room temperature, and it is non-toxic, odorless, non-flammable, easy to refine, and has anti-oxidation and sterilization effects, so it can avoid the decomposition of heat-sensitive substances caused by high-temperature operation, and ensure the high quality and stability of the product. sex.

High value-added products

Supercritical extraction is a high-pressure technology, resulting in high technical requirements, relatively expensive equipment processing, and higher product quality than traditional methods. Therefore, supercritical extraction is mainly suitable for the extraction and separation of high value-added products.

Supercritical carbon dioxide extraction

Solubility of solute in supercritical carbon dioxide

Studying the solubility of solutes in supercritical CO2 has theoretical and practical application significance. The CO2 solubility rules summarized by the research results of Francisi, Stahl, Hyatt and other scholars,
It is helpful for people to initially understand the solubility and selectivity of various solutes in supercritical CO2. The solubility rules are as follows

Rule 1

The solubility value of solute in subcritical CO2 and supercritical CO2 generally differs by about an order of magnitude, but it has never been found that any kind of substance does not dissolve in subcritical CO2 but dissolves in a supercritical state, indicating that the substance is in subcritical CO2. The solubility behavior of CO2 and supercritical CO2 is continuous

Rule 2

CO2 has a strong homogenization effect. Studies have shown that at least 140 compounds can form a homogeneous miscible state with CO2 at moderate pressure and room temperature, that is, liquid CO2 and supercritical CO2 can be miscible with many non-polar and weakly polar solutes, such as carbon Normal alkanes with less than 12 atoms, normal alkenes with less than 10 carbon atoms, lower alcohols with less than 6 carbon atoms in the main chain, and lower fatty acids with less than 10 carbon atoms in the main chain. Ester compounds that are equal to 12 and the number of carbon atoms in the alcohol backbone is less than or equal to 4, low-carbon aldehydes with carbon atoms less than 7, low-carbon ketones with carbon atoms less than 8, low-carbon ethers with carbon atoms less than 4, etc.

Rule 3

Although liquid CO2 and supercritical CO2 under medium pressure have excellent solubility for the aforementioned aliphatic hydrocarbons and low-polar lipophilic compounds, as the number of carbon atoms increases, that is, with the increase of the chain length and molecular weight With the increase of, its solubility in liquid CO2 and supercritical CO2 will change from miscible state to partial solubility, and the solubility will gradually decrease

Rule 4

Strong polar functional groups (such as the introduction of O11COD will reduce the solubility of compounds, so polyols, polyacids, and multiple hydroxyl and carboxy And the solubility in supercritical CO2 is extremely low.

Rule 5

Liquid CO2 and supercritical CO2 are suitable for most mineral inorganic salts and highly polar substances (such as sugars, amino acids, starches, proteins, etc.) are almost insoluble, so they will not be extracted during sub-supercritical and supercritical CO2 extraction processes

Rule 6

Liquid CO2 and supercritical CO2 are almost insoluble to compounds with a molecular weight of more than 500

Summary

The above rules provide a reference for the preliminary qualitative judgment of whether a substance can use supercritical CO2 as the extraction solvent, and also provide a basis for the range of substances that can be selected as a co-reagent when a co-reagent (cden) needs to be added in the CO2 extraction process.

To be continued