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 Feature||Mode||Pressure p||Temperature T||Extraction state||Separated state|
|Buck||2||p1>pc>p₂||T1<Tc>T₂||Subcritical fluid||Vapor-liquid mixture|
T & p
Basic types of supercritical fluid extraction process
Depressurization separation process
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.
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.
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.
It should be (1’→2’→3→3°→4→-5→1′) on the temperature-entropy graph.
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.
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
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.
Supercritical fluid extraction (SFE) is the process of separating one component (the extractant) from another (the matrix) using supercritical fluids as the extracting solvent.
Supercritical CO2 extraction (SCFE) is used particularly in the food, beverage, cosmetics and pharmaceutical industry for extracting natural substances, aromas, fats, oils, waxes, polymers, enzymes and colourants in their supercritical physical state.
CO2 is a natural and environmentally-friendly solvent which has advantages over synthetic and harmful media such as n-hexane when it comes to sustainability.