Supercritical CO fluid extraction technology is mostly used for the extraction of solid raw materials, but a large number of research practices have proved that supercritical CO2 fluid fractionation technology has more advantages in the extraction and separation of liquid materials.
- The main reason is that the extraction process can be continuously operated, because the extraction raw materials and products are liquid, there is no problem of solid material feeding and slag discharge, and the extraction process can be continuously operated, which greatly increases the processing capacity of the device and reduces the process energy consumption accordingly And gas consumption to reduce production costs;
- The second is to realize the integration of extraction process and rectification process, which can continuously obtain high purity and high value-added products.
Liquid phase mixture extraction and separation can basically be processed by continuous countercurrent supercritical extraction device.
The technical feature is the coupling of CO2 extraction and distillation and rectification, which can effectively play the separation role of the two and improve the purity of the product.
Composition of fractionation system:
The system of supercritical CO2 fluid fractionation of liquid materials is roughly the same in composition, but for continuous feeding, there are differences in the flow direction, operating parameters, internal structure, etc. of solvents and solutes.
Classified by the flow direction of solvent and soluteAccording to the different flow directions of solvent and solute
The supercritical CO2 fluid fractionation process of liquid materials can be divided into countercurrent extraction, cocurrent extraction and mixed flow extraction.
Generally, the solvent is fed from the bottom of the column extraction in. Then, countercurrent extraction refers to liquid material entering from the top of the fractionation kettle, cocurrent extraction means entering from the bottom, and mixed flow extraction means entering from the middle
According to different classification of operating parameters
Since temperature has a greater influence on the solubility of solutes in supercritical CO fluids, in this case temperature gradient can be set in the axial direction of the column fractionation kettle.
So according to the different operating parameters, it can be divided into isothermal column and non-isothermal column operation.However, in order to achieve accurate separation, in many cases, a rectification column is installed at the back of the extraction kettle, and the rectification column is also provided with an axial temperature gradient, but the rectification column is a column extraction relative to the rear separator.
According to the different classification of the internal structure of the column fractionation column
In order to make the liquid materials fully contact with the solvent, it is generally necessary to fill the column extraction kettle with packing, which is called packing column.
Sometimes the packing is not installed, but the tray (tray) is used to form the tray (tray) column. In the existing supercritical CO2 fluid extraction process of liquid materials, most of them use packed columns. The type of packing in the packing column is an important factor affecting the separation effect
The fractionator is equipped with a set of perforated trays, and the heavier liquid mixture to be extracted is supplied from the top of the tower, and extraction is performed in a reverse flow state. The porous disk relies on thin-walled tubular spacer elements to keep its position unchanged. The outer diameter of this spacer element is slightly smaller than the inner diameter of the tube constituting the extraction column. All trays are the same, and are installed alternately by 180 rotations. The capacitive sensor on the top of the tower is used to control the position of the liquid level.
During extraction, the liquid material flows continuously down the downcomer, and the dispersed phase flows from top to bottom through the tray holes.
Under this condition, the dispersed phase divides the liquid material into bubbles and forms a continuous phase again in the area under the adjacent tray.
In order to maintain the bubbling effect of the supercritical COb2 fluid, the height of the liquid column under the hole should be kept at a certain value. Supercritical CO2 fluid undergoes multiple liquid-material contacts, which greatly enhances the mass transfer process.
Liquid material fractionation column
The height of the general column extraction kettle is between 3~7m. The extraction column is often composed of multiple sections, which can be divided into 4 sections according to their role
In the separation section, the mass transfer between the material and the supercritical CO2 fluid. Jacket for the outside of the separation sectionInsulate or form a temperature gradient along the column height for selective separation of certain components
Figure 2.26 Connection mode of connecting section and separation section
It is used to connect two separation sections and set supporting packing in them. In general, the length of the connecting section is about 0.25m. Each connecting section has multiple openings, which are used for feeding, temperature measurement and sampling, etc., and there are two ways to connect with the separation section, see Figure 2.26. Through the effective combination of the connection section and the separation section, as well as the change of the feed position, the separation requirements of different systems of extraction can be met.
Its design should consider the separation of extractant and solute, it is best to have an enlarged section, and use a jacket to keep warm.
For the collection of raffinate, jacket insulation can be used, and its design should be convenient for the release and cleaning of certain viscous materials