Supercritical CO2 fluid extraction technology is a new separation technology in modern chemical separation. It has the characteristics of fast mass transfer rate, strong penetration ability, high extraction efficiency and low operating temperature. This technology has been widely used in medicine, food, chemical industry and other industries.
Supercritical CO2 fluid extraction device is a device used for supercritical fluid extraction. The device is mainly composed of a freezing part, a pressurizing part, an extraction part, a separation part and a recovery part. However, the composition of the device varies depending on the production process and the fluid medium.
Structural requirements of the extraction kettle in the supercritical CO₂ extraction device
The actual application of supercritical CO₂ fluid extraction in production is achieved through a safe and reliable extraction device.
Among them, the extraction kettle is an important part of the extraction device system and a key equipment in supercritical fluid extraction technology. Due to the high operating pressure of supercritical fluid extraction, its pressure resistance requirements are higher than those of equipment used in other traditional separation methods. According to the characteristics of the extraction process, the extraction kettle used for supercritical CO₂ fluid extraction must have high pressure resistance and be equipped with a quick-opening cover and sealing device.
Pressure resistance design of extraction kettle
The internal pressure vessel is divided into four pressure levels according to the design pressure, which are specifically divided as follows:、
- Low pressure (code L) vessel 0.1Mpa≤p≤1.6Mpa;
- Medium pressure (code M) vessel 1.6Mpa≤p≤10.0Mpa;
- High pressure (code H) vessel 10.0Mpa≤p≤100Mpa;
- Ultra-high pressure (code U) vessel p≥100Mpa.
The extraction kettle needs to withstand high pressure, and the steel used determines the limit of its maximum working pressure. Different forms of materials need to be selected to withstand different pressures.
Cylinder structure of extraction kettle
Common extraction kettle cylinders are divided into two categories: single-layer integral forging cylinder and combined cylinder. Combined cylinders can also be divided into multi-layer wrapping type and belt winding type.
- Single-layer integral forging cylinder For intermittent extraction kettles with a volume of less than 100L, the cylinder structure mostly adopts a single-layer integral forging cylinder structure. The advantage of this cylinder structure is that it has a simple structure and no weak deep girth welds and longitudinal welds; the disadvantage is that the forgings caused by forging have large differences in mechanical properties in different directions, which increases the possibility of low-stress brittle failure.
- Multi-layer wrapped cylinder The multi-layer wrapped cylinder is a multi-layer high-pressure container without deep girth welds. It is currently the most widely used, manufactured and used combined cylinder in the world. The multi-layer wrapped cylinder generally consists of an inner cylinder with a thickness of 12~25mm and a multi-layer plate with a thickness of 4~12mm. In order to avoid cracks from extending along the thickness direction, the longitudinal welds between the layers should also be staggered by 75°. Its advantages are simple manufacturing process, no need for large and complex processing equipment, and higher safety and reliability compared with single-layer cylinders; its disadvantages are many manufacturing processes, low efficiency, long cycle, and low material utilization.
- Wrap-around barrel The wrap-around barrel is also a multi-layer high-pressure vessel without deep circumferential welds.
Quick-opening device of the extraction kettle
The quick-opening cover device is mostly used for medium and low pressure vessels. Supercritical CO2 fluid extraction requires the quick-opening cover device to be used for high pressure or even ultra-high pressure vessels.
The intermittently operated supercritical fluid extraction kettle requires frequent material replacement operations. It takes a lot of time to open and close the container cover. The use of a quick-opening device can reduce the operation time and improve production efficiency. The quick-opening device mainly has the following forms.
- Single-bolt structure The characteristic of the structure is that the entire kettle cover is screwed into the seal as a large bolt, and the thread is a component that bears axial force. Its advantages are simple structure and convenient processing. It does not require separate processing of flanges. The thread that matches the kettle cover can be directly machined on the barrel body to reduce welds. However, as the volume of the extraction kettle increases, the labor intensity of tightening the kettle cover is very high. Therefore, this structure is only suitable for extraction kettles below 20L. Internationally, the Swiss NOVA company often uses this structure, and small and medium-sized devices produced in my country also often use this structure.
- Multi-layer spiral bayonet lock structure This structure is an improved version of the single-bolt structure. Three locks are opened on the large threads of the kettle cover and the cylinder respectively. The kettle cover can be directly placed in the threaded opening of the cylinder through the lock. The kettle cover can be locked by rotating the kettle cover 30°, avoiding the situation where the single-thread structure needs to rotate multiple turns of threads. It is often used in large-scale production equipment.
- Clamp-type structure This is a more common structure. There are flanges on the kettle cover and the cylinder. When the kettle cover is placed in the cylinder, the kettle cover and the kettle body are clamped with two semicircular clamps to achieve the purpose of pressure bearing. This structure is more convenient for loading and unloading, and the kettle cover can be mechanically opened and closed by hydraulic devices. It is suitable for large-scale extraction devices. UHDE, a world-renowned supercritical device manufacturer, adopts this structure. Industrial extraction devices manufactured in my country also adopt this structure.
- Wedge-type structure This structure is to decompose the integral load-bearing component on the kettle cover that bears the axial force into several movable wedge-shaped blocks. These wedges are inserted into the groove of the cylinder body or pulled out from the groove by hydraulic or pneumatic means to complete the opening and closing actions. The advantage of this structure is that the locking and lifting are both on the kettle cover component, and there is no need to generate force with other fixed objects, so it is easy to realize mechanization, but the structure is relatively complex and is generally suitable for large-scale devices.
Sealing structure and sealing material of extraction device
There are many forms of sealing for extraction kettles, mainly clamp-type structure sealing and tooth-engaging quick-opening sealing, while supercritical CO2 fluid extraction kettles mostly use clamp-type structures.
- Clamp-type structure sealing The inner surface of the clamp of the clamp structure has two conical surfaces, which are in contact with the conical surfaces of the top cover and the end flange of the cylinder respectively. The sealing elements are O-rings and sealing rings. The sealing rings are in contact with the top cover and the end flange of the cylinder through the lining ring. There is an O-ring between the sealing ring and the top cover and the end flange of the cylinder. During assembly, the O-ring is squeezed to produce a pre-tightened seal. When in normal working state, the O-ring can achieve self-tightening sealing under the action of internal pressure. Clamp-type seals have been widely used in intermittent operation devices. Due to the complex structure and force, there are currently few domestic application examples.
- Tooth-meshing structure seal The tooth-meshing structure is processed into evenly distributed meshing teeth in the circumferential direction of the top cover and the end flange of the container. The O-ring is installed on the flange at the bottom of the top cover so that it moves with the top cover. The initial sealing of the container is achieved by the cooperation of the O-ring, the end flange and the top cover. After the pressure rises, the O-ring can achieve self-tightening sealing. The inner diameter of the end flange is the same as the inner diameter of the container cylinder, and the inner diameter of the meshing teeth of the end flange is slightly larger than the outer diameter of the top cover to facilitate the loading and unloading of the top cover.
- Sealing materials Commonly used sealing materials are non-metallic sealing elements and metal sealing elements. When the pressure in the extraction kettle increases, the sealing specific pressure (sealing specific pressure) on the sealing contact surface will be very high. Generally, non-metallic sealing elements are often difficult to bear such a large sealing specific pressure. According to my country’s industrial practice, due to the extremely strong penetration and dissolution ability of CO₂, ordinary rubber sealing rings can only be used once, and the swelling of the rubber ring will also affect the loading and unloading of the extraction kettle. Metal sealing components can fundamentally solve this problem. They have the characteristics of completely realizing quick opening, reliable sealing, almost eliminating swelling, and can be used continuously for about one year as long as there is no accidental damage.
Conclusion
Supercritical CO2 fluid extraction technology has been widely used in the fields of chemistry, petroleum, food, medicine, health care products, etc.