Supercritical CO2 extraction technology is a new type of separation technology in the modern chemical industry. Supercritical CO2 extraction uses CO2 gas as a solvent. The fluid density and dielectric constant of CO2 in a supercritical state are large, and the solubility of substances is very large and changes sharply with changes in pressure and temperature. Therefore, not only for some substances The solubility is selective, and the solvent and extract are very easy to separate. Supercritical CO2 extraction equipment is especially suitable for the extraction of fat-soluble, high boiling point, and heat-sensitive substances. It is widely used in the fields of biology, pharmaceuticals, and food.
In this paper, taking the CO2 extraction vessel of the 500L supercritical CO2 extraction system as an example, the finite element analysis of the clamp-type quick-opening structure of the flat cover, clamp, and cylinder end is carried out using AN-SYS software.
CO2 Extraction Vessel Structure and Parameters
The supercritical CO2 extraction vessel adopts a clamp-type quick-opening structure, operates 4 times a day at full pressure cycle, and has a design service life of 15 years, which is a fatigue vessel. The cylinder body of the inner container is made of 16Mn forging, the flat bottom head of the inner container is made of 16Mn forging, the upper flat cover, the end of the cylinder, and the clamp form a quick-open connection structure, and the inner wall is double-layer surfacing welding E309L + E308L, with a simple skirt support. Part of the material of the jacket is Q345R, and the medium is hot water, which traces the heat of the inner vessel.
Main design parameters
- Container category: Class III
- Chinese Design Code: JB 4732-1995 “Steel Pressure Vessels — Analysis and Design Standards”
- Extraction kettle structure: upper flat cover, cylinder end, clamp quick-open connection, forged and welded cylinder, skirt support structure with jacket
- Design fatigue times: 21 900 times
- Material of upper flat cover and cylinder end: 16MnIV forging, inner wall surfacing welding 30408
- Clamp material: 16MnIV forged
- Nominal volume: 500L
- Cylinder inner diameter: 550mm
- Pressure range: 0 ~ 32MPa
- Frequency of pressure changes: 4 times a day
- Design pressure: 35MPa
- Design temperature: 90℃
Main component materials and mechanical properties of the CO2 extraction vessel
| Part | Metal | Design stress intensity/MPa | Yield Strength/MPa | Poisson’s ratio |
| Cylinder( T≤100) | 16Mn Ⅳforging | 164 /178 | 305 /279 | 0．3 |
| Upper flat cover( T＞200 ~ 300) | 16MnⅣforging | 164 /167 | 275 /253 | 0．3 |
| Clamp, barrel end( T＞100 ~ 200) | 16MnⅣforging | 164 /174 | 295 /269 | 0．3 |
CO2 extraction vessel size
According to the design conditions and actual engineering experience, the size of the CO2 extraction vessel is as follows:
- O-shaped rubber ring self-sealing ring outer diameter DG = DC = 574mm, flat cover, clamp, cylinder end clamping ring inner diameter D1 = D3 = 826mm, flat cover, cylinder end outer diameter D0 = D2 = 880mm, the thickness of the middle part of the flat cover δp = 215mm, the thickness of the clamping ring of the flat cover h = h1 = 80mm, the root transition arc radius r1 = 14mm;
- Clamp outer diameter D4 =1100mm, clamp ring characteristic length l1 = 95mm, root transition arc radius r2 = 16mm;
- The characteristic length of the clamping ring at the end of the cylinder is l2 =88mm, and the radius of the transition arc at the root is r3 =14mm.
Fully self-tightening sealing structure, the clamping ring has no inclination angle and no preload.
- The bending stress σm of the longitudinal section of the flat cover: 116． 9 MPa;
- The equivalent stress of the ring section a-a at the root of the clamping ring of the flat cover: 116． 9 MPa;
Stress Analysis of CO2 Extraction Vessel
Finite element analysis of flat cover, clamp, and CO2 extraction cylinder end
The flat cover and cylinder end of the CO2 extraction vessel is subjected to the internal pressure of the supercritical CO2 fluid extraction system, and the flat cover of the CO2 extraction vessel tends to move axially. The force is transmitted to the clamp through the contact surface between the flat cover and the clamp. After the clamp is stressed, it tends to move axially, and the contact surface between the end of the cylinder and the clamp provides the restraint reaction force of the clamp so that the clamp is subjected to axial tension and bending force from the axial contact surface with the flat cover and the end of the cylinder. moment.
The flat cover and clamp of the CO2 extraction container have a tendency to move, the key consideration is the contact pressure between the clamp and the flat cover and the end of the cylinder, and the final contact setting is as follows:
- The flat cover of the CO2 extraction container and the axial contact surface of the clamp is provided with frictional contact, and the friction coefficient is set to 0.1;
- There is a frictional contact between the end of the cylinder and the axial contact surface of the clamp, and the friction coefficient is set to 0.1;
- The flat cover and the end of the cylinder are provided with frictionless unilateral contact;
- The two radial contact surfaces of the flat cover and the clamp are provided with friction-free unilateral contact;
- The end of the cylinder of the CO2 extraction container and the two radial contact surfaces of the clamp are provided with frictionless unilateral contact.
The above contacts are all contact types with separable contact surfaces, which simulate the real force of the components to the greatest extent. The contact surface setup is shown in Figure 3.
Analysis Results
According to the analysis of the stress cloud diagram of the third intensity theory, there is compressive stress on the axial contact surface of the cylinder body, flat cover and clamp of the CO2 extraction vessel, bending stress exists on the flat cover and clamp, and a large stress concentration occurs locally, but it is limited to the parts The surface layer only affects the fatigue life and has no effect on the structural strength, and the cylinder of the CO2 extraction vessel has local film stress and bending stress.
Final Words
The non-standard structure of the quick-opening clamp of the supercritical CO2 extraction vessel was designed and studied, the three-dimensional model was established according to the design conditions, and the static analysis of the assembly structure under the design conditions was carried out by using the finite element analysis ANSYS 15.0 software Workbench platform. analyze.
Based on the stress analysis results, analyze and check each dangerous cross-section where each stress peak occurs, determine the size of the designed clamp quick-opening structure, and double seal through the O-shaped rubber seal ring and the spring energy storage seal ring, The structure has a self-tightening effect, reliable sealing, and the extraction kettle is easy to open and close, and the structure is optimized during the design process, which makes the equipment simple in structure and compact in size, and reduces the difficulty of manufacturing and installation. The one-time hydraulic pressure test was qualified during the manufacturing process of the 1500L supercritical CO2 extraction equipment, and the quick-opening structure was tested and verified.