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CO2 extraction vessel materials and the top two structural forms

The pressure of the supercritical CO2 fluid extraction process is generally 8~35Mpa, sometimes higher. In GB1501998 “Steel Pressure Vessels”, the design specifications are unified for pressure vessels with a design pressure of less than 35Mpa. CO2 extraction vessel is high-pressure vessels with a working pressure greater than 35Mpa and can be regarded as pressure vessels working under special conditions. Read More: Pressure Vessel – Wikipedia

Material Selection of CO2 Extraction Vessel

Read More: CO2 Extraction Vessels: 4 Basics, 6 Special Requirements, 7 Important Parts

Medium-carbon nickel-chromium-molybdenum (vanadium) steel is widely used in the manufacture of ultra-high-pressure vessels.

The higher the pressure of the high-pressure vessel, the harsher the working conditions, and the higher the requirements for the strength of the material. Therefore, when selecting materials, the plasticity and toughness of the material should be improved as much as possible under the condition of satisfying the strength.

The content of impurities such as sulfur and phosphorus in steel for high-pressure vessels is very strictly controlled, and the general requirement is less than 0.015%. Medium-carbon nickel-chromium-molybdenum (vanadium) steel is widely used in the manufacture of ultra-high-pressure vessels. Its strength index is higher than that of low-carbon steel and ordinary low-alloy steel, and it has good hardenability and toughness matching.

Cylinder structure of two CO2 extraction vessels

Read More: CO2 Extraction Machine: 3 Types, 4 Tips, 5 Notes, Top 6 Apps

At present, most of the cylinders of supercritical CO2 fluid extraction high-pressure vessels have small diameters and large thicknesses. There are mainly two types of cylinder structures:

Cylinder structure 1: single-layer forged cylinder

The supercritical CO2 extraction high-pressure vessel with a volume of less than 100L generally adopts a single-layer integral forged cylinder structure.

Manufacturing method: Forge the steel ingot into a cylindrical shape, then perforate the steel ingot, and then perform heat treatment, and machine the inner and outer walls, sealing steps and end threads, etc.

  • Advantages: Simple structure, no weak deep girth welds, and longitudinal welds.
  • Disadvantages: Forgings will cause large differences in mechanical properties in different directions during the forging process, the ductile-brittle transition temperature is high, the possibility of low-stress brittle failure increases, and the requirements for processing equipment are also high.

For the supercritical CO2 fluid extraction container, it is necessary to meet the requirements of health regulations. Therefore, in order to maintain the sanitation of the inner surface of the container, the single-layer cylinder needs to use stainless steel forgings.

The single-layer cylinder structure of the supercritical CO2 extraction vessel with too large a volume is too expensive, and the combined cylinder should be used.

Cylinder structure 2: Combined cylinder

CO2 extraction vessels of platform arrangement
Combined cylinder of CO2 extraction vessels

As the pressure increases and the thickness of the pressure vessel increases, the distribution of stress along the wall thickness becomes more and more uneven. The stress on the inner wall of the cylinder is large and the stress on the outer wall is small. When the ratio K of the outer diameter to the inner diameter of the cylinder is greater than 1.5, the inhomogeneity of stress distribution is more prominent.

In order to improve the bearing capacity of supercritical CO2 extraction vessel cylinders, double-layer and multi-layer cylinder structures appeared to achieve the purpose of uniform stress distribution along the wall thickness.

Summary

In the design of supercritical co2 extraction high-pressure vessels, compared with the single-layer cylinder, the main advantage of the combined cylinder is that only the inner cylinder must be made of stainless steel, while the outer cylinder, laminate, or steel belt can be made of ordinary carbon steel.

Read More: CO2 Extraction Process: Basics, 5 Advantages, 7 Features, 4 Applications

Connection and sealing structure

The high-pressure vessel cylinder and the end cover are generally detachably connected. The self-tightening sealing structure bears the axial force acting on the end cover by the load-bearing element, and there is a screw plug or a clamp; the sealing element sealing ring is used to ensure pressure. According to the structural shape, there are wedge rings, B-rings, triangular rings, O-rings plus triangular rings, etc. for tight surface sealing.