Austenitic stainless steel: the best heat resistance, the most widely used. It has certain strength and toughness, high-temperature resistance, and no pollution to the medium. It is the most widely used material in supercritical CO2 fluid extraction.
Reasonable selection and correct use of CO2 extraction equipment materials should not only consider the CO2 extraction equipment structure, manufacturing process, service conditions (medium, design pressure, and temperature), and service life, but also consider its corrosion resistance, mechanical properties, sources, and prices And other factors.
Three basic requirements for manufacturing supercritical CO2 extraction equipment materials
How to choose the material for supercritical CO2 extraction equipment
The mechanical strength of the material is the basis for determining the allowable stress value. In the design, the strength limit σb and the yield limit σs are mainly considered. At high temperatures, the creep limit σn and the durability limit σd are also considered. Plastic elongation δ and impact toughness are also important factors in selecting materials.
On some occasions, there are special requirements for materials, for example, heat exchange equipment requires materials with high thermal conductivity. The low-temperature working environment has special requirements for low-temperature impact toughness, and the low-temperature Charpy V-notch impact test must be performed on the material.
It is also necessary to consider the manufacturing processability of materials, including weldability, enforceability, and machinability, etc., to minimize equipment manufacturing costs and select economical and reasonable materials.
Since the supercritical CO2 fluid extraction process is under high pressure (generally 8~35Mpa, sometimes higher), on the other hand, pure CO2 fluid is not particularly corrosive, but if trainers such as water and organic acids are added to it, Equipment will have fatigue/corrosion problems, so the correct selection of materials is an important part of equipment design.
Read more: Supercritical carbon dioxide
Commonly used three types of pressure vessel steel
Supercritical CO2 extraction equipment is generally made of alloy steel, that is, some alloying elements are added to carbon steel, such as Mn, Gr, Ni, Mo, Ti, Al, etc.
Low-alloy steel is based on the addition of a small amount of alloying elements (total alloying elements <5%) on the basis of ordinary carbon steel to improve strength, toughness, corrosion resistance, high temperature, and low-temperature resistance, and other properties.
16Mn is the most commonly used type of ordinary low-alloy steel, with a carbon content of about 0.16% and an Mn content of about 1.5%. Compared with Q235-A, the strength of 16Mn is significantly improved, and the yield limit is increased by 30% to 40%. It is used to make pressure vessels or stressed parts, which can reduce the quality by 30% to 40%. Another outstanding advantage of 16Mn is that it has high impact toughness at low temperatures and can be used at a temperature of -40°C. The welding performance of 16Mn is also better. 15Mni and 15MnV are also commonly used low alloy steels.
The total amount of alloying elements in alloy steel is >5%, and the grade composition is the same as that of low alloy steel. Its comprehensive mechanical properties, heat resistance, and corrosion resistance are superior to carbon steel, and the corresponding price is also more expensive. It is generally used for high-pressure vessels and various stressed parts.
Three Types of Stainless steel or stainless acid-resistant steel
The main alloying elements in stainless steel are Cr and N, which are used to improve its corrosion resistance. Some add a small amount of Mo and Ti to improve the ability to resist chloride ions and intergranular corrosion.
Stainless steel is divided into three categories according to metallographic structure
- Types 1#: Austenitic stainless steel – The best heat resistance, the most widely used. It has certain strength and toughness, high-temperature resistance, and no pollution to the medium. It is the most widely used material in supercritical CO2 fluid extraction.
- Types 2#: Ferritic stainless steel – Corrosion resistance is poor, and it is still useful for less corrosive media. Mainly include 0Cr13, generally used for equipment resistant to water vapor, ammonium bicarbonate, hot sulfur-containing corrosion products, etc.; 0Cr17Ti, mainly used for equipment in the nitric acid industry and food factory man-made fiber factories.
- Types 3#: Martensitic stainless steel – The corrosion resistance is poor, but the strength is high, such as 1Cr13, and 2C13, which are mainly used for parts with weak corrosion resistance and high toughness and impact resistance.