The cryogenic process was first developed by Carl von Linde in 1895 and improved by George Cloud in the 1900s to produce oxygen on a small scale to meet the needs of various industrial processes such as welding, cutting and as a medical gas. Cryogenic air separation began on an industrial scale at the beginning of the 20th century and fueled the development of metallurgy and other branches of industry heavily dependent on the availability of oxygen, nitrogen, and eventually argon.
Air Separation System (ASU) is characterized by high product quality, large capacities and reliability. Despite other emerging air separation technologies, cryogenic air separation technology is the most common and standard technology for producing gaseous products in liquid form. Cryogenic air separation devices are mostly used to produce products with high amount and purity, in this method, products are produced in liquid and gas form.
This separation method includes various processes, which include:
- Air compression
- air purification
- Heat exchange
- distillation
- Product compression
First stage:
Ambient air is compressed using a multi-stage turbo compressor with internal coolers. Dust particles are removed using a mechanical air filter as the air enters the compressor.
Second step:
The second step involves the removal of impurities, especially residual water vapor, carbon dioxide (CO2). These components are removed to meet product quality specifications and before air enters the distillation section of the plant. There are two basic methods for removing water vapor and CO2.
-
- Molecular sieve units
- Inverse converters
are.
Most new air separation plants use a molecular sieve pretreatment unit to remove water vapor and CO2 from the incoming air. Reverse converters for steam and CO2 removal are more cost-effective for smaller plants.
Third step:
Counterflow heat exchangers cool the process air to a temperature close to the liquid temperature.
Fourth step:
In the distillation process, trays are used to convert air into its components. The main function of the trays is to provide effective contact between the descending liquid and the rising gas.
Hence, the context tray for:
(1) cooling and partial condensation of the rising gas;
(2) heating and partial evaporation of the descending liquid
provides
Nitrogen from the top of the column as gas and oxygen to The liquid title is removed at the bottom of the column. A condenser at the top is used to liquefy pure nitrogen gas, and a boiler at the bottom is used to boil oxygen for greater product purity.
Argon can also be separated by withdrawing a stream in the middle of the column at the point where the concentration of argon is higher and separating it into another column where almost pure argon is separated from other gases.
Fifth step:
The products are usually removed at relatively low pressures, often just over one atmosphere (absolute). In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process.
Production of liquid products:
When liquid products are produced in a cryogenic air separation plant, an additional cooling unit is usually added to the basic air separation plant. This unit is called liquefier.
Non-cryogenic systems:
Non-cryogenic air separation is performed near ambient temperature, so the product, oxygen or nitrogen, is always in the gas phase. Production scale and purity in air separation by non-cryogenic methods span style=”font-weight: var( –e-global-typography-text-font-weight ); font-size: 1rem;”>
The scale and purity cannot be achieved by cryogenic air separation. They are designed through different processes such as: surface absorption technology and membrane technology. Non-cryogenic air separation processes use differences in physical properties such as molecular size and mass to produce nitrogen and oxygen of sufficient purity. While argon can be produced only by separating air by cryogenic method.
Surface Adsorption: Adsorption process technology is based on the ability of some natural and synthetic materials to absorb nitrogen or oxygen. This technology is used to produce nitrogen or oxygen by passing compressed air at several atmospheric pressures through a container containing absorbent materials. Adsorbents are selected based on their absorption characteristics. Special adsorbent materials are used as molecular sieves that preferentially absorb the desired gas species.
Membrane: Gas separating membranes are very fine hollow fibers through which clean and dry compressed air is fed. As the gases move through the tubes, a process called selective diffusion (diffusion-adsorption) allows us to separate the gases using Oxygen (O2), water vapor (H2O), and carbon dioxide (CO2) move faster through the membrane tube walls than argon (Ar) and nitrogen (N2). Nitrogen and argon gases, with lower diffusivity, remain longer in the fiber tubes and therefore can be used as containment gas.
