Cryogenic plants principle of operation is based on the air liquefaction and subsequent air separation into nitrogen, oxygen and argon. This method for the gas production is called air separation by deep cooling method.
1. First, air is compressed by compressor, after passing through heat-exchange units it expands in the expansion machine or throttling valve and cools to the temperature 93 K and then turns into liquid.
2. Further separation of liquid air that mainly consists of liquid nitrogen and liquid oxygen is based on the difference in the boiling points of its components: oxygen – 90,18 K, nitrogen – 77,36 K. This process numerously repeats on the special sieve trays and ends with the production of liquid oxygen, argon, nitrogen.
Gas separation principle
The operation of cryogenic air separation plants is based on the method of low-temperature rectification, which is based on the difference in the boiling points of air components and the difference in the composition of equilibrium liquid and vapor mixtures. In the process of air separation at cryogenic temperature between liquid and vapor phase in contact, consisting of air components, mass- and heat exchange is carried out. As a result, a vapor phase is enriched with low-boiling component and liquid phase – with high-boiling component. Vapor is enriched with low-boiling component – nitrogen, and flowing down liquid is enriched with high-boiling component – oxygen.
Cryogenic method is the only method that at any product quantity provides high purity of the separation products with high extraction coefficient, what is very important for providing high efficiency. This method also allows to get several separation products simultaneously and to get products both in form of gas and in form of liquid. The more products produced at the output the more efficient the plant works, providing higher technology flexibility.
Cryogenic air separation plant technology
Operation description of cryogenic air separation plant
Air separation plant has turbo-expander and is adsorption-type plant of normal temperature with following flow process:
1. Filtration, compression, pre-cooling and purification
Through the suction port air flows into the air filter where dust and mechanical admixtures (impurities) removed from the air and then it flows into centrifugal compressor for compression. Compressed air gets into the turbo expander. Expanded air flows into air cooling column, the system of preliminary cooling, where the air is cooled by interacting with chilled water that comes from the cooling tower. At the top of the air cooling column installed inertial separator and made of wire gauze separator to prevent water from getting into the process air.
Process air from precooling system enters purification system to remove moisture, carbon dioxide and hydrocarbon. Purification system consists of two double vertical adsorbers and when one is working, the other is regenerated by electric heater and heated waste nitrogen from cold block.
Pure process air from the purification system gets into the main heat exchanger of the cold block and is cooled by reverse flow gas. When cooling process reaches certain temperature, the part of pure air enters expander for the expansion, and then air flows back to the main heat exchanger for new heating. Then remaining air cools and liquefies in the main heat exchanger. Liquefied air gets into the bottom part of lower column. At the lower column increasing amount of gas fully mixes with the descending liquid resulting in heat exchange. The result of this heat exchange is the release of pure nitrogen from the air and its concentration in the upper part, while in the lower part oxygen-enriched air is generated. Pure nitrogen enters the main evaporator of the condenser, gaseous nitrogen compresses, and at the same time, liquid oxygen evaporates. Part of liquid nitrogen is used in lower and upper columns, while the rest of the nitrogen is supplied to the consumer as a final product.
Produced in the lower column liquid oxygen is cooled and fed to the upper column for rectification. In the upper column liquid oxygen, gaseous oxygen, gaseous nitrogen and liquid oxygen, liquid argon are produced.
Extracted from the upper column waste nitrogen is reheated through the pre-cooler and the main heat exchanger and then it enters the purification system for adsorbers recovery.
3. Cooling system
Turbo-expander provides cold generation that is required for air separation plant. Compression of dry air to 1.3 MPa is carried our in the centrifugal compressor stage of the turbo-expander. After compression stage dry air cools in the intermediate water cooler. From water cooler, air enters main heat exchanger and after cooling to the temperature of 152 K, one part of the air is sent to the turbo-expander, the other part of the air is choked into the lower column. In the turbo-expander, air expands to the pressure 0,14 MPa and discharges into the atmosphere through the main heat exchanger.
Applications of technology:
—Production of pure oxygen
—Production of pure nitrogen
—Production of pure rare technical gases and gas mixtures
—Separation of end blowdown gases from the ammonia production units
Following systems are used as parts of cryogenic technologies and as individual units:
— Storage systems
— Vaporizing systems
— Transportation systems
— Valves and piping components