1.Overview of the overall development of air separation unit technology
In the past decade, low-temperature air separation units have made great progress and development in technology: the extraction rates of oxygen and argon have been further improved, the unit energy consumption of products has been reduced, and the reliability of the unit has been more guaranteed.
2.Classification and characteristics of air separation processes
Process classification: Air separation processes are usually divided into two categories: metallurgical and chemical.
Metallurgical process: an air separation process that uses an oxygen compressor to compress the pressure of oxygen out of the cold box from about 0.13MPa to about 3.0MPa.
Chemical type (internal compression process): an air separation process that uses a cryogenic liquid pump to compress the liquid oxygen in the main condenser to a working pressure of 6.5-9.5MPa and then sends it out of the cold box, that is, cryogenic air separation.
Development and characteristics of internal compression process
Development status: In recent years, the internal compression process (chemical type) has been greatly developed. It is not only used in chemical enterprises, but also rapidly promoted by steel enterprises. For example, Taiyuan Iron and Steel Company, Baoshan Iron and Steel Company and Yangzi Petrochemical have all selected the internal compression air separation process.
Main features:
Since the expensive oxygen turbine compressor is replaced by the liquid oxygen pump and booster or the combined liquid oxygen pump and high-pressure air compressor, the investment can be reduced.
Since the spare parts of the liquid oxygen pump and booster are cheaper than those of the oxygen compressor, the maintenance cost is reduced.
After the liquid oxygen pump is compressed, there is no high-temperature gas oxygen, so the safety is better and the operation of the device is more reliable.
After the oxygen turbine compressor is replaced by the booster, the floor space is reduced, the installation cost is lower, the operation is convenient, and the control is simpler.
3.Improvement of technical and economic indicators of air separation units
After the process organization and the improvement of single machine and unit equipment, the technical and economic indicators of air separation units at home and abroad have been greatly improved:
The oxygen extraction rate can reach more than 99%.
In the conventional process of expanded air entering the lower tower, the argon extraction rate can reach 92%--94%.
For the process of expanded air entering the upper tower, when the amount of expanded air accounts for less than 8% of the processed gas volume, the argon extraction rate can reach about 83%.
With the improvement of oxygen-argon extraction rate and the application of new structures such as packed tower, the power consumption index has further decreased.
4. Details of the progress and development of air separation technology
Improvements in precooling and purification systems
For users with less requirements for nitrogen products (low-pressure process requires N₂: O₂≤1.1), it is reasonable to cancel the chiller. On the one hand, it can reduce a set of equipment, reduce plant area, investment and maintenance costs. On the other hand, due to the generally poor quality of domestic cooling water, the fouling coefficient reaches 0.0004 or even 0.0006m²・h・℃/kcal, causing the chiller to operate unstably and the maintenance cycle is short. Therefore, this improvement is very suitable for my country's national conditions.
Automatic variable load operation technology
On the premise of ensuring stable operation of the device, the oxygen content can be changed between 70% and 105% according to instructions in a short time. The reduction of oxygen content is achieved by reducing the exhaust volume of the air turbine compressor. Modern air turbine compressors reduce the exhaust volume while reducing the shaft power by controlling their inlet guide vanes, thereby achieving the purpose of energy saving and consumption reduction. Air separation units are large power consumers, with electricity costs accounting for about 70% of the cost. Therefore, the use of automatic variable load operation not only has considerable economic benefits, but also considerable social benefits.
Liquid filling start-up method
The liquid filling start-up method is to fill liquid oxygen or liquid nitrogen into the main cooler during the start-up phase of the air separation unit when the upper and lower towers and the main cooler are cooled to the specified temperature to accelerate the cooling and liquid accumulation process of the air separation unit and shorten the start-up time of the air separation unit. It can shorten the start-up time of the air separation unit by 30% to 50%, which has considerable economic benefits.
Progress and improvement on a single machine
Improve the design of the air turbine compressor so that its isothermal efficiency can reach about 75%. Most large compressors abroad are equipped with an impeller backwash system. It consists of a condensate tank, a condensate pump, a water wash pump, a nozzle, and an instrumentation and control system. The backwash system can maintain the flow and efficiency of the compressor and extend the working time of the compressor.
A two-stage shell-and-tube final cooler made of copper-nickel tubes containing 10% nickel is used to replace the air cooling tower to prevent the water-carrying phenomenon when the air passes through the air cooling tower.
A falling film condenser evaporator is used, which has the advantages of high performance and small heat transfer temperature difference. It can reduce the exhaust pressure of the air compressor by 0.3~0.4bar and reduce the shaft power of the air compressor by 3%~4%, achieving the purpose of energy saving.
A radial flow purifier is used to reduce the size of the purifier for easy transportation.
A block-type cold box is used. The cold box is divided into several large blocks that can be transported and hoisted. After the equipment and pipelines in the cold box are welded in the manufacturing plant, they are connected according to the diagram to speed up the installation, ensure the welding quality, ensure the construction period, and have higher safety.
5.Development history and trend of air separation process
After the era of stone refrigerant, switching plate heat exchanger, plate-fin heat exchanger, and normal temperature molecular sieve purification, the development of air separation process has developed a new era of full distillation of argon in regular packing tower. Air separation technology is constantly updated and improved with the advancement of science and technology, the improvement of technical level and the forward development of society. Therefore, it is particularly important to grasp the development direction and trend of air separation and keep abreast of its pulse anytime and anywhere.
