In the field of industrial gas production, cryogenic air separation technology is the cornerstone, providing many industries with indispensable high-purity oxygen, nitrogen and other gases. In the cryogenic air-division complex and exquisite process system, the gas expansion phenomenon is like a key gear in a precision instrument. Although it is small, it plays a decisive role in the efficient operation of the entire system.
What is gas expansion
The gas expansion link occupies a core position in the entire cryogenic air separation process. Based on the basic principles of thermodynamics, gas expansion is a key means to obtain cold volume. According to the first law of thermodynamics, during the adiabatic expansion process, there is no heat exchange between the gas and the outside world, and the gas does work externally, and its internal energy decreases, and the temperature decreases. Taking the changes in air in cryogenic air separation equipment as an example, when high-pressure air (pressure usually ranges from 10 - 20MPa) enters the expander for adiabatic expansion, the pressure drops rapidly. During this process, the gas temperature can drop sharply from normal temperature to -150℃ or even lower, successfully entering the cryogenic category.
Relevant research data show that in a typical cryogenic air separation device, each cubic meter of high-pressure air can generate about 200-300kJ after each cubic meter of high-pressure air is expanded by the expander. These cold amounts play a crucial role in maintaining the low-temperature distillation environment in the air separation tower and promoting accurate separation of various components of the air according to the boiling point difference.
Equipment operation
From the perspective of equipment operation, gas expansion plays an irreplaceable role in driving equipment operation and energy recovery. As the core refrigeration equipment of the cryogenic air separation system, the turbine expander vividly interprets this function. Transformer expanders mostly adopt a centripetal radius reactionary structure. High-pressure gas is first accelerated through the flow guide (nozzle), and high-speed airflow impacts the impeller, driving the impeller to rotate at high speed. During this process, the gas expands and cools, generating a large amount of cooling. At the same time, the rotation of the impeller can drive the supercharger to work, or generate electricity by connecting the generator. In actual operation, the cooling capacity generated by the air separation expander usually accounts for 80%-90% of the cooling capacity of the entire air separation system, and its energy recovery efficiency can reach more than 80%.
Application case
In the cryogenic air separation device of a large steel enterprise, through the efficient gas expansion process, the electricity can be recovered by millions of kWh each year, which not only significantly reduces the energy consumption of the device itself, but also saves the company a lot of electricity bills, fully demonstrating the great value of gas expansion in the energy utilization of equipment.
Process flow
In the cryogenic air separation process, gas expansion runs through the whole process to ensure smooth connection of all links. After the air is compressed and purified, the purified air enters the main heat exchanger and is cooled to the saturation temperature by the refluxed product nitrogen, waste gas, etc. At this time, some of the gas needs to enter the expander for expansion and refrigeration, providing the necessary cooling capacity for the subsequent air separation tower. Part of the expanded gas is used for regeneration and cooling of molecular sieves, and then discharged into the atmosphere through a silencer; the other part participates in the distillation link in the air separation tower, interacts with other gases, and helps the separation and purification of product gases such as oxygen and nitrogen. Effect of expansion on liquefaction efficiency
In cryogenic air separation, gas expansion is not just a simple physical process, it also directly affects the energy efficiency of the entire system. The expansion ratio setting can increase the liquefaction efficiency by 10%-20%.
Gas expansion during cryogenic air separation is not only a key step in achieving low temperature liquefaction, but also has an important impact on the energy efficiency of the entire system. By rationally designing expansion parameters, the gas volume can not only be greatly increased and the temperature can be reduced, but also effectively improved the product output rate. In actual operation, precise control of the working conditions of the expander and ensuring the best expansion effect is one of the important factors to ensure the efficient operation of the deep cryogenic sub-device.
