Due to the large demand for cryogenic liquid of the market, and in order to ensure the normal operation of the production line when the air separation plant stops abnormally or the air supply is interrupted due to failure, the air separation users with cryogenic liquid production capacity will maximize the production and storage of liquid products while ensuring that the gas production is not affected, and try their best to improve the production of liquid products.
The following is an example of the air separation plant in the process of adsorption and pre purification of molecular sieve at room temperature, refrigeration of booster turbo expander, and external compression of oxygen and nitrogen. There are two working conditions of improving liquid products are introduced for reference:
1.Double air expansion liquid working condition
Two expanders are operated for double air expansion at the same time. Because the expansion air quantity entering the upper column is limited by the distillation condition of the upper column, the expansion air of two turbo expanders can’t all enter the upper column, only half of them enter the upper column, and the other part of the expansion air only provides cooling capacity and does not participate in the distillation. It enters the sewage nitrogen passage through the bypass valve, and then enters the main heat exchange and regeneration as the molecular sieve regeneration gas source, or enters the water cooling column as the chilled water preliminary cooling.
2.Nitrogen expansion liquid working condition
Another way to increase the liquid output is to run two expanders at the same time, one for air expansion and one for nitrogen expansion. The nitrogen comes from the low-pressure nitrogen pipe network. After being pressurized at the booster end of the expander, it enters the nitrogen heat exchanger for cooling. Most of the nitrogen is extracted from the middle of the nitrogen heat exchanger, enters the expansion end of the expander for cooling, and then enters the nitrogen heat exchanger as a cold source to cool the nitrogen pressurized at the booster end, and enters the atmospheric nitrogen pipe network after being reheated. The other part of pressurized nitrogen, which is completely heat exchanged by the nitrogen heat exchanger, enters the upper column directly through the throttle valve, providing cooling capacity for the distillation of the upper column to increase the liquid output.
In contrast, under the same conditions, the plant under the nitrogen expansion liquid working condition produces more liquid than under the double air expansion liquid working condition, but the energy consumption increases correspondingly. Moreover, the nitrogen expansion is a nitrogen heat exchange system independent of the air heat exchange system, and the medium nitrogen can only be used when the nitrogen consumption is low and the nitrogen pressure is high. Due to the limitation of distillation conditions in the upper column, a large amount of expanded air is vented, which results in the reduction of oxygen extraction rate and the impact on oxygen production.
Therefore, the increasing production of nitrogen expansion liquid working condition is suitable when the nitrogen consumption is small and the nitrogen pressure is high, while the increasing production of double air expansion liquid working condition is suitable when the oxygen consumption is small and the oxygen pressure is high.
