When the air separation method is used to prepare oxygen, it is always desirable to separate as much of the oxygen in the process air as a product as possible. In order to evaluate the completeness of separation, the concept of oxygen extraction rate is introduced.
The oxygen extraction rate is expressed as the ratio of the total oxygen in the product oxygen to the total oxygen in the processing air entering the tower. which is
In the formula ψ-the extraction rate of oxygen;
Vo2, Vk---oxygen output and processing air volume, m3/h;
yo2, yk---product oxygen and the volume fraction of oxygen contained in the air.
It can be seen from the above formula that for a certain location, the oxygen content in the air is basically unchanged. When the amount of air entering the tower and the purity of the product oxygen are constant, the oxygen extraction rate depends on the amount of oxygen output. The amount of oxygen output is mainly determined by the level of oxygen in the polluted nitrogen under the condition of a certain air intake for the full low pressure oxygen generator. Now take the 3200m3/h air separation plant as an example. When the inlet air is 18100m3/h, the waste gas nitrogen amount is 60.2% of the processing air volume, and the volume fraction of oxygen in the waste nitrogen is 5.5%, the oxygen output is 3200m3/h , The oxygen purity is 99.6%. It can be calculated that the oxygen extraction rate at this time is
ψ=3200×99.6/(18100×20.9)=0.842, which is 84.2%
At the same time, it can be calculated that the amount of oxygen running away with the polluted nitrogen is 18100 (m3/h)×60.2/%×5.5%=599.2m3/h. If the oxygen content in the polluted nitrogen increases to 7.5%, the amount of oxygen running away with the polluted nitrogen is:
It can be seen that the oxygen output will be reduced by (817.2-599.2) m3/h=218m3/h, that is, the oxygen output will be (3200-218)m3/h=2982m3/h. At this time, the oxygen extraction rate ψ=2982×99.6/(18100×20.9)=0.718=71.8%.
Therefore, efforts should be made to reduce the oxygen content in the polluted nitrogen, so that more oxygen can be produced and the oxygen extraction rate should be improved.
The oxygen extraction rate of the all-low-pressure rectification tower used to be only 80%-85%, but now it has increased to 90%-95%, and the most advanced ones can even reach about 99%.