A process that uses nitrogen as the working medium in a turbo-expander is called nitrogen expansion. Nitrogen expansion can be classified into two types: pure nitrogen expansion and waste nitrogen expansion.
1. Pure Nitrogen Expansion Process
The pure nitrogen expansion process involves extracting pure nitrogen from the top of the lower column and the top of the nitrogen side of the main condenser.
A portion of the nitrogen is reheated through the regenerative passages of the reversing heat exchanger. The streams are then combined and fed into the turbo-expander.
After expansion, the nitrogen passes through the plate-fin heat exchanger and is withdrawn as product nitrogen.
2. Waste Nitrogen Expansion Process
The waste nitrogen expansion process involves extracting waste nitrogen containing approximately 1% oxygen from the middle of the lower column.
A portion of the waste nitrogen is reheated through the regenerative passages of the reversing heat exchanger. The streams are then combined and fed into the turbo-expander.
After expansion, the waste nitrogen passes through the subcooler-liquefier and the reversing heat exchanger to recover refrigeration. It is then sent to the evaporative cooling tower, where it absorbs the heat of evaporation from the cooling water, before being vented to the atmosphere.
3. Effect of Nitrogen Expansion on the Distillation Process
Since nitrogen is extracted from the lower column, the amount of liquid nitrogen condensed in the main condenser is reduced. Consequently, the amount of liquid nitrogen reflux supplied to the upper column decreases, allowing the reflux ratio of the distillation column to be adjusted to a more reasonable level.
This makes better use of the distillation potential of the upper column and improves the oxygen extraction rate.
4. Advantages and Disadvantages of Nitrogen Expansion
1) Higher refrigeration capacity per unit of gas
The gas after nitrogen expansion does not enter the upper column and therefore does not directly affect the distillation operating conditions of the upper column.
The superheat of nitrogen after expansion can be higher than that of air after expansion, and its discharge pressure can also be slightly lower.
Consequently, the unit refrigeration capacity is greater than that of air expansion. For a given equipment cold loss, a smaller expansion flow rate is required.
2) Smaller column diameter and simpler structure
Since the expanded gas does not enter the upper column and the heat load on the main condenser is reduced, the ascending gas flow rate in the upper column is lower than that in an air expansion process.
As a result, the column diameter can be reduced and the equipment structure can be simplified.
3) Potential impact on oxygen extraction rate
An increase in the nitrogen expansion flow rate has a certain effect on the temperature difference in the main condenser. It may increase the oxygen content in the waste nitrogen and reduce the oxygen extraction rate.
However, its direct impact on oxygen purity is relatively small, and its effect on the crude argon fraction is even smaller.
Nitrogen expansion has already been adopted in large-scale air separation units overseas.
4) Cleaner working medium and safer expander operation
The working medium used in nitrogen expansion is cleaner than that used in air expansion, allowing the expander to operate under safer conditions.
The nitrogen used for expansion is extracted from the upper part of the lower column and therefore does not carry solid carbon dioxide or similar impurities.
In contrast, the working medium for air expansion is extracted from below the first bubble-cap tray. Carbon dioxide in the air may therefore be carried into the expander, potentially causing wear.
Furthermore, during nitrogen expansion, when the discharge pressure is 0.12 MPa, liquid droplets do not appear until the temperature reaches 78.7 K. Therefore, liquid formation inside the expander is less likely.
5) Limited nitrogen extraction capacity
The minimum reflux ratio of the lower column is greater than that of the rectifying section of the upper column. In other words, the lower column has less surplus reflux liquid available than the upper column.
Therefore, the amount of gaseous nitrogen that can be extracted from the lower column is smaller than the amount of air that can be fed into the upper column.
Typically, the nitrogen expansion flow rate does not exceed 16% of the processed air flow rate, while the air expansion flow rate does not exceed 25% of the processed air flow rate.
