You probably heard LNG, Liquefied Natural Gas. Did you ever hear Liquefied Nitrogen Gas? Methane can be liquefied at a temperature of -259 F, while to liquefy nitrogen, the temperature is even lower down to -320 F. In this video, a simulation is modeled in HYSYS to demonstrate how nitrogen gas is turned into nitrogen liquid.
Let us start with gaseous nitrogen at 15 psia. The gaseous nitrogen is first compressed to about 70 psia and then further compressed into about 410 psia in the MP and HP compressor.
A fraction of gas 109 is boosted to about 630 psia in the warm booster, which is driven by the warm expander. Before feeding to the main cryogenic heat exchanger MHE, it is further boosted to about 940 psia in the cold booster, which is driven by the cold expander. After being cooled to about -135 F, a portion of the cold gas is fed into the cold expander, which drives the cold booster. The other portion is completely liquefied to the liquid form in the third section of the main heat exchanger MHE. Both of these two streams are sent into the MP separator.
The liquid stream of 119 is further sub-cooled in the sub-cooler. The majority of the liquid is sent to the product storage, where the product Liquid Nitrogen (LIN) can be trucked for sale. The small portion is sent back to the sub-cooler as a cold source to prevent too much vaporization in the storage tank. The low pressure low temperature gas is introduced to the main heat exchanger MHE to recover the cold before mixing with the gaseous nitrogen feed.
As you recall, the other fraction of gas 201 is sent to the main heat exchanger MHE as well. But it is withdrawn at a temperature of about 41 F and expanded to about 70 psia in the warm expander. The exhaust is sent back to the main heat exchanger, after mixing with the vapor from the MP Separator. It is then sent to the HP compressor.
There are two recycle loops. The low pressure recycle loop is the nitrogen at about 15 psia. It reaches the highest pressure at about 940 psia after the cold booster. Then the pressure is dropped to about 70 psia at the MP Separator. Further the pressure is reduced to about 15 psia at the LP Separator to provide sub-cooling to the liquid nitrogen product. The medium pressure loop is at about 70 psia. The nitrogen is compressed to about 410 psia and then expanded back to about 70 psia in the warm expander.
To minimize the liquefaction power, the key is to optimize the main heat exchanger MHE. As you can see from the heat release curve, the cold and the hot composite curves are very tight. By change the plot type into Delta T Vs. Temperature, there are four temperature pinch points at about 4 F!
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