What is the purpose of a separator placed after the expansion turbine?
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The air separation unit in our plant uses nitrogen expansion cooling; a certain amount of medium-pressure nitrogen is drawn from the intermediate stage of the nitrogen compressor, expanded to become low-pressure nitrogen, and after passing through a gas-liquid separator, the gaseous nitrogen goes through a high-pressure heat exchanger, where its temperature rises from -176 degrees to 38 degrees above zero, before circulating back to the first stage of the nitrogen compressor. The low-pressure nitrogen coming out of the expansion turbine is in gaseous form; it passes through a separator to separate some of the liquid nitrogen. I wonder if it’s possible to operate without using such a separator
Is it okay if no separator is used? No, if it enters the tube directly, it will expand and cause the tube to burst ； A separator is provided to act as a buffer, ensuring safe and stable pressure.
Expansion? Why does it expand as soon as it enters the tube? The pressure has already been reduced after passing through the expander; how could the pipe burst? I’d appreciate it if you could explain it a bit more clearly. Thank you. Oh, it’s because the pressure decreases, the distance between gas molecules increases, and thus the volume expands, right? Last edited by nmlingling on 2009-2-11 21:57.]
If liquid nitrogen gets into the heat exchanger, and the temperature rises from minus 176 degrees to 38 degrees, your heat exchangers still won’t explode; P. So it’s essential to separate the liquid nitrogen.
Is there a large amount of liquid nitrogen in the separator? If there is a lot of it, then it’s necessary; but if there’s hardly any liquid level, I don’t think it’s necessary. A small amount of liquid nitrogen entering the heat exchanger isn’t a problem, as heat exchange is a slow process and the liquid won’t vaporize instantly. So a small amount of liquid nitrogen is fine, while a large amount might cause issues. However, the amount of liquid nitrogen produced can be calculated. I’ve seen processes where there is no gas-liquid separator after the expansion machine, with the fluid going directly into the cryogenic tank for heat exchange; but this is somewhat different from your operating conditions. To conduct a more thorough analysis, it would be helpful to specify the pressure after expansion at stage 1, which will facilitate a more precise analysis of the issue.
For further clarification: the inlet pressure of the expansion turbine in our plant is 4,700 kPa, while the outlet pressure is 560 kPa. The high-pressure heat exchanger is a plate-fin type heat exchanger. Liquid oxygen is pumped to a pressure of 6600 kPa using a liquid oxygen pump and then fed into the high-pressure heat exchanger; its temperature rises from minus 180 degrees Celsius to plus 38 degrees Celsius, undergoing a phase change from liquid to gas. So, there shouldn’t be any problems with this heat exchanger, right?