The refrigeration systems used for LNG liquefaction depend on removing enthalpy from natural gas by the use of compression and expansion. However, the impact of pressure on enthalpy is often not considered. For two gas streams with the same composition and temperature, but different pressures, which stream generally has the lower enthalpy?
A. They have the same enthalpy
B. Lower pressure stream
C. Higher pressure stream
D. Give me a minute to try it out in ProMax...
The enthalpy of an ideal gas is independent of pressure, so one possible answer would be A.
For gases such as hydrogen, helium, or neon that might have a negative Joule-Thomson coefficient (∂T/∂P)H, the lower pressure gas will have lower enthalpy. This would make the correct answer B.
However, for most gases at moderate temperature and pressure, enthalpy goes down for an increase in pressure ((∂H/∂P)T < 0). In LNG processing, higher pressure gases are easier to liquefy and make more effective refrigerants because they are at a lower enthalpy state.
Now you might be thinking, “The higher pressure stream has had all that compression energy forced into it. It has to be a higher enthalpy level.” Try this thought experiment: You have two gas streams with the same composition and temperature, but at different pressures. Now isenthalpically expand (J-T) the higher pressure stream down to the lower stream pressure. What happens to the temperature? For natural gas at normal conditions, the temperature decreases with expansion. The previously higher pressure stream is now at a lower temperature than the lower pressure stream. Its enthalpy must be lower.
If you still have doubts, D is always a good answer.