Throttling process (Joule Thomson coefficient)

[theo dsouza]

This is a graph obtained during throttling process by keeping pressure and temperature at the inlet of the valve fixed and varying them at the outlet to get different isenthalpic curves for different inlet pressures and temperatures.

Is it true that the sudden change in temperature of this isenthalpic graph at the inversion point is due to the deviated nature of a real gas from the ideal gas? Will the graph for an ideal gas be lines parallel to the abscissa?

 

[Chestermiller]

View attachment 79412
This is a graph obtained during throttling process by keeping pressure and temperature at the inlet of the valve fixed and varying them at the outlet to get different isenthalpic curves for different inlet pressures and temperatures.

Is it true that the sudden change in temperature of this isenthalpic graph at the inversion point is due to the deviated nature of a real gas from the ideal gas?

I don't see any sudden changes at the inversion point.

Will the graph for an ideal gas be lines parallel to the abscissa?

Yes. The lines of constant enthalpy for an ideal gas undergoing a throttling process are horizontal on the graph. Note that, even in the low pressure region (where we usually expect ideal gas behavior), the isenthalpic lines are not horizontal. This is because the Joule Thompson coefficient is the result of deviation from ideal gas behavior.

Chet

 

[theo dsouza]

In some graphs of the Isenthalpic curves (not the one above) I've seen that the isenthalpic curves touch the ordinate axis to give a positive temperature value and a zero pressure value on the exit side of the throttle valve. Is this possible?

 

[Chestermiller]

In some graphs of the Isenthalpic curves (not the one above) I've seen that the isenthalpic curves touch the ordinate axis to give a positive temperature value and a zero pressure value on the exit side of the throttle valve. Is this possible?

The zero pressure value doesn't really mean zero pressure; it just means "at low pressures," where deviations from ideal gas behavior are negligible. Obviously, if you go all the way to zero pressure, the gas is not longer present, and its temperature is meaningless.

Chet