Joule – Thomson Effect (Download PDF)
Porous Plug Experiment: When any gas is subjected from a region of higher pressure to a lower pressure through a porous plug adiabatically, cooling takes place except for H, and He. At the same conditions of temperature and pressure when these gases are subjected from a higher-pressure region to a lower-pressure region through a porous plug adiabatically, heating takes place.
This effect is known as the Joule-Thomson effect'. It is an isenthalpic and adiabatic process.
As Joule – Thomson effects is an adiabatic Process, therefore,
dQ = dU + dW
0 = dU + dW
-dW = dU …(1) ('. dQ = 0)
From the above expression, it can be said that the work done by the gas is equal to the change in internal energy.
– PdV = dU
– P(V2 – V1) = U2 – U1
– PV2 + PV1 = U2 – U1
U1 + PV1 = U2 + PU2
H1 = H2
H2 – H1 = 0
H = constant
So, Joule – Thomson effect is an isenthalpic process.
Joule Thomson Coefficient:
H = H (T, P)
Joule – Thomson Coefficient and Ideal gas equation of state:
The expression of Joule-Thomson coefficient can be expressed as:
Inversion temperature (Ti):
The temperature at which the value of becomes zero which means neither heating nor cooling takes place and gas behaves ideally is called inversion temperature.
Joule-Thomson Coefficient for a Real Gas:
Joule-Thomson coefficient may be positive or negative or zero depending upon the temperature and pressure of the gas. Since the change in pressure, dp, in Joule-Thomson experiment is always negative, it follows that dT will be negative when is positive, and vice-versa.
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