Calculate the Joule-Thompson coefficient of hydrogen when it
is expanded from 20 bars to 5 bars at 100 degree Kelvin. Critical pressure is 12.5 atmospheres. Critical temperature is 23.25 degrees
Kelvin. One can use Table 248 b, Perry
to obtain the properties of compressed n-hydrogen.
Solution:
Consider a
gas, at an upstream temperature and pressure T1 and P1,
which flows through a porous plug, valve or similar device which offers
resistance to flow. At the exit section
the pressure will have decreased to P2. Suppose that the device is
insulated from the surroundings, no shaft work is produced, and that the
overall changes in kinetic and potential energy are negligible. Then the steady-state energy relation reduces
to deltaH is equal to zero or H2 is equal
to H1, and the process is one of constant enthalpy. Usually there is a change in temperature
accompanying the decrease in pressure, and the ratio of these changes is called
the Joule-Thompson coefficient. This
coefficient is a property of the gas, and hence, is a function of temperature
and pressure.
Enthalpy of hydrogen at 20 bars and 100 degrees Kelvin is
1546 joules per gram. The same enthalpy
value at final pressure of 5 bars corresponds to 98 degrees Kelvin. Now, Joules-Thompson coefficient can be
calculated as a ratio of delta T to delta P.
It is equal to the ratio of (98 minus 100) to (5 minus 20) or 0.1313
degrees Kelvin per bar.