Expansion Driven Heating or Cooling of Gases

A Topic Brief About the Joule-Thomson Effect & its Industrial Applications When a compressed real gas expands irreversibly and adiabatically across  a flow restriction element such as an orifice or valve, not only does the pressure of the gas decrease but its temperature can also either spontaneously cool or heat.[1]  Such an occurrence is commonly referred to as the Joule-Thomson (J-T) effect or expansion named after the 19^th century physicists James Prescott Joule and William Thomson (a.k.a. Lord Kelvin) who first collaboratively described this physical phenomenon. 

 

Since no work is done on or by the gas and the expansion occurs without significant heat transfer, a J-T process is essentially isenthalpic indicating that the temperature and pressure of a real gas both must change as its enthalpy depends on both the system T and P.  Conversely, J-T expansion of an ideal gas is not possible since ideal gas enthalpy only depends on temperature, and, hence, expansion of an ideal gas under the same conditions is always isothermal.  Consequently, the temperature and pressure of a real gas undergoing J-T expansion is related through a P-T envelope or inversion curve dependent on the J-T coefficient, m, while for an ideal gas mis always zero.  For a real gas, when m> 0, the expansion causes auto-refrigeration of the gas as it is throttled across the differential pressure device while for m< 0 the gas will self-heat and have a higher downstream temperature similar to an adiabatic exothermic reaction.  

 

Isothermal expansion occurs when m= 0 and the process lies on the J-T inversion curve.  Accordingly, closed loop process schemes can be designed to utilize such expansion driven auto-refrigeration or self-heating behavior of gases and even highly volatile liquids to manipulate the thermal state or quality of a fluid stream and facilitate a process change. 

 

For example, pressure maintenance systems for liquefied natural and petroleum gas (LNG / LPG) holding terminals often rely on auto-refrigeration from the J-T expansion of circulating liquid to control its pressure during diurnal heating.  In such a scheme, liquid is drawn from the bottom of the tank using a pump, passed through a process cooler to remove heat added from bulk fluid conveyance and night-to-day heating of the tank contents, and then back into the tank through a control valve followed by a distributor lance or spray nozzle with it flashing by J-T expansion to a vapor having a temperature substantially lower than the saturation temperature of the mixture in the tank which cools the vapor space and reduces the tank pressure through vapor condensation.  Similarly, liquefied propane is often used as a refrigerant in a closed loop refrigeration cycle relying on J-T expansion for liquefying natural gas.[2] 

 

Furthermore, the generation of super-heated steam can be achieved via J-T expansion when saturated or wet steam is flashed to a lower pressure through a control valve where a lower pressure and temperature steam will be produced with the temperature above the saturation value.[3]  Alternatively, high ratio expansion of purified hydrogen gas during filling of industrial hydrogen storage tanks causes the H₂ to heat which can be utilized for other process heating by cross-stream heat exchange.[4]

---