Liquefaction: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  1. Only a fraction of the gas entering a Joule-Thomson expansion is converted to liquid. Thus the mass flow rate of gas recycled in a liquefaction process is significantly greater than the mass flow rate of gas entering the process.
  2. The compressor work required to form 1 kg of liquid in a liquefaction process can be large.
  3. Several compressors are required to reach high pressures since the exit temperature from one compressor would be too high (above what the components could experience) and because significantly more work would be required for one compressor than the total of several compressors.
  4. The heat exchanger that cools the room-temperature, high-pressure gas using the low-temperature, low-pressure gas is critical for efficient formation of liquid from gas.
  5. The enthalpy of real gases depends on both pressure and temperature.
  6. The high-pressure gas exiting the heat exchanger must have a low-enough temperature and a high-enough pressure so that the exit from Joule-Thomson expansion is two phases.
  7. The cycle calculations assume reversible compressors and ideal heat exchangers.
From studying this module, you should now be able to:
  • Solve mass and energy balances for a liquefaction process to determine the fraction of fluid entering the Joule-Thomson expansion that is converted to liquid.
  • Solve energy balances to determine the compressor work required to form one kg of liquid.
  • Explain how gas is converted to liquid in a steady-state process.
  • Draw a liquefaction cycle on a pressure-enthalpy diagram and a temperature-entropy diagram.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder