In processes using supercritical fluids, especially in carbon dioxide separation or regeneration processes, high-pressure fluids are decompressed. To determine the phase and temperature of carbon dioxide during this time, the Mollier diagram (below) is used. From the initial state to the final pressure, draw a vertical line downwards to check. This diagram allows us to determine not only the temperature but also the ratio of gaseous carbon dioxide to liquid or dry ice. This diagram utilizes the fact that enthalpy does not change during free expansion.