What are supercritical fluids?

A SCF is defined as a substance above its critical temperature (TC) and critical pressure (PC). The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. The phenomenon can be easily explained with reference to the phase diagram for pure carbon dioxide (Figure 1).


Figure 1. Phase diagram of carbon dioxide.

This shows the areas where carbon dioxide exists as a gas, liquid, solid or as a SCF. The curves represent the temperatures and pressures where two phases coexist in equilibrium (at the triple point, the three phases coexist). The gas-liquid coexistence curve is known as the boiling curve. If we move upwards along the boiling curve, increasing both temperature and pressure, then the liquid becomes less dense due to thermal expansion and the gas becomes more dense as the pressure rises. Eventually, the densities of the two phases converge and become identical, the distinction between gas and liquid disappears, and the boiling curve comes to an end at the critical point. The critical point for carbon dioxide occurs at a pressure of 73.8 bar and a temperature of 31.1C. These parameters make equipment design and reaction set-up relatively simple.

The process of carbon dioxide undergoing a phase transition to form a supercritical fluid can be seen here:

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