Supercritical Carbon Dioxide

Solvents are used to dissolve reactants, and also to extract and purify products. For economic reasons, solvents are usually recycled.

Unfortunately many organic solvents are volatile, flammable and may pose a risk to health and the environment. All traces of these solvents must therefore be captured, as vapour cannot simply be vented to the atmosphere.

Alternative approaches are being developed, and one with growing areas of application is the use of supercritical fluids, of which the most common is carbon dioxide.
diagram: carbon dioxide


Supercritical conditions
Materials can generally exist in three states: solid, liquid and gas. A change from one state to another occurs if pressure or temperature is changed sufficiently. The relationship can be shown in a phase diagram.

The lines on the diagram represent the temperatures and pressures at which two states can exist together in equilibrium. At the triple point, all three states co-exist.

As temperature and pressure are increased along the liquid/gas line, the distinction between these two states eventually disappears, and the phases become identical. This happens at the critical point. For carbon dioxide, it occurs at a pressure of 72.9 atmospheres but a temperature of only 31.1°C. Above this point the carbon dioxide is said to be in the supercritical state.

Supercritical fluids show properties of both liquids and gases. It will fill any size of container, like a gas, and dissolve materials like a liquid. Its power as a solvent can also be "tuned" very easily by changing the pressure.
For a movie clip of carbon dioxide being heated and compressed to its critical point, see the pages at the Leeds Cleaner Synthesis Group (University of Leeds, UK)

Carbon dioxide has the advantage over other supercritical fluids that its critical temperature is remarkably low at only 31.1°C, so high temperatures are not necessary. This means scCO2 can be used as a solvent with materials that would decompose at higher temperatures.

Making Carbon Dioxide
Concerns over atmospheric levels of carbon dioxide mean that making it by burning fossil fuels would not be acceptable, and it would be far too expensive. Carbon dioxide is, however, a by-product of fermentation to make ethanol. Rather than simply venting the gas, it can be collected and used. It is also a by-product of ammonia production, though using carbon dioxide from this route will increase atmospheric levels as ammonia is manufactured from petrochemical feedstock.

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