Supercritical Carbon Dioxide

A substance becomes supercritical when both its temperature and pressure exceed its critical point. For carbon dioxide, NIST tabulates critical temperatures around 304.1–304.35 K and critical pressures around 73.4–73.825 bar, with minor variation among compiled measurements. Above this point, liquid and gas do not exist as separate phases. The properties of scCO₂ can change substantially with temperature and pressure; its density can be much higher than that of an ordinary gas, while it can flow through materials as a fluid.

Supercritical fluid extraction uses a fluid above its critical point to remove selected compounds from a material. Carbon dioxide is widely studied for this purpose. In food processing research, scCO₂ has been investigated for extracting compounds and for microbial-spore inactivation. The outcome depends on operating conditions and the material being treated; it is not a universal substitute for other processing methods.

In a closed Brayton power cycle, scCO₂ circulates as the working fluid, receiving heat, expanding through a turbine, and then being cooled and compressed before the cycle repeats. Near its critical point, CO₂ can reach relatively high density, which can reduce compression work in suitable cycle designs. DOE and NETL describe the potential for compact, efficient power systems, while also identifying substantial engineering work in turbines, recuperators, materials, reliability, and system integration. The potential benefits therefore depend on design and demonstration rather than being guaranteed for every plant.

Working with scCO₂ requires pressure-rated vessels, seals, pumps or compressors, and controls suited to the selected temperatures and pressures. Changes in operating conditions can alter density and heat-transfer behavior, so systems must be engineered for their specific purpose. In extraction, the choice of pressure, temperature, and co-solvent can affect which substances are removed; in energy systems, component performance and materials durability are key development issues.