Compressing and Heating the Liquid to Reach Supercritical Operating Conditions

2 Compressing and Heating the Liquid to reach Supercritical Operating Conditions 

Liquid carbon dioxide at hquid-hke densities can be used as an effective extraction solvent. In fact, the viscosities and diffusivities of a near-critical liquid (i.e., one that is within 30-40°C of the critical temperature) are nearly as advantageous as those of the fluid in the supercritical region. What is lost with a compressible near-critical liquid is the adjustable solvent power. Furthermore, as temperature is increased in order to support increased solvation of compounds with higher melting points and/or molecular weights, the user moves into the supercritical region an3rway. 

The mechanics of increasing the pressure on the fluid can he done in a couple of ways. One would he to increase the temperature surrounding the hquid carbon dioxide [14]. As noted above, the pressme exerted on the hquid carbon dioxide in the cyhnder is a function of its ambient temperature. Only at higher ambient temperatures would the resultant system pressures be enough to reach the nearly hquid-hke densities of 0.1 to 1.0 g/mL that are required to use supercritical fluid carbon dioxide as an extraction solvent for extraction temperatures ranging from 40 C to 150°C. This approach is not widely used at this time. Liquid pumps (such as those used in liquid chromatographs) are more commonly used. 

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