specialists in production of supercritical fluids systems.

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CO2 from liquid to supercritical state

The supercritical fluid commonly utilized is carbon dioxide (CO2). The state diagram of CO2 visualizes the various phases (solid, liquid, gas) depending on pressure and temperature. CO2, at 31,1°C and 73,8 bar, is in its supercritical state, in which there is no distinction between liquid and gaseous phases (as shown in the following picture).

Increasing the temperature and keeping the pressure constant (73,8 bar), CO2 remains in the supercritical state, and so happens when pressure is increased and temperature is constant (31,1°C): this individuates two rays – respectively parallel to to the pressure and temperature axes – defining the zone in which CO2 is in the supercritical state; in particular, within this state, the possible combinations of pressure and temperature are shownto variate CO2 solubilizing properties.The reasons for the choice of this supercritical solvent are of economic (CO2 is cheap), environmental (CO2 is not toxic, it does not harm the ozone layer, it does not pollute and it does not contaminate the extracts) and technical (CO2 critical conditions can be reached easily) concern.SC-CO2 assumes the characteristics of a non polar solvent and it is comparable to n-Hexane; it has the characteristic to solubilize compounds which are scarcely soluble in water due to their nature.

 

                                           CO2 from liquid to supercritical state

 

If essential oils are wanted, the presence of water in the matrix interferes negatively on the process, because it is extracted together with the oil, hence it is necessary to remove it in a second moment. In order to avoid this problem, vegetable matrices are usually dried before extraction, unless extracts containing also polar substances are wanted. In this case it is necessary to add other solvents (entrainers or co-solvents) directly to the matrix or to the CO2, like ethanol or water, able to extract those compounds. CO2 is chemically inert, so isomerization, oxidation or components hydrolysis are avoided.The advantage of this technique is that at the end of the extraction, the solvent can be removed as a gas, offering the possibility to recover the extracted concentrated compounds. In the industrial processes, CO2 can be recycled minimizing its consume. This technique finds several applications such as oil extraction from seeds, caffeine extraction from coffee, nicotine extraction from tobacco, etc.; it is also very convenient at industrial level.

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