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Why is carbon dioxide used as solvent for supercritical fluid extraction?

The so-called supercritical extraction ofcarbon dioxide is to pressurize carbon dioxide into a supercritical state as asolvent, with its very high solubility extraction of ordinary difficult toextract substances, the following are several instructions on extraction:

I. dissolution

In the supercritical state, the solubilityof CO2 to different solutes varies greatly, which is closely related to thepolarity, boiling point and molecular weight of the solutes. Generallyspeaking, there are the following rules: lipophilic, low boiling pointcomponents can be extracted below 104KPa (about 1 atmospheric pressure), suchas volatile oils, hydrocarbons, esters, ethers, epoxides, and natural plants.And the aroma components in fruit, such as eucalyptus, thymol, low boilingpoint esters in hops, etc. The more polar groups of compounds, the moredifficult to extract. Components are easy to extract, some low molecularweight, volatile components can even be directly extracted with CO2 liquid;high molecular weight substances are difficult to extract with carbon dioxide.

II. Characteristics

Supercritical carbon dioxide is widely usedfor extraction because it has the following technical characteristics:

1. Supercritical CO2 fluid is a colorless,odorless and non-toxic gas under normal conditions. After separation from theextraction components, there is no solvent residue at all. It can effectivelyavoid the residual toxicity of solvent under traditional solvent extractionconditions. It also prevents the toxicity of extraction process to human bodyand environmental pollution. It is a natural and environmentally friendlyextraction technology. Surgery.

2. The extraction temperature is low, thecritical temperature of CO 2 is 31.265 C, and the critical pressure is 72.9atm, which can effectively prevent the oxidation, evaporation and reaction ofthe thermosensitive components, and completely preserve the biological activityof the biomass. At the same time, the substances with high boiling point, lowvolatility and easy pyrolysis can be extracted below the boiling pointtemperature.

3. Extraction and separation are two inone. When the supercritical fluid of carbon dioxide saturated with dissolvedmatter flows through the separator, CO2 and the extract are quickly recoveredto separate two phases (gas-liquid separation) due to the pressure drop, andare separated immediately. There is no phase transition process of thematerial, no solvent recovery, and the operation is convenient. And less energyconsumption, cost saving, and in line with the trend of environmentalprotection and energy conservation.

4. Extraction is easy to operate. Bothpressure and temperature can be used as parameters to adjust the extractionprocess. In the vicinity of the critical point, small changes in temperatureand pressure will cause significant changes in the density of CO2, resulting inchanges in the solubility of the substance to be extracted. Temperature canseparate substances; Conversely, temperature can be fixed and pressure can belowered to separate the extracts; therefore, the technological process isshort, time-consuming, small, and environmentally friendly. Extraction fluidCO2 can be recycled, and will not emit carbon dioxide resulting in greenhouseeffect! To become a truly "green" production process.

5. The polarity of supercritical fluid canbe changed. Under certain temperature conditions, different polar substancescan be extracted by changing pressure or adding suitable entrainment agent. Theselection range is wide.