Applications of Dense Gases to Extraction and Refining

Abstract

Schultz and Randall published in 1970 the results of qualitative investigations into the extractability of lipophilic natural substances with liquid carbon dioxide [1]. They described the influence of molecular weight and functional groups of aroma compounds from fruit juices on the solubility in the liquid carbon dioxide. In 1976 Stahl and Schilz studied the extractability with supercritical carbon dioxide of numerous individual compounds from various classes (e.g. polyaromatics, phenols, aromatic carboxylic acids, anthraquinones, pyrones, hydrocarbons and other lipids [2]; this furnished the first clear picture of the solubilites of natural substances in dense carbon dioxide. This information, together with experience from recent years, has enabled the rules of thumb given below to be suggested for extraction with dense carbon dioxide: 1) easily extractable (up to 300 bar) are lipophilic compounds with a molecular mass of up to 300–400, such as hydrocarbons, ethers, esters, ketones and similar compounds. 2) the presence of polar functional groups lowers, and may even completely prevent, extraction. 3) not extractable are polar substances, such as sugars, glycosides, amino acids, lecithins, etc.; and polymers, including proteins, cellulose, polyterpenes and plastics. Non-polar oligomers are only sparingly soluble. 4) water is poorly soluble in liquid carbon dioxide (ca. 0.1% by weight, at 20 °C) but shows increasing solubility in supercritical CO2 with temperature increase (ca. 0.3% by weight at 50 °C). 5) fractionation is possible when the substances display differences in molecular mass, vapour pressure or polarity.