Supercritical Fluids in Natural Product and Biomass Processing – An Introduction

MANUEL NUNES DA PONTE

1.1 The Early History of Supercritical CO2 Extraction

Thomas Andrews's 1869 Bakerian Lecture "On the Continuity of the Gaseous and Liquid States of Matter" is widely credited to have established the term "critical point" to define the point in phase space where a liquid and its vapour attain the same density and become indistinguishable. In his lecture Andrews described, in detail, his experiments on carbonic acid (carbon dioxide). He stated that "On partially liquefying carbonic acid by pressure alone, and gradually raising at the same time the temperature to 88 °F, the surface of demarcation of the liquid and the gas became fainter, lost its curvature, and at last disappeared." Andrews goes on to establish the critical temperature of carbon dioxide as 30.92 °C, and to describe how, above this temperature there are no signs of phase separation, although the volume becomes extremely sensitive to pressure. He presents, in detail, the volume contractions obtained by small increases of pressure in the temperature region above the critical, up to 48.3 °C. He concludes that the volume exhibits a much greater contraction than it would if the perfect gas law was followed. He further concludes that, at higher pressures, the volume of the fluid takes similar values to those that might be calculated by using the thermal expansion of the liquid from below the critical temperature, that is, it behaves like a liquid.

This work inspired van der Waals to formulate his famous equation of state in his doctoral thesis "On the continuity of the gas- and liquid-state", presented in Leiden in 1873. This thesis had a profound effect in the development of molecular sciences in the late 19th century, ultimately leading to the award of the 1910 Nobel Prize for Physics to van der Waals.

A fluid in the pressure–volume–temperature region above the critical point where high compressibility and thermal expansivity were detected by Andrews is nowadays called a supercritical fluid. The high sensitivity of the density to small changes in pressure (or temperature) is the most distinctive property of supercritical fluids, and it forms the basis for their technological applications developed in the last forty years. It took, however, about a hundred years for Andrews's work on carbon dioxide to be translated into the industrial process known these days as supercritical fluid extraction.

Truly, through the years, processes like the high-pressure polymerization of ethylene, discovered by ICI in the 1930s, and the so-called ROSE (Residuum Oil Supercritical Extraction) process, developed in the petrochemical industry and using a light paraffinic solvent, like pentane, in supercritical conditions, can be counted as using supercritical fluid solvents. The massive work of Francis on mixtures of liquid carbon dioxide with hundreds of compounds, published in 1954 in one single paper, certainly contains much relevant information in conditions that may be deemed "near critical". But it was not until Zosel,working in the 1970s in the Max Planck Institute in Mulheim, Germany, used carbon dioxide to extract caffeine from coffee, and the technique went commercial, with an industrial plant inaugurated in 1978 by Kaffee HAG, that the field was really launched. The first symposium dedicated to the subject was held in Essen, Germany, in the same year, and influential books started to appear at the beginning of the 1980s, by Schneider, Wilke and Stahl and by McHugh and Krukonis. The development of the field was rapid, as it attracted researchers from many different areas. Ten years after the Essen Symposium, in 1988, a totally dedicated journal (The Journal of Supercritical Fluids) appeared, and the first International Symposium on Supercritical Fluids was held in Nice, France, with widespread international participation.

To a large extent most investigators were drawn to the area by the credentials of carbon dioxide as an environmentally safe solvent that could replace volatile organic solvents. This was the case with the first above-mentioned application, decaffeination, and in the second large scale commercial use, the extraction of hops for the beer industry. These days, most large-scale applications remain in natural products. One of the last processes to attain commercial status uses extraction volumes of the size (or bigger) of coffee decaffeination, and it cleans cork in the Spanish plant of the company Diam, described by Lack. The plant has recently undergone a duplication of capacity and the company is building a new plant in France, which might turn this process into the largest one in terms of the overall volume of the installed extractors.

Supercritical CO2 extraction has been thoroughly reviewed and explained in the 1994 book of Brunner. At about the same time, the second edition of the book of McHugh and Krukonis was published and a book edited by King and Bott described in detail extraction processes like caffeine from coffee or hops for the beer industry. With the appearance of these books, it may be said that the field attained full maturity and entered the array of well-known separation operations that can be applied to natural products and biomass.

1.2 The Role of Water in Supercritical CO2 Extraction

Extraction processes use carbon dioxide in essentially two sets of conditions: at high density (higher pressure), where it can dissolve the target solutes, and at low density (lower pressure), where it precipitates the solutes. The high compressibility of carbon dioxide at supercritical conditions allows its solvent power to be rapidly varied with relatively small changes in pressure (and with temperature, but changes in temperature are usually more difficult to implement).

Carbon dioxide is itself a bad solvent. As a small molecule with little polarity (no permanent dipole moment, a small quadrupole), it...