The point is that substances as common as water, methanol or carbon dioxide, when subjected to pressures and temperatures above certain values, simultaneously acquire properties of liquids -they dissolve many substances- and gases – they easily penetrate into porous materials and drag out the substance that we are interested in extracting.
To understand something so rare, let’s think about how water evaporates. You have a full bucket, you put it on the fire and when it reaches 100º C it starts to boil. Now, does this mean that water always boils at 100º? No. Like all liquids, water boils at a certain pair of temperature and pressure values . Under normal conditions, with typical atmospheric pressure, the boiling temperature is what we all know. If we want to prevent the water from boiling at that temperature, what we must do is increase the pressure on it, forcing the water molecules to be so tight that none of them can escape from the surface of the liquid.
If we continue to increase the temperature we will also have to increase the pressure to prevent the water from starting to boil. This fight between the temperature, which insists on making the water boil, and the pressure, which intends the opposite, ends when a point called the critical point of water is reached. It is, so to speak, the point at which the pressure is already unable to stop the boiling. In the case of water, if the temperature rises above 374.2º C, nothing can stop it from boiling. For this specific temperature value, the pressure maintained by liquid water is 218.3 times the ordinary atmospheric pressure. These values are called critical pressure and temperature . Above them we have supercritical water . Like steam, supercritical water will occupy the entire volume of the container that contains it. But the most amazing thing is that this water dissolves substances, just like liquid water.
This that we have just described happens to all liquids, only that the value of the critical temperature and pressure depends on what it is. In the case of carbon dioxide, its critical values are 31º C and 73 times the ordinary atmospheric pressure. As supercritical fluids dissolve some substances better than others, they become the ideal assistants for obtaining, separating, purifying or treating many products . Thus, for example, supercritical methanol is used to obtain biodiesel, since it eliminates the use of a catalyst.
In industry, the most widely used is carbon dioxide , largely because it is the easiest to handle. The greatest commercial success of supercritical fluids has been in the food processing industry. In the 1970s, researchers at the German Max Planck Institute studied the possibilities of these fluids and eventually developed a technique to remove caffeine from coffee with supercritical carbon dioxide. In 1978, the first European industrial plant for decaffeination began to function, which was soon followed by one for extracting hops and another for decaffeinating tea. In the United States, supercritical fluid extraction on an industrial scale began decades later. Today the annual decaffeinated coffee market represents, in the United States alone, more than 3,000 million dollars.
Supercritical carbon dioxide is also used in the extraction of spices and aromas , to obtain valuable unsaturated fats from fish oils or in the removal of cholesterol from butter. It has even been possible to eliminate 80% of the fat and 95% of the cholesterol from eggs . And everything, using that gas that we are so afraid of because it is the source of the greenhouse effect…
Brunner, G. (2010) “Applications of Supercritical Fluids”. Annual Review of Chemical and Biomolecular Engineering . 1: 321–342. doi : 10.1146/annurev-chembioeng-073009-101311