Greener Industry - Supercritical Carbon Dioxide

Methods for decaffeinating coffee have been available since the early 20th Century. The aim is to remove caffeine without also removing the compounds that give the coffee its flavour, and without leaving a toxic residue in the beans. Caffeine extraction generally takes place on "green" beans, before roasting. Extracted caffeine is sold to manufacturers of soft drinks and pharmaceuticals.
Several different solvents have been used:

Dichloromethane (CH₂Cl₂)	Removes caffeine but very little flavouring	A suspected carcinogen - not generally used now
Ethyl Ethanoate	Removes some flavouring	Still mildly toxic, even though it occurs naturally in some fruits
Water	Non-toxic	Complex method required to avoid removing flavour
Supercritical carbon dioxide	Selectively removes caffeine and very little flavouring	Little or no residue left in beans - no concern over toxicity

The Supercritical Solution
Supercritical carbon dioxide (scCO₂) is now widely used for decaffeination. It was one of the first commercial applications for scCO₂ and has been in use since the 1980s. Advantages over other methods include:

Precise control of pressure can selectively dissolve the caffeine, leaving the flavour intact
Once the beans are at normal pressure, any residual carbon dioxide will be lost easily, and this can safely be vented into the atmosphere
Between 97-99% of the caffeine is removed by this method

The Process
Green coffee beans (before roasting) are first soaked in water to make them swell and allow the scCO₂ to penetrate more easily. It is also thought water may be needed to help free the caffeine from chemical complexes in the bean.

ScCO₂ is circulated for up to 10 hours through pre-soaked beans in a high-pressure extraction chamber. In the second vessel, water and the caffeine-rich carbon dioxide are passed in opposite directions and the caffeine dissolves in the water. The carbon dioxide is re-pressurised and re-used.

Caffeine is recovered from the water as a concentrated solution using one of a range of processes, including reverse osmosis. In reverse osmosis pressure is used to make solvent pass through a semi-permeable membrane from high to low concentration of solute, rather than the other way round. This concentrates the caffeine further.