Analox Sensor Technology

Supercritical carbon dioxide – from coffee to dry cleaning

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Courtesy of Analox Sensor Technology

Sometimes I can’t start the day without my morning coffee, and I’m sure most adults would agree that consuming caffeine is a part of their daily routine.

A regular cup of coffee can contain 95 to 200mg of caffeine depending on what bean is used and how it has been prepared.

In moderation, caffeine can elevate your mood, reduce tiredness and make you more alert, however it can also affect you negatively as consuming too much can lead to headaches, restlessness and anxiety; so it is understandable that some coffee drinkers would rather opt for a decaffeinated version.

Methods for decaffeinating coffee have been available since the early 20th Century, and the aim is to remove caffeine without also removing the compounds that give the coffee its flavour, and not leave a toxic residue in the beans.

One of the solvents that is now widely used for decaffeination is supercritical carbon dioxide (scCO2).

Supercritical carbon dioxide is a fluid state of carbon dioxide where it is held at or above its critical temperature and critical pressure.

Carbon dioxide usually behaves as a gas in the air at standard temperature and pressure (STP), or as a solid called dry ice when frozen. If the temperature and pressure are both increased from STP to be at or above the critical point for carbon dioxide, it can adopt properties midway between a gas and a liquid.

How is supercritical carbon dioxide used to decaffeinate coffee?

The method to extract caffeine from coffee beans using carbon dioxide (CO2) and water was discovered by Kurt Zosel, a chemist at Max Planck Institute for Coal Research, Germany.

By using this method the caffeine can be dissolved, leaving the flavour intact, and once the beans are at normal pressure, any residual carbon dioxide will be lost easily, and this can be safely vented into the atmosphere.

Between 97-99% of the caffeine is removed by this method, and the steps for the scCO2  process are as follows:

  • 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 molecules from chemical complexes in the bean.
  • The water soaked coffee beans are placed in a stainless steel container called an extraction vessel. The extractor is then sealed and liquid CO2 is forced into the coffee at a high pressure to extract the caffeine.
  • The CO2 acts as a solvent to dissolve and draw the caffeine from the coffee beans, and leaves the carbohydrates and proteins behind, as they are the ones that give the coffee the flavour and smell.
  • The caffeine is recovered from the water and transferred to another container called the absorption chamber. Here the pressure is released and the CO2 returns to its gaseous state, leaving the caffeine behind.
  • The caffeine-free CO2 gas is pumped back into a pressurised container for reuse and the extracted caffeine is sold to manufacturers of soft drinks and pharmaceuticals.

What else can supercritical carbon dioxide be used for?

Supercritical carbon dioxide can also be used as a more environmentally friendly solvent for dry cleaning compared to more traditional solvents such as hydrocarbons and perchloroethylene.

The process works by placing clothes in a specialised machine, and then taking the gas form of carbon dioxide and pressurising it into a clear liquid, to act as the solvent. This is used in combination with other cleaning agents which are inserted into the machine.

Clothes are rotated in a cycle that lasts five to 15 minutes at room temperature. The liquid CO2 dissolves dirt, fats, and oils in the clothing, and at the end of the cleaning cycle, the liquid CO2 is pumped back into a storage tank to possibly be reused again, and the remaining CO2 is released in the air.

While CO2 is a main greenhouse gas, no new CO2 is generated with this technology, as the CO2 is a byproduct of other industrial processes, which is recycled into the liquid solvent for cleaning clothes. This means there is no added contribution to global warming.

Now, as well as removing dirt from clothes, and decaffeinating coffee, this type of solvent can also be used to create the masses of aromatherapy products and perfumes that are typically bought by individuals for themselves or as presents for others.

The essential oils in these products have traditionally been derived using steam distillation, or extracted using chemical solvents such as hexane, heptane or ethanol.

The CO2 extraction process consists of pumping pressurised carbon dioxide into a chamber filled with plant matter, such as Jasmine flowers.

When CO2 is subjected to pressure it becomes supercritical and has liquid properties of the gas, and functions as a solvent, pulling the oils and other substances such as pigment and resin from the plant matter.

There are several positive aspects of using CO2 as a form of extraction compared to steam distillation and solvents.

The steam distillation method produces varying oil qualities dependent upon the temperature, pressure and time used for distillation. The use of heat will also change the molecular composition of the essential oil, while the supercritical extraction process will produce an oil that has not been altered by temperature.

The most common technique for extracting aromatics in the perfume industry is using organic solvents. Solvents such as hexane extract compounds from the plant material, and after the solvent is removed, a waxy substance known as a concrete is obtained. The wax is treated with alcohol to separate it from the perfume to produce a substance called an absolute, which is mostly comprised of concentrated aromatic oil and some wax and pigments.

The CO2 extracts typically contain more components found in the plant than just the essential oil portion, so they closely resemble the original plant’s chemical make up, including the genuine aroma, so depending on what type of extraction process is used, a variation of the original smell can be produced from the same plant matter.

Protecting yourself from the risks of supercritical carbon dioxide

Despite the several uses of CO2 as a solvent, as a gas it can be dangerous in high concentrations. In November, an employee was killed in a gas leak at a coffee company in east Houston. They were using CO2 to decaffeinate coffee, and despite alarms sounding immediately when the leak happened, a man who was closest to the leak was affected.

In response to similar incidents of carbon dioxide leaks, the Analox Ax60 CO2 detector is available worldwide. The system incorporates visual strobes and audible alarms that are set to immediately trigger if the Ax60 detects a level of CO2 beyond pre-defined limits, thereby alerting management and crew to any leak of CO2 gas.

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