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Supercritical Way to Remove Caffeine
Coffee is an American favorite....perhaps a worldwide favorite. Every morning, people everywhere enjoy a fresh cup of coffee to help them start the day. A few years ago, people would shy away from coffee which had been decaffeinated. It seemed to lack the robust flavor and smell that regular coffee had. With the introduction of "naturally" decaffeinated coffee, it was given a second chance to succeed in the market place. It was indeed very successful. Suddenly, people had difficulty telling the difference between coffee that had been decaffeinated and that which still contained its usual amount of caffeine. What happened? How did engineers pull off this seemingly impossible task?
While some people may have started to believe that by removing the caffeine, engineers were destroying the quality of the coffee. While this is not entirely untrue, it was not the absence of the caffeine that was altering the coffee, it was the absence of the flavorful and aromatic compounds that lower the coffee's appeal.
Traditionally, caffeine was removed with a solvent called methylene chloride. Methylene chloride was contacted with the coffee and the caffeine was dissolved and removed later. Unfortunately, many of the other compounds in coffee (those responsible for its flavor and smell) were also soluble in methylene chloride. Thus, during decaffeination, engineers were "killing" the coffee.
In addition to the coffee quality concerns, these was additional motivation to replace methylene chloride as a solvent. Most governments began targeting environmentally hazardous solvents (such as methylene chloride) as the subjects of tight regulations. Not to mention concerns about residual solvent being left in the coffee.
In the search for another alternative, the use of solvents termed "supercritical" solvents looked like an attractive solution. "Supercritical" refers to solvents that are at temperatures and pressures beyond their critical points. For example, the critical pressure of a substance is the highest pressure at which the substance can exist as a true liquid, no matter what the temperature. However, the gases at supercritical conditions are nearly as dense as some liquids due to the extreme temperature and pressure. This makes these gases ideal for solvents.
The most popular supercritical fluid (gas) is supercritical carbon dioxide. The reason for its popularity is it's relatively mild critical points of 31.1 0C and 73.9 bars. It is also abundant and mostly benign. Other solvents, such as water, have much higher critical points (374 0C and 222.3 bars). Thus it would cost much more to use water as a supercritical fluid because larger compressors and thicker vessels would be necessary. Luckily for engineers, they also found that the most popular supercritical solvent (carbon dioxide) also had another advantage in the decaffeination process. While it easily removed most of the caffeine from the raw coffee beans, the aromatic compounds responsible for taste and smell were largely left in the beans.
The raw beans are placed into a batch decaffeinator vessel. Supercritical carbon dioxide is run through the beans for about 10 hours, which removed nearly 97% of the caffeine. The caffeine-rich carbon dioxide is contacted with water in he caffeine scrubber. This transfers nearly all of the caffeine in the carbon dioxide to the water. The caffeine is easily removed from the water in a reverse osmosis unit. Once the carbon dioxide is cleaned of caffeine, it can be recycled and the same can be said for the water.