When choosing a solvent for a specific application, among the most important characteristics to consider are density and viscosity, says William A. Peters, associate director for fuels and environmental research at the Massachusetts Institute of Technology's Energy Laboratory. In supercritical carbon dioxide, those properties can be varied greatly by changing temperature and pressure. They give you handles you can turn readily, especially on a commercial scale. Carbon dioxide's safety makes it a choice solvent for the food industry, which already uses it on a large scale, notably for decaffeinating coffee. As coffee beans move through huge vats filled with supercritical carbon dioxide and a little water, the caffeine leaches out for later extraction and recovery.
Carbon dioxide can also remove cholesterol and fat from milk and meat.
For most industrial applications, however, operating at lower temperatures and pressures reduces energy costs, says DeSimone, making liquid carbon dioxide a better option. Carbon dioxide is cheap and abundant. Moreover, it can be collected from the waste generated by industrial processes such as ammonia manufacture, DeSimone says. No special disposal procedures are necessary because carbon dioxide can be easily recaptured and reused. Despite its rosy profile, carbon dioxide suffers from some drawbacks that have hampered its usefulness for cleaning.