SO, WHAT DID YOU LEARN ABOUT CO2 AT SCHOOL?
CARBON DIOXIDE (CO2) IS A POPULAR (NOTORIOUS, ACTUALLY!) GAS AND EVERYONE SEEMS TO BE TALKING ABOUT IT. WE HEAR A LOT ABOUT IT IN THE CONTEXT OF GREENHOUSE EFFECT AND CLIMATE CHANGE. BUT DID YOU KNOW THAT BEYOND THE REGULAR PRESENCE IN THE ATMOSPHERE, CO2 IS STEADILY GAINING POPULARITY IN A NUMBER OF INDUSTRIAL APPLICATIONS?
For long, CO2 has been used in its solid form (also known as dry ice) as a cooling agent. Dry ice has an advantage over regular ice. Wondering what? Well, for starters, it’s dry and therefore, it sublimes from solid state directly to gaseous state without any puddles or mess. Recently, “Supercritical” CO2 has been making waves.
What is Supercritical CO2?
To understand the concept of ‘supercriticality’, let us quickly look at a few laws of physics. We all know that matter has three basic states: solid, liquid and gaseous. Gases differ from the other states in one very specific and important way — they lack a specific volume. This means that gas takes up the volume of the container it has been placed in. Now, the two factors that affect the state of gas are pressure and temperature. Increasing pressure and decreasing temperature can convert gas into liquid (and further to solid).
Let us look at an example. The LPG cylinder in your kitchen contains butane, a gas at regular pressure (called ‘one atmosphere pressure’). But due to the pressure inside the cylinder, the gas turns into a liquid. Conversely, decreasing pressure or increasing temperature of a liquid turns it into gas. Oh, here’s an example from everyday. Boiling of water and its conversion into steam!
Now, ‘critical point’ for a substance is one where at particular temperature and pressure, the boundaries between states cease to exist. Beyond the critical point, the substance is said to be in a critical state, where it shows properties of both liquid and gas. Every fluid (liquid or gas) has its own critical point and with varying temperature and pressure levels, it can be
brought to a supercritical level.
Supercritical substances do occur naturally too, most commonly near the undersea volcanoes. The high pressures at sea beds, which are at the depths of over 2-3 kilometres, set the water into its supercritical state.
Putting Supercritical CO2 to use
The use of supercritical CO2 started with coffee manufacturers looking for solvents for decaffeination. The basic process of decaffeination is to pass a solvent through coffee — the caffeine gets dissolved in the solvent and gets removed. Now coffee is a highly complex substance chemically, containing many compounds which give it its distinct aroma, flavour, colour etc. The trick is not to damage any of these compounds while removing the caffeine. Supercritical CO2 has replaced the solvents which were earlier used. The advantage that it provides is that it does not affect any other compounds and safely remove the caffeine.
2. Industrial solvent
Another characteristic that supercritical fluids show is that with varying pressure, the solubility of the fluid changes. Hence, tuning the pressure levels allow the separation of specific compounds. This property is making supercritical CO2 popular as an industrial solvent. The fact that it is lower on toxicity as compared to most of the already existing industrial solvents makes it a much more environmentally friendly option. The operating temperature of Supercritical CO2 is very low at about 30 degrees Celsius. This makes it very popular in a wide range of industries — perfume extraction, pharmaceuticals, food industry as well as electronic chips manufacturing.
3. Thermal power generation
Most of the modern power generation is done though turbines which are rotated using steam. Replacing steam with Supercritical CO2 provides for much better efficiency levels for power generators reducing the amount of fuel used per unit of electricity generated, leading to reduction in emissions.
4. Compressor gas
CO2 is also being seen as the future compressor gas in air-conditioning and refrigeration units that currently use either the ozone depleting Chlorofluorocarbons (which are also many times more greenhouse than CO2) or hydrocarbons like iso-butane. This usage is especially being promoted in car air conditioning and it has been tested to show superior performance
over current refrigerants apart from being cheaper.
5. Sterilisation agent
Another new application being discovered for Supercritical CO2 is in the medical industry where it is used for sterilisation of medical equipment.
6. Cleaning agent
Finally, CO2 can also be used as a cleaning agent especially for dry cleaning, replacing many chemicals which are derived from petroleum and are largely toxic.