How to Make Liquid CO2

Carbon dioxide (CO2) is a gas at standard temperature and pressure and comprises about 0.04 percent of the Earth's atmosphere. Liquid carbon dioxide is more easily transported than carbon dioxide gas, and has applications in refrigeration and industrial chemistry. It's an unusual compound in that solid carbon dioxide sublimates (evaporates) directly from the solid to the gaseous state under standard conditions. Carbon dioxide liquefaction requires repeated applications of cooling and pressure.

Difficulty: Moderate

Instructions

1. Identify the critical temperature for carbon dioxide as 88 degrees Fahrenheit. This is the maximum temperature at which carbon dioxide can be a liquid. The process of liquefying carbon dioxide must therefore keep the liquid below 88 degrees Fahrenheit.

2. Determine the minimum pressure needed to keep carbon dioxide a liquid. Carbon dioxide must be pressurized to at least 5.1 atm to remain a liquid, no matter how cold it is.

3. Compress carbon dioxide to about 56 atm at room temperature. This method is the simplest to demonstrate, but is also the most inefficient and is therefore rarely used commercially.

4. Make the carbon dioxide work against some external force. Cool the carbon dioxide and drive it through a turbine. The gas loses energy as it performs work and its temperature drops further. The gas can eventually lose enough energy to become a liquid.

5. Use the Joule-Thomson effect to turn carbon dioxide gas into a liquid. Pump the gas into a container under pressure through a one-way valve. Release the valve so that the gas expands, cooling as it does so. Repeat this process until the gas becomes a liquid.

Tips & Warnings

• The liquification of carbon dioxide requires very high pressure and should only be attempted by an expert with professional equipment.

References

• Argonne National Laboratory

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