Carbon dioxide (CO2) is typically encountered as a gas, but the pressure required to convert it into a liquid is not a single, fixed value. Instead, the necessary pressure is entirely dependent on the specific temperature of the gas. The relationship between these two variables defines the boundary where the gas-to-liquid transition occurs. Understanding this boundary is fundamental for controlling CO2 in industrial and commercial applications.
The Critical Point
The upper temperature limit for CO2 liquefaction is the critical temperature (\(T_c\)), which is 31.1 degrees Celsius (87.98 degrees Fahrenheit). Above this temperature, CO2 cannot be turned into a liquid, regardless of the pressure applied. This temperature marks the apex of the vaporization curve, where the distinct physical properties of the liquid and gas phases merge into a single, uniform state.
The pressure at this junction is the critical pressure (\(P_c\)), which for CO2 is approximately 73.8 bar (1,070 pounds per square inch, or psi). When both the temperature and pressure exceed these critical values, carbon dioxide transforms into a supercritical fluid. This state of matter is neither a true gas nor a true liquid, exhibiting characteristics of both, such as expanding like a gas but maintaining a density comparable to a liquid.
Because the critical temperature of CO2 is close to common room temperature, it is frequently encountered in this supercritical state in industrial settings. The existence of this absolute boundary means that any practical liquefaction process must occur at temperatures below 31.1 degrees Celsius.
Understanding the CO2 Phase Diagram
The relationship between the pressure and temperature required for CO2 liquefaction is best visualized using a phase diagram, which maps the regions where the solid, liquid, and gas phases exist. The lines separating these regions represent the combinations of pressure and temperature where two phases can coexist in equilibrium. The pressure needed to liquefy the gas dramatically increases as the temperature approaches the critical point.
The phase diagram includes the triple point, where all three phases—solid, liquid, and gas—coexist simultaneously. For carbon dioxide, this occurs at a temperature of -56.6 degrees Celsius (-69.88 degrees Fahrenheit) and a pressure of 5.11 atmospheres (atm). This specific pressure is the absolute minimum required for liquid CO2 to exist.
At standard atmospheric pressure (1 atm), which is significantly below the triple point pressure, carbon dioxide cannot exist as a liquid at any temperature. This explains why solid CO2, commonly known as dry ice, skips the liquid phase entirely, transitioning directly into a gas in a process called sublimation. Dry ice sublimates at a temperature of -78.5 degrees Celsius at atmospheric pressure.
The pressure required for liquefaction changes along the vaporization curve, which extends from the triple point up to the critical point. For instance, at a temperature of 0 degrees Celsius, the pressure needed to liquefy CO2 is about 34.8 bar. This value is substantially lower than the critical pressure, illustrating the strong inverse relationship between temperature and the required liquefaction pressure.
A practical example is the liquefaction of carbon dioxide at standard room temperature, often considered to be 20 degrees Celsius (68 degrees Fahrenheit). At this temperature, the gas must be compressed to a pressure of approximately 57 atmospheres (atm), or 57.2 bar. This required pressure is nearly 57 times greater than the pressure of the atmosphere around us.
Real-World Applications of CO2 Liquefaction
Manipulating carbon dioxide into its liquid or supercritical state is essential for many commercial processes. Liquid CO2 is routinely used for bulk transportation and storage because it is far more compact than the gaseous form. It is typically stored in high-pressure cylinders or refrigerated tanks, where conditions are maintained below the critical point.
The rapid expansion of liquid CO2 is the basis for producing dry ice, which is used for cooling and refrigeration due to its extremely low temperature and clean sublimation. Liquid CO2 also serves as a solvent in applications like specialized cleaning processes and extraction.
When heated and pressurized beyond the critical point, supercritical CO2 becomes a highly versatile solvent. Its solvent power can be precisely controlled by small adjustments in pressure, making it a valuable alternative to traditional organic solvents. Supercritical CO2 is used in several industries:
- To decaffeinate coffee beans in the food industry.
- For the extraction of natural compounds in pharmaceuticals.
- For cleaning precision electronic components in manufacturing.
- For the production of specialty materials.