At What Pressure Does CO₂ Liquify?

CO₂ liquefies at pressures ranging from 5.11 bar (at -56.6°C) to 73.8 bar (at 31.1°C), with the exact pressure requirement depending on temperature. Above 31.1°C, no amount of pressure can liquefy CO₂ as it becomes a supercritical fluid.

In this guide, we’ll explore the specific pressure requirements for CO₂ liquefaction at different temperatures. You’ll then learn about the critical factors that affect these requirements and what this means for industrial gas applications.

Understanding CO₂’s Critical and Triple Points

The ability to liquefy CO₂ is fundamentally limited by its critical point at 31.1°C and 73.8 bar. Beyond this temperature and pressure combination, CO₂ transforms into a supercritical fluid where liquid and gas phases become indistinguishable.

This supercritical state has unique properties  – the fluid maintains high density like a liquid while flowing like a gas.

At the other extreme, CO₂’s triple point occurs at -56.6°C and 5.11 bar. This is where solid, liquid, and gas phases coexist simultaneously.

Below 5.11 bar pressure, liquid CO₂ cannot exist at any temperature. Instead, solid CO₂ (dry ice) sublimes directly to gas, which is why dry ice doesn’t melt into a liquid at normal atmospheric pressure.

Temperature-Dependent Pressure Requirements

The pressure needed to liquefy CO₂ increases dramatically as temperature rises toward the critical point. At -50°C, only 6.82 bar is required for liquefaction.

Moving to 0°C, the required pressure jumps to 34.8 bar – a fivefold increase for a 50-degree temperature rise.

At room temperature (20°C), CO₂ requires 57.2 bar to remain liquid. Industrial systems often operate at 70-80 bar to provide safety margins and account for temperature fluctuations.

Near the critical temperature at 30°C, the pressure requirement reaches 72.8 bar. This steep pressure curve near the critical point makes temperature control crucial for efficient liquefaction operations.

Impact of Impurities on Liquefaction Pressure

While pure CO₂ has well-defined liquefaction pressures, real-world CO₂ streams often contain impurities that increase pressure requirements. For example, amine-captured CO₂ typically requires 10-15% higher pressure than pure CO₂ due to residual moisture and solvents.

Membrane-captured CO₂ presents even greater challenges. Contaminants can increase liquefaction costs by up to 34% by necessitating additional compression and purge systems.

Common impurities include:

• Nitrogen
• Oxygen
• Various hydrocarbons from industrial processes

Each contaminant affects the phase behavior differently, making purification an important consideration for efficient liquefaction.

California’s industrial facilities often deal with varying impurity levels depending on their CO₂ source (fermentation, combustion, or chemical processes).

Safety Considerations in Industrial Settings

Industrial CO₂ liquefaction systems must account for both operational efficiency and safety requirements. At 25°C, systems typically operate at 70-80 bar rather than the minimum 64 bar to handle temperature variations and impurities.

Low-temperature storage offers an alternative approach. Cooling CO₂ to -20°C reduces the required pressure to only 20 bar, though this requires refrigeration equipment and energy.

Safety becomes critical at elevated temperatures. At 80°C, CO₂ exists only as a supercritical fluid or gas, and system pressures can exceed 97 bar, requiring comprehensive pressure relief systems and appropriate materials.

Finally, storage vessels must use stainless steel or specialized alloys to withstand these high pressures while preventing corrosion. Regular pressure monitoring and maintenance are essential for safe operations.

Understand CO₂ Phase Behavior for Effective Use

CO₂ liquefaction requires careful balance of temperature and pressure, with industrial operations typically running between 20-80 bar depending on specific conditions.

Understanding these requirements helps optimize system design, energy efficiency, and safety protocols for industrial gas applications where effective liquid CO₂ storage and handling are critical.