How Cold is Liquid CO₂?

The temperature range of CO₂ is  between -56.6°C and 31.1°C, but only under specific pressure conditions above 5.11 atmospheres. Unlike water, which remains liquid at atmospheric pressure across a wide temperature spectrum, liquid carbon dioxide requires carefully controlled pressure and temperature to prevent it from turning into solid dry ice or gas.

In this guide, we’ll explore the precise temperature boundaries that define liquid CO₂. You’ll also learn how CO₂’s temperature affects its properties and why these thermal characteristics matter for industrial applications.

What Temperature Range Enables Liquid CO₂?

Liquid CO₂ can only exist between two critical temperature boundaries: the triple point at -56.6°C and the critical point at 31.1°C. Below the triple point temperature, CO₂ cannot form a liquid phase regardless of pressure conditions.

The triple point is the only temperature and pressure combination where solid, liquid, and gaseous CO₂ coexist in equilibrium. This happens at exactly -56.6°C and 5.11 atmospheres of pressure.

Below this pressure threshold, solid CO₂ sublimes directly to vapor at -78.5°C under standard atmospheric conditions, which is why we see dry ice “smoke” without leaving liquid residue.

At the upper boundary, the critical point occurs at 31.1°C and 72.8 atmospheres. Beyond this temperature, the distinction between liquid and gas phases disappears entirely, creating a supercritical fluid.

How Does Temperature Affect Liquid CO₂ Properties?

Temperature dramatically influences the physical properties of liquid CO₂, especially its density. At -50°C, liquid CO₂ has a density of 1,156 kg/m³, but this drops to just 598 kg/m³ at 30°C—a 48% reduction.

This density relationship follows a nonlinear pattern, with more dramatic changes occurring near the critical temperature. For example, a 10°C increase from -30°C to -20°C reduces density by only 4.2%, while the same temperature rise from 20°C to 30°C causes a 22.6% density decline.

These density variations have direct practical implications for storage and handling systems. A tank filled with liquid CO₂ at -30°C will experience 103% volumetric expansion if warmed to 30°C without pressure relief, potentially exceeding design limits and creating safety hazards.

Thermal conductivity also varies with temperature, peaking at -30°C (0.112 W/m·K) before declining to 0.087 W/m·K at 20°C. This affects heat transfer efficiency in cooling systems, where lower temperatures enhance heat conduction but require more energy to maintain due to higher density.

Finally, the latent heat of vaporization decreases from 571 kJ/kg at -78.5°C to nearly zero at the critical point. This means that pressure-release refrigeration systems become less effective at higher temperatures, yielding 25% less cooling capacity per unit mass when operating at 20°C compared to -20°C.

What CO₂ Uses Require Strict Temperature Control?

Low-temperature processing applications typically operate liquid CO₂ between -50°C and -30°C to maximize thermal mass transfer capabilities. At these temperatures, the high density (1,000-1,100 kg/m³) and low viscosity (0.115-0.119 mm²/s) make it ideal for processes like:

• Food freezing
• Pharmaceutical lyophilization
• Metal treatment processes

The phase change during depressurization generates dry ice snow at -78.5°C, which is used for specialized processes like aerospace component testing and medical sample preservation. This extreme cold provides rapid temperature reduction for applications requiring precise thermal shock or cryogenic preservation.

Near-critical liquid CO₂ operating between 20-30°C functions as a tunable solvent in extraction processes. Temperature adjustments within this narrow range modify solvent density by 22%, altering polarity and solvation power without requiring chemical additives.

This thermal tuning enables selective compound isolation in applications like caffeine extraction, which preferentially occurs at 28°C and 60 atmospheres while preserving volatile compounds. System designs must maintain isothermal conditions within ±0.5°C to prevent unintended supercritical transitions that would disrupt separation efficiency.

What CO₂ Safety and Handling Considerations Should You Keep In Mind?

One of the key CO₂ handling considerations is material compatibility. It becomes critical when working with liquid CO₂ at low temperatures due to ductile-brittle transition effects in standard steels.

Carbon steel equipment requires specialized grades resistant to embrittlement below -45°C, as standard materials exhibit fracture toughness reduction below -29°C.

Using automated pressure-temperature compensation systems is another best practice, as it prevents unintended phase transitions during storage and transfer operations.

These systems use proportional-integral-derivative controllers to maintain ±0.5°C stability, accounting for non-linear property changes near critical conditions.

For example, a storage tank at 15°C requires approximately 50 atmospheres pressure stabilization, while 25°C operation demands 64 atmospheres to avoid vapor bubble formation. Pressure relief valves must incorporate dual-phase sensing to distinguish between normal vapor pressure increases and hazardous overfilling conditions.

Insulation systems combine vacuum-jacketed piping with multilayer wraps to maintain stable temperatures, achieving thermal losses below 0.4 W/m·K even in ambient environments. This prevents temperature fluctuations that could trigger solidification or create pressure variations exceeding safe operating limits.

Finally, real-time density monitoring enables automatic composition correction during transfer operations, ensuring consistent filling ratios across temperature variations. This monitoring becomes especially important near critical conditions where small temperature changes cause large property variations.

Understand CO₂’s Unique Temperature Profile

Liquid CO₂’s narrow temperature window between -56.6°C and 31.1°C creates both opportunities and challenges for industrial applications. Understanding these boundaries and their effects on physical properties enables better system design and safer handling procedures.

As industries continue developing more sophisticated CO₂-based processes, precise temperature control will remain essential for maximizing efficiency while maintaining safety.

Tyler O'Brien

Tyler is a results-driven marketing professional specializing in the industrial gases and equipment industry, bringing his 10 years of technical expertise and digital marketing acumen to the complex industrial gas B2B environment.