CO2 Snow Freezing

Fundamental Principles

CO2 snow freezing utilizes solid carbon dioxide (dry ice) particles produced through rapid expansion of liquid CO2. The system exploits the sublimation phase transition where solid CO2 converts directly to gas at atmospheric pressure without passing through a liquid phase.

Phase Behavior

Carbon dioxide exhibits unique phase characteristics critical to freezing applications:

Critical Properties:

• Critical temperature: 31.1°C (87.98°F)
• Critical pressure: 7.38 MPa (1,071 psia)
• Triple point temperature: -56.6°C (-69.9°F)
• Triple point pressure: 0.518 MPa (75.1 psia)
• Sublimation temperature at 1 atm: -78.5°C (-109.3°F)

At atmospheric pressure, CO2 cannot exist as a liquid. Liquid CO2 stored at approximately 2.0 MPa (290 psia) and -20°C (-4°F) rapidly converts to solid snow and vapor when expanded through a nozzle to atmospheric conditions.

Snow Formation Mechanism

The expansion process from liquid to solid follows the Joule-Thomson effect with significant cooling:

Mass Distribution: Approximately 45-50% of liquid CO2 converts to solid snow particles, with the remainder forming cold vapor at -78.5°C.

Energy Balance:

h_liquid = h_solid + h_vapor

Where:
h_liquid = enthalpy of liquid CO2 at storage conditions (≈ 150 kJ/kg)
h_solid = enthalpy of solid CO2 at -78.5°C (≈ 25 kJ/kg)
h_vapor = enthalpy of CO2 vapor at -78.5°C (≈ 575 kJ/kg)

The latent heat of sublimation for solid CO2 is 571 kJ/kg (245.4 BTU/lb), providing substantial cooling capacity.

Sublimation Cooling Mechanism

Solid CO2 particles provide cooling through two mechanisms operating simultaneously:

Primary Cooling: Sublimation

The dominant cooling effect occurs as solid CO2 sublimes directly to vapor:

Heat Absorption:

Q_sublimation = m_CO2 × ΔH_sublimation

Where:
Q_sublimation = cooling capacity (kJ)
m_CO2 = mass of CO2 snow (kg)
ΔH_sublimation = 571 kJ/kg

Cooling Rate: The sublimation rate depends on:

• Surface area of snow particles (particle size distribution)
• Temperature differential between product and CO2
• Air velocity over snow surface
• Relative humidity of surrounding atmosphere

Secondary Cooling: Sensible Heat

Additional cooling occurs as CO2 vapor warms from -78.5°C to ambient temperature:

Sensible Heat Absorption:

Q_sensible = m_CO2 × c_p,vapor × ΔT

Where:
c_p,vapor = specific heat of CO2 vapor ≈ 0.85 kJ/kg·K
ΔT = temperature rise from -78.5°C to ambient

For vapor warming from -78.5°C to 20°C:

Q_sensible = m_CO2 × 0.85 × 98.5 = 83.7 kJ/kg

This represents approximately 15% additional cooling capacity beyond sublimation.

Total Cooling Capacity:

Q_total = 571 + 83.7 = 654.7 kJ/kg (281.4 BTU/lb)

Snow Production Equipment

Expansion Nozzles

CO2 snow generators employ specialized nozzle designs to optimize solid phase conversion:

Nozzle Types:

Design Parameters:

• Inlet pressure: 1.8-2.2 MPa (260-320 psia)
• Outlet pressure: 0.101 MPa (14.7 psia)
• Expansion velocity: 200-400 m/s
• Nozzle throat diameter: 2-10 mm
• Discharge angle: 15-60° depending on application

Snow Horns and Applicators

Horn Design:

• Conical expansion chamber following nozzle
• Insulated construction to prevent ice buildup
• Snow separation from vapor stream
• Adjustable discharge pattern

Typical Dimensions:

• Entry diameter: 25-50 mm
• Exit diameter: 100-300 mm
• Length: 300-600 mm
• Expansion angle: 15-30°

Top Icing Applications

Top icing applies CO2 snow directly to product surfaces for temperature reduction and preservation during transport or processing.

Application Methods

Direct Snow Discharge: CO2 snow sprayed directly onto product surface through horn assemblies positioned above conveyor or container.

Coverage Rate:

Application rate = 0.5-2.0 kg CO2/m² product surface

Typical settings:
- Light icing: 0.5-0.8 kg/m²
- Standard icing: 0.8-1.2 kg/m²
- Heavy icing: 1.2-2.0 kg/m²

Temperature Depression:

ΔT = (m_CO2 × Q_total) / (m_product × c_p,product)

Where:
ΔT = temperature change (°C)
m_CO2 = CO2 mass applied (kg)
m_product = product mass (kg)
c_p,product = specific heat of product (kJ/kg·K)
Q_total = 654.7 kJ/kg

Product Categories

Fresh Produce:

• Temperature maintenance: -1 to 4°C
• Application: 0.5-1.0 kg CO2/kg produce
• Typical products: lettuce, berries, grapes
• Residence time: minimal (flash cooling)

Bakery Products:

• Target temperature: -18 to -10°C
• Application: 0.8-1.5 kg CO2/kg product
• Products: bread, rolls, pastries
• Benefits: prevents staling, moisture retention

Meat and Poultry:

• Temperature range: -2 to 2°C
• Application: 1.0-2.0 kg CO2/kg product
• Products: ground meat, poultry parts, sausages
• Critical: surface freezing without deep penetration

System Configuration

Conveyor-Based Systems:

• Belt speed: 2-15 m/min
• Snow horn spacing: 300-600 mm
• Number of application zones: 1-4
• Enclosure for vapor containment

Box/Container Icing:

• Manual or automated snow injection
• Dosing: 0.5-3.0 kg per container
• Application time: 5-30 seconds
• Vapor venting provisions required

Tumbler Freezing Systems

Tumbler systems continuously mix product with CO2 snow for uniform temperature reduction and surface freezing.

Equipment Design

Rotating Drum Configuration:

• Drum diameter: 0.5-2.0 m
• Length: 1.0-4.0 m
• Rotation speed: 5-20 rpm
• Internal flights for product lifting
• Insulated construction

CO2 Injection System:

• Multiple injection points along drum length
• Radial or axial injection patterns
• Flow control for each injection point
• Snow distribution manifold

Operating Parameters

Residence Time:

t_residence = L_drum / (v_axial × 60)

Where:
L_drum = drum length (m)
v_axial = axial velocity (m/min)

Typical range: 2-10 minutes

Mixing Intensity: Froude number characterizes mixing:

Fr = ω² × R / g

Where:
ω = angular velocity (rad/s)
R = drum radius (m)
g = gravitational acceleration (9.81 m/s²)

Optimal Fr = 0.3-0.7

Fill Level:

• Typical: 20-40% of drum volume
• Higher fill reduces mixing efficiency
• Lower fill increases residence time variability

Application Categories

Diced/Chopped Products:

• Product size: 5-25 mm cubes
• CO2 ratio: 0.3-0.6 kg CO2/kg product
• Residence time: 3-6 minutes
• Examples: diced vegetables, fruit pieces

Ground Products:

• Particle size: 3-10 mm
• CO2 ratio: 0.4-0.8 kg CO2/kg product
• Residence time: 4-8 minutes
• Examples: ground meat, shredded cheese

Coated Products:

• Product size: varies
• CO2 ratio: 0.5-1.0 kg CO2/kg product
• Residence time: 5-10 minutes
• Purpose: crust freezing, coating solidification

Temperature Profile

Typical Freezing Progression:

CO2 Consumption Rates

Theoretical Consumption

Minimum CO2 required based on thermodynamic calculations:

m_CO2,min = (m_product × c_p × ΔT) / Q_total

Where:
m_CO2,min = minimum CO2 mass (kg)
m_product = product mass (kg)
c_p = specific heat of product (kJ/kg·K)
ΔT = temperature change required (K)
Q_total = 654.7 kJ/kg

Example Calculation: Freezing 1000 kg of ground beef from +5°C to -18°C:

c_p,beef = 3.5 kJ/kg·K (above freezing) and 1.8 kJ/kg·K (below freezing)
Latent heat = 250 kJ/kg

Q_required = (1000 × 3.5 × 5) + (1000 × 250) + (1000 × 1.8 × 18)
Q_required = 17,500 + 250,000 + 32,400 = 299,900 kJ

m_CO2,min = 299,900 / 654.7 = 458 kg

Actual Consumption

Real-world consumption includes inefficiencies:

Loss Factors:

Efficiency Factor:

η_system = 0.50-0.75 (typical range)

m_CO2,actual = m_CO2,min / η_system

Practical Ratios:

Consumption Optimization

Strategies to Improve Efficiency:

1. Pre-cooling: Reduce product inlet temperature using mechanical refrigeration

   • Effect: 10-20% reduction in CO2 usage
   • Break-even: typically 3-6 months

2. Vapor Containment: Minimize escape of cold CO2 vapor

   • Enclosure design with controlled venting
   • Effect: 5-15% improvement

3. Snow Particle Size Control: Optimize for maximum product contact

   • Smaller particles: better distribution but faster sublimation
   • Optimal range: 50-150 μm

4. Injection Staging: Apply CO2 in multiple zones with decreasing rates

   • Matches cooling load profile
   • Effect: 8-15% improvement

Equipment Specifications

CO2 Supply Systems

Bulk Storage Tanks:

Tank Features:

• Vacuum-insulated construction
• Pressure building coil for vapor withdrawal
• Liquid withdrawal dip tube
• Pressure relief valves (set at 2.4 MPa)
• Level gauges and telemetry

Distribution System:

• Piping: Schedule 80 steel or stainless steel
• Insulation: cellular glass or polyurethane foam
• Heat tracing not required (self-refrigerating)
• Pressure regulators at point of use
• Flow meters for consumption monitoring

Snow Generation Units

Capacity Ratings:

Control Systems:

• PLC-based process control
• Mass flow control with feedback
• Product temperature monitoring
• Automated dosing algorithms
• Data logging and SCADA integration

Material Specifications

Construction Materials:

Safety Considerations

Asphyxiation Hazards

CO2 displaces oxygen in confined spaces, creating serious asphyxiation risk:

Physiological Effects by Concentration:

Ventilation Requirements:

Q_vent = (G_CO2 × 10⁶) / (C_limit × 60)

Where:
Q_vent = ventilation rate (m³/hr)
G_CO2 = CO2 release rate (kg/hr)
C_limit = acceptable CO2 concentration (ppm)
Factor 10 provides safety margin

Example:
For G_CO2 = 100 kg/hr, C_limit = 5,000 ppm:
Q_vent = (100 × 10⁶) / (5,000 × 60) = 333,333 m³/hr minimum

Safety Measures:

• Continuous CO2 monitoring with alarms
• Low-oxygen sensors with automatic shutdown
• Mechanical ventilation with backup power
• Area access controls and warning signage
• Emergency procedures and training

Cold Contact Hazards

Solid CO2 at -78.5°C causes severe cold burns on contact:

Exposure Limits:

• Brief contact (<1 second): minor discomfort
• Contact 1-5 seconds: frostbite likely
• Contact >5 seconds: severe tissue damage

Protection Requirements:

• Insulated gloves (rated to -80°C minimum)
• Face shields for direct handling
• Protective aprons for bulk handling
• Closed-toe safety shoes
• Long sleeves and full-length pants

Pressure Hazards

Vessel Rupture Risk: CO2 expands significantly when warming from liquid to gas:

Expansion ratio = 535:1 (liquid to gas at 15°C, 1 atm)

A sealed container partially filled with liquid CO2 will develop extreme pressure if warmed:

Pressure Rise:

For fully constrained liquid CO2:
ΔP/ΔT ≈ 1.4 MPa/°C near critical point

Safety Provisions:

• Pressure relief devices on all vessels
• Burst discs sized for maximum credible release
• Thermal relief valves on isolated piping
• Operator training on pressure hazards
• Regular inspection and testing

Regulatory Compliance

OSHA Requirements:

• 29 CFR 1910.146: Permit-required confined spaces
• 29 CFR 1910.134: Respiratory protection
• 29 CFR 1910.1000: Air contaminants (CO2 PEL: 5,000 ppm TWA)

FDA Requirements:

• 21 CFR 184.1240: CO2 GRAS status for food contact
• Current Good Manufacturing Practices (cGMP)
• HACCP plan integration

Building Codes:

• IBC Chapter 53: Compressed gases (CO2 is Group A-1)
• IFC Chapter 53: Operational permits required
• NFPA 55: Compressed gases and cryogenic fluids

Comparison with Liquid Nitrogen (LN2)

Thermodynamic Comparison

Fundamental Properties:

Cooling Efficiency: Despite lower temperature, LN2 provides less total cooling per unit mass due to lower latent heat. CO2 delivers approximately 67% more cooling capacity per kilogram.

Economic Comparison

Cost Analysis (typical industrial pricing):

Break-Even Analysis: For typical food processing applications, CO2 snow offers 40-60% lower operating costs than LN2 when temperature requirements permit (-80 to -40°C range).

Performance Comparison

Freezing Rate:

LN2’s lower temperature produces faster surface freezing:

Freezing time ratio: t_CO2 / t_LN2 ≈ 1.5-2.0 for surface crust formation

However, for bulk temperature reduction, CO2’s higher total cooling capacity can achieve similar results with proper application.

Temperature Capability:

Product Quality:

• CO2: Gentler temperature gradient reduces thermal stress, better for delicate products
• LN2: Faster freezing improves ice crystal structure, better for high-water products

Operational Considerations

Handling Characteristics:

Safety Profile:

• CO2: Higher asphyxiation risk (heavier than air, accumulates low)
• LN2: More severe cold burns, lower asphyxiation risk (lighter than air)
• CO2: Less dramatic vapor plumes (warmer vapor)
• LN2: Extensive vapor generation creates visibility challenges

Application Selection Criteria

Choose CO2 Snow when:

• Target temperatures: -80 to -30°C
• Cost is primary driver
• Tumbler/mixing application required
• Solid phase application preferred
• High total cooling load per unit time
• Gentler freezing rate acceptable

Choose LN2 when:

• Ultra-low temperatures required (<-80°C)
• Fastest possible freezing essential
• Cryogenic grinding application
• Precise liquid dosing needed
• Minimal equipment footprint required
• Very small-scale application

Applications

Meat and Poultry Processing

Ground Meat Chilling:

• Application: tumbler freezing
• CO2 ratio: 0.3-0.5 kg/kg product
• Temperature: +5 to -2°C
• Benefits: prevents bacterial growth, maintains texture, extends shelf life

Poultry Parts Surface Freezing:

• Application: conveyor top icing
• CO2 ratio: 0.4-0.7 kg/kg product
• Temperature: +4 to -5°C surface crust
• Benefits: prevents moisture loss, improves handling

Sausage and Formed Products:

• Application: tumbler crust freezing
• CO2 ratio: 0.6-1.0 kg/kg product
• Temperature: -15 to -25°C surface
• Benefits: enables slicing, prevents smearing

Bakery Products

Bread and Roll Freezing:

• Application: conveyor or batch system
• CO2 ratio: 0.8-1.2 kg/kg product
• Temperature: +25 to -18°C
• Benefits: rapid freezing prevents staling, maintains structure

Pastry and Dough:

• Application: top icing
• CO2 ratio: 0.5-0.9 kg/kg product
• Temperature: +5 to -10°C
• Benefits: prevents spreading, maintains shape

Produce and Prepared Foods

Fresh-Cut Vegetables:

• Application: top icing during packaging
• CO2 ratio: 0.2-0.4 kg/kg product
• Temperature: +10 to 0°C
• Benefits: extends shelf life, maintains crispness

Prepared Meals:

• Application: post-filling crust freeze
• CO2 ratio: 0.7-1.3 kg/kg product
• Temperature: +70 to -18°C
• Benefits: prevents moisture migration, enables handling

Limitations

Temperature Constraints

Minimum Temperature Achievable: Product surface cannot reliably reach temperatures below -60°C due to:

• CO2 vapor temperature of -78.5°C
• Heat transfer resistance from product to snow
• Sublimation rate limitations

Applications Excluded:

• Cryogenic grinding (requires <-100°C)
• Ultra-low temperature storage
• Specialty products requiring <-80°C

Product Contact Considerations

Direct Contact Concerns:

1. Surface Moisture Freezing:

   • Snow particles stick to wet surfaces
   • Can create handling issues
   • May affect product appearance

2. Carbonation Effects:

   • CO2 dissolves in liquid water and fats
   • Creates slight pH reduction
   • Can affect flavor in sensitive products

3. Surface Desiccation:

   • Sublimation removes moisture from product surface
   • More pronounced than LN2 due to longer contact time
   • Requires monitoring on delicate products

Equipment Complexity

System Requirements:

• Bulk CO2 storage with pressure maintenance
• Snow generation and distribution equipment
• Vapor containment and ventilation
• Control systems for dosing
• More complex than LN2 liquid spray

Maintenance:

• Snow horn cleaning (product buildup)
• Nozzle replacement (erosion/plugging)
• Valve servicing (low temperature operation)
• More intensive than LN2 systems

Operational Limitations

Efficiency Sensitivity:

• Lower efficiency than LN2 in poorly designed systems
• Requires good snow-to-product contact
• Vapor losses can be significant without enclosure
• Performance varies with ambient conditions

Capacity Constraints:

• Bulk snow production rate limited by pressure drop
• Not suitable for very high-capacity continuous operations exceeding 5,000 kg/hr product throughput
• LN2 spray tunnels better for ultra-high volume

Regulatory and Quality Considerations

Food Safety:

• CO2 source must be food-grade (beverage quality)
• Impurities can affect product quality
• Requires verification of supplier specifications

Process Validation:

• Temperature distribution monitoring required
• Validation more complex than single-nozzle LN2 spray
• HACCP critical control point determination

Product Labeling:

• Modified atmosphere implications
• CO2 treatment disclosure requirements vary by jurisdiction
• Organic certification considerations

Related Topics:

• Cryogenic Freezing Fundamentals
• Liquid Nitrogen Freezing Systems
• Mechanical vs. Cryogenic Freezing Comparison
• Food Processing Refrigeration Load Calculations