Preparation of Dry Ice (Solid CO₂)

Dry ice is solid carbon dioxide (CO₂). It is produced commercially on a large scale and can also be made in small quantities in laboratories.

Industrial / Commercial Production Method

1. Purification of CO₂ gas
   CO₂ is captured as a byproduct from fermentation (breweries, ethanol plants), ammonia production, or natural underground wells. It is purified to ≥99.9%.
2. Compression & Cooling
   The purified CO₂ gas is compressed to about 70–100 bar and cooled, turning it into liquid CO₂.
3. Liquid CO₂ Storage
   Liquid CO₂ is stored in insulated, pressurized tanks at approximately –20 °C and 20 bar.
4. Expansion (Joule-Thomson effect)
   Liquid CO₂ is released suddenly through a nozzle into an expansion chamber at atmospheric pressure.
   Rapid pressure drop causes about 46% of the CO₂ to freeze instantly into fine snow-like particles at –78.5 °C.
5. Pressing into Blocks or Pellets
   The CO₂ "snow" is collected and compressed hydraulically into: - Blocks (e.g., 10 kg slabs) - Pellets (3 mm, 10 mm, 16 mm diameter – most common for dry ice blasting) - Slices or nuggets
6. Packaging & Storage
   Finished dry ice is packed in insulated containers. It sublimates at ≈5–10% per day even in good insulation.

Laboratory / Small-Scale Method (Demonstration)

1. Obtain a CO₂ fire extinguisher or a small CO₂ cylinder with a siphon tube (liquid CO₂).
2. Attach a cloth bag (or heavy-duty plastic bag) tightly over the nozzle.
3. Briefly discharge the extinguisher into the bag.
   The sudden expansion turns liquid CO₂ into dry ice snow, which collects inside the bag.
4. Quickly compress the snow with gloved hands or a mold to form small blocks before it sublimates.

Safety Warnings
• Never store dry ice in a sealed airtight container – pressure buildup can cause explosion.
• Always use insulated gloves – direct contact causes frostbite in seconds.
• Work in well-ventilated area – CO₂ gas displaces oxygen.
• Never ingest or place in mouth – extreme cold can cause severe injury.

Physical Principle

The formation of dry ice relies on the Joule-Thomson effect: when CO₂ expands rapidly from high to low pressure, it cools dramatically and part of it solidifies directly (sublimation in reverse).

Read Advantages and Disadvantages of Dry Ice