Dry ice is solid carbon dioxide. When CO2 liquefied at about 2 MPa is expanded to atmospheric pressure, part of it turns to gas and the rest solidifies as snow, which is pressed into pellets or blocks. Pressing reduces the surface area and so slows sublimation. The CO2 used is mostly recovered from the gas released by processes such as ammonia production.
Where to buy and what to check
Dry ice brands belong to industrial gas companies; Penguin, for example, is Airgas, an Air Liquide company. Dry ice sublimes at -78.5 °C and cannot be stockpiled: Airgas gives a sublimation rate of 10 lb (4.5 kg) a day in a 25-quart, roughly 24-litre cooler; a block loses mass more slowly than pellets. Part of what comes from a distant supplier has turned to gas before delivery, so quotes are compared on the net kilos weighed at delivery.
The storage container must not be airtight: 1 lb of dry ice gives about 250 litres of gas, and pressure building up in a sealed container can burst it. Use an insulated box that does not close tightly, in a well-ventilated place.
What to have in the quote
- Form (pellets or block), net weight and weighing method
- Delivery window, order cut-off and delay notice
- Packaging and any container return
- Delivery and other charges
Divide the total cost by the kilos weighed before use to get the real unit cost. Where supply is unreliable, reusable eutectic plates can do the same job.
