Dry Ice

Dry ice is the solid form of carbon dioxide.

Dry ice is used as a cooling agent, because it is about 80°C (78.5K) colder than frozen water under normal conditions, and only about half as dense. Moreover, for some applications, the convenience its sublimation "into thin air," in contrast to the melt-water left by warming water ice, may outweigh other costs.

Manufacture

Small pellets of dry ice sublimating in airDry ice is easily manufactured. Generally, there are common steps taken in producing dry ice.

First, gases containing a high concentration of carbon dioxide are produced. Such gases can be a byproduct of some other process, such as producing ammonia from nitrogen and natural gas, or large-scale fermentation.[13] Second, carbon dioxide-rich gas is pressurized and refrigerated until it changes into its liquid form.

Next, the pressure is reduced. When this occurs some liquid carbon dioxide vaporizes, and this causes a rapid lowering of temperature of the remaining liquid carbon dioxide. As a result, the extreme cold causes the liquid to solidify into a snow-like consistency.

Finally, the snow-like solid carbon dioxide is compressed into either small pellets or larger blocks of dry ice.[14]

Dry ice is typically produced in two standard forms: blocks and cylindrical pellets. A standard block weighing approximately 30 kg is most common. These are commonly used in shipping, because they sublimate slowly due to a relatively small surface area. Pellets are around 1 cm (0.4 in) in diameter and can be bagged easily. This form is suited to small scale use, for example at grocery stores and laboratories

references: www.wikipedia.org