Dry ice

Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO₂ does not have a liquid state at normal atmospheric pressure and sublimes directly from the solid state to the gas state. It is used primarily as a cooling agent, but is also used in fog machines at theatres for dramatic effects. Its advantages include lower temperature than that of water ice and not leaving any residue. It is useful for preserving frozen foods where mechanical cooling is unavailable.
Dry ice sublimes at at Earth atmospheric pressure. This extreme cold makes the solid dangerous to handle without protection from frostbite injury. While generally not very toxic, the outgassing from it can cause hypercapnia due to a buildup in confined locations.

Properties

Dry ice is the solid form of carbon dioxide, a molecule consisting of a single carbon atom bonded to two oxygen atoms. Dry ice is colorless, odorless, and non-flammable, and can lower the pH of a solution when dissolved in water, forming carbonic acid.
At pressures below 5.13 atm and temperatures below , CO₂ changes from a solid to a gas with no intervening liquid form, through a process called sublimation. The opposite process is called deposition, where CO₂ changes from the gas to solid phase. At atmospheric pressure, sublimation/deposition occurs at.
The density of dry ice increases with decreasing temperature and ranges between about below. The low temperature and direct sublimation to a gas makes dry ice an effective coolant, since it is colder than water ice and leaves no residue as it changes state. Its enthalpy of sublimation is 571 kJ/kg.
Dry ice is non-polar, with a dipole moment of zero, so attractive intermolecular van der Waals forces operate. The composition results in low thermal and electrical conductivity.

History

It is generally accepted that dry ice was first observed in 1835 by French inventor Adrien-Jean-Pierre Thilorier, who published the first account of the substance. In his experiments, he noted that when opening the lid of a large cylinder containing liquid carbon dioxide, most of the liquid carbon dioxide quickly evaporated. This left only solid dry ice in the container. In 1924, Thomas B. Slate applied for a US patent to sell dry ice commercially. Subsequently, he became the first to make dry ice successful as an industry. In 1925, this solid form of CO₂ was trademarked by the DryIce Corporation of America as "Dry ice", leading to its common name. That same year the DryIce Co. sold the substance commercially for the first time, marketing it for refrigeration purposes.

Manufacture

Dry ice is easily manufactured. The most common industrial method of manufacturing dry ice starts with a gas having a high concentration of carbon dioxide. Such gases can be a byproduct of another process, such as producing ammonia from nitrogen and natural gas, oil refinery activities or large-scale fermentation. Second, the carbon dioxide-rich gas is pressurized and refrigerated until it liquefies. Next, the pressure is reduced. When this occurs some liquid carbon dioxide vaporizes, causing a rapid lowering of temperature of the remaining liquid. 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 small pellets or larger blocks of dry ice.
Dry ice is typically produced in three standard forms: large blocks, small cylindrical pellets and tiny cylindrical, high surface to volume pellets that float on oil or water and do not stick to skin because of their high radii of curvature. Tiny dry ice pellets are used primarily for dry ice blasting, quick freezing, fire fighting, oil solidifying and have been found to be safe for experimentation by middle school students wearing appropriate personal protective equipment such as gloves and safety glasses. A standard block weighing approximately covered in a taped paper wrapping is most common. These are commonly used in shipping, because they sublime relatively slowly due to a low ratio of surface area to volume. Pellets are around in diameter and can be bagged easily. This form is suited to small scale use, for example at grocery stores and laboratories where it is stored in a thickly insulated chest. Density of pellets is 60–70% of the density of blocks.
Dry ice is also produced as a byproduct of cryogenic air separation, an industry primarily concerned with manufacturing extremely cold liquids such as liquid nitrogen and liquid oxygen. In this process, carbon dioxide liquefies or freezes at a far higher temperature compared to that needed to liquefy nitrogen and oxygen. The carbon dioxide must be removed during the process to prevent dry ice from fouling the equipment, and once separated can be processed into commercial dry ice in a manner similar to that described above.

Applications

Commercial

The most common use of dry ice is to preserve food, using non-cyclic refrigeration.
It is frequently used to package items that must remain cold or frozen, such as ice cream or biological samples, in the absence of availability or practicality of mechanical cooling.
Dry ice is critical in the deployment of some vaccines, which require storage at ultra-cold temperatures along their supply line.
Dry ice can be used to flash-freeze food or laboratory biological samples, carbonate beverages, make ice cream, solidify oil spills and stop ice sculptures and ice walls from melting.
Dry ice can be used to arrest and prevent insect activity in closed containers of grains and grain products, as it displaces oxygen, but does not alter the taste or quality of foods. For the same reason, it can prevent or retard food oils and fats from becoming rancid.
When dry ice is placed in water, sublimation is accelerated, and low-sinking, dense clouds of smoke-like fog are created. This is used in fog machines, at theatres, haunted house attractions, and nightclubs for dramatic effects. Unlike most artificial fog machines, in which fog rises like smoke, fog from dry ice hovers near the ground. Dry ice is useful in theatre productions that require dense fog effects. The fog originates from the bulk water into which the dry ice is placed, and not from atmospheric water vapor.
It is occasionally used to freeze and remove warts. However, liquid nitrogen performs better in this role, as it is colder, thereby requiring less time to act, and needs less pressure to store. Dry ice has fewer problems with storage, since it can be generated from compressed carbon dioxide gas as needed.
In plumbing, dry ice is used to cut off water flow to pipes to allow repairs to be made without shutting off water mains. Pressurised liquid CO₂ is forced into a jacket wrapped around a pipe, which in turn causes the water inside to freeze and block the pipe. When the repairs are done, the jacket is removed and the ice plug melts, allowing the flow to resume. This technique can be used on pipes up to 4 inches or 100 mm in diameter.
Dry ice can be used as bait to trap mosquitoes, bedbugs, and other insects, due to their attraction to carbon dioxide.
It can be used to exterminate rodents. This is done by dropping pellets into rodent tunnels in the ground and then sealing off the entrance, thus suffocating the animals as the dry ice sublimates.
Tiny dry ice pellets can be used to fight fire by both cooling fuel and suffocating the fire by excluding oxygen.
The extreme temperature of dry ice can cause viscoelastic materials to change to glass phase. Thus, it is useful for removing many types of pressure sensitive adhesives.

Industrial

Dry ice can be used for loosening asphalt floor tiles or car sound deadening material, making them easy to prise off, as well as freezing water in valveless pipes to enable repair.
One of the largest mechanical uses of dry ice is blast cleaning. Dry ice pellets are shot from a nozzle with compressed air, combining the power of the speed of the pellets with the action of the sublimation. This can remove residues from industrial equipment. Examples of materials removed include ink, glue, oil, paint, mold and rubber. Dry ice blasting can replace sandblasting, steam blasting, water blasting or solvent blasting. The primary environmental residue of dry ice blasting is the sublimed CO₂, thus making it a useful technique where residues from other blasting techniques are undesirable. Recently, blast cleaning has been introduced as a method of removing smoke damage from structures after fires.
Dry ice is also useful for the de-gassing of flammable vapours from storage tanks the sublimation of dry ice pellets inside an emptied and vented tank causes an outrush of CO₂ that carries with it the flammable vapours.
The removal and fitting of cylinder liners in large engines requires the use of dry ice to chill and thus shrink the liner so that it freely slides into the engine block. When the liner then warms up, it expands, and the resulting interference fit holds it tightly in place. Similar procedures may be used in fabricating mechanical assemblies with a high resultant strength, replacing the need for pins, keys or welds.
Dry ice has found its application in construction for freezing soil, serving as an effective alternative to liquid nitrogen. This method reduces the soil temperature to approximately -70 to -74 °C, rapidly freezing the groundwater. As a result, the soil's strength and impermeability significantly increase, which is essential for the safe execution of underground construction projects.
It is also useful as a cutting fluid.

Scientific

In laboratories, a slurry of dry ice in an organic solvent is a useful freezing mixture for cold chemical reactions and for condensing solvents in rotary evaporators. Dry ice and acetone forms a cold bath of, which can be used for instance to prevent thermal runaway in a Swern oxidation.
The process of altering cloud precipitation can be done with the use of dry ice. It was widely used in experiments in the US in the 1950s and early 1960s before it was replaced by silver iodide. Dry ice has the advantage of being relatively cheap and completely non-toxic. Its main drawback is the need to be delivered directly into the supercooled region of clouds being seeded.