Getting technical - When cold becomes cryo-cold

Getting technical - When cold becomes cryo-cold

By Charles Nicolson

The range over which temperatures decrease, from the lower limits of general commercial and industrial refrigeration down to absolute zero, covers a zone described as ‘cryogenic’ — a rather old word for an area of technology that became commercially active only fairly recently. 

Like many other long-established words, cryogenic is derived from Greek, specifically cryo, meaning cold, and genic, which indicates production. The definition of cryogenics therefore is that it describes the production and also the behaviour of materials at very low temperatures. However, there is not yet complete agreement on the actual upper temperature boundary of ‘cryogenic’.

Up to about the early 1960s, scientists tended to regard −150°C as the transition temperature between ‘refrigerated’ and ‘cryogenic’, even though the US National Institute of Standards and Technology had chosen −180°C on the basis that the normal boiling points of the gases helium, hydrogen, neon, nitrogen, and oxygen (which automatically includes normal air) are below −180 C, whereas manufactured gases, such as freon refrigerants, hydrocarbons, and many other refrigerants, have boiling points above −180 C. In addition, liquefied gases, particularly hydrogen, nitrogen, and oxygen, are widely used on a practical basis for cooling down materials to temperatures where they become electrically superconducting. However, over the past 50 years or so, as methods of working at low temperatures have become more widely available (and affordable), many of the practical applications that have been developed are now regarded as cryogenic at temperatures below −50°C, although it can get a bit confusing when specialists working in this field continue to use terms such as ‘high-temperature cryogenic’ for the range from −50 C down to the boiling point of liquid nitrogen at −195.79 C.

Stored gases such as liquid nitrogen are now used for a large number of diverse speciality chilling and freezing applications.

Prior to the 1940s, previous experimental work done to achieve and maintain cryogenic temperatures, was by only a few specialised companies, mainly to develop methods of producing hydrogen, nitrogen, and oxygen stored in cylinders at high pressures. Increased availability of these gases, along with backing by governments for technical innovations under wartime conditions, led to more advances across the field of cryogenics during World War II, in particular when metallurgists determined that many metals refrigerated down to temperatures between −80 C and −150 C demonstrated substantial increases in resistance to wear, which was described as ‘cryogenic hardening’.

In 1966, Ed Busch, who had a background in the metals heat-treating industry, started one of the first cryogenic processing industries when he founded a company in Detroit called CryoTech for cryogenic hardening on a production basis. Busch originally experimented with the possibility of increasing the life of metal tools by up to 400% of the original life expectancy using cryogenic tempering instead of lengthy traditional heating and controlled cooling treatment protocols. After expansions and merging with another specialist company, 300 Below, in 1995, CryoTech reportedly became the world’s largest and oldest commercial cryogenic processing company.

Stored gases, such as liquid nitrogen, are now used for a large number of diverse speciality chilling and freezing applications. Some chemical reactions, like those used to produce the active ingredients for the popular statin drugs, must occur at low temperatures approaching −100°C. Special cryogenic chemical reactors are used to remove reaction heat and provide low-temperature environments for these reactions. Fast freezing of foods and biotechnology products like vaccines, require liquid nitrogen for blast freezing in immersion freezing systems. Certain soft or elastic non-metallic materials become hard and brittle at very low temperatures, for which a machining process, cryogenic milling (or cryomilling) was developed as an option for some materials that cannot be milled at higher temperatures.

Various sections of cryogenics have become established under headings such as the following:

Cryobiology

Biology involving the study of the effects of low temperatures on organisms. Much of the work done has been to develop procedures for achieving cryopreservation and, in particular, for establishing practical methods for the conservation of animal genetic resources. When the objective of cryopreservation is for conserving specific breeds, it is referred to as ‘cryoconservation’.

Cryosurgery

Now frequently used or included in standard surgical procedures to remove undesirable tissue materials, particularly the malignant tissues of cancer cells.

Cryoelectronics

The word is self-descriptive, involving studying electronic phenomena at cryogenic temperatures. Examples include superconductivity and variable-range hopping. Practical applications of cryoelectronics are now known as ‘cryotronics’.

Cryonics

A recently adopted cryo word for long-held beliefs that cryopreserving humans and animals can be done by methods and procedures that will enable future revival.

Probably due to the similarities of the words ‘cryogenics’ and ‘cryonics’, they are often erroneously used interchangeably in popular culture and the media. The list of cryo headings continues to increase as further work is done at low temperatures across the spectrum of physical and chemical research and applied technologies. Regarding people working and carrying out studies in any of the various cryogenic fields, the single title of ‘cryogenicist’ is most commonly used.

To simplify and standardise temperatures, many scientists, as well as other people working in the field of cryogenics, no longer use Celsius or Fahrenheit temperature scales with arbitrary zero points, but rather Kelvin or Rankine scales, which both measure from absolute zero. Therefore, both scales begin at −273.15°C as 0°R and 0K. Rankine uses Fahrenheit degree intervals, so taking an example of a cryogenic temperature at −196°C, the Rankine reading would be 140 R and Kelvin would be 78K. Note that degrees Rankine are denoted as R in the same way that Celsius is shown as °C, whereas there is no degree symbol used for Kelvin; just the number of degrees above absolute zero followed by K. In practice, however, it will probably take several years or even decades before the use of Celsius reduces to negligible levels.

Among the many notable low temperatures previously not regarded as cryogenic prior to adoption of the upper cryogenic limit at −50 C, is the coldest temperature ever recorded on earth: −89.2°C measured at Vostok, Antarctica, on 21 July 1983. Another substance found at a similar temperature is solid carbon dioxide (CO2), commonly known as ‘dry ice’, which, under normal atmospheric pressure, changes phase at −78.5°C from solid directly to gas by sublimation (or solidifies directly from gas, which is called ‘deposition’).

By all accounts, the French chemist Thilorier was the first to record the appearance of solidified CO2 as dry ice when in 1835, he opened a cylinder containing liquid CO2 to observe it in liquid form. Almost immediately, virtually all the liquid evaporated, leaving a solid dry ice block at the bottom of the cylinder. Over the following 80–90 years, Thilorier’s discovery was used in experiments at universities and privately owned research establishments, but no practical applications were developed.

In 1923, a company in New York City, Prest Air Devices, made solid dry ice for demonstration purposes, and in 1924, proposed dry ice to the railroad companies to use for cooling in place of normal water ice currently used for refrigerating food and perishables carriages, on the grounds that dry ice with double the cooling power of water would be far more efficient. After initial successful trials, a dry ice production plant was constructed in 1925 and the company was incorporated as DryIce Corporation of America, which trademarked the name DryIce. By 1932, there were eight major manufacturers of dry ice producing an estimated amount of approximately 50 000 tons per year due to rapidly accelerating growth of demand over only seven years, which included the Great Economic Depression starting in 1929. Today, there are several thousand dry ice producers all around the world, most of which sell dry ice as a commercial product, but many of the larger organisations manufacture dry ice solely for their own use.

Dry ice is relatively easy to manufacture from either CO2 gas produced in fermentation plants or CO2-rich gases occurring as by-products of other processes, such as production of ammonia from nitrogen and natural gas fermentation. CO2 or CO2-rich gas is pressurised and refrigerated until it liquifies and then, just as Thilorier did in 1835, pressure is reduced. When this occurs, some liquid CO2 vaporises, 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 CO2 is compressed into small pellets or larger blocks of dry ice.

Small (16mm-diameter) pellets and smaller cylindrical particles (0.2mm diameter) have high surface to volume ratios, so that they float on oil or water and do not stick to skin because of their high radii of curvature. The smaller cylindrical dry ice pellets are used for ice blasting, quick freezing, flame extinguishers for firefighting, and containment of oil slicks by oil solidifying.

Charles001Small pellets and cylindrical particles.
Image credit: Ice Factory Online

Mixtures of small and cylindrical particles are widely used for blast cleaning. The pellets are shot from nozzles in compressed air, combining the power of the speed of the pellets with the action of the sublimation, which is highly efficient in removing residues from industrial and other equipment. Examples of materials removed include ink, glue, oil, paint, mould, and rubber. Dry ice blasting can replace sandblasting, steam blasting, water blasting, and solvent blasting. There is also an environmental benefit of dry ice blasting in that the residue is only sublimed CO2, thus making it a useful technique where residues from other blasting techniques are undesirable. One of the more recent applications of dry ice blasting is to remove smoke damage from structures after fires.

One of the more recent applications of dry ice blasting is to remove smoke damage from structures after fires.

Standard blocks of dry ice are manufactured in sizes of between 20 and 30kg, which are most commonly used in shipping because they sublime relatively slowly due to having low ratios of surface area to volume. Blocks are also less costly to use when large volumes of sublimated CO2 are required, for example when cleaning out flammable vapours from oil and fuel tanks. Other speciality applications of dry ice are to chill steel and other metals during mechanical assemblies so that strong interference fits result when temperatures rise back to ambient and to create slugs of ice in valveless pipes needing repairs or modifications.

Charles002Dry ice blocks.
Image credit: Wikipedia

In laboratories, making slurries of dry ice in organic solvents provides mixtures of cold baths at temperatures down to close to −100 C. This practical application of dry ice has been exceptionally useful in preventing what used to be a common nuisance called ‘thermal runaway’ occurring in many experimental research programmes.

Overall, however, the most common use of dry ice is still to preserve food, perishables, and items such as biological samples that must remain cold or frozen without the use of mechanical cooling. No other readily available and essentially non-toxic material will reduce the needed preservation temperatures down into the cryogenic region.

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