It is generally agreed that cryogenic fluids are those whose boiling points (bp) at atmospheric pressure are about 120 K or lower, although liquid ethylene with its boiling point of 170K is often included. A list of the cryogenic fluids, together with some selected properties, is given in Table 1. Detailed properties are available commercially on computer disc. (Cryodata Inc.)
Perhaps the most important and widely-used fluids are liquefied natural gas or LNG (bp = boiling point about 120 K), liquid oxygen (bp 90.2 K) and liquid nitrogen (bp 77.3 K).
The availability of cryogenic fluids forms an essential part of the infrastructure of a modem industrialized and civilized society. One of the major reasons for using liquid cryogens is to allow transport and storage as liquid at atmospheric pressure, rather than as high-pressure gas in thick-walled vessels, although there is an energy penalty involved in Refrigeration. However, the distillation of liquid air (air separation) enables the production of very high-purity oxygen and nitrogen. Plants producing up to several hundred tonnes per day and more of oxygen are commonplace, sometimes connected permanently to a chemical plant or steel works. Liquid nitrogen—formerly a by-product of the process—is now a product in its own right, being used principally as a convenient source of refrigeration, especially in the frozen food industry.
The other important by-product of air separation is liquid argon, which again can be produced at a very high purity. For welding, it is increasingly being stored as liquid at the factory rather than being delivered in high-pressure cylinders.
All cryogenic fluids except Helium and Hydrogen behave as 'normal' fluids, their common distinguishing features in general being a low specific heat and enthalpy of vaporization. All gaseous cryogens are odorless and all liquid cryogens are colorless apart from Oxygen, which is pale blue, and Fluorine, which is pale yellow. They are all diamagnetic except oxygen, which is quite strongly paramagnetic.
With the exception of oxygen, all the gases are asphyxiants, and even oxygen will not support human life in concentrations greater than about 60%. Fluorine and oxygen are powerful oxidizers even in liquid form. Some cryogens are flammable; hydrogen is especially delicate to handle.
Hydrogen is an unusual fluid in that the molecule exists in two forms known as ortho and para, with somewhat different properties. The ratio of ortho to para is determined by conventional thermodynamics and is dependent on temperature. There are also different forms of isotopes (deuterium and tritium).
An explanation of the behavior of the hydrogen molecule requires a knowledge of quantum mechanics and will not be discussed here. At low temperatures, equilibrium hydrogen (e-H2) is entirely para. At room temperature, the ortho: para ratio is 3. The equilibrium state at room temperature is often known as normal hydrogen or n-hydrogen. The transition from the ortho to the para state involves a heat of conversion — which can be greater than the enthalpy of vaporization — so that the vaporization rates of hydrogen are often much larger than expected. It is for this reason that a catalyst is often included in a hydrogen liquefier to ensure that only para hydrogen is present in the liquid.
Helium is the one cryogenic fluid which can be claimed to be unique. Because of its low molecular weight and chemical inertness, quantum mechanical effects are important. There are two isotopic forms: the natural form He4, which has a nucleus consisting of two protons and two neutrons; and the comparatively rare man-made form He3, with only one neutron. The two isotopes have markedly different properties due to their different nuclear spins. He3 is not considered here.
Below 2.2 K, He4 becomes 'superfluid', and is often known as HeII, the 'normal' liquid being known as 'HeI'. The locus of the HeI/HeII transition is known as the 'lambda line' from the shape of the curve of specific heat as a function of temperature. The phase diagram of He4 is shown in Figure 1, in which features of particular interest are the absence of a triple point and the fact that the liquid can only be solidified under pressure (greater than about 26 bars.)
The temperature of the norma/superfluid transition depends somewhat on pressure. One end of this boundary forms with solid HeI and HeII the "upper lambda point” (at 1.77 K and 30.2 bars). The other end of the line (at 2.18 K, 0.005 bar) where vapor, HeI and HeII coexist is known as the 'lower lambda point.'
HeI behaves as a conventional liquid (except when near the λ line) but requires much more care in handling than other cryogenic fluids, principally because of its extremely low latent heat of vaporization. HeII is quite different, having a variety of properties quite different from those of any other liquid. It will, for instance, climb up over the edge of a container and drip off the bottom; it has a small or zero viscosity and a very large thermal conductivity. Flow velocity through fine capillaries is independent of the pressure head and is greater in tubes of smaller diameter. Flow may be induced by a temperature gradient in the absence of any pressure gradient. A consequence of the very high thermal conductivity is that below the λ point, boiling ceases and the liquid becomes "quiescent;" although the rate of heat transfer remains very high. Vinen has published a brief but useful review of the properties of superfluid helium.
Cryogenic Engineering, ed. B.A. Hands (1986) Academic Press. Cryodata Inc. PO Box 558 Niwot, CO 80544 U.S.A. DOI: 10.1016/S0011-2275(96)90058-2
Vinen, W. F. 'Physical Properties of Superfluid Helium — A General Review', Proc. Workshop on Stability of Superconductors in Helium I and Helium II, 43-51, Saclay, France (International Institute of Refrigeration, 1981).