Helium, condensed

An experimental liquid helium condensation cryopump has been constructed as a model for a large-scale pumping system to be used for handling high H2/D2 gas loads. 

Helium condenses to a liquid at 4.224 K under atmospheric pressure and remains a liquid down to the absolute zero of temperature. (It is used as a coolant to reach very low temperatures.) The vapor pressure of liquid helium at 2.20 K is 0.05256 atm. Calculate the volume occupied by 1.000 mol helium vapor under these conditions and compare it with the volume of the same amount of helium at standard temperature and pressure. 

Figure 19. The home-made pressure cell used in Ref. 35 consists of a small volume chamber with two windows. The chamber has a capillary which connects it to a pressure gauge and two valves, one connected to a helium gas reservoir and the other to a vacuum pump. During operation. the cell is immersed in liquid helium. The capillary reaches out of the cryostat, where the gauge and the two valves are located. Liquid helium condenses in the sample chamber, and acts as pressure transmitting medium. The pressure changes in the helium gas phase in the upper part of the capillary, which are controlled by the vacuum pump and the helium reservoir, are transmitted to the sample by the liquid.

The home-built pressure cell, used for pentacene, consists of a small volume chamber made of copper. The chamber has two windows sealed with indium and a capillary (Fig. 19). The cell fits in the bottom of the cryostat and is connected through the steel capillary to the outside, where a pressure gauge, the helium supply, and the vacuum pump are located. During an experiment the cell is immersed in superfluid liquid helium at a temperature of 1.8 K. Helium condenses in the lower part of the cell and the liquid is used as pressure transmitting medium. Its magnitude is determined by the pressure of the helium gas in the upper part of the capillary, which can be controlled by the gas supply and the vacuum pumps. Hydrostatic pressures from 10 to 10 hPa can be applied to the sample. 

It has long been known from statistical mechanical theory that a Bose-Einstein ideal gas, which at low temperatures would show condensation of molecules into die ground translational state (a condensation in momentum space rather than in position space), should show a third-order phase transition at the temperature at which this condensation starts. Nonnal helium ( He) is a Bose-Einstein substance, but is far from ideal at low temperatures, and the very real forces between molecules make the >L-transition to He II very different from that predicted for a Bose-Einstein gas. 

Helium, plentiful in the cosmos, is a product of the nuclear fusion reactions that are the prime source of stellar energy. The other members of the hehum-group gases are thought to have been created like other heavier elements by further nuclear condensation reactions occurring at the extreme temperatures and densities found deep within stars and in supernovas. 

Liquid Helium-4. Quantum mechanics defines two fundamentally different types of particles bosons, which have no unpaired quantum spins, and fermions, which do have unpaired spins. Bosons are governed by Bose-Einstein statistics which, at sufficiently low temperatures, allow the particles to coUect into a low energy quantum level, the so-called Bose-Einstein condensation. Fermions, which include electrons, protons, and neutrons, are governed by Fermi-DHac statistics which forbid any two particles to occupy exactly the same quantum state and thus forbid any analogue of Bose-Einstein condensation. Atoms may be thought of as assembHes of fermions only, but can behave as either fermions or bosons. If the total number of electrons, protons, and neutrons is odd, the atom is a fermion if it is even, the atom is a boson. 

Because of their very low boiling points, helium, neon, and hydrogen are noncondensable under the conditions at the top of the nitrogen column, and they concentrate in the nitrogen gas there. Because they cut down on the rate of condensation of nitrogen and thereby reduce the thermal efficiency of the process, they must be withdrawn. The noncondensable stream withdrawn may have a neon, helium, or hydrogen content that varies from 1 to 12%... 

I eon—Helium Separation and Purification. As indicated eadier, neon, heHum, and hydrogen do not Hquefy in the high pressure (nitrogen) column because these condense at much lower temperatures than nitrogen. As withdrawn, the noncondensable stream has a neon—helium content that varies 1—12% in nitrogen, depending on the rate of withdrawal and elements of condenser design and plant operation. 

The coolant for the HTGR is helium. The helium is not corrosive has good heat properties, having a specific heat that is much greater than that of CO2 does not condense and can operate at any temperature has a negligible neutron absorption cross section and can be used in a direct cycle, driving a gas turbine with high efficiency. 

Cmde helium (containing 50—70% helium, associated hydrogen and neon, 1—3% methane, and the balance nitrogen) can easily be obtained by minor enhancements to the nitrogen rejection unit, particularly with natural gases containing 0.5% or more helium. For example, by operating the double-column condenser in a partial condensation mode, a stream of uncondensed vapor at about 50% helium concentration can be obtained. This cmde helium stream can be fed directly to helium purification and Hquefaction units. 

A Bureau of Mines system for the separation of hehum from natural gas is shown in Fig. 11-119. Since the major constituents of natural gas have boiling points very much different from that of helium, a distillation column is not necessary and the separation can be accomphshed with condenser-evaporators. 

LNG—consisting of ethane, propane, butane, and natural gasoline (condensate)—arrives at the plant for upgrading before it is sent to petrochemical plants and refineries as feedstock. Residue gas is sold to the interstate and intrastate pipeline network. MESA, one of the world s major crude helium producers, also delivers helium to a pipeline operated by the U.S. Bureau of Mines. 

L. D. Landau (Moscow) pioneering theories for condensed matter, especially liquid helium. 

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