Cooling, by adiabatic expansion

ABSTRACT Radiative forces on atoms can be used both to cool the atoms to temperatures on the order of microkelvlns and to trap them in a periodic array of microscopic potential wells formed by the interference of multiple laser beams, i.e., an optical lattice. The quantum motion of such lattice-trapped atoms can be studied by spectroscopic techniques. Atoms trapped in this way may be further manipulated so as to be cooled by adiabatic expansion, localized by sudden compression or driven into mechanical oscillations. 

In 1898 J. J. Thomson produced gaseous ions in moist air by exposure to X-rays, and then produced a cloud of water droplets by cooling by adiabatic expansion. He calculated the size of the droplets by the rate of settling of the cloud, using Stokes s law (see p. 745). The total weight of water precipitated was known from the cooling effect and he measured the current produced in the gas by an applied potential difference. The charge on the drop was found to be 6 5 X 10 e.s.u., later corrected to 3 4 x e.s.u. By a similar method H. A. Wilson found 3 1 x io e.s.u. 

Measurements at temperatures below 160 K have been achieved in a cooled Penning trap, a drift tube and a flow reactor that utilizes cooling by adiabatic expansion. A free jet-flow reactor has been employed to study ion-molecule reaction rate coefficients at temperatures as low as 0.3 to 30 K. 

Under special conditions sulfur cations with up to 56 atoms have been observed [209]. Evaporation of liquid sulfur and cooling the vapor in an atmosphere of a cold buffer gas (He) at low pressures followed by adiabatic expansion into the vacuum of a mass spectrometer and El ionization produced mass spectra of clusters of sulfur molecules with m/e ratios up to ca. 1800. The intensity pattern shows that the species (Ss)h are most abundant n = 1-7) followed by (Sy)(S8)n-i clusters and (S6)(Ss)h-i clusters. The latter have the same mass as (Sy)2(S8) -2 clusters see Fig. 34. Thus, the composition of the clusters reflects the composition of hquid sulfur near the melting point which contains Sg, Sy and Se molecules as the majority species [34, 210]. 

Fig. 34 Mass spectrum of sulfur clusters obtained by evaporation of liquid sulfur followed by cooling and adiabatic expansion of the vapor [209]. The figures at the peaks give the number of Sg molecules which represent this particular mass...

The ability to cool (and eventually liquefy) gases by adiabatic expansion underlies industrial gas liquefaction processes. Adiabatic cooling of gaseous nozzle-jet expansions is also an important technique in modem molecular beam and mass spectrometric research. Thermodynamicist John Fenn, winner of the 2002 Nobel Prize in Chemistry, pioneered many of the techniques of adiabatic nozzle-beam cooling. 

When transformation of a less stable into a more stable phase occurs, the change does not take place at one moment throughout the whole phase, but proceeds from definite points or growth centres (nuclei). Such nuclei may form spontaneously in a supercooled phase, as is seen, for example, in the cloud formation produced on the cooling of a vapour by adiabatic expansion. The influence of dust particles and of gaseous ions in increasing the number of condensation nuclei, is well known. 

An unsaturated or saturated vapor may become supersaturated by undergoing various thermodynamic processes, such as isothermal compression, isobaric cooling, and adiabatic expansion. In the first of these processes the vapor temperature remains constant, whereas it decreases in the latter two processes. 

Where they have a positive slope, water cools on adiabatic expansion and warms if adiabatically compressed, and the two regions are separated by the Joule-Thompson inversion curve. Much the same information is contained in the enthalpy-pressure diagram (Figure 8.6), where it can be seen that constant enthalpy changes in pressure lead to increases in temperature in one region and decreases in another. The effect of dissolved NaCl on the Joule-Thompson coefficient has been calculated by Wood and Spera (1984), and the effect will be similar for other electrolytes. Because the addition of most electrolytes to water results in a decrease in V and in a, fijT is smaller, and the net effect is to move the inversion curve to higher temperatures, as shown in Figure 8.5. 

The inversion temperature of Nj is 850 K. Inversion temperature is the temperature below which a gas cools down by adiabatic expansion (Joule-Thomson-Effect). Therefore, Nj can be liquefied from room temperature by means ofcounter[Pg.10]

Vacuum-cooling crystallisation is the preferred cooling crystallisation method under continuous operation. Because cooling is generated by adiabatic expansion of the solvent no cooling surfaces can be incrusted. Vacuum-cooling becomes uneconomical only if cooling has to be effected at very lotv temperatures. 

This rapid adiabatic expansion is sufficient to cool the nitrogen to below its boiling point of 77°K, so this is a way to make liquid nitrogen. There is a temperature for each gas called the Joule-Thomson inversion temperature and cooling occurs if the initial temperature is below that temperature but the gas heats upon expansion if the initial temperature is above the inversion temperature. At room temperature He is above its inversion temperature and will actually heat up upon expansion. Although there is also a pressure effect, there are absolute temperatures for this effect. For He the temperature is 51°K, for H2 202°K, for N2 621°K, and for O2 it is 764°K (see discussion at http //en.citizendium.org/wiki/Joule-Thomson effect). Thus, air (N2 + O2) can be liquefied by adiabatic expansion starting from room temperature and 1 atm, but He and H2 must be precooled to below their Joule-Thomson inversion temperatures. 

Figure 3.45. Cumulus clouds are created by adiabatic expansion, and thus cooling, of air in thermal upwind when the air temperature has fallen to the dew point, the water vapour will condense into this well-known cloud formation.

USA in 1872. Thermodynamically, the cooling version consists of an adiabatic (isentropic) compression followed by heat transfer to the surroundings, then adiabatic expansion and cooling. 

The fact that a gas can be cooled (/xJT > 0) or warmed (/zJT < 0) by merely expanding under adiabatic (adiabatic conditions, A U =w, so the work performed by the gas in reversible adiabatic expansion must be compensated by the change AU in internal energy, that is, by a temperature change (since heat capacity is nonzero). 

When water-wet gas expands rapidly through a valve, orifice or other restriction, hydrates form due to rapid gas cooling caused by adiabatic (Joule-Thomson) expansion. Hydrate formation with rapid expansion from a wet line commonly occurs in fuel gas or instrument gas lines. Hydrate formation with high pressure drops can occur in well testing, start-up, and gas lift operations, even when the initial temperature is high, if the pressure drop is very large. 

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