Helium solubility

As shown in Fig. 20.4-3, this added effect can improve helium selectivity relative to methane by a factor of 1.73. At the same time, a factor of 2.0 loss in helium solubility and a factor of2.S [eduction in heiium diffusivjty due to the crystallinity produces a 5,6-fold reduction in helium productivity relative to the hypothetical amorphous film case. Interestingly, the effects of chain restriction on different sizes of peaetranls ware found by Michaels to be effective only for rubbery materials, not glassy ones. A logical explanation for this fact was ndvanced in terms of the already low mobility of chain segments in the glass compared to the rubbery stale ... 

Barkan, E.S., Yakutseni, V.R (1976). Change in methane, nitrogen and helium solubility ratios as an important factor in the formation of helium content of gas accumulations. Gas indust.. No. 12, p. 50-53. (in Russian). 

Several methods of helium purification from nitrogen are used. The washing the concentrate with liquid hydrocarbons, particularly with propane, is based on abnormal helium solubility in liquid propane. There are also continued intensive studies and implementation works on helium purification using membranes [19,20]. The purification by washing does not, however give a product of a sufficient purity and efficiency of the diffusive separation on membranes is still too low. For these reasons, the basic process of helium denitrification is adsorption on solid adsorbents, mainly on active carbon. 

Fig. 6.6 shows two isothermal p — Xi) sections of the phase diagram. The two-phase region is located above the helium solubility curves of phases ( ) and ("), on the left- and right-hand sides of the critical line. 

Another consequence of the effect of pressure on gas solubility is the painful, sometimes fatal, affliction known as the bends. This occurs when a person goes rapidly from deep water (high pressure) to the surface (lower pressure), where gases are less soluble. The rapid decompression causes air, dissolved in blood and other body fluids, to bubble out of solution. These bubbles impair blood circulation and affect nerve impulses. To minimize these effects, deep-sea divers and aquanauts breathe a helium-oxygen mixture rather than compressed air (nitrogen-oxygen). Helium is only about one-third as soluble as nitrogen, and hence much less gas comes out of solution on decompression. 

FIGURE 8.21 The variation of the molar solubilities of oxygen, nitrogen, and helium gases with partial pressure. Note that the solubility of each gas doubles when its partial pressure is doubled. 

Divers avoid the bends by returning to the surface slowly, taking short decompression stops at intermediate depths to allow excess gas to escape from their blood without forming bubbles. Another way divers reduce the risk of the bends is by using helium-oxygen gas mixtures instead of compressed air. Helium is only half as soluble in water as nitrogen is, so less gas dissolves in blood. 

Assuming a cooler temperature for helium, this means the amount of heat energy available for pyrolysis is less, which would produce less hydrocarbon products. This ultimately would lead to a relatively smaller SL quenching compared to that observed in argon saturated solutions. The relative change in bubble temperature would also be less in helium saturated solutions due to the lower amount of hydrocarbon products generated. Solubility differences would re-enforce this. 

Meanwhile, computational methods have reached a level of sophistication that makes them an important complement to experimental work. These methods take into account the inhomogeneities of the bilayer, and present molecular details contrary to the continuum models like the classical solubility-diffusion model. The first solutes for which permeation through (polymeric) membranes was described using MD simulations were small molecules like methane and helium [128]. Soon after this, the passage of biologically more interesting molecules like water and protons [129,130] and sodium and chloride ions [131] over lipid membranes was considered. We will come back to this later in this section. 

Research Centers (IUCRC), 24 395 Inelastic mean free path (IMFP), 24 87 Inert fluids, 11 877 properties of, 11 879 Inert gas dilution, 11 456 Inert gases, 13 456 17 376-377. See also Helium- group elements Noble gases narcotic potency and solubility of, 17 377 Inert gas generators, 17 280 Inertial confinement fusion targets, microcapsules as, 16 460 Inertial impaction, in depth filtration theory, 11 339... 

Fig. 11.15. Gas chromatography interfaces (jet separator, top membrane separator, bottom). In the jet separator, momentum of the heavier analyte molecules causes them to be sampled preferentially by the sampling orifice with respect to the helium carrier gas molecules (which diffuse away at a much higher rate). In the membrane separator, the analyte molecules are more soluble in the silicone membrane material leading to preferential permeability. Helium does not permeate the membrane with the same efficiency and is vented away.

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