Helium membranes

Helium Helium is a veiy fast gas, and may be recovered from natural gas through the use of membranes. More commonly, membranes are used to recover He after it has been used and become diluted. 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

As the pore size is reduced to 1 nm or less, gas permeation may exhibit a thermally activated diffusion phenomena. For example, in studies at Oak Ridge National Laboratory, for a certain proprietary membrane material and configuration, permeation of helium appeared to increase much faster than other gases resulting in an increase in Helium to C02 selectivity from 5 at 25°C to about 48.3 at 250°C (Bischoff and Judkins, 2006). Hydrothermal stability of this membrane in the presence of steam, however, was not reported. 

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. 

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.

In this last section some recent developments are mentioned in relation to gas separations with inorganic membranes. In porous membranes, the trend is towards smaller pores in order to obtain better selectivities. Lee and Khang (1987) made microporous, hollow silicon-based fibers. The selectivity for Hj over Nj was 5 at room temperature and low pressures, with permeability being 2.6 x 10 Barrer. Hammel et al. 1987 also produced silica-rich fibers with mean pore diameter 0.5-3.0nm (see Chapter 2). The selectivity for helium over methane was excellent (500-1000), but permeabilities were low (of the order of 1-10 Barrer). 

The most common separators include the Ryhage or jet diffusion separator (74), the Watson-Biemann or pore diffusion separator (75), and the membrane solution diffusion separator originally developed by Llewellyn (75). The first two separators involve direct passage of the sample into the mass spectrometer the low molecular weight helium diffuses more readily and is pumped away. The membrane separator involves diffusion of the sample through a silicone membrane while the carrier gas vents to the atmosphere carrier gas is thus not confined to helium. There is no best separator the choice depends on the nature of the compounds, the temperature range over which it will be operated, and most usually what is available in a particular laboratory. A convenient configuration for a double-beam mass spectrometer such as the AEI MS-30 is two different separators, one into each beam, which permits rapid evaluation of separator performance. 

Alternatively, helium may be separated from natural gas by diffusion through permeable barriers, such as high silica glass or semipermeable membranes. The gas is supplied commercially in steel cylinders or tanks. The United States is the largest producer of helium in the world. 

Fig. 1.5.3 Aerosol generator assembly for the production of composite or coated particles (a) carrier gas (e.g., helium) tank, (b) drying column containing silica gel and molecular sieve, (c) Millipore membrane of 0.1 p.m pore size, (d) flow meters, (e) nuclei (e.g., NaCI, AgCI) generator, (f) and (m) boilers containing different reactant liquids, (g) and (n) condensation chambers, (h) heater, (i) chamber for recondensation of droplets, (q) and (e) coreactant containers, (k) and (p) vapor injection ports, (I) and (g) reaction chambers, (r) powder collector. (From Ref. 39.)...

Xenon is an odourless, colourless, non-explosive gas present in the atmospheres of both Earth and Mars in concentrations of approximately 0.08 ppm. Its density is approximately three times and its viscosity twice that of nitrous oxide. Like other noble gases, such as helium and argon, its outer electron shell contains the maximum number of electrons (8) making the molecule highly stable chemically. Despite this, its anaesthetic activity indicates that xenon binds to cell proteins and cell membrane constituents. 

To verify the membrane integrity prior to attempting separations, pure gas permeation rates for nitrogen and helium were determined and compared to the vendor s data supplied with the membrane. Figure 4 and Table V verify the vendor s data reasonably well for the only membrane which survived shipment and startup. The agreement of the nitrogen values is particularly indicative of the membrane s integrity. 

In these experiments, the measured helium flux through the membrane was less than the flux predicted on the basis of the average bulk concentrations. Consequently, the helium permeability coefficients calculated from observed membrane flux and the bulk partial pressures are lower than the pure gas values obtained by the membrane supplier or independently by us. At the same time, observed nitrogen coefficients are higher than predicted. 

Figure 11 and the accompanying material balances in Figure 12 show that five membrane separation stages are required in the hypothetical system to meet the specified levels in Table I for a 58% helium mixture. 

The design proposed here is a hypothetical preliminary design assembled to determine the technical feasibility of using semipermeable membranes to recover helium. 

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