Helium concentration

For gases a part per million usually is a volume ratio. Thus, a helium concentration of 6.3 ppm means that one liter of air contains 6.3 pL of He. 

The principal source of helium is certain natural gas fields. The helium contents of more than 10,000 natural gases in various parts of the world have been measured (9). Helium concentrations of a few are Hsted in Table 2. In the United States, recovery of helium is economical only for helium-rich gases containing more than about 0.3 vol % belium. Most of the United States helium resources are located in the midcontinent and Rocky Mountain regions, and about 89% of the known United States supply is in the Hugoton field in Kansas, Oklahoma, and Texas the Keyes field in Oklahoma the Panhandle and Cliffside fields in Texas and the Riley Ridge area in Wyoming (11). 

Table 2. Helium Concentrations of Helium-Bearing Natural Gases ...

Helium from Natural Gas. Recovery of helium from a given natural-gas stream depends almost entirely on the total economic picture of the stream. In the United States, the lowest practical helium level that is recovered is most frequendy 0.3 vol %, although helium is frequendy ignored, and hence wasted, in streams containing somewhat high concentrations. In other parts of the wodd where political considerations sometimes interact with the economic, the use of helium concentrations lower than 0.3 vol % maybe dictated. 

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. 

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

To determine overall leakiness of a test object pressurized with helium the object shall be enclosed in an envelope vi/hich is either rigid or deformable (plastic). The test gas leaving the leaks accumulates so that the helium concentration in the envelope rises. Following an enrichment period to be... 

Dispersion polymerizations of methyl methacrylate ntUizing poly(l,l,-dihydroper-fluorooctyl acrylate) as a steric stabilizer in snpercritical CO2 were carried out in the presence of helium. Particle size and particle size distribution were found to be dependent on the amonnt of inert helium present. Particle sizes ranging from 1.64 to 2.66 pm were obtained with varions amounts of helium. Solvatochromic investigations using 9-(a-perflnoroheptyl-p,p-dicyanovinyl)julolidine indicated that the solvent strength of CO2 decreases with increasing helium concentration. This effect was confirmed by calcnlations of Hildebrand solubility parameters (Hsiao and DeSimone, 1997). 

It leads to more extreme surface compositions in WC stars, i.e. to smaller surface helium concentrations but higher C- and O-abundances, The surface helium mass fraction Y can be roughly estimated to be larger than a certain value, depending on the mass of the convective core Mcc Y > (Mwc — Mcc)/ Mwc VHe), where Mwc is the mass of the WC star, Mwc its average mass loss rate, and rne its He-burning lifetime. 

More detailed data concerning the effects of humidity, membrane area and lower helium concentrations in the feed stream on helium recovery must be obtained before a final design can be performed with confidence. 

These assumptions are discussed in the following sections. Examples of helium concentrations in groundwaters of the world are given in Table 14.1. 

Case Studies and Observed Trends in the Distribution of Helium Concentrations in Groundwater Systems... 

Example In the Milo Holdings 3 well, 1421 m deep, in the Jurassic rock sequence of the Great Artesian Basic, east Australia, the helium concentration of 1000 x 10 x cc/cc water was found (Mazor and Bosch, 1990). The following local parametric values were applied to calculate the water age H = 1 U = 1.7 ppm Th = 6.1 ppm d= 2.6 effective porosity of 0.2 or a rock water ratio of 4. Thus the age of this groundwater was found to be... 

The low helium concentrations of rocks of known ages indicate emanation efficiencies of 0.8-1 (Podosek et al., 1980 Zaikowski et al., 1987). The concentrations of uranium and thorium vary by a factor of 10 in common rocks, but knowledge of the lithologies of the main aquifer rocks can narrow the uncertainty to a factor of three or less. The rock water ratio is more difficult to assess, but reasonable values are 4-20. Local information... 

Helium can be reliably measured in waters with an age of 104 years, but the age of groundwaters varies widely and may reach 108 years. Thus the measurable helium concentrations in groundwater may vary over four orders of magnitude due to differing ages therefore, helium concentration data are the most variable parameter of the age equation. 

Torgersen and Clarke (1985) found for the confined Jurassic aquifer in the Great Artesian Basin, Australia, that the observed helium concentrations were 70 times higher than expected for their calculation of the hydraulic ages. 

Helium Concentration Increases with Depth Groundwater Acts as a Sink of Helium... 

In confined groundwater systems deeper water strata often contain higher helium concentrations. This pattern is observed with depth increase along the dip of confined aquifers or in samples obtained from a sequence of aquifers of different depths sampled at one location. In some examples, presented in Fig. 14.1, the depth of the water is known from direct measurements, and in others it is reflected in the temperature, which increases with depth. 

The increase in helium concentration with depth is taken as an indication that the deeper waters are older, that is, they have been stored for longer times as compared to shallower waters. In this context it should be... 

Fig. 14.1 Helium concentrations as a function of aquifer depth or temperature (following Mazor and Bosch, 1992a) Bunter sandstone aquifer, eastern England (data from Andrews et al., 1984) Stripa granite, Sweden (data from Andrews et al., 1982) Blumau, Austria (data from Andrews et al., 1984) Molasse Basin, Austria (data from Andrews et al., 1981) Great Artesian Basin, Australia (data from Torgersen and Clarke, 1985).

Effect of Mixing of Groundwaters of Different Ages on the Observed Helium Concentration... 

The original helium concentration in the confined water can be reconstructed by dividing the observed concentration by the fraction of gas retained, as calculated from the accompanying atmospheric gases. These corrected helium concentrations, plotted as a function of the distance from the recharge area (Fig. 14.4), reveal a reasonable correlation, which provides confirmation of the reliability of the corrected helium values. 

Fig. 14.4 Helium concentration (corrected for gas losses) as a function of distance from the recharge area along two transects through the Great Artesian Basin (following Mazor and Bosch, 1992a). The good correlation provides a positive check on the validity of the corrected helium data, as the deeper inland groundwater must have been trapped earlier.

In order to date groundwaters, they have to be studied for possible mixing of waters of different ages. Helium concentrations should be applied for the calculation of meaningful ages only for nonmixed water or for reconstructed end members. 

Observed helium concentrations should be applied for water dating only in conjunction with the ANG, to correct for gas losses or gains. 

Internal checks are necessary for reliable water dating—for example, correlations of helium concentration with distance from the recharge area, water depth, temperature, or concentration of accompanying radiogenic 39Ar. 

Exercise 14.1 Study Fig. 11.26. Take the average helium concentration in group C as 26 x 10 8cc He/cc water, and assume that the hosting rocks contain 2 ppm U, 2 ppm Th, the density of the rocks is 2.5, the rock water... 

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