Helium molecular diameter

An alternative method of representing the movement of an individual molecule by computational techniques is shown in Figure 2.4 [9], This figure shows the movement of three different permeate molecules over a period of 200 ps in a silicone rubber polymer matrix. The smaller helium molecule moves more frequently and makes larger jumps than the larger methane molecule. Helium, with a molecular diameter of 2.55 A, has many more opportunities to move from one... 

Gas-viscosity measurements yield molecular diameters of 2.58 A (angstroms) for helium, 3.42 A for argon, and 4.00 A for carbon dioxide. With these values, calculate and c/i3 from Eq. (V-36), and calculate D i and Z i3 using Eq. (V-34). Compare with your experimental values. 

Molecular diameter of helium is less than 0.2 nm, and it is almost not adsorbed by any sample. Hence, it is an ideal gas to measure the total volmne including the volume of pores in particles and stacking gaps between particles. The testing equipments are conventional static gas adsorption devices. It detects the pressure difference of helium gas during the test, and then to Vsk of the sample is calculated by gas law, getting the value of skeletal density as shown in Eq. (7.42). 

Apparent density (pa)- Skeletal density is measm-ed by the medium of benzene, isopropanol etc, (not helium gas) and is not considered as true density but rather apparent density. Because their molecular diameters are bigger than helium and absolutely cannot enter into the iimer pores of catalyst (especially of microp-orous), the obtained skeletal volume is just an approximate value. 

Uranus The temperature in the Uranus atmosphere, which consists of molecular hydrogen containing around 12% helium, is close to 60 K. A methane cloud layer has been detected in the lower layers of this atmosphere. The planet is surrounded by a magnetosphere which extends into space for about ten times the diameter of Uranus. The planet has 27 moons of various sizes and is surrounded by a ring system which consists of thin dark rings. The planet is unusual in two respects its tilted axis and retrograde rotation. 

The diameter and volume of the micropores were also determined by the measurement of the density using as displacement molecules with different sizes, e g., helium, water, benzene, decaline. It was found that in the case of CMS-Kl and CMS-K2 sieves, the micropores with the pore size within the range 0.255-0.528 are dominated and that the used measurements enable characterisation of the structure of carbon molecular sieves. For equilibrium sieve the analysis of the micropores volume with the use of the pycnometric technique does not give proper results. 

Effective density. The diameter and volume of the micropores were determined by the measurement of the density using as displacement molecules with different sizes of effective diameter, e g., helium (0.25 nm), water (0.264 nm), benzene (0.370 x 0.528 nm), and decaline (0.472 X 1.01 X 0.624 nm). All pycnometric fluids are non-polar, except water. This adsorbate was used for the sake of the little diameter of its molecule. In the case of CMSs studied - not including of oxygen surface groups [8] - water molecule is good molecular probe. 

On the other hand, actual binary mixture tests using porous alumina and glass membranes show separation factor values for helium recovery from oxygen that are lower than what Knudsen diffusion provides, as indicated in Table 7.15. Only Koresh and Soffer [1983a 1983b] show an ideal separation factor of 20 to 40 with a low permeability of 1.2x10 barrers when molecular sieve membranes with a reported pore diameter of 0.3 to 0.5 nm are used. 

The so-called permanent gas fraction was routinely analyzed on a 6-ft X 1/8-inch diameter Supelco Porapak Q column using a Perkin-Elmer Model 3920 gas chromatograph (G.C.). The flowrate of the G.C. carrier, helium, was 30 ml/min. The bridge current was set for 175 mA and the thermal conductivity detector temperature was maintained at 200°C. CO and CO2 peaks were quantitatively analyzed at room temperature while the assymmetry of the acetylene peak necessitated elution at 100"C. The presence of hydrogen was determined on a molecular sieve 13X column at room temperature. Since acetylene was not separated from ethylene, confirmation of acetylene was made on a Supelco Porapak T column (10 ft x A inch) at room temperature. 

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