Pure gases

Gas mixtures are subject to the same degree of non-ideality as the one-component ( pure ) gases that were discussed in the previous section. In particular, the second virial coefficient for a gas mixture can be written as a quadratic average... 

The use of group contribution methods for the estimation of properties of pure gases and Uquids [20, 21] and of phase equilibria [22] also has a long history in chemical engineering. 

How many samples are taken can be of importance. One sample often suffices where it is known that the material in question is homogeneous for the parameter(s) to be tested, such as for pure gases or bulk solvents. If this is not the case, then statistical sampling should be considered. Samples should be taken from various points within the material, if the material stratifies. 

For pure gases above atmospbeHc pressure, the method of Stiel and Thodos may be used ... 

The ideal-gas-state heat capacity Cf is a function of T but not of T. For a mixture, the heat capacity is simply the molar average X, Xi Cf. Empirical equations giving the temperature dependence of Cf are available for many pure gases, often taking the form... 

Contact Electrification This form of charging involves the contact and separation of solid-sohd, sohd-hquid, or hquid-liqmd surfaces. Pure gases do not cause charging unless they cany droplets or dust particles. 

Oxidation of SO2 is slow in a mixture of pure gases, but the rate is increased by light, NO2, oxidants, and metallic oxides which act as catalysts for the reaction. The formed acids can react with particulate matter or ammonia to form salts. 

With correct experimental procedure TDS is straightforward to use and has been applied extensively in basic experiments concerned with the nature of reactions between pure gases and clean solid surfaces. Most of these applications have been catalysis-related (i. e. performed on surfaces acting as models for catalysts) and TDS has always been used with other techniques, e.g. UPS, ELS, AES, and LEED. To a certain extent it is quantifiable, in that the area under a desorption peak is proportional to the number of ions of that species desorbed in that temperature range, but measurement of the area is not always easy if several processes overlap. 

Fig. 7.18 Scaling rates for cast iron in pure gases (Fe-3-20Cioiai-l 13Si-0-72Mn-0-125S-...

With a mixture of gases, the cooling effect was less than the average effect for the pure gases. 

Dymond, J. H. Smith, E. B. The Virial Coefficients of Pure Gases and Mixtures Oxford Press New York, 1980. 

In comparison with hydrocarbon and polymeric matrices, which have their own absorptions in the IR and can react chemically with the intermediates, inert gas matrices are free of these shortcomings. Neon, krypton and xenon have been used as matrix substances in some studies. However, only argon and nitrogen matrices are widely adopted because of the availability of the pure gases and the fact that there is a variety of cryostats that can provide the optimal temperature conditions for the formation of rigid and transparent matrices from these elements. 

In the zeolite-alumina composite the behaviour of different gases (permeance of pure gases as a function of the temperature. Figure 8) behave very differently from those predicted by ideal Knudsen diffusion processes. 

However this is not dways the case, especially when the two components weakly interact with the surface When using the membrane to separate a H2/ isobutane mixture, the permeation of isobutane, due to its size, is restricted over the entire temperature range and the transmembrane fluxes of the two components of the mixture better follow the permeabilities of the pure gases. Separation factors are here much higher (factors up to 80 have been measured). 

Gasteiger HA, Markovic NM, Ross PN. 1995. H2 and CO electrooxidation on well-characterized PL Ru, and Pt-Ru. 1. Rotating disk electrode studies of the pure gases including temperature effects. J Phys Chem 99 8290-8301. 

Approximate values for the thermal conductivity of pure gases, up to moderate pressures, can be estimated from values of the gas viscosity, using Eucken s equation, Eucken (1911) ... 

Nucleation in a pure liquid. According to the kinetic theory for pure gases and liquids, there are local fluctuations of densities, which are clusters of molecules in a gas and holes (or vapor clusters) in a liquid. Frenkel (1955) established the population distribution of such holes of phase B in a liquid of continuum phase A by Boltzmann s formula,... 

The dimensionless second term in (4.1), Iprop(wg), expresses the role of propagation in mixed gases. As in the case of HHG in pure gases [23], the optimization conditions 1/3 > La and Lc > 5La ensure that the macroscopic response is more than half of the maximum response. The asymptotic value in L —> 00 and Lc —> 00 is independent of ptot and is a monotonically increasing... 

To produce polymer on a planar CrOx/SiO2/Si(100) model catalyst in a flow of ethylene, one needs to consider that a sample of 1 cm2 exposes no more than 1014 highly reactive chromium ions, which makes the model extremely sensitive to deactivation by impurities such as water or acetylene. Hence, working with ultra-pure gases, cleaned by filters close to the position of the model catalyst, is crucial [90],... 

Permeabilities measured for pure gases can serve as a rough guide for selection of membrane materials. For design, data must be obtained on gas mixtures, where selectivities are often found to be much lower than those calculated from pure-component measurements. This effect is often due to plasticisation of the membrane by sorption of the most soluble component of the gas. This allows easier penetration by the less-permeable components. The problem of concentration polarisation, which is often encountered in small-scale flow tests, may also be responsible. Concentration polarisation results when the retention time of the gas in contact with the membrane is long. This allows substantial depletion of the most permeable component on the feed side of the membrane. The membrane-surface concentration of that component, and therefore its flux through the membrane, decreases. 

A steady source of gas supply is necessary, which is hard to come by for rare or pure gases. 

For pure gases, a good indication of which mechanism is dominant, Knudsen diffusion or laminar flow, is given by the Knudsen number... 

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