Phase equilibria impurity effects

The s-H hydrates have potential to become a better medium for natural-gas (mixed-gas of CH4 and impurities) storage and transportation . The s-H hydrates helped by CH4 can be generated under lower pressure condition than the pure CH4 hydrate. That is, we can handle the natural-gas hydrate (NGH) under more moderate condition by generating the s-H hydrate. The pressure reduction from pure help-gas hydrate depends largely on the kind of LGS. Some literatures reported that both the molecular size and the molecular shape of LGS have much effect on the equilibrium pressure of s-H hydrate. A lot of phase equilibrium data for the s-H hydrate systems are required to develop the effective storage and transportation system. 

Impure metals and alloys exhibit all the structural features and crystal defects of the pure meteils already discussed. In addition, however, impure metals and alloys exhibit many structures which are not observed in pure metals, and which, in many instances, have an extremely important effect on the properties, particularly the corrosion resistance. However, before dealing with the structure of impure metals and alloys, it is necessary to consider the concept of metallurgical components, phases, constituents and equilibrium phase diagrams. 

The presence of impurities between phases oc and j3 in contact implies the introduction of a new phase (y). If current I passes and this new phase has resistance / , then a further potential drop RI appears in the system. However, at equilibrium the presence of phase y has no effect on the measured results, as it still holds that fic(a) = jUe(/J) = jUe(y). Similarly, the final equation is not affected by the fact that the leads and coils of the potentiometer are not made of the same metal as phase oc. It can also be readily seen that the validity of Eq. (3.1.31) is retained even in the presence of a new phase y between phase oc or phase oc and the potentiometer. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

Most measurements of densities of liquids below their normal boiling points are made in the presence of air. Densities reported here refer to liquids in equilibrium with a gas phase consisting of a mixture or air and vapor at a total pressure of one atmosphere below the normal boiling point and of vapor at the equilibrium vapor pressure above the boiling point. Thus air is not regarded as an impurity. The effect of dissolved air and other gases on the densities of liquid hydrocarbons has been reported by Ashcroft and Ben Isa [97-ash/ben], The differences they observed between the density of a liquid saturated with air at 1 atm and 298.15 K and the air free liquid are shown in Table 1. 

Effect of Impurities. In the previous sections we have considered calcite dissolution 1) far from equilibrium in various solutions, and 2) both far from and near equilibrium but only in pure C02 water solutions. We now investigate calcite dissolution in solutions where backward reaction must be considered in the presence of impurities. Impurities can be defined as constituents present in the aqueous phase that are not part of the original stoichiometric composition of the reactant ( 2). The presence of impurities can have a profound effect on the rate of dissolution or precipitation, but, unfortunately, there is no simple way of predicting, a priori, the total effect that an impurity or combination of impurities can have on the rate of reaction. 

In fact, thermal equilibrium is not attained in the vapor phase osmometer, and the foregoing equations do not apply as written since they are predicated on the existence of thermodynamic equilibrium. Perturbations are experienced from heat conduction from the drops to the vapor and along the electrical connections. Diffusion controlled processes may also occur within the drops, and the magnitude of these effects may depend on drop sizes, solute diffusivity, and the presence of volatile impurities in the solvent or solute. The vapor phase osmometer is not a closed system and equilibrium cannot therefore be reached. The system can be operated in the steady state, however, and under those circumstances an analog of expression (3-6) is... 

Speeds of sound were measured at 30° C and 1.5 GPa at frequencies of 1.3, 0.77 and 0.27 GHz. Velocities matched to within the uncertainties, i.e. 0.2% for the higher frequency and 0.5% for the two lowest. The ISLS velocities fair nicely with those of the W S model and are lower than the extrapolation of W S. More dispersion may exist at lower frequencies. Between 22° C and 122° C the fluid j8-phase boundary is well fit by the straight line P(GPa) = 0.0270 T(°C) 5.153 with a two o uncertainty on the slope of 10 GPa/°C. Each point of equilibrium was established by a visual observation of the simultaneous presence of both phases. Among observations, the volume of solid varied from approximately 5 to 95% of the sample no correlation was apparent between the deviations of the data from the fit and the fraction of solid. Since one expects that any impurities will be concentrated in the fluid, this fact suggests strongly that impurities had no significant effect on the measurements... 

The effect of substitutional impurities on the stability and aqueous solubility of a variety of solids is investigated. Stoichiometric saturation, primary saturation and thermodynamic equilibrium solubilities are compared to pure phase solubilities. Contour plots of pure phase saturation indices (SI) are drawn at minimum stoichiometric saturation, as a function of the amount of substitution and of the excess-free-energy of the substitution. SI plots drawn for the major component of a binary solid-solution generally show little deviation from pure phase solubility except at trace component fractions greater than 1%. In contrast, trace component SI plots reveal that aqueous solutions at minimum stoichiometric saturation can achieve considerable supersaturation with respect to the pure trace-component end-member solid, in cases where the major component is more soluble than the trace. 

There is one striking difference between the two sources of impurities. The contamination present in the surfactant sample appears as a minor component in the solution bulk with a concentration proportional to that of the main surfactant. If the origin of the impurity is the organic phase, its concentration is constant and independent of the surfactant concentration. Therefore, the effects of these two types of impurities on the adsorption kinetics and adsorption equilibrium are different. 

The accumulation of impurities in the interfacial region causes an effective increase in incorporation in the crystalline phase relative to that predicted by Eq. (3.5) alone. J.A. Burton et al. developed an expression that quantitatively relates the effective impurity distribution coefficient, K f/, to the equilibrium distribution coefficient, K... 

There are five established allotrdpic forms of cerium and some evidence for two or three others which actually may be metastable or impurity stabilized. The pressure-temperature relationships of the stable phases, their crystal structures and valences are discussed first under equilibrium conditions. Then non-equilibrium and hysteresis effects relative to the five established phases are reviewed. Finally the known information on the less reliably established phases is examined. 

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