Gaseous impurities

In cases in which a relatively large weight of catalyst, compared with that of the poison, is present, e.g., for the adsorption of a gaseous poison, or for an impure gaseous system containing a poison, in an adsorption bulb filled with platinum black, the values for the partition factor Ki may be far greater than those given in the table, which were measured... 

On compression, a gaseous phase may condense to a liquid-expanded, L phase via a first-order transition. This transition is difficult to study experimentally because of the small film pressures involved and the need to avoid any impurities [76,193]. There is ample evidence that the transition is clearly first-order there are discontinuities in v-a plots, a latent heat of vaporization associated with the transition and two coexisting phases can be seen. Also, fluctuations in the surface potential [194] in the two phase region indicate two-phase coexistence. The general situation is reminiscent of three-dimensional vapor-liquid condensation and can be treated by the two-dimensional van der Waals equation (Eq. Ill-104) [195] or statistical mechanical models [191]. 

High Purity Aluminum Trifluoride. High purity anhydrous aluminum triduoride that is free from oxide impurities can be prepared by reaction of gaseous anhydrous HF and AlCl at 100°C, gradually raising the temperature to 400°C. It can also be prepared by the action of elemental fluorine on metal/metal oxide and subsequent sublimation (12) or the decomposition of ammonium duoroaluminate at 700°C. 

Commercial boron trifluoride is usually approximately 99.5% pure. The common impurities are air, siUcon tetrafluoride, and sulfur dioxide. An excellent procedure for sampling and making a complete analysis of gaseous boron trifluoride has been developed (57). 

Anhydrous FeF is prepared by the action of Hquid or gaseous hydrogen fluoride on anhydrous FeCl (see Iron compounds). FeF is insoluble in alcohol, ether, and ben2ene, and sparingly soluble in anhydrous HF and water. The pH of a saturated solution in water varies between 3.5 and 4.0. Low pH indicates the presence of residual amounts of HF. The light gray color of the material is attributed to iron oxide or free iron impurities in the product. 

Because of the development of electronic appHcations for WF, higher purities of WF have been required, and considerable work has been done to improve the existing manufacturing and purification processes (20). Most metal contaminants and gaseous impurities are removed from WF by... 

As opposed to gaseous, pure formaldehyde, solutions of formaldehyde are unstable. Both formic acid (acidity) and paraformaldehyde (soHds) concentrations increase with time and depend on temperature. Formic acid concentration builds at a rate of 1.5—3 ppm/d at 35°C and 10—20 ppm/d at 65°C (17,18). Trace metallic impurities such as iron can boost the rate of formation of formic acid (121). Although low storage temperature minimizes acidity, it also increases the tendency to precipitate paraformaldehyde. 

Impurities that can negatively affect the physical and electrical properties of the metallisation layer can originate from several sources, particularly the deposition source and the gaseous environment. Impurities stemming from the source bombard the surface of the growing film and get trapped in the metal layer. 

Impurities can be removed by formation of a gaseous compound, as in the fire-refining of copper (qv). Sulfur is removed from the molten metal by oxidation with air and evolution of sulfur dioxide. Oxygen is then removed by reduction with C, CO, in the form of natural gas, reformed... 

The iodide or van Arkel-de Boer process is a volatilization process involving transfer of an involatile metal as its volatile compound. It is used for the purification of titanium. The reaction of iodine gas with impure titanium metal at 175°C yields gaseous titanium iodide and leaves the impurities in the sohd residue. 

The calculated half-life of 1 mol % (1.5 wt %) of pure gaseous ozone diluted with oxygen at 25, 100, and 250°C (based on rate constants from Ref. 19) is 19.3 yr, 5.2 h, and 0.1 s, respectively. Although pure ozone—oxygen mixtures are stable at ordinary temperatures ia the absence of catalysts and light, ozone produced on an iadustrial scale by silent discharge is less stable due to the presence of impurities however, ozone produced from oxygen is more stable than that from air. At 20°C, 1 mol % ozone produced from air is - 30% decomposed ia 12 h. 

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

The epitaxy reactor is a specialized variant of the tubular reactor in which gas-phase precursors are produced and transported to a heated surface where thin crystalline films and gaseous by-products are produced by further reaction on the surface. Similar to this chemical vapor deposition (CVE)) are physical vapor depositions (PVE)) and molecular beam generated deposits. Reactor details are critical to assuring uniform, impurity-free deposits and numerous designs have evolved (Fig. 22) (89). 

Phase diagrams can be used to predict the reactions between refractories and various soHd, Hquid, and gaseous reactants. These diagrams are derived from phase equiHbria of relatively simple pure compounds. Real systems, however, are highly complex and may contain a large number of minor impurities that significantly affect equiHbria. Moreover, equiHbrium between the reacting phases in real refractory systems may not be reached in actual service conditions. In fact, the successful performance of a refractory may rely on the existence of nonequilibrium conditions, eg, environment (15—19). 

For adding dopiag impurities duriag vapor-phase growth, a gaseous or easily vaporizable Hquid compound is metered, added to the siUcon source gas stream, and reduced along with the siUcon compound. Typical examples are diborane, 2 phosphine, and boron tribromide, BBr. ... 

Vacuum Tubes. In the manufacture of vacuum tubes for use in polarized ion sources, vaporized cesium is used as a getter for residual gaseous impurities in the tube and as a coating to reduce the work function of the tungsten filaments or cathodes of the tube. The cesium vapor is generated by firing, at about 850°C within the sealed and evacuated tube, a cesium chromate pellet and zirconium (12) (see Vacuum technology). 

Reagents similai to those used in the analysis of chloiine are commonly employed in the quantitation of gaseous and aqueous chloiine dioxide as well as its reaction coproducts chlorine, chlorite, and chlorate. The volatihty of the gas from aqueous solutions as well as its reactivity to light must be considered for accurate analysis. Other interferences that must be taken into account include other oxidizers such as chloramine, hydrogen peroxide, permanganate, and metal impurities such as ferrous and ferric iron. 

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

Countercurrent gas flow is preferred in pollution control when removal of gaseous impurities is desired. 

The anticipated content of impurities in the refined metal may be calculated a priori by assuming thermodynamic equilibrium at both metal/gas interfaces, and using the relevant stabilities of tire gaseous iodides. Adequate thermodynamic data could provide the activities of the impurities widr that of zirconium close to unity, but tire calculation of tire impurity transport obviously requires a knowledge of activity coefficients in the original impure material, which are not sufficiently well known. 

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