Vanadium vapor pressure

Next, let the example of vanadium, which, in the as-reduced condition, may contain a variety of impurities (including aluminum, calcium, chromium, copper, iron, molybdenum, nickel, lead, titanium, and zinc) be considered. Vanadium melts at 1910 °C, and at this temperature it is considerably less volatile than many of the impurity metals present in it. The vapor pressure of pure vanadium at this temperature is 0.02 torr, whereas those of the impurity elements in their pure states are the following aluminum 22 torr calcium 1 atm, chromium 6 torr copper 23 torr iron 2 torr molybdenum 6 1CT6 torr nickel 1 torr lead 1 torr titanium 0.1 torr and zinc 1 atm. However, since most of these impurities form a dilute solution in vanadium, their actual partial pressures over vanadium are considerably lower than the values indicated. Taking this into account, the vaporization rate, mA, of an element A (the evaporating species) can be approximated by the following free evaporation equation (Langmuir equation) ... 

V metals, vanadium has the least tendency to deoxidize by carbon monoxide evolution. This means that, at a given temperature and a given value of Pco, the residual carbon and/or oxygen contents in vanadium will be compared more to niobium and tantalum. In other words, the removal of carbon and/or oxygen from vanadium will occur to a much lesser extent than in the cases of niobium or tantalum. The effect of carbon deoxidation can be quite complicated if there is a significant loss of the metal by vaporization. The requirement of a low vapor pressure is also better satisfied by niobium and tantalum than by vanadium. 

In the refining of the Group V metals (which are more accurately represented as metal-carbon-oxygen alloys), carbon deoxidation is not the only method by which oxygen is removed, because sacrificial deoxidation also occurs simultaneously. The relative extents to which each of these two deoxidation modes contributes to the overall removal of oxygen can be assessed by calculating the ratio of the vapor pressures of carbon monoxide and the metal monoxide over the M-C-0 alloy. The value of this ratio for vanadium at 2000 K is given by the expression... 

The best results were obtained with compound 21 that exhibited high vapor pressure and low decomposition temperature (<523 K). Various CVD conditions were applied and gave in all cases shiny, dark-brown deposits.43 XRD and XPS analyses of the deposits indicated the presence of a vanadium carbonitride phase with little contamination from oxygen and free carbon. The films were less adherent on steel substrates than on silicon ones. The steel substrates seemed to suffer corrosion due to the presence of Cl-containing species. We had noticed the same feature in the case of Cl-containing precursors to vanadium carbide. Therefore, in order to increase the volatility of compound 23 and to reduce the Cl content of the molecule, we prepared compounds 24 and 25. Unfortunately, the yields obtained in their syntheses were much too low to permit TG and CVD experiments. 

Vanadium oxytrichloride is a lemon-yellow liquid. Its boiling point is 124.5°C. at 736 mm. and 127.16°C. at 760 mm. It remains liquid at —77°. The vapor pressure at —77° is 4.1 mm. at 0°, 21 mm. and at 85°C., 270 mm. Its density in grams per milliliter is 1.854 at 0° and 1.811 at 32°C. At ordinary temperatures, it neither dissolves nor reacts with carbon, hydrogen, nitrogen, oxygen, silicon, tellurium, or metals except the alkali metals and antimony. The reactions with the alkali metals are explosive at characteristic temperatures, varying from 30°C. for cesium to 180°C. for sodium (lithium not determined). Small... 

Vanadium oxytrichloride is a lemon-yellow liquid with a boiling point of 127°1,2 and a freezing point of — 79.5°.2 Vapor-pressure data are also given in reference 2. Its chemical properties are those to be expected of a covalent, anhydrous metallic halide. It is very readily hydrolyzed and should be protected from moisture at all times. If the liquid has been exposed even briefly to moisture, its color will be orange or red, and it will contain an orange-red precipitate. 

Hydrothermal synthesis does not require the water to be above its critical point. Huan, et al. published a synthesis of VOC6H5PO3XH2O prepared from phenylphosphonic acid, CeH5PO(OH)2 and vanadium(III) oxide, V2O3 (Huan et al., 1990). The two reagents were added to water, sealed in a Teflon acid digestion bomb, and heated to 200°C. Pure water has a vapor pressure of 225 PSI at 200°C, well within the bursting pressure of the bomb (1800 PSI). Unlike the quartz example, in this case, the solvent became incorporated into the final product. 

The calculated vapor pressure of vanadic acid is a factor of 30 lower than the equilibrium vapor pressure of vanadic acid over pure V2O5. At this vapor pressure of vanadic acid, the transfer of V to trap is rapid while the removal of V by transpiration is negligible. Approximately 64% would be transferred from catalyst to trap, and about 0.05% removed by transpiration. This is rationalized by noting that the velocity of the vanadic acid vapor (calculated from the kinetic theory of gasesX 4.4 x 10" cm s, is four orders of magnitude higher than the superficial velocity of the fluidizing gas. Equation 3a From these calculations it is clear that mass transfer of vapor phase vanadic acid in a fluid bed is sufficient to account for the transport of vanadium from catalyst to trap. 

On the contrary these results support the interpretation that gas phase vanadic acid is in pseudo-equilibrium with vanadium on the FCC catalyst, and the rate limiting step in vanadium transfer is the adsorption of vanadic acid vapor onto the trap. Since the rate of vanadium transfer is independent of catalyst particle size, intraparticle vanadium transfer must be very fast compared with interparticle vanadium transfer. The vanadium transfer experiments can provide information on the vapor pressure of vanadic acid, using the same techniques as outlined above. Equation 4 can be rewritten as. 

It is also possible that, because the surface of the catalyst is inhomogeneous, the equilibrium between H3VO4 vapor and surface vanadium may not follow the Langmuir isotherm. In this case, the vapor pressure can be modeled by a Freundlich isotherm. 

The results of this work show that even though the vapor pressure of vanadium is low, the transfer velocity of vanadium vapor is high and the rate of mass transfer in a fluidized bed is high. A high rate of vanadium transport to traps and a low rate of vanadium transport by transpiration are consistent with the vapor phase transport model. The vapor pressure of the vanadic acid follows a second order Freundlieh isotherm, which reflects a coverage dependent heat of adsorption. The rate of vanadium transfer from catalyst to trap is only weakly dependent on the number density of the catalyst or trap particles. This lack of dependence suggests that inter-particle collisions are not the dominant mechanism for vanadium transfer. Vanadium mobility in FCCU s is a complex issue dependent on many operating variables. 

The vapor pressure over vanadium was measured by Edwards et al. (j ) using the Knudsen method and by Farber and Snvastava (6) using effusion-mass spectrometric techniques. Our 2nd and 3rd law analysis is tabulated below, where reaction (A) refers to the sublimation V(cr) = V(g) and reaction (B), the vaporization V(t) = V(g). 

Mixed carbide fuels eliminate undesirable second phases or render them harmless. Alloying UC with ZrC increases the melting point and lowers the vapor pressure. Chromium and vanadium improve the compatibility of the carbide fuel with stainless steel cladding. 

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