Vanadium transfer

Although vapor phase mass transfer of vanadium is consistent with all our data, further experiments were performed to test the particle-to-particle mechanism. In this case, the rate of vanadium transfer is expected to be proportional to the collision frequency in the bed. 

Catalyst and trap blends were steamed in a fluid bed using the same conditions as in Table I. Weight blend ratios with 95/5, 90/10, 85/15 of catalyst trap for both the fine and the coarse fractions were steamed, and the sink/float technique was again used to measure the rate of vanadium transfer. The results (Table III) show that there is not a strong dependence of the vanadium transfer rate on particle size. The rate enhancement, which is the rate of the fine catalyst / rate of the coarse catalyst, only varies by 10%, whereas the number density varied by a factor of 6.5. This experiment shows that the rate of vanadium transfer from catalyst to trap is only weakly dependent on the particle density, and it implies that particle-to-particle collisions are not the dominant mode of vanadium transfer. 

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. 

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. 

Figure 3. Effect of oxidative vs. inert atmosphere on vanadium transfer.

Mechanism of Vanadium Transfer. In previous literature two mechanisms of vanadium migration were postulated Uquid or gas phase vanadium transport. The results in this study lead us to propose a third alternative mechanism It appears from our data that a more appropriate mechanism involves interparticle vanadium transport which occurs as a result of a solid state interaction between vanadium containing particles. 

Vanadium is transferred by interpaiticle solid state transport. The combination of oxygen or air plus steam promotes surface migration and enrichment of vanadia species which are not crystalline V2O5. Interpaiticle contact is a requirement for vanadium transfer from particle to particle. Evidence for a volatile vanadic acid species could not be found. 

A trap was then blended with 90wt% fresh catalyst and steamed by cyclic propylene steam (CPS) for 20 hours. After steaming, the catalyst and trap were density separated and analyzed for vanadium. Results are presented in Table 8. As shown in the table, less than 6% of the vanadium migrates back to the catalyst. This represents an insignificant amount of the total vanadium transferred. Additionally, since the vanadium on the catalyst may migrate back to the trap over time, the degree of reversibility may actually decrease with time. 

Vanadium pentoxide, vanadium(V) oxide, V2O5, is the most important compound in this oxidation state. It is a coloured solid (colour due to charge transfer, p. 60), the colour varying somewhat (red -> brown) with the state of subdivision it is formed when vanadium (or some of its compounds) is completely oxidised, and also by heating ammonium vanadate)V) ... 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

Figure 6.5 Proposed structure for the vanadium complex prior to the oxygen transfer from peroxide to the allylic olefin.

Aqueous electron transfer reactions vanadium(V) as reductant compared to iron(II). D. R. Ros-seinsky, Chem. Rev., 1972, 72, 215-229 (116). 

The adsorption spectrum of aerosil containing admixture vanadium ions exhibits a maximum within the band 290 - 380 nm which was attributed by authors of [97] to the charge transfer transitions in oxygen-containing complexes of five valance vanadium = O -... 

Similar to molybdenum oxide catalyst the capability to emit singlet oxygen is inherent to Si02 doped by Cr ions as well. Similar to the case of vanadium oxide catalysts in this system the photogeneration occurs due to the triplet-triplet electron excitation transfer from a charge transfer complex to adsorbed oxygen. 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

I) axial heat transfer of fixed bed packed with vanadium catalyst. J. Chem. lnd. Eng. (China) 46,416-423 (1995). 

---