Vanadium transfer requirements

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. 

The H transfer in Equation 1.13, to a vanadium oxo ligand, is surprisingly slow. The transfer requires a great deal of structural reorganization, as the V-O distance increases by 0.264A when V=0 is converted to V-OH. The barrier to transfer of an H from V-OH to V=0 (in Marcus theory, the intrinsic barrier to selfexchange) is thus large, and its size is reflected in the size of the barrier to the C—>0 H" transfer in Equation 1.13 [46],... 

One of the most important requirements that must be met is the membrane s ability to prevent excessive transfer of water from one half cell to the other. The preferential transfer of water can be a problem in the vanadium battery as one half-cell (the negative half cell in the case of cation exchange membranes) is flooded and becomes diluted, while the other becomes more concentrated, adversely affecting the overall operation of the cell. Most of the membranes show good initial water transfer properties, but their performance deteriorates with exposure to the vanadium solutions. Sukkar et al. ° evaluated various polyelectrolytes to determine whether they could improve the selectivity and stability of the membranes in the vanadium redox cell solutions. Both the cationic and anionic polyelectrolytes evaluated improved the water transfer properties of the membranes, although upon extended exposure to the vanadium electrolyte the modified membranes did not maintain their improved water transfer properties. The solvent based Nuosperse 657 modified membrane displayed exceptional properties initially but also failed to maintain its performance with extended exposure to the vanadium solutions. 

The stoichiometry (equation 2) requires eight electrons transferred from vanadium(II) ions to dinitrogen atoms (six electrons) and hydrogen atoms (two electrons) and this kinetic equation (3) was interpreted as evidence for a polynuclear structure of the transition state during the reduction of dinitrogen. An alternative mechanism was suggested.145... 

The electron transfer role of vanadium has possible relevance to vanadium bro-moperoxidase, although this system and V-BrPO differ in that V-BrPO requires dihydrogen peroxide for catalytic activity. A speculative catalytic cycle has been proposed to be... 

Hiatt et a/.34a-d studied the decomposition of solutions of tert-butyl hydroperoxide in chlorobenzene at 25°C in the presence of catalytic amounts of cobalt, iron, cerium, vanadium, and lead complexes. The time required for complete decomposition of the hydroperoxide varied from a few minutes for cobalt carboxylates to several days for lead naphthenate. The products consisted of approximately 86% tert-butyl alcohol, 12% di-fe/T-butyl peroxide, and 93% oxygen, and were independent of the catalysts. A radical-induced chain decomposition of the usual type,135 initiated by a redox decomposition of the hydroperoxide, was postulated to explain these results. When reactions were carried out in alkane solvents (RH), shorter kinetic chain lengths and lower yields of oxygen and di-te/T-butyl peroxide were observed due to competing hydrogen transfer of rm-butoxy radicals with the solvent. 

There is an apparent paradox from these experiments vanadium moves readily from catalyst to trap in a fluid bed, yet vanadium transpiration is negligible under the same conditions. This result would suggest that the mechanism of transport is by particle-to-particle collisions in the bed. Still, an alternate question arises, what are the requirements for mass transfer of vapor phase vanadium in a fluid bed Following the correlation of Richardson and Szekely (72), the mass transfer coefficient, km, in a fluid bed can be calculated from the Sherwood number, Sh, and a knowledge of the particle Reynolds number. Re, in the bed by. 

In addition, exhaustive of a solution of the V(IV) complex with the thiol in a molar ratio 1 8 requires the overall transfer of 9 (1+8) electrons per vanadium and the resulting CV exhibits the expected cathodic reversible wave for the final V(V) product (Fig.2 e), which, upon a subsequent cathodic ca. one-electron CPE regenerates the parent V(IV) complex (f). 

Solution processes use autoclave, tubular, or loop reactors. As compared to slurry and gas-phase polymerization, solution processes are commonly operated at a much higher temperature to keep the polymer dissolved in the reaction medium, and at much lower average residence times (5-20 min, as opposed to 1-4 h). Since polymerization conditions are more uniform in solutions reactors - there are no inter- and intraparticle heat- and mass-transfer resistances, for instance - this configuration is commonly used for the production of EPDM rubbers with soluble Ziegler-Natta vanadium-based catalysts. Composition homogeneity is a require-... 

Overview A large number of catalysts based on vanadium [35-37], titanium [38 0], zirconium [41], hafnium [42], lanthanides (in particular neodymium, samarium, and ytterbium) [43], cobalt [44, 45], niobium [46], chromium [47], nickel [48], and palladium [49] provide well-defined polyolefins. Many of these systems are able to meet the requirements for living polymerizations by suppressing P-hydride and P-methyl eliminations, as well as chain transfer to cocatalysts, such as alkyl aluminums or methylaluminoxane (MAO). Since MAO is usually obtained as a liquid solution with residual trimethylalumi-num, drying MAO to a white powder and removing residual trimethylaluminum can help minimize chain transfer to cocatlysts. 

The catalyst is a mixture of vanadium and molybdenum oxides on an inert support. Typical inlet reaction tenperatures are in the range of 350°C to 400°C. The catalyst is placed in 25 mm diameter tubes that are 3.2 m long. The catalyst pellet diameter is 5 mm. The maximum tenperature that the catalyst can be exposed to without causing irreversible damage (sintering) is 650°C. The packed-bed reactor should be costed as a shell-and-tube exchanger. The heat transfer area should be calculated based on the total external area of the catalyst-filled tubes required from the simulatioa Because of the high tenperatures involved, both the shell and the tube material should be stainless steel. An overall heat transfer coefficient for the reactor should be set as 100 W/m °C. (This is the value specified in the simulation.)... 

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