Reoxidation, heat

V/y-Al203 of different loadings contained different vanadia species (45). Therefore, the variation in reoxidation heat (and thus dehydrogenation selectivity) could also be interpreted in terms of different properties of different vanadia species. In addition to V/y-AFCb, it has been shown that the vanadia species in V/Si02 also depended on the loading. 

The heats of oxidation of the reduced oxides can be further measured using O2 adsorption. Large variations of the reoxidation heat can be sometimes observed when any further oxidation is limited by the diffusion of oxygen into the reduced portion of the particle [61]. 

Vanadium trioxide, V2O3. Black powder (V2OS plus Hj under heat). Readily reoxidized to VjOj. Stable down to VO,.35. 

Reoxidation occurs when the metallic iron in hot DRI reacts with oxygen in the air to form either Ee O or Ee202. The reaction continues as long as the DRI remains hot and sufficient oxygen is avadable. Because reoxidation reactions are exothermic and DRI is a good insulator, it is possible that once reoxidation begins inside a pde, the DRI temperature increases and accelerates the reoxidation rate. Although the inner core of the pde may reach temperatures up to the fusion point of iron, the maximum temperature of the outer parts of the pde will be much lower because of heat dissipation. 

Allowing DRI to become wet does not necessatily cause it to overheat. When large pdes of DRI are wetted with rain, the corrosion reactions are limited to the outer surface area of the pde and the resultant heat from the corrosion reactions is dissipated into the atmosphere. However, if water penetrates into the pde from the bottom, or if wet DRI is covered with dry DRI, the heat from corrosion reactions can budd up inside the pde to the point where rapid reoxidation begins. Corrosion occurs significantly faster with salt water than with fresh water. DRI saturated with water can cause steam explosions if it is batch charged into an electric arc furnace. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

These formerly involved the use of banks of externally heated, horizontal retorts, operated on a batch basis. They were replaced by continuously operated vertical retorts, in some cases electrically heated. Unfortunately none of these processes has the thermal efficiency of a blast furnace process (p. 1072) in which the combustion of the fuel for heating takes place in the same chamber as the reduction of the oxide. The inescapable problem posed by zinc is that the reduction of ZnO by carbon is not spontaneous below the boiling point of Zn (a problem not encountered in the smelting of Fe, Cu or Pb, for instance), and the subsequent cooling to condense the vapour is liable, in the presence of the combustion products, to result in the reoxidation of the metal ... 

The reason for this is that reoxidation of NADH via the alternative electron transport chain (not coupled to oxidative phosphorylation) liberates heat. 

Fig. 55. TG curve showing the oxidation of BaO to BaC>2 followed by decomposition and incomplete reoxidation during cooling. Sample weight 555.19 mg, heating rate 10°C/min, atmosphere ...

Fig. 62. Decomposition and reoxidation of Egyptian Blue and of the isostructural Sr and Ba compound in air, heating rate 2 °C/min...

Remote control model, 40 183-184 Reoxidation, 41 198-199 heat of, 40 19-22 Resolution, 33 251-254 Resonance-enhanced multiphoton ionization, in detection of surface-generated gas-phase radicals, 35 181-182... 

Selectivity and Metal-Oxygen Bond Strength. The metal-oxygen bond strength could be measured as the differential heats of reoxidation of the reduced... 

We have measured the differential heats of reoxidation and the reaction characteristics of a number of V2O5/Y-AI2O3 catalysts. The chemical transformation corresponding to the heat measurement in this system is ... 

Figure 3. Differential heat of reoxidation and selectivity for oxidative dehydrogenation of butane on V2O5/Y-AI2O3 samples, a 8.2 V/nm sample, reaction at 400°C and b 2.9 V/nm sample, reaction at 480°C.

The portion of this work on the vanadates has been supported by the Department of Energy, Basic Energy Sciences, Division of Chemical Sciences, and that on the heats of reoxidation by the National Science Foundation. PM acknowledges support by the Battelle s NASA Advanced Materials Center for the Commercial Development of Space, and LO acknowledges fellowship support from the 3M Company and a Faculty Minority Internship from the Monsanto Company. 

The production of sulphur compounds is believed to be very important in the development of Cheddar cheese flavour. Residual sulphydryl oxidase activity may play a role in initially reoxidizing sulphydryl groups exposed upon heating cheesemilk the sulphydryl groups thus protected may be reformed during the ripening process. 

Fig. 7. Differential heat of reoxidation and selectivity for oxidative dehydrogenation of butane on V2Ov y -AFO, samples. For the 2.9 V/nm2 sample, the selectivity was calculated for the detected gaseous products, (a) 8.2 V/nm2 sample, reaction at 400°C (b) 2.9 V/ntn2 sample, reaction at 480°C (c) 8.2 V/nm2 sample, reduction by CO at 530°C, butane reaction at 400°C and (d) 2.9 V/nm2 sample, reduction by CO at 400°C, butane reaction at 480°C. (a) and (b) are from Ref. 50 (c) and (d) and from P. J., Andersen, Ph D. thesis, Northwestern University, 1992.

Other Processes. Other production processes are suggested in the patent literature, but have not so far gained industrial importance. For instance, sodium dichromate can be mixed with heating oil and reacted at 300 °C. The soda formed must be washed out prior to calcining at 800 °C to avoid reoxidation in the alkaline melt [3.56],... 

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