Reactive zone

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

The gradient of species concentration is in a direction opposite to the thermal gradient, see the green arrows. At places where the front is concave toward the unburnt gas, the species flux is locally divergent. The flux of reactive species into the reactive zone decreases, leading in turn to... 

With fixed-bed updraft gasifiers, the air or oxygen passes upward through a hot reactive zone near the bottom of the gasifier in a direction countercurrent to the flow of solid material. Exothermic reactions between air/oxygen and the... 

This could be solved by trial, using a numerical integration, to find C as a function of t. However, the graph is of a direct numerical solution of Eq (1). The degradation of the catalyst Is very rapid. In practice the catalyst will proceed from the transfer line to a reactivation zone and will be recycled. 

Previous exposure to benzoate, but not to acetate, shortened the acclimation period to BTX degradation and enhanced the short-term bio-attenuation potential of the indigenous consortium, suggesting that benzoate could potentially be used to establish and sustain in situ reactive zones to attenuate BTX migration and protect downgradient groundwater resources. 

The meehanism of Mo removal in suboxie systems is unelear, and so the fundamental nature of this fraetionation requires further study. However, the effeet may be rmderstood in terms of a two layer diffusion-reaetion model in whieh a reaetion zone in the sediment (where Mo is ehemieally removed) is separated from seawater by a purely diffusive zone in which there is no chemical reaction (Braudes and Devol 1997). The presence of a diffusive zone is likely because Mo removal presumably occurs in suMdic porewaters that lie a finite distance L below the sediment-water interface (Wang and van Cappellen 1996 Zheng et al. 2000a). If HjS is present in the reactive zone such that Mo is removed below this depth, then Mo isotope fractionation in the diffusive zone may be driven by isotope effects in the reactive zone. 

Figure 3. Reactive Zone or Permeable Reactive Barrier to treat a contaminant plume in- situ.

Many industrial processes involve mass transfer processes between a gas/vapour and a liquid. Usually, these transfer processes are described on the basis of Pick s law, but the Maxwell-Stefan theory finds increasing application. Especially for reactive distillation it can be anticipated that the Maxwell-Stefan theory should be used for describing the mass transfer processes. Moreover, with reactive distillation there is a need to take heat transfer and chemical reaction into account. The model developed in this study will be formulated on a generalized basis and as a consequence it can be used for many other gas-liquid and vapour-liquid transfer processes. However, reactive distillation has recently received considerable attention in literature. With reactive distillation reaction and separation are carried out simultaneously in one apparatus, usually a distillation column. This kind of processing can be advantageous for equilibrium reactions. By removing one of the products from the reactive zone by evaporation, the equilibrium is shifted to the product side and consequently higher conversions can be obtained. Commercial applications of reactive distillation are the production of methyl-... 

T0052 ARCADIS Geraghty and Miller, In Situ Reactive Zones Using Molasses T0057 Arctech, Inc., Ozo-Detox... 

T0052 ARCADIS Geraghty and MiUer, In Sitn Reactive Zones Using Molasses... 

In situ reactive zones using molasses is an in situ bioremediation technology that precipitates heavy metals and destroys organic contaminants in groundwater. The technology uses... 

The vendor states that technology has been tested in pilot- and full-scale field demonstrations at over 85 sites in the United States and Europe. In situ reactive zones using molasses is commercially available. 

The toxicity of the degradation products may exceed the toxicity of the parent compounds. Heavy metals are converted to less soluble forms they are not removed from the subsurface. Heterogeneities in the subsurface may cause the uneven distribution of nutrients during direct-injection applications. Injection may be slower in formations with low hydraulic conductivities. Smaller reactive zones may also form in areas with low hydraulic conductivities. 

At an abandoned manufacturing facility in Emeryville, California, the groundwater was contaminated with hexavalent chromium and up to 12,000 micrograms per liter (p,g/liter) of trichloroethene (TCE). A pilot-scale demonstration of in situ reactive zones using molasses was conducted in 1995 and 1996. In 1997, the project was expanded to a full-scale application. The full-scale system used 91 temporary injection points to deliver molasses to the subsurface. The overall project cost was approximately 400,000 (D210571, p. 90). 

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