Thermal conversion front

Gort R., On the Propagation of a Reaction Front in a Packed Bed - Thermal conversion of municipal solid waste and biomass, PhD Thesis, University Twente in Enschede, Dept of Environmental Sciences, Energy Research, and Process Innovation of TNO, the Netherlands, (1995). 

The identification of prismanes 6 and 7 followed from spectral data and their thermal conversions inLo pyridines 8 and 9 (front 6) and 10 (from 7). The azaprismanes are quite stable and rearrange only slowly at 175 C. The formation of 6 is evidence of the phototransformation represented in Scheme 11,31 which is also observed for perfluoro(alkylpyridazincs) (vide infra). 

Figure 9.6 shows the evolution of temperature and conversion profiles when the specimen thickness is reduced i.e., for W, =0.1. Again, the material located close to the wall polymerizes at a fast rate, originating thermal and conversion fronts that travel to the core and to the wall. The maximum temperature is obtained at an intermediate location and is higher than that attained in the previous case T0 + ATad < Tmax < Tw + ATad. 

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

At places where the front is concave toward the unburnt gas, the heat flux is locally convergent. The local flame temperature increases and the local propagation velocity also increases, see the red arrows in Figure 5.1.5. The converse holds for portions of the front that are convex. The effect of thermal diffusion is to stabilize a wrinkled flame. 

Peters et al. (46) utilized their fourth-order approximation of the fountain flow velocity field, Eqs. 13.1-9 and 13.1-10, and the particle tracking numerical technique they incorporated, to calculate the temperature and conversion fields in that region. They assumed that the very flow front material particles experience an adiabatic thermal history, which is reasonable. 

Flat-plate PV collectors contain an array of individual cells connected in a series/parallel circuit and encapsulated within a sandwich structure, the front of which is glass or plastic. Unlike thermal collectors, the back of these collectors is not insulated, because for best performance, they need to be cooled by the atmosphere. If this energy could be used and thereby this loss could be eliminated in new designs, the conversion efficiency could be much improved. 

It should be noted that the temperature gradient in the front at 4.2 K is about 2 times that at 77 K (see Fig. 8), that is, the degree of chemical conversion increases with decreasing initial temperature. This is also atypical of the thermal mechanism based on the Arrhenius law, but can be explained by the above hypothesis, considering that the lifetime of a new surface in the active state increases with decreasing temperature, which results in the enhanced conversion. 

As a rule, an increase in temperature in the course of polymerization is accompanied, by various kinetic effects. For example, in the radical polymerization of vinyl monomers changes can take place in the concentration of radicals and the time when the gel effect sets in. In addition a process of degradation can be superimposed on the polymerization process. The temperature and conversion non-uniformities occurring in the course of polymerization can change the thermal process itself, converting bulk polymerization into a reaction with propagating front, and vice versa. 

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