Desorption, factors that influence temperature

The activation energy and pre-exponential factor may influence the desorption rate, as shown in Fig. 6.14. Note that when the activation energy is high, the peak moves toward higher temperatures. With increasing heating rate, peaks are shifted... 

In 1996, the vendor stated that soil treatment costs using the Enco-Tec low-temperature thermal desorption system began at approximately 40 per metric ton of soil treated. These costs include excavation and backfilling. Factors listed as influencing costs were the nature and concentration of site contaminants in the soil (D187717, p. 2). 

It has already been reported that the weight loss of as-synthesized MMSs depends on the kind of the template used in the synthesis [17]. This is an obvious consequence of the fact that different templates decompose and thermodesorb at different temperatures. However, it was somewhat unexpected that the decomposition/desorption of the same kind of the template may be dramatically influenced by the framework composition of materials [4,10-14]. This can be understood as an influence of the framework structure on the process of Hoffmann elimination of alkylammonium to the corresponding alkene and low molecular weight amine [4,8], This decomposition process leads not only to the elimination of the electrostatic framework-template interactions but also to the formation of decomposition products of lower molecular weight than that of the surfactant. Thus, the framework-surfactant interactions are crucial factors determining the thermogravimetric behavior. 

Chemical adsorption on metals is usually considered as nonacti-vated. For H2 on Ni/Al203, however, the isobars pass through a maximum at about 100°C 144), meaning that at 25°C kinetic factors (adsorption time) have an important influence on the amount of H2 adsorbed. For Rh/Si02 144) the amount of H2 adsorbed is found to depend very little on time the isobar descends smoothly as the temperature is increased. Thus each metal has its own behavior, which must be taken into account. The thermodynamics and kinetics of adsorption and desorption are discussed in the literature 145, 146). 

Temperature, which is an important influence in vapor systems, is only a minor factor in the desorption of solutes. With liquid systems, a change in temperature within the range that can be employed seldom alters the adsorption sufficiently to provide effective desorption. Temperature can be an adjunct to other means, but when so used it must be kept in mind that solutes vary in their response to changes in temperature. Some show increased adsorption as the temperature rises others decrease, and still others show no significant change. 

The characteristics of furnace and sample holder that may influence the desorption profile are bed length, diameter and porosity, while the characteristics of the sample that could be important are its weight, particle size radius, sample density, particle porosity and number of active sites. In order to avoid temperature gradients, the reactor can not be of big size hence, mass of the sample is limited by its size and geometry. Diameters of sample particles are another important factor - small particles decrease the possibility of intra-granular diffusion. 

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