Fronts broadening

From these publications, workers interested in exploring the microwave technique perceived it to be simultaneously beneficial through increased rates, yet hazardous in the presence of flammable organic solvents. Subsequently, a vast body of work was carried out with domestic microwave ovens, but under solvent-free conditions and without recourse to sample mixing or temperature measurement. This continued across a broadening front on the laboratory scale. These and other developments in microwave chemistry have been reviewed extensively in journals, book chapters4-20 and in a recent monograph21. 

To discern whether a front is sharpening, we compare the velocities at the initial concentration and final concentration. If the velocity at the final concentration is higher than that at the initial concentration, the front will be a sharpening front, or shock. In the opposite situation, in which the velocity at the final concentration is lower than that at the initial concentration, the front will be a spreading front or broadening front. If the velocity is independent of the concentration, the front will be an indifferent front. To discern the types of fronts, some (e.g.. Pope, 1980) compare the upstream and downstream velocities that correspond to the velocities at the final and initial concentrations, the terms used here. 

Preparative chromatography involves the collection of individual solutes as they are eluted from the column for further use, but does not necessarily entail the separation of large samples. Special columns can be designed and fabricated for preparative use, but for small samples the analytical column can often be overloaded for preparative purposes. Columns can be either volume overloaded or mass overloaded. Volume overload causes the peak to broaden, but the retention time of the front of the peak... 

Figure 2.5 Schematic representation of a loop-interface scheme for concunent eluent evaporation. The sample is first loaded in a loop and then, after switching the valve, directed by the caiiier into the GC column. The solvent evaporates from the front end of the liquid, thus causing band broadening. Since the column is not flooded, very large amount of liquid can be inti oduced.

We may contrast this behavior to that found for AOT. As shown in Figure 1, the chromatograms for AOT exhibit sharp fronts and somewhat diffuse tails, intermediate in shape between the symmetrical peaks typical of conventional solutes and the highly asymmetric chromatograms obtained for sodium dodecyl sulfate micelles in water (15). In addition, the concentration dependence of Mp" for AOT is gradual, not abrupt as for lecithin. These differences may be attributed to the lability of the AOT micelles which makes the observed retention time quite sensitive to the initial concentration (12) and leads to broadened chromatograms. 

Figure 7.11 Separation of a vixture of PTH-anino acid derivatives by unidinensional Multiple development (right) illustrating the use of the spot reconcentration nechanisn to control zone broadening. (left). A spot size after the first development and solvent front of the second development (line). B spot area (black oval) after reconcentration by the second development. C spot size after the second development. (Reproduced with permission from ref. 117. Copyright Dr. Alfred Huethlg Publishers).

Our data indicate that ammonia acts as a mild inhibitor for hydrogen absorption in Ni-containing AB5 alloys. The measured heat of adsorption of NH3 on Ni is about -11 kcal/mol NH3, suggesting weak, physical adsorption. Since Ni is viewed to be a catalyst for the hydriding reaction, weak physical adsorption of NH3 at these Ni sites would retard the reaction and promote a broadening of the reaction front as shown in Figures 12 and 13. 

In the case of an unfavorable isotherm (or equally for desorption with a favorable isotherm) a different type of behavior is observed. The concentration front or mass transfer zone, as it is sometimes called, broadens continuously as it progresses through the column, and in a sufficiently long column the spread of the profile becomes directly proportional to column length (proportionate pattern behavior). The difference between these two limiting types of behavior can be understood in terms of the relative positions of the gas, solid, and equilibrium profiles for favorable and unfavorable isotherms (Fig. 7). 

Macrostructuring measures are a well-known concept for the suppression of hotspots by local dilution of catalyst in multitubular reactors, for example [53]. It is also intuitively obvious that the need for adsorption to enhance an equilibrium reaction is much lower at the front end of the reactor, where kinetics rather than equilibrium are decisive, than downstream. The converse, however is not true in the outlet of such an adsorptive reactor both catalyst and adsorbent are required for maximal conversion. By a judicious selection of the ratio between the two, one can ameliorate the front broadening effects that arise in the reactor outlet. 

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