Adsorption sojourn time

The first approach to understanding the regularities of gas-solid IC and TC was based on the laws of molecular kinetics. Within this framework, the time which a molecule spends in the adsorbed state when migrating down the column is seen as a sum of the random elementary adsorption sojourn times which accompany each adsorption event. The number of adsorption events is individual of major interest is,... 

We start with the case of d(z) injection profile into an isothermal column under laminar gas flow regime. The factors which cause broadening of the initial profile are longitudinal diffusion and radial diffusion across streamlines. These continuously change the concentration distribution and gradients. In the absence of any adsorption sojourn time of molecules on the wall (i.e. negligible adsorption-desorption energy), and only in this case, the two above diffusion processes yield ... 

The actual random flight mechanism behind VTC deserves more detailed consideration. During a typical experiment a molecule must experience some 104 free flights. It enables direct simulation of each flight and of the sojourn time by a Monte Carlo technique. Modem computer workstations make it easy. The advantage is that both the position and shape of the peak are obtained in a consistent straightforward way. Besides assuming the cosine law of reflection (see below), there is another substantial assumption which we will examine in Sect. 5.4.1 - the absence of any lateral diffusion. In other words, we imply localized adsorption. 

The above considerations concerning the special potential wells might appear to be the required rationale for the similar values of energies, which characterize macro and microvolatility. In reality, the deepest adsorption pockets should not be very abundant. Let us estimate whether there is enough time to establish equilibrium for a nuclide living a second or so. In the experiments, the characteristic temperature (at the TC peak or the 50 percent yield in IC) is such that E /RT 20 then the mean adsorption sojourn is 10 12xexp(ed/kBT) 10 4s. Assume that the surface... 

The evaluation of the RPLC chromatograms of nonionizable analytes shows that the number of sorption events varies between n = 13000 and 20000 on a 150 X 3.9-mm column and that it is not affected strongly by the retention time of the analytes [107]. Fly-times in the mobile phase between a desorption and the subsequent adsorption vary roughly between Tm = 3 to 5 ms. During that fly-time, the mobile phase travels a distance that is 1.5 to 2.3-times the particle diameter. The sojourn time in the stationary phase, however, strongly correlates with the retention of the analytes, so the retention of analytes and the selectivity of their separation are mainly due to the variations of Tg, between 8.4 ms (for k = 1.75) and 47 ms (for fc = 12.7). 

Thus the p.d.f for nonadsorbing tracers can be used directly to estimate the p.d.f. of sojourn times on the catalyst surface if the partition coefficient, K, for the reactant is known and adsorption-desorption is fast. 

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