Critical residence time

Figure 1 Schematic energy diagram for the DIET process due to the MGR model illustrating the relaxation and desorption processes. Electronic excitation due to laser irradiation occurs via the Franck-Condon transition. After a residue time t at the intermediate excited state, relaxation occurs with an excess energy ZA surpassing the surface barrier for desorption. The value of depends strongly on t, and no desorption occurs when t is shorter than the critical residence time tc. The Absicissa is the adsorbate-substrate distance.

Table 5 Estimated lifetime (t) and critical residence time (tc) in the intermediate excited state for NO desorption of hep hollow species from Pt(l 11).

Finally, we would like to make a scenario of the desorption activity for NO and CO desorption from Pt(l 1 1) and Pt(l 1 1)-Ge surface alloy. This scenario will be extended to a general concept of desorption in the DIET process of simple molecules from metal surfaces. The lifetime and the critical residence time in the intermediate excited state followed by desorption are important keys for solving what is the origin of the desorption activity in the DIET process from metal surfaces. The excited molecules are not desorbed, if the residence time in the excited state is shorter than the critical residence... 

Free Volume Fraction and Critical Residence Time... 

As polynuclear aromatic hydrocarbons show a high tendency towards undesired gas-phase nucleation and soot formation, they should be avoided by choosing a suitable critical residence time and other appropriate processing parameters. 

Figure 2 shows the dependence of the rate of carbon deposition with initial hydrogen partial pressure. The rate depends on the hydrogen partial pressure with a negative order, i.e., the rate of deposition decreases with increased hydrogen pressure. The dependence of the rate on the initial benzene partial pressure is first order as revealed by the data on Figure 1. For fixed temperatures, benzene and hydrogen partial pressures, there is a critical gas phase residence time (t ) below which no appreciable deposition occurs. Above the critical residence time t, deposition occurs and its rate increases linearly with gas residence time. This is depicted in Figure 3.

By letting h = hi it is also possible to derive a formula for the critical residence time if the residence time of the bed at the heated wall is less than there is no difference in the value of U whether the bed is stirred or not ... 

As the critical residence time is approached, s increases much more sharply... 

The distribution of tracer molecule residence times in the reactor is the result of molecular diffusion and turbulent mixing if tlie Reynolds number exceeds a critical value. Additionally, a non-uniform velocity profile causes different portions of the tracer to move at different rates, and this results in a spreading of the measured response at the reactor outlet. The dispersion coefficient D (m /sec) represents this result in the tracer cloud. Therefore, a large D indicates a rapid spreading of the tracer curve, a small D indicates slow spreading, and D = 0 means no spreading (hence, plug flow). 

The gas flow rate is usually presented as a deposition parameter however, it is much more instructive to report the gas residence time [6], which is determined from the flow rate and the geometry of the system. The residence time is a measure of the probability of a molecule to be incorporated into the film. The gas depletion, which is determined by the residence time, is a critical parameter for deposition. At high flow rates, and thus low residence times and low depletion [303], the deposition rate is increased [357, 365] (see Figure 39) and better film quality is obtained, as is deduced from low microstructure parameter values [366],... 

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