Residence times step diffusion

Bacterial inactivation is achieved by CO2 absorption in the liquid phase, even though the reason why it happens is still not clear. In this respect, batch- and semi-continuous operating modes are substantially different. In the batch system the residence time, i.e., the time of contact between gas- and liquid phase, must be sufficient to allow the diffusion of CO2 in the liquid, and is therefore a fundamental parameter to assure a desired efficiency. In the semi-continuous system the contact between the phases is localized in the surface of the moving micro-bubbles. In this second case, the efficiency of the process is influenced by temperature, pressure, gas flux, bubble diameter, and other parameters that modify the value of the mass-transfer coefficient. Therefore, it is not correct to use the residence time as a key parameter in the semi-continuous process. In fact, a remarkable microbial inactivation is reached even with an exposure time of 0 min (i.e., pressurizing and immediately depressurizing the system) these two steps are sufficient to allow CO2 to diffuse through the liquid phase. 

The process of molecular diffusion may be viewed conceptionally as a sequence of jumps with statistically varying jump lengths and residence times. Information about the mean jump length /(P and the mean residence time t, which might be of particular interest for a deeper understanding of the elementary steps of catalysis, may be provided by spectroscopic methods, in particular by quasielastic neutron scattering (see next Section) and nuclear magnetic resonance (NMR). 

It has been demonstrated that the combined application of various NMR techniques for observing molecular rotations and migrations on different time scales can contribute to a deeper understanding of the elementary steps of molecular diffusion in zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffiisivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. 

The decomposition of tetramethyltin [Sn(CH3)4] was studied in detail and discussed recently by Gordon and coworkers [173,174,182). Their proposed mechanism is shown in Scheme 3-5. The decomposition does not take place on the surface in the adsorbed state, but the rate limiting step occurs in the gas phase. The species formed by this mechanism diffuse to the surface and are rapidly oxidized after adsorption on the substrate surface. The overall reaction is very complicated and their model contains 27 gas-phase species and 96 chemical reactions. Addition of more and more oxygen leads to a saturation of the deposition rate at about 48 nm/min for a substrate temperature of 470 °C. The apparent activation energy of the decomposition process was 170 kJ/mol in the temperature range of 370-470 °C. A significantly smaller value of 106 kJ/mol was determined by Vetrone and coworkers [170]. The difference may be explained by different residence times of the precursors in the reaction chamber. 

The effect of the residence time on the IBA conversion and products selectivity at the temperature of 235C is drawn in Figure 1, for the Ko sample (ammoniacal salt) calcined at 320C. The proportionality between conversion and residence time allows to exclude diffusion as the rate-determining step. Moreover, it is shown that the selectivities to the various products were substantially independent on the conversion. This is in favour of a reaction network constituted of parallel reactions (probably sharing a common reaction intermediate, obtained by IBA activation) for the formation of methacryhc add, acetone plus CO2, propylene plus CO, and carbon oxides from combustion (1,4,7). 

In vapour growth a new effect enters at high supersaturations. Transport of molecules to the growing steps is not primarily by direct collision rather, the vapour molecules are adsorbed on the crystal surface and then diffuse across it to be added to the step. There is thus a characteristic diffusion distance upon the surface, determined by the mobihty and residence time of the adsorbed molecules, from which each step can be considered to draw its molecules for growth. If the step separation d, determined by... 

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