Mass transport rate-decay kinetics

In FPTRMS, transport of the reactive species of interest from the reactor to the detector can make a contribution to the observed time dependence such that the chemical kinetics becomes convoluted with mass transport rates. This will have to be accounted for in data analysis if reliable rate coefficients are to be obtained. If the physical rate processes are sufficiently fast they will make a negligible contribution to the kinetics. In this section we examine the above four factors to see when they influence the chemical kinetics. The first, third, and fourth items put an upper limit on the rate at which decays and growths can be reliably determined, and the second one sets a lower limit on the decay rate. 

Some other catalysts, such as Cr /aluminophosphate, exhibit polymerization rates that do decay with time. In these systems, at least, polymer accumulation over time might cause the declining activity, because of increasing resistance to mass transport. However, this interpretation would mean that the polymerization rate would be a function, not of time itself, but of the amount of polymer accumulation. Investigation of the kinetics variables makes it clear that the rate is dependent not on polymer build up but on the reaction time. Similar rate-decay kinetics can be obtained with a high or a low polymer yield, by variation of ethylene concentration and other variables. In one experiment, the ethylene in the reactor was removed just as the peak reaction rate was reached, and not... 

Theoretical growth transients for various values of normalized rate constant are shown in Fig. 22 for an electrode located at the cavity center. For fast mass transport (or sluggish kinetics), the transient is governed by both the mass transport of material throughout the cavity and by the kinetic decay or the growth of the radical. As the normalized rate constant, K, increases (or as the mass transport decreases), the time taken for the transient to reach a steady state value decreases, and the transient changes shape until they attain a simple exponential form, which... 

From the kinetic point of view SPR experiments have the advantage that both the association and dissociation processes can be measured from the two phases in one sensogram. However, it is possible for artifacts to arise from refractive index mismatch during the buffer change and, for this reason, in general the initial parts of the association and dissociation phases are excluded from the kinetic analysis.73 When multiexponential decays are observed it is important to distinguish between kinetics related to the chemistry and potential artifacts, such as conformational changes of the bound reactant or effects due to mass transport limitations.73,75 The upper limit of detectable association rate constants has been estimated to be on the order of... 

The scope of kinetics includes (i) the rates and mechanisms of homogeneous chemical reactions (reactions that occur in one single phase, such as ionic and molecular reactions in aqueous solutions, radioactive decay, many reactions in silicate melts, and cation distribution reactions in minerals), (ii) diffusion (owing to random motion of particles) and convection (both are parts of mass transport diffusion is often referred to as kinetics and convection and other motions are often referred to as dynamics), and (iii) the kinetics of phase transformations and heterogeneous reactions (including nucleation, crystal growth, crystal dissolution, and bubble growth). 

The kinetic equations serve as a bridge between the microscopic domain and the behavior of macroscopic irreversible processes through the description of hydrodynamics in terms of intermolecular collisions. Hydrodynamics can specify a large number of nonequilibrium states by a small number of reproducible properties such as the mass, density, velocity, and energy density of a fluid conserved during the collision of molecules. Therefore, the hydrodynamic equations can describe a wide range of relaxation processes of nonequilibrium states to equilibrium state. We call such processes decay processes represented by phenomenological equations, such as Fourier s law of heat conduction. The decay rates are determined by the transport coefficients. Reliable transport coefficients provide microscopic and macroscopic information, and validate the results of molecular dynamics. 

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