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The transport of linearly interacting solutes in porous media is investigated with the help of residence time distributions, transfer functions, methods of system dynamics, and time-moment analyses. The classical one-dimensional convection-dispersion equation is extended to two-region (mobile-immobile water) transport by including diffusional mass transfer limitations characteristic of aggregated soils. The two-region model is further revised by incorporating the effects of multiple retention sites (in parallel or in series), multiple porosity levels, and arbitrary but steady flow fields. It is shown that different physical situations can be represented by a relatively small number of transfer functions containing only two types of parameters: distribution coefficients to account for equilibrium properties and characteristic times reflecting kinetic processes. Relevant kinetic processes include convective transport, hydrodynamic dispersion, adsorption-desorption, and physical or chemical mass transfer limitations. In most situations, theoretical breakthrough curves are found to be relatively insensitive to the mathematical structure of the transfer function, irrespective of the physical interpretation of the distribution coefficients and the characteristic times in the model. This means that alternative physical and chemical interpretations of model parameters can lead to nearly identical breakthrough curves. Certain transfer time distributions can lead to quite unusual shapes in the breakthrough curves; these curves strongly depend on the characteristic times and a few operational variables. Results of this study show that the transfer time distribution is an extremely useful tool for explaining some unexpected experimental results in the solute transport literature. t' :ms, and the ability or inability of the transport equations to ratch observed solute distributions versus time or distance. the soil and groundwater literature [Valocchi, 1985, 1989, 1990; Sposito et al., 1986; Go!tz and Roberts, 1987]. At the same time, the study of reactors in chemical engineering led to the development of alternative methods for simulating the transport of reactive solutes during nonideal flow, particularly dispersed plug flow [Himmelblau and Bischoff, 1968; Levenspiel, 1972; Wen and Fan, 1975; Villermaux, 1982; Nauman and Buffham, 1983]. Many researchers applied principles of linear system dynamics and ideal reactors to more efficiently model increasingly complex chemical systems. At present, one of the most powerful tools in chemical reaction engineering is the concept of residence time distribution which permits one to describe the main properties of a transport system with only a few key parameters, especially when the solute interacts linearly with the porous medium. This approach was first introduced in hydrology by Jury and coworkers [Jury, 1982; Jury and Sposito, 1986; White et al., 1986; Sposito et al., 1986; Jury and Roth, 1990].Although the prediction of water flow in a soil requires a detailed physical or mechanic...

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Modelling approaches of biomass steam gasification are investigated that take into account both chemical and physical kinetic limitations. The gas phase can be described by two independent reactions: (i) the steam reforming of CH 4 , which is kinetically limited under the operating conditions (1073 < T < 1273 K, p = 1 bar), (ii) the water-gas shift reaction, which would be close to equilibrium under the operating conditions (1073 < T < 1273 K, p = 1 bar). Concerning solid, a time scale analysis of the main phenomena has been performed. For particles of 500 lm, the transformation can be seen as two successive steps: (i) pyrolysis of biomass, which is both chemically and heat-transfer controlled; (ii) steam gasification of residue, which is chemically controlled.

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A three-step sintering mechanism is proposed for Co-based catalysts under Fischer–Tropsch reaction conditions. This mechanism includes an intermediate formation of oxide layer on cobalt metal nanoparticles in the presence of water. The partially reversibly oxidized surface accelerates sintering by both reducing the surface energy and enhancing the diffusion rates of cobalt particles. The proposed mechanism is then employed for a fixed-bed unsteady state reactor. The effect of particle growth on the catalytic activity was analyzed within a diverse range of operating conditions (syngas ratio = 1.5–4, water co-feed ratio = 0–6, inert co-feed ratio = 0–6). It is found that, at the same gas space velocity, sintering proceeds faster at higher H2/CO ratios. At the same initial conversion, a low H2/CO syngas ratio increases sintering severity, i.e., catalyst deactivation due to the crystallite growth, as it brings about higher relative water partial pressure. Dilution of syngas with different amounts of inert gas does not affect the cobalt sintering rate. Cobalt sintering proceeds more rapidly if water is co-fed during the reaction.

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