The effect of pore structure on fluid‐solid reactions: Application to the SO₂‐lime reaction

Abstract

The effect of varying pore structures on the kinetics of fluid-solid reactions is investigated through the random pore model developed in prior papers Perlmutter, 1980, 1981). By considering several idealized pore-size distributions it is shown that a solid having a uniform pore size is intrinsically less reactive than one possessing a pore-size distribution. For solids with bimodal pore size distributions optimal structures are shown to exist for which the reactivity is a maximum.Numerical solutions were obtained to the model equations for various values of the parameters characterizing the pore structure, the diffusion, and the chemical kinetics. The results show that the conversion-time behavior and the expected ultimate conversion can be very sensitive to variations in surface area and porosity for reactions accompanied by an increase in volume of the solid phase.These findings are in agreement with experimental literature on the SO 2-1' ime reaction (Ulerich e t al., 1978; Borgwardt and Harvey, 1972;Potter, 1969; Falkenberry and Slack, 1968) and the model is shown to fit the data of Borgwardt (1970), and of Coughlin (1974, 1976). It is seen that this reaction is diffusion controlled under the conditions of Hartman and Coughlin, in consonance with their own finding using the grain model, and a prior Pigford and Sliger (1973) interpretation. The temperature behavior of the diffusion coefficient in the product layer suggests the participation of an activated process, possibly a solid state diffusion step. SCOPEResults are reported on the effects of pore structure differences on the reaction behavior of solids, particularly in terms of variations in surface area, porosity, and dispersion in the pore size distribution. The analysis utilizes the random pore model developed by Perlmutter (1980,1981), representing a further development of these prior treatments.To demonstrate its use the model is applied to the data of Borgwardt (1970) and Coughlin (1974, 1976) on the lime-S02 reaction. The evaluation includes examination of the relative diffusional and kinetic resistances, of the incomplete conversions arising from pore closure in the solid, and of optimal pore distributions. Further comparisons are made with experimental reports of Ulerich et al. (1978), Borgwardt (1970), and Falkenberry and Slack (1968), to relate porestructure to the frequently observed differences in reaction behavior. CONCLUSIONS AND SIGNIFICANCEAn analysis of various simple pore size distributions shows that of all the pore structures with the same surface area and porosity, the one with pores of uniform size offers the least intrinsic reactivity. For bimodal pore size distributions optimal structures exist for which the reactivity is a maximum. Such results may offer guidance in the compaction of microporous solids by suggesting the desired pore structure.As expected, reactivity generally increases with surface area, but such variation can reduce ultimate conversion by more rapidly plugging pores a t the particle surface. Practical...

Modelling of the kinetics of sulphation of CaO particles under CaL reactor conditions

“…A typical range of inlet concentrations of SO 2 in the entire CaL system can change by about one order of magnitude between 500-3000 ppmv. In the literature [33,34] a variety of reaction orders generally in the range 0.6-1 have been reported depending on the experimental parameters, although there is relatively general consensus about an apparent first order reaction with respect to the SO 2 reactant. Figure 5a is an example of sulfation experiments carried out with highly cycled materials (N=50) to investigate the effect of the SO 2 concentration at the calciner conditions.…”

“…There is a large background of literature on the sulphation of CaO particles under combustion conditions [26][27][28][29][30][31][32][33][34][35][36][37][38][39]. However, there are several novel features in the CaL system of Figure 1 that have not yet been sufficiently dealt with in the literature.…”

“…One of the most widely used pore models is the model developed by Bhatia and Perlmutter [52,53]: the random pore model (RPM), which assumes that the particle is traversed by random size cylindrical pores with intersecting and overlapping surfaces as reaction proceeds. This model was applied successfully to gas solid reactions [54] including the carbonation and sulphation of CaO [33,[54][55][56][57].…”

Determinação dos parâmetros de arrhenius da reação de sorção do dióxido de enxofre por calcário

“…Nesse caso resultariam energias de ativação em torno de 12 kJ mol -1 para controle por difusão molecular e 4 kJ mol -1 para controle por difusão de Knudsen 20,[27][28][29] . Este trabalho insere-se no contexto do programa de pesquisas em energia e poluição do NETeF.…”

A Plausible Model for the Sulfidation of a Calcium-Based Core-in-Shell Sorbent

“…32 Perhaps the most realistic model is the random pore model of Bhatia and Perlmutter. [33][34][35][36] This model assumes that the actual reaction surface of the reacting solid is the result of the random overlapping of a set of cylindrical pores. Equations for calculating the evolving reaction surface have been developed.…”