APPLICATIONS OF PERCOLATION THEORY TO MODELING OF NONCATALYTIC GAS-SOLID REACTIONS.

摘要

A comprehensive structural pore model for describing the behavior of noncatalytic gas-solid reactions has been developed. The model is based on concepts of percolation theory, which provides a natural framework for describing and evaluating the role of reaction and transport processes in changing porous structures.; A Bethe network was used to represent the internal pore structure. By randomly deleting a fraction of bonds and distributing sizes to the rest, this regular branching network could be conveniently transformed to simulate the essential geometrical and topological properties of the pore structure. The fraction and size of the bonds were taken as the porosity and pore size distribution of the solid, respectively. The appropriate connectivity of the network structure could be implied from mercury porosimetry experiments via a Monte Carlo technique. This technique also resulted in improved characterizations of the pore size distributions. Based on this network representation, simple but fundamental relationships for evaluating physical, fragmentation and transport properties of the evolving pore structure were derived.; The desired properties of the pore structure were then incorporated into a comprehensive particle reaction and transport model that described the behavior of noncatalytic reactions. Gasification and sulfation reactions were taken as case studies. For gasification reactions the description included the role of pore opening, enlargement and coalescence in the evolution of porosity and internal surface area as well as the influence of pore topology on particle fragmentation. Effective transport coefficients were evaluated without resorting to adjustable factors while fundamentally accounting for the role of tortuous paths, narrow necks and dead ends. The sulfation analysis successfully accounted for the isolation of partially reacted pores caused by pore plugging.; Because of the detailed treatment of basic topological properties of the pore structure, the model exhibited the correct trends under a wide variety of initial and operating conditions. Successful predictions of available experimental data further supported the ability of the network model in properly describing the essential mechanisms by which reaction and transport processes take place in changing pore structures.