Fluid-solid reactions models

Use a combination of regime analysis and a fluid-solid reaction model... 

Strictly speaking, the validity of the shrinking unreacted core model is limited to those fluid-solid reactions where the reactant solid is nonporous and the reaction occurs at a well-defined, sharp reaction interface. Because of the simplicity of the model it is tempting to attempt to apply it to reactions involving porous solids also, but this can lead to incorrect analyses of experimental data. In a porous solid the chemical reaction occurs over a diffuse zone rather than at a sharp interface, and the model can be made use of only in the case of diffusion-controlled reactions. 

The mechanism of many of the noncatalytic fluid-solid reactions can be described by a model in between unreacted core and homogeneous reactions models. Ishida and Wen (9) formulated such a model using the zone reaction concept of Ausman and Watson (10). In this model the reaction is not restricted to the surface of the core as in the unreacted core model but occurs homogeneously within a retreating core of reactant. Wen and Ishida (11) combined the grain concept with the zone reaction model and analyzed the reaction of SO2 with CaO particles. In the study conducted by Mantri, Gokarn and Doraiswamy (12) the concept of finite reaction zone model was further developed. 

Fluid-solid reactions include thermal decomposition of minerals, roasting (oxidation) of sulfide ores, reduction of metal oxides with hydrogen, nitridation of metals, and carburization of metals. Each t3 e of reaction will be discussed finm the thermodynamic point of view. Then reaction kinetics for all of the various rate determining steps in fluid-sohd reactions will be discussed for two general models shrinking core and shrinking particle. 

T.T. Tsotsis, M. Sahimi, and G. Gavalas, Membrane and Membrane Processes, in Statistical Modeling of Fluid Solid Reaction Systems. Oxford Press, 1997, in press. 

Here the reactor equation is general and independent of the rate equation, which depends on the gas-solid reaction model used. It can also be adapted to other reactors with appropriate modifications. However, these involve unacceptable assumptions for the fluid-bed model. 

Heterogeneous Reaction Systems The mathematical treatment is quite different if the exchanger is neated as a solid phase. The conceptual models are then similar to (hose developed for noucatalytic fluid-solid reactions.17 18 These kinetic models have been applied successfully to some ion-exchange processes. I9"51... 

Figure 15.8 The expanding core or rotting apple (because the apple rots from the center) model for fluid-solid reactions.

Figure 19.7 A comprehensive model for SLPTC with provision for including any existing model for fluid-solid reactions (such as the sharp interface, volume reaction, or zone model) (Naik and Doraiswamy, 1997)...

Bhatia SK, Perlmntter DD (1980) A random pore model for fluid-solid reactions I. Isothermal, kinetic control. AIChE J 26 379-386... 

Dissolution of mineral is considered based on non-steady state perfectly mixing-fluid flow-reaction model by Fuji (1977). The change in concentration in aqueous solution by the dissolution of solid phase is given by... 

Sahimi M, Gavalas GR, Tsotsis TT. Statistical and continuum models of fluid-solid reactions in porous media. Chem Eng Sci 1990 45(6) 1443-502. 

Bhatia, S.K. and Perlmutter, D.D. (1980) "A random pore model for fluid-solid reactions", AIChEJ, 26, 379-85. 

As an alternative to film models, McNamara and Amidon [6] included convection, or mass transfer via fluid flow, into the general solid dissolution and reaction modeling scheme. The idea was to recognize that diffusion was not the only process by which mass could be transferred from the solid surface through the boundary layer [7], McNamara and Amidon constructed a set of steady-state convective diffusion continuity equations such as... 

In this chapter, we develop matters relating to the process design or analysis of reactors for fluid-solid noncatalytic reactions that is, for reactions in which the solid is a reactant. To construct reactor models, we make use of ... 

In the laboratory experiments of Seyfried et al. (1998), naturally altered sea floor basalt (5 Li = +7.4) was reacted with Li-free alkali-chloride aqueous fluid at 350°C for 890 hours (initial fluid/solid mass ratio 2). Samples of the fluid were taken throughout the experiment, and showed initial rapid influx of isotopically heavy-enriched Li released by early-dissolving alteration minerals. However, with progressive reaction, isotopic composition of the fluid decreased and Li concentration reaehed apparent steady state. Although an equilibrium model applies best to the synthetic results, Rayleigh distillation was considered most likely to apply in hydrothermal reactions occurring in nature. 

First-order adsorption kinetics model A simple first-order reaction model is based on a reversible reaction with equilibrium state being established between two phases (A— fluid, B—solid) ... 

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