Models for fluid-solid reactions

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... 

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

This chapter has briefly described a series of models for gas-solid reactions. The literature contains several more and many more could be developed. It would be hard, if not impossible, to assess these models as to their respective merits since careful and detailed experimentation is seriously lagging behind. In the few cases in which it was possible to check the theoretical results with experimental data the lack of fit has mainly been ascribed to inaccuracies in the models. Insufficient attention has been devoted to the kinetic equations proper there is no reason for limiting the kinetics of the reaction between a fluid and the component of a solid to zero- or first-order expressions. 

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 ... 

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. 

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... 

The following steps are involved in the overall reaction between a solid and a fluid (the fluid can be a liquid or a gas) (1) diffusion through the fluid film adjacent to the solid and (2) diffusion and reaction in the solid according to several possible models. The analysis is very similar to that for gas-solid reactions (Levenspiel, 1972, 1993, 1998 Szekely et al., 1976 Ramachandran and Doraiswamy, 1982 Doraiswamy and Sharma, 1984 (vol. 1) Sahimi et al., 1990 Bhatia and Gupta, 1993). 

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. 

Shrinking core model The shrinking core model has been derived for noncatalytic solid-fluid reactions (Levenspiel, 1972). However, it has been successfully used for specific ion-exchange systems—those using synthetic ion exchangers, mainly chelating resins (Cortina et al, 1998 Juang, 1999). 

Figure 8. Schematic representation of our technique for recovering solid products from reactions in supercritical fluids (a) shows the supercritical reactor connected to a computer controlled syringe pump (Lee Scientific Model 501) filled with scC02. (b) shows how the scC02 is used to drive the supercritical reaction mixture (colored) through an expansion valve (Jasco 880/81 Back-pressure Regulator) where the products dissolved in the fluid are precipitated.

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