Reactions Involving Solids

In Chapter 7 we considered catalytic reactions on solid surfaces and foimd that transport steps are essential in describing these reactions because mass transfer of reactants and products between phases must accompany reaction. In this chapter we consider the reactions of solids in which the solid enters the stoichiometry of the reaction as a reactant or product or both. We remark that the texts of Levenspiel give excellent and thorough descriptions of the reactions of solids, and we will only summarize some of the features of reactions involving solids here. 

The reaction of solids is not regarded as a traditional aspect of chemical reaction engineering, which usually concerns mostly fluids (gases and liquids). However, chemical processing of solids represents an increasingly important part of reaction engineering. The material in this chapter also introduces gas-liquid and liquid-liquid processes because similar geometries are involved. 

A major point we emphasize is that, from a reaction engineering viewpoint, these are all essentially identical problems. The fluid may be a gas or a liquid, and the particle may be a liquid, gas, or a solid, but the geometries and the reactions are very similar. The interfaces may be gas-liquid, liquid-liquid, gas-soHd, or liquid-solid. For gas-Hquid problems we have drops and bubbles rather than particles, but the geometries are identical. These systems are the subject of Chapter 12. The applications are also quite different, but, once one realizes the similarities, the same ideas and equations unify all these problems. 

Reactions involving solids are very important in many technologies such as microelectronics processing, ceramics, ore refining, electrochemical deposition and etching, chemical vapor deposition and etching, and food processing. We will consider some of these applications in problems, but we first note several important examples. 

The previous examples were all for drops, bubbles, or particles, all objects small in three directions. The other examples in this chapter are for solids that are small only in one direction. These are solid films, a class of solids with many important applications. 

---

Equilibrium constants for complexation reactions involving solids are defined by combining appropriate Ksp and K expressions. Eor example, the solubility of AgCl increases in the presence of excess chloride as the result of the following complexation reaction... 

The main distinguishing features of reactions involving solids are that (i) the chemical transformation occurs within a restricted zone of the solid,... 

The reaction interface can be defined as the nominal boundary surface between reactant and the solid product. This simple representation has provided a basic model that has been most valuable in the development of the theory of kinetics of reactions involving solids. In practice, it must be accepted that the interface is a zone of finite thickness extending for a small number of lattice units on either side of the nominal contact sur-... 

Although the literature contains a very large number of research articles concerned with the kinetics and mechanisms of reactions involving solids, there are comparatively few authoritative, critical and comprehensive reviews of the formidable quantity of information which is available. Probably the most important general account of the field is the book Chemistry of the Solid State, edited by Gamer [63]. Chapters 7—9 are particularly relevant in the present context as they provide a systematic exposition of the kinetic equations applicable to the decomposition of single solids (Jacobs and Tompkins [28]) and their application to endothermic [64] and exothermic [65] reactions. 

No unifying theoretical concepts have been recognized which can be used to provide satisfactory criteria for the comprehensive classification of the kinetics and mechanisms of reactions involving solids. Thus the scope and treatment of the subject cannot be entirely systematic. General problems encountered during any attempt to review the field include the following. 

The packed bed reactor is particularly lu ul for reactions involving solid-phase reagents such as cati Psts, immobilized enzymes,and metallic reducing agents [641,642,646,647,658]. Since solutions are added in this isase, there are no... 

Several types of reactions involving solids with gases or liquids occur at the interface between the two phases. The most important reaction of this type is corrosion. Efforts to control or eliminate corrosion involve research that spans the spectrum from the coatings industry to the synthesis and production of corrosion-resistant materials. The economic ramifications of corrosion are enormous. Although there are numerous types of reactions that can be represented as taking place at an interface, the oxidation of a metal will be described. Figure 8.6 represents the oxidation of a metal. 

In Section 9.3, we focus more on the intrinsic rates for reactions involving solids, since there are some modem processes in which mass transport rates play a relatively small role. Examples in materials engineering are chemical vapor deposition (CVD) and etching operations. We describe some mechanisms associated with such heterogeneous reactions and the intrinsic rate laws that arise. 

Intrinsic Kinetics of Heterogeneous Reactions Involving Solids 255... 

INTRINSIC KINETICS OF HETEROGENEOUS REACTIONS INVOLVING SOLIDS... 

The mechanisms, and hence theoretically derived rate laws, for noncatalytic heterogeneous reactions involving solids are even less well understood than those for surface-catalyzed reactions. This arises because the solid surface changes as the reaction proceeds, unlike catalytic surfaces which usually reach a steady-state behavior. The examples discussed here are illustrative. 

Most thermochemical calorimetric methods are used to determine enthalpy changes of chemical reactions. The reaction may give the enthalpy of interest directly or may represent a step in a thermodynamic cycle needed to obtain an enthalpy of interest. These techniques are also very suitable for direct determination of enthalpy of mixing in the liquid state or indirect determination of enthalpy of mixing in the solid state. Calorimetric methods for studies of chemical reactions involving solids can be divided into three main categories ... 

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... 

Just as with the metal surface reactions described above, the efficiency of heterogeneous reactions involving solids dispersed in liquids will depend upon the available reactive surface area and mass transfer. 

This example reveals one of the most desirable features in investigating reactions involving solids, an intelligent use of the microscope Without visueil observations (with optical or electron microscope), it is readily possible to wholly misunderstand the phenomena involved. 

The evolution of chemical engineering from petroleum refining, through petrochemicals and polymers, to new applications is de.scribed so that students can see the relationships between past, present, and future technologies. Applications such as catalytic processes, environmental modeling, biological reactions, reactions involving solids, oxidation, combustion, safety, polymerization, and multiphase reactors are also described. 

Previously mentioned properties can be conveniently investigated by studying the adsorption of a suitably chosen probe molecule on the solid. Adsorption influences all phenomena depending on the properties of the surface (e.g., it constitutes the primary step in corrosion as well as the prerequisite for every catalytic reaction involving solid catalysts). 

Pore-diffusion resistance Reactions involve solid particle size greater than about 1.6 mm All fast, noncatalytic gas-solid (G/S) reactions such as combustion and gasification Reactors with particle size lower than 100 pm to 0.1 mm Catalytic bubbling fluidized beds (BFB) Slurry reactors... 

This device has been used for many years to measure diffusion coefficients and, more recently, to study the rates of chemical reactions involving solids. In the latter case, a circular pellet of reactant is embedded in the disc. For a dissolving solid which reacts with first-order kinetics (rate constant ki) in the receiving phase (Fig. 5.18) the flux j is given by Equation 5.13 [13] ... 

Ionic substances are written in the ionic form only if the ions are separated from each other in the reaction medium (most often water). NaCl would be the conventional notation for reactions involving solid salt, because the ions in the solid are bound together in the crystal. Salt reactions in solution, however, would... 

As is common to general topochemical reactions involving solids, the observed rate of reduction, R, can usually be expressed in terms of a rate r per unit area of reaction interface, multiplied by a surface area, S, which may evolve in many possible ways in time, t, viz. 

In a general way, two main families of reactions involving solids must be distinguished ... 

Suppose that 650 mL of hydrogen gas are produced through a replacement reaction involving solid iron and sulfuric acid, H2SO4, at STP. How many grams of iron(II) sulfate are also produced ... 

---