Solid phase kinetic aspects

Sublimation. During sublimation, the lattice constituents of the solid are directly transferred to the gas phase without the intervention of liquefaction, though there may be mobile intermediates at the surface of the heated solid. Various features of the sublimation process have been reviewed by Somorjai [18] and by Rosenblatt [19] who included consideration of kinetic aspects. Rhead [ 20] has discussed diffusion processes at surfaces. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

Reports of kinetic studies do not always include an explicit statement as to whether or not the reactant melted during reaction or, indeed, if this possibility was investigated or even considered (cf. p. 1). This aspect of behaviour is important in assessing the mechanistic implications of any data since reactions in a homogeneous melt, perhaps a eutectic, usually proceed more rapidly than in a crystalline solid. It is accepted that the detection of partial or localized melting can be experimentally difficult, but, in the absence of relevant information, it is frequently impossible to decide whether a reported reaction proceeds in the solid phase. 

The main goal of this chapter is to review the most widely used modeling techniques to analyze sorption/desorption data generated for environmental systems. Since the definition of sorption/desorption (i.e., a mass-transfer mechanism) process requires the determination of the rate at which equilibrium is approached, some important aspects of chemical kinetics and modeling of sorption/desorption mechanisms for solid phase systems are discussed. In addition, the background theory and experimental techniques for the different sorption/ desorption processes are considered. Estimations of transport parameters for organic pollutants from laboratory studies are also presented and evaluated. 

The main reasons for investigating the rates of solid phase sorption/desorption processes are to (1) determine how rapidly reactions attain equilibrium, and (2) infer information on sorption/desorption reaction mechanisms. One of the important aspects of chemical kinetics is the establishment of a rate law. By definition, a rate law is a differential equation [108] as shown in Eq. (32) ... 

Ideally, a phase transformation should be investigated using a combination of techniques which enable changes in composition, structure, surface area, morphology and porosity of the solid phases and in the composition of the solution to be monitored, together with the reaction kinetics. This type of comprehensive investigation is rare for iron oxide interconversions in most cases only one or two of the above aspects of the transformation have been considered. 

In discussing the mechanisms of the formation of monodispersed colloids by precipitation in homogeneous solutions, it is necessary to consider both the chemical and physical aspects of the processes involved. The former require information on the composition of all species in solution, and especially of those that directly lead to the solid phase formation, while the latter deal with the nucleation, particle growth, and/or aggregation stages of the systems under investigation. In both instances, the kinetics of these processes play an essential role in defining the properties of the final products. 

The thermodynamics of formation and transformation of a solid phase into another are characterized by two aspects, both of them explaining the difficulty to produce solids of homogeneous composition. The more important of these is nucleation The other is the tendency of certain components of the solid to diffuse to, or away from, surfaces. These aspects, however, cannot be considered in isolation. Chemical reactions involve the breaking of bonds and formation of new ones. This involves kinetically limited processes. In many cases, diffusion brings about additional kinetic limitations. The final result is the combination of the effects of all these processes. 

The number of components is the minimum number of independent species required to define the composition of all of the phases in the system. The simplest example usually cited to demonstrate the concept of components is that of water, which can exist in various equilibria involving the solid, liquid, and gas. In such a system there is one component. Likewise for acetic acid, even though it associates into dimers in the solid, liquid, and gaseous state, the composition of each phase can be expressed in terms of the acetic acid molecule and this is the only component. The important point for such a system is that the monomer-dimer equilibrium is established very rapidly, that is, faster than the time required to determine, say, the vapour pressure. In the cases in which the equilbrium between molecular species is established more slowly than the time required for a physical measurement, the vapour pressure, for example, will no longer be a function only of temperature, but also of the composition of the mixture, and the definition of a component acquires a kinetic aspect. 

The particular chemical form in which an element exists in water is its speciation. For example, an element can be present as a simple hydrated ion, as a molecule, as a complex with another ion or molecule, and so forth. From what was said previously, bare ions or bare polar molecules do not exist in water. At the least, they would be solvated species. Species of an element are distinguishable from one another stoichibmetrically, structurally, and energetically. In addition to aqueous species, one can distinguish elements in different phases, for example, as gaseous species, as solid phases, or in adsorbed states, and on the basis of particle sizes. In the atmosphere, for example, speciation extends over liquid, gas, and aerosol phases (Seinfeld, 1986). The notion of chemical speciation is central to equilibrium and kinetic aspects of aquatic chemistry, as will be evident throughout this book. 

EIA often makes use of solid phases. The relative merits and disadvantages of such techniques, as well as ways to optimize them will be discussed. An important aspect, which has not yet been investigated in detail for EIA, is the influence immobilization of enzymes has on enzyme kinetics. The solid-phase may cause strong local differences in the microenvironment of the enzyme, the implications of which can only be inferred from studies on immobilized enzymes in solid-phase biochemistry (Chapter 9). Major or minor flaws in EIA design, which may discredit an otherwise perfectly valid EIA, will be discussed. 

Solid-state electrochemistry is an important and rapidly developing scientific field that integrates many aspects of classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis, and other areas of physical chemistry. This field comprises - but is not limited to - the electrochemistry of solid materials, the thermodynamics and kinetics of electrochemical reactions involving at least one solid phase, and also the transport of ions and electrons in solids and interactions between solid, liquid and/or gaseous phases, whenever these processes are essentially determined by the properties of solids and are relevant to the electrochemical reactions. The range of applications includes many types of batteries and fuel cells, a variety of sensors and analytical appliances, electrochemical pumps and compressors, ceramic membranes with ionic or mixed ionic-electronic conductivity, solid-state electrolyzers and electrocatalytic reactors, the synthesis of new materials with improved properties and corrosion protection, supercapacitors, and electrochromic and memory devices. 

Liquid-liquid-solid reactors are commonly used for biphasic reactions catalyzed by immobilized phase-transfer catalysts (which form the third, solid phase). Certain basic aspects of such reactors were considered in Chapter 19. Three-phase reactions of this type are also encountered in biological reactions, for example, the enzymatic synthesis of amino acid esters in polyphasic media (Vidaluc et al., 1983), and the production of L-phenylalanine utilizing enzymatic resolution in the presence of an organic solvent (Dahod and Empie, 1986). Predictably, the performance of these reactors is influenced by the usual kinetic and mass transfer aspects of heterogeneous systems (see Lilly, 1982 Chen et al., 1982 Woodley et al., 1991a,b). Additionally, performance is also influenced by the complex hydrodynamics associated with the flow of two liquids past a bed of solids (Mitarai and Kawakami, 1994 Huneke and Flaschel, 1998). It is noteworthy, for instance, that phase distribution within the reactor is different from that in the feed and is also a function of position within the reactor and within the voids of each pellet in the bed. More intensive research is needed before these reactors can be rationally designed. 

Liquid-liquid and solid-liquid dispersed systems are complicated forms from a physicochemical point of view, because of the presence of two phases. Their formulation therefore necessitates comprehension of fundamental aspects controlling the behavior of these systems. With this end in view, we begin this volume with theoretical considerations concerning pharmaceutical surfactants, formulation concepts, and emulsion properties, and the related know-how to attain them. As the text progresses, each chapter becomes more advanced and specific. Thermodynamic and kinetic aspects of suspension formulations, as well as... 

There are two main aspects that distinguish materials kinetics (1) the involvement of at least one solid phase and (2) the (frequent) presence of heterogeneity. 

Liquid-solid and solid-solid phase transformations are also known as condensed-matter phase transformations. Condensed-matter phase transformations, like other kinetic processes, are driven by thermodynamics. When a region of matter can lower its total free energy by changing its composition, structure, symmetry, density, or any other phase-defining aspect, a phase transformation can occur. 

For proper design and simulation of HDT reactors, kinetic and reactor modeling are aspects that need to be deeply studied however, this is not a trivial task due to the numerous physical and chemical processes that occur simultaneously in the reactor phase equilibrium, mass transfer of reactants and products between the gas-liquid-solid phases, diffusion inside the catalyst particle, a complex reaction network, and catalyst deactivation. Ideally, the contribution of all these events must be coupled into a robust reactor performance model. The level of sophistication of the model is generally defined based upon the pursued objectives and prediction capability [4]. 

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