Homogeneous reactions, solids

However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

The basic phenomenon was observed in modeling studies by Bjoreskov and Slinko (1965) that sudden increase in inlet temperature caused a transient drop of the peak temperature. The wrong-way response name was given by Mechta et al (1981) after they experienced the opposite a sudden of inlet temperature resulted in an increase of the peak temperature (which may eventually cause a runaway.) The work used a pseudo-homogeneous reaction model and explained the phenomenon by the different speeds of transient response in gas and solid. The example in the last part of Chapter 7.4 explained the speed difference by the large difference in heat capacity of gas and solid phases. For this a two-phase model is needed and spatial and time changes must be followed. 

The concentrations of reactants are of little significance in the theoretical treatment of the kinetics of solid phase reactions, since this parameter does not usually vary in a manner which is readily related to changes in the quantity of undecomposed reactant remaining. The inhomogeneity inherent in solid state rate processes makes it necessary to consider always both numbers and local spatial distributions of the participants in a chemical change, rather than the total numbers present in the volume of reactant studied. This is in sharp contrast with methods used to analyse rate data for homogeneous reactions in the liquid or gas phases. 

Very active, not too expensive, catalysts may be left in the product if they are used in such small quantities that they are not detrimental to the properties of the product. An example is the gas-phase polypropylene process. When the desired product is formed as a solid, i.e. crystallizes from the homogeneous reaction mixture, the product can be removed in relatively simple way. 

Table 5.4-3 summarizes the design equations and analytical relations between concentration, C/(, and batch time, t, or residence time, t, for a homogeneous reaction A —> products with simple reaction kinetics (Van Santen etal., 1999). Balance equations for multicomponent homogeneous systems for any reaction network and for gas-liquid and gas-liquid-solid systems are presented in Tables 5.4-7 and 5.4.8 at the end of Section 5.4.3. 

Application Homogeneous reactions Liquid-Solid reactions Gas-Liquid-Solid reactions Gas-Liquid reactions Liquid-Liquid reactions... 

It may be recalled that in homogeneous reactions all reacting materials are found within a single phase, be it gas, liquid or solid if the reaction is catalytic, then the catalyst must also be present within the phase. Thus, there are a number of means of defining the rate of a reaction the intensive measure based on unit volume of the reacting volume (V) is used practically exclusively for homogeneous systems. The rate of reaction of any component i is defined as... 

The progress of this category of reactions is expected to depend on the composition of the materials within the phase as well as the temperature and pressure of the system. The rate of homogeneous reaction should not be affected by the shape of the container, the surface properties of the solid materials in contact with the phase, and the diffusion characteristics of the fluid. Thus the rate of reaction of component i may be expressed as... 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

The solid state reactions are extremely complex due to intervention of many physical parameters and it becomes often necessary to make some generalizations in the complex reactions. The rate in solid state reactions cannot be defined in the same way as that for a homogeneous reactions because the concept of concentration in solid state reactions has no significance. The energy of activation in a solid state reaction has also no significance, except in some rare cases. 

Homogeneous reactions Homogeneous noncatalytic reactions are normally carried out in a fluidized bed to achieve mixing of the gases and temperature control. The solids of the bed act as a heat sink or source and facilitate heat transfer from or to the gas or from or to heat-exchange surfaces. Reactions of this type include chlorination of hydrocarbons or oxidation of gaseous fuels. 

The behavior of carbonates will be used to illustrate heterogeneous processes, with emphasis upon the formation of inorganic surface coatings and solid solutions. This is a vital topic in the study of solid-solution interactions since it is coatings rather than bulk phases which are sensed by liquid solutions. Homogeneous reactions will be studied in terms of the competition of coulombic ion pairs with true complexes for anions. An extended form of the phase rule will be used. 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

The structure of a reacting molecule can be used as the chemical probe for the reaction mechanism in several ways. Ample experience is available with these methods from the research of noncatalytic homogeneous reactions, and their possibilities and limitations are well known. However, the solid catalyst restricts the scope to some extent on the one hand, but opens new applications on the other. For this reason, the methods of physical organic and inorganic chemistry developed for noncatalytic reactions cannot simply be transferred into the field of heterogeneous catalysis. The following remarks should identify some of the problems. 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

As has been mentioned previously, one is most likely to find analogies to catalytic reactions on solids with acidic and/or basic sites in noncatalytic homogeneous reactions, and therefore the application of established LFERs is safest in this field. Also the interpretation of slopes is without great difficulty and more fruitful than with other types of catalysts. The structure effects on rate have been measured most frequently on elimination reactions, that is, on dehydration of alcohols, dehydrohalogenation of alkyl halides, deamination of amines, cracking of the C—C bond, etc. Less attention has been paid to substitution, addition, and other reactions. 

When more than one reaction affects the mass of the ion i in solution (e.g., dissolution and/or precipitation of heterogeneous solid assemblages homogeneous reactions in solution), the overall change in mass of component i is given by the summation of individual absolute reaction velocities Vj, multiplied by their respective stoichiometric reaction coefficients ... 

The above example considered a homogeneous reaction where all of the reactants and products are in the same state, i.e., in an aqueous solution. However, many reactions involve mixtures of gases, liquids, solids, etc. In this case they are defined as heterogeneous reactions. 

Since the main use of these and many other forms of multistage operations is with solid catalyzed gas-phase reactions we discuss these operations in Chapter 19. Design for homogeneous reactions parallels that for catalytic reactions, so the reader is referred to Chapter 19 for the development. 

Table 1-1 Homogeneous reaction rate coefficients Reactions in aqueous solutions Gas-phase reactions Solid-phase reactions...

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