Reactions between gases and solids

Reactions between gases and solids. We shall now remove the restriction imposed in paragraph 1, p. 297, namely, that all the reacting substances shall belong to the same phase and shall consider reactions between substances in different states of matter. Let us first consider the simplest case of a reaction between solids and gases. 

Examples of such gaseous-solid reactions are comparatively common, and the fact that they are susceptible of simple thermodynamical treatment was discovered relatively early. Many solid compounds dissociate when the temperature is raised, giving off one or more gases. Examples of this are calcium carbonate, according to the equation 

2NH3 = AgClH-2NH3 oxides of heavy metals, such as 

Besides these there are a large number of reactions which are of great importance in the preparation of chemical substances and in technical chemistry, such as the reduction of metallic oxides by carbonic oxide, the action of water vapour on carbon or on metals, etc. It is, therefore, not surprising that reactions between solids and gases have been investigated so thoroughly both theoretically and experimentally in the last few decades. 

We shall place at the head of the following thermodynamical discussion the almost obvious theorem that the vapour pressme 

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A very valuable feature of PulseTA is the possibility of in-situ investigation of the course of reactions. This offers an important tool for elucidating mechanisms of complicated decomposition processes such as the decompositions of oxalates and for investigating, in differential mode, reactions between gases and solids such as occur in heterogeneous catalysis. 

The conclusion that can be drawn is that the inverse temperature profile can be exploited successfully to allow much faster processing, particularly where reactions between gases and solid phases is required. 

Thermochemical measurements on reactions between gases and solids, other than combustions, are uncommon. Gross and his co-workers measured the enthalpy of reaction between alkali-metal fluorides MF (M = Li or Na) and BFg to form MBF4 using an isoperibol calorimeter operated at 110 The stream of argon flowing over the heated fluoride was replaced by BF for a period of 10 min. Duus and Mykytiuk used a flow method to determine the enthalpy of the reaction... 

Non-catalytic gas-solid reaction process. Reactions (5.1) and (5.2) are not common chemical reactions these are non-catalytic gas-solid reaction processes. It is necessary to consider the heat and mass transfer process of reactants and products, and the reaction between gases and solid and between solids. The process follows the general rule of non-catal3dic gas-solid reaction process. 

The third part (Chapters 12 to 16) is devoted to the application of the general concepts of modeling to a certain number of families of transformations such as the transformations of coalescence of grains (Chapter 12), decompositions of solids (Chapter 13), reactions between solids (Chapter 14), and reactions between gases and solids (Chapter 15). Finally, we approach the treatment of transformations involving solid solutions, a field still largely in the waste land (Chapter 16). Essentially, this part is concerned with the function reactivity. 

Processes that involve the reaction of gases and solids are extremely difficult to handle mainly due to solid flow difficulties (Knowlton, 2000). The difficulty in the scale-up of these reactors constitutes then main disadvantage. The maximum scale-up factor for fluidized beds is usually between 50 and 100, whereas for fixed beds it could reach the value of 10,000. This is due to the fact that the flow characteristics are very different in the small and the large reactor the bubble diameter does not change upon scale-up, whereas reactor diameter does. 

Reactions between gases and liquids may involve solids also, either as reactants or as catalysts. Table 17.9 lists a number of examples. The lime/limestone slurry process is the predominant one for removal of S02 from power plant flue gases. In this case it is known that the rate of the reaction is controlled by the rate of mass transfer through the gas film. 

The development of solvent-free organic synthetic methods has thus become an important and popular research area. Reports on solvent-free reactions between solids, between gases and solids, between solids and liquid, between liquids, and on solid inorganic supports have become increasingly frequent in recent years. 

The best solvent from an ecological point of view is without doubt no solvent. There are many great reactions that can already be carried out in the absence of a solvent, for example numerous industrially important gas-phase reactions and many polymerizations. Diels-Alder and other pericyclic reactions are also often carried out without solvents. Reports on solvent-free reactions have, however, become increasingly frequent and specialized over the past few years. Areas of growth include reactions between solids [5], between gases and solids [6], and on supported inorganic materials [7], which in many cases are accelerated or even made possible through microwave irradiation [8]. 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

In recent years increasing interest has been shown in the mechanism of reactions between gases at solid interfaces and between gases and solids at elevated temperatures. These reactions include many of interest to catalysis, the protection of metals and the combustion of solid fuels. From the practical viewpoint these are some of the most useful reactions to mankind in our present state of industrial development. These reactions have been studied extensively in the past but owing to lack of precise experimental techniques and the lack of adequate theoretical interpretation, progress in the understanding of the mechanism of the reactions has been slow. 

A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface. The principles that govern heterogeneous catalytic reactions can be applied to both catalytic and noncatalytic fluid-solid reactions. These two other types of heterogeneous reactions involve gas-liquid and gas-Hquid-solid systems. Reactions between gases and liquids are usually mass-transfer limited. 

The computer program accepts as input the mechanisms of gas-phase reactions and of reactions between gases and a solid surface. It gives all the information necessary to calculate reactors using the SURFACE PSR and SPIN computer programs. 

The precipitation of a solid product as the result of the chemical reaction between gases and/or liquids is a standard method for the preparation of many industrial chemicals. Precipitation occurs because the gaseous or liquid phase becomes supersaturated with respect to the solid component. A crude precipitation operation, therefore, can be transformed into a crystallization process by careful control of the degree of supersaturation. 

Lewis Acids and Bases— The Lewis acid-base theory views an electron-pair acceptor as a Lewis acid and an electron-pair donor as a Lewis base. The addition compound of a Lewis acid-base reaction is referred to as an adduct. The theory is most useful in situations that cannot be described by means of proton transfers, for example, in reactions involving gases and solids and in reactions between organic compounds (considered in Chapter 27). 

Chemical reactions may result from interactions among and between the three phases of matter solid, liquid, and gas. The major interactions that occur in the deep-well environment are those between different liquids (injected waste with reservoir fluids) and those between liquids and solids (injected wastes and reservoir fluids with reservoir rock). Although gases may exist, they are usually dissolved in liquid at normal deep-well pressures. 

The rate of reaction between fluorine and hydrogen varies a great deal with conditions. Solid fluorine and liquid hydrogen explode even at 21 K but mixing of the gases at room temperature in the dark may preclude any reaction however a reaction can... 

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