Gas-solid reaction kinetics

Barbour. L.J. Achleitner. K. Green. J.R. A system of studying gas-solid reaction kinetics in controlled atmospheres. Thermochim. Acta 1992. 205, 171-177. 

ABSTRACT. Characteristics and fluid dynamics of gas phase recirculation in a novel Riser Simulator Reactor have been investigated using constant temperature hot wire anemometry. In situ concentration and velocity measurements enabled to evaluate the mixing time and the inner recirculation ratio of the gas phase. In addition, fibre optic techniques allowed to characterize the degree of fluidization of the catalyst particles and the effect of gas phase density changes. By combining the anemometry and the fibre optic techniques, mixing patterns in the Riser Simulator have been evaluated. The importance of the study can be realized in the context of the potential use of the Riser Simulator for gas-solid reaction kinetics. 

The key role played by gas-solid reaction kinetics in SO2 removed by limestone should be obvious from the above remarks particularly important is the rate of the reaction... 

As is perhaps natural, in the preparation of this manuscript we have leaned heavily on our own experience and published work in the fields of gas-solid reaction kinetics and reaction engineering. It is fitting, however, to make a specific acknowledgment here of the important contributions made to this field by Professors J. M. Smith, E. E. Petersen, C. Y. Wen, R. L. Pigford, W. O. Philbrook, G. Bitsianes, and Drs. E. T. Turkdogan, R. W. Bartlett, and N. J. Themelis, in addition to the numerous citations that will appear in the text. 

Important aspects of all the models include limestone-S02 kinetics, combustion kinetics and other gas-solids reaction kinetics, gas phase material balances, solid phase material balances, gas exchange between the bubble and emulsion phases, heat transfer, and bubble hydrodynamics. 

Barea, A. G., Ollero, R, Leckner, B. (2007). Mass transport effects during measurements of gas-solid reaction kinetics in a fluidised bed. Chemical Engineering Science, 62, 1477-1493. doi 10.1016/j.ces.2006.10.018. 

A. K. Galwey, Reactions in the Sohd State, in Bamford and Tipper, eds.. Comprehensive Chemical Kinetics, vol. 22, Elsevier, 1980. Galwey, A. K., Chemistry of Solids, Chapman and Hall, 1967. Sohn, H. Y, and W. E. Wadsworth, eds.. Rate Frocesses of Extractive Metallurgy, Plenum Press, 1979. Szekely, J., J. W. Evans, and H. Y. Sohn, Gas-Solid Reactions, Academic Press, 1976. Uhmann, ed., Enzyklopaedie der technischen Chemie, Uncatalyzed Reactions with Solids, vol.. 3, 4th ed., Verlag Chemie, 1973, pp. 395-464. 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

A kinetics study was performed to examine the rate-controlling steps in a gas-solid reaction governed by the shrinking-core model ... 

Various types of reactor configuration may be employed to effect non-catalytic gas—solid reactions. Events occurring during such reactions (see Sect. 5) are complex and industrial equipment for particular applications has evolved with operating experience rather than as a result of analytical design. Those factors which influence the course of the reaction are the reaction kinetics (as observed for a single particle), the size distribution of the solid reactant feed and the flow pattern of both solid and gas phases through the reactor. An excellent account of gas—solid reactions and... 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Phase transformations in heterogeneous catalysis have been described recently by topochemical kinetic models [111-115]. These models were taken from solid chemistry, where they had been developed for "gas-solid reactions. The products of such reactions are solids. When gas is in contact with the initial solid, the reaction rate is negligible. But as nucleates of the phase... 

A gas-solid reaction usually involves heat and mass transfer processes and chemical kinetics. One important factor which complicates the analysis of these processes is the variations in the pore structure of the solid during the reaction. Increase or decrease of porosity during the reaction and variations in pore sizes would effect the diffusion resistance and also change the active surface area. These facts indicate that the real mechanism of gas-solid noncatalytic reactions can be understood better by following the variations in pore structure during the reaction. 

The reduction of U03 to U02 has drawn much attention in connection with the preparation of fuels for nuclear reactors. Above 400° C, in which temperature range most of the reductions have been studied, the oxides of uranium take the forms, U02+, U409, U308-x(U02 6) and U03, as in Fig. 5 [65], In common with other gas—solid reactions, the details of the kinetics vary depending on the origin of the sample [69], the surface area [68], etc. Some authors claim that the reduction by hydrogen proceeds stepwise U03 -> U3Og - U02 [66—69], but there is a report [70] that no evidence for the existence of the intermediate oxides was found by X-ray diffraction at the reaction boundary of U03 and U02. 

Such questions are answered empirically all too often. A more fundamental approach is needed. In the area of gas-phase kinetics, the developments in the chemistry of large sets of elementary reactions and diffusion in multi-component mixtures in a combustion context are now finding applications in chemical engineering, as mentioned above. In the area of gas-solid reactions, the information flow will be in the opposite direction. A need exists... 

The kinetics of the process is important in view of economic viability, because it is the crucial factor for productivity. For an assessment the complete macrokinetics of both reaction steps have to be considered (i.e. heat and mass transfer as well as the rates of the gas-solid reactions). Thermal reduction of an oxide and its reoxidation by CO2 are reversible gas-solid reactions. The reaction rate of a solid reacting with a gas to form another solid material is described by Equation (8) ... 

In some cases, adsorption of analyte can be followed by a chemical reaction. The Langmuir-Hinshelwood (LH) and power-law models have been used successfully in describing the kinetics of a broad range of gas-solid reaction systems [105,106]. The LH model, developed to describe interactions between dissimilar adsorbates in the context of heterogeneous catalysis [107], assumes that gas adsorption follows a Langmuir isotherm and that the adsorbates are sufficiently mobile so that they equilibrate with one another on the surface on a time scale that is rapid compared to desorpticm. The power-law model assumes a Fre-undlich adsorption isotherm. Bodi models assume that the surface reaction is first-order with respect to the reactant gas, and that surface coverage asymptotically approaches a mmiolayer widi increasing gas concentration. 

Present research efforts aim mainly at obtaining the important parameters from a study of the macrokinetics of combustion. It is important to estimate the effects of flame retardants, chemical structure of the polymer and polymer composition on variations of the solid- and gas-phase reaction kinetics. 

In order to be able to represent the behaviour of fluidized bed reactors with confidence, one must have a thorough understanding of the bed hydrodynamics and of the reaction kinetics. Almost all of the reactions carried out in fluidized beds are either solid-catalysed gas phase reactions or gas-solid reactions. (We will not consider here homogeneous gas phase reactions, reactions in liquid fluidized beds or reactions in three phase fluidized beds.) While the chemical kinetics can often be highly complex. 

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