Solid reaction rates

The retarding influence of the product barrier in many solid—solid interactions is a rate-controlling factor that is not usually apparent in the decompositions of single solids. However, even where diffusion control operates, this is often in addition to, and in conjunction with, geometric factors (i.e. changes in reaction interfacial area with a) and kinetic equations based on contributions from both sources are discussed in Chap. 3, Sect. 3.3. As in the decompositions of single solids, reaction rate coefficients (and the shapes of a—time curves) for solid + solid reactions are sensitive to sizes, shapes and, here, also on the relative dispositions of the components of the reactant mixture. Inevitably as the number of different crystalline components present initially is increased, the number of variables requiring specification to define the reactant completely rises the parameters concerned are mentioned in Table 17. 

Section 3 deals with reactions in which at least one of the reactants is an inorganic compound. Many of the processes considered also involve organic compounds, but autocatalytic oxidations and flames, polymerisation and reactions of metals themselves and of certain unstable ionic species, e.g. the solvated electron, are discussed in later sections. Where appropriate, the effects of low and high energy radiation are considered, as are gas and condensed phase systems but not fully heterogeneous processes or solid reactions. Rate parameters of individual elementary steps, as well as of overall reactions, are given if available. 

The gas-solid reaction rate is modeled by the nonlinear expression... 

Power-law kinetic rate expressions can frequently be used to quantify homogeneous reactions. However, many reactions occur among species in different phases (gas, liquid, and solid). Reaction rate equations in such heterogeneous systems often become more complicated to account for the movement of material from one phase to another. An additional complication arises from the different ways in which the phases can be contacted with each other. Many important industrial reactors involve heterogeneous systems. One of the more common heterogeneous systems involves gas-phase reactions promoted with porous solid catalyst particles. 

In practice, the thickness of liquid films in trickle beds has been estimated to vary between 0.01 and 0.2 mm (0.004 and 0.008 in). The dynamic liquid holdup fraction is 0.03 to 0.25, and the static fraction is 0.01 to 0.05. The high end of the static fraction includes the liquid that partially fills the pores of the catalyst. The effective gas-liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high liquid loading. This results in an increase of reaction rate as the amount of wetted surface increases (i.e., when the gas-solid reaction rate is negligible). 

A similar treatment may be used to describe the gas-solid reaction rate when there is no solid product formed such as encountered in solid fuel combustion. However, in that case, the product layer diffusion resistance is always zero. 

For equilibrium measurements in Reaction 12, the system was brought to nearly the eutectic conditions and was held for about 1/2 hour to ensure that both CaO and Ca(OH)2 were present. The system pressure and charge temperature then were adjusted to the desired levels while staying on the Ca(OH)2 side of equilibrium. The charge temperature was increased by about 5°F. and held constant. Since the solid-solid reaction rates were much slower than when a liquid phase was present, both CaO and Ca(OH)2 assuredly were present at all times. The system pressure became constant in about 1 hour. 

Gas plus catalytic solid Reaction rates very fast and very rapid deactivation of catalyst. Solid particle diameter 0.007-1.5 mm. 

Ra (bubble) transfer rate from the bubble Ra (surface) transfer rate of catalyst Ra (solid) reaction rate in the catalyst Thus ... 

FIG. 6.1. Schematic diagram of the gravimetric apparatus for measuring gas-solid reaction rates. 

When a chemical reaction takes place at the solid surface, we expect a smooth variation in gas composition in the macropores on a scale comparable with the whole pellet, provided the reaction rate is not too high. 

POLYRATE can be used for computing reaction rates from either the output of electronic structure calculations or using an analytic potential energy surface. If an analytic potential energy surface is used, the user must create subroutines to evaluate the potential energy and its derivatives then relink the program. POLYRATE can be used for unimolecular gas-phase reactions, bimolecular gas-phase reactions, or the reaction of a gas-phase molecule or adsorbed molecule on a solid surface. 

Sodium cyanide does not dissolve m butyl bromide The two reactants contact each other only at the surface of the solid sodium cyanide and the rate of reaction under these con ditions IS too slow to be of synthetic value Dissolving the sodium cyanide m water is of little help because butyl bromide is not soluble m water and reaction can occur only at the interface between the two phases Adding a small amount of benzyltrimethyl ammonium chlonde however causes pentanemtnle to form rapidly even at room temper ature The quaternary ammonium salt is acting as a catalyst it increases the reaction rate How7... 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

Adiabatic. Control gas flow and/or solids feed rate so that the heat of reaction is removed as sensible heat in off gases and solids or heat supphed by gases or solids. 

Rate of Reaction Rate equations of fluid reactions catalyzed by solids are of two main types ... 

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. 

Poor distribution Implement appropriate procedures and training of solids or liquid charge. Potential for excessive reaction rates due to localized over-concentrations of reactants. CCPS G-22... 

In most circumstances, it can be assumed diat die gas-solid reaction proceeds more rapidly diaii die gaseous transport, and dierefore diat local equilibrium exists between die solid and gaseous components at die source and sink. This implies diat die extent and direction of die transport reaction at each end of die temperature gradient may be assessed solely from diermodynamic data, and diat die rate of uansport across die interface between die gas and die solid phases, at bodi reactant and product sites, is not rate-determining. Transport of die gaseous species between die source of atoms and die sink where deposition takes place is die rate-determining process. 

The above discussion relates to diffusion-controlled transport of material to and from a carrier gas. There will be some circumstances where the transfer of material is determined by a chemical reaction rate at the solid/gas interface. If this process determines the flux of matter between the phases, the rate of transport across the gas/solid interface can be represented by using a rate constant, h, so that... 

The value of tire heat transfer coefficient of die gas is dependent on die rate of flow of the gas, and on whether the gas is in streamline or turbulent flow. This factor depends on the flow rate of tire gas and on physical properties of the gas, namely the density and viscosity. In the application of models of chemical reactors in which gas-solid reactions are caiTied out, it is useful to define a dimensionless number criterion which can be used to determine the state of flow of the gas no matter what the physical dimensions of the reactor and its solid content. Such a criterion which is used is the Reynolds number of the gas. For example, the characteristic length in tire definition of this number when a gas is flowing along a mbe is the diameter of the tube. The value of the Reynolds number when the gas is in streamline, or linear flow, is less than about 2000, and above this number the gas is in mrbulent flow. For the flow... 

The solid iron ore is formed into pellets, which are presented to the gas in a vertical shaft containing the pellets in tire form of a packed bed. The reducing gas enters the shaft at the bottom and rises tluough the packed bed reacting to form gaseous oxidation products, CO2 and H2O. The heat required to raise the reactants to a temperature at which the reaction rate is fast enough is usually canied by the inlet gas phase. 

We will consider flow through a solid element. Introducing the notations for molar flow density, partial density, and the reaction rate gives an equation for the mass balance ... 

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