Heterogeneous reactions transport control

The effect of temperature on the rate of a typical heterogeneous reaction is shown in Figure 3.25. At low temperatures the reaction is chemically controlled and at high temperatures it is diffusion or mass transport controlled. 

If the interface reaction rate is extremely small so that mass/heat transfer is rapid enough to transport nutrients to the interface, then interface reaction rate (Equation 4-33) is the overall heterogeneous reaction rate (Figure 1-lla). If the interface reaction is relatively rapid and if the crystal composition is different from the melt composition, the heterogeneous reaction rate may be limited or slowed down by the mass transfer rate because nutrients must be transported to the interface and extra junk must be transported away from the interface (Figures 1-llb and 1-llc). If the crystal composition is the same as the melt composition, then mass transfer is not necessary. When interface reaction rate and mass transfer rate are comparable, both interface reaction and mass transfer would control the overall heterogeneous reaction (Figure 1-lld). 

The determination of mechanistic rate laws for soil chemical processes is very difficult since microscopic heterogeneity is pronounced in soils and even for most soil constituents such as clay minerals, humic substances, and oxides. Heterogeneity can be enhanced due to different particle sizes, types of surface sites, etc. As will be discussed more completely in Chapter 3, the determination of mechanistic rate laws is also complicated by the type of kinetic methodology one uses. With some methods used by soil and environmental scientists, transport-controlled reactions are occurring and thus mechanistic rate laws cannot be determined. 

The rate of calcite dissolution is known to depend on the hydrodynamic conditions of the environment and on the rate of heterogeneous reaction at the mineral surface. Numerous laboratory studies demonstrate transport and surface-controlled aspects of calcite reactions in aqueous solutions, but until recently, no study has been comprehensive enough to enable comparison of kinetic results among differing hydro-chemical systems. 

Bircumshaw, L.L., and Riddiford, A.C. Transport control in heterogeneous reactions. Quart. Rev. Chem. Soc. London 6, 157-185 (1952). 

Corrosion is fundamentally a chemical reaction between a metal and its environment. As such it is a heterogeneous reaction between a fluid and a solid. At higher temperatures (when the environment is a gas rather than a liquid), the reaction is typically a direct reaction between oxygen and the metal to form the metal oxide. The oxide will form as a solid on the metal surface," and oxidation will be controlled by the transport of oxygen and metal ions through the corrosion product. 

Apart from the alkali metals, the reactions of which have been reviewed in Section 2, very few metals are sufficiently volatile to enable gas-phase diffusion flame studies to be undertaken. The high temperatures required for vaporization would destroy organic materials, so only reaction with inorganic gases can be considered. The combustion of some metals has been studied, as they are of importance as possible rocket propellant systems. The kinetics, however, are complex. Metals burn predominantly, and in some cases exclusively, by heterogeneous reactions [305], since both the fuel and the products are usually in the condensed state. In metal combustion, transport processes exert at least a partially controlling influence, and so information on reaction kinetics is difficult to obtain. The reaction may occur at the surface of the metal, or on the surface of... 

In a fixed-bed reactor the catalyst pellets are held in place and do not move with respect to a fixed reference frame. Material and energy balances are required for both the fluid, which occupies the interstitial region between catalyst particles, and the catalyst particles, in which the reactions occur. For heterogeneously catalyzed reactions, the effects of intraparticle transport on the rate of reaction must be considered. Catalytic systems operate somewhere between two extremes kinetic control, in which mass and energy transfer are very rapid and intra-partide transport control, in which the reaction is very rapid. Separate material and energy balances are needed to describe the concentration and temperature profile inside the catalyst pellet. The concentrations... 

The most prominent feature of homogeneous transition metal catalysts are the high selectivities that can be achieved. Homogeneously catalyzed reactions are controlled mainly by kinetics and less by material transport, because diffusion of the reactants to the catalyst can occur more readily. Due to the well-defined reaction site, the mechanism of homogeneous catalysis is relatively well understood. Mechanistic investigations can readily be carried out under reaction conditions by means of spectroscopic methods (Fig. 1-3). In contrast, processes occurring in heterogeneous catalysis are often obscure. 

The interfacial concentration of Fe " is in good agreement with solubility data. The increase of pH by H+ migration is reflected in the profile of SO . It is concluded that no barrier layer is involved in the mass transport control of iron dissolution. The heterogeneous reaction rate is reported as not dependent on the HSO4 concentration therefore, the only acceptor species likely to limit the dissolution rate is water [74]. This is compatible with the modified form (29) of the initial step of dissolution in which one water molecule is dissociated and the depletion of free water at the electrode surface by Fe(II) hydration and electromigration of hydrated Fe(II) away from the surface. 

Diagnostic experiments can be carried out to determine whether a heterogeneous catalytic reaction is being affected by external transport. These experiments are based on a very simple concept. If a heterogeneous catalytic reaction is controlled by intrinsic kinetics. 

Diagnostic experiments can be designed to determine whether a heterogeneous catalytic reaction is controlled or influenced by external transport, based on the above idea. Consider a tubular reactor with an inner diameter A loaded with a weight of catalyst W. The height of catalyst in the reactor is L. An experiment is carried out with a volumetric inlet flow rate of Mb-The concentrations of the components of the feed stream are Cg), the inlet temperature is To, and the reactor is operated either isothermally or adiabatically. The conversion of A leaving the reactor xa is measured. This situation is shown schematically in Figure 9-15... 

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