Kinetics heterogeneous reactions

Heterogeneous reactions occur in gas or liquid phase or in both phases in the presence of a solid as a catalyst or as reactant, which depends on the process in use. Usually, the reactions in gas-solid phase or gas-liquid-solid phases depend on the industrial reaction conditions. In catalytic processes, the catalyst promotes the reaction rate in 

The ammonia synthesis is one of the most important processes in the production of fertilizers. The reaction is exothermic (AH= —46kJ/molNH3) and runs at relatively low temperatures ( 400°C) and high pressures ( 60-100atm). The kinetics is an example of a heterogeneous system occurring on iron-based catalyst. 

The limiting step of the kinetics is the chemical reaction occurring at the surface. However, there are diverse physical and chemical phenomena, besides mass transfer and internal diffusion inside pore particles affecting the global reaction rate. There are different steps  

Diffusion of reactant molecules through the fluid in the direction of the surface. 

Diffusion of molecules from the surface through pores. 

Regarding gasification, four distinct reactions have to be considered to describe the conversion of carbon. Data are most abundant for the carbon oxidation and the Boudouard reaction, as O2 and CO2 are easily handled reactants. There are 

Most data are provided in terms of extended Arrhenius expressions according to Equation (5.26). 

Mon and Amundson [19] recalculated data from Field and Roberts [11] and Dutta et al. [20] and adjusted it in order to eliminate the linear temperature dependency of the Arrhenius expression. 

Other reaction kinetics as reported by Kajitani et al. [21,22] and by Deng et al. [23] are not related to surfece area but to mass loss. These so-called appent kinetics are shown in Table 5.5. Hence, this data is limited to the particle structure investigated and cannot be used for direct comparison without knowledge of the surfece area. 

Chen et al. [24] provided a new data set for numerical simulation, publishing the wrong unit for activation energy. Recently, these data were cited [9], correcting the activation energy unit but altering the value for carbon oxidation. 

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. 

One approach to describe the kinetics of such systems involves the use of various resistances to reaction. If we consider an irreversible gas-phase reaction A — B that occurs in the presence of a solid catalyst pellet, we can postulate seven different steps required to accomplish the chemical transformation. First, we have to move the reactant A from the bulk gas to the surface of the catalyst particle. Solid catalyst particles are often manufactured out of aluminas or other similar materials that have large internal surface areas where the active metal sites (gold, platinum, palladium, etc.) are located. The porosity of the catalyst typically means that the interior of a pellet contains much more surface area for reaction than what is found only on the exterior of the pellet itself. Hence, the gaseous reactant A must diffuse from the surface through the pores of the catalyst pellet. At some point, the gaseous reactant reaches an active site, where it must be adsorbed onto the surface. The chemical transformation of reactant into product occurs on this active site. The product B must desorb from the active site back to the gas phase. The product B must diffuse from inside the catalyst pore back to the surface. Finally, the product molecule must be moved from the surface to the bulk gas fluid. 

To look at the kinetics in heterogeneous systems, we consider the step of adsorbing a gaseous molecule A onto an active site s to form an adsorbed species As. The adsorption rate constant is ka. The process is reversible, with a desorption rate constant kd  

Since we are dealing with gaseous molecules, we usually write the rate of adsorption in terms of the partial pressure of A (PA) rather than molar concentration. The net rate of adsorption and desorption is 

If we define 6 as the fraction of total sites covered by the adsorbed molecules, then 

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Apart from pyrometallurgy, the various ingredients of heterogeneous reaction kinetics are also very pertinent to several hydrometallurgical operations. This will be discussed in Chapter 5. 

For a reaction to produce a new phase, the new phase must first form (nucleate) from an existing phase or existing phases. Nudeation theory deals with how the new phase nucleates and how to predict nudeation rates. The best characterization of the present status of our understanding on nudeation is that we do not have a quantitative understanding of nudeation. The theories provide a qualitative picture, but fail in quantitative aspects. We have to rely on experiments to estimate nudeation rates, but nudeation experiments are not numerous and often not well controlled. In discussion of heterogeneous reaction kinetics and dynamics, the inability to predict nudeation rate is often the main obstacle to a quantitative understanding and prediction. The nudeation theories are... 

Experimental investigations will continue to play a critical role in understanding heterogeneous reaction kinetics, such as mineral dissolution rates in silicate melts and in aqueous solutions, the melting rates at the interfaces of two... 

A Simple Electrochemical Approach to Heterogeneous Reaction Kinetics 261... 

In DESIGNER, different ways of taking account of heterogeneous reaction kinetics are available, depending on the reaction rate and character. One further possibility is to use a detailed model for the heterogeneous catalyst mass transfer... 

To analyze reaction mechanisms in complex catalytic systems, the application of micropulse techniques in small catalytic packed beds has been used. Christoffel [33] has given an introduction to these techniques in a comprehensive review of laboratory reactors for heterogeneous catalytic processes. Mtlller and Hofmann [59,61] have tested the dynamic method in the packed bed reactor to investigate complex heterogeneous reactions. Kinetic parameters have been evaluated by a method, which employs concentration step changes and the time derivatives of concentration transients at the reactor outlet as caused by a concentration step change at the reactor inlet. 

Selected Elementary Reactions Additional Sections HETEROGENEOUS REACTIONS KINETICS AND TECHNOLOGICAL PROCESSES... 

Section 6. oxidation and COMBUSTION REACTIONS (2 volumes) Section 7. SELECTED ELEMENTARY REACTIONS (1 volume) Volume 18 Selected Elementary Reactions Additional Sections HETEROGENEOUS REACTIONS KINETICS AND TECHNOLOGICAL PROCESSES... 

To facilitate our discussion of heterogeneous reaction kinetics let us consider the oxidation of CO on Pd, an fee metal The following mechanism has been proposed for the oxidation reaction over Pd(l 11 [9]... 

The simulator is able to treat different heterogeneous reaction kinetics, depending on the reaction rate and its character. For example, a detailed model for the heterogeneous catalyst mass-transfer efficiency can be used, which is based on the approach of [99]. When applying this type of kinetic model, the intrinsic kinetics data are needed (see Section 10.3.1). Another way is the pseudohomoge-neous approach with effective kinetics expressions, by which the kinetics description is introduced as source terms into the balance equations (see Eqs. (10.3) and (10.4)). 

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