Multiple oxidation reactions and diffusion in the catalytic layer of monolith reactors

Oxidation reactions of mobile exhaust gas in a catalytic monolith are considered in this paper. The exhaust gas is described by four concentrations, namely those of the carbon monoxide, fast- and slow-oxidizing hydrocarbons and hydrogen. The problem of multiple reactions and diffusion in the catalytic layer of the monolith walls is studied first. The diffusional resistance of the catalytic layer is often significant in spite of its thinness and it seems important to allow for both the hydrogen and hydrocarbon oxidation reactions in analyzing the system for steady-state. The influence of hydrogen on the multiplicity of steady-state wall temperatures is also studied and it is found to be important, because hydrogen reacts rapidly at lower temperatures.     

Activation energy variation for catalytic oxidation of aqueous SO2

SO₂ oxidation in aqueous solutions catalysed with manganous sulphate was studied to determine temperature dependencies of the reaction rate. The process was carried out at relatively high sulphuric acid (reaction product) concentrations with regard to its application for SO₂ removal from waste gases. Variation of the apparent activation energy has been linked with alteration of reaction rate determining steps.     

Kinetics of the catalytic dehydrogenation of cyclohexanol to cyclohexanone on a zinc oxide catalyst in a gradientless reactor

The catalytic dehydrogenation of cyclohexanol to cyclohexanone on a ZnO catalyst has been studied in a gradientless reactor (GLR) in the temperature range of 550–650 K. The data were fitted to several kinetic models using a non-linear least squares fitting computer programme. From model discrimination procedures, the most suitable model was found to be a Langmuir-Hinshelwood model, involving associative adsorption of cyclohexanol, cyclohexanone and hydrogen on the same kind of sites. No effects of film diffusion or intraparticular temperature gradients could be detected, while a certain influence of pore diffusion has to be taken into account at temperatures above 620 K. The mixing characteristics of the GLR have been investigated by measuring the mean residence time distribution and the recirculation ratio. Furthermore, the possibility of temperature gradients along the catalyst bed are discussed. The limitations of application of the GLR are shown and some suggestions to overcome the shortcomings are presented.     

Kinetics of catalytic hydrodesulfurization of a petroleum residue in a batch-recycle trickle bed reactor

The kinetics of catalytic hydrodesulfurization of an atmospheric residuum were investigated in a batch-recycle trickle bed reactor with a commercial catalyst. The global rate equation determined was R = Catalyst effectiveness factors η, remaining activity ε, effective pore diffusivities D_e and activation energies E have been determined quantitatively. The data have also been interpreted in terms of the two parallel, first-order reactions model and the respective parameters have been determined. Solid—liquid chromatography fractions of three hydrocarbon and sulfur compound groups have been determined in addition to the asphaltenes content of the feedstock and the hydrotreated product. These data are discussed and conclusions are drawn with respect to their behaviour during HDS and a reaction network is proposed which explains the complicated interrelations involved.     

A kinetic model for methanol conversion to hydrocarbons

We developed a detailed kinetic model for methanol conversion to hydrocarbons over zeolites. The dimethylether at equilibrium with methanol, generates carbene; it attacks the oxygenates giving light olefins, and the latter forming higher olefins. The olefins, through carbenium ions, form aromatics and paraffins. The kinetic parameters, identified by nonlinear optimization techniques, and the model predictions, agree quite well with the experiment and literature available data.     

Kinetics of the reaction between steam and the basal plane of graphite

The gold decoration/transmission electron microscopy technique has been used to measure the rate of removal of carbon atoms from the basal plane of gra processes have been determined: removal of edge atoms surrounding the monolayer etch pits, and abstraction of carbon atoms within the basal plane which for edge carbon removal ranges from 0.02 at 600°C to 26 atom/atom active site/s at 900°C. The activation energy of 52 kcal/mole is close to the val of the saturated basal atoms (with 3 sp² bonds) may be expressed in terms of the probabilities of successful collisions of O atoms which are fo for both CO₂ and CH₂O reactions, are of the order of 10^?10 in the above temperature range. Comparison of the two reaction oxygen and water is the species responsible for basal atom abstraction.The gasification of graphite by steam is immensely anisotropic. At 23 torr H₂O the turnover frequency in the prismatic or edge directions, {10$\text{1}$0} and {11$\text{2}$0} faces, is higher than that on the basal {0001} plane by a factor of 4 × 10¹⁰ at 900°C and 4 × 10¹² at 700°C.     

Modeling of fixed bed catalytic reactors

The simulation of a fixed-bed catalytic reactor requires the selection of a model, which is a set of balance equations that describes the reactor, as well as correlations for the model parameters involved. In this work general criteria, leading to a better choice of a model that fulfills the objectives of the simulation, are established. Different ways in which the parameters can be obtained are analyzed, and the numerical methods for solving the model equations are discussed.     

Isothermal multiplicity on catalytic surfaces

The necessary condition for the existence of multiplicity has been obtained for the case of a first-order surface reaction, on the assumption that the     

Modelling forced periodic operation of catalytic reactors

The kinetic adsorption/desorption or AD models used to predict product selectivities obtained through forced-feed concentration cycling have been re-examined. Selectivity improvement results from model structure and nonlinear rates of reaction. The relaxed-steady state bound to periodic operation was found to be predictable from steady state rates for the kinetic model used. Adsorption/desorption models predict no rate improvement for periodic operation. Also they fail to predict experimental observations that are now fairly extensive.     

A Quasi-steady-state analysis of the dynamics of two-species heterogeneous catalytic reactions

This study demonstrates how low-order algebraic non-linearities that exist in a simple two-component Langmuir-Hinshelwood type reaction kinetics (CO oxidation) are sufficient to produce rate multiplicities and oscillatory steady states (periodic solutions). A singular perturbation analysis is employed wherein certain quasi-steady-state considerations are made which lead to the definition of system manifolds and invariants along which the large-time dynamics of the system can be discerned without recourse to numerical integration. New results, confirmed by simulation, include an explanation for such experimentally observed pathological phenomena as the coexistence of oscillatory and stationary steady states and multi-peak oscillations. It is shown how the existence of oscillatory states and a plausible “buffering” physisorption surface reaction mechanism causing a periodic switching between these states and coexisting stationary ones can give rise to these multi-peak oscillations.     

A revised model for predicting the performance of a fluidised-bed catalytic reactor

In a gas fluidised-bed, bubble coalescence gives rise to a very rapid mass exchange rate with the dense phase. The extent of this exchange rate is calculated from the observed changes in bubble size with height in the bed. It is calculated that with a catalytic reaction in progress, the difference in composition between the dense phase and bubble phase rapidly falls to a negligible value for all cases where the reaction rate and throughput are su that a significant height of bed is required for nearly complete conversion. A corresponding model of the fluidised-bed catalytic reactor is analysed f which it transpires that its behaviour is very close to that of a CSTR, as is apparent when published data on reactor performance is examined. The stat mixing of the dense phase, if it were to depart from the completely mixed condition assumed, would influence this conclusion.     

Optimal temperature profiles for heterogeneous catalytic parallel reactions

The problem of determination of the optimal temperature profiles in a fixed-bed catalytic reactor for the catalytic parallel reactions including transport phenomena (external or internal diffusion) has been worked out.Special attention was given to predict the shapes of the optimal temperature profiles. The general considerations are illustrated by some numerical examples. It has been found that for every studied case the optimal temperature profile need not to end by the isotherm T = T_max.     

Inhomogeneities and surface structures in oscillatory catalytic kinetics

The existence of inhomogeneous surface states is investigated for three oscillatory kinetic models, using realistic values of rate and communication parameters. The analysis shows that local oscillators are effectively synchronized if the autocatalytic variable is a gas-phase concentration. If a surface concentration is the autocatalytic variable and the second variable is a gas-phase reactant, the system will admit an inhomogeneous solution in the form of two surface regions separated by a stationary or undulating front. Oscillations may be translated into either a train of trigger waves or into a stationary inhomogeneous state in a system including two surface reactants.The emergence of inhomogeneous surface states leads to falsification of the apparent reaction kinetics, and may induce dependence of the reaction rate on geometrical factors. It may also facilitate formation of permanent surface structures thereby strongly influencing evolution of the clalyst under the influence of reaction conditions.     

Effectiveness of catalytic archipelagos—I Regular arrays of regular islands

The interplay of the adsorption and desorption of reactants onto a catalytically active surface and their diffusion in that surface is an important process in catalysis. A surface with regularly distributed circular or rectangular catalyst islands in a plane has been investigated here under the assumption that the adsorbed reactant diffuses to the boundaries of the islands and disappears there by a first-order reaction. With the aid of the finite element method, effectiveness factors of the model systems have been calculated in terms of two dimensionless parameters ψ and μ, one measuring the ratio of the adsorption and desorption rates to that of surface diffusion and the other the rate of reaction to the rate of surface diffusion. It is found that the effectiveness factor for arrays whose symmetry elements are not too elongate can be approximated closely by that of an equivalent annulus whose inner and outer radii are calculated in terms of the geometrical parameters of the various systems. The equivalent annulus approximation is fairly accurate for these arrays in the parameter range 0.01 ?ψ, μ?10. Furthermore, outside this parameter range, asymptotic formulas are given which are accurate for all systems considered.     

Films by anodic oxidation of mercaptoanilines AT Cu electrodes

The anodic oxidation of mercaptoanilines at different metal anodes proves that coherent coatings may be formed in situ on copper only, using the meta isomer. These coatings probably from through the oxidation of an intermediate Cu-3-mercaptoaniline complex, which leaves high copper content in the polymer deposit: however, such materials impart outstanding corrosion protection to the substrate. It is suggested that, after easy dimerization of the monometer to disulfide, C—S coupling reactions play a basic role in the constitution of the polymeric network.