LHHW kinetic model

For SR of higher hydrocarbons, Rostrup-Nielsen " and Tottrup " postulated a Langmuir-Hinshelwood-Houghen-Watson (LHHW) kinetic model. It was assumed that the hydrocarbon chemisorbs on a dual catalytic site, followed by successive a-scission of the C-C bond. The resulting Ci species react with adsorbed steam to form H2 and CO. The expressions were lit to data for SR of n-Cv on a Ni/MgO catalyst at 500°C the overall rate expression is " " ... 

Table 5.9 Parameters in LHHW kinetic model for cyclohexanol dehydrogenation.

In general, a reaction kinetics following a LHHW model is suitable, but the identification of parameters remains demanding. For some catalysts power-law models may be appropriate, for others not. For example, reaction orders identical with stoichiometric coefficients were suitable for Pd/Al203 doped with different metals. On the contrary, for Pd/MgO reaction orders with respect to phenol ranging from -0.5 to 0.5 were observed [17]. However, the bibliographic search was not able to find a quantitative kinetic model for Pd-type catalysts suitable for reactor design. 

Recently a rigorous quantitative model was developed in order to describe promotional and, more generally, catalytic kinetics [130,147]. The model can be viewed as an extension of classical Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics. 

Although the kinetics of reforming of pure paraffins may be satisfactorily represented by LHHW-type models, such models are difficult to apply for real fuels, such as diesel or gasoline. For such complex systems, it may be more practical to use pseudo-homogeneous kinetics. For example, hydrocarbon fuel components can be lumped in groups with similar properties and kinetic behaviors for example, paraffins are also grouped into lumped reactions. However, the levels of simplification must be carefully evaluated to make them consistent with the final aim of the kinetic model. 

An equally important system of reactions is one where the catalyst becomes deactivated either intrinsically or throngh deposition of carbonaceons products. The modeling of such systems becomes involved where deactivation has to be conpled with LHHW kinetics. Another complicated situation can arise where the reaction occnrring on the deactivating catalyst is complex but each step can be represented by power law kinetics. In the present case study, we consider such a reaction to illustrate the application of rigorons statistical methods to complex reacting systems. ... 

Extensive experimental and kinetic modelling work on these reactions carried out by Zmcevic and Rusic (1988) and Takahashi etal. (1986) using a nickel catalyst supported on different materials gives strong evidence to suggest that the LHHW mechanism is more probable. 

Kinetic models can be used to link the reactor design with its performance. The reaction rate may be expressed by power law functions, by more complex expressions, as Langmuit-Hinselwood-Hougen-Watson (LHHW) correlations for catalytic processes, or by considering user kinetics. There are two ideal models, continuous stirred tank reactor (CSTR) or plug flow (PFR), available in rating mode (reaction volume fixed) or design mode (conversion specified). 

An interesting feature of LHHW kinetics is worth noting. Many reactions on surfaces known to be nonideal surprisingly follow the ideal LHHW models, a situation that can only be described as the placebo effect or the paradox of heterogeneous kinetics (Boudart et al., 1967 Boudart, 1986). In the same vein but with less justification, it has also been argued for more than four decades—... 

A major factor in modeling hydroformylation reactions is that two gases (H2 and CO) are present compared to one gas (CO) in carbonylation. The most commonly used complexes are those of Co and Rh. A typical kinetic model for hydroformylation has the following hyperbolic (LHHW) form (Martin, 1954) ... 

The kinetics of methane combustion over a perovskite catalyst (Lao.9Ceo.iCo03) has been studied in Micro-Berty and fixed bed reactors. Discrimination among twenty-three rival kinetic models from Eley-Rideal, LHHW and Mars-van Krevelen (MVK) types has been achieved by means of (a) the initial rate method as well as by (b) integral kinetic data analysis. Two MVK type models could be retained as a result of the two studies, with a steady-state assumption implying the equality of the rate of three elementary steps. 

The kinetic model contains concentration terms given as activities. The overall reaction rate is not a simple power law but has absorption terms in LHHW form. This type of chemical kinetic cannot be used in the standard Aspen Plus reactive distillation model. The various formulas follow for the overall reaction rate (mol/s) ... 

Most standard chemical engineering tests on kinetics [see those of Car-berry (50), Smith (57), Froment and Bischoff (19), and Hill (52)], omitting such considerations, proceed directly to comprehensive treatment of the subject of parameter estimation in heterogeneous catalysis in terms of rate equations based on LHHW models for simple overall reactions, as discussed earlier. The data used consist of overall reaction velocities obtained under varying conditions of temperature, pressure, and concentrations of reacting species. There seems to be no presentation of a systematic method for initial consideration of the possible mechanisms to be modeled. Details of the methodology for discrimination and parameter estimation among models chosen have been discussed by Bart (55) from a mathematical standpoint. 

In general, the use of Langmuir-Hinshelwood-Hougen-Watson (LHHW)-type of rate equation for representing the hydrogenation kinetics of industrial feedstocks is complicated, and there are too many coefficients that are difficult to determine. Therefore, simple power law models have been used by most researchers to fit kinetic data and to obtain kinetic parameters. 

In this project, we make use of platinum-type catalyst on silica gel. Although this is less selective than more modem palladium-based catalysts, kinetic data are available in the literature as an LHHW model [2], better suited for flexible reactor design. The reaction rate equations are ... 

In heterogeneous catalysis these models are generally referred to as the Langmuir-Hinshelwood-Hougen-Watson (LHHW) models. The term Michaelis-Menten kinetics is often used in homogeneous catalysis, enzyme reactions and reactions of microbial systems. 

In homogeneous catalysis, enzyme reactions, and reactions of microbial systems the same types of equations are used as in the LHHW models. In the latter disciplines, however, they are often referred to as Michaelis-Menten kinetics. 

That the theory of complex reactions kinetics went beyond LHHW treatment is due in major part to Horiuti and Temkin. Up to now it serves as a basis for mathematical modeling of catalytic processes and reactors at stationary conditions. The reaction and the process are considered to be stationary, if the concentration of all reactants and products in any element of the reactor space, including the active catalyst surface, do not change in time. At stationary conditions, concentrations of the intermediates are time independent as the rates of their generation in elementary steps are equal to the rates of consumption in other elementary reactions. 

It was pointed out earlier that regime 3 is controlling (for simple power law kinetics) if /M 1. It can similarly be shown (Chaudhari, 1984) that regime 3 is controlling for LHHW models if... 

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