Separable Kinetics Research Articles

Effective Ethyl Carbamate Prevention in Red Wines by Treatment with Immobilized Acid Urease.

Climate change poses several challenges in the wine industry, including increasing risks related to chemical food contaminants such as biogenic amines and ethyl carbamate (EC). In this work, we focused on urea removal in red wines by immobilized acid urease aiming at limiting EC formation during wine storage. By considering separable kinetics of catalyst deactivation and urea hydrolysis, it was possible to model the time course of urea removal in repeated uses in stirred batch reactors. Treatments based on immobilized urease of red wine enriched with 30 mg/L of urea allowed the reduction in the contaminant concentration to <5 mg/L. After 28.5 h of treatment, the observed urea level was reduced to about 0.5 mg/L, corresponding to a decrease in the potential ethyl carbamate (PEC) from 1662 μg/L to 93 μg/L, below the level of the non-enriched wine (187 μg/L). As a comparison, when treating the same wine with the free enzyme at maximum doses allowed by the EU law, urea and PEC levels decreased to only 12 mg/L and 415 μg/L respectively, after 600 h of treatment. These results show that, for red wines, urease immobilization is an effective strategy for urea removal and, thus, effective reduction in ethyl carbamate as a process contaminant. This study provides the scientific background for the future scaling-up of the process at an industrial level.

Modeling catalyst deactivation in heterogeneous Fenton-like oxidation reactions

Abstract One of the most crucial problems in catalysis is the loss of catalytic activity. When analyzing or designing a reactive system involving a decaying catalyst, a rate law adjusted for the catalyst deactivation should be utilized. An example of deactivating catalysts is supported-iron catalysts used in the heterogeneous Fenton-like oxidations of organic compounds. In this study, the observed reaction rate of such reactive systems was modeled – through separable kinetics – with a general rate expression that was written in terms of a rate law, describing the reaction kinetics on the fresh catalyst, and an activity term, accounting for the catalyst deactivation. The model was developed and verified for the catalytic wet peroxide oxidation of selected dyes (Reactive Yellow and Red) over Fe-Y zeolite and drugs (Diclofenac and Naproxen) over magnetite/multi-walled carbon nanotubes, which experienced notable deactivation during the reaction. It was found that the reaction rate followed Langmuir-Hinshelwood kinetics while the catalyst decay rate was second order in the present activity and first order in the concentration of the fouling species, which presumably resulted from the degradation of the organic compounds. The effect of the temperature on the rate expression parameters was also studied. The model included parameters such as the reaction and decaying rate constants, reaction and decaying activation energies, adsorption constant, and enthalpy of adsorption. The rate expression proved to satisfactorily describe the concentration–time trajectories in all the studied reactions despite their differences in the utilized catalysts, reacting organic compounds, and/or conditions, which verified the robustness of the model

Integrated modelling of reaction and catalyst deactivation kinetics—Hydrogenation of sitosterol to sitostanol over a palladium catalyst

Catalyst deactivation is traditionally described with semi-empirical models involving separable kinetics and time-dependent activity factors, but the rational approach is to treat catalyst deactivation as a chemisorption process among other surface processes. The catalytic process A→R along with surface poisoning P+⁎→P⁎ (⁎ is an active site) was modelled for batch and continuous reactors. Simple semi-analytical mathematical solutions were obtained, which avoid the use of surface coverage in computations. Numerical simulations revealed the behaviour of the model in the parameter space. For batch reactors, the approach to a limit conversion as well as the increase of the apparent reaction order due to catalyst poisoning was illustrated. The model was applied to batchwise hydrogenation of sitosterol to sitostanol on a dispersed platinum catalyst and in the presence of a sulphurous poison. The behaviour of the system was successfully described with a simple, mechanistic model based on the adsorption-reaction-poisoning concept.

Deactivation Modeling through Separable Kinetics of Coking On Ni/CZ Catalyst Used In SMR

Steam methane reforming (SMR) is a very significant technique to produce hydrogen from fossil fuels. In this particular work, nickel is used as the active metal and ceria-zirconia (CZ) bi-metallic oxide is used as the support. The foremost challenge to this process is sooting or coking over the catalyst surface and blocking the active sites. For the economic viability of the catalyst, it is very significant to make it coke deposition resistant. This is the reason that the kinetic modeling of the deactivation is very important. Therefore, this paper is aimed to model the deactivation and activity of the catalyst. A rate model of the deactivation process is also developed using separable kinetics. A comparison with commercial catalyst is also reported to show that the Ni/CZ catalyst is much more stable towards the coking. Keywords -Coking, Deactivation, Methane, Separable kinetics, Steam reforming.

Reaction and Deactivation Rates of Methane Catalytic Cracking over Nickel

The kinetics of methane catalytic cracking over nickel supported on porous and nonporous aluminas was modeled using a separable kinetics approach in order to develop initial rate and activity decay equations. The model parameters were estimated using a set of experiments conducted in an electrobalance. The experimental work covered the 500–650 °C temperature range, using pure methane, as well as different partial pressures of CH4/N2 and CH4/H2 mixtures at atmospheric pressure. The model results showed a good match with the experimental data, and the estimated kinetic parameters agreed well with those reported in the literature. The morphology of the support affected the initial reaction rate and catalyst deactivation. The methane cracking activation energy was estimated to be 88 and 75 kJ/mol for the porous and nonporous catalysts, respectively. The activation energy for the encapsulating carbon formation was estimated to be 147 and 149 kJ/mol for the porous and nonporous catalysts, respectively. The deac...

Kinetic studies on vapour phase pyridine synthesis and catalyst regeneration studies

AbstractThe kinetics of vapour phase aminocylisation of acetaldehyde, formaldehyde, and ammonia over deactivating HZSM‐5 catalyst is studied. A series‐parallel reaction scheme has been employed in which coke formation from its precursors has been lumped. Applying the concept of separable kinetics, the catalyst deactivation and reaction kinetics unconstrained and constrained by it are studied independently. Employing experimental time on stream studies, the rate of catalyst decay has been computed from the latter. A catalyst deactivation function based on experimental decay of its activity has been evaluated separately. Its magnitude and trajectory matched well with that of the computed catalyst decay function. It facilitated to obtain a direct relationship between coke formation and product distribution as a function of time on stream for aminocyclisation. An attempt has also been made to study the regeneration characteristics of the catalyst.

Pyridine hydrogenation and piperidine hydrogenolysis on a commercial hydrocracking catalyst I. Reaction and deactivation kinetics

The vapor phase kinetics of pyridine hydrogenation over a commercial fresh hydrocracking catalyst were studied in a continuous-flow fixed-bed reactor and the feasibility of this reaction as a probe for characterizing the catalyst was examined. Kinetic experiments at total pressures of 13.01 to 13.48 atm, temperatures of 312 to 334°C, and initial pyridine partial pressures of 0.116 to 0.483 atm indicated that pyridine hydrogenation to piperidine was the predominant reaction and that the reaction rate was first order in pyridine. The feasibility of piperidine hydrogenolysis as a probe for characterizing dual-functional catalysts was also examined and the reaction and deactivation kinetics of this system were studied in the above reactor operating at integral conversions. The kinetic studies were conducted at total pressures of 15.86 to 16.14 atm, temperatures ranging from 281 to 321°C, and initial concentrations of piperidine from 4.03 to 11.84 × 10 −3 mol/liter. Product distributions revealed that the predominant reactions were only those converting piperidine to other nitrogen-containing compounds. Both the metallic and acidic catalyst functions were active simultaneously in the conversion reactions and both were deactivated under conditions of the experiments. To offset catalyst deactivation effects, the conversion data were extrapolated to zero time on stream. The kinetic parameters were determined using a reaction-deactivation model based on separable kinetics. The reaction rate data were best fit to Langmuir-Hinshelwood type expressions proposing two different catalytic sites for hydrogen and nitrogen compound adsorption.

Kinetic studies on catalytic methanation by means of a concentration‐controlled recycle reactor

AbstractThe rate behaviour of multi‐step reaction systems is difficult to model. Concentration‐controlled investigations in a recycle reactor provide new tools for this purpose, e.g. the possibility to decompose a network into single reaction steps. Methanation of carbon monoxide over a supported Ni catalyst could be shown to be accompanied to a considerable extent by the shift reaction, and to a lesser extent by direct hydrogenation of carbon dioxide. Deactivation of the catalyst was measured at definite gas compositions. The two main steps show different responses to deactivation: Separable kinetics was found only for the methanation step. Power law rate equations were fitted to the rate data of the individual reaction steps to represent the main features of their kinetic behaviour. Some aspects of deactivation kinetics are also discussed.

Experimental study on the separability of reaction‐deactivation kinetics: Thermal desorption of alcohols from fresh and Na‐poisoned γ–Al2O3

AbstractTemperature programmed desorption (TPD) of methanol and ethanol from fresh and Na‐poisoned γ–Al2O3 has been employed to investigate experimentally the separability of reaction‐deactivation kinetics. The analysis of the TPD spectra has provided both an energetic characterization of the nonhomogeneous catalytic surface and the kinetic parameters of the desorption reaction, and has allowed quantitative discussion of the separability of the expression for the rate of desorption. The results obtained have been related to the chemical causes of nonseparability of reaction‐deactivation kinetics; they also demonstrate the influence of catalyst decay on selectivity. For the systems considered, it appears that the assumption of separable deactivation kinetics is generally not a satisfactory approximation.

Simulation of isothermal catalytic fixed-bed reactors operated in successive reaction-regeneration cycles

The reaction time and catalyst initial activity that maximize the apparent production rate of isothermal fixed-bed reactors operated in reaction-regeneration cycles have been computed. The calculated results have been compared with those from experiments on reaction and regeneration carried out for a system where kinetic equations for the main reaction, deactivation and regeneration, and the activity-coke relationship are known. The proposed calculation procedure has been employed for several reaction systems with separable kinetics. The effects of the reaction orders in the main and deactivation reactions and of the kinetic constants of both reactions on the optimum operating conditions have been analysed. The effects of space time and dead time between two cycles have also been studied.

Kinetics of deactivation of Re2O7‐Al2O3 metathesis catalyst

AbstractThe deactivation of 9% Re2O7‐Al2O3 catalyst during the metathesis of propene is investigated. Based on integral kinetic measurements and assuming separable deactivation kinetics, a deactivation rate equation has been developed which fits the experimental results well.

Optimization of reactors with catalyst decay and the constant conversion policy

For continuous stirred-tank and plug-flow catalytic reactors in which the catalyst activity decreases with time, Szepe [1] showed that the optimal temperature-time policy can lead to a policy of constant exit conversion under specific conditions. These conditions included a single irreversible reaction with a separable rate equation and a rate of decay of catalyst activity also separable and independent of composition. It is the purpose of this paper to extend the results of Szepe and to establish: (a) A necessary and sufficient condition for a constant-conversion optimal policy in a continuous stirred-tank reactor, and (b) A proof of the constant-conversion policy for a plug-flow reactor with distributed control of temperature for a general irreversible reaction with separable kinetics. In both cases, the rate of decay is assumed to depend on composition.