Isothermal Reaction Conditions Research Articles

Studies on hydriding kinetics of some La-based metal hydride alloys

In this paper, the hydriding kinetics of LaNi5, LaNi4.7Al0.3 and LmNi4.91Sn0.15 is presented. Experiments were carried out by maintaining the pressure ratio (supply pressure to equilibrium pressure at the mid-point of the pressure–concentration–isotherm) equal to 2 and by maintaining nearly isothermal reaction conditions. Two widely used reaction kinetics models, namely Johnson–Mehl–Avrami (JMA) model and Jander diffusion model (JDM) are considered for the analysis. Two JMA models are considered; in the first model, the order of the reaction is assumed as unit and in the second model, the rate constant is calculated by estimating the order by fitting the reaction kinetics data with a reaction kinetics equation. The activation energy and pre-exponential constants of the above-mentioned alloys are estimated by constructing the Arrhenius plot. Activation energies estimated from the different models are compared and the accurate values of activation energy for the different alloys are determined by comparing the reaction kinetics data obtained from the models with the experimental data. The rate-controlling step of the hydriding reaction is obtained for all the alloys investigated.

Development of a microstructured reactor for heterogeneously catalyzed gas phase reactions: Part II. Reactor characterization and kinetic investigations

Abstract A new type of microstructured reactor was characterized and tested in the oxidative dehydrogenation of propane (ODP) with respect to reaction engineering aspects. Residence time behavior was measured using O 2 /N 2 step injection experiments and theoretically analyzed by applying the axial dispersion model, resulting in Bodenstein numbers in the range of 70. Catalytic performance in the ODP was predicted on the basis of a kinetic model from the literature, showing good agreement for low degrees of propane conversion. In order to elucidate discrepancies between experimental and forecasted model data at higher degrees of propane conversion (i.e., >12.5%), possible sources of error were systematically investigated. Specifically, heat and mass transport limitations were excluded as well as possible inaccuracies of the applied kinetic model were examined. It could be shown that microstructured reactors are well suited to be applied for strongly exothermic heterogeneously catalyzed gas phase reactions since they allow isothermal reaction conditions over a wide range of concentrations and temperatures.

The influence of ultrasound on the RuI3-catalyzed oxidation of phenol: Catalyst study and experimental design

The influence of ultrasound at 24kHz on the heterogeneous aqueous oxidation of phenol over RuI3 with hydrogen peroxide (H2O2) was studied isothermally at 298K. Effect of ultrasound irradiation on catalytic properties and performance of RuI3 has been studied in details by means of scanning electron microscopy (SEM), X-ray powder diffraction (XRD), dispersion analyzer and a surface analyzer. Turn over frequency of the catalyst was also calculated. In this work, experimental design methodology was applied to optimize the degradation of phenol in aqueous solution, while minimizing an excessive consumption of chemical reagents. The independent variables considered were the catalyst load and oxidant concentration. The multivariate experimental design allowed the development of empiric non-linear quadratic models for total organic carbon (TOC) removal after 120 and 240min of the reaction, and the time needed for total hydrogen peroxide consumption, which were adequate to predict responses in all of the range of experimental conditions used. Ruthenium leaching was not detected from samples studied at different stages of the reaction, indicating stability of the chosen catalyst. A reaction scheme involving radical species (OH, HO2) was proposed to explain phenol conversion. Ultrasound-assisted catalytic oxidation demonstrated nearly two-fold increase in phenol conversion (up to 70%), contrary to 31% obtained during silent process. High catalytic activity of RuI3 associated with isothermal reaction conditions at circum neutral pH was capable to extend the applicability of such catalyst in ultrasound-assisted oxidation processes.

Interactions between reaction kinetics in ATR-reactors and transport mechanisms in functional ceramic membranes: A simulation approach

In this investigation the interaction between the mass transport in separation membranes and the autothermal steam-reforming in a membrane reactor was studied with a special focus on the consequences for designing smart materials that influence the reactor performance. A steady-state 1D-model was used to determine optimal thicknesses of Pd/Ag-layers for separating hydrogen gas from the reactor. The results indicate that a 400 nm layer is optimal for reactor temperatures ranging from 450 to 600 °C. To attain ideal isothermal reaction conditions, a spatial dynamic model containing an oxygen feeding controller was employed. It showed that depending proportionality of temperature with a small differential part, a controller is best suited for minimize deviations from the isothermal behaviour. In this work, an oxygen flux characteristic of the controller was used to simulate a functionalized membrane with the same characteristic. The obtained results are almost as good as in the ideal case with values up to 99% reaction yield at 600 °C in a 100 mm tube reactor.

Estimation of kinetic triplet of cellulose pyrolysis reaction from isothermal kinetic results

This paper presents a new approach to estimate the Arrhenius parameters as well as the reaction model function of cellulose pyrolysis reaction. Reduced time plot (RTP) was employed to choose a proper form of reaction model function for cellulose pyrolysis reaction. A state-of-the-art thermobalance (TB) that is able to form real isothermal reaction conditions was introduced to construct RTPs from isothermal decomposition kinetic data. The reaction model function of cellulose pyrolysis reaction would be accounted for by Avrami-Erofeev function, n(1-α){-ln(1-a)}1-1/n where n is determined to be 3.69.

DNA Polymerase-Catalyzed Elongation of Repetitive Hexanucleotide Sequences: Application to Creation of Repetitive DNA Libraries

We demonstrate the elongation of various hexanucleotide sequences with thermophilic DNA polymerase, under isothermal or thermal cyclic reaction conditions. We prepared 10 types of double repeat hexanucleotide duplexes with various GC compositions containing between 0 and 6 GC nucleotides per repeat and incubated these duplexes with thermophilic Taq DNA polymerase and dNTPs at various temperatures. All of the model repetitive short duplexes were elongated under the isothermal incubation conditions, although there were some differences in the elongation efficiencies derived from the GC composition in the repetitive sequences. It was also found that all of the model repetitive duplexes were extended more effectively by a 3-step thermal cyclic reaction involving denaturation, annealing, and extension. On the basis of this technique, we prepared a glutamate-encoding short repetitive duplex and created long repetitive DNAs under isothermal and thermal cyclic reaction conditions. DNA sequencing analysis of the cloned repetitive DNA revealed that well-ordered long repetitive DNAs of various chain lengths were created by this DNA polymerase-catalyzed ligation method, and these were easily cloned into vectors by the TA-cloning method. This method could be useful for obtaining DNAs encoding arbitrary long repetitive amino acid sequences more effectively than the conventional T4 ligase-catalyzed ligation method.

Simulation of acid-catalysed organosolv fractionation of wheat straw

Organosolv fractionation allows the separation of lignocellulosic biomass in its three major components; sugars from hemicelluloses, cellulosic fibres (pulp), and solid, low molecular weight, lignin. The kinetics of fractionation of wheat straw in aqueous ethanol was studied and a new generalised kinetic model was proposed to simulate the experimental results. This model was applied to correlate the kinetic results for non-isothermal experimental conditions, incorporating the effects of reaction time, temperature and acid concentration. Using the proposed model, optimal fractionation time, in terms of the quality of the resulting cellulose fraction, can be calculated for the conditions of a given experiment. As an example, for isothermal reaction conditions at 140 °C and catalyst concentration equal to 0.2 N H 2SO 4, minimum lignin content (2% w/w on the resulting pulp) would be obtained for 52 min reaction time. At these conditions, pulp yield 42% (w/w on dry straw), maximum solubilized lignin 96% (w/w on original lignin), and soluble total sugars 27% (w/w on the original straw polysaccharides) would be obtained.

Reaction kinetics modeling and thermal properties of epoxy–amines as measured by modulated‐temperature DSC. I. Linear step‐growth polymerization of DGEBA + aniline

AbstractA mechanistic approach including both reactive and nonreactive complexes can successfully simulate both nonreversing (NR) heat flow and heat capacity (Cp) signals from modulated‐temperature DSC in isothermal and nonisothermal reaction conditions for different mixtures of diglycidyl ether of bisphenol A + aniline. The reaction of the primary amine with an epoxy–amine complex initiates cure (E1A1 = 80 kJ mol−1), whereas the reactions of the primary amine (E1OH = 48 kJ mol−1) and secondary amine (E2OH = 48 kJ mol−1) with an epoxy–hydroxyl complex are rate determining from about 2% epoxy conversion on. The reliability of the proposed mechanistic model was verified by experimental concentration profiles from Raman spectroscopy. When cure temperatures are chosen inside or below the full cure glass‐transition region, vitrification takes place partially or completely, respectively, as can be concluded from the magnitude of the stepwise decrease in Cp. The effect of the epoxy conversion (x) and mixture composition on thermal properties such as the glass‐transition temperature (Tg), the change in heat capacity at Tg [ΔCp(Tg)], and the width of the glass transition region (ΔTg) are considered. The Couchman relationship, in which only Tg and ΔCp(Tg) of both the unreacted and the fully reacted systems are needed, was evaluated to predict the Tg– x relation by using simulated concentration profiles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91:2798–2813, 2004

Catalytic Wall Reactor as a Tool for Isothermal Investigations in the Heterogeneously Catalyzed Oxidation of Propene to Acrolein

The vapor phase oxidation of propene to acrolein is a highly exothermic reaction. To ensure isothermal reaction conditions, a catalytic wall reactor was used for detailed investigations on the reaction behavior on a multicomponent bismuth−molybdate oxide catalyst. The reaction temperature showed only small influence on the selectivity to acrolein, but a significant optimum at about 360 °C is observed. The results further indicate a change in the rate-determining step: while at low temperatures (<360 °C) catalyst reoxidation is rate determining, with increasing oxygen content accelerating the formation of acrolein considerably, its influence disappears at higher temperatures. Not only does water increase the selectivity to acrolein, but also, at low temperatures, it improves catalyst reoxidation remarkably. Additionally, the formation of the most important side products (acrylic acid, carbon oxides, acetaldehyde, formaldehyde, and acetic acid) was observed, depending on the reaction parameters.

Modeling of particle segregation phenomena in a gas phase fluidized bed olefin polymerization reactor

In a gas phase fluidized bed olefin polymerization reactor, it is generally assumed that polymer particles are well mixed and near isothermal reaction conditions prevail. When a highly active Ziegler–Natta catalyst or other type of high activity supported catalyst is used in a gas phase fluidized bed reactor, a certain heterogeneity in the polymer properties is often observed in different size particles. Moreover, particle agglomeration and sheeting phenomena may also occur due to irregular particle growth, internal particle segregation or nonisothermal effect. In many of the past reports, particle residence time distribution has been commonly used as a tool to calculate polymer particle size distribution in a fluidized bed polyolefin reactor. In this paper, a multi-compartment population balance model is presented to directly model the particle segregation phenomena and particle size distribution in a gas phase olefin polymerization reactor. To model the particle segregation effects, size dependent particle transfer constants are employed. The effects of various fluidized bed operating conditions on the particle size distribution are investigated through model simulations.

Mechanism of N 2 formation during coal char oxidation

In spite of many studies, the reaction mechanism of the evolution of nitrogen-containing gases during coal oxidation is not fully understood yet. One reason is that the nitrogen mass balance has not been well established in most studies. In the present study, we attempted to clarify the N 2 formation mechanism during the reaction between coal char and O 2 by paying special attention to nitrogen mass balance. The closure of the nitrogen mass balance was 100±6%. Blair Athol coal char was used as a sample, and the formation of N-containing species was determined during temperature-programmed reaction as well as isothermal reactions at 700 and 850°C in a packed-bed reactor. In all the cases, N 2 was the major product, with a little amount of NO as a minor product. Under the isothermal reaction conditions, the ratio of NO/N 2 was low in the initial stage and increased in the later stage. The effect of bed height on the product gas distribution was examined, and it was found that the shallower the bed, the higher the ratio of NO/N 2 . When the O 2 /He gas mixture was switched to He, the formation of N 2 and NO became almost negligible. From these observations, we proposed the following reaction mechanism for the N 2 formation: the formation of NO as a primary oxidation product, and the reaction of NO with surface nitrogen species to form N 2 . The NO capture on carbon surface to form surface nitrogen species is thought to be an important step during the course of this reaction.

Kinetic investigations of the deactivation by coking of a noble metal catalyst in the catalytic hydrogenation of nitrobenzene using a catalytic wall reactor

The vapour phase hydrogenation of nitrobenzene to aniline is a highly exothermic reaction deactivated by coking of the palladium catalyst supported on α-alumina carrier. For studying the deactivation of the catalyst a catalytic wall reactor was used in order to ensure isothermal reaction conditions for the kinetic measurements. Furthermore, the catalytic wall reactor allowed the determination of axial coke profiles by total carbon analysis of different wall segments. On the assumption that the main reaction and the deactivation of the catalyst can be assumed separable both the steady state and the unsteady state kinetics were studied. Nitrobenzene was identified as the relevant coke precursor whereas aniline has neither an influence on the main reaction nor on the deactivation. It could be shown that the hydrogenation of nitrobenzene to aniline follows a Langmuir–Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step. The differentiation of coke on the active sites and coke on the support must be taken into account to model the kinetics of coke formation with sufficient accuracy.

Characterization of Catalytic Surfaces by Isotopic-Transient Kinetics during Steady-State Reaction

Steady-state isotopic-transient kinetic analysis (SSITKA) is a very useful technique for the kinetic study of heterogeneous catalytic reactions. The technique is based upon the detection of isotopic labels in the reactor effluent species versus time following a switch (step change) in the isotopic labeling of one of the reactant species in the reactor feed. In addition to maintaining isothermal and isobaric reaction conditions, the reactant and product concentrations and flow rates remain undisturbed during the step change. Thus, in the absence of isotopic mass effects, steady-state reaction conditions are maintained under isotopic-transient operation. The reaction intermediates present on the catalyst surface do not change, and unlike for other transient techniques, analysis of the steady-state kinetic behavior of the catalyst surface is possible. An isotopic-step input is typically used so that the transient behavior of the less-active catalytic sites is included in the kinetic determinations, information which may be lost when using an isotopic-pulse technique. Steady-state kinetic information which has been obtained from SSITKA includes concentrations of different types of adsorbed reaction intermediates, coverages, surface lifetimes, site heterogeneity, activity distributions, and identification of possible mechanisms. This overview of SSITKA includes discussions of the technique of steady-state isotopic-transient labeling for kinetic study,more » the mathematical formalisms used in transient analysis, the kinetic parameters which can be obtained, the experimental considerations of the technique, and the reactions to which SSITKA has been applied. 98 refs.« less

Reactivity of steam in exhaust gas catalysis I. Steam and oxygen/steam conversions of carbon monoxide and of propane over PtRh catalysts

Pt (1 wt.-%), Rh (0.2 wt.-%) and Pt (1%) Rh (0.2%) catalysts supported on A 2O 3 (A) and on 12 wt.-% CeO 2/Al 2O 3 (CeA) were prepared and characterized by the chemisorption and the titration of probe molecules. Their catalytic behavior in steam and oxygen/steam conversions of carbon monoxide and of propane was investigated. The reactions were carried out in oxygen-deficient medium, under isothermal or temperature-programmed reaction (TPR) conditions. Light-off temperatures as well as turnover frequency ( N t) deduced from the low-conversion branch of the TPR curves were used to evaluate the catalytic activities. In oxygen/steam conversion of carbon monoxide, the relative N t values were: PtCeA, 2500 > PtA, 240 > RhCeA, 75 > RhA, 1 for carbon monoxide oxidation at 100°C and PtCeA, 43500 > RhCeA, 100 > PtA, 75 > RhA, 1 for the water-gas shift (WGS) at 200°C. This reaction, on Al 2O 3 and to a lesser extent on CeO 2/Al 2O 3, was inhibited by oxygen, WGS activities being one to three orders of magnitude higher in the CO+H 2O mixture. In oxygen/steam conversion of propane, the relative N t values were: PtA, 76 > PtRhA, 5.4 > PtCeA, 3.6 > RhCeA, 1.8 > RhA, 1 > PtRhCeA, 0.8 for propane oxidation at 300°C while the activity order was: PtRhCeA > PtRhA > RhCeA > PtA > RhA for the propane steam reforming by 400°C. This reaction started when oxygen was consumed and was inhibited by carbon monoxide, which explains why there was a definite cooperation effect between RhCeA intrinsically active in steam reforming and platinum active in oxidation and in WGS.

Problems in use of supported liquid‐phase catalysts in fluidized bed reactors

AbstractSince SLP catalysts can become irreversibly deactivated due to evaporation of the solvent used to prepare the contacts, a saturation technique is proposed to stabilize the catalysts' activity. Application of this technique requires isothermal reaction conditions which can be achieved directly in fluidized bed reactors. Although the fluidization properties of SLP catalysts are mainly determined by interparticle forces, it can be shown that this type of reactor is suitable for an SLP‐catalyzed gas reaction provided that a number of prerequisites are fulfilled. Taking a hydroformylation reaction catalyzed by an Rh complex dissolved in dimethyl glycol phthalate as an example, it is demonstrated that the activity of the prepared SLP catalyst can, indeed, be stabilized by saturation of the reactant gas mixture with the employed solvent.

Formation of ethene and propene from methanol on zeolite ZSM-5: I. Investigation of Rate and Selectivity in a Batch Reactor

The catalytic conversion of methanol to hydrocarbons on zeolite ZSM-5 was investigated in the range 550 to 625 K in a well mixed batch reactor system, which permits observation of product composition as a function of conversion accurately for isothermal reaction conditions. Interconversion of methanol and dimethyl ether rapidly attains equilibrium. Conversion to hydrocarbons is preceeded by an induction period. This induction period and the selectivity for formation of ethene and propene increase with the Si:Al ratio in the zeolite and decrease with crystal size. Ethene and propene appear simultaneously as gaseous products if water vapour is present.