Parallel Reaction Scheme Research Articles

Photocatalytic Oxidation of Phenol: Reaction Network, Kinetic Modeling, and Parameter Estimation

The photocatalytic degradation of phenol and other phenolic compounds can follow different pathways depending on the reaction conditions. It is found that the photocatalytic oxidation of phenol is faster in acidic pHs with an optimum pH value of 3.2. On the basis of experimental data, it is concluded that the photocatalytic oxidation of phenol, ortho-dihydroxybenzene (o-DHB), para-dihydroxybenzene (p-DHB), and 1,4-benzoquinone (1,4-BQ) can all be described with a series−parallel reaction scheme. The present study reports a detailed reaction network incorporating possible reaction steps based on data obtained for the oxidation of phenol and its three aromatic intermediates. Four carboxylic acids (fumaric acid, maleic acid, oxalic acid, and formic acid) are detected as intermediates in the photocatalytic oxidation of phenol, o-DHB, p-DHB, and 1,4-BQ, suggesting that, in the oxidation of any phenolic compounds, these acids are part of the oxidation breakdown of more complex molecules. Additionally, two kinet...

Kinetics and Mechanism of Cyclodextrin Inclusion Complexation Incorporating Bidirectional Inclusion and Formation of Orientational Isomers

The kinetics of cyclodextrin (CD) inclusion complexation has been usually analyzed in terms of a one-step reaction or a consecutive two-step reaction involving intracomplex structural transformation as a second step. These schemes presume the inclusion of guest molecules through only one side of the CD cavity and the formation of unidirectional CD complexes. However, there has been increasing experimental evidence for the inclusion of guests through both sides of the CD cavity and the formation of orientational isomers for noncentrosymmetric guest molecules. This article presents a novel parallel reaction scheme for CD inclusion complexation, incorporating bidirectional inclusion and the formation of orientational isomers into the scheme. It is shown that the parallel reaction scheme gives the same concentration versus reaction time relationship as the consecutive two-step reaction scheme. The experimental methods for determining the microscopic directional rate constants are presented. The kinetic parameters of the two-step reaction scheme are expressed as functions of the directional rate constants. The ratios of orientational isomers of alpha-CD-based [2]-pseudorotaxanes and the microscopic directional rate constants of the threading and dethreading reactions are estimated from the reported thermodynamic and kinetics data obtained by using either the one-step or two-step reaction scheme. It is shown that the thermodynamic preference of an isomer over the other is mainly due to the slow dethreading rate of the isomer.

Structural Determinants of Factor IX(a) Binding in Nitrophorin 2, a Lipocalin Inhibitor of the Intrinsic Coagulation Pathway

Nitrophorin 2 (NP2) is a salivary lipocalin from Rhodnius prolixus that binds with coagulation factors IX (fIX) and IXa (fIXa). Binding of NP2 with fIXa results in potent inhibition of the intrinsic factor Xase complex. A panel of site-directed surface mutants of NP2 was generated to locate determinants of high affinity fIX(a) binding. The locations of the mutations were based on comparisons with the related, but less potent, inhibitor nitrophorin 3 (NP3). Three point mutants (K21A, K92A, and V94A) were found that clearly reduced the inhibitory potency as measured by the activity of a reconstituted factor Xase system. Binding of NP2 with fIXa and fIX as measured by surface plasmon resonance and isothermal titration calorimetry was reduced in a similar manner. Of the three mutants, two (K92A and V94A) were located on the loop connecting beta-strands E and F of the lipocalin beta-barrel. The largest changes were seen with the K92A mutation, which lies at the apex of the loop, with a smaller effect being seen with mutation of Val(94). Combination of four E-F loop mutations (K92A, A93K, V94A, E97A) in a single mutant reduced the inhibitory potency and binding to levels similar to those seen with NP3 without affecting heme or histamine binding.

Friedel–Crafts alkylation properties of aluminosilica SBA-15 meso/macroporous monoliths and mesoporous powders

The catalytic activities of SBA-15 aluminosilica meso/macroporous monoliths (Si/Al=72) and mesoporous powders (Si/Al=70) have been investigated using batch Friedel–Crafts alkylation of single-ring aromatic compounds, including toluene, ethylbenzene, cumene, and styrene, with benzyl alcohol. The toluene alkylation activities of the meso/macroporous monolith catalysts were compared with nanoporous zeolite Beta (Si/Al=75, average particle size ∼10 μm) and mesoporous SBA-15 aluminosilica (average particle size ∼25 μm), as powders and pellets (0.5 cm thick × 1 cm diameter) to study the effects of framework crystallinity, particle size, pore size, and pore structure on the overall conversion. Apparent reaction rate coefficients were quantified based on a model for the alkylation of toluene with benzyl alcohol via two parallel reaction schemes to benzyl toluene. Moderate toluene alkylation rates (∼10 −4 s −1) and high selectivities (>90%) for benzyl toluene were observed for meso/macroporous aluminosilica SBA-15 monoliths, compared to the mesoporous SBA-15 and zeolite Beta powders, which displayed faster alkylation rates (∼10 −3 s −1), though were less selective (79 and 59%, respectively). In pellet form, both the mesoporous SBA-15 and the zeolite Beta materials yielded lower rates of alkylation (∼10 −4 s −1), due to slower internal diffusion of reactants and products within the smaller pellet macropores, compared to the meso/macroporous monoliths. Diffusive resistances to mass transfer were quantified by estimating Thiele moduli and effectiveness factors for the different catalysts, based on benzyl alcohol diffusivities measured by PFG NMR. The aluminosilica meso/macroporous SBA-15 monoliths deactivated more slowly, retaining 92% of their original activities after one use, compared to 75, 76, and 0.6% for SBA-15 powder, SBA-15 pellet, and zeolite Beta powder catalysts, respectively.

Fundamental insights into the oxidative dehydrogenation of ethane to ethylene over catalytic materials discovered by an evolutionary approach

An evolutionary optimisation procedure combined with high-throughput synthesis and testing has been applied for discovering new catalytic materials for the oxidative dehydrogenation of ethane to ethylene (ODHE). After 10 generations comprising a total of 600 vanadium-free catalytic compositions supported on α-Al 2O 3, Cr/Co/Sn/W and Cr/Mo mixed oxides have been identified as new well performing catalytic materials for the ODHE reaction; ethylene yields of up to 20% and selectivities of 55–65% (500 °C; C 2H 6:O 2:Ar=20:10:70) were achieved. The surface and bulk characteristics of the new catalytic materials derived from near-surface composition, binding energies of the elements (XPS), and phase analysis (XRD) as well as from their surface area (BET) lead to the assumption that for both systems (Cr/Mo or Co/Cr/Sn/W oxide) the active phase is located in the X-ray amorphous part of the material. Based on transient and isotopic experiments it was concluded that the ODHE reaction follows the Mars–Van Krevelen mechanism. The different selectivities towards CO 2 and CO between the two systems have been attributed to parallel and consecutive reaction schemes of the product formation, derived from transient experiments. From the fundamental insights obtained improvements of the catalytic performance in the ODHE reaction are suggested.

Treatment of industrial oily wastewaters by wet oxidation

In the present work, the homogeneous wet oxidation (WO) of an oily wastewater (COD≈11,000 mg l −1), composed mainly of alcohols and phenolic compounds, was studied in a high-pressure agitated autoclave reactor in the temperature range of 180–260 °C and oxygen pressure 1 MPa. Temperature was found to have a significant impact on the oxidation of the contaminants in the wastewater. Among the compounds contained in the wastewater, ethylene glycol showed great resistance to wet oxidation. Temperatures above 240 °C were required for its effective degradation. Organic acids, mainly acetic acid, were the intermediate products of the wet oxidation process and their conversion to carbon dioxide was very slow. A generalised model based on a parallel reaction scheme was used to interpret the experimental data obtained. The activation energies obtained were in the range of 90–130 kJ mol −1.

The hydrogen economy, the creation of the worldwide energy web and the redistribution of power on earth

The study of hydrogen fuel cell vehicles is currently one of the popular research topics. However, a vital issue for hydrogen fuel cell vehicles is the hydrogen tank safety when hydrogen tank catch fires. In this paper, the carbon fibre-epoxy composite from hydrogen tank was investigated by thermogravimetric analysis. Thermogravimetric experiments were carried out at three heating rates (20, 30 and 40 K/min) in a nitrogen atmosphere. A two-step parallel reaction scheme was developed for the decomposition of the carbon-fibre epoxy composite based on the K-K method and previous studies. The kinetic reaction scheme was coupled to Shuffled Complex Evolution (SCE) method to obtain optimized kinetic parameters. The predicted values using the optimized kinetic parameters show a good agreement with the experimental data, which shows the potential promising application of the reaction scheme and optimized kinetic parameters of the carbon-fibre epoxy composite.

Yield Effects in Single and Coupled Nonlinear Thermokinetic Reaction Systems

The influence of nonlinear dynamic effects on yield is considered for consecutive-parallel reaction schemes in a single phase CSTR. In the first part, the behavior of a single uncoupled reactor is investigated. Emphasis is on the reaction dynamics and the yield of the intermediate ethanal using continuation and optimization techniques. We characterize regions of periodic, complex periodic, and chaotic oscillations. The chaotic region is reached by type III intermittency. In all cases, the global optimum yield of the intermediate is a steady state, which is unstable or stable due to the location of the stability boundary. It is found that autonomous periodic operation is only locally better than steady state operation. In the second part, different types of mass and energy coupling between two reactors are studied. The results obtained for a simple consecutive parallel reaction scheme are validated by a more detailed model of the ethanol oxidation by hydrogen peroxide under iron(III) catalysis, which can be described by an extended consecutive-parallel reaction scheme. Analogies are found with respect to the dynamics and the yield of the intermediate.

Simulation of structured catalytic reactors with enhanced thermal conductivity for selective oxidation reactions

The replacement of conventional-packed beds of pellets with “high conductivity” honeycomb catalysts in industrial externally cooled multitubular fixed-bed reactors is investigated by modeling and simulation for the oxidation of methanol to formaldehyde and for the epoxidation of ethylene to ethylene oxide, which involve a consecutive and a parallel reaction scheme, respectively. Results suggest that near-isothermal operation of the fixed-bed reactors can be achieved using monolithic catalyst supports based on relatively large volume fractions of highly conductive materials. Pressure drops are reduced to less than 1%. The selectivity is favored by the excellent control of the intraporous diffusional resistances resulting from the thin catalytic washcoats. Reactor designs based on larger tubes are feasible at the expense of greater volume fractions of catalyst support. A critical aspect is represented by the restrictions on the specific load of catalyst per reactor volume resulting from the poor adhesion of very thick catalyst layers onto metallic surfaces. Such a difficulty can be circumvented by maximizing the geometric surface area of the monolith (e.g. minimizing the honeycomb pitch), enhancing the catalytic activity (e.g. increasing the load of active components), and increasing the coolant temperature (if the selectivity is not adversely affected).

Kinetics of the low-temperature pyrolysis of chromated copper arsenate-treated wood

In this paper, a kinetic evaluation of non-isothermal thermogravimetric (TG) data is carried out by the method of least squares for both untreated and chromated copper arsenate (CCA)-treated wood. The type of wood used is Pinus sylvestris and the slow-heating decomposition in an inert (nitrogen) atmosphere (low-temperature pyrolysis) is studied. In the case of untreated wood, best fit parameters are determined for the three independent parallel reactions scheme, proposed by several other researchers. For CCA treated wood a comparative study is performed, in which five different kinetic models are considered. These are based on the assumption of: (1) a single reaction; (2) independent parallel reactions; (3) competitive reactions; (4) a chain of successive reactions and (5) a combination of two independent parallel reactions and one subsequent reaction. The choice between which of these models offer the best fit, is based partly on chemical considerations and partly on the quality of the fit. Altogether, the DTG curve of CCA treated wood is well described by assuming two independent parallel reactions at low temperature, which may be catalyzed by the metals, followed by the successive decomposition of the residue. A comparison is made with the kinetic parameters derived for untreated wood, resulting in the following observations and conclusions drawn therefrom: hemicellulose decomposition is catalyzed by the metals (lower activation energy, pre-exponential factor and peak temperature and higher contribution to volatiles production); cellulose decomposition starts once the major part of chromium(III) arsenate is decomposed and volatilised (higher activation energy) and may then even be catalyzed by the metals (higher pre-exponential factor) and lignin decomposes very slowly at higher temperatures together with intermediate products from hemicellulose and cellulose decomposition (low pre-exponential factor and activation energy). During this slow charring process the metals act no more as catalysts. This hypothesis is confirmed by a TG analysis, performed on a chromium(III) arsenate precipitate, which is the major arsenic compound present in CCA treated wood, and by a study concerning the microdistribution of copper, chromium and arsenic in the pyrolysis residue of CCA treated wood.

Oxidation of aqueous Cr(III) at birnessite surfaces: constraints on reaction mechanism

X-ray Photoelectron Spectroscopy (XPS) was used to investigate oxidation of aqueous Cr(III) at the surface of 7 Å-birnessite [MnO1.75(OH)0.25]. Special emphasis was placed on detection of intermediate oxidation states of chromium due to their critical environmental significance. No previous studies have been able to identify these intermediate oxidation states of chromium (namely, Cr[IV] and Cr[V]) on mineral surfaces or in natural solutions. Mn(2p3/2), Cr(2p3/2) and O(1s) spectra of the reacted surfaces reveal that Mn(IV) of synthetic birnessite undergoes reductive dissolution in two steps. The first step involves Mn(IV) reduction to Mn(III),that forms at the oxide surface probably as an oxyhydroxide (MnOOH), and in the second step Mn(III) is reduced to Mn(II) that is subsequently taken into solution. Each reductive reaction step involves transfer of only one electron to the Mn ion. After Cr(III)aq is adsorbed onto the MnO2 surface, it undergoes oxidation in three separate steps, each involving the loss of one electron to Mn ions, so that Cr(IV), Cr(V) and Cr(VI) are produced. The intermediate reaction products, namely Mn(III), and Cr(V) were positively identified by XPS spectral analyses. Similarity in XPS binding energy values of Cr(III) and Cr(IV) as well as that of Cr(V) and Cr(VI), however, preclude separate identification of Cr(III) from Cr(IV) and Cr(VI) from Cr(V) multiplets on the near-surface of the solid. A parallel reaction scheme (exclusive of sorption reactions) best describes the birnessite-Cr(III)aq redox reactions. The two parallel reactions proceed by separate mechanisms with a monodentate complex formed in one mechanism and a bidentate complex in another. The bulk of Cr(IV) probably is formed via the monodentate complex and Cr(V) via the bidentate complex. The rate expressions associated with these reactions display near-perfect correlation with changing surface abundances of Cr(IV) and Cr(V) as a function of reaction time. Copyright © 1999 Elsevier Science Ltd.

Kinetic modeling of biomass pyrolysis

The thermal decomposition of lignocellulosic biomass materials and their major components is discussed. Thermogravimetric and DSC curves at different T( t) heating programs were evaluated by the method of least squares. Pseudo-first order models, parallel, successive and competitive reaction schemes and complex reaction networks were employed in the modeling. The following topics are treated: thermal decomposition of cellulose at low (2 °C min −1) and high (50–80 °C min −1) heating rates; low temperature phenomena; the validity of the Broido-Shafizadeh model; effects of mineral catalysts; cellulose pyrolysis in closed sample holders; thermal decomposition kinetics of xylan, lignin and lignocellulosic plant samples.

First‐order closure theories for series‐parallel reaction in simulated homogeneous turbulence

AbstractSimple first‐order closure models for covariances of concentration fluctuations, for use in modeling turbulent flow reactors, were tested by direct numerical simulations. Concentration covariances and other statistical functions were evaluated for a series parallel reaction scheme in decaying, homogeneous turbulent flow. The simulations involve solving the unsteady Navier‐Stokes and mass conservation equations by a pseudo‐spectral method in a 643 wavenumber domain, with initially segregated reactants, for an initial turbulence Reynolds number of 29.9. Simulation results show that covariances of concentration fluctuations normalized with respect to mean concentration values are almost constant and that the time dependence of concentration covariances can be estimated if the mean concentrations are known at any one time after the initial time. Predictions of the first‐order closure models of Bourne and Toor, Brodkey and Lewalle, Li and Toor, and Dutta and Tarbell were compared to simulation results. While none of these closures are satisfactory for all the conditions tested, the Brodkey‐Lewalle closure agrees best with the simulations.

Kinetic model development for single-stage coal coprocessing with petroleum waste

Coal coprocessing with waste petroleum materials achieves the dual purpose of upgrading both the coal and the waste materials economically. The kinetics of coal coprocessing with petroleum waste has been studied experimentally and modeled mathematically. A combined parallel and series reaction scheme was assumed for the coal liquefaction. Experiments were carried out in a tubing bomb microreactor at temperatures of 375 to 425°C, at reaction times of 15–120 min, with 10% coal loading and with 1250 psig pressure. Coal conversions as high as 98% were obtained during coprocessing. More than 70% conversion to oil is achieved at high temperatures and grease loadings. The effect of temperature and grease loading on coal liquefaction were also studied. A rigorous parameter estimation technique was employed to determine the parameters involved in the model. The model predictions are good for experimental conditions other than those used to determine the model parameters. Higher conversion and higher rate of liquefaction obtained during coprocessing were explained from the obtained kinetic rate parameter values.

Two-Stage Kinetic Model of Primary Coal Liquefaction

A mathematical model enabling the calculation of reaction rate parameters for coal weight loss during liquefaction has been formulated. The model explicitly accounts for (i) product release during heatup as well as during holding at peak experimental temperature and (ii) stages of the process prior to and following the onset of extensive covalent bond scission (around 350 °C); the procedure thus allows the calculation of distinct activation energies for the two stages. In the initial phase of the calculation, both stage A processes (prior to the onset of extensive bond cleavage) and stage B processes (following the onset of extensive bond cleavage) have been modeled as single, irreversible, first-order processes. In the fully developed model, both stage A and stage B were considered to proceed by means of multiple parallel, independent, irreversible, first-order reactions with a Gaussian distribution of activation energies. Calculations were matched with liquefaction experiments in tetralin, carried out in a flowing-solvent reactor using seven Argonne PCSP coals and Point of Ayr (U.K.) coal. For stage A processes, the single-reaction model gave activation energies between 35 and 80 kJ mol-1. Energies of activation for the higher temperature range stage B processes (about 350−450 °C) were found to be significantly higher, between 124 and 238 kJ mol-1. This result confirms the validity and necessity of describing coal liquefaction in terms of a two-stage process. For stage A, the multiple parallel independent reaction model gave mean activation energies that were only slightly larger than those calculated from the single-reaction model. Activation energies for stage B processes, however, were greater, between 160 and 275 kJ mol-1, indicating that the single-reaction model significantly underestimates energies of activation for coal thermal breakdown. For individual coals, the standard deviation of the distribution of activation energies progressively decreased with increasing coal rank, apparently reflecting increasing structural uniformity and possibly also increasing degrees of cross-linking accompanying coal maturation. Activation energies reported in the literature for the liquefaction of coal, mostly based on isothermal kinetics, range from 18 to 358 kJ mol-1. Qualitative agreement has been found between values from the present work and results from a multiple, parallel independent reaction scheme fitted to data from the pyrolysis of the Argonne PCSP coals.

Hydrotreatment process kinetics for bitumen and bitumen-derived liquids

Hydrodenitrogenation, hydrodesulfurization and resid conversion data for the Whiterocks bitumen and bitumen-derived liquid were analysed using a modified power rate law model. The data were obtained with a fixed-bed reactor in the upflow mode. A space velocity (WHSV α) term was included to account for deviations from plug flow behaviour. The upflow mode was preferred to the trickle-bed mode to ensure complete wetting of the catalyst and nearly isothermal operation. The apparent kinetic parameters were obtained by combined non-linear regression and ODE solver techniques for the analysis of laboratory data. A simple nth-order power rate law expression for hydrodenitrogenation and hydrodesulfurization was examined. The higher than first-order kinetics for hydrodenitrogenation and hydrodesulfurization of the bitumen and bitumen-derived liquids were explained by invoking two parallel first-order reactions for the facile and refractory fractions or lumps. Parallel and consecutive reaction schemes were used to examine the extent of conversion of the resid fraction to middle distillate, gas oil and naphtha. The apparent kinetic parameters were also determined.

Deamination of sec-butylamine over acidic zeolites

The conversion of sec-butylamine (BA ) has been investigated over MFI, MOR, FAU, BEA and MAZ zeolites. Faujasites were used in the H and Ce forms. The products of the reaction are 1- and 2-butenes, and di-sec-butylamines (di-sec-BA). Butene formation follows a zero order relative to the pressure of butylamine, while dibutylamine formation follows a first order. The catalytic activity for deamination to butenes is related to the total number of acid sites. An effect of the zeolite structure is observed: at similar aluminium content, MFI and MOR appear more active than FAU and BEA for butene formation. The formation of dibutylamines is observed on large pore zeolites only, and results from two different pathways, di-sec-butylamine appearing as a secondary product in the presence of non-dealuminated H-form of Y zeolites, and as a primary product in the presence of Ce 3+ exchanged and dealuminated Y zeolites. This change is interpreted in terms of a classical consecutive reaction scheme in the former case, through addition of sec-butylamine to butene intermediates catalyzed by Brønsted acids, and in terms of a parallel reaction scheme in the latter case, through condensation of two sec-butylamine molecules catalyzed by Lewis acids.

Temperature runaway in fixed bed reactors: online and offline checks for intrinsic safety

Abstract Recent trends in the online detection of thermal runaway precursors in exothermic reaction systems and its applicability to fixed bed tubular reactors are briefly discussed. While online techniques are favoured for restablization purposes, offline predictive methods are suggested for locating the parametrically sensitive zones. A computer-aided method for a priori assessment of parametric sensitivity based on the two-dimensional heterogeneous transport model for packed beds is formulated for a series—parallel reaction scheme. Its utility is demonstrated for two commercially important partial oxidation processes with different levels of reaction vigour and exothermicity.

Hydrocracking of n-heptane with a nickel monoxide-molybdenum trioxide/HY ultrastable zeolite as catalyst. The network of the reaction

The hydrocracking of n-heptane has been studied in a continuous, tubular, plug flow reactor using a 4 wt NiO-8 wt% MoO/sub 3//HYUS zeolite as catalyst, to try to obtain a network of reactions to account for the formation of the various products observed. In view of the products obtained and depending on whether they are primary or secondary, a series of simultaneous parallel reaction schemes have been proposed to explain the network of the reaction. The kinetic parameters of these reactions have been obtained from the initial selectives of the products. The values of the apparent activation energies obtained for the isomerization, hydrogenolysis, cracking, and disproportionation reactions were 99.1, 169.6, 221.4, and 195.9 kJ/mol, respectively.

Ethylene oxide oxidation over a supported silver catalyst: I. Kinetics of uninhibited oxidation

The kinetics of the complete oxidation of ethylene oxide over a supported silver catalyst in the absence of dichloroethane was studied by the flow circulation method. The temperature range 100–300°C was investigated under steady-state conditions and at atmospheric pressure. The kinetic measurements showed that ethylene oxide oxidation starts at 190°C. Below this temperature, the ethene oxidation over a silver catalyst proceeds according to the parallel reaction scheme. At 300°C and an oxygen: ethylene oxide feed ratio of 1:2 catalytic decomposition of ethylene oxide to ethene and oxygen was observed. A sequential experimental design was applied for the determination of the parameters and the simultaneous discrimination of rival kinetic models. A kinetic model is proposed that describes the rate of uninhibited ethylene oxide oxidation in the temperature range 250–300°C. The model corresponds to a mechanism in which ethylene oxide and oxygen adsorption (dissociative and non-dissociative) are fast steps at equilibrium. The rate-limiting steps are ethylene oxide oxidation to formaldehyde and further to carbon dioxide and water with the participation of both atomic and molecular adsorbed oxygen. These steps are irreversible. The calculated kinetic constants offer the possibility of evaluating tentatively the rate constants of the irreversible steps and the equilibrium constants of the fast reversible steps.