Reaction Kinetic Expressions Research Articles

Review of hydrogen ortho–para conversion: experimental data and reaction kinetics

This work reviews ortho–para hydrogen conversion data for various types of catalysts at cryogenic temperatures and evaluates different reaction kinetic expressions employed to describe these experimental data.

Analysis of the overall pressure balance loop for a methanol-to-olefins dual fluidized bed reactor-regenerator

Development of a comprehensive reactor model is of paramount importance for design and scale-up of methanol-to-olefins (MTO) process in a dual fluidized bed reactor (DFB). These models must integrate suitable reaction kinetic expressions with hydrodynamic models properly descriptive of gas-solid contact in fluidized bed reactors. In this modeling study, our previously developed kinetic models of MTO fluidized bed reactor and regenerator are coupled with overall mass, energy and pressure balances to ensure smooth circulation of catalyst particles between the two fluidized beds. This integrated model was then applied to determine geometric dimensions of a demo-scale MTO DFB configuration and to obtain the mass distribution of catalyst particles throughout the entire system including the pipes connecting the two reactors. Our model is capable of being integrated into simulation software such as Aspen Plus for plant-wide optimization and scale-up studies.

A review on mathematical modeling of packed bed membrane reactors for hydrogen production from methane

The most widely used process for hydrogen production is steam methane reforming. It can be carried out using a membrane reactor in which simultaneous hydrogen production and purification occur. Mathematical modeling of these reactors plays a key role in the selection of the design and operating parameters that yield high performance for the reactor. This review study discusses, synthesizes, and compares different mathematical modeling studies on the packed bed membrane reactors for hydrogen production from methane found in the literature. Different approaches used in these modeling studies for the hydrogen permeation steps, reaction kinetic expressions, phases involved (pseudo-homogeneous and heterogeneous), and spatial dimensions (one, two, and three dimensional) are given.

Integrated Method of Monitoring and Optimization of Steam Methane Reformer Process

Reforming of natural gas with steam represents the most energy-intensive part of ammonia production. An integrated numerical model for calculating composition of primary reforming products with cross-checking of outlet methane molar concentration, heat duty, maximum tube wall temperature, tube pressure drops, and approach to equilibrium was set up involving production parameters. In particular, the model was used for continuous monitoring and optimization of a steam methane reformer (SMR) catalyst in ammonia production. The calculations involve the solution of material and energy balance equations along with reaction kinetic expressions. Open source code based on Matlab file was used for modelling and calculation of various physical properties of the reacting gases. One of the main contributions is development of the rapid integrated method for data exchange between any distributed control system (DCS) and the model to accomplish continuous monitoring and optimization of SMR catalyst and reformer tubes. Integrated memory block was proposed for rapid synchronization between commercial DCS with the model solver. The developed model was verified with the industrial top-fired SMR unit in ammonia production located in Petrokemija, Croatia. Practical application of proposed solution can ensure overall energy savings of up to 3% in ammonia production.

Invariants for optimal operation of a reactor-separator-recycle process

Globally optimal invariants for the one unconstrained degree of freedom in an A + B → C reactor-separator-recycle process are derived for four alternative reaction kinetic expressions from an analysis of the overall plant material balance. The derivation is performed for the simplified economic objective of minimizing recycle rate (Mode I) at given production and maximizing production (Mode II) with maximum recycle rate as the capacity bottleneck. For power law kinetics, holding the reactor A/B ratio constant is obtained as the invariant while a more complex non-linear expression is obtained for Langmuir-Hinshelwood kinetics. For the more practical economic objective of minimizing separation column boilup or maximum column boilup as the capacity bottleneck, the invariant as the controlled variable (CV) achieves near optimal operation with the loss being <0.1% over the envisaged operating space in all cases. The invariants are thus good self-optimizing CVs (SOCVs). The work provides a physical basis for using reactant ratio as an SOCV in plantwide control.

Determination of coal gasification kinetics from integral drop tube furnace experiments with steam and CO2

Lab scale drop tube furnace reactors with conditions similar to large-scale gasification reactors are able to generate applicable reaction-kinetic expressions for reactor simulation and dimensioning. In the present work, a method is introduced to determine reaction kinetic parameters from integral drop tube furnace experiments. A one-dimensional model of the gasification process was developed incorporating the influence of reaction gas composition, char particle structure change, pore diffusion and gas phase reaction. Suitable kinetic parameters were determined by fitting the gasification model to experimental data. Gasification experiments with gas mixtures of steam, carbon dioxide, carbon monoxide and hydrogen were carried out in an isothermal pressurized drop tube furnace reactor at 6 bar total pressure and temperatures of 900–1100 °C. It was shown that the determined kinetic expressions fit the experimental data. The best fit to the experiments was achieved by application of a Langmuir-Hinshelwood type rate expression with respect to product gas inhibition. Integrating a pore diffusion model did not improve the fitting quality.

1-Butanol dehydration in microchannel reactor: Kinetics and reactor modeling

A structured microchannel reactor (MCR) coated with pure γ-Al2O3 was applied to investigate 1-butanol dehydration under atmospheric and isothermal conditions. Kinetic experiments were carried out and the advantages related to mass transfer properties with the application of MCR were explored under different reaction parameters and conditions. It was revealed that with the average coating thickness of 25μm, the operation was free of diffusional limitations and hence intrinsic kinetics could be determined by describing the reactor as pseudohomogeneous PFR with reaction kinetic expressions incorporated. Experimental data was regressed using power-law kinetics with a simplified reaction scheme for 1-butanol dehydration to butenes and dibutylether as well as with mechanistic model involving surface butoxies as relevant species. Both models were able to describe the experimental observations and the estimated values for kinetic parameters were in physically meaningful order of magnitude. A dynamic mathematical model was developed including diffusional mass transport of components and reaction inside coating, whereas, in free channel, plug flow mass transport was considered. The proposed model besides reproducing the experimental results was also able to predict presence of diffusional limitations when coating thickness is increased beyond 40μm. Furthermore, the simulation results show that for specific operation regimes Microchannel Reactor (MCR) outperforms packed-bed reactor, as mass transfer limitations can be appreciably reduced for 1-butanol dehydration.

Oxidative cleaning of reverse osmosis membranes during reclamation of steel wastewater

Oxidants in alkaline solutions were developed to regenerate reverse osmosis membranes that had been severely fouled during the reclamation of wastewater from steel production. The cleaning efficiency and kinetics of the processes were evaluated in parallel with the fouling formation. Analysis of primary foulant constituents using scanning electron microscope (SEM)-electron-dispersive X-ray microanalysis spectrometry and Fourier transform infrared spectroscopy (FTIR) suggested that organic fouling might be dominant. Analysis of wastewater constituents using gas chromatography–mass spectrometry indicated that aliphatic acids and long chain alkanes were likely to be the predominant foulants. The cleaning efficiency increased initially, and then decreased with increasing concentrations of the oxidant. Micro-analyzed by FTIR, SEM, and atomic force microscope, the membrane integrity was not destroyed by the oxidant under the proper cleaning conditions. Surface reaction kinetic expression based on fouling resistance decline was developed to delineate the process.

Photoreactor scale-up for degradation of aqueous poly(vinyl alcohol) using UV/H2O2 process

Although advanced oxidation technologies (AOTs) are effective methods of degradation for recalcitrant pollutants in wastewater, the photoreactor scale-up is still a major challenge in photochemical reaction engineering. In this study, a methodology for the photoreactor scale-up, including all reaction mechanisms for the binary degradation of aqueous polyvinyl alcohol (PVA) by UV/H2O2 process is proposed. In this procedure, intrinsic kinetic parameters are determined using experimental data obtained in a laboratory-scale batch recirculating photoreactor. The obtained kinetic parameters are used to predict the performance of a pilot-scale continuous flow photoreactor without any adjustable parameter. Three compulsory conditions are necessary in this approach: (1) to have a validated kinetic scheme including the binary degradation of the polymer; (2) to have a validated intrinsic reaction kinetic expression considering continuous distribution kinetics to describe the dynamics of the polymer degradation; and (3) to apply and appropriately solve a rigorous mathematical model for the pilot-scale photoreactors using the computational fluid dynamics (CFD) model development. The CFD model for the scaling-up methodology is validated experimentally in a pilot-scale photoreactor.

A robust methodology for kinetic model parameter estimation for biocatalytic reactions

Effective estimation of parameters in biocatalytic reaction kinetic expressions are very important when building process models to enable evaluation of process technology options and alternative biocatalysts. The kinetic models used to describe enzyme-catalyzed reactions generally include several parameters, which are strongly correlated with each other. State-of-the-art methodologies such as nonlinear regression (using progress curves) or graphical analysis (using initial rate data, for example, the Lineweaver-Burke plot, Hanes plot or Dixon plot) often incorporate errors in the estimates and rarely lead to globally optimized parameter values. In this article, a robust methodology to estimate parameters for biocatalytic reaction kinetic expressions is proposed. The methodology determines the parameters in a systematic manner by exploiting the best features of several of the current approaches. The parameter estimation problem is decomposed into five hierarchical steps, where the solution of each of the steps becomes the input for the subsequent step to achieve the final model with the corresponding regressed parameters. The model is further used for validating its performance and determining the correlation of the parameters. The final model with the fitted parameters is able to describe both initial rate and dynamic experiments. Application of the methodology is illustrated with a case study using the ω-transaminase catalyzed synthesis of 1-phenylethylamine from acetophenone and 2-propylamine.

Reaction kinetics of bacteria disinfection employing hydrogen peroxide

The inactivation reaction of Escherichia coli bacteria employing hydrogen peroxide at 20 °C and pH = 7 was studied in a well-mixed batch reactor. The proposed objective, as far as the extent of inactivation is concerned, was obtained for H 2O 2 concentrations above 100 ppm (1 ppm = 2.94 × 10 −5 mmol cm −3) but, compared with other disinfection technologies, for too long reaction times. Below 40 ppm of the oxidant concentration inactivation was practically ineffective. Results were analyzed employing Modified forms of the Series-Event and Multitarget mechanistic models. At concentrations above 100 ppm the induction time in the semi-logarithmic plot of bacteria concentration versus time was reduced. With both modified models it was found that the reaction order with respect to the hydrogen peroxide concentration was different than one. Both mathematical descriptions provide a good representation of the experimental results in an ample range of the disinfectant concentrations and confirm a methodology that renders the starting point of a reaction kinetic expression useful for further studies regarding the optimization of the operating conditions (pH and temperature, for example), including also combination with other advanced oxidation technologies. An interpretation of the data in terms of a Weibull-like model [1] is also included.

Catalytic oxidation of naphthalene using a Pt/Al2O3 catalyst

Polycylic aromatic hydrocarbons (PAHs) are listed as carcinogenic and mutagenic priority pollutants, belonging to the environmental endocrine disrupters. Most PAHs in the environment stem from the atmospheric deposition and diesel emission. Consequently, the elimination of PAHs in the off-gases is one of the priority and emerging challenges. Catalytic oxidation has been widely used in the destruction of organic compounds due to its high efficiency (or conversion of reactants), its economic benefits and good applicability.This study investigates the application of the catalytic oxidation using Pt/γ-Al2O3 catalysts to decompose PAHs and taking naphthalene (the simplest and least toxic PAH) as a target compound. It studies the relationships between conversion, operating parameters and relevant factors such as treatment temperatures, catalyst sizes and space velocities. Also, a related reaction kinetic expression is proposed to provide a simplified expression of the relevant kinetic parameters.The results indicate that the Pt/γ-Al2O3 catalyst used accelerates the reaction rate of the decomposition of naphthalene and decreases the reaction temperature. A high conversion (over 95%) can be achieved at a moderate reaction temperature of 480K and space velocity below 35,000h−1. Non-catalytic (thermal) oxidation achieves the same conversion at a temperature beyond 1000K. The results also indicate that Rideal–Eley mechanism and Arrhenius equation can be reasonably applied to describe the data by using the pseudo-first-order reaction kinetic equation with activation energy of 149.97kJ/mol and frequency factor equal to 3.26×1017s−1.

Design and simulation of a reactor for the chlorination of acetone in gaseous phase

The aim of this work was to design a reactor for the production of monochloroacetone in the gaseous phase. To obtain a high selectivity and yield of the process, the reactor was proposed as a part of an atmospheric packed rectification column. The principal reactions take place on the feed stage of the column without packing. The recycling of the unconverted acetone was carried out from both the rectifying (in liquid phase) and the stripping sections (in gas phase). The reaction heat was removed by evaporation of the liquid acetone. The simulation of such a column was performed using the simulation program HYSYS. Because of non-standard reaction kinetic expressions, not allowed in the HYSYS program, the reactor had to be described by using fictive streams. The simulation program is very flexible and useful for the simulation and design of this type of column.

Calcite precipitation in seawater using a constant addition technique: A new overall reaction kinetic expression

A simple “constant addition” system was developed to study calcite precipitation reaction kinetics in seawater under steady state conditions. It can be applied to carbonate-trace element coprecipitation studies and may also provide an interesting alternative for kinetic studies of calcite dissolution reactions and other mineral-solution interactions. Calcite precipitation in seawater can be represented by a reversible overall reaction: Ca 2+ + CO 3 − ∗ag CaCO 3(S). The measured precipitation rate, R, is adequately described by a classic kinetic model of the form: R = R f − R b = κ f ( γCa[ Ca 2+]) n1 ( γCO 3[ CO 3 2− n 2 − κ b , where R f , R b and κ f , κ b are the forward and backward reaction rates and rate constants for the overall reaction, respectively; [ i], γ i , and n i are the total concentration, activity coefficient and reaction rate order, respectively, for each species involved in the reaction. If [Ca 2+] is held constant throughout the precipitation experiments, the above equation reduces to R = K f [ CO 3 2−] n 2− κ b . The equation was used to fit calcite precipitation rate data measured over a wide range of saturation states and extending to near saturation conditions. The least-squares fit to the above expression yields values of K f = 10 3.5 μmol(kg) 3/m 2h(mmol) 3, n 2 = 3, and κ b = 0.29 μmol/m 2 h with a correlation coefficient of 0.99 at 25°C, when P CO 2 = 0.0031 atm. and [ Ca 2+] ≈ 10.5 mmol/kg sw. The partial reaction order for the carbonate ion confirms that calcite precipitation in seawater proceeds through a complex mechanism, as suggested by previous calcite-seawater interaction studies. The calcite dissolution rate constant derived from this study is significantly lower than values obtained in dilute solutions. This observation is in agreement with results of previous studies which indicate that calcite dissolution is much faster in dilute solutions than in seawater under identical saturation conditions.

Thermodynamic restrictions on non-isothermal chemical reaction rates

Using arguments derived from classical chemical thermodynamics, it is found that the isobaric, non-isothermal chemical reaction rate is in general a function of heating rate as well as of temperature and composition. However, non-isothermal reaction rate expressions have usually been identified with the corresponding isothermal rate expressions in the literature on non-isothermal chemical reaction kinetics. Generalizing from a specific example, a method is proposed according to which changes in isothermal reaction kinetic expressions may be made to agree with thermodynamics.

Unified Model for Ballistic Transport and Gas-Solid Reactions in Features

ABSTRACTThe three dimensional equations which govern free molecular or ballistic transport and heterogeneous gas-solid reactions in features on patterned wafers during low pressure processing are reviewed. The governing equations specific to long rectangular trenches are extended to include nonuniform molecular velocity distributions in the source volume above the wafer as well as surface diffusion of adsorbed species or intermediates. Introduction of reaction stoichiometries and kinetic expressions for general deposition and etch reactions completes the specification of the problem. Local generalized sticking factors arise in the process of nondimensionalizing the governing equations.

Reaction kinetics of the gasification of Michigan Antrim oil shale char

Both CO 2 and steam gasification kinetics were determined for Michigan Antrim shale char using TGA techniques in combination with on-line gas chromatography. While the reaction kinetic expressions were similar to those previously derived for Green River Formation (western USA) and Swedish Ranstad shale chars, the gasification rates were found to be on the same order as those of Ranstad char but significantly lower than those of western shale char. In addition, the gas produced from steam gasification was found to be significantly higher in CO than the corresponding gas from western shale char. This and the presence of significant quantities of CO 2 lower the attractiveness of Antrim char as a source of synthesis gas or hydrogen.

Selectivity improvement in the solvent refined coal process 2. Detailed second-stage reaction studies: Hydrotreating of coal liquids

A two-stage process has been investigated for the production of a low-sulfur solid SRC-I type boiler fuel with a minimum consumption of costly hydrogen. The process employs a coal mineral with scavenging action in the first stage dissolver, followed by second stage hydrotreating of the dissolver effluent. This paper reports a detailed investigation of second-stage hydrotreating conditions designed to optimize the process with respect to maximum sulfur removal with minimum hydrogen consumption. The process variable space is experimentally mapped and modeled using appropriate reaction kinetic expressions. This model is then used to select optimum conditions for operation of the second-stage hydrotreating process for the production of a low-sulfur solid SRC product with minimum hydrogen consumption.

Char gasification in a countercurrent reactor

AbstractA model is developed for the countercurrent char gasifier of Lurgi form. Char particles are fed to the top and oxygen, inert, and steam in the bottom. The combustion zone is defined as the zone in which there is a nonzero molfraction of oxygen. In the gasification zone, the particles are assumed to be a lumped system in which the Johnson reaction kinetic expressions are assumed to be valid with the water gas shift reaction at equilibrium. All of the reactions between oxygen and carbon and water and carbon are assumed to take place in the combustion zone but with the carbon‐oxygen reaction predominating to form a shell progressive system within the particles. Radiation is taken into account, and parametric computations are made on temperatures, compositions, flow rates, and char reactivity. The maximum temperature is strongly influenced by radiation and its position is a sensitive function of the solid flow rate producing an ash layer at the bottom of varying thickness.