Controlled Reaction Rate Research Articles

Synthesis of 1,2‐Propylene Glycol in a Continuous Down‐Flow Fixed‐Bed Reactor With Cu/Al2O3Catalyst

AbstractGlycerol hydrogenolysis to 1,2‐propylene glycol in a continuous down‐flow fixed‐bed reactor with 60 %Cu/Al2O3catalyst was investigated. The routes of main, intermediate and by‐products formation were proposed. It is shown that an increase in the catalyst particle size from 0.1–0.16 to 1.6–2.5 mm transfers the process from the kinetic mode to an intermediate mode between the kinetic and internal diffusion controlled reaction rate. In this case, the observed activation energy decreased by 30 %. The optimal conditions for the process have been determined.

A modified acid system to enhance carbonate matrix acid stimulation: An experimental study

AbstractIn carbonate matrix acidizing, the critical design parameters are interstitial velocity and pore volume to breakthrough for wormhole propagation (vi, opt and PVBT,opt). Hydrochloric acid (HCl), the most commonly used acid in carbonate acidizing, sometimes shows low efficiency because the injection rate needed for optimal wormhole propagation is not attainable, especially for when long completion intervals, low permeability carbonates, or coiled tubing operations are involved. It also raises safety, corrosion, and environmental concerns. When conventional retarded acid systems are used to overcome these challenges, the efficiency of acid stimulation often suffers from low reaction rate. This study presents testing results of a modified acid system that has controlled reaction rate with favourable wormhole propagation characteristics, especially at low interstitial velocities. Due to the increased activation energy barriers utilized in these modified‐acid systems, it is possible to control the reaction rate of the hydrogen proton and optimize the wormholing effect based on the completion method and formation specifications. Laboratory linear core flooding experiments and acid jetting experiments were conducted to study the wormhole efficiency with the new acid systems. The experimental results showed clear advantages of the modified acid systems. The modified acids have similar or better wormhole efficiency parameters compared with HCl having comparable dissolving power. When combined with acid jetting, further improvement in wormhole growth in low permeability limestone is achieved. In addition to minimizing the hazardous exposure levels and corrosion rates compared with HCl, the new acid system provides the positive aspects of solubilizing ability, thus improving optimal wormhole conditions.

Transcription Factors and DNA Play Hide and Seek.

Transcription factors (TFs) bind to specific DNA motifs to regulate the expression of target genes. To reach their binding sites, TFs diffuse in 3D and perform local motions such as 1D sliding, hopping, or intersegmental transfer. TF-DNA interactions depend on multiple parameters, such as the chromatin environment, TF partitioning into distinct subcellular regions, and cooperativity with other DNA-binding proteins. In this review, how current understanding of the search process has initially been shaped by prokaryotic studies is discussed, as well as what is known about the parameters regulating TF search efficiency in the context of the complex eukaryotic chromatin landscape.

GASFLOW-MPI: A new 3-D parallel all-speed CFD code for turbulent dispersion and combustion simulations: Part I: Models, verification and validation

The objective of the presented work is to develop an efficient and validated approach based on a multi-dimensional computational fluid dynamics (CFD) code for predicting turbulent gaseous dispersion, conjugated heat and mass transfer, multi-phase flow, and combustion of hydrogen mixtures. Applications of interest are accident scenarios relevant to nuclear power plant safety, renewable energy systems involved in hydrogen transport, hydrogen storage, facilities operating with hydrogen, as well as conventional large scale energy systems involving combustible gases. All model development is conducted within the framework of the high-performance scientific computing software GASFLOW-Multi-Physics-Integration (MPI). GASFLOW-MPI is the advanced parallel version of the GASFLOW sequential code with many newly developed and validated models and features. The code provides reliability, robustness and excellent parallel scalability in predicting all-speed flow-fields associated with hydrogen safety, including distribution, turbulent combustion and detonation. In the meanwhile, it has been well verified and validated by many international blind and open benchmarks.The recently developed combustion models in GASFLOW-MPI code are based on the transport equation of a reaction progress variable. The sources consist of turbulence dominated and chemistry kinetics dominated terms. Models have been implemented to compute the turbulent burning velocity for the turbulence controlled combustion rate. One-step and two-step models are included to obtain the chemical kinetics controlled reaction rate. These models, combined with the efficient and verified all-speed solver of the GASFLOW-MPI code, can be used for simulations of deflagration, detonation and the important transition processes like flame acceleration (FA) and deflagration-to-detonation-transition (DDT), without additional need for expert judgment and intervention. It should be noted that the major goal is to develop a reliable and efficient numerical tool for large-scale engineering analysis, instead of resolving the extremely complex physical phenomena and detailed chemistry kinetics on microscopic scales. During the course of this development, new verification and validation studies were completed for phenomena relevant to hydrogen-fueled combustion, such as shock wave capturing, premixed and non-premixed turbulent combustion with convective, conductive and radiation heat losses, detonation of unconfined hydrogen–air mixtures, and confined detonation waves in tubes. Excellent agreements between test data and model predictions support the predictive capabilities of the combustion models in GASFLOW-MPI code. In Part II of the paper, the newly developed CFD methodology has been successfully applied to a first analysis of hydrogen distribution and explosion in the Fukushi Daicchi Unit 1 accident.The major advantage of GASFLOW-MPI code is the all-speed capability of simulating laminar and turbulent distribution processes, slow deflagration, transition to fast hydrogen combustion modes including detonation, within a single scientific software framework without the need of transforming data between different solvers or codes. Since the code can model the detailed heat transfer mechanisms, including convective heat transfer, thermal radiation, steam condensation and heat conduction, the effects of heat losses on hydrogen deflagrations or detonations can also be taken into account. Consequently, the code provides more accurate and reliable mechanical and thermal loads to the confining structures, compared to the overly conservative results from numerical simulations with the adiabatic assumptions.Predictions of flame acceleration mechanisms associated with turbulent flames and flow obstacles, as well as DDT modeling and their comparisons to available data will be presented in future papers. A structural analysis module will be further developed. The ultimate goal is to expand the GASFLOW-MPI code into an integral high-performance multi-physics simulation tool to cover the entire spectrum of phenomena involved in the mechanistic hydrogen safety analysis of large scale industrial facilities.

Denitrification in a low-temperature bioreactor system at two different hydraulic residence times: laboratory column studies

ABSTRACTNitrate removal rates in a mixture of pine woodchips and sewage sludge were determined in laboratory column studies at 5°C, 12°C, and 22°C, and at two different hydraulic residence times (HRTs; 58.2–64.0 hours and 18.7–20.6 hours). Baffles installed in the flow path were tested as a measure to reduce preferential flow behavior, and to increase the nitrate removal in the columns. The nitrate removal in the columns was simulated at 5°C and 12°C using a combined Arrhenius-Monod equation controlling the removal rate, and a first-order exchange model for incorporation of stagnant zones. Denitrification in the mixture of pine woodchips and sewage sludge reduced nitrate concentrations of 30 mg N L−1 at 5°C to below detection limits at a HRT of 58.2–64.0 hours. At a HRT of 18.7–20.6 hours, nitrate removal was incomplete. The Arrhenius frequency factor and activation energy retrieved from the low HRT data supported a biochemically controlled reaction rate; the same parameters, however, could not be used to simulate the nitrate removal at high HRT. The results show an inversely proportional relationship between the advection velocity and the nitrate removal rate, suggesting that bioreactor performance could be enhanced by promoting low advection velocities.

Possibility of H2O2 decomposition in thin liquid films on Mars

In this work the pathways and possibilities of H2O2 decomposition on Mars in microscopic liquid interfacial water were analyzed by kinetic calculations. Thermal and photochemical driven decomposition, just like processes catalyzed by various metal oxides, is too slow compared to the annual duration while such microscopic liquid layers exist on Mars today, to produce substantial decomposition. The most effective analyzed process is catalyzed by Fe ions, which could decompose H2O2 under pH<4.5 with a half life of 1–2 days. This process might be important during volcanically influenced periods when sulfur release produces acidic pH, and rotational axis tilt change driven climatic changes also influence the volatile circulation and spatial occurrence just like the duration of thin liquid layer.Under current conditions, using the value of 200K as the temperature in interfacial water (at the southern hemisphere), and applying Phoenix lander׳s wet chemistry laboratory results, the pH is not favorable for Fe mobility and this kind of decomposition. Despite current conditions (especially pH) being unfavorable for H2O2 decomposition, microscopic scale interfacial liquid water still might support the process. By the reaction called heterogeneous catalysis, without acidic pH and mobile Fe, but with minerals surfaces containing Fe decomposition of H2O2 with half life of 20 days can happen. This duration is still longer but not several orders than the existence of springtime interfacial liquid water on Mars today. This estimation is relevant for activation energy controlled reaction rates. The other main parameter that may influence the reaction rate is the diffusion speed. Although the available tests and theoretical calculations do not provide firm values for the diffusion speed in such a “2-dimensional” environment, using relevant estimations this parameter in the interfacial liquid layer is smaller than in bulk water. But the 20 days׳ duration mentioned above is still relevant, as the activation energy driven reaction rate is the main limiting factor in the decomposition and not the diffusion speed.The duration of dozen(s) days is still longer but not with orders of magnitude than the expected duration for the existence of springtime interfacial liquid water on Mars today. The results suggest such decomposition may happen today, however, because of our limited knowledge on chemical processes in thin interfacial liquid layers, this possibility waits for confirmation – and also points to the importance of conducting laboratory tests to validate the possible process. Although some tests were already realized for diffusion in an almost 2-dimensional liquid, the same is not true for activation energy, where only the value from the “normal” measurements was applied.Even if H2O2 decomposition is too slow today, the analysis of such a process is important, as under volcanic influence more effective decomposition might take place in thin interfacial liquids close to the climate of today if released sulfur produces pH<4.5. Large quantity and widespread occurrence of bulk liquid phase are not expected in the Amazonian period, but interfacial liquid water probably appeared regularly, and its locations, especially during volcanically active periods, might make certain sites than others more interesting for astrobiology with the lower concentration of oxidizing H2O2.

Effect of Reaction Rate and Calcination Time on CaNb2O6 Nanoparticles

The properties of CaNb2O6 nanoparticles synthesized by coprecipitation method under controlled reaction rate and extended calcination time were studied. Analysis of the X-ray diffraction pattern shows single orthorhombic phase of the material with lattice parameters: a=15.0147 Å, b=5.74148 Å, and c=5.30296 Å. The morphology and size of particles was found to be improved due to the controlled reaction rate and extended calcination time. The average sizes of the particles were estimated as 40 nm and 90 nm for sintering temperatures 650°C and 800°C, respectively. The material was found to possess dielectric constant which is inversely proportional to the frequency. Surprisingly, the material shows ferroelectric behavior, the possible origin of which is discussed here.

Bio-geochemical reactive transport modeling of microbial induced calcite precipitation to predict the treatment of sand in one-dimensional flow

Microbial induced calcite precipitation (MICP) has been well studied to date in the laboratory as a viable alternative soil improvement technique that harnesses a natural bacterial process to induce cementation. Specifically, MICP utilizes the microbial process of hydrolysis of urea to induce pH increase leading to calcite precipitation. The study presented herein demonstrates the utility of a simple bio-geochemical reactive transport model to predict MICP in one-dimensional column experiments. The mathematical model was originally developed in the framework of the TOUGHREACT code to include kinetically controlled reaction rates for urea hydrolysis and calcite precipitation. Inverse modeling, via UCODE-2005, is utilized to calibrate and verify the model to experimental data including aqueous and mineral chemistry. Results indicate good agreement between data and simulated results for capturing the trends and magnitudes of a variety of MICP treatment schemes in half meter, one-dimensional flow columns. A design procedure is presented for predicting MICP in one-dimensional flow by sequentially coupling UCODE-2005 with TOUGHREACT.

Please, Get in My Way So That I Can Be More Efficient!

Please, Get in My Way So That I Can Be More Efficient!

Glass transition temperature of polyurethane foams derived from lignin by controlled reaction rate

Sodium ligninosulfonate (LS)-based polyurethane (PU) foams were prepared using three kinds of ethylene glycols, diethylene glycol, triethylene glycol or polyethylene glycol. Two kinds of industrial NaLS, acid-based and alkaline-based NaLS, were mixed with various ratios, and foaming reactions were controlled. Mixing, cream, and rise time were used as an index of foaming reaction. Mixing time was defined as the time interval from adding isocyanate to detection of evolved heat under stirring, cream time as the time interval from termination of stirring to starting of foaming, and rise time as the time interval from starting to completion of foaming. The above reaction time increased with increasing amount of acid base NaLS content in polyols. Apparent density, compression strength and compression modulus of PU foams linearly increased with reaction time. Thermal decomposition temperature was measured by thermogravimetry and glass transition temperature by differential scanning calorimetry. Glass transition temperature can be controlled in a temperature range from 310 to 390 K by changing the mixing rate of two kinds of LS and molecular mass of ethylene glycols. It was found that mechanical and thermal properties of PU foams are controllable through the foaming reaction rate using two kinds of industrial lignin.

Kinetics and modeling studies of the CaH2/LiBH4, MgH2/LiBH4, Ca(BH4)2 and Mg(BH4)2 systems

Borohydrides of alkali and alkaline-earth elements are of interest because of their high hydrogen holding capacities. However, their usefulness for hydrogen storage applications is often limited by high thermodynamic stabilities and slow kinetics. In this study comparisons have been made of the physical and chemical characteristics of Mg(BH4)2, Ca(BH4)2, and mixtures of LiBH4/MgH2 and LiBH4/CaH2. Temperature programmed desorption, TPD, analyses showed that samples containing Mg generally had lower onset temperatures than the corresponding samples containing Ca. Pressure Composition Temperature, PCT, isotherms showed that these materials displayed well defined two phase plateau regions. Therefore, kinetics measurements were done to determine the hydrogen desorption rates in the two phase plateau region. All kinetics determinations were done using constant pressure thermodynamic forces in which the ratios of the plateau pressure to the applied hydrogen pressure were kept the same for all samples. In all these measurements the pressure ratio was set at 3 and the temperature was 450°C. Under these conditions the reaction rates were in the order: Mg(BH4)2>Ca(BH4)2>LiBH4/MgH2>LiBH4/CaH2. An attempt was also made to determine the rate-controlling process in these samples by doing kinetic modeling. The results showed that diffusion was the rate-controlling process in Mg(BH4)2 whereas reaction at the phase boundary controlled reaction rates in Ca(BH4)2. In LiBH4/MgH2 and LiBH4/CaH2, reaction at the phase boundary controlled reaction rates initially whereas diffusion controlled rates in the latter stages.

Identifying heat and mass transfer characteristics of metal hydride reactor during adsorption: Improved formulation about parameter analysis

Transport process affects the performance of a metal hydride reactor significantly. Therefore in a former paper presented by the same authors, two parameters, which are known as heat transfer controlled reaction rate and mass transfer controlled reaction rate, were defined to account for this effect and assist the design of the reactors. However, a few simplifications were adopted in that article, which may result in some errors. In order to achieve better accuracy and clarity, more factors such as the external convection heat transfer and propagation of reaction front were considered here in the formulation of the parameters. Then numerical simulations for the adsorption in a tubular reactor were carried out and the situation under which parameter analysis can be applied was discussed. More characteristics in the process were revealed by the newly formulated parameters, which could be seen from the comparison of the results by parameter analysis and numerical simulation.

Identifying heat and mass transfer characteristics of metal hydride reactor during adsorption—Parameter analysis and numerical study

The performance of a metal hydride reactor is highly dependent on its transport process. It is important to know how the transport process affects the performance, therefore two key parameters—heat transfer controlled reaction rate and mass transfer controlled reaction rate were introduced to explain this kind of effect. Moreover, a brief discussion about how to use the new parameters was given. In order to analyze the reactor performance and heat/mass transfer characteristics, a non-local thermal equilibrium model describing the actual adsorption process was formulated, and numerical simulations were carried out. Then the two parameters were applied for the same purpose. The result of the parameter analysis coincided well with that of the numerical simulation, which approved the validity of the two parameters in identifying heat and mass transfer characteristics of the metal hydride reactor.

Energy diffusion controlled reaction rate of reacting particle driven by broad-band noise

The energy diffusion controlled reaction rate of a reacting particle with linear weak damping and broad-band noise excitation is studied by using the stochastic averaging method. First, the stochastic averaging method for strongly nonlinear oscillators under broad-band noise excitation using generalized harmonic functions is briefly introduced. Then, the reaction rate of the classical Kramers' reacting model with linear weak damping and broad-band noise excitation is investigated by using the stochastic averaging method. The averaged Ito stochastic differential equation describing the energy diffusion and the Pontryagin equation governing the mean first-passage time (MFPT) are established. The energy diffusion controlled reaction rate is obtained as the inverse of the MFPT by solving the Pontryagin equation. The results of two special cases of broad-band noises, i.e. the harmonic noise and the exponentially corrected noise, are discussed in details. It is demonstrated that the general expression of reaction rate derived by the authors can be reduced to the classical ones via linear approximation and high potential barrier approximation. The good agreement with the results of the Monte Carlo simulation verifies that the reaction rate can be well predicted using the stochastic averaging method.

A variable reaction order model for prediction of curing kinetics of thermosetting polymers

Based on the study of the curing reaction of an fluorinated aliphatic cyanate ester resin using an isothermal Differential Scanning Calorimetric (DSC) method, a new kinetics model with variable reaction order was proposed to describe the curing of thermosetting resins in both the chemical controlled and diffusion controlled regions. α–Tg relationship was used to calculate the diffusion controlled reaction rate constant during cure. Then the chemical and diffusion rate constants were combined in the Simon–Gillham equation to model the overall rate constant for the entire reaction. By using the overall rate constant combined with the variable reaction order, a simple model can be built up for prediction of curing kinetics of thermosetting polymer in both the chemical and the diffusion controlled regions. The new model is simple, easy to use and has very good agreement with the experimental results.

Energy diffusion controlled reaction rate in dissipative Hamiltonian systems

In this paper the energy diffusion controlled reaction rate in dissipative Hamiltonian systems is investigated by using the stochastic averaging method for quasi Hamiltonian systems. The boundary value problem of mean first-passage time (MFPT) of averaged system is formulated and the energy diffusion controlled reaction rate is obtained as the inverse of MFPT. The energy diffusion controlled reaction rate in the classical Kramers bistable potential and in a two-dimensional bistable potential with a heat bath are obtained by using the proposed approach respectively. The obtained results are then compared with those from Monte Carlo simulation of original systems and from the classical Kramers theory. It is shown that the reaction rate obtained by using the proposed approach agrees well with that from Monte Carlo simulation and is more accurate than the classical Kramers rate.

Determination of diffusion controlled reaction rates at a solid/liquid interface using scanning electron microscopy

A high-resolution method has been developed for the determination of localized values of interfacial reaction rate and mass transfer coefficient in aqueous solution. Scanning electron microscopy has been successfully applied to this problem through the measurement of electroplated film thickness formed under limiting current conditions. The method involves the calculation of local values of reaction rate via Faraday's laws and subsequent conversion of the data to absolute values of mass transfer coefficient. The technique has been verified in an undisturbed, turbulent flow regime (rotating cylinder electrode) through the use of Sherwood group dimensionless analysis. The resulting relationship shows comparable accuracy relative to electrochemical measurements. Favourable comparison has also been made with the generally accepted rotating cylinder correlation of Eisenberg, Tobias and Wilke. Differential rates of mass transfer to a single surface under conditions of disturbed flow have also been examined at a high spatial resolution using the stepped rotating cylinder electrode geometry. In this case, reaction rates have been measured as a function of circumferential distance within a recirculation zone situated immediately downstream of a backward-facing step.

Proton Reaction with a Mild Base in Ice Studied by Proton−Photoacid Dynamics

We have studied the kinetics of the reaction of a proton with a mild base in ice. The proton was injected to the ice crystal by a proton-transfer reaction from an electronically excited photoacid. Mild bases react with the transferred proton with a large intrinsic rate constant. We used our diffusion assisted geminate recombination model based on the Debye−Smolochowski equation with an additional term to account for the proton scavenging by the base. We find that in liquid water the measured scavenging effect is slightly smaller than expected from a diffusion controlled reaction rate constant. In ice, the scavenging effect is rather large, and the diffusion controlled reaction rate constant underestimates the measured proton scavenging effect in a wide range of concentrations and temperature. We explain the large effective proton scavenging in ice by the tendency of ice formation to concentrate the impurities to confined volumes in order to minimize the ice crystal energy.

Diffusion controlled reaction rate, survival probability, and molecular trajectory characteristics in the bulk, transition and Knudsen regime

Random walk simulation results are presented for the diffusion controlled reaction rate coefficient, molecular survival probability, average track length and relative collision frequency in the bulk, transition and Knudsen diffusion regime, in porous media formed by randomly oriented cylindrical fibers. The survival probability and mean survival distance are found to increase with the porosity in all diffusion regimes. The diffusion controlled reaction rate coefficient attains higher values at lower porosities, obeying an upper bound of the literature and approaching its value in the bulk regime. The values computed for all three parameters in the transition regime depend strongly on the Knudsen number, whereas in the Knudsen regime they are in line with analytical predictions based on the chord length distribution of the fiber structures. A harmonic average approximation based on Bosanquet's equation for the effective diffusivities is found to link successfully the values of the diffusion controlled reaction rate coefficient in the three diffusion regimes. The simulation results for the average molecular track length and relative collision frequency deviate in the biggest part of the transition regime from earlier analytical and numerical results obtained from long diffusion time random walks, in agreement with an analysis centered on the basic differences of molecular trajectories in diffusion controlled reaction systems from those involving molecular reflection from the pore surface.

On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity

Effects of pyrolysis pressure on char reactivity remain a poorly understood aspect of the coal gasification process. In an attempt to address this problem, effects of pyrolysis pressure on char structure and reactivity are being investigated. In this paper, chemical reactivities to O2, CO2, and H2O of chars made from three Australian black coals were measured in a pressurized thermogravimetric analyzer, under conditions where chemical processes alone controlled reaction rates. These chars were prepared at a range of pyrolysis pressures, using pressurized flow reactors and an atmospheric pressure tube furnace, to ascertain any effects of devolatilization pressure and heating rate on the chemical reactivity of the resultant coal chars. It was found that while the apparent (as measured) reaction rate can be affected by pyrolysis pressure, the rate normalized to the char surface area (intrinsic rate) is much less affected, because of large effects of pyrolysis pressure on char micropore surface area. This fin...