Ratio Of Reaction Rate Research Articles

Dimethyl Ether to Olefins over Modified ZSM-5 Based Catalysts Stabilized by Hydrothermal Treatment

The reaction of dimethyl ether to olefin over HZSM-5/Al2O3 catalysts modified by Zr and Mg and stabilized by hydrothermal treatment has been studied. Regardless of the introduction method and the nature of the metal, the dependence of the key products selectivity on X(DME) over hydrothermally treated steady-state catalysts does not change, and the experimental points are described by the same curves. Metal introduction and the corresponding changes in the acid sites distribution do not change the ratio of main reaction rates, only the absolute values of the formation rate of the products are changed. Zr doping leads to the greatest activity in the DME conversion due to an equable decrease in the total acidity of the sample. On the other hand, the Mg-modified sample has a higher amount of weak acid sites, which reduces activity. At low DME conversion, methanol is one of the primary reaction products which formed from DME simultaneously with propylene in alkene cycle. At high DME conversion, the methanol acts as a main reagent which leads to ethylene formation in the arene cycle. Based on the results, the role of the metal in the reaction chemistry is considered and the mechanism of product formation from DME over steady-state catalyst is proposed, which describes both the participation of DME and the methanol produced.

On Thermal Acceleration of Medical Device Polymer Aging

An empirical rule, the 10 °C rule, states that chemical reaction rates are doubled for every 10 °C temperature increase. This is often used in thermally accelerated medical device polymer aging studies. Here, theoretical evidence and limitations for the rule are analyzed. Thus, a new and more accurate rule based on averaging Arrhenius chemical reaction rate ratios over typical activation energies 0.1 eV–0.9 eV in the normal medical device accelerated test temperature interval 25 °C–70 °C is proposed. A comparison of the 10 °C rule shows that the 10 °C rule provides similar estimates, but only at the reference temperature 25 °C. Fitting the reaction rate ratio based on the Arrhenius equation using the reference temperature 25 °C to the 10 °C rule data reveals that the best agreement is achieved with thermal aging activation energy of 0.67 eV.

Analysis of activation energy in magnetohydrodynamic flow with chemical reaction and second order momentum slip model

This paper is communicated theoretically to study the collective effects of Arrhenius activation energy and binary chemically reactive species in the presence of the second order momentum slip model which has not been studied so far. To support these declaration in addition with electrically conducting boundary layer flow and heat transport have considered towards an exponential stretching sheet. The current study incorporates the impact of activation energy, temperature difference ratio parameter, 1st and 2nd order slip parameter, chemical reaction rate on fluid velocity, fluid temperature and concentration of chemical species are elaborated through graphically and discussed in detail. Appropriate transformations are betrothed to acquire nonlinear highly coupled ordinary differential equations (ODE's) from partial differential equations which are then solved numerically by employing finite difference collocation process that apply three-stage Lobatto IIIa scheme. The obtained results confirm that an excellent agreement is achieved with those available in open literature. It is found that concentration profile decreases in the presence of chemical reaction rate and temperature difference ratio parameter whereas opposite demeanour is seen for activation energy.

Effect of particle size and enzyme load on the simultaneous reactions of lactose hydrolysis and transgalactosylation with glyoxyl-agarose immobilized β-galactosidase from Aspergillus oryzae

Abstract The simultaneous reactions of lactose hydrolysis and transgalactosylation in the production of galacto-oligosaccharides (GOS) were evaluated with immobilized Aspergillus oryzae β-galactosidase in glyoxyl-agarose of different particle sizes (fine and macro) and enzyme loads (1, 10 and 30 mgprotein/gsupport) to produce biocatalysts subjected to different magnitudes of internal diffusional restrictions. The ratio of initial reaction rates depended on the initial concentration of lactose: at values higher than 800 mM, the ratio of hydrolysis to total reaction rate (rhydr/rtotal), which reflects the hydrolytic potential, had values of 16 and 30%, and the ratio of transgalactosylation to total reaction rates (rtransgal/rtotal), which reflects the transgalactosylation potential, had values of 84 and 70% with the biocatalysts of smaller size and enzyme load, and bigger size and enzyme load, respectively. Results obtained highlight the fact that the biocatalyst should be optimized with regard to its intended use; β-galactosidase biocatalysts that have been optimized for lactose hydrolysis can be quite inadequate for performing GOS synthesis.

SENSMG: First-Order Sensitivities of Neutron Reaction Rates, Reaction-Rate Ratios, Leakage, keff, and α Using PARTISN

SENSMG is a tool for calculating the first-order sensitivities of reaction-rate ratios, keff, and α in critical problems and reaction-rate ratios, reaction rates, and leakage in fixed-source problems to multigroup cross sections, isotope densities, material mass densities, and interface locations using the PARTISN multigroup discrete-ordinates code by implementing Generalized Perturbation Theory. SENSMG can be used for one-dimensional spherical and slab (r) and two-dimensional cylindrical (r-z) geometries. For fixed-source (leakage) problems, SENSMG relies on the MISC and/or SOURCES4C codes to compute neutron source rate densities from spontaneous fission and (α,n) sources. SENSMG is a combination of Python and Fortran and was developed under Linux. This computer code abstract describes all user inputs, the input file, and output files. This computer code abstract describes how SENSMG can be modified to support different computer platforms, PARTISN versions, or cross-section availability. Several verification problems are presented in which SENSMG results are compared with MCNP6, SCALE6.2, and direct perturbations (central differences). SENSMG is available at https://github.com/jafavorite/SENSMG. SENSMG can be modified to accommodate other deterministic transport codes that have an adjoint capability.

Modeling Cascading Failure of Thermal Runaway in Stacks of Li-Ion Pouch Cells with Variation in Cooling Attributes

The propagation of a local thermal runaway event to adjacent cells will determine the severity of potential local thermal abuse events in large battery packs relevant to transportation or grid-scale storage. Propagation can occur or heat can be dissipated depending on the ratio of the thermochemical reaction rates to the heat transfer rates. Also of importance is the thermal capacity of a system relative to the thermal energy released. We demonstrate these phenomena with recently developed thermochemical models for thermal abuse scenarios. These models have been developed with greater fidelity for high temperature heat release rates that we will show are important. Predictions are compared with recent experiments where thermal runaway was initiated through thermal abuse of one cell and subsequent cascading propagation, or failure to propagate, through the remaining cells in a pack, depending on the energy stored and the thermal characteristics of the battery pack. Implications for cascading failure in larger systems are discussed. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2017-12561 C

Quantifying the Trade-Offs between Energy Consumption and Salt Removal in Membrane-Free Cation Intercalation Desalination

Recent work has predicted that Na-ion batteries with symmetric electrodes can be used for desalination with low energy consumption and high desalination degree from brackish to seawater salinity level.1 This technology, referred to as cation intercalation desalination (CID), shows much promise, but its wide-scale application will be dependent on capital cost. Similar to other electrochemical desalination technologies,2 initial embodiments of the CID concept used ion-exchange membranes (IEMs) to separate desalinated and concentrated streams in electrodes.1,3,4 IEMs in capacitive deionization cells can consume 50% of their expense, and IEMs have a finite lifespan.5 One solution to these challenges is to replace IEMs with chemically inert separators. Since the 1920s, non-selective diaphragms have been used in brine electrolysis. Furthermore, environmental-friendly diaphragms have shown the property of limiting ionic transfer between electrodes. Building on these concepts, we consider a novel CID cell design that omits IEMs by the use porous, fluid-blocking diaphragms as economical and stable alternatives to IEMs. In this study we quantify how the loss of selectivity incurred by using a diaphragm impacts the resultant salt removal and energy consumption levels. We consider two different flow arrangements for these devices, with or without electrolyte recycling, among which we compare energy consumption and salt removal. We also explore the effect of the operating conditions using a reduced-order analytical model. With our analytical model we predict salt removal efficiency (SRE, the ratio of the amount of salt removed relative to that of a perfect diaphragm) and degree of desalination (DoD, defined as the fraction influent concentration removal as a percentage) as a function of two dimensionless numbers: (1) the Peclet number Pe (the ratio of advective mass transfer rate over diffusive mass transfer rate) and (2) the Damköhler number Da (the ratio of reaction rate to the diffusive mass transfer rate). Figures 1a and 1b show contours of SRE in the space of Pe and Da for the non-recycling and recycling cell, respectively. Based on the definition of Pe and Da, these results reveal the influence of flow rate and current density on SRE, thus, operating conditions can be selected to reach a specific efficiency. Using the optimized operating conditions determined from the analytical model, we simulate the two-dimensional charge transport processes within porous electrodes and the diaphragm as source water is desalinated. The two-dimensional model employed here is based on porous electrode theory. To produce 60% DoD and 90% SRE from influent with 500 mM NaCl we use the conditions marked by α and β in Figs. 1a and 1b, respectively, for our two-dimensional simulations: Da=6.31 and Pe=109 for the recycling cell and Da=6.31 and Pe=6.31 for the non-recycling cell. Simulations with these operating conditions and 90% SRE reveals that the diaphragm cell can desalinate 500 mol/m3 solution to 260 mol/m3 (with recycling) and 244 mol/m3 (without recycling) solution (see Figs. 1c and 1d). For these two cases both diaphragm cells consume approximately 4.7 kWh energy when 1 m3 of desalinated water is produced. Figure 1c shows effluent salt concentration as a function of time for the non-recycling cell. Effluent salinity within the negative electrode decreases to a steady value after the process starts For the recycling cell effluent salinity continuously decreases during the charging processes (Fig. 1d). Salt concentration is distributed differently within each cell depending on whether or not effluent recycling is used. This effect is shown in Fig. 1e, where the concentration distribution in the recycling cell (lower) is more uniform than that in the non-recycling cell (upper), because a faster flow speed is used in the recycling cell. The simulated DoD (50%) agrees well with the predicted 60% DoD, and hence the analytical model can be used as a tool for performance predictions. The CID cells simulated here using diaphragms demonstrate the potential to replace IEMs with diaphragms. These findings motivate the future application of membrane-less CID technology in ion separation and/or removal processes. Acknowledgements: We are grateful for funding and support from the College of Engineering and the Mechanical Science and Engineering Department at UIUC.

The isomerization of cytosine: Intramolecular hydrogen atom transfer mediated through formic acid

AbstractThe catalytic ability of H2O and HCOOH to facilitate the tautomerism of KA to KIt isomer has been studied. It is shown that the direct tautomerism (path A) is unlikely because of the high activation free energy, whereas the presence of H2O and HCOOH (paths B and C) significantly contributes to decreasing the activation free energy. Meanwhile, the conventional transition state theory followed by Wigner tunneling correction is applied to estimate the rate constants. The rate constant with Wigner tunneling correction for direct tautomerization is obviously smaller than that of HCOOH‐mediated tautomerization, which is the most plausible mechanism. Finally, another important finding is that the ratio of reaction rates between direct tautomerism reaction and catalyst‐induced tautomerism increases with the increase of the catalyst concentration at a given temperature. The results of the present study demonstrate the feasibility of acid catalysis for DNA bases isomerization reaction that would otherwise be forbidden.

Effect of internal diffusional restrictions on the selectivity of α-chymotrypsin in a series-parallel reaction of peptide synthesis

Abstract The kinetically-controlled synthesis of the kyotorphin precursor dipeptide with immobilized α-chymotrypsin was used as a model system to show that diffusional restrictions, though reducing reaction rates, can benefit product formation in the case of series-parallel reactions because the competing reactions of hydrolysis of the acyl donor substrate and the peptide product are more affected by mass transfer limitations than the reaction of peptide synthesis. Six immobilized α-chymotrypsin catalysts were prepared with different particle diameters and enzyme loads. More than a fourfold increase in selectivity (from 10 to 45), defined as the ratio of reaction rate of peptide synthesis to the reaction rates of unwanted side product formation, was obtained in the case of the catalyst with larger particle size and higher protein load (effectiveness factor of 0.4) with respect to one with the smallest size and lower protein load that was free of diffusional restrictions (effectiveness factor close to 1).

A combined theoretical-experimental investigation on the mechanism of lignin pyrolysis: Role of heating rates and residence times

This work provides a combined theoretical-experimental investigation on the relationship between structural features and energetic requirements of pyrolytic breakdown of solid lignin, with a focus on the loss of aromaticity in the likely products. The breaking of β-O-4 linkages was identified as the most likely route for the fragmentation of solid lignin from both theoretical and experimental standpoints. The kinetic approach indicates that both the degradation mechanism and kinetic parameters (between 200 and 350 °C) are independent of the temperature and conversion degree. The yield of aromatic and/or aliphatic products was found is influenced by the heat and mass transfer phenomena in the gas phase (e.g., heat transfer rate and residence times), which affect the ratio of the individual reaction rates, and therefore, the relative abundance of products. The thermochemical approach suggests that when the heating process is fast and the residence time is short, the relative amount of aromatic fragments is favored.

Copper adsorption reaction rate and ion exchange ratio during the copper activation of sphalerite

Copper adsorption reaction rate and ion exchange ratio during the copper activation of sphalerite

Evaluation of reaction kinetics of CO2 and Li4SiO4 by thermogravimetry under various CO2 partial pressures

The CO2 absorption kinetics of Li4SiO4, which attracted interest as CO2 absorbent, under P(CO2) of 1.0, 0.1 and 0.03bar was evaluated using thermogravimetry at fixed temperatures between 460°C and 600°C. Reaction rate and saturated reaction ratio increased with increasing temperature and P(CO2). Under P(CO2) of 1.0bar and 0.1bar, reaction rate at initial period was enhanced by decreasing particle size and variation of reaction ratio by time could be successfully explained by using Jander model assuming Li+ diffusion process induced by difference of CO2 concentration as rate determining step. Under P(CO2) of 0.03bar, Jander model assuming superficial reaction between CO2 gas and Li4SiO4 solid surface as rate determining step was more applicable, indicating change of rate determining step varied from diffusion process to superficial one by decreasing P(CO2) from 0.1bar to 0.03bar.

Development of continuous-energy sensitivity analysis capability in OpenMC

The iterated fission probability (IFP) method and the Contributon-Linked eigenvalue sensitivity/Uncertainty estimation via Track length importance CHaracterization (CLUTCH) method have been implemented in several Monte Carlo codes to perform sensitivity calculations. In this work the unified adjoint and generalized adjoint calculation framework combining the IFP method and the CLUTCH method is proposed, and corresponding capabilities for calculating nuclear data sensitivity to the effective multipilication factor and reaction rate ratios are implemented in OpenMC. Results are presented on Godiva, Jezebel and a PWR fuel pin system which demonstrate that OpenMC results agree well with reference direct perturbation sensitivity coefficients. In addition, two main problems of the IFP method related to the number of latent generations, namely convergence and statistical fluctuations are investigated theoretically and numerically. This paper further demonstrates that the fission matrix approach is better suited to generate the adjoint and generalized adjoint source distribution in the CLUTCH method to overcome the drawbacks of the IFP method.

Synthesis of PNVP-Based Copolymers with Tunable Thermosensitivity by Sequential Reversible Addition⁻Fragmentation Chain Transfer Copolymerization and Ring-Opening Polymerization.

Through the reversible addition–fragmentation chain transfer (RAFT) copolymerization of 3-ethyl-1-vinyl-2-pyrrolidone (C2NVP) and N-vinylpyrrolidone (NVP), a series of well-defined P(C2NVP-co-NVP) copolymers were synthesized (Mn = ca. 8000 to 16,000 and Mw/Mn <1.5) by using a difunctional chain transfer agent, S-(1-methyl-4-hydroxyethyl acetate) O-ethyl xanthate (MHEX). Copolymerizing kinetics and different monomer ratio in feeds were conducted to study the apparent monomer reaction rate and reactivity ratios of NVP and C2NVP, which indicated similar reaction rates and predominantly ideal random copolymers for the two monomers. The Tgs of the obtaining P(C2NVP-co-NVP) copolymers significantly corresponded to not only molecular weights MWs but also copolymer compositions. These copolymers presented characteristic lower critical solution temperatures (LCST) behavior. We then studied the cloud points (CPs) of the copolymers with varying MWs and compositions. With different MWs, the CPs were linearly decreased from ca. 51 to 45 °C. With different compositions, the CPs of the copolymers decreased from ca. 48 to 29 °C with C2NVP content (i.e., from 60.8 to 89.9 mol %). Fitting the CPs by the theoretical equation, the result illustrated that the introduction of more hydrophobic units of C2NVP suppressed the hydrophilic interaction between the polymer chain and water. We then successfully proceeded the chain extension through the ring-opening polymerization (ROP) of ε-caprolactone (CL) to the synthesis of a novel P(C2NVP-co-NVP)-b-PCL amphiphilic block copolymer (Mn,NMR = 14,730 and Mw/Mn = 1.59). The critical micelle concentration (CMC) of the block copolymer had a value of ca. 1.46 × 10−4 g/L. The block copolymer micelle was traced by dynamic light scattering (DLS), obtaining thermosensitive behaviors with a particle size of ca. 240 nm at 25 °C and ca. 140 nm at 55 °C, respectively.

Reaction Rates in Chemically Heterogeneous Rock: Coupled Impact of Structure and Flow Properties Studied by X-ray Microtomography.

We study dissolution in a chemically heterogeneous medium consisting of two minerals with contrasting initial structure and transport properties. We perform a reactive transport experiment using CO2-saturated brine at reservoir conditions in a millimeter-scale composite core composed of Silurian dolomite and Ketton limestone (calcite) arranged in series. We repeatedly image the composite core using X-ray microtomography (XMT) and collect effluent to assess the individual mineral dissolution. The mineral dissolution from image analysis was comparable to that measured from effluent analysis using inductively coupled plasma mass spectrometry (ICP-MS). We find that the ratio of the effective reaction rate of calcite to that of dolomite decreases with time, indicating the influence of dynamic transport effects originating from changes in pore structure coupled with differences in intrinsic reaction rates. Moreover, evolving flow and transport heterogeneity in the initially heterogeneous dolomite is a key determinant in producing a two-stage dissolution in the calcite. The first stage is characterized by a uniform dissolution of the pore space, while the second stage follows a single-channel growth regime. This implies that spatial memory effects in the medium with a heterogeneous flow characteristic (dolomite) can change the dissolution patterns in the medium with a homogeneous flow characteristic (calcite).

Review of Early 21st-Century Monte Carlo Perturbation and Sensitivity Techniques for k-Eigenvalue Radiation Transport Calculations

Since the 1960s, Monte Carlo methods have been used to compute the effect of perturbations on system responses and for computing sensitivity coefficients. This review article focuses on 21st-century developments specific to k-eigenvalue calculations. The theory of correlated sampling, differential operator sampling, and adjoint-based approaches and their historical methods from the 20th century are briefly summarized. Specific focus is given to four recent and significant developments: fission source correction using the correlated sampling and differential operator sampling methods, adjoint-based perturbations for the k eigenvalue using the iterated fission probability method, an extension to reaction rate ratios using generalized perturbation theory, and a recent development using a collision history approach allowing for the calculation of sensitivity coefficients of bilinear ratios and generalized responses. Differences and similarities of the four methods are discussed along with a comparison to the 20th-century approaches. A perspective on future developments is also given.

Scaling analysis of the In-Situ Upgrading of heavy oil and oil shale

The In-Situ Upgrading (ISU) of heavy oil and oil shale is investigated. We develop a mathematical model for the process and identify the full set of dimensionless numbers describing the model. We demonstrate that for a model with nf fluid components (gas and oil), ns solid components and k chemical reactions, the model was represented by 9+k×(3+nf+ns-2)+8nf+2ns dimensionless numbers. We calculated a range of values for each dimensionless numbers from a literature study. Then, we perform a sensitivity analysis using Design of Experiments (DOE) and Response Surface Methodology (RSM) to identify the primary parameters controlling the production time and energy efficiency of the process. The Damköhler numbers, quantifying the ratio of chemical reaction rate to heat conduction rate for each reaction, are found to be the most important parameters of the study. They depend mostly on the activation energy of the reactions and of the heaters temperature. The reduced reaction enthalpies are also important parameters and should be evaluated accurately. We show that for the two test cases considered in this paper, the Damköhler numbers needed to be at least 10 for the process to be efficient. We demonstrate the existence of an optimal heater temperature for the process and obtain a correlation that can be used to estimate it using the minimum of the Damköhler numbers of all reactions.

Precision requirement of the photofission cross section for the nondestructive assay

Principle of the new NDA technique based on the photofission reaction rate ratio (PFRR) has been developed by Kimura et al for measurement of uranium enrichment by using the only relative measured counts of neutron produced by photofission reactions of 235 U and 238 U at different specific incident photon energies. In the past analysis, no attentions have been paid for relatively large uncertainty of photonuclear cross section of special nuclear materials around 10%. In the present paper, quantitative analysis was performed to reveal the impact of photonuclear cross section uncertainty to predicted value of the uranium enrichment by the PFRR methodology. And also, the requirement of photofission cross section precision was evaluated as less than 3%, to satisfy the uncertainty of PFRR methodology to within 5%.

Oxidative pyrolysis reforming of methanol in warm plasma for an on-board hydrogen production

To achieve on-board hydrogen production with high energy efficiency and low energy cost, the oxidative pyrolysis reforming (OPR) of methanol using air as an oxidant in a heat-insulated gliding arc plasma reactor is explored. Effects of dioxygen/methanol (O2/C) ratio, steam/methanol (S/C) ratio and specific energy input (SEI) on the OPR are investigated. The reaction rate ratio (α) of pyrolysis reforming to oxidative reforming in the OPR is deduced. The OPR of methanol strongly depends on the O2/C ratio, with which methanol conversion increases rapidly. In the OPR, methanol conversions occur mainly by the oxidative reforming (partial oxidation) at the O2/C ratios below 0.20, but by the oxidative reforming and the promoted pyrolysis reforming at the O2/C ratios above 0.20, which is confirmed by the enthalpy change for the overall reaction of OPR. Higher O2/C ratio results in higher energy efficiency and lower energy cost, however, higher S/C ratio or larger SEI leads to lower energy efficiency and higher energy cost. Under conditions of O2/C = 0.30, S/C = 0.5, SEI = 24 kJ/mol, energy efficiency of 74% and energy cost of 0.45 kWh/Nm3 with methanol conversion of 88% are achieved.

Application of MCNP nonanalog techniques for calculations of reaction rate measurements at the BFS facilities

An analysis of measurements of criticality, central reaction rate ratios, reaction rate distributions and reactivity coefficients performed at the BFS critical assemblies is important both for obtaining information required to refine neutron data and assessing the accuracy of neutron-physical characteristics. The high core heterogeneity of critical assemblies dictates the need for using codes capable of reproducing an accurate description of the geometry and detailed representation of the energy dependence of neutron data. At the same time, serial verification tests of evaluated nuclear data libraries and processing and transport codes require significant computer time to obtain results of sufficient precision. In such cases, it is possible to achieve the required accuracy in measuring neutron-physical characteristics by using non-analogous calculation methods (variance reduction methods). The paper presents the algorithms for improving the efficiency of calculations based on non-analogous methods implemented in the MCNP code as exemplified by an analysis of experiments on measuring reaction rates in critical multiplying systems with a complex heterogeneous composition. The results of the analysis have shown that the combination of mesh-based weight window with energy splitting/roulette leads to a significant increase in the calculation efficiency and a reduction of computational time from a few days to a few hours at a statistical error in the spectral indexes of less than 2%.