Application of wavefront analysis for kinetic investigations of watergas shif reaction

It has been demonstrated that during dynamic operation of the low temperature watergas shift reaction on the wavefront the CO₂—production is faster than the H₂—production. For a catalyst pretreated for several hours with a mixture of CO/N₂ its re-oxidation by a mixture of H₂O/CO/N₂ is a slow process, whereas on the wavefront the H₂—concentration is twofold the CO₂—concentration. The quantitative analysis of CO₂—wavefront data for a catalyst pretreated for several hours with a H₂O/N₂—Mixture has given a reaction order n_co = 0.625 and an activation energy E = 46.27 kJ/mole for the CO oxidation. It is shown that H₂O/N₂inhibits the CO oxidation. For a catalyst pretreated for a long time period with a H₂O/N₂—mixture and then reduced in a short time period by a H₂O/CO/N₂—mixture the reoxidation by a H₂O/N₂—mixture is a slow process. The wavefront analysis has also been applied to clear-up the H₂O-sorption on the low temperature catalyst, suggesting that a physisorption of Langmuir type is combined with a second mechanism saturating the H₂O—capacitance for low values of P_H2O.     

Investigation of reaction kinetics by wavefront analysis

A new technique for kinetic investigations in fixed bed reactors, wave front analysis, is introduced in this paper. Use is made of Riemann's integration method to solve the linearized hyperbolic equations representing the dynamics of a homogeneous reaction in a packed tubular reactor. It is shown how the analysis of the front of the faster disturbance, in this system the concentration step, can be used for kinetic investigations. Wave front analysis is generalized for non-linear systems through Picard's iteration technique.     

Kinetic models of complex reaction systems

In this work, the desired N-oxidation of β-picoline (3-methylpyridine), 2,6-lutidine, 3,5-lutidine and 2,4,6-collidine and the undesired hydrogen peroxide decomposition accompanying the syntheses reactions have been studied by means of reaction calorimetry. The power generation measurements have been used for the development of a kinetic model that will represent satisfactorily the N-oxidation of this family of reactions and it can be used towards the prediction of selected runaway scenarios of the respective reactions. Their kinetic study was based on the kinetic model of Sempere et al. [Sempere, J., Nomen, R., Rodriguez, J. L., & Papadaki, M. (1998). Modelling of the reaction of 2-methylpyridine using hydrogen peroxide and a complex metal catalyst. Chemical Engineering and Processing, 37, 33–46.], which was improved and refined. It was found that the model represents well the N-oxidation of methylpyridines. Due to limited miscibility of di- and trimethylpyridines with hydrogen peroxide, water and catalyst only partial agreement with the model was achieved. These reactions need further study under conditions where homogeneous mixtures can be formed. A simple methodology was employed for the evaluation of the model coefficients. Accurate evaluation of the kinetic model constants can be achieved if the noise of measurements is reduced, or conditions where it is less pronounced are employed.     

Kinetic analysis of NO-Char reaction

Two Chinese coals were used to prepare chars in a flat flame flow reactor which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Acid treatment on the YB and SH chars was applied to obtain demineralized chars. Kinetic characterization of NO-char reaction was performed by isothermal thermogravimetry in the temperature range of 973–1,573 K. Presence of catalytic metal matter can increase the reactivity of chars with NO, which indicates that the catalytic effects of inherent mineral matter play a significant role in the NO-char reaction. The discrete random pore model was applied to describe the NO-char reactions and obtain the intrinsic kinetics. The model can predict the data for all the chars at various temperatures well, but underestimate the reaction rates at high carbon conversions for the raw YB and SH chars, which can be attributed to the accumulation of metal catalyst on char surface. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

On imperfect contacting in batch heterogeneous chemical reaction systems

Large scale probabilistic mixing in series with small scale molecular diffusion is proposed as the contacting mechanism operative between the bulk of the reactant phase and the surface of the catalyst particles in a suspension heterogeneous chemical reaction system. The model describes well-mixed systems and provides a correction to the limit of perfect contacting ordinarily assumed in the analysis of such systems.Departures from perfect contacting cause the reaction rate in a batch process to become dependent upon the entire history of the state of the process and thereby induce a hereditary integral into the dynamic equation for the chemical state of the process. The solution of the limiting vector integro-differential equation representing small departures from perfect contacting is given for the special case of a spherical diffusion operator. Predictions of the model are reported which quantify the interpretation of imperfect contacting as a kinetic disguise.     

异相反应法合成纳米硫化铅

以硫化氢和硝酸铅为原料,在有络合剂的醇/水介质中,采用气/液异相化学反应法合成了纳米硫化铅。对产物硫化铅粒度的影响因素进行了探讨,并通过XRD、SEM和激光粒度仪分别对产物纳米硫化铅的结构、形貌、粒度和粒度分布进行了表征。结果表明：增大EDTA与硝酸铅的初始浓度比（cEDTA /c硝酸铅）、乙醇与水体积比（V乙醇/ V水）和溶液pH,以及减小硫化氢气体流量,都可以减小硫化铅颗粒粒度。当cEDTA /c硝酸铅为1、V乙醇/V水为1、pH为3.5、硫化氢气体流量为20 mL/min时,所得到的纳米立方晶型硫化铅的粒度为69 nm,粒度分布较窄,颗粒形状为球型。     

Selectivity enhancement in heterogeneous catalysis induced by reaction modifiers

The application range of solid catalysts can be greatly extended by reaction or process modifiers, that is by simple addition of an inorganic or organic compound to the reaction mixture. The modifier, used in catalytic amounts, ideally interacts strongly with the active sites in a fashion which induces favorable changes in the outcome of the reaction. Evolution of the actual modified metal catalyst during reaction and the importance of in situ characterization in understanding these processes are illustrated using the examples of promotion by metal ions and nitrogen-containing bases. The major part of the review describes the advantages and limitations of employing N-base modifiers for tuning the performance of solid catalysts. Reactions discussed include chemo-, stereo-, enantio- and diastereoselective hydrogenations over metal catalysts, aerobic oxidation of alcohols with Pt and Pd, and epoxidation of allylic alcohols with titania–silica mixed oxides and alkyl hydroperoxides.     

Kinetic analysis of phenol adsorption from aqueous systems

The kinetic adsorption of mixtures of phenolic compounds onto a polymeric adsorbent from aqueous solutions was studied. Experimental data for single‐component solutions were obtained and fitted to a parallel diffusion model. Effective diffusion coefficients were related to liquid‐phase diffusion and surface diffusion. The effect of solute concentration and salinity on these parameters was also analysed. Van Laar's equation is proposed to evaluate the influence of concentration on diffusion. The adsorption kinetics for phenol and pcresol mixtures at different initial concentration ratios were determined and correctly adjusted to the mentioned kinetic model. Results confirm that adsorption from multicomponent aqueous solutions is a surface diffusion‐controlled process.     

Kinetic analysis and thermodynamic simulation of alkali-silica reaction in cementitious materials

This work aims to study the alkali-silica reaction (ASR) with incorporated kinetic-thermodynamic analysis. The model reactant tests were first conducted to study the silica dissolution in the simulated pore solution under different temperatures. Then, the kinetic model for silica dissolution was further improved by considering the influence of effective surface area. The improved kinetic model was also incorporated into the GEMS thermodynamic simulation. The model analysis demonstrated the silica dissolution rate is decreased mainly caused by reduced hydroxyl activity with increased saturation degree. The dissolved silica first reacted with the portlandite phase and formed the Calcium–Silicate–Hydrate (CSH) gel. The ASR gel can only be generated under high silicate ion concentration after the consumption of portlandite.     

Ignition of heterogeneous systems by radiant energy

Translated from Fizika Goreniya i Vzryva, Vol. 24, No. 4 pp. 3–10, July–August, 1988.     

CFD modelling of stirred tank chemical reactors: homogeneous and heterogeneous reaction systems

The presented work is a part of studies carried out to develop a practical method to prevent runaway events in chemical reactors. The use of computational fluid dynamics (CFD) techniques is proposed here to indicate in advance the local hot-spots in the reaction mixture, so as to elaborate an efficient and sensitive method for early warning detection of runaway. The CFD method has been chosen to support, elaborated by Bosch et al. (Comput. Chem. Eng. 28 (2004) 527), the on-line runaway detection method in batch reactors, which is based on the divergence criterion. Series of CFD simulations have been executed for two exothermic reactions, being the esterification reaction of the 2-butanol and the propionic anhydrite catalysed by the sulphuric acid (homogeneous system) and the hydrolysis of the propionic anhydrite catalysed by the sulphuric acid (heterogeneous system). For both the considered reactions the results of calculations have been verified with the results of experimental measurements performed in a bench scale RC1 Mettler-Toledo reaction calorimeter. The elaborated conclusions can be further employed in the parametric sensitivity analysis (Comput. Chem. Eng. 28 (2004) 527; Am. Inst. Chem. Eng. J. 45 (1999) 2429) to indicate a number of temperature sensors and their location inside the reactor, which will help to apply efficiently the divergence criterion method.     

非均相UV/Fenton催化降解苯酚研究

研究了UV/H2O2/改性海藻酸铁非均相催化降解苯酚时H2O2的投加量、溶液初始pH、苯酚初始浓度、改性海藻酸铁的含铁量等因素对苯酚降解率的影响.结果表明:该非均相体系在pH 2.0～11.1范围内都能高效催化氧化降解不同浓度苯酚废水.在基准条件下,若增加改性海藻酸铁的含铁量,则在反应时间2 min时,苯酚降解率可接近100%;以日光代替紫外光,苯酚降解率仍可达68.9%.     

Kinetic analysis of benzyl alcohol oxidation by alcohol oxidase in aqueous—organic two-phase systems

Alcohol oxidase-catalyzed oxidation of benzyl alcohol to benzaldehyde was studied in three two-liquid phase systems involving butyl acetate, xylene and n-decane. A more highly hydrophobic solvent resulted in more favorable partitioning of the substrate into the aqueous phase and less deleterious effect on the enzyme activity. As a result, the overall production of aldehyde was highest with the n-decane system and lowest with the butyl acetate system. In addition, the organic solvent served as an extractant for the product, which acted both as a competitive inhibitor and as a denaturant of the enzyme. The time courses of benzaldehyde production were simulated by a simple mathematical model assuming Michaelis–Menten kinetics in the aqueous phase, enzyme deactivation by organic solvents and the equilibrium partitioning of the reagents between the two phases.     

Dependence of the kinetics of the low-temperature water-gas shift reaction on the catalyst oxygen activity as investigated by wavefront analysis

The systematic investigation by means of wavefront analysis of the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of a $\text{CuO}\text{ZnO}$ industrial catalyst is reported. The catalyst was pretreated for a long period of time in a mixture of H₂O, H₂, and N₂ at definite $\text{ψ =}\text{P}\text{H}{}_2\text{O}\text{P}\text{H}{}_2$ ratios, from ψ = 0.1 to ψ = 10.0 and at 230 °C. The fitting procedure at different ψ levels suggested a three-path reaction model consisting of two formal Eley-Rideal-type mechanisms which are relevant for the microkinetic shift conversion through adsorption intermediates of CO and H₂O, and a redox mechanism which regulates the oxygen activity on the catalyst surface and accounts for the interaction catalyst/reaction medium. In this investigation, an Elovich-type rate dependence of the redox mechanism has been suggested.     

Distance determination in heterogeneous DNA model systems by pulsed EPR

Many biological systems, especially those based on nucleic acids, are structurally heterogeneous in solution. We demonstrate here the ability to measure multiple distances, of between 2 and 7 nm, from a heterogeneous mixture of double-spin-labeled DNA duplexes. We have constructed a DNA distance ruler based on the attachment of nitroxide spin labels to 2'-amino-modified nucleosides. The distribution of distances between the spin labels was obtained by Tikhonov regularization analysis of the dipolar coupling evolution data measured by using the electron paramagnetic resonance method, pulsed-electron double resonance (PELDOR). Optimization of the conditions and techniques used in the preparation of the samples has allowed us to increase the sensitivity and reduce aggregation artifacts. As a result, we have been able to demonstrate deconvolution of distances from structurally heterogeneous samples and show the limits of the technique by examining data derived from up to five DNA duplexes, in a single mixture, in which the concentration of each species was as low as 5 microM.     

Distributional uncertainty analysis and robust optimization in spatially heterogeneous multiscale process systems

Multiscale models have been developed to simulate the behavior of spatially‐heterogeneous porous catalytic flow reactors, i.e., multiscale reactors whose concentrations are spatially‐dependent. While such a model provides an adequate representation of the catalytic reactor, model‐plant mismatch can significantly affect the reactor's performance in control and optimization applications. In this work, power series expansion (PSE) is applied to efficiently propagate parametric uncertainty throughout the spatial domain of a heterogeneous multiscale catalytic reactor model. The PSE‐based uncertainty analysis is used to evaluate and compare the effects of uncertainty in kinetic parameters on the chemical species concentrations throughout the length of the reactor. These analyses reveal that uncertainty in the kinetic parameters and in the catalyst pore radius have a substantial effect on the reactor performance. The application of the uncertainty quantification methodology is illustrated through a robust optimization formulation that aims to maximize productivity in the presence of uncertainty in the parameters. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2374–2390, 2016