Dynamics of catalytic reactions and reactors

Heterogeneous catalytic reactions and catalytic reactors are known to exhibit complex dynamic behavior and significant progress has been made, in the past two decades, in understanding this complexity using the tools of nonlinear dynamics and advanced experimental methods. This article presents an overview of complex dynamic behavior in heterogeneous catalysis and points out their intriguing nature along with practical implications. The issues discussed are the source of oscillatory behavior, classification of complex motions, transitions to chaotic solutions and spatiotemporal patterns in reactors of various geometries.     

化学反应体系中温度场自组织类型和产生阈值的多样性研究

选择和控制反应过程的温度是反应器设计和控制中的重要问题。线性稳定性分析说明，在Lindemann单分子化学反应-扩散-热传导体系中，由于非线性动力学方程组具有诸多控制参数，导致温度场自组织类型及其产生阈值的多样性。数值模拟结果还说明，Turing分支因其产生阈值的不同，致使温度场空间周期结构的波数出现了多值性。毫无疑问，这些重要结果是实施化学反应器的合理设计选型及最佳控制所必须考虑的内容。     

Chaos and synchronisation in heterogeneous catalytic systems: CO oxidation over Pd zeolite catalysts

The influence of experimental parameters on the structure of global reaction rate oscillations and the coupling of local oscillators on a catalyst bed in a continuous stirred tank reactor is studied for the oxidation of CO on zeolite supported palladium catalysts. Global coupling can be achieved via mass transfer through the gas phase or via heat transfer in the case of a support of high heat conductivity. Characteristic differences in the activity of catalysts as well as in the period and the amplitude of the oscillations are related to the size of the palladium clusters and can be simulated by adding the state of the oxidation of the metal surface as a parameter to a common kinetic model. The analysis of observed chaotic behaviour leads to the conclusion that diffusional chaos characteristic of a distributed system is observed on the level of the zeolite crystallite that supports the palladium clusters.     

微通道内气泡和液滴自组织行为的研究进展

微流体技术良好的可控性为制备高通量的单分散性气泡或液滴提供了新的途径,气泡和液滴的流动行为因在材料领域具有较大的应用前景而受到关注。综述了近年来微通道内气泡和液滴自组织行为的研究进展。气泡或液滴自组织晶格具有周期性的流动特征,自组织行为受分散相体积分数、液滴或气泡尺寸、聚并效应和通道构型的影响。展望了气泡和液滴自组织行为研究过程中待解决的关键科学问题,为进一步的模拟和实验研究提供了参考。     

Dynamics of Capillary-Viscous Dewetting

Abstract

In this work the kinetics of partial dewetting was investigated for the case where dewetting, driven by capillary forces, is resisted by viscous effects. Both axisymmetric and unidirectional dewetting cases were considered. The analysis extended previous investigations using the Brochard-Wyart and de Gennes (1992) relation, having a cut off of molecular size beyond which the continuum approach is no longer valid, to obtain the dynamic contact angle condition at the receding contact line, and an analogous relation at the advancing contact line at the constant liquid film thickness side. The dynamic contact angle conditions at both sides of the ridge along with the viscous dissipation in the system were considered. A general formulation that can be used for different dynamic contact angle relations was developed. Simplification led to an analytical solution for the dynamic contact angle, size of the dry zone, and width of the ridge. The validity of the approximation was considered. The dynamics of dewetting for the simplified formulation was found to be consistent with the one developed by de Gennes et al. (2010). The dewetting dynamics based on Hoffman, and Brochard-Wyart and de Gennes contact angle relations were together found to yield results in good agreement with available experimental data over the whole viscosity range. The use of the receding contact angle in the model was found to be of primary importance for the case involving contact angle hysteresis.     

Dynamics of catalytic reactions on microdesigned surfaces

We report experimental and computational studies of reaction dynamics on Pt/Rh and Pt/TiO₂ microcomposite catalytic surfaces. Reaction fronts initiated at the material interface dominate both steady and dynamic behavior of the composite catalytic material. Our analysis links the transient phenomenon of front initiation to the bifurcations of reaction–diffusion systems with active boundaries.     

Kinetic instabilities in man-made and natural reactors

Kinetic instabilities and self-organization phenomena in chemical and biological systems are reviewed and discussed from the point of view of the unified qualitative theory of non-equilibrium processes.     

Intramolecular Charge Transfer of 1-Aminoanthraquinone and Ultrafast Solvation Dynamics of Dimethylsulfoxide

The intramolecular charge transfer (ICT) of 1-aminoanthraquinone (AAQ) in the excited state strongly depends on its solvent properties, and the twisted geometry of its amino group has been recommended for the twisted ICT (TICT) state by recent theoretical works. We report the transient Raman spectra of AAQ in a dimethylsulfoxide (DMSO) solution by femtosecond stimulated Raman spectroscopy to provide clear experimental evidence for the TICT state of AAQ. The ultrafast (~110 fs) TICT dynamics of AAQ were observed from the major vibrational modes of AAQ including the ν_C-N + δ_CH and ν_C=O modes. The coherent oscillations in the vibrational bands of AAQ strongly coupled to the nuclear coordinate for the TICT process have been observed, which showed its anharmonic coupling to the low frequency out of the plane deformation modes. The vibrational mode of solvent DMSO, ν_S=O showed a decrease in intensity, especially in the hydrogen-bonded species of DMSO, which clearly shows that the solvation dynamics of DMSO, including hydrogen bonding, are crucial to understanding the reaction dynamics of AAQ with the ultrafast structural changes accompanying the TICT.     

Periodic behaviour of a class of unstructured kinetic models for continuous bioreactors

The periodic behaviour of a large dass of unstructured kinetic models for continuous bioreactors is analyzed using elementary concepts of singularity theory and continuation techniques. The class consists of models for which the utilization rate of the limiting substrate is linearly related to the rates of cell growth and product formation. The model kinetics are allowed, on the other hand, to depend on substrate, biomass and product. The stability analysis allows the derivation of general analytical conditions for the occurrence of periodic behaviour in these models. It is shown that for a number of important cases, the occurrence of oscillatory behaviour is conditioned mainly by the kinetics of product formation. The singularity theory also allows the construction of a useful picture in the multidimensional parameter space delineating the different behaviour these models can predict including bistability and stable oscillatory behaviour.     

Dynamics of inertial dewetting

The dynamics of fast dewetting is considered in both unidirectional and axisymmetric dewetting cases. The analysis provides the front and dewetting velocities along with the wave speed (Froude number larger than 1) based on the momentum and continuity equations. The model does not require any fitting parameter. In the unidirectional case, the present model results are direct, while in the axisymmetric case, a simple trial-and-error procedure is required. Culick’s law (derived for the rupture of soap films and extended to unidirectional fast dewetting velocity by other investigators) is found to be a limiting case for both the dewetting and front velocities at small values of the film thickness for both unidirectional and axisymmetric dewetting geometries. The proposed model results show favorable agreement with published experimental front velocity data at large values of the film thickness.     

Dynamics of irreversible uncoiling of a polymer chain

Using a simple dynamic model, the growth of rigidity and of the mean size of the polymer chain induced by a chemical reaction irreversibly fixing extended conformations of units has been investigated. When the reaction rate is controlled by the rotational diffusion of segments, the change in the mean length of a rigid segment and mean radius of gyration of the chain with time are described by power dependencies. For a macromolecule in a constant viscosity medium, the time needed to complete the reaction (i.e. to transform the coil into an extended rigid rod) is t_comp N³, where N is the initial number of segments in the chain. The cases of a kinetically controlled reaction and a mixed type of reaction have also been considered. The slowing down of the reaction caused by increasing the viscosity of the reaction medium has been evaluated, giving t_comp N⁷.     

Molecular Dynamics at Extended Timescales

The use of short time trajectory fragments to enhance sampling of kinetics is discussed. It is argued that partitioning of phase space with interfaces that are called milestones and conducting a large number of short time trajectories between these interfaces is a highly efficient and accurate method to calculate thermodynamics and kinetics. The method of Milestoning, which is a theory and an algorithm, enables the rapid calculations of fluxes, stationary distributions, and moments of the mean first passage time. It is based on sub-nanosecond trajectories between cell partitions (milestones) that are defined in coarse space. While the method was applied in the past to highly complex systems, we illustrate it here on a pedagogical two-dimensional system using dividers of Voronoi cells as milestones.     

Kinetic study of polyurethane synthesis using different catalytic systems of Fe,Cu, Sn,and Cr

This work presents a comparative study between alternative catalytic systems, metal‐β‐diketones complexes (iron, copper, chromium, and tin), and the commercial catalyst dibutyltin dilaurate, DBTDL, in the polyurethanes synthesis obtained from isophorone diisocyanate (IPDI) and polyols as polypropyleneglycol/diethyleneglycol and 1,6‐hexanodiol polyadipate (polyester A‐Mn = 2000 g/mol and polyester B‐Mn = 1000 g/mol) reactions. The polyurethanes synthesis was followed by the IPDI consumption in time, verified by infrared spectroscopy (FTIR) through the decrease of free NCO characteristic band at 2300–2200 cm^?1. The FTIR data was used to determine the polyurethanes formation kinetic behavior. It was verified that for the reactions with polyethers excess, DBTDL catalyst was more effective when compared to metal‐β‐diketones complexes, while for the reactions with polyester, A and B, the metal‐β‐diketones complexes were more effective. © 2009 Wiley Periodicals, Inc. JAppl Polym Sci, 2010     

Population balance models of autonomous periodic dynamics in microbial cultures. Their use in process optimization

Oscillations in continuous microbial cultures are well documented for baker's yeast. The oscillations can be modeled by unstructured population balance models in which a key cell cycle parameter, such as the division age, is a function of the substrate or product concentration. The state variables of the periodic solutions exhibit average values different from those of steady state cultures. This suggests that oscillating cultures can be used to optimize microbial processes. Models analyzed here indicate that the possible improvements of oscillating cultures over steady state cultures are insignificant when the cultures are harvested continuously but not when harvested periodically.     

Composite core-double sheath fibers based on some biodegradable polyesters obtained by self-organization during electrospinning

Obtaining core-sheath fibers by single-spinneret electrospinning is a recent and straightforward approach to prepare composite fibers. Fibers of more complex architecture consisting of poly(ethylene oxide) (PEO) core, inner poly(l -lactide) sheath sd and outer beeswax (BW) sheath may also be obtained using this method. In the present study we report its applicability for a large series of (bio)degradable polyesters such as poly(ε-caprolactone), poly(d ,l -lactide-co-glycolide), poly(butylene succinate), poly(3-hydroxybutyrate), and poly(l -lactide-co-d ,l -lactide). The fibers have a well-differentiated PEO core, polyester inner sheath and BW outer sheath. The possibility for targeted location of hydrophilic or hydrophobic substances in the core or in the sheaths of the PEO/polyester/BW fibers has been demonstrated using nanosized zinc oxide with unmodified (hydrophilic) or silanized (hydrophobic) surface. PEO/polyester/BW fibrous materials loaded with a model drug (5-nitro-8-hydroxyquinoline) exhibit antimicrobial activity. The obtained results show that single-spinneret electrospinning is a novel and versatile method to prepare core-double sheath composite fibers prospective for various applications such as biomedicine, cosmetics, and food packaging.     

中低负荷下脉动热管壁温信号的分形特性分析

运用功率谱、小波分解、相空间重构、关联维数计算等非线性分析方法对脉动热管壁面温度波动信号分析。表明:功率谱呈现连续谱线,时间序列存在自相似性,展现温度波动信号的混沌行为;小波分解表现出温度波动的分形特性;重构吸引子展现出4种不同的空间分形结构,说明温度信号波动属于分形下的混沌行为。计算分形维数与传热性能的关系,发现系统存在3~6个分形维;随嵌入维数的增加,分形速度关系为R134a大于丙酮大于去离子水,基本表现为分形维数越大,传热性能越好。并发现关联维和热阻成负相关关系。     

Spatial simulations in systems biology: from molecules to cells

Cells are highly organized objects containing millions of molecules. Each biomolecule has a specific shape in order to interact with others in the complex machinery. Spatial dynamics emerge in this system on length and time scales which can not yet be modeled with full atomic detail. This review gives an overview of methods which can be used to simulate the complete cell at least with molecular detail, especially Brownian dynamics simulations. Such simulations require correct implementation of the diffusion-controlled reaction scheme occurring on this level. Implementations and applications of spatial simulations are presented, and finally it is discussed how the atomic level can be included for instance in multi-scale simulation methods.     

Butadiene polymerization with vanadium–titanium–aluminum catalytic systems

Butadiene polymerization in the presence of mixed vanadium–titanium–aluminum catalytic systems containing various organoaluminum compounds (OACs) was investigated. The main factors influencing the activity and stereospecificity of the [VOCl₃–TiCl₄–OAC₁]–(heating)–OAC₂ catalysts [where OAC₁ and OAC₂ were Al(i‐Bu)₃, Al(i‐Bu)₂H, or Al(i‐Bu)₂Cl] were considered. The kinetic parameters of the process were determined. The high activity and stereospecificity of the multicomponent systems probably accounted for the formation of polymerization active sites with both transition‐metal derivatives in their structure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 211–217, 2004