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The embodiment of the NOx selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so-called SCR-on-Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu-Chabazite and Fe-ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NOx conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60-80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu-Chabazite and Fe-ZSM5 catalysts, as well as in-series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238-248, 2017 [ABSTRACT FROM AUTHOR]

The relationship between fuel penalty and precious metal loading in coupled LNTSCR systems for NOx reduction at low temperatures is determined. Simulations of adiabatic, aerobic NOx reduction determine the impact of catalyst architecture, precious metal loading, cycle time, catalyst length and the support material. These revealed that (a) high NOx conversion can be achieved for a given precious metal loading at low temperatures (510 K) with about 0.3% fuel penalty, (b) shortened cycle time and increased pulse duty reduce the overall NOx slip from the coupled catalyst and lead to significant improvement in the NOx conversion, (c) alternate arrangement of the LNT/SCR catalysts increases the NOx conversion, (d) a monotonic decreasing PGM loading in LNT leads to higher NOx conversion compared to uniform loading, (e) for a fixed catalyst volume, there is an optimal aspect ratio that maximizes the nonisothermal effect, and (f) metal based monolith supports improve NOx conversion. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3421-3431, 2013 [ABSTRACT FROM AUTHOR]

The article discusses the findings of a study which simulated and predicted the dependence of temporal temperature increases on changes in the exhaust feed temperature of a diesel particulate filter (DPF). The study determined the effect of exhaust temperature changes in oxygen concentration, filtration velocity, and the development of control protocols. The simulations predicted that the magnitude of the wrong-way behavior depended on the initial bed temperature, amplitude of the temperature decrease and initial particulate matter (PM) loading.

The article presents a study which aimed to determine the magnitude of the temperature rise of individual supported metallocene catalyst particles and its dependence on the reactor temperature, initial catalyst particle size and the addition of hydrogen to the feed. It has been found that the maximum transient temperature rise of single commercial catalyst particle during ethylene polymerization is lower than that which can lead to local melting and polymer sheet formation. However, the addition of hydrogen to the feed significantly increased the initial particle temperature rise.

Recent analysis revealed that a pseudohomogeneous model of a uniformly active, adiabatic packed bed reactor can predict formation of stable hot zones in the cross-section of the reactor if the kinetic rate expression can lead to isothermal rate oscillations. This prediction was confirmed by simulations of CO oxidation. We show that hot zone formation in a shallow reactor can be predicted also for C2H4 hydrogenation, the kinetic model of which is structurally different from that of CO oxidation. Qualitatively different spatiotemporal temperature patterns may form under the same operating conditions. Their number increases as the reactor diameter is increased. An increase in the reactor diameter increases the time constant of the transversal heat dispersion and decreases the temperature synchronization among points on the same reactor cross-section. The interaction and conjugation among qualitatively different moving temperature patterns can lead to formation of complex motions. [ABSTRACT FROM AUTHOR]

The processability of various polyolefins may be enhanced by producing one having a bimodal molecular weight distribution. Such polymers can be produced by loading the reactor with a catalyst on which two different catalytic sites are impregnated. The catalysts must be loaded with a proper ratio between the two catalytic sites to produce the desired polymer. The total loading should enable a maximum polymer production in the specified residence time while keeping the maximum transient temperature below a specified maximum, in order to avoid local melting and sheet formation. A model is presented that enables this catalyst design based on kinetic information about the initiation, propagation, and deactivation reaction rates of the two individual catalytic sites. The model simulations provide insight about the factors affecting the maximum safe loading of the two catalytic sites. We show here how to utilize kinetic information about each of the two catalytic sites to predict the optimum loading of dual-site metallocene catalysts. As the maximum temporal temperature occurs during the initial stages of the reaction, the site that leads to a faster generation of a temperature peak, exerts a stronger bound on the total polymer production than the second site. The larger the propagation and/or the initiation rate constants or their activation energies are, the lower should be the catalyst loading. The deactivation rate constants affect both the relative site loading of the two sites as well as the total production. The total polymer production can be increased by decreasing the deactivation rate of the more active site. [ABSTRACT FROM AUTHOR]

Transversal hot zones have been observed in many industrial and laboratory reactors. Yet, previous models which utilized rate expressions that depended only on the temperature and concentration of the limiting reactant failed to predict formation of stable hot zones. We previously proved that pseudohomogeneous models of a uniformly active shallow adiabatic packed-bed reactor cannot predict a bifurcation to stable hot zones when the transversal heat dispersion is larger than that of the species, and the reaction rate depends only on the temperature and concentration of the limiting reactant. We prove here that the same is true also for a two-phase model of a uniformly active shallow, adiabatic packed-bed reactor. It is shown that a shallow reactor model which uses more detailed kinetic expression, may attain stable, transversal hot zones. [ABSTRACT FROM AUTHOR]

Transversal hot zones have been observed in industrial and laboratory packed-bed reactors. Yet, previous modeling attempts failed to predict them without making the unrealistic assumption that the transversal heat dispersion exceeds that of the reactants. It is shown that the formation of transversal hot zones in a uniformly active catalytic reactor is strongly dependent on the reaction kinetics. For example, transversal spatiotemporal concentration and temperature patterns can be predicted to form in a shallow adiabatic packed-bed reactor using realistic parameters for a catalytic reaction, the rate of which may oscillate under constant ambient conditions. Various experimentally tested rate expressions, such as those describing the oxidation of CO, exhibit this feature. A large number of different types of stable, transversal patterns may form for a sufficiently large reactor diameter, most of which do not exhibit azimuthal symmetry. Surprisingly, the time-averaged effluent reactant concentration and the period of the different spatiotemporal patterns are rather similar and close to those obtained under a uniform oscillating state. A systematic procedure is presented for finding the initial conditions leading to formation of the different patterns. Numerical simulations show that, because of their homoclinic features, the spatiotemporal patterns have a long period (order of hours), in agreement with various reported laboratory experiments. [ABSTRACT FROM AUTHOR]

A transient electric voltage of the order of 1 V may form during the combustion of a metal particle. The electrical field may form due to the different diffusion rate of the positive and negative charge carriers through a growing mixed-ionic-electronic conducting oxide shell. Unlike previous models, the effect of ambient gaseous oxygen transport to the particle surface is accounted for and steady-state and charge neutrality in the oxide layer is not a priori assumed. Four key parameters affect the voltage-temperature formation. The ratio between charge carries diffusion coefficients determines the surface electric potential amplitude, polarization and the time delay between the maximums of particle potential and temperature. The larger the difference between the diffusion coefficients, the greater is the voltage and the longer is the combustion time and the shorter is the time delay. The convection and radiation heat-transfer coefficients exert a major impact on the time delay between the maximum of the surface electric potential and that of the temperature. The maximum electric potential and temperature are achieved almost at the same time when the transport coefficients are sufficiently large. This time difference decreases as either the heat-transfer coefficients or the difference between the diffusion coefficients increase. Increasing the activation energy of the adsorption strongly decreased the oxidation, temperature and surface electric potential rate of rise during the early stage of the oxidation. Increasing the activation energy of the desorption increased the duration of the temperature and the potential drop in the later stage of the reaction. [ABSTRACT FROM AUTHOR]

Experiments revealed that a transient electric voltage of the order of 1 V may form during the combustion of a single metal particle. We present a model showing that such an electrical field may form as a result of the different diffusion rates of the positive and negative charge carriers through a growing mixed-ionic-electronic-conducting oxide shell. Simulations of the evolution of the electric charge carriers and temporal potential distribution reveal that three key parameters affect the voltage formation. The ratio of the characteristic oxygen ion diffusion rate within the particle to the maximum oxygen transport rate to the particle surface affects the formation of a diffusion layer and the carrier distributions inside the oxide shell. The ratio between the thickness of an exterior oxide layer, formed before the reaction has started, and the Debye screening length has a strong impact on the amplitude and temporal evolution of the voltage. The thinner this initial oxide shell is, the larger is the voltage amplitude. The ratio between the diffusion coefficients of the positive and negative charge carriers determines the sign of the excess electrical charge on the surface. [ABSTRACT FROM AUTHOR]

The combustion of metal particles may generate a temporal electrical field due to a difference in the electrochemical potential across the growing oxide shell, which usually is a mixed ionic electronic conductor. A novel measurement technique enables simultaneously measurement of the temporal electrical field and temperature generated by a planar reaction wave during highly exothermic gas-solid reactions. The voltage formed after the rate of the temperature rise reached its maximum value, but before the temperature attained its maximum. For both simple (Ti02, Fe2O3, MgO) and complex oxides (LiMn204, SrFe12O19, PbFe12O19 BaTiO3) the voltage grew to its maximum value within 0.2-0.6 s, irrespective of the distance between the measuring probes. The decay of the voltage signal was slower than its rise. Increasing the distance between the measuring probes increased the period during which the voltage decayed. The difference between the time at which the temperature attained its maximum and the electrical signal vanished depended on both the distance between the two probes, and on the difference in time at which the voltage and temperature attained their maximum. Changing the exothermicity of the reaction (by dilution of the reactant mixture) can shift the time of the voltage decay relative to that at which the maximum temperature occurs. [ABSTRACT FROM AUTHOR]

The article presents a study which discussed oxygen flux increases via mixed ionic-electronic conductor (MIEC) membranes by enhanced surface exchange. According to the authors, oxygen flux through MIEC membrane is influenced by both surface exchange and bulk diffusion processes. Model analysis and experiments show that under mixed transport control, the outwards flux exceeds the inwards flux through a bare MIEC tube.