Step Response Experiments Research Articles

Investigation of NO dissociation and reduction by H 2 on nanosized W 2N surface with transient-response techniques

Nanosized W 2N was prepared by NH 3-temperature-programmed reaction and characterized by XRD, transmission electron microscopy (TEM), BET, X-ray photoelectron spectroscopic (XPS) and temperature-programmed desorption (TPD) techniques. Transient-response experiments were applied in order to improve understanding of the mechanism of NO reduction with H 2 on W 2N surface. Switches of He into 1% NO/0–7% H 2/He gas streams on W 2N surface were performed at 500 °C. The 1% NO/He step-response experiment showed that NO dissociation in the absence of H 2 was not effective on W 2N surface because the surface oxygen from dissociated NO was strongly bond to the catalyst surface, poisoning the NO dissociation sites and inhibiting further NO dissociation. In the 1% NO/1–7% H 2/He step-response experiments, N 2, NH 3, H 2O and N 2O (little or none) were detected as reaction products. The formation of H 2O on the catalyst surface was the most crucial step for NO dissociation to proceed and that it depended on the NO:H 2 ratio in the reaction mixture. Reasonable NO:H 2 ratio of 1:5 in feed gas was determined to keep the catalyst surface sites active for the catalytic dissociation of NO. Reaction pathways explaining the reduction of NO with H 2 were proposed.

On the different roles of isocyanate and cyanide species in propene-SCR over silver/alumina

Silver/alumina is one of the most promising catalyst materials for HC-SCR. The reaction mechanism remains incompletely understood, however. Isocyanate species ( NCO) and cyanide species ( CN), important intermediates in the reaction path in HC-SCR, were investigated by gas-phase FTIR and DRIFT spectroscopy in step-response experiments. In accordance with the literature, the corresponding gas-phase species, HNCO and HCN, were always detected at the same time at the different steps of the experiment; however, the accumulation and consumption of surface NCO and CN species did not show a similar correlation. Consequently, in conflict with the literature, we propose two parallel, independent reaction pathways for NCO and CN species. In one of these reaction pathways, surface nitrates react with C-containing surface species either directly to NCO or via R NO 2 species. In the other pathway, C-containing surface species react with gas-phase NO x or short-lived N-containing surface species to R NO species and subsequently to CN species.

Dynamic methods for catalytic kinetics

The application of various transient techniques in heterogeneous catalysis (TAP, step-response experiments, SSITKA, TEOM), with the aim to determine reaction kinetics for design purposes, is presented for several cases. These cases, comprising catalytic cracking, diffusion in zeolites, simultaneous NO, and SO, removal, syngas production from methane by chemical looping and selective catalytic reduction of NOx, show that transient techniques can be well used for the purpose of rapid determination of the reaction kinetics without the laborious classical approach of steady-state kinetic measurements and without the need of high levels of sophistication to interpret and process the experimental data. In this respect transient kinetics deserve, next to fundamental catalysis studies, more frequent application in design Studies for industrially relevant reaction systems. Topics and challenges for further developments in transient studies are indicated. (c) 2008 Elsevier B.V. All rights reserved.

Detection of 3rd mechanism in renal blood flow via high‐resolution, time‐varying spectral analysis of records subjected to chirp forcing of blood pressure

It is generally agreed that renal autoregulation is mediated by myogenic (0.01–0.3 Hz) and tubuloglomerular feedback (0.02–0.05 Hz) mechanisms. Recently a potential 3rd mechanism that operates at ~0.01 Hz has been reported based upon step response experiments. To begin its characterization, 6 anesthetized, normotensive rats were instrumented for blood pressure (BP) and renal blood flow (RBF) measurements. We forced BP with linearly increasing frequencies (frequency range 0.001 – 0.02 Hz, amplitude ± 7 mmHg). The motivation for using linearly increasing frequencies in BP forcing is that if such a third mechanism exists, it should resonate at its characteristic frequency in the resultant RBF recordings and that RBF power at that frequency should be amplified beyond that in BP. Comparison between the time‐varying spectral amplitudes of BP versus RBF shows that when the BP forcing frequency passed ~0.01 Hz, there was greater spectral amplitude at the same frequency in RBF than in BP. The significance of this increase of spectral amplitude in RBF was verified by testing against white noise signals. Any spectral power above white noise spectral amplitude suggests that such oscillation does exist and not a consequence of some random occurrence. We conclude that a third mechanism contributes to autoregulation of RBF and that it resonates at ~0.01 Hz. Funded by CIHR & NHLBI

Optimizing Control of Hot Blast Stoves in Staggered Parallel Operation

An optimizing control scheme is designed for the minimization of the energy consumption in the hot blast stoves. By an appropriate selection of the control structure, the problem of energy minimization at steady-state in the presence of constraints is formulated as a constrained optimal control problem. A model predictive control (MPC) scheme is designed based on a simple linear control model, which is obtained from step response experiments on a detailed dynamical model of the process. The control model is augmented with an integrating disturbance model to compensate the steady-state offset. The performance of the MPC scheme is tested on the detailed dynamical model.

Modelling and Control of Consumable Double-Electrode Gas Metal Arc Welding Process

Consumable double-electrode gas metal arc welding (DE-GMAW) is an innovative process, which offers unique advantages to increase productivity and reduce heat distortion. However, it must be appropriately feedback controlled for any practical use. To this end, the authors identified two critical variables as outputs for the control system to be developed. The control variables were then selected to ease the modeling and control design through establishing two decoupled SISO subsystems. Each SISO subsystem was modeled as an interval model, whose parameters were unknown but bounded by known intervals. The intervals were obtained through a set of designed step response experiments. A prediction based interval model control algorithm was then implemented to control the resultant interval models. Closed-loop control experiments verified the effectiveness of the developed control system for this novel welding process.

Catalytic oxidation of CO over ordered mesoporous platinum

Hexagonal phase mesoporous (H 1 Pt) was recently reported to have different catalytic properties compared to conventional platinum catalysts. To further investigate this observation the catalytic activity of H 1 Pt/Al 2O 3 for CO oxidation was compared with the activity of a corresponding catalyst prepared from Pt-black (Pt-black/Al 2O 3). The H 1 Pt/Al 2O 3 catalyst showed ignition at lower temperatures but extinction at higher temperatures compared to Pt-black/Al 2O 3. These observations were further supported by oxygen step-response experiments at constant temperature, where the H 1 Pt/Al 2O 3 catalyst showed ignition at lower oxygen concentrations when starting from a CO poisoned surface and extinction at higher O 2 concentrations when starting from the high-reactive state. Furthermore, adsorption of CO on the catalysts was studied in situ using infrared spectroscopy in absence and presence of oxygen after pre-oxidation and pre-reduction, respectively. At 150 °C the H 1 Pt/Al 2O 3 sample showed activity for CO oxidation in the presence of oxygen regardless of pretreatment, whereas Pt-black/Al 2O 3 was inactive due to CO self-poisoning. The differences observed in the low reactive state are suggested to be due to structural differences of the platinum surface in the catalysts resulting in a lower sensitivity of the H 1 Pt/Al 2O 3 catalyst towards CO self-poisoning and a higher capacity to activate oxygen, and thus a higher activity for CO oxidation. During the high reactive state, the observed higher sensitivity to the concentration ratio between CO and oxygen, and to the temperature is likely due to less optimal ratio between the sticking coefficients of the reactants on the H 1 Pt catalyst and to higher mass-transport limitations in its narrower pores during the initial stage of the extinction.

Liquid-Phase Residence Time Distribution for Two-Phase Flow in Coiled Flow Inverter

The coiled flow inverter (CFI) is an innovative device, which has potential for the intensification of processes currently carried out in conventional mixers. Step response experiments were carried out in a CFI to study liquid-phase residence time distribution (RTD) for gas-liquid flow under the conditions of both negligible and significant molecular diffusion using Newtonian fluids. A total of 16 CFIs of different curvature ratios ranging from 6.7 to 20, dimensionless pitch from 1 to 2.5, and number of bends from 1 to 15 were investigated. The range of Dean numbers for the gas and the liquid phase was varied from 235 to 1180 and 3.16 to 1075, respectively. The reduction in dispersion number is nearly 2.6 times for two-phase flow in CFI with 15 number of bends as compared to a straight helix under identical process conditions. A modified axial dispersion model is proposed to describe the RTD. The efficiency of the device is characterized by a mixing criterion.

Voltage-Loop Power-Stage Transfer Functions With MOSFET Delay for Boost PWM Converter Operating in CCM

Open-loop transfer functions can be used to create closed-loop models of pulsewidth-modulated (PWM) converters. The closed-loop small-signal model can be used to design a controller for the switching converter with well-known linear control theory. The dynamics of the power stage for boost PWM dc-dc converter operating in continuous-conduction mode (CCM) are studied. The transfer functions from output current to output voltage, from duty cycle to output voltage including MOSFET delay, and from input voltage to output voltage are derived. The derivations are performed using an averaged linear circuit small-signal model of the boost converter for CCM. Experimental Bode plots and step responses were used to test the accuracy of the derived transfer functions. The theoretical and experimental responses were in excellent agreement, confirming the validity of the derived transfer functions

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature ( T < 250 °C) where NO chemisorption takes place and high temperature ( T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and 15NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N 2 and CO 2 to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N 2 emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.

Magnetic characteristics of electromagnetic levitation system for short‐radius curve track

AbstractA magnetically levitated (MAGLEV) vehicle using electromagnets and iron rails generates lateral guiding force naturally by controlling levitation force to maintain the air gaps between the magnets and the iron rails. A MAGLEV system without guide magnets offers simple design and cost advantages. But such a system has little lateral damping force. In order to improve damping characteristics, an experimental apparatus using salient‐pole‐type magnets and laminated iron rails was made. The iron cores were placed in two rows. The suspension coils surrounded both of the magnet iron cores and the damping coils surrounded every magnet core separately. The damping coils were excited by currents proportional to the lateral displacement velocity. According to a step response experiment, the lateral motion dissipated after a cycle or two of the swing. The self‐inductance of the suspension coil of the magnet was almost unchanged against variation of the lateral displacement. The influence of damping current on the levitation was very small. This paper discusses the electromagnetic characteristics of the levitation system. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(3): 71–80, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20372

Catalytic and mechanistic study of lean NO 2 reduction by isobutane and propane over HZSM-5

In this study, the selective catalytic reduction (SCR) of NO 2 by two saturated hydrocarbons, propane and isobutane, has been investigated over acidic zeolite HZSM-5. The activity for NO 2 reduction and the selectivity for N 2 formation correlate well with the intracrystalline acidity (Brønsted acid sites) of the zeolite. Isobutane is a more efficient reducing agent than propane due to its tertiary hydrogen being more readily activated than the secondary hydrogen in propane. In situ DRIFT step–response experiments with NO 2 and isobutane show formation of surface bound NO +, isocyanate, unsaturated hydrocarbons and amine species. The NO + species seems to play an important role in the reduction of NO 2 probably by reacting with alkenes formed from carbenium ion adsorbates. These species likely form isocyanates, amine species and finally nitrogen.

The positive effect of NO on the N 2O decomposition activity of Fe-ZSM-5: A combined kinetic and in situ IR spectroscopic study

NO-assisted N 2O decomposition over four different Fe-ZSM-5 samples prepared by wet ion exchange (WIE) or chemical vapor deposition (CVD) of FeCl 3 was investigated by steady-state kinetics, in situ infrared (IR) spectroscopy, and transient response methods. Despite their lower iron loading, the samples prepared by WIE had the highest activity for N 2O decomposition in the presence of NO. The in situ IR experiments showed that the most active sample was characterized by a high concentration of adsorbed NO, as well as adsorbed NO 2 under reaction conditions. Step response experiments proved that NO 2 is an intermediate of the catalytic cycle and functions as intermediate oxygen storage. IR and transient kinetic experiments showed that WIE catalysts behave qualitatively different in NO-assisted N 2O decomposition than CVD catalysts. These differences are discussed in terms of the different structure of the iron species in the two types of samples.

A look into the surface chemistry of N 2O decomposition on iron zeolites by transient response experiments

The mechanism of decomposition of N 2O on Fe-ZSM-5 prepared by chemical vapor deposition was studied by step and pulse response experiments and by isotope labeling. The results show that only a very small fraction of the iron sites is taking part in the catalytic cycle. N 2O deposits its oxygen atoms on these sites, the deposited oxygen atoms migrate over the iron cluster, exchange with other oxygen atoms and then desorb by recombination to O 2. On strongly dehydroxylated iron sites also a direct recombination of two oxygen atoms, which were deposited from N 2O, is possible. The concentration of these sites increases after treatment of the samples at high temperatures in inert gas. The relation between the sites involved in N 2O decomposition and the α-sites, which were first described by the group of Panov, is discussed. We can conclude that the deposition of N 2O on Fe-ZSM-5 CVD catalysts creates active oxygen species on the catalyst surface (in particular, after pretreatments at high temperature), but their properties are different from those of α-oxygen.

Mechanistic study of the influence of surface acidity on lean NO 2 reduction by propane in HZSM-5

This study focuses on the mechanism of lean NO x reduction by propane, over acidic zeolites (HZSM-5), and the influence of surface acidity. In situ FTIR measurements of ammonia adsorption indicate a higher number of Brønsted acid sites for a sample with a low SiO 2/Al 2O 3 ratio. The activity for NO x reduction and the selectivity for N 2 formation correlate well with the Brønsted acidity. Step-response experiments with NO 2 and propane show the formation of surface-bound NO +, isocyanate, unsaturated hydrocarbons, and amine species. Formation of the latter two seems to be closely related to the Brønsted acidity. In the NO 2 reaction with propane, the NO + species seem to play a vital role, probably reacting with carbenium ions (from propane cracking) to form isocyanates, which may be hydrolysed to amine species. Step-response experiments with isopropylamine and NO 2 indicate a fast reaction, where the amine and NO + species react over Brønsted acid sites. Hence, amine species are possible reaction intermediates in the lean reduction of NO 2 by saturated hydrocarbons, such as propane, over HZSM-5.

小半径軌道用磁気浮上車の支持案内兼用電磁石の電磁力特性

A magnetically levitated (MAGLEV) vehicle using electromagnets and iron rails generates lateral guiding force naturally by controlling levitation force to maintain the air gaps between the magnets and the iron rails. A MAGLEV system without guide magnets offers simple design and cost advantages. But such a system has little lateral damping force. In order to improve damping characteristics, an experimental apparatus using salient-pole-type magnets and laminated iron rails was made. The iron cores were placed in two rows. The suspension coils surrounded both of the magnet iron cores and the damping coils surrounded every magnet core separately. The damping coils were excited by currents proportional to the lateral displacement velocity. According to a step response experiment, the lateral motion dissipated after a cycle or two of the swing. The self-inductance of the suspension coil of the magnet was almost unchanged against variation of the lateral displacement. The influence of damping current on the levitation was very small. This paper discusses the electromagnetic characteristics of the levitation system. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(3): 71–80, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20372

Effect of periodic operation on the low-temperature activity for propane oxidation over [formula omitted] catalysts

We report experimental results for the oxidation of propane over alumina supported platinum catalysts with varying Pt dispersions. Flow-reactor experiments introducing 0.15 vol% C 3 H 8 while changing the oxygen concentration step-wise (step-response experiments) from rich to lean composition or periodically (pulse-response experiments) switching between rich and lean O 2 concentrations, at a constant inlet gas temperature of 250 °C have been performed. Complementary in situ FTIR spectroscopy experiments for surface analysis have also been carried out. The results reveal a strong correlation between the reactant composition and the catalytic activity, showing an optimum reaction rate for compositions close to the stoichiometric value for complete oxidation of propane. For lean gas compositions the activity is low probably due to a high O-coverage (O self-poisoning) which in turn is likely to also cause platinum oxide formation. However, by periodically introducing rich periods (periodic operation) the activity can temporarily be restored and the surface composition can be kept close to the optimum, leading to an overall improvement of the oxidation rate. Generally, the introduction of short rich periods to a slightly lean mixture (lean–rich cycling) seems to be more efficient than the corresponding rich–lean cycling since the latter suffers from oxygen deficiency limiting the reaction during the long periods.

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O 2 concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature. As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO 2 production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO 2 peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO 2 production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO 2 production rate drops abruptly as the reaction “quenches”. Catalyst layer thickness was changed over the range 0.06–1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO 2 production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.

The surface chemistry of N 2O decomposition on iron-containing zeolites (II)—The effect of high-temperature pretreatments

The effect of pretreatment conditions on the N 2O-decomposition activity of Fe-ZSM-5 prepared by chemical vapor deposition (CVD) was studied by steady-state kinetics and step-response experiments. Pretreatment at 873 K in H 2 or He significantly increased the N 2O-decomposition activity of Fe-ZSM-5 CVD. In addition to the increased steady-state activity, the catalyst exhibited a very high initial activity in the step-response experiments, which slowly decayed to steady state. This “transient” activity was particularly high after pretreatment of the samples in He, whereas it vanished completely after pretreatment in O 2 or exposure to water vapor. This led to the conclusion that the transient activity is related to strongly dehydroxylated Fe 2+ sites created by autoreduction at high temperatures. On the basis of isotope-labeling data a mechanism for N 2O decomposition is proposed. The first step is the reaction of Fe 2+ with N 2O to give a Fe 3+ O − species, which quickly transforms into a Fe 2+–peroxo complex. Desorption of O 2 occurs upon reaction of the Fe 2 + O 2 2 − complex with another N 2O molecule (mainly during the transient) or via the recombination of two peroxo species after their migration over the iron oxide cluster (mainly during steady state).

Low-Temperature CO Oxidation over a Pt/Al2O3Monolith Catalyst Investigated by Step-Response Experiments and Simulations

The ignition–extinction processes for CO oxidation over a Pt/Al2O3 monolith catalyst have been studied by flow-reactor experiments and simulations. The study was performed by stepwise changes of the inlet O2 concentration ranging 0–20 vol% while the CO concentration and the inlet gas temperature were kept constant at 1.0 vol% and 423 K, respectively. Several features observed experimentally are qualitatively simulated with our model: (i) the ignition of the CO oxidation demands 8.0vol% O2 (ii) corresponding to a catalyst ignition temperature of 433 K (due to the exothermicity of the reaction) and (iii) occurs in the rear part of the monolith where (iv) a local reaction zone is formed which (v) moves towards the reactor inlet as a function of time on stream. Additionally, the simulations show first order kinetic phase transitions, i.e. rapid adsorbate concentration changes, where the catalyst surface is predominantly CO covered in the low reactive state and almost completely oxygen covered in the high reactive state. For the ignition process the kinetic phase transition occurs after the actual catalytic ignition. However, the extinction process is more difficult to simulate dynamically without changing the model parameters for O2 adsorption in the low and high reactive state, respectively. The influence of diffusion limitations and the role of formation of a less reactive Pt state under oxidising conditions is discussed.