Oscillations, glow and ignition in carbon monoxide oxidation in an open system. I. Experimental studies of the ignition diagram and the effects of added hydrogen

1985 ◽  
Vol 397 (1812) ◽  
pp. 21-44 ◽  
Keyword(s):  
Carbon Monoxide ◽  
Open System ◽  
Reaction Rate ◽  
Experimental Studies ◽  
Reaction Rates ◽  
Step Change ◽  
Stable Node ◽  
Stationary States ◽  
Show Evidence ◽  
Oxidation Of Carbon Monoxide

The oxidation of carbon monoxide in equimolar mixtures (CO + O 2 ) has been studied in a well-stirred open system (0.5 dm 3 ) at vessel temperatures in the range 700-840 K, and reactant pressures up to 100 Torr ( ca . 13.3 kPa) at a mean residence time of 8.5 s. Stationary states are established and oscillatory states sustained indefinitely in this system. The effect of small quantities of added hydrogen is studied by a carefully controlled, continuous supplement to the principal reactants. Four different modes of reaction (I-IV) have been characterized, and conditions for their occurrence mapped on a reactant pressure-vessel temperature ( p - T a ) ignition diagram. Most boundaries are quite sharp, and some show evidence of hysteresis. Close to the axes, reaction is slow, non-luminous and non-oscillatory (I). Within a first broad promontory (II) reaction is accompanied by steady luminescence. Crossing the boundary is not accompanied by a step change in reaction rate, but there is a change in character from stable node (in I) to stable focus (in II). Auto-oscillatory luminescence occurs in a closed region (III) wholly within the promontory II. The effects of adding hydrogen on all these modes is to increase the reaction rates markedly and to make them non-isothermal; the boundaries between I, II and III are not as greatly affected. However, systems to which more than 0.10% H 2 have been added also display a new mode, of oscillatory ignition. This appears at first in a region (IV) of high temperatures and pressures but as more H 2 is increased its realm expands and it eventually dominates the ignition diagram, invading the region of luminescence and soon obliterating the oscillatory part completely.

Oscillations, glow and ignition in carbon monoxide oxidation in an open system II. Theory of the oscillatory ignition limit in the c. s. t. r

1985 ◽  
Vol 402 (1823) ◽  
pp. 187-204 ◽  
Keyword(s):  
Carbon Monoxide ◽  
Open System ◽  
Flow System ◽  
Ignition Limit ◽  
Critical Conditions ◽  
Self Heating ◽  
Algebraic Expressions ◽  
Radical Concentration ◽  
The Mean ◽  
Oxidation Of Carbon Monoxide

The oxidation of carbon monoxide in the presence of hydrogen can produce a single ignition pulse in a closed vessel and repetitive, i. e. oscillatory, ignition in an open system. It is possible to predict the locus of critical conditions on a map of reactant pressure, p , against vessel temperature, T a , in a flow system by a treatment based on the change in local stability of the stationary state. Even the very simplest kinetic model for the CO + H 2 + O 2 reaction allows satisfactory predictions of the dependence of the critical pressure on T a , and of the displacement of such p – T a peninsulae as the mixture composition (CO : H 2 ratio) is varied. Many of the results can be obtained in terms of simple algebraic expressions. The relation between this approach and classical treatments of criticality based on the unbounded growth of the steady-state radical concentration or on tangency conditions (chain–thermal theory) is investigated. Oscill­atory periods (the interval between successive ignition pulses) are calcu­lated, and the variation in the mean residence time arising from the change in the number of moles during reaction and the accompanying self-heating is discussed.

EFFECTS OF FINITE REACTION RATES ON THE KINETIC PHASE TRANSITIONS IN THE CATALYTIC OXIDATION OF CARBON MONOXIDE

2002 ◽  
Vol 09 (05n06) ◽  
pp. 1735-1739
Author(s):  
L. D. LÓPEZ-CARREÑO
Keyword(s):  
Carbon Monoxide ◽  
Phase Transitions ◽  
Steady State ◽  
Catalytic Oxidation ◽  
Active Sites ◽  
Nearest Neighbor ◽  
Reaction Rates ◽  
Neighbor Site ◽  
State Configuration ◽  
Oxidation Of Carbon Monoxide

Oxidation of carbon monoxide is one of the most extensively studied heterogeneous catalysis reactions, being important among other applications in automobile-emission control. Catalytic oxidation of carbon monoxide on platinum (111) surface was simulated by the Monte Carlo technique following an extended version of the model proposed by Ziff, Gulari and Barshad (ZGB). In the simulation, a simple square two-dimensional lattice of active sites replaces the surface of the catalyst. Finite reaction rates for (i) diffusion of the reactive species on the surface, (ii) reaction of a CO molecule with an oxygen atom in a nearest neighbor site, and (iii) desorption of unreacted CO molecules, have been taken into account. The produced CO 2 desorbs instantly. The average coverage of O, CO and the CO 2 production rate for a steady state configuration, as a function of the normalized CO partial pressure (P CO ), shows two kinetic phase transitions. In the ZGB model these transitions occur at P CO ≈ 0.39 and P CO ≈ 0.53. For 0.39 < P CO < 0.53 a reactive ( CO 2 production) steady state is found. Outside of the interval, the only steady state is a poisoned catalyst of pure CO or pure O. Our results show that finite reaction rates shift the values in which these phase transitions occur.

CFD Modeling Considering Different Kinetic Models for Internal Reforming Reactions in an Anode-Supported SOFC

2010 ◽  
Author(s):  
Hedvig Paradis ◽  
Martin Andersson ◽  
Jinliang Yuan ◽  
Bengt Sunde´n
Keyword(s):  
Fuel Cells ◽  
Kinetic Models ◽  
Reaction Rate ◽  
Experimental Studies ◽  
Reaction Rates ◽  
Transport Processes ◽  
Cfd Modeling ◽  
Internal Reforming ◽  
Partial Pressures ◽  
Shift Reaction

Fuel cells are electrochemical devices that transform chemical energy into electricity. Solid oxide fuel cells (SOFCs) are particularly interesting because they can handle the reforming of hydrocarbon fuels directly within the cell. This is possible due to their high operating temperature. The purpose of this study is to develop an anode-supported SOFC model, to enhance the understanding of the internal reforming and effects on the transport processes. In this study, a CFD approach, based on the finite element method, was implemented for the analysis to unravel the interaction between internal reforming, momentum, heat and mass transport. COMSOL Multiphysics is used to analyze the effects of different global kinetic models available for the steam reforming reaction. The three different reaction rates applied in this study were developed and correlated through experimental studies found in the literature. An equilibrium equation is implemented for the reaction rate for the water-gas shift reaction. The partial pressures and the related reaction order of the pressure are found to affect the reaction rate.

Oscillatory ignitions and cool flames accompanying the non-isothermal oxidation of acetaldehyde in a well stirred, flow reactor

1981 ◽  
Vol 374 (1758) ◽  
pp. 313-339 ◽  
Keyword(s):  
Reaction Rate ◽  
Light Emission ◽  
Flow Reactor ◽  
Open Systems ◽  
Negative Temperature ◽  
Stable Node ◽  
Stationary States ◽  
Residence Times ◽  
Self Heating ◽  
Cool Flames

Oxidation, cool flames and ignition in equimolar mixtures of acetaldehyde and oxygen have been studied in a well stirred, continuous-flow reactor (an open system), over temperatures from 450 to 625 K and pressures from 5 to 25 kN m -2 . The reactor is a 500 cm 3 , spherical, glass vessel, ca . 10 cm in diameter, and is stirred mechanically at a revolution rate of up to 1200 min -1 . Residence times can be varied down to a few seconds; our work relates to 10, 7, 5 and (mainly) 3 s. These conditions broadly resemble those in which transient cool-flame phenomena can be seen in closed vessels, and for which steady-state calculations have been made for open systems. Open systems, however, allow unlimited numbers of oscillations to be observed, and stationary states to be maintained indefinitely. The emphasis in the present work is on establishing the variety of behaviour, and on characterizing the new modes of reaction possible in open systems by quantitative and continuous measurements. Concentrations of reactants, products and molecular intermediates, and hence rates of reaction, are monitored continuously by a mass spectrometer; light emissions are monitored instrumentally; self-heating and hence rates of heat evolution are detected and recorded by measuring excess temperatures with fine-wire thermocouples. An unprecedented variety of behaviour has been encountered. Nine chemically and physically distinct, stable modes of reaction have been observed. There are stable oscillations (limit cycles) of seven clearly differentiated forms, and two stationary states. The conditions for the occurrence of each mode have been mapped for various residence times on a traditional ignition diagram of reactant pressure and vessel temperature. They occupy five regions (representative, overlapping reactortemperature ranges in parentheses): I, steady reaction without light emission (up to 550 K); II, oscillatory ignition (500-520 K); III, five modes of oscillatory ignition interspersed with cool flames (520-540 K); IV, oscillatory cool flames (500-600 K); V, steady reaction with chemiluminescence (580-650 K). At the various boundaries between the five regions, sharp jumps occur from one kind of behaviour to another. At three segments of the boundary, there is hysteresis, the jumps occurring at different temperatures during heating (I -> II, III or IV) and cooling (II, III or IV -> I) traverses. There are thus regions of bistability, where identical external conditions - vessel temperatures, reactant pressures and flow rates - can give rise to alternative states inside the reactor. The two non-oscillatory, stationary states have different characters: I is a stable node and V is a stable focus. In region I, the reaction rate increases with temperature; but in region V, both reaction rate and extent of self-heating show a near-zero or negative temperature-coefficient.

Eclogitization kinetics of continental granulites : quantification and implications

2020 ◽  
Author(s):  
Marie Baisset ◽  
Loic Labrousse ◽  
Alexandre Schubnel
Keyword(s):  
Rheological Behavior ◽  
Reaction Rate ◽  
Experimental Studies ◽  
Reaction Rates ◽  
First Order ◽  
Piston Cylinder ◽  
Ductile Flow ◽  
Convergence Zones ◽  
Kinetics Of ◽  
Type Apparatus

&lt;p&gt;&lt;span&gt;When implicated in convergence zones, granulites of the lower continental crust are expected to eclogitize at depth.When exposed in the field such units show a bimodal rheological behavior between fracturing of the protolith rock (granulites) and ductile flow of the transformed parts (eclogites). It seems therefore that a competition exists between the rate at which the rocks are loaded in stress and the rate at which they transform, i.e. the overall eclogitization kinetics. The aim of the work presented here is to quantify the kinetics of the metamorphic reactions involved in eclogitization by estimating the reaction rates in plagioclase-bearing assemblages&lt;span&gt;&amp;#160; &lt;/span&gt;submitted to different P-T conditions over different time spans. For this, experiments have been performed in piston-cylinder apparatus on aggregates derived from natural granulites. Special attention is paid to the location where nucleation starts and how it propagates in and between the grains. In this prospect, the presence of garnet and cpx in the plagioclase matrix is a first order control on the reaction process. This work follows previous experimental studies (e.g. Shi et al., 2017, Incel et al., 2018) which show that reaction-enhanced embrittlement may be key for fracturing at high pressure. It has been proposed that transient properties of the rocks induced by the very beginning of the reaction (e.g. volume change, small grain size nucleation products) can lead to brittle instabilities. As we assume that the rheological behavior of the crust is controlled by a competition between reaction rate and strain rate, experiments involving deformation of granulites while undergoing eclogitization are required. Preliminary results performed on Griggs-type apparatus, which constitutes the best tool for that, will also be presented.&lt;/span&gt;&lt;/p&gt;

scholarly journals CFD Modeling: Different Kinetic Approaches for Internal Reforming Reactions in an Anode-Supported SOFC

2011 ◽  
Vol 8 (3) ◽  
Author(s):  
Hedvig Paradis ◽  
Martin Andersson ◽  
Jinliang Yuan ◽  
Bengt Sundén
Keyword(s):  
Fuel Cells ◽  
Reaction Rate ◽  
Experimental Studies ◽  
Reaction Rates ◽  
Transport Processes ◽  
Cfd Modeling ◽  
Chemical Compositions ◽  
Internal Reforming ◽  
Shift Reaction ◽  
Reforming Reactions

Fuel cells are electrochemical devices that convert chemical energy into electricity. Solid oxide fuel cells (SOFCs) are a particularly interesting type because they can reform hydrocarbon fuels directly within the cell, which is possible, thanks to their high operating temperature. The purpose of this study is to develop an anode-supported SOFC theoretical model to enhance the understanding of the internal reforming reactions and their effects on the transport processes. A computational fluid dynamics approach, based on the finite element method, is implemented to unravel the interaction among internal reforming reactions, momentum, and heat and mass transport. The three different steam reforming reaction rates applied were developed and correlated with experimental studies found in the literature. An equilibrium rate equation is implemented for the water-gas shift reaction. The result showed that the reaction rates are very fast and differ quite a lot in size. The pre-exponential values, in relation to the partial pressures, and the activation energy affected the reaction rate. It was shown that the anode structure and catalytic composition have a major impact on the reforming reaction rate and cell performance. The large difference between the different activation energies and pre-exponential values found in the literature reveals that several parameters probably have a significant influence on the reaction rate. As the experiments with the same chemical compositions can be conducted on a cell or only on a reformer, it is important to reflect over the effect this has on the kinetic model. To fully understand the effect of the parameters connected to the internal reforming reaction, microscale modeling is needed.

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