Runaway Conditions Research Articles

Stability of the Be—steam reaction and its impact on safety

Beryllium is one of the major candidate materials for the plasma-facing components of ITER. At high temperatures it represents, however, a considerable safety issue because of its high toxicity and its highly exothermic and fast oxidation in case of steam or air ingress into the vacuum vessel of the reactor. Such conditions might occur after a plasma disruption. The work presented here attempts to analyze the transient behavior of the thermochemical system of a reacting plasma-facing surface, identify the reaction runaway conditions, and assess its impact on overall ITER safety.

Self heating and thermal failure of polymers sustaining a compressive cyclic loading

Self heating of polymers sustaining a compressive cyclic loading is assessed by theoretical analyses and experimental measurements. Steady-state solutions are obtained in the case of isotropic linear viscoelastic temperature sensitive materials and negligible inertia effects. The conditions of thermal failure associated with the nonexistence of steady-state solutions are discussed. The stability of steady states is also analysed, and conditions of thermal runaway are obtained. The extent to which these theoretical analyses are in agreement with experimental results is evaluated.

Thermal Safety of Batch Reactors and Storage Tanks. Development and Validation of Runaway Boundaries

This paper addresses the problem of predicting the critical parameters that characterize thermal runaway in a stirred-tank batch reactor or a storage tank with wall-cooling. For first-order reactions, four parameters are required for defining criticality. For second-order reactions, a further parameter is necessary (the stoichiometric excess of a reactant). The set of parameters used here are those used by early workers in the field. Runaway and safe conditions are identified based on the pattern of temperature−conversion trajectories near full conversion. The resulting heuristics is useful for establishing runaway boundaries on the N−S parameter plane, using CAPSA, a computer software developed ad hoc for thermal hazard prognosis. The boundaries based on CAPSA successfully predict runaway in five homogeneous and pseudohomogeneous, bench-scale, sensitive reactor systems.

Process dynamics and temperature control of fed-batch reactors

The dynamics of a fed-batch reactor was studied using the detailed first principle model of the system. The conditions of reaction runaway, the typical sources of hazardous operation in fed-batch techniques are determined. Considering these conditions a model-based control scheme is proposed to ensure robustness. When the stability criterion predicts that the range of operation is safe the cooling is set to its maximum and the temperature is controlled by the reactant feed. Otherwise the reactant feed is stopped and the flow rate of the cooling medium is the control variable.

Runaway limits for homogeneous and catalytic reactors

A unified approach that relates and combines the concepts of parametric sensitivity, ignition, extinction, hysteresis and isola formation is presented and used to derive a single universal runaway criterion valid for all homogeneous reactor models. It is also shown that the heat and mass dispersion have very little impact on this runaway locus. Next, the approach is extended to heterogeneous reactor models where interphase gradients strongly influence the runaway boundary. It is shown that the model of a well-mixed two-phase reactor may be used to derive runaway boundaries for a cooled multitubular catalytic reactor influenced by interparticle and intraparticle transport limitations. The validity of the approach is verified by numerical simulations of temperature profiles under safe and runaway conditions for some selected cases. The analysis of the heterogeneous model shows that the heat transfer resistance between the solid and the fluid introduces a qualitative change in the shape of the stability boundary. There exists a limiting value of the dimensionless heat of reaction parameter B, above which the reactor is in a runaway region regardless of the rate of external cooling. This limiting value of B corresponds to the ignition of catalyst particles. It is also shown that within the safe region of operation, there exists an optimal catalyst particle diameter that maximizes the reactor productivity. The main concepts involved and computation of the runaway boundaries for multiple reactions are illustrated by discussing three specific examples of consecutive, simultaneous and consecutive-simultaneous reactions. The important result is that the runaway locus for any multiple reaction network differs significantly from that of the single reaction only when the product λbψ > 0.1. Here, λ, ψ and b, represent, respectively, the ratio of rate constants at feed conditions, heats of reaction and feed concentrations of the primary reaction to that of the main competing reaction. This work also presents slightly conservative analytical results for the determination of the runaway limits of homogeneous and catalytic reactors for most cases of practical interest.

Results from Computational and Experimental Modeling of Runaway Electron Damage on Plasma Facing Components

The purpose of this research was to extend the theoretical and experimental knowledge of runaway electron damage-impact-bombardment on plasma facing components and materials in magnetic fusion devices. The emphasis of this work involved computational modeling and experimental studies to investigate runaway electron energy deposition and thermal response in plasma facing materials. The goals were: (1) to develop a computational model to study and analyze runaway electron damage; (2) to characterize runaway electron parameters; and (3) to perform experiments to analyze runaway electron damage. These goals were accomplished by first assembling the PTA code package. PTA is a unique application of PATRAN, the Integrated TIGER Series (ITS), and ABAQUS for modeling high energy electron impact on magnetic fusion materials and components. The PTA code package provides a three-dimensional, time dependent, computational code package which predicts material response from runaway bombardment under most runaway conditions (i.e., electron energy, incident angle, energy density, and deposition time). As part of this research, PTA was used to study energy deposition and material response in several design applications, to analyze damaged material, and to analyze several experiments. Runaway electron characterization was determined through parametric studies, analysis of damaged materials, and analysis of experimental results. Characterization providedmore » information on electron energy, incident angle, current, deposition time, and volume of material impacted by runaway electrons. Finally an experiment was performed on the Advanced Toroidal Facility (ATF) at Oak Ridge National Laboratory to study runaway electron damage. The experiment provided information on the runaway electron energy and current in ATF, as well as supplemented the existing experimental knowledge of runaway electron damage.« less

The theoretical and experimental investigation of a catalyst-controlled semi-continuous process: An alternative approach

In catalyst-controlled reactions, an alternative reaction procedure is possible, namely adding the catalyst to the educt solution. This is demonstrated by investigating the reaction of phenol with formaldehyde, catalysed by sodium hydroxide. The reaction enthalpy was found to be −35kJ mol −1 and the activation energy was 80 kJ mol −1 in good agreement with the literature values. The reaction procedure of adding the catalyst can be described by a simple set of differential equations, and was simulated with a computer to investigate the influence of different parameters on the course of the reaction. The relation between runaway conditions for the primary reaction and the secondary reaction is discussed.

On runaway conditions of orbital debris environment

An empirical function for the growth of the orbital debris particle density has been formulated. A growth coefficient in the function, based on the theoretical derivation of the evolution of the orbital debris environment, is obtained from the computer modeling. This empirical function can be used to calculate the evolution of the orbital debris and to examine whether the critical spatial density of the debris particles at certain regions in space have been reached or not. This report shall use this growth function to analyze the evolution of the current space debris environment without any future traffic input. The result indicates that the debris particles with sizes larger than 10 cm are very unstable only at regions around 1500 km altitude, in support of Kessler's /1/ conclusion.

Nova V2214 Ophiuchi 1988 - A magnetic nova inside the period gap

view Abstract Citations (25) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nova V2214 Ophiuchi 1988: A Magnetic Nova inside the Period Gap Baptista, R. ; Jablonski, F. J. ; Cieslinski, D. ; Steiner, J. E. Abstract The discovery of a coherent photometric modulation in Nova Oph 1988 with period 0.117515 +/- 0.000002 d, which is associated with the orbital period of the underlying binary, is reported. On the basis of photometric observations, it is concluded that Nova V2214 Oph 1988 is a magnetic nova with an orbital period inside the period gap. The inclusion of this system in the statistics of novae suggests that there is no period gap for novae and that there is a clear correlation between the occurrence of novae with short orbital periods and the presence of magnetic white dwarfs. It is suggested that funneling of the accretion flow onto the magnetic poles favors the conditions for a thermonuclear runaway, increasing the frequency of eruptions for magnetic systems. Publication: The Astrophysical Journal Pub Date: April 1993 DOI: 10.1086/186788 Bibcode: 1993ApJ...406L..67B Keywords: Cataclysmic Variables; Novae; Stellar Magnetic Fields; Stellar Orbits; Stellar Spectrophotometry; Light Curve; Time Series Analysis; Astrophysics full text sources ADS | data products SIMBAD (6)

BEHAVIOUR OF AN ADIABATIC PACKED BED REACTOR PART 2: MODELLING

Abstract The steady state and dynamic behaviour of an adiabatic packed bed reactor for the selective hydrogenation of mixtures of ethyne and ethene is studied. A heterogeneous model with axial dispersion of heat is solved numerically by means of a fully implicit discretisation scheme. From an analysis of the inlet and outlet boundary conditions, it follows that in order to avoid an influence of the type of boundary condition on the reactor behaviour, sufficiently long inert zones before and after the active bed should be present. Using preliminary kinetic expressions adapted from Mcn'shchikov ct al. (1975), the model calculations show a reasonable agreement with the experiments performed in a laboratory scale reactor and demonstrate the importance influence of small amounts of carbon monoxide both on the steady state as well as on the transient behaviour of the reactor. However, using the rate expressions derived from our own kinetic experiments in a Berty reactor, the behaviour under runaway conditions a...

Violent oxidation of lithium-containing aluminum alloys in liquid oxygen

A strong exothermic and quite well known thermite reaction involving aluminum, oxygen and transition metals (Fe, Cr, Ni, etc.) has apparently been initiated during impact testing of Alcoa aluminum alloy #2090 in liquid oxygen at NASA-MSFC. In some instances, this reaction, essentially an oxidation process, has been so intense that the Inconel 718 cup containing the aluminum alloy disk and associated impacter has melted raising certain safety concerns in the use of this alloy. Reaction products as well as the test specimen surfaces have been studied with surface science techniques like XPS/ESCA, SIMS and AES. Typically, in order to initiate the thermite reaction a temperature of approximately 1000°C is necessary. The mechanism responsible for this oxidation is of great interest. The analysis of the reaction products together with a theoretical analysis, including digital modeling has been pursued. There is strong evidence that the large relaxation energy of the aluminum oxide coating, formed during the aluminum alloy cleaning process, is causing a highly localized energy release during fracture or lattice deformation which is enhancing the oxidation process to a runaway condition. The presence of alkali atoms (Li) enhances the likelihood and intensity of the oxidation reaction. The details of the surface studies will be discussed.

A GENERAL RUNAWAY CRITERION FOR FIXED-BED REACTORS

The safe design and operation of fixed bed reactors require the consideration of potential runaways. The steep temperature rise may affect the selectivity, activity and stability of catalysts and in certain cases the reactor safety. Mathematical models simulate the runaway conditions. Such simulations require extensive calculations to define the range of safe operation. A new runaway criterion was developed to replace the need to solve the differential reactor balances. The criterion is simple, defined by one algebraic equation. It can be applied to any kinetic expression. Extensive testing shows an excellent agreement between the runaway criterion and the two-dimensional model predictions. It also predicts the runaway conditions in agreement with experimental data.

Metal-oxide surge arrester operating limits defined by a temperature-margin concept

The problem of ascertaining the absence of possible thermal runaway of metal-oxide surge arresters when they are used in applications requiring a very low protective level is addressed. An electrothermal model of arresters is used to predict the temperature-time profile of their active elements in severe (near thermal runaway) conditions. Experimental evidence is produced to validate the proposed method and justify a criterion for the operating limit of any arrester design. This operating limit is proposed as a maximum acceptable temperature for the arrester metal-oxide elements. >

Radial flow reactor optimization for highly exothermic selective oxidation reactions

Radial flow reactors (RFR's) with catalyst external to cooling tubes are compared with fixed bed, axial flow cases for highly exothermic, selective oxidation reactions. Steady state, one dimensional mass, heat and momentum balances were integrated for the butane to maleic anhydride (ma) reaction at commercially relevant conditions. “Triangular” kinetics for the main, total oxidation and product decomposition reactions were utilised (Wellauer et al., 1986). From this work, diffusion restrictions in the particles and interphase heat transfer were approximated making the reactor model quasi-heterogeneous. From the literature, heat transfer for radial flow through a packed bed with cooling tubes was estimated in addition to pressure drop for radial flow using flow parameters based on “local” values and including the bed voidage occupied by the cooling tubes. Comparison of the optimal cases at the same ma yields showed significant advantages for RFR's with respect to an increase in productivity, a decrease in reactor volume, lower hotspot temperatures and steam production and a significantly lower pressure drop. Stable operation was predicted for butane concentrations (5 mol %) within flammability limits but below ignition temperatures and well within runaway conditions for axial flow.

Impact of Fuel Conservation Measures on Safe Truck Downgrade Speeds

Runaway truck accidents are primarily due to brake fade and failure resulting from excessive temperature of the truck system brake components generated while descending severe downgrades. A grade severity rating system (GSRS) was developed in 1982 that determined the maximum safe downgrade speeds for various categories of truck gross weights. Heavy trucks, without retarders, that adhere to the maximum recommended descent speed have sufficient braking capacity to prevent a runaway condition. Since the development of the GSRS model, the use of radial truck tires and the manufacture of streamlined truck tractors have increased substantially. These fuel conservation measures reduce the nonbraking forces and place a greater load on the truck system brakes when it is necessary to maintain truck speed on downgrades. The impact of fuel conservation measures on truck system brake temperature is the equivalent of descending a downgrade which is 0.34% steeper than actual conditions. In addition, fuel conservation measures lower the threshold of truck weight susceptible to excessively high brake temperatures, and subsequent runaway conditions, from 45,000 to 35,000 lb.

Compartment model concept used in the design of fermentation with recombinant microorganisms

Simple models are of great value in order to simulate cell dynamics, and a class of potentially useful models are the so-called compartment models, which are a simple extension of the unstructured approach. Compartment models have earlier been used with great success to describe cell dynamics. In this article the authors present a 4-compartment model that is able to describe a majority of the observations that have been made through fermentations with recombinant microorganisms. The compartment model is especially suited for description of dynamic changes in plasmid copy number, e.g., runaway replication. Simulation of batch fermentation with runaway conditions corresponds very well with experimental results. Finally the model is used in the design of an optimal fermentation strategy in a CSTR.

A Thermal Analysis of a Spirally Wound Battery Using a Simple Mathematical Model

A two‐dimensional thermal model for spirally wound batteries has been developed. The governing equation of the model is the energy balance. Convective and insulated boundary conditions are used, and the equations are solved using a finite element code called TOPAZ2D. The finite element mesh is generated using a preprocessor to TOPAZ2D called MAZE. The model is used to estimate temperature profiles within a spirally wound D‐size cell. The model is applied to the lithium/thionyl chloride cell because of the thermal management problems that this cell exhibits. Simplified one‐dimensional models are presented that can be used to predict best and worst temperature profiles. The two‐dimensional model is used to predict the regions of maximum temperature within the spirally wound cell. Normal discharge as well as thermal runaway conditions are investigated.

Two-phase flow venting from reactor vessels

This paper provides an overview of the emergency relief requirements for reactor vessels under runaway conditions. The importance of utilizing appropriate thermokinetic data in providing the necessary energy release rate and pressure generation rate is emphasized with experimental styrene data. The effect of two-phase flow venting on reducing the volumetric flow capacity of a pressure relief device is clearly elucidated. In addition, the paper presents a concise and unified treatment of two-phase flashing flow in the form of formulae and design charts covering single component fluids, multicomponent mixtures, nozzles, long pipes, elevation changes, and back pressure effects. Finally, the effect of allowing for the overpressure in lowering the required relief area during two-phase venting relative to the no overpressure case can be quickly assessed via the proposed normalized relief sizing formula.

THE “UCKRON-1” TEST PROBLEM FOR REACTION ENGINEERING MODELING

Abstract Testing kinetic models against a “true” and detailed kinetic expression was the aim of the Workshop on Kinetic Model Development at the Denver AIChE Meeting in August, 1983. For this purpose an artificial reaction mechanism was created, based on the known thermodynamics of the methanol synthesis as a framework. The kinetic rate laws, that were derived from this mechanism, were made thermodynamically consistent by achieving agreement between equilibrium constants calculated at various temperatures from the given, real original thermodynamic relationship and those calculated from the detailed reversible kinetic expressions. Using the artificial kinetics as the "true" one, CSTR experiments were simulated. The results of a statistically designed set of experiments were published after 5% random error was added to the data. Participation was invited for all interested to correlate the data, develop kinetic models and to calculate the performance of the specified reactor. The results of 19 submitted entries are summarized with the conclusion that the models had more differences than were expected, but their predictive values were not as different as was anticipated, if the extreme high production rates due to thermal runaways are not considered. This in turn points out the necessity to check models experimentally, in pilot plant, not only for predicted optimum, but also for calculated runaway conditions. Models which did not capture the true character of this reaction failed to predict the onset of runaway reactions.

Velocity distribution of H+ ions under runaway conditions in helium

The velocity distribution of H+ ions in helium has been obtained using the Monte Carlo method for ratios of the electric field to gas number density E/N from 5.65-56.6 Td. The discrepancies in the velocity distributions between runaway conditions and non-runaway conditions are considered. In the case of the runaway conditions, the velocity distribution has elongated tails in all directions and the tail in the direction of the electric field is especially elongated.