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Ex situ transmission electron microscopy (TEM) was used to study the crystal morphology in sputtered amorphous Ge4Sb1Te5, Ge2Sb2Te5, and Ag0.055In0.065Sb0.59Te0.29 thin films used for phase change recording. Tilting of plan view samples revealed that each crystallized growth formation is a bent single crystal. Cross-sectional TEM showed that crystals only nucleate heterogeneously at the (naturally oxidized) film surface. These findings allow the determination of nucleation parameters around 150°C from earlier experiments [J. Kalb, F. Spaepen, and M. Wuttig, Appl. Phys. Lett. 84, 5240 (2004)]. The time lag for nucleation has an activation energy of (2.74±0.13)eV for Ge2Sb2Te5 and (2.33±0.18)eV for Ag0.055In0.065Sb0.59Te0.29. The activation energies for the steady-state nucleation rate were (4.09±0.20)eV for Ge4Sb1Te5 and (3.50±0.17)eV for Ge2Sb2Te5. With the activation energy for the crystal-growth velocity found in the earlier article the critical work for formation of the nucleus was found to be (1.35±0....

The operating conditions of a turbulent contact absorber (TCA) can be divided into four regimes. The characteristic behaviour of a TCA in each of the regimes is discussed. The condition below the flooding point is the usual operating regime. Correlations are developed to relate the pressure drop and the liquid holdup of a TCA by the operating variables. A terminal velocity model is developed to explain the flooding condition of a TCA. With this model, the hydrodynamic behavior of the TCA at the flooding point can be identified. The total pressure drop and the pressure drop across a single stage are distinguished to clarify the controversy whether the gas velocity can affect the pressure drop of a TCA. The total pressure drop of a TCA is composed of the pressure drop due to distributor loss at the entrance and the pressure drop of the bed itself. The pressure drop of a single stage bed of a TCA is independent of gas velocity. On peut diviser en 4 regimes les conditions operatoires d'un absorbeur a contact turbulent (ACT); on discute le comportement caracteristique d'un ACT dans chacun d'eux. La condition qui prevaut au-dessous du point d'engorgement constitue le regime habituel d'operation. On a mis au point des correlations entre la chute de pression et la retention du liquide d'un ACT par les variables operatoires. On a mis au point un modele terminal de vitesse pour expliquer la condition d'engorgement d'un ACT; on peut grâce a celui-ci identifier le comportement hydrodynamique de l'ACT au point d'engorgement. On fait une distinction entre la chute totale de pression et la chute de pression dans un simple etage, pour clarifier la controverse relative a l'influence possible de la vitesse du gaz sur la chute de pression d'un ACT. La chute totale de pression d'un ACT comprend la chute de pression qui resulte de la perte du distributeur a l'entree et la chute de pression du lit meme. La chute de pression d'un lit a etage simple d'un ACT est independante de la vitesse du gaz.

A comprehensive model for the simulation of fluidized bed coal combustors (FBC) is developed, capable of predicting the combustion efficiency, char and limestone elutriation and the corresponding particle size distribution in the bed and in the entrained materials, solids withdrawal rate from the bed, bed temperature profile, sulfur dioxide retention, sulfur dioxide and NOx emissions, concentrations of oxygen, carbon monoxide, carbon dioxide, volatiles, sulfur dioxide and NOx along the combustor height. The model can also simulate combustors with varying cross section along the bed height. The performance of the model is compared with the data obtained from four different combustors. Agreement between the computed results and the data is good. The salient features in the model which need further investigation are pointed out.

A one-parameter volume reaction model is developed by a simplified analytical approach to describe reaction of a gas and a consumble solid in a porous particle. In this model, a modified Thiele modulus M is introduced to take into account the effect of solid reactant depletion during the reaction. Experimental results of an isothermal carbon-carbon dioxide reaction in the temperature range from 1 130° to 1 365°K under atmospheric pressure were analyzed by the working diagrams derived from this model. A satisfactory agreement between the experimental results and those predicted by the model indicated the applicability of this model.

A method for estimating entropies of both solid and liquid fuels is proposed. An example showing calculation of availability of coal is presented. Entropy and availability of coal calculated are compared with those obtained by other methods. Deviations among different methods examined are discussed. On propose une methode pour evaluer les entropies de combustibles solides et liquides. On montre un exemple de calcul de l'accessibilite au charbon. On compare l'entropie et l'accessibilite du charbon obtenues par calcul avec celles obtenues en employant d'autres methodes. On discute les ecarts qu'on trouve entre les resultats des differentes methodes examinees.

Bubble size is one of the most important parameters in the design and simulation of a fluidized-bed reactor. A correlation of the bubble size and growth in fluidized beds of various diameters is developed. A maximum bubble diameter determined from the bubble coalescence is incorporated in the correlation to relate the effect of the bed diameter on the bubble size. Experimental data of bubble size reported are used to develop and test the validity of the correlation. The bubble diameters calculated using this correlation show good agreement with the observed bubble diameters.

A model describing the entrainment of solid particles in the freeboard region of a fluidized bed is proposed. Published experimental data have been used to correlate the entrainment rate near the bed surface and the elutriation rate above the bed. It has been shown that the column diameter can have an important influence on the amount of fines elutriated.

This work presents a methodology for the shape control of structures undergoing inelastic deformations through the use of an active pulse/force system. Large flexible structures such as tall buildings, long bridges, and offshore platforms tend to develop large deformations under the action of intense dynamic loads such as wind, waves, or earthquakes. Unless these deformations are controlled in some fashion, the structure will sustain extensive damage that may lead to total collapse. One possible solution to this problem is to apply external forces to the structure through cables, air jets, or other devices in order to assure that the deformations are kept below the limits set for serviceability at all times. The methodology presented herein is based on an active control algorithm derived from standard numerical integration methods that uses corrective pulses or forces to limit the response of a structure that has already entered the inelastic range. Material nonlinearities are accounted for by use of stan...

The present study describes neuronal degeneration and its accompanying non-neuronal cellular reaction in the hypoglossal nucleus following an intraneural injection of Ricinus communis agglutinin-60 (RCA-60) into the hypoglossal nerve. The first noticeable structural changes were observed in neurons in hamsters killed 3 days after the RCA injection. Drastic alterations occurred in the period extending from the 5th to the 15th postoperative day. Two forms of neuronal degeneration were observed: light and dark types. In the light type, masses of free ribosomes were observed; other changes included the dilation of Golgi saccules and the presence of abnormal mitochondria. In the dark type of degeneration, the cells became condensed with vacuoles in their cytoplasm. Axon terminals presynaptic to the degenerating cells during this period appeared to be normal. A massive influx of mononuclear leucocytes by diapedesis occurred at the large venules. Some of the infiltrated cells were clearly lymphocytes, while others were monocytes which became indistinguishable from indigenous microglia once they were in the neuropil. Neural macrophages, most probably derived both from microglia and the infiltrated monocytes, were engaged in the phagocytosis of neuronal debris. A remarkable finding in the present study was the wide-spread occurrence of dark axon terminals in the neuropil in longer surviving animals (90 and 120 days). The structural alterations, e.g., clumping and swelling of some of the synaptic vesicles in the enhanced cytoplasmic density, suggest that these were undergoing atrophic changes resulting from the long period of dysfunction following the death of postsynaptic neurons induced by RCA.

Presynaptic dendrites (PSDs) are described in the cuneate nucleus of the monkey. These PSDs establish dendro-dendritic synapses with dendrites of variable size. They contain small numbers of pleomorphic synaptic vesicles which are clustered together near the presynaptic membrane. Such PSDs have also been observed to be postsynaptic to medium-sized or large axon terminals containing round vesicles.

A comprehensive model incorporating elutriation, entrainment and reaction is presented to describe the chemical reaction and hydrodynamics occurring in the freeboard of a fluidized bed. A comparison of experimental data on the amount of particles elutriated and the concentration profiles of SO2 and NOx obtained from the freeboard of fluidized bed coal combustion (FBC) by the Babcock & Wilcox Company with the calculation based on the proposed model to include the effect of interaction among the entrained particles is desirable. More experimental data with chemical reactions in the freeboard are needed to validate the model for applications to fluidized bed catalytic reactors.

Transient equations based on impulse, step, ramp, parabolic and sinusoidal inputs for reactors of various flow models are presented. The flows accompanied by the first order irreversible chemical reaction are considered. The response expressions are useful for the selection of a proper model and for the evaluation of the residence time distributions in a given reactor. On preesente les equations transitoires pour divers modeles d'ecoulement dans des reacteurs selon des inputs varies tels que: en escalier, a variation constante, par impulsion, sinusoidal et parabolique. On considere les ecoulements accompagnes d'une reaction chimique irreversible du premier ordre. Les expressions sont utiles pour l'evaluation des distributions de temps de residence dans un reacteur donne.

Mechanical and dynamical characteristics of semifluidized beds of single-size particles in solid-liquid system were investigated. A semifluidized bed is a type of fluidized bed in which the bed expansion is partially restricted. The emphasis was placed on the study of packed bed formation and pressure drop increase when the semifluidized beds were formed by the compression of fluidized beds. The data showed good agreement with the theoretical and semiempirical equations based on a simple model of fluidized beds. The results of this investigation would also contribute to the understanding of fluidized beds.

Two progression models which can be employed to represent flow systems with varying degree and mode of mixing along the direction of flow such as a shallow fluidized bed, radial flow reactors, and short tubular reactors, etc. are proposed. Residence time distributions are derived from the standpoint of stochastic processes. The use of progression models is illustrated with a second order, isothermal reaction. On propose deux modeles de progression qu'on peut utiliser pour representer des systemes d'ecoulement qui possedent des degres et modes differents de melange suivant le plan de direction de l'ecoulement, tels que, par exemple, un lit fluidise et peu profond, des reacteurs a ecoulement radial et des reacteurs tubulaires de faible longueur. On deduit les distributions du temps de sejour moyen dans le reacteur, en se basant sur les methodes conjecturales et on illustre l'emploi des modeles de progression au moyen d'une reaction isothermique de second ordre.

Solid-fluid reactions often involve two or more simultaneous reactions because either the feed contains multiple fluid reactants or the fluid products are reactive with the solid. Three types of simultaneous reactions, independent, parallel, and consecutive, are examined in terms of the selectivity and the effectiveness factor based on the unreacted core shrinking model under isothermal conditions. Criteria for high selectivities are derived, and effects of diffusion and chemical reaction are discussed.

A consecutive, first-order, irreversible, biochemical reaction, , taking place in a series of N reactors with product recycle is considered. A discrete version of the maximum principle is used to derive general equations necessary for maximizing the production of (1) the final product, C, by choosing the temperature or the pH value in each reactor, and (2) the intermediate product, B, by choosing the reactor volume. A numerical computation for a series of three reactors with recycle is illustrated. The effects of varying the recycle rates on the optimal state and decision variables are also presented.

The flow characteristics of dense solid-gas mixtures transported through horizontal pipes were studied with glass beads and coal powders of various sizes (0.0028 to 0.0297 in.) in 1/2-, 3/4- and 1-in. glass pipes and a 1/4-in. steel pipe. Fluidized-bed feeders were utilized, thus permitting solid-gas ratios considerably higher (range 80 to 750) than those possible with conventional pneumatic transport. When such high solid-gas ratios are used, the flow of mixtures in transport lines is characterized by a large amount of slippage between gas and solids. The flow pattern is dicussed on the basis of visual observation through glass pipes. A simple and interesting velocity relationship was noted, namely that the average gas velocity is about twice as large as the average solid-particle velocity. The solid-particle velocities and solid loadings in the pipe line were found to be primary factors affecting pressure drops, and the particle sizes and shapes, on the other hand, exerted a very slight effect on the pressure drops. This is apparently due to the fact that the solids move predominantly in the bottom of the pipes as agglomerated masses rather than as individually suspended particles. A pressure-drop correlation for the dense solid-gas mixtures is proposed, and applications and limitations of the correlation are shown.

The coal char hydrogasification reaction may be regarded as two simultaneous reactions differing considerably in their rates. The rate of coal hydrogasification, particularly that for the initial rapid reaction, was studied. The initial reaction rates in semiflow tests were analyzed and found to be proportional to the amount of unreacted carbon remaining in the particles as well as the effective partial pressure of hydrogen. The rate constant for small conversion of carbon was found to be 2.0 (atm.-hr.)/sup -1/. The reaction model used for semiflow test analysis was satisfactorily applied to continuous free-fall reactor tests.

Longitudinal liquid mixing in fluidized and fixed beds was studied using sinusoidal and pulse response techniques. The tracer used was light emissive fluorescein dye. A systematical study of liquid phase dispersion by varying particle size, fluid velocity, fraction voids, and particle density was conducted. A generalized correlation applicable to both fixed bed and fluidized bed was obtained. The application of the correlation in predicting the effect of the dispersion on reactor performance was discussed.

Kinetic and diffusion models for gas-solid reactions in spherical particle, comparing core shrinking mechanism and activation energies

Various coal-gasification processes for pipeline-gas production are studied to select those processes which offer some future possibility and attractiveness for commercial development. The gasification processes are mathematically simulated and evaluated, using a methodology of system analyses which optimizes the operating and design criteria of each process alternative with regard to lowest cost or highest energy-utilization efficiency. In using systems methodology, a generalized method of economic comparison is established and problems of fundamental importance are identified. This study specifies those steps having significant effect so that a more precise and detailed evaluation can be made of the critical problem areas. The study also indicates areas of serious deficiency in the present-day technical knowledge needed to develop successfully the proposed coal-conversion process. The results are interpreted in terms of the most advantageous reaction path to follow for the coals having specific...

A study on the flow of power-law fluids through a multi-particle system including both fixed bed and fluidized bed is presented. Equations for the pressure drop, the minimum fluidization velocity, and the bed expansion are obtained by extending the Blake-Kozeny's equation for the pressure drop through packed beds to power-law fluids. Bed expansion equations are also obtained by extending the Richard-son-Zaki's theory for the drag force in a multiparticle suspension to power-law fluids. These equations are compared with experimental data. On presente une etude de l'ecoulement des fluides d'entrainement dans un systeme a particules multiples forme a la fois de lits fixes et fluidises. On obtient des equations qui ont trait a la chute de pression a la velocite minimum de fluidisation et a l'expansion des lits, en etendant a des fluides d'entrainement l'equation de Blake-Kozeny relative a la chute de pression dans des lits garnis. On obtient egalement des equations pour l'expansion des lits en etendant a des fluides d'entrainement la theorie de Richardson-Zaki relativement a la force de trainee dans une suspension de particules multiples. On compare les dites equations avec des donnees experimentales.

A new fluid-solid contacting technique known as Semifluidization has been studied. Mass transfer data were obtained for a benzoic acid–water system, and correlations in terms of mass transfer factors and over-all void fractions (for both packed and fluidized sections) are presented. Previous mass transfer correlations for fixed and fluidized beds are critically evaluated with respect to the limitation and applicability and are discussed in relation to semifluidized-bed mass transfer correlations.

The kinetics of simultaneous, noncatalytic solid-gas reaction systems has been analyzed in terms of the selectivity and effectiveness factor, based on the unreacted core shrinking model. The reaction systems discussed include the first-order independent, parallel, and consecutive reactions with the simultaneous influences of heat and mass transfer. The effects of various chemical and transport properties on the stability, selectivity, and effectiveness factor of the system are discussed. The pseudosteady state approximation is employed to reduce the mathematical difficulties.

The nonisothermal unreacted-core-shrinking model with unsteady state heat transfer analysis has been experimentally evaluated. The system studied is the combustion of a single agglomerated carbon-fire clay sphere in a thermobalance. By comparing the experimental results with those predicted by the model it is concluded that the model adequately describes the reaction system under the experimental conditions such that the ash layer diffusion is rate controlling. (40 refs.)

The axial dispersion correlations in liquid-solid fluidized beds presented by Chung and Wen(1) and Krishnaswamy and Shemilt(8) are compared and the applicability of the correlations are discussed. On compare les correlations sur les dispersions axiales dans des lits fluidises formes de liquides et de solides qu'ont presentees Chung et Wen(1) et Krishnaswamy et Shemilt(8), on discute aussi l'applicabilite desdites correlations.

Equations are derived for the transient activity of flow medium in multistage continuous flow reactors. Activity behavior of linear, exponential, and step change is considered. Complete mixing in each stage is assumed. Application to a fluidized bed reactor is presented and a method of obtaining mixing coefficients is given. Les equations exprimant l'activite d'un catalyseur circulant dans un reacteur continu a effets multiples en regime d'etablissement sont developpees pour les cas ou la diminution d'activite en fonction du temps est lineaire, exponentielle et “en gradins” en supposant un melange parfaitement homogene dans chacune des parties du reacteur. A titre d'exemple, ces equations sont appliquees au cas d'un reacteur a lit fluidise et on titre une methode d'estimation du degre d'homogeneite.