Wen-Ching Yang

Criteria for choking in vertical pneumatic conveying lines

Abstract Choking in vertical pneumatic conveying lines is not a clear-cut phenomenon. Different definitions and criteria are available in the literature. They are first reviewed in this paper. A mathematical definition for choking first proposed by the author was refined and presented with additional data. The concept of continuity wave and cluster formation, which were successfully applied to predict the transition between the bubbling and the turbulent fluidization, were then introduced to derive the basic relationship expected at choking. A semi-theoretical equation similar to the Richardson and Zaki equation for particulate fluidization was obtained from the available literature data. Hopefully, this mechanistic approach will eventually lead to a unified theory for choking in vertical pneumatic transport lines.

L-valve equations

Abstract Non-mechanical valves, especially the L-valves, have been used extensively in fluidized beds and circulating fluidized beds for solids recycling or to serve as a pressure seal. Despite their widespread use, reliable characterization equations for L-valves are still not available, though design principles have been proposed by Knowlton of the Institute of Gas Technology. This paper presents a set of L-valve equations which relate the solids flow rate to the L-valve design, aeration rate, and pressure drop across the L-valve. The equations were developed by visualizing a L-valve as a pneumatically-operated pseudo-mechanical valve. The valve opening is activated pneumatically by L-valve aeration. The physical interpretation of the L-valve operation corresponds to that observed by Knowlton and other researchers, and the proposed equations correlate well with the extensive data base. The data base covers L-valve diameter from 38 mm to 152 mm for particles ranging from 175 μm to 509 μm, particle density ranging from 1230 kg m −1 to 4150 kg m −3 , and shape factor ranging from 0.56 to 0.915.

Comparison of jetting phenomena in 30-cm and 3-m diameter semicircular fluidized beds

Jetting phenomena in a 30-cm and a 3-m diameter semicircular transparent cold flow models were compared. Momentum dissipation, jet penetration depth, and jet velocity profiles were studied by visual observation and pitot tube traverse. Gas interchange between the jet and the emulsion phase of the fluidized bed was investigated by injection of tracer gas helium or carbon dioxide. The two-phase Froude number first proposed empirically to correlate the jet penetration depth was found to be still applicable for large jets up to 255-mm in diameter. A theoretical basis based on buoyancy is suggested for the applicability of the two-phase Froude number. Extension of the Froude number to apply in the case of two-phase jets where jets also carry solids and in the case of concentric jets is also outlined. Not only the gas velocity profiles in the jet are similar, tracer gas injection data indicated that gas concentration profiles in the jet are also similar as well.

Solid entrainment rate into gas and gas—solid, two-phase jets in a fluidized bed☆

Abstract A mathematical model for solid entrainment into a permanent flamelike jet in a fluidized bed was proposed. The model was supplemented by particle velocity data obtained by following movies frame by frame in a motion analyzer. The experiments were performed at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings ranging from 0 to 2.75. The particle entrainment velocity into the jet was found to increase with increases in distance from the jet nozzle, to increase with increases in jet velocity and to decrease with increases in solid loading in the gas—solid, two-phase jet.

Survey of characterization techniques of dry ultrafine coals and their relationships to transport, handling and storage

Abstract Handling of fine materials is always problematic because of the great variability in system configurations and in interacting properties that control the behavior of the solids in these systems. Three dry ultrafine coals having mass-mean diameter from 7 to 25 microns have been subjected to a variety of tests to explore their characteristics and suitability for transport, handling and storage. Flow and storage tests were conducted to evaluate the relationships between the coal characteristics and their performance under these conditions. The characterization was carried out experimentally by measuring the following coal properties: a1. flooding — related to bin/feeder performance; a2. absorption of pressure pulses — related to bin performance; b1. Hausner ratio — cohesive factor — related to fluidization, bin/feeder performance, conveyability; b2. de-aeration — related to bin/feeder performance, conveyability; b3. fluidization — related to bin/feeder performance, conveyability; c1. pickup velocity — related to conveyability; d1. shear stress — related to bin performance; e1. size and size distribution —general classification; e2. shape — general classification. The measured values of the various parameters are reported and related to the flow observations. Of particular interest is the relation between flooding and degree of aeration and between pressure wave absorption and aeration, which leads to the suggestion that several flow problems can be modified by controlling the degree of aeration in the bin or hopper.

Momentum Dissipation of and Gas Entrainment into a Gas-Solid Two-Phase Jet in a Fluidized Bed

Gas velocity profiles in a gas-solid two-phase jet inside a fluidized bed were determined at five different horizontal planes perpendicular to jet direction using a pitot tube. The experiments were conducted at three nominal jet velocities (35, 48, and 63 m/s) and with solid loadings (weight of solid/weight of gas) ranging from 0 to 2.75. The velocity profiles were integrated graphically, and gas entrainment into a jet was found to occur primarily at the base of the jet. The gas velocity profiles in the jet and the gas entrainment into the jet are reported.

Operational analysis of ash-agglomerating fluidized bed gasifiers

The operational characteristics of ash-agglomerating fluidized bed gasifiers are analyzed through a simplified approach. Based on the simple theoretical analysis, true steady state operations in ash-agglomerating fluidized bed gasifiers require that the ash concentration in the bottom ash withdrawal be equal to the steady state ash concentration dictated by the kinetics of coal combustion and gasification, and other process variables in the fluidized bed gasifiers. There are only three steady state operational modes possible depending on the coupled effects of ash agglomeration rate and the particle separation efficiency of char and ash agglomerates. Without precise knowledge of the ash agglomeration rate, the combustion and gasification kinetics, and the rate and degree of particle separation between char and ash agglomerates, experimental trial and error is required to approach the true steady state. Locating the true steady state or the lack of it may seem unimportant in a pilot plant operation, where solids samples are obtained hourly so that correctional steps can be implemented quickly, its implication for the commercial operation can be serious.

Large-scale fluidized bed physical model: methodology and results

Abstract Fluidized bed physical modeling principles are identified and applied to simulate a large-jetting fluidized bed. Physical (cold) model results from 30-cm and 3-m gasifier simulation units for initial bubble diameter, bubble frequency, gas leakage, bubble velocity, jet penetration depth and jet half-angle are correlated and compared with previous studies.

Energy losses and particle—wall interactions on rough surfaces

Abstract The development of coal combustion and gasification units, in which gas—solid phases are present, has created the necessity for refractory materials resistant to particle impact. Refractory concretes appear as promising materials for pipe and vessel linings, since they meet cost and construction criteria. The design and modeling of conveying lines in these units requires data on materials degradation and particles energy losses upon impact. An erosion study was performed in a commercial-scale pneumatic test facility, in which the erosion rates of different refractory concretes were measured at high temperatures under various operating conditions. River sand, coal slag and tabular alumina were used as eroding materials. The effects of gas velocity and solids concentration were measured. The erosion process generated extremely rough surfaces, whose topology is determined by the concrete microstructure and projectile characteristics. The mechanisms of erosion were identified and the resulting surfaces were characterized using fractal analysis. The effect of this high roughness on the pressure drop in pipes was determined at high temperatures. Particle energy losses upon impact on eroded surfaces were determined in a different set-up, using high-speed photography. As a result, rebounding angles and velocities were correlated for the different materials and surface conditions.

A study of fine particles residence time in a jetting fluidized bed

Abstract Experiments were carried out to study the residence time of fine particles in a 30-cm dia. laboratory fluidized bed. Fines, − 170 +230 mesh with an average size of 75 μm, were injected both radially and coaxially to simulate different fines recycle possibilities in an actual operating unit at two different bed heights and three different bed operating conditions. It was found that the transitory time of fines from the bed surface to the collecting cyclone dipleg and filter dominated the characteristic particle collection times. The experimental results were corrected for this transitory time to arrive at an estimate of the fines residence time in the bed. The true fines residence time in the bed is of the order of 10 to 20 s. However, the experimental uncertainties in the current experiments do not allow exact determination of the residence time distribution.