Cylindrical Bed Research Articles

Radial Thermal Conductivity in Cylindrical Beds Packed by Shaped Particles

Experimental data on radial thermal conductivity of beds formed of solid cylinders, Rashig rings, six-spoke wheels, four- and 52-hole cylindrical pellets are presented. The use of shaped particles in packed beds can significantly improve radial heat transfer. Thermal conductivity of the bed increases with the increase of the channel cross-section in the particle. The hydrodynamic approach to the description of convective radial thermal conductivity in packed beds of cylindrical particles with arbitrary number and form of channels is proposed. The approach is based on the account of mean fluid velocities around and through the hollow cylinders in the packed bed.

The fission power of a conceptual fluidised bed thermal nuclear reactor

The fluidised bed thermal nuclear reactor investigated in this paper is an innovative reactor design in which I mm diameter TRISO-coated fuel particles are fluidised by helium gas coolant in a 2.5 m diameter and 6 m high cylindrical bed. The coolant flow rate provides part of the reactivity control mechanism. The TRISO-coated particles have an enriched uranium oxide kernel surrounded by layers of porous carbon, pyrolytic carbon and silicon carbide. This paper presents detailed transient modelling results of this conceptual fluidised bed thermal nuclear reactor obtained using the FETCH nuclear criticality model. Previous work has provided evidence to suggest that such a reactor can be dynamically stable for low power outputs of ∼20 MWt. This work focuses on a reactor with a much higher thermal output of 100 MWt. To simulate the fluidised bed reactor the FETCH model has been used to solve the neutron transport equation in full-phase space, coupled to multi-phase gas-particle fluid dynamics. The main difficulty in modelling such a reactor is that its reactivity is a sensitive function of the fuel particle distribution inside the inner fluidised bed reactor cavity. This fuel particle distribution varies chaotically with time which is the root cause of the reactor's power variability.

The combustion of oil sand beds containing an axial fracture

The processes within simplified laboratory cylindrical beds of specially reconstituted oil sand during oil recovery are described. The beds contained a small diameter cylindrical axial passage of high permeability representing a fracture where hot vitiated air of known composition was fed steadily. The fuel sand cores tested contained either pure hydrocarbon or extracted oil off naturally occurring oil sands. It was shown that combustion appeared to have been sustained mainly by the oxidation of the light fuel fractions rather than by the coke deposits produced by the cracking reactions. Most of the burning took place, for the conditions examined, within the fracture rather than the main granular bed.

Radial heat transfer in packed beds of spheres, cylinders and Rashig rings: Verification of model with a linear variation of λer in the vicinity of the wall

Experimental data on the effective radial thermal conductivities and wall heat transfer coefficients of cylindrical beds formed of spheres, cylinders and Rashig rings are presented. The obtained heat transport parameters are compared with literature data. A model with a linear variation of λ er in the vicinity of the wall is proposed to the description of the radial heat transfer in the packed bed. The model allows simple correlation between the wall Nusselt number and the bed core effective radial thermal conductivity. The model is verified by experiments.

Experimental verification of the scaling relationships for bubbling gas-fluidised beds using the PEPT technique

An experimental verification of the scaling relationships for bubbling gas-fluidised beds has been carried out by measuring particle cycle frequency. The Positron Emission Particle Tracking technique was used, which enabled non-intrusive measurements of solids motion in three dimensions with a spatial resolution of ∼2 mm under the conditions of this work. Three cylindrical beds (70, 141 and 240 mm ID) equipped with multiple orifice-type distributors were tested. It was shown that for geometrically similar beds, the pair of Froude numbers based on the minimum fluidisation and excess gas velocities was sufficient for similarity at the viscous limit. The experimental results showed little influence of the gas-to-particle density ratio, but a small effect of particle-to-bed diameter ratio when the change in the ratio was significant. The experimental results also indicated a possible relaxation of the viscous limit up to a particle Reynolds number of about 100. More experimental studies are needed to confirm the validity of such a relaxation.

Structural Properties of Beds Packed with Ternary Mixtures of Spherical Particles: Part I–Global Properties

An experimental investigation of the global properties of cylindrical beds packed with ternary mixtures of spherical particles is described. The water displacement technique is used to determine the influence of different ternary mixtures of spherical particles on the mean voidage of such beds, as well as the effect of bed ends and different packing procedures. Over the range of beds and packings examined, the mean voidage of beds containing ternary mixtures is found to be affected by the tube-to-equivalent particle diameter ratio, the relative percentages of particles in a mixture, and the size ratio of the particle diameters, with ternary mixtures generally resulting in lower mean voidage values than for beds packed with mono-sized or binary mixtures of particles. Bed end, or entrance, effects on mean voidage are found to be significant for values of the ratio of bed length-to-diameter less than 3, with compression during packing resulting in the lowest overall mean voidage values. Complementary studies to evaluate the local properties of such beds are considered in a companion paper.

Thermal Conductivity of Oil Shale Particles in a Packed Bed

A steady-state one-dimensional heat transfer method was used to measure the effective thermal conductivity of Sultani and El-Lajjun oil shale particles. The particles were packed in a cylindrical bed and heated uniformly over a temperature range of 25 to 250°C. The effect of particle sizes of < 250, 500-710, and 1000-1400 w m on thermal conductivity was also investigated. The effective thermal conductivity of the samples was observed to decrease with increasing temperature. Moreover, the values of thermal conductivity were found to increase with increasing oil content of the shale samples. Over the tested range, particle size showed little effect on thermal conductivity.

Formation Mechanism of AlN–SiC Solid Solution by Combustion Nitridation in Si3N4–Si‐Al‐C System

The synthesis of solid solutions of AlN–SiC was investigated through the combustion reaction between Si3N4, aluminum, and carbon powders and nitrogen gas at pressures ranging from 0.1 to 6.0 MPa. The combustion reaction was initiated locally and then the wave front propagated spontaneously, passing through the cylindrical bed containing the loose powder. In the presence of Si3N4 as a reactant, it was feasible to synthesize solid solutions at an ambient pressure (0.1 MPa). The relationship between nitrogen pressure and full‐width at half‐maximum of the (110) peak of the product showed that lower pressures produced more‐homogeneous solid solutions. Some aspects of formation of the AlN–SiC solid solutions were discussed with special emphasis on the influence of nitrogen pressure and reactant stoichiometry.

Impact of tube‐to‐particle‐diameter ratio on pressure drop in packed beds

AIChE JournalVolume 46, Issue 5 p. 1084-1088 Fluid Mechanics and Transport Phenomenon Impact of tube-to-particle-diameter ratio on pressure drop in packed beds M. Winterberg, M. Winterberg Thermal Process Engineering, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, GermanySearch for more papers by this authorE. Tsotsas, E. Tsotsas Thermal Process Engineering, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, GermanySearch for more papers by this author M. Winterberg, M. Winterberg Thermal Process Engineering, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, GermanySearch for more papers by this authorE. Tsotsas, E. Tsotsas Thermal Process Engineering, Otto-von-Guericke-University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, GermanySearch for more papers by this author First published: 16 April 2004 https://doi.org/10.1002/aic.690460519Citations: 74AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume46, Issue5May 2000Pages 1084-1088 RelatedInformation

Heat and mass transfer during drying of granular products — simulation with convective and conductive boundary conditions

Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with specific boundary conditions: evaporation, which occurs with forced air flowing through the bed, is intensified with a conductive heat flux from the wall. We consider two mathematical models of heat and mass transfer through granular medium: two temperature model (No Local Temperature Equilibrium Model: NLTEModel) and one temperature model (Local Temperature Equilibrium Model: LTEModel). The air–grain mass transfer is expressed with a drying rate equation outcome from experiments. The resolution of the equations set is carried out, using a finite volume method. Different effects, such as the effect of the porosity variation on energy and mass transport, are discussed. Sensitivity to external conditions is determined. Finally, validity of local thermal assumption is examined.

Gas spouting hydrodynamics of fine particles

AbstractHydrodynamic measurements for the spouting of fine particles were obtained in a conical‐based cylindrical bed of diameter 0.186 m, inlet orifice diameters of 6 to 14 mm, and included cone angles of 45 to 80 degrees. Four different types of fine particles with diameters ranging from 0.286 to 0.861 mm were investigated. Four different flow regimes—fixed bed, stable spouting, unstable spouting and slugging—were identified and mapped. The termination of the spouting of fine particles was caused by choking rather than by fluidization of particles in the upper bed. Stable spouting could be achieved when the ratio of the orifice diameter to mean particle diameter was less than 20. The measured minimum spouting velocities were within ±15% of those predicted by the Mathur—Gishler equation. Solids density measurements obtained by a gamma ray densitometer showed that the solid density in the spout increases with bed height and in the annulus is 3% to 10% higher than in a loosely packed bed.

Two-dimensional mathematical model and numerical simulation describing the melting process of cylindrical basalt bed

A two-dimensional mathematical model was built to describe the melting process of cylindrical basalt particle bed in a crucible. The melting processes with respect to the factors of thermal boundary conditions and particle sizes of basalt were simulated by using the numerical method (FDM). The governing equations were discretized in tridiagonal matrix form and were solved by using the tridiagonal matrix algorithm (TDMA) as well as the alternative direction implicit (ADI) solver. The temperature distribution, the moving law of the two dimensional phase-change boundaries, the thermal current distribution were given through the numerical simulation.

The effect of moisture on the adsorption of xenon by activated carbon.

The following procedure was used to determine the effect of moisture on the adsorption of xenon from air onto activated carbon: A known amount of water is added to a sample of dried activated carbon and allowed to equilibrate. This activated carbon is then split between two cylindrical beds placed in a temperature controlled water bath, and air is passed through the beds sequentially. Because the beds contain pre-moistened activated carbon from the same sample, the first bed acts as a buffer, maintaining a constant humidity in the second. The mean holdup time of a pulse of 133Xe injected into the second bed is used to determine the adsorption coefficient for xenon under these conditions. Measurements were made for three carbons activated to 35, 40, and 59%, respectively, at temperatures of 25 degrees C and 55 degrees C. The effect of moisture on the same activated carbon at these two temperatures shows an affine relationship that could be helpful in extending these results to other temperatures. At low moisture uptakes, a plot of the log(adsorption coefficient) vs. moisture uptake gives a straight line.

HEAT AND MASS TRANSFER DURING DRYING OF GRANULAR PRODUCTS BY COMBINED CONVECTION AND CONDUCTION

Numerical simulation of grain drying in a vertical cylindrical bed has been carried out with an imposed hot air flow and a conductive heat flux at the wall. The model equations are numerically solved using a finite volume method. The numerical simulation gives the time and space evolution of temperature when the lateral area of the cylinder is heated by a constant density flux and a constant temperature. The influence of different parameters (essentially the ratio of heat flux to the heat capacity of flow, and the dryer geometry) on the relative moisture content and the drying time is examined.

Thermal Analysis of Energy Storage in Packed Beds of Multilayer Storing Medium

Thermal analysis of energy storage in layers of various materials used as a packed bed storage system is presented. It is more economical to minimize insulation costs, to store more energy from the same input conditions, to lengthen the time of charging that the energy could be kept stored with acceptable losses, and to obtain higher storage capacity. These are the focus of this work. A cylindrical bed is formed from-three layers of different materials, equal in length, as heat-absorbing media. The bed is charged with flowing hot air in the axial direction, the hot air representing the heat source. A transient one-dimensional model is used to describe the thermal behavior of the system. The partial differential equations that govern the flow and heat transfer for both the air and the solid, constituting the bed and their boundary conditions, are driven. The numerical solution of two partial differential equations is obtained using the finite difference method through a computer program especially constructed for this purpose. The temperature field for the air and the solid are obtained, and the energy stored inside the bed is computed. A wide range of layers of storage media of different materials with different thermal properties is selected. Comparison is made between the present system and packed bed systems using one storing medium. The results show that the bed packed with different layers has higher storage capacity and there is step stratification of the temperature between the material layers. Also, the bed that includes metal material (such as steel) has a maximum storage capacity, and less considerable temperature than other beds, so such a bed can realize the claimed function.

Thermal Storage in Ammonium Alum/Ammonium Nitrate Eutectic for Solar Space Heating Applications

Ammonium alum and ammonium nitrate in the weight ratio of 1:1 form a eutectic that melts at 53°C and solidifies at 48°C. The thermophysical properties of this eutectic were measured in detail and the eutectic was found to have properties desirable for energy storage for solar space heating applications. The eutectic was encapsulated in 0.0254-m diameter high-density polyethylene (HDPE) balls and packed into a cylindrical bed in a scale model for testing its heat transfer characteristics when exposed to an air flow. Test results indicate that the thermal extraction efficiency of the model was 89 percent with an uncertainty of ±8.0 percent. The packed bed had a Stanton number value in close agreement with that predicted with an empirical equation for sensible heat extraction from the eutectic in the solid phase. This Stanton number was increased by about 74 percent for sensible heat extraction from the eutectic in the liquid phase, a phenomenon not previously reported in the literature.

An x-ray study of bubbles in fluidised beds

Synopsis Gas-fluidised beds are characterised by the formation of bubbles which rise through them and determine the pattern of particle movement and gas flow within. The basic nature and shape of the bubbles are of interest not only in predicting the overall behaviour of the bed (from a gas-solids contacting point of view, for example) but also when considering the hydrodynamic characteristics of fluidised systems. In this paper the previous methods of studying bubbles are briefly discussed and the results are presented of observations of bubbles in cylindrical beds using the technique of X-ray cine photography. Measurements of the shapes, sizes, and velocities of bubbles in beds of spherical ballotini, irregular natural sand, and several other materials are given. It is observed that stable bubbles in all these materials have an essentially spherical form with the particle-wake region occupying the lower part of the sphere. Both the size of the particle wake and the bubble velocity depend upon the size and shape of the fluidised particles. Cine sequences showing the events (splitting and coalescence, for example) experienced by bubbles as they rise through the bed are presented and some statistical evidence for the frequency of these events is given. The size of bubble produced by a given volumetric injection of gas was measured. Only under special circumstances were bubble and injection volume equal.

Temperature Profiles in a Wood Packed Bed Heated by Hot Inert Gases

Presented in this paper are temperature profiles in a green-wood packed bed heated by hot inert gases for the sensible heating stage of wood carbonization. They give insight into the heat transfer during the initial carbonization stage and hence are useful for carbonization retort design. Gas temperatures within a cylindrical bed, packed with green wood particles, were measured at different heights across the diameter by means of thermocouples. The wood bed was heated by inert hot gases from a charcoal combustion chamber. There was a high temperature drop which happenedmostly in the first one third of the bed. This was attributed to the high moisture content of the wood. The measured radial temperature variations showed a symmetrical profile between each side of the bed; the centre being the highest with strong local horizontal variations. This uneven heat distribution could affect the quality and uniformity of the charcoal produced. It was deduced that for uniform charcoal production, a short and wide bed was more appropriate. An attempt was made to predict the axial temperatures profiles using the one-dimensional axial mathematical model, initially proposed by Schumann 6 and extended by Rosen 10 and Jeffreson1 11 . The bed was physically modelled as thin independent elements similar to that proposed by Parti 5 . The predictions were within 14% of the measured values. Using the axial temperatures at every measurement height, an empirical relationship for radial temperatures was suggested.

COMPARISON OF SIMULATED PERFORMANCE OF FIXED ION EXCHANGE BEDS IN LINEAR AND RADIAL FLOW

ABSTRACT Simulations are performed using models developed to predict ion exchange performance in fixed cylindrical resin beds. One model predicts performance in a conventional bed in which the flow is parallel to the bed's axial, while the other model predicts performance in a wedge-shaped radial bed. In this radial bed the injected solution flows radially from the inlet located at the annular bed's axis to the outlet located at the bed's periphery. The simulated breakthrough curves for the exhaustion and regeneration cycles for the Ca-H cation exchange system are compared to study the effects of operating conditions. It is found that because dispersion grows less in radial flow than in linear flow, radial ion exchange processes are more efficient than the conventional ion exchange column processes under identical operating conditions for the geometries simulated.

Spouted bed electrowinning of zinc: Part II. Investigations of the dynamics of particles in large thin spouted beds

The behavior of particles in thin spouted beds, mostly equipped with draft tubes, has been investigated. Three apparatuses have been used: a laboratory-scale cylindrical bed, a 2-m-tall “flat” (rectangular cross section) bed and a 2-m-wide flat bed, the last equipped with multiple draft tubes. Most of the results were obtained on the tall bed. Minimum spouting flow rate, pressure distribution, particle velocities, and solid circulation rates were determined as a function of bed geometry (including draft tube dimensions and position). Observations were made of the direction of liquid flow in the bed outside the draft tube and of the occurrence of zones in the bed where the particles appeared stationary. The wide bed was used to determine that there is a maximum separation between draft tubes beyond which particles cannot be kept in motion across the whole width of the bed.