Axial Solids Research Articles

The sedimentation‐dispersion model for slurry bubble columns

Slurry bubble columns have been widely used in petrochemical and coal conversion industries, and increasingly applied in fermentation and wastewater treatment fields. A slurry bubble column operated under the expanded bed regime is characterized by an axial solids concentration distribution in the column. Quantitative description of this distribution has been based mainly on the sedimentation-dispersion (S-D) model. In this paper, the physical nature of the S-D model in the literature is discussed in light of a rigorous analysis of the axial solids concentration distribution based on a macroscopic balance of the solid phase. General validity of the model for various modes of operation of slurry bubble column systems is also discussed.

Hydrodynamics of a three‐phase fluidized bed containing low‐density particles

AbstractExperiments were conducted to study the hydrodynamics of a gas‐liquid‐solid fluidized bed containing low‐density particles. The density and size of the particles employed were comparable to those ordinarily encountered in fluidized‐bed bioreactors for either wastewater treatment or fermentation. A dual resistivity probe and a conductivity probe were employed to study the axial holdup of gas and liquid phases and the bubble size distribution. The behavior of the axial holdup distribution for solid, gas, and liquid phases is reported. A mechanistic model, based on solids entrainment and deentrainment mechanisms by bubbles, is developed to describe the behavior of the axial solids holdup distribution. The model is shown to satisfactorily account for the experimental data.

Melting mechanism in single screw extrusion

AbstractFlow visualization experiments of polycarbonate and polystyrene resin extrusion were performed to observe the melting mechanism and the flow kinematics around the solid‐bed in the melting zone of a single screw extruder. The axial solids content and pressure profile calculations of the Tadmor melting model were modified as a result of variable solid‐bed velocity and solid‐bed temperature observations.

A bubble model describing the influence of internals on gas fluidization

Pipes, baffles, perforated plates, and other types of internals are often employed in multiphase flow systems in order to improve important performance parameters such as staging, bubble size and mass transfer. Previous studies in our laboratory on gas/liquid bubble flow (Roes et al., 1984) have demonstrated the efficacy of horizontal perforated plates with a free area of ca. 70 % for reducing bubble size. We have now employed the same type of plates for the fluidization, by air, of a Geldart class-A powder in beds of 0.3 and 0.6 m diameter. The bubble hold-up was found to be larger than in the same bed without internals at the same gas superficial velocity. The solids dispersion coefficient was reduced by a factor of about 5 by the action of the internals. The experimental observations are quantitatively described with a mathematical model containing only one adjustable parameter, namely the maximum stable bubble size. The success of this model suggests that the effect of the internals can be described solely in terms of a reduction in maximum bubble size. The model can furthermore be used for scaling studies, for predicting the effect of an increase in equipment size on fluidization properties such as bubble hold-up and axial solids dispersion.

Gas and solids behavior in a baffled and unbaffled slurry bubble column

AbstractNew data and analyses are presented for describing gas and solids behavior in a slurry bubble column. Axial solids concentration and bubble size distributions were measured in a 0.108 m ID slurry bubble column apparatus operated at steady state conditions. The column was operated with and without internal baffles. The slurry and gas superficial velocities ranged from 0.0077 to 0.009 m/s and from 0.031 to 0.24 m/s, respectively. The solid phase consisted of glass spheres in a narrow size range with a mean diameter of 96.5 μm; the liquid phase consisted of tap water.With a one‐dimensional sedimentation‐dispersion model, the data have been used to predict average solids loadings and axial distributions of the solids for specified operating conditions. Local bubble size and concentration measurements indicate that the slurry bubble column was operated in the churn‐turbulent flow regime throughout the entire range of operating conditions used in this study. The impact of internal baffles and solids concentration on the gas and solids behavior is discussed.

Dispersed solid dynamics in a slurry bubble column

New data and theory are presented for describing dispersed solids in slurry bubble columns. Axial solids concentration distributions were measured in a 0.108 m.i.d. slurry bubble column apparatus operated at steady-state conditions. Slurry and gas superficial velocities ranged from 0.0 to 0.02 m/s and 0.03 to 0.20 m/s, respectively. The liquid-phase was either water or ethanol, and the solid-phase consisted of narrow-sized fractions of glass spheres. With a one-dimensional sedimentation-dispersion model, the data have been used to develop a method for predicting average solids loading and axial distribution of solids in a bubble column. Correlations are given for the hindred settling velocity, the solids dispersion coefficient, and the solids concentration at the top and bottom of the column. A solids distribution function is derived using one parameter that contains the hindred settling velocity and the solids dispersion coefficient. The new method is applied to data from the literature for three slurry bubble columns with diameters in the range of 0.066–0.286 m. The new method predicts axial distribution of solids with improved accuracy over existing methods, especially for non-aqueous systems and for those with low slurry feed concentration or high settling velocity.

Oxygen enrichment in rotary lime kilns

AbstractThe operation of a pilot‐scale rotary lime kiln, fired with natural gas, was studied to determine the effects of enriching the combustion air with oxygen. The variables of interest were the temperature, calcination and heat flow patterns within the kiln as functions of the fuel rate, oxgyen concentration and limestone feed rate. All tests were conducted with essentially a stoichiometric fuel/oxygen ratio and employed two different limestones. Data are presented to show the increase in kiln throughput that can be achieved using oxygen enrichment. The results are interpreted using detailed measurements of axial solids, gas and wall temperatures and axial calcination profiles. These permit the evaluation of the driving forces for the various heat exchange processes, and of local heat flow to the solids bed.

Axial solids mixing and bubble characteristics in gas-fluidized beds with vertical internals

Axial solids diffusivity (D sa) in batch gas-fluidized beds with an assembly of longitudinal springs/rods as internals is evaluated by the tracer technique using the one-dimensional simple diffusion model. Empirical correlations are presented. D sa decreases in the presence of internals, and further with a reduction in the voidage of the internal. These values are used in conjunction with the data on bed expansion to investigate the bubble characteristics such as the effective bubble size and the degree of interaction.

The mixing of solids in slugging gas fluidized beds

AbstractMeasurements have been made of axial solids mixing in fluidized beds of 0.05, 0.1, and 0.22 m diameter with a variety of materials at atmospheric pressure. Round nosed slugs, raining slugs, and turbulent bubbling have been investigated. Raining slugs occur with coarser materials. With cracking catalyst round nosed slugs form readily in the smaller diameter beds, but in the largest bed, breakdown to the turbulent state was normal. A model has been developed to predict the axial solids mixing in a slugging fluidized bed containing round nosed slugs. The model supposes that each slug is followed by a well‐mixed wake and a piston flow region. The model predicts well the scale effect noted by W. G. May. Axial mixing is fast for round nosed slugs, falls for turbulent bubbling, and is low for raining slugs.

Role of the elastic constants in negative thermal expansion of axial solids

In cubic solids the elastic properties cannot affect the sign of the thermal expansion, but in axial solids negative thermal expansion can arise not only from anisotropy in the Gruneisen functions, but also from elastic anisotropy. The origin of negative thermal linear expansion is discussed for arsenic, graphite, selenium, tellurium and indium. A negative linear compressibility is neither necessary nor sufficient to ensure negative thermal expansion. The extension of the treatment to solids of lower symmetry is outlined.