Spout-fluid Bed Research Articles

Solid circulation and gas bypassing in spout-fluidized bed with draft tube at elevated pressure

Solid circulation and gas bypassing in spout-fluidized bed with draft tube at elevated pressure

Voidage and particle velocity profiles in a spout‐fluid bed

AbstractDetailed hydodynamic measurements have been obtained for fully cylindrical spout‐fluid beds of 1.3 mm, 1.8 mm and 2.5 mm glass beads in a fully cylindrical column of diameter 152 mm using three different types of optical fibre probes. The reallocation of up to 43% of the air to auxiliary air introduced through the conical base caused some decrease in spout voidages, but remarkably little change in spout diameter compared with spouted beds where there was no auxiliary air addition. Auxiliary air led also to some decrease in particle velocities in the spout and to a modest decrease in the net solids circulation rate.

Experimental Investigations of Catalytic Partial Oxidation of Methane to Synthesis Gas in Various Types of Fluidized-Bed Reactors

The partial oxidation of methane to synthesis gas over Ni/α-Al 2 O 3 catalysts (1 and 5 wt% Ni loading, 71-160 and 250-355 μm particle diameter) was investigated in different types of fluidized-bed reactors, i.e., the bubbling fluidized bed (FIB), the spout fluid bed (SFB) and the internally circulating fluidized bed (ICFB). A methane-to-oxygen ratio of 2:1 was used in all experiments and the temperature was varied between 700 and 800°C. Gas velocities and catalyst masses were adjusted to assure a stable and controllable reactor operation. A nearly isothermal operation was established in all reactors. The thermodynamic equilibrium values were achieved in the FIB and SFB reactor whereas in the ICFB reactor slightly lower conversions and selectivities were obtained. Taking the direct scale-up concept of the ICFB reactor into account, significant higher space-time yields were obtained in this reactor than in the industrial-scale bubbling fluidized-bed reactor. No increase of the space-time yield in comparison to the FIB was obtained in the SFB reactor.

The application of computer-based imaging to the measurements of particle velocity and voidage profiles in a fluidized bed

Computer-based video imaging techniques were utilized in order to measure the axial and radial components of particle velocity and voidage profiles in the draft tube region of a semi-circular spouted fluid bed coating device. Computer images were obtained using standard RS-170 video signals (30 frames/s) from an image processing board and a CCD variable shutter speed camera. When measuring particle velocities, low shutter speeds of 1–2 ms were utilized. Due to rapid motion of particles, the images obtained were blurred streaks several millimeters in length. The length and direction of streaks were proportional to the magnitude and direction of the projected velocity vector. By using dispersed back lighting, the particle velocity images appeared as light streaks on a dark background. Custom software was written in order to automatically search for and to identify bright streaks and calculate particle velocities. The velocity measurements obtained from the software were compared with data taken with a high speed Kodak video system (1000 frames/s with a 20-μs strobe). The results were in close agreement. The video images for analysis of voidage measurements were obtained using the same techniques as for the velocity measurements except with a shutter speed of 0.1 ms. The images thus obtained showed the particles to have a dark outer ring with a light center. The search algorithm for identifying particles involved establishing the gray level for nine different locations: eight compass points and a center point. By searching the frame for the presence of particles, and then comparing the gray levels at the circumference and the center of the particle image it was possible to distinguish between in-focus and out-of-focus particles. A comparison of the results using the software with those of standard models made with glass beads, and by visually counting the particles, was in close agreement.

Solid circulation in a liquid spout-fluid bed with multi draft tubes

Considering the application of the technique of liquid spouting with multi draft tubes to heat exchangers, the solids circulation rate was studied both experimentally and theoretically. It was found that the solids circulation rate increased with the liquid velocity and with the amount of solids used. Experimental results were found to be lower than the calculated values. It was concluded that the friction forces between closely packed particles and between particles and the wall of the return pipe at the zone of entrainment retard the solids circulation rate. The rate of the calculated to the experimental circulation rates was observed to be lower as the concentration of solids increased. The effect of the slip velocity on the particle velocity and the calculated values of solids circulation rate was also investigated. © 1998 SCI

BEAN DRYING IN FIXED, SPOUTED AND SPOUT-FLUID BEDS: A COMPARISON AND EMPIRICAL MODELING

ABSTRACT In this work the drying of IAC-Carioca bean was analyzed experimentally in fixed, spouted and spout-fluid beds. A laboratory scale, batch operated apparatus was used. To obtain experimental data, a factorial design technique was used and the influences of the following variables on the process were evaluated: the solids toad, initial grain moisture and drying air temperature. This analysis was done for each dryer by means of drying curves, evolution of grain temperature over time, drying rate and statistical results obtained from factorial design. An energy evaluation was also done and used as a means of comparison between the dryers studied. The three dryers operating under the same conditions showed very similar behavior and the fixed bed dryer showed a performance slightly better. With this work the potential of using spouted bed grain dryerswas confirmed and the possibility of using the spout-fluid bed as a dryer was verified. Two empirical models were proposed and their parameters were correlated with experimental variables. A sensory evaluation was also performed for the purpose of evaluating the effects that the high drying temperatures had on the grain's potential value as food.

Modelling of a plasma reactor for the synthesis of calcium carbide

Plasma synthesis of calcium carbide was investigated using calcium oxide and graphite powders. A semi-batch spout-fluid bed reactor with a DC plasma torch was used for the study. Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reactor consisted of two different zones : a high temperature reaction zone and a well mixed isothermal bed zone. The size of the reaction zone and the particle flow rate into the jet depended on the net plasma power. The reaction rate was correlated by a shrinking core, reaction control model and showed excellent fit for conditions where hydrogen was present in the plasma gas and heat transfer limitations were negligible. The apparent activation energy of the reaction was determined to be 377 kJ/mol (90 kcal/mol). Extrapolation of the reaction model and the experimental results indicated that a plasma fluid bed process could be a technically viable and a more efficient alternative for the production of calcium carbide.

SOLID-PHASE SYNTHESIS OF CALCIUM CARBIDE IN A PLASMA REACTOR

A laboratory-scale spout-fluid bed reactor with a dc plasma torch was used to study the solid-phase synthesis of calcium carbide. Calcium oxide powder with a mean particle size of 170 μm was reacted with graphite powder (130 μm). Argon was used to initiate the plasma and hydrogen gas was then added to increase power and raise the plasma jet enthalpy. Experimental results showed that the reaction took place in the vicinity of the plasma jet and that conversion to calcium carbide increased linearly with reaction time. The rate of conversion increased exponentially with plasma jet temperature, indicating that chemical reaction was the controlling mechanism. Microscopic analysis of the solid product showed that calcium carbide was formed around both reactants, and that the reaction followed a shrinking core model. Although melting and agglomeration of partially reacted particles occurred at high temperature, resulting in instability of the bed and impeding the reaction progress, high conversions are expected in a continuous process with optimized reactor design.

Hydrodynamic modeling of vertical accelerating gas-solids flow

The one-dimensional model of accelerating turbulent gas-solids dilute phase flow of coarse particles is formulated and experimentally verified by measuring the pressure distribution along the transport tube. The theoretical basis of the model is the continuity and momentum equations for the fluid and particle phases of Nakamura and Capes (Can. J. Chem. Eng., 51 (1973) 39) and the authors' variational model for calculating fluid-particle interphase drag coefficient (Grbavcic et al., Powder Technol., 68 (1991) 199). The main model assumptions are that the constituted relations for fluid-particle interphase drag coefficient, fluid-wall friction coefficient and particle-wall friction coefficient remain the same regardless of the acceleration of the fluid and particle phases. In addition, the transport line inlet voidage required for closure has been derived for the cases where top-closed spouted or spout-fluid beds were used as a feeding device to the transport tube. Experiments were performed by transporting spherical glass particles 1.94 mm in diameter in a 30 mm i.d. glass tube. Reynolds number for the particles, based on slip velocity, ranged from 1524 to 2562 and loading ratio was varied between 4.8 and 26.4. Model predictions for the pressure distribution along the transport tube are in good agreement with experimental measurements.

Gas Holdup and Axial Dispersion Coefficient in Gas-Liquid Cocurrent Spout-Fluid Beds

Abstract An experimental investigation was carried out to determine the effect of gas velocity, liquid velocity, and column diameter on the gas holdup and axial dispersion coefficients in cocurrent spout-fluid beds in three columns with diameters of 74, 114, and 144 mm, all with a height of 1.20 m, at superficial gas and liquid velocities of 0.001–0.186 m·s−1 and 0.002–0.06 m·s−1, respectively. The axially dispersed plug flow model equation was solved by using the finite difference technique and compared with the analytical solution proposed by Uysal and Anabtawi. Gas holdup was found to increase with increases of both the gas velocity and column diameter. The effect of liquid velocity on the gas holdup was found to be insignificant. The axial dispersion coefficient was found to increase with increasing gas velocity, liquid velocity, and column diameter. New correlations for predicting the gas holdup and axial dispersion coefficient in a spout-fluid bed and based on large data for a two-phase system are p...

Gas Holdup and Volumetric Liquid-Phase Mass Transfer Coefficient in a Spout-Fluid Bed.

The effects of gas velocity, liquid velocity and nozzle diameter on the gas holdup and the volumetric liquid phase mass transfer coefficient, kLa, were studied a experimentally in a spout-fluid bed by absorption of oxygen in air into tap water. Gas holdup was found to increase with increasing gas velocity and to decrease with increasing liquid velocity and not to depend on nozzle diameter. kLa was found to increase with increasing gas velocity and liquid velocity and to decrease with increasing nozzle diameter. kLa was found to be strongly dependent on the liquid velocity. Two correlations for gas holdup and kLa in spout-fluid bed were presented with maximum deviation of 8% and 7% respectively.

Coal gasification in a pressurized spouted bed

Gasification characteristics are described for five coals of different rank which were gasified with air-steam or oxygen-steam mixtures in a 5 kg coal h −1 continuous spouted bed reactor at pressures to 1440 kPa. Results show the suitability of the different coals for dry ash spouted-fluid bed gasification, the processing requirements for low and medium calorific value gas, and the approach to equilibrium expected for various coals. For one of the coals, a comparison is made between results from the bench scale reactor and those for a forty times larger demonstration plant.

Minimum Spouting Velocity, Minimum Spout-fluidized Velocity and Maximum Spoutable Bed Height in a Gas-solid Bidimensional Spout-fluid Bed.

Hydrodynamic studies in a bidimensional spout-fluidized bed was conducted using four materials of seven particle sizes spout-fluidized with air. Four nozzles of different diameters were used and their effect on minimum spouting velocity, minimum spout-fluidizing velocity and maximum spoutable bed height were determined. Three correlations, for predicting the minimum spouting velocity, minimum spout-fluidized velocity and maximum spoutable bed depth, were developed and compared with corresponding equations in the literature.

Flow characteristics in a rectangular spout-fluid bed

Hydrodynamic studies are reported on six different materials, spouted and spout-fluidised with air in a column of square cross-section, using nozzles of five different sizes. Correlations for minimum spout-fluidised flow rate in cylindrical columns were investigated for this geometry. New correlations for predicting the minimum spouting velocity and the minimum spout-fluidising velocity are developed. The velocity profile in the bed agreed well with the observations of Lim and Grace [1], and Mamuro and Hattori [2].

Spouted and spout-fluid bed combustors 1: Devolatilization and combustion of coal volatiles

The devolatilization and volatile combustion of a single coal particle in spouted and spout-fluid beds have been studied. The results showed that the flame extinction time increases with the particle diameter, and decreases with the bed temperature. When the bed temperature and the air flow rate were fixed, the operation modes (spouted or spout-fluid bed) showed less effect on the mean flame extinction time. A mathematical model of the spouted bed mode for preignition and postignition periods has also been developed assuming the devolatilization rate to be controlled by heat transfer and multireaction pyrolysis kinetics based on volatile products. Ignition, heat transfer back from the volatile flame to the particle surface, variation in flame temperature, and the hydrodynamics of SB are taken into account. The model predictions, with some adjusting parameters, were in good agreement with experimental results.

Combustion behaviour of coal and carbon in spouted and spout-fluid beds

This paper presents the experimental study of batch combustion of Thai lignite and pyrolysed electrode carbon in spouted bed and spout-fluid bed combustors. The combustion behaviours of evolved coal volatiles and char particle fragmentation are also investigated and compared with pyrolysed electrode carbon

Spouted and spout-fluid bed combustors. 2: Batch combustion of carbon

SUMMARY A batch combustion model of carbon particles in a spouted bed hasheen developed based on the concept of carbon residence time in each region of the spouted bed (annulus, spout and fountain). Both isothermal and nonisothermal particles are considered, with the assumption of no temperature gradient inside the particle. Model predictions in terms of carbon conversion and burnout time were in good agreement with experimental data obtained from a laboratory scale spouted bed combustor. For comparison, an experimental study on a spout-fluid mode was also carried out.

Prediction of the maximum spoutable bed height in spout-fluid beds

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An axisymmetric model for fluid flow in the annulus of a spout-fluid bed

The axisymmetric model previously developed for spouted beds is adapted to describe fluid flow in the annulus of a spout-fluid bed at the minimum spout-fluid flow rate. Experimental axial pressure profiles provide the boundary condition at the spout-annulus interface. The fluid streamlines, residence time distribution, average residence time, average axial fluid velocity in the annulus and the fluid flow rate entering the annulus from the spout are all calculated. The fluid streamlines in spout-fluid beds are shown to be very different from those of spouted beds in the lower part of the annulus. Although the residence time distribution of the fluid percolating through the annulus of a spout-fluid bed is more uniform than that of a spouted bed, the average residence time does not vary significantly with the inlet fluid flow to the annulus.

SOLIDS CIRCULATION IN SPOUTED AND SPOUT-FLUID BEDS WITH DRAFT-TUBES

A 20 cm semi-cylindrical spout-fluid bed with a draft-tube has been used to investigate the effect of various physical parameters on the solids circulation rate, A new method has been developed by which these data can be easily and accurately collected. Equations are presented which can be used to predict the maximum attainable solids circulation rate and the spouting velocity required to achieve this maximum for the unit operating in spouting mode. The circulation trends are shown to be a direct result of the phenomena of entrainment and jetting occurring in the entrainment zone investigated visually with a video camera. Preliminary gas bypassing experiments were also performed which show the ability to almost eliminate spout gas bypassing to the annulus with appropriate auxiliary gas addition.