Ind. Eng. Chem. Res. 1987,26, 763-767
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Direct Imaging of Time-Averaged Flow Patterns in a Fluidized Reactor Using X-ray Computed Tomography William F. Banholzer,* Clifford L. Spiro, Philip G. Kosky, and Donald H. Maylotte General Electric Company, Corporate Research and Development, KI-CEB 410, Schenectady, New York 12301
X-ray computed tomography (CT), a technique which has revolutionized medical imaging diagnostics, has been applied t o the study of fluidization. In particular, a model fluidized bed reactor containing powdered coal was placed inside a medical CT device and scanned under a range of experimental conditions. The CT technique enables nondestructive, noninvasive visualization based on differences in the X-ray attenuation. For coal, X-ray attenuation is dominated by Compton scattering so that visualization is essentially that of the localized bed density to a n excellent approximation. Spatial resolution is 0.087 mm3, while density resolution is better than 30 kg/m3. Density differences within the confines of the fluidized bed were observed and quantified. Gradients in static, fluidized, and spouted beds were observed. At the highest superficial velocity tested (0.288m/s), channeling and/or a stream of bubbles along a central vortex region was noted. Wall effects were identified which would be obscured by visual examination. General agreement between actual and expected flow patterns was found. Reactions and processes involving contact of a gas and a solid, particularly those involving highly exothermic reactions, are conveniently carried out in fluidized beds. Fluidized beds are unsurpassed in maximizing both gassolid contact and solids mixing. However, since fluidized beds involve at least two phases as well as poorly characterized dynamic processes, such as bubble formation, they are very difficult to analyze and model. Characterization of the fluid dynamics inside a fluidized bed reactor is essential in forming an accurate picture of the reaction processes involved. Experimental verification of flow patterns in a fluidized bed is very difficult. Solids are usually opaque to most low-energy electromagnetic radiation, preventing observation of the flow patterns by conventional techniques (i.e., visual observation using tracers and laser dopplerimetry). Three techniques have provided experimental verification of flow patterns. The first is based on pressure fluctuations between two vertical locations and provides information on the frequency and velocity of bubbles (Zhang et al., 1982; Fan et al., 1983; Ghate, 1983; Nicastro and Glicksman, 1984). A more widely used technique uses capacitance probes which must be inserted into the bed. Changes in the capacitance of the bed can be correlated with bed density but only give information at one point. This has proven useful in obtaining local information such as heat-transfer rates near heated and submerged surfaces (Biyikli et al., 1983). To determine the overall flow pattern, a capacitance probe must be moved to a large number of locations or a series of probes must be used (Homsy, 1981; Yutani et al., 1983; Almstedt and Olsson, 1982; Fasching et al., 1982). The procedure is laborious and gives poor spacial resolution. In addition, it is always possible to disturb the flow in the proximity of the probe, producing artifacts. The third technique is X-ray shadow photography. Rowe and co-workers have used this technique to visualize flow patterns inside fluidized beds (Barreto et al., 1983; Rowe and MacGillivray, 1980; Rowe et al., 1978; Rowe and Yacono, 1976). The technique mimics the familiar medical X-ray transmission technique. Differences in electron density in the exposed subject appear as shadows on a photographic film. Shadow radiography produces a two-
* Author
t o whom correspondence should be addressed.
0888-5885/87/2626-0763$01.50/0
dimensional projection of a three-dimensional object. The shadowing in a radiograph removes the ability to discern patterns at different depths, complicating image analysis. However, an important advantage is that images are recorded with very high temporal resolution and can often reveal individual components of bed structure. This paper describes the use of X-ray computed tomography (CT) as a nonintrusive technique to directly observe the time-averaged flow pattern inside a laboratory scale fluidized bed. The details of the technique as applied to nonmedical applications appear elsewhere (Maylotte et al., 1982). There are essential differences in the CT technique and the shadow-graph method. In particular, CT produces images taken as a two-dimensional (2D) slice through the object rather than as a projection of a 3D object onto a plane. Therefore, images are not degraded by adjacent planes within the object. The data are given in digitalized form so that the field of view is divided into discrete “pixels”, elements of area out of which an image slice is reconstructed. Each slice is of a thickness determined by the X-ray beam collimation, the volume of each pixel slice being a “voxel”. Therefore, the size of a voxel determines the spacial resolution and is determined by the physical construction of the instrument. Since a unique attenuation coefficient is thus attributed to each voxel, the reconstructed field of view is a map of the local density. These slices can be pieced together to produce a true three-dimensional image. A General Electric 8800 Series whole body medical scanner was employed in this work. This instrument uses an 88-kW rotating anode X-ray tube mounted on a rotatable gantry. Attached to the gantry, opposite the object X-ray tube, are 517 gas-filled X-ray detectors. The fluidized bed was placed in the middle of the gantry where a patient would usually be located. The X-ray beam was collimated to produce a flat fan 1500-1m thick with an angle of 30 deg. This fan beam passed through the fluidized bed, and the X-rays, attenuated by the fluidized bed, were measured by the detector array. One set of detector readings was a “view”. After each view, the gantry was rotated a small amount, and another view was taken. The process was repeated 576 times until the gantry made close to a 360-deg arc around the fluidized bed. One complete rotation required 9 s and was called a “scan”. This time is determined by the mechanics of the gantry 8 1987 American Chemical Society
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which support the rotating X-ray tube and detector array and for a given machine is not variable by the operator. The fastest rotating anode machine has a scan time of 2.5 s. Electronically simulated rotational CT machines with millisecond capability do exist (Schlosser et al., 1980); however, we did not have access to these devices. The 9 s required to take a scan determine the time frame of events one can examine. Events occurring in less than 9 s will be unresolved, with the final picture representing a 9-s average of the higher frequency motion. The CT technique therefore is complementary to the X-ray shadow-graph method, the former emphasizing spacial resolution, the latter stressing temporal resolution. Individual bubbles in motion cannot be viewed in CT image; rather the density averaged over 9 s is determined. Notwithstanding this difficulty, CT is being used in the general area of two-phase flow. Considerations of resolution and discrimination of individual two-phase-flow features have been discussed (Seshadri et al., 1986). The 576 X 517 attenuation coefficients produced by rotating the gantry are reconstructed to give a map of the attenuation experienced by the X-rays during their passage through the fluidized bed. With the instrumentation used in this study, each voxel was 0.086 mm3 with a pixel resolution of 0.24 X 0.24 mm. Attenuation of the X-rays results from three causes: Compton scattering, photoelectron scattering, and coherent scattering. Above 40 keV, Compton scattering is responsible for the majority of the attenuation experienced by the X-rays. The 88-kW rotating anode produces X-rays with energies from 20 to 120 keV. For this discussion, all the attenuation may be considered due to Compton scattering, although the contributions from all three mechanisms are determined in analyzing the data. Assuming that attenuation is caused by Compton scattering, the attenuation in each voxel is proportional to the number of electrons in that voxel (electron density). For low 2 elements, such as C and 0, the atomic weight is approximately twice the atomic number. Thus, the electron density is roughly proportional to the mass density. By use of organic liquid phantoms of known density, the CT scans can be calibrated and converted into mass density maps. Plots of density vs. CT number are very nearly linear in the range p = 600-1300 kg/m3 (Maylotte et al., 1982) with a slope of 9.4 kg/m3/CT number. The attenuation coefficients, in the form of CT numbers, are arranged in a scale of 4096 units for the full-scale range of the instrument. The parameters used in this study produced a density resolution of 400 0 210-115 3.0 400-305 4.7 115-20 47.1 305-210 8.6
