Ind. Eng. Chem. Res. 2010, 49, 5891–5899
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Magnetic Resonance Studies of Fluidization Regimes D. J. Holland,* C. R. Mu¨ller, J. S. Dennis, L. F. Gladden, and J. F. Davidson Department of Chemical Engineering and Biotechnology, UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom
Measurements of the voidage and velocity distributions in a gas-fluidized bed operating in the bubbling, turbulent, and core-annular fluidization regimes were acquired using magnetic resonance imaging (MR). The bed studied was contained in a column 50 mm in diameter and was fluidized with air. The particles were silica-alumina catalyst support loaded with water doped with gadolinium (diameter 63 µm, density of the water loaded particles 1530 kg m-3). Both time-averaged and ultrafast measurements are presented and provide the first noninvasive measurements of the velocities of particles and local voidage in a fluidized bed in each of the flow regimes. Measurements of the pressure fluctuations were also recorded as a function of the superficial velocity of the fluidizing gas to compare with the MR measurements. Ultrafast measurements of the voidage were used to examine the void structures present in the different flow regimes and to provide a means of studying the dynamics. A novel MR technique was used to measure the velocities of the particles in a diametral region of 15 mm × 15 mm square cross-section through the center of the bed every 7.7 ms. These measurements confirmed that the highest particle velocities in the bubbling fluidization regime occurred in the wakes of bubbles. The distribution of particle velocities in a bubbling bed is highly skewed; however, it approaches a Gaussian distribution and appears to scale with the superficial gas velocity in the turbulent fluidization regime. Finally, a simple model to infer the slip velocity in core-annular fluidization indicates that the particles in the center of the column group together in clusters with a diameter of between 4 and 8 particles. 1. Introduction Despite a large body of work concerning the transitions from bubbling to turbulent fluidization and beyond to the onset of core-annular flow, there is still a lack of understanding surrounding factors affecting these transitions owing to the challenge associated with making noninvasive measurements in fluidized beds. This is especially difficult in turbulent fluidized beds where the flow structures are stable only for very short times. Magnetic resonance (MR) imaging has previously been used to provide noninvasive measurements of detailed voidage and flow structures in the center of a 3-dimensional (3D) fluidized bed in the bubbling regime.1 This study reports the first application of MR to make measurements in turbulent fluidized beds and in core-annular flow and demonstrates how the results can be used to provide new information on the flow behavior occurring in them. Turbulent fluidized beds are widely used in industry owing to the excellent gas-solids contacting, for example in FCC regeneration, Fischer-Tropsch synthesis or fluidized bed driers. There is an excellent review by Bi et al.,2 which summarizes the research on turbulent fluidization prior to 2000. Measurements of turbulent fluidization have generally been limited to point measurements, with the exception of the work of Du et al., which used electrical capacitance tomography, requiring significant axial averaging of the voidage. More recently, 3D reconstruction techniques have been developed for ECT that obviate the axial averaging,4 but these have not yet been applied to turbulent flow. Turbulent fluidization occurs at superficial gas velocities between those giving bubble or slug flow and core-annular flow. In bubble and slug flow, the void and dense phases are distinct and void structures (viz. bubbles or slugs) remain stable for extended periods of time. In core-annular flow, the system has two distinct regionssa relatively uniform * To whom correspondence should be addressed. E-mail: djh79@ cam.ac.uk.
region with a high solids density at the wall and a region of much lower solids density in the core. In turbulent fluidization, neither the dense phase nor the gas phase predominates and the flow behavior is characterized by unstable void structures. These structures govern the behavior of the bed and thus are essential to understanding and characterizing the bed as a whole.5 Core-annular flow has been more widely studied than turbulent fluidization owing to its relatively stable and welldefined flow structures.6 These studies have identified the key parameters characterizing core-annular fluidization: (i) a falling film of particles at the wall, (ii) a core consisting of upwardflowing gas containing a relatively low concentration of solids in the form of clusters or streams of particles, and (iii) small fluctuations in the pressure, compared with bubbling or turbulent beds. Clusters in core-annular flow are defined as collections of particles with a lower voidage than the average value in the core region, although there is no precise definition of the value of the voidage distinguishing a cluster. Clusters arise because the flow of air around a particle can reduce the drag on particles further down the flow field. This causes the particles to approach one another and hence group together in the form of a cluster. Determining the characteristics of these clusters of particles is critical for understanding the behavior of circulating fluidized beds and core-annular flow, as these will determine the residence time of the particles and the mass transfer between the gas and solid phases. Despite this, few noninvasive measurements of features such as the cluster size, cluster velocity, and voidage have been reported. Measurements of these features are essential in the development of phenomenological models (e.g. that of Petersen and Werther7) and numerical models (e.g. that of Enwald et al.8) of circulating fluidized beds. In industry, fluidized beds are commonly used when the desired reaction is highly exothermic. In such systems, intraparticle and bed-scale heat transfer are important;9 in particular, the transfer of heat to the surroundings is often critical to the
10.1021/ie901450q 2010 American Chemical Society Published on Web 05/24/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
Figure 2. Particle size distribution measurements before and after fluidization studies. A slight change in the size distribution was observed due to elutriation of fine particles.
Figure 1. Schematic showing the layout and characteristic dimensions of the fluidized bed. The broken lines indicate the positions at which MR measurements were acquired and correspond to (A) 50, (B) 100, and (C) 150 mm above the distributor.
safe operation of these reactors. This heat transfer is governed by the solids transport and heat transfer from the solids to the walls or heat exchanger surfaces.9 As a result, the heat transfer properties of fluidized beds are often governed by the velocity distribution of the particles and how this influences the mixing of the solid and gas. For example, if the transfer of solids from the bed to the wall is very rapid then heat transfer will be governed by this exchange. By contrast, if the exchange is slow, then the heat transfer will be governed by conduction through the bed material. Therefore, a better understanding of the particle velocity distribution in a fluidized bed could provide a better understanding of heat transfer mechanisms, and in particular, how these vary in the different fluidization regimes. In this paper, two types of measurement are presented. First, measurements of absolute fluctuations in pressure characterize the flow behavior, using a conventional technique. Second, timeaveraged and ultrafast MR measurements give voidage and void structures and determine the local spatial average particle velocity in the fluidized bed. Both types of measurement were applied to a fluidized bed operating at gas velocities covering bubbling, turbulent, and core-annular flow. 2. Experimental Section 2.1. Fluidized Bed. A schematic diagram of the fluidized bed used is shown in Figure 1. It consisted of a Perspex tube 1.3 m long and of internal diameter 50 mm; this was the maximum feasible diameter, limited by the internal diameter of the radio frequency (rf) coil available for the MR experiments. A disengaging device at the top limited the elutriation of particles: it was a conical expansion to a diameter of 250 mm over a height of 600 mm; see Figure 1. The distributor was a porous glass frit (porosity ∼40%, pore size range 100-160 µm). The pressure drop across the distributor was 100-3000 Pa, depending on the superficial gas velocity. The particles employed were silica-alumina catalyst particles (Sigma); they were sieved prior to the experiments to remove particles passing
through a 45 µm sieve. The particle-size distribution was then measured using a Coulter LS light scattering device; results are shown in Figure 2. The surface mean diameter for the prefluidization particles (see Figure 2) was 63 µm. As discussed in earlier work,1 to obtain a MR signal from the solid particles, they must contain nuclei, preferably 1H nuclei, with favorably long (>1 ms) nuclear spin relaxation times (T1 and T2). Since the silica-alumina particles used here have nuclei with far shorter relaxation times, the particles were doped with water, readily absorbed because the particles have a large internal surface area. The particles were loaded with 40 wt % deionized water, which remains in the internal pores of the particles and does not impair their free-flowing nature. The relaxation times of deionized water loaded onto the internal pores of these particles were T1 ∼100 ms and T2 ∼10 ms. For the ultrafast MR imaging techniques used here, the signal-to-noise ratio can be improved by using water doped with 0.5 kg m-3 of gadolinium chloride hexahydrate (GdCl3.6H2O); this reduces the T1 relaxation time to 30 ms and the T2 relaxation time to 3 ms. These relaxation times are sufficiently long that the signal is detectable but sufficiently short to enable ultrafast measurements of voidage and velocity. Prior to an experiment, the loaded particles were vigorously fluidized for 4 h using humidified air to ensure that the water was evenly distributed throughout the particles and that they remained free-flowing. Humidified air has the further advantage of reducing electrostatic attraction between the particles. The bed was initially loaded with 220 g of saturated particles (particle density, Fs ) 1530 kg m-3). The superficial velocity of the air at the onset of fluidization for these particles, Umf, was ∼2 mm s-1 measured at 298 K and 1 atm. The air was supplied at ambient temperature by a compressor and the flow rate was measured by a rotameter maintained at 1 barg. For air velocities in excess of ∼0.1 m s-1, particles enter the disengaging zone, but mostly return to the bed; the maximum superficial air velocity that could be used without significant elutriation was 0.64 m s-1 (at 298 K and 1 atm). To characterize the transition to core-annular flow, superficial gas velocities up to 0.7 m s-1 were used, elutriating 50 g of particles during an experiment lasting about 1 h. The size distribution was measured after the experiments. The results in Figure 2, before and after fluidization, show an increase in surface mean particle size from 63 to 70 µm. This increase is attributed to the elutriation of a greater fraction of fine particles (
