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Fluidized-bed reactors are widely used in the biofuel industry for combustion, pyrolysis, and gasification processes. In this work, a lab-scale fluidized-bed reactor without and with side-gas injection and filled with 500-600 lm glass beads is simulated using the computational fluid dynamics (CFD) code Fluent 6.3, and the results are compared to experimental data obtained using pressure measurements and 3D X-ray computed tomography. An initial grid-dependence CFD study is carried out using 2D simulations, and it is shown that a 4-mm grid resolution is sufficient to capture the time-and spatial-averaged local gas holdup in the lab-scale reactor. Full 3D simulations are then compared with the experimental data on 2D vertical slices through the fluidized bed. Both the experiments and CFD simulations without side-gas injection show that in the cross section of the fluidized bed there are two large off-center symmetric regions in which the gas holdup is larger than in the center of the fluidized bed. The 3D simulations using the Syamlal-O'Brien and Gidaspow drag models predict well the local gas holdup variation throughout the entire fluidized bed when compared to the experimental data. In comparison, simulations with the Wen-Yu drag model generally over predict the local gas holdup. The agreement between experiments and simulations with side-gas injection is generally good, where the side-gas injection simulates the immediate volatilization of biomass. However, the effect of the side-gas injection extends further into the fluidized bed in the experiments as compared to the simulations. Overall the simulations under predict the gas dispersion rate above the side-gas injector. V V C 2009 American Institute of Chemical Engineers AIChE J, 56: 1434AIChE J, 56: -1446AIChE J, 56: , 2010 Keywords: biofuel processing, fluidized-bed reactor, gas holdup, multiphase flow, tomography IntroductionFluidized beds are commonly used by the chemical, mineral, pharmaceutical, and energy industries because of low Correspondence concerning this article should be addressed to R. O. Fox at rofox@iastate.edu. PARTICLE TECHNOLOGY AND FLUIDIZATION pressure drops, uniform temperature distributions, and high energy-and mass-transfer rates. In the biofuel industry, fluidized-bed reactors are central components for combustion, pyrolysis, and gasification processes. The local gas holdup (or solids voidage) distribution in the fluidized bed is an extremely important parameter in practical systems, determining the uniform mass and energy distributions and gasification efficiency. Because of the rapid time scales associated with the fluidization dynamics, time-averaged (as opposed to instantaneous) data are often more meaningful than transient data in the design of commercial-scale fluidized-bed reactors.Because the bed material (solid phase) is typically opaque in gas-solid systems, it is difficult to obtain detailed local data (either instantaneous or time-averaged) for the entire bed. As fluidization is a dynamic process, invasive monitoring meth...

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Fluidized beds are commonly found in the chemical and energy processing industries because of their low pressure drop, uniform temperature distribution, and high heat transfer rates. For example, in biomass gasification, biomass particles are injected into a heated bubbling bed of inert material (typically refractory sand) that volatilizes to form a flammable gas. However, the movement of the biomass particle through the bubbling bed is difficult to quantify because the systems are opaque. This paper describes X-ray particle tracking velocimetry (XPTV) applied to fluidized beds, where X-ray flow visualization is used to track the location of a single fabricated tracer particle as a function of time in a fluidized bed to study the bed/particle hydrodynamics. Using stereoscopic X-ray imaging, the 3D position of the tracer particle as a function of time is determined, from which tracer particle velocity can be calculated. Details and challenges of the XPTV process are also summarized.

Fluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. Knowing how the fluidized bed hydrodynamics vary as reactor dimensions are scaled up is vital for improving reactor efficiency. This study utilizes 10.2 cm and 15.2 cm diameter fluidized beds with added side port air injection to investigate column diameter effects on fluidized bed hydrodynamics. Both inert (glass beads) and biomass (ground walnut shell and ground corncob) bed materials are used and the hydrodynamic differences with side port air injection are recorded. Minimum fluidization velocity is determined through pressure drop measurements. Time-averaged local and global gas holdup are recorded using X-ray computed tomography imaging. Results show that by varying the side port air flow rate as a percentage of the minimum fluidization flow rate, partial and complete fluidization is observed in both fluidized beds. Local gas holdup trends are also similar in both fluidized beds. These results will be used in future studies to validate computational fluid dynamics models of fluidized beds.Keywords fluidized bed, gasification, minimum fluidization velocity, particle injection, x-ray computed tomography ABSTRACTFluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. Knowing how the fluidized bed hydrodynamics vary as reactor dimensions are scaled up is vital for improving reactor efficiency. This study utilizes 10.2 em and 15.2 em diameter fluidized beds with added side port air injection to investigate column diameter effects on fluidized bed hydrodynamics. Both inert (glass beads) and biomass (ground walnut shell and ground corncob) bed materials are used and the hydrodynamic differences with side port air injection are recorded. Minimum fluidization velocity is determined through pressure drop measurements. Timeaveraged local and global gas holdup are recorded using X-ray computed tomography imaging. Results show that by varying the side port air flow rate as a percentage of the minimum fluidization flow rate, partial and complete fluidization is observed in both fluidized beds. Local gas holdup trends are also similar in both fluidized beds. These results will be used in future studies to validate computational fluid dynamics models of fluidized beds.