Local Time-average Gas Holdup Research Articles

Hydrodynamics Study in a fluidized bed reactor Using Gamma Ray Computed Tomography (CT) Technique

The main goal of this study is to further understand the fluidized bed hydrodynamics by determining the effects of superficial gas velocity and bed density on the hydrodynamics of gas-solid fluidized beds by using a 14-cm-diameter Plexiglas cylindrical fluidized bed column, air at room temperature was used as the fluidizing gas, two different Geld art type-B particles were used glass bead and copper particle with material densities of 2500, 5300 Kg/m3 respectively with the same size particle 210 µm. Three different superficial gas velocities were investigated Ug=25, 30, and 35 cm/s. Non-invasive flow measurement technique gamma ray computed tomography (CT) has been used to measure the time-averaged cross-sectional and radial profiles of solids and gas holdups distribution. The results show that local time-averaged gas holdup is a function of bed material density, that as material density decreases, gas holdup increases, gas holdup increases with increasing superficial gas velocities

Probe effects on the local gas holdup conditions in a fluidized bed

Fluidized beds are found in the energy production industry, and knowing their hydrodynamic conditions, such as local gas holdup or volumetric gas fraction, is important for effective operation. Local measurement probes are commonly used to measure local gas holdup. Although the probes are typically as small as possible to mitigate their influence on the local conditions, the invasive nature of this measurement technique has the potential to alter the fluidized bed behavior. This study used a noninvasive measurement technique, X-ray computed tomography (CT) imaging, to determine the local time-average gas holdup everywhere within a 10.2cm diameter fluidized bed filled with 500–600μm glass beads in which simulated probes were inserted. Three different simulated probe tips (pointed, rounded, and flat) and two different orientations (horizontal and vertical) were investigated at four different bed heights and two different fluidization velocities. The results show increases in local time-average gas holdup when the probes were present, but the scale of the effects was dependent upon the height at which the probes were placed; the lower the probe within the fluidized bed, the larger the local effects. The shape of the probe tip did not show any differences in the local gas holdup. On the other hand, probe orientation was an important factor, with the horizontal orientation showing a sustained increase in gas holdup values farther from the probe. The two different flow rates used in this study (1.5Umf and 3Umf, where Umf is the minimum fluidization velocity) also showed differences in the amount of variation in gas holdup, but the trends were inconsistent. Overall, the local hydrodynamic effects of the simulated invasive probes of this work were minimized when: (i) the probes were placed in the central part of the bed, outside the aeration region and top of the bed, (ii) the fluidized bed was operated at higher superficial gas velocities, and (iii) the probes were mounted in a vertical orientation.

Characterizing Jetting in an Acoustic Fluidized Bed Using X-Ray Computed Tomography

Understanding the jetting phenomena near the gas distributor plate in a fluidized bed is important to gas–solid mixing, heat and mass transfer, and erosion to any bed internals, which can all affect the performance of the bed. Moreover, acoustic vibration in a fluidized bed can be used to enhance the fluidization quality of the particulate matter and influence the jetting behavior. Characterizing the jetting structure using X-ray computed tomography (CT) in a three-dimensional (3D) fluidized bed, with and without acoustic intervention, is the focus of this study. A 10.2 cm ID fluidized bed filled with glass beads and ground walnut shell, with material densities of 2500 kg/m3 and 1440 kg/m3, respectively, and particle sizes ranging between 212 and 600 μm, is used in these experiments. X-ray CT imaging is used to determine local time-average gas holdup. From this information, qualitative and quantitative characteristics of the hydrodynamic structure of the multiphase flow system are determined. Local time-average gas holdup images of the fluidized bed under acoustic intervention at a high superficial gas velocity show that jets produced near the aeration plate merge with other jets at a higher axial position of the bed compared to the no acoustic condition. Acoustic fluidized beds also have a fewer number of active jets than the no acoustic fluidized bed, which allowed for a more homogeneous gas holdup region deep in the bed. Hence, the acoustic presence has a significant effect on the jetting phenomena near the aeration plate in a fluidized bed.

Local time-averaged gas holdup in fluidized bed reactor using gamma ray computed tomography technique (CT)

Many invasive and non-invasive techniques have been used to analyze the hydrodynamics of fluidized beds. In this study, the effect of superficial gas velocity and bed particle density on the hydrodynamics of gas–solid fluidized beds was investigated by using a cylindrical plexiglas fluidized bed column, 14 cm in diameter. Air at room temperature was used as the fluidizing gas and two different Geldart type-B particles were used: glass beads and copper particles with material densities of 2.5 and 5.3 g/cm3, respectively, with the same size particle, 210 µm. To measure the time-averaged cross-sectional gas and solid holdup distribution, gamma ray computed tomography was used for the first time as a non-invasive technique instead of using X-rays (due to the height attenuation of the copper particles). The results show that gas holdup increases by increasing the superficial gas velocity, and decreasing the particle density increases the gas holdup in the bed.

Acoustic fluidized bed hydrodynamics characterization using X-ray computed tomography

Acoustic vibration in a fluidized bed can be used to enhance the fluidization quality of particulate matter; this particular noninvasive method provides no internal changes to the bed material structure. However, due to the complexity of this multiphase flow system, characterizing the hydrodynamics of a fluidized bed has become critical in understanding this system behavior. The local void fraction behavior in a cold flow 3D fluidized bed with and without acoustic intervention is investigated in this research. Several noninvasive techniques like gamma-ray computed tomography (GRT), X-ray computed tomography (XCT), or electrical capacitance tomography (ECT), can be used to study the void fraction distribution in multiphase flow systems. In this study, XCT imaging is used to determine the time-average local void fraction or gas holdup. Experiments are implemented in a 10.2cm ID fluidized bed filled with glass beads or ground walnut shell, having a material density of 2500kg/m3 and 1440kg/m3, respectively, and particle size ranges between 212 and 600μm. In this study, three different bed height-to-diameter ratios are examined: H/D=0.5, 1 and 1.5. The loudspeaker’s frequency, used as the acoustic source, is fixed at 150Hz with a sound pressure level of 120dB for glass beads, and 200Hz and 110dB for ground walnut shell. Local time-average gas holdup results show that the fluidized bed under the presence of an acoustic field provides a more uniform fluidization, the bed exhibits less channeling, and the jetting phenomena produced by the distributor plate is less prominent when compared to no acoustic field. Thus, acoustic intervention affects the local hydrodynamic behavior of the fluidized bed.

Comparisons of Annular Hydrodynamic Structures in 3D Fluidized Beds Using X-Ray Computed Tomography Imaging

Fluidized beds are common equipment in many process industries. Knowledge of the hydrodynamics within a fluidized bed on the local scale is important for the improvement of scale-up and process efficiencies. This knowledge is lacking due to limited observational technologies at the local scale. This paper uses X-ray computed tomography (CT) imaging to describe the local time-average gas holdup differences of annular hydrodynamic structures that arise through axisymmetric annular flow in a 10.2 cm and 15.2 cm diameter cold flow fluidized bed. The aeration scheme used is similar to that provided by a porous plate and hydrodynamic results can be directly compared. Geldart type B glass bead, ground walnut shell, and crushed corncob particles were studied at various superficial gas velocities. Assuming axisymmetry, the local 3D time-average gas holdup data acquired through X-ray CT imaging was averaged over concentric annuli, resulting in a 2D annular and time-average gas holdup map. These gas holdup maps show that four different types of annular hydrodynamic structures occur in the fluidized beds of this study: zones of (1) aeration jetting, (2) bubble coalescence, (3) bubble rise, and (4) particle shear. Changes in the superficial gas velocities, bed diameters, and bed material densities display changes in these zones. The 2D gas holdup maps provide a benchmark that can be used by computational fluid dynamic (CFD) users for the direct comparisons of 2D models, assuming axisymmetric annular flow.

Local time-average gas holdup comparisons in cold flow fluidized beds with side-air injection

Fluidized beds are interesting and useful processing systems that are employed in many industries such as processing biomass into biofuels or the coating of pharmaceuticals. Knowledge of fluidized bed hydrodynamics is necessary for the design and scale-up of such devices. This paper describes the local time-average differences of gas holdup in a 10.2cm and 15.2cm diameter cold flow fluidized bed that were recorded using 3D X-ray computed tomography. Three different Geldart type B bed materials are studied at various superficial gas velocities and side-air injection flow rates, where the side-air injection simulated the immediate volatilization of a fuel like coal or biomass particles. Variations in side-air injection flow rate have little influence on global bed hydrodynamics, but significantly affects local gas holdup. Axial annular flow dominates over all flow conditions for each material and bed diameter. Wall effects increasingly influence hydrodynamics as bed diameter decreases for all materials.

The repeatability and uniformity of 3D fluidized beds

Hydrodynamic characterization of fluidized beds is vital to many industrial processes, including the production of biofuels, pharmaceuticals, and numerous petroleum products. Local time-average phase fractions can be quantified in these multiphase three-dimensional systems using X-ray computed tomography (CT). This paper describes repeatability and uniformity measurements in a 3D 15.2 cm diameter cold flow fluidized bed using CT imaging to quantify gas holdup. Two different bed materials and a range of superficial gas velocities are investigated. Five tests over different days are completed to show repeatability for each test condition. Highly repeatable local time-average gas holdup is recorded at various locations within the bed for each material over all flow conditions. Uniformity is determined by analyzing the local time-average gas holdup at 12 azimuthal locations for both materials and various flow conditions. Uniform axi-symmetric fluidization is observed for each material and U g when the bed height is greater than h = 0.25D, where the effects of individual aeration holes dissipate.

Bed height and material density effects on fluidized bed hydrodynamics

Characterizing the hydrodynamics of a fluidized bed is of vital importance to understanding the behavior of this multiphase flow system. Minimum fluidization velocity and gas holdup are two of these key characteristics. Experimental studies addressing the effects of bed height and material density on the minimum fluidization velocity and gas holdup were carried out in this study using a 10.2 cm diameter cylindrical fluidized bed. Three different Geldart type-B particles were tested: glass beads, ground walnut shell, and ground corncob, with material densities of 2600, 1300, and 1000 kg/m 3, respectively. The particle size range was selected to be the same for all three materials and corresponded to 500–600 μm. In this study, five different bed height-to-diameter ratios were investigated: H/ D=0.5, 1, 1.5, 2, and 3. Minimum fluidization velocity was determined for each H/ D ratio using pressure drop measurements. Local time-average gas holdup was determined using non-invasive X-ray computed tomography imaging. Results show that minimum fluidization velocity is not affected by the change in bed height. However, as the material density increased, the minimum fluidization velocity increased. Finally, local time-average gas holdup values revealed that bed hydrodynamics were similar for all bed heights, but differed when the material density was changed.

Local time-averaged gas holdup in a fluidized bed with side air injection using X-ray computed tomography

Local time-averaged gas holdup in a 10.2 cm diameter fluidized bed is determined using X-ray computed tomography (CT) over a range of superficial gas velocities ( U g), side air injection flow rates ( Q side), and fluidized bed material. Without side air injection, only small variations in the local time-averaged gas holdup are observed for beds composed of glass beads, ground walnut shell, or ground corncob. With the introduction of side air injection, which simulates the immediate volatilization of biomass in a fluidized bed gasifier, a distinctive plume is observed along the reactor wall above the side injection port. The plume gradually expands toward the center of the bed as height increases; the expansion is found to increase with increasing Q side. As U g increases, fluidization becomes more uniform and the effect of the side air injection on the fluidization hydrodynamics is less pronounced. Additionally, increasing U g increases overall gas holdup and bed expansion. Of the three bed materials examined, ground corncob fluidization is the least affected by side air injection and shows the highest overall gas holdup while glass bead fluidization is much more affected by side air injection and has the lowest overall gas holdup. This study demonstrates the usefulness of X-ray CT in noninvasively visualizing detailed internal features of fluidized beds. These results will be used in future studies to validate computational fluid dynamics (CFD) models of fluidized beds.

Hydrodynamics of Bubble Column Reactors at High Gas Velocity: Experiments and Computational Fluid Dynamics (CFD) Simulations

This paper focuses on the modeling of flow and mixing in a bubble column reactor operated at high gas velocities (up to 0.40 m/s). A dual-tip conductivity probe was used to measure local void properties such as local time-averaged gas holdup, chord length distribution, bubble velocity distribution, and interfacial area. Chord length distribution was converted to bubble size distribution, using the backward transformation method. Liquid-phase mixing time measurements were conducted using a conductivity probe. A computational fluid dynamics (CFD) model was developed to simulate the unsteady gas−liquid flow in a bubble column using commercial code FLUENT 6.2. The time-averaged flow properties predicted by CFD simulations were compared with the experimental data. The role of unsteady flow structures in mixing was studied. The “multiple snapshots” approach was used to simulate the mixing time using CFD. The mixing times that were predicted for all superficial gas velocities compared favorably to the measured values. This study of the hydrodynamic behavior of a bubble column at higher gas velocity provides a basis for understanding and simulating solid suspension (or solid mixing) in slurry bubble column reactors.