Impacts of dense heat exchanging internals on gas holdup cross-sectional distributions and profiles of bubble column using gamma ray Computed Tomography (CT) for FT synthesis (original) (raw)
Related papers
Chemical Engineering Science, 2018
The effects of the presence of the vertical internals of different sizes at a wide range of superficial gas velocity on the overall, local gas holdup distributions and their profiles have been studied and quantified in a 6-in. (0.14 m) Plexiglas bubble column with air-water system using a non-invasive advanced gamma-ray computed tomography (CT) technique. In this study, two sizes of Plexiglas vertical internals, having the same occupying area (25%) of the column’s cross-sectional area (CSA) that represents those used in Fischer-Tropsch synthesis, have been used within a range of superficial gas velocities that cover bubbly and churn turbulent flow regimes (0.05–0.45 m/s). The reconstructed CT scan images revealed that the bubble columns equipped with or without internals displayed a uniform cross-sectional gas holdup distribution (symmetric) for all studied superficial gas velocities. However, the bubble column equipped with 1-in. vertical internals exhibited more uniform gas holdup distribution than the column with 0.5-in. internals. Also, the visualization of the gas-liquid distributions for bubble columns with and without internals reveal that the well-known phenomenon of the core-annular liquid circulation pattern that observed in the bubble column without internals still exists in bubble column packed densely with vertical internals. Moreover, a remarkable increase in the gas holdup values at the wall region was achieved in the churn turbulent flow regime based on the insertion of the vertical internals inside the column as compared with using a bubble column without obstacles. Furthermore, the values of the gas holdup in the core region of the bubble column with vertical internals are similar to those of the bubble column without vertical internals when they are operated at high superficial gas velocity (churn turbulent flow regime), based on the free cross-sectional area (CSA) for the flow. In general, the magnitude of the gas holdup increased significantly with increasing superficial gas velocity for the bubble columns with and without internals. However, the gas holdup profile was shaped like a wavy line in the bubble column with vertical internals, whereas it exhibited a parabolic gas holdup profile in the bubble column without obstacles.
Experimental Thermal and Fluid Science, 2018
The impact of dense vertical internal tubes and their configurations on the gas holdup distributions and their diametrical profiles in pilot-scale bubble column is visualized and quantified for the first time ever using an advanced gamma-ray computed tomography (CT) technique. Two arrangements of vertical internals (circular and hexagonal configurations) occupying the same cross-sectional area (CSA) of the column (about 25% of the total cross-sectional area to represent the heat exchanging tubes that are used in the Fischer-Tropsch synthesis), were examined in addition to the measurement in the bubble column without vertical internals. Moreover, the gas holdup distribution results of the 18-inch (0.46 m in outer diameter, O.D.) bubble column are compared with an available data of 6-inch (0.15 m in O.D.) bubble columns with and without vertical internals. CT scans have been conducted for 18-inch bubble columns with and without vertical internals for the air-water system under a wide range of superficial gas velocity (0.05–0.45 m/s). The experimental results indicate that an improvement in the gas holdup distribution over the column's cross-sectional area is obtained when the vertical internal tubes (arranged in either a circular or a hexagonal configuration) were used. However, better cross-sectional gas holdup distribution was achieved in the bubble column with vertical internals arranged in a hexagonal configuration as compared to the bubble column without and with vertical internals arranged in a circular arrangement. Additionally, the averages of the cross-sectional gas holdup and their profiles for bubble column with and without vertical internals are close to each other when the bubble column with vertical internals is operating at a high superficial gas velocity, which is calculated based on the free cross-sectional area for the flow. Furthermore, the gas holdup distributions are further improved when the larger bubble column with vertical internals was used as compared to the 6-inch bubble columns with and without vertical internals.
International Journal of Multiphase Flow, 2018
An advanced gamma-ray computed tomography (CT) technique was used for the first time to visualize and quantify the impacts of the presence of heat-exchanging tubes and their configurations on the gas-liquid distributions and their profiles in a 6-inch (0.1524 m O.D.) Plexiglas® bubble column in an air-water reactor. Two superficial gas velocities (i.e., 0.2 and 0.45 m/s) were employed to simulate the churn turbulent flow regime. To investigate the impact of vertical internals configurations, three arrangements (i.e., hexagonal, circular without a central internal, and circular with a central internal) were employed in addition to the column with no internals. Using the same sized vertical internals and the same oc-cluded cross-sectional area (CSA), it was found that the configuration of the vertical internals significantly impacted the gas holdup distribution over the CSA of the column. All studied superficial gas velocities resulted in symmetrical gas holdup distributions over the CSA of the bubble columns without vertical internals; however, the columns equipped densely with vertical internals did not have symmetrical gas holdup distributions. The presence of an extra central tube in the circular configuration played a key role in the gas-liquid distribution over the CSA of the bubble column. The hexagonal configuration had the advantage of providing the best spread of the gas phase over the entire CSA of the column. Gas holdup values at the wall region of the bubble column increased with the addition of vertical tubes in all investigated configurations. However, a remarkable increase in the gas holdup values was obtained with the hexagonal configuration. The experimental data (i.e., gas holdup distributions and their diametrical profiles) can help to evaluate and validate three-dimensional (3-D) computational fluid dynamics (CFD) simulations to better predict the hydrodynamic parameters involved in these types of reactors. Published by Elsevier Ltd.
Impact of Internals on the Gas Holdup and Bubble Properties of a Bubble Column
Industrial & Engineering Chemistry Research, 2009
Most industrial applications involving bubble columns require the addition of internals for heat exchanging. However, most academic research on bubble columns has been performed in hollow (empty) ones. Hence, the impact of internals on the hydrodynamics of a bubble column has been insufficiently studied in the open literature. Accordingly, this study focuses on investigating the effect of internals that mimic those used in methanol synthesis (5% covered cross-sectional area) and the Fischer-Tropsch (FT) process (22% covered cross-sectional area) on the local gas holdup, gas-liquid interfacial area, bubble chord length, and bubble velocity distributions. The investigations have been performed using a four point optical probe in an 8 in. diameter column. An air-water system was used with superficial gas velocities up to 20 cm/s. An increase in gas holdup and interfacial area was obtained upon insertion of the internals, although the impact of the less dense internals (5% covered area) was not significant. However, in the case of dense internals, the bubble chord length decreased, yielding a decrease in the bubble velocity.
International Journal of Multiphase Flow, 2019
The effect of the vertical heat-exchanging tube bundle configurations on local gas holdup and bubble dynamics, including specific gas-liquid interfacial area, bubble chord length, bubble rise velocity, and bubble passage frequency have been studied using the 4-point optical fiber probe technique. Two different tube bundle configurations were investigated, circular tube bundle and hexagonal tube bundle. 30 internal tubes, each with a diameter of 0.5 inches (0.013 m), were used in each configuration occupying 25% of the column cross-section area to represent the heat exchanging tubes utilized in the Fischer–Tropsch process. The experimental work was performed in a 0.14 m inner diameter Plexiglas bubble column using an air-water system. The applied superficial gas velocities were based on the free cross-sectional area of the column available for fluid flow and were in the range of 0.02 to 0.45 m/s covering bubbly, transition, and churn turbulent flow regimes. Although the size and the number of the tubes in both configurations were similar, their effects on the hydrodynamics were found to be different. When compared to bubble column without internals, the circular tube bundle showed a significant increase in the local gas holdup in the core region and a decrease in the wall regions. Simultaneously, a substantial increase in the bubble chord length and bubble velocity was seen. Another important observation was the decrease in the interfacial area while using circular tube bundle. A distinct asymmetrical effect on the radial profiles of gas holdup and the specific interfacial area was observed when the hexagonal configuration was used. The gas holdup and interfacial area significantly increased on one side of the column and decreased on the other side. The bubble chord length and bubble rise velocity decreased, exhibiting a narrower distribution with smaller values, in comparison to the bubble column without internals.
Experimenental study of the effect of orifice inclination on gas holdup in bubble columns
Chemical Engineering Research Bulletin, 2015
Gas holdup in the column section of bubble columns is determined by a large number of operating-design variables-parameters. In this study the effect of orifice inclination on the gas holdup is investigated at various superficial gas velocities. It is observed that the effect depends largely on the gas velocities. At very low gas velocities such as 0.015-0.076 cm/s where truly dispersed-bubble regime prevails, an inclination of the orifice keeping the size unchanged tends to decrease the gas holdup. However this effect diminishes with increasing gas velocity and no remarkable effect is found at relatively higher gas velocities like 2.20-8.50 cm/s where dispersed-transition-coalesced bubble regimes prevail. Therefore when bubble columns are intended to operate at very low superficial gas velocities, the orifice inclination is also an alternative option to control the gas holdup at the desired level. The experimental results produced in this study would be useful for the development and validation of proper mathematical model.
Gas holdup in a trayed cold-flow bubble column
Chemical Engineering Science, 2001
An experimental study was performed to investigate the e!ect of sieve trays on the time-averaged gas holdup pro"les and the overall gas holdup in a cold-#ow bubble column that was scaled-down from a commercial unit.-ray computed tomography (CT) was used to scan the column at several axial locations in the presence and absence of trays from which the local variation of the gas holdup was extracted. The overall gas holdup was also determined using the same con"guration by comparing the expanded and static liquid heights. Air and water were used as the gas}liquid system. The super"cial gas and liquid velocities were selected to span the range of the commercial system using gas spargers having multiple lateral distributors that were also scaled-down from the commercial design. To investigate the impact of sparger hole density on the local and overall gas holdup, two di!erence sparger designs were used in which the hole density per lateral was varied. The gas hole velocity was maintained constant at ca. 245 m/s, which approached that used in the commercial reactor. It is shown that the local gas holdup determined by CT is generally higher in the tray down comer region and exhibits an asymmetric pattern when trays are present. The use of increased sparger hole density at a constant gas super"cial velocity leads to steeper gradient in the gas holdup near the column centerline and a higher overall gas holdup. These "ndings suggest that the performance of bubble column reactors for various applications is sensitive to both sparger and tray design.
Influence of heat-exchanging tubes diameter on the gas holdup and bubble dynamics in a bubble column
Fuel, 2019
The effects of the presence of vertical internal tubes and their diameters on the local gas holdup and bubble dynamics, including the specific interfacial area, bubble chord length, and bubble velocity were investigated in a 6 in. bubble column for the air-water system by using a four-point optical fiber probe technique. Two different diameters, 0.5-inch, and 1-inch, of vertical internals equally covering 25% of the column's cross-sectional area (CSA) were used to represent the heat-exchanging tubes utilized in the Fischer Tropsch (FT) process. For both sizes, the vertical internals were uniformly distributed over column CSA. The experiments were performed using the air-water system, in a 6-inch bubble column at superficial gas velocities of 20, 30, and 45 cm/s. The experimental results indicated that the presence of vertical internals and their diameters have a significant effect on the hydrodynamic properties of the bubble column reactor at high superficial gas velocities. The local gas holdup significantly increased in the core region and decreased at the wall regions when the 0.5-inch vertical internals were used. Contrarily, the 1-inch vertical internals enhanced the gas holdup near to the wall regions. Additionally, the bubble chord length and the bubble rise velocity were found to be larger in the presence of vertical internals, especially at high superficial gas velocities. The specific interfacial area with the 0.5-inch internal was much lower than bubble column without vertical internals, but while using 1-inch internals, it was enhanced in the wall regions.
Gas-holdup measurements in bubble columns using computed tomography
AIChE Journal, 1997
Page 1. Gas-Holdup Measurements in Bubble Columns Using Computed Tomography Sailesh B. Kumar, Davood Moslemian, and Milorad P. DudukoviC Chemical Reaction Engineering Laboratory, Dept. of Chemical Engineering, Washington University, St. Louis, MO 63130 ...