SLURRY BUBBLE COLUMN HYDRODYNAMICS
Tenth Quarterly Report
Budget Year 2: July 1 -September 30 1997
Submitted to
Air Products and Chemicals
Contract #DOE-FC 22 95 PC 95051
Chemical Reaction Engineering Laboratory
Chemical Engineering Department
Washington University
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SLURRY BUBBLE COLUMN HYDRODYNAMICS
Tenth Quarterly Report
Chemical Reaction Engineering Laboratory
Contract #DOE-FC 22 95 PC 95051
Budget Year 2: July 1 -September 30, 1997
TABLE OF CONTENTS
Page No.
Executive Summary / i1. / Objective for the Second Budget Year / 1
2. / Liquid Recirculation Velocity Profiles Measurements by
Computer Automated Radioactive Particle Tracking (CARPT)
for Air-Drakeoil System in an 18” Diameter Bubble Column
with Internals / 2
2.1 Outline of the Experiment / 2
2.2 Experimental Results and Discussion / 3
2.2.1 The Two-dimensional Vector Velocity Profile / 3
2.2.2 The Radial Profiles of Liquid Recirculation Velocity / 3
2.2.3 Comparison with the Results Obtained without Internals / 4
2.3 References / 4
3. / Evaluation of the dominant Terms in the Radial Momentum Balance Equation Using Gas Holdup Profiles Measured by CT and Stresses Measured by CARPT
3.1 Turbulent Drag Model
3.2 Experimental Validation
3.3 Discussion
3.4 Conclusion
3.5 References
SLURRY BUBBLE COLUMN HYDRODYNAMICS
Tenth Quarterly Report
Chemical Reaction Engineering Laboratory
Contract #DOE-FC 22 95 PC 95051
Budget Year 2: July 1 -September 30, 19
EXECUTIVE SUMMARY
The main purpose of this subcontract from the Department of Energy via Air Products and Chemicals to the chemical Reaction Engineering Laboratory (CREL) at Washington University is to study the fluid dynamics of bubble/slurry bubble column and address issues related to scale-up and design.
The second budget year was ended on September 30, 1997. The objectives and other accomplishments that exceed the objectives set for budget year 2 were completed successfully.
In this report, we summarize the research progress that has been made during the tenth quarter (July 1 -September 30, 1997) according to the objectives set for the second budget year. The accomplishments of the past quarter are as follows:
Computer Automated Radioactive Particle Tracking (CARPT) experiments, data filtering and data processing for the time averaged velocity profiles using air-drakeoil system in the 44.0 cm (18”) diameter column with internals (16 tubes of 1 inch diameter) and with a perforated plate distributor (0.7 mm hole size and 0.076% open area) at superficial gas velocities of 2, 5 and 10 cm/s were completed. These experimental conditions are similar to these conditions in the same column without internals.
The time averaged axial velocity profiles in the column with internals exhibit similar trends to those obtained in the column without internals. As the superficial gas velocity increases, the velocity inversion moves radially inward (further from the wall), whereas in the column without internals the velocity inversion occurs almost at the same radial position (i.e. same r/R). In the center of the column, the time averaged axial velocity in the column with internals is larger than that in the column without internals. However, the middle region of the column is represented by the smallest compartments for counting of the CARPT particle visits and has the highest statistical variation due to the particle occurrences. Hence, in the first approximation, at the range of superficial gas velocities used , the internals used at AFDU does not affect much the overall liquid circulation.
Gas holdup measured by Computer Tomography (CT) and Reynolds Stresses measured by CARPT are used to determine the dominant terms in the momentum equation. In Case of neglecting the overall drag force, the comparison between the left hand side (LHS) and right hand side (RHS) of the momentum balance equation exhibits a systematic deviation close to the wall. As the superficial gas velocity increases, the discrepancy between LHS and RHS of the momentum equation becomes very large. When the turbulent drag force is included by using the form proposed by Sannaes (1997) and Jakobsen (1991) and by assuming a gas velocity profile based on the measured time averaged liquid velocity profile and constant slip, the difference between the LHS and RHS of the momentum balance equation in the region between the mid-radius of the column and the wall is still large but is smaller compared to the difference obtained without accounting for the turbulent drag force. A low sensitivity of the radial momentum balance has been shown with respect to the shape and magnitude of the gas velocity profile.
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SLURRY BUBBLE COLUMN HYDRODYNAMICS
Tenth Quarterly Report
Chemical Reaction Engineering Laboratory
Contract #DOE-FC 22 95 PC 95051
Budget Year 2: July 1 -September 30, 1997
1. Objectives for the Second Budget Year
The main goal of this subcontract from the Department of energy via Air Products to the Chemical Reaction Engineering Laboratory (CREL) at Washington University is to Study the fluid dynamics of slurry bubble columns and address issues related to scale-up and design. The objectives for the second budget year were set as follows:
1. Complete review of gamma ray tomography and densitometry for obtaining density profiles with emphasis on applications in the La Porte AFDU reactor.
2. Develop phenomenological models in interpretation of tracer runs at La Porte.
3. Extend the Computer Aided Radioactive Particle Tracking/Computed Tomography (CARPT/CT) data base.
4. Continue the evaluation of closure schemes for Computational Fluid dynamic (CFD) codes. (It should be noted that this objective and objective No. 3 are coupled).
The second budget year was ended on September 30, 1997. The goals set to the second budget year and other accomplishments that exceed these goals were completed successfully. The results and achievements were reported in the previous quarterly and topical reports.
In the following sections, the research progress and achievements that have been accomplished in the tenth quarter (July 1 -September 30, 1997) are discussed.
2. Liquid Recirculation Velocity Profiles Measurements Obtained by Computer Automated Radioactive Particle Tracking (CARPT) for Air-Drakeoil System in an 18” Diameter Bubble Column With Internals
Liquid recirculation velocities and turbulent parameters in bubble columns are important parameters for design and performance of bubble column reactors. Many studies have been reported on this subject. However, most studies were limited to measurements of several single points or a small part of the column. Only recently, the Computer Automated Radioactive Particle Tracking (CARPT), developed in the Chemical Reaction Engineering Laboratory (CREL) makes it possible to measure the time averaged flow patterns in the whole column (Devanathan, 1991; Devanathan et al., 1990; Moslemian et al., 1992; Yang et al 1993; Limtrakul, 1996; Roy et al, 1997). However, previous studies for liquid recirculation velocity were limited in small diameter columns with air-water systems and without internals. Very few studies have been reported for large bubble columns with more viscous liquid than water. Moreover, time averaged liquid recirculation velocities in bubble columns with internals are rarely reported. Hence, it is very important to investigate the effect of internals on the liquid recirculation velocity. In the ninth quarterly report we reported the results of the time averaged gas holdup distribution obtained using air-drakeoil system in an 18” diameter column with internals. A comparison between the gas holdup measured in an 18” diameter column with internals and those measured in the same size column without internals were reported as well.
In this work, we used our unique facility, Computer Automated Radioactive Particle Tracking (CARPT), to measure the time averaged liquid recirculation velocity in the 18” diameter column with internals for air-drakeoil system. The internals were designed to simulate the heat exchanger tubes used in the AFDU reactor in La Porte, Texas.
2.1 Outline of the Experiments
In the present work, the time averaged liquid recirculation velocity is measured by CARPT in the 18” diameter bubble column with internals using air-drakeoil system. The experimental set-up is shown in Figure 2.1(a). The column is made of Plexiglas with a diameter of 18 inches and a height of 8 feet (L/D=5.3). The distributor used is a perforated plate with the hole diameter of 0.7 mm and an open area of 0.076% as shown in Figure 2.1(b). The internals are composed of two bundles of 1 inch aluminum tubes. Each bundle has 8 equally distributed tubes. The configuration of the internals, shown in Figure 2.1(c), simulates the heat exchanger tubes in the 18” diameter slurry bubble column reactor used in La Porte, Texas. The details of the CARPT facility used in the investigation can be found elsewhere (Devanathan, 1991). The radioactive particle Sc 46 used in CARPT experiments was about 350 Ci in strength and thirty NaI scintillation detectors were employed. Data collecting time were totally forty hours in order to get good statistical results. For comparison, the experiments were conducted at the same superficial gas velocities of 2, 5, 10 cm/s as used in the CARPT experiments for the same size column without internals. Bellow are some results for the time averaged liquid recirculation velocity for the air-drakeoil system with internals and some comparison between them and those for without internals.
2.2 Experimental Results and Discussion
2.2.1 The Two-dimensional Vector Velocity Profile
Devanathan et al. (1990) have shown that the gas holdup in a bubble column is high in the center and low at the wall and this leads to gross liquid circulation throughout the column with liquid flowing up in the center and down near the wall. Figure 2.2(a) and (b) display typical two dimensional time averaged velocity vector plots at superficial gas velocities of 2.0 and 10.0 cm/s for the column with internals while Figure 2.3(a) and (b) display the ones for the column without internals, respectively. It is obvious that from Figure 2.3 that there is only one liquid recirculation cell through out the column with upflow in the center and downflow near the wall of the column. However, it is also clear that the at low gas superficial gas velocity the velocity vectors exhibit an asymmetric flow pattern throughout the whole column with higher down flow velocity near one side of the wall (r = -22 cm) and lower down flow velocity near the other side of the wall (r = 22 cm). The asymmetry in liquid flow is caused by the maldistribution of the gas holdup which was observed from CT scans and was reported in the previous reports. From Figure 2.2(a) it is seen that at superficial gas velocity of 2 cm/s the liquid flow up from one side of the column and done from the other side of the column. This is different from what is observed in the column without internals as shown in Figure 2.3(a). This behavior is probably caused by the presence of the internals which prevent the flow from developing. Comparing the vector plots for the columns with and without internals at the same gas superficial velocity, one can see that the velocity vectors are not uniform in both magnitude and direction in the bottom part of the column (almost half of the height of the column) which means that more height is needed for exclude the entrance effect in the column with internals. At high gas superficial velocity 10 cm/s, the velocity vectors in the column with internals become similar to those in the column without internals and the flow is developed at the upper part of the column. However, strong entrance effect is also observed.
The maldistribution of gas holdup and the asymmetry in recirculation velocity pattern of liquid may be caused by the large diameter of the column and small holes in the distributor, which are prone to plugging. This needs further investigation and interpretation and will be reported later.
2.2.2 The Radial Profiles of Liquid Recirculation Velocity
Figures 2.4(a) and (b) show the azimuthally averaged radial profiles of liquid recirculation velocities in axial direction and radial direction, respectively, at different gas superficial velocities for the column with internals. Note that at 2.0 cm/s gas superficial velocity, it is impossible to do azimuthal averaging for the axial velocity since the flow near one side of the wall is upward and downward near the other side of the wall. Therefore, only the axial velocities at gas superficial velocities of 5.0 and 10.0 cm/s are plotted in Figure 2.4(a). It should be also mentioned that the axial and radial velocities were averaged throughout the column height. For comparison, the axial and radial velocity profiles in the column without internals are also shown in Figures 2.5(a) and (b). Figure 2.4(a) shows the time averaged axial velocity profiles in the column with internals that exhibit similar trends to those obtained in the column without internals. As the superficial gas velocity increase, the velocity inversion moves radially inward (further from the wall) as indicated in Figure 2.4(a), whereas in the column without internals the velocity inversion occurs almost at the same radial position (same r/R) as shown in Figure 2.5(a). The time averaged radial velocities in the column with internals are a little larger than those obtained in the column without internals as shown in Figure 2.4(b ) and 2.5(b).Nevertheless, the magnitude of radial velocities is very small compared to the axial ones indicating that the flow is close to fully developed. The profiles in Figure 2.4(b) indicate that the liquid flow toward the wall near the wall region and toward the center near the center part of the column. It seems that the internals (two bundle of tubes) divide the column into three parts in the radial direction. From the center of the column to the first bundle of the tubes, the radial flow is toward the center. Between the two bundle of the tubes, the radial flow is almost zero and from the second bundle of the tubes to the wall, the radial flow is toward the column wall.
Note that in Figures 2.4(a) and (b), the dash lines indicate the radial positions of the tubes.
2.2.3 Comparison between the Results Obtained with Internals to Those Obtained in
the Same Column without Internals
Figures 2.6(a) and (b) illustrate the comparison between the velocity profiles in the column with internals and those obtained in the same column without internals at superficial gas velocities of 5 and 10 cm/s, respectively. The trend of the time averaged axial liquid velocity profiles in the column with and without internals are very similar. For the superficial gas velocity of 5 cm/s most of the differences between the two profiles are well within the experimental error except in the center of the column where larger velocity is observed in the column with internals. For the higher gas superficial velocity of 10 cm/s the liquid velocity is a little lower for the column with internals except close to the center where it is significantly higher (47 cm/s with internals and 39 cm/s without internals). However, the center region of the column is represented by the smallest compartments for counting of CATPT particle visits and has the highest statistical variation due to the particle occurrences. Therefore, in the first approximation (for the range of superficial gas velocities used, up to 10 cm/s) we can state that the type of internals used at AFDU does not affect the overall liquid circulation much.
Data processing and analysis to estimate the turbulence parameters in the columns with and without internals are in progress and will be reported later.
2.3 References
- Devanathan, N., “Investigation of Liquid Hydrodynamics in Bubble Columns via Computer Automated Radioactive Particle Tracking”, D.Sc. Thesis, Washington University in St. Louis, 1990.
- Devanathan, N., Moslemian, D., and Dudukovic´, M. P., “Flow Mapping in Bubble Columns Using CARPT”, Chem. Eng. Sci.,45, 2285, (1990).
- Limtrakul, S., “Hydrodynamics of Liquid Fluidized Beds and Gas-Liquid Fluidized Beds”, Ph.D. Thesis, Washington University in St. Louis, 1996.
- Moslemian, D., N. Devanathan and M. P. Dudukovic´, “Radioactive Particle Tracking Technique for Investigation of Phase Recirculation and Turbulence in Multiphase Systems”, Rev. Sci. Instrum. 63(10), 4361 (1992).
- Roy, S., J. Chen, S. Kumar, M. H. Al-Dahhan, and M. P. dudukovic, “Tomography and Particle Tracking Studies in a Liquid-Solid Riser”, I & E C, in press, (1997).
- Yang, Y. B., N. Devanathan, and M. P. Dudukovic, “Liquid Backmixing in Bubble Columns via Computer Automated Radioactive Particle Tracking (CARPT)”, Exp. Fluids, 16, 1(1993)
Figure 2.1(a). CARPT Experimental set-up for the 18” column with internals
Figure 2.1(b). Perforated plate distributors used in the experiments
Figure 2.1(c). Configuration of the internal
(a) Ug=2.0 cm/s (b) Ug=10.0 cm/s
Figure 2.2. Velocity vector Plots for the 18” diameter column with internals
(a) Ug=2.0 cm/s (b)Ug=10.0 cm/s
Figure 2.3. Velocity vector plots for the 18” diameter column without internals
(a) (b)
Figure 2.4. Time averaged velocity profiles in the column with internals
(dash lines illustrate the radial positions of the internals)
(a) (b)
Figure 2.5. Time averaged velocity profiles in the column without internals