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Goals

Data Base Expansion for Bubble Column Flows reported for DOE in period from 1995 to 2001 & summarized by Peter Spicka. Goals. Contract DE-FC 22-95 PC 95051 Development of reliable data base for evaluation of parameters in CFD based models

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Goals

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  1. Data Base Expansion for Bubble Column Flowsreported for DOE in period from 1995 to 2001& summarized by Peter Spicka

  2. Goals • Contract DE-FC 22-95 PC 95051 • Development of reliable data base for evaluation of parameters in CFD based models • Development of improved engineering models for flow mixing and mass transfer in bubble columns • Qualitative answer regarding the effect of specific variable on observed flow patterns Outline Topical review of the data reported for DOE since 1995 • Gas Holdup and Liquid Recirculation Effect of Ug, Column Diameter, Internals, Liquid Properties Sparger Effect and Solid Loadings • Eddy diffusivity Effect of Ug, Column Diameter and Solids Loading • Scale-Up of Bubble Columns • Summary

  3. Reference Axial Liquid Velocity Overall gas holdup Reilly et al. (1986) Hammer et al. (1984) Gas holdup radial profile Luo& Svendsen (1991) Centerline axial liquid velocity Joshi&Sharma (1979) Zehner (1982b) Axial liquid velocity profile Garcia-Calvo et al. (1994) Gas holdup and Liquid Recirculation Main aspects of multiphase flow • Hydrodynamics is driven by: • buoyancy, drag, inertia, pressure, viscous, interface forces… • strong coupling between the forces • many different scales Commonly used correlations The scale-up is tricky and only very sophisticated CFD simulations can resolve all the aspects - feasible in future ? Simplifications: • Steady–state one-dimensional flow • Only gas holdup, liquid velocity and turbulence radial profiles are determining factors

  4. Gas Holdup & Liquid Recirculation I.Effect of Ug and Liquid Properties (DOE Quarterly Reports 7-11, 1996) Axial velocity profiles Gas holdup profiles air-water air-Drakeoil air-water air-water vs. air-Drakeoil Air-water air-Drakeoil • Observations: • Increased Ug results in higher holdup and less uniform holdup profile • Gas holdup is lower in air-Drakeoil system due to higher liquid viscosity (20 cP) • The higher holdup in air-water system results in higher recirculation rate

  5. Gas Holdup & Liquid Recirculation II.Effect of Column Diameter & Internals (DOE Quarterly Reports 8 & 9, 1996) Overall gas holdup in 6”, 8” and 18” columns Axial velocity profiles 18” 8” 6” Internals layout 6”& 8” columns with and without internals in 18” columns • Observations: • Overall gas holdup and liquid recirculation increases with column diameter • Effect of internals on axial velocity is less pronounced • Internals reduce radial eddy diffusivity ( Chen et al., 1999)

  6. Ug = 14 cm/s Ug = 30 cm/s Z+ D1 D2 Z- Porosity = 0.1 % 163 holes of 0.4 mm ID Equilateral triangle 1 cm apart Porosity = 0.1 % 4 holes of 2.6 mm ID Distributed on a cross D6 D4 D5 D3 Porosity = 0.15 % 163 holes of 0.5 mm ID Equilateral triangle 1 cm apart Porosity = 1.0 % 163 holes of 1.25 mm ID Equilateral triangle 1 cm apart Porosity = 0.04 % 61 holes of 0.4 mm ID Circular rings 1.5 cm apart Porosity = 0.1 % Single hole of 5.1 mm ID Located in the center Gas Holdup & Liquid RecirculationIII.Gas Distributor Effect (DOE Quarterly Report 18, 1999) Gas distributors: • Observations: • Gas distributor effect is visible only at low Ug (14 cm/s) and near the column bottom • Flow stabilizes faster when single nozzle distributors are used • At Ug of 30 cm/s, the gas distributor effect is negligible

  7. Gas Holdup & Liquid Recirculation III.Gas Distributor Effect (DOE Quarterly Report 14, 1996; Degaleesan et al., 1997) Turbulent kinetic energy profile Liquid Velocity profile 8” column, Ug=12.0 cm/s. Distributors are: Cone (8C), Bubble Cap (8B), and Perforated Plate (8A) • Observations: • Single nozzle distributors • produce larger bubbles flow is less organized with large spiraling structures • suppressed recirculation and higher turbulent kinetic energy (about 40% higher compared to multiple nozzles distributors) • Multiple holes distributors • Smaller bubbles, less violent flow and enhanced recirculation

  8. Ug cm/s 14 8 2 Gas Holdup & Liquid Recirculation IV.Pressure Effect (DOE Quarterly Report 22, 2000; Kemoun et al., 2001) 6.4” column • Findings • Almost uniform gas holdup profiles, which are not pressure dependent were observed at lower Ug. This finding is in good agreement with Letzel (1997) • In churn-turbulent regime, gas holdup increases with pressure (in the interval from 1 to10 bars) as well as the steepness of the profile • Single hole distributor D3 provides visibly higher gas holdup than perforated plate distributors at 4 bars and Ug of 30 cm/s due to increased dispersion and break up of the gas jet produced by the single nozzle (Kling, 1962; Nauze et al., 1974) • 6.4” column, axial level z/D = 5.5, distributor D4 6.4” column, 4 bars, Ug= 30 cm/s

  9. G-L system G-L-S system Dc inch(cm) 4 (10.2) 6 (14) 4 (10.2) 6 (14) Composition air - 50% iso-propanol air - water air - 50% iso-propanol - alumina air - water - glass beads Ug [cm/s] 4-12 2.4-12 2-8 2-14 Solids [wt. %] ‑ ‑ 10  7, 14 and 20 Particle size [mm] -   - 40-106 125-177 Sparger perf.plate, bubble cap perf. plate sintered plate perforated plate perforated plate Gas Holdup & Liquid Recirculation V.Effect of Solids Loading (DOE Quarterly Report 12, 1998) • Comparison of G-L and G-L-S systems • Ug has smaller effect on axial velocity profiles in slurries and its effect decreases with increased concentration of slurries • All observed differences can be attributed to altered viscosity and density of the pseudo slurry phase Influence of Solids Loading Axial Distance

  10. Eddy DiffusivitiesEffect of Ug, solids loading (DOE Quarterly Reports 13 & 14, 1998) • Eddy diffusivity • Flow in bubble columns is of transient nature • Fluctuating character of flow and backmixing can by captured by eddy diffusivity, defined in Langrangian framework as: • Findings of the study • With increased Ug, the axial eddy diffusivity increases • Maximum Dzz occurs at r/R = 0.75 • Solids loading does not affect the axial eddy diffusivity profiles significantly Effect of Ug Effect of solids loading 6” column , GLS system, glass particles of 150 mm

  11. Eddy DiffusivitiesEffect of Ug and Dc(DOE Quarterly Report 13, 1998; Degaleesan, 1997) These correlation are valid for air-water systems in large columns size (Dc> 10 cm) and churn-turbulent regime ( Ug > 5 cm/s)

  12. CREL contribution II In a separated effort ( not directly funded by DOE), new correlations for radial gas holdup and axial liquid velocity were proposed Scale–Up Issues I.(DOE Quarterly Report 13, 1998) • Chronology: • Energy balance models: • Whalley & Daviddson (1974) • extended by Joshi & Sharma (1979) who proposed circulation structures • Momentum balance models: • Rietema & Ottengraf (1970) • Zehner (1980) • Implementation of turbulence: • universal mixing length by Ueyama & Miyauchi (1979) • Anderson and Rice (1989) proposed a‘three zones’ concept • Geary and Rice (1992) proposed model which depends on bubble size CREL contribution I based on CARPT/CT, a new correlation for centerline axial velocity was developed: Wu et al. (2001b) Wu et al. (2001a)

  13. Scale –Up Issues II.(DOE Quarterly Report 13, 1998; Degaleesan, 1997) CREL contribution III Correlations for radial profiles of axial and radial eddy diffusivities were proposed (Valid for churn-turbulent regime and Dc> 10 cm)

  14. Summary I. Extensive data base has been created which encompass broad range of operating conditions: • Column diameters: 4, 6, 8 and 18 inch column • Range of UG: from 2 to 30 cm/s • Range of pressure: 1, 4, and 10 bars for 6.4” column • Distributors: perfor. plate (various porosities), sintered plate,cross sparger, cone, buble cap • Liquids : water, Drakeoil, 50% isopropanol in water • Internals in 18”column)

  15. Summary II. • Gas holdup and liquid recirculation is affected primarily by superficial gas velocity • Secondary effects are due to: liquid physical properties; column diameter and; solids concentration • Heat exchange tubes do not affect significantly gas holdup and liquid recirculation profiles • Gas distributor does has minimum effect gas holdup and liquid velocity in fully developed region • Gas holdup is not affected by pressure in bubbly regime but it rises with pressure in churn-turbulent regime and becomes increasing parabolic • Correlations have been developed for the gas holdup, liquid velocity and eddy diffusivity radial profiles Future work • Data base extension to higher pressure and temperature • Experiments at low H/Dc to identify sparger with most desirable properties • New models need to be proposed as to which variable is dominant one that governs the establishment of gas holdup and liquid velocity profile

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