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Laboratory of Chemical Process and Plant DesignDepartment of Chemical EngineeringAristotle University of Thessaloniki
43rd European Two-Phase Flow Group Meeting, Prague, Czech Republic, 11-13 May 2005
STUDY OF COALESCENCE AND BREAKAGE IN BUBBLE COLUMNS
WITH FINE PORE SPARGERS
N.A. KazakisA.A. MouzaS.V. Paras
Bubble Columns
Bubble columns are used as gas-liquid contactors in:AbsorptionsFermentationsBio-ReactionsCatalytic ReactionsWaste Water Treatment
Advantages
High mass and energy transfer
Low operation cost
Simple construction
2
Bubble column design
Important design parameters:Important design parameters:
Bubble size
Gas holdup (εεgg )
Porous SpargerPorous Sparger
Column geometry
Operating conditions
Physical properties
Type of gas sparger
32
6 gadε
=
Limited informationLimited information
Greater gas-liquid interfacial area
Numerous and small bubbles
Affected by:Affected by:
Previous workEffect of liquid properties on bubble size distribution and gas holdup
in a bubble column reactor with fine pore sparger [1]
[1] Mouza, A.A., Dalakoglou, G.K. & Paras, S.V. 2005 Chem. Eng. Sci. 60, 5, 1465-1475.
0.1 m
1.5 m
Various gas flow rates (focused on
the homogeneous regime)
Liquids with properties in the range
• viscosity: 1.0-22.5 mPas
• surface tension: 48-72 mN/m
Sparger pore diameter (20 & 40 μm)
Photographic technique
3
Resultsnew experimental data (εεgg & dd)
concerning bubble columns with
porous sparger
gas holdup prediction in the
homogeneous regime
2/3
0.5 0.1 2.20.001 sg
c
dFr Ar Eo
dε
⎡ ⎤⎛ ⎞= ⎢ ⎥⎜ ⎟
⎢ ⎥⎝ ⎠⎣ ⎦
0
10
20
30
40
50
0 1 2 3 4 5 6
1.0 3.5 8.2 22.5
num
ber f
requ
ency
, %
d, mm
h=40 cm
μL, mPa.s
bubble size distributions for
various liquids depend on
coalescencecoalescence--breakage rates breakage rates
which in turn are influenced by
liquid phase properties
Results
Bubble size distribution is mainly determined by coalescence/breakageoccurring in the vicinity of the porous sparger
0
5
10
15
20
25
0 1 2 3 4 5 6
340 cm
num
ber f
requ
ency
, %
d, mm
h, cmwaterμ=1.0 mPasσ=72 mN/m
0
10
20
30
40
50
0 1 2 3 4 5 6
340
num
ber f
requ
ency
, %
d, mm
h, cmglycerol solutionμ=8.2 mPasσ=68 mN/m
4
gain insight into coalescence/breakage phenomena onto the sparger surface
develop reliable predictive tools for bubble column design
formulate correlations of general validity for CFD calculations
acquire experimental data in microscopic scale
investigate ifif and howhow bubble size and coalescence/breakage mechanisms
are affected by:
Operating conditions (gas flow rate, hydrostatic pressure)
Liquid phase properties (viscosity, surface tension)
Sparger pore diameter
The scope of this study is to:
Motivated by the previous results and in order to:
Porous sparger simulationPorous sparger
100μm
Porous sparger simulation
5
Experimental set-upColumn (cell)
Square cross section(side length 4cm)Sparger2 stainless steel capillary tubes:60 and 110 μm i.d. ~70 μm apart
Gas Phase
Nitrogen
Liquid Phase
Liquids with various physical properties (viscosity 1-20 mPas, surface tension 48-72 mN/m)
Measurement technique
Fast video recording (up to 1000 fps)Various flow rates by changing chamber pressureVarious hydrostatic pressures
Liquids studied
68117014.0glycerol solution (g2)
6811235.1glycerol solution (g1)
489901.0butanol solution
729981.0water
Surface tension, σL (mN/m)
Density,ρL (Kg/m3)
Viscosity,μL (mPas)
Physical properties ofliquid phase
6
• Effect of pore diameter-hydrostatic pressure
• Effect of physical properties
Results
Pore diameter – Hydrostatic pressure
Bubble size distribution is practically independent of the capillary tube diameter
Preliminary results with liquid height 10-100 cm and capillary tube diameter 60 and 110 μm indicate that:
Liquid pressure does not significantly influence bubble size and coalescence/breakage mechanisms on the sparger surface
Rest of the experiments were conducted with:
Liquid height 80 cm (as in the bubble column)
Capillary tube diameter 110 μm
7
water
Low flux
High flux
Rapid coalescence
Equally-sized bubbles
Larger bubbles
Formation of small bubbles
bubble columnbubble column
cellcellμ=1.0 mPasσ=72 mN/m
Low flux homogeneous
High flux heterogeneous
100m
m
3mm
water
Mono-dispersed distribution with d~1.5mm, i.e. same as the lower part of the distribution in the bubble column
The broaderbroader distribution in the bubble column can be attributed:
to the formation of larger bubbles due to strong coalescence in the vicinity of the sparger, resulting from the existence of numerous pores,
to the fact that larger bubbles are more susceptible to breakage
0
5
10
15
20
25
0 1 2 3 4 5 6
340 cm
num
ber f
requ
ency
, %
d, mm
h, cm
bubble columnbubble column cellcell
d32~ 3.8 mmSimilar fluxes in column and cell
8
butanol solution
No coalescence
Formation of a number of smaller bubbles for each large one that attains the criticalcritical size
μ=1.0 mPasσ=48 mN/m
Smaller and more frequent bubbles
Low flux homogeneous
High flux heterogeneous
bubble columnbubble column
cellcell
Low flux
High flux
butanol solution
No distribution obtained
Limited data give d32~ 1.1 mm
Same bubble size as that of bubble column (d32~ 1.0mm)
No coalescence occurs
Bubble size during/after detachment is not affected by the existence of other capillary tubes
Bubble sizes at the vicinity of the sparger surface determine thBubble sizes at the vicinity of the sparger surface determine the distribution e distribution in the entire columnin the entire column
bubble columnbubble column cellcell
Similar fluxes in column and cell
9
glycerol solution
Spherical bubbles
Size increase due to coalescence between rising and under formation bubbles
Coalescence between forming bubbles
Less rapidLess rapid coalescence compared to water due to viscosity increase
μ=14.0 mPasσ=68 mN/m
Low flux homogeneous
High flux heterogeneous
bubble columnbubble column
cellcell
Low flux
High flux
glycerol solution
d32~ 3.5 mm
0
10
20
30
40
50
0 1 2 3 4 5 6
340
num
ber f
requ
ency
, %
d, mm
h, cm Mono-dispersed distribution with d~1.7mm
The larger bubbles in the bubble column can be attributed to coalescence in the vicinity of the sparger resulting from the presence of numerous pores
More stablestable bubbles since the increase of viscosity reduces breakage rate due to turbulence decrease
bubble columnbubble column cellcell
Similar fluxes in column and cell
10
glycerol solution (g1)
Less rapid coalescence compared to water
Small bubbles escape
At low viscosities, an increase of viscosity inhibits coalescence
Coalescence is more pronounced due to interaction between the leading and the under-formation bubbles
μ=5.1 mPasσ=68 mN/m
Low flux homogeneous
High flux heterogeneous
bubble columnbubble column
cellcell
Low flux
High flux
glycerol solution (g1)
0
10
20
30
40
50
0 1 2 3 4 5 6
340
num
ber f
requ
ency
, %
d, mm
h, cm
d32~ 2.8 mm
Bubble size distribution with lower d32 (~1.4mm)
The larger bubbles in the bubble column can be attributed to coalescence in the vicinity of the sparger due to the existence of more pores
Increase of viscosity reduces breakage rate due to turbulence decrease
bubble columnbubble column cellcell
Similar fluxes in column and cell
11
Higher viscosity systems
A more distinct bimodal distribution is observed at a height of 40cm above the sparger (possibly due to bubble breakage)
0
10
20
30
40
0 1 2 3 4 5 6 7
3cm40cm
Num
ber f
requ
ency
, %
D, mm
More pronounced bubble interaction for systems with higher viscosity
glycerol solutionμ=22.5 mPasσ=68 mN/m
Away from the sparger
Coalescence and breakage occur in some extent away from the sparger in the column after bubble detachment
Experiments above the capillary tubes are essential in order to gain a complete knowledge about the mechanisms occurring at columns with porous sparger
50m
m fr
om
spar
ger
glycerol solutionglycerol solution
12
Future work
h
ri
rj
film thickness
Faster video recording would allow us to extract information about the coalescencecoalescence process:
Film thicknessCoalescence time
1000 fps1000 fps
CFD - Coalescence model
1/ 23
ln16ij l o
ijf
r ht
hρσ
⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠
2 / 3
1/ 3ij
ij
rτ
ε=
( )tijije τ−
( ) ( )tijijT B LS
ij ij ij ijQ e τθ θ θ−
= + +
Collision efficiency:
Contact time:
Coalescence time:
initial film thicknessinitial film thicknesswhen collision occurswhen collision occurs
final film thickness whenfinal film thickness whenrupture of the film occursrupture of the film occurs
Initial and final film thickness have been determined only for the air-water system and are available in the literature
h
ri
rj
film thickness
Prince & Blanch, 1990
13
Concluding remarks
More experimental work is needed and is currently in progress:
Thorough study of the videos is expected to give information about the initial and final film thickness during bubble coalescence
Coalescence is the governing mechanism for all liquids studied except for the low-surface tension butanol solution
No breakage was observed in the cell
Involving more than 2 adjacent tubes for the gas injection in order to simulate more accurately the porous spargerStudying the area just above the sparger to investigate whether coalescence/breakage occur
2 capillary tubes are not enough to simulate accurately the porous sparger
14
Forces on under-formation bubble
( )b gF gVρ ρ= −
b g p d iF F F F F Fσ+ + = + +
Buoyancy force
Gas momentum force
Pressure force
Drag force
Inertial force
Surface tension force
2 2 2, 4 /( )4g a g g g aF d W W Q dπ ρ π= =
2 ( )4p a gF d P Pπ
= −2
212 4d d
dF W Cπρ=
giF a V
ρρ γ
ρ⎛ ⎞
= +⎜ ⎟⎝ ⎠
aF dσ π σ=
Just before detachment
SnabreSnabre && MagnifotchamMagnifotcham, 1998, 1998
dα
d
Forces on under-formation bubble
Pg : gas pressure in the bubble
P : average liquid pressure
W : average velocity of bubble expansion
Cd : drag coefficient
αρV: added fluid mass
γ : average bubble acceleration
σ : surface tension
Pg=P at the plane of the bubble base
ρg<<ρ
15
15124233111149232208Re final
26238468187289415475Re initial
1.81.721.571.370.711.011.231.5d32, mm
1.81.721.511.170.610.91.031.5dave, mm
3.11.231.372.851.3931.34Flux, m/s
HighLowHighLowHighLowHighLowvideo
glycerol solution (g2)
glycerol solution (g1)
butanolsolutionwater
Summary of results
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