STUDY OF COALESCENCE AND BREAKAGE IN BUBBLE … et al.Prague.pdfPreliminary results with liquid...

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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

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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

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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

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• 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

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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

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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




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


Similar fluxes in column and cell

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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





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



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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





cellcell

Low flux

High flux


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



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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

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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

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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

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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

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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



butanolsolutionwater

Summary of results

STUDY OF COALESCENCE AND BREAKAGE IN BUBBLE … et al.Prague.pdfPreliminary results with liquid...

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