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X-ray tomography :
a tool for revealing local distribution of
liquid in structured packed columns
Dominique TOYE
Products, Environment, and Processes (PEPs) Department of Chemical Engineering
Université de Liège www. chemeng.ulg.ac.be
Contents
Introduction
Experimental setup
Results
Global hydrodynamic quantities
Influence of viscosity
Flow morphology
Conclusions
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Structured packed columns
Gas/Vapor Gas/ Vapor Liquid LiquidPacked column applications
Gas-liquid absorption CO
2capture
Solvent stripping regeneration
Distillation
High performance structured packings
High void fraction
low pressure drop high capacity
High specific surface area
high mass transfer properties
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Structured packed columns
Gas/VaporGas/ Vapor
Liquid
Liquid
Liquid flow hydrodynamics
Liquid holdup
Gas – liquid interfacial surface area
Liquid flow morphology
(films, rivulets…)
Reliable predictive models
Characterisation of the fluid distribution
down to a very local scale
X-ray tomography
5 X-ray source Voltage : 30 - 420 kV Current : 2 - 8 mA Focal spot : 0.8 mm Collimator : Pb Fan beam geometry Angle : 40° - Thickn.: 1 mm
Linear detector
1280 high energy photodiodes Pitch : 0.4 mm – H : 0.6 mm 12-bit (4096 intensity levels)
Rotating table 360° object rotation (tmes= 45 s)
Max diam. : 0.45 m Max. height: 3.80 m Toye et al., Meas. Sci. Technol.,
2005,16, 2213-2220
ULG X-ray tomograph
Packed column Operating modes 1- tomographic mode 2- radiographic mode LIEGE - 11 May 2017
Packed column
Liquid distributor Pall rings (HPall= 800 mm) 4 MellapakPlus 752.Y elements rotated by 90° (HMellapak= 800 mm)Radiography of the column
(transparent PVC) Dbed= 100 mm Hbed= 1600 mm Operating conditions Liquid - Water (µ = 1 mPa.s) - Glycerine solutions (µ = 10 – 20 mPa.s) Flowrate : uL= 0 – 25 m3/m2.h Gas = Air LIEGE - 11 May 2017 6
Structured packing
MellapakPlus 752.Y geometrical properties
Packing element height 0.2 m Packing element diameter 0.10 m
(0.09 m) Specific surface area 510 m²/m³
Void fraction 97.5 % Corrugation angle 41° Corrugation base 9.85 mm Corrugation height 6.50 mm (Sulzer Chemtech) LIEGE - 11 May 2017 7 8
Image of dry packing image
Mellapak 752Y
(1)Column wall (2)Corrugated sheet (3) Wall wiper
(4) Hole Aferka et al., 2010, Chem. Eng. Sci., 65, 511–516
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Methodology
uL= 23 m³m-²hWater -1no gas flow
Grey = solid (binary image)
Blue = liquid (binary image) Aferka et al., 2007, Chem. Eng. Sci., 62, 6076-6080
Images of liquid distribution
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Liquid distribution in cross sections
LIEGE - 11 May 2017 Uniform liquid distribution Film flow predominates At contact points between sheets, rivulets and/or flooded channels Mellapak 752Y Water uL= 23 m³m-²h-1 No gas flow H =500 mm H =800 mm H =600 mm H =700 mm
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Image processing
Objective = quantitative analysis
1. Normalisation(partial volume effect)
convert grayscale values into liquid holdup values divide each pixel value by the value of a pixel
completely filled by liquid
Distribution of liquid holdup
2.
Counting of interfacial pixels
(liquid – gas) Distribution of gas-liquid interfacial areaLIEGE - 11 May 2017
Viva et al., 2011, Flow Meas Instrum, 22, 279-290
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Method validation : liquid holdup
Comparison to experimental data
(global values averaged on 70 sections)
Viva, 2008, PhD thesis, University of Pisa
Very different methods: - quick closing valves + weighing - Tomo + geometrical LIEGE - 11 May 2017
13 LIEGE - 11 May 2017 Very different methods: - chemical - geometrical
Method validation : G-L interfacial area
Comparison to experimental data
(global values averaged on 70 sections)
Aferka et al., 2011, Chem. Eng. Sci. , 66, 3413-3422
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Influence of liquid viscosity
Mellapak 752Y H = 300 mm uL= 17 m³m-²h-1 uG= 0 Water µ = 1 mPa.s
Aqueous solution of glycerine µ = 10 mPa.s Uniform liquid distribution Thicker liquid films Larger number of flooded channels between sheets
0 0,05 0,1 0,15 0,2 0 5 10 15 20 25 hL,t o t (-) uL(m³/m²h)
Water Glycerine 10 cP Glycerine 20 cP
On liquid holdup
µ
Each point = one operating condition
= averaged value over the whole packed bed (70 sections)
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Bradtmöller et al., 2015, Ind. Eng. Chem. Res., 52, 2015, 2803-2815
On G-L interfacial area
0 100 200 300 400 500 0 5 10 15 20 25 aG L (m ²/ m ³) uL(m³/m²h)Water Glycerine 10 cP Glycerine 20 cP
µ
Each point = one operating condition
= averaged value over the whole packed bed (70 sections)
On axial profiles
Liquid hold-up uL= 17 m³/m²h E le va ti o n [m m ] Hold-up [-]1 mPas 10 mPas 20 mPas
-17-K o lo n n e n h ö h e [ m m ] aeff[m²/m³]
1 mPas 10 mPas 20 mPas
uL= 17 m³/m²h
Gas-liquid interfacial surface area
E le va ti o n [ m m ]
70 cross-section images for each operating condition LIEGE - 11 May 2017
Liquid flow morphology
Flow structures
Films
Ideal morphology
Contact point flow
Liquid mixing
Flooded channels
No transfer
-18-Liquid distribution Binary image LIEGE - 11 May 2017 Janzen et al., 2013, Chem. Eng. Sci., 102, 451-460= Iterative method
Step 1 = Separation and labelling of each flow structure Step 2 = Classification based on the size and shape
-19-SIZE
Minimum and maximum Feret diameters (Fminand Fmax) computed on all flow structures
Fmin and Fmax= dimensions of the minimal enclosing parallelogram
LIEGE - 11 May 2017 Janzen et al., 2013, Chem. Eng. Sci., 102, 451-460
Flow structure classification
Flow structure classification
Step 1 = Separation and labelling of flow structures
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Janzen et al., 2013, Chem. Eng. Sci., 102, 451-460
Limited spatial resolution of the ULG X-ray CT Distinct flow structures may be adjacent « seen » as a single structure
Criterion for structure splitting
Elimination of pixels with the smallest number of neighbors
(erased pixels not lost = arbitrarily added to the main flow pattern = film flow)
5 . 0 _ < rect Feret t Flow_struc S S
Step 2 = Classification based on the size and shape
Flooded channels
Thickness ≥ distance between packing sheets
F
min> 9 mm
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Flow structure classification
Step 2 = Classification based on the size and shape
Contact point flow
« Round » shape and
Fmin< 9 mm
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Flow structure classification
2 min max < F F
Step 2 = Classification based on the size and shape
Film flow
Elongated thin shape
and
Fmin< 9 mm
+ all remaining pixels (including erased pixels from structure separation)
-23-2 min max > F F LIEGE - 11 May 2017
Flow structure classification
Flow morphology: results
- 24 µ= 1 mPa.s uL= 17 m³/m²h µ= 10mPa.s uL= 17 m³/m²h µ= 20 mPa/s uL= 17 m³/m²h Films
Flooded channels Contact points
Flow morphology
-25-Films
µ µ LIEGE - 11 May 2017Flooded
channels
Contact
points
Janzen et al., 2013, Chem. Eng. Sci., 102, 451-460
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Influence of liquid flowrate
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Flooded channels Contact points Films
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Influence of liquid viscosity
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Flooded channels Contact points Films
uL= 17 m/h
Hydrodynamic Analogy model
dH α α α α zL zG
G-L interfacial surface area
Wetting efficiency
⇒ fraction of irrigated andnon-irrigated channels
Liquid holdup
=> total amount of liquid
Liquid flow morphology
⇒ Flooded channels fraction ⇒ Contact point flow fraction( mixing length : ZL)
zG= gas mixing length (packing geometry) fraction flow point contact points contact between dist. = L z LIEGE - 11 May 2017 28
0 0,002 0,004 0,006 0,008 0,01 0,012 5 10 15 20 25 30 kL a [ 1 /s ] Flüssigkeitsbelastung [m³/m²h]
F-Faktor=0,63
Experiment SimulationModel validation :
mass transfer
Liquid load (m3/m2.h) F-factor = uG.ρρρρG0.5= 0.63 Pa0.5 CO2desorption from water − − = * * ln bot bot top top pack L L x x x x H u a k Satisfying agreement LIEGE - 11 May 2017 29
Conclusions
Tomographic measurements
Quantitative assesment of the influence
of liquid load and liquid viscosity on
G-L interfacial surface area
Liquid holdup
Flow morphology
(films, rivulets, flooded)
Mass-transfer model
-
Based on an Hydrodynamic Analogy
-Validated by mass transfer experiments
Global values
Spatial distributions
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Acknowlegments
Saïd Aferka, Michel Crine, Pierre Marchot, Aurora Viva, Elisabetta Brunazzi, Julia Steube, Anna Janzen, Evgeny Kenig
