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chemical engineering research and design 8 6 ( 2 0 0 8 ) 585–591

Contents lists available at ScienceDirect

Chemical Engineering Research and Design

journa l homepage: www.e lsev ier .com/ locate /cherd

iquid distribution and liquid hold-up inodern high capacity packings

ascal Alixa,∗, Ludovic Raynalb

Chemical Engineering, IFP Lyon, BP 3, Solaize 69360, FranceChemical Engineering, IFP, Solaize, France

a b s t r a c t

In order to model and optimise industrial gas/liquid contactors such as those used for distillation or for post-

combustion capture of CO2, liquid hold-up and liquid distribution have been measured for two modern high capacity

packings, a structured packing and a random packing. A gamma-ray tomographic system has been used to obtain

liquid flow maps over a cross section of a 400 mm internal diameter column from which liquid hold-up values can be

deduced. It is observed that the liquid flow is homogeneously distributed for both packings, the structured packing

giving better results. Correlations are proposed to estimate the liquid hold-up, the effect of the liquid flowrate and

the liquid viscosity being taken into account. A non-negligible static liquid hold-up is considered for the structured

packing, which can be explained by the texture on the packing walls. As long as there is a little effect of the counter

current gas, then below the loading point, results can be extrapolated to larger columns.

© 2008 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.

Keywords: Liquid hold-up; Structured packing; Random packing

since it gives the liquid weight in operation (Suess and Spiegel,

. Introduction

o reduce greenhouse gases emissions, the E.U. CASTORroject1 has been set to develop the Carbon Capture andequestration technology. For that purpose, the world’s largestilot plant for the capture of CO2 from a conventional coal-red power station has been built in Denmark; it is underperation since March 2006. The process selected for the pilot

s a 30 wt% MEA amine process using columns equipped withhe IMPT 50 random packing. The treatment of less than 1% ofhe flue gas requires a 1.1 m diameter column. In order to treat00% of power plant flue gases, one would thus have to con-ider very large size capture plants. The optimisation of suchigh volume reactor design is thus of great importance to min-

mize investments. This calls for the development of reliableodels for pressure drop and mass transfer characteristics

etermination.Since capture process operates downstream the power

lant, it requires very low pressure drops (<100 mbar for the

bsorber). To meet these requirements of size optimisationnd pressure drop limitation, efficient high capacity packings

∗ Corresponding author. Tel.: +33 4 78 02 21 82; fax: +33 4 78 02 20 08.E-mail address: pascal.alix@ifp.fr (P. Alix).Received 25 September 2007; Accepted 13 February 2008

1 www.co2-castor.com.263-8762/$ – see front matter © 2008 Published by Elsevier B.V. on behoi:10.1016/j.cherd.2008.02.021

are needed. The recent Sulzer Chemtech high capacity struc-tured packing MellapakPlus 252.Y (Fig. 1a and b), and the KochGlitsch third generation random packing IMTP50 (Fig. 2a andb), have been selected. These packings are essentially pro-posed for revamps of distillation columns and are much lessdocumented than more common packings proposed for grass-roots columns. Then, to build-up models, tests are highlyneeded to characterize these packings in terms of hydrody-namic and mass transfer.

The aim of the present study is to determine the liquidhold-up and the liquid distribution for the selected packings.The liquid hold-up is an important hydrodynamic parameterfor gas-liquid flow in packed beds. It enables the determina-tion of the pressure drop and the fluid effective velocity withinthe packing (Iliuta and Larachi, 2001). The latter is further usedfor the determination of the liquid-side mass transfer coeffi-cient, kL, via the Higbie theory (Bravo et al., 1985). The liquidhold-up is also used to design support devices for the column

1992). The liquid distribution is also an important parame-ter. First, it is required to ensure that all geometric surface is

alf of The Institution of Chemical Engineers.

586 chemical engineering research and design 8 6 ( 2 0 0 8 ) 585–591

Nomenclature

ag geometric area of the packing (m2 m−3)B channel base (m)C1 empirical constant (s m−2)dp drip point density of the liquid distributor (m−2)D column inner diameter (m)h crimp height (m)hL averaged liquid hold-up over 80% of the cross

section of the column (% vol/vol)hLi

averaged liquid hold-up over zone i (% vol/vol)hLi,av averaged liquid hold-up over the five defined

zones (% vol/vol)hL0 static liquid hold-up (% vol/vol)hL,dyn dynamic liquid hold-up (% vol/vol)KP empirical constant (s m−2)QL liquid load (m3 m−2 s−1)ReL liquid Reynolds numberS channel side (m)VSG superficial gas velocity (m s−1)VSL superficial liquid velocity (m s−1)

Greek symbolsıi mal-distribution parameter (%)ε bed void fraction (% vol/vol)� linear liquid flow, based on perimeter

(kg s−1 m−1)�L liquid viscosity (Pa s)�w water viscosity (Pa s)�P packed bed density (kg m−3)�L liquid density (kg m−3)� surface tension (N m−1)

Fig. 1 – Zoom of channels for the metallic structured

homogeneously fully wetted which further ensures high inter-facial area, then high efficiency. Second, it is required to ensurethat present results can be directly applied for larger columnsif wall effects are found to be negligible.

In this work, the influence of the liquid load and liquid vis-cosity has been studied. In the following, the experimentalset-up is first described. Second, results are shown and anal-ysed. Last, results are discussed and correlations to predictliquid hold-ups are proposed. The advantage of using suchpackings for post-combustion CO2 capture is finally discussed.

2. Methods and materials

2.1. Operating conditions

A transparent column is used, the inner diameter, D, is closeto 400 mm. The pressure is close to the atmospheric pressure,the temperature is the room temperature, the gas is air, andthe liquid is water or water with additives. By adding polyacry-lamides (FA920) in water (0.1 wt%), the liquid viscosity, �L, hasbeen varied from 1 to 2.5 cP, which covers the reference indus-trial operating conditions for MEA at 30% wt. It was checkedthat the surface tension of these solutions, �, were identical(Sidi-Boumedine and Raynal, 2005). Thus, the surface tensionhas no effect on the present results, the effect of viscosity onlybeing looked at.

3 −2 −1

Liquid load, QL, varies from 4 to 160 m m h . For a largerange of gas flowrate, the effect of the counter current gas flowcan be neglected. Above a limit value of the gas flowrate, which

packing MellapakPlus 252.Y, 400 mm diameter column. (a)Top view. (b) Side view.

is called the loading point and is dependent on the liquid load,the effect of the gas becomes non-negligible (Billet, 1995). Thiswork concerns operating conditions under the loading point,then superficial gas velocity, VSG, can be fixed to 0 m s−1. Gaseffect on the liquid flow has been partly studied and will bediscussed in further publication. The drip point density of theliquid distributor, dp, which is the number of liquid injectorsby surface area, is higher than 347 dp m−2. According to Fairand Bravo (1990) or Aroonwilas et al. (2001), it is high enoughto ensure that the distributor does not influence the results.

2.2. Beds characteristics

The bed height is close to 1.5 m for the two selected packings.For the structured packing, the bed includes seven elementsturned by 90◦ relative to each other inside the column. Thegeometric characteristics of the structured packing (h, S, B),given in Table 1, are those used by Bravo et al. (1986).

For the random packing, the density of the bed has beenmeasured equal to 159 kg m−3. This value is close to the man-ufacturer one (IMTP Brochure, 2003) and to Esbjerg (DK) projectCASTOR pilot (D = 1.1 m, Knudsen et al., 2006) one, respectivelyequals to 156 and 159 kg m−3. This means that a diameter of400 mm is large enough to be representative of larger beds. Thegeometric characteristics of the random packing are given inTable 1.

2.3. Tomographic measurements procedure

Measurements consist in mapping the liquid hold-up, or liquidvolume fraction, across the packed bed. This is done via a high-resolution gamma-ray system developed at IFP, as described

chemical engineering research and design 8 6 ( 2 0 0 8 ) 585–591 587

Table 1 – Geometric characteristics of the structuredpackings

Packing MellapakPlus 252.Y IMTP50

Crimp height, h;channel side, S;channel base, B

11; 19; 30 mm –

Characteristic sizes ofan element

– 20 mm width

40 mm large

Geometric area, ag 250 m2 m−3 110 m2 m−3

Void fraction, ε 0.98 0.98Bed density, �p – 159 kg m−3

Channel flow anglefrom horizontal (◦)

45◦ –

State of surface Perforated, textureon the walls

Smooth

Material Stainless steel 316 L

i(lmv1T

Frb

n details by Boyer and Fanget (2002). The radioactive sourceCesium 137, 300 mCi) rotates all around the column, then aocal density map can be obtained from tomographic measure-

ents, which is further converted into local liquid hold-upalues. The use of 32 BgO detectors enables to build 64 × 64 or

28 × 128 maps. For the present study 64 × 64 maps are used.his allows for reducing time consumption and corresponds

ig. 2 – Single element and packed bed of stainless steelandom packing IMTP 50. (a) Single elements. (b) Packeded.

Fig. 3 – Column cross section decomposition.

to a spatial accuracy of ±6 mm for a 400 mm diameter column,which is considered to be sufficient. The absolute error on theliquid hold-up is ±0.6%. Measurements can be realised fromthe level (+1), near the top of the bed, down to level (−1) nearthe bottom of the bed.

It has to be noticed that the spatial accuracy can be reducedto 300 �m by using X ray tomography (Marchot et al., 1999,2001; Green et al., 2007). This enables to study the liquid flowat a local scale. However, the X ray radioactive source mustbe very powerful compared to the gamma ray one to makemeasurements with metallic packings at a big scale. For exam-ple, Green et al. (2007) use a 6 MeV radioactive source for a150 mm diameter column. Then, the corresponding plant isbuilt in a special building and the packed column rotates sincethe radioactive source is fixed. Gamma tomography is muchmore convenient to use, then both methods looks to be verycomplementary.

The goal of present experiments is to estimate the homo-geneity of the liquid flow at a meso scale for metallic packings.A 400 mm diameter column is used to extrapolate results toindustrial plants. Then, gamma-ray technique is first moreadapted than X ray one. Second we defined the averaged liquidhold-up over a 360 mm diameter circle, hL. This corresponds to80% of the column cross section. Third we defined five zones(Fig. 3). For each zone, i, an averaged liquid hold-up, hLi

, is cal-culated and a mal-distribution parameter, ıi, is calculated viathe following relation:

⎧⎪⎪⎪⎨⎪⎪⎪⎩

ıi = hLi− hLi,av

hLi,av× 100

hLi,av = 15

∑i

hLi

i = [1; 5]

(1)

3. Results and analyses

3.1. Flow homogeneity, influence of the axial position

Fig. 1a gives an upper view of an element of the struc-tured packing, and Fig. 4a gives a tomographic picture of thecross section at the bottom of the bed (level −1). First, the

structure of packing channels can be recognized on the tomo-graphic picture. Second, one observes that some channels

588 chemical engineering research and

Fig. 4 – Tomographic picture and local differences forMellapakPlus 252.Y. QL = 120 m−3 m−2 h−1, �L = 1 cP. (a)

firms that the liquid distributor does not influence presentresults.

Fig. 5 – Tomographic picture and local differences for

Tomographic picture, level (−1). (b) Local differences.

are almost empty while in some others the liquid volumefraction can reach more than 20%. Fig. 4b gives the localdifferences ıi (as given by relation 1) measured at the top(level +1), at the middle (level 0) and at the bottom of thebed (level −1) for the five zones considered. It is seen thatıi are less than 10%. It has to be noticed that hLi,av andhL are similar since differences between the two are lowerthan 3%, which is comparable to the experimental error (0.6%absolute). hL is systematically higher than the liquid hold-upaveraged for the overall section, however relative differencesare lower than 7%. Then, despite the local inhomogeneitiespreviously discussed at small scale, and despite a little walleffect, the liquid flow can be considered homogeneous ata scale significantly larger than the geometric scale of thepacking. ıi values are measured to be in the same range atthe top and at the bottom of the packed bed, which meansthat there is not a strong axial evolution in liquid distribu-tion.

Fig. 5a gives a tomographic picture of the cross sectionat the bottom of the bed for IMTP50 (level −1), and Fig. 5bgives the corresponding local differences at different levelsof the bed. The liquid flow is homogeneous if one consideredthe packed bed at a “meso” scale, but local inhomogeneitiesmust be taken into account if one considered the packedbed at the geometric scale of a single element. ıi can reachup to 20% for this random packing, and evolution of theliquid flow distribution all along the bed is stronger than

for the structured packing (Fig. 4b). However, since the liq-uid hold-up is much lower for the IMTP50 than for the

design 8 6 ( 2 0 0 8 ) 585–591

MellapakPlus 252.Y (Fig. 6a and b), the relative error for mea-surements is higher for the random packing and can penalizeIMTP50. Differences between hLi,av and hL, and between hL

and the overall liquid hold-up are lower than 3%. Then, bothpackings give good results even if the structured packingseems to give better liquid distribution despite stronger walleffects.

3.2. Liquid hold-up

For the present study, the liquid flow is considered homoge-neous for both packings. Then, the lower section (level −1)averaged liquid hold-up, hL, can be used to estimate the over-all liquid hold-up across the entire bed. Fig. 6a and b gives asa function of QL, for MellapakPlus 252.Y and IMTP50. For thelatter, present experiments are compared to results of Lineket al. (2001), obtained at 1 cP for RMSR50 packing which is sim-ilar to IMTP50. These authors use a 0.29 m diameter columnwith a very elaborate liquid distributor (dp = 2500 dp m−2), andinject a liquid tracer (NaCl solution) to estimate the res-idence time which is further used to calculate the liquidhold-up. For both packings, hL is proportional to QL. Thistrend is similar to the one observed by Sidi-Boumedine andRaynal (2005) for SMV structured packing, and present resultsare in agreement with Linek et al. data. It indicates thata plug liquid flow could be assumed for IMTP50, and con-

IMTP50. QL = 120 m−3 m−2 h−1, �L = 1 cP. (a) Tomographicpicture, level (−1). (b) Local differences.

nd design 8 6 ( 2 0 0 8 ) 585–591 589

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4

4

FccaeibsC1d2mtol

u

FV

chemical engineering research a

For both packings, there is a little influence of �L, on theiquid hold-up. A power law of 0.13 and 0.17 is followed forhe MellapakPlus 252.Y and the IMTP50 respectively. This isn agreement with Sidi-Boumedine and Raynal (2005) andtichlmair et al. (1989) for �L lower than 5 cP.

. Discussion

.1. Structured packing

or the MellapakPlus 252.Y structured packing, experimentsan be compared to SULPAK 3.0 manufacturer software cal-ulations based on the correlation of Suess and Spiegel (1992),nd to correlations of Billet and Schultes (1999) and Stichlmairt al. (1989) developed for Mellapak 250.Y structured pack-ng. The latter is very similar to the tested structured packingut at packing junctions: junctions of the high capacity ver-ion are smooth thanks to the metal sheets curvature (Sulzerhemtech Brochures). Results are also compared to a simpleD model which assumes a fully wetted packing and a fullyeveloped laminar falling film (Sidi-Boumedine and Raynal,005). No empirical constant is needed for the 1D model. In thisodel, the liquid hold-up is a function of the packing geome-

ry (Table 1), the film thickness, e, and the operating conditionsnly. According to the laminar film theory, hL follows a power

aw of 0.33 for � .

L

Fig. 7a and b compares experimental versus calculated val-es of hL for different liquid viscosities. For a viscosity of 1 cP,

ig. 6 – Liquid hold-up as a function of the liquid load for

SG = 0 m s−1. (a) MellapakPlus 252.Y. (b) IMTP50.

Fig. 7 – Liquid hold-up: model versus experiments,

MellapakPlus 252.Y, VSG = 0 m s−1. (a) �L = 1 cP. (b) �L = 2.5 cP.

calculations from Billet and Schultes (1999) correlation are alittle bit lower than those from Stichlmair et al. (1989) one, andboth correlations are in agreement with present data. Thesedifferences can be partly explained by the fact that, for thispacking, hL is slightly higher than the one calculated for theoverall cross section as discussed in Section 3.1. SULPAK 3.0and 1D model underestimate strongly measurements what-ever the liquid load.

For a viscosity of 2.5 cP, correlations of Billet and Schultesand Stichlmair et al. give similar calculated values. This is dueto the fact that, for the first correlation, hL follows the laminarfilm theory for �L, while for the second liquid viscosity effectis neglected below 5 cP. Once again, these two correlations arein agreement with present data. As for the liquid viscosity of1 cP, SULPAK 3.0 and 1D model underestimate measurementswhatever the liquid load.

Correlations of Billet and Schultes and Stichlmair et al. canbe used to estimate hL, however they do not fully agree withpresent results for the viscous term. This calls for new corre-lations for hL.

A linear fit can be done to estimate hL (Fig. 6a). AtQL = 0 m3 m−2 h−1, such relation leads to a non-negligible so-called static hold-up, hL0 . The latter can be explained bytexture on packing walls (Fig. 1b) which generate recircu-lation zones, as discussed by Raynal and Royon-Lebeaud(2007). Above ReL = 800, recirculation zones fill up cavities

of the packing, and the static hold-up can be consideredconstant. A non-negligible static hold-up can explain why

and

590 chemical engineering research

1D model, which assumes smooth surface, underestimatesstrongly experimental results. If it is assumed that hL0 is afunction of the geometry only, following relationships can befirst proposed:

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩

hL = hL0 + KP �

�L

(�L

�w

)1/3

� = �LVSL1ag

KP = 691 [s m−2]hL0 = 6.3%

(2)

Relation (2) is similar to the one of Sidi-Boumedine andRaynal (2005) for SMV structured packings, and the viscousterm follows the laminar film theory. From relation (2) onecan estimate the liquid residence time via the dynamic liq-uid hold-up (hL,dyn = hL − hL0 ). Since at low liquid load hL

is close to hL0 , hL,dyn and the residence time tend to zerowhich is not physical. Based on CFD calculations (Raynaland Royon-Lebeaud, 2007), it is first assumed that hL0 is afunction of the ReL, instead of being constant like for rela-tion (2). Second, it is assumed that hL follows a power lawof 0.4 for the liquid load, which is comparable to resultsobtained at low liquid load by Suess and Spiegel (1992). Then,following relationships can be also proposed to estimatehL:

⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

hL = hL0 + C1� 0.4(

�L

�w

)1/3

ReL = 4�

�L= 4�LVSL

ag�L

C1 = 0.2683 [s m−2]

ReL0 = 4�0

�L= 800

hL0 = 0.032�

�0for ReL ≤ ReL0

hL0 = 0.032 for ReL > ReL0

(3)

As for relation (2), relation (3) is in agreement with lami-nar film theory for the viscous term. At low liquid loads, hL0

becomes negligible according to relation (3). This means thathL,dyn becomes similar to hL instead of becoming close to zerowith relation (2). Relation (3) seems to be more physical thanrelation (2) since the residence time does not tend to zero atlow liquid load. But CFD calculations need further validationto valid hL0 expression.

For the two proposals differences between calculated andmeasured values are lower than 12%, that means that bothrelations predict very well experimental data. According tothe selected relation, the calculated liquid residence time candiffer strongly. However this parameter is not critical for achemical system like CO2/MEA since it leads to fast reactionregime.

4.2. Random packing

To our knowledge, there is no available correlation in the liter-ature for the IMTP50. As for the structured packing, a linear fit(Fig. 6b) leads to a non-negligible so-called static hold-up and

design 8 6 ( 2 0 0 8 ) 585–591

to similar relationships:

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩

hL = hL0 + KP �

�L

(�L

�w

)1/3

� = �LVSL1ag

KP = 164 [s m−2]hL0 = 3%

(4)

Relation (4) is in agreement with the laminar film theory forthe viscous term. The non-negligible static hold-up could beexplained by contact points between single elements of pack-ing. However, as observed with MellapakPlus 252.Y, the prob-lem is that hL0 is close to 0% for low liquid loads, which is notphysical. Then, hL0 is not a real static hold-up for the IMTP50.

Previous discussion leads to propose another correlationfor hL, similar to relation (5) for the structured packing.Because the IMTP50 surface is metallic and smooth, it isassumed that hL0 is negligible. Following relation can beretained:

hL = 0.119(

�L

�w

)0.17� 0.4 (5)

Relation (5) is not in agreement with the film theory for theviscous term. hL is proportional to the liquid viscosity with apower law of 0.17 instead of 0.33. This is in contradiction withresults obtained on structured packing. These results couldbe explained by at least three reasons. First, the assumption offully developed flow used in the 1D model is probably not validfor a packing which has almost no plane walls. Second a non-negligible fraction of the liquid flow can be made of droplets,the size of droplets mainly depends on the surface tension andnot on the liquid viscosity. Third, hL0 can be non-negligible.

As for the structured packing, differences between calcu-lated and measured values are lower than 10% for the twoproposals (relations (4) and (5)), that means that both relationspredict very well experimental data.

Future work is needed to improve the description of phe-nomena and conclude on these points, in particular by varyingthe liquid surface tension.

5. Conclusions

To reduce the size of future post-combustion capture plantshigh capacity and low pressure drop packings are highlyneeded. In this study, two high capacity packings had beenselected: a random packing (IMTP50) and a structured packing(MellapakPlus 252.Y). The liquid distribution found to be goodfor both packings, that means that there is no wall effect andthat experimental results can be extrapolated to large sizecolumns, and that these packings should be used at theiroptimum.

Correlations are proposed to estimate the liquid hold-upof the packed beds while there is no influence of the countercurrent gas flow. For the structured packing, texture on thepacking walls can increase the liquid amount in the bed, andthe viscous term is in agreement with the film theory. For therandom packing, if a negligible static hold-up is assumed, theviscous term is not in agreement with the film theory.

Both packings seem to be well adapted for capture plants,for which good liquid distribution is required to maximise con-

tact between gas and liquid. This conclusion will be furtherconfirmed by measuring pressure drops and mass transfercoefficients, and study the impact of the gas.

nd de

pi

A

Tt

R

A

B

B

B

B

B

F

G

chemical engineering research a

Future work is needed to improve physical description ofhenomenon, in particular by varying liquid properties and

ncreasing superficial gas velocity.

cknowledgment

he authors would like to thank the European Commission forheir financial support.

eferences

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