Liquid distribution and liquid hold-up in modern high capacity packings

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

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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Liquid distribution and liquid hold-up in modern high capacity packings Pascal Alix a,∗ , Ludovic Raynal b a b

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

1.

Introduction

To reduce greenhouse gases emissions, the E.U. CASTOR project1 has been set to develop the Carbon Capture and Sequestration technology. For that purpose, the world’s largest pilot plant for the capture of CO2 from a conventional coalfired power station has been built in Denmark; it is under operation since March 2006. The process selected for the pilot is a 30 wt% MEA amine process using columns equipped with the IMPT 50 random packing. The treatment of less than 1% of the flue gas requires a 1.1 m diameter column. In order to treat 100% of power plant flue gases, one would thus have to consider very large size capture plants. The optimisation of such high volume reactor design is thus of great importance to minimize investments. This calls for the development of reliable models for pressure drop and mass transfer characteristics determination. Since capture process operates downstream the power plant, it requires very low pressure drops (<100 mbar for the absorber). To meet these requirements of size optimisation and pressure drop limitation, efficient high capacity packings

∗

are needed. The recent Sulzer Chemtech high capacity structured packing MellapakPlus 252.Y (Fig. 1a and b), and the Koch Glitsch third generation random packing IMTP50 (Fig. 2a and b), have been selected. These packings are essentially proposed for revamps of distillation columns and are much less documented than more common packings proposed for grassroots columns. Then, to build-up models, tests are highly needed to characterize these packings in terms of hydrodynamic and mass transfer. The aim of the present study is to determine the liquid hold-up and the liquid distribution for the selected packings. The liquid hold-up is an important hydrodynamic parameter for gas-liquid flow in packed beds. It enables the determination of the pressure drop and the fluid effective velocity within the packing (Iliuta and Larachi, 2001). The latter is further used for the determination of the liquid-side mass transfer coefficient, kL , via the Higbie theory (Bravo et al., 1985). The liquid hold-up is also used to design support devices for the column since it gives the liquid weight in operation (Suess and Spiegel, 1992). The liquid distribution is also an important parameter. First, it is required to ensure that all geometric surface is

Corresponding author. Tel.: +33 4 78 02 21 82; fax: +33 4 78 02 20 08. E-mail address: [email protected] (P. Alix). Received 25 September 2007; Accepted 13 February 2008 1 www.co2-castor.com. 0263-8762/$ – see front matter © 2008 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers. doi:10.1016/j.cherd.2008.02.021

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Nomenclature ag B C1 dp D h hL hLi hLi ,av hL0 hL,dyn KP QL ReL S VSG VSL

geometric area of the packing (m2 m−3 ) channel base (m) empirical constant (s m−2 ) drip point density of the liquid distributor (m−2 ) column inner diameter (m) crimp height (m) averaged liquid hold-up over 80% of the cross section of the column (% vol/vol) averaged liquid hold-up over zone i (% vol/vol) averaged liquid hold-up over the five defined zones (% vol/vol) static liquid hold-up (% vol/vol) dynamic liquid hold-up (% vol/vol) empirical constant (s m−2 ) liquid load (m3 m−2 s−1 ) liquid Reynolds number channel side (m) superficial gas velocity (m s−1 ) superficial liquid velocity (m s−1 )

Greek symbols ıi mal-distribution parameter (%) ε bed void fraction (% vol/vol)  linear liquid flow, based on (kg s−1 m−1 ) L liquid viscosity (Pa s) water viscosity (Pa s) w P packed bed density (kg m−3 ) liquid density (kg m−3 ) L  surface tension (N m−1 )

perimeter

homogeneously fully wetted which further ensures high interfacial area, then high efficiency. Second, it is required to ensure that present results can be directly applied for larger columns if wall effects are found to be negligible. In this work, the influence of the liquid load and liquid viscosity has been studied. In the following, the experimental set-up is first described. Second, results are shown and analysed. Last, results are discussed and correlations to predict liquid hold-ups are proposed. The advantage of using such packings 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 close to 400 mm. The pressure is close to the atmospheric pressure, the temperature is the room temperature, the gas is air, and the liquid is water or water with additives. By adding polyacrylamides (FA920) in water (0.1 wt%), the liquid viscosity, L , has been varied from 1 to 2.5 cP, which covers the reference industrial operating conditions for MEA at 30% wt. It was checked that the surface tension of these solutions, , were identical (Sidi-Boumedine and Raynal, 2005). Thus, the surface tension has no effect on the present results, the effect of viscosity only being looked at. Liquid load, QL , varies from 4 to 160 m3 m−2 h−1 . For a large range of gas flowrate, the effect of the counter current gas flow can be neglected. Above a limit value of the gas flowrate, which

Fig. 1 – Zoom of channels for the metallic structured 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). This work concerns operating conditions under the loading point, then superficial gas velocity, VSG , can be fixed to 0 m s−1 . Gas effect on the liquid flow has been partly studied and will be discussed in further publication. The drip point density of the liquid distributor, dp, which is the number of liquid injectors by surface area, is higher than 347 dp m−2 . According to Fair and Bravo (1990) or Aroonwilas et al. (2001), it is high enough to 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 elements turned by 90◦ relative to each other inside the column. The geometric 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 been measured equal to 159 kg m−3 . This value is close to the manufacturer one (IMTP Brochure, 2003) and to Esbjerg (DK) project CASTOR pilot (D = 1.1 m, Knudsen et al., 2006) one, respectively equals to 156 and 159 kg m−3 . This means that a diameter of 400 mm is large enough to be representative of larger beds. The geometric characteristics of the random packing are given in Table 1.

2.3.

Tomographic measurements procedure

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

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

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Table 1 – Geometric characteristics of the structured packings Packing

MellapakPlus 252.Y

IMTP50

Crimp height, h; channel side, S; channel base, B Characteristic sizes of an element

11; 19; 30 mm

–

–

20 mm width

Geometric area, ag Void fraction, ε Bed density, p Channel flow angle from horizontal (◦ ) State of surface

250 m2 m−3 0.98 – 45◦

110 m2 m−3 0.98 159 kg m−3 –

Perforated, texture on the walls Stainless steel 316 L

Smooth

40 mm large

Material

Fig. 3 – Column cross section decomposition. in details by Boyer and Fanget (2002). The radioactive source (Cesium 137, 300 mCi) rotates all around the column, then a local density map can be obtained from tomographic measurements, which is further converted into local liquid hold-up values. The use of 32 BgO detectors enables to build 64 × 64 or 128 × 128 maps. For the present study 64 × 64 maps are used. This allows for reducing time consumption and corresponds

to a spatial accuracy of ±6 mm for a 400 mm diameter column, which is considered to be sufficient. The absolute error on the liquid hold-up is ±0.6%. Measurements can be realised from the level (+1), near the top of the bed, down to level (−1) near the bottom of the bed. It has to be noticed that the spatial accuracy can be reduced to 300 ␮m by using X ray tomography (Marchot et al., 1999, 2001; Green et al., 2007). This enables to study the liquid flow at a local scale. However, the X ray radioactive source must be very powerful compared to the gamma ray one to make measurements with metallic packings at a big scale. For example, Green et al. (2007) use a 6 MeV radioactive source for a 150 mm diameter column. Then, the corresponding plant is built in a special building and the packed column rotates since the radioactive source is fixed. Gamma tomography is much more convenient to use, then both methods looks to be very complementary. The goal of present experiments is to estimate the homogeneity of the liquid flow at a meso scale for metallic packings. A 400 mm diameter column is used to extrapolate results to industrial plants. Then, gamma-ray technique is first more adapted than X ray one. Second we defined the averaged liquid hold-up over a 360 mm diameter circle, hL . This corresponds to 80% of the column cross section. Third we defined five zones (Fig. 3). For each zone, i, an averaged liquid hold-up, hLi , is calculated and a mal-distribution parameter, ıi , is calculated via the following relation:

⎧ hL − hLi ,av ⎪ ıi = i × 100 ⎪ ⎪ hLi ,av ⎨  1 hLi hLi ,av = ⎪ 5 ⎪ ⎪ i ⎩

(1)

i = [1; 5]

Fig. 2 – Single element and packed bed of stainless steel random packing IMTP 50. (a) Single elements. (b) Packed bed.

3.

Results and analyses

3.1.

Flow homogeneity, influence of the axial position

Fig. 1a gives an upper view of an element of the structured packing, and Fig. 4a gives a tomographic picture of the cross section at the bottom of the bed (level −1). First, the structure of packing channels can be recognized on the tomographic picture. Second, one observes that some channels

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MellapakPlus 252.Y (Fig. 6a and b), the relative error for measurements is higher for the random packing and can penalize IMTP50. Differences between hLi ,av and hL , and between hL and the overall liquid hold-up are lower than 3%. Then, both packings give good results even if the structured packing seems to give better liquid distribution despite stronger wall effects.

3.2.

Liquid hold-up

For the present study, the liquid flow is considered homogeneous for both packings. Then, the lower section (level −1) averaged liquid hold-up, hL , can be used to estimate the overall liquid hold-up across the entire bed. Fig. 6a and b gives as a function of QL , for MellapakPlus 252.Y and IMTP50. For the latter, present experiments are compared to results of Linek et al. (2001), obtained at 1 cP for RMSR50 packing which is similar to IMTP50. These authors use a 0.29 m diameter column with a very elaborate liquid distributor (dp = 2500 dp m−2 ), and inject a liquid tracer (NaCl solution) to estimate the residence time which is further used to calculate the liquid hold-up. For both packings, hL is proportional to QL . This trend is similar to the one observed by Sidi-Boumedine and Raynal (2005) for SMV structured packing, and present results are in agreement with Linek et al. data. It indicates that a plug liquid flow could be assumed for IMTP50, and confirms that the liquid distributor does not influence present results.

Fig. 4 – Tomographic picture and local differences for MellapakPlus 252.Y. QL = 120 m−3 m−2 h−1 , L = 1 cP. (a) Tomographic picture, level (−1). (b) Local differences. are almost empty while in some others the liquid volume fraction can reach more than 20%. Fig. 4b gives the local differences ıi (as given by relation 1) measured at the top (level +1), at the middle (level 0) and at the bottom of the bed (level −1) for the five zones considered. It is seen that ıi are less than 10%. It has to be noticed that hLi ,av and hL are similar since differences between the two are lower than 3%, which is comparable to the experimental error (0.6% absolute). hL is systematically higher than the liquid hold-up averaged for the overall section, however relative differences are lower than 7%. Then, despite the local inhomogeneities previously discussed at small scale, and despite a little wall effect, the liquid flow can be considered homogeneous at a scale significantly larger than the geometric scale of the packing. ıi values are measured to be in the same range at the top and at the bottom of the packed bed, which means that there is not a strong axial evolution in liquid distribution. Fig. 5a gives a tomographic picture of the cross section at the bottom of the bed for IMTP50 (level −1), and Fig. 5b gives the corresponding local differences at different levels of the bed. The liquid flow is homogeneous if one considered the packed bed at a “meso” scale, but local inhomogeneities must be taken into account if one considered the packed bed at the geometric scale of a single element. ıi can reach up to 20% for this random packing, and evolution of the liquid flow distribution all along the bed is stronger than for the structured packing (Fig. 4b). However, since the liquid hold-up is much lower for the IMTP50 than for the

Fig. 5 – Tomographic picture and local differences for IMTP50. QL = 120 m−3 m−2 h−1 , L = 1 cP. (a) Tomographic picture, level (−1). (b) Local differences.

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

4.

Discussion

4.1.

Structured packing

For the MellapakPlus 252.Y structured packing, experiments can be compared to SULPAK 3.0 manufacturer software calculations based on the correlation of Suess and Spiegel (1992), and to correlations of Billet and Schultes (1999) and Stichlmair et al. (1989) developed for Mellapak 250.Y structured packing. The latter is very similar to the tested structured packing but at packing junctions: junctions of the high capacity version are smooth thanks to the metal sheets curvature (Sulzer Chemtech Brochures). Results are also compared to a simple 1D model which assumes a fully wetted packing and a fully developed laminar falling film (Sidi-Boumedine and Raynal, 2005). No empirical constant is needed for the 1D model. In this model, the liquid hold-up is a function of the packing geometry (Table 1), the film thickness, e, and the operating conditions only. According to the laminar film theory, hL follows a power law of 0.33 for L . Fig. 7a and b compares experimental versus calculated values of hL for different liquid viscosities. For a viscosity of 1 cP,

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.

Fig. 6 – Liquid hold-up as a function of the liquid load for VSG = 0 m s−1 . (a) MellapakPlus 252.Y. (b) IMTP50.

calculations from Billet and Schultes (1999) correlation are a little bit lower than those from Stichlmair et al. (1989) one, and both correlations are in agreement with present data. These differences can be partly explained by the fact that, for this packing, hL is slightly higher than the one calculated for the overall cross section as discussed in Section 3.1. SULPAK 3.0 and 1D model underestimate strongly measurements whatever the liquid load. For a viscosity of 2.5 cP, correlations of Billet and Schultes and Stichlmair et al. give similar calculated values. This is due to the fact that, for the first correlation, hL follows the laminar film theory for L , while for the second liquid viscosity effect is neglected below 5 cP. Once again, these two correlations are in agreement with present data. As for the liquid viscosity of 1 cP, SULPAK 3.0 and 1D model underestimate measurements whatever the liquid load. Correlations of Billet and Schultes and Stichlmair et al. can be used to estimate hL , however they do not fully agree with present results for the viscous term. This calls for new correlations for hL . A linear fit can be done to estimate hL (Fig. 6a). At QL = 0 m3 m−2 h−1 , such relation leads to a non-negligible socalled static hold-up, hL0 . The latter can be explained by texture on packing walls (Fig. 1b) which generate recirculation 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 considered constant. A non-negligible static hold-up can explain why

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1D model, which assumes smooth surface, underestimates strongly experimental results. If it is assumed that hL0 is a function of the geometry only, following relationships can be first proposed:

to similar relationships:

⎧   1/3 L P  ⎪ h = h + K ⎪ L L 0 ⎪ L w ⎪ ⎨ 1  = L VSL

⎧    L 1/3 ⎪ hL = hL0 + KP ⎪ ⎪ L w ⎪ ⎨ 1  = L VSL

ag ⎪ ⎪ P = 691 [s m−2 ] ⎪ K ⎪ ⎩

ag ⎪ ⎪ ⎪ KP = 164 [s m−2 ] ⎪ ⎩ hL0 = 3%

(2)

hL0 = 6.3%

Relation (2) is similar to the one of Sidi-Boumedine and Raynal (2005) for SMV structured packings, and the viscous term follows the laminar film theory. From relation (2) one can estimate the liquid residence time via the dynamic liquid hold-up (hL,dyn = hL − hL0 ). Since at low liquid load hL is close to hL0 , hL,dyn and the residence time tend to zero which is not physical. Based on CFD calculations (Raynal and Royon-Lebeaud, 2007), it is first assumed that hL0 is a function of the ReL , instead of being constant like for relation (2). Second, it is assumed that hL follows a power law of 0.4 for the liquid load, which is comparable to results obtained at low liquid load by Suess and Spiegel (1992). Then, following relationships can be also proposed to estimate hL :

⎧  1/3 0.4 L ⎪ h = h + C  ⎪ L L 1 0 ⎪ w ⎪ ⎪ ⎪ ⎪ ReL = 4 = 4L VSL ⎪ ⎪ L ag L ⎪ ⎨ −2 C1 = 0.2683 [s m

]

40 ⎪ ⎪ ReL0 = = 800 ⎪ ⎪ L ⎪ ⎪  ⎪ ⎪ for ReL ≤ ReL0 hL0 = 0.032 ⎪ ⎪  0 ⎩ hL0 = 0.032

(3)

for ReL > ReL0

As for relation (2), relation (3) is in agreement with laminar film theory for the viscous term. At low liquid loads, hL0 becomes negligible according to relation (3). This means that hL,dyn becomes similar to hL instead of becoming close to zero with relation (2). Relation (3) seems to be more physical than relation (2) since the residence time does not tend to zero at low liquid load. But CFD calculations need further validation to valid hL0 expression. For the two proposals differences between calculated and measured values are lower than 12%, that means that both relations predict very well experimental data. According to the selected relation, the calculated liquid residence time can differ strongly. However this parameter is not critical for a chemical system like CO2 /MEA since it leads to fast reaction regime.

4.2.

(4)

Random packing

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

Relation (4) is in agreement with the laminar film theory for the viscous term. The non-negligible static hold-up could be explained by contact points between single elements of packing. However, as observed with MellapakPlus 252.Y, the problem is that hL0 is close to 0% for low liquid loads, which is not physical. Then, hL0 is not a real static hold-up for the IMTP50. Previous discussion leads to propose another correlation for hL , similar to relation (5) for the structured packing. Because the IMTP50 surface is metallic and smooth, it is assumed that hL0 is negligible. Following relation can be retained:

  0.17

hL = 0.119

L

w

 0.4

(5)

Relation (5) is not in agreement with the film theory for the viscous term. hL is proportional to the liquid viscosity with a power law of 0.17 instead of 0.33. This is in contradiction with results obtained on structured packing. These results could be explained by at least three reasons. First, the assumption of fully developed flow used in the 1D model is probably not valid for a packing which has almost no plane walls. Second a nonnegligible fraction of the liquid flow can be made of droplets, the size of droplets mainly depends on the surface tension and not on the liquid viscosity. Third, hL0 can be non-negligible. As for the structured packing, differences between calculated and measured values are lower than 10% for the two proposals (relations (4) and (5)), that means that both relations predict very well experimental data. Future work is needed to improve the description of phenomena and conclude on these points, in particular by varying the liquid surface tension.

5.

Conclusions

To reduce the size of future post-combustion capture plants high capacity and low pressure drop packings are highly needed. In this study, two high capacity packings had been selected: a random packing (IMTP50) and a structured packing (MellapakPlus 252.Y). The liquid distribution found to be good for both packings, that means that there is no wall effect and that experimental results can be extrapolated to large size columns, and that these packings should be used at their optimum. Correlations are proposed to estimate the liquid hold-up of the packed beds while there is no influence of the counter current gas flow. For the structured packing, texture on the packing walls can increase the liquid amount in the bed, and the viscous term is in agreement with the film theory. For the random packing, if a negligible static hold-up is assumed, the viscous 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 contact between gas and liquid. This conclusion will be further confirmed by measuring pressure drops and mass transfer coefficients, and study the impact of the gas.

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Future work is needed to improve physical description of phenomenon, in particular by varying liquid properties and increasing superficial gas velocity.

Acknowledgment The authors would like to thank the European Commission for their financial support.

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