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Effect of a new high porosity packing on hydrodynamics of bubble columns
Chemical Engineering and Processing 41 (2002) 419– 426 www.elsevier.com/locate/cep
Effect of a new high porosity packing on hydrodynamics of bubble columns S. Moustiri, G. Hebrard *, M. Roustan Laboratoire d’Ingenierie des Proce´de´s de l’En6ironment (LIPE, EA 883), Department Genie des Procedes Industriels, 135 A6enue de Rangueil, Dpt GPI-INSA-31077 Toulouse Cedex 4, France Received 20 April 2001; received in revised form 31 July 2001; accepted 31 July 2001
Abstract The purpose of this work was to study the effect of a new high porosity packing (solid fraction corresponds to 0.5%) on the hydrodynamics of two bubble columns of different diameters (15 and 20 cm) operating with co-current up-flow of gas and liquid. This specific packing is a stainless steel welded grid with a mesh size of 12.5 mm. Two types of gas sparger were used. Gas hold up, gas hold up profiles bubble size, slip velocity and liquid axial dispersion coefficients were determined with and without packing. The results obtained show that this type of packing has a considerable effect: it delays regime transition and can maintain a homogeneous regime over a large range of superficial gas velocities. Liquid axial mixing was also greatly affected by the presence of this packing, tending toward plug flow. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bubble column; Gas hold-up; Bubble size; Slip velocity; Residence time distribution; Liquid phase mixing; High porosity packing
1. Introduction Bubble columns currently have widespread applications in industrial process such as absorption, catalytic reactions and wastewater treatment. The efficiency of these different processes depends strongly on the hydrodynamics of the reactor. Bubble columns offer many advantages, such as simplicity of construction, low operating costs and high mass transfer efficiency [1,2]. However, the complexity of their flow patterns, linked to different design parameters such as column diameter and gas sparger, makes their scale-up difficult and their hydrodynamic behaviour difficult to define. It is now accepted that liquid mixing in bubble columns is a result of various mechanisms. These include global convective re-circulation of the liquid phase, which is induced by the non-uniform gas radial hold-up distribution [3], turbulent diffusion due to the eddies generated by the rising bubbles [4], and molecular diffusion which is negligible in comparison to the * Corresponding author. Tel.: + 33-0561-55-9789; fax: +33-056155-9760. E-mail address: [email protected] (G. Hebrard).
other factors. According to Ityokumbul et al. [5], the liquid is assumed to be mixed by the motion of gas bubbles in it. The specific high porosity packing used in this study affects greatly bubbles moving. In fact, it canalises the rise of the bubbles by the formation of vertical ‘fairways’. Since the packing modifies the gas hydrodynamics in bubbles columns, it is reasonable to suppose that it can induce modifications in liquid mixing. The results presented in this paper show that the liquid become less back-mixed when this type of packing is used.
2. Materials and methods The schematic diagram of the experimental apparatus is shown in Fig. 1. The experiments were conducted using two vertical bubble columns operating with cocurrent up-flow of gas and liquid. The columns were, respectively 15 and 20 cm in diameter and 4.25 and 4.50 m in height. The liquid was tap water at room temperature and the gas used for all experiments was compressed air. Two types of gas spargers, a perforated plate and flexible membrane discs, which covered the
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 6 2 - 3
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
420 Table 1 Gas sparger characteristics Gas spargers
Square pitch arrangement (mm)
Orifice diameter (mm)
Holes (cm−2)
Thickness (mm)
Membrane Perforated plate
5 5
(initial 0.5) 2.5
4 4
2.5 20
%(db 3m)
%nm(db 3m) m
dbs =
=
m
(1)
%nm(db )
%(db )
m
m
2 m
2 m
The Residence Time Distribution (RTD) curve of the liquid phase was determined by the classical tracer method. A shot tracer (NaCl, 100 g 1 − 1) was added at the bottom of the bubble column, into the liquid inlet, and the time variation of tracer concentration was detected in the upper part of the column, at the liquid outlet. A conductance probe (Tacussel XE 100) connected to a conductimeter and to a computer was used to follow this time variation. The response curve obtained represents the RTD curve and the reactor is considered as closed to dispersion. The selected model was the axial dispersion model characterised by the Peclet number (Pe). The Peclet number was calculated calculated using the variance | 2 of the RTD curve in the following equation: Fig. 1. Experimental plant.
whole cross section of the column, were used. The sparger characteristics and their connection with the columns are described in Table 1. The packing used was a stainless steel welded grid with 12.5 mm mesh size; the diameter of wire forming the grid was 0.63 mm. The welded grid was then corrugated as shown in Fig. 2, and folded to entirely fill the bubble column volume. Experiments were performed at superficial gas velocities ranging from 0.52 to 5.5 cm s − 1 and at superficial liquid velocities from 0.62 to 2.16 cm s − 1. Pressure taps, located 0.4 m apart (column I; Dc =15 cm) or 0.5 apart (column II; Dc =20 cm), were used to determine the average gas hold-up values. An Optoflow optical probe used with a Y-type Optoelectronic module and linked to a computer was used to determine the local gas hold-up and to establish the radial gas hold-up profiles. Bubble size was determined using a video camera (Sony XC-75/CE) connected to a computer. A program, using Vislog Software 5.2, was used to obtain the average diameter (dbm) of each bubble. All bubble sizes were measured in homogeneous flow at a height of 2 m above the gas distributor. For each gas velocity, 150– 200 bubbles were measured. The average Sauter diameter, dbs, was calculated from the following relation:
|2=
2 2 − 2(1−e − Pe) Pe Pe
(2)
The axial liquid dispersion coefficient (Ezl) can be deduced from the value of the Peclet number: Ezl =
U1 × H c Pe
Fig. 2. Packing device (welded grid used).
(3)
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
421
Fig. 3. Gas hold-up versus superficial gas velocity with and without packing for the membrane and the perforated distributors.
3. Results and discussion
3.1. Hydrodynamics of the gas phase 3.1.1. A6erage gas hold-up Gas hold-up, which is one of the most important parameters characterising the hydrodynamics of bubble columns, depends mainly on gas velocity, column diameter, the physical properties of the liquid and the type of gas sparger. The introduction of our particular stainless steel packing into the column led to a modification of average gas hold-up. Fig. 3 shows the experimental results obtained on both column with and without packing using two types of gas sparger. 3.1.1.1. Effect of superficial gas 6elocity. The gas holdup values were found to increase with the superficial gas velocity with and without packing. In the range of low superficial gas velocities (Ug B1 cm s − 1), the presence of packing led to a small reduction of average gas hold-up values. Visual observations showed that the packing created channels in the bubble displacement, increasing their rise velocities and leading to a slight decrease of gas hold-up. For superficial gas velocities Ug \1 cm s − 1, the packing became an obstacle to the
rise of the bubbles; they took more time to cross the column, which led to an increase in the average gas hold-up.
3.1.1.2. Effect of superficial liquid 6elocity. Fig. 3 shows that, without packing, an increase in liquid velocity Ul resulted in a slight decrease in the gas hold-up whatever the column diameter. These results are in good agreement with the work of Kara et al. [6]. With the packing, the gas hold-up value decreased with increasing Ul, In fact, in this case, the packing effect produces a decrease of slip velocity, which is reduced by the presence of superficial liquid velocity. 3.1.1.3. Effect of column diameter. Column diameter also has an effect on the overall gas hold-up decreased with the increase in the column diameter (wall effect). At low bubble column diameters the liquids down flow near the wall hindered the ascension of the bubbles and mg increased. Under similar operating conditions (Ug =3 cm s − 1 for example) up to 15% difference was obtained. 3.1.1.4. Gas flow regime. Fig. 3 shows the difference in bubble column behaviour with and without packing. Without packing, a linear relationship exists between
422
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
the gas hold-up superficial gas velocity up to Ug = 4.5 cm s − 1. Above this value, the dependence of mg on Ug become weak, indicating the transition of gas flow from a homogeneous to a heterogeneous regime. With packing, the relationship between mg and Ug remain linear up to Ug =6 cm s − 1. On the other hand, with the packing, the homogeneous regime can be maintained over a wide range of superficial gas velocities indicated by the linearity between mg and Ug.
3.1.1.5. Effect of gas sparger. Fig. 3 shows that the trends in average gas hold-up observed in bubble columns equipped with the perforated plate are similar to those with the membrane. However, without packing, the heterogeneous regime with the perforated plate occurs earlier (Ug =4.1 cm s − 1) compared with the results obtained with the membrane (Ug =4.7 cm s − 1). With the perforated plate, in the range of high superficial gas velocities, the increase in average gas holdup caused by the packing presence was strong. The effect of the packing on average gas gold-up was greater when the bubble column was fitted with the perforated plate or, in other words, when the gas sparger provided large bubbles. The values shown in Fig. 4 present the experimental results obtained on column II. 3.1.2. Radial gas hold-up profiles Hebrard et al. [7] have shown the relation between flow patterns and the radial gas hold-up profiles. In fact the homogeneous regime occurs at relatively low superficial gas velocity. It is characterised by almost uniformly sized bubbles and a uniform concentration of bubbles. The heterogeneous regime occurs at relatively high superficial gas velocities. This regime is character-
ised by the presence of parabolic radial gas hold-up profiles as against a flat profile in the homogeneous regime. Fig. 4 shows the radial gas hold-up profiles obtained on column II fitted with the membrane and the perforated plate with and without packing. Radial gas hold-up profiles show that the presence of packing delayed regime transition. In presence of packing, flat profiles were obtained, which are characteristic of a homogeneous bubble flow regime (Fig. 4p). In contrast, without packing, parabolic profiles characteristic of a heterogeneous regime were obtained at high superficial gas velocities. With packing, in the range of superficial gas velocities studied, the gas flow regime was maintained homogeneous.
3.1.3. Bubble diameter Bubble diameter is one of the most important parameters characterising the hydrodynamics of bubble columns. Together with the gas hold-up, the bubble diameter determines the gas–liquid interfacial area, which directly characterises the mass transfer efficiency. The variation of the mean bubble Sauter diameter with the superficial gas velocity with and without packing is shown in Fig. 5. It has, however, to be considered that the video camera technique used in this study is valid only in the case of a homogeneous regime, since only bubble diameter in the vicinity of the column wall can be observed. As previously noted, the packing delays regime transition and maintains a homogeneous gas flow regime over the whole range of superficial gas velocities tested. So, with packing, the bubble size was determined over a wider range of superficial gas velocities than without packing. As shown in Fig. 5, the bubble size provided by the membrane was smaller than that obtained from the
Fig. 4. Gas hold-up profiles with and without the packing, Ul =0 cm s − 1, column II.
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
Fig. 5. Bubble size gas evolution versus superficial gas velocity, with and without the packing, Ul = 0 cm s − 1, column II.
perforated plate for both cases, with and without packing. It can also be noticed that the packing greatly influenced bubble size in the case of the perforated plate. Visual observation (Fig. 6) showed that the nozzles of this type of gas sparger generated large bubbles which split into small bubbles on arriving at the packing. With the perforated plate bubble size distribution obtained with the packing was less widely spread than that obtained without the packing. Under similar operating conditions, without packing the bubble diameter dbm ranged from 1.5 to 7.5 mm. With the packing, the bubble population was less heterogeneous, about 73% of the bubble were characterised by diameters ranging from 2.5 to 3.5 mm, (Fig. 7).
3.1.4. Slip 6elocity Slip velocity is useful parameter to characterise the performance of bubble columns. It can be defined as the relative velocity between the phases and can be calculated using the following equation applicable in co-current flow: Vs =
Ug Ul − mg 1− mg
(4)
Fig. 8 presents the slip velocity variations obtained on both columns with and without packing. It appears that, without packing, the higher slip velocities were
423
Fig. 7. Bubble size distribution for the perforated plate, with and without the packing, Ul =0 cm s − 1, Ug =0.90 cm s − 1, column II.
obtained for the perforated plate. In fact, the bubbles provided were larger and rose faster than those obtained with a membrane. One can likewise notice that, in the case of the membrane, the slip velocity seems to have a constant value of about 23–25 cm s − 1 which corresponds to the terminal velocity of a bubble of 3–4 mm diameter as found in the literature. For the three cases studied here, the slip velocity increased with Ug from a superficial gas velocity corresponding to the transition from homogeneous to heterogeneous regime. With the packing, slip velocity decreased with increasing superficial gas velocity. In fact, the number and the diameter of bubbles provided by the sparger increased with Ug and the presence of the packing formed an obstacle to the rise of the bubbles. The effect of the packing was greater on large bubbles. In that case the slip velocity obtained with the membrane could be higher than that provided by the perforated plate. In Fig. 8, it is shown that, at low superficial gas velocity (Ug B 1 cm s − 1), the slip velocity without packing is smaller. Under these operating conditions, the packing constructs ‘fairways’ for the bubble displacement increasing their rise velocities. For Ug \1 cm s − 1 with packing, slip velocity is lower than that obtained without packing. In the range of superficial gas velocities 4–6 cm s − 1 it can be lower than 20 cm s − 1. These slip velocity values confirm the assumption that the packing stops the bubbles rising.
Fig. 6. Interactions between bubbles and packing.
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
424
cm s − 1), the packing has a negative effect on Peclet number. In fact in these conditions, the presence of the packing, creates local turbulence which involves an increase of mixing: the packing acts as static mixer. It can be seen in Table 3 that, in the absence of gas, the Peclet number is divided by two for a large superficial liquid velocity (Ul = 2.15 cm s − 1). Fig. 9 presents an example of the RTD curve shapes with and without packing obtained under similar operating conditions. The shape of the RTD curve with the packing is relatively narrow, indicating that the resi-
3.2. Hydrodynamics of the liquid phase According to the experimental results in Tables 2 and 3 and Table 4, the liquid shows an intermediate behaviour between plug flow and perfectly mixed flow depending on the gas and liquid velocities, column diameter and the type of gas sparger used. Generally, the Peclet number with the packing is larger than that obtained without packing, except in column II at low gas velocity. In column II at low superficial gas velocity (Ug B 1
Fig. 8. Slip velocity versus superficial gas velocity, with and without the packing, Ul =0.64 cm s − 1. Table 2 Perfect number value versus superficial gas and liquid velocities, with and without packing, column I, membrane Ul (cm s−1)
0.92
Ug (cm s−1) Pe
0 0.54 1.11 2.22 3.17 4.65 5.67
Ul (cm s−1)
Without packing
With packing
233 9.6 7.3 7.8 5.3 3.6 2.1
45.2 19.6 22.8 22.1 28.9 21.0 21.0
1.54
Ul (cm s−1)
Pe Without packing
With packing
84.4 15.8 11.5 9.2 7.8 6.2 3.0
70.5 33.4 32.7 43.2 40.5 41.1 35.2
2.16
2.16
Pe Without packing
With packing
84.9 18.5 12.6 11.5 9.4 7.7 4.7
77.7 23.1 37.2 51.5 55.5 68.9 63.0
Table 3 Peclet number value versus superficial gas and liquid velocities with and without packing, column II, membrane Ul (cm s−1)
0.99
Ug (cm s−1) Pe
0 0.52 1.06 2.29 3.19 4.71 5.49
Ul (cm s−1)
Without packing
With packing
85.0 5.2 4.2 3.9 3.6 3.3 1.6
63.1 3.5 3.9 6.7 6.3 6.2 5.3
1.54
Ul (cm s−1)
Pe Without packing
With packing
59.1 6.1 5.4 3.8 3.9 4.4 2.2
51.7 4.3 5.3 9.0 9.4 7.3 7.5
2.15
Pe Without packing
With packing
101 9.4 8.5 7.5 5.8 4.8 –
57.1 5.1 6.5 12.1 13.4 14.2 11.0
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
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Table 4 Peclet number value versus superficial gas and liquid velocities, with and without packing, column II, perforated plate Ul (cm s−1)
0.99
Ug (cm s−1) Pe
0.49 0.90 1.93 2.69 4.14 4.86
Ul (cm s−1)
Without packing
With packing
3.5 2.5 2.4 2.7 2.8 –
2.2 4.2 5.2 6.7 8.4 7.9
1.54
dence times of different liquid elements are similar to one another. This result confirms the previous statements about the liquid re-circulation being reduced in the presence of this presence of this type of packing.
Ul (cm s−1)
Pe Without packing
With packing
4.4 4.2 3.2 2.2 3.3 –
2.7 3.9 7.7 8.8 10.5 10.9
2.15
Pe Without packing
With packing
4.8 3.9 3.7 4.0 – –
2.6 5.0 9.2 11.8 12.2 8.6
order to obtain more homogeneous hydrodynamics behaviour.
4. Conclusion
3.2.1. Effect of superficial gas and liquid 6elocities Without packing and at constant Ul, if Ug increases, Pe decreases indicating that the flow is more backmixed. At constant Ug, Pe increases with an increase of Ul, which means that the liquid flow tends towards a plug flow [8]. The experimental results obtained with the packing show that the behaviour of the bubble column is completely modified. The Peclet number increases with superficial gas velocity; as previously seen, the packing reduces the size and distribution of the bubbles, leading to a flat radial gas hold-up profile. So the packing reduces the liquid re-circulation generally encountered at the wall. In this case, the presence of the packing canalises bubbles movement and, therefore, the residence time distribution of the liquid elements becomes narrow indicating that the liquid regime is closer to plug flow (Fig. 9). In this range of Ul, superficial liquid velocity maintains the same effect and Pe increases with increasing Ul. So with the packing, an increase of superficial gas and liquid velocities involves an increase of Pe number. 3.2.2. Effect of column diameter In agreement with Field and Davidson [9], Whalley and Davidson [10] and Towell and Ackerman [11], our experimental results shows that the liquid mixing increases with column diameter and, therefore, the Peclet number decreases with Dc. This is due to the formation and increasing size of gross liquid circulation cells with increasing column diameter, leading to greater axial mixing (see Tables 2 and 3). This trend is observed with and without packing. Although the negative effect of packing on mixing is reduced when bubbles columns diameter is increased, the use of such packing in large industrial bubbles columns can be recommended in
The effect of stainless steel packing on the gas and liquid hydrodynamics of bubbles columns has been studied in two columns using two types of gas sparger. The following conclusions can be drawn from this work. At low gas velocity (UgB 1 cm s − 1), the packing canalises the bubbles, their rise velocity increases, leading to a small reduction in average gas hold-up compared with the results obtained without packing. At high gas velocity (Ug\ 1 cm s − 1), the packing becomes an obstacle to the bubbles ascension, which increases the average gas hold-up. The curve mg =f (Ug) and gas hold-up profiles show that the packing delays regime transition. Less effect of superficial liquid velocity on gas holdup is found without the packing. With the packing, the liquid flow accelerates the ascension of bubbles help back by the packing. This induces a decreases of mg with increasing Ul.
Fig. 9. Example of ‘Residence Time Distribution’ curve with and without packing, Dc =15 cm, Ug =4.63 cm s − 1, Ul =2.16 cm s − 1.
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
426
Column diameter has the same effect on the gas hold-up, in presence or in absence of the packing. In both cases, a decrease of bubbles column leads to an increase of the down-flow of liquid near the wall, which reduces the bubbles rise velocity leading to an increase in average gas hold-up. Generally, Peclet number is larger in presence of the packing than without packing. With the packing, the Peclet number is more important because it canalises bubbles ascension, and the liquid flow becomes closer to plug. The packing effect is more noticeable in column I (Dc = 15 cm). So the present packing can be introduced to control the hydrodynamics of bubbles columns. Compared with a conventional packing, because of its high porosity, this packing creates very low pressure drop limiting all problems of flow. Moreover, it can be used to support catalysis at high pressure and temperature. The first results show the interest to use this type of packing in order to improve the hydrodynamic of bubble columns. This work will be followed by, testing the effect of packing at high superficial gas velocity and for different liquid properties.
Appendix A. Nomenclature dbm dbs Dc E(q) Ezl Hc MB PP R Ug
mean bubble diameter (mm) Sauter bubble diameter (mm) column diameter (cm) Residence time distribution function versus reduced time (−) liquid axial dispersion (cm2 s−1) column height (cm) flexible membrane (−) Perforated plate (−) radial position (cm) superficial gas velocity (cm s−1)
Ul Vs
superficial liquid velocity (cm s−1) slip velocity (cm s−1)
Dimensionless numbers Peclet number (Ul Hc/(1−mg)Ezl) Pe Greek symbols mg average time (−) |2 variance of distribution (−)
References [1] Y.T. Shah, B.G. Kelkar, S.P. Godbole, W.D. Deckwer, Design parameters estimations for bubble column reactors, AIChE J. 28 (3) (1982) 3417 – 3422. [2] M.V. Kantak, S.A. Shetty, B.G. Kelkar, Liquid phase backmixing in bubble column reactors — a new correlation, Chem. Eng. Commun. 127 (1994) 23 – 34. [3] N. Devanathan, D. Moslemian, M.P. Dudukovic´ , Flow mapping in bubble columns using CARPT, Chem. Eng. Sci. 45 (1990) 2285. [4] L.S. Fan, K. Tsuchiya, Bubble Wake Dynamics in Liquids and Liquid– Solid Suspensions, Butterworth-Heinemann, Boston, 1990. [5] M.T. Ityokumbul, N. Kosaric, W. Bulani, Gas hold-up and liquid mixing at low and intermediate gas velocities. I. Air –Water system, Chem. Eng. J. 53 (1992) 167 – 172. [6] S. Kara, B.G. Kelkar, Y.T. Shah, N.L. Carr, Hydrodynamics and axial mixing in a three-phase bubble column, Ind. Eng. Chem. Process 21 (1982) 584 – 594. [7] G. Hebrard, D. Bastoul, M. Roustan, Influence of the gas sparger on the hydrodynamic behaviour of bubble columns, Trans. IChemE. 74 (1996) 406 – 414. [8] S. Moustiri, G. Hebrard, S.S. Thakre, M. Roustan, A unified correlation for predicting liquid axial dispersion coefficient in bubble columns, Chem. Eng. 56/3 (2001) 1041 – 1047. [9] R.W. Field, J.F. Davidson, Axial dispersion in bubble columns, Trans. IChemE. 58 (1980) 228 – 235. [10] P.B. Whalley, J.F. Davidson, Liquid circulation in bubble columns, Proc of the symposium on multiphase flow systems, Symp ser, No. 38.J.5, Ichem, London (1974) 1 – 5. [11] G.D. Towell, G.H. Ackerman, Axial Mixing of Liquid and Gas in Large Bubble Reactors, Proc. Of Fifth European/Second International symposium (1972).
Effect of a new high porosity packing on hydrodynamics of bubble columns S. Moustiri, G. Hebrard *, M. Roustan Laboratoire d’Ingenierie des Proce´de´s de l’En6ironment (LIPE, EA 883), Department Genie des Procedes Industriels, 135 A6enue de Rangueil, Dpt GPI-INSA-31077 Toulouse Cedex 4, France Received 20 April 2001; received in revised form 31 July 2001; accepted 31 July 2001
Abstract The purpose of this work was to study the effect of a new high porosity packing (solid fraction corresponds to 0.5%) on the hydrodynamics of two bubble columns of different diameters (15 and 20 cm) operating with co-current up-flow of gas and liquid. This specific packing is a stainless steel welded grid with a mesh size of 12.5 mm. Two types of gas sparger were used. Gas hold up, gas hold up profiles bubble size, slip velocity and liquid axial dispersion coefficients were determined with and without packing. The results obtained show that this type of packing has a considerable effect: it delays regime transition and can maintain a homogeneous regime over a large range of superficial gas velocities. Liquid axial mixing was also greatly affected by the presence of this packing, tending toward plug flow. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bubble column; Gas hold-up; Bubble size; Slip velocity; Residence time distribution; Liquid phase mixing; High porosity packing
1. Introduction Bubble columns currently have widespread applications in industrial process such as absorption, catalytic reactions and wastewater treatment. The efficiency of these different processes depends strongly on the hydrodynamics of the reactor. Bubble columns offer many advantages, such as simplicity of construction, low operating costs and high mass transfer efficiency [1,2]. However, the complexity of their flow patterns, linked to different design parameters such as column diameter and gas sparger, makes their scale-up difficult and their hydrodynamic behaviour difficult to define. It is now accepted that liquid mixing in bubble columns is a result of various mechanisms. These include global convective re-circulation of the liquid phase, which is induced by the non-uniform gas radial hold-up distribution [3], turbulent diffusion due to the eddies generated by the rising bubbles [4], and molecular diffusion which is negligible in comparison to the * Corresponding author. Tel.: + 33-0561-55-9789; fax: +33-056155-9760. E-mail address: [email protected] (G. Hebrard).
other factors. According to Ityokumbul et al. [5], the liquid is assumed to be mixed by the motion of gas bubbles in it. The specific high porosity packing used in this study affects greatly bubbles moving. In fact, it canalises the rise of the bubbles by the formation of vertical ‘fairways’. Since the packing modifies the gas hydrodynamics in bubbles columns, it is reasonable to suppose that it can induce modifications in liquid mixing. The results presented in this paper show that the liquid become less back-mixed when this type of packing is used.
2. Materials and methods The schematic diagram of the experimental apparatus is shown in Fig. 1. The experiments were conducted using two vertical bubble columns operating with cocurrent up-flow of gas and liquid. The columns were, respectively 15 and 20 cm in diameter and 4.25 and 4.50 m in height. The liquid was tap water at room temperature and the gas used for all experiments was compressed air. Two types of gas spargers, a perforated plate and flexible membrane discs, which covered the
0255-2701/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 1 ) 0 0 1 6 2 - 3
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
420 Table 1 Gas sparger characteristics Gas spargers
Square pitch arrangement (mm)
Orifice diameter (mm)
Holes (cm−2)
Thickness (mm)
Membrane Perforated plate
5 5
(initial 0.5) 2.5
4 4
2.5 20
%(db 3m)
%nm(db 3m) m
dbs =
=
m
(1)
%nm(db )
%(db )
m
m
2 m
2 m
The Residence Time Distribution (RTD) curve of the liquid phase was determined by the classical tracer method. A shot tracer (NaCl, 100 g 1 − 1) was added at the bottom of the bubble column, into the liquid inlet, and the time variation of tracer concentration was detected in the upper part of the column, at the liquid outlet. A conductance probe (Tacussel XE 100) connected to a conductimeter and to a computer was used to follow this time variation. The response curve obtained represents the RTD curve and the reactor is considered as closed to dispersion. The selected model was the axial dispersion model characterised by the Peclet number (Pe). The Peclet number was calculated calculated using the variance | 2 of the RTD curve in the following equation: Fig. 1. Experimental plant.
whole cross section of the column, were used. The sparger characteristics and their connection with the columns are described in Table 1. The packing used was a stainless steel welded grid with 12.5 mm mesh size; the diameter of wire forming the grid was 0.63 mm. The welded grid was then corrugated as shown in Fig. 2, and folded to entirely fill the bubble column volume. Experiments were performed at superficial gas velocities ranging from 0.52 to 5.5 cm s − 1 and at superficial liquid velocities from 0.62 to 2.16 cm s − 1. Pressure taps, located 0.4 m apart (column I; Dc =15 cm) or 0.5 apart (column II; Dc =20 cm), were used to determine the average gas hold-up values. An Optoflow optical probe used with a Y-type Optoelectronic module and linked to a computer was used to determine the local gas hold-up and to establish the radial gas hold-up profiles. Bubble size was determined using a video camera (Sony XC-75/CE) connected to a computer. A program, using Vislog Software 5.2, was used to obtain the average diameter (dbm) of each bubble. All bubble sizes were measured in homogeneous flow at a height of 2 m above the gas distributor. For each gas velocity, 150– 200 bubbles were measured. The average Sauter diameter, dbs, was calculated from the following relation:
|2=
2 2 − 2(1−e − Pe) Pe Pe
(2)
The axial liquid dispersion coefficient (Ezl) can be deduced from the value of the Peclet number: Ezl =
U1 × H c Pe
Fig. 2. Packing device (welded grid used).
(3)
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
421
Fig. 3. Gas hold-up versus superficial gas velocity with and without packing for the membrane and the perforated distributors.
3. Results and discussion
3.1. Hydrodynamics of the gas phase 3.1.1. A6erage gas hold-up Gas hold-up, which is one of the most important parameters characterising the hydrodynamics of bubble columns, depends mainly on gas velocity, column diameter, the physical properties of the liquid and the type of gas sparger. The introduction of our particular stainless steel packing into the column led to a modification of average gas hold-up. Fig. 3 shows the experimental results obtained on both column with and without packing using two types of gas sparger. 3.1.1.1. Effect of superficial gas 6elocity. The gas holdup values were found to increase with the superficial gas velocity with and without packing. In the range of low superficial gas velocities (Ug B1 cm s − 1), the presence of packing led to a small reduction of average gas hold-up values. Visual observations showed that the packing created channels in the bubble displacement, increasing their rise velocities and leading to a slight decrease of gas hold-up. For superficial gas velocities Ug \1 cm s − 1, the packing became an obstacle to the
rise of the bubbles; they took more time to cross the column, which led to an increase in the average gas hold-up.
3.1.1.2. Effect of superficial liquid 6elocity. Fig. 3 shows that, without packing, an increase in liquid velocity Ul resulted in a slight decrease in the gas hold-up whatever the column diameter. These results are in good agreement with the work of Kara et al. [6]. With the packing, the gas hold-up value decreased with increasing Ul, In fact, in this case, the packing effect produces a decrease of slip velocity, which is reduced by the presence of superficial liquid velocity. 3.1.1.3. Effect of column diameter. Column diameter also has an effect on the overall gas hold-up decreased with the increase in the column diameter (wall effect). At low bubble column diameters the liquids down flow near the wall hindered the ascension of the bubbles and mg increased. Under similar operating conditions (Ug =3 cm s − 1 for example) up to 15% difference was obtained. 3.1.1.4. Gas flow regime. Fig. 3 shows the difference in bubble column behaviour with and without packing. Without packing, a linear relationship exists between
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the gas hold-up superficial gas velocity up to Ug = 4.5 cm s − 1. Above this value, the dependence of mg on Ug become weak, indicating the transition of gas flow from a homogeneous to a heterogeneous regime. With packing, the relationship between mg and Ug remain linear up to Ug =6 cm s − 1. On the other hand, with the packing, the homogeneous regime can be maintained over a wide range of superficial gas velocities indicated by the linearity between mg and Ug.
3.1.1.5. Effect of gas sparger. Fig. 3 shows that the trends in average gas hold-up observed in bubble columns equipped with the perforated plate are similar to those with the membrane. However, without packing, the heterogeneous regime with the perforated plate occurs earlier (Ug =4.1 cm s − 1) compared with the results obtained with the membrane (Ug =4.7 cm s − 1). With the perforated plate, in the range of high superficial gas velocities, the increase in average gas holdup caused by the packing presence was strong. The effect of the packing on average gas gold-up was greater when the bubble column was fitted with the perforated plate or, in other words, when the gas sparger provided large bubbles. The values shown in Fig. 4 present the experimental results obtained on column II. 3.1.2. Radial gas hold-up profiles Hebrard et al. [7] have shown the relation between flow patterns and the radial gas hold-up profiles. In fact the homogeneous regime occurs at relatively low superficial gas velocity. It is characterised by almost uniformly sized bubbles and a uniform concentration of bubbles. The heterogeneous regime occurs at relatively high superficial gas velocities. This regime is character-
ised by the presence of parabolic radial gas hold-up profiles as against a flat profile in the homogeneous regime. Fig. 4 shows the radial gas hold-up profiles obtained on column II fitted with the membrane and the perforated plate with and without packing. Radial gas hold-up profiles show that the presence of packing delayed regime transition. In presence of packing, flat profiles were obtained, which are characteristic of a homogeneous bubble flow regime (Fig. 4p). In contrast, without packing, parabolic profiles characteristic of a heterogeneous regime were obtained at high superficial gas velocities. With packing, in the range of superficial gas velocities studied, the gas flow regime was maintained homogeneous.
3.1.3. Bubble diameter Bubble diameter is one of the most important parameters characterising the hydrodynamics of bubble columns. Together with the gas hold-up, the bubble diameter determines the gas–liquid interfacial area, which directly characterises the mass transfer efficiency. The variation of the mean bubble Sauter diameter with the superficial gas velocity with and without packing is shown in Fig. 5. It has, however, to be considered that the video camera technique used in this study is valid only in the case of a homogeneous regime, since only bubble diameter in the vicinity of the column wall can be observed. As previously noted, the packing delays regime transition and maintains a homogeneous gas flow regime over the whole range of superficial gas velocities tested. So, with packing, the bubble size was determined over a wider range of superficial gas velocities than without packing. As shown in Fig. 5, the bubble size provided by the membrane was smaller than that obtained from the
Fig. 4. Gas hold-up profiles with and without the packing, Ul =0 cm s − 1, column II.
S. Moustiri et al. / Chemical Engineering and Processing 41 (2002) 419–426
Fig. 5. Bubble size gas evolution versus superficial gas velocity, with and without the packing, Ul = 0 cm s − 1, column II.
perforated plate for both cases, with and without packing. It can also be noticed that the packing greatly influenced bubble size in the case of the perforated plate. Visual observation (Fig. 6) showed that the nozzles of this type of gas sparger generated large bubbles which split into small bubbles on arriving at the packing. With the perforated plate bubble size distribution obtained with the packing was less widely spread than that obtained without the packing. Under similar operating conditions, without packing the bubble diameter dbm ranged from 1.5 to 7.5 mm. With the packing, the bubble population was less heterogeneous, about 73% of the bubble were characterised by diameters ranging from 2.5 to 3.5 mm, (Fig. 7).
3.1.4. Slip 6elocity Slip velocity is useful parameter to characterise the performance of bubble columns. It can be defined as the relative velocity between the phases and can be calculated using the following equation applicable in co-current flow: Vs =
Ug Ul − mg 1− mg
(4)
Fig. 8 presents the slip velocity variations obtained on both columns with and without packing. It appears that, without packing, the higher slip velocities were
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Fig. 7. Bubble size distribution for the perforated plate, with and without the packing, Ul =0 cm s − 1, Ug =0.90 cm s − 1, column II.
obtained for the perforated plate. In fact, the bubbles provided were larger and rose faster than those obtained with a membrane. One can likewise notice that, in the case of the membrane, the slip velocity seems to have a constant value of about 23–25 cm s − 1 which corresponds to the terminal velocity of a bubble of 3–4 mm diameter as found in the literature. For the three cases studied here, the slip velocity increased with Ug from a superficial gas velocity corresponding to the transition from homogeneous to heterogeneous regime. With the packing, slip velocity decreased with increasing superficial gas velocity. In fact, the number and the diameter of bubbles provided by the sparger increased with Ug and the presence of the packing formed an obstacle to the rise of the bubbles. The effect of the packing was greater on large bubbles. In that case the slip velocity obtained with the membrane could be higher than that provided by the perforated plate. In Fig. 8, it is shown that, at low superficial gas velocity (Ug B 1 cm s − 1), the slip velocity without packing is smaller. Under these operating conditions, the packing constructs ‘fairways’ for the bubble displacement increasing their rise velocities. For Ug \1 cm s − 1 with packing, slip velocity is lower than that obtained without packing. In the range of superficial gas velocities 4–6 cm s − 1 it can be lower than 20 cm s − 1. These slip velocity values confirm the assumption that the packing stops the bubbles rising.
Fig. 6. Interactions between bubbles and packing.
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cm s − 1), the packing has a negative effect on Peclet number. In fact in these conditions, the presence of the packing, creates local turbulence which involves an increase of mixing: the packing acts as static mixer. It can be seen in Table 3 that, in the absence of gas, the Peclet number is divided by two for a large superficial liquid velocity (Ul = 2.15 cm s − 1). Fig. 9 presents an example of the RTD curve shapes with and without packing obtained under similar operating conditions. The shape of the RTD curve with the packing is relatively narrow, indicating that the resi-
3.2. Hydrodynamics of the liquid phase According to the experimental results in Tables 2 and 3 and Table 4, the liquid shows an intermediate behaviour between plug flow and perfectly mixed flow depending on the gas and liquid velocities, column diameter and the type of gas sparger used. Generally, the Peclet number with the packing is larger than that obtained without packing, except in column II at low gas velocity. In column II at low superficial gas velocity (Ug B 1
Fig. 8. Slip velocity versus superficial gas velocity, with and without the packing, Ul =0.64 cm s − 1. Table 2 Perfect number value versus superficial gas and liquid velocities, with and without packing, column I, membrane Ul (cm s−1)
0.92
Ug (cm s−1) Pe
0 0.54 1.11 2.22 3.17 4.65 5.67
Ul (cm s−1)
Without packing
With packing
233 9.6 7.3 7.8 5.3 3.6 2.1
45.2 19.6 22.8 22.1 28.9 21.0 21.0
1.54
Ul (cm s−1)
Pe Without packing
With packing
84.4 15.8 11.5 9.2 7.8 6.2 3.0
70.5 33.4 32.7 43.2 40.5 41.1 35.2
2.16
2.16
Pe Without packing
With packing
84.9 18.5 12.6 11.5 9.4 7.7 4.7
77.7 23.1 37.2 51.5 55.5 68.9 63.0
Table 3 Peclet number value versus superficial gas and liquid velocities with and without packing, column II, membrane Ul (cm s−1)
0.99
Ug (cm s−1) Pe
0 0.52 1.06 2.29 3.19 4.71 5.49
Ul (cm s−1)
Without packing
With packing
85.0 5.2 4.2 3.9 3.6 3.3 1.6
63.1 3.5 3.9 6.7 6.3 6.2 5.3
1.54
Ul (cm s−1)
Pe Without packing
With packing
59.1 6.1 5.4 3.8 3.9 4.4 2.2
51.7 4.3 5.3 9.0 9.4 7.3 7.5
2.15
Pe Without packing
With packing
101 9.4 8.5 7.5 5.8 4.8 –
57.1 5.1 6.5 12.1 13.4 14.2 11.0
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Table 4 Peclet number value versus superficial gas and liquid velocities, with and without packing, column II, perforated plate Ul (cm s−1)
0.99
Ug (cm s−1) Pe
0.49 0.90 1.93 2.69 4.14 4.86
Ul (cm s−1)
Without packing
With packing
3.5 2.5 2.4 2.7 2.8 –
2.2 4.2 5.2 6.7 8.4 7.9
1.54
dence times of different liquid elements are similar to one another. This result confirms the previous statements about the liquid re-circulation being reduced in the presence of this presence of this type of packing.
Ul (cm s−1)
Pe Without packing
With packing
4.4 4.2 3.2 2.2 3.3 –
2.7 3.9 7.7 8.8 10.5 10.9
2.15
Pe Without packing
With packing
4.8 3.9 3.7 4.0 – –
2.6 5.0 9.2 11.8 12.2 8.6
order to obtain more homogeneous hydrodynamics behaviour.
4. Conclusion
3.2.1. Effect of superficial gas and liquid 6elocities Without packing and at constant Ul, if Ug increases, Pe decreases indicating that the flow is more backmixed. At constant Ug, Pe increases with an increase of Ul, which means that the liquid flow tends towards a plug flow [8]. The experimental results obtained with the packing show that the behaviour of the bubble column is completely modified. The Peclet number increases with superficial gas velocity; as previously seen, the packing reduces the size and distribution of the bubbles, leading to a flat radial gas hold-up profile. So the packing reduces the liquid re-circulation generally encountered at the wall. In this case, the presence of the packing canalises bubbles movement and, therefore, the residence time distribution of the liquid elements becomes narrow indicating that the liquid regime is closer to plug flow (Fig. 9). In this range of Ul, superficial liquid velocity maintains the same effect and Pe increases with increasing Ul. So with the packing, an increase of superficial gas and liquid velocities involves an increase of Pe number. 3.2.2. Effect of column diameter In agreement with Field and Davidson [9], Whalley and Davidson [10] and Towell and Ackerman [11], our experimental results shows that the liquid mixing increases with column diameter and, therefore, the Peclet number decreases with Dc. This is due to the formation and increasing size of gross liquid circulation cells with increasing column diameter, leading to greater axial mixing (see Tables 2 and 3). This trend is observed with and without packing. Although the negative effect of packing on mixing is reduced when bubbles columns diameter is increased, the use of such packing in large industrial bubbles columns can be recommended in
The effect of stainless steel packing on the gas and liquid hydrodynamics of bubbles columns has been studied in two columns using two types of gas sparger. The following conclusions can be drawn from this work. At low gas velocity (UgB 1 cm s − 1), the packing canalises the bubbles, their rise velocity increases, leading to a small reduction in average gas hold-up compared with the results obtained without packing. At high gas velocity (Ug\ 1 cm s − 1), the packing becomes an obstacle to the bubbles ascension, which increases the average gas hold-up. The curve mg =f (Ug) and gas hold-up profiles show that the packing delays regime transition. Less effect of superficial liquid velocity on gas holdup is found without the packing. With the packing, the liquid flow accelerates the ascension of bubbles help back by the packing. This induces a decreases of mg with increasing Ul.
Fig. 9. Example of ‘Residence Time Distribution’ curve with and without packing, Dc =15 cm, Ug =4.63 cm s − 1, Ul =2.16 cm s − 1.
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Column diameter has the same effect on the gas hold-up, in presence or in absence of the packing. In both cases, a decrease of bubbles column leads to an increase of the down-flow of liquid near the wall, which reduces the bubbles rise velocity leading to an increase in average gas hold-up. Generally, Peclet number is larger in presence of the packing than without packing. With the packing, the Peclet number is more important because it canalises bubbles ascension, and the liquid flow becomes closer to plug. The packing effect is more noticeable in column I (Dc = 15 cm). So the present packing can be introduced to control the hydrodynamics of bubbles columns. Compared with a conventional packing, because of its high porosity, this packing creates very low pressure drop limiting all problems of flow. Moreover, it can be used to support catalysis at high pressure and temperature. The first results show the interest to use this type of packing in order to improve the hydrodynamic of bubble columns. This work will be followed by, testing the effect of packing at high superficial gas velocity and for different liquid properties.
Appendix A. Nomenclature dbm dbs Dc E(q) Ezl Hc MB PP R Ug
mean bubble diameter (mm) Sauter bubble diameter (mm) column diameter (cm) Residence time distribution function versus reduced time (−) liquid axial dispersion (cm2 s−1) column height (cm) flexible membrane (−) Perforated plate (−) radial position (cm) superficial gas velocity (cm s−1)
Ul Vs
superficial liquid velocity (cm s−1) slip velocity (cm s−1)
Dimensionless numbers Peclet number (Ul Hc/(1−mg)Ezl) Pe Greek symbols mg average time (−) |2 variance of distribution (−)
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