Hydrodynamics and mass transfer in a suspended solid bubble column with polydispersed high density particles

Pergamon

Chemtcal Engtneermq Stience. Vol 52. Nos 21.22, pp 3827 3834, 1997 t 1997 Elsevier Science Ltd. All rights reserved Pnnted in Great Britain

PII: S0009-2509(97)00286-8

0009-250997 s)700 - 000

Hydrodynamics and mass transfer in a suspended solid bubble column with polydispersed high density particles J. Garcia-Ochoa, R. Khalfet, S. Poncin and G. Wild* Laboratoire des Sciences du G6nie Chimique, C.N.R.S., E.N.S.U.C. 1, Rue Grandville, B.P. 451. 54001 Nancy Cedex, France (Accepted 26 July 1997) Abstract--Hydrodynamics and gas-liquid mass transfer are studied in a bubble and slurry bubble column. The investigations are made in a small scale (ID 0.1 m; height: 2 m) column using two batches of polydispersed and dense pyrite particles (p, = 4500 kg/m3). These experiments were also made with glass beads of different sizes (38, 85 and 160 ~m), to compare with literature results. The evolution of the gas holdup and the volumetric gas-liquid mass transfer coefficient with the particle diameter obtained in the case of glass beads indicates the existence of a critical particle diameter corresponding to the maximum values of these parameters. ~(" 1997 Elsevier Science Ltd

Keywords: Bubble column; slurry; hydrodynamics; gas-liquid mass transfer.

I. I N T R O D U C T I O N

Slurry bubble column reactors are used in many industrial processes, e.g. in catalytic reactions, coal conversion, waste water treatment, etc. In industrial mineral ore treatment, bioleaching of sulfides is usually implemented in very large mechanically agitated reactors. Suspension of the finely ground pyrite particles and aeration to ensure the gas-liquid mass transfer rates allowing bioleaching to take place, require considerable amounts of mechanical energy and only low solid loading may be used. Airlift reactors in which higher solid loading is possible, have already been used for bioleaching; however, the suspension of high density particles has caused many problems in industrial practice because of difficulties at shut-down and start-up. The use of slurry bubble columns may be an alternative to avoid this problem since the particles are supported by the gas as well as by the liquid flow in continuous operation. However, the design of such a reactor requires the knowledge of the hydrodynamic behaviour (critical gas velocity for complete suspension of the particles, holdup and mixing of phases), of mass transfer (volumetric gas-liquid mass transfer coefficient kLa) and of heat transfer (heat transfer coefficient h to a tube) parameters. Unfortunately, there are no data available with such untypical

*Corresponding author. Tel.: 00 33 83 175206: fax: 00 33 83 322975; e-mail: [email protected]_nancy.fr.

solid systems (polydisperse high density particles). In addition to this, as reported by Charinpanitkul et al. (1993), contradictory results have been reported concerning the effect of small solid particles on the gas holdup and the volumetric gas-liquid mass transfer coefficient, although no bubble disintegration is expected for the concerned particle diameters. The gas distributor, the particle diameter and the solid concentration may also play an important role even if these aspects have not often been investigated up to now.

The present work concerns hydrodynamics and gas-liquid mass transfer in continuous bubble and slurry bubble columns. Different sizes of glass beads and two batches of pyrite particles are used.

2. E X P E R I M E N T A L

Experiments are carried out in a cylindrical glass column as shown schematically in Fig. 1. The column has 0.1 m ID and a total height of 2 m. Pressure sensors and sampling taps are positioned at regular intervals along the column wall. The water or slurry employed in the experiments is pumped through the system by a peristaltic pump. The flow is regulated using a valved bypass line and measured with an electromagnetic flow meter. After passing through the column, the liquid returns to the feed tank. The gas used is air, its flow rate is measured by a rotameter. The air and the liquid are introduced at the bottom of the column. A ring sparger with

3827

3828

J. Garcia-Ochoa et al. Z

1 m

m

(I) (2) (3) (4) (5) (6) (7) (8)

!

_T 5

9

Feed tank Thermostated bath Saturation column Peristaltic pump Sampling taps Pressure sensor and/or oxygen probe Rotameter Flow-meter

air

mtroge~

Fig. 1. Experimental apparatus. Table 1. Pro ~erties of solids used Particles t~pe

code

rs (kg/m 3)

d~/~m)

Size d i s t r i b u t i o n 20-

38

BV38

0

i

I

1,32

Glass beads

BV85

2450

85

'

9,48

68,3

• 0

492

dp ~

i

i

1,32

9,48

r

i

68,3

492

68,3

492

40-1%

160

BVI60

20. 0

~

1,32 20

~CF

% ¸

4700

89 0

Pyrite

•

L

1,32

PyPU

i

9,48

4482

10-

68,3

492 7

%

[

71 1,32

62 orifices of 0.7 mm diameter is used; the solid is supported by a perforated plate with orifices of I mm diameter. The properties of the solids used are shown in Table 1.

9,48

9,48

68,3

492

The superficial gas velocity varies from 0 to 0.129 m/s for the two-phase system and from 0.004 to 0.132 m/s for the three-phase systems; the superficial liquid velocity ranges from 1.8 x 10- 3 to 3.5 x 10- 3 m/s.

3829

Hydrodynamics and mass transfer in bubble column The overall gas holdup is obtained by measuring the gas volume in the column after a simultaneous shutdown of gas and liquid feeds and by measuring the static pressure and solid concentration along the column: In steady state the static pressure gradient can be written as a function of the phase holdups as follows: dP (dp) d--~ = (r,.,p~ + ~:tPl * %P~) + ~ ;

(l,

where the friction pressure drop (dp/dz)c is negligible. The ratio ~:~/e,t is determined by measuring the liquid and solid volume in samples; it is then easy to show that g ~

Pg

rq =

(2) P~ - P~

Pl----

~:~

+~

1¢)

.q (P~ - Pq)

g'l

{Ps - P~)

e,.,(p~ - Pl) + - Pt - P#

(3)

The RTD is determined by the tracer pulse input technique, injecting an NaCI concentrated solution and then measuring the tracer concentration at the inlet and the outlet of the reactor. The results obtained show that the behaviour of the liquid is well represented by the axial dispersed plug flow (ADM) model. The volumetric gas-liquid mass transfer coefficient (kLa) is determined by the static (Nguyen-Tien et al., 1985) and dynamic physical absorption/desorption technique. The oxygen concentrations are measured at five axial positions along the column and at the entrance of the liquid (below the gas sparger) by Clark probes connected to a data acquisition system. The time constants of the probes are taken into account by deconvolution with the impulse response of the probes. In the case of the static method, only the liquid phase balance is considered, due to the small values of gas dispersion coefficient generally reported for bubble columns (Deckwer, 1992) and the small solubility of oxygen in water (Yo2 ~ const.). However, for the dynamic technique, a plug flow model is assumed for the gas phase in order to account for the high oxygen concentration variation at the beginning of measurements. A plug flow model with axial dispersion is generally used for the liquid phase. The mass balance equations are as follows. For the gas phase: ~73, ?t

, ~y --

ug (?z

R T k~a ff " ~,~

(C*-C).

(4)

For the liquid phase: OC - - = -

?~t

, ?C =-+

ut c z

g2C D ~

kta +

"" ( C * - C ) .

~t

(5)

For the resolution of the equations of the model, values of the phase hold-ups determined experimentally are used. In order to get the best precision of kta values, we used values of dispersion determined during the study of the RTD using the salt tracer technique. However, the values of dispersion coefficients obtained by fitting directly the oxygen concentration profiles of the mass transfer experiments are the same as those determined during the study of the RTD, which confirms the validity of these results. 3. R E S U L T S

3.1. Gas holdup and solid concentration profile Values of the overall gas holdup obtained for different gas velocities and particle type and diameter are shown in Fig. 2. For gas velocities lower than about 4 cm/s, i.e. in the homogeneous regime, the gas holdup increases approximately linearly with uq but, for higher values of the velocity (in the heterogeneous regime), the rise of % is not so steep. According to literature (e.g. Khare and Joshi, 1990; Charinpanitkul et al., 1993), the presence of smallish solid particles (1011m < dp < 150/~m) usually has a small influence on the gas holdup: however, this effect depends on particle size and concentration: in the presence of solids, bubble coalescence is promoted or inhibited, depending on the particle size. The results obtained here with glass beads confirm these observations: as a first approximate, one can consider the gas holdup as the same as in the solidfree bubble column. If the results are considered more precisely, one can note, as observed also by the authors mentioned above, first a slight increase and than a decrease of the gas holdup with increasing particle diameter, the results obtained with the largest particles being only very slightly smaller than those obtained in a solid free bubble column: gg. 85~4m ~> E#.38,um ~ E~/. no solid ~ g~/. 160urn-

With the polydisperse pyrite PyPU particles (dp = 71/~mj, the gas holdup is in the same order of magnitude as the one obtained with the 85 #m glass beads. This phenomenon may be related to the large amount of very fine particles (order of magnitude 1/2m) of this batch of pyrite particles (Table 1) and to a rigidification of the bubbles, hindering bubble coalescence. The pyrite PyCF is less polydispersed and presents a behaviour similar to the 160/~m glass beads. The experimental results obtained in two-phase and in three-phase flows are compared to those predicted by literature correlations. The latter have generally been established in batch operation. In the case of cocurrent flow of gas and liquid, Hughmark (1967) proposed the following relation to connect the gas holdup obtained in batch operation %° to the gas holdup in continuous gas-liquid cocurrent flow ~:~: ~:~ =

U0 /-,',!,%, - Ud(1 --

(6) t:9,,)

3830

J. Garcia-Ochoa et al. 0,18] 0,16

s,- S'" ,.,o'

0,141

"

.0 ....

/ 0,12 1

BC

x

•

+GB38

/ O,l I

£g

. . . - - ' o ' ~ G B85 - - - O " - - - G B 160

0,08

...x... PyPU

0,06 1

--. ~-. FyCF

0.04 0,02 0 10

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

Ug (m/s) Fig. 2. Influence of the presence and the type of particles on gas holdup.

0,2

- --

0,18

•

•

{ {

0.16

~ ,.<~

0,14

~"

,j., ~ , , ~

<

I

"

j,,v-,¢¢~ £g

o, I

0,08 0,06

present work " " - Hughmark -- ~ Akitaand Yoshida Sada et al. - - ~ R e i l l y et al. . . . . . . Kawaseet al.

1 • ~ • _ • • ,~P"~ ••

0,04

•~ ,-~'ilb# •

0,02 0

i ......

0

0,01

J_._

0,02

{

0,03

__

,

,

0,04 0,05 Ug (m/s)

.....

t. . . .

0.06

i

0,07

_ x _ _ _

0,08

0,09

Fig. 3. Comparison of cg values predicted by literature correlations and measured with water-air system (U~ = 0.007 m/s).

For the two-phase system, Fig. 3 shows that the correlations of Akita and Yoshida (1973), Hughmark (1967), Kawase et al. (1992) and Sada et al. (1984) fit the best our experimental results, while the other correlations usually overestimate the gas holdup (e.g. the predictions of the correlation of Reilly et al., 1986). For the three phase system, in presence of glass beads (dr = 38, 85 and 165#m) the best prediction obtained (Fig. 4) is that of the correlations of Haque

et al. (1986), K u m a r et al. (1976) and Roy et al. (1963),

while with the pyrite particles (PyPU), the best correlation proves to be that of Reilly et al. (1986) as shown in Fig. 5. All these correlations were based on experiments obtained in systems similar to those of the present work (water and small glass beads). For the three-phase systems studied, the volume fraction of the solid is not homogeneous along the column, except with the smallest (38/~m) glass beads the concentration of which is quasi independent of

Hydrodynamics and mass transfer in bubble column

3831

0,14 ;

0,12 -

°°'°~°°°'°°°°''°''*°

0,1i

//

i

l

£g

0,08

.

0,06

-

0,04

-

J

=

/

Royet al. . . . . . . Kumar et al.

~

•

Hakeetal.

•

0,02

0

.i . . . . . . . . . .

0

.~....

0,01

.

o,o2

0,03

0,04

0,05

n

0,06

.

.

0.07

Ug (m/s) Fig. 4. Comparison of e~ values predicted by literature correlations and measured with water air glass beads system (hi = 0.007 m/s).

0,2 -I 0,18 ! 0,16 J 0,]4 0,12 -

Eg

0,1 0,08 ; 0,06

-~

0,04

"

~,'f ~1~"

i I

. ~ •

0,021 0

"

"

. . . . . . Royet el. Kumar et al, ° " ° Hakeetal. Reilly et al. Kawase et al,

.....

i

0,01

i

0,02

!

.....

0,03 Ug (m/s)

0,04

0,05

0,06

0,07

Fig, 5. Comparison of ~:qvalues predicted by literature correlations and measured with water air PyPU particle system (/.:~ = 0.007 m/s).

the height a n d the gas velocity. F o r the other systems studied, the c o n c e n t r a t i o n profile depends on the gas velocity, but is almost i n d e p e n d e n t of the liquid velocity in the range of velocities studied (Fig. 6).

3.2. Volumetric gas-liquid mass tran.~[er coefficient (kLa) The effect of the gas velocity a n d the particle type and diameter on k~.a is shown in Fig. 7. While the gas

3832

J. Garcia-Ochoa et al. 0,45 7 ~ ' " - - - . ~ _ ' ~N x ~ 0,4 i

U g = 0,01 m/s

............ ""6"--GB38 ----t>--GB85

" 0,3 !

Cs

---or- GB 160 I PyPU X PyCF

--

0,2 0,15 0,1 0,05 0

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

Z

0,45 .

.

.

.

.

.

. Ug = 0,116 m/s

0,4 - ~

---./X--GB38 •---o-- GB85

0. i % .

+o.,6o

'

il PyPU ~ PyCF

0,3. 0,25

i

Cs 0,2 0,15 ] 0,1 0,05 0

. . . . . .

0

~

i

0, I

0,2

.-.

L. . . . . . . .

0,3

±. . . . .

0,4

i . . . . .

0,5

~

i

0,6

0,7

..

I

a

0,8

0,9

Z

Fig. 6. Concentration profile in the column (G = 0.007 m:sL

0,12" 0,I

...~ °4°°'''

•

0,08

BC

---4g-- BV38 ----o-- BV85

kt.a (s'l)

0,06

K."- ' "

O

"~

--'o--- BV 160 • -- x - - • P y P U

0,04

o 0,02' 0 0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

Ug (m/s)

Fig. 7. Influence of the presence and the type of particles on kta.

PyCF

1

3833

Hydrodynamics and mass transfer in bubble column

(but should also be apparent in the gas holdup); another mechanism cited in literature (the 'grazing' effect described e.g. by Kars et al., 1979) is difficult to imagine in this case. the particles being not porous and having a priori no adsorption capacity for oxygen or nitrogen. In order to compare our results with predictions of different literature correlations, equivalent fluid properties (p and p) of the slurry (viscosity calculated using Einstein's correlation cited by Wild and Poncin, 1996) are used in the correlations established for two-phase systems. For the two-phase system, the correlation proposed by Akita and Yoshida (1973) is valid at lower gas velocities ( < 0.03 m."s). At larger velocities, values predicted by this correlation are lower than the experimental values of this work. For thc three-phase system (Fig. 8) in presence of glass beads, the best prediction is that of the correlation proposed by Sada and Kumasawa (1988). In fact, the correlation proposed by these authors is of the type

holdup was approximately equal for the solid free bubble column and in the presence of particles and did not depend much on the particles used, the mass transfer coefficient is definitely smaller in the presence of particles than in the bubble column. Furthermore. strong differences in the values of kLa between the different kinds of particles are observed: the values obtained with the glass beads are much smaller than those obtained with the pyrite particles; one can also observe, that. concerning the influence of the glass bead particle size, kta follows the same order as the gas holdup kt.ass,,,,, > kl.a3~um > kt.al~,o,~. It is interesting to note that. while glass beads and pyrite particles had similar values of the gas holdup, this is not true for the volumetric mass transfer coefficient. The difference between values of kLa for the twophase and for three-phase systems in presence of glass beads is probably caused by an increase of the bubble coalescence, which may be, according to Koide et al. (1984), due to the increase of the viscosity of the slurry phase; this explanation is not totally satisfactory. since it would lead to differences in ~:q of the same order of magnitude; changes of the behaviour of the gas-liquid interface should also play a role. On the other hand. the reasons of the observed increase of kLa in the three-phase system in presence of pyrite PyPU particles compared with glass beads are not yet well understood: the decrease of the bubble size in the presence of fine pyrite particles (dp < 10 lim) as has been described by Koide (1996) may play a role

0,035 i 0,03 ~

0,025

k~.a = b,~:,~

(7)

where b =0.193 and n =0.83. These authors have worked with glass beads of 100 tim. The other correlations usually overestimate (e.g. Nigam and Shumpe, 1987) or underestimtac (Hikita et al., 1981) the values of kta. For the three-phase system in presence of pyrite PyPU particles, most correlations yield values of kLa lower than our experimental results, but by using the general expression (7) we obtain a good agreement

............................

.-.--- / 7

• presentWork . . . . . Hikitael al. Nigam and Shump Sadaand Kumasa

j 0,015

0o S..

S n . . . .

o,®5

0

/ / .d 4r _

0,01

•

0,02

,,. . . .,.

0.03

0,04 Ug (m/s)

,,,,

0,05

. . . .

"

0.06

0,07

0,08

Fig. 8. Comparison between the values ofkta predicted by literature correlations and those measured with water-air-glass beads (GBI60) system.

3834

J. Garcia-Ochoa et al.

with our experimental results for b =0.862 and n =0.83. 4. CONCLUSIONS

The effect of different types and sizes of particles on hydrodynamics and mass transfer experiments is studied. The study of hydrodynamics confirms the qualitative behaviour proposed by Charinapanitkul et al. (1993) of the different suspensions used, as well with glass beads as with pyrite particles. The behaviour of the volumetric mass transfer coefficient is quite different, depending on whether the particles are glass beads or pyrite. The reason for this behaviour is not yet clear. Further experiments are necessary to understand the effect of particle characteristics on the hydrodynamics and mass transfer. NOTATION

b

C C* Cs

dp d5 D kLa n

P R

T U'

U Y Z

coefficient defined in eq. (7) oxygen concentration in bulk of the liquid, mol/m 3 oxygen concentration at the gas-liquid interface, mol/m 3 volumetric solid concentration particle diameter, #m Sauter diameter,/~m liquid phase dispersion coefficient, m/s 2 volumetric mass transfer coefficient, s- 1 exponent defined in eq. (7) total pressure, Pa gas constant, J/(K mol) temperature, K interstitial velocity, m/s superficial velocity, m/s oxygen mole fraction in the gas dimensionless axial position

Greek letters average phase holdup p~ solid density, kg/m 3 Subscripts g gas phase l liquid phase s solid phase REFERENCES

Akita, K. and Yoshida, F., (1973) Gas holdup and volumetric mass transfer coefficient in bubble column. Ind. Engng Chem. Process Des. Dev. 12, 76-80. Charinpanitkul, T., Tsutsumi, A. and Yoshida, K. (1993) Gas-liquid mass transfer in a three-phase reactor. J. Chem. Engng Japan 26, 440-442. Deckwer, W. D. (1992) Bubble Column Reactors. Wiley, Chichester.

Haque, M. W., Nigam, K. D. P. and Joshi, J. B. (1986) Hydrodynamics and mixing in highly viscous pseudo-plastic non-newtonian solutions in bubble columns. Chem. Engng Sci. 41, 2321-2331. Hikita, H., Asai, S., Tanigawa, K., Segawa, K. and Kitao, M. (1981) The volumetric liquid-phase mass transfer coefficient in bubble columns. Chem. Engng J. 22, 61-69. Hughmark, G. A. (1967) Holdup and mass transfer in bubble columns. Ind. Engng Chem. Process Des. Dee. 6, 218-220. Kars, R. L., Best, R. J. and Drinkenburg, A. A. H. (1979) The sorption of propane in slurries of active carbon in water. Chem. Engny J. 17, 201-210. Kawase, Y., Umeno, S. and Kumagai, T. (1992) The prediction of gas holdup in bubble column reactors: Newtonian and non-newtonian fluids. Chem. Engng J. 50, 1-7. Khare, A. S. and Joshi, J. B. (1990) Effect of fine particles on gas holdup in three-phase sparged reactors. Chem. Engng J. 44, 11-25. Koide, K. (1996) Design parameters of bubble column reactors with and without solid suspensions. J. Chem. Engny Japan 29, 745-759. Koide, K., Tekazawa, A., Komura, M. and Matsunaga H. (1984) Gas holdup and volumetric mass transfer coefficient in solid-suspended bubble columns. J. Chem. Engng Japan 17, 459-466. Kumar, A., Degalessan, T. E., Laddha, G. S. and Hoelscher, H. E. (1976) Bubble swarm characteristics in bubble columns. Can. J. Chem. Engng 54, 503-508. Nguyen-Tien, K., Patwari, A. N., Schumpe, A. and Deckwer, W.-D. (1985) Gas-liquid mass transfer in fluidized particle beds. A.I.Ch.E.J. 31, 194-201. Nigam, K. D. P. and Schumpe, A. (1987) Gas-liquid mass transfer in a bubble column with suspended solids. A.I.Ch.E.J. 33, 328-330. Reilly, I. G., Scott, D. S., De Bruijn, T., Jain, A. and Piskorz. J. (1986) Correlation of gas holdup in turbulent coalescing bubble columns. Can. J. Chem. Engng 64, 705-717. Roy, N. K., Guha, D. K. and Rao, M. N. (1963) Fractional gas holdup in two-phase and threephase batch-fluidized bubble-bed and foam-bed systems. Indian Chem. Engr, Trans. 27, 31. Sada, E., Katoh, H., Yoshii, H., Yamanishi, T. and Nakanishi, A. (1984) Performances of the gas bubble column in molten salt systems. Ind. Engng Chem. Process Des. Dev. 23, 151-154. Sada, E. and Kumasawa, H. (1988) Gas holdup and mass-transfer characteristics in a slurry bubble column, In Proc. Asian Conf. on Fluidized-bed and Three-phase Reactors, eds K. Yoshida and S. Morooka. University of Tokyo, Tokyo, 1988, pp. 511- 519. Wild, G. and Poncin, S. (1996) Hydrodynamics in three-phase sparged reactors. In Three-Phase Sparged Reactors, eds K. D. P. Nigam and A. Schumpe, pp. 84-85. Gordon & Breach, Amsterdam.