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Hydrodynamics, Heat and Mass Transfer in Inverse and Circulating Three-Phase Fluidized-Bed Reactors for WasteWater Treatment
159 Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) © 2006 Elsevier B.V. All rights reserved
103 103
Hydrodynamics, Heat and Mass Transfer in Inverse and Circulating Three-Phase Fluidized-Bed Reactors for WasteWater Treatment Sang Done Kim"* and Yong Kangb "Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea(* [email protected]) b School of Chemical Engineering, Chungnam National University, Daejoen 305-764, Korea 1. INTRODUCTION Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The threephase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gasliquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. In wastewater treatment processes, solid materials including catalyst supporter, bio-media, food particles, adsorbent or absorption media are normally porous, small and light particles. These relatively low density solids are easily floating in the continuous liquid medium due to the buoyance force of solid particles. To fluidize these low density particles, the inverse fluidized bed reactor has been employed in which the liquid phase flows downward against the buoyance force acting on the floating particles and the upward gas flow. This countercurrent gas/liquid flow mode can increase the gas holdup. With increasing gas holdup, the performance of reactors for wastewater treatment could be improved, since the bubbles can provide the microorganism with oxygen and the liquid with gas reactants [5-9]. In spite of attractive advantages of conventional three-phase fluidized bed reactors, the range of UL has to be extremely low and narrow, when the small or porous particles are fluidized by viscous liquid medium, which is often encountered in the processes of wastewater treatment. To overcome this limitation and increase the treatment efficiency, the three-phase circulating fluidized-bed reactor has been devised in order to recycle solid particles from the reactor through a downcomer to the reactor. The circulation mode can minimize the dead zone by increasing the contacting efficiency among three (gas-liquidsolid) phases with sufficiently high UL. In addition, the deactivated solid media can be regenerated continuously by means of the circulation mode of particles [2-4,10,11]. To provide the prerequisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrodynamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve
104 104
the state of the present knowledge have been defined with recommendations. 2. THREE-PHASE INVERSE FLUID1ZED-BED REACTORS 2.1. Hydrodynamics The inverse fluidization of low density particles in wastewater treatment reactors can be divided into two modes ; the one is gas-liquid countercurrent flow mode comprising a continuous flow system and the other is gas only flow mode in a batch system, since the floated low density particles can be easily fluidized by the upward gas flow. For the wastewater treatment in bioreactors, the biomass is usually growing on the surface of the fluidized particles to form a biofilm. In a batch system, the collision frequency and pressure on the particles increase with increasing gas velocity (uG) but show their maxima with increasing particle holdup. The effects of UG are similar to those of other studies [9, 12, 13], but the effects of solid holdup are not consistent with the results of other investigations in which the particle fluctuation frequency increases with increasing solid holdup up to 0.6. It is interesting to note that the increase trend of fluctuation frequency and dispersion coefficient of particles with increasing uG in a continuous inverse beds [14] is very similar to that of particle collision frequency and pressure in the batch system. However, the unified correlations to predict the frequency of collision or fluctuations of fluidized particles have not been proposed up to now due to the limited research works. It has been reported that the gas phase holdup or bubble size(Lv) increases with increasing UG, UL or liquid viscosity (UL); its rising velocity(UB) increases with increasing UG or ML, but decreases with increasing UL; the frequency(Fe) of rising bubbles increases with increasing UG or UL but decreases with increasing UL- The gas (or bubble) holdup in the beds of relatively heavier particles is higher than that in the beds of relatively lighter particles, since the bubble size in case of the former is smaller than that of the latter. The bubble properties and liquid axial dispersion coefficient (Dz) have been correlated with the experimental variables as [8,15-17]: (1);
( x FB = \2AUoaO52U°Lmju?m\
&-] KP)
UB=0A2QU0Gl>*Ul0A1^0Lm\-^\
(2)
( V'26
1
(3);
Dz = 0.157C/»-263l/f2 V' 0 1 8 £ -
(4)
\P)
with correlation coefficients of 0.92, 0.95, 0.94 and 0.92 for Eqs. (l)-(4), respectively. 2.2 Heat and Mass Transfer Very limited data on the heat and mass transfer in three-phase inverse fluidization systems is available up to now. For the wastewater treatment, the reactor temperature should be controlled and maintained within a certain level to optimize the reactor performance, since the temperature of reactor or process can provide the microorganisms with favorable circumances. The heat transfer coefficient (h) has been determined by measuring the temperature difference between the immersed heater and the bed. The h value increases with increasing Uc (Fig. l(a)), but exhibits a maximum value with increasing UL (Fig. 2(a)). The effects of Uc on h is dominant, since the bubbling phenomena become more vigorous due to the
105 105
increase of gas holdup and turbulence intensity with increasing Uc. The reason why the h value exhibits a maximum with uL can be due to the considerable decrease in the solid holdup (es) in the higher range of UL. As in the conventional beds, in the lower range of UL, the h 4500
(a)
h, [W/m K]
4000
m A
2
3500 3000 2500
2000 0.000 /s] , [1 0.0100.005 a kL 0.015 0.020
(kgfm')
UL (m/s)
4
Liquid
Solid
966.6
00
Water
PE
ft ,
4
966.6
0.03
Water
PE
m
4
966.6
00
Water
PE
4
877.3
0.01
Water
PP
•
4
877.3
00
Water
PP
4
PP
877.3
0.005
Water
• oA
4
966.6
0.01
Water
PE
4
966.6
0.020
Water
PE
4
877.3
0.02
Water
PP
4
877.3
0.00
Water
PP
O
3.5
650
0.04
Water
PS
(b)
0.025 0.00
dp (mm)
Author
Cho etal. [6]
Kim etal. [18]
Nikolovetal. [5]
0.08
3.5
0.02 0.04
U , 0.06 G [m /s]
Fig. 1. Effect of UG on h (a) and kLa (b) in three-phase inverse fluidized beds.
0.08 0.10 U.1U
value increases with increasing UL due to the increases of turbulence intensity of fluid element as well as mobility of fluidized particles, however, the turbulence and particle mobility would decrease with a further increase in uL due to the considerable decrease of es, compensating for the increase of gas and liquid holdups. Relatively heavier particles would be more effective for the heat transfer due to their potential for bubble breaking and consequent increase in contacting frequency with heater surface. The h value can be predicted from Eq. (5), with a correlation coefficient of 0.95[6].
2
h, [W/m K]
3500
/s] , [1 0.010 k La
(a)
dp (mm)
P. (kg/m)
(m/s)
966.6
0.002
Water
PE
966.6
0.004
Water
PE
966.6
0.006
Water
PE
877.3
2000
Water
PP
877.3
0.004
Water
PP
877.3
0.006
Water
PP
4
966.6
0.015
Water
PE
4
966.6
0.020
Water
PE
4
877.3
0.00
Water
PP
V
4
877.3
0.02
Water
PP
O
3.5
650
0.04
Water
PS
3.5
650
0.06
Water
PS
3000
2500
•
• o
2000 0.005
A
0.015
(b)
0.020 0.00
Liquid
Solid
Cho et al. [6)
Kim etal. [18]
NUtolovetal. [5]
0.02
U , 0.04 L [m /s]
Fig. 2. Effect of UL on h (a) and k L a (b) in three-phase inverse fluidized bed.
0.06 0.08
Nu
=
kLss
(5)
The volumetric gas-liquid mass transfer coefficient (kLa) has been obtained by fitting the concentration profile of dissolved oxygen to the axial dispersion model [8, 18]. The value of
106 106
kLa increases with increasing UG(Fig. l(b)). With increasing uL, the kLa value increases initially but approaches to an asymptotic value with a further increase in UL (Fig. 2(b)). This trend of kLa is very similar to that of gas holdup; the gas holdup increases initially and approaches to an asymptotic value with increasing VL. That is, although the value of ss decreases considerably in the higher range of uL, the value of kLa does not decrease but maintains an asymptotic value due to the effects of gas holdup and bubbling phenomena. The values of kLa in the beds of relatively higher density particles (polyethylene) are higher than those in the beds of relatively lower density particles(polypropylene). The kLa value can be correlated based on the isotropic turbulence theory as Eq. (6) with a correlation coefficient of 0.95.
uLj
(6)
{uG+u
3. THREE-PHASE CIRCULATING FLUIDIZED-BED REACTORS 3.1. Hydrodynamics Comparing with the conventional three-phase beds, the axial solid holdup distribution is much more uniform and the radial distribution of gas holdup (EG) is much flatter in circulating beds, due to the relatively high UL and solid circulation. The values of EG and bed porosity can be predicted by Eqs. (7) and (8) with a correlation coefficient of 0.94 and 0.95, respectively. f\[\1TT
0.492,-r -0.023,-, -0.047
ec=0.07UG
UL
Gs
,„
(7);
,
n
,,
eG + sL=\.2WG
;
-0.003 rr 0.279,-, -0.023
UL
Gs
,o.
(8)
Bubble size in the circulating beds increases with UG, but decreases with UL or solid circulation rate (Gs); bubble rising velocity increases with UG or UL but decreases with Gs; the frequency of bubbles increases with UG, UL or Gs. The axial or radial dispersion coefficient of liquid phase (Dz or Dr) has been determined by using steady or unsteady state dispersion model. The values of Dz and Dr increase with increasing UG or Gs, but decrease (slightly) with increasing UL. The values of Dz and Dr can be predicted by Eqs.(9) and (10) with a correlation coefficient of 0.93 and 0.95, respectively[10].
D)
{ UU
)
Pe r_ "\
3.2. Heat and Mass Transfer The available data on the heat and mass transfer coefficients in three-phase circulating fluidized-bed reactors are comparatively sparse. The heat transfer coeffieient(h,cir) has been measured in the immersed heater-to-bed system. The value of h Cir increases with increasing UQ or Gs but exhibits a local maximum with increasing UL(Figs. 3(a)-5(a)). The mass transfer coefficient (kLa>Cjr) has been recovered by employing the similar dispersion model as in the case of inverse or conventional fluidized beds[18]. The k^ack value increases with increasing UG, GS or dp, but does not change considerably with increasing UL. The effects of Gs on kLa,Cir are not consistent; Yang et al.[2] reported that the value of kLa>Cir showed a maximum with increasing Gs, but Kim et al.[18] pointed out that the value increased gradually with Gs.
107 107
dp p, G^ U,, (mm) (kg/m1) (kg/m"s) (m/s) 2.1 2500 2 0.27 2.1 2500 4 0.27 2.1 2500 6 0.27
2600
(a)
2
h, [W/m K]
2400
,[ k La
1/s
2200
2000
]
0.01
•
0.00
o
0.02
A
0.03
0.04
V
(b)
o
0 .00
ft
0.02
U , 0.04 G [m /s ]
0.06
0.05(eJ
0.02
1130
0.1(eJ
0.02
0.401
1130
0.15(e.)
0.02
I 1.7 2.1 3 0.4 0.4
2500 .00 2500 2500 2460 2460
2 2
0.17 0.22 0.02 0.33 0.0812 0.0632
2 2
dp P, Gs^ U(1 (mm) (kg'm*) (kg'nA) (m/s) 2.1 2500 2 0.01 2.1 2500 2 0.03 2.1 2500 2 0.05
2600
(a) 2400 2
h, [W/m K]
1130
0.401
•
0.401
1130
0.05(E,)
0.003
•
0.401
1130
0.1(E,)
0.003
2000
*
0.401
1130
0.15(eJ
0.003
0.00
• o
1 1.7 2.1 3 0.4 0.4
2500 2500 2500 2500 2460 2460
2 2 2 2
0.04
2200
A
0.03
(b)
0.04 0.0
V
o *
0.1 0.2
U , 0.3 L [m /s ]
Solid
Water Glass bead Water Glass bead Water Glass bead Waste Polymer resin water Waste Polymer resin water Waste Polymer water resin Water Glass bead Water Glass bead Water Glass bead Water Glass bead Water Glass bead Water Glass bead
Cho et al. [4]
Kang etal. [21]
Kim et al. [22]
Srtal. [2]
Fig. 3. Effects of UG on h (a) and kLa (b) in three-phase circulating fluidized bed.
0.08
/s] 0.01 , [1 k La 0.02
0.401
Liquid
0.4
0.1
0.2 0.3 0.0361 0.0542
Liquid Water Water Water Waste water Waste water Waste water Water Water Water Water Water Water
Cho etal. [4] Polymer resin Kang Polymer resin etal. [21] Polymer resin Glass bead Glass bead Kim et al. [22] Glass bead Glass bead Glass bead Yanj et al. [2] Glass bead
Fig. 4. Effect of UL on h (a) and kLa (b) in three-phase circulating fluidized bed
2600
(a)
i
iI
1I
2
h, [W/m K]
2400
I
2200
1
kL
/s]
[ _ i
0.00 0.01
0.02
0.03 0 5
G , S [k g
J (b)
0.04
10
/ m 2s SJ]
"
l5 15
A V
2000
1 a, [
•
0
II I
i
D O A V
O
dr P< u,. (mm) (kg'mJ) (m/s) 2.1 2000 0.25 2.1 0.27 2500 0.01 2500 0.01 2.1 2500 0.02 I 2500 0.21 0.24 1.7 2500 2.1 0.28 2500 3 2500 0.40 0.4 2460 0.0722 0.4 2460 0.0722
(m/s) 00 0.03 0.01 0.02 0.03 0.03 0.07 0.07 0.0542 0.0361
Liquid Water Water Water Water Water Water Water Water Water Water
Glass bead Glass bead Cho et al. [4] Glass bead Glass bead Glass bead Glass bead Kim et al. [22] Glass bead Glass bead Glass bead . Glass bead Yang et al. [2]
Fig. S. Effects of G s on h (a) and kLa (b) in three-phase circulating fluidized bed.
Although the values of kLa Cjr in the literature are reasonable and comparable each other, the different trend mentioned above may be due to the different operating conditions. The gasliquid interfacial area(a) and liquid side mass transfer coefficient(ki) have been determined from the knowledge of measured values of gas holdup and kLaiCir [11]. The values of a and k] increase almost linearly with increasing UG or UL- The values of h>Cir and kr,aCir in circulating beds can be predicted by Eqs.(ll) and (12) with a correlation coefficient of 0.92 and 0.93,
108 108
respectively[19]. ,
T7T/-J-T
0 . 0 8 0 r r 0 . 0 3 2 - - , 0.065
hcir=2776UG
UL
Gs
, , , ,
,
(11);
kLacil. =0.263C/ o
r, ~r*>TT
0.275r
T
UL
0 . 0 1 7 , - , 0.155 , 0.097
Gs
dp
, , „ -
(12)
4. RECOMMENDATIONS FOR THE FUTURE STUDY Although some correlations have been proposed to predict the hydrodynamics and heat and mass transfer characteristics in three-phase inverse as well as circulating fluidized beds, unified correlations covering wide range of operating conditions cannot be drawn due to the extremely limited data reported in the literature. With increasing potential for the application of these reactors in the fields of wastewater treatment processes, the following features have to be examined in the future study. Effects of solid (dp, shape, ps etc.) and liquid properties (u L, OL, PU etc) on the hydrodynamics and heat and mass transfer have to be determined for designing the reactors. Unified correlations to predict the hydrodynamics and heat and mass transfer characteristics are essentially needed in the future study. For modeling of these reactors, the model study with reasonable and practical assumptions is also required. Since the reactors are composed of multiphase-dynamic system, a stochastic approach in addition to the deterministic one has to be considered to analyze, diagnose the fault and conduct on-line control of the system. In addition, various kinds of modifications of three-phase inverse as well as circulating fluidized beds have to be investigated for the development of more effective and economic wastewater treatment reactors.
REFERENCES 1. L. S. Fan, Gas-Liquid-Solid Fluidization Engineering, Butterworth, Boston, MA(1989) 368. 2. W. G. Yang, J. F. Wang, W. Chem and Y. Jin, Chem. Eng. Sci., 54(1999)5293. 3. J. Zhu, Y. Zheng, D. G. Karamanev and A. S. Bassi; Can. J. Chem. Eng., 78(2000)82. 4. Y. J. Cho, P. S. Song, S. H. Kim, Y. Kang and S. D. Kim, J. Chem. Eng. Japan, 34(2001)254. 5. V. Nikolov, I. Farag, I. Nikov, Bioprocess Bioeng., 23(2000)427. 6. Y. J. Cho, H. Y. Park, S. W. Kim, Y. Kang and S. D. Kim, Ind. Eng. Chem. Res., 41(2002)2058. 7. P. Buffire and R. Moletta, Chem. Eng. Sci., 54(1999)1233. 8. S. W. Kim, H. T. Kim, P. S. Song, Y. Kang and S. D. Kim, Can. J. Chem. Eng., 81(2003)621. 9. P. Buffiere and R. Moletta, Chem. Eng. Sci., 55(2000)5555. 10. Y. Kang, Y. J. Cho, C. G. Lee, P. S. Song and S. D. Kim, Can. J. Chem. Eng., 81(2003)1130. U . S . Han, J. Zhou, Y. Jin, K. C. Loh and Z. Wang, Chem. Eng. J., 70(1998)9. 12. M. P. Comte, D. Bastoul, G. Hebrard, M. Roustan and and V. Lazarova, Chem. Eng. Sci., 52(1997)3971. 13. C. L. Brieno, Y. A. A. Ibrahim, A. Margaritis and M. A. Bergougnou, Chem. Eng. Sci., 54(1999)4975. 14. S. M. Son, S. H. Kang, Y. Kang and S. D. Kim, J. KIChE, 42(2004)332. 15. S. M. Son, S. H. Jung, S. W. Kim, Y. Kang and S. D. Kim, Prof. 9th Asian Conf. on FluidizedBed and Three-Phase Reactors(2004)265. 16. H. T. Kim, P. S. Song, S. W. Kim and Y. Kang, J. KIEC, 13(2002) 691. 17. S. M. Son, P. S. Song, C. G. Lee, S. H. Kang, Y. Kang and K. Kusakabe, J. Chem. Eng. Japan, 37(2004)990. 18. S. W. Kim, P. S. Song, H. T. Kim, Y. Kang and S. D. Kim, J. KIChE, 40(2002)482. 19. Y. J. Cho, S. J. Kim, S. H. Nam, Y. Kang and S. D. Kim, Chem. Eng. Sci, 56(2001)6107. 20. H. T. Kim, P. S. Song, S. W. Kim and Y. Kang, J. KIEC, 13(2002) 691. 21. T. Kang, P. Song, G. Choi, Y. Cho, Y. Kang, H. Choi and S. D. Kim, J. KIChE, 40(2002)640. 22. S. J. Kim, J. S. Kim, C. K. Lee, Y. Kang and S. D. Kim, Proc. 14th symp. on Chem. Eng. (2001)187.
103 103
Hydrodynamics, Heat and Mass Transfer in Inverse and Circulating Three-Phase Fluidized-Bed Reactors for WasteWater Treatment Sang Done Kim"* and Yong Kangb "Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea(* [email protected]) b School of Chemical Engineering, Chungnam National University, Daejoen 305-764, Korea 1. INTRODUCTION Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The threephase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gasliquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. In wastewater treatment processes, solid materials including catalyst supporter, bio-media, food particles, adsorbent or absorption media are normally porous, small and light particles. These relatively low density solids are easily floating in the continuous liquid medium due to the buoyance force of solid particles. To fluidize these low density particles, the inverse fluidized bed reactor has been employed in which the liquid phase flows downward against the buoyance force acting on the floating particles and the upward gas flow. This countercurrent gas/liquid flow mode can increase the gas holdup. With increasing gas holdup, the performance of reactors for wastewater treatment could be improved, since the bubbles can provide the microorganism with oxygen and the liquid with gas reactants [5-9]. In spite of attractive advantages of conventional three-phase fluidized bed reactors, the range of UL has to be extremely low and narrow, when the small or porous particles are fluidized by viscous liquid medium, which is often encountered in the processes of wastewater treatment. To overcome this limitation and increase the treatment efficiency, the three-phase circulating fluidized-bed reactor has been devised in order to recycle solid particles from the reactor through a downcomer to the reactor. The circulation mode can minimize the dead zone by increasing the contacting efficiency among three (gas-liquidsolid) phases with sufficiently high UL. In addition, the deactivated solid media can be regenerated continuously by means of the circulation mode of particles [2-4,10,11]. To provide the prerequisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrodynamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve
104 104
the state of the present knowledge have been defined with recommendations. 2. THREE-PHASE INVERSE FLUID1ZED-BED REACTORS 2.1. Hydrodynamics The inverse fluidization of low density particles in wastewater treatment reactors can be divided into two modes ; the one is gas-liquid countercurrent flow mode comprising a continuous flow system and the other is gas only flow mode in a batch system, since the floated low density particles can be easily fluidized by the upward gas flow. For the wastewater treatment in bioreactors, the biomass is usually growing on the surface of the fluidized particles to form a biofilm. In a batch system, the collision frequency and pressure on the particles increase with increasing gas velocity (uG) but show their maxima with increasing particle holdup. The effects of UG are similar to those of other studies [9, 12, 13], but the effects of solid holdup are not consistent with the results of other investigations in which the particle fluctuation frequency increases with increasing solid holdup up to 0.6. It is interesting to note that the increase trend of fluctuation frequency and dispersion coefficient of particles with increasing uG in a continuous inverse beds [14] is very similar to that of particle collision frequency and pressure in the batch system. However, the unified correlations to predict the frequency of collision or fluctuations of fluidized particles have not been proposed up to now due to the limited research works. It has been reported that the gas phase holdup or bubble size(Lv) increases with increasing UG, UL or liquid viscosity (UL); its rising velocity(UB) increases with increasing UG or ML, but decreases with increasing UL; the frequency(Fe) of rising bubbles increases with increasing UG or UL but decreases with increasing UL- The gas (or bubble) holdup in the beds of relatively heavier particles is higher than that in the beds of relatively lighter particles, since the bubble size in case of the former is smaller than that of the latter. The bubble properties and liquid axial dispersion coefficient (Dz) have been correlated with the experimental variables as [8,15-17]: (1);
( x FB = \2AUoaO52U°Lmju?m\
&-] KP)
UB=0A2QU0Gl>*Ul0A1^0Lm\-^\
(2)
( V'26
1
(3);
Dz = 0.157C/»-263l/f2 V' 0 1 8 £ -
(4)
\P)
with correlation coefficients of 0.92, 0.95, 0.94 and 0.92 for Eqs. (l)-(4), respectively. 2.2 Heat and Mass Transfer Very limited data on the heat and mass transfer in three-phase inverse fluidization systems is available up to now. For the wastewater treatment, the reactor temperature should be controlled and maintained within a certain level to optimize the reactor performance, since the temperature of reactor or process can provide the microorganisms with favorable circumances. The heat transfer coefficient (h) has been determined by measuring the temperature difference between the immersed heater and the bed. The h value increases with increasing Uc (Fig. l(a)), but exhibits a maximum value with increasing UL (Fig. 2(a)). The effects of Uc on h is dominant, since the bubbling phenomena become more vigorous due to the
105 105
increase of gas holdup and turbulence intensity with increasing Uc. The reason why the h value exhibits a maximum with uL can be due to the considerable decrease in the solid holdup (es) in the higher range of UL. As in the conventional beds, in the lower range of UL, the h 4500
(a)
h, [W/m K]
4000
m A
2
3500 3000 2500
2000 0.000 /s] , [1 0.0100.005 a kL 0.015 0.020
(kgfm')
UL (m/s)
4
Liquid
Solid
966.6
00
Water
PE
ft ,
4
966.6
0.03
Water
PE
m
4
966.6
00
Water
PE
4
877.3
0.01
Water
PP
•
4
877.3
00
Water
PP
4
PP
877.3
0.005
Water
• oA
4
966.6
0.01
Water
PE
4
966.6
0.020
Water
PE
4
877.3
0.02
Water
PP
4
877.3
0.00
Water
PP
O
3.5
650
0.04
Water
PS
(b)
0.025 0.00
dp (mm)
Author
Cho etal. [6]
Kim etal. [18]
Nikolovetal. [5]
0.08
3.5
0.02 0.04
U , 0.06 G [m /s]
Fig. 1. Effect of UG on h (a) and kLa (b) in three-phase inverse fluidized beds.
0.08 0.10 U.1U
value increases with increasing UL due to the increases of turbulence intensity of fluid element as well as mobility of fluidized particles, however, the turbulence and particle mobility would decrease with a further increase in uL due to the considerable decrease of es, compensating for the increase of gas and liquid holdups. Relatively heavier particles would be more effective for the heat transfer due to their potential for bubble breaking and consequent increase in contacting frequency with heater surface. The h value can be predicted from Eq. (5), with a correlation coefficient of 0.95[6].
2
h, [W/m K]
3500
/s] , [1 0.010 k La
(a)
dp (mm)
P. (kg/m)
(m/s)
966.6
0.002
Water
PE
966.6
0.004
Water
PE
966.6
0.006
Water
PE
877.3
2000
Water
PP
877.3
0.004
Water
PP
877.3
0.006
Water
PP
4
966.6
0.015
Water
PE
4
966.6
0.020
Water
PE
4
877.3
0.00
Water
PP
V
4
877.3
0.02
Water
PP
O
3.5
650
0.04
Water
PS
3.5
650
0.06
Water
PS
3000
2500
•
• o
2000 0.005
A
0.015
(b)
0.020 0.00
Liquid
Solid
Cho et al. [6)
Kim etal. [18]
NUtolovetal. [5]
0.02
U , 0.04 L [m /s]
Fig. 2. Effect of UL on h (a) and k L a (b) in three-phase inverse fluidized bed.
0.06 0.08
Nu
=
kLss
(5)
The volumetric gas-liquid mass transfer coefficient (kLa) has been obtained by fitting the concentration profile of dissolved oxygen to the axial dispersion model [8, 18]. The value of
106 106
kLa increases with increasing UG(Fig. l(b)). With increasing uL, the kLa value increases initially but approaches to an asymptotic value with a further increase in UL (Fig. 2(b)). This trend of kLa is very similar to that of gas holdup; the gas holdup increases initially and approaches to an asymptotic value with increasing VL. That is, although the value of ss decreases considerably in the higher range of uL, the value of kLa does not decrease but maintains an asymptotic value due to the effects of gas holdup and bubbling phenomena. The values of kLa in the beds of relatively higher density particles (polyethylene) are higher than those in the beds of relatively lower density particles(polypropylene). The kLa value can be correlated based on the isotropic turbulence theory as Eq. (6) with a correlation coefficient of 0.95.
uLj
(6)
{uG+u
3. THREE-PHASE CIRCULATING FLUIDIZED-BED REACTORS 3.1. Hydrodynamics Comparing with the conventional three-phase beds, the axial solid holdup distribution is much more uniform and the radial distribution of gas holdup (EG) is much flatter in circulating beds, due to the relatively high UL and solid circulation. The values of EG and bed porosity can be predicted by Eqs. (7) and (8) with a correlation coefficient of 0.94 and 0.95, respectively. f\[\1TT
0.492,-r -0.023,-, -0.047
ec=0.07UG
UL
Gs
,„
(7);
,
n
,,
eG + sL=\.2WG
;
-0.003 rr 0.279,-, -0.023
UL
Gs
,o.
(8)
Bubble size in the circulating beds increases with UG, but decreases with UL or solid circulation rate (Gs); bubble rising velocity increases with UG or UL but decreases with Gs; the frequency of bubbles increases with UG, UL or Gs. The axial or radial dispersion coefficient of liquid phase (Dz or Dr) has been determined by using steady or unsteady state dispersion model. The values of Dz and Dr increase with increasing UG or Gs, but decrease (slightly) with increasing UL. The values of Dz and Dr can be predicted by Eqs.(9) and (10) with a correlation coefficient of 0.93 and 0.95, respectively[10].
D)
{ UU
)
Pe r_ "\
3.2. Heat and Mass Transfer The available data on the heat and mass transfer coefficients in three-phase circulating fluidized-bed reactors are comparatively sparse. The heat transfer coeffieient(h,cir) has been measured in the immersed heater-to-bed system. The value of h Cir increases with increasing UQ or Gs but exhibits a local maximum with increasing UL(Figs. 3(a)-5(a)). The mass transfer coefficient (kLa>Cjr) has been recovered by employing the similar dispersion model as in the case of inverse or conventional fluidized beds[18]. The k^ack value increases with increasing UG, GS or dp, but does not change considerably with increasing UL. The effects of Gs on kLa,Cir are not consistent; Yang et al.[2] reported that the value of kLa>Cir showed a maximum with increasing Gs, but Kim et al.[18] pointed out that the value increased gradually with Gs.
107 107
dp p, G^ U,, (mm) (kg/m1) (kg/m"s) (m/s) 2.1 2500 2 0.27 2.1 2500 4 0.27 2.1 2500 6 0.27
2600
(a)
2
h, [W/m K]
2400
,[ k La
1/s
2200
2000
]
0.01
•
0.00
o
0.02
A
0.03
0.04
V
(b)
o
0 .00
ft
0.02
U , 0.04 G [m /s ]
0.06
0.05(eJ
0.02
1130
0.1(eJ
0.02
0.401
1130
0.15(e.)
0.02
I 1.7 2.1 3 0.4 0.4
2500 .00 2500 2500 2460 2460
2 2
0.17 0.22 0.02 0.33 0.0812 0.0632
2 2
dp P, Gs^ U(1 (mm) (kg'm*) (kg'nA) (m/s) 2.1 2500 2 0.01 2.1 2500 2 0.03 2.1 2500 2 0.05
2600
(a) 2400 2
h, [W/m K]
1130
0.401
•
0.401
1130
0.05(E,)
0.003
•
0.401
1130
0.1(E,)
0.003
2000
*
0.401
1130
0.15(eJ
0.003
0.00
• o
1 1.7 2.1 3 0.4 0.4
2500 2500 2500 2500 2460 2460
2 2 2 2
0.04
2200
A
0.03
(b)
0.04 0.0
V
o *
0.1 0.2
U , 0.3 L [m /s ]
Solid
Water Glass bead Water Glass bead Water Glass bead Waste Polymer resin water Waste Polymer resin water Waste Polymer water resin Water Glass bead Water Glass bead Water Glass bead Water Glass bead Water Glass bead Water Glass bead
Cho et al. [4]
Kang etal. [21]
Kim et al. [22]
Srtal. [2]
Fig. 3. Effects of UG on h (a) and kLa (b) in three-phase circulating fluidized bed.
0.08
/s] 0.01 , [1 k La 0.02
0.401
Liquid
0.4
0.1
0.2 0.3 0.0361 0.0542
Liquid Water Water Water Waste water Waste water Waste water Water Water Water Water Water Water
Cho etal. [4] Polymer resin Kang Polymer resin etal. [21] Polymer resin Glass bead Glass bead Kim et al. [22] Glass bead Glass bead Glass bead Yanj et al. [2] Glass bead
Fig. 4. Effect of UL on h (a) and kLa (b) in three-phase circulating fluidized bed
2600
(a)
i
iI
1I
2
h, [W/m K]
2400
I
2200
1
kL
/s]
[ _ i
0.00 0.01
0.02
0.03 0 5
G , S [k g
J (b)
0.04
10
/ m 2s SJ]
"
l5 15
A V
2000
1 a, [
•
0
II I
i
D O A V
O
dr P< u,. (mm) (kg'mJ) (m/s) 2.1 2000 0.25 2.1 0.27 2500 0.01 2500 0.01 2.1 2500 0.02 I 2500 0.21 0.24 1.7 2500 2.1 0.28 2500 3 2500 0.40 0.4 2460 0.0722 0.4 2460 0.0722
(m/s) 00 0.03 0.01 0.02 0.03 0.03 0.07 0.07 0.0542 0.0361
Liquid Water Water Water Water Water Water Water Water Water Water
Glass bead Glass bead Cho et al. [4] Glass bead Glass bead Glass bead Glass bead Kim et al. [22] Glass bead Glass bead Glass bead . Glass bead Yang et al. [2]
Fig. S. Effects of G s on h (a) and kLa (b) in three-phase circulating fluidized bed.
Although the values of kLa Cjr in the literature are reasonable and comparable each other, the different trend mentioned above may be due to the different operating conditions. The gasliquid interfacial area(a) and liquid side mass transfer coefficient(ki) have been determined from the knowledge of measured values of gas holdup and kLaiCir [11]. The values of a and k] increase almost linearly with increasing UG or UL- The values of h>Cir and kr,aCir in circulating beds can be predicted by Eqs.(ll) and (12) with a correlation coefficient of 0.92 and 0.93,
108 108
respectively[19]. ,
T7T/-J-T
0 . 0 8 0 r r 0 . 0 3 2 - - , 0.065
hcir=2776UG
UL
Gs
, , , ,
,
(11);
kLacil. =0.263C/ o
r, ~r*>TT
0.275r
T
UL
0 . 0 1 7 , - , 0.155 , 0.097
Gs
dp
, , „ -
(12)
4. RECOMMENDATIONS FOR THE FUTURE STUDY Although some correlations have been proposed to predict the hydrodynamics and heat and mass transfer characteristics in three-phase inverse as well as circulating fluidized beds, unified correlations covering wide range of operating conditions cannot be drawn due to the extremely limited data reported in the literature. With increasing potential for the application of these reactors in the fields of wastewater treatment processes, the following features have to be examined in the future study. Effects of solid (dp, shape, ps etc.) and liquid properties (u L, OL, PU etc) on the hydrodynamics and heat and mass transfer have to be determined for designing the reactors. Unified correlations to predict the hydrodynamics and heat and mass transfer characteristics are essentially needed in the future study. For modeling of these reactors, the model study with reasonable and practical assumptions is also required. Since the reactors are composed of multiphase-dynamic system, a stochastic approach in addition to the deterministic one has to be considered to analyze, diagnose the fault and conduct on-line control of the system. In addition, various kinds of modifications of three-phase inverse as well as circulating fluidized beds have to be investigated for the development of more effective and economic wastewater treatment reactors.
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