- Email: [email protected]
Hysteresis effects of minimum fluidization velocity in a draft tube airlift reactor
Shorter Communications
996
cessive band-spreading due to mass transfer or other rate processes. Other heterogeneities not considered in the present analysis may also contribute to band-shape alteration. For example, no allowance was made for the effect of heterogeneity of binding sites, i.e. when different sites have different affinities for identical molecules. Nor have dynamic conformational changes of adsorbing species, for which molecules continually change between different conformations, e.g. by folding and unfolding, been considered. All these effects are likely to influence protein separations, and merit further investigation. Department ofChemical Engineering University of California Davis, CA 95616, U.S.A.
BENJAMIN
J. MCCOY
I
-M(C)
input distribution (continuous), i.e. c(<, f, z = 0) = a(@b(t) s+(l--s)K, E+(l -E)K, input distribution (discrete), i.e. c&, z=O) = aid(t) total concentration [eq. (10) or (2311 concentration of species i concentration of species with property in the range (r, r + de) axial dispersion coefficient equilibrium coefficient for adsorption species of property c equilibrium adsorption coefficient for species i constant value equilibrium adsorption coeficien t 03 c(t, z)t”dt, nth moment I 0 m cXt, z)t”dt, nth moment for species i s0
Chemical Engineering Science, Vol. 44, No. 4. pp. 99&%X3, Printed
in GreatBritain.
Hysteresis
c(& t, z)t” dt, nth moment for species of
p:ooperty < 0A or aA time superl%ial velocity (volumetric flow rate/ column cross-sectional area) length coordinate measured from column inlet
T t v z
Greek
letters parameter in the Gaussian distribution parameter in distributions defined by eqs (32) and (36) parameter in the exponential distribution variable in the continuous distributions m,/m,, normalized first moment
:
NOTATION &+(I
a’
1 m - 1 (t -pJ’c(t, m 0 central moment
Pz
z)dt,
1 m (t -pJjc(t, z)dt, normalized “1 s 0 central moment parameter in the input boundary dition, e.g. cq. (4)
(Received
7 June
velocity
1988; accepted
Minimum fluidiition velocity is one of the most relevant parameters for proper design and operation of three-phase duidized bed reactors. Although airlift reactors can be successfully used for gas-liquid-solid reactions, only a few papers report on minimum gas velocity needed for fluidization of solid particles in this type of reactor (Koide et al., 1984: Muroyama et al., 1985; Heck and Onken, 1987.1988; Pogarac and PetroviC, 1988). As is well known the driving force for liquid recirculation in an airlift reactor is the difference of dispersion densities in
third con-
REFERENCES
0
fluidization
second
Dal Nogare, S. and Juvet, R. S., Jr., 1962, Gas Liquid Chromatography: Theory and Practice. Wiley-Interscience, New York. Kang, K. and McCoy, B. J., 1988, Protein separation by ion exchange chromatography: a model for gradient elution. Biotechnol. Bioengng. Regnier, F. E., 1987, The role of protein structure in chromatographic behavior. Science 238, 319-323. Suzuki, M., and Smith, 5. M., 1975, Transport and kinetic parameters by gas chromatographic techniques. Adu. Chromut. 13,213-263.
1989.
effects of minimum
normalized
aoo%2509/89 s3.cm + 0.00 1989 Pergamon Press pb
in a draft tube airlift reactor
25 August
1988)
the riser and downcomer of the reactor. At gas flow rates sufficient to induce high liquid recirculation rate the fluidization of solid particles is achieved. Therefore the hydrodynamic behaviour of this type of reactor is considerably different than that of reactors with externally forced liquid flow. Recent investigations have shown some specific hysteresis effects of minimum fluidization velocity (U,,) in airlift reactors. Namely, two U,, were observed: one to reach fluidization and the other one to maintain the solid particles in fluidized state. Heck and Onken (1987) have observed
Shorter Communications these hysteresis phenomena in both bubble column and draft tube airlift reactor, working with particles of 0.308 mm. PoSarac and Petrovit (1988) have found another type of hysteresis behavlour of U,, with periodical expansion and contraction of the bed (“breathing”) in the airlift reactor with an external loop working on the system air-butanol solution-3-6 mm solid spheres. In this note hysteresis effects of U, of solid particles 1, 3 and 6 mm in a draft tube airlift reactor are presented. The experiments have been carried out in a glass column 200 mm i.d., 3 m in height with a conical bottom and a draft tube 2 m in height and 80 mm i.d. (Fig. 1). The vertical clearance between draft tube and gas distributor was 40 mm. Air was used as a gas phase in all experiments. The air was sparged into the draft tube through a perforated plate, 70mm in diameter, with 19 holes of 1 mm arranged in a triangular pitch. As a liquid phase tap water, a 0.5% wt aqueous solution of n-butanol and a 46% wt aqueous solution of glycerol were used. Glass spheres of 1, 3 and 6 mm diameter, with a density of 2550kg/m3, were used. Minimum fluidization velocity was determined by static pressure measurement at the gas distribution level (piezometric tube) and visually (applying l-s criterion). In all experiments the gas flow rate was first increased up to and above the fluidization velocity and then decreased, observing the U,, again, until the bed became steady. In the three-phase airlift reactors liquid recirculation rate strongly depends on hydraulic resistances in the circulation path. At low gas flow rates a solid layer on the bottom of the column entirely plugged the entry of the draft tube so that the liquid recirculation was completely hindered. In this case the drafl tube operated as a bubble column. In the solid layers of 1 and 3 mm spheres bubble coalescence was evident even in the presence of n-butanol. When the solid layer in the draft tube was higher than that at the bottom of the annulus bubbles went through the annulus. This reverse flow and greater pressure in the draft tube
997
caused a slow leakage of the solids from the draft tube. At the moment when the pressure in annular section became greater than in the draft tube gas preferably went through it, solid phase was suddenly drawn in the draft tube and during a short time a fluidization of the solids was established. At low gas flow rates the fluidized bed was not stable, it gradually fell down and the described behaviour periodically repeated. By increasing gas flow rates a steady liquid recirculation rate was achieved and stable fluidization of solids occurred. A further increase of gas flow rate produced circulation of solids through the reactor. On the contrary, during the decrease of gas flow rates stable liquid recirculation was maintained even at lower gas flow rates so that U,, at decreasing order of velocities was lower. The described behaviour explains the hysteresis effects shown in Fig. 2 which have been found in the water and glycerol solution. One can see an increasing hysteresis effect with an increase of initial solids concentration. It is in accordance with the previous results of the hysteresis effects of unlr in airlift reactors (Heck and Onken, 1987; PoZarac and Petrovik, 1988). The solid layer of 1 mm particles had greater hydraulic resistance to liquid flow and tendency to promote coalescing of bubbles, resulting in more pronounced hysteresis effects. Greater viscosity and slight surface active properties of glycerol solution explain less pronounced hysteresis effect and minimum fluidization velocity in comparison with water. In experiments with 6 mm spheres and n-butanol solution we also found another hysteresis effect previously reported by PoSarac and Petrovik (1988). It was observed that the solid layer of 6 mm spheres acted as a good additional distributor in all investigated liquids, producing small bubbles. In the buranol solution the bubbles were 2-3 mm in diameter (visual estimation). Even at low gas flow rates only a small part of gas flow went through the annulus, flowing mostly
a. =
Irnnl
0
-
WATER
q
-
‘6
V.
OLYCEROL P
b)
dr = 3mm
1
10
0
-
w*TER
0
-
16%
I
20
OLYCEROL
I
30
P
I
I
10
50 Cs
-
AIR
Fig. 1. Experimental apparatus. (1) glass column, (2) draft tube, (3) gas distributor, (4) solid bed and (5) spacer.
60
t
70
1
(kg/m’)
Fig. 2. Hysteresis effects of minimum fluidization velocity for (a) ds = 1 mm, (b) ds = 3 mm. Open symbols-increasing order of velocities; solid symbolsaecreasing order of velocitles.
998
Shorter
Communications
through the solid layer in the draft tube, producing dense bubbly Bow. Further increase of gas flow rate caused an accumulation of small bubbles in the fixed bed. In the moment when the difference between gas holdup in the draft tube and the annulus induced a sufticient driving force the fluidization occurred. The expansion of solid layer and the flow of liquid quickly pushed the accumulated gas out of the draft tube, the difference in gas holdups suddenly decreased and expanded solid layer fell down. Such expansion and contraction of the bed periodically and regularly repeated. At higher gas flow rates the periodical eruptions of solids were very intense (see Fig. 3a) with shorter periods. The described “breathing” disappeared at the moment when the eruption threw out a certain amount of solids from the draft tube so that a stable liquid and solid recirculation was enabled. Gradual decrease of gas flow rate below the starting point of circulated bed mode resulted in the appearance of a stable fluid&d bed in the draft tube in a certain range of gas flow rates. Below that range the breathing of bed occurred again. It is known that the gas distributor design in the presence of bubble coalescence inhibitors has strong influence on the hydrodynamics of gas-liquid reactors. We assume that the perforated plate with 1 mm holes as a relatively efficient primary distributor, combined with a layer of 6 mm spheres as an additional gas distributor, play an important role in described breathing behaviour of the bed. Initially formed small bubbles tended to accumulate in the fixed bed causing the breathing of bed
To check this assumption we performed an experiment with an inefficient gas distributor of single nozzle type 8 mm i.d. At low gas flow rates (Uo=O.8 to 1 cm/s) large bubbles generated by this distributor, moved fast through the bed, causing very intense oscillations of solid particles which in turn caused the disintegration of the bubbles. As a result, there was no accumulation of gas in bed, no breathing occurred and a stable guidization was reached. An increase in gas flow rate increased the height of the bed causing less intense disintegration of bubbles and a decrease of gas hold up in the draft tube. As a consequence the height of the bed decreased with more intense disintegration of bubbles and the expansion of the bed occurred again. With further increase of gas flow rate (Uo> 1.8 cm/s) these contractions and expansions of the fluidized bed almost completely disappeared. The presented results show a significant influence of particle diameter and concentration, gas distributor design and liquid phase properties on minimum fluidization velocity and observed hysteresis effects. We hope that this note will contribute to better understanding of hydrodynamics of the three phase fluid&d bed airlift reactors. D. PETROVI@ D. POSARAC Faculty OJ Technology 21000 Novi Sad Yugoslavia D. SKALA
4
PERFORATED
PLATE 16 I 1 mm 6s = 6 mm
a)
1
Faculty of Technology 11000 Beograd Yugoslavia
Cs ds H H0 uo I7 mf
and MeraNurgy
NOTATION concentration of solid particles, kg/m3 solid particle diameter, mm height of the fluid&d bed, m height of the static bed, m superficial gas velocity calculated on the crosssectional area of the outer column, m/s LJ, at the beginning of fluidization, m/s REFERENCES
Heck, .I. and Onken, U., 1987, Hysteresis effects in suspended solid particles in bubble columns with and without draft tube. Chem. Engng Sci. 42, 1211-1212. Heck, J. and Onken, U., 1988, Eintluss der Partikelform auf die Feststoffsuspendierung in BlasensHule and AirliftSchlaufenreaktor. Chem.-Zng.-Techn. 60, 40&4Ol. Koide, K., Horihe, K., Kawabata, H. and Ito, S., 1984, Critical gas velocity required for complete suspension of solid particles in solid-suspended bubble column with draught tube. J. them. Engng Japan 17, 368-374. Muroyama, K., Mitani, Y. and Yasunishi, A., 1985, Hydrodynamic characteristics and gas-liquid mass transfer in a draft tube slurry reactor. Chem Engng Commun. 34,87-98. PoHarac, D. and PetroviC, D., 1988, An experimental study of the minimum fluidization velocity in the three-phase external loop air-lift reactor. Chem. Engng Sci. 43,1161-l 165. Fig. 3. The influence of the distributor design on the bed expansion in the system: air-0.S% n-butanol solution-6 mm glass spheres. Cs = 70 kg/m3. Open symbols-increasing order of velocities; solid symbols-decreasing order of velocities.
+Author to whom correspondence
should be addressed.
996
cessive band-spreading due to mass transfer or other rate processes. Other heterogeneities not considered in the present analysis may also contribute to band-shape alteration. For example, no allowance was made for the effect of heterogeneity of binding sites, i.e. when different sites have different affinities for identical molecules. Nor have dynamic conformational changes of adsorbing species, for which molecules continually change between different conformations, e.g. by folding and unfolding, been considered. All these effects are likely to influence protein separations, and merit further investigation. Department ofChemical Engineering University of California Davis, CA 95616, U.S.A.
BENJAMIN
J. MCCOY
I
-M(C)
input distribution (continuous), i.e. c(<, f, z = 0) = a(@b(t) s+(l--s)K, E+(l -E)K, input distribution (discrete), i.e. c&, z=O) = aid(t) total concentration [eq. (10) or (2311 concentration of species i concentration of species with property in the range (r, r + de) axial dispersion coefficient equilibrium coefficient for adsorption species of property c equilibrium adsorption coefficient for species i constant value equilibrium adsorption coeficien t 03 c(t, z)t”dt, nth moment I 0 m cXt, z)t”dt, nth moment for species i s0
Chemical Engineering Science, Vol. 44, No. 4. pp. 99&%X3, Printed
in GreatBritain.
Hysteresis
c(& t, z)t” dt, nth moment for species of
p:ooperty < 0A or aA time superl%ial velocity (volumetric flow rate/ column cross-sectional area) length coordinate measured from column inlet
T t v z
Greek
letters parameter in the Gaussian distribution parameter in distributions defined by eqs (32) and (36) parameter in the exponential distribution variable in the continuous distributions m,/m,, normalized first moment
:
NOTATION &+(I
a’
1 m - 1 (t -pJ’c(t, m 0 central moment
Pz
z)dt,
1 m (t -pJjc(t, z)dt, normalized “1 s 0 central moment parameter in the input boundary dition, e.g. cq. (4)
(Received
7 June
velocity
1988; accepted
Minimum fluidiition velocity is one of the most relevant parameters for proper design and operation of three-phase duidized bed reactors. Although airlift reactors can be successfully used for gas-liquid-solid reactions, only a few papers report on minimum gas velocity needed for fluidization of solid particles in this type of reactor (Koide et al., 1984: Muroyama et al., 1985; Heck and Onken, 1987.1988; Pogarac and PetroviC, 1988). As is well known the driving force for liquid recirculation in an airlift reactor is the difference of dispersion densities in
third con-
REFERENCES
0
fluidization
second
Dal Nogare, S. and Juvet, R. S., Jr., 1962, Gas Liquid Chromatography: Theory and Practice. Wiley-Interscience, New York. Kang, K. and McCoy, B. J., 1988, Protein separation by ion exchange chromatography: a model for gradient elution. Biotechnol. Bioengng. Regnier, F. E., 1987, The role of protein structure in chromatographic behavior. Science 238, 319-323. Suzuki, M., and Smith, 5. M., 1975, Transport and kinetic parameters by gas chromatographic techniques. Adu. Chromut. 13,213-263.
1989.
effects of minimum
normalized
aoo%2509/89 s3.cm + 0.00 1989 Pergamon Press pb
in a draft tube airlift reactor
25 August
1988)
the riser and downcomer of the reactor. At gas flow rates sufficient to induce high liquid recirculation rate the fluidization of solid particles is achieved. Therefore the hydrodynamic behaviour of this type of reactor is considerably different than that of reactors with externally forced liquid flow. Recent investigations have shown some specific hysteresis effects of minimum fluidization velocity (U,,) in airlift reactors. Namely, two U,, were observed: one to reach fluidization and the other one to maintain the solid particles in fluidized state. Heck and Onken (1987) have observed
Shorter Communications these hysteresis phenomena in both bubble column and draft tube airlift reactor, working with particles of 0.308 mm. PoSarac and Petrovit (1988) have found another type of hysteresis behavlour of U,, with periodical expansion and contraction of the bed (“breathing”) in the airlift reactor with an external loop working on the system air-butanol solution-3-6 mm solid spheres. In this note hysteresis effects of U, of solid particles 1, 3 and 6 mm in a draft tube airlift reactor are presented. The experiments have been carried out in a glass column 200 mm i.d., 3 m in height with a conical bottom and a draft tube 2 m in height and 80 mm i.d. (Fig. 1). The vertical clearance between draft tube and gas distributor was 40 mm. Air was used as a gas phase in all experiments. The air was sparged into the draft tube through a perforated plate, 70mm in diameter, with 19 holes of 1 mm arranged in a triangular pitch. As a liquid phase tap water, a 0.5% wt aqueous solution of n-butanol and a 46% wt aqueous solution of glycerol were used. Glass spheres of 1, 3 and 6 mm diameter, with a density of 2550kg/m3, were used. Minimum fluidization velocity was determined by static pressure measurement at the gas distribution level (piezometric tube) and visually (applying l-s criterion). In all experiments the gas flow rate was first increased up to and above the fluidization velocity and then decreased, observing the U,, again, until the bed became steady. In the three-phase airlift reactors liquid recirculation rate strongly depends on hydraulic resistances in the circulation path. At low gas flow rates a solid layer on the bottom of the column entirely plugged the entry of the draft tube so that the liquid recirculation was completely hindered. In this case the drafl tube operated as a bubble column. In the solid layers of 1 and 3 mm spheres bubble coalescence was evident even in the presence of n-butanol. When the solid layer in the draft tube was higher than that at the bottom of the annulus bubbles went through the annulus. This reverse flow and greater pressure in the draft tube
997
caused a slow leakage of the solids from the draft tube. At the moment when the pressure in annular section became greater than in the draft tube gas preferably went through it, solid phase was suddenly drawn in the draft tube and during a short time a fluidization of the solids was established. At low gas flow rates the fluidized bed was not stable, it gradually fell down and the described behaviour periodically repeated. By increasing gas flow rates a steady liquid recirculation rate was achieved and stable fluidization of solids occurred. A further increase of gas flow rate produced circulation of solids through the reactor. On the contrary, during the decrease of gas flow rates stable liquid recirculation was maintained even at lower gas flow rates so that U,, at decreasing order of velocities was lower. The described behaviour explains the hysteresis effects shown in Fig. 2 which have been found in the water and glycerol solution. One can see an increasing hysteresis effect with an increase of initial solids concentration. It is in accordance with the previous results of the hysteresis effects of unlr in airlift reactors (Heck and Onken, 1987; PoZarac and Petrovik, 1988). The solid layer of 1 mm particles had greater hydraulic resistance to liquid flow and tendency to promote coalescing of bubbles, resulting in more pronounced hysteresis effects. Greater viscosity and slight surface active properties of glycerol solution explain less pronounced hysteresis effect and minimum fluidization velocity in comparison with water. In experiments with 6 mm spheres and n-butanol solution we also found another hysteresis effect previously reported by PoSarac and Petrovik (1988). It was observed that the solid layer of 6 mm spheres acted as a good additional distributor in all investigated liquids, producing small bubbles. In the buranol solution the bubbles were 2-3 mm in diameter (visual estimation). Even at low gas flow rates only a small part of gas flow went through the annulus, flowing mostly
a. =
Irnnl
0
-
WATER
q
-
‘6
V.
OLYCEROL P
b)
dr = 3mm
1
10
0
-
w*TER
0
-
16%
I
20
OLYCEROL
I
30
P
I
I
10
50 Cs
-
AIR
Fig. 1. Experimental apparatus. (1) glass column, (2) draft tube, (3) gas distributor, (4) solid bed and (5) spacer.
60
t
70
1
(kg/m’)
Fig. 2. Hysteresis effects of minimum fluidization velocity for (a) ds = 1 mm, (b) ds = 3 mm. Open symbols-increasing order of velocities; solid symbolsaecreasing order of velocitles.
998
Shorter
Communications
through the solid layer in the draft tube, producing dense bubbly Bow. Further increase of gas flow rate caused an accumulation of small bubbles in the fixed bed. In the moment when the difference between gas holdup in the draft tube and the annulus induced a sufticient driving force the fluidization occurred. The expansion of solid layer and the flow of liquid quickly pushed the accumulated gas out of the draft tube, the difference in gas holdups suddenly decreased and expanded solid layer fell down. Such expansion and contraction of the bed periodically and regularly repeated. At higher gas flow rates the periodical eruptions of solids were very intense (see Fig. 3a) with shorter periods. The described “breathing” disappeared at the moment when the eruption threw out a certain amount of solids from the draft tube so that a stable liquid and solid recirculation was enabled. Gradual decrease of gas flow rate below the starting point of circulated bed mode resulted in the appearance of a stable fluid&d bed in the draft tube in a certain range of gas flow rates. Below that range the breathing of bed occurred again. It is known that the gas distributor design in the presence of bubble coalescence inhibitors has strong influence on the hydrodynamics of gas-liquid reactors. We assume that the perforated plate with 1 mm holes as a relatively efficient primary distributor, combined with a layer of 6 mm spheres as an additional gas distributor, play an important role in described breathing behaviour of the bed. Initially formed small bubbles tended to accumulate in the fixed bed causing the breathing of bed
To check this assumption we performed an experiment with an inefficient gas distributor of single nozzle type 8 mm i.d. At low gas flow rates (Uo=O.8 to 1 cm/s) large bubbles generated by this distributor, moved fast through the bed, causing very intense oscillations of solid particles which in turn caused the disintegration of the bubbles. As a result, there was no accumulation of gas in bed, no breathing occurred and a stable guidization was reached. An increase in gas flow rate increased the height of the bed causing less intense disintegration of bubbles and a decrease of gas hold up in the draft tube. As a consequence the height of the bed decreased with more intense disintegration of bubbles and the expansion of the bed occurred again. With further increase of gas flow rate (Uo> 1.8 cm/s) these contractions and expansions of the fluidized bed almost completely disappeared. The presented results show a significant influence of particle diameter and concentration, gas distributor design and liquid phase properties on minimum fluidization velocity and observed hysteresis effects. We hope that this note will contribute to better understanding of hydrodynamics of the three phase fluid&d bed airlift reactors. D. PETROVI@ D. POSARAC Faculty OJ Technology 21000 Novi Sad Yugoslavia D. SKALA
4
PERFORATED
PLATE 16 I 1 mm 6s = 6 mm
a)
1
Faculty of Technology 11000 Beograd Yugoslavia
Cs ds H H0 uo I7 mf
and MeraNurgy
NOTATION concentration of solid particles, kg/m3 solid particle diameter, mm height of the fluid&d bed, m height of the static bed, m superficial gas velocity calculated on the crosssectional area of the outer column, m/s LJ, at the beginning of fluidization, m/s REFERENCES
Heck, .I. and Onken, U., 1987, Hysteresis effects in suspended solid particles in bubble columns with and without draft tube. Chem. Engng Sci. 42, 1211-1212. Heck, J. and Onken, U., 1988, Eintluss der Partikelform auf die Feststoffsuspendierung in BlasensHule and AirliftSchlaufenreaktor. Chem.-Zng.-Techn. 60, 40&4Ol. Koide, K., Horihe, K., Kawabata, H. and Ito, S., 1984, Critical gas velocity required for complete suspension of solid particles in solid-suspended bubble column with draught tube. J. them. Engng Japan 17, 368-374. Muroyama, K., Mitani, Y. and Yasunishi, A., 1985, Hydrodynamic characteristics and gas-liquid mass transfer in a draft tube slurry reactor. Chem Engng Commun. 34,87-98. PoHarac, D. and PetroviC, D., 1988, An experimental study of the minimum fluidization velocity in the three-phase external loop air-lift reactor. Chem. Engng Sci. 43,1161-l 165. Fig. 3. The influence of the distributor design on the bed expansion in the system: air-0.S% n-butanol solution-6 mm glass spheres. Cs = 70 kg/m3. Open symbols-increasing order of velocities; solid symbols-decreasing order of velocities.
+Author to whom correspondence
should be addressed.