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Volumetric gas-liquid mass transfer coefficients in a rectangular bubble column with a rubber aeration pad
The Chemical Engineering
Journal, 41 (1989)
B51 - B54
B51
Volumetric Gas-Liquid Mass Transfer Coeffkients in a Rectangular Bubble Columti With a Rubber Aeration Pad L. MEDIC and A. CEH~VIN Department
for Wastewater
Treatment,
IMP, 61000 Ljubljana
(Yugoslavia)
T. KOLOINI and A. PAVKO Department (Yugoslavia) (Received
of Chemistry
and Chemical Technology,
E. Kardelj
University
of Ljubljana,
61000 Ljubljana
in revised form February 27, 1989)
ABSTRACT
The aeration performance of a bubble column with 2 m* cross sectional area at water depths of 2, 3, 4 and 5 m was investigated. A perforated rubber aeration pad was used as a gas distributor. A range of superficial air velocities of 2.5 X 1 0e4 to 4.5 X 1 0V3 m s-l which is usual for this type of aerator was covered. A design correlation is proposed, in accordance with the results which show that k Ia’ decreases with increasing liquid height at constant aeration rate.
1. INTRODUCTION
Bubble columns are widely used in the chemical and fermentation process industries, as well as in aerobic biological wastewater treatment, due to their simple construction and energy efficient mass transfer. A large amount of design data is available from laboratory pilot plant units of circular cross section. For example, Aikita and Yoshida [l] and Schiigerl et al. [2] studied the performance of units with internal diameters of 0.15 m and 0.14 m, respectively. However from the review articles of Deckwer [3] and Heijnen and van’t Riet [4] it is apparent that data from industrial size reactors are very rare. Bubble formation is very much dependent on sparger type, while superficial gas velocity and liquid properties determine the gas liquid inter-facial area per unit volume. Media for industrial use or wastewater usually contain salts and therefore coalescence is not pronounced. Formation of fine bubbles at low 0300-9467/89/$3.50
aeration rates to achieve high k 1a values is thus suitable, especially when energy for aeration represents a considerable part of total costs. Porous plates are commonly used for this purpose, though in recent years, special perforated rubber or plastic sheets, stretched over a solid support have gained significance. This latter type of distributor is recognized to be very efficient for aerobic waste-water treatment (Passavant [5], GVA [6], Messner [7]). Since efficient effluent disposal using reactor units and storage tanks in rather expensive industrial areas often leads to rectangularly shaped vessels, gas liquid mass transfer in a rectangular bubble column with a perforated rubber distributor was investigated in the present study.
2. EXPERIMENTAL
The rectangular bubble column was made of steel plates. It was 1.0 m wide, 2.0 m long and 6.0 m high and was painted to prevent corrosion. An aeration pad (IMP, Ljubljana) was used as an air distributor. Reinforced rubber of 1.5 mm thickness, perforated with a cold needle (4 holes per cm*, size of a hole approx. 1 mm), was stretched by a stainless steel frame on a 1.0 m wide and 2.0 m long stainless steel carrying plate, with an air inlet in the centre. The bubble column investigated here is shown in Fig. 1, while the distributor is shown in Fig. 2. This aerator uniformly distributed air over the whole cross section of the reactor investigated. Furthermore, this type of construction enables the combination 0 Elsevier Sequoia/Printed
in The Netherlands
B52
Fig. 1. Bubble column with a rubber aeration pad. stonk.s corrylng
steel plate stainless steel , frame
air
Inlet
I
permmted wbber
3. RESULTS AND DISCUSSION
pod
! / t---i 1’1!i l-
ducted at the following liquid heights: 2.0 m, 3.0 m, 4.0 m and 5.0 m. Tap water was used in the experiments. Six experiments were conducted in one water batch. The last experiment was always the repetition of the first one and the results agreed within the experimental error of 10%. The electrical conductivity of one batch of water was in the range 250 - 650 ~.lscm-l and was considered to be intermediate between coalescing and noncoalescing media.
axlo
Fig. 2. Schematic diagram of aeration pad.
The volumetric mass transfer coefficient as a function of superficial gas velocity at different liquid heights is shown in Fig. 3. The equation for coarse bubble systems, proposed by Heijnen and van’t Riet [4] k,a = 0.32~2~
of a number of distributors to aerate large areas, for example basins for wastewater treatment. The air/water system was investigated at 15 “C. Air pressure, temperature and volumetric flow rate were measured at the inlet to determine the superficial gas velocity and mass flow rate. For the volumetric gas-liquid mass transfer coefficient determination, the unsteady gassing-in method was used. After the aeration was adjusted, approximately 300 g mP3 of Na2S03*7H20 together with 2 g mW3CoCl, - 6H,O was added to the reactor to achieve approximately zero oxygen concentration for a few minutes [8]. Two oxygen probes were used during each run, located at the top and the bottom of the reactor. The deviation between the probes was within experimental error, therefore a well mixed liquid phase was assumed. The response of the YSI Model 58 oxygen meter was followed on a chart recorder and used for data evaluation. The response time of the probes rE was less than 10 s. Since the condition k la < 1/7x was fulfilled, the electrode dynamics were neglected in the calculations and the k t a value for each run was obtained from the slope of the straight line in the plot In((C* - C,)/ (c* - CL)) versus t [9, lo]. This is a standard method for the measurement of oxygen transfer in equipment for aerobic wastewater treatment [ 111. The experiments were con-
(1)
which correlates the data of several investigators and holds for a wide range of superficial gas velocities, liquid heights and types of gas distributor, is also shown in this figure. Heijnen and van’t Riet [4] believe that this equation slightly overestimates the measured values at low vg. Taking this into account, the results of our investigation are higher, compared to the mentioned correlation. This is due to the special gas distributor investigated here: its design enables effective distribution of fine gas bubbles over the entire cross section of the bubble column. The dependency of liquid height is also indicated in Fig. 3; the coefficients at lower liquid heights are higher. Accordingly, the
0
1
2
3 Yg x Id,
L
5
6
Ill s-1
Fig. 3. Volumetric mass transfer coefficient.
B53
OTE=
V-H=Zm O-H=3m 0 -Hz&m
-t
FJ=
“g ,
ills-’
Fig. 4. Volumetric mass transfer coefficient.
data were correlated taking into account the liquid height (H) as a parameter. The resulting equation 3&$.83p.33
(2)
is shown in Fig. 4. The experiments were carried out in the range of liquid heights 2.0 m < H < 5.0 m. Significant coalescence occurs at the bottom region of the column close to the gas distributor, but it also takes place throughout the rest of the column [ 121. Since the coalescing bubbles reduce the effective inter-facial area per unit of liquid volume in the whole reactor and so gas-liquid mass transfer, the diminishing effect of coalescence on gas-liquid mass transfer is more obvious at higher liquid levels. This effect has already been mentioned by Heijnen and van’t Riet [ 41. In Fig. 5 the effectiveness of the bubble column investigated here is shown: oxygen transfer efficiency (OTE) is plotted versus air flux (u,). Oxygen transfer efficiency is defined by the equation [13]
(3)
4 5
8 9 10 11 12 5 4
6
6
10
v. , m?mzh
Fig. 5. Oxygen transfer coefficient.
12
14
16
Q&WdW
In ~21~~
(4)
REFERENCES
7
2
(k,c)*(C*)V P
where M is the molar mass of air, R is the gas constant and p2 and pz are the inlet and barometric air pressures. It is seen from Fig. 5 that in the range of air fluxes u, = 1 - 15 Nm3 me2 h-i at which bubble columns with this type of gas distributor usually operate, oxygen transfer efficiency in the range of 14.0 - 7.0 kg O2 kWh_’ can be achieved. From eqn. (3) it is seen that the value of OTE depends on the evaluation of power input P. For example, Zlokamik [13] reportedvalues of OTE to be 3.0 - 4.0 kg O2 kWh’ for injectors in the range of u, up to 5 Nm3 me2 h-l. His values are based on the calculation of compressor power input, where the efficiency of the compressor is 77= 0.6.
6
0
P
=
where the oxygen transfer rate (OTR)* and klu* were evaluated under standard conditions [14, 151. Net power input (P) at the investigated liquid height and air mass flow rate (Q,) were calculated from
A-H:5m
h,c = 1
(OTR)*
-
13
K. Aikita and F. Yoshida, Ind. Eng. Chem. Process Des. Dev., 12 (1973) 76. K. Schiigerl, J. Lucke and 0. Oels, Adv. Biochem. Eng., 7 (1977) 1. W. D. Deckwer, Bubble Column Bioreactors, Biotechnology, Fundamentals of Biochemical Engineering, Vol. 2, Verlag-Chemie, Weinheim, 1983, p. 445. J. J. Heijnen and K. van? Riet, Chem. Eng. J., 28 (1984) B21. Passavant-Werke, 6209 Aarbergen 7, BRD, Bioflex Belufter, Technische Znformationen, 1988. GWA, 4020 Mettmann, BRD, Elastox, Technische Znformationen, 1988. Messnertechnik, 8551 Adelsdorf, BRD, Plattenbelufter, Technische Informationen, 1988. V. Linek and V. Vacek, Chem. Eng. Sci., 36 (1981) 1747. G. Ruchti, I. J. Dunn, J. R. Bourne and U. von Stockar, Chem. Eng. J., 30 (1985) 29. M. Y. Chisti and M. Moo-Young, Biotechnol. Bioeng., 31 (1988) 487. C. R. Baillod, W. L. Paulson, J. J. McKeown, H. J. Campbell, Jr., J. Water Pollut. Control Fed., (1986) 290. G. Marrucci and L. Nicodemo, Chem. Eng. Sci., 22 (1967) 1257. M. Zlokarnik, Tower Shaped Reactors for Aerobic Biological Waste Water Treatment, Biotechnology, Vol. 2, Verlag-Chemie, Weinheim, 1985, p. 537.
B54
14 AMT fiir Gewasserschutz und Waaserbau, Direktion der Offentlichen Bauten des Kantons Ziirich, Garantiebestimmungen von Beluftungseinrichtungen, Ziirich, 1980. 15 Lehr und Handbuch der Abwassertechnik, Band 2, Wilhelm Ernst & Sohn, Berlin, 1975.
APPENDIX A: NOMENCLATURE
C* CL Ccl H klc M
liquid oxygen concentration in equilibrium with air (kg me3) concentration of oxygen in liquid (kg mw3) initial concentration of oxygen in liquid (kg me3 ) liquid height (m) volumetric gas-liquid mass transfer coefficient (s-l) air molar weight (g mol-‘)
OTE OTR
P P
Q,
R t Tw V %7
oxygen transfer efficiency (kg O2 kWh-‘) oxygen transfer rate (kg O2 h-l) pressure (bar) net power input for aeration (kW) air mass flow rate (kg s-l) gas constant (8314 J kmol-’ K-‘) time (s) water temperature (K) liquid volume ( m3) superficial gas velocity at water temperature (m s-‘) superficial gas velocity at 0 “C and 1.0 bar (Nm3 rnw2h-l)
Greek symbols efficiency r) first order electrode time constant (s) 73 Superscript * conditions: Tw = 10 “C, p = 1 bar
Journal, 41 (1989)
B51 - B54
B51
Volumetric Gas-Liquid Mass Transfer Coeffkients in a Rectangular Bubble Columti With a Rubber Aeration Pad L. MEDIC and A. CEH~VIN Department
for Wastewater
Treatment,
IMP, 61000 Ljubljana
(Yugoslavia)
T. KOLOINI and A. PAVKO Department (Yugoslavia) (Received
of Chemistry
and Chemical Technology,
E. Kardelj
University
of Ljubljana,
61000 Ljubljana
in revised form February 27, 1989)
ABSTRACT
The aeration performance of a bubble column with 2 m* cross sectional area at water depths of 2, 3, 4 and 5 m was investigated. A perforated rubber aeration pad was used as a gas distributor. A range of superficial air velocities of 2.5 X 1 0e4 to 4.5 X 1 0V3 m s-l which is usual for this type of aerator was covered. A design correlation is proposed, in accordance with the results which show that k Ia’ decreases with increasing liquid height at constant aeration rate.
1. INTRODUCTION
Bubble columns are widely used in the chemical and fermentation process industries, as well as in aerobic biological wastewater treatment, due to their simple construction and energy efficient mass transfer. A large amount of design data is available from laboratory pilot plant units of circular cross section. For example, Aikita and Yoshida [l] and Schiigerl et al. [2] studied the performance of units with internal diameters of 0.15 m and 0.14 m, respectively. However from the review articles of Deckwer [3] and Heijnen and van’t Riet [4] it is apparent that data from industrial size reactors are very rare. Bubble formation is very much dependent on sparger type, while superficial gas velocity and liquid properties determine the gas liquid inter-facial area per unit volume. Media for industrial use or wastewater usually contain salts and therefore coalescence is not pronounced. Formation of fine bubbles at low 0300-9467/89/$3.50
aeration rates to achieve high k 1a values is thus suitable, especially when energy for aeration represents a considerable part of total costs. Porous plates are commonly used for this purpose, though in recent years, special perforated rubber or plastic sheets, stretched over a solid support have gained significance. This latter type of distributor is recognized to be very efficient for aerobic waste-water treatment (Passavant [5], GVA [6], Messner [7]). Since efficient effluent disposal using reactor units and storage tanks in rather expensive industrial areas often leads to rectangularly shaped vessels, gas liquid mass transfer in a rectangular bubble column with a perforated rubber distributor was investigated in the present study.
2. EXPERIMENTAL
The rectangular bubble column was made of steel plates. It was 1.0 m wide, 2.0 m long and 6.0 m high and was painted to prevent corrosion. An aeration pad (IMP, Ljubljana) was used as an air distributor. Reinforced rubber of 1.5 mm thickness, perforated with a cold needle (4 holes per cm*, size of a hole approx. 1 mm), was stretched by a stainless steel frame on a 1.0 m wide and 2.0 m long stainless steel carrying plate, with an air inlet in the centre. The bubble column investigated here is shown in Fig. 1, while the distributor is shown in Fig. 2. This aerator uniformly distributed air over the whole cross section of the reactor investigated. Furthermore, this type of construction enables the combination 0 Elsevier Sequoia/Printed
in The Netherlands
B52
Fig. 1. Bubble column with a rubber aeration pad. stonk.s corrylng
steel plate stainless steel , frame
air
Inlet
I
permmted wbber
3. RESULTS AND DISCUSSION
pod
! / t---i 1’1!i l-
ducted at the following liquid heights: 2.0 m, 3.0 m, 4.0 m and 5.0 m. Tap water was used in the experiments. Six experiments were conducted in one water batch. The last experiment was always the repetition of the first one and the results agreed within the experimental error of 10%. The electrical conductivity of one batch of water was in the range 250 - 650 ~.lscm-l and was considered to be intermediate between coalescing and noncoalescing media.
axlo
Fig. 2. Schematic diagram of aeration pad.
The volumetric mass transfer coefficient as a function of superficial gas velocity at different liquid heights is shown in Fig. 3. The equation for coarse bubble systems, proposed by Heijnen and van’t Riet [4] k,a = 0.32~2~
of a number of distributors to aerate large areas, for example basins for wastewater treatment. The air/water system was investigated at 15 “C. Air pressure, temperature and volumetric flow rate were measured at the inlet to determine the superficial gas velocity and mass flow rate. For the volumetric gas-liquid mass transfer coefficient determination, the unsteady gassing-in method was used. After the aeration was adjusted, approximately 300 g mP3 of Na2S03*7H20 together with 2 g mW3CoCl, - 6H,O was added to the reactor to achieve approximately zero oxygen concentration for a few minutes [8]. Two oxygen probes were used during each run, located at the top and the bottom of the reactor. The deviation between the probes was within experimental error, therefore a well mixed liquid phase was assumed. The response of the YSI Model 58 oxygen meter was followed on a chart recorder and used for data evaluation. The response time of the probes rE was less than 10 s. Since the condition k la < 1/7x was fulfilled, the electrode dynamics were neglected in the calculations and the k t a value for each run was obtained from the slope of the straight line in the plot In((C* - C,)/ (c* - CL)) versus t [9, lo]. This is a standard method for the measurement of oxygen transfer in equipment for aerobic wastewater treatment [ 111. The experiments were con-
(1)
which correlates the data of several investigators and holds for a wide range of superficial gas velocities, liquid heights and types of gas distributor, is also shown in this figure. Heijnen and van’t Riet [4] believe that this equation slightly overestimates the measured values at low vg. Taking this into account, the results of our investigation are higher, compared to the mentioned correlation. This is due to the special gas distributor investigated here: its design enables effective distribution of fine gas bubbles over the entire cross section of the bubble column. The dependency of liquid height is also indicated in Fig. 3; the coefficients at lower liquid heights are higher. Accordingly, the
0
1
2
3 Yg x Id,
L
5
6
Ill s-1
Fig. 3. Volumetric mass transfer coefficient.
B53
OTE=
V-H=Zm O-H=3m 0 -Hz&m
-t
FJ=
“g ,
ills-’
Fig. 4. Volumetric mass transfer coefficient.
data were correlated taking into account the liquid height (H) as a parameter. The resulting equation 3&$.83p.33
(2)
is shown in Fig. 4. The experiments were carried out in the range of liquid heights 2.0 m < H < 5.0 m. Significant coalescence occurs at the bottom region of the column close to the gas distributor, but it also takes place throughout the rest of the column [ 121. Since the coalescing bubbles reduce the effective inter-facial area per unit of liquid volume in the whole reactor and so gas-liquid mass transfer, the diminishing effect of coalescence on gas-liquid mass transfer is more obvious at higher liquid levels. This effect has already been mentioned by Heijnen and van’t Riet [ 41. In Fig. 5 the effectiveness of the bubble column investigated here is shown: oxygen transfer efficiency (OTE) is plotted versus air flux (u,). Oxygen transfer efficiency is defined by the equation [13]
(3)
4 5
8 9 10 11 12 5 4
6
6
10
v. , m?mzh
Fig. 5. Oxygen transfer coefficient.
12
14
16
Q&WdW
In ~21~~
(4)
REFERENCES
7
2
(k,c)*(C*)V P
where M is the molar mass of air, R is the gas constant and p2 and pz are the inlet and barometric air pressures. It is seen from Fig. 5 that in the range of air fluxes u, = 1 - 15 Nm3 me2 h-i at which bubble columns with this type of gas distributor usually operate, oxygen transfer efficiency in the range of 14.0 - 7.0 kg O2 kWh_’ can be achieved. From eqn. (3) it is seen that the value of OTE depends on the evaluation of power input P. For example, Zlokamik [13] reportedvalues of OTE to be 3.0 - 4.0 kg O2 kWh’ for injectors in the range of u, up to 5 Nm3 me2 h-l. His values are based on the calculation of compressor power input, where the efficiency of the compressor is 77= 0.6.
6
0
P
=
where the oxygen transfer rate (OTR)* and klu* were evaluated under standard conditions [14, 151. Net power input (P) at the investigated liquid height and air mass flow rate (Q,) were calculated from
A-H:5m
h,c = 1
(OTR)*
-
13
K. Aikita and F. Yoshida, Ind. Eng. Chem. Process Des. Dev., 12 (1973) 76. K. Schiigerl, J. Lucke and 0. Oels, Adv. Biochem. Eng., 7 (1977) 1. W. D. Deckwer, Bubble Column Bioreactors, Biotechnology, Fundamentals of Biochemical Engineering, Vol. 2, Verlag-Chemie, Weinheim, 1983, p. 445. J. J. Heijnen and K. van? Riet, Chem. Eng. J., 28 (1984) B21. Passavant-Werke, 6209 Aarbergen 7, BRD, Bioflex Belufter, Technische Znformationen, 1988. GWA, 4020 Mettmann, BRD, Elastox, Technische Znformationen, 1988. Messnertechnik, 8551 Adelsdorf, BRD, Plattenbelufter, Technische Informationen, 1988. V. Linek and V. Vacek, Chem. Eng. Sci., 36 (1981) 1747. G. Ruchti, I. J. Dunn, J. R. Bourne and U. von Stockar, Chem. Eng. J., 30 (1985) 29. M. Y. Chisti and M. Moo-Young, Biotechnol. Bioeng., 31 (1988) 487. C. R. Baillod, W. L. Paulson, J. J. McKeown, H. J. Campbell, Jr., J. Water Pollut. Control Fed., (1986) 290. G. Marrucci and L. Nicodemo, Chem. Eng. Sci., 22 (1967) 1257. M. Zlokarnik, Tower Shaped Reactors for Aerobic Biological Waste Water Treatment, Biotechnology, Vol. 2, Verlag-Chemie, Weinheim, 1985, p. 537.
B54
14 AMT fiir Gewasserschutz und Waaserbau, Direktion der Offentlichen Bauten des Kantons Ziirich, Garantiebestimmungen von Beluftungseinrichtungen, Ziirich, 1980. 15 Lehr und Handbuch der Abwassertechnik, Band 2, Wilhelm Ernst & Sohn, Berlin, 1975.
APPENDIX A: NOMENCLATURE
C* CL Ccl H klc M
liquid oxygen concentration in equilibrium with air (kg me3) concentration of oxygen in liquid (kg mw3) initial concentration of oxygen in liquid (kg me3 ) liquid height (m) volumetric gas-liquid mass transfer coefficient (s-l) air molar weight (g mol-‘)
OTE OTR
P P
Q,
R t Tw V %7
oxygen transfer efficiency (kg O2 kWh-‘) oxygen transfer rate (kg O2 h-l) pressure (bar) net power input for aeration (kW) air mass flow rate (kg s-l) gas constant (8314 J kmol-’ K-‘) time (s) water temperature (K) liquid volume ( m3) superficial gas velocity at water temperature (m s-‘) superficial gas velocity at 0 “C and 1.0 bar (Nm3 rnw2h-l)
Greek symbols efficiency r) first order electrode time constant (s) 73 Superscript * conditions: Tw = 10 “C, p = 1 bar