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Medium effects on oxygen mass transfer in a plunging jet loop reactor with a downcomer
Chemical Engineering and Processing 38 (1999) 259 – 265
Medium effects on oxygen mass transfer in a plunging jet loop reactor with a downcomer A.H. Fakeeha, B.Y. Jibril, G. Ibrahim, A.E. Abasaeed * Chemical Engineering Department, King Saud Uni6ersity, P.O. Box 800, Riyadh 11421, Saudi Arabia Received 15 October 1998; accepted 19 January 1999
Abstract The rate of oxygen delivery determines, in many cases, the efficiency of aerobic processes used in wastewater treatment. In this work, a plunging jet loop system with a perforated downcomer was designed and operated to study oxygen transfer rate in aqueous solutions of glucose (1–4 wt.%) and a low foam surfactant (0.037 – 0.074 wt.%). At normal temperature and pressure, KLa decreases with increasing the weight percent of glucose while it increases with increasing the surfactant concentration. Simultaneous presence of glucose and surfactant in water was found to result in intermediate values of KLa. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Mass transfer coefficient; Plunging liquid; Jet-flow; Surfactants; Downcomer; Wastewater treatment.
1. Introduction Aeration is a very important factor in many chemical and biological systems. Treating of wastewater is one of these processes requiring proper aeration to sustain the growth of the microorganisms responsible for biodegrading organic contaminants. Systems which are employed to supply the oxygen to the microbial environment are also important. Sparging, free-jet flow, bubbling column, trickling of air are some of the systems used to supply the needed oxygen to the system [1 – 3]. After 1970, a great deal of research effort was expended on studying the phenomenon of gas entrainment in plunging liquid jets. Recently, Bin [1] presented a comprehensive review on the restricted-geometry plunging jet entrainment. In this review, the effect of factors such as nozzle geometry, liquid phase properties, jet velocity, jet length, impact angle, jet diameter... etc. on gas entrapment in the liquid was analyzed [1,4 –10]. In this investigation, a continuous re-circulation, constant liquid holdup system was used to study oxygen * Corresponding author. Fax: +966-1-4678770. E-mail address: [email protected] (A.E. Abasaeed)
mass transfer. The effect of pollutant type on the transfer rate was investigated. For that purpose, aqueous solutions of glucose and/or surfactant are used. This type of approach provided the basis for most of the mass transfer studies [11,12].
2. Experimental The experimental setup as shown in Fig. 1, is composed of: (1) a cylindrical vessel with conical bottom, (the details and dimensions are shown in Fig. 2). (2) A pump, (3) a dissolved oxygen meter, (4) a source of nitrogen gas for purging, (5) a manometer to measure the pressure drop across an orifice and (6) a cooling system to maintain a constant temperature. The jet loop reactor consists of an ejector that discharges, through a perforated downcomer, into a vessel; liquid is circulated through the system via an external loop. The ejector can be subdivided into three sections: (a) Liquid is supplied to the system via a nozzle (situated below the perforation) that allows consequently the gas to be sucked into suction chamber (downcomer), (b) In the throat a so-called mixing shock occurs causing an intensive mixing of the two phases and (c) Mass transfer then takes place in the diffuser section. As shown in
0255-2701/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 9 9 ) 0 0 0 0 7 - 0
260
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Fig. 2, the liquid is ejected through a nozzle (dn = 1.0 cm) housed inside the top of a downcomer (dd = 6 cm, Hd =180 cm) which is perforated at the top. The downcomer is concentric with the tank (dt =25 cm, Ht = 205 cm). The total length of the external-circulation tube is 290 cm and its diameter (dt) is 4 cm as shown on the figure. The dead liquid volume (liquid in external tube) is about 6% of the total volume of the working liquid. In the experimental procedure, the cylinder was filled with known amount of water. Nitrogen gas was used to purge-off dissolved oxygen from the water until a steady state oxygen concentration (close to zero) was reached. This steady concentration was considered as a starting concentration for the controlled dissolution of oxygen. The recirculation pump was started (this results in sucking of oxygen into the system through the perforations in the inner cylinder) and the dissolved oxygen concentration was then measured using a fast response DO meter until another steady state concentration (corresponding to oxygen saturation concentration in water) was reached. This (purging with nitrogen and recording of oxygen concentration) was repeated for the different flow rates. Measurement of oxygen concentrations at different positions between the downcomer (inner cylinder) and the tank (outer cylinder) showed insignificant variations, therefore, perfect mixing in the system was assumed. The viscosity of glucose solution was experimentally measured.
A solute balance for the liquid phase yields the following familiar equation. dc =KLa(cs − c) dt cs − c = − KLa t In cs − co
(1) (2)
3. Results and discussion The experimental data were obtained for the dissolution of oxygen in a variety of predesigned contaminated water, namely: aqueous solutions of glucose of 1, 2, 3 and 4% by weight (i.e. 0.056, 0.11, 0.17 and 0.22 mol l − 1 respectively), aqueous solutions of a low foam surfactant (lfs) of 0.0371 and 0.074% and aqueous solutions of glucose (2%) mixed with surfactant of 0.0185, 0.0371 and 0.0741%. As a reference, the dissolution of oxygen in distilled water is also reported. Several runs of each experiment were performed, therefore the reported results represent the average of these runs. As given by equation [1], the rate of oxygen transfer is proportional to the driving force, with the mass transfer coefficient (KLa) being the proportionality constant. The traditional linear regression was performed on equation [2] at different flow rates for the contaminated waters to obtain the values of the KLa. Very good correlations (R 2 = 0.999) were obtained in all cases. The values of the mass transfer coefficient with the corresponding flow rates are listed in Tables 1 and 2.
Fig. 1. Schematic diagram of the experimental set-up.
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
261
Fig. 2. Detailed dimensions of the tank, downcomer and external tube.
Table 1 Experimental values of KLa at different flow rates and glucose/surfactant (lfs) concentrations Surfactant (wt.%) in water at different flow rates (cm3 s−1) 295 348 553 685 845
0.00% 0.72 0.87 1.45 1.97 2.17
0.037% 2.23 2.77 3.10 3.29 3.41
Glucose (wt.%) in water at different flow rates 0.074% 3.00 3.73 4.20 4.53 4.82
(cm3 s−1) 317 543 724 940 1066 1166
3.1. Effect of glucose Fig. 3 shows the variation of the KLa with the flow rate at glucose concentrations of 1, 2, 3 and 4%. Glucose is usually used to resemble organic water pollutants from domestic sources. The essential effect of the addition of glucose is the change of the viscosity of
0.00% 0.60 1.50 1.72 1.76 2.43 3.86
1.00% 0.57 1.25 1.69 2.33 2.71 3.14
2.00% 0.49 1.12 1.51 1.95 2.50 2.65
3.00% 0.29 0.91 1.33 1.76 2.07 2.46
4.00% 0.19 0.84 1.17 1.29 1.70 2.03
water. The viscosity of the solutions was measured at 24°C. It changed from 0.91 cp for distilled water to 1.07 cp for the 4% glucose solution (a 17.5% increase in viscosity). It is clear from the figure that the rate of oxygen transfer, as depicted by the mass transfer coefficient, increases linearly with flow rates for the range used in this work. This trend is maintained despite the
262
Flowrate (cm3 s−1)
Glucose (2%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0185%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0371%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0741%)
315 540 720 935 1060 1160
0.49 1.12 1.51 1.95 2.50 2.65
295 348 553 685 845 1086
1.60 2.02 2.83 2.90 4.41 4.87
295 348 553 685 845 1086
1.62 2.07 2.43 2.97 4.54 5.18
295 348 553 685 845 938
2.12 2.07 3.01 3.77 4.86 5.82
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Table 2 Experimental values of KLa at different flow rates and surfactant (lfs) and /or glucose concentrations
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
increase in viscosity due to addition of glucose. However, continuous decrease in the magnitude of KLa with increase in percent glucose (i.e. viscosity) for each flow rate was observed. The curves for the aqueous solution of glucose are lower than the distilled water curve, which is plotted as a reference. For instance, at a flow rate of 540 cm3 min − 1, the KLa is about 1.5 min − 1 in water while it is approx. 1.0 min − 1 in aqueous solution of 2% glucose. It is even lower at higher percentages of glucose. The decrease could be attributed to the decrease in mobility of oxygen molecules due to higher viscosity. Taking this effect into isolation, it suggests that the presence of a viscosity increasing agent would make an aerobic process requires more power for the supply of oxygen. The decrease in KLa with weight
263
percent glucose is clearly shown in Fig. 4. The same figure indicates that higher flow rate, and hence more power, is needed to improve the value of KLa.
3.2. Effect of surfactant Wastewater could also contain surface active agents which generally reduce its static surface tension. Addition of a lfs has shown a tremendous increase in the value of KLa at the same value of flow rate as depicted by Fig. 5. At a flow rate of 540 cm3 min − 1, the KLa is about 1.5 min − 1 in water while it is 3.0 min − 1 in a 0.0371 wt.% of a lfs. This suggests that the presence of surface active agent (in the concentration range considered in this investigation) enhances the rate of oxygen
Fig. 3. KLa versus flow rate for distilled and contaminated water of 1 – 4 wt.% glucose. ( + ) distilled water, (") 1%, ( ) 2%, () 3% and () 4% glucose.
Fig. 4. KLa versus glucose concentration (wt.%) at different flow rates. ( +) 316, (2) 732 and () 1166 cm3 s − 1.
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A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Fig. 5. Effect of flow rate on KLa at different percentages of low foam surfactant. ( + ) distilled water, () 0.037% and (") 0.0741% lfs.
Fig. 6. KLa versus flow rates at different concentrations of lfs in a 2 wt.% aqueous solution of glucose. ( +) 2% glucose, (") 2% glucose +0.0185% lfs, ( ) 2% glucose+ 0.0371% lfs and () 2% glucose+ 0.0741% lfs.
mass transfer. The large increase of the KLa in the presence of a surface active agent is perhaps due to rapid increase in the interfacial area. This brings about the reduced coalescence effect. However, the concentrations used in this work may not necessarily be obtained in a typical wastewater. The observations here, agree well with the results obtained with other gas –liquid dispersion equipment [13,14].
increasing (glucose) and the surface active (low foam surfactant) agents. At a flow rate of 540 cm3 min − 1 the value of KLa is about 2.5 min − 1. This shows an intermediate value between that for aqueous solutions of the low foam surfactant (3 min − 1) which enhances KLa and the glucose solutions (1 min − 1) which decreases it.
4. Conclusion
3.3. Combined effect of glucose and surfactant The aerobic bacteria in wastewater suffer conflicting effects due to different pollutants in the water. In order to approximate a conflicting microbial environment, Fig. 6 shows the combined effects of both the viscosity
Efficient treatment of wastewater could be achieved biologically by the use of aerobic microorganisms. In such processes, the transfer rate of oxygen is a very important and many cases a limiting factor to the success of the treatment process. The transfer rate is
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
influenced by the type of pollutants present in the wastewater, glucose and low foam surfactants were used in this study to simulate pollutants. Glucose, a viscosity increasing agent is found to decrease the value of KLa while the low foam surfactant enhances the transfer of oxygen. This work has shown that the combined and conflicting effects of different pollutants must be understood for proper design and operation of aerobic processes.
5. Nomenclature a c co cs dd de dn dt Hd Ht KL t
specific mass transfer area (m−1) instantaneous oxygen concentration (mol/m3) initial oxygen concentration (mol/m3) saturation, oxygen concentration (mol/m3) downcomer diameter (cm) external circulation tube diameter (cm) nozzle diameter (cm) tank diameter (cm) downcomer height (cm) tank height (cm) mass transfer coefficient (m/s) time (s)
References [1] A.K. Bin, Gas entrainment by plunging liquid jets, Chem. Eng. Sci. 48 (1993) 3585. [2] D. Birch, N. Ahmed, The influence of sparger design and location on gas dispersion in stirred vessels, Trans IChemE, (A) 75 (1997) 487.
.
265
[3] A. Ohkawa, D. Kusabiraki, Y. Kawai, N. Sakai, Some flow characteristics of a vertical liquid jet system having downcomers, Chem. Eng. Sci. 41 (1986) 2347. [4] A.K. Bin, Effect of nozzle on gas entrainment characteristics of a vertical liquid jets, Japan 21 (1988) 326. [5] G.M. Evans, G.J. Jameson, B.W. Atkinson, Prediction of the bubble size generated by a plunging liquid jet bubble column, Chem. Eng. Sci. 47 (1992) 3265. [6] N.H. Thomas, T.R. Auton, K. Sene, J.C. Hunt, Entrapment and transport of bubbles by plunging water, in: W. Brutsaert, G.H. Jirka (Eds.), Gas Transport at Water Surfaces, Reidel, Dordrecht, 1984, pp. 255 – 268. [7] K. Yamagiwa, D. Kusabiraki, A. Ohkawa, Gas holdup and gas entrainment rate in downflow bubble column with gas entrainment by a liquid jet operating at high liquid throughput, J. Chem. Eng. Japan 23 (1990) 343. [8] D. Kusabiraki, K. Yamagiwa, M. Yasuda, A. Ohkawa, Gas entrainment behavior of vertical plunging liquid in terms of changes in jet surface length, Canad. J. Chem. Eng. 70 (1992) 181. [9] D. Kusabiraki, M. Murota, S. Ohno, K. Yamagiwa, M. Yasuda, A. Ohkawa, Gas entrainment rate and flow pattern in a plunging liquid aeration system using inclined nozzles, J. Chem. Eng. Japan 23 (1990) 704. [10] R.M. Detsch, R.N. Sharma, The critical angle for gas bubble entrainment by plunging liquid jets, Chem. Eng. J. 44 (1990) 157. [11] K. Funatsu, Y.-ch. Hsu, M. Noda, S. Sugawa, Oxygen transfer in the water-jet vessel, Chem. Eng. Commun. 73 (1988) 121. [12] A. Ohkawa, D. Kusabiraki, Y. Shiokawa, N. Sakai, M. Fujii, Flow and oxygen transfer in a plunging water system using inclined short nozzles and performance characteristics of its system in aerobic treatment of wastewater, Biotechnol. Bioengng. 28 (1986) 1845. [13] R.W. Sneath, Performance of a plunging jet aerator and aerobic treatment of pig slurry, Water Pollut. Control 77 (1978) 408. [14] J.A.C. van de Donk, R.G.J.M. Van der Lans, J.M. Smith, The effect of contamination on oxygen transfer rate achieved with a plunging jet contactor, in: Proceedings of the 3rd European Conference on Mixing, University of York, F1 (1979) 289.
Medium effects on oxygen mass transfer in a plunging jet loop reactor with a downcomer A.H. Fakeeha, B.Y. Jibril, G. Ibrahim, A.E. Abasaeed * Chemical Engineering Department, King Saud Uni6ersity, P.O. Box 800, Riyadh 11421, Saudi Arabia Received 15 October 1998; accepted 19 January 1999
Abstract The rate of oxygen delivery determines, in many cases, the efficiency of aerobic processes used in wastewater treatment. In this work, a plunging jet loop system with a perforated downcomer was designed and operated to study oxygen transfer rate in aqueous solutions of glucose (1–4 wt.%) and a low foam surfactant (0.037 – 0.074 wt.%). At normal temperature and pressure, KLa decreases with increasing the weight percent of glucose while it increases with increasing the surfactant concentration. Simultaneous presence of glucose and surfactant in water was found to result in intermediate values of KLa. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Mass transfer coefficient; Plunging liquid; Jet-flow; Surfactants; Downcomer; Wastewater treatment.
1. Introduction Aeration is a very important factor in many chemical and biological systems. Treating of wastewater is one of these processes requiring proper aeration to sustain the growth of the microorganisms responsible for biodegrading organic contaminants. Systems which are employed to supply the oxygen to the microbial environment are also important. Sparging, free-jet flow, bubbling column, trickling of air are some of the systems used to supply the needed oxygen to the system [1 – 3]. After 1970, a great deal of research effort was expended on studying the phenomenon of gas entrainment in plunging liquid jets. Recently, Bin [1] presented a comprehensive review on the restricted-geometry plunging jet entrainment. In this review, the effect of factors such as nozzle geometry, liquid phase properties, jet velocity, jet length, impact angle, jet diameter... etc. on gas entrapment in the liquid was analyzed [1,4 –10]. In this investigation, a continuous re-circulation, constant liquid holdup system was used to study oxygen * Corresponding author. Fax: +966-1-4678770. E-mail address: [email protected] (A.E. Abasaeed)
mass transfer. The effect of pollutant type on the transfer rate was investigated. For that purpose, aqueous solutions of glucose and/or surfactant are used. This type of approach provided the basis for most of the mass transfer studies [11,12].
2. Experimental The experimental setup as shown in Fig. 1, is composed of: (1) a cylindrical vessel with conical bottom, (the details and dimensions are shown in Fig. 2). (2) A pump, (3) a dissolved oxygen meter, (4) a source of nitrogen gas for purging, (5) a manometer to measure the pressure drop across an orifice and (6) a cooling system to maintain a constant temperature. The jet loop reactor consists of an ejector that discharges, through a perforated downcomer, into a vessel; liquid is circulated through the system via an external loop. The ejector can be subdivided into three sections: (a) Liquid is supplied to the system via a nozzle (situated below the perforation) that allows consequently the gas to be sucked into suction chamber (downcomer), (b) In the throat a so-called mixing shock occurs causing an intensive mixing of the two phases and (c) Mass transfer then takes place in the diffuser section. As shown in
0255-2701/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 9 9 ) 0 0 0 0 7 - 0
260
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Fig. 2, the liquid is ejected through a nozzle (dn = 1.0 cm) housed inside the top of a downcomer (dd = 6 cm, Hd =180 cm) which is perforated at the top. The downcomer is concentric with the tank (dt =25 cm, Ht = 205 cm). The total length of the external-circulation tube is 290 cm and its diameter (dt) is 4 cm as shown on the figure. The dead liquid volume (liquid in external tube) is about 6% of the total volume of the working liquid. In the experimental procedure, the cylinder was filled with known amount of water. Nitrogen gas was used to purge-off dissolved oxygen from the water until a steady state oxygen concentration (close to zero) was reached. This steady concentration was considered as a starting concentration for the controlled dissolution of oxygen. The recirculation pump was started (this results in sucking of oxygen into the system through the perforations in the inner cylinder) and the dissolved oxygen concentration was then measured using a fast response DO meter until another steady state concentration (corresponding to oxygen saturation concentration in water) was reached. This (purging with nitrogen and recording of oxygen concentration) was repeated for the different flow rates. Measurement of oxygen concentrations at different positions between the downcomer (inner cylinder) and the tank (outer cylinder) showed insignificant variations, therefore, perfect mixing in the system was assumed. The viscosity of glucose solution was experimentally measured.
A solute balance for the liquid phase yields the following familiar equation. dc =KLa(cs − c) dt cs − c = − KLa t In cs − co
(1) (2)
3. Results and discussion The experimental data were obtained for the dissolution of oxygen in a variety of predesigned contaminated water, namely: aqueous solutions of glucose of 1, 2, 3 and 4% by weight (i.e. 0.056, 0.11, 0.17 and 0.22 mol l − 1 respectively), aqueous solutions of a low foam surfactant (lfs) of 0.0371 and 0.074% and aqueous solutions of glucose (2%) mixed with surfactant of 0.0185, 0.0371 and 0.0741%. As a reference, the dissolution of oxygen in distilled water is also reported. Several runs of each experiment were performed, therefore the reported results represent the average of these runs. As given by equation [1], the rate of oxygen transfer is proportional to the driving force, with the mass transfer coefficient (KLa) being the proportionality constant. The traditional linear regression was performed on equation [2] at different flow rates for the contaminated waters to obtain the values of the KLa. Very good correlations (R 2 = 0.999) were obtained in all cases. The values of the mass transfer coefficient with the corresponding flow rates are listed in Tables 1 and 2.
Fig. 1. Schematic diagram of the experimental set-up.
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
261
Fig. 2. Detailed dimensions of the tank, downcomer and external tube.
Table 1 Experimental values of KLa at different flow rates and glucose/surfactant (lfs) concentrations Surfactant (wt.%) in water at different flow rates (cm3 s−1) 295 348 553 685 845
0.00% 0.72 0.87 1.45 1.97 2.17
0.037% 2.23 2.77 3.10 3.29 3.41
Glucose (wt.%) in water at different flow rates 0.074% 3.00 3.73 4.20 4.53 4.82
(cm3 s−1) 317 543 724 940 1066 1166
3.1. Effect of glucose Fig. 3 shows the variation of the KLa with the flow rate at glucose concentrations of 1, 2, 3 and 4%. Glucose is usually used to resemble organic water pollutants from domestic sources. The essential effect of the addition of glucose is the change of the viscosity of
0.00% 0.60 1.50 1.72 1.76 2.43 3.86
1.00% 0.57 1.25 1.69 2.33 2.71 3.14
2.00% 0.49 1.12 1.51 1.95 2.50 2.65
3.00% 0.29 0.91 1.33 1.76 2.07 2.46
4.00% 0.19 0.84 1.17 1.29 1.70 2.03
water. The viscosity of the solutions was measured at 24°C. It changed from 0.91 cp for distilled water to 1.07 cp for the 4% glucose solution (a 17.5% increase in viscosity). It is clear from the figure that the rate of oxygen transfer, as depicted by the mass transfer coefficient, increases linearly with flow rates for the range used in this work. This trend is maintained despite the
262
Flowrate (cm3 s−1)
Glucose (2%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0185%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0371%)
Flowrate (cm3 s−1)
Glucose (2%) and lfs (0.0741%)
315 540 720 935 1060 1160
0.49 1.12 1.51 1.95 2.50 2.65
295 348 553 685 845 1086
1.60 2.02 2.83 2.90 4.41 4.87
295 348 553 685 845 1086
1.62 2.07 2.43 2.97 4.54 5.18
295 348 553 685 845 938
2.12 2.07 3.01 3.77 4.86 5.82
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Table 2 Experimental values of KLa at different flow rates and surfactant (lfs) and /or glucose concentrations
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
increase in viscosity due to addition of glucose. However, continuous decrease in the magnitude of KLa with increase in percent glucose (i.e. viscosity) for each flow rate was observed. The curves for the aqueous solution of glucose are lower than the distilled water curve, which is plotted as a reference. For instance, at a flow rate of 540 cm3 min − 1, the KLa is about 1.5 min − 1 in water while it is approx. 1.0 min − 1 in aqueous solution of 2% glucose. It is even lower at higher percentages of glucose. The decrease could be attributed to the decrease in mobility of oxygen molecules due to higher viscosity. Taking this effect into isolation, it suggests that the presence of a viscosity increasing agent would make an aerobic process requires more power for the supply of oxygen. The decrease in KLa with weight
263
percent glucose is clearly shown in Fig. 4. The same figure indicates that higher flow rate, and hence more power, is needed to improve the value of KLa.
3.2. Effect of surfactant Wastewater could also contain surface active agents which generally reduce its static surface tension. Addition of a lfs has shown a tremendous increase in the value of KLa at the same value of flow rate as depicted by Fig. 5. At a flow rate of 540 cm3 min − 1, the KLa is about 1.5 min − 1 in water while it is 3.0 min − 1 in a 0.0371 wt.% of a lfs. This suggests that the presence of surface active agent (in the concentration range considered in this investigation) enhances the rate of oxygen
Fig. 3. KLa versus flow rate for distilled and contaminated water of 1 – 4 wt.% glucose. ( + ) distilled water, (") 1%, ( ) 2%, () 3% and () 4% glucose.
Fig. 4. KLa versus glucose concentration (wt.%) at different flow rates. ( +) 316, (2) 732 and () 1166 cm3 s − 1.
264
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
Fig. 5. Effect of flow rate on KLa at different percentages of low foam surfactant. ( + ) distilled water, () 0.037% and (") 0.0741% lfs.
Fig. 6. KLa versus flow rates at different concentrations of lfs in a 2 wt.% aqueous solution of glucose. ( +) 2% glucose, (") 2% glucose +0.0185% lfs, ( ) 2% glucose+ 0.0371% lfs and () 2% glucose+ 0.0741% lfs.
mass transfer. The large increase of the KLa in the presence of a surface active agent is perhaps due to rapid increase in the interfacial area. This brings about the reduced coalescence effect. However, the concentrations used in this work may not necessarily be obtained in a typical wastewater. The observations here, agree well with the results obtained with other gas –liquid dispersion equipment [13,14].
increasing (glucose) and the surface active (low foam surfactant) agents. At a flow rate of 540 cm3 min − 1 the value of KLa is about 2.5 min − 1. This shows an intermediate value between that for aqueous solutions of the low foam surfactant (3 min − 1) which enhances KLa and the glucose solutions (1 min − 1) which decreases it.
4. Conclusion
3.3. Combined effect of glucose and surfactant The aerobic bacteria in wastewater suffer conflicting effects due to different pollutants in the water. In order to approximate a conflicting microbial environment, Fig. 6 shows the combined effects of both the viscosity
Efficient treatment of wastewater could be achieved biologically by the use of aerobic microorganisms. In such processes, the transfer rate of oxygen is a very important and many cases a limiting factor to the success of the treatment process. The transfer rate is
A.H. Fakeeha et al. / Chemical Engineering and Processing 38 (1999) 259–265
influenced by the type of pollutants present in the wastewater, glucose and low foam surfactants were used in this study to simulate pollutants. Glucose, a viscosity increasing agent is found to decrease the value of KLa while the low foam surfactant enhances the transfer of oxygen. This work has shown that the combined and conflicting effects of different pollutants must be understood for proper design and operation of aerobic processes.
5. Nomenclature a c co cs dd de dn dt Hd Ht KL t
specific mass transfer area (m−1) instantaneous oxygen concentration (mol/m3) initial oxygen concentration (mol/m3) saturation, oxygen concentration (mol/m3) downcomer diameter (cm) external circulation tube diameter (cm) nozzle diameter (cm) tank diameter (cm) downcomer height (cm) tank height (cm) mass transfer coefficient (m/s) time (s)
References [1] A.K. Bin, Gas entrainment by plunging liquid jets, Chem. Eng. Sci. 48 (1993) 3585. [2] D. Birch, N. Ahmed, The influence of sparger design and location on gas dispersion in stirred vessels, Trans IChemE, (A) 75 (1997) 487.
.
265
[3] A. Ohkawa, D. Kusabiraki, Y. Kawai, N. Sakai, Some flow characteristics of a vertical liquid jet system having downcomers, Chem. Eng. Sci. 41 (1986) 2347. [4] A.K. Bin, Effect of nozzle on gas entrainment characteristics of a vertical liquid jets, Japan 21 (1988) 326. [5] G.M. Evans, G.J. Jameson, B.W. Atkinson, Prediction of the bubble size generated by a plunging liquid jet bubble column, Chem. Eng. Sci. 47 (1992) 3265. [6] N.H. Thomas, T.R. Auton, K. Sene, J.C. Hunt, Entrapment and transport of bubbles by plunging water, in: W. Brutsaert, G.H. Jirka (Eds.), Gas Transport at Water Surfaces, Reidel, Dordrecht, 1984, pp. 255 – 268. [7] K. Yamagiwa, D. Kusabiraki, A. Ohkawa, Gas holdup and gas entrainment rate in downflow bubble column with gas entrainment by a liquid jet operating at high liquid throughput, J. Chem. Eng. Japan 23 (1990) 343. [8] D. Kusabiraki, K. Yamagiwa, M. Yasuda, A. Ohkawa, Gas entrainment behavior of vertical plunging liquid in terms of changes in jet surface length, Canad. J. Chem. Eng. 70 (1992) 181. [9] D. Kusabiraki, M. Murota, S. Ohno, K. Yamagiwa, M. Yasuda, A. Ohkawa, Gas entrainment rate and flow pattern in a plunging liquid aeration system using inclined nozzles, J. Chem. Eng. Japan 23 (1990) 704. [10] R.M. Detsch, R.N. Sharma, The critical angle for gas bubble entrainment by plunging liquid jets, Chem. Eng. J. 44 (1990) 157. [11] K. Funatsu, Y.-ch. Hsu, M. Noda, S. Sugawa, Oxygen transfer in the water-jet vessel, Chem. Eng. Commun. 73 (1988) 121. [12] A. Ohkawa, D. Kusabiraki, Y. Shiokawa, N. Sakai, M. Fujii, Flow and oxygen transfer in a plunging water system using inclined short nozzles and performance characteristics of its system in aerobic treatment of wastewater, Biotechnol. Bioengng. 28 (1986) 1845. [13] R.W. Sneath, Performance of a plunging jet aerator and aerobic treatment of pig slurry, Water Pollut. Control 77 (1978) 408. [14] J.A.C. van de Donk, R.G.J.M. Van der Lans, J.M. Smith, The effect of contamination on oxygen transfer rate achieved with a plunging jet contactor, in: Proceedings of the 3rd European Conference on Mixing, University of York, F1 (1979) 289.