Flow and Oxygen-Transfer Characteristics in an Aeration System

Dec 28, 2012 - aeration. The entrainment ratio decreased with the primary water flow rate ... flow rate in the ejector aeration, while the times decre...
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Flow and Oxygen-Transfer Characteristics in an Aeration System Using an Annular Nozzle Ejector Sangkyoo Park and Heicheon Yang* School of Mechanical and Automotive Engineering, Chonnam National University, 50 Daehak-Ro, Yeosu, Jeonnam, 550-747, Korea ABSTRACT: This paper is aimed at the investigation of flow and oxygen-transfer characteristics in an aeration system using an annular nozzle ejector, and the experimental evaluation of the oxygen-transfer characteristics in the ejector aeration and blower aeration. The entrainment ratio decreased with the primary water flow rate of the annular nozzle ejector, with ratios ranging between 6.8 and 0.4. It was found that the turbulence level and entrainment ratio strongly affected the air bubble size and the volumetric mass-transfer coefficient. The saturation times and volumetric mass-transfer coefficients varied with the suction air flow rate in the ejector aeration, while the times decreased and the coefficients increased with the blowing air flow rate in the blower aeration. The average mass-transfer coefficient of the ejector aeration was about 3.7 times higher than that of the blower aeration. It was found that the high turbulence level and optimum entrainment ratio were needed to increase the oxygen-transfer rate.

1. INTRODUCTION In many industries, the demand for optimum contacting processes or reactions of gas−liquid has increased. Among multiphase contacting devices, gas−liquid ejectors can be used effectively as they offer large interfacial area and high masstransfer rate. The absorption of gas into liquid is of crucial importance for multiphase reactions because the diffusion of a low-soluble gas across a gas−liquid interface generally limits reaction rates. In wastewater treatment processes, for example, an efficient transfer of oxygen into water is very important to maintain optimum dissolved oxygen (DO) concentration because the processes require proper aeration to sustain the growth of microorganisms necessary for biodegrading organic contaminants. The transfer of air to water increases the level of oxygen dissolved in the water. This process is called aeration, and it may be accomplished by artificially constructed aeration systems. Oxygen is often supplied by means of air or pure oxygen bubbles introduced to water to create additional air− water interfaces. Because of the low solubility of oxygen and the consequent low rate of oxygen transfer, sufficient oxygen to meet the requirements of reaction processes is not available through a normal surface air−water interface. To transfer the optimum quantities of oxygen that are needed, additional interfaces must be formed.1 Many different types of oxygen-transfer system have been employed in the field, depending on specific treatment requirements. Conventional oxygen-transfer systems consist of air compressors or blowers, an air-distributing pipe network, and diffusers mounted above the pipe network. The most commonly used diffuser consists of a matrix of perforated membranes or porous plates arranged near the bottom of the aeration tank to provide the maximum amount of oxygen to the water. The membrane diffusers are the most essential elements of the conventional systems, and thus their design and dimensions define to a great extent the efficiency of the oxygen-transfer process with respect to dissolved oxygen.2−4 © 2012 American Chemical Society

Ejector-type oxygen-transfer systems that use the kinetic energy of a high-velocity liquid jet to entrain and disperse a gas phase have attracted interest in recent years.5 An ejector essentially consists of an active (primary) nozzle, a passive (suction) nozzle, a parallel mixing tube, and a diffuser. Ejectors are gas−liquid contactors that directly transfer energy and momentum from a high-energy primary fluid to a low-energy suction fluid and lead to intimate mixing of the two fluids. After passing through the mixing tube, the mixed fluid expands and the velocity is reduced, which results in recompressing the mixed fluids by converting the velocity energy back into the pressure energy. The dispersion of the entrained gas into the primary fluid causes continuous formation of fresh interfaces and generation of large interfacial area. Ejectors are being used as gas−liquid dispersion devices for many purposes in many industries, since they have high mass-transfer and mixing rates.5,6 Many reports have researched the gas−liquid flow and mixing in ejectors, but the results are different and have individual scope of application due to the different ejector geometries, study methods, and species of fluids.7 The effects of different operating conditions such as nozzle velocity, pressure drop, and ejector geometry parameters on the performance of ejectors have been experimentally investigated by several researchers.8−14 Mass-transfer and hydrodynamic characteristics of ejectors using air or water as the primary fluid or the entrained fluid have been investigated by Balamurugan et al.,15 Kim et al.,16 Mandal et al.,17 Cramers and Beenackers,18 and Havelka et al.19 An ejector works like a vacuum pump without usage of piston, rotor, or any other moving components. In general, the ejector is a less efficient device, because most of the fluid machineries are operated by the normal stresses on the rotating Received: Revised: Accepted: Published: 1756

August 17, 2012 September 30, 2012 December 28, 2012 December 28, 2012 dx.doi.org/10.1021/ie302208e | Ind. Eng. Chem. Res. 2013, 52, 1756−1763

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blades, while the ejector is driven only by a pure shear action between the primary and the secondary streams.20 However, due to its simple design and lack of moving components, ejectors are very reliable devices with practically no maintenance cost and relatively low installation cost. There are two configurations of ejectors as shown in Figure 1.21 The

Therefore, the objective of this study is to experimentally investigate the flow and oxygen-transfer characteristics of an aeration system using an annular nozzle ejector. First of all, the flow characteristics of the annular nozzle ejector were studied. Then, the effect of suction or blowing air flow rate on the oxygen-transfer characteristics was investigated.

2. EXPERIMENTAL SETUP AND METHOD 2.1. Experimental Setup. A schematic view of the experimental system used in the present study is shown in Figure 2. The system consists of two oxygen-transfer lines. One

Figure 1. Schematic diagram of a central-driven ejector and an annular-driven ejector.

first is the conventional central-driven ejector, in which the primary fluid passes through the inner nozzle inside the ejector and the suction fluid passes through the annular periphery surrounding the nozzle.5−19 Lima Neto et al.22 and Baylar and Ozkan23 investigated the behavior of horizontal gas−liquid injection using a venture nozzle in a water tank as this study. Gas−liquid mass transfer in a pumped circulation loop reactor using a venture-type sparger was investigated by Fadavi and Chisti.24 However, the principle of the venture nozzle and sparger is similar to that of the central-driven ejector. The second is the annular-driven ejector, the annular nozzle ejector, in which the suction fluid passes through the inner tube of the ejector and the primary fluid passes through the annular nozzle on the periphery of the suction tube. It is well-known that because the annular nozzle ejector has relatively large airhandling capacity, the ejector is well suited for priming large pumps, such as dredging pumps, where air pockets can cause these pumps to lose their prime. However, compared to the conventional central-driven ejector, it seems that there are not so many publications on the annular nozzle ejector. Chen et al.20 investigated the effect of the secondary flow on the starting pressure of a second-throat supersonic ejector by adapting the height of the secondary flow inlet. Park et al.,25 Kim and Kwon,26 and Kim et al.27 experimentally investigated the effects of the geometric parameters of an annular injection supersonic ejector on the performance parameters including secondary flow pressure, starting pressure, and unstarting pressure. The development of new aeration systems for high-efficiency oxygen transfer at low operating costs is a very important issue in many applications. Because of the low solubility and transfer rate of oxygen, to achieve a higher oxygen-transfer rate, it is necessary to have a large amount of entrainment air and to increase the contact time and area of the air−water bubbles by decreasing the size of the entrained air bubbles. Because the annular nozzle ejector has high suction fluid handling capacities, it is particularly well suited for aeration systems. However, few experimental studies examining the effect of flow characteristics in an annular nozzle ejector on the oxygen-transfer rate of an aeration system have been reported in the literature. Also, as reviewed by Lima Neto et al.,22 there are only limited experimental studies on horizontal gas−liquid injection.28

Figure 2. Schematic diagram of experimental setup.

of them is the ejector line that consisted of an electric motor pump, an annular nozzle ejector assembly, an aeration tank, a control panel, and measuring and controlling accessories such as a dissolved oxygen meter, water and air flow meters, pressure and vacuum gauges, and control valves. The other is the blower line for comparative experiments, consisting of a motorized blower, an air flow meter, a pressure gauge, and a control valve. A perforated diffusing pipe was installed at the bottom of the aeration tank. All experiments were carried out in a 1.46 m3 (0.9 m wide × 1.8 m long × 0.9 m height) aeration tank. An electromagnetic flow meter (Kometer, KTM-800) was used to measure the circulating water flow rate at the primary flow inlet with accuracy of ±1.0% of full scale. The volumetric flow rate of the circulating water was controlled by an electric motor pump (Wilo, HL 805-1). An air flow meter (Kometer, DPE-S) was used to measure the entrained air flow rate at the air suction inlet of the ejector with accuracy of ±2.0% of full scale, while the difference in pressure caused by the suction air was measured by a differential pressure transducer (Autrol, APT3100-D5) with accuracy of ±0.5%. The volumetric flow rate of the blowing air in the blower line was controlled by a motorized blower (Inha, IHB-1000) and measured by another air flow meter (Testo 435) with accuracy of ±(0.2 m/s + 2%). The air entrainment rates of the ejector were adjusted by varying the circulating water flow rate. Moreover, the flow rates of the blowing air of the blower line were adjusted for comparison with the ejector line. The schematic diagram of the annular nozzle ejector with variable primary nozzle used in this study is shown in Figure 3, 1757

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transfer, the overall volumetric mass-transfer coefficient, and the concentration driving force: dCt = KLa(Cst − Ct ) dt

where Ct is the instantaneous dissolved oxygen concentration at time t, Cst is the saturated dissolved oxygen concentration, and KLa is the volumetric mass-transfer coefficient for oxygen. The difference Cst −Ct between the saturation value and actual concentration of oxygen in the aerating water is called the oxygen deficit. To evaluate the mass-transfer coefficient from the un-steadystate DO response curve, a number of assumptions have to be made. With the assumptions of a well-mixed system and that the mass-transfer coefficient is liquid-side-controlled, integration of eq 1 yields

Figure 3. Schematic diagram of an annular nozzle ejector.

and the detailed geometric dimensions of the ejector are listed in Table 1. The convergent cone angle (α) of the primary fluid

⎛ C − C0 ⎞ ln⎜ st ⎟ = KLat ⎝ Cst − Ct ⎠

Table 1. Geometric Dimensions of the Ejector dimension value (mm)

dp, ds, dm 26

dd 35

dc 50

Lc 50

Lt 45

Lm 290

Ld 130

(1)

Ln 10

(2)

where C0 is the initial DO concentration. KLa is calculated as the slope of the graph of ln[(Cst − C0)/(Cst − Ct)] versus t. For the mass-transfer measurements, the water in the aeration system was deaerated by addition of sodium sulfite (Na2SO3) with a cobalt chloride (CoCl2) catalyst until the DO concentration fell to zero.1,3,29 The deaeration process can be represented by the following equation:

chamber is 29.5°, the divergent cone angle (β) of the diffuser is 3.84°, and the periphery tip angle (θ) of the suction fluid tube is 5.14°. The key components of the ejector are a primary fluid chamber and nozzle, a suction fluid tube, a parallel mixing tube, a diffuser, and a screw apparatus. The screw apparatus can control the suction fluid tube (or annular nozzle tip) position. The tip position (Ltn) is defined as the distance between the suction tube (or annular nozzle) exit plane and the parallel mixing tube inlet plane as shown in Figure 3. The initial position (or zero pitch of the screw) of the annular nozzle tip is 1.0 mm from the inlet of the parallel mixing tube. As the suction fluid tube moves backward (or as the screw pitch increases), the distance between the annular nozzle tip and the inlet of the parallel mixing tube and the annular nozzle area (Atn) increase as listed in Table 2. One pitch of the screw is 1.5 mm. All ejector pieces were manufactured from stainless steel. An O-seal was used to eliminate leakage between the fixed and movable pieces of the ejector. The outlet of the diffuser was connected to the perforated diffusing pipe at the bottom of the aeration tank by means of a flexible pipe. The diffusing pipe with a number of holes spaced uniformly and arranged in three straight lines faced downward. The inner diameter and length of the diffusing pipe are 51.6 mm and 1.6 m, and the diameter and number of holes are 6.0 mm and 108, respectively. 2.2. Experimental Method. There are several methods for the experimental determination of oxygen mass-transfer coefficients. The clean water un-steady-state method, which is also the ASCE standard method,1,3,29 is presently the most broadly accepted experimental procedure and has been selected for determination of the overall mass-transfer coefficients. A mass balance on the DO in the air bubbles contained within the aerating water gives the relationship between the rate of oxygen

O2 + 2Na 2SO3 → 2Na 2SO4

(3)

The total amount of sodium sulfite required for each test run was calculated from the theoretical demand for sodium sulfite by use of eq 3. In this study, the experiments were based on the un-steadystate reaeration technique, in which clean tap water was first deaerated and then reaerated back to steady-state (saturation) conditions. A continuous circulating technique of aerating water was used to investigate oxygen-transfer characteristics in the ejector aeration line. Meanwhile, only air was blown into the constant-volume aerating water by the motorized blower in the blower aeration line. The changes in DO concentration with time were recorded until the water became saturated with air. The temperatures of circulating water and suction air were 23.5 and 26.5 °C. The saturated dissolved oxygen concentration with the temperature of circulating water at standard atmospheric conditions is 8.48 mg/L.30 The dissolved oxygen concentration (Ct) was measured at 10 s time intervals with a DO meter (YSI model 5B). In each experiment, measurements were repeated three times at each operating condition. The average values of the measured experimental data were calculated and were used in evaluation of the volumetric mass-transfer coefficients. To qualitatively predict the mixed flow behavior within the ejector, typical images of mixed flow ejecting from the exit of the ejector were captured by a Canon EOS 60D camera with a frame rate of 5 frames per second (fps) and an exposure time of

Table 2. Dimensions of Tube Tip Position and Annular Nozzle Area pitch n Ltn (mm) Atn (mm2)

1

2

3

4

5

6

7

8

9

10

2.5 56

4 91

5.5 126

7 163

8.5 200

10 238

11.5 276

13 316

14.5 357

16 398

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1

/100 s. The exit of the ejector was placed at the aeration tank center line, 0.2 m above the bottom of the tank, and the tank was filled with tap water to a depth of 0.55 m. The experimental parameter in the ejector aeration line was the screw pitch of the annular nozzle ejector, while in the blower aeration line the parameter was the blowing air flow rate, which was similar to the suction air flow rate of 3−7 screw pitch in the ejector aeration line.

3. RESULTS AND DISCUSSION 3.1. Flow Characteristics in an Annular Nozzle Ejector. The measured variables in the annular nozzle ejector were the pressures at the primary flow inlet and the suction flow inlet and the flow rates of the primary and suction fluids (water and air). Figure 4 shows the variation of pressure in the primary and suction inlets of the ejector with the screw pitch or the distance

Figure 5. Variations of (a) primary flow rate, (b) suction flow rate, and (c) entrainment ratio with screw pitch.

ejector aeration system. This quantity is the amount of suction air flow that can be drawn in by a given primary flow. The entrainment ratio is calculated by q= Figure 4. Experimental results for (a) inlet pressure of primary flow and (b) inlet pressure of suction flow.

Qs Qp

(4)

where Qs is the suction air flow rate and Qp is the primary water flow rate. As the screw pitch increases, the distance between the annular nozzle tip and the inlet of the parallel mixing tube increases as listed in Table 2. As a result, the annular nozzle outlet area increases, and consequently, the primary water flow rate increases as shown in Figure 5a. The primary water flow rate increases from 3.0 to 17.5 m3/h as the distance increases from 2.5 to 16 mm. Figure 5b shows that the suction air flow rate in the pitch range 1−3 increases, while the air flow rate in the pitch range 3.5−10 decreases. This may be ascribed to the variation of the suction inlet vacuum pressure in the suction fluid tube, as shown in Figure 4b. When the screw pitch is at 3, the suction air flow rate has the maximum value of 35.8 m3/h. In contrast, after the pitch range, the air suction rate decreases with increasing primary water flow rate due to the decrease in primary water velocity. This may be due to the fact that the increasing rate in the annular nozzle area is higher than the increasing rate of the primary water flow. As a result, the air suction rate decreases with increasing screw pitch due to the decrease of the primary water jet velocity and subsequent vacuum pressure of the suction tube inlet. The amount of air

between the annular nozzle tip and the throat of the parallel mixing tube. As the screw pitch increases (or as the annular nozzle tip moves backward), the inlet pressure of the primary water flow decreases, as shown in Figure 4a. This is because the increase in primary nozzle annular area reduces the flow resistance. The primary inlet pressure decreases from 8.3 to 2.1 kgf/cm2 in the pitch range from 1 (2.5 mm) to 10 (16 mm). The variation of suction air inlet pressure with screw pitch is shown in Figure 4b. As the annular nozzle tip moves backward from the initial position, the suction inlet vacuum pressure increases rapidly from −0.40 kgf/cm2 at pitch 1 to −0.80 kgf/ cm2 at pitch 2. As the screw pitch increases further, the suction inlet vacuum pressure increases slightly and reaches to its maximum value of −0.86 kgf/cm2 at pitch 3.5. After the maximum value, the suction inlet vacuum pressure decreases gradually to −0.27 kgf/cm2 at pitch 10. Figure 5 shows the variation of primary water flow rate, suction air flow rate, and entrainment ratio with screw pitch. The entrainment ratio is a very important parameter in the 1759

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seen in Figure 6, dissolved oxygen concentrations increase with reaeration time. The increase of dissolved oxygen concentration with reaeration time may be attributed to the longer contacting time of air bubbles in aerating water. It took approximately 5−7 min reaeration time to reach saturation in the pitch range 3− 10, while it took about 14 min at pitch 2 as summarized in Table 3. When pitches 2 and 4 are compared, the suction air flow rates are nearly equal (32.7 m3/h) as shown in Figure 5b, but the saturation time at pitch 4 is 2.8 times shorter than that at pitch 2. Therefore, on average, the dissolved oxygen concentration increased by about 1.7 (mg/L)·min−1 at pitch 4, while the corresponding value was about 0.61 (mg/L)·min−1 at pitch 2. Because the entrainment ratios were 5.7 and 2.6 at pitch 2 and 4 as shown in Figure 5c, the primary water flow rate at pitch 4 was 2.2 times larger than that at pitch 2, thereby decreasing the transfer resistance in more turbulent conditions. In the pitch range 4−7, the entrainment ratio decreases about 26.5%, owing to a 20% decrease of the suction air flow rate and an 8.7% increase of circulating water flow rate, but the saturation time increased only about 6%. Figure 7 shows the percentage dissolved oxygen concentrations at five blowing air flow rates with reaeration time for

drawn in depends on the amount of suction created by the primary annular water jet due to pressure reduction at the suction tube tip. A hydraulic energy balance is applied between the atmosphere and the suction tube tip. The pressure head between the atmosphere and the suction tube tip acts as the driving force for the air suction. When the annular nozzle velocity increases, the momentum generated by the primary water jet increases, increasing the air suction in the ejector. It is found that the entrainment ratio is higher when the annular nozzle tip position is closer to the inlet of the parallel mixing tube than when the tip position is farther from the inlet, as shown in Figure 5c. The maximum entrainment ratio of 6.8 can be found in the pitch range 1−1.5 with suction air flow rates of 20.5−28 m3/h. This is because the ejector has an optimum area ratio for maximum entrainment ratio in this pitch range. 3.2. Oxygen Transfer Characteristics. Figure 6 shows the percentage dissolved oxygen concentrations at nine different

Figure 7. Percentage dissolved oxygen concentrations at five blowing air flow rates with reaeration time for the blower aeration line.

the blower aeration line. The blowing air flow rates are similar to the suction air flow rate in the screw pitch range from 3 (35.8 m3/h) to 7 (16.8 m3/h) in the ejector aeration line. As shown in Figure 7, dissolved oxygen concentrations increase with reaeration time and air flow rate. An increase in dissolved oxygen concentration with increasing blowing air flow rate may be ascribed to the increase in interfacial area between air bubbles and aerating water. The times required to reach saturation increase from 15.5 to 30+ min as the blowing air flow rate decreases from 35.8 to 16.8 m3/h. In the blowing air flow rate range 35.8−21.6 m 3 /h, on average, the time is approximately 4 times longer than that of the ejector aeration line. This result may be attributed to the longer persistence of

Figure 6. Percentage dissolved oxygen concentrations at nine pitches with reaeration time for the ejector aeration line.

pitches with reaeration time for the ejector aeration line. The suction air and aerating water temperatures are 26.5 and 23.5 °C. The percentage dissolved oxygen concentrations are calculated as the measured dissolved oxygen concentration divided by the saturated dissolved oxygen concentration (Cst), 8.48 mg/L at 23.5 °C aerating water temperature. As can be Table 3. Saturation Time with Pitch in the Ejector Aeration Line pitch Tst(min)

2 14.0

3 7.2

4 5.0

5 5.5

6 5.7 1760

7 6.0

8 5.3

9 6.9

10 8.3

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Figure 8. Typical images of the mixed flow of water/air bubbles with time in the aeration tank.

the outside of the jet core are observed visually and any vertical motion of the air bubbles no longer appears. The behavior of the quasi- and/or horizontal bubbly jet is due to higher Re number of the primary water flow and lower entrainment ratio. The turbulence in the liquid phase in the ejector appears to help generating bubbles with relatively uniform and small diameters.22,28 At pitch 1, 4, and 8, Re numbers are about 1.7 × 104, 7.8 × 104, and 1.0 × 105, where Re is defined as

air bubbles due to the reduced buoyancy force resulting from their smaller size, and the increased contact area between smaller air bubbles and aerating water in the ejector aeration line as shown in Figure 8. Air that is entrained into water from the suction inlet of the ejector is forced downstream in the form of small air bubbles. The dissolution of oxygen into water is usually greater in systems with smaller air bubbles than in systems with larger air bubbles. This is because smaller air bubbles present a greater surface area to the surrounding water than larger air bubbles. The persistence time of entrained air bubbles in the aerating water is an important parameter: it not only directly affects the gas phase residence time but also is related to oxygen-transfer efficiency. At the conditions of 21.6 and 35.8 m3/h, the average dissolved oxygen concentrations were 4.25 and 5.87 mg/L for 6 min of reaeration time in the ejector aeration line, whereas the average values were 4.15 and 5.54 mg/L for 7 min of reaeration time in the blower aeration line. Therefore, as the suction or blowing air flow rate increases about 65%, the dissolved oxygen concentrations increase only about 38% and 33% in the ejector and blower aeration lines, respectively. This result may be due to the low solubility of oxygen and the slow speed of oxygen transfer compared to increase of the air flow rate. Figure 8 show typical images of the mixed jet of water/air bubbles with time. At pitch 1, the mixed jet tends to move upward as like a buoyancy jet and then becomes a surface jet. This behavior is due to the buoyancy force resulting from larger air bubbles and dominating the momentum of the primary water flow. At pitch 4, the mixed jet behaves like a quasihorizontal bubbly jet, where relatively larger air bubbles separate from the dominant bubbly jet and move to the surface vertically. This is explained by the fact that the momentum of the primary water flow dominates buoyancy force resulting from relatively smaller air bubbles compared to pitch 1. At pitch 8, the mixed jet behaves as a horizontal momentum jet, where mistlike bubbles at the leading edge and

Re =

d hu p ν

(5)

where dh is the hydraulic diameter of the annular nozzle [dh = 2(r2 − r1)], up is the superficial velocity of the primary water flow at the annular nozzle exit, and ν is the kinematic viscosity of the water. From the qualitative visualization test, it can be deduced that the higher the Re number and the lower the entrainment ratio, the smaller the air bubble size. Figure 9 presents the experimentally determined volumetric mass-transfer coefficients with screw pitch for the ejector aeration line. The volumetric mass-transfer coefficients were calculated from the experimental data of dissolved oxygen concentration by eq 2. It is well-known that the volumetric

Figure 9. Oxygen-transfer coefficients with screw pitch for the ejector aeration line. 1761

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mass-transfer coefficients compared to the blower aeration line. It can be seen that the average mass-transfer coefficients of the two lines are about 5.8 and 21.4 h−1, so the value of the ejector line is about 3.7 times higher than that of the blower aeration line. This phenomenon can be explained by a decrease of the transfer resistance in more turbulent conditions and by the increase of contact time and area of the two phases in the ejector aeration line, as described in Figure 8.

mass-transfer coefficient increases with increasing suction air flow rate. The result may be ascribed to the increase of contact between air bubbles and aerating water per reaeration time. In the pitch range 1−3, the volumetric mass-transfer coefficient increases owing to increasing suction air flow rate as shown in Figure 5b. At screw pitch 4, the volumetric mass-transfer coefficient reaches the maximum value of 28.1 h−1 in spite of about an 8.4% decrease of the suction air flow rate compared to the maximum suction air flow rate at screw pitch 3. This can be explained by the optimum entrainment ratio and decrease of the transfer resistance and increase of contact area of the two phases in more turbulent conditions as described in Figure 8. In the pitch range 5−7, the volumetric mass-transfer coefficient decreases only about 5%, while the suction air flow rate decreases about 21% as shown in Figure 5b. At screw pitch 8, the volumetric mass-transfer coefficient increases again and reaches the second highest value of 24.1 h−1. Then, at screw pitch 9, the volumetric mass-transfer coefficient decreases about 9.7% compared to the value of screw pitch 5, while the suction air flow rate decreases about 68.4% as shown in Figure 5b. The first reason is a decrease in the resistance to oxygen transfer at the air bubble−water interface as a result of increased turbulence. It is well-known that oxygen from air bubbles is transferred through the gas−liquid interface, and the major resistance to oxygen transfer is in the liquid film surrounding the gas bubble.7 The second is an increase in the contact area of two phases due to smaller air bubbles as a result of increased turbulence as shown in Figure 8 (pitch 4 and 8). The third is an increase in the contact time of two phases due to longer penetration in the diffusing pipe resulting from increasing the circulating water, as can be deduced from the result of Figure 8 (pitch 8). We can thus conclude that the high turbulence level and optimum entrainment ratio are needed to increase the oxygen-transfer rate. In Figure 10, the experimentally determined volumetric mass-transfer coefficients for the ejector and blower aeration

4. CONCLUSIONS An experimental study was carried out to investigate the flow and oxygen-transfer characteristics in an aeration system using an annular nozzle ejector, and to evaluate the oxygen-transfer characteristics in the ejector aeration and blower aeration. The primary water flow rate increased with the screw pitch, while the suction air flow rate increased initially and then decreased with the screw pitch due to the variation of suction inlet vacuum pressure. The entrainment ratio was higher when the annular nozzle tip was closer to the inlet of the mixing tube than when the nozzle tip was farther from the inlet. It was found that the turbulence level and entrainment ratio strongly affected the air bubble size and volumetric mass-transfer coefficient. The saturation times and volumetric mass-transfer coefficients varied with the suction air flow rate in the ejector aeration, while the saturation times decreased and the volumetric mass-transfer coefficients increased with the blowing air flow rate in the blower aeration. The average oxygen masstransfer coefficient of the ejector aeration was about 3.7 times higher than that of the blower aeration. It can be concluded that the high turbulence level and optimum entrainment ratio were needed to increase the volumetric oxygen-transfer rate. In future work, more detailed investigation and comparison of flow and oxygen-transfer characteristics between the central nozzle ejector and annular nozzle ejector aeration systems will be conducted.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +82-61-6597223. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Special Research Program of Chonnam National University (2009) and tha Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (2011-0014640).

Figure 10. Comparison of the oxygen-transfer coefficients with air suction rate for the ejector and blower aeration lines.

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lines are plotted with air suction rates. It can be found that the volumetric mass-transfer coefficient is an increasing function of the blowing air flow rate in the blower aeration line. The volumetric mass-transfer coefficient increases from 3.2 to 8.2 h−1 as the blowing air flow rate increases from 16.8 to 35.8 m3/ h. On the other hand, the volumetric mass-transfer coefficient is not a linear function of the suction air flow rate in the ejector aeration line, as shown in Figure 9. Under the same operating conditions of suction air flow rate in the ejector aeration line, the minimum and maximum mass-transfer coefficients are 17.8 and 28.5 h−1. When the coefficients for the two aeration lines are compared, the ejector aeration line has higher volumetric 1762

NOMENCLATURE A = annular nozzle area (mm2) C = dissolved oxygen concentration (mg/L) d = diameter (mm) DO = dissolved oxygen KLa = volumetric mass-transfer coefficient (1/h) L = length (mm) P = pressure (kgf/cm2) Q = air or water flow rate (m3/h) q = entrainment ratio Re = Reynolds number T = saturation time of oxygen (min) dx.doi.org/10.1021/ie302208e | Ind. Eng. Chem. Res. 2013, 52, 1756−1763

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(20) Chen, J.; Wu, J.; Wang, Z.; Luo, W. The Effect of Secondary Flows on the Starting Pressure for a Second-Throat Supersonic Ejector. J. Therm. Sci. 2011, 20, 503. (21) El-Ghandour, M.; El-Sawaf, I. A.; El-Ottla, F. M. Solid-MaterialsHandling Central-Type Jet Pump. 6th International Water Technology Conference, Alexandria, Egypt, 2001; p 328. (22) Lima Neto, I. E.; Zhu, D. Z.; Rajaratnam, N. Horizontal Injection of Gas-Liquid Mixtures in a Water Tank. J. Hydraul. Eng. 2008, 134, 1722. (23) Baylar, A.; Ozkan, F. Applications of Venturi Principle to Water Aeration Systems. Environ. Fluid Mech. 2006, 6, 341. (24) Fadavi, A.; Chisti, Y. Gas-Liquid Mass Transfer in a Novel Forced Circulation Loop Reactor. Chem. Eng. J. 2005, 112, 73. (25) Park, G.; Kim, S.; Kwon, S. A Starting Procedure of Supersonic Ejector to Minimize Primary Pressure Load. J. Propul. Power 2008, 24, 631. (26) Kim, S.; Kwon, S. Experimental Determination of Geometric Parameters for an Annular Injection Type Supersonic Ejector. J. Fluid Eng. 2006, 28, 1164. (27) Kim, S.; Jin, J.; Kwon, S. Development of an Ejector System for Operation of Chemical Lasers (II) - Optimal Design of the SecondThroat Type Annular Supersonic Ejector. Trans. Korean Soc. Mech. Eng. B 2004, 28, 1231. (28) Rainer, B.; Fonade, A.; Moser, A. Hydrodynamics of a New Type of Ejector. Bioprocess Eng. 1995, 13, 97. (29) Chern, J. M.; Chou, S. R.; Shang, C. S. Effects of Impurites on Oxygen Transfer Rates in Diffused Aeration Systems. Water Res. 2001, 35, 3041. (30) YSI. The Dissolved Oxygen Handbook: A Practical Guide to Dissolved Oxygen Measurements; YSI.com, 2009.

t = time (s) u = superficial velocity (m/s) ν = kinematic viscosity (m2/s) Subscript

a = suction or blowing air d = discharge flow or diffuser p = primary flow s = suction flow w = aeration water

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REFERENCES

(1) Al-Ahmady, K. K. Analysis of Oxygen Transfer Performance on Sub-Surface Aeration Systems. Int. J. Environ. Res. Public Health 2006, 3, 301. (2) Gillot, S.; Capela-Marsal, S.; Roustan, M.; Heduit, A. Predicting Oxygen Transfer of Fine Bubble Diffused Aeration Systems: Model Issued from Dimensional Analysis. Water Res. 2005, 39, 1379. (3) Chern, J. M.; Yang, S. P. Oxygen Transfer Rate in a CoarseBubble Diffused Aeration System. Ind. Eng. Chem. Res. 2003, 42, 6653. (4) Wagner, M. R.; Popel, H. J. Oxygen Transfer and Aeration Efficiency: Influence of Diffuser Submergence, Diffuser Density, and Blower Type. Wat. Sci. Technol. 1998, 38, 1. (5) Balamurugan, S.; Lad, M. D.; Gaikar, V. G.; Patwardhan, A. W. Hydrodynamics and Mass Transfer Characteristics of Gas-Liquid Ejectors. Chem. Eng. J. 2007, 131, 83. (6) Liao, C. Gas Ejector Modelling for Design and Analysis. Ph.D. Thesis, Texas A&M University, College Station, TX, 2008. (7) Garcia-Ochoa, F.; Gomez, E. Bioreactor Scale-Up and Oxygen Transfer Rate in Microbial Processes: An Overview. Biotechnol. Adv. 2009, 27, 153. (8) Yan, J.; Cai, W.; Li, Y. Geometry Parameters Effect for AirCooled Ejector Cooling Systems with R134a Refrigerant. Renewable Energy. 2012, 46, 155. (9) Teaima, I. R. A Central-Type Jet Pump Model for Wheat Grains Removing from Water Channels. 16th International Water Technology Conference, Istanbul, Turkey, 2012; p 1. (10) Lima Neto, I. E. Maximum Suction Lift of Water Jet Pumps. J. Mech. Sci. Technol. 2011, 25, 391. (11) Ma, X.; Zhang, W.; Omer, S. A.; Riffat, S. B. Experimental Investigation of a Novel Steam Ejector Refrigerator Suitable for Solar Energy Application. Appl. Therm. Eng. 2010, 30, 1320. (12) Yapici, R.; Ersoy, H. K.; Aktoprakoglu, A.; Halkaci, H. S.; Yigit, O. Experimental Determination of the Optimum Performance of Ejector Refrigeration System Depending on Ejector Area Ratio. Int. J. Refrig. 2008, 31, 1183. (13) Yapici, R. Experimental Investigation of Performance of Vapour Ejector Refrigeration System Using Refrigerant R123. Energy Convers. Manage. 2008, 49, 953. (14) Eames, I. W.; Ablwaifa, A. E.; Petrenko, V. Results of an Experimental Study of an Advanced Jet-Pump Refrigerator Operating with R245fa. Appl. Therm. Eng. 2007, 27, 2833. (15) Balamurugan, S.; Lad, M. D.; Gaikar, V. G; Patwardhan, A. W. Effect of Geometry on Mass Transfer Characteristics of Ejectors. Ind. Eng. Chem. Res. 2007, 46, 8505. (16) Kim, M. I.; Kim, O. S.; Lee, D. H.; Kim, S. D. Numerical and Experimental Investigations of Gas-Liquid Dispersion in an Ejector. Chem. Eng. Sci. 2007, 62, 7133. (17) Mandal, A.; Kundu, G.; Mukherjee, D. Energy Analysis and Air Entrainment in an Ejector Induced Downflow Bubble Column with Non-Newtonian Motive Fluid. Chem. Eng. Technol. 2005, 28, 210. (18) Cramers, P. H. M. R.; Beenackers, A. A. C. M. Influence of the Ejector Configuration, Scale and the Gas Density on the Mass Transfer Characteristics of Gas-Liquid Ejectors. Chem. Eng. J. 2001, 82, 131. (19) Havelka, P.; Linek, V.; Sinkule, J.; Zaharadnik, J.; Fialova, M. Hydrodynamics and Mass Transfer Characteristics of Ejector Loop Reactors. Chem. Eng. Sci. 2000, 55, 535. 1763

dx.doi.org/10.1021/ie302208e | Ind. Eng. Chem. Res. 2013, 52, 1756−1763