Microbubble Size Distribution Measurement in a DAF System

Article pubs.acs.org/IECR

Microbubble Size Distribution Measurement in a DAF System Wen-Hui Zhang,* Jinzhao Zhang, Bo Zhao, and Penghui Zhu Tianjin Key Laboratory of Pulp & Paper, College of Material Science & Chemical Engineering, Tianjin University of Science &Technology, Tianjin 300457, China ABSTRACT: Microbubble size distribution is an important parameter of DAF in water and wastewater treatment. In this study, a new image method with a sampling tube was developed to measure the microbubble size in the DAF process. In addition, the effects of saturator pressure, TX-100 concentration, and polyDADMAC concentration in the saturator on bubble size distribution were investigated. The results showed that the bubble size decreased with the increase of the saturator pressure when the pressure was below 0.4 MPa and the change of bubble size was insignificant when the pressure increased further. The bubble size decreased with the increase of TX-100 concentration, while polyDADMAC had a minimal effect on the bubble size in the range of 0−5 ppm. This study indicated that the method was a rapid and robust one, which may be applied in the industrial-scale DAF.

1. INTRODUCTION Bubble size is an important parameter for gas−liquid or gas− liquid−solid multiphase flow. Microbubbles can improve the process performance due to the increase of the gas−liquid interfacial area or the bubble−particle collision rate.1,2 In recent years, the study of microbubbles is a very attractive topic in the flotation process, such as water treatment and mineral process. In a dissolved air flotation (DAF) system for water treatment, air is usually dissolved into the recycle flow under pressure (0.4−0.6 MPa) in a saturator and microbubbles are released through nozzles or special valves at the bottom entrance to the contact zone. In the contact zone, microbubbles attach to flocs to produce bubble−floc aggregates. Then the bubble−floc aggregates are separated from water due to the density difference in the separation zone. In the earlier application, microbubbles in mineral particles recovery were not successful due to the low lifting force for the coarse or dense particles.3 Recent studies found that microbubbles have improved separation efficiency compared to macrobubbles in the traditional condition.1,4 One key factor needed to be considered in these applications is the distribution of microbubble sizes. Thus far there are many methods to measure macrobubble size, such as the optical probe method and the conductivity probe method.5 However, these methods cannot be used or applied directly to estimate microbubble size. The methods to measure microbubble size are few, mainly including image analysis and laser-based methods. Rodrigues and Rubio6 developed an image analysis system, including a bubble capture cell, a microscope, and a CCD camera. They applied an intermittent measurement to estimate the microbubble size from photographs that were taken immediately after the flow was stopped. A limitation of the intermittent measurement may be subjective to determine the shooting time. Han et al.7 studied the feasibility of the commercially available batch-type and online particle counter on microbubble size estimation in DAF and electroflotation processes. The first method that they investigated was based on electrical resistance, and the second one was based on laser light obscuration. Compared to the image analysis method, it was found that the online particle © 2015 American Chemical Society

counter can produce reasonably accurate results in a shorter time (10 min). However, the particle counter method also had two disadvantages: bubble coalescence when bubbles moved to the sensor during the sampling process and bubble overlapping which may result in counting fewer but larger bubbles. Couto et al.8 applied a laser diffraction technique in which the laser diffraction angle was inversely proportional to the particle size to measure size distribution of microbubbles. Though it was a fast and reliable method, the change of bubble size may also occur in the sampling tube and the laser diffraction particle size analyzer. In addition, it is not suitable for the industrial situation. Moruzzi and Reali9 applied a nonintrusive image acquisition system to estimate bubble size distribution (BSD). The system included a 100 mW laser system to establish a nice illuminated vertical plane. Images were taken by a digital camera outside a pilot plant. The greatest advantage of the method was nonintrusive due to no sampling unit, but it was only suitable for the transparent DAF or reactor. Leppinen and Dalziel10 used a Perspex device with a vertical tube to measure the BSD. Though the device can measure bubble size and bubble cluster at the same time, many microbubbles may be out of focus in images since the width of the viewing channel was larger than 2 cm. In addition, bubble coalescence may occur in the sampling tube. Pérez-Garibay et al.11 applied a CCD with a peephole on the wall to measure microbubble size in a flotation column. The disadvantage of this image method was similar to Moruzzi and Reali’s method, which is only suitable for the transparent reactor. Bubble size depends on several factors in a DAF process, such as saturation pressure, temperature, nozzle type, and chemical conditions. In general, bubble size in a DAF system is in the range of 10−150 μm.2 When the surfactant is added, bubble size can decrease due to low surface tension.8,12 This study developed a new measurement device based on image analysis to estimate microbubble size distribution in the Received: Revised: Accepted: Published: 5179

January 9, 2015 April 13, 2015 April 17, 2015 April 17, 2015 DOI: 10.1021/acs.iecr.5b00109 Ind. Eng. Chem. Res. 2015, 54, 5179−5183

Article

Industrial & Engineering Chemistry Research

chamber and the vertical direction was 5° in order to provide an unambiguous plane of focus and facilitate microbubbles to rise along the glass plate rapidly. The observation side and illumination side were made of glass to minimize bubble adhesion, and the other sides were made of acrylic. The viewing chamber was connected to the top sampling tube whose inner diameter (i.d.) was 4 mm. The i.d. of the bottom sampling tube was 1.8 mm, which was 10 times greater than the maximum size of the microbubbles. At the same time, a small sampling tube can decrease the bubble concentration in the viewing tube and minimize bubble coalescence in the sampling tube. A nonionic surfactant TX-100 (1.0 g/L) was added into the top sampling tube by a mixing part (see Figure 2). The buffer tank was used to minimize the flow pulsation caused by the peristaltic pump. A 1 W LED was placed behind the viewing chamber for illumination. The images were recorded by a CCD camera (Beijing Join Hop Image Technology Ltd., OK-AM 1530, resolution 1024 × 1024 pixels, maximum sampling rate 25 Hz) equipped with a camera Len (Computar, MLM-3XMP) and 2× focal length extender (Computar, EX2C). The captured images were automatically analyzed by Zhang’s method.15 According to the image analysis method, the images were first converted into the binary image and the contours were extracted and smoothed. Then the dominant points on the smoothed contour were detected for segmentation, and the candidate segments were grouped by an average distance deviation criterion finally. Though the shape of the microbubble was very close to spherical,16 it was regarded as an elliptical shape in the study. The Feret diameter,13 which was used to estimate the microbubble size, was calculated by according eq 1

DAF process. The effects of saturation pressure, surfactant concentration, and cationic polymer concentration on bubble size in the DAF processes were investigated.

2. EXPERIMENTS AND METHODS 2.1. Bench-Scale DAF. The schematic diagram of the experiment is shown in Figure 1. A bench-scale DAF unit

Figure 1. Schematic diagram of the experiment.

included an oil-free air compressor, a 4.5 L stainless steel unpacked saturator, a circulating water pump, and a flotation column. The flotation column was constructed of a 0.04 m i.d. Plexiglas column that is 0.28 m in height. Tap water or other solutions containing surfactant or cationic polymer was pressurized with the air compressor. The circulating pump was used to promote the gas−liquid transfer at a defined time (30 min) which can achieve about 80% saturator efficiency. The liquid containing dissolved air was released through a needle valve to produce microbubbles at 8 cm height above the base of the column. 2.2. Measurement Device for Microbubble Size Estimation. A new method for microbubble size estimation was developed based on the combination of Grau’s method13 and Hernandez-Aguilar’s method.14 The device mainly consisted of a viewing chamber, a sampling tube, a CCD camera, a LED light, a peristaltic pump, a buffer tank, a centrifugal pump, and a storage tank (see Figure 1). The viewing chamber was 2.5 cm wide, 7.5 cm high, and 0.5 mm deep. There was a graduated structure at the end of the viewing chamber (Figure 2), which can minimize the change of bubble size as the flow channel change. The angle between the viewing

deq =

3

F2 2F1

(1)

where F1 and F2 are the short and long axis lengths of the fitted ellipse, respectively. Before sampling, the viewing chamber was filled by the TX100 solution. The usage of surfactant solution was to minimize bubble coalescence problems in the device and bubble adhesion in the viewing chamber. During the sampling process, the TX100 (1.0 g/L) solution was added by the centrifugal pump from the storage tank into the mixing part and the sample speed (sucking speed) was controlled by the peristaltic pump. After a steady flow regime in the viewing chamber was available, an image capture process was launched. The number of captured images depended on bubble concentration in the measurement device. In general, the image sampling rate was 10 fps, and 100 captured images were used to analyze the BSD. A typical image is shown in Figure 3. All experiments were conducted at 18 ± 2 °C. All experimental conditions were tested at least 6 times for measurements in each experimental set for the BSD estimation.

3. RESULTS AND DISCUSSION 3.1. Bubble Size Measurement. Figure 4 presents a typical BSD measured by the microbubble measurement device. The possible sources of error for the measurement are the change of bubble size in the sampling tube due to coalescence or breakup, equal probability sampling for each bubble class, the change of bubble size caused by pressure difference, and camera record and image analysis. In order to estimate the effect of the change of bubble size in the sampling tube due to coalescence or breakup on BSD, we investigate the effect of the sample speed (Vsample), the length of

Figure 2. Detail of the sampling device. 5180

DOI: 10.1021/acs.iecr.5b00109 Ind. Eng. Chem. Res. 2015, 54, 5179−5183

Article

Industrial & Engineering Chemistry Research

Table 1. Effect of the Measurement Device Parameters on the Bubble Size no.

Vsample [cm/s]

Lbottom [cm]

Ltop [cm]

1 2 3 5 6 7 8 9

6.5 6.5 6.5 1.3 3.2 6.5 13.1 6.5

4 8 12 4 4 4 4 4

20 20 20 20 20 20 20 50

d10 [μm] 29.1 30.4 30.2 30.3 29.5 29.3 28.6 30.5

± ± ± ± ± ± ± ±

0.5 0.3 0.4 0.5 0.5 0.6 0.7 0.5

d32 [μm] 45.1 45.6 44.7 44.4 45.0 45.5 46.5 43.5

± ± ± ± ± ± ± ±

0.8 0.5 0.7 0.9 0.9 1.0 1.2 0.9

the sample speed have a minimal effect on the measurement results under the experiment conditions. Hence, the addition of surfactant and the design of the sampling tube are effective to minimize the change of bubble size in the sampling tube. As far as we are concerned, an equal probability sampling condition for bubble was ambiguous in the literature. Rodrigues and Rubio6 suggested the sample speed was better at 17−21 cm/s,19 which may be above the maximum microbubble rise velocity. Other researchers14,18,20 applied the positive displacement principle to sampling, which implied that the sample speed was zero. From Table 1 it is observed that the sample speed has little effect on the BSD. The change of bubble size caused by the pressure difference mainly comes from the different pressure between the sampling port and the viewing chamber. The flow rate in the bottom sampling tube, the top sampling tube, and the viewing chamber is less than 13.1, 2.7, and 2.2 cm/s, respectively. Hence, the pressure difference is mainly caused by the height difference between the two positions. In the paper, the bubble size is modified by the height difference according to eq 2

Figure 3. Typical microbubble image captured by the measurement device.

Figure 4. Typical BSD for microbubble.

D b = D b*

⎛ 10.3 + H * − ΔH ⎞⎛ 298 ⎞ ⎜ ⎟⎜ ⎟ ⎝ ⎠⎝ T + 273 ⎠ 10.3 + H *

(2)

where D*b is the bubble size in the viewing chamber, Db is the bubble size in the sampling port, H* is the height below the water level, ΔH is the height between the viewing chamber and the sampling port, and T is the measurement temperature. In general, about 8000−20 000 bubbles are counted for estimating the BSD. The number of bubbles is large enough to minimize the statistical error. The relatively low sample speed and slightly tilted (5°) viewing chamber can minimize the bubble overlapping and bubble shape deformation, which may occur easily in the horizontal viewing chamber. 3.2. Effect of the Saturation Pressure on the Microbubble Size. Figure 5 shows the effect of the saturation pressure on the mean diameters under different flow conditions. When no auxiliary water is added, the arithmetic mean diameter (d10) and the Sauter mean diameter (d32) decreases approximately from 56 and 75 μm, respectively, to 30 and 43 μm as the saturation pressure increases. When the pressure is above 0.4 MPa, the change of bubble diameters is insignificant, especially the arithmetic mean diameter. It is in accordance with the results by other researchers.6,8 Since the experimental condition is different, such as bubble concentration (or gas holdup) in the column or reactor and valve type for air releasing,2 the BSD is also different. Here, in order to explain the effect of bubble concentration on the BSD in the column, the auxiliary water is added to reduce bubble concentration. When the flow rate of auxiliary water is 45 L/ min, d10 and d32 both decrease. It is because a low bubble

the top sampling tube (Ltop), and the length of the bottom sampling tube (Lbottom). The sample speed affects the bubble size in two ways. On one hand, the bubble number concentration in the sampling tube increases as the speed increased, which can increase the bubble collision rate; on the other hand, the increase of flow rate can also increase the bubble−eddy collision, which is likely to result in bubble breakup.17 In addition, high bubble concentration can lead to more overlapped bubbles in an image, make bubble recognition more difficult, and reduce the speed and accuracy of calculation finally. Since bubble coalescence is dominant in the low turbulence condition, the surfactant is usually added into the sampling tube to increase the coalescence time and minimize bubble coalescence.13,18 Instead of a T-tube, a concentric tube structure is used in this study for surfactant mixing to minimize bubble circulation in the mixing part (see Figure 2). As the length of the sampling tube increases, it may increase bubble size due to the increase of probability of bubble collision. In this study, the length of the top sampling tube is 20−50 cm, the length of the bottom sampling tube is 4−12 cm, and the sample speed is 1.3−13.1 cm/s (based on the cross-sectional area of the bottom sampling tube). Here, the arithmetic mean diameter d10 (=(∑nidi)/(∑ni)) and the Sauter mean diameter d32 (=(∑nidi3)/(∑nidi2)) are used for comparison. It is shown that the relative deviation of d10 and d32 is both less than 7% for different parameters of the measurement device (Table 1). Hence, the length of the sampling tube (top or bottom) and 5181

DOI: 10.1021/acs.iecr.5b00109 Ind. Eng. Chem. Res. 2015, 54, 5179−5183

Article

Industrial & Engineering Chemistry Research

σ (3) ΔP where σ is the surface tension and ΔP is the pressure difference across the nozzle. Equation 3 shows that the addition of surfactant can decrease the critical bubble diameter. Takahashi et al.25 indicated that the pressure difference and the surface tension affect the lowest free energy to generate bubbles by cavitation, which can be formulated by eq 4 dcr =

ΔF =

16πσ 3 3ΔP 2

(4)

In addition to decreasing the bubble size in the first step from eq 3 or 4, the surfactant also affects bubble growth, which can reduce the bubble coalescence rate by the increase of the coalescence time. 3.4. Effect of the polyDADMAC Concentration in the Saturator on the Microbubble Size. Bubble charge in aqueous solution is important in mineral processing and water treatment. Air bubbles in water without the addition of chemical additives exert a negative charge, which causes the electrostatic repulsive forces between bubbles.2 Karhu et al. found that the application of the PosiDAF technique with polyDADMAC in the treatment of real oily wastewater was very successful.26 Yap et al. used polyDADMAC as a bubble modifier to treat with cyanobacteria cell and found that cell removal could be above 90%.27 In our view, bubble size was as important as zeta potential to study the effect of cationic polymer on DAF. However, to our knowledge, there is no report about the effect of polyDADMAC in the saturator on the microbubble size. Figure 7 shows the effect of polyDADMAC concentration in the saturator on BSD. It is shown that d10 and d32 change little

Figure 5. Effect of the saturation pressure on d10 and d32 under different flow conditions. Measurement conditions: Lbottom = 4 cm, Ltop= 20 cm, Vsampling = 6.5 cm/s.

concentration leads to a low probability of bubble coalescence under the low turbulent condition. 3.3. Effect of Surfactant Concentration in the Saturator on the Microbubble Size. The effect of surfactant on the bubble characteristics in the gas−liquid flow has drawn the attention of researchers in the last decades.8,21,22 In many cases, the surfactant is added into gas−lqiuid flow, which can hinder bubble coalescence and drastically reduce bubble size.6,8 Recently, Henderson et al.23 applied a PosiDAF technique, in which the surfactant was added into the saturator to modify the bubble surface and improve the DAF efficiency. Obviously, the surfactant changed not only the bubble surface and but also the bubble size. Here, we investigate the effect of surfactant concentration in the saturator on the microbubble size (Figure 6). From Figure 6 it is observed that d10 decreases about 12.2%

Figure 6. Effect of the TX-100 concentration in the saturator on d10 and d32 under different flow conditions. Experimental conditions: Lbottom = 4 cm, Ltop = 20 cm, Vsampling = 6.5 cm/s, saturation pressure = 0.4 MPa.

Figure 7. Effect of the polyDADMAC concentration in the saturator on d10 and d32 under different flow conditions. Experimental conditions: Lbottom = 4 cm, Ltop = 20 cm, Vsampling = 3.2 cm/s, saturation pressure = 0.4 MPa.

from about 34 μm and d32 decreases 22.8% from 56 μm as TX100 concentration increases on the condition of no auxiliary water. When the flow rate of auxiliary water is 45 L/h, d10 decreases about 13% from 33 μm and d32 decreases 9.5% from 43 μm with TX-100 concentration. The results are in in accordance with Couto et al.’s results.8 Rykaart and Haarhoff24 developed a simple bubble growth model, in which the microbubbles were formed in two consecutive steps: bubble nucleation and bubble growth. In the bubble nucleation step, bubbles can be formed by liquid eddies or solid surfaces. The former is called homogeneous nucleation. The critical bubble diameter is predicted from eq 32

as the polyDADMAC concentration increases from 0 to 5 ppm. polyDADMAC has a high charge density (6.5 mequiv/g) and relative high molecular weight (viscosity-average molecular weight 42 000). According to Rykaart et al.’s view, polyDADMAC mainly affects microbubble growth due to the change of the viscosity of the liquid phase. Since the polyDADMAC concentration is low (no more than 5 ppm, which is in commonly used range for wastewater treatment), the viscosity increases slightly and hardly affects the BSD in the DAF. 5182

DOI: 10.1021/acs.iecr.5b00109 Ind. Eng. Chem. Res. 2015, 54, 5179−5183

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Industrial & Engineering Chemistry Research

(13) Grau, R. A.; Heiskanen, K. Visual Technique for Measuring Bubble Size in Flotation Machines. Miner. Eng. 2002, 15, 507. (14) Hernandez-Aguilar, J. R.; Coleman, R. G.; Gomez, C. O.; Finch, J. A. A Comparison between Capillary and Imaging Techniques for Sizing Bubbles in Flotation Systems. Miner. Eng. 2004, 17, 53. (15) Zhang, W.-H.; Jiang, X.; Liu, Y.-M. A Method for Recognizing Overlapping Elliptical Bubbles in Bubble Image. Pattern Recognit. Lett. 2012, 33, 1543. (16) Clift, R.; Grace, J. R.; Weber, M. E. Bubbles, Drops, and Particles; Academic Press: New York, 1978. (17) Prince, M. J.; Blanch, H. W. Bubble Coalescence and Break-up in Air-Sparged Bubble Columns. AIChE J. 1990, 36, 1485. (18) Bhondayi, C.; Moys, M. H. Determination of Sampling Pipe (Riser) Diameter for a Flotation Bubble Load Measuring Device. Miner. Eng. 2011, 24, 1664. (19) Parkinson, L.; Sedev, R.; Fornasiero, D.; Ralston, J. The Terminal Rise Velocity of 10−100 Mm Diameter Bubbles in Water. J. Colloid Interface Sci. 2008, 322, 168. (20) Seaman, D. R.; Franzidis, J. P.; Manlapig, E. V. Bubble Load Measurement in the Pulp Zone of Industrial Flotation Machinesa New Device for Determining the Froth Recovery of Attached Particles. Int. J. Miner. Process. 2004, 74, 1. (21) Alves, S. S.; Maia, C. I.; Vasconcelos, J. M. T.; Serralheiro, A. J. Bubble Size in Aerated Stirred Tanks. Chem. Eng. J. 2002, 89, 109. (22) Xu, J. H.; Li, S. W.; Chen, G. G.; Luo, G. S. Formation of Monodisperse Microbubbles in a Microfluidic Device. AIChE J. 2006, 52, 2254. (23) Henderson, R. K.; Parsons, S. A.; Jefferson, B. Surfactants as Bubble Surface Modifiers in the Flotation of Algae: Dissolved Air Flotation That Utilizes a Chemically Modified Bubble Surface. Environ. Sci. Technol. 2008, 42, 4883. (24) Rykaart, E. M.; Haarhoff, J. Behaviour of Air Injection Nozzles in Dissolved Air Flotation. Water Sci. Technol. 1995, 31, 25. (25) Takahashi, T.; Miyahara, T.; Mochizudi, H. Fundamental Study of Bubble Formation in Dissolved Air Pressure Flotation. J. Chem. Eng. Jpn. 1979, 12, 275. (26) Karhu, M.; Leiviskä, T.; Tanskanen, J. Enhanced Daf in Breaking up Oil-in-Water Emulsions. Sep. Purif. Technol. 2014, 122, 231. (27) Yap, R. K. L.; Whittaker, M.; Diao, M.; Stuetz, R. M.; Jefferson, B.; Bulmus, V.; Peirson, W. L.; Nguyen, A. V.; Henderson, R. K. Hydrophobically-Associating Cationic Polymers as Micro-Bubble Surface Modifiers in Dissolved Air Flotation for Cyanobacteria Cell Separation. Water Res. 2014, 61, 253.

4. CONCLUSIONS BSD was an important parameter of DAF in water and wastewater treatment. However, it was not easy to measure microbubbles accurately. In this study, a new image method with a sampling tube was developed to measure the microbubble size in the DAF process. Our results showed that the length of the sampling tube and the sample speed had a minimal effect on the measurement. Hence, the method was a rapid and robust one, which may be applied in the industrialscale DAF. In addition, the effects of saturator pressure, TX-100 concentration, and polyDADMAC concentration in the saturator on BSD were investigated. The results showed that the bubble size decreased with the increase of the saturator pressure when the pressure was below 0.4 MPa, and the change of bubble size was insignificant when the pressure increased further. The bubble size decreased with the increase of TX-100 concentration in the range of 0−50 ppm, and polyDADMAC had minimal effect on the microbubble size in the range of 0−5 ppm.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 022 60602199. Fax: +86 022 60601854. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by Foundation (No. 201303) of Tianjin Key Laboratory of Pulp & Paper (Tianjin University of Science & Technology), P. R. China, and Foundation of Tianjin University of Science & Technology (No. 20130115).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b00109 Ind. Eng. Chem. Res. 2015, 54, 5179−5183