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`5
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`CoN E-7 Pe 7~—™ |
`
`APPLICATION OF DIGITAL IMAGE ANALYSIS FOR SIZE DISTRIBUTION
`MEASUREMENTS OF MICROBUBBLES
`
`Susan E. Burns, Sotira Yiacoumi, and J. David Frost
`School of Civil and Environmental Engineering
`Georgia Institute of Technology
`Atlanta, Georgia 30332-0512
`
`Costas Tsouris
`Chemical Technology Division
`Oak Ridge National Laboratory*
`P. O. Box 2008
`Oak Ridge, Tennessee 37831-6226
`
`pe 4’
`
`Fra i2 ty
`OsTi!
`
`MASTER
`
`Submitted for publication in proceedings:
`IMAGING TECHNOLOGIES: Techniques and Civil Engineering Applications
`Engineering Foundation
`Davos, Switzerland
`May 25-30, 1997
`
`The submitted manuscript has been authored
`by a contractor ofthe U.S. Government
`under contract No. DE-ACO5-960R22464.
`Accordingly, the U.S. Government retains a
`nonexclusive, royalty-free license to
`publish or reproduce the publishedform
`ofthis contribution, or allow others to
`
`doso,forU.S. Governmentpurposes.
`
`USTRIGUTION OF WEB DOCUMENT iS Deir
`
`y
`
`*QOak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. for the U.S. Departmentof
`Energy under contract number DE-AC05-960R22464.
`
`Tennant Company
`Exhibit 1031
`
`
`Tennant Company
`Exhibit 1031
`
`

`

`DISCLAIMER
`
`This report was prepared as an account of work sponsored by an agency of the United
`States Government. Neither the United States Government nor any agency thereof, nor
`any of their employees, make any warranty, express or implied, or assumes any legal liabili-
`ty or responsibility for the accuracy, completeness, or usefulness of any information, appa-
`ratus, product, or process disclosed, or represents that its use would notinfringe privately
`owned rights. Reference herein to any specific commercial product, process, or service by
`trade name, trademark, manufacturer, or otherwise does not necessarily constitute or
`imply its endorsement, recommendation, or favoring by the United States Government or
`anyagency thereof. The views and opinions of authors expressed herein do not necessar-
`
`ily state or reflect those of the United States Government or any agency thereof.
`
`

`

`DISCLAIMER
`
`Portions of this document may beillegible
`in electronic image products.
`Images are
`produced from the best available original
`
`document.
`
`

`

`Application of Digital Image Analysis for Size Distribution
`Measurements of Microbubbles
`
`Susan E. Burns', Sotira Yiacoumi', David Frost’, and Costas Tsouris”
`
`Abstract
`
`This work employs digital image analysis to measure the size distribution of
`microbubbles generated by the process of electroflotation for use in solid/liquid
`separation processes. Microbubbles are used for separations in the mineral
`processing industry and also in the treatment of potable water and wastewater. As
`the bubbles move upward in a soHd/liquid column due to buoyancy,particles collide
`with and attach to the bubbles andare carried to the surface of the column where they
`are removed by skimming. The removalefficiency ofsolids is strongly affected by
`the size of the bubbles. In general, higher separation is achieved by a smaller bubble
`size.
`The primary focus of this study was to characterize the size and size
`distribution of bubbles generated in electroflotation using image analysis. The study
`found that bubble diameter increased slightly as the current density applied to the
`system was increased. Additionally, electroflotation produces a uniform bubblesize
`with narrow distribution which optimizes the removalof fine particles from solution.
`
`Introduction
`
`Manyenvironmental and industrial treatment processes rely on the separation
`of solid particles from liquid solutions. Traditionally, solid particles are removed by
`sedimentation; however, sedimentation does not work well for low density particles
`like clay minerals, spores, and coagulated fulvic acids (Edzwald et al., 1992; Malley
`and Edzwald, 1991; Letterman, 1987). As a result, a method known as flotation,
`which floats rather than sediments low density particles,
`is being used more
`commonly.
`In flotation, small gas bubbles are generated at the bottom of the water
`column to be treated. The microbubbles thenrise to the surface of the liquid through
`buoyancy. As the bubbles rise, they collide with and adsorb to particles in the
`
`_
`
`' Graduate Research Assistant, Assistant Professor, and Associate Professor, respectively, Georgia
`Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA 30332-0355
`2 Chemical Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN
`37831-6226
`
`1
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`Burns etal.
`
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`
`

`

`solution; consequently, the low density solids are floated to the top of the column for
`removal by skimming. Theprocessofflotation is known as dissolvedair flotation if
`the microbubbles are produced by pressurizing air into water, as dispersed air
`flotation if the bubbles are produced by forcing gas through a sparger, and as
`electroflotation if the bubbles are produced through the electrolysis of water. This
`study will focus on bubbles generated by electroflotation.
`
`Electroflotation has been used by the mineral processing industry for the
`recovery of mineral particles (Ketkar et al., 1991), and in environmental and
`industrial processes for the separation ofoil from oil/water emulsions (Hosny, 1992;
`Balmer and Foulds, 1986), and for the removal of coagulated heavy metals from
`solution (Srinivasan and Subbaiyan, 1989; Ramadorai and Hanten, 1986).
`In all of
`these applications, the removal efficiency is strongly affected by the size of the
`generated bubbles. Smaller bubbles have a longer residence time in the system, have
`a larger surface area, and are morelikely to adhereto solids after a collision (de Rijk
`et al, 1994). Consequently, the treatment process is optimized by generating the
`smallest diameter bubbles possible.
`
`This paper examines the effect of the process variables of voltage, current,
`and ionic strength on the size of the bubbles generated during electroflotation.
`Bubble images were recorded with a long-distance, high-magnification microscope,
`and were printed and imported into a digital image analysis for measurement of
`equivalent bubble diameter.
`The average equivalent circular diameter for was
`calculated for each experimental condition; additionally, the volume distribution of
`the bubbles was calculated for each experiment.
`
`Experimental
`
`The rectangular test cell used in the experiments of this study was madeof
`Plexiglas with dimensions of 58.4 cm by 7.6 cm by 2.5 cm.
`Inflow and outflow
`ports were drilled in the top and bottom ofthe cell and it was mounted vertically in
`order to allow gas flow out the top. After the cell was filled with the test solution, a
`soap-film flow meter was attached to one outflow port to measure gas flow rate and
`the remaining outflow ports were sealed. The electrodes used in the experiments
`were polished graphite electrodes (7.6 cm by 2.5 cm by 1.3 cm) and were mounted at
`the bottom of the cell with a separation of 13 cm. Electrical leads were attached to
`the electrodes using conductive epoxy, and then connected to an external power
`supply.
`
`Twenty eight experiments were performed using aqueous solutions of
`deionized water mixed with Na,SO,at ionic strengths of 0.1, 0.01, and 0.001 M.
`In
`the experiments, the voltage was varied from 15 to 86 V and the current was varied
`from 11 to 288 mA. Current was applied to the system using a high-voltage, high-
`current power supply and the generated bubbles were videotaped using a long-
`
`
`
`2
`
`Burnsetal.
`
`

`

`distance microscope with a magnification of approximately 230 times attached to a
`video camera, monitor, and VCR. The calibration factor was a wire of known
`diameter. The experiments performed at 0.001 M did not produce a significant gas
`flow rate; consequently, only the results for the experiments performed at 0.1 and
`0.01 M are reported in this paper.
`
`The gas bubbles produced in electroflotation form throughthe electrolysis of
`water by the following redox reactions:
`
`H,O-— 2H* +40, (g)+2e7
`2H,O+2e —+20H +H,(g)
`
`Anode(+)
`Cathode(—)
`
`The oxygen and hydrogen gas dissolve into the liquid surrounding the electrodes;
`when the liquid becomes supersaturated with gas, bubbles begin to form on the
`electrode surface (Verhaart et al., 1980). The camera was focused on the bottom
`electrodein the test cell and two experiments were performed at each powerlevel. In
`the first experiment, the bottom electrode was the anode, and in the second
`experimenttheelectrical leads were reversed making the bottom electrode the
`cathode. This configuration wasused becauseit prevented the mixing of oxygen and
`hydrogen bubbles during videotaping. A light source was set up behind the cell to
`produce contrast between the bubbles and the solution in the recorded images.
`
`Image Processing and Analysis
`
`After the experiments were completed, the images were printed to hard copy
`using a video copy processor and imported into a Quantimet Q570 Digital Image
`Processor. The printed images were gray-scale pictures of dark bubbles on a light
`background because the light source behind the cell was blocked by the bubbles but
`passed through the aqueoussolution. After the images were acquired by the image
`processor, they were converted from gray-scale into digital images and a minimal
`amount of image processing was performed.
`In some instances, background noise
`occurred on the images and was erased. Additionally, sometimes two bubbles
`touched each other on the images. In this case, the bubbles were either separated and
`analyzed individually, or were eliminated from the image. No other image
`processing was performed on the bubble pictures. Because not all of the bubbles
`were circular in cross-section, the image analyzer measured the area of each bubble
`and converted that to an equivalent circular diameter for the output.
`
`A statically valid sample size was chosen for analysis by first determining the
`error in the measurement. Measurements of one experiment were performed twice
`and the difference was 1.9 um between the average diameters measured. Assuming a
`normal distribution, and choosing a 95 % confidence interval, the sample size was
`calculated by the following (Hines and Montgomery, 1990):
`
`
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`

`

`n=(227] .
`
`(1)
`
`where n = sample size, Z,/? = confidence interval, o = standard deviation, and E =
`error.
`
`Results
`
`Figure 1 showsa plot of the equivalent circular diameter of oxygen bubbles
`as a function of current density. The figure shows a trend ofslightly increasing
`bubble diameter with increasing current density applied to the system. This is
`consistent with other electroflotation results using metallic electrodes (Brandon and
`Kelsall, 1985; Janssen and Hoogland, 1973; Landolt et al., 1970). The formation of
`bubbles in electroflotation is an inhomogeneous, or surface controlled, process rather
`than a homogeneous process where the bubbles form out of solution without the
`presence of a surface. Previous research has found that bubbles will form at the
`location of scratches and pits on the electrode surface (Janssen and Hoogland, 1973;
`Glas and Westwater, 1964) which illustrates the importance of the surface
`characteristics of the electrodes.
`In this study, the electrodes were not polished
`between successive experiments which mostlikely explains the scatter seen in the
`average bubble diameter measurements because the application of current will affect
`the surface characteristics of the electrode. Average oxygen and hydrogen bubble
`diameters measured in the experiments ranged from 17.1 to 37.9 um, which is
`consistent with the size of bubbles produced on stainless steel and platinum
`electrodes (Ketkar et al., 1991).
`
`35
`3
`®E
`30
`25
`D
`oi: 20
`Ss 15
`
`10
`
`5
`0
`
`0
`
`80
`60
`40
`20
`Current Density (mA/cm’)
`
`100
`
`m®
`
`o$
`
`<x
`
`Figure 1. Oxygen bubble diameteras a function of current density: I= 0.1 M.
`
`
`
`4
`
`Bumsetal.
`
`

`

`No clear trends in bubble size were seen as a function of voltage or as a
`function of ionic strength; however, volume of gas generated was a strong function
`of both powerinput to the system and ofionic strength of the surrounding medium.
`As would be expected, the generation of hydrogen and oxygen gas increased as both
`the powerinput to the system was increased andas theionic strength of the aqueous
`medium was increased.
`
`The volume distribution of the generated bubbles was also plotted for
`comparison between the experimental conditions. Figures 2 and 3 show the volume
`
`0
`
`50
`
`100
`
`150
`
`Bubble Diameter(:m)
`
`Figure 2(a). Cumulative volumedistribution for hydrogen bubbles: I = 0.1 M.
`
`Distribution
`Distribution 0.0
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`+-smifi
`
`
`
`CumulativeVolume
`
`
`
`CumulativeVolume
`
`0
`
`50
`
`100
`
`150
`
`Bubble Diameter(1:m)
`
`Figure 2(b). Cumulative volumedistribution for oxygen bubbles: I = 0.1 M.
`
`
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`
`

`

`distribution of both the oxygen and hydrogen bubbles producedat ionic strengths of
`0.1 and 0.01 M and at powerlevels ranging from 0.255 to 2.070 W. The distribution
`plots show that electroflotation produces bubbles with fairly uniform diameters. The
`most common range in bubble diameters is approximately 60 um, whichis a rather
`narrow distribution for flotation methods. Additionally, the majority of the bubbles
`produced have diameters smaller than 50 um which increases the removalefficiency
`of fine particles from the solution. However, as was seen with average bubble
`diameter, no distinct trendsin the distribution are seen as functions of either power or
`ionic strength.
`
`
`
`CumulativeVolume
`
`
`
`CumulativeVolume
`
`a 0.560 W
`4 0.900 W
`
`0
`
`100
`50
`Bubble Diameter(1:m)
`
`150
`
`Distribution o 0.360 W
`Distribution 0.0
`
`Figure 3(a). Cumulative volume distribution for hydrogen bubbles: I = 0.01 M.
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`oS
`
`0
`
`50
`
`100
`
`150
`
`Bubble Diameter (um)
`
`Figure 3(b). Cumulative volume distribution for oxygen bubbles: I = 0.01 M.
`
`
`
`6
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`Burnset al.
`
`

`

`Conclusions
`
`image analysis provided an efficient method for measuring the
`Digital
`average equivalent diameter of microbubbles produced byelectroflotation. The use
`of automated measuring processes allowed for the rapid determination of bubble size
`and eliminated the measurement errors which can occur in manual measurements.
`Because the measurement process is faster using an image analyzer than manual
`measurements, a larger sample space can be used and/or more experiments can be
`performed allowing for more complete data sets.
`
`In these experiments, the average bubble diameter increased as the current
`density applied to the system was increased. No trends were identified in bubble
`diameter as a function of either power or ionic strength of the aqueous medium;
`however, gas flow rate increased as both power and ionic strength were increased.
`The volume distribution of the generated bubbles showed that electroflotation
`produces bubbles with fairly uniform diameters and narrow ranges. Again, no trends
`were identified in distribution as functions of either powerorionic strength.
`
`Acknowledgments
`
`The authors thank Ken Thomas for his assistance with equipment preparation.
`Partial financial support from the Division of Chemical Sciences, Office of Energy
`Sciences, US Department of Energy, under contract DE-AC05-960R22464 with
`Lockheed Martin Energy Research Corp., is gratefully acknowledged.
`
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`(1986). Separating Oil
`Balmer, L. M. and Foulds, A. W.
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`(1985). Growth Kinetics of Bubbles
`Brandon, N.P.
`and Kelsall, G.H.
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`DE Rijk, S.E, vAN per Graaf, J.H.J.M., and pen Blanken (1994). Bubble Size in
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`and Montgomery, D.C.
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`Letterman, R.D. (1987). An Overview of Filtration, Journal of the American Water
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`Malley, J.P. and Edzwald, J.K.
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`from Effluents by Flotation, Minerals and Metallurgical Processing, August,
`pp. 149-154.
`Srinivasan, V. and Subbaiyan, M.(1989). Electroflotation Studies on Cu, Ni, Zn, and
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`Technology, Vol. 24, No. 1&2, pp. 145-150.
`Verhaart, H.F.A., pe Jonge, R.M., and van Stralen, S.J.D. (1980). Growth Rate of a
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`Burnsetal.
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