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Separation of solid-liquid suspensions with ultrasonic acoustic energy
Pergamon PII: S0043-1354(97)00088-2
War. Res. Vol. 31, No, 10, pp. 2543 2549, 1997 © 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00 + 0.00
SEPARATION OF SOLID-LIQUID SUSPENSIONS WITH ULTRASONIC ACOUSTIC ENERGY M. C. BEKKER, J. P. MEYER*, L. PRETORIUS and D. F. VAN DER MERWE Research Groap for Cooling and Heating Technology, Department of Mechanical and Manufacturing Engineering, Rand Afrikaans University, Laboratory for Energy, P.O. Box 524, Auckland Park, 2006, Republic of South Africa (Received December 1995; accepted in revised form March 1997)
Abstract--A theoretical and experimental study was carried out to investigatethe possibility of separating solid-liquid suspensions of water and talcum powder, as well as in a process cooling water system, by means of a standing ultrasonic acoustic wave. Separation was successfullyachieved with suspensions of water and talcum powder. The separation of suspended solids from the process cooling water was unsuccessful owing to the extremely small sizes of the suspended solid particles. © 1997 Elsevier Science Ltd Key words--acoustic, standing waves, separation, suspension, sound waves, transducer, ultrasonic
NOMENCLATURE diameter of particles (mm) velocity of sound in water (m/s) E = energy density of the acoustic field (Nm/m3) F(po/ p, ) density fi~ctor f = frequency (Hz) h= reference position measured from node or antinode (m) K(k)2/n amplitude factor k = 2rt/wavelength (m ~) P = radiation pressure (N) r..~. radius of particles (mm) T = time per~tod for movement of a particle to a node or antinode(s) U ~ particles velocity (m/s) 2 = wavelen~;th (mm) /t= viscosity (kg/m.s) po density of fluid (kg/m3) p l ---= density of particles (kg/m3~ a_~_ e~
=
=
INTRODUCTION Fouling of suspended solids on equipment in process cooling water causes severe heat losses and high maintenance activities. Numerous solid-qiquid separation methods have been applied to remove or decrease the effect of suspended solids ill process cooling water systems. These methods include centrifuging, sand filtration, sedimentation, etc. However, the suspended solids content in cooling water systems, after the mentioned separation methods have been applied individually or combined, is still high, owing to the ever-changing physical properties of the suspensions. An alternative
*Author to whom all correspondence should be addressed [Fax: + 2711 489 2466].
method of removing the suspended solids from the cooling water system needed to be investigated. The use of acoustic energy to separate solid-liquid suspensions was investigated. Not much work had been done in the past on this topic, and from literature it was learned that most work was done on the dewatering of specific substances. In the dewatering of substances, electric energy was used in addition to acoustic energy. (Muralidhara et al., 1988). This type of separation is referred to as EAD (Electro-acoustic Dewatering). Acoustic energy, in combination with filtration processes, was also investigated to a large extent in the past (Bongert, 1976; Kowalski et al., 1987; Swamy et al., 1983), specifically for the reduction of the moisture content of sludges. Only one patent was registered (Furedi, 1977) to remove solids from water by acoustic energy only. To form a basis of the principles of acoustic separation, two co-authors (Beard and Muralidhara, 1985) provided some insight into some mechanisms that influence the physical separation process. Oakley et al. (1954) investigated the use of the propagation of waves in water at ultrasonic frequencies and applied it on three public health engineering problems. It dealt with the effect of the vibrations on the aeration of water, on bacteria and algae, and on the sedimentation of suspended solids. This study concentrates only on the use of standing acoustic waves to achieve separation. The basic principles, background and mathematical description of acoustic waves, with special emphasis on standing waves, were investigated. The hydrodynamic behaviour and movement of particles were investigated separately, and the synergistic behaviour of applied acoustic energy and hydrodyn-
2543
2544
M.C. Bekker et al.
amic effects were then studied. The behaviour of small particles in suspension, under the influence of an acoustic wave, was based initially on the findings of King (1934), which were used as basis for the sizing of acoustic separation equipment.
In any separation application it is important to know at what rate the separation of the solid-liquid suspension will take place. An equation was derived (King, 1934) from which the time period for movement of a solid particle, from an arbitrary position to a node or antinode, can be calculated:
THEORY
T = (2/V)F(po/p, )K(k)2/~z
A standing wave is formed when a sinusoidal wave is reflected by a fixed point and the resulting motion gives no evidence that there are two waves travelling in opposite directions (Lighthill, 1990). This fixture results then in the forming of nodes and antinodes as shown in Fig. 1. At the nodes a very low energy level occurs and. particle velocity is close to zero. At the antinodes the opposite occurs and high energy and velocity levels is the result. These phenomena are very important and form the backbone of acoustic separation. When an acoustic wave is introduced into a suspension, the force acting on a spherical particle is referred to as the radiation pressure (King, 1934) and is given below:
where K(k)2/~ is the amplitude factor. Calculations proved that the time period for movement of a particle from an arbitrary position to a node or antinode is almost instant (less than a second), depending on the density ratio. It was calculated that, for a suspension of water and talcum powder with particle size 10#m and density ratio 1.1, an acoustic transducer with a frequency of 32 kHz will be required to achieve proper separation.
P = 2na2(ka)sin(2kh)F(po/pOE
(1)
where k = 27t/2 In order to move a particle through a fluid a force is required to overcome the viscous force. This force is also called the Stokes' force, attributed to Sir George Stokes who first formulated it in the 1850s, and is described in detail by Landau and Lifshitz (1987), as well as Happel and Brenner (1983). The Stokes' force is given below: F=6n~ru
(2)
By setting equation (1) equal to equation (2), the wavelength can be calculated. With the wavelength known the frequency required from the acoustic transducer can be calculated from the equation (3): f=c/2
ANTINODES
(3)
or LOOPS
Fig. 1. Illustration of the position and occurrence of nodes and antinodes in a standing acoustic wave.
(4)
EXPERIMENTS
The first series of experimental work was done to establish a workable acoustic separation process, the second series to measure the effectiveness of separation under different conditions and the third series to test different suspensions under established optimum conditions. No special equipment was designed or manufactured for the experiments. All experiments were carried out with available equipment. The experimental set-up consisted of a function generator, an amplifier, oscilloscope, a glass tube (20 mm diameter, 150 mm long) with flat bottom and a piezoelectric transducer. Two transducers were used during the execution of the experimental work, a 41-kHz and a 36-kHz transducer. The piezoelectric transducers were the limiting equipment. The transducers were not specifically manufactured for the experimental work owing to cost. Both transducers were limited to a maximum of 12V input voltage. Owing to the temporal availability of both transducers, the first series of experiments was done with the 41-kHz and the rest of the experiments with the 36-kHz transducer. A schematic layout of the experimental set-up is shown in Fig. 2. The piezoelectric transducer was glued onto the flat-bottomed glass tube. The glass-tube/transducer assembly was fitted in a vertical position to a stand. The function generator output splits, with the one end connected to the amplifier input, and the other end connected to the oscilloscope channel number 1. The amplifier output was connected to the transducer and a tie-off from the transducer was connected to the oscilloscope channel number 2. The glass tube was filled with water up to a level of 44 ram. All the equipment was switched on. The function generator was pre-set to a frequency of 41 kHz and an amplitude of 0.3 V. All the equipment was switched off. A small quantity of talcum powder was added to
Separation with ultrasonic acoustic energy
2545
..----q Glass tube with suspension t I
1 J
I~
~w,=c~1 Amplifier
Piezoelectric transducer
t
I Functiongenerator Oscilloscope
Fig. 2. Experimental set-up.
the water and the suspension stirred. All the equipment was switched on again. With a small adjustment to the frequency, clear separation of the solid(powder)/liquid(water) suspen-
sion could be observed, as illustrated in Fig. 3. As discussed, nodes and antinodes are formed at every half wavelength in a standing wave. When separation takes place at a specific frequency the solid particles
STANDING WAVE td
~
!
J
:i~.d~.~~
SuSI~nsion level _
~ q " ~ l . . - ' , , I / " . H. " ~ : b / '..k.' l I "~ & v o ~ , l g g s
"I
Fig. 3. The formation of separation levels and the measurement of wavelengths.
2546
M . C . B e k k e r et al.
will accumulate either at the nodes or antinodes depending on the density ratio. The areas where the solid particles accumulate can clearly be seen, with the naked eye, in the form of white "rings". These "rings" are referred to as "separation levels". In this case the accumulations were at the antinodes because the density of the powder is less than that of water. With the above the first objective, namely to establish a workable acoustic separation process with available equipment, was achieved. With a basic separation process established, an experiment was carried out to compare experimental results to some elementary calculations used with acoustic waves in water. The distance between every separation level reflects half a wavelength and can be measured. From the measured distance between two separation levels the wavelength can be calculated from equation (3) and the velocity of sound can be calculated. The same experimental set-up was used. The glass tube was filled with different levels of the powder-water suspension. The first fill was up to a level of 25 mm and tested. The second fill was up to a level of 48 mm, the third up to 71 ram, etc. The frequency was adjusted after every fill until clear separation could be observed. The number of separation levels was counted in each case and listed in Table 1. Since powder is less dense than water, separation takes place at the antinodes. Therefore, when two separation levels are counted it means one wavelength is formed as illustrated in Fig. 3. The sound velocity was calculated in each case and compared. The results from the experiment are given in Table 1. The velocity of sound in pure water is 1 478 m/s (Sears et al., 1982). The results obtained in the last column of Table 1 are in the region of this figure, which shows that the experimental work correlates acceptably enough with the theory to continue with further work on the established set-up. The glass tube was filled again with the suspension, clamped in a horizontal position and a frequency was applied as in the previous experiments. Separation again occurred and the powder started to settle out at the separation levels, forming "powder rings" at the bottom of the tube at intervals equal to the applied wavelength. The above experiments formed the first series of experiments in which a workable acoustic separation process was established with available equipment.
Table 1. Results from measured wavelengths and calculated wave speed Suspension level (mm) 25 48 71 92 121 152
Frequency (kHz)
Separation levels
Wavelength (mm)
44.2 42.2 43 42.2 41.2 39.9
2 3 5 6 8 9
25 32 28.4 30.7 30.3 33.8
Wave speed (m/s) 105 350 221 549 246 348
In the second series of experiments a fixed ratio of powder to water was prepared to carry out controlled tests to quantify the effect of flow and acoustic power on the effectiveness of separation. For these experiments a piezoelectric transducer with an optimum frequency of 36 kHz was used. The experimental set-up was modified to cater for the flow of suspensions through the separation section. The modification consists of the introduction of an inlet and an outlet nozzle to which the suspension supply source, in this case the suspension container, and outlet sample bottle were connected. A schematic layout of the revised set-up and procedure is explained below and shown in Fig. 4. A 25-1itre glass container was filled with a fixed concentration of powder-water (1.2 g/litre powder) suspension and placed at an elevated position. A magnetic stirring device was placed at the bottom for continuous mixing of the suspension to ensure a uniform concentration. The container was coupled to the same glass tube used before in a horizontal position via a rubber tube, and flow was controlled with a tube clamp. The outlet was routed to a sample bottle. Filter papers were numbered, dried in a laboratory drying oven and weighed. Each numbered filter was allocated to a specific sample to be taken. Before initializing the separation equipment, a control sample was taken in order to calibrate the suspension. A sample of 17 ml of the suspension was taken and filtered through the dried filter paper. The filtered paper was dried again and weighed. The net mass difference was divided by the sample volume to get the concentration of the sample. The result of this calibration test agreed with the original make-up suspension of 1.2g/litre. The accuracy of the concentration measurement was sufficient for this type of experiment; if higher accuracy is required the concentration could be measured turbidimetrically. Although it might have been easier to measure suspension concentrations turbidimetrically, such equipment was not available to the authors. All the separation equipment was activated and the experiments commenced. Samples were taken approximately every 30 s from the glass tube outlet in sample bottles of 17 ml each. The samples were filtered and the filtered samples were dried again for a minimum of 4 h. The filter samples were weighed again and the difference in mass was tabled. From this information the concentration at the outlet was calculated, since the sample volumes were known. From the results it could be seen whether separation took place and whether any solids were contained in the glass tube at the separation levels. Initially the effect of the voltage applied to the separation effectiveness was measured. A fixed flow rate of 16.7ml/s was established and different voltages were applied. Tests were repeated with 2, 4, 6, 8, 10 and 12 V. The maximum allowable input
Separation with ultrasonic acoustic energy
2547
25 litm Suspension contoiner
:~
Hose
Magnetic s#mng
Clamp r~.~,,~
Rubber hole
Piezoelectric transducer coupled to signal generator Glass tube w~n inlet end ou~et
+?
ii .,+,
. . . . . . . . . . i!i~ ii
Sample bollle
Fig. 4. Experimental set-up to establish flow through the separating equipment.
voltage for the transducer is 12 V (it must be noted that acoustics are generally measured in voltages as output from transducers, therefore explaining the use of volt instead of watts). The results from the experiments indicated that separation did take place and that the separation process does indeed work. The best results were with the voltage amplitude set at 12V. A graphical summary of the res,lts is given in Fig. 5,
1.5 1.4 1.3 1.2 z 1.1 O i1 < 0.9
wP 0,8 w zu 0.7 oO 0.6 0.5
0
2:
4
6
8
10
12
AMPLITUDE (volts)
Fig. 5. Separation concentration at applied voltage.
From the comparison it is clear that maximum separation took place with an amplitude of 12 V and with an average concentration of 0.683 g/litre, which reflects a separation effectivity of 43% when compared to the original concentration of 1.2g/litre. However, a high voltage does not necessarily mean better separation., Depending on the solids' characteristics, agglomeration can be disturbed, due to shear stress, at certain voltages, which could explain the uneven trend in Fig. 5. This phenomenon was also experienced by Muralidhara et al. (1988). For further experimental work, 12 V was used in all cases since it is the amplitude at which the best results were obtained, although it may not be the optimum value, and experiments with higher voltages would have been preferred. The rate at which the suspension flows through the separation glass tube could influence the effectivity of separation. The next series of experiments investigated the effect of flow rate (all in the laminar flow region). The voltage was fixed at 12 V and the same procedures as previously used were followed to measure the initial and final concentration levels at different flow rates. From the results obtained in these experiments it was clear that the best separation took place at the lowest flow of 2.5 ml/s, with an average concentration
2548
M.C. Bekker et
A
al.
0.7
~0.6 E ,~0.5 Z 0 0.4 I-~0.3
°
°
°
°
°
°
°
°
°
°
°
°
=
.
°
°
.
.
.
°
.
.
°
I
~0.2 o
~0.1 o 0
2.5
i
i o
4
-
i
16.7
FLOW RATE (rrgsec) Fig. 6. Separation concentration at different flow rates.
level of 0.425 g/litre (65% separation). A graphical summary of the results is given in Fig. 6. From these experiments it was clear that the most effective separation takes place at the lowest possible flow. From the second series of experiments it was clear that the most effective separation, for the available equipment, was at an amplitude of 12 V and as low as possible flow rate. In the third series of experiments the optimum separation settings, as were found in the first and second series of experiments, were applied to effluents from a process cooling water system. These effluents were process cooling water, cooling water blow-down and ash water. The experiments were carried out following the same procedure as previously where the concentrations were established before and after separation. The separation of process cooling water was complicated by the size of the suspended solids. Suspended particles in the cooling water are extremely small in size and can hardly be seen. The filtration process, to measure the solids' concentration, takes twice as long as with the powder suspension due to the particles that block the pores of the filter paper. The filter paper pore size is approximately 0.1/am and is generally used in similar applications in water research laboratories. The suspension concentration of process cooling water was measured at 1.696 g/litre average. The flow rate was set at 2.5ml/s and 12V applied. Samples of 25 ml were taken and the suspension concentration at the outlet was measured again. The results proved that effectively no separation took place. This could be because of the fact that the solids in suspension are too fine to be concentrated at the separation points in the glass tube. The solids are in
fact so small that they can hardly be observed with the naked eye. The same procedures as before were followed with cooling water blow-down and again the results were "disappointing", as effectively no separation took place. The average measurement even showed an increase in concentration and the question arose whether there was maybe a concentration at a separation point which "came loose" and went through at a high concentration level. With ash water, again no separation took place. As with the above two suspensions the suspended solids could hardly be seen with the naked eye, owing to the extremely small sizes. The third and final series of experiments proved to be disappointing, in the sense that virtually no separation took place in any of the tested substances. The highest applied voltage was used and a very low flow of 2.5ml/s was maintained. The lack of separation can be due to the extremely small sizes of the suspended particles, in some cases so small that they could hardly be observed visually. CONCLUSION From the experiments, it is evident that solidliquid separation can be achieved by means of a standing acoustic wave. Separation takes place at the nodes or antinodes, dependent on the density ratio. If the solids are less dense than the fluid, the solid particles accumulate at the antinodes and vice-versa. The actual distances between the separation levels compare well with the theoretical distances, which confirm the practical application of the experimental apparatus. With a 36-kHz piezoelectric transducer and a control suspension of water and talcum powder, the
2549
Separation with ultrasonic acoustic energy best separation results were achieved at a voltage amplitude of 12 V and a flow rate of 2.5 ml/s. A separation effectivity of 65% was achieved under those conditions. The 2.5-ml/s was the lowest measurable flow rate that could be achievext. It could be possible to achieve even better separation results with lower flow rates. The 12-V amplilmde was the highest possible voltage that could be applied through the transducer. However, higher voltage does not necessarily mean better separation. A too high voltage can break up agglomerated partic]les at the separation levels owing to imposed covibration. The optimum separation voltage may differ between suspensions and care must be taken when selecting the voltage amplitude. Separation of solid-liquid suspension in a cooling water system proved to be unsuccessful due to the very small sizes of t:he solids in suspension. In order to migrate the solids towards the nodes or antinodes, higher voltage amplitude should be experimented with. As the particle sizes decrease it becomes more "difficult" to move the particles through the fluid, owing to the increased effect of fluid viscosity and smaller exposure area for acoustic force. Higher input voltage should assist in overcoming the viscous, or Stokes' force, in the fluid. From the theoretical and experimental studies it is clear that the selecl:ion of separation equipment is vital to achieve efl'ective separation. Not enough information is currently available to optimize
acoustic separation equipment selection, and extensive research could still be done. REFERENCES
Beard R. E. and Muralidhara H. S. (1985) Mechanistic Considerations of Acoustic Dewatering Techniques, Ultrasonic Symposium. Bongert W. (1976) Method and means for separation of liquids from a mixture of solids and liquids. U.S. Patent No 3,970,552. Furedi P. (1977) Apparatus and method for removing fine particles from a liquid medium by ultrasonic waves. U.S. Patent No 4,055,491. Happ¢l J. and Brenner H. (1983) Low Reynolds Number Hydrodynamics. Kluwer Academic, King L. V. (1934) Proceedings of the Royal Society of London, Vol. CXLVII. Harrison & Sons, London. Kowalski E., Chmura K. and Bien J. (1978) Ultrasonics in the dehydration of sludge. Ultrasonics 16, 183-185. Landau L. D. and Lifshitz E. M. (1987) Fluid Mechanics, 2nd edn, Vol. 6, Course of Theoretical Physics, Pergamon Press, Lighthill J. (1990) Waves in Fluids. Cambridge University Press, Cambridge, UK. Muralidhara H. S., Senapati N. and Beard R. (1988) Electro-acoustic dewatering (EAD) a novel approach for food processing and recovery. Sep. Sei. Teehnol. 23, 2143-2158. Sears F. W., Zcmansky M. W. and Joung H. D. (1982) University Physics, Edition 6, Addison-Wesley, Swamy K. M., Rao A. R. K. and Narasimhan K. S. (1983) Acoustics aids dewatering. Ultrasonics 21, 280-286. Oakley H. R., Philpott J. A. and Abdaila Z. B. (1954) The use of ultrasonic vibrations in public health engineering. Proc. Inst. Civil Engrs (Part III) 3(1), 283-288.
War. Res. Vol. 31, No, 10, pp. 2543 2549, 1997 © 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/97 $17.00 + 0.00
SEPARATION OF SOLID-LIQUID SUSPENSIONS WITH ULTRASONIC ACOUSTIC ENERGY M. C. BEKKER, J. P. MEYER*, L. PRETORIUS and D. F. VAN DER MERWE Research Groap for Cooling and Heating Technology, Department of Mechanical and Manufacturing Engineering, Rand Afrikaans University, Laboratory for Energy, P.O. Box 524, Auckland Park, 2006, Republic of South Africa (Received December 1995; accepted in revised form March 1997)
Abstract--A theoretical and experimental study was carried out to investigatethe possibility of separating solid-liquid suspensions of water and talcum powder, as well as in a process cooling water system, by means of a standing ultrasonic acoustic wave. Separation was successfullyachieved with suspensions of water and talcum powder. The separation of suspended solids from the process cooling water was unsuccessful owing to the extremely small sizes of the suspended solid particles. © 1997 Elsevier Science Ltd Key words--acoustic, standing waves, separation, suspension, sound waves, transducer, ultrasonic
NOMENCLATURE diameter of particles (mm) velocity of sound in water (m/s) E = energy density of the acoustic field (Nm/m3) F(po/ p, ) density fi~ctor f = frequency (Hz) h= reference position measured from node or antinode (m) K(k)2/n amplitude factor k = 2rt/wavelength (m ~) P = radiation pressure (N) r..~. radius of particles (mm) T = time per~tod for movement of a particle to a node or antinode(s) U ~ particles velocity (m/s) 2 = wavelen~;th (mm) /t= viscosity (kg/m.s) po density of fluid (kg/m3) p l ---= density of particles (kg/m3~ a_~_ e~
=
=
INTRODUCTION Fouling of suspended solids on equipment in process cooling water causes severe heat losses and high maintenance activities. Numerous solid-qiquid separation methods have been applied to remove or decrease the effect of suspended solids ill process cooling water systems. These methods include centrifuging, sand filtration, sedimentation, etc. However, the suspended solids content in cooling water systems, after the mentioned separation methods have been applied individually or combined, is still high, owing to the ever-changing physical properties of the suspensions. An alternative
*Author to whom all correspondence should be addressed [Fax: + 2711 489 2466].
method of removing the suspended solids from the cooling water system needed to be investigated. The use of acoustic energy to separate solid-liquid suspensions was investigated. Not much work had been done in the past on this topic, and from literature it was learned that most work was done on the dewatering of specific substances. In the dewatering of substances, electric energy was used in addition to acoustic energy. (Muralidhara et al., 1988). This type of separation is referred to as EAD (Electro-acoustic Dewatering). Acoustic energy, in combination with filtration processes, was also investigated to a large extent in the past (Bongert, 1976; Kowalski et al., 1987; Swamy et al., 1983), specifically for the reduction of the moisture content of sludges. Only one patent was registered (Furedi, 1977) to remove solids from water by acoustic energy only. To form a basis of the principles of acoustic separation, two co-authors (Beard and Muralidhara, 1985) provided some insight into some mechanisms that influence the physical separation process. Oakley et al. (1954) investigated the use of the propagation of waves in water at ultrasonic frequencies and applied it on three public health engineering problems. It dealt with the effect of the vibrations on the aeration of water, on bacteria and algae, and on the sedimentation of suspended solids. This study concentrates only on the use of standing acoustic waves to achieve separation. The basic principles, background and mathematical description of acoustic waves, with special emphasis on standing waves, were investigated. The hydrodynamic behaviour and movement of particles were investigated separately, and the synergistic behaviour of applied acoustic energy and hydrodyn-
2543
2544
M.C. Bekker et al.
amic effects were then studied. The behaviour of small particles in suspension, under the influence of an acoustic wave, was based initially on the findings of King (1934), which were used as basis for the sizing of acoustic separation equipment.
In any separation application it is important to know at what rate the separation of the solid-liquid suspension will take place. An equation was derived (King, 1934) from which the time period for movement of a solid particle, from an arbitrary position to a node or antinode, can be calculated:
THEORY
T = (2/V)F(po/p, )K(k)2/~z
A standing wave is formed when a sinusoidal wave is reflected by a fixed point and the resulting motion gives no evidence that there are two waves travelling in opposite directions (Lighthill, 1990). This fixture results then in the forming of nodes and antinodes as shown in Fig. 1. At the nodes a very low energy level occurs and. particle velocity is close to zero. At the antinodes the opposite occurs and high energy and velocity levels is the result. These phenomena are very important and form the backbone of acoustic separation. When an acoustic wave is introduced into a suspension, the force acting on a spherical particle is referred to as the radiation pressure (King, 1934) and is given below:
where K(k)2/~ is the amplitude factor. Calculations proved that the time period for movement of a particle from an arbitrary position to a node or antinode is almost instant (less than a second), depending on the density ratio. It was calculated that, for a suspension of water and talcum powder with particle size 10#m and density ratio 1.1, an acoustic transducer with a frequency of 32 kHz will be required to achieve proper separation.
P = 2na2(ka)sin(2kh)F(po/pOE
(1)
where k = 27t/2 In order to move a particle through a fluid a force is required to overcome the viscous force. This force is also called the Stokes' force, attributed to Sir George Stokes who first formulated it in the 1850s, and is described in detail by Landau and Lifshitz (1987), as well as Happel and Brenner (1983). The Stokes' force is given below: F=6n~ru
(2)
By setting equation (1) equal to equation (2), the wavelength can be calculated. With the wavelength known the frequency required from the acoustic transducer can be calculated from the equation (3): f=c/2
ANTINODES
(3)
or LOOPS
Fig. 1. Illustration of the position and occurrence of nodes and antinodes in a standing acoustic wave.
(4)
EXPERIMENTS
The first series of experimental work was done to establish a workable acoustic separation process, the second series to measure the effectiveness of separation under different conditions and the third series to test different suspensions under established optimum conditions. No special equipment was designed or manufactured for the experiments. All experiments were carried out with available equipment. The experimental set-up consisted of a function generator, an amplifier, oscilloscope, a glass tube (20 mm diameter, 150 mm long) with flat bottom and a piezoelectric transducer. Two transducers were used during the execution of the experimental work, a 41-kHz and a 36-kHz transducer. The piezoelectric transducers were the limiting equipment. The transducers were not specifically manufactured for the experimental work owing to cost. Both transducers were limited to a maximum of 12V input voltage. Owing to the temporal availability of both transducers, the first series of experiments was done with the 41-kHz and the rest of the experiments with the 36-kHz transducer. A schematic layout of the experimental set-up is shown in Fig. 2. The piezoelectric transducer was glued onto the flat-bottomed glass tube. The glass-tube/transducer assembly was fitted in a vertical position to a stand. The function generator output splits, with the one end connected to the amplifier input, and the other end connected to the oscilloscope channel number 1. The amplifier output was connected to the transducer and a tie-off from the transducer was connected to the oscilloscope channel number 2. The glass tube was filled with water up to a level of 44 ram. All the equipment was switched on. The function generator was pre-set to a frequency of 41 kHz and an amplitude of 0.3 V. All the equipment was switched off. A small quantity of talcum powder was added to
Separation with ultrasonic acoustic energy
2545
..----q Glass tube with suspension t I
1 J
I~
~w,=c~1 Amplifier
Piezoelectric transducer
t
I Functiongenerator Oscilloscope
Fig. 2. Experimental set-up.
the water and the suspension stirred. All the equipment was switched on again. With a small adjustment to the frequency, clear separation of the solid(powder)/liquid(water) suspen-
sion could be observed, as illustrated in Fig. 3. As discussed, nodes and antinodes are formed at every half wavelength in a standing wave. When separation takes place at a specific frequency the solid particles
STANDING WAVE td
~
!
J
:i~.d~.~~
SuSI~nsion level _
~ q " ~ l . . - ' , , I / " . H. " ~ : b / '..k.' l I "~ & v o ~ , l g g s
"I
Fig. 3. The formation of separation levels and the measurement of wavelengths.
2546
M . C . B e k k e r et al.
will accumulate either at the nodes or antinodes depending on the density ratio. The areas where the solid particles accumulate can clearly be seen, with the naked eye, in the form of white "rings". These "rings" are referred to as "separation levels". In this case the accumulations were at the antinodes because the density of the powder is less than that of water. With the above the first objective, namely to establish a workable acoustic separation process with available equipment, was achieved. With a basic separation process established, an experiment was carried out to compare experimental results to some elementary calculations used with acoustic waves in water. The distance between every separation level reflects half a wavelength and can be measured. From the measured distance between two separation levels the wavelength can be calculated from equation (3) and the velocity of sound can be calculated. The same experimental set-up was used. The glass tube was filled with different levels of the powder-water suspension. The first fill was up to a level of 25 mm and tested. The second fill was up to a level of 48 mm, the third up to 71 ram, etc. The frequency was adjusted after every fill until clear separation could be observed. The number of separation levels was counted in each case and listed in Table 1. Since powder is less dense than water, separation takes place at the antinodes. Therefore, when two separation levels are counted it means one wavelength is formed as illustrated in Fig. 3. The sound velocity was calculated in each case and compared. The results from the experiment are given in Table 1. The velocity of sound in pure water is 1 478 m/s (Sears et al., 1982). The results obtained in the last column of Table 1 are in the region of this figure, which shows that the experimental work correlates acceptably enough with the theory to continue with further work on the established set-up. The glass tube was filled again with the suspension, clamped in a horizontal position and a frequency was applied as in the previous experiments. Separation again occurred and the powder started to settle out at the separation levels, forming "powder rings" at the bottom of the tube at intervals equal to the applied wavelength. The above experiments formed the first series of experiments in which a workable acoustic separation process was established with available equipment.
Table 1. Results from measured wavelengths and calculated wave speed Suspension level (mm) 25 48 71 92 121 152
Frequency (kHz)
Separation levels
Wavelength (mm)
44.2 42.2 43 42.2 41.2 39.9
2 3 5 6 8 9
25 32 28.4 30.7 30.3 33.8
Wave speed (m/s) 105 350 221 549 246 348
In the second series of experiments a fixed ratio of powder to water was prepared to carry out controlled tests to quantify the effect of flow and acoustic power on the effectiveness of separation. For these experiments a piezoelectric transducer with an optimum frequency of 36 kHz was used. The experimental set-up was modified to cater for the flow of suspensions through the separation section. The modification consists of the introduction of an inlet and an outlet nozzle to which the suspension supply source, in this case the suspension container, and outlet sample bottle were connected. A schematic layout of the revised set-up and procedure is explained below and shown in Fig. 4. A 25-1itre glass container was filled with a fixed concentration of powder-water (1.2 g/litre powder) suspension and placed at an elevated position. A magnetic stirring device was placed at the bottom for continuous mixing of the suspension to ensure a uniform concentration. The container was coupled to the same glass tube used before in a horizontal position via a rubber tube, and flow was controlled with a tube clamp. The outlet was routed to a sample bottle. Filter papers were numbered, dried in a laboratory drying oven and weighed. Each numbered filter was allocated to a specific sample to be taken. Before initializing the separation equipment, a control sample was taken in order to calibrate the suspension. A sample of 17 ml of the suspension was taken and filtered through the dried filter paper. The filtered paper was dried again and weighed. The net mass difference was divided by the sample volume to get the concentration of the sample. The result of this calibration test agreed with the original make-up suspension of 1.2g/litre. The accuracy of the concentration measurement was sufficient for this type of experiment; if higher accuracy is required the concentration could be measured turbidimetrically. Although it might have been easier to measure suspension concentrations turbidimetrically, such equipment was not available to the authors. All the separation equipment was activated and the experiments commenced. Samples were taken approximately every 30 s from the glass tube outlet in sample bottles of 17 ml each. The samples were filtered and the filtered samples were dried again for a minimum of 4 h. The filter samples were weighed again and the difference in mass was tabled. From this information the concentration at the outlet was calculated, since the sample volumes were known. From the results it could be seen whether separation took place and whether any solids were contained in the glass tube at the separation levels. Initially the effect of the voltage applied to the separation effectiveness was measured. A fixed flow rate of 16.7ml/s was established and different voltages were applied. Tests were repeated with 2, 4, 6, 8, 10 and 12 V. The maximum allowable input
Separation with ultrasonic acoustic energy
2547
25 litm Suspension contoiner
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voltage for the transducer is 12 V (it must be noted that acoustics are generally measured in voltages as output from transducers, therefore explaining the use of volt instead of watts). The results from the experiments indicated that separation did take place and that the separation process does indeed work. The best results were with the voltage amplitude set at 12V. A graphical summary of the res,lts is given in Fig. 5,
1.5 1.4 1.3 1.2 z 1.1 O i1 < 0.9
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From the comparison it is clear that maximum separation took place with an amplitude of 12 V and with an average concentration of 0.683 g/litre, which reflects a separation effectivity of 43% when compared to the original concentration of 1.2g/litre. However, a high voltage does not necessarily mean better separation., Depending on the solids' characteristics, agglomeration can be disturbed, due to shear stress, at certain voltages, which could explain the uneven trend in Fig. 5. This phenomenon was also experienced by Muralidhara et al. (1988). For further experimental work, 12 V was used in all cases since it is the amplitude at which the best results were obtained, although it may not be the optimum value, and experiments with higher voltages would have been preferred. The rate at which the suspension flows through the separation glass tube could influence the effectivity of separation. The next series of experiments investigated the effect of flow rate (all in the laminar flow region). The voltage was fixed at 12 V and the same procedures as previously used were followed to measure the initial and final concentration levels at different flow rates. From the results obtained in these experiments it was clear that the best separation took place at the lowest flow of 2.5 ml/s, with an average concentration
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level of 0.425 g/litre (65% separation). A graphical summary of the results is given in Fig. 6. From these experiments it was clear that the most effective separation takes place at the lowest possible flow. From the second series of experiments it was clear that the most effective separation, for the available equipment, was at an amplitude of 12 V and as low as possible flow rate. In the third series of experiments the optimum separation settings, as were found in the first and second series of experiments, were applied to effluents from a process cooling water system. These effluents were process cooling water, cooling water blow-down and ash water. The experiments were carried out following the same procedure as previously where the concentrations were established before and after separation. The separation of process cooling water was complicated by the size of the suspended solids. Suspended particles in the cooling water are extremely small in size and can hardly be seen. The filtration process, to measure the solids' concentration, takes twice as long as with the powder suspension due to the particles that block the pores of the filter paper. The filter paper pore size is approximately 0.1/am and is generally used in similar applications in water research laboratories. The suspension concentration of process cooling water was measured at 1.696 g/litre average. The flow rate was set at 2.5ml/s and 12V applied. Samples of 25 ml were taken and the suspension concentration at the outlet was measured again. The results proved that effectively no separation took place. This could be because of the fact that the solids in suspension are too fine to be concentrated at the separation points in the glass tube. The solids are in
fact so small that they can hardly be observed with the naked eye. The same procedures as before were followed with cooling water blow-down and again the results were "disappointing", as effectively no separation took place. The average measurement even showed an increase in concentration and the question arose whether there was maybe a concentration at a separation point which "came loose" and went through at a high concentration level. With ash water, again no separation took place. As with the above two suspensions the suspended solids could hardly be seen with the naked eye, owing to the extremely small sizes. The third and final series of experiments proved to be disappointing, in the sense that virtually no separation took place in any of the tested substances. The highest applied voltage was used and a very low flow of 2.5ml/s was maintained. The lack of separation can be due to the extremely small sizes of the suspended particles, in some cases so small that they could hardly be observed visually. CONCLUSION From the experiments, it is evident that solidliquid separation can be achieved by means of a standing acoustic wave. Separation takes place at the nodes or antinodes, dependent on the density ratio. If the solids are less dense than the fluid, the solid particles accumulate at the antinodes and vice-versa. The actual distances between the separation levels compare well with the theoretical distances, which confirm the practical application of the experimental apparatus. With a 36-kHz piezoelectric transducer and a control suspension of water and talcum powder, the
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Separation with ultrasonic acoustic energy best separation results were achieved at a voltage amplitude of 12 V and a flow rate of 2.5 ml/s. A separation effectivity of 65% was achieved under those conditions. The 2.5-ml/s was the lowest measurable flow rate that could be achievext. It could be possible to achieve even better separation results with lower flow rates. The 12-V amplilmde was the highest possible voltage that could be applied through the transducer. However, higher voltage does not necessarily mean better separation. A too high voltage can break up agglomerated partic]les at the separation levels owing to imposed covibration. The optimum separation voltage may differ between suspensions and care must be taken when selecting the voltage amplitude. Separation of solid-liquid suspension in a cooling water system proved to be unsuccessful due to the very small sizes of t:he solids in suspension. In order to migrate the solids towards the nodes or antinodes, higher voltage amplitude should be experimented with. As the particle sizes decrease it becomes more "difficult" to move the particles through the fluid, owing to the increased effect of fluid viscosity and smaller exposure area for acoustic force. Higher input voltage should assist in overcoming the viscous, or Stokes' force, in the fluid. From the theoretical and experimental studies it is clear that the selecl:ion of separation equipment is vital to achieve efl'ective separation. Not enough information is currently available to optimize
acoustic separation equipment selection, and extensive research could still be done. REFERENCES
Beard R. E. and Muralidhara H. S. (1985) Mechanistic Considerations of Acoustic Dewatering Techniques, Ultrasonic Symposium. Bongert W. (1976) Method and means for separation of liquids from a mixture of solids and liquids. U.S. Patent No 3,970,552. Furedi P. (1977) Apparatus and method for removing fine particles from a liquid medium by ultrasonic waves. U.S. Patent No 4,055,491. Happ¢l J. and Brenner H. (1983) Low Reynolds Number Hydrodynamics. Kluwer Academic, King L. V. (1934) Proceedings of the Royal Society of London, Vol. CXLVII. Harrison & Sons, London. Kowalski E., Chmura K. and Bien J. (1978) Ultrasonics in the dehydration of sludge. Ultrasonics 16, 183-185. Landau L. D. and Lifshitz E. M. (1987) Fluid Mechanics, 2nd edn, Vol. 6, Course of Theoretical Physics, Pergamon Press, Lighthill J. (1990) Waves in Fluids. Cambridge University Press, Cambridge, UK. Muralidhara H. S., Senapati N. and Beard R. (1988) Electro-acoustic dewatering (EAD) a novel approach for food processing and recovery. Sep. Sei. Teehnol. 23, 2143-2158. Sears F. W., Zcmansky M. W. and Joung H. D. (1982) University Physics, Edition 6, Addison-Wesley, Swamy K. M., Rao A. R. K. and Narasimhan K. S. (1983) Acoustics aids dewatering. Ultrasonics 21, 280-286. Oakley H. R., Philpott J. A. and Abdaila Z. B. (1954) The use of ultrasonic vibrations in public health engineering. Proc. Inst. Civil Engrs (Part III) 3(1), 283-288.