PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Comparison of Cavitation Effect in Case of Fixed and Free Fibers in
an Ultrasound Beaker
Taraka PAMIDI 1; Örjan JOHANSSON1; Torbjörn LÖFQVIST 1
1Luleå University of Technology, Sweden

ABSTRACTThis study investigate the impact of high-intensity ultrasound treatment on the mechanical properties of pulp
fibers. The pulp fiber samples are sonicated in an acoustically optimized beaker where high-intensity
ultrasound is generated using a tuned sonotrode device. The idea is to create a controlled resonance toefficiently enhance the sound pressure in the beaker. Input power was 90 W. The objective is to define thedifference between freely suspended fibers in a beaker compared to keeping fibers in a fixed position. The
hypothesis is that fiber treatment at a specific input power will be more efficient in the case when fibers are
kept in a high pressure zone. Since the fiber wall is a layered structure, it is likely to delaminate internallywhich will affect the mechanical properties of the fiber. The effect on fiber properties is verified by measuring
the ultrasound attenuation spectra for the treated fibers. The attenuation measurements are based on
measurements of a low-intensity ultrasound pulse-echo technique. On a macroscopic scale, changes in theattenuation spectra relates to a change in mechanical properties of the fiber wall, since the suspended fibers
more or less retain their diameter and length distributions.
Keywords: Ultrasonic, Cavitation, Paper pulp, Cellulose fibers

1. INTRODUCTION

Paper is used worldwide as an everyday commodity that is around us in a spectra of applications,from packaging to construction materials and from bills to receipts. Paper is nowadays generally madefrom different species of wood but has historically been made from other fibrous materials ever since
the paper was first produced around 200 years BC. Today, paper making is a complex, highly
automated large scale process industry to continuously evolve to meet the increasing demand for
different pulp and paper products.
Pulp and paper production requires a significant part of the total electrical power usage in Sweden.
In the production process the most energy-intensive step is refining, which is used to develop desirable
fiber properties of the pulp before it is used for production in a paper machine. Therefore, methods
achieving a reduction of energy use in the process of refining and development of appropriate fiber
properties are highly interesting. The primary idea behind this study is to investigate if new, moreenergy efficient, techniques for fiber development could be realized by the use of hydrodynamic andacoustic cavitation. To study this, a flow through cavitation reactor has been developed in a previous
project (1), as a potential alternative to conventional refining.
In the refining process, the proposed ultrasound technology shows potential improvement sincecavitation is relatively gentle to fibers and lead to increased fibrillation without fiber cutting action.
Ultrasound refining could be an alternative to conventional refining but is not yet used by the industry,see e.g. (2,3). However, despite some promising results, earlier research show a non-consistency andfurther investigations are therefore motivated.
The main objective of this paper is to evaluate two different principles of ultrasonic treatment for1

taraka.pamidi@ltu.se
orjan.johansson@ltu.se
tlt @ltu.se

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fibrillation of cellulose fibers. The first principle refers to an ultrasound batch reactor where fibers
are freely floating in the beaker. The second principle requires fibers to be fixed in the pressure
antinode of the beaker volume. The fiber sample is concentrated to a small container fixed in the mostintensive cavitation zone of the beaker. One important aspect is to optimize the reactor setup, which
combines several resonance phenomena. The idea is to minimize loss of energy transfer from thepiezo-driven horn through water volume to cavitation on fiber surfaces. One challenge, in particular,
was to optimize the cavitation reactor concerning the effect fiber material has on sound propagation.
The aim is to develop an optimization methodology for the design of new reactor types for
improvement of energy efficiency by controlled cavitation (1).

2. EXPERIMENTAL SETUP
2.1 Sonoreactor
Figure 1 shows the experimental set-up of the reactor which is composed of a cylindrical glass
beaker of 71 mm in inner diameter. The beaker is filled with water representing 5/4th of the acoustic
wavelength of the fluid. The water volume was 330ml. The middle plane of the sample container isplaced 40mm from the bottom. The sample container is used to keep the fibers under test fixed in the
acoustic anti-node of the fluid volume. The beaker volume is acoustically excited by a sonotrode at afrequency of 20.8 kHz. The distance between the tip of the sonotrode and bottom corresponds
approximately to one wavelength in the fluid (74 mm). The dimensions of the beaker are chosen toenhance the vertical mode and suppress the radial mode (26.5 kHz). The breathing mode of the beakeris at 18.5 kHz. The exact value of the sonotrode is tuned for each experiment since the wavelength
varies depending on the experimental conditions.

+Figure 1 Sonoreactor used for pulp treatment with pressure mode highlighted.
2.2 Apparatus used
The assessment of the reactor cavitation performance requires measurements of the frequency
response of sound and vibration signals; input electrical power, electrical impedance; and verificationof sound pressure mode shapes. Frequency response measurement in the time and frequency domains
were made by the software and hardware (CLIO, f s=192 kHz). The pressure response was measured
at 1-80VA electric input power. The pressure sensor (Dytran model 2200V1) and miniature shock
accelerometer (PCB 353M15) were conditioned by a B&K NEXUS 2692 amplifier. The pressuretransducer has a frequency range of 300 kHz. The pressure response in the time domain has also beenmonitored using a 1 MHz bandwidth oscilloscope.

3. Experimental Methods
3.1 Aluminum foil test
The aluminum foil test is a common method for characterizing the sound field in an ultrasoundreactor. Hydrophones or chemical dosimeters (4) are other methods for measuring cavitation activities.

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In this study aluminum foil and hydrophone tests were used.
For the aluminum foil test the beaker was filled with 0.330 liters of water prior to treatment. The
test material consisted of 128 circular aluminum film discs, 5 mm in diameter and a thickness of 16µm. The foil discs were dropped into the beaker to freely float around during the first test. In thesecond test the foil discs were placed in the sample container (see Figure 1). The aluminum foil discs
were taken out and photographed for observation of the damage pattern after exposure. The aluminum
foil discs were subjected to the same optimized excitation frequency and electrical energy input.
3.2 Calorimetric test
The input acoustic power in the liquid can be determined by a number of methods. Calorimetry is
a common but precise method (5 - 7) to measure the power dissipation in liquids. The dissipated power
due to cavitation depends on the acoustic and thermal properties of the fluid. The temperature increase
with time (dT/dt) is proportional to the input power. By compensating for heat transfer losses to thesurrounding, the power dissipation and efficiency of the ultrasound reactor can be determined, seeequations 1-3.
𝑃𝑃𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 = 𝐶𝐶𝑝𝑝 𝑀𝑀

𝑑𝑑𝑑𝑑
𝑑𝑑𝑑𝑑

(1)

Where C P is the specific heat capacity (Jkg −1 K−1 ) of the liquid, M is the liquid mass (kg), and dT
represents the temperature change over the time interval dt.
The power density of ultrasound is defined by Equation (2)
𝛱𝛱 =

𝑃𝑃𝐴𝐴
𝑉𝑉

(2)

The volume of the ultrasonic vessel or reactor is defined either by the liter (L) or (cm3 ). Pdiss is
proportional to the applied acoustic power (PA). The power density unit can, therefore, be either W/Lor W/cm3 . Subsequently, the reactors power conversion efficiency is determined byƞ=

𝑃𝑃𝐴𝐴
𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃(𝑊𝑊)(3)

The properties of the material used in the calorimetric test are shown in the following table. The
water absorbed most of the heat dissipated through the cavitation. Moreover, the beaker was heatinsulated to minimize the heat transfer to the materials in contact with water. The heat dissipation to
the surrounding materials was measured with the ultrasound off. This is an approximation of the true
heat dissipation to the surrounding materials. During cavitation, the fluid is subjected to motion which
increases the heat transfer efficiency. Therefore, true power conversion efficiency was slightly higher
than the estimated values.
Table 1. Physical properties of the material in use for a calorimetric test.
Material used in a calorimetric setup

Specific heat capacitanceDensity of material

Mass(JKg/K)

3(kg/m )

(kg)

Aluminum

336.3

2700

0.02

Water

4190.2998.270.33

Quartz glass

700

2200

0.05

3.3 Fiber qualities
The fibers used in the experiments were hardwood fibers in never-dried form. Hardwood pulp
(birch), totally chlorine-free (TCF) having a mean length of 0.932 mm (8). These fibers were not pretreated before exposure to ultrasound. After ultrasonic cavitation processing, the samples wereanalyzed with respect to various fiber properties and the energy used for processing.
The fiber experiments were carried out in three ways: (i) fibers freely floating in the beaker, (ii)fibers fixed in the small container and (iii) no treatment. Only one pulp quality was used. The pulp
was industrially manufactured hardwood pulp, sampled at the end of the bleaching process. The detailsof the experiments are tabled in Table 2.

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Table 2 the experimental design for beaker reactor (335 ml) where fibers are free and fixed.
Test

Type of treatment

Nr

Conc

fUS

%

kHz

signalTemp

EXP Time

Pinput*

°C

sec

VArms

REF

free

0.43

20.9

-

-

-

-

BEXP1

free

0.32

20.9

sine

50-52

167

45

BEXP2

free

0.49

20.5-21.4

chirp

50-51

163

43

BEXP3

free

0.33

20.5-21.5

chirp

51-52

322

43

BEXP4

free

0.40

20.9

sine

50-52

295

45

BEXP5

free

0.33

20.5-21.5

chirp

50-51

92

76

BEXP6

fixed

0.55

20.6

sine

49-50

188

40

BEXP7

fixed

0.55

20.5-21.4

chirp

50-51

150

47

BEXP8

fixed

0.64

20.5-21.5

chirp

51-52

360

38

BEXP9

fixed

0.59

20.6

sine

50-52

333

40

fixed
0.50
20.5-21.5
chirp
51-52
* The chirp signal peak power is more than two times higher than the rms power.

91

77

BEXP10

4. Results and discussions
4.1 Aluminium foil test
. The aluminum foil test was performed for both of the test conditions (free/fixed) at the same levelof electrical power input to the sonotrode. The exposure time was 33 seconds and the electrical input
power was 75 WRMS. Irradiation of the water filled beaker was based on an excitation signal with apure sinusoidal frequency of 20.8 kHz at 22 oC.
The beaker experiments, both for freely floating foil discs and in a fixed position, gave very minuteerosion, which can almost not be seen with the naked eye. However, the foil pieces in a fixed position
were slightly more affected. When an aluminum foil is exposed longer time, a greater erosion effectwill be observed. In case of a flow-through type of reactor evaluated in parallel, a more pronounced
effect was found (1).

Figure 2 Aluminum foil discs sonicated by 33 sec in the beaker reactor. (a) Freely dropped (b) In container.
4.2 Calorimetric test
The glass beaker was filled with 330 ml of water at 55o C and the fluid was sonicated with puresine signal at 20.8 kHz and an input power of 51.5 Wrms to the sonotrode. The heat insulated beakerremained static until a thermal balance at 45o C was achieved. The ultrasound was then activated for
20 minutes and at each fifth minute, the beaker's temperature rise was recorded with a precision of
0.1o C by using two thermocouple thermometers. Figure 3 showed a nearly linear 20 minutes rise in8204

temperature and exponential 20-minutes decay in temperature when the ultrasound was turned off.
The acoustic power was then determined by Eq. 1. The experiment was repeated to verify the reaction
stability. With a power conversion efficiency of 25%, the acoustic input power was found to be 13.2
W (Eq 3). The corresponding power density was 0.16 W/cm3 (Eq 2). However, the acoustic pressure
varies extensively within the water volume due to the standing wave, and acoustic properties of the
beaker.

Figure 3. Electric input power to the beaker reactor and corresponding internal temperature variations.
4.3 Fiber treatment
The fiber treatment conditions are summarized in Table 3. The average fiber geometrical properties
were characterized by a pulp analyzer. The impact on pulp quality was determined by measuring theultrasound absorption according to a validated method (9). The experimental pulp used for theverification of the cavitation effect on fibers properties was a never dried type of birch fiber. The fiber
suspension concentration was 0.3-0.7%, see Table 3.
The results in Figures 4 to 7 shows that the ultrasound controlled cavitation can alter the fiber
properties, both in the case of fixed fibers and free fibers. Figure 4 and 5 shows the results of allcompleted absorption measurements characterizing changes in the fibers mechanical properties.
Table 3 Energy conversion in the conducted experimental series
Test

Fiber

fUS

signalposition
Nr

kHz

Ref

Temp

EXP-

Electic

conc

interval

time

Power

fibers

°C

s

W

%

EnergyEnergy/ton
dry fiber

kWs

kWh/bdt

0.43

21

BExp1

Free

20.9

sine

50-52

167

45

0.32

7515

1941

BExp2

Free

20.5-21.4

chirp

50-51

163

43

0.49

6993

1191

BExp3

Free

20.5-21.4

chirp

51-52

322

43

0.33

13813

3452

BExp4

Free

20.9

sine

50-52

295

45

0.40

13275

2711

BExp5

Free

20.5-21.4

chirp

50-51

92

76

0.33

6992

1773

BExp6

Fixed

20.6

sine

49-50

188

40

0.55

7520

1140

BExp7

Fixed

20.5-21.4

chirp

50-51

150

47

0.55

7050

1068

BExp8

Fixed

20.5-21.4

chirp

51-52

360

38

0.64

13788

1784

BExp9

Fixed

20.6

sine

50-52

333

40

0.59

13320

1851

BExp10Fixed

20.5-21.4

chirp

51-52

91

77

0.50

7007

1166

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Figure 4 shows that the spread within a specific pulp sample is small, except for sample "BExp2"
where absorption is reduced. Test BExp1 and BExp5 have approximately the same attenuation. BExp3
has the greatest attenuation, which indicates a clear change of fiber properties at an energy equivalent
to 3452 kWh / bdt. In case of Test BExp3 the fiber suspension was exposed by a chirp signal within
the frequency band of 20.5-21.5 kHz. Increased attenuation relates most likely to internal and external
fibrillation, as other fiber parameters do not change significantly by ultrasonic treatment. However,test BExp2 has a significantly lower absorption which may be related to lower sonification energy,
since the sonification signal type is the same. However, Figure 5 shows that BExp2 deviates from theother samples by having a greater mean width and a shorter mean length. In some earlier experiments,
absorption values lower than the reference was related to damage of fiber walls. If that the case here,
it may be related to the chirp signal, which creates a pulse like rise of acoustic power. At 60°C thecellulose structure is still brittle and could be damaged by misguided and too powerful cavitation.

Figure 4. Measured absorption of fiber suspensions treated in a beaker. The change of the normalized
attenuation rate indicates a change of the fiber mechanical properties.

Figure 5. Fiber length and width distribution of fibers moving freely during sonification.
The fiber types tested have been analyzed with Lorentzen&Wettre pulp analyzer, see Figure 5.
After the cavitation treatment, fiber suspensions were tested and analyzed to see define the size andshape distributions. Figure 5 shows that all samples in case of freely moving fibers except BExp 4 has
an increase fiber width. BExp2 has the most pronounced change in fiber properties likely related to a

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destruction of the fibers walls, since the absorption was lower than reference. Test BExp1, BExp3 andBExp5 were almost equal in geometrical distribution, see Figure 6.

Figure 6. Length and width distribution of test samples: Ref, BExp1, BExp3, and BExp 5.
Figure 7 shows the result of sonification of fixed fibers, denoted test BExp6 to Bexp10. Test BExp6
has the greatest attenuation, which indicates a clear change of fiber properties at an energy equivalent
to 1140 kWh/bdt. In case of Test BExp6 the fiber suspension was exposed by a sinusoidal signal at
20.6 kHz. Increased attenuation relates most likely to internal and external fibrillation, as other fiber
parameters do not change significantly by ultrasonic treatment. Test BExp10 has a lower absorption,that could be related to a delamination of the fibers walls, caused by the chirp signal with highest rms
power. The chirp signals characteristics having a sudden increase of power when hitting the systems
resonance frequency, may have exceeded the suitable cavitation intensity and caused fiber wall
destruction.

Figure 7. Measured absorption of fiber suspensions sonicated in a container fixed in the pressure anti-nodeof the beaker. The change of the normalized attenuation rate indicates a change of the fiber mechanicalproperties. Reduced absorption indicates deteriorated fiber properties.
Figure 8 shows the comparison of the cavitation effect with fixed and free fibers in an ultrasoundbeaker. The maximum absorption was with test BExp3 and BExp6. The power used for theseexperiments was 3452 kWh/bdt (BExp3) and 1140 kWh/bdt (BExp6). This proves that with fibers at a fixed
position and using pure sinusoidal signal give most energy efficient change of fiber properties.

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Figure 8. Comparison of measured absorption at 5MHz. Free fibers (BExp1 - 5). Fixed fibers (BExp6 - 10).

5. CONCLUSIONS

The results of a small scale experiment on the effectiveness of resonance amplified cavitation withrespect to hardwood fiber fibrillation gave promising results. The hypothesis that fixed fibers in an
acoustic antinode would be more effectively processed is validated. Foil test results indicated that
there is only a minor difference between free and fixed in the acoustic antinode. The reason for smaller
effect using foil discs is because they were not as fixed as the fibers were, and by that, the discs weremoved to the circumference by the max pressure in the center of the container. In the fiber tests, thecontainer was packed and only allowed for minor movement of the fibers. The change in absorptionof the fiber mass verifies a significant modification of the fibers mechanical properties for all samples.
The increase was most pronounced in the case when fibers are fixed in the acoustic pressure anti-nodein the beaker. The fibers were also more evenly modified when fixed in the beaker as indicated by theabsorption spectrum.

ACKNOWLEDGEMENTS

The project was funded by Swedish Energy Agency and VINNOVA, and has been supported by ÅF,
SCA, Stora Enso, Holmen, Innventia AB, TU Dresden and Mid Sweden University. The pulp samples
and the fiber analysis were supplied by Smurfit Kappa Kraftliner, Piteå. Sweden.

REFERENCES

1. Ö. Johansson, T. Pamidi, T. Löfqvist. Design of a high-intensity ultrasound reactor. 2017 IEEE
International Ultrasonics Symposium (IUS) 6-9 Sept. 2017 Washington, DC, USA
2. Thompson R, Manning A. A review of ultrasound and its applications in papermaking. Prog Pap Recycl.
2005;14(2):26–42.
3. Laine J, MacLeod J, Bolker H, Goring D. Applications of ultrasound in pulp and paper technology. Pap
ja puu, 1977.
4. Ley S V., Low CMR. Ultrasound in Synthesis . Berlin, Heidelberg: Springer Berlin Heidelberg; 1989.
5. Contamine RF, Wilhelm AM, Berlan J, Delmas H. Power measurement in sonochemistry. Ultrason
Sonochem. 1995 Jan 1;2(1):S43–7.
6. Margulis IM, Margulis MA. Measurement of acoustic power in studying cavitation processes. Acoust
Phys. 2005 Nov;51(6):695–704.
7. Kimura T, Sakamoto T, Leveque J-M, Sohmiya H, Fujita M, Ikeda S, et al. Standardization of ultrasonicpower for sonochemical reaction. Ultrason Sonochem. 1996 Nov 1 3(3):S157–61.
8. Paper and textile from pulp. Available from: https://www.sodra.com/en/pulp/products/
9. Aitomäki, Yvonne, and Torbjörn Löfqvist. "Estimating suspended fibre material properties by modelling
ultrasound attenuation." AIP Conference Proceedings. Vol. 834. No. 1. AIP, 2006.

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