- Email: [email protected]
Detection and quantification of insoluble particles by ultrasound spectroscopy
Ultrasonics 43 (2005) 231–235 www.elsevier.com/locate/ultras
Detection and quantification of insoluble particles by ultrasound spectroscopy q T.I.J. Goodenough *, V.S. Rajendram, S. Meyer, D. Preˆtre Nestle´ Research Centre, Vers-chez-le-Blanc, Lausanne CH-1000, Switzerland Available online 23 July 2004
Abstract The Ultrafood system, a custom-built diagnostic ultrasound device, is used to accurately measure the concentration of particulate matter in a fluctuating high temperature liquid system. The two main problems, of thermal expansion and thermal variation in ultrasonic outputs were tackled by multi-distance measurement and low frequency spectroscopy, respectively. The resulting techniques have application at laboratory, scale for investigation of particulate suspensions and for online process monitoring. 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafood; Attenuation spectroscopy; Insoluble particles
1. Introduction The development of a non-destructive, rapid and reliable analytical method that is sensitive to the detection and quantification of particulate suspensions is of considerable industrial interest. Filtration and centrifugal separation methods are widely practiced, however these methods cannot be used for real time measurement or in situ, these techniques are also time consuming and difficult to implement on-line. This paper details the application of ultrasound techniques for quantification of particulates suspended in a liquid substrate for the purpose of investigating the kinetics of production and solution on a laboratory scale, but also with a view to potential online application. The restrictions on the systems are to:
q This article is based on a presentation given at the Ultrasonics International 2003. * Corresponding author. E-mail address: [email protected] (T.I.J. Goodenough).
0041-624X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2004.06.009
• Accurately quantify the amount of insolubles present in the liquid. • Function up to 150 C and under the resulting pressure, a maximum of approximately 6 bar. These conditions are industrially relevant and interesting. • Maintain accuracy whilst the cell temperature is increased.
2. Experimental 2.1. System description The Ultrafood system is a broadband pulse device for diagnostic studies of liquid food materials at laboratory scale, which was designed, built and tested at the Nestle´ Research Centre in Lausanne [1]. The system measures amplitude and time of flight at multiple distances and the rate of change of these is used to calculate attenuation and speed of sound. A PC based MATLAB workspace controls the transducer movement, operation and calculation and then presents the results graphically to
232
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
the user. The results produced by the Ultrafood system are unaffected by thermal expansion of the cell and so is perfect for monitoring a system which is subjected to fluctuating temperatures. The phase velocity, c(x), and attenuation spectrum, a(x), are calculated from the Fourier transforms of pulses received from multiple distances, for example taking two distances with pulses A(x1, t), A(x2, t), at angular frequency x: argfAðx1 ; xÞg argfAðx2 ; xÞg x 1 m s1 ðx2 x1 Þ Aðx1 ; xÞ 1 Np m1 aðxÞ ¼ ln ðx2 x1 Þ Aðx2 ; xÞ
c1 ðxÞ ¼
ð2Þ
Buffer rod x2
A1 or 2 Moveable reflector plate Cooling flow Transducer
It is well know that attenuation is the better property for characterising dispersed phase composition and particle size, while speed of sound is better for characterising chemical compositions at a molecular level [2]. The attenuation value at any particular frequency has two component parts, intrinsic, a1, and excess, a2, attenuation relating to the dispersed and continuous phases, respectively. aTotal ¼ a1 þ a2 ð1 /Þ
ð1Þ
where arg {. . .} is the angle and j. . .j is the magnitude of the complex frequency signal. The hardware described in [1] was customised to allow operation at high temperatures; an insulating buffer rod (Solid polymer PEEK, 20 mm in diameter and 5 mm in thickness) and cooling system were added to prevent the transducer from being damaged, Fig. 1. To reduce the complexity of cooling a reflector plate replaced the second, moveable, transducer and the system set to pulse/echo mode. The seals and fittings were upgraded to withstand the high pressure and further insulation added to prevent the motor and gearing units from over-heating. The signal generation and reception was by a UTEX UT340 pulser (UTEX, Canada, set to 25 dB gain, 100 V, width 5 nS) connected to a commercially available 10 MHz transducer (Krautkramer, Germany, 10 MHz, 10 mm diameter). The temperature was varied by an external bath circulating oil through the double wall of the cell; direct temperature measurement was restricted to less than 100 C, the limit of the PT100 probe, and above this the set temperature of the bath was taken as that of the cell with a slight offset. The cell self pressurised with the increase in temperature.
x1
2.2. Measurement principles
Heating flow
Fig. 1. Schematic of the Ultrafood system. A pulse, A(t), travels distance x through the sample, reflects off the moveable reflector plate and is received at the transducer. The cell is heated by an oil flow, while the transducer is cooled by a separate water flow.
ð3Þ
where / is the volume fraction of the dispersed phase. Assuming that the intrinsic attenuation remains constant the total attenuation value is proportional to the concentration of insolubles [2]. Speed of sound is inversely proportional to the density, q, and compressibility, e, of the sample: 1 c ¼ pffiffiffiffiffi eq
ð4Þ
Therefore this value can be correlated to the amount of dissolved solids in the sample [3]. The ability to use a single technique to investigate the insoluble, dissolved and total solids is one of the strengths of diagnostic ultrasound. ItÕs worth noting that both these parameters vary with temperature, which must be accounted for in any calculation.
2.3. Sample preparation and method The aim of this work was to validate the system and to establish a reliable method for measuring the concentration of particles formed during industrial processing. To achieve this varying dilutions of two particulate samples were used; sample A, example industrial particles and sample B, reference spherical silica. All percentage values quoted are weight-by-weight measures. Sample A was obtained in the form of a moist paste, the product of industrial centrifugation, which had 50.8% total solids of which 49.56% were insoluble at room temperature with a mono-dispersed particles size distribution of average 0.3 lm. Distilled water was added to make a range of samples 0–2% of insolubles and the amount of soluble solids kept constant at 0.5% by adding sucrose, this was done to prevent any ambiguity of correlation, see Table 1. Sample B, the reference silica colloid was a standard reference material from Malvern (UK) with 40% total solids and a mono-dispersed particle size distribution of 0.3 lm, a good match to the other sample. The colloid was diluted and sucrose added to have a mix of insoluble and soluble components, this could be used to test the accuracy of attenuation and speed of sound at measuring solid concentration in the different phases. The dilu-
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
233
Table 1 Components of the sample fluids used in experiments, sample A is predominantly a mix of industrially obtained particles while sample B is solution of silica and sucrose Name
Constant component
Varying component
Sample A Industrial
0.5% Su + Ips
0% Ip
0.24% Ip
0.54% Ip
0.81% Ip
1.08% Ip
1.62% Ip
Sample B Varying silica
5% Su
0% Si
4.4% Si
5.5% Si
7.4% Si
8.4% Si
12% Si
Sample B Varying sucrose
4.4% Si
0% Su
5% Su
7% Su
10% Su
11% Su
12% Su
2.16% Ip
13% Su
14% Su
15% Su
Su = sucrose, Si = silica, Ip = industrial particle, Ips = soluble component of the industrial particle mix. The soluble component of sample A was kept constant by adding enough sucrose to make the total soluble solids 0.5%. All solutions were mixed with distilled water. Bold enteries indicate the solutions used to produce the results displayed in Fig. 4.
tions were done so that the amount of silica varied between 0% and 12% with 5% sucrose and the amount of sucrose ranged between 0% and 15% with 4.4% silica, see Table 1. The solutions were all prepared at room temperature and mixed thoroughly before being added to the test cell, where a magnetic stirrer kept the particulate matter in suspension. Temperature changes were carried out incrementally with readings taken only when the temperature had stabilised, which at high temperatures could be observed using the speed of sound. Each point displayed is an average of 30 readings taken over a 20 min period.
3. Results and discussion The attenuation when measuring sample A, industrial particles, increases with concentration and decreases with temperature, with greater effect at higher frequency, see Fig. 2. Therefore there is the ambiguity that once the cell temperature is increased from room temperature, is the change in attenuation representative
of a change in concentration, due to dissolving or settling particles, or a thermal variation in the ultrasonic output? Conventional solids measurements are only possible just above room temperature and so it is impossible to check. Therefore it was desirable to find a way of removing the temperature dependency of the attenuation value, so that any variation represents a physical change in the sample. Comparison of the two parts of Fig. 2 indicates that below 6 MHz the attenuation value is independent of temperature but responsive to particulate concentration; therefore the attenuation value at 5 MHz was used as a single representative value. The stability of this value for a constant solution type increasing in temperature, but responsiveness to concentration change is discussed later in this section. The phase velocity was found to be approximately frequency independent with both sample types and so the single 5 MHz value was also taken as the output. The variation in speed of sound with temperature is significant [4], which makes this output a less sensitive measure of the system and so attenuation is the primary tool for directly measuring the concentration of particles.
60
60 0% 0.24 % 0.81 % 1.08 %
40 30 20 10 0 -10
25ºC 30ºC 50ºC
50
Attenuation (Np/m)
Attenuation (Np/m)
50
40 30 20 10 0
2
4
6
8
10
Freq (MHz)
12
14
16
-10 2
4
6
8
10
12
14
16
Freq (MHz)
Fig. 2. Attenuation spectra with sample A, solutions made up with the industrial particles. Left: varying concentration of particles from 0% to 1.08% measured at 25 C. Note the increase in attenuation with increased concentration, with greater effect at higher frequencies. Right: the variation with temperature of the 1.08% particle solution. Increased temperature reduces attenuation with greater effect at higher temperatures.
234
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235 1530 16
1525
Attenuation (Np/m)
12
Speed of Sound (m/s)
Sample A, Industrial particles Sample B, Varying Silica Sample B, Varying sucrose
14
10 8 6 4
1520 1515 1510 1505 1500 1495
2
1490
0
1485
-5
0
5
10
15
Sample A, Industrial particles Sample B, Varying Silica Sample B, Varying sucrose
1480 -5
20
0
5
10
15
20
Solids (% wt/wt)
Solids (% wt/wt)
Fig. 3. 5 MHz attenuation and speed of sound measurements for varying concentrations of samples A and B. The X-axis is the % solids of the varying component of each sample. Left: attenuation. Right: speed of Sound.
therefore making real time investigation of the system easy and graphical. Comparison between the results from silica and industrial particles indicates that different particle types can have very different attenuation characteristics, with the industrial particles significantly more attenuative and so any calibration must be particle type specific. An example application of this system, to monitor the physical properties of particulate systems during heating is illustrated in Fig. 4. The temperature range was 25– 150 C with samples of 0.81% industrial particles, 4.4% silica and distilled water. The attenuation values produced with water as a sample highlights the stability of the 5 MHz value with less than 0.2 Np/m change over the temperature range with acceptable repeatability. The attenuation from the silica sample increases slightly with temperature, which was attributed to changes in the attenuative properties of the silica with temperature. However the industrial particles give a marked decrease in attenuation from 70 C onwards in temperature, due
Sample B, made with silica and sucrose, was used to test the independence of the experimental variables of concentration of insoluble and soluble solids with the ultrasonic parameters of attenuation and speed of sound, respectively. Fig. 3 displays these results, and shows that the attenuation is proportional to the concentration of silica, but independent to that of sucrose. Similarly the speed of sound only increases proportionally to the sucrose concentration. The values shown are taken from the attenuation spectrum at 5 MHz. The results with sample A are also displayed in Fig. 3, the attenuation increases proportionally to the particulate concentration and a linear relationship could be achieved and used as a calibration. The soluble solids were kept constant and so there is no trend in the speed of sound response, the scatter comes from the slightly varying composition of an industrial sample. The MATLAB GUI, which controls the Ultrafood system, was customised so that, if desired, the output displays physical quantities of components and not ultrasonic values, 7
1550 Sample A, 0.81% Industrial particles Sample B, 4.4% Silica Colloid Distilled water
1540
Speed of Sound (m/s)
Attenuation (Np/m)
6 5 4 3 2 1 0 20
1530 1520 1510 1500 1490 Sample A, 0.81% Industrial particles Sample B, 4.4% Silica Colloid Distilled water
1480 40
60
80
100
Temp (ºC)
120
140
160
1470
20
40
60
80
100
120
140
160
Temp (ºC)
Fig. 4. Ultrasonic measurement of the samples over temperature, up to a maximum of 150 C. Sample A is the 0.81% insoluble particles solution while sample B is 4.4% silica particles with no added sucrose. Left: attenuation, note the decrease in attenuation of the industrial particles once the temperature is >70 C, this was attributed to a change in solubility. Right: speed of sound.
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
dissolution of the particles at higher temperature to the extent that at 150 C the industrial particles are almost completely soluble, with an attenuation value close to that of water. 5 MHz speed of sound for these samples are also displayed in Fig. 4, with the values from all three samples displaying the characteristic maximum at approximately 70 C and reducing at higher temperatures. The silica and water values are very similar while the industrial particles sample is slightly offset; this is due to the small density difference of the continuous phase from the dissolved solids that may increase at higher temperatures as the insoluble particles dissolve. The speed of sound output could be normalised using the water value, but this is very susceptible to temperature control and measurement, which is difficult to achieve accurately once the temperature is greater than 100 C and the temperature probe ceases to give an accurate output.
4. Conclusion The judicious use of the ultrasonic output from the Ultrafood system allows the user to accurately measure the concentration of particulate matter in fluctuating
235
temperature systems. This is made possible by a number of factors: firstly, the measurement method of the Ultrafood system, which does not require reference fluid calibration and so is not susceptible to thermal expansion of the cell. Secondly, the good correlation between attenuation and particulate concentration and lastly, the consistency of the ultrasonic output at lower frequencies for a given concentration of particulates over a range of temperatures. This system was used to investigate the change in solubility of particles formed during an industrial process and can be used further to investigate other physical properties at laboratory scale, or online.
References [1] T.I.J. Goodenough, V.S. Rajendram, S. Meyer, D. Preˆtre, Development of a multi frequency pulse diagnostic ultrasound device, Ultrasonics 43 (2005) in press. [2] A.S. Dukhin, P.J. Goetz, Ultrasound for characterising liquid based food products, Dispersion Technology Inc. Publication, 1999. [3] J.N. Coupland, Low intensity ultrasound, Food Res. Int. 37 (2004) 537–543. [4] V.A. Del Grosso, C.W. Mader, Speed of sound in pure water, J. Acoust. Soc. Am. 52 (1972) 1442–1446.
Detection and quantification of insoluble particles by ultrasound spectroscopy q T.I.J. Goodenough *, V.S. Rajendram, S. Meyer, D. Preˆtre Nestle´ Research Centre, Vers-chez-le-Blanc, Lausanne CH-1000, Switzerland Available online 23 July 2004
Abstract The Ultrafood system, a custom-built diagnostic ultrasound device, is used to accurately measure the concentration of particulate matter in a fluctuating high temperature liquid system. The two main problems, of thermal expansion and thermal variation in ultrasonic outputs were tackled by multi-distance measurement and low frequency spectroscopy, respectively. The resulting techniques have application at laboratory, scale for investigation of particulate suspensions and for online process monitoring. 2004 Elsevier B.V. All rights reserved. Keywords: Ultrafood; Attenuation spectroscopy; Insoluble particles
1. Introduction The development of a non-destructive, rapid and reliable analytical method that is sensitive to the detection and quantification of particulate suspensions is of considerable industrial interest. Filtration and centrifugal separation methods are widely practiced, however these methods cannot be used for real time measurement or in situ, these techniques are also time consuming and difficult to implement on-line. This paper details the application of ultrasound techniques for quantification of particulates suspended in a liquid substrate for the purpose of investigating the kinetics of production and solution on a laboratory scale, but also with a view to potential online application. The restrictions on the systems are to:
q This article is based on a presentation given at the Ultrasonics International 2003. * Corresponding author. E-mail address: [email protected] (T.I.J. Goodenough).
0041-624X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2004.06.009
• Accurately quantify the amount of insolubles present in the liquid. • Function up to 150 C and under the resulting pressure, a maximum of approximately 6 bar. These conditions are industrially relevant and interesting. • Maintain accuracy whilst the cell temperature is increased.
2. Experimental 2.1. System description The Ultrafood system is a broadband pulse device for diagnostic studies of liquid food materials at laboratory scale, which was designed, built and tested at the Nestle´ Research Centre in Lausanne [1]. The system measures amplitude and time of flight at multiple distances and the rate of change of these is used to calculate attenuation and speed of sound. A PC based MATLAB workspace controls the transducer movement, operation and calculation and then presents the results graphically to
232
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
the user. The results produced by the Ultrafood system are unaffected by thermal expansion of the cell and so is perfect for monitoring a system which is subjected to fluctuating temperatures. The phase velocity, c(x), and attenuation spectrum, a(x), are calculated from the Fourier transforms of pulses received from multiple distances, for example taking two distances with pulses A(x1, t), A(x2, t), at angular frequency x: argfAðx1 ; xÞg argfAðx2 ; xÞg x 1 m s1 ðx2 x1 Þ Aðx1 ; xÞ 1 Np m1 aðxÞ ¼ ln ðx2 x1 Þ Aðx2 ; xÞ
c1 ðxÞ ¼
ð2Þ
Buffer rod x2
A1 or 2 Moveable reflector plate Cooling flow Transducer
It is well know that attenuation is the better property for characterising dispersed phase composition and particle size, while speed of sound is better for characterising chemical compositions at a molecular level [2]. The attenuation value at any particular frequency has two component parts, intrinsic, a1, and excess, a2, attenuation relating to the dispersed and continuous phases, respectively. aTotal ¼ a1 þ a2 ð1 /Þ
ð1Þ
where arg {. . .} is the angle and j. . .j is the magnitude of the complex frequency signal. The hardware described in [1] was customised to allow operation at high temperatures; an insulating buffer rod (Solid polymer PEEK, 20 mm in diameter and 5 mm in thickness) and cooling system were added to prevent the transducer from being damaged, Fig. 1. To reduce the complexity of cooling a reflector plate replaced the second, moveable, transducer and the system set to pulse/echo mode. The seals and fittings were upgraded to withstand the high pressure and further insulation added to prevent the motor and gearing units from over-heating. The signal generation and reception was by a UTEX UT340 pulser (UTEX, Canada, set to 25 dB gain, 100 V, width 5 nS) connected to a commercially available 10 MHz transducer (Krautkramer, Germany, 10 MHz, 10 mm diameter). The temperature was varied by an external bath circulating oil through the double wall of the cell; direct temperature measurement was restricted to less than 100 C, the limit of the PT100 probe, and above this the set temperature of the bath was taken as that of the cell with a slight offset. The cell self pressurised with the increase in temperature.
x1
2.2. Measurement principles
Heating flow
Fig. 1. Schematic of the Ultrafood system. A pulse, A(t), travels distance x through the sample, reflects off the moveable reflector plate and is received at the transducer. The cell is heated by an oil flow, while the transducer is cooled by a separate water flow.
ð3Þ
where / is the volume fraction of the dispersed phase. Assuming that the intrinsic attenuation remains constant the total attenuation value is proportional to the concentration of insolubles [2]. Speed of sound is inversely proportional to the density, q, and compressibility, e, of the sample: 1 c ¼ pffiffiffiffiffi eq
ð4Þ
Therefore this value can be correlated to the amount of dissolved solids in the sample [3]. The ability to use a single technique to investigate the insoluble, dissolved and total solids is one of the strengths of diagnostic ultrasound. ItÕs worth noting that both these parameters vary with temperature, which must be accounted for in any calculation.
2.3. Sample preparation and method The aim of this work was to validate the system and to establish a reliable method for measuring the concentration of particles formed during industrial processing. To achieve this varying dilutions of two particulate samples were used; sample A, example industrial particles and sample B, reference spherical silica. All percentage values quoted are weight-by-weight measures. Sample A was obtained in the form of a moist paste, the product of industrial centrifugation, which had 50.8% total solids of which 49.56% were insoluble at room temperature with a mono-dispersed particles size distribution of average 0.3 lm. Distilled water was added to make a range of samples 0–2% of insolubles and the amount of soluble solids kept constant at 0.5% by adding sucrose, this was done to prevent any ambiguity of correlation, see Table 1. Sample B, the reference silica colloid was a standard reference material from Malvern (UK) with 40% total solids and a mono-dispersed particle size distribution of 0.3 lm, a good match to the other sample. The colloid was diluted and sucrose added to have a mix of insoluble and soluble components, this could be used to test the accuracy of attenuation and speed of sound at measuring solid concentration in the different phases. The dilu-
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
233
Table 1 Components of the sample fluids used in experiments, sample A is predominantly a mix of industrially obtained particles while sample B is solution of silica and sucrose Name
Constant component
Varying component
Sample A Industrial
0.5% Su + Ips
0% Ip
0.24% Ip
0.54% Ip
0.81% Ip
1.08% Ip
1.62% Ip
Sample B Varying silica
5% Su
0% Si
4.4% Si
5.5% Si
7.4% Si
8.4% Si
12% Si
Sample B Varying sucrose
4.4% Si
0% Su
5% Su
7% Su
10% Su
11% Su
12% Su
2.16% Ip
13% Su
14% Su
15% Su
Su = sucrose, Si = silica, Ip = industrial particle, Ips = soluble component of the industrial particle mix. The soluble component of sample A was kept constant by adding enough sucrose to make the total soluble solids 0.5%. All solutions were mixed with distilled water. Bold enteries indicate the solutions used to produce the results displayed in Fig. 4.
tions were done so that the amount of silica varied between 0% and 12% with 5% sucrose and the amount of sucrose ranged between 0% and 15% with 4.4% silica, see Table 1. The solutions were all prepared at room temperature and mixed thoroughly before being added to the test cell, where a magnetic stirrer kept the particulate matter in suspension. Temperature changes were carried out incrementally with readings taken only when the temperature had stabilised, which at high temperatures could be observed using the speed of sound. Each point displayed is an average of 30 readings taken over a 20 min period.
3. Results and discussion The attenuation when measuring sample A, industrial particles, increases with concentration and decreases with temperature, with greater effect at higher frequency, see Fig. 2. Therefore there is the ambiguity that once the cell temperature is increased from room temperature, is the change in attenuation representative
of a change in concentration, due to dissolving or settling particles, or a thermal variation in the ultrasonic output? Conventional solids measurements are only possible just above room temperature and so it is impossible to check. Therefore it was desirable to find a way of removing the temperature dependency of the attenuation value, so that any variation represents a physical change in the sample. Comparison of the two parts of Fig. 2 indicates that below 6 MHz the attenuation value is independent of temperature but responsive to particulate concentration; therefore the attenuation value at 5 MHz was used as a single representative value. The stability of this value for a constant solution type increasing in temperature, but responsiveness to concentration change is discussed later in this section. The phase velocity was found to be approximately frequency independent with both sample types and so the single 5 MHz value was also taken as the output. The variation in speed of sound with temperature is significant [4], which makes this output a less sensitive measure of the system and so attenuation is the primary tool for directly measuring the concentration of particles.
60
60 0% 0.24 % 0.81 % 1.08 %
40 30 20 10 0 -10
25ºC 30ºC 50ºC
50
Attenuation (Np/m)
Attenuation (Np/m)
50
40 30 20 10 0
2
4
6
8
10
Freq (MHz)
12
14
16
-10 2
4
6
8
10
12
14
16
Freq (MHz)
Fig. 2. Attenuation spectra with sample A, solutions made up with the industrial particles. Left: varying concentration of particles from 0% to 1.08% measured at 25 C. Note the increase in attenuation with increased concentration, with greater effect at higher frequencies. Right: the variation with temperature of the 1.08% particle solution. Increased temperature reduces attenuation with greater effect at higher temperatures.
234
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235 1530 16
1525
Attenuation (Np/m)
12
Speed of Sound (m/s)
Sample A, Industrial particles Sample B, Varying Silica Sample B, Varying sucrose
14
10 8 6 4
1520 1515 1510 1505 1500 1495
2
1490
0
1485
-5
0
5
10
15
Sample A, Industrial particles Sample B, Varying Silica Sample B, Varying sucrose
1480 -5
20
0
5
10
15
20
Solids (% wt/wt)
Solids (% wt/wt)
Fig. 3. 5 MHz attenuation and speed of sound measurements for varying concentrations of samples A and B. The X-axis is the % solids of the varying component of each sample. Left: attenuation. Right: speed of Sound.
therefore making real time investigation of the system easy and graphical. Comparison between the results from silica and industrial particles indicates that different particle types can have very different attenuation characteristics, with the industrial particles significantly more attenuative and so any calibration must be particle type specific. An example application of this system, to monitor the physical properties of particulate systems during heating is illustrated in Fig. 4. The temperature range was 25– 150 C with samples of 0.81% industrial particles, 4.4% silica and distilled water. The attenuation values produced with water as a sample highlights the stability of the 5 MHz value with less than 0.2 Np/m change over the temperature range with acceptable repeatability. The attenuation from the silica sample increases slightly with temperature, which was attributed to changes in the attenuative properties of the silica with temperature. However the industrial particles give a marked decrease in attenuation from 70 C onwards in temperature, due
Sample B, made with silica and sucrose, was used to test the independence of the experimental variables of concentration of insoluble and soluble solids with the ultrasonic parameters of attenuation and speed of sound, respectively. Fig. 3 displays these results, and shows that the attenuation is proportional to the concentration of silica, but independent to that of sucrose. Similarly the speed of sound only increases proportionally to the sucrose concentration. The values shown are taken from the attenuation spectrum at 5 MHz. The results with sample A are also displayed in Fig. 3, the attenuation increases proportionally to the particulate concentration and a linear relationship could be achieved and used as a calibration. The soluble solids were kept constant and so there is no trend in the speed of sound response, the scatter comes from the slightly varying composition of an industrial sample. The MATLAB GUI, which controls the Ultrafood system, was customised so that, if desired, the output displays physical quantities of components and not ultrasonic values, 7
1550 Sample A, 0.81% Industrial particles Sample B, 4.4% Silica Colloid Distilled water
1540
Speed of Sound (m/s)
Attenuation (Np/m)
6 5 4 3 2 1 0 20
1530 1520 1510 1500 1490 Sample A, 0.81% Industrial particles Sample B, 4.4% Silica Colloid Distilled water
1480 40
60
80
100
Temp (ºC)
120
140
160
1470
20
40
60
80
100
120
140
160
Temp (ºC)
Fig. 4. Ultrasonic measurement of the samples over temperature, up to a maximum of 150 C. Sample A is the 0.81% insoluble particles solution while sample B is 4.4% silica particles with no added sucrose. Left: attenuation, note the decrease in attenuation of the industrial particles once the temperature is >70 C, this was attributed to a change in solubility. Right: speed of sound.
T.I.J. Goodenough et al. / Ultrasonics 43 (2005) 231–235
dissolution of the particles at higher temperature to the extent that at 150 C the industrial particles are almost completely soluble, with an attenuation value close to that of water. 5 MHz speed of sound for these samples are also displayed in Fig. 4, with the values from all three samples displaying the characteristic maximum at approximately 70 C and reducing at higher temperatures. The silica and water values are very similar while the industrial particles sample is slightly offset; this is due to the small density difference of the continuous phase from the dissolved solids that may increase at higher temperatures as the insoluble particles dissolve. The speed of sound output could be normalised using the water value, but this is very susceptible to temperature control and measurement, which is difficult to achieve accurately once the temperature is greater than 100 C and the temperature probe ceases to give an accurate output.
4. Conclusion The judicious use of the ultrasonic output from the Ultrafood system allows the user to accurately measure the concentration of particulate matter in fluctuating
235
temperature systems. This is made possible by a number of factors: firstly, the measurement method of the Ultrafood system, which does not require reference fluid calibration and so is not susceptible to thermal expansion of the cell. Secondly, the good correlation between attenuation and particulate concentration and lastly, the consistency of the ultrasonic output at lower frequencies for a given concentration of particulates over a range of temperatures. This system was used to investigate the change in solubility of particles formed during an industrial process and can be used further to investigate other physical properties at laboratory scale, or online.
References [1] T.I.J. Goodenough, V.S. Rajendram, S. Meyer, D. Preˆtre, Development of a multi frequency pulse diagnostic ultrasound device, Ultrasonics 43 (2005) in press. [2] A.S. Dukhin, P.J. Goetz, Ultrasound for characterising liquid based food products, Dispersion Technology Inc. Publication, 1999. [3] J.N. Coupland, Low intensity ultrasound, Food Res. Int. 37 (2004) 537–543. [4] V.A. Del Grosso, C.W. Mader, Speed of sound in pure water, J. Acoust. Soc. Am. 52 (1972) 1442–1446.