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Influence of electronic antifouling on agglomeration of calcium carbonate
Powder Technology 206 (2011) 201–207
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Influence of electronic antifouling on agglomeration of calcium carbonate W.N. Al Nasser a,b,⁎, A.H. Al Ruwaie b, M.J. Hounslow a, A.D. Salman a a b
Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield-S1 3JD, United Kingdom Saudi Aramco Company, Dhahran 31311, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 7 July 2010 Accepted 15 July 2010 Available online 24 July 2010 Keywords: Electronic antifouling Inline Calcium carbonate Precipitation Agglomeration Scaling
a b s t r a c t Scaling represents a serious problem to many industries. Scaling in pipes leads to an increased pressure drop and often to complete blockage. On heat exchangers, it reduces the heat transfer. It may also lead to unstable operation which could result in unscheduled shutdown and loss of revenue. One of the most common types − of scaling is due to calcium carbonate. Calcium carbonate scaling occurs when Ca++ and CO− ions in water 3 react to form an insoluble solid. This research is focused on understanding the behaviour of calcium carbonate agglomeration and scaling in the presence and absence of electronic antifouling (EAF). The novelty of this work is that an inline in situ monitoring technique is utilized to obtain real-time data of the calcium carbonate deposition on a surface. The calcium carbonate crystals are obtained by standard precipitation and agglomeration processes and the formation and deposition of the crystals is monitored using an inline technique known as focused beam reflectance measurement (FBRM). The experimental work has been designed to understand the effect of EAF on precipitation, agglomeration and scaling of calcium carbonate at given calcium ion concentration and solution temperature. The scaling is characterized by measuring the rate of number of crystals deposited on a surface of the FBRM which, unlike mass measurement as used by previous workers, makes this method unique. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Scaling causes loss of production time, deterioration of equipment, increased energy consumption and loss of turnover. Scale formation is the precipitation of sparingly soluble salts, most commonly calcium carbonate which form an insoluble deposit on surfaces. Preventing scaling using magnetic treatment could not only reduce the scaling potential of the water but also cause the existing scale to dissolve over an extended period of time [1]. However, at very high levels of supersaturation, Antiscale magnetic treatment (AMT) became ineffective [2]. The effect of magnetic field strength on the scaling rate has also been studied [3]; the precipitation rate and agglomeration increased with increasing magnetic flux. Other than inducing direct magnetic flux, the use of solenoids has also been reported for scale mitigation. Electronic antifouling (EAF), a green method of scale mitigation, has potential to reduce the major problems associated with scale formation. Therefore, understanding the characteristics of calcium carbonate scaling in the presence and absence of EAF is crucial in this study. Adapting the experimental setup used for the reference as shown in Al Nasser et al. [4], an experimental program has been designed to understand the effect of EAF on the precipitation, agglomeration and scaling of calcium ⁎ Corresponding author. Saudi Aramco Company, Dhahran 31311, P.O Box 9761, Saudi Arabia. Tel.: +96650582 5152; fax: +9663 876 8870. E-mail address: [email protected] (W.N. Al Nasser). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.07.010
carbonate at a given calcium ion concentration and solution temperature. The scaling was characterized by measuring the rate of the number of crystals deposited on a window of the FBRM which, unlike mass as used by previous workers, makes this method unique. FBRM has been used to monitor the progress of bulk solution crystallization, nucleation, growth rate and agglomeration in standard process such as crystallization, flocculation and emulsification. It is manufactured by METTLER TOLEDO and has D600L model. FBRM can measure particles in the range of 0.5–1000 μm allowing superior quantification of the rate and degree of change to particle dimensions shape and number. It focuses a 780 nm laser near the instrument's sapphire window and rotates at an angular velocity of 2 ms−1. Particle dimensions and particle counts are measured in terms of chord length distributions (CLD) based on the laser beam reflectance technology and FBRM detects and transforms the reflected light into an electronic signal. By characterizing the real size distribution in the system, the crystal size distribution can be attained in real-time for batch process. Further, SEM will be used to study the morphology and scale shapes of CaCO3 in the presence and absence of EAF. 2. Experimental setup and procedure The experimental setup is according to the schematic diagram in Fig. 1. It consists of a 2-l beaker placed on a hot plate with inline instrument FBRM immersed at an angle along with a temperature probe. The sensors are connected to a PC to record the number of
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W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
The supersaturation values in this study were calculated by Oklahoma scale software. Experiments with & without electrical field application were carried out for duration of 60 minutes. The experiments in absence of ultrasound are termed as Blank experiments henceforth.
2.2. Electronic antifouling procedure
Fig. 1. Schematic of the experimental setup.
crystals and temperature of the solution. A cover plate was used to cover the beaker to avoid any evaporation. It consists of two metal plates with a rubber plate in the middle. The metal plate has several openings to allow probes access to the solution. The position of probes in the solution was fixed for each experiment. A magnetic stirrer was used to mix the solution and hence keep the crystals suspended in the solution.
An electronic antifouling (EAF) device (Water Descaler device) was developed and produced in-house. The device was able to generate a current through the solution using a custom made solenoid. The descaler device could generate a 0–300 mA current in the frequency range of 50 to 500 Hz. One hundred meters of wire was required to fully wrap around the 2000 ml beaker, as shown in Fig. 2; the wire was 0.9 mm in diameter. The experimental procedure has already been detailed above, the EAF experiments mixing, solution temperature and calcium ion concentration were similar to that for the reference experiments for effective comparison, but they were conducted in the presence of an EAF device and solenoid wire around the crystallizer. The system was worked at current of 300 mA and 50–500 Hz frequency.
3. Determining precipitation and scale kinetics by online measurement 3.1. Precipitation Kinetics
2.1. Experimental procedure The experimental procedure is as follows: first, solutions required for the precipitation of calcium carbonate were prepared. The sources of calcium ions and carbonate ions were (CaCl2 ·2H2O) and (NaHCO3). The chemicals were provided by Analar and the purity was N99%. The chemicals for the above solutions were weighed using Sartorius weighing machine with accuracy up to four decimal places and were put in a separate beaker. One liter of deionized water was added to each beaker and the salts were dissolved. The solutions were filtered using a 2.5 μm filter paper (Whatman) to prevent any impurities that could act as nuclei and hence affecting the crystallization process. The beaker was placed on a hot plate as shown in Fig. 1 set at the required solution temperature. Solutions A (calcium ions) and B (bicarbonate ions) were poured into the crystallizer. At this moment, the FBRM and temperature measurements were started simultaneously. The number and size of particles within the solution were recorded after every 2 seconds by FBRM. Particle dimensions and counts are measured in terms of CLD and they are based on the focus or boundary of FBRM which is 2 mm range that covers the area of reflection. This area generally represents the area of particles in the system. Mixing was brought about by a magnetic stirrer at a speed 1100 rpm to give good mixing in order to keep all the crystals in suspension without settling. The experiments were conducted at the same stirring speed and various calcium ion concentrations from 0.01 to 0.05 mol/l at 25 °C temperature. S is the supersaturation ratio of these solutions. For calcium carbonate, S can be expressed as
S=
" #1=ν aCa2+ ⋅aCO2− 3
Ksp
aCa2 + = activity of calcium ion aCO23 − = activity of carbonate ion Ksp = solubility product of calcium carbonate ν = number of ions = 2
The mixing of two solutions resulted in spontaneous precipitation of calcium carbonate. FBRM measures chord size distribution of the crystals present in the solution. This is reflected by the instantaneous rise in number of chord counts of crystals detected by FBRM. FBRM is studied for many particulate applications. Various authors has reported the use of FBRM in crystallization process [5,6]. The data obtained from FBRM can be further processed to obtain the total number of chord counts, mean size and other statistics. The detailed method of obtaining the mean size and growth rate of calcium carbonate crystals from the chord size distribution using population balance equation and method of moments is reported by [4]. In summary the precipitation kinetics of calcium carbonate can be defined by two rate processes known as nucleation and growth rate.
ð1Þ
Fig. 2. Solenoid wrapped around the 2000 ml beaker.
W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
The moment form of population balance equation for nucleation and growth is given as min dmj 0 j j −mj = −jGmj−1 −B l0 θ dt
ð2Þ
th moment at the inlet of system min j =j mj = jth moment at the outlet of system θ = space velocity B0 = nucleation rate G = growth rate l0 = size of a crystal j = moment index t = time The above equation can be further simplified for Batch system as follows
dmj 0 j −jGmj−1 −B l0 = 0 dt
ð3Þ
3.1.1. Nucleation rate The expression of nucleation rate in terms of moment form can be obtained by substituting the moment index j = 0 in Eq. 3. Hence, the rate of change of zeroth moment with time is indication of nucleation rate (B 0). The nucleation rate ( J) in terms of chord size is written as Eq. 4. 0
B =
dm0 d α0 mC0 =J = dt dt
Scale rate =
ΔNi ti + 1 −ti
203
ð8Þ
where m0 is the number of chords before cleaning, m0′ the number of chords after cleaning, and (ti + 1−ti) is the total time of the scaling period. The FBRM probe was taken out at certain intervals to measure the scaling on the probe and it appears that the probe stopped measuring the actual particles in the solution. Once the probe was cleaned from the particles sticking on the sapphire, it measures the particles in the solution. As a result, the supersaturation will be changed due to scaling. More details about the procedure and measuring the scale rate was describe by Al Nasser et al. [8]. 4. Results The total number of chord counts measured by the FBRM probe for the reference and EAF cases has been displayed in Fig. 3A–B; the calcium ion concentrations were 0.02 and 0.04 mol/l, respectively. The results show that by increasing the calcium ion concentration, the chord count also increased. Further, the chord counts for the EAF case were lower for both solutions during the experiments. For the 0.04 mol/l case, it was evident that the number of chord counts would remain the same after 60 minutes. The chord (1,0) mean size has been plotted in Fig. 4A–B under the same conditions. The EAF case had a higher mean chord size, and after
ð4Þ
α0 = proportionality constant mC0 = chord-based zeroth moment 3.1.2. Growth rate Randolph and Larson [7] showed that the growth rate G can be determined from following equation: P d l1;0 d m1 G= = ð5Þ dt m0 dt m1 = first moment of particle size distribution m0 = zeroth moment of particle size distribution P l1;0 = 1,0 mean size based on particle size distribution The growth rate (Gc) based on chord size can be written as [4,5,7] 1 0 P dl d @α1 mC1 A = C ð1;0Þ Gc = dt α m dt 0
ð6Þ
C0
α1 = proportionality constant mC0 = zeroth moment based on chord size distribution mC1 = first moment based on chord size distribution 3.2. Deposition kinetics The calcium carbonate formed during precipitation readily deposits on the sapphire window of the FBRM probe, thus affecting the FBRM measurements. In order to measure the actual number of chord counts in the solution, the FBRM probe was removed from the solution and the probe surface was cleaned. The total number of chords measured after the cleaning was monitored. The number of crystals deposited on the surface of the probe can be determined by the difference in number of chords measured before cleaning and after cleaning. The scaling rate is calculated by following relationship: ′
Total number of particles deposited = ΔNi = m0 −m0
ð7Þ
Fig. 3. A-B. The total number of chords for 0.02 and 0.04 mol/l for the reference and EAF case.
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Fig. 6. The mean chord size for 0.01-0.05 mol/l for the reference and EAF case.
size, where its presence results in a little larger chord size compared to the reference case. 4.1. Effect of EAF on chord counts in various size classes
Fig. 4. A-B. The mean chord size for 0.02 and 0.04 mol/l for the reference and EAF case.
60 minutes of testing it was 19 μm at 0.02 mol/l and 22 μm at 0.04 mol/l. With increasing calcium ion concentration, the total number and mean size of chords are derived at the end of 60 minutes of testing. The results are presented in Fig. 5. With increasing calcium ion concentration, the total number of chord counts also increased. Examining the reference case and EAF case at any calcium ion concentration, the chord counts measured in the presence of EAF were found to be lower. In addition, examining Fig. 6, the effect of EAF is more apparent for the chords (1,0) mean
The number of chord counts were split into three different size classes that ranged between 1 and 5 μm, 5 and 23 μm and 23 and 86 μm. Fig. 7A–B shows the results for the reference case and with EAF treatment. For the reference case in Fig. 7A, the number of chord counts increased with time for all three size classes; the 5–23 μm size class dominated the results. However, for the EAF case in Fig. 7B, the number of chord counts for the same size class was lower at 5000 (#). The 23–86 μm size class was higher for the EAF case as was the difference. This may be due to the molecular agitation caused by the electrical field. Fig. 8 shows a representative plot for the total number of chord counts versus time measured by FBRM probe during calcium carbonate precipitation. The data has been smoothed according to the methodology reported by Al Nasser et al. [4]. The number of chord counts increased with time for the reference experiment at a 0.02 mol/l calcium ion concentration; a similar trend was observed in the presence of EAF. However, the total number of chord counts in the presence of EAF was found to be less than for the reference case. After repeating the experiments, the difference in the number of chord counts between the reference (B) and EAF (E) conditions was not significant; this can be seen in Fig. 9. The total number on the ordinate after 15 minutes of the process and the vertical bars indicate the average values of the numbers measured for 21 experiments. The average values show that the reference case contained a slightly higher number of chord counts, the error bars were calculated as the ratio of standard deviation to root of number of measurements. The error bars for each condition overlapped each other; hence, any significant difference in the results could not be concluded. 4.2. Effect of EAF on scaling rate
Fig. 5. The total number of chords for 0.01-0.05 mol/l for the reference and EAF case.
In the present work, the scaling rate was calculated from online measured data using FBRM. The detailed procedure to calculate online scale rate is detailed in the work done by Al Nasser et al. [8]. It shows the online scaling rate obtained from FBRM in terms of number of crystals deposited per minute. The experiment was performed at 0.02 mol/l calcium ion concentration and at 25 °C solution temperature. The values presented in Fig. 10 were the averages obtained from 21 experiments. The difference in the average results shows a
W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
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Fig. 9. Effect of EAF on total number of chord counts for 0.02 mol/l calcium ion concentration at 25 °C.
4.3. Nucleation rate Using the number of chord counts obtained from the FBRM and utilizing Eq. 4, the nucleation rate, J, was plotted against relative supersaturation, as shown in Fig. 11. With increasing relative supersaturation, the nucleation increased linearly. Examining the results further, there was little variation between the reference case and EAF case at 300 mA and 50–500 Hz. 4.4. Growth rate
Fig. 7. A-B. Number of chords for various size classes at 0.04 mol/l for the reference and EAF case.
significant difference in the scaling rate between the reference (B) and EAF (E) conditions. In the presence of EAF, the number of particles per minute which is the unit of the scale rate of calcium carbonate deposited is lower than the reference case, where the scale rate decreased to 2075 particles/min; agreeing with the literature [9].
Fig. 8. Effect of EAF for 0.02 mol/l calcium ion concentration at 25 °C.
Using Eq. 6, the growth rate was calculated. The average growth rate was plotted against relative supersaturation with varying concentration and solution temperature, as shown in Fig. 12. With increasing relative and square relative supersaturation, the growth rate was found to increase. The growth rate was higher for the EAF case, confirming the result that the mean chord size was also higher when compared to the reference case. 4.5. Scale rate The scaling rate was calculated using Eqs. 7 and 8 and is plotted in Figs. 13 and 14 against time and relative supersaturation, respectively. Due to crystal deposition on the FBRM probe, the scaling rate for the reference case was high for the first 10 minutes of testing; what followed was an exponential decrease with time, as shown in Fig. 13.
Fig. 10. Effect of EAF on scaling rate for 0.02 mol/l calcium ion concentration at 25 °C.
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Fig. 11. Nucleation rate versus relative supersaturation for the reference and EAF case at 25 °C.
Fig. 13. Scale rate for the reference and EAF case.
5. Discussion and conclusion Additionally, with increasing relative supersaturation, the scaling rate also increased for the reference case and EAF. Also, it shows that in the reference case, the scale rate is higher than in the presence of EAF at any given relative supersaturation, as shown in Fig. 14.
4.6. The Effect of EAF on the morphology of crystals Previous literature has documented that calcite forms at temperatures up to 40 °C, aragonite has been found to form at temperatures exceeding 40 °C, and vaterite was seen to form under at all conditions [10,11]. In this study, the crystal morphology of calcium carbonate under various conditions for the reference case and in the presence of EAF is presented in Fig. 15. At 25 °C, a noticeable difference was observed in crystal morphology between the reference case and EAF case. Utilizing EAF, calcite was converted to vaterite, therefore becoming easier to remove—if required. When the temperature was increased to 60 and 80 °C, no significant difference was found between the two cases. At 60 °C, SEM images revealed flower-shaped vaterite with minor traces of calcite. At 80 °C, the needle-shaped crystals were indicative of aragonite and furthermore, vaterite was also observed. Finally, at higher temperatures, aggregation of the crystals was witnessed, which may have been a result of the induced molecular agitation conducted by the EAF.
Fig. 12. Growth rate versus relative supersaturation for the reference and EAF case.
The present results show that there was a negligible difference in the total number of chord counts measured under EAF and reference conditions. However, the difference in the scaling rate, as measured by FBRM in terms of number of deposition of crystals per unit time, was considerable. The effect of EAF on the scaling rate was in agreement with the literature, which showed considerable evidence of scaling rate decreasing with increasing EAF [12]. The reduction in scale formation can be explained as follows [9]. The pulsing current creates a magnetic field inside the system, which induces an electrical field. This resulted in solenoid-induced molecular agitation, increasing the frequency of collision between calcium and bicarbonate ions, thereby favoring precipitation and agglomeration in the solution [13]. EAF had a more pronounced effect on the scaling process compared to the precipitation process. This could be verified further by examining the morphology of the crystals in the presence and absence of EAF. However, the number of crystals was not measured directly using SEM and as a result, the effect of EAF on the number of crystals was not very clear. The present results do not conclusively show the effect of EAF on the number of chord counts measured. Yet, EAF has a similar trend to the reference case at higher concentrations and temperatures, where the particle sizes and numbers increase as temperatures and concentrations increase in the presence and absence of EAF.
Fig. 14. Scale rate versus relative supersaturation for the reference and EAF case at 25 °C.
W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
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Fig. 15. Various SEM image of calcium carbonate crystals obtained from reference and EAF experiments.
Acknowledgments The authors are grateful to Dr. Kate Pitt, Oz McFarlane and Keith Penny for their assistance and cooperation in presenting this work. Also, much thanks to Saudi Aramco for their permission to publish this paper. References [1] J.D. Donaldson, Scale prevention and descaling, Tube InternationalJan. ed., , 1988, pp. 39–49. [2] D. Hasson, D. Bramson, Effectiveness of magnetic water treatment in suppressing CaCO3 scale deposition, Industrial and Engineering Chemistry Process Design and Development 24 (1985) 588–592. [3] F.T. Ellingsen, H. Kristiansen, Does magnetic treatment influence precipitation of calcium carbonate from supersaturated solutions, Vatten 35 (1979) 309–315. [4] W.N. Al Nasser, A. Shaikh, C. Morriss, M.J. Hounslow, A.D. Salman, Determining kinetics of calcium carbonate precipitation by inline technique, Chemical Engineering Science 63 (5) (2008) 1381–1389. [5] A. Shaikh, A.D. Salman, S. Mcnamara, G. Littlewood, F. Ramsay, M.J. Hounslow, In situ observation of the conversion of sodium carbonate to sodium carbonate
[6] [7] [8]
[9]
[10] [11]
[12]
[13]
monohydrate in aqueous suspension, Industrial and Engineering Chemistry Research 44 (2005) 9921–9930. P. Barrett, B. Glennon, In-line FBRM monitoring of particle size in dilute agitated suspensions, Particle and Particle Systems Characterization 16 (1999) 207–211. A.D. Randolph, M.A. Larson, Theory of Particulate Processes: Analysis and Techniques of Continuous Crystallization, Academic Press, London, 1971. W.N. Al Nasser, A. Shaikh, M.J. Hounslow, A.D. Salman, Determining calcium carbonate scaling kinetics by online technique, PSA 2008 Conference and Exhibition, 2nd- 4th September 08, Paper No.113, Stratford upon-Avon, UK, 2008. Y.I. Cho, Chunfu Fan, Byung-Gap Choi, Theory of electronic antifouling technology to control precipitation fouling in heat exchangers, International Communications in Heat and Mass Transfer 24 (6) (1997) 757–770. G.S. Theophylaktos, P.G. Koutsoukos, The crystallization of calcium carbonate in artificial seawater; role of the substrate, Journal of Crystal Growth 133 (1–2) (1993) 13–22. N.K. Abdel- Aal, S.K. Sawada, Study of the adhesion mechanism of CaCO3 using a combined bulk chemistry/QCM technique, Journal of Crystal Growth 245 (2002) 87–100. Chunfu Fan, Young I. Cho, Microscopic observation of calcium carbonate particles: validation of and EAF technology, International Communications in Heat and Mass Transfer 24 (1997) 747–756. J.S. Baker, S.J. Judd, Magnetic amelioration of scale formation, Water Research 30 (2) (1996) 247–260.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Influence of electronic antifouling on agglomeration of calcium carbonate W.N. Al Nasser a,b,⁎, A.H. Al Ruwaie b, M.J. Hounslow a, A.D. Salman a a b
Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield-S1 3JD, United Kingdom Saudi Aramco Company, Dhahran 31311, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 18 December 2009 Received in revised form 7 July 2010 Accepted 15 July 2010 Available online 24 July 2010 Keywords: Electronic antifouling Inline Calcium carbonate Precipitation Agglomeration Scaling
a b s t r a c t Scaling represents a serious problem to many industries. Scaling in pipes leads to an increased pressure drop and often to complete blockage. On heat exchangers, it reduces the heat transfer. It may also lead to unstable operation which could result in unscheduled shutdown and loss of revenue. One of the most common types − of scaling is due to calcium carbonate. Calcium carbonate scaling occurs when Ca++ and CO− ions in water 3 react to form an insoluble solid. This research is focused on understanding the behaviour of calcium carbonate agglomeration and scaling in the presence and absence of electronic antifouling (EAF). The novelty of this work is that an inline in situ monitoring technique is utilized to obtain real-time data of the calcium carbonate deposition on a surface. The calcium carbonate crystals are obtained by standard precipitation and agglomeration processes and the formation and deposition of the crystals is monitored using an inline technique known as focused beam reflectance measurement (FBRM). The experimental work has been designed to understand the effect of EAF on precipitation, agglomeration and scaling of calcium carbonate at given calcium ion concentration and solution temperature. The scaling is characterized by measuring the rate of number of crystals deposited on a surface of the FBRM which, unlike mass measurement as used by previous workers, makes this method unique. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Scaling causes loss of production time, deterioration of equipment, increased energy consumption and loss of turnover. Scale formation is the precipitation of sparingly soluble salts, most commonly calcium carbonate which form an insoluble deposit on surfaces. Preventing scaling using magnetic treatment could not only reduce the scaling potential of the water but also cause the existing scale to dissolve over an extended period of time [1]. However, at very high levels of supersaturation, Antiscale magnetic treatment (AMT) became ineffective [2]. The effect of magnetic field strength on the scaling rate has also been studied [3]; the precipitation rate and agglomeration increased with increasing magnetic flux. Other than inducing direct magnetic flux, the use of solenoids has also been reported for scale mitigation. Electronic antifouling (EAF), a green method of scale mitigation, has potential to reduce the major problems associated with scale formation. Therefore, understanding the characteristics of calcium carbonate scaling in the presence and absence of EAF is crucial in this study. Adapting the experimental setup used for the reference as shown in Al Nasser et al. [4], an experimental program has been designed to understand the effect of EAF on the precipitation, agglomeration and scaling of calcium ⁎ Corresponding author. Saudi Aramco Company, Dhahran 31311, P.O Box 9761, Saudi Arabia. Tel.: +96650582 5152; fax: +9663 876 8870. E-mail address: [email protected] (W.N. Al Nasser). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.07.010
carbonate at a given calcium ion concentration and solution temperature. The scaling was characterized by measuring the rate of the number of crystals deposited on a window of the FBRM which, unlike mass as used by previous workers, makes this method unique. FBRM has been used to monitor the progress of bulk solution crystallization, nucleation, growth rate and agglomeration in standard process such as crystallization, flocculation and emulsification. It is manufactured by METTLER TOLEDO and has D600L model. FBRM can measure particles in the range of 0.5–1000 μm allowing superior quantification of the rate and degree of change to particle dimensions shape and number. It focuses a 780 nm laser near the instrument's sapphire window and rotates at an angular velocity of 2 ms−1. Particle dimensions and particle counts are measured in terms of chord length distributions (CLD) based on the laser beam reflectance technology and FBRM detects and transforms the reflected light into an electronic signal. By characterizing the real size distribution in the system, the crystal size distribution can be attained in real-time for batch process. Further, SEM will be used to study the morphology and scale shapes of CaCO3 in the presence and absence of EAF. 2. Experimental setup and procedure The experimental setup is according to the schematic diagram in Fig. 1. It consists of a 2-l beaker placed on a hot plate with inline instrument FBRM immersed at an angle along with a temperature probe. The sensors are connected to a PC to record the number of
202
W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
The supersaturation values in this study were calculated by Oklahoma scale software. Experiments with & without electrical field application were carried out for duration of 60 minutes. The experiments in absence of ultrasound are termed as Blank experiments henceforth.
2.2. Electronic antifouling procedure
Fig. 1. Schematic of the experimental setup.
crystals and temperature of the solution. A cover plate was used to cover the beaker to avoid any evaporation. It consists of two metal plates with a rubber plate in the middle. The metal plate has several openings to allow probes access to the solution. The position of probes in the solution was fixed for each experiment. A magnetic stirrer was used to mix the solution and hence keep the crystals suspended in the solution.
An electronic antifouling (EAF) device (Water Descaler device) was developed and produced in-house. The device was able to generate a current through the solution using a custom made solenoid. The descaler device could generate a 0–300 mA current in the frequency range of 50 to 500 Hz. One hundred meters of wire was required to fully wrap around the 2000 ml beaker, as shown in Fig. 2; the wire was 0.9 mm in diameter. The experimental procedure has already been detailed above, the EAF experiments mixing, solution temperature and calcium ion concentration were similar to that for the reference experiments for effective comparison, but they were conducted in the presence of an EAF device and solenoid wire around the crystallizer. The system was worked at current of 300 mA and 50–500 Hz frequency.
3. Determining precipitation and scale kinetics by online measurement 3.1. Precipitation Kinetics
2.1. Experimental procedure The experimental procedure is as follows: first, solutions required for the precipitation of calcium carbonate were prepared. The sources of calcium ions and carbonate ions were (CaCl2 ·2H2O) and (NaHCO3). The chemicals were provided by Analar and the purity was N99%. The chemicals for the above solutions were weighed using Sartorius weighing machine with accuracy up to four decimal places and were put in a separate beaker. One liter of deionized water was added to each beaker and the salts were dissolved. The solutions were filtered using a 2.5 μm filter paper (Whatman) to prevent any impurities that could act as nuclei and hence affecting the crystallization process. The beaker was placed on a hot plate as shown in Fig. 1 set at the required solution temperature. Solutions A (calcium ions) and B (bicarbonate ions) were poured into the crystallizer. At this moment, the FBRM and temperature measurements were started simultaneously. The number and size of particles within the solution were recorded after every 2 seconds by FBRM. Particle dimensions and counts are measured in terms of CLD and they are based on the focus or boundary of FBRM which is 2 mm range that covers the area of reflection. This area generally represents the area of particles in the system. Mixing was brought about by a magnetic stirrer at a speed 1100 rpm to give good mixing in order to keep all the crystals in suspension without settling. The experiments were conducted at the same stirring speed and various calcium ion concentrations from 0.01 to 0.05 mol/l at 25 °C temperature. S is the supersaturation ratio of these solutions. For calcium carbonate, S can be expressed as
S=
" #1=ν aCa2+ ⋅aCO2− 3
Ksp
aCa2 + = activity of calcium ion aCO23 − = activity of carbonate ion Ksp = solubility product of calcium carbonate ν = number of ions = 2
The mixing of two solutions resulted in spontaneous precipitation of calcium carbonate. FBRM measures chord size distribution of the crystals present in the solution. This is reflected by the instantaneous rise in number of chord counts of crystals detected by FBRM. FBRM is studied for many particulate applications. Various authors has reported the use of FBRM in crystallization process [5,6]. The data obtained from FBRM can be further processed to obtain the total number of chord counts, mean size and other statistics. The detailed method of obtaining the mean size and growth rate of calcium carbonate crystals from the chord size distribution using population balance equation and method of moments is reported by [4]. In summary the precipitation kinetics of calcium carbonate can be defined by two rate processes known as nucleation and growth rate.
ð1Þ
Fig. 2. Solenoid wrapped around the 2000 ml beaker.
W.N. Al Nasser et al. / Powder Technology 206 (2011) 201–207
The moment form of population balance equation for nucleation and growth is given as min dmj 0 j j −mj = −jGmj−1 −B l0 θ dt
ð2Þ
th moment at the inlet of system min j =j mj = jth moment at the outlet of system θ = space velocity B0 = nucleation rate G = growth rate l0 = size of a crystal j = moment index t = time The above equation can be further simplified for Batch system as follows
dmj 0 j −jGmj−1 −B l0 = 0 dt
ð3Þ
3.1.1. Nucleation rate The expression of nucleation rate in terms of moment form can be obtained by substituting the moment index j = 0 in Eq. 3. Hence, the rate of change of zeroth moment with time is indication of nucleation rate (B 0). The nucleation rate ( J) in terms of chord size is written as Eq. 4. 0
B =
dm0 d α0 mC0 =J = dt dt
Scale rate =
ΔNi ti + 1 −ti
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ð8Þ
where m0 is the number of chords before cleaning, m0′ the number of chords after cleaning, and (ti + 1−ti) is the total time of the scaling period. The FBRM probe was taken out at certain intervals to measure the scaling on the probe and it appears that the probe stopped measuring the actual particles in the solution. Once the probe was cleaned from the particles sticking on the sapphire, it measures the particles in the solution. As a result, the supersaturation will be changed due to scaling. More details about the procedure and measuring the scale rate was describe by Al Nasser et al. [8]. 4. Results The total number of chord counts measured by the FBRM probe for the reference and EAF cases has been displayed in Fig. 3A–B; the calcium ion concentrations were 0.02 and 0.04 mol/l, respectively. The results show that by increasing the calcium ion concentration, the chord count also increased. Further, the chord counts for the EAF case were lower for both solutions during the experiments. For the 0.04 mol/l case, it was evident that the number of chord counts would remain the same after 60 minutes. The chord (1,0) mean size has been plotted in Fig. 4A–B under the same conditions. The EAF case had a higher mean chord size, and after
ð4Þ
α0 = proportionality constant mC0 = chord-based zeroth moment 3.1.2. Growth rate Randolph and Larson [7] showed that the growth rate G can be determined from following equation: P d l1;0 d m1 G= = ð5Þ dt m0 dt m1 = first moment of particle size distribution m0 = zeroth moment of particle size distribution P l1;0 = 1,0 mean size based on particle size distribution The growth rate (Gc) based on chord size can be written as [4,5,7] 1 0 P dl d @α1 mC1 A = C ð1;0Þ Gc = dt α m dt 0
ð6Þ
C0
α1 = proportionality constant mC0 = zeroth moment based on chord size distribution mC1 = first moment based on chord size distribution 3.2. Deposition kinetics The calcium carbonate formed during precipitation readily deposits on the sapphire window of the FBRM probe, thus affecting the FBRM measurements. In order to measure the actual number of chord counts in the solution, the FBRM probe was removed from the solution and the probe surface was cleaned. The total number of chords measured after the cleaning was monitored. The number of crystals deposited on the surface of the probe can be determined by the difference in number of chords measured before cleaning and after cleaning. The scaling rate is calculated by following relationship: ′
Total number of particles deposited = ΔNi = m0 −m0
ð7Þ
Fig. 3. A-B. The total number of chords for 0.02 and 0.04 mol/l for the reference and EAF case.
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Fig. 6. The mean chord size for 0.01-0.05 mol/l for the reference and EAF case.
size, where its presence results in a little larger chord size compared to the reference case. 4.1. Effect of EAF on chord counts in various size classes
Fig. 4. A-B. The mean chord size for 0.02 and 0.04 mol/l for the reference and EAF case.
60 minutes of testing it was 19 μm at 0.02 mol/l and 22 μm at 0.04 mol/l. With increasing calcium ion concentration, the total number and mean size of chords are derived at the end of 60 minutes of testing. The results are presented in Fig. 5. With increasing calcium ion concentration, the total number of chord counts also increased. Examining the reference case and EAF case at any calcium ion concentration, the chord counts measured in the presence of EAF were found to be lower. In addition, examining Fig. 6, the effect of EAF is more apparent for the chords (1,0) mean
The number of chord counts were split into three different size classes that ranged between 1 and 5 μm, 5 and 23 μm and 23 and 86 μm. Fig. 7A–B shows the results for the reference case and with EAF treatment. For the reference case in Fig. 7A, the number of chord counts increased with time for all three size classes; the 5–23 μm size class dominated the results. However, for the EAF case in Fig. 7B, the number of chord counts for the same size class was lower at 5000 (#). The 23–86 μm size class was higher for the EAF case as was the difference. This may be due to the molecular agitation caused by the electrical field. Fig. 8 shows a representative plot for the total number of chord counts versus time measured by FBRM probe during calcium carbonate precipitation. The data has been smoothed according to the methodology reported by Al Nasser et al. [4]. The number of chord counts increased with time for the reference experiment at a 0.02 mol/l calcium ion concentration; a similar trend was observed in the presence of EAF. However, the total number of chord counts in the presence of EAF was found to be less than for the reference case. After repeating the experiments, the difference in the number of chord counts between the reference (B) and EAF (E) conditions was not significant; this can be seen in Fig. 9. The total number on the ordinate after 15 minutes of the process and the vertical bars indicate the average values of the numbers measured for 21 experiments. The average values show that the reference case contained a slightly higher number of chord counts, the error bars were calculated as the ratio of standard deviation to root of number of measurements. The error bars for each condition overlapped each other; hence, any significant difference in the results could not be concluded. 4.2. Effect of EAF on scaling rate
Fig. 5. The total number of chords for 0.01-0.05 mol/l for the reference and EAF case.
In the present work, the scaling rate was calculated from online measured data using FBRM. The detailed procedure to calculate online scale rate is detailed in the work done by Al Nasser et al. [8]. It shows the online scaling rate obtained from FBRM in terms of number of crystals deposited per minute. The experiment was performed at 0.02 mol/l calcium ion concentration and at 25 °C solution temperature. The values presented in Fig. 10 were the averages obtained from 21 experiments. The difference in the average results shows a
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Fig. 9. Effect of EAF on total number of chord counts for 0.02 mol/l calcium ion concentration at 25 °C.
4.3. Nucleation rate Using the number of chord counts obtained from the FBRM and utilizing Eq. 4, the nucleation rate, J, was plotted against relative supersaturation, as shown in Fig. 11. With increasing relative supersaturation, the nucleation increased linearly. Examining the results further, there was little variation between the reference case and EAF case at 300 mA and 50–500 Hz. 4.4. Growth rate
Fig. 7. A-B. Number of chords for various size classes at 0.04 mol/l for the reference and EAF case.
significant difference in the scaling rate between the reference (B) and EAF (E) conditions. In the presence of EAF, the number of particles per minute which is the unit of the scale rate of calcium carbonate deposited is lower than the reference case, where the scale rate decreased to 2075 particles/min; agreeing with the literature [9].
Fig. 8. Effect of EAF for 0.02 mol/l calcium ion concentration at 25 °C.
Using Eq. 6, the growth rate was calculated. The average growth rate was plotted against relative supersaturation with varying concentration and solution temperature, as shown in Fig. 12. With increasing relative and square relative supersaturation, the growth rate was found to increase. The growth rate was higher for the EAF case, confirming the result that the mean chord size was also higher when compared to the reference case. 4.5. Scale rate The scaling rate was calculated using Eqs. 7 and 8 and is plotted in Figs. 13 and 14 against time and relative supersaturation, respectively. Due to crystal deposition on the FBRM probe, the scaling rate for the reference case was high for the first 10 minutes of testing; what followed was an exponential decrease with time, as shown in Fig. 13.
Fig. 10. Effect of EAF on scaling rate for 0.02 mol/l calcium ion concentration at 25 °C.
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Fig. 11. Nucleation rate versus relative supersaturation for the reference and EAF case at 25 °C.
Fig. 13. Scale rate for the reference and EAF case.
5. Discussion and conclusion Additionally, with increasing relative supersaturation, the scaling rate also increased for the reference case and EAF. Also, it shows that in the reference case, the scale rate is higher than in the presence of EAF at any given relative supersaturation, as shown in Fig. 14.
4.6. The Effect of EAF on the morphology of crystals Previous literature has documented that calcite forms at temperatures up to 40 °C, aragonite has been found to form at temperatures exceeding 40 °C, and vaterite was seen to form under at all conditions [10,11]. In this study, the crystal morphology of calcium carbonate under various conditions for the reference case and in the presence of EAF is presented in Fig. 15. At 25 °C, a noticeable difference was observed in crystal morphology between the reference case and EAF case. Utilizing EAF, calcite was converted to vaterite, therefore becoming easier to remove—if required. When the temperature was increased to 60 and 80 °C, no significant difference was found between the two cases. At 60 °C, SEM images revealed flower-shaped vaterite with minor traces of calcite. At 80 °C, the needle-shaped crystals were indicative of aragonite and furthermore, vaterite was also observed. Finally, at higher temperatures, aggregation of the crystals was witnessed, which may have been a result of the induced molecular agitation conducted by the EAF.
Fig. 12. Growth rate versus relative supersaturation for the reference and EAF case.
The present results show that there was a negligible difference in the total number of chord counts measured under EAF and reference conditions. However, the difference in the scaling rate, as measured by FBRM in terms of number of deposition of crystals per unit time, was considerable. The effect of EAF on the scaling rate was in agreement with the literature, which showed considerable evidence of scaling rate decreasing with increasing EAF [12]. The reduction in scale formation can be explained as follows [9]. The pulsing current creates a magnetic field inside the system, which induces an electrical field. This resulted in solenoid-induced molecular agitation, increasing the frequency of collision between calcium and bicarbonate ions, thereby favoring precipitation and agglomeration in the solution [13]. EAF had a more pronounced effect on the scaling process compared to the precipitation process. This could be verified further by examining the morphology of the crystals in the presence and absence of EAF. However, the number of crystals was not measured directly using SEM and as a result, the effect of EAF on the number of crystals was not very clear. The present results do not conclusively show the effect of EAF on the number of chord counts measured. Yet, EAF has a similar trend to the reference case at higher concentrations and temperatures, where the particle sizes and numbers increase as temperatures and concentrations increase in the presence and absence of EAF.
Fig. 14. Scale rate versus relative supersaturation for the reference and EAF case at 25 °C.
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Fig. 15. Various SEM image of calcium carbonate crystals obtained from reference and EAF experiments.
Acknowledgments The authors are grateful to Dr. Kate Pitt, Oz McFarlane and Keith Penny for their assistance and cooperation in presenting this work. Also, much thanks to Saudi Aramco for their permission to publish this paper. References [1] J.D. Donaldson, Scale prevention and descaling, Tube InternationalJan. ed., , 1988, pp. 39–49. [2] D. Hasson, D. Bramson, Effectiveness of magnetic water treatment in suppressing CaCO3 scale deposition, Industrial and Engineering Chemistry Process Design and Development 24 (1985) 588–592. [3] F.T. Ellingsen, H. Kristiansen, Does magnetic treatment influence precipitation of calcium carbonate from supersaturated solutions, Vatten 35 (1979) 309–315. [4] W.N. Al Nasser, A. Shaikh, C. Morriss, M.J. Hounslow, A.D. Salman, Determining kinetics of calcium carbonate precipitation by inline technique, Chemical Engineering Science 63 (5) (2008) 1381–1389. [5] A. Shaikh, A.D. Salman, S. Mcnamara, G. Littlewood, F. Ramsay, M.J. Hounslow, In situ observation of the conversion of sodium carbonate to sodium carbonate
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