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Investigating surface crystal growth using an intrinsic exposed core optical fibre sensor
Sensors and Actuators B 157 (2011) 581–585
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigating surface crystal growth using an intrinsic exposed core optical fibre sensor M. Boerkamp a , D.W. Lamb a,∗ , P.G. Lye b a b
Physics and Electronics, School of Science & Technology, University of New England, Armidale, NSW 2351, Australia Chemistry, School of Science & Technology, University of New England, Armidale, NSW 2351, Australia
a r t i c l e
i n f o
Article history: Received 30 November 2010 Received in revised form 4 May 2011 Accepted 12 May 2011 Available online 19 May 2011 Keywords: Optical fibre sensor Scale Heterogeneous crystal growth Quantification Growth rate
a b s t r a c t Surface, or heterogeneous crystallisation processes, also known as scale formation, has been monitored directly using an intrinsic, exposed core optical fibre sensor (IECOFS). Optical attenuation was found to be linearly correlated with scale layer height and average crystal contact area and the optical signal was found to be restored following chemical removal of deposited crystals (without physical intervention). The IECOFS was found to be insensitive to bulk-solution (homogenous) crystallisation processes, making it a potentially powerful tool for the study of heterogeneous-only crystallisation processes. Kinetic parameters extracted from the obtained crystallisation measurements showed that the IECOFS is capable of providing a reliable means of monitoring surface crystal growth. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fouling of surfaces by inorganic salts is known as scaling or crystallisation fouling and is a common problem in domestic, commercial and industrial processes [1]. The inorganic salts responsible for scale formation are largely referred to as ‘inverse solubility’ salts because a large number of these salts exhibit diminished solubility at elevated temperatures. Consequently scale formation is often found in industrial processes that involve heat exchangers [2], and the low thermal conductivity of the crystallised salts reduces their performance [3]. Also surface blockages reduce exchanger through-put, reducing process efficiency [4]. Scale deposition can cost industry millions of dollars per annum [5,6] due to equipment failure and the necessity for regular de-scaling which results in lost or reduced production. Calcium carbonate (CaCO3 ) and calcium sulfate (CaSO4 ) are common scalants found in industry due to the relative high concentrations of calcium, sulfate (SO4 2− ) and bicarbonate (HCO3 − ) ions in natural waters [4]. Calcium oxalate (CaC2 O4 ) is another scalant responsible, for example, for much of the intractable scale deposited in evaporators found in sugar mill refineries [7,8]. Scale development is a multistage process, of which adhesion of the scaling agent to a surface is an essential step. The surface crystal growth process (scale formation) is termed heterogeneous crystallisation, whilst the growth of crystals
∗ Corresponding author. E-mail address: [email protected] (D.W. Lamb). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.05.026
in the bulk liquid is known as homogenous crystallisation. Many factors affect the formation of scale including the super-saturation concentration, which is the concentration exceeding the saturation concentration, pH, reactant flow velocity and temperature [1]. The chemical treatment of the process waters using anti-scalants or inhibitors, which either disrupt the thermodynamic stability of growing nuclei or interfere or block the subsequent crystal growth, prevent or inhibit scale formation [9]. Whilst proven effective, the efficient use of anti-scalant chemicals requires detection methods capable of quantifying scale formation. Electrical conductivity (eC) and turbidity measurements are two commonly used methods for monitoring scale formation [4], however both techniques are significantly influenced by the nature of the bulk fluid. The former responds to the presence of non-scale-forming ions in solution and the latter is influenced by light scattering as a result of suspended particulates. In effect, these two techniques are not able to differentiate between heterogeneous and homogeneous crystallisation processes. It is known that there is a significant difference in the crystal growth rates between heterogeneous and homogeneous crystallisation [10,11], therefore it is necessary to apply a detection method that is capable of differentiating between the two crystallisation processes in order for scale preventative measures to be effective. An alternative method for scale detection involves the use of optical fibres. Optical fibres, which are generally made of fused silica or a polymer, comprise a central fibre core, of higher refractive index surrounded by concentric cladding of lower refractive index. The difference in refractive indices of the core and cladding is such
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that optical radiation, when appropriately directed (or coupled) into the core of an optical fibre, is trapped and propagates along the fibre core via total internal reflection at the core-cladding interface. Apart from their important role in telecommunications, optical fibres also find many uses as extrinsic and intrinsic components of chemical and physical sensors [12,13]. In terms of detecting heterogeneous crystallisation, the optical fibre can be modified to expose a section of the core to the crystal-growing liquid [14]. This configuration, hitherto referred to as an Intrinsic Exposed Core Optical Fibre Sensor (IECOFS) [15,16], allows the radiation guided within the core to interact with any surface crystals. Radiation incident at the core-crystal interface, where the crystal has a higher refractive index than the core, will be refracted out into the crystal resulting in a net loss of radiation propagating along the fibre [16]. The sensitivity of the optical fibre sensor is highly dependant on the number of core/crystal interactions that the guided radiation makes and this is a direct consequence of the mode distribution of the optical fibre (i.e. number of skips at the core surface) and coverage of the core surface by the crystals [16]. Other workers have shown that propagating radiation in an IECOFS is not attenuated when this sensor configuration is immersed in liquids containing suspended particulates [17]. Thus the potential advantage of IECOFS is that it may be capable of responding to only heterogeneous crystal growth. In this paper, we demonstrate a number of characteristics of an IECOFS for detecting heterogeneous crystallisation, including sensor recoverability (or re-usability), confirm the insensitivity to homogenous crystal formation, the ability to quantify heterogeneous crystal (scale) deposition and the potential of the IECOFS to provide information of the rate of scale formation on a surface of interest.
2. Materials and methods Calcium carbonate (CaCO3 ) crystals were chemically deposited onto the exposed cores of silica and poly methyl methacrylate (PMMA) optical fibres, by immersing them into a solution comprising equal volumes of a CaCl2 (0.0035 M) and Na2 CO3 (0.0035 M) solution. The silica fibres used were PUV-600T or a PUV-200T fibre (Ceram Optec, MA, USA), with 600 and 200 m diameter fused silica cores, respectively. The cores, of refractive index 1.457 were surrounded by a silicone cladding with refractive index of 1.408. The cladding, in turn was surrounded by a Tefzel® jacket. Typically a 6 cm section of exposed fibre core was prepared by physically removing both the jacket and cladding using a scalpel. The exposed section of core was further treated with a tissue soaked in ethanol to remove oil residue and persistent fragments of cladding. The PMMA optical fibres (LG265-48, Regal Lighting Systems, NSW, Australia) had a core diameter of 250 m and an overall diameter, with cladding of 265 m. The refractive index of the core and cladding was 1.492 and 1.405, respectively. Cladding was removed from the PMMA fibres following the method described by Merchant et al. [18]. Fibres used for crystal deposition measurements were inserted in one of two reaction cells. A reaction cell (Cell A in Fig. 1) was made from Perspex and capable of simultaneously supporting IECOFS as well as conventional measures of homogeneous crystallisation and temperature in response to progressive crystal formation. The cell, with connections fitted at each end to permit the reactants to enter and exit the cell, positioned the fibre vertically and had a capacity of 265 ml. The turbidity of the cell solution was monitored by measuring the attenuation of an unbound beam from a 5 mW He–Ne laser ( = 632.9 nm, Uniphase, CA, USA) directed through windows in the side of the cell at 90◦ to the solution flow. The scalant solution temperature was measured using a thermistor (EC95F502W, Vishay Americas, CT, USA). All experiments were conducted at 28 ◦ C unless otherwise advised. A second,
Thermistor probe Photodetectors
Laser access windows Cell A
He-Ne lasers Photodetector IECOFS fibres
Cell B Fig. 1. Schematic diagram of the two alternate reaction cells used in this study showing the arrangement of optical components.
smaller reaction cell with a capacity of 150 ml, Cell B (Fig. 1), was fabricated from stainless steel and supports an IECOFS only. Radiation from 5 mW He–Ne lasers ( = 632.9 nm, Uniphase, CA, USA) was coupled into each fibre examined using a precision optical fibre coupler (F916, Newport Corporation Irvine CA, USA). Fibre coupling was optimised to ensure maximum coupling efficiency. The continuous measurement of fibre output signal was facilitated using photovoltaic detectors (UDT PIN 10DP, United Detector Technologies, CA, USA) connected to an A-D converter (USB6009 A-D converter, National Instruments, TX, USA) with the data recorded to file using an in-house program written in LabVIEWTM 7 Express (National Instruments, TX, USA). When single optical fibre output measurements were required, a photometer (20 W–20 mW, Industrial Fibre Optics, AZ, USA) was used. In both cases, optical power attenuation (AttdB ) was calculated using AttdB = −10 log10
P P0
(1)
where P and P0 were the initial (no crystal) and attenuated (with crystal) output power, respectively. In order to test IECOFS recoverability, and in order to conduct successive sets of experiment using the same optical fibre, the calcium carbonate crystals were chemically removed from the exposed cores following the methods of Gill [19]. The extent of the scale layer growing on the fibre surface was determined using the diffraction pattern produced by a He–Ne laser (5 mW, = 632.9 nm, Uniphase, CA, USA) directed at right angles across the fibre cross-section. The thickness was measured using D=
l x
(2)
where x is the distance between the diffraction minima, l is the distance between the screen on which the diffraction pattern was projected and the fibre, was the wavelength of the laser radiation and D is the combined fibre diameter comprising core diameter and scale layer thickness. The distribution and size of crystals deposited on the surface of 600 m fibre segments were observed using a Scanning Electron Microscope (SEM, Joel JSM5800-LV, Japan). Fibres were prepared for SEM analysis by drying over silica gel for 2 days and subsequently covered with a gold coating using a gold sputter coating device (E5100 Polaron, Quorum Technologies, UK), 4 min at 2.2 kV. The growth and surface coverage of crystals on the surface of an optical fibre core was investigated by immersing a 200 m diameter silica core IECOFS in a calcium carbonate crystallising solution. Short, bare core segments of 600 m diameter silica fibre were also placed in the solution. At specific time intervals, a set of three bare core segments were removed from
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0.5 1,5
Attenuation (dB)
Attenuation (dB)
0.4 0.3 0.2
1,0
IECOFS 0,5
Unbound laser beam
0.1 -0,1 0 0.0 0
40
80
120
Time (min) Fig. 2. Plot of optical power attenuation of guided radiation for a silica core fibre, diameter = 600 m. The clean fibre was exposed to a reacting solution depositing CaCO3 at t = 0. The fibre core was cleaned at t = 40 and 80 min, each time followed by re-immersion in the reactant solution.
the crystallising solution and images of the fibre surface recorded using scanning electron microscopy (SEM). Approximately 10 crystals were examined per SEM image. For each crystal the area of the fibre-contacting face was calculated by measuring the planimetric width and length of the crystal face. 3. Results and discussion 3.1. IECOFS recovery Fig. 2 depicts the measured optical attenuation of the guided radiation as a function of time for a segment of exposed fibre core immersed in a thermostated (28 ◦ C) reacting solution depositing CaCO3 commencing at t = 0 (initially clean fibre). At t = 40 and 80 min the deposited crystals were chemically removed (without physical intervention) from the fibre and a new cycle of reaction initiated. The recovery of the IECOFS resulting from cleaning the surface of the exposed core is clearly evident. From a chemical point of view, the low pH environment created during the recovery phase results in the dissolution of the crystals. The removal of crystals from the surface of the exposed fibre core is accompanied by a return of the fibre to its original state of optical power propagation and no residual (or hysteresis) effects are evident from the repeated cycle of deposition, cleaning and deposition (Fig. 2). Similar results of sensor recovery were observed in other work utilising calcium sulfate [20] and calcium oxalate [15] as the scaling agent. 3.2. IECOFS insensitivity to homogenous crystallisation processes Fig. 3 compares the progressive optical attenuation of an unbound laser beam directed through the crystallising solution (homogeneous crystallisation) and the IECOFS attenuation resulting from immersion of a clean fibre in the same solution. In this
20
40
60
80
100
120
Time (min) Fig. 3. Measured optical power attenuation of unbound laser beam and IECOFS (600 m diameter silica) as a function of time when periodically exposed to a solution containing suspended CaCO3 crystals.
experiment heterogeneous crystallisation on the fibre surface was avoided by conducting the crystallisation reaction in a separate cell and then exposing both unbound laser beam and IECOFS to a sample of the solution at 10 min intervals. Due to the progressive growth of the crystals in solution, the attenuation of the unbound laser beam increases due to scattering of the laser beam. The measured attenuation rapidly increases with time and then eventually plateaus as the chemical reaction reaches an equilibrium state. Unlike the unbound laser beam, the IECOFS is not responding at all to the presence of the suspended crystals in solution. The reason for this insensitivity is two-fold. Firstly the refractive index of the solution throughout the entire reaction remained constant (1.3332 ± 0.0002, measured for a duration of 90 min at intervals of 5 min) and therefore there is no perturbation to the mode structure of the radiation guided through the exposed fibre core. Secondly, the only other interaction between guided radiation and the external solution would be via interaction of the evanescent field with the solution containing the suspended crystal particles. However, in this present work, the maximum crystal size of 6 m [21] leads to an estimated average inter-crystal distance of 116 m, which largely exceeds the maximum penetration depth of the evanescent field (222 nm for a fused silica fibre immersed in solution) and as demonstrated earlier by Lamb et al. [17], the evanescent field penetration depth is only small compared to the inter-particle (or in this case inter-crystal) separation. 3.3. Effect of fibre core surface coverage on heterogeneous crystal growth measurements Fig. 4 shows scanning electron microscope (SEM) images of a silica fibre surface at 3 different time intervals during exposure to a scale forming solution. The SEM images reveal the growth and surface coverage of the calcium carbonate polymorph, vaterite. The increase in attenuation from an IECOFS subjected to surface crystal growth correlates to an increase in the relative surface coverage
Fig. 4. Scanning electron microscope images of fibre surface sections after different time intervals (a 3 min, b 17 min and c 35 min) of exposure to a crystallising solution (600 m silica optical fibre, magnification 8000×).
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PMMA
6
3
Attenuation (dB)
Attenuation (dB)
y = 1.12x - 0.46 2
4
R = 0.97
2
Silica
2
1
0
0
0 0
2
4
10
Fig. 5. Attenuation as a function of the scale layer height, as determined from the diffraction patterns of a 200 m diameter crystallised (calcium carbonate) silica optical fibre. Note error bars result from the 5% uncertainty in measuring the distance between diffraction minima.
of crystals (i.e. more interaction of the guided radiation with the crystals). Previous work involving rotating disk electrodes have also found a link between increasing surface coverage and progressive crystal growth [10,11]. Fig. 5 shows the optical attenuation of the IECOFS as a function of the measured scale layer height and Fig. 6 depicts the optical attenuation as a function of the average crystal contact area as determined by SEM using silica segments extracted at the same time as the attenuation measurements. Within experimental uncertainty, both figures demonstrate a linear trend, suggesting the vertical (out from fibre) growth of crystals is linearly related to the horizontal (along plane fibre surface) growth. Comparing Figs. 5 and 6 for a given attenuation level for example 6 dB, coincides with a crystal contact area of 29 ± ∼4 m2 and a scale layer height of 5.5 ± 0.6 m. This suggests crystal growth to be isotropic, and is consistent with the observations of Lamb et al. [16] who concluded the surface coverage of the exposed fibre core to be the major factor for the loss of guided radiation. 3.4. Heterogeneous crystal growth rates on different surfaces Fig. 7 shows that both silica (200 m diameter) and polymethyl-methacrylate (PMMA) (250 m diameter) optical fibres are capable of detecting and monitoring heterogeneous crystallisation processes. However, Fig. 7 also shows that there is a noticeable difference in the attenuation profiles between the two fibre materials, even though the optical attenuation associated with the smaller diameter fibre (silica) should be more sensitive [17]. The increased
8
Attenuation (dB)
40
50
Time (min)
Scale layer height (µm)
y = 0.31x - 2.94
6
30
20
6
2
R = 0.91 4
Fig. 7. Temporal attenuation curves associated with calcium carbonate crystal growth on silica (200 m diameter) and PMMA (250 m) fibre cores exposed, sideby-side, to the same reactants. The solution temperature was 22 ◦ C.
sensitivity of the PMMA optical fibre could be explained by the higher refractive index of the PMMA core; that is the smaller difference in refractive index between the fibre core and that of the deposited crystal material. As the PMMA fibre core has a higher refractive index than that for silica this will result in a smaller critical angle at the core-liquid interface and hence the potential for supporting propagating modes with a higher number of internal reflections per unit length along the core. This ultimately increases the probability of guided-ray/crystal interactions [16]. The increased sensitivity of the PMMA fibre could also be due to surface roughness introduced as a result of exposing PMMA to acetone during the cladding removal process. Greater surface roughness has been observed to result in a higher degree of nucleation [22], and would result in a higher degree of nucleation on the PMMA core fibre in respect to the smoother silica core fibre surface. It is not clear which of the above may have the largest impact on the crystal nucleation, however all are consistent with the observation of enhanced sensitivity of the PMMA optical fibre to heterogeneous crystallisation. Notwithstanding the difference in surface type or structure, or the differences in the optical ‘sensitivity’ of the IECOFS to crystal growth, Table 1 confirms there is no impact of the two fibre surfaces on parameters that describe the dynamics of crystal growth. Here the IECOFS attenuation profiles for silica and PMMA optical fibres shown in Fig. 7 were fitted with a modified form of the widely used Avrami equation [23,24]:
AttdB (t) = Att∞ − B exp(−k t n )
(3)
where k and n are the optical equivalent of the Avrami crystal growth rate constant and the Avrami exponent, respectively, and B is a pre-exponential factor, and Att∞ is the end-point attenuation (t = ∞), otherwise known as the saturation value. The value of Att∞ is assumed to represent the optical equilibrium state for the system under study. Table 1 shows that there is no significant difference in the calculated Avrami crystal growth rate constant, k , between the heterogeneous crystal growth on silica and PMMA, however, there are differences in the Avrami exponent, n , and the end-point attenuation levels (Att∞ ). These latter quantities reflect the different optical
2
0 0
10
20
30
40
2
Average crystal contact area (µm ) Fig. 6. Attenuation of a 200 m silica optical fibre as a function of the average (calcium carbonate) crystal contact area on 600 m diameter, bare core segments of silica optical fibres as determined from SEM images.
Table 1 Fitted Avrami parameters for 250 m diameter PMMA and 200 m diameter silica fibre cores exposed to calcium carbonate crystal growth. The solution temperature was 22 ◦ C. Avrami parameter
PMMA
Silica
k n Att∞
0.0064 ± 0.0002 1.44 ± 0.01 5.2 ± 0.1
0.0061 ± 0.0003 1.14 ± 0.03 2.2 ± 0.3
M. Boerkamp et al. / Sensors and Actuators B 157 (2011) 581–585
‘sensitivity’ of the PMMA versus silica IECOFS. Even if nucleation conditions differ, as influenced by surface type and/or structure, the ensuing crystal growth does not rely on the type and structure of the surface [25] and hence the growth rate parameters are likely to be reflective of growth processes on nearby different surfaces. 4. Conclusion An intrinsic, exposed core optical fibre sensor (IECOFS) has been used to monitor, in situ surface or heterogeneous crystallisation processes in a laboratory environment. Through the progressive loss of guided modes due to the growth of randomly orientated surface nucleated crystals, with a higher refractive index than the fibre core, optical power output was found to be linearly correlated with crystal contact area and scale layer height (in the current experimental setup the measurement precision of the scale layer height was determined to be ±0.5 m). The optical attenuation was found to be restored following chemical removal of deposited crystals (without physical intervention), pointing to the potential application of this technique as the basis of a self-cleaning sensor. The IECOFS was found to be insensitive to bulk-solution (or homogenous) crystallisation processes, rendering it a useful tool for monitoring surface-only phenomena. The extraction of kinetic parameters from the crystal growth measurements showed no significant dependence of the crystal growth kinetics on surface type or structure, which shows the capability of the IECOFS to reliably estimate the scale formation rate on nearby, different, surfaces of interest such as heat exchanger surfaces. Acknowledgments The authors wish to acknowledge the technical support of the Faculty of the Sciences Engineering Workshop, in particular Mr. Brad Dawson and Mr. Mike Beveridge for construction of the sample cell, and the electronics/photonics support provided by Mr. Graham Hyde (Physics & Electronics, UNE). The assistance of Mr. Pat Littlefield (Physics & Electronics, UNE) with the acquisition of SEM photographs is also gratefully acknowledged. Furthermore, the primary author gratefully acknowledges receipt of a Postgraduate Research scholarship (UNERS) and a Keith and Dorothy Mackay Travelling Scholarship (2009), both from the University of New England. References [1] J. MacAdam, S.A. Parsons, Calcium carbonate scale formation and control, Rev. Environ. Sci. Bio/Technol. 3 (2004) 159–169. [2] P.G. Klepetsanis, E. Dalas, P.G. Koutsoukos, Role of temperature in the spontaneous precipitation of calcium sulphate dihydrate, Langmuir 15 (1999) 1534–1540. [3] Q. Yang, J. Ding, Z. Shen, Investigation of calcium carbonate scaling on ELP surface, J. Chem. Eng. Jpn. 33 (2000) 591–596. [4] W.Y. Shih, K. Albrecht, J. Glater, Y. Cohen, A dual-probe approach for evaluation of gypsum crystallization in response to antiscalant treatment, Desalination 169 (2003) 213–221. [5] J.K. Smith, M. Yuan, T.H. Lopez, J.L. Przybylinski, Real-time and in-situ detection of calcium carbonate scale in a west Texas oil field, SPE Prod. Facil. 19 (2004) 94–99.
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[6] V. Tantayakom, H.S. Fogler, P. Charoensirithavorn, S. Chavadej, Kinetic study of scale inhibitor precipitation in squeeze treatment, Cryst. Growth Des. 5 (2005) 329–335. [7] W.O.S. Doherty, C.M. Fellows, S. Gorjian, E. Senogles, W.H. Cheung, Inhibition of calcium oxalate monohydrate by poly(acrylic acid)s with different end groups, J. Appl. Polym. Sci. 91 (2003) 2035–2041. [8] W.O.S. Doherty, Effect of calcium and magnesium ions on calcium oxalate formation in sugar solutions, Ind. Eng. Chem. Res. 45 (2006) 642–647. [9] N.P. Tung, N.T.P. Phong, N.H. Duy, N.T.T. Trang, Study of scale inhibition mechanism of phosphonates by quantum mechanical method ab initio, in: Proc. of the 8th German-Vietnamese Sem. on Phys. & Eng., 2005, pp. 3–8. [10] A.P. Morizot, A. Neville, T. Hodgkiess, Studies of the deposition of CaCO3 on a stainless steel surface by a novel electrochemical technique, J. Cryst. Growth 198 (1999) 738–743. [11] A.P. Morizot, A. Neville, Barium sulfate deposition and precipitation using a combined electrochemical surface and bulk solution approach, Corrosion 56 (2000) 638–645. [12] B. Culshaw, J. Dakin, Optical Fiber Sensors Volume 2: Systems and Applications, Artech House, London, 1989. [13] J. Dakin, B. Culshaw, Optical Fiber Sensors Volume 4: Applications, Analysis and Future Trends, Artech House, London, 1997. [14] R. Philip-Chandy, P.J. Scully, D. Thomas, A novel technique for on-line measurement of scale using multimode optical fibre sensor for industrial applications, Sens. Actuators B 71 (2000) 19–23. [15] A.D. Wallace, M. Boerkamp, P.G. Lye, D.W. Lamb, W.O.S. Doherty, C.M. Fellows, Assessment of an intrinsic optical fiber sensor for in-situ monitoring of scale-forming salts, Ind. Eng. Chem. Res. 47 (2008) 1066– 1070. [16] D.W. Lamb, M. Boerkamp, P.G. Lye, Guided-mode refraction model for optical fiber sensing of surface crystal growth, Opt. Lett. 35 (2010) 3625–3627. [17] D.W. Lamb, Y. Bunganaen, J. Louis, G.A. Woolsey, R.L. Oliver, G. White, Fibre evanescent field absorption (FEFA): an optical fibre technique for measuring light absorption in turbid water samples, Mar. Freshwater Res. 55 (2004) 533–543. [18] D.F. Merchant, P.J. Scully, N.F. Schmitt, Chemical tapering of polymer optical fibre, Sens. Actuators A: Phys. 76 (1999) 365–371. [19] J.S. Gill, A novel inhibitor for scale control in water desalination, Desalination 124 (1999) 43–50. [20] M. Boerkamp, Intrinsic Exposed Core Optical Fibre Sensors for Monitoring Heterogeneous Crystal Growth, PhD thesis, University of New England, 2009. [21] D. Chakraborty, V.K. Agarwal, S.K. Bhatia, J. Bellare, Steady-state transitions and polymorph transformations in continuous precipitation of calcium carbonate, Ind. Eng. Chem. Res. 33 (1994) 2187–2197. [22] S. Keysar, R. Semiat, D. Hasson, J. Yahalom, Effect of surface roughness on the morphology of calcite crystallizing on mild steel, J. Colloid Interface Sci. 162 (1994) 311–319. [23] W. Lee, H.L. Chen, T.L. Lin, Correlation between crystallization kinetics and microdomain morphology in block copolymer blends exhibiting confined crystallization, J. Polym. Sci. B: Polym. Phys. 40 (2002) 519–529. [24] K.L. Dangaran, J.M. Krochta, Kinetics of sucrose crystallization in whey protein films, J. Agric. Food Chem. 54 (2006) 7152–7158. [25] J.J. De Yoreo, P.G. Vekilov, Biomineralization, Mineralogical Society of America, Washington, 2003.
Biographies Martijn Boerkamp graduated from the Hanze University in the Netherlands. He received his Ph.D. degree from the University of New England in Australia. He is currently employed as a research scientist at the Dutch Organisation for Applied Scientific Research (TNO). His research area interests are; optical fibre sensing, applied spectroscopy and sensor development. David Lamb is a research scientist with 22 years experience investigating optical fibre sensors for use in ‘hostile’ gaseous and liquid media, including high voltage environments. He also leads a Precision Agriculture Research Group engaged in developing remote and proximal electromagnetic (including optical) sensors for environmental and agricultural applications. Peter Lye is a research chemist with 18 years experience covering both industrial and academic projects. His current area of research interest is optical fibre sensors appended with chelating ligands to enhance their chemical sensing applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigating surface crystal growth using an intrinsic exposed core optical fibre sensor M. Boerkamp a , D.W. Lamb a,∗ , P.G. Lye b a b
Physics and Electronics, School of Science & Technology, University of New England, Armidale, NSW 2351, Australia Chemistry, School of Science & Technology, University of New England, Armidale, NSW 2351, Australia
a r t i c l e
i n f o
Article history: Received 30 November 2010 Received in revised form 4 May 2011 Accepted 12 May 2011 Available online 19 May 2011 Keywords: Optical fibre sensor Scale Heterogeneous crystal growth Quantification Growth rate
a b s t r a c t Surface, or heterogeneous crystallisation processes, also known as scale formation, has been monitored directly using an intrinsic, exposed core optical fibre sensor (IECOFS). Optical attenuation was found to be linearly correlated with scale layer height and average crystal contact area and the optical signal was found to be restored following chemical removal of deposited crystals (without physical intervention). The IECOFS was found to be insensitive to bulk-solution (homogenous) crystallisation processes, making it a potentially powerful tool for the study of heterogeneous-only crystallisation processes. Kinetic parameters extracted from the obtained crystallisation measurements showed that the IECOFS is capable of providing a reliable means of monitoring surface crystal growth. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fouling of surfaces by inorganic salts is known as scaling or crystallisation fouling and is a common problem in domestic, commercial and industrial processes [1]. The inorganic salts responsible for scale formation are largely referred to as ‘inverse solubility’ salts because a large number of these salts exhibit diminished solubility at elevated temperatures. Consequently scale formation is often found in industrial processes that involve heat exchangers [2], and the low thermal conductivity of the crystallised salts reduces their performance [3]. Also surface blockages reduce exchanger through-put, reducing process efficiency [4]. Scale deposition can cost industry millions of dollars per annum [5,6] due to equipment failure and the necessity for regular de-scaling which results in lost or reduced production. Calcium carbonate (CaCO3 ) and calcium sulfate (CaSO4 ) are common scalants found in industry due to the relative high concentrations of calcium, sulfate (SO4 2− ) and bicarbonate (HCO3 − ) ions in natural waters [4]. Calcium oxalate (CaC2 O4 ) is another scalant responsible, for example, for much of the intractable scale deposited in evaporators found in sugar mill refineries [7,8]. Scale development is a multistage process, of which adhesion of the scaling agent to a surface is an essential step. The surface crystal growth process (scale formation) is termed heterogeneous crystallisation, whilst the growth of crystals
∗ Corresponding author. E-mail address: [email protected] (D.W. Lamb). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.05.026
in the bulk liquid is known as homogenous crystallisation. Many factors affect the formation of scale including the super-saturation concentration, which is the concentration exceeding the saturation concentration, pH, reactant flow velocity and temperature [1]. The chemical treatment of the process waters using anti-scalants or inhibitors, which either disrupt the thermodynamic stability of growing nuclei or interfere or block the subsequent crystal growth, prevent or inhibit scale formation [9]. Whilst proven effective, the efficient use of anti-scalant chemicals requires detection methods capable of quantifying scale formation. Electrical conductivity (eC) and turbidity measurements are two commonly used methods for monitoring scale formation [4], however both techniques are significantly influenced by the nature of the bulk fluid. The former responds to the presence of non-scale-forming ions in solution and the latter is influenced by light scattering as a result of suspended particulates. In effect, these two techniques are not able to differentiate between heterogeneous and homogeneous crystallisation processes. It is known that there is a significant difference in the crystal growth rates between heterogeneous and homogeneous crystallisation [10,11], therefore it is necessary to apply a detection method that is capable of differentiating between the two crystallisation processes in order for scale preventative measures to be effective. An alternative method for scale detection involves the use of optical fibres. Optical fibres, which are generally made of fused silica or a polymer, comprise a central fibre core, of higher refractive index surrounded by concentric cladding of lower refractive index. The difference in refractive indices of the core and cladding is such
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that optical radiation, when appropriately directed (or coupled) into the core of an optical fibre, is trapped and propagates along the fibre core via total internal reflection at the core-cladding interface. Apart from their important role in telecommunications, optical fibres also find many uses as extrinsic and intrinsic components of chemical and physical sensors [12,13]. In terms of detecting heterogeneous crystallisation, the optical fibre can be modified to expose a section of the core to the crystal-growing liquid [14]. This configuration, hitherto referred to as an Intrinsic Exposed Core Optical Fibre Sensor (IECOFS) [15,16], allows the radiation guided within the core to interact with any surface crystals. Radiation incident at the core-crystal interface, where the crystal has a higher refractive index than the core, will be refracted out into the crystal resulting in a net loss of radiation propagating along the fibre [16]. The sensitivity of the optical fibre sensor is highly dependant on the number of core/crystal interactions that the guided radiation makes and this is a direct consequence of the mode distribution of the optical fibre (i.e. number of skips at the core surface) and coverage of the core surface by the crystals [16]. Other workers have shown that propagating radiation in an IECOFS is not attenuated when this sensor configuration is immersed in liquids containing suspended particulates [17]. Thus the potential advantage of IECOFS is that it may be capable of responding to only heterogeneous crystal growth. In this paper, we demonstrate a number of characteristics of an IECOFS for detecting heterogeneous crystallisation, including sensor recoverability (or re-usability), confirm the insensitivity to homogenous crystal formation, the ability to quantify heterogeneous crystal (scale) deposition and the potential of the IECOFS to provide information of the rate of scale formation on a surface of interest.
2. Materials and methods Calcium carbonate (CaCO3 ) crystals were chemically deposited onto the exposed cores of silica and poly methyl methacrylate (PMMA) optical fibres, by immersing them into a solution comprising equal volumes of a CaCl2 (0.0035 M) and Na2 CO3 (0.0035 M) solution. The silica fibres used were PUV-600T or a PUV-200T fibre (Ceram Optec, MA, USA), with 600 and 200 m diameter fused silica cores, respectively. The cores, of refractive index 1.457 were surrounded by a silicone cladding with refractive index of 1.408. The cladding, in turn was surrounded by a Tefzel® jacket. Typically a 6 cm section of exposed fibre core was prepared by physically removing both the jacket and cladding using a scalpel. The exposed section of core was further treated with a tissue soaked in ethanol to remove oil residue and persistent fragments of cladding. The PMMA optical fibres (LG265-48, Regal Lighting Systems, NSW, Australia) had a core diameter of 250 m and an overall diameter, with cladding of 265 m. The refractive index of the core and cladding was 1.492 and 1.405, respectively. Cladding was removed from the PMMA fibres following the method described by Merchant et al. [18]. Fibres used for crystal deposition measurements were inserted in one of two reaction cells. A reaction cell (Cell A in Fig. 1) was made from Perspex and capable of simultaneously supporting IECOFS as well as conventional measures of homogeneous crystallisation and temperature in response to progressive crystal formation. The cell, with connections fitted at each end to permit the reactants to enter and exit the cell, positioned the fibre vertically and had a capacity of 265 ml. The turbidity of the cell solution was monitored by measuring the attenuation of an unbound beam from a 5 mW He–Ne laser ( = 632.9 nm, Uniphase, CA, USA) directed through windows in the side of the cell at 90◦ to the solution flow. The scalant solution temperature was measured using a thermistor (EC95F502W, Vishay Americas, CT, USA). All experiments were conducted at 28 ◦ C unless otherwise advised. A second,
Thermistor probe Photodetectors
Laser access windows Cell A
He-Ne lasers Photodetector IECOFS fibres
Cell B Fig. 1. Schematic diagram of the two alternate reaction cells used in this study showing the arrangement of optical components.
smaller reaction cell with a capacity of 150 ml, Cell B (Fig. 1), was fabricated from stainless steel and supports an IECOFS only. Radiation from 5 mW He–Ne lasers ( = 632.9 nm, Uniphase, CA, USA) was coupled into each fibre examined using a precision optical fibre coupler (F916, Newport Corporation Irvine CA, USA). Fibre coupling was optimised to ensure maximum coupling efficiency. The continuous measurement of fibre output signal was facilitated using photovoltaic detectors (UDT PIN 10DP, United Detector Technologies, CA, USA) connected to an A-D converter (USB6009 A-D converter, National Instruments, TX, USA) with the data recorded to file using an in-house program written in LabVIEWTM 7 Express (National Instruments, TX, USA). When single optical fibre output measurements were required, a photometer (20 W–20 mW, Industrial Fibre Optics, AZ, USA) was used. In both cases, optical power attenuation (AttdB ) was calculated using AttdB = −10 log10
P P0
(1)
where P and P0 were the initial (no crystal) and attenuated (with crystal) output power, respectively. In order to test IECOFS recoverability, and in order to conduct successive sets of experiment using the same optical fibre, the calcium carbonate crystals were chemically removed from the exposed cores following the methods of Gill [19]. The extent of the scale layer growing on the fibre surface was determined using the diffraction pattern produced by a He–Ne laser (5 mW, = 632.9 nm, Uniphase, CA, USA) directed at right angles across the fibre cross-section. The thickness was measured using D=
l x
(2)
where x is the distance between the diffraction minima, l is the distance between the screen on which the diffraction pattern was projected and the fibre, was the wavelength of the laser radiation and D is the combined fibre diameter comprising core diameter and scale layer thickness. The distribution and size of crystals deposited on the surface of 600 m fibre segments were observed using a Scanning Electron Microscope (SEM, Joel JSM5800-LV, Japan). Fibres were prepared for SEM analysis by drying over silica gel for 2 days and subsequently covered with a gold coating using a gold sputter coating device (E5100 Polaron, Quorum Technologies, UK), 4 min at 2.2 kV. The growth and surface coverage of crystals on the surface of an optical fibre core was investigated by immersing a 200 m diameter silica core IECOFS in a calcium carbonate crystallising solution. Short, bare core segments of 600 m diameter silica fibre were also placed in the solution. At specific time intervals, a set of three bare core segments were removed from
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0.5 1,5
Attenuation (dB)
Attenuation (dB)
0.4 0.3 0.2
1,0
IECOFS 0,5
Unbound laser beam
0.1 -0,1 0 0.0 0
40
80
120
Time (min) Fig. 2. Plot of optical power attenuation of guided radiation for a silica core fibre, diameter = 600 m. The clean fibre was exposed to a reacting solution depositing CaCO3 at t = 0. The fibre core was cleaned at t = 40 and 80 min, each time followed by re-immersion in the reactant solution.
the crystallising solution and images of the fibre surface recorded using scanning electron microscopy (SEM). Approximately 10 crystals were examined per SEM image. For each crystal the area of the fibre-contacting face was calculated by measuring the planimetric width and length of the crystal face. 3. Results and discussion 3.1. IECOFS recovery Fig. 2 depicts the measured optical attenuation of the guided radiation as a function of time for a segment of exposed fibre core immersed in a thermostated (28 ◦ C) reacting solution depositing CaCO3 commencing at t = 0 (initially clean fibre). At t = 40 and 80 min the deposited crystals were chemically removed (without physical intervention) from the fibre and a new cycle of reaction initiated. The recovery of the IECOFS resulting from cleaning the surface of the exposed core is clearly evident. From a chemical point of view, the low pH environment created during the recovery phase results in the dissolution of the crystals. The removal of crystals from the surface of the exposed fibre core is accompanied by a return of the fibre to its original state of optical power propagation and no residual (or hysteresis) effects are evident from the repeated cycle of deposition, cleaning and deposition (Fig. 2). Similar results of sensor recovery were observed in other work utilising calcium sulfate [20] and calcium oxalate [15] as the scaling agent. 3.2. IECOFS insensitivity to homogenous crystallisation processes Fig. 3 compares the progressive optical attenuation of an unbound laser beam directed through the crystallising solution (homogeneous crystallisation) and the IECOFS attenuation resulting from immersion of a clean fibre in the same solution. In this
20
40
60
80
100
120
Time (min) Fig. 3. Measured optical power attenuation of unbound laser beam and IECOFS (600 m diameter silica) as a function of time when periodically exposed to a solution containing suspended CaCO3 crystals.
experiment heterogeneous crystallisation on the fibre surface was avoided by conducting the crystallisation reaction in a separate cell and then exposing both unbound laser beam and IECOFS to a sample of the solution at 10 min intervals. Due to the progressive growth of the crystals in solution, the attenuation of the unbound laser beam increases due to scattering of the laser beam. The measured attenuation rapidly increases with time and then eventually plateaus as the chemical reaction reaches an equilibrium state. Unlike the unbound laser beam, the IECOFS is not responding at all to the presence of the suspended crystals in solution. The reason for this insensitivity is two-fold. Firstly the refractive index of the solution throughout the entire reaction remained constant (1.3332 ± 0.0002, measured for a duration of 90 min at intervals of 5 min) and therefore there is no perturbation to the mode structure of the radiation guided through the exposed fibre core. Secondly, the only other interaction between guided radiation and the external solution would be via interaction of the evanescent field with the solution containing the suspended crystal particles. However, in this present work, the maximum crystal size of 6 m [21] leads to an estimated average inter-crystal distance of 116 m, which largely exceeds the maximum penetration depth of the evanescent field (222 nm for a fused silica fibre immersed in solution) and as demonstrated earlier by Lamb et al. [17], the evanescent field penetration depth is only small compared to the inter-particle (or in this case inter-crystal) separation. 3.3. Effect of fibre core surface coverage on heterogeneous crystal growth measurements Fig. 4 shows scanning electron microscope (SEM) images of a silica fibre surface at 3 different time intervals during exposure to a scale forming solution. The SEM images reveal the growth and surface coverage of the calcium carbonate polymorph, vaterite. The increase in attenuation from an IECOFS subjected to surface crystal growth correlates to an increase in the relative surface coverage
Fig. 4. Scanning electron microscope images of fibre surface sections after different time intervals (a 3 min, b 17 min and c 35 min) of exposure to a crystallising solution (600 m silica optical fibre, magnification 8000×).
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PMMA
6
3
Attenuation (dB)
Attenuation (dB)
y = 1.12x - 0.46 2
4
R = 0.97
2
Silica
2
1
0
0
0 0
2
4
10
Fig. 5. Attenuation as a function of the scale layer height, as determined from the diffraction patterns of a 200 m diameter crystallised (calcium carbonate) silica optical fibre. Note error bars result from the 5% uncertainty in measuring the distance between diffraction minima.
of crystals (i.e. more interaction of the guided radiation with the crystals). Previous work involving rotating disk electrodes have also found a link between increasing surface coverage and progressive crystal growth [10,11]. Fig. 5 shows the optical attenuation of the IECOFS as a function of the measured scale layer height and Fig. 6 depicts the optical attenuation as a function of the average crystal contact area as determined by SEM using silica segments extracted at the same time as the attenuation measurements. Within experimental uncertainty, both figures demonstrate a linear trend, suggesting the vertical (out from fibre) growth of crystals is linearly related to the horizontal (along plane fibre surface) growth. Comparing Figs. 5 and 6 for a given attenuation level for example 6 dB, coincides with a crystal contact area of 29 ± ∼4 m2 and a scale layer height of 5.5 ± 0.6 m. This suggests crystal growth to be isotropic, and is consistent with the observations of Lamb et al. [16] who concluded the surface coverage of the exposed fibre core to be the major factor for the loss of guided radiation. 3.4. Heterogeneous crystal growth rates on different surfaces Fig. 7 shows that both silica (200 m diameter) and polymethyl-methacrylate (PMMA) (250 m diameter) optical fibres are capable of detecting and monitoring heterogeneous crystallisation processes. However, Fig. 7 also shows that there is a noticeable difference in the attenuation profiles between the two fibre materials, even though the optical attenuation associated with the smaller diameter fibre (silica) should be more sensitive [17]. The increased
8
Attenuation (dB)
40
50
Time (min)
Scale layer height (µm)
y = 0.31x - 2.94
6
30
20
6
2
R = 0.91 4
Fig. 7. Temporal attenuation curves associated with calcium carbonate crystal growth on silica (200 m diameter) and PMMA (250 m) fibre cores exposed, sideby-side, to the same reactants. The solution temperature was 22 ◦ C.
sensitivity of the PMMA optical fibre could be explained by the higher refractive index of the PMMA core; that is the smaller difference in refractive index between the fibre core and that of the deposited crystal material. As the PMMA fibre core has a higher refractive index than that for silica this will result in a smaller critical angle at the core-liquid interface and hence the potential for supporting propagating modes with a higher number of internal reflections per unit length along the core. This ultimately increases the probability of guided-ray/crystal interactions [16]. The increased sensitivity of the PMMA fibre could also be due to surface roughness introduced as a result of exposing PMMA to acetone during the cladding removal process. Greater surface roughness has been observed to result in a higher degree of nucleation [22], and would result in a higher degree of nucleation on the PMMA core fibre in respect to the smoother silica core fibre surface. It is not clear which of the above may have the largest impact on the crystal nucleation, however all are consistent with the observation of enhanced sensitivity of the PMMA optical fibre to heterogeneous crystallisation. Notwithstanding the difference in surface type or structure, or the differences in the optical ‘sensitivity’ of the IECOFS to crystal growth, Table 1 confirms there is no impact of the two fibre surfaces on parameters that describe the dynamics of crystal growth. Here the IECOFS attenuation profiles for silica and PMMA optical fibres shown in Fig. 7 were fitted with a modified form of the widely used Avrami equation [23,24]:
AttdB (t) = Att∞ − B exp(−k t n )
(3)
where k and n are the optical equivalent of the Avrami crystal growth rate constant and the Avrami exponent, respectively, and B is a pre-exponential factor, and Att∞ is the end-point attenuation (t = ∞), otherwise known as the saturation value. The value of Att∞ is assumed to represent the optical equilibrium state for the system under study. Table 1 shows that there is no significant difference in the calculated Avrami crystal growth rate constant, k , between the heterogeneous crystal growth on silica and PMMA, however, there are differences in the Avrami exponent, n , and the end-point attenuation levels (Att∞ ). These latter quantities reflect the different optical
2
0 0
10
20
30
40
2
Average crystal contact area (µm ) Fig. 6. Attenuation of a 200 m silica optical fibre as a function of the average (calcium carbonate) crystal contact area on 600 m diameter, bare core segments of silica optical fibres as determined from SEM images.
Table 1 Fitted Avrami parameters for 250 m diameter PMMA and 200 m diameter silica fibre cores exposed to calcium carbonate crystal growth. The solution temperature was 22 ◦ C. Avrami parameter
PMMA
Silica
k n Att∞
0.0064 ± 0.0002 1.44 ± 0.01 5.2 ± 0.1
0.0061 ± 0.0003 1.14 ± 0.03 2.2 ± 0.3
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‘sensitivity’ of the PMMA versus silica IECOFS. Even if nucleation conditions differ, as influenced by surface type and/or structure, the ensuing crystal growth does not rely on the type and structure of the surface [25] and hence the growth rate parameters are likely to be reflective of growth processes on nearby different surfaces. 4. Conclusion An intrinsic, exposed core optical fibre sensor (IECOFS) has been used to monitor, in situ surface or heterogeneous crystallisation processes in a laboratory environment. Through the progressive loss of guided modes due to the growth of randomly orientated surface nucleated crystals, with a higher refractive index than the fibre core, optical power output was found to be linearly correlated with crystal contact area and scale layer height (in the current experimental setup the measurement precision of the scale layer height was determined to be ±0.5 m). The optical attenuation was found to be restored following chemical removal of deposited crystals (without physical intervention), pointing to the potential application of this technique as the basis of a self-cleaning sensor. The IECOFS was found to be insensitive to bulk-solution (or homogenous) crystallisation processes, rendering it a useful tool for monitoring surface-only phenomena. The extraction of kinetic parameters from the crystal growth measurements showed no significant dependence of the crystal growth kinetics on surface type or structure, which shows the capability of the IECOFS to reliably estimate the scale formation rate on nearby, different, surfaces of interest such as heat exchanger surfaces. Acknowledgments The authors wish to acknowledge the technical support of the Faculty of the Sciences Engineering Workshop, in particular Mr. Brad Dawson and Mr. Mike Beveridge for construction of the sample cell, and the electronics/photonics support provided by Mr. Graham Hyde (Physics & Electronics, UNE). The assistance of Mr. Pat Littlefield (Physics & Electronics, UNE) with the acquisition of SEM photographs is also gratefully acknowledged. Furthermore, the primary author gratefully acknowledges receipt of a Postgraduate Research scholarship (UNERS) and a Keith and Dorothy Mackay Travelling Scholarship (2009), both from the University of New England. References [1] J. MacAdam, S.A. Parsons, Calcium carbonate scale formation and control, Rev. Environ. Sci. Bio/Technol. 3 (2004) 159–169. [2] P.G. Klepetsanis, E. Dalas, P.G. Koutsoukos, Role of temperature in the spontaneous precipitation of calcium sulphate dihydrate, Langmuir 15 (1999) 1534–1540. [3] Q. Yang, J. Ding, Z. Shen, Investigation of calcium carbonate scaling on ELP surface, J. Chem. Eng. Jpn. 33 (2000) 591–596. [4] W.Y. Shih, K. Albrecht, J. Glater, Y. Cohen, A dual-probe approach for evaluation of gypsum crystallization in response to antiscalant treatment, Desalination 169 (2003) 213–221. [5] J.K. Smith, M. Yuan, T.H. Lopez, J.L. Przybylinski, Real-time and in-situ detection of calcium carbonate scale in a west Texas oil field, SPE Prod. Facil. 19 (2004) 94–99.
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Biographies Martijn Boerkamp graduated from the Hanze University in the Netherlands. He received his Ph.D. degree from the University of New England in Australia. He is currently employed as a research scientist at the Dutch Organisation for Applied Scientific Research (TNO). His research area interests are; optical fibre sensing, applied spectroscopy and sensor development. David Lamb is a research scientist with 22 years experience investigating optical fibre sensors for use in ‘hostile’ gaseous and liquid media, including high voltage environments. He also leads a Precision Agriculture Research Group engaged in developing remote and proximal electromagnetic (including optical) sensors for environmental and agricultural applications. Peter Lye is a research chemist with 18 years experience covering both industrial and academic projects. His current area of research interest is optical fibre sensors appended with chelating ligands to enhance their chemical sensing applications.