An intrinsic exposed core optical fiber sensor as a quantitative surface crystallization monitoring sensor

Sensors and Actuators B 177 (2013) 964–969

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An intrinsic exposed core optical fiber sensor as a quantitative surface crystallization monitoring 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 10 June 2012 Received in revised form 26 November 2012 Accepted 4 December 2012 Available online 13 December 2012 Keywords: Surface crystallization Scale formation Exposed core optical fiber sensor Stainless steel surface

a b s t r a c t An intrinsic exposed core optical fiber sensors has been described that is capable of monitoring surface crystallization, also known as scale formation. The optical fiber sensor is a more reliable sensor of scale growth than other scale sensing methods, such as turbidity measurement, due to its ability to discriminate between bulk and surface crystallization. When the sensor was subjected to the same crystal growth conditions as a stainless steel surface, the optical fiber sensor showed the capability to follow the scale formation on the scale affected stainless steel surface. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The crystallization of inorganic salts on surfaces, also known as scale formation, is a common problem in domestic, commercial and industrial processes [1]. These inorganic salts are known as ‘inverse solubility’ salts, meaning that their solubility decreases at elevated temperatures. This causes major problems to industrial processes that make use of heat exchangers [2], with the low thermal conductivity of the scale severely reducing the heat exchanger’s efficiency [3]. Furthermore, the formation of scale results in a permanent flux decline, which reduces the efficiency of industrial processes [4]. It is estimated that scale formation costs the industry yearly millions of dollars [5,6]. Calcium carbonate (CaCO3 ) and calcium sulfate (CaSO4 ) are among the most common scaleforming minerals found in industry due to the presence of calcium, sulfate (SO4 2− ) and bicarbonate (HCO3 − ) ions in many process waters.[4] Calcium oxalate (CaC2 O4 ) is another important scalant found in sugar mill evaporators [7,8]. Many factors affect the formation of scale, including the super-saturation concentration, which is defined as the concentration exceeding the saturation concentration, reactant flow velocity, temperature and solution pH [1]. Scale forms as a result of heterogeneous crystallization on a surface and may therefore show marked thermodynamic and kinetic differences from crystal growth within the bulk solution, known as homogeneous crystallization. Scale inhibition can be achieved

∗ Corresponding author. E-mail address: [email protected] (P.G. Lye). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.12.020

through the addition of chemical compounds known as inhibitors. Inhibitors interfere with the thermodynamic stability of growing nuclei or block crystal growth [9]. Commonly used methods for monitoring scale formation are the measurement of electrical conductivity and turbidity [4]. The electrical conductivity of a solution is determined by the concentration of ions in solution whereas the turbidity of a solution is dependent on the optical transparency of the bulk solution. In effect, neither technique is able to differentiate between the two crystallization processes. The efficiency of scale inhibitors is known to differ significantly between the two crystallization types [10,11]. Therefore, to study scale inhibitors and their ability to prevent scale formation, it is critical to use a detection method that is able to differentiate between the two crystallization processes and is able to quantify the scale deposition on a given surface. Calorimetric methods are also frequently used to monitor scaling by measuring the decline in the heat transfer coefficient [12], however; calorimetric techniques require heated solutions which precludes it from monitoring scale formation under condition found, for example, in a desalination plant. Optical fibers, which are effectively circular dielectric waveguides capable of guiding light through a solid core surrounded by a cladding of lower refractive index via the process of ‘total internal reflection’ are often employed as chemical and physical sensors [13,14]. An alternative method for monitoring heterogeneous crystallization processes involves the use of an exposed section of optical fiber core. When the exposed fiber core is inserted in a medium of lower refractive index such as water, the fiber core remains capable of guiding light via the process of total internal reflection. However, when inserted in a super-saturated solution

M. Boerkamp et al. / Sensors and Actuators B 177 (2013) 964–969

Cell A

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Thermistor probe Photodetectors

Laser access windows

He-Ne lasers Photodetector IECOFS fibres

Cell B Fig. 1. Schematic diagram of the two reaction cells (for IECOFS and turbidity measurements) used in this study showing the arrangement of optical components.

(of lower refractive index), the fiber core provides a surface for heterogeneous crystallization. The crystals formed on the fiber core surface typically have a refractive index higher than the core, and the core/crystal interfaces do not allow total internal reflection and light refracts out of the core. This causes a measurable reduction in optical fiber output and can be linked to the formation and growth of heterogeneous crystals (scale formation). This configuration was first proposed by Philip-Chandy et al. [15] and was capable of measuring silver chloride crystallization. This configuration is termed here an ‘intrinsic exposed core optical fiber sensor’ (IECOFS) [16–19]. In a previous publication it was shown that the IECOFS response to heterogeneous crystallization together with its insensitivity toward homogeneous crystallization contrasted with the current detection methods of scale formation that are largely affected by homogeneous crystallization; electrical conductivity and turbidity [19]. The aim of this paper is to show the capability of the IECOFS in measuring heterogeneous crystal growth only, whilst remaining unaffected by the homogeneous crystallization, hereby providing a realistic picture of the scale formation process. The ability of the IECOFS as a scale monitoring sensor for a stainless steel surface under investigation is shown when the IECOFS is present in the same scale forming solution. 2. Materials and methods Calcium carbonate (CaCO3 ) crystals were chemically deposited onto the exposed cores of silica optical fibers, by immersing the fibers into a solution comprising equal volumes of a CaCl2 (0.0035 M) and Na2 CO3 (0.0035 M) solution made of laboratory grade chemicals and demineralised filtered water. Mixing of the chemicals was performed after immersion of the fiber into the solution. Between successive experiments, the calcium carbonate crystals were chemically removed from the exposed cores following the methods of Gill [20]. The silica fibers used were PUV-600T or a PUV-200T fiber (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 fiber 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. Fibers 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 conventional measures of homogeneous crystallization 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 fiber 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 scalent solution temperature was measured using a thermistor (EC95F502W, Vishay Americas, CT USA). All experiments were conducted at 28 ◦ C unless otherwise advised. A second, 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 fiber examined using a precision optical fiber coupler (F916, Newport Corporation Irvine CA USA). Fiber coupling was optimized to ensure maximum coupling efficiency. The continuous measurement of fiber output signal was facilitated using photovoltaic detectors (UDT PIN 10DP, United Detector Technologies, CA USA) connected to an A-D converter (USB-6009 AD 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 fiber output measurements were required, a photometer (20 ␮W–20 mW, Industrial Fiber Optics, AZ USA) was used. In both cases, optical power attenuation (AttdB ) was calculated using AttdB = −10log10

P P0

(1)

where P and P0 were the initial (no crystal) and attenuated (with crystal) output power, respectively. The Avrami equation is often used to characterize crystal growth over time at a constant temperature and limited reactant concentrations [21,22]. The obtained IECOFS attenuation profiles can be fitted using a modified form of the Avrami equation [16] where optical attenuation (AttdB ) in dB over time (t), to give: Attdb = Att∞ − B exp(−kt n )

(2)

where k and n are the optical equivalent of the Avrami crystal growth rate constant and Avrami exponent, respectively and B is a

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pre-exponential factor. 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. In order to compare the growth of calcium carbonate crystals on an optical fiber surface and a stainless steel surface, a 200 ␮m diameter stainless steel wire (316 SS, Hengyang Stainless Steel Products Co., Hebei, China) was inserted in the same crystallizing solution as the optical fiber. The optical fiber was measured with the method mentioned above. The extent of the scale layer growing on the stainless steel 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 wire cross-section. The thickness was measured using; D=

l x

(3)

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 wire,  was the wavelength of the laser radiation and D is the combined wire diameter comprising wire diameter and scale layer thickness. The distribution and size of crystals deposited on the surface of 600 ␮m fiber segments were observed using a Scanning Electron Microscope (SEM, Joel JSM5800-LV, Japan). Wires 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 by crystals on the surface of a stainless steel wire was investigated by immersing short segments of 200 ␮m diameter stainless steel wire in a calcium carbonate crystallizing solution. At specific time intervals, a set of three stainless steel segments were removed from the crystallizing solution and images of the wire surface recorded using scanning electron microscopy (SEM). Approximately 10 crystals were examined per SEM image and 10 images were taken per segment. For each crystal the area of the fiber-contacting face was calculated by measuring the planimetric width and length of the crystal face. 3. Results and discussion 3.1. The thermodynamics of crystal growth The solutions often found in industrial applications have an adequate super-saturation ratio to sustain crystallization. Scale forming compounds are also known as inverse solubility salts, meaning their super-saturation ratio increases with increasing temperature. Therefore, the process temperature has a great influence on the total crystallization kinetics [23]. The crystal growth in these experiments involves both surface and bulk crystallization processes. Both processes obey different crystallization kinetics [10,11]. The kinetics of crystal growth for each process is largely affected by the availability of reactants for each process. Furthermore, if the temperature of the crystallizing solution is varied the equilibrium between the two crystallization phases shifts as the two processes are competing for the same reactants. The reason for the shift in balance between the bulk and surface crystal phases is that with surface crystallization the effective transport of material toward the growing crystals on the surface is limited [24] whilst material present in the bulk can easily be transported toward the bulk crystallized particles. It is known that, even at equilibrium, there is a constant attachment and detachment of reactants at the crystal surface [25] and these reactants diffuse either from the bulk to the surface of the crystal or from the surface of the crystal to the bulk [23,24]. The separation between the layer surrounding the surface [26] and the bulk solution is known as a diffusion boundary layer [24,27]. If the super-saturation ratio is sufficiently increased,

16

IECOFS Att∞ Turbidity

Attenuation (dB)

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12

8 4 0 25

35

45

55

65

Temperature (°C) Fig. 2. The change in turbidity and IECOFS Att∞ as a function of temperature.

i.e. at higher temperatures, the reaction becomes bulk controlled and less material is available to crystallize at the surface [24]. At lower concentrations, i.e. when limited bulk crystallization occurs, the bulk serves as a reservoir for surface crystallization [27]. To test the diffusion boundary layer characteristics, both surface and bulk crystallization was measured at various temperatures. The change in attenuation at equilibrium, for the IECOFS and the turbidity sensor (Perspex cell) as a function of the system temperature, is shown in Fig. 2. A decrease in the end-point attenuation (Att∞ ) with increasing temperature for the IECOFS measurements is shown while turbidity increases with increasing temperature. The Att∞ was determined by fitting the IECOFS profiles with the modified Avrami equation (Eq. (2)). It should be noted that the results shown in Fig. 2 do not imply that the IECOFS is incapable of monitoring surface crystal growth at elevated temperatures, the IECOFS is subject to the same degree of crystallization as any other non-heated surface found in the solution. The result shows that the relation between the turbidity and surface crystal growth is a dynamic process between the surface and the bulk with a great complexity due to the large number of factors involved. However, the result shown in Fig. 2 can be used to analyze the overall system response of the change in temperature and the total crystallization experienced in the solution, therefore the equilibrium between these two phases can be represented by: CaCO3 ,bulk ↔ Ca2+ bulk + CO3 2− bulk ↔ Ca2+ surface + CO3 2− surface ↔ CaCO3 ,surface where the CaCO3 ,bulk is represented by the turbidity and the CaCO3 ,surface is represented by the IECOFS measurement. As the concentrations of the Ca2+ and the CO3 2− are representative for the amount of crystallized CaCO3 at both the bulk and the surface, the availability of these ions for either surface or bulk crystallization could be thought as a boundary or equilibrium constant describing the phase separation process: KB =

][CO2− ] [Ca2+ 3,bulk bulk [Ca2+ ][CO2− ] 3,surface surface

(4)

If it is assumed that the turbidity is a valid representation of the bulk crystallization and that Att∞ represents the surface crystallization, both at equilibrium, then KB describes the equilibrium between the two phases. It is known from chemical thermodynamics that the equilibrium constant is temperature dependant, with the temperature dependence described by the following expression: ln Keq =

H ◦ S ◦ − RT R

(5)

where R is the gas constant (8.314 J/Kmol), H◦ is the standard enthalpy change (kJ/mol) and S◦ is the standard entropy change

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967

4.5

2

4

R = 0.99

Scale layer height (μm)

ln KB

2.5

0.5 2.9 -1.5

3.0

3.1 -1 T (× 1000)

3.2

3.3

-3.5 Fig. 3. Plot of the natural logarithm of the equilibrium constant as a function of the inverse absolute temperature.

3

y = 0.38x - 0.99 R2 = 0.97

2 1 0 0

2

4

6

8

10

12

2

It was previously shown by Boerkamp et al. [16] that if the conditions for crystal growth remain the same, after nucleation any differences in surface material and structure do not significantly influence the crystal growth kinetics. The surface crystal growth on the optical fiber core should therefore be representative for the surface crystal growth on the surface of interest, for example stainless steel, if the conditions for heterogeneous crystal growth on both surfaces remain the same. Stainless steel’s use is widespread and it is commonly affected by the formation of scale [28,29]. The crystal growth process on a surface of interest, such as stainless steel, can be investigated by means of a laser diffraction method or the measurement of crystal size by investigating Scanning Electron Microscope (SEM) photographs, both described in a previous publication [16]. In this publication it was shown by the clear trend between the optical attenuation of the IECOFS with the size of the crystal layer growing perpendicular from the fiber core surface, termed here ‘scale layer height’, together with the average crystal face size, termed the ‘crystal contact area’ [16] and to a lesser extend with the scale mass [18] that the optical attenuation of the IECOFS could follow the crystal growth process on its fiber surface. Both measurements of the scale layer height and the average crystal contact area are a measure of the average surface crystal growth process, which is similar to the sensing principle of the IECOFS,

which is also a direct result of the nucleation and growth of a large number of crystals on the optical fiber core [16,17]. Both methods can therefore give a good indication of the validity of the IECOFS to determine the surface crystal growth process on the surface of interest. The crystal growth process, measured as the scale layer height and the average crystal contact area, measured at set time intervals, on stainless steel is shown in Fig. 4. This result shows that the dynamics of crystal growth is well represented by these direct measures of the average crystal growth process. The intercept of the line of best fit, shown in Fig. 4, is negative which implies that the measurement of the scale layer height at the early stages of crystal growth lacks sensitivity, i.e. when the crystals are too small to measure via laser diffraction. Fig. 5 shows such a measurement where the IECOFS was placed inside the same scaling solution as a stainless steel surface and at set intervals the scale formation process was monitored, by means of laser diffraction on the stainless steel surface and the IECOFS optical attenuation. Therefore, if the IECOFS is subject to the same conditions of crystal growth as the surface of interest, the crystal growth process on the fiber core surface, and therefore the optical attenuation, should correlate to the scale layer height and average crystal contact area of the surface nucleated crystals on the surface of interest. Fig. 6 shows a clear trend between the IECOFS attenuation and the average crystal contact area on the stainless steel surface. The scale layer height shows that the increase in the size of the crystal faces is responsible for the extraction of radiation out of the fiber

5

3

Stainless steel IECOFS

4

2

3 2

1

Attenuation (dB)

3.2. The efficiency of IECOFS as a scale monitoring device

Fig. 4. Scale layer height as a function of the surface coverage on stainless steel surface.

Scale layer height (μm)

(J/K). A plot of ln Keq as a function T−1 should give a straight line with a slope of −H◦ /R and an intercept of S◦ /R. Fig. 3 shows the experimentally derived values of ln KB plotted as a function of the inverse temperature. Fig. 3 shows a straight line confirming the existence of the phase separation and its effect on the amount of crystallized material either in the bulk or on the surface. From this plot a value of the standard enthalpy (H◦ ) for the phase transition is given as 136.1 ± 9.5 kJ/mol. The fitted standard entropy term is not reported as the chemical basis of this value is not clear and the error contained in the value will be large. No literature data could be found for the thermodynamics relating to the phase separation discussed in this work. Thus no comparison can be made with the enthalpy value determined in this work. Due to the complexity of the crystallizing system in these measurements the enthalpy value for the phase separation should only be considered as an estimate. While the kinetics of diffusion between reactants in a bulk solution and a surface is generally well understood [24,27], much less is known about the thermodynamics of such processes. The work presented here can be thought as a first step in our understanding of the thermodynamics involved in phase separation processes. However, these preliminary results show that the IECOFS may prove to be a valuable tool for investigating such complex systems by providing a real-time picture of surface crystallization.

surface coverage (μm )

1

0

0

20

40

60

80

100

120

0

Time (min) Fig. 5. Scale layer height profile determined from stainless steel wire and IECOFS attenuation profile in similar crystal growth conditions. Error bars result from the 2% uncertainty from determining the average distance between diffraction minima.

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affected surface that is present in the same solution as the IECOFS and is subjected to the same conditions of scale build up. Therefore, the IECOFS could function as a reliable and intrinsic sensor of scale growth and the close relationship of the IECOFS attenuation with the scale growth would allow quantitative measurements on the scale growth on the surface of interest.

Attenuation (dB)

12 yy== 4.32x 4,32x –- 1.78 1,78 R2 =R² 0.99 = 0,99

8

4 References

0 0

1 2 Scale layer height (μm)

3

4

Fig. 6. IECOFS attenuation as a function of the scale layer height on stainless steel surface when both are subjected to the same crystallizing solution.

Attenuation (dB)

12

y = 1.65x – 6.13 R2 = 0.97

8

4

0 0

2

4

6

8

Average crystal contact area

10

12

(μm2 )

Fig. 7. IECOFS attenuation as a function of the surface coverage of crystals on stainless steel surface when both are subjected to the same crystallizing solution.

core by increasing the interaction with the guided radiation. Fig. 6 shows that the average increase in calcium carbonate crystal size on the stainless steel surface is similar to the average crystal growth process on the optical fiber core. Similarly, Fig. 7 shows the clear trend between the optical attenuation and the surface coverage of the crystals on the stainless steel surface. This result again shows that the subsequent average increase in crystal size on the stainless steel surface is similar to the average crystal growth process on the optical fiber core. In combination the results shown in Figs. 6 and 7 show that by monitoring the optical attenuation of the IECOFS a clear picture of crystal growth on a surface of interest can be obtained. A negative intercept is observed for the line of best fit in Figs. 6 and 7. This does not represent the inability of the IECOFS to determine the very early stage of crystal growth. This phenomenon was previously seen in the scale monitoring results of Wallace et al. [19], where solutions of different super-saturations caused the optical attenuation signal to decrease, before the optical attenuation increased due to the nucleation and growth of crystals. This process is most likely explained by the formation of cat-ionic surface layers on the optical fiber core surface, as can be explained with the work of Atkin and Warr [30] and the subsequent refraction of lossy modes back into the fiber. However, a more thorough treatment of this effect is beyond the scope of this paper. 4. Conclusion It was shown that the IECOFS could provide a more realistic picture of the scale formation process, also known as surface crystallization, due to its ability to discriminate between the bulk and surface crystallization. It was also shown that the IECOFS could provide a realistic picture of the scale formation process on a scale

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Biographies Dr. Martijn Boerkamp received his Ph.D. degree at the University of New England in Australia in the area of optical fiber sensing. He is now working as a postdoctoral researcher at the University of Technology Delft in the Netherlands on optical trapping of bacteria using photonic integrated circuits. His research area interests are; optical fiber sensing, integrated photonics, applied spectroscopy and sensor development. Prof. David Lamb is a research scientist with 23 years experience investigating optical fiber 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. Dr. Peter Lye is a research chemist with 19 years experience covering both industrial and academic projects. His current area of research interest is optical fiber sensors appended with chelating ligands to enhance their chemical sensing applications.