Detecting and Monitoring Industrial Scale Formation Using an Intrinsic

Apr 13, 2010 - Measurements of heterogeneous crystallization (scaling) have been performed using an intrinsic exposed- core optical fiber sensor (IECO...
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Ind. Eng. Chem. Res. 2010, 49, 4682–4686

Detecting and Monitoring Industrial Scale Formation Using an Intrinsic Exposed-Core Optical Fiber Sensor Martijn Boerkamp,† David W. Lamb,† Peter G. Lye,*,‡ Christopher M. Fellows,‡ Ali Al-Hamzah,‡ and Andrew D. Wallace‡ School of Science & Technology, UniVersity of New England, Armidale, NSW 2351, Australia

Measurements of heterogeneous crystallization (scaling) have been performed using an intrinsic exposedcore optical fiber sensor (IECOFS). The IECOFS showed several advantages over the conventional techniques of scale detection: turbidity and electrical conductivity, including insensitivity to the presence of suspended crystals caused by homogeneous crystallization and the ability to monitor scale deposition under continuous supply conditions. Calibration of the IECOFS, to quantify scale deposited on a given surface, and the limitations of the technique are also presented. Introduction The deposition of inorganic salts on surfaces, or scale formation, is a common problem in domestic, commercial, and industrial processes.1 Some of these inorganic salts are known as “inverse solubility” salts, meaning that their solubility decreases at elevated temperatures. This can cause 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, scale formation results in a permanent flux decline, which reduces the efficiency of industrial processes.4 It is estimated that scale formation costs industry millions of dollars per annum.5,6 Calcium carbonate (CaCO3) and calcium sulfate (CaSO4) are among the most common scale-forming minerals found in industry owing to the presence of calcium, sulfate (SO42-), and bicarbonate (HCO3-) ions in many process waters.4 Among other significant scale-forming minerals, calcium oxalate (CaC2O4) is responsible for much of the intractable scale deposited in sugar mill evaporators.7,8 Many factors affect the formation of scale, including the supersaturation concentration, which is defined as the concentration exceeding the saturation concentration, temperature, reactant flow velocity, and solution pH.1 Scale forms as a result of heterogeneous crystallization on an affected surface and may therefore show marked thermodynamic and kinetic differences from crystal growth within the bulk solution, also referred to as homogeneous crystallization. Scale inhibition can be achieved through the addition of chemical compounds known as inhibitors or antiscalants. Inhibitors interfere with the thermodynamic stability of growing nuclei or block crystal growth.9 Commonly used methods for monitoring scale formation are electrical conductivity and turbidity measurements.4 However the response from both these techniques are dominated by crystallization events in the bulk solution. 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, heterogeneous and homogeneous crystal growth. The efficiency of scale inhibitors is known to differ significantly between * To whom correspondence should be addressed. E-mail: Peter.Lye@ une.edu.au. † Physics and Electronics. ‡ Chemistry.

heterogeneous and homogeneous crystallization.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; however, this requires a scaling surface with good thermal conductivity and instrumentation whose size and cost, in general, make it unsuitable for bench work.12 Furthermore, calorimetric techniques require heated solutions, which precludes them from monitoring scale formation under conditions found in a desalination plant, for example. Optical fibers are 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” and are often employed as extrinsic and intrinsic components of chemical and physical sensors.13,14 An alternative method for monitoring heterogeneous crystallization processes involves the use of an “optical fiber sensor”. To detect heterogeneous crystal growth, an exposed section of optical fiber core is used. 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 supersaturated solution (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, resulting in light incident at the core/crystal interface to refract 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).16-21 In a previous publication it was shown that the IECOFS response to calcium oxalate crystallization correlated to the response of the current detection methods of scale formation, electrical conductivity, and turbidity.17 However, as the IECOFS only responds to heterogeneous crystal growth, it has advantages over the existing techniques. Furthermore, because of an increase in the surface coverage that accompanies the crystal growth process, the IECOFS is capable of monitoring the average crystal growth process, which allows

10.1021/ie901471p  2010 American Chemical Society Published on Web 04/13/2010

Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010

Figure 1. The two alternate reaction cells used in this study showing the arrangement of optical components.

the technique to be used to quantify the scale deposited on a given surface.16 A more detailed description regarding the optical characteristics of the IECOFS can be found elsewhere.21 The aim of this paper is to show the response of three crystallization sensors (electrical conductivity, turbidity, and IECOFS) exposed to the same crystallizing solution under varying conditions. The ability of the three techniques to assess scale formation will be investigated as well as the ability to calibrate the IECOFS to allow quantification of the scale deposition process. Materials and Methods Calcium carbonate (CaCO3) crystals were chemically deposited onto the exposed cores of silica target fibers by immersing the fibers in a solution made by mixing equal volumes of CaCl2 (0.0035 M) and Na2CO3 (0.0035 M). All chemicals used were of laboratory grade. The silica fibers were PUV-600T or a PUV200T fiber (Ceram Optec, MA USA), with 600 and 200 µm diameter fused silica cores, respectively. The fiber cores, 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 cell arrangement was made from Perspex and capable of simultaneously performing measurements of fiber signal (optical power attenuation), measures of homogeneous crystallization (electrical conductivity and turbidity), and temperature in response to crystal formation and deposition (cell A in figure 1). The reaction 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 was measured by monitoring the attenuation of a 5 mW He-Ne laser (λ ) 632.9 nm, Uniphase, CA) directed through windows in the side of the cell. The electrical conductivity was measured by means of two platinum electrodes, placed in direct contact with the solution and connected to a conductivity meter (Beta 81, CHK Engineering, Australia). The solution temperature was measured using a thermistor (EC95F502W, Vishay Americas, CT). All experiments were conducted at 28 °C unless otherwise indicated. Cell B, which has a capacity of 150 mL, was fabricated from stainless steel with the fiber immersed horizontally (Figure 1). Radiation from 5 mW He-Ne lasers (λ )

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632.9 nm, Uniphase, CA) was coupled into each fiber using a precision optical fiber coupler (F916, Newport Corp., Irvine, CA). In order to create continuous supply conditions, the reactants were pumped from separate containers into the cell using a peristaltic pump (minipuls 2, Gilson, France) and combined just before entering the cell, at an overall rate of 15 mL/min. Continuous measurement of fiber output signals was performed using photovoltaic detectors (UDT PIN 10DP, United Detector Technologies, CA) connected to an A-D converter (USB-6009 A-D converter, National Instruments, TX) and data recorded to file using an in-house program written in LabVIEW 7 Express (National Instruments, TX). When single optical fiber output measurements were required, a photometer (20 µW20 mW, Industrial Fiber Optics, AZ) was used. Optical power attenuation (AttdB) was calculated using AttdB ) -10 log10

( ) P P0

(1)

where P and P0 are the output and input optical power, respectively. To conduct successive sets of experiments using the same optical fiber, calcium carbonate crystals were chemically removed from the exposed cores after each experiment following the method of Gill.22 The extent of scale growth on the fiber surface was determined using the diffraction pattern produced by a He-Ne laser (5 mW, λ ) 632.9 nm, Uniphase, CA) directed at right angles to the fiber. The thickness was measured using d + 2η )

lλ 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 fiber, λ was the wavelength of the laser radiation, d is the fiber diameter, and η is the scale layer thickness. Over a period of 90 min, the mass of calcium carbonate scale accumulating on 600 µm diameter silica optical fiber cores was periodically determined by treating five 3 cm fiber segments using 10 mL of a 0.03 M HCl solution. Each group of five segments was extracted from the solution at 5 min intervals up to 40 min, then every 10 min up to 90 min. The dissolved calcium ion concentration was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES) (Varian, Vista-MPX CCD simultaneous ICP-OES, Australia). The 616.217 nm line was used for the calcium analysis. The mass of scale deposited at each time interval was determined assuming the scale was entirely composed of calcium carbonate. The mass of the scale is given as a relative mass, represented as microgram (µg) of scale per gram (g) of clean optical fiber core. At each extraction time, the IECOFS optical attenuation was recorded, using an optical fiber of equal diameter immersed in the crystallizing solution. Results Response of Optical Fiber, Turbidity, and Electrical Conductivity Sensors to Heterogeneous and Homogeneous Crystallization. Previously we have shown that the responses of the three scale sensors (IECOFS, turbidity, and electrical conductivity) to the crystallization of calcium oxalate in a similar system were highly correlated.17 However, the turbidity and electrical conductivity (eC) responses should be dominated by the

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Figure 2. Measured optical power attenuation of a turbidity sensor and the IECOFS and the change in electrical conductivity as functions of time in response to exposure to suspended crystals of CaCO3 and the absence of heterogeneous (surface) crystal growth.

homogeneous crystallization process, while the IECOFS should only respond to the heterogeneous crystallization process. The similar responses observed by Wallace et al.17 for the sensors may be explained by the high supersaturation ratio of the solutions used in the experiments. A lower supersaturation ratio should result in a very low response for the turbidity measurement, as a high supersaturation ratio is required for homogeneous crystallization to occur.23 If the eC signal is present in a low supersaturation ratio environment, it would then also be possible to deconvolute the eC response to the heterogeneous and homogeneous crystallization processes. In contrast to these techniques of scale detection, the IECOFS should be insensitive to the presence and growth of homogeneously nucleated, suspended particles and be only sensitive to particles that adhere to the fiber surface.21 These particles can come from heterogeneous nucleation and secondary nucleation, and this process resembles closely the total crystallization process of scale formation on any of the surfaces present in solution and therefore the IECOFS would be more suitable for acquiring a more accurate description of the build up of scale on surfaces. To test the relative sensitivity of the three techniques toward suspended crystals created by homogeneous crystallization, the three sensors were exposed to a crystallizing solution while measuring the presence of suspended crystals only. Preventing heterogeneous crystallization can be achieved by performing the crystallization in a separate container and introducing the solution to the measurement cell at set time intervals. Hereby the effect of the presence and growth of large particles (homogeneously nucleated crystals) on the existing techniques, and the IECOFS can be investigated. Figure 2 compares the response of conventional turbidity and eC measurements (plotted as -∆eC), and the IECOFS attenuation resulting from exposure to a suspension of CaCO3 crystals. With homogeneous crystal growth in the solution, the attenuation of the turbidity sensor, due to scattering of the laser beam, rapidly increases with time and then eventually plateaus as the system comes to equilibrium. The eC measurement also shows a similar response as ions are consumed in the crystal forming process. Unlike the turbidity and eC sensors, the IECOFS gave no response to the presence of the suspended crystals in solution. Lamb et al.24 explained this effect in a similar system of suspended clay particles in terms of the relative size of the suspended particles and the penetration of the evanescent field of the optical fiber. The space separation between the scattering particles was large enough for the optical fiber’s evanescent field to pass through the measurement solution without significant attenuation of the guided radiation from the fiber core.24 The insensitivity of the IECOFS to the suspended crystals observed is consistent with the hypothesis of Lamb et al.24 and shows a

Figure 3. Plot showing the difference between IECOFS and conventional techniques of turbidity and electrical conductivity in response to CaCO3 crystallization under continuous supply conditions. Solution temperature was 25 °C.

major advantage of IECOFS over the alternate methods of monitoring scale formation. Continuous Supply Conditions. Previously reported crystallization studies have used single batch crystallizing solutions, where the reactants are limited, and ultimately exhausted as the crystal growth process moves toward thermodynamic equilibrium.25 However, industrial processes often operate under “continuous supply conditions” in which the reactants are continuously replenished. Operations under continuous supply conditions can result in increased scale deposition, as the crystal growth process will never reach thermodynamic equilibrium. The comparison between the IECOFS optical power attenuation and the conventional measurements of turbidity and electrical conductivity (eC), as a function of time, for a system under continuous supply conditions (i.e., flow cell), is shown in Figure 3. In a flow system, the continuous supply of scale forming ions will lead to a “steady state condition” in which there is no noticeable decline in the electrical conductivity, even though the scale continues to increase. A similar steady state will be observed in the turbidity measurement. Here the crystals are continuously removed from the sensing region (i.e., laser beam) and replenished by newly formed crystals formed below the sensing region. In contrast, the continuous onset of scale on the optical fiber core surface shows a constant increase in attenuation of the IECOFS output signal. The result shown in Figure 3 is consistent with the earlier findings of Hasson et al.,26 which showed a continuous deposition of scale in continuous supply conditions. As the IECOFS has been shown to be insensitive to homogeneous crystallization, its response is a true representation of scale deposition and allows quantitative data of scale deposition to be obtained. Although Figure 3 appears to show a linear response of the IECOFS to deposition of scale on the fiber surface, the fact that the surface coverage of crystals on the exposed fiber core cannot exceed 100% sets a maximum limit on the amount of heterogeneous crystal growth able to be detected. The effect of the continuous scale buildup on the fiber core surface can be investigated by means of laser diffraction, a technique that has previously been used to show that the IECOFS attenuation closely follows the crystal growth process.16 Figure 4 shows a graph of IECOFS attenuation as a function of scale layer height determined by laser diffraction) for a 200 µm silica core fiber under continuous-supply conditions. The IECOFS attenuation increases linearly with increasing scale layer height until approximately 13.5 µm. Beyond this point further crystal growth, as indicated by continuously increasing scale layer height, results in no change in the IECOFS output. At a scale layer height of 13.5 µm the crystals must cover the entire fiber core surface. The key point is that the IECOFS ability to respond to heterogeneous crystal growth is effectively limited to crystal

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Figure 4. Maximum attenuation of guided radiation as a result from the continuous deposition of the calcium carbonate scale on a 2 cm exposed core section of a 200 µm diameter silica optical fiber. Error bars result from a 2% uncertainty in measuring the distance between diffraction minima.

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by homogeneous crystallization, which is an important aspect in the evaluation of the IECOFS as a scale only sensor. This is in contrast with the conventional techniques, which have been shown to respond to the presence of suspended crystals. The continuous deposition of scale on the optical fiber core under flow conditions showed a continuous increase in attenuation of the guided radiation, unlike the conventional techniques. It was further shown that continuous supply conditions can saturate the fiber core surface, after which the core surface needs to be reinitiated, that is, cleaned, before scale measurements are able to be continued. Quantifying scale deposition is an interesting issue and one which we have shown may be possible with the IECOFS after calibration of the sensor’s response to the scale deposited on the fiber core surface. Acknowledgment The authors acknowledge the support of the University of New England’s Science Engineering Workshop in constructing the apparatus. Furthermore, the primary author gratefully acknowledges receipt of a Postgraduate Research scholarship (UNERS) and a Dorothy Mackay Travelling Scholarship (2009), both from the University of New England. Literature Cited

Figure 5. IECOFS attenuation as a function of the relative calcium carbonate scale mass, determined from ICP-AES analyses on 600 µm diameter silica core segments. Error bars result from the uncertainty in the repeat measurement of the ICP-AES.

growth up to 100% surface coverage of the exposed fiber core. Beyond this limit there appears to be no further optical response of the IECOFS. Therefore, the core surface needs to be reinitiated, that is, cleaned before scale measurements can continue. Quantifying Scale Deposition. Scale sensors may be used not only to detect scale formation but to estimate the total scale mass deposited.27 The ability of the IECOFS to monitor heterogeneous crystallization suggests the potential of the technique to quantify the amount of scale deposited on a surface. Figure 5 shows the measured attenuation of the 600 µm diameter silica IECOFS as a function of the relative calcium carbonate scale mass on 600 µm diameter silica fiber core segments. The correlation between the IECOFS attenuation and the relative scale mass is likely to be controlled by the effect of the crystal growth process on the IECOFS measurements, in which the surface coverage by the scale growth results in the reduction of optical fiber output.16 This result confirms earlier reports on the correlation of scale mass deposition on a surface of a rotating disk electrode27 and the linearity shown in Figure 5 suggests that the sensor could be calibrated to quantify the extent of scale deposition; a point of interest in managing remedial or preventative strategies in an industrial context. Conclusion The results presented in this paper show that the intrinsic exposed core optical fiber sensors (IECOFS) have several advantages over conventional turbidity and electrical conductivity measurements for monitoring scale deposition. The IECOFS has been shown to be insensitive to suspended crystals caused

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ReceiVed for reView September 18, 2009 ReVised manuscript receiVed March 18, 2010 Accepted March 25, 2010 IE901471P