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Ind. Eng. Chem. Res. 2008, 47, 1066-1070
MATERIALS AND INTERFACES Assessment of an Intrinsic Optical Fiber Sensor for in Situ Monitoring of Scale-Forming Salts Andrew D. Wallace,† Martijn Boerkamp,† Peter G. Lye,† David W. Lamb,† William O. S. Doherty,‡ and Christopher M. Fellows*,† School of Science and Technology, UniVersity of New England, Armidale, NSW 2351, Australia, and Sugar Research and InnoVation, Queensland UniVersity of Technology, Q 4001, Australia
An exposed fiber core (EFC) method employing poly(methyl methacrylate) optical fibers was used to monitor the formation of calcium oxalate monohydrate scale under laboratory conditions using the phenomenon whereby development of a high-refractive-index scale coating on the fiber surface attenuated the guided radiation in the fiber core. Formation of scale on the optical fibers was verified by scanning electron microscopy. The results obtained by this method were comparable to results obtained from turbidity and conductivity measurements, but whereas these latter methods rely on phenomena in the bulk that might or might not be relevant to the kinetics of scaling per se, the optical fiber method directly interrogates phenomena at the solid/liquid interface. Introduction Scaling can be defined as the accumulation of undesired soluble and insoluble materials at phase interfaces. Scale usually consists primarily of one or more sparingly soluble ionic salts in a matrix of organic matter and silica and causes increased downtime in many industries including oil production, sugar production, geothermal drilling, desalination, pulp and paper production, and alumina production.1-4 Fouling is generally classified according to the principal process giving rise to the phenomenon: crystallization, particulate deposition, chemical reaction, corrosion, or biological fouling. Most industrial processes involve a combination of these processes, with crystallization and particulate fouling of most significance. Many techniques are currently used to study these processes such as visual observation, electrical conductivity measurements, and constant composition monitoring through pH control and light scattering (e.g., turbidity). Light-scattering techniques depend on the fact that the logarithm of the ratio of the intensities of light passing into and out of a suspension, measured under conditions where absorption is not important, is proportional to the sum of the scattering cross sections for all of the particles present in the suspension. For scattering particles that are large compared to the wavelength of light used, this is simply twice the cross-sectional area of the particles.5 The shortcomings of techniques such as turbidity and electrical conductivity monitoring are that they are unstable and insensitive in concentrated solutions (e.g., sucrose) or in solutions with high background conductivity (e.g., brine). Furthermore, they are unsuitable for use to determine the amount of material that actually deposits on a surface. Continuous in situ measurement of the heat-transfer coefficient of the fouled * To whom correspondence should be addressed. Tel.: +61 2 6773 2470. Fax: +61 2 6773 3268. E-mail:
[email protected]. † University of New England. ‡ Queensland University of Technology.
surface is the best current method for monitoring the process of scale formation, but this is not always readily adaptable to many applications.6 As such, there is a need to develop new methods for monitoring fouling. In recent years, modified optical fibers have been used as efficient carriers of spectroscopic light to and from a test sample.7,8 The light is transmitted via total internal reflection at the interface of a higher-refractive-index core, commonly silica or poly(methyl methacrylate), and a lower-refractive-index cladding. If polymer optical fibers are used, low-cost measurement probes can be realized.7 Of particular interest are methods that remove a section of the fiber cladding and use the test liquid as a pseudocladding: the exposed fiber core (EFC) method. If the exposed core is subjected to an environment with a lower refractive index than the core, the fiber remains capable of guiding light via total internal reflection. In optical fibers, a small component of the guided radiation penetrates into the cladding material; this is known as the evanescent field. The use of the evanescent field in exposed core sensors is the basis of fiber evanescent field absorption (FEFA) spectroscopy, which has been used to measure the absorption characteristics of chromophores in solution.9-11 Notwithstanding evanescent fieldrelated phenomena, any physical modification of the surface of the exposed core that results in a change in effective refractive index outside the core, such as the growth of crystals on the surface, will also affect the guided radiation. The higher refractive index of the crystals bound to the exposed fiber core will result in refraction of this radiation out of the core and into the crystals. This loss of light, measured as an absorbance signal, can be related to the surface crystal growth. One immediate advantage of this technique over conventional methods of scale detection is that the obtained signal is a direct measurement of crystal growth or deposition on the surface. The evanescent field penetrates on the order of
