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Sensors 2012, 12, 12519-12544; doi:10.3390/s120912519 sensors ISSN 1424-8220 Review Fiber-Optical Sensors: Basics and Applications in Multiphase Reactors Xiangyang Li, Chao Yang *, Shifang Yang and Guozheng Li Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; E-Mails: (X.L.); (S.Y.); (G.L.) * Author to whom
  Sensors   2012 , 12 , 12519-12544; doi:10.3390/s120912519  sensors ISSN 1424-8220  Review Fiber-Optical Sensors: Basics and Applications in Multiphase Reactors   Xiangyang Li, Chao Yang *, Shifang Yang and Guozheng Li Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; E-Mails: (X.L.); (S.Y.); (G.L.) *  Author to whom correspondence should be addressed; E-Mail:; Tel.: +86-10-6255-4558; Fax: +86-10-8254-4928.  Received: 30 July 2012; in revised form: 7 September 2012 / Accepted: 7 September 2012 /  Published: 13 September 2012 Abstract:  This work presents a brief introduction on the basics of fiber-optical sensors and an overview focused on the applications to measurements in multiphase reactors. The most commonly principle utilized is laser back scattering, which is also the foundation for almost all current probes used in multiphase reactors. The fiber-optical probe techniques in two-phase reactors are more developed than those in three-phase reactors. There are many studies on the measurement of gas holdup using fiber-optical probes in three-phase fluidized beds, but negative interference of particles on probe function was less studied. The interactions between solids and probe tips were less studied because glass beads etc . were always used as the solid phase. The vision probes may be the most promising for simultaneous measurements of gas dispersion and solids suspension in three-phase reactors. Thus, the following techniques of the fiber-optical probes in multiphase reactors should be developed further: (1) online measuring techniques under nearly industrial operating conditions; (2) corresponding signal data processing techniques; (3) joint application with other measuring techniques. Keywords:  fiber-optical sensor; probe; multiphase reactor; local flow characteristics   OPEN ACCESS  Sensors   2012 , 12  12520   Abbreviations BSD bubble size distribution CCD charge-coupled device CFD computational fluid dynamic CLD chord length distribution DAF dissolved air flotation DSD drop size distribution ECT electrical capacitance tomography EMI electromagnetic interference immunity ERT electrical resistance tomography FBR forward–backward ratio FBRM focused beam reflectance measurement GLSCFB gas-liquid-solid circulating fluidized bed IAC interfacial area concentration LED light emitting diode LSCFB liquid-solid circulating fluidized bed ORM optical reflectance measurement PBE population balance equation UV-VIS ultraviolet-visible 1. Introduction  Multiphase reactors are the most important equipment in the chemical industry, where chemical reactions take place involving several reactants in different phases. To describe and design multiphase reactors, traditional approaches based on empirical rules and correlations rely to a large extent on the measurements made under conditions as relevant as possible to industrial practice. Modern computational fluid dynamics (CFD), which has been extensively used for the numerical simulation of multiphase reactors [1–5], also requires the information on local and transient flow characteristics to  build precise physical models. Reliable measuring techniques are therefore needed for the rational description and design of multiphase reactors. The measurement techniques for multiphase reactors can be classified as invasive (such as fiber-optical  probes [6,7], impedance probes [8,9], heat transfer probes [10,11] and ultrasound probes [12,13]) and non-invasive techniques (including optical techniques [14–18] and tomography [19–23]). Boyer et al  . [24] have reviewed and compared them in detail. Invasive measuring techniques cannot be avoided though non-invasive techniques are intensively developed for the analysis of multiphase flows. This is  particularly true for highly turbulent systems, due to two main reasons: (i) in case of nearly industrial operating conditions (particular physico-chemical environment, opaque walls, high gas holdups or solid concentrations, etc .), non-invasive techniques become ineffective; (ii) non-invasive techniques are often difficult and expensive for industrial applications. In all of non-invasive techniques, fiber-optical probes may be the most promising ones because of their inherent advantages such as harsh environment tolerance and very small size, which will be discussed in Section 2.1. Benefitting from great developments in the optoelectronic and fiber-optical  Sensors   2012 , 12  12521  communications industries, great progress has also been made in fiber-optical sensor technology with vastly improved optical and mechanical properties and lower cost of the components over the past 30 years. As a result, the ability of fiber-optical sensors to displace traditional sensors for rotating, accelerating, electric and magnetic field measurements, temperature, pressure, acoustics, vibration, linear and angular positions, strain, humidity, viscosity, chemical measurements, and a host of other sensor applications has been enhanced [25]. A number of useful reviews such as those by Kersey [26], Grattan and Sun [27] and Lee [28], and monographs such as those by Yin et al  . [29] and Udd et al  . [30] have been produced over the years. Progresses in fiber-optical sensor technique open a door for the measurements of multiphase reactors and can offer many important measurement opportunities and great potential applications in this area. The aim of this paper was to review the most significant developments and applications of fiber-optical probes for multiphase reactors. The remainder of this paper is organized as follows: in the next section, the basics of fiber-optical sensors are presented. Then, significant developments and applications of fiber-optical sensors/probes for multiphase reactors (involving gas-solid, liquid-solid, gas-liquid, liquid-liquid, gas-liquid-solid systems) will be introduced. Finally, the future research trends in the field of fiber-optical sensors/probes for multiphase reactors will be discussed and summarized. 2. Fiber-Optical Sensor Basics 2.1. Why Fiber-Optical Sensors? The inherent advantages of fiber-optical sensors range from their: (1) harsh environment capability to strong EMI (electromagnetic interference immunity), high temperature, chemical corrosion, high  pressure and high voltage; (2) very small size, passive and low power; (3) excellent performance such as high sensitivity and wide bandwidth; (4) long distance operation; and (5) multiplexed or distributed measurements, were heavily utilised to offset their major disadvantages of high cost and end-user unfamiliarity [29]. 2.2. Compositions of Fiber-Optical Sensors As shown in Figure 1, a fiber-optical sensor system consists of an optical source (laser, LED, laser diode, etc .), optical fiber, sensing or modulator element transducing the measurand to an optical signal, an optical detector and processing electronics (oscilloscope, optical spectrum analyzer, etc .) [25]. The advent of laser opens up a new world to researchers in optics. Light sources used to support fiber-optical sensors produce light that is often dominated by either spontaneous or stimulated emission. A combination of both types of emission is also used for certain classes of fiber-optical sensors. Figure 1.  Basic components of a fiber-optical sensor system [25].  Sensors   2012 , 12  12522   2.3. Fiber-Optical Sensor Classifications Fiber-optical sensors are often loosely grouped into two basic classes referred to intrinsic, or all-fiber and extrinsic, or hybrid sensors. The intrinsic fiber-optical sensor has a sensing region within the fiber and light never goes out of the fiber. In extrinsic sensors, light has to leave the fiber and reach the sensing region outside, and then comes back to the fiber [29]. Furthermore, fiber-optical sensors can also be classified under three categories [25]: the sensing location, the operating principle and the application, as seen in Table 1.  Table 1.  Fiber-optical sensor classifications under three categories.   Category Class Trait sensing location point sensors with a sensitized tip in the measurand field distributed sensors to measure along the length of the fiber itself quasi-distributed sensors “in between” point and distributed sensors operating principle intensity sensors  phase sensors frequency sensors  polarization sensors application physical sensors for temperature, stress, velocity, etc . chemical sensors for pH, gas analysis, spectroscopic studies, etc .  bio-medical sensors for blood flow, glucose content, etc . 2.4. Current Applications Fiber-optical sensors have been the topic of considerable amounts of research for the past 30 years and their application fields are being extended continuously in two major fields, i.e. , as a direct replacement for existing sensors and the development/deployment of fiber-optical sensors in new areas. To date, the most highlighted application fields of fiber-optical sensors are in large composite and concrete structures, electrical power industry, medicine, chemical sensing, and gas and oil industry. A wide range of environmental parameters such as position, vibration, strain, temperature, humidity, viscosity, chemicals, pressure, current, electric field and several other environmental factors have been widely monitored. More detailed information on the applications can resort to references [25] and [29]. 3. Application of Fiber-Optical Probes in Multiphase Reactors A multiphase system with gas as the dispersed phase may be a gas-liquid or gas-liquid-liquid or gas-liquid-solid system. Due to the too small differences in refractive index between gases and organic liquids, fiber-optical probes are rarely utilized in experimental studies on the measurement of gas-phase characteristics in a gas-liquid-liquid system. So the discussion on this system is combined with the gas-liquid one. The gas-liquid-solid system has two dispersed phases and the complicated effects between different phases make the experimental studies more difficult. So in Section 3.1, the main concern is the measurements in gas-liquid reactors, and the studies on gas-liquid-solid reactors
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