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• W287169241 abstract "This work presents the progress made in the last years at the Vienna University of Technology on the design and optimization of a non-intrusive method, based on inductance changes of a coil and able to quantitatively detect fluid-dynamically similar ferromagnetic tracer particles in a fluidized bed cold flow model. The method finds application not only in RTD determination but also in the measurement of solids circulation rate and bed density in similar units. INTRODUCTION The use of gas-solid reactors is widely spread; more precisely, circulating beds are increasingly present in many applications and processes due to their advantages regarding mass and heat transfer and their operational flexibility. The efficiency and extent of the thermal and chemical exchange processes that take place in a fluidized bed reactor strongly depend on the contact efficiency and the contact time between the phases. These factors are particularly important in catalytic processes and in those where high reactivity is critical. Investigations on residence time distribution (RTD) as well as on distribution of solids aid in understanding the fluid-dynamics, and are essential for reactor design, plant operation and optimization of existing circulating fluidized beds. Finding adequate tracer materials and detection methods to make these analyses is difficult. This work presents the progress lately made at the Vienna University of Technology on the design, construction and optimization of a non-intrusive online method able to detect, with high resolution, ferromagnetic tracer particles in a fluidized bed cold flow model. IDENTIFICATION OF REQUIREMENTS After a careful literature review on existent tracer methods used to determine residence time distribution of solids in fluidized beds (1), (2), and considering that the system was planned to be implemented in cold flow models, the requirements and expected features of the RTD measurement system were determined. An impulseresponse tracer measurement was selected. Tracer particles were expected to hold very similar fluid-dynamically-relevant properties in comparison with the bulk of bed material and not to involve health risks. With regard to the measurement system, some of the most important conditions desired were: ⋅ Sensitivity of the measurement should be as high as possible; use of a small mass of tracer is advantageous. ⋅ The measurement should be reproducible and repetitions should be possible within reasonable periods of time. ⋅ Ideally, calculation of tracer concentration (and hence RTD) should be possible without need of intricate assumptions or corrections. ⋅ The detection device or method should not imply any disruption of the internal flow pattern (particularly, pressure and inventory should not be effected). ⋅ Measurement should be possible under steady state operation conditions. ⋅ The response of the detection method should be fast enough to capture the features of RTD in fast beds, which change in very short time. On this basis, ferromagnetic particles were chosen as good candidates to be used as tracer in a cold flow model where heavy particles, such as bronze, are used (this is to maintain the fluid-dynamic similarity with the hot unit e.g. combustors, gasifiers or chemical looping units). Ferromagnetic particles exhibit a number of advantages for their use in such tests (3), (4): they neither require special handling nor are they toxic; the density and size can be modified to fit fluid-dynamic requirements by forming a composite with a polymer; the magnetic properties of the material do not deteriorate with time or use, and temperature has only a slight influence as long as kept below certain limits (Curie temperature of Fe 770°C); the particles can be easily separated from bed material by means of magnets; and most importantly, a simple coil inductor can be used for the detection of tracer particles. MEASUREMENT PRINCIPLE If a magnetic field is generated by means of a coil of N turns, a length l, a cross section area of the conductor A, and carrying a current of magnitude I, then the magnetic field strength designated by H is given by Equation 1 (5). The magnetic induction, or magnetic flux density, denoted by B, represents the magnitude of the internal field strength within a substance that is subjected to a magnetic field of strength H (Figure 1). The magnetic field strength and flux density are related according to Equation 2, where μ is the permeability. In vacuum B0=μ0⋅H, where μ0 is the permeability of a vacuum, a universal constant. The ratio of the permeability in a material to the permeability in a vacuum is as shown in Equation 3, where μr is called the relative permeability. For a cylindrical coil with a core different than vacuum (Figure 1) the inductance is given by Equation 4. Thus, the inductance L will change proportionally to μr (6). In this way, the presence of dispersed ferromagnetic particles in the core area of a coil would influence the coil's inductance in certain proportionality with respect to the concentration of ferromagnetic material in the core volume. The chance in inductance could be analyzed in order to allow quantitative concentration measurement." @default.
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• W287169241 date "2013-01-01" @default.
• W287169241 modified "2023-09-27" @default.
• W287169241 title "Non-Intrusive Online Detection of Ferromagnetic Particles for Measurement of Bed Density and Residence Time Distribution in Circulating Fluidized Bed Systems" @default.
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