137 research outputs found

    A novel sensor measuring local voidage profile inside a fluidised bed reactor

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    Liquid-solid fluidisation is frequently encountered in drinking water treatment processes, often to obtain a large liquid-solid interfacial surface area. A large surface area is crucial for optimal seeded crystallisation in full-scale softening reactors. Due to crystallisation, particles grow and migrate to a lower zone in the reactor which leads to a stratified bed. Larger particles adversely affect the surface area. To maintain optimal process conditions in the fluidised beds, information is needed about the distribution of particle size, local voidage and available surface area, over the reactor height. In this work, a sensor is developed to obtain the hydraulic state gradient, based on Archimedes’ principle. A cylindrical heavy object is submerged in the fluidised bed and lowered gradually while its weight is measured at various heights using a sensitive force measuring device. Based on accurate fluidisation experiments with calcite grains, the voidage is determined and a straightforward empirical model is developed to estimate the particle size as a function of superficial fluid velocity, kinematic viscosity, suspension density, voidage and particle density. The surface area and specific space velocity can be estimated accordingly, which represent key performance indicators regarding the hydraulic state of the fluidised bed reactor. The prediction error for voidage is 5 ± 2 % and for particle size 9 ± 4 %. The newly developed soft sensor is a more time-effective method for obtaining the hydraulic state in full-scale liquid-solid fluidised bed reactors

    Non-invasive and non-intrusive diagnostic techniques for gas-solid fluidized beds – A review

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    Gas-solid fluidized-bed systems offer great advantages in terms of chemical reaction efficiency and temperature control where other chemical reactor designs fall short. For this reason, they have been widely employed in a range of industrial application where these properties are essential. Nonetheless, the knowledge of such systems and the corresponding design choices, in most cases, rely on a heuristic expertise gained over the years rather than on a deep physical understanding of the phenomena taking place in fluidized beds. This is a huge limiting factor when it comes to the design, the scale-up and the optimization of such complex units. Fortunately, a wide array of diagnostic techniques has enabled researchers to strive in this direction, and, among these, non-invasive and non-intrusive diagnostic techniques stand out thanks to their innate feature of not affecting the flow field, while also avoiding direct contact with the medium under study. This work offers an overview of the non-invasive and non-intrusive diagnostic techniques most commonly applied to fluidized-bed systems, highlighting their capabilities in terms of the quantities they can measure, as well as advantages and limitations of each of them. The latest developments and the likely future trends are also presented. Neither of these methodologies represents a best option on all fronts. The goal of this work is rather to highlight what each technique has to offer and what application are they better suited for

    Assessment of experimental methods for measurements of the horizontal flow of fluidized solids under bubbling conditions

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    Dual fluidized bed systems are indispensable for future energy systems that require solids cycling between different atmospheres. However, controlling the residence time of solids in the reactor, which is crucial for controlling the heat and mass transfer of the fuel, is a significant challenge. This study investigates four experimental techniques to quantify the horizontal flow of solids fluidized under bubbling conditions: integral mass accumulation; differential mass accumulation; thermal tracing; and magnetic solids tracing. Integral mass accumulation entails collecting bed material using a defluidized box within a given time period. Differential mass accumulation measures the material accumulation rate in a section of the bed that is monitored using pressure measurements. Thermal tracing calculates the solids flow rate by solving the heat balance to match the temperature field captured by a thermographic camera. Magnetic solids tracing involves injecting a batch of magnetic tracer solids into the reactor and then measuring the residence time distribution using impedance coils. The experiments were conducted under down-scaled conditions that resemble large-scale operations with a length scaling factor of 0.12. For this study, three operational parameters were varied: the fixed bed height; the volumetric flow rate of the conveying air; and the fluidization velocity in the bed. The horizontal solids circulation rates achieved ranged from 1.7 710−4 to 10 kg/m\ub7s, corresponding to 1.2 710−3 to 70 kg/m\ub7s on a hot up-scaled basis, which is a relevant range to indirect biomass gasification in an industrial setting. The three selected operational parameters led to increases in the horizontal solids flow. While all four methods replicated the trends, quantitative variations in the measured circulation rates occurred due to the inherent characteristics of the methods. High circulation rates resulted in a continuous decrease in the solids inventory, leading to an underestimation of the circulation rate when using the integral mass accumulation method. The accuracy of the differential mass accumulation method relied on transient pressure measurements, which were less-effective at low solids flow rates. Conversely, the accumulation time required for pressure measurements was reduced at high circulation rates, resulting in uncertainties in the analysis. The accuracy of the thermal tracing method decreased drastically with higher solids circulation, resulting in an overestimation of the circulation rate. Moreover, low circulation rates adversely affected the accuracy of the magnetic solids tracing by producing barely discernible tracer concentration gradients

    Experimental, numerical and analytical study for the improvement of biomass fluidized bed gasifiers

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    The gasification of biomass is considered one of the most important sources of renewable energy due to the sustainability of agriculture waste around the world. There are many types of gasification systems depending on the mechanism of gasification. BFBG is one of the powerful gasifiers due to the mixing mechanism between the solid materials (biomass and the inert material) and the gas phase (air). Gasification process in the BFBG involves three main interactive factors: hydrodynamics, heat transfer and chemical reaction. The present work focuses on improving the hydrodynamic performance and the product gas quality of a new BFBG developed at Cardiff University. Hydrodynamics has been analysed experimentally and numerically using four different distributors designed to improve the fluidized bed fluidic patterns. The tests have been performed experimentally using a representative perspex prototype, while an isothermal 3D unsteady-state CFD simulation by using OpenFOAM software based on multiphase resolution was employed in order to select the optimal design that can improve the system performance. The post improving of the BFBG product gas with catalyst has been analysed numerically by using ASPEN PLUS software. The hydrodynamic behaviour of the BFBG with four different air distributors was studied experimentally in terms of pressure drop and bubble formation. Two design factors were observed as the major contributors towards the impact on the BFBG performance, i.e. the orifice size and the distribution of orifices. Small orifices with triangular arrangement have demonstrated superior performance than large orifice size with square arrangement. Similar findings were obtained from the CFD simulation of the BFBG with the four distributors with an accepted comparison with the experimental results and literature. Regarding the post -gasification improvement, ASPEN PLUS analysis showed the using of BFBG product gas with suitable amount of N2 and Ar can increase the H2 and CO selectivity, H2/CO ratio and decrease the heat duty. The analysis results were compared with literature
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