1,252 research outputs found

    Hydrodynamic and solid residence time distribution in a circulating fluidized bed: experimental and 3D computational study

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    Vertical profiles of local pressure, horizontal profiles of net vertical solid mass flux, and residence time distributions (RTD) of the solid phase are experimentally assessed in the riser of a small scale cold Circulating Fluidized Bed of 9 m high having a square cross section of 1111 cm. Air (density 1.2 kg/m3, dynamic viscosity 1.8Ă—10-5 Pa.s) and typical FCC particles (density 1400 kg/m3, mean diameter 70 mm) are used. The superficial gas velocity is kept constant at 7 m/s while the solid mass flux ranges from 46 to 133 kg/m2/s. The axial dispersion of the solid phase is found to decrease when increasing the solid mass flux. Simultaneously, 3D transient CFD simulations are performed to conclude on the usability of the eulerian-eulerian approach for the prediction of the solid phase mixing in the riser. The numerical investigation of the solid mixing is deferred until later since the near-wall region where the solid phase downflow and mixing are predominant is not well predicted in spite of well-predicted vertical profiles of pressure

    Hydrodynamic modelling of circulating fluidised beds

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    A one-dimensional model for the riser section of a circulating fluidised bed has been developed which describes the steady-state hydrodynamic key variables in the radial direction for fully developed axisymmetric flow. Both the gas and the solid phase are considered as two continuous media, fully penetrating each other. As a first approximation gas phase turbulence has been incorporated in our hydrodynamic model by applying a slightly modified version of the well-known Prandtl mixing length model. To solve the resulting set or transport equations, the solids distribution along the tube radius is required. Several strategies are given to obtain this information. In addition the effect of clusters on the momentum transfer between both phases has been modelled using an empirical correlation. Theoretically calculated results agree well with reported experimental data of different author

    Estimation and experimental validation of the circulation time in a 2D gas-solid fluidized beds

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    The circulation time is defined as the time required for a group of particles to reach the freeboard from the bottom of a fluidized bed and return to their original height. This work presents an estimation and validation of the circulation time in a 2D gas solid bubbling fluidized bed under different operating conditions. The circulation time is based on the concept of the turnover time, which was previously defined by Geldart [1] as the time required to turn the bed over once. The equation tc,est =2Ah′/Qb is used to calculate the circulation time, where A is the cross section of the fluidized bed, h′ is the effective fluidized bed height and Qb is the visible bubble flow. The estimation of the circulation time is based on the operating parameters and the bub ble phase properties, including the bubble diameter, bubble velocity and bed expansion. The experiments for the validation were carried out in a 2D bubbling fluidized bed. The dense phase velocity was measured with a high speed camera and non intrusive techniques such as particle image velocimetry (PIV) and digital image analysis (DIA), and the experimental circulation time was calculated for all cases. The agreement between the theoretical and experimental circulation times was satisfactory, and hence, the proposed estimation can be used to reliably predict the circulation time.Publicad

    Dynamic model of circulating fluidized bed

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    Circulating fluidized beds (CFBs) are used in many processes in the chemical industry to reduce pollution and increase efficiency. Optimization and control of CFBs are very important and require an accurate, real time, dynamic model to describe and quantify the process.;The present work focuses on modeling the transient behavior of large CFB units, whose flow characteristics were shown to yield C-shaped voidage profiles using cork as the fluidized material and air at ambient conditions.;The riser is modeled in two ways: (1) as a set of well-mixed tanks connected in series; (2) as a 1-D axisymetric cluster flow. The tanks-in-series model visualizes the riser as consisting of a series of well-mixed vessels. Using this method, the dynamic response time at different locations along the riser was estimated successfully. The cluster flow model assumes that gas and solids flows are unidirectional with no mixing in the axial direction, and the solids move upward in the riser as clusters. This model can be used to predict the smooth changes in voidage profiles for transient processes. The influence of exit is also considered and a modified cluster model can be extended to the entire riser which includes an acceleration region, developed flow region and exit region. It can also be applied to a reacting system.;A model based on the Ergun equation is developed to predict the solids flow rate and voidage in the dense phase of the standpipe. The profile of solids flow rate under unsteady state is also presented. Using this method, the dynamic response time at different locations along the standpipe is estimated successfully.;Using the pressure balance analysis, the above models are combined into an integrated CFB model. It can be applied to CFB real-time simulation under transient conditions

    Hydrodynamics and Micro Flow Structure of Gas-Solid Circulating Turbulent Fluidized Beds

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    Circulating Turbulent Fluidized Beds (CTFB) refer to fluidized beds integrated into high density circulating systems to simultaneously achieve highly efficient gas-solid interactions existing in turbulent fluidized beds and low solids backmixing featured by circulating fluidized beds. Hydrodynamics and micro flow structure were experimentally studied in a CTFB (3.6 m high and 0.104 m id) using 76 µm FCC particle with air velocities of 0.5 ~ 5.0 m/s and solids circulation rates of 0 ~ 420 kg/m2s The distributions of solids holdup were acquired using optical fibre probes and pressure transducers at sampling frequencies of 50 kHz × 131 s and 1 kHz × 400 s respectively. A Pseudo Bubble-Free Fluidized Bed was developed to dynamically calibrate the optical fibre probes. Based on statistical parameters, a Moment Consistency Data Processing Method (MCDPM) was proposed to calculate solids holdups of the dense and dilute phases from the experimental data. A Divided Phase Cross-Correlation Method (DPCCM) was adopted in cross-correlating the solids holdup signals of the dense and dilute phases to obtain the phase particle velocities. MCDPM provided average solids holdups of the dense and dilute phases and the phase fractions over bubbling (BFB), turbulent (TFB), circulating turbulent (CTFB), high density circulating (HDCFB) and circulating (CFB) fluidized beds. The flow structure in terms of phase division and the micro flow characteristics were studied across all five regimes from low to high velocities, CTFB was found to have strong similarities with TFB. Study on the detailed hydrodynamics and transition characteristics of the CTFB demonstrated that solids holdup distribution in CTFB was more homogeneous both axially and radially than that of other regimes, and the local solids flux and the local particle velocity were both proportional to the solids circulation rate. Microscopically, CTFB was characterized by dilute phase dominating flow in the centre and dense phase dominating flow in the annular region. Such flow structure was different from either dense phase dominating flow in BFB or dilute phase dominating flow in CFB. New criteria for the transition air velocities were proposed for CTFB. The results demonstrated that the onset transition velocity from BFB to CTFB remained nearly unchanged, and the ending transition air velocity from CTFB to CFB increased, with increasing solids circulation rate

    Distributor effects near the bottom region of turbulent fluidized beds

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    The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, Uc, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor platesPublicad

    Lattice Boltzmann based discrete simulation for gas-solid fluidization

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    Discrete particle simulation, a combined approach of computational fluid dynamics and discrete methods such as DEM (Discrete Element Method), DSMC (Direct Simulation Monte Carlo), SPH (Smoothed Particle Hydrodynamics), PIC (Particle-In-Cell), etc., is becoming a practical tool for exploring lab-scale gas-solid systems owing to the fast development of parallel computation. However, gas-solid coupling and the corresponding fluid flow solver remain immature. In this work, we propose a modified lattice Boltzmann approach to consider the effect of both the local solid volume fraction and the local relative velocity between particles and fluid, which is different from the traditional volume-averaged Navier-Stokes equations. A time-driven hard sphere algorithm is combined to simulate the motion of individual particles, in which particles interact with each other via hard-sphere collisions, the collision detection and motion of particles are performed at constant time intervals. The EMMS (energy minimization multi-scale) drag is coupled with the lattice Boltzmann based discrete particle simulation to improve the accuracy. Two typical fluidization processes, namely, a single bubble injection at incipient fluidization and particle clustering in a fast fluidized bed riser, are simulated with this approach, with the results showing a good agreement with published correlations and experimental data. The capability of the approach to capture more detailed and intrinsic characteristics of particle-fluid systems is demonstrated. The method can also be used straightforward with other solid phase solvers.Comment: 15 pages, 11 figures, 2 tables. In Chemical Engineering Science, 201

    Reactor Performances and Hydrodynamics of Various Gas-Solids Fluidized Beds

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    The reactor performances and hydrodynamics were systematically studied in a multifunctional fluidized-bed system which included a bubbling fluidized bed (BFB), a turbulent fluidized bed (TFB) as well as a newly identified circulating turbulent fluidized bed (CTFB) using the same batch of activated FCC particles. Catalytic ozone decomposition was employed as the model reaction to experimentally investigate the reactor performances of BFB, TFB and CTFB. The complete mappings of ozone concentration were obtained in these fluidized beds, showing close relationship with the solids holdup distributions. In the BFBs and TFBs, the study of scale-up effect revealed that the static bed height had almost no influence on the ozone concentration distributions, whereas the bed diameter affected the concentration distributions, especially in the bubbling regime. This work, for the first time, examined the reactor performance of a CTFB: the ozone concentration decreased along the axial direction with a large portion of reaction happening in the entrance section, while the “centre-high” and “wall-low” radial profile of ozone concentration was presented. Comprehensive evaluations on reactor performance were then conducted across the full spectrum of the commonly used fluidized-bed reactors, including BFB, TFB, CTFB, riser and downer, in order to illustrate the superior and inferior features for each. The clear correlations between ozone concentrations and solids holdups confirmed that the reactor performances were essentially controlled by the flow structures including gas/solids behaviour and distributions. Furthermore, the CTFB and downer showed a comparable reactor performance that was very close to that of a plug-flow reactor, resulting from the uniform flow structures with little backmixing. While the TFB demonstrated favourable reactor performance, the CTFB is still superior in reactor efficiency. The further deviation of the BFB and riser from a plug-flow reactor was due to the significant gas bypassing and backmixing. The performances of the various fluidized beds were then quantitatively characterized by gas-solids contact efficiencies. The hydrodynamics of the BFB, TFB and CTFB were also studied in order to help understand their reactor performances. An optical fibre probe was used to obtain the spatial distribution (i.e., axial and radial profiles of time-average data) and the temporal variation (i.e., time-serial data) of solid holdup. By analysing the instantaneous signals of solids holdup, the BFB was found to be dominated by a dense (solid) phase with a discrete dilute (bubble) phase, while the TFB exhibited a dynamic flow structure with the comparable dense (cluster) and dilute (void) phases. The CTFB experienced even more transient behaviour over the TFB, causing more interfacial activities. In addition, the CTFB successfully achieved a gas/solids upflow with solids circulation rates as high as 300 kg/m2s while maintaining a dense bed with solids holdup ranging from 0.25 to 0.35. The CTFB possesses many hydrodynamic advantages, such as uniform axial flow structure, homogeneously inter-diffused dilute/dense phases and no net solids downflow, leading to very favourable mass transfer and highly efficient gas-solids contact

    Modeling of non-uniform hydrodynamics and catalytic reaction in a solids-laden riser

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    The riser reactors are widely used in a variety of industrial applications such as polymerization, coal combustion and petroleum refinery because of the strong mixing of gas and solids that yields high heat and mass transfer rates, and reaction rates. In a Fluid Catalytic Cracking (FCC) process, the performance of riser reactor is strongly dependent on the interaction between the fluid and catalysts, since the reaction takes place on the active surface of the catalysts. This is why, the local coupling between hydrodynamics and reaction kinetics is critical to the development of riser reaction models. The local gas-solids flow structure in riser reactors is highly heterogeneous both in axial and radial direction with back-mixing of catalyst. The radial non-uniform gas-solid flow structure is presented as core-annulus regime, with up-flow of dilute suspension of fresh catalyst and hydrocarbon vapor in the core regime, which is surrounded by dense down-flow of deactivated catalyst in the wall regime. As a result, the reaction characteristics in core and wall regions are strikingly different. The performance of the riser reactor is also strongly dependent on the vaporization and reaction characteristics in the feed injection regime of the riser reactors. From the modeling point of view, to predict the reaction characteristics in riser reactors, there is a need to develop hydrodynamics model, which can predicts both axial and radial nonuniform distribution of hydrocarbon vapor and catalyst and back-mixing of catalyst. There is also need for reasonable description of mechanistic coupling between nonuniform flow hydrodynamics and the cracking kinetics. This dissertation is aimed to develop the mechanistic model for nonuniform hydrodynamics and catalytic reactions in a FCC riser reactor. A mechanistic model for multiphase flow interactions, vaporization of droplets and reactions in the feed injection regime is developed for to decide proper input boundary conditions for FCC riser reaction models. The dissertation is divided into the three major parts: 1) development of governing mechanisms and modeling of the axial and radial nonuniform distribution of the gas-solids transport properties in riser reactors 2) development of mechanistic model that gives a quantitative understanding of the interplay of three phase flow hydrodynamics, heat/mass transfer, and cracking reactions in the feed injection regime of a riser reactor 3) modeling of nonuniform hydrodynamics coupled reaction kinetics in the core and wall regime of the riser reactors. For the modeling of the axial nonuniform distribution of gas-solids transport properties, a new controlling mechanism in terms of impact of pressure gradient along the riser on the particles transport is introduced. A correlation for inter-particle collision force is proposed which can be used for any operation conditions of riser, riser geometry and particle types. For simultaneous modeling of axial and radial nonuniform distribution of the gas-solids phase transport properties, a continuous modeling approach is used. In this dissertation, governing mechanisms for radial nonuniform distribution of gas-solids phase is proposed based on which a mechanistic model for radial nonuniform distribution of the gas and solid phase transport properties is proposed. With the proposed model for radial nonuniform phase distribution, the continuous model can successfully predicts both axial and radial nonuniform distribution of phase transport properties. As the performance of the riser reactor is strongly influence by the vaporization and reactions in the feed injection regime, in this dissertation, a detailed mechanistic model for the multiphase flow hydrodynamics, vaporization and reaction characteristics in feed injection regime is established. To simulate the conditions of industrial riser reactor, the four nozzle spray jets were used, while overlapping of the spray jets is also considered. Finally, in this dissertation, a modeling concept for the reactions in the core and wall regime of the riser reactor is explored. The proposed modeling concept takes into the account very important missed out physics such as, non-thermal equilibrium between the hydrocarbon vapor and the feed, back mixing and recirculation of the deactivated catalyst, activity of catalyst in core and wall regime, and coupling between the flow hydrodynamics and reaction kinetics

    Heterogeneous flow structure and gas-solid transport of riser

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    This study aims to understand physical mechanisms of gas-solid transport and riser flow, investigate heterogeneous flow structures of gas-solid transport and their formation mechanism of the in riser flows, both in axial and radial directions. It provides sound interpretation for the experimental observation and valuable suggestion to riser reactor design. Chemical reaction is also coupled with flow hydrodynamics to board the industrial applications. This study mainly focuses on mathematical modeling approach based upon physical mechanism, and endeavor to validate model prediction against available experimental data. First of all, most important physical mechanisms including inter-particle collision force, gas/solid interfacial force and wall boundary effects, which are believed to be most important aspects of the flow hydrodynamics, have been investigated in this part. An energy-based mechanistic model was developed to analyze the partitions of the axial gradient of pressure by solids acceleration, collision-induced energy dissipation and solids holdup in gas-solid riser flows. Thought this part of study, important understanding of the inter-particle collision force (Fc), gas/solid interfacial force (FD) inside the momentum equations and energy dissipation (F), especially in dense and acceleration region, has been reached, Based on these understandings, a mechanistic riser hydrodynamic model was developed on the basis of gas-solid continuity and momentum equations, along with the better formulated drag force correlation and new formulation for moment dissipation of solids due to solids collisions. The proposed model is capable of yielding the coupled hydrodynamic parameters of solid volume fraction, gas and solid velocity, and pressure distribution along the whole riser. At the same time, special considerations are given to solids back-mixing and resultant cross- section area variation for the upward flow, which is especially prominent for low solids mass flow condition. With the further understanding of solid collision, gas/solid interfacial and wall boundary effects, in order to soundly interpret the well-known core-annulus 2-zone flow structure, newly discovered core-annulus-wall 3-zone structure and provide reasonable explanation for the choking phenomena, a comprehensive modeling of continuous gas-solids flow structure both in radial and axial directions has been presented. This model, assuming one-dimensional two-phase flow in each zone along the riser, consists of a set of coupled ordinary-differential equations developed from the conservation laws of mass, momentum, and energy of both gas and solids phases. This part of study not only provides reasonable explanation for the 2-zone and 3-zone structure , but also finds out the potential reasons for the choking phenomenon. In order to investigate the different riser inlet configuration\u27s effects on gas-solid mixing in dense region and improve the uniform inlet condition assumption in above models, a systemically study regarding with different inlet conditions have been done based on commercial package, Those simulation results are directly combined with model approach which reached the conclusion that riser flow structure an flow stability are weakly dependent on the type of solids feeding configuration. This part of study is specifically focused on chemical reaction coupled gas-solid transport flow hydrodynamics. The aim of this work is to develop a generic modeling approach which can fully incorporate multiphase flow hydrodynamics with chemical reaction process. This modeling approach opens up a new dimension for making generic models suitable for the analysis and control studies of chemical reaction units. The chemical reaction model was represented by a relatively simple four-lump based FCC reaction kinetic model, which will not bring us too complicated mathematical derivation without losing its popular acceptance. As a first endeavor to consider the significant mutual coupling between the flow hydrodynamics and cracking reaction, a localized catalyst to oil ratio is introduced. The new developed chemical reaction coupled hydrodynamic model was capable of quickly evaluating the flow parameters including gas and solid phase velocity and concentration, temperature and reaction yield profiles as the function of riser height
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