Residence Time Distribution Of Solids Research Articles

From circular to square‐based section spouted beds: Scale‐up and design overview of a multistage thermal unit

AbstractThis work summarizes the results of several research projects to demonstrate the possibility of scaling‐up spouted beds by assembling a number of square‐based jet units. Cylindrical vessels with a bottom cone and parallelepiped ones with a frustum base do not differ to a large extent in terms of hydrodynamic features. The existing correlations can be used to predict these square‐based spouted beds. Spouted beds can be very reasonably described as well‐mixed reactors. By assembling several units, it is possible to approach a plug flow state for solids residence time distribution (RTD) in continuous processes. Spouting stability is fully reached when the modules do not interfere each other: this goal is obtained by positioning the downstream units at a certain lower level and having the solids move by overflow; placing internal vertical baffles between contiguous units is the solution. Two hydrodynamical models are proposed to describe single and multiple spouted beds; their predictions help in choosing the best geometrical configuration for the assemblage. A possible application of a multiple spouted bed reactor was envisaged for gasifying pelletized textile residues to generate syngas which can be directly used in situ as a co‐fuel. Two units were built: a single 0.2 m side square‐based spouted bed furnace and the consequent dual stage scale‐up apparatus suitable for an auto thermal process. In the bottom stage, four units work independently to run combustion/gasification of low‐quality solid residues, while the upper stage had a cascade of four modules to run pyrolysis/gasification of selected biomass and obtain valuable secondary products.

From circular to square‐based section spouted beds: Scale‐up and design overview of a multistage thermal unit

AbstractThis work summarizes the results of several research projects to demonstrate the possibility of scaling‐up spouted beds by assembling a number of square‐based jet units. Cylindrical vessels with a bottom cone and parallelepiped ones with a frustum base do not differ to a large extent in terms of hydrodynamic features. The existing correlations can be used to predict these square‐based spouted beds. Spouted beds can be very reasonably described as well‐mixed reactors. By assembling several units, it is possible to approach a plug flow state for solids residence time distribution (RTD) in continuous processes. Spouting stability is fully reached when the modules do not interfere each other: this goal is obtained by positioning the downstream units at a certain lower level and having the solids move by overflow; placing internal vertical baffles between contiguous units is the solution. Two hydrodynamical models are proposed to describe single and multiple spouted beds; their predictions help in choosing the best geometrical configuration for the assemblage. A possible application of a multiple spouted bed reactor was envisaged for gasifying pelletized textile residues to generate syngas which can be directly used in situ as a co‐fuel. Two units were built: a single 0.2 m side square‐based spouted bed furnace and the consequent dual stage scale‐up apparatus suitable for an auto thermal process. In the bottom stage, four units work independently to run combustion/gasification of low‐quality solid residues, while the upper stage had a cascade of four modules to run pyrolysis/gasification of selected biomass and obtain valuable secondary products.

Catalyst-enhanced autothermal chemical looping reforming: Intrinsic SMR kinetics and numerical simulation

In this work, we propose a catalyst-enhanced autothermal chemical looping reforming (CE-aCLR) to address the environmental concerns associated with toxic Ni-based oxygen carrier (OC) and the low fuel conversion related to the low reactivity of Fe-based OC. In this process, a non-toxic noble-metal-based steam methane reforming (SMR) catalyst is employed for methane conversion, while a Fe-based OC is employed for oxygen transfer between two reactors. Firstly, a commercial Rh-based SMR catalyst is studied in a micro-fixed bed reactor and its intrinsic reaction kinetics are determined. Then, a 1-D CE-aCLR model is developed based on a verified multi-scale aCLR model to facilitate analyzing commercial-scale feasibility. This model accounts for the essentials of the reactor hydrodynamics, mass and heat transfer, detailed reaction kinetics, and the influence of the solids residence time distribution in the fuel reactor. The main dimensions of a commercial CE-aCLR unit are determined and this unit is simulated using the developed model. The results demonstrate the feasibility of the novel, environmentally friendly process that enables high methane conversion using Fe-based OC within realistically sized reactors and an acceptable solid circulation rate. Moreover, sensitivity study shows that the CE-aCLR allows using OCs with low activity (potentially low cost) while maintaining high CH4 conversion.

Determination residence time distribution of the solid phase in a dry anaerobic semi-continuous horizontal reactor of 0.5 m3 using innovative 3D printed tracers

Solid residence time distribution (SRTD) analysis is important in the design, modeling, and operation of solid biomass in continuous dry anaerobic digestion (D-AD) reactors. SRTD analysis could allow the appropriately adjust of the applicable organic charge and the study of unusable volume due to solid accumulation and preferential pathways. The lack of sustainable, environmentally friendly, and cost-effective solid tracers limits the study of solid flow patterns in D-AD digesters. In this study, 3D printing enabled the fabrication of customized Poly Lactic Acid (PLA) tracers with a specific shape to perform SRTD analysis of the semi-continuous digestion of straw-cattle manure (SCM) in a horizontal reactor of 0.5 m3. The recovery of tracers at the reactor outlet allowed the determination of the average SRTD at 29.43 days. Solid convection was predominant over diffusion, indicating that the reactor behaves closer to a plug flow reactor (PFR) rather than a complete stirred tank reactor.

Coil-shaped rotating spiral gas-solid reactor – design procedure for catalytic reaction

A novel coil-shaped rotating spiral gas-solid contacting device was proposed. The spiral can be made by welding “long-elbows”, which are available as parts of boiler tubes. In the bottom of the spiral, particles form bed, through which whole gas flows without gas bypassing. The bed is transported with the rotation of the spiral. The authors have made transparent cold models and heat-resistant metal models, carried out experimental works, and revealed the properties of this type of reactor for transportation rate of solids, residence time distribution of solids, pressure drop when gas was fed, the maximum limit of gas feed rate to maintain stable bed, gas-solid contact efficiency, heat transfer between wall and solids, and influence of reactor size on above-mentioned properties. This work reveals the influence of scale-up on the maximum limit of gas flow rate at which stable bed is maintained. Gas-solid contact efficiency and heat transfer coefficient were also evaluated using models larger than that used in previous studies. By summarizing the past findings and the present results, a procedure to design a gas-solid catalytic reactor, i.e., determination of tube diameter, rotating rate, number of spiral cycles, and heat transfer rate, is discussed.

Study of Flow Patterns in a Moving Bed Reactor for Chemical Looping Combustion Based on Machine Learning Methods

Abstract A tower-type moving bed can be used as the air reactor in a chemical looping combustion system because of its low-pressure drop and smooth operation. In our previous simulation, a quasi-two-dimensional numerical model was established using discrete element method (DEM) approach to investigate the velocity and solid residence time distributions in the moving bed. In this work, the flow patterns under different operating and structural parameters are studied and optimized via machine learning methods. The random Forest regression model is applied to evaluate the importance of each variable to the solid flow pattern, while the feed forward neural network is applied to buildup a high-accuracy model to predict the solid axial velocity in the moving bed without the requirement to understand the physical mechanisms. Results show that the solid mass flux has the least impact on the mass flow index, while the axial position has the dominant influence and what comes next is the wedge angle, reactor angle, and ratio of down-comer diameter to reactor diameter. Further, based on the established feed forward neural network model, relation between the effective transition position and structural parameters of the moving bed is built, which provides valuable guidance for optimization of the reactor configuration.

Correlating measurement qualities of cross-correlation based solids velocimetry with solids convection-mixing competing mechanism in different gas fluidization regimes

Solids velocimetry based on cross-correlation algorithm has been widely used in multiphase systems. Recent studies have revealed that its measurement qualities depend critically on fluidization regimes, and the underlying mechanism was supposed to be the competition between solids convection and mixing. In this study, a method was proposed to analyze the local solids residence time distribution (RTD) from the results of Eulerian-Lagrangian simulations. Systematic simulations were then conducted to acquire the local solids RTDs in various gas-solids fluidization regimes. Afterward, the local Péclet numbers were quantified. It was found that the measurement qualities are intimately related to the local Péclet numbers and the solids convection-mixing competing mechanism. Present study revealed the underlying mechanism of the success and failure of cross-correlation based solids velocimetry, and proved that the cross-correlation algorithm is ideal for solids convection-dominated systems or regions in a system, but caution is needed when solids mixing is important.

Particle-scale study of coal-direct chemical looping combustion (CLC)

The hydrodynamic and thermochemical characteristics in the coal-direct chemical looping combustion (CLC) process are studied by a self-developed computational fluid dynamics - discrete element method (CFD-DEM) approach featuring particle-scale simulations of collisions, heat and mass transfer, drying process, coal pyrolysis, gasification, and heterogeneous reactions between gas species and oxygen carriers. A polydisperse drag model is adopted to accurately calculate gas-solid interactions. After comprehensive model validations, the flow pattern, pressure drop, gas products composition, particle temperature, combustion efficiency, and solid residence time (SRT) are qualitatively and quantitatively analyzed. The results show that increasing the coal feeding rate slightly improves the temperature of coal particles and accelerates the release of moisture in coal particles. Finer oxygen carriers promote the conversion of intermediate gas products into CO2 and H2O, therefore improving the combustion efficiency in the CLC process. The SRT distribution with an early peak and a long tail is profoundly revealed under different operating parameters. A dual-side coal feeding arrangement remarkably improves the uniformity in the CLC system.

Archimedes tube crystallizer: Design and characterization for small-scale continuous crystallization

Various continuous crystallizer concepts have been developed for the small-scale production (V˙≤100mLmin−1) of APIs and fine chemicals. The existing concepts differ in terms of achievable residence times and residence time distribution (RTD) width and can be classified qualitatively in a portfolio on the basis of these characteristics. The classification demonstrates, that it is still challenging to achieve long, flexible residence times in combination with narrow RTD width leading to narrow particle size distributions. Thus, an air-segmented coiled tube crystallizer was developed based on the Archimedes screw principle: The Archimedes tube crystallizer (ATC). In this device, coiled tubing is employed to convey individual fluid batches by rotation. Thereby, the air-segmented flow is generated by a specifically designed inlet tank mounted on the rotational axis. As a first step towards the implementation of the ATC concept in industry, characterization experiments were focused in this work. In a Design of Experiment, liquid phase RTD with tracer sodium chloride/water and solid phase RTD with material system L-alanine/water were investigated. The experimental results proved narrow RTD close to ideal plug flow for both liquid and solid phase over the complete operating window. Hence, the ATC represents a reasonable expansion of the continuous crystallizer portfolio.

Residence time distribution and solids mixing of sawdust in a horizontal pulsed fluidized bed

A prototype horizontal pulsed fluidized bed (HPFB) has been built for continuous biomass torrefaction in our lab. Solids residence time distribution (RTD) of hemlock sawdust in the HPFB reactor was measured to study particle transport and mixing using a pulse injection of particle tracer. Effects of weir height, gas pulsation frequency, solids feed rate and gas velocity were investigated. A modified axial dispersion model was adopted to fit the experimental RTD curves, where horizontal dispersion coefficients were obtained. It is found that horizontal dispersion coefficients were slightly higher in the deeper bed. In the shallow bed, horizontal dispersion coefficients increased with increasing solids feed rate. The solids mixing intensity also found to increase with increasing the gas velocity. Horizontal solids dispersion coefficients obtained in the HPFB were significantly smaller than values calculated by literature correlations. A correlation in the shallow HPFB was established based on our experimental data.

Dynamics of heat-sensitive pharmaceutical granules dried in a horizontal fluidized bed combined with a screw conveyor

Continuous manufacturing can greatly benefit the pharmaceutical industry for improving production yields, size flexibility, and transferability among sites.This study investigated the dynamics of pharmaceutical granules dried in a continuous horizontal fluidized bed dryer (FBD) equipped with a screw conveyor to evaluate the feasibility of the process.Pharmaceutical granules are often irregularly shaped and feature a large particle size distribution, thus complicating the operation of traditional continuous FBDs with smooth fluidization regimes. The presence of screw conveyor enhanced plug flow, decreased agglomeration, and improved fluidization stability and homogeneity.The residence time distribution (RTD) of solid flow was evaluated by the pulse-tracer injection method. The dispersion model and the tanks-in-series model were both suitable for describing the particle flow through the dryer. The corresponding Peclet numbers vary between 9.8 and 13.3 while the tank number better fitting the model are between 5.4 and 7.2.The influences of different parameters were studied. The internal screw conveyor rotation was the most critical parameter affecting the solid RTD, followed by air velocity. The air temperature also had a moderate impact. The solid throughput rate had a negligible effect on the solid RTD, but it did have a direct impact on the overall drying rate.

MP-PIC simulation of the effects of spent catalyst distribution and horizontal baffle in an industrial FCC regenerator. Part II: Effects on regenerator performance

Improving distribution uniformity of spent catalyst and adding horizontal baffles are two effective measures to improve the regeneration performance of a fluid catalytic cracking (FCC) unit. In this study, the effects of the spent catalyst distribution and horizontal baffle on the regeneration performance of an industrial coaxial compact FCC regenerator is simulated using an Eulerian-Lagrangian multi-phase particle in cell (MP-PIC) method with coupled hydrodynamic and coke-burning kinetics models. The coke-burning kinetics model developed by Arbel et al. (Ind Eng Chem Res, 1995, 34(4): 1228–1243) is used in the simulation. The MP-PIC simulations successfully predicted agreeable temperatures in the industrial regenerator. Serious afterburning in the freeboard was also successfully predicted. The effectiveness of the two intensification measures in enhancing the performance of an industrial FCC regenerator were proved, as indicated by the restrained afterburning in the freeboard, the increased coke burning efficiency, the lower coke content in the regenerated catalysts. Comparatively, adding a properly design horizontal baffle was proved to be more effective and should be a primary intensification measure used in industrial regenerators. A better spent catalyst distributor was suggested to be used to help the inserted horizontal baffles further improve the performance of the regenerator. By the simulation, more flow-reaction coupling mechanisms during the regeneration process are capable to be understood in more depth. The effectiveness of different intensification measures depends both on the resulted matching quality between the distributions of spent catalyst and oxygen in the main air, and the solids residence time distribution.

CFD simulation of solids residence time distribution for scaling up gas‐solid bubbling fluidized bed reactors based on the modified structure‐based drag model

AbstractSolids residence time distribution (RTD), which reflects the degree of solids mixing in bubbling fluidized bed (BFB), has become an essential parameter for the evaluation of reactor performances. In this paper, a modified structure‐based drag model was established to investigate the effect of the bed size and the correlation of bubble dimension on the gas‐solids hydrodynamics and solids RTD in three different scales of BFB. The result obtained from the modified structure‐based drag model shows better agreement with the experimental values and it can be used to predict the RTD properly. The deviation of flow field will cause a large difference in the RTD prediction. An accurate flow field is a prerequisite for calculating the RTD. For the cases of Geldart B particles, the Darton correlation is the best choice for low gas velocities. When scaling up a BFB, the RTD calculated by the modified structure‐based drag model is reduced in comparison with the experimental value, which is related to the increase of the bed size and the excessive estimation of the bubble size. On the other hand, the lack of consideration for the friction on both front and back walls in 2D simulation may lead to the over‐prediction of particle velocity and result in the deviation between calculated and experimental RTD value.

Fluid dynamic analysis and residence time distribution determination for rectangular based spouted beds

Fluid dynamic characterization for a set of three serial rectangular based spouted beds (RBSB) using sawdust or turnip seeds, were made. In batch experiences, the minimum spouting velocity (Ums) and the maximum pressure drop (ΔPmax) were determined based on the bed height and moisture content. With respect to the continuous solid feed experiences, initially the conditions that allowed the stationary operation for 1, 2 and 3 beds connected in series, were determined. Following, injecting colored particles and with an image analysis it was possible to determine the residence time distribution (RTD). When compared with expressions from literature, good agreement was found for Ums, whereas for ΔPmax an expression is proposed that includes the sphericity of the particles. The RTDs were adjusted with the tank in series model, finding that the effects of solids feed rate, air flow and moisture content are significant for each solid RTD.

Validation of a filtered drag model for solid residence time distribution (RTD) prediction in a pilot-scale FCC riser

Computational fluid dynamics (CFD) is a powerful tool for prediction and analysis of complex multiphase flow hydrodynamics and residence time distribution (RTD) in chemical reactors. This study presented the validation and application of a filtered drag model for solid RTD prediction in a pilot-scale fluid catalytic cracking (FCC) circulating fluidized bed (CFB) riser with Geldart A particles. First, the filtered drag model implemented in the open-source MFiX-TFM solver was validated for flow hydrodynamics simulation in a FCC CFB riser. After that, the model was further employed to validate its ability for solid RTD prediction in the same riser by comparing the simulation results with the experimental data. The simulation with the filtered drag model well reproduced the tracer response experiment which is more accurate than that with the Gidaspow drag model. Simulations with both pulse and step tracer injection methods were compared, which reveals the limitation of solid RTD measurement using a pulse tracer injection method in the experiment.

Experimental Study on the Solids Residence Time Distribution in Multiple Square-Based Spouted Beds

The present work aims at investigating the residence time distribution (RTD) of a multiple spouted bed reactor, which will be applied for the pyrolysis and gasification of residual biomass. The unit is composed of square-based spouted beds, placed in series and at descending heights, and communicating with each other through an opening in the lateral wall. The gas is fed evenly in parallel. The experimental analysis is based on tracer experiments in cold-flow units, assessing the influence of the number of units and the bed height. The tests proved the good mixing properties of the spouted beds, which create a stable fluidization regime and do not feature dead zones. Each spouted bed can generally be well assimilated to an ideal continuous stirred tank reactor (CSTR). The RTD of the device seems adequate for the application, and also seems to be well tuneable through the selection of the bed height and number of units. Given the good similarity with ideal reactor networks, these represent a valid tool to estimate the final behavior in terms of RTD.

Analysis of flexible ribbon particle residence time distribution in a fluidised bed riser using three-dimensional CFD-DEM simulation

Fluidised bed technology is a very common industrial process and has a wide range of applications in process engineering. Though indirectly, a common way to infer the end product uniformity is through investigating the solid residence time distribution (RTD) in the system. A good understanding of the particle RTD is crucial for the proper design, optimisation, and scale-up of a fluidisation process. Flexible ribbon particles have a very different flow behaviour in a fluidised bed compared to that of spheres which in turn influences the RTD of the particles. However, this issue is still not very well addressed to date. In this paper, a double chain model is adopted to model cut tobacco strips in a fluidised bed riser. The spring constants of this chain model are calibrated, and the model is validated against experimental datum. Finally, the particle RTD as a function of different process parameters is investigated.

Eulerian-Lagrangian simulation of the full-loop gas-solid hydrodynamics in a pilot-scale circulating fluidized bed

The full-loop gas-solid hydrodynamics in a pilot-scale circulating fluidized bed (CFB) is numerically investigated by using multi-phase particle-in-cell (MP-PIC) method. In this method, fluid motion is resolved under Eulerian framework while particle motion is tackled under Lagrangian framework. The numerical results agree well with experimental data, demonstrating the reasonability of the MP-PIC method in simulating dense gas-solid systems. The main features of gas-solid flows in the full-loop CFB are qualitatively predicted, with a typical core-annulus flow structure in the riser. A large pressure gradient of gas phase in the bottom region and top region of riser reflects the turbulent and chaotic gas-solid flows in these two regions. Enlarging superficial gas velocity leads to a more non-symmetrical pattern of axial gas velocity, indicating the enhanced influence of recycle structure on gas-sold uniformity under the higher superficial gas velocity. The solid residence time (SRT) distribution in the riser exhibits a feature of early peak and a long tail, which can be well fitted by a lognormal probability distribution function. Enlarging superficial gas velocity gives rise to a decreased mean value and narrower distribution of the solid residence time in the riser, reflecting the depressed solid back-mixing intensity in this condition.

CFD-DEM simulations of a continuous square-based spouted bed and evaluation of the solids residence time distribution

CFD-DEM is an established approach to simulate spouted beds, but researchers usually focus on batch units, without commenting on the residence time distribution of the particles. In this work, we employed our validated CFD-DEM approach to simulate a continuously-fed square-based spouted bed, in the framework of the development of a scaled-up multiple unit. After analysing several flow patterns of the spouted bed, we propose three methods that permit evaluating the residence time distribution curve of the particles on the basis of the results of the simulations. These methods allow exploiting CFD-DEM's great reliability without reproducing the actual tracer experiment, which would be too time-consuming. Each method has advantages and disadvantages, but all of them were able to provide a good match of the experimental E(t) curve and may be applied to other devices as well.

Validation and application of a multiphase CFD model for hydrodynamics, temperature field and RTD simulation in a pilot-scale biomass pyrolysis vapor phase upgrading reactor

Accurate prediction of transport phenomena is critical for VPU reactor design, optimization, and scale-up. The current study focused on the validation and application of a multiphase CFD model within an open-source code MFiX for hydrodynamics, temperature field, and residence time distribution (RTD) simulation in a non-reacting circulating fluidized bed riser for biomass pyrolysis vapor phase upgrading (VPU). First, an Eulerian-Eulerian approach three-dimensional CFD model was employed to simulate the pilot-scale VPU riser on the supercomputer Joule. Excellent quantitative agreement between experimental and simulated results was achieved for pressure drops and temperature field in a range of operating conditions. Then the validated multiphase CFD model was applied to predict gas and solid residence time distributions (RTDs) since prediction and analysis of RTD is an important tool to study the complex multiphase flow behavior and mixing inside chemical reactors. The predictions show that solid mean residence time is 3.5 times the gas residence time; the solid RTD is more sensitive to the process gas flow rate than the solids circulation rate.