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Solids holdup as one of the main parameters in characterizing the performance of fluidized bed reactors is widely concerned. With its development and improvement, visualization technology has been applied in fluidization because of its little disturbance to the flow. In this study, four types of particles with different properties are tested in a narrow rectangular fluidized bed equipped with a high-speed video camera. Calibration curves of these different types of particles are achieved by correlating the grayscale of the digital images with the corresponding solids holdup. These calibration curves are further applied to obtain the average solids holdup across the sectional area and local solids holdup from the center towards the wall in both a gas-solids turbulent fluidized bed and a circulating fluidized bed to verify the results. The calibration method works well for solids holdup of different types of particles in both dense and dilute fluidization systems. This method is important to characterize the fluidization quality and reactor performance with a wide operating condition. [ABSTRACT FROM AUTHOR]

Particle attrition is a prevalent problem in fluidized beds due to continuous moving of catalyst particles. It is always operated at high temperature either for lab- or industrial-scale fluidized beds. The influence of temperature on the attrition behavior of commercial methanol-to-olefins (MTO) and fluid catalytic cracking (FCC) catalysts is analyzed in a three-orifice lab-scale fluidized bed device from room temperature to 600 °C. The two catalysts are found to be alike in attrition mode. Both change from a combination of abrasion and fragmentation to main abrasion with increasing temperature, but differ greatly in the variation of attrition index with temperature which may be attributed to the difference of material and particle properties. An empirical correlation of attrition index to attrition temperature for all test samples is proposed. [ABSTRACT FROM AUTHOR]

In fluidized bed methanation reactors, gas volumetric flow reduction due to the reaction can cause serious problems for fluidization quality, and in extreme cases, unwanted defluidization may occur. Note that the fluidization quality is closely linked to the expansion of emulsion phase, this paper present a numerical study on the emulsion phase expansion of Geldart A type of particles in fluidized bed methanation reactors by use of the well-established CFD–DEM model. The effects of operation conditions such as reaction rate, superficial gas velocity, pressure and bed inventory are discussed and compared qualitatively with experiments in literature. It is found that a higher gas velocity, based on the reactant gas at the inlet, is required to fully fluidize the particles and maintain good fluidization quality in a methanation reactor. Fast bubbles, considered as an important feature in the fluidization of Geldart A type of particles, were observed. Close check shows that the gas circulation around fast bubbles is responsible for the worsened gas exchange between gas bubbles and emulsion phase and consequently the apparent contraction of emulsion phase for Geldart A type of particles. Higher pressure and lower bed height can improve emulsion phase expansion in the methanation fluidized bed reactors, which is in accordance with previous experimental findings. The superficial gas velocity affects the emulsion phase expansion in a complicated way, where two mechanisms, i.e., large bubble size and short gas residence time, will work together. [ABSTRACT FROM AUTHOR]

In this work, a comparative study on methanol to olefins (MTO) kinetics in fixed and fluidized bed reactors was conducted. Based on the analysis of experimental observations, a modeling approach that can be used to simulate both reactor configurations was proposed. The coke deposition was shown responsible for the difference of product selectivities for these two reactor configurations. In a fluidized bed reactor, the distribution of the coke deposition is spatially uniform, and evolves with time on stream. However, the coke deposition shows a distribution along the bed height in a fixed bed reactor, and olefins conversions were taken into consideration to account for the selectivity changes with time on stream. With the proposed model, we established a linkage between fixed bed reactors and fluidized bed reactors through coke deposition. By use of this model, it is possible to predict the performance of MTO reaction over SAPO-34 catalyst in fluidized bed reactor with experimental data obtained in fixed bed reactor, or verse vice. [ABSTRACT FROM AUTHOR]

Attrition of catalyst in a fluidized bed reactor is an inevitable issue especially in a commercial unit. Methanol to olefins (MTO) is becoming one of the main stream technologies for light olefins production. The attrition of MTO catalyst, however, received little attention. This study is focused on the attrition behavior of MTO catalyst in jet cup at high temperature. The influence of test time, inlet gas velocity, and temperature on MTO catalyst attrition was studied. It is found that the Gwyn formulation can well represent the relation between attrition index and test time. Our results show that jet cup can retrieve results quantitatively comparable to high velocity gas jets method while significantly shortening test time. It is also found that the inlet gas velocity has considerable influence on the MTO catalyst attrition, and the relation between inlet gas velocities and attrition index can be described by a power index of 3.7. Similar to high velocity gas jets experiments the attrition index manifests a maximum with the increase of temperature. But the temperature corresponding to the maximum attrition index shifts from 300 °C in high velocity jets tests to 100 °C in jet cup experiments. An analysis based on SEM pictures indicates that the transition of attrition mechanism is responsible for this shift. An empirical correlation has been presented for MTO catalyst attrition in jet cup, which shows good agreement with experimental data for inlet gas velocity from 88 to 158 m/s, and temperature from 100 to 500 °C. [ABSTRACT FROM AUTHOR]

Previous simulations of different-sized MTO fluidized beds indicated that the reactive predictions gradually deviate from the experiment with the reactor scale-up. This study aimed to find the underlying reasons by investigating effects of kinetic models and coke content. Two kinetic models were separately integrated into the multi-fluid model, and a reactor model was then established considering coke distribution. It showed that the ratio of ethylene to propylene is still over-predicted even the kinetic model considers the conversion of ethylene to propylene if using average coke content in simulation, but considering a reasonable coke content distribution both models can give better predictions in methanol conversion, hydrocarbon products and selectivity to light olefins. It indicated that coke content distribution plays a critical role in reaction predictions and the kinetic models with similar component classification mainly affect the variation of products along the bed height due to different reaction routes. [Display omitted] • The role of kinetic models are investigated in reactive simulation of an industrial MTO reactor. • The coke content plays a more critical role in reaction predictions. • The kinetic model mainly affects the variation of products along space position in reactor. • Both kinetic models considering coke distribution can give reasonable reaction predictions. [ABSTRACT FROM AUTHOR]

In the methanol-to-olefins (MTO) process, coke deposition is closely related to the selectivity of light olefins. Previous simulations of different-sized MTO reactors using two-fluid model (TFM) combined with the EMMS (energy minimization multi-scale)-based drag well predict the hydrodynamic behaviors but poorly predict the product distribution of large reactors due to the unreasonable prediction in coke distribution. In this study, the TFM integrated with the EMMS-based drag is still employed, but the solid phase is treated as a mixture of a series of species with different coke contents. Because the coke content depends on the age of catalyst particles inside the reactor, a continuous stirred tank reactor (CSTR) model mimicking turbulent fluidization while considering the age distribution of catalysts is established to predict the initial coke distribution for speeding up simulation. Compared with the previous simulation without consideration of coke distribution, this approach increases the computational time by a factor of 36% and shows no influence on hydrodynamic predictions, but the reaction quantities such as methanol conversion, mass fractions of gaseous products and selectivity of light olefins, are better predicted. [ABSTRACT FROM AUTHOR]

Scale-up of fluidized bed reactors has long been regarded as a big challenge in chemical reaction engineering. While traditional scaling theories are mostly based on hydrodynamics similarity, computational fluid dynamics (CFD) aided approach allows direct coupling between hydrodynamics and reaction factors and is expected to speed up the experiment-based scale-up process with lower cost. In this study, we aim to investigate the scale-up effects through simulations of a series of methanol-to-olefins (MTO) reactors of different sizes. The two-fluid model and energy-minimization multi-scale (EMMS)-based drag models are combined in simulations. The fluidization characteristics in terms of flow structures, velocity distribution, mass fractions of gaseous product and coke distribution are presented against available experimental data for different-sized reactors. It is found that typical hydrodynamic features can be reasonably predicted, while the prediction of reaction behavior shows growing discrepancy with increasing reactor size. Possible reasons are discussed in the last section along with future work presented for scale-up studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Quantitative analysis of the cluster characteristics in gas–solid turbulent fluidized bed by image processing. • Volume/mass fraction of each particle phase are characterized based on area statistics. • Distribution of the particle cluster characteristics are obtained under wide operating conditions. • Cluster cores movement brings a good solids mixing in the lateral direction. • Most cluster cores moved downward in turbulent fluidized bed. A two-step Otsu's method was applied to process the images of gas-solids flow field at multiple axial levels in a TFB with a H 0 / D ratio from 0.5 to 2.0. The flow field was segmented to a discrete particle phase, a dense particle phase, and a cluster core and a cluster cloud phases. The solids holdup, volume and mass fractions, and the velocity vector field of each phase were analyzed by postprocessing. The frequency, size, shape, and velocity of the cluster cores were also extracted. The increasing H 0 / D ratio results in an increase in solids holdup in each phase and a decrease in the mass and volume fraction of cluster clouds. A strong disturbance in the local flow field was resulted from the formation and movement of cluster cores and a more dynamic velocity change was found inside cluster clouds. The clusters were found to travel downwardly and with equal chances to move towards the center and the wall of the TFB at a low H 0 / D ratio with a high U g. [ABSTRACT FROM AUTHOR]

[Display omitted] • A population balance model is proposed for describing the evolution of coke content. • The PBM and multiscale CFD are efficiently combined to simulate an industrial-scale MTO reactor. • The primary light olefins and their selectivity are better predicted. • The relaxation time for expressing the effects of induction period in catalysts are considered. The coke distribution over shape-selective catalysts in fluidized bed reactors is critical to the selectivity of target products. A population balance model (PBM) is developed in this study to describe the evolution of coke distribution due to both reactions and circulation of catalysts. The PBM is coupled with multiscale computational fluid dynamics (CFD) to simulate a methanol-to-olefins fluidized bed reactor. A wide, uneven distribution of coke content can thus be predicted. The mass fractions of the desired products, ethylene and propylene, show better agreement with the experimental data than those without PBM. And the selectivity ratio of ethylene to propylene is also better predicted. Furthermore, the relaxation time of fresh catalyst is considered to express the effects of the induction period in MTO reactions. The reasonable prediction suggests that coupling PBM with multiscale CFD is helpful to understand MTO reactors and may find more application with respect to its optimization and scale-up. [ABSTRACT FROM AUTHOR]

The methanol to olefins (MTO) process opens an economical and important route to produce light olefins. The design of MTO reactor borrows ideas from the reaction–regeneration configuration of the modern fluid catalytic cracking (FCC) units. However, their hydrodynamic behaviors are quite different in the sense that the fluidized bed for MTO reactions operates in different flow regime from that of FCC, calling for new modeling for scale-up. In addition, the coke deposited on catalysts greatly affects the MTO reaction while its generation is very slow. It normally takes tens of minutes or even hours for catalysts to reach the desired level of coke content. Time-dependent computational fluid dynamics (CFD) simulation of such a long process poses a big challenge to reactive multiphase flow modeling. To speed up it, we try to integrate the classic chemical reaction engineering (CRE) model with CFD. In particular, the continuous stirred tank reactor (CSTR) model is established to estimate the steady state distribution of coke content, which is then set as the initial distribution for CFD simulation to shorten the time to reach the steady state of reactive flows. Comparison with experimental data shows good agreement and also great speed-up ratio compared to traditional CFD simulation. [ABSTRACT FROM AUTHOR]