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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]

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]

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]