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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]

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] • 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]