Bed density and circulation mass flowrate in a novel annulus-lifted gas–solid air loop reactor

Air loop reactors (ALR) have been widely used as promising and high-efficiency gas–liquid and gas–liquid–solid reactors. Extensive research on ALR has been conducted, but mostly limited to gas–liquid and gas–liquid–solid systems. Work associated with gas–solid systems has been rare and mainly focused on draft tube-lifted spouted bed treating coarse Geldart B, D particles. The present paper proposed a novel gas–solid air-loop reactor treating fine Geldart A particles and operating in a new annulus-lifted mode, with bubbling or turbulent bed upward flow in the annulus in parallel with bubbling bed downward flow in the draft tube. In view of these differences, distinct hydrodynamic behaviour can be anticipated for the gas–solid annulus-lifted air-loop reactor. The influence of operating conditions and geometric configuration on the distribution of bed density is discussed in a cold model annulus-lifted air loop reactor. A mechanistic model for the circulation mass flowrate is established based on an energy balance and resistance analysis. Nearly 50% and 30% of the energy dissipation rate occurs in the bottom and top regions, respectively. With increasing draft tube height, the energy dissipation rate increases in the annulus and draft tube regions, while it decreases in the top and bottom regions. The circulation mass flowrate decreases with increasing draft tube height. Analysis of the distribution of bed density and energy dissipation rate leads to suggestions regarding optimization of the design and axial location of the ring distributor and gap height.     

Fundamental studies on the hydrodynamics and mixing of gas and solid in a downer reactor

The effect of flow direction on hydrodynamics and mixing in the upflow and downflowcirculating fluidized beds is discussed in details.Similar profiles of gas and solids velocities andsolids concentration are found in both risers and downers.When the flow is in the direction ofgravity(downer),the radial profiles of gas and particle velocity are more uniform than that inthe riser,the solids mixing is very small and the flow pattern approaches plug flow,while theflow is against gravity(riser),the solids backmixing significantly increase and the flow pattern isfar from plug flow.Among many of factors the flow direction has the largest influence onhydrodynamics and axial mixing of gas and solids.     

Scale and structure dependent drag in gas–solid flows

Drag plays a crucial role in hydrodynamic modeling and simulations of gas–solid flows, which is significantly affected by particle Reynolds number, solid volume fraction, heterogeneity, granular temperature, particle-fluid density ratio, and so on. To clarify and quantify the multiscale effects of these factors, large-scale particle-resolved direct numerical simulations of gas–solid flows with up to 115,200 freely moving particles are conducted. Both domain-averaged kinetic properties and local averaged dimensionless drag are sampled and analyzed. It is revealed that the complex scale-dependence of drag is attributed to the multiscale effects of heterogeneous structures and particle fluctuating velocity. The granular temperature and the scalar variance of solid volume fraction are also found to be scale-dependent. On account of these, a new drag correlation as the function of Froude number is proposed with consideration of scale-dependence.     

Experimental investigation and CFD simulation of liquid–solid–solid dispersion in a stirred reactor

For understanding the monosodium aluminate hydrate crystallization from the supersaturated aluminate solution containing red mud as the leaching liquor of bauxite, the liquid–solid–solid dispersion of a simulant system, i.e. glycerite, red mud and sand, in a stirred reactor has been experimentally investigated as well as simulated using computational fluid dynamics model (CFD) for the first time. The computational model is based on the Eulerian multi-fluid model along with RNG k–ε turbulence model, where Syamlal–obrien-symmetric drag force model (Syamlal, 1987) of the inter-phase momentum transfer between two dispersed solid phases is taken into account. A good agreement is obtained between the experimental data of solid distributions and the simulation results in the flow fields of liquid–solid–solid as well as liquid–solid systems. The solid suspension qualities of both liquid–solid and liquid–solid–solid systems in the stirred reactors with and without draft tube were also studied in detail based on mixing time, the standard deviation of solid concentration proposed by Bohnet and Niesmak (1980), the flow pattern and power number. The influence of the interaction between two dispersed solid phases on the suspension of red mud is found significantly greater than that of sand. The holdup of sand below the impeller is considerably larger than that above the impeller and the red mud dispersion approaches homogeneous in the reactor. The mixing time of liquid–solid–solid suspension is longer than that of liquid–solid suspension under the same conditions, and the mixing times of both systems in the stirred reactor with draft tube are longer than that in the reactor without draft tube. Furthermore, the distributions of sand and red mud in the reactor with draft tube were found less homogeneous than those without draft tube in most cases.     

Horizontal gas and gas/solid jet penetration in a gas–solid fluidized bed

In this paper, the real time, dynamic phenomena of the three-dimensional horizontal gas and gas/solid mixture jetting in a 0.3 m (12 in) bubbling gas–solid fluidized bed are reported. The instantaneous properties of the shape of the jets and volumetric solids holdup are qualified and quantified using the three-dimensional electrical capacitance volume tomography (ECVT) recently developed in the authors’ group. It is found that the horizontal gas jet is almost symmetric along the horizontal axis during its penetration. As the jet width expands, the total volume of the gas jet increases. A mechanistic model is also developed to account for the experimental results obtained in this study. Comparison of jet penetration length and width between the model prediction and ECVT experiment shows that both the maximum penetration length and the maximum width of the horizontal gas jet increase with the superficial gas velocity. When the horizontal gas jet coalesces with a bubble rising from the bottom distributor, it loses its symmetric shape and can easily penetrate into the bed. For the horizontal gas/solid mixture jet penetration in the bed, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which are different from the characteristics exhibited by the gas jet. The solids holdup in the core region of the gas/solid mixture jet is higher than that in the gas jet. The penetration length of the horizontal gas/solid mixture jet is also larger than that of the gas jet.     

Micromixing in a gas–liquid vortex reactor

The gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas-only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10⁻⁴ ~ 10⁻³ s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.     

Classification and identification of gas–liquid dispersion states in a jet bubbling reactor

Three gas–liquid dispersion states including flooding, loading, and complete dispersion are observed sequentially in a jet bubbling reactor with an increase of the liquid jet velocity at the nozzle outlet (u_j). The gas–liquid dispersion states are identified through the slope (k) of the curve of fluctuation distribution index (FI) versus u_j as follows: (a) under the flooding, k = 0; (b) under the loading, k > 0; (c) under the complete dispersion, k < 0. In particular, the u_j at the transition points from flooding to loading and from loading to complete dispersion are referred to flooding jet velocity (u_jf, the transition point between k = 0 and k > 0) and complete dispersion jet velocity (u_jcd, the transition point from k > 0 to k < 0), respectively. The average relative deviations of the u_j at the transition points obtained through the acoustic emission measurement and visual observation are less than 5%.     

Nature and characteristics of gas–liquid flow regimes in a micro-packed bed reactor

Micro-packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale-up, is still insufficient due to the limitation of micro-scale and complexity of capillary force. In this work, the flow regimes in the two-dimensional μPBR were systematically studied by visual method utilizing a high-performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo-static flow in the μPBR was provided for the first time based on the summary of the current and previous published results.     

Hydrodynamics of gas–solid flow in the circulating fluidized bed reactor for dry flue gas desulfurization

Process design and scale-up require a fundamental understanding of the hydrodynamics of gas–solid flow in the circulating fluidized bed flue gas desulfurization (CFB-FGD) reactor although the CFB system has been widely used in flue gas desulfurization and flue gas cleaning processes. The hydrodynamics in the CFB-FGD reactor model was investigated by pressure measurements and specially designed sampling probe based on three dimensionless groups for practicable similarity of industrial CFB-FGD process. The results show that the pressure drop in the venturi section is predominant as high as 60% of the total pressure drop and the total pressure drop significantly increases with the increasing external solid circulating rates at the same superficial gas velocity. Moreover, the measurements of radial solid mass fluxes show that the flow pattern in the CFB-FGD reactor is a typical core–annulus flow and this flow structure prevails until the top of the reactor. Reflux ratios are used to quantitatively evaluate the internal solid reflux in the reactor and the values in the low section of the reactor are much higher than those in the upper section.     

Modelling and gas–solid mixing characterization in the jiggled bed reactor (JBR)

The jiggled bed reactor (JBR) is a new multiphase laboratory-scale microreactor consisting of a sealed container attached to a piston that is rapidly moved up and down by a pneumatically powered actuator. Particles and fluids in the container are mixed by this up and down motion instead of mechanical agitators or a fluidizing gas. This alternating motion provides intense mixing of all phases (gas, liquid, or solid) and intense contact between phases. Small rods inside the solids bed are heated by induction, allowing for excellent control of bed temperature and heating rate. The JBR is inexpensive and easy to operate, and it has been applied to catalytic gasification of bio-oil, biomass pyrolysis, activated carbon production, high-pressure oil hydrogenation, and hydrocarbons adsorption. Experiments demonstrated that solids mixing depends on the reactor platform maximum accelerations during both up and down strokes. A minimum acceleration, 55 m²/s for the tested JBR, was required to achieve good solids mixing. A physical model was developed to predict the reactor platform motion and its maximum acceleration. It requires a few preliminary experiments (around 10) to obtain its four empirical parameters. The model can then determine how to adjust the actuator compressed air pressure or modify the equipment to eliminate performance bottlenecks.     

Experimental investigation on multiscale hydrodynamics in a novel gas–Liquid–Solid three phase jet-Loop reactor

Multiphase flow hydrodynamics in a novel gas–liquid–solid jet-loop reactor (JLR) were experimentally investigated at the macroscales and mesoscales. The chord length distribution was measured by an optical fiber probe and transformed for bubble size distribution through the maximum entropy method. The impacts of key operating conditions (superficial gas and liquid velocity, solid loading) on hydrodynamics at different axial and radial locations were comprehensively investigated. JLR was found to have good solid suspension ability owing to the internal circulation of bubbles and liquid flow. The gas holdup, axial liquid velocity, and bubble velocity increase with gas velocity, while liquid velocity has little influence on them. Compared with the gas–liquid JLRs, solids decrease the gas holdup and liquid circulation, reduces the bubble velocity and delays the flow development due to the enhanced interaction between bubbles and particles (Stokes number >1). This work also provides a benchmark data for computational fluid dynamics (CFD) model validation. © 2019 American Institute of Chemical Engineers AIChE J, 65: e16537, 2019     

EMMS-based modeling of gas–solid generalized fluidization: Towards a unified phase diagram

Hydrodynamic features of gas-solid generalized fluidization can be well expressed in the form of phase diagrams, which are important for engineering design. Mesoscale structure presents almost universally in generalized fluidization and should be considered in such phase diagrams. However, current phase diagrams were mainly proposed for cocurrent upward flow according to experimental data or empirical correlations with homogeneous assumption. The energy-minimization multiscale (EMMS) model has shown the capability of capturing mesoscale structure in generalized fluidization, so EMMS-based phase diagrams of generalized fluidization were proposed in this article, which describe more reasonable global hydrodynamics over all regimes including the important engineering phenomena of choking and flooding. These characteristics were also found in discrete particle simulation under various conditions. For wider range of application, the typical hydrodynamic parameters of the phase diagrams were correlated to non-dimensional numbers reflecting the effects of material properties and operation conditions. This study thus shows a possible route to develop a unified phase diagram in the future.     

TG–DSC analysis of straw biomass pyrolysis and release characteristics of noncondensable gas in a fixed-bed reactor

Under the double pressures caused by the energy shortage and environmental damage, to exploit the agricultural wastes and convert into available clean fuels are becoming more and more urgent in modern society. The aim of learning the pyrolysis characteristics of soybean straw and corn straw, the nonisothermal thermogravimetry and differential scanning calorimetry (TG-DSC) method was used in this work. The results showed that both of biomass feedstocks all underwent four different pyrolysis stages, with the increase in heating rate, the peak temperature shifted toward the high-temperature interval, and that the yield of bio-char also increased correspondingly; potassium had an influence on the thermal cracking of biomass, and that the existence form of potassium and impregnation increment of sylvite would result in the yield of bio-char was distinct. In addition, temperature and catalyst had a significant impact on the gaseous products of biomass pyrolysis. Increasing the pyrolysis temperature could enhance the yield of CO and H₂ and CH₄ content reached the maximum at 600°C. For both of the biomass, sylvite had a negative effect on the formation of CH₄, and H₂ content of soybean straw reached a maximum with 5% K₂CO₃ and corn straw with 5% KCl.     

Development and verification of coarse-grain CFD-DEM for nonspherical particles in a gas–solid fluidized bed

Computational fluid dynamics coupled with discrete element method (CFD-DEM) has been widely used to understand the complicated fundamentals inside gas–solid fluidized beds. To realize large-scale simulations, CFD-DEM integrated with coarse-grain model (CG CFD-DEM) provides a feasible solution, and has led to a recent upsurge of interest. However, when dealing with large-scale simulations involving irregular-shaped particles such as biomass particles featuring elongated shapes, current CG models cannot function as normal because they are all developed for spherical particles. To address this issue, a CG CFD-DEM for nonspherical particles is proposed in this study, and the morphology of particles is characterized by the super-ellipsoid model. The effectiveness and accuracy of CG CFD-DEM for nonspherical particles are comprehensively evaluated by comparing the hydrodynamic behaviors with the results predicted by traditional CFD-DEM in a gas–solid fluidized bed. It is demonstrated that the proposed model can accurately model gas–solid flow containing nonspherical particles, merely the particle dynamics are somewhat lost due to the scaleup of particle size. Finally, the calculation efficiency of CG CFD-DEM is assessed, and the results show that CG CFD-DEM can largely reduce computational costs mainly by improving the calculation efficiency of DEM. In general, the proposed CG CFD-DEM for nonspherical particles strikes a good balance between efficiency and accuracy, and has shown its prospect as a high-efficiency alternative to traditional CFD-DEM for engineering applications involving nonspherical particles.     

Methanol synthesis in a countercurrent gas—solid—solid trickle flow reactor. An experimental study

The synthesis of methanol from CO and H₂ was executed in a gas—solid—solid trickle flow reactor. The reactor consisted of three tubular reactor sections with cooling sections in between. The catalyst was Cu on alumina, the adsorbent was a silica—alumina powder and the experimental range 498–523 K, 5.0–6.3 MPa and 0.2–0.33 molar fraction of CO. Complete conversion in one pass was achieved for stoichiometric feed rates, so that the gas outlet could be closed. The experimental results are compared with the model presented in the previous paper [Westerterp, K.R. and Kuczynski, M. (1987) Chem. Engng Sci.42,]; agreement is close over the entire conversion range from 15% to 100%.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Local solid mixing in gas–solid fluidized beds

Diffusivity of the solid particles in a 152-mm ID gas–solid fluidized bed was determined at different regimes of fluidization. The gas was air at room temperature and atmospheric pressure and the solids were 385 μm sand or 70 μm FCC particles. The experiments were done at superficial gas velocities from 0.5 to 2.8 m/s for sand and 0.44 to 0.9 m/s for FCC (in both bubbling and turbulent regimes). Movement of a tracer was monitored by radioactive particle tracking (RPT) technique. Once the time-position data became available, local axial and radial diffusivity of solids were calculated from these data. Calculated diffusivities are in the range of 3.3×10⁻³ to 5.6×10⁻² m²/s for axial and 2.6×10⁻⁴ to 1.5×10⁻³ m²/s for radial direction. The results show that the diffusivities, both axial and radial, increase with superficial gas velocity and are linearly correlated to the axial solid velocity gradient. Solid diffusivity in a bed of FCC was found to be higher than that of a bed of sand at the same excess superficial gas velocity.     

Flow regimes in a gas–liquid–solid three-phase moving bed

The gas–liquid–solid three-phase moving beds could supply a potential solution for multiphase reactions with catalyst easily deactivated, and the flow regimes in it were studied by optical method and pressure drop measurement. Results showed that taking the trickle flow as the initial flow regime, the flow channels were more obvious as the particle velocity increased. When the initial flow regimes were pulse flow and bubble flow respectively, the pulse-to-trickle and bubble-to-pulse flow transitions mainly occurred at moderate-to-high particle velocities (0.01–0.04 m s⁻¹ under conditions used in this work). Moreover, the flow regime map in the three-phase moving bed was constructed and shown that the region of trickle flow increased and the region of bubble flow decreased. Finally, the application of three-phase moving beds was discussed, and it could be suitable for those reactions, which had to operate in the pulse flow, bubble flow, and transition zone.     

Modeling steady-state heterogeneous gas–solid reactors using feedforward neural networks

A new method for solving gas–solid heterogeneous reactors is proposed. Mass balance inside the pellet (numerical integration of a differential equations system) is replaced by an analytical function, which functionality corresponds to an adequate trained three-layer feedforward neural network. The global reaction rate evaluated by using this function includes the complex phenomena of simultaneous diffusion and chemical reaction into the solid. The methodology was successfully applied to the steam reforming of methane. Both methods are compared. Results of the reactor simulation are very similar in both cases but the one that used neural networks is about 20 times faster. The method proposed could also be applied to any type of two-phase heterogeneous reactors.