Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that U_Lmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, U_Lmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, U_Lmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of U_Lmf. An empirical correlation was suggested to predict U_Lmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016     

Volume‐of‐fluid‐based model for multiphase flow in high‐pressure trickle‐bed reactor: Optimization of numerical parameters

Aiming to understand the effect of various parameters such as liquid velocity, surface tension, and wetting phenomena, a Volume‐of‐Fluid (VOF) model was developed to simulate the multiphase flow in high‐pressure trickle‐bed reactor (TBR). As the accuracy of the simulation is largely dependent on mesh density, different mesh sizes were compared for the hydrodynamic validation of the multiphase flow model. Several model solution parameters comprising different time steps, convergence criteria and discretization schemes were examined to establish model parametric independency results. High‐order differencing schemes were found to agree better with the experimental data from the literature given that its formulation includes inherently the minimization of artificial numerical dissipation. The optimum values for the numerical solution parameters were then used to evaluate the hydrodynamic predictions at high‐pressure demonstrating the significant influence of the gas flow rate mainly on liquid holdup rather than on two‐phase pressure drop and exhibiting hysteresis in both hydrodynamic parameters. Afterwards, the VOF model was applied to evaluate successive radial planes of liquid volume fraction at different packed bed cross‐sections. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Hydrodynamics in a pilot‐scale cocurrent trickle‐bed reactor at low gas velocities

Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m²s and 4.5 – 45 kg/m²s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018     

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor

Turbulent mixing in the confined swirling flow of a multi‐inlet vortex reactor (MIVR) was investigated using planar laser induced fluorescence (PLIF). The investigated Reynolds numbers based on the bulk inlet velocity ranged from 3290 to 8225, and the Schmidt number of the passive scalar was 1250. Measurements were taken in the MIVR at three different heights (¼, ½, and ¾ planes). The mixing characteristics and performance of the MIVR were investigated using instantaneous PLIF fields and pointwise statistics such as mixture fraction mean, variance, and one‐point concentration probability density function. It was found that the scalar is stretched along velocity streamlines, forming a spiral mixing pattern in the free‐vortex region. In the forced‐vortex region, mixing intensifies as the turbulent fluctuations increase significantly there. The mixing mechanisms in the MIVR were revealed by identifying specific segregation zones. At Re = 8225 the mixing in the free‐vortex region was dominated by both large‐scale structures and turbulent diffusion, while in the forced‐vortex region mixing is dominated by turbulent diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2409–2419, 2017     

Statistical analysis of the phase holdup characteristics of a gas–liquid–solid fluidized bed

Experiments have been carried out to study the individual phase holdup characteristics in a cocurrent three‐phase fluidized bed. An antenna type modified air sparger has been used in the gas–liquid distributor section, for uniform mixing of the fluids with the gas moving as fine bubbles to the fluidizing section. This arrangement also reduces the pressure drop encountered through a conventional distributor used for the purpose. To overcome the non‐uniformity of flow through the column (i.e., the central region), a distributor plate with 20% open area has been fabricated with concentric circular punched holes of increased diameter from centre to the wall. Model equations have been developed by factorial design analysis for predicting various individual phase holdups.     

Three-Dimension imaging of particle clusters in dilute gas—solid suspension flow

The three dimensional flow structure of dilute gas—solid suspensions in a small-scale circulating fluidized bed (0.200 m riser diameter) was visualized by applying the laser sheet technique. FCC particles were fluidized with sustained solid loading at gas velocities corresponding to the turbulent and the fast fluidization regimes in cases where the solid circulation was sufficient. Three typical shapes of clusters in the core section of the riser were observed. Clusters characterized by a paraboloidal shape heading downward were connected to neighboring clusters at their tail part, forming a three dimensional network structure.     

Probability density function simulation of turbulent reactive gas‐solid flow in a FCC riser

A newly developed hybrid solution algorithm applicable on turbulent multiphase, gas‐solid, reactive flows is presented. A finite volume (FV) and a probability density function (PDF) method are combined. The FV technique is used to solve the total mass and momentum conservation equations together with the k‐ε turbulence equation for the gas phase and the granular temperature equation for the solid phase. The PDF method is applied to solve the species continuity equations and avoids the need to model the chemical source terms in the latter. Stochastic differential equations (SDEs) are introduced as replicas of the transported composition PDF equations. The notional‐particle‐based Monte‐Carlo technique is used to solve the PDF model equations. The hybrid FV/PDF solution algorithm is applied to simulate a fluid catalytic cracking (FCC) riser using a 12‐lump kinetic model. A good agreement between simulated results and available plant data is obtained. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Cyclic mass transport phenomena in a novel reactor for gas–liquid–solid contacting

This study introduces a novel reactor concept, referred to as the Siphon Reactor, for intensified phase contacting of gas–liquid reactants on heterogeneous catalysts. The reactor comprises a fixed catalyst bed in a siphoned reservoir, which is periodically filled and emptied. This serves to alternate liquid–solid and then gas–liquid mass transfer processes. As the duration of each phase can be manipulated, mass transfer can be deliberately harmonized with the reaction. Residence time experiments demonstrate that, in contrast to periodically operated trickle‐bed reactors, the static liquid hold‐up is exchanged frequently and uniformly due to the complete homogeneous liquid wetting. A mathematical model describing the siphon hydrodynamics was developed and experimentally validated. The model was extended to account for a heterogeneously catalyzed gas–liquid reaction and capture the influence of siphon operation on conversion and selectivity of a consecutive reaction. © 2016 American Institute of Chemical Engineers AIChE J, 63: 208–215, 2017     

Preparation of sorbitol from D‐glucose hydrogenation in gas–liquid–solid three‐phase flow airlift loop reactor

A new process for D ‐glucose hydrogenation in 50 wt% aqueous solution, into sorbitol in a 1.5 m³ gas–liquid–solid three‐phase flow airlift loop reactor (ALR) over Raney Nickel catalysts has been developed. Five main factors affecting the reaction time and molar yield to sorbitol, including reaction temperature (T_R), reaction pressure (P_R), pH, hydrogen gas flowrate (Q_g) and content of active hydrogen, were investigated and optimized. The average reaction time and molar yield were 70 min and 98.6% under the optimum operating conditions, respectively. The efficiencies of preparation of sorbitol between the gas–liquid–solid three‐phase flow ALR and stirred tank reactor (STR) under the same operating conditions were compared. Copyright © 2004 Society of Chemical Industry     

Borescopy in pressurized gas‐solid fluidized beds

A borescopic technique was used for finding the effect of pressure on the hydrodynamics of gas‐solid fluidized beds. The results showed that solids radial distribution may become more or less uniform with increasing pressure depending on the superficial gas velocity. Moreover, it is found that the solids volume fraction of the emulsion phase may decrease at relatively high pressures, only in the central region of the bed. Additionally, it is observed that with increasing pressure the bubble size generally decreased in the central regions and increased near the wall regions. This trend was more complicated at low excess gas velocities. The number of bubbles increased for the central regions and near the walls for all the performed experiments. However, this parameter showed a different trend at other radial positions. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 3303–3311, 2018     

Effects of pressure and temperature on flow regimes in gas—Solid fluidization systems

The effects of operating temperature and pressure on the flow regimes of gas—solids fluidization are predicted based on the recently published flow regime diagram of Bi and Grace (1995b). The predictions are consistent with the evidence available in the literature. At constant superficial gas velocity, increasing system pressure and decreasing system temperature shift the behaviour toward higher velocity flow regimes. The opposite trends are realized if, instead, the gas mass flowrate is held constant.     

Dynamic delayed detached eddy simulation of a multi‐inlet vortex reactor

The multi‐inlet vortex reactor (MIVR) is used for flash nanoprecipitation to manufacture functional nanoparticles. A validated computational fluid dynamics model is needed for the design, scale‐up, and optimization of the MIVR. Unfortunately, available Reynolds‐averaged Navier‐Stokes methods are unable to accurately model the highly swirling flow in the MIVR. Large‐eddy simulations (LES) are also problematic, as excessively fine grids are required to accurately model this flow. These dilemmas led to the application of the dynamic delayed detached eddy simulation (DDES) method to the MIVR. In the dynamic DDES model, the eddy viscosity has a form similar to the Smagorinsky sub‐grid viscosity in LES, which allows the implementation of a dynamic procedure to determine its model coefficient. Simulation results using the dynamic DDES model are found to match well with experimental data in terms of mean velocity and turbulence intensity, suggesting that the dynamic DDES model is a good option for modeling the turbulent swirling flow in the MIVR. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2570–2578, 2016     

Gas‐solid flow distribution through identical vertical passages: Modeling and stability analysis

As previous evidence shows, the distribution of gas‐solid flow traveling through identical parallel paths can be significantly nonuniform, often with harmful operating consequences in practice. A fundamental analytical and numerical study is performed on the distribution of gas‐solid pneumatic flow passing through a “Y branch”. While many steady‐state gas‐solid distribution solutions, including a uniform distribution, satisfy the governing equations, linear stability analysis indicates that the uniform distribution is stable the most likely solution of the system. Both 2‐D (two‐dimensional) and 3‐D multiphase computational fluid dynamic simulations and stability analyses confirm the analytical conclusions. © 2010 American Institute of Chemical Engineers AIChE J, 2010