Contraction behavior of a liquid-solid fluidized bed has been investigated numerically. Based on a simple hydrodynamic model proposed by Brandani and Zhang （2006）, a case study for solid particles with a density of 3,000 kg/m^3 and a diameter of 2.5× 10^-3 m is simulated in a two-dimensional fluidized bed （0.50 m height and 0.10 m width）. Due to the continuity of numerical computation, there is a transition region between two zones of different solid holdups when the liquid velocity is suddenly changed. The top, middle and bottom interfaces are explored to obtain a reasonable interface height. The simulated results show that the steady time of the middle interface is more close to Gibilaro＇s theory and suitable for describing the contraction process of a phase interface. Furthermore, the effect of liquid velocity and particle diameter is simulated in the other two-dimensional fluidized bed （0.10 m height and 0.02 m width） where the solid particles are glass beads whose properties are similar to those of the catalyst particles used in the alkylation process. The results also show good agreement with Gibilaro＇s theory, and that larger particles lead to a more obvious bed contraction.
Yao XiuyingGuan YanjunChang JianZhang KaiJiang Jianchun
This paper presents a computational investigation of hydrodynamics, heat transfer and cracking reaction in a heavy oil riser operated in a novel operating mode of low temperature contact and high catalyst-to-oil ratio. Through incorporating feedstock vaporization and a 12-lump cracking kinetics model, a validated gas-solid flow model has been extended to the analysis of the hydrodynamic and reaction behavior in an industrial riser. The results indicate that the hydrodynamics, temperature and species concentration exhibit significantly nonuniform behavior inside the riser, especially in the atomization nozzle region. The lump concentration profiles along the riser height provide useful information for riser optimization. Compared to conventional fluid catalytic cracking （FCC） process, feedstock conversion and gasoline yield are respectively increased by 1.9 units and 1.0 unit in the new FCC process, the yield of liquefied petroleum gas is increased by about 1.0 unit while dry gas yield is reduced by about 0.3 unit.
Under the Eulerian-Eulerian framework of simulating gas-solid two-phase flow, the accuracy of the hydrodynamic prediction is strongly affected by the selection of rheology of the particulate phase, for which a detailed assessment is still absent. Using a jetting fluidized bed as an example, this work investi- gates the influence of solid rheology on the hydrodynamic behavior by employing different particle-phase viscosity models. Both constant particle-phase viscosity model （CVM） with different viscosity values and a simple two-fluid model without particle-phase viscosity （NVM） are incorporated into the classical two- fluid model and compared with the experimental measurements. Qualitative and quantitative results show that the jet penetration depth, jet frequency and averaged bed pressure drop are not a strong func- tion of the particle-phase viscosity. Compared to CVM, the NVM exhibits better predictions on the jet behaviors, which is more suitable for investigating the hydrodynamics of gas-solid fluidized bed with a central jet.
Particle suspension characteristics are predicted computationally in a stirred tank driven by a Smith turbine. In order to verify the hydrodynamic model and numerical method, the predicted power number and flow pattern are compared with designed values and simulated results from the literature, respectively. The effects of particle density, particle diameter, liquid viscosity and initial solid loading on particle suspension behavior are investigated by using the Eulerian-Eulerian two-fluid model and the standard k-ε turbulence model. The results indicate that solid concentration distribution depends on the flow field in the stirred tank. Higher particle density or larger particle size results in less homogenous distribution of solid particles in the tank. Increasing initial solid loading has an adverse impact on the homogeneous suspension of solid particles in a low-viscosity liquid, whilst more uniform particle distribution is found in a high-viscosity liquid.
A simple hydrodynamic model based on two-fluid theory, taking into account the effect of discrete particles on both the gas- and solid-phase momentum equations, was used to numerically investigate the pressure fluctuation characteristics in a gas-solid fluidized bed with the aid of CFX 4.4, a commercial CFD software package, by adding user-defined Fortran subroutines. Numerical simulations together with typical experimental measurements show that pressure fluctuations originate above the distributor when a gas pulse is injected into the fluidized bed. The pressure above the bubble gradually increases due to the presence of a rising bubble. When the bubble passes through the bed surface, the pressure near the bed surface gradually decreases to a lower value. Moreover, the pressure signals in the bubbling fluidized beds show obviously periodic characteristics. The major frequency of pressure fluctuations at the same vertical position is affected slightly by the operating gas velocity, and the amplitude of pressure fluctuations is related to both the operating gas velocity and the vertical height. In this study, the influence of the operating gas velocity on the pressure wave propagation velocity can be ignored, and only two peak frequencies in the power spectrum of the pressure fluctuations are observed which are associated with the bubble formation above the distributor and its eruption at the bed surface.
A computational fluid dynamics （CFD） model is used to investigate the hydrodynamics of a gas-solid fluidized bed with two vertical jets. Sand particles with a density of 2660 kg/m3 and a diameter of 5.0 × 10^-4 m are employed as the solid phase. Numerical computation is carried out in a 0.57 m ×1.00 m two-dimensional bed using a commercial CFD code, CFX 4.4, together with user-defined Fortran subroutines. The applicability of the CFD model is validated by predicting the bed pressure drop in a bubbling fluidized bed, and the jet detachment time and equivalent bubble diameter in a fluidized bed with a single jet. Subsequently, the model is used to explore the hydrodynamics of two vertical jets in a fluidized bed. The computational results reveal three flow patterns, isolated, merged and transitional jets, depending on the nozzle separation distance and jet gas velocity and influencing significantly the solid circulation pattern. The jet penetration depth is found to increase with increasing jet gas velocity, and can be predicted reasonably well by the correlations of Hong et al. （2003） for isolated jets and of Yang and Keairns （1979） for interacting jets.
