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
Flow characteristics, such as flow pattern, gas holdup, and bubble size distribution, in an internal loop reactor with external liquid circulation, are simulated to investigate the influence of reactor internals by using the computational fluid dynamics （CFD）-population balance equations （PBE） coupled model. Numerical results reveal that introducing a downcomer tube and a draft tube can help to improve the mass and heat transfer of the reactor through enhanced liquid circulation, increased gas holdup and reduced bubble diameter. The hydrodynamic behavior in the internal loop reactor with external liquid circulation can be managed effectively by adjusting the diameter and axial position of the draft tube.
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.
The lignite pre-drying process plays an important role in modern lignite power plants and the fluidized bed dryer with internal heat utilization is a promising drying method which has both high efficiency and cost-effectiveness. After conducting an in-depth analysis of a typical lignite pre-drying power plant, this work proposed a novel lignite pre-drying system with low-grade heat integration. Through system integration, the low-temperature evaporation of the lignite was recovered to heat the combustion air, while the residual heat from the flue gases was used to heat the feed or condensed water, thereby saving a portion of heat from the steam bleeds of the high and intermediate pressure turbines. The results for a1,000 MW lignite-fired power plant showed that, the proposed pre-drying system could yield an increase in net plant efficiency of approximately 3.6 % points and a reduction in the cost of electricity of $1.83/(MW h). The thermodynamic and economic performances were each superior to those of the existing pre-drying system, convincingly demonstrating that the research of this paper may provide a promising integrated lignite pre-drying method for the next-generation of lignite-fired power plants.
Cheng XuGang XuYu HanFeifei LiangYaxiong FangYongping Yang
