提升管反应器介尺度结构影响规律的数值模拟研究

提升管反应器存在典型的颗粒聚团介尺度结构,其分布特性对气固流动、反应有重要影响,对介尺度结构影响规律进行分析有助于为反应器的设计与优化操作提供基础信息。采用基于能量最小多尺度（EMMS）方法的曳力模型建立了提升管气固两相流动模型,考虑了颗粒聚团对气固相间动量传递的影响。此外,进一步通过考虑颗粒聚团的存在以及颗粒聚团的非均匀性对化学反应的影响,提出了描述介尺度结构对反应速率影响的修正因子,与气固流动模型进行耦合,建立了基于介尺度结构的流动-反应综合数学模型,并进行了模型验证。进一步应用该模型,对工业催化裂化提升管反应器的流动-反应特性进行了模拟分析。结果表明,该模型可以合理描述提升管气固相互作用,能够预测出壁面附近存在较多介尺度结构的分布特性,由于聚团的存在使得重油组分难以与催化剂充分接触,生成汽柴油的反应速率较低,转化较慢,聚团的分布特性导致靠近边壁处的重油组分浓度较高,汽柴油组分浓度较低;汽柴油在聚团内部的流动阻力较大,在聚团内发生过量的二次反应生成较多焦炭,导致壁面处焦炭浓度较高。与传统基于平均化而未考虑聚团影响的模型相比,基于介尺度结构的模型所预测的汽油收率最佳值与工业实际相接近。因此,基于介尺度结构的流动-反应综合数学模型可以合理描述提升管内进行的流动-反应耦合特性,并能揭示介尺度结构对催化裂化反应过程的影响,有望为工业提升管装置反应终止剂技术的开发提供重要的基础信息。     

基于气泡和聚团的结构模型及其应用

综合考虑鼓泡流化床内气泡及聚团对床内细颗粒流动的影响,建立基于气泡和聚团的结构曳力模型及结构参数模型,同时,借助计算流体软件预测细颗粒在鼓泡床中流动状态。首先,基于细颗粒在鼓泡流化床的流动状态,在介观尺度上将床内气固流动结构划分为3个子结构,即气泡相、相间相及乳化相(聚团相)。然后,综合考虑细颗粒鼓泡流化床中气泡和聚团对气固流动的影响,根据力平衡、质量守恒建立基于气泡和聚团的结构参数模型及结构曳力模型。通过对结构参数模型封闭求解,得到11个结构参数值(f_b,U_b,d_b,U_(gb),f_i,U_(gi),ε_i,f_e,U_(ge),ε_e,d_c)。对结构参数计算结果进行分析,结构参数模型能够很好反映床内流动情况及床内各结构参数之间的关系,并能有效地预测颗粒聚团直径。此外借用非均匀因子,耦合结构曳力模型及结构参数模型到欧拉双流体模型对气固在床内流动行为进行数值模拟。模拟结果表明,使用基于气泡和聚团的结构曳力模型能够较好地预测细颗粒在鼓泡流化床中的流动行为。在对模拟结果中颗粒径向浓度比较时,可以发现,相对比基于气泡模型的结构曳力模型,使用基于气泡和聚团的结构曳力模型的模拟结果与实验结果更一致。     

磁场对湿颗粒流化床系统中介尺度结构影响机制研究

湿颗粒系统在自然界及工业过程非常普遍,例如喷雾造粒、反应器中矿物黏结、催化以及制药等,这其中含有大量典型介尺度结构如颗粒聚团、结块以及气泡等结构,这些结构的存在导致颗粒系统的流动及热质传递特性发生明显改变。针对鼓泡流化床湿颗粒系统中颗粒聚团以及气泡等介尺度结构,应用离散单元模型并引入外加磁场,研究磁场作用下湿颗粒系统中介尺度结构的演化机制,探究磁场力、液桥力、接触力对气泡演化过程的影响。研究发现,在不考虑磁场的条件下,颗粒易形成聚团并存在气泡边界不规则等现象,引入外加匀强磁场后,磁场力对鼓泡流化床内气泡结构存在破坏和抑制作用。     

气固流化系统多尺度跨流域EMMS建模

气固流化床反应器是典型的具有多尺度非均匀动态结构的复杂系统。实现对该类反应器定量描述和定向调控的关键是深入了解系统内介尺度结构的形成和演化特征。能量最小多尺度（EMMS）方法为气固非均匀系统的量化表征提供了一种通用的建模思路。首先回顾了EMMS理论在构建曳力本构关系方面的应用,重点介绍了本课题组在EMMS曳力模型普适化方面所做的部分工作;随后对介尺度结构时空动态演化行为的群平衡建模方法进行了论述,并给出了群平衡和结构曳力模型相耦合的连续介质模拟框架;最后讨论了EMMS原理在预测反应器宏尺度动力学方面的应用,包括模型在不同流域的拓展、操作相图的绘制以及循环流化床的全回路稳态建模方法等。     

基于重置温度方法的双参数介尺度气固传热模型构建

颗粒聚团等介尺度结构在气固两相流中普遍存在,这些介尺度非均匀结构直接影响气固流动特性及气固接触效率,进而影响气固相间传热、传质及化学反应过程。在更适合工业大尺度气固传热模拟的粗网格方法中缺乏准确度高、简单易用且可以考虑介尺度非均匀结构影响的传热模型。采用计算流体力学-离散单元法（CFD-DEM）研究了气固两相流相间传热,为了保证气固相间持续传热,采用了两种维持气固相间传热温差的方法,并讨论了两种方法的优缺点。方法一：给气相能量方程添加热源项;方法二：每间隔一段时间重置气相温度,重置温度后气固两相自由传热,两种方法中均保持固相温度不变。结果表明聚团界面位置的局部单位体积气固传热量最大,重置温度方法在稀相和界面位置的局部单位体积传热量与总单位体积传热量之比大于热源项方法,而在浓相位置的局部单位体积传热量与总单位体积传热量之比小于热源项方法。通过过滤CFD-DEM计算数据,为重置温度方法构建了双参数（过滤固含率、过滤尺度）传热系数修正因子模型,通过先验分析评价了模型的表现,研究表明所构建模型在过滤网格尺度为5~40倍颗粒直径范围内优于文献中已有的双参数模型。     

气固流化床介尺度结构形成机制及过滤曳力模型研究进展

气固流化床中,介于颗粒与宏观尺度间的复杂的时空多尺度结构（介尺度结构）将完全改变气固相间作用规律,加大了流态化系统调控及预测的难度。为此,需要构建考虑结构影响的相间本构关系。其中,曳力作为影响流态化动力学特征的主导因素,对其研究尤为重要。从结构产生演化的机制出发,概述结构影响曳力的机理,以模型构建流程的角度对结构和过滤两类模型进行总结,并重点综述过滤模型构建在提升准确性、有效性、通用性和考虑更多物理机制方面的最新进展。研究表明：提升模型通用性和考虑真实系统中更丰富的物理机制仍是建模中亟待解决的问题,结合结构演化机制理性建模和充分发挥机器学习数据分析处理优势或是曳力建模进一步发展的关键。     

基于EMMS介尺度模型的双分散鼓泡流化床的模拟

双分散气固鼓泡流化床中颗粒通常具有不同粒径或密度,导致产生颗粒偏析等现象,影响传递和反应行为。颗粒分离和混合与气泡运动密不可分,其中相间曳力起关键作用。最近Ahmad等提出了一种基于气泡结构的双分散介尺度曳力模型,能成功预测双分散鼓泡流化床的床层膨胀系数。本研究耦合该曳力模型与连续介质方法,模拟了两种不同的双分散鼓泡流化床,通过分析不同流化状态下的气泡运动、颗粒浓度比的轴向分布等参数,进一步检验模型的适用性。研究表明,当双分散颗粒处于完全流化状态时,耦合双分散介尺度曳力模型可合理预测不同颗粒的分离现象;而其处于过渡流化状态时,新曳力模型和传统模型均无法获得合理结果,此时调节固固曳力可改进模拟结果。     

基于EMMS介尺度模型的双分散鼓泡流化床的模拟

双分散气固鼓泡流化床中颗粒通常具有不同粒径或密度,导致产生颗粒偏析等现象,影响传递和反应行为。颗粒分离和混合与气泡运动密不可分,其中相间曳力起关键作用。最近Ahmad等提出了一种基于气泡结构的双分散介尺度曳力模型,能成功预测双分散鼓泡流化床的床层膨胀系数。本研究耦合该曳力模型与连续介质方法,模拟了两种不同的双分散鼓泡流化床,通过分析不同流化状态下的气泡运动、颗粒浓度比的轴向分布等参数,进一步检验模型的适用性。研究表明,当双分散颗粒处于完全流化状态时,耦合双分散介尺度曳力模型可合理预测不同颗粒的分离现象;而其处于过渡流化状态时,新曳力模型和传统模型均无法获得合理结果,此时调节固固曳力可改进模拟结果。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

介尺度视角下干法重介流态化分选过程强化

煤炭在保障我国能源安全中具有重要作用。干法重介流态化分选是煤炭分选加工领域的重要组成部分,有助于推动我国煤炭资源的高效洁净利用。流态化分选密度的稳定调控是实现高效分选的必要条件,调控的核心是如何削弱气泡扰动作用,关键是理解流态化分选过程中介尺度结构的演变及调控机制。从介尺度视角分析了气固流态化干法分选调控过程的关键科学问题,梳理了单元干法分选设备以及系统放大过程中介尺度结构的研究进展,分析了介尺度结构的演变规律,提出了介尺度结构的精准调控策略,对干法流态化的工业推广应用及煤炭的分选提质具有重要意义。     

Discrete particle simulation of gas fluidization of ellipsoidal particles

Fluidization is widely used in industries and has been extensively studied, either experimentally or theoretically, in the past decades. In recent years, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) has been successfully developed to study the gas–solid flow and heat transfer in fluidization at a particle scale. However, to date, such studies mainly deal with spherical particles. The effect of particle shape on fluidization is recognized but not properly quantified. In this paper, the CFD–DEM approach is extended to consider the fluidization of ellipsoidal particles. In the simulation, particles used are either oblate or prolate, with aspect ratios varying from very flat (aspect ratio=0.25) to elongated (aspect ratio=3.5), representing cylinder-type and disk-type shaped particles, respectively. The commonly used correlations to determine the fluid drag force acting on a non-spherical particle are compared first. Then the model is verified in terms of solid flow patterns. The effect of aspect ratio on the flow pattern, the relationship between pressure drop and gas superficial velocity, and microscopic parameters such as coordination number, particle orientation and force structure are investigated. It is shown that particle shape affects bed permeability and the minimum fluidization velocity significantly. The coordination number generally increases with aspect ratio deviating from 1.0. The analysis of particle orientations shows that the bed structures for ellipsoids are not random as that for spheres. Oblate particles prefer facing upward or downward while prolate particles prefer horizontal orientation. Spheres have the largest particle–particle contact force and fluid drag force under the comparable conditions. With aspect ratio deviating from 1.0, particle–particle interaction and fluid drag become relatively weak. The proposed model shows a promising method in examining the effect of particle shape on different flow behaviour in gas fluidization.     

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.     

Assessment of mesoscale solid stress in coarse‐grid TFM simulation of Geldart A particles in all fluidization regimes

This study focused on assessing the effect of mesoscale solid stress in the coarse grid two‐fluid model (TFM) simulation of gas–solid fluidized beds of Geldart Group A particles over a broad range of flow regimes, including bubbling, turbulent, fast, and pneumatic transport fluidization regimes. Particularly, the impact of mesoscale solid pressure, mesoscale solid viscosity, and mesoscale solid stress anisotropy were investigated by comparing six different coarse‐grid TFM settings. Compared with the available experimental data, it is found that both the kinetic theory‐based TFM with only drag correction and the filtered TFM can predict the flow behavior in all fluidization regimes. Mesoscale solid pressure and viscosity have the opposite impact on flow hydrodynamics; they compete and offset each other, which confirms the assumption employed in many previous studies that the mesoscale solid stress could be neglected in coarse‐grid TFM simulation. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. AIChE J, 64: 3565–3581, 2018     

Aggregation and fragmentation of agglomerates in a fluidized bed of mixed nanoparticles by adding FCC coarse particles

In this study, the influence of fluid cracking catalyst (FCC) on the fluidization behavior of ZnO-CuO binary nanoparticles was systematically investigated by varying FCC size. High-speed camera was employed to analyze the collision and fragmentation process of agglomerates with adding FCC coarse particles. It can be found from photographs by the camera that fluidization performance improved by the agglomerate variation that is bound to be shaped a compact and spherical structure. Furthermore, the result of agglomeration composition analysis showed that uniform mixing of nanoparticles remarkably affected the fluidization behavior of ZnO-CuO binary system. Finally, the improvement of fluidization performance can be justified by the analysis of inter-cohesive force between the two agglomerates with sharp reduction of the newly-formed agglomerates.     

Numerical investigation of gas-solid heat transfer in rotating fluidized beds in a static geometry

Gas-solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas-solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas-solid slip velocities and resulting gas-solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed “width” to “height” allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas-solid slip velocities drastically increase the rate of gas-solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas-solid drag force being influenced by the fluidization gas flow rate in a similar way, rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed.     

Characterizing the catalyst fluidization with field synergy to improve the amine absorption for CO_2 capture

There are great interests to capture the CO_2 to control the greenhouse gas emission. Amine absorption of CO_2 is being taken as an effective way to capture CO_2 in industry. However, the amine absorption of CO_2 is cost-ineffective due to great energy consumption and solution consumption. In order to reduce the capture cost, catalyst fluidization is proposed here to intensify the mass transfer and heat transfer. Catalyst fluidization with field synergy and DFT model is developed by incorporating the effects of catalyst reaction kinetics, drag force and multi-field into the mass transfer, heat transfer, fluid flow and catalyst collision. Experiments with an improved distributor are performed well to validate the model. The reaction kinetics is determined by the DFT simulation and experiment. The mass transfer coefficient in the fluidized reactor is identified as 17% higher than the conventional packed reactor. With the field synergy of catalyst fluidization, the energy consumption for CO_2 desorption is reduced by 9%. Stepwise operation and inclination reactor are used to improve catalyst fluidization process.     

气固两相流内中空多孔催化剂性能的数值模拟

气固流态化过程中流体和颗粒分别聚集,形成稀密两相,严重限制其传质效率和反应速率的提高。针对此问题,本工作设计了一种中空多孔结构的催化剂颗粒,通过模拟方法研究该颗粒对稀密两相气相传质与反应的影响,及其在稀密相间转换的时间尺度。结果表明,一定的流动强度时,在颗粒稀密相转换的时间尺度内,中空多孔结构的颗粒能够有效地在稀相存储反应气体,并在密相释放,为密相提供额外的反应气体,增强体系的整体反应效率。当催化反应速率高于传质速率时,在所研究的流动条件下中空多孔颗粒体系的反应效率比实心球形颗粒体系高出26.92%~29.55%。可以预见在稀密相分布更广的大型气固流化床反应器中,中空多孔结构的催化剂颗粒能够更为有效地提高反应器的整体效率。