拟泡-乳曳力模型在大型高温费托流化床反应器中的CFD模拟研究

以带冷却盘管的大型高温费托流化床反应器为研究对象,开展三维计算流体力学模拟研究。传统双流体模型基于局部平均的假设,认为单位控制体内气固两相均匀分布,网格尺寸必须足够小才能正确揭示局部非均匀结构的所有细节。采用双流体模型模拟大型工业化流化床装置时,将导致网格数量过于庞大,远超现有计算能力。为提高计算效率的同时不损失模拟精度,提出了基于局部非均匀假设、适用于粗网格的拟泡-乳三相非均匀曳力(PBTD)模型。该模型将流化床分为乳化相气体、乳化相颗粒以及气泡三相,分别建立守恒方程,体现气泡的非均匀特性对气固曳力的影响。乳化相内气固曳力以及气泡相与乳化相内颗粒的曳力分开考虑。采用PBTD模型耦合传质和反应模型,建立基于局部非均匀假设的高温费托合成反应器三维流动-传递-反应模型,包括各相守恒控制方程、气泡尺寸模型、相间物质和动量交换模型、高温费托合成反应动力学模型以及初始和边界条件,预测反应器内的流场和组分浓度分布。研究结果表明:在粗网格条件下,非均匀曳力模型可以预测床层内相含率的分布情况,预测的床层膨胀高度与经验公式计算值接近,偏差为1.2%。反应器出口气体组分的质量分数与试验测量值相近,偏差在1.5%~16.0%。模拟结果证实,基于非均匀假设的PBTD模型适用于模拟工业规模的鼓泡流化床反应器,对其设计开发和工业运行具有指导价值。

CFD simulation of pseudo bubble-emulsion drag model in a large-scale high-temperature Fischer-Tropsch fluidized bed reactor

In this paper,three-dimensional computational fluid dynamics(CFD)simulation of a large-scale high-temperature Fischer-Tropsch fluidized bed reactor was carried out.The traditional two-fluid model is based on the assumption of homogeneity at the level of computational cells,which assumes that the gas and solid phases are uniformly distributed within a cell and the mesh size must be small enough to properly reveal all the details of the local heterogeneous structure.When the two fluid model is used to simulate large-scale industrial fluidized bed reactors,the number of grids will be too large,which is far beyond the existing computing capacity.In order to improve the simulation efficiency without losing the simulation accuracy,a pseudo bubble-emulsion triple-phase drag(PBTD)model based on the assumption of local heterogeneity and applicable to coarse meshes was proposed.In this model,the fluidized bed is divided into three phases:emulsified phase gas,emulsified phase particle and bubble.A conservation equation is established for each of the three phases to reflect the effect of the heterogeneous characteristics of the bubbles on the gas-solid drag force.The gas-solid drag force within the emulsion phase and the drag force between the bubble phase and the particles in the emulsion phase are considered separately.A three-dimensional flow-transfer-reaction model of the high-temperature Fischer-Tropsch reactor based on the assumption of local heterogeneity is established using the PBTD model coupled with the mass transfer and reaction models,including the conservation control equations of each phase,the bubble size model,the interphase mass and momentum exchange model,the high-temperature Fischer-Tropsch reaction kinetics model,and the initial and boundary conditions to predict the flow hydrodynamics and gas component concentration distribution.The results show that under the condition of coarse grid,the proposed heterogeneous drag model can obtain good results of the distribution of phase holdup in the bed,and the predicted bed expansion height is in good agreement with empirical correlation with a deviation of 1.2%.The mass fraction of the gas compositions at the reactor outlet is close to measurement values,and the deviations are in the range of 1.5%-16.0%.The simulation results show that the PBTD model is suitable for simulating industrial scale bubbling fluidized bed reactors,which has guiding value for application in the design and operation of industrial reactor.