CFD Solution for Low Tube-to-Particle Diameter Ratio Packed Beds

Genong LiChi-Yang ChengAnsys Inc.Lebanon, NH 03766Email: Genong.Li@ansys.comEmail: Chi-Yang.Cheng@ansys.comAniruddha MukhopadhyayYi DaiAnsys Inc.Lebanon, NH 03766Email: Aniruddha.Mukhopadhyay@ansys.comEmail: Yi.Dai@ansys.comABSTRACTPacked beds are the dominantreactor type for industrial het-erogeneous catalytic reactions. Packing pattern affects the ﬂow,heat and mass transfer characteristics within the beds. With thedevelopment of high performance computing and advances inCFD, it becomes possible to resolve ﬂow/reactions in detail inpacked beds, especially for beds with low tube-to-particle diam-eter ratio. In this paper a whole set of CFD solutions in dealingwith such packed beds from packing, meshing, solving and post-processing is discussed. The solution process is illustrated byconsidering cylindrical beds packed with spherical balls. As anexample, a plant-scale bed packed with about 1000 balls is sim-ulated in the paper.INTRODUCTIONPacked beds are widely used in chemical processes such asheat exchangers and catalytic reactors. Most existing numericalanalysis of packed beds is based on the homogeneous porousmedium approach. In that approach, packing structure is de-scribed by a number of statistical, lumped parameters such asporosity, tortuosity and permeability, and the ﬂow characteris-tics including pressure, heat and mass transfer are computed bysome semi-empirical correlations based on those lumped param-eters. The approach is acceptable if all dimensions of the packedzone are much larger than the dimension of the packing element.For highly exothermic catalytic reactions, reactors with lowtube-to-particle diameter ratio are often used. The aforemen-tioned homogenous porous-medium approach will cause largeerrors. For example, high porosity near walls allows ﬂow toby-pass, so the Ergun equation developed for the homogenousporous medium will not hold any more. To ﬁx this shortcoming,at least a non-constant porosity proﬁle needs to be used even fora simple ﬂow calculation [1–4]. In addition, the occurrence ofrun-away reactions and existence of hot spots are local phenom-ena, so the homogenous porous model cannot capture them. Asa result, there is a need to resolve the packing structure directlyand simulate the ﬂow within the packed bed with high ﬁdelity.With the development of high performance computing, itbecomes possible to resolve ﬂow/reactions in detail in packedbeds, especially for beds with low tube-to-particlediameterratio.The ﬁrst challenge of doing so is to prepare a packing geometryfor the CFD simulation. For a structured bed, no special tool isneeded and the geometry can be created according to the packingpattern directly [5]. Dixon et al. [6] studied the ﬂow and temper-ature ﬁeld within a structured packed bed, and they showed thevalue of doing such simulations. Salari et al. [7] simulated thecatalytic cracking of Naphtha in a small structured packed bed.For a random packed bed, preparing the packing geometry itselfpresents a challenge. In general a dynamic simulation code canbe used. However, dynamic simulations are usually very com-putationally expensive. Bai et al. [8] used a Discrete ElementMethod (DEM) in their simulation of a packed bed. To packspherical balls into a cylindrical bed, a numerical packing algo-rithm can be used directly instead. Mueller [9] described a geo-metric packing scheme in his paper. However, the detail of thepacking algorithm was not clearly presented. A direct geometricpacking scheme will be discussed in detail in this study.1 Copyright © 2013 by ASME