Influence of Macroscopic Wall Structures on the Fluid Flow and Heat Transfer in Fixed Bed Reactors with Small Tube to Particle Diameter Ratio

Abstract

Fixed bed reactors are widely used in the chemical, nuclear and process industry. Due to the solid particle arrangement and its resulting non-homogeneous radial void fraction distribution, the heat transfer of this reactor type is inhibited, especially for fixed bed reactors with a small tube to particle diameter ratio. This work shows that, based on three-dimensional particle-resolved discrete element method (DEM) computational fluid dynamics (CFD) simulations, it is possible to reduce the maldistribution of mono-dispersed spherical particles near the reactor wall by the use of macroscopic wall structures. As a result, the lateral convection is significantly increased leading to a better radial heat transfer.This is investigated for different macroscopic wall structures, different air flow rates (Reynolds number Re = 16 ...16,000) and a variation of tube to particle diameter ratios (2.8, 4.8, 6.8, 8.8). An increase of the radial velocity of up to 40%, a reduction of the thermal entry length of 66% and an overall heat transfer increase of up to 120% are found.

Assessing the wall effects of packed concentric cylinders and angular walls on granular bed porosity

“…This elucidates the geometric interactions needed to improve throughput in a filter cake. Furthermore, a homogeneous porosity distribution was observed in the annular spaces of this system, showing potential for applications demanding porosity control, such as the freeze-drying of bulky solids [43] and heat transfer in a fixed-bed reactor [44]. Additionally, variations of the coefficient of friction and insert thickness were considered but did not show significant effects.…”

Particle-Resolved Computational Fluid Dynamics as the Basis for Thermal Process Intensification of Fixed-Bed Reactors on Multiple Scales

“…As discussed, particle-resolved CFD is a valuable tool to support process intensification on the meso-scale level, e.g., by finding optimized particle shapes [4,[10][11][12] or new reactor concepts, e.g., by applying macroscopic wall structures [20,21] or using internals [19]. However, for process intensification on a macroscopic scale, e.g., by running plants under dynamic operation conditions, developing process integration strategies, or doing plant optimization, different numerical tools are necessary.…”

“…Here, the radial heat transport was characterized by the wall heat transfer coefficient α w and the effective radial thermal conductivity λ eff,r , which was assumed to be uniform everywhere in the reactor. By extracting the axial core temperature profile and average inlet/outlet temperatures from the CFD simulations, both parameters were determined by using Equations ( 17), (19), and (20). The results are summarized in Table 2.…”

“…The three necessary parameters of the Winterberg correlation were determined by conducting a parameter optimization study. The basis of this study was the transport equation described by Equation (21). A summary of the optimized model parameters, including the mean squared error MSE, is given in Table 2.…”

“…In the last few years, particle-resolved computational fluid dynamics (CFD) was heavily used by numerous authors to develop process intensification strategies with the focus on the effects on the mesoscopic pellet scale. The range of works extends from investigations of the influence of particle shape on bed morphology and fluid dynamics [2][3][4], heat transport [5][6][7][8], and mass transfer processes [9][10][11][12] to the development of novel reactor concepts, such as packed foams [13][14][15], periodic open-cell structures [16][17][18], finned reactors [19], or the use of random macroscopic wall structures [20,21]. However, particleresolved CFD is a numerically very demanding method, and its applicability is currently limited to systems with a few thousand particles [22].…”