In the article, a model is proposed for a single-phase flow in a random packed bed of arbitrary particles Raschig rings in particular. The Navier-Stokes flow equations are solved using a projection method on a structured grid and the complex geometry of the bed is captured with the immersed boundary method and direct forcing approach. The model allows for analysis of flows in complex geometries with classical and widely available finite volume or finite difference solvers for Cartesian grids. The generation of unstructured computational grids fitted to the bed particles is not necessary. The realistic bed geometry has been obtained in the simulation of the filling process in which the particles are sequentially introduced into a cylindrical container and move until mechanical equilibrium is attained. The pressure drop in the bed as the function of gas velocity has been found to be in good agreement with empirical formulas for beds of Raschig rings and sample results on detailed flow characteristics are presented. (C) 2016 Elsevier Ltd. All rights reserved.