A numerical study was made in order to investigate whether the radial dispersion mechanism dependent on the packing structure can be represented by means of a continuum model based on a two-dimensional porosity distribution. This distribution is generated from a computer-generated sphere packing which resembles closely the characteristics of a real packing. The radial Peclet numbers are obtained by simulating the radial spread of a tracer and fitting the data to a dispersion model with a constant porosity. A comparison to the predictions of Gunn [Trans. Inst. Chem. Engng. 47 (1969) T3513] using a stochastic model yields a qualitative agreement in the turbulent regime but with an asymptotic value for the Peclet number more than twice as high than the experimentally obtained value Pe(r) approximate to 11. Reasons for the discrepancy and possible improvements are discussed in detail.