Abstract Granular flow in the rapid flow regime is dominated by particle–particle collisions and the constitutive relations for the solid stress are obtained from the classic kinetic theory of granular flow. In the dense flow regime, on the other hand, particles interact via enduring contacts and the solid stress can be deduced from soil mechanics theories. In this paper, constitutive equations, recently proposed by Tardos et al. [2003. Slow and intermediate flow of frictional bulk powder in the Couette geometry. Powder Technology 131, 23–39.] has been incorporated in the simulation of gas–solid flow in a horizontal duct. These equations smoothly merge the rapid granular flow solution with the so-called “intermediate” regime (where both kinetic/collisional and frictional contributions might play a role) and reduce to Coulomb yield condition for slow frictional flow (shear rate → 0). The results of this new modelling approach have shown good qualitative agreement with the reported experimental observation on wide range of gas–solid flow conditions. In this study, we also present the definition of boundaries between rapid–intermediate–dense flow regimes based on the dimensionless shear rate ( λ ), and a modified Reynolds number ( Re). We have shown that the intermediate flow regime can be classified at approximately 0.1 < λ < 1.0 and 100 < Re < 3000 .