Flow behavior of gas and micro-particles in riser is predicted within the framework of the classical Euler-Lagrange approach. The Newtonian equation of motion considering the effect of van der Waals forces is solved for each simulated particle in the system. Particle collisional dynamics is modeled by means of the direct simulation Monte Carlo (DSMC) method. The influence of van der Waals forces on the particle collisions is investigated. The effect of interparticle collisions on the particle concentration and velocity distributions is presented. These numerical results allow one to understand the effect of the considered parameters on the flow behavior of micro-particles agglomerations. (c) 2007 Elsevier B.V. All rights reserved.