For the calculation of gas-solid flow in circulating fluidized beds, a Eulerian-Eulerian approach is taken. An integration scheme based on dual time stepping and a finite volume technique is developed and implemented in 3D. The inviscid part of the equations is treated following an extension of the preconditioned advection upstream splitting method (AUSMP) to two-phase flows. Calculations on an industrial size straight riser are performed. The influence of the inelasticity of particle-particle collisions on the stability of the flow is investigated. Further, the effects of a double abrupt side outlet configuration are shown.