This work focuses on the 3D full-loop CFD isothermal simulation of a transparent plexi-glass small-scale CFB carbonator built by IFK for the hydrodynamic investigation of the CFB reactor, utilized for the investigation of the calcium looping (Ca-L) process. This work couples the state of the art TFM approach with the advanced EMMS scheme for the calculation of drag coefficient exerted in the solid phase by the gas one. The CFB loop is discretized by 286,753 elements. In contrast to the majority of the published work, the flow characteristics of the re-circulation system of the unit, i.e. cyclone, down-comer and the pneumatic valve type of loop seal working as a flow regulator, is as well simulated and allows for the in-depth investigation and understanding of the whole CFB operation by means of CFD. In order to achieve this, the new proposed Pitman-Schaffer-Gray-Stiles yield criterion forms the basis, upon which the stresses exerted in the particulate phase within the loop-seal are calculated. The numerical results are averaged over a time period of 120 QUOTE s and the predicted flow patterns are compared against corresponding visual observations. Simulation results agree quite well with the experimental data, regarding the re-circulation flux and the pressure profile along the full-loop. (C) 2012 Elsevier Ltd. All rights reserved.