The paper presents the results of numerical simulations of the fluid flow in milli torus reactor operated as airlift without agitation. CFD simulations were performed in 3D using FLUENT 6.2.16 numerical software. Unstructured mesh for prediction of gas hold up inside the milli torus reactor was investigated. An Eulerian-Eulerian multi-fluid approach was applied with a k - epsilon turbulence model. The inter-phase drag forces were used. For simplification of the solution, the bubbles inside the reactor were assumed to have the same size. The computed gas hold up was compared with the experimental data, a good agreement was obtained for unstructured meshing. For such mesh, simulations were validated with experimental residence time distribution (RTD) determination. (C) 2011 Elsevier B.V. All rights reserved.