Trickle bed reactors (TBRs) are commonly used in chemical industries. Scale-up of TBR is difficult and simple scaling rules often lead to poor design. Flow mal-distribution, channeling, wetting of catalyst and local temperature variation are some of the important parameters which controls the overall performance of the TBRs. Fluid dynamics of the TBRs is complex and very sensitive to the scale of the reactors. Conventional modeling techniques are unable to account these key design issues. Recent advances in computational fluid dynamics (CFD) show promising results in understanding fluid dynamics and its interactions with chemical reactions. In this study we have developed a CFD model for simulating flow and reactions in the laboratory scale and commercial scale reactors. A case of hydro-processing reactions was considered. The CFD models were first evaluated by comparing the model predictions with the published experimental data. The models were then used to understand the influence of porosity distribution, particle characteristics and reactor scale on overall performance. Validated model was used to predict the performance of the commercial scale reactor. Various critical issues of scaling of TBR and how CFD modeling can help in reducing uncertainties associated with it are discussed in details. The approach, model and results presented here will be useful for understanding the complex hydrodynamics, its interaction with chemical reactions and influence of different reactor scales on performance of the TBRs. (c) 2007 Elsevier Ltd. All rights reserved.