Worldwide hydro-processing capacity is increasing several folds. Existing/new refineries demand an improvement in performance or advanced designs to meet stringent environmental regulations. Hydro-processing and hydro-cracking are carried out in trickle bed reactors. In trickle bed reactors, gas and liquid reactants flow co-currently downward through a packed catalyst bed. Complex interaction of gas, liquid and solid phases lead to different flow regimes. Wide range of spatiotemporal scales exists and complex interaction among these scales complicates the design, scale-up and operation of trickle bed reactors. Scale-up from hourly space velocity often does not work and necessitates time consuming pilot plant experiments. Residence time distribution and mixing varies in complex fashion with scale-up. Most of the correlations developed on laboratory scales may not work for the large scale reactors (operated at high pressure/ temperature). Various such modeling and scale-up issues are recently revisited by Dudukovic and Al-dahhan1. Conventional modeling methods were relying on phenomenological models or 1D modeling by approximating flow and mixing through empirical models. However recent developments in CFD models help to reduce efforts in experiments and empiricism in modeling. These models account for detailed reactor geometry and predict hydrodynamic parameters, phase distribution and flow fields3 with good accuracy. Therefore, applicability of such detailed models in designing hydro-processing units where desired product is of high purity (~ below 100 ppm sulfur concentration) is investigated in this work. A comprehensive CFD model was developed for hydro-processing trickle bed reactor (TBR). Porosity variation in axial and radial direction was considered in the model and Eulerian-Eulerian model was used to simulate the flow, mixing and reactions in TBR. In hydro-processing unit, hydro-desulfurization and hydro-dearomatization was considered and detailed kinetic rate parameters and properties of the compounds were taken from the literature2. Obtained flow field information along with mass and energy balance equations were solved using FLUENT 6.2 (of Fluent Inc., USA) for wide range of operating conditions. Model results were first validated with the experimental data in literature2. Validated model was used to understand influence of various design and operating parameters. Various critical issues of scale-up of TBR such as bypassing & channeling, partial wetting and flow mal-distribution were studied using the CFD model. The approach, CFD model and presented results will have significant implications in enhancing performance of hydro-processing reactors.

References: 1. Dudukovic M. P. and M. H. Al-dahhan, Why Scaleup Still Matters? AICHE Annual Meeting, Cincinnati, Ohio, Nov 1, (2005). 2. Chowdhury R., E. Pedernera and R. Reimert, AIChE Journal, 48, 126, (2002).