(55c) Assesment of an Eulerian-Eulerian Multiphase CFD Framework for A Three Dimensional Modeling of A Trickle BED Reactor in Counter-Current Operation

Commercial HDT processes usually operate in a trickle-bed regime, with co-current downward flow of gas and liquid over a randomly fixed bed of catalyst particles where reactions take place. It is well known that sulfur removal is strongly inhibited by the competitive adsorption effect of H₂S at the sulfided active sites of the catalyst. As a consequence, it is important in these process to maintain the concentration of H2S as low as possible during the reaction to achieve a low sulfur content product at the reactor outlet. According to Ancheyta et al. (2007), a more convenient profile of H2S concentration can be provided by operating the reactor in counter-current mode, for instance, introducing the feed at the top and H₂ at the bottom of the reactor. Thus, in the lower section of the reactor, the H₂S concentration is lower and H₂ is greater, promoting higher reaction rates. The main problems on the counter-current operation are a lower efficiency on the gas-liquid contact; a difficulty in preventing flooding conditions and a higher pressure drop. In order to investigate the above processes in more detail, a three-dimensional fluid-dynamic study of a counter-current trickle bed reactor was carried out using CFD (Computational Fluid Dynamics) techniques with emphasis on determining the pressure drop and the distribution of liquid hold-up in the bed. A CFD tridimensional model based on an Eulerian – Eulerian multiphase approach was used to model the hydrodynamics of the pseudo two-phase flow in a trickle bed reactor (TBR) in counter-current operation. The closure terms for the phase interactions have been based on the fluid-fluid interfacial force balance concept (Attou et al. 1999), which was used and validated to simulate co-current HDT reactors by Gunjal e Ranade (2007). Radial variation of porosity was estimated using the Klerk (2003) distribution. The above set of model equations were implemented inside the commercial software ANSYS-CFX Release 13. The model provided information about phase distribution, velocity profiles, pressure drop liquid hold up for a isothermal trickle bed reactor at laboratory scale in counter-current operation.