Assessment of Liquid Distribution in a VGO-Hydroprocessing Trickled Bed Reactor with Computational Fluid Dynamics

Assessment of liquid distribution in a VGO-hydroprocessing trickled bed reactor with computational fluid dynamics

J. J Arias B., jjariasb@unal.edu.co

A. Molina, amolinao@unal.edu.co

CFD simulations of the liquid distribution in a Trickled Bed Reactor (TBR) hydro-processing Vacuum Gas Oil (VGO) were carried out. TBRs are vertical columns with fixed-bed reactors within. Inside the reactor liquid and gaseous phases pass through the bed on a concurrent, down-flow mode. This kind of reactor has the capacity to host different kind of catalytic reactions. Frequently they are used to refine crude oil cuts, as is the case in hydro-processing, where VGO reacts with hydrogen to eliminate heteroatoms by hydrogen substitution (hydro-treating) and saturate broken bonds generated by catalytic cracking with hydrogen atoms (HydroCracking (HC). Uniform liquid distribution at the top of this kind of reactors, while fundamental to guarantee proper operation, is very difficult to achieve.

To understand the effect of liquid flow mal-distribution at the top of TBR, four sections of an industrial-size HC reactor were modeled. The first section involves the surroundings of the gas-liquid distributor, itself the second section. A third simulation considered the distribution tray. The bed was the fourth section modeled considering a reaction mechanism particularly designed for the analysis of HC processes. To simulate each of these sections ANSYS Fluent was used. Sections two and three were modeling using a VOF model while the fourth section used an Eulerian model. The meshes used were built on ICEM with cell numbers not below 200000 with non-uniform sizes.

The simulations showed that the velocity profile at inlet of the gas-liquid distributor unit has velocities values larger than 15 m/s. In the gas-liquid distributor there is an expansion of the gas while flows through it, also the gas-liquid distributor disperses the liquid across the distributor's cross-section. The effect of liquid distribution in the reactor performance did not show a significant coupling between injection points. A maldistribution factor was used to evaluate the dispersion of the liquid at the top of the bed and at the outlet compartment of the gas-liquid distributor, this factor was used like a measurement of the improvement of the liquid distribution. Finally the catalytic bed simulation shows that there is an inefficient use of the catalyst bed and therefore a lower performance of the reactor when a non-uniform liquid profile is used.

Volume liquid fraction profiles, velocity profiles at different location and kinetic performance of the TBR were analyzed on each of the simulations to enhance the liquid distribution over the TBR.