MODELING OF MULTIPHASE FLOW IN TRICKLE BED REACTOR

Abstract

Three-phase reactors (G-L-S) comprising a fixed bed of catalyst with flowing liquid and gaseous phases have various applications, particularly in the petroleum industry for hydroprocessing of oils (e.g. hydrotreating, hydrocracking). Trickle-bed reactors (TBR) are one of the most extensively used three-phase reactors. With a view towards developing more efficient TBR units in the future, for meeting stringent environmental and profitability targets, it is crucial that we develop the know-how for tailoring the flow patterns in them to optimally match the demands made by the kinetics of these reaction processes. Design and scale up of TBRs continues to be a major challenge for chemical engineers. The design and scale-up of trickle bed reactors depend on key hydrodynamic variables such as liquid volume fraction (liquid saturation), particle scale wetting and overall gas—liquid distribution. These variables are difficult to determine experimentally and interactions between these are as yet poorly understood. Most of the previous studies on quantifying holdup and pressure drop have been conducted under atmospheric pressure, whereas the desired conditions of investigation are industrial operating pressures of 20-30 MPa. A basic understanding of the hydrodynamics of trickle bed reactors at the operating conditions of interest is essential to their design, scale-up, scale-down, and performance prediction. Theoretical and phenomenological aspects of trickle-bed reactors as reported in a wide number of papers of the last 35 years have been reviewed. In the present scenario, the fact that even the two-fluid CFD models have a lot of questionable assumptions (even though deceptively at a micro-scale), the wisdom of using a highly computationally intensive model for predicting global profiles in a TBR may be questioned. In light of that, in this work, a less computationally intensive, yet first-principle based CFD model has been presented using the porous media concept. The idea was to test this model against the state of the art in TBR modeling and offer it as a viable modeling approach. In this dissertation, to investigate hydrodynamics in trickle bed reactor, a 2D computational domain based on the different experimental operating conditions has been simulated using commercial computational fluid dynamics (CFD) code Fluent. For validation of the simulated results, different drag models are used and results are compared. It is found that for the present application, Satz and Carbonell drag model provided the best result. The iii hydrodynamics of two-phase flow in trickle-bed reactors (TBRs) under high pressure conditions was predicted and the effect of various operating parameters such as reactor pressure, particle size, phase velocity etc. were studied. Using Carbonell drag model, several simulations were performed based on the operating conditions in a pilot plant trickle bed reactor and the results were validated against the extensive experimental database available in the open literature. The results showed that CFD simulation offers better agreement with the results of theoretical and experimental investigations within maximum deviation of 10%. It was also found that the porous media model is advantageous to handle gas—liquid interaction terms due to its ability to lump the adjustable parameters as compared to the conventional k-fluid CFD treatment of the problem. This work has proved that this CFD model can productively be implemented for high-pressure operation (most of the commercial TBRs operate) which is cumbersome to account for in case of three-phase Eulerian simulation.