(395c) CFD Modeling and Simulation of Forced Periodic Operation of Trickle Bed Reactors

Recently introduced regulatory requirements for ultra-low-sulfur diesel (ULSD) have created incentives for refiners to build new refining units or revamp existing ones to meet ULSD specifications. Hydrotreating is one of the most important processes for ULSD production in which contaminants (such as sulfur, nitrogen, and metals) are removed and aromatics are partially hydrogenated. Hydrotreating is usually conducted in trickle bed reactors (TBRs) operated in steady-state mode with constant gas and liquid throughputs. In recent years, forced periodic operation of trickle bed reactors has gained growing interest in view of the resulting performance enhancements (increased conversion and selectivity). As opposed to conventional steady-state operation, periodic operation involves far more complex time-dependent hydrodynamic behaviors. Obviously, understanding the two-phase flow dynamics in a TBR is a prerequisite for practical design and implementation of periodic operation. In this regard, computational fluid dynamics (CFD) modeling and simulation can serve as an important tool to provide insight into the two-phase flow dynamics and catalyst performance in the reactor operated under periodic mode. Due to kinetic and/or thermodynamic constraints, industrial hydrotreating reactors are usually operated at relatively high temperatures and pressures (320?420°C, 30?120 atm). In contrast to the industrial operating conditions, most research work reported in the literature concerning the hydrodynamics of TBRs has been performed at atmospheric pressure and room temperature. Recently, a few investigations have been published on liquid holdup and pressure drop using pressurized TBRs under mild temperatures. However, no experimental work has been published so far to address the hydrodynamics of TBRs employing high pressure and temperature simultaneously. In order to close this fundamental knowledge gap, the present work utilizes CFD modeling and simulation to explore the flow dynamics in TBRs under industrially relevant conditions. An Eulerian approach is used in this study to perform CFD modeling and simulation of periodic operation of TBRs for middle distillate hydrotreating. The two-phase flow dynamics under different operating scenarios (high pressure with either cold flow or hot flow) are systematically studied. The time-dependent hydrodynamic behaviors are investigated under forced periodic operation, along with the implementation of different cyclic modulation strategies (slow and fast base/pulse or on/off modulation). The details of the CFD modeling and simulation results will be given in the extended abstract and in the presentation.