(97e) Gas Phase Mixing Behavior of Gas-Liquid Upflow in a Moving Catalytic Packed/Expanded Bed Hydrotreating Reactor Using Advanced Gas Tracer Technique

Authors: 
Alexander, V., Missouri University of Science and Technology
Al-Bazzaz, H., Kuwait Institute for Scientific Research
Al-Dahhan, M., Missouri University of Science and Technology
Gas-liquid upflow moving catalytic packed bed reactor are widely used in industries for hydrotreating of feeds with a higher level of contaminants including heavier feeds. In these reactors spent catalyst are replaced periodically by adding fresh catalyst at the top and removing spent catalyst from the conical bottom which supports catalyst bed. While the catalyst moves downwards systematically, gas and liquid phase move upwards. The catalyst is removed in small increments once a week. The other times the reactor operates in upflow packed or expanded bed condition. The problem associated with these reactors is maldistribution, which causes hotspots, sintered carbon deposition and reduces expected conversions. To address such issues, detailed studies to enhance the understanding of the hydrodynamics and mixing behavior of gas in this reactor are still required. The lab scale reactor is developed based on the combination of hydrodynamic and geometrical similarity with the industrial reactor. This reactor consists of plenums and catalyst bed section. In this work, a new methodology has been developed using advanced gas tracer technique to remove the external dispersion due to additional volume (sampling line, the top section of the reactor) and to deconvolute gas mixing characteristics of the gas phase in catalyst bed from the gas mixing characteristic of the whole reactor. The gas tracer system consists of the gas analyzer (TCD), in-house developed gas-liquid separator, water pump, air pump, sampling line and helium as a tracer. As part of the methodology, the helium is injected at three location (inlet, below the catalyst bed, above the catalyst bed) and sampled at the top of the reactor. These injection-sampling networks characterize residence time distribution (RTD) of various section of the reactor. Dispersion coefficient (Dg) quantifies the mixing characteristics of gas in catalyst bed section, and it is obtained using developed model for this parts. The RTDâ??s gave the indication of ideal behavior of plenums and not much deviation from plug flow behavior in the catalyst bed. Hence, the plenums are modeled as ideal CSTR-PFR and catalyst bed as Axial Dispersion Model (ADM). Dispersion parameter estimation is done using convolution and parameter regression with the experimental data. We focus in this work on the catalyst bed section, which is a Plexiglas column of 11 inches (ID) and 30-inch height, filled with extrudate catalyst of 3mm diameter till 24-inch height. The measurements were conducted at superficial liquid (water) velocity of 0.017 cm/sec to 1.78 cm/sec and superficial gas (air) velocity of 1.27 cm/sec to 8.8 cm/sec. At low liquid superficial velocity, it is seen that the bed behaves as packed bed or slightly expanded bed and Dg increases with increasing gas superficial velocity. At higher liquid superficial velocity the bed acts as extended bed and Dg decreases with increasing gas superficial velocity. The Dg decreases with increasing liquid superficial velocity at constant gas superficial velocity. These kind of information are essential at industrial scale, to improve the performance of the real plant reactor. In this presentation, results and findings are discussed.