Phase Distribution, Local Maldistribution and Back Mixing Behavior Identification in Upflow Moving Packed Bed Hydrotreating Reactor Using Two Tip Optical Probe
Vineet Alexander1, Hamza Al-Bazzaz2 and Muthanna Al-Dahhan*
1,*Chemical Engineering and Biochemical Engineering Department
Missouri University of Science and Technology, Rolla, MO 65409-1230. USA
2Kuwait Institute of Scientific Research, P.O Box 24885, 13109 Kuwait
Upflow moving packed bed hydrotreating reactors have been used to process feeds with higher level of contaminants including heavier feeds, to increase catalyst cycle and life of the downstream reactors by providing better protection from fouling metals of the feedstock and to improve in general the downstream reactors product quality. In these reactors spent catalyst are replaced periodically by adding fresh catalyst at the top and removing spent catalyst from the bottom, while the catalyst move downwards periodically, gas and liquid phase move upwards. The problem associated with these reactors is maldistribution, which causes hotspots, sintered carbon deposition and reduces expected conversions. To address such problems, detailed studies to enhance the understanding of the hydrodynamics in the upflow moving packed bed reactor are still required. In this work two point optical probe technique has been developed and used to determine the phase distribution in terms of local liquid and gas velocities, holdup and there time series fluctuations , local maldistribution and back mixing behavior of phases. Two optical fiber tips are placed at distance of 1mm and principle behind to distinguish gas and liquid is total internal reflection phenomena. The experimental work was carried out in 11 inch (ID) Plexiglas column for an air-water system flowing over a packed bed of extrudate catalyst 3mm in diameter. The measurements were conducted in various axial and radial positions with the superficial liquid flow rate of 0.017 cm/sec and superficial gas flow rate of 8.8cm/sec. The parameter measured are local holdups and local velocities of gas and liquid at the void space inside the catalyst bed packing. This is the first time that these kind of study has been implemented on upflow moving bed hydrotreaters. Local holdups gives the phase distributions and local zero velocity of phases gives the local maldistribution at locally occupied void space inside the catalyst bed packing. Zero velocity of phase's condition is an undesirable event in which the void space is either occupied by continuous flow of gas, stagnant liquid or gas bubble deviation. All these conditions will lead to ineffective working of catalyst at that region. Measurement of negative velocities quantify the back mixing behavior characteristics of the phases at a specified local point. Negative velocity are generated when the flow of liquid or gas is in opposite direction to general flow of the phases, this occurs due to back pressure at that point and hence back mixing. The results indicate that this technique can determine the local, radial, cross sectional and along the height of the bed phase distribution, local maldistribtuion and back mixing behavior profile. These kind of information are very essential at industrial scale, to improve the performance of the real plant reactor. In this presentation results and findings are discussed.
Keyword: Two Point Optical Probes, Phase distribution, Local Maldistribution, Back Mixing, Zero Velocity, Negative velocity, Gas Holdup, Liquid Holdup.