(485c) Phase Distribution, Local Maldistribution and Back Mixing Behavior Identification in Upflow Moving Packed Bed Hydrotreating Reactor Using Two Tip Optical Probe

Authors: 
Alexander, V., Missouri University of Science and Technology
Al-Bazzaz, H., Kuwait Institute for Scientific Research
Aldahhan, M. H., Missouri University of Science and Technology

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

Email:
vaxt8@mst.edu

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.