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University of Alberta
The Impact of Multiphase Behaviour on Coke Deposition in Heavy OilHydroprocessing Catalysts
by
Xiaohui Zhang
A thesis submitted to the Faculty of Graduate Studies and Research in partialfulfillment of the requirements for the degree of Doctor of Philosophy
in
Chemical Engineering
Department of Chemical and Materials Engineering
Edmonton, Alberta
Fall 2006
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Abstract
Coke deposition in heavy oil catalytic hydroprocessing remains a serious problem. The
influence of multiphase behaviour on coke deposition is an important but unresolved
question. A model heavy oil system (Athabasca vacuum bottoms (ABVB) + decane) and
a commercial heavy oil hydrotreating catalyst (NiMo/-Al2O3) were employed to study
the impact of multiphase behaviour on coke deposition.
The model heavy oil mixture exhibits low-density liquid + vapour (L1V), high-density
liquid + vapour (L2V), as well as low-density liquid + high-density liquid + vapour
(L1L2V) phase behaviour at a typical hydroprocessing temperature (380C). The L2
phase only arises for the ABVB composition range from 10 to 50 wt %. The phase
behaviour undergoes transitions from V to L2V, to L1L2V, to L1V with increasing
ABVB compositions at the pressure examined. The addition of hydrogen into the model
heavy oil mixtures at a fixed mass ratio (0.0057:1) does not change the phase behaviour
significantly, but shifts the phase regions and boundaries vertically from low pressure to
high pressure.
In the absence of hydrogen, the carbon content, surface area and pore volume losses for
catalyst exposed to the L1 phase are greater than for the corresponding L2 phase despite a
higher coke precursor concentration in L2 than in L1. By contrast, in the presence of
hydrogen, the carbon content, surface area and pore volume losses for the catalyst
exposed to the L2 phase are greater than for the corresponding L1 phase. The higher
hydrogen concentration in L1 appears to reverse the observed results. In the presence of
hydrogen, L2 was most closely associated with coke deposition, L1 less associated with
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coke deposition, and V least associated with coke deposition. Coke deposition is
maximized in the phase regions where the L2 phase arises. This key result is inconsistent
with expectation and coke deposition models where the extent of coke deposition, at
otherwise fixed reaction conditions, is asserted to be proportional to the nominal
concentration of coke precursor present in the feed.
These new findings are very significant both with respect to providing guidance
concerning possible operation improvement for existing processes and for the
development of new upgrading processes.
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Acknowledgements
This project would not have been a success without the unselfish contribution and help of
many people. Professor John M. Shaw gave me insightful supervision and endlessencouragement and support throughout the entire project. Not only is he a fantastic
supervisor, but also a good friend in my whole life. Professors Murray R. Gray and Alan
E. Nelson gave me most constructive criticism and valuable suggestions, which made this
project successful and shining. Thank you is far from what I want to say to them.
I would like to thank Drs. Yadollah Maham and Richard McFarlane for their willingness
to spend time on polishing and revising my presentations and thesis. Thanks also go to
Bei Zhao, Jessie Smith, Martin Chodakowski and Xiangyang Zou for their contribution tothis project.
I would like to thank all colleagues in the Petroleum Thermodynamics and Bitumen
Upgrading group and all support staff in the Department of Chemical and Materials
Engineering. Without your help and cooperation, this undertaking would not have been
possible.
I gratefully acknowledge the Scholarship programs at the University of Alberta and
research funds from the sponsors of the NSERC Industrial Research Chair in PetroleumThermodynamics (Alberta Energy Research Institute, Albian Sands Energy Inc.,
Computer Modelling Group Ltd., ConocoPhillips Inc., Imperial Oil Resources, NEXEN
Inc., Natural Resources Canada, Petroleum Society of the CIMM, Oilphase-DBR,
Oilphase a Schlumberger Company, Schlumberger, Syncrude Canada Ltd., NSERC).
No words could describe my debt to my parents and my siblings for their sacrifices and
for giving me the opportunity to pursue higher education. My success is attributed to their
incessant encouragement and support.
Special thanks to my lifes partner, Chongguo, for her love, her sacrifice, her
encouragement, , to make one of the most important steps in my life successful.
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Table of Contents
1. INTRODUCTION...........................................................................................1
1.1 Background..............................................................................................................................................1
1.2 Coke Deposition Phenomena..................................................................................................................2
1.2.1 Coke Deposition Models on Catalyst ................................................................................................2
1.2.2 Coke Deposition along Catalyst Beds in Trickle-Bed Reactors ........................................................3
1.2.3 Coke Suppression by Addition of Solvents .......................................................................................4
1.3 What Can Be Learned from Carbon Rejection Processes?.................................................................4
1.4 Preceding Work.......................................................................................................................................5
1.5 Hypothesis................................................................................................................................................6
1.6 Objectives.................................................................................................................................................8
1.7 Thesis Outline ..........................................................................................................................................9
2. LITERATURE REVIEW...............................................................................10
2.1 Heavy Oils ..............................................................................................................................................10
2.1.1 Heavy Oils/Bitumen-Our Future Energy Resource .........................................................................10
2.1.2 Characteristics of Heavy Oils/bitumen............................................................................................11
2.1.3 Asphaltenes......................................................................................................................................12
2.2 Heavy Oil Catalytic Hydroprocessing .................................................................................................15
2.2.1 Introduction .....................................................................................................................................15
2.2.2 Fixed-Bed Processes........................................................................................................................17
2.2.3 Moving-Bed Processes ....................................................................................................................19
2.2.4 Ebullated-Bed Processes..................................................................................................................21
2.2.5 Slurry Phase Process........................................................................................................................24
2.2.6 Catalyst Characteristics ...................................................................................................................25
2.3 Catalyst Coking .....................................................................................................................................27
2.3.1 Catalyst Fouling Mechanisms..........................................................................................................27
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2.3.2 Fouling by Coke..............................................................................................................................27
2.3.3 Coke Formation Chemistry..............................................................................................................29
2.3.4 Coke Deposition Modes ..................................................................................................................31
2.3.5 Coke Formation Kinetics.................................................................................................................322.3.6 Minimisation of Catalyst Fouling....................................................................................................35
2.4 The Role of Phase Behaviour in Coke Formation .......................................................................
