Fluid Catalytic Cracking Handbook 2nd Ed
Fluid
Catalytic
Cracking
Handbook
SECOND EDITION
This page intentionally left blank
Fluid
Catalytic
Cracking
Handbook
Handbook Design, Operation and
Troubleshooting of
FCC Facilities
SECOND EDITION
GP Gulf Professional Publishing
I'M
an imprint of ButterworthHeinemann
uid
atalytic
racking
andbook
gn, Operation, and
bleshooting of
Facilities
OND EDITION
yright © 2000 by ButterworthHeinemann. All rights
rved. Printed in the United States of America. This book,
arts thereof, may not be reproduced in any form without
mission of the publisher.
ginally published by Gulf Publishing Company,
ston. TX.
information, please contact:
nager of Special Sales
erworthHeinemann
Wildwood Avenue
bum,MA01801–2041
7819042500
:7819042620
information on all ButterworthHeinemann publications
lable, contact our World Wide Web home page at:
://www.bh.com
9 8 7 6 5 4 3 2
ary of Congress CataloginginPublication Data
ghbeigi, Reza.
Fluid catalytic cracking handbook / Reza Sadeghbeigi.—2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 0884152898 (alk. paper)
1. Catalytic cracking. 1. Title.
P690.4.S23 2000
65.533 dc2l
00035361
ted in the United States of America.
ted on acidfree paper (°°).
This book is dedicated to
our respected clients who have
contributed to the success of RMS Engineering, Inc.
and to the content of this book
This page intentionally left blank
Contents
cknowledgments
xi
reface to the Second Edition
xii
HAPTER 1
rocess Description
1
Feed Preheat, 6. Riser—Reactor—Stripper, 7, Regenerator
Heat/Catalyst Recovery, 13. Main Fractionator, 22. Gas Plant,
25. Treating Facilities, 31. Summary, 39. References, 39.
CC Feed Characterization
40
Hydrocarbon Classification, 41. Feedstock Physical Properties,
45. Impurities, 54. Empirical Correlations, 68. Benefits of
Hydroprocessing, 81. Summary, 82. References, 82.
HAPTER 3
CC Catalysts
84
Catalyst Components, 84. Catalyst Manufacturing
Techniques, 96. Fresh Catalyst Properties, 99. Equilibrium
Catalyst Analysis, 102. Catalyst Management, 109.
Catalyst Evaluation. 115. Additives, 117. Summary, 123.
References, 124.
HAPTER 4
Chemistry of FCC Reactions
Thermal Cracking, 126. Catalytic Cracking, 128. Thermo
dynamic Aspects, 136. Summary, 136. References, 138
References, 134.
125
HAPTER 5
Unit Monitoring and Control
_.._ 139
Material Balance, 140. Heat Balance, 158. Pressure
Balance, 166. Process Control Instrumentation, 177.
Summary, 180. References, 181.
HAPTER 6
Products and Economics
182
FCC Products, 182. FCC Economics, 202. Summary, 205.
References, 205.
HAPTER 7
Project Management and
Hardware Design
206
Project Management Aspects of an FCC Revamp, 206.
Process and Mechanical Design Guidelines, 212.
Summary, 232. References, 232.
HAPTER 8
roubleshooting
234
Guidelines for Effective Troubleshooting, 235. Catalyst
Circulation, 236. Catalyst Losses, 244. Coking/Fouling, 248.
Flow Reversal, 251. High Regenerator Temperature, 256.
Increase in Afterburn, 259. Hydrogen Blistering, 260. Hot
Gas Expanders, 263. Product Quantity and Quality, 264.
Summary, 275.
HAPTER 9
Debottlenecking and Optimization
Introduction, 276. Approach to Debottlenecking, 277.
Reactor/Regenerator Structure, 281. Flue Gas System, 296.
FCC Catalyst, 296. Instrumentation, 304. Utilities/Offsites,
305. Summary, 306.
276
Emerging Trends in Fluidized Catalytic Cracking _
307
Reformulated Fuels, 308. Residual Fluidized Catalytic
Cracking (RFCC), 323. Reducing FCC Emissions, 327.
Emerging Developments in Catalysts, Processes, and
Hardware, 232. Summary, 335. References, 336.
PPENDIX 1
emperature Variation of Liquid Viscosity
338
PPENDIX 2
Correction to Volumetric Average Boiling Point
339
PPENDIX 3
OTAL Correlations
340
PPENDIX 4
dM Correlations _.._._ ._.._ __._
_
341
PPENDIX 5
Estimation of Molecular Weight of
Petroleum Oils from Viscosity Measurements
342
PPENDIX 6
Kinematic Viscosity to
aybolt Universal Viscosity _._
_
344
PPENDIX 7
API Correlations
345
PPENDIX 8
Definitions of Fluidization Terms
_._..._..._ _
Conversion of ASTM 50% Point
o TBP 50% Point Temperature
_
347
350
PPENDIX 10
Determination of TBP Cut
Points from ASTM D86
351
PPENDIX 11
Nominal Pipe Sizes
Conversion Factors __
Glossary
ndex
__._
_
..._...
....._.
_._ _
About the Author
_
353
355
357
363
..__ _ ._
.__. 369
Acknowledgments
am grateful to the following individuals who played key roles in this
ook's completion: Warren Letzsch of Stone & Webster Engineer
ng Corporation; Terry Reid of Akzo Nobel Chemicals, Inc.; Herb
elidetzki of KBC Advanced Technologies, Inc.; and Jack Olesen of
Grace/Davison provided valuable input. My colleagues at RMS
ngineering, especially Shari Gauldin, Larry Gammon, and Walt Broad
went the "extra mile" to ensure the book's accuracy and usefulness.
Preface to the
Second Edition
The first edition of this book was published nearly five years ago.
he book was well received and the positive reviews were over
whelming. My main objective of writing this second edition is to
rovide a practical "transfer of experience" to the readers of the
nowledge that I have gained in more than 20 years of dealing with
arious aspects of the cat cracking process.
This second edition fulfills my goal of discussing issues related to
he FCC process and provides practical and proven recommendations
o improve the performance and reliability of the FCCU operations.
he new chapter (Chapter 9) offers several "notolow" cost modifica
ons that, once implemented., will allow debottlenecking and optimiza
on of the cat cracker.
I am proud of this second edition. For one, I received input/feedback
rom our valued clients, industry "FCC gurus," as well as my colleagues
t RMS Engineering, Inc. Each chapter was reviewed carefully for
ccuracy and completeness. In several areas, I have provided additional
iscussions to cover different FCCU configurations and finally, both
he metric and English units have been used to make it easier for
eaders who use the metric system.
Unfortunately, the future of developing new technologies for petro
eum refining in general, and cat cracking in particular, is not promis
ng. The large, multinational oil companies have just about abandoned
heir refining R&D programs. The refining industry is shrinking
apidly. There is no "farm system" to replace the current crop of
echnology experts. In cat cracking, we begin to see convergence and
imilarity in the number of offered technologies. Even the FCC
atalyst suppliers and technology licensers have been relatively quiet
n developing "breakthrough" technologies since the introduction of
eolite in the late 1960s. More and more companies are outsourcing
heir technical needs. In the next several years, refiners will be
pending much of their capital to reduce sulfur in gasoline and diesel,
n the area of cat cracking, the emphasis will be on improving the
erformance and reliability of existing units, as well as "squeezing"
more feed rate and/or conversion without capital expenditure. In light
these developments, this book is needed more than ever.
Reza
Houston, Texas
This page intentionally left blank
CHAPTER
Process Description
Fluid catalytic cracking (FCC) continues to play a key role in an
ntegrated refinery as the primary conversion process. For many
efiners, the cat cracker is the key to profitability in that the successful
peration of the unit determines whether or not the refiner can remain
ompetitive in today's market.
Approximately 350 cat crackers are operating worldwide, with a
otal processing capacity of over 12.7 million barrels per day [1]. Most
f the existing FCC units have been designed or modified by six major
chnology licensers:
1. ABB Lummus Global
2. Exxon Research and Engineering (ER&E)
3. Kellogg Brown & Root—KBR (formerly The M.W. Kellogg
Company)
4. Shell Oil Company
5. Stone & Webster Engineering Corporation (SWEC)/IFP
6. UOP (Universal Oil Products)
Figures 11 through 13 contain sketches of typical unit configura
ons offered by some licensers. Although the mechanical configuration
f individual FCC units may differ, their common objective is to
pgrade lowvalue feedstock to more valuable products. Worldwide,
bout 45% of all gasoline comes from FCC and ancillary units, such
s the alkylation unit.
Since the startup of the first commercial FCC unit in 1942, many
mprovements have been made. These improvements have enhanced
e unit's mechanical reliability and its ability to crack heavier, lower
alue feedstocks. The FCC has a remarkable history of adapting to
ontinual changes in market demands. Table 11 shows major develop
ents in the history of the process.
The FCC unit uses a microspheroidal catalyst, which behaves like
liquid when properly aerated by gas. The main purpose of the unit
Fluid Catalytic Cracking Handbook
Products
Regen
Flue
Gas
Transfer
Line
Reactor
Air Blower
Figure 11. Typical schematic of Exxon's flexicracker,
to convert highboiling petroleum fractions called gas oil to high
alue, highoctane gasoline and heating oil. Gas oil is the portion of
rude oil that commonly boils in the 650+°F to 1,050+°F (330° to
50°C) range. Feedstock properties are discussed in Chapter 2.
Before proceeding, it is helpful to examine how a typical cat cracker
ts into the refinery process. A petroleum refinery is composed of
everal processing units that convert raw crude oil into usable products
uch as gasoline, diesel, and jet fuel (Figure 14).
The crude unit is the first unit in the refining process. Here, the
aw crude is distilled into several intermediate products: naphtha,
erosene, diesel, and gas oil. The heaviest portion of the crude oil,
(text continued on page 6)
Process Description
Flue Gas
To Fractionator
Reactor
^Stripping
Steam
Figure 12. UOP FCC (courtesy of UOP).
Second stage
regenerator
Riser termination
device
Combustion Air
First stage
regenerator
Combustion
Air
Lift air
r
Feed Injection
Figure 13. SWEC stacked FCC unit (courtesy of Stone & Webster Engi
eering Corporation),
Fluid Catalytic Cracking Handbook
915
936
938
942
943
947
948
950s
951
952
954
Mid50s
956
961
964
972
974
975
981
983
985
994
996
Table 11
The Evolution of FCC
McAfee of Gulf Refining Co. discovered that a FriedelCrafts
aluminum chloride catalyst could catalytically crack heavy oil.
Use of natural clays as catalyst greatly improved cracking
efficiency.
Catalyst Research Associates (CRA) was formed. The original
CRA members were: Standard of New Jersey (Exxon), Stan
dard of Indiana (Amoco), Anglo Iranian Oil Company (BP
Oil), The Texas Company (Texaco), Royal Dutch Shell,
Universal Oil Products (UOP), The M.W, Kellogg Company,
and I.G. Farben (dropped in 1940).
First commercial FCC unit (Model I upflow design) started up
at Standard of New Jersey's Baton Rouge, Louisiana, refinery.
First downflow design FCC unit was brought online. First
thermal catalytic cracking (TCC) brought online.
First UOP stacked FCC unit was built. Kellogg introduced the
Model III FCC unit.
Davison Division of W.R. Grace & Co. developed micro
spheroidal FCC catalyst.
Evolution of bedcracking process designs.
M.W. Kellogg introduced the Orthoflow design.
Exxon introduced the Model IV.
High alumina (A12 O2) catalysts were introduced.
UOP introduces sidebyside design.
Shell invented riser cracking.
Kellogg and Phillips developed and put the first resid cracker
onstream at Borger, Texas.
Mobil Oil developed USY and ReY FCC catalyst. Last TCC
unit completed.
Amoco Oil invented hightemperature regeneration.
Mobil Oil introduced CO promoter.
Phillips Petroleum developed antimony for nickel passivation.
TOTAL invented twostage regeneration for processing residue,
Mobil reported first commercial use of ZSM5 octane/olefins
additive in FCC
Mobil started installing closed cyclone systems in its FCC units.
Coastal Corporation conducted commercial test of ultrashort
residence time, selective cracking.
ABB Lummus Global acquired Texaco FCC technologies.
GASOLINE
f
•XL
TAR
!j§
I
DELAYED
COKER
HEATING OIL
DECANT OIL
3AS( gasoline to
REFORMER
Figure 14. A typical high conversion refinery.
NO. 6 OIL
Fluid Catalytic Cracking Handbook
ext continued from page 2)
which cannot be distilled in the atmospheric tower, is heated and sent
o the vacuum tower where it is split into gas oil and tar. The tar from
he vacuum tower is sent to be further processed in a delayed coker,
easphalting unit, or visbreaker, or is sold as fuel oil or road asphalt.
The gas oil feed for the conventional cat cracker comes primarily
om the atmospheric column, the vacuum tower, and the delayed
oker. In addition, a number of refiners blend some atmospheric or
acuum resid into the feedstocks to be processed in the FCC unit.
The FCC process is very complex. For clarity, the process descrip
on has been broken down into six separate sections:
• Feed Preheat
• Riser—Reactor—Stripper
« Regenerator—Heat/Catalyst Recovery
• Main Fractionator
• Gas Plant
« Treating Facilities
EED PREHEAT
Most refineries produce sufficient gas oil to meet the cat crackers'
emand. However, in those refineries in which the gas oil produced
oes not meet the cat cracker capacity, it may be economical to
upplement feed by purchasing FCC feedstocks or blending some
esidue. The refineryproduced gas oil and any supplemental FCC
eedstocks are generally combined and sent to a surge dram, which
rovides a steady flow of feed to the charge pumps. This drum can
lso separate any water or vapor that may be in the feedstocks.
From the surge drum, the feed is normally heated to a temperature
f 500°F to 700°F (260°C to 370°C). The main fractionator bottoms
umparound and/or fired heaters are the usual sources of heat. The
eed is first routed through heat exchangers using hot streams from
he main fractionator. The main fractionator top pumparound, light
ycle oil product, and bottoms pumparound are commonly used (Fig
re 15). Removing heat from the main fractionator is at least as
mportant as preheating the feed.
Most FCC units use fired heaters for FCC feed final preheat. The
eed preheater provides control over the catalysttooil ratio, a key
ariable in the process. In units where the air blower is constrained.
Process Description
Vent to Main Column
cSi—^
Slurry
I
Feed Preheater
Figure 15. Typical feed preheat system.
ncreasing preheat temperature allows increased throughput. The effects
f feed preheat are discussed in Chapter 6.
RISER—REACTOR—STRIPPER
The reactorregenerator is the heart of the FCC process. In a modern
at cracker, virtually all the reactions occur in 1.5 to 3.0 seconds
before the catalyst and the products are separated in the reactor.
From the preheater, the feed enters the riser near the base where it
ontacts the regenerated catalyst (see Figure 16). The ratio of catalyst
ooil is normally in the range of 4:1 to 9:1 by weight. The heat
bsorbed by the catalyst in the regenerator provides the energy to heat
he feed to its desired reactor temperature. The heat of the reaction
ccurring in the riser is endothermic (i.e., it requires energy input). The
irculating catalyst provides this energy. The typical regenerated catalyst
emperature ranges between 1,250°F to 1,350°F (677°C to 732°C).
Fluid Catalytic Cracking Handbook
To Reactor or Cyclone
Catalyst
From
Regenerator
(Typical for
Multiple Nozzles)
Figure 16. Typical riser "Y".
The catalytic reactions occur in the vapor phase. Cracking reactions
egin as soon as the feed is vaporized. The expanding volume of the
apors that are generated are the main driving force to carry the
atalyst up the riser.
Catalyst and products are quickly separated in the reactor. However,
ome thermal and nonselective catalytic reactions continue. A number
Process Description
9
f refineries are modifying the riser termination devices to minimize
hese reactions.
The riser is a vertical pipe. It usually has s 4 to 5inch (10 to 13
m) thick refractory lining for insulation and abrasion resistance.
Typical risers are 2 to 6 feet (60 to 180 cm) in diameter and 75 to
120 feet (25 to 30 meters) long. The ideal riser simulates a plug flow
eactor, where the catalyst and the vapor travel the length of the riser
with minimum back mixing.
Efficient contacting of the feed and catalyst is critical for achieving
he desired cracking reactions. Steam is commonly used to atomize
he feed. Smaller oil droplets increase the availability of feed at the
eactive acid sites on the catalyst. With highactivity zeolite catalyst,
irtually all of the cracking reactions take place in three seconds or less.
Risers are normally designed for an outlet vapor velocity of 50 ft/sec
o 75 ft/sec (15.2 to 22.8 m/sec). The average hydrocarbon residence
me is about two seconds (based on outlet conditions). As a consequence
f the cracking reactions, a hydrogendeficient material called coke is
eposited on the catalyst, reducing catalyst activity.
Catalyst Separation
After exiting the riser, catalyst enters the reactor vessel. In today's
CC operations, the reactor serves as a housing for the cyclones. In
he early application of FCC, the reactor vessel provided further bed
racking, as well as being a device used for additional catalyst separation.
Nearly every FCC unit employs some type of inertial separation
evice connected on the end of the riser to separate the bulk of the
atalyst from the vapors. A number of units use a deflector device to
urn the catalyst direction downward. On some units, the riser is
irectly attached to a set of cyclones. The term "rough cut" cyclones
enerally refers to this type of arrangement. These schemes separate
pproximately 75% to 99% of the catalyst from product vapors.
Most FCC units employ either single or twostage cyclones (Figure
7) to separate the remaining catalyst particles from the cracked
apors. The cyclones collect and return the catalyst to the stripper
hrough the diplegs and flapper/trickle valves (See Figure 18). The
roduct vapors exit the cyclones and flow to the main fractionator
or recovery. The efficiency of a typical twostage cyclone system
s 99.995+%.
0
Fluid Catalytic Cracking Handbook
igure 17. A twostage cyclone system. (Courtesy of Bill Dougherty, BP Oil
efinery, Marcus Hook, Pa.)
It is important to separate catalyst and vapors as soon as they enter
he reactor. Otherwise, the extended contact time of the vapors with
he catalyst in the reactor housing will allow for nonselective catalytic
ecracking of some of the desirable products. The extended residence
me also promotes thermal cracking of the desirable products.
Process Description
11
Pivot
Cyclone Dipleg
Restraint
PLAN
Cyclone Dipleg*
Pivot
Restraint
ELEVATION
Figure 18. Typical trickle valve (courtesy of Emtrol Corporation),
tripping Section
As the spent catalyst falls into the stripper, hydrocarbons are adsorbed
n the catalyst surface, hydrocarbon vapors fill the catalyst pores, and
he vapors entrained with the catalyst also fall into the stripper.
tripping steam, at a rate of 2 to 5 Ibs per 1,000 lbs (2 kg to 5 kg
er 1,000 kg,) is primarily used to remove the entrained hydrocarbons
etween catalyst particles. Stripping steam does not address hydro
arbon desorption and hydrocarbons filling the catalyst pores. How
ver, reactions continue to occur in the stripper. These reactions are
2
Fluid Catalytic Cracking Handbook
riven by the reactor temperature and the catalyst residence time in
he stripper. The higher temperature and longer residence time allow
onversion of adsorbed hydrocarbons into "clean lighter" products.
oth baffled and unbaffled stripper designs (Figure 19) are in com
mercial use. An efficient stripper design generates intimate contact
etween the catalyst and steam. Reactor strippers are commonly
esigned for a steam superficial velocity of 0.75 ft/sec (0.23 m/sec)
nd a catalyst flux rate of 500 to 700 lbs per minute per square foot
.4 kg to 3.4 kg per minute per square meter). At too high a flux,
UPPER STEAM DISTRIBUTOR
LOWER STEAM DISTRIBUTOR
Figure 19. An example of a twostage stripper.
Process Description
13
he falling catalyst tends to entrain steam, thus reducing the effective–
ess of stripping steam.
It is important to minimize the amount of hydrocarbon vapors
arried through to the regenerator, but not all the hydrocarbon vapors
an be displaced from the catalyst pores in the stripper. A fraction of
hem are carried with the spent catalyst into the regenerator. These
ydrocarbon vapors/liquid have a higher hydrogentocarbon ratio than
he coke on the catalyst. The drawbacks of allowing these hydrogen
ch hydrocarbons to enter the regenerator are as follows:
* Loss of liquid product. Instead of the hydrocarbons burning in the
regenerator, they could be recovered as liquid products.
« Loss of throughput. The combustion of hydrogen to water pro
duces 3.7 times more heat than the combustion of carbon to
carbon dioxide. The increase in the regenerator temperature caused
by excess hydrocarbons could exceed the temperature limit of the
regenerator internals and force the unit to a reduced feed rate
mode of operation.
* Loss of catalyst activity. The higher regenerator temperature
combined with the formation of steam in the regenerator reduces
catalyst activity by destroying the catalyst's crystalline structure.
The flow of spent catalyst to the regenerator is typically controlled
y a valve that slides back and forth. This slide valve is controlled
y the catalyst level in the stripper. The catalyst height in the stripper
rovides the pressure head, which allows the catalyst to flow into the
egenerator. The exposed surface of the slide valve is usually lined
with refractory to withstand erosion. In a number of earlier FCC
esigns, lift air is used to transport the spent catalyst into the regener
tor (Figure 110).
REGENERATOR–HEAT/CATALYST RECOVERY
The regenerator has two main functions: it restores catalyst activity
nd supplies heat to crack the feed. The spent catalyst entering the
egenerator contains between 0.4 wt% and 2.5 wt% coke, depending
n the quality of the feedstock. Components of coke are carbon,
ydrogen, and trace amounts of sulfur and nitrogen. These burn
ccording to the following reactions.
4
Fluid Catalytic Cracking Handbook
Products
Reactor
Regen
Catalyst
Standpipe
LiftAir
Air Blower
Figure 110. A typical Model II using lift air to transfer spent catalyst.
+ 1/2 CX,
O + 1/2 02
+ O2
H2, + 1/2 02
+ xO
+ xO
_>
— >
— >
>
— »
>
CO
CO2
CO2
H2O
sox
NO y
K Cal/kg of
C, H2, or S
BTU/lb of
C, H2, or S
2,200
5,600
7,820
28,900
2,209
3,968
10,100
14,100
52,125
3,983
(11)
(12)
(13)
(14)
(15)
(16)
Process Description
15
Air provides oxygen for the combustion of coke and is supplied by
ne or more air blowers. The air blower provides sufficient air velocity
nd pressure to maintain the catalyst bed in a fluid state. The air enters
he regenerator through an air distributor (Figure 111) located near
he bottom of the vessel. The design of an air distributor is important
n achieving efficient and reliable catalyst regeneration. Air distributors
re typically designed for a 1.0 psi to 2.0 psi (7 to 15 Kpa) pressure
rop to ensure positive air flow through all nozzles.
There are two regions in the regenerator: the dense phase and the
ilute phase. At velocities common in the regenerator, 2 ft/sec to 4 ft/sec
0.6 to 1.2 m/sec), the bulk of catalyst particles are in the dense bed
mmediately above the air distributor. The dilute phase is the region
bove the dense phase up to the cyclone inlet, and has a substantially
ower catalyst concentration.
Standpipe/Slide Valve
During regeneration, the coke level on the catalyst is typically
educed to 0.05%. From the regenerator, the catalyst flows down a
ransfer line commonly referred to as a standpipe. The standpipe
rovides the necessary pressure to circulate the catalyst around the
nit. Some standpipes extend into the regenerator, and the top section
s often called a catalyst hopper. The hopper, internal to the regener
tor, is usually an inverted cone design. In units with "long" catalyst
tandpipes, external withdrawal hoppers are often used to feed the
tandpipes. The hopper provides sufficient time for the regenerated
atalyst to be "debubbled" before entering the standpipe.
Standpipes are typically sized for a flux rate in the range of 100 to
00 lb/sec/ft2 (500 to 1,500 kg/sec/m2) of circulating catalyst. In most
ases, sufficient flue gas is carried down with the regenerated catalyst
o keep it fluidized. However, longer standpipes may require external
eration to ensure that the catalyst remains fluidized. A gas medium,
uch as air, steam, nitrogen, or fuel gas, is injected along the length
f the standpipe. The catalyst density in a welldesigned standpipe is
n the range of 35 to 45 lb/ft3 (560 to 720 kg/m3).
The flow rate of the regenerated catalyst to the riser is commonly
egulated by either a slide or plug valve. The operation of a slide valve
s similar to that of a variable orifice. Slide valve operation is often
ontrolled by the reactor temperature. Its main function is to supply
6
Fluid Catalytic Cracking Handbook
igure 111. Examples of air distributors. (Top: courtesy of Enpro Systems,
nc., Channelview, Texas; bottom: courtesy of VALVAMP, Incorporated,
ouston, Texas.) Note: These distributors are upside down for fabrication.
Process Description
1?
nough catalyst to heat the feed and achieve the desired reactor
emperature. In Exxon Model IV and flexicracker designs (see Figure
11), the regenerated catalyst flow is mainly controlled by adjusting
he pressure differential between the reactor and regenerator.
Catalyst Separation
As flue gas leaves the dense phase of the regenerator, it entrains
atalyst particles. The amount of entrainment largely depends on the
lue gas superficial velocity. The larger catalyst particles, 50}i90p, fall
ack into the dense bed. The smaller particles, 0|J,50ji, are suspended
n the dilute phase and carried into the cyclones.
Most FCC unit regenerators employ 4 to 16 parallel sets of primary
nd secondary cyclones. The cyclones are designed to recover catalyst
articles greater than 20 microns diameter. The recovered catalyst
articles are returned to the regenerator via the diplegs.
The distance above the catalyst bed at which the flue gas velocity
as stabilized is referred to as the transport disengaging height (TDH).
At this height, the catalyst concentration in the flue gas stays constant;
one will fall back into the bed. The centerline of the firststage cyclone
nlets should be at TDH or higher; otherwise, excessive catalyst entrain
ment will cause extreme catalyst losses.
Flue Gas Heat Recovery Schemes
The flue gas exits the cyclones to a plenum chamber in the top of
he regenerator. The hot flue gas holds an appreciable amount of
nergy. Various heat recovery schemes are used to recover this energy.
n some units, the flue gas is sent to a CO boiler where both the
ensible and combustible heat are used to generate highpressure
team. In other units, the flue gas is exchanged with boiler feed water
o produce steam via the use of a shell/tube or box heat exchanger.
In most units, about twothirds of the flue gas pressure is let down via
n orifice chamber or across an orifice chamber. The orifice chamber is
vessel containing a series of perforated plates designed to maintain a
iven backpressure upstream of the regenerator pressure control valve.
In some larger units, a turbo expander is used to recover this
ressure energy. To protect the expander blades from being eroded by
atalyst, flue gas is first sent to a thirdstage separator to remove the
8
Fluid Catalytic Cracking Handbook
ines. The thirdstage separator, which is external to the regenerator,
ontains a large number of swirl tubes designed to separate 70% to
5% of the incoming particles from the flue gas.
A power recovery train (Figure 112) employing a turbo expander
sually consists of four parts: the expander, a motor/generator, an air
blower, and a steam turbine. The steam turbine is primarily used for
tartup and, often, to supplement the expander to generate electricity.
The motor/generator works as a speed controller and flywheel; it can
roduce or consume power. In some FCC units, the expander horsepower
xceeds the power needed to drive the air blower and the excess power
s output to the refinery electrical system. If the expander generates less
ower than what is required by the blower, the motor/generator provides
he power to hold the power train at the desired speed.
From the expander, the flue gas goes through a steam generator to
ecover thermal energy. Depending on local environmental regulations,
n electrostatic precipitator (ESP) or a wet gas scrubber may be placed
ownstream of the waste heat generator prior to release of the flue
as to the atmosphere. Some units use an ESP to remove catalyst fines
n the range of 5|i20ji from the flue gas. Some units employ a wet
as scrubber to remove both catalyst fines and sulfur compounds from
he flue gas stream.
Partial versus Complete Combustion
Catalyst can be regenerated over a range of temperatures and flue
as composition with inherent limitations. Two distinctly different
modes of regeneration are practiced: partial combustion and complete
ombustion. Complete combustion generates more energy when coke
ield is increased; partial combustion generates less energy when the
oke yield is increased. In complete combustion, the excess reaction
omponent is oxygen, so more carbon generates more combustion. In
artial combustion, the excess reaction component is carbon, all the
xygen is consumed, and an increase in coke yield means a shift from
CO2 to CO.
FCC regeneration can be further subdivided into low, intermediate,
nd high temperature regeneration. In low temperature regeneration
about 1,190°F or 640°C), complete combustion is impossible. One
f the characteristics of low temperature regeneration is that at 1,190°F,
ll three components (O2, CO, and CO2) are present in the flue gas at
UE GAS FROM REGENERATOR
ELECTRO
PRECIPIT
C
F
REGENERATOR
Figure 112, A typical flue gas power recovery scheme.
0
Fluid Catalytic Cracking Handbook
ignificant levels. Low temperature regeneration was the mode of
peration that was used in the early implementation of the catalytic
racking process.
In the early 1970s, high temperature regeneration was developed.
High temperature regeneration meant increasing the temperature until
ll the oxygen was burned. The main result was low carbon on the
egenerated catalyst. This mode of regeneration required maintaining
n the flue gas, either a small amount of excess oxygen and no CO,
r no excess oxygen and a variable quantity of CO. If there was excess
xygen, the operation was in a full burn. If there was excess CO, the
peration was in partial burn.
With the advent of combustion promoter, the regeneration tem
erature could be reduced and still maintain full burn. Thus, intermediate
emperature regeneration was developed. Intermediate regeneration is
ot necessarily stable unless combustion promoter is used to assist in
he combustion of CO in the dense phase. Table 1 2 contains a 2 x 3
matrix summarizing various aspects of regeneration.
The following matrix of regeneration temperatures and operating
modes shows the inherent limitations of operating regions. Regenera
on is either partial or complete, at low, intermediate, or high tern
Table 12
A Matrix of Regeneration Characteristics
Operating Region Regenerator
Combustion
Partial Combustion
Mode
Full Combustion
Mode
ow temperature (nominally
,190°F/640°C)
Stable (small
afterburning) O2,
CO, and CO2 in
the flue gas
Not achievable
ntermediate temperature
nominally 1,275 °F/690°C)
Stable (with
combustion
promoter); tends to
have high carbon
on regenerated
catalyst
Stable with
combustion
promoter
igh temperature (nominally
,350°F/730°C)
Stable operation
Stable operation
Process Description
21
eratures. At low temperatures, regeneration is always partial, carbon
n regenerated catalyst is high, and increasing combustion air results
n afterburn. At intermediate temperatures, carbon on regenerated
atalyst is reduced. The three normal "operating regions" are indicated
n Table 12.
There are some advantages and disadvantages associated with full
nd partial combustion:
dvantages of full combustion
• Energy efficient
• Heatbalances at low coke yield
• Minimum hardware (no CO boiler)
• Better yields from clean feed
Disadvantages of full combustion
• Narrow range of coke yields unless some heat removal system
is incorporated
• Greater afterburn, particularly with an uneven air or spent catalyst
distribution system
• Low cat/oil ratio
The choice of partial versus full combustion is dictated by FCC feed
uality. With "clean feed," full combustion is the choice. With low
uality feed or resid, partial combustion, possibly with heat removal,
s the choice.
Catalyst Handling Facilities
Even with proper operation of the reactor and regenerator cyclones,
atalyst particles smaller than 20 microns still escape from both of
hese vessels. The catalyst fines from the reactor collect in the frac
onator bottoms slurry product storage tank. The recoverable catalyst
nes exiting the regenerator are removed by the electrostatic pre
ipitator or lost to the environment. Catalyst losses are related to:
« The design of the cyclones
« Hydrocarbon vapor and flue gas velocities
• The catalyst's physical properties
• High jet velocity
• Catalyst attrition due to the collision of catalyst particles with the
vessel internals and other catalyst particles
2
Fluid Catalytic Cracking Handbook
The activity of catalyst degrades with time. The loss of activity is
rimarily due to impurities in the FCC feed, such as nickel, vanadium,
nd sodium, and to thermal and hydrothermal deactivation mechanisms.
o maintain the desired activity, fresh catalyst is continually added to
he unit, Fresh catalyst is stored in a fresh catalyst hopper and, in most
nits, is added automatically to the regenerator via a catalyst loader.
The circulating catalyst in the FCC unit is called equilibrium
atalyst, or simply Ecat. Periodically, quantities of equilibrium catalyst
re withdrawn and stored in the Ecat hopper for future disposal. A
efinery that processes residue feedstocks can use goodquality Ecat
om a refinery that processes light sweet feed. Residue feedstocks
ontain large quantities of impurities, such as metals and requires high
ates of fresh catalyst. The use of a goodquality Ecat in conjunc
on with fresh catalyst can be costeffective in maintaining low
atalyst costs.
MAIN FRACTIONATOR
The purpose of the main fractionator, or main column (Figure 113),
to desuperheat and recover liquid products from the reactor vapors.
he hot product vapors from the reactor flow into the main fractionator
ear the base. Fractionation is accomplished by condensing and
evaporizing hydrocarbon components as the vapor flows upward
hrough trays in the tower.
The operation of the main column is similar to a crude tower, but
with two differences. First, the reactor effluent vapors must be cooled
efore any fractionation begins. Second, large quantities of gases will
avel overhead with the unstabilized gasoline for further separation.
The bottom section of the main column provides a heat transfer
one. Shed decks, disk/doughnut trays, and grid packing are among
ome of the contacting devices used to promote vapor/liquid contact.
he overhead reactor vapor is desuperheated and cooled by a pump
round stream. The cooled pumparound also serves as a scrubbing
medium to wash down catalyst fines entrained in the vapors. Pool
uench can be used to maintain the fractionator bottoms temperature
elow coking temperature, usually at about 700°F (370°C).
The recovered heat from the main column bottoms is commonly
sed to preheat the fresh feed, generate steam, serve as a heating medium
or the gas plant reboilers, or some combination of these services.
Process Description
23
Figure 113. A typical FCC main fractionator circuit.
The heaviest bottoms product from the main column is commonly
alled slurry or decant oil. (In this book, these terms are used inter
hangeably.) The decant oil is often used as a "cutter stock" with
acuum bottoms to make No. 6 fuel oil. Highquality decant oil (low
ulfur, low metals, low ash) can be used for carbon black feedstocks.
Early FCC units had soft catalyst and inefficient cyclones with
ubstantial carryover of catalyst to the main column where it was
bsorbed in the bottoms. Those FCC units controlled catalyst losses
wo ways. First, they used high recycle rates to return slurry to the
eactor. Second, the slurry product was routed through slurry settlers.
4
Fluid Catalytic Cracking Handbook
ither gravity or centrifugal, to remove catalyst fines. A slipstream
f FCC feed was used as a carrier to return the collected fines from
he separator to the riser. Since then, improvements in the physical
roperties of FCC catalyst and in the reactor cyclones have lowered
atalyst carryover. Most units today operate without separators. The
ecant oil is sent directly to the storage tank. Catalyst fines accumulate
n the tank, which is cleaned periodically. Some units continue to use
ome form of slurry settler to minimize the ash content of decanted oil.
Above the bottoms product, the main column is often designed for
hree possible sidecuts:
* Heavy cycle oil (HCO)—used as a pumparound stream, some
times as recycle to the riser, but rarely as a product
* Light cycle oil (LCO)—used as a pumparound stream, sometimes
as absorption oil in the gas plant, and stripped as a product for
diesel blending; and
* Heavy naphtha—used as a pumparound stream, sometimes as
absorption oil in the gas plant, and possible blending in the
gasoline pool
In many units, the light cycle oil (LCO) is the only sidecut that
eaves the unit as a product. LCO is withdrawn from the main column
nd routed to a side stripper for flash control. LCO is sometimes
eated for sulfur removal prior to being blended into the heating oil
ool. In some units, a slipstream of LCO, either stripped or unstripped,
sent to the sponge oil absorber in the gas plant. In other units,
ponge oil is the cooled, unstripped LCO.
Heavy cycle oil, heavy naphtha, and other circulating side pump
round reflux streams are used to remove heat from the fractionator.
hey supply reboil heat to the gas plant and generate steam. The
mount of heat removed at any pumparound point is set to distribute
apor and liquid loads evenly throughout the column and to provide
he necessary internal reflux.
Unstabilized gasoline and light gases pass up through the main
olumn and leave as vapor. The overhead vapor is cooled and partially
ondensed in the fractionator overhead condensers. The stream flows
o an overhead receiver, typically operating at
Catalytic
Cracking
Handbook
SECOND EDITION
This page intentionally left blank
Fluid
Catalytic
Cracking
Handbook
Handbook Design, Operation and
Troubleshooting of
FCC Facilities
SECOND EDITION
GP Gulf Professional Publishing
I'M
an imprint of ButterworthHeinemann
uid
atalytic
racking
andbook
gn, Operation, and
bleshooting of
Facilities
OND EDITION
yright © 2000 by ButterworthHeinemann. All rights
rved. Printed in the United States of America. This book,
arts thereof, may not be reproduced in any form without
mission of the publisher.
ginally published by Gulf Publishing Company,
ston. TX.
information, please contact:
nager of Special Sales
erworthHeinemann
Wildwood Avenue
bum,MA01801–2041
7819042500
:7819042620
information on all ButterworthHeinemann publications
lable, contact our World Wide Web home page at:
://www.bh.com
9 8 7 6 5 4 3 2
ary of Congress CataloginginPublication Data
ghbeigi, Reza.
Fluid catalytic cracking handbook / Reza Sadeghbeigi.—2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 0884152898 (alk. paper)
1. Catalytic cracking. 1. Title.
P690.4.S23 2000
65.533 dc2l
00035361
ted in the United States of America.
ted on acidfree paper (°°).
This book is dedicated to
our respected clients who have
contributed to the success of RMS Engineering, Inc.
and to the content of this book
This page intentionally left blank
Contents
cknowledgments
xi
reface to the Second Edition
xii
HAPTER 1
rocess Description
1
Feed Preheat, 6. Riser—Reactor—Stripper, 7, Regenerator
Heat/Catalyst Recovery, 13. Main Fractionator, 22. Gas Plant,
25. Treating Facilities, 31. Summary, 39. References, 39.
CC Feed Characterization
40
Hydrocarbon Classification, 41. Feedstock Physical Properties,
45. Impurities, 54. Empirical Correlations, 68. Benefits of
Hydroprocessing, 81. Summary, 82. References, 82.
HAPTER 3
CC Catalysts
84
Catalyst Components, 84. Catalyst Manufacturing
Techniques, 96. Fresh Catalyst Properties, 99. Equilibrium
Catalyst Analysis, 102. Catalyst Management, 109.
Catalyst Evaluation. 115. Additives, 117. Summary, 123.
References, 124.
HAPTER 4
Chemistry of FCC Reactions
Thermal Cracking, 126. Catalytic Cracking, 128. Thermo
dynamic Aspects, 136. Summary, 136. References, 138
References, 134.
125
HAPTER 5
Unit Monitoring and Control
_.._ 139
Material Balance, 140. Heat Balance, 158. Pressure
Balance, 166. Process Control Instrumentation, 177.
Summary, 180. References, 181.
HAPTER 6
Products and Economics
182
FCC Products, 182. FCC Economics, 202. Summary, 205.
References, 205.
HAPTER 7
Project Management and
Hardware Design
206
Project Management Aspects of an FCC Revamp, 206.
Process and Mechanical Design Guidelines, 212.
Summary, 232. References, 232.
HAPTER 8
roubleshooting
234
Guidelines for Effective Troubleshooting, 235. Catalyst
Circulation, 236. Catalyst Losses, 244. Coking/Fouling, 248.
Flow Reversal, 251. High Regenerator Temperature, 256.
Increase in Afterburn, 259. Hydrogen Blistering, 260. Hot
Gas Expanders, 263. Product Quantity and Quality, 264.
Summary, 275.
HAPTER 9
Debottlenecking and Optimization
Introduction, 276. Approach to Debottlenecking, 277.
Reactor/Regenerator Structure, 281. Flue Gas System, 296.
FCC Catalyst, 296. Instrumentation, 304. Utilities/Offsites,
305. Summary, 306.
276
Emerging Trends in Fluidized Catalytic Cracking _
307
Reformulated Fuels, 308. Residual Fluidized Catalytic
Cracking (RFCC), 323. Reducing FCC Emissions, 327.
Emerging Developments in Catalysts, Processes, and
Hardware, 232. Summary, 335. References, 336.
PPENDIX 1
emperature Variation of Liquid Viscosity
338
PPENDIX 2
Correction to Volumetric Average Boiling Point
339
PPENDIX 3
OTAL Correlations
340
PPENDIX 4
dM Correlations _.._._ ._.._ __._
_
341
PPENDIX 5
Estimation of Molecular Weight of
Petroleum Oils from Viscosity Measurements
342
PPENDIX 6
Kinematic Viscosity to
aybolt Universal Viscosity _._
_
344
PPENDIX 7
API Correlations
345
PPENDIX 8
Definitions of Fluidization Terms
_._..._..._ _
Conversion of ASTM 50% Point
o TBP 50% Point Temperature
_
347
350
PPENDIX 10
Determination of TBP Cut
Points from ASTM D86
351
PPENDIX 11
Nominal Pipe Sizes
Conversion Factors __
Glossary
ndex
__._
_
..._...
....._.
_._ _
About the Author
_
353
355
357
363
..__ _ ._
.__. 369
Acknowledgments
am grateful to the following individuals who played key roles in this
ook's completion: Warren Letzsch of Stone & Webster Engineer
ng Corporation; Terry Reid of Akzo Nobel Chemicals, Inc.; Herb
elidetzki of KBC Advanced Technologies, Inc.; and Jack Olesen of
Grace/Davison provided valuable input. My colleagues at RMS
ngineering, especially Shari Gauldin, Larry Gammon, and Walt Broad
went the "extra mile" to ensure the book's accuracy and usefulness.
Preface to the
Second Edition
The first edition of this book was published nearly five years ago.
he book was well received and the positive reviews were over
whelming. My main objective of writing this second edition is to
rovide a practical "transfer of experience" to the readers of the
nowledge that I have gained in more than 20 years of dealing with
arious aspects of the cat cracking process.
This second edition fulfills my goal of discussing issues related to
he FCC process and provides practical and proven recommendations
o improve the performance and reliability of the FCCU operations.
he new chapter (Chapter 9) offers several "notolow" cost modifica
ons that, once implemented., will allow debottlenecking and optimiza
on of the cat cracker.
I am proud of this second edition. For one, I received input/feedback
rom our valued clients, industry "FCC gurus," as well as my colleagues
t RMS Engineering, Inc. Each chapter was reviewed carefully for
ccuracy and completeness. In several areas, I have provided additional
iscussions to cover different FCCU configurations and finally, both
he metric and English units have been used to make it easier for
eaders who use the metric system.
Unfortunately, the future of developing new technologies for petro
eum refining in general, and cat cracking in particular, is not promis
ng. The large, multinational oil companies have just about abandoned
heir refining R&D programs. The refining industry is shrinking
apidly. There is no "farm system" to replace the current crop of
echnology experts. In cat cracking, we begin to see convergence and
imilarity in the number of offered technologies. Even the FCC
atalyst suppliers and technology licensers have been relatively quiet
n developing "breakthrough" technologies since the introduction of
eolite in the late 1960s. More and more companies are outsourcing
heir technical needs. In the next several years, refiners will be
pending much of their capital to reduce sulfur in gasoline and diesel,
n the area of cat cracking, the emphasis will be on improving the
erformance and reliability of existing units, as well as "squeezing"
more feed rate and/or conversion without capital expenditure. In light
these developments, this book is needed more than ever.
Reza
Houston, Texas
This page intentionally left blank
CHAPTER
Process Description
Fluid catalytic cracking (FCC) continues to play a key role in an
ntegrated refinery as the primary conversion process. For many
efiners, the cat cracker is the key to profitability in that the successful
peration of the unit determines whether or not the refiner can remain
ompetitive in today's market.
Approximately 350 cat crackers are operating worldwide, with a
otal processing capacity of over 12.7 million barrels per day [1]. Most
f the existing FCC units have been designed or modified by six major
chnology licensers:
1. ABB Lummus Global
2. Exxon Research and Engineering (ER&E)
3. Kellogg Brown & Root—KBR (formerly The M.W. Kellogg
Company)
4. Shell Oil Company
5. Stone & Webster Engineering Corporation (SWEC)/IFP
6. UOP (Universal Oil Products)
Figures 11 through 13 contain sketches of typical unit configura
ons offered by some licensers. Although the mechanical configuration
f individual FCC units may differ, their common objective is to
pgrade lowvalue feedstock to more valuable products. Worldwide,
bout 45% of all gasoline comes from FCC and ancillary units, such
s the alkylation unit.
Since the startup of the first commercial FCC unit in 1942, many
mprovements have been made. These improvements have enhanced
e unit's mechanical reliability and its ability to crack heavier, lower
alue feedstocks. The FCC has a remarkable history of adapting to
ontinual changes in market demands. Table 11 shows major develop
ents in the history of the process.
The FCC unit uses a microspheroidal catalyst, which behaves like
liquid when properly aerated by gas. The main purpose of the unit
Fluid Catalytic Cracking Handbook
Products
Regen
Flue
Gas
Transfer
Line
Reactor
Air Blower
Figure 11. Typical schematic of Exxon's flexicracker,
to convert highboiling petroleum fractions called gas oil to high
alue, highoctane gasoline and heating oil. Gas oil is the portion of
rude oil that commonly boils in the 650+°F to 1,050+°F (330° to
50°C) range. Feedstock properties are discussed in Chapter 2.
Before proceeding, it is helpful to examine how a typical cat cracker
ts into the refinery process. A petroleum refinery is composed of
everal processing units that convert raw crude oil into usable products
uch as gasoline, diesel, and jet fuel (Figure 14).
The crude unit is the first unit in the refining process. Here, the
aw crude is distilled into several intermediate products: naphtha,
erosene, diesel, and gas oil. The heaviest portion of the crude oil,
(text continued on page 6)
Process Description
Flue Gas
To Fractionator
Reactor
^Stripping
Steam
Figure 12. UOP FCC (courtesy of UOP).
Second stage
regenerator
Riser termination
device
Combustion Air
First stage
regenerator
Combustion
Air
Lift air
r
Feed Injection
Figure 13. SWEC stacked FCC unit (courtesy of Stone & Webster Engi
eering Corporation),
Fluid Catalytic Cracking Handbook
915
936
938
942
943
947
948
950s
951
952
954
Mid50s
956
961
964
972
974
975
981
983
985
994
996
Table 11
The Evolution of FCC
McAfee of Gulf Refining Co. discovered that a FriedelCrafts
aluminum chloride catalyst could catalytically crack heavy oil.
Use of natural clays as catalyst greatly improved cracking
efficiency.
Catalyst Research Associates (CRA) was formed. The original
CRA members were: Standard of New Jersey (Exxon), Stan
dard of Indiana (Amoco), Anglo Iranian Oil Company (BP
Oil), The Texas Company (Texaco), Royal Dutch Shell,
Universal Oil Products (UOP), The M.W, Kellogg Company,
and I.G. Farben (dropped in 1940).
First commercial FCC unit (Model I upflow design) started up
at Standard of New Jersey's Baton Rouge, Louisiana, refinery.
First downflow design FCC unit was brought online. First
thermal catalytic cracking (TCC) brought online.
First UOP stacked FCC unit was built. Kellogg introduced the
Model III FCC unit.
Davison Division of W.R. Grace & Co. developed micro
spheroidal FCC catalyst.
Evolution of bedcracking process designs.
M.W. Kellogg introduced the Orthoflow design.
Exxon introduced the Model IV.
High alumina (A12 O2) catalysts were introduced.
UOP introduces sidebyside design.
Shell invented riser cracking.
Kellogg and Phillips developed and put the first resid cracker
onstream at Borger, Texas.
Mobil Oil developed USY and ReY FCC catalyst. Last TCC
unit completed.
Amoco Oil invented hightemperature regeneration.
Mobil Oil introduced CO promoter.
Phillips Petroleum developed antimony for nickel passivation.
TOTAL invented twostage regeneration for processing residue,
Mobil reported first commercial use of ZSM5 octane/olefins
additive in FCC
Mobil started installing closed cyclone systems in its FCC units.
Coastal Corporation conducted commercial test of ultrashort
residence time, selective cracking.
ABB Lummus Global acquired Texaco FCC technologies.
GASOLINE
f
•XL
TAR
!j§
I
DELAYED
COKER
HEATING OIL
DECANT OIL
3AS( gasoline to
REFORMER
Figure 14. A typical high conversion refinery.
NO. 6 OIL
Fluid Catalytic Cracking Handbook
ext continued from page 2)
which cannot be distilled in the atmospheric tower, is heated and sent
o the vacuum tower where it is split into gas oil and tar. The tar from
he vacuum tower is sent to be further processed in a delayed coker,
easphalting unit, or visbreaker, or is sold as fuel oil or road asphalt.
The gas oil feed for the conventional cat cracker comes primarily
om the atmospheric column, the vacuum tower, and the delayed
oker. In addition, a number of refiners blend some atmospheric or
acuum resid into the feedstocks to be processed in the FCC unit.
The FCC process is very complex. For clarity, the process descrip
on has been broken down into six separate sections:
• Feed Preheat
• Riser—Reactor—Stripper
« Regenerator—Heat/Catalyst Recovery
• Main Fractionator
• Gas Plant
« Treating Facilities
EED PREHEAT
Most refineries produce sufficient gas oil to meet the cat crackers'
emand. However, in those refineries in which the gas oil produced
oes not meet the cat cracker capacity, it may be economical to
upplement feed by purchasing FCC feedstocks or blending some
esidue. The refineryproduced gas oil and any supplemental FCC
eedstocks are generally combined and sent to a surge dram, which
rovides a steady flow of feed to the charge pumps. This drum can
lso separate any water or vapor that may be in the feedstocks.
From the surge drum, the feed is normally heated to a temperature
f 500°F to 700°F (260°C to 370°C). The main fractionator bottoms
umparound and/or fired heaters are the usual sources of heat. The
eed is first routed through heat exchangers using hot streams from
he main fractionator. The main fractionator top pumparound, light
ycle oil product, and bottoms pumparound are commonly used (Fig
re 15). Removing heat from the main fractionator is at least as
mportant as preheating the feed.
Most FCC units use fired heaters for FCC feed final preheat. The
eed preheater provides control over the catalysttooil ratio, a key
ariable in the process. In units where the air blower is constrained.
Process Description
Vent to Main Column
cSi—^
Slurry
I
Feed Preheater
Figure 15. Typical feed preheat system.
ncreasing preheat temperature allows increased throughput. The effects
f feed preheat are discussed in Chapter 6.
RISER—REACTOR—STRIPPER
The reactorregenerator is the heart of the FCC process. In a modern
at cracker, virtually all the reactions occur in 1.5 to 3.0 seconds
before the catalyst and the products are separated in the reactor.
From the preheater, the feed enters the riser near the base where it
ontacts the regenerated catalyst (see Figure 16). The ratio of catalyst
ooil is normally in the range of 4:1 to 9:1 by weight. The heat
bsorbed by the catalyst in the regenerator provides the energy to heat
he feed to its desired reactor temperature. The heat of the reaction
ccurring in the riser is endothermic (i.e., it requires energy input). The
irculating catalyst provides this energy. The typical regenerated catalyst
emperature ranges between 1,250°F to 1,350°F (677°C to 732°C).
Fluid Catalytic Cracking Handbook
To Reactor or Cyclone
Catalyst
From
Regenerator
(Typical for
Multiple Nozzles)
Figure 16. Typical riser "Y".
The catalytic reactions occur in the vapor phase. Cracking reactions
egin as soon as the feed is vaporized. The expanding volume of the
apors that are generated are the main driving force to carry the
atalyst up the riser.
Catalyst and products are quickly separated in the reactor. However,
ome thermal and nonselective catalytic reactions continue. A number
Process Description
9
f refineries are modifying the riser termination devices to minimize
hese reactions.
The riser is a vertical pipe. It usually has s 4 to 5inch (10 to 13
m) thick refractory lining for insulation and abrasion resistance.
Typical risers are 2 to 6 feet (60 to 180 cm) in diameter and 75 to
120 feet (25 to 30 meters) long. The ideal riser simulates a plug flow
eactor, where the catalyst and the vapor travel the length of the riser
with minimum back mixing.
Efficient contacting of the feed and catalyst is critical for achieving
he desired cracking reactions. Steam is commonly used to atomize
he feed. Smaller oil droplets increase the availability of feed at the
eactive acid sites on the catalyst. With highactivity zeolite catalyst,
irtually all of the cracking reactions take place in three seconds or less.
Risers are normally designed for an outlet vapor velocity of 50 ft/sec
o 75 ft/sec (15.2 to 22.8 m/sec). The average hydrocarbon residence
me is about two seconds (based on outlet conditions). As a consequence
f the cracking reactions, a hydrogendeficient material called coke is
eposited on the catalyst, reducing catalyst activity.
Catalyst Separation
After exiting the riser, catalyst enters the reactor vessel. In today's
CC operations, the reactor serves as a housing for the cyclones. In
he early application of FCC, the reactor vessel provided further bed
racking, as well as being a device used for additional catalyst separation.
Nearly every FCC unit employs some type of inertial separation
evice connected on the end of the riser to separate the bulk of the
atalyst from the vapors. A number of units use a deflector device to
urn the catalyst direction downward. On some units, the riser is
irectly attached to a set of cyclones. The term "rough cut" cyclones
enerally refers to this type of arrangement. These schemes separate
pproximately 75% to 99% of the catalyst from product vapors.
Most FCC units employ either single or twostage cyclones (Figure
7) to separate the remaining catalyst particles from the cracked
apors. The cyclones collect and return the catalyst to the stripper
hrough the diplegs and flapper/trickle valves (See Figure 18). The
roduct vapors exit the cyclones and flow to the main fractionator
or recovery. The efficiency of a typical twostage cyclone system
s 99.995+%.
0
Fluid Catalytic Cracking Handbook
igure 17. A twostage cyclone system. (Courtesy of Bill Dougherty, BP Oil
efinery, Marcus Hook, Pa.)
It is important to separate catalyst and vapors as soon as they enter
he reactor. Otherwise, the extended contact time of the vapors with
he catalyst in the reactor housing will allow for nonselective catalytic
ecracking of some of the desirable products. The extended residence
me also promotes thermal cracking of the desirable products.
Process Description
11
Pivot
Cyclone Dipleg
Restraint
PLAN
Cyclone Dipleg*
Pivot
Restraint
ELEVATION
Figure 18. Typical trickle valve (courtesy of Emtrol Corporation),
tripping Section
As the spent catalyst falls into the stripper, hydrocarbons are adsorbed
n the catalyst surface, hydrocarbon vapors fill the catalyst pores, and
he vapors entrained with the catalyst also fall into the stripper.
tripping steam, at a rate of 2 to 5 Ibs per 1,000 lbs (2 kg to 5 kg
er 1,000 kg,) is primarily used to remove the entrained hydrocarbons
etween catalyst particles. Stripping steam does not address hydro
arbon desorption and hydrocarbons filling the catalyst pores. How
ver, reactions continue to occur in the stripper. These reactions are
2
Fluid Catalytic Cracking Handbook
riven by the reactor temperature and the catalyst residence time in
he stripper. The higher temperature and longer residence time allow
onversion of adsorbed hydrocarbons into "clean lighter" products.
oth baffled and unbaffled stripper designs (Figure 19) are in com
mercial use. An efficient stripper design generates intimate contact
etween the catalyst and steam. Reactor strippers are commonly
esigned for a steam superficial velocity of 0.75 ft/sec (0.23 m/sec)
nd a catalyst flux rate of 500 to 700 lbs per minute per square foot
.4 kg to 3.4 kg per minute per square meter). At too high a flux,
UPPER STEAM DISTRIBUTOR
LOWER STEAM DISTRIBUTOR
Figure 19. An example of a twostage stripper.
Process Description
13
he falling catalyst tends to entrain steam, thus reducing the effective–
ess of stripping steam.
It is important to minimize the amount of hydrocarbon vapors
arried through to the regenerator, but not all the hydrocarbon vapors
an be displaced from the catalyst pores in the stripper. A fraction of
hem are carried with the spent catalyst into the regenerator. These
ydrocarbon vapors/liquid have a higher hydrogentocarbon ratio than
he coke on the catalyst. The drawbacks of allowing these hydrogen
ch hydrocarbons to enter the regenerator are as follows:
* Loss of liquid product. Instead of the hydrocarbons burning in the
regenerator, they could be recovered as liquid products.
« Loss of throughput. The combustion of hydrogen to water pro
duces 3.7 times more heat than the combustion of carbon to
carbon dioxide. The increase in the regenerator temperature caused
by excess hydrocarbons could exceed the temperature limit of the
regenerator internals and force the unit to a reduced feed rate
mode of operation.
* Loss of catalyst activity. The higher regenerator temperature
combined with the formation of steam in the regenerator reduces
catalyst activity by destroying the catalyst's crystalline structure.
The flow of spent catalyst to the regenerator is typically controlled
y a valve that slides back and forth. This slide valve is controlled
y the catalyst level in the stripper. The catalyst height in the stripper
rovides the pressure head, which allows the catalyst to flow into the
egenerator. The exposed surface of the slide valve is usually lined
with refractory to withstand erosion. In a number of earlier FCC
esigns, lift air is used to transport the spent catalyst into the regener
tor (Figure 110).
REGENERATOR–HEAT/CATALYST RECOVERY
The regenerator has two main functions: it restores catalyst activity
nd supplies heat to crack the feed. The spent catalyst entering the
egenerator contains between 0.4 wt% and 2.5 wt% coke, depending
n the quality of the feedstock. Components of coke are carbon,
ydrogen, and trace amounts of sulfur and nitrogen. These burn
ccording to the following reactions.
4
Fluid Catalytic Cracking Handbook
Products
Reactor
Regen
Catalyst
Standpipe
LiftAir
Air Blower
Figure 110. A typical Model II using lift air to transfer spent catalyst.
+ 1/2 CX,
O + 1/2 02
+ O2
H2, + 1/2 02
+ xO
+ xO
_>
— >
— >
>
— »
>
CO
CO2
CO2
H2O
sox
NO y
K Cal/kg of
C, H2, or S
BTU/lb of
C, H2, or S
2,200
5,600
7,820
28,900
2,209
3,968
10,100
14,100
52,125
3,983
(11)
(12)
(13)
(14)
(15)
(16)
Process Description
15
Air provides oxygen for the combustion of coke and is supplied by
ne or more air blowers. The air blower provides sufficient air velocity
nd pressure to maintain the catalyst bed in a fluid state. The air enters
he regenerator through an air distributor (Figure 111) located near
he bottom of the vessel. The design of an air distributor is important
n achieving efficient and reliable catalyst regeneration. Air distributors
re typically designed for a 1.0 psi to 2.0 psi (7 to 15 Kpa) pressure
rop to ensure positive air flow through all nozzles.
There are two regions in the regenerator: the dense phase and the
ilute phase. At velocities common in the regenerator, 2 ft/sec to 4 ft/sec
0.6 to 1.2 m/sec), the bulk of catalyst particles are in the dense bed
mmediately above the air distributor. The dilute phase is the region
bove the dense phase up to the cyclone inlet, and has a substantially
ower catalyst concentration.
Standpipe/Slide Valve
During regeneration, the coke level on the catalyst is typically
educed to 0.05%. From the regenerator, the catalyst flows down a
ransfer line commonly referred to as a standpipe. The standpipe
rovides the necessary pressure to circulate the catalyst around the
nit. Some standpipes extend into the regenerator, and the top section
s often called a catalyst hopper. The hopper, internal to the regener
tor, is usually an inverted cone design. In units with "long" catalyst
tandpipes, external withdrawal hoppers are often used to feed the
tandpipes. The hopper provides sufficient time for the regenerated
atalyst to be "debubbled" before entering the standpipe.
Standpipes are typically sized for a flux rate in the range of 100 to
00 lb/sec/ft2 (500 to 1,500 kg/sec/m2) of circulating catalyst. In most
ases, sufficient flue gas is carried down with the regenerated catalyst
o keep it fluidized. However, longer standpipes may require external
eration to ensure that the catalyst remains fluidized. A gas medium,
uch as air, steam, nitrogen, or fuel gas, is injected along the length
f the standpipe. The catalyst density in a welldesigned standpipe is
n the range of 35 to 45 lb/ft3 (560 to 720 kg/m3).
The flow rate of the regenerated catalyst to the riser is commonly
egulated by either a slide or plug valve. The operation of a slide valve
s similar to that of a variable orifice. Slide valve operation is often
ontrolled by the reactor temperature. Its main function is to supply
6
Fluid Catalytic Cracking Handbook
igure 111. Examples of air distributors. (Top: courtesy of Enpro Systems,
nc., Channelview, Texas; bottom: courtesy of VALVAMP, Incorporated,
ouston, Texas.) Note: These distributors are upside down for fabrication.
Process Description
1?
nough catalyst to heat the feed and achieve the desired reactor
emperature. In Exxon Model IV and flexicracker designs (see Figure
11), the regenerated catalyst flow is mainly controlled by adjusting
he pressure differential between the reactor and regenerator.
Catalyst Separation
As flue gas leaves the dense phase of the regenerator, it entrains
atalyst particles. The amount of entrainment largely depends on the
lue gas superficial velocity. The larger catalyst particles, 50}i90p, fall
ack into the dense bed. The smaller particles, 0|J,50ji, are suspended
n the dilute phase and carried into the cyclones.
Most FCC unit regenerators employ 4 to 16 parallel sets of primary
nd secondary cyclones. The cyclones are designed to recover catalyst
articles greater than 20 microns diameter. The recovered catalyst
articles are returned to the regenerator via the diplegs.
The distance above the catalyst bed at which the flue gas velocity
as stabilized is referred to as the transport disengaging height (TDH).
At this height, the catalyst concentration in the flue gas stays constant;
one will fall back into the bed. The centerline of the firststage cyclone
nlets should be at TDH or higher; otherwise, excessive catalyst entrain
ment will cause extreme catalyst losses.
Flue Gas Heat Recovery Schemes
The flue gas exits the cyclones to a plenum chamber in the top of
he regenerator. The hot flue gas holds an appreciable amount of
nergy. Various heat recovery schemes are used to recover this energy.
n some units, the flue gas is sent to a CO boiler where both the
ensible and combustible heat are used to generate highpressure
team. In other units, the flue gas is exchanged with boiler feed water
o produce steam via the use of a shell/tube or box heat exchanger.
In most units, about twothirds of the flue gas pressure is let down via
n orifice chamber or across an orifice chamber. The orifice chamber is
vessel containing a series of perforated plates designed to maintain a
iven backpressure upstream of the regenerator pressure control valve.
In some larger units, a turbo expander is used to recover this
ressure energy. To protect the expander blades from being eroded by
atalyst, flue gas is first sent to a thirdstage separator to remove the
8
Fluid Catalytic Cracking Handbook
ines. The thirdstage separator, which is external to the regenerator,
ontains a large number of swirl tubes designed to separate 70% to
5% of the incoming particles from the flue gas.
A power recovery train (Figure 112) employing a turbo expander
sually consists of four parts: the expander, a motor/generator, an air
blower, and a steam turbine. The steam turbine is primarily used for
tartup and, often, to supplement the expander to generate electricity.
The motor/generator works as a speed controller and flywheel; it can
roduce or consume power. In some FCC units, the expander horsepower
xceeds the power needed to drive the air blower and the excess power
s output to the refinery electrical system. If the expander generates less
ower than what is required by the blower, the motor/generator provides
he power to hold the power train at the desired speed.
From the expander, the flue gas goes through a steam generator to
ecover thermal energy. Depending on local environmental regulations,
n electrostatic precipitator (ESP) or a wet gas scrubber may be placed
ownstream of the waste heat generator prior to release of the flue
as to the atmosphere. Some units use an ESP to remove catalyst fines
n the range of 5|i20ji from the flue gas. Some units employ a wet
as scrubber to remove both catalyst fines and sulfur compounds from
he flue gas stream.
Partial versus Complete Combustion
Catalyst can be regenerated over a range of temperatures and flue
as composition with inherent limitations. Two distinctly different
modes of regeneration are practiced: partial combustion and complete
ombustion. Complete combustion generates more energy when coke
ield is increased; partial combustion generates less energy when the
oke yield is increased. In complete combustion, the excess reaction
omponent is oxygen, so more carbon generates more combustion. In
artial combustion, the excess reaction component is carbon, all the
xygen is consumed, and an increase in coke yield means a shift from
CO2 to CO.
FCC regeneration can be further subdivided into low, intermediate,
nd high temperature regeneration. In low temperature regeneration
about 1,190°F or 640°C), complete combustion is impossible. One
f the characteristics of low temperature regeneration is that at 1,190°F,
ll three components (O2, CO, and CO2) are present in the flue gas at
UE GAS FROM REGENERATOR
ELECTRO
PRECIPIT
C
F
REGENERATOR
Figure 112, A typical flue gas power recovery scheme.
0
Fluid Catalytic Cracking Handbook
ignificant levels. Low temperature regeneration was the mode of
peration that was used in the early implementation of the catalytic
racking process.
In the early 1970s, high temperature regeneration was developed.
High temperature regeneration meant increasing the temperature until
ll the oxygen was burned. The main result was low carbon on the
egenerated catalyst. This mode of regeneration required maintaining
n the flue gas, either a small amount of excess oxygen and no CO,
r no excess oxygen and a variable quantity of CO. If there was excess
xygen, the operation was in a full burn. If there was excess CO, the
peration was in partial burn.
With the advent of combustion promoter, the regeneration tem
erature could be reduced and still maintain full burn. Thus, intermediate
emperature regeneration was developed. Intermediate regeneration is
ot necessarily stable unless combustion promoter is used to assist in
he combustion of CO in the dense phase. Table 1 2 contains a 2 x 3
matrix summarizing various aspects of regeneration.
The following matrix of regeneration temperatures and operating
modes shows the inherent limitations of operating regions. Regenera
on is either partial or complete, at low, intermediate, or high tern
Table 12
A Matrix of Regeneration Characteristics
Operating Region Regenerator
Combustion
Partial Combustion
Mode
Full Combustion
Mode
ow temperature (nominally
,190°F/640°C)
Stable (small
afterburning) O2,
CO, and CO2 in
the flue gas
Not achievable
ntermediate temperature
nominally 1,275 °F/690°C)
Stable (with
combustion
promoter); tends to
have high carbon
on regenerated
catalyst
Stable with
combustion
promoter
igh temperature (nominally
,350°F/730°C)
Stable operation
Stable operation
Process Description
21
eratures. At low temperatures, regeneration is always partial, carbon
n regenerated catalyst is high, and increasing combustion air results
n afterburn. At intermediate temperatures, carbon on regenerated
atalyst is reduced. The three normal "operating regions" are indicated
n Table 12.
There are some advantages and disadvantages associated with full
nd partial combustion:
dvantages of full combustion
• Energy efficient
• Heatbalances at low coke yield
• Minimum hardware (no CO boiler)
• Better yields from clean feed
Disadvantages of full combustion
• Narrow range of coke yields unless some heat removal system
is incorporated
• Greater afterburn, particularly with an uneven air or spent catalyst
distribution system
• Low cat/oil ratio
The choice of partial versus full combustion is dictated by FCC feed
uality. With "clean feed," full combustion is the choice. With low
uality feed or resid, partial combustion, possibly with heat removal,
s the choice.
Catalyst Handling Facilities
Even with proper operation of the reactor and regenerator cyclones,
atalyst particles smaller than 20 microns still escape from both of
hese vessels. The catalyst fines from the reactor collect in the frac
onator bottoms slurry product storage tank. The recoverable catalyst
nes exiting the regenerator are removed by the electrostatic pre
ipitator or lost to the environment. Catalyst losses are related to:
« The design of the cyclones
« Hydrocarbon vapor and flue gas velocities
• The catalyst's physical properties
• High jet velocity
• Catalyst attrition due to the collision of catalyst particles with the
vessel internals and other catalyst particles
2
Fluid Catalytic Cracking Handbook
The activity of catalyst degrades with time. The loss of activity is
rimarily due to impurities in the FCC feed, such as nickel, vanadium,
nd sodium, and to thermal and hydrothermal deactivation mechanisms.
o maintain the desired activity, fresh catalyst is continually added to
he unit, Fresh catalyst is stored in a fresh catalyst hopper and, in most
nits, is added automatically to the regenerator via a catalyst loader.
The circulating catalyst in the FCC unit is called equilibrium
atalyst, or simply Ecat. Periodically, quantities of equilibrium catalyst
re withdrawn and stored in the Ecat hopper for future disposal. A
efinery that processes residue feedstocks can use goodquality Ecat
om a refinery that processes light sweet feed. Residue feedstocks
ontain large quantities of impurities, such as metals and requires high
ates of fresh catalyst. The use of a goodquality Ecat in conjunc
on with fresh catalyst can be costeffective in maintaining low
atalyst costs.
MAIN FRACTIONATOR
The purpose of the main fractionator, or main column (Figure 113),
to desuperheat and recover liquid products from the reactor vapors.
he hot product vapors from the reactor flow into the main fractionator
ear the base. Fractionation is accomplished by condensing and
evaporizing hydrocarbon components as the vapor flows upward
hrough trays in the tower.
The operation of the main column is similar to a crude tower, but
with two differences. First, the reactor effluent vapors must be cooled
efore any fractionation begins. Second, large quantities of gases will
avel overhead with the unstabilized gasoline for further separation.
The bottom section of the main column provides a heat transfer
one. Shed decks, disk/doughnut trays, and grid packing are among
ome of the contacting devices used to promote vapor/liquid contact.
he overhead reactor vapor is desuperheated and cooled by a pump
round stream. The cooled pumparound also serves as a scrubbing
medium to wash down catalyst fines entrained in the vapors. Pool
uench can be used to maintain the fractionator bottoms temperature
elow coking temperature, usually at about 700°F (370°C).
The recovered heat from the main column bottoms is commonly
sed to preheat the fresh feed, generate steam, serve as a heating medium
or the gas plant reboilers, or some combination of these services.
Process Description
23
Figure 113. A typical FCC main fractionator circuit.
The heaviest bottoms product from the main column is commonly
alled slurry or decant oil. (In this book, these terms are used inter
hangeably.) The decant oil is often used as a "cutter stock" with
acuum bottoms to make No. 6 fuel oil. Highquality decant oil (low
ulfur, low metals, low ash) can be used for carbon black feedstocks.
Early FCC units had soft catalyst and inefficient cyclones with
ubstantial carryover of catalyst to the main column where it was
bsorbed in the bottoms. Those FCC units controlled catalyst losses
wo ways. First, they used high recycle rates to return slurry to the
eactor. Second, the slurry product was routed through slurry settlers.
4
Fluid Catalytic Cracking Handbook
ither gravity or centrifugal, to remove catalyst fines. A slipstream
f FCC feed was used as a carrier to return the collected fines from
he separator to the riser. Since then, improvements in the physical
roperties of FCC catalyst and in the reactor cyclones have lowered
atalyst carryover. Most units today operate without separators. The
ecant oil is sent directly to the storage tank. Catalyst fines accumulate
n the tank, which is cleaned periodically. Some units continue to use
ome form of slurry settler to minimize the ash content of decanted oil.
Above the bottoms product, the main column is often designed for
hree possible sidecuts:
* Heavy cycle oil (HCO)—used as a pumparound stream, some
times as recycle to the riser, but rarely as a product
* Light cycle oil (LCO)—used as a pumparound stream, sometimes
as absorption oil in the gas plant, and stripped as a product for
diesel blending; and
* Heavy naphtha—used as a pumparound stream, sometimes as
absorption oil in the gas plant, and possible blending in the
gasoline pool
In many units, the light cycle oil (LCO) is the only sidecut that
eaves the unit as a product. LCO is withdrawn from the main column
nd routed to a side stripper for flash control. LCO is sometimes
eated for sulfur removal prior to being blended into the heating oil
ool. In some units, a slipstream of LCO, either stripped or unstripped,
sent to the sponge oil absorber in the gas plant. In other units,
ponge oil is the cooled, unstripped LCO.
Heavy cycle oil, heavy naphtha, and other circulating side pump
round reflux streams are used to remove heat from the fractionator.
hey supply reboil heat to the gas plant and generate steam. The
mount of heat removed at any pumparound point is set to distribute
apor and liquid loads evenly throughout the column and to provide
he necessary internal reflux.
Unstabilized gasoline and light gases pass up through the main
olumn and leave as vapor. The overhead vapor is cooled and partially
ondensed in the fractionator overhead condensers. The stream flows
o an overhead receiver, typically operating at