Microporous and Mesoporous Materials 254 (2017) 45e58

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Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso

Nanoporous materials forge a path forward to enable sustainable
growth: Technology advancements in fluid catalytic cracking
Melissa Clough a, Jacqueline C. Pope a, Lynne Tan Xin Lin b, Vasileios Komvokis c,
Shuyang S. Pan d, Bilge Yilmaz d, *
a

BASF Refinery Catalysts, 11750 Katy Fwy. #120, Houston, TX 77079, USA
BASF Refinery Catalysts, 7 Temasek Blvd, 038987, Singapore
BASF Refinery Catalysts, SK8 6QG Cheadle, United Kingdom
d
BASF Refinery Catalysts, 25 Middlesex-Essex Tpk., Iselin, NJ 08830, USA
b
c

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 7 November 2016
Received in revised form
8 March 2017
Accepted 31 March 2017
Available online 2 April 2017

An overview is presented on the central role that zeolites and other nanoporous materials currently play
in the Fluid Catalytic Cracking (FCC) process as well as how this role evolved over the course of the years
since its inception. Today, utilization in FCC constitutes the vast majority of global zeolite catalyst consumption by volume. FCC is the main conversion process in a typical fuels refinery, and as the most
critical ingredient of the catalyst, zeolites are responsible for producing majority of the gasoline used
around the world as well as taking an important role in the production of other transportation fuels (e.g.,
diesel, jet fuel) and building blocks for the petrochemical industry (e.g., propylene, butylenes). Therefore,
it can be stated that zeolite catalysts fuel our industrialized society and provide the building blocks for its
advancement; consequently, zeolites have a direct impact on the future of the global economy and its
sustainability. Strategies that involve zeolites and other nanoporous materials for improving performance of FCC operation and ensuring its environmental sustainability were reviewed. Zeolite modifications were examined with each leading to an improvement in zeolite stability under severe conditions
in an FCC unit. The importance of diffusion pathways within an FCC catalyst particle, leading to higher
accessibility of the active zeolite sites, were explored, and the importance of a well-designed catalyst
architecture, allowing FCC feed, intermediates, and final products to diffuse freely in and out of the
catalyst particle were discussed. The role of contaminant metals in FCC was investigated, and some
mitigation strategies for the most common FCC contaminants, nickel and vanadium, were presented. The
impact of contaminant iron was discussed alongside catalyst architecture, particularly surface porosity of
the catalyst particle. Utilization of other nanoporous materials in FCC, especially as environmental additives, was summarized. Testing considerations were screened with an emphasis on matching laboratory deactivation to refinery FCC observations.
© 2017 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND
license ( />
Keywords:
Fluid catalytic cracking
FCC
Nanoporous materials
Zeolite catalysis
Catalyst testing
FCC additives
Catalyst deactivation

1. Introduction
The fluid catalytic cracking (FCC) process represents an integral

part of a refinery complex, providing the majority of gasoline
consumed throughout the world in addition to other important
transportation fuels. The FCC, in its current design, utilizes a wellfluidizable catalyst that is continually added and withdrawn (and/
or lost). The catalyst facilitates the cracking of crude oil into

* Corresponding author.
E-mail address: (B. Yilmaz).

important products including gasoline, diesel, jet fuel, and liquefied
petroleum gases (LPGs).
The FCC has been utilized in the industry for over 70 years, going
from a single FCC unit in the USA to over 400 in operation today
around the world. Since FCC's inception, the technology has undergone major transformations, including changes in both hardware and catalyst technologies. Importantly, these transformations
have ripple effects e as a result of the flexibilities afforded by
technology changes, the FCC is continually being pushed to its
limits. Ancillary units have been implemented and have undergone
dramatic changes as well, including crude oil desalters and particulate matter collectors. Regulatory requirements have been put

/>1387-1811/© 2017 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license ( />

46

M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

in place to push FCC limits even harder. For instance, improvements
in hardware and catalyst technologies have enabled increased
control of emissions, including particulate matter, NOx, and SOx.
Hardware changes over the years have allowed for step changes
by improved feed atomization and catalyst distribution and fluidization. Catalyst changes over the years have allowed for step
changes in conversion and selectivities. The two, hardware, and

catalyst, have constantly undergone iterative changes as the other
has allowed. For instance, improved catalyst activity through enhancements in zeolitic cracking led to drastically different unit
designs such as the short contact time unit configuration.
Hardware and catalyst changes not only respond to each other,
but also to the market. The availability and diversity of crude oils on
the market represents another force to which FCC technology must
adapt. Processing of residue-containing (i.e. resid) and shalederived (i.e. tight oil) feeds require optimized hardware and catalyst technologies. Product demand, such as diesel over gasoline,
also drives hardware revamps and catalyst selection criteria.
The past 70 years demonstrate the compounding improvements
in FCC catalyst and additive technologies e a result of both push
and pull forces in the market and from process licensors, hardware
vendors, catalyst suppliers, and government agencies. The goal of
this contribution is to review FCC catalyst and additive technology
developments in detail starting with the inception of the FCC
concept.
2. FCC background and history
The FCC process was a result of the combination of an urgent
global need, political dynamics of that period, focused collaboration, and scientific advancements in related fields (fluidization,
heterogeneous catalysis, materials science, etc.). In 1938, a consortium called Catalytic Research Associates (CRA) was formed
between multiple firms with varying functions (operating,
licensing, chemical manufacturing) in order to develop a catalytic
cracking process [1]. Houdry held patents that covered fixed-bed
cracking using oxides of silicon and aluminum; the consortium
was tasked with inventing a novel process that did not infringe on
these patents. Houdry's patent was industrially important because
of the regeneration ability of the catalyst via coke burning. Two
years later, the structure and members of the consortium were
reorganized amid World War II politics [2].
Early on, the transfer of fluidized solids was investigated at a
Baton Rouge pilot plant. Shortly after, investigations into fluidized

catalysts over catalyst pellet systems commenced. In 1940, with
political and national pressures to provide jet fuel during World
War II, funds to build the first powdered/fluidized catalyst plant
were approved. The first catalyst used at the Louisiana plant, called
PCLA for powdered catalyst Louisiana, was an amorphous aluminabased catalyst. The operation was publicly announced, albeit carefully with political tensions world-wide, on February 11, 1941, in
which a new “continuous catalytic cracking process” was unveiled
[3]. Within three years, 34 additional units were built and
commissioned due to its overwhelming success in producing high
quality gasoline and aviation fuel, among other products.
Since the first continuous catalytic cracking process, hundreds
of FCC units have been built around the world. Major hardware and
catalyst changes have taken place during that time. On the catalyst
side, changes have also occurred since the 1940's. Originally, Houdry utilized acid-activated bentonite. Natural and synthetic aluminosilicates had good cracking properties and strong Lewis acid
sites. Early catalysts had low alumina content, around 13%, and by
the 1950's, this figure increased to about 25% [4]. The early FCC
catalysts were finely ground materials, i.e. no spray-drying process
was employed. Spray drying was first utilized in FCC catalyst

production in 1948 and improved the fluidization and attrition
properties of the catalyst via sphericity improvements. However,
the biggest technological change in the history of the FCC catalyst
was the implementation of zeolites. Zeolites were first introduced
to FCC catalysts in 1964 and represented a major step change in
catalyst stability and selectivity as a result of their strong Brønsted
and Lewis acid sites. Zeolites not only provided strong solid acidity
for high activity and stability, but also introduced a high surface
area pore/channel network that brought size and shape selectivity
as well as higher density of acid sites (per unit weight of catalyst).
By the late 1990's, additional modifications were being made to
the FCC catalyst. Special alumina matrices for nickel passivation

and additional metals traps for vanadium passivation were introduced. The stability of the zeolite was improved for maximum
conversion via the introduction of rare earth oxides. Ion exchange
technology in the FCC catalyst manufacturing process allowed for
the reduction of sodium, a catalyst poison for the acid sites. Lower
sodium enabled higher catalyst activity and stability. Finally,
attention to catalyst porosity was heightened in order to take
advantage of resid processing [5]. Resid processing became
important in the 1980's with the oil crisis at the time, when refiners
were looking to take advantage of opportunity (i.e. low cost) crude
oils.
In the 2000's, FCC catalyst additives aimed at improving not only
yields off the FCC unit, but also at minimizing its environmental
impacts. ZSM-5, which was patented and introduced by Mobil
much earlier, was being employed for octane improvement by
increasing light olefins [6]. Other additives developed targeted
emissions and product specifications, such as SOx, NOx, and gasoline sulfur reduction additives. Also in the 2000's, DMS (distributed
matrix structures) technology represented another breakthrough
for the FCC catalyst with improved pore architecture for the most
advanced zeolitic cracking in the industry, which is explained in
more detail in section 5.1. This resulted in immediate market
acceptance via utilization at numerous refineries around the world
as shown in Fig. 1 below. DMS technology provided a step out in
performance via state of the art porosity modifications, partially
enabled by a novel technique of in situ zeolite crystallization. The
new technique allowed for optimized porosity, including surface
porosity and channel modifications.
Activity impacts of the major catalyst and hardware improvements are summarized in Fig. 2, as estimated by historical conversion levels in FCC units. The second order activity as a function of
major step change technologies improved dramatically from 1950
when amorphous catalysts were still used. Subsequent improvements via zeolites, ultra-stable Y-zeolite (USY), short contact time
(SCT) unit designs, and DMS are illustrated.

The proceeding sections of this manuscript detail the important
catalyst and hardware improvements described in this section with
an emphasis on advancements in materials science and chemistry
that were integral in FCC improvements.
3. Hardware developments
The first commercial FCC operation commenced in 1942 when
the Model I FCC unit started in Baton Rouge, Louisiana, processing
13,000 barrels of crude oil per day [7]. Since this first FCC unit,
hundreds of FCC units have been built and designs have evolved to
adapt to changing market demands. These changes effectively
enabled the FCC to maintain its dominance as the key conversion
process for gasoline and light olefins production within a refinery
complex.
In recent years, the use of opportunistic crude feeds, the greater
emphasis on light olefins production, and the concerns around
environmental emission regulations have driven a host of


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

47

DMS USER BASE GROWS Q UICKLY AFTER MARKET
INTRODUCTION

NUMBER OF USERS

140
120
100

80
60
40
20
0
0

1

2
3
YEARS AFTER MARKET INTRODUCTION

4

5

Fig. 1. Number of DMS technology users 5 years after its market introduction.

2.1
1.5

a.u.

3.3

4

CATALYST ACTIVITY AS A FUNCTION OF IMPORTANT
TECHNOLOGY IMPROVEMENTS


1950: AMORPHOUS
CATALYSTS

1970: ZEOLITES

1990: USY AND SCT

2000: DMS AND SCT

Fig. 2. Activity impacts of major catalyst and hardware changes since 1950.

innovations in FCC unit hardware designs. On the reactor side,
improvements in the feed injection systems, riser internals, and
standpipe designs have enabled continuous improvement. Designs
of the atomizers have evolved from single fluid atomizers to twinfluid atomizers, which require lower feed pressure to form a more
homogenous droplet distribution. The use of feed injectors with
adjustable angles has also enabled a more uniform radial riser
temperature profile and improves mixing with the regenerated
catalyst. In addition, the introduction of ceramic feed distributors,
which have higher temperature tolerance and erosion resistance
relative to the metallic counterparts, has enabled more severe operations [8].
In risers where numerous chemical reactions take place,
improved designs of the riser termination device have also enabled
quicker and more efficient separation of catalyst and product gases
to minimize post-riser cracking. The use of direct-coupled cyclones
preserves olefins for refineries in maximum olefin operations.
Catalyst circulation enhancement technology in the standpipe inlet
has also helped to prevent gas entrainment by removing excess gas
bubbles from the fluidized bed before the catalyst enters the

standpipe [9], which can improve catalyst circulation rate by as
much as 50%.
Following the riser, developments in the regenerator to further
improve heat balance have taken place. External catalyst coolers are
an option to the basic FCC design to reduce regenerator heat load to
process heavier feeds and to reduce catalyst deactivation under
high temperatures. The use of dense-phase downflow exchangers
to replace older upflow designs can also improve catalyst circulation without the use of a slide valve [10].

In order to comply with more stringent environmental regulations, hardware modifications were made to separate particulate
matter (PM) from flue gas. A typical regenerator is designed with
multiple pairs of two-stage cyclones, but it is often relatively large
in diameter and not effective in capturing small catalyst particles,
especially particles smaller than 20 mm. The innovation of a third
stage separator as an improved cyclonic technology uses a swirl
tube as an axial flow separator to induce fast rotating motion,
which results in improved separation efficiencies [11].
The FCC unit continues to serve a central role in the refining
industry due to its flexibility to meet current demand while processing a wide variety of feeds. Hardware modifications are a key
consideration to enable operation objectives to be met at a relatively low cost. Though most changes tend to be incremental, the
move to more desirable product yield slates can be substantial over
time as history has demonstrated.

4. Zeolites in FCC catalysts
4.1. Zeolite basics
The term zeolite was first coined in 1756 when Baron Cronstedt,
a Swedish mineralogist noted a new class of silicate materials with
curious physical properties [12]. The term zeolite is derived from
two Greek words: zeo meaning “to boil” and lithos meaning “a
stone”. Cronstedt coined the term after exposing the materials to a

flame and observing intumescence. Cronstedt's first zeolite discovery was stilbite, later corrected to be stellerite [13].


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M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

Since then, various authors around the world have discovered
and described the properties of zeolite materials, including
adsorption properties, ion exchange capabilities, and dehydration
characteristics. By the 1940s, the first examples of synthetic zeolites
were seen from the laboratories of Union Carbide Corporation. This
work was initially conducted to find new ways to separate and
purify air. By the early 1950s, zeolites A, X, and Y were discovered
[14].
Zeolites are complex crystalline structures made up of an uninterrupted framework of tetrahedra [15]. Zeolites can be chargebalanced with group IA and IIA elements such as sodium, potassium, magnesium, and calcium [16]. The charge balancing ion can
be exchanged for others in solution, making zeolites very versatile
as an industrially relevant material [17]. Zeolites containing AlO4
tetrahedra lead to a negative charge in the zeolitic framework,
which require balancing by cations. The framework structures are
made up of primary (PBU), secondary (SBU), composite (CBU), or
periodic (PerBU) building units. Today, 232 known framework
structures of zeolites exist. The zeolite framework is somewhat
flexible, allowing for minor adjustments in the size of pores based
on changes in temperature as well as cationic species in the
framework [14,18].
4.2. Zeolites in FCC
Zeolites are used in a wide range of applications, including as
detergents, adsorbents, and catalysts. The vast majority (ca. 95%) of
the global zeolite catalyst consumption by volume is for FCC [19].

Today, zeolites are the primary source for activity in FCC catalysts.
The typical concentration of zeolite in the catalyst can range anywhere from 15 to 50% depending on the catalyst technology and
application. The most common type of zeolite used in the FCC industry is zeolite Y, which also makes it the highest volume of
synthetic zeolite for catalysis. Zeolite Y has the faujasite (FAU)
framework, which is made up of tetrahedral Si and Al joined by
oxygen bonds. The tetrahedral structures form small building
blocks, i.e. sodalite cages. The sodalite cages are joined together in
hexagonal prisms to form the FAU framework. This framework has
a distinctive lattice structure that controls the size of hydrocarbon
molecules that can enter the framework. The inner pore size of the
framework is approximately 0.74 nm, which is precisely imparted
by the 12-membered ring, therefore feed molecules that are larger
than this must be pre-cracked by the matrix component of the
catalyst or by the acid sites on the outer zeolite surface [20].
In the 1960s and 1970s, synthetic zeolites were first incorporated into FCC catalysts, which proved to be a pivotal step for the
refining industry. The zeolites were added to enhance catalytic
activity and selectivity of the FCC catalysts, leading to exceptional
cracking properties. The catalysts contained a significantly higher
density of strong Brønsted and Lewis acids sites through incorporation of the zeolites. This is key for cracking and subsequent
hydrogen transfer reactions, which dictates the commercial
outcome of this process.
In 1962, Mobil Oil introduced a new FCC catalyst containing
Zeolite Y [21]. This catalyst was prepared by adding a small amount
of zeolite into the matrix of the SieAl catalyst. This new zeolitecontaining catalyst outperformed all existing FCC catalysts at the
time. It also resulted in a remarkable increase in gasoline yields.
This was the first commercial zeolite-based FCC catalyst and truly
revolutionized the industry. The initial zeolite Y was a Mgstabilized structure and combined high surface area/pore volume
solid acidity with sufficient room to allow cracking to take place.
The catalysts, however, were quickly switched to rare earth stabilization, which is often seen today. Zeolite Y has been the main
cracking component of FCC catalysts since 1964 [17].


Furthermore, in the early 1970s, Argauer and Landolt at Mobil
Oil Corporation synthesized a new zeolite known as ZSM-5. This
zeolite has since found large-scale application in the FCC industry,
currently with the main application being increased propylene
yields [22] (discussed in more detail in the additives section).
Today, both zeolite Y and ZSM-5 are used extensively in FCC and
related applications.

4.3. Zeolite modifications
In the FCC unit, zeolites are subjected to harsh hydrothermal
conditions, leading to a loss in zeolite surface area, zeolite dealumination, and acid site loss. Without modifications, zeolite Y is not
stable enough against the hydrothermal conditions that occur in
the FCC regenerator. Most post-synthetic modifications of zeolite
aim to improve activity retention. The magnitude of the increase in
activity retention depends on how the zeolite is modified, i.e. type
of treatment, severity of treatment, and initial acid site density.
In the late 1960s, post synthetic modifications to zeolite-Y-based
catalysts were introduced. One modification involved steaming of
the catalyst after synthesis, forming ultra-stable zeolite Y (USY
zeolite) [23]. Ever since, zeolite Y, in its various modified forms, has
been the main cracking component of FCC catalysts [21].
Dealumination represents a pathway to the introduction of
mesoporosity, which can be achieved by one of two ways. The first
method is USY, described above. The second method involves
chemical treatments, such as acid leaching or reaction with
chelating agents (e.g. EDTA) to remove alumina. Both dealumination pathways lead to a lower number of acid sites and an
initial loss in the integrity of the framework of the zeolite. However,
these disadvantages are offset by the creation of new types of acids
sites and enhanced diffusion properties, which can lead to

increased catalytic activity in the FCC [17].
Another method to create zeolites with tunable mesopores in
addition to zeolitic micropores is the creation of an ultra-stable
zeolite with iron (Fe). This involves the synthesis of a Fecontaining zeolite and subsequent ion exchange to replace the
sodium cations with ammonium ions. The resulting zeolites then
undergo a deferrization-dealumination process with steaming in
order to create the ultra-stable zeolite. The pore diameter and
volume of mesopores depend on the content of framework Fe. Such
modifications of zeolites have shown that in the catalytic cracking
of 1,3,5-triisopropylbenzene, the activity retention increased with
the improvement in the mesoporosity compared to the typical USY
[24]. Modifications like these allow for increased hydrothermal
stability, which is critical in conditions such as in the FCC regenerator [25].
Furthermore, post-synthesis modifications can reduce sodium
content of the zeolite, which increases activity retention. For
instance, pathways to lower sodium include ion exchange with
ammonium or rare earth cations. Calcination allows sodium to
easily migrate to accessible sites in order for the later critical ion
exchange steps to take place [26]. Regarding rare earth stabilization, lanthanum and cerium stabilize the framework aluminum
atoms in the zeolite lattice when exposed to hydrothermal conditions (e.g. in an FCC regenerator). Rare earth stabilization ranges
from partially or fully exchanged. IR studies demonstrate that rare
earth stabilization imparts zeolite stability indicated by a shift to
higher frequencies of framework vibrations [27]. Presence of rare
earth cations in the zeolite framework not only improves thermal
and hydrothermal stability of the zeolites, but also increases the
gasoline selectivity in an FCC unit. Thus, the rare earth level must be
tailored to control selectivities of the catalyst based on refinery
needs.



M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

5. Residual feed processing
5.1. Residual feed in the market
Globally, FCCs process more residue containing feedstocks (i.e.,
resid feeds) than in the past. This is especially true in areas of the
world where access to tight/shale oil is limited. While the processing of resid feeds can be economically profitable, it can lead to
issues during the cracking process such as significant increases in
contaminant metals, which negatively impact zeolite-based catalyst [28].
To maximize profitability of a refinery, the FCC catalyst is
specially designed to meet specific requirements of each unit
including architectural concepts such as metals tolerance, surface
area, rare earth on zeolite, matrix type, and carefully designed pore
architecture. First, the pore architecture requires optimization to
facilitate the diffusion of heavy feed molecules and to help improve
heavy molecule (i.e. bottoms) cracking. Second, the relative zeolite
and matrix content require optimization to prioritize conversion or
distillate yield. Third, a moderate to high zeolite content provides
coke-selective cracking. Lastly, strong tolerance towards contaminant metals such as nickel (Ni), vanadium (V), sodium (Na), iron
(Fe), and calcium (Ca) improves performance in the FCC unit. Due to
the destructive effects of the synergistic interaction between Na
and V, a low Na content in the fresh catalyst is especially desirable
to reduce zeolite deactivation [17].
Due to the higher contaminant metals content of resid feeds,
higher hydrogen and coke production results from undesired
metal-catalyzed dehydrogenation reactions. Utilization of
advanced catalyst technologies can mitigate these risks. An open
pore architecture allows for optimized diffusion of the feed, intermediate products, and products, allowing for coke selectivity. The
distributed matrix structure (DMS) FCC catalyst technology enables
open pore structure through carefully tailored in situ

manufacturing [29]. In DMS, the pore architecture is designed to
facilitate diffusion, allowing access to the zeolite crystals and coke
selective upgrading. The open pore architecture also facilitates the
movement of products out of the catalyst to reduce over-cracking,
i.e. further cracking of gasoline to liquefied petroleum gas (LPG),
dry gas, and coke. Likely the most important aspect of porosity,
DMS enables significant surface porosity, leading to lower diffusion
limitations and resistance to pore-blocking contaminants.
Left uncontrolled, metal contaminants increase undesired side
reactions and effects [30]. Specifically, Ni leads to increased H2 and
coke yields. Vanadium also has mild dehydrogenation activity and
severely impacts catalyst activity by poisoning the zeolite active
sites. In the presence of steam and high temperatures in the FCC
unit, V deactivates the catalyst by destroying the zeolite framework
structure [26]. Vanadium first deposits on the catalyst and is
oxidized in the regenerator. The oxidized form can undergo further
reactions to form highly mobile vanadic acids, which react with Naproducing sodium vanadate. Sodium vanadate can undergo hydrolysis to form sodium hydroxide (NaOH). The hydroxyl group
(OHÀ) can attack the zeolite framework, leading to the destruction
of the zeolite and deactivation of the catalyst [26]. Lastly, iron accumulates on the surface of the catalyst, which can potentially
block the surface pores. This is especially detrimental if the surface
porosity of the catalyst is not optimized. Pore blockage results in
lower conversion and higher slurry yields.
5.2. Mitigating the effects of contaminant metals
Contaminant metals contribute to dehydrogenation reactions
and can hinder operation in an FCC unit. The contaminant metals
described here include Ni, V, Fe, and Na. The total dehydrogenation

49

activity of contaminant metals in an FCC can be expressed in terms

of equivalent Ni through the formula: Ni ỵ V/4 ỵ Fe/10 ỵ 5Cu e 4/3
Sb. This equation suggests that both V and Fe have lower dehydrogenation activity compared to Ni, while Cu has higher dehydrogenation activity. Mitigation can be accomplished by
introducing a crystalline alumina into the matrix of the catalyst,
which traps and passivates Ni, effectively lowering its dehydrogenation activity. Electron microscopy studies suggest that V is highly
mobile and evenly distributed within the catalyst particle, while Ni
mainly deposits on the outer edges of the catalyst [31]. Vanadium
passivators can be incorporated into the catalyst either directly to
the catalyst particle or as a separate particle in the form of an additive. These specialized materials effectively passivate V, mitigating the harmful interaction between V and the active zeolite
acid site. Furthermore, the Na content in the fresh catalyst can be
reduced during the manufacturing process via consecutive calcination and ion exchange steps, which allow for improved resistance
of zeolite deactivation. Sodium below 0.2 wt% is achievable by
employing these steps in the in situ manufacturing process.
Catalyst designed with high zeolite surface area or small zeolite
crystal size protect against V poisoning. An increased amount of
zeolite and overall accessibility to the zeolite due to smaller zeolite
crystals provides a larger number of accessible active centers
available for catalysis. In this context, more V would be required to
deactivate the increased number of zeolite active sites. Furthermore, a decrease in the zeolite crystallite size can also lead to other
benefits via diffusion improvements. This strategy requires an
optimized Si to Al ratio for the catalyst to remain stable in the
presence of resid feeds under the high temperatures and steam
environment in the regenerator [32].
When the Ni content on equilibrium catalyst (Ecat) is above
1000 ppm, Sb, a well-known Ni passivator, can be injected into the
feed to add additional Ni passivation. The efficiency of Sb as a Ni
passivator can be monitored by looking at the absorber off-gas H2/
methane ratio. It is theorized that Sb forms an alloy with Ni,
effectively passivating Ni [33].
A new approach to contaminant metal passivation, boron-based
technology (BBT), has recently been developed [34]. This new

technology exploits the mobility of boron under FCC conditions and
the selective interaction with un-passivated Ni. As a result, catalysts
employing this technology are capable of achieving a much higher
degree of Ni passivation than earlier technologies. Significant reductions in hydrogen and delta coke and improvements in yield
selectivity with BBT have been successfully demonstrated in refinery operations [34].
Iron can be found in the fresh catalyst as well as in the feed.
Added Fe is calculated by subtracting fresh Fe content from Ecat Fe.
(i.e. Added Fe ¼ Ecat Fe - fresh Fe). Fe can affect the FCC catalyst
both chemically and physically. Chemically, Fe can act as a dehydrogenation catalyst, a CO promoter, and an inverse SOX additive,
with the latter resulting in increased SOX emissions. Fe can physically impact the catalyst by forming nodules on the surface, not
allowing the particles to pack as tightly, as well as causing vitrification, or the formation of a glassy surface on the catalyst. The latter
effects can lead to a loss in conversion via blockage of surface pores
in extreme cases [35]. To combat the effects of added Fe, catalysts
with optimized surface porosity should be employed.
5.3. Surface porosity and its role with respect to metals
Two industrial process to manufacture FCC catalyst exist: the
“incorporated” and “in situ” routes. In the incorporated
manufacturing process, zeolite and matrix are fabricated separately. These two components are then spray dried together with
fillers and binder to form the catalyst. The binder amount can vary,


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M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

but a minimum amount is needed to sustain the physical structure
of the catalyst. The binder becomes important when processing
resid feeds. Since the process involves spray drying, the binder has
a tendency to migrate towards the surface of the catalyst during the
evaporation process. This process can result in a densification at the

catalyst surface, in which the edge of the catalyst is more rich in
alumina or silica. This phenomenon can lead to decreased surface
porosity, which limits diffusion of feed molecules into the catalyst
particle.
The in situ manufacturing process allows for additional flexibility. First, a catalyst microsphere is manufactured from kaolin clay
and functional matrix raw materials. Then, zeolite is grown within
the microsphere. Nutrients for zeolite growth are provided by the
microsphere itself, giving rise to the term in situ. Because the zeolite
is directly grown on the microsphere, an epitaxial layer is formed,
eliminating the need for a separate binder. By using the in situ
manufacturing process, higher levels of zeolite can be included in
the catalyst. Also, this manufacturing process leads to a more open
pore architecture, allowing for improved metals tolerance.
The phenomenon of surface densification can be visualized via
SEM EDS mapping, in which alumina or silica is color mapped. In
Fig. 3, the in-situ catalyst maintains a homogeneous density of
alumina throughout the catalyst particle. However, the incorporated route (both alumina sol or silica sol catalysts), a densification
of alumina or silica can be seen at the edge of the catalyst. This
suggests that the surface porosity is not optimized as a result of
binder migration.

6. FCC additive technologies
In addition to significant catalyst developments since FCC's
inception, additives have also played a large role in advancing FCC
technology and capabilities. From the 1970's to present, the industry has seen remarkable development of FCC additives that aim
to combat some of the challenges facing the refining industry. Two
categories of additives will be discussed in this contribution:
environmental and performance additives.
Environmental additives aim to meet strict global regulations
and to minimize the emission of pollutants; this category includes

sulfur oxide (SOx) reduction additives, nitrogen oxide (NOx)
reduction additives, and low NOx carbon monoxide (CO) promoter.
Performance additives were designed to meet moving economic
and market trends and aim at shifting the yield slate off the FCC
unit; this category includes olefins maximization additives, octane
enhancement additives, CO promoters, gasoline sulfur reduction
additives, and a variety of others including bottoms cracking,
metals traps, fluidization aids, and co-catalysts. The largest additive

segments globally include SOx, ZSM-5, and gasoline sulfur reduction additives and will be further discussed in this contribution in
addition to CO promoter and NOx reduction additives.

6.1. SOx reduction additives
SOx reduction additives, which were first developed in the
1980's, aim to mitigate the emission of SOx from the FCC unit. SOx
is a mixture of SO2 and SO3 and is a dangerous atmospheric
pollutant, causing acid rain formation and contributes to the
destruction of the ozone layer. Among the sources of SOx emissions,
power plants contribute ca. 65% of the total emissions, followed by
petroleum refinery processes, and more specifically the FCC, which
contributes ca. 7% [36].
In FCC units, most of the feed sulfur is either converted to H2S or
retained in the liquid stream (gasoline, bottoms, and the diesel
range). A detailed breakdown of sulfur distribution as a function of
feed hydrotreating in an FCC is shown in Table 1. Five to 30% of the
sulfur is in coke formed on the catalyst, depending on whether the
feed is hydrotreated or not. During coke burn-off, all sulfur in the
coke is converted to SOx. The S that ends up in coke and slurry is
primarily thiophenes. Therefore, feed thiophenes or slurry sulfur
are good predictors of uncontrolled SOx emissions [37].

Two strategies exist to reduce SOx emissions: hardware improvements and the use of SOx reduction additives. For hardware
solutions, a scrubber can remove SOx from flue gas. This solution
involves high capital costs, depending on type, size, and the need
for supporting infrastructure. SOx reduction additives provide an
alternative approach to control SOx without the heavy up-front
capital investment.
A typical SOx additive contains multiple active ingredients that
can include magnesium oxide, cerium oxide, and vanadium pentoxide. The relative amounts of the key ingredients are optimized for
the best performance. A potential SOx removal mechanism is
illustrated in Fig. 4. During coke combustion in the regenerator,
sulfur is oxidized to form SO2. In the first step, SO2 is oxidized to
SO3, which then reacts with the magnesium oxide in the additive to
form magnesium sulfate. When the additive reaches the riser, the

Table 1
Distribution of FCC feed sulfur in different FCC products.

H2S
Gasoline
Bottoms ỵ LCO
Coke

Non-hydrotreated feed

Hydrotreated feed

40e50%
5e15%
40e50%
5e10%


20e50%
2e10%
30e50%
15e30%

Fig. 3. Elemental mapping images collected via SEM-EDX (Scanning Electron Microscopy e Energy Dispersive X-Ray Spectroscopy) depicting the surface densification in incorporated catalysts and absence thereof in in situ catalysts.


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

51

Fig. 4. Possible reaction pathways for SOx reduction in the presence of SOx reduction additives.

magnesium sulfate is reduced by hydrogen and other reducing
gases to regenerate the magnesium oxide and/or produce a magnesium sulfide [38]. In the stripper, the magnesium sulfide is hydrolyzed in the presence of steam and vanadium pentoxide to form
the magnesium oxide.
An effective SOx additive oxidizes SO2 to SO3, forms stable
magnesium oxide, and effectively releases sulfate species to
regenerate the activity of the additive. Two compounds are
commonly used commercially in SOx additives: spinel [39] and
hydrotalcite [40]. A hydrotalcite is a layered double hydroxide and
can be created by replacing some divalent cations with trivalent
cations forming a positively charged layered array. Anions, located
in the interlayer region, compensate for the positive charges.
MgAl2O4 spinels have also been studied and show effective commercial application.
Pick-up factor (PUF), shown in Equation (1), is a unitless number
and is indicative of the efficacy of a SOx additive. Average PUFs
range from 15 to 40. PUF for a typical partial burn FCC unit is

approximately half of full-burn units. When partial burn is very
deep, the PUF factor can fall as low as 3e4. The PUF is influenced by
the SOx additive, SOx partial pressure, catalyst circulation rate,
regenerator temperature, stripper efficiency, and excess O2.
Equation (1): SOx additive PUF calculation

Pick À up Factor ¼

SOx CapturedðlbsÞ
Additive UsedðlbsÞ

(1)

Another important property for SOx additives is the attrition
resistance after multiple cycles of absorption and desorption.
During the FCC process, magnesium oxide in the additive interacts
with sulfur containing species to form magnesium sulfate. This
chemical interaction imparts physical stress on the additive particle. In subsequent steps, magnesium oxide is regenerated,
continuing the stress profile. Repetitive cycles as such can cause
additive particle fracture and degrade the attrition resistance over
time. As demonstrated in Fig. 5, SOx additive attrition resistance
deteriorates over laboratory simulated regeneration cycles. Excessive degradation can cause severe loss of SOx additive from the
circulating catalyst inventory and reduced SOx control performance. Therefore, the initial attrition loss rate as well as the loss
rate after many cycles needs to be considered.

6.2. ZSM-5 additives
ZSM-5 was first developed by Mobil in the 1960's and its utilization has been explored for various purposes in the FCC process.

ATTRITION AS A FUNCTION OF
REGENERATION CYCLES

AIR JET ATTRITION RATE

SOx Additive A

SOx Additive B

4
3
2
1
0
0

5

10

15
NUMBER OF CYCLES

20

25

30

Fig. 5. Increase in the air-jet attrition rate (AJAR) with increasing SOx regeneration cycles as a function of SOx additive.


52


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

Fig. 6. Pathway for the formation of C3 and C4 in the presence of Zeolites Y and ZSM-5.

Propylene Yield as a Function of ZSM-5
Additive
Propylene Yield (wt%)

12
10
8

90% of incremental propylene
is from the first 20% of ZSM-5

6
4
2
0
0

10

20

30

40


50

60

ZSM-5 Additive (wt%)
Fig. 7. Effect of a ZSM-5 additive on propylene yield based on FCC catalytic lab testing at 70% conversion.

This type of additive falls under the performance category and aims
at improving yields off the FCC (e.g., increase propylene make and/
or improve gasoline octane). Mobil investigated the synergism
between ZSM-5 zeolite and zeolite-Y and found no benefit to having
the two in the same particle [41]. For this reason, common industry
practice is to use ZSM-5 as a true additive.
Utilization of ZSM-5 additives in FCC results in higher gasoline
octane ratings and increased C3 and C4 olefin yields. The pathway
for the formation of C3 and C4 molecules in the presence of ZSM-5 is
illustrated in Fig. 6. Large hydrocarbon feed molecules are first
cracked by the FCC catalyst resulting in gasoline-range molecules.
The gasoline-range olefins are then cracked by the ZSM-5 additive
to form C3 and C4 molecules. However, with increasing ZSM-5 additive content, the propylene yield reaches a plateau; thus, there
exists a diminishing return to the ZSM-5 additive as shown in Fig. 7.
The plateau is typically attributed to the depletion of gasoline range
olefins. Further gasoline olefin analysis reveals that while C7eC9
olefins are mostly upgraded by ZSM-5, residual C6 olefins remain
(Fig. 8).
To further improve the performance of the ZSM-5 additive,
pathways to stabilize the zeolite have been investigated in the industry. Phosphorous proves to be a good stabilizing agent for ZSM5 zeolite by retarding aluminum from leaving the zeolite framework and by producing a zeolite that retains a larger fraction of its
acidity [41]. The increased effectiveness of phosphorus-modified

ZSM-5 translates into reduced additive usage and reduced cost

for refiners. In addition to the improvement in stability, phosphorus
can generate Brønsted acid sites that are active in propylene formation and improve the binding of the ZSM-5 additive particles
and its mechanical integrity. Traditional ZSM-5 additive is produced using a single phosphorus treatment. Recently, a multi-stage
phosphorus treatment method was developed to further improve
zeolite activity [42], leading to a more effective ZSM-5 additive.
6.3. Gasoline sulfur reducing catalysts/additives
Environmental Tier III regulations come into effect in
2017e2020 in the United States and mandate a 10 ppm gasoline
sulfur limit. Since 90% of gasoline sulfur comes from FCC gasoline,
much effort has been devoted to reduce the sulfur level of gasoline
produced from FCC units.
Different approaches have been used to reduce the gasoline
sulfur content. Hydrotreatment of FCC gasoline is an effective way
to reduce the sulfur content; however, this approach leads to a loss
of octane [32,43]. Pre-treatment of FCC feed to reduce sulfur content is another effective approach. Both methods employ high
capital costs. Modifications to FCC operating parameters such as cut
point adjustment between gasoline and LCO have also been
employed; however, this approach can lead to deterioration of
yields and product quality [44]. FCC gasoline sulfur reduction


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

53

Fig. 8. Effect of a typical ZSM-5 additive at 3 loadings on gasoline olefin distribution with the majority of C7eC9 being cracked into smaller molecules, still leaving uncracked C6
olefins in the gasoline.

catalyst solutions and additives have also been used by refiners to
reduce FCC gasoline sulfur without an added capital cost. Up to 40%

reduction in gasoline sulfur has been observed by refineries utilizing this technology [45]. FCC catalyst solutions and additives
have multiple advantages including a) octane preservation through
by-passing a portion of light gasoline in the post-hydrotreater, b)
continued operation during turnaround of pre-treatment or posttreatment hydroteating units, c) improved flexibility of crude selection, d) reducing severity of existing hydrotreating equipment
thus extending the hydrotreating catalyst life, and e) reducing
hydrogen consumption in the hydrotreating process. This last
benefit is especially relevant in periods of hydrogen outages.
A sulfur speciation method was employed to understand sulfur
reduction chemistry and to support the development of sulfur
reduction additive technology [46]. An example from a speciation
study for FCC gasoline sulfur compounds is shown in Fig. 9. Reactive
compounds such as mercaptans, sulfides, and disulfides are cracked
to H2S, whereas refractory compounds such as thiophenes, alkyl
thiophenes, and benzothiophenes remain in FCC gasoline. Compounds like dibenzothiophenes (DBT) and substituted DBT's
remain in the LCO and bottoms fractions.
Various technologies available in the field have the ability to
crack S-containing species into coke (with S leaving the unit ultimately as SOx) or into light gases (i.e. H2S) [47]. Catalyst-based
solutions aimed at maximizing conversion and minimizing gasoline sulfur have demonstrated a significant reduction in gasoline
sulfur across the gasoline boiling point range in refinery applications [45]. This technology employs a zeolite Y based substrate with
reactants aimed at sulfur cracking incorporated into the material.

6.4. CO promoter
During coke combustion in the FCC regenerator, both CO and
CO2 are present. CO oxidation in the flue gas line or upper section of
the regenerator, where the catalyst density is low, can lead to
localized high temperatures, which can easily cause mechanical
damage in addition to severely deactivating the FCC catalyst. The
extent to which this occurs depends on regenerator design, air and

spent catalyst distribution, dense bed temperature, and excess

oxygen content [48]. The minimization of such CO oxidation in low
catalyst density environments is desired.
To facilitate the oxidation of CO in the dense phase of the
regenerator, CO combustion promoters can be used, which assist
the reaction of CO to CO2 in the presence of oxygen. The additive
consists of a support loaded with highly dispersed active components. Platinum is the most commonly used active component in
CO combustion promoters, however the promotion reaction can
also be catalyzed by a wide variety of noble and base metals. The
use of CO promoter allows for more efficient oxygen utilization in
the dense phase of the regenerator, thus reducing the availability of
CO in the dilute phase.

6.5. NOx reduction additives
NOx has been identified as the primary cause for formation of
ground level ozone (smog), which forms when NOx reacts with
volatile organic compounds in the presence of heat and sunlight. As
a result, emissions of NOx are highly regulated. In order to meet
government imposed regulations, NOx additives were introduced
to the market in the early 2000's. NOx in the regenerator is formed
via two mechanisms with one being more dominant. Thermal NOx
is produced from the reaction of molecular nitrogen with oxygen
and is not a significant contributor to total NOx in a FCCU regenerator [49]. The second mechanism, which produces fuel NOx, results from the oxidation of nitrogen-containing coke species. The
latter mechanism has been shown to predominantly contribute to
the NOx emissions from an FCC unit.
Hardware options, such as selective non-catalytic reduction,
selective catalytic reduction (SCR), and ozone injection in scrubbers
(e.g. LoTOx), have been used to control NOx emissions. Alternatively, additive solutions have been developed to reduce NOx
emissions. To reduce NOx emissions from an FCCU regenerator, two
approaches can be taken. The first approach prevents the formation
of NOx, while the second converts NOx into nitrogen. NOx formation involves a series of reactions, as shown in Fig. 10. A test method

can be used to study the reaction pathways during nitrogen-


54

M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

Fig. 9. Typical contribution of sulfur compounds to gasoline sulfur and their respective conversion abilities in the FCC.

containing coke combustion (i.e. Lambda Sweep Method) [50]. This
study demonstrates that:

7. Testing and catalyst evaluation techniques
7.1. Testing insights

 The presence of metal oxides (MOx), such as cerium oxide, in the
additive lowers NOx selectivity for NH3 oxidation. MOx compounds also enhance selective catalytic reduction (SCR) of NOx
with NH3.
 CuO reduces NOx by improving HCN oxidation selectively to N2.
 The often-proposed NOeCO and NO-coke pathways are not
observed.
NOx reduction additives and low NOx CO promoters have
shown commercial success. An effective commercial example is
illustrated in Fig. 11, reducing NOx emissions at equivalent excess
O2 levels.

Laboratory testing for catalyst evaluation has been widely used
to understand the FCC cracking mechanism to support the research
and development of new catalysts and how compositional changes
affect catalyst performance (e.g. REO, zeolite-to-matrix ratio, effect

of additives, etc.). Testing aims to closely mimic the commercial
performance of the catalyst under a wide range of operational
conditions using varying quality feeds. Lastly, testing of aliquots of
circulating catalyst inventory, or Ecat, allows for the continual
monitoring of catalyst performance.

Fig. 10. Reaction pathways during nitrogen-containing coke combustion.


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

55

Fig. 11. Commercial reduction of NOx emissions by CLEANOx additive.

7.2. Deactivation
Realistically mimicking the Ecat properties via accelerated
deactivation in the laboratory is critical for the development of
catalyst technologies as well as the selection of the optimal catalyst
for each FCC unit [51].
A proper deactivation protocol should be adopted to mimic the
transformations that occur in the zeolite due to elevated temperatures in the presence of steam [52], including the effect of
contaminant metals [53], which cause the undesirable formation of
hydrogen and coke [26]. Metals effects have been widely recognized in literature [26,53e55] as discussed previously. Furthermore, catalyst inventory (and thus Ecat) contains particles of
different ages. The freshly added catalyst particles have undergone
the fewest cycles of reaction, stripping, and regeneration. They have
the highest surface area and activity as well as lowest levels of
contaminant metals. On the other hand, the oldest particles have
gone through many cycles of reaction, stripping, and regeneration.
Therefore, they typically also have the lowest surface area and activity as well as the highest contaminant metal levels.

Two approaches to deactivation are often used in laboratory
analyses. For low metal FCC operations, steam deactivation without
the addition of contaminant metals is employed. For high metal
operations, contaminant metals (usually V and Ni) are deposited
onto the catalyst particles using the Mitchell impregnation method
for adding metals and deactivating via cyclic propylene steaming
(CPS) [54]. Also, cyclic metals deactivation (CMDU) [55] can be used
with the contaminant metals depositing on the catalyst during the
cracking step.
During steam deactivation, fresh catalyst is hydrothermally
treated at high temperatures in the absence of metals causing
zeolite dealumination and acid site loss. The steam temperature
and duration are controlled to match the target Ecat's UCS, activity,
and to some extent, selectivity. Hydrothermal deactivation leads to
permanent loss of catalytic activity due to zeolite dealumination
and zeolitic surface area reduction. To incorporate age distribution
into deactivation and better mimic the Ecat selectivity, a continuous age distribution model has been developed [56].
Deactivation of FCC catalysts in small-scale units in the presence
of contaminant metals has been the industry workhorse for a

number of years due to the robustness and simplicity of such
methods. The first approach for metalation and deactivation of
fresh FCC catalysts was the Mitchell method [54]. The Mitchell
method consists of incipient wetness impregnation of FCC catalysts
with V and Ni naphthenate or octoate solutions. This is followed by
a subsequent deactivation performed in small-scale laboratory or
pilot units by exposing the metallated FCC catalysts to hydrothermal conditions. The CPS method, which is based on the Mitchell
technique of adding metals, was introduced with the objective of
better simulating, on a small or pilot scale, the performance of
commercial FCC catalysts compared to Mitchell technique followed

by steaming. In the CPS method, the FCC catalyst is impregnated by
the Mitchell method with V and Ni species prior to deactivation in
reductioneoxidation cycles using propylene as the reducing medium [54].
The deactivation procedures for the metal-loaded catalyst in the
CPS protocol have been modified continuously over the years
[57e60], while the Mitchell type impregnation had not been
modified substantially except some recent efforts [60]. In traditional Mitchell-type methods, the FCC catalysts are artificially
metallated with V and Ni naphthenates that are dissolved in an
organic solvent. Most literature studies conclude that catalyst
metallated by the Mitchell method and deactivated by CPS or
steaming led to the following visible discrepancies with the Ecat
[55,61e63]: a) improper age distribution, b) misleading distribution of metals (mainly Ni), and c) inaccurate activity of the metals
(both Ni and V) compared to Ecat (and the CMDU method,
described below).
Vincz et al. have recently studied the metals' deposition in both
FCC operation and laboratory simulation [64]. By employing spot
analyses of a statistically meaningful set of Ecat samples, the variation in Ni deposition profiles was studied. The peripheral deposition
index (PDI) was defined to quantify the variation observed in
deposition profiles. Ni deposits toward the periphery of the catalyst
particle in an FCCU, evidenced by PDI values greater than 1 and
ranging as high as 5. Vincz et al. also developed a novel Ni source. By
employing the new Ni-containing molecule for FCC impregnation,
the importance of the source molecule in determining the final
metal distribution profile during laboratory simulation of Ni deposition was highlighted. By using this molecule after impregnation, Ni


56

M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58


was deposited mostly on the periphery of the particle as in the case
of Ecat vs using a Ni-naphthenate source. The development of the
PDI as a meaningful way to quantify the contaminant metals'
deposition profiles coupled with novel metal sources for impregnation represent significant new methodologies for better simulating FCC Ecat. It also leads to an independent understanding of the
impacts of metals under more realistic conditions.
In another recent study, Wu et al. studied the effect of the solvent used during impregnation [65]. They observed that by
changing the solvent, the deposition profile can change significantly from a PDI value of 1e4.3. This reflects a fourfold increase in
the amount of Ni deposited toward the periphery of the catalyst
particles. The endpoints of this range of PDI values have been
observed in Ecat as well as intermediate values. It is believed that
using mixtures of solvents or sequential treatments with different
solvents can achieve such intermediate PDI values. The use of
different solvents to alter the deposition profile of Ni also impacts
the catalytic activity, with increases in hydrogen and coke as well as
improved accessibility of the catalyst with lower PDI values. These
findings can be incorporated into existing methods of catalyst
deactivation to give more realistic portrayals of Ecat and to better
evaluate catalysts designed for resid-FCC applications.
In order to improve the simulation of commercial deactivation
in lab testing, cyclic metals deactivation unit (CMDU) was designed.
In this method, the aging of FCC catalyst is simulated by deactivation in a fixed fluidized bed through repeated cycles of reaction,
stripping, and regeneration. During the cracking step, metals are
deposited on the catalyst with a metal-spiked FCC feedstock. Volatile hydrocarbons are then stripped from the catalyst with nitrogen and/or steam and the temperature is raised to the regeneration
temperature. The catalyst is regenerated with a gas mixture of
steam, including oxygen and nitrogen and optionally other gases
(e.g. SOx). The CO combustion mode (full or partial) can be
controlled with the gas flow and composition. After regeneration,
the catalyst temperature is reduced for a new cycle of reaction,
stripping, and regeneration. Fresh catalyst addition and/or withdrawal can be employed during the run in order to simulate the age
distribution of Ecat. Also, this can be done in order to collect

catalyst samples for the determination of physical and chemical
properties as well as catalytic performance. The metal tolerance can
thus be determined easily upon progressive metal buildup and
(hydro) thermal deactivation [66,67].
The key differences between CMDU and Mitchell/CPS deactivation methods include:
As the metal deposition process in CMDU is more representative
of the deposition process in the commercial FCC units, CMDU better
mimics the metal distribution of Ecat samples [51,54,65e68]. If the
CPS method is employed, the metal loading should be ca. 1/3 or 1/4
that of Ecat. This method ensures that the activity of the impregnated metals are not exaggerated, as has been seen when 100% Ecat
metals are loaded onto a deactivated sample.

CMDU

Mitchell/CPS

Metal deposition during the
cracking step at the riser
temperature by using a metalspiked feedstock
Improved metals deposition profile
(closer to actual distribution of
metals in refinery)
Low-temperature deactivation
during stripping followed by
high-temperature deactivation
during regeneration

Metal impregnation by using a metalspiked solution at low temperatures

Uniform metals distribution


Constant high-temperature
deactivation

7.3. Cracking evaluation
Several test methods for the evaluation of the performance of
FCC catalysts are in use: Micro Activity Test (MAT), Short Contact
Time Resid Test (SCT-RT), Advanced Cracking Evaluation (ACE), and
Circulating Riser Unit (CRU).
The MAT is a small scale fixed bed unit, which was widely used
in the past to characterize performance of FCC catalysts due to its
relative simplicity and low cost. Some drawbacks include [69]:
a) Long catalyst residence times in the reactor, which are far
from commercial operations
b) Development of non-homogeneous temperature and coke
profiles along the fixed catalytic bed
c) Collection of products during a certain time period that gives
average conversion and product distribution with high contributions of early phase cracking
d) Possibility of further reaction occurring during the stripping
step
ACE was introduced in 1997 as a fixed-fluid bed laboratory
cracking system [70]. ACE units have since rapidly taken over as the
industry standard for bench scale FCC testing. Compared to fixed
bed MAT units, ACE offers a number of advantages:
a)
b)
c)
d)
e)
f)

g)

Improved accuracy and repeatability
More automation e less operator-induced variability
Heavier feed capability
Better heat transfer/lower temperature drop
Better feed/catalyst contacting
Uniform coke depositioneno bed gradients
Improved stripping in fluid bed

The ACE maintains the small scale and high workload as well as
the practicality benefits of the MAT relative to larger pilot-scale test
units such as the CRU, described below. Most FCC catalyst manufacturers and research laboratories have implemented ACE technology into all aspects of FCC testing.
The SCT-RT is a small-scale fluidized bed unit in which the
catalytic performance of fluid cracking catalysts is measured [71].
Realistic simulation of commercial operation is achieved by the
unique reactor and unit design. The SCT-RT simulates the adiabatic
behavior of a commercial riser. Different from the traditional smallscale tests like MAT and the related fluid bed performance tests,
preheated oil (typically 85  C) is injected into a fluidized bed of hot
catalyst particles, at the typical catalyst return temperature of
approximately 700  C. The special reactor design provides optimal
mixing and cracking at very short and realistic contact times. This is
followed by fast disengagement of the cracked products and
stripping of the catalyst. The oil time on stream is only 1 s and
provides performance measurements at realistic cat-to-oil ratios
and vapor contact times.
Most FCC catalyst testing laboratories and catalyst manufacturers have designed, built, and operated CRUs to evaluate catalyst
performance in conditions similar to commercial operations [72].
Since CRUs are designed as miniature FCC units, their operation
most closely mimics commercial operation. The data obtained are

generally considered to be more predictive of commercial units
compared to similar data generated from small scale units (e.g.
MAT, ACE, SCT-RT).
8. Conclusions
The FCC process was first commercialized more than 70 years
ago. It quickly became the main conversion process in refining that


M. Clough et al. / Microporous and Mesoporous Materials 254 (2017) 45e58

produces the majority of gasoline we use today. However, the
technology continues to evolve to adapt to changing demands of
the refining industry. New challenges such as processing opportunistic feedstocks, maximizing light olefins production, and more
stringent environmental regulations have also pushed refiners,
catalyst manufacturers and technology providers to overcome the
existing limitations with innovative solutions.
Although the FCC is a mature process, the process design is a
flexible platform that enables changes without major overhauls.
With the rise of the new FCC hardware technologies, the development of FCC will be very interesting for the next coming century.
In addition to hardware improvements, the industry continues
to see advances in catalyst and additive development. From natural
clay to silica-alumina based materials, today's FCC catalysts' success
is enabled by zeolites. These catalysts have excellent pore structures for diffusivity of feedstocks and intermediate products and
strong acid sites for high overall conversion. In addition to cracking
feedstock, new generation catalysts have added functionalities such
as metal passivation to mitigate the effects of V and Ni from feed.
Such catalysts enable the FCC to process heavier and higher metalcontent feedstocks without pre-treating for commercially attractive conversions to valuable liquid hydrocarbons [73].
Although the past 70 years have demonstrated the agile nature
of both equipment licensors and FCC catalyst manufacturers, today's refiners demand more. As alluded in the text above, the field
of zeolite-based olefin maximization additives still has room for

improvement e namely in the ability to crack even more C6 olefins
into lighter olefins. Furthermore, the industry demands specific
selectivity; at the time, in certain parts of the world, butylenes are
more valued than propylene (and vice versa). Technology demands
include selective catalysts and additives that can achieve both. In
addition, feedstock challenges continue to demand technology
advancements. With the volatility in global crude oil prices, refiners
are processing different feeds than before; for instance, the
marketplace is seeing a surge in sulfur-rich crudes, which exacerbate the need for SOx emissions and gasoline sulfur mitigating
additives. Heavier feeds push the need for improved coke selectivity catalysts and the need to mitigate incoming contaminant
metals. Furthermore, exploration of biofuel upgrading via zeolitebased catalysis is also underway [74], including co-processing of
biofuels and conventional petroleum-based feedstocks [75]. For
both licensors and catalyst manufacturers, this emphasizes the
need to continue with intensive research and development efforts
to meet these technology needs.
FCC is an exciting technology and the development in both the
process and the catalyst enables a continuous adaptation to meet
the energy and chemical needs of the future. This evolution will
continue with new catalyst and process developments, which will
in turn open up new possibilities to meet the needs of the future.
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