Bart De Graaf, Johnson Matthey Process Technologies, USA, and Paul Diddams,
Johnson Matthey Process Technologies, Europe, review the journey and influence of the fluid
catalytic cracking unit in a refinery’s operations, and how its functions will develop into the future.
T
he worldwide refining landscape is far from a static
picture. Local economies have a tremendous impact on
shrinking or expanding refining capacity. Where in
Europe many refineries are fighting for their life, refining
capacity is declining and further consolidation is expected, the
situation in India is very different with a number of major
expansions currently underway.1 In China, new regulations and oil
import taxation for teapot refineries is part of a refinery
restructuring that leads to capacity concentration in big, ‘state of
the art’ refinery complexes, as currently refinery utilisation rate is
below 70%. In the US, the number of refineries has declined, but
over the past few years refining capacity has managed to increased
by 10%.
For many years the trends in fluid catalytic cracking (FCC) feed
were clear and a given: feeds will continuously become heavier,
more sour and contain higher levels of contaminant metals. The
recent tight oil (or shale oil) revolution broke with this trend, and
there are still many more vacuum gas oil (VGO) units than residual
units being built worldwide.
Challenges and opportunities: tight oil
revolution
In November 2014, Saudi Arabia announced a change in their
pricing strategy for oil.2 The decision was made to compete for
market share, rather than to support high oil prices – the move
clearly targeting shale oil production in North America.
Plummeting crude oil prices forced a revolution in tight oil
production technology, and while rig count dropped by 27%,
production per rig went up and overall US oil production
decreased by 9%.3, 4 Since then, for most of the world, Iran has
regained acceptance as an oil producer, due to changes in the
political climate, and is ramping up its oil production. Even though
Saudi Arabia has now engaged in discussions to curb OPEC output
in an effort to raise crude prices, the expectations of most experts
are low that this will significantly change oil prices in the near
future.5 All these moves have led to substantial changes in crude
oil prices over the past two years. Whereas in 2014, oil prices
peaked above US$100/bbl, early 2016 prices had plummeted by
70% (to about US$30/bbl), recovering through the summer
months to around US$50/bbl.
Though tight oil offers many benefits to US refiners by
offering access to relatively cheaply priced sweet crudes, tight
oils are not an automatic ‘hand in glove’ fit for many refineries.6
Tight oils typically contain high levels of light ends, some having
gravities of 55 API or higher, and composition is often highly
variable, even in the same tight oil formation. The high amount
of naphtha and low amounts of VGO and vacuum residue in
tight oils can derail crude distillation substantially, as crude units
are designed for heavier crudes. Pre-flash towers, tower upgrades
and optimisation of unit heat integration networks is required.
Many tight oils contain waxes, which can form deposits that foul
storage tanks and process units. Also, the blending of paraffinic
tight oil crudes with asphaltene rich crudes can result in the
asphaltenes precipitating out, increasing fouling in process
equipment such as heat exchangers. Tight oils contain low
concentrations of sulfur and nitrogen contaminants, so minimal
pre-treatment of feed or post-treatment of product is required.
The type of metal contaminants in tight oils differs
significantly compared to conventional crudes; iron, calcium, and
sometimes potassium, can be present in high concentrations, as
well as the more usual nickel and vanadium contaminants. Iron
and calcium especially have been proven to be ‘difficult to
remove’ contaminants from the feed, with substantial amounts
still present after treatment in de-salters. These metals
concentrate up in the FCC feed (VGO) and affect FCC unit
operation and impact upon product selectivities. This requires a
reformulation of the FCCU catalyst system (base catalyst and
additives).
Catalyst reformulation for FCC tight oils
Tight oils are very paraffinic, meaning coke yield is very low. This
can cause problems in the FCC heat balance where regenerator
temperatures can drop to lower than comfortable values. This can
be mitigated to some extent by increasing catalyst activity to
increase delta coke, or by increasing catalyst addition rates,
reformulation of the catalyst or a combination of both. When
active matrix, zeolite, and/or rare earths contents are increased,
catalyst activity is increased. Elevating regenerator bed
temperature by use of torch oil or reducing stripping steam rates
are also measures of last resort, as they will inherently force
increased catalyst deactivation rates, with inherent loss of product
selectivities. Co-processing heavier feeds can help as well, but
compatibility of the feeds has to be ascertained to prevent
precipitation of asphaltenes.
Additional catalyst modifications can be required to deal with
the adverse effects caused by contaminant metals. This typically
involves the use of more alumina type matrices, with more
mesopores, lower zeolite content, and lower sodium content on
catalyst. Specialised separate particle metal trapping additives have
successfully been used to mitigate iron poisoning effects.
Challenges: control of flue gas
environmental emissions
Environmental legislation is tightening worldwide. Where SOX and
NOX limits in the US are amongst the most stringent in the world,
many other countries have started to follow suit. In the US, SOX
limits specified in consent decrees are typically 25 ppm, and even
lower in California. Canada is considering implementing regulation
to further limit SOX emissions. Europe is slowly working to
implement stricter legislation for flue gas emissions, but with more
latitude than in the US: 100 - 800 mg/Nm3 monthly average for
existing full burn units, 100 - 1200 mg/Nm3 monthly average for
partial burn units. These new limits are expected to come in force
from October 2018, but there is some leeway as frequently bubble
limits apply (and might continue to exist in a compensation
mechanism) instead of point emissions.
Challenges: gasoline standards
An FCC unit’s primary product is gasoline. Regulation of gasoline is
evolving; currently Tier 3 is being phased in in the US. FCC cracked
naphtha is typically, by far, the largest contributor of sulfur in
gasoline. To reduce sulfur in cracked naphtha, either more severe
pretreatment of FCC feed, or post-treatment of FCC gasoline, is
required. This comes at the expense of gasoline octane.
Modification of the base catalyst by optimisation of matrix and
zeolite content, and zeolite unit cell size can help towards
mitigating this octane loss, while various refiners use gasoline sulfur
reduction additives to further minimise octane loss by reducing the
required severity of the pretreatment of feed7 or post-treatment of
product. Use of ZSM-5 additives can also help to boost FCC
gasoline octane. In addition, ZSM-5 additives can increase alkylate
feedstock by increasing the amount of butylenes produced in the
FCC unit. Alkylate is an extremely good gasoline blending stock as
it has high octane and low sulfur. A number of refining companies
are using, or considering using, amylene (C5 olefins), as well as
propylene, to extend the feed slate fed to the alkylation unit –
especially in North America where there is an abundance of
relatively cheap isobutene on the market from shale gases. Though
such extended alkylate octane values and acid consumption are
not as good as for butylenes, this helps to increase overall gasoline
quality on a crude oil consumed basis.
Biofuel quotas impose another burden on US refiners. Refiners
(or importers of gasoline) need to meet biofuel quotas set by the
government, either through blending fuel with ethanol or buying
renewable identification numbers (RINs). Trading in RINs is an
opaque market at best, and substantially adds to costs, especially
for refiners without a retail network. These refiners face tough
competition from imported gasoline from overseas refineries that
do not need to meet the same environmental standards. Imports
into the east coast accounted for 87% of total US gasoline imports
in 2015. One east coast refiner recently made news with having to
lay off up to 100 employees, forced by the need to spend
US$250 million dollar on RINs.8
Corporate Average Fuel Economy (CAFE) standards for gasoline
are only a few years away, targeting a reduction in gasoline
consumption of 2 million bpd in the US.9 The legislative drive for
lower gasoline consumption, with substantially higher octane
requirements, will change the refinery landscape and the role of
January 2017 70 HYDROCARBON
ENGINEERING
the FCC unit in the refinery. The light cracked naphtha will become
the most desirable part of the FCC naphtha as it has the highest
octane and lowest sulfur content. Use of ZSM-5 additives can help
to increase feed to alkylation units. As most of the liquefied
petroleum gas (LPG) will be cracked out of the light naphtha, and
not the medium or heavy naphtha, a careful balance has to be
struck to maximise the gasoline yields over all assets.10
Implementing CAFE standards will affect US refineries in various
areas differently. Currently, gasoline consumption on the east coast
(PADD 1) is 3.2 million bpd (2015 data), where refinery production is
only 550 000 bpd.11 Gulf Coast refineries (PADD 3) export
1.9 million bpd to the east coast (balance from other countries).
Gulf oast refineries produce 4 million bpd of gasoline, compared
with a local consumption of only 1.5 million bpd.
Petrochemical FCC
Strong growth in propylene demand has led to the FCC unit
becoming the second largest source of propylene today
(30 - 35% of worldwide demand); much of this propylene is used to
produce polypropylene. Propylene dehydrogenation and other
on-purpose propylene producing processes are beginning to
emerge, but are still very much minority sources. These processes
are both Capex and Opex intensive, but do yield excellent quality
propylene. High severity FCC units, especially at integrated
refinery/petrochemical complexes are, and will remain, a very
attractive source of propylene for many years to come.
Conclusion
FCC should perhaps be re-named to ‘flexible catalytic cracking’
considering how it has evolved from a simple gasoline machine to
a highly flexible process able to adapt to changes in feedstocks,
product slates and product qualities driven by political and
economic needs. Changes in process design, operation and
availability of new generations of catalysts and additives to
maximise residue processing, tight oil processing, gasoline, octanes,
propylene, alkylation unit feeds, and diesel, while minimising
bottoms, particulate emissions, SOX and NOX emissions and
gasoline sulfur allow this truly versatile process to remain one of
the most profitable and essential processes at the heart of the
modern refinery.
References
1. http://economictimes.indiatimes.com/industry/energy/oil-gas/indiasbiggest-oil-refinery-on-west-coast-to-cost-30-bn/articleshow/53030103.
cms.
2. 'Saudis block OPEC output cut, sending oil price plunging', Reuters,
28 November 2014, http://www.reuters.com/article/us-opec-meetingidUSKCN0JA0O320141128.
3. Baker Hughes North America Rig Count, http://phx.corporate-ir.net/
phoenix.zhtml?c=79687&p=irol-rigcountsoverview.
4. US Energy Information Agency, US Field Production of Crude Oil, https://
www.eia.gov/dnav/pet/hist/LeafHandler.ashx?n=PET&s=MCRFPUS1&f=M.
5. 'Saudi Arabia Says Many Nations Will Join OPEC Output Cuts', Bloomberg,
19 October 2016, http://www.bloomberg.com/news/articles/2016-10-19/
saudi-arabia-says-many-nations-back-opec-move-to-boost-oil-price.
6. LINDSAY, D., GLOVER, B., GRIFFITHS, M., SABITOV, A. and SIOUI, D., AFPM
Annual Meeting, 22 - 24 March 2015, San Antonio, AM-15-15 Adapting to
a Tight Oil World.
7. http://www.ogj.com/articles/print/volume-106/issue-14/processing/studyexamines-production-of-near-zero-sulfur-fcc-gasoline.html.
8. 'Another quarter of weak results looms for US refiners', Reuters,
19 October 2016, http://www.reuters.com/article/usa-results-refinersidUSL1N1CO21C.
9. 'Regulations & Standards: Light-Duty', EPA's Office of Transportation and Air
Quality (OTAQ), https://www3.epa.gov/otaq/climate/regs-light-duty.htm.
10. DE GRAAF, B., ALLAHVERDI, M., EVANS, M. and DIDDAMS, P., The Roles of
RE-USY, Matrix and ZSM-5 on FCC Gasoline Composition, PTQ.
11. https://www.eia.gov/todayinenergy/detail.php?id=27992.
TECHNICAL BULLETIN
CAT-AID FCC additive for high iron feeds
Many shale oil feeds contain high levels of metal
contaminants such as Nickel, Vanadium, Calcium, Iron,
Sodium and Potassium which can limit the refiner’s ability
to process these feeds and significantly impact unit
operation and the profitability of the Fluid Catalytic
Cracking Unit (FCC).
Base catalyst without CAT-AID:
Iron poisoning, in particular, can have an adverse effect on
FCC performance, causing many challenges for refiners
through diminished conversion, increased slurry, coke, dry
gas selectivity and increased regenerator flue gas SOx
emissions.
When the iron content is high enough it reacts with the
silica from the base catalyst and essentially seals off the
catalyst interior by forming a shell-like layer at the catalyst
surface that inhibits hydrocarbon diffusion into and out of
the catalyst particle interior. Often refiners will try to
tackle iron poisoning with increased fresh catalyst
make-up rate or use of added equilibrium catalyst to
dilute the iron by flushing it out of the FCC unit.
Base catalyst with CAT-AID:
Johnson Matthey’s INTERCATJMTM catalyst enhancement
additive, CAT-AIDTM, is an effective metals trap for
vanadium and other contaminants such as iron. CAT-AID
was originally designed to capture vanadium, a
permanent poison that accumulates on the catalyst
where it causes catalyst deactivation and promotes
undesirable dehydrogenation reactions, leading to
increased coke and gas make.
In recent commercial applications CAT-AID has been
found to be able to reverse the effects of iron poisoning of
the FCC catalyst. By breaking down the nodular iron-rich
shell on the surface of the catalyst, CAT-AID opens up
access to the inner core of the catalyst, allowing it to
become available for cracking once more.
This significantly increases the profitability of the FCC
operation by relieving operating constraint, improving
product yields, and reducing fresh and flushing
equilibrium catalyst consumption.
Many refiners who process residue and shale crudes have
turned to Johnson Matthey’s CAT-AID, as an effective way
to mitigate the effects of feed contaminants, and have
realized substantial benefits resulting from improved unit
operation.