Using additives to optimise the fluid catalytic cracking process could

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PETROCHEMICALS
Add it up
Using additives to optimise the fluid catalytic
cracking process could help address shortages of
propylene, say Bart de Graaf, Mehdi Allahverdi,
Charles Radcliffe, and Paul Diddams
T
HE increase in US oil and gas
production resulting from tight shale
reservoirs is changing the face of
the domestic refinery and petrochemical
industry, and the effects are now being felt in
Europe and other parts of the world.
One knock-on effect of this shale revolution
is a shortage in supply of high-value propylene,
and other products heavier than ethylene.
Here we discuss bench-scale experiments
that demonstrate how using additives in fluid
catalytic cracking (FCC) can increase yields of
petrochemicals.
the shale effect
There is an abundance of recoverable shale
oil and shale gas reserves in the US which are
being actively exploited. However, this is not
occurring to the same extent outside the US,
for example in European countries, mainly
because of environmental concerns, mineral
rights legislation and higher population
densities. The addition of new shale resources
has lifted total US gas and oil reserves by
35% and 38% respectively, and 11% and 47%
worldwide, since 2011.1
US-based chemical companies are
experiencing a renaissance as the availability
of shale gas has reduced the costs of both raw
materials and energy.1, 2 The US has boosted
recovery of natural gas liquids (NGL) along
with the extra production of methane, with
production expected to increase by more than
40% between 2011 and 2016 (from 2.2m bbl/d
to 3.1m bbl/d)3. For example, the wet gas from
the Marcellus play in the Appalachian Basin
consists typically of 75% methane, 16% ethane,
5% propane and 1% butane, pentane, hexane
and other gases. While the price of methane
Middle Eastern chemical
companies that have
previously enjoyed low-cost
feedstock and energy are
now facing competition from
US companies.
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alone is insufficient to justify the production of
natural gas; the presence of the NGL makes it
profitable.
It’s this glut of NGL that is revolutionising
the US petrochemicals industry. Ethane
prices have dropped from a peak of nearly
US$1.40/gallon in 2008 to US$0.30/gallon
in 2013.4 The result is that the US market for
steam cracker feed has shifted focus onto
ethane. Ethane is now the dominant feed in
the US and Middle East, while in Europe and
Asia naphtha remains the major feedstock.
Middle Eastern chemical companies that have
previously enjoyed low-cost feedstock and
energy advantage are now facing competition
from US companies on the European market.
European chemical producers meanwhile face
a substantial competitive disadvantage until
the differences in regional prices for crude oil
and natural gas narrow.
propylene demand grows
A major consequence of the shift from
heavier feeds to ethane as the main steam
cracker feedstock is a reduction in products
heavier than ethylene. Since 2005, the yield of
propylene from US crackers reduced by 50%
(similarly benzene and butadiene yields have
dropped by 50% and 30% respectively).3,4 Using
ethane as feedstock produces only minor
amounts of propylene, while naphtha feed
typically gives a propylene-to-ethylene ratio of
between 0.4 and 0.57. The shift from naphtha
in the US removes 1.5m t/y of propylene from
the US market, and an estimated additional
2m t/y is needed to keep up with the growing
demand.
While five new 2.75m t/y propylene units
have been announced in the US, there is a
large deficit in supply in the intervening years
before these units come online in the next five
years, and even after then a demand gap of
750,000m t/y will remain.
Approximately 60% of the worldwide
propylene production currently comes from
steam crackers, 30% from refineries, and
the balance from propylene on-demand
units. In a typical refinery configuration the
majority of the propylene comes from the
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CAREERS
PETROCHEMICALS
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FCC unit (FCCU) with minor contributions from
visbreakers and cokers.5 Only in a refinery with
an FCCU is there sufficient quantity of propylene
to warrant cost effective recovery. Compared to
a steam cracker, an FCCU is extremely flexible
with respect to feedstock and product slate, and
in recent years the role of FCCUs has shifted
from producing gasoline from heavy distillate
(eg vacuum gas oil, or VGO), to producing
petrochemicals, especially propylene, from
residual oils. Advances in the design, operation,
and catalysts have moved propylene yield to new
highs.
improving yields
There are two modes of FCCU operation that can
improve propylene yields. In the first, the main
objectives remain the maximum production
of gasoline and other fuel oils, with propylene
yield adjusted using FCC additives. In the
second mode, the FCCU is operated primarily
for petrochemical feedstock production. Various
process licensors have developed the process for
maximum propylene. Typically these involve high
conversion, and some include secondary risers
to crack gasoline. Operating in this mode has a
substantial effect on the process economics. The
gasoline yield is reduced and is not suitable for
direct fuels blending because of the high benzene,
toluene and xylene (BTX) content, and investment
and utility costs are significantly higher.
A typical FCCU feedstock consists of large
hydrocarbons with carbon number of C20–C40 for
vacuum gas oil, and even higher for residual feeds.
The catalyst provides acid sites to crack these
hydrocarbon molecules. While this is the primary
reaction, other reactions also take place, including
cyclisation, dehydrogenation, isomerisation,
hydrogen transfer and recombination. Although
the majority of these reactions are catalytic, a
small amount of thermal cracking does take place.
Catalytic cracking is by far the most important
reaction pathway to propylene in the FCCU,
and the higher the conversion, the higher the
propylene yield will be. Therefore propylene yield
can be increased by increasing the severity of the
FCC operation and typically a combination of
the following operating changes are used to do
this selectively: increased reactor temperature;
increased catalyst-to-oil ratio; and increased
catalyst activity.
Other operating parameters that can also be
optimised for maximum propylene yield include:
• Lowering hydrocarbon partial pressure. This
shifts the reaction equilibria towards light olefins,
by reducing propylene recombination reactions
to benzene. Lowering operating pressure and
the addition of steam both increase propylene
selectivity.
• Improving feed quality. High hydrogen content
of the feed helps conversion; aromatic cores
cannot be cracked).
• Reducing hydrogen transfer. Hydrogen transfer
improves gasoline stability as it saturates gasoline
june 2014 www.tcetoday.com
41
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PETROCHEMICALS
Figure 1 (Left): Propylene (C3) production is increased (relative to base catalyst alone) when ZSM-5 catalysts are added
Figure 2 (Right): Under the right conditions, C3 additives can increase aromatics in the gasoline pool. Eg 25% additive increases the
gasoline aromatics by a significant 10%.
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12
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Aromatic in gasoline, wt%
13
C3=, wt%
11
10
9
8
7
6
5
4
59
57
55
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49
47
45
55
60
65
70
75
55
60
Conversion, w%
Base
5% C3= additive
25% C3= additive
olefins. A catalyst system with minimum rare
earth helps to maximise propylene yield.
• Reducing contact time. Short contact time
helps to reduce hydrogen transfer.
• Reducing backmixing. One of the new
maximum propylene FCC processes makes
use of a downer (an FCCU that has an
unconventional down flow catlayst and
product flow) to minimise backmixing and
thus reduces hydrogen transfer reactions to
an extent not achievable in conventional FCC
designs.
• Using shape-selective zeolite additives.
ZSM-5 additives selectively crack gasoline
olefins and typically provide the biggest shifts
in propylene yield.
The quality of the feed is crucial in
determining the yield from an FCCU, and
understanding this is essential for maximum
propylene production. Feed characteristics
that improve conversion help increase
propylene production.
FCC feed is a complex mixture of various
hydrocarbon types (aromatics, paraffins
and naphthenes, predominately with low
levels of olefins) and impurities including
large complex coke precursors (asphaltenes,
carbon residues); other hetero-atomic species
(sulphur, nitrogen and oxygen compounds);
and metals (Ni, V, Na, Fe, Ca, Na). These feed
impurity levels increase with boiling point.
At a constant boiling range, feed density is
directly proportional to aromatic carbon, and
higher density feed will be less crackable, and
usually result in lower conversion and higher
coke production.
Asphaltenes, paraffin insoluables, and
carbon residues in the feed can substantially
decrease conversion, by adverse impact on
the heat balance (eg by reducing catalyst
circulation) due to increased ‘coke make’,
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70
75
Conversion, w%
50% C3= additive
Base
5% C3= additive
ie the amount of coke produced per ton of
feed. These molecules consist mainly of
polyaromatic rings that cannot be cracked
under FCC conditions. Dealkylation is the
main mechanism for any conversion of
carbon residue such as Conradson carbon
residue (CCR), but typically 50–75% will
end up as coke. Increasing CCR increases
regenerator temperatures and increases
regenerator air demand, thereby limiting the
amount of heavy feed that can be run to the
FCC unit – which in turn limits propylene
production.
Feed nitrogen can poison the active (acid)
sites in FCC catalysts and additives limiting
conversion and increasing coke. Feed metals
like Ni and V catalyse dehydrogenation
reactions increasing hydrogen and coke
selectivity: the high specific volume
of hydrogen uses much more wet gas
compressor capacity than the desired LPG
products, and coke blocks active sites and
may limit diffusion. Some feed metals (eg Na
and V) can also permanently lead to loss of
FCC catalyst activity by poisoning acid sites
or increasing the rate of zeolite destruction
under the hydrothermal conditions
present in the regenerator. This is clearly
disadvantageous for maximum propylene
operations.
FCC catalysts and additives
Typically an FCCU produces 3–5 wt%
propylene. Higher conversions mean higher
propylene yields (typically, the propylene
yield will increase by 0.2 wt% per wt%
conversion increase).
Adding shape-selective zeolites is the major
method of increasing propylene selectivity.
The optimal effect is obtained when used
with a high activity base catalyst with low
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25% C3= additive
50% C3= additive
hydrogen transfer activity, but ZSM-5 additives
(fluidisable powder catalysts containing ZSM-5
crystal) will boost propylene yields with every
type of base catalyst used.
Additives are used on top of base catalysts to
tailor the product yields and selectivities in an
FCCU. They are a flexible option to optimise
yields against unit constraints and within
refinery economics. As feed composition
and product prices can change, additives are
the quickest solution to maintain an FCCU
running at maximum profitability.
For residual feeds coke selectivity becomes
another important consideration. Separate
addition of additives and catalyst means the
catalyst system can be constantly adjusted to
optimise the operation to match the feedstock
and the propylene objectives, because
the FCCU requires continuous catalyst
replacement. This unique flexibility is a major
advantage for the FCCU process.
maximum propylene
FCCUs using high levels of ZSM-5 (>10%)
might be expected to see dilution of the
catalyst activity, but in practice so long as
the FCCU has capacity to increase catalyst
circulation there is no negative effect on
conversion. Coke is not produced on ZSM-5
additives, so the effect is to lower the coke
formation per reactor pass (delta coke) which
reduces the regenerator temperature. The
FCCU will respond to maintain the heat
balance by increasing catalyst circulation to
sustain the reactor temperature and restore
conversion to the additive-free level.
lab testing
Our bench scale study demonstrates how
ZSM-5 additives can be used to dramatically
shift yields towards petrochemicals. Here we
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PETROCHEMICALS
used an equilibrium catalyst obtained from
a VGO operation, with moderate rare earth
and hydrogen transfer, intermediate activity
and no ZSM-5 present. This catalyst has
been designed with the balance of matrix
and zeolite for maximum octane barrels and
bottoms conversion consistent with delta
coke constraints, rather than for maximum
petrochemical production. The tests
were carried out in an advanced cracking
evaluation (ACE) bench-scale reactor,
mimicking an FCCU reactor temperature of
550oC, which already favours high propylene
yields for the base catalyst.
The study simulated three different cases
on a common feed with the following
objectives: incremental increase in
propylene; major increase in propylene; and
major increase in butylene and propylene.
The base equilibrium catalyst, without
additive, produced 5.5 wt% propylene (see
Figure 1). Using 5% additive increased
the propylene yield by 1.8 wt%, a relative
increase of over 30%. And using 25% additive
boosted that figure to 5 wt%, ie a 90% relative
increase.
Adding 25% ZSM-5 might seem extreme,
in terms of catalyst composition, but several
units worldwide are already operating at
similar levels. Figure 2 shows that besides the
increase in propylene, there is a substantial
boost in aromatics in gasoline, including
BTX.
In addition to increasing propylene,
ZSM-5 additives also boost butylene yields.
Special butylene selective additives can
further enhance the butylene yield relative to
propylene. Figure 3 shows that this additive
increased butylene fraction in the LPG, with
butylene in the LPG increased from 6.5 to 8.5
wt%, a relative increase of over 30%.
The addition of 50% additives, either
all propylene-selective or 25% propyleneselective and 25% butylene-selective showed
there is a theoretical ceiling for what can be
achieved without further optimisation of the
base catalyst. Figure 4 shows that propylene
yield can be boosted to 12.5%, butylene yield
to 10%. Unit operation limits might prevent
these gains in catalyst selectivity to be fully
exploited, but it illustrates how changes
in catalyst composition can substantially
modify the LPG yield and composition.
By using additives the FCCU’s catalyst
selectivity can be quickly switched between
petrochemicals and fuels modes. Dedicated
addition systems allow the FCCU to be
operated as close as possible to its constraints
such as wet gas compressor and gas plant
capacity to maximise profitability.
ZSM-5 additives also allow the refiner
to make up the propylene shortfall from
running cheaper heavier feeds. This study
illustrates that when the FCCU has the
capacity to process and recover the increased
petrochemical products, the relative
propylene yield can be increased by more
than 100% within days.
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means that even small increases in yield are
significant, and they are already the source of
nearly a third of the worldwide propylene. tce
Corresponding author Bart de Graaf (bart.
[email protected]) is FCC R&D director
at Johnson Matthey
further reading
1. Technically Recoverable Shale Oil and Shale
Gas Resources: An Assessment of 137 Shale
formations in 41 Countries Outside the United
States, US Energy Information Administration
(EIA) report, June 2013
2. Shale Gas, Reshaping the US chemicals
Industry, PWC report, October 2012
3. Can Shale Gale Save the Naphtha Crackers?,
Platts special report: Petrochemicals by Jim
Foster, January 2013
4. The US Shale Gas Boom, Outlook and
Implications for Global Petrochemicals, ICIS
report by Joseph Chang, August 2013
5. The FCC Unit as a Propylene Source, PTQ
Q3 2007, Charles Radcliffe
conclusions
The FCCU is the ideal process to help
mitigate the shortfalls in petrochemicals
caused by the shale gas revolution. The
example shows that a 1%wt propylene
yield increase is easily achieved using
ZSM-5 additives, representing a relative
rise of 20%wt. Besides propylene, the FCC
can make a valuable contribution to BTX,
because aromatics in the gasoline pool can
also be significantly increased under special
conditions.
The volume of oil processed in FCCUs
Chemical Engineering Matters
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C3=(solid) or total C4= (dotted), wt%
Figure 3 (Left): Using C3 and C4 (butylene) additives increases LPG yield. With C3 it is propylene rich, with C4 additives the LPG is
more butylene rich. Using a blend of C3 and C4 additives can optimise LPG composition;
Figure 4 (Right): Yields of C3 (solid lines) and C4 (dotted lines). 25% C3 additive and C4 additive produce the same yield as with 50%
C3 addtive, and produces more butylenes.
Total C4=s/LPG, wt%
0.42
0.40
0.38
0.36
0.34
0.32
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5% C3= additive
25% C3= additive
25% C4= additive
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Conversion, w%
Conversion, w%
50% C3= additive
Base
25% C4= additive / 25% C3= additive
50% C3= additive
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