Propylene production process

1. On-demand propylene from naphtha
T
oday, steam crackers are
still the leading source of
propylene. The next larg-
est source of propylene comes
from refineries. However, the
share in propylene production
by these sources is decreasing.
On-purpose technologies have
been gaining ground rapidly
over the last 10 years. Current
front runners among on-pur-
pose technologies are propyl-
ene dehydrogenation (PDH)
and coal-to-olefins (CTO). The
main difference between con-
ventional propylene production
and on-purpose technologies is
feedstock: conventional technol-
ogies use mostly oil or naphtha,
whereas on-purpose produc-
tion technologies use propane
and coal.
The US market is experienc-
ing a substantial increase in
availability of naphtha with the
advent of very light tight oils.
This will be exacerbated by the
implementation of CAFE stand-
ards. These aim to reduce US
gasoline consumption by 2.5
million b/d. In Europe, export
refineries are experiencing
similar trends, combined with
the fact that over the past five
years US gasoline production
A new catalytic process targets economic production of propylene from
naphtha at relatively low capital and operating costs
BART DE GRAAF and RAY FLETCHER Inovacat B.V.
ANGELOS LAPPAS Chemical Process and Energy Resources Institute of the Center for Research andTechnology
www.digitalrefining.com/article/1001395 PTQ Q2 2017 1
has started to meet US gaso-
line demand. A substantial sur-
plus in naphtha and gasoline is
starting to emerge. Distressed
naphthas can be a very cheap
feedstock for petrochemical
production.
Propyleneproductionprocesses
Steam cracking utilises what is
likely the most severe condi-
tions of any chemical process
in industry. When sufficient
heat is supplied, molecules will
start to ‘fall apart’ into free rad-
icals. Steam cracking involves
networks of many different
free radical reactions, includ-
ing initiation, propagation and
termination steps. Cracking
light naphtha feeds in a steam
cracker produces high yields
of ethylene due to the free
radical chemistry involved.
Selectivities towards propylene
can be improved by reducing
the severity of operation but
steam crackers are not designed
to operate at propylene-to-eth-
ylene ratios much larger than
one.
Catalytic cracking creates a
pathway for molecules that
allows for a lower energy bar-
rier for the reactions to pro-
ceed. This requires an affinity
of the reactant for the cata-
lytic site and the creation of an
intermediary product that will
crack into the desired prod-
ucts. The lower energy barrier
means that reactions already
occur at a lower temperature.
This reduces energy losses
and helps to steer selectivity
to the most desired products.
Thermal cracking products
show a higher selectivity
towards ethylene, whereas cat-
alytic cracking occurring at
gentler temperatures favours
larger olefins such as propyl-
ene and butylene.1
Selectivities
can be optimised using zeo-
litic catalysts. Optimising the
pore geometry and affinity to
the intermediates may strongly
influence the selectivity
towards desired products.
Process design can help to
increase selectivities further.
Fluid catalytic cracking (FCC)
of naphtha has been commer-
cialised under various names
and is sometimes also applied
as a secondary riser added to a
conventional FCC unit. One of
the main challenges in this pro-
cess is the heat imbalance: there
is a gap between the highly

2. 2 PTQ Q2 2017 www.digitalrefining.com/article/1001395
sive ethane has replaced naph-
tha in steam crackers which has
resulted in a large decline in
propylene production in these
units. Inexpensive propane
has also been a benefit to PDH
operations.
Worldwide there are approxi-
mately 30 PDH units. The main
challenges of PDH unit opera-
tion are catalyst breakage and
transport from unit to unit,
chromium content of catalyst,
and limited possibility for turn-
down. These units tend to run
either at full capacity or are
idled.
Gasolfin catalysis
Naphtha is an attractive feed-
stock for propylene production.
Existing processes can convert
naphtha into 15-20% propylene.
Steam crackers cracking naph-
tha at high severity yield about
15% propylene. FCC processes
can produce up to 20% propyl-
ene, with some claims of up to
22%. Significant optimisation
of the catalyst and process con-
ditions are required to achieve
these propylene yields when
cracking naphtha.
Cracking of olefinic feeds
occurs readily via carbenium
ion mechanisms. For paraffinic
and naphthenic feeds, super
acid cracking is required. This
mechanism accurately describes
the initial stages of cracking
paraffins.2
However, crack-
ing of light paraffinic feeds
at temperatures below steam
cracking has been proven to
be rather challenging in exist-
ing processes and conversion is
limited. Therefore, to enhance
conversion of paraffinic and
naphthenic feeds, the Gasolfin
catalytic system has two dis-
tinct functionalities: in the first
step a pre-conditioning of the
endothermic deep conversion
of naphtha needed to produce
high propylene yields and the
very low coke make produced
by naphtha cracking.
There are a limited number
of FCC units operating world-
wide using these technologies.
Whereas secondary riser tech-
nology benefits from the much
more stable FCC heat balance,
it pays a penalty in selectivities
due to the presence of faujasites
in the catalyst mix which boosts
hydrogen transfer reactions.
A second challenge in current
fluidised cracking processes is
the high degree of back-mixing
reducing selectivities towards
propylene.
Addition of ZSM-5 additive
to the FCC unit is a simple
and effective way to increase
propylene yields. The propyl-
ene yield achievable utilising
ZSM-5 is a function of feed-
stock, FCC design and base
catalyst composition. ZSM-5
additives may be a relatively
cheap option to incrementally
increase propylene yields in
various refineries.
At present, the conversion of
methanol to propylene is com-
mercially unattractive due to
methanol and propylene prices.
The process starting with the
conversion of coal to metha-
nol, followed by methanol to
propylene, is commercially fea-
sible. The CTO process comes
at a high environmental price:
14-20 tonnes of CO2
is emitted
for every tonne of propylene
produced. Additionally, sub-
stantial amounts of wastewater
are produced. The environmen-
tal challenges make this pro-
cess less attractive compared to
competitive processes.
PDH technology has in recent
years begun to gain increasing
importance. Many PDH units
have been built over the past
five years, especially in China.
Typically, these units have a
capacity between 300 000 t/y
and 660 000 t/y of propylene.
In the US recently, one 750 000
t/y unit came online and a sec-
ond one is expected to start up
this summer.
The recent large surpluses in
natural gas liquids have bene-
fited both steam crackers and
PDH units. Relatively inexpen-
C2= C3= C4=
15
25
20
10
5
Yield,wt%
0
Catalyst System 2
Catalyst System 3
Catalyst System 1
Figure 1 Effect of catalytic system on olefin yields at total conversion for
constant naphtha feed and operating conditions.The Gasolfin process
makes use of a catalytic system that will be optimised as a function of feed
composition

3. feed molecules occurs, followed
by cracking over a second com-
ponent of the catalytic system.
Controlling reaction condi-
tions are critical in this pro-
cess. The graded bed promotes
the initial cracking step while
suppressing side reactions.
Optimising temperature and
hydrocarbon partial pressure
are key to steering selectivities
to maximum propylene or max-
imum aromatisation mode, or
any selectivities in between.
Testing of a variety of feed-
stocks over the Gasolfin cat-
alytic system has shown
propylene selectivities between
25% and 45%. Selectivities
depend on feed type. Different
yield patterns are achieved
while processing paraffinic vs
olefinic feeds. Propylene or aro-
matic yields may be optimised
for each feed stock modification
of operating conditions and
composition of the catalytic
system.
For the composition of the
catalytic system, a few consid-
erations are important: what are
the expected variations in feed
composition and what is the
range of desired product selec-
tivities? Changing the catalytic
system will result in different
olefin yields, and there is sub-
stantial room for optimisation
(see Figure 1; note that in all cat-
alytic systems a catalytic crack-
ing component was present,
with the intrinsic cracking activ-
ity of system 1 being the largest
and system 3 being the lowest).
Gasolfin process design
Minimising back-mixing is a
key to maximum propylene
yield. Fluid bed or fast fluid-
ised riser operations always
involve a substantial amount
of back-mixing. While this may
www.digitalrefining.com/article/1001395 PTQ Q2 2017 3
be preferential for feed distri-
bution, it does limit propylene
yield by increasing the likeli-
hood of propylene consuming
reactions such as oligomerisa-
tion and aromatisation. Limited
back-mixing is a typical fea-
ture of fixed bed operations.
Optimisation of catalyst layout
and functionalities in such a
fixed bed enables the cracking
of every type of naphtha feed
including paraffins.
Even though FCC is widely
used in refining, this type of
operation is easily the most
complex type of reactor oper-
ated today. Capital investment
cost of a FCC is approximately
$500 million. The capital invest-
ment for a steam cracker with
its exotic metallurgy is one of
the most expensive reactors in
the chemicals industry. In the
chemicals industry as a whole,
the most common type of reac-
tor used is fixed bed which is
also the least expensive type of
reactor.
When designing a new pro-
cess, it is essential to mini-
mise risk. The simplest type of
reactor with the easiest mode
of operation has the highest
chance of being successful at
start-up. Therefore, the Gasolfin
process has been designed with
reactors consisting of graded
fixed beds. All other equip-
ment required in the process is
standard equipment commer-
cially proven in many chemical
plants and refineries.
Feedstock selection
The Gasolfin catalytic system
has been tested with a wide
range of naphtha feeds, includ-
ing highly paraffinic, olefinic
and naphthenic feeds (see
Figure 2). The process does not
convert aromatic components.
All feedstocks are readily con-
verted into propylene and aro-
matics, depending on catalyst
composition and process condi-
tions. For example, for a typical
FCC gasoline, feed selectivities
of the primary products range
from 28% propylene to 72%
aromatics. For paraffinic feeds,
up to 45% propylene selectivity
has been obtained in testing.
Depending on the feed and
desired flexibility, the process
will consist of two or more
reactors with varying graded
beds. (A minimum of two reac-
tors will be required for con-
tinuous operations while one
C2= C3= C4=
30
40
35
25
20
15
10
5
0
Yield,wt%
Naphtha 2
Naphtha 3
Naphtha 1
Figure 2 Effect of feedstock on olefin yields at total conversion for constant
catalyst composition and operating conditions

4. reactor is off-line for regenera-
tion.) Generally, the most prof-
itable feeds are light straight
run (LSR) and delayed coker
naphthas (DCN).
Economics and benefits
The price of the feed is an
important variable in operat-
ing costs. Gasolfin converts
gasoline and naphthas into
petrochemicals. As discussed
in previous sections, naphtha
prices are expected to come
under pressure due to the
increase in light feed produc-
tion containing high fractions
of light straight run naphthas
(especially in the US),3
and a
reduction in gasoline use by
increasing fuel efficiency and
increased demand for elec-
tric cars.4
Competitive feeds
such as propane and coal face
different challenges. Propane
prices are expected to increase
slightly due to increased
demand, and environmental
challenges for CTO will make
it unlikely to gain much trac-
tion outside of China.
Open literature evaluations
of operating expenses on the
basis of feedstock costs for
CTO, PDH and steam cracking
are made more challenging by
the differences in attributing
the benefits of the by-prod-
ucts for each process. The esti-
mated operating margin for
each technology when taking
into account the prices of the
respective feedstocks and prod-
ucts for each process, together
with their manufacturing costs,
are shown in Figure 3. Naphtha
cracking via the Gasolfin pro-
cess clearly passes the profita-
bility hurdle.
The capital costs of the
Gasolfin process are limited
due to the simple process lay-
out and operation. The process
consists essentially of a feed
preheat section, a series of fixed
beds reactors, an aromatics
extraction unit, and a product
rectification section. Due to the
simple design and operation,
this process is much less expen-
sive to build and operate than
next best available technolo-
gies. The capital costs are in
the range of $60-100 million for
a 5-15 000 b/d (feed) unit, less
than half the costs of competi-
tive propylene producing tech-
nologies. As process conditions
are rather mild, operating costs
are at least 30% less than for
steam cracking or PDH units.
Steam crackers and PDH
units have a typical minimum
size to allow for profitable
operation, and have limited
possibilities for turndown. This
makes the decision to build a
new PDH unit dependent on
the possibility to sell the excess
propylene readily. The Gasolfin
process starts at a smaller size
and can comfortably bear a
smaller economy of scale due
to savings in energy and other
operational costs. The 70%
turndown ratio is much larger
than the 25% span for stand-
ard propylene production pro-
cesses. The process lends itself
well to the operator desiring to
incrementally increase propyl-
ene yield. Probably one of the
biggest advantages is the flex-
ibility of the process: the same
process can be used in maxi-
mum propylene mode, maxi-
mum aromatics mode or any
operational point in between.
Conclusions
Naphtha will remain a cheap
feed for making petrochemicals.
This makes naphtha a prime
feedstock for the growing pro-
pylene market. On-demand
propylene technologies have
thus far shunned naphtha as
a feedstock. The Gasolfin pro-
cess fills this gap. Next to low
capital and operational costs,
the biggest advantages of the
process are flexibility in prod-
4 PTQ Q2 2017 www.digitalrefining.com/article/1001395
H1 2014 H2 2014 H1 2015 H2 2015 H1 2016 H2 2016
Half years
1200
1400
1000
800
600
400
200
0
−200
−400
Operatingmargin,USD/t
MTO/MTP
Naphtha cracker (EU)
PDH
Propylene price
Inovacat Gasolfin
Figure 3 Propylene price and operating margins of various propylene
manufacturing processes for 2014-2016. Data have been derived from Platts,
Plastics Information Europe, HIS, ICIS and OPIS

5. ucts and the operational range
of the process, especially if a
standard size PDH unit is eco-
nomically unfeasible.
References
1 Greenfelder B S,Voge H H, Good G M,
Catalytic and thermal cracking of pure
hydrocarbons, Industrial and Engineering
Chemistry, 1949 (41) 2573.
2 Haag W O, Dessau R M, Proceedings
of8thInternationalCongressonCatalysis,
Berlin, 1984, Vol. II, 305, Dechema,
Frankfurt am Main.
3 www.eia.gov/analysis/studies/
petroleum/lto/?src=home-b6
4 www.bp.com/en/global/corporate/
energy-economics/energy-outlook.html
Bart de Graaf is a Consultant in the
refining and petrochemical industry.
He previously worked in research
and technical sales with Akzo Nobel/
Albemarle Catalyst Company and
www.digitalrefining.com/article/1001395 PTQ Q2 2017 5
Angelos Lappas is Research Director
of the Chemical Process and Energy
Resources Institute of the Center
for Research and Technology Hellas
(CPERI/CERTH) and is director of CPERI
laboratory that operates pilot and bench
scale facilities that are recognised as
unique on an international scale. He has
published 89 ISI papers, has presented
at 139 conferences, has authored
three books and contributed chapters
in seven additional books, and holds a
PhD in chemical engineering from the
University of Thessaloniki, Greece.
Johnson Matthey Process Technologies.
He has authored several papers on
various processes and the catalysis and
chemistry involved, and holds a MSc
in chemical engineering from Twente
University and a PhD in heterogeneous
catalysis from the University of
Amsterdam.
Ray Fletcher is a leading process
technology developer and Co-Founder
of Inovacat BV, a Netherlands based
petrochemical technology innovation
company. More than 28 years of hands-
on refinery operations include 22 years
of direct operating, troubleshooting,
optimisation and catalysis experience
with most FCC unit designs spanning
nearly all refining companies.The author
of 45 journal articles, he holds a bachelor
of science degree in chemical engineering
from the University of Washington.
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