Chemical Engineering Journal xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths
L. Basini a, S. Cimino b,⇑, A. Guarinoni a, G. Russo b, V. Arca c
a
eni, Refining & Marketing Division, Italy
Istituto Ricerche sulla Combustione CNR, Italy
c
eni Versalis S.p.A., Italy
b
h i g h l i g h t s
" CPO of ethane and n-butane to olefins was studied on Pt–Sn/LaMnO3 honeycombs.
" Bench scale testing showed high single pass yields of C2H4 + C3H6 around 55 wt.%.
" Stable reactivity demonstrated for 500 h.o.s. with ethane feed and sacrificial H2.
" Pt–Sn/LaMnO3 catalyst guaranteed a net hydrogen production across CPO reactor.
" Products quenching & catalyst overheating issues were identified during scale-up.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Olefins production
Catalytic partial oxidation
Light alkanes
Pt perovskite
Structured catalyst
Long-term stability
a b s t r a c t
The reactivity of a multi-layered monolith catalyst containing Pt and Sn over LaMnO3/La-c-Al2O3/cordierite, previously studied in a lab-scale plant for producing ethylene via Short Contact Time – Catalytic
Partial Oxidation of ethane, has been further and extensively investigated in a bench-scale plant with
higher production capacity. Ethylene yields exceeding 55 wt.% have been achieved and the reactivity performances have been maintained for more than 500 h.o.s. The experiments, while confirming the potential of the technology, have pointed out some weakness in catalyst stability and reactor design. The
bench-scale experimental study has also addressed the reactivity features of n-butane indicating that
ethylene + propylene yields approach 54 wt.% in a wide range of experimental conditions.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Light olefins are the most important building blocks for the
polymers and variety of intermediates industry. World demand
of ethylene and propylene is exceeding now 180 MTA (about 2/3
related to ethylene production), with an annual growth of 4–5%
in the next decade [1,2].
Steam cracking of hydrocarbons has been and still is the main
industrial technology for producing light olefins [1–4]. However,
despite the technological improvements occurred in more than
50 years, steam cracking remains the most energy-consuming process in the petrochemical industry.
It is expected that the possibility to perform oxidative dehydrogenation (ODH) or oxy-cracking of light alkanes through Short
Contact Time – Catalytic Partial Oxidation (SCT–CPO) would lead
to a novel technology with low capital investment, improved
⇑ Corresponding author. Address: Istituto Ricerche sulla Combustione CNR, P.le V.
Tecchio 80, 80125 Napoli, Italy. Tel.: +39 081 7682233; fax: +39 081 5936936.
E-mail address: [email protected] (S. Cimino).
energy efficiency [2–6] and reduced NOx and CO2 emissions. In particular, it has been shown that monolithic Pt-based catalysts, operated under autothermal conditions – i.e. wherein the feed is
partially combusted to drive the endothermic cracking processcan efficiently convert ethane to ethylene, propane and n-butane
to ethylene and propylene, isobutane to propylene, isobutene and
ethylene [3–20].
Recent experiments performed at laboratory scale utilizing ethane and a patented Pt(–Sn)/LaMnO3 catalysts [10,11], have produced olefins with yields exceeding 61 wt.% and selectivity above
75 wt.% per pass. It has been estimated that these reactivity features could result in reduced production cost of ethylene with respect to steam cracking [10]. Indeed the advantages resulting from
the high olefin yields and the compact reactor system [4–6,9] could
more than compensate the additional oxygen consumption costs,
not to mention the benefit of reducing the CO2 and NOx emissions
by avoiding large heating furnaces.
However, from a technical standpoint, a better definition of several key operating issues is required, including: catalyst activity
and stability as well as reactor design [5,8,9,12,13,16]. In fact, most
of the available experimental results were obtained with small
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.153
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
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2
L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
scale catalytic reactors operated with diluted streams for a rather
limited time on stream.
In this work we set out to validate previous ethane SCT–CPO
performance data utilizing the same Pt–Sn/LaMnO3 catalyst
[10,21] in a pre-pilot scale test rig with a 15 times larger production capacity, specifically designed and operated by eni in order
to explore the effect of the main operational parameters (space
velocity, preheating temperature, feed ratio, sacrificial fuel ratio,
pressure) on process performance and products distribution. With
the perspective of an industrial application, the target of our work
was to test the stability of the catalyst under relevant operating
conditions (i.e. adiabatic reactor, without feedstock dilution) for
at least 500 h on stream, while assessing key issues related to process scale-up. The catalyst, was fully characterized pre- and posttest by optical microscopy, SEM-BSE and XRD.
In addition, olefins production via SCT-CPO from n-butane was
tested for a preliminary assessment of the economics of this process, since the market demand for propylene is increasing faster
than for ethylene [1–4].
2. Material and methods
2.1. Catalyst preparation
Commercial cordierite honeycombs with straight and parallel
channels of roughly square section (600 cpsi by NGK [22]) were
cut in the shape of disks of 25 mm diameter and 10 mm long;
the monoliths were washcoated by dip-coating in an aqueous slurry of alumina powder (3% La2O3-c-Al2O3, SCFa140-L3 Sasol, 140 m2/g) and pseudobohemite (Disperal, Sasol) to obtain a
nominal thickness of 40 lm (Fig. 1b). A LaMnO3 layer (30% w/w
of the alumina washcoat) was deposited by repeated cycles of
co-impregnation with an equimolar solution of the precursor salts
(La-nitrate and Mn-acetate) and calcination in air at 800 °C for 3 h.
Finally, Pt (up to 4% w/w of the active layer, monolith substrate excluded) and Sn as dopant (Sn/Pt atomic ratio = 6) were added to the
structure by co-impregnation of the coated monoliths with a
H2PtCl6 and SnCl2 acid solution. More details on catalyst preparation and characterization can be found elsewhere [11,21–23].
Monolith catalysts with identical nominal composition were employed in the CPO of ethane or n-butane.
2.2. Bench-scale CPO plant
The tests were performed in a bench-scale plant with ethane
capacity of 1000 Nl/h (roughly 15 times larger than previous lab
scale [10,11]) and n-butane capacity of 390 Nl/h, composed by five
main zones:
1.
2.
3.
4.
5.
Feeding and Preheating zone.
Mixing zone.
Reaction zone.
Cooling zone.
Analysis zone.
The reactants ethane, nitrogen, oxygen and hydrogen were supplied by cylinders and their flowrates controlled by mass flow controllers (Brooks, Bronkhorst). n-Butane flowrate was measured and
controlled by a mass flow meter specific for liquids (Bronkhorst
mini CORI-FLOW).
The hydrocarbon feedstock, nitrogen and hydrogen were conveyed into a single line. Oxygen flew into a second, independent
line. Each line was equipped with an electric pre-heater. Mixing
of feedstock and oxidant streams was performed in a ‘‘tube in
tube’’ device, located on the top of the vertical steel vessel, featuring a thick internal refractory lining (tight fit concentric ceramic
tubes) to limit heat loss in the reaction zone, whose inner diameter
was 26 mm (Fig. 1a). Three electrical resistances surrounded the
reaction zone and were switched on only during the light-off of
the self-sustained SCT–CPO reactions.
The hot effluent from the catalytic reactor was transferred
through a water cooled line to heat exchangers and filters before
flowing downstream through a back-pressure valve and being
flared.
A side-stream of the effluent was collected and analyzed by an
online GC (HP 7890, equipped with FID and TCD detectors) and a
microGC (Agilent Quad), calibrated to measure CO, CO2, N2, O2,
H2 and hydrocarbons up to C6.
Fig. 1. (a) Schematic of the bench scale SCT–CPO mixing, reaction and cooling zones. (b) Optical microscopy and SEM-BSE images of the structured catalyst with Pt–Sn/
LaMnO3/Al2O3 active washcoat layer on a cordierite honeycomb (600 cpsi).
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
http://dx.doi.org/10.1016/j.cej.2012.06.153
L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Mass balances were calculated by adjusting the volumetric
flow-rate of the effluent to balance C atoms in and out and the
water flow to balance O atoms in and out. The error on N and H
atoms typically resulted in the range ±3 vol.%.
Results are generally reported in terms of C-atom selectivity of
species j here defined as:
v j F out
j
Sj ¼ in
n F C n Hm F out
C n Hm
where vj is the number of C atoms in species j, F out
its outlet molar
j
in
out
flow, F Cn Hm F Cn Hm is the difference between inlet and outlet molar flows of the specific feedstock CnHm containing n atoms of C.
2.3. Operating conditions
Catalytic tests were generally performed at the operating pressure of 1.5 bara; in a few tests this was set at 2 bara. According to
previous reports [5,9–15,23–25] hydrogen was fed to the reactor as
a sacrificial fuel in order to maximize the selectivity to olefins. The
inlet temperature of the pre-mixed stream was set at 250–270 °C
with ethane and 200–250 °C with n-butane, i.e. the maximum temperatures allowed by our experimental apparatus. Nitrogen was
used as an internal standard: its content in the feed stream (ca.
8 vol.% with ethane and 5 vol.% with n-butane) was kept as low
as possible according to the minimum stable flow rate of N2
achievable with the specific mass flow controllers employed in
each set of experiments.
Oxygen conversion in the CPO reactor was almost complete;
since in industrial units the presence of oxygen in the effluent cannot be tolerated, we performed specific tests to ascertain that the
eventual presence of residual O2 was caused by some lateral bypass between the honeycomb catalyst and the reaction tube. This
is a common issue with bench-scale CPO reactors, which has a direct negative impact on feed conversion and selectivity to olefins.
The experimental campaign with ethane was performed with a
single catalyst sample that was on stream for a total of 550 h. During the first 220 h the operating conditions were widely changed in
order to optimize the yield in C2 + C3 olefins while minimizing the
consumption of reactants:
O2 =C ratio
H2 =O2 ratio
0:20—0:25 v=v
1:00—3:10 v=v
Ethane load 300—600
Nl=h:
These values resulted in space velocities ranging from 230,000
to 510,000 Nl/kg/h (corresponding to a maximum GHSV of
400,000 h1 referred to the volume of the monolith catalyst).
A life test was then performed for 320 h at fixed inlet conditions:
O2/C = 0.22 v/v, H2/O2 = 1.53 v/v and GHSV = 360,000 Nl/kg/h.
With n-butane the main operating parameters were varied in
the following range:
O2 =C
0:15—0:27 v=v
H2 =O2
0—2:50
n Butane load 150—300
v=v
Nl=h:
The corresponding space velocities were comprised between
100,000 and 450,000 Nl/kg/h.
3. Results and discussion
3.1. CPO of ethane
Initial experiments, performed with ethane feedstock, examined the effects of process parameters on the overall process
performance.
3
The effects on ethane conversion and ethylene selectivity
played by: (i) O2/C and H2/O2 feed ratios, (ii) space velocity and
(iii) preheating temperature were strongly related: a single parameter variation determined an opposite effect on conversion and
selectivity; i.e. the increase of O2/C ratio and preheating temperature enhanced conversion and reduced selectivity, while the increase of the H2/O2 ratio and space velocity reduced conversion
but enhanced the selectivity towards ethylene.
As shown in Fig. 2a–c, in the conversion-selectivity diagram the
data obtained with the Pt–Sn/LaMnO3 catalyst in the bench scale
rig under optimized process conditions follow a single line and
are highly reproducible as already reported in previous works on
the same catalyst at smaller scale [10,11]. A maximum total Catom yield per pass to ethylene and propylene up to 60% (equivalent to 55.7% on a mass basis) was achieved for several operation
conditions. All these runs were carried out with H2 added to the
feed as sacrificial fuel and we always observed an overall net production of hydrogen from the catalytic reactor and a C-selectivity
to propylene of ca. 1–1.5%.
A direct comparison (Fig. 2a–c) of the products distribution obtained in the present study with previous experimental data at lab
scale [10,11] shows that the main species follow the same qualitative trends as a function of ethane conversion (i.e. process severity), whose increment is always accompanied by an increase of
C-atom selectivity to CO, CH4, C2H2 and CO2 and, consequently, a
reduction in C2H4 selectivity. However, with the bench-scale reactor a limited and constant loss of selectivity to ethylene was measured at any ethane conversion level. This effect was mainly
counterbalanced by a slightly higher selectivity to methane and
CO and a larger formation of C4–C6 hydrocarbons (C-sel. = 3–5%),
whose selectivity was estimated 62% in the lab-scale reactor. At
the same time, identical C-selectivity to acetylene, CO2 and to valuable propylene (not shown) were measured from the two reactors
for every ethane conversion level. The overall catalytic performance of the bench-scale reactor with Pt–Sn/LaMnO3 catalyst is
closer to the results obtained with the undoped (without Sn) catalyst at lab scale (Fig. 2a–c). However, it is pointed out that similar
ethane conversion levels were obtained with less oxygen in the
feed (13%) in the bench scale reactor, thanks to its higher degree of adiabaticity, related to the higher capacity and to a more
favorable surface/volume ratio limiting heat loss. Since pure oxygen is a significant cost component of the feedstream, the decrease
in oxygen employed per unit of C2H4 produced translates directly
into economic savings [4,5,10]. Moreover, since oxygen reacts with
hydrogen, lower amounts of oxygen lead to a decrease in hydrogen
consumed. As a consequence, a surplus of H2 was found in the
products, which was not always the case with previous reports
on Pt–Sn and Pt–Sn/LaMnO3 based catalysts evaluated at smaller
scale [10–13,23].
The analysis of the relationship between ethane ‘‘cracking conversion’’ (C-atom ratio of all hydrocarbon products over all hydrocarbon products including unconverted feed [9]) and ‘‘cracking
selectivity’’ (C-atom selectivity based on all hydrocarbon products,
excluding unconverted feed and COx [9]) is reported in Fig. 3.
Cracking selectivity data converge in single narrow curves for each
hydrocarbon product, insensitive to the wide range of experimental process conditions and largely following the trends predicted
for homogeneous isothermal steam cracking of the residual C2H6
feed not converted to COx [11]. However CH4 formation is underpredicted by the simplified isothermal steam cracking model, as
also reported by Lange et al. who compared data from several catalysts and attributed this effect to the temperature spike at the entrance of the catalytic reactor [9]. This analysis provides evidence
that most olefins are produced through homogeneous dehydrogenation and pyrolysis reactions driven by the heat generated by heterogeneous oxidation reactions [9–11,14]. The comparison
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
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L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
(a)
100
60 %
50 %
80 %
70 %
Pt/Perovskite
Pt-Sn/Perovskite
Pt-Sn/Perov. ENI
C-atom selectivity (%)
90
40 %
80
C2H4
70
60
C2H2
5
0
50
60
70
80
90
100
C 2 H 6 conversion (%)
(c)
C-atom selectivity (%)
25
20
15
CO
10
CO2
5
0
50
60
70
80
90
100
C2H6 conversion (%)
C-atom selectivity (%)
(b)
15
CH4
10
5
C2H2
0
50
60
70
80
90
100
C2H6 conversion (%)
Fig. 2. Process selectivity to (a) C2H4 and C2H2, (b) CO and CO2, (c) CH4 and C2H2 as a function of ethane conversion during the CPO reaction over Pt–Sn/LaMnO3 catalyst in the
bench scale reactor (eni) as compared to previous results at lab scale [10,11]. Dashed lines represent iso-yield curves.
ene just formed. This circumstance points out the importance of
the design of the post catalytic and cooling section to effectively
quench the valuable and highly reactive products, an easier task
to achieve with a small quartz reactor than with a larger ceramic
one [11].
Cracking selectivity (%)
100
90
80
70
60
20
Pt-Sn/Perov. ENI
Pt-Sn/Perov. labscale
model 1000°C
10
0
50
60
70
80
90
100
C2H6 Cracking conversion (%)
Fig. 3. Effect of the reactor size (eni bench scale vs. lab scale) on C-atom ‘‘cracking
selectivity’’ to ethylene, methane and acetylene as a function of ethane ‘‘cracking
conversion’’ over Pt–Sn/LaMnO3 catalyst after normalization to exclude COx
products. Lines represent predictions by a purely homogeneous steam cracking
model in an isothermal plug flow reactor operated at 1000 °C and 1.5 bara with
C2H6:H2O = 1:1 [11].
between lab-scale and bench-scale reactivity features in terms of
cracking selectivity and conversion (Fig. 3) confirmed a reduction
in ethylene selectivity in the latter case, which is due to a (slightly)
larger formation of methane, and mainly to heavier hydrocarbons
formed by undesired consecutive reactions consuming the ethyl-
3.1.1. Life test
As reported in Fig. 4a and b the Pt–Sn/LaMnO3 system displayed
a substantial stability of catalytic performance over a life test lasting for 330 h at fixed inlet conditions, with several start-up and
shut-down of the bench-scale rig. In particular, ethylene yield
was stable at the average value of 56.1 vol.% from 221 to
380 h.o.s., then it started a slow decrease settling down to
54.5 vol.% after 541 h.o.s. (end of the test), almost exclusively for
a corresponding reduction in ethane conversion. In fact, all main
products were formed with substantially unchanged selectivities:
CO selectivity, a measure of how inefficiently the catalyst utilizes
the oxygen in the feed, passed from 11.1 to 11.6 vol.% at the end
of the run; this is a remarkable result as opposed to the increase
of some points percent previously observed for CO on Pt–Sn on
a-alumina foams during only 24 h on stream [13].
The slight reduction in ethane conversion was accompanied by
a progressive increase in the pressure drops measured across the
reactor and the transfer line which increased from 50 mbar up to
77 mbar at the end of the test (Fig. 4a). Visual inspection of the
reactor revealed the presence of carbonaceous deposits in the lower section of the catalytic monolith and in the post catalyst zone
(heat shields and thermocouple sheath).
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
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L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
C2H6 conversion %
100
(a)
80
100
60
75
40
50
20
0
100
100
200
300
400
500
25
600
(b)
80
Selectivity %
125
60
15
10
5
0
100
200
300
400
500
600
time on stream, h
Fig. 4. Ethane CPO life test on Pt–Sn/LaMnO3 catalyst: (a) fuel conversion and
pressure drops across the reactor and (b) process selectivities to main products as
function of time on stream at fixed inlet conditions.
3.1.2. Catalyst investigation
Microscopic inspection of the catalytic monolith after the life
test (Fig. 5a and b) showed a partial collapse of the honeycomb
support structure in its top face but not of the oxide overlayers:
since the melting temperature of cordierite is higher than
1200 °C, this is a clear indication of strong hot spot formation in
the oxidation zone of the catalyst. This was never observed during
operation of the same monolith catalyst in the lab scale reactor for
a total of 100 h.
In fact, in two occasions the threshold value for the O2/C ratio
was exceeded (for reasons not dependent on the process) causing
the peaking of the inlet temperature for the time needed (10 s)
for the Emergency Shut Down (EDS) to trip the Unit. Apart from that,
due to the concentrated heat generation [25], the upper section of
the CPO catalyst is always exposed to very harsh conditions, which
are expected to be even more severe in the bench-scale rig (no dilution, higher flow rates, lower heat loss). Therefore, the robustness of
the catalyst is a key feature for the industrial application.
XRD analysis detected that, where the cordierite structure was
damaged, some Si had migrated and formed a mixed phase with
the oxide overlayers (Al/La/Mn + Si). Where the cordierite honey-
5
comb structure was preserved, the active washcoat layer appeared
partially consumed and thinner close to the entrance (Fig. 5c).
Looking at the back face of the honeycomb catalyst, roughly 30%
of the channels appeared partially blocked by carbonaceous deposits, that built-up forming thick overlayers above the catalytic
washcoat (Fig. 5d). These deposits contributed to the increase in
pressure drops across the reactor. However, some adjacent catalytic channels in Fig. 5d appeared free from carbon, suggesting that
carbon formation was strongly related to gas flow (mal-)distribution in the inlet section, due to the collapse of the cordierite structure that caused longer residence times at high temperatures at
specific locations. From a different point of view, since the partial
occlusion of the honeycomb catalyst did not cause a significant decrease of catalytic performance, it can be argued that the catalytic
system was efficiently operated at contact times significantly lower than the nominal ones (i.e. higher GHSV).
SEM-BSE analysis revealed that the residual active phase close
to the inlet section contained Pt clusters but was almost completely depleted in Sn, whereas Pt and Sn were still alloyed further
downstream along the channels of the monolith. In particular,
accordingly to a progressive consumption of oxygen along the catalytic reactor, SnO2 was recognized in the central zone of the channels whereas metallic Sn was detected in the last part.
Apart from the obvious issue of catalyst failure due to melting of
the ceramic substrate, which could be coped with a more heat
resistant support, high temperatures and high oxygen partial pressures caused a depletion of active phase due to volatilization of tin
(and possibly Pt), thus explaining the curves of Fig. 2a–c approaching the results obtained on the undoped Pt/LaMnO3 systems. Similar conclusions were drawn by Bodke et al. [13], who observed a
much faster decay in catalytic performance (in terms of both ethane conversion and ethylene selectivity) of their Pt–Sn system during only 20 h.o.s., and verified that a continuous addition of SnCl2
aqueous solution to the feed was able to restore activity by replenishing lost Sn in the catalyst.
These findings suggest that the main differences observed in the
bench scale reactor are due to the development of a different temperature profile along the catalytic monolith, most likely characterized by a steeper hot spot on the surface close to the entrance.
High temperatures (>1000 °C) are known to favor the formation
of methane over alkenes, and this was already identified as a drawback of adiabatic CPO reactors [9]. In fact CH4 and C2H2 formation
in gas phase are both enhanced by high temperatures (and indeed
they are used as an indication of process severity in the steam
cracking); however methane can be formed at comparable rates
also on the Pt surface [24,26]. In our case, the higher selectivity
to methane associated to the same levels of C2H2 (Figs. 2c and 3)
may be due to the occurrence of a hot spot on the inlet section
of the catalyst and not in the gas phase – due to heat transfer limitations [14,25] – speeding up the heterogeneous formation of CH4
from ethane.
Fig. 5. Optical microscopy and SEM-BSE images of the Pt–Sn/LaMnO3 monolith catalyst after the life test of ethane CPO. (a and b) Partially melted cordierite structure close to
the top inlet section and (c) loss/thinning of active washcoat. (d) Build-up of carbon overlayer above the active phase causing the partial occlusion of some channels in the
outlet section of the catalyst.
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
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L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
3.2. CPO of n-butane
Fig. 6 compares the results of two CPO tests performed with
ethane and n-butane under the same operating conditions. The
higher reactivity of n-butane with respect to ethane, as well as
the higher partial pressure of oxygen when operating with a heavier alkane at the same O2/C feed ratio, determined a significantly
higher process severity with fuel conversion reaching 91.2% for nbutane vs. 76.3% for ethane. As already reported [4,15] the CPO of
n-butane produced less ethylene, which was partly compensated
by the formation of propylene and butenes. The relevance of the
cracking and condensation reactions producing methane and
C4 + hydrocarbons increased. On the other hand, total COx selectivity did not change significantly, but CO/CO2 ratio decreased with nbutane feed. Since oxygenates represent a net loss of feedstock, low
CO/CO2 ratios are preferred because they imply a more advantageous use of the oxygen feed to produce the heat required to sustain the endothermic reactions leading to olefins formation.
Fig. 7a summarizes in the conversion-selectivity plane the datasets collected in a wide range of conditions. Yields in light olefins
(ethylene + propylene) can easily exceed 50% on a carbon atom basis, which is considered the minimum acceptable value for the economics of the process. The degree of feedstock conversion (i.e.
process severity) controlled the selectivity trends for all the products. Ethylene selectivity progressively increased with conversion
since C2H4 can be formed either via primary or secondary cracking
reactions, involving butenes and propylene. Accordingly, butenes
and propylene selectivities were adversely affected by the increase
in fuel conversion. For conversion values > 90%, propylene decrease
was no longer compensated by ethylene increase, hence the total
selectivity to C2 + C3 olefins dropped. In fact methane and acetylene were formed at a higher extent following the same trend already observed with the CPO of ethane .
The maximum C-atom yield of C2 + C3 olefins was 55.6% corresponding to 53.7% on a mass basis, which increases up to 55.1 wt.%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
C2H6
n-C4H10
HCn>4
C4H10
C4H8
C3H6
C3H8
C2H2
C2H6
C2H4
CO2
CH4
CO
FS Conv.
C2H6
n-C4H10
Fig. 6. Comparison of fuel conversion and C-atom selectivities in the SCT–CPO of
C2H6 and n–C4H10 over Pt–Sn/LaMnO3 catalyst operated at comparable inlet
conditions. O2/C = 0.2 v/v, H2/O2 = 1.0 v/v, space velocity 330,000 Nl/kg/h and
preheating temperature 250 °C.
80
50%
40%
C-Selectivity %
In order to reduce the extent of the hot spot formation, specific
strategies for the optimization of the thermal behavior and heat
management of the reactor have been proposed and could be considered [25 and ref. therein]. We mention the variation of channel
opening [22] or support morphology (foams, honeycombs, microchannels in stacked metal plates [20,27]), the enhancement of
the thermal conductivity of monolith support [20,28], and the
reduction of the catalytic bed aspect ratio L/D to increase back heat
dispersion [25].
60%
70%
(a)
C2 + C3 Olefins
60
30%
C2H4
40
20%
C3H6
20
10%
0
50
10
molar ratios
6
CH4
C4H8
C2H2
60
70
80
90
100
C4H10 Conversion %
(b)
8
C2H4 / C3H6
6
4
2
0
50
CH4 / C3H6
60
70
80
90
100
C4H10 Conversion %
Fig. 7. Results of n-butane CPO over Pt–Sn/LaMnO3 monolith catalyst. (a) C-atom
selectivities to C3H6, C2H4, C2H2 and CH4 (dashed lines represent iso-yield curves).
(b) C2H4/C3H6 and CH4/C3H6 molar ratios as a function of C4H10 conversion.
including butenes: C-selectivity to ethylene and propylene were
respectively 46.3% and 14.5% at 91.3% conversion. A maximum Catom yield of propylene around 18% was achieved for n-butane
conversions between 70% and 80%.
At low process severity ethylene and propylene were formed in
a molar ratio around 2 (Fig. 7b) whereas this value rapidly increased for conversion values above 80%. A similar trend was observed for the methane to propylene ratio, which was initially
close to 1, confirming that the valuable propylene is ‘‘overcracked’’ to smaller molecules at higher n-butane conversions.
Fig. 8a–d compares the effect of the O2/C ratio on conversion
and products selectivity at different values of H2/O2 ratio. Butane
conversion was directly controlled by the O2/C ratio (Fig. 8a), while
H2/O2 ratio showed almost no effect.
Ethylene selectivity was enhanced by increasing O2/C, while
propylene was adversely affected by this parameter (Fig. 8b). An
important difference with the case of ethane CPO is related to
the effect of the sacrificial fuel: H2 addition had no direct impact
on propylene formation curve, whereas it enhanced ethylene selectivity up to H2/O2 = 1. Similar results were found during the CPO of
propane over the same catalyst [10].
In fact, H2 addition raised the operating temperature on the catalyst since hydrogen has a higher net heat of combustion per mole
of O2 [10] and it is preferentially oxidized instead of the hydrocarbon feed [14], as demonstrated by the reduction observed in CO
and CO2 selectivities (Fig. 8d). However propylene formation does
not appear to be limited by the selectivity of the catalytic oxidation
reactions but only by the low selectivity of the homogeneous
cracking reaction.
Methane selectivity (Fig. 8c) increased with both the oxygen
and hydrogen content in the feed due to higher operating temperatures and faster CAC cleavage in the presence of larger amounts of
H2. C4+ selectivity (Fig. 8c) was slightly affected by O2/C and rather
insensitive to hydrogen addition.
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
http://dx.doi.org/10.1016/j.cej.2012.06.153
7
H2/O2=0.0
H2/O2=0.9
70
H2/O2=1.0
60
H2/O2=2.0
H2/O2=3.0
50
C-selectivity %
50
40
0.14
0.16
0.18
0.20
0.22
0.24
0.26
(b)
1
0
30
0
1
2
in
3
4
O2in
20
H2 /
10
Fig. 9. Moles of H2 in the outlet stream per mole of O2 fed vs. the feed H2/O2 ratio
during the CPO of ethane and n-butane over Pt–Sn/LaMnO3 catalysts. Points below
the dashed line indicate a net consumption of H2 throughout the catalytic reactor.
0.14
0.16
0.18
20
C-selectivity %
2
O2/C
0
0.20
0.22
0.24
0.26
O2/C
(c)
15
10
5
0
0.14
C-selectivity %
3
in
80
/ O2
90
4
(a)
out
100
H2
n-C4H10 conv. %
L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
14
12
10
8
6
4
2
0
0.16
0.18
0.22
0.24
0.26
0.22
0.24
0.26
O2/C
(d)
0.14
0.20
0.16
0.18
0.20
O2/C
Fig. 8. Effect of O2/C and H2/O2 feed ratios on fuel conversion and C-atom selectivity
to the main products during the CPO of n-butane over Pt–Sn/LaMnO3 monolith
catalyst. Data at H2/O2 = 0.9 were obtained at P = 2 bara. Arrows indicate trends for
increasing H2/O2 values.
Regarding the effect of pressure on the catalytic features, moving from 1.5 to 2 bara (data set at H2/O2 = 0.9 in Fig. 8) we observed
a small reduction in the process selectivity to ethylene whereas
fuel conversion and selectivity to propylene were almost
unaffected.
A net hydrogen production was identified as a key condition to
achieve favorable overall economics of the process. As seen in
Fig. 9, for ethane to ethylene the process produces more H2 than
fed for all the operating condition explored with H2/O2 up to 3.
On the contrary, for n-butane to olefins the CPO process produces
a H2 surplus only at low H2/O2 ratios, namely below 0.5 v/v. Similar
qualitative results were reported by Bodke et al. [29] but the crossover points depend strongly on the type of catalyst [21] as well as
on reactor design and process conditions such as the fuel to oxygen
ratio and preheat [29]. In fact, a further peculiar feature of the Pt/
LaMnO3 catalyst is the possibility to advantageously use CO or
mixtures of CO and H2 as sacrificial fuels to avoid the eventual
H2 unbalance [11,23].
However, since in the case of n-butane CPO the positive effect of
sacrificial H2 addition was modest and only limited to ethylene
selectivity, an upper limit to the H2/O2 feed ratio at 0.5 did not preclude the achievement of high olefin yields.
Though the identification of the reaction mechanism was not
the primary scope of these tests, the data-sets collected in a wide
range of operating conditions have much improved our understanding on the behavior of the system.
With ethane as well as with n-butane, the sacrificial fuel fed together with the feedstock is preferentially oxidized and produces
heat for following pyrolysis reactions. From this point of view,
the superior performance of the Pt–Sn/LaMnO3 catalyst can be explained considering the presence of an additional active phase (the
perovskite LaMnO3) able to selectively promote the combustion of
the sacrificial fuel without catalyzing undesired side reactions such
as steam reforming or hydrogenolysis consuming both reactant
and/or desired products [14]. In other words, an effective SCT–
CPO catalyst for olefins production must produce the maximum
heat release with the minimum oxygen consumption [9]. Though
we cannot exclude a partial contribution to olefins production
via heterogeneous dehydrogenation or oxidative dehydrogenation
reactions, the strong dependence of all hydrocarbon products on
feedstock conversion (directly related to the operating temperature [15,17]) is a clear indication that homogeneous reactions plays
a major role. As a general rule, all the parameters that enhance
conversion induce an increase in ethylene, acetylene, methane,
C4+ hydrocarbons and a corresponding decrease to butenes and
propylene. Since C3 and C4 olefins are destroyed through subsequent reactions (secondary cracking), it is likely that an optimized
plant design, with strong integration between reaction and cooling
zones, will reduce the incidence of this undesired effect.
4. Conclusions
The experimental campaigns performed so far have substantially confirmed, on a larger scale, that the SCT–CPO technology
over Pt–Sn/LaMnO3 monolith catalysts originally proposed for ethylene production from ethane, can be successfully applied to heavier alkanes feedstock.
Ethylene yields exceeding 55 wt.% were achieved from ethane
and, for the first time, the reactivity performance were demonstrated for more than 500 h.o.s. With n-butane feedstock, ethylene + propylene yields approached 54 wt.% in a wide range of
experimental conditions.
Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
http://dx.doi.org/10.1016/j.cej.2012.06.153
8
L. Basini et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Though the bench-scale unit was specifically designed for
obtaining high feedstock conversion and high olefins yield, wide
changes in the operating conditions confirmed that olefins are
mainly produced via gas phase reactions; however a proper choice
of active phase and catalytic reactor design are of paramount
importance for achieving and maintaining high performance due
to the strong interplay between hetero-homogeneous and exoendothermic reactions.
The understanding of the prevailing mechanisms leading to olefins formation indicated the way for maximizing light olefins yield.
However, high yields of light olefins could not be sufficient to
achieve favorable economics for the industrial application of the
technology. Other features are as much as important, such as the
net production of hydrogen, the minimization of oxygen consumption and of by-products formation. From this point on, it is of paramount importance that technical-economic evaluations support
and address the experimental work.
In conclusions, we deem that SCT–CPO technology has a great
potential for the production of light olefins; however, this novel
technology will be ready for the industrial application only after
completing an extensive experimental work in the range of operating conditions identified as the most promising by the technicaleconomic evaluations and after improving the reliability of the catalyst and the reactor design, particularly with respect to heat
management.
Those issues, which have been partly addressed in this work,
are currently under investigation.
Acknowledgments
The authors wish to thank Pierino Visioli and Luigi Romano, for
their patience and precision in performing the experimental work,
Eleonora Di Paola and Danila Ghisletti, for their extensive work
respectively on optical microscopy/SEM-BSE and XRD characterization of catalysts. Special thanks go to Giovanni Forlani, who
realized most of the technical devices utilized in this work, with
unique passion and devotion.
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Please cite this article in press as: L. Basini et al., Olefins via catalytic partial oxidation of light alkanes over Pt/LaMnO3 monoliths, Chem. Eng. J. (2012),
http://dx.doi.org/10.1016/j.cej.2012.06.153