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Synthesis, biological activities studies of ruthenium(II) polypyridyl complexes
Int J Ind Chem (2017) 8:221–233
DOI 10.1007/s40090-016-0106-8
RESEARCH
Reactivity of naphtha fractions for light olefins production
Aaron Akah1
•
Musaed Al-Ghrami1 • Mian Saeed2 • M. Abdul Bari Siddiqui2
Received: 9 May 2016 / Accepted: 7 November 2016 / Published online: 15 November 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract The catalytic cracking of naphtha fractions for
propylene production was investigated under high severity
catalytic cracking conditions (high temperatures and high
catalyst to oil ratio). Straight run naphtha and cracked
naphtha along with a with proprietary catalyst were used,
and reaction was carried out using a catalyst to oil ratio (C/
O) of 3–6 at 600–650 °C and 1 atm in a micro activity
testing (MAT) unit. The results from this experiments show
that light cracked naphtha (LCN) gave the highest propylene yield of 18% at 650 °C, and that propylene yield
depends on the naphtha fraction being used as feed. The
trend for reactivity and propylene yield was as follows:
light cracked naphtha [ heavy straight run naphtha [ light
straight run naphtha [ heavy cracked naphtha.
Keywords Naphtha cracking FCC catalysts Light olefin
production High severity catalytic cracking
Introduction
Light olefins such as ethylene and propylene are important
building blocks for many end products like polyethylene
and polypropylene. Recently, market analysis show that the
demand for propylene is outpacing that of ethylene and the
current supply cannot match the demand. A large proportion of propylene, about 65 wt%, is produced by steam
& Aaron Akah
aaron.akah@aramco.com
1
Research and Development Center, Saudi Aramco,
Dhahran 31311, Saudi Arabia
2
Center for Refining and Petrochemicals, Research Institute,
KFUPM, P.O. Box 807, Dhahran 31261, Saudi Arabia
cracking and about 30 wt% during the fluid catalytic
cracking (FCC) process as by product [1–3]. The propylene
to ethylene ratio produced by steam cracking of naphtha is
about 0.6, whereas the ethylene and propylene yields are
about 2 and 6 wt% from conventional FCC process.
During catalytic cracking, the heavier and more complex hydrocarbon molecules are broken down into simpler
and lighter molecules by the action of heat and catalyst. It is
through this way that heavy oils can be upgraded into
lighter and more valuable products (light olefin, gasoline
and middle distillate components). The FCC is one of the
most catalytic cracking technologies used widely in refinery for producing gasoline and diesel. However, current
direction is to maximize olefins such as propylene and
butylene by the addition of ZSM-5 to the catalyst formulation [4–14]. ZSM-5 shows high catalytic activity for the
cracking of C7? olefins into LPG range olefins and isomerization of n-olefins into i-olefins, while hydrogen
transfer (a bimolecular reaction) is not allowed because of
its small pore size [10, 15]. As a result, the entry of large
branched hydrocarbons is restricted, thereby making the
active sites accessible only to linear and monomethyl
molecules [16].
Synergetic effects through mixing of conventional FCC
catalyst (mostly USY zeolite) with ZSM-5 additive have
been observed by several authors, and show that there is an
increase in the yield of light olefins for the catalyst mixture,
compared to product yield on the individual catalysts,
suggesting that the reaction products are transferred
between USY zeolite and ZSM-5 [6, 7, 9, 10, 17, 18].
Improvements in FCC catalyst, process design, hardware,
and operation severity can boost high value light olefins
yields, with propylene yield that can increase from 6 wt%
up to 25 wt% or higher with VGO feed. However, additional efforts in the area of catalyst and process
123
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Int J Ind Chem (2017) 8:221–233
development are needed to be able to process light
hydrocarbon such as naphtha.
Naphtha is the predominant feed for steam crackers, as
more than half of the ethylene currently produced worldwide
is derived from cracking naphtha feed. However, propylene
production from steam crackers depends on the operating
rates of the steam cracker and the type of feedstock. The
propylene yield from steam cracking is directly proportional
to the average molecular weight of the feed [19].
In the past, propylene was produced from steam cracking of naphtha and, as a result, propylene was available in
substantial amounts. However, most modern steam crackers use ethane-based feed in place of heavy liquids leading
to less propylene [19]. Consequently, it is expected that
propylene production from steam crackers will further
decrease as a result of the shift to ethane-based feeds. The
knock-on effect is that more naphtha will be available as
feedstock for the production of propylene via catalytic
cracking and this can contribute to reducing the gap
between the high demand and low supply of the propylene.
The demand for propylene from FCC is growing at a faster
rate than global FCC capacity and, therefore, propylene
yields from FCC need to increase to keep up with demand.
This paper will discuss the evaluation of naphtha fractions as feedstock for the production of propylene and light
olefins via catalytic cracking under high severity conditions
which are high temperature and high catalyst/oil (C/O) ratio. The catalyst used in this study is made of ZSM-5 and
USY in order to draw on the synergetic effect of mixing
MFI and FAU zeolites to increase light olefin yield and
boost the octane number of the gasoline produced.
deactivation treatment according to ASTM D4463. In a
fluidized bed unit (Sakuragi Rikagaku, Japan), catalyst and
additive for this study were treated separately in 100%
steam environment at 810 °C for 6 h. The Y zeolite-based
commercial catalyst and ZSM-5-based commercial additive were physically mixed in 3:1 ratio for all the cracking
reactions presented in this paper.
Experimental procedure
Naphtha catalytic cracking was carried out in a fixed-bed
micro activity test (MAT) unit (Sakuragi Rikagaku, Japan),
using a quartz tubular reactor (I.D. 22 mm, and 38 cm in
length). A schematic representation of the experimental set
up is shown in Fig. 1. A low-temperature circulating bath
maintained at -10 °C was added to the unit instead of using
conventional ice water. All experiments were conducted in
the MAT unit at 30 s time-on-stream (TOS). The feed
injector and reactor assembly were placed in the heating
zone. Before feed injection, the system was purged with N2
flow at 30 mL/min for about 15 min. Liquid receiver with
the product vial was then connected to the bottom of the
reactor. The other end of the receiver was connected to the
burette for gas collection. A leak test was performed and a
low-temperature bath was raised to cover the liquid receiver.
The system was continuously purged with N2 gas for further
15 min. The reactor was charged with a known amount of
catalyst and about 1 g of naphtha was then fed to the reactor
during 30 s along with 30 mL/min of N2 flow. After the
reaction, stripping of catalyst was carried out for 5 min using
30 mL/min of N2 flow. The low-temperature bath was
removed and stripping of liquid was continued for three more
Materials and methods
Naphtha feeds
Catalytic cracking experiments were carried out using
naphtha fraction available at Saudi Aramco Refineries:
light straight run naphtha (LSRN), heavy straight run
naphtha (HSRN), light cracked naphtha (LCN) and heavy
cracked naphtha (HCN). Detailed hydrocarbon analysis
(PIONA) of the gasoline-range MAT liquid products was
conducted using a Shimadzu PIONA GC equipped with an
FID detector. The capillary column used was CP-Sil 5CB
(50 m long, 0.32 mm ID).
Catalyst
A proprietary catalyst was used for the evaluation of all
feedstocks. Prior to testing, the fresh low-activity Y zeolite-based commercial catalyst and ZSM-5-based commercial additive were subjected to hydrothermal
123
Fig. 1 Schematic diagram of the ASTM MAT Unit
Int J Ind Chem (2017) 8:221–233
223
minutes to remove the gas product dissolved in the liquid.
During the reaction and stripping modes, gaseous products
were collected in a gas burette by water displacement.
Weight of the feed syringe was taken before and after
experiments to obtain the exact weight of oil fed. Catalytic
cracking experiments were performed at temperatures
between 600 and 650 °C and the effect of catalyst/oil (C/
O) ratio for each temperature was studied.
mainly a mixture of pentanes and hexanes (C5 and C6
paraffins) which make up about 94 wt% of the feed. The
remaining 6% was made up of naphthenes and there were
no aromatics in LSRN. It also shows that about 69.9 wt%
of the HSRN feed consisted of paraffins, while the
remaining portion of HSRN was almost equally distributed
between naphthenes (14.4%) and aromatics (15.7%). While
LCN feed is mainly a mixture of iso-paraffins, olefins and
aromatics, HCN feed consists predominantly of aromatics
compounds.
Analysis of MAT products
Screening of naphtha feeds
MAT products comprised gas, liquid, and coke. Mass balance was considered acceptable within the limits of
95–103 wt%. A thorough gas chromatographic analysis of
all MAT products was conducted to provide detailed yield
patterns and information on the performance of the feed
being tested. Gases were analyzed using two Varian GCs
equipped with Flame Ionization Detector (FID) and Thermal
Conductivity Detector (TCD). This allowed the quantitative
determination of all light hydrocarbons up to C4, C5 paraffins, hydrogen and fixed gases. Hydrocarbons from C1 to C4,
and C5 paraffins, could be determined accurately. After gas
analysis, the weight of each gas component was added and
the weight of all components heavier than C4 was added to
gasoline fraction. The detailed composition of the product
was obtained from the gas analysis which was normalized to
account for the differences in mass.
Coke on spent catalyst was determined by Horiba Carbon–Sulfur Analyzer Model EMIA-220 V. About 1 g of
spent catalyst (with tungsten and tin added as combustion
promoters) was burnt in the high temperature furnace. The
resulting combustion gas (CO2) was passed through an
Infra-Red Analyzer and carbon content was calculated as a
percent of catalyst weight. All the results in this work are
presented as weight percent (wt%) of the product. The
conversion of naphtha feeds is defined as the total yield of
the hydrocarbons from C1 to C4, hydrogen and coke.
Naphtha conversion ðwt%Þ = Yield ðwt%Þ of total gas
þ coke ðwt%Þ:
ð1Þ
The terms and definitions used in this work are summarized in Table 1.
Results
PIONA Analysis of Naphtha Feeds
Detailed hydrocarbon (PIONA) analysis of naphtha feeds is
presented in Table 2. This table indicates that LSRN was
The naphtha fractions were initially screened at 650 °C,
using a C/O ratio of 6 to determine the reactivity for light
olefin production. The results are summarized in Table 3. It
can be seen that the conversion of HCN was very low
compared to the other three types of naphtha. Similar
trends were also observed for total light olefin yield and
LPG olefin. However, the coke yield on HCN was much
higher than that on all the other naphtha fractions. The low
reactivity of HCN and its tendency to produce more coke
can be attributed to its high aromatics content.
Using the C/O of 6, the naphtha fractions were further
screened at lower temperatures and the results are summarized in Fig. 2 below.
Figure 2 shows that while conversion increased with
increasing temperatures for different naphtha fractions,
conversion of HCN had insignificant increase. A similar
trend was observed for propylene yield. Based on the
screening results, which showed that HCN was the least
reactive feed, it was decided not to conduct further study
with HCN feed. The low reactivity and high coke yield of
HCN are attributed to the fact that HCN is made up of
predominantly aromatics which are highly stable and difficult to convert and they also act as coke precursors. The
hydrocarbon composition of each feed affects it reactivity
and it has been shown that a feed that is high in paraffin
and aromatic content shows low reactivity during catalytic
cracking, while a feed rich in olefins is very reactive.
For LSRN, HSRN and LCN, detailed cracking patterns
were obtained by varying C/O ratios in the range of 3–6 at
selected temperatures of 600, 625 and 650 °C.
Catalytic cracking of LSRN and HSRN
Figure 3 shows the change in conversion with increasing
C/O from 3–6 at different temperatures for both LSRN and
HSRN. Both feeds showed an increasing trend in conversion with increase in temperature. HSRN showed higher
conversion when compared to that of LSRN because it has
less thermal stability than the LSRN. For LSRN, the
increase in conversion with the increase in temperature
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Int J Ind Chem (2017) 8:221–233
Table 1 Definition of terms used in the estimation of performance
Term
Description
Naphtha Conversion
All non-condensable components in the product ? coke (H2, C1–C4 hydrocarbons and coke)
Dry gas
H2, C1 and C2 and C2=
LPG
All C3 and C4 hydrocarbons (paraffins ? olefins)
Light olefins
All olefins in C2–C4 range
LPG olefins
Propylene ? total butylenes
%Yield
Percentage of respective product based on total feed
% Selectivity
Percentage of respective product in the converted feed only
Table 2 Composition of
naphtha feeds
LSRN
Component/wt%
N-Paraffins
C-5
29.8
8.2
2.3
40.3
C-6
26.9
28.5
3.7
59.1
C-7
HSRN
LCN
Olefins
Aromatics
Total
0.6
Total
57.3
36.7
6.0
C-6
4.8
1.5
2.2
0.3
8.7
C-7
11.4
8.5
5.5
2.5
27.9
C-8
10.0
9.4
3.3
6.4
29.1
C-9
7.0
8.6
2.7
5.0
23.2
C-10
2.8
3.6
0.6
1.3
8.4
C-11
0.9
1.1
0.1
0.1
C-12
0.2
0.1
Total
37.0
32.9
C-4
100.0
2.3
0.3
14.4
15.7
0.6
100.0
0.6
C-5
4.5
24.7
22.2
1.6
C-6
C-7
1.5
0.8
9.4
2.9
4.2
0.9
3.5
3.7
0.5
0.3
0.18
Total
6.8
37.5
28.2
9.0
C-5
0.2
1.5
1.1
0.1
C-6
0.1
0.4
0.5
0.2
C-7
53.1
11.8
6.7
30.4
14.9
18.5
100
1.0
2.9
1.2
0.1
20.7
20.8
C-8
0.3
1.0
1.3
0.6
33.0
36.2
C-9
0.3
1.5
0.3
0.6
23.4
26.0
C-10
0.3
2.1
0.1
0.2
6.0
8.6
C-11
0.2
2.1
0.1
1.6
3.9
1.9
84.6
100.0
C-12
Total
0.2
1.4
from 625 °C to 650 °C was more pronounced compared
with the increase in conversion from 600 to 625 °C. This is
probably because the hydrocarbon molecules undergo
further cracking at 650 °C than when the temperature is
625 °C or 600 °C. Dry gas yield showed an increasing
trend with conversion at all temperatures for both LSRN
and HSRN. The rate of increase in dry gas with conversion
is higher for the LSRN feed. For both LSRN and HSRN,
total light olefins (C2= ? C3= ? C4=) yield showed a
linear trend with increasing conversion at all temperatures,
123
Naphthenes
0.6
C-8
HCN
iso Paraffins
8.8
0.2
3.3
whereas HSRN showed higher total light olefins than
LSRN.
Figure 4 shows the trend of each of the light olefins
yields with conversion. Propylene yield showed a linear
trend with increasing conversion at all temperatures for
both LSRN and HSRN. HSRN showed higher propylene
yields when compared to those of LSRN, because HSRN is
made up of molecules with longer chains which are more
reactive than those found in LSRN. Ethylene yields showed
an increasing trend with increasing conversion at all
Int J Ind Chem (2017) 8:221–233
225
Table 3 Comparison of naphtha cracking at 650 °C
Feed
HCN
LCN
HSRN
LSRN
CAT/OIL
5.98
6.04
5.71
5.95
CONV.(%)
14.71
40.07
34.79
26.55
Yields (wt%)
Methane
0.85
2.81
1.81
2.33
Ethylene (C2=)
4.75
9.14
5.45
4.79
Propylene (C3=)
4.78
17.75
12.94
10.12
Coke
0.79
0.07
0.66
0.70
6.60
13.48
9.04
9.78
more than that at 600 °C. Propylene and ethylene yields
showed a linear trend with increasing conversion at all
temperatures for LCN feed. Although butylenes yield
showed an increasing trend with conversion, this trend was
not very sharp at higher temperatures of 625 and 650 °C.
Dry gas yield showed an almost linear increase with conversion at all temperatures. Total light olefins
(C2= ? C3= ? C4=) yield with conversion showed a linear
trend with increasing conversion at all temperatures for
LCN.
Groups
H2–C2 (Dry gas) incl C2=
All C3–C4 (LPG)
7.32
26.53
25.09
16.08
C2= - C4= (Total light olefins)
C3= ? C4= (LPG olefins)
11.23
6.48
33.63
24.49
25.79
20.34
18.46
13.66
C4= (Butenes)
1.70
6.74
7.40
3.54
Dry gas
44.86
33.63
25.98
36.82
Propylene
32.49
44.28
37.19
38.12
Coke
5.36
0.16
1.89
2.64
Selectivities
temperatures for both LSRN and HSRN. Butylenes yield
showed a linear trend with increasing conversion at all
temperatures for both LSRN and HSRN. HSRN showed
higher butylenes yields when compared to those of LSRN.
For HSRN, the difference in butylenes yields at 625 and
650 °C was less pronounced.
Catalytic cracking of LCN
Figure 5 shows the change in conversion and yield pattern
with increasing C/O from 3 to 6 at different temperatures
for LCN. At all temperatures, cracked naphtha feed showed
an increasing trend in conversion. The conversion at the
highest C/O of 6 for temperature 650 °C was about 31%
45
30
Propylene yield (wt%)
Conversion (wt%)
35
The hydrogen transfer index (HTI) and the cracking
mechanism ratio (CMR) for the cracking of the three
naphtha fractions are shown in Fig. 6. The HTI describes
the degree of hydrogen transfer reaction, which reduces
the olefin yield in the products and, in this study, the
HTI was measured using the ratio of C4=/C4. From
Fig. 6, the general trend was that the HTI increased with
increasing temperature and, for the same temperature,
the HTI decreased with conversion. This is because
hydrogen transfer is an exothermic reaction with a
slower reaction rate and it is not favored by a high
reaction temperature and shorter reaction time, but being
a bimolecular reaction, it is promoted by a higher acid
site density, which is provided by increasing the C/
O ratio [20, 21].
The cracking mechanism ratio (CMR), which is defined
as the ratio of dry gases (methane, ethane, and ethylene) to
isobutane in the gas products, is used to measure the ratio
of monomolecular to bimolecular types of cracking, since
C1 and C2 are typical products from protolytic cracking,
while iC4 is a typical product formed by b-scission of
branched products [18, 22, 23].
20
HCN
LCN
HSRN
LSRN
40
Hydrogen transfer index (HTI) and cracking
mechanism ratio (CMR)
25
20
15
10
HCN
LCN
HSRN
LSRN
15
10
5
5
0
0
550
600
650
700
550
Temperature (C)
600
650
700
Temperature (C)
Fig. 2 Impact of temperature on various naphtha feeds cracking at C/O of about 6
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Int J Ind Chem (2017) 8:221–233
40
40
LSRN-600
LSRN-625
LSRN-650
30
HSRN-600
HSRN-625
HSRN-650
35
Conversion (wt%)
Conversion (wt%)
35
25
20
15
10
5
30
25
20
15
10
5
0
0
0
2
4
6
8
0
2
LSRN-600
LSRN-625
LSRN-650
8
20
30
40
30
40
HSRN-600
HSRN-625
HSRN-650
10
Dry gas yield (wt%)
10
Dry gas yield (wt%)
6
12
12
8
6
4
8
6
4
2
2
0
0
0
10
20
30
0
40
10
Conversion (wt%)
Conversion (wt%)
30
30
LSRN-600
LSRN-625
LSRN-650
25
Total Light Olefins yield (wt%)
Total Light Olefins yield (wt%)
4
C/O
C/O
20
15
10
5
0
HSRN-600
HSRN-625
HSRN-650
25
20
15
10
5
0
0
10
20
30
40
Conversion (wt%)
0
10
20
Conversion (wt%)
Fig. 3 Conversion and yield data for LSRN and HSRN fractions
A qualitative estimation of the relative importance of the
two cracking mechanisms can be made using the cracking
mechanism ratio (CMR):
P
ðC1 þ C2 Þ
CMR ¼
;
ð2Þ
iC4
123
where C1, C2, and iC4 denote the selectivities to methane,
ethane and ethylene, and i-butane, respectively.
A high CMR value ([1) reflects an important contribution of the protolytic cracking route, while a low value
(\1) indicates the prevalence of the classical b-scission
Int J Ind Chem (2017) 8:221–233
227
16
16
LSRN-600
LSRN-625
LSRN-650
12
HSRN-600
HSRN-625
HSRN-650
14
C3= yield (wt%)
C3= yield (wt%)
14
10
8
6
12
10
8
6
4
4
2
2
0
0
0
10
20
30
0
40
10
Conversion (wt%)
LSRN-600
LSRN-625
LSRN-650
6
40
30
40
30
40
HSRN-600
HSRN-625
HSRN-650
5
5
C2= yield (wt%)
C2= yield (wt%)
30
6
7
4
3
2
4
3
2
1
1
0
0
0
10
20
30
0
40
10
20
Conversion (wt%)
Conversion (wt%)
8
8
LSRN-600
LSRN-625
LSRN-650
6
HSRN-600
HSRN-625
HSRN-650
7
Total C4= yield (wt%)
7
Total C4= yield (wt%)
20
Conversion (wt%)
5
4
3
2
6
5
4
3
2
1
1
0
0
0
10
20
30
40
0
10
20
Conversion (wt%)
Conversion (wt%)
Fig. 4 Light olefins yields of LSRN and HSRN cracking at different temperatures
cracking mechanism. From Fig. 6, it can be seen that the
protolytic (monomolecular) cracking mechanism was more
predominant than the beta scission (bimolecular) cracking
mechanism for all the three naphtha fractions. It was also
found that for each reaction temperature, as the conversion
increased due to an increase in C/O, the CMR decreased as
the contribution of the wide pore zeolite was increased.
This shows that the production of light olefins is favored
when protolytic cracking mechanism becomes dominant
over classical bimolecular cracking reactions.
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Int J Ind Chem (2017) 8:221–233
16
45
LCN-600
LCN-625
LCN-650
Dry gas yield (wt%)
Conversion (wt%)
40
LCN-600
LCN-625
LCN-650
14
35
30
25
12
10
8
6
4
2
20
2
3
4
5
6
0
7
20
25
C/O
40
45
40
45
40
45
10
LCN-600
LCN-625
LCN-650
LCN-600
LCN-625
LCN-650
8
C2= yield (wt%)
18
C3= yield (wt%)
35
Conversion (wt%)
20
16
14
12
6
4
2
10
0
20
25
30
35
40
45
20
25
Conversion (wt%)
30
35
Conversion (wt%)
8
40
LCN-600
LCN-625
LCN-650
7
Total Light Olefins yield (wt%)
C4= yield (wt%)
30
6
5
LCN-600
LCN-625
LCN-650
35
30
25
20
15
4
20
25
30
35
Conversion (wt% )
40
45
20
25
30
35
Conversion (wt% )
Fig. 5 Impact of increasing C/O on the conversion and yields of LCN cracking
Thermal cracking index (TCI)
The contribution of thermal cracking to the cracking of
naphtha is illustrated in Fig. 7. The contribution of thermal cracking was measured using the thermal cracking
123
index (TCI), defined as the weight ratio of the sum of C1
and C2 yields to the sum of isobutane and isobutene
yields [21].
P
C1 þ C2
:
ð3Þ
TCI ¼
iC4 þ iC¼
4
Int J Ind Chem (2017) 8:221–233
229
8
30
7
25
6
20
4
CMR
HTI
5
3
10
2
LSRN-600
LSRN-625
LSRN-650
1
0
15
0
10
20
LSRN-600
LSRN-625
LSRN-650
5
0
30
0
10
Conversion (wt%)
5
8
HSRN-600
HSRN-625
HSRN-650
4
30
HSRN-600
HSRN-625
HSRN-650
7
6
5
CMR
3
HTI
20
Conversion (wt%)
2
4
3
2
1
1
0
0
0
14
10
20
30
Conversion (wt%)
0
40
20
30
40
Conversion (wt%)
25
LCN-600
LCN-625
LCN-650
12
10
LCN-600
LCN-625
LCN-650
20
15
8
CMR
HTI
10
6
4
5
2
0
10
0
20
25
30
35
40
45
20
25
30
35
40
45
Conversion (wt%)
Conversion (wt%)
Fig. 6 Impact of temperature on HTI and CMR
A value of TCI\0.6 means that catalytic cracking is the
main reaction, while a value of TCI [1.2 means that
thermal cracking is serious [21].
From Fig. 7, it can be seen that the TCI was much
greater than 1 under most conditions indicating that at these
temperatures, thermal cracking was serious and as such
contributed to the product yield.
The general trend for all three fractions was that TCI
increased as the reaction temperature increased. This is
expected because an increase in temperature will lead to
more thermal cracking contributions as less stable intermediates undergo further reaction.
Discussion
The cracking reactions can be categorized into two types,
namely catalytic cracking and thermal cracking. Catalytic
cracking is endothermic, occurs on the surface of the
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230
Int J Ind Chem (2017) 8:221–233
3
7
HSRN-600
HSRN-625
HSRN-650
6
2
4
3
TCI
TCI
5
2
LSRN-600
LSRN-625
LSRN-650
1
0
0
10
20
1
0
0
30
10
Conversion (wt%)
5
30
40
LCN-600
LCN-625
LCN-650
4
TCI
20
Conversion (wt%)
3
2
1
0
20
25
30
35
40
45
Conversion (wt%)
Fig. 7 Effect of temperature on TCI
catalyst and yields high C3 and C4 olefins. So, the reaction
temperature and acidic properties of the catalyst play a
major role in the activity and selectivity of the reaction.
The thermal cracking reaction is also endothermic and
follows the free radical mechanism which generates ethylene and dry gas. For increasing propylene production, the
reaction conditions have to be optimized to minimize
thermal cracking.
The results from this study (Figs. 2, 3, 4, 5) show that
reaction temperature, C/O ratio and feed characteristics
have an effect on light olefin yield during catalytic
cracking. The yield of light olefins and selectivity to
propylene when using HCN feed are the lowest compared
to the other feeds because of its high aromatic content. It
has been shown in the literature that hydrocarbon feeds
with a high C/H ratio (low hydrogen content) are difficult
to convert under FCC conditions [3]. The production of
propylene requires a disproportionate share of the
hydrogen and co-products such as propane, and dry gas
requires an even greater share of hydrogen. Therefore, the
amount of hydrogen available from the feedstock can
limit the potential to produce propylene. Subsequently,
propylene production is highly dependent on feed
properties.
123
In contrast to HCN, LCN is the most reactive and shows
the highest conversion. It also shows the highest yield for
light olefins as well as selectivity. This is because olefins
make up a great portion of this feed (28.5 wt%), and with
the right type of catalyst this can act as intermediate for
high yield of light olefins.
LSRN is predominantly C5–C6 while HSRN is mostly
C6–C10 with much smaller fractions of C11–C12. Their
compositions should explain why HSRN is more reactive
than LSRN, because HSRN is made up of longer chain
molecules, which are easier to crack than the short length
LSRN counterpart. This also has an effect on the product
yield, as HSRN shows a higher light olefin yield.
The overall classification in terms of reactivity is as
follows: LCN [ HSRN [ LSRN [ HCN.
Influence of catalyst-to-oil ratio on FCC naphtha
cracking
In order for propylene to be produced, the cracking of FCC
naphtha has to be carried out at high temperatures, but
increasing temperature favors thermal cracking which
leads to the formation of dry gas from hydrocarbon [24].
Int J Ind Chem (2017) 8:221–233
Raising the temperature is most effective in controlling
the exothermic hydrogen transfer reaction and accelerating
the catalytic cracking which is endothermic. In a commercial unit, the high reaction temperature is achieved by
raising the catalyst circulation rate. This helps to improve
the rate of reaction as well as the propylene yield. Since the
hydrogen transfer reaction is a secondary reaction, it is
better controlled using a short contact time to limit the
hydrogenation of olefin over cracking reactions [3, 8].
By increasing C/O ratio, contact opportunities between
catalyst active centers and hydrocarbon molecules are
improved leading to enhanced selectivity and yields of
light olefins such as propylene and butylene during catalytic cracking of hydrocarbons.
As can be seen from Figs. 3 and 5, when there is an
increase in C/O, the conversion, total light olefin yield and
the yields of dry gas increase. The reactions of olefins take
place more easily at high C/O due to its high reactivity over
catalyst active centers and this explains the high yield of
light olefins with LCN as feed. Compared to olefins,
paraffins have a lower reactivity towards cracking due to
energetically more difficult formation of carbenium ions.
Increasing the catalyst to oil ratio increases the severity
of the hydrogenation and cracking reaction leading to over
cracking and more saturates. Subsequently, more dry gas is
produced, which in this case is made up of mainly methane
and ethane. For thermal cracking, free radicals are formed
through hydrocarbons splitting their C–C bonds and H–C
bonds, and then inclined to undergoing alpha scission, beta
scission, and polymerization to produce H2, methane,
ethylene and coke [24]. However, at higher C/O, thermal
cracking is effectively inhibited as hydrocarbons have
more opportunities to contact with active centres of catalysts leading to catalytic cracking. Therefore, the yields of
C1 and H2 decrease as the C/O is enhanced. But because of
more small pore-zeolite (ZSM-5) being available at higher
C/O, which are more favorable for the formation of ethane,
therefore, mostly the yields of ethylene and ethane in dry
gas are enhanced. Also, a very high C/O does not necessarily lead to a significant increase of propylene and
butylene due to the enhanced hydrogen transfer reaction
stemming from the presence of more Y zeolite-based catalyst. As Y zeolite-based catalyst are used in FCC units, the
bimolecular mechanism competes with cracking to form
light olefins.
For maximizing light olefin production, FCC units
should be operated at high severity, which includes high C/
O ratios, high temperatures, and short contact time to
minimize the hydrogen transfer reaction.
It has been well established that the conversion of light
hydrocarbons occurs via carbenium ion chemistry
[10, 25–27] involving the monomolecular and bimolecular
mechanisms.
231
The steps for the catalytic cracking mechanism of
alkanes [10, 25, 26, 28] are summarized below and illustrated in Fig. 8:
1.
2.
3.
A carbenium ion is first generated by protonation of a
paraffin molecule on a Brönsted acid site to form a
carbonium ion (a pentacoordinated ion),
The carbonium ion then immediately cracks to give a
carbenium ion and a paraffin.
Then, the carbenium ion cracks to give an olefin and an
acidic proton on the surface of the catalyst.
For the Bimolecular mechanism, the carbenium ion in
step 3 acts as an active center by reacting with a feed
paraffin molecule to form a larger molecule that further
cracks according to the following steps:
4.
5.
A paraffin reacts with carbenium ion to form a smaller
paraffin and a bigger carbenium ion.
The bigger carbenium ion then cracks into an olefin
and a carbenium ion through beta scission.
The bimolecular mechanism explain the hydrogen
transfer reaction and the formation of heavier compounds
than those found in the feed such as coke, light cycle oil and
heavy cycle oil. Hydrogen transfer reaction usually occurs
from an olefin to a carbenium ion on the catalyst surface
giving rise to a paraffin and a hydrogen-deficient species
(allylcarbenium ion) [29]. The hydrogen-deficient species
can be further transformed into aromatics and coke via
further dehydrogenation and cyclization reactions. Hydrogen transfer reactions are prevalent at high conversions and
help to decrease the selectivity to olefins as shown in Fig. 8.
Thus, a bimolecular cracking mechanism will preferentially lead to the formation of aromatics from the
naphthenes, since the carbenium may be directly formed
from the feed molecule through a hydride transfer, while a
monomolecular cracking mechanism will preferentially
lead to short olefins. Then, the balance between hydrogen
transfer reactions and cracking reactions will be critical in
orienting the selectivity of olefins and naphthenes’ conversion towards either propylene or gasoline-range aromatics or paraffins.
The balance between these two mechanisms will depend
on the surface coverage by carbenium ions. The presence
of carbenium ions is influenced by reaction temperature
and catalyst acid site density. The higher number of acid
sites favors the bimolecular mechanism. The acid site
density may be decreased by dealumination or coking.
Catalysts with small pore sizes such as ZSM-5 favor the
monomolecular mechanism since there is limited space for
the formation of the intermediate species for the bimolecular mechanism. Propylene production is hampered by
hydrogen transfer reaction as it consumes the olefins generated by the catalytic cracking. This hydrogen transfer is
123
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Int J Ind Chem (2017) 8:221–233
RH
RH
1
R1H
RH
+
RH2
(1)
(4)
(2)
+
+
R2
H
desorpon
+
+
R
R
2
Beta-scission
(5)
(3)
alkene
Monomolecular Mechanism
alkene
Bimolecular Mechanism
Fig. 8 Illustration of cracking mechanism
generally known to have a bimolecular reaction and it
depends on the acidity of the catalyst, contact time and
reaction temperature.
Conclusions
The results from these experiments show that propylene
yield depends on the naphtha fraction being used as feed.
LCN which contains a high percentage of olefins showed
high reactivity and gave the highest propylene and light
olefin yields, while HCN which is made up of mostly
aromatics was the least reactive naphtha fraction. For
straight run naphtha, HSRN showed a higher reactivity
compared to LSRN. This is because, LSRN is made up of
mainly C5–C6 molecules and are difficult to crack compared to HSRN which is made up of mainly C6–C11.
The results from this study further provide a guideline
for processing naphtha fractions under high severity conditions for the production of light olefins. This will require
the fine tuning of the catalyst system and reaction conditions to maximize the yields of light olefins.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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