Abstract: A 10%Co/TiO2 catalyst has been synthesized by incipient wetness impregnation method and tested in a fixedbed reactor at 20 bar and various temperatures to assess the effect of operating temperature on Fischer-Tropsch (FT) reaction. Increasing the operating temperature (from 200oC - 220oC) led to an increase in methane and light product selectivity but increased the CO conversion rate and the net C5+ production rate. Apparent activation energy for CO conversion and light alkanes (up to C4) formation has also been estimated using Arrhenius plot

Kalala, Jalama

English

University of Johannesburg Institutional Repository

  Abstract—A 10%Co/TiO2 catalyst has been synthesized by incipient wetness impregnation method and tested in a fixed-bed reactor at 20 bar and various temperatures to assess the effect of operating temperature on Fischer-Tropsch (FT) reaction. Increasing the operating temperature (from 200oC - 220oC) led to an increase in methane and light product selectivity but increased the CO conversion rate and the net C5+ production rate. Apparent activation energy for CO conversion and light alkanes (up to C4) formation has also been estimated using Arrhenius plot.   Index Terms—Co/TiO2, Effect of temperature, Fischer-Tropsch  I. INTRODUCTION HE world’s energy demand has significantly increased in the last decade and is expected to increase as more countries are being industrialized and the world population is increasing. A significant portion of energy demand is from the transportation sector which mainly uses crude oil-derived petrol/diesel. Unfortunately, the reserves of crude oils are dwindling and strategies for alternative fuels production have become the focus of many research activities around the world. Fischer-Tropsch (FT) synthesis is one of many alternative processes for producing fuels. It converts synthesis gas (a mixture of H2 and CO) derived from various sources such as coal, natural gas or biomass into liquid fuels. The technology has been used at industrial scale mainly by SASOL in South Africa and SHELL in Malaysia. The most recently completed commercial FT projects include the Oryx GTL in Qatar by Sasol with Qatar Petroleum in 2007 [1] and the Pearl GTL by Shell and the government of the Qatar State in 2012 [2]. More projects are currently in various development stages and indicate an increased interest in FT processes than ever before. While all big projects completed to date use either coal- or natural gas-derived synthesis gas, research on using biomass-derived synthesis gas has also significantly developed.  A catalyst is always required for the FT reaction to occur. Cobalt and iron-based catalysts are the most used for industrial application [3]. The performance of these catalysts depends on a number of factors such as operating pressure, temperature, gas composition and flowrates. A  Manuscript received July 13, 2015; revised August 15, 2015. This work was supported in part by the National Research Foundation (NRF) and the Faculty of Engineering and the Built Environment at the University of Johannesburg.  K. Jalama is the Department of Chemical Engineering, University of Johannesburg, South Africa (corresponding author, e-mail: kjalama@uj.ac.za, phone: +27-11-559-6157).   better understanding of these factors can unfold opportunities for modeling FT processes more accurately and improve the development of new technologies. The present study aims at evaluating the effect of operating temperature on the CO conversion rate and product selectivity during FT reaction using a titania-supported cobalt catalyst. In addition, the apparent activation energy for CO conversion and light hydrocarbons formation has been estimated. II. EXPERIMENTAL DETAILS     The catalyst was prepared by incipient wetness impregnation method to achieve ca. 10 wt.% of Co on TiO2 support. Prior to the FT reaction, the catalyst was first activated around atmospheric pressure using a gas mixture containing 5% H2 in Ar at 350oC for 14 hours in a fixed-bed reactor. The FT reaction was performed at 20 bar and 200, 210 and 220oC respectively. The reaction feed was maintained at 10 Nml/min using an Aalborg mass flow controller and contained around 10% N2, 30% CO with an H2 balance. 0.5 g of catalyst was used for the reaction and loaded between two layers of quartz wool in the middle of the reactor. The rest of the reactor was filled with stainless balls of 3 mm in diameter to improve heat transfer. Two knock-out pots were mounted downstream the reactor for product collection and the outlet gas products were analyzed on-line using a Dani Master GC equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID). A 60/80 Carboxen 1000 column was connected to the TCD and was able to separate H2, N2, CO, CH4 and CO2 while the analysis of organic products was performed on a Supel-Q Plot fused silica capillary column connected to the FID. III. RESULTS AND DISCUSSION  The effect of operating temperature on CO conversion and methane selectivity is shown in fig. 1. Both the CO conversion and methane selectivity increased with an increase in temperature. CO conversion levels of ca. 13.2, 15.0 and 19.4% were measured at temperatures of 200, 210 and 220oC respectively. The methane selectivity followed a similar trend with values of ca. 6.1, 10.1 and 19.4% at 200, 210 and 220oC respectively.  Effect of Temperature on CO Rate and Product Distribution during Fischer-Tropsch Reaction over Co/TiO2 Catalyst Kalala JalamaT 0510152025195 200 205 210 215 220 225% CO conv. and CH4 select. Reaction temperature [oC]CO convCH4 select. Fig. 1 Effect of operating temperature on CO conversion and methane selectivity  The selectivity to C2-C4 and C5+ hydrocarbons was also significantly influenced by the operating temperature as shown in fig. 2. The selectivity to C2-C4 increased from ca. 3.4% to 8.3 and 13.5% when the operating temperature was increased from 200 to 210 and 220oC respectively. The selectivity to C5+ decreased from ca. 90.5% to 81.6 and 67.1% upon increasing the reaction temperature from 200oC to 210 and 220oC respectively.   0204060801001 2 3Select.[ %]Temperature [oC]C2-C4 C5+200                     210                    220 Fig. 2 Effect of operating temperature on C2-C4 and C5+ selectivities  These results suggest that an increase in operating temperature for the FT reaction has a negative effect on the product selectivity as it increases the methane and light hydrocarbons selectivities. On the other hand, higher operating temperatures result in higher catalyst activity. In order to determine the optimum operating temperature, in addition to the CO conversion and product selectivity, one must also consider a number of factors such as the rate of liquid hydrocarbons production (rC5+) and catalyst deactivation profile. The rate of C5+ hydrocarbons production as well as the chain growth probability (alpha) as function of the operating temperature are reported in table 1. The alpha value did not change when the temperature was increased from 200 to 210oC but slightly decreased from 0.52 to 0.50 when the temperature was increased from 210 to 220oC. However, the productivity for C5+ hydrocarbons increased from 0.031 g/gCat/h to 0.035 and 0.047 g/gCat/h when the temperature was increased from 200 to 210 and 220oC respectively. Even though the highest temperature used in this study led to undesirable high selectivity to light products, it however also led to higher CO conversion rates resulting in higher C5+ production rate. The effect of operating temperature on catalyst deactivation profile is part of a separate study and will be reported in future communications.   TABLE I EFFECT OF OPERATING TEMPERATURE ON CHAIN GROWTH PROBABILITY AND C5+ HYDROCARBONS FORMATION RATE  Temp. [oC] Alphaa rC5+ [g/gCat/h]200 0.52 0.031210 0.52 0.035220 0.50 0.047  a: based on gas phase product excluding methane and C2, i.e. based on C3 to uncondensed C7  Olefin to paraffin (O/P) ratios are also useful parameters that give an indication on the catalyst hydrogenation properties. The results in fig. 3 show that an increase in reaction temperature leads to more olefins hydrogenation to paraffins as suggested by the decrease in O/P ratio. For example, an ethylene to ethane ratio of ca. 0.85 was measured at 200oC, decreased to ca. 0.45 at 210oC and ethylene almost disappeared when the temperature was increased to 220oC as its corresponding GC peak was below the detection threshold. Similar trends can be observed for C3 and C4 O/P ratios.  00.40.81.21.621 2 3O/P ratioCarbon number200oC 210oC 220oC2                              3                               4 Fig. 3 Effect of operating temperature on light olefin-to-paraffin ratio  Arrhenius plots for CO conversion and light hydrocarbons formation have been generated to evaluate their apparent activation energies. The plot for CO conversion rate was fairly linear (fig. 4) with a calculated activation of ca. 39 kJ/mol. This value was significantly lower than values around 100 kJ/mol reported by other studies [4, 5] that also used cobalt-based catalyst probably due to the difference in operating conditions and type of catalyst used. For example Yan et al. [5] conducted their experiments at very low pressure (0.33 to 1 bar) and a H2/CO ratio of 1 over a model Co/SiO2 catalyst different from our conditions where the reaction was performed at 20 bar with a H2/CO of 2.   00.511.520.00202 0.00204 0.00206 0.00208 0.0021 0.00212ln (-rCO) 1/T [K-1] Fig. 4 Arrhenius plot for CO conversion   Arrhenius plots for light hydrocarbons are shown in fig 5. A linear trend line could not be properly fitted to light olefins (fig. 5a). However, satisfactory linear trends were obtained for light alkanes (fig. 5b) with their calculated apparent activation energies summarized in table 2. Methane had the lowest activation energy of ca. 106.8 kJ/mol and this can explain its higher selectivity compared to other light alkanes. The apparent activation energy for C2 was the highest at ca. 183 kJ/mol. Thereafter, it decreased with an increase in carbon chain length.   -3-2.5-2-1.5-1-0.50.00202 0.00204 0.00206 0.00208 0.0021 0.00212ln (Rate)1/T [K-1]C2C3C4Linear (C3)Linear (C4)-4-3-2-101230.00202 0.00204 0.00206 0.00208 0.00210 0.00212ln (Rate)1/T [K-1]C2 C3C4 C1Linear (C2) Linear (C3)Linear (C4) Linear (C1)a)b) Fig. 5 Arrhenius plot for light hydrocarbons: a) olefins, b) alkanes         TABLE II APPARENT ACTIVATION ENERGY FOR LIGHT SATURATED HYDROCARBONS FORMATION  Apparent Ea[kJ/mol]C1 106.8C2 182.9C3 168.2C4 152.3   IV. CONCLUSION     The effect of operating temperature on FT reaction over a cobalt-based catalyst has been evaluated. The data show that increasing the temperature from 200oC to 210 and 220oC leads to an increase in methane and light product selectivity but increases the CO conversion rate and the net C5+ production rate. An apparent CO activation energy of ca. 39 kJ/mol was determined using Arrhenius plot. Apparent activation energies for light alkanes (up to C4) have also been estimated and show the lowest value (ca. 106.8 kJ/mol) for methane and the highest (ca. 183 kJ/mol) for ethane. Arrhenius plots for light olefins did not give a satisfactory linear trend.   REFERENCES [1] Sasol projects: www.sasol.co.za/innovation/gas-liquids/projects [2] Pearl GTL- An overview: www.shell.com/global/aboutshell/major-projects-2/pearl/overview.html [3] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 (2004) 1-680. [4] C.G. Cooper, T-H. Nguyen, Y-J. Lee, K. M. Hardiman, T. Safinski, F. P. Lucien, A. A. Adesina, Catalysis Today 131 (2008) 255-261 [5] Z. Yan, Z. Wang, D. B. Bukur, D. Goodman, Journal of Catalysis 268 (2009) 196–200   

Effect of temperature on CO rate and product distribution during Fischer-Tropsch reaction over Co/TiO2 catalyst

Name not available

  
Abstract—A 10%Co/TiO2 catalyst has been synthesized by 
incipient wetness impregnation method and tested in a fixed-
bed reactor at 20 bar and various temperatures to assess the 
effect of operating temperature on Fischer-Tropsch (FT) 
reaction. Increasing the operating temperature (from 200oC - 
220oC) led to an increase in methane and light product 
selectivity but increased the CO conversion rate and the net 
C5+ production rate. Apparent activation energy for CO 
conversion and light alkanes (up to C4) formation has also been 
estimated using Arrhenius plot.  
 
Index Terms—Co/TiO2, Effect of temperature, Fischer-
Tropsch  
I. INTRODUCTION 
HE world’s energy demand has significantly increased 
in the last decade and is expected to increase as more 
countries are being industrialized and the world 
population is increasing. A significant portion of energy 
demand is from the transportation sector which mainly uses 
crude oil-derived petrol/diesel. Unfortunately, the reserves 
of crude oils are dwindling and strategies for alternative 
fuels production have become the focus of many research 
activities around the world. Fischer-Tropsch (FT) synthesis 
is one of many alternative processes for producing fuels. It 
converts synthesis gas (a mixture of H2 and CO) derived 
from various sources such as coal, natural gas or biomass 
into liquid fuels. The technology has been used at industrial 
scale mainly by SASOL in South Africa and SHELL in 
Malaysia. The most recently completed commercial FT 
projects include the Oryx GTL in Qatar by Sasol with Qatar 
Petroleum in 2007 [1] and the Pearl GTL by Shell and the 
government of the Qatar State in 2012 [2]. More projects 
are currently in various development stages and indicate an 
increased interest in FT processes than ever before. While 
all big projects completed to date use either coal- or natural 
gas-derived synthesis gas, research on using biomass-
derived synthesis gas has also significantly developed.  
A catalyst is always required for the FT reaction to occur. 
Cobalt and iron-based catalysts are the most used for 
industrial application [3]. The performance of these 
catalysts depends on a number of factors such as operating 
pressure, temperature, gas composition and flowrates. A 
 
Manuscript received July 13, 2015; revised August 15, 2015. This work 
was supported in part by the National Research Foundation (NRF) and the 
Faculty of Engineering and the Built Environment at the University of 
Johannesburg.  
K. Jalama is the Department of Chemical Engineering, University of 
Johannesburg, South Africa (corresponding author, e-mail: 
kjalama@uj.ac.za, phone: +27-11-559-6157).  
 
better understanding of these factors can unfold 
opportunities for modeling FT processes more accurately 
and improve the development of new technologies. 
The present study aims at evaluating the effect of operating 
temperature on the CO conversion rate and product 
selectivity during FT reaction using a titania-supported 
cobalt catalyst. In addition, the apparent activation energy 
for CO conversion and light hydrocarbons formation has 
been estimated. 
II. EXPERIMENTAL DETAILS 
    The catalyst was prepared by incipient wetness 
impregnation method to achieve ca. 10 wt.% of Co on TiO2 
support. Prior to the FT reaction, the catalyst was first 
activated around atmospheric pressure using a gas mixture 
containing 5% H2 in Ar at 350oC for 14 hours in a fixed-bed 
reactor. The FT reaction was performed at 20 bar and 200, 
210 and 220oC respectively. The reaction feed was 
maintained at 10 Nml/min using an Aalborg mass flow 
controller and contained around 10% N2, 30% CO with an 
H2 balance. 0.5 g of catalyst was used for the reaction and 
loaded between two layers of quartz wool in the middle of 
the reactor. The rest of the reactor was filled with stainless 
balls of 3 mm in diameter to improve heat transfer. Two 
knock-out pots were mounted downstream the reactor for 
product collection and the outlet gas products were analyzed 
on-line using a Dani Master GC equipped with a thermal 
conductivity detector (TCD) and flame ionization detector 
(FID). A 60/80 Carboxen 1000 column was connected to 
the TCD and was able to separate H2, N2, CO, CH4 and CO2 
while the analysis of organic products was performed on a 
Supel-Q Plot fused silica capillary column connected to the 
FID. 
III. RESULTS AND DISCUSSION  
The effect of operating temperature on CO conversion and 
methane selectivity is shown in fig. 1. Both the CO 
conversion and methane selectivity increased with an 
increase in temperature. CO conversion levels of ca. 13.2, 
15.0 and 19.4% were measured at temperatures of 200, 210 
and 220oC respectively. The methane selectivity followed a 
similar trend with values of ca. 6.1, 10.1 and 19.4% at 200, 
210 and 220oC respectively. 
 
Effect of Temperature on CO Rate and Product 
Distribution during Fischer-Tropsch Reaction 
over Co/TiO2 Catalyst 
Kalala Jalama
T
 0
5
10
15
20
25
195 200 205 210 215 220 225
%
 C
O
 c
on
v.
 a
nd
 C
H
4 
se
le
ct
. 
Reaction temperature [oC]
CO conv
CH4 select.
 
Fig. 1 Effect of operating temperature on CO conversion 
and methane selectivity 
 
The selectivity to C2-C4 and C5+ hydrocarbons was also 
significantly influenced by the operating temperature as 
shown in fig. 2. The selectivity to C2-C4 increased from ca. 
3.4% to 8.3 and 13.5% when the operating temperature was 
increased from 200 to 210 and 220oC respectively. The 
selectivity to C5+ decreased from ca. 90.5% to 81.6 and 
67.1% upon increasing the reaction temperature from 200oC 
to 210 and 220oC respectively.  
 
0
20
40
60
80
100
1 2 3
Se
le
ct
.[ 
%
]
Temperature [oC]
C2-C4 C5+
200                     210                    220
 
Fig. 2 Effect of operating temperature on C2-C4 and C5+ 
selectivities 
 
These results suggest that an increase in operating 
temperature for the FT reaction has a negative effect on the 
product selectivity as it increases the methane and light 
hydrocarbons selectivities. On the other hand, higher 
operating temperatures result in higher catalyst activity. In 
order to determine the optimum operating temperature, in 
addition to the CO conversion and product selectivity, one 
must also consider a number of factors such as the rate of 
liquid hydrocarbons production (rC5+) and catalyst 
deactivation profile. The rate of C5+ hydrocarbons 
production as well as the chain growth probability (alpha) as 
function of the operating temperature are reported in table 1. 
The alpha value did not change when the temperature was 
increased from 200 to 210oC but slightly decreased from 
0.52 to 0.50 when the temperature was increased from 210 
to 220oC. However, the productivity for C5+ hydrocarbons 
increased from 0.031 g/gCat/h to 0.035 and 0.047 g/gCat/h 
when the temperature was increased from 200 to 210 and 
220oC respectively. Even though the highest temperature 
used in this study led to undesirable high selectivity to light 
products, it however also led to higher CO conversion rates 
resulting in higher C5+ production rate. The effect of 
operating temperature on catalyst deactivation profile is part 
of a separate study and will be reported in future 
communications. 
 
 
TABLE I 
EFFECT OF OPERATING TEMPERATURE ON CHAIN GROWTH 
PROBABILITY AND C5+ HYDROCARBONS FORMATION RATE 
 
Temp. [oC] Alphaa rC5+ [g/gCat/h]
200 0.52 0.031
210 0.52 0.035
220 0.50 0.047  
a: based on gas phase product excluding methane and C2, i.e. 
based on C3 to uncondensed C7 
 
Olefin to paraffin (O/P) ratios are also useful parameters 
that give an indication on the catalyst hydrogenation 
properties. The results in fig. 3 show that an increase in 
reaction temperature leads to more olefins hydrogenation to 
paraffins as suggested by the decrease in O/P ratio. For 
example, an ethylene to ethane ratio of ca. 0.85 was 
measured at 200oC, decreased to ca. 0.45 at 210oC and 
ethylene almost disappeared when the temperature was 
increased to 220oC as its corresponding GC peak was below 
the detection threshold. Similar trends can be observed for 
C3 and C4 O/P ratios. 
 
0
0.4
0.8
1.2
1.6
2
1 2 3
O
/P
 ra
tio
Carbon number
200oC 210oC 220oC
2                              3                               4
 
Fig. 3 Effect of operating temperature on light olefin-to-
paraffin ratio 
 
Arrhenius plots for CO conversion and light hydrocarbons 
formation have been generated to evaluate their apparent 
activation energies. The plot for CO conversion rate was 
fairly linear (fig. 4) with a calculated activation of ca. 39 
kJ/mol. This value was significantly lower than values 
around 100 kJ/mol reported by other studies [4, 5] that also 
used cobalt-based catalyst probably due to the difference in 
operating conditions and type of catalyst used. For example 
Yan et al. [5] conducted their experiments at very low 
pressure (0.33 to 1 bar) and a H2/CO ratio of 1 over a model 
Co/SiO2 catalyst different from our conditions where the 
reaction was performed at 20 bar with a H2/CO of 2. 
 
 0
0.5
1
1.5
2
0.00202 0.00204 0.00206 0.00208 0.0021 0.00212
ln
 (-
rC
O
) 
1/T [K-1]
 
Fig. 4 Arrhenius plot for CO conversion  
 
Arrhenius plots for light hydrocarbons are shown in fig 5. A 
linear trend line could not be properly fitted to light olefins 
(fig. 5a). However, satisfactory linear trends were obtained 
for light alkanes (fig. 5b) with their calculated apparent 
activation energies summarized in table 2. Methane had the 
lowest activation energy of ca. 106.8 kJ/mol and this can 
explain its higher selectivity compared to other light 
alkanes. The apparent activation energy for C2 was the 
highest at ca. 183 kJ/mol. Thereafter, it decreased with an 
increase in carbon chain length. 
 
 
-3
-2.5
-2
-1.5
-1
-0.5
0.00202 0.00204 0.00206 0.00208 0.0021 0.00212
ln
 (R
at
e)
1/T [K-1]
C2
C3
C4
Linear (C3)
Linear (C4)
-4
-3
-2
-1
0
1
2
3
0.00202 0.00204 0.00206 0.00208 0.00210 0.00212
ln
 (R
at
e)
1/T [K-1]
C2 C3
C4 C1
Linear (C2) Linear (C3)
Linear (C4) Linear (C1)
a)
b)
 
Fig. 5 Arrhenius plot for light hydrocarbons: a) olefins, b) 
alkanes 
 
 
 
 
 
 
 
 
TABLE II 
APPARENT ACTIVATION ENERGY FOR LIGHT SATURATED 
HYDROCARBONS FORMATION 
 
Apparent Ea
[kJ/mol]
C1 106.8
C2 182.9
C3 168.2
C4 152.3  
 
IV. CONCLUSION 
    The effect of operating temperature on FT reaction over a 
cobalt-based catalyst has been evaluated. The data show that 
increasing the temperature from 200oC to 210 and 220oC 
leads to an increase in methane and light product selectivity 
but increases the CO conversion rate and the net C5+ 
production rate. An apparent CO activation energy of ca. 39 
kJ/mol was determined using Arrhenius plot. Apparent 
activation energies for light alkanes (up to C4) have also 
been estimated and show the lowest value (ca. 106.8 
kJ/mol) for methane and the highest (ca. 183 kJ/mol) for 
ethane. Arrhenius plots for light olefins did not give a 
satisfactory linear trend. 
  
REFERENCES 
[1] Sasol projects: www.sasol.co.za/innovation/gas-liquids/projects 
[2] Pearl GTL- An overview: www.shell.com/global/aboutshell/major-
projects-2/pearl/overview.html 
[3] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 
(2004) 1-680. 
[4] C.G. Cooper, T-H. Nguyen, Y-J. Lee, K. M. Hardiman, T. Safinski, F. 
P. Lucien, A. A. Adesina, Catalysis Today 131 (2008) 255-261 
[5] Z. Yan, Z. Wang, D. B. Bukur, D. Goodman, Journal of Catalysis 268 
(2009) 196–200 
 
 


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Effect of temperature on CO rate and product distribution during Fischer-Tropsch reaction over Co/TiO2 catalyst

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