Abstract: Fischer-Tropsch (FT) runs were performed using a titania-supported cobalt catalyst at total operating pressures of 1, 10 and 20 bar respectively, to determine the optimal pressure for C5+ hydrocarbons production. The reaction temperature and feed flowrates were kept constant as 220oC and 20 Nml/gCat/min respectively. Lower methane and higher C5+ hydrocarbons selectivities were measured at higher pressure. However the highest rate of C5+ hydrocarbons production was measured at an optimum pressure of 10 bar

Kalala, Jalama

English

Name not available

  
Abstract—Fischer-Tropsch (FT) runs were performed using a 
titania-supported cobalt catalyst at total operating pressures of 
1, 10 and 20 bar respectively, to determine the optimal 
pressure for C5+ hydrocarbons production. The reaction 
temperature and feed flowrates were kept constant as 220oC 
and 20 Nml/gCat/min respectively.  Lower methane and higher 
C5+ hydrocarbons selectivities were measured at higher 
pressure. However the highest rate of C5+ hydrocarbons 
production was measured at an optimum pressure of 10 bar. 
 
 
Index Terms— C5+ production rate, Fischer-Tropsch, 
Operating pressure 
I. INTRODUCTION 
HE central process in gas/coal/biomass to liquid 
technologies is Fischer-Tropsch (FT) synthesis. 
Through this process, gas-, coal- or biomass-derived 
synthesis gas (a mixture containing H2 and CO) is converted 
to hydrocarbons. This process yields a broad product 
spectrum with different carbon chain length. The main 
products formed include paraffins, olefins and some side 
products such as oxygenates (alcohols, aldehydes, ketones 
and carboxylic acids) and branched compounds [1]. A 
significant amount of water is also produced during an FT 
reaction. For industrial production, it is of economic interest 
to maximize the formation of liquid products (C5+). The 
product distribution can be influenced by operating 
conditions (temperature, pressure, etc.), the type of reactor 
and the catalyst used [1]. 
This study aims at determining the optimal operating 
pressure that maximizes the yield of liquid hydrocarbons 
(C5+) for an FT reaction over a Co/TiO2 catalyst at 220oC 
and a feed gas flowrate of 10 Nml /min. 
II. EXPERIMENTAL DETAILS 
    P25 Degussa TiO2 powder was first mixed with deionized 
water, dried and calcined in air to make the catalyst support. 
The latter was subsequently impregnated with a solution of 
cobalt nitrate, dried and calcined in air in order to obtain a 
fresh catalyst with ca. 10 wt.% cobalt loading. The details 
on conditions used in each of the catalyst preparation steps 
are summarized in fig. 1 
0.5g of fresh catalyst was loaded and maintained at the 
middle of a fixed-bed reactor (internal diameter: 16 mm, 
length: 260 mm) by two layers of quartz wool (one on top 
and the other at the bottom of the catalyst bed). The rest of 
the reactor was filled with stainless steel balls to improve 
heat transfer. The catalyst was reduced around atmospheric 
pressure using a gas mixture (30 Nml/min) containing 5% 
H2 in Ar at 350oC for 14 hours. After cooling the reactor 
below 100oC, pre-mixed synthesis gas containing ca. 10% 
N2, 30% CO with an H2 balance was introduced in the 
reactor at a flow of 10 Nml/min using an Aalborg mass flow 
controller. The FT reaction was first performed at 1 bar and 
220oC and the pressure was further increased to 10 and 20 
bar respectively while all other operating conditions 
(temperature, feed flow rate and feed gas composition) were 
kept unchanged. The catalyst stabilized within 10 hours 
after changing the pressure. The gas pressure in the reactor 
was built by using a back pressure regulator mounted 
downstream the reactor after two product knock-out pots 
used to collect condensed products. A portion of gas 
products was introduced through a six-port Valco sampling 
valve into a Dani Master gas chromatograph (GC) for on-
line analysis. H2, N2, CO, CH4 and CO2 were separated 
using a 60/80 Carboxen 1000 column connected to a 
thermal conductivity detector (TCD) and organic products 
(light hydrocarbons up to C8) were separated in a Supel-Q 
Plot fused silica capillary column connected to a flame 
ionization detector (FID). The GC was calibrated on a daily 
basis using a calibration mixture with the molar composition 
reported in table 1. 
 
TABLE I 
MOLAR COMPOSITION FOR GC CALIBRATION MIXTURE 
 
Component Mol.% 
C2H4 0.98 
C2H6 0.98 
CH4 5.2 
CO2 4.8 
N2 10.4 
CO 24 
H2 53.64 
 
 
 
Fischer-Tropsch Synthesis Over Titania-
supported Cobalt Catalyst: Optimum Pressure 
for C5+ Hydrocarbons Production 
Kalala Jalama
T
 Mixing
Calc. Drying Impreg.
SievingCalc.Drying
TiO2
H2O
1:1 wt. ratio 120oC, 1h
400oC, 
16h in air
120oC, 
overnight
400oC, 
6h in air
Fresh catalyst
0.1 - 0.5 mm
Cobalt 
nitrate 
solution  
                                                                   Fig. 1 Detailed catalyst preparation steps 
 
 
III. RESULTS AND DISCUSSION  
The effect of operating pressure on CO conversion and CH4 
selectivity is summarized in fig. 2. CO conversion of ca. 
22% was measured at 1 bar. This values increased to ca. 
28% at 10 bar and decreased to ca. 19% upon further 
increase of the operating pressure to 20 bar.  
The methane selectivity decreased from ca. 30% at 1 bar to 
ca. 21 and 19% when the operating pressure was increased 
to 10 and 20 bar respectively. 
 
 
Fig.2 CO conversion and methane selectivity as function of 
the operating pressure 
 
C2-C4 and C5+ selectivities as function of the operating 
pressure are reported in fig. 3. The C2-C4 selectivity 
increased from ca. 8.3 % at 1 bar to ca. 18.6% at 10 bar and 
decreased to ca. 13.5 at 20 bar. The C5+ selectivity did not 
significantly change as it remained around 60% when the 
operating pressure was increased from 1 to 10 bar and 
increased to the highest value of ca. 67% when the pressure 
was further increased to 20 bar. These data and those in fig. 
2 indicate that at higher operating pressure, for example 20 
bar in this study, the methane selectivity is minimized and 
the selectivity to C5+ is high. However, when the rate of C5+ 
hydrocarbons formation is plotted as function of the 
operating pressure (fig. 4), an operating pressure of 10 bar 
yields the highest productivity.  
 
0
10
20
30
40
50
60
70
80
90
1 2 3
S
el
ec
tiv
ity
 [%
]
Pressure [bar]
C2-C4 select. C5+ select.
1                         10                        20
 
Fig. 3 C2-C4 and C5+ selectivity as function of pressure 
 
 
 
 
Fig. 4 C5+ production rate as function of operating pressure 
 0.032 g of C5+/gCat/h were produced at 1 bar, increased to 
ca. 0.040 g/gCat/h at 10 bar and decreased to ca. 0.031 
g/gCat/h at 20 bar (fig. 4). Even though an operating 
pressure of 20 bar led to better CH4 and C5+ selectivity, it 
had the lowest rate of C5+ production. This is due to a low 
CO conversion measured at 20 bar as shown in fig. 1.  
Zennaro et al.[2] have conducted kinetics study on titania-
supported cobalt catalyst under conditions where pore-
diffusional mass transfer were avoided (3 - 5% CO 
conversion) and found that their data were best fitted by the 
following simple Langmuir-Hinshelwood rate equation: 
 
 2
74.0
2
1 CO
COH
CO bP
PaP
r 
                (1) 
 
Where –rCO is the CO turnover frequency [TOF], PH2 the 
hydrogen partial pressure, PCO the CO partial pressure and a, 
and b temperature-dependent constants. 
This model was used in this study to compare the predicted 
and measured trends of CO rate as a function of the 
operating pressure. No CO TOF was determined in our 
study but since our experiments were all performed on a 
single catalyst sample at different pressures but same 
temperature, and space velocity, the CO TOF as function of 
the operating pressure should yield a similar trend as that for 
CO conversion. The predicted CO rates are summarized in 
table 2. 
 
TABLE II 
PREDICTED CO RATE AS FUNCTION OF OPERATING PRESSURE 
 
Total P [bar] PCO [bar] PH2 [bar] a* b* -rCO [s
-1]
0.0011 0.04
1 0.3 0.6 0.0002
10 3 6 0.0099
20 6 12 0.0270  
* From [2] 
 
The model predicts a significant increase in CO rate as the 
partial pressures of CO and H2 (consequently the total 
operating pressure) are increased. This does not totally 
agree with the CO conversion trend measured in this study 
(fig 2). An increase in CO conversion was only observed 
when the pressure was increased from 1 to 10 bar and can 
be attributed to an increase in reactants (H2 and CO) 
pressures in the reactor. The decrease in CO conversion as 
the pressure was further increased to 20 bar could be 
explained by significant mass transfer limitations in the 
catalyst resulting from the condensation of product in the 
catalyst pores. A similar trend was observed by Feyzi et al. 
[3] on a 40%Fe/60%Ni/40%Al2O3 where a significant 
decrease in CO conversion was observed when the total 
operating pressure was increase above 7 bar. 
IV. CONCLUSION 
    The total operating pressure for an FT reaction over a 
Co/TiO2 catalyst was found to have an effect on catalyst 
activity and product distribution. Increasing the operating 
pressure respectively from 1 to 10 and 20 bar decreased the 
methane selectivity and increased the selectivity to C5+ 
hydrocarbons. However the maximum C5+ hydrocarbons 
production rate was obtained at an optimal pressure of 10 
bar. Further experiments are needed to determine whether 
this optimal pressure is also function of other parameters 
such space velocity, reaction temperature, catalyst type and 
pores distribution in the catalyst support. 
  
REFERENCES 
[1] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 
(2004) 1-680. 
[2] R. Zennaro, M. Tagliabue, CH. Bartholomew, Kinetics of Fischer-
Tropsch synthesis on titania-supported cobalt, catalysis Today 58 
(2000) 309-319 
[3] M. Feyzi, A.A. Mizraei, H.R. Bozorgzadeh, Effects of preparation and 
operation conditions on precipitated iron nickel catalysts for Fischer-
Tropsch Synthesis, Journal of natural Gas Chemistry 19 (2010) 341-
353 
 
 


Fischer-Tropsch synthesis Over titaniasupported cobalt catalyst : optimum pressure for C5+ hydrocarbons production

University of Johannesburg Institutional Repository

  Abstract—Fischer-Tropsch (FT) runs were performed using a titania-supported cobalt catalyst at total operating pressures of 1, 10 and 20 bar respectively, to determine the optimal pressure for C5+ hydrocarbons production. The reaction temperature and feed flowrates were kept constant as 220oC and 20 Nml/gCat/min respectively.  Lower methane and higher C5+ hydrocarbons selectivities were measured at higher pressure. However the highest rate of C5+ hydrocarbons production was measured at an optimum pressure of 10 bar.   Index Terms— C5+ production rate, Fischer-Tropsch, Operating pressure I. INTRODUCTION HE central process in gas/coal/biomass to liquid technologies is Fischer-Tropsch (FT) synthesis. Through this process, gas-, coal- or biomass-derived synthesis gas (a mixture containing H2 and CO) is converted to hydrocarbons. This process yields a broad product spectrum with different carbon chain length. The main products formed include paraffins, olefins and some side products such as oxygenates (alcohols, aldehydes, ketones and carboxylic acids) and branched compounds [1]. A significant amount of water is also produced during an FT reaction. For industrial production, it is of economic interest to maximize the formation of liquid products (C5+). The product distribution can be influenced by operating conditions (temperature, pressure, etc.), the type of reactor and the catalyst used [1]. This study aims at determining the optimal operating pressure that maximizes the yield of liquid hydrocarbons (C5+) for an FT reaction over a Co/TiO2 catalyst at 220oC and a feed gas flowrate of 10 Nml /min. II. EXPERIMENTAL DETAILS     P25 Degussa TiO2 powder was first mixed with deionized water, dried and calcined in air to make the catalyst support. The latter was subsequently impregnated with a solution of cobalt nitrate, dried and calcined in air in order to obtain a fresh catalyst with ca. 10 wt.% cobalt loading. The details on conditions used in each of the catalyst preparation steps are summarized in fig. 1 0.5g of fresh catalyst was loaded and maintained at the middle of a fixed-bed reactor (internal diameter: 16 mm, length: 260 mm) by two layers of quartz wool (one on top and the other at the bottom of the catalyst bed). The rest of the reactor was filled with stainless steel balls to improve heat transfer. The catalyst was reduced around atmospheric pressure using a gas mixture (30 Nml/min) containing 5% H2 in Ar at 350oC for 14 hours. After cooling the reactor below 100oC, pre-mixed synthesis gas containing ca. 10% N2, 30% CO with an H2 balance was introduced in the reactor at a flow of 10 Nml/min using an Aalborg mass flow controller. The FT reaction was first performed at 1 bar and 220oC and the pressure was further increased to 10 and 20 bar respectively while all other operating conditions (temperature, feed flow rate and feed gas composition) were kept unchanged. The catalyst stabilized within 10 hours after changing the pressure. The gas pressure in the reactor was built by using a back pressure regulator mounted downstream the reactor after two product knock-out pots used to collect condensed products. A portion of gas products was introduced through a six-port Valco sampling valve into a Dani Master gas chromatograph (GC) for on-line analysis. H2, N2, CO, CH4 and CO2 were separated using a 60/80 Carboxen 1000 column connected to a thermal conductivity detector (TCD) and organic products (light hydrocarbons up to C8) were separated in a Supel-Q Plot fused silica capillary column connected to a flame ionization detector (FID). The GC was calibrated on a daily basis using a calibration mixture with the molar composition reported in table 1.  TABLE I MOLAR COMPOSITION FOR GC CALIBRATION MIXTURE  Component Mol.% C2H4 0.98 C2H6 0.98 CH4 5.2 CO2 4.8 N2 10.4 CO 24 H2 53.64    Fischer-Tropsch Synthesis Over Titania-supported Cobalt Catalyst: Optimum Pressure for C5+ Hydrocarbons Production Kalala JalamaT MixingCalc. Drying Impreg.SievingCalc.DryingTiO2H2O1:1 wt. ratio 120oC, 1h400oC, 16h in air120oC, overnight400oC, 6h in airFresh catalyst0.1 - 0.5 mmCobalt nitrate solution                                                                     Fig. 1 Detailed catalyst preparation steps   III. RESULTS AND DISCUSSION  The effect of operating pressure on CO conversion and CH4 selectivity is summarized in fig. 2. CO conversion of ca. 22% was measured at 1 bar. This values increased to ca. 28% at 10 bar and decreased to ca. 19% upon further increase of the operating pressure to 20 bar.  The methane selectivity decreased from ca. 30% at 1 bar to ca. 21 and 19% when the operating pressure was increased to 10 and 20 bar respectively.   Fig.2 CO conversion and methane selectivity as function of the operating pressure  C2-C4 and C5+ selectivities as function of the operating pressure are reported in fig. 3. The C2-C4 selectivity increased from ca. 8.3 % at 1 bar to ca. 18.6% at 10 bar and decreased to ca. 13.5 at 20 bar. The C5+ selectivity did not significantly change as it remained around 60% when the operating pressure was increased from 1 to 10 bar and increased to the highest value of ca. 67% when the pressure was further increased to 20 bar. These data and those in fig. 2 indicate that at higher operating pressure, for example 20 bar in this study, the methane selectivity is minimized and the selectivity to C5+ is high. However, when the rate of C5+ hydrocarbons formation is plotted as function of the operating pressure (fig. 4), an operating pressure of 10 bar yields the highest productivity.   01020304050607080901 2 3Selectivity [%]Pressure [bar]C2-C4 select. C5+ select.1                         10                        20 Fig. 3 C2-C4 and C5+ selectivity as function of pressure     Fig. 4 C5+ production rate as function of operating pressure  0.032 g of C5+/gCat/h were produced at 1 bar, increased to ca. 0.040 g/gCat/h at 10 bar and decreased to ca. 0.031 g/gCat/h at 20 bar (fig. 4). Even though an operating pressure of 20 bar led to better CH4 and C5+ selectivity, it had the lowest rate of C5+ production. This is due to a low CO conversion measured at 20 bar as shown in fig. 1.  Zennaro et al.[2] have conducted kinetics study on titania-supported cobalt catalyst under conditions where pore-diffusional mass transfer were avoided (3 - 5% CO conversion) and found that their data were best fitted by the following simple Langmuir-Hinshelwood rate equation:   274.021 COCOHCO bPPaPr                 (1)  Where –rCO is the CO turnover frequency [TOF], PH2 the hydrogen partial pressure, PCO the CO partial pressure and a, and b temperature-dependent constants. This model was used in this study to compare the predicted and measured trends of CO rate as a function of the operating pressure. No CO TOF was determined in our study but since our experiments were all performed on a single catalyst sample at different pressures but same temperature, and space velocity, the CO TOF as function of the operating pressure should yield a similar trend as that for CO conversion. The predicted CO rates are summarized in table 2.  TABLE II PREDICTED CO RATE AS FUNCTION OF OPERATING PRESSURE  Total P [bar] PCO [bar] PH2 [bar] a* b* -rCO [s-1]0.0011 0.041 0.3 0.6 0.000210 3 6 0.009920 6 12 0.0270  * From [2]  The model predicts a significant increase in CO rate as the partial pressures of CO and H2 (consequently the total operating pressure) are increased. This does not totally agree with the CO conversion trend measured in this study (fig 2). An increase in CO conversion was only observed when the pressure was increased from 1 to 10 bar and can be attributed to an increase in reactants (H2 and CO) pressures in the reactor. The decrease in CO conversion as the pressure was further increased to 20 bar could be explained by significant mass transfer limitations in the catalyst resulting from the condensation of product in the catalyst pores. A similar trend was observed by Feyzi et al. [3] on a 40%Fe/60%Ni/40%Al2O3 where a significant decrease in CO conversion was observed when the total operating pressure was increase above 7 bar. IV. CONCLUSION     The total operating pressure for an FT reaction over a Co/TiO2 catalyst was found to have an effect on catalyst activity and product distribution. Increasing the operating pressure respectively from 1 to 10 and 20 bar decreased the methane selectivity and increased the selectivity to C5+ hydrocarbons. However the maximum C5+ hydrocarbons production rate was obtained at an optimal pressure of 10 bar. Further experiments are needed to determine whether this optimal pressure is also function of other parameters such space velocity, reaction temperature, catalyst type and pores distribution in the catalyst support.   REFERENCES [1] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 (2004) 1-680. [2] R. Zennaro, M. Tagliabue, CH. Bartholomew, Kinetics of Fischer-Tropsch synthesis on titania-supported cobalt, catalysis Today 58 (2000) 309-319 [3] M. Feyzi, A.A. Mizraei, H.R. Bozorgzadeh, Effects of preparation and operation conditions on precipitated iron nickel catalysts for Fischer-Tropsch Synthesis, Journal of natural Gas Chemistry 19 (2010) 341-353   

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Fischer-Tropsch synthesis Over titaniasupported cobalt catalyst : optimum pressure for C5+ hydrocarbons production

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