Abstract: The effect of space velocity on FT reaction has been evaluated over a 10%Co/TiO2 catalyst prepared by incipient wetness impregnation method. The catalyst was tested in a fixed-bed reactor at 220oC, 20 bar and various feed gas space velocities (20, 40 and 60 Nml/gCat/min). The CO consumption rate, olefin to paraffin ratio, C5+ selectivity and the chain growth probability (alpha) were found to increase with an increase in feed gas flow rate. The results were explained in terms of reactants partial pressures and mass transfer in the reactor as function of CO conversion levels

Kalala, Jalama

English

Name not available

  
Abstract— The effect of space velocity on FT reaction has 
been evaluated over a 10%Co/TiO2 catalyst prepared by 
incipient wetness impregnation method. The catalyst was 
tested in a fixed-bed reactor at 220oC, 20 bar and various feed 
gas space velocities (20, 40 and 60 Nml/gCat/min). The CO 
consumption rate, olefin to paraffin ratio, C5+ selectivity and 
the chain growth probability (alpha) were found to increase 
with an increase in feed gas flow rate. The results were 
explained in terms of reactants partial pressures and mass 
transfer in the reactor as function of CO conversion levels. 
 
Index Terms— Co/TiO2, Fischer-Tropsch, Space velocity 
I. INTRODUCTION 
ISCHER-TROPSCH (FT) reaction consists of 
converting synthesis gas into hydrocarbons. The 
synthesis gas can be derived from coal, biomass or natural 
gas. Where the synthesis gas is produced from coal, the 
technology is referred to as Coal to Liquid (CTL) and Gas 
to Liquid (GTL) when the synthesis gas is derived from 
natural gas. Biomass to Liquid (BTL) technologies are not 
as significantly developed as CTL and GTL but also now 
receive more attention as processes for renewable fuels 
production. A solid catalyst is required for the FT reaction 
to occur. Cobalt- and iron-based catalysts are the most used 
for commercial applications [1]. The reaction is also 
affected by a number of operating conditions such as 
temperature, pressure, feed gas composition and flow rates. 
In this study, the effect of feed gas space velocity on CO 
conversion rate and product selectivity during FT reaction 
over a cobalt-based catalyst has been evaluated.  
II. EXPERIMENTAL DETAILS 
A. Catalyst Preparation 
    The catalyst was prepared using the procedure 
described in an earlier study [2]. The support was first 
prepared by mixing titania (TiO2) powder (Degussa P25) 
with an equal amount (weight-based) of water and drying at 
120oC for 1 hour. The dried support was calcined in air at 
400oC for 16 h, cooled, crushed and sieved to retain 
particles between 0.1 and 0.5 mm. Cobalt was introduced in 
the support by incipient wetness impregnation method 
where a cobalt nitrate solution was added to the support in 
 
Manuscript received July 13, 2015; revised August 16, 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). 
order to achieve ca. 10% cobalt loading. The cobalt nitrate-
impregnated support was subsequently dried at 120oC in air 
overnight and calcined at 400oC in air for 6 hours to obtain 
a fresh catalyst. 
B. Catalyst Testing 
    The catalyst was tested in a fixed-bed reactor where 0.5g 
of fresh catalyst and an appropriate amount of stainless steel 
balls (3 mm in diameter) were loaded. Two layers of quartz 
wool separated the catalyst bed from the stainless steel balls 
as shown in fig. 1. 
 
 
Catalyst bed Quartz wool
Stainless steel balls ( 3mm diameter)
Stainless steel balls ( 3mm diameter)
 
 
Fig. 1 Details on catalyst loading in the FT reactor 
 
After the calcination process, the fresh catalyst contained 
cobalt in an oxide form. An activation process that usually 
consists of catalyst reduction in a hydrogen flow between 
250 and 400oC is required prior to the FT reaction [1]. 
Studies that used other reducing agents such as CO-
containing gas mixtures have also been reported in literature 
[2-9]. In this study, the catalyst was activated using a gas 
containing 5% H2 in Ar (30 Nml/min) at 350oC for 14 hours 
around atmospheric pressure. The reactor was cooled to 
room temperature, then synthesis gas (30% CO, 60% H2, 
balance N2) was introduced and the pressure was built up to 
20 bar using a back pressure regulator placed downstream 
the reactor. An appropriate feed gas flow rate was set on an 
Aalborg mass flow controller and the temperature of the 
reactor was raised to 220oC to perform the FT reaction. A 
hot pot was mounted at the bottom of the reactor for wax 
collection followed by a cold pot for water and light oil 
collection. The gas product was analyzed on-line using a gas 
chromatograph. An excel spreadsheet was developed to 
Effect of Space Velocity on Fischer-Tropsch 
Reaction over Co/TiO2 Catalyst 
Kalala Jalama 
F
 calculate the CO conversion, CO conversion rate and 
product selectivities. 
 
III. RESULTS AND DISCUSSION  
%CO conversion, rate of CO conversion and CH4 selectivity 
as function of space velocity are reported in fig. 2. The CO 
conversion decreased with an increase space velocity. 
Values of ca. 19.9, 16.9 and 16.5 were respectively 
measured at 20, 40 and 60 Nml/gCat/min. This was 
expected since an increase in feed gas flow rate decreases 
the residence time of reactants in the reactor. However, the 
rate for CO conversion linearly increased with an increase 
in feed gas flow rate. A CO conversion rate of ca. 3.3 
mmol/gCat/h was measured at 20 Nml/gCat/min compared 
to ca. 8.3 mmol/gCat/h at 60 Nml/gCat/min. On the other 
hand, the CH4 selectivity was found to decrease with an 
increase in space velocity. About 19.4, 13.7 and 10.7% CH4 
selectivity were respectively measured at 20, 40 and 60 
Nml/gCat/min. 
 
 
0
5
10
15
20
25
10 20 30 40 50 60 70
C
O
 c
on
v.
 a
nd
 C
H
4 
se
l. 
[%
], 
C
O
 
co
nv
.  
ra
te
 [m
m
ol
/g
C
at
/h
]
SV [Nml/gCat/min]
CO conv
CH4 sel
CO conv rate
 
Fig. 2 CO conversion, CH4 selectivity and CO conversion 
rate as function of space velocity 
 
The effect of feed space velocity on C2-C4 and C5+ 
selectivities is reported in fig. 3. The data show that C2-C4 
selectivity decreased while C5+ selectivity increased with an 
increase in space velocity. C2-C4 selectivities of ca. 13.2, 
11.4 and 8.3%, and C5+ selectivities of ca. 67.1, 74.8 and 
80.9 were respectively measured at 20, 40 and 60 
Nml/gCat/min. 
 
0
20
40
60
80
100
1 2 3
C
2-
C
4 
an
d 
C
5+
 s
el
ec
t. 
[%
]
SV [ml/gCat/min]
C2-C4 select. C5+ select.
20                         40                           60
 
Fig. 3 Effect of space velocity on C2-C4 and C5+ selectivity 
The effect of space velocity on olefin to paraffin ratios as 
function of carbon number is shown in fig. 4. All the data 
show a significant increase in olefin to paraffin ratio when 
the space velocity was increased from 20 to 40 
Nml/gCat/min and no significant change was observed 
when the space velocity was further increased to 60 
Nml/gCat/min.  
 
The overall olefin to paraffin ratio and alpha values as 
function of feed gas space velocity are summarized in table 
1.  
 
 
TABLE I 
EFFECT OF SPACE VELOCITY ON LIGHT OLEFINS TO PARAFFINS 
RATIO AND CHAIN GROWTH PROBABILITY 
 
SV [ml/gCat/min] O/P ratioa Alphab
20 0.18 0.50
40 0.35 0.52
60 0.34 0.54  
a and b: based on C3 to C7 hydrocarbons in gas product after 
cooling at room temperature and depressurized to 
atmospheric pressure. 
 
 
Olefin to paraffin ratios of 0.18, 0.35 and 0.34 were 
measured at 20, 40 and 60 Nml/gCat/min respectively. A 
significant change was observed when the space velocity 
was increased from 20 to 40 Nml/gCat/min with no 
significant effect when the space velocity was further 
increased. The alpha value was found to increase from 0.50 
at 20 Nml/gCat/min to 0.52 at 40 Nml/gCat/min and to 0.54 
at 60 Nml/min. 
 
The results can possibly be explained as follows: 
i) At low space velocity, the residence time for the 
reactants is long and leads to high levels of CO 
conversion; 
ii) The partial pressure of reactants, for example CO, is 
related to the fractional CO conversion by the 
following expression: 
  COCOinCO XPP  1            (1) 
 
Where PCOin is the partial pressure of CO as it 
enters the reactor and XCO the fractional CO 
conversion. At high space velocities, because 
the conversion (XCO) is low, the partial 
pressure of CO (PCO) and that for H2 will be 
relatively higher and can explain the high CO 
conversion rate measured at high space 
velocity. 
 
 
 
 
 
 
 0
0.3
0.6
0.9
1.2
1.5
1.8
1 2 3
O
/P
 ra
tio
SV [ml/gCat/min]
a)
20                       40                      60
0
0.3
0.6
0.9
1.2
1.5
1.8
1 2 3
O
/P
 ra
tio
SV [ml/gCat/min]
b)
20                       40                      60
0
0.3
0.6
0.9
1.2
1.5
1.8
1 2 3
O
/P
 ra
tio
SV [ml/gCat/min]
c)
20                       40                      60
0
0.3
0.6
0.9
1.2
1.5
1.8
1 2 3
O
/P
 ra
tio
SV [ml/gCat/min]
d)
20                       40                      60
0
0.3
0.6
0.9
1.2
1.5
1.8
1 2 3
O
/P
 ra
tio
SV [ml/gCat/min]
e)
20                       40                      60
 
 
 
Fig. 4 Effect of space velocity on olefin to paraffin ratios as function of carbon number: a) C3; b) C4; c) C5, d) C6 and e) C7 
 
 
iii) The proportion of products present in the reactor 
during FT reaction is also function of CO 
conversion. Because of low selectivity to 
oxygenates and limited water-gas-shift activity 
for cobalt-based FT catalysts, for every mole 
of CO converted to hydrocarbons, there will be 
approximately one mole of water formed. The 
water increases with the hydrocarbon products 
formed. From equation (1) we can 
approximate the partial pressure of water 
formed during FT reaction over a cobalt 
catalyst as: 
 
COCOinOH XPP .2                (2) 
 
As the space velocity is increased, the CO 
conversion XCO decreases, therefore the partial 
pressure of water and the proportion of 
hydrocarbon products also decrease in the 
reactor. This could result in better mass 
transfer that can also explain higher CO 
conversion rates measured at high space 
velocities. This could also lead to a change in 
composition of the liquid present in the pores 
of the catalyst and explain the changes in 
product selectivity observed upon increasing 
the space velocity. 
I. CONCLUSION 
    The CO consumption rate, olefin to paraffin ratio, C5+ 
selectivity and the chain growth probability (alpha) were 
found to increase with an increase in space velocity. The 
results were explained in terms of reactants partial pressures 
and mass transfer in the catalyst as function of CO 
conversion that in turn depends on space velocity. 
  
REFERENCES 
[1] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 
(2004) 1-680. 
[2] K. Jalama, N. J. Coville, D. Hildebrandt, D. Glasser, L. L. Jewell, 
Fuel 86 (2007) 73 
[3] P.N. Dyer, R. Pietantozzi, H.P. Withers, US Patent 4 670 472 (1987), 
to Air Products & Chemicals. 
[4] P.N. Dyer, R. Pietantozzi, H.P. Withers, US Patent 4 681 867 (1987), 
to Air Products & Chemicals. 
 [5] W.N. Mitchel, US Patent 5 292 705 (1994), to Exxon Research & 
Engineering. 
[6] B. Nay, M.R. Smith, C.D. Telford, US Patent 5 585 316 (1996), to 
British Petroleum.  
[7] B. Jongsomjit, J.G. Goodwin Jr., Catalysis Today 77 (2002) 191. 
[8] P.N. Dyer, R. Pietantozzi, US Patent 4 619 910 (1986), to Air 
Products & Chemicals. 
[9] K. Jalama, J. Kabuba, H. Xiong, L.L. Jewell, Catalysis 
Communications 17 (2012) 154 


Effect of space velocity on Fischer-Tropsch reaction over Co/TiO2 catalyst

University of Johannesburg Institutional Repository

  Abstract— The effect of space velocity on FT reaction has been evaluated over a 10%Co/TiO2 catalyst prepared by incipient wetness impregnation method. The catalyst was tested in a fixed-bed reactor at 220oC, 20 bar and various feed gas space velocities (20, 40 and 60 Nml/gCat/min). The CO consumption rate, olefin to paraffin ratio, C5+ selectivity and the chain growth probability (alpha) were found to increase with an increase in feed gas flow rate. The results were explained in terms of reactants partial pressures and mass transfer in the reactor as function of CO conversion levels.  Index Terms— Co/TiO2, Fischer-Tropsch, Space velocity I. INTRODUCTION ISCHER-TROPSCH (FT) reaction consists of converting synthesis gas into hydrocarbons. The synthesis gas can be derived from coal, biomass or natural gas. Where the synthesis gas is produced from coal, the technology is referred to as Coal to Liquid (CTL) and Gas to Liquid (GTL) when the synthesis gas is derived from natural gas. Biomass to Liquid (BTL) technologies are not as significantly developed as CTL and GTL but also now receive more attention as processes for renewable fuels production. A solid catalyst is required for the FT reaction to occur. Cobalt- and iron-based catalysts are the most used for commercial applications [1]. The reaction is also affected by a number of operating conditions such as temperature, pressure, feed gas composition and flow rates. In this study, the effect of feed gas space velocity on CO conversion rate and product selectivity during FT reaction over a cobalt-based catalyst has been evaluated.  II. EXPERIMENTAL DETAILS A. Catalyst Preparation     The catalyst was prepared using the procedure described in an earlier study [2]. The support was first prepared by mixing titania (TiO2) powder (Degussa P25) with an equal amount (weight-based) of water and drying at 120oC for 1 hour. The dried support was calcined in air at 400oC for 16 h, cooled, crushed and sieved to retain particles between 0.1 and 0.5 mm. Cobalt was introduced in the support by incipient wetness impregnation method where a cobalt nitrate solution was added to the support in  Manuscript received July 13, 2015; revised August 16, 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). order to achieve ca. 10% cobalt loading. The cobalt nitrate-impregnated support was subsequently dried at 120oC in air overnight and calcined at 400oC in air for 6 hours to obtain a fresh catalyst. B. Catalyst Testing     The catalyst was tested in a fixed-bed reactor where 0.5g of fresh catalyst and an appropriate amount of stainless steel balls (3 mm in diameter) were loaded. Two layers of quartz wool separated the catalyst bed from the stainless steel balls as shown in fig. 1.   Catalyst bed Quartz woolStainless steel balls ( 3mm diameter)Stainless steel balls ( 3mm diameter)  Fig. 1 Details on catalyst loading in the FT reactor  After the calcination process, the fresh catalyst contained cobalt in an oxide form. An activation process that usually consists of catalyst reduction in a hydrogen flow between 250 and 400oC is required prior to the FT reaction [1]. Studies that used other reducing agents such as CO-containing gas mixtures have also been reported in literature [2-9]. In this study, the catalyst was activated using a gas containing 5% H2 in Ar (30 Nml/min) at 350oC for 14 hours around atmospheric pressure. The reactor was cooled to room temperature, then synthesis gas (30% CO, 60% H2, balance N2) was introduced and the pressure was built up to 20 bar using a back pressure regulator placed downstream the reactor. An appropriate feed gas flow rate was set on an Aalborg mass flow controller and the temperature of the reactor was raised to 220oC to perform the FT reaction. A hot pot was mounted at the bottom of the reactor for wax collection followed by a cold pot for water and light oil collection. The gas product was analyzed on-line using a gas chromatograph. An excel spreadsheet was developed to Effect of Space Velocity on Fischer-Tropsch Reaction over Co/TiO2 Catalyst Kalala Jalama F calculate the CO conversion, CO conversion rate and product selectivities.  III. RESULTS AND DISCUSSION  %CO conversion, rate of CO conversion and CH4 selectivity as function of space velocity are reported in fig. 2. The CO conversion decreased with an increase space velocity. Values of ca. 19.9, 16.9 and 16.5 were respectively measured at 20, 40 and 60 Nml/gCat/min. This was expected since an increase in feed gas flow rate decreases the residence time of reactants in the reactor. However, the rate for CO conversion linearly increased with an increase in feed gas flow rate. A CO conversion rate of ca. 3.3 mmol/gCat/h was measured at 20 Nml/gCat/min compared to ca. 8.3 mmol/gCat/h at 60 Nml/gCat/min. On the other hand, the CH4 selectivity was found to decrease with an increase in space velocity. About 19.4, 13.7 and 10.7% CH4 selectivity were respectively measured at 20, 40 and 60 Nml/gCat/min.   051015202510 20 30 40 50 60 70CO conv. and CH4 sel. [%], CO conv.  rate [mmol/gCat/h]SV [Nml/gCat/min]CO convCH4 selCO conv rate Fig. 2 CO conversion, CH4 selectivity and CO conversion rate as function of space velocity  The effect of feed space velocity on C2-C4 and C5+ selectivities is reported in fig. 3. The data show that C2-C4 selectivity decreased while C5+ selectivity increased with an increase in space velocity. C2-C4 selectivities of ca. 13.2, 11.4 and 8.3%, and C5+ selectivities of ca. 67.1, 74.8 and 80.9 were respectively measured at 20, 40 and 60 Nml/gCat/min.  0204060801001 2 3C2-C4 and C5+ select. [%]SV [ml/gCat/min]C2-C4 select. C5+ select.20                         40                           60 Fig. 3 Effect of space velocity on C2-C4 and C5+ selectivity The effect of space velocity on olefin to paraffin ratios as function of carbon number is shown in fig. 4. All the data show a significant increase in olefin to paraffin ratio when the space velocity was increased from 20 to 40 Nml/gCat/min and no significant change was observed when the space velocity was further increased to 60 Nml/gCat/min.   The overall olefin to paraffin ratio and alpha values as function of feed gas space velocity are summarized in table 1.    TABLE I EFFECT OF SPACE VELOCITY ON LIGHT OLEFINS TO PARAFFINS RATIO AND CHAIN GROWTH PROBABILITY  SV [ml/gCat/min] O/P ratioa Alphab20 0.18 0.5040 0.35 0.5260 0.34 0.54  a and b: based on C3 to C7 hydrocarbons in gas product after cooling at room temperature and depressurized to atmospheric pressure.   Olefin to paraffin ratios of 0.18, 0.35 and 0.34 were measured at 20, 40 and 60 Nml/gCat/min respectively. A significant change was observed when the space velocity was increased from 20 to 40 Nml/gCat/min with no significant effect when the space velocity was further increased. The alpha value was found to increase from 0.50 at 20 Nml/gCat/min to 0.52 at 40 Nml/gCat/min and to 0.54 at 60 Nml/min.  The results can possibly be explained as follows: i) At low space velocity, the residence time for the reactants is long and leads to high levels of CO conversion; ii) The partial pressure of reactants, for example CO, is related to the fractional CO conversion by the following expression:   COCOinCO XPP  1            (1)  Where PCOin is the partial pressure of CO as it enters the reactor and XCO the fractional CO conversion. At high space velocities, because the conversion (XCO) is low, the partial pressure of CO (PCO) and that for H2 will be relatively higher and can explain the high CO conversion rate measured at high space velocity.        00.30.60.91.21.51.81 2 3O/P ratioSV [ml/gCat/min]a)20                       40                      6000.30.60.91.21.51.81 2 3O/P ratioSV [ml/gCat/min]b)20                       40                      6000.30.60.91.21.51.81 2 3O/P ratioSV [ml/gCat/min]c)20                       40                      6000.30.60.91.21.51.81 2 3O/P ratioSV [ml/gCat/min]d)20                       40                      6000.30.60.91.21.51.81 2 3O/P ratioSV [ml/gCat/min]e)20                       40                      60   Fig. 4 Effect of space velocity on olefin to paraffin ratios as function of carbon number: a) C3; b) C4; c) C5, d) C6 and e) C7   iii) The proportion of products present in the reactor during FT reaction is also function of CO conversion. Because of low selectivity to oxygenates and limited water-gas-shift activity for cobalt-based FT catalysts, for every mole of CO converted to hydrocarbons, there will be approximately one mole of water formed. The water increases with the hydrocarbon products formed. From equation (1) we can approximate the partial pressure of water formed during FT reaction over a cobalt catalyst as:  COCOinOH XPP .2                (2)  As the space velocity is increased, the CO conversion XCO decreases, therefore the partial pressure of water and the proportion of hydrocarbon products also decrease in the reactor. This could result in better mass transfer that can also explain higher CO conversion rates measured at high space velocities. This could also lead to a change in composition of the liquid present in the pores of the catalyst and explain the changes in product selectivity observed upon increasing the space velocity. I. CONCLUSION     The CO consumption rate, olefin to paraffin ratio, C5+ selectivity and the chain growth probability (alpha) were found to increase with an increase in space velocity. The results were explained in terms of reactants partial pressures and mass transfer in the catalyst as function of CO conversion that in turn depends on space velocity.   REFERENCES [1] A. Steynberg, M. Dry, Studies in Surface Science and Catalysis 152 (2004) 1-680. [2] K. Jalama, N. J. Coville, D. Hildebrandt, D. Glasser, L. L. Jewell, Fuel 86 (2007) 73 [3] P.N. Dyer, R. Pietantozzi, H.P. Withers, US Patent 4 670 472 (1987), to Air Products & Chemicals. [4] P.N. Dyer, R. Pietantozzi, H.P. Withers, US Patent 4 681 867 (1987), to Air Products & Chemicals.  [5] W.N. Mitchel, US Patent 5 292 705 (1994), to Exxon Research & Engineering. [6] B. Nay, M.R. Smith, C.D. Telford, US Patent 5 585 316 (1996), to British Petroleum.  [7] B. Jongsomjit, J.G. Goodwin Jr., Catalysis Today 77 (2002) 191. [8] P.N. Dyer, R. Pietantozzi, US Patent 4 619 910 (1986), to Air Products & Chemicals. [9] K. Jalama, J. Kabuba, H. Xiong, L.L. Jewell, Catalysis Communications 17 (2012) 154 

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Effect of space velocity on Fischer-Tropsch reaction over Co/TiO2 catalyst

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