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Fischer-Tropsch Synthesis and Product Upgrading Over Dual Bed Catalytic Reactor

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Conference Paper

Fischer-Tropsch Synthesis and Product Upgrading Over Dual Bed Catalytic Reactor

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Converting biomass derived synthesis gas (syngas) to liquid hydrocarbons using Fischer-Tropsch (F-T) process is an emerging area of research. Traditionally, iron- and cobalt-based catalysts have been used as F-T catalysts. The advantages of Fe-based catalysts are that they are inexpensive, durable and can be used over a wide range of H2:CO ratios; Fe catalysts are currently being used by Sasol to convert coal derived synthesis gas to a wide range of products. However, one of the problems with the use of Fe catalyst is that it forms hydrocarbon products rich with olefins. The resulting hydrocarbon product is not stable due to a variety of oxidation and polymerization reactions. This would require a post-treatment of the product to convert it to saturated hydrocarbons via a catalytic hydrogenation process. The objective of this work is to develop a catalytic system for the single-step conversion of syngas to saturated hydrocarbons (paraffins). To accomplish this goal we studied syngas conversion over a dual catalyst bed comprised of a Fe catalyst coupled with a hydrogenation catalyst. This allows the in-situ conversion of olefinic compounds produced by the Fe catalyst to paraffinic products which are more suitable for use as transportation fuel. A number of Ni- and Co-containing hydrogenation catalysts were tested in combination with the Fe catalyst. Operational parameters including temperature, pressure, space velocity and syngas composition were varied and their effect on syngas conversion, selectivities towards light hydrocarbon gases, and liquid hydrocarbons were determined. The effect of addition of CO2 to the H2-CO mixture was also studied. Among tested hydrogenation catalysts alumina-supported Ni-Mo catalyst demonstrated the highest activity and selectivity. Table 1. depicts the liquid product composition generated with the Fe catalyst alone and in a dual bed Fe + Ni-Mo/alumina arrangement. Table 1. Composition of syn-crude generated over Fe and Fe + Ni-Mo/alumina catalysts at syngas composition: [H2]/[CO]= 2, reactor temperature: 333oC, pressure: 200 psig, and space velocity of about 4100 h-1. Product fraction, % Catalyst Fe Fe+Ni-Mo/Alumina Gasoline range (C5-C10) 55.8 55.4 Kerosene/jet fuel range(C11-C12) 15.5 17.1 Diesel range (C13-C16) 15.3 16.8 Lube oil and wax range (C17-C26) 13.5 10.7 Olefin to paraffin molar ratio (for C8) 5.7 0.0 Results obtained, to date, indicate that: 1. A highly paraffinic liquid product is obtained when a Ni-Mo/alumina hydrogenation catalyst is used in a combination and downstream of an iron catalyst. 2. Presence of Ni-Mo catalyst did not markedly alter the distribution of liquid fractions in the product, but practically eliminated olefins from the raw product. 3. Presence of CO2 in the feed gas adversely affected syngas conversion and space time yield of liquid products. Upon further development, this approach can provide a more facile and direct route to production of high quality liquid hydrocarbons from syngas. In this paper, a detailed description of these and other significant findings will be presented and discussed.
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1
Fischer-Tropsch Synthesis and Product Upgrading over Dual
Bed Catalytic System
Amit C. Gujar, Nathaniel Garceau, Gary Bokerman, Nazim Muradov and Ali T-Raissi
Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL-32955-5703
Introduction
Converting biomass derived synthesis gas (syngas) to liquid hydrocarbons using
Fischer-Tropsch (F-T) process is an active area of research. Traditionally, iron- and
cobalt-based catalysts have been used as F-T catalysts. The advantages of Fe-based
catalysts are that they are inexpensive, durable and can be used over a wide range of
H2:CO ratios. Iron based catalysts are currently in use by Sasol to convert coal derived
synthesis gas to a wide range of products. However, one of the problems with the use of
Fe catalyst is that it yields olefin rich hydrocarbons. The resulting hydrocarbon product
is prone to undesirable oxidation and polymerization - requiring post-treatment to convert
it to saturated hydrocarbons via catalytic hydrogenation.
The objective of this work is to develop a catalytic system for the single-step
conversion of syngas to saturated hydrocarbons (paraffins). To accomplish this objective,
we have investigated syngas conversion over a dual catalyst bed comprised of Fe catalyst
paired with hydrogenation catalyst. This allows the in-situ conversion of olefinic fraction
produced by the Fe catalyst to paraffinic ones that are more desirable as transportation
fuel. Experimental results for the performance of Fe catalyst coupled with a cracking
catalyst in a dual bed configuration are also provided.
Several process parameters including temperature, pressure, space velocity and
syngas composition were varied and their effect on the syngas conversion, selectivities
towards light gaseous and liquid hydrocarbon formation were determined. The effect of
CO2 addition to H2-CO mixture was also investigated. Among the hydrogenation
catalysts evaluated, alumina-supported Ni-Mo catalyst demonstrated the highest activity
and selectivity.
Experimental
In a typical experiment, F-T catalyst (potassium-promoted Fe-based catalyst) was
mixed with an appropriate amount of silicon carbide (200-450 mesh, Aldrich) and packed
in a ½ inch stainless steel tube (ID.= 10 mm). The reactor was heated by a heating tape,
and its temperature controlled by a thermocouple placed on the reactor outside wall. A K-
type thermocouple was used to monitor the internal bed temperature. The catalyst bed
was pretreated by passing 300 mL/min of H2 at 400°C for 12 hrs followed by heating to
500°C for another two more hours. A syngas mixture (typically 33% CO and 67% H2)
entered the reactor at a set flow rate, temperature and pressure. The liquid product was
collected in a condenser and the hydrocarbon and aqueous phase fractions were separated
2
and analyzed via GC-FID (Shimadzu-14b) and an Agilent 6890 GC connected to a JEOL
GC Mate-II MS/MS. Two online GCs (SRI Instruments) were used to determine the
concentration of the gaseous products. The GCs were calibrated daily using certified
calibration gases. The conversion of the reactants (e.g., CO) is defined as follows:
00
00
CO
COCO CF
CFCF
COofConversion ×
××
=
Selectivity toward CO2 formation of a product was calculated as follows:
COCO
CO
CFCF
nCF
COofySelectivit ××
××
=00
2
2
Where,
F0 and F denote inlet and exit flow rates, respectively,
C0 and C are the inlet and exit concentration of the species respectively,
n is the number of carbon atoms in the species.
Results and Discussion
The Fe catalyst required pre-activation with the input syngas. This may be a result
of the formation of catalytically active iron species (e.g., carbide) during the activation
stage. Figure 1 shows the effect of syngas (33% CO and 67% H2 by volume) flow rate on
the Fe catalyst performance at reactor temperature of 333ºC and 200 psig pressure (only
gaseous products are shown).
It can be seen that the CO and H2 conversions don’t change markedly at higher
space velocities - implying a highly active catalyst. The selectivity toward methane seems
to slightly increase with increased space velocity.
Table 1 shows the distribution of liquid hydrocarbons produced by Fe-catalyzed
F-T reaction at the conditions specified in Fig. 1 along with olefin to paraffin molar ratio.
The data of Table 1 also includes that obtained for the dual-bed catalytic system
comprised of Fe and H+/ZSM-5 catalysts.
Table 1. Comparison of organic phase product composition for the Fe and
Fe+H+/ZSM-5 catalysts.
Product
Fe-catalyst
Fe-catalyst+H-ZSM-5
Gasoline range (C5-C10)
55.79
71.08
Kerosene/jet fuel range (C11-C12)
15.46
14.40
Diesel range (C13-C16)
15.30
11.74
Lube oil and wax range(C17-C26)
13.46
2.78
Olefin to paraffin molar ratio (for C8)
5.72
1.86
3
Figure 1: Conversions and selectivities for the reactants and products vs. syngas
flow rate (T= 333°C P= 200 psig).
It is apparent that addition of H-ZSM-5 increases the yield of gasoline range
hydrocarbons and decreases the yield of wax formation as well as the olefin/paraffin
ratio.
Table 2 shows the comparative results for the conversion of the reactants and
selectivity of the gaseous products obtained for Fe and Fe+ H+/ZSM-5 catalysts operating
at 340°C, 200 psi pressure at the syngas input flow rate of 290 mL/min.
Table 2. Conversion and selectivity comparisons for Fe and Fe+ H+/ZSM-5.
Catalyst
H
2
conversion
(%)
CO conversion
(%)
CO
2
selectivity
(%)
CH
4
selectivity
(%)
Fe
30.71
87.00
46.77
6.14
Fe+H-ZSM-5
27.06
89.16
47.96
11.75
The CO+H2 conversion and CO2 selectivity are essentially equal in both cases,
whereas the methane selectivity has increased when H+/ZSM-5 present. These results can
be attributed to the cracking activity of ZSM-5 catalyst that allows lower yields of higher
hydrocarbons (e.g., wax) and higher yields of gaseous hydrocarbons (e.g., methane).
Also, we have tested a dual-bed catalytic system consisting of Fe and Ni-
Mo/alumina catalysts. The objective was to hydrogenate unsaturated hydrocarbons
(olefins) produced by Fe catalyst in situ and in the presence of Ni-Mo/alumina catalyst.
4
Figure 2 depicts the effect of reaction temperature on the conversion of the reactants and
product selectivities for the Fe + Ni-Mo/alumina dual catalyst.
Temperature,
o
C
280 300 320 340 360 380
Conversion/selectivity, %
0
20
40
60
80
100
H2 conversion
CO conversion
CO2 selectivity
CH4 selectivity
Selectivity for light
gas formation
Figure 2. Effect of temperature on the syngas conversion and product selectivities
using Fe+Ni-Mo/alumina catalyst (X and S denote species conversion and selectivity,
respectively).
Data of Figure 2 imply that increasing the reaction temperature has a positive
effect on the CO conversion - reaching 94.35% at 360°C. The selectivity toward CO2
seems to decrease with increased temperature, whereas the selectivity toward light gases
(LG: C1-C4) seems to increase at higher temperatures.
The effect of total pressure on the reactant conversion and products selectivities is
shown in Figure 3.
Running the reaction at higher pressures has a positive effect on the conversion of
the reacting species. The selectivity toward CO2 formation decreases as the pressure
increases and the selectivity toward formation of the light gases remains almost the same
over the range of pressures examined.
A comparison of the composition of liquid products obtained for Fe and Fe+Ni-
Mo/alumina catalysts is shown in Table 3. It is evident that in the presence of Ni-
Mo/alumina catalyst, olefinic hydrocarbons are completely hydrogenated to paraffinic
compounds. At the same time, Fe+Ni-Mo/alumina catalyst does not significantly affect
the yield of gasoline, jet and diesel fractions.
5
Pressure, psig
80 100 120 140 160 180 200 220
Conversion/selectivity, %
0
20
40
60
80
100
H
2
conversion
CO conversion
CO
2
selectivity
CH
4
selectivity
Selectivity toward light
gas formation
Figure 3. Effect of pressure on CO and H2 conversion, and product selectivities. T=
359°C, catalyst: Fe+Ni-Mo/alumina.
Table 3. Comparison of liquid product slate obtained from F-T synthesis of CO/H2
in the presence of Fe and Fe+Ni-Mo/alumina catalysts.
Product
Fe
Fe+Ni-Mo/Alumina
Gasoline range (C5-C10), %
55.8
55.4
Kerosene/jet fuel range (C11-C12), %
15.5
17.1
Diesel range (C13-C16), %
15.3
16.8
Lube oil and wax range (C17-C26), %
13.5
10.7
Olefin to paraffin molar ratio (for C8)
5.7
0.0
Figure 4 depicts the picture of the liquid hydrocarbon product obtained from the
F-T synthesis of CO/H2 using Fe + Ni-Mo/alumina combined catalyst.
It is known that syngas generated in a biomass gasifier may contain appreciable
amounts of CO2. So it is of interest to determine the effect of CO2 on the yields of
products during F-T synthesis. Table 4 shows the effect of CO2 at two different CO:CO2
ratios using Fe+Ni-Mo/alumina as the F-T catalyst.
It can be seen that CO2 conversion is negative in both cases. In other words, there
is a net generation of CO2 as a result of water gas shift reaction. However, at higher
CO2:CO ratios, the selectivity for CO2 formation decreases allowing a better utilization of
CO for the formation of hydrocarbons.
6
Figure 4. Liquid hydrocarbon product obtained by the F-T synthesis of syngas in
the presence of combined Fe+Ni/Mo/alumina catalyst.
Table 4. Effect of feed CO2 concentration on the F-T reaction parameters.
Reaction conditions
CO:CO2= 1.0
CO:CO2= 0.5
Temperature, °C
370
370
Pressure, psig
200
200
WHSV, mL/gcat./hr
18000
18000
CO conversion, %
82.33
73.66
CO2 conversion, %
-17.06
-2.65
CO+CO2 conversion, %
32.54
23.13
CO2 selectivity, %
20.76
7.07
CH4 selectivity, %
11.86
7.87
STY, g of liq. HC/gcat./hr
0.3887
0.3608
Conclusions
1. The use of cracking catalyst H+/ZSM-5 in combination with Fe catalyst results in
an increase in the yield of gasoline fraction in the liquid product and a decrease in
the yield of olefin to paraffin.
2. A highly paraffinic liquid product is obtained when a dual-bed catalytic system
including Fe and Ni-Mo/alumina hydrogenation catalyst is employed in the F-T
synthesis.
3. Presence of Ni-Mo catalyst did not significantly alter the product slate for the
liquid fraction, but practically eliminated olefins from the raw product.
4. Presence of CO2 in the feed gas adversely affects syngas conversion and space
time yield of liquid products.
5. Upon further development, the process described here can provide a more facile
and direct route to production of high quality liquid hydrocarbons from biomass-
derived syngas.
7
Acknowledgements
The authors would like to acknowledge the Florida Department of Agriculture and
Consumer Services for funding of this project.
References
1. Lv, P., Yuan, Z., Wu, C., Ma L., Chen Y. and Tsubaki, N., Energy Conversion and
Management 48, 1132-1139(2007).
2. Kolasinski Kurt W., Surface Science-Foundations of catalysis and nanotechnology,
John Wiley & sons ltd. (2002).
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