Research Note pubs.acs.org/IECR
Control of Product Distribution of Fischer−Tropsch Synthesis with a Novel Rotating Packed-Bed Reactor: From Diesel to Light Olefin Jian-Feng Chen,*,‡ Yi Liu,† and Yi Zhang*,† †
State Key Laboratory of Organic−Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100029, People’s Republic of China ABSTRACT: A rotating packed-bed (RPB) reactor has been first applied to the Fischer−Tropsch synthesis (FTS) reaction. The main products of the FTS can be selectively formed by choosing the optimum high gravity level of the RPB reactor, which has opened a door to tailor the product distribution of FTS. The high selectivity of light olefins in RPB reactor provides a novel pathway to directly and effectively form light olefins from syngas, syngas to olefin (STO), which would avoid low-efficiency methanol synthesis and methanol-to-olefin (MTO) process.
1. INTRODUCTION Fischer−Tropsch synthesis (FTS) has received renewed interest, because of the worldwide demand for a decreased dependence on petroleum.1,2 Generally, FTS products are almost always normal aliphatic hydrocarbons, following the Anderson−Schultz−Flory (ASF) distribution, which is determined by the polymerization mechanism.3 It is a great challenge for FTS to produce the specific products selectively, especially to form light olefins effectively. FTS is a surface-catalyzed polymerization process that uses CHx monomers, to produce hydrocarbons with a broad range of chain length and functionality. Such a distribution is unselective, and the development of novel FTS catalysts,4−11 which can selectively produce hydrocarbons in a certain range (such as our previously published zeolite-encapsulated catalysts12a) has attracted much attention. Mass transfer is also considered to be a very important factor that affects both activity and selectivity.13 Even though in the gas-phase FTS, the catalyst pores may be filled with liquid products. The mass transfer in the liquid phase is typically 4−5 orders of magnitude slower than in the gas phase. Some reactions are even perceived slow, because of slow and limited diffusion in the liquid phase.14 However, applying monolith catalysts8a or supercritical fluids9 in the FTS reaction has significantly improved the mass transfer of products, resulting in a narrow products distribution. Meanwhile, Liu8b reported that the use of appropriate material structures and flow conditions enables manipulation of external and internal mass transfer steps to enhance the reaction activity and selectivity. Herein, the rotating packed-bed (RPB)15a,b reactor was used for first time for the FTS reaction to control the product distribution. The basic principle of the RPB reactor is to create a high-gravity environment via the action of centrifugal force. The rate of mass transfer in a rotating bed is 1−3 orders of magnitude larger than that in a conventional packed bed, allowing a dramatic reduction in residence time. On the other hand, the strong shearing action between the catalysts bed and gas flow would significantly influence the stagnation layer of catalyst particles, resulting in the intensified mass transfer of © XXXX American Chemical Society
reactants and products. In this study, the FTS reaction was carried out in a rotating catalyst bed, and the product distribution was significantly adjusted by the high gravity environment. Therefore, the main products of the FTS can be selectively formed by choosing the optimum high gravity level of the RPB reactor, which has opened a door to tailor the product distribution of FTS. It is considered that the intensified mass transfer of products fastens the specific products flushing out of the catalyst and catalyst bed and obstructs the secondary reaction, such as chain growth and hydrogenation of formed 1olefin, resulting in high selectivity of specific hydrocarbons, as illustrated in Scheme 1.
2. EXPERIMENTAL SECTION The aqueous solution of cobalt nitrate was impregnated onto the silica support (specific surface area = 460 m2 /g, pore volume = 1.22 cm3 /g, average pore diameter = 6.7 nm) via the incipient wetness method. The loading of cobalt was 10 wt %. The catalysts were dried at 393 K for 12 h and calcined at 673 K for 2 h. The preparation details of conventional supported Co/SiO2 FTS catalysts were described in our previous publication.12b Thermal analysis was carried out with the catalysts after 10 h of FTS reaction on a Model DTG-60 system (Shimadzu) to investigate the possible coke and FTS wax, which formed on the catalyst. It was implemented in an air flow of 50 mL min−1. The temperature increased from 303 to 700 K at a rate of 5 K min−1. The FTS reaction was conducted in a continuous-flow-type fixed-bed reactor and a RPB reactor, in which the catalyst was loaded on the rotor, using 1 g of catalyst for all reactions. Before the reaction, the catalyst was reduced by H2 at 673 K for 10 h. During the reaction, effluent gas released from the reactor was analyzed by online gas chromatography. CO and CO2 were Received: January 10, 2012 Revised: May 16, 2012 Accepted: June 11, 2012
A
dx.doi.org/10.1021/ie300079j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Research Note
Scheme 1. Control of the Products Distribution of FTS Reaction via the RPB Reactor
analyzed using an active charcoal column equipped with a thermal conductivity detector (TCD). The hydrocarbons were also analyzed using FID for C1−C5 (Porapak Q, online) and for C6−C25 (SE-30, uniport), respectively. The selectivity is described in terms of carbon mole percentage (c-mol %). Argon was employed as an internal standard, with a concentration of 3% in the feed gas. All FTS reactions reached steady state after 3 h, and the selectivity and CO conversion were calculated using reaction data at 10 h. The reaction conditions for all reaction were pressure (P) (total) = 1.0 MPa, CO/H2 = 1/2, gas hourly space velocity (GHSV) = 2500 h−1, and temperature (T) = 513 K. For the RPB reactor, as described in ref 15b, the high gravity level (gr)15b was set at gr = 50 m/s2 (RPB 1) and gr= 300 m/s2 (RPB 2), respectively.
Figure 1. Product distribution of various FTS reaction using different reactors: (A) fixed bed, (B) rotating packed bed (RPB 1) (gr = 50 m/ s2), and (C) rotating packed bed (RPB 2) (gr = 300 m/s2).
C10−C20 was 47.5%, which is 2 times higher than that in a conventional fixed-bed reactor, as compared in Table 1. Table 1. Reaction Performance of FTS Using Different Reactorsa C2−C4 Sel. (%)
reactor fixed bed RPB 1d RPB 2e
3. RESULTS AND DISCUSSION The FTS reaction was carried out under different high-gravity environments in a RPB reactor using a conventional supported Co/SiO2 catalyst. As a comparison, a fixed-bed reactor was also used to test catalytic performance of the same catalyst under the same reaction conditions. All FTS reactions reached steady state after 3 h, and the selectivity and CO conversion were calculated using reaction data at 10 h. The product distribution of different FTS reactions is illustrated in Figure 1. The conventional FTS reaction using a fixed-bed reactor presents a very typical product distribution of the gas phase reaction, as shown in Figure 1A. The selectivity of methane is 16.1%, and the selectivity of ethane is very low. Only 0.45% ethylene was formed. Beside the chain growth mechanism of conventional FTS,16a,b the formed ethylene or other 1-olefins always readsorb onto the cobalt metallic sites as intermediates of FTS to form other hydrocarbons,16c resulting in low ethylene selectivity in fixed-bed reactors. For the RPB FTS reaction, when the high gravity level was relatively lower (gr = 50 m/s2), the main product of the FTS reaction was diesel, as shown in Figure 1B. The selectivity of
CO Conv. (%)b
CO2 Sel. (%)b
CH4 Sel. (%)
olefin
parrafin
C10−C20 Sel. (%)
□αc
46.3
2.30
16.1
5.21
9.89
25.8
0.83
43. 6
2.10
8.34
4.01
5.88
47.5
0.91
44.2
1.95
14.2
27.5
32.6
4.99
0.69
Reaction conditions: P = 1.0 MPa; T = 513 K; GHSV = 2500 h−1. Selectivities are presented in terms of carbon mole percent (c-mol %). b From TCD. cChain growth probability. dgr = 50 m/s2. egr = 300 m/s2. a
Generally, the liquid-phase product formed from the FT reaction could be trapped due to capillary forces and therefore could not be flushed away from the catalyst external surface promptly by the convective flow. As a result, molecules in the stagnant liquid phase continue to react and grow into longer chains to form waxes, because of secondary reactions. As reported, the monolith FTS catalyst8a realized high C5−C18 selectivity in a gas-phase fixed-bed FTS reaction, because the liquid-phase products can be readily flushed out of the straight channels of monolith catalyst, to prevent the formation of wax. Using the same mechanism, for the RPB FTS reaction, because of the advantageous mass-transfer effects, the formed liquidphase products, which would be trapped inside or outside of B
dx.doi.org/10.1021/ie300079j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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the catalyst, were accelerated to flush them away from catalyst and catalyst bed, preventing the formation of wax, leading to the formation of more C10−C20 hydrocarbons. On the other hand, as the basic principle of chemical reaction, when one product is quickly removed from reaction system, this reaction would tend to form more of this type of product. Therefore, RPB FTS realized a C10−C20 selectivity that was 2 times higher than that of conventional fixed-bed FTS. Amazingly, when the high gravity level of RPB was as high as gr = 300 m/s2, not only did the product distribution deviate from the ASF law, but also the heavy hydrocarbons (C14+) were completely suppressed. Furthermore, the C2−C4 hydrocarbons became the main products, and the selectivity of methane is slightly lower than that of a fixed-bed reactor, as shown in Figure 1C. Meanwhile, the selectivity of C2=−C4= reached 27.5%, which is 5 times higher than that obtained from a fixedbed reactor, and the total selectivity of C2−C4 was as high as 60.1%, as compared in Table 1. Furthermore, the CO conversion and selectivity of C2=−C4= were stable during the RPB FTS reaction at gr = 300 m/s2, as shown in Figure 2.
catalysts. CH4 selectivity varied as a function of pellet size with the maximum methane selectivity obtained for the largest catalyst pellets. The ratio of 1-hexene/hexane decreased as the pellet size increased. It is demonstrated that the fastened mass transfer of products enhanced the formation of 1-olefin and decreased the methane selectivity, because of the obstructing secondary reaction of 1-olefins. Therefore, the light olefin formation was accelerated in the high gravity level RPB reaction, as a consequence of its enhanced mass-transfer efficiency. Furthermore, Hunger et al.17 compared the methanol-to-olefin (MTO) reaction on SAPO-34 in the fixed bed with that in the MAS NMR rotor reactor, and obtained two times higher yields of propylene and ethylene from the MAS NMR rotor reactor than those from a fixed-bed reactor at 625 K. Based on the information given above, it is indicated that the rotating-bed reactor could strongly improve the mass transfer of light hydrocarbon products, such as propylene and ethylene, leading the FTS reaction tends to form much more light hydrocarbons, contributing to high selectivity of light hydrocarbons in RPB reactor. The higher selectivity of C2 products than that of C3 might be relative to obstructing the secondary reaction and/or mass-transfer effects. The thermogravimetric analysis (TGA) results are shown in Figure 3 for the catalysts used in different reactors. The weight
Figure 2. Time-on-stream of the RPB FTS reaction at gr = 300 m/s2. Reaction conditions: P = 1.0 MPa; T = 513 K; GHSV = 2500 h−1. The selectivity is expressed in terms of carbon mole percent.
Generally, the decrease in the overall concentration of reagents inside the catalyst particles is often accompanied by an increase in the H2/CO ratio. This seems to be the reason why the apparent rate of CO consumption could be relatively constant, even when severe internal mass-transfer limitations occur; depletion of the CO intraparticle concentration leads to a higher reaction rate, because of more activated hydrogen on the catalyst surface.13 In contrast, for RPB, since the mass transfer is ∼100 times faster15b than that of the conventional fixed bed, it is believed that the increased concentration of reactants inside the catalyst particles would decrease the H2/CO ratio, because CO is more sensitive to diffusion limitation than H2, resulting in slightly decreased CO conversion for the RPB 2 reaction. The decreased H2/CO ratio inside the pores of the catalyst and decreased CO conversion would be advantageous to produce light hydrocarbons. Meanwhile, inside the RPB, the improved mass transfer leads to the fast flush out of the formed light hydrocarbons of the catalyst and catalyst bed; as a result, the FTS tends to produce more light hydrocarbons, because of the decreased concentration of light hydrocarbons inside the catalyst. On the other hand, Eglesia et al.13 reported the effect of internal mass transfer on the syngas consumption rate and selectivity in an isothermal fixed-bed reactor with Co/SiO2
Figure 3. Thermogravimetric analysis (TGA) of different used catalysts after 10 h of a FTS reaction: (a) RPB 2 (gr = 300 m/s2), (b) RPB 1 (gr= 50 m/s2), and (c) a fixed bed.
loss below 423 K is due to the water removal, whereas that above 473 K is ascribed to the coke or the heavy FTS products combustion. Because the same catalysts are used and the reaction performance is the same for different reactions, the oxidation of supported cobalt is believed to be the same in each TGA curve. It is clearly illustrated that the accumulation of coke and heavy FTS products, which was as high as 30% (Figure 3c), is serious for the catalyst used in the conventional fixed-bed reactor. For the catalyst used in reaction RPB 1, the weight loss decreased from 30% to 13%, and the exothermic temperature of weight loss decreased from 633 K to 613 K based on DSC analysis, compared to that of fixed-bed reaction. Furthermore, for the catalyst used in reaction RPB 2, the weight loss decreased from 30% to 7%, and the exothermic temperature of weight loss decreased from 633 K to 563 K based on DSC analysis, compared to that of a fixed-bed reaction. These results coincide with the product distribution results shown in Figures 1B and 1C, which indicate that the coke and heavy hydrocarbon is significantly suppressed, C
dx.doi.org/10.1021/ie300079j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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(13) Iglesia, E.; Reyes, S. C.; Madon, R. J.; Soled, S. L. Adv. Catal. 1993, 39, 221−302. (14) Hilmen, A.; Bergene, E.; Lindvag, O.; Schanke, D.; Eri, S.; Holmen, A. Catal. Today 2001, 69, 227−232. (15) (a) Ramshaw, C. The Chemical Engineer 1983, 13−14. (b) Chen, J.; Gao, H.; Zou, H.; Chu, G.; Zhang, L.; Shao, L.; Xiang, Y.; Wu, Y. AIChE J. 2010, 56, 1053−1062. (16) (a) Liu, Z.; Hu, P. J. Am. Chem. Soc. 2002, 124, 11568−11569. (b) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Martin Lok, C. J. Catal. 2008, 257, 221−228. (c) Schulz, H. Catal. Today 2003, 84, 67− 70. (17) Hunger, M.; Seiler, M.; Buchholz, A. Catal. Lett. 2001, 74, 61− 68.
resulting in little accumulation of coke or FTS heavy products on the catalyst used in the RPB reactor. On the other hand, because of the little accumulation of FTS heavy products on the catalyst, the surface of the catalyst would become dry and hot, which are advantageous to forming smaller hydrocarbons, contributing to high selectivity of light hydrocarbons.
4. CONCLUSION In summary, the RPB reactor has been first applied to the Fischer−Tropsch synthesis (FTS) reaction. The intensified and controllable mass transfer of both reactants and products amazingly adjusted the product distribution of FTS. The main products of the FTS can be selectively formed by choosing the optimum high gravity level (gr) of the RPB reactor, which has opened a door to tailor the product distribution of FTS via selected mass-transfer efficiency. Meanwhile, avoiding lowefficiency methanol synthesis and MTO process, direct and effective formation of light olefins from syngas, syngas to olefin (STO), is feasible by utilizing the RPB reactor and iron-based catalyst. It is believed that this novel method of syngas to olefin would obtain great attention of industry application, because of the significant savings in equipment investment and energy consumption.
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AUTHOR INFORMATION
Corresponding Author
*E-mails:
[email protected] (J.-F.C.), yizhang@mail. buct.edu.cn (Y.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Nos. 51174259, 21121064, and 20990221), Ministry of Education (20110010120002, LX2011002), National “863” program of China (No. 2009AA033301), and the Foundation of State Key Laboratory of Coal Conversion (No. 10-11-902-1) is greatly appreciated.
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REFERENCES
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dx.doi.org/10.1021/ie300079j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
