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Energy & Fuels 1996, 10, 1067-1073
1067
Separation of Fischer-Tropsch Catalyst/Wax Mixtures Using Dense-Gas and Liquid Extraction C. M. White,* K. L. Jensen, P. C. Rohar, J. P. Tamilia, L. J. Shaw, and R. F. Hickey Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 Received February 16, 1996X
This paper describes a catalyst/wax separation technique based on dense-gas and/or liquid extraction of the soluble hydrocarbon components from the insoluble inorganic catalyst particles. The separation by extraction can also be performed in conjunction with magnetic separation of iron catalyst particles. Extractions of 4.91 wt % catalyst in wax were performed with n-butane, n-pentane, and n-hexane. Up to 91 wt % of the catalyst/wax feed mixture to the extractor could be recovered as a catalyst-free wax (combined yield of the second- and third-stage separators). High-temperature gel permeation chromatography was used to measure the average molecular weight of the extraction fractions. The extraction process separates the wax according to molecular weight. The lower molecular weight wax components are extracted from the catalyst/wax mixture and accumulate in the second-stage separator, while the higher molecular weight wax components remain in the first-stage separator. The low molecular weight wax fraction can be remixed with the catalyst and pumped back into the slurry reactor, reducing the average molecular weight, reducing the viscosity, and improving the transport properties of the reaction media while minimizing the chances of reactor gelation due to buildup of high molecular weight waxes.
Introduction A slurry-phase reactor is preferable to a fixed-bed reactor in the Fischer-Tropsch (FT) reaction because it eliminates hot spots within the catalyst bed and, in some cases, minimizes coke formation on the catalyst particles. Further, maintaining a uniform reactor temperature improves the operator’s ability to control both the reaction and the product distribution. Slurry-phase Fischer-Tropsch synthesis gives a high level of catalyst productivity.1 Slurry-phase Fischer-Tropsch operation allows the use of low H2 to CO ratios and both addition and withdrawal of catalyst during operation. The liquid level in the slurry reaction zone is maintained by continuous or batch withdrawal of liquid product which contains catalyst particles. Therefore, slurry-phase Fischer-Tropsch reactors require separation of the catalyst from the liquid product and recycle of the recovered catalyst to the reactor. The separation of Fischer-Tropsch catalyst from wax products is an important issue when the synthesis is conducted in a slurry-phase reactor. The wax and catalyst can be separated using a variety of techniques including settling tanks, filtration, centrifugation, and, when an iron-containing catalyst is employed, magnetic separation.2 This paper describes a catalyst/wax separation technique based on dense-gas and/or liquid extraction of the soluble hydrocarbon components from the insoluble inorganic catalyst particles. A summary * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Bhatt, B. L. Liquid Phase Fischer-Tropsch Demonstration In The LaPorte Alternate Fuels Development Unit; DOE Contract DE-AC2291PC90018, June 1994; Vol. 1 and 2. (2) Zhou, P. Z. Status Review Of Fischer-Tropsch Slurry Reactor Catalyst/Wax Separation Techniques; prepared for U.S. DOE Pittsburgh Energy Technology Center in Feb 1991 by Burns and Roe Services Corp. under Contract DE-AC22-89PC88400 Subtask 43.02.
of the extraction process as it is applied to the fractionation of petroleum has been published by Gearhart and Garwin,3 while Brule and Corbett have provided an excellent description of the physical and chemical phenomena that make the process work.4 The separation by extraction may also be performed in conjunction with magnetic separation of iron catalyst particles as is shown in several examples. The Alternative Fuels Development Unit at LaPorte, TX, was operated in the slurry-phase Fischer-Tropsch mode during the summer of 1992.1 The operators planned to separate the catalyst from wax by filtration and return the separated catalyst to the reactor. The filters clogged on the first day of operation. Consequently, the reactor continued to lose catalyst over the duration of the experiment. The average particle size of the catalyst before charging to the reactor was about 10 µm.1 Significant attrition occurred during use due in part to poor mechanical stability, resulting in a large amount of fines that clogged the filter. There are two potential solutions to this problem. One is to develop an iron catalyst with improved mechanical stability; the other is to develop a low-cost, highly efficient catalyst/ wax separation system. The Pittsburgh Energy Technology Center (PETC) is pursuing both solutions. In addition to problems related to the separation of catalyst from wax, another potential problem encountered during non-steady-state slurry-phase FischerTropsch operation is reactor gelation.2 As the slurryphase Fischer-Tropsch reaction continues, the average molecular weight of the organic constituents in the reactor increases. Light ends are lost via the vapor state from the reactor; the higher molecular weight (3) Gearhart, J. A.; Garwin, L. Oil Gas J. 1976, 74, 63-66. (4) Brule, M. R.; Corbett, R. W. Hydrocarbon Process. 1984, 63, 7377.
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1068 Energy & Fuels, Vol. 10, No. 5, 1996
White et al.
Figure 1. Diagram of the laboratory-scale semicontinuous extractor used to separate Fischer-Tropsch catalyst from wax.
components remaining in the reactor undergo additional chain growth.5-8 As the average molecular weight of the reaction media increases, viscosity increases, eventually resulting in reactor gelation and the involuntary termination of the reaction. The technology developed at PETC and described here both addresses the FischerTropsch catalyst/wax separation problem and positively impacts the problem of reactor gelation. Experimental Section The feed to the semicontinuous extractor was a catalyst/ wax mixture containing about 4.91 wt % catalyst from a recent slurry-phase Fischer-Tropsch reaction performed at the Alternative Fuels Development Unit at LaPorte, TX, where the separation by filtration failed due to filter clogging from very fine catalyst particles.1 It is expected that similar results would be obtained on mixtures containing higher concentrations of catalyst. The experiments conducted at PETC used a laboratory-scale semicontinuous extraction system designed and built at PETC to fit into a walk-in hood. The system is capable of independent continuous feeding of the melted catalyst/wax slurry mixture and solvent into an oven containing a static tube mixer (5) Pichler, H.; Schulz, H. Chem.-Ing. Tech. 1970, 42, 1162-1174. (6) Pichler, H.; Schulz, H.; Elstner, M. Brennst.-Chem. 1967, 48, 7887. (7) Schulz, H.; Rao, B. R.; Elstner, M. Erdoel Kohle, Erdgas, Petrochem. Brennst.-Chem. 1970, 23, 651-655. (8) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: Orlando, FL, 1984; pp 190-192.
and two or more stages of separation (separation vessels called separators). The flow rate of the n-alkane solvent was constant during each experiment at between 2 and 4 mL/min, while the flow rate of the melted catalyst/wax mixture was between 0.1 and 0.2 mL/min. The pressures and temperatures of the stages are independently controlled. The system can process approximately 100 g/day of catalyst/wax mixture, but normally processed between 30 and 50 g. A diagram of the laboratory-scale semicontinuous extraction system appears in Figure 1. A summary of the n-butane extraction conditions is shown in Table 1. The n-butane extractions shown in Table 1 were performed above the critical pressure and below the critical temperature with liquid n-butane. The separation process described in Figure 1 works by mixing the catalyst/wax mixture with hot liquid or supercritical solvent in a static mixer. The effluent from the mixer enters the bottom of the first stage separator. The inorganic catalyst particles are not soluble in the organic solvent and remain in the first-stage separator while most of the organic wax components dissolve in the flowing solvent stream and flow out of the first-stage separator through a back pressure regulator, where the pressure is reduced, and into the secondstage separator. When the pressure is reduced, the dense gas or liquid becomes a low-density gas and looses its solvation properties, causing the dissolved wax to precipitate in the second-stage separator. The catalyst and some insoluble wax constituents remain in the first-stage separator. The lowdensity gas used as a solvent is vented in the laboratory system shown in Figure 1, while in a commercial system it would be collected, repressurized, and reused.
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Catalyst/Wax Separation
Energy & Fuels, Vol. 10, No. 5, 1996 1069
Table 1. Extraction with n-Butane [Tc ) 152 °C, Pc ) 37.47 atm (536.0 psig)]9 as the Solvent operating conditionsa
exp
1st sep temp, °C
2nd sep temp, °C
1st sep pressure, psig
1 2 3 4
138 145 145 107
138 145 145 107
1018 628 620 750
results, wt % 2nd sep pressure, psig
1st sep % feedb
100 180 174 42
2.0 39.3 33.9 6.6
2nd sep % feedc
feed rec during cleaningd
total feed rece
1st sep catalystf
2nd sep catalystg
feed wax rec
feed catalyst rec
82.1 51.2 51.3 76.8
12.8 7.9 7.8 22.2
96.9 98.4 93.0 105.5
22.6 7.9 6.8 72.6
0.11 0.04 0.05 0.04
101.5 97.7 92.7 105.5
10.9 110.3 98.9 106.0
a Experiments 1-3 were performed without any magnets, while experiment 4 employed magnets external to the first-stage separator. Feed was 4.91 wt. % catalyst in wax. b Weight percent of catalyst/wax feed material that was recovered from the first-stage separator after extraction. c Weight percent of catalyst/wax feed material that was recovered from the second-stage separator after extraction. dWeight percent of catalyst/wax feed material that was recovered from the first-, second-, and third-stage separators after the system was flushed with several void volumes of hot toluene. eSum of the weight percent of material recovered from the first- and second-stage separators after extraction plus the amount of material recovered after hot toluene cleaning. f Weight percent of catalyst in the material recovered from the first-stage separator as determined by ashing as described under Experimental Section. The higher the weight percent catalyst in the first-stage separator, the better the separation. gWeight percent of catalyst in the material recovered from the second-stage separator as determined by ashing as described under Experimental Section. The lower the weight percent catalyst in the second-stage separator, the better the separation.
Table 2. Extraction with n-Pentane [Tc ) 196.5 °C, Pc ) 33.25 atm (473.9 psig)]9 as the Solvent and Magnets External to the First-Stage Separatora operating conditions
exp 1 2 3
feed rec total feed feed 1st sep 2nd sep 3rd sep 1st sep 2nd sep 3rd sep temp, temp, temp, pressure, pressure, pressure, 1st sep 2nd sep 3rd sep during feed 1st sep 2nd sep wax catalyst recj °C °C °C psig psig psig % feedb % feedc % feedd cleaninge recf catalystg catalysth reci 204 204 204 xk (sl (rsdm 204
4
results, wt %
204 204 204
>30 >30 >30
800 800 800
43 33 36
0 0 0
204
>30
800
39
0
5.9 4.2 3.4 4.4 1.3 29.5 5.5
73.1 69.6 76.3 73.0 3.4 4.7 74.7
9.8 15.3 3.9 9.7 5.7 58.8 5.1
9.4 6.9 3.5 6.6 3.0 45.5 10.0
98.2 95.9 87.1 93.7 5.8 6.2 95.3
63.6 50.6 60.8 58.3 6.8 11.7 57.3
0.05 0.01 0.09 0.05 0.04 80.0 0.00
98.4 97.6 88.7 94.9 5.4 5.7 95.1
93.2 64.7 57.2 71.7 19.0 26.5 99.2
a
Feed was 4.91 wt % catalyst in wax. The first three experiments were performed under identical conditions using the static tube mixer. The fourth experiment was performed using the same conditions without the mixer. b Weight percent of catalyst/wax feed material that was recovered from the first-stage separator after extraction. c Weight percent of catalyst/wax feed material that was recovered from the second-stage separator after extraction. d Weight percent of catalyst/wax feed material that was recovered from the third-stage separator after extraction. e Weight percent of catalyst/wax feed material that was recovered from the first-, second-, and third-stage separators after the system was flushed with several void volumes of hot toluene. f Sum of the weight percent of material recovered from the first-, second-, and third-stage separators after extraction and the amount recovered after hot toluene cleaning. g Weight percent of catalyst in the material recovered from the first-stage separator as determined by ashing as described under Experimental Section. The higher the weight percent catalyst in the first-stage separator, the better the separation. h Weight percent of catalyst in the material recovered from the second-stage separator as determined by ashing as described under Experimental Section. The lower the weight percent catalyst in the second- stage separator, the better the separation. i Total weight percent wax recovered. j Total weight percent catalyst recovered. k Average value. l Standard deviation. m Relative standard deviation.
The solvent was delivered to the extractor using two ISCO 260 D pumps (labeled pumps A and B in Figure 1) operated in tandem such that when one emptied, the second began feeding solvent to the extractor while the first pump refilled. Pumping the melted catalyst/wax slurry was accomplished using a third heated (135 °C) ISCO 260 D pump (labeled pump C in Figure 1). The extraction gas was 99.5% n-butane [Tc ) 152 °C, Pc ) 37.47 atm (536.0 psig)].9 The catalyst-free wax was collected in the second-stage separator, where the pressure was reduced, causing the extracted wax to precipitate from solution. Catalyst and any undissolved wax components were collected in the first-stage separator. The volume of the first- and secondstage separators was about 75 mL each. The temperature of both separators was the same during the n-butane experiments. Two additional series of extractions using the PETC laboratory-scale semicontinuous extractor, shown in Figure 1, were conducted using n-pentane (97.3%) [Tc ) 196.5 °C, Pc ) 33.25 atm (473.9 psig)]9 from Fisher Scientific and n-hexane (99.1%) from Ashland Chemical [Tc ) 234.2 °C, Pc ) 29.9 atm (425 psig)]9 as illustrated in Tables 2 and 3, respectively. The experiments performed with n-pentane and n-hexane were above both the critical temperature and the critical pressure of the solvent in the first-stage separator. In most extraction (9) Kudchaker, A. P.; Alani, G. H.; Zwolinski, B. J. Chem. Rev. 1968, 68, 659-735.
experiments, an upper pressure of about 55.4 atm (800 psig) was selected because that pressure is expected to be the approximate upper operational pressure of a slurry FischerTropsch reactor. In both the n-pentane and n-hexane experiments, a third-stage separator (not shown in Figure 1) was incorporated to collect the lowest molecular weight wax components. Similarly, in the n-pentane and n-hexane experiments and in the fourth n-butane experiment shown in Table 1, magnets were added to the exterior of the first-stage separator to minimize carry-over of iron catalyst to the secondstage separator. The magnets employed were high field strength rare earth/cobalt permanent magnets from Edmund Scientific. In all of the experiments reported here, solvent was pumped through the system for approximately 2 h after the flow of catalyst/wax mixture was stopped in an attempt to prevent any unprocessed catalyst/wax mixture from remaining in the system. At the end of each experiment, the system was cooled and slowly depressurized. The first-, second-, and third-stage separators were detached from the system, and their contents were removed and weighed. In the case of the n-hexane experiments, the third-stage separator contained liquid nhexane, which was evaporated to dryness, and the wax was weighed. The transfer lines, valves, mixer, and other components of the system contain feed at various degrees of processing. Any material flushed from the system was collected and weighed at the end of each experiment. Specifically, after the
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1070 Energy & Fuels, Vol. 10, No. 5, 1996
White et al.
Table 3. Extraction with n-Hexane [Tc ) 234.2 °C, Pc ) 29.9 atm (424.8 psig)]9 as the Solvent and Magnets External to the First-Stage Separatora operating conditions
results, wt %
feed rec total 1st sep 2nd sep 3rd sep 1st sep 2nd sep 3rd sep feed 1st sep 2nd sep temp, temp, temp, pressure, pressure, pressure, 1st sep 2nd sep 3rd sep during b c d exp °C °C °C psig psig psig % feed % feed % feed cleaninge recf catalystg catalysth 1 2 3
240 240 240 xk (sl (rsdm
197 198 194
