Evaluation of Laboratory Methods for the Acquisition of Catalyst from

Oct 18, 2012 - Catalyst breakdown by physical attrition and chemical stresses is a problem of great and continuing concern in the slurry phase Fischer...
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Evaluation of Laboratory Methods for the Acquisition of Catalyst from Fischer−Tropsch Wax/Catalyst Mixtures Tie-jun Lin, Meng Xuan, and Li Shi* The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: Catalyst breakdown by physical attrition and chemical stresses is a problem of great and continuing concern in the slurry phase Fischer−Tropsch Synthesis. To reach a better understanding of the morphology and composition of a working catalyst, it requires first the removal of wax from the catalyst. In this work, Improved Soxhlet Extraction (ISE) in comparison of Standard Soxhlet Extraction was evaluated concerning their separation efficiency and relevancy for characterization of working catalysts. ISE extracted a greater amount of wax and recovered nearly all of the catalyst. In addition, ISE does not change the morphology and particle size distribution of working catalysts whereas Soxhlet extraction fails to reflect the actual information on the catalyst. The ISE is appropriate for the rapid and accurate analysis in plant operation.

1. INTRODUCTION Attrition of Fischer−Tropsch catalyst has been a major problem in commercial implementation of slurry bubble column reactors (SBCR) or stirred tank slurry reactors (STSR).1,2 Generation of fine particles caused mainly by the physical and chemical attrition of the original catalyst leads to the loss of catalytic agents, product contamination, downstream filter plugging, increase in viscosity, reduction in heat and mass transmission, and increase the difficulties in wax/catalyst separation.3,4 To understand the nature of attrition, it is necessary to perform a study that monitors the particle size distribution (PSD), microstructure, catalyst physical properties, element distribution, and support structure of working iron catalysts.5 Before these analyses, the first step requires the removal of Fischer−Tropsch (F-T) hydrocarbon wax from the surface and inner pores of working catalysts, and then the waxfree catalysts can be submitted for characterization. Standard Soxhlet extraction, a commonly used technique by laboratories worldwide to realize this solid−liquid separation process,6,7 however, has many problems such as adverse time consumption,8,9 failure to completely strip the residual wax,3 lower catalyst recovery yield,10 and limit in the gross weight of sample treated at a time. These troubles would mainly cause agglomeration of small particles and affect characterization results. In view of the above, the aim of this work is to design a novel separation device (defined as Improved Soxhlet Apparatus, ISA) and establish its separation procedures, and then study the effect of two different separation methods on the recovery yield and particles size distribution of the catalyst separated from F-T wax/catalyst solid mixtures.

samples were prepared by adding catalysts to clean F-T wax. The properties reported for the average size of the catalyst were 30−200 μm, surface area of 160−210 m2·g−1, and bulk density of 0.6−0.8 g·cm−3. 2.2. Apparatus. Figure 1 shows that an Improved Soxhlet Apparatus (ISA) comprises a filter (pore size less than 0.6 μm), a porous PTFE lid, a vessel with a volume of 1000 mL, an outer columnar heating mantle, and a wire mesh filter cartridge with multiple layers of mesh, whose mesh sizes vary from 80 to 200 mesh. The whole ISA is shown in Figure 1b. Figure 1c

Figure 1. (a) Elements of ISA; (b) Schematic diagram of ISA; (c) Vacuum drying apparatus refitted from ISA; (1) Extraction cell; (2) Filter; (3) Tap funnel; (4) Gas distributor; (5) Porous PTFE lid; (6) Wire mesh filter cartridge with variable mesh sizes; (7) Columnar heating mantle; (8) Pipeline of inert gas; (9) Glass tube; (10) buffer; (11) Vacuum pump; (12) Flask; (13) Plug.

2. EXPERIMENTAL SECTION 2.1. Samples. Two slurry samples were withdrawn at different Time-On-Stream (TOS) from a STSR during Fischer−Tropsch Synthesis (FTS) at 523 K, 2.5−3 MPa, a superficial gas velocity of 0.1−0.35 m·s−1, a H2/CO ratio of 1.4−1.9, a syngas/catalysts ratio of 5000−15000 m3·t−1. Other © 2012 American Chemical Society

Received: Revised: Accepted: Published: 14511

June 11, 2012 October 16, 2012 October 18, 2012 October 18, 2012 dx.doi.org/10.1021/ie301535e | Ind. Eng. Chem. Res. 2012, 51, 14511−14516

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2.5.2. Percentage of F-T Wax Extracted. The F-T wax extracted by the two methods described in this work was dried in a heater to constant weight (under a gentle stream of nitrogen), and the percentage of extractives content of wax was calculated as follows: a W = × 100% p (1)

describes another apparatus refitted from ISA, which can be used to dry the sample under the vacuum or N2 breezing. A classical Soxhlet extraction apparatus with cellulose extraction cartridges (33 mm ×120 mm) was used to extract the same slurry samples at ambient pressure and a temperature according to the boiling points of the solvent. 2.3. Materials. Solvents of AR grade, such as n-heptane, toluene, xylene, petroleum ether, carbon disulfide, carbon tetrachloride, and anhydrous ethanol were purchased from Sinopharm Chemical Reagent Co.Ltd. The aromatic solvent used in this work was obtained from a commercial catalytic reforming unit of Sinopec Zhenhai Refining & Chemical Company. The main components of this aromatic solvent were toluene (0.2, m%), xylene (60.1, m%), C9+ aromatics (39.7, m %). PTFE filter paper and Cellulose thimble filter were purchased from Toyo Roshi Kaisha, Ltd. 2.4. Separation Procedure. 2.4.1. Standard Soxhlet Extraction. Different slurry samples were extracted in a conventional Soxhlet extraction apparatus. The sample was placed in a cellulose thimble filter, which fit in the Soxhlet extraction apparatus. The temperature of extraction depended on the boiling point of the solvent; the actual extraction temperature was adjusted so that the rate of solvent refluxing through the sample was 6 cycles/h. Cooling the fresh extract liquid samples periodically, one can decide that the extraction is terminated if there is no precipitation of wax. The extraction solvent used then was reclaimed by rotary evaporation, and the percentages of extracted wax or recovered catalyst were calculated by weight. 2.4.2. Improved Soxhlet Extraction (ISE). An ISE apparatus was used for the separation of catalyst from F-T wax/catalyst solid mixtures. The sample was placed into a 1000 mL extraction cell, and a quantity of solvent of about 150 mL was preintroduced into the filter. Once the sample was impregnated by the refluxing solvent, the valve of tap funnel was subsequently closed, and the vacuum filtration began. The valve was opened after the tap funnel was filled, so the waxladen solvent could flow into the flask. The procedure was repeated until the wax was completely stripped, which can be judged by observing whether wax crystals precipitate from the solvent in the tap funnel or not. At the end of the separation, anhydrous ethanol was used to rinse the catalyst particles several times, and vacuum drying or drying with slight N2 breezing was performed in the vacuum drying apparatus (Figure 1c). In other case, the wax-free catalyst could be kept in the ethanol. The wax-free catalyst particles were submitted for PSD measurements or other analysis. Several parameters were investigated for the separation process of ISE in this work.11 2.5. Analytical Methods. 2.5.1. Particle Size Distribution. The PSD of catalyst particles was determined using a MALVERN Mastersizer 2000 laser particle size analyzer instrument, whose particle diameter detection ranged from 0.02 to 2000 μm. The details about particle size measurement and calculation can be found in the MALVERN Web site.12 Multiple measurements (three to five) were made, and the average particle size was represented using the value of the volume moment. An ultrasonic bath was selected to better disperse the catalyst particles in anhydrous ethanol. Our studies showed that the short application of ultrasonic bath has a negligible impact on the particle distributions. The PSD provided by Mastersizer 2000 helps to compare the separation efficiency of the two separation methods described in this work.11

where W = F-T wax extracted; a = the weight of dried extract; and p = the total weight of original sample. 2.5.3. Catalyst Recovery. The catalyst particles obtained by standard Soxhlet extraction and ISE were dried in a vacuum apparatus to constant weight and then cooled at room temperature. The (relative) recovery yield of catalyst was calculated as follows c Y1 = × 100% (2) e

Y2 =

d × 100% f

(3)

where Y1 is the recovery yield of catalyst, Y2 is the relative recovery yield of catalyst, c or d is the weight of catalyst recovered, e is the weight of catalyst added in clean wax, and f is the weight of actual F-T samples.

3. RESULTS AND DISCUSSION 3.1. Effect of Solvent Used. Because of the easy solidification of slurry samples at room temperature and the high viscosity of F-T wax, it is vital to dissolve and extract the hydrocarbon wax from the solid mixtures with an organic solvent. Just as Zhou and Srivastava13 had pointed out, the solvent-assisted wax/catalyst separation is an effective way to reduce the viscosity of hydrocarbon wax and deserves to be investigated systematically. Generally, light hydrocarbon solvents, such as F-T naphtha, pentane, hexane, heptane, toluene, xylene, and so forth, are thought to be suitable solvents to dilute the catalyst coated with hydrocarbon wax. These solvents comprise molecules containing 5 to 12 carbon atoms.13,14 In this work, 7 solvents, namely, n-heptane, toluene, xylene, petroleum ether, carbon disulfide(CS2), carbon tetrachloride(CCl4), aromatic solvent, were employed for the ISE. From the results, given in Figure 2, one can find that the wax has different solubilities in the various solvents . Among them the aromatic solvent shows the maximal percentage of F-T wax extracted (91%), while the carbon disulfide gives the minimum result. The toluene showed a result in agreement with the n-heptane, between 59% and 63%. With xylene, the result showed recoveries in 78%. The aromatic solvent and xylene are superior solvents of F-T wax. We suppose that solvents mixed with different carbon atoms possess a higher solubility because of the synergistic effect. Because the aromatic solvent used in this study is commercially available and cheaper than xylene, it is selected as solvent for treating wax/catalyst mixtures. 3.2. Effect of Temperature. Temperature is a key parameter in the removal of hydrocarbon waxes from catalyst particles. It is found that raising the temperature is favorable to reduce the viscosity of wax, avoid the wax from plugging the pore size of the filter medium, and increase the solubility of hydrocarbon solvents. So, in particular, a columnar heating mantle around the filter was set up. 14512

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Figure 2. Interval of mean extraction yield (95% confidence) of F-T wax using different solvents. Figure 3. Effect of gas flow rate on PSD of an iron catalyst.

The freezing point and melting point of these F-T waxes is about 369 and 400 K, respectively. Once the treating temperature approaches the melting point of the wax, one would expect a significant increase in the solubility of wax, and the melting point of the aromatic solvent is about 393−423 K, so the optimal separation time should be controlled within this range. 3.3. Effect of Gas−Liquid Contact State on PSD. To improve the effectiveness of wax removal, especially the wax remaining in the pores of catalyst particles, an inert gas (e.g., N2) was introduced into the bottom of the extraction cell to mix the solvent intimately with the catalyst particles coated with wax, which is similar to the three phase bubble column reactor. Because of the resistance of gas, the catalyst particles have to pass through the wire mesh filter cartridge first at a very low rate, so adequate time can be guaranteed for the suspended catalysts coated with wax to contact the solvent. Other than mechanical agitation, such a gas-agitation, a milder treatment technology is not supposed to further cause the catalyst particles to break apart. To verify our estimation, the influence of three gas flow rates (0.6 L/min, 1.2 L/min, 2.2 L/min) on particle sizes distribution were investigated in this experiment. Figure 3 indicates that there are no significant differences among these PSDs after 4 h of different gas flow rates (corresponding to different gas−liquid contact states) treatment. The gas-agitation treated in the ISE did not cause breaking up of the catalyst particles. In the actual separation process, the gas−liquid contact state is permitted to vary based on the amount of residual wax in the extraction cell. For example, the flow rate should be controlled to avoid the dissolution of a large amount of wax at the beginning. As the extraction progresses, the flow rate can be increased to improve the removal of wax from the pores of catalyst particles. However, it should be noted that excessive flow rate would carry away the solvent steam. 3.4. Comparison of the Separation Efficiency Between the Two Methods. 3.4.1. Comparison of Separation Time. The typical feature of Soxhlet extraction separation is that it is time consuming, which is adverse to the rapid and accurate analysis in the plant operation. In this work, experiments were conducted to compare the separation time needed by the two methods. With regard to the model sample (40 g, 7.7% solid content, particle sizes ranging from 10 to 76 μm), the Soxhlet

extraction system needs at least 10 h, while the ISE requires only 2−3 h. For the slurry sample (40 g, about 2.7% solid content, particle sizes were unknown), not less than 72 h are needed to purify the catalyst particles by Soxhlet extraction, while just 4−5 h are enough by ISE. The properties of catalyst wrapped in the hydrocarbon waxes account for the differences in time needed between the model samples and the real slurry samples. Fine particles are generated when the iron catalysts undergo attrition in the working slurry reactor, the surfaces and pore sizes of catalyst are filled with heavy wax, and sometimes the pores of catalyst may be blocked by waxes or coking. Therefore, it is not so easy to remove the wax of real slurry samples with static Soxhlet extraction. In the contrast, the ISE is not affected obviously owing to the use of gas-agitation. Consequently, it is not difficult to explain why the ISE required less time to acquire particles from F-T wax/ catalyst mixtures than did the Soxhlet extraction method. 3.4.2. Comparison of the Effect on the Catalyst Characterization. Slurry samples with different particle sizes and solid contents were used to compare the recovery yield of catalyst and the effect on the PSD of catalyst applying the two methods. Figure 4 presents the percentage of wax extracted by Soxhlet extraction and ISE for different samples. As can be seen, nearly all of the extraction yields by ISE coincide with each other, while the yields obtained by Soxhlet extraction are quite lower, especially for the samples with smaller particles (Figure 4 a) and lower solid contents (Figure 4 b), indicating that the ISE acquired higher recovery yield of catalyst from wax-catalysts samples. To better understand this difference and learn more about the effect of each method on PSD, the individual catalysts recovered were submitted for particle size analysis. Figure 5 shows the relative amount of catalyst within each size range. It is obvious that the Soxhlet extraction and ISE acquire similar volume frequency, which is in good agreement with the PSD of the original catalyst, for the samples with particle size larger than 40 μm (Figure 5a). However, when there are fine particles in the system, the results are quite different between Soxhlet extraction and ISE (Figure 5b). The volume frequency under 40 μm obtained by Soxhlet extraction is less than that obtained by ISE. Notice that the volume frequency between 40 and 200 μm is relatively larger; we suspect that these differences are caused by some fine agglomeration to larger particles because 14513

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Figure 4. Effect of different methods on the percentage of catalyst extracted for different samples.

in the stirred tank slurry reactor (STSR); while the other conclusion may be that the catalyst has undergone severe attrition, and the 60% of the observed increase in fraction of particles smaller than 10 μm is a disadvantage to be used in STSR. In fact, the catalyst indeed suffered severe physical attrition and chemical stress. In other words, the Soxhelt fails to reflect the actual information about the working catalysts, and the inaccurate result may bring about a series of serious consequences. It is well-known that the catalysts for use in FTS need to be resistant to both fracture and abrasion/erosion. The generation of fine particles can result in downstream filter plugging and eventual reactor shutdown.15 Therefor, it is necessary to monitor the attrition behavior of catalysts during the FTS, and the accurate PSDs of working catalyst are required. Note that Bukur et al.3 have stated that they cannot obtain the accurate PSD of catalysts, because the smaller particles are held together by the hydrocarbon wax, which acts like a glue and it is extremely difficult to be removed using traditional separation methods. In our study, this problem is sure to be solved by using ISE, and it is appropriate for rapid analysis in plant operation of FTS.

of the incomplete stripping of wax after the long extraction time by Soxhlet extraction. To further verify the better separation efficiency of ISE, two real F-T wax/catalyst slurry samples, namely, that, TOS = 72 h, 236 h, were employed for the separation. The relative recovery and PSD of catalyst are compared, respectively. Just as expected, each separation technique gets similar results of catalyst recovery when TOS = 72 h (2.59% for Soxhlet extraction, and 2.61% for ISE). But when the TOS = 236 h, the relative recovery of catalyst obtained by ISE is 1.5 times that of Soxhlet extraction. The majority of these catalyst particles withdrawn from TOS = 72h have yet not undergone obvious attrition and are able to retain their original particle size (30−200 μm). At the end of FTS (TOS = 236 h), the catalyst has changed its particle morphology because of the severe physical and chemical attrition, and the number of smaller particles has increased. Figure 6 gives the comparison result of the PSD curve of the sample at TOS = 236 h; Figure 7 presents the specific changes in each size range of this catalyst purified by Soxhlet extraction and ISE. As can be seen from these comparison results, the two methods reflect the bimodal distribution of catalyst at the end of FTS, but the peak value is quite opposite. We can suppose that several classic parameters used to describe the extent of attrition are also quite different. For example, the D(0.5), the particle size corresponding to 50% of the cumulative PSD frequency, is 85 μm for the original catalyst, and 40 μm for the catalyst after being handled by Soxhlet extraction, while it is 5.5 μm for this sample treated by ISE. On the basis of these PSD measurement results, we would deduce contrary conclusions: the extent of this catalyst is moderate, and it is adequate for use

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CONCLUSION

Two separation technologies, Soxhlet extraction and ISE, have been applied to separate catalyst particles from F-T wax/ catalyst solid mixtures. Among the solvents investigated, a solvent primarily composed of xylenes and higher molecular weight aromatic solvents is best for wax extraction, and the application of N2 agitation does not affect the PSD of the catalyst. The recovery yield of wax or catalyst obtained by ISE is greater than those acquired using an aromatic solvent in the 14514

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Figure 5. Effect of different methods on the PSD of catalyst for different samples.

Figure 6. Effect of different methods on the volume frequency of catalysts withdrawn at TOS = 236 h.

Figure 7. Effect of different methods on the relative PSD of catalysts withdrawn at TOS = 236 h.

Soxhlet extraction. Analysis of the catalysts recovered indicates that the ISE can reflect the actual state of catalysts, retain unchanged their catalyst morphology, while the Soxhlet extraction needed more total time and would cause fine particle loss and may mislead one to reach inaccurate conclusions. The ISE is more suitable for the purpose of rapid analysis of the PSD of working catalysts in an FTS running unit.

In conclusion, this study provides an efficient alternative way to replace the Soxhlet extraction technique for the acquisition of fine particles from F-T wax/catalyst solid mixtures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 021-64252274. 14515

dx.doi.org/10.1021/ie301535e | Ind. Eng. Chem. Res. 2012, 51, 14511−14516

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Notes

The authors declare no competing financial interest.

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REFERENCES

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