ARTICLE pubs.acs.org/IECR
Removal of Trace Olefins from Aromatics over Metal-Halides-Modified Clay and Its Industrial Test Guo-liang Li, Jin-ning Luan, Xian-song zeng, 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: Commercial clay modified with metal halides was prepared to remove olefins from aromatics in the laboratory, and its industrial test was conducted. Different amounts of metal-halides-modified clay were made and characterized by XRD and pyridine adsorption followed by FT-IR. The type and nature of active sites in the removal of olefins over the prepared catalysts have been examined. The study of acidic properties of the modified clay revealed that the L acid, especially the weak L acid plays an important role in the reaction. The experimental results revealed that commercial clay modified with metal halides had much better catalytic activity. During Sinopec Qilu company industrial test, modified clay had an effective running time six times as long as that of untreated commercial clay.
1. INTRODUCTION Benzene, toluene, and xylene (BTX), which are called primary organic raw materials, play an important role in the chemical industry. In petroleum processing, aromatic streams can be obtained from reforming and cracking processes. These aromatic streams also contain undesirable olefin impurities which are very deleterious to the followed technological processes and applications of aromatics. To meet aromatic purity standards, trace quantities of olefins must be removed before aromatic streams are sent to the petrochemical processes.1 Two processes have been discovered for removing trace olefins from aromatics which are (1) clay treating and (2) catalytic hydrogenation treating. In clay treating, the acidic nature of the clay causes the olefins to react with the aromatics present via an alkylation reaction.2 Catalytic hydrogenation treating saturates the olefins via hydrogenation action on selective hydrogenation catalysts. Clay treating was first used in removal of trace olefins from aromatics and is the predominant treatment method used in aromatics refining currently. A 130kt/a PX device (Sinopec Qilu Company) was employed, using the Parex unit of UOP company. Simulated moving bed technology was used to separate and produce PX with a highly selective adsorbent ADS-7. The process stream contained olefins in an amount from about 300 to about 700 as measured by bromine index (BI). To protect the adsorbent, clay treating was used to remove olefinic contaminants before the aromatics hydrocarbons enter into the adsorption tower. The product stream will have a bromine index less than 20. Since commercial clay has very limited lifetime in aromatics treatment service, some type of effective catalyst that has longer lifetime must be applied in this field to protect the environment and improve economic efficiency.3 Since clay treatment mainly depends on clay adsorption and alkylation reaction, regulating the acidity of the clay surface may increase its ability to remove olefin. The effect of catalysts supported with different Lewis acids in a FriedelCrafts alkylation has been studied.4 Researchers in our group studied metal r 2011 American Chemical Society
halides (AlCl3, CeCl3, ZnCl2, etc.) modified clay as a catalyst, applied in removal of trace olefins in aromatics, and conducted an industrial test at Sinopec Qilu, Co.
2. EXPERIMENTAL SECTION 2.1. Materials. Commercial clay used in Sinopec Qilu company, which was taken from Inner Mongolia, was used as the support in this study. The metal halides (AlCl3, CeCl3, ZnCl2) were taken from Sinopharm Chemical Reagent Co., Ltd. Just like in the industrial test, the aromatic hydrocarbons used in the laboratory were obtained from the bottom of the naphtha reforming column at Sinopec Qilu company. The BI is about 300700, and the components are shown in Table 1. 2.2. Catalysts Preparation. The metal halides solution was prepared by mixing deionized water with AlCl3, CeCl3, and ZnCl2 at a certain proportion. The acidic clay was mixed with different amounts of metal halides solution, kneaded with some additive and the proper amount of water, and then squeezed into the form of strips to make a series of modified clay. The catalysts were removed with moisture in an oven, in an air atmosphere at 393 K for 6 h, and finally activated at 423 K for 1 h. After being cooled, the catalysts were crushed and screened to 2040 mesh for use. 2.3. Catalytic Tests in Laboratory. The catalytic activity tests were carried out in a fixed-bed tubular microreactor, equipped with flow controllers and a heating system. Two milliliters of the synthesized catalyst was placed between two quartz sand (4060 mesh) packed areas and inserted into the reactor. The reaction was carried out under the following conditions: reaction temperature, 448 K; reaction pressure, 1 MPa; and weight hourly space velocity (WHSV), 30 h1. Inlet and effluent liquids to and Received: January 22, 2011 Accepted: May 3, 2011 Revised: April 21, 2011 Published: May 03, 2011 6646
dx.doi.org/10.1021/ie2001567 | Ind. Eng. Chem. Res. 2011, 50, 6646–6649
Industrial & Engineering Chemistry Research from the reactor were analyzed using a bromine index analyzer. The conversion of olefins was calculated according to the following equation: ðXÞ ¼ ½ðn0 ni Þ=n0 100 where n0 is initial content of olefins and ni is the content of olefins in sample i (i = 1, 2, 3, ...). Bromine Index. The bromine index (BI) is an indicator of the olefins content in an aromatics stream. According to ASTM Standard D 2710-92, the bromine index is defined as the number of milligrams of bromine consumed by 100 g of hydrocarbon sample. 2.4. Characterization. The commercial clay and the best modified clay were characterized by X-ray powder diffraction (XRD) on a Rigaku-3014 diffractometer with a monochromator using Cu KR (λ = 0.154 nm) radiation. The diffractograms were recorded in the 2θ range 1080° in steps of 0.04° with a scan rate of 2° per min. The surface acidity was studied via Fourier transform infrared (FT-IR) spectroscopy, with pyridine adsorbed on the solid surface as the probe molecule. About 16.5 mg of the sample was pressed into a self-supported wafer. Prior to pyridine adsorption, the sample wafer was evacuated at 753 K under high vacuum, followed by pyridine adsorption at room temperature. Subsequently, the spectra were recorded after evacuation of the
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sample at 473 K and 723 K. All the FT-IR spectra were measured using a FT-IR6700 spectrometer (Nicolet Company) in the range 4000400 cm1. 2.5. Industrial Test: Process Flow and Catalysts Loading. Figure 1 shows the flow scheme of industrial test at Sinopec Qilu company. A big clay tower and two small ones are used to remove olefins in the PX device. The industrial test was conducted in one of the small clay towers. Six tons of commercial clay (original use) were loaded in Tower B, and six tons of modified commercial clay (for test) were loaded in tower C. Aromatics stream enter into the reactors from the top and flow out from the bottom of reactors after being treated in the reactor. The reactor operating temperature was about 433443 K, controlled by regulating the steam flow. The reaction pressure was 1.0 MPa, and the volume space velocity was about 2.1 h1. The samples were collected from the bottom of the reactor.
3. RESULTS AND DISCUSSION 3.1. Effect of Metal Halides Proportion. First, we compared the catalytic activity of commercial clay modified with different
Table 1. Aromatic Hydrocarbon Components component
content W (%)
nonaromatics
2.5
benzene
0.06
toluene ethylbenzene
0.09 10.2
p-xylene
9.41
m-xylene o-xylene
20.98 13.09
C9 and C9þ
43.65
Figure 2. Packing structure of the clay tower.
Figure 1. Flow scheme of the Sinopec Qilu company industrial test process Figure 2 shows the packing structure in the clay tower. Catalysts were loaded in the center of the fixed-bed reactor, between two quartz sand packed areas. The upper and lower parts of the tower were loaded with Φ19 ceramic balls. The internal reactor was purged by nitrogen before loading. 6647
dx.doi.org/10.1021/ie2001567 |Ind. Eng. Chem. Res. 2011, 50, 6646–6649
Industrial & Engineering Chemistry Research contents of metal halides (MH). The commercial clay alone was used as a reference. The modified clay with different amounts of
Figure 3. Effect of commercial clay modified with different amounts of metal halides.
Figure 4. XRD characterization of commercial clay and modified commercial clay.
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metal halides are designated as clayþX%MH (X = 1, 4, 7, 10). As shown in Figure 3, the olefin conversions of clay modified with 1%, 4%, 7%, and 10% metal halides are higher than those of commercial clay alone. Furthermore, the activity of catalyst increases as the amount of metal halides increases. The olefin conversions of 7% and 10% metal halides modified clay remained above 50% for 8 h, whereas commercial clay was only 3 h. For all cases, the best catalytic activity was observed with the 7% metal halides containing catalyst. Considering the performance and cost of the catalyst, we took 7% metal halides supported commercial clay as the catalyst using in the industrial test. 3.2. XRD Analyses of Two Samples. Figure 4 shows the XRD diffraction patterns obtained for commercial raw clay and also for the commercial clay after the modification procedure. Both diffractograms are very similar, which suggest that the modification procedure did not change the structure of the commercial clay. There is no characteristic peak of metal halides in the diffractogram of modified Commercial clay, because it is present in a small quantity and highly dispersed on the large surface areas.5 3.3. Pyridine FT-IR Spectra of Five Samples. Figure 5 represents the IR spectra for pyridine adsorption of five samples at 473 and 723 K, and the corresponding calculated concentration of acid sites is summarized in Table 2. The spectra displayed many bands during 1400 to 1600 cm1, which is attributed to the interaction of pyridine with Lewis (L) and Brønsted (B) acid sites on the sample surfaces.6 We mainly observed the band at around 1450 cm1, arising due to the 19b ν(CC) vibration of pyridine adsorbed at the Lewis acid sites. Another band in this spectrum at 1490 cm1 arose due to contributions of both the Lewis and Brønsted acid sites.7 There is no band observed at 1540 cm1, so the amount of B acid could not be calculated. As shown in Table 2, the amount of L acid sites in commercial clay was very little. The clay mainly provided mesopores, in which metal halides largely reside, while the metal halides loaded on clay provided most of the active sites. When the metal halides on the commercial clay support increased from 1% to 10%, there was an increase in the total L acid and the weak L acid. However, while increasing the amount of metal halides from 7% to 10%, the amount of L acid increased slowly. It was in accordance with the catalytic activity data. While the number of weak L acid sites increased fast, there’s little change with the strong L acid. From
Figure 5. IR spectra of different amount of metal halides loading catalysts after pyridine desorption at 473 and 723 K. 6648
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Industrial & Engineering Chemistry Research
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from aromatics. Catalyst activity increased with an increase in the metal halides amount; the optimum catalyst is obtained for catalysts which are loading 7 wt % metal halides. (2) The XRD results suggested that the modification of metal halides did not change the chemical and crystalline structures of commercial clay. (3) The FT-IR analyses revealed that the L acid of the catalysts, especially with the weak L acid, played an important role in removal of trace olefins from aromatics. (4) During the industrial test, the effective reaction time of modified commercial clay lasts about 22 days, 6 times as long as that of untreated commercial clay. Owing to the modified clay’s potential contribution to environment protection and economic efficiency, we expect a large scare application of modified clay.
Table 2. Acidic Properties of Different Metal Halides Loading Catalysts acidity ( 104 mol 3 g1) sample
total L acid sites
strong L acid sites
weak L acid sites
commercial clay
0.34316
0.11936
0.2238
clayþ1%MH
0.67886
0.2984
0.38046
clayþ4%MH clayþ7%MH
1.28312 3.11455
0.39165 0.1492
0.89147 2.96535
clayþ10%MH
3.42414
0.11563
3.30851
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel. 021-64252274.
’ REFERENCES
Figure 6. Bromine index of aromatics after the reaction over commercial clay and modified clay at the Sinopec Qilu industrial test.
the catalytic activity data, we could see that the increase in the conversion of the olefins was apparently due to increase of the total L acid sites, in particular with the weak L acid sites. Therefore, modification of metal halides mainly increased the amount of weak L acid, thus improving the capability of removing olefins from aromatics. 3.4. Industrial Test Analyses. Figure 6 shows the bromine index of aromatics after reaction over commercial clay and modified Commercial clay at an industrial test. The reaction was carried out under the following conditions: reaction temperature, 433443 K; reaction pressure, 1 MPa; and volume space velocity, 2.1 h1. To satisfy the production demand, the olefin-free product stream should have a bromine index less than 20 mg/100 g. The bromine index of aromatics with commercial clay remained less than 20 for less than 4 days. As for the modified commercial clay, the effective reaction time of modified commercial clay lasts about 22 days, 16 days of which bromine index of aromatic streams stayed less than 5 mg/100 g. While greatly increasing the lifetime of catalyst, treating of such process stream also does not significantly change the quantity or distribution of the aromatic hydrocarbons treated. Thus, modified commercial clay is an effective catalyst in removal of trace olefins from aromatics. By reducing the frequency of clay replacement and landfill quantity, modified clay will significantly reduce production cost and protect the environment.
(1) Stephen, H. B.; Terry, E. H.; Arthur, P. W. Decreasing BIreactive contaminants. U.S. Patent 6,368,496 B1, 2002. (2) Sachtler, J. W. A.; Barger, P. T. Removal of trace olefins from aromatic hydrocarbons. U.S. Patent 4,795,550, 1989. (3) Chen, C. W.; Wu, W. J.; Zeng, X. S.; Jiang, Z. H.; Shi, L. Study on several mesoporous materials catalysts applied to the removal of trace olefins from aromatic and commercial sidestream tests. Ind. Eng. Chem. Res. 2009, 48, 10356–10363. (4) Saïd, S.; Rachid, T.; Rachid, N.; Saïd, B. Comparison of different Lewis acids supported on hydroxyapatite as new catalysts of Friedel Crafts alkylation. Appl. Catal. A 2001, 218, 25–30. (5) Kasztelan, S.; Moffat, J. B. The oxidation of methane on heteropolyoxometalates. III. Effect of the addition of cesium on silicasupported 12-molybdo- phosphoric acid, molybdena, vanadia, and iron oxide. J. Catal. 1988, 112 (1), 54–65. (6) Hoang, V. T.; Qing, l. H.; Adrian, U.; Mladen, E.; Do, T. On.; Serge, K. Effect of the acid properties on the diffusion of C7 hydrocarbons in UL-ZSM-5 materials. Microporous Mesoporous Mater. 2006, 92, 117–128. (7) Pranjal, Kalita; Narendra, M. Gupta; Rajiv, Kumar. Synergistic role of acid sites in the Ce-enhanced activity of mesoporous CeAl-MCM-41 catalysts in alkylation reactions: FTIR and TPD-ammonia studies. J. Catal. 2007, 245, 338–347.
4. CONCLUSIONS (1) Modification of metal halides on commercial clay provokes a considerable effect in the removal of trace olefins 6649
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