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Heavy aromatics upgrading using noble metal promoted zeolite catalyst
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
H e a v y aromatics upgrading using noble metal p r o m o t e d zeolite
887
catalyst
S.H. Oh a, S.I. Lee a, K.H. Seonga, Y.S. Kim a, J.H. Lee a, J. Woltermannb, W.E. Cormierb, Y. F. Chub,*
aSK R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-712, South Korea bZeolyst International, PO Box 830, Valley Forge, PA 19482-0830, USA
Heavy C9§ aromatics can be converted to more valuable benzene, toluene and xylene products (BTX) such as in the traditional Tatoray unit. However, existing commercial catalysts not only deactivate rapidly but also are difficult to regenerate. A precious metal promoted zeolite catalyst is developed to maximize the conversion of the heavy aromatics (HA) and to extend the life of the catalyst. The catalyst developed exhibited not only high C9+ conversion and high xylene yield over a wide range of feedstock compositions but also showed a high stability and regenerability for the transalkylation of C9§ aromatics with toluene. The catalyst has been commercialized and has been on stream for two years without yet requiring regeneration. In this study, experimental data obtained in the laboratory and kinetic explanations in accordance with catalytic results are presented. Some commercial data are also provided.
1. INTRODUCTION BTX are valuable petrochemical feedstocks that can be produced from less valuable HA generated in the refinery process. These materials include naphtha reformate and pyrolysis gasoline which have considerable of C9+ aromatics content. These heavy aromatics can be used as a gasoline blending stock but they can be more economically transalkylated with toluene to increase xylene production over zeolite catalysts [1]. Transalkylation of toluene with C9+ aromatics over large pore zeolites with 12-membered rings such as mordenite, beta and Y zeolite have been studied extensively [1-6] under laboratory conditions. Particular attention was given to the transalkylation of toluene with 1,2,4-trimethylbenzene. However, a commercial C9+ feed would typically contain other trimethylbenzene isomers (TMB), methylethylbenzenes (MEB), and propylbenzenes (PB) as well as C10+ aromatics that are very difficult to convert. Hence, the results reported do not completely represent the commercial processes. For an effective catalyst, it is also necessary to examine the maximum C9+ aromatics concentration in feed, feed impurity tolerance level, product purity, yield pattern of mixed xylenes, cycle length and regenerability of the catalyst [7]. In recent years, information on commercial transalkylation catalysts even though limited has become available [7,8] but * To whom all correspondences should be addressed.
888 few catalysts seem to meet all the criteria mentioned above. Stable, regenerable catalysts that achieve higher HA conversion activity are desired. The aim of this work is to develop an efficient transalkylation catalyst that could overcome the drawbacks of the existing catalysts. As mentioned, zeolite catalysts have been shown to be useful for this application. Studies have also shown that the performance of the catalysts are affected by the acidity and pore structure of the zeolite [4]. Large pore zeolites with 12membered rings, such as mordenite, are useful for this reaction due to their larger pore size but unfortunately they also tend to deactivate rather rapidly [1]. As constituted, the strong acidity of these materials will also result in excessive cracking [2]. A bi-functional catalyst with modified zeolite acidity and promoted with a balanced amount of noble metal to obtain high activity, selectivity and stability has been developed for the commercial transalkylation process. In this paper, we will discuss catalytic performance and the distinctive features of the new catalyst and present the commercial experience of SK Corporation in Korea.
2. EXPERIMENTAL The catalyst was manufactured by Zeolyst International based on technology developed by SK Corporation. Catalytic experiments were carried out in a high pressure catalyst testing apparatus under a pressure of 27 kg/cm 2 with WHSV=2.3 ht and H2/HC molar ratio of 3.6 and various feed compositions. All catalysts compositions were pre-reduced with 1-12flow at 400 ~ for 2 h and then cooled down to the reaction temperature of 360 ~ Feed composition was varied with C9§ aromatic contents of 0 wt%, 70 wt% and 100 wt%, balance toluene. The composition of the commercial C9§ feed used consists of TMB, MEB and PB and more than 15% of C10§ The products were analyzed with HP 5890 gas chromatography equipped with FID and the HP-PONA capillary column. All mass balances were within • of closure. The reaction results were obtained at normal operating conditions as described above. Subsequently, the catalyst was subjected to accelerated aging at high severity operating conditions of 420 ~ WHSV=2.3 h1 and H2/HC molar ratio of 1.0 for 40 h under the pressure of 10 kg/cm2. Regeneration experiments were done by calcining the accelerated aged catalyst samples in the laboratory at 450 ~ for 100 h under air and the balance Ar flow to obtain an oxygen concentration of 1-3 mol% at the pressure of 5 kg/cm2. The regenerated catalyst was subjected to a further reduction at 400 ~ for 2 h under H2 flow before testing again.
3. RESULTS AND DISCUSSION 3.1. Discussion of results and proposed reaction network
The results shown in Table 1 demonstrates that the catalyst to be very effective in the transalkylation reaction (TA) of C9§ aromatics. Xylene yield is high, particularly in the case of a toluene to C9§ aromatics weight ratio of 30/70 (case 2). The catalyst also exhibits strong hydrodealkylation activity (HDA). Ethyl and propyl groups are easily dealkylated to provide high BTX yield and a low concentration of EB and PB in the product (cases 1-3). Toluene disproportionation (TDP) has also occurred as evidenced by the high conversion of 100% toluene (case 1). As the concentration of Cg§ increases in the feed (cases 2,3) the TDP reaction is suppressed and hence the toluene conversion and benzene yield decreases
889 Table 1 Test results over the novel metal promoted zeolite catalyst with various feed compositions Reaction conditions Case Feed, wt% Toluene C9+ H A
Yield on feed, wt% Olefins Paraffins Benzene Mixed xylenea Conversion, wt% Toluene
360 ~ 27 kg/cm2, WHSV=2.3 hI, H~-IC=3.6 1
2
3
100 0
30 70
0 100
trace 1.78 21.60 24.32
trace 12.95 7.49 34.06
trace 13.50 3.10 32.97
52.73
3.84 74.20 51.75 97.38 78.45 0.91
66.00 44.80 95.89 57.40 1.30
C9+ H A
Trimethylbenzene Methylethylbenzene Clo+ HA Ethylbenzene/Mixed xylene, wt%
1.97
a Mixed xylene: p-xylene + m-xylene + o-xylene + ethylbenzene
while xylene yield increases. Olefin saturation is evidenced by the trace concentration of olefins in the product. These results can be explained by a complex reaction network as shown in Figure 1. Stoichiometrically, two moles of xylene can be obtained by the transalkylation of one mole of toluene with one mole of TMB. Thus an equal molar ratio of toluene and TMB will be the ideal feed composition for xylene production. In addition to the transalkylation of toluene and TMB, other reactions such as toluene disproportionation and hydrodealkylation of ethylbenzene (EB), MEB and PB and dimethylethylbenzene (DMEB) have taken place to produce the desirable products of BTX. A sequence of reactions may also take place. For instance, MEB dealkylates into toluene and some of the toluene formed could be further reacted to form xylene through the transalkylation with TMB. This could explain the high xylene yield at low apparent toluene conversion for C9+-rich feedstocks (cases 2,3).
890 ITransalkylafiol~
~)isproportionation t
!Hydrodealkylation / 01efin saturatior~
C3H8
I
~r
C_~
IN
M e t ~ C2H6
Figure 1. Proposed reaction network of C9+HAand toluene over newly developed catalyst. 3.2. Comparison to a traditional catalyst
The newly developed catalyst has a much higher C9§ aromatic conversion compared to traditional catalysts. High conversion of C9§ aromatics implies the reduction of these compounds in the recycle. This enables refineries to save utility cost or to treat additional flesh HA. The catalyst also produces mixed xylenes with lower EB content even at 100 wt% HA feedstock. It has been reported in previous work that EB content in mixed xylenes increases as C9+ aromatics in the feedstock increases [2,7]. Low EB content in mixed xylenes (- 2.0%) is desirable since it reduces p-xylene recovery cost in the xylene isomerization loop [9]. High purity benzene (99.85%) can also be obtained in the process. The selectivity of catalyst can be attributed to the well-balanced acid/metal function of the catalyst. Saturation of ethylene or propylene that might be generated from the dealkylation of EB, MEB and PB and DMDB is rapid inhibiting secondary ethylation of benzene to EB and oligomerization of light olefins into coke precursors. Low coke formation ensures longer catalyst cycle length. Hydrogenation of aromatic tings to naphthenes is minimal It is also clear from Table 2 that
891 Table 2 Comparison between traditional catatyst and new catalyst developed ATA-11
Caalyst
Feed, wt% Toluene/C9 HA/Clo + HA Product, wt% Xylene EB C9 HA C10+ HA
Traditional
ATA- 11
66/32/2
20/66/14
28~1 2.8 12_8 4.0
34.8 0.16 22.6 4.1
Reaction conditions: Reactor inlet temperature=330~380 ~ WHSV=I.5-~2.5 hl, H~I-IC=3.0 -~7.0 mole/mole
Pressure=25-~30 kg/cm2,
there is also a net loss of C l o + aromatics in contrast to a traditional catalyst system that would produce a net increase of these compounds.
3.3. Regeneration of catalyst During normal operation, coke will gradually accumulate on the catalyst with increased Table 3 Regenerability of metal promoted zeolite catalyst developed Catalytic Performances
Feed, toluene wt~ + H A wt~ Mixed xylene yield, wt% Toluene conversion, wt% Cg+HA conversion, wt% Ethylbenzene/Mixed xylene, wt%
Fresh"
Before regenerationu
Atter regeneration"
30/70 35,11 8.90 72.52 0.40
30/70 31.66 1.85 72.07 1.26
30/70 34.93 9.10 72.23 0.40
"Reaction conditions: 360 ~ 28kg/cm2, WHSV=2.3 hq, H:/HC=3.6 mole/mole b Obtained atter high severity operation at 420 ~ 101~cm2, WHSV=2.3 h"1, H2/HC=I.0 mole/mole for 40 h
892 time on stream until it will be no longer possible to maintain desired activity. The catalyst is then regenerated. The regeneration of zeolite catalyst is generally carried out by coke combustion under air or oxygen flow [10]. An accelerated aging test was conducted to ascertain the regenerability of the catalyst. The catalyst was deactivated at high severity operating conditions, which are high temperature, low pressure and low H2/HC molar ratio. The catalyst then was regenerated by coke removal at 450 ~ in air for about 100 h. The catalyst has been regenerated successfully in the laboratory as shown in Table 3. Toluene conversion, C9+ aromatics conversion and xylene yield were almost completely recovered to their original levels over the regenerated catalyst_ More than 98% of the catalyst's activity was recovered. This regenerability has not yet been demonstrated commercially since the commercial catalyst has retained activity after more than+two years on stream. 3.4. Commercial experience The catalyst of this study has been successfully commercialized and is marketed under the tradename ATA-11. A transalkylation unit in the SK refinery at Ulsan, Korea was loaded with ATA- 11 and the run started in July 1999. The catalyst remains very active after two years TOS requiring only about half of the amount of catalyst compared to a traditional catalyst. As a result the throughput is very high even exceeding design capacity. The catalyst has been very stable for more than two years of operation even with H2/HC molar ratio as low as 2.8. The stability of the catalyst might be ascribed to the well-balanced acid/metal function of catalyst. High gas make due to overcracking of C9+ aromatics has not been observed. Aromatics ring loss through hydrogenation of BTX and consecutive hydrocraeking is less than 2%. Benzene produced is of sufficient high purity for use as chemical grade without the need of further purification by extraction. Reactor temperature is the prime, variable for control of reaction severity. During the run, an accidental sulfur poisoning of the catalyst occurred that deactivated the catalyst at the end of the year 2000. As a resulL the temperature was increased to compensate for the deactivation. Figure 2 shows the reactor inlet temperature trend during two years of commercial operation. Feed was introduced+ at low temperature to compensate for the reaction
100
500 o
2400
~" 300
~
80
o
60
m
,..
O 41o 9
9
. ,...~
~ 4o
o 200 -
O
. ,...,~ 0
+-. 100 ~
9 ~
+~
r~ 20
-
0
0
Jul-99
Jan-00
Jul-00 Jan-01 Jul-01 Date
Figure 2. Reactor inlet temperature changes during two years of commercial operation.
Jul-99
I
Jan-00
I
I
~
[
Jul-00 Jan-01 Jul-01 Date
Figure 3. Flexible C9 + HA concentration in feedstock during the commercial operation.
893 exotherm. After initial start-up, the temperature was adjusted to maximize xylene yield and benzene purity. On the whole, reactor inlet temperature is low and the temperature increase is also steady. An actual deactivation rate is below 20 ~ Low operating temperature has a beneficial effect on cycle length. The moderate increase of reactor inlet temperature during two years of run demonstrates the stability of this catalyst. The feed composition has deliberately been varied over the course of the commercial run as shown in Figure 3. C9§ aromatics concentration in the feed has been adjusted in response to BTX unit balance or market situation. The catalyst has been shown to be very tolerant to heavy aromatics. The feedstock is flexible for 100% toluene to 100% C9§ HA. The capability to accommodate a C9§ aromatics-rich feed is highly desirable. C9+ aromatics are converted into more valuable BTX. Increasing the HA blending ratio to toluene somewhat enhances xylene yield. However, C9+ aromatics content in the feed influences the stability of zeolite catalyst. HA, particularly C10§ can be trapped in the pore of zeolite catalyst causing catalyst deactivation involving probably relatively bulky biphenylmethane intermediate [11] in a large pore zeolites. The present catalyst is capable of handling as high as 20% concentration of C10+aromatics with proper selection of zeolite pore size and a wen balanced acidity and metal function built in the catalyst. The most desirable product of heavy aromatics up~ading is xylene. The reactor effluent is separated into gas and liquid product through separator and stripper. Figure 4 shows xylene percentage in liquid products treated by the stripper_ Some abrupt changes in the xylene content are due to variation of feedstock compositions. According to in-out mass balance, high xylene yield, which is in the range of 30-36 wt%, is achieved. Transalkylation of C9§ aromatics with toluene and hydrodealkylation of C9§ aromatics give rise to the high xylene yield. Relatively stable xylene production has been maintained throughout the two years of operation. Low EB content in mixed xylene has also resulted in increase of p-xylene productivity at the PX unit. The operating temperature and H2/HC molar ratio are low,
o•
50 45 40
A
It~
9
A
~
35 o
30 25 20 o,..~ 15 = 10 0
~A
t
r
I
I
I
I
Jul-99 Nov-99 Mar-00 Jul-00 Nov-00 Mar-01 Jtfl-01 Date Figure 4. Xylene content in the stripper bottom stream during the commercial operation.
894 resulting in energy savings. The cost savings are estimated to be more than $5MM/yr for the company from this process with a daily throughput of 6000 barrels/day.
4. CONCLUSIONS A commercial transalkylation catalyst has been developed which is capable of processing of Cg+HA at a high throughput, producing excellent xylene yield (30-36%) and very low EB yield. The catalyst exhibits strong ~ansalkylation and hydrodealkylation activity. Acid/metal function of the catalyst is controlled to enhance its activity, selectivity and stability. Well-balanced hydrogenation activity of metal retards the formation of coke via rapid saturation of olefins formed by dealkylation of heavy aromatics. Saturation of aromatics is minimal. The catalyst responds well to changes in feedstock composition ranging from concentrations of 100% toluene to 100% C9§ aromatics. The capability of processing C9§ aromatic-rich feed is a key feature of this catalyst. The catalyst is both stable and regenerable. Regenerability was confirmed by repeated accelerated aging tests in the laboratory. Heavy aromatics upgrading over ATA-11 has been operated commercially for more than two years without regeneration and its performance has exceeded expectations based on pilot tests. The operation has been smooth and high purity BTX has been produced over this catalyst.
REFERENCES
1. Y.K. Lee, S.H. Park and H.K. Rhee, Catal. Today, 44 (1998) 223. 2. J.C. Wu and L.J. Leu, Appl. Catal., 7 (1983) 283. 3. J. Das, Y.S. Bhat and A.B. Halgeri, Catal. Lett., 23 (1994) 161. 4. I. Wang, T.C. Tsai and S.T. Huang, Ind. Eng. Chem. Res., 29 (1990) 2005. 5. K.J. Chao and L.J. Leu, Zeolites, 9 (1989) 193. 6. E. Dumitriu, V. Hulea, S. Kaliaquine and M.M. Huang, Appl. Catal. A, 135 (1996) 57. 7. T.C. Tsai, S.B. Liu and I. Wang, AppL Catal. A, 181 (1999) 355. 8. C.R. Marcilly, Topics in Catal., 13 (2000) 357. 9. A. Smieskova, P. Hudec, M. Paciga andZ~ Zidek, Appk CataL A, 149 (1997) 265. 10. M. Guisnet and P. Magnoux, Catal. Today, 36 (1997) 477. 11. S.M. Csicsery, Zeolites, 4 (1984) 202.
H e a v y aromatics upgrading using noble metal p r o m o t e d zeolite
887
catalyst
S.H. Oh a, S.I. Lee a, K.H. Seonga, Y.S. Kim a, J.H. Lee a, J. Woltermannb, W.E. Cormierb, Y. F. Chub,*
aSK R&D Center, SK Corporation, 140-1 Wonchon-dong, Yusung-gu, Taejon 305-712, South Korea bZeolyst International, PO Box 830, Valley Forge, PA 19482-0830, USA
Heavy C9§ aromatics can be converted to more valuable benzene, toluene and xylene products (BTX) such as in the traditional Tatoray unit. However, existing commercial catalysts not only deactivate rapidly but also are difficult to regenerate. A precious metal promoted zeolite catalyst is developed to maximize the conversion of the heavy aromatics (HA) and to extend the life of the catalyst. The catalyst developed exhibited not only high C9+ conversion and high xylene yield over a wide range of feedstock compositions but also showed a high stability and regenerability for the transalkylation of C9§ aromatics with toluene. The catalyst has been commercialized and has been on stream for two years without yet requiring regeneration. In this study, experimental data obtained in the laboratory and kinetic explanations in accordance with catalytic results are presented. Some commercial data are also provided.
1. INTRODUCTION BTX are valuable petrochemical feedstocks that can be produced from less valuable HA generated in the refinery process. These materials include naphtha reformate and pyrolysis gasoline which have considerable of C9+ aromatics content. These heavy aromatics can be used as a gasoline blending stock but they can be more economically transalkylated with toluene to increase xylene production over zeolite catalysts [1]. Transalkylation of toluene with C9+ aromatics over large pore zeolites with 12-membered rings such as mordenite, beta and Y zeolite have been studied extensively [1-6] under laboratory conditions. Particular attention was given to the transalkylation of toluene with 1,2,4-trimethylbenzene. However, a commercial C9+ feed would typically contain other trimethylbenzene isomers (TMB), methylethylbenzenes (MEB), and propylbenzenes (PB) as well as C10+ aromatics that are very difficult to convert. Hence, the results reported do not completely represent the commercial processes. For an effective catalyst, it is also necessary to examine the maximum C9+ aromatics concentration in feed, feed impurity tolerance level, product purity, yield pattern of mixed xylenes, cycle length and regenerability of the catalyst [7]. In recent years, information on commercial transalkylation catalysts even though limited has become available [7,8] but * To whom all correspondences should be addressed.
888 few catalysts seem to meet all the criteria mentioned above. Stable, regenerable catalysts that achieve higher HA conversion activity are desired. The aim of this work is to develop an efficient transalkylation catalyst that could overcome the drawbacks of the existing catalysts. As mentioned, zeolite catalysts have been shown to be useful for this application. Studies have also shown that the performance of the catalysts are affected by the acidity and pore structure of the zeolite [4]. Large pore zeolites with 12membered rings, such as mordenite, are useful for this reaction due to their larger pore size but unfortunately they also tend to deactivate rather rapidly [1]. As constituted, the strong acidity of these materials will also result in excessive cracking [2]. A bi-functional catalyst with modified zeolite acidity and promoted with a balanced amount of noble metal to obtain high activity, selectivity and stability has been developed for the commercial transalkylation process. In this paper, we will discuss catalytic performance and the distinctive features of the new catalyst and present the commercial experience of SK Corporation in Korea.
2. EXPERIMENTAL The catalyst was manufactured by Zeolyst International based on technology developed by SK Corporation. Catalytic experiments were carried out in a high pressure catalyst testing apparatus under a pressure of 27 kg/cm 2 with WHSV=2.3 ht and H2/HC molar ratio of 3.6 and various feed compositions. All catalysts compositions were pre-reduced with 1-12flow at 400 ~ for 2 h and then cooled down to the reaction temperature of 360 ~ Feed composition was varied with C9§ aromatic contents of 0 wt%, 70 wt% and 100 wt%, balance toluene. The composition of the commercial C9§ feed used consists of TMB, MEB and PB and more than 15% of C10§ The products were analyzed with HP 5890 gas chromatography equipped with FID and the HP-PONA capillary column. All mass balances were within • of closure. The reaction results were obtained at normal operating conditions as described above. Subsequently, the catalyst was subjected to accelerated aging at high severity operating conditions of 420 ~ WHSV=2.3 h1 and H2/HC molar ratio of 1.0 for 40 h under the pressure of 10 kg/cm2. Regeneration experiments were done by calcining the accelerated aged catalyst samples in the laboratory at 450 ~ for 100 h under air and the balance Ar flow to obtain an oxygen concentration of 1-3 mol% at the pressure of 5 kg/cm2. The regenerated catalyst was subjected to a further reduction at 400 ~ for 2 h under H2 flow before testing again.
3. RESULTS AND DISCUSSION 3.1. Discussion of results and proposed reaction network
The results shown in Table 1 demonstrates that the catalyst to be very effective in the transalkylation reaction (TA) of C9§ aromatics. Xylene yield is high, particularly in the case of a toluene to C9§ aromatics weight ratio of 30/70 (case 2). The catalyst also exhibits strong hydrodealkylation activity (HDA). Ethyl and propyl groups are easily dealkylated to provide high BTX yield and a low concentration of EB and PB in the product (cases 1-3). Toluene disproportionation (TDP) has also occurred as evidenced by the high conversion of 100% toluene (case 1). As the concentration of Cg§ increases in the feed (cases 2,3) the TDP reaction is suppressed and hence the toluene conversion and benzene yield decreases
889 Table 1 Test results over the novel metal promoted zeolite catalyst with various feed compositions Reaction conditions Case Feed, wt% Toluene C9+ H A
Yield on feed, wt% Olefins Paraffins Benzene Mixed xylenea Conversion, wt% Toluene
360 ~ 27 kg/cm2, WHSV=2.3 hI, H~-IC=3.6 1
2
3
100 0
30 70
0 100
trace 1.78 21.60 24.32
trace 12.95 7.49 34.06
trace 13.50 3.10 32.97
52.73
3.84 74.20 51.75 97.38 78.45 0.91
66.00 44.80 95.89 57.40 1.30
C9+ H A
Trimethylbenzene Methylethylbenzene Clo+ HA Ethylbenzene/Mixed xylene, wt%
1.97
a Mixed xylene: p-xylene + m-xylene + o-xylene + ethylbenzene
while xylene yield increases. Olefin saturation is evidenced by the trace concentration of olefins in the product. These results can be explained by a complex reaction network as shown in Figure 1. Stoichiometrically, two moles of xylene can be obtained by the transalkylation of one mole of toluene with one mole of TMB. Thus an equal molar ratio of toluene and TMB will be the ideal feed composition for xylene production. In addition to the transalkylation of toluene and TMB, other reactions such as toluene disproportionation and hydrodealkylation of ethylbenzene (EB), MEB and PB and dimethylethylbenzene (DMEB) have taken place to produce the desirable products of BTX. A sequence of reactions may also take place. For instance, MEB dealkylates into toluene and some of the toluene formed could be further reacted to form xylene through the transalkylation with TMB. This could explain the high xylene yield at low apparent toluene conversion for C9+-rich feedstocks (cases 2,3).
890 ITransalkylafiol~
~)isproportionation t
!Hydrodealkylation / 01efin saturatior~
C3H8
I
~r
C_~
IN
M e t ~ C2H6
Figure 1. Proposed reaction network of C9+HAand toluene over newly developed catalyst. 3.2. Comparison to a traditional catalyst
The newly developed catalyst has a much higher C9§ aromatic conversion compared to traditional catalysts. High conversion of C9§ aromatics implies the reduction of these compounds in the recycle. This enables refineries to save utility cost or to treat additional flesh HA. The catalyst also produces mixed xylenes with lower EB content even at 100 wt% HA feedstock. It has been reported in previous work that EB content in mixed xylenes increases as C9+ aromatics in the feedstock increases [2,7]. Low EB content in mixed xylenes (- 2.0%) is desirable since it reduces p-xylene recovery cost in the xylene isomerization loop [9]. High purity benzene (99.85%) can also be obtained in the process. The selectivity of catalyst can be attributed to the well-balanced acid/metal function of the catalyst. Saturation of ethylene or propylene that might be generated from the dealkylation of EB, MEB and PB and DMDB is rapid inhibiting secondary ethylation of benzene to EB and oligomerization of light olefins into coke precursors. Low coke formation ensures longer catalyst cycle length. Hydrogenation of aromatic tings to naphthenes is minimal It is also clear from Table 2 that
891 Table 2 Comparison between traditional catatyst and new catalyst developed ATA-11
Caalyst
Feed, wt% Toluene/C9 HA/Clo + HA Product, wt% Xylene EB C9 HA C10+ HA
Traditional
ATA- 11
66/32/2
20/66/14
28~1 2.8 12_8 4.0
34.8 0.16 22.6 4.1
Reaction conditions: Reactor inlet temperature=330~380 ~ WHSV=I.5-~2.5 hl, H~I-IC=3.0 -~7.0 mole/mole
Pressure=25-~30 kg/cm2,
there is also a net loss of C l o + aromatics in contrast to a traditional catalyst system that would produce a net increase of these compounds.
3.3. Regeneration of catalyst During normal operation, coke will gradually accumulate on the catalyst with increased Table 3 Regenerability of metal promoted zeolite catalyst developed Catalytic Performances
Feed, toluene wt~ + H A wt~ Mixed xylene yield, wt% Toluene conversion, wt% Cg+HA conversion, wt% Ethylbenzene/Mixed xylene, wt%
Fresh"
Before regenerationu
Atter regeneration"
30/70 35,11 8.90 72.52 0.40
30/70 31.66 1.85 72.07 1.26
30/70 34.93 9.10 72.23 0.40
"Reaction conditions: 360 ~ 28kg/cm2, WHSV=2.3 hq, H:/HC=3.6 mole/mole b Obtained atter high severity operation at 420 ~ 101~cm2, WHSV=2.3 h"1, H2/HC=I.0 mole/mole for 40 h
892 time on stream until it will be no longer possible to maintain desired activity. The catalyst is then regenerated. The regeneration of zeolite catalyst is generally carried out by coke combustion under air or oxygen flow [10]. An accelerated aging test was conducted to ascertain the regenerability of the catalyst. The catalyst was deactivated at high severity operating conditions, which are high temperature, low pressure and low H2/HC molar ratio. The catalyst then was regenerated by coke removal at 450 ~ in air for about 100 h. The catalyst has been regenerated successfully in the laboratory as shown in Table 3. Toluene conversion, C9+ aromatics conversion and xylene yield were almost completely recovered to their original levels over the regenerated catalyst_ More than 98% of the catalyst's activity was recovered. This regenerability has not yet been demonstrated commercially since the commercial catalyst has retained activity after more than+two years on stream. 3.4. Commercial experience The catalyst of this study has been successfully commercialized and is marketed under the tradename ATA-11. A transalkylation unit in the SK refinery at Ulsan, Korea was loaded with ATA- 11 and the run started in July 1999. The catalyst remains very active after two years TOS requiring only about half of the amount of catalyst compared to a traditional catalyst. As a result the throughput is very high even exceeding design capacity. The catalyst has been very stable for more than two years of operation even with H2/HC molar ratio as low as 2.8. The stability of the catalyst might be ascribed to the well-balanced acid/metal function of catalyst. High gas make due to overcracking of C9+ aromatics has not been observed. Aromatics ring loss through hydrogenation of BTX and consecutive hydrocraeking is less than 2%. Benzene produced is of sufficient high purity for use as chemical grade without the need of further purification by extraction. Reactor temperature is the prime, variable for control of reaction severity. During the run, an accidental sulfur poisoning of the catalyst occurred that deactivated the catalyst at the end of the year 2000. As a resulL the temperature was increased to compensate for the deactivation. Figure 2 shows the reactor inlet temperature trend during two years of commercial operation. Feed was introduced+ at low temperature to compensate for the reaction
100
500 o
2400
~" 300
~
80
o
60
m
,..
O 41o 9
9
. ,...~
~ 4o
o 200 -
O
. ,...,~ 0
+-. 100 ~
9 ~
+~
r~ 20
-
0
0
Jul-99
Jan-00
Jul-00 Jan-01 Jul-01 Date
Figure 2. Reactor inlet temperature changes during two years of commercial operation.
Jul-99
I
Jan-00
I
I
~
[
Jul-00 Jan-01 Jul-01 Date
Figure 3. Flexible C9 + HA concentration in feedstock during the commercial operation.
893 exotherm. After initial start-up, the temperature was adjusted to maximize xylene yield and benzene purity. On the whole, reactor inlet temperature is low and the temperature increase is also steady. An actual deactivation rate is below 20 ~ Low operating temperature has a beneficial effect on cycle length. The moderate increase of reactor inlet temperature during two years of run demonstrates the stability of this catalyst. The feed composition has deliberately been varied over the course of the commercial run as shown in Figure 3. C9§ aromatics concentration in the feed has been adjusted in response to BTX unit balance or market situation. The catalyst has been shown to be very tolerant to heavy aromatics. The feedstock is flexible for 100% toluene to 100% C9§ HA. The capability to accommodate a C9§ aromatics-rich feed is highly desirable. C9+ aromatics are converted into more valuable BTX. Increasing the HA blending ratio to toluene somewhat enhances xylene yield. However, C9+ aromatics content in the feed influences the stability of zeolite catalyst. HA, particularly C10§ can be trapped in the pore of zeolite catalyst causing catalyst deactivation involving probably relatively bulky biphenylmethane intermediate [11] in a large pore zeolites. The present catalyst is capable of handling as high as 20% concentration of C10+aromatics with proper selection of zeolite pore size and a wen balanced acidity and metal function built in the catalyst. The most desirable product of heavy aromatics up~ading is xylene. The reactor effluent is separated into gas and liquid product through separator and stripper. Figure 4 shows xylene percentage in liquid products treated by the stripper_ Some abrupt changes in the xylene content are due to variation of feedstock compositions. According to in-out mass balance, high xylene yield, which is in the range of 30-36 wt%, is achieved. Transalkylation of C9§ aromatics with toluene and hydrodealkylation of C9§ aromatics give rise to the high xylene yield. Relatively stable xylene production has been maintained throughout the two years of operation. Low EB content in mixed xylene has also resulted in increase of p-xylene productivity at the PX unit. The operating temperature and H2/HC molar ratio are low,
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Jul-99 Nov-99 Mar-00 Jul-00 Nov-00 Mar-01 Jtfl-01 Date Figure 4. Xylene content in the stripper bottom stream during the commercial operation.
894 resulting in energy savings. The cost savings are estimated to be more than $5MM/yr for the company from this process with a daily throughput of 6000 barrels/day.
4. CONCLUSIONS A commercial transalkylation catalyst has been developed which is capable of processing of Cg+HA at a high throughput, producing excellent xylene yield (30-36%) and very low EB yield. The catalyst exhibits strong ~ansalkylation and hydrodealkylation activity. Acid/metal function of the catalyst is controlled to enhance its activity, selectivity and stability. Well-balanced hydrogenation activity of metal retards the formation of coke via rapid saturation of olefins formed by dealkylation of heavy aromatics. Saturation of aromatics is minimal. The catalyst responds well to changes in feedstock composition ranging from concentrations of 100% toluene to 100% C9§ aromatics. The capability of processing C9§ aromatic-rich feed is a key feature of this catalyst. The catalyst is both stable and regenerable. Regenerability was confirmed by repeated accelerated aging tests in the laboratory. Heavy aromatics upgrading over ATA-11 has been operated commercially for more than two years without regeneration and its performance has exceeded expectations based on pilot tests. The operation has been smooth and high purity BTX has been produced over this catalyst.
REFERENCES
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