Ind. Eng. Chem. Process Des. Dev. 1906, 25, 151-155
M2 Forming-A
151
Process for Aromatization of Light Hydrocarbons
Nai Y. Chen' and Tsoung Y. Yan Mobil Research and Development Corporation, Princeton, New Jersey 08540
Effective aromatization processes for converting light hydrocarbons to BTX aromatics could improve the availability of aromatics as chemicals and high octane gasoline-blending components. To this end, M2 forming offers a potential new route. The process can convert olefins and paraffins to aromatics. The aromatic yields are only limited by the stoichiometric constraint imposed by the hydrogen content of the feed and the products. The heart of this process is the HZSM-5-type catalyst. The process operates in the cyclic mode at about 538 "C and 1-20 atm. The reactions involved in the M2 forming process are complex, consecutive, acid-catalyzed reactions, including (1) conversion of olefinic and paraffinic molecules to small olefins via acidic cracking and hydrogen-transfer reactlons, (2) formation of C2-C,o olefins via transmutation, oligomerization, cracking, and isomerization reactions, and (3) aromatic formation via cyclization and hydrogen transfer.
As high octane blending components, BTX aromatic hydrocarbons, namely, benzene, toluene, and xylenes, make up about 21% of the gasoline pool. In 1980,5,29, and 36 million metric tons per year of BTX, respectively, were consumed as gasoline in the US. Aromatic hydrocarbons from petroleum are also an important source of petrochemicals, even though the volume of BTX for chemical use is relatively small compared to their use in gasolines. Most petroleum-derived aromatics are obtained by catalytic reforming of naphthas. In the catalytic reforming process, low octane naphthas, boiling between about 65 and 200 "C, are converted to high octane gasolines containing a high concentration of aromatics. The important reactions which lead to the production of aromatics in catalytic reforming are (1)dehydrogenation of six-membered ring naphthenes, (2) dehydroisomerization of five-membered ring naphthenes, and (3) dehydrocyclization of paraffins. However, present day catalytic reforming processes are incapable of converting light hydrocarbons with carbon numbers of five or less to aromatics. Effective processes to convert the light hydrocarbons to aromatics would improve the availability of high octane gasoline in the future. The synthesis of aromatics from light hydrocarbons was reviewed by Bragin (1981). Russian workers reported in 1946 that light olefins, such as n-butene, can be catalytically oligomerized, cyclized, and dehydrogenated to aromatics. The polymerization of isobutene in the presence of an alumina/molybdenum oxide catalyst was investigated, and mixed xylenes were detected in the reaction products and isolated by Rapoport et al. (1957). The aromatization of C3-C5alkanes in the presence of a number of metal oxide catalysts was reported by Csicsery (1970, 1979). The formation of aromatics from hexane over HZSM-5 was reported by Wang et al. (1979). More recently, UOP and BP jointly announced the development of the Cyclar process, a dehydrocyclodimerization process converting LPG to aromatics (Johnson et al., 1984). Studies directed toward the development of a new aromatization process based on the ZSM-5 catalyst are presented in this paper.
Experimental Section Catalyst. The catalyst used in this study was a ZSM-5 with an SiOz/A1,O3ratio of 70 prepared according to the method of Argauer and Landolt (1972). The zeolite was calcined to decompose occluded organics and subsequently ammonium ion exchanged. Prior to use, the catalyst was 0196-4305/86/1125-015 1$01.50/0
heated from room temperature to 538 "C over an 8-h period and held at 538 "C for 10 h. Feeds. Feedstocks used in this study included propene, n-pentane, n-hexane, and a number of commercial streams, viz., naphthas and light FCC gasolines. The pure hydrocarbons used were the highest purity grade available. They were used without further purification. Experimental Procedure. The aromatization reactions were studied in a microreactor system containing about 2 cm3of 1/16-in.extrudate broken to lengths of about 2-3 mm. The reaction conditions, unless otherwise stated, were 425-575 "C, 1-75 LHSV, and atmospheric pressure. The reaction products were analyzed by gas chromatography for conversion and product yields.
Results and Discussions Efficacy of HZSM-5Catalyst for Aromatization. ZSM-5 converts light paraffin, olefins, and naphthenes to aromatics and light gases. As shown by the data in Tables I and 11, at 538-575 OC and 1WHSV, the aromatics yield from n-pentane and n-hexane is more than 30% by weight; similar yield is obtained from the more reactive propene at 68 LHSV. A virgin naphtha which contains 47% paraffins, 41% naphthenes, and 12% aromatics gives 44% aromatics or a net gain of 32 % , and a light FCC gasoline which contains 41% olefins and 10% aromatics gives 54% aromatics or a net gain of more than 44% aromatics. By varying the reaction temperature, the product yield pattern for the conversion of n-hexane at a constant space velocity can be changed. The data, as shown in Figure 1, clearly indicate that the initial reaction products are that of cracking to mostly C3and C4fragments. The aromatics start to form as the temperature and the severity are increased. This figure also clearly shows that under the reaction conditions used, all the nonaromatic cracked products are aromatizable. Only methane and ethane are not aromatized, and their concentrations are continuously increased as the reaction severity is increased. If these aromatizables were reacted to extinction, they would give an ultimate aromatics yield or an extinction yield which can be estimated from the product distribution. The extinction yield is defined as the sum of the observed aromatics yield and the estimated aromatics yield from the aromatizables. For the purpose of estimating this extinction yield from the observed yield, a value of 50% 0 1985 American Chemical Society
152
Ind. Eng. Chern. Process Des. Dev., Vol. 25, No. 1, 1986
Table I. Aromatization of Light Hydrocarbons" reaction
c%
feed n-pentane n-hexane propene
temp, "C 575 538 538
83.3 83.72 85.7
WHSV 1.0 1.0 68
obsd 31 32 33
aromatics yield, % extinction 45 45 58
"Catalyst, ZSM-5; pressure, atmospheric. FEED. CATALYST. PRESSURE : LHSV:
Table 11. Aromatization of FCC Gasoline and Naphtha feed boiling range, "C initial end elemental analysis C
H type analysis, % paraffin olefin naphthene aromatics reaction conditions temp, "C WHSV, wjw h C5+ product, wt % on feed total yield aromatics yield aromatics in C,+, wt %
light FCC gasoline
virgin naphtha
c5
C6
110
110
85.4 14.6
84.8 15.2
34 41 15 10
47
56 54
i
20k
i
41 12
390 1.2
550 1.3
550
53.2 43.6 82.0
54.3 54.0 99.4
47.3 43.8 92.6
1.3
aromatics yield was assigned to Czf olefins and C4+paraffiis, and a value of 30% was assigned to propane. These are conservative values derived from stoichiometric considerations, assuming that the aromatic products have an average molecular formula of C7H8and the nonaromatizables have an average molecular formula of CH,,,. The results are also shown in Table 11. These yields compare favorably with that reported by Bragin et al. (1977) and Csicsery (1970) and are comparable to that reported by Johnson et al. (1984). The detailed product distribution from the conversion of a light FCC gasoline as a function of LHSV is shown in Table 111. As the space velocity was decreased to 2.5, the aromatics yield increased to 60.7 wt %. If the severity was further increased, more aromatics could have been made from the aromatizable nonaromatics, including ethene and higher olefins and propane and higher paraffins. Figure 2 shows that the concentration of these lower molecular weight aromatizables increases first at low severities and decreases to lower levels at higher severities. The other products are nonaromatizables, HZ,CH4, and C2&. These nonaromatizables all increase with decreasing space velocities. Depending on the reaction conditions, the composition of the nonaromatizables may vary, and its composition, together with the composition of the
925 450 475
500 525 550 575
10.8 2.0 8.2 1.5 9.5 1.5 4.3 60.7 14.5 30.5 12.1 3.6
4.3 6.5 7.2 7.6 11.7 6.9 3.2 51.1 12.5 25.1 10.8 2.7
5.3 3.8 8.6 3.7 17.5 5.0 5.0 49.7 11.4 23.3 10.6 4.4
TEMPERATURE, t
TEMPERATURE, O C
Figure 1. Effect of temperature on product distribution: feed, n-hexane; catalyst, HZSM-5; pressure, atmospheric; LHSV, 1.0.
6o
8
t
yo
301/
A\/o-AROMATIZABLES
20
0-
FEED
0.1
I
I
0.2
03
04
IVLHSV, HR-I
Figure 2. Product distributions vs. l/LHSV.
Table 111. Effect of Soace Velocity on Product Distribution-Light FCC Gasoline wt 9" LHSV 2.5 5.0 6.25 12.5 1.9 1.4 1.9 1.9 H? methane ethece ethane propene propane butanes pentanes total aromatics benzene toluene xylenes Cy+ aromatics
n-HEXANE HZSM-5 ATKSPHERIC 1.0
3.0 8.1 4.9 10.2 11.6 7.7 3.8 48.7 8.4 22.3 13.2 4.8
25.0 1.3 1.7 8.6 3.5 13.5 11.3 13.3 7.8 39.4 5.9 17.9 11.8 3.8
37.5 1.0 1.4 10.0 2.3 15.7 10.4 14.8 10.4 34.3 5.5 15.3 10.3 3.2
75.0 0.5 0.8 9.5 1.7 19.2 8.8 14.4 13.5 26.9 3.1 14.2 6.5 3.1
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
aromatics, determines the extinction yield of aromatics from a given feed. Reaction Mechanism. The data presented in Figures 1and 2 clearly indicate that the feed rapidly converted and reached high conversion at relatively mild conditions. The aromatizable o l e f i c and paraffmic intermediates, C2=-C5, reach a maximum and then decreases slowly. Meanwhile, the aromatics and nonaromatizable (Hz, CH4, and C2H6) yields start to increase significantly only after the feed was nearly exhausted. Figure 2 shows that the concentration of the aromatizable intermediates reached a maximum of 5070, suggesting that the first step, Le., the cracking of hexane to the intermediates, is very much faster than the last step, i.e., the aromatization reaction. The first-order rate constant for the cracking of olefins was found to be over 200 times higher than that for paraffins (Haag et al., 1982). The rate constant for paraffins is, in turn, 3-4 times higher than the aromatization rate, which is the rate-controlling step. These data clearly indicate that the aromatics are formed in a typical consecutive reaction scheme, paraffin small olefins via cracking and hydrogen transfer C2-CI0 olefins via transmutation/oligomerization (Chen et al., 1979)/cracking and isomerization aromatics via cyclization and hydrogen transfer, all acid-catalyzed reactions (Haag et al., 1982). It is noted that all the reaction steps are catalyzed by an acid catalyst, and a monofunctional acidic catalyst will be sufficient to catalyze the full reaction scheme. The reaction pathway can be represented with the scheme
-
-
-
N
t F
h ---!-0 --!!..-
A t N
t P
Where F is the feed, 0 and P are aromatizable low molecular weight olefins and paraffins, i.e., ethene and heavier olefins and propane and heavier paraffins, respectively, and A and N are aromatics and nonaromatizables, respectively. The nonaromatizables are H2, CHI, and C2H6. The k's are the corresponding rate constants. The relative rates k 1 / k 2vary significantly depending on the reaction conditions and the nature of the feed, in particular. For paraffinic feed, the olefiic intermediates are mainly derived from the catalytic cracking step. With feedstocks, containing olefins, diolefins, naphthenes, and hydroaromatic compounds, and at relatively low temperatures, Haag and Mitchell (1984) found that without cracking, hydrogen-transfer reactions may provide an alternate route to aromatics and saturates. At higher temperatures, however, catalytic cracking again dominates the first reaction step. A similar consecutive mechanism was proposed by Csicsery (1970) for the dehydrocyclodimerizationof C3-C5 paraffins over a dual-functional catalyst (hydrogenation/acid) rather than the monofunctional acidic catalyst used in this study. The difference in the reaction mechanism between these two types of catalysts is manifested in the difference in the effect of feed composition on reactivity and product yield. The first step for the dual-functional catalyst is the dehydrogenation of the alkanes over Pt to generate the olefins. The rapid establishment of the paraffin/olefin equilibrium over the Pt/AI2O3 catalyst erases any major difference between an olefinic feed and a paraffinic feed.
153
n-HEXANE
Figure 3. Reaction pathways.
The ZSM-5 catalyst has been shown to be an excellent acidic catalyst for olefin oligomerization (Chen et al., 1979; Garwood, 1982, 1983). Along with oligomerization, the olefins disproportionate rapidly and redistribute to an equilibrium composition. Since these reactions involving the olefins are so much faster than the aromatization reaction, it is not surprising to find that a similar aromatics composition is obtained irrespective of the feed composition (Haag et al., 1982). In contrast to the HZSM-5 catalyst, the Pt/A1203catalyst lacks a strong and stable acid function for the olefin transmutation and hydrogen-transfer reactions; the observed reactions appear to proceed by olefin dimerization followed by the dehydrocyclization of the dimer. As a result, the aromatics formed depend mostly on the feed composition and are predictable. It is noted that at the high temperature and the low operating pressure required for the aromatization step, the Pt/A1203catalyst deactivated rapidly. As a result, the overall reaction cannot be sustained for an extended period of time. The aromatization of light hydrocarbons over HZSM-5 differs from the classical paraffin dehydrocyclization over such catalysts as Pt/A1203, Crz03/A1203,CoMo/A1203,and Te/NaY (Lang et al., 1971). A comparison of the reaction pathways of hexane aromatization over these catalysts is shown in Figure 3. The three apexes represent n-hexane (the feed), C1-Cs nonaromatics, and aromatics (the final product). The reaction pathway for the dehydrocyclization catalyst is nearly a direct path from the feed to the aromatics. On the other hand, the reaction pathway for HZSM-5 is curved to the right toward the cracked intermediates and then to the final product of aromatics. Major differences in the distribution of aromatics made over these two types of catalysts are evident from the data obtained with n-hexane as the feed, as shown in Figure 4. Dehydrocyclization yielded almost excluisvely benzene, while aromatization over HZSM-5 at 538 OC, yielded a spectrum of C6-clO aromatics. Because of its difference from conventional dehydrocyclization, the aromatization of light hydrocarbons over the ZSM-5-type catalysts has been named "M2 forming". Compared with other zeolites, the HZSM-5 catalyst is unique in its ability to catalyze the aromatization reaction with sustained activity. Its stability is attributed to the shape and size of the pore openings and the tortuosity of the channels which inhibit the formation of coke precursors
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986
154
-
0 EHYWOCYCUZATION
z
d
L
60
a
E
I
l
2o 0
a
t 6
7
S
9
IO
AROMATIC CARBON NUMBER
Figure 4. Aromatics distribution for n-hexane conversion.
(Chen and Garwood, 1978; Walsh and Rollmann, 1979). Stoichiometric Limitation and Extinction Yield. The desired products for M2 forming are the BTX aromatics with an average hydrogen content of about 8.7 wt % . The aromatizables are much higher in hydrogen content, ranging up to 18.3 wt 90for propane. The excess hydrogen in the reactant molecules must be rejected in the form of hydrogen-rich products, such as H2, CH4, and C2Hs,to satisfy the stoichiometric constraint. To obtain high aromatics yields, hydrogen-deficient feedstocks, are therefore preferred. Another approach to increase the aromatics yield is to increase the reaction severity to reduce the aromatizables to extinction. Still another approach is to alter the composition of the nonaromatizables and increase the yield of H2production. As shown by the data in Table 111, a significant amount of molecular H2 is produced when using the HZSM-5 catalyst. The H2 production is believed to be an acid-catalyzed hydrogentransfer reaction as proposed by Haag and Dessau (1984). Effects of Feed Composition. The feed composition has a profound effect on the reaction in four ways: (1) reactivity, (2) aromatics yield, (3) rate of catalyst deactivation, and (4) heat of reaction. (1) Reactivity. The more unsaturated, and the higher the carbon number of, the feed, the more it is reactive for conversion to the aromatics. The reaction severity required tp achieve the extinction yields for a hydrocarbon type of the same carbon number increases in the order diolefins
