Ind. Eng. Chem. Res. 1988,27,401-409
Conclusions On-line plasma optical emission analysis as an element-specific detector for SEC is a viable tool for comparing residual oils for HDV and HDS. SEC can determine small and large vanadium molecules for feeds with vanadium concentrations less than 120-150 ppm. The large and small vanadium molecules can be treated as fast and slow reactants in a parallel, first-order, kinetic model. The parallel, kinetic model indicates that kinetics and not reactor hydrodynamics may be the explanation for deviations from ideal plug flow, first-order reactor models observed for many hydrotreating reactions. In feeds containing high metal and/or asphaltene content, the differentiation between large and small molecules becomes lost. High sulfur concentration in residual oils also impairs the resolution between large and small molecules. Desulfurization of large sulfur molecules is a combination of size reduction and removal of thiophenic sulfur. Registry No. V, 7440-62-2.
Literature Cited Akimoto, D.; Iwamoto, Y.; Kodama, S.; Takeuchi, C. “Pilot-Plant Automation for Catalytic Hydrotreating of Heavy Residua”. Prepn.-Am. Chem. SOC.,Diu. Pet. Chem. 1983,28(4),1010-1026. Baldi, G.; Gianetto, A.; Sicardi, S.; Specchia, V.; Mazzarino, I. Can. J. Chem. Eng. 1985, 63(1), 62-66. Bridge, A. G.; Green, D. C. “DiffusionalConsiderations in Residuum Hydrodemetallization”. 178th National Meeting of the American Chemical Society, Washington, D.C., Sept 1979. Coombs, D. C.; Willers, G. P. U.S.Patent 4 482 453, 1984. Drushel, H. V. “Analytical Characterization of Residua and Hydrotreated Products”. 164th National Meeting of the American Chemical Society, New York, Aug 1972. Givens, E. N.; Venuto, P. B. ‘Hydrogenolysis of Benzothiophenes and related intermediate over Cobalt Molybdena catalyst”. 160th
401
National Meeting of the American Chemical Society, Chicago, Sept 1970. Hall, G.; Herron, S. P. “Size Characterization of Petroleum Asphaltenes and Maltenes”. ACS Adu. Chem. Ser. 1981, 195, 137-153. Hausler, D. W. Spectrochim. Acta 1985, 40B, 389-396. Jaffe, S. B. U.S.Patent 4267071, 1981. Johnston, H. D.; Hogan, R. J.; McMurtrie, D. E. “An Integrated Testing Facility for Bench Scale Catalyst Research”. Prepn.Am. Chem. SOC.,Diu. Pet. Chem. 1983,28(4), 960-972. Kilanowski, D. R.; Teeuwen, H.; de Beer, V. H.; Gates, B. C. J.Catal. 1978, 55, 129-137. Oleck, S. M.; Sherry, H. S. Ind. Eng. Chem. Process Des. Deu. 1977, 16, 525-528. Paraskos, J. A.; Frayer, J. A.; Shah, Y.T. Ind. Eng. Chem. Process Des. Deu. 1975, 14, 315-322. Rajagopaian, K.; Luss, D. Ind. Eng. Chem. Process Des. Deu. 1979, 18,459-465. Reynolds, J. G.; Biggs, W. R. “Analysis of Residuum Demetalation by Size Exclusion Chromatography with Element Specific Detection”. 190th National Meeting of the American Chemical Society, Chicago, 1985. Reynolds, J. G.; Biggs, W. R. “Analysis of Residuum Desulfurization by Size Exclusion Chromatography with Element Specific Detection”. 193rd National Meeting of the American Chemical Society, Denver, 1987. Richardson, R. L.; Alley, S. K. “Consideration of Catalyst Pore Structure and Asphaltenic Sulfur in the Desulfurization of Resids”. ACS Symp. Ser. 1975,20, 136-149. Shimura, M.; Shiroto, Y.; Takeuchi, C. “Effect of Catalyst Pore Structure on Hydrotreating of Heavy Oil”. National Meeting of the American Chemical Society, Las Vegas, March 1982; Div. Coll. Surf. Chem. Spry, J. C., Jr.; Sawyer, W. H. “ConfigurationalDiffusion Effects in Catalystic Demetallization of Petroleum Feedstocks”. 68th Annual AIChE Meeting, Los Angeles, 1975; Paper 30C.
Received for review November 12, 1986 Revised manuscript received August 13, 1987 Accepted November 2 , 1987
Competitive Reaction in Intrazeolitic Media Jan-Ku Chen,Alison M. Martin, Young Gul Kim, and Vijay T. John* Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118
T h e effects of an aromatics cofeed on paraffin hydroisomerization and hydrocracking over Pt/ mordenite are studied as an example of competitive reaction in intrazeolitic medium. Preferential adsorption by the aromatic (benzene) modifies catalytic site distributions available for the paraffin (n-hexane) reaction and also affects diffusional characteristics in the one-dimensional channel structure of the zeolite. A significant inhibition of the hexane reaction is observed. The hexane isomer distribution in the presence of the aromatic is rationalized using concepts of coreactant induced size and shape selectivity modifications. In all catalytic reactions taking place in porous Catalysts, diffusion and adsorption play important roles in determining overall reaction rates and product selectivities. This is especially true for crystalling zeolite catalysts wherein intraparticle diffusivities are often orders of magnitude smaller than the Knuden-type diffusivities encountered in amorphous materials (Weisz, 1973). The fact that zeolite pore dimensions and hydrocarbon kinetic diameters are of the same order of magnitude leads to the interesting concept of shape selectivity wherein molecular species are discriminated on the basis of their ability to enter zeolite pores and to access intrazeolitic catalytic sites. In this paper, we describe another aspect of transport and reaction in zeolite pores. We consider the case of an induced species discrimination based on the ability of a secondary species in the feed to suppress reaction of the 0888-5885/88/2627-0401$01.50/0
primary species. Thus, strong adsorption of a secondary species may cause a diffusional barrier to transport the primary reactant; this induced transport barrier may modify the intrazeolitic residence times of the reacting species and thus affect reaction rates and product selectivities. The phenomenon is especially relevant to the realistic situations of complex feeds processing. In such situations, competitive reaction between the feed components results in overall rates and selectivities that cannot be deduced from pure-component experiments alone. On the other hand, the nature of the competitive reaction becomes difficult to understand if actual multicomponent feeds are used, due to the complexity in analysis. Insight into competitive reaction in intrazeolitic media can be gained through experiments using binary model compound feeds, and this is the approach followed. In this 0 1988 American Chemical Society
402 Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988
paper, we discuss the competitive hydroisomerizations of n-hexane (the primary reactant) and benzene (the secondary coreactant) taking place in synthetic mordenite, a zeolite with an effective one-dimensional channel structure. The use of the binary feed mixture facilitates analysis of the competitive reaction. The choice of nhexane hydroisomerization as the model reaction is based on the ease of product identification, and the commercial importance of paraffii isomerization to increase fuel octane numbers (Kouwenhoven, 1973). The choice of benzene as a representative cofeed is twofold. First, the basicity of the aromatic implies strong adsorption on acid sites and therefore the potential creation of an induced diffusional barrier. Second, as will shown in the discussion, isomer products of the benzene reaction are distinct from those of the n-hexane reaction; thus the effects of the combined feed on the individual reactions of hexane and benzene can be clearly defined. The choice of synthetic mordenite is due to commercial significance (the Shell Hysomer Process) and to resemblance with the ideal one-dimensional pore structure. The hydroisomerization of n-hexane over Pt/zeolites has been studied extensively. The catalyst operates through classical dual functional mechanisms wherein the noble metal behaves as a hydrogenation/dehydrogenationagent and the zeolite has acidic functions of isomerization and cracking through carbenium ion mechanisms (Kouwenhoven and van Helden, 1970). The distinct roles of the acid and metal functions have been detailed by Ribeiro et al. (1982),Perot et al. (1980),and Gianneto et al. (1986); dual-site LangmuirHinshelwood models have been suggested as appropriate for the reaction kinetics (Voorhies and Bryant, 1968; Beecher and Voorhies, 1969; Nagy et al., 1984). The synergism between different hydrocarbon species in a feed mixture is also a phenomenon that has been reported as far back as 1948 when Mavity et al. observed that the addition of benzene to a feed of n-pentane treated over an aluminum chloride-hydrogen chloride catalyst reduced disproportionation of n-pentanes to butanes and hexanes. Voorhies and Hatcher (1969) have briefly reported inhibition in n-hexane hydrocracking over Pt/mordenite, by cyclohexane, and have attributed the result to preferential adsorption of cyclohexane. In another relevant study, Steijns and Froment (1981) have reported that the hydrocracking of n-heptane over P t / Y is significantly inhibited by the addition of decane to the feed. Thus, the present study attemps to explore in depth the phenomena of competitive reaction within zeolitic pores and the resultant concept of coreactant-induced modifications in rate and selectivities. Modern advances in carbenium ion chemistry (Brouwer, 1980) have helped elucidate the isomerization mechanisms of paraffins and to identify the slow steps in the isomerization network. This helps us understand the influence of an aromatics cofeed on the rates of reaction between individual isomers in the hexane isomerization network. This study examines isomer selectivities between mono and dibranched species and amongst species with the same degree of branching. The results are discussed using both mechanistic arguments and arguments based on transport characteristics within zeolitic pores. Experimental Section Synthetic mordenite in the H+ form was obtained from Norton Chemical Company (Zeolon 900-H).Synthetic mordenite has an effective one-dimensional channel-type structure with a Si02/A1203ratio of about 1011. While sodium mordenite has a main pore diameter at 6.7 X 7.0 A (Breck, 1974; Meier and Olson, 1978), protonation and
Table I. Product Distributions from n -Hexane and Benzene Hydroisomerization/Hydrocracking feed n-hexane benzene 573 T,K 573 P, MPa 0.76 0.76 W/F, g hr/mol 130.5 130.5 72.55 31.61 conversion, % uroduct distribution mol % mol 7i methane 0.55 0.30 ethane 0.60 0.34 4.89 propane 2.90 1.69 2MP 0.27 1.44 1.86 n-butane 1.20 2MB 0.18 0.24 0.57 n-pentane 0 10.73 2,2DMB 0 10.08 2,3DMB 40.75 0 2MP 0 27.08 3MP a 0.35 n-hexane 0 38.12 methylcyclopentane 0 55.86 cyclohexane a 0 benzene a
Feed.
some aluminum leaching increases the effective diameter to 8-9 A (Norton Catalogue; Satterfield (1980));this is the material supplied to us. The zeolite was extensively washed with NH40H and then heated to remove NH3, to make sure that the H+ form was used in experiments. Pt2+ loading (0.6 w t 9%) by ion exchange into the zeolite was carried out by using the procedure reported by Chick and co-workers (1977). Thus, Pt(NH3)*Cl2(Strem Chemicals) was dissolved in 1N NH,N03 solution and slowly added to a slurry of mordenite in 1 N NH4N03at pH 10. The Pt-containing mordenite was filtered and washed to remove all C1- ions. The catalyst was dried at 383 K and calcined at 623 K for 3 h and a t 823 K for 10 h. Catalyst reduction was carried out in situ by passing H2 through the reactor which was slowly heated at 1K/min to 673 K and maintained at 673 K for 10 h. Pulse chemisorption results on the reduced catalyst showed a dispersion level of 0.85. n-Hexane (Aldrich Chemicals, >99 mol % purity), benzene (Fluka purum grade), and hydrogen (Matheson ultrahigh purity) were used as reactants. The experiments were carried out in a computer-controlled microreactor system (CDS 8100) incorporating a 5/8-in.-i.d.tubular reactor into which was packed 6 g of the catalyst. Liquid hydrocarbon feed was pumped into a mixing chamber by a Beckman HPLC metering pump where it was vaporized and mixed with hydrogen and fed to the reactor. Reaction products were maintained in the vapor phase through heated lines and automatically injected through sampling valves into a Quadrex fused silica capillary column with methyl silicone as the liquid phase. The products were analyzed by using FID after initial identification through mass spectrometry. Base reaction conditions of 573 K, 0.76 MPa, 1 LHSV (mL of liquid hydrocarbon/g of catalyst/h), and a 10/1 H,/hydrocarbon mole ratio were used in the experiments. Changes in these operating conditions are listed in the Results and Discussion and in the accompanying figures and tables. Results and Discussion Mechanistic Aspects. Table I lists product distributions obtained from the reactions of pure n-hexane and benzene. The experiments were conducted at the base conditions listed in the Experimental Section. The data
Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 403
-
Table 11. Equilibrium Distributions
A. hexane isomers temp, K 523 548 573 598
n-hexane, mol % 18.04 19.48 20.82 22.07
2MP, mol % 37.36 37.37 37.26 37.06
3MP, mol % 22.0 22.64 23.17 23.60
of Table I serve to illustrate the reaction mechanisms of hexane and benzene hydroisomerization, a brief mention of which is necessary to interpret subsequent results of competitive reaction between benzene and n-hexane. Hydroisomerization of n-hexane can occur through the classical bifunctional scheme (Weisz, 1962) shown below
Pt
acid
-
iCs y +iH Z C
6=
ic6=
I\
7 -n+
ic6+
Cracked Products
/
diffusion
where carbenium ion formation is through protonation of olefins formed over metal sites. Carbenium ions can also be formed directly from paraffins through Lewis acid catalyzed hydride ion removal (Pines, 1981). Depending on the strength of the acid site, the carbenium ion either rearranges to the isomeric form or cracks to lower molecular weight hydrocarbons. I t has been well established (Weisz, 1962) that the bifunctional mechanism operates efficiently only when the acid and metal sites are in close proximity so that olefins formed at the metal sites can be easily removed, thus continuously displacing the paraffin-olefin equilibrium. In the “ideal” bifunctional mechanism (Jacobs et al., 1980),the roles of the metal and acid sites are completely distinct, with hydrogenation/dehydrogenation occurring on metal sites and the acid sites being responsible for isomerization and cracking. In reactions of hexane and smaller hydrocarbons, hydrogenolyses and skeletal isomerization can also occur through metal-catalyzed mechanisms (Gates et al., 1979). We next consider the distribution of branched isomers from n-hexane hydroisomerization. Table I1 lists the equilibrium distribution obtained from the work by Condon (1958) and is included to help identify deviations from equilibrium in our experimental runs. On examination of the isomer distribution in Table I, we see that the experimentally observed relative distribution of 2-methylpentane (2MP):3-methylpentane (3MP):2,3-dimethylbutane (2,3DMB):2,2-dimethylbutane (2,2DMB) is 45.7:30.4:11.7:12.2 and is in close similarity with the corresponding distribution at equilibrium (47.1:29.3:10.413.3; see Table 11). Mechanisms proposed to explain the reaction network between n-hexane and its branched isomers include a bimolecular-type mechanism within zeolitic pores (Bolton and Lanewala, 1970) and mechanisms based on carbenium ion rearrangement and scission (Christoffel, 1982; Marin and Froment, 1982) generally accepted as the prevalent mechanisms of branching in hexane reforming over Pt/SiO2-Al2O3. Determination of carbenium ion stabilities in superacids have led to theories of carbenium ion formation and transformation (Brouwer, 1980),which
2,2DMB, mol % 2,3DMB, mol % 13.78 8.83 11.97 8.54 10.50 8.26 9.28 7.99
B. methylcyclopentane cyclohexane MCP, mol % CH, mol % 74.29 25.71 80.69 19.31 83.04 16.96 84.98 15.02
have been used to rationalize the products of hydrocracking and hydroisomerization of long-chain alkanes in Pt/zeolites (Weitkamp, 1975; Weitkamp et al., 1983) and to deduce characteristics of intrazeolitic void structure through such probe reactions (Martens et al., 1984). We therefore discuss n-hexane isomerization under the framework of rules for carbenium ion rearrangement and cleavage. While a comprehensive description of these rules is found in the work by Brouwer (1980), a brief ennumeration here helps in the subsequent interpretation of our experimental results. Thus, (i) stabilities of carbenium ions decrease in the order R3C+ > R&H+ >> RCHz+ >> CH3+,(ii) carbenium ion isomerizations that lead to no change in the degree of branching occur through 1,2hydride and alkyl shifts (these include the transformations between 2- and 3-methylpentane and between 2,2- and 2,3-dimethylbutane),and (iii) carbenium ion isomerizations that lead to a change in the degree of branching occur through protonated cyclopropane ring intermediates. These considerations lead to a generally accepted network for hexane isomerization (Marin and Froment, 1982) shown below. It must be pointed out that although alkyl hydride 2MP y 2 , 3 D M B
nC6 \
AI
.-
2,2DMB
\ .1
3MP
shifts are considered faster than transformations involving protonated cyclopropane, the slow step in the sequence is the transformation from 2,3-dimethylbutane to 2,2-dimethylbutane. This is because the step involves transition from a tertiary carbenium ion, the 2,3-dimethylbutyl carbocation, to a secondary carbenium ion, the 2,2-dimethylbutyl carbocation. The data at 573 OC for n-hexane hydroisomerization corroborates to some degree the reaction mechanisms described. Of the cracked products, propane is formed in the largest amounts, validating the carbenium ion mechanism of hydrocracking through @-scission.C1, Cz,C4,and C5 species are formed either through hydrogenolysis over metal sites or through acid-catalyzed carbenium ion transfer or proton-transfer mechanisms (Brouwer, 1980). It is interesting to note that the isobutaneln-butane ratio of 0.91 and the 2-methylbutane/n-pentaneratio of 2.1 are close to the equilibrium ratios of 0.92 and 2.6, respectively, thus implying rapid equilibration between products of the same molecular weight. The reaction of benzene over Pt/mordenite reveals an isomer product distribution distinct from that observed in n-hexane hydroisomerization. Cyclohexane, the primary product of benzene hydroisomerization, is typically obtained by hydrogenation over Pt, and methylcyclopentane is subsequently formed by rearrangement of cyclohexane over the acid sites (Christoffel, 1982). Figure 1 illustrates the temperature plots for the formation of cyclohexane, methylcyclopentane, and cracked products from the benzene reaction. A t low temperatures, C6 ring rearrangement is the slow step, while at higher temperatures
404 Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988
-
1000
X
z
;E
0 W
40-
\.
/
1 I
500
100.
U
Y9
i
= !O/!
P = 0 78 MPa W/F = 130 C Hr/Mal
3
I
525
I
k
550
I
1.00
I
575
TEMPERATURE
I
I
I
800
6
U
0
3
A
i!
f K)
E
-
-
0.00% 7.14% 15.9%
0'1°
' v
37.0% 73.4%
0.01
hydrogenation appears to become rate limiting. The maximum observed in the production rate of cyclohexane with temperature is typical of a sequential reaction (A B C) in which a change in rate-controlling step from B C at low temperatures, to A B at higher temperatures, results in a maximum production rate of species B at an intermediate temperature. It is important to note from Table I that trace amounts of n-hexane represent the total acyclic c6 isomers formed during the benzene reaction. n-Hexane is formed either through hydrogenolysis of cyclohexane or through acid-catalyzed protonolysis and rupture of the methylcyclopentane ring (Christoffel, 1982). The distributions of cracked products from the benzene reaction are also listed in Table I. The fact that substantially more cracked products are formed than acyclic c6 hydrocarbons implies that at least two bonds of the ring structures undergoing cleavage are broken before desorption. The distribution of cracked products shows some similarity to the distribution obtained from n-hexane hydroisomerization in that propane is again the predominant cracked species. However, the isobutaneln-butane ratio (0.19) and the i-methylbutaneln-pentane ratio (0.75) are much lower than equilibrium values. The observations (Table I) that negligible acyclic c6 hydrocarbons are formed from the benzene reaction and negligible cyclic c6 compounds from the n-hexane reaction significantly simplify the analysis of products obtained from benzene + n-hexane cofeeding experiments, since the individual reactions can now be easily distinguished. Thus, carbon balances on the feed, and effluent streams yield overall conversions of n-hexane and benzene. Conversions of n-hexane to branched isomers and of benzene to cyclohexane and methylcyclopentane are then obtained by assuming that the branched isomers are formed exclusively from n-hexane and the cyclic c6 compounds from benzene. The difference between overall conversions and conversions to isomers then gives conversions of n-hexane and benzene to cracked products. Results of the aromatic cofeeding experiments are discussed next. Mixed Feed Experiments. Figure 2 illustrates the effect of benzene addition on the hydroisomerization rates and conversion of n-hexane. The reaction was again carried out at different temperatures with all other operating conditions held at the base values. As the figures indicate, increasing amounts of benzene inhibit the n-hexane re-
\ \
Concentration
E
PURE BENZENE FEED H?/HC
10.0
v
Figure 1. Effect of temperature on the products of benzene hydroisomerization/ hydrocracking.
--
130 G Hr/Mol
?
CYCLOHEXANE
3E
5
=
ew
CRACKED PRODUCTS
A M-CYCLOPENTANE
t
W
W/F
3
tll \ ii cl
-
H2/HC = 10/1 P = 076 MPa
h
m
1.8
1.7
1.8
1.9
2.0
Hz/HC = 10/1
-
P = 0.78 MPa W/F 130 C Hr/Mol
I1 525
I
I
550
575
TEMPERATURE
I I 600
(
K)
Figure 2. Effect of temperature and benzene concentration level on n-hexane conversion and reaction rate.
action significantly. For example, at 523 K, Figure 2a indicates that the rate of hexane reaction drops as much as 2 orders of magnitude for a 37% decrease in hexane concentration; Figure 2b indicates that the corresponding hexane conversion level drops from 17.4% to less than 1%. It is clear that competitive adsorption and reaction of benzene is responsible for the decreased reaction rate of n-hexane. The basicity of aromatics toward acids (Pines, 1981) may lead to preferential adsorption of benzene on acid sites, thus decreasing the catalytic site availability for the paraffin reaction. In addition to such site suppression, the presence of strongly adsorbed c6 cyclic compounds within the pore structure may retard pore access to hexane. The kinetic diameter of hexane is 4.8 A; with an increase
Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 405 Table 111. Effect of Process Variables on n -Hexane and Benzene Conversions and Isomer Distributions: Temperature Effect (P= 0.76 MPa, W/F = 130.5 g h/mol) feed: pure n-hexane at T,K feed: 7.14% benzene at T,K 523 535 548 560 573 523 535 548 560 573 hexane convers., % 17.35 26.71 43.95 58.82 72.53 7.90 17.84 32.66 48.83 59.78 benzene convers., % 100.0 100.0 100.0 100.0 100.0 2MP/3MP 1.62 1.66 1.61 1.57 1.51 1.79 1.75 1.74 1.59 1.56 2MP/2,3DMB 3.97 4.05 3.95 4.05 4.04 4.01 3.91 4.13 3.92 3.86 2,3DMB/2,2DMB 2.00 1.96 1.61 1.32 0.94 1.79 1.89 1.67 1.45 1.18 CH/MCP 1.15 0.63 0.41 0.34 0.29 feed 15.9% benzene at T,K feed: 37.0% benzene at T,K 523 535 548 560 573 523 535 548 560 573 hexane convers., % 2.21 5.47 14.07 22.96 37.85 0.59 1.23 2.50 5.04 8.72 benzene convers., % 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 2MP / 3MP 1.85 1.79 1.75 1.75 1.66 1.64 1.65 1.79 1.76 1.76 2MP/2,3DMB 5.94 4.69 4.55 4.44 4.44 4.14 4.31 5.07 5.31 5.33 2,3DMB/2,2DMB 0.74 1.05 1.32 1.35 1.33 0.77 0.77 0.89 1.16 1.64 CH/MCP 7.15 3.16 0.97 0.50 0.37 11.63 6.50 3.39 1.82 0.93 feed: 73.4% benzene at T,K feed uure benzene at T.K 523 535 548 560 573 598 523 548 573 598 2.24 11.64 hexane convers., % 0.12 0.18 0.34 0.93 benzene convers., % 37.07 40.19 47.84 54.83 60.83 69.15 20.13 26.02 31.00 36.26 2MP/3MP 1.40 1.55 1.51 1.66 1.77 1.84 2MP/2,3DMB 4.91 5.27 4.18 4.38 2,3DMB/2,2DMB 4.00 3.70 CH/MCP 10.8 5.78 3.26 1.95 1.22 0.57 11.39 3.44 1.50 0.67
of about 0.7 A for every additional increase in branch length, the kinetic diameters of both 2MP and 3MP are 5.5 A, and those of 2,2DMB and 2,3DMB are 6.2 A. The kinetic diameters of benzene and cyclohexane are 6.75 and 6.9 A, respectively (Moore and Katzer, 1972). The fact that the hydrocarbons have kinetic diameters greater than half the effective pore diameter of mordenite implies that any strongly adsorbed c 6 hydrocarbon would create a barrier to diffusion of other (26 hydrocarbons through the pore. Thus, strongly adsorbed c 6 cyclics can induce resistance to the mass transfer of n-hexane to the pore interior, resulting in an induced pore blockage. It is also instructive to consider the fate of the aromatic during the cofeeding experiments. Figure 3 illustrates the Arrhenius plots for the benzene reaction. The flat lines at lower benzene concentrations in the feed reflect almost complete conversion at all temperatures. It is interesting to note that at higher benzene concentrations, some selfinhibition of the benzene reaction is observed. The initial hydrogenation of benzene to cyclohexane is a bimolecular reaction; from a mechanistic viewpoint, self-inhibition in a bimolecular reaction could be due to decreased adsorption of one of the reactants. The hydrogenation mechanism occurs essentially through dissociative chemisorption of H2on Pt (Gates et al., 1979). At high benzene concentrations, saturation of Pt sites by benzene may inhibit this dissociative chemisorption, thereby reducing the rate of the initial hydrogenation step and thus causing selfinhibition. An alternate interpretation stems from the fact that in the presence of an alkane which serves as a hydrogen-transfer intermediate, benzene can be hydrogenated by superacids (Wristers, 1975; Pines, 1981). A reduction in alkane concentrations could thus inhibit such superacid-catalyzedmechanisms of benzene hydrogenation, if prevalent in this zeolitic system. To examine the effect of benzene on the individual steps of the hexane isomerization network, the rates of formation of the different isomers were monitored as a function of temperature, total pressure, and residence time, both with benzene addition and without. Results of these experiments are presented in Figures 4-7 and in Tables 111-V and are discussed in some detail.
HZ/HC
=
10/1
P = 0.76 MPa W/F = 130 G Hr/Mol
c
s
s
$
4
Benzene Concentration 7.14% A 15.Q% 37.0% v 73.4% 1oo.x I
1
1/T
( K-lXl.0
I
-')
Figure 3. Benzene reaction rate as a function of temperature and concentration level.
Figure 4 illustrates temperature effects on the rates of formation of hexane isomers. A clear inhibition of the rates of formation as a function of benzene concentration is observed. T o study the effect of benzene addition and temperature on the relative amounts of the different isomers, we examine Figure 4 in conjunction with Table I11 where the ratios 2MP/3MP, 2MP/2,3DMB, and 2,2DMB/2,3DMB are listed. At 0 mol % benzene, the 2MP/3MP ratio varies from 1.62 at 523 K to 1.51 at 573 K in fairly good agreement with equilibrium values of 1.7 and 1.61 over the same temperature range (Table I). Allowing for experimental errors in chromatogram interpretation and errors in predicting equilibrium values, it
406 Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988
:I a
4
\
a
l 590
580
560
540
Tr U P E R A T L R E
590
#
/
l
540
#
TtMPrriA’iRF
