Ind. Eng. Chem. Prod, Res. Dev. 1981, 20, 654-660
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Hydroisomerization and Hydrocracking. 2. Product Distributions from n-Decane and n-Dodecane Matt SteiJnsand Gllbert Froment‘ Laboratorium voor Petrochemische Techniek, Rijksuniversiteit Gent, Krlgslaan 27 1, 59000 Gent, Belgium
Peter Jacobs and Jan Uytterhoeven Centrum vmr Oppervlaktescheikunde,Katholieke Universlteit Leuven, De Croylaan 42, 8-3030 Leuven, Belgium
Jens Weltkamp Engler-Bunte-Institut, Universitat Karlsruhe (TH), Richard-Wilktaffer-Allee 5, 137500 Karlsruhe, Federal Republic of Germany
The hydroisomerization and hydrocracking of ndecane and ndodecane on an ultrastable zeolite Y, containing 0.5 wt % platinum, at T = 130-250 OC and P = 5-100 bar was investigated. The product distributions are a unique function of the total conversion only and are typical for a carbenium ion mechanism. Primary products are the monobranched isomers of the nalkane feed. Multibranched feed isomers and cracked products are formed in consecutive reactions. Secondary cracking reactions start at high conversion levels (>90%) only. The energetically favored cleavage of some multibranched feed isomers is more probable than the previously proposed exclusive cracking of monobranched feed isomers.
Introduction Hydrocracking is practiced in modern petroleum refming for the production of light fuels (Le., gasoline, diesel, and jet fuel) from heavy distillates and residua (Choudhary and Saraf, 1975; Bolton, 1976). Other applications of this process comprise the upgrading of petrochemical feedstocks (Mavity et al., 1978; Kelley et al., 1979), the improvement of the gasoline octane number (Chen, 1968), and the production of high quality lubricants (Bolton, 1976). Zeolite-based catalysts for the hydrocracking of distillates were shown to be superior to the more conventional amorphous catalysts with respect to activity, selectivity, and resistance to poisons (Bolton, 1976; Ward et al., 1973). The processing of a broad r a g e of heavy feedstocks with high concentrations of sulfur, nitrogen, and polycyclic aromatics is feasible. In the exothermic hydrocracking process, where problems such as hot-spots and run-away temperatures may be encountered (Jaffe, 1974), it is essential that the catalysts are thermally and hydrothermally stable. Ultrastable Y zeolites (US-Y), first described by McDaniel and Maher (1968),meet these requirements and were used in the present study. Hydrocracking catalysts are dual functional and contain a well-dispersed metal in the pores of an acidic support (Jacobs, 1977). The relative strength of the two functions determines the nature and the distribution of the produds. When a strong hydrogenation function such as platinum is used a high selectivity for hydroisomerization as well as pure primary cracking can be obtained. In such a case considerable insight may be obtained into the reaction mechanism and for this reason Weitkamp (1975) introduced the term “ideal” hydrocracking. Recently, we reported that Pt/US-Y shows the ideal “behavior” with n-decane hydrocracking as a test reaction (Steijns et al., 0196-4321/81/1220-0654$01.25/0
1978). Another feature of this catalyst is the absence of catalyst deactivation,even with a hydrogen partial pressure as low as 1 bar. Hydroisomerization and hydrocracking essentially proceed through carbenium ions (Poutsma, 1976). (“Carbenium ions” is synonymous with “carbonium ions”. The former nomenclature has been recommended by IUPAC in its Information Bulletin No. 39 of 1974.) Alkanes may be converted into carbenium ions, either directly by of hydride abstraction (-H-) or by protonation (+H+) olefinic intermediates formed by dehydrogenation on the metal. The reaction path via the olefins is still generally accepted for reforming, hydroisomerization, and hydrocracking of alkanes, but this route is subject to discussion (Poutsma, 1976). The carbenium ions can either rearrange or undergo @-scission. Cleavage gives rise to a smaller carbenium ion and an olefinic fragment, which is immediately hydrogenated. The fast hydrogenation of unsaturated hydrocarbons is a major reason for the absence of catalyst deactivation by coking. Many parallel and consecutive hydrogenation, isomerization, and cracking reactions take place in hydroisomerization and hydrocracking. Even with a pure model hydrocarbon such as n-decane, a large number of products is formed. The kinetic modeling of such a complex reaction network is a difficult task. The common approach in the literature is to consider only the global disappearance rate of the model hydrocarbon (Beecher et al., 1968; Engler, 1976). Yet the product distributions yield sufficient insight into the mechanisms of isomerization and cracking to permit a more rigorous kinetic treatment. The detailed investigations of Weitkamp et al. (1975, 1978) on the distributions of the products from long-chain alkane hydroisomerization and hydrocracking over zeolites led to important information on the rearrangement and 0 1981 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 655
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Figure 1. Hydrocracking equipment: 1, thermal mass flow controller/meter (two channels); 2, hydrocarbon reservoir and metering buret; 3, liquid pump; 4, evaporator/preheater; 5, safety valve; 6, reactor; 7, magnet drive stirrer; 8, sampling valve; 9, cooler; 10, separator/collector; 11,back-pressure regulator; 12, wet gas meter; 13, gas chromatograph; 14, reactor temperature controller; 15, vent; 16, liquid products; --, heated line; N, check valve; cross-hatched areas, filters.
cleavage reactions of alkylcarbenium ions. In the present study n-decane and n-dodecane were chosen as model hydrocarbons. Both the partial pressures of the feed n-alkane and the hydrogen were varied over the maximum attainable range to provide information on the structure of the reaction network and the relative importance of the various reactants and reactions. Special attention was paid to the relation between the distribution of feed isomers and cracked products because it is relevant for the construction of the cracking routes in the reaction network. Experimental Section Catalyst Preparation and Pretreatment. The starting base, a Na Y zeolite (Type 30-200) was obtained from Union Carbide Corp., Linde Division. In a first stage 90% of the sodium ions were replaced by ammonium ions by ion exchange. After a calcination at 550 "C, in deep-bed geometry (McDaniel and Maher, 1968), a second ammonium exchange was performed. Ultrastable Y zeolite (US-Y) was obtained after a final calcination a t 750 "C. Chemically it has a very low sodium content (xO.1wt %) and the same Al/Si ratio as the original Y zeolite. X-ray characterization of the material showed the typical features of ultrastable Y zeolite, as described by McDaniel and Maher (1968). The ultrastable Y zeolite was exchanged with an aqueous solution of Pt(NH3)&12, so as to contain 0.5% by weight of platinum on an anhydrous base. The powder was fiist compressed into plates which were then crushed. The kinetic experiments were carried out on the sieve fraction 0.4-1.0 mm. The platinum loaded zeolite (Pt/US-Y) was pretreated in situ: first in oxygen at 400 "C for 1 h and subsequently in hydrogen at 400 "C for 1 h. Feedstock. n-Decane (95+ %) and n-dodecane (99.5+%) were obtained from Johann Haltermann, Antwerp. The n-decane was further purified up to 99.5+% by distillation. Hydrogen was obtained from l'Air Liquide and was purified by passing it over a Pt/A1203catalyst and a CaA zeolite to drastically reduce oxygen and water impurities. A Panametrix Model 2000 hygrometer was installed to monitor the water content of the feed and product streams. To avoid deactivation the water content
had to be lower than 10 ppm, especially for high-pressure work. Apparatus. A flow scheme of the high pressure equipment (maximum 100 bar) is given in Figure 1. Liquid hydrocarbons can be fed with a high-pressure pump (Lewa type FLK-1). The hydrogen feed is controlled and metered with a Brooks thermal mass flow controller. The high-pressure reactor is of the Berty type (Berty, 1974) and is manufactured by Autoclave Engineers. Special precautions were taken to avoid condensation in the magnetic drive shaft. Gaseous product samples were taken at high pressure and high temperature directly after the exit of the reactor. A capillary column OV-101 (length 100 m, diameter 0.25 mm) was used for the gas chromatographic separation of the products. The product identification was based on Matukuma's (1969) retention indices. The individual multibranched isododecanes could not be identified. Procedure. One and the same batch of catalyst (9 g) has been used over a period of more than a year with a total on stream time of at least 700 h. Between experiments the catalyst was kept under hydrogen pressure (>5 bar) at 130-250 "C. A standard test for the reproducibility of catalyst activity and selectivity was performed periodically. There was no catalyst deactivation. Data were collected over a wide range of experimental conditions: temperature, 130-250 "C;pressure, 5-100 bar; hydrogen flow, 10-600 Nl/h; n-alkane flow, 2-80 cm3/h. Discussion of the Results The selectivity of the reaction between hydroisomerization and hydrocracking will be treated first. Subsequently, the product distributions of hydroisomerization and hydrocracking will be considered separately. Finally, cleavage routes of carbenium ions will be discussed. Selectivity for Hydroisomerization. The isomerization conversion is a unique function of the total conversion, as shown in Figure 2 for n-decane as feed. The isomerization selectivity is close to 100% at low conversion levels. This observation rules out the possiblity of a direct cracking of the n-decane without structural rearrangement. The maximum conversion into decane isomers amounts
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Figure 2. Isomerization conversion of n-decane on Pt/US-Y as a function of the total conversion of n-decane. TPC)
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*/* n - C12 CONVERSION Figure 3. Isomerization Conversion of n-decane on Pt/US-Y as a function of the total conversion of n-dodecane.
to 40% and is only dependent on the type of catalyst or reactor (Steijns et al., 1978). With n-dodecane as feed again a unique relation was found between isomerization conversion and total conversion (Figure 3). The maximum conversion into isomers amounts only to about 32% in this case. This is in line with the observations of Weitkamp (1971,1975,1978),who reported similar high isomerization conversions for Pt/CaY, i.e., 61% for n-decane and 48% for n-dodecane. The uniqueness of the selectivity curves of Figures 2 and 3 implies that isomerization and cracking have approximately the same activation energy. Finally, it should be mentioned that Gol'dfarb et al. (1977) also obtained a
unique curve for the hydroisomerhtion and hydrocracking of n-decane over nickel-molybdenum-alumina catalysts. The Distribution of the Hydroisomerization Products. Figure 4 gives the composition of the decane fraction vs. the total conversion of n-decane. This product distribution too is a function of the total conversion only. With n-dodecane as feed similar results were obtained, as shown in Figure 5. It is clear from the Figures 4 and 5 that dibranched respbctively multibranched structures are formed in consecutive reactions from the monobranched isomers of the feed. When the experimental distribution of the decanes is compared with a calculated thermodynamic equilibrium distribution using literature data (Stull
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 857 T
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Figure 4. Composition of the decane fraction as a function of the total n-decane conversion; (multibranched
