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Ind. Eng. C h e n . Res. 1987, 26, 706-711
M-Forming Process Nai Y.Chen,* William E. Garwood, and Roland H. Heck Mobil Research and Development Corporation, Princeton, New Jersey 08540, and Mobil Research and Development Corporation, Paulsboro, New Jersey 08066
M-Forming is a zeolite-based catalytic process which, when used in conjunction with naphtha reforming, produces a higher octane, lower aromatic content gasoline by selectively converting low octane normal and singly branched chain paraffins. I t also allows increased reformer throughput and increased process flexibility to meet changing product quality demands. The process may be employed to process either full-range reformates from a low-pressure reformer or light reformates, boiling below 120 "C, which are difficult to reform. The reactions involved in the M-Forming process include (1)selective cracking of normal and singly branched paraffinic molecules, (2) formation of alkyl aromatics by the alkylation of benzene and toluene with olefinic cracked fragments, (3) aromatics formation via cyclization and hydrogen transfer, and (4) redistribution of alkyl aromatics to produce a spectrum of aromatic products.
I. Introduction The development of the platinum reforming process to produce high octane gasolines from naphthas is one of the major achievements in the history of industrial catalysis (Ciapetta et al., 1958). Naphtha reforming involves a number of desirable reactions, including converting six-ring and five-ring naphthenes to aromatics via dehydrogenation and dehydroisomerization and converting paraffins to higher octane isoparaffins and aromatics via hydroisomerization and dehydrocyclization. Also taking place are the undesirable hydrogenolysis and hydrocracking reactions, which reduce liquid yield with only minimal octane increase. Over the years, continued research in catalyst and process technology has improved the process performance with respect to product octane, liquid yield, and catalyst life. However, the presence of low octane paraffins in the product limits the maximum octane obtainable by this process. Although some paraffins have relatively higher octane than others, reformates contain an equilibrium mixture of both low and high octane paraffins. Octane numbers (lead free) greater than about 98 (research clear, R + 0)generally require substantial hydrocracking of this equilibrium mixture of paraffins to gases. The result is an increased concentration of aromatics and an increase in octane but at a large loss of liquid yield. The selective conversion of the lowest octane components in a reformate offers a new approach to boost octane. One such process, Selectoforming (Chen et al., 1968), uses an eight-memberedoxygen ring small-pore zeolite, erionite, as the catalyst to selectively hydrocrack n-paraffins out of a reformate. The product is a higher octane gasoline blending stream with propane as the major byproduct. Catalyst stability in Selectoforming is maintained by incorporating a weak metal function, such as nickel, into the catalyst to saturate olefins and prevent coke formation without hydrogenating the aromatics present in the feed (Chen and Garwood, 1968; Chen and Rosinski, 1971; Heinemann, 1981). However, the octane boosting potential of this process is limited by the concentration of nparaffins present in the reformate. A further increase in octane would necessitate the conversion of singly branched paraffins as well. The availability of ZSM-5 (Argauer and Landolt, 1972), a 10-membered oxygen ring medium-pore zeolite, offers a potential for such a process. ZSM-5 has a larger pore opening than erionite. It can admit singly branched paraffins as well as simple aromatics, such as benzene and toluene (Argauer and Landolt, 1972; Chen and Garwood, 1978). It also possesses some 0888-5885/87/2626-0706$01.50/0
unusual shape-selective characteristics. Chen and Garwood (1978) showed that when benzene and toluene are present in the reaction mixture, the rate of benzene alkylation surpasses the rate of toluene alkylation when the reaction is carried out under pressure. The observation was attributed to the unique channel openings in ZSM-5, which either impose a stereospecific effect on the alkylation reaction or the rate of alkylation is inhibited by the slow diffusion of some of the dialkylbenzene isomers. Aromatics alkylation in ZSM-5 also exhibits shape-selective effects due to configurational diffusion constraints of the channel structure of the zeolite. p-Dialkylbenzenes, the smallest of the dialkylbenzene isomers, are preferentially formed (Chen et al., 1979~). Presented in this paper are results from studies directed toward the development of a new postreforming process, M-Forming (Chen, 1973; Heinemann, 1981), which uses ZSM-5 as the catalyst. In this process, the catalyst performs two major functions. It selectively cracks low octane paraffins and utilizes the olefinic fragments to alkylate the benzene and toluene present in the reformate. The resulting alkyl aromatics contribute to octane boosting and reduce the loss of cracked products to gas, thus increasing liquid yield. 11. Experimental Section A. Catalyst. The catalyst used in this study was a ZSM-5 with a Si02/A120,ratio of 70 prepared according to the method of Argauer and Landolt (1972). The zeolite was extruded with 35% alumina to form 1/16-in.extrudates, which were calcined to decompose occluded organics and subsequently ion exchanged with ammonium and nickel chloride solutions. The finished catalyst contained about 0.2% nickel. B. Feed. Feedstocks used in this study included nheptane, n-octane, benzene, toluene, ethylbenzene, xylenes, cumene, cymene, and n-butylbenzene, either as pure components or in mixtures, and a number of commercial refinery streams. The pure hydrocarbons used were the highest purity grade available. They were used without further purification. The commercial refinery streams were as follows: a full-range Persian Gulf naphtha (C6-165 "C) with composition and selected properties as shown in Table I; a full-range midcontinent naphtha (c6-185 "C), which had been reformed in the laboratory to a clear octane of 99.5 (the composition of the reformate is shown in Table 11) a light reformate from a midcontinent crude, which had been distilled in the laboratory to yield a C6-80 "C fraction and reformed to yield a C,+ reformate of 84 octane (R + 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 707 Table I. Composition and Selected Properties of C6-165 "C Persian Gulf Naphtha wt % c51.5 C6+ 98.5 paraffins 70.4 naphthenes 18.8 aromatics 10.8 av molecular wt research octane, R
+0
104.6 46.5
Table 11. ComDosition of Full-Range Reformatea wt%
paraffins c5 c6 c 7
C8+ naphthenes benzene toluene C8+ alkylbenzenes
5 5 8 I 2 1 7 20 50
0
a Obtained by reforming a full-range midcontinent naphtha (c6-185 "C) to 99.5 clear octane (R + 0).
+
Table 111. Composition of Light Reformate, 84 R 0 wt % wt % i-C, 10.05 c3 0.89 i-C4 1.45 C8+ paraffins 0.16 cyclopentanes 1.46 n-C4 1.48 i-C5 4.46 benzene 18.45 to1u en e 23.40 n-C5 3.54 n-C6 9.50 C8 aromatics 0.60 Cg aromatics 0.23 i-C6 21.40 n-C7 2.86 Clo+ aromatics 0.03
0). The concentration of key chemical components in the reformate is summarized in Table 111. C. Procedure. The reactions with pure hydrocarbons were studied in a microreactor system containing about 2 mL of 1.6" (1/16-in.)extrudate broken to lengths of about 2-3 mm. The reaction products were analyzed by gas chromatography to determine conversion and product yields. Experiments with commerical feeds were conducted in a 1.7-cm-i.d. downflow stainless steel reactor containing 62 cm3 of catalyst. The reaction was carried out at 1828-atm total pressure, at a 7:l molar ratio of H2/hydrocarbon, and over a temperature range of 315-482 "C. The gaseous product was analyzed by on-stream gas chromatography. The liquid product was analyzed by gas chromatography and mass spectroscopy. 111. Chemistry of M-Forming
A. Shape-Selective Conversion of Paraffins. Spatiospecific and configurational diffusion effects influence the relative rate of cracking of paraffins over ZSM-5. The rate of cracking of paraffins over ZSM-5 increases with the length of the molecules and decreases with the bulkiness of the molecules (Chen and Garwood, 1978). For octane boosting, this is a most desirable feature, because the octane rating of paraffinic molecules increases in a similar fashion. Figure 1 shows the relationship between the relative cracking rate of C5-C7 paraffins and their octane ratings (Chen et al., 1979b; Weisz, 1980). ZSM-5 has been shown to be an excellent acidic catalyst for olefin oligomerization (Chen et al., 1979a; Garwood, 1982; 1983). Along with oligomerization, the olefins disproportionate rapidly and redistribute to an equilibrium
20
40 60 80 Research Octane Number (R t 0 )
100
Figure 1. Relationship between relative cracking rate of C5-C7 paraffins and their octane ratings. Table IV. Products from n -Octane Cracking at 275 " C , 35 atm, 1 LHSV, and No Hydrogen wt%
methane ethane propene propane butenes isobutane n-butane isopentane n-pentane paraffins c6
c7 C8 monoolefins diolefins benzene
