Energy & Fuels 2006, 20, 439-445
439
Distillate Production by Oligomerization of Fischer-Tropsch Olefins over Solid Phosphoric Acid Arno de Klerk* Fischer-Tropsch Refinery Catalysis, Sasol Technology Research and DeVelopment, P.O. Box 1, Sasolburg 1947, South Africa ReceiVed October 21, 2005. ReVised Manuscript ReceiVed December 21, 2005
The conversion of Fischer-Tropsch derived olefins over solid phosphoric acid (SPA) is practiced commercially. Although it is mostly used for motor-gasoline production, it is also used to produce distillates to increase refinery flexibility. During distillate production, the distillate yield can be increased by recycling some of the naphtha from the process or by cofeeding naphtha-range Fischer-Tropsch olefins. The SPA catalyzed conversion of Fischer-Tropsch olefins and other olefinic naphtha feed materials was investigated at pilot plant scale in the range of 175-260 °C and 3.8 MPa. Contrary to expectations based on propene tetramermode operation, it was found that the distillate yield was limited to 70% when processing olefinic feed mixtures containing propene and naphtha. The weak interaction of long chain olefins with phosphoric acid mechanistically limited distillate production, and naphtha-range olefin oligomerization was disrupted by short chain olefins, like propene, which became the main carbocation source. Inhibition of the oligomerization reaction was also found when processing some of the oxygenate containing Fischer-Tropsch material. A processing methodology could be suggested to improve distillate selectivity by separate processing of the short chain and naphtharange olefins, but no productivity advantage was demonstrated.
Introduction Solid phosphoric acid (SPA) catalysts are not widely used for the production of distillates (C11-C22) by the oligomerization of olefins, apart from the conversion of propene to distillaterange products, which is also known as the tetramer-mode of operation.1,2 The propene tetramer is used for the manufacture of alkyl benzene sulfonate detergents, but its usage has declined considerably as a result of its replacement by detergents manufactured from linear alkyl benzenes.3 It is known that a distillate (tetramer) yield of 25-30% per pass can be expected4 and that the overall distillate yield is mainly controlled by recycling.5 In this way, an overall distillate yield of close to 90% can be obtained by recycling the propene dimers and trimers.1 The selectivity to the distillate is influenced by both the operating temperature and the SPA catalyst hydration state.4 Accounts of olefin oligomerization using SPA with the aim to produce distillates are almost absent from the literature. Yet, Sasol commercially uses SPA for the production of distillates from mixed C3-C5 Fischer-Tropsch olefins using the tetramer mode of operation.6 This mode of operation is used to increase * Corresponding author. Tel: +27 16 960-2549; fax: +27 11 522-3517; e-mail:
[email protected]. (1) Jones, E. K. Cumene and tetramer production. Petroleum Refin. 1954, 33(12), 186. (2) Asinger, F. Mono-olefins chemistry and technology; Pergamon: Oxford, 1968; p 188. (3) Kocal, J. A.; Vora, B. V.; Imai, T. Production of linear alkylbenzenes. Appl. Catal. A 2001, 221, 295. (4) Zhirong, Z.; Zaiku, X.; Yongfu, C.; Refeng, W.; Yaping, Y. Free phosphoric acid of diatomite-phosphate solid acid and its catalytic performance for propylene oligomerization. React. Kinet. Catal. Lett. 2000, 70, 379. (5) McMahon, J. F.; Bednars, C.; Solomon, E. Polymerization of olefins as a refinery process. In AdVances in Petroleum Chemistry and Refining; Kobe, K. A., McKetta, J. J., Eds.; Wiley: New York, 1963; Vol. 7; pp 319-320.
refinery flexibility with respect to the ratio of motor-gasoline to distillate production, which is driven by local market requirements. This paper explores the possibilities and describes the limitations of SPA as a catalyst for the production of distillates from Fischer-Tropsch olefins. The stability and reactivity of distillate-range products at high temperatures, conversion of naphtha-range Fischer-Tropsch derived olefins to distillates, and propene oligomerization in the presence of naphtha-range olefins will be covered. All work described in this paper was performed at a pilot plant scale at 175-260 °C and 3.8 MPa, which are of interest commercially. Experimental Procedures Materials. All feed materials were obtained from the Sasol Synfuels refineries at Secunda, South Africa (Table 1), and the origin of the various streams is shown in Figure 1. The olefinic distillate (feeds 1 and 2) as well as the C7-C9 olefinic naphtha (feed 3) were taken from the product stream of a CatPoly unit (SPA oligomerization of C3-C5 olefins). Feed 4 is a C5-C6 fraction of stabilized light oil (SLO), which is part of the primary FischerTropsch product. SLO is obtained after condensation of the oil fraction from Fischer-Tropsch synthesis, and it contains various oxygenates, like alcohols, carbonyls, esters, and carboxylic acids.7 In feed 4, the most abundant oxygenate is 2-butanone (MEK). The cold box (CB) condensate derived feeds (feeds 5 and 6) are also primary Fischer-Tropsch products but were obtained by cryogenic cooling after knock-out of the SLO8 and contain much less (6) Dry, M. E. High yield high quality diesel from Fischer-Tropsch processes. ChemSA 1984, 10, 286. (7) Steynberg, A. P.; Dry, M. E. Eds. Fischer-Tropsch technology; Elsevier: Amsterdam, 2004. (8) Steynberg, A. P.; Espinoza, R. L.; Jager, B.; Vosloo, A. C. Hightemperature Fischer-Tropsch synthesis in commercial practice. Appl. Catal. A 1999, 186, 41.
10.1021/ef0503459 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/28/2006
440 Energy & Fuels, Vol. 20, No. 2, 2006
de Klerk
Table 1. Mass Percentage Composition of Olefinic Feed Materials Used compound propene propane butenes butanes pentenes pentanes hexenes hexanes heptenes heptanes octenes octanes nonenes decenes undecenes dodecenes C13 + olefins oxygenates
Base case (C3 only)
feed 1 (distillate)
82 18
feed 2 (distillate + C3)
feed 3 (C7-C9 naphtha)
feed 4 (C5-C6 SLO)
feed 5 (C5-C6 CB condensate)
feed 6 (C5 + CB condensate)
5 1 27 5 48 9 8
1 2 2 24 1 52 5 13 6 31 32 31
1 2 52 3 29 5 6.5 1
37 10 26 2 6.5 1 6 0.5 5.5 3 2
6 29 30 29 3
0.5
0.5
Table 2. Base Case Propene and Olefinic Distillate Conversion over SPA at 3.8 MPa run
base case
feed (Table 1) temperature (°C) LHSV (h-1) propene conversion (%) C11-C14 converted to naphtha (%)a distillate content of product (%) paraffin content, excluding C3 (%) T95 boiling point (°C)
C3 only 185 1.4-1.5 65-68
a
Figure 1. Flow diagram of the high-temperature Fischer-Tropsch plant of Sasol Synfuels in Secunda, South Africa, showing the origin of the various feed streams used in this study.
oxygenates (0.5% vs 3% in the SLO-cut). The mixture of propene and propane used for the base case was also Fischer-Tropsch condensate derived. In all tests, a commercial solid phosphoric acid catalyst was used, which consists of a kieselguhr support (silica), which is coated with phosphoric acid. The active phase is the phosphoric acid, which is present as a glassy layer on the kieselguhr. The phosphoric acid that is bound to the support to form silicon phosphate is not catalytically active.9 The acid-type and content of the SPA catalysts were determined by X-ray diffraction and titration methods.10 The catalyst had the following properties: an ortho- to pyrophosphate ratio of 370:130, a total acid content of 68%, and a free acid content of 26% by mass. Equipment and Procedure. The pilot plant reactor consisted of a 1.0 m stainless steel tube with a 25 mm internal diameter. The reactor was encased in aluminum blocks and heated by three independently controllable electric heaters. The temperature in the catalyst bed was measured by a movable thermocouple inside a 6 (9) Krawietz, T. R.; Lin, P.; Lotterhos, K. E.; Torres, P. D.; Barich, D. H.; Clearfield, A.; Haw, J. F. Solid phosphoric acid catalyst: A multinuclear NMR and theoretical study. J. Am. Chem. Soc. 1998, 120, 8502. (10) Cavani, F.; Girotti, G.; Terzoni, G. Effect of water in the performance of the solid phosphoric acid catalyst for alkylation of benzene to cumene and for oligomerisation of propene. Appl. Catal. A 1993, 97, 177.
34 4 254
A
B
C
D
1 1 2 2 230 260 230 260 0.7 0.7 0.7 0.7 99 99 19 29 31 31 75 65 63 62 0.5 2.5 1 4 233 241 241 257
Feed contained 6% C10 naphtha.
mm centered thermowell. The catalyst (100 g) was loaded in its original physical form, and the catalyst bed was supported on a fine mesh stainless steel gauze. To avoid maldistribution, the voids were filled with sand (0.3-0.5 mm). Coarse sand (1.2-3.4 mm) was loaded on top of the catalyst bed to provide a preheating zone. The feed was delivered to the reactor by a positive displacement pump, and the liquid hourly space velocity (LHSV) was controlled by the pump rate. The pressure was controlled by a back-pressure regulator and was kept at 3.8 MPa for all the tests. The product was cooled and collected for analysis by gas chromatography. Distillate yields were obtained by batch fractionation of the product in glass columns. Unless otherwise stated, density values were determined at 20 °C and viscosity values at 40 °C. Mass balance closure for all runs was between 97 and 103%. Calculations. The distillate is defined as the material boiling in the range of 177-360 °C, which roughly corresponds to the carbon number range C11-C22. Catalyst hydration has been calculated from the feedwater content and operating temperature using the vapor pressure data of Brown and Whitt11 since it is still the recommended data set for process design.12 This is an important parameter in SPA catalysis since the activity and selectivity of the catalyst are both influenced by the level of catalyst hydration.4,10,13-14
Results Propene Feed. A propene once-through operation (i.e., without a recycle like in tetramer-mode) was used as a base case (Table 2). The feed was a propene (82%) and propane (18%) mixture, and the reactor was operated at 185 °C, 3.8 MPa, and LHSV of 1.4-1.5 h-1. Propene conversion was 65-68%, with a distillate selectivity of 34% and therefore a distillate yield of 22-23%. The catalyst was stable over the test period lasting (11) Brown, E. H.; Whitt, C. D. Vapor pressure of phosphoric acids. Ind. Eng. Chem. 1952, 44, 615. (12) Chowdhury, A. K.; Liu, M.-B.; Gulbenkian, A. Mass spectrometric studies of vaporization of phosphoric acids. Ind. Eng. Chem. Res. 1993, 32, 989.
Oligomerization of Fischer-Tropsch Olefins
Energy & Fuels, Vol. 20, No. 2, 2006 441
Table 3. Olefinic Naphtha (Feed 3) Conversion over SPA at LHSV 0.6 h-1 and 3.8 MPa
Table 4. Conversion of Different Fischer-Tropsch Derived Olefinic Naphtha Materials over SPA at 3.8 MPa
run
E
F
G
H
run
I
J
K
L
temperature (°C) catalyst hydration (% H3PO4) carbon on catalyst after 180 h (mass %) C7 olefin conversion (%) C8 olefin conversion (%) distillate yield (%) distillate selectivity (%) T95 boiling point (°C) distillate density, hydrogenated (kg m-3) distillate viscosity, hydrogenated (cSt) distillate cetane number, hydrogenated
200 104 2.5 39 26 18 75 303 798 2.1 30
200 109 1.5 48 38 24 75 300 797 2.1 29
230 107 3.9 50 42 26 76 284 796 1.8 31
260 108 2.6 59 50 36 88 281 809 1.8 29
feed (Table 1) feed type temperature (°C) LHSV (h-1) C5 olefin conversion (%) C6 olefin conversion (%) distillate yield (%) distillate selectivity (%) T95 boiling point (°C) distillate density, olefinic (kg m-3) distillate viscosity, olefinic (cSt) distillate cetane number, hydrogenated
4 C5-C6 SLO 175 1.4 40 14 6 36 271 827
5 C5-C6 CB 180 1.6 65 26 28 67
6 C5 + CB 180 1.4 83 42 20 49 244 803
6 C5 + CB 210 1.6 93 69 26 50 241 801
1.7
1.8
4 weeks, despite the low hydration level (108% H3PO4) of the catalyst. The product had a distillate range T95 boiling point (temperature at which 95 vol % of the product had been distilled off), and no gums or other heavy organic material were detected. Distillate Feed (C11-C14 Olefins). The stability of olefinic distillate against cracking at temperatures above 225 °C, the recommended upper limit for tetramer-mode operation,1 was investigated to determine the possibility of operating at higher temperatures. The ability of distillate-range olefinic feed to incorporate propene without producing naphtha was also of commercial interest, as well as the extent of distillate conversion to heavy oligomers (gums) and subsequent catalyst deactivation.15 The conversion of C11-C14 olefinic distillate (feeds 1 and 2), which contained 6% naphtha, was evaluated at 230 and 260 °C in both the absence and the presence of propene (Table 2). Propene addition had a negative influence on the distillate yield, causing more of the distillate-range products to crack to naphtharange products (at 230 °C, the naphtha increased from 19 to 31%). Increased hydrogen transfer activity could be seen when the operating temperature was increased from 230 to 260 °C and when propene was co-fed. The catalyst was monitored for deactivation by returning to standard conditions and monitoring propene conversion after each run. No catalyst deactivation was observed over the 4 week test period, despite the catalyst being operated at a hydration level of 108% H3PO4 and higher. Naphtha Feed (C7-C9 Olefins). In the tetramer-mode of operation, the naphtha-range product (C9 trimer) was recycled to the reactor to react with propene to produce more distillaterange product (C12 tetramer). This is done under the tacit assumption that the propene would react with the naphtha-range olefins as is indeed required to boost distillate production. To verify this assumption, the contribution of olefinic naphtha-range material to the reaction was considered in the absence of propene. The conversion of the C7-C9 olefinic naphtha, produced from short chain olefin oligomerization (feed 3), was evaluated at 200-260 °C without any propene addition (Table 3). The conversion values listed for C7 and C8 olefins should be seen as estimates because the number of compounds and isomers formed during the reaction makes it difficult to unambiguously interpret the chromatograms. It is nevertheless clear that distillate formation from the olefinic naphtha took place and that both high temperature and low catalyst hydration benefited distillate (13) Langlois, G. E.; Walkey, J. E. Improved process polymerizes olefins for high-quality gasoline. Petroleum Refin. 1952, 31(8), 79. (14) Vinnik, M. I.; Obraztsov, P. A. Determination of water vaporization rate from phosphoric acid catalyst according to the change of its catalytic activity. Kinet. Catal. 1983, 24, 752. (15) Egloff, G.; Weinert, P. C. Polymerization with solid phosphoric acid catalyst; Proceedings of the Third World Petroleum Congress: 1951; Vol. IV, p 201.
1.9 25
production. The carbon content on the catalyst after 180 h of operation for each run showed no clear trend, and no catalyst deactivation was noticeable. The distillate fractions of the products were hydrogenated over a sulfided base metal catalyst to saturate the olefins. The hydrogenated distillates had similar viscosities (1.8-2.1 cSt at 40 °C), cetane numbers (29-31), and densities (796-798 kg m-3), except for the product of run H at 260 °C, which had a higher density (809 kg m-3). Naptha Feed (C5-C6 Fischer-Tropsch Olefins). The cracking propensity of C7+ material is high compared to that of C5-C6 material.16 The ability of a C5-C6 naphtha fraction to produce distillates in the absence of propene would therefore depend on its ability to oligomerize since direct cracking of C5-C6 to produce short chain olefins was shown not to proceed over SPA at the operating temperatures investigated.17 The reaction of different Fischer-Tropsch derived C5-C6 and C5+ olefinic naphthas in the absence of propene was therefore evaluated (Table 4). The highest distillate yield was obtained from the C5-C6 rich condensate feed (run J). The SLO derived material (run I) produced a significantly lower distillate yield than the condensate derived material, which can be related to the difference in oxygenate content. The oxygenates present in the different feed materials tested were converted over the SPA catalyst, and the oligomerization product had only traces of oxygenates present. Some oxygenates can produce water by acid-catalyzed reactions, like dehydration of alcohols, and cause the catalyst hydration to increase significantly. However, it needs to be pointed out that the most abundant oxygenate in the feed materials tested was 2-butanone. A test reaction with condensate-derived feed spiked with 2% 2-butanone was done and compared to the conversion data without the oxygenate addition. At 180 °C, 3.8 MPa, and LHSV of 1.4 h-1, all of the 2-butanone was converted, but no difference could be seen between the results with and without addition of this oxygenate. Further work is currently in progress in our laboratories to determine the effect of oxygenates on acidcatalyzed olefin conversion processes. Propene and Olefinic Naphtha Mixed Feed. From the preceding results, it was clear that olefinic naphtha on its own is capable of being converted to distillates over SPA. However, when a feed containing mixed olefins of different chain lengths was contacted with the SPA catalyst, the shorter chain olefins (16) Buchanan, J. S.; Santiesteban, J. G.; Haag, W. O. Mechanistic considerations in acid-catalyzed cracking of olefins. J. Catal. 1996, 158, 276. (17) De Klerk, A. Oligomerization of 1-hexene and 1-octene over solid acid catalysts. Ind. Eng. Chem. Res. 2005, 44, 3887.
442 Energy & Fuels, Vol. 20, No. 2, 2006
Figure 2. Conversion of propene and different olefinic naphtha mixtures over SPA at LHSV of 1.4-1.6 h-1, 3.8 MPa, 180 °C (9), and 195 °C ()).
always had a higher conversion (Tables 3 and 4). Propene is more reactive than the C5 and heavier olefins,18 and it was therefore reasoned that when propene is co-fed with olefinic naphtha, conjunct polymerization would result and the propene would have the highest conversion. A series of test runs was done with different mixtures of propene, C5-C6 condensate derived naphtha (feed 5), and CatPoly C7-C9 naphtha (feed 3) at 180-195 °C, 3.8 MPa, and LHSV of 1.4-1.6 h-1 (Figure 2). Catalyst hydration was 104-105% H3PO4. It was found that olefin conversion decreased with increasing carbon number, with the average conversion over all the test runs being propene 83%, pentenes 80%, and hexenes 49%. The distillate correlated best with the propene-derived oligomer content of the product. This does not imply that the distillates were only derived from propene but that the distillates were formed predominantly by propene reacting with propene and longer chain olefins. Discussion Reactions and Mechanism. The network of acid-catalyzed olefin reactions is well-documented in literature.19 The discussion that follows will therefore deal with this reaction network only in general terms and will focus instead on SPA specific aspects. The main acid-catalyzed reactions of olefins taking place over SPA in order of increasing difficulty are double bond isomerization, skeletal isomerization (C5 and heavier), oligomerization, cracking (C7 and heavier), and hydrogen transfer. Thermodynamically, double bond isomerization (-∆Hr ) 6-12 kJ mol-1), skeletal isomerization (-∆Hr ) 3-16 kJ mol-1), and oligomerization (-∆Hr > 60 kJ mol-1), which are exothermic, will be favored by low temperature, while cracking, which is endothermic, will be favored by high temperature. However, these reactions are not equilibrium limited at the reaction conditions studied. The rate of these reactions is determined by kinetic considerations, and the reaction mechanism over SPA plays an important role in determining which reactions are favored at a given set of operating conditions. Catalytic conversion over SPA does not follow the classic Brønsted acid-catalyzed mechanism but rather a mechanism involving the formation of phosphoric acid esters,20,21 with (18) De Klerk, A.; Engelbrecht, D. J.; Boikanyo, H. Oligomerization of Fischer-Tropsch olefins: Effect of feed and operating conditions on hydrogenated motor-gasoline quality. Ind. Eng. Chem. Res. 2004, 43, 7449. (19) Corma, A. Inorganic solid acids and their use in acid-catalyzed hydrocarbon reactions. Chem. ReV. 1995, 95, 559. (20) Farkas, A.; Farkas, L. Catalytic polymerization of olefins in the presence of phosphoric acid. Ind. Eng. Chem. 1942, 34, 716. (21) Schmerling, L.; Ipatieff, V. N. The mechanism of the polymerization of alkenes. AdV. Catal. 1950, 2, 21.
de Klerk
Brønsted acidity contributing to a lesser degree.22 Over SPA, branched olefins are more reactive than linear olefins, similar to the situation with Brønsted acid-catalyzed reactions, but unexpectedly, linear C4 olefins are more reactive than either shorter or longer chain olefins.13,18 This can be explained in terms of the stability of the phosphoric acid esters, which determine how strongly the olefin is bonded to the phosphoric acid, and the time period that the phosphoric acid can polarize the olefin.22 The steric bulk of the molecule and the apolar nature of the alkyl chain decreases its adsorption on the polar catalyst surface. Both of these effects increase with carbon chain length. The lifetime of the polarized molecule can be extended if the double bond is attached to a tertiary carbon because a tertiary carbon results in a more stable polarized intermediate, as would be expected from relative carbocation stability, increasing in the order primary < secondary < tertiary.23 This has implications for the carbon number distribution obtainable over SPA. Long chain olefins have a short period of interaction and polarization with SPA. As a result, the monomolecular cracking reaction of the olefins is favored over oligomerization, which is a bimolecular reaction. It is therefore expected from the mechanism over SPA that the carbon number distribution will be capped and that it would be difficult to form very heavy oligomers. The present data supports this interpretation (Tables 2-4) since the T95 boiling point seldom exceeds 300 °C, which is in line with previous reports published by UOP.15,24-28 Furthermore, because the interaction of the olefins with SPA becomes increasingly weaker and more short-lived with increasing chain length, oligomerization to produce distillates is not favored. Ester stability also has implications for the competitive reactions of the olefin mixtures. It is known that propene forms very stable esters with phosphoric acid.29 In an olefin mixture containing propene, the propene will be strongly adsorbed onto the catalyst, and as a result, it is likely that the surface concentration of propene would be higher than anticipated from its bulk concentration. The probability of propene on propene reactions is therefore high. This was indeed seen in the test runs with mixed propene and olefinic naphtha feeds. Effect of Hydration and Temperature. In a SPA catalyst, the acid strength and proton density does not have a simple relationship with the phosphoric acid concentration. The composition of the active phase, the phosphoric acid layer on the kieselguhr support, is determined by its water content. The phosphoric acid concentration is expressed as an equivalent percentage H3PO4 (% H3PO4 ) mass % P/0.316). However, 100% H3PO4 does not consist of a single species but a mixture of water, orthophosphoric acid (H3PO4), and pyrophosphoric acid (H4P2O7).30-33 This has been ascribed to the persistence (22) De Klerk, A. Reactivity differences of octenes over Solid Phosphoric Acid. Ind. Eng. Chem. Res. 2006, 45, 578. (23) March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley: New York, 1985; pp 942-954. (24) Ipatieff, V. N.; Egloff, G. Polymer gasoline has higher blending value than pure isooctane. Oil Gas J. 1935, May 16, 31. (25) Ipatieff, V. N.; Corson, B. B.; Egloff, G. Polymerization, a new source of gasoline. Ind. Eng. Chem. 1935, 27, 1077. (26) Egloff, G. Polymer gasoline. Ind. Eng. Chem. 1936, 28, 1461. (27) Egloff, G.; Morrell, J. C.; Nelson, E. F. Motor fuels from polymerization. Refin. Nat. Gasoline Manuf. 1937, 16, 497. (28) Shanley, W. B.; Egloff, G. Midget Poly units. Oil Gas J. 1939, May 18, 116. (29) Ipatieff, V. N. Catalytic polymerization of gaseous olefins by liquid phosphoric acid. I. Propylene. Ind. Eng. Chem. 1935, 27, 1067. (30) Bell, R. N. Composition of strong phosphoric acids. Ind. Eng. Chem. 1948, 40, 1464. (31) Huthi, A.-L.; Gartaganis, P. A. The composition of the strong phosphoric acids. Can. J. Chem. 1956, 34, 785.
Oligomerization of Fischer-Tropsch Olefins
of the hydrate, (H3PO4)2‚H2O,34 but it is more likely due to the self-dissociative behavior of phosphoric acid (2 H3PO4 T H4P2O7 + H2O).35,36 The self-dissociative behavior of phosphoric acid is also responsible for creating Brønsted acid sites on the catalyst, which causes reactions to occur via both carbocation and ester formation pathways. Furthermore, the water content is not fixed but determined by the vapor-liquid equilibrium between water and phosphoric acid.11,37-38 The active phase of the catalyst consequently changes during the reaction, depending on the feedwater content and operating conditions, with both temperature and degree of hydration affecting the catalyst at a fundamental level. The distillate yield from olefinic naphtha increased with increasing temperature and decreasing degree of catalyst hydration (Table 3), but the distillate selectivity of runs E-G was similar (75-76%) and did not follow the upward trend expected from previous studies with propene as feed.4 Neither temperature nor catalyst hydration affected distillate selectivity during the conversion of olefinic naphtha in the range of 200-230 °C and 104-109% H3PO4. Higher temperatures did have an effect though, and the product of run H at 260 °C had a higher density, indicative of hydrogen transfer and aromatic formation, as well as higher distillate selectivity (88%). Much lower distillate selectivities were recorded for the oligomerization of FischerTropsch derived naphtha and propene (Figure 2), but this was not due to hydration or temperature, but the feed, as will be explained in the following section. Effect of Feed Composition. In commercial tetramer-mode operation,1 the reactor is typically operated at 170-225 °C and a combined feed space velocity of 1.0-4.7 h-1. The propene derived dimers and trimers are recycled to achieve an overall tetramer yield of 87% based on the fresh propene feed, and a propene based tetramer selectivity close to 95% is achievable at 92% propene conversion. It can therefore be said that in tetramer-mode, the motor-gasoline fraction can be recycled to extinction. It should also be noted that propene oligomers are quite resistant to cracking (depolymerization),39-41 whereas the oligomers of butene, for example, are known to crack quite readily.40-42 This is due to the skeletal structure of the products,43 with propene derived oligomers likely to crack (32) Ohashi, S.; Sugatani, H. The composition of strong phosphoric acids. Bull. Chem. Soc. Jpn. 1957, 30, 864. (33) Jameson, R. F. The composition of the strong phosphoric acids. J. Chem. Soc. 1959, 752. (34) Fontana, B. J. The vapor pressure of water over phosphoric acids. J. Am. Chem. Soc. 1951, 73, 3348. (35) Munson, R. A. Self-dissociative equilibria in molten phosphoric acid. J. Phys. Chem. 1964, 68, 3374. (36) Munson, R. A. Kinetics of the ortho-pyro interconversion in 100% phosphoric acid. J. Phys. Chem. 1965, 69, 1761. (37) Fontana, B. J. The vapor pressure of water over phosphoric acids. J. Am. Chem. Soc. 1951, 73, 3348. (38) MacDonald, D. I.; Boyack, J. R. Density, electrical conductivity, and vapor pressure of concentrated phosphoric acid. J. Chem. Eng. Data 1969, 14, 380. (39) Sharrah, M. L.; Feighner, G. C. Synthesis of Dodecylbenzene. Synthetic detergent intermediate. Ind. Eng. Chem. 1954, 46, 248. (40) Lee, R. J.; Knight, H. M.; Kelly, J. T. Production of t-butyltoluene by depolyalkylation. Ind. Eng. Chem. 1958, 50, 1001. (41) Patinkin, S. H.; Friedman, B. S. Alkylation of aromatics with alkenes and alkanes. In Friedel-Crafts and related reactions. Vol. II. Alkylation and related reactions; Olah, G. A., Ed.; Wiley: New York, 1964; pp 5456. (42) Ipatieff, V. N.; Pines, H. Alkylation accompanying depolymerization. J. Am. Chem. Soc. 1936, 58, 1056. (43) Terres, E. Beitrag zur kenntnis der chemischen struktur der polymerisationsprodukte von olefinen niedrigen molekulargewichtes mittels phosphorsa¨ure. (Contribution to the knowledge of the structure of the polymerization products of olefins of low molecular weight with phosphoric acid). Brennstoff Chem. 1953, 34, 355.
Energy & Fuels, Vol. 20, No. 2, 2006 443
yielding fragments with integral multiples of propene units. This creates the impression that propene oligomers are resistant to cracking, with only the products that skeletally rearranged yielding cracking products that can be distinguished from the propene oligomers. The addition of propene to olefinic naphtha, rather than propene derived oligomers, significantly affects the distillate yield (compare Tables 3 and 4 with Figure 2). At 180-185 °C, 3.8 MPa, and LHSV of 1.4-1.6 h-1, the distillate yield of the propene only reaction (22-23%) and condensate-derived olefinic naphtha only reaction (20-28%) are both higher than that obtained with a mixed propene and olefinic naphtha feed (
