Ind. Eng. Chem. Res. 2000, 39, 267-271
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Dimerization of n-Butenes for High Octane Gasoline Components Michael Golombok* and Jacques de Bruijn Shell International Oil Products, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
Dimerization of linear olefins represents an attractive route for the production of high octane number blending components. The oligomerization needs not only to be high conversion and to produce mainly dimers but also to be selective within the dimer range, as only certain isomers have advantageous blending octane numbers. Moreover, the octane numbers of these dimers blend in a highly nonlinear fashion. Normally the focus for such gasoline polymerization processes is on maximizing branching; however, in this study we seek to maximize the fraction of allylic hydrogens, because we have shown that these are directly linked to the blending octane number of the species. 1-Butene and 2-butene produce different quality dimer gasoline. This unexpected result can be explained from a consideration of the isomerization kinetics between the butene monomers. 1. Introduction The manufacture of high octane gasoline blending components is particularly attractive when it is coupled to the upgrading of a relatively low value component such as one of the subsidiary products of a refinery or chemical plant process stream. In addition, gas to liquid conversions are desirable for reasons of transport and storage. One way of adding value is to convert a gas sidestream into a high octane number liquid. Typical feeds are the butene (so-called “BB”) streams emerging from steam or catalytic crackers. These have become particularly attractive because very recently methyl tertbutyl ether (MTBE)sthe standard high octane oxygenate blending componentshas been outlawed in California, and the trend is expected to continue the usual expansion eastward. This is resulting in a return to previously evaluated processes for manufacturing high octane gasoline componentssand one of the most favored of these are butene dimers in the form of branched octenes. One of the oldest commercial processes for making “polygasoline” is the CATPOLY process of UOP using phosphoric acid on kieselguhr.1 Temperatures are sufficiently high (200 °C) that the oligomerization is clearly not selective to the particular isomers associated with high octane value (see below). The high acid conditions and the use of phosphoric acid were superseded by the OCTOL and CATCON processes which convert a mixture of isobutene and n-butene to C8 olefins with a relatively high degree of branching.2 The process has a high conversion (up to 90%) yet maintains a high selectivity to dimers of 85%. Work on various zeolites has been directed at optimizing catalyst preparation to get the best gasoline yield,3,4 and recent work has successfully modeled the oligomer distribution that is obtained.5 However, there has been little focus on optimizing the product mixture for best product end use in an engine. The main issues that emerge from a survey of previous work is that much of it is chemicals directed.6-8 The octane properties do not emerge as important parameterssat least in explicit * Corresponding author currently with Shell International Chemicals at the same address. Tel: 31 20 630 2794. Fax: 31 20 630 8004. E-mail:
[email protected].
Figure 1. Relationship between octane number of a pure component, blend, and base fuel.
descriptions of this work. The OCTOL process is geared toward high isoindex.2 Isoindex is a parameter developed by UOP-Huls to describe the average properties of mixtures of gasoline molecules. It measures the branching of species be weighting it by the mole content xi of species i
η)
∑ηixi
(1)
where the average branching is defined by the presence of primary, secondary, or tertiary carbons on a molecule. Thus, for n-octane η ) 1, for methylheptane (and ethylhexane) η ) 2, and for trimethylpentane η ) 3. A 1:1:1 mixture would have η ) 2, thus representing the average branching of the molecules. However, we have previously shown that, whereas branching is a good measure of octane number for paraffinic components, it fails for olefins.9 This is because the best value of the octane boost provided by an olefin comes from its blending octane number, which describes the nonlinear blending as shown in Figure 1. From this, we obtain the blending octane number as
1 BON ) ONref + (ONbl - ONref) f
(2)
where ONref is the octane number of the base fuel and ONbl is the octane number of the blend containing fraction f of the component whose pure octane number is ON. (There are two ways of measuring octane
10.1021/ie9906060 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/07/2000
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Figure 2. Distribution of BRON for C8 olefins as a function of branching.
number: the research octane number RON and the motor method MONsin this study we use the former; thus, we use the measurement of research octane number RON and also an extrapolation measurement to blending research octane number BRON.) High octane number is the real goal for gasoline dimerization, but because it is difficult to measure, a number of alternative measures have been used, such as the isoindex. The problem with isoindex is that it is more a measure of the pure octane number ON, and fuels with quite different BON values have nearly identical ON values, even within the range of octene isomers. This is shown in Figure 2 where we have collated data for BRON as a function of the isoindices of 72 octene isomers.10 This clearly indicates that the much quoted isoindex is not a good indicator of blending octane number. The goal in our current study is to see if lower value linear butenes can be dimerized to give a distribution of dimethylhexenes in the upper range of the indicated BRON values. This would then compete with the diisobutylenes but would involve a more economic feedstock. There has been little reported work to date on such a selective dimerization. This is because the main difficulty is the barrier set by finding an effective temperature range for the reaction. If the temperature is too low, then conversion is low. If the temperature is too high, then skeletal isomerization occurs and the selectivity to high BRON isomers is low. In this study we take as our starting point the result of a screening of numerous catalysts using a 1-butene feed. Amorphous silica alumina is a good beginning point for such a study as it produces a lower amount of linear dimers than, for example, zeolite systems. The major problem in optimizing a reaction for BRON is that this is not a quantity which can be measured rapidlys this is the reason for the use of other indicators such as the isoindex as well as the allylic hydrogen fraction, which we have shown9 to be a rapid basis for assessing the blending octane number as opposed to engine tests which are time-consuming and require on the order of 1 L. We establish optimum conditions for obtaining a high yield of high BRON product and compare this for a number of different feeds. 2. Experimental Section Linear butenes were fed undiluted to a microflow reactor column consisting of a stainless steel tubular diameter of length 300 mm and diameter 10 mm. Typical conditions were 35 bar and 120 °C. The elevated pressure is necessary as the monophase conditions also increase selectivity to dimers.6 A further pressure increase does not affect the reactions. Between 80 and 180 °C, all C4’s are present as liquids. The reactor was operated in downflow mode as this minimizes the effect of backmixing. Helium was added as a tracer gas for mass balancing purposes.
The amorphous silica alumina catalyst (ASA) as used in this study was prepared from a batch of ASA powder. The powder was compressed to large particles of ASA which were subsequently ground to a mesh size fraction of 30-80 because there were no diffusion limitations on mass transfer for that size. The catalyst was calcined for 1 h at 500 °C and had a BET surface area of 390 m2/g and a pore volume of 0.66 mL/g. Impurities such as iron cations are at a negligible level compared to the number of acidic sites on the catalyst surface. A total of 0.5-3.0 g of catalyst was used in the butene oligomerization experiments. The reactor was placed in an oven which was preheated to the desired reaction temperature. After the reactor was filled with catalyst and put into place, it was purged with 6 NL/h of nitrogen for approximately 1 h, during which the reactor was heated from room temperature to the reaction temperature. Components up to C5 were separated using a KCI plot column and detected by FID with a preliminary thermocouple detector for helium and methane. After the analyses the column was backflushed to remove heavier components injected with the gas sample onto the column. The liquid products were analyzed by GC. We used the standard true boiling point analysis (TBP ASTM D2887) and a more sophisticated GC analyses where the components were also separated by boiling point but where the separation was more highly resolved so that individual components could be identified. We defined the dimer fraction as the fraction having a boiling point between 60 and170 °C. We took the trimer fraction between 170 and 230 °C and the tetramer fraction between 230 and 300 °C. The integration areas agreed with the results from the more sophisticated GC analyses. For branching analysis the olefins were hydrogenated to identify the mono-, di-, and tribranched paraffins through a high-resolution GC analysis. The blending octane number was determined by engine tests. As mentioned above, this requires blending in a primary reference fuel followed by an engine rating procedure. Apart from being time-consuming, this also requires a considerable amount of samplesrather more than can be readily obtained from a microflow reactor in a short time. We have previously shown that the allylic hydrogen fraction is a good measure of the BRON. This parameter is defined by
R ) nal/ntot
(3)
where nal is the number of allylic hydrogens and ntot is the total number of hydrogens on a molecule. R is easily obtained from an NMR spectrum, as we have previously described.9 This requires a much smaller amount of sample than an engine test and makes it possible to assess the BRON resulting from particular reaction conditions very quickly. 3. Results and Discussion Table 1 summarizes the runs on pure C4 feeds over the ASA-13 catalyst at 120 °C and weight hourly space velocity (WHSV) 8 kg/kg h. The important parameters are the BRON and single-pass conversion. From Table 1 we see that 1-butene gives a good conversion to a high quality dimer mix of BRON in the high 130s. (We have also shown that this performance can be maintained for a period of 150 h.) Figure 3 shows the relationship between the WHSV of the butene feeds at constant
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Figure 3. Conversion with dimer selectivities and the allylic hydrogen fraction (a) for 1-butene at various WHSVs (T ) 120 °C) and (b) for 2-butene conversion at 140 °C and various WHSV values. Table 1. Summary Data from This Study for Comparing Dimerization of C4 Olefins over 13% Alumina Containing Amorphous Silica Alumina at 120 °C and WHSV 8 kg/kg h component conversion (% wof) BRON
1-C4d
2-C4d
i-C4d
85 138
35 110
95 145
temperature and (a) the mass conversion per pass, (b) the mass selectivity for the split between dimers, trimers, and tetramers, and (c) the allylic hydrogen fraction R as a measure of BRON. Up to a WHSV of 20 kg/kg h or so the yield of dimers from 1-butene is reasonably constant. If we wish to optimize the process, we need to take into account the BRON quality as represented by Rsand we find that the conversion at WHSV 8 kg/kg h is optimum for obtaining the best yield of a good quality dimerate. Depending on the catalyst batch preparation, we may thus say that optimum conversions of typically 70-80% yield the total liquid product of BRON of 135-138 (confirmed from engine rating) over protracted periods. On the other hand, we expect there to be a difference in the reactivity of the butene monomers. 2-Butene is more stable than 1-butene. To get comparable levels of conversion for 2-butene, the temperature had to be raised to 140 °C. Over the whole range the conversions are lower than those for 1-butene. Similarly the yield of the dimers remains relatively constant as WHSV is increased up to 25 kg/kg h but thereafter decreases, although the conversion decreases throughout. These differences (between linear butene monomer performance) are surprising. In any butene stream, the internal linear olefin 2-butene predominates for thermodynamic reasons. Given the acidic nature of the ASA catalyst and the relative ease of isomerization, it is surprising to find that 2-butene yields a considerably
Figure 4. Conversion, dimer fraction, and allylic fraction for (a) 1-butene dimerization with temperature variation at WHSV ) 50 kg/kg h and (b) 2-butene dimer product as a function of temperature at constant WHSV ) 8 kg/kg h.
poorer quality dimer product (of around BRON 110) for gasoline purposes. Given the acidity of the catalyst, we expect the adsorbed form of both linear butene isomers to be identicalsnamely, the secondary butene carbenium ion. In addition, we would expect immediate equilibration of both pure feeds to the equilibrium double bond distributionsnamely, 95% internal olefins following contact with a heated acidic catalyst. Subsequent reactions should be the same irrespective of which butene double bond isomer was the monomer feed. However, we see that there is a clear difference, which is explored below. The effect of temperature is seen in Figure 4 where we vary the temperature during runs of both butene monomers. The conversion of 1-butene is seen to be very sensitive to temperatures between 100 and 160 °C. Both the dimer mass fraction and the allylic ratio fall off above 120 °Csthe two, in fact, will be related because decreasing the mass fraction of dimers means that heavier oligomers are being formed with the result that the BRON is degraded. The slight increase we observe at the top of the temperature range over 200 °C could be due to the onset of catalytic cracking of trimers and tetramers. It does not actually indicate a better dimer yield as the allylic ratio continued to decrease throughout. Over the range of temperatures studied, we see a drop of BRON from 137 at 120 °C to 106 at 206 °C, as obtained from engine tests. The reason for the falloff in BRON quality is that, as the temperature increases, the dimerization gets less selective; i.e., more lower BRON species are produced. Similarly, the dimer selectivity goes down much more quickly with increasing temperature for 2-butene than for 1-butene. The responses are similar in form to 1-butene but occur at a somewhat higher temperature so are less selective and result in poorer quality gasoline components. Is there any significant change in the range of structural (as opposed to double bond isomers of the same structure) dimer isomers produced within the
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Figure 5. Branching and double bond isomer composition for the dimerate product at 70% conversion as a function of WHSV and temperature. MH ) methylheptenes; TMP ) trimethylpentenes; DIB ) diisobutylene; DMH ) dimethyl hexenes.
possible 72 octenes discussed above? We address this problem by hydrogenating the molecules and looking at their saturated structural isomerssbecause as the number of double bond isomers identifiable without a very exhaustive analysis is limited. Samples were taken from a run where the conversion was held constant at 70% and WHSV and temperature were varied. The analysis of the dimer component of these varying runs, with the associated reaction conditions, is shown in Figure 5. Over all of these runs, the dimer selectivity only varied from 65 to 71% so that it will only be a second-order effect. This is partially reflected in the allylic hydrogen fraction R, showing that the BRON quality of the fuel decreases somewhat even though the turndown ratio on the feed rate was on the order of 50 whereas the temperature only increased overall by 80 °C. (A corresponding check on the BRON values showed that these were in the range 133-135.) It is presumably the temperature, therefore, which is responsible for the relative decrease in R of the dimers which are 3,4dimethylhexenes. From thermal equilibrium considerations, we expect that higher temperatures will tend to reduce the concentration of higher branched isomers, leading to a decrease in the isoindex. Figure 5 shows that, in general, over the temperature regimes of interestsnamely, the region 120-160 °C where significant linear butene conversion occurs then there is relatively little variation in the dimer composition makeup. 3,4-Dimethylhexenes (DMH) predominate with a small amount of trimethylpentenes (TMP) split and methylheptenes (MH)sno linear octenes were found. In summary, lower temperatures preserve the primary products (3,4-dimethylhexenes) while temperatures above 120 °C give rise mainly to methyl shift reactions which are relatively harmless from the BRON point of view. The most important aspect of the studies with 1- and 2-butene is the fact that, whereas the liquid product is different under similar conditions, the off gas composition remains relatively constantsclose to the equilibrium level of a 2-butene/1-butene ratio of 13, as one would expect for a temperature of 140 °C. Figure 6 shows the result of an equilibrium calculation where we have plotted the equilibrium linear butene composition over a range of temperatures. As qualitatively expected, 1-butene is a minority component. Double bond isomerization is a relatively facile process so it is not surprising that the offgas is at equilibriumswhat is surprising is that the liquid-phase products are different.
Figure 6. Butene isomer equilibrium distributions as a function of temperature.
A similar phenomenon (from the chemical not the octane viewpoint) has been seen with NiO catalysts, however, that is attributed to two alternative routes for dimerizationsin addition to the usual carbenium ion mechanism, there is also a coordination of the double bond to the metal.11 This latter mechanism would preserve a unique conformational pathway for the external as opposed to the internal olefin. However, this possibility does not exist for the ASA catalyst because no metal coordinating center is present. Thus, the different dimerization behaviors of 1-butene and 2-butene are related to the different thermodynamics of adsorption of the codimerizing monomers (e.g., based on heats of adsorption in zeolites, the isobutene reactivity is 8 times that of 2-butene). There are numerous examples in the literature of this having been observed12-14 (e.g., preferential formation of cis-2-butene during isomerization), and the simplest explanation is provided by a consideration of the thermochemical kinetics.15 Of the two reactions it is the first which has
the lower activation energy because the 1,2 methyl shift involves breaking only a normal paraffinic C-C bond whereas a similar shift for 2-butene involves breaking an allylic C-C bond which means an extra roughly 30 kJ/mol activation energy increase to reach the delocal-
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ized allylic intermediate. 1-Butene thus forms isobutene much more quickly because the drive to equilibrium is also much faster than that for 2-butene (see Figure 6). Both homo- and codimerization of the isobutene and 1-butene will produce a very high BRON quality dimerate. Figure 6 shows that a mixture already containing 2-butene will not change that much in composition. This is the reason that 1-butene produces a much better product than 2-butene as shown by the high BRON values of the diisobutylenes. It just so happens that we are operating in a zone where selective dimerization to a favorable octene isomer can take place. We are currently exploring the limits of severity of operation, whereby the selectivity of the distribution of octene isomers formed during the process can be optimized to give the highest blending octane number. Literature Cited (1) Chauvel, A.; Lefebvre, G. Petrochemical Processes; Technip: Paris, 1992; Vol. 1. (2) Nierlich, F. Integrated tert-butyl alcohol/di-n-butenes production from FCC C4’s. Erdoel, Erdgas, Kohle 1987, 103 (11), 486. (3) Kojima, M.; Rautenbach, M. W.; O’Connor, Cryil. T. Butene oligomerization over ion-exchanged mordenite. Ind. Eng. Chem. Res. 1988, 27, 248. (4) Tabak, S. A.; Krambeck, F. J.; Garwood, W. E. Conversion of propylene and butylene over ZSM-5 catalyst. AIChE J. 1986, 32 (9), 1526. (5) Ngandjui, L. M. T.; Thyrion, F. C. Kinetic study and modelization of n-butene oligomerization for H-mordenite. Ind. Eng. Chem. Res. 1996, 35, 1269.
(6) Chauvin, Y.; Hennico, A.; Leger, G.; Nocca, J. L. Upgrading of C2, C3, C4 olefins by IFP Dimersol technology. Erdoel, Erdgas, Kohle 1990, 106 (7/8), 309. (7) Oldham, W. J. 3,4-Dimethylhexenes are prepared from n-butenes. U.K. Patent GB 1,164,474, 1969. (8) IFP. Gasoline jet fuel and hydraulic fluid production from light olefin(s)sby polymerisation with silica-alumina catalyst followed by fractionation and hydrogenation. U.K. Patent GB 2,006,262, 1979. (9) Golombok, M.; de Bruijn, J. N. H.; Morley, C. Kinetically based NMR method of measuring blending octane number of olefins. Chem. Eng. Res. Des. 1995, 73A, 849. (10) Knocking characteristics of pure hydrocarbons; API Research Project 45, ASTMS no. 225; API: Washington, DC, 1958. (11) Kiessling, D.; Wendt, G.; Hagenau, K.; Schoellner, R. Dimerization of n-butenes on amorphous nickel oxide-alumina/ silica catalysts. Appl. Catal. 1991, 71, 69. (12) Lucchesi, P. J.; Baeder, D. L.; Longwell, J. P. Stereospecific isomerization of butene-1 to butene-2 over SiO2-Al2O3 catalyst. J. Am. Chem. Soc. 1959, 81, 3235. (13) Mackenzie, K. Alkene rearrangements. In The Chemistry of Alkenes; Zabicky, J., Ed.; Wiley: London, 1970. (14) Holmes, J. L.; Ruo, L. S. M. A gas kinetic investigation of the n-butene equilibria. J. Chem. Soc., Part A 1969, 58, 1924. (15) Benson, S. W. Thermochemical kinetics; Wiley: London, 1970.
Received for review August 9, 1999 Revised manuscript received October 28, 1999 Accepted November 22, 1999 IE9906060
