Ind. Eng. Chem. Res. 2005, 44, 5291-5297
5291
Influence of Linear Butenes in the Dimerization of Isobutene Maija L. Honkela and A. Outi I. Krause* Laboratory of Industrial Chemistry, Helsinki University of Technology, P.O. Box 6100, FI-02015 HUT, Finland
Isomerization and dimerization of linear alkenes under isobutene dimerization conditions with tert-butyl alcohol as the selectivity-enhancing component were studied in experiments with pure 1-butene and cis-2-butene feeds and with a mixture of isobutene and cis-2-butene. Conversion of cis-2-butene to 1-butene was low, but both linear butenes were found to isomerize to trans2-butene, especially at high temperatures and low tert-butyl alcohol concentrations. Codimers of isobutene and 2-butene were formed in the experiments with the isobutene-cis-2-butene mixture. Isobutene conversions were a little higher in these experiments than in the experiments with the pure isobutene feed, while the selectivities for the dimers were about the same. Kinetic modeling of the data was carried out, and a Langmuir-Hinshelwood-type kinetic model was constructed for the codimerization reaction. Introduction Isobutene (iB) dimerization as part of the isooctane production process is being successfully applied industrially in Edmonton, Canada.1 Isooctane is produced for the Californian market because groundwater contamination has led to a ban there on methyl tert-butyl ether (MTBE) in gasoline.2,3 Dimerization of isobutene and further hydrogenation of the produced diisobutenes are carried out to obtain MTBE-free isooctane with good gasoline blending properties. Isobutene dimerization can be carried out on acidic ion-exchange resins. Polar components such as methanol4 and tert-butyl alcohol (TBA)5 improve the selectivity for diisobutenes, but the activity of the resin is decreased. When methanol is used, MTBE is produced as an undesired side product; TBA, in turn, does not react with isobutene6 and allows high diisobutene selectivities even at low TBA concentrations.5 When the feed for the dimerization process is the C4 stream from an oil refinery, it includes various amounts of 1- and 2-butenes. A typical C4 feed from steam cracking (SC) contains the same amounts of n-butenes and isobutene and less than 11 wt % alkanes.4,7 The C4 flow from fluid catalytic cracking (FCC) contains more 1- and 2-butenes than isobutene. Feed from isobutane dehydrogenation contains only isobutene and C4 alkanes.4 The side reactions of isobutene dimerization in the presence of TBA include the dehydration of TBA and further oligomerization of the produced diisobutenes. Furthermore, the linear butenes, if present, will react with isobutene to a varying degree4 and thus influence the total conversion and selectivity of the process. While isobutene dimerization produces mainly 2,4,4trimethyl-1- and 2-pentenes, other trimethylpentenes are formed from 2-butenes (2B) and isobutene and dimethylhexenes from 1-butene (1B) and isobutene. 1-Butene and 2-butene dimerizations produce less branched alkenes. Zeolites8 and amorphous silica alumina catalysts9,10 have been used to obtain high conversion and selectivity for C8 alkenes in the dimerization of n-butenes. Klepel * To whom correspondence should be addressed. Tel.: +3589-451 2613. Fax: +358-9-451 2622. E-mail:
[email protected].
et al.8 studied the skeletal isomerization of 1-butene to isobutene on zeolite H-ZSM-5 at 548-673 K and observed simultaneous dimerization and cracking of the products, leading to C3-C7 alkenes. In a study of n-butene dimerization on amorphous silica alumina, Golombok and de Bruijn9 obtained 85% conversion of 1-butene and 35% conversion of 2-butene to dimers at 393 K. The 1-butene dimer mixture had better gasoline blending properties, which they attributed to the fast skeletal isomerization of 1-butene to isobutene under the applied conditions. In another study in which they used also the ion-exchange resin Amberlyst XN1010 in the dimerization of 1-butene at 393 K, Golombok and de Bruijn10 observed catalyst deactivation and less than 50% conversion. Amorphous silica alumina (13% alumina) catalyst gave the best conversion, product quality, and catalyst stability. O’Conner et al.11 studied ion-exchange resins in the dimerization of butenes and obtained a maximum conversion of about 70% with 40 wt % dimers in the product on Amberlyst 15 at 373 K with a feed containing almost 60 wt % 1-butene and only 7 wt % isobutene. Increasing the weight hourly space velocity was found to decrease the conversion. Yet, when linear butenes are fed together with isobutene to the isobutene dimerization system, they react with isobutene only to a limited extent.12,13 Di Girolamo et al.4 observed that linear butenes increase the selectivity for dimers because they adsorb on the active sites more readily than the dimers do. Alkanes, on the other hand, do not adsorb on the catalyst. Although linear butenes have been fed together with isobutene to the isobutene dimerization system,11,4 no study has been made of their effect in a system where TBA is used as the selectivity-enhancing component. The purpose of the present work was to study the effect of linear butenes on the yield and kinetics of the isobutene dimerization in the presence of TBA. Experiments were carried out with pure 1-butene and cis-2butene (c-2B) feeds and with an isobutene and cis-2butene mixture. Alkene conversions and dimer selectivities were calculated from the results. To supplement our previous kinetic model for isobutene dimerization,14 we constructed a kinetic rate equation for the codimerization reaction.
10.1021/ie0491903 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005
5292
Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005
Experimental Section Reactor System. The reactor was a continuous stirred tank reactor (CSTR) that operated in the liquid phase. The volume of the reactor was 50 cm3, and it was stirred with a six-blade mechanical stirrer. The reactor included a mixing baffle and a metal gauze basket, in which the catalyst was placed. Two feeding tanks (2 dm3) were used in the reactor system, one containing the pure alkene or alkene mixture as a liquid and the other a mixture of TBA (or 2-butanol in some experiments) and isopentane. The feeding tanks were pressurized with nitrogen to 1.8 MPa. The flow through the reactor was achieved with a pressure difference, as the pressure in the reactor was kept at 1.5 MPa. The tubes in which TBA flowed were heated to 306-308 K to keep TBA in the liquid state (mp 296-298 K). The reactor was heated with hot oil flowing through its jacket. An automation system made with Wonderware FactorySuite 2000 software was used to gather measured data online and to control the reactor temperature according to a predetermined plan. Analytical Methods. The flows to and from the reactor were analyzed with an online Hewlett-Packard 5890 series II gas chromatograph (GC). The GC was equipped with an HP-1 capillary column of length 60 m, film thickness 1.00 µm, and diameter 0.25 mm. Helium (Aga, 99.996%) was used as the carrier gas. A flame ionization detector was used in the GC. Isopentane was considered as the internal standard. In all of the experiments, the error in the alkene balance was less than 4 mol % and that in the solvent balance less than 3 mol % during the steady state. Chemicals and Catalyst. Experiments were carried out with pure 1-butene (Aga, 99%), cis-2-butene (Aga, 99%), and isobutene (2-methylpropene; Aga, 99%). 1-Butene and isobutene could not be separated from each other with the GC column employed, and experiments with mixtures were carried out only with isobutene and cis-2-butene (49.5 wt % iB and 49.4 wt % c-2B). In most cases, TBA (Riedel-de Hae¨n, G99.5%) was the selectivity-enhancing component; 2-butanol (2-BuOH; Riedel-de Hae¨n, G99%) was used in some experiments. TBA was dissolved in the isopentane (iP; Fluka Chemika AG, G99%) used as the solvent. The catalyst was a commercial acidic ion-exchange resin consisting of a styrene-divinylbenzene-based support to which sulfonic acid groups had been added as active sites. The surface area of the dried catalyst was measured to be 37 m2/g (Brunauer-Emmett-Teller analysis), the acid capacity was 5.1 mmol/g (by titration15), and the particle size was between 0.42 and 1.0 mm. Before use, the catalyst was dried overnight in an oven at about 373 K. Procedure. At the beginning of each experiment, the mixed flow of alkene, isopentane, and TBA (or 2-BuOH) was analyzed by gas chromatography. After the flow had stabilized and the feed had been accurately analyzed, the flow was fed to the reactor. The reaction at each temperature was continued until a steady state was reached. The temperature was then changed, while the feed rate and compositions were kept the same. Calculation. In our previous study14 with an isobutene feed, on average 99.5% of the dimers formed were either 2,4,4-trimethyl-1-pentene (TMP-1) or 2,4,4-trimethyl-2-pentene (TMP-2). All of the dimers were summed together and used as one pseudocomponent in
the kinetic modeling. In the present study, only TMP-1 and TMP-2 were considered as isobutene dimers (diB), and the other C8 alkenes were considered as codimerization products (dim). In the steady state, the ratio of the diisobutenes TMP-1 and TMP-2 was very close to thermodynamic equilibrium.16 The compositions of the flows to and from the reactor were analyzed, and the alkene conversion and the selectivity for dimers were calculated. The equation used for isobutene conversion was
XiB )
n˘ iB,in - n˘ iB,out × 100% n˘ iB,in
(1)
The selectivity for diisobutenes was calculated as the ratio of diisobutenes to all of the products
sdiB )
2n˘ diB 2(n˘ diB + n˘ dim) + 3n˘ triB + 4n˘ tetraB
× 100% (2)
and the total dimer selectivity as the ratio of all C8 dimers to all of the products
s)
2(n˘ diB + n˘ dim) 2(n˘ diB + n˘ dim) + 3n˘ triB + 4n˘ tetraB
× 100% (3)
The equation used for the conversion of 2-butenes was
X2B )
n˘ c-2B,in - (n˘ c-2B,out + n˘ t-2B,out) × 100% (4) n˘ c-2B,in
Results and Discussion Intraparticle resistances and external diffusion limitations were not observed in our previous study on isobutene dimerization.14 Experiments were now carried out with the unsieved catalyst (about 1 g) and at a mixing speed of 900 rpm. Experiments with Pure 1-Butene and Pure cis2-Butene. Experiments were carried out with pure 1-butene and pure cis-2-butene with an alkene to solvent molar feed ratio of 0.6 and TBA contents of 0.45 and 0.9 wt % at temperatures of 353, 373, and 393 K. C4 isomer contents in the steady state in the experiments with 0.45 wt % TBA are presented in Figure 1. At 353 K, the isomerization rates are low. At the higher temperatures, 1-butene reacts to both cis- and trans-2butene, while the conversion of cis-2-butene to 1-butene remains low. The same trans-2-butene contents are obtained independently of the alkene feed, but steadystate contents of 1-butene and cis-2-butene are dependent on the feed. At higher TBA contents (0.9 wt %; data not shown), the isomerization rates were even lower, and at 373 K, less than 4 wt % of the alkenes fed to the reactor isomerized. In a study of the isomerization of C4 alkenes on ionexchange resins in the gas phase, Słomkiewicz17 observed that 1-butene preferentially isomerizes to trans2-butene rather than cis-2-butene, while the isomerization rates of the 2-butenes to 1-butene are very low. Also in our experiments, 1-butene appears to react preferentially to trans-2-butene rather than cis-2-butene (at 373 K with 0.45 wt % TBA and 1-butene feed, a t-2B content of 14.7 wt %, and a c-2B content of 8.5 wt %). Isomerization of 1-butene and cis-2-butene to trans-2butene does not reach thermodynamic equilibrium18,19
Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5293
Figure 1. Amounts of 1-butene (a), trans-2-butene (b), and cis2-butene (c) in the steady state with pure 1-butene or pure cis-2butene as the feed. The alkene content in the feed was about 32-34 wt % and the TBA content 0.45 wt %.
Figure 2. Dimer contents (wt %) in the steady state with 1-butene and cis-2-butene feeds and 0.45 wt % TBA.
in the experiments. At 353 K, the ratio of trans-2-butene to 1-butene varied from 0.6 to 4.5 (depending on the TBA content) while the equilibrium ratio was 11.3 (at 355 K)19 and the ratio of trans-2-butene to cis-2-butene from 0.01 to 0.1 while the equilibrium ratio was 2.6,19 again at 353 and 355 K, respectively. Dimer contents in the product with 1-butene and cis2-butene feeds and with 0.45 wt % TBA are presented in Figure 2. The maximum dimer content was 3.1 wt % with the cis-2-butene feed at 393 K. Evidently, 1-butene and cis-2-butene dimerizations are very slow under the applied conditions.
At 353 K about 0.3 wt % and at 393 K 0.2 wt % 2-butanol and less than 0.05 wt % TBA were obtained in the experiments with the cis-2-butene feed and an initial TBA content of 0.45 wt %. Also, in the 1-butene experiments, almost all TBA dehydrated and part of the water that was formed reacted further with the linear alkenes to 2-butanol. The reason for the extensive TBA dehydration is the lack of isobutene in the feed and the disappearance of the formed water to alcohols corresponding to the alkenes in the feed. All linear butenes (1-butene, cis-2-butene, and trans-2-butene) give the same sec-butylcarbenium ion, which may react with water to 2-butanol.20 According to Petrus et al.,20 the chemical equilibrium of the hydration of linear butenes at 393-423 K is strongly on the side of the alcohol. Experiments were carried out with pure cis-2-butene with 2-butanol as the additive to compare the effects of TBA and 2-butanol. The feed and product compositions with cis-2-butene and with 0.4 wt % TBA or 2-butanol as the additive are presented in Table 1. At 353 K, slightly higher trans-2-butene contents were obtained with 2-butanol than with TBA (6 wt % vs 3 wt %). The alcohol and dimer contents in the steady state are about the same independent of the type of alcohol in the feed. There is virtually no isomerization or dimerization of linear butenes at 353 K. Isomerization of cis-2-butene to trans-2-butene is more significant at higher temperatures. The results of experiments where pure isobutene was fed to the reactor with TBA or 2-butanol as an additive are shown in Table 2. In the 2-butanol experiments, almost all of the 2-butanol dehydrates, and the water formed reacts to TBA. Although the product distribution is about the same for the two additives, isobutene conversions are slightly higher with 2-butanol than with TBA. Although most of the 2-butanol reacted to TBA, a small part probably reacted with isobutene to form ethers. The loss of alcohol (not present as 2-butanol or TBA) in the 2-butanol experiments was 20% at 353 K and 17% at 373 K, while with TBA the losses due to the dehydration of TBA were 6% and 10%, respectively. Even minor conversion of alcohols to components other than water may increase the conversion with this low alcohol content. Quantitative analysis of the ethers is not, however, possible because of their small amounts, and thus the separation of the effects of the polar components cannot be carried out. Experiments with an Isobutene and cis-2-Butene Mixture. Experiments were carried out with a mixture of isobutene and cis-2-butene (49.5 wt % iB and 49.4 wt % c-2B), with an isobutene to isopentane and cis-2butene molar feed ratio [iB/(iP + c-2B + TBA)] of 0.6, and with molar flows corresponding to conditions of our previous experiments with the isobutene feed.5 This means that the isobutene flow was the same as that used earlier. The maximum 1-butene concentration was 2.1 wt % under the steady state in the experiments with the pure cis-2-butene feed. In the experiments with a mixture of isobutene and cis-2-butene, the 1-butene contents are even lower because less 2-butene and more TBA are fed to the reactor, and thus it is assumed that the isomerization of 2-butenes to 1-butene is negligible under the applied conditions. No deactivation of the catalyst was observed in the experiments. At 353 K, about the same amount of trimer peaks was obtained in the experiments with the isobutene-cis-2-butene mixture as with pure isobutene,
5294
Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005
Table 1. Feed and Product Compositions in the Experiments with cis-2-Butene Feed and Either TBA or 2-Butanol as the Selectivity-Enhancing Component feed wt % TBA 2-BuOH TBA 2-BuOH
product wt %
T/K
iP
TBA
2-BuOH
c-2B
iP
TBA
2-BuOH
1B
t-2B
c-2B
dim
353 353 373 373
67.9 66.7 67.9 66.7
0.4 0.0 0.4 0.0
0.0 0.4 0.0 0.4
31.4 32.6 31.4 32.6
67.6 66.0 67.4 66.4
0.0 0.0 0.0 0.0
0.3 0.3 0.1 0.2
0.6 0.7 1.6 1.7
3.4 6.2 15.5 16.5
27.8 26.6 13.9 14.2
0.3 0.1 1.4 1.0
Table 2. Feed and Product Compositions in the Experiments with Isobutene Feed and Either TBA or 2-Butanol as the Selectivity-Enhancing Component feed wt % TBA 2-BuOH TBA 2-BuOH
product wt %
T/K
iP
TBA
2-BuOH
iB
iP
TBA
2-BuOH
iB
diB
triB
tetraB
353 353 373 373
67.0 67.8 67.0 67.8
0.6 0.1 0.6 0.1
0.0 0.6 0.0 0.6
32.3 31.5 32.3 31.5
67.1 68.4 67.2 68.0
0.6 0.4 0.6 0.5
0.0 0.1 0.0 0.1
13.8 10.8 9.1 8.0
15.2 15.8 17.5 17.1
3.2 4.2 5.5 6.1
0.1 0.1 0.1 0.2
whereas at higher temperatures, more trimer peaks were observed in the mixture experiments. The ratio of cis- to trans-2-butene molar flows as a function of the TBA content is presented in Figure 3. As can be seen, increasing the temperature and decreasing the TBA content result in more isomerization of cis-2-butene to trans-2-butene. The amounts of cisand trans-2-butenes are summed together and used as the amount of 2-butene in the following calculations and in the kinetic modeling.
tures and low TBA contents, the fraction of diisobutenes in the product decreases as more codimers are produced. At 333 and 353 K, virtually no 2-butanol was formed in the experiments with the mixture. The TBA and 2-butanol contents in the product at 373 K are presented
Figure 3. Ratio of cis- to trans-2-butene molar flows in experiments with the isobutene-cis-2-butene mixture as a function of the TBA content.
Figure 4 compares the conversion of isobutene, the diisobutene selectivity, and the total dimer selectivity with our previous results for the pure isobutene feed.5 At 333 K, no differences are observed in the isobutene conversions obtained with the isobutene and cis-2butene mixture and the pure isobutene feed. At higher temperatures (353 and 373 K), conversions are higher (3-8 percentage units depending on the TBA content) for the mixture. Diisobutene selectivities are lower for the mixture than for the pure isobutene feed. The fraction of diisobutenes increases with the TBA content and decreasing temperature. Total dimer (both diisobutenes and codimers included) selectivities as a function of the TBA content coincide independently of the feed and temperature. The conversion of 2-butenes is presented in Figure 5. 2-Butenes do not react at 333 K, and the conversion remains below 14% at 353 K. 2-Butene conversions as high as 35% are obtained at 393 K and low TBA contents. The 2-butene conversion exhibits the same kind of trend as a function of the TBA content as does the isobutene conversion. The diisobutene selectivities (Figure 4b) follow the 2-butene conversion so that with high 2-butene conversions, especially at high tempera-
Figure 4. Isobutene conversions (a), diisobutene selectivities (b), and selectivities for C8 dimers (c) with isobutene5 and isobutenecis-2-butene mixture feeds as a function of the TBA content with the molar feed ratio [iB/(iP + c2B + TBA)] of 0.6.
Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5295
Figure 5. Conversion of 2-butenes in experiments with an isobutene-cis-2-butene mixture as a function of the TBA content.
and 2-butenes, and trimerization of isobutene via diisobutenes. The net rates of formation for the different components (riB, r2B, rdiB, rdim, and rtriB) can be calculated using the reaction rate for diisobutene formation in the dimerization rdiB,j, the reaction rate for codimer formation in the codimerization rdim,j, and the reaction rate for triisobutene formation in the trimerization rtriB,j, where mechanism j is used to derive the rate equations. The net rates of formation for the components in the system are
riB ) -2rdiB,j - rdim,j - rtriB,j
(5)
r2B ) -rdim,j
(6)
rdiB ) rdiB,j - rtriB,j
(7)
rdim ) rdim,j
(8)
rtriB ) rtriB,j
(9)
The net equations are used in the optimization together with the mass balance for the CSTR reactor operating under the steady state:
n˘ IN,i ) n˘ OUT,i - rimcat Figure 6. TBA and 2-butanol contents in the product obtained with the isobutene-cis-2-butene mixture at 373 K.
in Figure 6. The amount of 2-butanol does not correspond to the amount of TBA that has converted, which means that there may be some water from TBA dehydration in the product or 2-butanol may have been consumed in the etherification with isobutene. Still, even with the highest TBA content (3.3 wt %), only 15% of TBA has converted to products other than 2-butanol, and thus possible ether amounts remain very small. Derivation of a Kinetic Model. The data obtained in the experiments with the mixture with TBA as the selectivity-enhancing component were used to optimize parameters for a codimerization rate equation that supplements our previous model for isobutene dimerization.14 Although TBA dehydration occurred in the experiments, this reaction was not included in the kinetic models because the TBA contents in the feed were low (
