Ind. Eng. Chem. Res. 1992,31,475-481
475
Alkylation of Isobutane with Pentenes Using Sulfuric Acid as a Catalyst: Chemistry and Reaction Mechanisms Lyle F. Albright* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Ken E . Kranz Stratco, Inc., Leawood, Kansas 66211
Isobutane was alkylated either with a pure C5 olefin, with a mixture of a pure C5 olefin and mixed C4 olefins, with C4olefins, or with propylene using sulfuric acid as the catalyst. Alkylates produced at operating conditions of commercial importance had research octane numbers of 90.7-91.7,97.8, 93.2, and 89.0, respectively, when C5 olefins, n-butenes, isobutylene, and propylene were used as olefins. The compositions of the alkylates produced from n-pentenes, and especially 2-pentenes, were significantly different than those of alkylates produced from isopentenes. Cyclopentene which is present in C5 mixtures of refinery products produces very poor quality alkylates in low yields. For both n-pentenes and isopentenes, isopentane is produced by self-alkylation in appreciable amounts. The alkylation chemistry differs significantly for various Csolefins, and reaction mechanisms are proposed and compared to those for C3 and C4 olefins.
Introduction During World War 11, alkylation of isobutane with propylene, butenes, and pentenes (or amylenes) was widely practiced. The primary product from this process was aviation gasoline. However, with the significant drop in demand for aviation gasoline after World War 11, refiners essentially eliminated pentenes from their alkylation unit and blended them into the gasoline pool. Butenes, and to a lesser extent propylene, have provided the olefin feedstock for increasing quantities of high-octane, low vapor pressure motor gasoline. Recent “clean air” legislation requires that many refiners soon reformulate their gasoline product. To meet these new gasoline specifications, most of the pentenes must be removed from the gasoline pool. Thus, attention and interest is once again being focused on adding pentenes to the alkylation unit’s feedstock. Past experimental alkylation data on the use of pentene mixtures or of the individual pentene isomers are too limited to understand clearly the basic chemistry or qualities of alkylates produced. Schmerling (1955) reports that when isobutane is alkylated with 2-pentenes in the presence of sulfuric acid at 10 “C, the alkylate contains 55-60 w t ?% Cgisoparaffins and 11-15 w t ?% isopentane. Cupit et al. (1961) and Durrett et al. (1963) reported similar results for alkylations with 2-pentenes, but they indicate that alkylations with 2-methylbutene-2 (2-Ml32) and 2-methylbutene-1 (2-MB1) result in greater production of isopentane, C6 and C, isoparaffins, and trimethylpentanes (TMP’s). In the latter alkylations, fewer Cg isoparaffins were produced. Iverson and Schmerling (1958) also present some information on alkylations with pentenes. Information on the effect of operating changes and on the other C5 olefins (l-pentene, 3-methylbutene-1(3-MBl)’ and cyclopentene) are essentially unavailable. All six C5olefins are produced in refinery cracking units and are available as feedstocks for alkylation. When isobutane is alkylated with butenes using sulfuric acid as a catalyst, most of the isoparaffins are produced by three mechanisms designated as mechanisms 1,2, and 3 by Albright and Spalding (Albright et al., 1988). Mechanism 1, which has been discussed in detail by Schmerling (1955), is the predominant one for the production of TMP’s when l-butene or 2-butenes are the olefin feedstocks since l-butene rapidly isomerizes in the
presence of sulfuric acid to 2-butenes before alkylating. When pentenes are used as olefins, Cg isoparaffins are produced by mechanism 1. In mechanism 2, CI2-Cmolefins and cations are initially formed. Fragmentation of these heavy species plus subsequent proton and hydride transfer results in the production of C4 to at least CI3 isoparaffins. Mechanism 2, proposed by Hofmann and Schriesheim (1962), is the predominant one when isobutylene is the olefin feed and sulfuric acid is the catalyst. Whenever C4 olefins are used, it accounts for most light ends (C5-C7 isoparaffins; LE’S) and dimethylhexanes (DMH’s). In the case of pentenes, mechanism 2 is the predominant mechanism for production of C6-C8 isoparaffii including both TMP’s and DMHs. Mechanism 3 accounts for the formation of some heavy isoparaffins. It is of minor importance for moderate- or high-quality alkylates that are produced in most refineries. A hydrogen-transfer or self-alkylation mechanism (mechanism 4) also occurs in which a pentene is hydrogenated (Schmerling, 1955; Hofmann, 1964) as follows: C5 olefin 2(isobutane) isopentane + C8 isoparaffin (often TMP)
+
-
Since little or no n-pentane is produced when 2-pentenes are used as olefins, they are probably first isomerized to produce an isopentene. Such a hydrogen-transfer mechanism is of importance when propylene and n-butenes are alkylated in the presence of hydrofluoric acid (HF), producing propane and n-butane, respectively, as by-products. Mechanism 4 is of minor importance when propylene and n-butenes are alkylated using sulfuric acid as the catalyst. In the present investigation, extensive data have been obtained for alkylations with several pure pentenes and with mixtures of pentenes and butenes. These results clarify the alkylation mechanisms for the different pentene isomers. Experimental Details A well-stirred reactor was used, to which various mixtures of isobutane, pentenes, and butenes plus sulfuric acid were continuously fed. The reactor has an internal volume of 450 mL and is provided with an impeller that was rotated at 1100 rpm to provide effective agitation. The flow rate of the hydrocarbon feed mixture from a storage tank to the reactor was carefully controlled by displacement with water pressurized using nitrogen at about 6 atm (see
0888-5885f 92f 263l-Q475$Q3.~Q f 0 0 1992 American Chemical Society
476 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 WATER C Y L I N D E R WATER FLOWMETER
ISOBUTANE EL OLEFIN F E E D CYLINDER ISOBUTANE C Y L I N D E R MOLECULAR SIEVE V A R I A B L E SPEED MOTOR 4 5 0 MIL R E A C T O R
EMULSION SIGHT O L A S S ACID FLOWMETER
@ @ @ @ @ @ @ @ @
SETTLER CAUSTIC BED FLOW C O N T R O L VALVE
EFFLUENT SAMPLE C Y L I N D E R
PRODUCT PLATFORM B A L A N C E
ACIDSAMPLE TEMPERATURE CONTROLLER
MULTI-POINT TEMPERATURE RECORDER
A C I D PUMP
Figure 1. Bench-scale alkylation pilot plant.
Figure 1). The instantaneous flow rate of water (and hence of the hydrocarbon feed) was measured by a rotameter. The total amount of feed hydrocarbons added to the reactor was measured to within about 0.4g using a calibrated sight glass on the water cylinder. The emulsified mixture of acid and hydrocarbons from the reactor was separated by gravity in a settler having an internal volume of 1300 mL. A settling leg was provided to measure the acid-to-hydrocarbon ratio of the emulsified mixture. The rate of acid being recycled from the settler to the reactor was measured by a rotameter. In all cases, the hourly space velocities in the reactor were maintained at about 0.3, which approximates those in many commercial units. A calibrated thermocouple in direct contact with the reaction mixture was used to provide temperature readouts. The temperature was controlled to within 0.6 "C of the desired value of 0-20 "C by adjusting the refrigerant flow to the cooling coil of the reactor. The acidity of the acid was measured by titration with standard caustic solutions. A temperature-programmed Hewlett Packard gas chromatographic unit equipped with a PONA capillary column provided separation of all major compounds in the isobutane-alkylate mixtures and the hydrocarbon feed mixtures of this investigation. It was not possible to identify positively the chromatographic peaks of the Clo and heavier isoparaffins. Tentative divisions between C9 and ClotClo and CI1,etc. isoparaffins are based on results of Durrett et al. (1963) and the available boiling points of the n-paraffins. The hydrocarbon analyses are reported here on a weight basis. Both research and motor octane numbers (RON'S and MONS) were calculated as weighted averages using published octane numbers of all isoparaffins in the C , + , range (Hutson and Logan, 1975; ASTM, 1956). For Cg isoparaffms, values have been reported for only two isomers: 2,2,5-trimethylhexane (2,2,5TMH), 91 RON and 88 MON and 2,3,5-TMH, 81 RON and 78 MON. A value of 40 RON was assumed for the Cg isoparaffins thought to be di-
methylheptanes. This value was determined by extrapolation using the octane numbers of dimethylbutanes, dimethylpentanes, and dimethylhexanes. For the Clo and heavier paraffins, octane numbers varying from about 40 to 120 have been reported. In the case of tetramethylhexanes, values are likely high, probably greater than 100, because of the high level of branching. For the remaining T M H s C1o)s,and higher isoparaffins, values of 80 RON and 80 MON were used. The calculated octane numbers were found in this investigation to be reproducible within 0.1 based on two duplicate runs and the smooth curves of octane numbers versus olefin composition as the relative amounts of C4 and C5 olefins were varied. Calculating octane numbers from weighted averages does not account for any blending synergism between the isoparaffins. Limited investigations using ASTM test engines indicate that the predicted octane numbers are often low by about 1.5.
Alkylates Produced from Pure C3-Cs Olefins Several runs were made at essentially identical conditions in which isobutane was alkylated with one of the following seven olefin feedstocks: propylene, n-butenes (a mixture of 33% 1-butene,35% truns-2-butene,and 32% cis-%butene),mixed butenes (a mixture containing 26.1% 1-butene, 25.4% isobutylene ,30.9% truns-&butene, and 17.1% cis-2-butene), isobutylene, 1-pentene, 2-pentenes (a mixture of 76% trans-2-pentene and 24% cis-2-pentene), and 2-MB2. These pentenes are the predominant ones in alkylation feeds in refineries. All hydrocarbon feeds used in these experiments contained less than 3% propane, n-butane, or n-pentane. The alkylations were made at temperatures varying from 4.5 to 10 "C. The substantial differences in the composition of the alkylates produced from these seven olefin feeds at essentially identical operating conditions are shown in Table I. Similar comparisons have previously been made for alkylates produced from propylene and C4 olefins (Li et
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 477 Table I. Comparison of Alkylates Produced When Isobutane Is Alkylated with C3,C,, or C5 Olefins Using Sulfuric Acid as Catalyst propylene
n-butenes
mixed butenes
isobutylene
'-pentene
2-pentenes ~
temp, "C 110 acidity, HzSOl isoparaffins, w t %
iC5 nC5
CS'S (37's TMPs DMH's methylheptanes
Cg's C10's
GI's
Cn's C13)s c141s
RON
MON
c6's/cs's TMP's/DMHs Cg's/heavier
2-MB2
~~
9 8.6 94.5
10 7.2 94.4
10 7.8 94.7
10 8.0 94.2
4.5 7.3 94.4
10 7.3 94.5
4.5 7.5 94.5
9.5 7.5 94.6
4.5 8.2 94.5
10 8.2 94.4
3.0 0.0 3.8 67.4 11.6 1.9 0.0 1.2 8.7 1.9 0.5 0.0 0.0 89.0 87.1 0.79 6.1 0.11
2.2 0.0 2.6 2.7 80.0 8.1 0.0 1.0 0.7 2.6 0.1 0.0 0.0 97.8 93.9 0.85 9.9 0.23
4.0 0.0 3.9 3.8 66.0 9.8 0.2 2.7 2.1 6.5 0.9 0.0 0.1 94.6 91.5 1.0 6.7 0.28
7.8 0.0 5.8 5.6 54.0 8.5 0.0 5.6 4.3 7.7 0.6 0.0 0.1 93.2 90.3 1.3 6.4 0.44
11.0 0.6 1.4 1.6 30.9 3.5 0.0 46.7 2.2 1.0 0.9 0.2 0.0 91.7 88.7 8.3 8.8 11.1
11.8 0.7 1.5 1.8 32.1 3.7 0.0 44.0 2.3 1.0 0.9 0.2 0.0 91.3 88.4 8.3 8.7 10.0
12.3 0.5 1.2 1.1 20.0 2.1 0.0 58.5 2.1 0.7 1.3 0.1 0.0 90.7 87.8 10.7 9.6 13.9
14.0 0.6 1.4 1.3 22.3 2.7 0.0 52.4 2.5 1.5 1.2 0.1 0.0 91.1 88.1 10.8 8.3 9.4
14.5 0.0 6.2 3.6 39.4 5.2 0.0 11.2 11.9 6.0 1.4 0.4 0.2 91.4 88.9 2.3 7.6 0.56
13.0 0.0 5.8 3.6 40.0 5.6 0.0 12.7 10.6 6.2 1.6 0.7 0.2 91.2 88.8 2.2 7.1 0.66
Table 11. ADDroximate RON of Isoparaffin Families with Various Olefins" olefin feed isoparaffin family c5
C6
c7
TMP DMH
C9 Cloand higher
propylene 93.7 98.8 88.1 102.1 65.1 82.7 80
n-butenes 93.7 96.6 85.2 102.7 64.1 88.5 80
mixed butenes 93.7 95.2 83.2 102.5 64.5 83.0 80
isobutylene 93.7 96.0 85.1 102.1 62.0 86.0 80
1-pentene 92.0 94.4 83.5 102.4 62.9 85.0
80
2-pentenes 92.0 92.4 82.5 102.2 62.8 87.2 80
2-MB2 93.7 89.9 81.0 102.2 62.6 85.7
80
Operating conditions: 10 OC;8:l isobutane/olefin; 94.5% sulfuric acid.
al., 1970a,b; Durrett et al., 1963). Considerably more information relative to the use of pentenes is however reported here. For the C4 olefins as shown in Table I, the alkylates contained 62.5-88.1 wt % C8 isoparaffins. In all cases, the major C8 isoparaffin was 2,2,4-TMP. The ratio of TMP'sfDMH's varied from a high of 9.9 in the case of n-butenes to 6.4 for isobutylene. When n-butenes were used, 80% of the alkylate was TMP's, but when isobutylene was used, T M P s accounted for only 54%. The alkylate produced from n-butenes had a RON that was 4.6 units higher than that of the alkylate produced from isobutylene. Li et al. (1970a,b) obtained similar results. They also found that similar quality and similar composition alkylates are produced from both 1-butene and 2-butenes. The RON of the alkylate produced from propylene was lower by 8.8 numbers, as compared to the RON of the alkylate produced from n-butenes. For the alkylate produced from propylene, 67% of the isoparaffins were C7%, which were mainly dimethylpentanes (DMP's). Similar results have been reported earlier (Li et al., 1970a, 1970b). Substantial differences in the compositions of the alkylates were found to occur when l-pentene, 2-pentenes, and 2-MB2 were used as the olefin feedstock. Alkylates produced using 2-pentenes contained 52.5-58.5% Cg isoparaffms, whereas only 11-13% of the alkylates produced using 2-MB2 were Cg's. The latter alkylates contained larger amounts of C6, C7,TMP, DMH, and Clo and higher isoparaffins. All alkylates produced from C5olefins contained relatively large amounts of isopentane. When either 1-pentene or 2-pentenes were used as a feedstock, a small amount of n-pentane was also produced. The compositions
of the alkylates produced from 1-pentene were intermediate to those of alkylates produced from the other two C5 olefins. Recent results obtained by am Ende (1991), who used a batch system, show closer similarity of the alkylates produced from 1-pentene and 2-pentenes as compared to the results of the present investigation. For alkylation made using 1-pentene or 2-pentenes, fewer Cg isoparaffins were produced as the temperature increased from 4.5 to 10 "C. Although the alkylate compositions varied greatly with the specific C5 olefin used, there were only fairly small differences in the alkylate quality (RONvalues varied from 90.7 to 91.7). For the three runs at 9.5-10.0 OC, the RON values were in the range of 91.1-91.3. Twelve peaks which are thought to be Cg isoparaffins were deteded on the chromatographs. Several of these peaks were identified using pure samples of several Cg isoparaffiis and the results of Durrett et al. (1963). The first four Cg peaks were 2,2,5-TMH, 2,2,4-TMH, 2,4,4TMH, and 2,3,5-TMH. For all olefins investigated, except for cyclopentene, approximately 65-77% of the Cg isoparaffins was 2,2,5-TMH and about 11-15% was 2,3,5TMH. In general, 8742% of the C,'s was TMH's, and the remainder was probably mostly dimethylheptanes. The ratios of TMH's to dimethylheptanes were almost identical in all cases, but may have been slightly higher when 2pentenes were used as the olefin as compared to 1-pentene. In Table 11, the compositions and calculated research odane numbers (RON'S)of the following C5, C6,C,, TMP, DMH, Cg, and Clo and heavier families were compared for the alkylates reported in Table I. Although the compositions and octane numbers of each of the families were
478 Ind. Eng. Chem. Res., Vol. 31, No. 2,1992
quite similar for all olefin feeds, relatively small but significant differences occur when comparing alkylates produced from isobutylene versus n-butenes or 2-MB2 (and the other isopentenes) versus n-pentenes. For example, the TMP's produced a t 10 "C from isobutylene, 2-MB2, and propylene contained about 56-5890 2,2,4-TMP whereas TMP's produced from n-butenes contained 5042%. TMP's produced from l-pentene and 2-pentenes contained 52.5 and 55% 2,2,4-TMP, respectively.
these heavy hydrocarbons were quite different as compared to those in other alkylates based on significant changes in the relative heights and locations of the chromatographic peaks. The concentration of 2,2,5-TMH was always low (in the 0.243% range). For the akylatea produced from cyclopentene mixtures, production of C,-C, isoparaffins (or LE'S) was very small (
