840
Ind. Eng. Chem. Res. 1994,33, 840-848
Degradation and Isomerization of Isoparaffins while in Contact with Sulfuric Acid in Alkylation Units: Chemistry and Reaction Kinetics David J. a m Ende and Lyle F. Albright' School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907 When isoparaffins such as those produced during the alkylation of isobutane with c3-C~olefins are contacted with H2SO4, both isomerization and degradation reactions occur. The degradation products consist of mainly C4-CI2 isoparaffins and acid-soluble oils. Experimental runs were made with the following hydrocarbons: 2,2,4-trimethylpentane, 2,3-dimethylpentane, two commercial alkylates, and mixtures of isobutane and alkylates. Both alkylate quality and quantity decreased significantly due primarily to the degradation reactions of both commercial alkylates and trimethylpentanes. Correlations were developed to predict the levels of degradation of alkylates by acids of different compositions. Methods to minimize these undesired reactions in commercial alkylation units are proposed. Introduction When isobutane is alkylated with C345 olefins using sulfuric acid as the catalyst in order to produce high-quality gasolines, the acid also acts to at least some extent as an isomerization, degradation, and oxidizing agent (Condon, 1958;Hofmann, 1964;Kramer, 1967a;Doshi and Albright, 1976). Isoparaffins and especially trimethylpentanes (TMPs) degrade to produce C4-& isoparaffins and acidsoluble oils (ASOs) which are sometimes referred to as conjunct polymers or red oil. Sulfur dioxide and water are also produced. Roebuck and Evering (1953) found that 3-methylpentane, 2-methylhexane, and 2,4-dimethylhexane isomerized when contacted with HzS04 with little degradation occurring. Oxidation and degradation reactions are, of course, undesired since the following occur: the octane number (or quality) of the isoparaffin product is lowered; part of the alkylate is destroyed; and acid consumption is increased. There is a need to clarify how important these reactions are in commercial units. Hofmann (1964) and Kramer (1967a) investigated the degradation and isomerization reactions that occur when 2,2,4-TMP or 2,3,4-TMP is contacted with concentrated fresh HzSO4 at 25 "C. Doshi and Albright (1976) investigated the reactions of all four TMPs using high rates of agitation. They found that the kinetics of these reactions were in the following order: 2,3,4-TMP > 2,3,3TMP > 2,2,4-TMP > 2,2,3-TMP. They extended the range of operating conditions investigated to include temperatures from -10 to 25 "C, several levels of agitation, and acids having compositions in the range of commercial importance. Significant amounts of ASOs were sometimes produced in addition to C4-Clz isoparaffins. Degradation reactions were predominant, but appreciable isomerizations also occurred. In measuring the kinetics of degradation plus the isomerization of a specific TMP, the results of the past investigations were not correlated to account for the ASOs produced. The rates of degradation for a specific TMP were found to be first order relative to the TMP content on a weight basis in the hydrocarbon phase and essentially second order relative to proton activity of the acid (Kramer, 1967a). Limited tests indicated that isobutane degrades at extremely slow rates. Condon (1951) investigated isomerization reactions that occur when 2,2,4TMP is contacted with AlBr3. Degradation reactions also occurred to a high degree although he refers to them as side reactions. Li (1969) found in several cases that alkylates degrade significantly in the presence of relatively concentrated
sulfuric acids. In one case when the alkylate-isobutane mixture from a continuous-flow alkylation reactor was agitated for an additional 10 min at 10 "C, the alkylate quality decreased from 96.7 RON to 95.1 RON (RON = research octane number). The level of agitation is considered as having been high, and the isobutane-toalkylate ratio was about 19:l. Insufficient information was reported to calculate the decrease in the quantity of the alkylate. Degradation, isomerization, and. alkylation reactions probably occur predominantly at or at least close to the interface between the acid and hydrocarbon phases (Doshi and Albright, 1976; Kramer, 1967a). Increased levels of agitation obviously result in higher interfacial surface areas. Relatively little information is, however, available on how changes in the acidlhydrocarbon ratios affect the interfacial surface area. Most experiments conducted by Doshi and Albright employed hydrocarbon-continuous emulsions. Yet acid-continuous emulsions are normally employed in commercial alkylation reactors. The latter emulsions which require a higher acidlhydrocarbon ratio probably have larger interfacial surface areas. In the present investigation, both degradation and isomerization reactions have been investigated further in order to clarify the chemistry, and improved kinetic equations have been developed. Methods to minimize these reactions are proposed. Experimental Details Two reactors were employed in the present investigations. The first reactor had an internal volume of 350 mL and was equipped with two thick glass windows so that the emulsion could be observed during a run. It was also provided with four baffles and with a six-blade squarepitch turbine impeller operated at 1750rpm. This reactor was used earlier by Albright et al. (1988a). The amounts of acid and hydrocarbon feeds were carefully metered to the reactor. The temperature of the bath for the reactor was controlled to *O.l "C by a Neslab immersion cooler. The hydrocarbon phase was sampled for this reactor by two methods depending on the reaction conditions. If the emulsions separated in less than 1 min, such as for hydrocarbon-continuous emulsions, the agitator was stopped. The needle of a gas-tight syringe was quickly inserted through a silicone septum which capped an opening on top of the reactor, and a hydrocarbon sample was obtained. When the emulsions separated slowly as with most acid-continuous emulsions, the agitator was not stopped and an approximately 0.5-mL sample of emulsion
0888-5885/94/26~3-084Q~Q4.5QIQ 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 84 1 was obtained in the syringe. This emulsion sample was quickly injected into a vial covered with a silicone septum. The emulsion was mixed with a dilute sulfuric acid solution in the vial quenching the reactions in the emulsion. After the hydrocarbon phase separated to the top of the liquids in the vial, it was sampled and analyzed. The second reactor was a glass vial with an internal volume of 8 mL. Approximately 7 mL of the acid to be investigated was added to the vial. Next, about 0.5 mL of the hydrocarbon mixture to be investigated was added to the vial. The vial was then covered with a silicone septum and screw cap. The weight of the hydrocarbon phase added was determined to f O . l mg by weighing the vial before and after the hydrocarbon was added. The interface between the acid and hydrocarbon phase was 1.62 cm2 as calculated from the internal diameter of the vial; no meniscus was noticed at the interface. Interfacial tension information obtained indicates that no significant curvature of the interface should be expected. The vial was then positioned in a constant temperature bath that was controlled to f O . l OC. Since the vapor volume in the vial was about 0.5 mL, only a negligible amount of hydrocarbons vaporized. Hydrocarbon liquid samples of less than 5 pL were removed from the vial and then analyzed. Two temperature-programmed gas chromatographic units were employed for analyzing the liquid hydrocarbon samples. A Varian 3700 chromatographic unit equipped with a 100-mPetrocol-DH (Supelco) capillary column and a flame ionization unit was used for detailed hydrocarbon analyses which were completed in about 60 min. 50-60 hydrocarbon peaks were usually obtained. All c4-C~peaks and most CSpeaks have been identified. The remaining peaks have been tentatively identified as to their carbon number by determining where CS,CIO,CII, etc., n-paraffin exit the chromatographs; isoparaffins exit before n-paraffins. A Varian 1400chromatographic unit equipped with a 1.5-m Petrocol-A (Supelco) column was employed for more rapid but less detailed analyses. Acid samples were titrated for acidity using a standardized NaOH solution and for water by a Karl Fischer method. The AS0 content of the acid was determined by difference assuming the acidity, water content, and AS0 content equal 100%. Sulfuric acids of the desired compositions were prepared by blending fresh acids, used alkylation acids, and water. Rhone-Poulenc provided a used acid from their Hammond, IN plant. Synthetic used acids were also prepared by bubbling isobutylene through 99 % fresh acid for extended periods of time. The insoluble hydrocarbon phase that formed on the top of acid phase was separated and discarded.
Degradation-Isomerization Results for 2,2,4-TMP Several degradation-isomerization runs were made at -5 to 30 OC using 2,2,4-TMP in contact withH2S04's having a relatively wide range of compositions. The acids employed contained 0.6-5.0 wt % water, 0-12 wt % ASOs, and the remainder H2S04. For runs in the agitated reactor, emulsions containing 10-80 % acid were investigated. These runs were continued until at least 10% of the 2,2,4TMP reacted, or usually for at least 1 h. Runs in the static reactor were, however, significantly longer. Plots were made for each run of ln(mo1epercent of 2,2,4TMP in the hydrocarbon phase) versus time. The top straight line of Figure 1is an example of such a plot. In this run, an emulsion with a 1:l volumetric ratio of 2,2,4TMP to acid was agitated at 10 OC to produce an acidcontinuous emulsion. The acid contained 2.1 % water and
90
80 .
Basis: 100 mots starting 2,2,4 TMP 70
1
,
~
1
1
,
1
,
,
,
~
,
,
1
4.3% ASOs. For all runs of this investigation, such plots resulted in straight lines, except for arather small induction period at the start of some runs. As indicated in Figure 1,the 2,2,4-TMP mole percent in the hydrocarbon phase decreased from 100 to 78 after 98 min. Kramer (1967a) and Doshi and Albright (1976) made similar plots except they plotted on a weight instead of a molar basis. For TMPs or commercial alkylates, there is only a small difference in the mole and weight fractions of CSisoparaffins. When degradation-isomerization runs are made, however, for lighter or heavier isoparaffins, weight and mole fractions differ significantly. In such cases, the slopes calculated using the two procedures differ substantially. The top straight line of Figure 1does not take account of the amount of ASOs also produced when 2,2,4-TMP degraded. The amount produced was calculated by the following procedure. First, using the analytical results for each hydrocarbon sample, the atomic ratio of carbon to hydrogen was calculated for the hydrocarbon phase. Second, the ASOs produced as a result of the degradation reactions were assumed to have an atomic ratio of hydrogen to carbon of 1.75. Such a ratio is in good agreement with information reported by Hengstebeck (1965), Miron and Lee (19641, and Den0 et al. (1962, 1964). When 2,2,4TMP was used as the starting hydrocarbon, its ratio of hydrogen to carbon is 2.25. Finally, carbon and hydrogen balances were made to determine the following for every 100 g of starting 2,2,4-TMP degraded: grams of ASOs produced and grams of TMP unreacted. The grams of TMP unreacted is of course equal to the percent of TMP unreacted. Plots of the ln(percent of TMP unreacted) vs time were also essentially straight lines. The bottom line of Figure 1shows such a plot. The following equation was found to correlate the kinetic data for all decompositionisomerization reactions of 2,2,4-TMP
where TMP = mol unreacted 2,2,4-TMP/100 mol feed 2,2,4-TMP
k = rate constant, cm/s
842 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 30:
A = interfacial area between phases, cm2
25
V = volume of hydrocarbon phase, cm3
4
10
Y,I, , ,, , ,, , , , , ~ ,, , ,
,
'
" " " / " " /
, , , , , , 1 , , , , ~
' I " " , " ' -
"
Basis lODg staning 2.2.4 TMP
] =-Ft
L
TMP TMPinitid TMP is related to XTMP,the fractional conversion of the trimethylpentane, by the following equation (3) 100 The bottom line of Figure 1is hence also the plot of ln(1 - XTMP)versus time, The slope of each straight line obtained equals kA/ V for the agitated runs. Approximately 3-9% of the 2,2,4-TMP that reacted by degradation reactions produced ASOs. The reliability of the method of calculating the amounts of ASOs produced by carbon and hydrogen balances was tested by comparing the acid analyses before and after a run. The amounts of ASOs produced were similar in all cases for the two calculation procedures, but the method using carbon and hydrogen balances is considered more reliable. Plots were also made for the grams of isoparaffins and grams of ASOs produced per 100 g of starting 2,2,4-TMP as a function of conversion. Figure 2 shows the results for several runs at 10 "C using 97.9 wt 5% H2S04 (and 2.1 % water) in the stirred reactor; the acid in the emulsion varied from 10 to 80% (by volume). When 30% of the TMPs had reacted, approximately 33%, 35%, 13%, 11%, and 8%, respectively, of the 2,2,4-TMP that reacted were converted to ( 2 4 4 7 isoparaffins, HEs (CS and heavier isoparaffins), other TMPs, dimethylhexanes (DMHs),and ASOs. The isoparaffins produced had, on the average, octane numbers that were in the range of 89-90. Figure 3 indicates that relatively more ASOs were produced at low conversion levels as compared to high levels. For example for runs with 40% conversion, 3050% of the ASOs produced had been formed during the 0-5 % conversion range. More ASOs were produced with acids having higher acidities. The weight ratios of isobutane produced to ASOs produced were calculated for all runs made at 10 "C; several acids and several acid/ hydrocarbon ratios were employed. This weight ratio varied from 2:l at low conversions up to 2.25:l at higher conversions. Assuming 2 g of isobutane and 1 g of ASOs were produced from 3 g of 2,2,4-TMP, good carbon and hydrogen balances are obtained if the hydrogen/ carbon ratio for the ASOs is 1.75. Condon (1958),in his discussion of maximum yields of isobutane produced during decomposition of TMPs, reports information that indicates a somewhat higher ratio of isobutane to ASOs, actually 2.44: 1. The relatively close agreement between the results of the two investigations is, however, considered important. The small differences noted could have been caused in part at least by the use of different catalysts. Condon had also assumed the ASOs of his investigation had a carbon/ hydrogen ratio equivalent to Cl.OHl.6. For acid-continuous emulsions that generally require several minutes to separate, significant degradationisomerizations occurred during separation. For a run using
" " 1 " " l " " .
" I " "
15
For runs in both the agitated and unagitated reactors, kA/ Vis assumed to be relatively constant during the entire run. As a result, eq 1 can be rearranged and integrated from tinitid = 0 to t = t and from TMPinitid to TMP (which is TMPfind). Equation 2 is hence obtained:
xTMP
' I
Basis lOOg staning 2,2,4 TMP
20
t = time, s
In[
'
"
'F
=
0
10
30
20
50
40
60
70
2,2,4 TMP Conversion, %
Figure 2. Degradation and isomerization products aa function of conversion when 2,2,4-TMP is contacted with 97.9 w t % HzSOb at 1750 rpm and 10 O C . The volumetric percent of acid in emulsion varies from 10 to 80%. '0
4.0 1
n
t
1
E a 3
!
I
!
I
I
I
OYoASO
/ I
3.0
AS0
j
2.0
1.o v)
0
v)
a I
0
5
10
15
1
1
20
25
1
30
35
, 40
2,2,4 TMP Conversion, YO
Figure 3. Amount of acid soluble oils (ASOs)produced aa function of 2,2,4-TMP conversion with four acids at 1:l volume ratio, 1750 rpm, and 10 "C.
a 7525 volumetric ratio of 99.1% H2S04 to 2,2,4-TMP, approximately 27 7% of the 2,2,4-TMP reacted during the first 5-min period of agitation, as shown in Figure 4. Agitation was then stopped, but no hydrocarbon phase separated during the first 15 min. During the next 25 min, approximately half of the hydrocarbon phase separated. It was then analyzed, and an additional 52 % of the 2,2,4-TMP had reacted. The agitation was then started, and the initial slope for the straight line correlation was again obtained. A series of 2,2,4-TMP degradation-isomerization runs were also made in the 8-mL vial with a known area (A) and hydrocarbon volume (V). For these runs, plots were made of [h(l-XTMP)] versus AtIV. In dl cases, straight lines resulted, and the slopes were equal to k expressed in cm/s. For a run using an acid with an acidity of 98.9% (and 1.1%water), about 30 h were required to obtain a 13% conversion whereas 10 min were needed for such a conversion in the agitated reactor. The large difference of the kinetics is of course caused by the much larger A/ V ratios in the agitated reactor.
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 843
\Agitation: Reaction No
still
3ccurring during phase separatio
On the basis of equilibrium values, about 99% of the water reacts with acid at 25 "C as shown in eq 4 (Sanders, 1985), or about 5.4 g of acid reacts per gram of water. ASOs contain both conjugated diene groups and nonconjugated C-C double bonds that form either cations or ASOsulfates (Albright et al., 1988b). Based on iodine values for the ASOs as reported by Miron and Lee (19631, it is estimated that about 0.3-1.1 g of acid react per gram of ASOs; this latter ratio is referred to as B in eq 6. Equation 6 was used to calculate the weight percent of unreacted H2S04 or (H2S04),.
Agitation
\
(H2S04),= acidity - 5.4(wt 5% H20) - B(wt % ASOs) (6) 10
20
30
40
50
60
Time (minutes)
Figure 4. Moles of 2,2,4-TMP reacted with and without agitation at 1750 rpm, 99.1% H&04 (0.9% HzO), 3:l acid/hydrocarbon ratio, 10 "C.
Tests were made in the static reactor to determine if mass transfer of either 2,2,4-TMP or acid to the acidhydrocarbon interface was a rate-controlling step or if the ratio of acid to hydrocarbons affected k values. Toperform these tests, a small magnetic stirrer was added to the vial in order to provide gentle circulation in the acid and to a lesser extent in the hydrocarbon liquid in contact with the acid. Visual observations indicated that the interfacial area between the liquid phases was essentially unchanged. In these tests, k values were unchanged within experimental accuracy indicating that neither mass transfer nor A/ V had a significant effect. As an example, for otherwise identical tests, with A/ V = 0.75 (stirred) and with A/ V = 3.214 (unstirred), the Iz values were 2.8 X 10-8 and 3.0 X 1O-a cm/s, respectively. The interfacial area between the two liquid phases in the agitated reactor can be determined by performing degradation runs in both the agitated and static reactors with the same acid and using the same operating temperature. For the static reactor, the value of k is calculated. This value of k and that of V are then substituted in the value of kA/ V as determined in the agitated reactor; the interfacial area ( A )of the stirred reactor is then calculated. For different acids, A values of about 400-1000 cm2/cm3 of emulsion were calculated with a 1:l volumetric ratio of the two phases. Table 1indicates for 30% conversions of 2,2,4-TMP in the agitated reactor that acid composition has alarge effect on the relative importance of the reaction products formed at 10 "C and with 1:1 volumetric ratio of acid to 2,2,4TMP. The major products formed in all cases were CS and heavier isoparaffins. The HEs accounted for 33-44 % of the products whereas the C r C 7 isoparaffins totaled about 25-37 5%. Production of isobutane and ASOs results in a loss of alkylate; ASOs dissolve in the acid, and isobutane is normally not included in the gasoline pool, especially with recent environmental regulations. As indicated by the results in Table 1, 15-30% of the 2,2,4TMP that degraded produced isobutane and ASOs. Values of unreacted H2SO4 on wt 5% basis were used for correlation purposes. In calculating the value, it is known that HzSO4 reacts with both water and ASOs as follows: H2S04+ H 2 0 * H30++ HSO, H2S04+ ASOs * ASO-sulfates (or ASO'
(4)
+ HSO;)
(5)
Acidity was determined by caustic titration of the acid as weight percent. Plots of (HzS04), using B = 0.30) versus other TMPs, DMHs, HEs, the percent isobutane, C~-C~'S, and ASOs as reported in Table 1resulted in all cases in correlations almost approximating straight lines, as shown in Figure 5. Higher values of (HzS04),, resulted in increased and ASOs but in production of isobutane, C&'s, decreased production of HEs, other TMPs, and DMHs. The kinetic results obtained at a given temperature with different composition acids result in straight-line correlations when In k , or sometimes ln(kA/ V), is plotted versus (H2S04),. Figure 6 showsfour such examples. The bottom two lines of Figure 6 are correlations of In k versus (H2SO4), for the degradations of 2,2,4-TMP at 10 and 23 "C as obtained in the static reactor of the present investigations. Straight-line correlations were also obtained when ln(kA/ V) was plotted versus (HzS04), using the results obtained by Doshi (1976) for the degradation of 2,2,4TMP at 10 " C in a stirred reactor and by Kramer (1967a) for the degradation of 2,3,4-TMP at 25 "C in a shaken reactor. The latter two sets of data were probably obtained using hydrocarbon-continuous emulsions. Nonlinear regression (SAS/NLIN) was employed to calculate the slopes of the straight lines, the best values of B to use in eq 6 for calculation of (HzS04),, and the correlation coefficients, as shown in Table 2. The logarithm of the rate constants for the isomerization of 3-methylpentane as determined by Roebuck and Evering (1953) also correlated well as a straight line with (HzS04),. As shown in Table 2, the slopes for the five sets of data are almost identical even though the temperature, reactant, and types of reactors sometimes differed. The B values used in eq 6 were lower for the runs in the static reactor; in these runs, Iz values were calculated using the molar conversion of 2,2,4-TMP in the hydrocarbon phase. Weight concentrations were, however, used by Doshi and Kramer for calculating kA/ Vvalues. A larger B value was needed to obtain the best correlation of Doshi's results. In this investigation for runs in a stirred reactor, the kA/ V values did not, however, correlate well with the above technique. All of these latter runs, however, employed acid-continuousemulsions,and the interfacial area differed greatly depending on acid composition. (H2SO4), as calculated here is related to the Hammett acidity function (Ho). For fresh acids with acidities (as determined by caustic titration) of 95-98.5 % , an almost straight line correlation results when -Hovalues (Jorgenson and Hartter, 1963)in the range 9.85-10.6 are plotted versus (HzSO&in the range 68-93. For stronger acids, Hovalues are much greater than the predicted values based on an extrapolation of the straight line to (HzS04),, = 100. Rather than using eq 6, the percent of unreacted H2S04 can be calculated on a mole percent basis. HzS04, HzO,
844 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 1. Reaction Products Obtained at 30% Conversion of 2,2,4-TMP for Several Acids: 10 O C , Volume Ratio of Phases 1.0, 1750 rpm Acid Composition (wt %) 90.6 92.7 97.9 96.2 99.4 91.9 93.3 acidity 2.1 1.2 0.6 3.0 2.2 1.4 1.3 water 5.2 4.6 8.0 6.0 0.0 2.6 0.0 ASOs 86.6 88.9 96.1 74.0 80.2 80.4 83.6 (HfiOdu Grams of Degradation Producta/100 g of 2,2,4-TMP Degraded isobutane 12.0 14.3 15.7 16.9 16.3 17.8 20.7 i-Cri-C.l's 12.7 15.3 14.4 15.7 17.0 17.0 16.9 total light ends 24.7 29.6 30.1 32.6 33.3 34.8 37.6 10.6 9.9 9.2 12.6 11.3 10.7 10.4 DMHs and other Cs's 13.4 12.8 10.4 15.5 14.7 14.7 14.0 other TMPs 24.0 22.7 19.6 28.1 26.0 25.4 24.4 total Cs isomers 34.5 33.2 38.6 35.7 35.2 44.1 38.7 HEs 8.0 9.6 5.9 7.3 7.5 3.1 5.7 ASOs 100.0 100.0 100.0 100.0 100.0 100.0 100.0 totala Total = total liiht ends + total Cs isomers + HEs 50
1
' " ,
"
I
" I
' ,
I
+ ASOs.
'"'j
B U
40
1
30
00 c
i
C8isomers
20;
on a mole basis also resulted in straight-line correlations. The Arrhenius equation correlates the k values for two sets of 2,2,4-TMP runs at temperatures from -1 to 30 "C. Two acids were used first an acid with 98% acidity and 2% water and second an acid with 93.3% acidity that contains 2.2% water and 4.5% ASOs. As indicated in Figure 7,the slopes of the two straight lines are essentially identical. The calculated energy of activation equals 70 kJ/(mol K). This finding suggests that the degradation mechanism is the same for both fresh acids (containing no ASOs)and used acids such as present in &lation reactore.
Reactions with 2,S-Dimethylpentane 75
70
80
85
90
95
100
Unreacted H,SO,, wt%
Figure 5. Correlations using (H&30Juof several families of products that result when 2,2,4-TMPdegrades and isomerizeswhile in contact with different acids: 10 "C, 30% conversion, 1750 rpm, acid/ hydrocarbons = 1. 1000000
E
'
'
'
'
The static reactor was employed for reactions of 2,3dimethylpentane (2,3-DMP)in the presence of 99.1 9% H2SO4 (and the remainder water) at 25 "C. After 378 h, approximately 25% of the starting 2,3-DMP had isomerized, and 1.5% had degraded, as shown in Table 3. The predominant isomer produced was 2,4-DMP, but other C, isoparaffins included 3-methylhexane, 2,3,3-TMB, and 2-methylhexane. Degradation products included C4-C13 isoparaffins plus trace amounts of propane and ASOs. Since 2,3-DMP has a higher RON value than 2,4-DMP, the above isomerizations result in lower quality fuels.
Reactions of Isoparaffin Mixtures
A mixture of 54.6 wt % (or 38 mol %) 2,2,4-TMP and the remainder isobutane was degraded-isomerized a t 10 "C in the agitated reactor. The acidity of the acid was 97.9% with the remainder water, and a volumetric 1:l ratio of acid to hydrocarbons was used. The kinetics of reactionswere first order relative to the 2,2,4-l" content. 23°C About 35 7% of the 2,2,4-TMPreacted in 95 min. The LAIV am €de: 2,2,4-TMP value was 37 % of that for the degradation of pure 2,2,410°C TMP, or the kAIV value was, within experimental accuracy, directly proportional to the mole percent of the 50 60 70 80 90 100 2,2,4-TMP in the starting feedstock. Unreacted HISO,, wt% The compositions of the degradation products were Figure 6. Correlation of rate constants as a function of ( H Z S O ~ ) ~ . compared a t the 30% conversion level with hnd without Rate constanta are (kA/V) for data obtained by Kramer and Doshi isobutane in the hydrocarbon phase. When isobutane was and are k for current data. mixed with 2,2,4-TMP, the amount of TMP isomers and DMHs produced was greater by factors of 2.8 and 1.9, HSO4-, H30+, and at least ASO+ are present in the acid. respectively; obviously isomerization reactions were of Insufficient information is currently available to estimate much greater importance. The productiorr of CS and the acid composition when these species are all included. heavier isoparaffins was reduced, however, by a factor of Based on what are thought to be reasonable estimates, 8.2. Production of C&,'s and ASOs was essentially values on a mole basis are likely to be about 10% smaller unchanged. Due to the presence of isobutane in the starting feedstock, the calculation procedure to determine than those on a weight basis. Plots of values estimated U.1
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 845 Table 2. Correlation of ln(k observed) or ln(kA/V) as a Function of (HISO& for Different Degradation Reactions and Isomerizations SAS/NLIN Used for Parameter Estimation investigation reaction temperature, OC range of H~SOI),, method of contact slope X 100 intercept value of B correlation coefficientR a
present investigationa 2,2,4-TMP degradation 10 72-96 quiescent interface 11.9 0.00026 0.32 0.997
present investigation0 2,2,4-TMP degradation 23 60-92 quiescent interface 12.6 0.00031 0.31 0.97
Kramerb 2,3,4-TMP degradation 25 51-91 shaken "Wig-L-Bug" 13.1 6.04 not applicable 0.994
Doshib 2,2,4-TMP degradation 10 53-92 agitated 12.3 0.086 1.15 0.998
Roebuck 3-Methylpentane isomerization 25 58-94 agitated 12.0 0.06 not applicable 0.996
kA/V calculated using mole percent of hydrocarbon phase. Calculated using weight percent of hydrocarbon feed.
Table 3. Isomerization and Degradation of 2,3-Dimethylpentane i n the Presence of 99.1% Sulfuric Acid at 25 O C : Static Reactor hydrocarbon compn, wt % Oh 138 h 378 h 0.00 0.00 0.05 propane isobutane 0.00 0.33 0.43 isopentane 0.00 0.30 0.35 0.00 0.07 0.09 2,3-DMB 0.00 0.11 0.15 2-MP 0.90 0.05 0.09 3-MP 22.28 0.11 11.92 2,4-DMP 0.36 0.00 0.19 2,2,3-TMB 85.58 73.52 2,3-DMP 99.72 2.23 0.08 1.11 3-MH 2,2,4-TMP 0.09 0.09 0.13 0.00 0.06 0.07 2,5-DMH 2,2,3-TMP, 2,4-DMH 0.00 0.04 0.05 2,3,4-TMP 0.00 0.02 0.02 2,3,3-TMP 0.00 0.03 0.02 0.00 0.00 0.00 2,3-DMH 0.00 0.00 0.00 2-MHept 0.00 0.00 0.00 4-MHept 0.00 0.00 0.02 3,4-DMH 0.00 0.00 0.00 3-MHept CS'S 0.00 0.08 0.07 (210's 0.00 0.01 0.02 Cll'S 0.00 0.00 0.00 C12's 0.00 0.00 0.00 ClS'S 0.00 0.00 0.05
Table 4. Degradation of 94.1 RON Alkylate in Static Reactor: 97.9% acidity; 10 OC 0h
isobutane n-butane isopentane n -p en tan e 2,3-DMB 2-MP 3-MP 2,4-DMP 2,2,3-TMB 2,3-DMP 3-MH 2,2,4-TMP 2,2,3-TMP 2,3,4-TMP 2,3,3-TMP 2,5-DMH 2,4-DMH 2,3-DMH 3,4-DMH 2-MHept 3-MHept 2,2,5-TMH 2,2,4-TMH 2,4,4-TMH 2,3,5-TMH other C i s
Go+ RON gASOs/100g alkylate
93.3% H,SO, 1 ' " " " " " " ' " ' " ' " "
3.2
3.3
3.4 1 n (K')
3.5 +
3.6
3.7
(1000)
Figure 7. Arrheniusplot of rateconstanta for 2,2,4-TMP degradation from -1 to 30 OC with two acids in a constant-area reactor.
isobutane and ASOs production is, however, considered to be less accurate. The calculated RON values of the C&-Clz isoparaffins produced are 91.2 for the mixture containing isobutane as compared to 89.2 when pure 2,2,4TMP was used.
0.04 2.81 4.06 0.00 2.04 0.42 0.20 7.51 0.04 21.33 0.21 29.69 1.29 10.08 5.46 1.39 1.29 1.95 0.27 0.02 0.03 0.89 0.05 0.16 0.15 8.64 94.0 0
hydrocarbon compn, wt % 37.2 h 63.2 h 116.3 h 169.1 h 1.31 2.23 3.33 3.62 2.62 2.98 2.78 2.35 4.10 3.74 3.92 3.63 0.03 0.08 0.02 o.Oo0 2.05 2.19 2.18 2.11 0.61 0.77 0.94 1.05 0.41 0.47 0.28 0.34 9.34 9.58 8.17 8.63 0.07 0.06 0.08 0.09 19.38 18.13 16.42 14.50 0.28 0.32 0.40 0.45 29.13 28.38 28.40 27.63 1.41 1.47 1.32 1.33 9.51 9.10 8.67 8.25 5.33 5.21 5.19 5.18 1.57 1.65 1.88 2.08 1.41 1.47 1.32 1.33 1.90 1.85 1.82 1.80 0.27 0.26 0.28 0.29 0.06 0.07 0.10 0.14 0.08 0.10 0.05 0.05 1.03 1.11 1.32 1.55 0.02 0.04 0.05 0.06 0.04 0.16 0.17 0.19 0.22 0.37 0.53 0.22 0.30 9.05 11.38 9.54 9.26 93.6 0.23
93.4 0.94
93.1 1.38
92.7 0.82
170.3 h 3.97 2.59 3.88 0.03 2.17 1.08 0.47 9.59 0.10 14.50 0.45 27.36 1.45 8.12 5.15 2.03 1.45 1.75 0.29 0.16 0.10 1.54 0.02 0.05 0.22 0.54 10.95 92.7 1.27
Four runs were made using either a 94.1 RON alkylate (prepared using mixed C3-C4 olefins) or a 96.4 RON alkylate (prepared using mainly (24's as olefins). Two runs were made using only pure alkylates while the other two were made using mixtures of isobutane and alkylate in order to simulate hydrocarbon mixtures in alkylation reactors. Table 4 indicates how the composition of the 94.1 RON alkylate changed in a 170-h run in the static reactor at 10 "Cwith an acid having a 97.9% acidity (and 2.1 % water). The concentrations in the hydrocarbon phase of the following compounds decreased on a relative basis after 7 days as follows: 2,3,4-TMP, 19%;2,3,3-TMP, 6% ;2,2,4TMP, 8 % ;and 2,3-dimethylpentane (DMP), 32%. The concentrations of the following isoparaffins increased as the run progressed: CSand heavier isoparaffins, DMHs, most light ends, 2,4-DMP, methylheptanes, and 3-methylhexane. The latter two isoparaffins have very low octane numbers; the increases for them on an absolute basis were
846 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994
small but on a relative basis were rather large. About 3.75% of the original alkylate was converted on a weight basis to isobutane and about 1.8% to ASOs; hence, the weight of the Cb-Ci2 isoparaffins decreased by about 5.5 % . The calculated RON of the alkylate mixture decreased from 94.0 to 92.7. Based on the results obtained for the two reactors, about 60 min would have been required in the agitated reactor to obtain the same extent of degradation as obtained in the 7-day run. One degradation run was made using pure 96.4 RON alkylate in the agitated reactor. This run employed an acid with 97.9 wt % acidity (and 2.1% water) and had a 1:lvolumetric ratio of acid to alkylate. The concentrations in the hydrocarbon phase of the following decreased on a relative basis in 60 min: 2,3,4-TMP, 42%; 2,3,3-TMP, 25 % ; 2,2,4-TMP, 15% ; 2,3-dimethylpentane, 137%; and 2,4-dimethylpentane, 5 % . The concentration of the heavy ends increased on a relative basis by 140%. With the exception of isopentane, isoparaffins containing only a single methyl side group increased appreciably on a relative basis in all cases. The compositions of the CS,C7, TMP, and DMH groups changed appreciably as a result of isomerization reactions. The calculated RON decreased during the run from 96.4 to 93.7. The production of isobutane and ASOs caused an alkylate loss of 9.0% by volume. Two degradation runs were made in the agitated reactor a t 10 “C using mixtures of isobutane and 96.4 RON alkylate. The first run was made with an acid having 97.9 % acidity and containing 2.1 % water. The second run used an acid having a 94.9% acidity and containing 1.7 % water and 3.4% ASOs. This acid had a composition such as could be present in an alkylation reactor. In both runs, there were relatively large decreases in the concentrations of 2,3,3-TMP and especially of 2,3,4-TMP. The concentrations of 2,2,4-TMP, however, increased somewhat. In the second run, the total TMP concentration decreased by 9 % . In this run, the concentrations of both CSand C7 isoparaffins decreased slightly. The concentrations of the following, however, all increased: DMHs, HEs, and isopentane. The increased amounts of HEs are, however, much less than those experienced with pure alkylates (containing little or no isobutane). The presence of isobutane clearly suppresses HE production and promotes isomerization of TMPs which results in the formation of additional 2,2,4-TMP. The loss of alkylate weight in the second run totaled about 6.1 % because of the production of isobutane and ASOs. There was also a substantial decrease in alkylate quality in both runs. It is estimated that the (RON)(barrel) of alkylate in the second run decreased about 7.1% or about 3.5%/h.
Discussion of Results Previous investigators have recognized that more than one type of reaction occurs when isoparaffins and concentrated H2SO4 are contacted. More details on the reaction have been obtained in the present investigation. These reactions can be categorized as isomerization of isoparaffins, as decomposition of isoparaffins, and as oxidation reactions with H2S04 acting as the oxidant. The isomerizations are both positional and structural ones. For structural isomerizations, the more branched isoparaffins are generally isomerized to less branched isoparaffins. Octane numbers apparently decrease in all cases because of isomerizations. To start the isomerizations, an isoparaffin must first be converted to a cation. Cations are initially formed by two methods (Kramer, 1967b;Doshi and Albright, 1976): first, ASOs may abstract H- to form
an isoakyl cation; or second, sulfuric acid acts as an oxidant to remove a H- and simultaneously produces water and S02. The isoalkyl cations then isomerize with methideor hydride-transfer steps and/or with structural rearrangements. A hydride transfer either directly or indirectly from an isoparaffin molecule results in the production of the isoparaffin isomer. Degradation reactions are the second major type of reactions that occur. Previous investigators (Condon,1951; Kramer, 1967a; Doshi and Albright, 1976) have discussed the mechanism of degradations. In the case of 2,2,4-TMP, 2,2,4-TMP+’sare produced initially. Some of these cations fragment to form t-C4H9+ and isobutylene. H- transfer, from both 2,2,4-TMP and perhaps ASOs, to t-C4Hg+ produces isobutane. When isobutylene reacts with TMP+’s, C12+-C2o+’s are produced. Fragmentation of these heavy cations followed by H- and H+transfer results in the production of C5-C12 isoparaffins. This mechanism also occurs during alkylations and is referred to as mechanism 2 by Albright et al. (1988a). For TMPs other than 2,2,4-TMP, the TMP+’s initially produced likely isomerize to form 2,2,4-TMP+’s before fragmenting. If TMP+’sother than 2,2,4-TMP+fragment,either propylene or i-C3+’swould presumably form. Propane would probably be a byproduct in such a case, but there is no evidence that propane is ever produced during the degradation of TMPs. The formation of trace amounts of propane in the experiment with 2,3-DMP suggests that a small fraction of one of the DMP+’sfragmented to form either propylene or i-C3+’s. Apparently DMH+’s do not fragment to any appreciable extent. Cg and heavier isoparaffins form i-Cg+’s and heavier cations that probably fragment and start degradation reactions. ASOs are also produced during degradation reactions. Relatively good carbon and hydrogen balances were obtained when both the c4-c12 isoparaffins and ASOs were included when making the calculations. The third type of reactions of isoparaffins, in the presence of H2S04, involves an oxidative-polymerization reaction sequence. Such oxidations are often needed to form isoalkyl cations, but such oxidations are relatively minor for most isoparaffins. Doshi (1976) found that a trace amount of some heavier isoparaffins were produced when isobutane was contacted with H2S04. Oxidation of isobutane leads to the formation of t-CdHg+’s. Some t-CqHg+’s deprotonate to form isobutylene which then reacts with t-CdHg+’sforming TMP+’sand heavier cations. Fragmentation of these heavy cations eventually results in the formation of c5-c12 isoparaffins. ASOs are also produced. Several previous investigators qualitatively recognized that the relative importance of isomerization to degradation reactions varied with different isoparaffins. The present investigation provides quantitative information for these two reactions. As shown in Table 1,19.6-28.1% of the 2,2,4-TMP that reacted produced other Cg isoparaffins. Hence, at least 71.9-80.457, of the 2,2,4-TMP that reacted produced degradation products. It should be emphasized that part of the 2,2,4-TMP that reacts produces C12-C20 cations and olefins as intermediates and some CS isoparaffins are then reformed. So the actual percent degraded is somewhat higher than 71.9-80.4%. The apparent percent degradation depends on the acid composition and also on other operating variables. For the reactions of 2,3-DMP in the presence of H2SO4, isomerizations are the predominant reactions. Using the results shown in Table 3 after 378 h in the static reactor,
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 847 approximately 93.9% of the 2,3-DMP that reacted produced isomers whereas about 6.1 % degraded. Based on literature information plus that obtained in this investigation, isomerization is the predominant reaction for C6, C7, and DMHs (Roebuck and Evering, 1953; Doshi and Albright, 1976). Cations formed from these isoparaffins do not generally fragment. The following sequence, however, explains the relatively minor amount of degradation that occurs. Some of the cations deprotonate to produce olefins. These olefins then enter into polymerization-type reactions to form relatively heavy cations such as Cm+, C12+,C14+,and CIS+ when Cg, CS,C7, and DHMs, respectively, are used. These heavier cations then fragment, and eventually C4-C12 isoparaffins are produced along with ASOs. For the run of 94.1 RON alkylate which contained a relatively high concentration of 2,3-DMP, isomerization of 2,3-DMP to 2,4-DMP obviously occurred. For the run with 96.4 RON alkylate which contained only a small concentration of 2,3-DMP, the concentrations of both 2,3DMP and 2,4-DMP decreased slightly during the run. Isomerization of DMPs is thought to have been of lesser importance in this run. Comparison of the results for the two alkylates indicates that isomerization of TMPs also occurred. For the 94.0 RON alkylate containing the higher ratio of 2,2,4-TMP to 2,3,3-TMP, the relative decrease in the 2,3,3-TMP content was smaller as compared to that for the 96.4 RON alkylate which had the higher ratio of the two TMP's. Isomerization reactions to produce 2,2,4TMP were apparently more important for the latter alkylate. The presence of isobutane and other light isoparaffins in the hydrocarbon mixtures is also important and promotes isomerization reactions producing 2,2,4TMP. Both isobutane and isopentane are sources of hydride ions. The availability of hydride ions would affect the relative importance of degradation and isomerization reactions. When n-butenes or propylene are used as olefins, the intermediates formed are sec-butyl or isopropyl sulfates respectively (Albright et al., 1988a). For isobutylene, (212C20 olefins and cations are intermediates. The current industrial consensus seems to be that withlonger residence times more intermediates react to form more alkylate and increase the octane number. The present results plus those of Li (1969), however, indicate that both the yield and quality of the alkylate pass through maxima as the residence time in the reactor increases. The possibility of severe degradation of isoparaffins in the decanter (where the acid and isoparaffin phases separate) is apparently also not recognized by many refineries. Estimates have been made of probable levels of degradation in typical large alkylation units, based on the results of the present investigation. The following approach and assumptions were made to extrapolate the results to a large alkylation plant. (a) Three alkylation reactors are operated with sulfuric acid flowing in a series from one reactor to the next. The acidities of the acid as measured by caustic titration are 96.0,93.5, and 91.0%, respectively, in the three reactors. The water content of the acid increases from 1.0 to 2.5 wt % as it passes through the reactors. Since the highest acidity occurs in the first reactor, the largest amount of degradation occurs in it, and the least degradation occurs in the third reactor. The effectiveness of the three acids to promote degradation reactions is assumed to be directly proportional to the k values for 2,2,4-TMP degradation at 10 "C, as determined by the correlation of In k as function of (HzS04),, see Figure 6.
(b) The rate of degradation also depends on the composition of the isoparaffin mixture, and the rates were estimated using the information obtained in this investigation and by Doshi (1976). When isobutane is mixed with alkylates, the rates of degradation decrease based on the results of the present investigation. (c) The level of agitation and the interfacial area per given volume between phases in the commercial alkylation reactors is identical to those in the stirred reactor used in the present investigation. (d) The residence times employed in these calculations a r e t h e design values a s reported by company representatives: 19 min in Stratco contactors and 45 min in Exxon's Cascade reactors. Each Cascade reactor was assumed to have five agitated compartments in series, and the average residence time of the hydrocarbon phase in a reactor was calculated to be 27 min. (e) Most of the alkylate forms during the first minute in the alkylation reactor. Degradation of alkylate was assumed to occur for 18 min in the Stratco contractor and for 26 min at the Cascade reactor. The following are predicted for a plant using Stratco reactors and assuming the alkylate produced was the 96.4 RON alkylate investigated in this study: (a) decrease of RON, 0.07; (b) decrease in alkylate quantity (or barrels), 0.25 % ;(c) increase of acid consumption due to degradation of hydrocarbons, 0.02 lb HzSOd/gal alkylate; (d) decrease in octane barrel (defined as (RON barrel)), 0.3%. Cascade reactors probably have 40-45 % more relative degradation since these reactors provide 40-45 % longer residence times. The undesired degradation reactions are generallymore important for higher quality alkylates which contain higher concentrations of TMPs. These predictions must be considered as only approximations since the degradations are highly dependent on the operating conditions. Predictions based on the results of this investigation are, however, substantially lower than those based on the results of Li (1969). He had agitated the acid-hydrocarbon emulsion in a laboratory continuous-flowalkylation reactor for several additional minutes after completion of the alkylation reactions. Using his results and correcting the reactivities of the acids because of their different compositions, a 96.4 RON alkylate would likely average a decrease of 0.55 RON in the Stratco reactors. RON drops averaging about 0.75 would be expected in Cascade units. The laboratory reactor employed by Li is thought to approximate that of industrial reactors. In the present investigation, isoalkyl cations must, however, be provided before degradation can occur. There likely were higher average concentrations of these cations in Li's experiments. Use of Li's technique in commercial alkylation reactors would provide valuable information on the importance of degradation-isomerization reactions in a refinery. Information could be obtained on losses of both alkylate quality and quantity and on acid consumption because of these undesired side reactions. Based on the results of Figure 4, a significant degradation may sometimes occur in the decanter especially if 60 rnin is required for separation of the phases. If 60 min for separation is equivalent to about 20 min of agitated contact as seems probable, degradation levels in the decanter may often be similar to those in the reactor. Simple experiments can be made to determine the importance of degradation reactions in the decanter. An emulsion sample leaving the reactor should be quickly separated such as with a small centrifuge, or it could be diluted by the method used in the present investigation. The resulting hydro-
848 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994
carbon and acid phases would both be analyzed. These analyses would be compared to the analyses of the phases leaving the conventional decanter in order to determine the decrease in alkylate quality, decrease in quantity of the alkylate, and the increase in acid consumption. Optimization of residence time in an alkylation reactor depends on several operating variables including agitation, as indicated by the results of the present investigation plus those of Li, Eckert, and Albright (1970a,b) and Li (1969). In one series of experiments using C4 olefins as a feedstock, a residence time of about 5-7 min was optimum with an agitator speed of 3000 rpm (which produced what is considered to be a high level of agitation). By far, the best quality alkylates were also produced at 3000 rpm. For alkylations using Cq olefins at an intermediate level of agitation (2000 rpm), the optimum residue time was about 10-12 min. Residence times greater than indicated above would result in lower yields and quality since the degradation reactions then increase significantly in importance. An alkylation process to produce the highest quality alkylates and highest yields should have the following three features. First, the reactor should use operating conditions that provide rapid rates of alkylation. Two possible methods to increase the rates of alkylation are outlined next. Increasing the ratio of acid to hydrocarbon phases to values from 1:l (which is commonlyused) up to perhaps 2:l promotes higher interfacial areas between the acid and dispersed hydrocarbon phases based on recent Purdue findings or increased levels of agitation which result in larger interfacial areas. Large areas are needed because the main alkylation reactions occur at such interfaces (Kramer, 1977a; Albright and Doshi, 1977). Second, the residence time in the reactor would need to be reduced simultaneously to minimize degradation reactions. Residence times at least as low as 5-7 min seem to be preferred. Third, the acid and hydrocarbon phases in the emulsion leavingthe reactor should be separated rapidly to minimize the undesired degradation reactions. At least two methods can be used to increase the rate of separation of the two phases. First, a mechanical separator such as centrifuge or perhaps a packed bed could be employed. The second method would involve recycling part of the hydrocarbon phase from the decanter. This recycle stream would decrease the ratio of the acid to hydrocarbon phases in the emulsion and hence promote coalescence. A centrifugal pump or a small well-agitated vessel would provide good contact between the two streams. Sufficient hydrocarbon phase might be recycled in order to convert the acid-continuous emulsion to a hydrocarboncontinuous one. These latter emulsions separate much quicker, often within several minutes, while acid-continuous emulsions may require 0.5-1.0 h for essentially complete separation. For this proposed separation procedure, additional research is needed to determine the preferred conditions including amount of hydrocarbon phase to be recycled, method of combining two streams, etc. Either of these two separation techniques would probably significantly reduce the amount of degradation reactions as compared to what is currently occurring in some, if not most, decanters.
Conclusions TMPs and DMPs degrade and isomerize when contacted with sulfuric acid to produce two organic materials: mixtures of C4-C14 isoparaffins with RON values of about 89-91 and ASOs. The rates of these reactions depend significantly on the compositions of both the acid and hydrocarbons, the temperature, and the interfacial area
between the acid and hydrocarbon phases. In at least some commercial alkylation plants, the alkylate product degrades and isomerizes to a significant extent causing a small but nevertheless significant drop in both quality and quantity.
Literature Cited Albright, L. F.; Spalding, M. A.; Faunce, J.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. Two-step Process Using Sulfuric Acid as Catalyst. Znd. Eng. Chem. Res. 1988a,27, 391-397. Albright, L. F.; Spalding,M. A.; Kopser, C. G.; Eckert, R. E. Alkylation of Isobutane with CI Olefins. Production and Characterization of Conjunct Polymers. Znd. Eng. Chem. Res. 1988b,27, 386-391.
Albright,L.F.;Spalding,M.A.;Nowinski,J.A.;Ybarra,R.M.;Eckert, R. E. Alkylation of Isobutane with C4 Olefins. First-Step Reactions Using Sulfuric Acid Catalyst. Znd. Eng. Chem. Res. 1988c,27, 381-386. Condon, F. E. Aluminum Bromide Isomerization of 2,2,4-Trimethylpentane. J. Am. Chem. SOC.1951,73,3938-3947. Condon, F. E. Isomerizationof Hydrocarbons. In Catalysis;Emmett, P. H. Ed.; Reinhold Publishing Corp.: New York, 1958;Vol. 6,pp 83-84. Deno, N. C.; Pittman, C. U. Carbonium Ions. XV. The Disproportionation of Cycloalkyl Cations to Cycloalkenyl Cations and Cycloalkanes. J. Am. Chem. SOC.1962,84,1744-1745. Deno, N. C.; Boyd, D. B.; Hodge, J. D.; Pittman, C. U.; Turner, J. 0. Carbonium Ions. XVI. The Fate of the t-butyl Cation in 96% HzSO,. J. Am. Chem. SOC.1964,86,1745-1748. Doshi,B. KineticandMechanisticStudyofAcidCatalyzedAlkylation of Isobutane with C4-Olefins at Low Temperatures. Ph.D. Thesis, Purdue University, 1975. Doshi, B. Albight, L. F. Degradation and Isomerization Reactions Occurringduring Alkylation of Isobutane with Light Olefins. Znd. Eng. Chem. Process Des. Deu. 1976,15,53-60. Hengstebeck, R. G. Here's an Easy Way to Figure Alkylate Yields Accurately. Oil Gas J. October 4, 1965,145-150. Hofmann, J. E. Ionic Reactions Occurring in Sulfuric Acid IV. Tritium Exchange between Tritiated Sulfuric Acid and Isoparaffine. J. Org. Chem. 1964,29,3039-3042. Joregenson, M. J.; Hartter, D. R. A Critical Re-evluation of the Hammett Acidity Function at Moderate and High Acid Concentrations of Sulfuric Acid. New Ho Values Based Solely on a Set of Primary Aniline Indicators. J. Am. Chem. SOC.1963,85,878883. Kramer, G. M. The Reaction of 2,3,4-Trimethylpentane in Concentrated Sulfuric Acid. J. Org. Chem. 1967a,32,920-923. Kramer, G.M. The Oxidation of Paraffins in Sulfuric Acid. J. Org. Chem. 1967b,32, 1916-1918. Li, K. W. Alkylation of Isobutane withvarious Olefins. Ph,D. Thesis, Purdue University, 1969. Li, K. W.; Eckert, R. E.; Albright, L. F. Alkylation of Isobutane with Light Olefins Using Sulfuric Acid: Operating Variables Affecting BothChemicalandPhysicalPhenomena. Znd.Eng. Chem.Process Des. Deu. 1970a,9,441-446. Li, K. W. Eckert, R. E.; Albright, L. F. Alkylation of Isobutane with Light Olefins Using Sulfuric Acid Operating Variables Affecting Physical Phenomena Only. Znd. Eng. Chem. Process Des. Dev. 1970b,9,434-440. Miron, S.; Lee, R. J. Molecular Structure of Conjunct Polymers. J. Chem. Eng. Data 1963,8,150-160. Roebuck, A. K.; Evering, B. L. Sulfuric Acid Isomerization of Methylalkanes, Dimethylalkanes, and Dimethylcyclohexanes. J. Am. Chem. SOC.1953,75,1631-1635. Sanders, S. J. Modeling Organics in AqueousSulfuric Acid Solutions. Znd. Eng. Chem. Process Des. Deu. 1985,24,942-948.
Received for review June 28, 1993 Revised manuscript received December 2, 1993 Accepted January 21, 1994' e
Abstract published in Advance ACS Abstracts, March 15,
1994.
