Alkylation of Isobutane with C3âC4 Olefins: Identification and

2110

Ind. Eng. Chem. Res. 1997, 36, 2110-2120

Alkylation of Isobutane with C3-C4 Olefins: Identification and Chemistry of Heavy-End Production Lyle F. Albright* and Karl V. Wood Purdue University, West Lafayette, Indiana 47907

The chemistry of the production of C10-C16 isoparaffins during the alkylation of isobutane has been significantly clarified based on the new analytical information obtained in this investigation. These heavy isoparaffins are produced when isobutane is alkylated with C3-C4 olefins using either sulfuric acid or HF as the catalyst: at least 14 families of isomers are present in this heavy-end fraction, and many C10-C12 isoparaffins present in larger quantities have now been identified. This new information also permits better evaluation of process conditions for alkylation and improved calculation of yields. Since 10-16 wt % of industrial alkylates often are C10-C16 isoparaffins, these heavier isoparaffins often have a significant effect on alkylate quality (or octane number) when they are left in motor fuels. To obtain this new analytical evidence, seven alkylates produced using different olefins and/or different acid catalysts were analyzed using a gas chromatographic unit, and the results were compared. In addition, mass spectrometric results and the normal boiling points of C10-C12 isoparaffins were employed. Introduction Alkylates produced in refineries often contain 1016% of so-called heavy-end isoparaffins in approximately the C10-C16 range. When pentenes are added to the olefin feedstocks, even higher amounts of heavy ends are often produced in addition to increased amounts of C9 isoparaffins. These heavy ends are often considered the less desired portion of the alkylate. Almost no information has previously been published on the composition in the C10-C16 range or on details of the chemistry involving their production. Alkylates are produced industrially when isobutane reacts with C3-C5 olefins using either sulfuric acid or HF as catalysts. Of all gasolines produced in a refinery, alkylates are the cleanest burning and have the highest octane numbers, often in the 94-97 research octane number (RON) range. About 1 million barrels of alkylate are produced daily in the U.S.A. Alkylate production is meanwhile increasing rapidly in many foreign countries. Durrett et al. (1963) analyzed several alkylates and reported the identity of six chromatographic peaks as being four trimethylheptanes and two tetramethylhexanes. No reasons for these choices were reported. More than six C10 isoparaffins would seem probable, and how and why tetramethylhexanes are produced were not explained. Other than this paper, relatively little is known about the composition of the heavier isoparaffins in alkylates or how changes of the operating conditions or of the olefin feedstocks affect the composition and the overall quality (or octane number) of the heavy ends in the alkylate. C4 olefins are most commonly used as feedstock olefins, but propylene and C5 olefins are also employed to an appreciable extent. When C4 olefins and propylene are used, more than 1 mol of olefin reacts on the average with each mole of isobutane for the production of C9 and heavier isoparaffins (Albright, et al., 1993). Yet little has been reported on the chemistry of the production of these heavier isoparaffins. The identification procedure initially reported by Li et al. (1970) for the trimethylpentane (TMP) family of isomers probably can be adapted to identify isomer * Author to whom correspondence should be addressed. S0888-5885(96)00265-5 CCC: $14.00

families in the C10-C16 range. They found that the compositions of the TMP family are often almost identical regardless of the quality of the alkylate. Such a finding is true for alkylates produced at the same temperature. Appreciable changes in composition, however, occur as the temperature varies in the range of -20 to 30 °C (Albright et al., 1977). The significant differences of the TMP compositions of commercial alkylates produced with sulfuric acid and HF are probably due mainly to temperature differences in the two processes. Small differences in TMP composition are, however, caused by variations of the sulfuric acid composition and the use of different olefins as feedstock. It has also been found that the compositions of the trimethylhexane (TMH) family are also similar in different alkylates. If the families of isomers in the C10-C16 range also have similar compositions in different alkylates, an important new identification technique will be available. More branched C8 isoparaffins generally elute from chromatographic columns before the less branched C8 isoparaffins, although there is significant intermingling of these two types. In general, more branched C8 isoparaffins are present at larger weight percents. Similar results also occur for C9 isoparaffins. Chromatographic results for various alkylates would suggest a similar phenomenon for isoparaffins heavier than C9’s. In the present investigation, families of C10, C11, C12, C13, C14, C15, and C16 positional isomers were identified for the first time. In addition, many major chromatographic peaks were identified as to the specific isoparaffin. This information permits a better understanding of the chemistry and the evaluation of process variables for alkylation. Identification Procedure Seven alkylates (alkylates 1-7) produced using different olefin feedstocks and/or different acid catalysts (see Table 1) were each analyzed with a gas chromatograph in the Stratco laboratory. A capillary column and flame ionization detector were used in this unit. This column was 50 m long, had a 0.32 mm i.d., and had a cross-linked phenyl methyl silicone coating. Hydrogen was used as a carrier for 0.06 µL samples. The column © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2111 Table 1. Seven Alkylates Produced from Isobutane and C3 and C4 Olefins alkylate no.a 1 2 3 4 5 6 7

olefin feed propylene 50:50 mixture of propylene and mixed C4 olefins isobutylene mixed C4 olefins n-C4 olefins approximately 50:50 mixture of propylene and mixed C4 olefins C4-rich olefins (some propylene and C5 olefins)

temp, °C acid catalystb 10 10

93.6% H2SO4 94.8% H2SO4

9.5 9.40% H2SO4 10 93.4% H2SO4 10 92.9% H2SO4 HF HF

a For alkylates 1-5, a space velocity of 0.29-0.30 and isobutane/ olefin ratio of 8 were used. b All acids contained acid-soluble oils and water.

was held at 35 °C for 5 min, then programmed with a 20 °C/min rise to 70 °C, held at 70 °C for 4 min, programmed with a 5 °C/min rise to 150 °C, held at 150 °C for 5 min, then programmed with a 10 °C/min rise to 280 °C, and held at 280 °C until the completion of the analysis. The C4-C9 isoparaffins in the alkylate separated into about 41 peaks in approximately the first 17 min. These isoparaffins were identified by comparison with published information for other alkylates including Durrett et al. (1963). The relative amounts of C4-C9 isoparaffins in the seven alkylates tested were similar to literature values. The compositions of the TMP family in the seven alkylates were essentially identical to those reported earlier by Li et al. (1970) and Albright et al. (1977). TMP and TMH compositions were similar to those reported earlier when different olefins or different acids are employed. About 100-120 peaks eluted for the heavier compounds in the time period from about 17 to 40 min. The same major peaks were generally detected in all alkylates. The elution time for a specific large peak agreed for all seven alkylates within about 0.05-0.10 min. For some alkylates at a given elution time, a relatively broad peak occurred, which suggests incomplete chromatographic separation of hydrocarbons, but in other alkylates, two distinct peaks were found at similar elution times. More chromatographic peaks were detected for alkylate 2 (produced using a mixture of C3-C4 olefins) than for alkylate 1 (produced using propylene) or for alkylate 5 (produced using n-butenes). First Identification Step. Small and known amounts (about 1-2 wt %) of each of the following pure isoparaffins were mixed in several cases with alkylate 2: 2,2,4,4-tetramethylpentane; 2,2,3,4-tetramethylhexane; 2,4,6-trimethylheptane; 3,3,5-trimethylheptane; 3,4,5-trimethylheptane; 3,3,4,4-tetramethylhexane; 2,2,6,6-tetramethylheptane; 2,2,7,7-tetramethyloctane; 2,3-dimethyldecane; and 2,2,3-trimethyldecane. The resulting mixtures were chromtographically analyzed to provide the following information: (1) Times to elute the known isoparaffins. The normal boiling points of the isoparaffins were correlated versus the chromatographic elution times to provide preliminary estimates of the approximate elution times for the C9, C10, ..., C13 families. Improved correlations were eventually developed, as will be discussed later. (2) Determinations if any of the pure isoparaffins added were actually present in the alkylate samples. (3) Correlation coefficients, if needed, to correct the weight percent value for a given isoparaffin. The known values were determined by measuring the amount of

each isoparaffin added to alkylate 2. The correlation coefficients of C5-C9 n-paraffins or isoparaffins were in all cases about 1.0. The coefficients increased, however, with increasing molecular weight from about 1.1 for C10’s to 2.0 for C16’s. Two explanations of the higher coefficients were considered. First, there is more of a problem in establishing a good baseline during the separation of heavier isoparaffins. These heavier isoparaffins often separate rather poorly from each other due to the large number of isomers present; as a result, broad peaks often occur that appear to raise the baseline. Second, partial decomposition or cracking of the heavier isoparaffins may occur, in the column of the chromatographic unit (Nurok, 1995). The heavier isoparaffins are subjected to higher temperatures and are in the capillary column for longer times. Determination of Isomer Families and Carbon Numbers. The chromatographic results of alkylates 1-7 were employed to identify for the first time 14 families of isomers and also provide key information on the carbon number for most peaks (or isoparaffins). A family is defined here as that group of peaks (or isoparaffins) in which the relative amounts of each peak were essentially identical for all or at least several of the alkylates investigated. The chemical reason that a family was present in only some alkylates will be discussed later. The families and their tentative carbon numbers are reported next. Families 1-3 were C10 families (see Table 2); families 4-6 were C11’s (see Table 3); families 7-9 were C12’s (see Table 4); and families 10 and 11 were C13’s (see Table 5). Table 6 indicates the results for families 12-14; they are postulated to be C14-C16 families, respectively. Additional information will be provided later as to the choice of carbon numbers for these families. Most larger peaks (or hydrocarbons with larger weight percents) were members of families. Some peaks were, however, not identified as belonging to a family, probably for at least three reasons. First, heavier isoparaffins are more difficult to separate than lighter isoparaffins, due to the increased number of isomers having similar boiling points. Some peaks may consist of two or more isoparaffins belonging to different families. Second, numerous peaks were very small which complicates family identification. Third, family composition sometimes varies depending on the chemical steps for production. For example, the composition of the dimethylpentane family differs significantly depending on the olefins used for production. For isoparaffins with a given carbon number, larger families (based on weight percent) tend to elute before smaller families. C10 and C11 isoparaffins are examples. Similar results occur for C8 and C9 isoparaffins. For C12 isoparaffins, two relatively large families tended to elute before the smallest family; the elution time of the larger two families overlapped to a high degree. The information obtained as a result of family identification was complemented by chemical information. C10-C16 isoparaffins are produced by either mechanism 2 or 3 (Albright et al., 1988). The analytical results of especially alkylates 1, 2, and 4 were next compared to determine the relative importance of these two mechanisms. In mechanism 2, i-C12H25 to i-C20H41 cations are first produced by reacting t-C4H9+ with two to perhaps five olefin molecules. These heavy cations having a wide range of isomeric structures can be produced by different sequences of chemical reactions. The following is a sequence in which several C4 olefins

0.029 0.004 0.006 0.003 0.042

4.904

total unknown C10’s

total C10’s

total diMeC8

unkown C10’s

0.078

0.198 0.022 0.220

90 10 100

3.8 20.5 30.3 25.6 3.9 15.9 100.0

0.174 0.937 1.381 1.170 0.177 0.725 4.564

0.046 0.032

wt % in family

wt % in alkylate

diMeC8 (tentative)

total tetraMeC6

tetraMeC6 (tentative)

2

3

2,4,6;2,2,4-triMeC7 2,2,6-triMeC7 2,2,5-triMeC7 2,5,5-triMeC7 2,4,5-triMeC7 2,3,4-triMeC7 total triMeC7

isoparaffin

1

family

alkylate 1: propylene: H2SO4

4.703

0.013

0.008 0.005

0.291 0.034 0.325

0.126

0.087 0.039

0.148 0.874 1.335 0.998 0.200 0.684 4.239

wt % in alkylate

90 10 100

3.5 20.6 31.5 23.5 4.7 16.1 99.9

wt % in family

alkylate 2: C3-C4 olefins: H2SO4

79 21 100

3.672

0.070

99.9

65.5 30.5 3.9

3.5 21.7 38.4 20.1 3.4 13.0 100.1

wt % in family

0.040 + 0.004 0.013 + 0.007 0.006

0.081 0.022 0.103

1.230

0.806 0.376 0.048

0.079 0.492 0.871 0.456 0.077 0.294 2.269

wt % in alkylate

alkylate 3: isobutlyene: H2SO4

Table 2. C10 Isoparaffins in Seven Alkylates Produced Using Different Olefins and/or Acids

1.523

0.011

0.007 0.004

0.010 0.010

0.543

0.347 0.169 0.027

0.040 0.191 0.333 0.209 0.033 0.143 0.949

wt % in alkylate

100.0

63.9 31.1 5.0

4.2 20.1 35.1 22.0 3.5 15.1 100.0

wt % in family

alkylate 4: mixed C4 olefins: H2SO4

0.840

0.003

0.003

0.005 0.005

0.297

0.190 0.092 0.015

0.023 0.108 0.188 0.118 0.017 0.081 0.535

wt % in alkylate

100.0

64.0 31.0 5.0

4.3 20.2 35.1 22.1 3.2 15.1 100.0

wt % in family

alkylate 5: n-C4 olefins: H2SO4

4.506

0.017

0.014 0.003

0.177 0.021 0.198

0.184

0.150 0.034

0.167 0.928 1.306 0.902 0.153 0.561 4.107

wt % in alkylate

89 11 100.0

100.0

81.5 18.5

4.1 22.5 31.8 24.2 3.7 13.7 100.0

wt % in family

alkylate 6: C3-C4 olefins: HF

1.342

0.133

0.094 0.036 0.003

0.014 0.014

0.164

0.132 0.032

0.043 0.281 0.291 0.198 0.043 0.175 1.031

wt % in alkylate

100.0

80.5 19.5

4.2 27.3 28.2 19.2 4.2 17.0 100.1

wt % in family

alkylate 7: C4-rich olefins: HF

2112 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997

0.004 + 0.047 0.006 + 0.039 0.003 + 0.008 0.031 0.138

3.826

unknown C11’s


total C11’s

a

0.011 0.023 0.043 0.077

5,5-diMeC9 2,4-; 4,4-diMeC9 2,5-diMeC9 total diMeC9

6

Sum of peaks with wt % of 0.012 and 0.011.

0.062 0.038 0.112 0.072 0.051 0.335

2,4,4-triMeC8 2,2,4-triMeC8 2,5,5-triMeC8 2,2,6-; 2,2,7-triMeC8 2,6,6-triMeC8 total triMeC8

5

0.049 0.130 0.022 0.066 1.972 0.007 0.423 0.397 0.210 3.276

wt % in alkylate

2,2,4,6-tetraMeC7 2,2,6,6-tetraMeC7 2,2,5,5-tetraMeC7 2,2,5,6-tetraMeC7 2,2,4,5-tetraMeC7 unknown 2,3,5,5-tetraMeC7 mixture mixture total tetraMeC7

isoparaffin

4

family

14 30 56 100

18.5 11.3 33.4 21.5 15.2 99.9

1.5 4.0 0.7 2.0 60.2 0.2 12.9 12.1 6.4 100.0

wt % in family


13 30 57 100

24.3 12.8 22.9 19.2 20.9 100.1

1.5 4.0 0.7 1.7 58.4 0.3 13.8 12.5 7.1 100.0

wt % in family

4.607

0.195

0.005 + 0.121 0.032 + 0.010 0.027

0.023 0.052 0.098 0.173

0.209 0.110 0.197 0.165 0.180 0.861

0.051 0.134 0.025 0.057 1.972 0.009 0.468 0.423 0.239 3.378

wt % in alkylate


0.171 3.693

1.949

0.055

0.015 0.014 0.026

0.013

0.005 + 0.034 0.029 + 0.070 0.033

0.021

0.061 0.031 0.101 0.069 0.043 0.305

0.031 0.086 0.013 0.025 0.880 0.003 0.242 0.195 0.101 1.576

wt % in alkylate

0.004 0.009

17.2 7.2 34.7 27.1 13.8 100.0

1.7 5.2 0.6 2.1 56.4 0.3 15.3 12.3 6.2 100.1

wt % in family

20.0 10.2 33.1 22.6 14.1 100.0

2.0 5.5 0.8 1.6 55.8 0.2 15.4 12.4 6.4 100.1

wt % in family


0.007 0.014

0.084 0.035 0.169 0.132 0.067 0.487

0.050 0.156 0.018 0.064 1.700 0.009 0460 0.371 0.186 3.014

wt % in alkylate

alkylate 3: isobutlyene: H2SO4


19.1 99.9

0.026 0.136

1.112

0.019

0.009 0.010

25.7 12.5 42.6

15.3 12.5 6.4 100.0

0.146 0.120 0.061 0.957 0.035 0.017 0.058

1.7 5.1 0.7 1.0 57.3

wt % in family

0.016 0.049 0.007 0.010 0.548

wt % in alkylate


14 31 55 100

26.9 12.9 19.7 21.9 18.6 100.0

1.1 6.0 0.5 1.2 57.1 0.3 14.5 12.6 6.8 100.1

wt % in family

6.697

0.005 + 0.003 0.002 + 0.055 0.002 + 0.029 0.002 + 0.111 0.209

0.023 0.051 0.092 0.166

0.235 0.113 0.712 0.192 0.163 0.875

0.058 0.327 0.026 0.066 3.109 0.014 0.792 0.686 0.369 5.447

wt % in alkylate


2.874

0.112

0.002 + 0.023 0.003 + 0.016 0.002 + 0.066

0.016 (0.023)a 0.027 0.066

0.125 0.063 0.101 0.052 0.087 0.428

0.341 0.278 0.157 2.268

0.043 0.154 0.015 0.054 1.226

wt % in alkylate

24 35 41 100

29.2 14.7 23.6 12.2 20.3 100.0

15.0 12.3 6.9 100.1

1.9 6.8 0.7 2.4 54.1

wt % in family


Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2113

pentaMeC7 2,2,4,5,6-; 2,2,3,6,6-pentaMeC7 2,2,4,4,6-pentaMeC7 2,2,3,4,6-pentaMeC7 2,3,4,4,6-pentaMeC7 2,3,3,5,6-pentaMeC7 2,2,3,3,6-pentaMeC7 2,2,2,3,6-pentaMeC7 2,3,4,5,6-pentaMeC7 2,2,4,4,5-pentaMeC7 2,3,4,5,5-pentaMeC7 total pentaMeC7

tetraMeC8 2,2,5,5-tetraMeC8 2,4,6,6-tetraMeC8 2,2,6,7-tetraMeC8 2,2,3,5-tetraMeC8 total tetraMeC8

triMeC9 family

7

8

9

0.244 1.019

total unknowns

total C12’s 2.545

0.584

0.093 0.035 0.456

0.035 0.015 0.145

0.010 0.020 0.055 0.041 + 0.175 0.013 0.015

0.045 0.050

99.9

0.224

0.087 0.358 0.264 0.046 0.755

0.253 0.091 0.018 0.024 0.087 0.234 0.037 0.237 0.038 0.042 1.061

wt % in alkylate

24 10 100

31 35

11.5 47.4 35.0 6.1 100.0

23.8 8.6 1.7 2.3 8.2 22.1 3.5 22.3 3.6 4.0 100.1

wt % in family


0.025 0.010

11.6 47.3 41.0

24.0 8.5 1.2 2.2 8.5 23.6 3.4 23.0 3.6 2.0 100.0

wt % in family

0.026 0.106 0.092

0.119 0.042 0.006 0.011 0.042 0.117 0.017 0.114 0.018 0.010 0.496

wt % in alkylate

unknown C12’s

total triMeC9

isoparaffin

family


9.815

0.609

0.090 + 0.032 0.010 + 0.020 0.172 + 0.285

0.129 0.080 0.047 0.074 0.038 0.368

0.367 2.715 1.996 0.336 5.414

0.871 0.360 0.067 0.075 0.270 0.755 0.025 0.739 0.089 0.173 3.424

wt % in alkylate

35.1 21.7 12.8 20.1 10.3 100.0

6.8 50.1 36.9 6.2 100.0

25.2 10.5 1.9 2.2 7.9 22.1 0.7 21.6 2.6 5.1 100.0

wt % in family

alkylate 3: isobutylene: H2SO4


6.477

0.410

0.305 0.027 0.078

0.103 0.149 0.032 0.043 0.018 0.345

0.175 1.241 0.802 0.136 2.354

0.854 0.304 0.061 0.075 0.259 0.752 0.082 0.710 0.090 0.181 3.368

wt % in alkylate

29.9 43.2 9.3 12.5 5.2 100.1

7.4 52.7 34.1 5.8 100.0

25.4 9.0 1.8 2.2 7.7 22.3 2.4 21.1 2.7 5.4 100.0

wt % in family


12 3 100

31 54

6.3 53.2 34.4 6.0 99.9

25.6 9.5 1.9 2.2 7.6 21.8 2.1 21.3 2.5 5.5 100.0

wt % in family

4.535

0.234

0.143 + 0.039 0.015 + 0.011 0.026

0.023 0.005 0.188

0.059 0.101

0.116 0.979 0.633 0.111 1.839

0.581 0.215 0.044 0.051 0.172 0.495 0.049 0.484 0.057 0.126 2.274

wt % in alkylate


27 38 6 20 9 100

8.1 41.0 44.7 6.2 100.0

29.5 10.2 2.5 2.3 7.5 20.1 1.8 19.1 2.6 4.4 100.0

wt % in family

3.006

0.125

0.010 + 0.026 0.083 + 0.006

0.042 0.061 0.010 0.031 0.014 0.158

0.086 0.434 0.473 0.066 1.059

0.490 0.169 0.042 0.039 0.124 0.335 0.030 0.318 0.044 0.073 1.664

wt % in alkylate


4.907

0.150 0.026 0.022 0.024 0.222

0.095 0.124 0.022 0.061 0.029 0.331

0.098 0.686 0.689 0.107 1.580

0.782 0.274 0.071 0.065 0.195 0.563 0.071 0.555 0.080 0.118 2.774

wt % in alkylate

29 37 7 18 9 100

6.2 43.4 43.6 6.8 100.0

28.2 9.9 2.5 2.3 7.0 20.3 2.6 20.0 2.9 4.3 100.0

wt % in family



a

0.237 0.025 0.039 0.301 0.019 0.043 0.014 0.009 0.085 0.807

C13 family probably moderately branched total

unknown C13’s


total C13’s

0.060 0.027 0.023 0.010 0.019 0.013 0.039 0.029 0.010 0.089 0.074 0.028 0.421

wt % in alkylate

Sum of peaks with wt % of 0.095 and 0.111.

11

C13 family probably highly branched

10

total

isoparaffin

family

78.7 8.3 13.0 100.0

14.3 6.4 5.5 2.4 4.5 3.1 9.3 6.9 2.4 21.1 17.6 6.6 100.1

wt % in family


2.163

0.245

0.068 0.107 0.052 0.018

0.477 0.054 0.095 0.626

0.180 0.087 0.076 0.032 0.056 0.041 0.119 0.094 0.033 0.267 0.221 0.086 1.292

wt % in alkylate

76.2 8.6 15.2 100.0

13.9 6.7 5.9 2.5 4.3 3.2 9.2 7.3 2.6 20.7 17.1 6.7 100.1

wt % in family


7.1 4.8 2.5 14.4 12.4 5.3 100.0

0.028 0.019 0.010 0.057 0.049 0.021 0.395

0.937

0.150

0.012 0.058 0.048 0.032

78.8 4.6 16.2 100.0

10.4 4.6 25.1

0.041 0.018 0.099

0.309 0.018 0.065 0.392

13.4

wt % in family

0.053

wt % in alkylate



0.331

0.034

0.008 0.019 0.007

0.095 0.007 0.031 0.133

71.4 5.3 23.3 100.0

99.9

14.6 11.6

0.024 0.019 0.164

6.1 3.0

20.1

0.033 0.010 0.005

14.0 20.1 10.4

wt % in family

0.023 0.033 0.017

wt % in alkylate


0.219

0.088

0.049 0.019 0.014 0.006

0.049 0.006 0.011 0.066

0.065

0.011 0.009

0.001

0.013

0.012 0.019

wt % in alkylate

74 9 17 100

14 100

17

2

20

18 29

wt % in family


1.015

0.199 0.014 0.026 0.019 0.028 0.286

0.202 0.018 0.040 0.260

0.026 0.016 0.114 0.065 0.035 0.469

0.066 0.038 0.016 0.014 0.028 0.051

wt % in alkylate

77.7 6.9 15.4 100.0

5.5 3.4 24.3 13.8 7.5 100.0

14.7 8.1 3.4 3.0 6.0 10.9

wt % in family


73.0 8.5 18.4 99.9

(0.206)a 0.024 0.052 0.282

0.894

0.154 + 0.012 0.020 0.048 0.043 0.005 0.282

6.7 3.3 13.6 13.0 5.5 100.0

12.7 14.2 7.6 3.3 11.2 8.8

wt % in family

0.022 0.011 0.045 0.043 0.018 0.330

0.042 0.047 0.025 0.011 0.037 0.029

wt % in alkylate



0.210 0.050 0.026 0.286

0.119

0.405

0.016 0.040 0.010 0.066

0.014

0.080

0.007 0.006 0.006 0.019

0.099

0.118

unknown C14’s

total C14’s

family 13 (C15 family)

unknown C15’s

total C15’s


unknowns C16’s

total C16’s

wt % in alkylate


isoparaffin

37 31.5 31.5 100

24 61 15 100

73 18 9 100

wt % in family


0.285

0.233

0.023 0.019 0.010 0.052

0.307

0.088

0.080 0.110 0.029 0.219

0.768

0.383

0.257 0.085 0.041 0.385

wt % in alkylate

44 37 19 100

37 50 13 100

67 22 11 100

wt % in family


0.765

0.498

0.063 0.161 0.043 0.267

0.483

0.194

0.088 0.145 0.056 0.189

0.781

0.273

0.406 0.058 0.044 0.508

wt % in alkylate

24 60 16 100

31 50 19 100

80 11 9 100

wt % in family


0.415

0.261

0.032 0.093 0.029 0.154

0.134

0.033

0.030 0.055 0.016 0.101

0.167

0.039

0.102 0.013 0.013 0.128

wt % in alkylate

21 60 19 100

30 54 16 100

80 10 10 100

wt % in family


Table 6. C14, C15, and C16 Isoparaffins in Seven Alkylates Produced Using Different Olefins and/or Acids

0.161

0.093

0.019 0.037 0.012 0.068

0.051

0.007

0.004 0.033 0.007 0.044

0.095

0.039

0.049 0.003 0.004 0.056

wt % in alkylate

28 54 18 100

9 75 16 100

88 5 7 100

wt % in family


0.147

0.097

0.015 0.028 0.007 0.050

0.245

0.111

0.033 0.074 0.027 0.134

0.524

0.283

0.186 0.035 0.020 0.241

wt % in alkylate

30 56 14 100

25 55 20 100

77 15 8 100

wt % in family


0.301

0.197

0.024 0.062 0.018 0.104

0.168

0.082

0.020 0.049 0.017 0.086

0.124

0.078

0.036 0.009 0.046

wt % in alkylate

23 60 17 100

23 57 20 100

wt % in family




react one at a time: C 4H 8

C 4H 8

C4H8

t-C4H9+ 98 i-C8H17+ 98 i-C12H25+ 98 i-C16H33+ The cations, however, can deprotonate to produce olefins. One example is as follows:

i-C12H25+ T i-C12H24 + H+ Heavy olefins also react with t-C4H9+, and the following is another method to produce i-C16H33+: C 4H 8

t-C4H9+ + i-C12H24 98 i-C16H33+ In mechanism 2, heavy cations fragment, producing C4C16 cations and olefins. Hydride transfer to the cations produces C4-C16 isoparaffins. Depending to some extent on the olefin feed, mechanism 2 is often the predominant or even the only route for production of light ends (C5-C7 isoparaffins), dimethylhexanes, and C9 isoparaffins (Hofmann and Schriesheim, 1962; Albright et al., 1988). With the proper choice of the olefin feedstock, C10 and heavier isoparaffins can also be produced by mechanism 3 (Albright et al., 1988). For example, mechanism 3 is a potential mechanism to produce a C10 isoparaffin if two propylene molecules react with t-C4H9+ to produce i-C10H21+(probably in most cases a trimethylheptyl cation). Hydride transfer would result in production of the C10 isoparaffin, often a trimethylheptane. The overall chemistry is as follows:

i-C4H10 + 2C3H6 f i-C10H22 C11 isoparaffins can also be produced by mechanism 3 if both propylene and C4 olefins are present. A i-C11H23+ would form as an intermediate when one molecule of propylene and one of a C4 olefin react with t-C4H9+; subsequent hydride transfer produces a C11 isoparaffin, which based on chemical considerations would probably be a tetramethylheptane. The overall reaction would be as follows:

i-C4H10 + C3H6 + C4H8 f i-C11H24 C12 isoparaffins are also produced by mechanism 3 when C4 olefins react with isobutane; the predominant C12 family would often be pentamethylheptanes.

i-C4H10 + 2C4H8 f i-C12H26 Based on the above chemical considerations, propylene promotes C10 production; a mixture of propylene and a C4 olefin promotes C11 production; and C4 olefins promote C12 production. Such a postulate has never been tested until now. A comparison of the chromatographic results for alkylates 1, 2, and 4 supports this postulate, as explained next. (a) The following comparison of the weight percent of family 1 (C10 family) suggests that mechanism 3 contributes significantly to its production when propylene is included as part of the feedstock; see Table 2:

alkylate 1 > alkylate 2 > alkylate 4 Production of families 2 and 3 will be discussed later. (b)Mechanism 3 also is of importance for the production of C11 isoparaffins including families 4-6 when a mixture of propylene and a C4 olefin is used as the olefin

feedstock; such a conclusion is based on the following comparisons of weight percent; see the results of Table 3:

alkylate 2 g alkylate 1 > alkylate 4 (c) Families 7-9, which are all C12 isoparaffins, plus several unidentified C12’s have the following weight percent comparison, indicating mechanism 3 is of importance for production of C12 isoparaffins from C4 olefins; see Table 4:

alkylate 4 > alkylate 2 > alkylate 1 The isoparaffins that eluted immediately after the C12 isoparaffins were present in the largest amounts in alkylate 2 as compared to alkylates 1 and 4; see Tables 5 and 6. A total of about 40 peaks showing such a behavior are thought to be C13-C15 isoparaffins; Families 10-13 were present in this group of peaks. C13’s could be produced by mechanism 3 if three molecules of propylene reacted with one molecule of isobutane; tetramethylnonanes based on chemical considerations would presumably then be the major C13 family produced. Mechanism 3 is apparently in this case, however, of lesser importance since C13 peaks are present in considerably smaller amounts in alkylate 1; see Table 5. Mechanism 2 was apparently the major route for production of C13’s in all alkylates investigated. Families 10 and 11 are almost certainly C13 isoparaffins, based on their time of elution from the chromatographic column. Both C14 and C15 isoparaffins can be produced by mechanism 3 when both propylenes and C4 olefins are in the feed stream, such as was used to produce alkylate 2 (see Table 6). Even larger weight percents of these isoparaffins are present in alkylate 3 (produced using isobutylene), which strongly suggests that mechanism 2 is then of major importance. Insufficient information is available, in part because of the small size of the peaks, to determine if mechanism 2 or 3 is predominant for the formation of C14’s and C15’s in alkylate 2. Families 12 and 13 are thought to be C14’s and C15’s, respectively, again based primarily on their times of elution. After the C14 and C15 isoparaffins eluted, seven relatively small peaks showed the following relative order in weight percents:

alkylate 4 > alkylate 2 > alkylate 1 This order suggests that at least seven C16 isoparaffins are produced to at least a high extent by mechanism 3, in which three molecules of C4 olefins react with a single isobutane molecule. Based on the likely mechanism, either heptamethylnonanes or hexamethyldecanes would be the predominant families. Family 14 (see Table 6) consists of three of these peaks. Following these C16 peaks, several small peaks eluted that were probably C17’s and C18’s; their concentrations were largest in alkylates 2 or 3. Family 2, a C10 family, was present in only alkylates 3-5, all of which were produced using C4 olefins. Hence, it was produced only by mechanism 2. Intermediate cations produced likely include i-C8H17+, i-C12H25+, i-C16H33+, etc. Most i-C8H17+’s are probably one of several TMP+’s, but several DMH’s also are present. Different i-C12H25+’s obviously form depending on the i-C8H17+’s that reacts with C4 olefins. Most i-C12H25+’s are probably pentamethylheptyl+’s but some


may have ethyl or isopropyl branches. For example, an isopropyl branch forms if 2,3,4-TMP+(with the charge on the third carbon) reacts with either isobutylene or a 2-butene. When propylene is the olefin used, the cations produced are mainly i-C7H15+, i-C10H21+, i-C13H27+, i-C16H33+, etc. These cations contain mainly methyl branches, but some ethyl branches are possible for C10H21+’s and heavier cations. Further, these latter cations would be less branched than those produced from C4 olefins. Based on chemical considerations, family 2 which forms exclusively by mechanism 2 might be tetramethylhexanes since it is produced from only C4 olefins. Family 3 was present only in alkylates 1, 2, and 6 for which the olefin feed is either pure propylene or mixtures of C3 and C4 olefins. The heavy cations needed to produce family 3 are apparently not produced from just C4 olefins. Possibly family 3 is a C11 family, but its elution times in comparison to neighboring families strongly suggest it is an i-C10 family, and it is probably less branched than trimethylheptanes. Based on information obtained by comparing the chromatographic results for alkylates 1-7, carbon numbers for the various chromatographic peaks were established with high probability for all C10-C13 isoparaffins. There is less confidence for the C14 and C15 families, which are present in only relatively small amounts. Tables 2-6 indicate carbon numbers based on the results of this investigation. The elution times for isoparaffins with a given carbon number are approximately as follows: C10’s, 16.4-19.8 min; C11’s, 19.2-23.4 min; C12’s, 23.2-27.3 min; C13’s, 26-31 min; and C14’s and C15’s, 30-36 min. Seven small peaks thought to be C16’s eluted from 34.5 to 36.7 min. Some small and unidentified peaks eluted after 37 min; these latter peaks had the largest weight percent values in alkylates 2-4. Identification of Chromatographic Peaks. The following additional information was employed to identify for the first time major C10-C12 isoparaffins in the alkylates, as reported in Tables 2-4. (a)Normal boiling points of C10-C12 isoparaffins (Das et al., 1993). The elution time of a compound in a chromatographic column is generally proportional to the normal boiling point, especially for families of isoparaffins. Such data are, however, extremely limited for C13 and heavier isoparaffins. (b) Mass spectrometric (MS) data obtained in Purdue University’s Campus-wide Mass Spectrometry Center for the major chromatographic peaks of alkylate 2 and for 12 pure C9-C13 isoparaffins. The analyses of these pure isoparaffins provided both MS and elution time information. (c) Limited literature MS data for C10 and heavier isoparaffins (Heller and Milne, 1978; McLafferty and Stauffer, 1989; Mass Spectral Data Centre, 1991; NIST/ EPA/NIH, 1992). The following three criteria were all met in the identification of the isoparaffin for a specific GC peak: (a) The isoparaffin was produced by a predicted chemical pathway. (b) The isoparaffin agreed well on plots of the boiling point versus elution time. Seven separate plots were obtainedsone each for C7, C8, C9, C10, C11, C12, and C13 isoparaffins. Construction of these plots was significantly aided by the chromatographic data obtained using alkylate 2 samples to which had been added small amounts of known heavy isoparaffins. The plot of the

isoparaffins with a given carbon number overlapped in all cases with the plot of the isoparaffins with a lower (or higher) carbon number. As an example at an elution time of 19.5 min, the C10 plot predicts a boiling point of 159 °C. At 19.5 min, some C11 isoparaffins had already eluted, and the C11 plot predicts a boiling point of about 164 °C. In addition for isoparaffins with a given carbon number, less branched isoparaffins, having only one or two methyl side groups, may have boiling points that are higher by perhaps 1 °C at a given elution time as compared to more branched isoparaffins. (c) The MS results obtained at Purdue University for different chromatographic peaks of alkylate 2 were compared to predicted MS results for the C10-C12 isoparaffins proposed in Tables 2-4. Predictions had to be made in most cases due to the limited amounts of MS data available for C10-C12 isoparaffins. These predictions are probably relatively good. For example, isoparaffins containing 2,2-dimethyl structures always have distinctive 43, 56, and 57 peaks. Tables 2-4 indicate the predicted identities of major peaks in the C10-C12 isoparaffin range. The identities of the largest peaks are considered to be relatively certain. The presence of more than one isoparaffin in a chromatographic peak can prevent identification. Another complicating factor is that at least some heavy isoparaffins exist as two or more stereoisomers which produce more than one chromatographic peak (Durrett et al., 1963; Lubeck and Sutton, 1983). For example, 2,3,4-TMH has two distinct peaks, which was confirmed in the present investigation. For several minor peaks, no reliable MS data were obtainable and/or the peak did not appear to be a family member. Such peaks are listed as unidentified. For alkylate 2, at least two minor peaks were olefins; they had fragmentation patterns that can be attributed only to olefins. Based on the elution times, they were probably a C9 and a C11 olefin. Possibly these two olefins were produced when isoalkyl sulfates decomposed. Insufficient boiling point information is currently available to identify C13 or heavier peaks. Boiling point data of C9-C12 isoparaffins might be extrapolated in the future to help identify C13 and heavier isoparaffins. Discussion of Results The present investigation has provided valuable new information on the amounts and compositions of heavy isoparaffins in alkylates produced when isobutane is reacted with light olefins. The present results disagree with those of Durrett et al. (1963) relative to two rather large chromatographic peaks identified by them as being tetramethylhexanes. These peaks were found in the present investigation to be trimethylheptanes and part of family 1. Their production was promoted when propylene was used as the olefin feed; based on chemical considerations, production of tetramethylhexanes would be highly unlikely. As indicated in Table 7 (which summarizes the results of Tables 2-6), the relative amounts of both the isoparaffins with a given carbon number and families 1-14 differ significantly in alkylates 1-5 (produced using H2SO4 as the catalyst). The sum of the weight percents of the C9-C16 isoparaffins is greatest when isobutylene is the olefin. The smallest sums occurred when propylene and especially n-butenes were used as olefin feeds. Table 7 indicates that C4 olefins promote the formation of C8, C12, and C16 isoparaffins. Propylene promotes the

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2119 Table 7. Summary of C8-C16 Compositions of Seven Alkylates Produced Using Different Olefins and/or Acids carbon no.

alkylate 1

alkylate 2

alkylate 3

alkylate 4

alkylate 5

alkylate 6

alkylate 7

8

trimethylpentanes dimethylhexanes methylheptanes unknowns total C8’s

8.833 3.061 0.064 0.055 12.013

42.686 5.956 0.147 0.024 48.813

55.370 7.690 0.048 0.003 63.111

63.087 11.257 0.082

72.677 10.990 0.069

74.426

83.736

37.306 7.979 0.062 0.025 45.372

48.116 9.795 0.109 0.014 58.034

9

trimethylhexanes dimethylheptanes methyloctanes total C9’s

1.973 0.330 0.075 2.378

1.887 0.472 0.096 2.455

7.383 0.490 0.094 7.967

3.238 0.352 0.052 3.642

2.030 0.157 0.029 2.216

3.783 0.561 0.097 4.441

8.655 1.446 0.289 10.390

10

trimethyl-C7’s tetramethyl-C6’s dimethyl-C8’s unknowns total C10’s

4.564 0.078 0.220 0.042 4.904

4.239 0.126 0.325 0.013 4.703

2.269 1.230 0.103 0.070 3.672

0.949 0.543 0.010 0.011 1.523

0.535 0.297 0.005 0.003 0.840

4.107 0.184 0.198 0.017 4.506

1.031 0.164 0.014 0.0133 1.342

11

tetramethyl-C7’s trimethyl-C8’s dimethyl-C9’s unknowns total C11’s

3.276 0.335 0.077 0.138 3.826

3.378 0.861 0.173 0.195 4.607

3.014 0.487 0.021 0.171 3.693

1.576 0.305 0.013 0.055 1.949

0.957 0.136 0.019 1.112

5.447 0.875 0.166 0.209 6.697

2.268 0.428 0.066 0.112 2.874

12

pentamethyl-C7’s tetramethyl-C8’s trimethyl-C9’s unknowns total C12’s

0.496 0.224 0.055 0.244 1.019

1.061 0.755 0.145 0.584 2.545

3.191 5.414 0.368 0.609 9.815

3.368 2.354 0.345 0.410 6.477

2.274 1.839 0.188 0.234 4.535

1.664 1.059 0.158 0.125 3.006

2.774 1.580 0.331 0.222 4.907

13

family 10 family 11 unknowns total C13’s

0.421 0.301 0.085 0.807

1.292 0.626 0.245 2.163

0.395 0.392 0.150 0.937

0.164 0.133 0.034 0.331

0.065 0.066 0.088 0.219

0.469 0.260 0.286 1.015

0.330 0.282 0.282 0.894

14

family 12 unknowns total C14’s

0.286 0.119 0.405

0.385 0.383 0.768

0.508 0.273 0.781

0.128 0.039 0.167

0.056 0.039 0.095

0.241 0.283 0.524

0.046 0.078 0.124

15


0.066 0.014 0.080

0.219 0.088 0.307

0.289 0.194 0.483

0.101 0.033 0.134

0.044 0.007 0.051

0.134 0.111 0.245

0.086 0.082 0.168

16


0.019 0.099 0.118

0.052 0.233 0.285

0.267 0.498 0.765

0.154 0.261 0.415

0.068 0.093 0.161

0.050 0.097 0.147

0.104 0.197 0.301

13.537

17.833

28.113

14.638

9.229

20.581

21.000

total C9’s and heavier

production of family 1 (trimethylheptanes). Mixtures of C3 and C4 olefins promote production of C11, and C13C15 isoparaffins. Isobutylene which alkylates to a high degree via mechanism 2 promotes formation of most heavy isoparaffins. Future investigations are, however, needed to quantify how various operating variables including temperature, level of agitation, isobutane/ olefin ratio, acid composition, and acid/hydrocarbon volumetric ratio affect heavy-end production. Increasing the isobutane/olefin ratio would, for example, with high certainity minimize the relative importance of both mechanisms 2 and 3 and produce fewer heavy isoparaffins. When n-butenes are used as olefins, one of the several modifications of the two-step alkylation process (Albright et al., 1988) can be used to obtain extremely high isobutane/olefin ratios in the reaction mixtures. secButyl sulfates are produced as intermediates from n-butenes. Only three peaks (or presumably three isoparaffins) were found in each of families 12-14 that are probably C14-C16 families respectively. Each family likely contains more members, and more families are probably present in the C14-C16 range. Such members or families were, however, not detected in the present investigation presumably because of the very low concentrations of C14 and heavier isoparaffins present in the alkylates. To test this hypothesis, various alkylates could be fractionated to obtain mixtures containing relatively high concentrations of C14 and heavier isoparaffins. For C14-C16 isoparaffins, families 12-14

probably have the largest concentrations on a weight percent basis and are also likely the most branched isoparaffins. The improved identification procedure developed here has resulted in improved material balances for pilotplant runs made by Stratco. Yields of alkylate based on both the moles of olefins and of isobutane reacted can, hence, be more accurately calculated since yield calculations require an accurate analysis of the alkylate (Albright et al., 1993). The C10 and heavier isoparaffins almost certainly have an important effect on the quality (or octane number) of alkylates since they are present in concentrations frequently in the 10-16 wt % range. Furthermore, the quality of the heavy ends likely differs significantly for each of the seven alkylates investigated here, based on the general rules developed for C7-C9 isoparaffins. Isoparaffins with more branches or with higher ratios of branches/carbon number have significantly higher octane numbers or higher blending octane numbers. Two examples are as follows: TMP’s versus dimethylhexanes and dimethylpentanes versus dimethylhexanes. Based on these comparisons, the average octane numbers of several families of heavy isoparaffins are probably in the following order: pentamethylheptanes > tetramethyloctanes; tetramethylheptanes > tetramethyloctanes; trimethylhexanes > trimethylheptanes > trimethyloctanes. C10 isoparaffins as a group likely have relatively low octane numbers. C11 and


especially C12 isoparaffins may have significantly higher octane numbers. The research octane number (and the motor octane number) of different members of the TMP or TMH family differs by about 4-9 units. Presumably significant differences in octane numbers also occur for different members of families 1-14. Additional data are obviously needed so that the overall quality of the heavy isoparaffins in different alkylates can be better quantified. Conclusions The amounts and compositions of C10 and heavier isoparaffins in typical alkylates differ significantly depending on the olefin feed employed and on the operating conditions. The improved identification of these heavy isoparaffins, as developed in this investigation, can be used to better optimize alkylation processes. Acknowledgment David Graves and Ken E. Kranz of Stratco, Inc., of Leawood, KS, provided valuable advice and arranged for gas chromatographic analyses of the alkylate samples. George Kramer provided important advice concerning the variety of families and branching of isoparaffins in the heavy ends. Literature Cited Albright, L. F.; Doshi, B. M.; Ferman, M. A.; Ewo, A. Two-Step Alkylation of Isobutane with C4 Olefins: Reaction of Isobutane with Initial Reaction Products. In ACS Symposium Series; Albright, L. F., Goldsby, A. R., Eds.; American Chemical Society, Washington, DC, 1977; Vol. 55, p 109. 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. Ind. Eng. Chem. Res. 1988, 27, 391.

Albright, L. F.; Kranz, K. E.; Masters, K. Alkylation of Isobutane with Light Olefins: Yields of Alkylates for Different Olefins. Ind. Eng. Chem. Res. 1993, 22, 2991. Das, A.; Frenkel, M.; Gadalla, N. M.; Marsh, K. N.; Wilhoit, R. C. TRC Thermodynamic Tables: Hydrocarbons; Thermodynamic Research Center, Texas A & M University, College Station, TX, 1993; Vol. 7. Durrett, L. R.; Taylor, L. M.; Wantland, C. F.; Dvoretzky, I. Component Analysis of Isoparaffin-Olefin Alkylate by Capillary Gas Chromatography. Anal. Chem. 1963, 35, 637. Heller, S. R.; Milne, G. W. A. EPA/NIH Mass Spectral Data Base; U.S. Department of Commerce, National Bureau of Standards: Washington, DC, 1978; Vols. 1 and 2. Hofmann, J. E.; Schriesheim, A. Ionic Reactions Occurring During Sulfuric Acid Catalyzed Alkylation. J. Am. Chem. Soc. 1962, 84, 953-961. Li, K. W.; Eckert, R. E.; Albright, L. F. Alkylation of Light Olefins Using Sulfuric Acid. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 434. Lubeck, A. J.; Sutton, D. L. Kovats Retention Indices of Select Hydrocarbon Through C10. J. High Resolut. Chromatogr. Chromatogr. Commun. 1983, 6, 328. Mass Spectrometry Data Centre. Eight Peak Index of Mass Spectra, 4th ed.; Royal Society of Chemistry: Cambridge, U.K., 1991. McLafferty, F. W.; Stauffer, D. B. Wiley/NBS Registry of Mass Spectral Data; John Wiley and Sons: New York, 1989. NIST/EPA/NIH Mass Spectral Library, NIST Mass Spectral Search Program, Washington, DC, 1992. Nurok, D. Personal communication, Purdue University, 1995.

Received for review May 9, 1996 Revised manuscript received March 12, 1997 Accepted March 17, 1997X IE960265F

X Abstract published in Advance ACS Abstracts, May 1, 1997.

Alkylation of Isobutane with C3âC4 Olefins: Identification and

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