Ind. Eng. Chem. Res. 1993,32, 2991-2996
2991
Alkylation of Isobutane with Light Olefins: Yields of Alkylates for Different Olefins Lyle F. Albright' Purdue University, West Lafayette, Indiana 47907
Ken E. Kranz and Kenneth R. Masters STRATCO, Znc., Leawood, Kansas 66211
For alkylation of isobutane with C3-C5 olefins using sulfuric acid as the catalyst, the yields of alkylates with different olefins are compared as the operating conditions are changed. The results of recent pilot plant experiments with propylene, C4 olefins, and c6 olefins permit such comparisons. The yields expressed as weight of alkylate produced per 100wt of olefin consumed varied from about 201:lOO to 220:lOO. Weight ratios of the isobutane consumed per olefin consumed vary from about 101:lOO t o 120:lOO. Differences of yield values are explained by the changes in the overall chemistry. The procedure employed t o calculate yields with good accuracy is based on the analysis of the alkylate and the amount of conjunct polymers produced. Based on literature data, yields are also reported for alkylations using HF as the catalyst. Introduction
Yield Calculations
Recent legislation requires many refineries to reformulate their gasoline in order to reduce its Reid vapor pressure and aromatic content and to meet new distillation requirements. As a result, more pentenes will almost certainly be alkylated in the future in addition to butenes and propylene. Yet relatively little has been published on how or to what extent the yields of alkylates vary for the different C3-C5 olefins. A detailed investigation Of C5 olefins and propylene has recently provided sufficient information (Albright and Kranz, 1992) so that accurate yield comparisons can now be made of all C3-C5 olefins. For Cq olefins, the theoretical yield is often reported as 203.6 wt of alkylate per 100 wt of C4 olefins. The basic assumption for such a yield value is that isobutane and a C4 olefin react on a 1:l molar ratio and that each alkylate molecule averages eight carbon atoms. Such a yield is seldom obtained. In the laboratory or pilot plant, yields are often determined by measuring the flow rates of the isobutane and olefin feeds and by measuring the fraction of the alkylate in the product stream. Relatively small errors in measuring this fraction can lead to substantial errors of the yield calculations. Many refineries cannot or do not accurately measure the flow rates of the feed and product streams. In addition, accurate analyses of the streams are often not obtained. Calculation of the yields is complicated, especially in a refinery, by the presence of propane, n-butane, andlor pentanes in the feed and/or product streams of many alkylation units. Furthermore, the olefin feed stream is often a mixture of C3-C5 olefins. In this paper, the yields of alkylates were calculated for different olefins using a carbon and hydrogen balance procedure that is somewhat modified as compared to the one used initially by Hengstebeck (1965). The only information needed for available pilot plant data was alkylate composition and acid consumption. The effects of temperature, isobutane-to-olefin ratio, and acidity on alkylate yields are reported for c&6 olefins when sulfuric acid is the catalyst. Yields are also reported for typical alkylation runs made with HF as catalyst, based on literature information (Hutson and Hayes, 1977).
Yields of alkylate can be calculated by obtaining balances of both the carbon and hydrogen atoms. The method proposed by Hengstebeck (1965)and used later by Hutson and Logan (1975) was modified somewhat in the present paper by incorporating the production of conjunct polymers (often referred to as acid-soluble oils or red oil) in the calculation procedure. On a material balance basis, the CS, CS, C,, etc., isoparaffins produced can be considered as the addition of one, two, three, etc., methylene groups (-CHz-), respectively, to isobutane. Propylene, C4 olefins, and Cg olefins supply three, four, and five methylene groups, respectively. Multipliers can be used to determine the average number of olefin molecules that react to form each molecule of a specific family. As an example for the production of a c6 isoparaffin, the multipliers for propylene, C4 olefins,and Cg olefins are 0.667,0.500, and 0.400, respectively. Table I indicates the multipliers for different families when isobutane is alkylated. When propylene is used as the olefin for alkylations with HF as catalyst, significant propane is produced via so-called self-alkylation reactions. Similarly, n-butane is formed when n-butenes are the olefins using HF as the catalyst. In such cases, the apparent chemistry based on material balances is as follows:
* Author to whom correspondence should be addressed.
HF
i-C4Hlo
C3H8+ -CH,- (or 0.333 C3H6)
-
(1)
HF
LC4H10
n-C,Hlo
The multipliers, as shown in Table 11, for these two reactions are -0.333 and 0.000, respectively. On the average, one isobutane molecule is required for the production of each alkylate molecule regardless of its molecular weight. Any propane or n-butane produced is part of the alkylate, even though all or at least most of these two volatile compounds are not included in the final gasoline pool. Table I1 summarizes the calculations for a pilot plant alkylation of isobutane with n-butenes at 10 OC using HzSOras the catalyst. When 100molof isobutane reacted, 100 mol of alkylate were produced.
oaaa-5aa519312632-299i$o~.ooio 0 1993 American Chemical Society
2992 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
Table I. Multiplier Used To Calculate Moles of Olefin That React To Form Family of Alkylate from Isobutane olefin feed. mol alkylate family propylene Ca's CS'S -0.333a propanea 0.00 0.00 0.000 n-butane0 0.25 0.20 0.333 Ca's 0.50 0.40 0.667 cis 0.75 0.60 1.000 C,'S 1.00 0.80 1.333 cis 1.25 1.00 Cis 1.667 1.50 1.20 2.000 Go's 1.75 1.40 2.333 Cll'S 2.00 1.60 2.667 ClZ'S a
Produced by self-alkylation reactions.
Table 11. Calculations Based on Analysis of Alkylate: Isobutane Alkylated with n-Butenes (io O C , Isobutane/ a-Butene Ratio of 7.2, Acidity of Ha804 94.4%) mol of olefin family of mol/ 100 mol reacted isoparaffins multiplier of alkylate 0.86 3.437 0.25 cs 1.72 0.50 3.432 c6 2.28 0.75 3.040 c7 86.70 86.698 1.00 C8 1.40 1.25 1.117 CS 0.61 1.50 0.404 ClO 3.17 1.75 1.814 c11 0.12 2.00 0.058 ClZ 2.25 c13 2 = 96.85 total mol 100.OOO of olefin reacted
(a)The molds of n-butenes that reacted to form alkylate were 96.85, which is the sum of the moles of n-butenes that reacted to produce CSthrough CIZisoparaffins. No self-alkylation reactions occurred; i.e., no n-butane was produced. (b) The total weight of alkylate produced was 11 224 g, which is the sum of 5800 g (for 100 mol of isobutane) and 5424 g (for 96.9 mol of n-butenes). Hence, 48.3 % by weight of the alkylate was produced from olefins and 51.7% was produced from isobutane. Ignoring the relatively small amount of olefin that reacted to form conjunct polymers, the yield was 207.0 wt of alkylate/100 wt of olefin. The yield is often reported in these units. To simplify these calculations, the atomic weights of carbon and hydrogen were assumed to be 12.0 and 1.0, respectively, rather than the more accurate values of 12.011 and 1.0079. The former weights result in only insignificant changes of the wt % or weight ratio values. (c)The average molecular weight of an alkylate molecule was 112.24, since there were 100 mol of alkylate. The average number of carbon atoms in an alkylate molecule was 7.87 since the molecules have a structure equivalent to CnH~n+2* The following calculation procedure can also be used, and was used, when mixtures of c3-C~olefins were used. The first step in the calculation is to determine the total number of carbon atoms in 100 mol of alkylate. This number is the C(mol% of each family)(number of carbon atoms in the family). Since 400 carbon atoms are contributed by isobutane, the carbon atoms contributed by the mixed olefins are calculated by difference. The composition of the olefin feed is determined as the weight fraction of C3, Cq, and CS olefins, respectively. These weight fractions are equivalent to the fractions of -CH2groups obtained from CB,C4, and CSolefins, respectively. The moles of propylene, C4 olefins, and Cs olefins that react per 100 moles of alkylate are next calculated.
In commercialalkylation units, some propane, n-butane, isopentane, and n-pentane are almost always present in the feed. Sufficient information must be obtained so that amounts of each can be determined in both the feed and product streams. Subtraction of the amounts in the two streams is used to determine the amount of each produced during alkylation. In the experiments reported here, these hydrocarbons were generally not present in the feed streams. Finally, calculations must be made to determine the moles of olefins that react to form conjunct polymers. Although isoparaffins including isobutane can form conjunct polymers (Doshi and Albright, 19761, there is considerable information that conjunct polymers are produced mainly from olefins. The amounts of conjunct polymers formed from the various olefins are often in the following order: n-butenes < isobutylene < n-pentenes < isopentenes I propylene. Conjunct polymers are highly unsaturated, contain Cg rings, and have about 10-20 carbon atoms per molecule (Miron and Lee, 1963; Carlson et al., 1966). These polymers react to a significant degree with sulfuric acid to form sulfate esters (Albright et al., 1988b). Hengstebeck (1965) reported that the polymers have hydrogen/carbon ratios of 1.52-1.75. Sung (1991) has found that after the acid is removed from the alkylation reactor the hydrogen/ carbon ratio of the conjunct polymers decreases with time because of the oxidation reactions of the acid. In the alkylation unit, the composition of the conjunct polymers is probably approximately CnH1.7sn. Conjunct polymers are produced from olefins in alkylation units by at least four mechanisms. The overall or apparent chemistry of the first mechanism is approximately as follows for a C4 olefin: (3) The hydrogen atoms produced above are transferred during alkylation to produce primarily light isoparaffins that have rather high H/C ratios. This mechanism is considered to be the main one in most commercial units. In the second mechanism, a normal Cq olefin frequently reacts to formsec-butyl acid sulfate. This sulfate is soluble in the acid phase. This and similar isoalkyl sulfates react rather rapidly, especially in the absence of isobutane, to produce sulfuric acid, conjunct polymers, and a mixture of isoparaffins having rather low octane numbers (Doshi and Albright, 1976). Such reactions can occur in the alkylation reactor but probably to a greater extent in the decanter when the acid and hydrocarbon phases are separated. Third, a rather small fraction of conjunct polymers is produced essentially as follows:
-
4C4H, + 2H,S04 16(-C1.0H1.76-)+ 4H,O + 250, (4) Finally, Doshi and Albright also found that various isoparaffins and especially trimethylpentanes react in the presence of HzS04 to form C4-C16 isoparaffins plus conjunct polymers and water. Conjunct polymers are also produced from butadiene, other dienes, acetylenic compounds, and cyclopentene; probably over 50% of this latter compound converts to conjunct polymers. In the present calculations, it was assumed that such impurities were not present. Probably 90% or more of the water that collects in the acid phase of commercial alkylation units is due to the water in the feed streams of the alkylation reactor. Most alkylation plants water wash the alkylate-isobutane mixture leaving the decanter. As a result, the isobutane recycled to the
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 2993 Table 111. Yield of Alkylates Produced with Different Olefins (H2904Used as Catalyst) mol of olefin wt % of in alkylate/ alkylate acidity, acid cons., 100 mol of produced T,"C 1/0 % RON lb/gal isobutane fromolefii olefin feed mixed C Aolefins 4.5 7.8 94.45 95.6 0.33 98.2 48.7 97.3 48.4 10 7.8 94.7 94.6 0.39 95.1 47.9 10 11.3 95.0 95.6 0.38 48.3 89.4 94.6 0.65 96.8 9.5 7.8 48.2 7.8 94.4 93.8 0.48 96.4 14.4 96.9 48.3 94.4 97.8 0.35 10 7.2 n-C4 olefins 47.4 93.2 0.52 93.3 7.8 94.7 10 isobutylene 109.5 1.00 44.2 9 8.6 93.2 89.0 propylene 116.4 45.7 93.1 (5.47)" 10 12.0 88.5 44.7 111.5 91.4 1.42 18 7.4 92.6 108.2 1.31 43.9 17 6.1 91.0 88.0 77.9 91.7 0.35 48.5 7.3 94.4 4.5 1-pentene 76.6 48.0 91.3 0.52 7.3 94.5 10 79.1 0.31 48.8 4.5 7.5 94.5 90.7 2-pentenes 47.9 76.1 91.1 0.43 94.6 10 7.5 72.1 46.5 91.1 0.44 4.4 8.2 94.5 2-MB2 74.2 47.2 91.2 0.53 8.2 94.4 10 48.9 88.3 0.38 4.5 7.6 94.5 92.9 55% 1-pentene + 45% mixed C4's 48.3 86.1 92.8 0.53 9.5 7.6 94.6 48.2 85.7 0.57 9.5 11.5 94.5 93.3 87.7 48.8 92.2 0.55 7.6 89.2 10 86.3 0.44 48.6 92.9 8.0 94.4 4.5 60% 2-pentenes + 40% mixed Cd's 84.7 48.2 93.4 0.30 10 8.0 93.5 48.3 85.2 93.0 0.34 10 11.6 94.8 49.2 88.2 91.8 0.46 8.0 89.2 10 47.8 84.4 92.7 0.31 7.8 94.1 4.5 55% 2-MB1+ 45% mixed Cl's 47.7 84.0 92.2 0.54 10 7.8 94.0 47.6 83.8 92.5 0.46 10 11.9 93.9 82.2 47.1 0.35 10 7.7 89.1 92.5 47.3 81.7 93.1 0.29 7.0 94.9 4.5 60% 2-MB2 + 40% mixed C i s 82.7 47.6 92.5 0.39 7.0 94.8 10 80.1 46.8 94.1 93.6 0.61 10 10.4 79.3 46.5 92.7 0.45 10 7.0 89.2 78.1 45.9 93.5 0.25 4.5 8.1 93.9 55% 3-MB1+ 45% mixed C i s 46.1 78.9 93.0 0.49 10 8.1 93.8 77.4 45.7 94.1 93.6 0.31 10 11.7 45.6 77.4 0.22 88.7 93.2 10 8.0
avg no. of carbon in alkylate molecule 7.92 7.89 7.80 7.87 7.86 7.87 7.73 7.29 7.49 7.35 7.25 7.90 7.83 7.96 7.81 7.61 7.71 7.97 7.87 7.85 7.94 7.92 7.85 7.87 8.01 7.79 7.77 7.77 7.69 7.71 7.76 7.64 7.60 7.51 7.54 7.48 7.47
wt of alkylate/100 wtof olefin 204 205 207 204 205 204 208 220 (190) 215 220 204 206 203 206 213 209 203 204 204 202 203 206 205 201 208 207 207 210 210 209 210 212 216 214 217 218
a This acid consumption value is considered to be only an apparent value. Acid consumption values for other propylene runs are also likely apparent.
alkylation reactor is saturated with water which is irreversibly absorbed in the acid in the alkylation reactor. This adsorbed water contributes to acid consumption. Impurities in the feed streams are probably a significant cause in all industrial alkylation units for acid strength reduction and hence for acid consumption. In the present paper, the conjunct polymers are assumed to be produced only from olefins. When the compositions of the inlet and spent acid from the reactor are known, then the fraction of olefins that react to produce conjunct polymers can be estimated. In two commercial reactors which are thought to be well operated, the feed acid contains about 99.5% acid (and 0.5% water); the spent acid contains approximately 2.5 % water and 6.5 % conjunct polymers. Assuming that all of the conjunct polymers are produced by the first mechanism (eq 31,109.3 g of used acid is produced per 100 g of feed acid. This used acid contains 99.5 g of HzS04,2.7 g of water, and 7.1 g of conjunct polymers. To produce these conjunct polymers, about 7.5 g of olefins reacts. The water added to the acid stream, about 2.2 g, is assumed to be the water present in the wet feed streams to the alkylation reactor. Approximately 0.52 % to 2.6 % of the olefin reacts to form CPs as acid consumption varies from 0.2 to 1.0 lb of acid/ gal of alkylate, respectively. Since about 48 wt 7% of the alkylate is often produced from olefins, as will be presented later, the following equation approximates the weight of olefins that react to form conjunct polymers for these
two refineries: wtof olefin lbs of acid (5) 100 wt of alkylate - 1*25(galof alkylate) When HF is used as the catalyst, the conjunction polymers are probably produced by the first mechanism shown above. HF is not an oxidizing agent as is sulfuric acid. Less information is available on the relative amounts of polymers produced when different olefins are used or when the operating conditions vary. Hutson (1978) reports, however, that conjunct polymer production increases rapidly as the isobutane/olefin ratio decreases, but he did not indicate the olefin (or olefins) used. Increased temperatures also promote the production of conjunct polymers. Information reported by Fenske (1956) and Hutson (1978) and a brochure of Phillips Petroleum Co. suggest that about 0.1-0.7% of the olefin feed to an HF alkylation unit is converted to conjunct polymers. Values of 0.2%, 0.3%, and 0.4% were used as typical values for alkylations employing n-butenes, isobutylene, and propylene, respectively.
Yield Results Table I11 indicates yield values and other results for 37 runs in which isobutane was alkylated with c3-C~using sulfuric acid as the catalyst. These runs were made in the pilot plant of STRATCO, Inc.; the hydrocarbon feeds were added continuously, but the acid was recycled during each
2994 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
run after separation from the hydrocarbon product stream. The acid composition shown in Table I11 is the average value in the run. The alkylation results, pilot plant, and analytical equipment employed were described earlier by Albright and Kranz (1992). Each alkylation run was from 6-8 h in length, and the operating conditions, alkylate quality, and acid consumption for each run are reported. The following were calculated: moles of olefins which reacted to form alkylate per 100mol of isobutane that also reacted, w t % of alkylate produced from olefin, average number of carbon atoms in an alkylate molecule, and weight of alkylate produced per 100 wt of olefins that reacted. For this latter weight ratio, account was taken for the relatively small amount of olefins that reacted to form conjunct polymers. Table I11 indicates that the w t % of the alkylate produced from mixed C4 olefins differs from 47.9 to 48.7. This wt 5% decreased as the temperature increased from 4.5 to 14.4 "C and as the isobutane/olefin ratio increased from 7.8:l to 11:l. Changing the acidity from 89.4% to 94.7% had only a small effect. For the alkylation run using isobutylene, the w t % was significantly lower as compared to the runs using n-butenes. Hence, less isobutane reacted for a given weight of alkylate when n-butenes were used. For alkylations with propylene, a much smaller percent (43.9-45.7 wt % ) of the alkylate was produced from the olefin, and the average molecular weight of the alkylate was much lower as compared to alkylations with C4 olefins. With propylene, however, on average more than one propylene molecule reacted for every molecule of isobutane that reacted. Heavier cations such as i-Cm+ 's or Cn+ 's were apparently being produced as intermediates toa fairly large degree. These heavier cations then fragmented, producing the light end products and Ca isoparaffins. For alkylations involving propylene, both increased temperature and increased isobutane/olefin ratios resulted in decreased consumption of isobutane; the reverse trends were noted with the mixed C4 olefins. Table I11 indicates that the yields of the alkylates produced from 1-pentene, 2-pentenes, and 2-methylbutene-2 (2-MB2) varied from 46.5 to 48.8 w t % . For the five mixtures of c4-C~olefins reported, they varied from 45.6 to 48.9 w t %. For the olefin mixtures containing n-pentenes, increased yields occurred as the operating temperature decreased from 10to 4.5 "C and as the acidity decreased from 94% to 89%. Changes in temperature and acidity had, however, little effect on the yields for olefin mixtures containing isopentenes. In addition, with all C4-C5 olefin mixtures, increased isobutane/olefin ratios generally had little effect on this wt %. For alkylations of isobutane, there is single correlation of the weight percent of alkylate produced from the olefins as a function of the average number of carbon atoms in an alkylate molecule, as indicated by Figure 1. The following ranges were noted in Table I11 for the different olefins: w t % alkylate
olefin propylene n-Cd olefins mixed C d olefins isobutylene 1-pentene 2-pentenes 2-methylbutene-2 (2-MB2)
produced from olefin 43.9-45.7 48.3 47.9-48.7 47.4 48.0-48.5 47.9-48.8 46.5-47.2
avg no. of carbon atoms in alkylate molecule 7.25-7.49 7.87 7.80-7.94 7.73 7.83-7.90 7.81-7.96 7.61-7.71
P 8.5 ' Muhanism #I with Butenes
8 n
g
7.5
-
+25
30
35
40
I
45
50
55
Weight Percent Alkylate Produced from Oleflns
Figure 1. Correlationof average number of carbon atoms in alkylate molecule aa a function of weight percent of alkylate produced from olefins for alkylation of isobutane.
The lowest wt % values of the alkylate produced from mixtures of C4-C5 olefins were for mixtures containing 3-methylbutene-1 (3-MB1). For these mixtures, greater amounts of isopentane were produced because of more self-alkylation reactions. Table I11 also shows the ratio of the weight of alkylate produced/100 w t of olefin reacted. In determining this ratio, the amount of olefin that reacted to form conjunct polymers was also determined. This ratio varied from about 201:lOO to about 220:100, expect for the second run with propylene shown in Table 111. Since the remainder of the alkylate was produced from isobutane, the weight ratio of the isobutane consumed per 100 wt of olefin consumed varied from about 101:lOO to 120:lOO. For the alkylation runs with propylene, the apparent acid consumption was in all cases high. Acid consumption for these runs is almost certainly caused in part by the buildup of isopropyl acid sulfates in the acid phase rather than the buildup of conjunct polymers. Such a buildup of sulfates is thought to have been the main reason for the large decrease in acidity noted for the second propylene run mentioned earlier. This run employed an acid having relatively low acidity. To minimize the buildup of dissolved isopropyl acid sulfate, there is a need to react more of this sulfate with isobutane to produce alkylate and HzS04. Knoble and Hebert (1959) reported amethod toreduce acid consumption to about 0.7 lbs/gal. Research is currently in progress to find methods to reduce further acid consumption. When the wt % values of the alkylate produced from olefins are plotted versus the moles of olefin that reacted per 100 mol of isobutane that reacted, the curves for propylene, C4 olefins, and C5 olefins differ as shown in Figure 2. The moles of olefins that reacted varied from about 109 to 116, from 93 to 99, and from 72 to 79 respectively for propylene, C4 olefins, and C5 olefins.Fewer moles of branched C4 or C5 olefins reacted as compared to unbranched C4 or C5 olefins. The curves for mixtures of C445 olefins lie between the curves for C4 and C5 olefins. Table IV compares yield information for what are considered to be typical alkylation runs using sulfuric acid and HF as the catalyst. Information for runs with sulfuric acid (Albright and Kranz, 1992) is reported in Table 111. Hutson and Hayes (1977) reported the composition of alkylates produced using HF. They did not indicate the operating conditions used, and the alkylate compositions were on a propane-free and n-butane-free basis. Yet both propane and n-butane are produced when propylene and n-butenes, respectively, are used as olefins. For every 100
Ind. Eng. Chem. Res., Vol. 32, No. 12,1993 2995
44
4
43
;----
--t
70
80
---------c---------
90
,.-------,
+---
100
110
120
Moles Olefin Reacted per 100 moles lsobutane Reacted
Figure 2. Correlation of moles of C S - Colefins ~ reacted per 100 mol of isobutane reacted as a function of weight percent of alkylate produced from olefin. Table IV. Comparison of Alkylate Yields When Using HsSO4 and HF as Catalysts on the Basis of 100 mol of Alkylate (CSand Heavier Isoparaffins) wt of alkylate per molof molof 100wtof l00wt T, isobutane olefin isobutane ofolefin olefin acid OC reacted reacted reacted reacted 204 n-butenes H2SO4 10 100 96.9 193 205 97.3 194 mixed C4's H2SO4 10 100 190 208 100 93.3 isobutylene H2SO4 10 220 HzSO, 10 100 108.7 184 propylene 215 111.2 181 100 propylene HzSO4 18 105b 203 100.5 188 HF 1-butene' 206 - 105b 97.8 185 cis-2-butenea HF 208 100 96.1 193 isobutylene0 HF 219 - llOb 104.4 HF 172 propylene' a Experimental data from Hutaon and Hayes, 1977. b 5 and 10 mol of isobutane were assumed to react via self-alkylationreactions with n-butenes and propylene, respectively. The alkylates were assumed to be free of n-butane and propane.
mol of propane-free and n-butane-free alkylate produced from these olefins when HF was used as catalyst, it was assumed in preparing Table IV that 10 mol of propane and 5 mol of n-butane were produced or 110 and 105 mol of isobutane reacted. Direct comparisons between the alkylates produced using HF and sulfuric acid were made on two bases: first, the amount of alkylate produced per 100 wt of isobutane that reacted and, second, the amount of alkylate produced per 100wt of olefin that reacted. The latter value is the amount of olefins that reacted to produce both the alkylate and conjunct polymers. As indicated by Table IV, 100 parts by weight of isobutane reacted in the presence of sulfuric acid to produce several percent more alkylate when both n-butenes and propylene were used as olefins as compared to the HFtype alkylations. These differences depend on the amounts of n-butane and propane produced and can vary significantly depending on the operating conditions used in the HF-type units (Hutson, 1978). When isobutylene is used as the olefin, self-alkylation is of no importance. In this case, there likely are only small differences in yields with the two acids.
Discussion of Results For especiallylaboratory and pilot plant alkylation units, alkylate yields and information such as that supplied in Table I11 can generally be calculated more accurately by the procedure employed in this paper as compared to the direct method of making material balances around the
reactor. For small alkylation units, the following are often not easy to measure accurately when the direct method is used flow rates of the hydrocarbon feeds and the acid; amount of alkylate produced (due to handling losses, partial evaporation, etc.); amount of conjunct polymer produced. For the proposed method of yield determination, the calculations are greatly simplified if the feed streams are pure (or essentially pure) isobutane and olefins (such as propylene, C4 olefins, or C5 olefins). The feed streams may not even need to be analyzed for each run except when mixtures of propylene, C4 olefins, and C5 olefins are used. A reliable analysis of the alkylate or of the product mixture of alkylate and unreacted isobutane is, however, required. Care must be taken to obtain a reliable alkylate sample. The analysis should be able to identify and group the isoparaffin products into different molecular weights (or carbon numbers). Sufficient analysis of the acid should also be taken to calculate acid consumption and conjunct polymer formation. Comparisons such as those shown in Table I11 are considered highly reliable since the same analytical procedures and assumptions were made for all 37 alkylation runs. Figure 1helps explain the yield results. The alkylates produced by mechanisms 1,2,and 4, as defined by Albright, Spalding et al. (1988a), and Albright and Kranz (1992), are plotted in this figure. For mechanism 1,exactly one olefin molecule reacts with one isobutane molecule, and C7,CS,and CSisoparaffins are produced when propylene, C4 olefins, and C5 olefins, respectively, are used as feed olefins. As indicated by Figure 1, 42-54.8 wt 7% of the alkylate is produced by mechanism 1 from these three olefins. The alkylates produced by self-alkylation reactions (mechanism 4) are also plotted in Figure 1when propylene, C4 olefins, and Cg olefins are used. In these cases, the alkylate products are considered to contain equimolar amounts of C8 isoparaffins and either propane, n-butane, or a Cg paraffin (n-pentane or isopentane), respectively. The average numbers of carbon atoms per molecule are 5.5, 6.0, and 6.5, respectively, with the three families of olefins. The weight percent of alkylate produced from the olefins is low, varying from about 26.5 to 37.5 wt % . The weight percent of the alkylate produced from isobutane is, however, high, varying from 62.5 to 73.5 wt 7%. For mechanism 4, one olefin molecule reacts with two isobutane molecules. The alkylates produced by mechanism 2 have on the average approximately 7.7-7.8 carbon atoms per molecule, as indicated by Figure 1. Intermediate C ~ O - Ccations ~O and olefins are produced as intermediates by polymerization-type reactions (Albright et al., 1988c; am Ende, 1990). These intermediates then fragment and form mainly C4+2 isoparaffins. Isobutylene and isopentenes produce especially large amounts of these intermediates. The compositions of the intermediates and the resulting alkylate presumably vary to some extent with the olefin feed and the operating conditions. For mechanism 2, approximately 1.23-1.27, 0.93-0.95, and 0.74-0.76 olefin molecules respectively react with an isobutane molecule when propylene, C4 olefins, and Cg olefins are used. Alkylates produced by mechanism 3 are not shown in Figure 1. These alkylates, produced by polymerizationtype reactions but involving no fragmentation, contain only Clo and heavier isoparaffins. Mechanism 3 is generally of minor importance when sulfuric acid is used as the catalyst except when the quality of the alkylate is extremely poor (and of no commercial interest). Alkylates produced by mechanism 3, however, consume relatively large
2996 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
amounts of olefins. Hutson and Hayes (1977) reported for alkylations using HF as the catalyst the analyses of the alkylates on both a weight and mole basis. Using that information, the heavy ends were found to contain on average 13.5-14.5 carbon atoms per molecule. Such carbon numbers are higher than those for heavy ends in alkylates produced using sulfuric acid in the STRATCO pilot plant. Mechanism 3 may then be of more importance for alkylations using HF as the catalyst. As previously reported (Albright et al., 1988a), mechanism 1is favored regardless of the olefin by low operating temperatures and high levels of agitation. Mechanism 2 is, however, promoted by high temperature and low levels of agitation. Hence, for C4 olefins, higher temperatures result in an increase of the ratio of the moles of isobutane that react per mole of olefin that reacts. For propylene, this ratio, however, decreases significantly as the temperature increases. As also previously reported, increased temperatures result in increased quality alkylates when propylene is used as the olefin, but the reverse is true when C4 olefins are used. When self-alkylation reactions (mechanism 4) are important in addition to mechanisms 1and 2, the explanation of yields is more complicated. First, the alkylate needs to be clearly defined as to whether it contains any propane and n-butane. Second, self-alkylation reactions result in relatively few carbon atoms per alkylate molecule when propane and n-butane are considered part of the alkylate. If the alkylate is to be considered on a propane- and n-butane-free basis, then weight ratios as reported in Table IV permit good comparisons based on the amounts of isobutane and olefins consumed. Table I11 indicates for the alkylations with mixed Cd olefins that the run with the higher isobutane/olefin ratio resulted in a lower molecular weight alkylate. It contained more ( 2 5 4 7 isoparaffins and fewer heavy isoparaffins. Since ( 2 5 4 7 isoparaffins are formed by mechanism 2, the heavy cations and olefins formed as intermediates apparently fragmented on averageto produce lower molecular weight isoparaffins. The higher isobutane/olefin ratio probably resulted in lower molecular weight intermediates or fewer polymerization-type reactions. Plots were constructed of the weight percent of the alkylate produced from olefins versus the mole percent of C5 olefin in C445 olefin mixtures. Data are available for three C5 olefins as indicated in Table 111. Essentially straight-line plots resulted for all three cases, which suggests that no large synergistic effects occur when C4 and C5 olefins are mixed. Alkylation quality and acid consumption (or conjunct polymer formation) have been known for many years to depend both on the olefin used and on the operating conditions known. It was also realized that yields differed somewhat on operating conditions, but the present results give the best quantitative comparisons known. Conclusions Refineries can vary to at least some degree the relative amounts of isobutane and olefins that react by varying
the operating conditions (such as temperature, isobutaneto-olefin ratio, acid composition, etc.). Bigger changes in the relative amounts are possible when propylene and C5 olefins are used relative to the changes possible with C4 and especially n-C4 olefins. When comparing alkylations of isobutane using either H2S04or HF as the catalyst, yields of alkylate are similar when based on the moles of n-butenes, isobutylene, and propylene that react. Yields of alkylate are, however, significantly less based on the moles of isobutane that react when HF is used as catalyst for either n-butenes or propylene olefin feeds. In these latter alkylations, self-alkylation reactions are relatively important. Literature Cited Albright, L. F.; Kranz, K. E. Alkylation of Isobutane with Pentenes Using Sulfuric Acid as Catalyst. Ind. Eng. Chem. Res. 1992,31, 475-481. Albright, L. F.; Spalding, M. A.; Faunce, J.; Eckert, R. E. Alkylation of Isobutane with Cd Olefins: Two-step Process Using Sulfuric Acid as Catalyst. Ind. Eng. Chem. Res. 1988a,27,391-397. Albright,L. F.; Spalding,M. A,;Kopser, C. G.; Eckert, R. E. Alkylation of Isobutane with C4 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 withC4 Olefins: First-Step Reactions Using Sulfuric Acid, Catalyst. Znd. Eng. Chem. Res. 1988c,27, 381-385. am Ende, D. J. The Effect of Sulfuric Acid Composition on TwoStep Alkylation. M.S. Thesis, Purdue University, W. Lafayette, IN, 1990. Carlson, E. G.; Coe, R. H.; Durrett, L. R.; Martin, J. M.; Slaymaker, S. C. Cyclic HydrocarbonIons in Acid-Catalyzed Isoparaffin-Olefin Alkylation. Symposium of Hydrocarbon Ions; Meeting of Amer. Chem. SOC.,New York, NY, Sept. 11-16, 1966. Doshi, B.; Albright, L. F. Degradation and Isomerization Reactions During Sulfuric Acid Catalyzed Alkylation. Ind. Eng. Chem. Process Des. Dev. 1976,15, 53-60. Fenske, E. R. Isoparaffin Alkylation Process, U.S. Patent 3,249,650, May-5, 1956. Hengstebeck,R. G.AnEasy WaytoFigure Alkylate Yields Accurately. Oil and Gas Journal 1965,Oct. 4, 145-150. Hutson, T. HF Alkylation in the 1980's: The Role of Isobutane/ Olefin Ratio. Symposium on Octane in the 1980's; Amer. Chem. SOC.Meeting, Miami Beach, FL, Sept. 10-15, 1978. Hutson, T.; Hayes, G. E. Reaction Mechanism for Hydrofluoric Acid Alkylation, in Industrial and Laboratory Alkylations; ACS Symposium Series 55; Albright, L. F., Goldsby, A. R., Eds.; American Chemical Society; Washington, DC, 1977; pp 96-108. Hutson, T.; Logan, R. S. Estimate Alkyl Yield and Quality. Hydrocarbon Processing 1975,No. 9, 107-110. Knoble, W. S.;Herbert, F. E. Key to Propylene Alkylation Found. Petroleum Refiner 1959,No. 12, 101-104. Miron, S.;Lee, R. J. Molecular Structure of Conjunct Polymers. J. Chem. Eng. Data 1963,8,150-160. Sung, S. The Decomposition Kinetics of Spent Sulfuric Acids Obtained from AlkylationPlants. M.S. Thesis, Purdue University, W. Lafayette, IN, Aug. 1992. Received for review March 1, 1993 Revised manuscript received August 2, 1993 Accepted August 13, 1993' Abstract published in Advance ACS Abstracts, October 15, 1993.
