I
J. A. RIDGWAY, Jr.
American Oil Co., Texas City, Tex.
Free Radical Alkylation of Isobutane with Ethylene Addition of a small amount of ethyl bromide nearly doubles neohexane yield at constant ethylene conversation T H E increasing octane number of gasoline has focused attention on premium quality components and means for their production. Such a component is neohexane (2,2-dimethylbutane) which can be produced by isomerization of other hexanes or by free radical I alkylation of isobutane with ethylene. This alkylation process was first developed by Frey and others (5, 74)) who used the severe conditions of 950’ F. and 4000 p.s.i.; neohexane yields were poor. Later O’Kelly and Sachanen (75) showed that less severe conditions (800’ F. and 2500 p s i . ) were feasible if organic halogen compounds were used as reaction promoters. Better results were obtained, presumably as the result of reduced pyrolytic side reactions, but the best yields were still only about 90 weight 70, based on ethylene-far short of the 307 weight % theoretical and too low to be of commercial interest. The possibility of further improvement was examined. Some preliminary runs carried out a t low temperature, where the primary processes would not be obscured by pyrolytic side reactions, defined the problem. Typical was a run in which a 4 to 1 isobutane-ethylene blend was caused to react batchwise; di-tert-butyl peroxide was used as initiator. The hexane and heavier prod-
uct consisted predominantly of a series of even carbon number paraffins. This is typical of an ethylene telomer (7) such as that obtained on the free radical alkylation of a secondary alcohol with ethylene (6). It indicates that the principal reactions controlling product yield and character are ethylene addition and chain transfer via hydrogen abstraction from a donor molecule (secondary alcohol or, in this case, isobutane). See bottom of column one. A tert-butyl radical adds ethylene to form a neohexyl radical (reaction A’), which in turn may add an ethylene to form a CS radical (reaction A), or abstract a hydrogen from isobutane to form neohexane and regenerate a tertbutyl radical (reaction B). The latter desired reaction; formation of adicals is reflected in high heavy alkylate production and low total alkylate yields. O n the basis of this reaction picture optimum yields require that reactions other than A’ and B be minimized; in particular, the rate ratio of B to A should be maintained as high as possible. Factors that may influence the relative rates of the competing reactions and therefore must be considered in a study of process improvement, are isobutane-olefin ratio, pressure, temperature, and materials capable of modifying the reaction. ~
C
c-c I .
+-
C
1
(A’)JC=C
I
I
C
C
(B)
I . c-c-c-c I
-----f
i C4
I I
c-c. +
neo-
C hexane
C
f
(A) i C = C
C
CS’ ClO ’ etc.
Apparatus and Methods
-+ c-c.I + (B)
i C4
_I
I heavy
C paraffins
The runs were all batch operations in 1400-ml. stainless steel reactors. The required quantity of blended isobutane and ethylene was withdrawn from a masterbatch stored over water a t 250 p.s.i., and conveyed via a calibrated gage glass and a dryer to the evacuated reaction bomb. I t was possible to charge the reactor repetitively with a blend of fixed composition. Free radical initiators and experimental reaction modifiers were added by appropriate techniques : Nonvolatile liquids and solids requiring no special treatment were added directly to the
bomb before charging. Volatile liquids were flushed in with the charge, and gases were charged from calibrated pressure containers. Solids requiring special pretreatment or protection from the air were charged in sealed glass ampoules. After charging, the reactor was placed in a rocking heater, heated to reaction temperature, and held for the desired time. Two-hour heating and 20-hour reaction periods were typical. At the conclusion of the run, heating was stopped, and after cooling, the reactor was drained to a water-jacketed atmospheric pressure receiver, where liquid product was collected. Gas passed to a proportional sampling device and a gas meter. Low temperature distillation of the liquid product provided information on content of neohexane and other hexanes and allowed the specific gravity of the bottoms heavier than hexanes to be determined. The gas sample, essentially 100% neohexane and lighter, was analyzed by mass spectrometer. At 665’ F. and above, formation of chain initiating radicals by pyrolysis of the charge was sufficiently rapid to give experimentally practical reaction rates. At lower temperatures auxiliary sources of initiating radicals were required. Di-tert-butyl peroxide and tetraethyllead (3, 70) were employed. Conditions under which these initiators were effective and performance are summarized in Table I. Reaction chain length is defined as moles of olefin reacting per mole of initiating radical. Ethylene (99.5y0) and isobutane (99%) were Matheson C.P. grade materials, tetraethyllead was supplied by the Ethyl Corp., and the di-tert-butyl peroxide initiator was purchased from Matheson, Coleman, and Bell, Inc. Materials examined as reaction modifiers (Table 11) were off the shelf. Experimental Data
’
i‘
The experimental data are presented in Table 111. Conversion of ethylene to ethane is serious with some reaction VOL. 50, NO. 10
OCTOBER 1958
1531
modifiers and the olefin conversion data must be corrected for this in analyzing reaction performance. The total ethylene converted includes this reaction. Weight per cent of total alkylate on converted olefin is the yield of total alkylate (after correction for the small estimated contribution of initiators and modifiers) based on total olefin converted. The minor amount of pentanes observed in some runs was assumed to derive from thermal cracking of heavy alkylate and therefore was included in total alkylate. Weight per cent of C6on total alkylate is thc hexane content of total alkylate corrected for initiator and modifiers. Volume per cent of neo-cs on total cg shows hexane isomer distribution, and K’ corresponds to the chain branching constant of Mayo (7, 73).
reactions control the yield and composition of the alkylate. The ratio of these rates can be expressed as (rate of chain transfer) (rate of ethylene addition)
where K’A is the rate constant for ethylene addition to a tertiary radical, K A is the rate constant with a primary radical, and (CF), ((24. ), and ((2,. +) represent moles of ethylene, tert-butyl, radical, and hexyl and heavier radicals in a given reactor volume. The rate of isobutane consumption is
-
r.
L.
I C
where ( i c , ) / ( C ~ ) is the reactant isobutane-ethylene mole ratio and K’ is the ratio of reaction rate constants. This relation between isobutane-ethylene ratio and the alkylation reactions has been assumed throughout the study. I t allows the intrinsic reaction rate ratio, as measured by K’, to be isolated for Discussion study. Operating Conditions. EFFECTOF K’ was evaluated from the run data ISOBUTANE-OLEFIN RATIOAND DERIVA- using equations developed as follows: TION OF K’. The relative rates of the The rate of ethylene consumption can chain transfer and ethylene addition be expressed as
Table 1.
Substituting for ((24.) in Equation 2 , then dividing by Equation 3, yields
Reaction Initiators Were Effective under These Conditions Moles
Technique of
Initiator/100
Addition
Moles Olefin
Initiator
Reaction Temp.. F.
Reaction ,Chain Length
Di-tert-butyl peroxide
Initially During run
1.6 1.6
255 350-390
...
TEL
Initially During run
0.4 0.4
483 483-662
...
Table 11. Water and water solution Water (258, 667) Buffered pH 7 (666) 5% NaOH (257) 10% Co acetate (252) 10% FeS04 (256) 10% soap (257) Solvents tert-C4OH (257,671) Light alkylate product (675) Heavy alkylate product (673) Benzene (673) Diisopropylxylene (668) Hydrogenation catalysts Platinum-alumina (257) Molybdena-alumina (258) Chromia-alumina (257) Nickel-kieselguhr (257) Copper chromite (258) Cobalt molybdate (256) Nickel-tungsten sulfide (257) Raney nickel (655) Molybdenum sulfide (256)
26
Metals and treatment for removal or neutralization Reduced copper (255) Metal deactivator“ (669) Bomb washed with boric acid (257) Sodium (666) Nonmetallic solids Silica gel (257) Alumina (257) Silica-alumina [cracking cat. ] (258) ZnO (257) Activated charcoal (669) Inhibitors and reactants Tename1ve-2~(258) p-Quinone (256) NO (257, 300) 02 (256) Miscellaneous Cobalt naphthenate (257) Copper naphthenate (255,675) CuCl (256,675) ZnClz (258) Phenyl mercuric acetate (257,667)
* N,N’-di-sec-butyl p-phenylenediamine (+ennessee Eastman). Figures in parentheses indicate test temp., F. INDUSTRIAL AND ENGINEERING CHEMISTRY
Converting to a weight basis gives the following relation :
37
Materials Examined as Modifiers
Halogen compounds Iodine (250, 615, 670, 760) Ethyl iodide (670,760) Ethyl bromide (260,670,760) tert-Butyl bromide (660, 750) tert-Butyl chloride (670, 760) Hydrogen bromide (670) a N,N’-disalicylidene-1,2-propanediamine (Du Pont).
1532
where K B is the rate constant for chain transfer from a primary radical to isobutane and (iC4) represents moles of isobutane in a given reactor volume. Under steady-state conditions
where X is weight of isobutane and Y is the weight of ethylene. This equation assumes no ethylene consumption by hydrogenation. The occurrence of hydrogenation requires a correction in calculating this. constant. This correction was obtained by assuming a constant correction factor, D, defined as the ratio: (rate of total ethylene consumption)/(rate of ethylene consumption by alkylation). When dY is defined as incremental ethylene consumption by alkylation plus hydrogenation, D applies as follows: dY
=
D
Y + 0.483 (rx
This differential form is not useful in determining K‘ in batch runs because of the wide variation in Y / X during the run. It is a linear differential equation, however, and can be integrated between the starting composition Xo, YO,and final composition X , Y , to give: D -
Y-7 YD - 0.483XD YO-
K
$ - 0.483XoD
=
(z)”‘
(9)
A trial and error solution was used in obtaining K’ values from individual run data.
I SOB UTAN E A 1K V LAT 10)I EFFECT OF REACTION PRESSURE. Pressure was studied over the 1200 to 2800 p.s.i. range in four runs (Table IV). A slight effect was observed on conversion and possibly on neohexane content of the hexane fraction; K' was es-
Table 111.
Neohexane Yield
sentially unaffected. I t is concluded that pressure is a variable of only minor importance. EFFECTOF TEMPERATURE. T h e experimental results from the study of temperature are presented as a log K'
-
Is Favored by Increase in Reaction Temperature and Use of Reaction Modifiers Run Performance Total alkylate , Total on Neo con- CS on % CZ= ethyl- verted total Cs on con- ene ethyl- dkyl- total verted conate, ene, C6, toCa verted wt. % wt. % vol. 70
Charge Initiator Moles/ 100 Moles Olefin , Means of Total Name addn. added Decomp.. 3.35 2.36 DTBP Feed
Modifier 110 G./100 charge Name olefin ratio None 4.5
... ... ... ...
4.5 4.8 3.1 3.5 3.5 3.8 3.8 3.9 3.9 4.1 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.5 3.5 3.5 4.1 4.1 4.1 4.1 4.1 3.9 4.1 3.9 3.9 3.5 3.5 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.9 3.5 4.5 3.9 3.9 3.5 3.5 3.5 3.9 3.9 3.5
... ...
... ... ... ... ...
... ... ... ... . I .
... ...
... ... ... ... ...
... ... ... ... ... ... ... ... ... . . I
... 2.2 1.5 0.6 2.0 1.4
12
Trace
Et1 HBr EtBr
tBuBr
tBuCl
0.5 4.6 2.2 2.3 2.7 2.4 1.3 23.8 1.8 1.7 1.6 2.0 2.0 1.6 1.5 1.3
Cont. TEL'
Feed Cont. TELc None
... ... ...
.
... ...
,
... ... ... ... ... ... . I .
DTBP
Feed
None
*.. ...
... ... ... ...
DTBP None
... ... ... Feed
... ... ... ... ..... ... ... ..I
None
*
reciprocal T relation in Figure 1. Considerable data scattering was observed and statistical study was undertaken. From this the cross-hatched area representing the 80% confidence limits was developed. Chain transfer is
3.06 3.18 2.16 2.42 1.43 1.61 1.63 1.58 2.82 2.36 2.69 0.40 0.41 0.87 0.66 0.37 0.46
... ... ... ... ... ... ...
2.15 2.23 1.52 1.70 0.98 1.10 1.11 1.08 1.98 2.36 2.69 0.40 0.41 0.87 0.66 0.37 0.46
... ... ... ... ... ...
... ...
I
...
...
... ... ...
... ... ...
...
2.9 3.1 2.9
... ... ... ... ... ... ... ... ...
3.2
... ... ... ... ... ... ...
...
... .
.
... ...
2.0 2.1 2.0
... ...
... ...
...
... ... ...
...
2.2
...
... ... ... ... ... ... ...
Run Conditions PresTime, Temp., sure, hr. ' F. p.s.i.g. 24 24 24 24 24 24 24 24 24 22 2.5 3 7 3 2 2.5 2 4 4.5 '22 22 22 22 22 22 22 22 22 22 22 22 5 3 4.5 22 22 22 22 22 22 22 22
3 24 3 22 22 24 22
3 22 22 3.5 22 3 22 3.8
258 258 255 258 257 256 257 257 257 257 347 392 483 533 545 572 572 662 665 669 669 669 669 673 673 673 675 676 676 680 682 707 752 752 255 257 255 675 666 669 667 669 757 613 759 667 669 257 666 759 669 662 752 667 759 669 752
0 0 0 0 0 0 0 0 0 0 0 0 0 0 3.7 0 0 0 0 5.1 1.3 0.8 1.4 0 0 0 0 0 0 0 0 0 2.6 3.6 0 0 0 11.8 5.0 3.3 3.6 32.6 20.1 8.2 57.5 31.5 1.1 0 0.8 9.gd 1.0 1.0 2.3 1.8 4.6 1.2 2.5
585 620 585 675 645 670 665 650 640 680 930 1150 1400 1450 1425 1650 1600 1500 1600 1975 1988 2075 1875 2350 1960 2100 2800 1200 1930 1600 2010 1950 2050 2075 725 695 680 2000 1878 1890 1915 1900 2100 1900 2300 2000 1735 635 1800 2 100 1830 1825 2000 1800 2100 1883 2075
81.6 82.7 80.7 74.5 71.9 59.0 57.5 57.7 67.3 83.3 63.9 58.5 59.5 69.4 70.1 81.9 66.1 91.7 59.9 97.3 91.2 88.2 96.81 99.2 98.6 98.6 99.1 95.7 99.2 97.5 96.7 91.3 96.0 99.0 65.6 61.4 81.7 97.3 91.5 89.6 91.1 92.0 96.0 81.9 97.0 96.6 96.5 81.1 88.8 96.7 86.9 90.0 98.1 87.8 96.8 88.2 97.6
105.6 113.8 109.1 109.1 116.2 89.3 110.4 97.4 110.5 104.1 94.4 95.0 113.7 131.7 119.9 142.7 133.8 137.9 149.3 156.7 152.1 148.5 151.1 180.7 180.0 171.5 181.3 172.4 162.9 178.9 162.1 167.1 173.5 158.1 115.7 120.5 129.6 196.9 206.7 180.5 185.6 167.5 190.9 183.7 141.6 161.8 185.1 97.0 219.6 220.8 201.0 189.7 203.0 192.3 195.0 168.5 172.0
12.9 11.2 7.9 8.2 5.2 6.9 7.3 6.9 5.0 9.3 13.6 17.4 15.2 22.6 23.4 27.7 21.7 35.9 29.2 38.9 29.8 28.0 31.8 34.8 36.4 37.4 37.0 36.8 36.1 37.4 38.2 37.6 38.0 40.0 6.5 6.9 10.5 57.2 43.8 36.2 38.7 65.5 62.4 47.2 51.3 58.8 43.0 15.7 57.9 69.6 40.8 39.3 56.3 46.1 55.1 36.2 41.3
... . . a
... ... ... ...
... ... ... .. 64.6 t
60.3 66.5 63.6 67.3 56.6 68.5 58.1 72.2 68.8 65.2 71.4 74.0 74.7 76.8 78.7 71.8 76.6 77.1 78.6 73.2 69.9 69.1
... ...
...
88.2 88.3 83.8 83.3 86.1 72.1 88.8 72.1 85.0 88.1 87.9 93.3 90.5 84.3 83.7 85.6 85.1 80.9 70.4 68.3
100K' 0.30 0.77 0.47 0.82 1.40 0 0.93 0 0.84 0.25 0 0 1.23 2.83 2.34 3.47 3.18 2.43 5.50 4.74 4.54 4.27 3.79 6.14 6.17 5.00 6.25 5.55 4.11 6.17 4.35 5.68 7.46 4.79 1.3 1.9 2.2 18.9 18.2 9.4 10.4 66.7 31.3 15.2
> 100
32.3 9.0 0 19.6 31.3 15.4 11.4 13.9 11.6 12.1 7.4 6.4
38
Kd(DTBP) 10'6 e m (18). ' Commercially Ethyl motor mix, contains 0.5 mole ethylene dibromide and 1mole ethylene dichloride per mole TEL, weight on TEL only. pure TEL. =
'
Probably result of bomb contamination by iodine.
VOL. 50, NO. 10
OCTOBER
1958
1533
0 10
0 08 0 06 0 05 0 04
0 03 0 02 X
0 01
0 008
0 006 0 005 0 004 ( g. ATOMS HALOGEN CHARGE ) ,ooo ( g,MOLS ETHYLENE CHARGE)
0 003
0 002
0 001
Figure 2. a t 670" F. 250
300
350
400 450 500 550 600 T E M PERATU RE,
Figure 1. Chain transfer temperatures
increased with increased iodine concentration
700
OF.
i s favored
favored by higher temperatures; the slope of the curve indicates an activation energy difference between the transfer of hydrogen from isobutane to a radical (reaction B), and the addition of ethylene to the radical (reaction A) of about 6 kcal. Reaction Modifiers. An advantage has been shown for higher temperatures; however, even at 760' F., the maximum studied, the rate ratio of chain transfer to ethylene addition was not above about 1 to 14. Operation a t substantially higher temperature is limited by the incidence of pyrolysis; hence it was concluded that, to attain better performance, reaction modifiers must be found capable of promoting hydrogen abstraction and/or inhibiting ethylene addition. There are numerous examples of materials affecting free radical reactions (2, 77). However, in no case is there demonstrated an action that is selective for only one type of free radical reaction, and such selective action would be necessary if the desired modifying effect was to be attained. I n the absence of definite leads the plan of research was essentially Edisonian; all the potential modifiers listed in Table I1 were examined. Only the halogen compounds showed activity. This was a rather surprising development, as in other studies using halogen compounds (9, 75) only initiating activity was reported. They were studied further. MODIFIERCOXCENTRATION, Iodine was the first halogen examined, and the only one with which the effect of quantity was studied. Increasing quantities had an increasing effect (Figure 2 ) . A halogen level of 0.004 gram atom
1534
K'
by higher
per gram mole of olefin was arbitrarily chosen as standard in subsequent studies. MODIFIER COMPOSITION. The effect at the 0.004-gram atom level of the several halogen compounds studied is shown in Figure 3. To facilitate comparison K' values have been estimated for a reaction temperature qf 700' F. (Table V). Correction to this standard temperature was accomplished by assuming the same activation energy as for the nonmodified reaction. Data on iodine, ethyl bromide, and tert-butyl bromide support this assumption. Free halogen, hydrogen halide, and tert-butyl halide are grouped, because hydrogen halide will probably be the predominant form of the modifier under reaction conditions, irrespective of which is charged. Ethyl halide is more stable and decomposes by a different mechanism than the tert-butyl halide (8, 72), so that some form other than the hydrogen halide may be present; a distinction between it and the hydrogen halide group is indicated by its modifying activity. I t is concluded that, at 700' F., the effectiveness of the halides increases in the order C1< Br < I. Further, the form of the halogen affects its performance, the ethyl halides being the most active of those studied. These results do not apply at lower temperatures. Iodine exhibits some activity at 250' F., but bromine compounds are without effect. Data indicating that bromine and chlorine become effective only at relatively high temperatures are provided by runs in which tetraethyllead motor mix was used as initiator. This mix contains 0.5-mole of ethylene dibromide and 1 mole of ethylene dichloride per mole of
INDUSTRIAL AND ENGINEERING CHEMISTRY
tetraethyllead. Through its use in the experimental runs about 0.007 gram atom of bromide and 0.014 gram atom of chlorine were added per gram mole of olefin charge. No substantial modifying effect was observed even a t 572' F.; these compounds have not been tested a t higher temperatures. Product Composition. Although K' is of major importance, the yield of neohexane is affected by other factors as well. This is seen in product composition and in reactant consumption through side reactions and must be considered in evaluating alkylation performance. ETHANE PRODUCTION.Minor side reactions resulting in conversion of ethylene to ethane were observed in 750' F. operations. This was not affected by the modifiers studied, except those containing iodine. I n that case ethane production was aggravated to serious proportions at both 670' and 750' F. I n view of this, iodine and its compounds do not appear to be practical modifiers. Effect of Temperature and Modifiers on Ethane Production
Temp., a F. 670
750
Ethane Production", Mole % of Converted Ethylene-
Modifier
i
Br compounds 3 C1 compounds None Iodine 13 18 Ethyl iodide 30 50 a At modifier concentration of 0.004 gram
atom halogen/gram mole ethylene.
The activity of iodine in hydrogenation is recognized ( 4 ) ; its effect on the ethane-producing reaction was not unexpected. More noteworthy is the marked difference in performance of iodine compared to ethyl iodide. This
I S 0 BUTAN E ALKYL AT10 N 0.40
0.30
1
m
I
100
600
640
680
720
TEMPERATURE
760
800
oc
I50
Figure 4. yield
Alkylate hexane content increases with alkylate
0 ferf-Butyl bromide &3 HBr 0 Ethyl bromide Iodine
A Ethyl iodide 0 ferf-Butyl chloride * Average of 4 ferf-butyl chloride runs
parallels the relation between compound type and modifying activity. ALKYLATE HEXANE CONTENT.I t follows that the hexane content of the alkylate will be a function of alkylate yield. At 100% alkylate yield, corresponding to ethylene polymerization (elimination of reaction B), there should be no hexane in the product; a t 307% alkylate yield, corresponding to straight alkylation (elimination of reaction A), the product should be all hexanes. An attempt to correlate the data in this manner (Figure 4) yielded a relation useful in predicting the composition of an alkylate, but differing from the theoretical in that it did not pass through zero at 100% alkylate yield. This indicates that hexanes are produced by routes involving side reactions such as radical pyrolysis and radical-olefin disproportionation as well as by simple alkylation. There appear to be slightly more side reaction source hexanes produced a t 755' F. than a t 670' F. QUALITY OF ALKYLATE HEXANE FRACTION. The desired hexane isomer 2,2dimethylbutane requires reaction between a tert-butyl radical and ethylene for its production. Other hexane isomers would be formed by reaction of radicals resulting from abstraction of primary hydrogen from isobutane or by pyrolysis or disproportionation. Accordingly, a t low alkylate yield, where a relatively high proportion of the c6 fraction is side reaction hexanes, the c6 fraction should be low in the 2,2dimethylbutane isomer. IJnder condi-
307
250
WT, % ALKYLATE ON ETHYLENE CONVERTEO TO ALKYLATE
Figure 3. Effectiveness of halides increases in order CI < Br < I
A
200
tions of high alkylate yield, side reaction hexanes will contribute but little and the neohexane concentration will approach a maximum controlled by the ratio of tertiary to primary radicals formed through hydrogen abstraction. This relation is demonstrated in Figure 5, where it serves as a basis for a correlation useful in predicting hexane fraction comuosition. This correlation is noteworthy in twp respects: There is a distinct quality loss on increasing ternperature, related to the increase, with temperature, of s?de reaction hexane production; and, the extrapolated neo-
hexane content a t high alkylate yields is surprisingly high. An ultimate concentration of 90%) which appears conservative, indicates that the tertiary hydrogen was attacked 90% of the time; this compares with 53% (76,77)) 73% (9), and 76% (79)calculated from literature data. The ability to attain this selectivity is important; why it is attained still is in doubt. POTENTIALCOMMERCIAL PERFORMa commercial flow unit adANCE. vantage can be taken of the high internal isobutane-olefin ratios attainable in a stirred or circulating reactor. This performance can be estimated using the K' values and the product composition correlations discussed. The alkylate yield is calculated in the following way: In a circulating reactor the internal or effective isobutaneolefin ratio is constant, so that the Y / x weight ratio of Equation 8 is a constant. If this is expressed as 0.483/R, where
Table IV.
Pressure Is a Variable of Minor Importance in Alkylation Performance (22 hours, 675' F., thermal initiation) Hexane in Neohexane in Total Hexane Pressure, % Ethylene Alkylate, Fraction, P.S.I. Converted K' Jvk % % 1200 1960 2350 2800
Table V.
95.7 98.6 99.2 99.1
0.056 0.062 0.061 0.062
36.8 36.4 34.8 37.0
71.8 74.7 74.0 78.7
700" F.
Effectiveness of Halides Increases in Order CI Halogen Halogen Compound None c1 Br At
K' at 700' F. 0.05-0.06
Base case
1
Free halogen Hydrogen halide tert Butyl halide
...
:::
... ... ... 0.09a ...
...
...
< Br < I I
... 0.20
0.10 e . . 0.12 . . a 0.20 0.35 Ethyl halide a Because of scatter of data, simple average of four tert-butyl chloride runs was used to obtain
..*
this value.
VOL. 50, NO. 10
OCTOBER 1958
1535
0
100
c
90 I
I I
ao
g
2501
601 200
I50
100
250
307
WT % ALKYLATE ON ETHYLENE CONVERTED TO ALKYLATE
Figure 5. Quality of alkylate hexane fraction increases with alkylate yield
R is the isobutane-olefin mole ratio, Equation 10 results. dX
=
0.4830 ( k R
+ 1)
Operating conditions 660 Temp., F. Internal If0 mole ratio 25 Modifier Name None Concn., g. atomsfg., mole Cz= a , .
(lo)
Integrating and solving for alkylate yield lead to Equation 11. Y
w=
100 x--+ D
Y
Conchs ions
In the free radical alkylation of isobutane with ethylene, neohexane yield is limited by competitive reactions leading to heavy alkylate. I t is favored by increase in reaction temperature but reaches only about 40% of the theoretial at 750’ F. Minor improvement through use of still higher temperatures is at the expense of increased losses to cracking Neohexane yield is also favored by use of halogen, hydrogen halide, or organic halide as reaction modifiers. Effec-
1536
750
750
25
25
25
EtBr
None
EtBr
0.004
...
0.004
Figure 6.
Under best conditions a neohexane yield % on converted ethylene can b e expected in a continuous unit
100 -- D +
This relation used in conjunction with K’, determined from Figure 3, and D, estimated from the table on page 1534, allows ethane and total alkylate yields to be calculated. The alkylate composition breakdown can be determined by using Figures 4 and 5. This procedure was used in calculating a number of cases designed to show the effect of temperature and of ethyl bromide modifier on performance at an isobutane-ethylene ratio of 25 (Figure 6). This ratio was assumed to allow a satisfactory compromise between neohexane yield and reaction rate and therefore to be representative of possible commercial practice. Under the best conditions a neohexane yield of 220 weight % on converted ethylene can be expected-over 70% of theoretical.
660
of 220 weight
tiveness varies with the halogen in the order C1 < Br < I and with the cornpound; the ethyl halides were the most active studied. Iodine and its compounds promote side reactions leading to ethane formation. Bromine does not have this fault; hence ethyl bromide is the preferred modifier. When this modifier is used a t 750’ F. the maximum practical neohexane yield is estimated a t 220 weight % based on converted ethylene; this is over 70% of the theoretical. Nomenclature D = (total ethylene consumed)/ (ethylene converted to alkylate) K’ = chain transfer constant = K B / K A K A = rate constant for reaction A (ethylene addition) K B = rate constant for reaction B (hydrogen abstraction) R = isobutane-ethylene mole ratio W = weight % total alkylate based on converted ethylene X = weight isobutane Y = weight ethylene d = differential operator t = time ( ) = moles in reactor-i.e., moles per reactor volume Subscript 0 = charge conditions Acknowledgment
The author wishes to acknowledge the very substantial assistance of J. M. Stuckey and W. C. Fulton in the experimental phases of this study.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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