Production of 2,3=Dimethylbutane bv Alkvlation J
J
R. B. THOMPSON AND J. A. CHENICEIC Universal Oil Products Company, Riverside, I l l .
A
process is described for the production of 2,3-dimethylbutane by the alkylation of isobutane with ethylene, using a liquid aluminum chloride-hydrocarbon complex catalyst and ethyl chloride promoter. The catalyst has a long life, is relatively inexpensive, and can be maintained at constant activity level such that high yields of 2,3-dimethylbutaneare obtained. Total alkylate yields of 270 to 300 weight yo, based on ethylene consumed, are obtained containing 68 to 75 volume qo of 2,3dimethylbutane. The catalyst life is approximately 85 gallons of total alkylate per pound of fresh aluminum chloride introduced from a-saturator to the catalyst as a solution in the isobutane charge. The following conditions are believed to be most desirable: a saturator temperature of 160' F., or lower, in order to maintain catalyst activity at optimum value; reactor temperatures in the range 110' to 140' F.;sufficient pressure for liquid phase operation; 0.5 to 2.5 mole ethyl chloride concentration; 15 to 25 minutes space time; and a molar ratio of isobutane to ethyleneof at least 4.5. The compositionof the catalyst affects the yield and quality of alkylate produced. A satisfactory alkylate may be obtained using catalyst with the followingrange of composition: 17 to 13qo aluminum, 50 to 42qo chlorine, and 23 to 349%carbon.
I
N THE past few years 2,3-dimethylbutane has been found t o be a desirable component of aviation fuels (1,6) because of its
high octane number, high blending value, and excellent richmixture response. I n addition, the boiling point of 136' F. (58" C.) is sufficiently low t h a t the quantity of isopentane used to meet vapor pressure requirements of aviation fuel can be reduced. 2,3-Dimethylbutane has been synthesized in several ways. I t can be prepared in the laboratory by the Wurtz reaction, using isopropyl iodide and sodium (9),by the electrolysis of potassium isobutyrate (9), or through dimolecular reduction of acetone to pinacol, followed by dehydration and hydrogenation (6). It has also been isolated from the distillation of crude petroleum ( 2 ) . More recently it has been found in the hexane fraction ( 7 ) formed in the alkylation of isobutane with butenes and in the products of hexane isomerization. However, 2,3-dimethylbutane forms only a smalI proportion of the total material produced by these methods, and its isolation involves laborious fractionation. Grosse and Ipatieff (4) found t h a t 2,3-dimethylbutane is a major product of the catalytic alkylation of isobutane with ethylene. This alkylation reaction appears promising from a commercial point of view since the raw materials are available in large quantity a t low cost. Consequently, in 1939 a comprehensive investigation of this reaction was begun in this laboratory. Extensive work has shown that materials containing aluminumchloride are the most satisfactory catalysts. A liquid aluminum chloride-hydrocarbon complex formed by the interaction of ethylene, isobutane, and aluminum chloride in the presence of a
promoter has been found to be a particularly desirable catalyst for 2,3-dimethylbutane. This catalyst is sufficiently active to utilize substantially all of the ethyleGe in a single pass, but not so active as t o promote undesirable side reactions. It has H long life, is relatively inexpensive, and can be maintained at constant activity level t o permit continuous operation. By use of this catalyst, a yield of alkylate closely approaching the theoretical may be obtained. The 2,3-dimethylbutane content of the alkylate is over 68%, and of the hexane fraction, over 90%. In the development of this reaction two points had t o be given special consideration-namely, continuity of operation and evaluation of the fundamental process variables. Continuous operation is most advantageous when uniform process conditions can be used. Constant operating conditions require that the catalyst have a uniform activity. Since the catalyst depreciates as i t is used, a method of introducing fresh aluminum chloride continuously is necessary. Although several methods are available to introduce make-up aluminum chloride, thisis most conveniently accomplished by passing all or a portion of the isobutane through % heated zone containing aluminum chloride. This method is flexible and exact, since the quantities introduced can be controlled by regulating temperature or flow of isobutane. Furthermore, the bulk supply of catalyst is not contaminated because isobutane does not react with aluminum chloride at the temperatures used in the absence of a promoter. The optimum values of the important process variables were determined in a small pilot plant. APPARATUS AND P R O C E D U R E
Figure 1 is a diagram of the continuous-flow laboratory apparatus used to study the variables involved in the alkylation reaction. It consists essentially of a tube containing granular aluminum chloride, a turbomixer reactor, a catalyst settling and recycle system, and a stabilizer. The flow may b e summarized as follows: Dissolved aluminum chloride is transported into the reactor by pumping isobutane charge stock through the saturator containing 4-t o 20-mesh aluminum chloride, with valves 1 and 4 open and valve 2 closed. The amount of aluminum chloride carried into the reactor is easily and accurately controlled by the temperature at which the saturator zone is maintained. Dowtherm A is the heat transfer medium, and temperature is regulated b y a Variaccontrolled electric heater. A previously determined amount of anhydrous hydrogen chloride may be admitted into the reactor through valve 3 b y a calibra.ced capillary tube ( 3 ) . The ethylene is also introduced into the reactor through valve 3. T h e desired ethylene rate is achieved by maintaining the flow from a cylinder, equipped with a pressure regulator, through a calibrated rotameter by a manually operated Hofer valve. The reactants thus introduced are stirred in the turboreactor which is maintained at the desired temperature by a n electrically heated water bath. From the alkylation zone the reaction product, aluminum chlogide-hydrocarbon complex formed during the reaction, unreacted hydrocarbons, and promoter flow ttirough valve 5 into the settler where the phases are separated; the complex sinking to the bottom while the hydrocarbons and promoter flow through valve 6 into the stabilizer. Here the promoter, butanes, and lighter hydrocarbons are separated from the alkylate under plant pressure. Alkylate is withdrawn through manually 1265
INDUSTRIAL AND ENGINEERING CHEMISTRY
1266
MECHANICALLY
and also would result in decreased 2,3-diniethylbutane content of the debutanized alkylate. I n most of the present work ethylme of 98% purity has been used. Horvenx, runs with dilute feed stocks containing as low as 31 mole % ethylene resulted in alkylates of similar quantity and hexane content to those obtained with w e n tially pure ethylene (Table I). The relatively low yields of hcxanei obtained in these two experiments mere duc to thta fact that catalyst complex of higher than optimum aluminum chloride content v a s used. The diluent gases had no apparent effect upon the rste of deterioration of the complex catalyst. However, experience with an ethylene feed stock containing 4 to 5 mole mo propylene indicates that the latter should not be present in appreciable quantity, inasmuch as it reduces the yield of debutanixed alkylate, hexane content, and octane number, and increases the volume percentage of higher boiling material.
S T A B I L I2E R
ETHYLENE PROMOTER
LATE STORAGE
SAT URAT0R
Figure 1.
Vol. 40, No. 7
Pilot Plant for 2,3-Dimethylbutane Production
CATALYST
operated valve 7, and the lighter overhead materials ai e collected in a pressure vessel after being passed through valve 8 and a pressure controller. As the complex forms and separates, it accumulates, and a level builds up in the calibrated Jerguson gage by flow through valves 9 and 10. The complex is recycled to the reactor through by means Of a pump' The rate is determined by closing valve 9 and then, with valve 10 open, measuring the volume pumped from the calibrated Jerguson gage during any convenient interval of time. Samples of catalyst for analysis may be collected by valve 12. The introducticm of aluminum chloride may be halted in order to studp the rate of deterioration of the complex by closing valve 4, opening valve 2, and thus by-passing the saturator zone. The plant-stabilized alkylate is debutanized and fractionally distilled in a modified Podbielniak column packed with a Nichrome spiral. The 2,3-dimethylbutane content of the hexane fraction is determined by precision distillation in a Lecky-Ewe11 column and by infrared analysis. All yields are reported with total penfor the pentanecontent of the charge tane retention stock and the plant-stabilizer overhead. !
COMPOSITION OF FEED STOCK
The isoparaffin charging stock contained 70% isobutane, the remainder being n-butane with lesser amounts of propane and pentane. Experience with a saturator indicates that the charge to this tube should contain less than 10 mole '% pentane to prevent deterioration of the aluminum chloride. If the catalyst does deteriorate, the solubility of the aluminum chloride is considerably decreased with resultant unpredictability of catalyst input at a given temperature. The propane content of the isoparaffin charge also should be sufficiently low that a liquid phase can be maintained in the saturator without application of excessive pressure. Olefins should be removed from the isoparaffin charge passing through the saturator to eliminate deterioration of the granular catalyst. I n all of this work a molar ratio of isobutane to ethylene of 4-5 t o 1 has been maintained with production of excellent quality alkylate in all cases where other reaction conditions 'rvere satisfactory. Unreacted olefins appeared in the. stabilizer overhead only occasionally, a n indication of virtually complete conversion of the ethylene. A slightly higher molar ratio of isobutane t o ethylene may be advisable to ensure compl$e alkylation of the olefin. charged. It is also possible that the use of considerably higher ratios might increase the specificity of the reaction and thus enhance the yield of 2,3-dimethylbutanee It is undoubtedly possible t o run at lower ratios in order to increase plant capacity, but this would mean a sacrifice of complete conversion per pass
The catalyst consists of a mobile aluminum chloride hydrocarbon complex used in the presence of a promoter. AklthouKh complexes Prepared from a number of different hydrocarbons are effective catalysts, it has been found most desirable t o use the complex produced in the reaction itself-namely, from chloride, ethylene, isobutane, and ethyl chloride. The catalpst can be Prepared outside and charged t o the reactor, or more conveniently by charging aluminum chloride plus the other ingredients to the reactor until sufficient material for recycle is formed. A product prepared in this way is a dark brown liquid, can be readily pumped, and settles rapidlv from the hydrocarbon phase.
TABLE I.
E F F E r T OF C'~MPL)IITIOP\I OB
(RUN1)
Period No. Olefin feed, mole % C2Ha Yield, wt. yo CzHa reacted Product, vol. yo Ca (68-104' F.) CI, (104-149" F.) C7t (> 149' F.)
ani P@El;;&:nfole
ETHYLENE FEED 7 98 316 6
74 20
RTOCK
8
31"
296
8 74 18
% rriethne, 54% ethane, 8 % propane, 104 Ilropylene,
The composition of the aluminum chloride-hydrocarbon complex used as catalyst has a decided effect upon the yield and quality of alkylate produced. The data in Table I1 were obtained in different plants under different reaction conditions, but in neither case was fresh aluminum chloride added to the sludge initially charged to the system. The results shorn that the aluminum chloride-hydrocarbon complex slowly decreases in aluminum chloride content, a n indication of gradual accumulation of hydrocarbons in the complex. This is substantiated by the increase in the carbon content of the catalyst with use. This increase does not seem to be due to replacement of either chlorine or aluminum since the atomic ratio of chlorine to aluminum is relatively constant a t 2.2 to 2.5 throughout the course of a run. The figures for aluminum chloride content are based upon gravimetric analysrs for aluminum, and the chlorine resultr are based upon determination of ionizable hdogen. The total chlorine content of the catalyst may be somewhat higher than the inorganic chlorine, as chlorine is probably present in the hydrocarbon part of the complex.
July 1948
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INDUSTRIAL AND ENGINEERING CHEMISTRY
longer period, both decreased rapidly t o values such as those Run 2 : Space Time 25 Min.. shown in columns 1 and 2, Temp. 140° F., 1.7 Mole % HCi: Table 111. Hydrogen chloride 250 Lh./Sq. In. Gage Run 1: Space Timea 30 Min: Temp. 140° F 2 Mole Isobutane/Ethylene, 4:1 % HCi, 250 Lb./Sq. In. Gage: Isobutane/Et&lene 4.6 concentrations from 0.5 to 5.2 Period No. 1 2 3 4 5 6 7 8 4 . 7-8 13 18 26-2 mole yowere satisfactory with Cumulative time, hr. 96 144 192 240 288 336 384 437 215 389 617 843 1197 Yield, wt. % C2H4 respect t o composition of the charged 257 276 253 283 312 299 316 29$ 234 229 226 218 192 product, but the yield of total Val. % in alkylate 49 55 57 73 73 73 74 74 73 80 78 82 82 Sludge composition alkylate decreased as the hyWt. 70 AiClib 92.1 92.7 89.8 8 3 . 8 85.4 84.9 81.7 81.5 76.8 74.9 72.1 71.5 67.6 Wt. 7" c 1 56.9 5 6 . 3 54.9 5 3 . 1 51.7 5 0 . 4 49.7 49.1 4 9 . 8 4 8 . 3 4 8 . 1 4 5 . 1 42.7 drogen chloride concentration Wt. % CC 20.5 21.8 23 4 23.5 26.2 25.8 27.9 31.3 ... increased (Table 111). Cl/Al 2134 2 . 2 7 2 : 2 a 2 . 3 8 2 : 2 8 2 . 2 2 2 : 2 8 2 . 2 5 2 . 4 3 2 . 4 2 2 . 5 0 2 . 3 6 2.37 The fact that the yield dea Space time = volume of catalyst in reactor/volume hydrocarbons charged per minute. b Calculated from AlzOs determination. creased when the hydrogfn C Determined by wet combustion of total catalyst. chloride concentration became high suggested t h a t the hydroCHLORIDE CONCENTRATION ON YIELD T ~ B L E111. EFFECTOF HYDROGEN gen chloride was reacting with --O, OaI ) , 4b --, --O. 6 --1.3--7--2.1---5.2-the ethylene to form ethyl HCI, mole yo chloride. This supposition was Yield Cat, wt. % CzH4 confirmed by the facts that reacted 137 100 205 176 183 222 234 220 210 208 194 167 158 Vol. % co 67 60 85 84 82 79 79 83 83 86 83 79 82 only a trace of hydrogen chloVol. % boilride (0.01 gram per hour when inq below 3OOOP. 94.8 92.2 98.8 8.9 9 9 . 1 . . . 99.1 99.0 98.5 9 9 . 0 98.6 99.3 92.1 1.8 grams per hour were being C ~ H plus I CZHRwas found in exit gas. charged) was in the exit gas, b CzHo was found in exit gas. and that ethyl chloride was found in the butane fraction of the exit gas. The ethyl chloride is found in the butane fraction because it forms a n azeoApparently a catalyst of excessively high aluminum chloride trope with n-butane. The feasibility of operating with ethyl content gives a product of low volume percentage of hexanes, chloride as a promoter is demonstrated by the data in Table IV. and then, as the catalyst decreases in aluminum chloride content Ethyl chloride is the desirable promoter, as none of the ethylthrough a considerable range, the hexane content remains fairly ene was removed from the reaction by combining with the constant. The data for run 2, Table 11,show that, if the reaction promoter, and the yield approached theoretical value of 3o7Y0. is continued until the aluminum chloride concentration is subThe results for the ethyl chloride balance indicate that, if any stantially decreased, the yield of crude alkylate is considerably ethyl chloride is consumed during the reaction, the amount is lower although the hexane content remains high. Comparison of runs 1 and 2 of Table I1 indicates that, under different reaction conditions, catalysts of different compositions may be used. On the basis of these results the optimum catalyst composition probably lies in the range 17 t o 13% aluminum (84 t o 64% aluminum chloride), 50 to 42% chlorine, and 23 to 34% carbon. a0 During the course of the present work, no excessive corrosion of equipment was noted; apparently the low temperatures 4.0 decrease the corrosion rate appreciably. Mild carbon steel test strips exposed t o the action of flowing alkylation sludge a t 130 O F. in the presence of promoter were corroded t o the extent 2.0 of only 0.009 inch a year in sludge of 87y0 aluminum chloride content, and 0.0024 inch a year in sludge of 65% aluminum chloride content. The walls of the turbomixer reactor were not i visibly corroded after approximately 750 hours of continuous m 1.0m operation at 140' F., and the Monel metal impeller lost only 0.3% in weight after operation for the same period. J 08COMPOSITION TABLE 11. EFFECTOF CATALYST
-
-
-
2 a
PROMOTER
W
Initially, anhydrous hydrogen chloride was used as a promoter for the reaction. Concentrations in the range 0.0 t o 5.2 mole 70 of the total charge were studied. When no promoter was used, the yield of alkylate and the percentage of hexanes in alkylate were initially high; however, as the reaction was continued over a
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TABLE IV. (Ron 1:
ETHYL CHLORIDE AS
A
-
PROMOTER
1.1 mole % CzHsC1 4 8 isobutane/CzH4, 29-minute space time, 250 Ib./sq. in. gale, i4Oo F. reactor temperature) >d No. 1 i.Cs+ (wt. % C2Ha reacted) 304
aos -
0.0
I
2.7
Based on chlorine analysis of stabilizer overhead, assuming total recovery of unreacted hydrocarbons. a
Figure 2.
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24
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i
2.n I / T X 103
29
I 3.1 ( KELVIN )
I 32
Solubility of Aluminum Chloride in Butane
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 40, No. 7 R E 4 C T O R TEMPERATURE
TABLE V. EFFECT OF SATURATOR TEMPERATURE
Reactor temperatures of 110" to 140" F. have been 28 30 used n i t h equal succcss, as 694 742 demolistrated by the results 110 110 By-passed in Table VI. There is some evidence that, if the aluminum 29 29 chloride concentration of the 279 276 2:5 214 ... 222 . . . . . . 177 ... 213 catalyst is too high for maxi214 219 ... ...... 202 144 171 72 185 mum production of 2,3-di8 5 5 10 4 8 9 16 30 3 4 methylbutane fraction when 78 80 81 68 84 74 81 59 41 83 87 14 15 14 22 12 11 17 25 29 14 9 operating a t 140" F , an alkyl98.2 98.6 ate of high hexane content 98.8 96.0 . . . . . . 98.3 97.2 96.5 98.2 . . . . Chlorine, wt. % Ca may tw obtained by lowering fraction 0.004 0.008 0.003 0.003 0.003 0,004 0.002 0,004 0 , 0 0 2 0.012 0.0033 Bromine No. of CT+ the reactor temperature and fraction 1 3
