Isomerization - Advances in Chemistry (ACS Publications)

Isomerization R. C. GUNNESS

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1951 | doi: 10.1021/ba-1951-0005.ch011

Standard Oil Co. (Indiana), Chicago, Ill.

Isomerization played a vital role during World War II, primarily by increasing the supply of isobutane for alkylation. Although less extensively employed, isomerization of pentanes and hexanes provided valuable blending stocks for high octane aviation fuels, and isomerization of naphthenes contributed to toluene production. Isomerization offers the best method of raising the octane levels of pentanes and hexanes, and this application can be expected to become important as performance requirements of gasolines increase. Butane isomerization would again be extensively used in a future war, and pentane and naphtha isomerization might also be employed. The current shortage of aromatics is reviving interest in naphthene isomerization.

Isomerization is one of the important ways i n which petroleum technologists have employed chemical synthesis to meet the demand for "tailor-made" fuels needed b y i n creasingly powerful internal combustion engines. Isomerization may be defined as the rearrangement of the molecular configuration of a hydrocarbon without change i n molecular weight. Although such rearrangements are known to occur i n cracking and reforming operations, the present discussion will consider only those processes i n which isomerization is the basic reaction. When applied to the low-boiling fractions of petroleum, containing predominantly straight-chain paraffin hydrocarbons, isomerization increases the proportion of valuable branched molecules. I n the case of butane, branching leads to increased chemical activity through the availability of a reactive tertiary hydrogen atom. T h i s is the basis for the most extensive commercial application of isomerization—the manufacture of isobutane for alkylation to high octane components of aviation gasoline. W i t h hydrocarbons i n the gasoline boiling range, rearrangement to more highly branched structures leads directly to improvement i n antiknock properties. Table I illustrates this trend b y relating the structures of the pentanes and hexanes to their octane ratings, as determined b y the A S T M M o t o r method. The cyclic structures, included here for comparison, occupy a n intermediate position i n each group. Isomerization has long been familiar to organic chemists, but reactions of this type have become important i n petroleum chemistry only i n relatively recent years. N o t until 1933 did the first paper on the isomerization of individual paraffinic hydrocarbons appear in the scientific literature (11). W i t h the growing interest during the 1930's i n the use of catalysts i n petroleum processing, extensive exploratory work was conducted on a variety of catalytic reactions, i n cluding isomerization. Subsequent development work carried isomerization to the point where several processes could be quickly brought to full scale operation when the situation warranted. E a r l y i n W o r l d W a r I I , the demand for high octane aviation gasoline provided the necessary incentive. 109 In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

no

ADVANCES IN CHEMISTRY SERIES

Table I.

Effect of Hydrocarbon Branching on Octane Number Motor Octane Number Unleaded +3 cc. T E L

Hydrocarbon

Pentanes n-Pentane

C—C—C—C—C

61.9

83.6

Cyolopentane

/ \

85.0

95.2

Isopentane

C—C—C—C ^

90.3

Iso-octane + 2 cc. T E L


Hexanes n-Hexane

C —- C C -— C — C — C — C

26.0

65.2

2- Methylpentane

C —- cC -— C — C — C

73.5

91.1

3- Methylpentane

C—C—C—C—C

74.3

91.3

77.2

87.3

80.0

93.0

93.4

Iso-octane + 2.1 cc. T E L

94.3

Iso-octane + 1.8 cc. T E L

Cyclohexane Methylcyclopentane

Neohexane

c C

ΓΤ C I

c — c — c — c

è Diisopropyl

C—C—C—C I "^ C

The first commercial isomerization plant was a butane unit at Shell's Houston re finery. I t began operation i n November 1941. B y the end of the war, a total of 43 isom erization units had been built and placed i n operation—38 i n the United States and the remainder i n Canada, the Caribbean area, and Arabia. The wartime role of isomerization i n the United States is summarized i n Figure 1. M o s t of the units were built to supplement the natural supply of isobutane for alkylation. Domestic production of synthetic isobutane began i n 1941 and rose i n four years to more than 40,000 barrels per day. M o s t of the isomerization units were shut down when the military need for aviation gasoline dropped after the close of the war i n Europe i n M a y 1945. Pentane isomerization was carried out on a much smaller scale. Isopentane, because of its high octane number and good lead response, was blended directly into aviation gasoline. I t also served to increase the volatility of blends containing such high-boiling components as alkylate. Isomerization of light naphtha—mainly pentanes and hexanes—was practiced to only a limited extent. A fourth type of petroleum isomerization, which was commercialized on a small scale, involves the rearrangement of naphthenes. I n the manufacture of toluene b y dehydrogenation of methylcyclohexane, the toluene yield can be increased by isomerizing to methylcyclohexane the dimethylcyclopentanes also present i n the naphtha feed. T h i s type of isomerization is also of interest i n connection with the manufacture of benzene from petroleum sources.

Basic Factors in Isomerization Isomerization of paraffins and naphthenes is a reversible first-order reaction limited by thermodynamic equilibria. I t is slightly exothermic i n nature and does not take place to any appreciable extent without a catalyst (4). Although the mechanism of the reaction

In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


GUNNESS—ISOMERIZATION

111

has been extensively studied (15), no clear-cut agreement has been reached among the various investigators. E q u i l i b r i a . T h e e q u i l i b r i u m distributions of butane, pentane, and hexane isomers have been experimentally determined (5, 16) and are diagrammed i n Figure 2. I n each case, lower temperatures favor the more highly branched structures. A t the approximately 200° F . temperature usually employed for isomerization, the butane equilibrium mixture contains about 7 5 % isobutane. T h a t for pentane contains about 8 5 % isopentane.. I n the case of hexane, the equilibrium product contains about 5 0 % neohexane and has a M o t o r octane rating of about 82. I n all cases, of course, the yield of the desired isomers can be increased by fractionation and recycle. For butane and hexane, the experimental equilibria agree fairly well with equilibria calculated from thermodynamic data (17). I n the case of pentane the isomer favored thermodynamically is neopentane. This material is not obtained, however, apparently for reasons connected with the reaction mechanism.

1942

Figure 1.

1

9

4

3

1

9

4

4

1

9

4

5

Wartime Growth of U. S. Commercial Isomerization

Side Reactions. T h e chemical problems encountered i n paraffin isomerization— particularly the suppression of side reactions—become more serious as the molecular weight of the hydrocarbon increases. I n addition to lowering the yield of the desired product, such side reactions shorten the effective life of the catalyst. Butane isomerization is relatively straightforward. The butanes show no appreciable tendency to crack or disproportionate under isomerization conditions (6). I n the case of pentanes, disproportionation to isobutane and hexane is pronounced. This undesirable side reaction can be suppressed by the addition of small amounts of cyclic hydrocarbons or by operation under hydrogen pressure (6). The hexanes undergo side reactions even more readily than do the pentanes. A l though disproportionation and cracking can be suppressed to some extent by the addition of cyclic hydrocarbons, this treatment is not effective enough to ensure satisfactory catalyst life, and hydrogen at relatively high pressure must be used as the inhibitor. The isomerization of paraffinic heptanes presents still more difficult problems, and no successful method has been found for suppressing the side reactions. I n the case of naphthenes, isomerization takes place under such mild conditions that side reactions do not interfere. Catalyst. I n a l l of the commercial isomerization processes applied to paraffins and naphthenes, the catalyst is a l u m i n u m chloride plus hydrogen chloride. I n the pure state, these two ingredients do not associate chemically (1), but they become associated i n the presence of certain hydrocarbons normally occurring i n petroleum stocks. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

112



M o s t of the mechanisms proposed for isomerization assume formation of a transitory compound of aluminum chloride, hydrogen chloride, and the hydrocarbon undergoing isomerization. This unstable compound, which cannot be isolated, should not be confused with the liquid aluminum chloride-hydrogen chloride-hydrocarbon complex formed during the isomerization process as a result of cracking, disproportionation, hydrogen transfer, and polymerization. T h i s liquid complex resembles heavy motor oil i n appearance and viscosity. However, i t has a specific gravity of about 1.5, and i t is immiscible with liquid hydrocarbons. The hydrocarbon component of the complex is of relatively high average molecular weight (300 and higher) and contains an average of at least two double bonds per molecule. There may be as many as two molecules of combined a l u minum chloride for each double bond. The complex is itself an active catalyst, and i t may, in addition, dissolve aluminum chloride. I n the presence of hydrogen chloride, this dissolved aluminum chloride increases the catalytic effect. BUTANES

100

200

300

PENTANES

400

100

200

300

TEMPERATURE,

Figure 2.

HEXANES

400 E

100

200

300

400

F

Experimental Equilibria

D u r i n g the isomerization process the complex gradually becomes more highly u n saturated because of hydrogen transfer to lower boiling hydrocarbons formed i n side reactions. A s this occurs, the A1C1 appears to become more tightly bound, and the complex gradually loses catalytic activity. 3

Commercial Operations T h i s consideration of basic factors tends to oversimplify the technical problems that had to be solved before isomerization could become a successful commercial operation. Several processes resulted from largely independent work carried out i n separate petroleum laboratories. A s would be expected, each of these laboratories devised a somewhat different scheme—particularly i n the case of butanes, which received the greatest amount of attention. A major share of the credit for the rapid commercialization of these processes can be traced to the free exchange of technical information and operating experience through the Isomerization Subcommittee of the wartime A v i a t i o n Gasoline Advisory Committee (12). Butane Isomerization. F i v e processes for butane isomerization were i n commercial use b y the end of W o r l d W a r I I . These processes differ p r i m a r i l y i n the method of contacting the hydrocarbon w i t h the catalyst. T w o are vapor-phase processes, which require periodic discard and replacement of the catalyst bed; the other three are carried out i n the liquid phase and are continuous with respect to catalyst addition and withdrawal. The essential features of the Shell Isocel process (2)—the first to reach commercialization—are shown in Figure 3. I n this process, the dried butane feed is vaporized and sent


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to the top of the reactor, where i t picks up anhydrous hydrogen chloride. The combined vapors are charged to the reactor, which is packed with Isocei—aluminum chloride on bauxite. T o recover the aluminum chloride picked up b y the hydrocarbon, the reactor product is passed through a guard chamber filled with bauxite. The product is then con densed, cooled, and passed to the accumulator, from which small amounts of by-product gases lighter than butanes are vented to prevent pressure build-up i n the system. T h e hydrocarbon is charged to the top of the stripper tower, which recovers the hydrogen chloride for recycle. Remaining traces are removed b y caustic and water washing, and the hydrocarbon stream is sent to an efficient fractionating tower. The η-butane recovered is generally recycled. The other vapor-phase butane isomerization process, developed cooperatively b y the Anglo-Iranian O i l C o . and the Standard O i l Development Co., is somewhat similar to the Isocei process. I n the AIOC-Jersey process (13), the reactor is initially filled with bauxite, and aluminum chloride is sublimed into the vaporized feed as necessary to maintain the desired catalyst activity. Upflow of vapor through the reactor is the customary arrange ment. Since carry-over of aluminum chloride is not excessive at the usual rates of catalyst addition, about half of the commercial plants employing this process were not equipped with guard chambers. BUTANE FEED

î

HCl HCl

DRIER

RECYCLE LIGHT GASES

[HEATER}—J

U1

ACCUMULATOR!

TO FRACTIONATION

2 Figure 3. Vapor-Phase Butane Isomerization Shell Isocei process

The butane isomerization process developed b y the Universal O i l Products C o . is shown i n Figure 4. I n this process (3), the feed is maintained essentially i n the liquid phase under pressure. Part of the feed is by-passed through a satura tor, where i t dissolves aluminum chloride. The feed later picks up hydrogen chloride and passes through the reactor, which is packed with quartz chips. Some insoluble liquid complex is formed, and this adheres to the quartz chips. The aluminum chloride i n the feed is preferentially taken up by the complex, which thus maintains a n active catalyst bed. The complex slowly drains through the reactor, losing activity en route. I t arrives at the bottom i n essentially spent condition and is discarded. Aluminum chloride carried overhead i n the reactor products is returned to the reactor from the bottom of the recovery tower. T h e rest of the process is the same as i n the vapor-phase processes. A second process employing complex as the catalyst carrier was independently developed b y the Standard O i l C o . (Indiana) and b y The Texas C o . I n this process (19,20), liquid butane containing make-up aluminum chloride and recycled hydrogen chloride is bubbled upward through a bed of preformed liquid complex about 20 feet i n depth. B e cause the aluminum chloride in the feed is effectively transferred to the complex, catalyst carry-over in the reactor effluent is low and no recovery tower is needed. The third liquid-phase butane-isomerization process, shown i n Figure 5, was developed by Shell as an improvement over the original intermittent vapor-phase process. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


114

The aluminum chloride is handled, i n this case, as an approximately 9 % solution i n rela tively inert molten antimony trichloride ; the solution has a solidification point of about 170° F . (10). A s in the other processes, a small amount of liquid aluminum chloride-hy drogen chloride-hydrocarbon complex is continuously formed.

BUTANE FEED

1

HCl HCl

_L

R E C Y C L E LIGHT GASES

DRIER

_ t _ ACCUMULATOI


3 - β

HEATER T O FRACTIONATION

AlCl, SAT.

P.AIIgTIP.

W A S H

C O M P L E X

Figure 4.

I

I

Liquid-Phase Butane Isomerization U.O.P. Process

In this process, the liquid butane feed is employed first to recover aluminum chloride and antimony chloride from spent catalyst. This is accomplished i n a scrubber, from which insoluble complex is continuously discarded. The butane stream then picks up re cycled hydrogen chloride and enters the reactor, where mechanical agitation causes i n t i mate contacting with an equal volume of catalyst. T h e undesirable complex formed i n

HCl

R E C Y C L E

LIGHT GASES ' |ACCUMULATQR| i

2USTIC CA WASH

TO FRACTIONATION

CATALYST RETURN Figure 5.

Liquid-Phase Butane Isomerization Shell antimony chloride process

the reactor dissolves i n the molten mixture of aluminum chloride and antimony chloride but is continuously rejected from the system b y a small catalyst side stream passing through the scrubber. A portion of the liquid catalyst is carried with the products from the top of the contactor to a distillation tower. Here the aluminum and antimony chlo rides are recovered from the products, which pass overhead and are treated as in the other processes. The proper aluminum chloride content of the molten catalyst is restored by a saturator through which part of it is passed before it is returned to the contactor. In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

115


A summary of commercial operations employing the five butane isomerization proc esses is given in Table I I . The data represent typical wartime process operations rather than characteristics of any specific commercial plant. Table II.

Typical Commercial Process Data for Butane Isomerization Vapor-Phase AIOCJersey

Process

Shell

Catalyst form Catalyst life, gal. isobutane per lb. AICI3

H C l concentration, wt. % Once-through conversion, % Reactor conditions Temp., F . Pressure, lb./sq. in. Space velocity, vol./hr./vol. Reactor material No. of U . S. plants


0

Impregnated bauxite 200 2-14 40

Sublimed on bauxite 200 4 35

270 210-300 235 200 0.5-1.0 0.5-1.0 Carbon steel 8 7

U.O.P.

Liquid-Phase IndianaTexas

Shell

Complex on quartz chips

Liquid complex

Dissolved in S b C l

50-120 5 38

50-120 4 38

50-120 5 45

200 235 0.5 Hastelloy Β or G unite 14

205 365 1.0 cement 3

3

180 300 2.5 Nickel 2

Since the vapor-phase processes employing supported aluminum chloride contain no provision for withdrawal of spent catalyst, the feed must be practically free of impurities that would form complexes and clog the pores of the catalyst, thereby reducing catalyst activity. High-quality feeds—obtained, for example, b y acid treating to remove olefins and rerunning—form virtually no complex and give substantially greater catalyst life than is obtained i n the liquid-phase processes, where the discard of spent catalyst eliminates the need for special equipment for pre treating the feed. Satisfactory conversion was pro longed i n the Shell vapor-phase units by gradually increasing the temperature and the hydrogen chloride concentration as the catalyst lost activity. I n the A l O C - J e r s e y plants, catalyst activity was maintained by the addition of fresh catalyst, and run length was l i m ited b y the catalyst capacity of the reactor. Hydrogen chloride concentrations were essentially the same i n all except the Shell vapor-phase units. Once-through conversions were generally similar, although slightly higher levels re sulted from the combination of liquid catalyst and mechanical stirring in the Shell liquidphase plants. A l l the processes except the Shell vapor-phase processes used constant temperatures ranging from 180° to 270° F . , depending on the process. Pressures ranged from 200 to 365 pounds. The reactor materials reveal the comparative magnitude of the corrosion problem encountered with the five processes (7). Aluminum chloride—hydrocarbon complex is a highly corrosive material, particularly i n the presence of hydrogen chloride. The vaporphase reactors could be built of ordinary carbon steel, since essentially no complex was formed. N o appreciable corrosion was encountered so long as moisture was excluded. I n plants employing complex as the catalyst carrier, i t was necessary to use corrosion-resist ant materials i n the reactor lining and i n other items of equipment where conditions of temperature, turbulence, and hydrogen chloride concentration would have contributed to high corrosion rates. Relatively minor corrosion problems were encountered i n the proc ess employing antimony chloride as the catalyst carrier, and nickel was successfully used as the reactor material. The 34 domestic commercial units employing these five butane-isomerization proc esses contributed substantially to the war effort. Pentane Isomerization. Pentane isomerization, although carried out on a much smaller scale, increased the critical supply of a v i a t i o n gasolines toward the end of the war. T w o pentane processes—one developed b y Shell and one b y Standard (Indiana) —were commercialized before the end of the war. T h e p r i n c i p a l differences between the butane and pentane processes are the use i n pentane isomerization of somewhat milder conditions and the use of an inhibitor to suppress side reactions, principally disproportion ation. In general, the problems of the butane processes are inherent also in pentane isom erization, but the quality of the feed stocks is less important. Catalyst life is much In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.

116


shorter than i n the butane processes; only about 30 to 50 gallons of isopentane are pro duced per pound of aluminum chloride. The Shell pentane process (10) is similar to the corresponding liquid-phase butane process. T o inhibit side reactions, a hydrogen partial pressure of 60 to 70 pounds per square inch is maintained, largely b y recycle. Make-up hydrogen is added as necessary to offset that lost from the system or consumed i n saturating any olefins present i n the feed. The molten catalyst contains only about 2 % of aluminum chloride and has a lower solidifi cation point than the butane catalyst. Typical reactor temperatures are slightly lower, but other process conditions are essentially the same as i n the butane process. The only commercial plant employing this process—that of Tide Water Associated—produced an average of about 500 barrels of isopentane per calendar day during the last year of the war.


NAPHTHA FEEO

X

HCl HCl RECYCLE

1

IDRIERI

.PRODUCTS

τ

TjHEATEwf

AT

IftAllRTIP. I

t

1 WASHΤρ—"

SPENT COMPLEX PENTANE OR HEXANE RECYCLE Figure 6. Naphtha Isomerization Isomate process

The other commercialized pentane isomerization process is that of the Standard O i l Co. (Indiana) (20). This process differs from the Indiana-Texas butane process i n that the aluminum chloride is introduced as a slurry directly to the reactor and that about 0.5% by volume of benzene is added continuously i n the feed to suppress side reactions. T e m perature, catalyst composition, space velocity, and hydrogen chloride concentration are generally similar to those i n the corresponding butane process, but the reactor pressure is about 100 pounds lower. The P a n American Refining C o . operated the Indiana pentane isomerization process commercially during the last nine months of the war and produced about 400 barrels of isopentane per calendar day. Napththa Isomerization. T h e only commercial isomerization of light naphtha was carried out i n two plants employing the isomate process developed b y the S t a n d ard O i l C o . (Indiana) (20). I n this process, a feed containing normal pentane and low octane number hexanes is converted to isopentane and to hexanes of higher octane num ber. Pentanes and hexanes i n any ratio may be processed. B y recycle of selected frac tions of the product, concentrates of isopentane or of neohexane and diisopropyl can be ob tained as the ultimate products. Essential features of the isomate process are shown i n Figure 6. The principal de partures from the Indiana pentane process are the necessary use of hydrogen as an inhibi tor and the use of higher temperature and pressure. Feed-stock quality is much less i m portant that i n any other of the isomerization processes. After the dried feed picks up about 5 % b y weight of hydrogen chloride and is heated to 250° F . , hydrogen is added at the rate of 40 to 80 cubic feet per barrel. A s most of the hydrogen is consumed i n a hydrocracking side reaction, no hydrogen recycle is employed. The feed enters the reactor u n der a pressure of 700 to 800 pounds per square inch, and the liquid hourly space velocity is about 1.3. Catalyst life varies from 50 to 100 gallons of product per pound of aluminum chloride. The presence of high-pressure hydrogen causes greater catalyst carry-over than


117



in the corresponding butane and pentane processes, and the reactor products are passed through hot and cold settlers to remove entrained catalyst. A s i n the other processes, hydrogen chloride is recovered for recycle and the products are washed with caustic and fractionated. During the last year of the war, the two commercial isomate units processed more than 5000 barrels of light naphtha per calendar day. The Standard O i l C o . (Indiana) unit at Whiting produced a mixture of hexane isomers directly and recycled Tir-pentane to convert 9 5 % of it to isopentane; light naphtha of 69 M o t o r octane number was u p graded to ultimate products with an average rating of about 81. The Salt Lake C i t y unit of the U t a h O i l Refining Co. also used a 69-octane feed. Pentanes were isomerized on a once-through basis, and hexanes were recycled to increase the production of neohexane and diisopropyi. The hexane fraction of the product amounted to 8 0 % of the total feed and had an octane number of 84. Naphthene Isomerization. I n addition t o the paraffin isomerization processes, naphthene isomerization also proved useful d u r i n g the war i n connection w i t h the manufacture of toluene. I n the Shell dehydrogenation process for the manufacture of toluene, good yields depend upon increasing the methylcyciohexane content of the feed by isomerization of dimethylcyclopentanes. This process was employed commercially at one refinery i n the Midwest and one on the Pacific Coast.

FEEO

CATALYST Fiqure 7.

TO OEHYDROGENATO IN

Shell Naphthene Isomerization Process

Essential features of the Shell naphthene isomerization process (18) are outlined i n Figure 7. Although the contactor principle employed i n the other liquid-phase Shell processes is used, the catalyst is handled i n the form of hydrocarbon complex. F o r the manufacture of methylcyciohexane, a carefully fractionated and dried concentrate of dimethylcyclopentanes is preheated to 200° F . , and about 0 . 1 % of anhydrous hydrogen chloride is added. The feed is joined b y a stream of catalyst complex and charged to the reactor u n der a pressure of 15 pounds per square inch gage. Isomerization takes place readily under the conditions of intimate mixing provided i n the stirred reactor. The catalyst carry-over is separated from the products i n the settler and recycled. I n the conventional manner, the hydrocarbon stream is freed of hydrogen chloride, caustic-washed, and fractionated. A portion of the unisomerized overhead is discarded from the system to prevent the buildup of paraffins, and the balance is recycled. I n commercial practice, an ultimate yield of methylcyciohexane of about 8 0 % was obtained, and catalyst life approximated 100 gallons of feed per pound of aluminum chloride. The naphthene isomerization process has been applied also to the conversion of methyicyclopentane to cyciohexane for subsequent dehydrogenation to benzene. Shell's W i l mington, Calif., refinery has been operating commercial equipment on this basis since M a r c h 1950 (18). In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.


118


O l e f i n Isomerization. One other type of hydrocarbon isomerization is on the threshold of commercialization—namely, that of olefins. Processes for olefin isom erization were first developed some 15 years ago (11, H, 20) after i t was recognized that highly branched olefins have higher octane numbers than do their straight-chain isomers, and that the octane numbers of olefins increase as the double bond moves toward the m i d die of the molecule. The new Brownsville, Tex., plant for the manufacture of synthetic liquid fuels from natural gas makes use of this reaction to increase the octane number of its product b y as much as 20 units. Synthetic naphtha produced over iron catalyst is highly oiefinic and contains substantial amounts of straight-chain isomers with terminal double bonds (8). The shifting of these double bonds toward the center of the molecule may be accom plished b y vapor-phase treatment employing synthetic cracking catalyst i n the fluid state, under mild catalytic cracking conditions. Oxygenated compounds also present are converted under the isomerization conditions to hydrocarbons and water.

Significance of Isomerization Although isomerization is now being operated commercially to only a limited extent, its significance i n petroleum technology should not be underestimated. During World W a r I I , the demand for Grade 100/130 aviation fuel brought into play every technique known to petroleum scientists for making high octane components of aviation gasoline. Isobutane obtained by isomerization of one barrel of normal butane led to an average of 1.3 barrels of aviation alkylate; since alkylate comprised the limiting 30 to 4 0 % i n the production of Grade 100/130 gasoline, butane isomerization increased the production of the desired fuel b y about four barrels for each barrel charged. I n the case of pentanes, the high octane number and good lead response of isopentane permitted an increase i n Grade 100/130 capacity of about three barrels for every barrel of w-pentane replaced b y isopentane. When naphtha isomerization was carried out to yield a neohexane concentrate, the leaded product was almost equivalent i n antiknock quality to leaded aviation alkylate. I n the event of another major war, i t is probable that a l l existing isomerization units would be reactivated and pushed to capacity. Although production of Grade 115/145 aviation fuel required b y newer aircraft engines may place somewhat greater emphasis on aromatics, there would still be a demand for maximum alkylate production, and butane isomerization would again play a n important role. Expansion of pentane and naphtha isomerization is somewhat less certain and would depend on future developments i n air craft fuels. Under peacetime conditions, the rising costs of petroleum and the upward trends i n motor-fuel quality are increasing the economic incentives for upgrading a i l gasoline com ponents. Alkylation and polymerization convert excess light ends to high-octane blend ing stocks and improve the quality of heavy gasoline components, but isomerization offers by far the best route for increasing the octane ratings of the pentanes and hexanes. Such increases may become necessary as over-all octane nunibers are raised. A t the present time, the national shortage of aromatics is also reviving interest i n naphthene isomerization. Wartime necessity brought about the commercial development of the isomerization processes well i n advance of the time when they would be needed under a peacetime econ omy; but either renewed military activity or increased peacetime demands for aromatics or for fuels of higher octane number may again bring isomerization into prominence.

Literature Cited (1) Brown, H . C., Pearsall, H . , and Eddy, L . P., J. Am. Chem. Soc., 72, 5347 (1950). (2) Cheney, Η. Α., and Raymond, C. L . , Trans. Am. Inst. Chem. Engrs., 42, 595 (1946).

(3) Chenicek, J . Α., Iverson, J . O . , Sutherland, R. E., and Weinart, P. C., Chem. Eng. Progress, 43, 210 (1947). (4) Egloff, G . , Hulla, G., and Komarewsky, V . I., "Isomerization of Pure Hydrocarbons," p. 28, New York, Reinhold Publishing Corp., 1942. (5) Evering, B .L.,and d'Ouville, E . L., J. Am. Chem. Soc., 71, 440 (1949).



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(6) Evering, B . L . , d'Ouville, E . L., Lien, A. P., and Waugh, R.C.,Preprint, Division of Petroleum Chemistry, pp. 285-306; 111th Meeting, AM. CHEM. SOC., Atlantic City, N . J . (7) Fragen, N . , Nysewander, C. W., and Hertwig, W. R., Ind. Eng. Chem., 40, 1133 (1948). (8) Grahame, J . H . , U . S. Patent 2,452,121 (Oct. 26, 1948). (9) Ipatieff, V . N . , Pines, H . , and Schaad, R.E.,J. Am. Chem. Soc., 56, 2696 (1934). (10) McAlister, S. H . , Ross, W . E., Randlett, H . E., and Carlson, G . J., Trans. Am. Inst. Chem. Engrs., 42, 33 (1946). (11) Nenitzescu, C. D., and Dragan, Α.,Ber.,66, 1892 (1933). (12) Perry, S. F., Ind. Eng. Chem., 40, 1624 (1948).


(13) Perry, S. F . , Trans. Am. Inst. Chem. Engrs., 42, 639 (1946). (14) Petroleum Refiner, 28, No. 9, 183 (1949).

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Isomerization - Advances in Chemistry (ACS Publications)

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