I O I L D E V E L O P M E N T C O M P A N Y , ELIZABETH, N. J.
FIE major industrial application of the isomerization reaction has been in the petroleum industry for t,he conversion of normal (straight chain) paraffins t o branched chain structure. %-Butane is isomerized for subsequent alkylat,ion with olefins to produce highly branched paraffins of higher molecular weight, primarily for use in aviation gasoline, 1%-hcreas pentane and hexane isomerization is employed for direct antiknock quality improvement of motor fuels. Because of t,he large amount of work done in this field in t,he past, few years, this review has been restricted ent.irely to paraffin isomerization, but it is expected that subsequent reviews can be broadened in scope. E A R L Y HISTORY AMiough the use of anhydrous aluminum chloride as a catalyst in organic synthesis and as a cracking and refining agent in the petroleum industry is old, the discovery of its ability to catalyze paraffin isomerization dates back only to 1933, when it mas reported t,hat n-hexane and n-heptane were partially converted to their branched chain isomers on refluxing with aluminum chloride (20). Other investigators promptly confirmed this finding (43, demonstrated the importance of anhydrous hydrogen chloride as a promoter (I$?), and showed that, n-butane ( I S ) and n-pentane ( 1 1 ) can be .isomerized readily. Aluminum bromide n-as also found to catalyze paraffin isomerization ( 1 1 ) . Snbsequent work in various laboratories resulted in the development of commercial processes for the iPomerizsiion of butane, pentane, and hexane; these are described in this reviem-. Practical processes for the isomerization of heptane and higher para.ffins have not, been developed, however, because of the increasing susceptibility of the longer paraffin chains to cracking in the presence of the isomerization catalyst; and because of -the increased complexity and lower paraffin content, of higher boiling virgin naphtha fractions. EQUILIBRIUM, R E A C T I O N RATE, M E C H A N I S M Paraffin isomerization is a reversible, first order reaction, and equilibrium favors the more branched, compact molecules, a t lo\\ t o moderate temperatures. Equilibria calculated from thermodynamic data by Rossini et al. ( 2 7 ) agree fairly well with the experimentally determined equilibria which were available for their comparison, and with more recently published experimental data (8, 16, 2 3 ) . A major exception, however, is t h a t neopentane, which is a favored isomcr thermodynamically, has not been produced by isomerization experimentally in appreciable quantity. I n hexane isomerization the corresponding isomer, neohexane, is formed a t a relatively slow rate (10) but appears in close to the calculated proportion a t equilibrium. Isomerization of paraffins does not occur t o any appreciable extent in t h e absence of catalyst even at cracking temperatures (7). The rate of isomerization is greatly dependent on the concentration and physical condition of the catalyst. Increasing the reaction temperature increases the specific reaction rates b u t shifts equilibrium toward the normal paraffin and increases the proportion of side reactions and the rate of catalyst degradation. Increasing the hydrogen chloride concentration also increases reaction rate; initial increments have the greatest effect. -4t least when using aluminum bromide, the reaction does not take place homogeneously v ith dissolved catalyst, but requires solid surface or a liquid-liquid interface (21).
The rneclianism of paraffin isomerization has been the subject of considerable study. It has been shown that in the presence of aluminum chloride under mild conditions, with rigidly purified butane or pentane feeds, the addition of a trace amount of hydrogen chloride, water, oxygen, or olefin is necessary to initiate isomerization ( 2 4 ) . It is postulated that the function of these various promoters is to provide a source of carbonium ions, through which isomerizat'ion then proceeds as a chain reaction. Several other investigators ( 2 , 14, $1) have also proposed this type of mechanism, which \vas first advanced by Whitmore (30) for various molecular rearrangements. Others (9, 12, a6) believe that, isomerization takes place by an intramolecular shift of an alkyl group (probably in the form of a carbonium ion) and a hydrogen atom, while thc niolecule is intimately associatcd with and rendered unstable by the catalyst. If the carbonium ion escapes from the molecule, a n accelerating chain reaction lcading to cracked products results (9). The effect of certain additives which may be used in the isomerization of pentane and hexane to inhibit side reactions also throivs some light on the reaction mechanism (Q,l4., 18). Molecular hydrogen and aromatic and naphthenic hydrocarbons have been shown to have this inhibiting effect and, in higher concentrations, to retard isonierizat'ion also. These inhibitors would tend to react, with and limit, the concentration of carbonium ions. Other interesting studies of the mechanism of isomerization have made usc of radioactive hydrogen (tritium) (26), heavy hydrogen (deuterium) (26) and also CIS ( 1 ) as tracers. It was found that butane isomerization v a s accompanied by a rapid exchange of hydrogen between hydrogen chloride or hydrogen bromide and bwtane, indicating that the hydrogen halide is involved directly in the rcact'ion mechanism. It has been proposed t h a t the actual catalyst is a combination of aluminuni halide and the hydrogen halide, WAlC14 (9, 26) or HAlBr4 118, 16). However, no evidence for the existence of the latter compound was found in vapor pressure studies (5). Thc isomerizat,ion of propane containing C I r in the end position, to propane containing Cia in the middle position, was found to proceed toward equilibrium a t a rate comparable to ordinary butane isomerization, in the presence of aluminum bromide ( 1 ) . There mas no detectable formation of propane containing two C,, atoms; this indicated t h a t the isoinerization reaction is intramolecular. The high selectivity of the butane, pentane, and hexane isomerization reactions under suitable conditions also points strongly in this direction and has discredited the early theory that isomerization occurs by a process of rupture and recombination of fragments (7). C O M M E R C I A L PROCESSES B U T A N E ISOMERIZATION
Because of the great wartime demand for isobutane (for the production of aviation gasoline components by alkylation with olefins) the butane isomerization reaction was commercialized rapidly and on a very large scale. Commerical production of isobutane by isomerization started in 1940 and rose to nearly 50,000 barrels (42 gallon capacity) per day in mid-1945. Total production included the output of some 40 plants in the United States, Canada, the Caribbean area, and the Middle East. This remarkable expansion was aided by the pooling of process and operating information through a government-sponsored industry
INDUSTRIAL AND ENG'INEERING CHEMISTRY
September 1948
committee (Isomerization Sub-committee, Aviation Gasoline Advisory Committee, Petroleum Administration for War) t o which the writer is indebted for background information on commercial operations. Since the end of the war only a few of these plants have continued in operation. The process flow in all commercial butane isomerization plants is essentially the same and consists of: feed preparation (fractionation and purification; the reaction system; hydrogen chloride recovery and recycle; and product fractionation. These plants may be grouped into four major types by the differences in the reaction system which are given in Table I. The use of aluminum chloride in the various forms shown in the table is the result of poor operability experienced in early work with granular aluminum chloride as catalyst, and provides an excellent illustration of the adage that there's more than one way to skin a cat. The use of bauxite as a catalyst support in the Shell vapor phase (5) and Anglo-Jersey (22) processes provides a large surface area for the aluminum chloride; this results in a more effective catalyst than granular aluminum chloride, minimizes loss of catalyst from the reaction zone, and prevents the degradation of the aluminum chloride to sludge which would otheiwise occur under the operating conditions. I n the Shell process, previously prepared catalyst is used until spent, and both temperature and hydrogen chloride concentration are raised to maintain conversion as catalyst activity declines. Guard chambers containing bauxite are placed in series with the reactors t o minimize the loss of aluminum chloride from the reaction system. I n the AngloJersey process, bauxite is charged to the reactors initially, and aluminum chloride is added by sublimation into the vaporized butane feed stream to establish and maintain catalyst activity. I n the Universal Oil Products Company process (6), aluminum chloride is introduced by solution in part of the butane feed and is gradually converted, under reaction conditions, to hydrocarbon complex or sludge, which deposits on the reactor packing and serves as the catalyst vehicle. I n the processes developed and used by the Standard Oil the liquid Company (Indiana) (8) and the Texas Company (18), feed containing dissolved hydrogen chloride is distributed at the base of the reactor and rises freely, as droplets, through a column of liquid aluminum chloride-hydrocarbon complex. The preferential solubility of aluminum chloride in the complex prevents its excessive loss by solution in the product. The catalyst in the Shell liquid phase process (I?') is a molten mixture of aluminum chloride in antimony trichloride; the latter acts.as an inert solvent. Because of the high activity of the isomerization catalyst in this form, a relatively high feed rate (small reaction volume) is employed and a high conversion level maintained. The antimony and aluminum chlorides which are soluble in the product t o the extent of about 10% and 1 to 2a/,, respectively, at reaction temperature, are separated from the product in a fractionator immediately following the reactor, and recycled, passing through a n aluminum chloride saturator to
Table Catalyst Means of contact Method of AlCla make-up Method of AlCh retention Typical reactor conditions Phase Temperature, F. Pressure Ib./sq. in gage 3 ," wt.' % based bn feed Feed rate, vol./vol./hr. b References Anglo-Jersey process only.
I.
b
the extent required for catalyst make-up. A small percentage of aluminum chloride-hydrocarbon sludge which is soluble in the catalyst but insoluble in hydrocarbon, is formed in the process and is rejected continuously by countercurrent scrubbing of a small stream of catalyst with the incoming feed. All of the above reaction systems are normally operated within the range of 35 to 55% conversion of normal to isobutane, well below equilibrium (about 75% isobutane at 212" F.), as the net rate of isomerization decreases as equilibrium is approached but the rate of side reactions does not. Isomerization selectivities of 95 t o 99% are reported for commercial operation. The life of the aluminum chloride catalyst appears to be governed chiefly by the amount of reactive impurities in the feed but has generally been in the range of 50 to 100 gallons of isobutane produced per pound of aluminum chloride for the sludge or complex type plants and 150 to 200 gallons per pound for the others. Feedstock specifications (6),applicable most strictly t o plants using bauxite supported catalyst, but desirable for the other type plants also, are as follows: Olcfins, mole yo,max. 0.01 Sulfur, wt. Yo,max. 0.005 Water, wt. %, max. 0.005 CS+, mole %, max. 0.5 Feedstocks meeting these specifications can be obtained, by fractionation and drying, from both natural butanes and the effluent from commercial isobutane-olefin alkylation plants. The drying is accomplished by distillation, the use of regenerative driers containing alumina, bauxite, or silica gel, or washing with concentrated sulfuric acid or caustic. Anhydrous hydrogen chloride promoter, which has about the same volatility as ethane, is recovered in all plants by stripping the reactor effluent in a conventional fractionating tower and is recirculated to the reactor. Trace amounts of noncondensable gases formed in the process are vented at a suitable point in the cycle, generally through a scrubber countercurrent to hydrocarbon feed or product to minimize loss of hydrogen chloride. The stripped product is caustic scrubbed to remove the last traces of hydrogen chloride and dissolved aluminum chloride before further fractionation. Feed and product fractionation equipment serves both a n isomerization and a n alkylation plant; it is used in separating isobutane feed for the alkylation plant, n-butane feed for isomerization, and the final alkylate product. Major difficulties encountered in the early operation of commercial butane isomerization plants were associated with: contamination of the feedstock; catalyst migration from the reaction zone; and corrosion. The first two items were largely overcome as previously discussed. Corrosion of two types W R S encountered: one was caused by extraneous water in the hydrogen chloride recovery system and the other by thc aluminum chloridehydrocarbon complex in the reaction system. Any moisture in the hydrogen chloride system tends to concentrate in the upper part of the hydrogen chloride stripper and causes localized cor-
Reaction Systems for Commercial Butane Isomerization Plants
.
Designation
1625
Anglo-Jersey Shell vapo; phase AlCls adsorbed on bauxite Fixed bed
Universal Oil Products
Indiana; Texas
Shell liquid phase
AlCla sludge. deposited on quartz chips Fixed bed
AlCla in molten SbCli
Sublimation in butane feeda Adsorption
Solution in butane feed
Liquid AlClr-hydrocarbon complex Flooded column (no packing) Solution in butane feed or injection as slurry Extraction
Vapor 210 to 300 160 t o 250 2 to 14 0 . 5 t o 1.0
Vapor and liquid 175 to 200 200 to 260 3 to 5
Liquid 180 to 200 350
Liquid 176 t o 194 300 5 2.5
(8,d 8 )
(17)
Fractionation
(6,3t) (6) Volumes liquid feed/reactor volume/hour.
,
3 to 5
Mechanical stirring Solution in circulated SbCla Fractionation
1626
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 40, No. 9
PENTANE ISOMERIZATION
Butane Isomerization Plant, Aruba Refinery, La30
Oil and Transport Company, Ltd.
rosion and fouling. Major sources of water were found to be: moisture initially in the system; rust in the system which reacted with hydrogen chloride to form water; leakage of steam or water through heat exchange equipment; and, in plants employing bauxite, the water and iron oxide contents of the bauxite charged. After operat,ing techniques were devised for the elimination of water from these sources, ordinary steel equipment was used throughout the hydrogen chloride system without further difficulty. I n plants employing the complex or sludge as catalyst', severe corrosion of steel was encountered where the catalyst was in motion a t reaction temperature. The only satisfactory alloy found for this service was Hastelloy B, which \vas used extensively for the lining of vessels and pipes and for valves and pump rods in contact with the catalyst. Cement lined reaction vessels also were employed with considerable success. The molten aluminum chloride-antimony trichloride mixture also was somewhat corrosive to steel, but the use of nickel-lined reactors was satisfactory for this service. N o published data have been found on the cost of bulane isomerization, but it appears to be low enough so that cost does not deter the use of the process to the extent -necessary to augment local isobutane supplies for alkylatioii.
The operation of two cominercial pentane isomerization plants has been reported, one using the Indiana (8, 29) and one the Shell liquid phase ( I ? ) isomerization proccss. The reaction systems are essentially the same as for the corresponding butane isomerization plants, as shonn in Table I, but, somewhat milder reaction conditions are used and the presence of an inhibitor is necessary to retard side reactions (the formation of lower and higher boiling paraffins and of aluminum chloride-hydrocarbon complex) which would otherwise predominate. I n the Indiana process, benzene in very small proportions is added as a n inhibitor; in the Shell process, the presence of 1.3 mole 7,of hydrogen in the feed serves the same purpose. Products containing 60 to 65yGisopentane (as compared with about 85YG isopentane in the equilibrium mixture at 212" F.) and 0.5 to 370 hydrocarbons boiling above and below pentane are typical. The Anglo-Jersey butane isomcrization process also has been applied successfully t o pentane isomerization on a pilot plant scale (32). Liquid phase reaction conditions are employed rather than vapor phase as in the butane process, at a lower temperature (200" F.), and aluminum chloride is added to the bauxite support by solution in the pentane feed rather than by sublimation; the same equipment is suitable for both processes. The addition of 0.2 volume 70 of benzene to the feed inhibits undesired reactions (the effect of benzene content on selectivity and catalyst life is very critical), and the presence of 1 mole 7, of hydrogen incrcases catalyst activity and life considerably. The hydrogen is dissolved in the feed and recycled along Tvith the hydrogen chloride.
HEXANE ISOMERIZATION
The isomerization of light virgin naphtha containing both pentanes and hexanes was brought t o a commercial scale by the Standard Oil Company (Indiana) with the construction of two plants utilizing the Isomate process (8, 99). Limited pilot plant data on the isomerization of hexane-containing fractions has also been reported for the Shell liquid phase (17) and Universal 011 Products ( 8 ) processes. I n the Isomate process a n aluminum chloride-hydrocarbon complex catalyst similar t o t h a t used for the Indiana type butane isomerization is employed. Hydrogen is added to repress cracking and maintain catalyst activity. Preferred operating conditions are 240" t o 250" F. and 700 to 800 pounds per square inch with 4 to 8 n.eight % hydrogen chloride and 50 to 180 cubic feet of hydrogen per barrel of feed. I n one-pass operation a product of about 80 octane number (A.S. T.SI. motor method, unleaded) is obtained; and by fractionation and recycling of n-pentane and a low octane Ca fraction-consisting of n-hexane and the methylpentanes-a product of about 91 unleaded octane number, predominantly isopentane and neohexane, is obtained in high yicld. Any methylcyclopcntane contained in the feed is largely isomerized to cyclohexane in the process. It is desirable to restrict the concentration of naph-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
September 1948
thenes and aromatics (benzene) in the feed because, while these inhibit cracking, they also tend to retard paraffin isomerization. The presence of heptanes is undesirable as they crack readily and accelerate catalyst consumption. Aluminum chloride addition a t a rate of 0.5 t o 1.0 pound per barrel of feed is required t o maintain catalyst activity. The cost of light naphtha isomerization by this process is in the range of 0.7 to 1.0 cent per gallon reactor charge; this is stated to be competitive with the use of tetraethyllead for raising octane number, at current motor gasoline quality levels, only when very low octane feedstocks, consisting largely of normal paraffins, are available (89). However, in view of the upward trend in gasoline octane requirements, as well as the increasing cost of crude petroleum (which favors high yield processes such as isomerization), the economics of naphtha isomerization undoubtedly will become more favorable.
LITERATURE CITED (1) Beeck, O., Otvos, J. W., Stevenson, D. P., and Wagner, C. D., presented before the Am. Phys. Soc. a t Los Angeles, Jan. 2-3, 1948: abstract. Bull. Am. Phvs. SOC..23. No. 1. 11 (1948). (2) Bloch, i%. S., Pines, H., and Schkerling; L.; J. A h . Chem. Soc., 68,153(1946). (3) Boedeker, E. R., Herold, R. J., and Oblad, A. G., presented before the AMERICAN CHEMICAL SOCIETY a t Dallas, Tex. (Dee. 1213, 1946); abstract, Petroleum Refiner, 26,No. 1, 149 (1947). (4) Calingaert, G., and Flood, D. T., J. Am. Chem. Soc., 57, 956 (1935). (5) Cheney, H. A., and Raymond, C. L., Trans. Am. i n s t . Chem. Engrs., 42,595 (1946). (6) Chenicek, J. A., Iverson, J. O., Sutherland, R. E., and Weinert, P. C., Chem. Eng. Progress, 43,210 (1947). (7) Egloff, G., Hulla, G., and Komarewski, V. I., “Isomerization of Pure Hvdrooarbons.” . -DD. - 24-8. New York. Reinhold Pub. Corp., 1942.
1627
(8) Evering, B. L., Fragen, N., and Weems, G. S., Chem. Eng. News,
22,1898 (1944). (9) Evering, B. L., d’ouville, E . L., Lien, A. P., and Waugh, R. C., Preprint, Div. Petroleum Chem., 111th Meeting AM. CHEM. SOC.,Atlantic City, N. J., pp. 285-306. (10) Evering, B. L., and Waugh, R. C., Preprint, Div. Petroleum Chem., 113th iweeting AM. CHEM.SOC., Chicago, Ill., pp. 75-82. (11) Glasebrook, A. L., Phillips, N. E., and Lovell, W. G., J. Am. Chcm. Soc., 58, 1944 (1936). (12) Heldman, J. D., Ibid., 66, 1786-91 (1944). (13) Ipatieff, V. N., and Grosse, A. V., IND.ENG. CHEM.,28, 461 (1936). (14) Ipatieff, V. N., and Bchmerling, Louis, Ibid., in press. (15) Koch, H., and Richter, H., Ber., 77, 127 (1944). (16) Leighton, P. A., and Heldman, J. D., J. Am. Chem. Soc., 65, 2276-80 (1943). (17) MoAllister, 5.H., Ross, W. E., Randlett, H. E., and Carlson, G. J., Trans. Am. Inst. Chem. Engrs., 42,33 (1946). (18) Mavity, J. M., Pines, H., Wackher, R. C. and Brooks, J. A,, IND.ENG.CHEM.,in press. (19) MoldavskiY, B. L., Kobuilskaya, M. V., and Livschitz, S. E., J . Gen. Chem. (U.S.S.R.), 5, 1791 (1935). (20) Nenitzescu, C. D., and Dragan, A.,Ber., 66,1892 (1933). (21) Oblad, A. G., and Gorin, M. H., IND.ENG. CHEM.,38, 822 (1946). (22) Perry, 5.F., Trans. Am. Inst. Chem. Engrs., 42, 639 (1946). (23) Pines, H., Kvetinskas, B., Kassel, L. S., and Ipatieff, V. N., J. Am. Chem. SOL, 67, 631 (1945). (24) Pines, H., and Wackher, R. C., Ibid., 68, 595-605 (1946). (25) Pines, H., and Wackher, R. C., Ibid., p. 2518. (26) Powell, T. M., and Reid, E. B., Ibid., 67, 1020 (1945). (27) Rossini, F. D., Prosen, E. J., and Pitzer, K. S.,J . Research Nutl. Bur. Standards, 27, 529 (1941). (28) Strawn, L. R., U. S. Patent 2,389,651 (Nov. 27, 1945). (29) Swearingen, J. E., Geckler, R. D., and Nysewander, C. W., Trans. Am. Inst. Chem. Engrs., 42,573 (1946). (30) Whitmore, F. C., J. Am. Chem. SOC.,54, 3274 (1932). RECEIVED June 14, 1948.
NITRATION WILLARD deC. CRATER HERCULES POWDER COMPANY, WILMINGTON, DEL.
T
HE scope of this review covers nitration in its broadest sense-that is, the treatment of organic compounds with nitric acid or its equivalent t o produce both nitrates and nitro compounds, as indicated by the following equations:
+ HO.NO2 * RO.NO2 + HOH RH + HO.NO2 R.NO2 + HOH ROH
-t
(1)
(2)
The nitration processes covered by this review deal with developments made and published since the start of World War 11. An over-all picture of the unit processes employed in nitration is presented, but no attempt is made to point out the merits of one process over the other, as this depends on individual requirements and the facilities available a t the point of manufacture.
M A N U F A C T U R E O F NITRIC A C I D ESTERS Nitration is the cornerstone of the explosive industry, which includes commercial blasting explosives, military explosives, and military and sporting smokeless powder, Nitration also plays an important role in the production of intermediates for the dye industry and of nitrocellulose for the protective coating and plastics fields. T h e agent most commonly employed for nitration is mixed acid, a mixture of nitric and sulfuric acids with or without water, although nitric acid in varying degrees of concentration may be used abne. Groggins ( 4 2 ) gives a rather complete list of the
agents used for nitration, depending on the procedure t o be followed and the compounds t o be nitrated. The mixture of nitric acid and acetic anhydride with or without acetic acid was used extensively during the war. Caesar ( 2 6 ) a n d Caesar and Goldfrank (17)describe a nitration process using nitrogen pentoxide (N205) with the addition of sodium fluoride for removal of by-product nitric acid, thus increasing the efficiency of the nitration. These authors also describe a method of preparing nitrogen pentoxide. Thomas, Anzilotti, and Hennion (91)state t h a t many organic compounds nitrate quickly and almost completely with stoichiometric amounts of nitric acid when boron fluoride is added. According to these authors, the amount of boron fluoride required indicates that the reaction proceeds as follows: R.H
+ HO.NO2 + BF,
-t
R.NOz
+ BFa.H.OH
A procedure for recovery of boron fluoride is also described. Recently there has been considerable interest in the use of acetic anhydride or a mixture of it and glacial acetic acid with nitric acid t o effect nitration of certain alcohols and amines. Secondary and tertiary alcohols are difficult t o nitrate with a nitric-sulfuric acid mixture. According t o Lufkin (65), such alcohols may be readily nitrated by introducing a stream of the alcohol simultaneously with a separate stream of nitric acid into acetic anhydride t o which 5 to 15% nitric acid has previously been added. Groggins (48)also mentions such mixtures. I n the explosives field sulfates reduce the stability of some
