UNIT P R O C E S S E S
Thermal and Catalytic Decomposition of Hydrocarbons H Y D R O C A R B O N pyrolysis at high temperatures is significant in reaction motors. \\,here only part of the fuel is burned and the remainder is used as reactant mass. T h e molecular weight of the cracked fragments governs the specific impulse obtained. New techniques, only beginning to be exploited, are the shock tube and plasma jet. They promise to open ne\v fields of hydrocarbon chemistry of both theoretical and practical significance. The pyrolysis of light hydrocarbons in the presence of steam to produce ethylene is currently the most significant thermal cracking process. The process has received the most attention in the literature. mainly as empirical correlations to aid in design and operation of heaters. The effect of gasoline composition on engine performance and smog forming tendencies will influence future processing methods. Olefins produced predominantly by catalytic cracking are being subjected to criticism on both counts; aromatics. to some extent from the performance point of view. when present in high concentrations. Although engine design and additives also enter the performance picture, the trend
A. J.
deROSSET received his B.S. from Lafayette College in 1936 and his Ph.D. from University of Wisconsin in 1939. He joined Universal Oil Products Co. in 1939 and was engaged in research and development work on cracking, isomerization, and alkylation catalysts. From 1942-46, he served in the U. S. Army as an air chemical officer. He returned to UOP in 1946 and i s now associate coordinator of hydrorefining research at the company's laboratory in Des Plaines, 111.
CHARLES V. BERGER i s a member of the UOP Engineering Research and Development Department, working on catalytic cracking and catalytic reforming. He received a B.Ch.E. degree from the University of Minnesota and joined Universal, working first in the Riverside Laboratories and later in the Patent and Service Departments.
\vi11 probably be toward a reduction in olefins and a ceiling on aromatics M hich will require retreating of catalvticallv cracked gasoline and increased use of isoparaffins from alkylation and isomerization. Interesting aspects of radiation chemistry of hydrocarbons deal with the effect of y-rays and neutrons on lubricants. The development of a radiation-resistant base stock. with inhibitors against radiation-induced decomposition. is another challenge presented bv the nuclear age.
Thermal Decomposition
Industrial. Ethane is pyrolyzed to ethylene at low pressure in the presence of a sream diluent. Although the mechanism does not seem to involve simple dehydrogenation of ethane, but rather a more complex series of free radical reactions, the approach to equilibrium in the ethane-ethylene-hydrogen system plays a major role in determining the side reactions which form methane and coke. T h e rate of coke formation determines the length of a commercial run and hence is significant. Schutt ( 2 5 4 ) has observed that the higher pressure units operating \vith coil outlet pressures of 27 to 28 p.s.i.g. are not so economical as low pressure units a t 8 to 10 p.s.i.g.. because of greater steam requirement for a given conversion. Commercial data are a better source of information on product distribution than laboratory or pilot plant data. Many of the laboratory data on reaction rates reported in the literature correlate reasonably well with these observed by Schutt on a commercial scale. Some of the earlier studies at 1050" to 1300' F.. lower than commercial, do not correlate and have led to design errors when extrapolated into the higher temperature region. Assuming a pseudo-firstorder mechanism based on ethane disappearance, a single Arrhenius-type equation represents the data nicely for the principal reaction. Side reactions. such as methane formation, do not correlate so well. Andrews and Pollock (7,4) have developed commercial rate equations expressing the decomposition of n-butane. propane. and ethane at 1200' to 1500' F. These equations have been com-
bined with equilibrium data, pressure drop equations, and heat transfer equations to permit analysis of commercial cracking furnaces using a digital computer. Literature laboratory data were used to establish the Arrhenius factors, but over-all conversions so calculated did not check commercial data until the frequency factors were changed by an unstated amount. Linden and Reid (78A) offer a general gas composition correlation which includes the effects of temperature, reaction time, and gas partial pressure on gas composition. A more tentative relationship between carbon-hydrogen ratio of the feed stock and ethylene yield permits the light gas structure to be estimated. A partial combustion process for the production of acetylene from natural gas or liquid hydrocarbons has been reported (23.4). High purity oxygen is used to simplify recovery. Maximum acetylene yields are obtained by utilizing maximum preheat and minimum feed combustion. Two specially devised burners account for the results (324). The liquid product from severe thermal cracking of hydrocarbons to produce ethylene is rich in aromatics which can be readily recovered ( 2 7 4 . Benzene content varies from 19 to 63%,! depending upon the feed stock and severity of cracking. Recovery is accomplished by rreatment with hydrogen over cobalt-molybdenum catalyst and extraction of aromatics by the Udex process. The rate of dissociation of molecular hydrogen is the limiting rate step in the thermal dealkylation of methylated polynuclear aromatics ( 4 A ) . The nuclear aromatic itself is essentially unreactive. T o maintain smooth circulation in a fluid coking unit, particle size of the coke must be held within certain bounds by selective withdrawal of coarse coke and by size reduction and rate of seed coke (7OA). Nonselective withdrawal results in accumulation of larger particles in the equilibrium coke and circulatory difficulties. However, withdrawal of too narrow a range of coarse particles may also cause circulation difficulties because of narrowing of the equilibrium coke size range. Experimentally, size reduction overcomes this problem. Research. Investigators have exploited vapor phase chromatography.
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UNIT PROCESSES isotope exchange, and radiotracer technique to study the mechanism of decomposition. Contact times have been shortened to identify primary products. Germain (77A) pyrolyzed methane at 0.1- to 16-second contact time, 1000' to 1100' C . ? both undiluted and in the presence of 50% nitrogen and hydrogen. Sitrogen behaved as an inert diluent, but hydrogen suppressed conversion. Activation energy was 87 kcal. in all three cases. suggesting that the rate-controlling step was the same. and that only the concentration of chain-initiating radicals was affected by dilution. Greene (72A) used shock tube technique to heat the C , and C:! hydrocarbons and benzene at 1600" to 2500" K . for 0.002 second. All hydrocarbons yielded acetylene. Acetylene gave diacetylene as a major product. These results suggested that coke is formed in vapor phase pyrolysis via acetylene and diacetylene and their corresponding radicals or diradicals. Heath ( 7 3 A ) found that hydrogen was not a n inhibitor of methane decomposition at these higher temperatures. and reasoned that chain propagation reactions were not involved. His activation energy of 93 kcal. agrees with that of Germain, and suggested that the rate-controlling step is a free radical split to methylene. Skinner (26A) found a higher activation energy, 101 kcal., in his shock tube experiments, and postulated a free radical split to methyl. Dahlgren (9.4) was unable to detect acetylene in the pyrolysis of ethylene at 582' C. His primary stable products were ethane, propene, butene, and butadiene. H e postulated the species C I H ~possibly , a diradical, as the reactive intermediate; addition to ethylene and subsequent cracking would account for propane. Hydrogen transfer to ethylene would give ethane and butadiene. Oxygen promoted and nitric oxide suppressed conversion. Hydrogen sulfide specifically inhibited propene formation. Butane (7A) decomposed at 560' to 600" C. to give propene and methane, or ethene and ethane. Reaction was homogeneous2 first order, with an activation energy of 40 kcal. per mole. Cyclobutane (77A) was cracked in the presence of an equal quantity of the octadeuterated species at 431 ' to 485' C. T h e heavier molecule required a higher activation No energy by 1.4 kcal. per mole. mixed ethylenes-e.g., CzH2D2-were produced: indicating the absence of free radicals in the reaction and confirming its molecular nature. Bryce and Kebarle (64) found the pyrolysis pattern of 1-butene and l-butene-4-d3 essentially the same a t 490" C . Introduction of methyl radicals (as mercury dimethyl) with either reactant increased decomposition rate by a factor of 10. The methyl radical added to the
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double bond of 1-butene-4-d3 and eliminated the terminal CD3. forming undeuterated butene. Isobutylene (79A) gave considerable isobutane as a primary product at 542" C . This hydrogen transfer resembles that noted by Dahlgren in pyrolysis of ethylene. However, the diolefin corresponding to Dahlgren's butadiene was not observed. n-Hexane (27.4) was thermally cracked at 290" to 412' C. in a tubular reactor internally illuminated with ultraviolet light. Mercury vapor in the reactor, activated by the radiation; provided an initiating source for radical chains. T h e hexyl radicals either dimerized or cracked to lower molecular weight olefins and alkyl radicals. No hexene was observed. indicating chat hexyl radical disproportionation did not occur. Hydrogen eliminated olefins from the final product at a hydrogen-hydrocarbon ratio greater than 25. However. olefins were among the primary products ( 2 2 4 ) . It was concluded that thermal cracking of the hexyl radical was the only important source of lower products. James and Steacie (74'4) demonstrated that an ethyl radical could abstract hydrogen from C7 hydrocarbons a t a rate increasing in the order: heptane. 1-heptene, and 1-heptyne. Rice and Vanderslice ( 2 4 A ) showed that methyl radicals could abstract tertiary hydrogen faster than secondary hydrogen, and secondary faster than primary. Pyrolysis of styrene ( 2 A ) at 710" C. yielded only small amounts of a suspected carcinogen, 3,4-benzopyrene. Vacuum decomposition of poly(a-methylstyrene) (5A) at 282.5' C. resulted in a high yield of monomer: at a rate increasing with the initial degree of polymerization. T h e clean-cut depolymerization reaction was attributed to blocking of the a-carbon with a methyl group, so that it cannot participate in chain-terminating or chain-transfer reactions. Newsworthy decomposition reactions include cleavage of benzene to acetylene by ultrasonics (8'4) ; degradation of methane. mineral oil. and polyethylene at 2000' to 5000' C. by intense millisecond light pulses ( Z O A ) ; and cracking of mineral oil at 6000" to 10.700" K . by small submerged electric arcs ( 7 5 4 , 76.4). It is claimed, in the last case, that the liquid hydrocarbon was either completely cracked or not at all. Products Lvere carbon black, hydrogen: and C1 to C 4 hydrocarbons. Reducing pressure reduced carbon formation and increased acetylene yield at the expense of other hydrocarbons.
Motor Fuel Production by Cracking and Reforming The tendency to regard catalytically cracked gasoline as a n inferior component
INDUSTRIAL AND ENGINEERING CHEMISTRY
of motor fuel arises from several sources. one of which points to the high sensitivity (spread between Research and Motor method octane numbers) o l catalytically cracked gasoline as undesirable; another to a purported relationship between olefins in catalytically cracked gasoline and smog-forming tendencies; and a third to other undesirablr tendencies of olefins and specific aromatics in very high compression engines. contributing to rumble and preignition. The criticism on performance applies also to a lesser extent to aromatic catalytic reformates, because aromatics are morc sensitive than paraftins. although less sensitive than olefins. ,4n excellent summary on fuel composition (73B) and engine performance showed that the factors entering into performance were complex. Not only were olefin and aromatic contents a factor. but also additives, lube oil type, and combustion chamber design. There is not complete agreement on the importance of these extraneous factors, but there is general agreement that engines having compression ratios greatly in excess o f the present levels are not likely to be satisfied merely by increase in Research octane number. Future planning among refiners is to limit the sensitivity and reduce dependence on catalytic cracking by hydrofining and catalytic reforming of the catalytically cracked gasoline. Limitation of aromatics is also necessary. In one study (76B) based on a typical midcontinent crude and utilizing modern isomerization, alkylation, and extraction processes in addition to cracking and reforming, ultimate production of a pool gasoline having a leaded Research number of 107, a Road rating of 106 to 107, and a Motor rating of 100 was discussed. I t would consist of about 45YGisoparafhs and 37y0 aromatics, with a small residue of olefins and naphthenes. The sequence of additional process steps necessary in a typical Gulf Coast refinery to keep pace with anticipated octane number increase during 1959-63 is described (79B). There are expected decreased reliance on catalytic cracking and reforming. increased importance of alkylation and isomerization. Yeither study takes advantage of the ne\ver processes developed to improve octane number by removal of normal paraffins: Molex ( F O P ) ( 4 B ) and Texaco Selective Finishing ( 6 B ) , ~vhich depend on the selective properties of Molecular Sieves to "denormalize" hydrocarbon screams. Denormalized reformates undergo a 3 to 4 leaded research increase, and generally permit a lolver aromatic content for the samc octane number in blending pools. The community, effort to reduce smog
N e w techniques probing hydrogen on catalyst surfaces include infrared, nuclear magnetic resonance, diborane adsorption
has resulted in research work dcsigned to determine the contribution of automobile exhaust fumes. I n one such effort i 72B) a n unleaded base stock was compared with the same stock leaded to commercial levels. T h e test engine is exhausted into glass chambers exposed to sunlight. in which eye irritation, rubber oxidation, and nitrogen oxides content could be measured. T h e method proved statistically significant when irradiation time was the variable. There was no significant difference bettveen leaded and unleaded fuel. Another study (923) on the effect or gasoline combustion on smog manifestation. particularly eye irritation, suggests that olefins are more objectionable Than aromatics or paraffins. At the same exhaust hydrocarbon level. the smogging characteristics of a blend of premium gasolines of 23.5% olefin content and catalytic cracking origin Lvere compared with a single nonolefinic catalytically reformed gasoline. As gasoline inspections are not included. it is difficult to eliminate differences in other variables between the fuels, particularly volatility. as a factor. A more correct comparison would be between fuels of essentially the same volatility at exhaust hydrocarbon levels that \vould result from comparable operation. Stephans and Schuck (2OB) concluded that predominantly olefinic gasolines (5257 olefins) are significantly more prone than nonolefinic fuels to produce eye irritation Lvhen the exhaust is irradiated. Again, volatility is a factor. T h e predominantly olefinic gasoline \vas far more volatile than the other gasolines tested in the front part of its distillation range, but had heavier boiling range components than the nonolefinic fuels, and much higher sulfur content than the other gasolines. T h e temaining gasolines varied from 0 to 167, olefins and from 3 to 34yc aromatics but ivere essentially similar in boiling range characteristics. There did not appear to be significant differences in eye irritation among the other four gasolines. For the remaining gasolines. the variation among autos appeared greater than among gasolines. T h e unusual distillation characteristics of the highly olefinic gasoline might have contributed to the differences in burning characteristics, perhaps to as great a n extent as composition. Variation in light scattering. or aerosol tendency. would correlate about as well \vith sulfur as \$-itholefin content. There is continual industrial emphasis on the handling of feed stocks high in contaminants. .4dditional data on im-
provement of catalytic cracking feed stocks by pretreatment in the presence of hydrogen have been reported (7B). As hydrogen consumption is increased! the sequence of reactions appears to be reduction of the polyaromatics to monoaromatics, which are converted to naphthenes if hydrogenation is carried far enough. For the heterocyclic compounds, the refractivity increases in the order sulfur, metallic compounds, and nitrogen compounds. T h e expected improvement in catalytic cracking yields is reported. Gasoline production by hydrocracking is trivial. Propane deasphalting is not so selective in removing metals from residual stocks as is desired for catalytic cracking feed preparation. An improved method ( 7 B ) depends on use of a wash oil as a selective solvent for the metallic compounds. A suitable \\.ash oil is a highly aromatic tar. insoluble in propane and low in metals content. obtained from the thermal cracking of catalytic cycle stock. T h e wash oil is continuously introduced, and leaves the e s tractor in the asphaltene residue. Methods for reducing Los Angeles basin crude bottoms to prepare the maximum acceptable catalytic cracking feed stock and minimum heavy fuel or coke (70B) include vacuum flashing. vishreaking of the feed plus vacuum flashing of the bottoms. vacuum flashing plus deasphalting of the bottoms. and vacuum flashing plus coking. -41though there appears to be no single best method at the same yield of catalytic cracking feed stock. vacuum flashing and deasphalting of the bottoms give less pilot plant coke in catalytically cracking the distillate plus extract than any other combination. .4cid treating of catalytic cracking feed stocks improves cracking susceptibility by removing both basic and nonbasic nitrogen compounds. Acid treating seems to be helpful even where onlba minor part of the nitrogen compounds are basic (78B). LVhen oxygen is injected into the catalytic cracking air supply, the throughput can be increased at constant conversion whenever the regenerating capacity has been limiting. An increase in conversion or ability to handle heavier stocks at the same feed rate and conversion is also possible. I n a commercial trial, addition of 100 tons of oxygen per day to a nominal 30.000barrels-per-day unit permitted a feed increase of approximately 15% (75B). Problems involved in building t\vo pilot plants for studies on the moving bed catalytic cracking process are discussed
by Ledley and Patterson ( 8 B ) . Solutions were obtained for temperature control of long slender reactors, differential pressure control of a few inches of water at high pressure levels, and differential thermal expansion. A means of reducing attrition of moving bed catalyst beads has been announced (73B) (no details given). T h e octane contributions to catalytic reformate vary from low values for the normal paraffins through intermediate values for isoparaffins and naphthenes to the highest values for the aromatics. T h e distribution of aromatics is concentrated toward the high boiling portion of catalytic reformates, so that fractionation is one means of obtaining a high octane component. Extraction is another commercial method of octane segregation ( 7 7B). Corrosion rates for low-chrome and austenitic chrome-nickel steels in the presence of HzS have been correlated and reported by Backensto and Sjoberg ( 2 3 ) . T h e correlation applies over a range of H?S levels and temperatures. in the presence of a large excess of hydrogen, and at pressures from 175 to 500 p.s.i.g., and is thus applicable to reformers or desulfurizing units. T h e regeneration of catalyst in a Sinclair-Baker reformer after 93 barrels per pound-life on a Coastal stock a t 97 leaded octane number is reported by Decker and Rylander ( 5 B ) . ?'he regeneration removed 7 to 1276 of coke from the reactors. T h e equilibrium constants for the hydrogenation of condensed ring aromatics as a function of temperature have been computed by Bondi (3B). T h e studies include the naphthalene-TetralinDecalin system. T h e difference in dielectric constant among aromatics. naphthenes, and parafEns is the basis of a n instrument for constantly monitoring reformer feed and product ( 7 7 B ) .
Catalytic Cracking Research Gunn (7C') used lo\v angle x-ray scattering to measure microscopic heterogeneity in cracking catalysts. H e obtained empirical correlations \vith BET surface area and pilot plant catalytic cracking activity. His parameter measuring the length of the small discontinuities in catalyst structure was of the same order of magnitude and varied in the same way as the pore radius, calculated from nitrogen absorption data. Ramser and Hill (73C) pointed out that the usual formula for the radius (twice
VOL. 51, NO. 9, P A R T II
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UNIT PROCESSES the volume-surface ratio) fails to give the true mean pore radius by an error which is directly proportional to the square of the standard deviation of the pore size distribution and inversely proportional to the mean value. T h e manner of chemical bonding of hydrogen in silica-alumina cracking catalysts was investigated by physical techniques. O'Reilly's nuclear magnetic resonance data (72C) showed that the chemical shift of the hydrogen in calcined silica-alumina and silica gel corresponded to that in dilute solutions of alcoholic-type protons in nonpolar solvents. His view that most of the hydrogen in both materials is firmly bound as rather isolated SiOH groups was supported by the infrared spectrum (3C) of silica gel containing chemisorbed water; the presence of bands due to hy-droxyl was confirmed by observing the isotopic shift when deuterium oxide was chemisorbed. Conductivity measurements on silica-alumina (2OC) also showed that the number of current carrying species was relatively small. T h e conductivity of cracking catalysts could be increased over 100-fold by absorption of ammonia, due to formation of N H 4 + ions. When chemical methods are used to investigate the nature of the cracking catalyst surface, it is postulated that the number of acid sites found will vary with the strength of the base absorbed. Thus, MacIver ( 7 IC) used the extremely weak base, isobutane, as the adsorbent. H e detected only the strongest acid sites corresponding to concentration of 1Olo sites per sq. cm. However. exchange with strong bases such as quinoline or potassium acetate gave surface concentrations of acid sites of the order of 1013 and 1Old, respectively. T h e ability of acid sites on silicaalumina to abstract a hydride ion was uniquely demonstrated by the appearance of the spectrum of triphenylmethyl carbonium ion when the parent hydrocarbon was adsorbed (70C). Webb (27C) extended the method by using 1.I-diphenylethylene as adsorbate. Absorption bands were observed under dry and hydrated conditions, associated with Lewis and Br@nsted sites, respectively. Weiss. Knight. and Shapiro (22C) introduce a novel technique to distinguish the hydroxyls attached to silicon atoms from those attached to aluminum. When diborane was adsorbed on silica gel, it hydrolyzed to generate 2 to 4 moles of hydrogen per mole of diborane reacted. T h e boron taken u p on the solid was completely exchangeable with isotopic boron-10. When diborane was adsorbed on alumina, only one mole of hydrogen was evolved per mole of diborane taken up. and the boron was only 5070 exchangeable with its isotope.
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As silica-alumina was heated to 450" C. the fraction of hydroxyls associated with aluminum increased. This trend reversed upon prolonged heating a t 500' C. I n catalytic cracking reactions, laboratory workers have concentrated largely on dealkylation or transalkylation of alkyl aromatics. Two methods of catalytic dealkylation by different mechanisms over different types of catalysts are under investigation : classic catalytic cracking employing acidic silicaalumina catalysts, and hydrocracking where acid catalysts are not required and in many cases are deliberately avoided. Kazansky and Georgiev (8C) dealkylated butylbenzenes over silica-alumina catalyst a t 370" to 490' C. T h e apparent activation energy for the cracking decreased in the order normal, secondary, the tertiary. in accordance with carbonium ion theory. Polyalkylbenzenes from coke oven distillate were cracked over silica-alumina at 480' C. in the presence of toluene diluent (78C). iMajor components in the feed stock were pseudocumene and mesitylene. At atmospheric pressure these cracked to benzene and xylene and the toluene did not participate in the reaction. At 15 atm. toluene was alkylated and contributed substantially to xylene yield. I n a study designed to produce alkyl styrenes (79C) ethylidene diisopropylbenzene was cracked over silica-alumina at 600' C. in the presence of steam at a 60 to 1 steam-hydrocarbon ratio. The anticipated products, isopropylstyrene and isopropylbenzene, were recovered in only small amounts. T h e authors claimed that steam deactivation of the catalyst did not reduce the rate of cracking because, at the temperarure used, diffusion of the reactants to the catalyst rather than extent of catalyst surface was rate controlling. When dealkylation is accomplished by hydrogenolysis. the molecular rearrangements characteristic of carbonium ion reactions need not take place. Shuikin ( 74C) processed butylbenzene with hydrogen at a 5 to 1 hydrogenhydrocarbon ratio over a 30% nickel on alumina catalyst at 340' to 465" C., 25 to 50 atm. T h e resulting Cg fraction was preponderantly ethylbenzene, and there \vas no substantial yield of the thermodynamically flavored xylenes. This indicated absence of effective ionic mechanisms to permit isomerization to the equilibrium mixture. He also demonstrated the absence of isomerization in the hydrocracking of n-heptane and n-hexane (75C). which yielded only lower normal paraffins when cracked in the presence of hydrogen over Raneytype nickel catalyst.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Doumani (5C) preferred to suppress ionic-type reactions in hydrodealkylation. H e demethylated toluene in the presence of steam and hydrogen a t 1050" to 1150' F., 1000 p.s.i.g. Impregnation of the cobalt-molybdenumalumina catalyst with alkali improved selectivity. H e attributed the beneficial effect of steam to deactivation of acid sites. Dehydrogenation investigations Lvere also directed to production of specific petrochemicals. T h e mechanism of formation of butadiene was investigated by radio tracer technique by Balandin ( 7C). H e dehydrogenated equimolar mixtures of butane and butene over chromium oxide catalyst at 635' C.? 9 to 1 steamhydrocarbon ratio. I n one mixture the butane was labeled with carbon-14; in the other the butene was labeled. H e concluded that butadiene was formed from butene adsorbed onto the catalyst from the gas phase, and not directly from butane nor from butene remaining on the catalyst as a result of butane conversion. T h e Russian literature (9C, 76C. 77C) on the dehydrogenation of pentanes evidences substantial interest in isoprene manufacture. Potassia-chromia-alumina catalysts are favored-for example: 88 A1203:9 C r ~ O s : 3 K20 as 6-mm. spheres. Temperatures of 500" to 575" C. are employed with process periods of 30 to 90 minutes between regenerations. Isopentane \vas less prone to give gaseous by-products and coke than npentane, although the pattern of isomers produced was more complicated. T h e selectivity for dehydrogenation of Ca olefins increased M.ith space velocity and decreased above 575' C. because of cracking reactions; 90Tc of the liquid product boiled from 20' tu 38' C. and analyzed 40% isopentenes, 47, isoprene. Isopentene was catalytically dehydrogenated a t 580" to 620' C. in the presence of steam to give 34% isoprene (2C). Goren ( 6 C ) reported extensive isomerization of the pentenes during this process. The nature of the catalyst was not stated. Corson (4C) dehydrogenated CS aromatic mixtures from which the o-xylenes had been removed, to recover styrene and unconverted m - and p xylenes. T h e aromatics were processvd with 10 parts of steam at 575' to 700' C . over 1707 catalyst ( M g 0 - F e 0 - K 2 0 CuO). Styrene yield per pass increased from 45 to 69% as the temperature was raised through the range studied. U1timate yield dropped with increasing temperature from 94 to 70%.
Reforming Research T h e dual function catalysts used in catalytic reforming offer the same prob-
DECOMPOSITION OF HYDROCARBONS lem as the silica-alumina cracking catalysts with respect to the nature of the acid sites. They pose questions as to the nature of the supported metal, its relationship to the base on which it is supported, and the bonds it forms with adsorbed reactants. These have been attacked by chemical and phy.sicochemical techniques. The view that acidic and hydrogen active sites of a dual function catalyst cooperate at a distance by vapor phase transfer of olefin intermediate was criticized by Starnes and Zabor (90). They argue that the synergism is more intimate. In lieu of the intermediate vapor phase olefin, Starnes and Zabor postulate partial dehydrogenation of the paraffin on the surface to give two adsorbed species. the half-hydrogenated olefin and hydrogen. T h e bonds are strongly polarized. so that the hydrogen behaves as a proton and the half-hydrogenated olefin as a carbonium ion. T h e latter can enter into reforming reactions while adsorbed. and revert to a paraffin by desorption and recovery of the hydrogen atom. Infrared spectra provided important information on the metal-gas interaction at catalytic surfaces. Pickering and Eckqtrom (60) used multiple reflecting mirrors. Pliskin and Eischens (70) used transmission through supported platinum-alumina and nickelsilica composites. Ethylene self-hydrogenated on rhodium mirrors to yield ethane in the gas phase and an adsorbed species which reduced the reflectance of the mirrors. Addition of hydrogen restored the reflectance and generated ethane equivalent to half that originally formed in the self-hydrogenation. O n the basis of stoichiometry the adsorbed species was postulated to be pairs of carbon atoms. Hydrogen adsorbed on fresh rhodium mirrors decreased the reflectivity irreversibly and gave a large number of low intensity adsorption bands; on platinumalumina composites. two adsorption bands. T h e latter effect \vas verified by noting rhe isotopic shift in both bands when deuterium was substituted for hydrogen. One band could be eliminated from the infrared spectrum by heating or evacuating the sample. This suggested I\VO types of chemisorption of hydrogen on platinum. one weak and one strong. corresponding to two types of bonds bet\veen the h>-drogenatom and the platinum surface. A similar investigation of nickel-silica composite revealed no adsorption bands. T h e catalytic chemist's generalized concept of at metal-hydrogen bond at the catalyst surface therefore does not account for the variations revealed by closer scrutiny of modern instrumental technique.
Hydrogen adsorption data on reforming catalysts have been used to estimate the platinum crystallite size. O n the assumption that hydrogen is adsorbed only on the exposed faces of the crystallite and on the bordering atoms of the support, and that one atom of hydrogen is adsorbed for each exposed platinum atom? Spenadel (80) arrived at 10 A. for the upper limit of crystallite size in fresh reforming catalysts. Heat treatment at 650" to 750' C. sintered the crystallites to 250 .4.measured by x-ray diffraction. The adsorbed hydrogenplatinum ratio dropped from near unity to less than 0.05. Keavney (70), using somewhat different assumptions and conditions for the adsorption, arrived a t similar conclusions. Steaming the cogelled catalyst for 5 hours at 705" C. increased the number of platinum atoms per crystallite from 20 to 1000; treatment of an impregnated catalyst increased the number from 50 to 100. Heat stability of platinum reforming catalysts at 730 to 985' C. was investigated by Waters (700). A catalyst prepared on y-alumina had a greater resistance to surface area loss and transformation than a catalyst prepared on 7-alumina, and it retained its activity better. T h e superior stability appeared to arise from the more random aggregation of primary particles in y-alumina. A variety of combinations of platinum, alumina, silica, silica-alumina, and halide were tested by Myers and hfunns (50) to evaluate their dehydrogenation activity and acid activity. These catalysts ivere used to hydrocrack n-pentane. hexane, and heptane. Two types of hydrocracking were distinguished. One involved the metal promoter and correlated with the dehydrogenation activity. T h e second involved an acid-catalyzed carbonium reaction. I n the case of n-pentane. the former predominated. With the higher hydrocarbons, acidictype cracking at the center of the carbon chain became more competitive, and predominated \vhen the base was silicaalumina. T h e contrast between hydrogenolysis over metallic and acidic sites was illustrated by Minachev ( 3 0 ) , who cracked normal nonane at 400' C., 20-atm. hydrogen pressure. Over silicaalumina catalyst conversion was 14y0 : with 1% platinum on silica-alumina conversion was loo%, with isomerization, hydrocracking. and dehydrocyclization reactions taking place. The role of hydrogen in supressing carbonium ion reactions was suggested by the cleavage of 1,1,2-trirnethylcyclopropaneover palladized carbon (20). A t 220' C., the cleavage yielded olefins corresponding to 2.3-dimethylbutane and 2-methyl-
pentane. Cleavage with hydrogen over platinized carbon at 100' C. gave neohexane, a structure not likely to be reached by a carbonium ion beta-scission mechanism. Mitchell ( 4 0 ) reported that dehydrocyclization of n - h e ~ t a n e - 1 - C 'produced ~ toluene with 27% of the labeled carbon in the methyl group. Catalyst was chromia-alumina-potassia-ceria and operating temperature was 490" C . Mechanisms in the literature would predict 50% of the carbon-14 to end u p in the methyl group. The lower value may be due to a mechanism involving a terpene-like bridged intermediate, or a rapid ring expansion-contraction step.
Radiation Gamma or electron bombardment of hydrocarbons decomposes them into fragments such as methane and hydrogen. The yield, or G value, of each fragment is generally expressed as molecules per 100 electron volts. The G value can be correlated with molecular structure and presence of inhibitors. Burton ( 4 E ) described the inhibiting effect of small amounts of benzene or iodine on the radiolysis of cyclohexane by cobalt-60 gamma radiation. Iodine at 0.01:M concentration reduced the hydrogen yield by almost 50yc. Benzene had a similar but smaller effect. It is postulated that the additive quenched excited molecules or radicals produced by irradiation. When deuterated benzene was used as an inhibitor, no deuterium was found in the product gas; it was inferred that inhibition by benzene did not involve hydrogen transfer. Product distribution from the irradiation of neopentane with cobalt-60 gamma radiation varied with temperature over the range -60' to +20° C. ( 6 E ) . At room temperature the methane yield was triple that at low temperature. T h e increase began at -20' C . near the melting point of neopentane. For cyclohexane the yield of both methane and hydrogen was constant over the temperature range which included the melting point of cyclohexane at 6.5" C. Dewhurst ( 5 E ) reported on the chemistry of branched alkanes irradiated with 800-kvp. electrons. I n the hexane series, hydrogen y-ield decreased with chain branching. Methane yield increased with methyl substitution, but a simple correlation with the number of methyl groups present was not observed. The gem structure: 2,2-dimethylbutane, gave the highest methane yield. Neutron bombardment ( 2 E ) of a series of higher paraffins, including n-decane? hexatriacontane, Vaseline, and polyethylene, dehydrogenated these materials at a yield substantially independent of molecular weight. Polyethylene
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UNIT PROCESSES gave vinylene unsaturation almost exclusively, lvhile a substantial amount of vinyl and vinylidene unsaturation was observed for n-decane and Vaseline. Bolt and Carroll (3E) measured the resistance of a number of hydrocarbons in the lube oil range to change viscosity when subjected to neutron or gamma radiation. Paraffin hydrocarbons showed the poorest stability and polycyclic aromatics the best. Iodine and its compounds, especially iodobenzene. were potent inhibitors. T h e methods of radiation processing with advantages and disadvantages of each, are discussed by Barr ( 7 E ) . If direct gamma or beta radiation accomplishes the reaction. this is the simplest means. Convenient sources of gamma radiation are cobalt-60 and cerium-137. Beta emission is best obtained from a n electron accelerator If neutron radiation is desired, the reactants may be employed as a coolant in the pile. This introduces certain complications. such as fouling the radiation surfaces, but more seriously, residual radiation induced by neutron capture in trace metals. Storage appears to be the only solution for the latter problem. An interesting application of radioactivity in refining is the use of cobalt-60 and receiver as a level-indicating device to determine the height of coke a n d foam in coke chambers ( 7 E ) . Literature Cited Historical a n d Review
Appell, H. R., Berger, C. V., IND.EKG. CHEM.50, 1330 (1958). Ibid., 50, Gohr, E. J., Fleming, C. L., Jr., Ibrd., 52A (1958). ( 1-~9 5--,. 81 52.4 HiGemann, H., Shalit, H., lVorld Petrol. Hcinemann, 29, N o . 5, 126 (1958). Ibid.,No. 8, p. p. 122. Ibid. No. 12. D. 90. I6 Petrol. Refine;jS, No. 1, 146 (1959). \
Thermal Decomposition
(1A) Andrews, A. J., Pollock, L. \V., IKD. ENG.CHEM.51, 125 (1959). (2.4) Badger, G. M . , Buttery, R . G.. J . Chem. -Sacoc., 1958, 2458.‘. (34) Barry, M . J., others, 134th Meeting ACS, Chicago, Ill., September 1958. (4.4) Betts, W. D Popper, F., J . Appi. Chem. (London) 8,’?09 (1958). (5.4) Brown, D. W., J . Phjs. Chem. 62, 848 (1958). (6.4) Bryce, W. A,, Kebarle, P., T7ans. Faraday Sac. 54, 1660 (1958). (7A) Cunningham, J. .4.,others, 134th Meeting ACS, Chicago, Ill., September 1958. (8.4) Currell, L., Zechmeister, L., J . Am. Chem. Sac. 80, 205 (1958). (9.4) Dahlgren, G., Douglas, J. E . , Ibid., 80, 5108 (1958). (10.4) Dunlop, D. D., others, Chrm. Eng. Progr. 54, No. 8, 39 (1958). (11.4) Germain, J. E., Vaniscotte, C., Bull. SOC. chim. France 1958, 319. (12A) Greene, E. F., J . Phys. Chem. 62, 238 (1958); 135th Meeting ACS, Boston, Mass., April 1959.
1080
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(13.4) Heath, C. E., others, 135th Meeting ACS,Boston, Mass., April 1959. (14’4) James, D. G. L., Steacie, E. W.R., Proc. Roy. Soc. A 244, 289 (1958). (15.4) Kroepelin, H., Neumann, K., Optzk 14. 311 119571. (16.4) Kroepelin, H., others, Uechema Monograph. 29, 204 (1957). (17.4) Langrish, J., Pritchard, H. O., J . Phys. Ciieni. 62, 761 (1958). (18A) Linden. H . R., Reid. J. M .’. Chem. E n g . Progr. 5 5 , No. 3, 71 (1959). (19.4) Lyadova, Yu I., others, Proc. Acad. Sci. U.S.S.R., Sect. Phys. Chem. (Eng. trans.) 114, 433 (1957). i20A) Nelson, L. S., Lundberg, J. L., J . Phys. Cheni. 63, 433 (1959). (21A) Norrish, R . C. W., Purnell, J. H . , Proc. Roy. Sod. A 243, 435 (1958). (22’4) Ibid., p. 441. (23A) Patton, J . L., others, Petrol. Rejinpr 37, No. 11, 180 (1958). (24.4) Rice, F. O., Vanderslice, T. .A,, J . A m . Chem. Soc. 80, 291 (1958). (25.4) Schutt, H. C., Chem. E n g . Progr. 55, No. 1, 68 (1959). (26A) Skinner, G. B., Ruehrwein, R . .4., 135th Meeting .4CS. Boston, Mass., April 1959. (27’41 Swanson, W. hl., LVatkins, C. H., Chem. Eng. Progr. 5 4 , S o . 12, 56 (1958). Motor Fuel Production by Cracking a n d Reforming
(1B) Abbott, M. D., Xrchibald, R . C., Dorn, R . W,, Pe/rol. Rejiner 37, No. 5, 161 (1958). (2Bi Backensto, E. B., Sjoberg, J. W., Ibid., S o . 12, 119 (1958). i3Bi Bondi, X., Ibid.;38,No. 2 , 161 (1959). (,4B) Broughton, D. B., Carson, D. B., O i i G a s J . 57, S o . 15, 112 (1959). (5B) Decker, W.H . , Rylander, C., Ibzd., No. 6, p. 88 (1959). (6B) Franz, W. F., Christensen, E. R., May, J. E . , Hess, H. V., Ibid., 57, No. 15, 116 (1958). (7B) Lawson, J . E . , Peet, J. P., Peet, S . P., Pelrol. ReJTner38, No. 3, 117 (1959). (8B) Ledley, R . E., Jr., Patterson, W.B., Jr., Chem. E n g . Progr. 5 4 , N o . 9,71 (1958). (,9B) hfader, P. P., Gliksman, J., Eye, M., Chambers, L. A , , IKD. ENG. CHEM.50, 1173 (1958). (10B) Medlin, W.V., Ornea, J. C., Johnson, .A. J., Oil Gas J . 5 6 , No. 20, 153 (1958). (11B) Moore, F. E., Ibid.,5 6 , No. 36, 96 (1958).
(12B) Morriss, F. V., Bolze, E., Goodwin, J. T., Jr., IND.ENG. CHEM.50, 673 11958). (13B) Oil Gas J.56, No. 14, 105 (1958). (14Bi Ibid.,No. 24, p. 62. (15B) Ibid., No. 38, p. 59, (16B) Read, D., Sterba, M . J., .\darns, N. R., Preprint API Merting, Los Angeles, Calif., 1958. (17BI Samuels, S. C . , Ray, G. C., Reichle, .4.D., IND.ESG. CHEM.51, 73 (1959). (18B) Samuelson, G. J., M’oelflin, W., Petrol. Refiner 38, No. 3, 221 (1959). (19Bi Service, FV, J., Payne, R. E., Askey, W.E., Ibid.,37, hTo.4, 181 (1958). (20B) Stephans, E. R . , Schuck, E. .4., Chem. E n g . Progr. 54, NO. 11,71 (1958). Catalytic Cracking Research
( l C ) Balandin, .4.A , , others, Bull. Acad. Sci. U.S.S.R., Div. Chem. Sci. S.S.R. 1957, p. 907 (Eng. trans.). (2C) Balandin, A . X., others, Doklady Akad. ,vauk. S.S.S.R. 120, 297 (1958). (3C) Benesi, H . A , , Jones, -4.C., J . Phys. Chem. 63, 179 (1959).
INDUSTRIAL AND ENGINEERING CHEMISTRY
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(4Cj Corson, B. R., others, IND. C s c . CHEM.50, 621 (1958). (5C) Doumani, T., Ibid.,50, 1677 (1958). (6C) Gorin, Y. A , , others, Kkim. Prom. 1958, p. 1. ( 7 C ) Gunn, E. L., J . Phys. Cheni. 62, 928 (,1 -9 5~R -) ~ , . (8‘2) Kazansky, B. A , , Georgiev, Kh. D. Proc. Acad. Sei. U.S.S.R. 116. 833 (195-1 (Eng. trans.). c9C) Kazansky, B. A , , others, Ibzd., 117, 1041 11957). (l0C) Leftin,‘ H. P., Hall, R. LV., 134th Meeting ACS, Chicago, Ill., Septernbrr 1958. (11C) MacIver, D. S., others, J . Phjr. Chem. 62, 935 (1958). t12C) O’Reilly, D. E., others, J C h e m Phys. 29, 970 (1958). (13C) Ramser, J. H., Hill, P. B., I s o . ENG.CHEM.50, 117 (19581. j14C) Shuikin, N. I., Berdnikova, S.G . , Bull. Acad. Sci. U.S.S.R., Diu. Cheni. Sci. S.S.R. 1957, p. 493 (Eng. trans.). 115C) Shuikin, N. I., Cherkashin, ?rf. I.. Ibid., 1957, p. 907. (16C) Shuikin, N. I., others, Ibzd.. 1957, p. 903 (Eng. trans.). (17C) Shuikin, N. I., others, Ircest. d i ; a d . 1Vauk. S.S.S.R., Otdel. Khim. Nauh‘. 1958, pp. 570, 726. (18‘2) Topchiev, A . V., Mamedaliev, C . hf., Proc. Acad. Sci. U.S.S.R. Set&. Chrm. 117, 1105 (1957); 112, 173 1,19571 (Eng. trans.). il9CI Vaiser, V. L., others. I6id, 112, 95 (1957). ( 2 0 C ) Voltz, S. E.. Weller. S . J Phis. Chem. 62.574 (19581. iZlC’) Webb, A: N., 135th Meetirig .!CS. Boston, h.lass., -4pril 1959. (22C) Weiss, H . G., others, 135th hleeting ACS, Boston, Mass., April 1959. Reforming Research (1D) Keavney, J. J., Adler, S. F., 135th
Meeting ACS, Boston, Mass., April 1959. (2D) Lukina, N. Yu.; others, Proc. .4cad. Sci. U.S.S.R., Sect. Chem. 114, 573 (1957) (Eng. trans.). 13D) Minachev, K . hf., others, H i d / . Acad. Sci. U.S.S.R., Ilia. Cheni. Ski, S.S.R. 1957, p. 1241 (Eng. trans.). i4D) Mitchell, J. J., J . Am. Cheni. Soc. 80, 51 (1958). (5D) Myers, C. G., Munns, G. \V., I X D . ENG.CHEM.50,1727 (1958). (6D) Pickering, H. L., Eckstrom, H . C . , 134th h4eeting ACS, Chicago, Ill., September 1958. (7D) Pliskin, W. .A,, Eischens, R. P., 135th Meeting .4CS, Boston, Mass., April 1959. t8D) Spenadel, L., Boudart, M., 135th Meeting XCS, Boston, Mass., .ipril 1959. (9D) Starnes, TV. C., Zabor, R. C., 135th Meeting ACS, Boston, Mass., April 1959. (10D) Waters, R. F., others, 134th Merting .4CS, Chicago, Ill., September 1958. Radiation
(1Ei Barr,-F. Barr, F. T., T. others, Rejnzng Engr. 30, Nn 12, 1 - C-48 (1958). ’ NO. (2E) Black, R. M . , J . Appl. Chem. !London1 8,159 (1958). (3E) Bolt, R. U., Carroll, J . G., I K D . ENG.CHEM.50, 221 (1958). (4E) Burton, M., others, Radiation ResParch 8, 203 (1958). (5E) Dewhurst, H . A , , J . Phys. Chem. 62, 246 (1958). (6E) Hamashima, M., Ibzd., Ibid., 62,246 (19581. (7E) Wright, P. G., 0 Oil 21 Gas J . 56, N o . 32, 931 (1958) 9 (1958). ’^
