Destructive Hydrogenation of Petroleum Hydrocarbons W. J. SWEEKEY AND ALEXIS VOORHIES,JR.,Standard Oil Company of Louisiana, Baton Rouge, La.
T
The foregoing equations are HE catalytic hydrogenaThe catalytic hydrogenation of petroleum oils plotted in Figure 1. The temtion of p e t r o l e u m s as is discussed in the light of thermodynamic data peratures of 750" and 1000" F. practiced in this country f o r pure hydrocarbons. Reactions of simple were chosen as fairly well enhas already been fully described hydrogenation (i. e., without attendant pyrolysis) compassing the hydrogenating in the literature (1, 4-7). It and reactions of destructive hydrogenation (i. e., and c r a c k i n g t e m p e r a t u r e is the purpose of this paper to ranges. From Figure 1 it is evishorn briefly how these results with attendunt pyrolysis) are interpreted with the dent that the equilibriurn ratio on petroleum oils may be inaid of ihe free energy relations. Graphs are p c y cl o h e xa n e / p b e nz e n e interpreted in the light of thermopresented shouirig the effect of hydrogen pressure creases quite sharply as the hydynamic data. The recently puband of reartion temperature in both simple and drogen pressure is i n c r e a s e d . lished monograph by Parks and destructive hydrogenation. Destructive hydroThus a t 750" F. this ratio inHuffman (8) was especially usecreases from 0.0001 at 1 atmosful in this work. genation o j petroleum oils is compared with phere of hydrogen to approxiThe historical development of ordinary cracking, particularly in respect to mately 300 a t 150 atmospheres. the hydrogenation process as apgasoline yields. The hydrogen content of virgin, It follows that t h e o r e t i c a l l y plied to coal and petroleum oils cracked, and hydrogenated petroleum fractions is any degree of hydrogenation is too well known to require regraphically presented as a function of boiling point may be obtained a t any temview. However, a simple enuperature by appropriate choice meration of the principal apand A . P . I . gravity. Recent experimental results of the hydrogen pressure. plications of hydrogenation in in hydrogenation of petroleum oils show over 115 As another example of simple this country is desirahle. These per cent uolumetric yield of stable gasoline hydrogenation may be taken the are: (1) gasoline production, (2) from refractory, hydrogen-deficient gas oils. gaseous reaction: lubricating oil improvement, (3) burning oil improvement, (4) upCsHlz(1 atm.) Hz (1 atm.) = CBHl4 (1 atm.) grading of heavy oils and residues, and ( 5 ) specialty products Hexylene Hexane such as petroleum cuts of very high solvency and safety fuel - A t ' " = 30,400 - 33.9 T K (9) (approximate) of high octane number. According to the procedure indicated for the hydrogenaThis paper is largely restricted to a discussion of destructive hydrogenation which for the present purpose will be tion of benzene, the following general equation may be defined as hydrogenation accompanied by extensive pyrolysis. developed for the hydrogenation of hexylene a t equilibrium: Gasoline production by hydrogenation is probably the best illustration of destructive hydrogenation. With certain simplifying assumptions it is believed possible to interpret This equation is plotted in Figure 1 for temperatures of the results obtained in the production of gasoline by hydrogenation in the light of simple thermodynamic considerations. 672" K. (750" F.) and 812" K. (1000" F.), Primarily it is shown that for all pressures of hydrogen above 1 atmosphere SIMPLEHYDROGENATION EQUILIBRIA and for all temperatures below about 1200" F., hexylene is By simple hydrogenation is meant solely the addition of unstable toward hexane. This indicates the extreme ease hydrogen to the molecule without any derangement of its with which olefins are hydrogenated. It will be observed that the slope of the reaction isotherms basic structure. As examples may be cited the hydrogenation of benzene to cyclohexane and the hydrogenation of is independent of the temperature and dependent solely on hexylene to hexane. Destructive hydrogenation is a com- the number of hydrogen molecules entering the reaction. bination of simple hydrogenation and pyrolysis. the simple This of course is in conformity with the LeChatelier-Braun hydrogenation taking place before and/or after the rupture of principle. Although the exact mechanism of pyrolysis PYROLYSIS, the molecule. As an example of simple hydrogenation may is not too well known a t the present time as far as the combe taken the gaseous reaction: plex feed stocks in commercial cracking coil operation are conC6H6 (1 atm.) f 3H2 (1 atm.) = CsH12(1 atm.) cerned, the reactions taking place may be briefly summarized Benzene Cyclohexane - A F o = 51,500 - 94.7 T e (9) (approximate) as follows ( 3 ): I n general the stability to heat of the various hydrocarbons .4t other psrtial pressures ( p ) of the three reactants, increases in the order of paraffins, olefins, diolefins, naphPCs& thenes, and aromatics. From this it follows that the paraf- AF = 51,500 - 94.7 T K - RTK In -pC~Hfi.p~Hz finic chains break first, the point of scission being nearly always between carbon atoms with the resulting formation of whence a t equilibriuni, one saturated and one unsaturated molecule. These new molecules may in turn break down into still smaller units. Likewise, naphthene rings may split to form olefins, and, At temperatures of 672" K. (750" F.) and 812" K. in general, it appears that the higher the temperature the ((1000" F.) this equation becomes: greater is the tendency for hydrogen to split off. By such dehydrogenation, naphthene rings tend to become converted into aromatics, while olefinic bonds are formed in paraffin chains. log = --6.8 + 3 log pHz ( l O O O o F.) (b) It therefore follows that unsaturated compounds always
+
195
INDUSTRIAL AND ENGINEERING
196
result from the initial decomposition reactions. These unsaturated molecules, particularly the larger ones, polymerize readily, and in this manner there are produced new compounds whose molecular weight tends to exceed that of the original feed stock. Since the unsaturated molecules possess less hydrogen per carbon atom than the original hydrocarbons from which they are derived, it follows that such polymerization results in the formation of products of lower hydrogen to
HYDROGEN P R E S S U R E - A T M O S P H E R E S
FIGURE 1. EFFECT OF TE~MPERATURE AND HYDROGEN PRESSURE IN SIMPLE HYDROGENATION
carbon ratio. On prolonged exposure to high temperature, the polymerized material suffers recracking with the formation of new unsaturated bonds which again give rise to further polymerization. Progressive reactions of this type lead to the formation of the typical products of petroleum cracking: (1) gaseous and low-boiling liquid compounds of high hydrogen content; ( 2 ) liquid material of intermediate molecular weight and a hydrogen to carbon ratio differing more or less from that of the original feed stock, depending upon the mode of operation, and (3) liquid material of high molecular weight, tar and petroleum coke, possessing a lower ratio of hydrogen to carbon.
DESTRUCTIVE HYDROGENATION EQUILIBRIA An essential difference between pyrolysis and destructive hydrogenation of petroleum oils lies in the fact that in pyrolysis there is always formed along with the light products (gasoline and gas) a certain amount of polymerized heavier products (tar and coke), whereas, in destructive hydrogenation, polymerization may be partly or even entirely prevented so that only light products (gasoline and gas) are made. Inasmuch as the prevention of tar and coke formation results in additional gasoline yield, it is well to focus attention on the condensed type of molecule which is probably most responsible for the formation of tar and coke. The following is an example of the destructive hydrogenation of a condensed ring molecule: CloHa(g) (1 atm.)
Naphthalene
+ 4H2 (1 atrn.) = CBenzene B H(g) ~ (1 atm.) + C4%0 (R) (1 atm.)
Butane - AF" = 51,300 - 79.7 T K (approximate)
ilnthracene furnishes another example of destructive hydrogenation of condensed ring molecules: Cl4HlD(g) (1 atm.) 3H2 (1 atm.) = CeHs (g) (1 atrn.) Anthracene Benzene GHlo (9) (1 a h . ) m-Xylene
+
+
CHEMISTRY
- A F " = 23,800
Vol. 26, No. 2
- 41.2T (9) (approximate)
These two equations are plotted in Figure 2 for teniperatures of 672" and 812" K. (750" and 1000" F.). At high hydrogen pressures considerable driving force is available for the production of benzene and butane from naphthalene, and for the production of benzene and m-xylene from anthracene. At very low hydrogen pressures, however (which exist in the ordinary cracking coil), i t is theoretically impossible to form any appreciable amounts of benzene and butane from naphthalene, or of benzene and m-xylene from anthracene. Katurally more important than this are the corollary truths that in the presence of sufficient hydrogen pressure (1) naphthalene cannot be formed from benzene and butane and ( 2 ) anthracene cannot be formed from benzene and m-xylene (i. e., referring to the temperature range under consideration). The above two reactions of pure hydrocarbons are merely taken as illustrative of the untold number of reactions possible during pyrolysis of petroleum oils. They serve to establish the thermodynamic reason for the fact that, in the destructive hydrogenation of petroleum oils, polymerization and condensation may be entirely prevented or effectively controlled in a manner not possible in the ordinary cracking reaction.
DESTRUCTIVE HYDROPETROLEUM OILS Figure 3 shows a correlation between percentage hydrogen, A. P. I. gravity, and average boiling point' of petroleum oils. This plot is based on analyses of a wide variety of straight-run, cracked, and hydrogenated products. I t COMPARISON O F PYROLYSIS A S D
GENATION OF
HYDROGEN PRESSURE -ATMOSPHERES
FIGURE2. EFFECT OF TEMPERATURE AND HnDROGEN PRESSURE IN DESTRUCTIVE HYDROGENATION
may be noted that the simple linear relation between percentage hydrogen and density (per cent H = 26 - 15 d ) ( 2 ) which has previously been found useful for straight-run products, is not applicable to heavily cracked or very aromatic petroleum fractions. The gasoline yield in pyrolysis of petroleum oils is probably more nearly a direct function of the percentage of hydrogen in the charging stock than of its gravity. It is well known that cracked gas oils of a gil-en gravity produce more tar and less gasoline than virgin gas oils of a corresponding gravity. This is in conformity with the lower hydrogen content of the cracked gas oil. 1 By average boiling point is meant a weighted average boiling point which in many case8 is approximately equivalent t o the 60 per cent point on the A S. T. 51. distillation rurve
February, 1931
INDUSTRIAL AND ENGINEERING
Typical cracking and hydrogenation results with two gas oils of nearly the same A. P. I. gravity but of considerably different hydrogen content are shown in Trtble I. The naphtha bottoms is a typical cracked stock while the Midcontinent gas oil is representative of rirgin stocks. TABLEI. COMPARISON OF GASOLINE YIELDSBY CRACKING AND HYDROGENATION (Hydrogenation results given are typical of low-temperature operation) Feed stock: NkPHTH.4 BorTox8 M I D C O N T I N E GAS ~ . T OIL 33.4 Gravity, A. P. I. 30.8 I56 Aniline point, ',F 110 50% boiling point, O - F. ld7 540 13 8 Hvdroaen 12.0 - - contentHYDROGENATION CRACKING IIYDHOGESITION Results: CRACKING 109.5 57 108.5 Yield gasoline, % ' b 41,O i'ione 29.5 None Yield t a r , yo 49.5 Gasoline octane NO.^ 78 5 64 69.6 64 Estimated from d a t a on siniilar stocks b Based on 420° F end-point gasolines 0 Series 30 motor
--.
I t is evident from Table I that the gasoline yield advantage of destructive hydrogenation as opposed to pyrolysis is in-
creasingly great, the loxer the hydrogen content of the original charging stock. hIoreover, the hydrogen-deficient molecules are converted into a gasoline of deqirable cyclic structure. RESULTSO B T A I X E D IS ~ A h O L I K EP R O D 1 CTION (I)ESTRUCTIVE I ~ Y D R O G E i X l T I O h - ) FROM PETROLEUM OILS The statement is true, of course, that a catalyst does not affect the equilibrium for a given reaction. On the other hand, in a complicated series of reactions it is possible that, by speeding one of the intermediate reactions, the catalyst will markedly affect the composition of the end product. Specifically such a chain of reactions is dealt with in the destructive hydrogenation of petroleum oils. Inasmuch as there is an ever-present tendency to polymerize even in the presence of hydrogen, i t is important to have not only a favorable hydrogen pressure but also a n active hydrogenating catalyst to convert the "tar-breeding" molecules into the desired light products. The prime requisite of a catalyst for hydrogenating petroleum oils is that it shall be sulfur-resistant. The general history and character of these catalysts have been discussed in the previously cited references. Two types of hydrogenation are available for the production of gasoline from gas oils; one may be roughly characterized as high-temperature, the other as low-temperature operation. Results obtained in high-temperature hydrogenation for gasoline production have been fully out lined in the previously cited references. I n summation it may be said that the volumetric yield of stabilized gasoline of high octane number is usually around 85 per cent and that the more aromatic feed stocks give higher antiknock quality. Catalysts for low-temperature hydrogenation have been available for some tirne, but this mode of operation has gained considerably in attractiveness recently by the demonstration of results from a highly active catalyst by the I. G. Farbenindustrie. More recently an even more active catalyst has been developed by the I. G. Farbenindustrie v, hich is now being used in commercial gasoline production in Germany and in experimental operation a t Baton Rouge, La. This new catalyst permits low-temperature (e. g., 650" to 800' F.) operation a t high reaction rate. Thus, in gasoline production from gas oils by recycle Operation, 60 per cent fresh feed and 122 per cent volumetric yield of gasoline have been obtained from a cracked charging stock (21.5' A. P. I., 50' F. aniline point) a t a production rate of one and a half volumes of gasoline per volume of catalyst per hour. This high rate of
CHEMISTRY
197
gasoline production a t low temperature obviously means that the efficient hydrogenating catalyst is also an efficient splitting catalyst. Moreover, a t low temperature the hydrogen-deficient refractory molecules are hydrogenated to more saturated, and hence more "crackable," molecules. Obviously the hydrogenating catalyst retains its efficiency for splitting only by virtue of being continually maintained clean (i. e., free of coke and polymers) owing to the hydrogenation reaction. In accordance with Is t h e p r e v i o u s l y developed picture of the 150 effect of temperature and hydrogen partial 140 pressure in simple and g in destructive hydro130 genation, it is possible with a given charging 5 stock to make a gasoa l i n e e i t h e r of a n a r o m a t i c o r of a 11: naphthenic-paraffinic type, while polymeri3o zation is either pre30 -10 50 50 7C vented or effectively A P I GRAiITY controlled. It is obViOUS that the tendFIGURE3. CORRELATION O F P E H ency t o polymerize CENTAGE HYDROGEN, A. P. I. GRAVITY, AND AVERAGE BOILINGPOINT JTill be greater at the (AT 760 MM. OR 29.9 IN. PRESSURE) higher tenlperatureOF VIRGIN, CRACKED, AND HYDROlower hydrogen presG E N . ~ T E D PETROLEUM PRODUCTS sure condition in conformity with the higher aroniatic content of the gasoline produced. Some of the recent experimental results on low temperature operation are given in Table 11. TABLE11. RESULTSON LOW-TEMPERATURE OPERATIOX Feed stock:
MIXED VIRQIN AND
CRACKED GAS OIL 25.1 0.9036 448 536 684 0.864
Gravity, A. P. I. Sp. gr.,(60;/6O0 F.) 5% point, F. 50% point, F. R&b.g., F.
;
HEAVILY CRACKED GABOIL 21.5 0.9248 440 47 1 565 0.500
60 60 Fresh ieed. % Hydrogenatea product: 116 122 Yield of gasoline, % by vol. Gasoline production rate, vol. of 1.5 gasoline/vol. of catalyst/hr 1.3 66.1 58.9 Gravity, A . P. I. 0.7161 0.7432 Sp. gr. ( 6 0 " / 6 0 " F.) 8.5 7.0 Distn. loss at 122' F. 11.0 7.5 at 140' F. 39.6 29.5 at 212' F. 339 383 90% point, ," F. 374 416 Final b. p., F. 95.0 Recovery % 97.0 60.4 Octane do (c. F. ~ . a ) 58.7 72.8 72.4 Octane No. (C. F. R.a) I 0 c c . Pb 0 Copper dish gum, mg.b 0 240+ Breakdown, min./2-lb. drop 240+ Color 4-30 30 < 0.02 < 0.02 Sulfur a CoOperative Fuel Research Steeriog Committee motur meth ,d. b Aiter receiving a soda wash t o reniuve dissolved 11 3.
7
+
+
+
The gasoline yields listed above (over 115 per cent) are, of course, superior to anything obtainable on cracking such aromatic, low hydrogen content feed stocks. Both the percentage fresh feed (60) and the gasoline production rate (1.5 volumes of gasoline per volume of catalyst per hour) are extremely high when the refractory character of the feed stocks is considered. These are outstanding examples of the role played by hydrogen a t high pressure and in the presence of very active catalyst-viz., the rapid rate and completeness of conversion of tar-breeding molecules into gasoline. The exceptional cleanliness (f30 color), low sulfur, and good
INDUSTRIAL AND ENGINEERING CHEMISTRY
198
stability of these distillates is a reflection of the intensity of hydrogenation. I n spite of this intense hydrogenation the octane numbers indicate that the gasolines are not of the paraffinic type but rather of the paraffinic-naphthenic type in conformity with the structure of the crudes from which the charging stocks were obtained. A word might be said of the important function of hydrogenation in the removal of sulfur, oxygen, and nitrogen. These elements are removed as hydrogen sulfide, water, and ammonia, respectively. The removal of sulfur may be represented by diethyl sulfide: S(g) + 2H2 = &S + 2C2Hs (9) + 6800 + 0 = -7840 - 21,400 f ( - AF02es) or ( - AFo29s) = 36,040 (C2Hs)t
Thus a t 298" K. there is a large driving force for the conversion of alkyl sulfides to hydrogen sulfide. Data on the . free energy of organic sulfur compounds at high temperature
Vol. 26, No. 2
are not complete. However, a similar driving force undoubtedly exists, inasmuch as results on all types of petroleum hydrogenation always indicate effective removal of sulfur.
LITERATURE CITED Boyer, M . R., Chern. & Met. Eng., 37, 741 (1930). Cragoe, C. S., Bur. Standards, Miscellaneous Pub. 97, 15 (1929). Frolich, P. K., Chem. & Met. Eng., 38, 343 (1931). Gohr, E . J., and Russell, R. P., J. Inst. Petroleum Tech., 18. 602 (1932). Haslam, R. T., and Bauer, W. C., S.A. E . Journal, 28,307 (1931). Haslam. R. T., and Russell, R. P., IND.ENQ.CHEM.,22, 1030 (1930). Howard, F. A., Oil Gas J.,30, No. 46, 90 (1932). Parks and Huffman, "Free Energies of Some Organic Con]pounds," A. C. S. Monograph No. 60, Chemical Catalog, 1932. lhid.,p. 93. RECEIVED September, 1933
-
Pyrolysis of Saturated Hydrocarbons F. E. FREY,Phillips Petroleum Company, Bartlesville, Okla.
S
IKCE petroleum is comheat is less intense than by such The literature is reviewed with special attention means as electric discharge or posed chiefly of saturated to the primary decomposition reactions. Parafin radioactive disintegration. The hydrocarbons, the study hydrocarbons decompose chiefly into simpler of the behavior of saturated hyp r e s e n c e of but two atomic complementary olefins and parafins. High dedrocarbons under cracking conspecies, carbon and hydrogen, composition temperatures favor the concomitant and the new knowledge of the d i t i o n s forms an appropriate nature of the valence bonds and starting point for unraveling the formation of complementary olefins and hydrocharacteristic v i b r a t i o n f r e complex reactions taking place gen, and in some cases more than two hydroquencies of hydrocarbon molein the several types of cracking carbon product molecules are formed. Two recules impart an added fundaoperations in use. For m a n y action mechanisms in accord with these observamental significance to work in years cracking or pyrolysis of tions haDe been proposed. The primary decomthis field. petroleum has been practiced for the purpose of making illumiposition of cycloparafins has not been invesnating gas, benzene, kerosene, tigated extensively. DECOMPOSITION OF PARAFFINS and gasoline. It has come to be B y surface catalysis the parafins are dehydroB e c a u s e of the analytical appreciated that many pyrolytic genated to the corresponding olefins or degraded difficulties, only the lower parafr e a c t i o n s are involved. The to carbon, methane, and hydrogen. The cyclofins have as yet been studied primary reaction has been shown for the purpose of determining to be a splitting of the saturated hexanes are converted into the corresponding the exact decomposition reach y d r o c a r b o n molecules into aromatics: the other cycloparafins enter various t i o n s . Decomposition t a k e s smaller complementary paraffins rearrangements. place homogeneously in silica, and olefins, sometimes accomPyrex glass, and copper vesse1s.I p a n i e d b y hydrogen. These primary products may be converted in turn by prolonged Temperatures of 500" to 700" C. and convenient pressures heating into smaller molecules, the olefins first formed may in the neighborhood of atmospheric usually have been empolymerize to large molecules, and, when very drastic heat- ployed (8, 20, 28).2 If the heating is interrupted when a ing is employed, only the heat-stable aromatic hydrocarbons few per cent of decomposition has taken place, secondary desurvive. However, the initial decomposition into paraffins composition is minimized. The pyrolysis of methane has interested many investigaand olefins is common to all pyrolytic processes applied to saturated hydrocarbons. Recent investigators have at- tors but because of its dissimilarity to the other paraffins tempted to select experimental conditions which permit the will not be discussed here (8, 20, 24, 28, 43A).2 Ethane decomposes into equivalent amounts of ethylene and hydrogen. study of primary decomposition itself. From the scientific standpoint the investigation of hydro- Propane undergoes an analogous decomposition into procarbon decomposition has become a more attractive field pylene and hydrogen, but an equal amount decomposes by for research in recent years, since methods of analysis for fracture of the carbon-carbon bond to produce equivalent hydrocarbon mixtures have been greatly improved, par- amounts of the complementary ethylene and methane. ticularly by the development of precise laboratory methods Isobutane yields isobutylene and hydrogen, together with propylene and methane. The other paraffins studied defor fractional distillation. I n the decomposition of saturated hydrocarbons by the compose to only a small extent by the dehydrogenation 1 Travers and Hockin ( 4 4 A ) report that the decomposition of ethane in a agency of heat, the complexity of the situation is somewhat tube may be accelerated by hydrogen atoms resulting from inward alleviated by the absence of polymerizing tendency and also silica diffusion of oxygen by the endothermic nature of the reactions, which minimizes * The literature in this field is voluminoua, specific reference is not made t o the possibility of thermal chains. Moreover, activation by all pertinent earlier work.
