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C. S. CARLSON, A. W. LANGER, JOSEPH STEWART, and R. M. HILL Esso Research and Engineering Co.,Linden, N. J.
Thermal Hydrogenation Transfer of Hydrogen from Tetralin to Cracked Residua Crude residua can be thermally cracked to more valuable products by using a reaction diluent such as Tetralin, which gives up hydrogen to cracked residua. The Tetralin is dehydrogenated, to naphthalene, and the residua are converted into lower boiling products with little coke and dry gas. Naphthalene can be catalytically hydrogenated to Tetralin and recycled. Paraffin and single-ring naphthenic compounds are ineffective hydrogen transfer agents; a condensed ring naphthenic compound such as Decalin is somewhat effective; a mixed naphthenic-aromatic condensed-ring compound such as Tetralin is much more effective under conditions investigated. Hydrogen transfer occurs in the absence of added catalyst
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R E s I D u u M is the major fraction of low value in the refining of crude oil. This product, used primarily as low grade fuel, may be less valuable to a refiner than the crude from which it is made. Residuum can be as much as 60y0of the crude run to a refinery; the national average is probably 12 to 15%. Refiners have given considerable attention to upgrading crude residua ( 5 ) . Vacuum distillation has been widely used for reducing residuum and increasing yield of heavy gas oil for catalytic or thermal cracking. Deasphalting or treatment with a light hydrocarbon solvent separates the residuum into a gas oil and an asphalt fraction; the gas oil can be processed to gasoline and middle distillates. A number of high temperature pyrolysis or coking processes have been developed; most recent is continuous fluid coking. A major portion of the residuum is converted to lighter products, and a minor portion is converted to coke. Residua can also be hydrogenated to lighter products. Vacuum distillation, deasphalting, and visbreaking are only partial conversion processes ; coking, catalytic: cracking, catalytic hydrocracking, and hydrogenation are capable of completely con-
verting residuum to lower boiling products. Both coking and catalytic cracking degrade excessive amounts of feed to coke and dry gas. Hydrocracking and hydrogenation have not been economically attractive because of the high cost of hydrogen and high pressure hydrogenation equipment. All catalytic processes suffer catalyst contamination from both metallic and coke-forming components in residua.
Process Vacuum distillation Deasphalting Visbreaking Coking Catalytic cracking Catalytic bydrocracking Catalytic hydrogenation
Limitation Volatility Solubility Coke plugging, fuel oil quality Degradation to coke and gas Degradation to coke and gas, catalyst contamination Catalyst contamination Catalyst contamination
Crude residua have hydrogen-carbon atomic ratios of 1.4 to 1.6, sufficiently high to allow formation of gas oil, heating oil, and gasoline components if the hydrogen is properly utilized. The processes discussed are nonselective as far as cracking is concerned, and a relatively high proportion of hydrogen is lost as low value products such as hydrogen, methane, and ethane. A selective cracking process, wherein the available hydrogen in residua is utilized, thermally cracks the residua in the presence of a condensed ring aromatic-naphthenic compound such as Tetralin (tetrahydronaphthalene). Only naphthenic hydrogens activated by adjacent, fused aromatic rings are sufficiently reactive to minimize nonselective reactions leading to coke and gas during thermal cracking. Compounds like Tetralin, here termed hydrogen donor diluents, efficiently transfer over 1000 cu. feet of hydrogen per barrel of residuum under mild thermal cracking conditions. With gaseous hydrogen, high pressure catalytic hydrocracking is required for such hydrogenation. Pott and Broche (6-9) were the first to use Tetralin in the absence of hydrogen for thermal dissolution of coal up to about 825’ F. D’Yakova and Malent’eva (7, 2) used Tetralin for thermal dissolution of coal, peat, and asphaltites.
Greensfelder ( 3 ) treated crude rrsiduum noncatalytically with Tetralin and hydrogen in the first stage, followed by catalytic hydrogenation. These processes do not prevent contamination of the catalyst by the metals and resins in the residuum. Early investigation did not appreciate the ability of condensed ring naphthenearomatics to donate large amounts of hydrogen readily in a manner which prevents coke formation. Use of Tetralin diluent in thermal cracking of a high asphalt Hungarian crude oil greatly reduced coke formation compared with cracking in the absence of diluent ( 7 7). Theoretical Considerations Although crude residua are extremely complex mixtures ofrelatively high molecular weight compounds, they can be fractionated by physical methods into nonaromatic waxes and oils, aromatic oils, and asphaltenes. Chemically, the nonaromatic fraction consists of paraffins and substituted naphthenes. The aromatic fraction contains substituted aromatic nuclei which may have naphthene rings in the substituent chains. There is no sharp transition between “aromatics” and “asphaltenes.” The latter probably contain more condensed aromatic ring structures linked by paraffinic chains or possibly sulfur, oxygen, or nitrogen, although the ratio of’hydrogen to carbon atoms in the asphaltenes is too high to permit visualization of the asphaltene structure as a molecule containing only condensed ring aromatics. Typical atomic ratios are :
H/C Ratio 16% West Texas residuum 1.58 Nonaromatic fraction5 1.84 Aromatic fractionD 1.39 Asphaltene fractiona 1.12 CB condensing ring aromatics 0.40-0.57 Thermal coke 0.65 Separated on alumina by successive elutions with heptane, benzene,and pyridine. Fractions amounted to 40, 40, and 20%, respectively, based on residuum.
The asphaltene fraction has approximately twice the hydrogen content shown O ring for unsubstituted C ~ condensed aromatics and thermal coke. A hypothetical structure for asphaltenes, based on West Texas asphaltenes isolated by heptane precipitation, illustrates that even at as low as 1.1 hydroVOL. 5 0 , NO. 7
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gen-carbon atomic ratio the structure can contain a number of alkyl groups and saturated rings without completely condensed aromatic rings. It should be crackable into distillate products, if further hydrogen was not lost, as this would result in condensation and polymerization reactions leading to coke formation.
Alkyl radicals -/- H-donor + presence of hydrogen. However, only Aromatic radicals hydrocarbons saturated diluents were studied and they dehydrogenated donor Asphaltene radicals do not contain sufficiently reactive This is the basis for the thermal crackhydrogens. This emphasizes the outing, hydrogen transfer process described. standing effect obtained with Tetralin, Failure to consider this requirement was which possesses hydrogens in the saturesponsible for lack of success in prerated ring highly activated by the advious extensive hydrogen transfer work jacent aromatic ring. Further support using paraffins, olefins, naphthenes, or was reported recently in a study of the rate of abstraction of hydrogen from paraffins, isoparaffins, aromatics, alkylated aromatics, cyclohexane, Decalin, and Tetralin by tert-butoxy radicals (72). More concrete evidence that aromatic radicals undergo condensation and polymerization during thermal cracking and that Tetralin prevents such reactions was obtained with clarified oil-the refractory, highly aromatic bottoms from catalytic cracking, which consists preMolecular weight ~000-100,000 selected refinery streams rich in saturated dominantly of condensed ring structures. H/C atomic ratio 1.1 naphthenes as sources of hydrogen. When clarified oil is thermally cracked, Sulfur, wt. 7% 4-6 many aromatic radicals are formed. Oxygen nitrogen, wt. % 1-3 Conradson carbon, wt. yo 40-60 Effectiveness of Diluents These radicals polymerize and dehydrogenate further, leading to high coke A proposed mechanism of coke forSix types of pure hydrocarbons were yields. As shown in Table 111, 20Yo mation is: evaluated for their ability to donate coke was formed in the nondiluent experiment, but essentially none in the heat presence of Tetralin under the same Residuum free radicals cracking conditions. The high 455" to Aromatics aromatic radicals 4-alkyl radicals -+ saturates Asphaltenes} 850' F. liquid yield and the elimination asphaltene radicals of coke in the Tetralin run are taken as Aromatic radicals dehydrogenation * asphaltenes evidence that the Tetralin effectively Condensation Asphaltene radicals polymerization coke prevented condensation of aromatics to products of higher molecular weight; Pyrolysis of residuum produces free 34.7% of the Tetralin was dehydrohydrogen to prevent coke formation durradicals which abstract hydrogen from genated to naphthalene. Based on the ing thermal cracking. Properties of the the aromatics and asphaltenes to pronaphthalene yield, 670 cu. feet of hydro16770 West Texas residuum selected are duce aromatic and asphaltene radicals. gen was transferred thermally from Tetgiven in Table IV. Cracking was These radicals then undergo dehydroralin per barrel of clarified oil. This carried out in a standard Aminco rockgenation, condensation, and polymerizais in the range that might be obtained ing bomb, using about equal amounts of tion reactions, aromatic radicals leading by catalytic hydrogenation. Under catresiduum and diluent. T h e mixture was to asphaltenes and asphaltenes to coke. alytic hydrogenation conditions, apheated rapidly to temperature and mainCondensation of aromatics to products of preciable hydrogen may be consumed by tained at 840' F. for 2.5 hours. At the higher molecular weight during thermal combination with sulfur usually present conclusion of the run, the bomb was alin residua. With thermal hydrogen cracking has been shown in a study of lowed to cool in the shaker. The prestransfer, sulfur is distributed throughout pyrolysis reactions of pure aromatic and sure attained was that exerted by the the boiling range of the cracked products. heterocyclic compounds ( 4 ) . cracked products, no extraneous gas For example, the naphtha produced from This mechanism suggests that conbeing added to the reaction mixture. the cracking of Bachaquero residuum densation of free radicals to high molecDetailed results obtained with these (3.36% sulfur) contained 0.94% sulfur. ular weight compounds and to coke pure hydrocarbon diluents are given in With hydrogen donor diluent cracking, could be minimized by providing a Table I. Reduction of coke is marked the distillate products may be desulfursource of readily available hydrogen with the fused ring aromatic-naphthene, ized by conventional methods, avoiding to satisfy the radicals as they form, and Tetralin, and less so with the fused contamination of the desulfurization that relatively mild cracking conditions ring naphthene, Decalin. A single-ring catalyst with residuum ash components. t o minimize the formation of low molecnaphthene, methylcyclohexane, had relaular weight radicals (methyl, ethyl, tively little effect a t the indicated conBatch Cracking of Residual etc.) would reduce coke formation. T o version level. Summarized data on Stocks with Tetralin Diluent meet these requirements, hydrogen transcoke yields are given in Table 11. fer must take place readily a t 750" to Because alkyl radicals abstract hydroThe use of Tetralin as a hydrogen 850" F. Transfer of hydrogen to alkyl gen from these diluents, the low coke donor during thermal cracking of various radicals will occur with any hydrocarbon yield with Tetralin was taken as evidence residua, asphalts, and lubricating oil diluent under these conditions. Therethat it was donating hydrogens to the extract was investigated to determine its fore, condensation can be prevented only aromatic and asphaltene radicals, general utility (Table 111). Feed stock by transfer of hydrogen to aromatic type whereas the other diluents did not. inspections are given in Table IV. radicals; the donor must possess highly The intermediate result with Decalin Conversions ranged from 50 to 94y0 with activated hydrogen positions, reactive was believed to be due to the formation of very little coke and low dry gas yields. enough to give up hydrogen to the relaTetralin under these severe cracking Kuwait Asphalt. A sample repretively stable aromatic and asphaltene conditions. T h e ineffectiveness of Decasenting 40% by volume of a 26y0 radicals, and must be more reactive than lin was reported recently (70), and it was Kuwait residuum was treated with an the hydrogens in the aromatic and stated that diluents had no effect in the equal weight of Tetralin at 750" to asphaltene structures: thermal cracking of residuum in the 800" F. for 0.5 hour and 400 to 500
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INDUSTRIAL AND ENGINEERING CHEMISTRY
THERMAL H Y D R O G E N A T I O N Table I.
Batch Cracking" of 16% West Texas Residuum in Diluent Systems at 840" F. Reduction of coke is particularly marked with Tetralin
Diluent
None
wt. % Pressure during cracking, p.s.i.g. Time, hours Conversion to 430' F. and coke, wt. % Yields, wt. % on residiumC Coked C3 and lighter
l00&250 2.5 57
+Heptane 58 950-1400 2.5 56
17 6.1 1.8 22.0 43.0'
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Methylcyclohexaneb 60 1000-1400 2.5 51
Benzene 64 1300-1500 2.5 56
19 8.6 4.1 28.0 44. OB
16 3.6 2.9 31.0 44. OB
15 5.5 2.8 26.0 49.0'
Decalin 47 800-2600 2.5 58
Tetralin 50 620-2240 2.5 53
Naphthalene 67
6 15 5.7 3lf
2.0 9.8 4.8 36.01
18 7.1 3.3 23.0'
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2.5 47
' CS-430' F. 370-455' C. 455-850' C. 22 24 29 850' C. bottoms 20 23 24 Product recovery, wt. '%a 90 104.0 98.0 98.0 95 95 97 a A 300-cc. stainless steel reactor charged t o 4 0 4 5 % of capacity, flushed with nitrogen, and heated t o indicated temperature. Reaction conducted in presence of 0.25 part ALOs-CrzOa catalyst/l part residuum. e Include cracking products from diluent. Unchanged diluent deterCoke recovered quantitatively from reactor walls by brush drill mined by mass spectrographic analysis and deducted from gasoline yields. after first rinsing reactor with benzene. Dry coke yields are reported. E 430' F. bottoms. f Cs-370° F. Q Based on residuum.
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Table II. Diluent Structure Has a Marked Effect on Coke Yields in Cracking of 16% West Texas Residuum Coke Yield,
Diluent None +Heptane Benzene Naphthalene Methylcyclohexane Decalin Tetralin a 430a F. and coke.
wt. % 17 19 16 18 15 6 2
Conversion,O Wt. % 57 56 56 47 60 58 53
p.s.i.g. pressure. Approximately 50y0 was converted to material boiling below 850" F. with only a trace of coke. Hawkins Asphalt. A sample of asphalt, from Hawkins crude, having a softening point of 198' F., was cracked Table 111. Clarified Oil 840 840 2.5 2.5
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with an equal volume of Tetralin at 840' F. to a conversion of 50% product boilingbelow 850'F. Only about 0.1% coke formation was observed, based on asphalt. Recycling of the 1000' F. product to Tetralin cracking gave an additional conversion of 28% with only 0.1 weight 7 0 coke. McMurray Tar. A sample of McMurray tar, extracted from Athabasca tar sands from Alberta, Canada, was cracked with Tetralin at 820' F. for 1 hour. Seventy-six per cent of the tar was converted to products boilin below coke 850' F., with only 1.6 weight formation based on tar. Elk Basin Residuum. A sample of Elk Basin vacuum residuum, representing about 12% on the original crude, was cracked with Tetralin a t 800 to 840' F. to a conversion of Gl% to materials boiling below 1000" F. Coke
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formation was only 0.6 weight % based on residuum. Coleville Residuum. A sample from Coleville crude (Canada) was cracked with Tetralin at 750" to 815" F. to a conversion of 6G% to products boiling below 1000" F. Coke formation was only 0.05y0 based on residuum. Bachaquero Residuum. Bachaquero residuum (900' F.+), a Venezuelan crude product, was cracked with and without Tetralin a t about 800' F. The yield of products boiling below 850' F. when Tetralin was used was 78Q/, accompanied by a coke yield of 5.2 weight %, compared to 67% conversion and 13% coke in the absence of Tetralin. Lubricating Oil Extract. As an example of a refractory stock, a phenol extract from a coastal lubricating oil distillate was cracked with Tetralin a t 840' F. for .1.2 hours. A remarkably
Batch Cracking" with Tetralin Diluent Produced Low Coke Kuwait Asphalt 750-800 0.5
Hawkins Asphalt 840 2.5
McMurray Tar 820 1.0
Elk Basin 800-840 0.5
Coleville 750-815 0.75
Bachaquero 800-843 1.25
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Lube Oil Extract 840 1.2
Temperature, F. Time, hr. Pressure during crack600-1600 950-2300 400-500 900-1200 600-1300 500-1250 410-665 700-2650 ing, p.s.i.g. 650-1700 Diluent, wt. % on feed 100 None stock 50 50 50 50 50 50 50 Conversion to 850' F. 45 55 and coke, wt. % ' 50 55 76 61b 66 78 94 Yields, wt. % on feed stockC
