A AND MECHANISM CHAIM WEIZMANN, EKNST BERGMANN, W. E. HLGGETT, HERBERT STEINER, AND MAX SULZBACHER WITH D Q N A I B PARKER, K. 0. MICHAELIS, SYDNEY WHINCUP, AND E M A N EL ZIMKIN The Weizmann Institute of Science, Rehozoth, Israel
&kromatichydrocarbons have been lcnoivn to be byproducts of cracking processes; from a study of the mechanism of their formation by cracking, it was hoped to develop a method for their industrial production from petroleum or petroleum fractions. The present four papers show t h a t the high temperature aromatization of hydrocarbons proceeds largely through degradation to small unsaturated units, outstanding among them butadiene, and through subsequent "diene reactions" leading to hydroaromatic hydrocarbons which are eventually dehydrogenated to the aromatics. For naphthenic charging atocks, direct thermal dehydrogenation represents a second source of aromatic hydrocarbons. A process is described which converts a nonaromatic (or only partly aromatic) charging stock intoliquidand gaseous products. The gases are predominantly unsaturated (ethyl-
T
HE cyclization of paraffinic hydrocarbons into aromatics,
discovered over a decade ago (10, 21, 24-27) has been described (16, 68, 36,36,39) as a three-step process: dehydrogenation of the paraffin hydrocarbon t o a corresponding olefin, cycloisomerization of this olefin to a hexahydrobenzene, and simultaneous removal of the supernumerary hydrogen atoms from the latter to form the aromatic hydrocarbon. This process is catalyzed by various oxides which are used either in the pure state or supported on carriers such as alumina. This reaction mechanism, however, cannot account for the formation of aromatics by thermal cracking in empty or packed tubes (9, 31). Two alternative mechanisms have been proposed. Both suggest that in the first stage of the reaction the starting material undergoes considerable breakdown, followed by resynthesis. Groll (9) assumes that the intermediate is acetylene or, rather, a diradical form of that hydrocarbon, whereas other workers (1.2,30, 4 7 ) believe this intermediate is butadiene. The latter is known to combine with other unsaturated hydrocarbons-for example, with ethylene to form cyclohexene (19), or with a second butadiene molecule to form vinylcyclohexene (2). This reaction is an extension of the classical Diels-Alder synthesis. The present paper describes the second type of aromatization The distinction between them is that in the first type the starting material and the end product have the same number of carbon atoms. In the present work any hydrocarbonaceous starting material will result in the complete series of aromatics, from benzene to the most complicated polycyclic system (53, 46, 46, 46). Under the following conditions a pure, or almost pure, aromatic liquid product can be obtained from a given petroleum fraction or other hydrocarbonaceous raw material: temperature, 630 ' to 680" C.; pressure, 1 to 3 atmospheres; space velocity, 0.05 to 0.5 (litera of liquid charging stock per liter catalyst volume per
ene, propylene, butylene, isobu tylene, and some butadiene), and the liquid product is more than 90%, and up to 98%, of aromatic nature and largely free of sulfur and nitrogen, even in cases in which the charging stock contained organic compounds of these elements. Separation and purification of the constituents are, therefore, comparatively easy; one obtains the whole range of aromatics from benzene to the most complicated polycyclics, practically independent of the molecular size of the charging stock. The process which thus yields practically all the hydrocarbons which the modern organic chemical industry requires can supplement and/or replace both the usual cracking processes and the coal tar industry, as far as they aim a t the production of chemicals. It is being successfully applied a t the present time on an industrial scale in England.
hour); and use of packed tubes. Metals are efficient as packing materials; copper is preferred because it gives rise t o a minimum of carbon formation. Refractory materials, however, can also be used as packing materials. They are rapidly covered by a layer of carbon which in itself has no adverse influence on the process. Under these conditions an almost completely aromatic (over 90%) product can be bbtained at 650" to 680" C. According to Groll ( 9 ) , 89 and 95% aromatization, respectively, in an empty tube requires temperatures of 750 O and 800 C. a t a space velocity of 1.8. The present authors found that temperatures above 680" produced no significant change in the quantity of liquid aromatized product; only secondary changes, such as dealkylation of alkyl aromatics, were found to occur. Two factors make the packed tubes more advantageous than empty ones. They are reduction in reaction temperature, small as it is in absolute value, and simultaneous increase in heat transfer. The lower temperature is advantageous also in the practical consideration of plant construction-those metals and alloys which have desirable mechanical properties in this temperature range are more readily available. APPARATUS
It was found that 18:8 nickel chromium steel is a satisfactory material which does not deteriorate either chemically or mechanically on prolonged use. The experiments were carried out f i s t in laboratory furnacea similar to those described recently by Henriques (14). The thermocouple well had practically the same length as the reaction tube. In view of the relatively low space velocity employed, no preheater was employed, but in calculating the space velocity only the part of catalyst space which actually had the desired reaction temperature was taken into account. In the further development of the process, a pilot plant waa constructed which had the form indicated in Figure 1. On this plant, which had automatic recording contro18, up to 1 ton of
2312
October 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
charging stock was processed per 24 hours and runs of up to 200 hours' length have been performed on it. It has now been in operation for more than 6 years. The vaporization section consisted of a helical coil with about ' twenty-five 2-foot diameter turns of 0.5-inch tubing, all placed in a gas-fired furnace. The heating was effected by direct radiation, and artly also by convection from upward passing flue gases. As t i e temperature in this section was not to exceed 450' C., ordinary (low-carbon) steel was employed. I n the U tubes following the vaporization section, the material is first brought up to reaction temperature and then subjected to the heat input necessary for the breakdown. The material then passes into a long soaking zone where the actual aromatization takes place without further substantial heat requirements. The U tubes and the Boaking zone are filled with packing material.
Figure 1. Pilot Plant for Aromatizing Cracking 1. Charging tanks 2. Charging p u m p
3. Vaporization section 4.
Tubereactors
5. Gas burners 6. Soaking seation 7. Tar separators 8. S t e a m inlet 9. Condenser and cooler 10. Run-down t a n k s 11. Vent
12. Absorber oil stripper 13. Cooler 14. Transfer pump 15. Absorber oil t a n k 16. Slops tank 17. Absorbar oil pump 18. Absorber 19. Finished product t a n k 20. Pressure oontrolling a n d safety liquid seals 11. Gas t o burners
Before passing into the condenser zone the finished product has to be freed from high-molecular pitch which is likely to solidify in the condenser lines. This is done by steam injection in a special tar trap. After condensation of the liquid reaction product, the gases pass through a tower with absorber oil and from there to the gas storage tanks. From the absorber oil, a further quantity of low boiling reaction product is obtained by stripping. [For further engineering data, see reference ( d ) ] . GENERAL CHARACTERIZATION OF T H E PRODUCT
Table I gives the composition of the aromatization product obtained from a naphthenic naphtha. The data regarding the fractions boiling above 180' C. are not complete; some as yet unidentified hydrocarbons are present. Even when derived from a charging stock containing sulfur, nitrogen, and oxygen compounds, the liquid product is almost completely hydrocarbonaceous in nature; sulfur, for example, is eliminated in the form of hydrogen sulfide. This fact, together with the high aromaticity of the product, is responsible for the facile separation of the liquid product into its constituents. This is easier, indeed, than for the products of similar composition originating from coal carbonization. The batch distillation curve of such a product consists of a number of plateaus, very sharply set off against each other. In general, a combination of fractional distillation and, for the higher fractions, crystallization permits the isolation of many of the constituents of the aromatization product. In addition to the liquid product, a gas is formed which is highly unsaturated, as one would expect for thermochemical reasons.
2313
TABLE I. AROMATIZATION OF A NAPHTHENIC NAPHTHA Charge Boiling range (5-9570), C. 110-190 S p i f i c gravity, 20° 0,798 omposition, % Paraffins 8 Olefins 4 Naphthenes 77 Aromatics 11" Process temp., O C. 680 Space velocity, liter/liter /hour 0.24 Liquid product Recovery 70by wt. of charge 50 Composition % by wt. of charge Boiling beiow benzene 1.1 Benzene 11.0 Toluene 11.0 Xylenes, ethylbenzene, styrene b 6.0 Alkylbenzenea (150-180° C.)C 2.6 200-300' C. (naphthalene fraction)d 3.5 Up to 140' C./2 mm. (alkylnaphthalene fraction) 4.0 140-190e C./2 mm. anthracene fraction)/ 2.7 190-230' C./2 mm. [chrysene fraction)g 2.4 Residue/2 mm.h 6.0 Gaseous product Recovery! % by wt. of charge 50 Composition, yo by wt. of charge Hydrogen 0.5 Methane 18.0 Ethane 6.5 Ethylene 9.5 Propane 1.5 Propylene 9.0 Butanes 0.8 Butenes and butadiene 4.2 The charge was free of sulfur and nitrogen compounds. b This fraction consists approximately of 20% styrene, 10% ethylbenzene, 16 o-xylene, 20% m-xylene, 26% p-xylene, 8% olefine and paraffins. Vaxes for m- and P-xylene are aDDroximations, based on soectronraohic . . .- . investigation. a-Methylstyrene (0.52% by wt. of char ing stock), indene (0.65% by wt. of charging stock) mesitylene isopropyl%enzene, and a few other substituted benzene derivktives have been identified in this fraction. Fraction consists of 45% of polymerizable materials. d Mainly naphthalene and tetrahydronaphthalene. e Following hydrocarbons have been isolated from this fraction: 1methylnaphthalene. %methylnaphthalene. 2 6-dimethylnaphthalene. 1 7dimethylna hthalede. 1 6-dimethy1naphth;le~k 1,2-dimethylnaph~hklede: 1,4,6-trimet?1ylnaphtdal&1e. 2 3 6-trimethy!nsphtholene; 1,4,5-trimethylnaphthalene. fluorene, icedphthene; biphenyl. From a quantitative point of vied, this fraciion is approximately composed as follows: ?% by Vol. Monomethylnaphthalene 40 Dimethylnaphthalene 30 Trimethylnaphthalene 18 Acenaphthene 4 Biphenyl 4 Fluorene 4 I The following hydrocarbons have been isolated: phenanthrene (0.47 by wt. of charging stock). anthracene (0.15% by wt. of charging stockf; 2-methvlanthracene: 2.7-bimethvlanthracene: 1-methvhhenanthrene: 3_ methylhorene. The followin hydrocarbons have been isolated: 1,2-benzanthracene. 3,4-benaophenant%rene; chrysene; pyrene; 1,2-benzofluorene. 2,3-benzo: 5uorene' 5uoranthene. 3,4-benzopyrene; 1,2-benzopyrene; iriphenylene; l'-meth;l-l 2-beneantdracene, h From this residue (pitch) picene and 1,2,7,8-dibenrochrysene were isolated. i The C4 fraction of the gas consists (in 7 by vol.) of 64% isobutylene 117 butenes lOv butadiene, and 15% sat2rated C4 s. The prevalence ob the780 compdundlis remarkable and may be due to the fact that the strai ht ohain components of the fraction participate more easily in the Diels-Alser reaction.
INFLUENCE OF OPERATIONAL FACTORS
In discussing the influence of various operational factors on the reaction, the part of the product boiling up to 180" C. is considered. In this discussion it should be remembered that it is the purpose of the process to produce pure aromatic hydrocarbons, and that, thereby, from a practical point of view the variability of the operating conditions is limited to such as will achieve this result. Otherwise, the isolation of the aromatics would be unduly complicated by the complex refining operations involved. TEMPERATURE. An increase in temperature of 30" C. from 650" to 680" C. increases the aromaticity of the product from 75 to 94%, if the space velocity is held constant at about 0.3 (Figure 2). If the temperature exceeds 680" C. at the ~ a m espace velocity, a certain decomposition of some of the aromatic hydrocarbons takes place, such as the elimination of side chains containing more than one carbon atom, This is not uncommon (86). PRESSURE.If the pressure is increased--e.g., up to 30 or 40 atmospheres-higher space velocities or lower temperatures can
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
2314
Vd. 43, No. 10 REACTION MECHANISM
TABLE 11. INFLUENCE OF SPACEVELOCITY, V , ON AROMATIZING CRACKING AT 680" C. A N D ATMOSPHERIC PRESSURE
DIEKE SYNTHESIS.The observations regarding the concentration of butadiene (Charging stock as in Table I, but boiling range (5-9570) 88-233' C.; specific gravity, 0.790) and of mono-olefins--e.g., ethylene-as Gas, Liquid a function of the contact time confirm Cc. Aromatics, Wt. % G./100 Cc./lOO G./100 Benl;ene Toluene Xylenes the hypothesis put forward by Hague CC. CC. cc. cut cut cut V Charge charge charge % and Wheeler ( 1 2 ) and by Schneider and 77 12.4 97.5 53.2 46.71 0 . 2 32.29 12.2 97.4 9.35 90.0 Frolich (SO). The hydrocarbons of the 76 14.8 55.7 47.64 0 . 4 31.36 70.0 11.4 81.6 6 . 7 5 74.7 46.71 82 14.25 6 4 . 5 56.2 0 . 6 32.29 11.05 75.4 9 . 6 65.1 starting material are broken down to 57.6 47.22 79 11.05 2 3 . 8 0.7 31.78 12.55 36.0 10.7 44.1 small molecules. By dimerization of 62.6 50.61 78 12.8 38.6 0 . 8 28.39 11.75 4 8 . 4 16.8 31.0 83 12.5 45.8 79.1 62.82 1.0 16.18 15.8 29.8 15.26 26.5 dienes and by their reaction with olefins 81.1 64.14 80 16.65 3 5 . 0 1 . 5 14.86 17.2 27.7 1 3 . 6 21.5 (and later with the aromatic hydrocarbons formed) cyclic compounds are produced, tihich in the last stage of be used to achieve a satisfactory degree of aromatization (Figure the process are dehydrogenated to the aromatic systems. Perhaps it is in this step that the metallic packing material of 3). However, under these conditions the formation of coke is facilitated. Under pressure the relative amount of olefins in the the reaction tubes plays a part. gas is reduced; also, more methane is formed than a t atmospheric The xylene fraction contains a relatively large amount of pressure, The composition of the liquid product, on the other ethylbenzene and styrene which may be formed through the hand, is hardly affected by a change in pressure. dimerization of butadiene t o a vinylcyclohexene. This disSPACEVELOCITY.Table I1 summarizes a number of experitinguishes the mechanism from the process of ilIoldavskT(Z4,2~7) ments in which, a t constant temperature (680" C.) and pressure and hlorrell and Grosse (10, 26, d 7 ) , as cyclization of n-octane (1 atmosphere), the space velocity was varied. leads mainly to o-xylene and not ethyl benzene or styrene. The first result of these investigations was that the cracking The condemation of butadiene and ethylene to tetrahydroreaction proceeds with great velocity; the final quantity of gau benzene constitutes the source of benzene, and with propylene (C, to C,) is obtained in about 20% of the time required for comand 2-butene one obtains the skeleton of toluene and o-xylene. plete aromatization. This is also illustrated in Figure 4. In the For the formation of naphthalene, one could assume analosubsequent 80% of that time, synthesis of aromatics from the gously that it oiiginates from the combination of benzene and primary cracking products takes place together with further butadiene cracking, a t ever-diminishing rate, of the still nonaromatic constituents of the reaction mixture which in the end are present only in traces.
z,.,
ETHYLENE, AXD BUTADIENE TABLE 111. YIELDSOF METHAXE, AS FUNCTION OF COXTACT TIME Contact Time, Sec. 0.55 1.67 2.78 4.17 10.28 11.94 13.61 20.00 25.55 40.00
Methane 0.63 2.00 4.63 5.39 8.84 9.79 9.05 14.63 13.90 16.00
Products, % by Wt. Ethylene 1.58 1.58 2.42 4.21 6.10 6.84 7.68 8.32 6.53 7.11
Butadiene 0.15 0.35 0.73 3.50 2.48 3.35 1.65 1.20 0.20
Table I11 shows the quantity of methane, ethylene, and butadiene as function of the contact time. While methane increases steadily throughout the operation (although, toward the end, a t a very slow rate), ethylene is formed rapidly only a t first. A stationary state is soon reached, and in the end the ethylene yield diminishes again, though slowly. The other mono-olefins present the same picture. Butadiene, on the other hand, shows a very marked maximum at an early stage, after which its quantity decreases rapidly. BO~LING RAXGEOF THE CHARGING STOCK. While the whole series of aromatic hydrocarbons from benzene to the most complicated ones is formed independently of the boiling range of the starting material, there is always a prevalence in the aromatized product of those hydrocarbons which boil in the same range as the starting material. Table IV gives some pertinent figures; their significance lies in the fact that in the lowest boiling naphtha which gives 13% (by weight of the charging stock) polycyclics, only a small amount of hydrocarbons of that high molecular size can be present. Similar conclusions can be drawn from the experiments performed with narrow cuts of East Texas naphtha (Table V).
if the formation of the 1,2,9,10-tetrahydronaphthalene system is accompanied by (catalytic) dehydrogenation to naphthalene and/or by isomerization to lJ2,3,4-tetrahydronaphthalenewhich is one of the constituents of the naphthalene fraction of the aromatization product. The resonance energy of the naphthalene and the tetrahydronaphthalene molecule would compensate for the loss of energy in the first step of the reaction Alternatively, one may think of a synthesis of the naphthalene system from butadiene and cyclohesene. The application of the butadiene rule to the higher products of the aromatization process will be discussed in the following paper. Three other observations support the diene mechanism of the aromatization,
-TEMP?
Figure 2.
'C.
Aromatization as Function of Temperature Space Telocity, 0.3 vol./vol./hour
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
TABLEIV. COMPOSITION OF AROMATIZED PRODUCT FROM NAPHTHENIC AND PARAFFINIC CHARGING STOCKAS FUNCTION OF THE
BOILINQRANGE
(Cracking temp., 680' C.; space velocity, 0.2) Light Heavy Naphtha Kerosene Kerosene Type of Charge Naphthenic Charging Boiling range (5-95%), O C. Specific eravity Liquid yield 7 b y wt. of char e Product boiiine! below 200° C. ?rnonoovclics), % b y wt: pf-charge Product boiling above 200' C. (polycyclics), % b y wt. of charge
Stock 102-228 0.79 45
162-255 0.84 52
194-302 0.86 53
32
29
21
13
23
32
2315
periments by us that, under the operational conditions applying, side chains with more than one carbon atom are broken off from aromatic ring systems in form of the corresponding olefins. One may enlarge the scope of this rule by adding: unless they are stabilized b resonance. Examples are styrene, a-methylstyrene, and iniene. Herington and Rideal (16) have already pointed out that aromatization of propylene (oxide catalyst) leads to 8-methylstyrene and indene. It is interesting to note that in both the MoldavskI-Grosse process and in the present aromatization process, methyl is the prevailing side chain occurring in the reaction products, but that the reason for this phenomenon is fundamentally different in the two cases.
Gasoline Naphtha Kerosene Para5nic Charging Stock 58-186 Boiling range (5-95'%), C. 0.72 Specific gravity 31 Liquid yield, % b y wt. of charge Product boiling below 180' C., % by wt. of 21.7 charge Product boiling above 180' C.,% b y wt. of 9.3 charge
115-184 0.76 37
178-218 0.80 40
26.0
23.8
11.0
16.2
OF NARROW CUTSFROM EAST TEXAS TABLE V. AROMATIZATION
NAPHTHA
-
Boiling range (5-95%), O'C. 31-84 124 143 132-157 140-169 156-180 0.785 0 65 0.766 0.774 0.758 8 ecific gravity 680 680 685 680 680 d a c k i n g temp., e C. 0.15 0.26 0.20 0.25 0.14 Space velocity Specifi-c gravity of liquid 0.912 0.912 0.750 0,898 0.890 product Liquid prqduct, 7 by wt. 42 34 24 36 39 of charging stocg Analysis of liquid product, % b y wt. of charge 0.8 13.5 0.8 1.5 0.6 Pre-benzene 7.0 5.2 3.4 7.0 8.0 5.6 6.7 7.7 2.1 7.9 4.4 4.0 7.7 6.2 1.5 0.6 3.6
4.0 9.7
5.9 8.2
7.2 11.6
8.9 14.5
-
Figure 4.
1
10
CONTACT
20
Yield of Gas (Including C,) as Function of Contact Time
3. The tendency to form highly unsaturated hydrocarbons expresses itself also in the fact that in the pre-benzene fraction such substances as cyclopentadiene occur and that in the naphthalene and methylnaphthalene fractions blue to violet contaminations of azulene structure appear (compound formation with phosphoricacid). This recalls the observation (16)that in the Moldavski-Grosse process similar intensely colored by-products are obtained which belong to the class of fulvenes. In both processes a certain amount of unsaturated cyclopentane compounds is formed along with the benzene derivatives, but again the formal analogy is not based on a mechanistic parallelism.
DIRECT AROMATIZATION. Even if the Hague-Wheeler mechanism appears to be verified, one should not assume that it is the only one operative. The above-mentioned influence of the boiling range of the charging stock shows that hydroaromatic compounds present in the starting material can also be simply dehydrogenated and open-chain hydrocarbons cyclized by a mechanism similar to that in the MoldavskI-Grosse process. There are some other observations which point in the same direction. If one plots against the contact time the refractive index (as indicative of the content in aromatics) of the benzene and toluene fractions (69 'to 95 and 95 to 125 C., respectively) (Figure 5) one finds two regions of rapid increase of the refractive index between which a rest zone is interposed. It, is reasonable to assume that the first zone represents the formation of aromatics by straight dehydrogenation of naphthenes in the charge, the second their formation by the three-step mechanism of breakdown, diene synthesis, and dehydrogenation. A similar but less pronounced effect is observed if one plots the density of the total liquid product against the contact time (Figure 6). THERMOCHEMICAL CONSIDERATIONS. The cracking of paraffinic hydrocarbons to small (saturated and unsaturated) molecules is strongly endothermic (about 300 to 500 kg.-cal. per kg.), while the condensation reactions of dienes and olefins with simultaneous liberation of hydrogen to give aromatic hydrocarbons are thermoneutral or only slightly exothermic (by up to 100 kg.-cal. per kg). From an engineering point of view, the conclusion is important that the short first cracking stage of the process is strongly endothermic, the relatively long second one practically thermoneutral. From the weight per cents of the various products in Table I, their mole fractions and partial pressures in the charge leaving O
I
'
20 -PRESSURE
Figure 3.
30
I 40
(ATK)
Aromatization as Function of Pressure
Space velocity, 0.75 vol./vol./houri 90% aromatization
1. The relative composition of the fractions of the gaseous product is almost independent of the de ree of aromatization, the only exception being the unsaturate$ C4 fraction which decreases in uantity with the increase in aromatics in the reaction product. I n going from 75 to 94% aromatization-eg., the unsaturated C4 (in per cent of charging stock) decreases from 6.2 to 3.4. 2. Apart from the unsubstituted aromatic hydrocarbons, only two types of alkyl derivatives appear: methyl derivatives and aromatics with an unsaturated side chain of the styrene type (and together with them, a certain amount of their hydrogenation products). It has been known ( 3 7 ) and confirmed by ex-
30
TIME (SEC.)
INDUSTRIAL AND ENGINEERING CHEMISTRY
2316
the cracking plant are calculated in Table VI (mean niolecwlar weight, 42; total pressure a t the exit, 2.4 atmospheres). From these figures, the percentages of olefins in the various fractions are calculated. For CZand Ca the figures are in gooci accord with those obtained from Pitzer's (89) equilibrium ronstante. Calcd
Found 54 84 71
c2
cs
c4
43 90 07
Regarding the styrene-ethylbenzene fraction, 85% of the latter should have dissociated to styrene and hydrogen, according to Guttman, Westrum and Pitzer (11); the experimental figure is 57% (0.75% ethylbenzene and 1.2% styrene, calculated on the charging stock). Perhaps some polymerization occurs during the lengthy process of batch distillation; this effect may also account for the fact that in the higher fractions not more than trares ot vinyl aromatics have been observed.
TABLEVI, MOLEFRACTIOWS AND PARTIAL PRESSURES OF PRODUCTS IN THE CHARGE LEAVIKG THE CRACKING PLANT Wt. 0.3 12.4 10.7 9.3 1.8 2.4 1.0 a b
h4o)e
Partial Pressure, Atmosphere
0.0636 0.186 0.143 0 003 0.017
0.152 0.45
0.018 0.0073
0.043 0.018
Fractiona
0.34
0.22 0.041
Based on total reaction product. Butadiene subtracted (-0.3).
Comparison of yield data with data derived from rate measurements ( 7 , 54) on the reactions (at 680' C.) a. Butadiene
+ ethylene = cyclohexene;
Koyoiahexeue =
1.5 X lo4 mole-' cc. see. b. 2-Butadiene = vinylcyclohexcnP;
Kv,,,~.ioycioherenp=
4 X 104 mole--' cc. ser.
-I
can give a rough indication of the extent t o which the HagueWheeler mechanism and the straight dehydrogenation of naphthenic hydrocarbons in the charging stock are responsible for the formation of aromatics, if one makes the reasonable assumption that the dehydrogenation of cyclohexene and vinylcyclohexene to the corresponding aromatics is w r y rapid as compared with t,he speed of the above two reactions. If one assumes constant average concentrations, c, of the various reactants, the total concentration of cyclohexene, C1, and vinylcyclohexme, C2, in the reaction product can very roughly he estimated as C1 =
tKicycloheronr
7 rthylene
Vol. 4% No. 10
undei identical conditions, 50% of the liquid aromatization product consisted of benzene, and 50% of higher hydrocarbons, so that in this case a t best 50% of the benzene wax formed by direct dehydrogenation. EXPERIMEN'rS
AASLYTICALPROCEDVRIL The crude reaction product w8.e fieparated into the constituents boiling up to 180" C. and thc. higher boiling parts. The former were then fractionated in a Gooderham column (8) and the following fractions were tnlten: pre-benzene, up to 75" C.; benzene, 75" to 95' C.: toluene, 95' to 125" C.; CSfraction, 125" to 150" C. Their composition wa.s determined from their bromine number, refractive index, and density. The analysis of the gaseous products was carried out by liquefaction and fractionat,ion in the Podbielniak low temperature column. The following fractions were secured: methane and hydrogen, ethylene, ethane, propylene and propane, and C;s, and analyzed in the Bone-Wheeler apparatus. For the determination of butadiene, the method of Tropsch and Mattox (38)was adopted Acetylene was present in the gases only in minor quantities-e.g., in a gas obtained a t 680' C. and with space velocity of 0.1, to t h c extent of 0.12% by weight (0.075% by weight of the charging stock).
PRE-BENZESE FRACTION. The fraction boiling up to 75' C. iE highly unsaturated. Fractionation on the Gooderham colum~i (8) (which for this purpose had to be fitt,ed Iyith a cooled reflux head and a low temperature condenser) revealed the presence of a hydrocarbon C8H8, boiling at 46" C., which proved to be cyclopentene [boiling point, 45" to 46" C. (IS)]. By analysis the following was obtained: Found: carbon, 87.8%; hydrogen, 12.0%. Calculated for C5H8: carbon 88.2%; hydrogen, 11.8%. The occurrence of cyclopentane derivative8 in the pre-benzene fraction was also demonstrated by the isolation of the addition product of cyclopentadiene and maleic anhydride ( 5 ) ,when the fraction was heated with maleic anhydride: melting point, 169" to 170' C.; melting point of the free acid (after recrystallization froin butyl acetate), 185" C. (decomposes). However, the total quantity of cyclopentadiene present in this fraction is small. In order to obtain more quantitative data, a sample (20 grams) of the pre-benzene fraction, boiling a t 30" to 65" C. (mostly 35" tu 45" C.) and having a bromine number of 150, was subject>ed to catalytic hydrogenation (in presence of Raney nickel) a.nd subsequent treatment with 90% sulfuric acid. The saturated product had 84.0% carbon and 15.3% hydrogen, and was therefow, ;i mixture of 30% cyclopent>ane (calculated for CsHlo: C, 85.7: H,
butadiene
c!z = tKvinylcyolohorene c2 butsdiene As Cbutadlene = 3.15 X 10-7 mole per cc., ?ethylene = 1.8 X 10-7 mole per cc., and the contact time, t, = 30 seconds, one obtains the theoretical weight percentages of 1.8% cyclohexene and 1.7% vinylcyclohexene, while the actual benzene yields are in the order of 10%; those of ethylbenzene plus styrene are in the order of 2% (per cent by weight of charging stock). Thus it appears that only about 20% of the benzene, but about 85% of the ethylbenzene plus styrene, are derived from the diene reaction, the balance being from straight dehydrogenation. These figures must be taken as no more than rough approximations. I n some orientative experiments with pure cyclohexane which was cracked
-
10 CONTACT
Y BENZENE
FRACTION
0 TOLUENE
$1
20
30
TIME (SEC)
Figure 5. Refractive Indexes of Benzene and Toluene Fraction as Function of Contact Time
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
14.3) and 70% pentanes (calculated for C6H12: carbon, 83.3%; hydrogen, 16.7%). The dependency of the purity of BENZENEAR'D TOLUENE. benzene and toluene cuts (for fractionation see Figure 7) on the operating conditions is illustrated by the following table. Specific gravity of liquid aromatization product 0.860 0 915 0 935 68 85-90 92-97 Purity of benzene, % Purity of toluene, % 74 92-97 95-99
r
2317
TABLEVII. DISTILLATION OF AN ALKYLBENZENE FRACTION (160-lSO0 C.) O F THE AROMATIZATION PRODUCT Aromatized liquid, specific gravity 0.950 0.935 0.935 0.940 Pressure, in distillation 760 mrn. 760 rnm. Vacuum Vacuum 160-180° fraction by wt. of original charge 1.8 2.3 5.5 5.0 pecific gravity 0,890 0.916 0.916 0,886 n CP 1.511 1.525 1.501 1.526 BFomine No. 48 105 98 58 Resin roindion from distilled cut, 20 56 28 56 % Alkylbensenes recovered after resin formation, % of distilled cut 61 71 38 39 High boilers recovered after resin formation, % of distilled cut 3 1.5 0 0
?
Isopropylbenzene (cumene): boiling point, 152 ' C.; 2,4diacetamino derivative: melting point, 216' C. ( 1 7 ) . 1,3,5-Trimethylbenzene (mesitylene): boiling point, 162 ' to 164" C.; 2,4,6-trinitro derivative: melting point, 265" C. (3, 6). tert-Butylbenzene: boiling point, 168' to 168.5' C.; 2,4-diaceti amino derivative: melting point, 210" C. ( 1 7 ) . 1,2,4Trimethylbenzene: boiling point, 170.5 to 171' C.; 3,5,6-tribromo derivative: white needles, melting point, 233 ' C. (18). sec-Butylbensene: boiling point, 174.5 to 175" C.; 2,bdiacetamino derivative: melting oint, 182" C. ( 1 7 ) . 1,4-Diethylbenzene: boitng point, 183-184" C. This fraction, which contained indene, was washed repeatedly with cold concentrated sulfuric acid, neutralized, and redistilled. Bromination with 4 moles of bromine gave the 2,3,5,6-tetrabromo derivative, needles of melting point of 112" C. from alcohol (20,40), but contaminated with a higher melting, alcohol-insoluble bromine I compound. O
-
I
CONTACT
Figure 6.
1
20
10
30
36
TIME (SEC)
Density of Liquid Product as Function of Contact Time
Treatment with 5% (by volume) of 90% sulfuric acid and refractionation in a good column (removal of a small amount of polymers) gives satisfactory products, free from olefinic material. In the case of toluene, for example, the refractionated product (ng = 1.49521)had the following characteristics:
Density dm Specificgravity 15.5'/15.5' DiBtillation, ' 6.
Obsvd. 0.8554 C . 0.870 110.75 110.88 Passes
O
-
Specification for Nitration Toluene 0.864-0.867 0.869-0.872 5-97% within 0.4O between 110" and 111" Not darker than 0.3 gram KzCrsO7 in 1000 cc. 50% (by vol.)
-
TOLUENE 33% BY VOL
BENZENE 35% BY VOL
I
Ca FRACTION. In this fraction (boiling point, 135' to 145" C . ) , too, the isolation of the aromatics is simpler, if one starts from a high density product. As an example, it is pointed out that for a crude aromatization product of the density 0.900 and 0.935, respectively, the CSfraction contains Density Paraffins, % Olefins, 7 Styrene S, Xylenes' and ethylbenzene, %
0.800 24 13 15 48
0.935 4 7 22 66
ALKYLBENZENES(145' TO 195" C. FRACTION).This fraction, which is usually deep yellow in color, roritains considerable quantities of unsaturated, polymerizablr substances which are no doubt aromatics with a conjugated double bond in the side chain. It was possible to isolate indene from a fraction boiling at 180" C. in the form of its picrate (melting point, 96" C.) (W) or of 1-(ahydroxybenzyl)-3-benzylideneindene (melting point, 135' C. ) (@), and from a fraction boiling at 165' to 167' C., a-methylstyrene (liquid dibromide, boiling point, 140-150"/15 mm.) (I,36'). As in the case of the CSfraction polymerization of these (and similar) substances takes place dur ng fractionation under atmoapheric pressure and is largely suppressed by distillation in vacuo (93)(Table VII). As to the saturated constituents of the alkylbenzene fraction, indications of the presence of the following substances have been obtained:
25%
50%
751
loo%
Figure 7. Distillation Curve of a Crude Aromatization Product Distillate up to 180' C.: 66%
RECYCLING OF THE ALKYLBENZENES.Further support for the assumption that the alkylbenzene fraction contains both polymethylbenzenes and benzene derivatives containing longer side chains is afforded by recycling experiments carried out with this material. It has been known that methyl groups are preserved under these conditions, while longer side chains are split off in form of olefins (37,44). Twenty-two per cent by weight of the charge was converted into gas. Forty-eight per cent of the liquid product boiled below, 34% above the boiling range of the charge, and 18% had retained the original boiling range, consisting, therefore, presumably of polymethylbenzenes. The following substances have been isolated from the lower and higher boiling fractions (all figures in per cent by weight of charge and calculated on pure substances in the appropriate cuts): Pre-benzene fraction Benzene Toluene, Cs fraction Naphthalene Alk lnaphthalene cut Ant5racene cut Chrysene
0.7 3.5 8.0 15.5 2.4 2.4 1.5 5.8
INDUSTRIAL AND ENGINEERING CHEMISTRY
2318
The source of the surprisingly large quantity of chrysene will be discussed in the following paper (48). ACKKOWLEDGMMENT
The authors wish to express their gratitude to H. E. Charlton, Chief Engineer of Petrocarbon Ltd., who collaborated in all questions of engineering and who was responsible for the design and construction of the pilot plant on which many of the results recorded in this and the following papers were obtained. LITERATURE CITED
Beilstein, “Handbuch der organischen Chemie,” Val. V, p. 483, Berlin, J. Springer, 1922; Suppl. Val. V, p. 233 (1930). Ibid., Suppl. Vol. V, p. 63 (1930). Blanksma, J. J., Rec. trav. chim.,21,327 (1902). Charlton, H. S., Manchester Association of Engineers,-1950. Diels. 0.. and coworkers. Ann.., 460. 98. (1928). ~ , Fittig, R:,Ibid.,141, 134 (1867). Geniesse, J. C., and Reuter, R., ISD. ENG.CHEM.,22, 1274 (1930); 24, 219 (1932). Gooderham, W. J., J . SOC.Chem. Ind.. 54,297T (1935). Groll, H. P. A., IND.ENG.CHEM.,25,784 (1933). Grosse, A. V.,U. S. Patents 2,124,566, 2,124,567 (July 26, 1938). Guttman, I,., Westrum, E. F., and Pitser, K. S., J . Am. Chem SOC.,65, 1247 (1943). Hague, E. S . ,and Wheeler, R. V., J . Chem. SOC.,1929,378. Harries, C., and T a n k , L., Ber. deut. chem. Ges., 41, 1701 (1908). Henriques, H . J., IND. ESG.CHEM.,39, 1564 (1947). Herington, E. F. G., and Rideal, E. K., Proc. Roy. SOC.(London) 184A, 447 (1945). Hooa, H . , Verheus, J., and Zuiderweg, F. J., Trans. Faraday Soi.,,35, 993 (1939). Ipatieff, V. N., and Schmerling, L., J . Am. Chem. Soc., 59, 1056 (1937 1. ,Jacobsen, O., Rm.deut. chem. Ges., 19, 1218 (1886). Joshel. L. AI.. and Butz, L. M., J . Am. Chem. Soc., 63, 3350 (1941). Klages, A . , and Keil, R., Ber. deut chem. Ges., 36, 1632 (1903). Koch, H., Brennsfof-Chem., 20, 1 (1939). Kraemer, G., and Spilker, A,, Ber. deut. chem. Ges., 23, 3276 (1890).
Vol. 43, No. 10
(23) Lummus Co., Brit. P a t e n t 590,590 (July 23, 1947). (24) MoldavskI, B. L., and Kamusher, H., Compt. rend. acad., sci. U.R.S.S., I , 355 (1936); Chem. Zentr., 1 9 3 6 , I I , 2339. (25) Moldavskl, B. L., etal., J . Gen. Chem. (U.S.S.R.), 7, 169, 1835, 1840 (1937); Chem. Zentr., 1 9 3 7 , I I , 1546; 1 9 3 8 , I I , 1023. (26) Morrell, J. C. (to Universal Oil Products Co.), U. S. P a t e n t s 2,124,583,2,124,585 (July 26, 1938). (27) Morrell, J. C., and Grosse, A. V., Ibid., 2,124,584, 2,124,586 (July 26, 1938). (28) Pitkethly, R. C., and Steiner, H., Trans. Faraday Soc., 35, 979 (1939). (29) Pitser, K. S., J . Chem. Phys., 5 , 469, 473 (1937). (30) Schneider, V., and Frolich, P. K., IND. ENG.CHEM.,23, 1405 (1931) (31) Shuikin, N. J., Uspekhi K h i m . , 15, 343 (1946). (32) Steiner, H., J. Am. Chem. SOC.,67,2052 (1945). (33) Steiner, H., J. I n s t . Petroleum, 33, 410 (1947). (34) Steiner, H., and Rowley, D., Trans. Faraday SOC., in press. (35) Taylor, H. S., and Turkevich, J., Ibid., 35, 921 (1939). (36) Tiffeneau, M.,Compt. rend.. 134, 846 (1902); Ann. chini., [8I 10,166 (1907). (37) Tilicheev, M. D., and coworkers, Ber. deut. chem. Ges., 62, 658 (1929); Khim. Tverdogo Toplica, 8 , 548, 617, 876 (1937); reviewed in Oil Gas J . , 39, No. 40, 41. 45, 46 (1941). (38) Tropsch, H., and Mattox, W.J., IND.ENG.CHBM.,AN.IL. ED., 6, 104 (1934). (39) Turkovich, J., Fehrer, H., and Taylor, H. S., J . Am. Chern. Soc., 63,1129 (1941). (40) Voswinkel, A,, Ber. deut. chem. Ges., 22, 315 (1889). (41) Weger, M., and Billmann, A,, Ibid.,36, 640 (1903). (42) Weizmann, C., et al., Brit. P a t e n t 552,216 (March 29, 1943): Ibid., 574,963, 574,973 (Jan. 29, 1946); Ibid., 575,383 (Feb. 15, 1946); Ibid., 575,766, 575,768, 575,771 (March 5, 1946). (43) Weismann, C., et al., ISD.ENG.CHEM.,43,2318 (1951). (44) Ibid., p. 2322. (45) Weismann, C . , et al.,J . SOC.Chem. Ind., 67, 114 (1948). (46) Weismann, C., et aZ., U. S.P a t e n t 2,349,781 (May 23, 1944); Ibid., 2,397,715 (April 2, 1946). (47) Wheeler, R . V., and Wood, W. L., J . Chem. SOC.,1930, 1819.
.
RECEIVED December.18, 1947. Contribution from the laboratories of The Weizmann Institute of Science, Rehovoth, Israel: Petrocarbon Ltd., London Bridge, London E.C. 4, England; and the Grosvenor Laboratory, 25 Grosvenor Crescent Mews, London S.W. 1, England.
( A romatizing Cracking of Hydrocarbon Oils)
POLYCYCLIC FRACTIONS WITH
CHAIM WEIZMANN, ERNST BERGMe4NN, H. S. BOYD-BARRETT, HERBERT STEINER, AND MAX SULZBACHER, J. R. HOLKER, ERNA MANDEL, JOHN PORGES, AND DERRICK ROWLEY The Weiamann Institute of Science, Rehowoth, Israel
T
HIS paper deals with those constituents of the aromatization
product of a naphthenic naphtha described in the preceding paper, which boil above approximately 200’ C. [see Table I in (64)],with their nature, their identification-as far as it has been achieved-and the presumable mechanism of their formation. NAPHTHALENE AND ALKYLNAPHTHALENE FRACTIONS
Table I lists the constituents of these fractions. The tetrahydronaphthalene contained in this fraction was accompanied by other hydrocarbons, some of them saturated, some unsaturated and polymerieahle t o a viscous oil, possibly 1,2-dihydronnphthalene (22, 60). The naphthalene itself had a high degree of purity; when washed free from adherent oil with isopropyl alcohol, treated with a small amount of aluminum chloride (0.1 to 0.2% by weight) at 200 C., washed with sodium hydroxide solution, and redistilled, it met the strictest specifications.
For the isolation of the alkylnaphthalenes, use was made of the picrates. Hoxever, it has to be borne in mind that, more frequently than generally known, picrates of similar hydrocarbons tend to form mixed crystals. This has been observed for the following pairs: Naphthalene and 1-methylnaphthalene 1-Methylnaphthalene and 2-methylnaphthalene 2-Methylnaphthalene and 1,2-dimethylnaphthalene 1,2-Dimethylnaphthalene and 1,2,3-trimethylnaphthalene 1,2,5-Trimethylnaphthalene and 1,2,5,&tetramethylnaphthalene (60) Phenanthrene and 9-methylphenanthrene (9, SI, 61) Regarding the di- and trimethylnaphthalenes, experience has shown that the 2,6-compound, a mixture of the 1,6- and 1,T-compound, and the 1,2-compound can be separated from each other to a considerable extent by fractional distillation, the 1,2-compound-like o-xylene-showing the highest boiling point.
