CATALYTIC CRACKING OF PURE HYDROCARBONS Cracking of Olefins B. S . GREENSFELDER AND H. H. VOGE
Shell Development Company, Emeryville, Cal(f.
Cracking over a silica-zirconia-alumina catalyst at 350300" C. of seven aliphatic olefins, two diolefins, two cyclic olefins, and two aromatic olefins was studied. The principal conversions of aliphatic mono-olefins were isomerization, cracking, saturation, and formation of higherboiling materials and coke. Diolefins and aromatic
olefins were extensively saturated and transformed to higher-boiling materials and coke. Catalytic and thermal cracking of olefins at 4OO-50Oo C. are compared. The catalyzed reactions, which are of the order of 1,OOO to 10,000 times as fast, yield larger fragments, involve much more isomerization, and lead to greater saturation of product.
C
distilled; it boiled at about 177-182" C., d:' 0.9952, na: 1.5748, and bromine number 135. Triisobutenes were from "cold acid" polymerization (Shell Chemical Company); they had a Cottrell boiling point of 183' C., di0 0.768, nai 1.4325, and bromine number about 86. Cetene (Bataafsche Petroleum Mmtschappij laboratories, Delft) had a boiling point of about 280' C., melting point of 4.05' C., dt0 0.7813, n y 1.4409, and bromine number 69.3.
RACKING of paraffins over a silica-zirconia-alumina catalyst under conditions similar t o those employed in the commercial cracking of petroleum fractions was described in the 6rst article of this series (6). The behavior of olefins was also studied because olefins are important primary products of the cracking of saturated hydrocarbons, and because olefin-containing fractions from either thermal or catalytic cracking are sometimes reprocessed over a cracking catalyst. The first tests with olefins over a cracking catalyst showed them to be very reactive, in accord with the findings of Egloff and co-workers (4). Because of this high reactivity, the relatively low temperature of 400"C. was employed for most of the experiments with olefins, rather than 500 C. which was used for cracking paraffins. The apparatus, cataIyst, procedure, and terminology were the same as described previously ( 6 ) . The catalyst, obtained from Universal Oil Products Company, analyzed 86.2% silica, 9.4% zirconia, and 4.3% alumina by weight. As stated before ( 6 ) ,it has been found to give essentially the same results as the synthetic silica-alumina catalysts used in present commercial practice. Properties and sources of hydrocarbons follow, with compounds arranged in the order of increasing molecular weight. Bromine numbers are grams bromine per 100 grams hydrocarbon by the Rosenmund method. Ethylene (Ohio Chemical Company) was over 99% by gas analysis. Butadiene (Shell Development Company's pilot plant) contained 98.3q;b conjugated dienes. +Butenes were made by dehydration of see-butyl alcohol; they analyzed 8.0% 1-butene, 92.0% 2-butenes, and O.Oyoabove C,. Isoprene was a crude material containing 70% conjugated diolefins and 26% other olefins. n-Pentenes were from the dehydration of sec-amyl aIcohol over alumina; the boiling range was 90-37' C., d:' 0.6519, n y 1.3792, bromine number 233. Cyclopentene was derived from cyclopentyl chloride, obtained by chlorination of a petroleum CSfraction. It had a boiling point of 44.4" C.,':u 0.7701, n22 1.4217, and bromine number 237. Cyclohexene (Dow Chemical Company) had a boiling range of 83.184.2' C.,u:' 0.8107, n2," 1.4467, and bromine number 195. Styrene (Dow Chemical Company) had a boiling point of 145.5' C., d:' 0.9055, n2: 1.5455, and bromine number 150.5. Diisobutenes were from "cold acid" polymerization (Shell Chemical Company), The A.S.T.M. 5 and 95% distillation temperatures were 101.4"and 104.3" C., ':u 0.7204, n'f 1.4105, and bromine number 123. n-Octenes (Eastman Kodak Company) were fractionally redistilled. The boiling range was 123.4-126' C., di0 0.7205, n2i 1.4135, and bromine number 144. Indene (Kahlbaum) RMfreshly
CRACKING BEHAVIOR
Aliphatic olefins with carbon numbers from 2 to 16 were examined, as well as several diolefins and cyclic olefins. The term "carbon nuwber" is employed to designate the number of carbon atoms per molecule. A temperature of 400" C. was normally used for aliphatics and of 500" C. for others. Discussion of results with the diolefins and cyclics follows that for the aliphatic olefins. The basic data for aliphatic mono-olefins are given in Tables I and 11, and those for the remaining compounds in Table 111. ETHYLENE. Ethylene did not react extensively a t 400" C.; there ww about 6% saturation and 3% polymerization to C4, No liquid product was obtained. The carbon deposit was hi&, 9.6% by weight of the feed. Thus the chief reactions were decomposition to carbon, saturation, and polymerization. ~-BUTENES.Experiments with mixed n-butenes agreed in a general way with those reported by Egloff and co-workers (4). The reactions over the catalyst involve cracking, isomerization, saturation, and formation of higher-boiling materials. The run at 500" C. is of interest in showing extensive conversion to isobutane. Isomerization and saturation were less marked a t 400 " C. and the longer process period. n-PENTENES. Data for the cracking of n-pentenes show extensive reaction. Olefin contents of the 20-30" C. and 35-38' C. fractions were 42 and 91%, respectively; since these fractions were 18 and 25 volume % of the total liquic!, there was substantial production of isopentane and much less of n-pentane. This provides another example of preferential saturation of isodefins (6). The higher-boiling liquid product above 90" C. was 20% of the total liquid and had na: of 1.4733, which indicates the presence of aromatics. I n general, reactions of pentenes resemble those of butenes. ~QCTENES. Products from n-octenes were about equally divided among gas, lower-boiling liquid, and higher-boiling material. Decomposition was extensive, since only 28% of C, material was recovered. The main gas products were propylene, bu983
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
984
t,enes, and butanw. The large amount of isobutene is noteworthy. Lower-boiling liquid was largely C', 72% olefinic. The uncracked Ch fraotion was 40% saturated and boiled below the original octenes, whioh indicates isomerization as well as saturation. Material above Cp had the moderately high refractive index of 1.4534, indicative of aromatics. Cracking characteristics of the n-octenes are. rapid decomposition under mild conditions, saturation of uncracked octenes and cracked products, isomerization, and formation of higher-boiling hydrocarbons. The experiments of Egloff and co-workers (4) led to similar conclusions, except for the saturation which was not examined. A later paper from the same laboratories by Thomas demonstrates the extensive saturation in detail (14).
TARLE I. CAZ'ALYTIC CRACKlNG L'emperature, O C. LHSV" Flowrate, moles/l:/hr. Process period, min. Gaseous roduct MolesPmole charge Vol. % ' HI C He CaHc CaHe CIHI CaH( iso-CkHa n-CIHs ino-CkHis n-CiHia Material balance. wt. % of charge LowerCNo.,asgas Lower C No., aa
unc liPd an
Ethvlene 400
... 7.05 60
.. .
0.5 1.0 91.3 5.5 0.2 0.3
.
OF
LOWERALIPHATIC OLEFINS
n-Butenes n-Pentsnes 400 5'00 400 0.77 0.63 0.74 8.3 6.8 6.9 30 15 60
... ...
1.4 9.5 0.2 13.7 0.2 3.3 0.0 3.7 6.1 18.3 1.3 7.2 7.2 4.1 75.7 8.3
0.5
cracking, and the major portion of the cetene not saturated wa.. cracked. This is interpreted as meaning an extensive saturation of higher olefins, together with rapid decomposition of the higher olefins which have not been saturated. The presence of isobutene in the gas again indicates extensive isomerization accompanying the cracking process. Further evidence is found in the lowered and wider boiling range of the recovered CISfraction. I n distillations at 15 mm., cetene boiled at 156" C. and cetane a t 158" C.; but 60 to 80% of the recovered Cla material from cetene cracking experiments boiled at 135150" C. Such isomerization does not occur with paraffins; when cetane was cracked, the recovered Cle material boiled at the same temperature as the feed. The study of cetene cracking by Egloff and co-workers (4) yields more information on the branched compounds, which usually exceed 70% of the individual product fractions. Their work indicated that the isomerized CISolefin cracks more rapidly than cetene, and led to a proposed description of the cracking [nvolving isomerization to a branched olefin, cracking of the isomer at all bonds except those near t o the extremities, and further reaction and isomerization of cracked products. BUTADIENE.The reactions of 1,3-butadiene at 400' C. (Tablr III) were extensive, and only 4.870 of the butadiene charged wa* recovered as such. Saturation, carbon formation, and polymerization were pronounced. The liquid product (33% of the
TABLE 11. CATALYTIC CRACKING OF HIGHER ALIPHATIC OLEFINS
. 4.4 22.8
15.4
edCNo. 87:O 69:2 38.2 6i*,2 Higher No.,liquid or gas 2.9 20.3 20.7 27.4 Carbon Qi6 6.1 14.4 BiO Loss d 2.0 0 Liquid hourly space velocity. b Total olefins C No. 3.7. C Total saturaiea, C No. 3.8. d Input not known accurately; balance on output bask.
8
n-Octenee 400 1.07 6.9 60
Vol. 37, No. 10
22.4 18.6 28.0 19.5 3.8 7.7
~~
DIISOBUTENES. Several experiments were made with diisobutenes, which are chiefly 2,4,4trimethyl-l-penteneand 2,4,4 trimethyl-2-pentene. At 450" C. and a flow rate of 6.6 moles per liter of catalyst per hour, decomposition exceeded 90%. Table I1 gives data for another run at 400" C. with a flow rate of 12.9 moles per liter per hour. Even under these milder conditions, decomposition to gas was extensive, and the liquid recovery was o d y 20.4% by weight. The principal product was isobutene, but a moderate amount of higher-boiling product was also formed. TRIISOBUTENES. At the low temperature of 350" C. triisobutenes (presumably 2,2,4,6,6-pentamethylheptenes)were almost entirely decomposed. The chief product was gas, largely isobutene. There was also a moderate amount of lower-boiling Liquid, distilling mostly in the Ce-C? range and containing 90% olefins. Remaining liquid above 150" C. was only 7.7% by weight of the charge and could have contained little unchanged ,''n was 1.5002. triisobutenes because the refractive index, ~ E T E N E(12-HEXADECENE). Table I1 shows results of cracking at 400' and 450' C. Data obtained by Egloff and co-workers (4) are also given. Using higher flow rates and a longer process period, they observed a similar extent of cracking, but their conditions caused a lower production of gas, a higher olefin content in the gas, and greater formation of gasoline. Cetene cracks readily over the catalyst and is much more susceptible to the catalysis than the corresponding paraffin, cetane. Olefin contents of fractions from cetene cracking show an overall decline as the molecular weight increases. Table IV gives the distributions and olefin contents of fractions. Of particular significance is the low olefin content of the recovered CISfraction. Saturation of cetene evidently proceeded concurrently with the
Diisobutenes Origin of data This work Tern erature, C. 400 LH& 2.0 Flow rate, moles/lJhr. 12.9 Process period, nun. 60 Gaseous roduct Molearmole aharge 1.27 0.3 Vol. % Ha CHd 0.6O CaHc 0.1 CaHI CIHI .4.0 0.5 CIH~ iso-CeH~ 78.0 n-CdHs 7.5 14.0 CIHI~ Material balance, wt. yo of charge Gas 63.1 Liauid, lower C No. 5.4 LI uid unchanged Nb. Liquid,higherCNo. Carbon 1 Loss 14.7 Including C:Ha. b Not including CO.
...
'9:
;:!
Triisobutenes
350 2.9 13.4 60
I:{
Y C e t e n e -This work 400 450 2.0 2.0 6.8 6.8 60 60
(4) 450 4.0 13.9 167
81.4 13.6 4.4
0.384 2.2 4.7 1.4 1.0 23.6 4.9 11.7 22.7 27.8
0.935 2.8 2.2 1.5 2.0 29.2 5.1 14.8 22.6 10.8
32.8 3.2 27.4 29.8 6.9
61.2 25.1
8.5 49.4
20.2 59.4
14.1 67.6
1.80
{7.7) 2.2 3.8
'96:; 1.2, ::!1 . l b 4.7
4.7
0.615 0.0 0.0 0.0 0.0
{I4.'
...
4.3
charge) was about 5Oy0 CS material, and the CI material contained 36% olefins, computed as mono-olefins (perhaps the butadiene dimer, 4vinylcyclohexene) and about 50% aromatics. IsoPRENE. An isoprene-olefin mixture containing 70% conjugated diolefins likewise underwent extensive reaction when treated over the cracking catalyst at 400" C. The recovered CS material was 31.7% of the charge but contained no conjugated diolefin and 64.5 % mono-olefin. The higher-boiling material (32.1% of the charge) consisted of 25% Ce-Cs, evidently a mixture of olefins, saturates, and aromatics; 55% Clo with n' of 1.4980 and containing only 5% olefins; arid about 20% above Ctowith ;'n of 1.5980. The complete disappearance of all diolefin, the heavy carbon formation, and the high yield of aromatic condensation products are the chief features of interest in this experiment. STYRENE. When styrene was passed over the catalyst at 500" C., a small amount of gas waa produced and some benzene resulted, but the principal reactions were deposition of carbon on the catalyst and saturation of styrene to ethylbenzene, together with formation of higher-boiling condensation products.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
October, 1945
boiling at 45-50 " C. contained 48% olefins, whereas the uncracked fraction from cyclohexene contained only 15%. The material boiling above 110' C., recovered in yield of 32%, waa quite aromatic (w*i 1.5508) and had a bromine number of 20.
TABLE 111. CATALYTIC CRACKINQ OF CYCLIC OLEFINS,STYRENE, AND DIOLEFINS Tern erature.'" C. LHS\ Flow rate, moles/l./ hr. lroceas period, min. Oweow roduct Mo~es?mole charge Vol. % Ha CHb C:H4 CaHs CaHa CIHI CdH* k-CbH1 n-CdH, CcHie Total olefins Total saturates Msterial balance. wt. % of charge LowerCNo.,aagaa Lower C No., aa li uid Unc%an ed C No. Higher No. Carbon
8
LOSS
Cyclo-
Cyclopentene
styrene
Indene
500 1.4
500
1.2
600 1.6
1.4
13.9 60
13.8 60
14.1
12.0
0.410 39.5 16.3 4.9 5.8 9.4 7.a
0.263 43.0 12.0 6.7 7.8 9.4 7.8
1.4 3.9 11.6
1.5 1 .8 10.0
...,
.... ....
....
.... ..
0.036 41.7
0.&7 65.1
.... ....
. , ., .... .. .,
0.458 1.2 3.0 1.8 5.6 7.3 2.9 10.4
..(. .... .... .... .... ....
15:s'
.... .... .... ... .... I
8.3
0.4
0.0
4.8 66.7
1.2 2.9 48.8
36.8
8.1
l!:i
6.5
S.6 64.3 10.9
ii.'a
ai.6
4::;
8.1
QO
42.6
3::;
7.3
48
60
,.
11.7 6.1 37.0
:$$400
400 0.65
0.72
REACTIVITY OF OLEFINS
7.2
60
The normal aliphatic olefins are remarkably reactive over the cracking catalyst, and it is clear that, when formed aa primary products from the cracking of saturates, they must undergo extensive secondary reaction. Figure 1 shows the variations of reactivity and products within the homologous series of n-olefins, CnHn,. Under the k e d conditions employed-namely, temperature of 400' C. and flow rate of about 6.8 moles per liter per hour formation of products with a lower carbon number than the feed increased rapidly with molecular weight. The formation of products with a higher carbon number reached a maximum at about CS. Carbon formation was relative11 high throughout but the percentage of the feed converted to carbon declined with increasing carbon number. Ac the molecular weight of the feed stock rose, the olefir) content of the recovered material with unchanged carbon number decreased rapidly, as shown below:
0.191 9.3
.,,. .... .... ..... . .. , . .. .... .. ..
... . ,...
3i:i* 65.1
6,g
12.4
,..,
985
....
>:!:
!:;,
37.3
31.7
