Reactions of Hvdrocarbons over Cracking Catalysts E. R4. GLADROW, R. W. KREBS, AND C. N. BIMBERLIN, JK. Esso Laboratories, Esso Standard Oil Co., Louisiana Division, Baton Rouge,La.
T
HE increased demands placed on the petroleum industry in recent years for high quality motor fuels have emphasized the need for the development of catalysts which give greater yields of the desired products. A fuller understanding of the mechanisms involved in catalytic cracking and a closer examination of the differences among the products of cracking with presently available catalysts would be of aid in this development. The present investigation of the cracking characteristics of catalysts employs the use of individual hydrocarbon feeds of several types and catalysts of different composition.
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and coke are low. The thermal type of decomposition can be catalyzed by several materials, for example, silica gel ( 3 ) . The product distribution and product quality when using silica gel approximate that from the straight thermal process. Mixed metal oxide cracking catalysts, such as silica-alumina, silica-magnesia, alumina-boria, acid-treated clays, etc., are generally considered to catalyze reactions through a carbonium ion mechanism. This notion was prompted by the acidic nature of these ratalysts and the fact that they promote a number of the same reactions at high temperatures that are catalyzed by strong acids a t relatively low temperatures. Furthermore, there has recently been some experimental evidence of carbonium ions formed between strong acids and hydrocarbons ( 5 ) . The catalytic property of the mixed oxides is ascribed to their surface acidity which apparently arises from the combination of the components; for example, neither pure silica gel nor magnesia are acidic, but when combined in a manner to make a catalyst, the composite displays surface acidity which can be titrated (9). When the same feed stock is used, the distribution of the products according to carbon number shows a marked difference between the free-radical and the carbonium ion-type catalysts (5). This is shown in Figure 1, drawn from the data of Table I, rhich gives the distribution of the lighter products from cracking cetane thermally and catalytically. The gasolines produced by these two methods from the same feed stock also differ markedly in their molecular weight distribution and in chemical composition. Although the mixed oxide catalysts closely approximate each other concerning the primary cracking reaction and the types of secondary reactions they promote, certain fundamental differences must exist to account for the well-known differences in yield and quality of gasolines produced by these catalysts from the same feed stock. Because of these differences, an investigation has been made to determine their source. The experimental work comprised cracking individual hydrocarbon feeds over a variety of natural and synthetic cracking catalysts. A detailed analysis of the data indicates that the differences between catalysts lie in the intensity of the action a t the “active catalytic centers.” It will be shown that all of the observed differences can be explained from a hypothetical picture of the
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TABLEI. PRODUCT DISTRIBUTION FROM THERMAL AND CATALYTIC CRACKING OF CETANE
CI c2 c3 c4 c5 CARBON ATOMS PER MOLECULE
(Temperature, 950’ F. ; 2-hour process period) Catalyst Silica None gel Space velocity, vol./vol./hr. 0.33 1.0 Conversiona, % 22.4 29.2 Product distribution, moles per 100 moles of cetane cracked 6 6 Ht 66 37 CH4 164 Q9 C2 64 62 c3 26 39 C4 15 25 CS
Figure 1. Molecular Weight Distribution of Products from Cracking Cetane with Various Catalysts
I n general, most cracking reactions are believed to proceed by one of two reaction paths. Thermal decomposition of hydrocarbons has been shown to folloIv a path in which the reactant forms a free radical which then decomposes to give a primary olefin and another free radical (6, 7 ) . I n this process, the products from cracking normal paraffin feeds are predominantly olefinic, with the Cz and other gaseous fractions being the most abundant. The yields of branched-chain isomers, aromatics,
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January 1953
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quantitative identification of as many of the cracked products as practical. The individual components in the molecular weight range from hydrogen to CbHlz were determined by a combination of mass spectrometer, low temperature Podbielniak distillation, and infrared absorption analyses. I n this manner, a complete analysis for the individual CCand Cg olefin and paraffin isomers was effected. The Ce+ liquid product was fractionally distilled in a 30-plate column. Cut points were appropriately chosen with each feed so that reasonably accurate values for the composition of the c6+ material were obtained. Hydrocarbon-type analyses of the liquid fractions were made by mass spectrometer and a modification of the method of Grosse and Wackher ( 4 ) , using correlations involving boiling point range, specific gravity, bromine number, specific dispersion, and refractive index. The carbon deposit on the catalyst was determined by combustion.
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20% CONVERSION
TABLE 11. CRACKING OF CETANE OVER MIXEDOXIDE H2
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Figure 2.
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cIo-,2 CUT Product Distribution from Cetane Cracking over Siliea-Alumina Catalyst IS O N E - T H I R D OF
%hourprocess period; temperature, 950' F.
carbonium ion complex which is compatible with the general reaction pattern. EXPERIMENTAL
MATERIALS.Of the many catalysts examined, the following are considered as typical of the mixed oxide type: 1. Silica-alumina (13% A12O3): surface area, 417 square meters per gram (BET); pore volume, 0.35 cc. per gram; specific acidity, 0.25 meq. per gram; bulk density, 0.70 (pellets). 2. Silica-magnesia: surface area, 408 square meters per gram; pore volume, 0.25 cc. per gram; specific acidity, 0.48 meq. per gram; bulk density, 0.82 (pellets). 3. Acid-treated clay: surface area, 332 square meters per gram; pore volume, 0.31 cc. per gram; specific acidity, 0.26 meq. per gram; bulk density, 0.79 (pellets). 4. Alumina-boria: surface area, 340 square meters per gram; pore volume, 0.40 cc. per gram; bulk density, 0.79 (pellets). The catalyst used for promotion of the thermal-type cracking reaction was silica gel, specially purified, from the Davison Chemical Co. This material had the following properties: surface area, 652 square meters per gram; pore volume, 0.38 cc. per gram; specific acidity, 0 meq. per gram; and bulk density, 0.70 (pellets). The catalysts were used in the form of 3/16 X '/I6 inch cyiindrical pellets. The hydrocarbons used are tabulated in the following table: Hydrocarbon
Source
Westvaco Chlorine Products n-Heptane Eastman Kodak Co. n-Decane Hydrotrimera Laboratory preparation D u Pont Co. n-Cetane a Hydrogenated triisobutylene.
Spec. Gr. Bro- Refractive (60" F./ mine Index, 60° F.) No. ngo 0.6886 0.7304 0.7401 0.7796
.00. 0
1.3878 1.4110 1.'4349
CRACKING APPARATUS.The principal piece of equipment was a stainless steel reactor having an inside diameter of 1.6 inches and an isothermal heated zone of about 12 inches. Heat was supplied to the reactor by a metal block heater so that temperatures within the reactor were constant within 10' F. This size of the reactor enabled use of a 200-ml. charge of pelleted catalyst and allowed a suitable vaporizing and preheating zone for the feed. The feed was passed through the reactor a t the desired flow rate and the reaction products were quenched in a cold receiver. ANALYTICAL PROCEDURES. Emphasis was placed on the
CATALYSTS
(Temperature, 950' F.; space velocity, 2 vol./vol./hour; 2-hour period) Catalyst AcidSilicaSilicatreated Aluminaalumina magnesia clay boria Conversiona, % 45.0 61.2 42.1 57.2 Product distribution, weight % of converted feed 0.11 0.06 0.10 0.31 HZ 1.6 1.2 1.9 1.5 C Ha 3.1 3.3 6.5 3.3 C2 23.6 11.9 16.2 17.4 CS 32.7 23.3 22.8 26.8 C4 17.8 18.3 16.0 17.5 CS 18.0 40.8 35.3 29.5 Ce-430' F 3.1 1.1 Carbon 1.2 3.7 Analysis of Cs-430' F. fraction, volume % Olefins 46.2 53.4 46.8 37.2 Aromatics 10.5 4.5 5.3 6.3 i-C4Ha 0.42 0.36 0.39 0.47 Ratio t o t a l 8 i-C4Hio 0.72 0.63 0.68 0.69 Ratio t o t a l 0 a Per cent of feed converted to coke and products boiling below 430' F
PRESENTATION OF DATA. The cracking behavior of the saturated hydrocarbons was determined a t 950' F. and atmospheric pressure. Unless otherwise specified, the process period was 2 hours. Most of the experiments were conducted with cetane feed; the other hydrocarbons were used to test hypotheses suggested by the results with cetane. The silica-alumina catalyst previously described was chosen as the standard catalyst and suitable correlation curves mere made to cover a conversion range from 13 to 73%. Linear relationships were found between the yields of the CS,Cd, Cg, and Ca to 430' F. fractions and the degree of conversion of the cetane feed to coke and products boiling below 430' F. Data from these correlations have been composited to construct the family of curves shown in Figure 2. The most striking features are the similarity among the curves over the entire conversion range and the definite existence of two maxima. The smaller peak shown a t CS may actually have its maximum a t C ~ Oas, the value plotted a t C ~ is O arbitrarily taken as one third the total Cl0-1~ composite fraction. Of the products formed, only those postulated as being formed by the carbonium ion mechanism in large amounts showed the linear yield-conversion relation. The CI and CZfractions, which have greater energy requirements for their formation by beta fission from the carbonium ion, were produced in relatively decreasing aniounts as conversion increased. Similar sets of curves have been constructed from results obtained with other catalysts. Typical results of cracking cetane using the mixed oxide catalysts are shown in Table 11. These experiments were all conducted with a feed rate of 2 volumes of hydrocarbon per hour per volume of catalyst. The product distributions from these
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catalysts are similar in that pronounced yield maxima are obtained at carbon number four. It appears that the silica-alumina and silica-magnesia catalysts represent tm-o extremes among oxide-type catalysts in product yields and quality. I n most instances, the other catalysts are intermediate.
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PER Figure 3. Product Distribution from Cracking Cetane over Silica-Alumina and Silica-Magnesia Catalysts 2-hour process period; 8.0 volumes/volume/hour; 950' F. Correlated data a t 20 Yo conversion
temperature,
Because the silica-alumina and silica-magnesia compositions apparently represent extremes as mixed oxide-type catalysts, experiments with additional feeds were conducted with only these two materials. The activities of these two catalysts for the conversion of paraffins of different chain lengths are compared in Table 111. The degree of isomerization in the Cq fraction from cracking cetane over a wide range of conversions with these two catalysts is s h o r n in Table IV. Tests were also made with hydrogenated triisobutylene to compare the degree of isomerization in the cracked products from this highly branched feed, These results are contained in Table I-. The results of the cracking tests will be discussed in the succeeding section. DISCUSSION OF EXPERI3fEYTAL RESULTS
Vol. 45, No. 1
more titratable acidity for the same catalytic activity in the conversion of cetane. If the lower average nlolecular weight of the cracked products, from silica-alumina results from a greater intensity of action as postulated above, it should follo~v t,hat the relative activity of silica-alumina as compared to silica-magnesia for the conversion of feed molecules should increase as the refractory nature of t h e feed is increased. It is conceivable that a very refractory feed might be converted to some extent by a catalyst containing only a small number of highly active centers and not at all by a catalyst containing a large number of weakly active centers; on the other hand, a very easily cracked feed might be more extensively convert,ed by the' catalyst containing the greater number of active centers. To test this hypothesis n-heptane, n-decane, and cetane were cracked under identical conditions over silicaalumina and silica-magnesia catalysts. With normal paraffin feeds, refractoriness increases as the chain length diminishes. The conversion data given in Table 111, presented graphicalljin Figure 4,show the inversion of the relative activities of these two catalysts as the refractoriness of the feed is increased. I t is believed Ohat these ob~ervationslead to the hypothesis of a more intensive, in contrast to extensive, cracking action on the silicaalumina surface and fully explain the behavior of these catalysts in commercial cracking relative to the degree of conversion of feed and the yield and volatility of the gasoline product (8). The quality of the products from cetane cracking ca.n be partially judged by their olefin and aromatic contents. If the assumption is correct that the initial cetyl carbonium ion cracks to form an a-olefin and a new carbonium ion, then that catalyst which produces a high olefin, low aromatic content product ciin be regarded as being less active than the catalyst which produces relatively less olefins and more aromatics. This follows from the belief that aromatization is a multistep process including polymerization, cyclization, and dehydrogenation of olefins (IO). It can be seen from Figure 5 that silica-alumina catalyst produces a more aromatic naphtha while silica-magnesia catalyst yields a more olefinic product. The presence of branched-cl~ainisomers among the products from cracking straight-chain hydrocarbons over mixed oxide catalyst,s vas first reported by Egloff (1). It has been postulated
TABLE
111. CONVERSIOK
O F S O R M S L ~'ARAPFINS OXIDE CATALYSTS
Tempesature, 950' I:.; spaci: 7-rlocity, 1 vol./vol./hr.;
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Conversiona, % A close examination of the experimcntal data reveals a number Silica-alumina Silica-magnesia of differences among the catalysts. The cetane results with catalyst catalyst l'eed silica-alumina and silica-magnesia catalysts show a pronounced 18.2 17 -Heptane 26 8 ,?-Decane 39 3 37.7 difference in the molecular weight distribution of the products. 73 4 79.7 Cetane as is illustrated in Figure 3. This figure employs 100 minus 7*of reco\ered fecd. correlated data for each catalyst a t a 2070 conversion level. The fact that silica-alumina gives a T A B L E II-, ~10121
