Development of a Reforming Catalyst M. J. FOWLE, R. D. BENT, F. G. CIAPETTA, P. M. PITTS, and L. N. LEUM
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The Atlantic Refining Co., Philadelphia, Pa.
A catalyst for the reforming of naphthas has been developed which provides an excellent octane-yield relationship and can be operated without regeneration for long periods of time. This development started with the discovery of a catalyst for the isomerization of paraffin and naphthene hydrocarbons which was composed of a hydrogenating component, such as nickel or platinum, deposited on silica-alumina. The early catalyst would promote isomerization at nominal temperatures but was much too active as a cracking agent at temperatures required for aromatic formation. By markedly decreasing the surface area of the silica-alumina component, a reforming catalyst was developed that gives the proper balance and direction for the main reactions of reforming: isomerization, dehydrogenation, and hydrocracking.
In the last 15 years of petroleum technology, reforming has moved from an operation of questionable necessity to a must for most refiners. This paper summarizes part of the chemistry of reforming and then shows how the authors have developed a new catalyst for this operation. Reforming a naphtha to a higher octane rating must involve at least one of the following chemical reactions: (a) production of aromatics, (6) production of highly branched paraffins, (c) production of olefins, or (d) lowering the molecular weight of the hydrocarbons i n the naphtha. Thermal reforming relies largely on molecular weight reduction to obtain antiknock improvement. I t forms some aromatics and olefins, but, i n general, i t is a brute force method of bringing about chemical transformations and does so at the expense of forming undesirable quantities of gas and high boiling material. The early hydroforming catalysts were a big step forward i n directing the chemical reactions of reforming to a desired end—namely, the formation of aromatics. These catalysts, however, were far from perfect. They required frequent regeneration with the attendant high plant investment cost and they fell short i n octane number improvement due to a lack of isomerization ability. The ability of a catalyst to promote isomerization plays two roles i n reforming: i t increases the amount of branched chain paraffins i n the product and it converts naphthene hydrocarbons with cyclopentane rings into cyclohexane ring naphthenes which are necessary for the formation of aromatics by dehydrogenation. C C—C—C—C—C—C 5 = ± C—C—C—C—C
C C C—C—C—C
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In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.
FOWLE et α/.—DEVELOPMENT OF A REFORMING CATALYST
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C
The octane number improvement obtained by isomerization of paraffin hydrocarbons is not great since the amounts of the more highly branched paraffins formed at equilibrium are small at the temperatures employed in catalytic reforming (6). Naphthene isomer ization, on the other hand, plays a more important role i n reforming. I n most naphthas about 5 0 % of the naphthene hydrocarbons are of the cyclopentane type (4) so that i n order to obtain the maximum aromatic formation, isomerization of these rings to cyclohexane rings must be promoted by the catalyst.
500
600
700 800 900 1000 Temperature °F. Figure 1. Methylcyclopentane-Cyclohexane and Cyclohexane-Benzene Equilibria A t temperatures where substantial dehydrogenation of naphthene hydrocarbons to aromatics can take place, the isomerization equilibrium of cyclopentane naphthenes to cyclohexane naphthenes is greatly i n favor of the cyclopentane compounds. However, the dehydrogenation of the cyclohexane rings superimposes its equilibrium on the former reaction giving an over-all conversion of the alkyl cyclopentane hydrocarbons into aro matics. This is illustrated i n Figure 1. I n this figure, the calculated equilibrium concen trations of cyclohexane formed from methylcyclopentane (1) and of benzene from cyclo hexane (7) are plotted against the reaction temperature. Experimental data have i n d i cated that the benzene-cyclohexane curve i n Figure 1 may be somewhat i n error since higher yields of benzene from cyclohexane have been obtained at 800° to 900° F . than that predicted b y this curve. The Platforming catalyst is a vast improvement over the older hydroforming cata lysts. I t produces less carbon so that frequent regeneration is unnecessary and i t pro motes the isomerization required for extensive aromatic formation. I n one way of looking at the development of this catalyst, assuming that its composition is that described i n the patent literature (6), one can say that Haensel and his associates started with a dehydrogenating component on a carrier that is relatively inert for isomerization and crack ing and added an isomerization-mild cracking component. Over the past several years, the authors have also developed a reforming catalyst that simultaneously isomerizes, In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.
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ADVANCES IN CHEMISTRY SERIES
dehydrogenates, and cracks. The history of its development, which will be the subject of the remainder of this paper, is about the reverse of that of the Platforming catalyst i n that it started with a discovery of a good isomerization and dehydrogenating agent, but one which was much too active i n its cracking behavior, and decreasing the latter to suit the needs of reforming. Several years ago, one of the authors found that nickel, platinum, and some other hydrogenating agents, when deposited on fresh synthetic silica-alumina cracking cata lyst, made a new catalyst that would isomerize paraffin and naphthene hydrocarbons i n the presence of hydrogen at elevated pressures and nominal temperatures. Table I shows some early typical results calculated from mass spectrometer analyses of the products obtained by passing methyl cyclopentane, cyclohexane, and n-hexane over a catalyst com posed of 5 % nickel i n silica-alumina at the indicated reaction conditions. Isomerization of a number of other hydrocarbons has also been studied and reported elsewhere (2). Table I.
Isomerization of Saturated Hydrocarbons in the Presence of Nickel-SilicaAlumina Catalysts Catalyst: 5% nickel on silica-alumina Pressure: 350 lb./sq. inch gage Liquid space velocity: 1.0 vol./vol./hour H : H C , mole ratio: 4.0 2
Hydrocarbon
-Hexane
Reaction temp., ° F . Products, mole % of charge Ce's and lighter 2.2- Dimethylbutane 2.3- Dimethylbutane 2- Methylpentane 3- Methylpentane n-Hexane Cyclohexane Methylcyclopentane Benzene Conversion, mole % of charge Isomer yield, mole % of charge
700 2.5 3.9) 5.5 29.7} 22.0 37.5 J
62.5 61.1
Methylcyclopentane
Cyclohexane
650
650
0.9
0.4
2.6
0.0
14.9 82.1 0.3 17.9 14.5
30.7 68.9 0.5 69.3 68.9
Platinum was found to be the most efficient hydrogenating component for the isomerization catalyst from the standpoint of amount required and resistance to sulfur poisoning. F r o m this beginning, an extensive study of the isomerization of η-heptane was made with platinum on silica-alumina catalysts. Figure 2 shows curves plotted from the data obtained illustrating the total isomer yield versus conversion and the temperatures that produced these conversions. The conversion-isomer yield curve follows closely the 45° theoretical yield line, goes through a maximum at about 6 5 % isomer yield, and then drops sharply because of cracking. The temperature at which the maximum yield of isomers was obtained was about 660° F . If this catalyst were to be used i n the temperature range of 850° to 950° F . where extensive aromatic formation may be obtained, one would be operating i n a range far to the right of the peak isomer yield and i n a region where very severe cracking of paraffin hydrocarbons occurs. I n fact, i t is questionable whether much of the paraffin hydro carbons i n a naphtha would survive such treatment. Hence, the problem became one of decreasing the cracking activity of the catalyst without harming its isomerization ability. This was accomplished by reducing the surface area of the silica-alumina component prior to platinization b y treating i t with super heated steam (5). Figure 3 shows the results obtained on a study of the total isomer yield obtained from passing η-heptane over five catalysts of decreasing surface area plotting the isomer yields against the reaction temperature. These data show that the temperatures required for isomerization increase as the surface area of the catalysts decreases and that the extent of isomerization obtained by catalysts of reduced surface area is essentially the same as that obtained with an undeactivated base within the limits of accuracy of the analytical methods employed. Figure 4 illustrates the surface area-reaction temperature relationship for maximum isomer yield i n the η-heptane isomerization experiments. The curve shows the reaction In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.
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FOWLE ef of.—DEVELOPMENT OF A REFORMING CATALYST
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temperatures at which the maximum isomer yields were obtained—i.e., the inflection points of the curves in Figure 3—as a function of the surface area of the catalysts. This curve points out that the catalyst should have a surface area below 65 or 70 square meters per gram, as determined b y the Brunauer-Emmett-Teller technique, in order to serve the purpose of catalyzing both isomerization and dehydrogenation without overcracking.
4001 0
20 40 60 80 100 N- Heptane Conversion (Wt. % Chg.) Figure 2. Isomerization of n-Heptane
Corves plotted from data obtained illustrating total isomer yield vs. conversion and temperatures that produced these conversions
The fact that the catalyst can promote dehydrogenation as well as isomerization is shown in Figure 5, where the weight per cent benzene in the products of experiments on dehydrogenation of cyclohexane is shown plotted against the reaction temperature. These results are compared to V o n Muffling's (7) calculated equilibrium line. The Table II.
Hydrocracking of Pure Hydrocarbons in the Presence o f Nickel-SilicaAlumina Catalysts Catalyst: 5% nickel on silica-alumina Pressure: 350 lb./sq. inch gage Liquid space velocity: 1,0 vol./vol./hour H : H Ç , mole ratio: 4.0 a
Hydrocarbon Reaction temp., ° F , Products, mole % of charge Methane Ethane Propane Butanes Pentanes Hexanes Conversion, mole % of charge Isomer yield, mole % of charge
n-Heptane 700 0.6 0.7 30.5 33.6 0.6 0.7 78.8 44,4
n-Ootane 690 0.7 0.0 22.6 56.8 22.8 0.8 100 47.5
In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.
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ADVANCES IN CHEMISTRY SERIES
authors' yields of benzene are definitely higher than those predicted by V o n Muffling's calculations, indicating an error i n the latter. The authors' calculations of this equi librium fall very close to the experimental data.
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Figure 3.
700 800 900 1000 Reoction Temp. · F Isomerization of n-Heptane
Total isomer yield obtained from passing η-heptane overfivecatalysts of decreasing surface vs. reaction temperature
Besides dehydrogenation of naphthenes and the isomerization of both naphthenes and paraffins, a reforming catalyst should be capable of controlling the hydrocracking reaction to minimize the formation of ethane and methane. The catalysts developed i n this work
a
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Catalyst Surface Area in Square Meters per Gram
Figure 4.
Isomerization of n-Heptane
Surface area-reaction temperature relationship for maximum isomer yield in n-heptane isomerization experiments
promote cleavage of the paraffin chains near the central carbon atoms, and hence help maintain a high yield of useful hydrocarbons. This is most clearly demonstrated i n Table I I which shows the products obtained from η-heptane and η-octane when operating at high conversions with an undeactivated catalyst. This observation has been confirmed In PROGRESS IN PETROLEUM TECHNOLOGY; Advances in Chemistry; American Chemical Society: Washington, DC, 1951.
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FOWLE et σ/.—DEVELOPMENT OF A REFORMING CATALYST
750 Figure 5.
800 850 900 950 Temperature (°F.)
1000
Benzene-Cyclohexane Equilibrium
Pressure, 500 lb./sq. inch gage, H :HC mole ratio, 10/ liquid space velocity, 2.0 vol./vol, /hour 2
by subsequent pilot plant operation on deactivated catalysts which has consistently shown a 97 to 9 8 % hydrogen purity on the recycle gas stream formed by an equilibrium flash separation at 500 pounds per square inch gage at 100° F . The purpose of a reforming catalyst is, after all, to provide as favorable a yield-octane number relationship as possible, and all the foregoing can be summed up b y the yield-.
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