US2847363A - Aromatization of straight run naphthenic gasolines - Google Patents

Description

couvaRaon, MOLE U1 Aug. 12, 1958 I. G. IYNIXON v AROMATIZATION OF STRAIGHT RUN NAPHTHENIC GASOLINES Filed Aug. 23, 1955 2 Sheets-Sheet 1 CYCLOH EXANE METHYLCYCLOPENTANE TEMPERATURE, c

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lnven'l'or; \vor 612:9 Nixon I. G. NIXON Aug. 12, 1958 AROMATIZATION OF STRAIGHT RUN NAPHTHENIC GASOLINES 2 Sheets-Sheet 2 Filed Aug. 23, 1955 lnvenTor: \vor rcu Nixon 23M PIQZMPW owNi/ibmmo United. State AROMATIZATION F STRAIGHT RUN NAPHTHENIC GASOLINES Ivor Gray Nixon, The Hague, Netherlands, assignor to Shell Development Company, New York, N. Y., a corporafion of Delaware pressure.

REFORMING It is well known that various gasolines having pooranti-knock characteristics can be improved by a catalytic reforming treatment. In the usual catalytic reforming process the gasoline is passed in vapor phase in contact with one of a number of dehydrogenation type reforming catalysts such for example as molybdenum oxide supported on an alumina carrier. The operation is usually carried out in the presence of added hydrogen. In this process, the main improvement is due to dehydrogenation of cyclohexane homologues to the corresponding aromatic hydrocarbons. Also, if the hydrogen pressure is not too high, some minor amount of aromatic hydroe carbons is produced from paraifin hydrocarbons by dehydrocyclization. The improvement in anti-knock properties obtained by these processes increases with increasing severity of the treating conditions until the available hydroaromatic naphthenes are substantially completely converted to aromatic hydrocarbons. The F-2 octane number of the product at this point is generally between 70 and 80. A further improvement in octane number may be obtained by increasing the severity of the treating conditions but this further increase is obtained largely by concentration of the aromatic hydrocarbons already formed through cracking out the less refractory nonaromatic hydrocarbons; such further improvement is, therefore, accompanied by a large loss in yield of liquid product. In commercial practice an 80% yield of a product of about 80 F-2 octane number is considered to be about the upper limit for economical operation. The high octane numbers obtainable by this process are only possible because the process is regenerative, i. e., the catalyst is regenerated after each short period of use. If it is desired to operate in a non-regenerative manner, relatively mild conditions must be used and the attainable improvement is less. By careful fractionation of the product a fraction suitable for use as aviation base stock can be obtained but the yield is poor. A similar product can, of course, be obtained by severe thermal reforming followed by fractionation but the yield is so low that the process is clearly of no practical interest under normal conditions.

PLATFORMING Recently a reforming process known as platforming has come into use for reforming motor 'gasolines. In this process gasolines of poor anti-knockcharacteristics are improved by treatment in the vapor phase and in the presence of hydrogen under pressure with a catalyst consisting of a cracking catalyst promoted with platinum. Aside from thetotally different catalyst used, this process differs from the previous reforming process in that it is non-regenerative, i. e., a single charge of the catalyst is out regeneration.

hydrocarbons and hydrogen are continuously passed.

atent "ice - cyclopentane homologues, which are usually present in appreciable concentrations, are likewise converted to a substantial extent to aromatic hydrocarbons. Also, some of the normal paraflins are isomerized to isoparafiins and this contributes to a minor extent to the improvement.

While platforming has been rather broadly described it is limited in practical application to the treatment of certain feed stocks. This is due to the nature of the reactions which are known to take place. On the one hand, the final boiling point of the feed must not be too high since higher boiling materials tend to coke the catalyst. A final boiling point around the end point of motor gasoline is generally considered about the maximum. On the other hand, the process is most amenable to the higher boiling gasoline constituents. It is therefore generally applied to the high boiling fraction of the gasoline. This is due largely to two factors. The first is that in platforming hydrocracking is one of the most important reactions. When treating 200400 F. naphtha this hydrocracking leads to the production of high octane components which still boil in the gasoline range and therefore tend to increase the volumetric yield. If lower boiling fractions are treated or included in the feed this hydrocracking leads to the production of materials which boil below gasoline or in the lighter fraction thereof and must be removed to reduce the volatility to the usual range thereby considerably decreasing the volumetric yield. The second is that the improvement in the octane number which may be obtained by platforming sharply declines as the boiling range of the feed islowered so that a point is soon reached where the minor improvement is insufiicient to justify the considerable cost of the platforming operation.

In motor gasoline production it is therefore the practice to platform only the high boiling portion of straight run gasoline boiling between about ZOO-400 F. In the production of aviation gasoline, which has a lower boiling range than motor gasoline, a fraction of somewhat lower boiling range is treated, e. g., 185-270 F. In either case it is considered undesirable to include hydrocarbons boiling below hexanes and it is therefore the practice to top the straight run gasoline to remove all such material boiling below at least about F. Thus, for example Fulton (Petroleum Refiner, vol. 29, pp. 109-112 (1935)) describes the platforming of various natural gasoline fractions and states that platforming is useful only for the treatment of the hexanes plus material and does not apply to the treatment of butanes and pentanes.

The present practice of excluding pentane in the platforming of straight run naphthenic gasoline fractions is based on the following considerations. Pentane, since it contains only 5 carbon atoms, cannot be converted by dehydrogenation or dehydrocyclization to any aromatic hydrocarbon. Hydrocracking is an important reaction in platforming and the catalyst is consequently compounded to produce substantial hydrocracking. The hydrocracking of pentane can lead only to the production of undesirable gases, to lowering of the volumetric yield, and

used continuously over an extended period of time withto consumption of valuable hydrogen.

The catalyst is, therefore, employed ent in straight run distillates is not pure normal pentane 7 in the form of one or more fixed beds through which the 7 7 but a mixture of normal pentane and lsopentane.

The pentane pres- The only advantage which could be expected by including F-Z-O octane number of the treated C fractiontherefore' would be less than about 76 i. 3 which is lower than the octane number of the higher boiling portion of the platformate. Its presence would therefore tend to lower the octane number of the total product.

The exclusion of lower boiling straight run components, and especially pentane, from the platforming feed is based primarily on the above reasons. Experimental runs using straight run feeds of initial cuflpoint to include the small amounts of normally occurring pentanes could not be expected to show any beneficial results for the above mentioned reasons and also because the pentane content of such feeds would not amount to more than about 7%.

ADDITION OF PENTANE Contrary to the belief and teachings hitherto advanced regarding the desirability of excluding pentane in the platforming feed, it has now been found that the inclusion of sizeable amounts of pentane has a totally un-' expected beneficial effect. Thus, it has been found that in the presence of sizeable amounts of added pentane the elficiency of conversion of the naphthenes to aromatics (whichis the chief octane-raising reaction in the process) is appreciably increased. It was at one time thought that this improvement was due to the relatively high heat carrying capacity of the added normal pentane which acted as a diluent and possibly also to a small extent by the small heat of the limited isomerization of part of the pentane, which isomerization is slightly exothermic and is known to take place.

Thus, as pointed out, when the reforming operation-is carried out with the mentioned acid platinum catalyst, the C -ring naphthenes which are normally present in about equal amounts with the (Z -ring naphthenes are largely converted to aromatics by dehydroisomerization. The total reaction is, therefore, quite endothermic. It is, in fact, so endothermic that it is difficult to carry out the process without a temperature drop in the catalyst bed. The major amount of the endothermic heat supplied must be supplied by preheating the reactant feed stream. However, the maximum temperature at which the material can be preheated without causing excessive thermal cracking and/ or excessive hydrocracking in the forepart of the catalyst bed is strictly limited. In normal operation with the preferred and enerally used adiabatic reactors the temperature drop through the catalyst bed is usually over 50 C., and may be as much as 110 C. or more. (See British Patent No.f662,002.)

The main desired reaction is the conversion of the naphthenic hydrocarbons in the feed to aromatic hydrocarbons and this is an equilibriumlimited reaction which is influenced by the temperature. The maximum degree of this conversion is, therefore, limited by the'equiv lib'riurn conversions of cyclohexane and methylcyclopentane to benzene plotted against the temperature at a pressure of 30 atmospheres and a hydrogen-to-hydrocarbon mole ratio of 3. The curves are shifted toward the right if the pressure is increased and are shifted downward slightly if the hydrogen-to-hydrocarbon mole ratio is increased. The curves for the equilibrium conversions of the C and C naphthenes are not identical but from the limited data available, they are of similar shape and relative positions.

Referring to Figure 1, it will be noted that under the moderate pressure of 30 atmospheres, and with the low -hydrogen-to-hydrocarbon mole ratio of 3 it is possible to obtain over conversion of the cyclohexane at a temperature of 475 C. However, the maximum possible conversion of the methylcyclopentane is less than 60%. Such conversions, if attainable, would not be bad. However, if 'a temperature drop of 50 C. takes place in the catalyst bed, the exit temperature is 425 C. and at this temperatureit is seen that the maximum possible conversions are only 53% and 13%, respectively. The usual means for overcoming this disadvantage is to reheat the product back to a high temperature and retreat it. In the second treatment, the temperature drop is less and, consequently, a higher outlet temperature with a higher limiting equilibrium is possible. Even two such treatments are usually not sufficient to obtain a really efficient conversion and in commercial practice three and often four such treatments are normally used.

If appreciable amounts of normal pentane are added to the feed the temperature drop, although still appreciable will be less and, as will be seen from the curves in the graph, this could result in a more favorable limiting equilibrium conversion.

The above explanation for the improved results appears plausible but it does not fit the facts since the improved conversions have now been obtained under carefully controlled temperature conditions where this heat effect is negligible. Thus, for some unexplained reason the inclusion of appreciable amounts of added pentane results in increased conversion and conversion efiiciency, particularly with respect to the C -ring aromatic precursors, even under carefully controlled temperature conditions and even at the same base feed throughput rate (hence increased liquid hourly space velocity).

This is illustrated by the data in the following Tables I, -II, and III in which are shown the results obtained in platforming the same feed stock containing different amounts of added normalpentane. In order to show the effect most clearly with the least difficulties and uncertainties due to analytical errors these experiments were carried out with a. synthetic feed prepared with relatively pure starting materials. To this base stock various amounts of normal pentane were added to give feeds having. the compositions shown in the following Table I.

Table I Feed Pound moles per hundred pounds of feed Normal Pentane Methylcyclopentane;

Tolueneofzeasmay mol) 0. 5198 (437 mol) 0. 8316 66 mol 0.4040 0 0.3125 0.1s9s

The platforming of these blends was carried out at a liquid hourly space velocity of 2 with a commercial platforming catalyst and with recycled hydrogen. There were 2 reactors in the series, each provided with means for careful temperature control. The pressures in the reactors were 325 and 275 p. s. i. g. The pertinent results are shown in the following Table II.

It will be noted that the equilibrium conversion of methylcyclopentane (MCP), which is the most sensitive indicator in this case, is the same within experimental limits in all three cases so that loss of conversion due to an unfavorable equilibrium at lower temperatures is excluded. It will be noted furthermore that the percent of equilibrium conversion obtained, i. e. the percentage approach to the equilibrium, was high in all cases and increased significantly with increasing dilution with pentane. Thus, the mols of benzene formed per mol of methylcyclopentane charged increased from 0.456 to 0.615 or an increase of 33%. The methylcyclohexane was in each case converted substantially completely to toluene. These data show that the improved results are not dependent upon any temperature effect and that they are due primarily to improved conversion of the C -ring naphthenes, which, as pointed out, constitute approximately half of the aromatic precursors in the usual feed stocks.

The above results were obtained by operating with a I constant total feed space velocity (LHSV=2). This advantage however is still maintained if the space velocity, based on the total feed, is increased so that the same amount of base feed is processed. The corresponding results on this basis (calculated from the same data using previously determined conversions versus space velocity correlation curves) are shown in the following Table III.

Table III Concentration normal pentane, mol percent- 23 43 66 Liquid hourly space velocity 2 2. 5 3. 5 Conversion MOP to benzene, percent 42 48 52 Relative benzene production rate, welght/ tirns 1. O 1. 14 1. 24

In order to produce aviation base stock of high quality,

a narrow boiling straight run fraction is separated for treatment. The cut points should be chosen so that the majority of the useful naphthenes are concentrated. On the other hand, the end point should be low enough to meet the boiling range requirements of the composite containing the isopentane. For the reason stated above, the C hydrocarbons should be excluded as far as practicable. The fraction ca. 85 C. to ca. 130 C. (cut point temperature) is suitable for aviation base stock production. All straight run gasoline fractions of this boiling range are, of course, not suitable. The straight run gasoline fractions should be one in which naphthenic hydrocarbon content is high, preferably at least 50%. The

naphthene content of the fraction is dependent upon the naphthenicity of the petroleum from which it is derived.

The amount of added normal pentane must be considerable. It should be borne in mind that the equilibrium between norm-a1 pentane and isopentane is not particularly favorable under the reaction temperature conditions and also that complete conversion to the equilibrium is not usually obtained. The amount of normal pentane required is, therefore, considerably above the normal pentane concentration in the straight run gasoline cuts regardless of their initial boiling point. In general, the amount of normal pentane required to be added is between about 10% and 70% of the narrow boiling fraction to be treated and preferably the amount is between 25% and 70%.

Upon completion of the conversion the normal pentane is removed. The recovered normal pentane is advantageously recycled. On the other hand, it is important that no appreciable amount of isopentane be recycled.

The process of the invention will be described in more detail in connection with Figure II of the accompanying drawing which schematically shows a flow diagram of one application of the process for the production of an aviation base stock.

I Referring to Figure 11, a debutanized straight run gaso-' line derived from a naphthenic petroleum and entering via line 1 is passed to fractionating column C Column C is operated to take overhead all of the pentanes. The overhead product is essentially normal pentane but contains a small amount of isopentane. The bottom product from column C is passed by line 2 to fractionating column C Column C is operated to take overhead all of the hexanes. The overhead product is withdrawn by line 3. The bottom product from column C is passed by line 4 to fraction-ating column C Column C is operated to take overhead the fraction desired for the production of aviation base stock. As pointed out, the recommended cut points for this fraction are C. and C. The bottom product from column C is withdrawn from the system by line 5.

The pentane fraction from column C is combined with an isopentane-rich fraction from line 6, later to be described, and the mixture passed to fractionating column C Column C is operated to separate isopentane from normal pentane. The overhead isopentane fraction is withdrawn by line 7 and combined with the reformed stock as later described. The normal pentane fraction withdrawn by line 8 is combined with the overhead fraction from column C As previously pointed out, the amount of pentane normally occurring in straight run gasolines is not suflicient to supply the needs for the process. Normal pentane or a pentane fraction from an exterior source, e. g., from a straight run gasoline of low naphthenicity is therefore introduced by line 9. The mixture of the selected straight run fraction, the recycled normal pentane, and the normal pentane introduced by line 9, is mixed with recycled hydrogen from line 10 and passed to the platforming unit indicated simply in the figure by the labelled rectangle. In the platforming unit the material is preheated to a reaction temperature of about 890 F. and then passed through a bed of catalyst consisting of from about 0.1 to about 0.4% of platinum supported upon a cracking catalyst such as alumina treated with hydrofluoric acid or an equivalent fluo-rinating agent, or a synthetic silica-alumina composite cracking catalyst. The pressure is normally between about 200 and 700 p. s. i. g. The space velocity, i. e. contact time, is adjusted such that the effiuent is substantially at equilibrium for the exit temperature. The vaporous effiuent is cooled somewhat to condense the major portion of the normally liquid hydrocarbons and then passed by line 11 to column C The purpose of column C is to separate product gas consisting essentially of hydrogen while retaining all of the isopentane in the bottom product. To

this end cooled higher boiling product fromline 12 is passed into the top of column C This cooled higher boiling product passing downward through the column eifectively removes isopentane from the overhead prod uct. The product gas consisting essentially of hydrogen is recycled to the platform unit by line as described. The amount of hydrogen thus recycled is generally between about 2 and about 10 moles per mole of hydrocarbon feed to the platforming unit. Any product gas in excess of this amount may be withdrawn by line 13. The bottom product from column C is passed by line 14 and valve 15 to separator 16 which normally operates at a lower pressure than column C Low pressure gas released upon reduction of the pressure is withdrawn by line 17. The liquid product from the separator is passed by line 18 to fractionating column C Column C is operated to take overhead all of the pentanes. The bottom product from column C is a highly aromatic material consisting almost exclusively of C and C hydrocarbons. Part of this cooled product is recycled to column C as described. The remaining part is combined with the overhead fraction from column 0.; consisting substantially of isopentanes. The mixing containing the correct ratio of isopentane to reformed stock to provide the required volatility is withdrawn by line 18 as product aviation base stock. The pentane fraction withdrawn as overhead product from column C is recycled back to column O; as described. The volatility of the aviation base stock withdrawn by line 18 depends upon the amount of the isopentane fraction recovered as top product from column C The desired correct amount is obtained by adjusting the amount of normal pentane from an outside source introduced by line 9. In a typical case the 85130 C. straight run fraction separated had the following composition:

Percent by weight Naphthenes 62 Aromatics 3 Paraffins 35 The amount of normal pentane required for the distillate was 35 parts by weight for each 65 parts by Weight of the distillate. This mixture was treated with a commercial platforming catalyst at a space rate of 2.5 kg. per liter of catalyst per hour at 430 C. and atmospheres pressure. The amount of hydrogen used was 1000 liters (standard conditions) per kilogram of the hydrocarbon feed. There was no noticeable cracking of the pentanes. The pentanes were separated from the product and separated into normal pentane (55%) and isopentane (45%). The isopentane was blended back with the reformate and was just suflicient to provide the necessary volatility.

I claim as my invention:

1. In the production of gasoline from straight run naphthenic distillates boiling in the gasoline boiling range the improvement which comprises adding to the straight run distillate from 25 to about of normal pentane, reforming the resulting mixture in the presence of added hydrogen with an acid platinum catalyst, and fractionating the resulting product to separate normal pentane.

2. In the production of gasoline from straight run naphthenic distillates boiling in the gasoline boiling range the improvement which comprises removing from the distillate all of the material boiling up through hexane and material boiling above the gasoline boiling range, adding to the remainder from 25 to about 70% of the amount thereof of normal pentane, reforming the resulting mixture in the presence of added hydrogen with an acid platinum catalyst, and fractionating the resulting product to separate normal pentane.

3. In the production of gasoline from straight run naphthenic distillates boiling in the gasoline boiling range the improvement which comprises fractionating the naphthenic distillate to remove therefrom material boiling up through hexane and material boiling above about 130 C. leaving a naphthenic fraction boiling between about C. and C., adding to said naphthenic fraction from 25 to about 70% of the amount thereof of normal pentane, reforming the resulting mixture in the presence of added hydrogen with an acid platinum catalyst and fractionating the resulting product to separate normal pentane.

4. Process according to claim 3 further characterized in that isopentane is separated from the pentanes from the straight run naphthentic distillate and from the pentane recovered from the reformed product, said separation being effected together by fractional distillation whereby the isopentane present in the straight run distillate is recovered with the isopentane from the reformed distillate and the combined isopentane is re-mixe-d with the depentanized reformed product.

References Cited in the file of this patent UNITED STATES PATENTS Read: .Petroleum Refiner, vol. 30 (March 1951), pp. l30-l36.

Claims (1)

1. 2. IN THE PRODUCTION OF GASOLINE FROM STRAIGHT RUN NAPHTHENIC DISTILLATES BOILING IN THE GASOLINE BOILING RANGE THE IMPROVEMENT WHICH COMPRISES REMOVING FROM THE DISTILLATE ALL OF THE MATERIAL BOILING UP THROUGH HEXANE AND MATERIAL BOILING ABOVE THE GASOLINE BOILING RANGE, ADDING TO THE REMAINDER FROM 25 TO ABOUT 70% OF THE AMOUNT THEREOF OF NORMAL PETANE, REFORMING THE RESULTING MIXTURE IN THE PRESENCE OF ADDED HYDROGEN WITH AN

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