IMPROVEMENT OF THE CETANE NUMBER OF DIESEL ENGINE FUELS

Abstract: Disclosed is a novel cetane number improver additive for diesel engine fuels comprising tetralin hydroperoxide as the active ingredient. A diesel engine fuel composition having a high cetane number may be obtained by adding this additive to a diesel engine fuel in such an amount as to adjust its peroxide number to a value of 100 to 1,000. Also disclosed is a process for the production of a diesel engine fuel which comprises the steps of hdyrogenating a naphthalene- or alkylnaphthalenes-containing hydrocarbon oil and partially oxidizing the hydrogenated oil to yield a hydrocarbon oil containing tetralin hydroperoxide.

Description: This invention relates to a process for the production of a deisel engine fuel having a high cetane number. Furthermore, it also relates to a technique for utilizing cycle oil obtained by fluidized catalytic cracking of a heavy hydrocarbon oil, tar oil obtained as a by-product by cracking of naphta, and oil obtained by lequefaction of coal, as diesel engine fuel components. If diesel engine fuels (i.e, gas oil and fuel oil) do not have an adequately high cetane number, the so-called diesel knock phenomenon will take place. This may produce loud engine noises and cause a reduction in combustion efficiency. If the cetane number becomes still lower, it may be impossible to start the engine. In order to fully utilize light cycle oil obtained by fluidized catalytic cracking of a heavy hydrocarbon oil and having a high aromatics content (hereinafter referred to as FCC-LCO), oil obtained by liquefaction of coal, and tar oil obtained as a by-product during the production of ethylene by cracking of naphtha, attempts have recently been made to hydrogenate such oils and use the resulting oils as diesel engine fuel components. However, these oils have a very low cetane number of the order of 15 to 25 and contain a considerable amount of polycyclic aromatic compounds (mainly comprising naphthalene and its derivatives), so that it is impossible to mix a high proportion of these oils with other straight-run petroleum oils (i.e., straight-run fractions obtained by distillation of petroleum, such as gas oil and the like). Conventionally, a large number of additives for improving the cetane number of diesel engine fuels have been proposed and some of them are now on the market. US-A-2011297 discloses the use of tetralin peroxide which is admixed to liquid base fuels in quantities of less than 5%, preferably 0.5 to 1.5%. Among these cetane number improver additives are nitrate compounds such as tetrahydro-2,5-furan dimethanol nitrate (US-A-4,522,630), alkyl and alkoxy nitrates (US-A-4,549,883 and 4,448,587), cyclododecyl nitrate (US-A-4,420,311), polysaccharide nitrate esters (JP-A-25387/′83) and the like. However, these additives are disadvantageous from an economical point of view, because the desired effect cannot be obtained unless they are added in an amount of as much as 1,000 to 3,000 ppm. In addition to the above-described additives, other techniques for improving combustibility have also been proposed. For example, DE-A-3136030 discloses a method in which dialkoxyalkanes are partiallly oxidized with ozone or H₂O₂, and US-A-4,330,304 discloses a combustion improver additive comprising nitroparaffin, cumene hydroperoxide and propylene oxide. Furthermore, the present inventors have previously found that the ignitability of a diesel engine fuel can be improved by oxidizing the fuel partially and then feeding it to a diesel engine (JP-A-45765/′85, 233703/′83 and 24386/′84) and that cumene hydroperoxide can act as a cetane number improver additive) (JP-A-27767/′85). Diesel engines have wide applications including vehicles (such as trucks, buses, passenger cars, locomotives, etc.), construction machinery, electric power generators, ships and the like. In these diesel engines, gas oil and fuel oil are used as fuels. Specifically, the fuels for high-speed and small-sized diesel engines used in trucks, buses and the like have conventionally been produced chiefly from an oil obtained by hydrogenating a petroleum distillate having a boiling range of about 230 to about 400°C. On the other hand, the heavy fuels for medium- to large-sized diesel engines used in ships and the like have been produced chiefly from an oil obtained by hydrogenating reduced crude. These fuels have a cetane number of about 45 to about 60 and are suitable for use in diesel engines. However, crackers for heavy hydrocarbon oils and naphthas are now being widely used in petroleum refining industry and petrochemical industry, so that cracked oils and tar oils containing a high proportion of aromatics (such as naphthalene and the like) and having a low cetane number are being produced in large amounts. In addition, the development of coal liquefaction equipment has led to the production of fuel oils having a high aromatics content and a low cetane number. In order to use these oils as diesel engine fuels, a technique for improving their cetane number is required. The present inventors have made an intensive study in the above-described background and have completed this invention. The present invention has been completed on the basis of the discovery that tetralin hydroperoxide, which is a partial oxidation product of tetralin, is effective in improving the cetane number of diesel engine fuels. When cetane numbers were measured using a cetane number measuring engine (CFR engine), the present inventors noticed that the performance of tetralin used as the standard fuel varied from lot to lot. As a result of careful search for its causes, it was found that the tetralin contained a slight amount of tetralin hydroperoxide and its treatment with clay gave a stabilized value. This has led to the discovery that tetralin hydroperoxide can be effectively used as a cetane number improver additive. On the basis of this discovery, the present inventors have investigated the method of forming tetralin hydroperoxide in a fuel production process and have thereby established a process for the production of a fuel having a high cetane number from a fuel material having a low cetane number. According to the present invention there is provided a process for improving a cetane number of a diesel engine fuel which comprises the steps of (a) hydrogenating a hydrocarbon oil containing 3 to 16% by weight of naphthalene or alkylnaphthalenes in the presence of a hydrogenation catalyst to convert at least a part of the naphthalene or alkylnaphthalenes into tetralin, wherein the hydrogenation conditions and catalyst are selected so that the content of tetralin or alkyltetralins in the hydrogenated oil is not less than 0.5% by weight and the content of decalin or alkyldecalins is not greater than 5% by weight, and then (b) partially oxidizing the hydrogenated oil, wherein the partial oxidation conditions are selected so that the peroxide number of the partially oxidized oil is increased to a value of not less than 100, to yield a hydrocarbon oil containing tetralin hydroperoxide and having an improved cetane number. Detailed Description of the Invention The cetane number improver additive for the present invention comprises, as the active ingredient, tetralin hydroperoxide having a structure represented by the formula This compound can be readily formed by exposing tetralin to ultraviolet light in air, or by heating tetralin in an atmosphere of oxygen (preferably in the presence of copper wire). For example, when tetralin is placed in an atmosphere of pure oxygen having a pressure of 4 to 11 bar and allowed to stand at a temperature of 8 to 100°C for about 6 hours or so, a part of the tetralin is oxidized to form tetralin hydroperoxide in such an amount as to give a peroxide number of about 1,000 to 2,000. Peroxide number can be measured according to the procedure described in ASTM D1563-84, "Standard Test Method for Peroxide Number of Mineral Insulating Oils". In order to use the resulting oil as an additive, it is convenient for blending purposes to dilute the oil with an n-paraffin having 6 to 16 carbon atoms. In this connection, the cetane number improving effect of peroxides is reported in SAE Quarterly Transactions, Vol. 5, No. 3, pp. 404-417 (1951), but no mention of tetralin hydroperoxide is found therein. This publication discloses that dibutyl peroxide, when added to a diesel engine fuel in an amount of 1.5% by volume, increases its cetane number by 20. In contrast, the additive of the present invention exhibits a marked effect at considerably lower concentrations, has good storage stability, and is easy to handle. More specifically, the additive of the present invention may be added to a diesel engine fuel in such an amount as to adjust its peroxide number to a value of about 100 to 1,000, thus increasing its cetane number by about 3 to 15. The present invention relates to a process for the production of a diesel engine oil which comprises the steps of hydrogenating a naphthalene-or alkylnaphthalenes-containing hydrocarbon oil to convert at least part of the naphthalene or alkylnaphthalenes into tetralin or alkyl derivatives thereof, and partially oxidizing the hydrogenated oil to yield a hydrocarbon oil containing tetralin hydroperoxide. The raw oil used in the present invention is a hydrocarbon oil containing a considerable amount of naphthalene or alkylnaphthalenes. Specific examples thereof include light cycle oil obtained by fluidized catalytic cracking of a heavy hydrocarbon oil (i.e., FCC-LCO), oil obtained by liquefaction of coal, and tar oil obtained as a by-product by cracking of naphtha. FCC-LCO useful as a raw oil has a boiling range of about 150 to 350°C and is characterized by n-d-M ring analysis values of 45-65% C A (C-aromatic), 0.5-5% C N (C-naphthenic) and 35-50% C P (C-paraffinic), an aniline point of 20 to 30, and a cetane number of about 10 to 20. Useful oil obtained by liquefaction of coal is a gasoil-to-fuel oil A fraction having a boiling range of about 150 to 450°C. This oil is recovered from coal liquefaction equipment and has a cetane number of 15 to 20 and an aromatics content of about 60% by weight. Useful tar oil obtained as a by-product by cracking of naphta is a fraction having a boiling range of about 150 to 400°C and containing a high proportion of aromatics. In these raw oils, naphthalene or alkylnaphthalenes having one or more alkyl substituents of 1 to 5 carbon atoms are present in an amount of about 4 to 20% by weight. In the process of the present invention, these raw oils are subjected to hydrogenation and partial oxidation under mild conditions. The hydrogenation is carried out in the presence of a hydrogenation catalyst commonly used in petroleum refining. Specific examples thereof are catalysts formed by supporting one or more metals (such as Ni, Co, Mo, W, V, Fe and the like) on a suitable carrier (such as alumina, silica, silica-alumina and the like). In particular, it is preferable to use a catalyst and reaction conditions suitable for partial nuclear hydrogenation of naphtalene and derivatives thereof, i.e., suitable for their conversion into tetralin and/or alkyl derivatives thereof. Among others, W-Ni catalysts are especially preferred for nuclear hydrogenation of naphtalene. Although the hydrogenation conditions can vary acccording to the types of raw oil and catalyst, used, the hydrogenation is usually carried out in the presence of an Ni-W catalyst and under conditions including a reaction pressure (hydrogen pressure) of 40 to 120 bar, a temperature of 250 to 350°C and preferably 300 to 340°C, an LHSV of 0.4 to 4.0 hr _1 and preferably 1.5 to 2.0 hr _1 , and a hydrogen feed rate of 300 to 1,000 m³/m³ of the raw oil. The hydrogenation conditions should be selected so that a reaction can take place in which the naphtalene or alkylnaphtalenes present in the raw oil undergo partial nuclear hydrogenation and form tetralin and/or alkyl derivatives thereof. Although the hydrogenation conditions can vary according to the type of catalyst used, they should be selected with due consideration to the criterion that the content of tetralin and/or alkyl derivatives thereof present in the hydrogenated oil should be as high as possible, i.e., not less than 0.5% by weight, preferably not less than 1.0% by weight and more preferably not less than 1.5% by weight. The tetralin and/or alkyl derivatives thereof are formed by partial nuclear hydrogenation of the naphthalene or alkylnaphthalenes. Accordingly, no upper limit is placed on the content of such naphthalene compounds in the raw oil, and higher contents are more effective. However, a content of about 3 to about 16% by weight will suffice. In the aforesaid FCC-LCO and oil obtained by liquefaction of coal, such naphthalene compounds are present in an amount of not less than 4% by weight. As used herein, the term "alkyl derivatives of tetralin" means derivatives of tetralin having 1 to 3 alkyl substituents each containing 1 to 5 carbon atoms. The hydrogenation conditions which cause complete nuclear hydrogenation of the naphthalene or alkylnaphthalenes present in the raw oil (i.e., those which convert the naphthalene or alkylnaphthalenes into decalin or derivatives thereof) should be avoided. For this reason, severe hydrogenation conditions should be avoided because they convert the naphthalene into decalin. Accordingly, the hydrogenation conditions, such as type of catalyst, reaction temperature, liquid space velocity and the like, should be selected so that the hydrogenated oil will contain decalin or derivatives thereof in an amount of not greater than 5% by weight, preferably not greater than 3% by weight and more preferably not greater than 1% by weight and, at the same time, will contain tetralin or derivatives thereof in an amount of not less than 0.5% by weight and preferably not less than 1.0% by weight. The formation of a large amount of decalin or derivatives thereof causes a decrease in the content of tetralin or derivatives thereof and, therefore, is undesirable for subsequent partial oxidation. Another criterion useful in determining the hydrogenation conditions is that the n-d-M ring analysis values of the hydrogenated oil should be 20-35% C A , 30-45% C N and 30-35% C P , and preferably 25-35% C A and 30-40% C N . Oil obtained by liquefaction of coal and tar oil obtained by cracking of naphtha contain a large amount of naphthalene compounds. These oils should likewise be hydrogenated under such conditions that the naphthalene compounds are selectively converted into tetralin or derivatives thereof and their conversion of into decalin or derivatives thereof is suppressed. At the hydrogenation conditions become severer, the hydrogenated oil has a higher cetane number. However, it is desirable that the cetane number of the hydrogenated oil be limited to at most 45 or so. The oil obtained by the above-described hydrogenation is then subjected to partial oxidation. The term "partial oxidation" as used herein means a process for increasing the peroxide number of the oil to a value of not less than 100. The peroxine number of the oil can be measured according to the procedure described in ASTM D1563-84. The partial oxidation may be carried out by placing the oil under an oxigen pressure of 3 to 8 bar and allowing it to stand at a temperature of 60 to 100°C for a period of 3 to 10 hours, or by adding a copper of nickel catalyst to the oil and stirring it under milder conditions. However, no particular limitation is placed on the partial oxidation conditions. Thus, the peroxide number of the oil can be increased to a value of not less than 100, preferably not less than 150 and more preferably not less than 300. As the peroxide number becomes higher, the oil has a higher cetane number. In particular, the use of an oil having a high tetralin content provides a peroxide number of not less than 1,500, resulting in a fuel having a high cetane number. More specifically, as a result of the partial oxidation, the tetralin or derivatives thereof present in the oil are readily converted into tetralin hydroperoxide, which functions to markedly improve the cetane number of the oil. The partial oxidation may cause the oil to assume a yellow to brown color, but this color may be improved by treatment with clay or the like. Prior to the partial oxidation, the tetralin content of the hydrogenated oil may be further increased by adding a small amount of tetralin. This serves to further improve the cetane number of the partially oxidized oil. It is to be understood in this connection that, if the hydrogenation is carried out under severe conditions which convert naphthalene into decalin, subsequent partial oxidation will fail to bring about an improvement in cetane number. The above-described treatment causes the cetane number of the resulting fuel oil to be increased by 30 to 60, as compared with that of the starting oil. For example, when FCC-LCO having a cetane number of 14 is used as the starting oil, its hydrogenation and partial oxidation makes it possible to produce a fuel oil having a cetane number of 50 to 75. This fuel oil may be used directly as a diesel engine fuel or mixed with other fuel components to yield a diesel engine fuel. The present invention and its effects will be more specifically explained with reference to the following examples. Example 1 Light cycle oil obtained by fluidized catalytic cracking of a heavy hydrocarbon oil and having the properties shown in Table 1 (i.e., FCC-LCO) was used as the starting oil. According to the following procedure, this starting oil was hydrogenated and than partially oxidized. In the hydrogenation step, a W-Ni catalyst was used, the reaction pressure (hydrogen pressure) was 100 bar, and the temperature and liquid space velocity (LHSV) were as shown in Table 1. As a result of this hydrogenation, there were obtained hydrogenated oils H-1, H-2 and H-3 as shown in Table 1. A glass container was placed in a pressure vessel and charged with 50 ml of each of the hydrogenated oils. Then, the pressure vessel was filled with pure oxygen to a pressure of 7.0 kg/cm² (7 × 10⁵ Nm _2 ), placed in a thermostatic chamber kept at 100°C, and allowed to stand for 8 hours. Thereafter, the pressure vessel was allowed to cool and taken out. In this manner, 100 ml of a partially oxidized oil was obtained from each of the hydrogenated oils. The peroxide number and cetane number of the partially oxidized oil obtained from each of the hydrogenated oils were measured and the results thus obtained are shown in Table 2. Cetane number measurements were made in the following manner: 100 ml of each of the partially oxidized oils was mixed with 900 ml of a commercially available gas oil (having a cetane number of 48.4). Using a CFR engine, the cetane number of this mixed oil was measured according to the procedure described in ASTM D613. In Table 2, the measured cetane numbers of the mixed oils are given in the upper row and the estimated cetane numbers of the partially oxidized oils in the lower row. Peroxide number measurements were made according to the procedure described in ASTM D1563-84. It is evident from Table 2 that, when the starting oil FCC-LCO having a cetane number of 14 as shown in Table 1 was hydrogenated and partially oxidized under appropriate conditions, the resulting products (i.e., the partially oxidized oils from H-1 and H-2) exhibited an increased cetane number of 60-70. In contrast, when the starting oil FCC-LCO was hydrogenated under severe conditions and then partially oxidized, the resulting product (i.e., the partially oxidized oil from H-3) showed no appreciable improvement in cetane number. Thus, the hydrogenation should be carried out under such conditions that the content of tetralin is increased and the formation of decalin is suppressed, and this enhances the effect of improving the cetane nunber by subsequent partial oxidation.

Claims: 1. A process for improving a cetane number of a diesel engine fuel which comprises the steps of (a) hydrogenating a hydrocarbon oil containing 3 to 16% by weight of naphthalene or alkylnaphthalenes in the presence of a hydrogenation catalyst to convert at least a part of the naphthalene or alkylnaphthalenes into tetralin, wherein the hydrogenation conditions and catalyst are selected so that the content of tetralin or alkyltetralins in the hydrogenated oil is not less than 0.5% by weight and the content of decalin or alkyldecalins is not greater than 5% by weight, and then (b) partially oxidizing the hydrogenated oil, wherein the partial oxidation conditions are selected so that the peroxide number of the partially oxidized oil is increased to a value of not less than 100, to yield a hydrocarbon oil containing tetralin hydroperoxide and having an improved cetane number. 2. A process as claimed in claim 1 wherein the partial oxidation step is carried out under such conditions that the resulting oil has a peroxide number of not less than 150. 3. A process according to claim 1 or claim 2 wherein the oil to by hydrogenated is selected from cycle oil obtained by fluidized catalytic cracking of a heavy hydrocarbon oil derived from petroleum, a diesel fuel fraction obtained by liquefaction of coal, and tar oil obtained by cracking naphta. 4. A process according to any preceding claim which includes the step of mixing the partially oxidised hydrogenated oil with another diesel engine fuel.

Inventors: AKADA, TAKUMI, DATE, KAZUTO C/O KYODO OIL TECHNICAL RESEARCH, KAWATE, AKIHIRO, KUDOH, MASUO C/O KYODO OIL TECHNICAL CENTER, SASAKI, TAKANOBU, TAMANOUCHI, MITSUO C/O KYODO OIL TECHNICAL, TANAKA, HIDEO C/O TECHNICAL RESEARCH CENTER,
Applicants: KYODO OIL TECHNICAL RESEARCH CENTER CO., LTD., NIPPON MINING COMPANY LIMITED
Application year: 1988
Main classification: C10L1
IPC codes: F02B3, C10L1, C10G65, F02B47, C10G69, C10L10