PROCESS FOR PRODUCTION OF AROMATIC HYDROCARBONS

Abstract: A process for producing aromatic hydrocarbons in a high yield by introducing a naphtha fraction the cyclopentane content of which is controlled to not more than 1% by weight, or the methylpentane content is controlled to not more than 10% by weight, in a catalytic reaction tower, and contacting with a catalyst comprising a large pore zeolite containing at least one of Group VIII metals of the Periodic Table. In this process, the catalyst cycle is greatly lengthened.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a process for producing aromatic hydrocarbons, more particularly it relates to a process for producing aromatic hydrocarbons by contacting a naphtha fraction with a catalyst comprising a large pore zeolite containing at least one of group VIII metals of the Periodic Table, which can be carried out efficiently over a long period of time. 2. DESCRIPTION OF RELATED ART For petrochemical production of aromatic hydrocarbons such as benzene, toluene and xylene, a method for catalytic reforming of a naphtha fraction with a strong acid zeolite catalyst, e.g., ZSM-5 has been known (see, for example, JP-B-42 639/1981, 23 368/1983, JP-A-92 717/1978, 140 934/1981). This method, however, has disadvantages in that cracked gases are formed in a large amount and therefore the yield of aromatic hydrocarbons is low. In order to overcome the above problems, a method using a zeolite L catalyst with platinum deposited thereon has been proposed (see, for example, JP-A-80 333/1984). Although this method has the advantage that aromatic hydrocarbons can be produced with high selectivity, it suffers from problems that the catalytic activity is rapidly reduced and the catalyst cycle is short. To lengthen the catalyst cycle, it is necessary to maintain the yield of aromatic hydrocarbons at a low level such as below 50 %. In EP-A-0 184 450 a reforming process for enhanced benzene yield has been disclosed wherein firstly a C₅-fraction is removed from the feed of a full boiling range naptha,then the remaining C₆+ fraction is separated into a C₆ fraction containing at least 10 % by volume of C₇+ hydrocarbons and a C₇+ fraction and then the C₆ fraction is introduced into a catalytic aromatization process and the formed aromatics are recovered therefrom after sending the C₇+ fraction to a catalytic reforming process. As a catalyst for aromatization in this process a K zeolite L catalyst containing 0,6 wt.% of platinum is used. This method has the disadvantage that cyclopentane and methylpentane which effect the life-time of the catalyst are contained in the used feed and therefore the activity of the catalyst rapidly decreases and life-time of the catalyst becomes markedly short. In US-A-3 853 745 a two-step process is disclosed wherein in the first step a catalytic reforming of a hydrocarbon feed-stock comprising naphthenes and paraffins is carried out, and in the second step a liquid product is adsorbed and desorbed to a zeolite or other suitable substance. The reforming catalyst and solid adsorbent are charged in one reactor and the above two steps are carried out alternately. Also in this case the feed used contains naphthenes and paraffins. The reformation of the feed comprising naphthenes is carried out at 316 to 370°C. Within this temperature the naphthenes are converted into aromatic hydrocarbons, but not the paraffins. Furthermore, also in this case the life-time of the used catalyst is shortened by paraffins contained in the feed. In order to overcome the above prior art problems, the present inventors have made extensive investigations from various view points. As a result, it has been found that cyclopentane or methylpentane contained in a naphtha fraction feed is responsible for the deposition of a large amount of coke becoming a catalyst poison for zeolite catalysts with large pore radius such as L-zeolite containing Group VIII metals of the Periodic Table, such as platinum and the like, and thus markedly shortens the catalyst cycle. The object of the present invention is to provide a process for efficiently producing aromatic hydrocarbons from a naphtha fraction with high selectivity, in which the catalyst life is lengthened. SUMMARY OF THE INVENTION Based on the findings, it has been found that if the catalytic reaction is carried out at a specific temperature and if the cyclopentane content of the naphtha fraction feed is controlled to not more than 1 % by weight or the methylpentane content is controlled to not more than 10 % by weight by distillating the naphtha fraction feed or separation of high purity normal paraffins, the catalyst cycle (catalyst life) can be lengthened and aromatic hydrocarbons can be produced efficiently. Subject-matter of the present invention is a process for producing aromatic hydrocarbons by contacting a naphtha fraction with a catalyst comprising a large pore zeolite containing at least one of Group VIII metals of the Periodic Table, which is characterized in that a) the catalytic reaction is carried out at a temperature of from 400 to 550°C and b) the cyclopentane content or the methylpentane content of the naphtha fraction to be introduced in the catalytic reaction region is previously controlled to not more than 1 % by weight, and to not more than 10 % by weight, respectively. The characteristic features of the claimed process, as described, are not only the temperature, at which the catalytic reaction is carried out, but furthermore the previous control of the composition of the feed-stock to be introduced into the reaction process whereby life-time of the catalyst can be considerably elongated. In the claimed process the reason why the content of cyclopentane or methylpentane in the naphtha fraction used as feed-stock is controlled is to maintain activity of the zeolite catalyst and to elongate life-time of the catalyst. Also, according to the present invention the reaction temperature is high as 400 to 550°C so that aromatics can easily be formed from paraffins. According to a preferred embodiment of the-claimed process the methylpentane content of the naphtha fraction to be introduced into the catalytic reaction region is previously controlled to not more than 5 % by weight, especially not more than 2 % by weight. According to a further preferred embodiment of the present invention as large pore zeolite a zeolite L, in particular a zeolite L with platinum deposited thereon, is used. According to a further preferred embodiment of the present invention a large pore zeolite L having platinum deposited thereon is used which has been subjected to a Fluorocarbon treatment. In the description and the claims the large pore zeolite means zeolite having large pores with a diameter in the range of from 0,7 to 0,9 nm (7-9 Å). BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 4 are graphs showing the relation between the reaction time of catalytic reforming and the yield of aromatic hydrocarbons in Examples and Comparative Examples following below. DESCRIPTION OF PREFERRED EMBODIMENTS The naphtha fraction to be used as the feed in the present invention may be a full range naphtha, i.e., a hydrocarbon mixture having 4 to 10 carbon atoms. In general, it is a hydrocarbon mixture having 4 to 7 carbon atoms, called a light naphtha, or a mixture containing a small amount of hydrocarbons having 8 carbon atoms. More specifically, a mixture of i-pentane, n-pentane, cyclopentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, methylcyclopentane, benzene and n-heptane is used. In accordance with the process of the present invention, the above naphtha fraction feed is contacted with a catalyst comprising a large pore zeolite containing at least one of Group VIII metals of the Periodic Table to produce aromatic hydrocarbons. In the practice of the process of the present invention, it is necessary that the cyclopentane content of the naphtha fraction feed be controlled to not more than 1% by weight, or the methylpentane content be controlled to not more than 10% by weight, preferably not more than 5% by weight and more preferably not more than 2% by weight, prior to the catalytic reforming reaction. In order to control the cyclopentane content of the naphtha fraction feed to not more than 1% by weight, it suffices that the naphtha fraction feed is distilled to separate it into a hydrocarbon fraction having 5 or less carbon atoms (C₅⁻ fraction) as a tower head oil (an overhead fraction) and a hydrocarbon fraction having 6 to or more carbon atoms (C₆⁺ fraction) as a tower bottom oil. Since cyclopentane has a boiling point near those of 2,2-dimethylbutane and the like in the C₆ fraction and thus they are difficult to separate from each other, it is preferred that the number of stages of fractionator be increased, or the reflux ratio be increased. The methylpentane content of the naphtha fraction feed can be easily controlled to not more than 10% by weight with, for example, (1) a method in which the naphtha fraction feed is contacted with a suitable adsorption separator (e.g., a molecular sieve) to selectively separate paraffins through adsorption, thereby removing methylpentane, or (2) a method comprising a step in which the naphtha fraction feed is distillated (distillation step) and a step in which the tower bottom oil obtained at the distillation step is contacted with a suitable adsorption separator to selectively separate paraffins through adsorption, thereby removing methylpentane (adsorption separation step). That is, in accordance with the method (2) above, the naphtha fraction feed is separated into a C₅⁻ fraction as tower head oil and a C₆⁺ fraction as the tower bottom oil through distillation at the distillation step, and the C₆⁺ fraction thus obtained is introduced in the adsorption separation step where paraffins are selectively separated through adsorption with a suitable adsorption separator (e.g., a molecular sieve) to thereby control the methylpentane content of the naphtha fraction feed to be fed to the catalytic reactor to not more than 10% by weight. The reaction at the adsorption separation step can be carried out under conditions commonly employed in the usual adsorption separation, for example at a temperature of 200 to 300°C under a pressure of 1,1 to 4,1 MPa (10 to 40 kg/cm²G). The order in which the distillation step and the adsorption separation step are carried out is not critical, and the adsorption separation step may be provided before the distillation step. The naphtha fraction feed thus obtained, having a cyclopentane content of not more than 1% by weight or a methylpentane content of not more than 10% by weight, preferably not more than 5% by weight and more preferably not more than 2% by weight, and particularly preferably having a methylpentane content of not more than 10% by weight and a cyclopentane content of not more than 1% by weight at the same time is introduced in the catalytic reactor where it is subjected to catalytic reforming. The catalyst to be used in the process of the present invention is a large pore zeolite containing at least one of Group VIII metals of the Periodic Table. This large pore zeolite includes zeolite X, zeolite Y, zeolite L and the like. Of these, zeolite L is preferred. This zeolite L is represented by the composition formula: 0.9 - 1.3 M 2 / n O · Al₂O₃ · 5.0 - 7.0 SiO₂ · 0 - 9 H₂O (wherein M indicates an alkali metal or an alkaline earth metal, and n indicates the atomic valency of M). More specifically, the zeolites described in JP-A-133835/1983 (p. 9 to 10) and 80333/1984 (p. 5) can be used. The catalyst to be used in the present invention is a lange pore zeolite, such as zeolite L, with at least one of Group VIII metals, such as platinum, iron, cobalt, and nickel, deposited thereon. Particularly preferred is a lange pore zeolite with platinum deposited thereon. The amount of at least one of Group VIII metals being deposited is not critical. It is usually 0.1 to 5.0% by weight, preferably 0.3 to 1.5% by weight based on the total weight of the catalyst. Deposition of metals such as platinum on a giant fine pore zeolite such as zeolite L can be carried out by various methods such as the vacuum impregnation method, the ordinary pressure impregnation method, the dipping method, and the ion exchange method. As a platinum source when platinum is used as the metal to be deposited, various compounds, specifically platinum ammine chloride, chloroplatinic acid, chloroplatinic acid salts, platinum tetraammine hydroxide, and platinum dinitrodiammine can be used. In accordance with the process of the present invention, the naphtha fraction (preferably the C₆⁺ fraction) as obtained above is introduced in a catalytic reactor charged with the above metal-deposited zeolite catalyst as a dehydrogenation cyclization catalyst and contacted with the catalyst under high temperature and high pressure conditions in the presence of hydrogen to produce aromatic hydrocarbons and hydrogen through the catalytic reforming reaction. In this reaction, the temperature is 350 to 600°C and preferably 400 to 550°C, the pressure is 0,1 to 4,1 MPa (0 to 40 kg/cm²G) and preferably 0,1 to 1,1 MPa (0 to 10 kg/cm²G) and the liquid hourly space velocity (LHSV) is 0.1 to 20 h⁻¹ and preferably 1 to 10 h⁻¹. Much better results can be expected when the ratio of hydrogen gas to the naphtha fraction feed, i.e., hydrogen/naphtha fraction feed ratio, is controlled to 1:1 to 50:1 (by mole) and preferably 2:1 to 10:1 (by mole). Although the reaction can be performed outside the above specified reaction conditions, problems sometimes occur in that aromatic hydrocarbons cannot be produced in the desired yield and the catalytic activity abruptly drops. As described above, the process of the present invention enables to produce aromatic hydrocarbons from a naphtha fraction feed with high selectivity. Further, since the catalyst cycle (catalyst life) is markedly lengthened, the number of regeneration of the catalyst can be decreased. As a result, the operation can be carried out continuously for a long period of time, which is greatly advantageous from an economic standpoint. Accordingly the process of the present invention is of the high practical value as a process for efficient production of aromatic hydrocarbons which are useful as starting materials for various products, or solvents and the like in the petrochemical industry. The present invention is described in further detail with reference to the following examples. EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 AND 2 A full range naphtha was distilled to obtain a feed oil having the composition shown in Table 1. A fluorocarbon-treated zeolite L with 0.5% by weight of platinum deposited thereon was used as a catalyst. The catalyst was previously subjected to pre-treatment for 24 hours at 540°C under reduced pressure in a stream of hydrogen. A catalytic reactor was charged with 0.5 g of the catalyst subjected to the above pre-treatment, and the above feed oil was introduced in the catalytic reaction tower to cause the catalytic reaction. This reaction was carried out continuously under conditions of temperature 500°C, pressure 0,6 MPa (5 kg/cm²G), liquid hourly space velocity 2 h⁻¹, hydrogen/feed oil ratio (by mole) 5. A relation between the reaction time and the yield of aromatic hydrocarbons is shown in Fig. 1 (Examples 1, 2, and Comparative Example 1) and Fig. 2 (Examples 3, 4 and Comparative Example 2). It can be seen from Figs. 1 and 2 that when the cyclopentane content of the naphtha fraction feed is not more than 1% by weight, the catalyst cycle can be greatly lengthened. EXAMPLE 5 A desulfurized light naphtha fraction was subjected to adsorption separation treatment by contacting with a molecular sieve (an adsorption separator) to obtain a feed oil having the composition shown in Table 2. This feed oil was introduced in a fixed bed reactor charged with 0.5 g of a Furon-treated zeolite L catalyst with 0.5% by weight of platinum deposited thereon, and was subjected to the catalytic reforming reaction under conditions of temperature 500°C, pressure 0,6 MPa (5kg/cm²G), LHSV 2 h⁻¹, hydrogen gas/feed oil ratio (by mole) 5. A relation between the reaction time and the yield of aromatic hydrocarbons is shown in Fig. 3. EXAMPLE 6 The same desulfurized light naphtha fraction as used in Example 5 was introduced in a distillation tower where it was distilled. The tower bottom oil (C₆⁺ fraction) thus obtained was subjected to the adsorption separation treatment under the same conditions as in Example 5 to obtain a feed oil having the composition shown in Table 2. The feed oil thus obtained was subjected to the catalytic reforming reaction under the same conditions as in Example 5. A relation between the reaction time and the yield of aromatic hydrocarbons is shown in Fig. 3. It can be seen from Fig. 3 that when the methylpentane content of the feed oil to be introduced in the catalytic reactor is not more than 10% by weight, the yield of aromatic hydrocarbons is high and the catalyst cycle is greatly lengthened. EXAMPLE 7 The catalytic reforming reaction was carried out under the same conditions as in Example 6 except that the pressure was changed from 0,6 MPa (5 kg/cm²G) to 0,1 MPa (0 kg/cm²G). A relation between the reaction time and the yield of aromatic hydrocarbons is shown in Fig. 4 along with that of Example 6. It can be seen from Fig. 4 that when the reaction pressure is decreased in the catalytic reforming reaction of the feed oil having the composition as controlled according to the present invention, the catalyst life is more lengthened. EXAMPLE 8 A desulfurized light naphtha fraction was subjected to the adsorption separation treatment under milder conditions than in Example 5 to obtain a feed oil having the composition shown in Table 2. The feed oil thus obtained was subjected to the catalytic reforming reaction under the same conditions as in Example 5. A relation between the reaction time and the yield of aromatic hydrocarbons is shown in Fig. 3.

Claims: A process for producing aromatic hydrocarbons by contacting a naphtha fraction with a catalyst comprising a large pore zeolite containing at least one of Group VIII metals of the Periodic Table, characterized in that a) the catalytic reaction is carried out at a temperature of from 400 to 550°C and b) the cyclopentane content or the methyl pentane content of the naphtha fraction to be introduced in the catalytic reaction region is previously controlled to not more than 1 % by weight, and to not more than 10 % by weight, respectively. The process as claimed in Claim 1 wherein the methylpentane content of the naphtha fraction is not more than 5 % by weight. The process as claimed in Claim 1 or 2 wherein the methylpentane content of the naphtha fraction is not more than 2 % by weight. The process as claimed in any of claims 1 to 3 wherein the large pore zeolite is a zeolite L. The process as claimed in any of Claims 1 to 4 wherein the large pore zeolite containing at least one of group VIII metals of the Periodic Table is a zeolite L with platinum doposited thereon. The process as claimed in any of Claims 1 to 5 wherein the large pore zeolite containing at least one of Group VIII metals of the Periodic Table is a platinum deposited zeolite L subject to Fluorocarbon treatment.

Inventors: HISASHI, KATSUNO, NOBUYUKI, TAKAHASHI, TAKASHI, MURAKAWA, TAKASHI, YAMAMOTO, TOSHIKAZU, YONEDA
Applicants: RESEARCH ASSOCIATION FOR UTILIZATION OF LIGHT OIL
Application year: 1988
Main classification: C10G35
IPC codes: C10G35