Title of Invention

"PROCESS FOR CONVERSION OF AROMATIC HYDROCARBONS"

Abstract

The present invention relates to a process for the conversion of aromatic hydrocarbons using the catalyst composition comprising by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a SiO2/Al2O3 molar ratio of 10 to 100, 0.05 to 10 parts of metal bismuth or oxides thereof, M being selected from the group consisting or molybdenum, copper, zirconium, strontium, rhenium, iron, cobalt, nickel, and silver, and 10 to 60 parts of alumina as an adhesive to effect disproportionation and transalkylation of aromatic hydrocarbon reactants comprising substantially toluene and C9 aromatic hydrocarbons and/or C10 aromatic hydrocarbons as well as hydrodealkylation and transalkylation of heavy aromatic hydrocarbons containing C9 aromatic hydrocarbons and/or aromatic hydrocarbons of more than nine carbon atoms under conversion conditions.

Full Text

FIELD OF THE INVENTION The present invention relates to a process for the conversion of aromatic hydrocarbons with a catalyst composition and uses thereof in the production of aromatic hydrocarbons. In particular, process for the conversion of aromatic hydrocarbons using the catalyst composition which is the subject matter of co-pending divisional application No. 1344/DEL/2004, and uses thereof in the production of aromatic hydrocarbons. BACKGROUND OF THE INVENTION A large quantity of aromatic hydrocarbons such as benzene, toluene, xylene and C9 aromatic hydrocarbons (C9 A) may be obtained from the reforming and cracking processes of the petroleum distillates. The contents of toluene and C9A generally range from 40 to 50% of the total amount of the aromatic hydrocarbons dependent on different boiling ranges of the distillate feedstock and different processing methods. Normally C9A, C1O aromatic hydrocarbons (C1O A) and aromatic hydrocarbons of more than ten carbon atoms are referred to as heavy aromatic hydrocarbons in the past. Heavy aromatic hydrocarbons are mainly derived from the side products of the cracking process of light oil for producing ethylene, the aromatic hydrocarbons extraction process in the catalytic reforming in refinery, as well as toluene disproportionation and transalkylation process. For various sources of the feedstock oil and different processing methods, an aromatic hydrocarbon combination unit of 225 thousand ton xylene output per year may produce 10 to 30 thousand tons of heavy aromatic hydrocarbons each year. C10A and aromatic hydrocarbons of more than ten carbon atoms are of little use due to their complicated compositions and high boiling points. These aromatic hydrocarbons are not suitable for use as additive components in gasoline or diesel. Only some of them may be used as solvent oil or as the feedstock for separating durene, and most of the rest are used as burning fuel, causing waste of the resources. With the development of plastic, synthetic fiber and synthetic rubber industries in the recent years, demand for benzene and xylene increases, market prices of which are higher than that of toluene and C9A. It is an important research subject in many countries to increase the production of aromatic hydrocarbons of high value from less valuable aromatic hydrocarbons through conversion processes of aromatic hydrocarbons including hydrodealkylation, toluene disproportionation and transalkylation reactions, thus making full use of the aromatic hydrocarbon resources. Toluene disproportionation is a process in which one mole of benzene and one mole of xylene are produced from two moles of toluene. Toluene may undergo transalkylation reaction with C9A to form xylene. Toluene may undergo transalkylation reaction with C1OA to form C9A. Alkyl aromatic hydrocarbons such as C9A and C1OA may undergo hydrodealkylation reaction to form aromatic hydrocarbons of fewer carbon atoms. A series of catalyst compositions and processes for such reactions have already been developed. In the processes for toluene disproportionation and transalkylation of the aromatic hydrocarbon feedstock substantially comprising toluene and C9A, mordenite is frequently used as the catalyst composition. For example, U.S. Pat. 2,795,629, 3,551,510, 3,729,521; 3,780,122 and 3,849,340 disclose catalyst compositions, feedstock compositions and reaction conditions for toluene disproportionation and transalkylation process, in which catalyst compositions used are not mentioned to comprise bismuth. Japanese patent 49-46295 discloses a catalyst composition for preparing alkyl benzene, which comprises a mordenite with, supported thereon, a zirconium cocatalyst composition anid optionally one or more components selected from silver, bismuth, copper and leacl. The catalyst compositions in the above patents have restricted performances, thus can not resist stringent reaction conditions. Therefore, in the toluene disproportionation and transalkylation processes where the above catalyst compositions are used, C9A and heavy aromatic hydrocarbons of more than nine carbon atoms are not convertted adequately, hence yields of desired products relatively low, energy and material consumption on industrial scale units relatively high. So they are not economical. Catalyst compositions for converting C1OA and heavy aromatic hydrocarbons of more than ten carbon atoms have been reported. For example, Japanese patent publication 51-29131 discloses a catalyst composition, MoO3-NiO/A12O3 (13 wt % Mo, 5 wt % Ni) composition, and a process for treating C9A and C1OA feedstock with this catalyst composition. U.S. Pat. 4,172,813 discloses a catalyst composition composition comprising 3 wt % WO3, Swt % MoO3 and a support consisting of 60 wt % mordenite and 40 wt % A12O3; over this catalyst composition selective hydrodealkylation and transalkylation reactions of heavy reformate are effected, among which the main reaction is the transalkylation reaction between toluene and trimethylbenzene. U.S. Pat. No. 4,341,914 discloses a process for the conversion of C1OA. In these references no catalyst composition containing bismuth is mentioned, contents of C1OA in the feedstock entering the reactor is relatively low, no more than 20%, and the main disproportionation and transalkylation reaction is between toluene and C9A. SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to provide a novel catalyst composition for the conversion of aromatic hydrocarbons. The catalyst composition can be used in (1) disproportionation and transalkylation of aromatic hydrocarbon reactants comprising substantially toluene and C9A and/or C10A as well as in (2) hydrodealkylation and transalkylation of heavy aromatic hydrocarbons containing C9A- and/or aromatic hydrocarbons of more than nine carbon atoms. The catalyst composition has better catalytic capacity for various kinds of conversion reactions of aromatic hydrocarbons and can be employed under stringent reaction conditions. The catalyst composition increases the yields of desired products such as benzene and xylene. Thus, contents of heavy aromatic hydrocarbons in the aromatic reactants to be converted can be highly increased, allowing drying and pre-purifying procedures omitted or simplified. The catalyst composition can improve the conversion of heavy aromatic hydrocarbons, enhance the selectivity and yields of benzene and xylene, make full use of the C9A and heavy aromatic hydrocarbon resources, lower material and energy consumption, and decrease expense. Another object of the present invention is to provide a process for the conversion of aromatic hydrocarbons. The process overcomes the disadvantages of conventional disproportionation, transalkylation and hydrodealkylation processes that heavy aromatic hydrocarbons are restricted under a low content in the aromatic hydrocarbon reactants and that they are not suitable under stringent reaction conditions. Still another object of the present invention is to apply said catalyst composition ancl process to the production of aromatic hydrocarbons, mainly benzene, xylene ancl C9A. The catalyst composition for the conversion of aromatic hydrocarbons according to the present invention comprises by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a Si02/A1203 molar ratio of 10 to 100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of one or more types of metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. The present invention also provides a process for the conversion of aromatic hydrocarbons, in which the aromatic hydrocarbon reactants contact the catalyst composition of the present invention to effect the conversion reaction. The present invention further relates to the use of the catalyst composition and process of the present invention for the conversion of aromatic hydrocarbons in the production of aromatic hydrocarbons such as benzene, xylene and C9A from toluene, C9A, C10A and aromatic hydrocarbons of more than ten carbon atoms. zeolite with a SiO2/AI2O3 molar ratio of 10 to 100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of one or more types of metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. The present invention also provides a process for the conversion of aromatic hydrocarbons, in which the aromatic hydrocarbon reactants contact the catalyst composition of the present invention to effect the conversion reaction. The present invention further relates to the use of the catalyst and process of the present invention for the conversion of aromatic hydrocarbons in the production of aromatic hydrocarbons such as benzene, xylene and C9A from toluene, C9A, C10A and aromatic hydrocarbons of more than ten carbon atoms. According to the present invention there is provided a process for the conversion of aromatic hydrocarbons, comprising contacting said aromatic hydrocarbon feedstock with a catalyst composition wherein said conversion comprises disproportionation and transalkylation of aromatic hydrocarbon reactants comprising substantially toluene and C9 aromatic hydrocarbons and or C10 aromatic hydrocarbons as well as hydrodealkylation and transalkylation of aromatic hydrocarbons containing C9 aromatic hydrocarbons and/or aromatic hydrocarbons of more than nine carbon atoms under a temperature from 300 to 600°C and a pressure from 1.5 to 4.0 Mpa, at a feedstock weight hourly space velocity within the range from 0.5 to 3.0 hr-1 , and a molar ratio of hydrogen to hydrocarbons within the range from 2 to 10, wherein the catalyst composition comprise by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a Si02/Al2O3 molar ratio of 10-100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of at least one of metal (s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. DETAILED DESCRIPTION OF THE INVENTION 1. Catalyst composition of the present invention for the conversion of Aromatic Hydrocarbons and its preparation. The catalyst composition according to the present ivention for the conversion of aromatic hydrocarbons comprises by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a Si02/Al2O3 molar ratio of 10 to 100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of one or more types of metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. The catalyst composition of the present invention may be prepared by weighing the starting materials in amounts corresponding to the predetermined composition of the final product, said starting materials including zeolite, metal bismuth or its compound, metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and alumina, mixing the starting materials thoroughly, followed by extruding, drying, pelleting and calcining for activating. The zeolite used may be natural or synthesized. Non-limiting examples of the zeolite include mordenite, ZSM-5 zeolite and ß-zeolite or a mixture thereof. DETAILED DESCRIPTION OF THE INVENTION 1. Catalyst composition of the Present Invention for the Conversion of Aromatic Hydrocarbons and Its Preparation. The catalyst composition according to the present invention for the conversion of aromatic hydrocarbons comprises by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a SiO2/A1203 molar ratio of 10 to 100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of one or more types of metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. The catalyst composition of the present invention may be prepared by weighing th( starting materials in amounts corresponding to the predetermined composition of the final product, said starting materials including zeolite, metal bismuth or it; compound, metal(s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and alumina, mixing the starting materials thoroughly, followed by extruding, drying, pelleting and calcining for activating. The zeolite used may be natural or synthesized. Non-limiting examples of the. zeolite include mordenite, ZSM-5 zeolite and P-zeolite or a mixture thereof, preferably mordenite, and more preferably hydrogen-form mordenite. The SiO2-to-A1203 molar ratio of the zeolite is within the range from 10 to 100, for example 10 to 30. In one preferred embodiment hydrogen-form mordenite with a sodium content less than 0.2 wt % is used, which mordenite may be a aluminum-lean mordenite prepared by extracting aluminum from low silica mordenite with an inorganic acid, or a hydrogen-form mordenite prepared bv ion-exchanging the direct-crystallized high silica Na-mordenite with ammonium chloride or nitrate solution. Non-limiting examples of bismuth compounds are bismuth oxide and bismuth nitrate, preferably bismuth nitrate. Non-limiting examples of metal NM compound(s) may be oxide(s) or salt(s) thereof, such as M nitrate. When M comprises molybdenum, the molybdenum compound in the starting material may take the form of ammonium molybdate. Said mixing procedure may be carried out by kneading the starting materials or impregnating the solid materials with an aqueous solution. Said extruding, drying, pelleting and calcining procedures may be proceeded by traditional methods in the prior art. 2. Process for the Conversion of Aromatic Hydrocarbons According to the Present Invention The present invention provides processes for the conversion of aromatic hydrocarbons, in which the aromatic hydrocarbon reactants contact the novel catalyst composition of the present invention to effect the conversion reactions. The reaction conditions in said processes may be as follows: In the presence of hydrogen, the aromatic hydrocarbon reactants flow through a gas-solid fixed bed reactor and contact the catalyst composition inside at a reaction temperature within the range from 300 to 600° C., a reaction pressure within the range from 1.5 to 4.0 MPa, an aromatic hydrocarbon reactant weight hourly space velocity within the range from 0.5 to 3.0 hrand a hydrogen-to-hydrocarbon molar ratio within the range from 2 to 10. [0025] The aromatic hydrocarbon reactants comprise one or more aromatic hydrocarbons selected from toluene, C9A, C1OA and aromatic hydrocarbons of more than ten carbon atoms or mixtures thereof, may contain a certain amount of impurities, such as water, indane, trace naphthalene, methylnaphthalene, dimethylnaphthalene and non-aromatic compounds. The aromatic hydrocarbon reactants contact the novel catalyst composition of the present invention under the reaction conditions and there may mainly occur the following reactions: (1) Toluene Disproportionation Reaction: C6H5CH3 + C6HCH3 > C6H6+C6H4(CH3)2 (2) Hydrodealkylation Reactions of Aromatic Hydrocarbons: C6H(CH3)5+H2 > C6H2(CH3)4+CH4 C6H2(CH3)4+H2 > C6H3(CH3)3+CH4 C6H3(CH3)3+H2 > C6H4(CH3)2+CH4 C6H4(CH3)2+H2 > C6H5CH3+CH4 C6H5CH3+H2 > C6H6+CH4 (3) Transalkylation Reactions of Aromatic Hydrocarbons: C6H6+C6H3(CH3)3 > C6H5CH3+C6H4(CH3)2 C6H5CH3+C6H3(CH3)3 > 2C6H5(CH3)2 C6H6+C6H2(CH3)4 > C6H5CH3+C6H3(CH3)3 C6H5CH3+C6H2(CH3)4 > C6H4(CH3)2+C6H3(CH3)3 Conventional processes for toluene disproportionate! and transalkylation from toluene and C9A reactants are carried out in a fixed bed reactor in the presence of hydrogen and a mordenite catalyst composition to produce C610A, C15 alkanes and a small amount of Cl1 aromatic hydrocarbons (Cl 1 A). Toluene and C9A in the reaction zone effluent are separated, recycled, and combined with fresh toluene and C9A outside to enter the reactor as feedstock. In toluene disproportionation and transalkylation processes or hydrodealkylation processes, heavy aromatic hydrocarbons, especially C10A and aromatic hydrocarbons of more than ten carbon atoms, may undergo accompanying side reactions such as (1) hydrocracking reactions to form saturated hydrocarbons and (2) aromatic condensation reactions to form polycyclic or fused ring compounds. The higher the reaction temperature is, the more serious the side reactions are, the more large molecule condensation products are formed, the more coke deposits on the catalyst composition and the quicker the activity of the catalyst composition decreases. C10A fraction contains trace polycyclic compounds such as naphthalene, methylnaphthalene and dimethylnaphthalene, which readily deactivate the catalyst composition. Therefore, as for the conversion reactions such as disproportionation and transalkylation of aromatic hydrocarbons, in order to slow the coke deposit rate on the catalyst composition and prolong catalyst composition life, it is required to run the reactions in the presence of hydrogen and to limit C10A contents in the aromatic hydrocarbon reactants to generally less than 4%, at the most no higher than 8%, and less than 2% in industrial practices. Indane is a poison to the catalyst composition for disproportionation and transalkylation reactions and usually controlled at less than 0.5%. The known catalyst compositions for disproportionation and transalkylation reactions are of limited performances and can not be used for treating reactants containing high contents of C10A and aromatic hydrocarbons of more than ten carbon atoms. Since the boiling point of indane is very close to that of trimethylbenzene (TMB) in C9A, and the indane content in C9A from the top of the heavy aromatic hydrocarbon tower which provides fresh C9A for the disproportionation unit, generally must be less than 1.0% so as to meet the processing requirements, about 515% of C9A from the tower bottom of the heavy aromatic hydrocarbon tower is removed and can not be fully utilized. It is surprising that the bismuth-containing zeolite catalyst composition of the present invention has much better catalytic properties than known catalyst compositions. Not only does it loosen the limit for the indane content in the reaction feedstock so that the indane content may be up to 05 wt % of the reaction feedstock and thereby it is no longer compulsory to remove most of the indane from the starting feedstock through heavy aromatic hydrocarbon tower, thus loss of C9A during separating indane is avoided; but also does it have stronger catalytic capacity for hydrodealkylation and transalkylation reactions of Cll and C1OA and can resist the poisonous impurities in heavy aromatic hydrocarbon feedstock, so that C1OA can be passed into the reactor or recycled as feedstock instead of being removed from the heavy aromatic hydrocarbon tower bottom, so the utilization ratio of the heavy aromatic hydrocarbons increases, effecting good results. Hence, in the process of the present invention for the conversion of aromatic hydrocarbons, the aromatic hydrocarbon reactants may comprise substantially a mixture of toluene and C9A in which the weight ratio of toluene to C9A is within the range from 90/10 to 10/90. In the process of the present invention for the conversion of aromatic hydrocarbons, the aromatic hydrocarbon reactants may comprise substantially heavy aromatic hydrocarbons, such as C9A, C1OA and aromatic hydrocarbons of more than ten carbon atoms or a mixture thereof. In the process of the present invention for the conversion of aromatic hydrocarbons, the aromatic hydrocarbon reactants may comprise substantially a mixture of toluene, C1OA and aromatic hydrocarbons of more than ten carbon atoms, in which the weight ratio of toluene to C1OA is within the range from 90/10 to 10/90. It is also surprising that, for the catalyst composition of the present invention, water content in the reactant mixture is not required to be very low. In U.S. Pat. No. 3,780,122, water content in the toluene feedstock has remarkable effect on the activity and stability of the catalyst composition for the toluene disproportionation reaction; even very low water content (15 ppm) can influence the toluene conversion. In this patent water content in toluene is required to be less than 25 ppm. U.S. Pat. No. 4,665,258 (1987) provides a novel improved toluene disproportionation process, in which aluminum-lean mordenite is used as a catalyst composition, can be carried out under stringent reaction conditions. The mordenite used in this catalyst composition is of a silica-to-aliunina molar ratio more than 30, preferably within the range from 40 to 60. Feedstock of more than 25 ppm water content may be directly passed into the reaction zone; yet permitted water content may be within the range from 50 and 250 ppm. In the process of the present invention, the bismuth-containing zeolite catalyst composition used has substantially improved water resistance and can even maintain high activity and stability when the feedstock contains up to 500 ppm water. For an industrial scale unit the dehydrating procedure for the feedstock can therefore be omitted or simplified. In addition, the high activity of the present catalyst composition can achieve a hydrocarbon conversion ratio of, for example, up to 45% at a low reaction temperature, meanwhile preserving excellent stability, effecting very good results. Therefore, in the process of the present invention for the conversion of aromatic hydrocarbons, water content in the aromatic hydrocarbon reactants may be up to 500 ppm. 3. Use of the Catalyst composition and Process of the Present Invention in the Production of Aromatic Hydrocarbons. By the process of the present invention for the conversion of aromatic hydrocarbons, benzene and toluene may be produced from feedstock substantially comprising toluene and C9A; and benzene, xylene and C9A may be produced from feedstock containing toluene, C10A and aromatic hydrocarbons of more than ten carbon atoms. A small amount of C14 aliphatic hydrocarbons may be formed in each case above. Thereby, the process of the present invention may be applied to the production of benzene, xylene and C9A from feed materials of various complex compositions. One embodiment of applying the process of the present invention to the production of benzene and xylene comprises the following steps of: (a) separating an aromatic feedstock comprising indane, Cs aromatic hydrocarbons (C8A), C9A, C10A and Cll aromatic hydrocarbons (C11A) in a first separation zone comprising a first and a second separation tower, where a stream rich in C8A is separated from the top of the first separation tower and the bottoms product of the first tower is passed into the second separation tower, where a stream comprising indane, C9A and C1OA, with an indane content of 0 to 5 wt % and a C10A content of 0 to 50 wt %, is separated from the top of the second separation tower and Cl1A are removed from the second tower bottom; (b) passing the effluent stream from the top of the second separation tower along with toluene into a conversion reaction zone for the aromatic hydrocarbons, where said reaction zone is packed with the catalyst composition of the present invention, and the aromatic hydrocarbons are transformed, upon contacting the catalyst composition under conversion conditions, into a converted effluent rich in benzene and C8A; and (c) passing said converted effluent into a second separation zone and separating them into benzene, toluene, C8A and heavy aromatic hydrocarbons containing C10A, According to the above embodiment, toluene separated from the second separation zone can be fed into the reaction zone. A part of benzene separated from the second separation zone can be recycled into the reaction zone to increase C8A yield; however, it may be removed directly as a product instead of being recycled because recycling of benzene will lower the conversion ratio of feed toluene. The heavy aromatic hydrocarbons containing C10A separated from the second separation zone may be passed into the second separation tower of the first separation zone with or without o-xylene separated therefrom. In the feed stream entering into the conversion reaction zone, the weight ratio of toluene to C9A is within the range from 90/10 to 10/90. By employing the novel catalyst composition of the present invention, limit to indane content in the aromatic feedstock is loosened, allowing it to range from 0 to 5 wt %. Therefore, it is not compulsory to separate and remove a major part of indane, which is of small amount, accompanied by the C9A feedstock through the heavy aromatic hydrocarbon tower, hence loss of C9A in indane separation process can be eliminated. Since the catalyst composition of the present invention enjoys strong capacity of converting C10A, ClOAper se may be recycled and it is no longer necessary to remove them from the heavy aromatic hydrocarbon tower bottom, thus increasing CIoA utilization ratio. FIG. 1 is a schematic diagram of a preferred embodiment of the use of the process of the present invention in the production of aromatic hydrocarbons. The process represented by FIG. 1 comprises a first separation zone, a second separation zone and an aromatic hydrocarbon conversion reaction zone 3. The first separation zone comprises an xylene tower 1 and a heavy aromatic hydrocarbon tower 2. The second separation zone comprises a benzene tower 4, a toluene tower 5 and an xylene tower 6 and/or an o-xylene tower 7. The reaction zone includes a reactor, a high pressure separation tank and a stripping tower. The effluent of the reaction zone, which contains C611A, is first passed into the benzene tower 4, where a product stream 11 rich in benzene is separated and removed or partially recycled into the reaction zone. The tower bottoms product of the benzene tower 4 is passed into the toluene tower 5. The toluene recycle stream 10 from the top of the toluene tower 5 is passed into the reaction zone along with fresh toluene, and the tower bottoms product of the toluene tower 5 is passed into the xylene tower 6. The stream -13 separated from the xylene tower 6, a mixture of ethylbenzene, m-xylene and p-xylene, is combined with stream 12 rich in C8A from the top of the xylene tower 1 and removed from the system. The tower bottoms products of the xylene tower 6 may be combined with the tower bottoms product of the xylene tower 1 which is rich in C9A and indane, and passed into the heavy aromatic hydrocarbon tower 2. Or, the tower bottoms product of xylene tower 6 may be first passed into an o-xylene tower 7 to separate therefrom a stream 14 rich in o-xylene stream, then combined with the tower bottoms product of the xylene tower 1 and then passed into the heavy aromatic hydrocarbon tower 2. A stream 15 separated from the top of the heavy aromatic hydrocarbon tower 2, rich in C9A, C10 hydrocarbons and containing all indane brought in, is fed into the reaction zone, while the tower bottoms effluent stream 11 rich in Cl1A, is removed from the system. The C1O hydrocarbons in the stream from the top of the heavy aromatic hydrocarbon tower of this process comprise C10A, C1O cyclic hydrocarbons and C1O fused ring hydrocarbons. Other advantages and features of the present invention will be apparent upon reading the following non-limiting examples. EXAMPLES "I. Catalyst compositions for the Conversion of Aromatic Hydrocarbons. Example 1 77.8 g of an ammonium-form mordenite (Na2O content: less than 0.15%, SiO2/A12O3 molar ratio: 12, weight loss after calcination at 550° C.: 30%) powder was mixed thoroughly with 42.8 g of pseudoboehmite ([alpha]-A12O3. H2O, Na2O content: less than 0.15%, weight loss after calcination at 550° C.: 30%). 0.10 g of bismuth nitrate [Bi(NO3)3.5H2O, chemically pure], 2 ml of nitric acid (chemically pure), 60 ml of water, 5.77 g of ammonium molybdate [(NH4)6Mo7024, chemically pure] were mixed to prepare a solution. This solution was added into the mixture of ammonium-form mordenite and pseudoboehmite, and the resultant mixture was mixed and kneaded thoroughly, extruded into strips, dried by oven, pelleted and calcined for activating to yield a catalyst composition A, the Bi2O3/MoO3/H-mordenite/A1203 weight ratio of which was 0.05/5/70/30. ExampFe 2 44.4 g of an ammonium-form mordenite powder and 85.7 g of pseudoboehmite in example 1 were mixed thoroughly. 0.14 g of bismuth nitrate [Bi(NO3)3.5H2O, chemically pure], 0.58 g of ammonium molybdate [(NH4)6Mo7O24, chemically pure], 19.4 g of nickel nitrate [Ni(NO3)2.6H20, chemically pure], 1.5 ml of nitric acid (chemically pure) and 60 ml of water were mixed to prepare a solution. This solution was added into the mixture of ammonium-form mordenite and pseudoboehmite, and the resultant mixture was mixed and kneaded thoroughly, extruded into strips, dried by oven, pelleted and calcined for activating to yield a catalyst composition B, the Bi2O3/MoO3/NiO/H-mordenitelA12O3 weight ratio of which was 0.07/0.5/5.0/40/60. Example 3 55.6 g of an ammonium-form mordenite (Na2O content: less than 0.15%, SiO2/A12O3 molar ratio: 26.1, weight loss after calcination at 550° C.: 10%) powder was mixed with 71.4 g of pseudoboehmite of the same type in example 1. 10.4 g of bismuth nitrate [Bi(NO3)3. 5H2O, chemically pure], 2.1 ml of nitric acid (chemically pure), 50 ml of water were mixed to prepare a solution. This solution was added into the mixture of ammonium-form mordenite and pseudoboehmite, and the resultant mixture was mixed and kneaded thoroughly, extruded into strips, dried by oven, pelleted and calcined for activating to yield a catalyst composition C, the Bi2O3H-mordenite/AI203 weight ratio of which was 5/50/50. Example 4 66.7 g of an ammonium-form mordenite (Na2O content: less than 0.15%, SiO2/A12O3 molar ratio: 19.2, weight loss after calcination at 550° C.: 10%) was mixed thoroughly with 57.1 g of pseudoboehmite of the same type in example 1. 1.88 g of bismuth nitrate [Bi(NO3)3.5H20, chemically pure], 3.88 g of nickel nitrate [Ni(NO3)2.6H2O, chemically pure], 1.8 ml of nitric acid (chemically pure) and 55 ml of water were mixed to prepare a solution. This solution was added into the mixture of ammonium-form mordenite and pseudoboehmite, and the resultant mixture was mixed and kneaded thoroughly, extruded into strips, dried, pelleted and calcined for activating to yield a catalyst composition D, the Bi2O3/NiO/H-mordenite/A12O3 weight ratio of which was 0.9/0.1/60/40. Example 5 A ZSM-5 zeolite with a SJO2/A12O3 molar ratio of 65 was synthesized by the method described in U.S. Pat. No. 3,702,886(1972), calcined at 550° C. for 2 hours under nitrogen atmosphere, then ion-exchanged with ammonium chloride or nitrate solution at 8098° C. for 110 hours, filtered to remove the mother liquor, then ion-exchanged repetitively for several times, washed with deionized water, and dried by oven at 110° C. to yield a H-ZSM-5 zeolite, the Na2O content of which was less than 0.1 wt %. H-ZSM-5 zeolite and pseudoboehmite ([alpha]-A12O3.H2O) were mixed at a weight ratio of 70/30, added with dilute nitric acid, bismuth nitrate and water, kneaded thoroughly, extruded into strips, dried by oven at 110° C., pelleted and calcined at 560° C. for 4 hours to yield a catalyst composition E , the Bi2O3 content of which was 1.0 wt %. Example 6 A catalyst composition F was prepared in the same manner as in example 5 except that a commercial [beta]-zeolite (SiO2A12O3 molar ratio: 35.0) was used in place of the synthesized ZSM-5 zeolite. Catalyst composition F is a bismuth-containing P-zeolite catalyst composition, the Bi2O3 content of which was 1.0 wt %. Example 7 (Comparative Example) A catalyst composition EC, a ZSM-5 zeolite catalyst composition containing no bismuth, was prepared in the same manner as in example 5 except that no bismuth nitrate was added. Example 8 (Comparative Example) A catalyst composition Fc, a [beta]-zeolite catalyst composition containing no bismuth, was prepared in the same manner as in example 6 except that no bismuth nitrate was added. Example 9 A high-silica H-mordenite zeolite was synthesized from a high-silica Na-mordenite, which was of a S1O2/A12O3 molar ratio of 1530 and prepared according to the method described in the Chinese patent ZL 89106793.0, by ion-exchanging with ammonium chloride or nitrate solution at 9098° C. for 18 hours and filtering to remove the mother liquor, then ion-exchanging repetitively for several times, washing, and drying by oven at 110° C. to obtain a high-silica H-mordenite. The high-silica H-mordenite and pseudoboehmite ([alpha]-A12O3.H2O) were mixed, added with dilute nitric acid, bismuth nitrate and water, kneaded thoroughly, extruded into strips, dried by oven at 110° C., pelleted and calcined at 580° C. to yield a catalyst composition Gl, the Bi2O3 content of which was 0.1 wt %. Catalyst compositions G2, G3, G4, G5 and G6, with different contents of mordenite, alumina and Bi2O3 as shown in Table 6, were prepared respectively in the same manner by varying the amounts of pseudoboehmite and bismuth nitrate used. Example 10 A high-silica H-mordenite and a commercial pseudoboebmite ([alpha]-A12O3 H2O) were mixed, added with dilute nitric acid and water, kneaded thoroughly, extruded, dried by oven at 110° C., pelleted and calcined at 400° C. to yield cylinder particles. The cylinder particles were impregnated with bismuth nitrate aqueous solution over night, dried by oven at 110° C., and calcined at 540° C. to yield a catalyst composition Hi, the Bi203 content of which was 0.1 wt %. Catalyst compositions H2, H3, H4 and H5, compositions of which were shown in Table 6, containing Bi2O3, respectively, 0.5, 1.0, 5.0, 10.0 wt %, were prepared correspondingly in the same manner by varying the amount of bismuth nitrate used. Example 11 In this example, a bismuth-containing aluminum-lean mordenite catalyst composition was prepared. An aluminum-lean mordenite was synthesized as follows: A commercial Na-mordenite with a SiO2/A12O3 molar ratio of 10 was refluxed with dilute nitric acid solution at 90° C. to extract aluminum from the mordenite, filtered, washed, and dried to obtain an aluminum-lean mordenite with a SiO2/AI2O3 molar ratio of 15.1, referred to hereinafter as aluminum-lean mordenite zeolite HM-15.1. The dealuminating procedure was repeated several times for HM-15.1 to obtain aluminum-lean mordenite zeolites with SiO2/A12O3 molar ratios of 19.8 and 24.9, referred to hereinafter as HM-19.8 and HM-24.9 respectively. Catalyst compositions II, 12 and 13 were prepared in the same manner as in example 9 except that HM-15.1, HM-19.8 and HM-24.9 were respectively used in place of the high silica H-mordenite zeolite. Example 12 (Comparative Example) A catalyst composition J, a mordenite zeolite catalyst composition containing no bismuth, was prepared in the same manner as in example 9 except that no bismuth nitrate was added. Example 13 High-silica H-mordenite and commercial pseudoboehmite ([alpha]-A12O3.H2O) were mixed, added with dilute nitric acid, water, bismuth nitrate and lanthanum nitrate, kneaded thoroughly, extruded into strips, dried by oven at 110°C, pelleted and calcined at 500°C to yield a catalyst composition Kl, the Bi2O3 content of which was 0.1 wt %. Catalyst compositions K2, K3, K.4 and K5, as shown in Table 8, containing Bi2O3 and an oxide of copper, zirconium, rhenium or strontium were prepared respectively in a similar manner. II. Process of the Present Invention for the Conversion of Aromatic Hydrocarbons. Catalyst compositions prepared in the above examples were employed in the conversion processes for aromatic hydrocarbons in the following examples. (1) Where the Aromatic Hydrocarbon Reactants Comprise Substantially Toluene and C9A. Example 14~17 A cylinder stainless steel reactor with an inner diameter of 25 mm, a length of 1000 mm was used. 20 g of catalyst composition G3 of example 9 were packed inside the reactor to form a catalyst composition bed, on top of which and below which were filled with glass beads of 5 mm diameter for the purposes of gas stream distribution, supporting the bed, preheating and vaporizing the feedstock. The feed toluene and C9A were derived from the aromatic hydrocarbon combination unit in petrochemical industry. The feedstock was mixed with hydrogen and passed through the catalyst composition bed from the top to the bottom. The hydrogen used was obtained from electrolysis. The reactor was heated electrically and the reaction temperature was controlled automatically. Feedstock compositions, catalyst compositions used, reaction conditions and experimental results were summarized in Table 1. Table 1 Reaction Conditions and Results for Feedstock Rich in Toluene and (Table Removed) Data in the above Table were processed according to the following equations: Toluene in Feedstock—Toluene in Product Effluent Toluene Conversion = — —x 100% (wt) Toluene in Feedstock C9A in Feedstock — C9A in Product Effluent C9A Conversion = — _ X100%(wt) C9A Ln Feedstock Indane in Feedstock — Indane in Product Effluent Indane Conversion = - XlOO%(wt) Indane in Feedstock Conversion of C9 Non-aromaric Hydrocarbons = Cg Non-aromatic Hydrocarbons in C9 Nnon-aromatic Hydrocarbons in Product Feedstock — Effluent X100%(wt) C9 Non-aromatic Hydrocarbons in Feedstock Benzene in Product Effluent -Benzene in Feedstock Benzene Selectivity =- — .. __ X 10uVo(wt) (Toluene + Cg A) Converted in Reaction C8A in Product Effluent - C8A in Feedstock C9A Selectivity = — —— : X100% (wt) (Toluene +C9A) Converted in the Reaction Results of example 15 obtained by processing the corresponding data in Table 1 were as follows: Toluene Conversion: 39.10% C9A Conversion: 60.02% Indane Conversion: 95.55% C9 Non-aromatic Hydrocarbon 89.42% Conversion: Benzene Selectivity: 20.32% C8A Selectivity: 74.63% As can be seen from the data in Table 1, while proportions of the different As can be seen from the data in Table 1, while proportions of the different product components varied with feedstock compositions and reaction conditions, quantities of benzene and C8A increased significantly, illustrating that benzene and C8A were produced in the reaction. From the data in Table 1 and the data processing results as to example 15, it can be seen that concentrations of Cg non-aromatic hydrocarbons and indane contained in the feedstock decreased substantially after reaction, illustrating that the catalyst composition of the" present invention enjoys very good capacities of converting C9 non-aromatic hydrocarbons and indane. Therefore, if an o-xylene production unit is provided in the separation flow for the product effluent of toluene disproportionation and transalkylation processes, o-xylene of high quality (purity higher than 98%) can be produced. The above examples showed that the feedstock are allowed to contain high concentrations of indane and CIO hydrocarbons. Whereby all C9A, indane and a part, or even all, of C1O hydrocarbons may be separated from the top of the heavy aromatic hydrocarbon tower in the industrial units and C9A resource can be fully utilized. Meanwhile, since C1O hydrocarbons in the feedstock can inhibit to a certain extent the disproportionation reaction of C9A, which favors transalkylation of C9A to form C8A, C8A selectivity is increased. Therefore, high purity o-xylene can be produced from the process of the present invention, and further, more o-xylene can be produced with the same reaction feedstock consumption. (II) Where the Aromatic Hydrocarbon Reactants Comprise Substantially Toluene and C10A. The reactor in example 14 was used, with 20 g of a catalyst composition packed therein. Feedstock rich in toluene and C10A was mixed with hydrogen and passed through the catalyst composition bed from the top to the bottom to effect aromatic hydrocarbon conversion reactions. The feed toluene and C10A were available from hydrocarbon combination units in petrochemical industry. Example 18-20 Catalyst composition A was used. The reaction conditions and results were summarized in Table 2. Table 2 Reaction Conditions and Results for Feedstock Rich in Toluene and C10A (Table Removed) Catalyst Composition B, Example 21—23 ••C, and D were used. The reaction conditions and results were Table 3 Reaction Conditions and Results for Feedstock Rich in Toluene and C10A (Table Removed) The following equations were employed to process the data in Table 3: Toluene into Reactor - Toluene out from Reactor Toluene Conversion = — — — X100% Toluene into Reactor C10A into Reactor - C10A out from Reactor C10A Conversion =— • ——: XI00% C10A into Reactor Benzene out from Reactor - Benzene into Reactor Benzene Selectivity =-. X100% (wt) (Toluene + C10 A) Converted in Reaction C9A out from Reactor - CgA into Reactor C9A Selectivity =• — —• • X100% (wt) (Toluene + C10 A) Converted in Reaction C8A out from Reactor - C8A into Reactor C8A Selectivity = X 100% (wt) (Toluene + C10 A) Converted in Reaction Results of example 19 by processing the corresponding data of feedstock compositions and product effluent compositions in Table 3 were as follows: Toluene Conversion: 40.78% C10A Conversion: 86.22% Benzene Selectivity: 12.49wl% C8A Conversion: 55.26 wt% C9A Selectivity: 21.66 wt% (Benzene + C6A) Selectivity: 67.75 wt% (Benzene + C8-C9A) Selectivity 89.41 wt% It can be seen that the proportions of benzene, toluene, C8A and C,A in the product effluent varied with the weight ratios of toluene and C10A in the feedstock. Yet the following features are shared in common: (1) Both toluene and C10A contents decreased after reaction in all cases of different toluene to C10A weight ratios, demonstrating that toluene and C10A were converted in the reactions; (2) Benzene, C8A and C9A contents increased simultaneously in all cases of different toluene to C10A weight ratios, demonstrating that benzene, C8A and C9A were formed in the reactions. It can be seen from Table 2 and the data processing results that product compositions and reaction results for disproportionation and transalkylation of toluene and C1OA are similar to that of toluene and C9A with the same weight ratio, except that the main products are benzene, C8A and C9A for disproportionation and transalkylation of toluene and C1OA, while the main products are benzene and C8A for disproportionation and transalkylation of toluene and C9A. It can be seen from Table 2 and Table 3 that the bismuth-containing zeolite catalyst compositions A, B, C and D of the present invention all enjoy fairly good effects in the conversion reactions of feedstock comprising substantially toluene and C1OA. It can be seen from the examples that the process of the present invention for the disproportionation and transalkylation of toluene and C1OA is an efficient way to produce benzene, C8 and C9A from toluene and C1OA. C1OA play substantially the same role in the process for the disproportionation and transalkylation of toluene and C1OA as C9A in the process for the disproportionation and transalkylation of toluene and C9A, i.e., C1OA may be used to produce benzene and C8A instead of C9A. (Ill) Where the Aromatic Hydrocarbon Reactants Comprise Substantially Heavy Aromatic Hydrocarbons. 1. Where the Heavy Aromatic Hydrocarbons Comprise Substantially C1OA C1OA feedstock, the composition of which is shown in Table 4, containing methylethylbenzene, trimethylbenzene, indane, diethylbenzene, dimethylethylbenzene, tetramethylbenzene, and other C1O and C11A, was derived from an aromatic hydrocarbons combination unit in petrochemical industry and its composition is shown in Table 4. TABLE 4 Composition of C1OA Feedstock (Table Removed) In the C1OA feedstock, the sum of the C1O and C11A contents was 95.02%, inclane content was 2.13% and C9A content was only 2.85%. The reactor of example 14 was used, with 20 g of a catalyst composition packed therein. Feedstock rich in C9A and C1OA was mixed with hydrogen and passed through the catalyst composition bed from the top to the bottom to effect hydrodealkylation and transalkylation reactions, in which aromatic hydrocarbons of less carbon atoms, for example, benzene, toluene, ethyl benzene, dimethylbenzene, methylethylbenzene, trimethylbenzene and the like, as well as small quantities of alkanes, for example, methane, ethane, propane, butane and the like, were produced. Hydrogen was used in this process because, on the one hand, the hydrodealkylation reactions per se consume hydrogen; on the other hand, presence of hydrogen can prolong catalyst composition life by inhibiting coke deposition thereon. Example 24-27 Feedstock rich in C1OA was subjected to hydrodealkylation and transalkylation reactions over the catalyst compositions A to D prepared in Examples 1 to 4 respectively. The experimental results were processed according to the following equations: C6-C9A Produced C6~C9A Yield = • X100% (C10 + C11A) Converted C10A into Reactor - C10A out from Reactor C10A Conversion =- into Reactor -Xl00'/o(wt) C,,A Conversion —- CUA into Reactor - CnA out from Reactor C,,A into Reactor -XlCD%(wt) Tabled Reaction Coaditions and Results for Feedstock Rich in C10 A (Table Removed) The evaluation results of catalyst compositions A to D illustrate that catalyst compositions comprising mordenite and a cocatalyst composition which comprises bismuth and one or more component,; selected from Fe, Co, Mi and Mo enjoy good catalytic capacities for hydrodealkylation and transalkylation reactions of C1O and C11A. Under preferred conditions, C10A conversion can reach 46.6%, C11A conversion can reach 80.807) and C6 to C9A yield can reach up to 94.1%. As can be seen from the product effluent composition, though Cl 1A content in the feedstock was as high as 18.22%, C11A content in the product stream was no higher, than 6%, illustrating that C11A undergo more readily than C10A hydrodealkylation and transalkylation reactions, therefore Cl 1A can be used effectively in the process of the present invention for the production of C6 to C9A. In the product effluents, benzene contents were significantly lower than that of toluene, C8A an C9A. For example, in the product effluent of example 24, the benzene/tuluene/C8A/C9A molar ratio was 1:3.35:5.57:4.28. The following reasons for low benzene contents are presumed: (1) Conversion of C10A and C11A to benzene required multi-step serial hydrodealkylation and transalkylation reactions, which led to little benzene generated; or (2) Benzene formed intermediately from the multi-step serial reactions, though the quantity of which might be considerable, was consumed in its transalkylation reactions with C10A to produce toluene and C9A as well as with C9A to produce toluene and xylene under the reaction conditions, whereby the benzene contents in the final products were significantly lower than that of toluene, C8A and C9A. No propylbenzene was detected in the product effluent. This can presumably be attributed to the facts that no propyl containing components were comprised in the feedstock, and that propylbenzene formed intermediately, the amount of which was small, underwent depropylation reactions and were consumed in its entirety. 2. Where the Heavy Aromatic Hydrocarbons Comprise Substantially C9A Example 28 Feedstock rich in C9A was passed into a reactor of example 14 with catalyst composition G3 packed therein, to effect reactions at a temperature of 390° C., a pressure of 1.0 MPa, a hydrogen to hydrocarbons molar ratio of 4.0, and a feedstock weight hourly space velocity of 2.0 hr1. The feedstock comprised 1.00 wt % of toluene, 1.30 wt % of indane, 97.0 wt % of C9A, 0.70 wt % of C10A. The product effluent comprised 2.07 wt % of benzene, 14.50 wt % of toluene, 27.14 wt % of C8A, 39.07 wt % of C9A, 9.85 wt % of C10A and 1.04 wt % of C11A. Data processing results were obtained in the same way as described in the previous examples. C9A Conversion=59.8 wt % (Toluene+CgA+Benzene) Selectivity=67.9 wt % It can be seen that catalyst compositions of the present invention enjoy good catalytic capacities for the conversion of heavy aromatic hydrocarbons comprising substantially C9A. (IV) Where the Aromatic Hydrocarbons Comprise High Content of Water Example 29 A series of experiments were conducted in this example. Feedstock comprising toluene and C9A with a 60/40 molar ratio and a water content of 500 ppm, was passed into a reactor of example 14, which was packed respectively with 20 g of the bismuth containing H-ZSM-5 zeolite catalyst composition E of example 5, the bismuth containing [beta]-zeolite catalyst composition F of example 6, the H-ZSM-5 zeolite catalyst composition EC without bismuth of example 7, the P-zeolite catalyst composition F without bismuth of example 8, the bismuth containing high-silica mordenite catalyst compositions of examples 9-11 and the mordenite catalyst composition J without bismuth of example 12, to effect reactions at a temperature of 385° C., a pressure of 3.0 MPa(gauge ), a hydrogen to hydrocarbons molar ratio of 5 and a feedstock weight hourly space velocity of 1.5 hr-1. Experimental results were summarized in Table 6. (Table Removed) As is shown in Table 6, all of the catalyst compositions of the present invention, whether in which bismuth was added by kneading or impregnating with an aqueous bismuth solution, demonstrate good activities and selectivity, especially enjoy much higher C9A conversion capacities than the comparative catalyst composition J without bismuth. Even among catalyst compositions in which the zeolite used were aluminum-lean mordenites, those containing bismuth enjoy higher activities than those without bismuth. As is also shown in Table 6, the ZSM-5-zeolite and [beta]-zeolite catalyst compositions containing bismuth demonstrate higher C9A conversion capacities than their counterparts Nvithout bismuth. These experiments illustrate that bismuth contained in the catalyst compositions of the present invention is a basic factor contributing to their distinguished performances. Example 30 Further experiments were conducted over catalyst composition G3 of example 9 and catalyst composition J of example 12 to evaluate their stability. The reactor and reaction conditions of example 29 were used, except that initial reaction temperatures for catalyst compositions G3 and J, respectively, were 380° C. and 400° C., and that C9A conversion was maintained at about 45 mol % by elevating the reaction temperatures gradually. Experimental results were summarized in Table 7. For catalyst composition G3 of the present invention, during a reaction cycle of 1000 hours with a mean C9A conversion of 45.2% and a mean (C8A+Benzene) selectivity of 96.1 mol %, reaction temperature was elevated from the initial of 380° C. to the final of 395° C.; while for the comparative catalyst composition J, in a reaction cycle of 500 hours with a mean C9A conversion of 45.1% and a mean (C8A+Benzene) selectivity of 96.0%, reaction temperature was elevated from the initial of 400° C. to the final of 460° C. Thus it can be seen that the catalyst compositions of the present invention are fit for reactants with a high water content, i.e., enjoy high water resistance in addition to their high activities, selectivity and stability. TABLE 7 Stability of Catalyst composition G3 of the Present Invention and Comparative Catalyst composition J (Table Removed) Example 31 Experiments were conducted, using the reactor and reaction conditions of example 29, except that catalyst compositions Kl to K5 of example 13 were packed in respectively, to evaluate their activities. Experimental results were summarized in Table 8. TABLE 8 (Table Removed) Note#: Mordenite: SiO2/Al203 = 27.0 mol/mol; weight ratio of mordenite to alumina = 70/30 Data in Table 8 demonstrate that the mordenite catalyst compositions with, supported thereon, Di203 and one or more optional components selected from CuO, La203, Zr203, Re2O3 and SrO, all enjoy high C9A conversion and (benzene+C8A) selectivity, i.e., enjoy high catalytic performances. The above examples are for the purposes of illustration only and shall not constitute limits to the present invention. It is to be understood by those skilled in the art that any variation and modification of the above embodiments fall within the scope of the appended claims. WE CLAIM: 1. A process for the conversion of aromatic hydrocarbons, comprising contacting said aromatic hydrocarbon feedstock with a catalyst composition wherein said conversion comprises disproportionation and transalkylation of aromatic hydrocarbon reactants comprising substantially toluene and C9 aromatic hydrocarbons and or C10 aromatic hydrocarbons as well as hydrodealkylation and transalkylation of aromatic hydrocarbons containing C9 aromatic hydrocarbons and/or aromatic hydrocarbons of more than nine carbon atoms under a temperature from 300 to 600°C and a pressure from 1.5 to 4.0 Mpa, at a feedstock weight hourly space velocity within the range from 0.5 to 3.0 hr" , and a molar ratio of hydrogen to hydrocarbons within the range from 2 to 10, wherein the catalyst composition comprise by weight 20 to 90 parts of a crystalline aluminosilicate zeolite with a SiO2/Al2O3 molar ratio of 10-100, 0.05 to 10 parts of metal bismuth or oxides thereof supported on the zeolite, 0 to 5 parts of at least one of metal (s) M or oxides thereof, M being selected from the group consisting of molybdenum, copper, zirconium, strontium, lanthanum, rhenium, iron, cobalt, nickel and silver, and 10 to 60 parts of alumina as an adhesive. 2. A process for the conversion of aromatic hydrocarbons as claimed in claim 1 comprising passing said aromatic hydrocarbon feedstock through a catalyst bed reactor and contacting said aromatic hydrocarbon feedstock with said catalyst composition inside the reactor in the presence of hydrogen to effect said conversion. 3. A process for the conversion of aromatic hydrocarbons as claimed in any one of the preceding claims, wherein said aromatic hydrocarbon feedstock comprises water in an amount of 0-500 ppm. 4. A process for the conversion of aromatic hydrocarbons as claimed in any one of the preceding claims, wherein the indane content in said aromatic hydrocarbon feedstock is within the range from 0-5 wt%. 5. A process for the conversion of aromatic hydrocarbons as claimed in any of the preceding claims wherein the said aromatic hydrocarbon feedstock comprises substantially toluene and C9 aromatic hydrocarbons. 6. A process for the conversion of aromatic hydrocarbons as claimed in any of the preceding claims wherein the said aromatic hydrocarbon feedstock comprises substantially toluene, C10 aromatic hydrocarbons and aromatic hydrocarbons of more than ten carbon atoms. 7. A process for the conversion of aromatic hydrocarbons as claimed in any one of the preceding claims wherein the said aromatic hydrocarbon feedstock comprises substantially C9 aromatic hydrocarbons, C10 aromatic hydrocarbons, heavy aromatic hydrocarbons of more than ten carbon atoms or a mixture of two or more types of aromatic hydrocarbons selected therefrom. 8. The process for the conversion of aromatic hydrocarbons as claimed in claim 1 used in the production of C6 to C9 aromatic hydrocarbons. 9. A process for the conversion of aromatic hydrocarbons, substantially hereinbefore described with reference to the foregoing examples and accompanying drawings.

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