PROCESS FOR PRODUCING P-XYLENE AND O-XYLENE

Abstract: A process for the production of p-xylene and o-xylene by catalytically isomerising alkyl aromatic hydrocarbons involves subjecting a substantially C₈ alkyl aromatic hydrocarbon mixture to a temperature of from 250°C to 550°C in the presence of an improved catalyst. This catalyst consists of a high silica zeolite composite in acid form containing small amounts of metals of Group VIII of the Periodic Table dispersed on a porous matrix. As a result of the subjection of the hydrocarbon mixture to high temperature in the presence of such catalyst, the xylene content of the mixture is isomerised to provide an isomeric mixture in equilibrium. Any ethyl benzene present is simultaneously dealkylated. Thereafter, the reaction medium is cooled, the lighter or top product consisting of a benzene-toluene mixture is separated, the heavier bottoms product rich in p-xylene and o-xylene isomers is withdrawn and such bottoms product is treated in any known manner to separate therefrom the p-xylene and o-xylene as individual products.

Description: The present invention relates to an improved process for the catalytic isomerisation of alkyl aromatic hydrocarbons in order to produce p-xylene and o-xylene. More particularly, the invention relates to an isomerisation process for the production of p-xylene and o-xylene wherein a mixture of xylenes and ethyl benzene is reacted with hydrogen over an improved composite high-silica zeolite catalyst. This catalyst which exhibits unique catalytic behaviour for the isomerisation of xylene comprises a variable mixture of amorphous and crystalline alumina, silica and aluminosilicates. The isomerisation of alkyl aromatics has become commercially important in view of its leading to the production of p-xylene and o-xylene. p-xylene is employed principally in the preparation of polyesters while o-xylene finds use mainly in the production of phthalic anhydride. m-xylene has fewer end-uses and accordingly is normally isomerised catalytically to the para and ortho forms which possess greater commercial application. For instance, after p-xylene and o-xylene have been separated from the isomerisation reaction, it is usual for m-xylene to be recycled to the isomerisation reactor along with any unreacted ethyl benzene and small amounts of p-xylene and o-xylene as well as any non-aromatic substances. In this way, the combined desired product and remaining alkyl aromatic hydrocarbons are recycled virtually to extinction. Any mixture of xylenes invariably includes ethyl benzene. This creates a problem during isomerisation since ethyl benzene is not easily isomerised. Furthermore, ethyl benzene can be present in a mixed xylene stream in an amount of from 15% to 40% which is greater than the equilibrium amount. Accordingly, it has been proposed first of all to remove ethyl benzene from the xylene mixture. Unfortunately, this is rather difficult since the boiling points of ethyl benzene and of xylenes are very close to each other. Nevertheless, it has become conventional to remove ethyl benzene present within the mixture of xylenes by means of distillation from pyrolysis gasolene. Two alternative approaches are normally taken to achieve this. According to the first approach, the problem is handled by removing ethyl benzene by disproportionation or hydrodealkylation to yield products such as benzene and heavier C₉ aromatics which can be easily separated from the alkyl aromatic mixture since the latter comprises compounds having only eight carbon atoms. Examples of this sort of prior art process are those described in U.S. Patents Nos. 3856871, 3856872, 3856873 and 3856874 and in European Patent Specification 57607-A in which special zeolite molecular sieves are used to isomerise xylene while converting ethyl benzene to benzene, toluene and C₉ aromatics and non-aromatics. This sort of processes is known as the Mobil vapour phase isomerisation process. As an alternative approach, the prior art discloses the reaction of ethyl benzene with hydrogen in the presence of a hydrogenation-dehydrogenation catalyst to form xylenes. Examples of such catalysts include platinum on alumina, silica alumina and a mixture of alumina and mordernite type zeolite. Typical processes of this nature are those disclosed in U.S. Patents Nos. 276332 and 4128591. The platinum based catalyst employed in such processes provides a hydrogenation-dehydrogenation function which is believed to be necessary for the conversion of ethyl benzene to xylene. Processes employing such catalyst have been found commercially successful and among such processes the one known as the 'Octafining Process' has been unique in its ability to convert ethyl benzene to xylene. Another such process having a similar effect is the one known as the 'Isomar Process'. There is no doubt, therefore, that both the isomerisation processes of the prior art and the catalysts employed thereby have met with a fairly high degree of success not only from the point of view of isomerisation of xylenes but also from that of the conversion of ethyl benzene to xylene. Nevertheless, there is still room for improvement both from the process angle as well as from the point of view of the catalyst employed. The prior art has disclosed a number of catalysts for the isomerisation of xylenes alone as well as for the combined isomerisation of xylenes and the conversion of ethyl benzene to xylene from a mixture of xylenes and ethyl benzene. However, the processes involved do not come anywhere near an approach to chemical equilibrium in the isomerisation of xylenes with minimum xylene loss. This invention provides a process for the isomerisation of xylenes employing a highly selective catalyst which permits the process to be operated at substantially chemical equilibrium with minimum loss of xylenes. The process operates at very low hydrogen to hydrocarbon mole ratios at a high weight hourly space velocity, and shows a high level of tolerance for non-aromatics present among the reactants. Moreover, the catalyst employed maintains its activity and selectivity for prolonged periods. In recent years, the art has witnessed the development of a catalyst by the National Chemical Laboratory, Pune, India or NCL as it is more popularly referred to. As a result, the catalyst in question is eponymously known as "encilite" and has represented a considerable improvement over catalysts hitherto known. The encilite catalyst, its production and use forms subject of Indian Patent Applications Nos. 159164 and 160212. The present invention has improved on the encilite catalyst through the provision of a high silica zeolite catalyst comprising a mixture of amorphous silica and crystalline aluminosilicate preferably provided on an alumina support and incorporating minor amounts of metals of Group VIII of the Periodic Table. In an alternative form, the catalyst components can be mixed with water to form an extrudable mass which is then extruded and the extrudates then dried and calcined, for instance at 500°C for about 16 hours. By employing the improved catalyst, it is possible according to the present invention to treat an ethyl benzene containing C₈ aromatic fraction so as to isomerise the xylene content thereof leading to the production of p-xylene and o-xylene with the simultaneous conversion of the ethyl benzene content to benzene. Accordingly, the present invention provides a process for the production of p-xylene and o-xylene by the catalystic isomerisation of alkyl aromatic hydrocarbons which comprises subjecting a substantially C₈ alkyl aromatic hydrocarbon mixture to a temperature of from 250°C to 550°C in the presence of hydrogen in a hydrogen to hydrocarbon mole ratio from 0.5 to 10 and a high silica zeolite catalyst in acid form, said catalyst comprising a mixture of amorphous silica and crystalline aluminosilicate in which the silica to alumina ratio is from 90 to 200 on an alumina support and containing small amounts of metals of Group VIII of the Periodic Table dispersed on a porous matrix whereby the xylene content of said mixture is isomerised to provide an isomeric mixture in equilibrium and the ethyl benzene present is simultaneously dealkylated, separating the reaction mixture into a lighter or top product consisting of a benzene-toluene mixture and a heavier bottoms product rich in p-xylene and o-xylene isomers, and treating said bottoms product to separate therefrom the p-xylene and o-xylene as individual products. The metal of Group VIII of the Periodic Table incorporated into said catalyst composite is preferably platinum either alone or in combination with one or more metals such as nickel or palladium. Reaction temperature can vary but is preferably in the range of from 350°C to 500°C. The process can be effected under a pressure of 981 to 1961 kPa (10 to 20 kg/cm²). More preferably, the reaction is carried out at a weight hourly space velocity (WHSV) of from 1 to 20. In accordance with a further feature of the invention the product stream remaining after separation of p-xylene and o-xylene which is rich in C₈ alkylaromatic hydrocarbons is recycled to the start of the reaction. The initial alkylaromatic hydrocarbon mixture and the recycled product stream can also contain from 4% to 25% non-aromatic aliphatic hydrocarbons. One particular embodiment of the invention is now described with reference to the accompanying drawing which illustrates a flow-diagram representative of the process of the invention. With reference to such drawing, a mixture of C₈ alkyl aromatics, e.g. obtained by the distillation of a C₅ reformate employing a platinum catalyst dispersed over alumina, is supplied through line 1 to ortho-meta splitting column 7. After the splitting reaction is complete, a top product stream comparatively rich in p-xylene is withdrawn from column 7 via line 3 to separation unit 9. At the same time, a bottoms product comprising a mixture of o-xylene, C₉ aromatics and other heavy end products is led out of column 7 by line 4 to re-run tower 8. Following treatment in tower 8, an o-xylene rich stream is withdrawn via line 5 as a top product from tower 8 while the heavier C₉⁺ aromatics are withdrawn from the bottom of such tower through line 6. Simultaneously, p-xylene is separated from the p-xylene rich stream in unit 9 by crystallisation or selective adsorption. The separated p-xylene is led off through line 10 and the remaining C₈ aromatic mixture which is lean in p-xylene content and therefore ideal as isomerisation feedstock is withdrawn via line 11. The withdrawn C₈ mixture is mixed with hydrogen entering through line 12, heated to the appropriate temperature and introduced at reaction conditions to isomerisation reactor 14 in the presence of the isomerisation catalyst. The essential reaction that takes place in reactor 14 is the isomerisation of the xylenes to an isomeric mixture in equilibrium. However, simultaneous secondary reactions also take place over the catalyst. These include dealkylation of ethyl benzene, disproportionation of ethyl benzene, transalkylation of xylenes and hydrocracking and indeed reaction conditions can be optimised to produce a desired product pattern. The reaction products of reactor 14 are cooled therein and then led by line 13 to isomer stabiliser 15. In stabilizer 15 the lighter products such as benzene, toluene, light saturates and gases are separated and withdrawn from the top via line 16. The residue which comprises a stream rich in C₈ aromatics is withdrawn as a bottoms product and recycled via line 2 as initial feed to ortho-meta splitting column 7 either directly of after blending with the fresh feed entering through line 1. Essentially, the improved catalyst employed in the process of the present invention fulfils the optimum requirements for such a catalyst, viz. it is active to isomerise xylene and convert ethyl benzene to benzene but is inactive for generating competing reactions particularly those which destroy C₈ aromatics. Where an isomerisation process is involved, the activity of the catalyst is expressed in terms of approach to equilibrium of the individual isomers. The equilibrium approaches of p-xylene and o-xylene and the disappearance of ethyl benzene [EB] are calculated according to the following equations: [i] [ii] % p-xylene approach to equilibrium expressed as where p x P is % p-xylene in the xylene product mixture p x F is % p-xylene in the feed, and p x E is % p-xylene equilibrium concentration at the reaction temperature. [iii] % o-xylene approach to equilibrium expressed as where o x P is % o- xylene in the xylene product mixture o x F is % o-xylene in the feed, and o x E is % o-xylene equilibrium concentration at the reaction temperature. The selectivity of the catalyst is expressed in terms of % xylene loss and % C₈ aromatics loss and these losses are represented hereafter by equations [iv] and [v], respectively: [iv] where XF = total xylenes in the feed, and XP = total xylenes in the product. [v] where C₈AF = total C₈ aromatics in the feed, and C₈AP = total C₈ aromatics in the product. The present invention will now be described at length and in greater detail in the following examples. In such examples the selectivity of the catalyst is indicated as a percentage of xylene and C₈ aromatics losses. In particular, the examples underline the fact that the reaction conditions under which the invention can be conducted can vary depending on the content of the feed and on the composition desired in the final product mixture. In Examples 1 to 4, the results of a series of test runs conducted in a pilot plant over the improved high silica zeolite catalyst in the form of 1/16 inch (1.6 mm) extrudates are set out. Conditions other than temperature were maintained constant with a pressure of 1569 kPa (16 kg/cm²), a weight hourly space velocity (WHSV) of 10 and a hydrogen to hydrocarbon molar ratio of 2.0. In each instance the charge was commercial naphtha reformate. EXAMPLE 1 A series of three test runs were conducted in a pilot plant reactor under simulated conditions which varied the reaction temperature of each of the runs but maintained the other parameters constant as follows: Pressure 1569 kPa (16 kg/cm²), WHSV 10 Hydrogen-hydrocarbon molar ratio 2.0 The charge which was a commercial C₈ aromatic fraction of a typical naphtha reformate comprised on a weight percent basis: 9.7% p-xylene, 51.9% m-xylene, 21.7% ethyl benzene, 3.2% toluene, 7.3% saturates. The results of liquid product analysis of each of the three runs conducted are set out below: Run 1 Run 2 Run 3 Temperature 410°C 430°C 435°C ANALYSIS wt. % Saturates 6.4 3.69 3.24 Benzene 6.11 9.53 10.06 Toluene 4.2 4.88 4.6 Ethyl benzene 12.5 10.9 10.02 p-xylene 15.85 16.10 16.19 m-xylene 37.35 36.6 36.4 o-xylene 15.62 15.78 16.12 C₉A aromatics 2.38 2.56 2.228 Ethyl benzene Conv % 40.0 48.6 52.4 Xylene loss % 1.44 1.67 1.88 C₈ A Loss % 11.5 12.0 12.5 ATE p-xylene % 100 100 100 ATE o-xylene % 92 95 96 The results given above indicate that the reaction temperature is a critical parameter and is particularly relevant to control ethyl benzene conversion which increases as the reaction temperature increases. It is observed that the approach of o-xylene to equilibrium will increase with the rise in temperature. A marginal increase in xylene loss and C₈ aromatics loss is also indicated with the rise in temperature. EXAMPLE 2 As in Example 1, a series of three test runs were conducted in a pilot plant reactor under simulated conditions but in this instance, it was pressure that was varied in respect of each run, the other parameters being maintained constant as follows: Temperature 410°C WHSV 10 Hydrogen-hydrocarbon molar ratio 2.0 The charge which was a commercial C₈ aromatic fraction of a typical naphtha reformate comprised on a weight percent basis: 9.36% p-xylene, 9. 7% o-xylene, 48.33% m-xylene, 21.25% ethyl benzene, 4.8% toluene, 6.56% saturates. The results of liquid product analysis of each of the three runs conducted are set out below: Run 1 Run 2 Run 3 Pressure, kPa (kg/cm²) 1569 (16) 1422 (14.5) 1275 (13) ANALYSIS wt. % Saturates 6.4 4.00 4.11 Benzene 6.11 6.6 6.65 Toluene 4.26 4.48 4.46 Ethyl benzene 12.26 14.11 4.46 p-xylene 16.00 15.98 16.14 m-xylene 36.93 37.26 37.06 o-xylene 15.66 15.45 15.61 C₉A aromatics 2.38 2.15 2.17 Ethyl benzene Conv % 40.3 32.03 33.75 Xylene loss % 1.44 1.25 1.29 C₈ A Loss % 11.5 9.53 9.7 ATE p-xylene % 100 100 100 ATE o-xylene % 92 88 90 The results of the three runs of this example establish that the p-xylene and o-xylene approach to equilibrium does not vary with pressure in the range study. The ethyl benzene conversion decreases with the decrease in pressure. There are marginal decreases in C₈ aromatics loss and xylene loss with decrease in pressure. EXAMPLE 3 Following Example 1, a series of four test runs were conducted in a pilot plant reactor under simulated conditions but in this instance, it was the hydrogen-hydrocarbon molar ratio that was varied in respect of each run, the other parameters being maintained constant as follows: Temperature 410°C Pressure 1569 kPa (16 kg/cm²) WHSV 10 The charge which was a commercial C₈ aromatic fraction of a typical naphtha reformate comprised on a weight percent basis: 9.36% p-xylene, 9.7% o-xylene, 48.33% m-xylene, 21.25% ethyl benzene, 4.8% toluene, 6.56% saturates. The results of liquid product analysis of each of the four runs conducted are set out below: Run 1 Run 2 Run 3 Run 4 H₂-HC molar ratio 1 2 3 4 ANALYSIS (wt. %) Saturates 5.44 6.4 6.33 6.41 Benzene 7.47 6.11 6.11 6.41 Toluene 4.52 4.26 4.33 4.23 Ethyl benzene 12.0 12.26 12.7 13.5 p-xylene 16.0 15.9 16.05 16.23 m-xylene 36.47 36.93 36.77 37.16 o-xylene 15.51 15.66 15.51 15.65 C₉A aromatics 2.69 2.38 2.21 1.62 Ethyl benzene conv. % 43.5 41.5 38.9 36.4 Xylene loss % 2.3 1.67 1.44 0.61 C₈ A Loss % 12.5 11.5 11.04 9.65 ATE p-xylene % 100 100 100 100 ATE o-xylene % 92 92 92.5 90 From the results of the four runs of this example, it will be observed that the percentage of ethyl benzene conversion decreases with the increase in hydrogen-hydrocarbon molar ratio. There is no significant change in the approach to equilibrium values. However, xylene loss and C₈ aromatics loss are reduced with the increase in the hydrogen-hydrocarbon molar ratio. EXAMPLE 4 As in Example 1, a series of three test runs were conducted in a pilot plant reactor under simulated conditions but in this instance, it was the weight hourly space velocity (WHSV) that was varied in respect of each run, the other parameters being maintained constant as follows: Temperature 410°C Pressure 1569 kPa (16 kg/cm²) Hydrogen-hydrocarbon molar ratio 2.0 The charge which was a commercial C₈ aromatic fraction of a typical naphtha reformate comprised on a weight percent basis: 9.36% p-xylene, 9.7% o-xylene, 48.33% m-xylene, 21.0% ethyl benzene, 4.8% toluene, 6.6% saturates. The results of liquid product analysis of each of the three runs conducted are set out below: Run 1 Run 2 Run 3 WHSV (hr⁻) 10 9 8 ANALYSIS wt. % Saturates 6.40 5.71 6.04 Benzene 6.11 6.82 6.76 Toluene 4.26 4.39 4.44 Ethyl benzene 12.26 12.24 11.90 p-xylene 16.00 16.04 16.09 m-xylene 36.90 36.71 36.74 o-xylene 15.66 15.73 15.71 C₉A aromatics 2.38 2.36 2.32 Ethyl benzene Conv % 42.3 42.4 44.0 Xylene loss % 1.67 1.8 2.0 C₈ A Loss % 11.5 11.66 12.0 ATE p-xylene % 100 100 100 ATE o-xylene % 92 95 96 The results of the runs conducted by this example establish that the percentage approach to equilibrium of p-xylene o-xylene remained more or less constant when varying the space velocity in the range of 8 to 10 whereas the ethyl benzene conversion percentage decreased marginally. Similarly, no significant change was noticed for xylene loss and C₈ aromatics loss in the range of WHSV that was studied. EXAMPLES 5, 6 and 7 Three separate test runs were conducted in the isomerisation reactor of a commercial aromatics plant employing the improved high silica zeolite catalyst. In respect of each run, temperature, pressure, WHSV and the hydrogen-hydrocarbon molar ratios were maintained constant but the non-aromatics content of the feed charge was varied from run to run by introducing into a C₈ aromatic fraction C₆ to C₈ non-aromatic hydrocarbon material. The constant parameters were as follows: Temperature 420°C Pressure 1569 kPa (16 kg/cm²) WHSV 10 Hydrogen-hydrocarbon molar ratio 2.0 In respect of each run, the performance of the catalyst was measured. The composition of the feed charge and the results of liquid product analysis for each example/run are set out hereafter: EXAMPLE 5 - Run 1 Composition Charge Product Analysis wt. % Saturates 10.1 9.5 Benzene 0.0 4.6 Toluene 2.7 3.2 Ethyl benzene 20.2 12.7 p-xylene 10.5 16.3 m-xylene 52.2 35.8 o-xylene 4.2 16.0 C₉A aromatics 0.0 1.9 Ethyl benzene Conv % 39.8 Xylene Loss % 2.5 C₈ A Loss % 11.2 ATE p-xylene % 106.5 ATE o-xylene % 96.5 EXAMPLE 6 - Run 2 Composition Charge Product Analysis wt. % Saturates 15.3 15.4 Benzene 0.0 3.8 Toluene 2.6 2.7 Ethyl benzene 19.0 11.9 p-xylene 9.9 15.3 m-xylene 50.8 33.3 o-xylene 2.4 15.0 C₉A aromatics 0.0 2.6 Ethyl benzene Conv % 39.4 Xylene Loss % 2.3 C₈ A Loss % 10.9 ATE p-xylene % 108.0 ATE o-xylene % 95.0 EXAMPLE 7 - Run 3 Composition Charge Product Analysis wt. % Saturates 16.0 15.1 Benzene 0.0 4.2 Toluene 2.8 2.9 Ethyl benzene 17.8 11.0 p-xylene 9.2 15.4 m-xylene 46.9 33.3 o-xylene 7.3 15.0 C₉A aromatics 0.0 3.1 Ethyl benzene Conv % 40.23 Xylene Loss % 2.82 C₈ A Loss % 11.02 ATE p-xylene % 107.69 ATE o-xylene % 92.76 A comparison of the results of the runs of Examples 5, 6 and 7 shows that although the saturates in the isomer feed charge varied from 10.1% to 16%, no significant variation was observed in respect of ethyl benzene conversion or in respect of the stability of the improved zeolite catalyst. Similarly, the p-xylene and o-xylene approaches were quite satisfactory throughout the runs. An interesting observation which emerges from these three examples is that the improved catalyst can tolerate amounts of saturates in the feed as high as 10% to 16% without any adverse effect on its overall activity and stability and the selectivity of the product. EXAMPLES 8, 9 and 10 In order to study the performance of the improved high silica zeolite catalyst employed by this invention at various severities of operation, three further test runs were conducted in the isomerisation reactor of a commercial aromatics plant employing such catalyst. The temperature for each run was varied over a range of from 400°C to 440°C, maintaining pressure, WHSV and hydrogen-hydrocarbon molar ratio constant. These constant parameters were as follows: Pressure 1569 kPa (16 kg/cm²) WHSV 10 Hydrogen-hydrocarbon molar ratio 2.0 In respect of each run, the performance of the catalyst was measured. The composition of the feed charge and the results of liquid product analysis for each example/run are set out hereafter: EXAMPLE 8 - Run 1 The temperature of the operation was 405°C. Composition Charge Product Analysis wt. % Saturates 12.2 11.7 Benzene 0.0 2.8 Toluene 1.8 2.1 Ethyl benzene 19.5 13.0 p-xylene 9.5 16.2 m-xylene 49.7 35.9 o-xylene 7.3 16.2 C₉A aromatics 0.0 1.9 Ethyl benzene Conv % 33.3 Xylene Loss % 1.3 C₈ A Loss % 8.9 ATE p-xylene % 105.0 ATE o-xylene % 94.0 EXAMPLE 9 - Run 2 The temperature of the operation was 420°C. Composition Charge Product Analysis wt. % Saturates 15.5 14.5 Benzene 0.0 3.3 Toluene 2.3 2.9 Ethyl benzene 19.1 11.6 p-xylene 9.8 15.7 m-xylene 49.4 34.0 o-xylene 3.9 15.4 C₉A aromatics 0.0 2.6 Ethyl benzene Conv % 40.75 Xylene Loss % 1.39 C₈ A Loss % 8.9 ATE p-xylene % 106.0 ATE o-xylene % 95.0 EXAMPLE 10 - Run 3 The temperature of the operation was 435°C. Composition Charge Product Analysis wt. % Saturates 9.2 8.82 Benzene 0.0 5.4 Toluene 2.3 3.76 Ethyl benzene 17.9 8.50 p-xylene 11.6 17.08 m-xylene 53.3 37.22 o-xylene 5.7 17.08 C₉A aromatics 0.0 1.75 Ethyl benzene Conv % 51.0 Xylene Loss % 1.5 C₈ A Loss % 10.6 ATE p-xylene % 104.4 ATE o-xylene % 97.3 A comparison of the results of the runs of Examples 8, 9 and 10 clearly establishes that reaction temperature controls the ethyl benzene conversion and is also a critical factor for severity of operation. In general, the severity of operation can be controlled by altering the reaction temperature. At low severity of operation around 30% ethyl benzene conversion, the formation of benzene is low but increases with increased severity at elevated temperatures. p-xylene approach is independent of the severity of operation but o-xylene approach increased with a rise in temperature. C₈ aromatics losses show an increasing trend with increased reaction temperature whereas xylene loss is correspondingly marginal. An interesting feature is that the rate of cracking of saturates is higher at high severity of operation and the build-up of saturates in the isomeriser loop can thus be reduced by operating the inlet reactor at elevated temperature. It is also possible to maximise xylene production by proper choice of the severity of operation. EXAMPLE 11 In order to assess the stability of the improved high silica zeolite catalyst employed by the invention, four test runs were carried out in respect of different specimens of the catalyst which had been in operation over varying periods of time. Each of these runs was effected under identical reaction conditions, viz. a temperature of 420°C., a pressure of 1569 kPa (16 kg/cm²), a weight hourly space velocity of 10 and a hydrogen-hydrocarbon molar ratio of 2. The results of each of the four runs conducted are set out below: Run 1 Run 2 Run 3 Run 4 Catalyst-Hours on Stream 750 1700 3500 5000 ACTIVITY (wt. %) Ethyl benzene conversion 40.50 40.60 41.2 42.0 ATE p-xylene 105 104 106 105 ATE o-xylene 95 96 96 96 The results presented above demonstrate the remarkable stability of the catalyst of the invention whereby even after 5000 hours on stream under identical operating conditions, the ethyl benzene conversion was maintained at approximately 40% throughout the run in question. The p-xylene and o-xylene approaches were well above 100% and 90% respectively. The catalyst stability during commercial run was good and the rise in temperature required to maintain the same severity at the end of 10 months operation was 2°C. to 3°C. This indicates that the catalyst is resistant to coking and has a predicted life of more than twelve months. EXAMPLE 12 In order to assess the comparative performance of the improved high silica zeolite catalyst of the invention both when employed as a fresh catalyst and after regeneration thereof, three tests were carried out in respect of the same catalyst. Each of these tests was effected under identical reaction conditions, viz. a temperature of 420°C., a pressure of 1569 kPa (16 kg/cm²), a weight hourly space velocity of 10 and a hydrogen-hydrocarbon molar ratio of 2. The results of each of the three tests conducted are set out below: Fresh Catalyst Catalyst regenerated once Catalyst regenerated twice Hours on Stream 100 1500 3000 ACTIVITY (wt. %) Ethyl benzene conversion 41.0 40.0 39.5 ATE p-xylene 101.0 100.6 39.5 ATE o-xylene 94.0 93.0 92.0 The data given above establish that even after successive regenerations, the catalyst of the invention does not show any appreciable loss in activity or selectivity. This is indicative of excellent catalysts stability. The results also suggest that, based on pilot test runs, the catalyst of the invention will have a longer average cycle length than earlier known catalysts. The foregoing description and the examples are provided for the purpose of guiding persons skilled in the art and it must be appreciated that the invention is not restricted only thereto. Other modifications and embodiments of the invention are clearly possible within the scope of what has been described herein.

Claims: A process for the production of p-xylene and o-xylene by the catalytic isomerisation of alkyl aromatic hydrocarbons which comprises subjecting a substantially C₈ alkyl aromatic hydrocarbon mixture to a temperature of from 250°C to 550°C in the presence of hydrogen in a hydrogen to hydrocarbon mole ratio from 0.5 to 10 and a high silica zeolite catalyst in acid form, said catalyst comprising a mixture of amorphous silica and crystalline aluminosilicate in which the silica to alumina ratio is from 90 to 200 on an alumina support and containing small amount of metals of Group VIII of the Periodic Table dispersed on a porous matrix whereby the xylene content of said mixture is isomerised to provide an isomeric mixture in equilibrium and the ethyl benzene present is simultaneously dealkylated, separating the reaction product into a lighter or top product consisting of a benzene-toluene mixture and a heavier bottoms product rich in p-xylene and o-xylene isomers, and treating said bottoms product to separate therefrom the p-xylene and o-xylene as individual products. A process as claimed in claim 1 wherein the metal of Group VIII of the Periodic Table incorporated into said catalyst is platinum or a combination of platinum with one or more metals selected from nickel and palladium. A process as claimed in claim 1 or 2 wherein said catalyst is employed in the form of an extrudate of the components thereof mixed with water to an extrudable mass, extruded, dried and calcined. A process as claimed in any of claims 1 to 3 wherein the isomerisation reaction is effected at a temperature in the range of from 350°C to 500°C. A process as claimed in any of claims 1 to 4 wherein the reaction is effected at a pressure in the range of from 10 to 20 kg/cm². A process as claimed in any of claims 1 to 5 wherein the reaction is effected at a weight hourly space velocity (WHSV) of from 1 to 20. A process as claimed in any of claims 1 to 6 wherein the product stream remaining after separation of p-xylene and o-xylene which is rich in C₈ alkylaromatic hydrocarbons is recycled to the start of the reaction. A process as claimed in any of claims 1 to 7 wherein the starting alkylaromatic hydrocarbon mixture contains from 4% to 25% non-aromatic aliphatic hydrocarbon.

Inventors: BHATT, RAMESH CHANDRA, HALGERI, ANAND BHIMARAO, KUMAR, SUSHIL, RAO, TURAGA SUNDARA RAMA PRASADA, SHARMA, RAMESH CHANDER
Applicants: INDIAN PETROCHEMICALS CORPORATION LTD.
Application year: 1988
Main classification: C07C15
IPC codes: C07C5, C07C15, B01J29