US3538173A - C8-alkylaromatic isomerization process - Google Patents

Description

United States Patent U.S. Cl. 260-668 4 Claims ABSTRACT OF THE DISCLOSURE Ethylbenzene contained in a C -aromatic stream is effectively isomerized to xylene isomers by carefully controlling the C -naphthene content in said stream in an amount of about 2 wt. percent to about 9 wt. percent of the C -aromatics contained in said stream.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of our copending application Ser. No. 692,655, filed Dec. 22, 1967, now abandoned.

BACKGROUND OF INVENTION This invention relates to a C alkylaromatic isomerization process. More particularly, this invention relates to an improved C -alkylaromatic process wherein ethylbenzene is effectively isomerized to xylenes without incurring excessive (l -aromatic cracking and hydrogenation loss.

Processes for the production of the various xylene isomers have acquired significant interest and importance within the petroleum and petrochemical industries. Currently, this interest stems from the demand for paraxylene as an intermediate for the synthetic fiber and fabric industry.

Several currently available processes are effective for isomerizing only the xylene isomers and not the ethylbenzene C -ar0matic isomer. Attempts to isomerize ethylbenzene to xylenes result in cracking losses which render the process economically unfeasible.

, DESCRIPTION OF "PRIOR ART In the prior art, Holm, US. Pat. No. Re. 25,753 describes a method for isomerizing a non-equilibrium mixture of xylenes and ethylbenzene towards equilibrium. Essentially, the method rests upon the hypothesis that the co-existance of the corresponding naphthenes is essential to the success of the method. Attention is directed toward a two-step process in which hydroisomerization and subsequent dehydrogenation are segregated, but data are also g ven for a one-step process in which from about 10 to 30 percent of the C cyclics are naphthenes. These data show varying degrees of approach to equilibrium para- Xylene in total xylenes which depends upon the depth of simultaneous hydrogenation of the aromatic. It would appear from the data presented, especially run numbers 3,538,173 Patented Nov. 3, 1970 4-1232 and 4-1214 of the Helm patent, that although para-xylene/xylene concentrations of about 21% and 25% respectively were obtained, saturation of C -ar0- matics was 10 and 35% respectively. It is noted that equilibrium para-xylene/xylene concentration at these conditions is approximately 24% SUMMARY OF INVENTION We have found, however, that a careful selection of processing conditions and catalyst will give substantially equilibrium para-xylene/xylene and ortho-xylene/Xylene concentrations, while saturating only a very minor portion of the C -aromatics to naphthenes, when attempting to isomerize ethylbenzene or an ethylbenzene-containing xylene stream. By substantial equilibrium we mean an approach to at least 90% of the para-xylene/xylenes in the reactor efiluent. Ortho-Xylene equilibrium is a little more diflicult to achieve. Hence, if ortho-xylene is the desired product from an ortho deficient feed, the term substantial equilibrium in this case would mean at least of the equilibrium value predicted for the orthoxylene mixture. The generally accepted equilibrium values (based on API sources) are tabulated below for our range of interest.

Temperature, K 600 700 800 Temperature, C 327 427 527 Mole percent of isomer (including ethylbenzene):

Ethylbenzene 6 8 l1 Paraerylene... 22 22 21 Meta-xylene l 50 48 45 Ortho'xylena 22 22 23 Mole percent of isom xylenes only) Para-xylene 24 24 23 Meta-xylene 54 52 51 Ortho-xylene 22 24 26 brium between the C -aromatics and the C -naphthenes in the presence of added hydrogen is readily adjusted by changing temperature, hydrogen ratio, or pressure. These conditions are so correlated that the naphthene content can be raised by either lowering temperature and/or raising pressure. Likewise, by either raising temperature and/ or lowering pressure, the naphthene content may be lowered.

When isomerization only among the xylenes is desired, without conversion of ethylbenzene to xylenes, only a trivial amount of C -naphthenes are required, but when ethylbenzene conversion to xylenes is desired, then the range of 2% to 9% C -nap'hthenes based on c -aromatics is essential for efficient operation. Thus, a feed stock rich in ethylbenzene, which is by far the most common feed available from catalytic or pyrolytic sources, can be utilized. The advantage of this in commercial operation is that, instead of having, for example, 30% additional feed as naphthenes, recycling through the isomerization system, a value of less than 10% naphthenes is completely adequate and makes for a more efiicient and economical process. The advantage of the process of our invention is that we have obtained very high efficiencies in converting ethylbenzene to xylene isomers without incurring naphthene formation of greater than 10%. This also will be shown in great detail in the accompanying examples.

Therefore in an embodiment, this invention relates to an isomerization process for the production of a specific xylene isomer which comprises contacting hydrogen and a C aromatic charge stock containing ethylbenzene at a concentration greater than equilibrium with respect to the xylenes present and containing C -naphthenes at isomerization conditions in a reaction zone, with a catalytic composite of alumina combined with about 0.01% to about 1.0% by weight of platinum component and about 0.1% to about 5.0% by weight of a halogen component, separating a specific Xylene isomer from the resulting reaction product effluent and providing a hydrocarbon stream containing unreacted ethylbenzene, C -naphthenes and xylene isomers, and recycling said hydrocarbon stream to said reaction zone; said process further characterized in that said isomerization conditions are selected to maintain a C -napthene content in said C -aromatic charge to said reaction zone of from 2.0% to about 9.0% by weight of the C -aromatics in said charge.

In further more limited embodiments said isomerization conditions include an LHSV of about 0.2 to about 8.0, a temperature of about 370 C. to about 450 C., a pressure of about 3 atmospheres to about 20 atmospheres, and a hydrogen concentration in a mole ratio of about 2.0:1 to about 20.0:1 with respect ot said C -aromatic charge. In addition, 1.0 to about 1000 p.p.m. by weight halogen may be added to said C -aromatic charge.

More specific embodiments and conditions will be found in the following more detailed description of this invention. As used herein, C -aromatic charge stock refers to the charge stock fed to the isomerization reaction zone. As is known to those trained in the art, the fresh feed, before being fed to the reactor may be first separated to recover a specific xylene isomer before it is passed to the reactor or the fresh feed may be combined with a recycle stream and fed directly to the reactor.

DESCRIPTION OF PREFERRED EMBODIMENTS The process of our invention is applicable to certain hydrocarbon fractions containing ethylbenzene in an amount above its equilibrium value where an object of the invention is to produce one or more of the xylene isomers therefrom. The charging stock may be ethylbenzene alone or ethylbenzene in admixture with ortho, meta-, or paraxylene, or a non-equilibrium mixture of the aforesaid C -aromatic hydrocarbons in which the ratio of isomers is other than the equilibrium portion of C aromatic components, said mixture being either exclusively C -aromatic components or accompanied by other classes of hydrocarbons such as paraflins, other aromatic hydrocarbons, olefins, naphthenes, etc., or a mixture of hydrocarbons including at least one of the aforesaid xylene isomers with other compounds inert in the present process. One of the preferred sources of the present charging stock is a fraction derived from certain petroleum conversion products containing aromatic hydrocarbons and including fractions boiling within the range of from about 120 to about 145 C. Suitable fractions utilizable in the process may be separated from gasoline produced by subjecting an appropriately boiling petroleum fraction to dehydrogenation as for example, a hydroformed gasoline boiling range fraction containing naphthenic hydrocarbons. Such gasoline boiling range fractions may be produced either thermally and/ or produced in a catalyzed cracking, reforming, or hydroforming reaction in accordance with procedures well known to the art.

The isomerization takes places in an isomerization zone maintained at a liquid hourly space velocity within the range of 0.2 to 8, and preferably within the range of 0.6 to 3, a temperature within the range of 370 C. to 450 C. and preferably within the range of 380 C. to 440 C., and a pressure within the range of 3 to 20 atmospheres and preferably within the range of to 18 atmospheres in the presence of not less than 2.0 nor more than 9 weight percent C -naphthenes, and preferably in the range of 3 to 8 weight percent. A molal excess of hydrogen shall be furnished to the isomerization zone, usually in the range of about 2.0 to 20 moles of hydrogen per mole of hydrocarbon feed.

The catalyst utilized in the present process comprises alumina, a platinum component, and combined halogen with said catalyst being disposed within the reaction zone. The alumina support may be a high surface area alumina such as gamma-, eta-, and theta-alumina, although these are not necessarily of equivalent suitability. By the term high surface area is meant a surface area measured by surface area techniques within a range of from 25 to 500 or more square meters per gram, and preferably a surface area of approximately to 300 square meters per gram.

The platinum component of this catalyst for use in our invention will normally be utilized in an amount of from about 0.01 to about 1.0% by weight based upon the solid support. Of the various halogens which may be utilized, both fluorine and chlorine can be used satisfactorily either separately or together. Normally, the halogen content of the catalyst ranges from 0.1% to 5% by weight, and preferably from 1% to 4%, also, it is contemplated within the scope of this invention that halogen in an amount of from 0.0001 to 0.1% by weight of feed, and preferably in an amount of from about 0.001 to about 0.05% by weight of feed may be continuously added to the isomerization zone. We have unexpectedly found, when using this p1atinum-alumia-combined fluorine catalyst, that chlorine, in the range of from 0.001 to 0.05% by weight of feed is of feed may be continuously added to the isomerization of our invention. This halogen can be added by any suitable halogen-containing compound such as t-butylchloride.

As will be recognized by one skilled in the art, the process of this invention utilizing the catalyst hereinbefore set forth may be effected in any suitable manner and may comprise either a batch or continuous operation, The preferred method by which the process of this invention may be effected is the continuous type operation. Thus, a particularly preferred method of the fixed bed operation is one in which a non-equilibrium C -aromatic hydrocarbon fraction is continuously charged to the reaction containing the fixed bed of the desired catalyst, said zone being maintained at the proper operating conditions of temperature and pressure as described above. The reaction zone may comprise an unpacked vessel or coil or may be lined with an adsorbent packing material. The charge may be passed through the catalyst bed in an upward, downward, or radial flow and the isomerized product may be continuously withdrawn, separated from the reactor effiuent and recovered, while any unreacted starting materials may be recycled to form a portion of the feed stock to the reaction zone. It is within the scope of this invention to recover one or more specific xylene isomers as a product; for example, the process may be designed to produce para-xylene and/or ortho-xylene.

EXAMPLES The following examples are introduced for the purpose of illustration only with no intention of unduly limiting the generally broad scope of the present invention.

Example I A catalyst comprising 0.375 weight percent platinum, 3.5 weight percent fluoride, 0.1 weight percent chloride on a gamma-alumina support was placed in an isomerization zone. A charge stock comprising 100% o-Xylene was charged to the isomerization zone at a liquid hourly space velocity of 5.0, a hydrogen to hydrocarbon mole ratio of 10:1, a pressure of 20.4 atmospheres and a temperature of 482 C. Charge stock and product analyses are shown in Table I below.

TAB LE I 6 tive to meta and each other. Ethylbenzene is not converted since the amount in the charge is below the equilibrium Char e Product value. As used herein, C ring retention is the percent of Ethylbenzene L2 the original C -aromat1cs rema1n1ng as C -armat1cs 0r m 2 converted to naphthenes wlthm the reactor. mgiylene- 38. 9 0- yene 46- C -uaphthenes 0. 6 Example In Other nonaromatics and benzene. 'oluene 1.0 A catalyst comprlsmg 0.375 welght percent platlnum, Q3 2.3 weight percent chloride on a gamma-alumina sup- Total, wt. percent 100.0 100.0 port was placed in the isomerization zone. A charge Caring retention 913 stock containing 23.6 weight percent ethylbenzene was charged to the isomerlzatlon zone at varying space velocihes, a hydrogen to hydrocarbon mole ratio of 5:1, a AS (2311 be $6611 from Tablfi t l w naphthene conpressure of 10.2 atmospheres, and a temperature of centrations, a very selectlve reaction occurs with substan- 400 C. to 402 C. tially equlhbrium conversion of para-xylene belng ob- The liquid hourly space velocities, as mentioned above tamed. The ethylbenzene which 1s produced is far less were varied for each of the tests to vary conversions than equllibrium amounts. In general, the rate of reaction as shown below.

TABLE 111 Product Charge stock Test 1 Test 2 Test 3 Ethylbenzene 23. 6 14. O 9. 9 16. 8 p-Xylene 10. 6 18. 3 19. 6 17. 5 m-Xylene 56. 1 42. 2 41. 5 43. 6 o-Xylene 9. 1 16. 2 17. 6 14. 9 Cg-uaphthenes 0 6. 1 6. 5 5. 1 Other nouaromatics and benzenes 0. 5 2. 0 2. 9 1. 3 Toluene ,r 0. 1 0. 4 0. 8 0. 3 011+ 0.8 1.2 0. 5

Total, wt. percent 100. 0 100.0 100. 0 100. 0

Total C -naphthenes, Wt. percent of charge... 6. 1 6. 5 5. 1 Liquid hourly space velocity 2 1 3 Total xylenes, wt. percent of charge 76. 7 78. 5 76. 0 Percent para/total xyleues 23. 9 24. 9 23. 0 Percent ortho/ total xylenes 21. 1 22. 4 19.6 05 ring retention 96. 9 95. 2 98. 0

involving ethylbenzene toward equilbrium is slower than the rate of xylene isomerization. This shows that the presence of naphthenes in a substantial concentration, as in the range of 2 to 9 weight percent is not necessary to isomerize a pure xylene feedstock.

Example 11 A catalyst comprising 0,375 weight percent platinum, 1.73 weight percent fluoride, 0.39 weight percent chloride on a gamma-alumina support was placed in an isomerization zone. A charge stock containing a minor amount of ethylbenzene, namely 5.2 weight percent was charged to the isomerization zone at a liquid hourly space velocity of 2.0, a hydrogen to hydrocarbon mole ratio of 6:1, a pressure of 12.1 atmospheres and a temperature of 398 C. Chloride addition to the feed was maintained at 0.0025 weight percent of the feed by utilizing t-butylchloride. Charge stock and product analyses are shown in Table II below.

TABLE 11 Charge stock Product Ethylbenzene. 5. 2 5. 2 pQiylcne-.-

11.5 19.1 n1-Xylene 71. 3 48. 4 o-Xylene 11.0 16. 5 Cg-naphthenes 0 8. 6 Other nonaromaties and benzene 0. 1 1. 4

Toluene r 0.3 09+ O. 5

Total, wt. percent 100.0 100.0

Percent para/total xylenes 22. 8 Percent ortho/total xylenes- 19. 6 (3 ring retention (percent) 97. 8

As can be seen from the product analyses, ethyl-benzene concentration remained the same but the orthoand para-xylene have come to substantial equilibrium rela- For all the tests shown, the paraand ortho-xylene are in substantial equilibrium with the meta-xylene.

Example IV A catalyst comprising 0.375 weight percent platinum, 1.7 weight percent fluoride on a gamma-alumina support was placed in the isomerization zone. A charge stock containing a high ethylbenzene concentration, namely 43.0 weight percent was charged to the isomerization zone at a liquid hourly space velocity of 2.0, a hydrogen to hydrocarbon mole ratio of 6:1, a pressure of 11.9 atmospheres and at a temperature of 405 C. Chloride addition to the feed was maintained at 0.0025 weight percent (of the feed). Charge stock and product analyses are shown in Table IV below.

TAB LE IV Charge stock Product Ethylbenzene 43. 0 23. 5 1e 8. 1 15. 6 41. 3 36. 0 7.1 13. 3 C -naphthenes 0 8. 3 Other nonarornatics and benzene 0. 4 2. 4 Toluene 0. 1 0. 3 CH" 9. 6

Total, Wt. percent 100.0 100. 0

Percent para/total Xylenes 24. 1 Percent ortho/total xylenes 20. 5 Total xylenes, weight percent of feed. 56. 5 64. 9 Cg ring retention 96. 8

The operation resulted in an appreciable conversion of the ethylbenzene to mixed xylenes which, in a recycle operation, would be more eflicient yet because the C naphthenes have already been produced in essentially equilibrium concentration and would not subtract further from the C -aromatics in the feed. Substantially equilibrium concentration of paraand ortho-Xylene was observed.

Example V The same catalyst utilized for the preceding example was utilized for a charge stock containing a slightly higher concentration of ethylbenzene, namely 46.0 weight percent. This charge stock was charged to the isomerization zone at a liquid hourly space velocity of 2.0, a hydrogen to hydrocarbon mole ratio of 6:1, a pressure of 12.1 atmospheres, and a temperature of 403 C. Chloride addition to the feed was maintained at 0.0025 weight percent (of the feed). The total weight percent C -naphthenes in the efiluent was 6.1 weight percent. Charge stock and product analyses are shown in Table V below.

TABLE V Charge stock Product Ethylbenzene 46. 31.0 p-Xylene 8. 1 14. 5 m-Xylenem 38. 7 33. 8 o-Xylene 6. 7 12. 2 Cs-naphthenes 0 6. 1 Other ncnaromatics and benzene 0. 2 1. 5 Toluene O. 1 0. 3 C 0. 2 0. 6

Total, wt. percent 100. 0 100. 0

Percent para/total xylenes 24. 0 Percent ortho/total xylenes 20. 1 Weight percent total xylenes Based on charge 53 5 60. 5 08 ring retention 98. 1

Again, the conversion of the ethylbenzene to mixed xylenes is efficient considering also the C -naphthenes found, and the paraand ortho-xylene are in substantial equilibrium with the meta-xylene.

Example VI A catalyst comprising 0.375 weight percent platinum, 1.7% chloride and 0.5% fluoride on a gamma-alumina support was placed in the isomerization zone. The charge stock containing 100.0 weight percent ethylbenzene was charged to the isomerization zone at a liquid hourly space velocity of 1.0, a hydrogen to hydrocarbon mole ratio of 5:1, a pressure of 6.8 atmospheres and at a temperature of 401 C. Chloride addition to the feed was maintained at 0.02 weight percent (of the feed). Charge stock and product analyses are shown in Table VI below.

As can be seen from the product analysis, it is apparent that the present catalyst and conditions of operation are greatly superior in accomplishing isomerization of any C -aromatic, including ethylbenzene, in spite of the low naphthene content of the system. It should also be noted that high yields of xylenes having substantially equilibrium concentration of paraand ortho-xylene were obtained.

Example VII The feed stock of Example VII was processed over the same catalyst at the same conditions as Example VI except that the temperature was about 405 C. and the chloride addition to the feed was held at 0.04% by weight. Charge and product are shown below.

TABLE VII Charge stock Product E tllylbenzene 100. O 75. 4 pXylene 3. 2 m-Xylene. 9. 6 o-Xylene 4.5 C -naphthenes 1. 4 Other nonaromatics and benzene. 4. 9 Toluene 0.4 09+ 0.6

Total, wt. percent 100. 0 100. 0

C3 ring retention, percent 94. 1

It will be noted that a greater ring loss and lesser conversion resulted than in Example VI, attributable to an insufiicient concentration of C -naphthenes in the system. This results even though the change in processing conditions appears to be relatively insignificant.

As noted in these examples, isomerization of a particular xylene (Example I) to the other xylenes is accomplished readily without necessitating the presence of significant amounts of C -naphthenes. On the other hand, when ethylbenzene is to be converted to xylenes in an efiicient manner, concurrent with establishing the essential isomerization equilibrium among the xylenes, some C napl1- themes are required.

In a typical recycle operation in which an average C aromatic fraction including ethylbenzene is to be converted entirely to specific xylene isomers, the combined feed rate through the isomerization zone is several times larger than the fresh feed. Consider, for example, the conversion of the following feed to para-and/or ortho-xylene.

Fresh feed composition, wt. percent:

Ethylbenzene 24.0 Para-xylene 20.5 Meta-xylene 40.5 Ortho-xylene 15.0

When 100 weight units of this feed is supplied to a reaction/separation circuit which includes a crystallizer for para-xylene removal and a fractionator for ortho-xylene removal, and these removal devices are then operated to recover a predetermined fraction of the specific isomers desired, the following yields are obtainable.

Case 1 2 3 4 5 Percent recovery of para in crystallizer feed 62. 4 60. 9 61.2 61.4 38. 1 0 Percent recovery of ortho in xylene fractionator feed 0 30 60 95 05 Combined teed ratio, based on unit weight of fresh feed 6.8 5.0 3. 9 3. 1 3. 7 5. 6 Yields, wt. percent:

Para-xylene. 82. 6 61. 8 49. 9 41. 2 30. 1 0

Ortho-xylene. 0 25. 4 40. 1 50. 8 60. 2 85. 7

C -naphthenes percent 4. 7 5. 5 5. 9 6. 1 5. 0 5. 2

C3 ring retention per pas 97 07 07 97 97 07 Reactor outlet condition Pressure, atm. gauge 10. 2 10.2 10. 2 10.2 10.2 10.2

Temperature, C 409 407 406 405 406 408 It will be noted that the combined feed ratio to the isomerization zone varies from about 3.1 to about 6.8 times the fresh feed. The C -naphthene concentration required in the reaction zone varies somewhat because the extent of conversion required per pass for the ethylbenzene depends upon the combined feed ratio. In all cases the C -naphthene level is chosen to result in a C ring retention of 97%. Small changes in pressure and/ or temperature accomplish this control because the C -naphthenes are in essential equilibrium with the C -aromatics at the reactor outlet. These naphthenes are automatically returned to the reactor with the unconverted ethylbenzene and xylenes to prevent the formation of additional amounts of naphthenes from the fresh feed. It is to be realized that some naphthenes must be formed in the reactor to oiiset losses incurred elsewhere in the total system. It has thus been shown that the presence of about 2 to about 9 weight percent naphthenes is critical to the eifective isomerization of ethylbenzene in both a recycle process and a once-through operation.

We claim as our invention:

1. An isomerization process for the production of a specific Xylene isomer which comprises contacting hydrogen and a C -aromatic charge stock containing ethylbenzene at a concentration greater than equilibrium with respect to the xylenes present and containing C -naphthenes at isomerization conditions in a reaction zone, with a catalytic composite of alumina combined with about 0.01% to about 1.0%, by weight, of a platinum component and about 0.1% to about 5.0%, by weight, of a halogen component, separating a specific xylene isomer from the resulting reaction product effluent and providing a hydrocarbon stream containing unreacted ethylbenzene, C -naphthenes, and xylene isomers, and recycling said hydrocarbon stream to said reaction zone; said process further characterized in that said isomerization conditions are selected to maintain a c -naphthene content in said C -arornatic charge to said reaction zone of from 2.0% to about 9.0% by weight, of the C -aromaties in said charge, said isomerization conditions including an LHSV of about 0.2 to about 8.0, a temperature of about 370 C. to about 450 C., a pressure of about 3 atmospheres to about.20 atmospheres, and a hydrogen concentration in a mole ratio of about 2.0:1 to about 20.011 with respect to said C -aromatic charge.

2. The process of claim 1 further characterized in that said specific xylene isomer is para-xylene.

3. The process of claim 1 further characterized in that said specific Xylene isomer is ortho-xylene.

4. The process of claim 1 further characterized in that from about 1.0 to about 10 00 p.p.m., by weight, of halogen is added to said C -aromatic charge.

References Cited UNITED STATES PATENTS 2/1963 Berger. 4/1968 Lovell et al.