US2890252A - Production of para-xylene - Google Patents

Description

June 9, 1959 J. E. co'rTLE PRODUCTION OF PARA-XYLENE 2 Sheets-Sheet 1 Filed .my so. 1954 INVENTOR. J. E. COTTLE ATTORNEYS June 9, 1959 J. E. coTTLE PRODUCTION 0F PARA-XYLENE 2 Sheets-Sheet 2 Filed July 50. 1954 HOLVNOLLDVHJ wNN,

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United States Patent() PRODUCTION OF' PARA-XYLENE lohn E. Cottle, Bartlesville, Ukla., assignor to Phillips PetroleumCornpany, a corporation of Delaware Application July 30, 1954,- Serial No. 446,772

9 Claims. (Cl.`26tl-668) This invention pertains to a process for maximum ultimate recovery of para-xylene from a mixture of xylene More particularly, it pertains to a process wherein a petroleum naphtha hydroformate is'separated into a para-xylene product, an ethyl benzene stream and a stream containing a mixture of ortho-xylene and meta-xylene and the subsequent isomerization of these two streams under conditions favorable to the maximum ultimate production of para-xylene.

'Para-xylene has for sometime been an article of commerce. The principal sources of para-xylene are now and have been from petroleum and coal tar xylene fractions, which normally consist of ortho-, meta-, and para- Yxylene and ethyl benzene, the content of the para-xylene varying between 10 and 20 percent. More recently, the

development ofsynthetic fabrics such as Dacron has increased the demand for para-xylene to such an extent vthat the supply of naturally occurring'xylenes is insufcient. Thus, considerable interest has been focused upon preferential conversion of other hydrocarbons to para-xylene.

It has been known that the is'om'eric 'forms of xylene, namely the ortho-xylene, meta-xylene and para-xylene, can be converted one into the other by isomerization. However, the isomerization has vbeen diicult to accomplish and has taken place at such a slow rate as to make "it impractical forcommercial operation. Even when the isomerization was carried out under relatively drastic conditions leading to appreciable degradation of the product, the reaction was quite slow. The xylenes have been isomerized by purely thermal means and by treatment under certain conditions Ywith a Freidel-Crafts type catalyst, e.g., aluminum chloride plus hydrogen chloride, `but the required conditions were severe, the reaction was slow and the yields were poor.

`Further development of the isomerization of xylenes resulted in the use of clay type cracking catalysts. Preferred catalysts of that typehave been the acid treated clays or synthetic materials composed largely of silica in combination with alumina and/or magnesia, zirconia, and boric oxide.

It is an object of this invention to provide an improved process for the production of para-xylene. Another obvject of this invention is to provide a process for the provduction of increased amounts of para-xylene by the isomverization of ethyl benzene lto mixed xylene over a supported platinum or palladium catalyst. A further object is to provide a process for the production of increased vamounts of para-xylene by the catalytic isomerization of ortho-xylene and meta-xylene to para-xylene. A more specific object is to provide a process for the production r'of para-xylene by the separate isomerization of ethyl ben- 'zene and a mixture of orthoand meta-xylene following theirseparation from a hydroforming process.

`*Theterm reforming is well known in the petroleum vir'rdustry and refers to the treatment of gasoline fractions to improve their anti-knock characteristics. Straight run gasolines contain naphthenic `hydrocarbons, `particularly ice of the type havinga cyclohexane nucleus, and paranic hydrocarbons which usually are of straight chain or slightly branched chain structure, as well as varying proportions of aromatic hydrocarbons. Reforming eiects dehydrogenation of the naphthenic hydrocarbons to aromatics, `cyclization of the straight chain paratiinic hydrocarbons to aromatics, cracking, and isomerization. The present invention makes use of the dehydrogenation and cyclizing action to convert part of the naphthenic feedstock to xylenes and makes use of the isomerizing action to convert orthoand meta-xylene to para-xylene. Catalytic reforming processes effected in the presence of hydrogen are known as hydro-forming and since the reforming process of this invention employs added hydrogen, it will be referred to as hydroforming throughout the subsequent specification and claims.

The presentprocess comprises the steps of hydroforming a petroleum naphtha fraction to produce in part, ethyl benzene and a mixture of xylene isomers, concentrating the aforesaid compounds, dividing the concentrate into three portions, viz. ethyl benzene, para-xylene, and a mixture of orthoand meta-xylenes, isomerizing the ethyl benzene in a separate unit under such conditions that at least part of the ethyl benzene is converted to xylene isomers, and passing the aforesaid orthoand meta-xylenes to the .feed to the hydroforrning zone. The preferred catalyst for the hydroforming step is supported platinum or palladium.

Figure l shows a specific embodiment of the invention. A -270 F. petroleum naphtha fraction is fed from line 1 to hydroforming zone 2 along with a mixture of recycled xylenes from 15. The catalyst is a conventional Platforming catalyst of the type described in U.S. 2,479,110. Operating conditions for this zone are 300- 550 p.s.i.a., temperature 880 F., LHSV (liquid hourly space velocity) of 2 3, and hydrogenzhydrocarbon ratio about 3:1. The efliuent, containing about 31/2-4% paraxylene, is passed through line 3`along with isomerizate from line 14 to a separation zone indicated generally at 4. This may comprise conventional separation equipment for separating from the xylene-ethyl benzene fraction the products which are lighter and those which are heavier than the fraction. This would normally include fractionators to separate light and heavy materials from the hydroforming ei'lluent, a solvent extraction zone for the separation of parans for aromatics, and additional fractionation equipment for the separation of light aromatics from heavy aromatics. This equipment forms no part of the present invention, however, and it is sulicient to indicate that light products are removed at 5 and heavy products atl 6. The eluent stream passed through line 7 to crystal purification zone S is essentially an equilibrium mixture of the three xylene isomers and ethyl benzene. Zone 8 includes a crystal forming step and crystal purification step, features described in detail in Serial No. 166,992, tiled June 9, 1950, now U.S. Patent No. 2,747,001, same assignee. In crystal purification zone 8, the para-xylene-is separated from the mixture and withdrawn in essentially pure form at 9. The mother liquor is` passed via line 10 to fractionator 11 from which ethyl benzene is taken overhead through line 12 to isomerization zone 13. This is operated at 900 F., a pressure of 365 p.s.i.a., and catalyzed by a platinum catalyst on HF- treated alumina. The isomerizate from 13 is passed through line 14 to line 3 entering the separation zone for reprocessing as described.

The above specific embodiment can be modied as illustrated by Figure 2. This modification parallels that of Figure 1 through the first separation step. Feedstock is fed vthrough line VV20 to hydroformer v21 along with recycled `mother liquor from line 34 containing mainly orthoand meta-xylene. The product passes through line 22, along with isomerizate from line 34, to separation zone 23 where light and heavy materials are removed at 24 and 25, respectively, and the xylene-ethyl benzene mixture withdrawn at 26. At this point, unlike the scheme of Figure 1, the mixture enters f ractionator 27 where ethyl benzene is removed overhead through line 28 to isomerizer 29, the effluent of which is returned to the system via line 30. The bottoms from fractionator 27, consisting essentially of a mixture of the three xylene isomers, is passed to crystal purification zone 32 through line 31 from which para-xylene is then withdrawn at 33 and the mother liquor recycled through line 34 to hydroformer 2. By operating in this manner considerable reduction in refrigeration cost of the crystallization zone is realized since the ethyl benzene fraction is removed prior to crystallization. The naphtha charge which is hydroformed in the present process is a fraction boiling in the range of 150-450 F., preferably in the range of 220 300 F. The preferred catalyst is HF-treated alumina impregnated with between 0.01 and weight percent platinum, preferably 0.1 to 1 weight percent platinum. Another platinum catalyst which may be used is platinum on silica-alumina which preferably has been heat treated to reduce the surface area to within the range of to 70 square meters per gram. These two types of catalyst are described further in U.S. 2,409,109 and U.S. 2,550,- 531, respectively. As stated before, however, the platinum may be replaced with palladium. The temperature employed in this step is in the range of 700 to 1000o F., preferably 800 to 950 F. A pressure within the range of atmospheric to 1000 p.s.i.g., preferably 200-600 p.s.i.g., is used to obtain the selective formation of xylenes while operating at an LHSV (liquid hourly space velocity) of 0.3-10, preferably 0.5-6. A hydrogen to hydrocarbon mol ratio of between 0.5 :1 to 20:1, preferably 1:1 to 10: l, is utilized.

The separation zone following the hydroformer may utilize both fractionation and solvent extraction or may even include extractive distillation and absorption processes in effecting the separation. In a preferred embodiment this separation is operated as follows: a hydrocarbon fraction lower boiling than the xylene, such as the C5s and lighter is taken overhead from a fractionator while hydrocarbons higher boiling than the xylenes are removed as kettle product. This leaves a hydrocarbon fraction composed largely of paraffms and aromatics which is then withdrawn as a side stream and solvent extracted to selectively remove the aromatics, leaving the paranic hydrocarbons as rainate. The extract comprising principally ethyl benzene, benzene, toluene, the three xylene isomers, and a small amount of heavy hydrocarbons, is passed to a fractionator along with a mixed xylene isomerizate introduced from the ethyl benzene isomerizer. The fractionator separates benzene and toluene as a light fraction, a heavier than xylene fraction as bottoms and a sidestream of xylene concentrate comprising principally the three xylene isomers and ethyl benzene.

The fractionation of the motor liquor (Figure 1) is by conventional means to effect the separation of ethyl benzene from the orthoand meta-Xylenes. The same means are used in the alternative embodiment (Figure 2) wherein this fractionation precedes the para-xylene crystallization step instead of following it. The xylene fraction passed to the isomerization zone from the fractionator undergoes carbon skeleton isomerization in which the orthoand meta-xylenes are converted in part to paraxylene.

The invention resides primarily in the separation from the xylene concentrate of ethyl benzene and the mixture of orthoand meta-xylene and the subsequent isomerization of these two streams under conditions favorable to increased production of para-xylene. Ethyl benzene is considerably more difficult to isomerize to para-xylene than is orthoor meta-xylene. But by separating the ethyl benzene and isomerizing it separately, conditions can be selected which give an increased conversion of ethyl benzene to isomeric xylenes, including para-xylene.

The catalyst used in the ethyl benzene isomerization step is preferably a supported platinum or palladium catalyst such as that used in the hydroforming zone, although other conventional isomerization catalysts are operative. This reaction is preferably conducted at temperatures of about 900 F. and pressures of 215 to 365 p.s.i.a. An undesirable side reaction of ethyl benzene isomerization is the cracking of the ethyl benzene to benzene and ethane. This reaction does not appear to be affected by pressure but the isomerization of ethyl benzene to para-xylene is favored by higher pressures. Hence, pressures at the upper end of the indicated range should be used since they favor the isomerization of ethyl benzene to para-xylene without at the same time accelerating the undesirable side reaction.

It has previously been proposed to isomerize the crystallizer mother liquor, consisting essentially of ethyl benzene and a mixture of orthoand meta-xylenes, to produce additional para-xylene. This conversion is based on the fact that the mother liquor contains a lesser amount of para-xylene than that which would be present in au equilibrium mixture of ethyl benzene and orthoand metaxylenes. Hence, there is a tendency for the mixture to isomerize in part to para-xylene and thus to approach equilibrium. However, the several components of the mixture do not isomerize to para-Xylene at the same rate under the same conditions. Assuming recycling to extinction, and that the components of the mother liquor behave as if they were pure components, the following ultimate yield of figures have been calculated for the components of the mother liquor.

From these figures it can be seen that meta-xylene is the component producing the highest yield of para-xylene. The meta-xylene and orthoxylene are converted to para-- xylene about 28 and 18 percent, respectively, more efticiently than is ethyl benzene.

When ethyl benzene is isomerized separately, however, it is converted to all three xylene isomers because ethyl benzene is the only component that is present in an amount in excess of equilibrium. Even under ideal conditions, it is not possible to convert much of the ethyl benzene to para-xylene per pass, but a considerable amount is converted to meta-Xylene. And, since metaxylene produces more para-xylene than the other isomers, once the ethyl benzene is converted to meta-xylene it is comparatively easy to re-isomerize the latter to paraxylene.

In the isomerization of the orthoand meta-isomers in the hydroformer, there is a tendency for some of these isomers to be converted to ethyl benzene; however, by leaving some ethyl benzene in the recycle stream, it is possible to minimize this tendency. In fact, it is difficult to prevent a certain amount of ethyl benzene from being recycled to the hydroformer along with the xylenes.

This case is related to S.N. 347,358, filed April 7, 1953, now U.S. Patent No. 2,741,646, and S.N. 345,167, filed March 27, 1953, same assignee.

I claim:

l. In a process for the production of para-xylene which comprises subjecting to catalytic hydroforming conditions a feedstock containing a mixture of xylene isomers to form as effluent a product containing a mixture of xylcne isomers and ethyl benzene, wherein the catalyst employed in hydroforming said feedstock is a supported noble metal selected from the group consisting of platinum and palladium, the improvement comprising dividing three portions out of the effluent, namely, para-xylene, ethyl benzene, and a mixture of orthoand meta-xylene; recycling the latter mixture to the hydroforming zone for partial isomerization to para-xylene, separately and catalytically somerzing in the presence of said catalyst the ethyl benzene portion to convert it in part to a mixture of xylene isomers, mixing the isomerizate with the hydroforming effluent for reprocessing, and recovering the para-xylene as product.

2. Process of claim 1 wherein the para-xylene is removed from the system by crystallization.

3. In a process for the production of para-xylene which comprises catalytically hydroforming a petroleum naphtha, to which has been added a mixture of xylene isomers, in the presence of a supported noble metal catalyst selected from the group consisting of platinum and palladium, at elevated temperature and pressure to secure as eluent a product containing a mixture of xylene isomers and ethyl benzene, concentrating said mixture by separating therefrom the other components of the eluent, and recovering para-xylene from the concentrated mixture, the improvement comprising fractionating the remainder of the mixture to separate the ethyl benzene as overhead and the residual orthoand meta-xylenes as bottoms, recycling the latter to the hydroforming zone for partial isomerization to para-xylene, separately and catalytically isomerizing in the presence of said catalyst the ethyl benzene to convert it in part to xylene isomers, and returning the isomerizate to the system for separation of the para-xylene therefrom.

4. Process of claim 3 wherein the para-xylene is separated by crystallization.

5. In a process for the production of para-xylene which comprises catalytically hydroforrning a petroleum naphtha fraction, admixed with recycled xylene isomers7 in the presence of a supported catalyst selected from the group consisting of platinum and palladium on a support selected from the group consisting of HF-treated alumina and heat-treated silica-alumina, and at an elevated temperature and pressure to secure as efuent a product containing a mixture of xylene isomers and ethyl benzene, concentrating said mixture by separating therefrom the other components of the eifluent, crystallizing para-xylene therefrom, and leaving as mother liquor a solution containing orthoand meta-xylenes and ethyl benzene, the improvement comprising fractionating said mother liquor to separate the ethyl benzene from the orthoand metaxylenes, returning the latter mixture to the hydroforming zone to effect partial conversion to para-xylene therein, catalytically isomerizing in the presence of said catalyst the ethyl benzene to form additional xylene isomers, and recycling the isomerization efuent to the system for recovery of the additional para-xylene.

6. In a process for the production of para-xylene which comprises passing into a hydroforming zone a feedstock consisting essentially of recycled orthoand meta-xylene and a petroleum naphtha fraction boiling within the range of 150450 F., contacting said feedstock with hydrogen and a platinum catalyst supported on HF- treated alumina at a temperature in the range of 700- 1000 F., a pressure within the range of atmospheric to 1000 p.s.i.g., a liquid hourly space velocity of 0.3-10, and a hydrogenzhydrocarbon mol ratio of between 0.5: 1 to :1, to form a product containing a substantial quan tity of xylene isomers and a minor amount of ethyl benzene, removing from the product substantially all products other than said xylene isomers and ethyl benzene, and crystallizing para-xylene from this mixture, the improvement comprising fractionating the residual mother liquor into an ethyl benzene overhead and a bottoms mixture comprising orthoand meta-xylene, recycling the latter mixture to the hydroforming zone to effect partial conversion therein to para-xylene, isomerizing the ethyl benzene in the presence of a platinum catalyst supported on HF-treated alumina and at elevated temperature and pressure to form additional para-xylene, and recycling the isomerizate to the system for recovery of para-xylene therefrom.

7. In a process for the production of para-xylene which comprises catalytically hydroforming petroleum naphtha, admixed with recycled orthoand meta-xylene, in the presence of a catalyst selected from the group consisting of platinum and palladium on a support selected from the group consisting of HF-treated alumina and heattreated silica-alumina, at an elevated temperature and pressure to secure as efluent a product containing a mixture of xylene isomers and ethyl benzene, and concentrating said mixture by separating therefrom the other components of the eiuent, the improvement comprising fractionating the concentrated mixture to remove ethyl benzene overhead, leaving `a mixture consisting essentially of the three xylene isomers, crystallizing para-xylene from the latter mixture, separating mother liquor from the para-xylene crystals, recycling the remaining isomers to the hydroforming zone to form additional para-xylene, isomerizing in the pressure of said catalyst the ethyl benzene in a separate zone, and recycling the isomerization effluent to the first separation step for reprocessing and recovery of the para-xylene.

8. In a process for the production of para-xylene which comprises passing into a hydroforming zone a feedstock consisting essentially of recycled orthoand meta-xylene and a petroleum naphtha fraction boiling within the range of -450 F., contacting said feedstock with hydrogen and a platinum catalyst supported on HF-treated alumina at a temperature in the range of 700-1000 F., a pressure within the range of atmospheric to 1000 p.s.i.g., a liquid hourly space velocity of 0.3-10, and a hydrogen:hydrocarbon mol ratio of from 0.5:1 to 20:1 to form a product containing a substantial quantity of xylene isomers and a minor amount of ethyl benzene, and removing from the product in la first separation zone substantially all products other than the ethyl benzene-xylene mixture, the improvement comprising fractionating said mixture to separate the ethyl benzene therefrom, leaving as bottoms a mixture comprising the three xylene isomers, crystallizing para-xylene from the bottoms mixture and returning the mother liquor to the hydroforming zone to effect isomerization of the orthoand meta-xylene content thereof to para-xylene, contacting the ethyl benzene with a catalyst consisting of platinum on HF-treated alumina at an elevated temperature and pressure to effect partial isomerization to xylenes, and recycling the isomerization eluent to the rst separation zone for reprocessing and subsequent recovery of para-xylene.

9. Process of claim 8 wherein the isomerization is conducted at -a temperature of about 900 F., a pressure of frf0g13211-365 p.s.i.a., and a liquid hourly space velocity o References Cited in the le of this patent UNITED STATES PATENTS 2,532,276 Birch Dec. 5, 1950 2,632,779 Pfennig Mar. 24, 1953 2,651,597 Corner et al Sept. 8, 1953 2,656,397 Holzman et al Oct. 20, 1953 2,741,646 Clark Apr. 10, 1956 2,837,581 Hill et al. June 3, 1958 2,837,582 Hill et al. June 3, 1958 FOREIGN PATENTS 1,058,336 France Nov. 10, 1953 OTHER REFERENCES Haensel: Oil and Gas Journal, Aug. 9, 1951, pages 80, 82, 84, 88, 101.

UNITED STATES PATENT @EETEE CERTIFICATE 0F CRRECTION Pel-bent No 2,890,252 June C), 1959 John E Cottle Column 2, line 46, for "pareffns for" reed "-1 pereffns from m3 column 3, line 59, 01* "motor" read. mother w; column 6, line 23, for "pressure" read w presence m- Signed and sealed this 29th dey o December 3.959o

(SEAL) Attest:

mi Ho AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents

Claims (1)

1. IN A PROCESS FOR THE PRODUCTION OF PARA-XYLENE WHICH COMPRISES SUBJECTING TO CATALYTIC HYDROFORMING CONDITIONS A FEEDSTOCK CONTAINING A MIXTURE OF XYLENE ISOMERS TO FORM AS EFFLUENT A PRODUCT CONTAINING A MIXTURE OF XYLENE ISOMERS AND ETHYL BENZENE, WHEREIN THE CATALYST EMPLOYED IN HYDROFORMING SAID FEEDSTOCK IS A SUPPORTED NOBLE METAL SELECTED FROM THE GROUP CONSISTING OF PLATINUM AND PALLADIUM, THE IMPROVEMENT COMPRISING DIVIDING THREE PORTIONS OUT OF THE EFFLUENT, NAMELY, PARA-XYLENE,ETHYL BENZENE, AND A MIXTURE OF ORTHO- AND META-XYLENE; RECYCLING THE LATTER MIXTURE TO THE HYDROFOIRMING ZONE FOR PARTIAL ISOMERIZATION TO PARA-XYLENE,SEPARATELY AND CATALYTICALLY ISOMERIZATION IN THE PRESENCE OF SAID CATALYST THE ETHYL BENZENE PORTION TO CONVERT IT IN PART TO A MIXTURE OF XYLENE ISOMERS, MIXING THE ISOMERIZATE WITH THE HYDROFORMING EFFLUENT FOR REPORCESSING, AND RECOVERING THE PARA-XYLENE AS PRODUCT.

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