CA2315101A1 - Meta-xylene production process - Google Patents

Abstract

In a process for producing meta-xylene, a hydrocarbon feedstream comprising at least 5 wt.% ethylbenzene and at least 20 wt.% meta-xylene is contacted with a molecular sieve catalyst under ethylbenzene conversion conditions sufficient to provide a primary product stream depleted of more than 50 % of the ethylbenzene present in the feedstream. The primary product stream is then stripped benzene and/or toluene by-products to provide a secondary product stream comprising at least 75 wt.% mixed ortho-xylene and meta-xylene. The secondary product stream is then split to remove substantially all of the ortho-xylene present therein to provide a tertiary product stream comprising at least 95 wt.% meta-xylene.

Description

META-XYLENE PRODUCTION PROCESS
This invention is direded to a process for producing mete-xylene and more specifically to a process for the catalytic conversion of ethylbenzene with minimized xylene isomerfzation, and subsequent purification to yield high-purity mete-xylene.
Mete-xylene is a valuable chemical intermediate useful in the produdion of specialty polyester resins for high-performance plastics, such as those used in blow-molded produds. These materials find use in various produds, inGuding plastic bottles, and this market is expelled to grow substantially in the coming years.
Meta-xylene may be derived from mixtures of Ce aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by seledive solvent extradion.
The Ce aromatics in such mixtures and their properties are:
Freezing Boiling Densit Point (C) Point (C) (Kg/m Ethylbenzene 95.0 138.2 869.9 Pare-xylene 13.2 138.5 883.9 Mete-xylene 47.4 138.8 886.3 Ortho-xylene 25.4 144.0 883.1 Principal sources of the mixtures of C8 aromatics are catalytically reformed naphthas and pyrolysis distillates. The C8 aromatic fradions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt.°r6 ethylbenzene (EB) with the balance, xylenes, being divided approximately 50 wt.% mete-xylene and 25 wt.% each of pare-xylene and ortho-xylene.
Individual isomer produds may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fradional distillation, and it is so produced commercially. Pare-xylene may be separated from the mixed isomers by fradional crystallization, seledive adsorption (e.g., the Parex process), or membrane separation.
As is evident in the table of properties above, the boiling point of ethylbenzene is very close to those of pare-xylene and mete-xylene, making separation of these components by distillation impractical. Ethylbenzene is therefore typically converted catalytically to benzene andlor additional xylenes during xylene isomerization to increase the yield of valuable xylene isomers. However, in most xylene isomerization processes, conversion of ethylbenzene is constrained by the need to hold conversion of xylenes to other compounds to acceptable levels. Thus, although catalytic removal of ethylbenzene is possible, operating conditions are generally seleded to balance the disadvantages of xylene loss by transaikylation with the conversion of ethyibenzene.
There is currently one commercially pradiced method for separation of mete-xylene, i.e., the Mitsubishi Gas Chemical Company (MGCC) process. See Klrfc-Othmer Encyclopedia of Chemical TechnoMgy, 3rd ell., Vol. 24, p. 727 (1984), and the publications cited therein. This separations process requires treating a mixture of isomerized xylenes with HF-BF3. Two layers are formed, i.e., a suesTtzv~ sHeEr ~RU~ a.~~

1:1 molecular complex, mete-xylene-HBF3 layer, and an organic layer containing the remaining xylenes. Mete-xylene is then recovered in 99°~ purity by thermal decomposition of the mete-xylene-HBF3 complex. Although this method is used commercially by MGCC, the difficulties in dealing with HF-BF3 make this process costly.
Thus, the state of the art of mete-xylene production is such that demand for this material continues to increase, while methods for preparing the material lag in commercial practicability.
Presently, the processes for producing mete-xylene are limited by cost and purity considerations. It would, therefore, be highly desirable to provide a commercially acceptable process for the production of mete-xylene in substantially pure forth.
Accorciing to the invention, there is provided a process for producing mete-xylene, comprising the steps of:
(a) converting a hydrocarbon feedstream comprising at least 5 wt.~o ethylbenzene, at least wt.96 mete-xylene, and less than 5 wt.°~6 pare-xylene over a molecular sieve catalyst under ethylbenzene conversion conditions sufficient to provide a primary product stream depleted of more IS than 5096, preferably more than 9596, of the ethylbenzene present in the feedstream;
(b) stripping benzene and/or toluene by-products from the primary product stream to provide a secondary product stream comprising at least 75 wt.°~ mixed orfho-xylene and mete-xylene; and (c) splitting the secondary product stream by removing substantially all of the ortho-xylene present therein to provide a tertiary product stream comprising at least 85 wt.°~, and preferably at 20 least 95 wt.~, mete-xylene.
Preferably, the process further comprises the step of distilling the tertiary product stream to obtain a distillate comprising at least 98 wt.~ mete-xylene.
Preferably, the process further comprises the preliminary step of separating pare-xylene from a mixed Cg hydrocarbon feed comprising pare-xylene, ortho-xylene, mete-xylene, and ethylbenzene to provide said hydrocarbon feedstream.
The process of the invention is capable of yielding high-purity mete-xylene in a straightforwarci and inexpensive process which can be incorporated into an existing xylene isomerization complex with minimal capital outlay but with high-purity metaxylene productivity.
The invention will now be more particularly described with reference to the accompanying drawings in which:
Figure 1 is schematic illustration of a prior art xylene isomerization processing operation.
Figure 2 is a schematic illustration of a mete-xylene production process according to one embodiment of the invention.
Referring to Figure 1, in the conventional xylene isomerization loop shown, a Ce aromatics stream, such as a heart cut consisting essentially of Ce aromatics, is fed through a xylene splitter 11 to remove ortho-xylene (and heavier C9' aromatics) from the mixed xylenes and ethylbenzene. The ortho-xylene and C9' aromatics are then fed to an ortho-xylene tower 12 where ortho-xylene is separated from the heavier components. The product from the xylene splatter 11 is fed to a pare-xylene recovery unit 13 where a substantial proportion of the pare-xylene is removed and SUBSTITUTE SHEET (RULE 2B) recovered. The remaining other Ce materials are then fed to a xylenes isomerization unit 14 together with a hydrogen cofeed. The isomerization product is fed to a distillation unit 15 to remove benzene and toluene byproducts, and the xyienes (with an increased proportion of paraxylene) is recycled to the xylene splatter 11.
Referring to Figure 2, in the process of said one embodiment of the invention, a portion of the pea-xylene effluent from the pare-xylene recovery unit 13 of the xylene isomerization loop shown in Figure 1, is fed to a catalytic conversion unit 18 where a diffusionally restricted catalyst selectively converts the ethyibenzene and the small amount of pare-xylene in the effluent at very high conversion levels. The resulting primary product stream is depleted of greater than 50°~, preferably greater than 95%, and more preferably greater than 99°~, of the ethylbenzene present in the feedstream. Optionally, benzene may be cofed to the conversion unit 18 to increase the conversion of pare-xylene to toluene and thereby increase the purity of the mete-xylene product thus afforded.
The ethylbenzene-depleted primary product stream exiting the unit 18 is fed directly to benzene/toluene splatter 17, where the lighter weight by-products (benzene, toluene and light gas) are removed, preferably by distillation. Typically, this stripping process will yield a secondary product stream which contains at least 75 wt.°h, preferably at least 85 wt.°~, and more preferably at least 95 wt.%, mixed ortho-xylene and mete-xylene. Thereafter, the secondary product stream is fed to a xylene splatter 18 to remove the o~fho-xylene and C9' aromatics. The light fraction from the splatter 18 is composed of very high purity mete-xylene. Typically this tertiary product stream comprises at least 85 wt.°~6, preferably at least 95 wt.%, and more preferably at least 98 wt.°~ mete-xylene. If desired, the tertiary product stream is distilled to further increase the purity of the mete-xylene.
It will be appreciated that in conventional xylene isomerization processes such as that shown in Figure 1, the xylene isomerization unit 11 may include an initial bed or reactor to effect conversion of ethylbenzene in the Ce aromatic feed before the feed undergoes xylene isomerization. In the process of the invention, however, the ethylbenzene-depleted product from the catalytic conversion unit 18 passes directly to the benzene/toluene splatter 17 without undergoing an intermediate xyiene isomeRzation step.
Feedsto"~C
In general, the feedstock to the process of the invention is aromatic C8 mixture having an ethylbenzene content of 5 to 60 wt.°~, an ortho-xylene content of 0 to 35 wt.°~, a mete-xylene content 20 to 95 wt.% and a paraxyaene content in the approximate range of 0 to 5 wt.%. Preferably, the feedstock will contain at least 30 wt.°~, more preferably at least 40 wt.%, mete-xylene. Feedstocks meeting this standard are termed "mete-xylene-rich' feedstocks. A preferred feedstock is obtained as the effluent stream from a pare-xyaene recovery unit.
In the process of the invention the ethyibenzene in the feedstock is selectively converted, largely by dealkylation to produce benzene and light gas, while isomerization of the mete-xylene and any oitiro-xylene is minimized so that the primary product stream exiting the catalytic unit 18 is substantially free of mete-xylene co-boilers.
SU6STITUTE SHEET (RULE 26) The Ce feedstream may also comprise added benzene as a cofeed to facilitate pare-xylene conversion by transalkylation to produce toluene. Moreover, in addition to the above, the aromatic C8 mixture may contain non-aromatic hydrocarbons, i.e., naphthenes and paraffins in an amount up to 30 wt.%. In a preferred embodiment, the invention provides means to process a mixture of CB
aromatics such as that derived after other known processing steps such as solvent extraction and distillation from catalytic reforming of a petroleum naphtha to a mixture of reduced ethylbenzene and pare-xylene content.
Process Conditions In accorcfance with the present invention, the above described feedstock is contacted with the catalyst system under su'ttable conversion conditions to effect ethylbenzene conversion and optionally pare-xylene conversion. Suitable conversion conditions include a temperature of 200° to 550°C, preferably 325° to 475°C, a pressure of 0 to 1000 psig (100 to 7000 kPa), preferably 50 to 400 psig (450 to 2850 kPa), a WHSV of 0.1 to 200 hr-', preferably 3 to 50 hr-', and an H2/HC molar ratio of 0.2 to 10, preferably 1 to 5.
Catalyst System The principal function of the catalyst system in the process of the invention is to effect a high degree of conversion of the ethylbenzene in the feed with minimal isomerization of the xylenes, particularly the mete-xylene and any ortho-xylene which also may be present, so that there is no net p-xylene make. This is preferably achieved by use of a molecular sieve catalyst which has limited diffusion for xylenes, particularly the larger isomers, mete-xylene and ortho-xylene. In this way the accessibility of the internal acid sites of the catalyst to the mete-xylene and ortho-xylene is limited. !n particular, it has been found that the catalyst preferably has an orfho-xylene sorption time (b.3), i.e., the time required to achieve 30°r6 of its equilibrium ortho-xylene sorption capacity, of greater than 50 minutes when measured at 120°C and an ortho-xylene pressure of 4.5 t 0.8 mm of mercury (493 Pa to 707 Pa). Such sorption measurements may be carried out gravimetrically in a thermal balance.
The catalyst used in the process of the invention is also preferably selected so as to have extremely low xylene isomerization activity, especially at its external acid sites. For example, the xylene isomerization activity of the catalyst is preferably such that the catalyst produces less than 12 wt.°~ pare-xylene when contacting a feed containing 60 wt.°~
mete-xylene, 20 wt.% ortho-xylene, and 20 wt.°~ ethylbenzene at a temperature of 426.7°C, a pressure of 150 psig (1136 kPaa), a weight hourly space velocity (WHSV) of 20 hr-', and a hydrogen to hydrocarbon (H2/HC) molar ratio of 1.
Catalysts useful in this invention comprise catalytic molecular sieves, such as zeolites and preferably intermediate pore size zeolites, that is zeolites having a pore size of 5A to 7 A. Preferably, the molecular sieve has a Constraint Index of 1 to 12 (as described in U.S.
Paient No. 4,016,218).
F~camples of intermediate pore size zeolites useful in this invention include ZSM-5 (U.S.
Patent No. 3,702,886 and Re. 29,948); ZSM-11 (U.S. Patent No. 3,709,979); ZSM-12 (U.S. Patent No. 3,832,449); ZSM-22 (U.S. Patent No. 4,556,447); ZSM-23 (U.S. Patent No.
4,076,842); ZSM-35 (U.S. Patent No. 4,016,245); ZSM-57 (U.S. Patent No. 4,046,685); and ZSM-58 (U.S. Patent No.
4,417,780).
SUBSTITUTE SHEET (RULE 2B) Other useful catalytic molecular sieves include MCM-22, MCM-36, MCM-49, MCM-56, silicoaluminophosphates such as SAPO-5, SAPO-11, and other zeolites including zeolite X, zeolite Y, and zeolite Beta.
As synthesized, the molecular sieves listed above may not exhibit the desired xylene 5 diffusion characteristics and xylene isomerization activity required to effect ethylbenzene conversion with minimal xylene isomerization. However, the requisite diffusional properties may be provided by controlling the crystal size of the sieve and/or by coating the catalyst with a seledivating agent which inhibits the diffusivity of the molecular sieve, particularly, the diffusivity of the molecular sieve to ortho-xylene and mefa-xylene. Preferably, the seledivating agent is silica and is produced by applying to the catalyst one or more coatings of an organosilicon seledivattng agent in a liquid canter followed, after each coating, by calcining the catalyst in an oxygen-containing atmosphere. Such a silicon seledivation process is described in detail in International Publication No. WO 98/16005.
Alternatively, the desired diffusionat properties may be achieved through the use of coke seledivation, as described in U.S. Patent No. 4,097,543, either alone or in combination with silicon seledivation. Seledivation with silica and/or coke reduces the external acid activity and hence the xylene isomerization activity of the catalyst.
The suitable molecular sieve may be employed in combination with a support or binder material such as, for example, a porous inorganic oxide support or a clay binder. For example, it may be desirable to formulate the catalyst of the invention with another material resistant to the temperature and other conditions of the hydrocarbon conversion process.
Iilusirative examples of binder materials include synthetic or naturally occurring substances as welt as inorganic materials such as clay, silica, andlor metal oxides, such as alumina, vanadta, beryllia, thoria, magnesia, titania, zirconia, bona, and combinations thereof. The preferred binder is primarily silica. The metal oxides may be naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides.
Naturally occurting clays that can be composited with the molecular sieve include those of the montmorillonite and kaolin families, which families include the subbentonites and the kaolins commonly known as Dixie, McNamee, Georgia, and Florida clays, or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite, or anauxite.
Suitable clay materials in-clude, by way of example, bentonite and kieselguhr. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment, or chemical modification.
The relative proportion of suitable crystalline molecular sieve 1o the total composition of catalyst and binder or support may be from 1 to 99 wt.°~, preferably from 30 to 90 wt.%, and more preferably from 50 to SO wt.°~, of the composition.
in order for the catalyst used in the process of the invention to be effective to convert ethylbenzene with minimal xylene isomerization, it is preferable that the catalyst has an alpha value of at least 5, more preferably from 75 to 5000 and typically from 100 to 2000.
The alpha value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of SUBSTITUTE SHEET (RULE 2B) catalyst per unit time). It is based on the activity of an amorphous silica-alumina cracking catalyst taken as an alpha of 1 (Rate Constant = 0.018 sec'). The Alpha Test is described in U.S. Patent No.

3,354,078 and in J. Catalysts 4:522-529 (August 1985): J. Catalysis 8:278 (1968); and J. Catalysis 81:395 (1980).
A hydrogenation-dehydrogenation functional metal can be incorporated into the catalyst of the invention. Such metals are known in the art to reduce ethylbenzene by product in hydrocarbon conversion processes, see, e.g., U.S. Patent No. 5,498,814.
Any metal possessing the desired hydrogenation-dehydrogenation function can be used in the modification method of the invention. These are termed "functional metals". Examples of such l0 functional metals include the oxide, hydroxide, sulfide, or free metal (i.e., zero valent) forms of metals in the Groups 3 to 15 of the periodic table. Preferred metals inGude Group 8, 9, and 10 metals (i.e., Pt, Pd, Ir, Rh, Os, Ru, Ni, Co, and Fe), Group 7 metals (i.e., Mn, Tc, and Re), Group 8 metals (i.e., Cr, Mo, and Vir7, Group 15 metals (i.e., Sb and Bi), Group 14 metals (i.e., Sn and Pb), Group 13 metals (i.e., Ga and In), Group 11 metals (i.e., Cu, Ag, and Au), and Group 12 metals (i.e., 15 Zn, Cd, and Hg). Noble metals (i.e., Pt, Pd, Ir, Rh, Os, Re, Ru, Mo, and V1r) are preferred.
Combinations or mixtures of catalytic forms of such noble or non-noble metal, such as combinations of Pt with Sn, may be used. The valence state of the metal is preferably reduced, e.g., when this component is in the form of an oxide or hydroxide. The reduced valence state of the functional metal may be attained, In situ, during the course of a reaction, when a reducing agent, 20 such as hydrogen, is inGuded in the feed to the reaction.
The functional metal may be incorporated into the catalyst by methods known in the art, such as ion exchange, impregnation, or physical admixture. For example, solutions of appropriate metal salts may be contacted with the remaining catalyst components, either before or after seledivation of the catalyst, under conditions sufficient to combine the respective components. The amount of 25 functional metal incorporated in the catalyst can vary widely depending, for example, on the hydrogenation activity of the metal employed. Generally, however, the amount of the functional metal is suitably from 0.001 to 10 wt.°~, preferably from 0.05 to 5 wt.°r°, more preferably from 0.1 to 2 wt.°~6, based on the total weight of the modified catalyst.
FJCAMPLE
30 An HZSM-5 catalyst preparation (65/35 zeolite/binder) was subjected to a four-fold silicone seledivation procedure. In each seledivation sequence, the catalyst was contacted with Dow-550 dissolved in decane and, after the decane was stripped, the catalyst was calcined at 540°C in nitrogen, and then in air.
Platinum was exchanged into the seledivated catalyst using a conventional exchange 35 technique to produce a modified catalyst containing 0.1 wt.36 Pt.
Specifically, 0.0271 g of Pt(NH3),CIZ Hz0 was dissolved in 80 mL distilled, deionized water in a beaker with a stirbar. The beaker was equipped to support a BOchner funnel (a "ceramic thimble'. Catalyst (15 g) was loaded into a BUchner funnel, which was then placed in this solution. The pH of the solution then dropped to pH 3 and ammonium hydroxide (0.1 N) was added dropwise to maintain the pH at between 4 and 7.
SUBSTITUTE SHEET (RULE 25) Following the exchange, the catalyst was dried and calcined at 350°C
for 2 hr. The calcined catalyst was then crashed and sized to 14/20 mesh.
A microunit evaluation was conducted on the resultant catalyst in an automated unit with on-line gas chromatograph (GC) sampling. Catalyst (0.75 g) was loaded into a 3/8-inch diameter, stainless steel tube reactor (with sand as inert packing material). The reactor was pressurized to 150 psig (1135 Kpa) with N2, and heated to 350°C under flowing nitrogen. The catalyst was then reduced by interrupting the NZ flow, and introducing HZ flow at a rate of 100 mUmin. After reducing for 2 hr, the reactor was heated to reaction temperature and feed was introduced. The feed was blended from mete-xylene, ortho-xylene, and ethyibenzene (Aldrich "HPLC
Grade's.
Catalytic evaluation was conducted at 10 hr'' WHSV, 1 H2/HC, 350°C and 150 psig (1135 Kpa) pressure. The results are presented in Table 1, below.

Yields (wt.%) Feed sups C5 - 5.3 Benzene - 14.3 Toluene - 1.6 Ethylbenzene 20 0.15 Pare-Xylene 0 0.7 Mete-Xylene 80 58.2 Ortho-Xylene 20 19.7 - 0.1 Ethylbenzene Conversion (~) - 99.3 Xylene Loss - 1.7 The results of this evaluation, as summarized above, demonstrate that extremely high ethylbenzene conversion can be achieved using the method of the invention.
Although pare-xylene was not present as a feed component, the amount of pare-xylene formed was very small.
Pare-xylene can also be selectively converted to toluene and trimethylbenzene (via disproportionation) or to toluene with benzene (via reverse toluene disproportionation). Given these results, we calculated mete-xylene purity, defined as the amount of mete-xylene divided by the sum of the amounts of mete-xylene and its coboilers (i.e., ethylbenzene, pare-xylene). In this case the mete-xylene purity was 98.8%.
These results clearly show that high-purity mete-xylene can be produced by catalytically converting ethylbenzene, coupled with distillation. Although this feed did not include pare-xylene, its inclusion in the feed would not be expected to significantly change these results. Furthermore, benzene may be cofed to facilitate the conversion of pare-xylene to compounds such as toluene and trimethylbenzene.
SUBSTITUTE SHEET (RUZ.E 26)

Claims (10)

CLAIMS:

1. A process for producing meta-xylene, comprising the steps of:
(a) converting a hydrocarbon feedstream comprising at least 5 wt.%
ethylbenzene, at least 20 wt.% meta-xylene, and less than 5 wt.% para-xylene over a molecular sieve catalyst under ethylbenzene conversion conditions sufficient to provide a primary product stream depleted of more than 50% of the ethylbenzene present in the feedstream;
(b) stripping benzene and/or toluene by-products from the primary product stream to provide a secondary product stream comprising at least 75 wt.% mixed ortho-xylene and meta-xylene; and (c) splitting the secondary product stream by removing substantially all of the ortho-xylene present therein to provide a tertiary product stream comprising at least 85 wt.% meta-xylene.

2. The process of Claim 1, further comprising the step of distilling the tertiary product stream to obtain a distillate having further increased meta-xylene content.

3. The process of Claim 1, further comprising cofeeding benzene with the hydrocarbon feedstream.

4. The process of Claim 1, wherein the ethylbenzene conversion conditions comprise a temperature of 200° to 550°C, a pressure of 0 to 1000 psig (100 to 7000 kPa), a WHSV of 0.1 to 200 hr-1, and an H2/HC molar ratio of 0.2 to 10.

5. The process of Claim 1, wherein the catalyst has an ortho-xylene t0.3 sorption time of greater than 50 min at 4.5 ~ 0.8 mm Hg and 120°C.

6. The process of Claim 1, wherein the catalyst produces less than 12 wt.%
para-xylene when contacting a feed containing 60 wt.% meta-xylene, 20 wt.% ortho-xylene, and 20 wt.%
ethylbenzene at a temperature of 426.7°C, a pressure of 150 psig, a WHSV of 20 hr-1, and a H2/HC
molar ratio of 1.

7. The process of Claim 1, wherein the molecular sieve is selected from the group consisting of ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-57, ZSM-58, SAPO-5, SAPO-11, zeolite Beta, zeolite X, zeolite Y, MCM-22, MCM-38, MCM-49, and MCM-56.

8. The process of Claim 7, wherein the molecular sieve is ZSM-5.

9. The process of Claim 1, wherein the catalyst has been modified by silicon and/or coke selectivation.

10. The process of Claim 1, wherein the catalyst includes a hydrogenation-dehydrogenation functional metal.