Paraxylene separation process

The invention relates to a p-xylene separation process wherein at least a portion of ethylbenzene present in an aromatics-containing feed is removed prior to isomerization. Aspects of the invention provide a process for producing p-xylene. The process includes providing a first mixture comprising ≧5.0 wt. % of aromatic C8 isomers, the C8 isomers comprising p-xylene and ethylbenzene. A p-xylene-containing portion and an ethylbenzene-containing portion are separated from the first mixture in a first separation stage to form a p-xylene-depleted raffinate. The first separation stage can include at least one simulated moving-bed adsorptive separation stage. At least a portion the p-xylene-depleted raffinate in the liquid phase is reacted to produce a reactor effluent comprising aromatic C8 isomers. The first mixture can be combined with ≧50.0 wt. % of the reactor effluent's aromatic C8 isomers. The combining can be carried out before and/or during the separating of the p-xylene and ethylbenzene portions.

FIELD OF INVENTION

Aspects of the invention relate to para-xylene (p-xylene) separation processes. In particular, aspects of the invention relate to xylene loop processes.

BACKGROUND OF INVENTION

Aromatic hydrocarbons, such as benzene, toluene, xylene, etc. are useful as fuels, solvents, and as feeds for various chemical processes. Of the xylenes, para-xylene is particularly useful for manufacturing phthalic acids such as terephthalic acid, which is an intermediate in the manufacture of synthetic fibers such as polyester fibers. Xylenes can be produced from naphtha, e.g., by catalytic reforming, with the reformate product containing a mixture of xylene isomers and ethylbenzene. Separating p-xylene from the mixture generally requires stringent separations, e.g., separations utilizing superfractionation and multistage refrigeration steps. Such separations are characterized by complexity, high energy-usage, and high cost.

Chromatographic separation is an alternative to more stringent separations, such as superfractionation, for removing p-xylene from a mixture of aromatic C8isomers. Chromatographic separation involves simulating a moving bed of selective adsorbent. Examples of commercial processes in which p-xylene is separated from aromatic C8isomers by simulated moving-bed separation include PAREX, available from UOP, ELUXYL, available from Axens, and AROMAX, available from Toray. Although a raffinate depleted in p-xylene can be recycled as a feed component to the p-xylene separation step, ethylbenzene will undesirably accumulate in the recycle stream.

In order to overcome this difficulty, p-xylene is conventionally produced in a continuous process (commonly referred to as a xylene loop), in which p-xylene-depleted raffinate is isomerized to reduce the amount of ethylbenzene therein. The isomerization reduces the amount of ethylbenzene in the stream by converting it into an equilibrium or near-equilibrium xylene mixture, e.g., a mixture comprising xylene isomers; diethylbenzene; benzene; and non-aromatics such as C2-C6olefins and C1-C6paraffins. One such process involves (a) providing a mixture of aromatic C8isomers containing p-xylene, (b) separating from the C8isomers a high-purity p-xylene extract and a p-xylene-depleted raffinate by simulated moving bed adsorption, crystallization, or a combination thereof, (c) catalytically isomerizing the p-xylene-depleted raffinate to produce an isomerate, and (e) recycling the isomerate to step (a).

Vapor-phase isomerization of the raffinate's ethylbenzene is generally needed to achieve an ethylbenzene content of ≦10.0 mole % ethylbenzene per mole of isomerate. However, vapor-phase isomerization has many disadvantages, including high energy consumption, costly and complex process equipment, and high xylenes loss due to conversion of the xylenes in the raffinate into undesirable products such as light gases and heavy aromatics, e.g., by one or more side-reactions such as one or more of cracking, transalkylation, or disproportionation. Attempts to overcome these disadvantages include reducing the quantity of raffinate going to the vapor-phase isomerization, e.g., removing ethylbenzene from the raffinate by (i) superfractionation, as disclosed in French Patent FR-A-2792632, or (ii) using chromatographic ethylbenzene separation in the p-xylene separation stage, as disclosed in U.S. Pat. No. 7,915,471. The separated ethylbenzene is isomerized in a vapor-phase isomerization stage, with the remainder of the raffinate being isomerized in a liquid-phase isomerization stage. The liquid-phase isomerization stage is operated under conditions which lessen undesired cracking, transalkylation, and disproportionation side-reactions. Isomerates from the vapor-phase and liquid-phase isomerization stages are then combined and recycled to stage (a) of the xylene loop.

Even when ethylbenzene is separated for vapor-phase isomerization, with the remainder of the p-xylene-depleted raffinate subjected to liquid-phase isomerization, the vapor-phase isomerization stage contributes to xylene-loop inefficiencies. Some of these inefficiencies result from one or more of (i) the need to vaporize the separated ethylbenzene and then re-condense the vapor-phase isomerate for combining with the isomerate derived from the liquid-phase isomerization stage, (ii) the need to separate unreacted molecular hydrogen vapor for re-use as an isomerization treat gas, and (iii) the need for removing non-aromatics formed during isomerization. Consequently, it is desired to further lessen or even eliminate the need for vapor-phase isomerization.

SUMMARY OF INVENTION

It has been found that xylene loop efficiency is unexpectedly improved by separating and removing from the loop at least a portion of the ethylbenzene present in the feed to the p-xylene separation stage and/or at least a portion of the non-aromatics present in the feed to the p-xylene separation stage, the ethylbenzene and/or non-aromatics removal being carried out upstream of raffinate isomerization. Separating and conducting away from the xylene loop at least a portion of the ethylbenzene increases xylene loop energy efficiency, e.g., by lessening the number of Joules consumed by the xylene loop to produce one kilogram of p-xylene by ≧20%, e.g., ≧25% when ≧90 wt. % of ethylbenzene present in the feed to the p-xylene separation stage is removed upstream of the isomerization. Moreover, removing at least a portion of the xylene loop's ethylbenzene decreases the xylene loop's complexity, e.g., by lessening or even eliminating the need for an energy-intensive and complex vapor-phase isomerization downstream of p-xylene separation. The ethylbenzene can be removed in the p-xylene separation stage, e.g., as a component of a second raffinate that is chromatographically separated in the p-xylene separation stage. It is also advantageous to remove at least a portion of any non-aromatics from the xylene loop, e.g., removing non-aromatics upstream of raffinate isomerization. The advantages can be attained even in aspects where ethylbenzene is not removed. Certain advantages result from the relatively high-value of gasoline boiling-range non-aromatics (e.g., those having an atmospheric-pressure boiling point in the range of from 30° F. to 430° F.), which can be removed from the loop for use in higher-value uses, e.g., as a blendstock for transportation fuels. Removing non-aromatics is also advantageous because doing so lessens the amount of hydrogen utilized in the process, and the associated compression costs.

In certain aspects, the invention relates to a process for producing p-xylene, the process comprising, (a) providing a first mixture comprising ≧5.0 wt. % of aromatic C8isomers, based on the weight of the first mixture, said aromatic C8isomers comprising p-xylene and ethylbenzene; (b) separating a p-xylene-containing portion and an ethylbenzene-containing portion from the first mixture in a first separation stage to form a p-xylene-depleted raffinate, wherein the first separation stage optionally includes at least one simulated moving-bed adsorptive separation stage; (c) reacting at least a portion the p-xylene-depleted raffinate in the liquid phase to produce a reactor effluent comprising aromatic C8isomers; and (d) combining with the first mixture ≧50.0 wt. %, preferably ≧90.0 wt. %, of the reactor effluent's aromatic C8isomers, preferably p-xylene, based on the weight of the reactor effluent's aromatic C8isomers, the combining being carried out before and/or during the separating of (b). At least part of the separated ethylbenzene-containing portion is conducted away.

A particular aspect relates to a process for producing p-xylene, the process comprising providing a first mixture comprising ≧5.0 wt. % of aromatic C8isomers, based on the weight of the first mixture, said C8isomers comprising p-xylene and ethylbenzene. The following components of the first mixture are separated in a first separation stage: (i) a p-xylene-depleted raffinate; (ii) a p-xylene-containing portion comprising ≧10.0 wt. % of the first mixture's p-xylene, based on the weight of the first mixture's p-xylene; and (iii) an ethylbenzene-containing portion comprising ≧10.0 wt. % of the first mixture's ethylbenzene, based on the weight of the first mixture's ethylbenzene. At least a portion of the p-xylene-containing portion is conducted away from the process, as is ≧50.0 wt. % of the ethylbenzene-containing portion, based on the weight of the ethylbenzene-containing portion. The process continues by isomerizing at least a portion the p-xylene-depleted raffinate in the liquid phase wherein ≦10.0 wt. %, e.g., ≦1.0 wt. % or ≦0.1 wt. %, of the p-xylene-depleted raffinate is in the vapor phase during the isomerizing, the weight percent being based on the weight of the p-xylene-depleted raffinate. The isomerizing produces a reactor effluent comprising ≧90.0 wt. % p-xylene, based on the weight of the reactor effluent's aromatic C8isomers. At least a portion of the reactor effluent is combined with the first mixture, the combining being carried out before and/or during the separating in the first separation stage.

In other aspects, the invention relates to an improved xylene loop, wherein the xylene loop comprises (a) providing a first mixture comprising aromatic C8isomers; (b) separating from the first mixture in a first stage: (i) a p-xylene-depleted raffinate; (ii) a p-xylene-containing portion comprising ≧10.0 wt. % of the mixture's p-xylene, based on the weight of the mixture's p-xylene; and (iii) an ethylbenzene-containing portion comprising ≧10.0 wt. % of the mixture's ethylbenzene, based on the weight of the mixture's ethylbenzene; wherein the first separation stage includes at least one simulated moving-bed adsorption chromatographic separation; (c) conducting away at least a portion of the separated p-xylene; (d) reacting at least a portion the p-xylene-depleted raffinate in the liquid phase to produce a reactor effluent comprising aromatic C8isomers; and (e) recycling to step (b) ≧50.0 wt. % of aromatic C8isomers of the reactor effluent, based on the weight of the aromatic C8isomers in the reactor effluent. The improvement comprises: (f) conducting away from the xylene loop ≧50.0 wt. % of the ethylbenzene separated in step (c), based on the weight of the separated ethylbenzene; and (g) exposing ≦10.0 wt. % of aromatic C8isomers in the xylene loop to vapor-phase isomerization, based on the weight of aromatic C8isomers in the xylene loop.

DETAILED DESCRIPTION

The first separation stage102is typically a simulated moving bed absorptive separation unit having solvent (sometimes referred to as “desorbant”) circulating therethrough via line106. The circulation can be carried out using one or more pumps, such as the pump shown schematically inFIG. 1as a component of stage102. In particular aspects the first separation stage102is a chromatographic separation stage. Solvent106should be selected to separate under the separation conditions, e.g., solvent flow, temperature, etc., the components of first mixture101as desired. Typical solvents include hydrocarbon solvents, e.g., toluene. Exemplary separation processes are described in U.S. Pat. No. 7,915,471, incorporated herein by reference in its entirety.

The process may include separating at least a portion of any non-aromatic hydrocarbon molecules from the first mixture101. This can be done upstream of stage107, e.g., in stage102(not shown). First separation stage102may remove from 5.0 to 100 wt. % of any non-aromatic hydrocarbons, based on the amount of such hydrocarbons in the first mixture. In particular aspects, the first separation stage102removes ≧50.0 wt. %, preferably ≧75.0 wt. %, or ≧90.0 wt. % of the non-aromatic hydrocarbons. Certain non-aromatic hydrocarbon molecules, e.g., C9non-aromatic molecules, have approximately the same volatility as p-xylene. It is conventional to ameliorate problems associated with non-aromatics separation from the xylene loop by cracking at least a portion of the non-aromatics during xylene isomerization. This approach lessens xylene loop efficiency (as a result of, e.g., bottlenecking of the isomerization stage), and leads to an increase in separation complexity as a result of the need to remove the cracked products. It has been found that this difficulty can be overcome by removing at least a portion of the non-aromatics upstream of the isomerization, e.g., by removing non-aromatics from the first mixture in the first separation stage. As in the case of ethylbenzene removal, separation of non-aromatics can be carried out using at least one simulated moving-bed adsorption chromatographic separation. Advantageously, non-aromatics separation and ethylbenzene separation can be carried out in the same simulated moving-bed adsorption chromatographic separation using the same desorbent. The chromatographic separation can be carried out using conventional methods, such as those described in U.S. Pat. No. 3,662,020, which is incorporated by reference herein in its entirety. One or more conventional desorbents can be used, e.g., toluene. Conventional configurations can be utilized for the simulated moving-bed adsorption chromatographic separation of the first separation stage, e.g., stacked-bed mode and/or multiple bed mode. Suitable configurations are disclosed in U.S. Pat. Nos. 2,985,589 and 3,310,486, which are incorporated by reference herein in their entirety. In certain aspects, four streams are conducted away from the first separation stage: (i) first and second extracts and (ii) first and second raffinates. Referring again toFIG. 1, the first extract corresponds to the p-xylene-containing portion removed from stage102. The second extract corresponds to the ethylbenzene portion removed from stage102. The first raffinate comprises at least a portion of the first mixture's m-xylene and o-xylene, e.g., ≧50.0 wt. % of the first mixture's m-xylene and ≧50.0 wt. % of the first mixture's o-xylene. The second raffinate comprises non-aromatics, e.g., ≧50.0 wt. % of the first mixture's non-aromatics. Although it is not required, the invention is compatible with removing at least a portion of any desorbent from one or more of the first extract, second extract, first raffinate, and second raffinate. For example, desorbent can be removed from the first raffinate upstream of the isomerization in order to further debottleneck the isomerizing. Since at least part of the first mixture's non-aromatics can be separated and conducted away, the process optionally includes subjecting to isomerization conditions <90.0 wt. % of the first mixture's non-aromatics, e.g., ≦50.0 wt. %, such as ≦25.0 wt. %. In certain aspects, the process includes subjecting ≦10.0 wt. % of the first mixture's non-aromatics to isomerization conditions, ≦5.0 wt. %, such as ≦1.0 wt. %.

The p-xylene-depleted raffinate105is provided to reactor107where raffinate105is reacted in the liquid phase to produce a reactor effluent comprising aromatic C8isomers. Reactor107may be any type of reactor or reactor process suitable for increasing the amount of C8isomers, relative to the amount of aromatic C8isomers in raffinate105. Reactor107typically performs at least one of (i) one or more reforming reactions, (ii) one or more disproportionation reactions, (iii) one or more transalkylation reactions, and (iv) one or more cracking reactions. While the reaction processes in reactor107are conducted in the liquid phase, some of raffinate105may be in the vapor phase. Thus, in particular aspects, ≦10.0 wt. %, e.g., ≦7.5 wt. %, ≦5.0 wt. %, ≦2.5 wt. %, ≦1.0 wt. %, ≦0.5 wt. %, −0.2 wt. %, ≦0.1 wt. %, of the p-xylene-depleted raffinate105in reactor107is in the vapor phase during the reacting, the weight percent being based on the weight of the p-xylene-depleted raffinate105.

Fourth separation stage304may be any suitable separation means, e.g., distillation tower, stabilization tower, flash drum, etc. At least a portion of the reactor effluent comprising the C8+hydrocarbons separated in second separation stage201via line203may be provided to fourth separation stage304, for removing and conducting away from the process at least a portion of one or more of toluene, o-xylene or C9+aromatics. A p-xylene containing portion exits third separation stage304via a stream306, which may be further processed or purified. In particular aspects at least a portion of p-xylene containing stream306may be recycled as described for streams203and204.

FIG. 4illustrates a process400encompassing aspects of the invention. First mixture401, comprising ethylbenzene, p-xylene, m-xylene, and o-xylene may be provided to optional distillation column402. Optional distillation column402separates an ethylbenzene containing distillate403comprising p-xylene and m-xylene and a residue404comprising xylenes and a minor amount of ethylbenzene. In particular aspects, however, distillation tower402may be absent. In certain aspects where distillation column402is absent, first mixture401may be provided directly to distillation tower409or to first simulated moving bed absorptive separation column418.

In aspects utilizing distillation tower402, residue404is provided to distillation tower409. Optional distillation tower409may be any separation means suitable for at least partially separating o-xylene from the residue404. In particular aspects, distillation tower409may be a xylene splitter. Optionally, residue404may be combined with isomerate from isomerization reactor427described below.

Distillation tower409delivers a distillate through a line410comprising an increased concentration of the m-xylene and p-xylene, based on the concentration of these compounds as provided to the distillation tower409. A residue comprising o-xylene exits the distillation tower409via line411.

The o-xylene in line411is further isolated in a distillation column412from which o-xylene-containing distillate is removed from line413. At least a portion of the o-xylene exiting the distillation tower412via line413may be recycled to an isomerization reactor427or carried away from the process for further isolation or processing. A residue containing C9+hydrocarbons exits distillation tower409via line415.

Distillate is provided to a first simulated moving bed absorptive separation column418having a desorbent, e.g., toluene, introduced therein from an external source (not shown) and recycled through column418via line418a. A p-xylene containing portion is typically withdrawn along with desorbent via line419. The p-xylene in line419may be directed to a distillation column421to separate the desorbent as a distillate fraction which is combined via line421awith the desorbent in line423for recycle to stage418. The p-xylene recovered as residue by line422generally has a high purity, e.g., 99.8%, or otherwise purified in at least one crystallization zone417at high temperature, as described European Patent EP-B-531 191, incorporated herein by reference in its entirety. The p-xylene conducted away via line424generally has a purity greater than 99.9%, for example. A liquid stream obtained from crystallization zone417can be withdrawn via line416, combined the xylene-containing distillate in line410and provided to the first simulated moving bed absorptive separation stage418.

An ethylbenzene-containing portion as described above is withdrawn from the column418via line420, and is preferably removed from the process. Non-aromatics can be removed in stage418and conducted away (not shown).

A p-xylene depleted raffinate as described above is withdrawn from the column418via line425. This raffinate typically comprises toluene and m-xylene. The raffinate of line425is combined with the o-xylene-rich line413, and introduced into isomerization reactor427via line426. In particular aspects the p-xylene depleted raffinate in line426comprises <10 wt. % ethylbenzene and >10 wt. % toluene, based on the total weight of the components in line426.

Isomerization reactor427may be any suitable reactor for isomerizing xylene/ethylbenzene mixtures. In particular aspects, the reactor427comprises a liquid phase isomerization process including a fixed bed of a zeolitic catalyst such as ZSM-5, under isomerization conditions the increase the content of p-xylene therein, preferably operated in the absence of hydrogen at a space velocity of 3 hr−1, for example, at a temperature of about 260° C. and a pressure <30 bar.

This reactor effluent exits the reactor427and is introduced into a distillation column428(for example a distillation column comprising about 30 plates). Distillation column428separates the reactor effluent into a light fraction (e.g., C1-C7hydrocarbon, particularly C1-C7non-aromatic hydrocarbon) recovered by a line429, a toluene fraction recycled by a line430to the adsorption column418, and a xylene-enriched raffinate that is provided to distillation tower409via line431. Optionally, residue404may be combined with distilled isomerate from the isomerization reactor427.

Optionally, distillate403comprising p-xylene and m-xylene from the distillation column402may be fed, optionally in combination with ethylbenzene provided via line420, to a catalytic vapor-phase isomerization reactor432of operating e.g., at a temperature of about 370-400° C. In aspects utilizing line420, ethylbenzene conveyed via line420can be obtained from an external source (not shown). Typically, the distillate403comprises ≦10.0 wt. %, e.g., ≦7.5.0 wt. %, ≦5.0 wt. %, ≦2.5 wt. %, ≦1.0 wt. %, of the aromatic C8isomers in the xylene loop, based on the weight of aromatic C8isomers in the xylene loop. The resulting isomerate conducted away from stage432is enriched in xylene, and may be passed to separation stage405. Light hydrocarbons i.e., C1-C7hydrocarbons are separated and carried away form the process via line407, optionally combined with the light hydrocarbons of line429. Benzene and toluene may be separated from the isomerate, and optionally carried away from the process, via line406. Isomerization effluent exits the separation stage405may be combined with the first mixture401and provided to the distillation column402.

Particular Aspects

Additionally or alternately, the present invention can include one or more of the following embodiments. The invention is not limited to these embodiments, and this description is not meant to foreclose other embodiments within the broader scope of the invention.

Embodiment 1. A process for producing p-xylene, the process comprising, (a) providing a first mixture comprising ≧5.0 wt. % of aromatic C8isomers, based on the weight of the first mixture, said C8isomers comprising p-xylene and ethylbenzene; (b) separating from the first mixture in a first separation stage one or more of (i) a p-xylene-containing portion, (ii) a non-aromatics containing portion, and (iii) an ethylbenzene-containing portion, to form a p-xylene-depleted raffinate, wherein the first separation stage includes at least one simulated moving-bed adsorptive separation stage; (c) reacting at least a portion the p-xylene-depleted raffinate in the liquid phase to produce a reactor effluent comprising aromatic C8isomers; and (d) combining with the first mixture ≧50.0 wt. %, preferably ≧90.0 wt. %, of the reactor effluent's aromatic C8isomers, preferably p-xylene, based on the weight of the reactor effluent's aromatic C8isomers, the combining being carried out before and/or during the separating of (b).

Embodiment 2. The process of Embodiment 1, wherein the separating step (b) includes separating from the first mixture: (A) the p-xylene-depleted raffinate; (B) the p-xylene-containing portion comprising ≧10.0 wt. % of the first mixture's p-xylene, based on the weight of the first mixture's p-xylene; and (C) the ethylbenzene-containing portion comprising ≧10.0 wt. % of the first mixture's ethylbenzene, based on the weight of the first mixture's ethylbenzene.

Embodiment 3. The process of Embodiment 1 or 2, further including conducting away at least a portion of the p-xylene-containing portion.

Embodiment 4. The process of any of Embodiments 1 to 3, further including conducting away ≧50.0 wt. % of the ethylbenzene-containing portion, based on the weight of the separated ethylbenzene.

Embodiment 5. The process of any of Embodiments 1 to 4, wherein the first mixture comprises ≧50.0 wt. % of a mixture of p-xylene, ethylbenzene, m-xylene, and o-xylene, based on the weight of the first mixture.

Embodiment 7. The process of any of Embodiments 1 to 6, wherein reacting at least a portion the p-xylene-depleted raffinate in the liquid phase includes at least one of (i) one or more reforming reactions, (ii) one or more disproportionation reactions, (iii) one or more transalkylation reactions, and (iv) one or more cracking reactions.

Embodiment 8. The process of any of the Embodiments encompassed by Embodiment 7, further comprising: (e) separating from the reactor effluent in a second separation stage at least a portion of any C1-C7compounds produced during the reacting step (c), the step (e) being carried out before the combining step (d); and (f) conducting the separated C7compounds away from the process.

Embodiment 9. The process of any of Embodiments 1 to 8, wherein the process further comprises: (g) removing a by-product from the first mixture, the by-product comprising (1) at least a portion of the first mixture's o-xylene and/or (2) at least a portion of any C9+aromatics in the first mixture; and/or (h) separating from the reactor effluent in a third separation stage one or more of toluene, o-xylene or C9+aromatics, and conducting away at least a portion of one or more of the separated toluene, the separated o-xylene, and the separated C9+aromatics.

Embodiment 10. The process of any of the Embodiments encompassed by Embodiment 9, further comprising separating from the first mixture in the first separation stage ≧50.0 wt. %, preferably ≧75.0 wt. %, or ≧90.0 wt. %, of any non-aromatic hydrocarbon molecules.

Embodiment 11. The process of any of Embodiments 1-10, wherein the reacting of step (c) includes liquid-phase isomerization, and wherein ≦10.0 wt. % of the p-xylene-depleted raffinate is in the vapor phase during the reacting, the weight percent being based on the weight of the p-xylene-depleted raffinate.

Embodiment 12. The process of Embodiment 11, wherein ≦1.0 wt. %, preferably ≦0.1 wt. %, of the p-xylene-depleted raffinate is in the vapor phase during the reacting.

Embodiment 13. The process of any of Embodiments 1-12, wherein (i) ≧90.0 wt. % of the first mixture's ethylbenzene is separated by chromatographic separation in the first separation stage, (ii) ≧90.0 wt. % of the separated ethylbenzene is conducted away from the process, and (iii) ≧90.0 wt. % of the reactor effluent's aromatic C8isomers are combined with the first mixture in step (d).

Embodiment 14. In a xylene loop, wherein the xylene loop comprises (a) providing a first mixture comprising aromatic C8isomers; (b) separating from the first mixture in a first stage: (i) a p-xylene-depleted raffinate; (ii) a p-xylene-containing portion comprising ≧10.0 wt. % of the mixture's p-xylene, based on the weight of the mixture's p-xylene; and at least one of (iii) an ethylbenzene-containing portion comprising ≧10.0 wt. % of the first mixture's ethylbenzene, based on the weight of the first mixture's ethylbenzene; or (iv) ≧10.0 wt. % of any non-aromatics in the first mixture; wherein the first separation stage includes at least one simulated moving-bed adsorption chromatographic separation; (c) conducting away at least a portion of the separated p-xylene; (d) reacting at least a portion the p-xylene-depleted raffinate in the liquid phase to produce a reactor effluent comprising aromatic C8isomers; and (e) recycling to step (b) ≧50.0 wt. % of aromatic C8isomers of the reactor effluent, based on the weight of the aromatic C8isomers in the reactor effluent; the improvement comprising: (f) conducting away from the xylene loop (i) ≧50.0 wt. % of the ethylbenzene separated in step (c), based on the weight of the separated ethylbenzene, and/or (ii) ≧50.0 wt. % of any non-aromatics separated in step (c); and (g) exposing ≦10.0 wt. % of aromatic C8isomers in the xylene loop to vapor-phase isomerization, based on the weight of aromatic C8isomers in the xylene loop.

Embodiment 15. The process of Embodiment 14, wherein the first mixture comprises ≧50.0 wt. % of a mixture of p-xylene, ethylbenzene, m-xylene, and o-xylene, based on the weight of the first mixture.

Embodiment 17. The process of any of Embodiments 14 to 16, wherein reacting at least a portion the p-xylene-depleted raffinate in the liquid phase includes at least one of (i) one or more reforming reactions, (ii) one or more disproportionation reactions, (iii) one or more transalkylation reactions, and (iv) one or more cracking reactions.

Embodiment 18. The process of any of Embodiments 14 to 17, further comprising: (h) separating from the reactor effluent in a second separation stage at least a portion of any C1-C7compounds produced during the reacting step (c), the step (e) being carried out before the combining step (d); and (i) conducting the separated C1-C7compounds away from the process.

Embodiment 19. The process of any of Embodiments 14 to 18, wherein the process further comprises: (j) removing a by-product from the first mixture, the by-product comprising (1) at least a portion of the first mixture's o-xylene and/or (2) at least a portion of any C9+aromatics in the first mixture; and/or (k) separating from the reactor effluent in a third separation stage one or more of toluene, o-xylene or C9+aromatics, and conducting away at least a portion of one or more of the separated toluene, the separated o-xylene, and the separated C9+aromatics.

Embodiment 20. The process of any embodiment encompassed by Embodiment 19, further comprising separating from the first mixture in the first separation stage ≧50.0 wt. %, preferably ≧75.0 wt. %, or ≧90.0 wt. %, of any non-aromatic hydrocarbon molecules.

Embodiment 21. The process of any of Embodiments 14 to 20, wherein the reacting of step (d) includes liquid-phase isomerization, and wherein ≦10.0 wt. % of the p-xylene-depleted raffinate is in the vapor phase during the reacting, the weight percent being based on the weight of the p-xylene-depleted raffinate.

Embodiment 22. The process of any embodiment encompassed by Embodiment 20, wherein ≦1.0 wt. %, preferably ≦0.1 wt. %, of the p-xylene-depleted raffinate is in the vapor phase during the reacting.

Embodiment 23. The process of any of Embodiments 14 to 22, wherein (i) ≧90.0 wt. % of the first mixture's ethylbenzene is separated by chromatographic separation in the first separation stage, (ii) ≧90.0 wt. % of the separated ethylbenzene is conducted away from the process, and (iii) ≧90.0 wt. % of the reactor effluent's aromatic C8isomers are combined with the first mixture in step (d).

Embodiment 24. A process for producing p-xylene, the process comprising, (a) providing a first mixture comprising ≧5.0 wt. % of aromatic C8isomers, based on the weight of the first mixture, said C8isomers comprising p-xylene and ethylbenzene; (b) separating from the first mixture in a first separation stage wherein the first separation stage includes at least one simulated moving-bed adsorptive separation stage: (i) a p-xylene-depleted raffinate; (ii) a p-xylene-containing portion comprising ≧10.0 wt. % of the first mixture's p-xylene, based on the weight of the first mixture's p-xylene; and (iii) an ethylbenzene-containing portion comprising ≧10.0 wt. % of the first mixture's ethylbenzene, based on the weight of the first mixture's ethylbenzene; (c) conducting away at least a portion of the p-xylene-containing portion; (d) conducting away ≧50.0 wt. % of the ethylbenzene-containing portion, based on the weight of the ethylbenzene-containing portion; (e) isomerizing at least a portion the p-xylene-depleted raffinate in the liquid phase wherein ≦10.0 wt. %, e.g., ≦1.0 wt. % or 0.1 wt. %, of the p-xylene-depleted raffinate is in the vapor phase during the isomerizing, the weight percent being based on the weight of the p-xylene-depleted raffinate, to produce a reactor effluent comprising ≧90.0 wt. % p-xylene, based on the weight of the reactor effluent's aromatic C8isomers; and (f) combining with the first mixture at least a portion of the reactor effluent, the combining being carried out before and/or during the separating of (b).

Embodiment 25. The process of Embodiment 24, further comprising: (g) removing a by-product from the first mixture, the by-product comprising (1) at least a portion of the first mixture's o-xylene and/or (2) at least a portion of any C9+aromatics in the first mixture; and/or (h) separating from the reactor effluent in a third separation stage one or more of toluene, o-xylene or C9+aromatics, and conducting away at least a portion of one or more of the separated toluene, the separated o-xylene, and the separated C9+aromatics; and wherein step (b) further includes (iv) a non-aromatic hydrocarbon portion, comprising ≧50.0 wt. %, preferably ≧75.0 wt. %, or ≧90.0 wt. %, of the first mixture's non-aromatic hydrocarbon molecules.

As demonstrated above, aspects of the invention provide methods of making p-xylene, particularly in a xylene loop. The new methods have one or more of the following advantages. For example, the vapor-phase isomerization stage may be further reduced in size when non-aromatics are separated and conducted away from the xylene loop upstream of isomerization. Vapor-phase isomerization stage(s) can be eliminated altogether when substantially all of the non-aromatics and substantially all of the ethylbenzene are separated and conducted away from the xylene loop upstream of isomerization. An advantage of some aspects is that the same type of chromatographic separation as is used for p-xylene and ethylbenzene separation from a mixture of aromatic C8isomers can also be utilized for separating and removing non-aromatics from the loop, e.g., as a component of a third raffinate, upstream of the isomerization stage. In the conventional process, vapor-phase isomerization is needed for cracking these molecules into lower molecular weight fragments. The invention overcomes this difficulty because the sufficient non-aromatics are conducted away from the xylene loop as a component of the third raffinate. Other characteristics and additional advantages are apparent to those skilled in the art.