Technical Field The present invention provides methods and systems for removing ethylbenzene from a mixed xylene stream and optimizing para-xylene separation.

A first method and system includes a pretreatment step of processing the feed stream through a reactor comprising a small version of an isomerization unit within which an isomerization catalyst is provided, the catalyst causing a high level of conversion of ethylbenzene to benzene while minimizing side reactions, such as cracking of xylenes. The benzene is then removed from the feed stream prior to cyclic para-xylene separation and isomerization, respectively.

A further method and system proposes directing flow of the para-xylene feed stream into the isomerization unit of the system rather than to the para-xylene separator of the system to avoid a bottleneck condition from being created at the separator. If desired, a pretreatment step as defined above also may be included. Although the systems and methods are proposed for use in para-xylene production they are equally well suited to use in production of ortho-xylene and meta-xylene.

Background

Para-xylene is a precursor in the manufacture of polyesters, which are used in creating clothing and other synthetic materials. Most para-xylene is produced in oil refineries downstream of catalytic reformers which manufacture gasoline.

Gasoline contains a mixture of hydrocarbons including C8 aromatics. The C8 aromatics include four chemical

compounds: para-xylene, meta-xylene, ortho-xylene and ethylbenzene. The para-xylene is isolated and extracted from the other C8 aromatics by one of two physical separation processes, crystallization or mol sieve technology. Once the para-xylene has been removed from the mixture of C8 aromatics, the remaining mixture of meta-xylene, ortho-xylene and ethylbenzene, commonly referred to as raffinate, is sent to an isomerization unit where further para-xylene is created by returning the xylene mixture to an equilibrium concentration, with the new ratio of xylene isomers, commonly referred to as effluent, being returned to the separation unit for reprocessing.

The concentration of para-xylene in the total feed to the separation unit determines the efficiency of the operation. For example, in a crystallizer, the mother liquor contains about 10% para-xylene. Therefore, if the total feed stream contains 19%, about 9% will be extracted and 10% will not be extracted. However, if the total feed stream contains 21% para-xylene, the production rate increases substantially due to an 11% extraction. Thus, increasing the para-xylene in the feed stream from 19% to 21% will increase the plant capacity by approximately 11/9, or 20%. An optimum goal is to provide an enriched 24% para-xylene effluent to a high efficiency separator.

In modern units, the ethylbenzene is dealkylated to benzene in the isomerization unit. This reaction proceeds at 50 to 60% conversion per pass. Thus, the feed stream provided to the separation unit always contains a substantial amount of ethylbenzene. This ethylbenzene builds up in the mother liquor/raffinate stream combination requiring processing equipment to be larger than necessary to process the para-xylene, ortho-xylene

and meta-xylene. Since the feed stream to the separation unit contains approximately 20% para-xylene, the mother liquor/raffinate stream will contain the unextracted para- xylene plus the remainder of the chemicals which are not para-xylene. Thus, the mother liquor/raffinate stream becomes quite large, further requiring large processing equipment. For a crystallizer, the mother liquor recycle is six times para-xylene flow rate. In a mol sieve extraction unit, the raffinate is about four times the para-xylene flow rate. If ethylbenzene were removed from the feed stream initially, the remaining stream would have substantially higher concentrations of para-xylene, meta- xylene and ortho-xylene therein.

Parameters of conditions required for an isomerization reaction to the point of equilibrium in the conversion of the meta-xylene and ortho-xylene to a combination thereof with created para-xylene become less severe if little or no ethylbenzene is fed to the isomerization unit. The severity is reduced because the point of equilibrium now revolves around only three xylenes, with no ethylbenzene contaminant needing to be considered. Also, the mother liquor flow rate per unit of para-xylene production is reduced, saving energy required for processing. Further, in certain circumstances described below, when the feed stream is directed into the para-xylene separator, together with the recycled feed returning to the separator from the isomerization unit, a bottleneck occurs within the separator, slowing the separation process, thereby producing a lesser amount of para-xylene over time.

Disclosure of Invention Accordingly, initially removing substantially all of the ethylbenzene from the feed stream would provide a considerable advantage inasmuch as the production of para- xylene would increase proportionally with the concentration of the para-xylene in the recycled feed stream and the system unit to which the feed stream is directed would be more efficient in processing due to lack of ethylbenzene in the feed stream. Such initial removal of ethylbenzene from the feed stream is accomplished by the methods and systems of the present invention.

Further, when separator parameters are less then optimal and a bottleneck occurs therein, one system and method proposes directing the feed to the isomerization unit rather than to the separator.

Finally, by manipulating the feed location and operating variables, some of the beneficial reactions can be accomplished in an isomerization reactor processing both fresh feed and mother liquor/raffinate stream.

Brief Description of Drawings Figure 1 is a block diagram of a prior art method and system for obtaining para-xylene product.

Figure 2 is a block diagram of one embodiment of the methods and systems of the present invention.

Figure 3 is a block diagram of a further embodiment of the methods and systems of the present invention.

Figure 4 is a block diagram of a further embodiment of the methods and systems of the present invention which is similar to the embodiment of Figure 3.

Modes for Carrying Out the Invention Turning now to Figure 1 there is illustrated therein a prior art method and system for the production of para- xylene product. As is known, a feed stream comprising a mixture of eight-carbon (C8) aromatics including para- xylene, meta-xylene, ortho-xylene and ethylbenzene is fed to a para-xylene separator. From the separator, existing para-xylene is shunted into a collection unit and a raffinate containing ethylbenzene, ortho-xylene and meta- xylene is passed on to an isomerization unit. In this isomerization unit, the ethylbenzene is dealkylated to benzene with the reaction proceeding at a 50 to 60% conversion per pass. The converted benzene is removed from the stream and an effluent comprising the remainder of the recycle feed stream, now containing somewhere in the range of 40 to 50% of the previous amount of ethylbenzene therein and equilibrium amounts of ortho- xylene, meta-xylene and para-xylene formed in the isomerization unit, is sent back to the feed line via which the feed stream is fed into the para-xylene separator.

Cyclic separation and isomerization continues until the feed is completely consumed.

It will be understood that the ethylbenzene builds up in the recycle feed stream since the recycle stream to the separation unit typically contains 20% para-xylene with the feed stream containing unextracted para-xylene plus the 80% comprising the other C8 aromatics defined above. Thus, the recycle stream is quite large, requiring processing equipment for this process to be extremely large to accommodate the flow therethrough that is being created. Thus, if one were capable of removing the ethylbenzene from the feed in a pretreatment step, rather

than removing it in the isomerization unit, approximately 15% of the feed stream bulk could be eliminated at the beginning of the process, a greater concentration of para- xylene would be obtained, and the isomerization unit as well as the separation unit would both work more efficiently. Not only would this increase the efficiency, requiring significantly decreased energy for processing, but further the size of the units could be decreased substantially while maintaining increased processing capacity. Still further, operating conditions in the isomerization unit would be less severe, significantly increasing yield and extending catalyst life.

Thus, it is proposed as shown in Figure 2, to produce a first system and method for the production of para- xylene product wherein substantially all of the ethylbenzene is converted to benzene in a pretreatment, or primary step, distilled, and removed with only para- xylene, ortho-xylene and meta-xylene being fed into the remainder of the processing complex. This system is generally identified by the reference numeral 10. Although the instant disclosure is provided using para- xylene as the chosen product, this is not to be construed as limiting inasmuch as the system 10 and method would be applicable to the production of ortho-xylene and/or meta- xylene and/or para-xylene. For purposes of brevity, application to a para-xylene production complex is set forth as a primary embodiment and a broader scope application would be feasible without undue experimentation. As shown, the system 10 includes a para-xylene separator 12 which functions to extract para-xylene product 13 for collection and an isomerization unit 14 which functions in a more efficient manner due to the

provision of a pretreatment reactor 20 in feed line 18, the function of which will be further defined hereinafter. In this respect, inasmuch as there will be almost no ethylbenzene being fed into the isomerization unit 14, none of the C8 aromatics being fed thereinto need to be eliminated from the feed stream. Thus, the isomerization unit 14 here strictly functions in a manner to produce para-xylene from ortho-xylene and meta-xylene in the raffinate 15 fed thereto. Obviously, inasmuch as several passes through the isomerization unit 14 will now be, and always have been, required, there will be an equilibrium concentration of meta-xylene and ortho-xylene in an effluent line 16 feeding into the feed line 18 to the para-xylene separator 12, downstream of the reactor 20. However, due to increased efficiency, the amount of meta- xylene and ortho-xylene will be significantly reduced, with a greater amount of para-xylene product being available in the effluent line 16, and there will be substantially no ethylbenzene in the effluent line 16. The ethylbenzene is proposed to be removed from the feed stream going into the para-xylene separator 12 by the provision of a pretreatment unit or reactor 20 within the feed line 18 for the fresh feed 21. This pretreatment unit 20 comprises a small version of an isomerization unit and includes therein a large amount of an isomerization catalyst 22, one form of which is sold under the mark I- 100 by UOP, Inc. of Des Plaines, Illinois. Other companies that sell similar catalysts are Criterion Co. of Houston, Texas, Englehardt Co., of New York, New York, IFP (Institute de Francaise Petroleum), Paris, France, and Toray (Toyo Rayon Company) of Tokyo, Japan. It is believed that such catalyst 22 is created of platinum and chloride supported on alumina. Further, a catalyst which

would be based on a molecular sieve base would also be functional, such catalyst being available through Mobil Oil Corporation of Princeton, New Jersey.

It has been found through empirical testing using the 1-100 catalyst that an approximately 90% conversion of ethylbenzene to benzene 24 may be achieved in the pretreatment reactor 20 at a liquid hourly space velocity of 1 to 4, as is known, with the benzene 24 being immediately purged from the system 10. Because the feed stream 18 rate is much lower than the recycle stream or effluent 16 rate, the catalyst 22 volume may be large in comparison to the hydrocarbon rate, such large volume of catalyst permitting a high rate of ethylbenzene conversion while avoiding side reactions such as cracking of xylenes. Based on calculations founded on the empirical testing performed, the following advantages, based on separation technique, are expected: 1. Mol Sieve Technolocrγ

For a para-xylene unit using mol sieve technology making 20 MT/hr of para-xylene product, the total flow rate to the para-xylene separation unit will be 86.3 MT/hr, and the feed rate to the isomerization unit will be about 60 MT/hr.

Using the system 10 and method defined above and holding constant the feed rate to the para-xylene separation unit, para-xylene production will increase to 27.2 MT/hr, an increase of 36%. At the current market price of $2,000/MT, the incremental production is worth over $115 million per year: (7.2 MT/hr) (8,000 hrs/yr) ($2,000/MT) =

$115,200,000/yr.

Because the raffinate 15 fed to the isomerization unit 14 will decrease to 55.3 MT/hr, the energy cost will

decrease by almost 8%:

(55.3 MT/hr) / (59.8 MT/hr) = 92% At an energy cost of roughly $100/MT of para-xylene presently existing, this would be worth over $1,000,000 per year:

($100/MT) (20 MT/hr) (8,000 hrs/yr) (8%) = $l,280,000/yr 2. Crystallization Technology

If the para-xylene separation uses crystallization technology, the circulation numbers change, but the results are just as dramatic. Holding the circulation rate at about 60 MT/hr, the production of para-xylene will be about one-half of that produced by the mol sieve unit, or 10 MT/hr.

By use of the system 10 and method defined above, the production of para-xylene will increase to 13.6 MT/hr, half of the above numbers. Likewise, the benefits will be about half of the dollars calculated above.

It will also be understood that some feed stocks for para-xylene production are prepared from low pressure reformers, or high pressure reformers followed by extraction. In low pressure reformers, non-aromatics in the xylene volatility range are reacted to either aromatics or to light non-aromatics. In reformers operating above 100 psig, the non-aromatics do not react to near extinction. As a result, the non-aromatics must be separated from the aromatics by extraction, an expensive process.

A further strong advantage of system 10 and method is that hydrocracking of the non-aromatics to light compounds occurs so that they can easily be removed from the xylenes.

Such system 10 has been found through empirical testing to be very functional in systems where the

separator parameters can accommodate combined feed 18 and recycle 16 flow.

However, if separator 12 parameters cannot accommodate the combined flow, a bottleneck condition has been found to occur at the separator 12, producing a less than optimum para-xylene product 13 output.

In such an instance, the system 100 and method of the present invention are proposed for use.

The system 10O includes a separator 112 and an isomerization unit 114. Here, however, to avoid a bottleneck condition from forming at the separator 112, the flow path 118 for the feed 121 has been modified to lead into the isomerization unit 114. To maximize isomerization, a pretreatment reactor 120 may be incorporated in the feed flow path 118, to remove substantially all ethylbenzene from the feed 121 as described hereinabove, again optimizing para-xylene production in the isomerization unit 114. Now, the isomerization unit 114 outputs an effluent 116 having an enriched optimized concentration of mixed xylenes.

The enriched mixed xylene effluent 116 is fed to the separator 112, and an optimized amount of xylene product 113 is extracted.

The pretreatment reactor 120 has heretofore been defined as a small version of an isomerization unit including therein a large amount of an isomerization catalyst. Operating parameters, which are unit specific, may be calculated to optimize ethylbenzene conversion.

Such pretreatment reactor 120 is proposed for use with an isomerization unit 114 which is small and/or efficient.

However, where an extremely large and efficient isomerization unit 114 is provided, if desired,

pretreatment of the feed 121 may be eliminated to save cost, as shown in Figure 4. Ethylbenzene conversion and extraction is still being initially performed, now within the efficient isomerization unit 114 by setting operating parameters thereof to produce an environment therein which mimics that of the reactor 120, with converted benzene 124 being immediately eliminated from the system 10. Again, only an enriched mixed xylene effluent 116 is provided to the separator 112. It will be understood that only a portion of ethylbenzene present has previously been converted to benzene in the isomerization unit 14 shown in Figure 1, as defined hereinabove. Inasmuch as it is desirable to produce maximized initial dealkylation, the known dealkylation control variables of liquid hourly space velocity, temperature, hydrogen partial pressure and/or catalyst amount for the specific efficient isomerization unit 114 being utilized may be calculated and set to produce substantially complete ethylbenzene conversion on the first pass of the feed 121 through the isomerization unit 114.

As defined above, the methods and systems of the present invention provide a number of advantages, some of which have been described above and others of which are inherent in the invention. Also, modifications may be proposed without departing from the teachings herein. For example, if the desired product were ortho-xylene, the separator used would be in the form of a distillation tower, which is considered within the scope of the invention. Further, if a functionally equivalent catalyst is used in place of an isomerization catalyst, this would also be construed as within the scope of the invention. Accordingly, the scope of the invention is only to be

limited as necessitated by the accompanying claims.