阅读说明：本技术 在使用变化的氢气吹扫流向的变压吸附装置中回收对二甲苯的方法 (Process for para-xylene recovery in a pressure swing adsorption unit using a varied hydrogen purge flow direction ) 是由 布莱恩·本杰明 于 2018-12-05 设计创作，主要内容包括：本发明公开了一种在使用变化的氢气吹扫压力的变压吸附装置中回收对二甲苯的方法。所述变压吸附区适合于在所述区中分压的周期性循环的基础上吸附和解吸对二甲苯。第一氢气吹扫与所述进料同向进料。第二氢气吹扫与所述进料反向流动。(A method for recovering paraxylene in a pressure swing adsorption unit using varying hydrogen purge pressures is disclosed. The pressure swing adsorption zone is adapted to adsorb and desorb paraxylene based on a periodic cycle of partial pressures in the zone. The first hydrogen purge is fed co-currently with the feed. The second hydrogen purge flows counter to the feed.)

1. A process for recovering a para-xylene product from a mixture of C8 aromatic hydrocarbons, the process comprising:

feeding a C8-rich aromatic hydrocarbon mixture to a pressure swing adsorption zone adapted to adsorb and desorb paraxylene based on a periodic cycle of partial pressure in the zone using a first hydrogen purge flowing co-currently with respect to the feed and a second hydrogen purge flowing counter-currently with respect to the feed to form a paraxylene-rich stream and a paraxylene-lean stream.

2. The process of claim 1, wherein an intermediate hydrogen purge occurs between the first and second hydrogen purges and flows co-currently with respect to the feed direction.

3. The process of claim 1, wherein an intermediate hydrogen purge occurs between the first and second hydrogen purges and flows counter-currently with respect to the feed direction.

4. The method of claim 1, wherein the first hydrogen purge is introduced at a pressure of 200-400 psia.

5. The process of claim 1, wherein the second hydrogen purge is introduced at a pressure of 40-400 psia.

6. The process of claim 1, further comprising isomerizing the para-xylene-depleted stream to form an isomer having a higher para-xylene concentration than the para-xylene concentration of the para-xylene-depleted stream.

7. The process of claim 1, further comprising crystallizing the paraxylene-rich stream to form a paraxylene product stream and a second paraxylene-lean stream.

8. The process of claim 7, further comprising isomerizing the second para-xylene-depleted stream in a second isomerization zone to form a second isomer having a higher para-xylene concentration than the para-xylene concentration in the second para-xylene-depleted stream.

9. The process of claim 8, further comprising converting ethylbenzene to other compounds in the second isomerization zone.

10. The method of claim 9, further comprising

Feeding the isomerate, the second isomerate, and a mixed xylene feed stream comprising C7, C8, and C9+ aromatic hydrocarbons to a fractionation zone and separating the C8 aromatic hydrocarbons from other components in the fractionation zone to form the C8-rich aromatic hydrocarbon mixture.

11. The method of claim 1, wherein the pressure swing adsorption zone comprises a plurality of adsorption vessels each containing adsorbent, each vessel controlled in a batch sequence of operations comprising the first hydrogen purge and the second hydrogen purge, the sequence of operations in each vessel being offset in time from the sequence of operations in the other vessels such that the plurality of vessels operate together in a pseudo-continuous manner.

Technical Field

The teachings of the present invention relate generally to processes for recovering para-xylene and, more particularly, to processes for recovering para-xylene using pressure swing adsorption.

Background

Para-xylene is a chemical intermediate that is oxidized to form terephthalic acid, a precursor used to obtain polyesters.

Para-xylene is typically produced and recovered from a stream comprising "mixed xylenes". In the industry, mixed xylenes refer to the narrow boiling distillation center fraction of C8 aromatic hydrocarbons, which contain the three xylene isomers (ortho-, meta-, and para-xylene) as well as the structural isomer ethylbenzene. Mixed xylenes may also contain non-aromatic compounds with boiling points close to xylene. They contain mainly C9 paraffins and naphthenes. Mixed xylenes also typically contain low levels of toluene as well as C9 and higher aromatics, which are present due to separation imperfections in the distillation column used to produce the mixed xylene center fraction. Mixed xylenes are typically obtained from the reformate of a refinery catalytic reforming unit or another unit for producing mixed xylenes, such as a non-selective Toluene Disproportionation (TDP) unit, a Selective Toluene Disproportionation (STDP) unit, a non-selective or selective toluene alkylation unit, a toluene/C9 + aromatic Transalkylation (TA) unit, or a C9+ aromatic transalkylation unit.

Para-xylene production plants typically have three sections in the recycle loop: 1) a reaction zone comprising a xylene isomerization catalyst and an ethylbenzene conversion catalyst; 2) a fractionation section for separating by-products produced in the reaction section and/or present in fresh feed; and 3) a para-xylene recovery section for recovering para-xylene from a mixture of xylene isomers and ethylbenzene. Recycle returning a paraxylene-lean stream formed as reject filtrate in the paraxylene recovery section to the reaction section.

The isomerization catalyst returns the para-xylene depleted stream to a near equilibrium ratio of 1:2:1 (para-xylene: meta-xylene: ortho-xylene). The ethylbenzene conversion catalyst also exists because the removal of ethylbenzene by distillation is impractical because of its boiling point very close to that of the xylene isomers. Therefore, ethylbenzene must be converted to xylenes or by-products that can be easily separated by distillation to prevent its gradual accumulation in the circuit. For example, an ethylbenzene isomerization type catalyst (also known as a naphthene pool catalyst) has the ability to convert a portion of ethylbenzene to xylene isomers via the C8 naphthene intermediate. Catalysts of the ethylbenzene dealkylation type convert ethylbenzene to form benzene and ethane, mainly by reaction with hydrogen. Ethylbenzene transalkylation type catalysts convert ethylbenzene primarily by transferring the ethyl group to another ethylbenzene or xylenes.

All these catalysts produce by-products from the ethylbenzene conversion reaction and/or side reactions which have to be separated in the fractionation section. These by-products include benzene, toluene and C9+ aromatics. The fractionation zone also removes C9+ aromatics and other heavy hydrocarbons present in the feed.

Two known methods for recovering paraxylene in the paraxylene recovery section are crystallization and selective adsorption. Selective adsorption methods include the UOP Parex method described in Handbook of Petroleum Refining Processes, R A Meyers, third edition (2004) and the Axens Eluxyl method described in G Ash et al, Oiland Gas Technology,49(5), 541-. However, crystallization is generally preferred over selective adsorption as it results in energy savings for the overall process. Although xylene isomers and ethylbenzene undesirably have similar boiling points (making distillation difficult), they have significantly different melting points. Pure para-xylene solidifies at 56 ° f (13 ℃), pure meta-xylene solidifies at-54 ° f (-48 ℃), pure ortho-xylene solidifies at-13 ° f (-25 ℃), and ethylbenzene solidifies at-139 ° f (-95 ℃).

In a typical crystallization zone for the recovery of para-xylene, liquid para-xylene is crystallized from a feed stream comprising xylene isomers and ethylbenzene. Typically, the para-xylene is caused to crystallize by cooling the feed stream to a temperature below the freezing point of para-xylene, but preferably above the freezing point of other components in the feed stream. More specifically, the temperature is selected in an attempt to optimize the crystallization of paraxylene, for example by selecting a temperature at which paraxylene solidifies but which is above the eutectic temperature (the eutectic temperature being the temperature at which xylene isomers other than paraxylene begin to co-crystallize). Depending on the composition in the crystallizer, the eutectic temperatures of para-xylene-meta-xylene and para-xylene-ortho-xylene may be close, so meta-xylene or ortho-xylene may both be the first isomers to begin co-crystallization. For non-selective feedstocks, the eutectic temperature is typically about-88 ° f (-67 ℃) to about-94 ° f (-70 ℃).

The low temperatures required to crystallize para-xylene from a xylene mixture are typically achieved by cascaded vapor compression refrigeration systems using a cryogenic refrigerant. A depth refrigerant is defined as a refrigerant whose vapor or gas compression to a pressure level at which condensation by air or water cooling is generally impossible or uneconomical. Ethylene is a deep refrigerant because its critical temperature is 49 ° f (9.5 ℃) and its critical pressure is 50.76 bar. Thus, for most parts of the world, ethylene is a gas at ambient temperatures above its critical temperature for at least part of the time of the year, and it is not possible to condense ethylene by air or water cooling. When used as a refrigerant, ethylene is typically condensed by transferring heat energy to the high level refrigerant. A high level of refrigerant is defined as a refrigerant whose vapor can condense against air or water. Thus, cascaded ethylene/propylene, ethylene/propane or ethylene/ammonia refrigeration systems can be used to achieve the low temperatures required for para-xylene crystallization.

The effluent from the crystallization zone contains paraxylene solids dispersed in the mother liquor, and it is therefore usually necessary to separate these solids in one or more solid-liquid separation devices, such as centrifuges. The separation of the effluent produces a filtrate and a filter cake relatively rich in para-xylene. The filter cake obtained by separating the effluent from the crystallization stage contains para-xylene crystals and an adherent mother liquor containing ethylbenzene, other xylene isomers, unrecovered para-xylene and other components of the feed stream. To increase purity, the filter cake is typically further treated in one or more resuspension zones, in which the filter cake is equilibrated with a dilute stream comprising liquid paraxylene to provide a slurry. The resuspended effluent is separated in a solid-liquid separator to form a relatively pure para-xylene solid product and a filtrate that can be recycled or used in other portions of the process.

Another method of recovering para-xylene from mixed xylenes is known as pressure swing adsorption and is disclosed in, for example, U.S. patent nos. 6,573,418, 6,600,083, 6,627,783, 6,689,929, and 7,271,305. In a pressure swing adsorption apparatus, a vapor phase containing mixed xylenes is fed at elevated temperature and pressure to a fixed bed of adsorbent containing selective molecular sieves. Para-xylene and ethylbenzene are preferentially adsorbed to the molecular sieve. The remaining stream is enriched in meta-xylene and ortho-xylene and exits the pressure swing adsorption unit. The pressure is then reduced and the para-xylene and ethylbenzene are desorbed to form an effluent stream rich in para-xylene and ethylbenzene. The effluent may then be sent to a crystallization zone to recover the para-xylene.

The existing pressure swing adsorption device effectively separates and recovers the paraxylene. However, one of the variable costs of the existing process is to compress the hydrogen for recycle to the pressure swing adsorption unit. There remains a need to develop a cost-effective process for making paraxylene.

Disclosure of Invention

In accordance with one aspect of the present invention, a process for recovering a para-xylene product from a mixture of C8 aromatic hydrocarbons is provided. The process includes feeding a C8-rich aromatic hydrocarbon mixture to a pressure swing adsorption zone to form a para-xylene-rich stream and a para-xylene-lean stream. The pressure swing adsorption zone is adapted to adsorb and desorb paraxylene based on a periodic cycle of pressure in the zone. The periodic cycling of pressure in the zone uses a first hydrogen purge moving in a co-current direction with respect to the xylene feed in the zone and a second hydrogen purge moving in a counter-current direction with respect to the xylene feed in the zone. An intermediate hydrogen purge moving in the same or opposite direction relative to the xylene feed in the zone may also be utilized.

Other aspects of the invention will become apparent to those skilled in the art from the following description.

Drawings

FIG. 1a shows a process flow diagram for making paraxylene according to one prior art method.

FIG. 1b shows a process flow diagram for making paraxylene according to one embodiment of the present invention.

FIG. 2 shows a process flow diagram for a pressure swing adsorption zone according to one embodiment of the present invention.

Figure 3 shows a process flow diagram for a para-xylene recovery zone according to one embodiment of the present invention.

Detailed Description

By way of general introduction, a process for recovering para-xylene is disclosed. The process includes a pressure swing adsorption zone for separating para-xylene and ethylbenzene from meta-xylene and ortho-xylene. The present invention improves the recovery and purity of the paraxylene-rich stream exiting the pressure swing adsorption zone. This results in cost savings, reduces impure recycle around the plant to save energy, and reduces unwanted xylene losses between reactors in design.

Referring now to fig. 1a and 1b, a process for recovering a para-xylene product is shown generally at 10. FIG. 1a illustrates a prior art process for making paraxylene and FIG. 1b illustrates a process according to the present invention. A fresh feed 12 comprising mixed xylenes is fed to a fractionation zone comprising a xylene recovery distillation column 20 for separating a C8-rich aromatic hydrocarbon mixture from other components. In one embodiment, the fresh mixed xylene feed comprises para-xylene, ortho-xylene, meta-xylene, and the structural isomer ethylbenzene. In other embodiments, the fresh feed also comprises C7 and C9+ aromatics and non-aromatics such as C9 paraffins and naphthenes. Typically, the fresh mixed xylene feed 12 is formed as a reformate for a refinery catalytic reforming unit or another unit for producing mixed xylenes, such as a non-selective Toluene Disproportionation (TDP) unit, a Selective Toluene Disproportionation (STDP) unit, a non-selective or selective toluene alkylation unit, a toluene/C9 + aromatic Transalkylation (TA) unit, or a C9+ aromatic transalkylation unit. The fresh mixed xylene feed 12 is typically at least 90 wt% mixed xylene. In certain embodiments, the fresh mixed xylene feed 12 is at least 95 wt%, 98 wt%, or 99 wt% mixed xylene.

In certain embodiments, a second fresh feed mixed xylene containing stream 14 is also fed to column 20. The second mixed xylene-containing stream 14 is typically a heavier fraction of reformate that contains a higher concentration of C9+ compounds and is fed on column 20 at a lower level than the first fresh feed 12. The second mixed xylene stream 14 typically contains at least 10 wt% C9+ compounds. In certain embodiments, the fresh mixed xylene feed 12 contains at least 15 wt%, 20 wt%, 25 wt%, 35 wt%, or 50 wt% of C9+ compounds.

At least one other xylene-containing stream resulting from the recycle loop in process 10 is fed to column 20. In the illustrated embodiment, two such feeds 16, 18 are shown. Those skilled in the art will recognize that other configurations of the recycle feed are possible. The xylene-containing feed stream 16 and the xylene-containing feed stream 18 typically contain a lower proportion of ethylbenzene than the mixed xylene feed streams 12, 14. Feed streams 16 and 18 also contain benzene. In certain embodiments, the feed stream contains at least 1 wt% or 2 wt% benzene. In other embodiments, feed stream 16 contains at least 5 wt.% benzene.

The xylene recovery column 20 is configured to separate the feed stream 12, 14, 16, 18 into one or more streams comprising a C8-rich aromatic hydrocarbon mixture, a C7-compound containing stream, and a C9+ compound containing stream. In the embodiment shown in fig. 1a or fig. 1b, the first side-cut stream 22 comprises a liquid phase C8-rich aromatic hydrocarbon mixture, while the second side-cut stream 24 comprises a vapor phase C8-rich aromatic hydrocarbon mixture. A first side draw stream 22 is withdrawn on the column at a location above the second side draw stream 24. Feed stream 18 is introduced to the column above vapor-phase sidedraw stream 24 such that gaseous components in feed stream 18 do not exit through vapor-phase sidedraw stream 24. The liquid phase side-cut phase is pressurized by pump 23. The vapor phase sidedraw stream is condensed by condenser 26 and the resulting condensate is pressurized by pump 27. The pressurized condensate of the vapor phase sidedraw stream and the pressurized liquid phase sidedraw stream are combined to form a combined C8-rich aromatic hydrocarbon mixture stream 28.

An overhead product stream 30 is withdrawn from the top of column 20 and contains C7-compounds including benzene, toluene, and ethane. The overhead product stream 30 is partially condensed by condenser 32 and the condenser effluent is separated into liquid and gaseous components in flash drum 34. The liquid phase is partially returned to column 30 as reflux stream 36b and partially removed from the process via stream 36 a. The gaseous components are removed from the process as a light co-product stream 38.

A bottoms stream 40 is removed from the bottom of column 20 and contains C9+ compounds, including trimethylbenzene and methylethylbenzene. A portion of the bottoms is recovered as a bottoms co-product stream 42 while another portion 44 of the bottoms stream 40 is reboiled by a reboiling furnace 46 and returned to the column 20. Reboiling furnace 46 provides the high temperature of column 20 operating at a temperature gradient between, for example, 500 ° f (260 ℃) and 50 ° f (10 ℃) and a pressure of 15-80 psia.

In the prior art process illustrated in fig. 1a, the combined C8-rich aromatic hydrocarbon mixture stream 28 is fed to a para-xylene recovery zone 72. However, in the present invention shown in fig. 1b, at least a portion of the C8-rich aromatic hydrocarbon mixture 28 recovered from the fractionation zone is preheated by furnace 50 and one or more heat exchangers (not shown) and routed to pressure swing adsorption zone 52 via line 51. In the pressure swing adsorption zone 52, the C8-rich aromatic hydrocarbon mixture is fed at elevated temperature and pressure to a fixed adsorbent bed containing a selective molecular sieve. Para-xylene and ethylbenzene preferentially adsorb to the molecular sieve. The remaining stream is enriched in meta-xylene and ortho-xylene and is discharged from the pressure swing adsorption unit as a para-xylene depleted stream 54. The partial pressure is then reduced and the para-xylene and ethylbenzene are desorbed to form a para-xylene-rich and ethylbenzene-rich effluent stream 56. The construction and operation of the pressure swing adsorption zone is described more fully below and with reference to FIG. 2. Stream 56 is then sent to condenser 57 and flash drum 62 where a C8-rich stream 70 is formed along with a hydrogen-rich stream 64. Stream 64 is then sent to compressor 66 for compression and return to the pressure swing adsorption zone 52.

A pressurized hydrogen purge gas stream is fed from compressor 66 to pressure swing adsorption zone 52 via lines 60a and 60 b. In one embodiment, a first hydrogen purge gas is fed via line 60a co-current to the feed direction, stream 54 being its destination. In another embodiment, the first hydrogen sweep is fed co-current to the feed direction via line 60a with stream 51 as its destination. The first hydrogen sweep through 60a feed co-current with the feed takes advantage of the fact that at the end of the feed step, the material near the point where the feed exits the bed is enriched in meta-xylene and ortho-xylene. By performing the first co-current hydrogen purge, such remaining meta-xylene and ortho-xylene can be removed while minimizing the loss of para-xylene and ethylbenzene, thereby increasing the purity and recovery of para-xylene and ethylbenzene and improving the overall performance of the pressure swing adsorption zone.

In one embodiment, the second hydrogen sweep is fed counter to the feed direction through line 60b with stream 56 as its destination. In another embodiment, an intermediate hydrogen purge is introduced through line 60a in the same direction as the feed between the first and second hydrogen purges. In another embodiment, an intermediate hydrogen purge is introduced through line 60b in a direction opposite to the feed between the first and second hydrogen purges. The para-xylene-rich and ethylbenzene-rich effluent 70 exiting the flash drum 62 is fed to a para-xylene recovery zone 72. In an embodiment, the C8-rich aromatic hydrocarbon mixture feed 74 to the para-xylene recovery zone 72 comprises a second portion of the combined C8-rich aromatic hydrocarbon mixture stream 28 exiting the column 20 and bypassing the pressure swing adsorption unit 52. In an embodiment, the second feed 74 comprises at least 10 wt% of the combined paraxylene-rich and ethylbenzene-rich stream 28. In other embodiments, the second feed 74 comprises at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt%, or at least 90 wt% of the combined C8-rich aromatic hydrocarbon mixture stream 28.

The para-xylene recovery zone 72 operates to produce a para-xylene product 76 and recycle a para-xylene depleted stream 78 for further processing. In one embodiment, the para-xylene recovery zone 72 is configured to recover para-xylene product by a selective adsorption process. In another embodiment, the para-xylene recovery zone 72 is configured to recover para-xylene by a crystallization process. One particular crystallization method is described below with reference to fig. 3.

The para-xylene depleted stream 54 exiting the pressure swing adsorption unit 52 is fed to an isomerization reactor 80. The isomerization reactor 80 is a packed bed reactor containing a bed of isomerization catalyst for converting meta-xylene and ortho-xylene to para-xylene at an approximate equilibrium ratio of 1:2:1 (para-xylene: meta-xylene: ortho-xylene). In one embodiment, hydrogen 82 is added to the paraxylene-lean stream 54 upstream of the isomerization reactor 80. In another embodiment, the para-xylene lean 54 contains sufficient hydrogen after exiting the pressure swing adsorption unit 52 such that no make-up hydrogen is added to the feed 54. In the illustrated embodiment, the isomerate stream 91 is fed to a condenser 120 and flash drum 122 where a C8-rich stream 126 is formed along with a hydrogen-rich stream 124. Stream 124 is then sent to compressor 66 for compression and return to the pressure swing adsorption zone 52. Bottoms stream 126 is sent to pump 128. All or a portion of the pump effluent can then be sent directly to furnace 50 via stream 130, or all or a portion of the pump effluent can be sent to tank 94 via stream 132.

The second para-xylene-lean stream 78 exiting the para-xylene recovery zone 72 is mixed with fresh hydrogen 84 and preheated with a furnace 86 and/or one or more heat exchangers (not shown). The preheated mixture 88 is fed to a further isomerization reactor 90. The isomerization reactor 90 contains an isomerization catalyst for converting meta-xylene and ortho-xylene to para-xylene in an approximate equilibrium ratio of 1:2:1 (para-xylene: meta-xylene: ortho-xylene). In certain embodiments, isomerization reactor 90 also contains an ethylbenzene conversion catalyst, such as a dealkylation catalyst, for converting ethylbenzene to benzene and ethane. Suitable isomerization catalysts and ethylbenzene catalysts are disclosed, for example, in U.S. patent nos. Re 31,782, 4,899,011, and 6,518,472.

In the embodiment shown, the isomerate stream 92 from the additional isomerization reactor 90 is fed to a high temperature separator 94 where the stream is flashed. The liquid-rich phase bottoms stream from the high temperature separator 94 is a xylene-containing feed stream 18 to the column 20. The vapor-rich phase stream 95 leaving the high temperature separator 94 is sent to a low temperature separator 96 where it is flashed. The liquid-rich phase stream leaving the cryogenic separator is another xylene-containing feed stream 16 to the column. The vapor phase stream 98 exiting the cryogenic separator 96 comprises hydrogen, ethane, and other light components, and can be recycled and used as a source of streams 82 and/or 84 or as fuel.

FIG. 2 illustrates one embodiment of a pressure swing adsorption zone 52 according to the present invention. The pressure swing adsorption zone 52 comprises one or more vessels 100a, 100b, 100c, 100d containing a para-xylene selective adsorbent. In the embodiment shown there are 4 containers, but a person skilled in the art will appreciate that other configurations are possible, for example configurations with different numbers of containers, e.g. 3 containers, 5 containers, 6 containers, 7 containers, 8 containers, etc. In one embodiment, the para-xylene selective adsorbent is a non-acidic mesoporous molecular sieve. In one embodiment, the molecular sieve is of the MFI structure type and the process is operated in the vapor phase at elevated temperature and pressure, wherein the temperature is substantially isothermal. Adsorbents useful in the present invention are based on molecular sieves that selectively adsorb para-xylene in the channels and pores of the molecular sieve without efficiently adsorbing meta-xylene and ortho-xylene C₈Isomers (i.e., greater rejection of m-xylene and o-xylene altogether or much slower adsorption rates than p-xylene).

Molecular sieves are ordered porous crystalline materials, usually made of silica, alumina and Phosphorus Oxide (PO)₄) Tetrahedra are formed, containing a crystal structure with cavities interconnected by channels. The crystal junctionThe cavities and channels in the structure are uniform in size and can allow selective separation of hydrocarbons according to molecular size. In general, the term "molecular sieve" includes a broad class of natural and synthetic crystalline porous materials, which are typically based on silica tetrahedra in combination with other tetrahedral oxide materials, such as aluminum, boron, titanium, iron, gallium, and the like. In these structures, the network of silicon and elements such as aluminum is cross-linked by the sharing of oxygen atoms. Substitution of silicon with elements such as aluminum or boron in the molecular sieve structure creates a negative framework charge that must be balanced with positive ions such as alkali, alkaline earth, ammonium or hydrogen ions. Molecular sieve structures can also be formed based on combinations of phosphates with other tetrahedral substitution elements such as aluminum.

Adsorbents useful in the present invention should not be taken to account for C₈The aromatic feed stream has catalytic isomerization or conversion activity. Thus, suitable molecular sieves should be non-acidic. If an element such as aluminum or gallium is substituted in the molecular sieve framework, the molecular sieve should be exchanged with a non-acidic counterion, such as sodium, to produce a non-acidic molecular sieve adsorbent.

Examples of molecular sieves suitable as adsorbents useful in the present invention include those having a pore size of from 5 to 6 angstroms (10 a) when measured on the transverse axis of the pore^-8Rice), typically in the range of 5.1 to 5.7 angstroms, preferably 5.3 to 5.6 angstroms. This range is commonly referred to as "mesopores" and typically contains a 10-ring tetrahedral structure. Typical examples of medium pore molecular sieves include molecular sieves having MFI and MEL framework structures, as classified in Meier and Olson, "Atlas of Zeolite Structure Types" (1987), the International Zeolite Association, which is incorporated herein by reference in its entirety. Small pore molecular sieves containing 8-ring structures, such as zeolite a, do not have sufficiently large pore openings to effectively adsorb para-xylene within the molecular sieve. Most large pore molecular sieves containing 12 ring structures, such as mordenite, Beta, LTL or Y zeolite, do not selectively adsorb para-xylene over ortho-xylene and meta-xylene. However, several 12-ring structures with smaller effective pore sizes may be useful in the present invention due to, for example, wrinkling, such as MTW (e.g., MTW)Such as ZSM-12) and ATO (e.g., ALPO-31) structure types.

Specific examples of molecular sieves include ZSM-5(MFI structure type) and ZSM-11(MEL structure type) and related isotypic structures. Since suitable adsorbents should not be catalytically reactive with the components in the feed stream, a preferred adsorbent useful in the present invention is silicalite (MFI structure type), a molecular sieve that is essentially all silica, containing very small amounts of aluminum or other substitutional elements. Generally, suitable silicalites have a silica/alumina ratio above 200, and may be in the range above 1000, depending on the contamination level of aluminum used in the preparation of the molecular sieve. Other MFI and MEL molecular sieves may be used in ranges where they are not catalytically active. Other potentially useful adsorbents include the structural types MTU, FER, EUO, MFS, TON, AEL, ATO, NES and other types with similar pore sizes.

Molecular sieves that are not catalytically reactive typically exhibit less than 10%, and in some embodiments less than 5%, and in other embodiments less than 1% conversion of para-xylene to meta-xylene and ortho-xylene at the operating temperatures used in the process of the present invention.

The C8-rich aromatic hydrocarbon mixture enters the pressure swing adsorption zone 52 through xylene piping 102 and is introduced into vessels 100a, 100b, 100C, 100d through respective feed control valves 102a, 102b, 102C, 102 d. The first hydrogen purge from 60a is passed through hydrogen manifold 106 and high pressure hydrogen feed control valves 106a, 106b, 106c, 106d into vessels 100a, 100b, 100c, 100d, respectively. The second hydrogen purge gas from 60a is passed through hydrogen manifold 112 and hydrogen feed control valves 112a, 112b, 112c, 112d into vessels 100a, 100b, 100c, 100d, respectively. A third hydrogen purge gas from 60b is passed through hydrogen manifold 104 and hydrogen feed control valves 104a, 104b, 104c, 104d into vessels 100a, 100b, 100c, 100d, respectively.

The pressure swing adsorption zone 52 also includes a paraxylene and ethylbenzene collection piping 108 and a set of outlet control valves 108a, 108b, 108c, 108d for removing the paraxylene-rich and ethylbenzene-rich streams 56 from each vessel 100a, 100b, 100c, 100d, respectively. The pressure swing adsorption zone 52 also comprises meta-xylene and ortho-xylene collection headers 110 and a set of outlet control valves 110a, 110b, 110c, 110d for removing the first para-xylene-depleted stream 54 from the vessels 100a, 100b, 100c, 100d, respectively. The pressure swing adsorption zone 52 also comprises a feed recycle line 114 that can direct a liquid stream back to the xylene line 102 through a set of control valves 114a, 114b, 114c, 114 d.

The vessels 100a, 100b, 100c, 100d in the pressure swing adsorption zone 52 are operated in a sequence of operations that is offset in time from the sequence of operations in the other vessels such that the vessels operate together in a pseudo-continuous manner.

The sequence of operations will now be described with reference to the first container 100 a. All valves are automatically controlled by a control system (not shown). The valve remains closed unless described as open for a particular operation below.

In the first operation, designated "fed", a C8-rich aromatic hydrocarbon stream is introduced to vessel 100a at high pressure through FEED header 102 and FEED control valve 102 a. Para-xylene and ethylbenzene molecules are adsorbed to the adsorbent, while meta-xylene and ortho-xylene molecules are blown through the bed and exit the process through outlet control valve 110a and meta-xylene and ortho-xylene collection header 110.

In a second operation, designated "HP 1" (hydrogen purge 1), high pressure purge gas from 60a (fig. 1b) is fed through the connection 106 and control valve 106a to purge the bed. This hydrogen displaces some of the meta-xylene and ortho-xylene left in the pore space of the bed after completion of the fed operation and continues to blow them out through control valve 110a and manifold 110. This allows extraction of para-xylene and ethylbenzene in a later operation without contamination by meta-xylene and ortho-xylene.

In a third operation, designated "HP 2" (hydrogen purge 2), high pressure purge gas from 60a (fig. 1b) is fed through headers 112 and 112a to purge the bed. This hydrogen displaces the remaining meta-xylene and ortho-xylene left in the pore spaces of the bed. In the act of removing the last remaining meta-xylene and ortho-xylene, some para-xylene and ethylbenzene also began to be displaced from the bed. Thus, the HP2 step exits the bed through valve 114a into the feed recirculation manifold 114 and back into the xylene feed manifold 102. This allows the remaining meta-xylene and ortho-xylene to be removed from the bed without loss of para-xylene and ethylbenzene along with them to the meta-xylene and ortho-xylene collection header 110. This improves the recovery and purity of the final product.

In a fourth operation, designated "HP 3" (hydrogen purge 3), a hydrogen stream from 60b (fig. 1b) is fed through hydrogen manifold 104 and valve 104a to vessel 100a at desorption pressure and the bed is purged. This bed purge reduces the partial pressure of para-xylene and ethylbenzene. This results in desorption of paraxylene and ethylbenzene from the adsorbent. The paraxylene and ethylbenzene rich stream then exits the bed through valve 108a and is sent to a paraxylene and ethylbenzene collection conduit 108.

All 4 vessels 100a, 100b, 100c, 100d undergo this same cycle of 4 operations, but at any given time each vessel is in a different phase of the cycle. The system is designed and operated so that there is always one vessel in FEED operation so that the FEED to the pressure swing adsorption zone 52 is generally constant. Table 1 illustrates one embodiment of a sequence of 4 operations of the pressure swing adsorption zone 52 in which multiple vessels are operated together in a pseudo-continuous manner. The table illustrates 4 time periods and shows what operations are performed in each container at each time period. A typical time period is 5 seconds to about 120 seconds. Those skilled in the art will recognize that the sequences in table 1 are exemplary and that other sequences are possible in order to carry out the present invention.

Table 1: pressure swing adsorption sequence of operations

Time period	Container a	Container b	Container c	Container d
					1	Feeding of the feedstock	HP3	HP2	HP1
2	HP1	Feeding of the feedstock	HP3	HP2
					3	HP2	HP1	Feeding of the feedstock	HP3
4	HP3	HP2	HP1	Feeding of the feedstock

Figure 3 illustrates one embodiment of a para-xylene recovery zone 72 according to the present invention. The paraxylene-and orthoxylene-rich effluent 70 enters a crystallization zone comprising a first crystallization zone comprising the first crystallizer 202 and a second crystallization zone comprising the second crystallizer 216. In one embodiment, effluent 70 comprises at least 25 wt% para-xylene. In other embodiments, effluent 70 comprises less than 75 wt%, less than 85 wt%, or less than 95 wt% para-xylene. The operating temperature of the first crystallizer 202 is controlled to solidify the para-xylene in the effluent 70. In one embodiment, the first crystalline region is operated at a temperature above-40 ° f (-40 ℃). In another embodiment, the first crystalline region is operated at a temperature below-30 ° f (-34.4 ℃). The first crystalline region is typically operated at a temperature between 40 ° f (4.4 ℃) and-40 ° f (-40 ℃). A first crystallizer effluent 204 is discharged from the first crystallizer 202 and sent to a first solid-liquid separator 206. The first solid-liquid separator 206 separates a para-xylene depleted filtrate stream 208 from a first para-xylene enriched filter cake stream 210. A portion 212 of the para-xylene-depleted filtrate stream may be recycled to the first crystallizer 202, while another portion 214 of the para-xylene-depleted filtrate stream is sent to a second crystallizer 216. The second crystallizer 216 is temperature controlled to further solidify any remaining para-xylene. The second crystallization zone operates at a lower temperature than the first crystallization zone. In one embodiment, the second crystallization zone is operated at a temperature below-70 ° f (-56.7 ℃). In another embodiment, the second crystallization zone is operated at a temperature below-90 ° f (-67.8 ℃). In another embodiment, the second crystallization zone is operated at a temperature below-110 ° f (-78.9 ℃). The second crystalline zone is typically operated at a temperature between-30 ° f (-34.4 ℃) and-130 ° f (-90 ℃). The second crystallizer effluent 218 is withdrawn from the second crystallizer 216 and directed to a second solid-liquid separator 220. In one embodiment, the effluent 218 exiting the second crystallizer is below-95 ° f (-70.6 ℃). The second solid-liquid separator 220 separates a second para-xylene-rich filter cake 222 from a second para-xylene-depleted filtrate stream 224. A portion 228 of the filtrate stream 224 may be recycled to the crystallizer 216 while another portion 78 of the para-xylene depleted filtrate stream 224 is recycled for further processing as described above and with reference to fig. 1 b.

The first and second solid-liquid separators 202, 216 may be any solid-liquid separation device known in the art, such as a centrifuge, a rotary pressure filter, a rotary vacuum filter, or a filter column. In a particular embodiment, the first solid-liquid separator 206 comprises a pusher centrifuge and the second solid-liquid separator 220 comprises a decanter centrifuge. In an embodiment, the second solid-liquid separator 220 removes additional para-xylene depleted filtrate 226 prior to discharging the second para-xylene depleted filtrate 224. The additional filtrate 226 has a higher para-xylene concentration than the second para-xylene depleted filtrate 224 and is recycled to the second crystallizer 216.

The first p-xylene-rich filter cake 210 and the second p-xylene-rich filter cake 222 enter one or more resuspension zones to remove any remaining impurities. The embodiment of fig. 3 shows two heavy suspension zones, each with a heavy suspension tank 224, 240. The para-xylene-rich filter cake 210 from the first solid-liquid separator is fed to either or both of the first resuspension tank 224 and the second resuspension tank 240 via streams 226 and 242, respectively. The second paraxylene-rich cake 222 is fed to a first resuspension tank 224. The para-xylene-rich filter cake is resuspended in a first resuspension tank 224 with a resuspension fluid to remove impurities from the para-xylene crystals, and the effluent 231 from the first resuspension tank 224 is sent to a third solid-liquid separator 232. The third solid-liquid separator 232 separates the effluent 231 into a third para-xylene-rich filter cake 238 and a third para-xylene-lean filtrate stream 234. A portion 228 of the third para-xylene-depleted filtrate stream 234 is recycled to the first resuspension tank 224 as a resuspension fluid, and another portion 236 can be recycled to the first crystallizer 202 for further recovery of para-xylene.

The third para-xylene-rich filter cake 238 is fed to a second resuspension tank 240 for further resuspension with one or more resuspension fluids for removal of impurities from the para-xylene crystals. The effluent 248 from the second suspension tank 240 is fed to a fourth solid-liquid separator 250. The fourth solid-liquid separator 250 separates the effluent 248 into a fourth paraxylene-rich filter cake 252 and a fourth paraxylene-lean filtrate stream 254. A portion 244 of the fourth para-xylene-depleted filtrate stream 254 is recycled to the second resuspension tank 244 as a resuspension fluid, and another portion 230 of the fourth para-xylene-depleted filtrate stream 254 can be recycled to the first resuspension tank 224 for use as a resuspension fluid.

The third and fourth solid- liquid separators 232, 250 may be any solid-liquid separation device known in the art, such as a centrifuge, a rotary pressure filter, a rotary vacuum filter, or a filter column. The fourth solid-liquid separator 250 may also be a wash column. Suitable filtration columns are disclosed, for example, in U.S. patent nos. 7,812,206, 8,211,319, 8,530,716 and 8,962,906. Suitable scrubbers are disclosed, for example, in U.S. patent nos. 4,734,102 and 4,735,781. In a particular embodiment, the third solid-liquid separator 232 comprises a pusher centrifuge and the fourth solid-liquid separator 250 comprises a pusher centrifuge. In an embodiment, the fourth solid-liquid separator 250 removes additional para-xylene-lean filtrate 246 before discharging the fourth para-xylene-lean filtrate 254. The additional filtrate 246 has a higher para-xylene concentration than the fourth para-xylene depleted filtrate 254 and is recycled to the second resuspension tank 240.

The fourth paraxylene-rich filter cake 252 is fed to a melt tank 256. The fourth para-xylene filter cake is completely melted and a para-xylene product stream 76 is recovered. A portion 258 of the molten para-xylene may be recycled to the fourth solid-liquid separator 250 to wash impurities from the filter cake. In one embodiment, the para-xylene product 76 contains at least 99 wt.% para-xylene. In other embodiments, the para-xylene product contains at least 99.5 wt%, 99.6 wt%, 99.7 wt%, or 99.8 wt% para-xylene.

The use of a pressure swing adsorption zone with an additional isomerization zone allows less total mass to be fed to the para-xylene recovery zone because a significant portion of the meta-xylene and ortho-xylene in the system is recycled via stream 54 (fig. 1 b). In one embodiment, the ratio of the total mass of the paraxylene-rich stream entering the paraxylene recovery zone to the total mass of the paraxylene-rich product stream is less than 6. In other embodiments, the ratio of the total mass of the paraxylene-rich stream entering the paraxylene recovery zone to the total mass of the paraxylene-rich product stream is less than 5, less than 4, less than 3, or less than 2. In other embodiments, the ratio of the total mass of the recycle stream 78 (fig. 1b) to the total mass of the paraxylene-rich product stream 76 is less than 5, less than 3, or less than 2. The feed to the para-xylene recovery zone also contains a higher concentration of para-xylene compared to a system without a pressure swing adsorption zone. This is because the pressure swing adsorption zone allows the para-xylene concentration to be higher than the equilibrium concentration resulting from the isomerization reaction.

According to another aspect of the present invention, a method for retrofitting a system for recovering paraxylene is provided. According to the retrofit method, a pressure swing adsorption zone 52 (FIG. 1b) is added to an existing system (FIG. 1a) that does not have a pressure swing adsorption zone. At least a first portion 51 of the combined C8-rich aromatic hydrocarbon mixture stream 28 is directed to a pressure swing adsorption zone 52 to form a paraxylene-rich intermediate stream 56 (which is flashed in drum 62 to form stream 70) prior to being fed to a paraxylene recovery zone 72. The retrofit process can also include adding a secondary isomerization zone 80 to an existing system that previously did not have a secondary isomerization zone 80. The conversion process may also include the addition of a bypass stream 74 to direct a second portion of the combined C8-rich aromatic hydrocarbon mixture stream 28 directly to the paraxylene recovery zone 72, bypassing the pressure swing adsorption zone 52. The amount of the C8-rich aromatic hydrocarbon mixture stream split by the bypass stream 74 is dependent on the throughput of the pressure swing adsorption zone 52 and the existing equipment. In one embodiment, the existing equipment does not have to be sized as a result of the retrofit, which allows for improved recovery of para-xylene without significant capital expenditure. By enriching the paraxylene additional isomerization capability in the combined stream 28 prior to routing to the paraxylene recovery zone, the retrofit process allows for increased recovery of paraxylene product as compared to prior systems. In one embodiment, the amount of para-xylene product recovered by the retrofit system is increased without increasing the throughput of the original isomerization zone 90. In another embodiment, the amount of para-xylene product recovered is increased without increasing the amount of hydrogen fed to the system. In another embodiment, the amount of para-xylene product recovered is increased without increasing the amount of refrigeration duty to the crystallization zone. In another embodiment, the amount of para-xylene product recovered is increased without increasing the amount of furnace load 86 of the original isomerization zone. In another embodiment, the amount of para-xylene product recovered is increased without increasing the amount of furnace duty 46 of the fractionation zone.

The foregoing detailed description and drawings are provided for purposes of explanation and illustration, and are not intended to limit the scope of the claims. Many variations in the presently preferred embodiments described herein will be apparent to those of ordinary skill in the art and still fall within the scope of the claims and their equivalents.

It should be understood that the elements and features recited in the claims may be combined in different ways to produce new claims which also fall within the scope of the invention. Thus, although the following dependent claims are only dependent on a single independent or dependent claim, it should be understood that these dependent claims may alternatively be dependent on any preceding claim, independent or dependent, and that such new combinations should be understood to form part of this specification.