Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C7/00—Purification; Separation; Use of additives
  • C07C7/005—Processes comprising at least two steps in series
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D15/00—Separating processes involving the treatment of liquids with solid sorbents; Apparatus therefor
  • B01D15/08—Selective adsorption, e.g. chromatography
  • B01D15/10—Selective adsorption, e.g. chromatography characterised by constructional or operational features
  • B01D15/18—Selective adsorption, e.g. chromatography characterised by constructional or operational features relating to flow patterns
  • B01D15/1814—Recycling of the fraction to be distributed
  • B01D15/1821—Simulated moving beds
  • B01D15/1828—Simulated moving beds characterised by process features
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D15/00—Separating processes involving the treatment of liquids with solid sorbents; Apparatus therefor
  • B01D15/08—Selective adsorption, e.g. chromatography
  • B01D15/10—Selective adsorption, e.g. chromatography characterised by constructional or operational features
  • B01D15/18—Selective adsorption, e.g. chromatography characterised by constructional or operational features relating to flow patterns
  • B01D15/1814—Recycling of the fraction to be distributed
  • B01D15/1821—Simulated moving beds
  • B01D15/185—Simulated moving beds characterised by the components to be separated
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
  • B01D3/14—Fractional distillation or use of a fractionation or rectification column
  • B01D3/143—Fractional distillation or use of a fractionation or rectification column by two or more of a fractionation, separation or rectification step
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
  • B01D3/14—Fractional distillation or use of a fractionation or rectification column
  • B01D3/32—Other features of fractionating columns ; Constructional details of fractionating columns not provided for in groups B01D3/16 - B01D3/30
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C15/00—Cyclic hydrocarbons containing only six-membered aromatic rings as cyclic parts
  • C07C15/02—Monocyclic hydrocarbons
  • C07C15/067—C8H10 hydrocarbons
  • C07C15/08—Xylenes
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C5/00—Preparation of hydrocarbons from hydrocarbons containing the same number of carbon atoms
  • C07C5/22—Preparation of hydrocarbons from hydrocarbons containing the same number of carbon atoms by isomerisation
  • C07C5/27—Rearrangement of carbon atoms in the hydrocarbon skeleton
  • C07C5/2767—Changing the number of side-chains
  • C07C5/277—Catalytic processes
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C7/00—Purification; Separation; Use of additives
  • C07C7/04—Purification; Separation; Use of additives by distillation
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
  • C07C7/00—Purification; Separation; Use of additives
  • C07C7/12—Purification; Separation; Use of additives by adsorption, i.e. purification or separation of hydrocarbons with the aid of solids, e.g. with ion-exchangers
  • C07C7/13—Purification; Separation; Use of additives by adsorption, i.e. purification or separation of hydrocarbons with the aid of solids, e.g. with ion-exchangers by molecular-sieve technique
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/50—Improvements relating to the production of bulk chemicals
  • Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts

Definitions

• TECHNICAL FIELD Paraxylene is mainly used for the production of terephthalic acid and polyethylene terephthalate resins, to supply synthetic textiles, bottles, and more generally plastics.
• the present invention relates to a process for obtaining high purity paraxylene using a specific sequence of steps making it possible to simplify the fractionation step.
• the overhead stream from this column which contains the C8-aromatic isomers, is then sent to the paraxylene separation process which is generally a separation step by adsorption in a simulated moving bed.
• the extract obtained at the end of the separation stage by adsorption in a simulated moving bed, which contains the paraxylene is then distilled by means of an extract column and then a toluene column, to obtain high paraxylene. purity.
• Patent FR2862638 describes a process for producing paraxylene from a hydrocarbon feed, using two stages of separation in simulated moving bed, and two stages of isomerization. The disadvantage of this process is that it requires two stages of separation in a simulated moving bed which results in a significant increase in the cost of production.
• Another advantage of the process according to the invention is that it can be used to advantage in revamping configurations of a xylene loop. Indeed, in this case the same raffinate column can be reused provided that it is supplied with the 2 raffinates introduced separately as described in step B according to the invention.
• the abbreviation EB denotes ethylbenzene.
• Metaxylene is designated by the abbreviation MX.
• xylenes is understood to mean a mixture of at least two isomers chosen from orthoxylene, metaxylene, and paraxylene.
• the abbreviation SMB is used to designate a simulated moving bed.
• C9 + hydrocarbons is understood to mean hydrocarbons containing at least 9 carbon atoms.
• C8 + hydrocarbons are understood to mean hydrocarbons containing at least 8 carbon atoms.
• C8Aromatics also denoted C8A, denote aromatic hydrocarbons containing 8 carbon atoms chosen from EB, PX, OX, MX.
• raffinate is intended to mean a mixture of C8A depleted in PX and which may contain desorbent, that is to say having a mass content of PX of less than 2.0%, preferably less than 1.5% and preferably 1.0%. .
• the term “free” means a mass content of a given compound relative to the total mass of the fraction considered, for example in EB, of less than 0.5% by weight, preferably of less than 0.1% and preferably less than 0.01%.
• residual quantity of a given compound is understood to mean an amount whose mass content relative to the total mass of the fraction considered is less than 5.0% by weight, preferably between 5.0 and 1.0, preferably between 4.0 and 1.0, and preferably between 3.0 and 1.0% by weight.
• distillation column means 2 sections or 3 sections, a distillation column allowing the production of 2 or 3 fractions respectively.
• FIG. 1a represents the general diagram of a Xylenes loop implementing a step of separation by adsorption, a step of fractionation, a step C of isomerization in the vapor phase.
• FIG. 1b represents the stage B of distillation of the raffinate resulting from stage A according to the prior art.
• FIG. 2 represents an implementation of the method according to the invention.
• the present invention relates to a process for obtaining paraxylene from a feed containing xylenes, ethylbenzene and C9 + hydrocarbons, said process comprising
• a single step A of separation in a simulated moving bed of said charge said step being carried out with a zeolite as adsorbent and a desorbent, at a temperature between 20 and 250 ° C., under a pressure comprised between the bubble pressure xylenes at operating temperature and 2.0 MPa, and with a volume ratio of the desorbent to the charge in the separation unit in simulated moving bed is between 0.4 and 2.5, and allowing obtaining at least three fractions,
• stage B of fractionation by distillation in a distillation column of the fractions A21 and A22 resulting from stage A in which the said fractions are introduced separately at separate injection points and makes it possible to obtain a fraction B2 containing ethylbenzene, orthoxylene and metaxylene, and a fraction B42 free of aromatic compounds containing 8 carbon atoms and containing desorbent.
• An advantage of the process according to the invention is to carry out a pre-fractionation during the step of separation into SMB which advantageously makes it possible to facilitate the step of fractionation of the raffinate without increasing the complexity of the process and without modifying the yield of Paraxylene.
• Another advantage of the process according to the invention is that it can be used to advantage in revamping configurations of a xylene loop. Indeed, in this case the same raffinate column can be reused provided that it is supplied with the 2 raffinates introduced separately as described in step B according to the invention.
• the method comprises a single step A of separation in a simulated moving bed implemented with a zeolite as adsorbent and a desorbent and making it possible to obtain at least three fractions, a fraction Al containing desorbent and paraxylene two fractions A21 and A22, also called "raffinate", depleted in paraxylene preferably comprising, in variable proportions, a mixture of EB, MX, OX, and the desorbent.
• the proportions of EB, MX, OX and desorbent in the fractions A21 and A22 are different.
• the fraction A21 has a mass content of desorbent lower than that of the fraction A22.
• the separation step of said charge is implemented in a unit operating in a simulated moving bed in at least one separation column containing a plurality of interconnected beds and circulating desorbent in a closed loop from which the three fractions come:
• the first is an Al extract comprising paraxylene and desorbent, so that after fractionation to remove the desorbent the PX reaches a commercial purity of minimum 99.0% and preferably 99.9% by weight.
• the extract Al has at least 30% by weight of the total mass of the extract.
• the depleted fraction A21 PX comprises, preferably consists, of a mixture of EB, MX, OX, and desorbent.
• the adsorbent used in the separation unit in a simulated moving bed is a zeolite X exchanged with barium or a zeolite Y exchanged with potassium or a zeolite Y exchanged with barium and potassium.
• the desorbent used in the separation unit in simulated moving bed is chosen from paradiethylbenzene, toluene, paradifluorobenzene or diethylbenzenes alone or as a mixture.
• the volume ratio of the desorbent to the load in the simulated moving bed separation unit is between 0.4 and 2.5, preferably between 0.5 and 2.0 and preferably between 0, 5 and 1.5.
• step A of separation in simulated moving bed is carried out at a temperature between 90 and 210 ° C, and more preferably between 160 and 200 ° C, and under a pressure between 1.0 and 2.2 MPa and preferably between 1.2 and 2.0 MPa.
• the adsorber contains a plurality of beds, interconnected and distributed over several zones delimited by the injections of the filler and the desorbent, as well as withdrawals of the extract, and of the raffinates.
• the total number of beds of the separation unit is between 10 and 30 beds, and preferably, between 15 and 18 beds distributed over one or more adsorbers, the number of beds being adjusted so that each bed has a height of between 0.70 and 1.40 m.
• the distribution of the quantity of solid adsorbent in each zone of the separation unit (SMB) is as follows:
• Each zone defines the injection and withdrawal points, fillers, desorbent, extract and raffinates as defined below:
• o Zone 1 is between the injection of the desorbent B4 and the withdrawal of the extract Al.
• o Zone 2 is between the withdrawal of the extract Al and the injection of the load o Zone 3A is between the injection of the load and the withdrawal of the raffmat A21 o Zone 3B, between the withdrawal of the raffinate A21 and withdrawal of raffmat A22 o Zone 4 is between the withdrawal of raffrnate A22 and the injection of desorbent B4
• step A of adsorption of the C8A cut free of C9 + are characterized by:
• the recovery rate of the PX in the extract Al defined by the ratio of PX in the extract Al / PX in the feed is at least equal to 97.0% and that after recovery of the desorbent, in the column of extract BC-l the minimum purity of the PX is 99.0% and preferably greater than 99.9%.
• R (C8A) the quantities of C8A in the raffinate A21 / the quantity of C8A in the total raffinate A2 is at least 60% and preferably greater than 75%
• R (Desorbent) the amount of desorbent in the raffinate A21 / the amount of desorbent in the total raffinate A2 the separation parameter "delta R" defined by the distribution difference R (C8A) - R (Desorbent).
• the minimum separation parameter is 10%, preferably 20% and more preferably 30%.
• the adsorption step A of the charge makes it possible to separate the PX, and also to prefragrate the raffinate A2 into two streams, one A21, enriched in C8A, and the other A22 enriched in desorbent.
• Step B of fractionation The process according to the invention comprises a step B of fractionation by distillation in a column of the fractions A21 and A22 from step A of separation.
• said fractions A21 and A22 are introduced separately into the raffinate column B-C2 at separate injection points.
• step B makes it possible to produce at the top of the column a fraction B2 free of desorbent and depleted of PX and containing MX, OX and ethylbenzene and at the bottom a fraction B42 free of C8A and consisting of desorbent.
• An advantage of this double feed is that it makes it possible to reduce the thermal load of the raffinate column by at least 2.0 to 15.0%, depending on the quality of the prefractionation of the raffinates obtained in step A.
• step A is implemented so as to carry out a pre-fractionation into two raffmates A21 and A22, one enriched in desorbent, the other enriched in C8A (OX, MX, EB).
• the desorbent used in step A is light, that is to say with a boiling point lower than that of C8A
• the fraction of the raffinate whose light desorbent content is the highest is introduced a few trays above the position of the raffinate feed with the lowest content of light desorbent.
• step B makes it possible to produce at the bottom of the column a fraction B2 free of desorbent and depleted in PX and containing MC, OC and ethylbenzene and at the top a fraction B42 free of C8A and consisting of desorbent.
• the distillation column used is chosen from a 2-section column and a 3-section column.
• said column has a number of theoretical plates of between 30 and 80, preferably between 35 and 75, preferably 40 and 70, very preferably between 45 and 65.
• the two injection points of the fractions A21 and A22 in the distillation column have a spacing of between 2 and 15 theoretical plates, preferably between 3 and 12 theoretical plates and more preferably between 4 and 9.
• the distillation column used is a 2-section column.
• said 2-section column has a number of theoretical plates between 30 and 70, preferably between 35 and 65, preferably 40 and 60, very preferably between 45 and 55.
• this spacing is correlated with the quality of the prefractionation of the raffinates carried out in step A.
• the injection of fraction A22 will be done by a distribution system on different trays so that the spacing between the injection points of the charges A21 and A22 increases when the parameter separation R increases.
• Said 2-section column further comprises a condenser and a reboiler, operated at 0.2 MPa with a reflux rate of 1.2.
• the desorbent used in step A is a heavier compound than the xylenes, that is to say having a higher boiling point than that of the xylenes
• the enriched raffinate by desorbing, ie the fraction A22 is introduced below the depleted raffinate by desorbing, ie the fraction A21.
• the desorbent of said separation step is a lighter compound than the xylenes, that is to say having a lower boiling point than that of the xylenes
• the raffinate enriched in desorbent that is to say the fraction A21
• the desorbing that is to say the fraction A22.
• the distillation column used is a 3-section column.
• said column comprises an internal wall preferably placed in the rectification zone and makes it possible to collect two fractions B2 and B3 free of desorbent and a fraction B42 free of C8A and comprising, preferably consisting of desorbent.
• the injection of the fractions A21 and A22 is carried out on either side of the internal wall.
• the positions of the two supplies of the fractions A21 and A22 are located on either side of the internal wall.
• the fraction Al originating from stage A containing, and preferably consisting of, a mixture of PX and desorbent is engaged in a fractionation stage by distillation in a distillation column (B-Cl) allowing the obtaining a fraction B1 free of desorbent consisting of PX and a fraction B41 consisting of desorbent.
• Said distillation is carried out according to the knowledge of a person skilled in the art.
• the desorbent fractions B41 and B42 free of C8A are recovered at the bottom of each distillation column, when the desorbent is heavy and at the head when the desorbent is light. Said fractions are then mixed and returned to step A of adsorption in a simulated moving bed by means of stream B4. Steam phase isomerization step C
• the process further comprises a step C of isomerization in the vapor phase of the fraction B2 comprising ethylbenzene, orthoxylene and metaxylene from step B of fractionation.
• step C of isomerization in the vapor phase allows the isomerization of the xylenes as well as of 1 ⁇ B, in a unit operating in the vapor phase, at high temperature and converting ethylbenzene into xylenes, to treat the raffinate B2 rich in EB from from step B.
• the vapor phase isomerization stage makes it possible to convert 1 ⁇ B into xylenes with a pass conversion rate of ethylbenzene generally between 10 and 50%, preferably between 20 and 40%, with a loss of C8 Aromatics ( C8A) less than 5.0% by weight, preferably less than 3.0% by weight and preferably less than 1.8% by weight.
• C8A C8 Aromatics
• said step C also makes it possible to isomerize the xylenes, so that the paraxylene (PX) has a concentration at thermodynamic equilibrium; defined by
• Ceff and Cin are the PX concentrations respectively in section C8A of the effluent and of the charge of the isomerization reactor,
• Ceq is the thermodynamic equilibrium concentration of PX in section C8A at reaction temperature; greater than or equal to 90%.
• the vapor phase isomerization step is carried out at a temperature above 300 ° C, preferably between 350 and 480 ° C, a pressure below 4.0 MPa, preferably between 0.5 and 2 0.0 MPa, a space velocity less than 10.0 h 1 , preferably between 0.5 and 6.0 h 1 , hydrogen to hydrocarbon molar ratio less than 10.0, preferably between 3.0 and 6, 0, and in the presence of a catalyst comprising at least one zeolite having channels whose opening is defined by a ring with 10 or 12 oxygen atoms (10 MR or 12 MR), and at least one metal from group VIII of content between 0.1 and 0.3% by weight.
• a catalyst is used containing an acidic zeolite, for example of the structural type MFI, MOR, MAZ, FAU and / or EUO. Even more preferably, a catalyst is used containing a zeolite of structural type EUO and at least one metal from group VIII of the periodic table.
• the catalyst used in step C comprises from 1 to 70% by weight of a zeolite of structural type Lt 10, preferably LU-1, comprising silicon and at least one element T preferably chosen among aluminum and boron whose Si / T ratio is between 5 and 100.
• the zeolite is in hydrogen form at least in part, and the sodium content is such that the atomic ratio Na / T is lower at 0.1.
• the catalyst comprises between 0.01 and 2% by weight of tin or indium, and sulfur in an amount of 0.5 to 2 atoms per atom of group VIII metal.
• step C having concentrations of the PX, OX and MX isomer close to the thermodynamic equilibrium concentrations, are recycled to step A of adsorption in simulated mobile ht.
• This example shows the advantage of the invention by comparing the performance of the distillation step B placed between an adsorption step A and an isomerization step C, said steps being part of an aromatic complex producing paraxylene at from a reformat.
• this section C8A is sent in a step A of adsorption in a simulated moving bed comprising an adsorber with 4 zones delimited by the injections of charges and desorbent B4 and the withdrawals of Raffmat A2 and of extract Al.
• This adsorber is composed of 15 beds containing X zeolite exchanged with barium distributed as follows:
• the temperature is 175 ° C.
• the desorbent used is Paradiethylbenzene, the level of solvent relative to the charge is 1.2 (vol / vol).
• the adsorption separation unit A is used to produce 2 streams Al and A2 supplying the distillation step B:
• said section C8A is sent in a step A of adsorption in a simulated moving bed comprising an adsorber with 5 zones delimited by the injections of fillers and desorbent ( B4) and the withdrawals of the raffmates A21, A22 and of extract Al.
• Said adsorber is composed of 18 beds containing zeolite X exchanged with barium distributed as follows:
• Zone 1 between the injection of the desorbent B4 and the withdrawal of the extract Al, o 6 beds in zone 2, between the withdrawal of the extract Al and the injection of the load, o 4 beds in zone 3A, between the injection of the load and the withdrawal of the A21 raffrnate, o 3 beds in zone 3B, between the withdrawal of raffinate A21 and the withdrawal of raffinate A22, o 2 beds in zone 4, between the withdrawal of raffinate A22 and the injection of desorbent B4.
• the temperature is 175 ° C.
• the desorbent used is Paradiethylbenzene, the level of solvent relative to the charge is 1.2 (vol / vol).
• step A of the adsorption separation unit according to the invention makes it possible to obtain three fractions Al, A21, and A22, according to the following distribution.
• the raffinates A21 and A22 are drawn off on either side of zone 3B of the unit A of adsorption in simulated moving bed, and have the following compositions:
• step B of fractionation by the distillation column B-C2 containing 47 theoretical plates the raffinate A21 is fed to the theoretical plate 24, and the raffinate A22 is fed to the theoretical plate 30.
• Said column further comprises a condenser and a reboiler, operated at 0.2 MPa with a reflux rate of 1.2. Said column makes it possible to produce the following 2 fractions:
• Example 2 This example illustrates the advantage of the invention, when the performances of the prefractionation of the raffinate during the adsorption stage A vary, as can happen for example during a change of setting of the operating parameters of the adsorber or a change of molecular sieve.
• the conditions of this example are identical to those of Example 1.
• the position of the feed of the light raffinate A21 in the raffinate column is not impacted, that of the heavy raffinate is very slightly modified, the gain in thermal load of the reboiler increases significantly with the quality of the pre-fractionation of the raffinate.
• the improvement of the adsorption capacities and selectivities of the molecular sieve can be used to reduce the energy consumption of the aromatic complex without modifying the configuration of the aromatic complex nor its performance in terms of productivity.

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