FR2773148A1 - A new process for the production of xylenes and para-xylene for the synthesis of terephthalic acid - Google Patents

Abstract

The process achieves a high yield of high purity para-xylene from an initial reformate which contains ethylbenzene and xylenes. A catalytically reformed hydrocarbon charge is subjected to a distillation to produce a mixture of 8C aromatic isomers in which: (a) the mixture is distilled (11) to give a fraction A enriched in ethylbenzene containing metaxylene and paraxylene, and a fraction B containing para-, meta- and ortho-xylene and C9+ aromatics; (b) the fraction A is hydrogenated (23) in a hydrogenating zone; (c) the hydrogenated fraction is isomerized (24) in hydroisomerizing conditions; (d) the isomerate is distilled (26) to produce a distillate (27) containing 1,3- and 1,4-dimethylcyclo-hexane and a residue containing 1,2-dimethylcyclohexane and ethylcyclohexane; (e) the distillate is recycled to the catalytic reforming zone (2); and (f) the residue is recycled to the hydrogenation/isomerization zone. The fraction B is treated as follows:- -the fraction is separated by distillation (14) to produce a distillate C containing ortho-, meta- and para-xylenes, and a residue D containing heavy hydrocarbons; -the distillate C is separated in an adsorption zone (17) comprising a mobile bed, in the presence of a desorbent, into a fractions depleted and enriched in para-xylene; -the depleted fraction is isomerized (20) in liquid phase conditions in the presence of toluene to nullify dismutation of the xylenes; and -the para-xylene rich fraction is distilled to eliminate desorbent and pure para-xylene recovered.

Description

The invention relates to a process for the production of paraxylene from

  of a hydrocarbon charge comprising aromatic isomers with 8 carbon atoms. She

  also relates to a process maximizing the production of xylenes.

  It applies in particular to the synthesis of terephthalic acid, a petrochemical intermediate for the manufacture of textile fibers. In the processing of aromatic hydrocarbons with eight carbon atoms for the production of paraxylene, all the isomers of xylenes including ethylbenzene must be completely converted into the desired isomer, paraxylene. While the mutual isomerization of xylenes (para-, metaet ortho-xylene, PX, MX, OX) is relatively easy to carry out with or without hydrogen in vapor or liquid phase, it is not the same for isomerization ethylbenzene to xylenes which requires a higher temperature, in the vapor phase, a significant partial pressure of hydrogen and a catalyst containing a hydro-dehydrogenating metal such as platinum. When ethylbenzene is not converted by isomerization, hydrodealkylation or disproportionation, it must be eliminated from the C8 aromatic loop to prevent it from circulating at an increasingly high concentration, which would be detrimental to the production of paraxylene. It is known that some of the industrially operating C8 aromatic hydrocarbon isomerization processes do not isomerize ethylbenzene to xylenes but rather convert ethylbenzene to benzene and / or diethylbenzene. Admittedly, these reactions remove ethylbenzene from the C8 aromatic loop, but they do not

  not maximize the production of paraxylene.

  To date, it is known to use superfractionation and adsorption on solids to remove ethylbenzene from an aromatic C8 charge to produce paraxylene. However, these two techniques are problematic:

  - superfractionation, very simple in design, is very expensive and expensive in energy.

  In fact, it turns out that you need a column of 400 trays with a reflux ratio of 55: 1 for example, to obtain ethylbenzene of sufficient quality to produce styrene, more than 95% conversion, taking into account the small difference (2 C)

  existing between the boiling points of the different isomers.

  - the column adsorption technique is not selective enough to separate

  effectively ethylbenzene from a mixture of xylenes containing it.

  An object of the invention is to remedy the drawbacks of the prior art.

  Another object is to convert ethylbenzene to xylenes in an original way and it

  therefore relates to a process for producing xylenes.

  Another object of the invention is to maximize the concentration of paraxylene in the feed at the inlet of the adsorption column and to facilitate its separation and recovery. Another object is to reduce the size of the isomerization reactor subsequent to the adsorption column and consequently to reduce the hydraulic flow rates of

  recycling to the adsorption column.

  Another object is to operate the isomerization reactor under more

economic.

  Another object is to produce paraxylene with very high purity in

very advantageous conditions.

  More precisely, the invention relates to a process for producing xylenes comprising paraxylene from a hydrocarbon feedstock comprising the catalytic reforming of said feedstock in a catalytic reforming zone and at least one distillation of the reformate obtained producing a mixture of isomers aromatic with 8 carbon atoms, in which: - a) a fraction A rich in ethylbenzene containing metaxylene and paraxylene is separated by a fraction B substantially free from said mixture, by distillation of said mixture, in a first distillation zone (11) ethylbenzene and containing paraxylene, metaxylene, orthoxylene and heavy aromatic hydrocarbons (C9g +); the process being characterized by the following stages: - b) the fraction A rich in ethylbenzene is hydrogenated in a hydrogenation zone (23) under suitable conditions; c) the hydrogenated fraction A isomerized in an isomerization zone (24) under hydroisomerizing isomerization conditions; - d) separating by distillation of the isomerate obtained, in a second distillation zone (26), a distillate (27) containing 1,3- and 1,4-dimethylcyclohexane from a residue containing 1,2- dimethylcyclohexane and ethylcyclohexane; - e) at least part of the distillate (27) from step d) is recycled in said catalytic reforming zone; and - f) at least part of the residue is recycled to the hydrogenation or isomerization zone. The invention also relates to a process for producing paraxylene from

xylenes produced.

  More precisely, - a distillate C containing metaxylene, orthoxylene and paraxylene is separated by fraction D from fraction B in a third distillation zone (14), from a residue D containing heavy hydrocarbons, - starting from distillate C in a zone (17) of adsorption in simulated moving bed containing an adsorbent, in the presence of a desorbent, a fraction poor in paraxylene and a fraction rich in paraxylene, - the fraction poor in isomerized (20) paraxylene under temperature and pressure conditions such that one operates in the liquid phase in the presence of toluene so that the disproportionation of the xylenes is substantially zero, - the fraction rich in paraxylene is distilled to eliminate the desorbent and recover

substantially pure paraxylene.

  By distilling at the outlet of the reforming reactor the reforming effluent, a distillate containing ethylbenzene but also meta- and para-xylene is separated from a residue not containing ethylbenzene but containing orthoxylene , paraxylene

and metaxylene.

  The distillate is hydrogenated respectively to ethylcyclohexane, to 1,3 dimethylcyclohexane and to 1,4 dimethylcyclohexane. The products obtained are isomerized under hydrogen pressure so that in particular the ethylcyclohexane derived from ethylbenzene is converted at least in part to 1,2 dimethylcyclohexane,

  1,3 dimethylcyclohexane and 1,4 dimethyl-cyclohexane.

  By distillation of the isomerate at an adequate rate of reflux, a distillate is produced containing the 1,3- and 1,4-dimethylcyclohexane which are recycled to the reaction reactor.

  catalytic reforming and are reconverted there by dehydrogenation to meta and para-

xylene respectively.

  The residue produced by distillation and containing ethylcyclohexane and 1,2 dimethylcyclohexane is recycled at least in part to the isomerization zone via

the hydrogenation zone.

  By the process according to the invention, the production of xylenes and in particular of paraxylene is maximized since the ethylbenzene via its transformation into ethylcyclohexane is recycled and reisomerized until extinction in the reactors

  hydrogenation and isomerization.

  Furthermore, the residue advantageously recycled in the hydrogenation reactor

  makes it possible to at least partially control the exothermicity of the reaction.

  The reforming of the hydrocarbon feedstock, in general a naphtha cut, is carried out under usual conditions known to those skilled in the art, for example those

  described in patents US 4,233,268, US 4,133,743 and US 4,210,519.

  According to another characteristic of the invention, step (a) of distillation of the mixture of aromatic hydrocarbons comprising the isomers of xylene and ethylbenzene is generally carried out under the following conditions: - Number of real plates: 20-100, preferably 50 to 80,

  - Rate of reflux relative to the distillate: 3 to 15, preferably 5 to 10.

  Under these conditions, 5 to 30% of meta- and para-xylene relative to the charge of

  distillation are present in the distillate and preferably 10 to 20% by weight.

  According to another characteristic of the process, step (b) of hydrogenation of the fraction A rich in ethylbenzene and containing meta- and paraxylene, at which

  are added by recycling the distillation residue, naphthenes mentioned above

  above, can be carried out in the liquid phase under the following conditions: - catalyst: alumina or silica containing from 0.1 to 3% by weight of platinum or 10 to 50% of nickel

  - pressure: 10 to 100 bar (1 bar = 105 Pa), preferably 15 to 30 bar.

  - temperature: 50 to 300 0C, preferably 150-200 0C.

  - ppH: 1 - 10 h-1, preferably 2 to 5 h-1

  - H2 / hydrocarbons: 1 to 5 times the stoichiometry and preferably 1.5 to 3 times.

  According to another characteristic of the process, step (c) of isomerization of the hydrogenated fraction containing ethylcyclohexane is usually carried out under the following conditions:

  - hydrogen pressure greater than 10 bar, preferably 15-30 bar.

  - temperature: 200 to 4500C, preferably 250 to 350 C0.

  - ppH: 0.1 to 10 h -1, preferably 0.5 to 5 h -1 - H2 / hydrocarbons (molar): 0.5 to 10, preferably 2 to 8 - catalyst containing a hydrodehydrogenating phase, preferably Pt (0.01 to 2% by weight, preferably 0.05 to 1% by weight) or Ni (0.01 to 10% by weight, preferably 0.05 to% by weight) and at least one acid phase, preferably a mordenite or a zeolite

  of EU-O structure such as for example the EU-1 zeolite (patent EP-B-42226).

  According to another characteristic, step (d) of distillation of the isomerized fraction containing ethylcyclohexane, 1,2-, 1,3- and 1,4-dimethylcyclohexane is generally carried out under the following conditions: - number real trays: 20 to 100 preferably 40 to 50,

  - reflux rate relative to the distillate: 3 to 15, preferably 5 to 8.

  Under these conditions, an amount of ethylcyclohexane less than 10% by weight and

  preferably between 1 and 4% by weight circulates in the distillate with the 1,3- and 1,4-

  dimethylcyclohexane, which is recycled to the reforming reactor.

  According to another characteristic of the process, the residual fraction B coming from the first distillation zone containing paraxylene (80% for example of the charge of said distillation), meta- and ortho-xylene and Cg9 + hydrocarbons are

  distilled in a conventional manner.

  A residue containing the Cg9 + is recovered while a distillate containing the para, ortho and meta-xylene is sent to the adsorption zone in a simulated moving bed, against the current according to US Patents 2,985,589 and US 3,558,732 or co-current according to US Patents 4,498,991 and US 4,402,832; The adsorbents can for example be one of those described in the patent

US 3,626,020.

  The adsorption step delivers a fraction enriched in paraxylene, after having been desorbed by the desorbent. This fraction freed from the desorbent can be purified by crystallization as described in US Patents 5,284,992, US 5,401,476 and

  WO 96/22262 incorporated as reference.

  It is thus possible to recover paraxylene of purity equal to 99.9%.

  A fraction depleted in paraxylene is also recovered from the adsorption step. This fraction after having been freed at least in part from the desorbent is generally isomerized under mild conditions, not favoring the conversion of ethylbenzene, all the more so since the isomerization charge contains substantially no ethylbenzene. An isomerate enriched in paraxylene is thus produced which can be recycled in the adsorption step via the third distillation zone to

  remove the heaviest oil.

  The conditions for the isomerization of this fraction poor in paraxylene described in particular in US Pat. No. 3,729,523, incorporated as a reference, can be as follows: any catalyst for isomerization of xylenes and / or ethylbenzene, in particular a zeolitic catalyst such as zeolite X, Y, mordenite or ZSM-4.

  - temperature: 100 to 500 C, preferably 180 to 260 C.

  - pressure: 1 to 140 bar, preferably 25 to 30 bar.

  - VVH: from 0.2 to 20 h -1, preferably 1 to 7 h 1.

  - H2 / hydrocarbons: 0 to 20 mol / mol, preferably 0 to 10.

  - injection of toluene from 5 to 30% by weight of the isomerization charge, preferably 15

at 25% by weight.

  The invention will be better understood in view of the single figure illustrating a mode of

carrying out the process.

  A charge 1 of naphtha comprising alkanes, naphthenes and aromatics is transformed in a reforming reactor 2 containing as catalyst, platinum and a second metal optionally, deposited (s) on a halogenated alumina, preferably chlorinated, into a reformate 3 enriched in hydrocarbons

  aromatic and hydrogen 29 which is recovered.

  The reformate is stabilized in a stabilization column 4 which releases an effluent

gas 6 (LPG) and fuel gas 7.

  The bottom 5 of the column containing C5 + hydrocarbons to which can be added via a line 5b a pretreated effluent from a steam cracking of a naphtha is introduced into another distillation column 8 to recover a distillate 9 of saturated hydrocarbons ( 05-C7), benzene, toluene and C8 aromatics, treated in a conventional manner and a residue 10 at the bottom of the column consisting of a mixture of aromatic hydrocarbons with 8 carbon atoms and hydrocarbons

heavier aromatics (Cg +).

  According to the invention and the figure, said mixture 10, which also comprises C8 aromatic hydrocarbons previously separated from the distillate 9 by an extractive distillation (not shown in the figure), is introduced into a first distillation column 11 operating at adequate rate of reflux. There is separated a distillate 12 at the top of the column comprising substantially all of ethylbenzene, paraxylene and metaxylene, and a residue 13 at the bottom of the column containing the majority of paraxylene (approximately 80%), substantially all of the orthoxylene, of metaxylene. ,

  C9 + aromatic hydrocarbons and substantially no ethylbenzene.

  This residue is sent to a third distillation column 14 (called xylenes splitter) which also receives a recycling 21 of an isomerate described below. This column makes it possible to recover a residue 15 containing the fraction C9 + at the bottom, and at the head a distillate 16 essentially comprising paraxylene, metaxylene and orthoxylene. The distillate is introduced into at least one adsorption column 17 containing an adsorbent such as a barium doped zeolite X and operating in the presence of a desorbent such as toluene, with simulated counter-current. A raffinate containing ortho- and meta-xylene is collected which is isomerized in the presence of a part of the desorbent (20% by weight for example) in a catalytic isomerization reactor operating under mild conditions. The collected isomerate is freed from benzene and the lightest compounds (line 22) which are recycled to column 4 for distillation

  while a heavier fraction of the isomerate containing ortho, meta- and para-

  xylene is recycled as indicated above in the third distillation column 14

by line 21.

  Is also collected from the adsorption column 17 an extract containing desorbent and paraxylene which is distilled. The paraxylene collected has a purity of more than 99%. If the purity is not sufficient, for example 90%, it is possible, as indicated in the figure, to introduce the paraxylene into at least one crystallizer 18 operating at high temperature (0 ° C. for example) and recover after centrifugation of the crystals (19) which, after possible resuspension and washing with

  molten paraxylene, have a purity of 99.9%.

  A collected mother liquor is partially recycled through a line 31 in the adsorption column. According to the invention, the distillate from the first distillation column 11 comprising substantially all of the ethylbenzene, paraxylene and orthoxylene, is introduced into a hydrogenation reactor 23 containing a catalyst (AI203-Ni for example), which receives hydrogen via a line 29 coming from the reforming reactor 2. The hydrogenate in naphthenic form (ethylcyclohexane, dimethylcyclohexane) is sent to a catalytic isomerization reactor 24 operating under hydrogen which comes from

  example of the reforming reactor by a line 32.

  The excess hydrogen and the light gases produced are evacuated from the hydrogenation reactor by a line 30 while the isomerate is sent by a line 25 to

  a second distillation column 26. A distillate containing 1,3- and 1,4-

  dimethylcyclohexane and methylcyclopentane intermediates is collected to be recycled into the catalytic reforming reactor via line 27 and to be dehydrogenated there. A distillation residue at the bottom of the column essentially contains hydrogenated ethylbenzene, that is to say ethylcyclohexane as well as 1,2 dimethylcyclohexane, which corresponds to the hydrogenation of a small amount of orthoxylene entrained in the first column 11 as a distillate and especially 1,2 dimethylcyclohexane originating from the isomerization step 24 of ethylcyclohexane according to proportions of the components (OX, MX and PX) corresponding to the thermodynamic equilibrium. This residue is recycled through a line 28 in the hydrogenation reactor 23 and therefore contributes to controlling the exothermicity of the hydrogenation reaction. The following example illustrates the invention. According to the figure, the hydrocarbon charge comprising hydrocarbons is reformed under conditions known to a person skilled in the art. The reformate (line 3) is conventionally stabilized and the residue obtained is distilled to remove (line 9) the non-aromatic hydrocarbons C5 to C7, benzene, toluene. The new residue (line 10) containing the isomers of xylenes OX, MX and PX, ethylbenzene is sent to the first distillation column 11. This column thus receives OX isomers,

  MX and PX and ethylbenzene which contaminated the distillate from line 9.

  The conditions of the first distribution column 11 are as follows - number of theoretical plates: 60 - reflux rate: 7 pressure at the head of the column: 1.2 bar Abs The conditions for the hydrogenation (23) of the fraction rich in ethylbenzene (line 12) are as follows

- pressure: 24 bar Abs.

  - temperature: 200 C - partial pressure of hydrogen: 15 bar H2 / hydrocarbons: 3 times the stoichiometry

  - catalyst: Alumina + nickel 16% by weight.

  The conditions for isomerization 24 of the hydrogenated fraction are as follows:

- H2 pressure: 23 bar Abs.

  - temperature: 300 C - ppH: 3 h-1 - H2 / hydrocarbons ratio (mole / mole): 5

  - catalyst: mordenite-Ni (3% by weight).

  The conditions of the second distillation column 26 are as follows - number of theoretical plates: 40 - reflux rate: 3 pressure at the top: 4 bar

  A material balance expressed in kilograms / hour is presented in the following table.

  It shows that the fraction rich in ethylbenzene (line 12) is converted to ethylcyclohexane then isomerized to give an isomerization effluent (line 25)

  containing 1,3-dimethylcyclohexane, 1,4-dimethylcyclohexane and 1, 2-

  dimethylcyclohexane as well as ethylcyclohexane not converted in proportions

  corresponding to those of equilibrium.

  After distillation in column 26 of this effluent, the residue comprising ethylcyclohexane and 1,2-dimethylcyclohexane is recycled in the hydrogenation step via line 28, which contributes to controlling the exothermicity of the

  reaction. On the other hand, the distillate (line 27) containing essentially 1,3- and 1,4-

  dimethylcyclohexane is recycled to the reforming reactor where it is dehydrogenated, which contributes to producing paraxylene, metaxylene and orthoxylene which are found in line 10, then in the residue line 13. This residue is distilled in column 14 and the distillate is adsorbed in simulated counter-current to produce an extract of

high purity paraxylene.

  The raffinate is isomerized under mild conditions in the liquid phase consisting of toluene: - Absence of hydrogen - catalyst: mordenite temperature: 2200C -pressure: 30 bar -VVH: 3 h- 1 - toluene: 10% w / w L ' isomerate containing substantially no ethylbenzene is recycled in step

  adsorption in simulated moving bed via the distillation step 14.

  Ugnes material balance Composition 10 12 13 29 28 25 30 27 Compositionî H2 - - 945 - 45 45 trace HO light - - - 1155 - 1421 1333 88

Toluene 334 334 - - - - - -

MCH - - - - - 355 88 267

EB 12692 12 534 158 - - - - -

ETCH - - - - 8,737 9,067 - 330

PX 15 564 936 14 628 _ - - - -

1.4 DMCH - - - - 157 5 382 - 5 225

MX 31 690 1 904 29 786 _ - - - -

1.3 DMCH - - - - 318 10908 - 10590

OX 15618 158 15460 _ - - - -

1.2 DMCH - - - - 5,090 5,090 - -

C9g + A 3,340 - 3,340 - - -

l

Claims (6)

Claims   1. A process for the production of xylenes comprising paraxylene from a hydrocarbon feedstock comprising the catalytic reforming of said feedstock in a catalytic reforming zone and at least one distillation of the reformate obtained producing a mixture of aromatic isomers with 8 atoms of carbon, in which: a) a fraction A rich in ethylbenzene containing metaxylene and paraxylene is separated by distillation from said mixture, in a first distillation zone (11), from a fraction B substantially free of ethylbenzene and containing paraxylene, metaxylene, orthoxylene and heavy aromatic hydrocarbons (Cg9 +); the process being characterized by the following stages - b) the fraction A rich in ethylbenzene is hydrogenated in a hydrogenation zone (23) under suitable conditions; c) the hydrogenated fraction A isomerized in an isomerization zone (24) under hydroisomerizing isomerization conditions; - d) separating by distillation from the isomerate obtained in a tenth distillation zone (26), a distillate (27) containing 1,3- and 1,4-dimethylcyclohexane from a residue containing 1,2-dimethylcyclohexane and ethylcyclohexane; e) at least part of the distillate (27) from step d) is recycled to said catalytic reforming zone; and -f) at least part of the residue is recycled to the hydrogenation or isomerization zone. 2. Method according to claim 1, in which step (b) of hydrogenation of the fraction A rich in ethylbenzene is carried out under the following conditions: Catalyst: alumina or silica containing from 0.1 to 3% by weight of platinum or 10 to 50% nickel weight.   Pressure: 10 to 100 bar, preferably 15 to 30 bar.   Temperature: 50 to 300 C, preferably 150-200 C ppH: 1 to 1 Oh-1, preferably 2 to 5 h-1   H2 / hydrocarbons: 1 to 5 times the stoichiometry and preferably 1.5 to 3 times.   3. Method according to one of claims 1 and 2 wherein Il'isomerization (c) of the   hydrogenated fraction is carried out under the following conditions   Hydrogen pressure greater than 10 bar, preferably 15-30 bar.   Temperature: 200 to 450 C, preferably 250 to 3500C.   ppH: 0.1 to 10 h -1, preferably 0.5 to 5 h -1 H2 / hydrocarbons (molar): 0.5 to 10, preferably 2 to 8 Catalyst containing a hydrodehydrogenating phase, preferably Pt (of 0.01 to 2% by weight) or Ni (from 0.01 to 10% by weight) and at least one acid phase, of preferably a mordenite.   4. Method according to one of claims 1 to 3, wherein said mixture of isomers   C8 aromatics further includes C8 aromatic isomers from   a steam cracking effluent from a naphtha.   5. Production method according to one of claims 1 to 4, in which:   -we separate by distillation of fraction B in a third distillation zone (14) a distillate C containing metaxylene, orthoxylene and paraxylene, from a residue D containing heavy hydrocarbons, -it is separated from distillate C in a simulated moving bed adsorption zone (17) containing an adsorbent, in the presence of a desorbent, a fraction poor in paraxylene and a fraction rich in paraxylene, - the fraction poor in paraxylene is isomerized (20) under conditions temperature and pressure such that one operates in the liquid phase in the presence of toluene so that the disproportionation of the xylenes is substantially zero, - the fraction rich in paraxylene is distilled to remove the desorbent and recover substantially pure paraxylene.   6. The method of claim 5 wherein the isomerization step in the liquid phase is carried out under the following conditions: - Temperature: 100 to 500 C, preferably 180 to 260 C - Pressure: 1 to 140 bar, preferably 25 at 30 bars, -VVH: 0.2 to 20 h-1, preferably 1 to 7 h-1 H2 / hydrocarbons: 0 to 20 mole / mole, preferably 0 to 10 - Toluene injection: 5 to 30% by weight the isomerization charge, preferably 15 to 25%.