The invention relates to a process for the production of xylenes by adsorption-reaction in a simulated moving bed and particularly a simultaneous process for dismutation and separation of a toluene feedstock into benzene and xylenes.
Paraxylene is a raw material that makes it possible to synthesize terephthalic acid and toluene terephthalate that are then transformed into resins and fibers, such as, for example, Tergal. The most effective process for the production and purification of paraxylene in terms of productivity is the adsorption in a simulated moving bed or countercurrent that may or may not be combined with a final purification by crystallization and necessarily coupled to a catalytic isomerization on a zeolite, for example an EUO-structural-type zeolite that is mentioned, for example, in the French patent application of the applicant 99/07 968. The paraxylene is produced mixed with the other isomers into C8-aromatic compounds primarily by the processes of isomerization, reforming and dismutation of toluene.
The dismutation of toluene consists in using two toluene molecules to produce a benzene molecule and a xylene molecule. This dismutation is referred to as paraselective when the paraxylene constitutes the majority of the xylenes that are produced.
The non-selective reaction is catalyzed by MFI-type zeolites (or pentasil) or mordenite or else metals of group VIII; examples will be found in Patents FR 2761904, U.S. Pat. Nos. 4,007,231, 4,011,276, 4,029,716 and 4,052,476. In the case of the selective dismutation, the catalysts that are used are pentasils of type ZSM5, ZSM 11, ZSM22 or ZSM23 or ZSM 35 on which a deposit of outside material such as carbon (coke) or silicon was placed, as described in U.S. Pat. Nos. 4,260,843, 4,274,982, 4,380,685, 4,908,342, 5,173,461 and EP26962. The Mobil Oil Company uses two commercial processes for dismutation of toluene: the MTDP (non-selective) (see Oil and Gas Journal Vol. 69 No. 48, 1971 by Grandio et al.), and the MSTDP (selective) (see Oil and Gas Journal Vol. 90 No. 41, 1992 by Gora et al. or Europ. Chem. News Vol. 54, p. 1418, 1990). For these two processes, the major drawback is that the conversion of toluene is limited to 30% by the thermodynamic equilibrium. At the outlet of the reactor, the typical composition of the mixture is therefore benzene 15%, toluene 70%, and xylenes 15%. A series of two columns to be distilled is therefore necessary to separate these three products and to recycle the largest portion of the toluene to the inlet of the reactor. In the case of the paraselective process, among the isomers of the xylene, it is possible to obtain up to 90% of paraxylene, but if a compromise between the conversion of toluene and the paraselectivity is sought, the proportion of paraxylene is smaller (from 75 to 85%).
Since the 1970""s, it has been known to use adsorption and reaction simultaneously (IandEC Fundam, 17 (1978) 1 D Sweich, J Villermaux). In the 1980""s, it was proposed by Barker and Ganestsos to use the adsorption reaction in a simulated moving bed and in liquid phase, for example, for the inversion of saccharose and its total separation into glucose and fructose. Since the beginning of the 1990""s, the idea of using adsorption and vapor-phase reaction simultaneously by using hydrogen as a vector fluid and as a reagent was first conceived by A. K. Ray and R. W. Carr (Chem Eng. Sci 50 (1995) n14, 2195-2202). It applied to the reaction for hydrogenation of mesitylene. Other authors adopted the idea of the separation reaction in a simulated moving bed and applied it to refining or to petrochemistry. It is thus possible to cite U.S. Pat. No. 5,530,173 that offers the additional original feature that the feedstock (mixture of aliphatic hydrocarbons with 6 to 8 carbon atoms) and the desorbent (normal pentane) are converted and separated simultaneously into isoparaffins with a high octane rating. Even more recently, U.S. Pat. No. 5,877,373 proposed the transalkylation of aromatic hydrocarbons into C9 and C10 by a mixture of toluene and benzene in a vapor phase and in the presence of hydrogen in such a way as to produce xylenes. In the latter document, it is not the implementation in a simulated moving bed but the circulation in opposite directions in the two reactors.
The object of the invention is the simultaneous production of benzene and xylenes by dismutation of toluene substantially without recycling toluene at the inlet of the reactor. To do this, the invention uses separation by adsorption, and the simultaneous reaction used in a simulated moving bed and in vapor phase.
More specifically, the invention relates to a simultaneous process for dismutation and separation of a feedstock that essentially consists of toluene, benzene and xylenes in the presence of a desorbent that is high in hydrogen in an adsorber-reactor that comprises at least three zones, characterized in that
a) the vapor-phase or supercritical feedstock is introduced at the inlet of a reaction and adsorption zone (zone III), and at the outlet of said zone, a raffinate that is high in benzene and desorbent is recovered,
b) the desorbent is introduced at the inlet of a desorption zone (zone I), and an effluent is recovered at the outlet of said zone from which a portion in the form of an extract that is high in xylenes and desorbent is drawn off,
c) the other portion of the effluent of zone I is introduced at the inlet of a reaction and desorption zone (zone II), and an effluent that is sent to the inlet of reaction and adsorption zone III is recovered at the outlet of said zone.
The process is also characterized in that each of the zones comprises at least one bed that contains an adsorbent that is adapted to separate benzene, toluene and xylenes and a catalyst that is adapted to dismutate toluene into benzene and xylenes, whereby the adsorbent and the catalyst are in solid form.
When one of the zones comprises several beds, the latter comprises at least one bed that contains an adsorbent and a catalyst, whereby the remaining beds can contain either only a catalyst or only an adsorbent.
According to a variant of the invention, the catalyst can be a non-selective catalyst for paraxylene, adapted to obtain in the extract a mixture of xylenes that is close to the thermodynamic equilibrium.
It may be selected from the group that is formed by MFI zeolites or pentasils, mordenite in acid form or a precious metal of group VIIIB that is deposited on mordenite, or the Y zeolite that is exchanged with nickel, sodium, lanthanum or in acid form, the X zeolite that is exchanged with sodium, lanthanum or in acid form.
According to another variant of the invention, the catalyst is selective for the paraxylene and adapted to obtain a mixture of xylenes containing at least 80% of paraxylene.
These catalysts have very few active sites on their surface; they can be selected from the group of MFI zeolites, in particular ZSM5, ZSM11, ZSM22, ZSM23 that are made selective by deposition of coke, or silicon, or magnesium, or germanium or a combination of these elements at the surface of the zeolite.
With regard to the adsorbent, it may be selected from the group that is formed by silicalite, MFI zeolites or pentasils, faujasites (X or Y zeolites) that are exchanged by a cation of group IA or a cation of group IIA, or a cation of group IA and a cation of group IIA, the mordenites that are exchanged by a cation of group IA or a cation of group IIA, and the preceding zeolites made selective by deposition at the surface of carbon or magnesium or germanium or silicon or a combination of these elements.
According to a first variant, it is possible to use the same solid both as adsorbent and as catalyst in each bed.
According to a second variant, it is possible to produce an approximately homogenous mixture of adsorbent and catalyst in each of the beds in proportions of 95:5 to 5:95.
According to a third variant, it is possible to use a layer of catalyst and a layer of adsorbent in each bed in proportions of 95:5 to 5:95.
The operating conditions in the adsorber-reactor are generally as follows:
the temperature is between 220 and 520xc2x0 C. and preferably between 275 and 350xc2x0 C.
the H2/feedstock molar ratio is in, for example, proportions of 0.5/1 to 50/1 and preferably between 2/1 and 10/1
the total pressure in the adsorber-reactor is between atmospheric pressure and 400 bar and preferably between 1.5 and 40 bar (1 bar=105 Pa)
the partial pressure of hydrocarbons at the injection point of the feedstock is generally between 0.1 and 40 bar and preferably between 0.2 and 10 bar
the volumetric flow rate is between a pph of 0.025 hxe2x88x921 and 25 hxe2x88x921, and preferably between 0.05 and 5 hxe2x88x921.
According to a first implementation of the process according to the invention that corresponds to a 3-zone adsorber-reactor, the fluid circulation between the last bed of zone III and the first bed of zone 1 can be interrupted with an all-or-nothing valve that is placed downstream from the sampling of the entire raffinate.
According to a second implementation of the process that corresponds to an adsorber-reactor with at least 4 zones, a recycling compressor that is located between the first bed and the last bed is adapted to control the flow of fluid that circulates successively in each of the zones.
According to a third implementation of the process that corresponds to an adsorber-reactor with at least four zones, the circulation of fluid between the last bed of zone IV and the first bed of zone I is interrupted with an all-or-nothing valve, and the fluid that contains the hydrogen that is obtained from zone IV is recovered and sent back to the intake of the hydrogen compressor that delivers this desorbent to the inlet of zone I.