Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce aromatic
Reexamination Certificate
1998-10-14
2001-08-07
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce aromatic
C585S315000, C585S481000, C585S482000, C585S478000, C585S477000
Reexamination Certificate
active
06271429
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of isomerising aromatic compounds containing eight carbon atoms.
BACKGROUND OF THE INVENTION
In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed, which is generally depleted in para-xylene with respect to the thermodynamic equilibrium of the mixture (i.e., where the para-xylene content is substantially lower than that of a mixture at thermodynamic equilibrium at the temperature under consideration, that mixture being constituted by meta-xylene, ortho-xylene, para-xylene and ethylbenzene) and generally rich in ethylbenzene with respect to that same mixture at thermodynamic equilibrium, is introduced into a reactor containing at least one catalyst, under suitable temperature and pressure conditions to obtain at the outlet from that reactor a composition of aromatic compounds containing eight carbon atoms which is as close as possible to the composition of that mixture at thermodynamic equilibrium at the reactor temperature.
From that mixture, the para-xylene and possibly ortho-xylene are separated out since they are the isomers which are sought as they are of importance, in particular to the synthetic fibre industry. The meta-xylene and ethylbenzene can then be recycled to the isomerisation reactor inlet so as to increase the production of para-xylene and ortho-xylene. When ortho-xylene is not to be recovered, it is recycled with the meta-xylene and the ethylbenzene.
Reactions for isomerising aromatic compounds containing eight carbon atoms per molecule, however, encounter a number of problems caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed such as hydrogenation of aromatic compounds to naphthenes, also naphthene ring opening reactions which lead to the formation of paraffins containing at most the same number of carbon atoms per molecule as the naphthenes from which they originate. Cracking reactions are also observed, such as paraffin cracking which leads to the formation of light paraffins typically containing 3 to 5 carbon atoms per molecule, and dismutation and transalkylation reactions which lead to the production of benzene, toluene, aromatic compounds containing 9 carbon atoms per molecule (for example trimethylbenzenes) and heavier aromatic compounds.
The aggregate of such secondary reactions substantially affects the yields of desired products.
The quantity of secondary products formed (primarily naphthenes containing 8 carbon atoms, paraffins containing 8 carbon atoms, benzene, toluene, and aromatic compounds containing 9 or 10 carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerisation reactor (temperature, partial pressures of hydrogen and hydrocarbons, feed flow rate).
The skilled person is aware that secondary reactions increase when the para-xylene content in the reactor is close to the amount of para-xylene at thermodynamic equilibrium under the given temperature and pressure conditions.
Optimising the operating conditions and optimising the formulation of the isomerisation catalyst can increase the para-xylene yield, but cannot overcome the losses. Further, research to obtain new catalysts is a long and expensive business.
SUMMARY OF THE INVENTION
We have surprisingly discovered that it is possible to arrive at para-xylene contents close to the para-xylene content at thermodynamic equilibrium while minimising xylene loss by combining at least two reaction steps.
Thus the present invention provides a process for isomerising a feed comprising aromatic compounds containing eight carbon atoms, comprising at least one isomerisation step a) and at least one hydrogenation step b). The invention also provides an apparatus for carrying out the process.
Isomerisation catalysts which can be used in step a) of the process of the invention are: all catalysts which, from a mixture comprising aromatic compounds containing eight carbon atoms, among them xylenes and/or ethylbenzene, can produce a mixture composition—xylenes and ethylbenzene—close to that of the composition of the mixture at thermodynamic equilibrium under the given temperature and pressure conditions, also catalysts which can effect dealkylating isomerisation of ethylbenzene and benzene.
Any catalyst which can dehydrogenate naphthene type compounds to aromatic compounds can be used in step b) of the process of the present invention. At the dehydrogenation reactor outlet, for a given number of carbon atoms per molecule, the aromatic compounds obtained are present in the proportions of thermodynamic equilibrium under the temperature and pressure conditions reigning at the outlet from the reactor.
The catalysts used in the first step of the process of the invention are supported alumina based catalysts which comprise at least one zeolite and at least one noble metal from group VIII of the periodic table (Handbook of Chemistry and Physics, 45
th
edition, 1964-1965); the metal is preferably platinum. The zeolites used are preferably mordenite, omega zeolite, zeolite with structure type MFI or zeolites with an activity as regards dealkylating isomerisation of ethylbenzene to benzene which is approximately of the same type as the activity of MFI zeolite.
Thus in a first step of the process of the present invention, the operating conditions in the isomerisation zone are selected so as to minimise the production of unwanted compounds from reactions which involve acid catalysis reactions (cracking, dealkylation, dismutation, . . . ). These operating conditions are such that production of naphthenes containing eight carbon atoms per molecule is significantly higher—about 10% to 30% by weight of the effluent at the outlet from the isomerisation zone—than the production obtained by conventional processes for isomerising aromatic compounds containing eight carbon atoms—which is generally about 5% to 10% by weight of the effluent at the outlet from the isomerisation zone.
The effluent obtained from the first reaction zone is treated in a second step in a reaction zone containing at least one dehydrogenation catalyst. The operating conditions for this second step may be identical to or different from the operating conditions in the first step; preferably the operating conditions in these two steps are different. The operating conditions in this second step are determined so as to obtain a xylene and ethylbenzene mixture composition which is as close as possible to the composition at thermodynamic equilibrium.
Catalysts for dehydrogenating paraffins and naphthenes are well known to the skilled person. The supports for these catalysts are generally refractory oxides, usually an alumina. These dehydrogenation catalysts comprise at least one noble metal from group VIII of the periodic table and at least one alkali or alkaline-earth element from groups Ia and IIa of the periodic table. The noble group VIII metal is preferably platinum, and the element from groups Ia or IIa of the periodic table is selected from the group formed by magnesium, potassium and calcium.
These dehydrogenation catalysts can also contain thorium and/or at least one element M from groups IVa or IVb of the periodic table. The group IVa or IVb elements are usually selected from the group formed by tin, silicon, titanium and zirconium. Certain dehydrogenation catalysts also contain sulphur and/or a halogen. More particularly, dehydrogenation catalysts described in U.S. Pat. Nos. 3,998,900 and 3,531,543 can be used in the dehydrogenation step of the process of the invention.
Without wishing to be tied to a particular theory, it can be noted that platinum has a hydrogenolysing activity which is expressed to the detriment of the activity for dehydrogenation of paraffins to aromatic compounds. This hydrogenolysing activity can be substantially reduced, and the selectivity of the catalyst as regards the dehydrogenation reaction can be increased, by adding additional element M.
The inorganic refractory supports used often have an
Alario Fabio
Coupard Vincent
Joly Jean-Francois
Magne-Drisch Julia
Miquel Gérard
Dang Thuan D.
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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