Chemistry of hydrocarbon compounds – Aromatic compound synthesis – By isomerization
Reexamination Certificate
1998-10-14
2001-03-06
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Aromatic compound synthesis
By isomerization
C585S477000, C585S478000, C585S481000, C585S482000, C585S479000
Reexamination Certificate
active
06198014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of isomerising aromatic compounds containing eight carbon atoms.
BACKGROUND OF THE INVENTION
this field has already been the subject of much study, described in particular in U.S. patents U.S. Pat. No. 3,538,173, U.S. Pat. No. 3,553,276, U.S. Pat. No. 4,128,591 and U.S. Pat. No. 4,255,606. In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed which is depleted in para-xylene with respect to the thermodynamic equilibrium of the mixture and which is rich in ethylbenzene (with respect to that same mixture at thermodynamic equilibrium) is introduced into a reactor containing at least one catalyst. That feed enters the reactor where the temperature and pressure conditions are suitable for obtaining a composition of aromatic compounds containing eight carbon atoms at the outlet from that reactor which is as close as possible to the composition of that mixture at thermodynamic equilibrium. Other documents describe processes for isomerising aromatic compounds containing eight carbon atoms in which the feed to be treated is rich in xylenes and depleted in ethylbenzene. The term “depleted in para-xylene” and “rich in ethylbenzene” as used in the present description mean that the para-xylene content is substantially lower and the ethylbenzene content is substantially higher than that of a mixture at thermodynamic equilibrium, under the temperature and pressure conditions under consideration (the mixture being constituted by meta-xylene, ortho-xylene, para-xylene and ethylbenzene).
From that mixture, the para-xylene and often the 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 possibly the ethylbenzene can then be recycled to the isomerisation reactor inlet so as to increase the production of para-xylene and ortho-xylene.
The reaction for isomerising aromatic compounds containing eight carbon atoms per molecule, however, encounters a number of problems caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed: 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. Those paraffins can undergo cracking reactions which lead to the formation of light paraffins typically containing 3 to 5 carbon atoms per molecule. Aromatic compounds undergo dismutation and transalkylation reactions which, in the case of aromatic compounds containing eight carbon atoms, 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 and deleteriously affects the yields of desired products.
The quantity of secondary products formed (essentially naphthenes, paraffins, 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).
An examination of the prior art documents reveals that recycling certain constituents contained in the effluent from the isomerisation reactor to the inlet of that reactor to minimise the production of secondary products has been envisaged.
As an example, U.S. Pat. No. 3,553,276, U.S. Pat. No. 3,558,173 and U.S. Pat. No. 4,255,606 recommend adding certain produces to the feed to be treated to reduce the loss due to secondary products.
U.S. Pat. No. 3,553,276 describes an apparatus in which toluene is recycled such that the concentration of toluene is kept at double the concentration which would be obtained without that recycling.
U.S. Pat. No. 3,558,173 describes recycling naphthenes containing eight carbon atoms produced by hydrogenation of the corresponding aromatic compounds to the reactor inlet.
In the description of U.S. Pat. No. 4,255,606, 1% to 10% by weight with respect to the total feed of an aliphatic hydrocarbon containing at least 5 carbon atoms per molecule is introduced into the reaction zone with or without adding toluene. This addition can be effected by recycling. The hydrocarbon introduced can also be a precursor of n-pentane.
SUMMARY OF THE INVENTION
The process of the invention for isomerising aromatic compounds containing eight carbon atoms per molecule comprises introducing into the reaction zone, with the feed containing the aromatic compounds to be isomerised and the hydrogen required for the reaction, a mixture containing compounds with a boiling point of about 80° C. to 135° C. comprising at least the following compounds:
at least one paraffin containing eight carbon atoms per molecule;
at least benzene;
at least toluene;
at least one naphthene containing eight carbon atoms.
This compound or these compounds are added to the fresh feed in the form of a recycle or in the form of fresh compounds, in quantities such that the percentages by weight of the added compounds with respect to the total feed which enters the reactor are as follows:
the percentage of paraffins containing eight carbon atoms per molecule is about 0.1% to 10% by weight, preferably about 0.2% to 2% by weight;
the percentage of naphthenes containing eight carbon atoms is about 0.5% to 15% by weight, preferably about 2% to 8% by weight;
the percentage of toluene is about 0.1% to 10% by weight, preferably about 0.2% to 5% by weight;
the percentage of benzene is about 01% to 10% by weight, preferably about 0.2% to 2% by weight.
The total percentage of added compounds represents about 0.8% to 20% by weight, normally about 2% to 15% by weight, with respect to the total feed entering the reactor.
Surprisingly, added this mixture to the feed to be treated reduces production of those very products. In particular, this addition reduces the production of paraffins containing eight carbon atoms per molecule, of aromatic compounds containing at least 9 carbon atoms per molecule, and can reduce or even stop the production of naphthenes containing eight carbon atoms per molecule.
Further, the process of the present invention enables the catalyst to be used under lower pressure and temperature conditions and at higher HSVs (weight of feed/weight of catalyst/hour), representing an advantage over prior art conditions.
The mixture containing paraffins containing eight carbon atoms per molecule, benzene, toluene and naphthenes containing eight carbon atoms per molecule can be obtained by recycling paraffins containing eight carbon atoms per molecule, benzene, toluene and naphthenes containing eight carbon atoms per molecule produced by the reaction. This mixture can also be obtained by specific addition of fresh products of by a combination of a recycle and adding fresh products.
The process of the invention has a number of advantages over the prior art, including a reduction in the loss of aromatic compounds containing eight carbon atoms per molecule by secondary side reactions of dismutation, transalkylation, hydrogenation and cracking.
In conventional processes for isomerising aromatic compounds containing eight carbon atoms, a mixture of xylenes and possibly ethylbenzene is brought into contact with a suitable catalyst generally containing a noble metal and a mineral support, to bring the mixture of aromatic compounds containing eight carbon atoms per molecule to a composition which is as close as possible to the composition corresponding to thermodynamic equilibrium at the temperature under consideration. The process of the present invention is suitable for application to all catalysts which are effective in isomerising a mixture of aromatic compounds containing eight carbon atoms per molecule, as well as catalysts which are effective in dealkylating isomerisation of ethylbenzene to benzene.
In the process
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
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