Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
2001-08-24
2004-08-31
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S822000, C585S825000, C585S826000, C585S827000, C585S831000
Reexamination Certificate
active
06784334
ABSTRACT:
The present invention relates to separating multibranched paraffins using at least one separation unit functioning by adsorption in which the adsorbent is a microporous zeolitic solid with a mixed structure, with channels with distinct sizes.
More particularly, the invention relates to the field of isomerisation of gasolines to improve their octane number. From the point of view of octane number, the hydrocarbons constituting the gasoline are preferably as highly branched as possible. Thus, a gasoline containing dimethylbutanes has a better octane number than a gasoline containing methylpentanes.
A number of techniques have been proposed in order to increase the octane number of a gasoline. Firstly, aromatic compounds, principal constituents of reforming gasolines, and isoparaffins produced by aliphatic alkylation or isomerisation of light gasolines, have compensated for the loss of octane number resulting from removing lead from gasolines, this removal being as a result of environmental constraints that are constantly being tightened up. As a result, oxygen-containing compounds such as methyl tertiobutyl ether (MTBE) or ethyl tertiobutyl ether (ETBE) have been introduced into the fuels. More recently, the acknowledged toxicity of compounds such as aromatics, in particular benzene, olefins and sulphur-containing compounds, and the desire to reduce the vapour pressure of gasolines, have resulted in the production of reformulated gasoline. As an example, since Jan. 1, 2000, the maximum olefin content, total aromatics content and benzene content of gasoline distributed in France are respectively 18% by volume, 42% by volume and 1% by volume. “Gasoline pools” comprise a number of components. The major components are reforming gasoline, which normally comprises 60% to 80% by volume of aromatic compounds, and FCC gasolines, which typically contain 35% by volume of aromatics but supply the majority of the olefinic and sulphur-containing compounds present in the gasoline pool. The other components can be alkylates, with neither aromatic compounds nor olefins, light isomerised or non isomerised gasolines, which contain no unsaturated compounds, oxygen-containing compounds such as MTBE, and butanes. Provided that the aromatics content is not reduced to below 35-40 vol %, the contribution of reformates to gasoline pools remains high, typically 40 vol %. In contrast, increased tightening of the maximum admissible aromatic compounds content to 20-25 vol % will cause a reduction in the use of reforming, and as a result will need straight run C7-C 10 cuts to be upgraded by methods other than reforming. Upgrading by hydroisomerisation is a possible route, as described in the patent application entitled “Process combining hydroisomerisation and separation using a zeolitic adsorbent with a mixed structure for the production of high octane number gasolines” deposited by the Applicant on the same day as the present application. The hydroisomerisation process results in the formation of multibranched compounds from compounds with lower octane numbers. It can only be carried out to recycle linear and monobranched C6-C10 paraffins, since the hydroisomerisation reaction is balanced and paraffins with low octane numbers cannot be sent to the “gasoline pool”
PRIOR ART
To increase the amount of multibranched paraffins in a feed from an isomerisation zone, it is possible to use molecular sieves that are selective because of the dimensions of their accessible pores.
Separation of linear, monobranched and multibranched paraffins by adsorption can be carried out by two different techniques that are well known to the skilled person: separation by differences in thermodynamic adsorption and separation by differences in the adsorption kinetics of the species to be separated. Depending on the technique employed, the adsorbent selected will have different pore diameters. Zeolites, composed of channels, are the adsorbents of choice to separate such paraffins.
The term “pore diameter” is known to the skilled person. It is used as a functional definition of pore size in terms of the size of the molecule that can enter into the pore. It does not define the actual dimension of the pore as that is often difficult to determine, since it often has an irregular shape (i.e., non circular). D. W. Breck provides a discussion on effective pore diameter in the book entitled “
Zeolite molecular sieves
(John Wiley & Sons, New York, 1974) on pages 633 to 641. The cross sections of the zeolite channels are rings of oxygen atoms, so the zeolite pore size can also be defined by the number of oxygen atoms forming the annular cross section of the rings, termed “member rings”, MR. This is shown, for example, in “The atlas of zeolite structure types”, W. M. Meier and D. H. Olson, 4
th
edition, 1996), which indicates that FAU structure type zeolites have a crystal channel network of 12 MR, i.e, the cross section is constituted by 12 oxygen atoms. This definition is well known to the skilled person and will be used below.
In the case of “thermodynamic” separation, the adsorbent has a pore diameter that is higher than the critical diameter of the molecules to be separated. A number of patents describe the separation of multibranched paraffins from linear and monobranched paraffins by selective thermodynamic adsorption. U.S. Pat. No. 5,107,052 proposes preferably adsorption of multibranched paraffins on SAPO-5, AIPO-5, SSZ-24, MgAPO-5 or MAPSO-5 zeolites. U.S. Pat. No. 3,706,813 proposes the same type of selectivity on barium-exchanged X or Y zeolites. U.S. Pat. No. 6,069,289, on the other hand, proposes the use of zeolites with selectivities that are inversely proportional to the degree of branching of the paraffins, such as beta, X or Y zeolites exchanged with alkali or alkaline-earth cations, SAPO-31, MAPO-31 zeolites. All of the zeolites cited above have pore diameters of 12 MR
In the case of “diffusional” separation, the separating power of the adsorbent is due to the difference in the diffusion kinetics of the molecules to be separated in the zeolite pores. In the case of separation of multibranched paraffins from monobranched and linear paraffins, the fact that the higher the degree of branching, the higher the kinetic diameter of the molecule, and thus the slower the diffusion kinetics, can be exploited. For the adsorbent to have a separating power, the adsorbent must have a pore diameter close to that of the molecules to be separated, which corresponds to zeolites with a pore diameter of 10 MR. Many patents describe the separation of linear, monobranched and multibranched paraffins by diffusional selectivity. U.S. Pat. Nos. 4,717,784, 4,804,802, 4,855,529 and 4,982,048 use adsorbents with channel sizes between 8 and 10 MR, the preferred adsorbent being ferrierite. U.S. Pat. No. 4,982,052 recommends the use of silicalite. U.S. Pat. Nos. 4,956,521, 5,055,633 and 5,055,634 describe the use of zeolites with elliptical cross section pores with dimensions in the range 5.0 to 5.5 Å along the minor axis and about 5.5 to 6.0 Å along the major axis, in particular ZSM-5 and its dealuminated form, or silicalite or with dimensions in the range 4.5 to 5.0 Å, in particular ferrierite, ZSM-23 and XZSM-11.
The zeolitic adsorbents proposed for diffusional separation of multibranched paraffins have a homogeneous channel size structure and are only composed of small channels (8 to 10 MR), which considerably reduces their adsorption capacity. Such materials, which suffer primarily from their low adsorption capacity, cannot result in optimum efficiency of the separation unit.
SUMMARY OF THE INVENTION
The present invention is based on the novel use of zeolitic adsorbents with a mixed structure, composed of two channel types with distinct sizes, in a section for separating multibranched paraffins comprised in a hydrocarbon feed comprising hydrocarbons containing 5 to 8 carbon atoms per molecule, in particular linear, monobranched and multibranched paraffins. The process of the invention comprises at least one separation unit function
Ducreux Olivier
Jolimaitre Elsa
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Nguyen Tam M.
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