Chemistry of hydrocarbon compounds – Saturated compound synthesis – By isomerization
Reexamination Certificate
2001-08-24
2004-10-26
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Saturated compound synthesis
By isomerization
C585S820000, C208S134000
Reexamination Certificate
active
06809228
ABSTRACT:
The present invention relates to the production of high octane number gasoline using a process combining at least one hydroisomerisation section and at least one section for separation by adsorption in which the adsorbent is a microporous solid zeolite with a mixed structure, with channels with distinct sizes.
More precisely, the process of the invention can produce a high octane number gasoline stock that forms part of the composition of the gasoline pool.
The quality of a gasoline is partially dependent on its octane number. From the point of view of the octane number, the hydrocarbons constituting the gasoline are preferably as highly branched as possible as shown by the research octane number (RON) and motor octane number (MON) of different hydrocarbon compounds (see table below).
Mono
Mono
Di
Di
Tri
Paraffins
nC8
nC7
C7
C6
C6
C5
C4
Tri C5
RON
<0
0
21-
42-
55-
80-
112
100-109
27
52
76
93
MON
<0
0
23-
23-
56-
84-
101
96-100
39
39
82
95
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-C10 cuts to be upgraded by methods other than reforming.
To this end, the production of multibranched isomers from slightly branched heptanes and octanes contained in naphthas, instead of producing toluene and xylenes from those compounds, appears to be a promising route. This justifies the search for catalysts that perform well for heptane isomerisation (also known as hydroisomerisation when carried out in the presence of hydrogen), octanes and, more generally, C5-C8 cuts and intermediate cuts, and the search for processes that can selectively recycle the low octane number compounds, namely linear and monobranched paraffins to the isomerisation (hydroisomerisation) stage.
In order to selectively recycle the linear and monobranched paraffins to the hydroisomerisation stage and to recover multibranched paraffins with a high octane number, to introduce them into the composition of the gasoline pool, the multibranched paraffins must be separated at least once. A separation unit, producing at least two distinct effluents, one with a high octane number and the other with a low octane number, and integrated into a process also comprising at least one hydroisomerisation unit to recycle the low octane number effluent to the hydroisomerisation unit, which converts linear paraffins and monobranched paraffins with a low octane number to multibranched paraffins with a high octane number.
The main difficulty in carrying out such a process combining hydroisomerisation and separation steps is separating the multibranched paraffins.
PRIOR ART
Techniques for separation by adsorption, using molecular sieves that are selective because of the dimensions of their accessible pores, are particularly suitable for separating linear, monobranched and multibranched paraffins. Conventional adsorption processes can result in procedures of the PSA (pressure swing adsorption), TSA (temperature swing adsorption), chromatographic (elution chromatography or simulated counter-current) type, for example. They can also result in a combination of these procedures. Such processes all bring a liquid or gaseous mixture into contact with a fixed bed of adsorbent to eliminate certain constituents of the mixture that may be adsorbed. Desorption can be carried out by different means. The common characteristic of PSA is that the bed is regenerated by de-pressurisation and in some cases by a low pressure flush. PSA type processes have been described in U.S. Pat. No. 3,430,418 or in the more general work by Yang (“Gas separation by adsorption processes”, Butterworth Publishers, US, 1987) In general, PSA type processes are operated sequentially and using all the adsorption beds in alternation. Such PSA processes have been successful in the natural gas field, for separating the compounds in air, for producing solvent and in different refining sectors.
TSA processes use temperature as the desorption driving force and were the first adsorption processes to be developed. The bed to be regenerated is heated by circulating a preheated gas in an open or closed loop in the reverse direction to that of the adsorption step. A number of variations of the schemes (“Gas separation by adsorption processes”, Butterworth Publishers, US, 1987) are used depending on local constraints and on the nature of the gas employed. This technique is generally used in purification processes (drying, gas and liquid desulphurisation, natural gas purification: U.S. Pat. No. 4,770,676).
Gas or liquid phase chromatography is a highly effective separation technique because of the very large number of theoretical plates (Belgian patent BE 891 522, Seko M., Miyake J., Inada K.; Ind. Eng. Chem. Prod. Res. Develop., 1979, 18, 263). It means that relatively low adsorption selectivities can be employed and difficult separations can be carried out. The competition from simulated moving bed or simulated counter-current processes for these processes is stiff. These latter processes have been developed to a great extent in the petroleum industry (U.S. Pat. No. 3,636,121, U.S. Pat. No. 3,997,620 and U.S. Pat. No. 6,069,289). The adsorbent is regenerated using the technique for displacement by a desorbent, which can optionally be separated by distillation of the extract and raffinate.
Linear, monobranched and multibranched paraffins can be separated by adsorption by different techniques that are well known to the skilled person: separation by thermodynamic adsorption difference, and separation by differences in the adsorption kinetics of the species to be separated. Depending on the technique used, the selected adsorbent 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
Ducreux Olivier
Jolimaitre Elsa
Griffin Walter D.
Institut Francais DuPetrole
Millen White Zelano & Branigan P.C.
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