Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2001-05-02
2004-06-01
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S646000, C585S647000, C585S671000
Reexamination Certificate
active
06743958
ABSTRACT:
The invention relates to a process for selective production of polymerization-quality propylene from an olefinic C
4
fraction.
The steam-cracking of feedstocks that consist of light paraffinic fractions produces the ethylene and the propylene that are necessary to petrochemistry. It also produces a certain number of other heavier products, and in particular a C
4
hydrocarbon fraction that contains mainly butadiene-1,3, isobutene, n-butenes and butanes, accompanied by traces of acetylenic hydrocarbons.
The catalytic cracking of heavy hydrocarbon feedstocks produces, alongside gasoline and gasoil fractions that are the main products, lighter products, including a C
4
hydrocarbon fraction that contains mainly isobutane, isobutene, n-butenes and butanes, accompanied by small amounts of butadiene-1,3 and acetylenic hydrocarbons.
Until recently, only butadiene-1,3 and isobutene were used in the polymer industry, in particular in the tire industry. The increase of the longevity of tires and a relative stagnation of the demand ensure that there is now excess butadiene that is not used or is poorly used. To date, isobutene was used, for example, for the synthesis of ethers with the use of additives in automobile fuels or as a monomer in the synthesis of polyisobutene. These uses, however, can lead to saturation and render the isobutene useless.
This invention proposes a process for treatment of a C
4
hydrocarbon fraction that contains primarily isobutene, n-butenes, butanes, and butadiene-1,3 in a variable amount that includes the skeletal isomerization of isobutene into n-butenes and that makes it possible to transform all of the C
4
unsaturated compounds into propylene that can be used for, for example, polymerization.
The fractions that are treated in the process according to the invention correspond to the C
4
fractions of conversion processes. They can correspond to, for example, the crude C
4
fraction for steam-cracking, the C
4
fraction for steam-cracking after extraction of the butadiene that is commonly called raffinate-1, or the C
4
fraction for catalytic cracking.
The relative proportions of ethylene and propylene that are produced in a steam-cracking operation can be modulated to a certain extent by changing the nature of the feedstock and by modifying the operating conditions (the degree of rigor) of the cracking. The operating method that is oriented toward a larger proportion of propylene, however, inevitably entails a decline in the yield of ethylene and a higher C
4
fraction and gasoline fraction production.
Another object of this invention is thus to increase the propylene production while maintaining a high ethylene yield with the treatment of the C
4
hydrocarbon fraction and therefore without it being necessary to reduce the rigorous conditions of the steam-cracking device.
The process that is the object of the invention is more specifically a process for converting into propylene an olefinic C
4
fraction, whereby said fraction comprises diolefins, primarily butadiene-1,3, butene-1, butene-2, isobutene and acetylenic impurities, and whereby said process comprises the following stages that take place successively:
1) the selective hydrogenation of diolefins and acetylenic impurities with isomerization of butene-1 into butenes-2, carried out in a reactor, in the presence of a catalyst, in order to obtain an effluent that contains for the most part butenes-2 and isobutene, and that contains virtually no diolefins or acetylenic compounds;
2) the separation by distillation of a top fraction that contains for the most part isobutene and unconverted butene-1 in the first stage, and a bottom fraction that contains essentially butene-2 and butane; and
4) the metathesis of the butenes-2 fraction that is obtained from stage 2 with the ethylene so as to obtain an effluent that contains propylene, whereby the metathesis is followed by a separation of the propylene;
whereby said process also comprises a stage 3 of skeletal isomerization of the isobutene into n-butenes in the top fraction, with recycling of at least a portion of the effluent in stage 1.
The isomerization of butene-1 into butenes-2 as carried out in stage 1 can also be carried out in part in association with the distillation (stage 2) by using an isomerization catalyst as described for stage 1 according to the teachings of French FR-B-2 755 130, in the name of the applicant.
The special conditions of the different stages of the process according to the invention, carried out from a C
4
hydrocarbon fraction that contains primarily isobutene, n-butenes, butanes, as well as butadiene in a variable amount, whereby said C
4
fraction is subjected to these stages to produce essentially propylene, are described in more detail below.
The main object of the first stage is to transform the butadiene and the n-butenes into butenes-2. Actually, the butenes-2 are the source of the propylene that is produced in stage 4 of metathesis in the presence of ethylene. It is therefore desirable to maximize the butenes-2 yield, i.e., to draw as close as possible to the ratio that is allowed by thermodynamics. The second object of this stage is to eliminate the acetylenic hydrocarbon traces that are always present in these fractions and that are poisons or contaminants for the subsequent stages.
In this first stage, the following reactions are thus carried out simultaneously in the presence of hydrogen:
the selective hydrogenation of butadiene into a mixture of n-butenes;
the isomerization of butene-1 into butenes-2 to obtain a distribution that is close to the thermodynamic equilibrium; and
the selective hydrogenation of the acetylenic hydrocarbon traces into butenes.
These reactions can be carried out with various specific catalysts that comprise one or more metals, for example from group 10 of the periodic table (Ni, Pd or Pt), deposited on a substrate. A catalyst that comprises at least one palladium compound that is fixed on a refractory mineral substrate, for example on an alumina, is preferably used. The palladium content in the substrate can be 0.01 to 5% by weight, preferably 0.05 to 1% by weight. Various pretreatment methods that are known to one skilled in the art optionally can be applied to these catalysts to improve the selectivity in the hydrogenation of butadiene into butenes at the expense of the total hydrogenation of butane that it is necessary to avoid. The catalyst preferably contains 0.05 to 10% by weight of sulfur. Advantageously, a catalyst is used that comprises palladium that is deposited on alumina, and sulfur.
The catalyst can advantageously be used according to the process that is described in Patent FR-B-2 708 596. According to this process, the catalyst is treated, before it is loaded into the hydrogenation reactor, by at least one sulfur-containing compound that is diluted in a solvent, then the catalyst that is obtained that contains 0.05 to 10% by weight of sulfur is loaded into the reactor and activated under a neutral atmosphere or a reducing atmosphere at a temperature of 20 to 300° C., a pressure of 0.1 to 5 MPa and a VVH of 50 to 600 h
−1
, and the feedstock is brought into contact with said activated catalyst.
The use of the catalyst, preferably with palladium, is not critical, but it is generally preferred to use at least one down-flow reactor through a catalyst fixed bed. When the proportion of butadiene in the fraction is large, which is the case, for example, of a steam-cracking fraction when it is not desired to extract the butadiene from it for specific uses, it may be advantageous to carry out the transformation in two reactors in series to better monitor the selectivity of the hydrogenation. The second reactor can have a rising flow and play a finishing role.
In some cases, it may be advisable to dilute the feedstock that is to be treated by said C
4
fraction in which the butadiene is partially or totally hydrogenated.
The amount of hydrogen that is necessary for all of the reactions that are carried out in this stage is adjusted based on the composition of the fraction
Commereuc Dominique
Travers Christine
Dang Thuan D.
Institut Francais du Pe'trole
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