Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2000-12-26
2004-02-03
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S671000, C585S332000, C585S644000, C585S646000, C585S647000, C585S259000, C585S262000
Reexamination Certificate
active
06686510
ABSTRACT:
The invention relates to a process for production of high-purity isobutene and propylene from a 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. On the contrary, interest was rekindled for isobutene, which can be used as, for example, a monomer in the synthesis of polyisobutene.
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 separation of isobutene by means of a catalytic distillation in which the butene-1 that is impossible to separate from the isobutene is isomerized in butenes-2, and that makes it possible to transform the butadiene-1,3 and the n-butenes into propylene that can be used for, for example, polymerization.
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 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 an olefinic C
4
fraction into high-purity isobutene and into propylene, whereby said fraction contains in particular diolefins, butene-1, butenes-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 in the presence of a catalyst, in order to obtain an effluent that contains n-butenes, whose ratio corresponds to the thermodynamic equilibrium and isobutene, and that contains virtually no diolefins or acetylenic compounds;
2) the separation, by distillation, of a top fraction that contains isobutene and a bottom fraction that contains essentially butenes-2 and butane; and
3) the metathesis of the butenes-2 fraction that is obtained from the preceding stage with the ethylene, in the presence of a catalyst, so as to obtain an effluent that contains propylene, whereby the metathesis is followed by a separation of the propylene;
whereby said process is characterized in that stage 2 is implemented in a column that integrates the hydroisomerization of butene-1 that remains in butenes-2, and in that the top fraction contains essentially the starting isobutene that is free of butene-1.
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 isobutene and propylene, will be described in more detail below.
The main object of the first stage is to transform the butadiene and the butene-1 into butenes-2. Actually, the butenes-2 are the source of the propylene that is produced in the last stage of metathesis in the presence of ethylene. It is therefore desirable to increase as much as possible 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 therefore carried out simultaneously in the presence of hydrogen:
the selective hydrogenation of butadiene into a mixture of n-butenes at thermodynamic equilibrium;
the isomerization of butene-1 into butenes-2 to obtain a distribution that is close to the thermodynamic equilibrium of the n-butenes, 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, and to promote the hydroisomerization of the n-butenes (from butene-1 into butenes-2). The catalyst preferably contains 0.05 to 10% by weight of sulfur. Advantageously, a catalyst is used that consists of 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, i.e., the catalyst was treated, before being 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.
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 advantageously to have only a slight hydrogen excess relative to the stoichiometry.
The operating conditions are selected such that the reagents and the products are in liquid phase. It may be advantageous, however, to select an operating mode such that the products are partially evaporated at the outlet of the reactor, which facilitates the thermal monitoring of the reaction. The temperature may vary from 20 to 200° C., preferably from 50 to 150° C. or better from 60 to 100° C. The pressure may be adjusted to a value of 0.1 to 5 MPa, preferably 0.5 to 4 MPa, and advantageously from 0.5 to 3 MPa, such
Commereuc Dominique
Didillon Blaise
Olivier-Bourbigou Helene
Saussine Lucien
Griffin Walter D.
Institut Français du Pétrole
Millen White Zelano & Branigan P.C.
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