Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2001-03-12
2003-06-17
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S643000, C585S646000, C585S647000
Reexamination Certificate
active
06580009
ABSTRACT:
The present invention relates to a process for converting olefinic C
4
-hydrocarbons, for example from steam crackers or FCC plants, into propene and hexene by means of a metathesis reaction.
Steam crackers represent the main source of basic petrochemicals such as ethene, propene, C
4
-olefins and higher hydrocarbons. In the cracking process, it is necessary to transfer large quantities of energy at high temperatures in a short time span which is sufficient to carry out cracking but does not permit further reaction of the cracking products. In the cracking of hydrocarbons, the yield of ethene and propene is therefore determined essentially by
the type of hydrocarbons used (naphtha, ethane, LPG, gas oil, or the like)
the cracking temperature,
the residence time
and the partial pressures of the respective hydrocarbons.
The highest yield of ethene and propene is achieved at cracking temperatures in the range from 800 to 850° C. and residence times of from 0.2 to 0.5 s. The main product in this range is always ethene, with the C
3
/C
2
product ratio of from about 0.5 to 0.7 being able to be increased slightly by varying the cracking conditions. The worldwide demand for propene is increasing more rapidly than that for ethene. This has the consequence, inter alia, that processes for downstream utilization of the higher hydrocarbons formed in the cracking process, e.g. C
4
, with a view to optimizing the propene yield are becoming increasingly important. A suitable process is olefin metathesis.
Olefin metathesis (disproportionation) is, in its simplest form, the reversible, metal-catalyzed transalkylidenation of olefins by rupture and reformation of C═C double bonds according to the following equation:
In the specific case of the metathesis of acyclic olefins, a distinction is made between self-metathesis in which an olefin is transformed into a mixture of two olefins having differing molar masses (for example: propene→ethene+2-butene) and cross-metathesis or cometathesis which is a reaction of two different olefins (propene+1-butene→ethene+2-pentene). If one of the reactants is ethene, this is generally referred to as an ethenolysis.
Suitable metathesis catalysts are, in principle, homogeneous and heterogeneous transition metal compounds, in particular those of elements of transition groups VI to VIII of the Periodic Table of the Elements, and also homogeneous and heterogeneous catalyst systems in which these compounds are present.
Different metathesis processes starting from C
4
streams have been described.
U.S. Pat. No. 5,057,638 relates to a process for preparing 1-hexene, comprising the process steps:
a) metathesis of 1-butene to give a mixture of 3-hexene and ethene,
b) separation of the 3-hexene from the product mixture obtained in step a),
c) reaction of the 3-hexene with an electrophile containing reactive hydrogen, preferably derived from water or carboxylic acid, under acid conditions which allow the addition of the electrophilic component to the C═C bond (e.g. hydration),
d) cracking of the product from step c), e.g. by dehydration, to produce a mixture of n-hexenes in which 1-hexene is present in economically acceptable amounts.
EP-A-0 742 195 relates to a process for converting C
4
or C
5
fractions into ethers and propylene. In the process starting from C
4
fractions, diolefins and acetylenic impurities present are firstly selectively hydrogenated, with the hydrogenation being associated with an isomerization of 1-butene to 2-butene. The yield of 2-butenes is said to be maximized in this way. The ratio of 2-butene to 1-butene after the hydrogenation is about 9:1. This is followed by etherification of the resulting isoolefins, with the ethers being separated from the C
4
fraction. Oxygen-containing impurities are then separated off. The resulting product, which comprises predominantly 2-butene in addition to alkanes, is then reacted with ethylene in the presence of a metathesis catalyst in order to obtain a reaction product comprising propylene as product. The metathesis is carried out in the presence of a catalyst comprising rhenium oxide on a support.
DE-A-198 13 720 relates to a process for preparing propene from a C
4
stream. Here, butadiene and isobutene are firstly removed from the C
4
stream. Oxygen-containing impurities are then separated off and a two-stage metathesis of the butenes is carried out. Firstly, 1-butene and 2-butene are converted into propylene and 2-pentene. The 2-pentene obtained is then reacted further with added ethylene to form propylene and 1-butene.
DE-A-199 32 060, which has earlier priority but is not a prior publication, relates to a process for preparing C
5
-/C
6
-olefins by reaction of a feed stream comprising 1-butene, 2-butene and isobutene to give a mixture of C
2-6
olefins. In this process, propene, in particular, is obtained from butenes. In addition, hexene and methylpentene are discharged as product. No ethene is added in the metathesis. If desired, ethene formed in the metathesis is returned to the reactor.
The world market prices of ethene and propene are subject to change. In addition, there is a demand for pentene and hexene olefin fractions which can be used as inexpensive alternative raw materials for plasticizer alcohols or surfactant alcohols. To be able to react to the price changes on the world market, there is a need for a process for the flexible preparation of propene and hexene which allows the product spectrum obtained to be appropriately matched to the price difference between ethene and propene. For example, it should be possible to obtain pentene and hexene olefin fractions and also propene flexibly using smaller to larger amounts of ethene.
It is an object of the present invention to provide a flexibly controllable catalytic process for obtaining propene and hexene from inexpensive olefin-containing C
4
-hydrocarbon mixtures. It should be possible to make a very flexible ethene addition by means of which the relative amounts of products obtained, in particular propene and hexene, can be influenced. In this way, the addition of value to steam cracker by-products should be improved, with the products having the greatest added value being able to be obtained.
We have found that this object is achieved by a process for preparing propene and hexene from a raffinate II feed stream comprising olefinic C
4
-hydrocarbons, which comprises
a) a metathesis reaction in which butenes present in the feed stream are reacted with ethene in the presence of a metathesis catalyst comprising at least one compound of a metal of transition groups VIb, VIIb or VIII of the Periodic Table of the Elements to give a mixture comprising ethene, propene, butenes, 2-pentene, 3-hexene and butanes, using, based on the butenes, from 0.05 to 0.6 molar equivalents of ethene,
b) firstly fractionally distilling the product stream obtained in this way to give a low-boiling fraction A comprising C
2
-C
3
-olefins and a high-boiling fraction comprising C
4
-C
6
-olefins and butanes,
c) subsequently fractionally distilling the low-boiling fraction A obtained from b) to give an ethene-containing fraction and a propene-containing fraction, with the ethene-containing fraction being returned to the process step a) and the propene-containing fraction being discharged as product,
d) subsequently fractionally distilling the high-boiling fraction obtained from b) to give a low-boiling fraction B comprising butenes and butanes, a pentene-containing intermediate-boiling fraction C and a hexene-containing high-boiling fraction D,
e) where the fractions B and C are completely or partly returned to the process step a) and the fraction D is discharged as product.
The individual streams and fractions can comprise or consist of the compounds mentioned. In the case of their consisting of the streams or compounds, the presence of relatively small amounts of other hydrocarbons is not ruled out.
In this process, in a single-stage reaction procedure, a fraction comprising C
4
-olefins, preferably n-butenes and butanes, is reacte
Huber Sylvia
Schulz Ralf
Schwab Peter
BASF - Aktiengesellschaft
Dang Thuan D.
Keil & Weinkauf
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