Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2002-03-07
2004-08-17
Dang, Thuan D (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S330000, C585S643000, C585S646000, C585S647000
Reexamination Certificate
active
06777582
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the processing of a C
3
to C
6
hydrocarbon cut from a cracking process, such as steam or fluid catalytic cracking, primarily for conversion of C
4
and C
5
olefins to propylene via auto-metathesis.
In typical olefin plants, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C
2
acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C
3
to C
6
. This mixture is separated into different carbon numbers typically by fractionation.
The C
3
cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene or for chemical synthesis such as propylene oxide, cumene, or acrylonitrile. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C
3
compounds end up as propylene thereby increasing the yield.
The C
4
cut consisting of C
4
acetylenes, butadiene, iso and normal butenes, and iso and normal butane can be processed in many ways. A typical steam cracker C
4
cut contains the following components in weight %:
C
4
acetylenes
trace
butadiene
33%
1-butene
15%
2-butene
9%
isobutene
30%
iso & normal butane
13%
Conventionally, it is common for some of the products of the stream to be separated and the balance recycled back to the olefins unit for pyrolysis or sent offsite as an olefinnic product. The C
4
acetylenes are first removed by selective hydrogenation followed by butadiene extraction. Alternately they are hydrogenated along with butadiene to form butenes. Isobutene can be removed by fractionation, by reaction to methyl tertiary butyl ether using methanol, or by reaction with itself and normal butenes in a catalytic C
4
dimerization unit. If the stream is to be recycle cracked, the butenes are further hydrogenated to butanes. An alternative processing option is metathesis. As practiced commercially in several units, conventional metathesis involves the reaction of normal butenes with ethylene to form propylene. The isobutene is typically removed before metathesis with ethylene. Isobutene does not react with ethylene or 2 butene under metathesis conditions. Thus isobutene will build up in the system as the C
4
fraction is recycled to obtain higher conversions. Isobutylene does however react with product propylene to form ethylene and 2 methyl-2-butene. In many cases this is not desired since it reduces propylene production. Typically after butadiene hydrogenation to normal butenes, over 50% of this stream is linear olefins.
The bottoms from the isobutene fractionation containing primarily the 1-butene and 2-butene are mixed with excess ethylene and passed through the metathesis or olefin conversion reacting step. In this conversion reaction step, the primary reaction is:
2-butene+ethylene→2 propylene
The unconverted butenes from the reaction are recycled to obtain a net high conversion of the butenes to propylene.
Typical molar ratios of ethylene/butenes are 1.5 or higher for metathesis with ethylene. Excess ethylene reduces the potential for the butenes to react with themselves thereby reducing the selectivity for propylene formation. The theoretical minimum ethylene required for maximum propylene is 1 mol/mol of 2-butene. The high concentrations of ethylene minimize the non-selective, in terms of propylene, reactions of the butenes with themselves by auto-metathesis. These reactions are shown below:
1-butene+2-butene→propylene+2 pentene
1-butene+1-butene→ethylene+3 hexene
2-butene+2-butene→no reaction
As can be seen, instead of 1 mol of butenes forming 1 mol of propylene and 1 mol of ethylene forming the other mol of propylene, in these auto-metathesis reactions, 2 mols of butene form less than 1 mol of propylene. In spite of the lower selectivity to propylene, this may be an economically desirable route dependent upon the relative values of feeds and products since ethylene is historically higher valued than propylene or butenes. Note however, when the metathesis reaction utilizes ethylene as a co-feedstock, the product of the C
5
and C
6
normal olefins are reduced.
The C
5
and heavier stream from the steam cracker is typically used in the production of gasoline but sometimes the C
5
's are separated and recycled to the cracking heaters. A typical steam cracker C
5
stream contains the following components in weight %:
pentanes
40%
1-pentene
5%
2-pentene
5%
isopentene
7%
cyclopentene
3%
cyclopentadiene
18%
n-pentadienes
8%
Isoprene
14%
This C
5
stream contains considerably lower amounts of linear components than the C
4
stream. After n-pentadiene hydrogenation, only about 20% of this stream is linear olefins. If the n-pentenes are processed through metathesis, the reactions are:
2-pentene+ethylene→propylene+1-butene
1-pentene+ethylene→no reaction
The C
5
stream and the C
6
stream are conventionally sent as a bottom product from a fractionation tower to gasoline. In some cases, after hydrogenation, the C
5
stream separated by fractionation and is recycled back to the cracking heaters. The C
6
+ stream after C
5
separation is typically sent to gasoline blending since it contains higher octane value aromatics such as benzene in addition to non-aromatic compounds.
For metathesis reactions, the catalyst is typically an oxide of Group VI B or Group VII B metals supported on either alumina or silica supports. In some cases, this oxide is physically admixed with a double bond isomerization catalyst such as MgO. In the reactor, the 2-butene and ethylene are metathesised to propylene. The 1-butene does not react with ethylene. The isomerization catalytic activity incorporated allows 1-butene to be isomerized to 2-butene which is then reacted with the ethylene. The effluent containing propylene, unreacted ethylene and butenes and some C
5
and heavier products is first passed through a deethylenizer for removal of that unreacted ethylene and then to a depropylenizer where product propylene is removed overhead. The bottoms may be sent to a debutylenizer where unreacted C
4
s are recovered and recycled. The C
5
and heavier fraction is typically sent to gasoline blending. Alternately, a C
4
stream is withdrawn from the depropyleneizer above the bottoms and recycled with the net bottoms of C
5
and heavier again being sent to gasoline blending.
In the conventional process for the metathesis of butenes to propylene such as generally described above, there are several problems or disadvantages. First, the reaction takes place with ethylene which not only consumes a valuable olefin but requires recovery for the excess using energy intensive refrigeration systems and then recirculation requiring compression. Secondly, to prepare the feed, there is a separate fixed bed hydrogenation units for butadiene. In the butadiene hydrogenation step, if high 2-butene concentrations are desired, additional hydrogenation is specified in order to maximize the hydroisomerization of 1-butene to 2-butene. High 2-butene concentration is desired because the reaction of 1-butene with ethylene will not occur and thus the 1-butene must be isomerized to 2-butene within the reaction bed itself by a double bond isomerization catalyst such as MgO. In the hydroisomerization of 1-butene to 2-butene in the selective butadiene hydrogenation unit, there is a substantial loss (10+%) of butenes to paraffins due to the added hydrogen which represents a considerable feed loss to the metathesis conversion step. Further, if fractionation is employed for the isobutene removal step, there is an additional loss of butenes since 1-butene is difficult to separate from isobutene without a very expensive fractionation tower.
In the prior U.S. patent application Ser. No. 09/769,871 filed Jan. 25, 2001, an improved process is disclosed and cla
Gartside Robert J.
Greene Marvin I.
Jones Quincy J.
ABB Lummus Global Inc.
Alix Yale & Ristas, LLP
Dang Thuan D
LandOfFree
Process for producing propylene and hexene from C4 olefin... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing propylene and hexene from C4 olefin..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing propylene and hexene from C4 olefin... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3283865