Selective hydrogenation process comprising partial...

Details Selective hydrogenation process comprising partial... Selective hydrogenation process comprising partial...

2001-03-08

2002-06-25

Griffin, Walter D. (Department: 1764)

Chemistry of hydrocarbon compounds

Adding hydrogen to unsaturated bond of hydrocarbon, i.e.,...

Hydrocarbon is contaminant in desired hydrocarbon

C585S260000, C585S262000, C585S264000, C585S265000, C585S324000, C585S818000, C208S092000, C208S144000, C208S145000

Reexamination Certificate

active

06410811

ABSTRACT:

The present invention relates to a process for selective hydrogenation of a hydrocarbon cut in a reactive column. The feed for the process of the invention essentially comprises hydrocarbons containing 1 to 6 carbon atoms, and hydrogen and possibly C
6
+
hydrocarbons, containing 6 or more carbon atoms. Said process can hydrogenate acetylenic compounds, and di- and poly-olefins, without significantly affecting the mono-olefins present in the feed.
PRIOR ART
A number of types of conversion process have been developed to produce unsaturated compounds from the hydrocarbons contained in petroleum cuts or in natural gas. In particular, they are steam cracking, catalytic cracking (FCC), visbreaking, coking or pyrolysis processes. Such processes produce gaseous or liquid hydrocarbons with a degree of unsaturation that increases as the treatment temperature is raised. At the end of such processes, a mixture of hydrocarbons containing mono-olefinic, di-olefinic or poly-olefinic and acetylenic compounds is obtained in variable proportions, possibly along with hydrogen in varying amounts.
In order to obtain the olefins desired to supply petrochemical or fine chemical processes, the effluents from such processes have to undergo a hydrogenation treatment to selectively hydrogenate the di- or poly-olefins and acetylenic compounds and to avoid hydrogenating mono-olefins as much as possible.
The selective hydrogenation step is generally carried out after fractionating said effluents into a plurality of cuts. Each of the separated cuts is then hydrogenated separately in a specific reactor. Thus International patent application WO-A-96/06900 describes a process for selective hydrogenation of cracked gas, in which the gas from a steam cracker is fractionated in order to remove the methane (C
1
) then the C
2
and C
3
compounds (i.e., compounds containing 2 or 3 carbon atoms per molecule). The fraction containing C
4
compounds (compounds containing 4 carbon atoms) and C
5
+
compounds (compounds containing 5 carbon atoms or more) is then hydrogenated and a fraction of the hydrogenated effluent is recycled to the fractionation section.
In general, the hydrogen necessary for the hydrogenation reaction and the feed containing the hydrocarbons to be hydrogenated arrive at the hydrogenation reactor separately. International patent application WO-A-95/15934 describes a process in which the stream of hydrogen on the one hand and a stream of hydrocarbons on the other hand are separately supplied to a hydrogenation reactor. Those two separate streams can optionally be mixed just before they enter the reactor.
U.S. Pat. No. 5,679,241 describes a process in which a feed containing all of the C
2
to C
6
hydrocarbons, heavier unsaturated hydrocarbons and hydrogen is sent to a reactive column for hydrogenation, with no prior hydrogen separation. The excess hydrogen over the quantity necessary for the reaction traverses the catalytic column without being converted then is separated downstream in a hydrogen recovery unit.
Thus, the prior art describes processes in which all of the hydrogen is separated from a gas stream either upstream or downstream of a hydrogenation unit.
Complete upstream separation requires cryogenic type units that are very expensive. Downstream separation involves the excess hydrogen circulating in the hydrogenation reactor. That excess hydrogen causes risks of runaway and renders control of the reaction more complex.
SUMMARY OF THE INVENTION
The present invention concerns a process for selective hydrogenation of a hydrocarbon cut in a reactive column (also termed a catalytic column). The feed for the process of the invention essentially comprises hydrocarbons containing 1 to 6 carbon atoms and possibly C
6
+
hydrocarbons. Said process can hydrogenate acetylenic compounds, and di- and poly-olefins, without significantly affecting the mono-olefins present in the feed. Said feed also comprises hydrogen in varying quantities, depending on the upstream process from which it originates (for example a steam cracking process or a thermal cracking process or catalytic cracking process or a pyrolysis process).
In the process of the invention, partial hydrogen separation is carried out upstream of the selective hydrogenation. Preferably, only a quantity substantially equal to or slightly greater than the quantity of hydrogen necessary for the reaction is left in the feed. This partial separation is preferably carried out using a membrane, rather than cryogenically. Cryogenic separation is better suited to substantially complete separation of hydrogen from the feed and investments are much higher.
Separating a fraction of the hydrogen present in the feed means that investments are much lower and the partial pressure of the hydrogen can be reduced, thus enabling the hydrogenation reaction to be better controlled. In the case of hydrogenation of light hydrocarbons, in particular C
2
or C
3
hydrocarbons, too high a partial hydrogen pressure can result in runaway and cause the temperatures to become very high, as well as causing hydrogenation of the mono-olefins present in the feed.
DETAILED DESCRIPTION OF THE INVENTION
The present invention concerns selective hydrogenation of a feed composed of C
2
+
hydrocarbons (hydrocarbons containing at least 2 carbon atoms per molecule), i.e., hydrogenation of acetylenic compounds or di- or poly-olefins contained in said feed.
The process of the invention is a process for selective hydrogenation of a hydrocarbon feed containing hydrogen and C
2
+
hydrocarbons, characterized in that it comprises at least one step for separating a fraction of the hydrogen contained in the feed by means of a membrane (step a)) and a step for selective hydrogenation of the effluent from step a) in a reactive column (step b)).
The feed for the process of the invention is a hydrocarbon feed also containing hydrogen and preferably originating from a steam cracking process, a heat cracking process, a catalytic cracking process or from a pyrolysis process.
The term “C
n
hydrocarbons” or “C
n
cut” will henceforth define a mixture of hydrocarbons containing n carbon atoms per molecule, and a “C
n
-C
m
cut” will define a mixture of hydrocarbons containing n to m carbon atoms per molecule. As an example, a C
1
-C
6
cut contains hydrocarbons containing 1 to 6 carbon atoms per molecule.
Preferably, the feed for the process of the invention is a C
1
-C
6
cut (i.e., a cut containing hydrocarbons containing 1 to 6 carbon atoms per molecule). Said feed can also be selected from C
1
-C
2
, C
1
-C
3
, C
1
-C
4
, C
1
-C
5
, C
1
-C
6
, C
2
-C
3
, C
2
-C
4
, C
2
-C
5
, C
2
-C
6
, C
3
-C
4
, C
3
-C
5
, C
3
-C
6
, C
4
-C
5
, C
4
-C
6
, C
5
-C
6
cuts or a mixture of said cuts, when said cuts or said mixture also contain hydrogen, optionally added or already present in said cut or said mixture. It is also possible to treat in the process of the invention any feed containing hydrogen and at least 2 cuts selected from the group formed by C
2
, C
3
, C
4
, C
5
or C
6
cuts.
Further, said feed can optionally contain C
6
+ hydrocarbons, preferably in an amount of less than 20% by weight, and methane (C
1
). Preferably, the feed for the process of the invention contains hydrogen, methane, C
2
-C
6
hydrocarbons and C
6
+ hydrocarbons. Highly preferably, said feed is from a steam cracking process or from thermal cracking or from a pyrolysis process.
Routinely used process layouts for steam cracking processes are described in “Ullman's Encyclopedia of Industrial Chemistry”, fifth edition, volume A10, pages 77 and 80. When the feed is from a steam cracking process, the hydrogenation process of the invention is preferably installed after the caustic soda and compression steps, more preferably after the cracked gas drying step. Such preferred implementations of the invention allow membrane separation and hydrogenation at a higher pressure.
It is also optionally possible to install the process of the invention after the de-ethaniser, when the latter is placed upstr

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