Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2000-01-14
2001-04-17
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S654000, C585S655000, C585S314000, C585S315000, C585S258000, C585S259000
Reexamination Certificate
active
06218589
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of art to which this invention pertains is the production of high purity propylene and a method for improving the operation of a propane-propylene splitter.
INFORMATION DISCLOSURE
Processes for the production of olefin hydrocarbons are very useful in the production of a great number of petrochemical products as well as motor fuel blending components. The short chain paraffins having from 2 to 5 carbon atoms per molecule are often subjected to dehydrogenation to form the corresponding olefin. The normal paraffin hydrocarbon having 2 to 5 carbon atoms per molecule is preferably selected from the group consisting of ethane, propane, butane and pentane. The olefins, in turn, are used, for example, in the alkylation of isoparaffins or in the etherification of alcohols to make motor fuel blending components.
U.S. Pat. No. 4,523,045 (Vora) discloses a process for the production of linear olefinic hydrocarbons wherein a feed stream comprising at least one C
4
to C
20
feed paraffinic hydrocarbon is dehydrogenated in a dehydrogenation zone and the resulting effluent is selectively hydrotreated in order to selectively hydrogenate diolefins and the resulting stream is stripped to remove light hydrocarbon gases and is introduced into an adsorptive separation zone to produce a stream comprising olefins and a stream containing paraffins and trace quantities of diolefins and other highly dehydrogenated hydrocarbon compounds. The resulting recovered paraffin stream from the adsorptive separation zone is then admixed with fresh feed and directly introduced into the paraffin dehydrogenation zone. The trace quantities of diolefins and methyl acetylene compounds are rapidly converted to coke in the dehydrogenation zone and this resulting coke is deposited on the charge heater tubes and dehydrogenation catalyst which shortens the life of the catalyst. This patent fails to disclose the recovery of a stream containing an enriched concentration of methyl acetylene and propadiene from a propane-propylene splitter column and the subsequent introduction of this stream into a selective diolefin hydrogenation zone to convert these impurities into propylene.
U.S. Pat. No. 4,761,509 (Vora et al) discloses a process for the catalytic dehydrogenation of paraffinic hydrocarbons.
U.S. Pat. No. 4,430,517 (Imai et al) discloses a catalyst for the conversion of hydrocarbons. The catalyst comprises a platinum group component, a Group IVA component, an alkali or alkaline earth component and a porous carrier material wherein the atomic ratio of the alkali or alkaline earth component to the platinum group component is more than 10. This catalyst is particularly useful for dehydrogenating paraffins having from 2 to 5 or more carbon atoms to the corresponding mono-olefins.
U.S. Pat. No. 4,438,288 (Imai et al) discloses a process for dehydrogenating hydrocarbons which comprises contacting a dehydrogenatable hydrocarbon in a dehydrogenation zone with a catalyst comprising a platinum group component, an alkali or alkaline earth component and a porous support material to produce a dehydrogenated hydrocarbon and a used catalyst; contacting the used catalyst in a catalyst regeneration zone with a halogen component to produce a regenerated catalyst containing added halogen component; and contacting a dehydrogenatable hydrocarbon in a dehydrogenation zone with the resulting regenerated catalyst to produce a dehydrogenated hydrocarbon and a used catalyst.
Currently, propane is dehydrogenated to produce propylene and hydrogen with the concomitant production of low levels of methyl acetylene and propadiene. The resulting effluent from a propane catalytic dehydrogenation reaction zone is subsequently fractionated to produce a high purity propylene product stream and a high purity stream of unconverted propane. The nature of the equilibrium of the propane and propylene with the methyl acetylene and propadiene allows the methyl acetylene and propadiene to concentrate in an identifiable central portion of the fractionator. This fractionator is frequently referred to as the propane-propylene splitter.
In the event of an upset in the fractionator, the high concentration of methyl acetylene and propadiene can impact either the marketability of the propylene product because of impurity contamination or cause severe coking if these impurities are recycled together with propane to the dehydrogenation catalyst.
In order to alleviate the buildup of highly unsaturated impurities such as methyl acetylene and propadiene, the dehydrogenation reactor effluent has been selectively hydrogenated to reduce the levels of these highly unsaturated impurities before any subsequent fractionation. Although the hydrogenation is relatively selective towards the highly unsaturated compounds, the very nature of the hydrogenation catalyst is to saturate compounds with hydrogen and if the olefin is hydrogenated there will necessarily be a loss of the desired olefin product. Therefore, the operating conditions of the selective hydrogenation zone are adjusted to balance the hydrogenation of the undesired highly unsaturated compounds including methyl acetylene and propadiene with the maximization of the olefin product.
In accordance with the present invention, it has been unexpectedly discovered that even the reduced levels of highly unsaturated compounds remaining after the selective hydrogenation can accumulate to undesirable levels in the product fractionator and still cause problems with the maximum production of high quality propylene. With the present invention, the accumulation of the highly unsaturated compounds is identified and removed from the fractionation zone and introduced into the selective hydrogenation zone to thereby permit the maximum production of high purity propylene product and to prevent excessive, premature coking of the dehydrogenation catalyst.
BRIEF SUMMARY OF THE INVENTION
It has now been found that an improved propane dehydrogenation process can be achieved by removing and recovering a small side-cut stream from the propylene product fractionator and introducing this side-cut stream containing methyl acetylene and propadiene compounds to a selective hydrogenation reaction zone. When the present invention is utilized, the high quality of the propylene product is ensured if not enhanced and any possible premature coking of the dehydrogenation catalyst is also ensured.
The invention provides a process for the dehydrogenation of propane by means of contacting the propane and hydrogen with a dehydrogenation catalyst to produce a stream containing propylene, hydrogen and trace quantities of methyl acetylene and propadiene. This resulting stream is selectively hydrogenated to hydrogenate at least a majority of the highly unsaturated impurities, i.e., methyl acetylene and propadiene without any significant hydrogenation of the desired propylene. The resulting selectively hydrotreated stream is then fractionated in a propane-propylene splitter to produce a high purity product stream containing propylene, a propane hydrocarbon stream which is recycled to the dehydrogenation zone and a small slip stream containing highly unsaturated impurities including methyl acetylene and propadiene which is introduced into the selective hydrogenation zone.
One embodiment of the present invention is a method for improving the operation of a propane-propylene splitter which method comprises the steps of: (a) contacting propane and hydrogen with a dehydrogenation catalyst at dehydrogenation conditions in a dehydrogenation zone to produce a first stream comprising propylene, propane, hydrogen and trace quantities of methyl acetylene and propadiene; (b) contacting at least a portion of the first stream comprising propylene, propane, hydrogen and trace quantities of methyl acetylene and propadiene with a selective hydrogenation catalyst at selective hydrogenation conditions in a selective hydrogenation zone to selectively saturate at least about 90% of the trace quantities of methyl acetylene and propadiene; (c) fractionating
Cutts, Jr. John G.
Dang Thuan D.
Griffin Walter D.
Spears, Jr. John F.
Tolomei John G.
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