Mineral oils: processes and products – Chemical conversion of hydrocarbons – With preliminary treatment of feed
Reexamination Certificate
1999-04-05
2001-07-17
Myers, Helane E. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With preliminary treatment of feed
C208S143000, C585S264000, C585S265000
Reexamination Certificate
active
06261442
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is related to applicants' concurrently filed application Attorney Docket No. Pet-1748, entitled “Process For Converting Hydrocarbons By Treatment In A Distillation Zone Comprising A Circulating Reflux, Associated With A Reaction Zone, And Its Use For Hydrogenating Benzene”, based on French Application 98/04.352 filed Apr.6, 1998, said applications being incorporated by reference herein.
The invention relates to a process for converting hydrocarbons. The process of the invention associates a distillation zone with a hydrocarbon conversion reaction zone which is at least partially external to the distillation zone. Thus this process can selectively convert hydrocarbons separated from a hydrocarbon feed by means of the distillation zone.
More particularly, the process of the invention is applicable to selective reduction of the quantity of light unsaturated compounds (i.e., containing at most six carbon atoms per molecule) comprising benzene and possibly olefins in a hydrocarbon cut essentially comprising at least 5 carbon atoms per molecule, with no substantial loss of octane number.
The general trend now is to reduce the quantity of benzenes and olefins (unsaturated compounds) in gasolines, because of their known toxicity.
Benzene has carcinogenic properties and thus the possibility of it polluting the air must be limited as far as possible, in particular by practically excluding it from automobile fuels. In the United States, reformulated fuels must not contain more than 1% by volume of benzene; in Europe, it has been recommended that a gradual decrease towards that value be made.
Olefins are known to be among the most reactive hydrocarbons in photochemical reactions with oxides of nitrogen, which occur in the atmosphere and which lead to the formation of ozone. A rise in the concentration of ozone in the air may be a source of respiratory problems. It is thus desirable to reduce the amount of olefins in gasolines, and more particularly of the lightest olefins which have the greatest tendency to vaporise when manipulating a fuel.
The benzene content of a gasoline is very largely dependent on that of the reformate component in that gasoline. The reformate results from catalytic treatment of naphtha intended to produce aromatic hydrocarbons, principally comprising 6 to 9 carbon atoms per molecule and the octane number of which is very high endowing the gasoline with antiknock properties.
Because of the toxicity described above, the amount of benzene in the reformate must be reduced by a maximum.
The benzene in a reformate can be hydrogenated to cyclohexane. Since it is impossible to selectively hydrogenate benzene in a mixture of hydrocarbons also containing toluene and xylenes, that mixture must first be fractionated to isolate a cut containing only benzene, which can then be hydrogenated.
International patent application WO 95/15934 describes a reactive distillation which aims to selectively hydrogenate diolefins and C2-C5 acetylenic compounds. The distillate can be separately recovered from the light compounds. The catalytic hydrogenation zone is completely internal to the distillation column, which means that the hydrogen cannot dissolve properly in the feed and the pressure cannot be increased.
A process has been described in which the catalytic benzene hydrogenation zone is internal to the distillation column has been described which separates benzene from other aromatic compounds (Benzene Reduction—Kerry Rock and Gary Gildert CDTECH—1994 Conference on Clean Air Act Implementation and Reformulated Gasoline—October 94), which cuts the cost of the apparatus. It appears that the pressure drop across the catalytic bed(s) in that process means that an intimate mixture between the liquid phase and the gaseous stream containing the hydrogen cannot be obtained. In that type of technology where the reaction and distillation proceed simultaneously in the same physical space, the liquid phase descends through every catalytic bed in the reaction zone in a trickle flow, and thus in threads of liquid. The gaseous fraction containing the fraction of vaporised feed and the gas stream containing hydrogen rise through the catalytic bed in columns of gas. In that arrangement, the entropy of the system is high and the pressure drop across the catalytic bed(s) is low. As a result, operating that type of technique cannot easily promote dissolution of hydrogen in the liquid phase comprising the unsaturated compound(s).
The Applicant's European patent application EP-A-0 781 830 describes a process for hydrogenating benzene using a distillation column associated with a reaction zone which is at least partially external. The effluent is recovered overhead from the column, then arrives in a drum via a condenser from which a new separation operation is necessary to recover the desired product. The overhead effluent comprises light gases such as excess hydrogen mixed with the reformate which is depleted in benzene and the liquid distillate contains a great deal of dissolved gas which risks requiring a supplemental separation step.
The process of the present invention is an improvement over the Applicant's patent application EP-A-0 781 830, the features of which are hereby included in the present description.
The invention provides a process for converting a hydrocarbon feed associating a distillation zone producing a vapour distillate and a bottom effluent, and a reaction zone which is at least partially external to the distillation zone. At least one reaction for converting at least a portion of at least one hydrocarbon takes place in a reaction zone comprising at least one catalytic bed in the presence of a catalyst and a gas stream comprising hydrogen. The feed for the reaction zone is drawn off at the height of a draw-off level and represents at least a portion of the liquid flowing in the distillation zone, and at least a portion of the effluent from the reaction zone is re-introduced into the distillation zone at the height of at least one re-introduction level, so as to ensure continuity of distillation. The process is characterized in that a liquid distillate is withdrawn from the distillation zone at the height of at least one withdrawal level, said level being located below the vapour distillate withdrawal level.
The term “liquid distillate” as used in the present description means a liquid fraction withdrawn from a distillation zone which is distinct from the feed for the reaction zone.
The particular application of the process of the invention to a process for reducing the benzene content in a hydrocarbon feed enables a reformate which is depleted in benzene or, if necessary, which is almost completely free of benzene and other unsaturated hydrocarbons containing at most six carbon atoms per molecule such as light olefins to be produced from a crude reformate, directly recovering a stabilised liquid distillate, with no significant loss in yield.
The process of the invention is characterized by dissociating the level from which the liquid distillate is withdrawn from the level from which the gaseous distillate is recovered, the liquid distillate being withdrawn from a withdrawal level beneath that for recovering the vapour distillate. Thus the desired product is recovered as a stabilised liquid distillate, i.e., free of the major portion of excess hydrogen and possibly light gases. Further, such distinct vapour distillate recovery can eliminate gases other than the hydrogen present in the gas stream comprising for the most part hydrogen introduced to carry out the conversion reaction via the gaseous distillate.
Thus, for example, this particular application of the process of the invention can directly recover, by withdrawal from the distillation zone, a stabilised liquid distillate in which at least partial selective hydrogenation of benzene and any other unsaturated compound containing at most six carbon atoms per molecule and other than benzene which may be present in the feed has been carried out, while limiting hydr
Ambrosino Jean-Louis
Didillon Blaise
Marache Pierre
Viltard Jean-Charles
Witte Gérald
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Myers Helane E.
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