Distillation: processes – separatory – Adding material to distilland except water or steam per se – Organic compound
Reexamination Certificate
1993-11-29
2001-03-06
Warden, Sr., Robert J. (Department: 1744)
Distillation: processes, separatory
Adding material to distilland except water or steam per se
Organic compound
C203S056000, C203S066000, C203S091000, C208S095000, C208S102000, C568S697000, C568S699000, C585S834000, C585S864000
Reexamination Certificate
active
06197163
ABSTRACT:
The present invention relates to improvements in removing impurities from hydrocarbons such as the hydrocarbons produced by the cracking of hydrocarbon feedstocks. In particular it is concerned with improvements in the preparation of feeds containing olefins for use in the preparation of ethers by reaction with alkanols.
BACKGROUND OF THE INVENTION
Hydrocarbon feeds derived from petroleum are commonly cracked to produce a product containing lower molecular weight hydrocarbons for use for various purposes. The cracked products generally contain olefins which are useful reactants for various purposes. Among olefins which may be present in cracked products are tertiary olefins, for example C
4
(iso-butene), C
5
and higher tertiary olefins.Iso-butene may be reacted with alkanols to give alkyl tertiary butyl ethers. Thus iso-butene may be reacted with methanol to give MTBE (methyl tertiary butyl ether). The C
5
and higher tertiary olefins may be reacted with alkanols to give alkyl tertiary alkyl ethers. Thus C
5
tertiary olefins may be reacted with methanol to give TAME (tertiary amyl methyl ether). These ethers are well-known to be useful as additives for gasoline.
Petroleum contains various sulphur and nitrogen compounds which can have an adverse affect on the activity of catalysts. In particular fractions obtained by cracking high boiling petroleum fractions can contain undesirable impurities. The etherification reactions mentioned above are generally carried out using acidic catalysts, for example acidic ion exchange resins. It has been found that basic nitrogen compounds present in cracked products used as feed to such etherification reactions can have an adverse effect on the activity of the acidic catalyst. Guard beds containing an acidic material, such as the acidic ion exchange resin, have been used to remove basic compounds before the olefin feed is reacted with the alkanol.
DESCRIPTION OF THE INVENTION
We have now found that despite the use of such guard beds the activity of catalysts used to react C
5
tertiary olefins with alkanols has declined due to the presence of one or more catalyst poisons which appear able to pass through the conventional guard beds. We have now found a new procedure for lowering the content of an undesirable impurity from fractions containing C
5
or C
6
hydrocarbons obtained by cracking materials derived from petroleum.
According to the present invention the process for reducing the content of an impurity in a fraction containing C
5
or C
6
tertiary olefins obtained by distillation of a cracked product obtained by the cracking of material derived from petroleum so as to recover a fraction containing C
5
or C
6
material as a distillate comprises feeding a lower alkanol to the distillation, and removing the impurity as a fraction with a higher boiling point than the fraction containing C
5
or C
6
olefins.
References to C
5
or C
6
tertiary olefins are to be understood as also including feeds containing both C
5
and C
6
tertiary olefins.
The impurity removed by the process of the present invention is a compound containing hetero atoms, i.e. atoms other than carbon and hydrogen, and in particular nitrogen-containing compounds. More than one impurity may be present and may be wholly or partially removed by the process of this invention. The process of the invention is particularly suitable for removing propionitrile, which we have found to be present in certain hydrocarbon streams resulting from the processing of petroleum.
Propionitrile can pass through guard beds which would remove basic materials. It has been found to have an adverse effect on the long term activity of certain catalysts. Propionitrile is a relatively high boiling material which on the basis of the boiling point of the pure material would not be expected to be found in the C
5
or C
6
overhead fraction.
The cracking step may conveniently be a catalytic cracking step, e.g. fluid catalytic cracking which may be applied to a gas oil fraction, or to feeds containing residues.
The process of the present invention can be usefully employed in removing impurities from streams containing C
4
, C
5
and C
6
tertiary olefins as well as feeds containing C
5
and C
6
tertiary olefins, but no substantial quantities of C
4
hydrocarbons. It can also be applied to streams containing C
6
tertiary olefins, but no substantial quantities of C
5
hydrocarbons. Alternatively, the process can be applied to streams containing C
5
tertiary olefins but no substantial quantities of C
6
hydrocarbons.
The feed may contain only 1 or 2% weight of C
5
or C
6
tertiary olefins, as in a gasoline range catalytically cracked spirit (CCS), but preferably contains at least 4% weight C
5
or C
6
, or C
5
and C
6
tertiary olefins as in a typical light catalytically cracked spirit (LCCS) or the feed to a depentaniser column. The feed to the distillation with alkanol preferably contains hydrocarbons with boiling points above those of the C
5
or C
6
olefins recovered as a distillate, for example C
7
and higher hydrocarbons, as these may provide a higher boiling fraction into which an impurity can be concentrated, while allowing the alkanol to be recovered separately from the impurity in a lower boiling fraction.
The fraction containing C
5
or C
6
tertiary olefins taken overhead in the distillation step may contain higher olefins, for example C
6
or C
7
olefins, provided the feed to the distillation process and the distillation conditions are chosen so as to leave a higher boiling hydrocarbon fraction in which the undesirable impurity, e.g. propionitrile, is concentrated.
The alkanol may be a methanol, ethanol, or a mixture of the two.
Persons skilled in distillation will understand that the distillate fraction containing C
5
tertiary olefins may not necessarily contain all the C
5
olefins fed to the distillation step. Depending on the distillation conditions used minor amounts of the olefin may be left in a higher boiling fraction. The same will be true for a distillate fraction containing C
6
tertiary olefins.
The distillation may be carried out to produce an overhead stream containing C
5
tertiary olefins and a bottoms stream in which the impurity, e.g. propionitrile, is concentrated. Alternatively a C
5
hydrocarbon fraction containing tertiary olefins may be recovered as an overhead stream, a fraction enriched in impurity, e.g. propionitrile, may be recovered as a side stream, and higher boiling materials with a low impurity content recovered as a bottoms product.
The alkanol may be added to the main distillation step in which the fraction containing C
5
or C
6
tertiary olefins is separated from higher boiling material. Alternatively it may be preferred to subject a lower boiling fraction from the main distillation to a second distillation step to which alkanol is added to recover a bottoms fraction, and an overhead fraction containing the C
5
or C
6
tertiary olefins and the alkanol.
The quantity of alkanol fed is preferably adjusted so that substantially all the alkanol is recovered in the distillate fraction. The use of large amounts of alkanol relative to the quantity of C
5
hydrocarbons present may lead to significant quantities of alkanol appearing in the boiling fraction in which the impurities are concentrated. This will make recovery of the alkanol for further use more difficult. The mole ratio of alkanol to C
5
hydrocarbon may, for example, be in the range 1:0.5 to 1:12, preferably 1:1 to 1:8, more preferably 1:2 to 1:4. For methanol weight ratio which may be used are for example 1:3 to 1:15, preferably 1:5 to 1:10.
The molar ratio of alkanol to C
6
hydrocarbons is preferably in the range 1:0.2 to 1:6, preferably 1:0.5 to 1:4, more preferably 1:1 to 1:2.
Where a mixture of C
5
and C
6
hydrocarbons is used then the alkanol used to satisfy the mole ratio requirement for C
5
hydrocarbon is not counted for the purposes of satisfying the mole ratio requirement for C
6
hydrocarbon.
Thus the molar ratio of alkanol to C
5
and C
6
hydrocarbons is based on a combination o
Catalytic Distillation Technologies
Johnson Kenneth H.
Warden, Sr. Robert J.
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