EP0552069A1 - Lowering of the benzene content in gasolines by isomerisation - Google Patents

Abstract

Process for reducing the benzene content and isomerising paraffins, in which an isomerisation is performed of the mixture (a) of the feedstock characterised by a weight composition included within the following ranges: - between 40 and 80 % of paraffins, - between 0.5 and 7 % of naphthenes, - between 6 and 45 % of aromatics and a maximum distillation temperature of between 70 and 90 DEG C, and (b) of a C5-C6 direct distillation cut, in the presence of an isomerisation catalyst.

Description

The invention relates to a process allowing the reduction of the benzene content in the gasoline pool by a process of isomerization of the light reformate and / or of a C₅-C₆ cut generally of direct distillation.

PRIOR ART

The environmental problems will lead jointly to the reduction of the lead content and to the reduction of the benzene content in gasoline fractions, preferably without reduction of octane number. This leads to the need for a rearrangement of the various hydrocarbons present in the gasoline fractions.

Catalytic reforming used under conditions of severe severity, and isomerization of normal C₅-C₆ paraffins with low octane number are the most common methods currently used to obtain high octane numbers without the addition of lead.

However, the catalytic reforming process produces significant amounts of high octane benzene. This is why it is necessary to develop new processes which make it possible to reduce the benzene content of gasolines while meeting the specifications for the octane number.

The combination of catalytic reforming and isomerization processes, consisting of separating the C₅-C₆ fraction from the reformate, isomerizing it and introducing it directly into the gasoline pool to improve the octane number is well known: it is described for example in patents US-A-4,457,832, US-A-4,181,599, US-A-3,761,392. The treatment by isomerization of the C₅-C₆ cut resulting from the direct distillation of crude oil is also well known. It leads to a considerable improvement in the octane number of this cut. The reduction of the benzene content of the reformate can also be carried out in various ways, such as for example the modification of the cutting point of naphtha between reforming and isomerization or the separation of the reformate into two fractions: a heavy reformate and a light reformate in which all the benzene is concentrated. This the light fraction is then sent to a hydrogenation unit which makes it possible to transform the benzene into naphthene, which is then decyclized in an isomerization unit working under severe conditions. The normal paraffins thus formed are isomerized by a conventional isomerization process (US-A 5,003,118). In the case of an isomerization catalyst based on chlorinated alumina, the naphthenes adsorb on the catalyst and thus contribute to deteriorating its activity.

US-A-3,611,117 also describes a process for the hydro-isomerization of cyclic hydrocarbons which uses a group VIII metal supported on zeolite as a catalyst for opening cycles under severe operating conditions and as an isomerization catalyst under mild operating conditions.

One of the major problems of reducing the benzene content by saturation followed by decyclization and isomerization of the paraffins formed is the possible reduction in the octane number of the charge.

The object of the present invention is to carry out jointly the reduction of the benzene content contained in the light reformate and the isomerization of the paraffins contained on the one hand in this light reformate and on the other hand in the C₅-C₆ cut, generally paraffinic and resulting from direct distillation. The process of the present invention therefore comprises the isomerization of the mixture of the filler defined below and of a C₅-C₆ cut. Surprisingly, the joint treatment of these two charges in an isomerization unit implemented under appropriate conditions leads to the production of an effluent practically completely free of benzene and having an octane number sufficiently high for that it can be directly incorporated into the gasoline fractions after stabilization.

DETAILED DESCRIPTION OF THE INVENTION

The conditions under which isomerization is carried out, namely the operating conditions, the catalyst, etc., are well known to those skilled in the art. They are nevertheless specified below.

The feed concerned by the present invention contains a light fraction of the reformate and a C₅-C₆ cut generally resulting from direct distillation.

• paraffines : entre 40,0 et 80,0 %,
• naphtènes : entre 0,5 et 7,0 %,
• aromatiques : entre 6,0 et 45,0 %.

The light fraction of the reformate is obtained by distillation of the said reformate. The maximum distillation temperature of this fraction is between 70 and 90 ° C, preferably between 77 and 83 ° C. The weight composition by hydrocarbon families of this light fraction of the reformate is variable according to the following intervals:

• paraffins: between 40.0 and 80.0%,
• naphthenes: between 0.5 and 7.0%,
• aromatic: between 6.0 and 45.0%.

Benzene is essentially the only aromatic compound included in said fraction.

Furthermore, said fraction can comprise between 1 and 3% of olefinic hydrocarbons.

• le poids moléculaire moyen est compris entre 70 et 90 g/mol.
• la masse volumique, mesurée à 15°C, est comprise entre 0,670 et 0,780 g/cm³.
• la valeur de l'indice d'octane recherche est généralement comprise entre 75 et 90.

On the other hand, the light fraction of the reformate as described above has the following characteristics:

• the average molecular weight is between 70 and 90 g / mol.
• the density, measured at 15 ° C, is between 0.670 and 0.780 g / cm³.
• the value of the research octane number is generally between 75 and 90.

Any other hydrocarbon feedstock from another process or set of processes and having characteristics similar to those described above can also be used.

The weight composition of the C₅-C₆ cut generally resulting from direct distillation is variable. It depends on the nature of the crude to be treated in the case where the C₅-C₆ cut is obtained from direct distillation.

It may contain very low contents of compounds containing 4 carbon atoms per molecule (less than 0.5% by weight).

The paraffin content of said cut is generally greater than 90% by weight, the naphthene content less than 10% by weight and the benzene content less than 1.5% by weight. Its research octane number is between 60 and 75.

According to the present invention these two fillers are mixed and then sent together to the isomerization zone, the content of light reformate or any other charge of similar characteristics in the mixture varying from 0 to 100% and preferably from 20 to 80% . The isomerization zone includes one or more isomerization reactors.

The isomerization zone is implemented under the usual conditions of isomerization: the temperature is generally between 150 ° C and 300 ° C and preferably between 230 and 280 ° C, and the partial pressure of hydrogen is included generally between atmospheric pressure and 70 bar and preferably between 5 and 50 bar. The space velocity is generally between 0.2 and 10 liters and preferably between 0.5 and 5 liters of liquid hydrocarbons per liter of catalyst per hour. The hydrogen / charge molar ratio is generally between 0.5 and 10 and preferably between 1 and 3.

The catalyst used in the isomerization zone according to the process of the present invention can be a catalyst based on platinum on chlorinated alumina, containing from 1 to 10% of chlorine and preferably from 2 to 9% of chlorine, but preferably uses a catalyst comprising at least one group VIII metal and a zeolite. Different zeolites can be used for said catalyst such as for example mordenite or Ω zeolite. Use is preferably made of a mordenite having an Si / Al (atomic) ratio of between 5 and 50 and preferably between 5 and 30, a sodium content of less than 0.2% and preferably of less than 0.1% ( relative to the weight of dry zeolite), a volume of mesh V of the elementary mesh comprised between 2.78 and 2.73 nm³ and preferably between 2.77 and 2.74 nm³, a benzene absorption capacity greater than 5% and preferably greater than 8% (relative to the weight of dry solid). The mordenite thus prepared is then mixed with a generally amorphous matrix (alumina, silica alumina, kaolin, etc.) and shaped by any method known to those skilled in the art (extrusion, pelletizing, coating). The mordenite content of the support thus obtained must be greater than 40% and preferably greater than 60% by weight.

At least one hydrogenating metal from group VIII, preferably chosen from the group formed by platinum, palladium, and nickel, is then deposited on this support, either in the form of a tetramine complex by cation exchange, or in the form of acid. hexachloroplatinic in the case of platinum or in the form of palladium chloride by anion exchange.

In the case of platinum or palladium, the content by weight is between 0.05 and 1% and preferably between 0.1 and 0.6%. In the case of nickel, the weight content is between 0.1 and 10% and preferably between 0, 2 and 5%.

The effluent thus obtained then has an octane number high enough to be incorporated into the gasoline fractions after stabilization and is practically completely free of benzene (content in the effluent less than 0.1%). The example which follows specifies the invention without limiting its scope. In this example, the isomerization is carried out in a single reactor (or unit).

Example 1 (according to the invention)

The light reformate obtained after distillation at 85 ° C, containing 21.5% of benzene and having an octane number of 80.3 is mixed in an amount of 50% by weight with a C₅-C₆ cut of direct distillation containing 0.7 % of benzene and having an octane number of 65. The compositions of these 2 products are shown in Table I. The charge from the mixture, the composition of which also appears in Table I, is sent to an isomerization unit at a temperature of 250 ° C, a pressure of 30 bar with an LHSV equal to 2 h⁻¹ and a hydrogen molar ratio on hydrocarbons with a charge equal to 4. The catalyst used in the isomerization unit contains 0.3% Pt deposited on a support composed of 80% mordenite with Si / Al ratio = 11 and 20% alumina. The effluent leaving the isomerization unit has the composition given in Table I; it contains very little benzene and has an octane number of 78.2. It can therefore be directly incorporated into the gasoline fractions after stabilization. Reformate C₅-C₆ distillation cup Isomerization charge Isomerization effluent light 6.5 1.0 3.7 4.6 iC5 9.9 18.9 14.4 18.8 nC5 7.1 25.4 16.25 12.5 22DMC4 3.0 0.4 1.7 6.3 23DMC4 4.1 1.85 3.0 4.8 2MC5 15.8 11.1 13.45 17.6 3MC5 12.5 9.4 11.0 11.55 nC6 12.1 19.6 15.9 8.75 C7 3.5 4.4 3.9 1.9 CC5 0.4 1.4 0.9 0.9 MCC5 3.6 4.1 3.85 9.85 Benzene 21.5 0.7 11.1 0.05 CC6 0 1.75 0.85 2.4 RON 80.3 65 72.9 78.2 Table I

Claims (6)

1 - Process for reducing the benzene content and for isomerization of paraffins in which an isomerization of the mixture (a) of a filler is carried out, characterized by
a weight composition in the following ranges: . between 40 and 80% paraffins . between 0.5 and 7% naphthenes . between 6 and 45% aromatics and a maximum distillation temperature between 70 and 90 ° C,
and (b) a C₅-C₆ cut, in the presence of an isomerization catalyst. 2 - Method according to claim 1 wherein the mixture comprises 20 to 80% of said load. 3 - Method according to one of claims 1 to 2 wherein the isomerization catalyst comprises a mordenite of Si / Al ratio between 5 and 50 and at least one group VIII metal. 4 - Process according to claim 3 wherein the metal of group VIII is selected from the group formed by platinum, palladium and nickel. 5 - Method according to one of claims 1 to 4 wherein the isomerization is carried out according to the following operating conditions: the temperature is between 150 and 300 ° C, the partial hydrogen pressure is between atmospheric pressure and 70 bar, the space velocity is between 0.2 and 10 liters of feed per liter of catalyst and per hour, and the hydrogen to charge molar ratio is between 0.5 and 10. 6 - Method according to one of claims 1 to 5 wherein the cut is a C₅-C₆ cut from direct distillation.