EP1417283B1 - Method for the isomerisation of a c5-c8 fraction involving two parallel reactors - Google Patents

Abstract

The invention relates to a method of isomerising a charge comprising hydrocarbons containing between 5 and 8 carbon atoms per molecule. According to the invention, said charge is separated into at least two fractions: fraction A mostly comprising hydrocarbons containing 5 or 6 carbon atoms and fraction B mostly comprising hydrocarbons containing 7 or 8 carbon atoms. Subsequently, said fractions A and B are treated separately under specific conditions in different isomerisation reaction zones.

Description

The present invention relates to an isomerization process in the presence of hydrogen (also sometimes called hydro-isomerization process), a feedstock comprising in majority hydrocarbons containing from 5 to 8 carbon atoms per molecule. In the remainder of the description, the abbreviation "C5-C8 cut" will be used to designate a feedstock comprising, for the most part, normal paraffins containing from 5 to 8 carbon atoms per molecule.

The suppression of lead alkyls in automotive gasolines, in particular for the purpose of protecting the environment, has led to the development of branched paraffin production processes which have a better octane number than linear compounds, and in particular the isomerization process of normal paraffins into branched paraffins. This process is becoming increasingly important in the oil industry.

• un naphta léger (coupe C5-C6) qui est envoyé à l'isomérisation,
• un naphta lourd (coupe C7-C10) qui est envoyé au reformage.

Current recovery schemes for naphtha (C5-C10 cut) from atmospheric distillation most often include fractionation producing:

• a light naphtha (C5-C6 cut) which is sent to isomerization,
• a heavy naphtha (C7-C10 cut) which is sent to reforming.

The product of the isomerization (or isomerate) is aromatic-free in contrast to the reformate which usually contains a significant amount (80 mol% and more). Isomerate and reformate are usually sent to the gasoline pool in which other bases or additives may also be involved: gasoline resulting from fluidized catalytic cracking (FCC), alkylation products, methyl-tertiobutyl-ether (MTBE), etc.

While aromatics usually have high octane numbers that are favorable for their use in spark ignition engines, increasingly stringent specifications lead to a reduction in the total aromatics content in gasolines. For example, a European specification foresees as of 2005 to reduce to a maximum of 35% in volume the total content of aromatics in the super-fuels whereas currently said content is of the order of 42% volume.

Thus, it is imperative to develop new processes for synthesizing new bases that are free of aromatics but have high octane numbers. The consideration of the octane numbers of the various C7 isomers shows, for example, that the isomers of normal-heptane (n-C7) having several branches have a sufficiently high octane number so that they can be sent directly to the gasoline pool. By however, isomers with only one branch have insufficient octane numbers to mix with the gasoline pool.

• les catalyseurs de type Friedel et Crafts, tels que les catalyseurs contenant du chlorure d'aluminium, qui sont utilisés à basses températures (environ 20 à 130°C),
• les catalyseurs à base de métaux du groupe VIII de la classification périodique des éléments ( Handbook of Chemistry and Physics, 45ème édition, 1964-1965 ) déposés sur alumine et contenant généralement un halogène, qui sont utilisés à des températures moyennes (environ 110 à 160°C). Les brevets US 2,906,798 , US 2,993,398 , US 3,791,960 , US 4,113,789 , US 4,149,993 , US 4,804,803 décrivent, par exemple, ce type de catalyseurs,
• les catalyseurs zéolithiques comprenant un métal du groupe VIII déposé sur une zéolithe, qui sont utilisés à des températures élevées (de 250°C à 350° C), ces catalyseurs conduisent à l'obtention d'un mélange d'hydrocarbures ayant un indice d'octane amélioré mais moins bon que celui obtenu par les procédés utilisant les catalyseurs cités ci-dessus, cependant ils présentent l'avantage d'être plus faciles à mettre en oeuvre et plus résistants aux poisons. Leur faible acidité ne permet pas de les employer pour l'isomérisation du n-butane. Le brevet US 4,727,217 décrit par exemple ce type de catalyseurs.

On the other hand, the isomerization of paraffins containing 5 and 6 carbon atoms per molecule also leads to the production of high octane gasoline bases which can also be directly incorporated into the gasoline fractions. The isomerization of C5-C6 paraffins has been the subject of many studies, three types of different catalysts are traditionally used to carry out this isomerization reaction:

• Friedel and Crafts type catalysts, such as catalysts containing aluminum chloride, which are used at low temperatures (about 20 to 130 ° C),
• Group VIII metal catalysts of the Periodic Table of Elements ( Handbook of Chemistry and Physics, 45th Edition, 1964-1965 ) deposited on alumina and generally containing a halogen, which are used at average temperatures (about 110 to 160 ° C). Licences US 2,906,798 , US 2,993,398 , US 3,791,960 , US 4,113,789 , US 4,149,993 , US 4,804,803 describe, for example, this type of catalysts,
• zeolite catalysts comprising a group VIII metal deposited on a zeolite, which are used at high temperatures (from 250 ° C. to 350 ° C.), these catalysts lead to the production of a mixture of hydrocarbons having a However, they have the advantage of being easier to implement and more resistant to poisons. Their low acidity does not allow them to be used for the isomerization of n-butane. The patent US 4,727,217 describes for example this type of catalysts.

Current processes for isomerizing paraffins containing 5 and 6 carbon atoms using chlorinated alumina type catalysts and including platinum are high activity catalysts. These processes are used without recycling or with partial recycling after fractionation of unconverted normal paraffins, for example on molecular sieve systems. These processes lead to the production of a base for fuels containing no aromatics and whose Research Octane Number (RON) is generally between 82 and 88, depending on whether the isomerization of normal paraffins used whether or not includes recycling.

Many patents relate to platinum-based mono-metal catalysts deposited on a halogenated alumina, and their use in isomerization processes of normal paraffins. We can mention the patent US 3,963,643 which requires treatment with a Friedel-Crafts compound followed by treatment with a chlorinated compound having at least two chlorine atoms, this treatment being more particularly applicable to linear chain hydrocarbons containing from 4 to 6 carbon atoms. The patent US 5,166,121 describes a catalyst comprising gamma-alumina shaped in the form of beads and having between 0.1 and 3.5% by weight of halogen on the support.

A major drawback of these processes, according to the current state of knowledge available to the public, is that they do not properly treat fillers having normal paraffin contents containing at least 7 carbon atoms per molecule greater than about 2% by weight. The known operating conditions for promoting the isomerization of paraffin-containing sections with 5 and 6 carbon atoms per molecule (that is to say more particularly the reaction temperature and the nature of the catalyst) lead to paraffin cracking rates containing at least one minus 7 carbon atoms per molecule too important.

The patent application EP-A1-922747 proposes, for limiting the cracking rate of paraffins containing at least 7 carbon atoms per molecule during the isomerization of a C5-C8 cut, the use of an acid catalyst associated with relatively mild operating conditions. This catalyst contains at least one halogen and at least one Group VIII metal, the reaction being carried out at a temperature between 30 and 150 ° C.

The patent application EP-A2-0256604 describes a process for the isomerization of paraffins of a feedstock comprising hydrocarbons containing from 5 to 7 carbon atoms per molecule without, in particular, the ratios R of the number of moles of hydrogen relative to the number of moles of hydrocarbons leaving from at least one reaction zone corresponds to the present invention.

The patent US3619408 describes a process for the isomerization of olefins, diolefins and aromatics of a feedstock comprising hydrocarbons containing from 5 to 8 carbon atoms per molecule without, in particular, the ratios R of the number of moles of hydrogen over the number of moles of hydrocarbons leaving at least one reaction zone corresponding to the present invention.

The patent US5994607 describes a process for the isomerization of paraffins of a feedstock comprising hydrocarbons containing from 5 to 8 carbon atoms per molecule without, in particular, the ratios R of the number of moles of hydrogen relative to the number of moles of hydrocarbons exiting from to at least one reaction zone corresponds to the present invention.

In its most general form, the present invention makes it possible to substantially improve the octane numbers of the products obtained by isomerization of a feed comprising mainly hydrocarbons containing from 5 to 8 carbon atoms, and possibly a significant amount of hydrocarbons containing from 2 to 4 carbon atoms (at least 0.1% by weight).

• Ladite charge est séparée en au moins deux fractions, une fraction A comprenant au moins 80 % d'hydrocarbures contenant 5 ou 6 atomes de carbone, et une fraction B comprenant au moins 80 % d'hydrocarbures contenant 7 ou 8 atomes de carbone et lesdites fractions A et B sont ensuite séparément traitées dans des zones réactionnelles d'isomérisation distinctes,
• dans lequel lesdites zones réactionnelles contiennent des catalyseurs d'isomérisation chimiquement différents, l'activité isomérisante du catalyseur traitant la fraction A étant supérieure à l'activité isomérisante du catalyseur traitant la fraction B,
• dans lequel soit l'isomérat B issu de l'isomérisation de la fraction B, soit le mélange des isomérats issus de l'isomérisation des fractions A et B est séparé en deux effluents, le premier desdits effluents comprenant la majeure partie de l'isopentane et des paraffines présentant au moins deux ramifications, le deuxième desdits effluents comprenant la majeure partie des normales paraffines et des paraffines contenant au moins six atomes de carbone et une seule ramification,
• dans lequel ledit deuxième effluent est recyclé et mélangé avec la charge ou avec la fraction B et
• dans lequel au moins l'une des réactions d'isomérisation est effectuée en présence d'un excès d'hydrogène tel que le rapport R du nombre de moles d'hydrogène sur le nombre de moles d'hydrocarbures sortant d'au moins une zone réactionnelle est compris entre 0,06 et 0,3 et ,
• au moins l'une des réactions d'isomérisation est effectuée en présence d'un excès d'hydrogène tel que ledit rapport R est compris entre 0,3 et 30 et supérieur à 0,3.

More specifically, the present invention relates to a process for the isomerization of a feedstock comprising hydrocarbons containing from 5 to 8 carbon atoms per molecule, in which:

• Said filler is separated into at least two fractions, a fraction A comprising at least 80% of hydrocarbons containing 5 or 6 carbon atoms, and a fraction B comprising at least 80% of hydrocarbons containing 7 or 8 carbon atoms and said Fractions A and B are then separately treated in separate isomerization reaction zones,
• wherein said reaction zones contain chemically different isomerization catalysts, the isomerizing activity of the fraction A catalyst being greater than the isomerizing activity of the fraction B catalyst,
• in which either the isomerization B resulting from the isomerization of the fraction B or the mixture of the isomerate resulting from the isomerization of the fractions A and B is separated into two effluents, the first of the said effluents comprising the major part of the isopentane and paraffins having at least two branches, the second of said effluents comprising most of the normal paraffins and paraffins containing at least six carbon atoms and a single branch,
• wherein said second effluent is recycled and mixed with the feed or with fraction B and
• in which at least one of the isomerization reactions is carried out in the presence of an excess of hydrogen such that the ratio R of the number of moles of hydrogen to the number of moles of hydrocarbons leaving at least one zone reaction is between 0.06 and 0.3 and
• at least one of the isomerization reactions is carried out in the presence of an excess of hydrogen such that said ratio R is between 0.3 and 30 and greater than 0.3.

In general, said feedstock is itself a naphtha (C5-C10 + cut), said fractionation then resulting in a fraction A (C5-C6) optionally also comprising C2-C4 hydrocarbons, a B fraction (C7-C8) and a heavy fraction C containing predominantly the heavier hydrocarbons of the initial naphtha fraction, ie containing at least 8 carbon atoms. According to the required specifications (as to the number of octane, for example), it is possible according to the invention to perform a fractionation for which between 0 and 100 mol%, preferably between 0.1 and 50 mol% and very preferred between 0.1 and 10 mol% of C8 present in the initial naphtha fraction are finally present in fraction B.

It was found by the applicant that the separation of said feedstock into two fractions and the treatment of each fraction under specific operating conditions allowed on the one hand the final increase in the conversion rate of paraffins containing five or six carbon atoms and on the other hand the reduction of the cracking rate of the hydrocarbons with 7 and 8 carbon atoms. As a result, the present process makes it possible to record substantial gains in the octane numbers of the bases obtained after isomerization.

According to the invention, said reaction zones contain chemically different isomerization catalysts.

The isomerizing activity of the catalyst treating fraction A is greater than the isomerizing activity of the catalyst treating fraction B.

The isomerization catalysts of the process according to the invention are typically included in the group consisting of: the catalysts most often supported by a mineral support, typically an oxide and containing at least one halogen and at least one group VIII metal, the zeolitic catalysts, containing at least one Group VIII metal, Friedel and Crafts catalysts, acid or super acid catalysts of the type HPA (heteropolyanions) on zirconia, WOx (tungsten oxides) on zirconia, sulphated zirconias. Said catalysts WOx (tungsten oxides) on zirconia are for example described in "Oxide Catalysts and Catalyst Development, PHILLIPS MJ and TERNAN M. eds" or in the patent US 5,422,327 . Said HPA type catalysts (heteropolyanions) are for example described in "Heteropoly and Isopoly Oxometalates, Springer-Verlag, Thor Pope, Berlin Heidelberg New York Tokyo 1983" or in the French patent FR2795340 . Other catalysts having an acidity comparable to that of catalysts mentioned above can also be used, for example catalysts comprising at least one mica and / or trioctaedric vermiculites with pillars.

The total pressure in the isomerization zones is typically about 0.1 to 10 MPa relative, the hourly space velocity being about 0.2 to 10 h ^-1 .

Thus, according to the invention, at least one of the isomerization reactions can be carried out in the presence of an excess of hydrogen such that the ratio R of the number of moles of hydrogen to the number of moles of hydrocarbons leaving the at least one reaction zone is between 0.06 and 0.3. In this case, it is not necessary to recycle the unconsumed hydrogen to the reactor inlet. We then operate on "lost hydrogen".

And, at least one of the isomerization reactions is carried out in the presence of an excess of hydrogen such that the ratio R of the number of moles of hydrogen to the number of moles of hydrocarbons leaving at least one zone reaction is between 0.3 and 30 and greater than 0.3. In this case, the excess hydrogen is recycled to the inlet of the reactor, for example by means of a recycle compressor after separation of the cracking gases.

According to the invention it is possible to mix the isomerate resulting from the isomerization of the fractions A and B. It is also possible to separate said mixture into two effluents, the first of said effluents comprising most of the isopentane and paraffins having at least two branches, the second of said effluents comprising most of the normal paraffins and paraffins containing at least six carbon atoms and a single branch, said second effluent being recycled and mixed with the feed or with the fraction B.

According to another embodiment, isomerization B is separated from the isomerization of fraction B into two effluents, the first of said effluents comprising most of the isopentane and paraffins having at least two branches, the second of said effluents comprising most of the normal paraffins and paraffins containing at least six carbon atoms and a single branch. Said second effluent is optionally recycled, ie for example mixed with the feedstock or with fraction B. The first effluent will advantageously be mixed with the isomerization resulting from the isomerization of the fraction A.

The isopentane fraction and paraffins having at least two branches and / or the fraction of normal paraffins and paraffins containing at least six carbon atoms and a single branch contained in each of the effluents in each of the embodiments of the invention. will obviously depend on the nature of the separation means used.

Said separation of said one or more isomerate in two effluents will be carried out according to any known technique or process of the prior art, preferably by gas phase adsorption, on molecular sieves by PSA methods, ie known to those skilled in the art. under the Anglo-Saxon term "pressure swing adsorption" or counter-current simulated. Said separation will advantageously employ at least one zeolitic adsorbent whose structural type is included in the group constituted, according to the reference " Atlas of Zeolite Structure Types, M. Meier and DH Olson, 4th edition (1996) ) ", by: MFI (silicate, ZMS-5), FAU (zeoliteX, Y, faujasite), MTT (ZSM-23, EU-13), EUO (Eu1, ZSM-50, TPZ-3), NES (Nu-87, ssz-37), BEA (beta), MWW (MCM-22, ERB-1, ITQ-1), MEL (ZSM-11), FER (ferrierite), MTW (ZSM-12), AFI (SAPO-5), ATO (SAPO-31), AEL (SAPO-11), ERI (eribite), MOR (mordenite), said zeolites can be exchanged with one or with a mixture of cations of columns IA and IIA of the periodic table.

It is also possible according to the invention to separate the isomerization A resulting from the isomerization of the fraction A into two effluents, the first of the said effluents comprising most of the isopentane and paraffins having at least one branch, the second said effluents comprising most of the normal paraffins, said second effluent being recycled and mixed with the feed or with the fraction A.

Advantageously, said first effluent resulting from isomerate A and said first effluent from isomerate B will be advantageously mixed.

Said separation of said one or more isomers resulting from the isomerization of the fraction A into two effluents will be carried out according to any known technique, in particular by a distillation column of the deisohexanizer type, by gas phase adsorption on a molecular sieve, for example according to the processes described. in patents US 5,233,120 or US 5,602,291 . Said separation will advantageously employ an adsorbent as described in the patent US 2,882,243 .

The invention will be better understood on reading the following examples which illustrate the invention without limiting its scope:

Example 1 (according to the prior art)

In this example, according to the method used in example 1 of the patent application EP-A1-922747 a catalyst based on chlorinated alumina marketed by Procatalyse under the reference IS 612 is used.

The operating conditions are as follows:
The isomerization reactor is fed with a feed comprising hydrocarbons containing from 5 to 7 carbon atoms (C5-C7) with a flow rate of 87 g / h, said feed containing 800 ppm of perchlorethylene. The mass of catalyst present in the reactor is 86 g, the PPH of 1.01 h ^-1 . The hydrogen flow rate is 4.5 l / h. The total pressure is 3 MPa relative. The isomerization is carried out at a temperature of 115 ° C. and the ratio R of the number of moles of hydrogen to the number of moles of hydrocarbons at the outlet of the reactor is equal to 0.11. The results obtained are shown in Table 1. <b> Table 1 </ b> compounds charge C5-C7
(% in weight) After isomerization C2-C4 0.74 7.15 Isopentane (iC5) 4.19 7.62 Normalpentane (nC5) 10.53 7.50 cyclopentane 0.28 0.28 isohexane (iC6) 4.01 4.73 normalhexane (nC6) 1.06 0.88 cyclohexane 1.40 2.60 methylcyclopentane 1.01 1.62 benzene 0.01 0 normalheptane (nC7) 65.07 17.35 Isoheptane (iC7) 11,70 50.27 Conversion rate nC5 29% nC6 17% nC7 73.6% C5 + 93.5%

Example 2 (according to the prior art)

The same IS 612 A catalyst is used as in Example 1 in two different isomerization reactors: reactor A for a synthetic feed containing predominantly hydrocarbons containing 5 or 6 carbon atoms (C5-C6 feedstock) and reactor B for a reactor. synthetic filler mainly containing hydrocarbons containing 7 carbon atoms C7). By mixing the charges A and B, an initial charge C5-C7 identical to that of Example 1 is obtained. The percentages by weight of the various hydrocarbons in the charge C5-C6 (reactor A), the charge C7 (reactor B ) and the total load are given in Table 2: <b> Table 2 </ b> compounds synthetic filler C5-C6 synthetic charge C7 total charge (% in weight) (% in weight) (% in weight) C2-C4 3.18 0 0.74 Isopentane (iC5) 18,04 0 4.19 Normalpentane (nC5) 45.33 0 10.53 cyclopentane 1.20 0 0.28 isohexane (iC6) 17.26 0 4.01 normalhexane (nC6) 4.56 0 1.06 cyclohexane 6.03 0 1.40 methylcyclopentane 4.36 0 1.01 benzene 0.04 0 0.01 normalheptane (nC7) 0 84.76 65.07 Isoheptane (iC7) 0 15.24 11,70

The isomerization reactor A is fed with the feed C5-C6 with a flow rate of 20.21 g / h (gram per hour), said feed containing 800 ppm by weight of perchlorethylene (C ₂ Cl ₄ ). The mass of catalyst present in reactor A is 20.01 g, the PPH (hourly space velocity) of 1.01 h ^-1 . The hydrogen flow rate is 1.41 / hr (liter per hour). The total pressure is 3 MPa relative. The isomerization reactor A operates at 140 ° C. This temperature makes it possible to optimize the conversion to C5 and C6 by approaching the thermodynamic equilibrium of the reaction. The ratio R _A of the number of moles of hydrogen over the number of moles of hydrocarbon at the outlet of reactor A is equal to 0.19.

Isomerization reactor B is fed with charge C7 with a flow rate of 66.79 g / h, said feed containing 800 ppm of perchlorethylene. The mass of catalyst present in reactor B is 66.13 g and the PPH is 1.01 h ^-1 . The hydrogen flow rate is 3.2 l / h. The total pressure is 3 MPa relative. The temperature of the isomerization reaction in reactor B is set at 115 ° C. The ratio R _B of the number of moles of hydrogen relative to the number of moles of hydrocarbon at the outlet of reactor B is equal to 0.19. The compositions of the isomerate A and B respectively from reactors A and B and that of the mixture of these two isomerate are shown in Table 3 below. <b> Table 3 </ b> compounds isomerate A isomer B (% by weight) isomer A + B (% by weight) (% in weight) (% in weight) (% in weight) C2-C4 4.32 8.15 7.26 iC5 43,14 0 10,02 nC5 18.22 0 4.23 cyclopentane 1.20 0 0.28 IC6 19.54 0 4.54 nC6 2.74 0 0.64 cyclohexane 5.94 0 1.38 methylcyclopentane 4.90 0 1.14 benzene 0 0 0 nC7 0 22.67 17.40 IC7 0 69.18 53.11 T. of reaction 140 ° C 115 ° C

For each of the n-paraffins present in the feed is calculated a conversion rate from the composition of the mixture of the two isomerate.

The total conversion rates are given in Table 4: <b> Table 4 </ b> Conversion rate nC5 59.8% nC6 39.6% nC7 73.3% C5 + 93.4%

The results reported in Table 4 indicate that, under optimized operating conditions compared with Example 1 and for an identical initial charge, the isomerization scheme makes it possible to significantly improve the conversion to nC5 and nC6 while maintaining a low conversion rate. conversion of normal-heptane and a yield of hydrocarbons having at least five carbon atoms (C5 +) substantially identical. The octane number of the isomerization products is thus improved.

Example 3 (according to the invention)

In this example, two different reactors, but catalysts of a different chemical nature, are used as in Example 2: an acidic catalyst (IS 612 A) having a high activity for reactor A and a zeolitic catalyst type Pt / H-beta minus acid but more selective for the isomerization of nC7. Said zeolite is marketed by the company Zeolyst and corresponds to zeolite beta in its protonic form.

The initial charges are similar to those given in Example 2 (Table 2).

The isomerization reactor A is fed with the feed C5-C6 at the rate of 20.21 g / h, said feed containing 800 ppm by weight of perchlorethylene (C ₂ Cl ₄ ). The mass of catalyst present in reactor A is 20.01 g, the PPH is 1.01 h ¹ . The hydrogen flow rate is 1.4 l / h. The total pressure is 3 MPa relative.

The isomerization reactor A operates at 140 ° C. The ratio R _A of the number of moles of hydrogen relative to the number of moles of hydrocarbon at the outlet of reactor A is equal to 0.19.

The isomerization reactor B is fed with charge C7 at the rate of 66.79 g / h. The mass of catalyst present in reactor B is 22.26 g, the PPH is 3 h ^-1 . The hydrogen flow rate is 33.45 l / h. The total pressure is 3 MPa relative. The isomerization reactor B operates at 240 ° C so as to optimize the yield of iso-paraffins.

The ratio R _B of the number of moles of hydrogen relative to the number of moles of hydrocarbon at the outlet of reactor B is equal to 1.86. The results obtained are reported in Table 5. <b> Table 5 </ b> compounds isomerate A isomer B isomer A + B (% in weight) (% in weight) (% in weight) C2-C4 4.32 5.39 5.14 iC5 43,14 0 10,02 nC5 18.22 0 4.23 cyclopentane 1.20 0 0.28 IC6 19.54 0 4.54 nC6 2.74 0 0.64 cyclohexane 5.94 0 1.38 methylcyclopentane 4.90 0 1.14 benzene 0 0 0 nC7 0 22.07 16.94 IC7 0 72.54 55.69 Reaction temperature 140 ° C 240 ° C Catalyst IS612A Pt / H-beta

The total conversion rates are given in Table 6: <b> Table 6 </ b> Isomerization A + B nC5 conversion 59.8% nC6 conversion 39.6% nC7 conversion 74.0% C5 + yield 95.6%

This embodiment of the invention also appears more favorable than the example according to the prior art. Indeed, for a comparable nC7 hydrocarbon conversion, the conversion nC5 and nC6 hydrocarbons are significantly increased as well as the C5 + yield. The present process thus makes it possible to improve the octane numbers of the isomerate compounds, on the one hand by increasing the conversion rates of hydrocarbons containing 5 to 6 carbon atoms, while minimizing the cracking rate of hydrocarbons containing 7 carbon atoms. .

Claims (6)

1. A process for isomerizing a feed comprising hydrocarbons containing 5 to 8 carbon atoms per molecule, in which:

   • said feed is separated into at least two fractions, a fraction A comprising at least 80% of hydrocarbons containing 5 or 6 carbon atoms, and a fraction B comprising at least 80% of hydrocarbons containing 7 or 8 carbon atoms and said fractions A and B are then treated separately in distinct isomerization reaction zones;

   • in which said reaction zones contain chemically different isomerization catalysts, the isomerization activity of the catalyst treating fraction A being higher than the isomerization activity of the catalyst treating fraction B;

   • in which either the isomerate B derived from isomerization of fraction B or the mixture of isomerates derived from isomerization of fractions A and B is separated into two effluents, the first of said effluents comprising the major portion of the isopentane and paraffins having at least two branches, the second of said effluents comprising the major portion of the normal paraffins and paraffins containing at least six carbon atoms and a single branch;

   • in which said second effluent is recycled and mixed with the feed or with the fraction B; and

   • in which at least one of the isomerization reactions is carried out in the presence of an excess of hydrogen such that the ratio R of the number af moles of hydrogen to the number of moles of hydrocarbons leaving at least one reaction zone is in the range 0.06 to 0.3; and

   • at least one of the isomerization reactions is carried out in the presence of an excess of hydrogen such that said ratio R is in the range 0.3 to 30 and more than 0.3.
2. A process according to claim 1, in which the isomerate B derived from isomerization of fraction B is separated into two effluents and said first effluent is mixed with the isomerate derived from isomerization of fraction A.
3. A process according to claim 2, in which in addition, the isomerate A derived from isomerization of fraction A is separated into two effluents, the first of said effluents comprising the major portion of the isopentane and paraffins having at least one branch, the second of said effluents comprising the major portion of the normal paraffins, said second effluent being recycled and mixed with the feed or with the fraction A.
4. A process according to claim 3, in which said first effluent derived from isomerate B is mixed with said first effluent derived from isomerate A.
5. A process according to one of claims 1 to 7, in which the or said isomerization catalysts are comprised in the group constituted by: supported catalysts containing at least one halogen and at least one metal from group VIII, zeolitic catalysts containing at least one metal from group VIII, Friedel-Crafts catalysts, and superacid catalysts of the HPA on zirconia, WOx on zirconia or sulphated zirconia type.
6. A process according to one of claims 1 to 8, in which the total pressure in the isomerization reaction zones is approximately 0.1 to 10 MPa relative, the hourly space velocity being approximately 0.2 to 10 h^-1.