EP3137583A1

EP3137583A1 - Petrol production method comprising an isomerisation step followed by at least two separation steps

Info

Publication number: EP3137583A1
Application number: EP15719168.5A
Authority: EP
Inventors: Jérôme PIGOURIER; Isabelle Prevost; Laurent Watripont; Pierre-Yves Martin
Original assignee: Axens SA
Current assignee: Axens SA
Priority date: 2014-04-29
Filing date: 2015-04-20
Publication date: 2017-03-08
Anticipated expiration: 2035-04-20
Also published as: US20170044447A1; FR3020374B1; SA516380157B1; CN106661460A; FR3020374A1; EP3137583B1; WO2015165763A1; MX2016013897A; US10113121B2; CN106661460B

Abstract

The invention relates to a method for the production of petrol with a high octane rating, by means of isomerisation of a light naphtha fraction, as well as comprising two separation steps performed downstream of the reaction step, which allow the energy efficiency of the method to be improved.

Description

METHOD FOR PRODUCING GASOLINE COMPRISING AN ISOMERIZATION STEP FOLLOWED BY AT LEAST TWO STEPS OF SEPARATION

FIELD OF THE INVENTION

The invention relates to the field of producing high octane gasoline. Naphthas from the atmospheric distillation of petroleum usually consist mainly of hydrocarbons comprising 5 to 10 carbon atoms (C5-C10 cuts). These naphthas are generally fractionated into a light naphtha cut (C5-C6 cut) and a heavy naphtha cut (C7-C10). Heavy naphtha cutting is usually sent in a catalytic reforming process. The light naphtha fraction, which essentially comprises hydrocarbons having 5 or 6 carbon atoms (C5 and C6) but may in addition comprise hydrocarbons having 4 or 7 or even 8 carbon atoms (C4, C7, C8), is generally isomerized in order to increase the proportion of branched hydrocarbons which have a higher octane number than linear hydrocarbons.

The isomerate and the reformate obtained are then sent to the gasoline pool with other bases or additives (catalytic cracking gasoline, alkylates, etc.). Given the gradual decrease in the maximum content of aromatic compounds allowed in gasolines (less than 35% volume in the Euro 5 specification for example), and significant aromatics contents of catalytic reforming gasolines, the isomerate which do not contain of aromatic compounds, are becoming increasingly important in the gasoline pool.

It is therefore important to have efficient isomerization processes, both in terms of yield and octane number. These processes must also be economically attractive in terms of level of investment and operating costs. It is therefore important to optimize the operation of both the isomerization reaction section and the fractionation sections of the feedstock or effluent.

EXAMINATION OF THE PRIOR ART Patent FR 2,828,205 describes a process for isomerizing a C5-C8 cut in which said cut is fractionated into a C5-C6 cut and a C7-C8 cut which are each isomerized separately in specific conditions for each cut. U.S. Patent 2,905,619 discloses an isomerization process in which the C5-C6 cut from a gasoline cut is separated into different fractions which are isomerized in two isomerization sections operated under specific conditions.

US Pat. No. 7,223,898 describes an isomerization process comprising in its fractionation section only a stabilization or stripping and a deisohexanizer producing 2 to 4 different slices. These process schemes do not include a deisopentaninser (DiP) and / or depentanizer ( DP). GB Patent 1,056,517 describes a process for the isomerization of a C5-C6 cut comprising a deisopentanizer (DiP), isomerization of the isopentane depleted cut (ISOM), a separation of the isomerized effluent to recover n-pentane (DP) which is recycled with the feedstock at the inlet of the deisopentanizer and a separation of the branched C6 hydrocarbons (deisohexanizer, DiH) for recovering branched high octane C 6 hydrocarbons, the balance being recycled to the reactor of isomerization. This diagram DiP / ISOM / DP / DiH corresponds to Figure 1 (according to the prior art) of this application.

SUMMARY DESCRIPTION OF THE FIGURES FIG. 1 represents a diagram of the process according to the closest prior art. This diagram shows the de-isopentanizer column [3], the isomerization reaction section [1], the stabilization column [2], the de-pentanizer column [4] and the de-isohexanizer column [5]. ].

These numbers are kept in the figures according to the invention to designate the same equipment.

FIG. 2 represents the method according to the invention in which the block denoted (3 + 4) represents the first separation step, and the block [5] the second separation step.

FIG. 3 represents a first variant of the method according to the invention in which the columns [3] and [4] are connected in series.

FIG. 4 represents a second variant of the method according to the invention in which the columns [3] and [4] are grouped together in a single column [3] allowing fractionation into 3 sections. FIG. 5 represents a third variant of the method according to the invention in which the columns [3] and [4] are in the reverse order, that is to say that it is the top flow of the column [4] which feeds the column [3]. FIG. 6 represents an example of thermal integration between the condenser of a first column and the reboiler of another column.

Equipment is marked with numbers in square brackets and flows by numbers in parentheses. The numbers of the conduits conveying the flows are the same as those of the flows conveyed.

SUMMARY DESCRIPTION OF THE INVENTION

The invention relates to the field of producing high octane gasoline. The naphthas resulting from the atmospheric distillation of petroleum usually consist mainly of hydrocarbons comprising from 5 to 10 carbon atoms (C5-C10 cut). The process according to the present invention processes a filler of the light naphtha type and preferentially a C5 and C6 cut (hydrocarbon cut comprising 5 or 6 carbon atoms), and aims at maximizing the branched molecules relative to the linear (or normal) molecules. . However, these fillers may optionally comprise other hydrocarbons, for example hydrocarbons comprising 4 or 7 or even 8 carbon atoms (C4, C7 and C8 cuts). However, it will preferably be sought to limit the amount of these hydrocarbons via, for example, a prior separation.

With regard to the C4 hydrocarbons, they can also be largely separated in the stabilization column [2].

The process according to the invention is more particularly applicable to fillers whose iso pentane content is less than 25% and preferably less than 20%. The process according to the invention comprises an isomerization section [1], a stabilization of the isomerized effluent [2] (denoted STAB), a separation of iso-pentane (denoted DiP), a separation of n-pentane (denoted by DP), (represented by the 3 + 4 block) and a separation of the remaining products, in particular C6 branched compounds (denoted DiH) (represented by block 5), according to the sequence ISOM / STAB / DiP / DP / IHL. In the process according to the invention, the separation of iso-pentane and n-pentane can also be carried out in the same column allowing a fractionation in 3 sections according to the ISOM / STAB / DiP / DiH sequence according to FIG. The process according to the invention is thus distinguished from the process according to the prior art (FIG. 1) in that it comprises the successive separation of iso-pentane, n-pentane and branched C 6 compounds in this order according to Figure 3, or the simultaneous separation of n-pentane and iso-pentane in the same fractionation column according to Figure 4, followed by the separation of C6 branched compounds; or the separation of a C5 cut, then that of n-pentane and iso-pentane and that of the C6 branched compounds according to FIG.

In the process according to the invention, said separations are all located downstream of the isomerization section [1], and more precisely downstream of the stabilization column [2], unlike the methods of the prior art which do not have only one column of DiH (de-isohexanizer), or 3 fractionation columns, but with the DiP (de-isopentanizer) column located upstream of the isomerization section according to the diagram of FIG.

More precisely, the present invention can be described as a process for the isomerization of a light naphtha, or preferentially of a C5-C6 essentially, said process comprising two distillation separation stages located downstream of the isomerization step: - a first distillation separation step (3 + 4 block) to separate the 5-carbon hydrocarbons from the heavier compounds sent to the second separation section [5]. This first separation step consists in producing the following 3 cuts: a) an iso-pentane enriched cut (15) which is a product of the process, b) an enriched n-pentane cut (16) which is recycled to the section reaction [1], and c) a heavier hydrocarbons enriched fraction than pentanes (17) which is directed to a second separation step [5],

a second separation step [5] consisting of a separation column whose top and bottom products are products of the unit, namely a head stream (19) rich in branched C 6 compounds, a flow of bottom (18), and an intermediate cut (20) enriched in n-hexane, taken off at the side and recycled to the reaction section [1].

According to a first variant of the method according to the invention, represented by FIG. 3, the first separation step comprises two columns (3 and 4) arranged in series, that is to say that the bottom flow of the isopentanizer [3] feeds the de-pentanizer [4] as shown in figure 3. The isopentane stream (15) leaves at the top of the column [3] and the heavier hydrocarbon stream than the pentanes (17) leaves at the bottom of the column [4] to feed the second stage splitting [5].

According to a second variant of the process according to the invention, represented by FIG. 4, the de-isopentanizer and the depentanizer are combined into a single column allowing fractionation into 3 streams (denoted [3] in FIG. 4). The isopentane stream (15) exits at the top of the column [3], and the heavier hydrocarbon stream than the pentanes (17) leaves at the bottom of said column to feed the second fractionation step [5]. Intermediate withdrawal (stream 16) is recycled to the isomerization unit [1]. According to a third variant of the method according to the invention, represented by FIG. 5, the first separation step comprises the two columns [4] and [3] arranged in series in this order. That is to say that the flow (12) coming from the bottom of the stabilization column [2] feeds the de-pentanizer [4] from which the flow (21) which supplies the de-isopentanizer [3] . The bottom flow (17) of the depentanizer [4] fed the de-isohexanizer [5]. The de-isopentanizer [3] produces the isopentane-rich stream (15) at the top and the normal pentane-rich stream (16), which is recycled to the isomerization [1].

- According to other variants of the process, it is possible to use the available calories in the condenser of one of the columns [3], [4] or [5] to bring calories to the reboiler of one of the columns [3] , [4] or [5], for example, according to the variant illustrated in Figure 6, it is possible to perform a heat exchange between the de-pentanizer condenser [4] and the de-isopentanizer reboiler [3] .

DETAILED DESCRIPTION OF THE INVENTION

In the process according to the invention, the charge (10) is generally constituted by a light naphtha, preferably a C5-C6 cut, which may optionally contain heavier hydrocarbons. This feedstock is sent to a catalytic isomerization section [1], and the effluent (11) is fractionated in a fractionation section comprising the following steps:

a stabilization [2] of the isomerized effluent which consists in separating at the top the lighter compounds than the pentanes (stream 13), and in bottom a stabilized effluent (12) - a first step of separation by distillation (block 3 + 4) to separate the hydrocarbons with 5 carbon atoms of the heavier compounds sent to the second separation section [5]. This first separation step consists in producing the following 3 cuts: a) an iso-pentane enriched cut (15) which is a first product of the process, b) an enriched n-pentane cut (16) which is recycled to the reaction section [1], and c) a heavier hydrocarbons enriched fraction than pentanes (17) which is directed to a second separation step [5],

a second separation step [5] consisting of a separation column whose top and bottom products are the products of the unit, namely a head stream (19) rich in branched C6 compounds, a stream of bottom (18), and an intermediate cut (20) enriched in n-hexane, taken off at the side and recycled to the reaction section [1].

The iso-pentane-enriched cut (15) from the first separation step and the head (19) and bottom (18) streams from the second separation step may optionally be subsequently mixed to provide the product or products. of the process. Description of Figure 1 according to the prior art:

FIG. 1 shows the prior art scheme which may be considered as closest to the present invention.

The charge (10) is fed into a de-isopentanizer [3] which makes it possible to leave an isopentane stream (15) at the head. The bottom flow (14) of the de-isopentanizer [3] is sent to the isomerization reaction section [1] via line 14.

The operating conditions of the reaction section [1] are chosen so as to promote the conversion of n-paraffins of low octane number (li-pentane, n-hexane) to iso-paraffins of higher octane numbers. (Isopentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane). The isomerization reaction section [1] is generally operated in the presence of an acid catalyst. The effluent of the isomerization section [1], once stabilized by separation of the light compounds (13) in the stabilization column [2], is directed via line (12) to a de-pentanizer [4]. The head flow (16) of the de-pentanizer [4] is recycled to the column [3] of the de-isopentanizer.

The recycling of n-pentane, the de-pentanizer top product [4], via line 16 to the de-isopentanizer [3], makes it possible to increase the proportion of isomerized n-pentane in the isomerization section [1]. ], and therefore to obtain products of higher octane numbers.

The stream (16) can be recycled to the de-isopentanizer [3], either by introducing it alone directly into the de-isopentanizer [3] (according to FIG. 1), or mixed with the load 10 (not shown) . The stream (16) also contains isopentane formed in the isomerization section which is separated in the de-isopentanizer [3].

The products (18) and (19) are respectively from the bottom and the head of the de-isohexanizer [5] which is fed by the bottom stream (17) from the de-pentanizer [4]. Isopentane is substantially absent from these two streams, being essentially present in stream 15.

The process of FIG. 1 has the disadvantage of mixing a recycled isopentane enriched fluid via the pipe (16) with the charge (10) resulting from the pipe 10, either before it is admitted into the de-isopentanizer [3] or as shown in FIG. 1, inside said de-isopentanizer [3].

This mixture entails significant investment and operating costs since it is necessary subsequently to separate this isopentane again during the isopentane / n-pentane separation of the de-isopentanizer [3], and during separation n- pentane / heavier compounds of de-pentanizer [4]. This is particularly disadvantageous when the feed contains little iso-pentane. The method according to the invention makes it possible, among other things, to circumvent this problem.

Description of the Figures According to the Invention (FIGS. 2, 3.4 and 5):

In its most general form, the method according to the invention comprises:

a) a catalytic isomerization section [1] operated under the conditions described below, b) a stabilization of the isomerized effluent (11) in a stabilization column [2] which consists in separating at the top the compounds lighter than pentanes, and in bottom a stabilized effluent (12), c) a first separation step carried out in the distillation block (3 + 4) for separating the 5-carbon hydrocarbons from the heavier compounds sent to the second separation section. This first separation step consists, by the use of one or two fractionation columns, in producing the following 3 sections: an iso-pentane enriched cut which is a product of the process (15),

an enriched n-pentane cut (16) which is recycled to the reaction section [1] and,

- a cut enriched in hydrocarbons heavier than pentanes (17), which is directed to the second separation step [5].

A second separation step [5] which can preferably be carried out by means of a de-isohexanizer consisting of a separation column whose top product (19) is rich in branched C6 compounds, and an intermediate cut ( 20), enriched in n-hexane, taken off at the side, is recycled to the reaction section [1]. The iso-pentane enriched stream (14), bottom product (18) and overhead product (19) can be blended to form the product (s) of the process. The isomerization reaction is preferably carried out on a high-activity catalyst, such as for example a chlorinated alumina and platinum catalyst, operating at low temperature, for example between 100 and 300 ° C., preferably between 110 and 300 ° C. 240 ° C, at high pressure, for example from 2 to 35 bar (1 bar = 0.1 MPa), and with a low molar ratio hydrogen / hydrocarbons, for example between 0.1 / 1 and 1/1, Suitable known catalysts preferably comprise an alumina support and / or high purity preferably containing 2 to 10% by weight of chlorine, 0.1 to 0.40% by weight of platinum, and possibly other metals. These catalysts can be used with a space velocity of 0.5 to 10 h ^-1 , preferably 1 to 4 h ^-1 .

Maintaining the rate of chlorination of the catalyst generally requires the continuous addition of a chlorinated compound such as injected carbon tetrachloride mixed with the filler at a concentration ranging from 50 to 600 parts per million by weight. The isomerization catalysts of the process according to the invention may be preferentially included in the group consisting of:

the catalysts supported, most often by a mineral support, typically an oxide (for example an oxide of aluminum or silicon or their mixture), and containing at least one halogen and a Group VIII metal.

zeolitic catalysts containing at least one Group VIII metal

the Friedel and Crafts type catalysts,

acid or super acid catalysts, for example of the heteropolyanion (HPA) type on zirconia, the oxides of tungsten on zirconia, the sulphated zirconias. The isomerization reaction is preferably carried out in the presence of a high activity catalyst, such as for example a chlorinated alumina and platinum catalyst, operating at low temperature, for example between 100 and 300 ° C., preferably between 110 and 240 ° C, at high pressure, for example between 2 and 35 bar (1 bar = 0.1 MPa), and with a low molar ratio hydrogen / hydrocarbons, for example between 0.1 / 1 and 1 / 1. The catalysts that are preferentially usable consist of a carrier of high purity alumina preferably containing 2 to 10% by weight of chlorine, 0.1 to 0.40% by weight of platinum and possibly other metals.

They can be implemented with a space velocity of between 0.5 and 10 h -1, preferably between 1 and 4 h -1. Maintaining the rate of chlorination of the catalyst generally requires the continuous addition of a chlorinated compound such as injected carbon tetrachloride mixed with the filler at a concentration of preferably between 50 and 600 parts per million by weight.

Other catalysts with comparable acidity to these catalysts can also be used. According to a first variant of the process according to the invention (represented by FIG. 3), the feed is sent to the isomerization section [1] via line 10. The conditions of the isomerization section [1] are chosen so as to promote the conversion of low octane n-paraffins (n-pentane, n-hexane) to iso-paraffins of higher octane numbers. high (isopentane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane).

The effluent from the isomerization section (11), once stabilized by separation of the light compounds in the stabilization column [2], is then directed via line (12) to a de-isopentanizer [3] so as to recover at the head, via the pipe (15), a stream enriched in isopentane and bottom via the pipe (14) a fluid depleted in isopentane.

The de-isopentanizer fractionation conditions [3] are preferably such that the isopentane recovery rate at the top (isopentane flow at the top of the de-isopentanizer divided by the isopentane flow rate in the de-isopentanizer feedstock). ) is typically greater than 70%. The n-pentane content in the top product (15) is then typically less than 15% by weight, preferably less than 10% by weight.

The bottom of the de-isopentanizer [3] is directed via line (14) to a de-pentanizer [4] so as to recover at its head (stream 16) a fluid enriched in n-pentane and containing little isopentane, which is recycled to the isomerization reaction section [1] via line (16). A stream (17) containing mainly hydrocarbons with 6 or more carbon atoms (C6 + cut) is fed into the bottom via line (17) and feeds the de-isohexanizer [5].

The de-isohexanizer [5] consists of a separation column whose top product (19) is rich in branched C 6 compounds, and an intermediate cut (20) enriched in n-hexane, taken off at the side withdrawal, is recycled. to the reaction section [1].

The iso-pentane enriched stream (14), the de-isohexanizer bottoms product [5], and the de-isohexanizer overhead product (19) can be blended to form the process product (s).

The sizing of the fractionation column [4] and the fractionation conditions are preferably such that the overall recovery rate of n-pentane (flow rate of n-pentane at the top of the condenser [4] divided by the flow rate of n -pentane at the outlet of the isomerization reaction section [1] is typically greater than 80% The content of hydrocarbons with 6 or more carbon atoms at the top of the de-pentanizer [4] is typically less than 15%, preferably less than 10% by weight. Compared with the state of the art illustrated in FIG. 1, this first variant reduces the energy consumption of the process since the isopentane produced in the isomerization reactor [1] is vaporized only once before to be exported, and the de-isopentanizer [3] splits a C5-enriched C5 cut, which facilitates said separation. According to a second variant of the process according to the invention (represented by FIG. 4), the de-pentanizer [4] and the de-isopentanizer [3] are replaced by a single column [3] which is a de-isopentanizer with 3 cuts to separate the n-pentane.

the head product (15) is a fluid enriched in isopentane,

the intermediate stream (16) withdrawn laterally, via line (16), is a fluid enriched in n-pentane, the bottom product (17) is a fluid depleted in iso and n-pentane containing essentially hydrocarbons for more of 6 carbon atoms. This bottom flow (17) feeds the de-isohexanizer [5]. The second separation step in the de-isohexanizer is performed identically to the first variant of the invention.

According to a third variant of the process according to the invention (represented by FIG. 5), the effluent of the isomerization reaction section [1], once stabilized by separation of the light compounds in the stabilization column [2], is directed via the pipe (12) to the de-pentanizer [4] so as to recover at the head, via the pipe (21), a C5 cut C6 depleted, and bottom via the pipe (17), a fluid containing mainly hydrocarbons with 6 or more carbon atoms, which feeds the de-isohexanizer [5]. The second separation step in the de-isohexanizer is performed identically to the first variant of the invention.

The cut C5 feeds, through the pipe (21), the de-isopentanizer [3] which allows to withdraw at the top of column iso-pentane (15), and bottom n-pentane (16) which is recycled to the reaction section [1].

Thermal integration

As in the prior art, the invention has other variants according to various thermal integrations. The principle of these thermal integrations consists in choosing the operating pressure of a first column so that the condensing temperature at the top of this column is greater than the reboiling temperature of one or more other columns of the process.

The exchange of heat between the top condenser of the first column to be cooled, and the bottom reboiler of another column to be heated, then replaces at least part, if not all, of the utility consumption. cold used at the top of the first column to ensure its cooling, and the hot utility used in the bottom of the second column to ensure its warming.

The terms "first column" and "other column" are generic since it is the choice of the column having the highest condenser temperature that defines it as the first column.

Thus, FIG. 6 shows an example of a mode of thermal integration between the de-pentanizer [4], considered as the first column and the de-isopentanizer [3] considered as the other column, according to the first variant (represented by FIG. FIG. 3) of the process according to the invention. FIG. 6 thus shows a heat exchange between the condenser of the column [4] (depentanizer) and the reboiler of the other column [3] (de-isopentanizer). Any other column coupling could be envisaged, for example an integration between the de-isohexanizer condenser [5] and the depentanizer reboiler [4] or between the de-isohexanizer condenser [5] and the reboiler of the de-isohexanizer [5]. isopentanizer [3] or between the condenser of the deisohexanizer [5] and the two reboilers depentanizer [4] and de-isopentanizer [3]. One of these columns may also include an intermediate withdrawal (fractionation column in 3 sections).

In summary, the invention relates to a process for the isomerization of a light naphtha.This process comprises an isomerization reaction step [1], followed by a stabilization step [2] of the reaction effluents, and two separation steps by distillation of the bottom stream from the stabilization stage [2]:

A first step of separation by distillation (block 3 + 4) making it possible to separate the hydrocarbons with 5 carbon atoms from the heavier compounds sent to the second separation section [5], said first separation step producing the 3 slices the following: a) an iso-pentane enriched cut (15) which is a product of the process, b) an enriched n-pentane cut (16) which is recycled to the reaction section [1], and c) an enriched cut in hydrocarbons heavier than pentanes (17) which is directed to a second separation step [5], 2 - a second separation step [5] consisting of a separation column of which the top and bottom products are the products of the unit, namely a head stream (19) rich in branched C6 compounds, a bottom stream (18), and an intermediate cut (20) enriched in n-hexane, taken off at the side rack which is recycled to the reaction section [1].

In a preferred manner, in the isomerization process according to the invention, the first separation step comprises two columns, a de-isopentanizer [3] and a de-pentanizer [4] arranged in series, that is to say say that the bottom stream (14) of the de-isopentanizer [3] feeds the de-pentanizer [4], the isopentane stream (15) comes out of the column [3], a stream enriched with heavier hydrocarbons the pentanes (17) exit at the bottom of the column [4] and supply the de-isohexanizer [5], and the top stream (16) of the column [4] is recycled to the isomerization unit [1] ]. According to another preferred variant of the isomerization process according to the invention, the first separation step comprises only one column [3], in which the flow of isopentane (15) leaves at the top of the column [3] , the stream enriched in heavier hydrocarbons than the pentanes (17) leaving at the bottom of said column [3] feeds the de-isohexanizer column [5], and the intermediate withdrawal (stream 16) is recycled to the unit of isomerization [1]. According to another preferred variant of the isomerization process according to the invention, the first separation step comprises the two columns [4] and [3] arranged in series in this order, in which the flow (12) coming from the column of stabilization [2] feeds the de-pentanizer [4] whose flow (21) which feeds the de-isopentanizer [3], and in which the bottom stream: enriched in hydrocarbons heavier than pentanes (17 ) of the de-pentanizer [4] feeds the de-isohexanizer [5], the de-isopentanizer [3] producing the isopentane-rich stream (15) at its head, and the flow (16), rich in normal pentane, in the background , which is recycled to the isomerization unit [1].

According to another variant of the isomerization process according to the invention, a heat exchange is carried out between the condenser of one of the columns [3], [4] or [5] and the reboiler of one of the columns [3], [4] or [5]. According to a first embodiment of this variant, the heat exchange is carried out between the de-isohexanizer condenser [5] and either the de-pentanizer reboiler [4] or the de-isopentanizer reboiler [3], either both. According to a second embodiment, the heat exchange is carried out between the de-pentanizer condenser [4] and the re-isopentanizer reboiler [3].

EXAMPLES ACCORDING TO THE INVENTION

EXAMPLE 1

This example is based on the load (10) whose composition is detailed in Table 1 below:

The reaction section consists of 2 isomerization reactors operating in series. The inlet temperature of the two reactors is 120 ° C.

The inlet pressure of reactor 1 is 35 bar absolute. The inlet pressure of the second reactor is 33 bar absolute.

The catalyst used consists of an alumina support containing 7% by weight of chlorine, and 0.23% by weight of platinum and optionally other metals.

The space velocity is 2.2h ^-1 . The hydrogen to hydrocarbon molar ratio is 0.1 / 1. The operating pressures of the columns are chosen so that the head temperature is compatible with the cooling means usually available (cooling water or air at room temperature).

The pentane recycle rate is defined as the flow rate of the recycled n-pentane enriched fluid to the isomerization reaction section divided by the fresh feed rate. The hexane recycle rate is defined as the flow rate of the recycled n-hexane enriched fluid to the isomerization reaction section divided by the fresh feed rate.

For both the process according to the prior art shown in FIG. 1, and for the process according to the invention represented by FIGS. 3 and 4, the recycling rates of the pentanes and of the hexanes are chosen so as to obtain a flow rate. constant at the isomerization reaction section [1], which corresponds to the same amount of catalyst for a given space velocity in the isomerization reactor [1].

The products (or outputs) of the processes are defined as the mixture of the top products (19) and bottom (18) of the de-isohexanizer [5], and the top product (15) of the de-isopentanizer [3] enriched in isopentane. The compositions of the products obtained are summarized in Tables 2 to 4 which follow:

Table 5 below compares the results obtained with the various schema variants according to the prior art and according to the invention. In this table 5, we use the following notions:

1: Efficiency defined as mass flow rate of product divided by fresh charge rate.

2: heat exchange with the bottom of the stabilization column

3: the feeding and racking trays are identified by their order number, all trays being numbered from 1 from top to bottom.

Table 5 draws the following conclusions:

1. The diagram with a de-isopentanizer and de-pentanizer according to the invention (FIG. 3) compared with the prior art with these same columns (FIG. 1) has a lower column size and needs for hot utilities. This necessarily leads to lower investment and operating costs. In addition, the octane number obtained is better.

2. The diagram according to the invention of FIG. 4, with a single column [3] fulfilling the role of de-isopentanizer and de-pentanizer, and 3 sections extracted from said column, has an advantage in terms of investment by compared to the implementation in two separate columns, and demonstrates, for an octane number and a yield close to the prior art, a sharp decrease in the requirements of hot utilities.

EXAMPLE 2

The operating conditions of the reaction section are unchanged with respect to Example 1.

Table 6 below presents the results of a thermal integration between the de-isohexanizer [5] and the de-isopentanizer [3] and de-pentanizer [4] according to the invention. In the diagram of Figure 3, the de-isohexanizer [5] is operated at a pressure of 8 bar absolute, the condensation temperature of the column head is then 127 ° C. A heat exchange is then possible between this column head and the de-pentanizer reboiler [4] operated at 87 ° C and the de-isopentanizer reboiler [3] operated at 109 ° C.

In the diagram of Figure 4, the de-isohexanizer [5] is operated at a pressure of 8 bar, the condensation temperature of the column head is then 127 ° C. A heat exchange is then possible between this column head and the de-isopentanizer reboiler [3] operated at 115 ° C. In Table 6 we use the following notions:

1: need covered by condensation of de-isohexanizer head [5] without resorting to a hot utility.

2: Efficiency defined as mass flow rate of product divided by fresh feed rate

3: heat exchange with the bottom of the stabilization column [2]

The requirements for hot utilities of the scheme according to Figure 3 are decreased by 10.2 M W (31.6 MW to 21.4 MW).

The requirements for hot utilities of the scheme according to Figure 4 are reduced by 4.8 MW (24.9 MW to 20.1 MW). With moderate over-investment for the DIH column [5], these thermal integrations make it possible to significantly reduce the operating costs without altering the performance of the unit.

EXAMPLE 3

The operating conditions of the reaction section [1] are unchanged with respect to Example 1. The flow diagram is that of FIG. 3 supplemented by the thermal integration detailed in FIG. 6. The de-pentanizer [4] is operated at a pressure of 11 bar absolute, the condensation temperature of the column head is then 123 ° C. A heat exchange is then possible between this column head and the de-isopentanizer reboiler [3] operated at 109 ° C. Table 7 details the results obtained.

In Table 7 we use the following concepts:

1: Of which 7.5MW covered by condensation of the de-isohexanizer head without the use of a hot utility.

2: need covered by condensation at the head of de-isohexanizer without resorting to hot utility.

3: heat exchange with the bottom of the stabilization column The hot utility requirements of Figure 6 are reduced by 5.6 MW (31.6 MW to 26.0 MW).

By means of: a moderate over-investment for the de-pentanizer [4], this thermal integration makes it possible, without altering the performance of the unit, to significantly reduce its operating cost.

Claims

1) Process for the isomerization of a light naphtha, said process comprising an isomerization reaction step [1], said step taking place under the following conditions:

- temperature between 100 and 300 ° C, preferably between 110 and 240 ° C,

pressure, from 2 to 35 bar (1 bar = 0.1 MPa), and

- molar ratio hydrogen / hydrocarbons, between 0.1 / 1 and 1/1,

- Space velocity of 0.5 to 10 h ^-1 , preferably 1 to 4 h ^-1 . the catalysts used being constituted by a high purity alumina support preferably containing 2 to 10% by weight of chlorine, 0.1 to 0.40% by weight of platinum, and possibly other metals, said step of isomerization being followed by a stabilization step [2] of the reaction effluents, and two steps of separation by distillation of the bottom flow from the stabilization step [2] placed downstream of the stabilization step (2 ), the two separation steps being as follows:

1- a first step of separation by distillation (block 3 + 4) for separating the 5-carbon hydrocarbons from the heavier compounds sent to the second separation section by distillation [5], said first separation step producing the 3 following cuts: a) an iso-pentane enriched cut (15) which is a product of the process, b) an n-pentane enriched cut (16) which is recycled to the reaction section [1], and c) a cross section. enriched in heavier hydrocarbons than pentanes (17) which is directed to a second separation step [5],

2- a second separation step [5] consisting of a separation column whose top and bottom products are the products of the unit, namely a head stream (19) rich in branched C6 compounds, a stream base (18), and an intermediate cut (20) enriched in n-hexane, taken off at the side and recycled to the reaction section [1], isomerization process in which a heat exchange is carried out between the condenser one of the columns [3], [4] or [5] and the reboiler of one of the columns [3], [4] or [5]. 2) The isomerization process of a light naphtha according to claim 1, wherein the first separation step comprises two columns a de-isopentanizer [3] and a de-pentanizer [4] arranged in series, that is to say that the bottom stream (14) of the de-isopentanizer [3] feeds the depentanizer [4], the isopentane stream (15) leaves the top of the column [3], a stream enriched in hydrocarbons more heavy as the pentanes (17) leaves at the bottom of the column [4] and feeds the de-isohexanizer [5], and the top stream (16) of the column [4] is recycled to the isomerization unit [ 1]. 3) Process for the isomerization of a light naphtha according to claim 1, wherein the first separation step comprises only one column [3], in which the flow of isopentane (15) leaves at the top of the column. [3], the stream enriched in hydrocarbons heavier than the pentanes (17) exiting at the bottom of said column [3] feeds the de-isohexanizer column [5], and the intermediate withdrawal (stream 16) is recycled to the isomerization unit [1]. 4) Process for the isomerization of a light naphtha according to claim 1, wherein the first separation step comprises the two columns [4] and [3] arranged in series in this order, the flow (12) from the column stabilization system [2] feeds the de-pentanizer [4], whose flow (21) feeds the de-isopentanizer [3], and the bottom flow of the de-pentanizer [4] enriched in heavier hydrocarbons. the pentanes (17) feeds the de-isohexanizer [5], the de-isopentanizer [3] producing the isopentane-rich stream (15) at its head, and the normal pentane-rich stream (16) at the bottom; is recycled to the isomerization unit [1]. 5) Process for the isomerization of a light naphtha according to claim 1, wherein a heat exchange is carried out between the condenser of the de-isohexanizer [5] and either the re-solvent reboiler [4] or the reboiler of the de-isopentanizer [3], or both. 6) Process for the isomerization of a light naphtha according to claim 1, wherein a heat exchange is carried out between the de-pentanizer condenser [4] and the de-isopentanizer reboiler [3].