TW201708526A - Process for cycling the thermal balance on a unit for the catalytic cracking of naphtha known as a NCC unit - Google Patents

Abstract

The present invention describes a process for the catalytic cracking of a naphtha cut in which the thermal balance is provided by combustion of a fraction of the C5 cut of the feed, which fraction contains only normal and iso-paraffins, while retaining the cyclopentane in said feed.

Description

Method for thermal equilibrium cycle of naphtha catalytic cracking unit (NCC unit)

This invention relates to the field of refining processes and petrochemicals, and in particular to the field of cracking light naphtha or straight run gasoline type hydrocarbon fractions.

The recent concern about cracking straight-run gasoline-type alkane feeds in the FCC (Acronym for Fluid Catalytic Cracking) unit to upgrade them to propylene and ethylene is relatively recent. A catalyst cracking process for ethylene and propylene from paraffin streams, The Advanced Catalytic can be cited in the field of patent WO 07043741 and 2007 at the Spring National Meeting of Houston (Texas) 2007. Olefins (ACO) Process.

The price difference between light olefins and gasoline is currently driving a strong push for this type of cracking. In addition, advances in improving zeolite catalysts are contributing to substantial production of light olefins. This novel type of FCC unit is currently commonly referred to as NCC, a naphtha catalytic cracking. This acronym will be used in the rest of this article.

Catalytic cracking of light alkane fractions poses a problem because this type of feed does not produce enough coke under FCC conditions to satisfy the heat generated by the feed vaporization (introduction of liquid) and endothermic cracking reaction by burning the coke demand.

The thermal balance of these units can only be obtained by supplying external heat to the process. The usual approach involves recycling the “slurry” fraction with high coke potential to regeneration. In the device.

The term "slurry" means a fraction having a distillation range generally in the range of from 250 ° C to 350 ° C, which fraction is relatively rich in aromatic coke precursor materials.

The invention proposes to feed a fraction of the C5 fraction of the feed, more precisely a fraction having a boiling point below 42 ° C (corresponding to a normal chain and an isoparaffin, but without cyclopentane), for use as an NCC The unit supplies the fuel of the required heat.

This selection is recommended due to the original test conducted on a pilot scale, which shows that this fraction is the most difficult fraction to lyse. The fraction is burned directly with the coke of the catalyst in a regenerator or as a fuel in a preheat furnace to preheat the feed or air to the regenerator to add the fraction to the heat balance of the unit.

1‧‧‧ Straight-running gasoline feedstock

2‧‧‧Separator

3‧‧‧Top stream / top fraction

4‧‧‧ bottom flow / bottom fraction

5‧‧‧fuel/C5 fraction

6‧‧‧fuel

7‧‧‧Preheating furnace

Section 8‧‧‧

9‧‧‧Feed

10‧‧‧ Part/Feed

11‧‧‧ Naphtha catalytic cracking (NCC) unit

12‧‧‧Reactor/lift tube

13‧‧‧Regenerator/catalyst regeneration zone

14‧‧‧Stripper/separator

15‧‧‧Heat Catalyst

16‧‧‧Cleaning products/material streams

17‧‧‧ coking catalyst

18‧‧‧ fractionation tower/fraction unit

19‧‧‧dry

20‧‧‧Ethylene

21‧‧‧C3-C4 fraction

22‧‧‧ material flow

23‧‧‧One part of the bottom fraction / material flow

24‧‧‧ material flow

25‧‧‧ burning air

26‧‧‧ material flow

27‧‧‧Some dry gas

28‧‧‧ gasoline pool

Figure 1 provides a flow chart of the process of the invention, emphasizing the following main elements: an NCC unit 11 comprising at least one riser 12 and a regenerator 13, a separator 2 for separating the positive and negative chains of the C5 fraction The paraffin fraction and the remainder of the feed, the positive and isoparaffin fractions of the C5 fraction exit from the top (stream 3), the remainder of the feed exits from the bottom (stream 4) and contains cyclopentane , furnace 7, for preheating feeds 4 and 10 and combustion air 25 for the regenerator, • fractionation unit 18 for separating the effluent from the NCC into different fractions 19, 20, 21, 22 And 23.

None of the prior art documents explicitly mentions that the fractions described in the present invention contribute to the thermal equilibrium of the catalytic cracking unit of the naphtha fraction.

NCC is a specific catalytic cracking unit that can be considered a branch of a FCC catalytic cracking unit; it is specifically characterized by cracking an extremely light paraffin feed (gasoline) to produce light olefins.

One of the key points of this type of method is the heat balance cycle of the unit. Actually, for In the case of the FCC, coke production can be used to generate enough heat to meet the heat demand of the unit, that is, the heat of the feed (the introduced liquid) and its endothermic cracking.

In the case of NCC units, due to the nature of the feedstock and catalyst, coke production is often insufficient to create a heat balance in the unit. Therefore, in the prior art, an external heat source is required.

The present invention uses one of the most difficult catalytic cracking in the feedstock (which consists of a positive or isoparaffinic compound containing 5 carbon atoms) as an energy source, by burning it directly in the regenerator, or by using it Used as a fuel in the furnace to preheat the feed and regenerate the air.

In addition to supplying the required heat to the unit, it has also surprisingly been found that when a C5 fraction having a boiling point below 42 ° C in the naphtha feed has been eliminated from the feed while retaining the cyclopentane in the feed The efficiency of NCC has been improved.

The present invention can be defined as a method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, comprising the steps of catalytically cracking the feed in the presence of a cracking catalyst, and depositing it by combustion a step of regenerating a catalyst on the surface of the catalyst to regenerate a catalyst, wherein at least a portion of the fraction comprising a normal or isoparaffinic compound having 5 carbon atoms is eliminated, and the cyclopentane in the feed is retained in a manner For example, the fractions that are eliminated are used, in whole or in part, as fuel in the catalytic cracking process to cycle the heat balance of the process.

According to a first variant of the method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, a fraction of the C5 compound (ie containing 5 carbon atoms) eliminated from the feed to be treated The fraction of the normal or isoparaffinic compound is transferred directly to the regenerator of the NCC unit.

According to a second variant of the method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, the fraction of the C5 compound eliminated from the feed to be treated is transferred as fuel to the furnace to Hot feed.

More precisely and with reference to Figure 1, the process of the invention can be described as follows: as claimed in claims 1 to 3 for the catalytic cracking of a straight range having a distillation range of 30 ° C to 220 ° C A method of refueling a gasoline feed in which the following operations are carried out: • Dividing feed 1 into two parts, 8 and 10, • transferring part 8 to C5 separator 2, which can be used to separate positive and extra-chain C5 alkanes and feeds For the remainder of the feed, the normal and off-chain C5 alkanes exit with the overhead stream 3, and the remainder of the feed exits from the bottom via stream 4. • Top stream 3 is primarily a hydrocarbon fraction consisting of C5 and consists of a positive chain and a different a chain C5 alkane composition, ● bottom stream 4 is a heavy fraction of the feed (cyclopentane and C6+ type compounds), which in particular contains cyclopentane, ● a portion of the feed that is not separated in separator 2 and The bottoms fraction 4 from the separator 2 is combined to form a stream 26 which, after preheating in the furnace 7, constitutes a feed to the reactor 12, • separates the cracked product 16 and the coking touch via the stripper/separator 14. The medium 17, ● burns coke by contacting the combustion air 25 supplied to the regenerator 13 in the regenerator 13 to regenerate the coking catalyst 17, and contains a fraction of the normal chain and the isobaric C5 alkane in the C5 hydrocarbon stream. Used as fuel 6 in the preheating furnace 7, or directly as the fuel 5 in the regenerator 13, or when heat balance is not required The entire fraction 3 is used by transferring it to the gasoline pool 28, • separating the cracked product 16 and the coking catalyst 17 via a stripper/separator 14 and transferring it to the fractionation column 18 for separation into a dry Gas 19, ethylene 20, C3-C4 fraction 21 and bottoms fraction (stream 22 and stream 23).

In the method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, a fraction of a positive or isoparaffin type compound having 5 carbon atoms (corresponding to stream 3) The boiling point having a boiling point of 42 ° C or less and having a cyclopentane and a C6 + compound in the feed (corresponding to stream 4) has a boiling point above 42 °C.

In the present invention for catalytic cracking of a straight-run steam having a distillation range of 30 ° C to 220 ° C In the oil feed method, a portion of the fraction composed of a positive or isoparaffin having 5 carbon atoms can be transferred to the gasoline pool 28.

In the method of the present invention for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, the combustion air may be preheated in the preheat furnace 7.

In the process of the present invention for catalytically cracking a straight run gasoline feed, a portion 23 of the bottoms fraction can be recovered to the inlet of the reactor 12.

In the method for catalytically cracking a straight-run gasoline feed of the present invention, the reactor may be of the "lift tube" type, that is, having a fluidized upflow.

In the method of the present invention for catalytically cracking straight-run gasoline feedstock, the reactor may be of the "downpipe" type, i.e., having a fluidized downflow.

Finally, in the process of the invention for catalytically cracking straight-run gasoline feedstocks, the catalyst may comprise ZSM-5 zeolite, or even all of ZSM-5 zeolite.

The catalytic cracking unit (FCC) typically treats heavy fractions (such as VGO (vacuum gas oil) or vacuum residue mixtures) or even atmospheric residue obtained from a vacuum distillation unit, either alone or in a mixture.

However, there may be situations where the feed to the FCC is greatly reduced due to previous pretreatment (e.g., VGO), or because it is derived from a conversion unit where the initial feed is already rich in hydrogen and some impurities have been eliminated.

Recent adaptation of FCC to even lighter feeds such as gasoline (distillation range 30 ° C to 220 ° C) is intended to convert these light alkane feeds to light olefins (ethylene and propylene), ie in the petrochemical market High value-added products.

The FCC is thus referred to as the NCC. In this case, the method must address the problems associated with low yields of coke from the feed, and thus the heat balance cycle.

In the present invention, a fraction having an exact boiling point (corresponding to the lightest fraction of the feed and corresponding to the most difficult catalytic cracking fraction) is used as a fuel for the heat balance of the NCC unit. The heat required should be increased and the production from the unit increased at the same time.

Figure 1 illustrates a layout of the process of the invention using a separator 2 that separates the fractions to be taken from the feed for the NCC unit.

Feed 1 is a straight run gasoline having paraffinic properties, i.e., consists of at least 50% paraffinic compounds. It contains a C5 compound (i.e., contains 5 carbon atoms) which can be classified into n-pentane, isopentane, and cyclopentane.

Divide it into two parts: 8 and 10. Part 8 is passed to separator 2, which can be used to separate the positive and extra-chain C5 alkanes from the remainder of the feed, with the normal and off-chain C5 alkanes exiting with the overhead stream 3, and the remainder of the feed via the feed stream 4 Leave at the bottom.

The overhead fraction 3 is a hydrocarbon fraction mainly composed of C5 having a boiling point of 42 ° C or lower and consists of a normal chain and an iso-chain C5 alkane.

The bottom fraction 4 is the heavy fraction of the feed (cyclopentane and C6+) having a boiling point above 42 °C. In particular, it contains cyclopentane having a boiling point of 49 °C.

The NCC unit 11 is represented schematically by a reactor 12 in which a feed 9 (corresponding to the bottom stream 4 heated in the preheat furnace 7) is brought into contact with the thermal catalyst 15 to cause cracking.

Reactor 12 can be operated in a parallel flow mode (referred to as "lift" mode) or in a descending parallel flow mode (referred to as "downpipe" mode).

The cracked product 16 is separated from the coking catalyst 17 via a stripper/separator 14.

The coking catalyst 17 is regenerated by burning coke in contact with the combustion air 25 supplied to the regenerator in the regenerator 13. This combustion air originates from the material stream 24 which has been preheated in the preheating furnace 7.

The portion 10 of the feed that has not been separated via separator 2 is reintroduced from bottom fraction 4 from separator 2 to form stream 26 which, after preheating in furnace 7, constitutes feed 9 into reactor 12.

The fraction 3 containing a positive chain and an isochain C5 alkane in the C5 hydrocarbon stream is used as the fuel 6 in the preheating furnace 7, or directly as the fuel 5 in the regenerator 13, or actually when the heat balance is not required The entire fraction 3 is transferred to the gasoline pool 28.

The cracked product 16 is passed to a fractionation column 18 for separation into dry gas 19, ethylene 20, C3-C4 fraction 21 and bottoms fraction; a portion 23 of the latter can be recovered to the inlet of riser 12.

A portion of the dry gas 27 can also be transferred to the regenerator 13 for use as a fuel.

The flow rate of the portion of the feed 8 that is delivered to the separator 2 can cover a wide range: the minimum corresponds to the value required to deliver sufficient C5 fraction 5 to the regenerator to produce a heat balance, and the maximum corresponds to feed 1 The total flow rate.

Characteristics of the feed entering the NCC

Part 8 of the stream 1 entering the separator 2 is a gasoline feed obtained directly from atmospheric distillation. The distillation range is usually from 30 ° C to 220 ° C. Depending on the crude oil used at the inlet of the fractionation column, the composition may vary widely. Its initial boiling point is usually above 30 ° C and its final boiling point is usually below 220 ° C.

The separator 2 functions to fractionate the feed by separating the lightest fraction containing the five carbon atoms (except cyclopentane) from the remainder of the feed. Since the separator 2 was operated at 42 ° C, cyclopentane having a boiling point of 49 ° C was found in the bottom stream 4 .

Characteristics of the material flow leaving the NCC

Stream 16 exiting separator/stripper 14 contains all of the reaction effluent produced in the NCC unit reactor, as well as the unconverted portion.

Operating conditions of the NCC unit

The NCC unit is a catalytic naphtha cracking unit having at least one primary reactor that acts in a forward flow mode ("lift") or a downward flow mode ("downpipe"). It has a separator-stripper zone 14 separating the catalyst from the hydrocarbon effluent.

It also has a catalyst regeneration zone 13 in which the coke formed during the reaction and deposited on the surface of the catalyst is burned, on the one hand to restore its activity as a catalyst, and on the other hand to produce a vaporization of the feed. It also covers the amount of heat required for the endothermic nature of the cleavage reaction.

The NCC unit itself has a zone for treating hydrocarbon effluents and in particular has a gas treatment Zone for separating light olefins (ethylene, propylene) from other gases: hydrogen, methane, ethane, propane, and for separating C4 fractions.

The heaviest portion of the hydrocarbon effluent is treated in a separation zone comprising at least one fractionation unit to recover different liquid fractions (not shown in Figure 1).

The NCC unit is preferably operated at high strength (i.e., high reactor outlet temperature (ROT) and high ratio of catalyst: feed mass flow rate (C/O)).

Table 1 below gives a wide range of operating conditions for NCC units:

Preferably, the reactor outlet temperature is in the range of from 600 ° C to 730 ° C and the C/O ratio is in the range of from 10 to 35, and still more preferably in the range of from 15 to 30. The catalyst can be any type of catalytic cracking catalyst, preferably containing a high proportion of zeolite.

ZSM-5 may or may not be supplemented, and may even be 100% composed of ZSM-5. The following examples are intended to illustrate the benefits of eliminating the normal and off-chain C5 alkane fractions from the straight run gasoline feed and retaining it for the combustion required for thermal equilibrium. On the other hand, it is stated that the elimination of all or a portion of this C5 fraction means that, in addition, the yield of light olefins from the unit can be increased.

Instance

Example 1 is consistent with prior art. Example 2 is consistent with the present invention.

Example 1 shows the yield structure obtained during the cracking of the straight run feed and during the cracking of the same feed which eliminated all C5 fractions (distillation has been carried out at 52 °C).

The cracking is carried out in a pilot unit representing an FCC unit with a single riser and two regenerators (denoted R2R).

Because the maximum propylene production is envisaged, the operating conditions are harsh: ● riser outlet temperature (ROT) = 611 ° C, ● C / O = 40.

The catalytic system consisted of "steaming" a commercially available ZSM-5, i.e., ZSM-5 which was subjected to maturation for 24 hours at 800 ° C under steam pressure.

Table 2 below shows the composition of the complete feed and the composition of the feed which removes the C5 fraction but retains the cyclopentane (and thus the feed of the present invention), and the corresponding distillation point.

Table 3 below gives the cracking results of the straight-run gasoline used as the feed for the NCC unit and the same feed obtained from the same feed of the C5 fraction removed:

The use of gasoline-based feeds to remove the NCC unit produces a very attractive light olefin production (20.9 T/h for ethylene and propylene). Table 3 also shows that the material containing 5 carbon atoms is not treated in the NCC unit ( The benefits of cyclopentane).

For each 100 T/h feed, an additional 1.92 T/h propylene was obtained with gasoline containing the C5 fraction but containing cyclopentane.

It should also be noted that ethylene production is increased by 0.3 T/h and butene production is increased by 0.4 T/h.

The reactivity of the compound of the feed:

Detailed analysis of the feed and reaction products can be used to calculate the reactivity of each of the compounds of the feed.

This reactivity of each compound is expressed as Cx and is defined by the following formula: reactivity = [% by weight of Cx in the feed - % by weight of Cx in the product] / % by weight of Cx in the feed, x 100.

Table 4 shows the reactivity of each of the compounds fed in the basic case under the operating conditions of Example 1.

It should be noted that the very low value of the normal chain C5 alkane (10.8) and the negative value of the C5 isoparaffin (-8.5). Therefore, there is no shortage of eliminating these two families from the feeds that will be processed using NCC. At the office.

Analysis of reactivity according to the chemical family also shows the benefit of retaining the cyclopentane in the feed to be treated by the NCC, and thus adjusting the distillation point to below 52 °C to eliminate all C5 compounds but to 42 ° C to retain the rings The benefits of pentane.

A distillation point of 42 ° C can be used to extract normal paraffins and isoparaffins containing 5 carbon atoms while retaining cyclopentane.

Table 5 provides a list of boiling points for paraffins, isoparaffins, and cycloalkanes containing 5 carbon atoms.

Example 2

To clarify the method of the present invention, we calculated the heat loss to ensure the thermal equilibrium of the NCC unit operating under the following conditions: ROT = 620 ° C and C / O = 40.

The straight-run gasoline feed has a flow rate of 100 T/h.

The coke production of this feed was estimated to be 1.6%.

Crack the feed and heat the feed from 150 ° C to 620 ° C and steam (estimated as feed) The heat required to heat up to 620 ° C from 150 ° C is about 40 MW.

The temperature of 650 ° C in the regenerator allows the heat balance of the unit to circulate.

In fact, the heat transferred by the catalyst from 650 ° C to 620 ° C is about 40 MW (the specific heat of the catalyst is estimated to be 1.2 kJ / kg / ° C).

In the regenerator, the amount of heat required to heat the catalyst from 620 ° C to 650 ° C is also 40 MW.

The heat required to raise the temperature of the gaseous effluent and compensate for the heat loss (estimated to be 3% of the heat of combustion) is approximately 5 MW. However, in my case, coke combustion only delivers 20 MW.

Therefore, the calculation shows that the heat loss corresponds to approximately 25 MW.

The combustion of the normal and iso-chain C5 alkanes (defined by their distillation point at 42 ° C) provided 48,600 kJ/kg.

Therefore, the C5 fraction must be burned at 1.9 T/h to balance the heat balance of the NCC unit under the conditions of the examples.

The ratio of this C5 fraction is approximately 15% of the initial feed, ie the flow rate is 15T/h, and thus covers most of the required heat, and therefore only a portion of the C5 fraction contained in the feed is required to balance The thermal balance of the unit.

The portion to which this heat balance applies is 10% of all C5 fractions. Therefore, it basically covers all positive and extra-chain C5 alkanes.

Thus, it can be concluded that the heat source comprised of the normal chain and the iso-chain C5 alkane fraction contained in the straight-run gasoline feed is substantially sufficient to provide a heat balance for the NCC unit.

1‧‧‧ Straight-running gasoline feedstock

2‧‧‧Separator

3‧‧‧Top stream / top fraction

4‧‧‧ bottom flow / bottom fraction

5‧‧‧fuel/C5 fraction

6‧‧‧fuel

7‧‧‧Preheating furnace

Section 8‧‧‧

9‧‧‧Feed

10‧‧‧ Part/Feed

11‧‧‧ Naphtha catalytic cracking (NCC) unit

12‧‧‧Reactor/lift tube

13‧‧‧Regenerator/catalyst regeneration zone

14‧‧‧Stripper/separator

15‧‧‧Heat Catalyst

16‧‧‧Cleaning products/material streams

17‧‧‧ coking catalyst

18‧‧‧ fractionation tower/fraction unit

19‧‧‧dry

20‧‧‧Ethylene

21‧‧‧C3-C4 fraction

22‧‧‧ material flow

23‧‧‧One part of the bottom fraction / material flow

24‧‧‧ material flow

25‧‧‧ burning air

26‧‧‧ material flow

27‧‧‧Some dry gas

28‧‧‧ gasoline pool

Claims (11)

A method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C, comprising the steps of catalytically cracking the feed in the presence of a cracking catalyst, and depositing on the catalyst surface by combustion a step of regenerating the catalyst to regenerate the catalyst, wherein at least a portion of the fraction comprising a normal or isoparaffinic compound having 5 carbon atoms is eliminated, and the cyclopentane in the feed is retained, such as in The eliminated fraction is used in the catalytic cracking process in whole or in part as a fuel to recycle the heat balance of the process. A method for catalytically cracking a straight-run gasoline feed having a distillation range of from 30 ° C to 220 ° C according to claim 1, wherein the fraction of the C5 compound eliminated from the feed to be treated, that is, comprising 5 carbon atoms The fraction of the normal chain or isoparaffinic compound is transferred directly to the regenerator of the NCC unit. A method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C according to claim 1, wherein the fraction of the C5 compound eliminated from the feed to be treated is delivered as a fuel to the furnace for pre-treatment Heat the feed. A method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C as claimed in claims 1 to 3, wherein the following operation is carried out: dividing the feed (1) into (8) and (10) In two parts, the part (8) is sent to a C5 separator (2) which can be used to separate the positive and extra-chain C5 alkanes from the rest of the feed, with the top and off-chain C5 alkanes flowing with the top (3 Leaving, the remainder of the feed exits from the bottom via stream (3), which is primarily a hydrocarbon fraction consisting of C5 and consists of a normal chain and an iso-chain C5 alkane, the bottom stream (4) Is a heavy fraction of the feed (cyclopentane and C6+ type compounds), which in particular contains cyclopentane, The portion (10) of the feed which is not separated in the separator (2) is combined with the bottom fraction (4) from the separator (2) to form a stream (26) which is in the furnace ( 7) After preheating, the feed entering the reactor (12) is separated from the coking catalyst (17) via a stripper/separator (14) by means of a regenerator (13) The combustion gas (25) supplied to the regenerator (13) is contacted to burn the coke to regenerate the coking catalyst (17), and the fraction of the normal chain and the isobaric C5 alkane in the C5 hydrocarbon stream is used (3) As the fuel (6) in the preheating furnace (7), or directly as the fuel (5) in the regenerator (13), or when the heat balance does not require the use of the entire fraction (3) Used in a gasoline pool (28), separating the cracking products (16) from the coking catalyst (17) via the stripper/separator (14) and transferring the cracking products (16) to the fractionation column (18) is separated into dry gas (19), ethylene (20), C3-C4 fraction (21) and bottom fraction (stream 22 and stream 23). A method for catalytically cracking a straight-run gasoline feed having a distillation range of 30 ° C to 220 ° C according to claim 4, wherein a positive or isoparaffin type comprising 5 carbon atoms corresponding to the stream (3) The fraction of the compound has a boiling point of 42 ° C or less, and the heavy fraction of the feed containing the cyclopentane and C6 + compounds corresponding to the stream (4) has a boiling point above 42 °C. A method for catalytically cracking a straight-run gasoline feed having a distillation range of from 30 ° C to 220 ° C according to claim 4, wherein a portion of the fraction consisting of a positive or isoparaffin having 5 carbon atoms is transferred to The petrol pool (28). A method for catalytically cracking a straight run gasoline feed having a distillation range of from 30 ° C to 220 ° C as claimed in claim 4, wherein the combustion air has also been preheated in the preheating furnace (7). A method for catalytically cracking a straight-run gasoline feed according to claim 4, wherein a portion (23) of the bottom fraction is recovered to the inlet of the reactor (12). A method for catalytically cracking a straight-run gasoline feed according to claim 4, wherein the reactor is of the "rising tube" type. A method for catalytically cracking a straight-run gasoline feed according to claim 4, wherein the reactor is of the "downpipe" type. A method for catalytically cracking a straight run gasoline feed according to claim 4, wherein the catalyst comprises a ZSM-5 zeolite.