EP0375547B1 - Process for upgrading the off-gases from a catalytic cracking process and lowering the benzene content of gasolines - Google Patents

Abstract

Process for the production of petrol having a low benzene content and a high octane number, characterised in that a. a material discharged from a reforming reaction is fractionated into a light fraction enriched in benzene and a heavy fraction depleted in benzene, b. the said light fraction is reacted with a cracking gas containing at least one C2-C5 monoolefin, in contact with a dealuminised mordenite catalyst having an overall atomic ratio Si/Al greater than 20, so as to give an alkylated fraction and c. the said alkylated fraction is mixed with the said heavy fraction. Use of the resulting mixture as a fuel. <IMAGE>

Description

The present invention relates to a process for upgrading gases from catalytic cracking and for producing gasoline with a low benzene content and an improved octane number. The gases from catalytic cracking, which can represent up to around 5% of the products from an FCC unit, are currently only little or not valued, apart from their use as fuel inside the refinery who produces them.

These gases are generally composed mainly of hydrogen, methane, ethane, ethylene and nitrogen, accompanied by carbon monoxide and dioxide, propane, propene and a little butane, butenes, pentanes and pentenes. The content of mono-olefins in these gases (ethylene, propene, butenes, pentenes) is of the order of 20 to 30% by weight. Given the large capacity of the catalytic cracking units, it is therefore economically very advantageous to transform these olefins into products with higher added value.

On the other hand, global and mainly European legislators will quickly impose, for reasons of public health, the limitation of the benzene content of gasolines and in particular of reformats, and it is likely that from the beginning of the 1990s, the permissible benzene content limit, currently around 5%, must be lowered considerably to reach at most 3% and may be less.

The method according to the invention makes it possible to jointly solve these two problems; it is therefore suitable for refiners with both a catalytic reforming unit and a catalytic cracking unit.

According to the method of the invention, a reformate is fractionated into a light fraction enriched in benzene and a heavy fraction depleted in benzene, the light fraction is reacted with a cracking gas containing at least one monoolefin from C₂ to C₅, contact with a mordenite catalyst defined below, so as to obtain an alkylated fraction, and said alkylated fraction is mixed with said heavy fraction.

This alkylation reaction is carried out in the presence of an acidic zeolitic catalyst arranged in the form of a fixed bed, at a temperature of approximately 50 to 350 ° C (preferably between 150 and 300 ° C), under a pressure of 2 to 10 MPa (preferably between 3 and 7 MPa), with a flow rate of liquid hydrocarbons (space velocity) of approximately 0.5 to 10 volumes per volume of catalyst and per hour, and with a molar ratio of benzene / olefins between 0.2 and 5 (preferably between 0.6 and 1.5).

We have already tried to recover the olefins contained in catalytic cracking gases by transforming them into petrol or jet fuel by oligomerization, in liquid phase or in heterogeneous phase, in the presence of either a catalyst formed by the action of an aluminum alkyl. on a nickel salt, ie a nickel-based catalyst deposited on alumina silica. In both cases, the presence of carbon monoxide (for example 0.1 to 2% by weight) is very annoying, the latter being a poison of nickel-based catalysts by the formation of nickel-carbonyls. The use of such catalysts therefore requires an expensive prior separation by cryogenics of the lighter fraction of the charge (hydrogen-CO-CO₂-methane).

On the contrary, the zeolitic catalyst used in the invention, namely a dealuminated mordenite with an overall Si / Al atomic ratio of between 30 and 75, makes it possible to treat the gases resulting from catalytic cracking without any prior separation. These gases may contain, for example, 0.1 to 2% by weight, of carbon monoxide.

Several zeolitic catalysts have already been proposed on the one hand, for the alkylation reaction of benzene with ethylene or on the other hand, for the alkylation reaction of benzene with propene.

In the years 1975-1980, the use of zeolites in H form was proposed, in particular the ZSM5 of MOBIL (EP 0104729).

More recently, other zeolites have been put forward, in particular certain mordenites, alone or in admixture with faujasite (US 3,436,432), possibly with a group VIII metal deposited thereon (US 3,851,004). Japanese patents (JP 582.16128 and JP 581 59427) recommend mordenites alone, of low Si / Al ratio (4.5 to 5), containing metal ions such as Mg, K, Ca, or in H⁺ form (JP 041670 ).

Furthermore, dealuminated mordenites and their preparation are known: EP-0084748, EP-0097552, EP-0196965 (small pore mordenites) and FR-87/12932. These mordenites were used to convert methanol or isomerize olefins.

These mordenites are preferably prepared as follows, to obtain the best results.

In a first step, the non-decomposable cations, generally Na⁺, present in the starting mordenite are eliminated. To do this, one or more exchanges can be carried out in dilute solutions of acids such as HCl or in NH₄ + solutions. The important point is that, at the end of this first step which can be described as decationization, almost all of the alkaline cations are eliminated (% Na between 150 and 1000 ppm, and preferably between 300 and 800 ppm ), and that the solid obtained is a form H or a precursor of form H (example NH₄ +) not dealuminated substantially (% dealumination 10% and preferably 5%). Preferably, the NH₄ + form will be chosen as the H form precursor.

In a second step, form H or the precursor of form H, little or no dealuminated, is subjected to a treatment under steam at a temperature above 450 ° C, for example at 450-650 ° C, and preferably 550 to 600 ° C. The water content (by volume) of the calcination atmosphere will advantageously be greater than 20% and preferably 40%.

At the end of this second step, a solid is obtained which is characterized by the presence of amorphous zones in small quantities, which are precursors of the secondary porous network, and by a crystalline structure practically free from structural defects. The presence of amorphous zones in zeolites treated at high temperature under water vapor is a known phenomenon. However, the operating conditions recommended make it possible to limit as much as possible the proportion of amorphous zone within the crystals. The crystallinity levels measured by X-ray diffraction are generally greater than 80% and, more specifically, 90%. Solids calcined under water vapor are also characterized by the presence, in the structural micropores, of extra lattice aluminum species, a subsequent acid attack is therefore necessary, because these micropores are practically plugged. However, to obtain a good alkylation catalyst, according to the invention, this acid attack must be optimized.

Acid attack is the third step in the preparation of the catalysts. At this point it is important to preserve or liberate the strong acid sites from the solids. The acid attack must therefore be strong enough to, on the one hand, eliminate the cationic aluminum formed during the treatment under water vapor, and which are poisons of strong sites, and, on the other hand, to release the microporosity. structural. The acid sites of the Brönsted type linked to extra-network species being of medium or low strength, it is not essential to proceed to their total elimination. The acid attack should not, however, be too strong, so as to avoid excessive dealumination of the structural aluminum. The force to be retained for the acid attack depends closely on the characteristics attained after calcination under steam and, in particular, on the crystal mesh. For Si / Al framework ratios varying between 30 and 40, concentrations of acid solutions (HCl, H₂SO₄) HNO₃, etc.) will be used between 0.5 and 5 N and preferably between 1 and 3 N. For higher Si / Al framing ratios, use concentrations of solutions acids between 5 and 20 N and preferably between 7 and 12 M (the Si / Al framework ratios are determined by infrared spectroscopy for ratios between 30 and 40, and by ² duSi NMR for higher ratios). Furthermore, to reach high Si / Al ratios, that is to say greater than 40 and more specifically greater than 60, it is advantageous to have recourse to several cycles of calcination under water vapor-acid attack.

• un réformat en provenance d'un unité de réformage catalytique est tout d'abord fractionné dans une première colonne à distiller ; en fond de colonne on recueille une fraction appauvrie en benzène, de point d'ébullition supérieur par exemple à 85 °C, qui est envoyée au stockage ; au sommet de cette première colonne on sort un réformat léger enrichi en benzène par exemple C₅-85 °C, qui est mélangé avec des gaz issus d'une unité de craquage catalytique,
• le mélange résultant est tout d'abord porté à la température de réaction, à la fois par échange thermique avec l'effluent issu du réacteur d'alkylation et par passage dans un four, puis envoyé dans la section d'alkylation renfermant le catalyseur mordénite précité,
• l'effluent de la section d'alkylation est alors envoyé dans une seconde colonne de strippage; on sort en tête une fraction légère constitutée d'hydrogène, de monoxyde de carbone, de dioxyde de carbone, d'azote, d'oxygène, de méthane et d'éthane, qui peut être éliminée à la torche, ou plutôt utilisée comme combustible dans le four de préchauffage de la charge allant à la section d'alkylation,
• la fraction soutirée en fond de cette seconde colonne de strippage est envoyée vers une troisième colonne de stabilisation ; on sort au sommet une fraction légère enrichie en propane et butanes, qui peut être commercialisée sous forme de G.P.L. (gaz pétroliers liquefiés),
• la fraction essence soutirée en fond de cette troisième colonne est dirigée vers le stockage pour y être mélangée avec la fraction de point d'ébullition supérieur à 85 °C environ issue du fond de la première colonne de fractionnement du réformat.

The solids prepared according to the invention have overall Si / Al atomic ratios between 30 and 75; they have a mesh volume between 2755 A³ and 2730 A³ (1A - 10⁻¹⁰m) and, preferably, between 2740 A³ and 2735 A³; they preferably have sufficient acid strength for the structural Al-OHs to interact with a weak base such as ethylene (infrared measurement at 77 ° K), or a weakly acidic compound such as H₂S (infrared measurement at 25 ° VS). These solids must also preferably be free of extra-lattice cationic species which can be detected by a fine signal (width at mid-height less than 5 ppm and preferably less than 2 ppm) located at 0 ppm ( reference Al (H₂O) ₆3 +) on an NMR spectrum of ²⁷Al, measured with the magic angle rotation technique,
The method according to the invention is designed so that:

• a reformate from a catalytic reforming unit is firstly fractionated in a first distillation column; at the bottom of the column is collected a fraction depleted in benzene, with a boiling point greater for example at 85 ° C., which is sent to storage; at the top of this first column, a light reformate enriched with benzene, for example C₅-85 ° C, is released, which is mixed with gases from a catalytic cracking unit,
• the resulting mixture is first brought to the reaction temperature, both by heat exchange with the effluent from the alkylation reactor and by passage through an oven, then sent to the alkylation section containing the mordenite catalyst above,
• the effluent from the alkylation section is then sent to a second stripping column; a light fraction consisting of hydrogen, carbon monoxide, carbon dioxide, nitrogen, oxygen, methane and ethane is taken out at the head, which can be eliminated by flaring, or rather used as fuel in the charge preheating furnace going to the alkylation section,
• the fraction drawn off at the bottom of this second stripping column is sent to a third stabilization column; a light fraction enriched with propane and butanes is released at the top, which can be marketed in the form of LPG (liquefied petroleum gases),
• the gasoline fraction drawn off at the bottom of this third column is sent to storage to be mixed there with the fraction of boiling point above about 85 ° C. coming from the bottom of the first reformate fractionation column.

The single figure illustrates a particular embodiment of the invention.

A reformate from a catalytic reforming unit, line 1, is sent to the distillation column 2; a fraction of boiling point above 85 ° C. is collected at the bottom of this first column, which is sent, via line 3, to the storage tank 4. At the top of this first column 2, it leaves via the line 5, a fraction of a boiling point below 85 ° C., which is mixed with gases arriving, via line 6, from a catalytic cracking unit. The resulting mixture is then sent, via line 7, to the heat exchanger 8, then, via line 9, to the preheating furnace 10, and finally, via line 11, to the inlet of the reactor. alkylation 12. At the outlet of the alkylation reactor 12, the effluent is introduced via line 13 into the heat exchanger 8, then at the outlet of the latter it is directed through line 14 in a second stripping column 15. At the head of this stripping column 15, a mixture of hydrogen, carbon monoxide, carbon dioxide, nitrogen, oxygen, methane and ethane is released, which can either be evacuated to the torch via line 16, or be sent, via line 17, to the preheating furnace 10 of the alkylation section, to be used as fuel there. At the bottom of this second stripping column 15, a product is drawn off which is sent, via line 18, to a third stabilization column 19. At the head of this third column, a mixture composed of propane and butanes is collected which is evacuated, via line 20, to a storage of liquefied petroleum gases (LPG) before being marketed. At the bottom of this third stabilization column 19, a gasoline is drawn off which is sent, via line 21, to the storage tank 4 where it is mixed with the fraction of boiling point above 85 ° C. coming from the line 3 from the bottom of the reformate fractionation column 2. The resulting mixture (line 22) is a gasoline fraction with a low benzene content and a high octane number.

The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1A

Three catalysts B1, B2, B3 are prepared.

The raw material used to prepare these various catalysts is a small pore mordenite from the Société Chimique de la Grande Paroisse, referenced Alite 150; its anhydrous chemical formula is Na, Al O₂ (SiO₂) _5.5 , its benzene adsorption capacity is 1% by weight relative to the weight of dry solid, and its sodium content is 5, 3% by weight. Five hundred grams of this powder are immersed in a 2M solution of ammonium nitrate, and the suspension is brought to 95 ° C for 2 hours. The volume of the ammonium nitrate solution used is equal to 4 times the weight of dry zeolite (V / P = 4). This cation exchange operation is repeated 3 times. After the 3rd exchange, the product is washed with water at 20 ° C. for 20 minutes, with a V / P ratio equal to 4. The sodium content expressed as a percentage by weight relative to the dry zeolite increases from 5, 5 to 0.2%. The product is then filtered, and different batches are subjected to calcination in a confined atmosphere, at a higher or lower temperature depending on the degree of dealumination of the frame that is to be obtained (Table 1). The duration of the calcination is fixed at 2 hours. The partial pressure of water vapor inside the reactor is around 90%. The crystallinity of the various solids, after this calcination step, is greater than or equal to 90%.

Then, on each of the solids, an acid attack is carried out with nitric acid, the higher the higher the dealumination rate of the frame obtained during the previous step (table 1). . During the acid attack, the solid is brought to reflux in the nitric acid solution for two hours, with a V / P ratio equal to 8. The product is then filtered, then washed thoroughly with distilled water.

The Si / Al atomic ratios obtained for each of the solids are shown in Table 1.

Each modified solid is then shaped by kneading with an aluminum type binder, at a rate of 20% by weight of binder, then passage through a die. The extrudates obtained, with a diameter of 1.2 mm, are then dried and calcined between 150 and 550 ° C. in stages of approximately one hour.

EXAMPLE 1B

Three catalysts B′1, B′2, B′3 are prepared. These catalysts differ from those described in Example 1A, in that the raw material used to prepare them is no longer Alite 150 mordenite from the Société Chimique de la Grande Paroisse, but a large pore mordenite from TOYO-SODA , referenced TSZ 600 NAA. Its chemical formula, in the anhydrous state, is Na, AlO₂ (SiO₂) _5.1 , and its sodium content of 5.7%.

All the steps of exchange, calcination, acid attack, shaping and calcination are carried out under the same conditions as those described in Example 1A. The Si / Al atomic ratios obtained are only very slightly different (Table II).

EXAMPLE 2A

The three catalysts B1, B2, B3, prepared in Example 1A, were tested by alkylation of a top fraction (boiling point below 85 ° C) of a reformate from a catalytic reforming unit , by gases from a catalytic cracking unit.

Before fractionation, the reformate had the weight composition and the characteristics presented in Table III.

• une fraction point initial -85 °C représentant 32 % en poids du réformat global,
• une fraction 85 °C - point final représentant 68 % du réformat global.

This reformate was then fractionated in an Oldershaw column of 60 theoretical plates, with a reflux rate of 5/1, and two fractions were collected:

• an initial point fraction -85 ° C representing 32% by weight of the overall reformate,
• a fraction 85 ° C - end point representing 68% of the overall reformate.

The main characteristics of these two fractions are also presented in Table III.

- hydrogène

: 1,30

- monoxyde de carbone (CO)

: 0,58

- dioxyde de carbone (CO₂)

: 0,17

- azote

: 5,50

- méthane

: 26,52

- éthane

: 20,90

- éthylène

: 19,30

- propane

: 3,55

- propène

: 17,75

- butanes

: 1,50

- butènes

: 2,50

- pentanes

: 0,43

The gases from the catalytic cracking unit had the following composition (% by weight):

- hydrogen

: 1.30

- carbon monoxide (CO)

: 0.58

- carbon dioxide (CO₂)

: 0.17

- nitrogen

: 5.50

- methane

: 26.52

- ethane

: 20.90

- ethylene

: 19.30

- propane

: 3.55

- propene

: 17.75

- butanes

: 1.50

- butenes

: 2.50

- pentanes

: 0.43

• température : 260°C
• pression : 4,5 MPa
• débit pondéral en réformat (fraction PI-85 °C) liquide égal à 1 fois le poids du catalyseur.
• rapport molaire benzène (contenu dans la fraction PI-85 °C du réformat)/oléfines (C₂+C₃+C₄) = 0,82.

The operating conditions used for the alkylation reaction were as follows:

• temperature: 260 ° C
• pressure: 4.5 MPa
• weight flow rate in reformate (PI-85 ° C fraction) liquid equal to 1 time the weight of the catalyst.
• benzene molar ratio (contained in the PI-85 ° C fraction of the reformate) / olefins (C₂ + C₃ + C₄) = 0.82.

At the outlet of the reactor, and after 120 hours of operation, the products obtained on the three catalysts B₁, B₂ and B₃ respectively had the main characteristics presented in Table IV.

On examination of the results, it can be seen that it is preferable to work with the catalysts recommended by the invention and to use dealuminated mordenites having a ratio global Si / Al atomic between 30 and 75. Indeed, mordenites having an overall Si / Al atomic ratio lower than 30 are much less selective, and they lead to the formation of paraffins (methane, ethane, propane, butanes and pentanes ); mordenites having an overall Si / Al atomic ratio greater than 75 are a little more selective, but less active since they do not completely transform the olefins (ethylene, propene, butenes, pentenes) contained in the charge.

In the case of catalyst B₂, the product obtained, at the bottom of column 19 of the single figure, was mixed with the fraction drawn off at the bottom of column 2 of this same figure; and 104 kg of petrol were thus obtained, ie a yield increase of 4%.

• densité à 20 °C : 0,790,
• courbe de distillation ASTM,

point initial °C

: 23°

30 % vol.

: 71°

50 % vol.

: 116°

70 % vol.

: 149°

90 % vol.

: 178°

95 % vol.

: 191°

point final °C

: 218°

• indice d'octane : 98 au lieu de 95 pour le réformat initial,
• la teneur en benzène de l'essence a été réduite de 8,62 à 3,19 % en poids soit une diminution de 63 %.

This species had the following characteristics:

• density at 20 ° C: 0.790,
• ASTM distillation curve,

initial point ° C

: 23 °

30% vol.

: 71 °

50% vol.

: 116 °

70% vol.

: 149 °

90% vol.

: 178 °

95% vol.

: 191 °

end point ° C

: 218 °

• octane number: 98 instead of 95 for the initial reformate,
• the benzene content of gasoline was reduced from 8.62 to 3.19% by weight, a decrease of 63%.

EXAMPLE 2B

The three catalysts B′1, B′2 and B′3, prepared in Example 1B, were also tested under the same operating conditions, and with the same charge as the catalysts B1, B2 and B3; the results obtained, presented in table V, are very close to those obtained with the catalysts B1, B2 and B3. As in the case of Example 2A, it can be seen that it is preferable to work with mordenites whose overall Si / Al atomic ratio is between 30 and 75.

Claims (4)

1. A process for producing a gasoline with a low benzene content and a high octane number, characterized in that:

a - a reforming effluent is fractionated into a light benzene-enriched fraction and a heavy benzene-impoverished fraction.

b - said light fraction is reacted with a cracking gas containing at least one C₂ to C₅ mono-olefin, by contacting it with a de-aluminized mordenite catalyst with a total SiAl atomic ratio ranging from 30 to 75, in order to obtain an alkyl fraction, and

c - said alkyl fraction is admixed with said heavy fraction.

2. A process according to claim 1 wherein the cracking gas comprises 0.1 to 2 % by weight of carbon monoxide.

3. A process according to one of claims 1 and 2 wherein the mordenite is the product resulting from :

1 - treating a H or NH₄ mordenite by a gas comprising steam at a temperature ranging from 450 to 650°C,

2 - treating the product from stage 1 by an acid with a concentration of 0.5 to 20 N.

4. A process according to claim 3 wherein the initial mordenite is a small-pore mordenite.

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