hydrocarbon feedstock

The present invention discloses a method for the conversion of low boiling point olefin containing hydrocarbon feedstock into higher boiling point cuts via olefin oligomerization.

More specifically, the processed hydrocarbon feedstock has an initial boiling point that is set between butanes to hexanes boiling points (included) and a final boiling point equal to or below 165°C.

Refineries of today have to adapt to a continuously evolving and fluctuating market, requiring always more flexibility. It is especially the case with the gasoline and middle distillate markets, which have largely evolved during the years: a shift in product focus from gasoline to middle distillates is being observed in the current and future European market demands.

To respond to the above-mentioned disequilibrium, a nice way of readjusting the gasoline/diesel balance according to the market needs consists in upgrading at least part of the gasoline into middle distillates (jet, diesel).

In a typical refinery today, most of the C4-C8 molecules end up in the gasoline-pool. It is important to note that only around 5% of these molecules were initially present in the crude oil as delivered, while cracking during refinery processing creates the rest. About 50% of the C4's and 40% of the C5's that are produced during Fluidized Catalytic Cracking (FCC) are olefinic in nature. Currently the C4 olefins are used as feed for the alkylation and etherification units to create gasoline components with high octane and the higher olefins are generally directly blended into the gasoline pool.

In that context, a convenient solution that allows a renewed equilibrium between gasoline and distillates would be to convert unsaturated molecules (olefins) contained in the gasoline feed into heavier molecules lying in the middle distillate range (i.e. diesel and kerosene) by selective oligomerization of these unsaturated molecules.

The present invention relates to a process for the manufacture of higher molecular weight organic molecules from a stream of lower molecular weight molecules which contains contaminants brought in by the feedstock.

Oligomerization of olefinic streams is largely documented and is a widely used commercial process, but is subject to limitations.

Typically, oligomerization processes involve contacting lower olefins (typically mixtures of propylene and butenes) coming from Fluid Catalytic Cracker (FCC) and/or steam crackers with a solid acid catalyst, such as Solid Phosphoric Acid (SPA) catalyst, crystalline molecular sieve, acidic ion exchange resin or amorphous acid material (silico-alumina).

With SPA catalyst, the pressure drop over the catalytic bed(s) increases gradually due to coking, swelling of the catalyst, and is therefore the limiting factor of a cycle (or run) duration, the unit being shutdown once the maximum allowable pressure drop has been reached.

With crystalline molecular sieve, acidic ion exchange resin or amorphous acidic material (silico-alumina), the limiting factor is usually no more the pressure drop along the catalytic bed but the reactor run length which is determined by the catalytic performances (shutdown when the catalytic activity has dropped to an unacceptably low level). The performances of such catalyst are therefore sensitive to poisons contained in the feedstock, which may considerably affect the cycle length of the said catalyst.

Certain impurities such as sulfur containing contaminants and basic nitrogen have an adverse effect in the useful lifetime of the catalyst.

Among the sulfur containing contaminants, low molecular weight sulfur species are especially troublesome, as described in US 2008/0039669, i.e. aliphatic thiols, sulfides and disulfides. For example dimethyl-, diethyl-, and ethyl-methyl-sulphides, n-propane thiol, 1 -butane thiol and 1 ,1 -methylethyl thiol, ethyl-methyl- and dimethyl- disulphides, and tetrahydrothiophene.

Among the basic nitrogen contaminants, we can distinguish:

- The organic Bronsted bases (characterized by at least one hydrogen atom bound to the nitrogen atom, and being proton acceptors), such as amines and amides, contribute to negatively affect the catalyst performances.

- The other organic nitrogen compounds, called Lewis bases, have free electron pair on the nitrogen atom such as nitriles, morphol ines or N-Methyl pyrrolidone. Though much weaker bases as compared to the Bronsted bases, they strongly deactivate the catalyst. The detrimental effect of such impurities has been discussed in US patent application publication 2008/031 2484.

In some specific cases, the purity of the olefinic stream is not an issue : It is the case when the stream involves very pure Fisher-Tropsch (FT) derived olefins (US2008/0257783 or WO2006/091986).

It is also the case in the fully integrated system MTG (Methanol-to-

Gasoline) where olefinic streams are produced from the Methanol-To-Olefin process and oligomerized through the so-called MOGD process (Mobil Olefin to Gasoline and Distillate). The MOGD process proposed by Mobil (US-4, 150,062; US-4,227,992; US-4,482,772; US-4,506,106; US-4,543,435) and developed between the seventies' and eighties', in fact used ZSM-5 zeolite as catalyst. The products obtained from the reaction of butenes are trimers and tetramers, characterized by a low branching degree. The gas oil fraction however is lower than that of the jet fuel fraction and consequently, even if this process offers good quality gas oil (cetane number > 50), it is more interesting for the production of jet fuel than gas oil. In US2004/0254413 patent application, ExxonMobil pursued the Mobil development and introduced the new generation of MOGD. This invention uses two or more zeolite catalysts. Examples of zeolite catalysts include a first catalyst containing ZSM-5, and a second catalyst containing a 10-ring molecular sieve, including but not limited to, ZSM-22, ZSM-23, ZSM35, ZSM-48, and mixtures thereof. The ZSM-5 can be unmodified, phosphorous modified, steam modified having a micropore volume reduced to not less than 50% of that of the unsteamed ZSM-5, or various mixtures thereof.

ZSM-5 stands for Zeolite Sieve of Molecular porosity (or Zeolite Socony Mobil) - 5, (structure type MFI-Mordenite Framework Inverted). ZSM-5 is an aluminosilicate zeolite mineral belonging to the pentasil family of zeolites. Its chemical formula is Na _n AlnSi96-nOi92- 16H ₂ O (0<n<27).

In a similar manner, Lurgi AG, Germany (WO2006/076942), has developed the Methanol to Synfuels (MTS) process, which is in principle similar to the MOGD process. The Lurgi route is a combination of simplified Lurgi MTP technology with COD technology from Sued Chemie (US5063187). This process produces gasoline (RON 80) and diesel (Cetane ~ 55) in the ratio of approximately 1 :4. Disclosed is a method for the production of synthetic fuels, wherein, in a first step, a gas mixture consisting of methanol and/or dimethyl ether and/or another oxygenate and water vapor is reacted at temperatures of 300-600 °C in order to form olefins with, preferably, 2-8 carbon atoms. In a second step, the olefin mixture thus obtained is oligomerized at an elevated pressure to form higher olefins with predominantly more than 5, preferably 10-20 carbon atoms. According to said method, a) the production of olefins in the first step is carried out in the presence of a gas flow which essentially consists of saturated hydrocarbons which are separated from the product flow of the second step and returned to the first step, and (b) the production of olefins is carried out in the second step in the presence of a flow of water vapor which is separated from the product flow of the first step and returned to the first step.

Above-discussed methods are hardly developable in the context of gasoline upgrading into distillates: Commonly available olefinic feedstocks cause rapid deactivation of existing oligomerization catalysts, due to the presence of contaminants in the feed, which is a critical issue. Catalytic cracking, usually fluid catalytic cracking (FCC), is a suitable source of cracked naphthas. Thermal cracking processes such as coking may also be used to produce usable feeds such as coker naphtha, pyrolysis gasoline, and other thermally cracked naphthas. The process may be operated with a part of, or the entire gasoline fraction, obtained from a catalytic or thermal cracking step.

To cope with the contaminants issue, different techniques have been proposed:

A first technique consists in contacting the nitrogen and sulfur contaminated feedstock either with a hydrotreating catalyst at oxidized state (US 6,884,916 - Exxon) or with a metal oxide catalyst (US 7,253,330) in the absence of hydrogen, ahead of the oligomerization section, thus limiting catalyst deactivation. The pretreatment is believed to convert small sulfur compounds into larger sulfur species, then into more sterically hindered molecules, no more entering the catalyst pores, and limiting catalyst deactivation.

Another convenient way (US 7,186,874 - Exxon) is to mitigate the adverse effect of the sulfur compounds on catalyst activity by appropriately adjusting the operating conditions of the process by, for example, temperature rising.

Removal of nitriles and other organic nitrogen-containing Lewis bases from the oligomerization feed may be achieved by a washing step with water (WO2007/104385 - Exxon). Removal of basic nitrogen and sulfur-containing organic compounds by scrubbing with contaminant removal washes such as caustic, methyl-ethyl-amine (MEA), or other amines or aqueous washing liquids, is discussed in WO2006/094010 (Exxon). This method allows contaminants to stand at acceptable levels (10-20 ppmwt S, trace levels for N) and therefore to limit catalyst deactivation prior to oligomerization reaction.

Sorption techniques are also reported for nitrogen components removal from the feed. US2005/0137442 (UOP) discloses the use of molecular sieves catalysts (such as Y-zeolite) to remove the nitrogen-based contaminants present in an olefinic stream to be alkylated. Specificity of US2005/0137442 (UOP) lies in operating conditions: adsorption is conducted at a temperature of at least 120°C to increase the nitriles adsorption capacity of the sorbent in the presence of water.

Purification section using molecular sieves is also reported in EP 1 433 835 (IFP), where shaped MOR catalyst having a Si/AI atomic ratio of 45 allows decreasing nitrogen content from l Oppmwt to 0.2ppmwt. US2008/0312484 (Exxon) shown that such a low nitrogen concentration can be tolerated in olefin-containing hydrocarbon feeds loaded in oligomerization sections.

WO2006/067305 (IFP) discloses a process for producing propylene from C4/C5 cut (from steam cracking or catalytic cracking). Prior to the steps of so-called "oligomerization/cracking", the following purification sequence is used to remove contaminants: a selective hydrogenation is used to convert the dienes and acetylenic compounds into mono-olefins, then drying and desulfurization steps are performed by the use of different sorbents (3A, 13X molecular sieves). Thus, state of the art processes do not use untreated refinery streams for oligomerization but rather pure streams (such as ex-FT or ex-MTO olefins). As such, existing commercial solutions cannot give satisfactory results for untreated refinery streams to be valorized. It has now been found an improved route to process untreated refinery streams (such as FCC, coker ...) into an oligomerization reaction. One aim of the inventors was the valorization of low value gasoline into high value middle distillate fractions, exhibiting good properties for fuel blendstocks to be used for the fabrication of kerosene and diesel fuels products achieving required specifications for commercial diesel, kerosene, and heating oil. In the following description, when a content specification is expressed in ppm only, it means implicitely "ppm in weight or ppm wt".

DESCRIPTION OF THE PROCESS

A first object of the invention concerns a method for the treatment of an olefin containing hydrocarbon feedstock, wherein said feedstock successively undergoes the following steps: (i) selective catalytic hydrogenation, (ii) a fractionation into at least a light cut, (iii) a treatment of the light cut on an adsorbent to obtain at least one nitrogen and sulfur depleted light cut, (iv) oligomerization of said nitrogen and sulfur depleted light cut to obtain a heavier cut effluent essentially consisting of a middle distillate fraction, (v) a fractionation of the effluent of step (iv) into at least two cuts essentially consisting of at least one unreacted material and the middle distillate fraction, and (vi) hydrotreating the obtained middle distillate fraction.

The first step (i) consists in selective catalytic hydrogenation especially of di-olefins included in the hydrocarbon feedstock as starting material, into mono-olefins to avoid gum formation in downstream catalyst, allowing at the same time conversion of low molecular weight sulfur containing molecules (aliphatic thiols, sulfides or disulfides being especially troublesome) into heavier molecular weight sulfur containing molecules.

The method of the invention permits to treat a hydrocarbon feedstock as starting material preferably issued from FCC, coker, flexi-coker, visbreaker, steam cracker, hydrocracker, for example from DHC (distillate hydrocracker) or MHC (mild hydrocracker) hydrocracker, preferably from FCC or coker.

The method of the invention notably permits to treat an olefin and nitrogen containing hydrocarbon feedstock which is substantially depleted in C4 and lower paraffinic and/or olefinic hydrocarbons. In other words, the feedstock may contain less than 50% wt of C4 and lower paraffinic and/or olefinic hydrocarbons, or may contain less than 30% wt or even less than 10% wt.

The final boiling point of the feedstock may be below 200°C, preferably below 165°C, more preferably below 65°C corresponding to a LCCS cut (IBP- 65°C).

The initial boiling point of the feedstock may be above -50°C, preferably above 0°C, more preferably above +25°C.

In the method of the invention, the selectively hydrogenated feedstock is split through the fractionation (step (ii)) into at least a light cut, advantageously representing a LCN (Light Catalytic Naphtha), preferably a heavier cut representing Heavy Catalytic Naphtha (HCN), and a fuel gas, prior to treatment of at least one of the foregoing on an adsorbent (step (iii)). Such fractionation permits to obtain a light cut essentially deprived of sulfur compounds (above mentioned "heavier molecular weight sulfur containing molecules"), those compounds remaining generally in the heavier cut

Particularly, the light cut may be a IBP-165°C, more particularly, the light cut may be a LCCS (Light Catalytic Cracked Spirit) cut (IBP-65°C) corresponding to the low boiling point fraction of a LCN cut. In some aspects, the light cut is a Light Catalytic Cracked Spirit (LCCS) or a Light Cracked Naphtha (LCN), with a final boiling point below 200°C, preferably below 165°C, more preferably below 65°C.

The further step (iii) consists in removing from the obtained light cut, the residual light N and S compounds by the use of different adsorbents, alone or in combination, such as 3A 13X molecular sieves and/or acid-treated clays, before undergoing the oligomerization step (iv).

Once purified in such a purification section, the obtained olefinic light cut, i.e. the nitrogen and sulfur depleted light cut, can then be valorized into an effluent essentially consisting in middle distillate fraction(s) by oligomerization

(step (iv)), as represented on figure 1 . For this step, classical appropriate catalysts are used.

Advantageously, the nitrogen and sulfur depleted light cut obtained by the treatment step (iii) is mainly constituted of C5 and C6 hydrocarbons, out of which C5 and C6 olefins are particularly desired.

The nitrogen and sulfur depleted light cut may contain 0-5% wt of C4 hydrocarbons, 0-15%wt of C8+ hydrocarbons, the remaining being C5-C7 hydrocarbons. For example, this nitrogen and sulfur depleted light cut may contain 20-60%wt olefins.

Then, the further step (v), which may be a standard distillation step, allows the separation of the previous effluent of step (iv) into at least two cuts essentially consisting in at least one unreacted material, containing paraffins in majority and certain olefins, and in said middle distillate fraction. Thus, this step allows a purification of the middle distillate fraction. In other words, step (v) permits to obtain a middle distillate fraction which is preferably a kerosene and/or a diesel cut, the kerosene cut being preferably a jet 145-245°C cut, and the diesel cut being preferably selected from the group consisting of Diesel 245°C+ and 165°C+ cuts.

Advantageously, said at least one unreacted material is recycled at step (iv) for at least one middle distillate fraction production.

The method includes a further step (step (vi)) of hydrotreating the obtained middle distillate fraction after step (v)), preferably carried out in one or more fixed bed reactors, at temperatures between 200°C and 400°C, and preferably between 200 and 400°C, and at pressures from 10 to 100 bars, and preferably from 20 to 90 bars, employing a standard hydrotreatment catalyst as used in a HydroDeSulfurization (HDS) unit, for example CoMo, NiMo, NiCoMo, NiW, CoMoP and/or NiMoP catalysts, and commercially available. The hydrogenation can be carried out in a dedicated unit or oligomerized effluents can be co-processed with Straight Run Gas Oil (SRGO). When the hydrotreating step (vi) is performed by diluting said middle distillate fraction with a sulfur containing gas oil, such as SRGO, the rate of middle distillate fraction introduced into the sulfur containing gas oil may be from 1 to 30% wt, preferably from 5 to 20% wt.

Said hydrogenation stage makes it possible to recover the middle distillate fraction, which is preferably a cut of diesel or kerosene fuel that is in the majority paraffinic (above 80wt% of paraffins). The hydrotreatment also removes the S compounds obtained in step (iii) to reach the specification of less than 10 ppm thereof in the middle distillate fraction.

Usually, step (vi) of hydrotreating is only performed for the diesel cut isolated from the middle distillate fraction exiting the fractionation step (v) because the sulfur specifications are much more constraining for the diesel product as compared to those of the kerosene product. In some embodiments, step (vi) may be omitted.

Preferably, the middle distillate fraction is at least partly incorporated in a pool of fuel blendstocks to produce Jet A-1 kerosene and/or diesel fuels compliant with their respective specifications (DEF STAN 91 -91 /Issue 7 and ASTM D1655 for Jet A-1 and EN590 for diesel fuel), preferably at a rate comprised between 1 to 30% in volume, especially between 5 to 20% in volume. Thus, preferably the middle distillate fraction is at least partly incorporated in a pool of fuel blendstocks to be used for the fabrication of kerosene and/or diesel products meeting the required specifications for the Jet A1 kerosene and/or the standard diesel. According to some embodiments, the incorporation of said middle distillate fractions is made into a commercial Diesel having a FAME content up to 10% in volume. FAME (fatty acid methyl ester) is made from vegetable oils and/or used fried oils and/or animal fats and is considered as a renewable diesel fuel.

According to the method of the invention, after step (vi), the resulting middle distillate fraction exhibits several advantages.

These fractions present such specifications so as to allow them to be preferably blended, as fuel blendstocks, with kerosene and/or diesel refinery fuels.

Advantageously, for the preparation of a blendstock for kerosene fuel incorporation or blending, the method includes the specific step of selecting amongst the characteristics of the middle distillate fraction obtained through the method, a combination of at least three, in particular at least four characteristics, among the following characteristics:

An aromatic content lower that 25% (v/v), preferably lower that 20% (v/v), and more preferably lower that 5% (v/v);

A density in the range of 740 to 840 kg/m ³ , preferably in the range of 750 to 820 kg/m ³ ;

A final boiling point (FBP) lower than 300°C, preferably lower than 260°C, more preferably lower than 240°C;

A Saybolt color higher than 25, preferably higher than 30;

A smoke point higher than 20 mm, preferably higher than 23 mm, more preferably higher than 25 mm;

A freezing point lower than -47°C, preferably lower than -50°C, and more preferably lower than -80°C;

A sulfur content lower than 3000 ppm wt, preferably lower than 300 ppm wt, and more preferably lower than 10 ppm wt;

A Flash point higher than 38°C according to IP170.

Accordingly, the middle distillate fraction obtained through the implementation of the method, as part of a jet kerosene blendstock, will improve the thermal stability (according to JFTOT test), the Saybolt color and the smoke point of a kerosene product, and decreases the sulfur content of said product that is in limit of compliance. The kerosene product is preferably a fraction meeting the Jet A1 specification known in the art. Advantageously, when the step (iv) of the claimed method includes an amorphous silica alumina (ASA) type catalyst, said Jet 145-245°C cuts have the following characteristics:

An aromatic content lower that 25% (v/v), preferably lower that 20% (v/v), and more preferably lower that 5% (v/v);

A density in the range of 770 to 820 kg/m ³ ;

A final boiling point (FBP) lower than 300°C, preferably lower than 260°C, more preferably lower than 240°C;

A Saybolt color higher than 25, preferably higher than 30;

A smoke point higher than 20 mm, preferably higher than 23 mm, and more preferably higher than 25 mm;

A freezing point lower than -47°C, preferably lower than -50°C;

A sulfur content lower than 3000 ppm wt, preferably lower than 300 ppm wt, and more preferably lower than 10 ppm wt;

A Flash point higher than 38°C, preferably higher than 40°C, and more preferably higher than 45°C, according to IP170.

Alternatively, when the step (iv) includes a zeolite-based catalyst, said Jet 145-245°C cuts have the following characteristics:

An aromatic content lower that 25% (v/v), preferably lower that 20% (v/v), and more preferably lower that 5% (v/v);

A density in the range of 740 to 800 kg/m ³ , preferably in the range of 755 to 800 kg/m ³ ;

A final boiling point (FBP) lower than 300°C, preferably lower than 260°C, more preferably lower than 240°C;

A Saybolt color higher than 25, preferably higher than 30,

A smoke point higher than 20 mm, preferably higher than 23 mm, and more preferably higher than 25 mm;

A freezing point lower than -47°C, preferably lower than -90°C, and more preferably lower than -100°C,

A sulfur content lower than 3000 ppm wt, preferably lower than 300 ppm wt, and more preferably lower than 10 ppm wt;

A Flash point higher than 38°C, preferably higher than 40°C, according to IP170. Advantageously, for the preparation of a blendstock for diesel fuel incorporation or blending, the method includes the specific step of selecting amongst the characteristics of the middle distillate fraction obtained through the method, a combination of at least four characteristics, among the following characteristics:

An aromatic content lower than 25% (v/v), preferably lower than 10% (v/v) and more preferably lower than 5% (v/v),

A density in the range of 780 and 845 kg/m ^3, preferably between 790 and 830 kg/m ³ ,

A Final boiling point lower than 380°C,

A Flash point (according to EN ISO2719) higher than 55°C, preferably higher than 60°C, more preferably higher than 70°C,

A cold filter plugging point (CFPP) lower than -15°C, preferably lower than - 30°C, and more preferably lower than -50°C,

A Cloud point lower than -5°C, preferably lower than -30°C, and more preferably lower than -70°C,

A Cetane index higher than 46, preferably higher than 53, and more preferably higher than 60, in particular between 55 and 75,

A sulfur content lower than 10 ppm wt.

According to the invention, the preferred set of selected characteristics is cloud point, density and cetane, and optionally CFPP.

Accordingly, the middle distillate fraction obtained through the implementation of the method allows middle to high blending value for cetane number, to decrease the cetane constraint in diesel fuels, for example by reducing the quantity of "cetane booster products", while improving cold properties of the diesel cuts and lowering the density thereof.

The combination of these three properties (optionally four properties) allows for a unique way for optimizing the diesel distillate pool with for example an increase of the Light Cycle Oil (LCO), known as such, incorporation in the final blend.

Advantageously, when the step (iv) includes an amorphous silica alumina or ASA type catalyst, said Diesel 165°C+ cut has the following characteristics:

An aromatic content lower than 25% (v/v), preferably lower than 10% (v/v) and more preferably lower than 5% (v/v), in particular lower than 2% (v/v), A density in the range of 800 and 845 kg/m ³ , preferably between 805 and 830 kg/m ³ ;

A Final boiling point lower than 360°C,

A Flash point (according to EN ISO2719) higher than 55°C, preferably higher than 57°C;

A cold filter plugging point (CFPP) lower than -15°C, preferably lower than - 30°C, and more preferably lower than -50°C,

A Cloud point lower than -5°C, preferably lower than -30°C and more preferably lower than -70°C,

A Cetane index higher than 46, preferably higher than 50;

A sulfur content lower than 10 ppm wt.

Alternatively, when the step (iv) includes a zeolite-based catalyst, said Diesel 165°C+ cut has the following characteristics:

An aromatic content lower than 25% (v/v), preferably lower than 10% (v/v) and more preferably lower than 5% (v/v),

A density between 780 and 830 kg/m ³ , preferably between 790 and 830 kg/m ³ , A Final boiling point lower than 360°C,

A Flash point (according to EN ISO2719) higher than 55°C, preferably higher than 60°C, more preferably higher than 70°C,

A cold filter plugging point (CFPP) lower than -15°C, preferably lower than - 30°C , and more preferably lower than -50°C,

A Cloud point lower than -5°C, preferably lower than -30°C, more preferably lower than -70°C,

A Cetane index higher than 46, preferably higher than 50, more preferably higher than55;

A sulfur content lower than 10 ppm wt.

Advantageously, when the step (iv) includes an amorphous silica alumina (ASA) type catalyst, said Diesel 245°C+ cut has the following characteristics:

An aromatic content lower than 25% (v/v), preferably lower than 10% (v/v) and more preferably lower than 2% (v/v),

A density between 820 and 845 kg/m ³ , preferably between 830 and 845 kg/m ³ , A Final boiling point lower than 380°C,

A Flash point (according to EN ISO2719) higher than 55°C, preferably higher than 100°C, and more preferably higher than 1 10°C, A cold filter plugging point (CFPP) lower than -15°C,

A Cloud point lower than -5°C, preferably lower than -25°C, more preferably lower than -35°C,

A Cetane index higher than 46, preferably higher than 50, more preferably higher than 65.

A sulfur content lower than 10 ppm wt.

Alternatively, when the step (iv) includes a zeolite-based catalyst, said Diesel 245°C+ cut has the following characteristics:

An aromatic content lower than 25% (v/v), preferably lower than 10% (v/v) and more preferably lower than 2% (v/v),

A density in the range of 800 to 840 kg/m ^3, preferably in the range of 810 to 830 kg/m ³ ;

A Final boiling point lower than 380°C,

A Flash point higher than 55°C, preferably higher than 100°C, and more preferably higher than 1 15°C,

A cold filter plugging point (CFPP) lower than -15°C preferably lower than - 20°C, more preferably lower than -30°C,

A Cloud point lower than -5°C, preferably lower than -50°C, more preferably lower than -70°C,

A Cetane index higher than 46, preferably higher than 55, more preferably higher than 65;

A sulfur content lower than 10 ppm wt. These characteristics are obtained and compared to EN590 and

AFQRJOS issue 25 and DEFSTAN 91 -91 issue 7 standards. The same applies for the Saybolt color and JFTOT test measured according to ASTM D 156, ASTM D 6045 and ASTM D 3241 standards respectively. Additionally, the cetane number is measured according to D 4737 standard and the flash point is measured according to EN ISO2719 for diesel cut and according to IP170 for kerosene cut. References to these standards are not limitative, these international standards being generally well-known by the one skilled in the art.

The obtained middle distillate diesel fractions (165°C+ and 245°C+ cuts) exhibit relatively low density, very good cold properties and relatively good cetane index and cetane number. These aforementioned properties allow increasing much more the diesel fuel volumes than in a classical case by authorizing the additional blending of lower quality constituents for diesel blend such as LCO cut (high density and low cetane product). The blended diesel fuel is fully compliant with the EN590 standard.

Moreover, as the obtained kerosene cut is more easily incorporable to diesel fuel than classical kerosene due to its higher cetane, most of the classical kerosene can be preserved to Jet-A1 production, resulting in an increase of the jet fuel volumes. The blended jet fuel is fully compliant with the AFQRJOS issue 25 and DEFSTAN 91 -91 issue 7 standards. The specific step of selection amongst the characteristics of the middle distillate fraction is classically carried out for example according to blending technologies known as such to the one skilled in the art.

DESCRIPTION OF THE PRETREATMENT STEP

The pretreatment step according to the invention is preferably corresponding to steps (i) - (iii).

In all of the embodiments of the invention, the adsorbent of step (iii) is advantageously selected from the group consisting of one or more of molecular sieves, such as 3A, 13X and HY, acidic ion-exchange resins, acid- treated clays, activated aluminas, spent FCC catalysts, MOF (Metal-Organic

Framework), amorphous alumina-silica (ASA), NiMo, catalysts guard beds and mixture thereof.

The acid-treated clays may be hydrated acid-treated smectite clays, such as montmorillonite, bentonite, vermiculite, hectorite, saponite, beidillinite and the like. In these clays, approximately every sixth aluminum ions has been replaced by a magnesium ion. This produces a crystal lattice with a negative charge which is neutralized by the absorption of metallic cations (such as Na+) on the surface. Their general chemical formula is (Na,Ca) ₀ ,3(AI,Mg)2Si ₄ Oio(OH)2-nH ₂ O.

Preferably, the adsorbent is selected among, or is a combination of one or more of 13X molecular sieve, ASA, NiMo, and MOF.

Advantageously, adsorbent used in any method of the invention is loaded into a purification section located in a guard bed capacity.

With regards to purification section, a guard bed reactor may be operated on a swing cycle with two beds, one bed being used on stream for contaminant removal and the other on regeneration in the conventional manner. If desired, a three-bed guard bed system may be used on a swing cycle with the two beds used in series for contaminants removal and the third bed in regeneration. With the three-bed guard bed system used to achieve low contaminant levels by the two-stage series sorption, the beds will pass sequentially through a three-step cycle of regeneration. A three-bed guard bed system allows better use of guard bed sorption capacity since non-used sorbent sent in regeneration is lowered, if not eliminated. Typically, a three-bed guard bed system may be operated as follows: Step 1 : feedstock flows into first then second guard bed, third one being isolated and under regeneration. Step 2: once first guard bed is saturated by impurities, the latter is isolated and regenerated, and feedstock now flows into second then third guard beds. Step 3: once second guard bed is saturated by impurities, the latter is isolated and regenerated, and feedstock now flows into third then first guard beds. Step 4: go to step 1 .

The hydrocarbon feedstock starting material is contacted with the adsorbent described above, within the reaction vessel, advantageously at a temperature comprised between the hydrocarbon feedstock freezing point and final boiling point, with a liquid hourly space velocity (LHSV) lower than 4h ^"1 , and a pressure comprised between 1 bar and 30 bar. Preferably, the hydrocarbon feedstock starting material is contacted with said adsorbent within said vessel at a temperature comprised between 0°C and 100°C, with a LHSV between 3 h ^"1 and 0.5 h ^"1 , and a pressure comprised between atmospheric pressure and 5 bar.

DESCRIPTION OF THE OLIGOMERIZA TION CATALYST

Classically, the oligomerization step (iv) is carried out with the following types catalysts, preferably being acid ones.

Amorphous or crystalline alumosilicate or silicaalumophosphate in inform, optionally containing alkali, alkali-earth, transition or rare-earth elements, selected from the group:

MFI (e.g. ZSM-5, silicalite-1 , boralite C, TS-1 ), MEL (Si/AI >25) (e.g. ZSM-1 1 , silicalite-2, boralite D, TS-2, SSZ-46), ASA (amorphous silica- alumina), MSA (mesoporous silica-alumina), FER (e.g. Ferrierite, FU-9, ZSM- 35), MTT (e.g. ZSM-23), MWW (e.g. MCM-22, PSH-3, ITQ-1 , MCM-49), TON (e.g. ZSM-22, Theta-1 , NU-10), EUO (e.g. ZSM-50, EU-1 ), ZSM-48, MFS (e.g. ZSM-57), MTW, MAZ, SAPO-1 1 , SAPO-5, FAU (e.g. USY), LTL, BETA, MOR, SAPO-40, SAPO-37, SAPO-41 , MCM-41 , MCM-48 and a family of microporous materials consisting of silicon, aluminum, oxygen and optionally boron, AI2O3, and mixtures thereof. Amorphous alumosilicates or silica-alumophosphates in H-form optionally modified by addition of halogens (Fluorine preferred) such as MSA (mesoporous silica-alumina) can also be used.

Above-mentioned catalysts can be subjected to an additional treatment before use, including activation, ion exchange, modification with metals such as alkali, alkali-earth and rare earth metals, steaming, treatment in an alkaline medium, acid treatment or other dealumination methods, phosphatation, surface passivation by silica deposition or combination thereof.

The amount of alkali, alkali-earth, transition or rare-earth elements is in the range 0.05-10wt%, preferably from 0.1 to 5wt%, more preferably from 0.2 to 3wt% (wt% stands for weight percent).

Preferred alkali, alkali-earth or rare-earth elements are selected among Na, K, Mg, Ca, Ba, Sr, La, Ce, and mixtures thereof.

Above-mentioned catalysts may be additionally doped with further metals. In this respect, and according to another embodiment of the invention, Me-catalysts (Me = metal) containing at least 0.1wt% are used. Preferably, the metal is selected from the group of Zn, Mn, Co, Ni, Ga, Fe, Ti, Zr, Ge, Sn, Cr, and mixtures thereof.

Those atoms can be inserted into the tetrahedral framework through a [MeO2] tetrahedral unit. Incorporation of the metal component is typically accomplished during synthesis of the molecular sieve. However, post- synthesis ion exchange or impregnation can also be used. In post-synthesis exchange, the metal component will be introduced as a cation on ion- exchange positions at an open surface of the molecular sieve, but not into the framework itself.

The selected materials could be subjected to a different treatment before use in the reaction, including introduction of phosphorous, ion exchange, modification with alkali, alkali-earth or rare earth metals, steaming, acid treatment or other dealumination methods, surface passivation by silica deposition or combination thereof.

The catalyst can be a blend of materials as depicted above, and/or can be further combined with other materials that provide additional hardness or catalytic activity to the finished catalyst product (binder, matrix).

According to preferred embodiments, the oligomerization catalysts are selected from the group consisting of alumosilicate type catalysts, such as amorphous alumosilicate type catalysts (ASA) or crystalline alumosilicate such as zeolite-based catalysts, or silicaalumophosphate in H-form, optionally containing alkali, alkali-earth, transition or rare-earth elements and modified by addition of halogens (Fluorine preferred) such as MSA (mesoporous silica- alumina).

Preferably, the oligomerization catalyst is a zeolite-based catalyst or an amorphous silica alumina type catalyst (ASA). In particular, it has been observed that a zeolite-based catalyst permits to improve the properties of the middle distillate, notably the measured cetane number. DESCRIPTION OF THE REACTION STEP

A plural reactor system may be employed with inter-reactor cooling for the oligomerization step (iv), whereby exothermal reaction can be carefully controlled to prevent excessive temperature above the normal moderate range.

The oligomerization reactor can be of isothermal or adiabatic fixed bed type or a series of such reactors or a moving bed reactor. The oligomerization may be performed continuously in a fixed bed reactor configuration using a series of parallel "swing" reactors. Herein used catalysts have been found to be stable enough. This enables the oligomerization process to be performed continuously in two parallel "swing" reactors wherein, when one or two reactors are in operation, the other reactor is undergoing catalyst regeneration. Catalysts of the invention may be regenerated. Regeneration may be done several times.

An object of the present invention is to convert olefins containing stream into heavier hydrocarbons enriched distillate, employing a continuous multistage catalytic technique. A plural reactor system may be employed with inter- reactor cooling, whereby the exothermal reaction can be carefully controlled to prevent excessive temperature above the normal moderate range. Preferably, the maximum temperature differential across only one reactor does not exceed 75° C. Optionally, the pressure differential between the two stages can be utilized in an intermediate flashing separation step.

When using a zeolite catalyst, in a oligomerization unit, the method is operated at temperatures between 150°C and 500°C, and preferably between 200°C and 350°C, and at pressures between 20 and 100 bar, and preferably between 30 and 65 bar.

When using a silica-alumina catalyst, in a oligomerization unit, the method is operated at temperatures between 20°C and 300°C, and preferably between 120°C and 250°C, and at pressures between 10 and 100 bar, and preferably between 20 and 65 bar.

The method of the invention permits to treat a feedstock issued from FCC, coker, flexi-coker, visbreaker, steam cracker, hydrocracker, for example from DHC (distillate hydrocracker) or MHC (mild hydrocracker) hydrocracker, preferably from FCC or coker.

The final boiling point of the feedstock may be below 200°C, preferably below 165°C.

The initial boiling point of the feedstock may be above -50°C, preferably above 0°C, more preferably above +25°C.

For example, the pretreatment step is carried out with a feedstock issued from FCC using a combination of 13X and 3A molecular sieves, leads to a LCCS cut, followed by the oligomerization step with a zeolite catalyst, such as ZSM-5, or amorphous alumino-silica type catalyst (ASA), at 150°C- 250°C and at pressures between 20 and 65 bar, then a hydrotreatment step is implemented as mentioned above.

The invention also relates to a middle distillate fraction obtainable according to the implementation of the above method, wherein said fraction is selected from the group consisting of a jet (kerosene) 145-245°C cut and the Diesel 245°C+ and 165°C+ cuts.

The obtained middle distillate fractions are mainly comprising a Diesel 165°C+ or 245°C+ cut, in other words wherein the true boiling point (TBP) of said fractions is Diesel 165°C+ or Diesel 245°C+, especially appropriated for incorporation in standard Diesel fuel blendings.

The obtained middle distillate fractions are also mainly comprising jet 145-245°C cuts, in other words wherein the true boiling point (TBP) of the said fraction is JET 145-245°C (kerosene), especially appropriated for incorporation in diesel fuels.

Further, the advantageous specifications or characteristics of the above- mentioned fractions, when using different catalysts, have already been described in details above. The invention also includes these fractions as such. The invention is described with further details according to the following non limitative examples, including a figure where, - Fig. 1 represents an implementation of the method using a purification section and optionally a recycle of an unreacted stream.

Fig. 1 shows the untreated refinery stream 1 10 fed to a Selective Hydrogenation Unit (SHU) 1 1 . The dedienized refinery stream 1 1 1 thus obtained is fed to a splitter 12 wherein it is separated in fuel gas 1 12, LCN 1 13 (preferably FBP = 165°C, more preferably FBP = 65°C) and HCN 1 14 (IBP ranges from 65 to165°C).

In the present application, FBP stands for Final Boiling Point and IBP stands for Initial Boiling Point.

LCN 1 13 is then fed to a purification unit 13 for removal of nitrogen by adsorption on adsorbents.

The purified LCN stream 1 15 obtained is fed to an oligomerization unit 14, the obtained products 120 being separated in a further splitter 15 into a gasoline 1 16 and middle distillate 1 17. The part of the gasoline 1 16, consisting of unreacted olefins, may optionally be recycled back into the oligomerization unit 14 via the line 1 19.

The middle distillate stream 1 17 can be fed to a hydrogenation unit 16 to eliminate sulfur compounds and to convert olefins into paraffins. The middle distillate streams 1 17' and 1 18 can be incorporated in a pool of fuel blendstocks.

EXAMPLES

In the following Examples 1 -3, the gasoline cut feedstock is a LCCS cut (Light Catalytic Cracked Stream) corresponding to the low boiling point fraction of a LCN (Light Cracked Naphtha) treated on a Prime-G 1 st stage unit, while a LCN cut is used as feedstock at Example 4 (Prime-G is a naphtha selective hydrogenation technology marketed by the French company Axens which is an IFPEN's affiliate, which hydrogenates most reactive alkenes, mainly di-olefins, in particular conjugated dienes (e.g. buta-1 ,3-diene into but-1 -ene) and eventually isomerizes n-olefins (end-chain double bond, e.g. n-hex-1 -ene) into sec-olefins (internalized double bond, e.g. n-hex-2-ene), so as to get rid of the di-olefins (by selective hydrogenation) and of the low molecular weight sulfur containing molecules by conversion into heavier ones.

The effluent which is issued from Prime-G is then passed though a pretreatment section for nitrogen removal, as described above, to obtain a nitrogen depleted gasoline cut.

Characteristics of the gasoline cut are reported below in table 1 . Table 1 - Characteristics of the LCCS cut used in Examples 1 -3

ASA as oligomerization catalyst

100 mL of ASA catalyst diluted with 100ml_ of inert material (SiC 0.21 mm) have been loaded in a fixed bed tubular reactor of 18 mm inner diameter. Before testing, catalyst has been activated at 250°C (10°C/h) under 135 NL/h Nitrogen during 8 hours. Temperature has then been decreased down to 40°C before starting the testing program. Zeolite as oligomerization catalyst

100 mL of ZSM-5 based catalyst diluted with 100mL of inert material (SiC 0.21 mm) have been loaded in a fixed bed tubular reactor of 18 mm inner diameter. Before testing, catalyst has been activated at 400°C (60°C/h) under 160 NL/h Nitrogen during 2 hours. Temperature has then been decreased down to 40°C before starting the testing program.

Table 2 - Operating conditions for both type of catalyst

Table 3 - Yields for both type of catalyst and both operating modes (weight %)

Example 1 (comparative example): without hydrotreatment

Analyses of the hydrogenated middle distillates reveal that:

- Sulfur content is around 70-150 ppm (above specification EN590)

- Presence of olefins is detrimental to stability to oxidation

Example 2: with hydrotreatment (dedicated unit)

After fractionation, the oligomerized 145+ and 165°C+ cuts are hydrotreated on a NiMo catalyst.

The following operating conditions were chosen: 80 barg, a LHSV (liquid hour space velocity) of 1 h ^"1 , a ratio H ₂ /hydrocarbons of 500 NL/L, once through, and temperature has been increased from 250°C to 270°C.

Analyses of the hydrogenated middle distillates reveal that:

- Sulfur content is found below 10 ppm

- Bromine index is found around 30 mg Br/100g

- Low sulfur content and good oxidation stability

The comparison of the results obtained without and with hydrotreatment (examples 1 &2) clearly underlines the beneficial effect of the use of a hydrotreatment step before blending: specifications in terms of sulfur content are reached and also good oxidation stability to oxidation.

Example 3: with hydrotreatment (co-processing) After fractionation, the oligome zed 145+ and 165°C+ cuts are incorporated into a SRGO (with high sulfur content) at 20 wt%.

The following operating conditions were chosen: 350°C, 30 barg, a LHSV (liquid hour space velocity) of 1 h ^"1 , a ratio H ₂ /hydrocarbons of 200 NL/L, once through.

The incorporation of oligonnerization effluents does not depreciate the quality of hydrotreated effluents. Since oligonnerization effluent contains less than 100 ppm of sulfur compounds (thiophenic type), it dilutes sulfur content in the HDS unit feed and allows an operability gain. However a small increase of H ₂ consumption can be noticed (from 0,51 wt% to 0,67 wt%).

Example 3 highlights the possibility not to use a dedicated unit to perform the hydrotreatment. Oligomerization effluents can be co-processed with a SRGO on a usual HDS unit. Example 4 Application of the method to LCN feedstock

Table 4 - General properties (2 LCN feedstock samples)

Below is an example of chemical composition and repartition thereof within an average LCN, not tested here. PIONA speciation (Paraffins, Iso- paraffins, Olefins, Naphthenes, Aromatics) is shown in table 5, below:

Table 5 - Avera e LCN Results in wei ht ercent

(C-nr = number of carbon atoms within considered compounds; Naph = naphthenes; i-par = iso-paraffins; n-par = normal-paraffins, Cycl. Ol. = cyclic olefins, i-olef. = iso-olefins, n-olef. = normal-olefins; Arom. = aromatics). For the purpose of Example 4, a LCN feedstock, further to pretreatment to remove excess nitrogen species, the composition of which is given at Table 6 below, is used under the similar conditions as for LCCS at example 1 , above, and produces distillates after ASA oligomerization (LHSV = 1 h ^"1 , 200°C and 25 barg) and hydrotreatment processing (as for Example 2) under the conditions specified at Table 2:

Table 6 :

For the purpose of Example 4, a LCN feedstock, further to pretreatment to remove excess nitrogen species, the composition of which is given at Table 7 below, is used under the same conditions as for LCCS at example 1 , above, and produces distillates after Zeolite oligomerization (LHSV = 1 h ^"1 , 240°C and 55 barg) and hydrotreatment processing (as for Example 2) under the conditions specified at Table 2:

Table 7:

Table 8 - Comparison of average conversion yields for ASA versus Zeolite:

Table 9 - Usual properties of kerosene and gas oil issued from HDS or from