Novel sulphur additives and Anti-fouling compositions for the production of biofuels

Abstract

The present invention relates to a method for producing biofuel from a stream comprising an unsaturated oil derived from a biosourced raw material and / or waste, characterised in that a mercaptan of the following general formula (I) is added upstream or at the inlet of the preheating device: X-R1-Y (I) wherein: - R1 is a linear or branched (C1-C30) alkylene; - X is selected from a hydrogen atom, HO-, HOOC- or HS-(CH2)n-C(O)-O-, n being an integer between 1 and 8; - Y is an -SH group or a hydrogen atom; and wherein, if Y is a hydrogen atom, X is an -O-C(O)-(CH2)n-SH group. The mercaptan may also be combined with another sulphur compound, in particular dimethyl disulphide.

Description

Description Title: NEW SULFUR ADDITIVES AND ANTI-FOULING COMPOUNDS FOR BIOFUEL PRODUCTION Scope of the invention The present invention relates to a biofuel production process comprising a step of adding a mercaptan as an antifouling additive, as well as the use of this mercaptan as an antifouling additive for a preheating device. The present invention also relates to a composition comprising a mercaptan according to the invention and dimethyl disulfide, particularly useful as an antifouling composition for a preheating device. Technical background Today, environmental issues related to fossil fuel consumption are paramount, and reducing this consumption is a major challenge on a global scale. Alternative fuels, called biofuels, derived from biomass or waste, have therefore been developed to reduce greenhouse gas emissions and anticipate the depletion of global fossil fuel reserves. These biofuels also allow for the valorization of previously unused waste such as cooking oil or plastic waste. Their use, alone or blended with fossil fuels, represents one of the renewable alternative solutions available to us for reducing our dependence on fossil fuels. Thus, there are two main biofuel production sectors: the "gasoline" sector and the "biodiesel" sector. The "gasoline" sector includes bioethanol, its derivative ETBE (ethyl tert-butyl ether), and synthetic biofuels. The diesel biofuel sector, often grouped under the name "biodiesel," includes various biofuels, which can be produced primarily by transesterification or hydrotreatment. In the latter case, they can also be called renewable diesels. The transesterification reaction consists of reacting a fat (the triglycerides contained in oils or fats) with an alcohol (methanol or ethanol) to obtain fatty acid esters (generally, fatty acid methyl ester or fatty acid methyl ester "FAME" in English), usable as a biofuel. Hydrotreating involves treating fats and oils, such as vegetable oils, animal fats, or waste oils, with hydrogen. This process yields what are generically called Hydrotreated Vegetable Oils for Diesel (HVHTG). Hydrotreating can be carried out in a dedicated unit such as a biorefinery or as a co-treatment within a refinery: the oil is blended upstream of The hydrotreating unit is connected to a diesel fuel oil stream. This process is called "coprocessing". Such processes are widely known and described, for example in US documents 3,425,810, FR 2 538 813 A1 and US 2022 / 0145193. As mentioned above, the raw materials used to produce biofuels by hydrotreating come from very diverse sources: biomass, agricultural waste, forestry waste, municipal waste, used cooking oil, and recycled plastics. Depending on the origin of the raw material used, the properties of the oils to be hydrotreated will vary considerably: different levels of free fatty acids and / or glycerides, different number and position of unsaturates, variable hydrocarbon chain lengths, and the presence of specific impurities. The nature and variability of the raw materials will have consequences for the biofuel production process by hydrotreating. Indeed, the oils used contain unsaturated fatty acids and / or glycerides and / or other types of unsaturated compounds. When heated, these compounds can react together, giving rise to dimers, which are again reactive, and oligomers. Their accumulation leads to the formation of fouling deposits, particularly in the raw material preheating devices. Organic debris (e.g., cellulose or lignin) or inorganic debris (e.g., sand) that are insoluble and contribute to the formation of these deposits can also be found in the raw material stream. It should be noted that other types of deposits can form in the hydrotreating reaction zone: these are generally linked to the formation of highly aromatic by-products (coke), resulting in a loss of catalyst activity. Fouling deposits that form in preheating devices require specific treatment and represent a real problem for (bio)refiners. They lead to raw material loss and reduced efficiency. These deposits can form a more or less thick and insulating layer inside the preheating devices, making it more difficult for the raw materials to reach the desired temperature. This necessitates increased energy consumption to preheat to the required temperature. Furthermore, as these deposits accumulate, they can eventually clog the equipment. Very costly shutdowns of the production unit for cleaning then become necessary. There is therefore a need for an improved biofuel production process, in which fouling of the elements upstream of the hydrotreating reactor, particularly of the preheating devices, is limited or even avoided. Document WO 2013 / 016256 describes the use of organic polysulfides to reduce this type of fouling. These polysulfides have, in particular, the general formula RS x - R, where R is a linear or branched alkyl containing 2 to 15 carbon atoms and x is an integer between 1 and 8. However, when using such compounds Over the long term, other types of deposits form. Above 170 °C, these polysulfides decompose, and this decomposition can lead to the formation of a deposit called "Carsul," which is composed of sulfur-containing polyaromatic molecules. This solution is therefore not entirely satisfactory. Thus, there is still a need for additives or antifouling compositions, particularly for heating devices, that are effective and suitable for biofuel production. An objective of the present invention is to provide a process for the production of biofuels in which fouling of the elements upstream of the hydrotreating reactor, in particular of the preheating devices, is limited or even avoided. Another objective of the present invention is to provide a new additive or a new composition which allows the treatment and / or limitation and / or prevention of fouling of elements upstream of the hydrotreating reactor, in particular preheating devices, of biofuel production units. Another objective of the present invention is to provide a new antifouling additive or composition that can be easily introduced into biofuel production units (biorefineries or refineries), and more particularly without modification of the production line. Brief description of the invention The present invention fulfills, in whole or in part, the objectives stated above. The inventors have discovered that mercaptans of general formula (I) as defined below have antifouling activity on preheating devices, particularly on preheating devices used upstream of hydrotreating reactors. They are notably more effective than the organic polysulfides described for this application. Furthermore, they exhibit even improved antifouling activity when combined with another sulfur compound, with the following general formula (II): R2-S m -R3(II) in which: R2 and R3, identical or different, are chosen from among the hydrogen atom, the methyl radical and the ethyl radical; m is an integer equal to 1 or 2; and on the condition that if R2 is an ethyl radical, then R3 is a hydrogen atom. The preferred sulfur compound with general formula (II) is dimethyl disulfide (hereinafter DMDS, with the formula CH3-SS-CH3). This combination offers improved antifouling properties and is ideally suited for industrial applications. The injection of these new anti-fouling additives can, for example, be carried out via the injection points of the general formula sulfur compound (II), particularly DMDS, which may sometimes already be present in the process. It can therefore be done without additional cost or adaptation required for (bio)refineries. Thus, the present invention relates to a process for producing biofuel from a stream comprising an unsaturated oil derived from a bio-based raw material and / or waste, said process comprising the following steps: a / the stream is introduced into a preheating device; b / the preheated stream is introduced into a hydrotreating reactor; c / the stream is hydrotreated with hydrogen, in the presence of a hydrotreating catalyst, to obtain a hydrotreated stream; d / optionally, the hydrotreated stream obtained at the outlet of the hydrotreating reactor is separated to obtain: - a hydrotreated organic stream, - a gaseous flow comprising H2, H2S and possibly CO and / or CO2, and - an aqueous stream; e / optionally, an isomerization and / or cracking reaction is carried out on the hydrotreated stream recovered at the end of step c) and / or step d), with hydrogen, in the presence of an isomerization and / or cracking catalyst; and f / the biofuel obtained at the end of one of steps c), d) or e); said process being characterized in that a mercaptan of the following general formula (I) is added upstream or at the inlet of the preheating device: X-Ri-Y (I) in which: Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched; - X is chosen from among a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is a -OC(O)-(CH2) group n- SH. The present invention also relates to the use of a mercaptan as an anti-fouling additive for a preheating device of a stream comprising an unsaturated oil, preferably derived from a bio-based raw material and / or waste, said mercaptan having the following general formula (I): X-Ri-Y (I) in which: Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched; - X is chosen from among a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is a -OC(O)-(CH2) group n - SH. The present invention relates to a composition comprising mercaptan of general formula (I) as according to the invention and dimethyl disulfide, and its use as an antifouling composition for a preheating device of a stream comprising an unsaturated oil, preferably from a bio-based raw material and / or waste. The present invention also relates to a method for combating fouling of a preheating device of a flow comprising an unsaturated oil, preferably from a bio-based raw material and / or waste, said method comprising a step of adding upstream or at the inlet of the preheating device a mercaptan of general formula (I) such as according to the invention or of a composition such as according to the invention. Detailed description of the invention Other features, aspects, objects and advantages of the present invention will become even clearer upon reading the description that follows. It is specified that the expressions "from ... to ..." and "between ... and ...." used in this description should be understood as including each of the mentioned limits. Biofuels are liquid or gaseous fuels obtained from the transformation of non-fossil organic raw materials. According to the invention, the unsaturated oil used can be derived from a bio-based raw material and / or waste. "Bio-based raw material" refers, in particular, to a raw material derived from biomass. "Biomass" refers, in particular, to organic matter of plant, animal, bacterial, or fungal origin (usable as an energy source). Biomass can, in particular, be considered a renewable raw material (when its consumption is at least equal to its regeneration). Preferably, the biofuels according to the invention are renewable fuels. More particularly, they are bionaphtha, renewable diesels, and / or sustainable aviation fuels, preferably sustainable aviation fuels. The biofuels obtained according to the invention can be blended with other petroleum fractions such as gasoline, kerosene or diesel. They can be "drop-in fuels", meaning they can completely replace petroleum-based fuels. As previously mentioned, they can be produced by hydrotreating processes that treat fats such as vegetable oils, animal oils or oils derived from animal fats, or even waste oils, with hydrogen. This yields Hydrotreated Vegetable Oils for Diesel (HVHTG). To produce biofuels using this method, two reactors are generally required: a hydrotreating reactor, similar to conventional petroleum hydrotreating units, and an isomerization and / or light cracking reactor. There are two main types of biofuel production units: single-stage units, where the two reactors are connected in series, and two-stage units, where the hydrotreated stream is purified before reaching the isomerization and / or light cracking reactor. Chemically, these processes hydrogenate the unsaturates in the fat chains and remove unwanted oxygen from the raw materials by hydrodeoxygenation (producing H2O), and / or by hydrogenation and decarbonylation (producing carbon monoxide C(O)), and / or by hydrogenation and decarboxylation (producing carbon dioxide CO2). Such processes are widely known to those skilled in the art. The biofuel production process according to the invention can be carried out in any type of hydrotreating unit equipped with a feedstock preheating device (i.e., located upstream of the hydrotreating reactor(s)). The preheating device can be a heat exchanger and / or a preheating furnace. The preheating furnace is particularly useful at startup. Subsequently, the exothermic nature of the reaction may allow the use of a heat exchanger alone (although the stream containing unsaturated oil can still pass through the preheating furnace). Thus, the stream containing the unsaturated oil is introduced into the preheating device (step a)). The stream containing the unsaturated oil may also include saturated hydrocarbons of fossil and / or bio-based origin. These mixtures reduce the number of functional groups requiring hydrogenation and thus limit the exothermic reaction in the hydrotreating reactor(s). The preheating device heats the stream containing the unsaturated oil to a temperature close to the hydrotreating reaction temperature or to the hydrotreating reaction temperature itself. "Temperature close to the hydrotreating reaction" refers, in particular, to a temperature between 0 °C and 100 °C lower than the hydrotreating reaction temperature.For example, the temperature of the stream containing unsaturated oil at the outlet of the preheating device is between 150°C and 430°C, for example between 200°C and 400°C, preferably between 250°C and 380°C, and more preferably between 250°C and 330°C. The stream containing unsaturated oil is thus preheated in the device. preheating then introduced into the hydrotreating reactor to be hydrotreated (step b)). Step c) of hydrotreating is known to those skilled in the art. It can be carried out at temperatures ranging from 150 to 430 °C, preferably from 200 to 400 °C, and / or at pressures ranging from 0.1 to 25 MPa, preferably from 1 to 20 MPa. When the hydrotreating reaction is carried out in a single reaction zone, the temperature can range from 200 to 400 °C, preferably from 250 to 380 °C. When there are two or more hydrotreating steps, the temperature in each reaction zone can be lower, as a gentler hydrotreating process can be performed. In such embodiments, the temperature can range from 150 to 300 °C, for example, from 200 to 300 °C. Furthermore, the temperature in the first reaction zone can be lower than the temperature in the second (or subsequent) reaction zone. Step c) of hydrotreating is carried out with hydrogen and in the presence of a hydrotreating catalyst. The hydrogen used can be fresh or recycled (from the process itself or from another unit of the (bio)refinery). It can be substantially pure or contain impurities and / or by-products, preferably such that the chemical nature and / or concentration of these impurities and / or by-products in the hydrogen does not cause a significant reduction in the activity and / or lifetime of the catalyst to which the hydrogen is exposed. The hydrogen content can be between 50 Nr¹⁷m 3 and 1000 Nm 3 / m 3 of catalyst. When relatively mild hydrotreating conditions are desired, the hydrogen content can be between 75 Nm 3 / m 3 and 300 Nm 3 / m 3 , preferably between 100 Nm 3 / m 3 and 250 Nm 3 / m 3When relatively more severe hydrotreating conditions are desired, the hydrogen content can be between 300 Nm 3 / m 3 and 650 Nm 3 / m 3 , preferably between 350 Nm 3 / m 3 and 550 Nm 3 / m. The catalyst used is known to those skilled in the art. It comprises, in particular, one or more metals from Group VIII (columns 8, 9, and 10 of the periodic table of elements) and / or one or more metals from Group VIA (column 6 of the periodic table of elements), for example, Ni and / or Co and / or W and / or Mo. Preferably, the catalyst comprises a NiMo or CoMo combination, or a ternary combination such as Ni, Co, and Mo, or Ni, Mo, and W. These metals may be in the form of oxides, sulfides, or oxysulfides. The hydrotreating catalyst is generally supported on an oxide such as alumina, silica, zirconia, titanium, or a combination thereof, or another known support material such as carbon. The catalyst may also contain other components such as promoters. By way of illustration, suitable hydrotreating catalysts are described in one or more of U.S. patents 6,156,695 and 6,299,760. 6,783,663; 7,288,182; 7,410,924 and 7,544,632, U.S. Patent Application Publications No. 2007 / 0084754 and 2008 / 012407, and International Publication WO 2007 / 084439. A combination of catalysts can be used, possibly in a mixture, in the first and / or second (or subsequent) hydrotreating reaction zones. These catalysts can be arranged in a stacked bed. Alternatively, one catalyst can be used in the first hydrotreating reaction zone and a second catalyst in the second (or subsequent) hydrotreating reaction zones. For example, the first hydrotreating reaction zone might consist of a stacked bed of NiMo catalyst, followed by a CoMo catalyst. The second reaction zone might consist of a CoMo catalyst. In another example, the NiMo catalyst in the first hydrotreating zone could be replaced by a catalyst containing Ni and W metals or a catalyst containing Ni, W, and Mo metals. Hydrotreatment can be carried out at liquid hourly space velocities (LHSV) of approximately 0.1 to approximately 10 h -1, for example from about 0.3 to about 5 h -1 or from approximately 0.5 to approximately 5 hours -1 In embodiments where there are two or more hydrotreating stages, the conditions in each reaction zone (or each reactor, when the reaction zones are in separate reactors) can be milder, and as noted above, this can be achieved using lower temperatures. In such an embodiment, the LHSV is preferably from about 1 to about 5 h -1 . The hydrotreated stream obtained at the outlet of the hydrotreating reactor can then be separated (step d)) in order to obtain: a hydrotreated organic stream (liquid stream), a gaseous stream comprising H2, H2S and possibly CO and / or CO2, and an aqueous stream. Such separation can be achieved by any means known to a person skilled in the art, for example by evaporation and / or decantation. The hydrotreated stream from step c) and / or step d) can then be recovered to carry out an isomerization and / or cracking reaction (step e)) with hydrogen, in the presence of an isomerization and / or cracking catalyst. This type of reaction is conventional and is used, in particular, to obtain liquid fuels. It is described in US document 2022 / 0145193, incorporated herein by reference (see in particular paragraphs

[0051] has

[0075] The catalyst can be either a NiW / zeolite type catalyst or a catalyst comprising a noble metal (e.g., Pt / Al₂O₃), preferably a sulfide. The temperature of the isomerization and / or cracking reaction can range from 150 °C to 450 °C, and the pressure can range from 1 MPa to 15 MPa. Unsaturated oil: The term "unsaturated oil" refers specifically to any oil containing at least one unsaturated organic compound, that is, an organic compound with at least one C=C double bond. Preferably, this organic compound contains one, two, or three C=C double bonds. This oil is derived, in particular, from raw materials of plant and / or animal origin and / or from waste. Advantageously, this unsaturated oil may be derived from renewable raw materials. In particular, unsaturated oil includes (as an unsaturated compound): - at least one unsaturated free fatty acid; and / or - at least one glyceride (formed from unsaturated fatty acid(s), which may be a mono-, di- or triglyceride), preferably a triglyceride; and / or - at least one unsaturated fatty acid ester other than a glyceride, for example C1-C5 alkyl esters of unsaturated fatty acids, such as methyl and / or ethyl esters of unsaturated fatty acids (known as FAME or FAEE in English); and / or - an unsaturated hydrocarbon chain. Preferably, said unsaturated oil comprises an unsaturated free fatty acid and / or a triglyceride formed from unsaturated fatty acid(s). Said fatty acids comprise in particular from 4 to 36 carbon atoms, preferably from 8 to 24 carbon atoms, preferably again from 12 to 18 carbon atoms. More specifically, said unsaturated oil comprises at least 40% by weight, preferably at least 50% by weight, and preferably even more at least 60% by weight of said unsaturated compound relative to the total weight of the oil. The said unsaturated oil can be chosen from vegetable oils, animal oils or oils derived from animal fat(s), oils derived from algae, used oils such as used cooking oils (for example frying oils), pyrolysis oils from plastic(s), oils derived from waste and their mixtures. Among the vegetable oils, we can mention: almond oil, peanut oil, babassu oil, camelina oil, hemp oil, canola oil, safflower oil, rapeseed oil, jatropha oil, jojoba oil, flaxseed oil, corn oil, mustard oil, coconut oil, olive oil, palm oil, palm kernel oil, grapeseed oil, castor oil, soybean oil, sunflower oil, rice bran oil, Jatropha curcas oil (Ratanjot, Wild Castor, Jangli Erandi), Madhuca indica oil (Mohuwa), Pongamia pinnata oil (Karanji, Honge), Calophyllum inophyllum oil, oil of Moringa oleifera and Azadirachta indica (Neem) oil, their derivatives and mixtures. Among animal oils and fats (hereinafter referred to as animal oils), we can mention: fish oils, lard, tallow, suet, beef fat, pork fat, whale fat, milk fats, and mixtures thereof. Among oils derived from algae, oils derived from microalgae such as diatoms and chlorophytes are preferred. Regarding oils derived from waste, examples include waste and recycled products from the plastics industry, the paper industry (such as tall oil), and forestry. This waste is generally pyrolyzed to obtain oil. For example, the pyrolysis of polystyrene can produce triphenylbenzene. In particular, said unsaturated oil is chosen from used cooking oils, linseed oil, rapeseed oil, soybean oil, corn oil, palm oil, animal oils, pyrolysis oils of plastic(s) and their mixtures. The process according to the invention may include a preliminary step of transforming a raw material into oil, for example, transforming animal fat into oil by melting it. It may also include a preliminary step of purifying and / or filtering the unsaturated oil. The process according to the invention includes a step of adding, upstream or at the inlet of the preheating device, a mercaptan of general formula (I) as defined below. Said mercaptan may be in the form of a composition comprising said mercaptan and DMDS. This step helps prevent unsaturated compounds present in the oil from forming dimers and / or oligomers. These compounds can form molecules with a high molar mass (the molar mass of these molecules can vary depending on the nature of the unsaturated compound). These dimers and oligomers can form deposits and thus foul the preheating device. In particular, the mercaptan of general formula (I) according to the invention, or the composition according to the invention, is added upstream or at the inlet of the preheating device to eliminate, reduce, and / or prevent fouling. It is added in sufficient quantity to eliminate, reduce, and / or prevent fouling of said preheating device. More specifically, said mercaptan or composition is added intermittently, semi-continuously, or continuously, preferably continuously. For its introduction, the mercaptan or composition may be pre-mixed with the stream containing the unsaturated oil. It is, for example, mixed upstream or at the inlet of the preheating device. In this case, preferably: - the quantity of said mercaptan is between 1 ppm and 1000 ppm, preferably between 10 ppm and 500 ppm, and more preferably between 25 ppm and 150 ppm, relative to the total weight of said stream; or - the quantity of said composition is between 1 and 5000 ppm, preferably between 1 ppm and 2000 ppm, preferably still between 50 ppm and 1800 ppm, and more preferably between 100 ppm and 1500 ppm, in relation to the total weight of said flux. Alternatively, the mercaptan or composition can be introduced directly at the inlet of the preheating device, without prior mixing with the stream comprising the unsaturated oil. The addition of mercaptan or the composition can be done by any technique known to a person skilled in the art (for example with a pump), who can adapt the quantities introduced according to the production conditions or the nature of the unsaturated oil. General formula mercaptan (I): The mercaptan according to the invention has the following general formula (I): X-Ri-Y (I) in which: Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched; - X is chosen from among a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is a -OC(O)-(CH2)n-SH group. By "alkylene", we mean in particular a divalent, saturated, linear or branched alkane group comprising from 1 to 30 carbon atom(s). Preferably, Ri is a (C4-C3o)alkylene, more preferably a (C4-Ci2)alkylene, and even more preferably a (C8-Ci2)alkylene. Most preferably, Ri is a branched alkylene as defined above. This branching is, in particular, a (Ci-C4)alkyl branch. For example, the alkylene is branched by the presence of at least one methyl and / or ethyl group attached to the main chain. In particular, the mercaptan in question corresponds to one of the following three general formulas: Ra-SH (la); HS-(CH2)nC(O)-OR a (Ib); HOOC-Ri-SH (the); in which: Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched (as defined above); R ais a (Ci-C3o)alkyl, linear or branched, preferably branched; n is an integer between 1 and 8, preferably between 1 and 4, preferably equal to 1 or 2. Preferably, R a is a (C4-C 30 )alkyl, more preferably a (C4-Ci2)alkyl, for example a (C8-Ci2)alkyl. Preferably, said mercaptan is of general formula (la) or (Ib), more preferably (la). Preferably, said mercaptan is chosen from the group consisting of: 2-mercaptoethanol, n-octylmercaptan (NOM), n-dodecylmercaptan (NDM), tert-dodecylmercaptan (TDM), tert-nonyl mercaptan (TNM), tert-butylmercaptan (TBM), 2-ethylhexyl thioglycolate (2-EHTG), isooctyl thioglycolate (IOTG), 2-methylheptyl thioglycolate (2-MHTG), methyl thioglycolate (MTG), ethyl thioglycolate, 3-mercaptopropionic acid, methyl 3-mercaptopropionate, ethyl 3-mercaptopropionate, butyl 3-mercaptopropionate, 2-ethylhexyl 3-mercaptopropionate, isooctyl 3-mercaptopropionate, octadecyl 3-mercaptopropionate, and thioglycolic acid, preferably tert-dodecyl mercaptan and 2-ethylhexyl thioglycolate. The structure of these compounds is given in Table 1 below: [Table 1] Preferably, the mercaptan is selected from the group consisting of: tert-dodecyl mercaptan (TDM), tert-nonyl mercaptan (TNM), tert-butyl mercaptan (TBM), 2-ethylhexyl thioglycolate (2-EHTG), isooctyl thioglycolate (IOTG), 2-methylheptyl thioglycolate (2-MHTG), butyl 3-mercaptopropionate, 2-ethylhexyl 3-mercaptopropionate, and isooctyl 3-mercaptopropionate. The particularly preferred mercaptans are tert-dodecyl mercaptan and 2-ethylhexyl thioglycolate. The mercaptan of general formula (I) as defined above can be used as an antifouling additive for a preheating device of a stream comprising an unsaturated oil, preferably from a bio-based raw material and / or waste. Sulphur compound of general formula (II): Surprisingly, the present inventors have discovered that the mercaptan of general formula (I) as defined above can be combined with a sulfur compound of the following general formula (II): R2-S m -R3(II) in which: R2 and R3, identical or different, are chosen from among the hydrogen atom, the methyl radical and the ethyl radical; m is an integer equal to 1 or 2; and on the condition that if R2 is an ethyl radical, then R3 is a hydrogen atom. In particular, the sulfur compound of general formula (II) is chosen from the group consisting of: hydrogen sulfide (H2S), methyl mercaptan (CH3-SH), dimethyl sulfide (CH3-S-CH3), dimethyl disulfide (CH3-SS-CH3) and ethyl mercaptan (CH3-CH2-SH), preferably hydrogen sulfide and dimethyl disulfide. Preferably, R2 and R3, whether identical or different, are a hydrogen atom or a methyl radical. Most preferably, the sulfur compound of general formula (II) is dimethyl disulfide (DMDS). This combination, in particular, improves the anti-fouling effect of the preheating device. Thus, the sulfur compound with the following general formula (II) can also be added to the process according to the invention. According to one embodiment, the mercaptan of general formula (I) and the sulfur compound of general formula (II) are introduced separately into the biofuel production process according to the invention. In this embodiment, the mercaptan is added independently of the sulfur compound, and its addition is carried out as described above. The sulfur compound of general formula (II) is sometimes used for the proper functioning of the process because it helps maintain the catalysts in a sulfuric state: it is preferably injected upstream or at the inlet of the preheating device. It can thus be injected into the stream containing the unsaturated oil. In this case, it can be introduced at concentrations ranging from 10 to 2000 ppm, preferably from 50 to 1000 ppm, and even more preferably from 75 to 800 ppm (relative to the total weight of the stream containing the unsaturated oil). According to another embodiment, the mercaptan of general formula (I) and the sulfur compound of general formula (II) are in the form of a composition, in particular such as defined below. Such a composition is preferably made with DMDS as a sulfur compound of general formula (II). Thus, the present invention also relates to a composition comprising a mercaptan of general formula (I) such as according to the invention and DMDS. In particular, said composition comprises at least 30%, preferably at least 65%, more preferably at least 85%, for example at least 95%, by weight of DMDS, relative to the total weight of the composition. Preferably, said composition comprises between 1 and 50% by weight, preferably between 5 and 45% by weight, preferably again between 10 and 40% by weight, of mercaptan of general formula (I) as defined above, relative to the total weight of the composition. Compositions including tert-dodecylmercaptan and DMDS or 2-ethylhexyl thioglycolate and DMDS are particularly preferred. For example, a composition according to the invention may include, among other things: - 5 and 45% by weight, preferably between 10 and 40%, for example 15% or 35% by weight, of mercaptan of general formula (I), preferably tert-dodecyl mercaptan, relative to the total weight of the composition; and - between 55 and 95% by weight of DMDS, preferably between 60 and 90% by weight, for example 65% or 85% by weight of DMDS, relative to the total weight of the composition. The composition may also contain one or more odor-masking agents (see, for example, international application WO 2011 / 012815A1). DMDS is commercially available. One example is DMDS Evolution® E2, marketed by ARKEMA. Such a composition can be introduced into the process according to the invention as mentioned above. The composition as defined above can be used as an antifouling composition for a preheating device of a flow comprising an unsaturated oil, preferably from a bio-based raw material and / or waste. The following examples are given for illustrative purposes only and do not limit the present invention. EXAMPLES Example 1: Anti-fouling effect of mercaptans according to the invention Operating procedure: Samples of 50 g of unsaturated oil used as raw material for biofuels are heated in an oven, possibly in the presence of 100 ppm of sulfur additive. The samples are heated in capsules that simulate the behavior of an unsaturated oil in the preheaters of vegetable oil hydrotreating units, where the gaseous air has been replaced by nitrogen. After being heated for 80 hours at 240 °C, the capsules are removed from the oven and samples are taken. The percentage by weight of high molar mass molecules (corresponding to the dimers and / or oligomers formed), relative to the total weight of the unsaturated oil, is measured by size-exclusion chromatography. The molar mass of the dimers and / or oligomers formed varies depending on the nature of the unsaturated oil tested. For linseed oil, the percentage by weight of molecules with a molar mass greater than 3500 g / mol is determined, relative to the total weight of the unsaturated oil (hereafter referred to as "% greater than 3500"). 1) Test 1: Linseed oil alone Linseed oil is heated without additives, as a control, according to the procedure mentioned above. The percentage by weight of molecules with a molar mass greater than 3500 g / mol is determined, relative to the total weight of the unsaturated oil. The results are given in Table 2 below. [Table 2] 2) Test 2: Linseed oil with anti-fouling additive In this series of tests, according to the procedure mentioned above, the heated oil was linseed oil and various anti-fouling sulfur additives were added at a level of 100 ppm: - 2-EHTG: 2-Ethylhexyl thioglycolate (marketed by Arkema) - DTDDS: Di-tert-dodecyl disulfide (marketed by Arkema) - DM DS: Dimethyl disulfide (marketed by Arkema) - Sulfrzol® 54: Di-tert-butyl polysulfide (marketed by Lubrizol) - TDM: Tert-dodecyl mercaptan (marketed by Arkema) - TPS® 20: Di-tert-dodecyl polysulfide (marketed by Arkema) - TPS® 54: Di-tert-butyl polysulfide (formerly marketed by Arkema) The percentage by weight of molecules with a molar mass greater than 3500 g / mol is determined, relative to the total weight of the unsaturated oil. The results are given in Table 3 below. [Table 3] The results show that mercaptans such as those according to the invention reduce the formation of high molar mass molecules (such as dimers and / or oligomers) when the oil is heated: they therefore have an anti-fouling activity. In particular, this anti-fouling effect is improved compared to the polysulfides described for this application in document WO 2013 / 016256. 3) Test 3: Linseed oil with anti-fouling additive and DMDS In this series of tests, according to the procedure mentioned above, the heated oil is linseed oil and the mercaptans according to the invention or DMDS were added at a level of 100 ppm, with 100 ppm of additional DMDS. The percentage by weight of molecules with a molar mass greater than 3500 g / mol is determined, relative to the total weight of the unsaturated oil. The results are given in Table 4 below. [Table 4] The results show improved anti-fouling efficacy, or even synergy, between the mercaptans according to the invention and DMDS: an even greater reduction in the presence of molecules with a molar mass greater than 3500 g / mol is observed. -1 .

Claims

DEMANDS 1. A process for producing biofuel from a stream comprising an unsaturated oil derived from a bio-based feedstock and / or waste, said process comprising the following steps: a / the stream is introduced into a preheating device; b / the preheated stream is introduced into a hydrotreating reactor; c / the stream is hydrotreated with hydrogen, in the presence of a hydrotreating catalyst, to obtain a hydrotreated stream; d / optionally, the hydrotreated stream obtained at the outlet of the hydrotreating reactor is separated to obtain: - a hydrotreated organic stream, - a gaseous flow comprising H2, H2S and possibly CO and / or CO2, and - an aqueous stream; e / optionally, an isomerization and / or cracking reaction is carried out on the hydrotreated stream recovered at the end of step c) and / or step d), with hydrogen, in the presence of an isomerization and / or cracking catalyst; and f / the biofuel obtained at the end of one of the steps c), d) or e); said process being characterized in that a mercaptan of the following general formula (I) is added upstream or at the inlet of the preheating device: X-Ri-Y (I) in which: - Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched; - X is chosen from among a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is an -OC(O)- (CH2) group n-SH.

2. A process for producing biofuel according to claim 1, wherein said mercaptan is pre-mixed with the stream comprising the unsaturated oil.

3. A biofuel production process according to claim 2, wherein the amount of said mercaptan is between 1 ppm and 1000 ppm, preferably between 10 ppm and 500 ppm, and more preferably between 25 ppm and 150 ppm, relative to the total weight of said flux.

4. A method for producing biofuel according to any one of the preceding claims, wherein the preheating device is a heat exchanger and / or a preheating furnace.

5. A process for producing biofuel according to any one of the preceding claims, wherein said mercaptan is selected from the group consisting of: 2-mercaptoethanol, n-octylmercaptan (NOM), n-dodecylmercaptan (NDM), tert-dodecylmercaptan (TDM), tert-nonyl mercaptan (TNM), tert-butyl mercaptan (TBM), 2-ethylhexyl thioglycolate (2-EHTG), isooctyl thioglycolate (IOTG), 2-methylheptyl thioglycolate (2-MHTG), methyl thioglycolate (MTG), ethyl thioglycolate, 3-mercaptopropionic acid, methyl 3-mercaptopropionate, ethyl 3-mercaptopropionate, butyl 3-mercaptopropionate, 2-ethylhexyl 3-mercaptopropionate, isooctyl 3-mercaptopropionate, octadecyl 3-mercaptopropionate, and thioglycolic acid.

6. A process for producing biofuel according to any one of the preceding claims, wherein said mercaptan is tert-dodecyl mercaptan or 2-ethylhexyl thioglycolate.

7. A process for producing biofuel according to any one of the preceding claims, wherein the unsaturated oil is selected from vegetable oils, animal oils or oils derived from animal fat(s), oils derived from algae, used oils such as used cooking oils, pyrolysis oils from plastic(s), oils derived from waste and mixtures thereof.

8. A process for producing biofuel according to any one of the preceding claims, wherein said unsaturated oil is selected from used cooking oils, linseed oil, rapeseed oil, soybean oil, corn oil, palm oil, animal oils, pyrolysis oils of plastic(s) and mixtures thereof. 11 9. A process for producing biofuel according to any one of the preceding claims, wherein a sulfur compound of the following general formula (II) is also added: R2-S m -R3(II) in which: R2 and R3, identical or different, are chosen from among the hydrogen atom, the methyl radical and the ethyl radical; m is an integer equal to 1 or 2; and on the condition that if R2 is an ethyl radical, then R3 is a hydrogen atom.

10. A process for producing biofuel according to any one of the preceding claims, wherein said biofuel is a bionaphtha, a renewable diesel and / or a sustainable aviation fuel.

11. Use of a mercaptan as an antifouling additive for a preheating device of a stream comprising an unsaturated oil derived from a bio-based raw material and / or waste, said mercaptan having the following general formula (I): X-Ri-Y (I) in which: Ri is a (Ci-C 30 )alkylene, linear or branched, preferably branched; - X is chosen from among a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is an -OC(O)- (CH2) group n -SH.

12. Use according to claim 11, wherein said mercaptan is selected from the group consisting of: 2-mercaptoethanol, n-octylmercaptan (NOM), n-dodecylmercaptan (NDM), tert-dodecylmercaptan (TDM), tert-nonyl mercaptan (TNM), tert-butyl mercaptan (TBM), 2-ethylhexyl thioglycolate (2-EHTG), isooctyl thioglycolate (IOTG), 2-methylheptyl thioglycolate (2-MHTG), methyl thioglycolate (MTG), ethyl thioglycolate, 3-mercaptopropionic acid, methyl 3-mercaptopropionate, ethyl 3-mercaptopropionate, butyl 3-mercaptopropionate, 2-ethylhexyl 3-mercaptopropionate, of the 3- isooctyl mercaptopropionate, octadecyl 3-mercaptopropionate, and thioglycolic acid.

13. Composition comprising: - a mercaptan of the following general formula (I): X-Ri-Y (I) in which: Ri is a (Ci-C3o)alkylene, linear or branched, preferably branched; - X is chosen from a hydrogen atom, HO-, HOOC- or HS-(CH2) n -C(O)-O- with n being an integer between 1 and 8, preferably between 1 and 4, preferably n is equal to 1 or 2; - Y is a -SH group or a hydrogen atom; and in which if Y is a hydrogen atom, then X is a -OC(O)-(CH2) group n - SH; and - dimethyl disulfide.

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