EA013998B1 - Process for the manufacture of diesel range hydrocarbons - Google Patents

Abstract

The invention relates to a process for the manufacture of diesel range hydrocarbons wherein a feed is hydrotreated in a hydrotreating step and isomerised in an isomerisation step, and a feed comprising fresh feed containing more than 5 wt% of free fatty acids and at least one diluting agent is hydrotreated at a reaction temperature of 200-400 DEG C, in a hydrotreating reactor in the presence of catalyst, and the ratio of the diluting agent/fresh feed is 5 - 30:1.

Description

The invention relates to an improved method for the production of hydrocarbons, especially hydrocarbons of a fraction of diesel fuel, from oils and fats of biological origin, in which the formation of compounds with a higher molecular weight is reduced. The invention also relates to the processing of feedstocks containing free fatty acids using a high ratio of recycle product to fresh oil at reduced reaction temperatures.

The scope of the invention

Environmental interests and the growing demand for diesel fuel, especially in Europe, are stimulating fuel producers to make more intensive use of available renewable sources. In the production of diesel fuel based on biological raw materials, the main interest has focused on vegetable oils and animal fats, including triglycerides of fatty acids. Long, linear and mostly saturated hydrocarbon chains of fatty acids chemically correspond to the hydrocarbons present in diesel fuels. However, vegetable oils in their natural form exhibit worse properties, especially excessive viscosity and poor stability, and therefore their use in motor fuels is limited.

Conventional approaches to converting vegetable oils or other derivatives of fatty acids to liquid fuels include, but are not limited to, transesterification, catalytic hydrotreating, hydrocracking, catalytic cracking without hydrogen and thermal cracking. Typically, the triglycerides that make up the main component of vegetable oils are converted to the corresponding esters by the transesterification reaction with alcohol in the presence of catalysts. The resulting product is a fatty acid alkyl ester, most often a fatty acid methyl ester (EAME). However, the poor low-temperature properties of ELME limit its wider use in regions with cold climatic conditions. The above properties are the result of the linear chain nature of the EAME molecule, and thus double bonds are required in order to create uniform acceptable cold flow characteristics. Carbon-carbon double bonds and ester groups, however, reduce the stability of fatty acid esters, which is the main disadvantage of transesterification technology. Further K. Schmidt and J.V. Herpen (δοϊιιηίάΐ. K., Ogrei, TU., 8AE (8еДе1у о £ А11отауе Едшеегз, Society of Automotive Engineers), article 961086), report that the presence of oxygen in esters leads to undesirable and increased emissions of NΟ _X compared to conventional diesel fuels.

Unwanted oxygen can be removed from fatty acids or their esters by deoxygenation reactions. The deoxygenation of biological oils and fats, which are oils and fats based on a biological material, to produce hydrocarbons useful as diesel fuel products can be accomplished using catalytic hydroprocessing, such as hydrocracking, but more controlled hydrotreating conditions can also be applied. .

In a hydrotreating process, especially during hydrodeoxygenation, oxygen-containing groups react with hydrogen and are removed by the formation of water, and therefore the reaction requires several large quantities of hydrogen. In view of the highly exothermic nature of these reactions, it is extremely important to control the thermal effect of the reaction. Contaminated vegetable oil / fat or animal fat / oil, high reaction temperatures, inadequate control of the reaction temperature or low concentration of hydrogen in the feed stream can cause unwanted side reactions such as cracking, polymerization, ketonization, cyclization and aromatization, and coking of the catalyst. These side reactions also reduce the yield and degrade the properties of the resulting diesel fraction.

Unsaturated raw materials and free fatty acids in biological oils and fats can also contribute to the formation of high molecular weight compounds that can clog the preheater and shorten the activity and life of the catalyst.

The composition of fatty acids, the size and degree of saturation of the fatty acid can vary significantly in the feedstock of various origins. The melting point of biological oil or fat is mainly a consequence of the degree of saturation. Fats are more saturated than liquid oils, and in this respect require less hydrogen to hydrogenate double bonds. Double bonds in the fatty acid chain also contribute to various side reactions, such as oligomerization / polymerization, cyclization / aromatization and cracking reactions, which deactivate the catalyst, increase hydrogen consumption and reduce the yield of the diesel fraction.

Vegetable oils / fats and animal oils / fats, as a rule, may contain 0-30% free fatty acids, which are formed in the method of enzymatic hydrolysis of triglycerides, especially when oilseeds are stored in a humid atmosphere. Free fatty acids can also be formed during the purification of biological oils and fats, especially in the alkaline washing method, i.e. alkali-catalyzed hydrolysis. The amount of free fatty acids present in vegetable oils is typically 1-5 wt.% And in animal fat 10-25 wt.%. Free fatty acids are corrosive in nature, they can destroy materials

- 1 013998 production plant or catalyst, and they may contribute to the occurrence of some adverse reactions. Free fatty acids interact very actively with metallic impurities to form metallic salts of carboxylic acids, which chemically contribute to the development of adverse reactions.

Fatty acids can also contribute to the formation of heavy compounds. The boiling range of these heavy compounds is different from the boiling range of diesel fuel and may shorten the life of the isomerization catalyst.

Due to the fact that free fatty acids are contained in biological oils and fats, the formation of compounds with a high molecular weight is significantly enhanced compared to triglyceride biological raw materials, which have only a small amount of free fatty acids (<1%).

Biological raw materials often contain metal compounds, organic nitrogen-containing, sulfur-containing and phosphorus-containing compounds, which are known inhibitors of catalytic activity and catalytic poisons, which inevitably shorten the life of the catalysts and create the need for more frequent regeneration or replacement of the catalyst. Metals in biological oils / fats inevitably accumulate on the surface of the catalyst and alter the activity and selectivity of the catalyst. Metals may favor some adverse reactions, but blocking the active sites of catalysts generally reduces activity, and therefore metal impurities, such as compounds Ia, Ca, and MD, should be removed as completely as possible.

The hydrolysis of triglycerides also produces diglycerides and monoglycerides, which are partially hydrolyzed products. Diglycerides and monoglycerides are surfactants that can form emulsions and make separation of the liquid phases of water and oil more difficult. Biological oils and fats may also contain other glyceride-like surfactants such as phospholipids (for example, lecithin), which contain phosphorus in their structures. Phospholipids are gummy materials that can be harmful to catalysts. Natural oils and fats also contain other types of components, such as waxes, stearins, tocopherols and carotenoids, some metals and organic sulfur compounds, as well as organic nitrogen compounds. These compounds may be harmful to the catalysts or create other problems in the process.

US patents 4992605 and 5705722 describe methods for producing diesel fuel additives by converting biological oils to saturated hydrocarbons under hydroprocessing with nickel-molybdenum (ымиο) and cobalt-molybdenum (CoMo) catalysts. The method is implemented at high temperatures of 350-450 ° C and produces n-paraffins and other hydrocarbons. The product has a high cetane number, but poor properties in cold conditions (melting point> 20 ° C), which limit the amount of product that can be mixed with conventional diesel fuels in the summer and prevent its use in the winter. The formation of heavy compounds with a boiling point of about 343 ° C was observed, especially when a fraction of fatty acids was used as a raw material. The reaction temperature with a lower limit of 350 ° C was adopted as a condition for uninterrupted operation.

A two-stage process is described in patent No. 100248 for the production of middle distillates from vegetable oil by hydrogenation of fatty acids or triglycerides derived from vegetable oil using commercial desulfurization catalysts such as nickel molybdenum (ΝίΜο) and cobalt-molybdenum (CoMo), with the formation of n- paraffins, followed by isomerization of the above n-paraffins with the use of metal-containing molecular sieves or zeolites to obtain branched paraffins. Hydrotreating was carried out at slightly higher reaction temperatures of 330-450 ° C, mainly 390 ° C. Hydrogenation of fatty acids at such high temperatures shortens the life of the catalyst due to coking and the formation of by-products.

Patent EP 1396531 describes a process comprising at least two steps, the first of which is a hydrodeoxygenation step, and the second step is a hydroisomerization step using the countercurrent principle and using biological raw material containing fatty acids and / or fatty acid esters as feedstock. The method includes, optionally, the stage of distillation of light fractions.

Deoxygenation of vegetable oils / fats and animal fats with hydrogen uses a large amount of hydrogen and at the same time produces a significant amount of heat. Heat is released in the reactions of deoxygenation and hydrogenation of double bonds. Different raw materials produce significantly different amounts of heat of reaction. The variation in the released heat of reaction mainly depends on the hydrogenation of double bonds. The average number of double bonds per triglyceride molecule can vary from about 1.5 to more than 5, depending on the source of biological oil or fat.

Patent No. 2607803 describes a method for hydrocracking vegetable oils or their derivatives

- 2 013998 fatty acids under pressure to form hydrocarbons and to some extent acid. The catalyst contains a metal dispersed on a support. The high reaction temperature of 370 ° C did not provide complete oxygen removal or high selectivity for the formation of n-paraffins. The resulting mixture of products also contained some intermediate derivatives of fatty acids.

The formation of water during hydrotreatment is caused by deoxygenation of triglyceride oxygen under the action of hydrogen (hydrodeoxygenation). Deoxygenation under hydrodeoxygenation conditions is to some extent accompanied by a decarboxylation reaction route and a decarbonylation reaction route. Deoxygenation of fatty acid derivatives by means of decarboxylation and / or decarbonylation reactions forms carbon oxides (CO ₂ and CO) and aliphatic hydrocarbon chains containing one less carbon atom than in the original fatty acid molecule. Decarboreaction here means decarboxylation and / or decarbonylation reactions.

The feasibility of decarboxylation varies greatly depending on the type of carboxylic acid or its derivative used as starting material. α-Hydroxy-, α-keto-and dicarboxylic acids are activated forms, and thus they are more easily deoxygenated during decarboration. Saturated aliphatic acids are not activated in this way, and in general they are difficult to deoxygenate by decarboreaction.

Decarboxylation of carboxylic acids to hydrocarbons in contact of carboxylic acids with heterogeneous catalysts was proposed by V.F. Meyer and colleagues (Malet, ^. P. s1 a1 .: Sysshschsys Vyspeys (1982), v. 115, No. 2, pp. 808-812). Meyer and co-workers tested Νί / Α1 ₂ Ο ₃ and Ρά / δίΟ ₂ catalysts for decarboxylation of several carboxylic acids. During the reaction, reactant vapors were passed through a catalyst bed with hydrogen. Hexane was the main product of decarboxylation of enanthic acid, taken as a test compound. When nitrogen was used instead of hydrogen, no decarboxylation was observed.

U.S. Patent 4,554,397 provides a process for the production of linear olefins from saturated fatty acids or esters, providing a catalyst system that consists of nickel and at least one metal selected from the group consisting of lead, tin and germanium. With other catalysts, such as S / C, low catalytic activity and cracking to saturated hydrocarbons, or the formation of ketones when Raney nickel was used, were observed.

Subject of invention

The subject of the invention is an improved method for the production of hydrocarbon fractions of diesel fuel from oils and fats of biological origin, with high selectivity, essentially without adverse reactions and with a high yield of diesel fraction.

A further subject of the invention is an improved method for the production of hydrocarbons of a diesel fraction from oils and fats of biological origin, in which the level of high molecular weight compounds formed in the hydrotreatment method is reduced and the stability of the catalyst is improved.

Another subject of the invention is an improved method for producing hydrocarbons of a diesel fraction from oils and fats of biological origin, in which hydrotreating a triglyceride feedstock containing free fatty acids is carried out using dilution of fresh feedstock and at a reduced reaction temperature.

Another subject of the invention is an improved method for the production of hydrocarbons of the diesel fraction from oils and fats of biological origin, this method produces a high-quality diesel component in high yield.

Distinctive features of the method according to the invention are presented in the claims.

Definitions

Here, hydroprocessing is understood as the catalytic treatment of organic material with molecular hydrogen in any way.

Here, hydrotreating is understood as a catalytic method that removes oxygen from organic oxygen-containing compounds in the form of water (hydrodeoxygenation, ΗΌΟ), sulfur from organic sulfur-containing compounds in the form of hydrogen sulfide (Η ₂ δ) (hydrodesulfurization, ΗΌδ), nitrogen from organic nitrogen-containing compounds in the form of ammonia (ΝΗ ₃ ) (hydrodenitrification, ΗΌΝ) and halogens, such as chlorine from organic chlorine compounds in the form of hydrogen chloride ЩС1) (hydrodechlorination, IOS1), usually under the action of sulphide илиο or sulphurous x CoMo catalysts.

Here, deoxygenation is understood in the sense of removing oxygen from organic molecules, such as derivatives of fatty acids, alcohols, ketones, aldehydes or esters, by any of the means previously described.

Here, the hydrodeoxygenation (ΗΌΟ) of triglycerides or other derivatives of fatty acids or fatty acids is understood to mean the removal of carboxylic oxygen in the form of water by the action of molecular hydrogen under the influence of a catalyst.

Here, the decarboxylation and / or decarbonylation of triglycerides or other derivatives of fatty acids or fatty acids is understood in the sense of removing carboxylic oxygen in the form

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CO ₂ (decarboxylation) or in the form of CO (decarbonylation) under the influence of molecular hydrogen or without it. The decarboxylation and / or decarbonylation reactions, together or separately, are referred to as decarboxylation.

Here, hydrocracking is understood as the catalytic decomposition of organic hydrocarbon materials using molecular hydrogen at high pressures.

Here, hydrogenation means the saturation of carbon-carbon double bonds with molecular hydrogen under the influence of a catalyst.

Here, n-paraffins mean normal alkanes or linear alkanes that do not contain side chains.

Here isoparaffins means alkanes having one or more C1-C9, typically _C1-C2 alkyl side chains, typically mono-, di-, tri- or tetrametilalkany.

Raw materials (raw materials in general) for a hydrotreating unit are here understood to include fresh raw materials and at least one diluent.

SUMMARY OF THE INVENTION

The present invention relates to an improved method for producing hydrocarbons from renewable sources such as vegetable oils / fats and animal oils / fats, comprising a hydrotreatment step and an isomerization step. In particular, the invention relates to the conversion of starting materials, including triglycerides, fatty acids and derivatives of fatty acids or combinations thereof, into n-paraffins with the reduced formation of high molecular weight hydrocarbons using dilution of fresh raw materials and a reduced reaction temperature in the hydrotreatment step, and conversion of the obtained -paraffins in branched alkanes of the diesel type using isomerization, with a high yield of diesel fraction. The hydrotreating step is carried out by contacting a feed comprising fresh feed and at least one diluent with a hydrotreating catalyst under hydrotreating conditions. Then, the resulting product is isomerized with an isomerization catalyst under isomerization conditions. The hydrocarbon oil formed in this way is a high quality diesel component.

DETAILED DESCRIPTION OF THE INVENTION

Surprisingly, it has been found that diluting fresh feed in the hydrotreating step, combined with lowering the reaction temperature, reduces undesired side reactions and improves reaction selectivity, especially when starting material containing free fatty acids is used. The diluent may be a hydrocarbon of biological origin and / or non-biological origin.

The diluent may also be a recycle product from the process (return product). The diluent / fresh feed ratio is 5-30: 1, preferably 10-30: 1, and most preferably 12-25: 1.

A preferred embodiment of the invention and a hydrotreating step are illustrated in FIG. 1, which shows the layout of a hydrotreating process comprising one or more successive catalyst beds, introducing a hydrotreated recycle product into the top of the first catalyst bed and introducing fresh feed, coolant and hydrogen into the top of each catalyst bed. This is manifested in improved control of the reaction temperature in the catalyst beds and thereby in the reduction of undesired side reactions.

In FIG. 1, a hydrotreating reactor 100 includes two catalyst beds 10 and 20. Fresh feed 11 is introduced as streams 12 and 13 into catalyst beds 10 and 20, respectively, and hydrogen in streams 22 and 23 into catalyst beds 10 and 20, respectively. The fresh feed stream 12 is first mixed with the hydrotreated recycle product stream 41 and the coolant stream 43, and the resulting mixture 31 diluted in the fresh feed concentration is then introduced onto the catalyst bed 10. In order to obtain the desired sulfur concentration in the feed stream 31, the desired a compensating amount of sulfur is added to the fresh feed stream 11 through stream 15. As the mixture 31 passes through the catalyst bed 10 with a stream of hydrogen 22, fatty acids and derivatives of fatty acids from the fresh stream raw materials 12 are converted into the corresponding reaction products. A two-phase stream 32 is discharged from the bottom of the catalyst layer 10 and mixed with a stream of fresh raw materials 13, a stream of cooling liquid 44 and a stream of hydrogen 23. The formed vapor-liquid mixture 33 diluted in the concentration of fresh raw materials is then introduced onto the catalyst layer 20 at a reduced temperature due to the cooling effect of hydrogen of coolant and fresh feed is passed through the catalyst bed 20 and finally removed from the catalyst bed as product stream 34. Stream 34 is separated into steam stream 35 and liquid stream 36 into ysokotemperaturnom separator 101. Vapor stream 35 is rich in hydrogen and is directed to further processing. A portion of the liquid stream 36 is returned to the reactor 100 as a recycle stream 40, which is further divided into a dilution stream 41 and a total coolant stream 42. The coolant stream 42 is cooled in the heat exchanger 102 to ensure proper cooling effect in the upper parts of the catalyst layers 10 and 20. The stream The hydrotreated product 51 is sent from the hydrotreatment step to further processing.

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The catalyst layers 10 and 20 can be placed in the same high-pressure chamber or in separate high-pressure chambers. In an embodiment in which the catalyst beds are in the same high pressure chamber, hydrogen flows 22 and 23 can alternatively be introduced into the catalyst bed 10 and then passed through the catalyst beds 10 and 20. In an embodiment in which the catalyst beds are located in different high-pressure chambers, the catalyst layers can operate in parallel with separate diluent streams, hydrogen streams and coolant streams. The number of catalyst layers may be one or two or more than two.

Sulfur replenishment in the hydrotreating step may be introduced with a fresh feed stream 11. Alternatively, the required amount of sulfur may be supplied with hydrogen streams 22 and 23 as a gaseous sulfur component, such as hydrogen sulfide.

Hydrogen is supplied to the hydrotreating reactor in excess of the theoretical consumption of hydrogen. In the method of the hydrotreatment step, triglyceride oils, fatty acids and their derivatives are converted, with a theoretical yield, to n-paraffins without or almost without adverse reactions. Additionally, propane is formed from the glycerol portion of triglycerides, water and CO and / or CO ₂ from carboxylic oxygen, H ₂ 8 from organic sulfur compounds and ΝΗ ₃ from organic nitrogen compounds. Using the above technologies at the hydrotreating stage, the temperature required to start the reactions is reached at the beginning of each catalyst layer, the temperature increase in the catalyst layers is limited, the formation of harmful and partially reacted intermediates can be avoided, and the service life of the catalyst is significantly increased. The temperature at the end of the catalyst bed is controlled by its own heat of reaction and, to some extent, by the use of a diluent. The diluent may be any available hydrocarbon of biological origin or non-biological origin. It may also be a recycle product from the method. The content of fresh raw materials from raw materials (raw materials in general) is less than 20 wt.%. If a recycle product is used, the ratio of recycle product to fresh raw materials is 5-30: 1, preferably 10-30: 1, most preferably 12-25: 1. After the hydrotreatment step, the product undergoes an isomerization step.

Feedstock

The biological oil and / or fat used as fresh raw materials in the method according to the present invention, come from renewable sources, such as fats and oils from plants and / or animals, and / or fish and compounds derived from them. The basic structural unit of a typical vegetable or animal oil / fat usable as a feedstock is triglyceride, which is a complex triester of glycerol and three fatty acid molecules having the structure shown in the following formula I

ABOUT

Formula 1. The structure of triglyceride

In formula I, the radicals Κι, K ₂ and K ₃ represent alkyl chains. The fatty acids found in natural triglycerides are almost exclusively fatty acids with an even number of carbon atoms. Therefore, the Κι, Κ ₂ and Κ ₃ groups typically represent C ₅ -C ₂₃ alkyl groups, for the most part Cc-C19 alkyl groups, and most typically C15 or C ₁₇ alkyl groups. Radicals Κ1, K ₂ and K ₃ may contain carbon-carbon double bonds. These alkyl chains may be saturated, unsaturated or polyunsaturated.

Suitable biological oils are vegetable oils and fats, animal fats, fish oils and mixtures thereof, containing fatty acids and / or fatty acid esters. Examples of suitable materials are wood-based fats and oils, and other plant-based fats such as rapeseed oil, rape oil, canola oil, tall oil, sunflower oil, soybean oil, hemp oil, olive oil, linseed oil, mustard oil, palm oil , peanut butter, castor oil, coconut oil, as well as fats found in genetically modified plants, animal fats such as pork, lard, blubber, and fats found in milk, as well as regenerated fats the food industry and mixtures of the foregoing thereof.

Biological oils or fats suitable as fresh raw materials may contain C ₁₂ -C ₂₄ fatty acids, derivatives thereof, such as anhydrides or esters of fatty acids, as well as triglycerides of fatty acids, or combinations thereof. Fatty acids or derivatives of fatty acids, such as esters, can be obtained by hydrolysis of biological oils or their fractionation

- 5 013998 or triglyceride transesterification reactions.

In order to avoid catalyst deactivation and unwanted side reactions, the feed must meet the following requirements: the amount of alkali and alkaline earth metals calculated on elemental alkaline and alkaline earth metals in the feed is less than 10, mainly less than 5 and most preferably less than 1 weight. ppm ^-1 . The amount of other metals calculated on elemental metals in the feed is less than 10, mainly less than 5 and most preferably less than 1 weight. ppm ^-1 . The amount of phosphorus per elemental phosphorus is less than 30, preferably less than 15 and most preferably less than 5 weight. ppm ^-1 .

In many cases, the feedstock, such as untreated vegetable oil or animal fat, is not suitable as such for processing due to the high content of impurities, and therefore, the feedstock is predominantly purified using one or more suitable generally accepted purification technologies before entering it into the hydrotreating stage of the process. Examples of a number of commonly accepted technologies are given below.

Dehumming vegetable oils / fats and animal oils / fats means removing phosphorus-containing compounds such as phospholipids. The solvent-extracted vegetable oils often contain significant amounts of gums, typically 0.5-3 wt.%, Which are for the most part phosphatides (phospholipids), and therefore a degumming step is required for crude vegetable oils and animal fats to remove phospholipids and metals present in crude oils and fats. Iron, as well as other metals, may be present in the form of metal phosphatide complexes. Even trace amounts of iron can cause catalytic oxidation of oil or fat.

Degumming is carried out by washing the raw materials at a temperature of 90-105 ° С and a pressure of 300-500 kPa (absolute) Н ₃ РО ₄ , ΝαΟΗ and softened water and separating the formed gums. Most of the metal components that are harmful to the hydrotreating catalyst are also removed from the feedstock during the degumming step. The moisture content in the degummed oil is reduced in the dryer at a temperature of 90-105 ° C and a pressure of 5-50 kPa (absolute).

The feedstock, which is optionally degummed or refined in some other well-known way, can be bleached. During bleaching, the degummed or refined feedstock is heated and mixed with natural or acid-activated bleaching clay. Bleaching removes a variety of trace impurities remaining after other stages of pre-treatment such as dehumming, such as chlorophyll, carotenoids, phospholipids, metals, soaps and oxidation products. Whitening is typically carried out in vacuo to minimize possible oxidation. In general, the goal of whitening is to reduce the amount of color pigments in order to produce an oil of an acceptable color and reduce the tendency of the oil to oxidize.

Optionally, the triglyceride structures of the feedstock can be cleaved by pre-hydrogenating the double bonds at a reduced reaction temperature on ΝίΜο or another catalyst before hydrodeoxygenation to prevent polymerization of unsaturated triglycerides by double bonds.

The method according to the invention is particularly advantageous when the fresh feed contains more than 5% free fatty acids and even more than 10% free fatty acids. Thus, naturally occurring fats and oils containing significant amounts of free fatty acids can also be processed without removing free fatty acids.

Below, the method according to the invention, comprising a hydrotreating step and an isomerization step, is described in more detail.

Hydrotreating biological oils and fats

In the first stage of the method, that is, in the hydrotreating stage, fatty acids, triglycerides and other derivatives of fatty acids contained in the feed are deoxygenated, denitrified and desulfurized.

Raw materials include fresh raw materials and at least one diluent, and the ratio of diluent to fresh raw materials is 5-30: 1, preferably 10-30: 1, most preferably 12-25: 1. The diluent is selected from hydrocarbons and a recycle process product, i.e., a return product, or mixtures thereof.

At the hydrotreating stage, the pressure range can vary between 20 and 150 bar, mainly between 50 and 100 bar and temperatures between 200 and 400 ° C, mainly between 250 and 350 ° C and most preferably between 280 and 340 ° C.

It has been found that the selectivity of decarboreaction and deoxygenation during decarboration can be promoted by hydrotreating over a hydrotreating catalyst using a sulfur content of 50-20000 weight. mn ^-1 (te-rrsh), preferably 1000-8000 w. mn ^-1, most preferably 2000-5000 w. ^-1 million of sulfur in the feedstock as a whole, based on elemental sulfur. In particular the sulfur content in the feed can double the yield of n-paraffins formed by removal of SO _X. The complete deoxygenation of triglycerides using decarboreaction can theoretically reduce hydrogen consumption by about 60% (maximum) compared to pure deoxygenation with

- 6 013998 hydrogen.

At least one organic or inorganic sulfur compound may optionally be introduced together with hydrogen or with the raw material to achieve the desired sulfur content. The inorganic sulfur compound may be, for example, H ₂ 8 or elemental sulfur, or the sulfur compound may be a readily decomposable organic sulfur compound such as dimethyldisulfide, carbon disulfide and butylthiol, or a mixture of readily decomposable organic sulfur compounds. It is also possible to use refinery gas or liquid streams containing decomposable sulfur compounds.

In the hydrotreating / hydrodeoxygenation step, known hydrogenation catalysts containing metals from Groups VIII and / or νίΒ of the Periodic System can be used. Advantageously, the hydrogenation catalysts are catalysts containing Pb, Ρΐ, ΝίΜ, ΝίΜο or CoMo on supports, where the supports may be alumina and / or silica, as described, for example, in patent No. 100248. Typically, ΝίΜο / Ά1 _{2 are used.} Θ ₃ and CoMo / A1 ₂ O ₃ catalysts.

In order to control the temperature increase due to the above reactions over the catalyst beds and the development of a side reaction, an improved reactor design is shown in FIG. 1. The hydrotreating unit includes one or more successive catalyst beds, introducing a diluent into the top of the first catalyst bed, and introducing fresh feed, recycle liquid, and hydrogen into the top of each of the catalyst beds. If the diluent is a return product, then the ratio of return product to fresh oil is 5-30: 1, preferably 10-30: 1, and most preferably 12-25: 1. The catalyst beds may be located in the same high-pressure chamber or each layer in a separate high-pressure chamber. Hydrogen is supplied in excess relative to the theoretical chemical flow rate of hydrogen, and the feedstock is completely or almost completely converted within each catalyst bed. Using these technologies, the formation of harmful and partially reacted intermediates can be avoided, the temperature necessary to initiate the reaction is reached at the beginning of each catalyst layer, the increase in the reaction heat generation is controlled in the catalyst layers, and the catalyst service life is significantly increased.

The hydrodeoxygenation of triglycerides facilitates the controlled decomposition of a triglyceride molecule in spite of uncontrolled cracking. Double bonds are also hydrogenated in a controlled hydrotreatment process. The resulting light hydrocarbons and gases, mainly propane, water, CO ₂ , CO, H ₂ § and ΝΗ ₃ , are removed from the hydrotreated product.

From the examples, it was unexpectedly observed that dilution with a recycle product can prevent or significantly reduce reactions between free fatty acids and the formation of high molecular weight compounds during hydrotreatment when a ratio of (recycle product) :( fresh oil) of at least 5: 1 is applied . The effect of the return product is based on two phenomena: the effect of dilution of the recirculation and more controlled and lower reaction temperatures maintained above the catalyst bed in the hydrodeoxygenation process. Higher temperatures and especially hot zones in the catalyst bed contribute to ketonization reactions. Thanks to this invention, it is possible to use a variety of sources of biological oils and fats without the need to remove fatty acids. After the hydrotreating step, the product is processed in the isomerization step.

Isomerization of n-paraffins formed in the hydrotreatment process

In the second stage of the method, that is, at the stage of isomerization, isomerization is carried out, which causes the branching of the hydrocarbon chain, resulting in improved performance of the obtained oil at low temperatures. Isomerization creates predominantly methyl branches. The stringency of the isomerization conditions and the choice of catalyst determine the number of methyl branches formed and the distance between them, and thereby the cold characteristics of the produced biodiesel fuel fraction. The product obtained from the hydrotreatment step isomerized under isomerization conditions with an isomerization catalyst.

In the method according to the invention, the feed to the isomerization reactor is a mixture of pure n-paraffins, and its composition can be predicted by the distribution of fatty acids in individual biological oils. During the hydrotreatment step of the process, triglyceride oils and other derivatives of fatty acids and fatty acids are converted to naraffins almost theoretically. Additionally, propane is formed from the glycerol portion of triglycerides, water and CO _X from carboxylic oxygen, H ₂ § from organic sulfur compounds and ΝΗ ₃ from organic nitrogen compounds. It is important for the process that these gas phase impurities are removed as completely as possible before the hydrocarbons come into contact with the isomerization catalyst.

The isomerization step may include, optionally, a step for evaporating the light fractions, in which the reaction product from the hydrotreating step can be purified by evaporation with water vapor or a suitable gas, such as light hydrocarbon, nitrogen or hydrogen. Optionally, the evaporation step is carried out in counterflow mode in a unit located upstream of the isomerization catalyst, in which gas and liquid are in contact with each other, or in front of a particular isomerization reactor in a separate evaporation unit, using the counterflow principle.

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At the isomerization stage, the pressure varies in the range of 20-150 bar, mainly in the range of 30-100 bar, and the temperature varies between 200 and 500 ° C, mainly between 280 and 400 ° C.

In the isomerization step, isomerization catalysts known in the art can be used. Suitable isomerization catalysts comprise molecular sieves and / or a metal selected from group VIII of the Periodic Table and / or a carrier. Advantageously, the isomerization catalyst contains 8ΆΡΘ-11 or 8ΆΡΘ-41, or Ζ8Μ-22 or Ζ8Μ-23, or ferrierite and Ρΐ, Ρ6 or N1 and A1 ₂ O ₃ or 81O ₂ . Typical isomerization catalysts are, for example, P1 / 8APO-11 / A1 ₂ O ₃ , Ρ1 / Ζ8Μ22 / A1 ₂ O ₃ , Ρί / Ζ8Μ-23 / Α1 ₂ Θ ₃ and Ρί / 8ΑΡΟ-11 / 8ίΟ ₂ . Many of these catalysts require the presence of hydrogen to reduce catalyst deactivation.

An isomerized product is obtained, which is a mixture of branched hydrocarbons and mainly branched paraffins boiling in the range of 180-350 ° C, i.e. in the diesel range, and having one carbon atom less than in the chain of the original fatty acid. Additionally, some gasolines and gasoline can be obtained.

Advantages of the Invention

The invention provides a method for reducing the formation of higher molecular weight compounds during hydrotreating feedstocks derived from vegetable oils and animal fats, and which may contain free fatty acids.

It has been unexpectedly discovered that the problems of the prior art process can be eliminated or at least significantly reduced by the method according to the invention, comprising a hydrotreating step and an isomerization step in which a return product or other diluent at the hydrotreating step combined with a reduced operating temperature leads to important improvements, especially when fresh raw materials contain more than 5 wt.% free fatty acids. The special design of the reactor and the strong dilution of fresh raw materials introduced into hydrotreating are used in the method. The level of adverse reactions decreases, and the stability of the catalyst in the hydrotreatment process increases during the hydrotreatment step.

In the examples, it can be seen that a ratio of at least 5: 1 (recycle: fresh) significantly reduces the formation of high molecular weight products when using feedstocks containing 10 wt.% Free fatty acids (based on fresh oil). With a recycle ratio of at least 5: 1 and a reduced reaction temperature, free fatty acids can be processed without the need for deoxidation. High-quality hydrocarbons were obtained, suitable for the entire range of diesel fuel with a high yield.

The invention is illustrated below by examples representing some preferred embodiments of the invention. However, it is obvious to all technologists that this does not mean limiting the scope of the invention to these examples.

Examples

All hydrotreating test experiments were performed in the presence of hydrogen.

Example 1. Comparative example.

Tall oil raw material (100% free fatty acids) with no return product.

Hydrotreating of tall oil (100% free fatty acids) with a катализο catalyst was carried out under a pressure of 50 bar, at a liquid hourly space velocity (ΠΗδν) of 1.5 and reaction temperatures of 340-360 ° С without a return product. The ratio of hydrogen to oil was 900 liters of H2 reduced to normal conditions per liter of oil supplied. Hydrotreating tall oil feeds with 100% free fatty acids caused a rapid deactivation of the catalyst, and the formation of high molecular weight compounds and aromatic compounds was observed. Bromine indices increased in the experimental method, even if temperature compensation of the catalyst was used (FIG. 8). The resulting oil contained about 7 wt.% Aromatic compounds and about 7 wt.% Heavy products (with a boiling point above 375 ° C). The density of the obtained oil (at 50 ° C) was high 777.1 kg / m ³ compared with typical values for the oil obtained by hydrotreating rapeseed oil (761-762 kg / m ³ ) using a lower reaction temperature and optimized reaction conditions.

Example 2. Comparative example.

Tall oil fatty acid feed (100% free fatty acids) at high reaction temperatures without return product.

Hydrotreating tall oil fatty acid feedstock (100% free fatty acids) at high reaction temperatures of 370-385 ° C was carried out without a return product. Rapid deactivation of the catalyst and the formation of compounds with a high molecular weight and aromatic compounds were observed. The density of the hydrotreated oil (Table 1) was significantly higher than in rapeseed oil experiments (typically 761-762 kg / m ³ ). Both oils contained mainly C18 fatty acids (~ 90 wt.%), And completely stationary formation of water was observed during the experiment. In the tall oil hydrotreatment process, approximately 7-8% by weight of higher molecular weight compounds and 8.1% by weight of aromatic compounds are formed. These adverse reactions were due to the supply of concentrated fatty acid and too high reaction rates.

- 8 013998 peraturats. Catalyst deactivation was clearly noticeable by the growth of bromine indices. In a satisfactory method of operation, the bromine index should be less than 50. Tab. 1 describes densities, bromine indices, reaction temperatures and water formed during test experiments for 2 to 14 days using tall oil fatty acid raw materials (100% free fatty acids) without product recycling.

Table 1

Test experiment duration 2 day 4 day 6 day 9 day 11 day 12 day 13 day 14 day Temperature ° C 370 375 378 381 385 385 385 385 Density, 50 ° C, kg / m ³ 771.8 773.1 773.7 776.5 779.1 779.8 780.5 781.2 Bromine Index 101 150 188 198 247 269 300 330 Formed water,% 9.37 9.5 9.81 10.3 10,2 10.0 10.1 10,2

Example 3. (Comparative example).

The effect of metallic impurities in biological oils on the effectiveness of the catalyst.

Tubing reactor hydrotreating test experiments were conducted using untreated rapeseed oil, untreated animal fat, and refined rapeseed oil. The analysis data of these raw materials are shown in table. 2. Raw raw materials contained significant amounts of metals, organic compounds of phosphorus, sulfur and nitrogen. The purified raw materials contained only trace amounts of these impurities.

table 2

Impurity levels in untreated and refined vegetable oils and animal fats

Raw animal fat

Impurity

Unit rapeseed oil

Refined rapeseed oil

Metals (overall) ppm (mn ^'1) 90 ~ 0 162 Organic nitrogen compounds ppm (mn ^'1) 33 7.2 1125 Free fatty acid gel permeation chromatography prcssnti no weight 0.8 0.7 10.8 Total acid number mg KOH / g 1,0 0.1 21.5 Phosphorus ppm (mn ^-1) 110 <1 86 Sulfur (originally) ppm (mn ^'1) 3 one 85

Test experiments using crude, crude oils / fats showed that the catalyst requires higher temperatures for proper efficiency, but gradually loses its activity (Fig. 5). Triglycerides and an increased bromine number of the resulting oil were detected. High amounts of metals were also detected on the catalyst. The temperature profile of the catalyst layer showed that the upper part of the catalyst layer was deactivated and the reaction zone moved forward (Fig. 4), while the heating of the reactor was kept constant. Metals adsorbed on the catalyst also contributed to side reactions such as decarboration.

The first hydrotreating test experiment was conducted using untreated rapeseed oil. Refined rapeseed oil was used as a control feed. Refined rapeseed oil achieved a complete hydrodeoxygenation conversion (ΗΌΟ) at 305 ° C with an average hourly feed rate (^ Н8У) of 2. Untreated rapeseed oil reached a total ΗΌΟ conversion only when the reaction temperature reached 330 ° С at a space velocity of ^ Н8У = 1 . However, it was evident from the temperature profile along the catalyst layer that the first part of the catalyst was deactivated very quickly. In FIG. 4 shows the reaction temperature profile over the catalyst bed and the conversion efficiency of untreated rapeseed oil.

A second hydrotreating test experiment was conducted using refined rapeseed oil and untreated animal fat. Refined rapeseed oil was used as a control feed. Refined rapeseed oil with a return product achieved a complete hydrodeoxygenation conversion (ΗΌΟ) at 305 ° C with an average hourly feed rate (^ Н8У) of 1. Untreated animal fat with a return product did not give a complete conversion

- 9 013998 versions at 305 ° С with ^ Н8У = 1. Analysis by gel permeation chromatography (CPC) shows that the oil obtained contained triglycerides, and the catalyst was also substantially deactivated by feeding untreated animal fat. When using untreated animal fat as a raw material, problems with pumping were also observed. The conversion efficiency of untreated animal fat is shown in FIG. 5.

Example 4. (Comparative example).

The effect of free fatty acids (10 wt.% In fresh raw materials) on the formation of high molecular weight hydrocarbons.

Hydro-purification was carried out using purified rapeseed oil as a control feed without a return product. The test experiment was carried out at 305 ° C and a pressure of 50 bar at an hourly average feed rate (\ UN8U). equal to 1, and the ratio H ₂ / oil = 1000. The sulfur content in the feed was 570 million ^-1. During the second test hydrotreatment period, stearic acid was supplied (10% by weight of rapeseed oil) together with purified rapeseed oil under the same reaction conditions without a recycle product. It was immediately found that the number of compounds with high molecular weight gradually increased from the initial level of ~ 3 to ~ 8 wt.%. These compounds with a high molecular weight (molecular weight twice or more than in the feed) do not fall within the boiling range of diesel fuel and thereby reduce the yield of diesel fraction and potentially shorten the life of the catalyst. Thus, free fatty acids in biological oils make their processing more difficult. In FIG. 2, one can see an increase in the formation of high molecular weight hydrocarbons when 10% by weight of free fatty acids were introduced along with the triglycerides of purified rapeseed oil without a return product.

Example 5. The effect of the return product on preventing the formation of undesirable heavy products of adverse reactions, when the feed contains 10 wt.% Free fatty acids.

A hydrotreating test experiment was carried out using purified rapeseed oil containing 10 wt.% Stearic acid as a control raw material without return product under the following reaction conditions: hourly average feed rate ^ Н8У = 1, pressure 50 bar, temperature 305 ° С, H ₂ / oil = 1000 and sulfur content of the raw material = 570 million ^-1. During the second test hydrotreatment period, the same feed was diluted with the hydrocarbons obtained as a product so that the ratio (fresh oil) / (return product) was 1: 5. The hourly average feed rate (\ UN8U) of fresh oil was maintained at 1, so that \ UN8U of the entire supplied oil was increased to 6. The reaction temperature was maintained at 305 ° C and the reaction pressure was 50 bar. The ratio of (H ₂ ) / (fresh oil) was maintained at 1000. The hydrodeoxygenation product (ΗΌΟ) (n-paraffins) simulated a recycle product that had been mixed with the fresh oil in advance. The initial heavy hydrocarbon content in the return product was ~ 0.4 wt.%.

It was unexpectedly found that the formation of heavy hydrocarbons was almost completely prevented or at least very significantly reduced when a return product was used (FIG. 3). In all likelihood, this is due to a significant suppression of adverse reactions of free fatty acids, in which a carboxylic acid molecule can react with another carboxylic acid molecule to form compounds with a higher molecular weight. In FIG. Figure 3 shows the effect of the return product on preventing the formation of unwanted higher molecular weight by-products. Tab. 3 presents the results of analyzes of raw materials and products.

- 10 013998

Table 3

Raw material and product analysis results

Raw material analysis Product Analysis Property Method Units LK Recirculation Product LK (10% 8A) AK (10% 8A) + CES 1: 5 AK + 10% by weight of stearic acid without a return product AK + 10% by weight 8A + return product after 196 hours AK + 10% by weight 8A + return product after 552 hours Density, 15 ° C, calculated Standard ϋ4052 kg / m ³ 920.4 788.1 915.8 807.2 790.8 788.3 788.3 Density, 50 ° C Standard O 4053 kg / m ³ 897.6 761.4 893.2 781.2 - 764.2 761.7 761.7 Bromine Index Standard 0 2710 mg / 100 g 53.7 21.5 26 Bromine number Standard ϋ 1159 g / 100 g 56 49.1 6.3 Iodine number Standard Ώ5554 g / 100 g 112 103 eighteen NA OPC area fraction (%) of chromatographic peak 99.6 83.0 94.3 99.6 99.6 Fatty acid CPC area fraction (%) of chromatographic peak 0.7 0 10.6 1.8 0 0 0 Neauuns OPC area fraction (%) of chromatographic peak 0 0.4 0.5 5.7 0.4 0.4 Diglycerides OPC area fraction (%) of chromatographic peak 2,3 0 2,4 0 0 0 Triglycerides CPC area fraction (%) of chromatographic peak 97 0 87 14.7 0 0 0

8L = stearic acid, LC = refined rapeseed oil, CEC = return product, HC = hydrocarbons, Leacu HC = high molecular weight hydrocarbons, CPC - gel permeation chromatography.

Example 6. (Comparative example).

The effect of reduced reaction temperature on the selectivity of n-paraffin formation and oil yield.

The studies were carried out with a catalyst using rapeseed oil as a raw material and reaction temperatures of 280-330 and 340-360 ° C at an average hourly feed rate of ^ Н8У = 1 and a reactor pressure of 50 bar. Rapeseed oil triglycerides after alkaline refining mainly contained C ₁₈ fatty acids. With _18, fatty acids comprised about 89% by weight of all fatty acids in rapeseed oil. The theoretical amount of n-paraffins formed from raw rapeseed oil is about 86.4% by weight (based on the rapeseed oil supplied).

The complete conversion of hydrodeoxygenation (ΗΌΟ) with an almost theoretical yield of n-paraffins was achieved when good control of reaction temperatures below 330 ° C was ensured. The almost theoretical yields of n-paraffins indicate complete ΗΌΟ conversion and highly controllable operation without significant adverse reactions. A large number of adverse reactions (cyclization, aromatization, and cracking) and a low yield of n-paraffins were observed when high reaction temperatures of 340-360 ° C were used unnecessarily. In FIG. Figure 6 shows the conversion of rapeseed oil triglycerides to n-paraffins.

Example 7. The stability of the catalyst.

The stability катализатораο of the catalyst using palm oil as a model raw material (with added impurities) along with the return product (test of the catalyst service life) was studied under the following reaction conditions: reaction temperature = 300-305 ° С, reactor pressure = 40 bar, hourly average feed rate of raw materials (fresh) ^ Н8У = 0.5, ^ Н8У (total) = 3, the ratio of Н ₂ / oil (fresh) = 900, sulfur in raw materials = 100 weight. ppm ^-1 . Palm oil was used as the main component of the feed, but it was modified with animal fat, free fatty acid fractions, untreated rapeseed oil and lecithin to provide useful details about the impurities in the test feed. Analysis of fresh raw materials is presented in table. 4. In addition, fresh oil was pre-diluted with a hydrodeoxygenation product (ΗΌΟ) (simulating the return product) in a 1: 5 ratio. The duration of the test experiment was over 9 months. The constancy of the operation conditions was maintained (Table 4 and FIG. 7), and the formation of heavy products was uniform throughout the entire test experiment, FIG. 7.

- 11 013998

Table 4

Catalyst stability

Experiment duration Fresh Raw Material Analysis Oil Analysis 383 hours 1898 hours 3408 hours 5601 hour Analysis Method Unit Density, 15 ° С Standard B 4052 kg / m ³ 804.9 787.4 785.6 785.3 784.9 'Density, 50 ° C Standard B 4052 kg / m ³ 778.8 760.7 758.9 758.6 758.1 Bromine Index Standard B 2710 mg / 100 g 29200 33 48 33 eleven Hydrocarbons CPC area fraction (%) of chromatographic peak 0 99.3 99,4 99.3 99,4 Fatty acid CPC area fraction (%) of chromatographic peak 1,2 0 0 0 0 Monoglycerides / hydrocarbons with CPC area share (%) 0.3 0.7 0.6 0.7 0.6 high molecular weight chromatographic peak Diglycerides CPC area fraction (%) of chromatographic peak 6.3 0 0 0 0 Triglycerides CPC area fraction (%) of chromatographic peak 92.1 0 0 0 0 Total acid number Standard B 664 mg KOH / g 2.1 ~ 0 ~ 0 ~ 0 -about Sulfur Standard B 5453 million ' ¹ (ppm) 3 1,2 2.0 2.7 2 Nitrogen Standard B 4629 mg / kg 6 <1 <1 1,2 <1 Sodium, oil ΑΆ3 mg / kg 3 0.4 <0.1 <0.1 <0.1 Calcium Oil ΆΑ3 mg / kg 2 0.3 <0.1 <0.1 <0.1 Magnesium oil AAZ mg / kg 0.3 <0.1 <0.1 <0.1 <0.1 Molybdenum oil ΑΆ3 mg / kg - <0.5 <0.5 <0.5 <0.5 Aluminum oil 1СР of metals mg / kg <2 <2 <2 <2 <2 Iron, oil 1СР of metals mg / kg <1 <1 <1 <1 <1 Nickel oil 1СР of metals mg / kg <1 <1 <1 <1 <1 Phosphorus oil 1СР of metals mg / kg 4 <1 <1 <1 <1

СРС - gel permeation chromatography, ΆΆ8 - atomic absorption spectrophotometry, 1СР metals - spectrometry of metals with inductively coupled plasma.

Claims (17)

CLAIM 1. Method for the production of hydrocarbons fraction of diesel fuel, in which the raw materials undergo hydrotreating at the stage of hydrotreating and isomerization at the isomerization stage, characterized in that the raw materials, including fresh raw materials, containing more than 5% by weight of free fatty acids, and at least one diluent is hydrotreated at a reaction temperature of 280-340 ° C in a hydrotreating reactor in the presence of a catalyst, and the ratio of diluent to fresh raw materials is 530: 1, and the fresh raw materials are of biological origin, selected and vegetable oils / fats, animal fats / oils, fish oils / oils, fats contained in plants derived from gene modification, regenerated fats from the food industry, and mixtures thereof. 2. The method according to claim 1, characterized in that the raw material contains more than 10 wt.% Free fatty acids. 3. The method according to claim 1 or 2, characterized in that the diluent is selected from hydrocarbons and recycled products of the method or mixtures thereof. 4. The method according to any one of claims 1 to 3, characterized in that the raw material contains less than 10 weight. ^-1 million alkali metal and alkaline earth metal calculated as elemental alkaline and alkaline earth metals, less than 10 wt. ppm ^{-1 of} other metals based on elemental metals and less than 30 weight. mn ^-1 phosphorus calculated as elemental phosphorus. 5. The method according to any one of claims 1 to 4, characterized in that the ratio of diluent to fresh raw materials is 10-30: 1, preferably 12-25: 1. 6. The method according to any one of claims 1 to 5, characterized in that the raw material contains 50-20000 weight. ^-1 million sulfur calculated as elemental sulfur. 7. The method according to any one of claims 1 to 6, characterized in that the fresh raw materials are selected from rapeseed oil, rapeseed oil, canola oil, tall oil, sunflower oil, soybean oil, hemp oil, olive oil, linseed oil, mustard oil, palm oil, peanut oil, castor oil, coconut oil, lard, lard, blubber or fats contained in milk. 8. The method according to any one of claims 1 to 7, characterized in that the mixture of raw materials of biological origin and hydrocarbon / hydrocarbons is used as a fresh raw material. - 12 013998 9. The method according to any one of claims 1 to 8, characterized in that the hydrotreating catalyst layer system comprises one or more catalyst layers. 10. The method according to any one of claims 1 to 9, characterized in that in the hydrotreating step the pressure varies in the range of 20-150 bar, preferably in the range of 30-100 bar. 11. The method according to any one of claims 1 to 10, characterized in that at the isomerization stage the pressure varies in the range of 2-15 MPa, preferably in the range of 3-10 MPa, the temperature varies between 200 and 500 ° C, preferably between 280 and 400 ° s 12. The method according to any one of claims 1 to 11, characterized in that the hydrotreating is carried out in the presence of a hydrogenation catalyst containing a metal from group VIII and / or SW of the Periodic system. 13. The method according to p. 12, wherein the hydrogenation catalyst is a catalyst of Ρά, Ρΐ, N1, ο or CoMo on the carrier, and the carrier is alumina and / or silicon oxide. 14. The method according to any one of claims 1 to 13, characterized in that the isomerization catalyst containing molecular sieves is used in the isomerization step. 15. The method according to p. 14, wherein the isomerization catalyst comprises a metal from elements of group VIII. 16. The method according to p. 14 or 15, characterized in that the isomerization catalyst contains A1 ₂ O ₃ or 8ΪΘ2. 17. The method according to any of paragraphs.14-16, characterized in that the isomerization catalyst contains 8APO-11 or 8APO-41, or Ζ8Μ-22 or 8Μ-23, or ferrierite and Ρΐ, or Ρά or N and A1 ₂ O ₃ or 8ΐΘ ₂ .