Method for decomposition of an oil shale kerogen with nitric acid into di- and TRI-carboxylic acids

Abstract

The present invention provides a method for decomposition of an oil shale kerogen into di- and tri-carboxylic acids, which is characterised in that the method is carried out in a flow reactor, into which the oil shale kerogen and a nitric acid are introduced, which forms a reaction mixture and which is heated in the flow reactor first at a temperature of 120-135 C and then at 140-165 C, resulting in a product mixture containing di- and tri-carboxylic acids and gases, which are discharged from the flow reactor.

Description

[0001] METHOD FOR. DECOMPOSITION OF AN OIL SHALE KEROGEN WITH NITRIC ACID INTO DI- AND TRI-CARBOXYLIC ACIDS

[0002] TECHNICAL FIELD OF INVENTION

[0003] The invention belongs to the field of chemical industry and more specifically, the invention provides a method for the decomposition of oil shale kerogen into di- and tri-carboxylic acids.

[0004] PRIOR ART

[0005] Oil shale is a valuable mineral resource that must be valorised to the maximum possible extent to produce valuable products. Oil shale is a sedimentary rock, which consists of the organic component - mainly kerogen and mineral part (depending on the origin, for example carbonate, silicate part, clay, pyrite, and / or feldspar). Oil shale kerogen is the organic part that is not soluble in ordinary organic solvents. The structural properties of kerogen in oil shale are related to the nature of the ancient bioorganic material (marine, lacustrine, or terrestrial) and the genesis of the material, for example the temperatures and pressures to which the organic components have been exposed.

[0006] Kerogen is a complex macromolecular organic matter consisting of several cross-linked aliphatic chains, aromatic rings and various heteroatoms such as oxygen, nitrogen and sulfur and their rings. The structure of kerogen varies to some extent but contains both saturated (aliphatic) and unsaturated (aromatic resorcinol) components, the ratio of which depends on the biological origin and thermal maturity.

[0007] In order to obtain valuable products from oil shale kerogen, it is necessary to oxidatively decompose it. During the oxidation of kerogen, most of its insoluble structure decomposes into compounds that are soluble in water or other solvents. Various oxidants can be used for this, such as potassium permanganate, air or oxygen, ruthenium tetroxide, chromic acid and nitric acid. Although the reaction mechanisms and conditions differ depending on the oxidant, there are no significant differences in the compositions of the oxidation products. Oxidation produces di- and tricarboxylic acids. Dicarboxylic acids can be used to produce plastics and polymers, in the pharmaceutical industry, as well as in the manufacture of body care products. Various tricarboxylic acids are also used, which can be used in the food and pharmaceutical industries, as well as in cosmetics and cleaning products.

[0008] For example, it is possible to decompose oil shale kerogen with potassium permanganate, and the oxidation products resulting from such decomposition contain dicarboxylic acids. Colorado oil shale has been studied by a two-step potassium permanganate digestion. The digestion produced o,(D-dicarboxylic acids (up to adipic acid, C6) and higher molecular weight (up to 800 Da) dicarboxylic acids. Also, the oxidation product obtained by the stepwise oxidation of Green River oil shale with alkaline potassium permanganate contains aliphatic dicarboxylic acids in the C4-C17 range. However, the use of potassium permanganate in large- scale industrial applications is not feasible due to its high cost.

[0009] An aqueous suspension of oil shale can also be oxidatively dissolved with atmospheric oxygen at elevated temperatures to obtain dicarboxylic acids. However, the disadvantages of this method are the low yield and the formation of a complex mixture of dicarboxylic acids and their homologues, from which it is very difficult to remove specific compounds of interest.

[0010] It is also possible to oxidise oil shale kerogen with RuC , but this method is unsuitable for large-scale industrial applications due to the very expensive oxidant.

[0011] Oil shale kerogen can be oxidised with chromic acid, which is a cheap oxidant, but it has high toxicity, environmental hazards, and requires strict handling regulations and costly waste management, making industrial use problematic. Kerogen decomposition with other cheaper chemicals such as nitric acid has also been used at various concentrations and temperatures. One example is the oxidative decomposition of kukersite kerogen with dilute nitric acid at high pressure of 50-100 atm, and at moderate temperature of 105-130 C for 6 to 8 hours. However, the disadvantages of such a method are the high pressure and long reaction time, which makes such a method technically complex and such a method cannot be carried out in a flow reactor. It is also possible to obtain dicarboxylic acid from oil shale by using concentrated nitric acid at a temperature of 105-130 C for 1.5 to 2 hours. However, the disadvantage of such a method is that it is an exothermic reaction, which is too dangerous to use in industry due to the risk of explosion.

[0012] The oxidation reaction of kukersite kerogen concentrate with nitric acid to obtain dicarboxylic acids has also been carried out in a cascade of four reactors with continuous air addition, where the reactors used in the cascade are acid-resistant chemical reactors with mixing paddle on top. Such cascade reactors and the mechanical stirring used in them do not allow for sufficiently intensive gas / liquid / solid phase mixing, which is necessary for the complex and multi-stage oxidation of a concentrate with a high kerogen content. Also, the reaction time in such a cascade reactor is long, 2.3-3.5 hours, which makes the productivity of the cascade low.

[0013] Furthermore, this method requires the use of nitric acid in an excess of >60%, meaning that the unreacted nitric acid must be regenerated after the process, which is difficult in the case of aqueous solutions and in the presence of organic material. Regeneration can be achieved, for example, by separating the unreacted nitric acid from the reaction mixture by distillation. This approach causes prolonged contact of the dicarboxylic acids with high concentrations of nitric acid and partial decomposition of the compounds, which in turn leads to a decrease in yield, and problems with explosiveness arise. The result of the process is a partially solid residue, from which dissolving the dicarboxylic acids with methanol is a time-consuming process. The nitric acid residue must be completely separated from the mixture so that explosive compounds are not formed when methanol is added. The relatively low concentration of nitric acid in the reactor extends the kerogen oxidation time, while the long reaction time reduces the productivity of the process.

[0014] Due to aforementioned circumstances, a new process for processing oil shale kerogen was needed, in which the oxidation of oil shale kerogen with nitric acid to di- and tri-carboxylic acids would take place efficiently in an flow reactor.

[0015] PURPOSE AND SUMMARY OF THE INVENTION

[0016] The purpose of the invention is to efficiently decompose oil shale kerogen into di-and tri-carboxylic acids in a flow reactor. The set objective is achieved by a technical solution, where the method of decomposition of oil shale kerogen with nitric acid is carried out in a flow reactor.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure FIG 1. shows a method for decomposition of oil shale kerogen into di- and tri-carboxylic acids in a flow reactor.

[0019] Figure FIG 2. shows a method for decomposition of oil shale kerogen into di- and tri-carboxylic acids in a flow reactor having a first section and a second section.

[0020] DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention describes a method for the decomposition of an oil shale kerogen into di- and tricarboxylic acids, wherein the method is carried out in a flow reactor and the method comprises the following steps:

[0022] -feeding the oil shale kerogen and a nitric acid into the flow reactor, where a reaction mixture is formed; -heating the reaction mixture in the flow reactor first at a temperature of 120-135 °C and then at a temperature of 140-165 °C, resulting in a product mixture and gas, wherein the product mixture contains di- and tricarboxylic acids;

[0023] - discharge of the product mixture and the gas from the flow reactor.

[0024] According to this invention, the oxidation of oil shale with nitric acid takes place in the flow reactor. The method is carried out in the flow reactor, because the reactions carried out in the flow reactors are more efficient. A flow reactor enables a continuous and automated production process, while also ensuring a more uniform performance of the decomposition process. In flow reactors, it is possible to optimise the reaction temperature and reaction time. Among other things, flow reactors are suitable for processing large quantities.

[0025] The oil shale kerogen is pumped into the reactor with a pump, the speed of which is controlled by a mass flow meter. A separate pump system adds the nitric acid to the flow reactor, the dosing rate of which can also be controlled. The pumps make it possible to control the speed, quantities and also the ratio of the oil shale kerogen and nitric acid fed into the reactor.

[0026] The oil shale kerogen and the nitric acid fed into the flow reactor form a reaction mixture, which is heated in the flow reactor at two temperatures, first at a lower temperature in the range of 120-135 °C, and then at a higher temperature of 140-165 °C. At the first temperature, the initial exothermic reaction takes place, and at the second temperature, the maximum possible oxidative decomposition of the oil shale kerogen takes place. Such two-stage heating allows for control of the composition of the product mixture. The first heating at a lower temperature helps to control the rate of the initial exothermic reaction, since an uncontrolled reaction rate can lead to overheating and undesirable side reactions. Thus, the lower initial temperature ensures the safety and the stability of the flow reaction. Heating at a higher temperature is necessary for the oil shale kerogen to form a product mixture containing di- and tri-carboxylic acids. The higher temperature used as the second heating helps to accelerate the reaction rate and complete decomposition, increasing efficiency and reducing the amount of undesirable compounds in the product mixture. At a lower temperature, the decomposition is partial and incomplete. Higher temperature also improves the mass transfer of the reaction mixture, accelerating mixing and reaction, which in turn ensures a more uniform reaction and greater yield.

[0027] As a result of heating, the product mixture and gas are formed from the reaction mixture and are discharged from the flow reactor. The product mixture contains di- and tri-carboxylic acids. Among other things, the two-stage temperature regime, together with the controlled ratio of the oil shale kerogen and nitric acid, allows for the removal of the undesirable distillation step of nitric acid from the mixture discharged from the flow reactor.

[0028] In an alternative embodiment, the method uses oil shale kerogen that contains carbonates less than 5% by weight. Alternatively, the oil shale kerogen concentrate with a kerogen content of 45-75 percent by weight is used. Such oil shale kerogen concentrate can be obtained by enriching oil shale kerogen using various methods. For example, the carbonates contained in the oil shale kerogen can be leached with formic acid or acetic acid. Such a pretreatment of the oil shale kerogen removes the carbonates and provides the oil shale kerogen concentrate with a kerogen content of 45-75% by weight. Since carbonates can also react at high temperatures to form carbon dioxide and water, the low content of carbonates in the oil shale kerogen helps to reduce the formation of byproducts in the reaction mixture. Also, the reaction of carbonates can form solid residues in the flow reactor, which can clog it, reducing the efficiency of the method. The carbon dioxide gases formed can also be dangerous, as they can cause a pressure increase in the flow reactor and create an explosion hazard.

[0029] In an alternative embodiment, the reaction mixture is heated to a temperature of 120-135 °C in a first section of the flow reactor and the heating to a temperature of 140-165 °C occurs in a second section of the flow reactor, wherein the reaction mixture moves from the first section of the flow reactor to the second section through a first connecting orifice, wherein a diameter of the first connecting orifice is smaller than a diameter of the flow reactor.

[0030] The advantage of this method is that the first section of the flow reactor and the second section of the flow reactor are separated from each other, and the first connecting orifice, the diameter of which is smaller than the diameter of the flow reactor, ensures that in each section the reaction mixture moves from the center upwards and from the edges downwards. This movement of the reaction mixture ensures uniform mixing of the reaction mixture and effective heat exchange.

[0031] Furthermore, the connecting orifice diameter that is smaller than the diameter of the flow reactor helps ensure that intermediates that may form in the first section only move to the second section when they have reached the desired level of conversion. This helps to increase the selectivity of the desired products and reduce the formation of byproducts. There is also better control of heat transfer and mass transfer between sections, allowing for a more uniform temperature and concentration profile, which is critical for maintaining stable and controlled reaction conditions.

[0032] The movement of the reaction mixture in the flow reactor can be controlled by how quickly the oil shale kerogen and the nitric acid are introduced into the flow reactor. The faster the oil shale kerogen and the nitric acid are fed into the flow reactor, the faster the reaction mixture moves from the first section of the flow reactor to the second section of the flow reactor. Optionally, additional sections can be added to the reactor.

[0033] The presence of multiple sections allows for precise control over the reaction conditions in each section, including temperature, pressure and reaction time. This allows for optimisation of process conditions for different heating conditions, increasing overall process efficiency and yield, increasing selectivity to desired products, and reducing by-product formation. Continuous and precise control of conditions in multiple sections helps ensure that the product mixture is of consistent quality.

[0034] To regulate the different temperatures of different sections, autonomous heat exchangers of the sections can be used, which can be used for both heating and cooling the reaction mixture if necessary. Such a solution allows the use of a specific temperature regime for each section, but only for that section, if necessary. Such a solution makes it possible to use, if necessary, a specific temperature regime that is suitable for each section.

[0035] In an alternative embodiment, the reaction mixture is further heated to a temperature of 140-165 °C in a third section of the flow reactor, wherein the reaction mixture moves from the second section of the flow reactor to the third section through a second connecting orifice, wherein a diameter of the second connection orifice is smaller than the diameter of the flow reactor.

[0036] The addition of the third section to the flow reactor helps to better control the process conditions before the product mixture and gases exit the flow reactor. This helps to ensure that the maximum decomposition of the oil shale kerogen has been completed. The third section can also act as a safety valve where the decomposition reaction can proceed to completion and the residues can be used up stably. This helps to reduce the likelihood of safety hazards and process instability. The heat exchanger jackets of the second and third sections of the flow reactor are connected and the temperature control of these sections is autonomous from the temperature of the first section. Water or thermal oil is used in the heat exchangers, and the oil flow rate in the thermal oil jacket is selected according to the required cooling and is, for example, 25 dm3 / min for a reactor volume of 3 I.

[0037] In an alternative embodiment, a residence time t of the reaction mixture in each section of the flow reactor is calculated using the formula t = (Vr- Vg) / vs, where Vris a volume of the corresponding section, Vgis a volume of gas in the section at each time point, and vsis a total volume of oil shale kerogen and the nitric acid fed into the flow reactor.

[0038] For the reaction mixture to react uniformly and for greater heat transfer to occur, it is necessary that the gases generated in the reaction are used to mix the mixture. The time, t, that the reaction mixture remains in each section of the reactor can be calculated using the volume of the corresponding section in cubic meters, Vr, the volume of gas in the section in cubic meters at each time point, Vg, and the total volume rate of the oil shale kerogen and nitric acid introduced into the flow reactor, vs, in m3 / s. If something else is introduced into the flow reactor in addition to the kerogen and nitric acid, then this is added to the total volume of the oil shale kerogen and nitric acid. The volume of gas in the section at each time point is a temperature- and pressure-dependent quantity.

[0039] The pressure in the flow reactor is kept below 2.5 MPa throughout the process. The time for the reaction mixture to pass through each section is approximately 10 minutes. It is important that the residence time in each section takes into account both the volume of the section, the volume of gas in the section, and the volume of the oil shale kerogen and the nitric acid introduced into the flow reactor, because in this case the conditions of the method can be chosen such that the decomposition is complete and efficient. In an alternative embodiment, the oil shale kerogen is a suspension comprising 25-50% by weight of oil shale kerogen and 50-75% by weight of water. The process typically uses the oil shale kerogen concentrate with a kerogen content of up to 75% by weight. Such concentrated oil shale kerogen is mixed with water so that the solids content is 25 to 50% by weight and the water content is 50 to 75% by weight, respectively. The oil shale kerogen and water suspension is pumped into a flow reactor by a pump controlled by a mass flow meter. The use of the suspension helps to control the reaction rate by reducing direct contact of the oil shale kerogen and nitric acid, which helps to reduce the risk of a sudden and uncontrolled reaction. This is particularly important because the first reaction that occurs in this process is an exothermic reaction where a large release of heat can cause hazardous situations. The water used in the suspension helps to stabilise the temperature and prevent overheating, reducing the risk of hot spots and localised overheating. In the case of suspension, the kerogen in the oil shale is more evenly distributed, which helps to achieve better yield and quality of the product mixture.

[0040] In alternative embodiments, a concentration of nitric acid introduced into the flow reactor is up to 68 percent by weight. The nitric acid of up to 68 percent by weight is of sufficient concentration to be an effective oxidiser of the oil shale kerogen, but not so concentrated as to be extremely hazardous and difficult to handle. The nitric acid of up to 68 percent by weight is of a suitable concentration to mix with the oil shale kerogen when fed into the flow reactor.

[0041] In alternative embodiments, the concentration of nitric acid in the reaction mixture is up to 35 percent by volume. In order to maintain the equilibrium of the exothermic reaction occurring during the first heating, it is important that the concentration of nitric acid introduced into the flow reactor in the first section of the reactor after mixing with the oil shale kerogen stream does not exceed 35 percent by volume. Such a maximum concentration of nitric acid in the flow reactor ensures a faster and cleaner decomposition reaction. Therefore, the residence time of the oil shale kerogen in the flow reactor is reduced, which allows a significant increase in process efficiency. Such a concentration of nitric acid also ensures that the product mixture leaving the flow reactor does not contain an excessively high concentration of nitric acid.

[0042] In alternative embodiments, the amount of nitric acid in the product mixture exiting the flow reactor is a maximum of 7 percent of the amount of nitric acid in the reaction mixture. The separation of excess nitric acid from the product mixture and gases and its regeneration is a complex process. In the present method, the need for regeneration is eliminated by adding nitric acid in such a concentration that the nitric acid reacts as completely as possible as it passes through the flow reactor sections and the amount of residual nitric acid at the outlet of the flow reactor is small. According to the preferred method, the ratio of nitric acid to the oil shale concentrate is selected such that the amount of residual nitric acid at the process output does not exceed 7 percent of the amount of nitric acid in the reaction mixture. This eliminates the need to separate nitric acid from the reaction mixture by distillation after passing through the flow reactor, and the small amount of nitric acid residue can be neutralised with a suitable alkali. The small amount of nitrates formed, together with the partially decomposed oil shale kerogen mass that has not completely reacted, are suitable for the manufacture of fertilisers and for use in agriculture. According to the preferred method, 2-5 g of nitric acid is used to oxidise 1 g of 100 percent oil shale kerogen.

[0043] In alternative embodiments, wherein when introducing the oil shale kerogen and the nitric acid into the flow reactor, an air is also introduced as a reaction gas. The use of the reaction gas ensures a more uniform reaction between the oil shale kerogen and the nitric acid and contributes to heat transfer. The use of air as a reaction gas helps to enrich the reaction mixture with oxygen, which helps to increase the rate of reactions, in turn promoting oxidation or decomposition. The advantage of using air compared to other reaction gases is that air is environmentally friendly, does not require separate transport and is safe to use, providing a moderate oxidation environment. Air is added to the first section as a reaction gas at up to 3000 cm3 / min at normal temperature and pressure per 1 I of reactor volume.

[0044] In alternative embodiments, the gases leaving the flow reactor are further directed to a gas / liquid separator. It is important to separate the gases from the product mixture and the gases leaving the flow reactor in order to stop unwanted reactions that could occur in the presence of gases in the product mixture. This provides better control over the composition of the product mixture and reduces the formation of byproducts. The separated gases can be more easily routed, which simplifies their waste management or further use.

[0045] In alternative embodiments, the introduction of the oil shale kerogen and the nitric acid into the flow reactor is carried out as a continuous process. The method allows the decomposition of the oil shale kerogen to be carried out as a continuous process, which means that it can be used efficiently and for automated processing of large volumes. The use of a continuous process allows the production of a product mixture that has a similar concentration of di- and tri-carboxylic acids. Among other things, continuous processes are less energy-intensive and allow for optimal use of the oil shale kerogen. The continuous process allows for better heat and mass transfer control, which is important for the stability and efficiency of the reactions. This helps to avoid the formation of hot spots and ensures that all reaction components are evenly mixed.

[0046] In alternative embodiments, the oil shale is kukersite. Kukersite is Estonian oil shale. Kukersite is rich in organic matter, containing large amounts of oil shale kerogen, which allows the use of kukersite to efficiently produce di- and tricarboxylic acids.

[0047] In alternative embodiments, the oil shale is of marine or lacustrine origin. Marine or lacustrine shale kerogen has a higher proportion of aliphatic chains compared to terrestrial shale kerogen, which allows the use of this type of kerogen to efficiently produce di- and tricarboxylic acids.

[0048] EXAMPLES

[0049] Example 1. Decomposition of kukersite kerogen with nitric acid into di- and tri-carboxylic acids.

[0050] Oil shale K60, with a kerogen content of 53 percent by weight, is mixed with water so that the solids content is 28 percent by weight, to obtain oil shale kerogen suspension. The suspension is pumped into a three- section flow reactor at a rate of 3300 g / h. 2250 g / h of nitric acid, with a weight percentage of 57, is added using a separate dosing pump. Air is continuously added to the flow reactor as a reaction gas at a rate of 3000 cm3 / min (at normal temperature and pressure). The reaction gas maintains the pressure in the flow reactor at up to 1.1 MPa. The reaction mass velocity in the reactors is such that the reaction time in each section is slightly less than 10 minutes. The reaction temperature in the first section is kept at 120-135 °C and in the upper two sections at 140- 165 °C.

[0051] The product mixture and gases exiting the reactor are cooled, the pressure is reduced to 0.1 MPa, and directed to the separator. The gases are directed to the absorber, where, by adding water and air, the NO gas is oxidised to NO2, which dissolves in water and forms nitric acid again. The resulting nitric acid is directed back to the inlet of the flow reactor. The product mixture leaving the flow reactor contains 22 percent by weight of dicarboxylic acids and 4 percent by weight of tricarboxylic acids, compared to the the amount of oil shale kerogen introduced into the flow reactor.

[0052] Example 2. Decomposition of Estonian kukersite, Green River USA, Irati Brazil, Kenderlyk Kazakhstan, El Lajjun Jordan and Yarmouk Syria oil shale kerogen into dicarboxylic acids with nitric acid

[0053] Estonian kukersite (EST), El Lajjun Jordan (JOR) and Yarmouk Syria (SYR) oil shales are of marine origin. Green River USA (GR), Irati Brazil (BRA), Kenderlyk Kazakhstan (KAZ), are of lacustrine origin.

[0054] Oil shales are analysed by elemental analysis. The highest total organic carbon (TOC) or kerogen content is in Brazilian oil shale (30.20%), followed by Estonian (24.30%) and Jordanian (21.41%) (Table 1). The lowest kerogen content is in Syrian oil shale (12.55%).

[0055] Table 1. Elemental composition / mass percentage of oil shale samples. aorganic carbon,binorganic carbon, calculated as Ctot-TOC The highest content of carbonate minerals, characterised by inorganic carbon (TIC), is highest in Estonian (6.10%) and Syrian (5.37%) oil shale (Table 1). The removal of carbonate minerals was carried out using formic acid - 5 ml of 30% HCOOH was added per 1 g of oil shale. The mixture was kept at 50 °C for 24 hours, then cooled and the liquid-solid phases separated. The results of the mass loss due to the removal of carbonate minerals and the elemental composition of the treated samples are shown in Table 2. Based on the minimum TIC values, it can be confirmed that the carbonate removal process was successful. Table 2. Composition of oil shale after removal of carbonate minerals.

[0056] Oil shale kerogen concentrates are oxidised. The solids content in the oil shale kerogen suspension is 25% by weight and is introduced into a three-section flow reactor, where the reaction mixture is formed. The reaction mixture is oxidised in a nitric acid solution with a weight percentage of 57, keeping the HNO3 / TOC molar ratio constant at 1.0. Air is continuously added to the flow reactor as a reaction gas at a rate of 3000 cm3 / min (at normal temperature and pressure). The reaction gas is used to maintain the pressure in the flow reactor at up to 1.1 MPa. The reaction mass velocity in the reactors is such that the reaction time in each section is 12 minutes. The reaction temperature is maintained at 120-135 °C in the first section and 140-165 °C in the upper two sections. The product mixture is separated in a separator and the yield of di- and tricarboxylic acids is determined by capillary electrophoresis, respectively di- and tricarboxylic acids (g / h) / organic carbon TOC (g / h) (Table 3).

[0057] Table 3. Yields of di- and tricarboxylic acids obtained from the oxidation of oil shale

[0058] DETAILED DESCRIPTION OF THE DRAWINGS

[0059] FIG. 1. illustrates a method for the decomposition of oil shale kerogen into di- and tri-carboxylic acids in a flow reactor 100. The oil shale kerogen 102 and the nitric acid 104 are fed into the flow reactor. A reaction mixture 106 is formed in the flow reactor. The reaction mixture is first heated to a temperature of 120-135 °C and then to a temperature of 140-165 °C, resulting in the formation of a product mixture and gas from the reaction mixture, where the product mixture contains di- and tri-carboxylic acids, and the product mixture 108 and gases 110 are discharged from the flow reactor.

[0060] FIG. 2. shows a method for the decomposition of oil shale kerogen into di- and tri-carboxylic acids in a flow reactor 200 having a first section 202 and a second section 204. The oil shale kerogen 106 and nitric acid 208 are fed into the flow reactor. A reaction mixture 210 is formed in the flow reactor. The reaction mixture 210 is heated to a temperature of 120- 135 C in the first section 202 of the flow reactor and then heated to a temperature of 140-165 C in the second section 204 of the flow reactor, wherein the reaction mixture moves from the first section of the flow reactor to the second section through a first connecting orifice 212, wherein the diameter of the first connecting orifice is smaller than the diameter of the flow reactor. Finally, the product mixture 214 and gases 216 are discharged from the flow reactor.

[0061] Modifications of the embodiments of the invention described above are possible without departing from the scope of the invention as defined by the claims. Expressions such as "comprises", "includes", "includes", "has", "is", used in describing and claiming the invention, should be interpreted in a non-exclusive manner, i.e., allowing the use of units, components or elements not specifically described. References to the singular should be interpreted as including the plural.

Claims

CLAIMS1. A method for decomposition of an oil shale kerogen into di- and tricarboxylic acids, wherein the method is carried out in a flow reactor and the method comprises the following steps:- feeding the oil shale kerogen and a nitric acid into the flow reactor, where a reaction mixture is formed;- heating the reaction mixture in the flow reactor first at a temperature of 120-135 °C and then at a temperature of 140-165 °C, resulting in a product mixture and gas, wherein the product mixture contains di- and tricarboxylic acids;- discharge of the product mixture and the gas from the flow reactor.

2. The method according to claim 1, wherein the oil shale kerogen contains carbonates less than 5% by weight.

3. The method according to any of the preceding claims, wherein the heating of the reaction mixture to a temperature of 120-135 °C occurs in a first section of the flow reactor and the heating to a temperature of 140-165 °C occurs in a second section of the flow reactor, wherein the reaction mixture moves from the first section of the flow reactor to the second section through a first connecting orifice, wherein a diameter of the first connecting orifice is smaller than a diameter of the flow reactor.

4. The method according to claim 3, wherein the reaction mixture is additionally heated to a temperature of 140-165 °C in a third section of the flow reactor, wherein the reaction mixture moves from the second section of the flow reactor to the third section through a second connecting orifice, wherein a diameter of the second connecting orifice is smaller than the diameter of the flow reactor.

5. The method according to any of the preceding claims, wherein a residence time t of the reaction mixture in each section of the flow reactoris calculated using the formula t = (Vr- Vg) / vs, where Vris a volume of the corresponding section, Vgis a volume of gas in the section at each time point and vsis a total volume rate of the oil shale kerogen and the nitric acid introduced into the flow reactor.

6. The method according to any of the preceding claims, wherein the oil shale kerogen is a suspension comprising 25-50% by weight of oil shale kerogen and 50-75% by weight of water.

7. The method according to any of the preceding claims, wherein a concentration of nitric acid introduced into the flow reactor is up to 68 percent by weight.

8. The method according to any one of the preceding claims, wherein the concentration of nitric acid in the reaction mixture is up to 35 percent by weight.

9. The method according to any of the preceding claims, characterised in that the amount of nitric acid in the product mixture exiting the flow reactor is a maximum of 7 percent of the amount of nitric acid in the reaction mixture.

10. The method according to any of the preceding claims, wherein when introducing the oil shale kerogen and the nitric acid into the flow reactor, an air is also introduced as a reaction gas.

11. The method according to any of the preceding claims, wherein the gases leaving the flow reactor are further directed to a gas / liquid separator.

12. The method according to any of the preceding claims, wherein the introduction of the oil shale kerogen and the nitric acid into the flow reactor is carried out as a continuous process.

13. The method according to any of the preceding claims, wherein the oil shale is a kukersite.

14. The method according to any of preceding claims, wherein the oil shale is of marine or lacustrine origin.

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