Method for treating aqueous effluent containing organic matter, in particular effluent from oil press

Abstract

Disclosed herein is a method (1; 1 ') for treating an aqueous effluent containing organics, in particular an oil press effluent; the invention relates to a process (1, 1 ', 100) for producing an aqueous effluent, comprising:-providing a stream (F1) of aqueous effluent containing organic matter,-pressurizing (2, F2) the stream of aqueous effluent to a pressure value equal to or greater than the pressure value of the critical point of water and heating the stream of aqueous effluent to a temperature value equal to or greater than the temperature value of the critical point of water, -treating the pressurized and heated stream (F2) of aqueous effluent by means of the supercritical water reactor,-subjecting the aqueous effluent (F4) treated by means of the supercritical water reactor to a catalytic hydrotreating.

Description

Technical Field

[0001] The present invention relates to the treatment of municipal or industrial effluents, in particular effluents from the food processing industry.The invention has been developed with particular, but not exclusive, reference to aqueous effluents from oil mills. Background Art

[0002] The wastewater effluent of the oil mill consists mainly of the olive vegetation water, inherently contained in the fruit of the olive tree up to 50% by weight, and water added during the extraction process.

[0003] The amount of wastewater produced depends on the extraction process implemented: for example, three-stage centrifugation produces at least twice the amount of water as the amount of oil produced, while two-stage treatment produces the least amount of waste (approximately equal to 10% of the mass of olives treated).

[0004] Furthermore, the composition of the waste depends greatly on the process technology used, but on average it consists of 80% water, 15-18% organic compounds (mainly sugars) and about 2% inorganic salts. The solution pH is generally below 5.5 and the chemical oxygen demand (COD) may be as high as 200,000 ppm; these are very high values, which, together with the presence of fairly stable phenolic compounds, make oil mill wastewater a particularly problematic production residue from a health and environmental point of view, due to its high toxicity to plants and animals.

[0005] Compared to conventional biological, chemical, physicochemical and thermal treatments, membrane-based methods have several advantages in the treatment of aqueous effluents from oil mills, namely low specific consumption, no additives and no phase changes.

[0006] On the other hand, such methods involve technical problems due to concentration polarization phenomena and membrane fouling caused by the specific composition of the aqueous effluent. Typically, membrane-based methods include a combination of various different technologies, characterized by a gradual decrease in the characteristic size of the filter elements from microfiltration and ultrafiltration in the first stage (capable of significantly reducing suspended solids) to nanofiltration and reverse osmosis in the second stage (capable of significantly reducing polluting organic compounds and recovering pure water).

[0007] Another problem with aqueous effluents from oil mills is the high energy consumption and low possibility of recycling the purified effluent. Aqueous effluents from oil mills consist essentially of water (although water may be introduced into the mains), which does not allow any substantial conversion or recovery of the energy consumed in the purification process. Summary of the invention

[0008] Purpose of the invention

[0009] The present invention aims to solve the above-mentioned technical problems. In particular, the present invention aims to provide a method for treating aqueous effluents containing organic matter, such as aqueous effluents from oil mills, which method is substantially free of the technical problems affecting the above-mentioned known treatment technologies, in particular membrane-based treatment technologies, and which is capable of recovering the aqueous effluent by converting it into a product suitable for further utilization.

[0010] The objects of the invention are achieved by a method having the features forming the subject of the appended claims, which form an integral part of the technical disclosure provided herein in relation to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will now be described with reference to the accompanying drawings, provided by way of non-limiting example only, in which:

[0012] - Figure 1 is a schematic block diagram of a first embodiment of the processing method according to the present invention.

[0013] - Figure 2 is a schematic block diagram of a second embodiment of the processing method according to the present invention, and

[0014] - Figure 3 is a schematic block diagram of other method steps according to the present invention.

[0015] Figures 1 to 3 In addition to the usual reference numerals that appear consistently throughout the specification, the schematic diagrams are accompanied by a brief text section that is intended only to help understand each schematic diagram by referring to some specific embodiments (however, these embodiments are described in detail in the specification). It is obvious from the following description that such text sections shall not be interpreted in any way as constituting an informative or limiting element of the object of the description or schematic diagram of the present invention, wherein the reference is always indicated by the specific embodiments provided below. DETAILED DESCRIPTION

[0016] As a general introduction, the treatment method according to the invention comprises two treatment sub-methods, a method for treating a stream of an aqueous effluent containing organic matter (e.g. effluent from an oil mill, but also intended to include other activities such as dairies and related aqueous effluents, as well as municipal effluents, leachates, etc.) under supercritical conditions (according to Figure 1 and Figure 2 ), which is used to convert complex organic molecules into simpler molecules and hydrogen, another method for further converting the products of supercritical processing into combustible products by catalytic hydrogenation of carbon oxides (such as catalytic methanation) (according to Figure 3), preferably with subsequent upgrading so that the product meets regulatory requirements for use as a motor fuel or for input into a distribution main.

[0017] The method according to the invention envisages the integrated management of two sub-methods for the treatment of aqueous effluents containing organic matter: the two sub-methods are not only complementary at the functional level, but they can also be applied both at a single territorial level (preferably without interruption from one sub-method to the other) and at two territorial levels.

[0018] Specifically, referring to a pair of territorial levels, considering the “micro” geographical scale (territory of individual municipalities and territory of small groups of municipalities), the first sub-method ( Figure 1 or Figure 2 ) is very suitable for use in the vicinity of a single local olive processing plant of small to medium size (reference is made to an oil mill by way of non-limiting example only), for on-site treatment of locally generated aqueous effluents, and for the local production of crude synthesis gas by a thermochemical supercritical gasification process (again, preferably using semi-batch operation to better adapt to the type of material flow from an oil mill or a dairy, but also suitable for continuous operation).

[0019] Thus, the first sub-method is conducted in a decentralized and distributed manner, whereas the second sub-method is applied in conjunction with and in parallel with the first sub-method at a "macro" geographic scale (at the county or inter-county level, up to the regional scale).

[0020] The second sub-process consists in the collection and catalytic hydroprocessing (which may or may not be centralized), in particular the catalytic hydroprocessing of carbon oxides (hereinafter referred to as "catalytic hydrogenation"), for example by catalytic methanation technology of all the raw synthesis gas produced at decentralized sites. The catalytic hydroprocessing of carbon oxides (mainly monoxides and dioxides) is used entirely or almost entirely for the production of (bio)fuels.

[0021] Catalytic hydroprocessing includes a wide range of specific processes depending on the desired end product. If the desired end product is methane, catalytic hydroprocessing includes catalytic methanation. If the desired end product is gasoline or light diesel, catalytic hydroprocessing is a Fischer-Tropsch process.

[0022] According to an advantageous aspect of the invention, by this method, for both decentralized plants and centralized plants (in general for both sub-processes), a significant part or even the entirety of the required electrical energy (which is almost exclusively used for compressing the supply stream and supporting the operating conditions in the supercritical water reactor) comes from renewable energy sources, preferably photovoltaic fields. Thus, it is possible to obtain as a final product a biofuel, which is a so-called "e-fuel", i.e. a fuel obtained by energy input from renewable sources.

[0023] In particular, a photovoltaic field with limited peak power, which may already exist or be newly installed at each selected oil mill (or local site), coupled with a backup energy system (electrical energy storage batteries), can meet the energy needs of the local mills, while retaining the possibility of access to the national transmission mains when more energy is needed. In any case, the photovoltaic field in the embodiment can be of the so-called "stand-alone" type, that is, completely independent of the electricity mains and operating only in combination with an electrical energy storage system (e.g. a battery bank). In this way, an important economic advantage is achieved, since distribution excise duties and fees no longer need to be paid.

[0024] The same thing can be said with reference to the second sub-process and the associated centralized plant. In this respect, the second sub-process can be implemented within the same plant where the first sub-process is carried out, so that in this case also a local or "micro" scale is envisaged, or it can be implemented in a plant that is physically remote and relocated to another geographical area, and / or operates at a different ("macro") scale. In the latter case, the synthesis gas produced with the first sub-process is input into pressurized gas cylinders and transported to the site where the second sub-process is implemented by gas cylinder trucks, which collect gas cylinders from individual oil mills, dairies or generally production sites that discharge aqueous effluents containing organic matter.

[0025] The preferred goal of the second sub-method is to centrally upgrade the fuel to produce fuel that meets legal regulations for use in vehicles or is fed into the national mains as natural gas (in the case of advanced biomethane gas). As mentioned above, the fuel produced includes biofuels and electronic fuels, depending on the energy input used.

[0026] As will become clear hereinafter, in order to enrich the synthesis gas obtained from the supercritical water treatment, in particular the synthesis gas obtained from the supercritical water gasification treatment, in order to produce fuels (such as advanced renewable fuels), the second sub-process envisages a catalytic hydrogenation treatment, which requires a supply of hydrogen as a second process reagent.

[0027] According to an advantageous aspect of the invention, the hydrogen required is preferably of the so-called "green" type (green hydrogen), ie hydrogen obtained from renewable sources. In this case, renewable electrical energy produced by a photovoltaic solar field associated with the plant implementing the second sub-method can be used almost exclusively to supply the system of electrolyzers and storage compressors. This enables the centralized production of green hydrogen from electrolysis of water by solar energy and its temporary storage for later use (in case of insufficient solar radiation).

[0028] In a preferred embodiment of the invention, with regard to aqueous effluents containing organic matter and originating from oil mills, the combination of the two sub-processes is suitable for semi-continuously treating the amount of aqueous effluent produced by one or more oil mills in a typical two operating months over an "operating year" of 2000-3000 nominal operating hours. Of course, the process according to the invention can be used for continuous treatment or for time-distributed treatment, so that in the latter case the plant size can be reduced.

[0029] In other words, a plant operating semi-continuously (or in semi-batch mode) must be sized to manage the processing requirements of the effluent stream from the semi-continuous source. This means that the equipment and filtration systems must be larger in size because it is not possible to provide delay and distribution treatment for the lower flow rates of effluent, nor is it possible to store excess flow.

[0030] In the case of continuous operation of the plant, the scale of the equipment and the filtration system may be smaller than in the case of semi-continuous operation mode; in other words, the system can be scaled for operation with smaller effluent flows: in this case, the excess flow can be stored and treated in a delayed and timely manner (for example, if the oil mill operates only two months a year, the plant can be operated twelve months a year by distributing the treatment of the effluent generated by the oil mill in two months to twelve months), when the oil mill (or the source of the aqueous effluent) is not in operation, hydrogen can be stored in tanks, and / or electrical energy can be stored in batteries (preferably the former solution is preferred) to ensure that short periods of autonomy are available when the sun is not shining and / or in case of temporary flow peaks of the aqueous effluent to be treated.

[0031] The storage of electrical energy is preferably secondary to the storage of hydrogen, which is generally considered to be a priority: when the storage tank is full, electrical energy starts to accumulate.

[0032] In any case, management solutions in which the reactor (and in general the plant) is operated on a continuous basis can minimize the size of the equipment, distribute the duty cycles and avoid repeated on-off switching. The limit of continuous operation will only be determined by the availability of small-scale equipment on an industrial scale.

[0033] refer to Figure 1 and Figure 2 , two implementations of the first sub-method will now be described with reference to the simplified block diagram provided in the figure.

[0034] refer to Figure 1 , reference numeral 1 generally indicates a schematic diagram of a first embodiment of a first sub-process, which is part of the general process according to the present invention. The main feature of sub-process 1 is the supercritical water oxidation treatment.

[0035] In the following description, sub-process 1 is applied to an oil mill OM by intercepting a stream F1 of aqueous effluent containing organic matter, which is generated in the oil mill OM as a result of the normal processing of olives. Of course, sub-process 1 can be applied to any activity of an industrial or other nature that generates a stream of aqueous effluent containing organic matter.

[0036] As a general rule, each block described below must be understood to represent a method step and an apparatus operating within a plant that implements a sub-method.

[0037] The material stream F1 is subjected to a treatment to increase its pressure and temperature, which is represented by block 2. Through the compression and preheating system, the material stream F1 obtains pressure and temperature conditions higher than the critical point of water (221 bar and 374° C.), preferably obtaining a nominal pressure of 240 bar and a nominal temperature of about 600° C. (both of which are significantly higher than the critical point of water).

[0038] For ease of description, the stream F1 thus treated is associated with the new reference F2. It must be remembered that, apart from the different thermo-dynamic conditions, the stream F2 may correspond completely to the stream F1, but that it generally corresponds to the sum of the stream F1 and a recycle stream F3, the composition of which will be described below.

[0039] Stream F2 is subsequently sent to a supercritical water treatment 4, which in this embodiment corresponds to a supercritical water gasification treatment carried out by a supercritical water gasification reactor. It is important to note that the water to be treated is the same as the aqueous effluent constituting stream F2 and therefore the water contained in stream F2.

[0040] The gasification reactor is preferably operated in semi-batch mode, since olive processing is also typically a batch operation.

[0041] In the gasification reactor, the reagent mixture, stream F2, is maintained at the same temperature and pressure for a residence time which maximizes the conversion of the organic components and the yield of methane, the most important component in the subsequent treatments. These treatments also involve the hydrogen formed in the mixture during the gasification process.

[0042] In the thermal (non-catalytic) gasification processes in aqueous supercritical environments that are widely described in the literature, the residence time in the reactor is typically between 100 seconds and several tens of minutes.

[0043] The conditions of temperature, pressure and residence time are chosen in a manner known per se in such a way that, when the chemical composition of the treated stream F2 is varied, a gas product is obtained characterized by a maximum hydrogen yield or a given ratio between hydrogen and carbon oxides, without taking into account the methane yield.

[0044] In fact, the latter condition may be more favorable as it enables the saving of “green” hydrogen in order to comply with the purity requirements of methane, which is the goal of the second sub-method.

[0045] Reference F4 leaving box 4 denotes the stream of supercritical water (in this embodiment, gasification) treated products, which corresponds substantially to stream F2, except for negligible internal leakage.

[0046] After leaving the reactor, the stream F4 is depressurized (block 6) to a moderate pressure level, generally to an amount of 40-60 bar, cooled and separated into a gaseous portion corresponding to the target intermediate product (i.e. wet synthesis gas) and a liquid portion, the liquid portion comprising an aqueous stream containing organic residues which have neither been completely oxidized and dissolved in water nor converted into gas due to kinetic or thermodynamic barriers.

[0047] The liquid fraction (stream F5), which is usually extracted at a pressure level in the range of 40-60 bar, is filtered (concentrated, block 8) by means of a conventional membrane separation system. The choice of the type and number of separation stages (microfiltration, nanofiltration, ultrafiltration, reverse osmosis) is made according to known criteria, in a manner that implements a configuration that minimizes problems of polarization, clogging and fouling of the membranes, and in a manner that supports the operation of regeneration and recovery of the filtration system.

[0048] Preferred conditions for satisfactory operation of a filtration system in which the final stage is performed by reverse osmosis are characterized by the absence of suspended solids in appreciable amounts, or in amounts that are incompatible with the membrane system.

[0049] In this way, it is possible to take advantage of the fact that the stream F5 is already separated under pressure and ready to be supplied to the reverse osmosis module, thereby saving energy for the pumping process. In this case, it may be necessary to add make-up water (stream F6) from the mains before input into the reverse osmosis unit, thereby simultaneously producing a certain amount of demineralized water that can be used for other purposes, for example, for recycling to the oil mill OM, stream F7, or for supply to the electrolyzer 9 (stream F8) to produce "green" hydrogen, as described below.

[0050] Furthermore, the (aqueous) stream of concentrated solid residue produced by reverse osmosis (stream F3 mentioned above) can have a concentration of organic residues up to 3-4 times that of stream F1 after each concentration cycle and can therefore be recycled to the supercritical gasification reactor (block 4) since it contains unreacted compounds.

[0051] For each batch of "fresh" effluent F1 from the oil mill, a single recycling operation can be repeated more than once, in order to maximize the conversion of organic matter into light gaseous substances. The whole process can be essentially schematized as a semi-batch process, since there is a first initial effluent (stream F1), which is treated, separated and partially recycled (only with respect to the concentrated residue of reverse osmosis, batch F5): these operations are carried out without interruption, the net effect being to extend the total residence time in the reactor (or more precisely, the total time that the reactor remains at the same temperature), although at each recycling the actual average composition of the stream (F1+F3) entering the reactor may be different - the concentration of organic residues becoming increasingly higher or being kept constant by diluting the concentrate F3 to the operating concentration of the gasification reactor (usually, effluent: water = 1: 10, in any case not higher than 1: 5). A booster pump is preferably provided for the recycling of stream F3, since an unsupported circulation would require pressure conditions that are incompatible with filtration.

[0052] To the gaseous portion of stream F4 (indicated by reference numeral F10) is added pure hydrogen (stream F9, in an amount sufficient to achieve a volume ratio of 4:1 relative to the carbon dioxide already present in portion F10), which has in the meantime been produced by the hydrolysis of demineralized water in electrolyser 9. The resulting mixture is temporarily stored under pressure in gas cylinders (block 10) before being transferred to gas cylinder trucks (F11) and transported to a central enrichment (catalytic hydrogenation) / upgrading site where the second sub-process is carried out.

[0053] The oxygen (stream F12 ) simultaneously produced by the electrolyser 9 can be released into the atmosphere without purification.

[0054] According to an advantageous aspect of the invention, the energy needs of the plant implementing method 1 can be met by a photovoltaic plant (or farm) FV, preferably equipped with batteries for storing electrical energy.

[0055] Reference numerals E2, E4, E8, E9 indicate the flows of electrical energy supplied to the devices of stage 2 (compression and preheating), stage 4 (supercritical water gasification treatment), stage 8 (concentration / filtration of the aqueous portion of the stream F4), stage 9 (electrolyzer), respectively. Each flow of electrical energy E2, E4, E8, E9 is part of the total flow of electrical energy EFV from the photovoltaic field FV, which can also supply the flow of electrical energy used to charge the battery. In fact, when the gasification reactor is not in operation, the electrical energy generated by the photovoltaic field FV can be used to charge the battery, that is, to supply the user device of the oil mill OM.

[0056] In general, it is not necessary to supply all of the aforementioned method stages with an electrical energy flow EFV coming from the photovoltaic field FV, but preferably, at least one (or at least one requiring more energy, such as the electrolyzer 9) should be supplied with an electrical energy flow, and even more preferably, all stages should be supplied with an electrical energy flow (i.e. all electrical energy flows E2, E4, E8, E9 should be supplied).

[0057] refer to Figure 2 , reference numeral 1 ' generally denotes a second embodiment of the first sub-method according to the present invention. Except for the different supercritical water treatment, the corresponding block diagram and the sub-method in general are the same as those of reference Figure 1 The sub-methods described are identical. Specifically, a supercritical water oxidation process is envisaged instead of a supercritical water gasification process and a corresponding reactor. For this purpose, Figure 1 Box 4 in the Figure 2 4' in (the electricity supply from the photovoltaic field FV is therefore renamed E4', and likewise the stream leaving the supercritical water treatment is renamed F4'). Further differences will be described below, but in general, reference is made to Figure 1 The description of still applies (and will not be repeated in full), and all previous Figure 1 The reference numerals are considered to have the same meaning (if not explicitly stated otherwise).

[0058] exist Figure 2 In the sub-process, the synthesis gas produced at a decentralized site (stream F10) is expected to consist almost entirely of pure carbon dioxide and hydrogen; it requires a larger amount of "green" hydrogen produced by electrolysis, but also leads to a higher purity of the final product.

[0059] In this case, the organic substances present in the vegetable water and the aqueous effluent of the oil mill (stream F1) are subjected to supercritical water oxidation by direct reaction using pure oxygen or compressed air at a nominal pressure of 240 bar and a nominal temperature of about 580° C. During the heating and compression steps of block 2, stream F1 is preferably brought to corresponding or slightly lower pressure values. Again, it should be noted that such temperature and pressure levels are well within the water supercritical range.

[0060] like Figure 2 As can be seen, if pure oxygen is used, the oxygen can be provided directly by the electrolyzer 9 (stream F12, which can optionally be supplied to box 2 together with stream F1), which already exists and is used to produce hydrogen; however, in this case, the electrolyzer 9 will have to be provided with a powerful, high-performance purification (dehydrogenation) section for the oxygen, which must be completely free of hydrogen traces in order to be safely compressed and supplied to the reactor 4'.

[0061] The main advantage of this solution is that a synthesis gas F10 is obtained, the composition of which is almost independent of the nature of the organic components of the supplied plant water, since the synthesis gas F10 is expected to contain substantially pure carbon dioxide and hydrogen.

[0062] On the other hand, since the amount of CO2 present in the main mixture F4' is higher in absolute terms than that produced in the case of the aforementioned supercritical water gasification treatment, it is necessary to add a larger amount of "green" hydrogen from electrolysis in order to comply with the volume ratio of 4:1 input to the enrichment (catalytic hydrogenation) / upgrading plant.

[0063] However, the quality of the high-grade renewable biomethane obtained after the methanogenic treatment will be higher and certainly more in line with the technical specifications imposed by current regulations, since the product will not contain hydrocarbon components with two or more carbon atoms (ethane, propane, butane and higher).

[0064] refer to Figure 3 , a second sub-process according to the invention will now be described, indicated by the reference numeral 100. This sub-process enables the enrichment (catalytic hydrogenation) and, if necessary, the centralized upgrading of the synthesis gas (F10) coming from local factories placed near the olive processing sites, indicated herein by the reference numerals OM1, OM2, OMn (the latter indicating the nth oil mill).

[0065] The second sub-process essentially consists in the catalytic hydrogenation (for example catalytic methanation) of the carbon oxides present in the synthesis gas to which "green" hydrogen has been previously added, also obtained by electrolysis of demineralized water through photovoltaic solar energy. Moreover, it must be remembered that the centralized enrichment / upgrading site can be equipped with its own electrolyzer in order to produce more hydrogen for supplementary operation.

[0066] refer to Figure 3In a preferred embodiment, the sub-process for the enrichment and upgrading of synthesis gas to obtain biofuels (or, in general, "electrofuels" of biological origin, depending on the catalyst used for hydrogenation), such as methane that can be used for distribution in the national natural gas mains (regulation UNI EN 16723-1 / 2016) or for automotive purposes (regulation UNI EN 16723-2 / 2017), is carried out at a centralized site that receives batches of synthesis gas produced in "local" sites distributed throughout the territory, where the first sub-process is implemented. The type of treatment implemented at the local level - oxidation or gasification - depends on the residual organic matter, on its strong or weak resistance to gasification, and on the kind it is desired to obtain (advanced fuel or "electrofuel").

[0067] For example, if the goal is to obtain high-grade biomethane or LPG, gasification is preferred, as long as the organic matter contained in the aqueous effluent is easy to gasify. On the other hand, if the goal is to obtain DME (dimethyl ether), gasoline, light diesel, etc., oxidation is preferred, which is combined with a catalytic hydrogenation reactor with a correspondingly selective catalyst. This also applies if the goal is to obtain pure methane, without the need to separate the GPL part, such as the case of synthesis gas from supercritical water gasification.

[0068] In the latter case, oxidation is preferred, although it requires a higher input of resources (particularly hydrogen), but leads to purer products.

[0069] The dual management on which the method according to the invention is based, carried out in a centralized way (second sub-method) and in a decentralized way (first sub-method, whatever the embodiment may be), allows the centralized enrichment and upgrading plant to receive as input a feed of synthesis gas or any gas mixture with a high content of carbon oxides (stream F10, possibly stored in cylinders), or even a stored carbon dioxide stream coming from other sources or from plants other than oil mills.

[0070] In any case, the centralized plant is designed for continuous operation, unlike the decentralized plant described above, which is preferably designed for semi-continuous operation with recirculation (semi-batch operation).

[0071] After checking the correct volume ratio between hydrogen and carbon dioxide through a sampling step, the syngas batch delivered by the cylinder truck is accepted and temporarily stored under pressure in a tank (block 101 ).

[0072] From these tanks, material is withdrawn at a known and constant flow rate F101 for preheating (block 102 , preferably after decompression) and for catalytic hydrogenation treatment 104 , which, in the embodiment shown in the figure, is a catalytic methanation treatment carried out by means of a catalytic methanation reactor.

[0073] During the catalytic methanation process, carbon monoxide and carbon dioxide present in large quantities in the synthesis gas participate in the water gas shift reaction and the methanation reaction due to their durability at high temperatures (300-350°C) in the presence of a catalyst, typically based on nickel or ruthenium supported on alumina or mixed aluminum and magnesium oxides (each of which has unique activity and selectivity in methane).

[0074] The former is a well-known reaction of chemical equilibrium between carbon monoxide and water on the one hand, and carbon dioxide and hydrogen on the other hand:

[0075]

[0076] This reaction is exothermic (thus low temperatures favor the equilibrium toward the desired product) and it typically occurs in one or more reactors other than the methanation reactor, with a different catalytic system and operating conditions, in order to maximize the amount of hydrogen (stream F102) in the mixture before it enters the methanation reactor and before it contacts the nickel / ruthenium catalyst.

[0077] The second reaction leads to the formation of methane and water starting from carbon dioxide and hydrogen, and actually involves the reduction of carbon from the +4 oxidation state to the -4 valence state:

[0078] CO2+4H2→CH4+2H2O

[0079] For a given supply and reaction conditions, the amount of methane produced depends on the activity and selectivity of the catalyst, as well as its yield in time and space. A good catalyst must preferably show high activity, even at relatively low temperatures (which is also useful for maximizing the yield at reaction equilibrium, since the process is exothermic), and must show high selectivity for the desired compound. In the case of the methanation process, the function of the catalyst is to bias the reaction towards preferentially producing methane over other higher hydrocarbons (ethane, propane) by so-called reaction kinetic control.

[0080] To cope with the high exotherm of the reaction, a possible operating option is to operate with a high excess of one of the two reagents (hydrogen or carbon oxides): in this case, multiple methanation stages are required, interspersed with cooling of the reaction mixture, removal of the water that may have been formed in the reaction, and replenishment of the consumed reagents in order to restore the stoichiometric excess.

[0081] At the outlet of the catalytic reactor, the gas mixture is cooled, with particular attention to energy recovery (heat integration between the streams in order to favor the preheating of the feed stream at block 102 ), which in any case plays a crucial role in the overall economy of the process.

[0082] The combination of the methanation reaction and the reaction to remove the gas from the water suggests that the overall process of conversion of carbon oxides to methane proceeds with a net consumption of hydrogen and a net formation of water, purely theoretically and ignoring other possible simultaneous reactions (e.g., the formation of coke deposits, which have an adverse effect on the catalytic system).

[0083] After cooling, the biomethane gas mixture is separated from the aqueous portion contained therein so as to separate a gas stream F104 which is subjected to upgrading (block 106, generally optional) if the biomethane produced is intended for domestic or automotive use. In this respect, stream F104 corresponds to the raw biofuel which can constitute the final product of the process according to the invention; of course, if a specific biofuel, i.e. an advanced biofuel, is desired, the upgrading of block 106 is necessary. The aqueous solution forms a stream F108 which is subjected to water recovery and reverse osmosis filtration, block 108.

[0084] In detail, the condensed water is removed by simple phase separation and subjected to reverse osmosis treatment in order to obtain demineralized water. This water is recycled to the electrolyzer 110 associated with the plant implementing the second sub-process (enrichment / upgrading plant); this electrolyzer 110 is used to produce "green" hydrogen, which in this case (stream F110) serves as a supplement to maintain a stoichiometric excess in the methanogenization plant.

[0085] The water required for the electrolysis process must be demineralized: for this purpose, a plant implementing method 100 can be equipped with a pretreatment (reverse osmosis) unit for the water coming from the mains (if it is newly supplied to the electrolyzer) and for the treatment of the water produced in the methanation reaction.

[0086] In this respect, although the overall process results in a net production of water and can therefore be self-sustaining, a more rational choice convenient in the overall economics does not imply using a certain amount of fresh water (stream W_IN) from the main pipeline to cool the synthesis gas mixture leaving the methanation reactor, thus having a diluted solution (such as ) as input to the subsequent reverse osmosis treatment 108. Figure 3 This results in a stream of demineralized water W_R which is supplied to the electrolyser and a concentrate which in any case complies with the regulations for direct discharge into the sewage system (stream W_OUT) according to Italian Legislative Decree 152 / 2006.

[0087] The biofuel or e-fuel (in the mixture leaving the catalytic hydrogenation reactor (in this case, the methanation, stream F104) is separated Figure 3Biomethane, in the embodiment described in the present application, contains a small percentage of water vapor and must therefore be dehumidified if the aim is to comply with the limits set out in the aforementioned regulations (UNI EN 16723-1 / 2016 and UNI EN 16723-2 / 2017).

[0088] Furthermore, always with the aim of complying with such regulations, it is necessary to remove the unreacted hydrogen from the mixture, for which two possibilities can be considered:

[0089] - In the case of operation with excess hydrogen, the carbon oxides will be completely converted and it will be necessary to separate the unreacted hydrogen from the biomethane (for example by pressure swing adsorption (PSA) technology) in order to comply with the maximum volume limit of 0,5% for input into the main pipeline required by regulation UNI EN 16723-1 and the maximum volume limit of 2% for automotive use required by regulation UNI EN 16723-2;

[0090] - In the case of operation with excess carbon oxides, the hydrogen and possibly also the carbon monoxide are completely converted and it is necessary to separate the unreacted carbon dioxide from the biomethane (e.g. by membrane separation technology) in order to comply with the maximum volume limits of 2% and 5% set by the two aforementioned regulations.

[0091] A system for final compression of the biomethane for input into the automotive consumption network or into the national distribution mains, after conditioning of the additives and odorants added to the biomethane, completes the process 100. However, the latter operation is optional, since, in an embodiment, it is possible to obtain only the biofuel of the stream F104 as product of the process according to the invention, and therefore only to carry out catalytic hydrogenation without having to carry out further treatment in order to comply with specific regulations.

[0092] Furthermore, as previously mentioned, it is not absolutely necessary for methods 1 and 1' to be carried out in separate locations: in some embodiments, the two methods can be implemented at a single location by supplying stream F4 directly to the catalytic methanation treatment of box 104, or by supplying stream F10 directly to the catalytic hydrogenation treatment of box 104. In this case, the addition of hydrogen to stream F10 (i.e., stream F9 from electrolyzer 9) can be conveniently adjusted so as to obtain the desired stoichiometric ratio (or a slightly higher ratio, with an excess of hydrogen) to produce the desired advanced biofuel as a final product. Any excess oxygen that may be present can be discharged to the atmosphere without further purification. This is true for the supercritical water treatment of sub-methods 1 and 1' (oxidation and gasification), but in the case of supercritical water oxidation, oxygen will only be discharged to the atmosphere when the biofuel required as a final product requires a certain amount of hydrogen (involving the production of oxygen by electrolyzer 9) that is higher than the amount required for oxidation alone.

[0093] As previously mentioned, biofuels produced by catalytic hydrogenation may or may not be upgraded for regulatory compliance, as desired.

[0094] As already anticipated, according to an advantageous aspect of the invention, the hydrogen required for the methanation process preferably comes from renewable energy sources, such as photovoltaic solar plants. To this end, the electrical energy can be obtained from a photovoltaic field FV placed near the plant implementing sub-method 100.

[0095] The use of hydrogen produced from renewable resources enables the entire plant to produce so-called e-fuels, which benefit from incentives from the Energy Services Management according to current regulations.

[0096] Reference numerals E102, E104, E106, E108, E110 denote the electrical energy flows supplied to the devices of stages 102 (decompression and preheating of the synthesis gas), 104 (catalytic methanation), 106 (biomethane upgrading), 108 (water recovery from the biomethane gas mixture), 110 (electrolyzer), respectively. Each electrical energy flow E102, E104, E106, E108, E110 is part of the total flow from the photovoltaic field FV, from which an electrical energy flow can also be derived for charging the battery. In periods when the gasification reactor is not in operation, the electrical energy generated by the photovoltaic field FV can actually be used to charge the battery.

[0097] In general, it is not necessary to supply all of the aforementioned method steps with an electrical energy flow EFV coming from the photovoltaic field FV, but preferably, at least one of them (or at least one that consumes the most energy, such as the electrolyzer 110) is supplied with an electrical energy flow, and even more preferably, all of them are supplied with an electrical energy flow (i.e., the photovoltaic field supplies all the electrical energy flows E102, E104, E108, E106, E110).

[0098] Furthermore, the electrical energy flow EFV from the photovoltaic field FV can be used to supply the battery of the electrolyzer 110, which produces hydrogen by splitting water molecules by electrolysis. This solution can be an alternative to storing the energy from the photovoltaic field in batteries, preferably using it directly by overproducing hydrogen and storing it in pressurized cylinders.

[0099] It will therefore be understood that the overall methods 1, 1' and 100 according to the invention are capable of treating aqueous effluents containing organic matter from any source efficiently and without any technical disadvantages by converting the effluent into a substance for further use (e.g. "raw" biofuel / e-fuel, or as a specific biofuel for automotive use or for input into the main pipeline (if upgrading is carried out in box 106)) for recycling.

[0100] Moreover, in addition to the choice of adjusting and scaling the plants implementing methods 1, 1' and 100 according to the activity generating the aqueous effluent and according to the region in which this activity is located, the possibility of using renewable energy sources on a large scale is provided. Compared to the treatment methods of the prior art, the purification of the stream of aqueous effluent does not rely exclusively on filtration through membrane separators / concentrators, but mainly on more efficient supercritical water reactors. As a result, the energy consumption of such devices - when they are sized to treat the entire stream - is significantly reduced, as are the production, installation and maintenance costs of such devices. This advantage is further increased if the method according to the invention is implemented in a continuous operation mode, since this solution further reduces the size of all plant components, including the membrane separation / concentration devices.

[0101] Of course, the implementation details and the embodiments may vary considerably with respect to what is described and illustrated herein, without departing from the scope of the present invention as defined in the appended claims.

Claims

1. A method (1; 1', 100) for treating an aqueous effluent containing organic matter, in particular an effluent from an oil mill, comprising: - providing a stream of aqueous effluent containing organic matter (F1), - pressurizing (2, F2) the stream of aqueous effluent containing organic matter (F1) to a pressure value equal to or greater than the pressure value of the critical point of water, and heating the stream of aqueous effluent containing organic matter (F1) to a temperature value equal to or greater than the temperature value of the critical point of water, - treating the pressurized and heated aqueous effluent stream (F2) through a supercritical water reactor, - subjecting the aqueous effluent (F4) treated by the supercritical water reactor to catalytic hydrogenation, in particular catalytic hydrogenation of carbon oxides.

2. The method (1; 1', 100) according to claim 1, wherein the catalytic hydrogenation comprises catalytic methanation.

3. The method (1; 1', 100) according to claim 1, further comprising depressurizing (6) the aqueous effluent treated by the supercritical water reactor (4) before subjecting the aqueous effluent (F4) treated by the supercritical water reactor to catalytic hydrogenation.

4. according to the described method (1; 1 ', 100) of claim 1 or claim 3, it also comprises before the aqueous effluent that will be processed by supercritical water reactor (4) is carried out catalytic hydrogenation, in the material stream (F4) of the aqueous effluent processed by supercritical water reactor, add hydrogen (F9).

5. The method (1; 1', 100) according to claim 4, further comprising providing an electrolyser (9) for producing hydrogen to be added to the stream (F4) of aqueous effluent treated by the supercritical water reactor.

6. according to the method (1; 1 ', 100) described in any one of claim 1, 3 or 4, it also comprises the gaseous part (F10) of the material stream (F4) of the aqueous effluent processed by supercritical water reactor being separated from the liquid part (F5) of the material stream (F4) of the aqueous effluent processed by supercritical water reactor, the gaseous part (F10) is subjected to the catalytic hydrogenation, and the liquid part is processed to separate the water-containing part (F8, F9) from the solid part (F3).

7. A method (1; 1', 100) according to claim 6, wherein the electrolyser (9) is supplied with an aqueous portion (F8) extracted from the liquid portion (F5) of the stream (F4) of the aqueous effluent processed by the supercritical water reactor, and wherein the solid portion (F3) of the stream of the aqueous effluent processed by the supercritical water reactor is aggregated into the stream (F1) of the aqueous effluent containing organic matter supplied to the supercritical water reactor (4).

8. The method (1; 1', 100) according to claim 1 or claim 6, further comprising providing an electrolyser (110) for producing hydrogen for catalytic hydrotreatment, wherein the stream (F4) of the aqueous effluent treated by the supercritical water reactor is subjected to the catalytic hydrotreatment.

9. A method (1; 1', 100) according to any of the preceding claims, wherein at least one of the supercritical water reactor (4), the electrolyzer (9, 110) and the concentrating device (8) is powered by electrical energy generated by a photovoltaic (PV) field, and the concentrating device (8) is configured to separate the aqueous portion (F8, F9) from the solid portion (F3) of the stream (F4) of the aqueous effluent treated by the supercritical water reactor.

10. The method (1; 1', 100) according to any one of the preceding claims, wherein the supercritical water reactor comprises one of the following: -Supercritical water oxidation reactor, -Supercritical water gasification reactor.

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