Drying composition for conditioning digestates

Abstract

The present invention relates to the technical aspects of digestate treatment, resulting from the methanation process, in general, and more particularly to its conditioning prior to recovery or disposal through appropriate channels. The drying composition of the invention allows for easy transport, mixing, storage, and handling. This drying composition comprises: - at least 50% and up to 99% by weight of a superabsorbent polymer - and from 1% to 50% by weight of a flocculating polymer. The invention relates to the use of this composition for conditioning digestate while reducing the volatilization of ammonia nitrogen once the digestate has been absorbed by the composition.

Description

Title of the invention: Drying composition for conditioning digestates

[0001] The present invention relates to the technical aspects of digestate treatment from the methanation process, in general, and more particularly to its conditioning prior to recovery or disposal through appropriate channels. The drying composition of the invention allows for easy transport, mixing, storage, and handling. This drying composition comprises: - at least 50% and up to 99% by weight of a superabsorbent polymer - and from 1% to 50% by weight of a flocculating polymer. The invention also relates to the use of this composition for conditioning digestate while reducing the volatilization of ammonia nitrogen. Technical field of the invention

[0002] At the end of the methanation process, while some of the organic matter has been transformed into biogas, approximately 70 to 80% of the incoming volume remains unbiodegraded, composed of mineral and organic matter: this is the digestate. Its composition depends on the digestion process used, the nature of the materials entering the digester, and any post-treatments. The composition of the digestate corresponds to an organo-mineral fertilizer with numerous advantages that can replace chemical fertilizers.

[0003] Among the various packaging methods used to prepare digestate for valorization, the following may be mentioned, but are not limited to:

[0004] - phase separation to separate a liquid digestate, used as fertilizer, from a solid fraction, used as amendment, - composting with other organic products or green waste, - various post-treatment processes (drying, evaporation-concentration...) to concentrate the digestate.

[0005] A person skilled in the art will understand that obtaining digestate in solid form is a major technical problem in order to facilitate its removal and its valorization as fertilizer.

[0006] The invention relates to a drying composition which, at very low dosages, allows the liquid part of digestates to be absorbed in order to be able to transport, mix, store and handle them easily. Technical background

[0007] Digestates are generally concentrated, where possible, using physical treatments, called post-treatments, which are energy-intensive and therefore costly, by:

[0008] - drying, - evaporation-concentration...

[0009] To date, only a very limited number of additives have been recommended for drying digestates due to their high concentration of organic and mineral matter, which makes this conditioning step difficult. The addition of a superabsorbent polymer (SAP) to the composition has been considered. SAPs are particularly effective at absorbing water from aqueous solutions. However, it is well known to those skilled in the art that the higher the ammonia concentration of the liquid to be absorbed, as in the case of digestates, the lower the SAP's absorption capacity becomes, thus requiring significant overconsumption. In addition to the increased cost, this overconsumption also negatively impacts the digestate concentration. Presentation of the invention

[0010] The applicant has carried out numerous tests and demonstrated that when a composition based on a superabsorbent polymer and a flocculant polymer is added to a digestate in the form of a dry mixture, the drying of the digestate is not only made possible in very small proportions but is also extremely rapid.

[0011] In the context of the invention, digestate is the residue from the (anaerobic) methanation process of organic matter not converted into biogas. It can be produced from agricultural products / residues and industrial and / or municipal waste. Digestate is in the form of a more or less pasty liquid and is composed of non-mineralized organic matter and minerals, including a very high concentration of ammoniacal nitrogen, which is easily assimilated by plants.

[0012] According to the present invention, it was found, unexpectedly, that a particularly effective drying composition for the conditioning of digestates can be used provided that it comprises - at least 50% and up to 99% by weight of a superabsorbent polymer powder - and from 1% to 50% by weight of a flocculant polymer powder. Summary of the invention

[0013] The present invention relates to a powdered drying composition for optimal conditioning of digestates prior to their recovery and / or disposal through appropriate channels. The composition of the invention allows for the rapid and easy drying of digestates. This drying composition comprises:

[0014] - at least 50% and up to 99% by weight of a superabsorbent polymer powder, preferably between 70% and 95% by weight, and advantageously between 75% and 90% by weight - and 1% to 50% by weight of a flocculant polymer powder, preferably between 5% and 30% by weight, and advantageously between 10% and 25% by weight

[0015] The invention also relates to the use of the composition for conditioning digestates.

[0016] The compositions according to the invention, when added to digestates, transform them into a dry form that is transportable, storable, and handleable, eliminating all the drawbacks of the prior art. In particular, the drying of the digestate is no longer influenced by its ammonia content. Moreover, the presence of a flocculating polymer in the composition makes it possible, quite surprisingly, to significantly reduce the amount of superabsorbent polymer required to obtain a dry form. Detailed description of the invention

[0017] One object of the invention is to provide new drying compositions with optimized properties to facilitate the transport, storage, and handling of digestates in order to promote their recovery or disposal through appropriate channels. These compositions are in dry form, namely a mixture of powders, so that they can be easily transported, stored, and applied.

[0018] The present invention therefore relates to a new drying composition for digestates comprising:

[0019] - at least 50% and up to 99% by weight of a superabsorbent polymer powder, preferably between 70% and 95% by weight, and advantageously between 75% and 90% by weight - and 1% to 50% by weight of a flocculant polymer powder, preferably between 5% and 30% by weight, and advantageously between 10% and 25% by weight

[0020] The drying compositions made according to the invention, in the form of a powder, are used at a rate of 2 to 25 kg per tonne of digestate, preferably 3 to 20 kg per tonne of digestate.

[0021] Unlike conventional sludge, digestate is a product sanitized by anaerobic fermentation, thus limiting the development of pathogens, weeds, and odors. The composition of digestates depends on the nature of the inputs used in the digester: agricultural or industrial effluents, domestic wastewater, etc. The fertilizing elements N, P, K, and trace elements are present in mineralized form. Digestate is also rich in ammoniacal nitrogen, which is more easily assimilated by plants. Digestate is therefore a product similar to synthetic fertilizers and an asset for crop fertilization. However, it is very susceptible to the volatilization of the ammoniacal nitrogen it contains, which can significantly reduce its value. Fertilizer by reducing its nitrogen content. In fact, the use of energy-intensive post-treatments is made difficult because it is likely to amplify this volatilization / evaporation phenomenon.

[0022] The present invention relates to the technical aspects of digestate treatment and its "conditioning" prior to recovery or disposal through appropriate channels. The drying composition of the invention allows for improved digestate management by facilitating its transport, storage, mixing, and handling.

[0023] In practice, the superabsorbent polymer used is a water-retaining agent of natural or synthetic origin that has a water retention capacity greater than or equal to 30 times its weight in demineralized water, preferably greater than or equal to 50 times, advantageously greater than or equal to 100 times. This type of polymer is generally known by the abbreviation: SAP ("superabsorbent polymer"). It is generally in powder form, agglomerated or not. Its structure, based on a three-dimensional network resembling a multitude of small cavities, each capable of deforming and absorbing water, gives it the property of absorbing very large quantities of water and thus swelling.

[0024] Superabsorbent polymers of natural origin, usable within the scope of the present invention, are for example those described in patents US358364, US1693890, US3846404, US3935099 or US3661815... Examples include, but are not limited to: guar gum, alginates, carboxymethyl cellulose, dextran, xanthan gum...

[0025] Synthetic SAPs usable within the scope of the present invention are, for example, crosslinked or crosslinkable water-soluble polymers. Many types exist. Such polymers are, for example, described in French patent FR 2559158, which describes crosslinked polymers of acrylic or methacrylic acid, crosslinked grafted copolymers of the polysaccharide / acrylic or methacrylic acid type, crosslinked terpolymers of the acrylic or methacrylic acid / acrylamide / sulfonated acrylamide type, and their alkaline earth or alkali metal salts.

[0026] In a preferred embodiment, the monomers used for preparing the superabsorbent polymers are selected from acrylamide and / or partially or totally salified acrylic acid and / or partially or totally salified ATBS (acrylamido tert-butylsulfonate) and / or NVP (N-vinylpyrrolidone) and / or acryloylmorpholine and / or partially or totally salified itaconic acid. In a preferred embodiment, the superabsorbent polymers are anionic crosslinked homopolymers or copolymers based on partially or totally salified acrylic acid.

[0027] Other hydrophilic monomers, such as cationic monomers, but also hydrophobic monomers, can be used to produce superabsorbent polymers. Examples of cationic monomers include diallyldialkyl ammonium salts and dialkyl-aminoalkyl (meth)acrylate and dialkylaminoalkyl (meth)acrylamide monomers, as well as their quaternary ammonium or acid salts. Quaternized or salified dimethylaminoethyl acrylate (ADAME) and / or dimethylaminoethyl methacrylate (MADAME), acrylamidopropyltrimethylammonium chloride (APTAC), and / or methacrylamidopropyltrimethylammonium chloride (MAPTAC) are particularly noteworthy.

[0028] Synthetic superabsorbent polymers are generally crosslinked with 100 to 6000 ppm of at least one crosslinking agent chosen from the group including acrylic compounds such as methylene bis acrylamide, allylic compounds such as tetra-allylammonium chloride, vinyl compounds such as divinyl benzene, diepoxy compounds, metallic salts... Some may also have double crosslinking such as with an acrylic crosslinker.

[0029] For cost reasons, superabsorbent polymers of synthetic origin of the crosslinked sodium polyacrylate type will be preferred.

[0030] SAP can be obtained by all polymerization techniques well known to those skilled in the art: gel polymerization, precipitation polymerization, emulsion polymerization (aqueous or reverse) followed or not by a distillation step, suspension polymerization, solution polymerization, these polymerizations being followed or not by a step allowing isolation of a dry form of the (co)polymer by all types of means well known to those skilled in the art.

[0031] According to the invention, the superabsorbent polymer can be post-crosslinked on the surface, namely that they:

[0032] - result from the polymerization with partial crosslinking of water-soluble ethylenically unsaturated monomers comprising at least one carboxylic function, in particular acrylic and methacrylic acids and their alkali salts, whether obtained by a solution, bulk or reverse suspension polymerization process, such as described for example in patent applications EP312952, EP441507 or EP742231... - and exhibit cross-linking on the exterior of the polymer grains. This surface cross-linking is called post-cross-linking because it occurs on the SAP powder after polymerization is complete and the SAP is partially dehydrated, during drying. Post-cross-linking allows the formation of a highly cross-linked shell around the SAP particles. The SAP particles then exhibit a "core-shell" structure.

[0033] The post-crosslinking steps of hydrophilic polymers possessing car- groupsBoxyl and / or carboxylates by a multifunctional agent are well known to those skilled in the art. The dry powder is typically post-crosslinked by reacting it with other crosslinking agents such as, for example, organic crosslinking agents and / or multivalent cations, to produce a surface layer that is more strongly crosslinked compared to the interior of the particles. Commonly used processes for surface post-crosslinking include a step of contacting the base polymer with a surface post-crosslinking agent followed by a subsequent heat treatment step. This latter step is intended to complete the surface post-crosslinking, and the heat treatment is generally carried out hot, usually using continuous dryers. Examples include documents GB 2126591, EP789048, FR2923830...Many molecules described in the literature are capable of reacting quickly enough with the carboxyl groups of superabsorbent polymers (SAPs) to bridge the polymer chains. A well-known method involves post-treating the base superabsorbent polymer. This is achieved by subjecting the dried powder of the base superabsorbent polymer to additional cross-linking on a surface layer of its particles through a surface post-cross-linking step. This surface post-cross-linking increases the cross-linking density on the surface of the superabsorbent polymer particles, thereby reducing the absorption capacity of the superabsorbent polymer in the surface layer.

[0034] The invention will advantageously be implemented with superabsorbent polymer particles, spherical or not, whose average diameter is generally between 5 µm and 5000 µm, and preferably between 100 and 250 µm. This water-retaining polymer (or the mixture of water-retaining polymers) represents from 50% to 99% by weight of the latter, preferably from 70% to 95% by weight, and advantageously more than 75% and less than 90% by weight of the drying composition according to the invention. Advantageously, the SAP will have a particle size similar to that of the flocculating polymer powder also present in the composition.

[0035] In practice, the flocculant polymer constituting one of the two essential elements of the composition according to the invention is a high molecular weight, water-soluble anionic or non-ionic (co)polymer in powder form, with a molecular weight greater than 10⁵ g / mol and preferably greater than 10⁶ g / mol. The high molecular weight, water-soluble polymer was obtained by polymerization of at least one monomer selected from:

[0036] - non-ionic monomers: water-soluble vinyl monomers. Preferred monomers belonging to this class include acrylamide and methacrylamide, acrylamide derivatives such as N-alkylacrylamide (e.g., N-isopropylacrylamide), N-tert-butylacrylamide, octylacrylamide, and N, N-dialkylacrylamides such as NN-dimethylacrylamide and N-methylolacrylamide can be used. Vinylformamide, N-vinylpyridine, N-vinylpyrrolidone, hydroxyalkyl acrylates, and methacrylates can also be used. Acrylamide is a preferred nonionic vinyl monomer.

[0037] - and / or anionic monomers exhibiting acrylic functionalities, vi nylic, maleic, fumaric, allylic, and contain a carboxy, phosphonate, sulfonate, or other anionically charged group, or the ammonium or alkali earth metal or corresponding alkali metal salt of such a monomer. These include acrylic acid, methacrylic acid, acrylamidomethylpropanesulfonic acid, acrylamidomethylbutanoic acid, maleic acid, fumaric acid, itaconic acid, vinylsulfonic acid, styrenesulfonic acid, vinylphosphonic acid, allylsulfonic acid, allylphosphonic acid, and their water-soluble salts of an alkali metal, an alkali earth metal, and ammonium.

[0038] The invention will advantageously be implemented with flocculant polymer particles, spherical or not, with an average diameter generally between 5 µm and 5000 µm, and preferably between 100 and 250 µm. The flocculant polymer (or the mixture of flocculant polymers) represents from 1% to 50% by weight of the latter, preferably from 5% to 30% by weight, and advantageously more than 10% and less than 25% by weight of the drying composition according to the invention. Advantageously, the flocculant polymer will have a particle size similar to that of the superabsorbent polymer powder also present in the composition.Advantageously, the flocculating polymer shall consist of the same monomers as the superabsorbent polymer used in the composition, these monomers being preferably chosen partly from among the monomers of acrylic acid and / or its water-soluble salts, and advantageously exclusively from the monomers of acrylic acid and / or its water-soluble salts.

[0039] The superabsorbent polymer and the flocculant polymer are in the form of a homogeneous mixture and are therefore introduced simultaneously. This constitutes a true “composition”.

[0040] The composition of the invention is added directly to the digestate by mechanical mixing (stirring screw, mechanical shovel, etc.) and provides an optimized drying and agglomerating effect. It has been observed that the composition produces combined effects far superior to the separate addition of the components. The drying composition of the invention has the property of dramatically increasing the retention capacity of liquid rich in ammoniacal nitrogen (NH4+) and significantly reducing the volatilization of the latter in the form of NH3 gas. This result was entirely unexpected since the presence of ammonia leads In most cases, the inactivation of SAPs (superabsorbent polymers) is achieved by limiting their swelling capacity, so those skilled in the art had no incentive to use this type of polymer to solve the problem at hand. The presence of an anionic or non-ionic flocculant polymer appears, for reasons that remain unclear, to have a beneficial effect on the superabsorbent polymer by reducing its sensitivity to ammonia. There is therefore a genuine and unexpected synergy between the two ingredients in the composition, especially since this combination helps reduce the volatilization of ammonia nitrogen once the digestate has been absorbed by the composition.

[0041] The present invention also relates to any variant or adaptation which will become clear to a person skilled in the art, if necessary by resorting to some routine trials. Examples

[0042] Comparative tests were carried out using the following compounds:

[0043] • Superabsorbent polymers (SAPs): as described in Table 1 below • Flocculant polymer (PF): brand “APROFLOC” ALP99: hy polymer high molecular weight (5.106 g / mol) anionic soluble copolymer of sodium acrylate and acrylic acid.

[0044] [Tables 1] Product Reference Superabsorbent Polymer Type Trade Name Anionicity Average Particle Sizes of SAP P150 Crosslinked Sodium Polyacrylate Brand “Apromud” P150 100% Anionic < 300 µm G300 Crosslinked and Post-Crosslinked Sodium Polyacrylate Brand “Apromud” G300 100% Anionic 500 - 850 µm

[0045] Drying tests (solidification / stabilization) were conducted on two digestates from methanization units from 2 different agricultural holdings annotated in the table dig-UM1 and dig-UM2.

[0046] For the realization of the examples presented in Table 2, the mixing of the drying agents with the digestates was carried out mechanically until visual homogenization.

[0047] [Tables2] Nature of digestate Composition of drying agent (SAP and PF) expressed by weight % Dosage of drying agent expressed in kg per tonne of sludge Digestate drying / Solidification (YES or NO) Counter-example 1 dig-UMl P150 (100%) 10 kg / tonne NO Counter-example 2 dig-UMl G300 (100%) 10 kg / tonne NO Counter-example 3 dig-UM2 P150 (100%) 10 kg / tonne NO Counter-example 4 dig-UM2 G300 (100%) 10 kg / tonne NO Counter-example 5 dig-UM2 G300 (100%) >30 kg / tonne YES Example 1 dig-UMl P150 (95%) / PF (5%) 10 kg / tonne YES Example 2 dig-UM2 P150 (88%) / PF (12%) 10 kg / tonne YES Example 3 dig-UMl P150 (80%) / PF (20%) 10 kg / tonne YES Example 4 dig-UM2 G300 (95%) / PF (5%) 10 kg / tonne YES Example 5 dig-UMl G300 (88%) / PF (12%) 10 kg / tonne YES Example 6 dig-UM2 G300 (80%) / PF (20%) 10 kg / tonne YES Counter-example 6 Sequenced addition dig-UM2 P150 (88%) added first then PF (12%) added secondly 8.8 kg / tonne of P150 then l,2kg / tonne of PF NO Counter-example pie 7 Sequenced addition dig-UM2 PF (12%) added first then P150 (88%) added secondly 1.2kg / tonne of PF then 8.8kg / tonne of P150 NO ,

[0048] As can be seen in Table 2, compared to the addition of superabsorbent alone, regardless of the superabsorbent polymer used, the compositions according to the invention allow the digestates to be dried while drastically reducing the amount of superabsorbent required to achieve this result. Examples They further clearly indicate that when the ingredients of the composition according to the invention are not added as a dry mixture but in fractions, drying of the digestate is no longer possible at the same dosages. It appears that, unexpectedly, a synergistic effect occurs when the SAP and the flocculating polymer are added as a mixture.

[0049] We were also able to visually observe that this drying process was extremely rapid. The drying compositions of the invention are simultaneously simple (only one mixing point is required), quick to prepare, and effective. They thus perfectly meet the needs of industry to facilitate the "conditioning" of the materials before their recovery or disposal through appropriate channels. The drying composition of the invention allows for easy transport, mixing, storage, and handling.

[0050] It was also observed during the tests that the digestates, naturally rich in ammoniacal nitrogen (NH4+), conditioned by the compositions of the invention have a very strongly attenuated ammonia (NH3) odor, unlike the counter-examples, thus confirming their positive effect in controlling the harmful phenomenon of ammoniacal nitrogen volatilization.

Claims

Demands

1. Use of a drying composition for the conditioning of digestates from methanation processes characterized in that said drying composition comprises a mixture consisting of: - at least 50% and up to 99% by weight of a superabsorbent polymer powder, preferably between 70% and 95% by weight, and advantageously between 75% and 90% by weight, characterized in that said superabsorbent polymer is a homopolymer or a crosslinked synthetic copolymer comprising partially or totally salified acrylic acid, of the crosslinked sodium polyacrylate type, having a water retention capacity greater than or equal to 30 times its weight in demineralized water, preferably greater than or equal to 50 times - and 1% to 50% by weight of a flocculant polymer powder, preferably between 5% and 30% by weight, and advantageously between 10% and 25% by weight,characterized in that the flocculant polymer is a water-soluble anionic or non-ionic (co)polymer with a high molecular weight greater than 105 g / mol and preferably greater than 106 g / mol.

2. Use of a drying composition for the conditioning of digestates from methanization processes according to any one of the preceding claims in which said superabsorbent polymer and flocculant polymer powders have a particle size between 5 pm and 5000 pm and preferably between 100 and 250 pm.

3. Use of the drying composition according to any one of claims 1 to 2 for the conditioning of digestates from methanization processes characterized in that the flocculant polymer is made up of the same monomers as the superabsorbent polymer used in the composition, these monomers being chosen preferably partly from acrylic acid monomers and / or its water-soluble salts, and advantageously exclusively from acrylic acid monomers and / or its water-soluble salts.

4. Use of the drying composition according to any one of claims 1 to 3 for conditioning digestates from methanization processes at a rate of 2 to 25 kg per tonne of digestates.