The present invention relates to the utilization of waste streams of microbial fermentation processes. It involves the production of biogas and its conversion to syngas.

Currently, the chemical industry worldwide strives to move away from fossil resources as raw materials and to use renewable feedstock instead. One major route to do so is the biotechnological production of basic chemicals based on crops, e. g. sugar, corn or lignin. Production is typically realized via fermentation using dedicated microorganisms.

However, those fermentation processes are inherently associated with the generation of liquid and solid waste. Wastewater origins from various sources, e. g. from the mother liquor of the product separation process (e. g. by crystallization), from washing stages, or from cleaning processes. Typically, it contains a significant amount of organic matter. The solid waste consists mainly of the fermentation biomass (microorganisms). Conventional discharge routes for the liquid as well as for the solid waste, e. g. drying / incineration for the fermentation residue and aerobic treatment for the wastewater, exhibit high costs as well as negative environmental effects.

Apart from the generation of liquid and solid waste, aerobic fermentation processes result in high CO2- emissions that reduce the advantage in terms of carbon footprint. Further CO2 emissions might result from downstream chemical processing.

A first step towards more cost efficient and environmentally benign fermentation processes would be the use of anaerobic digestion for the waste streams. For wastewater from Lysine production, this has been suggested (Moacir Messias de Araijjo Jr. and Marcelo Zaiat (2009): An upflow fixed-bed anaerobic-aerobic reactor for removal of organic matter and nitrogen from L-lysine plant wastewater. Can. J. Civ. Eng. 36, 1085-1094). However, an implementation is currently hindered by the relatively low value of the biogas which is used energetically, e. g. for electricity generation or in combined heat and power plants (CHP). It has been realized recently, that a higher value could be obtained from the biogas if it was used as feedstock for renewable raw materials. This is suggested by EP 2 257 633 for production of ethanol and by WO 2016/101076 for the production of synthetic fuels via 2-stage anaerobic digestion and Fischer-Tropsch synthesis. However, all these processes exhibit a high complexity while at the same time generating only a fuel substitute. Furthermore, these processes do not reduce the CO2 emissions of the fermentation processes. Even worse in this regard is the approach to convert the biogas to biomethane by separating CO2. This process increases the CO2 emissions further, in addition to exhibiting high costs and complexity as well.

US 10,640,793 discloses a process for increasing the efficiency of biofuel production. The CO2 which is emitted during the generation of a biofuel by microbial fermentation is reacted with hydrogen in order to convert the excess CO2 into a product as well. The source of the additional hydrogen is natural gas. Thus, this process requires the separation and permanent storage of the fossil CO2 which is generated as by-product of ^-production.

US 9,315,735 discloses a method for producing syngas using an A/C plasma reactor. This process also involves the production of biogas by anaerobic fermentation. This biogas may be used for heat generation or it may be fed into the process in order to increase the concentration of H2. The source of the biogas may be any biodegradable waste. A closed carbon circle, where a product is produced in a first fermentation and the carbon comprised by the by-products of this fermentation (microbial biomass, wastewater and / or CO2) is also used for product-formation is not disclosed.

Handler et al. (2016), Ind. Eng. Chem. Res. 55: 3253-3261 describe the use of bacteria for the conversion of CO-containing gas streams into ethanol. In this process carbon monoxide is an educt and not the desired product.

Compared to these state-of-the-art approaches, a new process was found that generates higher value out of the waste by converting it into a true building block for chemical production and at the same time uses at least part of the carbon dioxide.

Therefore, in a first embodiment, the present invention relates to a method comprising the steps of a) anaerobic treatment of at least one waste stream selected from the group consisting of solid, aqueous and gaseous waste streams originating from at least one microbial fermentation process, wherein said anaerobic treatment results in biogas formation; b) reforming the biogas so that a syngas comprising carbon monoxide and optionally hydrogen is generated; and c) using the carbon monoxide comprised by the syngas generated in method step b) for the synthesis of phosgene; wherein CO2 is added to the biogas prior to or during the reforming in method step b).

Fermentation process

The term "fermentation process" refers to any biotechnological production process which involves the growth of microbial cells on at least one substrate which can be utilized by said microbial cells to build new biomass. Aerobic and anaerobic fermentation processes are equally preferred as both generate waste streams as defined below in this application. Preferred are those fermentation processes, where the microbial biomass as such is not the desired product and only serves as catalyst for the production of a fermentation product other than microbial cells. In such processes the microbial biomass is a waste product which has to be disposed of. Typically, such fermentation processes generate in addition to the desired product at least one low molecular weight compound selected from the group consisting of alcohols, sugar alcohols, organic acids, sugars, antibiotics, amines and amino acids. These compounds may be excreted by the microbial cells during growth and/or production of the desired product or they may be nutrients originally added to the fermentation broth which were not fully taken up by the microbial cells. Proteins are further typical waste products due to the same mechanisms as described for small molecules.

Preferred amino acids are selected from the list consisting of ortho-amino benzoic acid, para-amino- benzoic acid, lysine, glutamic acid, methionine, threonine, tryptophan and tyrosine.

Preferred alcohols are selected from the list consisting of ethanol, butane diol, isopropanol and isobutanol.

Preferred amines are monoamines and diamines, particularly pentane diamine.

Preferred organic acids are selected from the list consisting of citric acid, lactic acid and acetic acid. Preferred vitamins are selected from the list consisting of B2, B12, A and C.

Waste stream

The term "waste stream" refers to any solid, liquid or gaseous product of the fermentation which is not the desired product or does not comprise the desired product in a concentration which warrants further treatment to isolate the desired product therefrom. Preferred waste streams are liquid or solid.

Liquid waste streams are derived from the fermentation broth used in the fermentation. After the isolation of the desired product from the fermentation broth a liquid with a high organic carbon content remains as the fermentation broth, which may comprise undesired by-products and/or residual nutrients. Depending on the downstream processing used for isolating the desired product the liquid waste stream may or may not comprise microbial cells.

Solid waste streams comprise the microbial biomass used or generated in the fermentation process. If said biomass is separated from the fermentation broth in the course of downstream processing after the fermentation, a solid waste stream originates. In the context of the present invention it does not matter if microbial biomass comprised by a waste stream is viable or not. It comprises organic carbon and, thus, its decomposition increases biological oxygen demand during wastewater treatment. Thus, it has to be disposed of.

In a preferred embodiment of the present invention at least one liquid or solid waste stream originating from the microbial fermentation process are subjected to the anaerobic treatment method step a).

Anaerobic treatment

The term "anaerobic treatment of a waste stream" refers to the process of incubating this waste stream under conditions which allow for the biological degradation of at least some of its organic carbon content resulting in the generation of biogas. The main requirement for such a process is the presence of viable microbial consortia capable of methane formation under physicochemical conditions such as temperature, salinity and pH conductive for metabolic activity of said consortia. Methods for the production of biogas from organic matter are well known to the person skilled in the art (Aivasidis, A. (2012): Wastewater, 3. Anaerobic Biological Treatment. Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, pp. 639-665; Rosenwinkel, K.-H.; Kroiss, H.; Dichtl, N.; Seyfried, C.-F.; Weiland, P. (Hrsg.) (2015): Anaerobtechnik. 3. Edition, Springer)

Biogas

The term "biogas" refers to the gaseous product of the anaerobic degradation of organic matter. In a preferred embodiment of the present invention at least one biogas stream is obtained from anaerobic digestion of at least one liquid and at least one biogas stream is obtained from anaerobic digestion of at least solid waste stream, wherein all waste streams originate from the same microbial fermentation process. Subsequently all biogas streams are combined and then subjected to further purification processes and the reforming step a).

Biogas comprises methane and carbon dioxide. Typically, hydrogen sulfide and ammonia are also present as by-products.

As hydrogen sulfide or ammonia may be detrimental to the reforming step, it is preferred to purify the biogas prior to the reforming step. Thus, in a preferred embodiment of the present invention, the content of at least one component selected from the group consisting of hydrogen sulfide, ammonia, organic amines, siloxanes, mercaptanes, halogenated compounds and hydrocarbons other than methane in the biogas is decreased before the reforming step. Preferably, the decreased content is lower than the concentrations defined in the next paragraph. It is preferred that the concentration of hydrogen sulfide in biogas entering the reforming step does not exceed 1 ppm. It is preferred that the concentration of ammonia in biogas entering the reforming step does not exceed 3 ppm. It is particular preferred that concentrations of both compounds do not exceed the aforementioned concentrations.

Reforming step

The reforming step involves the reformation of biogas resulting in the formation of syngas. Syngas comprises carbon monoxide and hydrogen. It is formed by the addition of steam:

ChU + H2O <- CO + 3 H2 (steam reforming reaction)

In reality, this reaction doesn't take place alone, but competes with several other reactions, the most important one being

CO + H2O <- CO2 + H2 (water gas shift reaction)

Compared to state of the art, in the present invention this reaction is shifted to the reverse direction by the presence of CO2 in the biogas.

In order to further influence the ratio of carbon monoxide to hydrogen in the syngas formed in the reforming step, in a preferred embodiment of the present invention additional carbon dioxide is added to the biogas before or during the reforming step, so that at least some of the formed hydrogen is converted to carbon monoxide and water.

CO2 + H2 <- CO + H2O (reverse water gas shift reaction)

The added carbon dioxide may be derived from the raw syngas after separation and recycling, or from any source. However, it is preferred to use carbon dioxide originating from a microbial fermentation process, more preferably a microbial fermentation process which yields one of the waste streams used in method step a). Thus, the net carbon dioxide emissions caused by the microbial fermentation process can be decreased by recycling the inevitably formed carbon dioxide.

In one particular preferred embodiment, the amount of carbon dioxide added before or during the reforming step is selected such that only carbon monoxide is produced. Thus, the amount of carbon dioxide added or recycled must be approximately equimolar to the amount of hydrogen generated from methane. Therefore, in a preferred embodiment of the present invention the molar ratio of C02 to CH4 comprised by the biogas at the beginning of method step b) is between 0.7 : 1 and 1.3 : 1, more preferably 0.8 : 1 to 1.1 : 1. In a preferred embodiment, any excess hydrogen, that might inevitably result from the reaction due to reaction thermodynamics and kinetics, is used to supply heat to the reaction in the reforming step.

In another particularly preferred embodiment of the present invention, the amount of carbon dioxide added or recycled before or during the reforming step is selected such that a carbon monoxide and hydrogen are produced in a molar ratio between 1 : 2 and 1 : 3. This ratio is particularly suitable for the manufacture of polyurethanes, related to the stoichiometric requirements of hydrogen for hydrogenation of nitrobenzene to aniline or dinitrotoluene to diaminotoluene and phosgenation of methylenedianiline to methylenediphenylisocyanate or toluenedianiline to toluenediisocyanate.

In a particularly preferred embodiment of the present invention, more than 90% of the carbon dioxide added before or during the reforming step originate from method step a) and at least 90 % of any carbon dioxide originating from method step a) not used in the reforming step are subjected to electrolysis to form carbon monoxide.

The method of the present invention does not necessarily depend on the combination of gas streams from different sources. It can provide a self-contained method for utilizing the biogas formed by anaerobic degradation of microbial biomass for the production of a chemical raw material. Therefore, in a preferred embodiment of the present invention at least 50 mol-% of the methane used in the reforming step are derived from the anaerobic digestion of biomass. More preferably, this share is at least 70 mol-%, even more preferably at least 80 mol-% and most preferably at least 90 mol-%. In a particularly preferred embodiment no methane from sources other than the anaerobic digestion of biomass is used. With regard to this embodiment, the term biomass may refer to any organic matter directly or indirectly derived from renewable sources and is not limited to microbial biomass, i.e. if the microbial biomass used in method step a) is not sufficient for an efficient use of a steam reformer, gas streams originating from other anaerobic digestions may be added. Such additional biomass is preferably plants or parts of plants. However, in a particular preferred embodiment the biomass originates from the anaerobic treatment in method step a).

Synthesis of chemical raw materials

The syngas resulting from the reforming step is preferably used for the production of chemical raw materials in an additional method step c). "Chemical raw materials" as understood in the present application are chemical compounds which are chemically converted to another compound in at least one reaction step. Preferred are compounds having a molecular weight between 80 g/mol and 1,000 g/mol. Preferred raw materials which can be produced using hydrogen and carbon monoxide are methanol, fatty acids, liquid fuels (medium to long chain alkane compounds to replace diesel, petrol or jet fuel, exhibiting identical combustion properties) and oxoalcohols.

In one preferred embodiment of the present invention, this production is performed using the Fischer- Tropsch-process.

In another preferred embodiment of the present invention, the syngas is converted to phosgene. For this embodiment it is particularly preferred to adjust the stoichiometry of carbon dioxide and methane in the biogas before or during the reforming step in such a way that the molar ratio of carbon monoxide to hydrogen in the syngas is at least 1 : 1.5, preferably at least 1 : 1, most preferably at least 2 : 1. For this embodiment it is particularly preferred to separate the hydrogen from the syngas and use it for heating of the reforming step, resulting in a final syngas with the aforementioned CO contents. For this embodiment it is particularly preferred that at least one microbial fermentation process produces an amine, preferably a diamine, and that at least a part of the phosgene produced in method step c) is used for phosgenation of said amine.

Advantages

The method of the present invention aims at providing a closed carbon cycle for biogas. In order to achieve this effect it is essential to convert as much carbon as possible to carbon monoxide. Steam reforming of biogas leads to the formation of carbon monoxide and hydrogen. These two compounds react in a water gas shift reaction to hydrogen and carbon dioxide. Thus, it is essential to prevent the second reaction as far as possible. To this end, carbon dioxide may be added to the biogas. This addition shifts the equilibrium of the water gas shift reaction towards carbon monoxide and water. Therefore, carbon dioxide emissions are decreased as the hydrogen generated during steam reforming is used to produce more carbon monoxide from externally added carbon dioxide. The resulting carbon monoxide can then be used as a chemical raw material, e.g. for the manufacture of phosgene (and further isocyanates or polycarbonates). This use of biogas creates a temporary sink for any carbon used in fermentation processes and increases the carbon balance for any such process

Modification of low molecular weight compounds

The term "modification" of low molecular weight compounds refers to the process of changing the chemical structure of a low molecular weight compound using at least one component of the syngas. Said component is preferably carbon monoxide and/or hydrogen, more preferably carbon monoxide. A "small molecule" as understood by the present invention is an organic molecule having a mass between 16 g/mol and 2,000 g/mol, more preferably between 16 g/mol and 1,000 g/mol. The term "modification" encompasses all structural changes of organic molecules which can be effected by carbon monoxide and/or hydrogen. Preferred modifications using carbon monoxide are the introduction of carbonyl and carboxyl groups. Preferred products are propionic acid, dimethyl formamide and acetic acid. Preferred modifications using hydrogen are hydrogenations of alkenes and alkynes as well as reductions of carboxylic or carbonyl groups.

Figure 1: General process scheme for the utilization of waste streams from fermentation for the manufacture of syngas

Figure 2: Results from the anaerobic degradation of concentrated wastewater obtained from the fermentative production of anthranilic acid; COD (centrif.): COD after centrifugation, i.e. dissolved COD; VFA: volatile fatty acids

Figure 3: Results from the anaerobic degradation of diluted wastewater obtained from the fermentative production of anthranilic acid; COD (centrif.): COD after centrifugation, i.e. dissolved COD; VFA: volatile fatty acids

The following examples are merely intended to illustrate the invention. They shall not limit the scope of the claims in any way.

Examples

Example la: Mass balance of industrial biobased aniline production

Calculation of a mass balance for a hypothetical 300 kt/a biobased aniline plant resulted in the following mass flow of gases set forth in table 1 below. The process involves the production of anthranilic acid by microbial fermentation followed by a chemical decarboxylation of the anthranilic acid.

Table 1: mass flow of gases in industrial-scale biobased aniline production

With these streams as feed gas, 11.000 Nm ³ /h of CO can be generated in a reformer set-up that is designed for maximized CO generation. Approx. 7.500 Nm ³ /h of CO2 and thus more than 50 % of the emissions from the biobased aniline process can be used in this case.

Example lb: Mass balance of industrial biobased aniline production

In another process configuration of an industrial biobased aniline plant the following mass balance as set forth in table 2 below is derived. The balance shows the relevant gas flows for the reforming step, including input streams from fermentation and anaerobic digestion and output stream to phosgene synthesis. The max. and normal case considers the variation in fermentation (CO ₂ generation) and anaerobic digestion (conversion factor of organic feed to biogas output and CH ₄ /CO ₂ ratio).

Table 2: mass balance of the reforming step for biobased aniline production As can be seen, not only all of the C02 from the biogas can be used in the reforming step, but additionally nearly half this amount from fermentation.

Example 2a: Degradability of concentrated waste water from aniline production (lab fermentation)

The anaerobic biodegradability of wastewater from biobased anthranilic production has been investigated with a sample as described in table 3 below. The water was obtained from laboratory- scale production of anthranilic acid.

Table 3: Composition of wastewater from biobased production of anthranilic acid

Non-adapted granular sludge (approx. 7.4 g VSS L ¹ ) was incubated with the pH-neutralized raw sample, diluted 4.7 times to an initial chemical oxygen demand (COD) of ca. 6 g L ¹ . Sodium bicarbonate (5 g L ^x ) was added to the batch bottles to ensure sufficient pH buffering during the course of the experiment. Figure 2 shows the result of the test and a substrate-free blank.

The curves of the test show rapid production of a limited volume of methane (digestion of ca. 30 % of the wastewater's COD) during the first days of incubation, corresponding to conversion the samples' SCOD. Following a certain lag period, methane production picked up again, which indicates adaptation of the seed sludge. A continuous-flow system will readily convert the whole fraction.

Example 2b: Degradation of diluted wastwater

A similar wastewater sample as described above was tested with the same procedure but diluted 15.2 times to an initial COD of 2 g L ¹ . Figure 3 shows the result of this test and a substrate-free blank

A similar pattern to example 2a is found, indicating that a combined treatment with a wastewater of 4.7 times the flow is sufficient for this wastewater from biobased aniline process.