Fermentation and product separation

Abstract

A fermentation process for producing a target organic acid is provided. The process comprises: (i) a fermentation step comprising fermentation of one or more carbon sources by a microorganism in a fermentation vessel to produce a first product stream comprising a salt of the target organic acid. The process further comprises: (ii) a first separation step comprising ion exchange of the first product stream to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin to produce a second product stream comprising the target organic acid. The process further comprises: (iii) a second separation step comprising separating the second product stream into an aqueous phase stream and an organic phase product stream by liquid-liquid extraction, the organic phase product stream containing the target organic acid. Also provided is a fermentation system.

Description

[0001] Fermentation and Product Separation

[0002] Field of the Invention

[0003] The present invention relates to a fermentation process for producing one or more organic acids, as well as a fermentation system for producing one or more organic acids.

[0004] Background

[0005] Many chemicals require fossil fuels for their production. At the same time, many industrial processes produce carbon dioxide (CO2), and solutions for effective carbon capture from such processes are being sought. Acetogenic bacteria are a group of bacteria capable of utilising CO2 (or carbon monoxide (CO)) as the sole carbon source, or a combination of CO2 and CO as carbon sources. The use of acetogenic bacteria has therefore been proposed for capturing CO2 from industrial processes. At the same time, these bacteria can also be used to produce useful chemicals, such as acetic acid.

[0006] Growth of such bacteria is typically conducted by fermentation processes using aqueous fermentation broths. During such fermentation processes, acid and / or base is often added to the fermentation broth in order to maintain the pH of the fermentation broth within a range suitable for growth of said bacteria. Once the concentration of a desired fermentation product in the fermentation broth has reached a target value, fermentation broth can be drawn from the fermentation vessel and the fermentation product extracted therefrom through one or more separation processes.

[0007] Fermentation broths typically contain a wide range of different compounds (e.g. salts, carbohydrates, alcohols, acids, cells, bases, water, etc.) that can make separation of the target product from the other compounds difficult and can restrict the ability to recycle waste streams back into the fermentation process due to insufficient purity of such streams.

[0008] The present invention has been devised in light of the above considerations.

[0009] Summary of the Invention

[0010] In a first aspect there is provided a fermentation process for producing a target organic acid, the process comprising: (i) a fermentation step comprising fermentation of one or more carbon sources by a microorganism in a fermentation vessel to produce a first product stream comprising a salt of the target organic acid; (ii) a first separation step comprising ion exchange of the first product stream to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin to produce a second product stream comprising the target organic acid; and (iii) a second separation step comprising separating the second product stream into an aqueous phase stream and an organic phase product stream by liquid-liquid extraction (LLE), the organic phase product stream containing the target organic acid.A fermentation process producing a conjugate base of an organic acid typically requires the addition of large quantities of base to the fermentation vessel in order to control the pH of the fermentation broth within appropriate bounds for the fermentation, because the endogenous production of the target organic acid otherwise results in the pH value dropping below the range suitable for fermentation by the microorganism. Addition of base to the fermentation vessel results in the formation of large quantities of salt of the target organic acid. As well as extracting the target organic acid product from the first product stream (where it is present in its conjugate base form), to reduce wastage of feedstocks for the process, it is desirable to recover the base from the first product stream. Accordingly, the process according to the first aspect provides efficient shifting of the organic acid into its free-acid form via the first separation step by exchanging the cations of the salt of the target organic acid in the first product stream with hydrogen ions, which in turn facilitates separation of the target organic acid by LLE from the inorganic byproducts (e.g. the (inorganic) base) in the second product stream.

[0011] It can be understood that the conjugate base of the target organic acid is the deprotonated, negatively-charged ion form of the target organic acid (e.g. where the target organic acid is acetic acid (CH3COOH), the conjugate base of the target organic acid is acetate (CH3COO)). It can be understood that the salt of the target organic acid is the conjugate base of the target organic acid, (notionally) paired with a positive ion / cation (e.g. Na+, Mg2+etc.) (in practice, in solution the salt will be almost entirely dissociated into its constituent ions). It can be understood that the salt of the target organic acid comprised in the first product stream is the salt form of the target organic acid comprised in the second product stream, and which the fermentation process is intended to produce.

[0012] The fermentation step produces a first product stream that predominantly comprises the target organic acid in its salt / conjugate base form. However, it can be understood that, depending on the pH of the first product stream, some of the target organic acid may also be present in its free acid form in the first product stream.

[0013] The fermentation process may be a gas fermentation process. Accordingly, the one or more carbon sources may comprise, or consist of, one or more gases.

[0014] The first product stream may comprise dissolved solids including biomass (e.g. the microorganism(s)) and one or more salts. The one or more salts and the biomass (e.g. the microorganism(s)) may constitute 85% or more of the dissolved solids on a mass basis, 90% or more of the dissolved solids on a mass basis, 95% or more of the dissolved solids on a mass basis, or 99% or more of the dissolved solids on a mass basis. The one or more salts may constitute 80% or more of the dissolved solids on a mass basis, 85% or more of the dissolved solids on a mass basis, or 90% or more of the dissolved solids on a mass basis. The one or more salts may constitute 95% or less of the dissolved solids on a mass basis, 90% or less of the dissolved solids on a mass basis, e.g. between 80% and 95% of the dissolved solids on a mass basis. The biomass (e.g. the microorganism(s)) may constitute 5% or more of the dissolved solids on a mass basis, 10% or more of the dissolved solids on a mass basis, or 15% or more of the dissolved solids on a mass basis. The biomass (e.g. the microorganism(s)) may constitute 20% or less of the dissolved solids on a mass basis, or less than or equal to 15% on a mass basis, e.g. between 5% and 20% on a mass basis.The gas fermentation process may comprise fermentation with a microorganism capable of metabolising carbon monoxide (CO) and / or carbon dioxide (CO2) into one or more target organic acids, optionally in the presence of hydrogen gas (H2). The process may comprise providing one or more gases to the fermentation as a substrate, e.g. the microorganism may grow on said one or more gases as the main carbon source. The one or more gases may comprise CO2, CO or a mixture thereof. The one or more gases may further comprise hydrogen (H2) (i.e. the one or more gases may comprise gases that are not carbon sources). The one or more gases may be syngas (i.e. a mixture comprising CO and H2, and optionally further comprising CO2, methane (CH4) and / or trace gases). In this way, the fermentation can produce the target organic acid(s) from byproduct gases from industrial processes and / or from gasification of biomass.

[0015] The fermentation process may comprise fermentation with a microorganism capable of metabolising C1 compounds (e.g. C1 hydrocarbons, such as methanol and / or formate), optionally in the presence of H2, CO2 and / or CO and, optionally, trace gases. The process may comprise providing one or more C1 compounds to the fermentation as a substrate, e.g. the thermophilic microorganism may grow on said one or more C1 compounds as the main carbon source.

[0016] Trace gases may have a volumetric / molar concentration of less than or equal to 1 % in the gas.

[0017] The gas fermentation may comprise gas fermentation with an organic acid-producing microorganism in a fermentation broth.

[0018] The microorganism may be thermophilic. The fermentation step may comprise maintaining the temperature of the fermentation broth above 45°C. Such temperatures may increase the production rate of the target organic acid in the fermentation step. For example, the thermophilic microorganism may be selected from the group consisting of: thermophilic acetogens; thermophilic propionibacteria; thermophilic butyric acid bacteria; thermophilic lactic acid bacteria; thermophilic succinic acid bacteria; and hydrogen oxidising bacteria; thermophilic pyruvic acid bacteria; thermophilic tartaric acid bacteria; thermophilic isobutyric acid bacteria; thermophilic hydroxybutyric acid bacteria; and thermophilic hydroxyvaleric acid bacteria. Preferably, the thermophilic microorganism may be selected from the group consisting of: thermophilic acetogens; thermophilic propionibacteria; thermophilic butyric acid bacteria; thermophilic lactic acid bacteria; thermophilic succinic acid bacteria; and hydrogen oxidising bacteria.

[0019] Alternatively, the microorganism may be mesophilic. For example, the microorganism may be selected from the group consisting of: mesophilic acetogens; mesophilic propionibacteria; mesophilic butyric acid bacteria; mesophilic lactic acid bacteria; mesophilic succinic acid bacteria; and hydrogen oxidising bacteria; mesophilic pyruvic acid bacteria; mesophilic tartaric acid bacteria; mesophilic iso-butyric acid bacteria; mesophilic hydroxybutyric acid bacteria; and mesophilic hydroxyvaleric acid bacteria. Preferably, the mesophilic microorganism may be selected from the group consisting of: mesophilic acetogens; mesophilic propionibacteria; mesophilic butyric acid bacteria; mesophilic lactic acid bacteria; mesophilic succinic acid bacteria; and hydrogen oxidising bacteria.

[0020] An acetogen may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into acetyl-coenzyme-A. A propionibacterium may be defined as a microorganism having a metabolismconverting CO2 (with H2) or CO into propionyl-coenzyme-A. A butyric acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into butyryl-coenzyme-A. A lactic acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into lactyl-coenzyme-A. A succinic acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into succinyl-coenzyme-A. A hydrogen-oxidising bacterium may be defined as a microorganism having a metabolism capable of using hydrogen as an electron donor. A pyruvic acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into pyruvyl-coenzyme-A. A tartaric acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into tartryl-coenzyme-A. An iso-butyric acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into isobutyryl-coenzyme-A. A hydroxybutyric acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into hydroxybutyryl-coenzyme-A. A hydroxyvaleric acid bacterium may be defined as a microorganism having a metabolism converting CO2 (with H2) or CO into hydroxyvaleryl-coenzyme-A.

[0021] In one example, a butyric acid bacterium may be defined as a bacterium capable of producing butyric acid. A lactic acid bacterium may be defined as a bacterium capable of producing lactic acid. A succinic acid bacterium may be defined as bacterium capable of producing succinic acid. A pyruvic acid bacterium may be defined as a bacterium capable of producing pyruvic acid. A tartaric acid bacterium may be defined as bacterium capable of producing tartaric acid. An iso-butyric acid bacterium may be defined as a bacterium capable of producing iso-butyric acid. A hydroxybutyric acid bacterium may be defined as bacterium capable of producing hydroxybutyric acid. A hydroxyvaleric acid bacterium may be defined as bacterium capable of producing hydroxyvaleric acid. The bacterium may be thermophilic or mesophilic, but preferably is thermophilic.

[0022] The microorganism may be selected from the group consisting of: thermophilic acetogens; thermophilic propionibacteria; thermophilic butyric acid bacteria; thermophilic lactic acid bacteria; thermophilic succinic acid bacteria; and thermophilic hydrogen oxidising bacteria. Such microorganisms are capable of producing organic acids and withstanding such acids in high concentrations in the fermentation broth. The microorganism may be selected from the genera consisting of: Moorella, Thermoanaerobacter, Thermoanaerobacterium, Acetogenum, Carboxydothermus, Clostridium, Butyribacterium, Eubacterium, Pyrococcus, Desulfobacterium, Parageobacillus, Geobacillus and Acetobaterium. In particular, the microorganism may be selected from the group consisting of: Clostridium Ljungdahlii, Clostridium autoethanogenum, Acetogenium kivui, Acetobacterium woodii, Acetoanaerobium noterae, Clostridium aceticum, Butyribacterium methylotrophicum, Clostridium acetobutylicum, Clostridium thermoaceticum, Eubacterium limosum, and Moorella thermoautotrophica. Preferably, the microorganism is Moorella thermoaceticum or Thermoanaerobacter kivui. Moorella thermoaceticum and T. kivui are known to be capable of producing acetic acid, for example.

[0023] The target organic acid productivity of the microbial fermentation (i.e. the productivity of the target organic acid) may be greater than or equal to 8 mM / hour, greater than or equal to 12 mM / hour, greater than or equal to 17 mM / hour, greater than or equal to 25 mM / hour, greater than or equal to 33 mM / hour, greaterthan or equal to 40 mM / hour, or greater than or equal to 50 mM / hour. The target organic acid productivity may be calculated based on the target organic acid produced by the microbial fermentation over a time period of 1 hour, 6 hours, 12 hours, 1 day, 2 days or 1 week. The target organic acid productivity may be defined as the productivity of both the free acid and salt / conjugate base forms of the target organic acid. Preferably, the productivity may be calculated based on the target organic acid produced by the microbial fermentation over a time period of 12 hours, and / or the target organic acid productivity may be greater than or equal to 33 mM / hour.

[0024] The target organic acid may be selected from the group consisting of formic acid; acetic acid; butyric acid; propionic acid; lactic acid; succinic acid; pyruvic acid; tartaric acid; iso-butyric acid; hydroxybutyric acid; and hydroxyvaleric acid. Preferably, the target organic acid is selected from the group consisting of formic acid; acetic acid, butyric acid; propionic acid; lactic acid; and succinic acid. More preferably, the target organic acid is a short chain fatty acid, e.g. selected from the group consisting of: formic acid; acetic acid; butyric acid and propionic acid. Most preferably, the target organic acid is acetic acid. A short-chain fatty acid may be defined as a fatty acid containing one to six carbon atoms.

[0025] The salt of the target organic acid may be selected from the group consisting of a salt of formic acid, a salt of acetic acid; a salt of butyric acid; a salt of propionic acid; a salt of lactic acid; a salt of succinic acid; a salt of pyruvic acid; a salt of tartaric acid; a salt of iso-butyric acid; a salt of hydroxybutyric acid; and a salt of hydroxyvaleric acid. Preferably, the salt of the target organic acid is selected from the group consisting of a salt of formic acid; a salt of acetic acid; a salt of butyric acid; a salt of propionic acid; a salt of lactic acid; and a salt of succinic acid. More preferably, the target organic acid is a salt of a short chain fatty acid, e.g. selected from the group consisting of: a salt of formic acid; a salt of acetic acid; a salt of butyric acid; and a salt of propionic acid. Most preferably, the organic acid salt is a salt of acetic acid salt, such as sodium acetate.

[0026] The fermentation step may further comprise adding one or more inorganic bases to the fermentation vessel, for example, sodium hydroxide. Addition of an inorganic base to the fermentation vessel may result in reaction of the target organic acid with the inorganic base to produce a salt of the target organic acid. In this way, it is possible to control the pH of the fermentation broth, for example, to between pH 4 and 7, in spite of the production of large amounts of target organic acid by the microorganism. The fermentation broth may have a pH less than 6, less than 5.2, less than 5 or less than 4. The fermentation broth may have a pH greater than 4, greater than 4.5, greater than 5, greater than 5.2 or greater than 6. The method may comprise maintaining the fermentation broth at a pH less than 7 and greater than 4, for example, maintaining the fermentation broth at a pH less than 6 and greater than 4.5, or maintaining the fermentation broth at a pH less than 6 and greater than 5. Preferably, the fermentation broth may have a pH less than 6 and greater than 5.5.

[0027] The ion exchange of the first separation step (ii) may be configured to convert a majority of the salt of the target organic acid in the first product stream into the free acid form of the target organic acid in the second product stream. Consequently, it can be understood that the majority of the acidification in the present process may be provided by ion exchange.The process may further comprise, following step (ii), step (iv): ion exchange resin regeneration using a regeneration acid (e.g. H2SO4) to extract the cation of the salt of the target organic acid from the ion exchange resin to a by-product salt stream. In this way, the resin in the ion exchange column can be regenerated for further use in step (ii), and the cation of the salt of the target organic acid can be recovered (e.g. for recycling).

[0028] Conventionally, ion exchange processes comprise washing of the ion exchange resin, both between adsorption (step (ii)) and regeneration (step (iv) and between regeneration (step (iv)) and further adsorption with the ion exchange resin. However, this can lead to dilution or waste of the product. The process may further comprise a post-adsorption wash step on the ion exchange resin (i.e. between step (ii) and step (iv)); a resultant wash stream from the post-adsorption wash step may be recycled to the fermentation vessel. In this way, any target organic acid washed from the resin during the post-adsorption wash step is recovered rather than sent to waste. The process may omit a post-regeneration wash step on the ion exchange resin (i.e. between step (iv) and a further step (ii)). Because the ion exchange step (ii) is used for protonation of the conjugate based of the target organic acid, rather than for purification, it is acceptable to pollute the second product stream formed during a subsequent absorption step (step (ii)) with any inorganic salt (e.g. sodium sulphate) that is left on the resin following a preceding regeneration step (step (iv)).

[0029] The process may further comprise, following step (iv), (v) a third separation step comprising separating the by-product salt stream into two or more streams. In some examples, e.g. where using bipolar membrane electrodialysis to effect the separation, the third separation step may separate the by-product salt stream into an acid stream, a base stream, and a diluate output stream. In other examples, e.g. where using a single-stage dialysis to separate the by-product salt stream, the third separation step may separate the by-product salt stream into (only) two streams, e.g. two streams selected from the group comprising, or consisting of, an acid stream, a base stream, and a diluate output stream; in some embodiments, the two streams may be an acid stream (e.g. an eluted output stream) and diluate output stream The third separation step may be conducted by a membrane-based separation process. The third separation step may be conducted by bipolar membrane electrodialysis (EDBM). In this way, the byproduct salt stream can be separated into the acid stream that can be used for further ion exchange resin regeneration (i.e. as at least part of the regeneration acid (e.g. H2SO4)), and the base stream that can be used for pH control in the fermentation step, thereby reducing the wastage of salt that was generated within the fermentation process as a consequence of pH control. In other examples, the third separation step may be conducted by a dialysis process. The dialysis process may produce an acid stream, comprising the regeneration acid (e.g. H2SO4), and a residual solution of a salt formed from the counterions of the base present in the (acidified) first product stream (e.g. Na+where NaOH is fed as the inorganic based to the fermentation vessel) fed to the first separation step (ii) and the acid present in the regenerant acid fed (e.g. SO42-where H2SO4 is fed as the regeneration acid) to the ion exchange unit during regeneration (iv).

[0030] At least a portion of the acid stream from step (v) may be recycled to the ion exchange regeneration step (iv) to provide regeneration acid. This recycling of the acid stream can reduce the need for addition ofvirgin acid feedstock into the process, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock.

[0031] At least a portion of the aqueous phase stream from step (iii) may be recycled to the ion exchange regeneration step (iv) to provide regeneration acid. The aqueous phase stream from the LLE step (iii) is typically highly acidic and thus can be used to supplement the acid stream recycled from the third separation step (v), and thereby reduce wastage of the acid contained in the aqueous phase stream. Moreover, where the acid stream from step (v) is recycled to the ion exchange regeneration step (iv) to provide regeneration acid, this additional recycling of the aqueous phase stream can reduce the need for addition of virgin acid feedstock into the process, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock. A portion of the aqueous phase stream from step (iii) may be bled off via a bleed line rather than being recycled to the ion exchange regeneration step (iv). This can provide control over the amount of acid entering the regeneration step (iv). Alternatively, or additionally, the process may further comprise supplementing the acid stream recycled to the ion exchange regeneration step (iv) with a supplementary (virgin) acid feed to provide the regeneration acid.

[0032] Step (iv) may be conducted such that the concentration of acid in the by-product salt stream is minimised (e.g. by appropriate control of the flow of regeneration acid to the ion exchange unit and the configuration of the ion exchange column). This is preferable, as where the by-product salt stream is separated by bipolar membrane electrodialysis, this separation process is most efficient when the pH of the by-product salt stream is close to neutral pH. By way of example, step (iv) may be configured such that the pH of the by-product salt stream is greater than or equal to 1 , more preferably around pH 7 (e.g. greater than or equal to pH 6 and less than or equal to pH 8). The pH of the by-product salt stream may be less than 11. The by-product salt stream may comprise one or more salts, and the concentration of the one or more salts in the by-product salt stream may be greater than or equal to 0.1 M. By way of example, the byproduct salt stream may have a concentration of the one or more salts that is greater than 1 M and less than or equal to 2.5 M, preferably greater than 1.5 M and less than 2 M.

[0033] The process may further comprise a step of concentrating the acid stream (e.g. prior to recycling the acid stream to the regeneration step (iv)). In this way, the volume of liquid in the process (e.g. in the regeneration step (iv), and the subsequent downstream separation stages) can be reduced, thereby reducing process equipment size requirements and pumping duty within the process. The step of concentrating the acid stream from step (v) may comprise concentrating the acid stream using a reverse osmosis unit, an evaporation unit, a concentration unit, ora precipitation unit.

[0034] The third separation step (v) may be conducted in a EDBM unit. The third separation step (v) may further comprise feeding a dilution water stream into the EDBM unit. The EDBM unit may comprise a three-compartment cell. The EDBM unit may comprise an anion exchange membrane (AEM), a cation exchange membrane (CEM), a first bipolar membrane (BPM), and a second BPM. The AEM and CEM may be positioned between the first and second BPMs within the EDBM unit. A first compartment may be defined between the AEM and CEM, a second compartment may be defined between the first BPM andAEM, a third compartment may be defined between the CEM and the second BPM. The first and / or second BPM may comprise an anion exchange layer and a cation exchange layer. The by-product salt stream may be fed into the first compartment. The dilution water stream may be fed into the second compartment and / or the third compartment. The cell may comprise a cathode positioned adjacent the first BPM and an anode positioned adjacent the second BPM. The diluate output stream may be drawn from the first compartment of the cell. The acid stream may be drawn from the second compartment of the cell. The base stream may be drawn from the third compartment of the cell. The third separation step (v) may be conducted in another type of unit configured to separate the by-product salt stream into an acid stream, a base stream and a diluate output stream. In some examples, the unit may be a membrane separation unit, e.g. a dialysis unit.

[0035] The diluate output stream may comprise a reduced concentration of acid (e.g. the inorganic acid), base (e.g. the inorganic base), and / or salt (e.g. an inorganic salt) compared to the respective concentrations in the by-product salt stream.

[0036] The diluate output stream may comprise one or more salts. The concentration of the one or more salts in the diluate output stream may be less than or equal to 5.0% by mass, less than or equal to 4.0% by mass, or less than or equal to 3.0% by mass. Preferably, the concentration of the one or more salts in the diluate output stream may be less than or equal to 3.0% by mass. This facilitates disposal of the diluate output stream. The one or more salts may comprise a sodium salt. The concentration of the one or more salts may be defined as the sum of the concentrations of any anions (e.g. SO42) and cations (e.g. Na+) in the diluate output stream. Anions and cations from the dissociation of water may not be included in the definition of the concentration of the one or more salts in the diluate output stream.

[0037] The process may further comprise (vi) an acidification step comprising acidifying the second product steam between step (ii) and step (iii) with an acid feed. This provides an acidified second product stream. In this way, the pH of the second product stream can be reduced further and accordingly a greater proportion of the target organic acid will be in its free acid form rather than in its conjugate base form, which in turn facilitates separation of the target organic acid from the inorganic aqueous compounds in the second product stream by LLE. The acidification step (vi) may comprise adding an inorganic acid to the second product stream. The inorganic acid may be sulfuric acid (H2SO4).

[0038] The total concentration of said target organic acid in the (acidified) second product stream may be defined as the sum of the concentrations of the free acid and salt forms of the target organic acid. The (acidified) second product stream may have a total concentration of the target organic acid of greater than or equal to 0.30 M, 0.50 M, 1.00 M, 1.25 M, 1.50 M, 2.00 M, 2.25 M, 2.50 M, 2.75 M, 3.00 M, 3.50 M, 4.00 M, 4.50 M, or 5.00 M. The (acidified) second product stream may have a total concentration of the target organic acid of less than or equal to 5.00 M, 4.50 M, 4.00 M, 3.50 M, 3.00 M, 2.75 M, 2.50 M, 2.25 M, 2.00 M, 1.75 M, 1.50 M, 1.25 M, 1.00 M, 0.75 M, or 0.50 M. The (acidified) second product stream may have a total concentration of the target organic acid of less than or equal to 5.00 M and greater than or equal to 0.30 M, for example, between 1.50 M and 3.00 M, e.g. less than or equal to 2.50 M and greater than or equal to 1.75 M. Preferably, the total concentration of the target organic acid in the (acidified) second product stream may be greater than or equal to 1.50 M and less than or equal to 3.00 M.In some examples, the process may not comprise a concentration (e.g. dewatering) step on the second product stream (i.e. downstream of the first separation step (ii)) and / or on the acidified second product stream (i.e. downstream of the acidification step (vi)). Concentration of the aqueous phase stream between the ion exchange and the liquid-liquid extraction (e.g. by evaporation or other de-watering techniques) is not practical for the short-chain fatty acids of particular interest, especially acetic acid. Consequently, it is preferable to perform any concentration (e.g. dewatering) steps upstream of the first separation step. The total concentration of the target organic acid in the second product stream fed to the second separation step (liquid-liquid extraction step) may be less than 150% of the total concentration of the target organic acid in the second product stream leaving the first separation step (step (ii)) and / or less than 150% of the total concentration of the target organic acid in the second product stream leaving the acidification step (step (vi)). In this way, no greater than two fold concentration occurs between the first separation step and / or acidification step and the liquid-liquid extraction step. In some examples, the total concentration of the target organic acid in the second product stream fed to the second separation step (liquid-liquid extraction step) may be less than 125% (more preferably less than 110%) of the total concentration of the target organic acid in the second product stream leaving the first separation step (step (ii)) and / or less than 125% (more preferably less than 110%) of the total concentration of the target organic acid in the second product stream leaving the acidification step (step (vi)). In some examples, the total concentration of the target organic acid in the second product stream fed to the second separation step (liquid-liquid extraction step) may be greater than 75% (more preferably greater than 90%) of the total concentration of the target organic acid in the second product stream leaving the first separation step (step (ii)) and / or greater than 75% (more preferably greater than 90%) of the total concentration of the target organic acid in the second product stream leaving the acidification step (step (vi)).

[0039] The total concentration of the inorganic base may be defined as the sum of the concentration of the undissociated inorganic base (e.g. NaOH) and the concentration of the cation of the dissociated inorganic base (e.g. Na+).The second product stream may have a total concentration of inorganic base of greater than or equal to 0.1 M, 0.5 M, 1.0 M, 1.5 M, 2.0 M, 2.5 M, 3.0 M, 3.5 M, 4.0 M, 4.5 M, 5.0 M, 5.5 M, 6.0 M, 6.5 M, 7.0 M, 7.5 M, 8.0 M, 8.5 M, 9.0 M, 9.5 M, or 10.0 M. The second product stream may have a total concentration of inorganic base of less than or equal to 10.0 M, 9.5 M, 9.0 M, 8.5 M, 8.0 M, 7.5 M, 7.0 M, 6.5 M, 6.0 M, 5.5 M, 5.0 M, 4.5 M, 4.0 M, 3.5 M, 3.0 M, 2.5 M, 2.0 M, 1.5 M, 1.0 M, or 0.5 M. Preferably, the total concentration of the inorganic base in the second product stream may be greater than or equal to 2.5 M and less than or equal to 5.0 M.

[0040] The fermentation step (i) may further comprise adding an inorganic base to the fermentation vessel. This can provide pH control for the fermentation step by counterbalancing the pH increase that would otherwise occur by production of the salt of the target organic acid in the fermentation step (i).

[0041] At least a portion of the base stream from step (v) may be recycled to the fermentation vessel, e.g. as a recycled base feed. In this way, the need for addition of virgin base feedstock into the process may be reduced, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock. The process may further comprise feeding a supplementary stream of base into the fermentation vessel in addition to the recycled base feed.The process may further comprise a step of concentrating the base stream following step (v) (e.g. prior to recycling the base stream to the fermentation vessel). In this way, the volume of liquid added to the reactor for pH control can be reduced, thereby reducing fermentation vessel size requirements and pumping duty within the process. The step of concentrating the base stream may comprise concentrating the base stream using a reverse osmosis unit, an evaporation unit, a concentration unit, or a precipitation unit.

[0042] The concentration of base in the base stream (e.g. prior to concentration) may be between 0.5 M and 2 M. The concentration (e.g. prior to concentration) of the anion of the inorganic acid (e.g. SO42-where the inorganic acid is H2SO4) in the base stream (e.g. prior to concentration) may be less than or equal to 0.1 M, e.g. less than or equal to 0.05 M.

[0043] The target organic acid may have an acid dissociation constant, pKa and the pH of the second product stream reaching step (iii) (e.g. after the acidification step (vi), where present, or absent any additional acidification step between the ion exchange and step (iii)) may satisfy the inequality: pKa - pH > 1.0. By way of example, where the target organic acid is acetic acid, pKa = 4.76 and thus the process may be configured (e.g. via the ion exchange step (ii) and / or the acidification step (vi), where present) such that the pH of the second product stream is less than or equal to 3.76. The (acidified) second product stream may satisfy the inequality pKa - pH > 1.5, pKa - pH > 2.0, or pKa - pH > 3.0. The pH of the first product stream may be less than or equal to pH 7 and greater than or equal to pH 4. The (acidified) second product stream predominantly comprises the target organic acid in its free acid form, as a result of the ion exchange step (ii) and / or the acidification step (vi) shifting the target organic acid to the free acid form by increasing the concentration of hydrogen ions in the solution. However, it can be understood that, depending on the pH of the (acidified) second product stream, some of the organic acid may also be present in its conjugate base / salt form in the (acidified) second product stream. The ion exchange of the first separation step may result in the pH of second product stream leaving the ion exchange (i.e the second product stream leaving the ion exchange column) satisfying the inequality pKa - pH > 1.0 . In this way, the acidification step (vi) may be omitted, or the acidification step (vi) may add less acid feed to the second product stream than would otherwise be the case.

[0044] The second separation step (iii) may comprise mixing the (acidified) second product stream with an organic solvent stream (e.g. in an LLE unit). In this way the target organic acid can be extracted into the organic solvent as a solute. The organic solvent may be immiscible with an aqueous solution (e.g. the (acidified) second product stream). The organic solvent stream may comprise ethyl acetate or methyl tertbutyl ether. Short-chain organic acids such as acetic acid have a high solubility in such organic solvents, and such organic solvents also provide low miscibility with the aqueous phase.

[0045] The second separation step (iii) may be configured such that the majority of the organic acid is separated into the organic phase product stream. However, a minor fraction of the organic acid (e.g. in its free acid or conjugate base / salt form) may be separated into the aqueous phase stream (i.e. the second separation step (iii) may be imperfect).The process may further comprise subjecting the organic phase product stream from step (iii) to a rectification step to extract an organic solvent therefrom. The process may further comprise subjecting the aqueous phase stream from step (iii) to a rectification step to extract an organic solvent and / or the target organic acid / salt of the target organic acid therefrom. At least a portion of the Organic solvent extracted from the organic phase product stream and / or the aqueous phase may be recycled to the LLE unit. The fermentation step may be at at least 1 m3scale, at least 5 m3scale, at least 50 m3scale, at least 150 m3scale, or at least 250 m3scale.

[0046] The process may further comprise a liquid-solid separation step comprising separation of the first product stream, e.g. separating out a solids stream from said stream. This may reduce the quantity of solid material (e.g. solid biomass) flowing into the downstream separation equipment, thereby reducing fouling and improving process reliability. At least a portion of the solid material extracted from the first product stream and / or the second product stream may be recycled to the fermentation vessel. The liquid-solid separation step may comprise a filtration step, a decanting step, a settling step or a centrifugation step. The filtration step may comprise microfiltration, nanofiltration or ultrafiltration of the first product stream and / or the second product stream. Ultrafiltration may be defined as filtration through a pore size of 0.01 to 0.05 microns. Nanofiltration may be defined as filtration through a pore size of 1 to 10 nanometres.

[0047] Microfiltration may be defined as filtration through a pore size of 0.05 to 1 microns.

[0048] The process may further comprise a concentration step comprising concentration (e.g. by reverse osmosis) of the first product stream. This may increase the concentration of the salt of the target organic acid in the first product stream prior to the first separation step (ii) and acidification step (vi) by removing water from the first product stream. The concentration step may be configured to increase the concentration of the salt by between 50% and 200% (i.e. 1.5 to three times the concentration of the first product stream prior to the concentration step). Where the process also comprises a polishing step (e.g. a filtration step comprising filtration of the first product stream), the concentration step may be downstream of the polishing (e.g. filtration) step along the first product stream. The step of concentrating the first product stream may comprise concentrating the first product stream using a reverse osmosis unit, an evaporation unit, a membrane separation unit, or a precipitation unit.

[0049] The process may further comprise a polishing step on the first product stream (or filtered first product stream). The polishing step may comprise comprising removal of divalent ions and / or halide ions from the first product stream (or filtered first product stream) (e.g. by ion exchange). This can remove divalent ions and / or halide ions from the first product stream prior to the first separation step (ii), thereby improving the efficiency of the first separation step in acidifying the second product stream and removing cations therefrom. Removal of divalent and halide ions can also improve the efficiency of the EDBM unit in separating the salt by-product stream (where these ions may end up following the first separation step and ion exchange resin regeneration) into an acid stream, a base stream and a diluate output stream. The polishing step may comprise removing organic contaminants and / or residues of growth medium from the first and / or second product stream (e.g. in addition to, or alternatively to, removing divalent ions and / or halide ions from the second product stream). The polishing step may comprise a filtration step,e.g. a nanofiltration step. A stream (e.g. comprising the organic contaminants and / or residues of growth medium) from the polishing step may be recycled to the fermentation vessel.

[0050] The numbering of steps (i) - (vi) above, and the references to “first separation step” and “second separation step” is only for the purpose of improving the ease of understanding the described process and does not limit the present invention to consisting of those steps, nor does it exclude the possibility of preceding, intermediate, and / or subsequent steps between steps (i) - (vi) recited above being included in the process. It can also be understood that the process may be operated as a continuous process, in the sense that each of steps (i) - (vi) may be being conducted contemporaneously with each other within the process.

[0051] In a second aspect, there is provided a fermentation system for producing a target organic acid, the system comprising: a fermentation vessel for producing a first product stream comprising a salt of the target organic acid; an ion exchange unit configured to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin to produce a second product stream comprising the target organic acid; and a liquid-liquid extraction unit configured to separate the second product stream into an aqueous phase stream and an organic phase product stream, the organic phase product stream containing the target organic acid.

[0052] Any one or more of the optional features set out in relation to the first aspect may be applied to the second aspect, except where such a combination is clearly impermissible or expressly avoided.

[0053] The fermentation vessel may have a volume of at least 1 m3, at least 5 m3, at least 50 m3, at least 150 m3, or at least 250 m3.

[0054] The system may further comprise an organic rectification unit (e.g. a distillation column) on the organic phase product stream. This can allow for recovery of the organic solvent from the target organic acid, allowing this organic solvent to be recycled to the LLE unit.

[0055] The system may further comprise an aqueous rectification unit (e.g. a distillation column) on the aqueous phase stream. This can allow for recovery of organic solvent and / or the target organic acid / salt of the target organic acid from the aqueous phase stream that is present therein due to imperfect separation of phases in the LLE unit, allowing this organic solvent / target organic acid / salt of the target organic acid to be recycled to the LLE unit.

[0056] The system may further comprise a liquid-solid separation unit on the first product stream. This may reduce the quantity of solid material (e.g. solid biomass) flowing into the downstream separation equipment, thereby reducing fouling and improving process reliability. The system may be configured to recycle the solids stream to the fermentation vessel. A fraction of the solids stream may be purged to control the accumulation of suspended solids in the fermentation vessel. The liquid-solid separation unit may be a filtration unit, a decanter, a settler or a centrifuge. The filtration unit may comprise a microfiltration unit, nanofiltration unit and / or an ultrafiltration unit.

[0057] The system may further comprise a polishing unit configured to condition the first product stream (e.g. downstream of the liquid-solid separation unit) upstream of the ion exchange unit configured to exchangea cation of the salt of the target organic acid in the first product stream with a hydrogen ion. The polishing unit may be configured to remove divalent ions and / or halide ions from the first product stream and / or second product stream. Such a step can remove halide and / or divalent ions (e.g. Ch, Mg2+) from the first product stream prior to ion exchange of cations of the salt of the target organic acid for hydrogen ions. The polishing unit may comprise a further ion exchange unit, such a unit can reduce the ionic strength of the first product stream prior to acidification. The polishing unit may be configured to remove organic contaminants and / or residues of growth medium from the second product stream (E.g. in addition to, or alternatively to, removing divalent ions and / or halide ions from the second product stream). The polishing unit may be a filter unit, e.g. a nanofiltration unit. The system may be configured to recycle a stream (e.g. comprising the organic contaminants and / or residues of growth medium) from the filter unit back to the fermentation vessel.

[0058] The system may further comprise a concentration unit configured to concentrate (e.g. by reverse osmosis) the first product stream. This may increase the concentration of the salt of the target organic acid in the first product stream prior to the first separation step (ii) and acidification step (vi) by removing water from the first product stream. The concentration unit may be configured to increase the concentration of the salt by between 50% and 200% (i.e. 1.5 to three times the concentration of the first product stream prior to the concentration step). Where the process also comprises a polishing unit (e.g. a filtration unit), the concentration unit may be downstream of the polishing (e.g. filtration) unit along the first product stream. The concentration unit may comprise a reverse osmosis unit, an evaporation unit, a membrane separation unit, or a precipitation unit.

[0059] The system may comprise an acidification unit configured for acidifying the second product stream from the ion exchange unit with an acid feed. The acidification unit may comprise a tank, e.g. a stirred tank. The second product stream may feed into the tank. An acid feed may feed into the tank. The acidified second product stream may be drawn from the tank. By conducting the acidification step in a tank (i.e. the acidification unit comprising a tank), a more homogenous acidified second product stream is provided than if other configurations were used for the acidification unit, in particular where the tank is stirred. The ion exchange unit may be configured to regenerate the ion exchange resin and extract the cation of the salt of the target organic acid therefrom to a by-product salt stream using a regeneration acid. In some configurations, the ion exchange unit may comprise a first ion exchange column and a second ion exchange column, and the system may be configured such that first product stream is fed into the first ion exchange column to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin whilst the second ion exchange column is fed with the regeneration acid to extract the cation of the salt of the target organic acid therefrom to a by-product salt stream using a regeneration acid. In such a case, the first ion exchange column can be regenerated by changing the feeding of the streams, such that the second ion exchange column is fed with the (acidified) second product stream and the first ion exchange column is fed with the regeneration acid. In this way, the ion exchange unit can be operated continuously. Alternatively, the process may be run batchwise and the ion exchange unit may be regenerated using the regeneration acid whilst the flow of (acidified) second product stream is halted. Alternatively, the system may comprise an exchange column fed withthe (acidified) second product stream and a regeneration column fed with the regeneration acid, and ion exchange resin may be transferred between the two as required to regenerate the resin (e.g. the first column and second column may be moving bed ion exchange columns configured to transfer the ion exchange resin therebetween).

[0060] The system may further comprise a bipolar membrane electrodialysis (EDBM) unit configured to separate the by-product salt stream into an acid stream, a base stream, and a diluate output stream.

[0061] The system may be configured to recycle at least a portion of the base stream to the fermentation vessel. In this way, the need for addition of virgin base feedstock into the process may be reduced, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock.

[0062] The system may be configured to recycle at least a portion of the acid stream to the ion exchange unit to provide regeneration acid. In this way, the need for addition of virgin acid feedstock into the process may be reduced, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock.

[0063] The system is configured to recycle at least a portion of the aqueous phase stream from the LLE unit to the ion exchange unit to provide regeneration acid. In this way, the need for addition of virgin acid feedstock into the process may be reduced, thereby reducing feedstock consumption by the process and the associated energy consumption in preparing such feedstock. Alternatively, or additionally, the system may be configured to supplement the acid stream recycled to the ion exchange unit with supplementary acid stream to provide regeneration acid.

[0064] In the present application, a microorganism being thermophilic may be defined as the microorganism being capable of growing at a temperature above 45°C.

[0065] In the present application, a microorganism being mesophilic may be defined as the microorganism being capable of growing at a temperature greater than or equal to 20°C and less than or equal to 45°C.

[0066] A fermentation vessel may be any tank that performs a biocatalytic conversion reaction (e.g. an enzymatic reaction or microorganism-based reaction). A fermentation vessel may have a set of monitored and (further optionally) controlled parameters; the set of parameters may include one or more of temperature, pH, liquid level, optical density, and pressure.

[0067] Summary of the Figures

[0068] Figure 1 provides a process flow diagram (PFD) of a first embodiment of a process according to the first aspect and a gas fermentation system according to the second aspect;

[0069] Figure 2 provides a process flow diagram (PFD) of a second embodiment of a process according to the first aspect and a gas fermentation system according to the second aspect; and

[0070] Figure 3 provides a schematic of a bipolar membrane electrodialysis (EDBM) unit for use in a process according to the first aspect.Detailed Description of the Invention

[0071] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0072] Figure 1 provides a PFD of a process according to the first aspect and a gas fermentation system according to the second aspect. The system comprises a fermentation vessel 110 for gas fermentation of a gas feed containing CO and / or CO2 and, optionally H2, with a fermentation broth in the vessel 110 comprising a microorganism, such as Moorella thermoacetica, that is capable of producing an organic acid (e.g. acetic acid in the case of Moorella thermoacetica) through fermentation of CO and / or CO2. The system and process are described below in the context of producing acetic acid, however, other organic acids (in addition, or alternatively, to acetic acid) may be produced by the fermentation, depending on the microorganism(s) present in the fermentation broth. It can also be understood that the process and system of Figure 1 may also be used for fermentation of C1 compounds (e.g. C1 hydrocarbons, such as methanol and / or formate).

[0073] The fermentation vessel is fed by a gas feed 9 comprising the CO and / or CO2, and a liquid feed 13 containing, for example, nutrients, process water and / or recycled media. Absent any process control, production of acetic acid in high concentrations by the microorganism would lead to a reduction in the pH of the fermentation broth, potentially reducing the pH to a level that inhibits the growth of the microorganism and / or the production of acetic acid. Accordingly, the system further comprises a nonrecycled base feed 10, such that an inorganic base (e.g. sodium hydroxide (NaOH)) can be added to the fermentation vessel 110 to control the pH of the fermentation broth. The system and process are described below using NaOH as the base, however, one or more alternative inorganic bases may be used. As is discussed further below, NaOH can be fed into the fermentation vessel 110 from a recycled base stream 8’ and / or the non-recycled base feed 10 that comprises virgin (i.e. non-recycled) NaOH. Addition of NaOH to the fermentation broth increases the pH of the fermentation broth to allow for continued growth of the microorganism, but also results in a large proportion of the acetic acid produced by Moorella thermoacetica being present in its salt form (Na+CH3COO) in the fermentation broth.

[0074] The system illustrated in Figure 1 is configured to facilitate both extraction of the primary product of acetic acid from the fermentation broth, and also the recycling of separated feedstocks to the process.

[0075] Specifically, fermentation broth is drawn from the fermentation vessel 110 to provide a first product stream 1 comprising acetic acid salt (Na+CH3COO). The first product stream 1 is passed to a filtration unit 180 configured to remove solid material (e.g. solid biomass) therefrom into a solids stream 14, and subsequently to a polishing unit 190 configured to remove divalent ions and halide ions from the first product stream 1. These first two processing steps thus provide a filtered first product stream 1’ that contains a reduced quantity of solid material, divalent ions and halide ions. The filtered first product stream T is passed to an ion exchange unit 120, where cations of the salt of acetic acid (e.g. Na+ions) in the filtered first product stream 1 ’ are exchanged with hydrogen ions on an ion exchange resin to providea second product stream 2. The second product stream 2 has a greater proportion of the acetic acid in its free acid form (as opposed to its conjugate base form) compared to the first product stream 1 , T as a result of the ion exchange.

[0076] Subsequently, the second product stream 2 passes to an acidification unit 170 where the second product stream 2 is acidified by the addition of an inorganic acid feed 11 (e.g. H2SO4) to provide an acidified second product stream 2’. The system and process are described below using H2SO4 as the inorganic acid, however, one or more alternative inorganic acids may be used. The acidification step ensures that the acetic acid in the acidified second product stream 2’ is predominantly in its free acid form (i.e.

[0077] CH3COOH), rather than its salt / conjugate base form (i.e. CH3COO). In the case of acetic acid, this is facilitated by the acidification step reducing the pH of the stream to less than or equal to 3.76 (e.g. down to pH 2.0), such that, acetic acid, with a pKa of 4.76, satisfies the inequality pKa - pH > 1.0.

[0078] Subsequently, because substantially all of the acetic acid in the acidified second product stream 2’ leaving the acidification unit 170 is in its free acid form as a result of the pH having been reduced to 2.0, the free acetic acid can be separated from the remainder of the aqueous fermentation broth by liquidliquid extraction (LLE) into organic and aqueous phases. The fermentation broth resulting from gas fermentation is particularly clean (e.g. comprising little-to-no carbohydrate and / or solid mass other than the microorganism), and accordingly, the acidified second product stream 2’ predominantly comprises water, the target organic acid (predominantly in its free acid form) and dissociated ions (e.g. ions of the inorganic base and inorganic acid). The acidified second product stream 2’ is passed to an LLE unit 130 for the second separation step. In the LLE unit 130, the acidified second product stream 2’ is mixed with an organic solvent (e.g. ethyl acetate) provided to the LLE unit 130 via an organic solvent line 12. As is discussed further below, the organic solvent line 12 comprises virgin organic solvent, and recycled organic solvent may also be fed into the LLE unit 130 to reduce the consumption of virgin organic solvent. Free acetic acid within the aqueous acidified second product stream 2’ is extracted into the organic solvent as a solute, with the organic phase (comprising the organic solvent and acetic acid) being immiscible with the remaining components of the aqueous phase second product stream (e.g. water, ions of the dissociated NaOH and H2SO4, and trace amounts of acetic acid (predominantly that in the salt / conjugate base form). An aqueous phase stream 3 is drawn from the LLE unit 130. The aqueous phase stream 3 contains the components of the acidified second product stream 2’, except the acetic acid extracted to the organic phase in the LLE unit 130, and may contain trace amounts of the organic phase (due to imperfect separation of phases in the LLE unit 130). An organic phase product stream 4 is also drawn from the LLE unit 130. The organic phase product stream 4 contains the organic phase in the LLE unit 130 (i.e. the organic solvent and extracted acetic acid).

[0079] The LLE unit 130 may, for example, be a Koch Modular Scheibel™ column or Karr™ column.

[0080] Downstream of the LLE unit 130, the aqueous phase stream 3 is passed to an aqueous rectification unit 140 wherein the trace amounts of the organic phase in the aqueous phase stream 3 are distilled from the aqueous phase, providing an organic solvent distillate 12” that can be recycled to the LLE unit 130 as shown in Figure 1 (or otherwise disposed of) and a purified aqueous phase stream 3’.Similarly, downstream of the LLE unit 130, the organic phase product stream 4 is passed to an organic rectification unit 150 wherein the acetic acid is distilled from the organic solvent. In this way, a concentrated organic phase product stream 4’ comprising predominantly acetic acid is provided, and an organic solvent distillate 12’ from the organic rectification unit 150 can be recycled to the LLE unit 130 as shown in Figure 1 (or otherwise disposed of).

[0081] Depending on the efficiency of the rectification units 140, 150 in recovering the organic solvent from their respective feed streams 3, 4, a certain amount of supplementary (e.g. non-recycled / virgin) organic solvent is provided to the LLE unit 130 alongside the recycled organic solvent.

[0082] The purified aqueous phase stream 3’ is still highly acidic because of the inorganic acid added to the second product stream 2’ in the acidification unit 170 to shift the acetic acid into its free acid form. This inorganic acid remains in the aqueous phase in the LLE unit 130 and thus enters the purified aqueous phase stream 3’. This highly acidic purified aqueous phase stream 3’ can also be repurposed within the process, as is discussed below in relation to the ion exchange unit 120 and the regeneration of the ion exchange resin.

[0083] As discussed above, the ion exchange unit 120 acts to shift acetic acid from its conjugate base form into its free acid form by exchanging Na+ions in the filtered first product stream T for H+ions on an ion exchange resin. After a period of operation, the ion exchange resin will be exhausted and require regeneration. Regeneration of the ion exchange resin can be conducted using a regeneration acid to replace Na+ions with H+ions such that the resin can then be used again in the first separation step to shift the acetic acid in the filtered first product stream 1 ’ into its free acid form. In the case of Figure 1 , the system is configured with two ion exchange columns in the ion exchange unit 120, such that a first column can be being regenerated whilst the second column is processing the filtered first product stream T, and once the first column is regenerated and the second column is exhausted, the column functions can be switched. Regeneration acid is fed into the top of the ion exchange column being regenerated, and a by-product salt stream 5 predominantly comprising an aqueous solution of a sodium salt is drawn from the bottom of the column. To provide the regeneration acid in the system in Figure 1 , the by-product salt stream 5 is passed to an EDBM unit 160 that is configured to separate the by-product salt stream 5 into an acid stream 6, a base stream 8, and a diluate output stream 7. The EDBM unit 160 is also fed with a dilution water stream 15. The EDBM unit 160 comprises a three-compartment cell configured to separate the purified aqueous phase stream 3’ into the base stream 8, acid stream 6 and the diluate output stream 7. The base stream 8 predominantly comprises water and dissociated NaOH (i.e. Na+and OH-ions), as well as trace quantities of SO42-(as the anion of the inorganic acid). The acid stream 6 predominantly comprises water and dissociated H2SO4 (i.e. H+and SO42), as well as trace quantities of Na+(as the cation of the inorganic base). The diluate output stream 7 predominantly comprises water, as well as small amounts of the Na+and SO42-ions from the dissociated inorganic base and inorganic acid. The EDBM unit 160 is described in further detail below with reference to Figure 3.

[0084] The acid stream 6 from the EDBM unit 160 is then recycled back to the ion exchange column being regenerated as an acid recycle stream 6’ to provide the regeneration acid. The base stream 8 is recycled to the fermentation vessel 110 to provide pH control for the fermentation.Because the separation by the EDBM unit 160 is not perfect (i.e. not all of the acid is recovered into the acid stream 6), the acid stream 6, the highly-acidic purified aqueous phase stream 3’ generated by the LLE unit 130 is used to supplement the acid stream 6. A portion of the purified aqueous phase stream 3’ can be bled off via bleed line 16 rather than being recycled. The ion exchange regeneration step is configured such that concentration of acid in the by-product salt stream is minimised, such that efficiency of the subsequent separation of this stream by EDBM is improved.

[0085] It can be appreciated that the PFD in Figure 1 is simplified, and that one or more additional steps in the process / system may be present. For example, the process may further comprise a further ion exchange step, comprising ion exchange of the first product stream 1 or filtered first product stream T, which can remove halide and divalent ions from the aqueous phase stream prior to ion exchange with the hydrogen ion-containing resin in the ion exchange unit 120.

[0086] Figure 2 provides a PFD of a second embodiment of a process according to the first aspect and a gas fermentation system according to the second aspect. The PFD in Figure 2 can be considered a modification to that shown in Figure 1. The differences between the PFDs in Figures 1 and 2 are discussed below. Where features of the PFDs in Figures 1 and 2 are the same, reference may be made back to the description of Figure 1 above. It can be understood that the process and system of Figure 2 may also be used for fermentation of C1 compounds (e.g. C1 hydrocarbons, such as methanol and / or formate).

[0087] Firstly, Figure 2 differs from that of Figure 1 in that the purified aqueous phase stream 3’ is not recycled to the ion exchange regeneration step to make up part of the regeneration acid.

[0088] Additionally, because the purified aqueous phase stream 3’ is not recycled to the ion exchange unit 120, a supplementary acid feed 16 is provided in Figure 2 to complement the acid recycle stream 6’. As discussed above in relation to Figure 1 , the separation provided by the EDBM unit 160 is not perfect and thus supplementary acid is provided by the supplementary acid feed 16 to maintain the ability to regenerate the ion exchange resin in the absence of the highly-acidic purified aqueous phase stream 3’ being recycled.

[0089] Finally, Figure 2 omits the polishing unit 190 of Figure 1 that is used to remove divalent ions and halide ions from the first product stream. Such a polishing unit / step is optional and the need for it is dependent on the composition of the fermentation broth and thus the first product stream.

[0090] Figure 3 provides a schematic of the EDBM unit 160 in Figures 1 and 2. As discussed in relation to Figure 1 , the EDBM unit comprises a three-compartment cell defined by an anion exchange membrane (AEM) 164, a cation exchange membrane (CEM) 163, a first bipolar membrane (BPM) 161 and a second BPM 162. A first compartment is defined between the AEM 164 and the CEM 163, a second compartment is defined between the first BPM 161 and the AEM 164, and a third compartment is defined between the CEM 163 and the second BPM 162. A cathode 165 is positioned adjacent the first BPM 161 and an anode is positioned adjacent the second BPM 162. Each BPM 161, 162 comprises a two layers: an anion exchange layer 161a, 162a and cation exchange layer 161b, 162b. At the interface of the two layers a bipolar junction is formed that results in the disproportionation of water supplied to the compartmentscontaining the BPM into hydrogen ions and hydroxide ions. These ions are then swept away from the bipolar junction by the electric field applied by the cathode 165 and anode 166. As illustrated in Figure 3, hydrogen ions from the first BPM 161 are swept from the first BPM 161 , through the cation exchange layer 161b, into the second compartment (i.e. away from the cathode 165) and hydroxide ions from the second BPM 162 are swept from the second BPM 161 , through the anion exchange layer 162a, into the third compartment. The salt by-product stream 5 comprising Na+and SO42-is fed to the first compartment, where the electric field applied by the cathode 165 and anode 166, and the arrangement of the AEM 164 and CEM 163 promotes the transport of the SO42-anions from the inorganic acid across the AEM 164 and into the second compartment containing the hydrogen ions from the first BPM 161 and the transport of the Na+cations from the inorganic base across the CEM 163 and into the third compartment containing the hydroxide ions from the second BPM 162. This dilutes the salt concentration of the liquid in the first compartment, whilst concentrating the acid and base in the second and third compartments, respectively. In this way, the diluate output stream 7 can be drawn from the first compartment of the cell, the acid stream 6 can be drawn from the second compartment of the cell, and the base stream 8 can be drawn from the third compartment of the cell, as illustrated in Figure 3.

[0091] The EDBM unit may, for example, be a Veolia Ionics™ 3CBPED Stack.

[0092] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0093] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0094] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0095] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0096] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0097] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particularvalue. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

[0098] Examples

[0099] Table 1 below provides a simplified stream table from process simulation of the gas fermentation process and system described in relation to Figure 1.

[0100] As illustrated by the results presented in Table 1 , the process and system of the present invention provides efficient separation of the fermentation broth comprising high concentrations of base and target

[0101] organic acid into an organic phase produc t stream predominantly comprising the target organic acid, and that provides base and acid streams that are can be recycled into the process in place of using virgin feedstock.

[0102] Table 1

[0103]

[0104]

Claims

Claims:

1. A fermentation process for producing a target organic acid, the process comprising:(i) a fermentation step comprising fermentation of one or more carbon sources by a microorganism in a fermentation vessel to produce a first product stream comprising a salt of the target organic acid;(ii) a first separation step comprising ion exchange of the first product stream to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin to produce a second product stream comprising the target organic acid; and(iii) a second separation step comprising separating the second product stream into an aqueous phase stream and an organic phase product stream by liquid-liquid extraction, the organic phase product stream containing the target organic acid.

2. The process according to claim 1 , the process further comprising, following step (ii):(iv) ion exchange resin regeneration using a regeneration acid to extract the cation of the salt of the target organic acid from the ion exchange resin to a by-product salt stream.

3. The process according to claim 2, wherein:the by-product salt stream comprises one or more salts, andthe concentration of the one or more salts in the by-product salt stream is greater than or equal to 0.1 M.

4. The process according to claim 2 or 3, the process further comprising, following step (iv):(v) a third separation step comprising separating the by-product salt stream into an acid stream, a base stream, and a diluate output stream;wherein, optionally, the diluate output stream comprises one or more salts, and the concentration of one or more salts in the diluate output stream is less than or equal to 5% by mass.

5. The process according to claim 4, wherein the third separation step is effected by bipolar membrane electrodialysis.

6. The process according to claim 4 or 5, wherein the process further comprises a step of concentrating the base stream following step (v) and / or a step of concentrating the acid stream following step (v).

7. The process according to any one of claims 4 to 6, wherein at least a portion of the acid stream from step (v) is recycled to the ion exchange regeneration step (iv) to provide the regeneration acid.

8. The process according to any of claims 2 to 7, wherein at least a portion of the aqueous phase stream from step (iii) is recycled to the ion exchange regeneration step (iv) to provide the regeneration acid.

9. The process according to claim 7, wherein the process further comprises supplementing the acid stream recycled to the ion exchange regeneration step (iv) with a supplementary acid stream to provide the regeneration acid.

10. The process according to any preceding claim, wherein the process further comprises:(vi) an acidification step comprising acidifying the second product stream between step (ii) and step (iii) with an acid feed.

11. The process according to any preceding claim, the fermentation step (i) further comprising adding an inorganic base to the fermentation vessel.

12. The process according to claim 11 as dependent on any of claims 4 to 7, wherein at least a portion of the base stream from step (v) is recycled to the fermentation vessel.

13. The process according to any preceding claim, wherein:the target organic acid has an acid dissociation constant, pKa andthe pH of the second product stream reaching step (iii) satisfies the inequality:pKa — pH > 1.

014. The process according to claim 13, wherein the ion exchange of the first separation step results in the pH of second product stream leaving the ion exchange satisfying the inequality pKa - pH > 1.0 .

15. The process according to any preceding claim, wherein process further comprises:a polishing step comprising removal of divalent ions and / or halide ions from the first product stream and / or removal of organic contaminants and / or residues of growth medium from the first product stream;a liquid-solid separation step comprising separating out a solids stream from the first product stream; and / ora concentration step comprising concentrating the first product stream.

16. The process according to any preceding claim, wherein the concentration of the target organic acid in the second product stream is greater than or equal to 0.3 M.

17. The process according to any preceding claim, further comprising subjecting the organic phase product stream from step (iii) to a rectification step to extract an organic solvent therefrom and / or further comprising subjecting the aqueous phase stream from step (iii) to a rectification step to extract an organic solvent and / or the target organic acid / salt of the target organic acid therefrom.

18. The process according to any preceding claim, wherein the fermentation step is at at least 1 m3scale, at least 5 m3scale, at least 50 m3scale, at least 150 m3scale, or at least 250 m3scale.

19. The process according to any preceding claim, wherein:the fermentation process is a gas fermentation;the one or more carbon sources comprise, or consist of, one or more or gases.

20. The process according to any preceding claim wherein the target organic acid is a short-chain fatty acid.

21. A fermentation system for producing a target organic acid, the system comprising:a fermentation vessel for producing a first product stream comprising a salt of the target organic acid;an ion exchange unit configured to exchange a cation of the salt of the target organic acid in the first product stream with a hydrogen ion on an ion exchange resin to produce a second product stream comprising the target organic acid; anda liquid-liquid extraction unit configured to separate the second product stream into an aqueous phase stream and an organic phase product stream, the organic phase product stream containing the target organic acid.

22. The system according to claim 21 , wherein the ion exchange unit is configured to regenerate the ion exchange resin and extract the cation of the salt of the target organic acid therefrom to a by-product salt stream using a regeneration acid.

23. The system according to claim 22, the system further comprising a unit configured to separate the byproduct salt stream into an acid stream, a base stream, and a diluate output stream.

24. The system according to claim 23, wherein the unit configured to separate the by-product salt stream into an acid stream, a base stream, and a diluate output stream is a bipolar membrane electrodialysis unit.

25. The system according to claim 23 or 24, wherein the system is configured to recycle at least a portion of the base stream to the fermentation vessel.

26. The system according to any one of claims 23 to 25, wherein the system is configured to recycle at least a portion of the acid stream to the ion exchange unit to provide the regeneration acid.

27. The system according any of claims 22 to 26, wherein:the system is configured to recycle at least a portion of the aqueous phase stream from the liquidliquid extraction unit to the ion exchange unit to provide the regeneration acid.

28. The system according to claim 26, wherein the system is configured to supplement the acid stream recycled to the ion exchange unit with a supplementary acid stream to provide the regeneration acid.

29. The system according to any of claims 21 to 28, wherein the system further comprises an acidification unit configured to acidify the second product stream from the ion exchange unit with an acid feed.

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