The present invention is directed to a process for converting organic acid salts produced in

5 bioprocesses into organic acids.

Background

Carboxylate product streams originating from a bioprocess are typically very dilute (usually with

10 less than 5-10 wt% of carboxylate salts) and often contain specific impurities, such as nutrient salts. As such, in addition to the acidification step for converting the carboxylate to the respective carboxylic acid, the downstream process also requires concentration and purification steps.

As the stream produced by a bioprocess typically has a low carboxylate concentration, water

15 must be removed to obtain a carboxylate containing stream that can be treated in an economical fashion. Evaporation techniques are commonly used to achieve this objective.

Electrodialysis (ED) is also a common technology used to perform the concentration step. ED converts the carboxylate product streams originating from a bioprocess into a concentrated

20 carboxylate solution that can be further acidified and a depleted carboxylate solution that can be recycled back to the bioprocess. It is known that an ED process can be more energy efficient in removing the first fraction of water compared to evaporation. The remaining water can subsequently be removed by evaporation, such as single stage evaporation, multistage evaporation, and Mechanical Vapor Recompression (MVR) evaporation.

25

The acidification step can be performed by adding inorganic acids, by ion-exchange, or by using electrodialysis with bipolar membranes (EDBM). Ion-exchange is commonly used due to its low cost and technological maturity. However, ion-exchange can be sensitive to impurities, such as those contained in carboxylate product streams originating from a bioprocess. Thus, such

30 streams must be purified before acidification can begin. Another issue with ion-exchange is that the resins need regenerating after saturation. This process not only consumes considerable acid and water but also brings about pollution (i.e. , streams with high chemical oxygen demand (COD) and high concentrations of salt).

35 In EDBM, bipolar membranes disassociate water into hydronium ions (acid) and hydroxyl ions (base) on application of an electrical field. These generated ions combine with cations and anions from a process stream, where the cations and anions are separated by one or more ion exchange membranes in the electrodialysis cell. The combination of the hydronium ions with the anions, and the hydroxyl ions with the cations, results in produced streams having acid and base. In literature, the production of carboxylic acids from their respective carboxylate salts using EDBM is described as a more environmentally friendly alternative for extraction processes or ion exchange.

An issue with EDBM however, is that it can be sensitive to impurities, such as those contained in carboxylate product streams originating from a bioprocess. In particular, the presence of multivalent cations, such as calcium and magnesium, can lead to precipitation of the corresponding hydroxides in the membranes. Thus, it is known to purify carboxylate feed streams before attempting acidification using EDBM.

Another issue with EDBM, is that a minimum conductivity of around 10-20 mS/cm is required for proper operation. In the case of producing carboxylic acids using EDBM, the conductivity of the solution drops to well below 10 mS/cm while converting the carboxylates into their respective carboxylic acids. This results in an increase of the electrical resistance and stack voltage leading to the risk of by-pass currents and local heat formation which deteriorates membranes and may cause reduced lifetime of expensive membranes. As a result, it becomes necessary to reduce the current density, which, in turn, limits the conversion rate and the achievable concentration of base that can be produced, due to back diffusion. Therefore, complete conversion of carboxylate to carboxylic acid using EDBM is technically unfavorable and not economically feasible.

Attempts to solve this issue are described in US 10,946,341 B2, which uses charged resin in the EDBM compartments, and in JP 4 778 308 B2, which describes using spacers composed of cation exchanger nets in the acid compartment. Charged resins can be added to the EDBM to improve conductivity of a compartment but will increase flow resistance and thereby pressure drop will increase. Charged spacer materials can also be used but are not readily and commercially available and need to be specifically designed and produced for an application.

Conductivity can also be improved by adding inert salts or supporting electrolytes to an EDBM process. However, as foreign components to the process, the added salts must be carefully selected in relation to the specific process and products, such that they are inert, so as not to generate additional byproducts, but give sufficient conductivity without precipitation. Furthermore, these salts need to be added in a separate dosing step and need to be removed in an additional separation step. As a result, adding supporting electrolytes to enhance conductivity presents a significant cost and complexity adding factor. Another issue with EDBM is the high level of water required by the system. For proper operation, the concentration of the produced acid and base streams is typically maximized at about 10 wt% (as operating at higher values results in highly inefficient processes with low current efficiency and shortens the life of the expensive membranes). This means that EDBM systems need high levels of water dosage, which ultimately has to be removed in order to generate the final, more concentrated product.

The previously known methods for converting organic acid salts produced in bioprocesses into organic acids therefore suffer from numerous drawbacks, including incomplete conversion to acid, insufficient energy efficiency, high water usage, issues with the water balance, and the need for specialized agents.

Accordingly, there is a need for a process wherein one or more of these issues is addressed.

Summary of the Disclosure

The present inventors have now found a process for converting carboxylic acid salts produced in bioprocesses into carboxylic acids, the process comprising: a) obtaining a carboxylate stream from a bioreactor, the carboxylate stream optionally comprising nutrient salt impurities, b) feeding the carboxylate stream to an electrodialysis (ED) unit and separating the carboxylate stream into a concentrated carboxylate stream and a dilute carboxylate stream, the concentrated carboxylate stream optionally comprising nutrient salt impurities, c) feeding the concentrated carboxylate stream to an acidic compartment of an electrodialysis with bipolar membranes (EDBM) unit, wherein at least a part of the dilute carboxylate stream is fed to a basic compartment of the EDBM unit and wherein at least a part of the dilute carboxylate stream is optionally returned to the bioreactor, and d) optionally purifying a product stream obtained from the acidic compartment of the EDBM unit, wherein at least a portion of a product stream obtained from a basic compartment of the EDBM unit is optionally returned to the bioreactor, and wherein at least a part of a water-based stream extracted during the purification procedure is optionally returned to the bioreactor and/or the EDBM unit. By using the dilute carboxylate stream from the ED unit instead of water in the basic compartment of the EDBM unit and optionally recycling at least part of the dilute carboxylate stream and/or at least part of the basic solution generated by the EDBM unit to the bioreactor, the water balance of the conversion process can be enhanced. Experiments have surprisingly found that the performance of the EDBM is not affected when using the dilute carboxylate stream instead of water in the basic compartment and that connecting these two units is therefore possible.

As demonstrated in the following worked examples, another surprising finding is that complete conversion of the carboxylate to the respective acid is possible by EDBM, without losing energy efficiency at the high conversion level, if nutrient salt impurities from the bioreactor are left to remain in the concentrated stream of the ED. Although experiments without nutrients could reach up to 99% conversion, they showed a decrease in energy efficiency at conversion levels above 90%, requiring increasing voltage above 90% conversion and the need to reduce current density (amperes per square meter membrane area) when maximum voltage was reached. This meant a larger membrane area was required and therefore increased capital expenditure for reaching the same production capacity. In the case where nutrient salts were present, the maximum current density could be maintained during the entire experiment and a conversion of up to 100% could be obtained. Therefore, the presence of bioprocess impurities in the form of nutrient salts (e.g., salts of cations like sodium, potassium and ammonium and anions like chloride, nitrate, nitrite, phosphate, phosphite and sulphate) is beneficial in the acidification step by EDBM, as the nutrient salts improve both conversion and energy efficiency.

The expression “high conversion”, as used herein, refers to conversion levels above 90%, for example, above 95%.

Thus, the integration of ED and EDBM per the presently disclosed process has the following key advantages:

• The water balance can be closed (i.e., minimal to no water addition required by the process due to the high level of water recycle);

• The basic solution generated by the EDBM can be directly recycled, leading to a circular, more efficient material usage;

• High or full conversion to 100% of the organic acid can be obtained; and

• The required energy for EDBM operation can be minimized. Figures

Figure 1 is a flow diagram of preferred embodiments of the present disclosure.

Detailed description

The various aspects of the present invention will be elucidated further below.

As indicated above, in a first aspect, the present invention provides a process for converting carboxylic acid salts produced in bioprocesses into carboxylic acids, the process comprising: a) obtaining a carboxylate stream from a bioreactor, the carboxylate stream optionally comprising nutrient salt impurities, b) feeding the carboxylate stream to an electrodialysis (ED) unit and separating the carboxylate stream into a concentrated carboxylate stream and a dilute carboxylate stream, the concentrated carboxylate stream optionally comprising nutrient salt impurities, c) feeding the concentrated carboxylate stream to an acidic compartment of an electrodialysis with bipolar membranes (EDBM) unit, wherein at least a part of the dilute carboxylate stream is fed to a basic compartment of the EDBM unit, and d) optionally purifying a product stream obtained from the acidic compartment of the EDBM unit.

Step a) comprises obtaining a carboxylate stream from a bioreactor. In this respect, the bioreactor and bioprocesses are not particularly limited. Any suitable bioreactor, such as, but not limited to, a continuous stirred tank bioreactor, a bubble column bioreactor, an airlift bioreactor, a fluidized bed bioreactor, a packed bed bioreactor, photo-bioreactors, or any type of fermenter, may be used. Examples of bioprocesses to which the presently disclosed process may be applied include, but are not limited to, enzymatic, fermentative and/or photo-initiated bioprocesses. As a particular example, the presently disclosed process may be applied to a glycolate stream obtained using the bioprocess described in WO 201 1/036213 A1 or WO 2020/152342 A1 .

In most cases, the carboxylate stream of step a) is an aqueous solution, suspension, slurry, or the like obtained from a bioreactor and comprising typically from 0.5 to 20 wt.%, more typically from 1 to 10 wt.% carboxylates (relative to the total weight of the carboxylate stream). When the carboxylate stream comprises solids, a filtration step may be incorporated between steps a) and b) so as to prevent damage to the ED unit. Preferably, the carboxylate stream comprises at least one, linear or branched, saturated or unsaturated, water-soluble C1-C10, preferably Ci-Cs, more preferably Ci-Ce mono- or polycarboxylate. The carboxylate may be substituted, for example with at least one hydroxyl group. Preferably, the carboxylate is selected from lactate, glycolate, acetate, formate, succinate, citrate, butyrate, tartrate, malate, gluconate, itaconate, propionate, or mixtures thereof. More preferably, the carboxylate is selected from lactate, glycolate, acetate or mixtures thereof. The counterion of the carboxylate is preferably sodium or potassium.

As explained above, the carboxylate stream of step a) may contain nutrient salt impurities. The term “nutrient salt” has its ordinary meaning, namely biological feed (i.e. , “nutrient”) salts found in the biological media used in the bioprocess (i.e., the media used to buffer and feed the biological agents, such as enzymes or cells, in the bioreactor). Such salts include, but are not limited to, salts of cations selected from sodium, potassium and ammonium and anions selected from chloride, nitrate, nitrite, phosphate, phosphite and sulphate. When present, the cations are preferably monovalent in order to avoid precipitation in the membrane of the EDBM unit. In addition, it is preferred that the anions originate from strong acids with a pKa value that is preferably 0.5, more preferably 0.7, and most preferably 1 unit less than the pKa value of the carboxylic acid. When present, it is typical that the carboxylate stream of step a) contains a mixture of two or more nutrient salt impurities, for example, three or more nutrient salt impurities, such as a combination of sodium sulphate, sodium chloride and sodium nitrate.

In a preferred embodiment, the amount of nutrient salt impurities in the carboxylate stream of step a) is from about 0.01 to about 2 wt.%, more preferably from about 0.02 to about 1 wt.%, and most preferably from about 0.05 to about 0.5 wt.%, in each instance relative to the total weight of the carboxylate stream.

Step b) of the process comprises separating the carboxylate stream of step a) into a concentrated stream and a dilute stream. This is achieved by the ED unit. Preferably, the concentrated carboxylate stream may comprise from about 10 to about 30 wt.%, preferably from about 15 to about 25 wt.% carboxylate, and the dilute carboxylate stream may comprise from about 0.1 to about 3 wt.%, preferably from about 0.5 to about 2 wt.% carboxylate, in each instance relative to the total weight of the respective carboxylate stream. The concentrated carboxylate stream may be further concentrated, preferably to about 25-40 wt.%, by incorporating a further concentration step between the ED and EDBM process steps. Step c) of the process comprises feeding the concentrated carboxylate stream of step b) (or optionally further concentrated carboxylate stream described above) to an acidic compartment of the EDBM unit. It is here that the acidification of the carboxylate salt takes place.

Step c) of the process further comprises feeding at least a part, and preferably all or substantially all, of the dilute carboxylate stream to a basic compartment of the EDBM unit. This gives rise to a more efficient process for the reasons set out above.

In addition, at least a part of the dilute carboxylate stream may be returned to the bioreactor. It is further preferred if at least a part, and preferably all or substantially all, of a product stream obtained from a basic compartment of the EDBM unit is returned to the bioreactor. By returning at least a part, and preferably all or substantially all, of a product stream obtained from a basic compartment of the EDBM unit to the bioreactor, the EDBM unit of the presently disclosed process can operate at lower than maximized base concentrations, that is, at base concentrations below 10 wt.% in the basic compartment, without upsetting the water balance and incurring the additional energy requirements associated with evaporation of surplus water when seeking to prepare more concentrated products (as is normally the case). Preferably, the EDBM unit is operated at a base concentration of from 1 to 8 wt.%, more preferably of from 2 to 5 wt.%, for example, at a concentration of from 1 to 8 wt.% NaOH and/or KOH. As demonstrated in the following worked examples, operating at lower base (caustic) concentrations improves the energy efficiency of the EDBM operation, thereby maximizing process efficiency whilst maintaining proper pH control of the bioreactor.

In addition, returning at least a part of the dilute carboxylate stream and/or at least a part, and preferably all or substantially all, of a product stream obtained from a basic compartment of the EDBM unit to the bioreactor, provides the further advantage of recycling any functional nutrient salts contained in these streams to the biological media used in the bioprocess.

In a preferred embodiment, multivalent cations are removed in the ED unit using selective membranes and/or in a separate process step before step b) or c), such as ion-exchange. In the concentrated carboxylate stream, multivalent cations such as calcium and magnesium are preferably below 5 ppm for proper operation of the EDBM to prevent precipitation of the corresponding hydroxides in the membrane.

As explained above, when the concentrated carboxylate stream of step b) contains nutrient salt impurities additional energy efficiency gains can be obtained. As will be appreciated, by concentrating the carboxylate salt in the ED unit, the nutrient salts can be concentrated to the same extent. Thus, in a preferred embodiment, the amount of nutrient salt impurities in the concentrated carboxylate stream is from about 0.05 to about 10 wt.%, more preferably from about 0.1 to about 5 wt.%, and most preferably from about 0.2 to about 2 wt.%, in each instance relative to the total weight of the carboxylate stream. These amounts of nutrient salt impurities have been found to maintain conductivity in the acidic compartment of the EDBM unit at or above 10 mS/cm, while converting the carboxylates into their respective carboxylic acids.

The process of EDBM is known in the art. An overview of the technology can be found in M. Bailly, Desalination, 144, 157-162 (2002). The EDBM unit of the presently disclosed process consists of a EDBM membrane stack and provisions (pumps, piping, vessels, sensors etc.) to allow flow of different fluids through the stack. The membrane stack consists of at least one anode in the anode compartment, multiple cells of consecutive acidic and basic compartments (this is a so-called ‘2-compartment’ or ‘2C’ stack) and at least one cathode in a cathode compartment. The acidic and basic compartments are separated by ion exchange membranes on either side. The membranes are positioned such that each acidic compartment has a bipolar membrane positioned at the anode side with the anion side of the bipolar membrane facing the anode. On the cathode side of the acidic compartment, a cation exchange membrane is positioned. The adjacent compartment of the acidic compartment is a basic compartment. The end cells adjacent to the anode and the cathode compartment are separated from the electrode compartments with a cation exchange membrane, most preferably with a chemically stable (e.g. made of Nation® or another brand of similar (fully or partly) fluorinated type of) cation exchange membrane.

The acidic compartments of the EDBM stack are flushed with the concentrated carboxylate containing stream from the ED process to be acidified and the basic compartments of the EDBM stack are flushed with the dilute carboxylate stream originating from the ED process and optionally with a dilute base stream (e.g., 2-8 wt.% KOH and/or NaOH). The electrode compartments are generally flushed with a dilute base stream from a separate fluid circuit. Gases produced at the electrodes are removed through the fluid circuits. Other electrolytes with suitable electrodes (e.g. NaaSC -aq with DSA® electrodes) can also be applied.

Under the influence of an electric potential difference applied between the anode and the cathode by means of a DC electric power supply, the membranes allow passing of cations (cation exchange membranes) and the splitting of water into acid and base (bipolar membranes) thereby converting the carboxylate in the acidic compartment into carboxylic acid and producing a base in the basic compartment.

Typical conditions of the EDBM process in the present invention are: electrical current density: between 300 and 1500 A/m ² , more preferably between 500 and 1200 A/m ² , most preferably between 700 and 1000 A/m ² .

- temperature: preferably as high as possible but typically a maximum of 60 °C and more preferably a maximum of 40 °C, for example, from 20 to 60 °C, preferably from 30 to 40 °C, to obtain a suitably high membrane lifetime.

- viscosity of the process fluids at process conditions: typically, below 15 mPa.s, more preferably below 10 mPa.s, for example, from 1 to 15 mPa.s, preferably from 1 to 10 mPa.s. Viscosity should be such that sufficient flow is achieved to allow for sufficient cooling of the membrane compartments to avoid (local) overheating of membranes.

The selection of suitable combinations of membranes and electrodes, process configurations and choosing of suitable process conditions is known by people skilled in the art of Electrodialysis.

When employed, the final step of the process, step d), comprises sending the product stream that exits the acidic compartment of the EDBM unit to a purification section. The purification includes removal of any nutrient salt impurities and water by means of generally known methods like ion exchange, membrane separation, crystallization, adsorption, evaporation and distillation. The final product is the carboxylic acid, ideally pure or close to its solubility in water, and preferably containing less than 5 wt.% impurities, more preferably less than 2 wt.% impurities and most preferably less than 1 wt.% impurities, as determined by general analytical methods, such as Inductively Coupled Plasma Optical Emission Spectrometry for determining the content of (residual) cations such as sodium, potassium and other metal ions such as calcium, magnesium, iron etc., and Ion Chromatography for determining the content of anions such as glycolate, chloride, phosphate, nitrate, sulfate, acetate, etc.

To determine the final content of carboxylic acid in the acidic compartment after the EDBM process, potentiometric titration is applied using calibrated sodium hydroxide solution and a combined pH/reference electrode to determine end-point.

In a preferred embodiment, at least a part of a water-based stream (optionally containing salts), extracted from the product stream in the purification section is returned to an earlier stage in the process, e.g., returned to the bioreactor and/or the EDBM unit. Preferably, the extracted water is returned to the basic compartment of the EDBM unit.

A preferred embodiment of the process of the first aspect is illustrated schematically in Figure 1 . Here: F is a feed stream (comprising a carbon containing source), A is a bioreactor, B is an ED unit, C is an EDBM unit and D is a purification section. In the process, a carboxylate stream comprising nutrient salt impurities is obtained from the bioreactor A and fed, via 1 , into the ED unit B. Here, the carboxylate stream is separated into a concentrated and a dilute carboxylate stream, both streams comprising nutrient salt impurities. The concentrated carboxylate stream comprising nutrient salt impurities is fed from the ED unit B into an acidic compartment of the EDBM unit C, via 2, while at least a part of the dilute carboxylate stream is fed from the ED unit B into a basic compartment of the EDBM unit C, via 4. In the EDBM unit C, the carboxylate in the acidic compartment is converted into carboxylic acid and a base is produced in the basic compartment. The carboxylic acid product stream of the acidic compartment of the EDBM unit C is fed, via 3, to the purification section D, where it is purified to a high purity carboxylic acid product stream P.

Further, optional, recycling steps in the process of the first aspect are shown by dashed lines 5, 6, 7 and 8. Here, line 5 is the return of at least a part, preferably all or substantially all, of the base product stream obtained from the basic compartment of the EDBM unit C to the bioreactor A, line

6 is the return of at least a part of the dilute carboxylate stream comprising nutrient salt impurities from the ED unit B to the bioreactor A, and lines 7 and 8 are the return of at least a part of a water-based stream, extracted during the purification procedure, to the bioreactor A and the EDBM unit C, respectively.

When the present process is used to convert sodium glycolate into glycolic acid, the streams in Figure 1 are as follows:

F = feed stream comprising a carbon containing source

1 = solution of sodium glycolate with nutrient salt impurities.

2 = concentrated solution of sodium glycolate with nutrient salt impurities.

3 = solution of glycolic acid with nutrient salt impurities.

4 = 6 = diluted solution of sodium glycolate (with possible nutrient salt impurities).

5 = sodium hydroxide solution with sodium glycolate (and possible nutrient salt impurities).

7 = 8 = water-based solution (with possible nutrient salt impurities).

P = high purity glycolic acid solution.

In a second aspect, the present invention provides a system for converting carboxylic acid salts produced in bioprocesses into carboxylic acid, the system comprising: a) an electrodialysis (ED) unit configured to receive a carboxylate stream optionally comprising nutrient salt impurities from a bioreactor and then separate the carboxylate stream into a concentrated carboxylate stream and a dilute carboxylate stream, the concentrated carboxylate stream optionally comprising nutrient salt impurities; and b) an electrodialysis with bipolar membranes (EDBM) unit, wherein an acidic compartment of the EDBM unit is configured to receive the concentrated carboxylate stream from the ED unit, wherein a basic compartment of the EDBM unit is configured to receive at least a part of the dilute carboxylate stream from the ED unit, and wherein the EDBM unit is optionally configured to return at least a part of a product stream from the basic compartment of the EDBM unit to the bioreactor.

The ED unit and the EDBM unit can be operated in batch, or in a ‘feed and bleed’ or continuous configuration.

In batch operation, a predetermined amount of the carboxylate stream or the concentrated carboxylate stream is fed into the ED or EDBM unit, respectively, and the processes are run until the desired product concentrations are achieved. The products are then sent for further treatment, as required.

In ‘feed and bleed’ operation, a predetermined flow of the carboxylate stream or the concentrated carboxylate stream is fed into the ED or EDBM unit, respectively, in a continuous manner, and, at the same time, the excess volume is continuously discharged from the process so as to maintain constant concentrations of desired levels in the product streams. The product streams are then sent for further treatment, as required.

Optionally, the concentrate and the diluate stream of the ED unit, and/or the acid and the base stream of the EDBM unit, can be operated individually in batch mode or in continuous mode or both. Depending on the desired operation, people skilled in the art of ED or EDBM can chose the optimal configuration and conditions for their process case.

In a preferred embodiment, the ED unit is configured to return at least a part of the dilute carboxylate stream to the bioreactor.

Preferably, the product stream obtained from the basic compartment of the EDBM unit comprises 1 to 8 wt.% caustic (being NaOH and/or KOH), preferably 2 to 5 wt.%.

Preferably, the system further comprises a purification section configured to receive a product stream from the acidic compartment of the EDBM unit and return a water-based stream, extracted from the product stream, to the bioreactor and/or the EDBM unit. Preferably, the purification section is configured to return water extracted from the product stream to the basic compartment of the EDBM unit.

Accordingly, in a most preferred embodiment, the system comprises: a. a bioreactor, b. an electrodialysis (ED) unit configured to receive a carboxylate stream optionally comprising nutrient salt impurities from a bioreactor and then separate the carboxylate stream into a concentrated carboxylate stream and a dilute carboxylate stream, the concentrated carboxylate stream optionally comprising nutrient salt impurities, wherein the ED unit is optionally configured to return at least a part of the dilute carboxylate stream to the bioreactor; c. an electrodialysis with bipolar membranes (EDBM) unit, wherein an acidic compartment of the EDBM unit is configured to receive the concentrated carboxylate stream from the ED unit, wherein a basic compartment of the EDBM unit is configured to receive at least a part of the dilute carboxylate stream from the ED unit, and wherein the EDBM unit is optionally configured to return at least a part of a product stream from the basic compartment of the EDBM unit to the bioreactor; and d. a purification section configured to return a water-based stream, extracted from a product stream obtained from the acidic compartment of the EDBM unit to the bioreactor and/or EDBM unit, preferably to the basic compartment of the EDBM unit.

It is noted that various elements of the presently disclosed process, including but not limited to preferred ranges for the various parameters, can be combined unless they are mutually exclusive.

The present invention will be elucidated by the following examples without being limited thereto or thereby.

Example 1 : Effect of dilute sodium glycolate in the EDBM basic compartment

The experiments were performed in a EURODIA 2-compartment electrodialysis membrane stack containing 3 cells with a bipolar membrane area of 3 times 0.02 m ² . The stack was connected to a DC power supply and an acid, a base and an electrolyte feed vessel. The acidic compartment was equipped with a pH and conductivity meter with an integrated thermometer and the basic compartment was equipped with a conductivity meter. The acid, base and electrolyte solutions were recycled with the help of three pumps. The electric current was kept constant at 14 Amperes (A) as long as the electrode potential difference was below a maximum value of 10 Volts (V). As soon as the potential exceeded 10 V, the current was decreased to maintain the potential at 10 V. The temperature of the EDBM was kept below 40 °C by the acid and base feed solutions, which were cooled below 40 °C by means of cooling water in the double wall of the feed vessels. The current and potential were monitored as well as the conductivity in the acidic and basic compartments. During the experiment deionized water was dosed in several portions to the base vessel to keep the conductivity, and as a consequence the base concentration, constant. The electrolyte feed vessel contained the same base at the same concentration level as the basic compartment to have a good conductivity and little to non-diffusional transport between the base and the electrode compartments.

At set time intervals, samples from the acidic compartment were taken and analysed on glycolic acid (GA) concentration by means of titration. The sodium ion concentration in the acidic compartment samples was determined by means of radially viewed Inductively Coupled Plasma Emission Spectrometry (ICP-ES, Agilent 5110). Samples of the base were taken as well to determine the actual NaOH concentration in the basic compartment by an acid-base titration method.

Experiment without dilute sodium glycolate in the EDBM basic compartment (A): The acid feed vessel was filled with 2500 g of a 25 wt.% sodium glycolate solution. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. During the experiment, 2592 g of water was dosed to the base feed vessel. At the end of the experiment the acid feed vessel contained 1707 g glycolic acid solution and the base feed vessel contained 5335 g NaOH solution.

Experiment with dilute sodium glycolate in the EDBM basic compartment (B): The acid feed vessel was filled with 2500 g of a 25 wt.% sodium glycolate solution. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. To the base solution, 1 wt.% of sodium glycolate was added. During the experiment, 2637 g of water was dosed to the base feed vessel. At the end of the experiment, the acid feed vessel contained 1860 g glycolic acid solution and the base feed vessel contained 5420 g NaOH solution.

Results of experiments (A) and (B) are shown in Tables 1 A and 1 B, respectively. The conversion is calculated according to: (Glycolic acid concentration measured [wt.%]/ 76 * total weight))/ (NaGlycolate concentration [wt.%]/ 98 * weight in acid feed vessel), where total weight = weight in acid feed vessel + weight in acidic compartment - water transported. At high conversion levels the acid conversion is more accurately calculated from the sodium ion concentration in the acid. As a sodium ion is replaced by a proton, the concentration of sodium ions is a measure for the presence of unconverted (non-protonated) carboxylate ions.

The current density is calculated according to: current / surface area membrane stack.

The energy needed per mole of acid formed in the interval between two sampling points is calculated according to: (average voltage x average current x Atime)/(Aconversion x moles glycolate in)

Table 1 A: Results of EDBM acidification without 1 wt. % glycolate added to basic compartment

Table 1B: Results of EDBM acidification with 1 wt.% glycolate added to basic compartment This example shows that the performance of the EDBM, with respect to conversion level, voltage and energy requirement, is unaffected by the presence of 1 wt.% sodium glycolate in the base solution. One can even observe a benefit in the energy requirement per mole acid formed. Therefore, it can be concluded that a dilute carboxylate salt stream, such as the dilute stream of the ED unit, can be used as feed stream for the base compartment of the EDBM. Example 2: EDBM performance without nutrient salts (A) and with nutrient salts (B)

In this example the procedure described in Example 1A was followed, but nutrient salts were added to the acid solution at a concentration of 0.4 wt%.

The experiments were performed in a EURODIA 2-compartment electrodialysis membrane stack containing 9 cells with a bipolar membrane area of 9 times 0.02 m ² . The electric current was kept constant at 16 Amperes (A) as long as the electrode potential difference was below a maximum value of 25 Volts (V). As soon as the potential exceeded 25 V, the current was decreased to maintain the potential at 25 V.

Experiment without nutrient salts (A): The acid feed vessel was filled with 3500 g of a 25 wt.% sodium glycolate solution. The base feed vessel was filled with 2000 g of a 8 wt.% NaOH solution. During the experiment, 4371 g of water was dosed to the base feed vessel. At the end of the experiment the acid feed vessel contained 2611 g glycolic acid solution and the base feed vessel contained 7237 g NaOH solution.

Experiment with nutrient salts (B): The acid feed vessel was filled with 3566 g of a 24.54 wt.% sodium glycolate solution. To this solution, 8.4 g sodium-nitrate, 3.9 g sodium-sulphate and 2.3 g sodium-chloride was added. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. During the experiment, 4680 g of water was dosed to the base feed vessel. At the end of the experiment, the acid feed vessel contained 2696 g glycolic acid solution and the base feed vessel contained 7523 g NaOH solution.

Results of experiments (A) and (B) are shown in Tables 2A and 2B, respectively. In case of nutrient salts being present, the calculation of the concentration of carboxylate ions from the sodium concentration is corrected for sodium ions from the nutrient salts. Thus, the conversion at levels above 90% is calculated according to: 1 - (Na concentration measured/23 [wt.%] * total weight - Na from nutrients)/(NaGlycolate concentration [wt.%]/98* weight in acid feed vessel).

Table 2A: Results of glycolate acidification in the absence of nutrient salts

Table 2B: Results of glyco late acidification in the presence of 0.4 wt% nutrient salts

The results show that when nutrient salts are present in the acid feed stream to the EDBM unit, total conversion of the acid is reached, as opposed to the experiment without nutrient salts (max. 99.4 % conversion).

In addition, it can clearly be seen that above 90% conversion the energy required per mole of acid formed increases in both cases, but significantly more when there are no nutrient salts present. At conversion levels above 95% the energy required is at least 25% less when nutrient salts are used. The comparative energy efficiency increases further at conversion levels near 100%.

Tables 2A and 2B also show that, in the absence of nutrient salts, the current density drops by 20-30% above a conversion level of 90% because the conductivity becomes too low when reaching a level well below 10 mS/cm. In contrast, when nutrient salts are present, the conductivity stays at or above 10 mS/cm and the current density can be maintained throughout the entire experiment. The current density is a measure for the capacity of the stack.

The time to reach 99.5% conversion is 150 min without nutrients salts and 135 min with nutrients salts. When comparing the two experiments the overall capacity is over 10% larger when nutrients salts present. Therefore, it can be concluded that the overall EDBM performance, and in particular the performance at the high conversion level, is increased in case nutrient salts are present in the acid feed stream to the EDBM.

Example 3: EDBM performance at higher nutrient concentrations

In this example, the procedure described in Example 2 was followed, but nutrient salts were added to the acid solution at a concentration of 1 wt.%.

The acid feed vessel was filled with 2500 g of a 35 wt.% sodium glycolate solution. To this solution, 16.8 g sodium-nitrate, 7.8 g sodium-sulphate and 4.6 g sodium-chloride was added. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. During the experiment, 4583 g of water was dosed to the base feed vessel. At the end of the experiment, the acid feed vessel contained 1844 g glycolic acid solution and the base feed vessel contained 7159 g NaOH solution. The results are shown in Table 3.

Table 3: Results of glycolate acidification in the presence of 1 wt.% nutrient salts

Again 100 % conversion to glycolic acid is reached. This example also shows that higher concentrations of nutrient salts in the acidic compartment, and as a result, a higher minimal conductivity of 15 mS/cm, is even more beneficial with respect to the energy requirement per mole of acid in the high conversion range of 90 -100%.

Example 4: Effect of base concentration on EDBM operation

In this example, the procedure described in Example 2 was followed, but the base concentration in the basic compartment was varied.

The acid feed vessel was filled with 3500 g of a 25 wt.% sodium glycolate solution. To this solution, nutrient salts were added: 16.8 g sodium-nitrate, 7.8 g sodium-sulphate and 4.6 g sodium-chloride. The base feed vessel was filled with 2000 g of either a 9 wt.% NaOH solution (Experiment A), 7 wt.% NaOH solution (Experiment B) or 5 wt.% NaOH solution (Experiment C). During the experiments, water was dosed in several portions to the base feed vessel to keep the NaOH concentration in the basic compartment within the specified ranges (i.e., 8 - 9 wt.% for Experiment A, 6.5 - 7 wt.% for Experiment B, and 4.5 - 5 wt.% for Experiment C).

The average energy needed per mole of acid formed was determined for conversion ranges of 0- 80 %, 0 - 90% and 0 - 100%. The results are shown in Table 4.

Table 4: Average energy needed per mole of GA formed vs EDBM base concentration

It is shown in Table 4 that the required energy/mole of acid formed decreases at lower NaOH concentrations. In this example, decreasing the NaOH concentration from 9 wt.% to 5 wt.% results in a decrease of required energy of 25%. Therefore, it can be concluded that operating the EDBM at lower base concentrations is beneficial with respect to energy use.

Example 5: Effect of dilute sodium glycolate in the EDBM basic compartment with nutrient salts present in the acidic compartment

In this example, the procedure described in Example 1 B was followed, but nutrient salts were added to the acid solution.

The experiments were performed in a EURODIA 2-compartment electrodialysis membrane stack containing 9 cells with a bipolar membrane area of 9 times 0.02 m ² . The electric current was kept constant at 16 Amperes (A) as long as the electrode potential difference was below a maximum value of 25 Volts (V). As soon as the potential exceeded 25 V, the current was decreased to maintain the potential at 25 V.

The acid feed vessel was filled with 3500 g of a 25 wt.% sodium glycolate solution. To this solution, 16.8 g sodium-nitrate, 7.8 g sodium-sulphate and 4.6 g sodium-chloride was added. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. To the base solution, 1 wt.% of sodium glycolate was added. During the experiment, 4476 g of water was dosed to the base feed vessel. At the end of the experiment, the acid feed vessel contained 2561 g glycolic acid solution and the base feed vessel contained 7417 g NaOH solution. The results are shown in

Table 5.

Table 5: Results of EDBM acidification in the presence of nutrient salts with 1 wt.% glycolate added to basic compartment

This example shows that the performance of the EDBM is comparable to Example, 2B and 3 with respect to conversion level, voltage and energy requirement, demonstrating that also in the case of nutrient salts in the acid solution, the presence of 1 wt.% sodium glycolate in the base solution does not have a negative impact on the operation of the EDBM. Therefore, it can be concluded that a dilute carboxylate salt stream, such as the dilute stream of the ED unit, can be used as feed stream for the base compartment of the EDBM.

Example 6: Performance for Lactate (A) and Acetate (B)

In this example, the procedure described in Example 2 was followed, but sodium lactate solution (A) or sodium acetate solution (B) was used instead of sodium glycolate solution.

Experiment with lactate (A): The acid feed vessel was filled with 3997 g of a 25 wt.% sodium lactate solution. To this solution, 16.8 g sodium-nitrate, 7.8 g sodium-sulphate and 4.6 g sodiumchloride was added. The base feed vessel was filled with 2001 g of an 8 wt.% NaOH solution. During the experiment 4107 g of water was dosed to the base feed vessel. At the end of the experiment the acid feed vessel contained 31 17 g lactic acid (LA) solution and the base feed vessel 7010 g NaOH solution. The results are shown in Table 6A.

Table 6A: Results of lactate acidification in the presence of nutrient salts

Experiment with acetate (B): The acid feed vessel was filled with 2928 g of a 25 wt.% sodium acetate solution. To this solution, 16.8 g sodium-nitrate, 7.8 g sodium-sulphate and 4.6 g sodiumchloride was added. The base feed vessel was filled with 2000 g of an 8 wt.% NaOH solution. During the experiment 3959 g of water was dosed to the base feed vessel. At the end of the experiment the acid feed vessel contained 2181 g acetic acid (AcA) solution and the base feed vessel 6715 g NaOH solution. The results are shown in Table 6B.

Table 6B: Results of acetate acidification in the presence of nutrient salts

These experiments show that processing a lactate or an acetate solution with nutrient salts in the EDBM unit, results in similar performance as shown for a glycolate solution with nutrient salts, and that 100 % conversion to their respective acids can be reached.

Whilst the invention has been described with reference to an exemplary embodiment, it will be appreciated that various modifications are possible within the scope of the invention.

In this specification, unless expressly otherwise indicated, the word ‘or’ is used in the sense of an operator that returns a true value when either or both of the stated conditions is met, as opposed to the operator ‘exclusive or’ which requires that only one of the conditions is met. The word ‘comprising’ is used in the sense of ‘including’ rather than to mean ‘consisting of’. All prior teachings acknowledged above are hereby incorporated by reference. No acknowledgement of any prior published document herein should be taken to be an admission or representation that the teaching thereof was common general knowledge in Europe or elsewhere at the date hereof.