Formic acid concentration

Abstract

The invention is in the field of formic acid production. In particular the present invention is directed to a method and system for the production of a concentrated aqueous formic acid solution. There is provided a process for the production of a concentrated aqueous formic acid solution, comprising the steps of: - providing a first aqueous solution comprising formate ions and one or more metal ions, - subjecting the first solution to a first electrodialysis step, wherein the first electrodialysis step comprises bipolar membrane electrodialysis, thereby obtaining an acidic second aqueous solution comprising formic acid, an alkaline third aqueous solution, and a diluate, - subjecting the second aqueous solution comprising formic acid to a second electrodialysis step, thereby obtaining a concentrated aqueous formic acid solution.

Description

[0001] P137474PC00

[0002] Title: Formic acid concentration

[0003] BACKGROUND OF THE INVENTION

[0004] The invention is in the field of formic acid production. In particular the present invention is directed to a method and system for the production of a concentrated aqueous formic acid solution.

[0005] Formic acid is a chemical that is expected to play an important role in the energy transition. Among others, formic acid can serve as a hydrogen carrier for e.g. fuel cells, or as an intermediate for the production of combustion fuels. Formic acid can be produced from non-fossil sources, for instance using electrochemical reduction of aqueous carbon dioxide solutions. Therefore, the use of formic acid in fuel applications can prevent release of carbon dioxide from fossil sources into the earth’s atmosphere, thereby contributing to a more sustainable future.

[0006] A challenge associated with electrochemical reduction of carbon dioxide to formic acid is that the electrochemical process typically produces an aqueous stream with relatively low concentrations of ionic product such as potassium formate. The concentration of formate is typically ca. 1-2 M. For further use or downstream processing of formic acid, the ionic product is typically treated in order to convert formate (e.g. in the form of potassium formate) to formic acid and subsequently concentrated.

[0007] In the art, different approaches for converting formate salts to formic acid are used in order to obtain a formic acid stream. For instance, a formate solution can be acidified with HC1, followed by chromatographic separation. However, acidification with HC1 leads to further dilution of the product stream and the process results in large amounts of waste. For instance, acidification of potassium formate with HC1 produces KC1, which needs to be regenerated or treated as a contaminated wastewater stream, which is a disadvantage in terms of energy efficiency and environmental impact.

[0008] Another approach for acidification is the use of ion exchange resins which can exchange the K+ in potassium formate for H+ in order to obtain formic acid. However, the use of ion exchange resins is not economically viable for large scale processes. In addition, regeneration of the ion exchange resins leads to a stream containing both HC1 and KC1, which are difficult to separate and to regenerate.

[0009] Concentrating an aqueous formic acid solution is difficult, because formic acid and water form an azeotrope. In such situations, special distillation processes such as extractive distillation, in which a solvent (e.g. toluene, MIBK, n-octanol) is added to aid separation between water and formic acid, or pressure swing distillation can be applied. However, these distillation methods are typically not energy efficient or need additional steps in order to obtain the concentrated formic acid.

[0010] An object of the present invention is to provide an energy-efficient process to obtain a concentrated formic acid solution from a dilute formate salt solution. Another object of the invention is to provide a process in which waste streams are minimized.

[0011] BRIEF SUMMARY OF THE INVENTION

[0012] In accordance with the present invention there is provided a process for the production of a concentrated aqueous formic acid solution.

[0013] The process comprises the steps of: - providing a first aqueous solution comprising formate ions and one or more metal ions, - subjecting the first solution to a first electrodialysis step, wherein the first electrodialysis step comprises bipolar membrane electrodialysis, thereby obtaining an acidic second aqueous solution comprising formic acid, an alkaline third aqueous solution, and a diluate, - subjecting the second aqueous solution comprising formic acid to a second electrodialysis step, thereby obtaining a concentrated aqueous formic acid solution.

[0014] The inventors have found that by performing bipolar membrane electrodialysis followed by a second electrodialysis step, a concentrated formic acid solution of ca. 15 wt.% or even higher can surprisingly be obtained using an electrodialysis process, without needing to resort to energy intensive processes such as distillation. This concentration is enough to perform downstream processes on the formic acid, such as microbial conversion of formic acid into hydrocarbons.

[0015] Because the process does not rely on the addition of other chemicals to the aqueous phase in the formation of formic acid or concentration, streams can easily be recycled, which also positively affects the energy usage and environmental impact of the entire process of formic acid production.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic representation of a process for the production of a concentrated aqueous formic acid solution.

[0018] Figures 2-5 show experimental results for electrodialysis and bipolar membrane electrodialysis processes for producing a concentrated formic acid solution.

[0019] DETAILED DESCRIPTION OF THE INVENTION

[0020] According to the invention there is provided a process for the production of a concentrated aqueous formic acid solution.

[0021] The process comprises a step of providing a first aqueous solution comprising formate ions and one or more metal ions. For instance, the solution may comprise sodium formate and / or potassium formate. In addition, the first aqueous solution may also comprise other compounds, for instance acidic and / or basic compounds such as CO2, bicarbonate, and / or their conjugated acids and / or bases.

[0022] In embodiments, the concentration of formate in the first aqueous solution is 0.1-3.0 M, preferably 0.5-2.0 M. These concentrations of formate can for instance be found in product streams coming from an electrolyzer cell in which carbon dioxide is electrochemically reduced into formate.

[0023] Negatively charged ions with a higher electronegativity than formate, for instance halide ions, may negatively influence the performance of the process. Therefore, the concentration of such ions in the first aqueous solution, if any, is preferably lower than the concentration of formate ions. For instance, the concentration of such ions is preferably 1.0 % by weight or less relative to the weight of entire first aqueous solution, more preferably 0.5 wt.% or less, such as 0.2 wt.% or less.

[0024] The first solution is subjected to bipolar membrane electrodialysis. The term electrodialysis, as used herein, refers to a process in which ions are transported through ion-exchange membranes under the influence of an applied electric field. In electrodialysis, anion and / or cationexchange membranes are used to create separate compartments between an electrode pair. When a potential difference is applied between the electrodes, the membranes selectively allow certain ions to permeate, thereby increasing the concentration of certain ions in some compartments, and decreasing the concentration of those ions in other compartments. Typically, a stack of alternation anion- and cation-exchange membranes is used, resulting in alternating compartments containing an increased concentration of ions (i.e., the concentrate) or containing a decreased concentration of ions (i.e., the diluate), respectively. Depending on the application, the diluate and / or the concentrate may be used for further process steps.

[0025] Bipolar membrane electrodialysis is a specific type of electrodialysis in which bipolar membranes are applied in the electrodialysis stack. Bipolar membranes comprise a cation-exchange layer and an anion-exchange layer, and can be used to split water into H+and OH-. In this way, bipolar membrane electrodialysis can be used to produce an acidic concentrate, an alkaline concentrate and a diluate.

[0026] By subjecting the first solution to bipolar membrane electrodialysis, formate ions are acidified to form a second aqueous solution comprising formic acid (i.e., the acidic concentrate comprising from BPMED). Preferably, in the bipolar membrane electrodialysis step, the conditions are chosen such that the concentration of acidic concentrate stays below 2.5 M, more preferably below 2.0 M, even more preferably below 1.5 M, such as 0.5-2.0 M or 0.75-1.5 M. At higher concentrations, the current efficiency of the bipolar membrane electrodialysis step may be negatively affected due to unwanted diffusion of formic acid and leakage of OH- in the bipolar membrane electrodialysis stack.

[0027] Advantageously, bipolar membrane electrodialysis can produce an acidic concentrate comprising formic acid from solutions comprising formate, without the need for adding additional chemicals to convert formate into formic acid. This means that the acidic concentrate, alkaline concentrate and / or diluate can be used further without the need to apply additional separation steps in order to remove chemicals that are otherwise often used for acidification.

[0028] The process further comprises a step of subjecting the second aqueous solution comprising formic acid to a second electrodialysis step, thereby obtaining a concentrated aqueous formic acid solution. Surprisingly, by subjecting the acidic concentrate comprising formic acid from the first bipolar membrane first electrodialysis step to a second electrodialysis step, a concentrated aqueous formic acid solution can be obtained in an energyefficient way. A concentrated formic acid solution having a formic acid concentration of 15 % or more by weight relative to the weight of the solution, or even 20 wt.% or more, can be achieved using electrodialysis, without significant reduction of the current efficiency which is observed in bipolar membrane electrodialysis step.

[0029] Without wishing to be bound by theory, the inventors believe that the improved current efficiency and higher formic acid concentrations that can be reached using (regular) electrodialysis of formic acid compared to bipolar membrane electrodialysis of formate are caused by the absence of OH- produced by bipolar membranes, meaning that there is less neutralization and efficiency loss due to OH- leakage. Therefore, by separating the steps of acidification of formate in a first bipolar membrane electrodialysis step and concentrating of formic acid in a second electrodialysis step, a more efficient way of producing a concentrated formic acid solution is possible.

[0030] The inventors also realized that such concentrations that can be achieved are enough to perform downstream processes on the formic acid such as microbial conversion of formic acid into hydrocarbons, and therefore there is less need to resort to more energy intensive concentration steps such as distillation. However, if higher concentrations of formic acid are desired, the concentrated formic acid solution from the second electrodialysis step can be subjected by one or more further concentration steps, for instance selected from freeze crystallization and / or distillation. If applied, the freeze concentration step can be driven by a refrigeration cycle comprising one or more stages in order to achieve the desired concentration of formic acid.

[0031] In order to prevent transport of water through the membranes in the first and / or second electrodialysis step, ion-exchange membranes designed for low water flux are preferred. This allows to operate the electrodialysis steps at high current density without transporting too much water, which would negatively affect the concentration. Preferably, ionexchange membranes with a water permeability of 5.0 mL / (bar x m2x h) or lower, more preferably below 3.0 mL / (bar x m2x h), even more preferably 2.5 mL / (bar x m2x h) or lower, preferably measured at NaCl concentrations between 0.1-0.7 M, are used in the first and / or second electrodialysis step. Suitably, water permeability of an ion-exchange membrane for a certain NaCl concentration can be measured by bringing a draw solution with that NaCl concentration in contact with a pure water solution through said membrane, and by measuring the rate of water permeation from the feed solution into the draw solution over time.

[0032] Preferably, the first and / or second electrodialysis steps are performed at current densities of 200-1000 A / m2, more preferably 300-500 A / m2.

[0033] Preferably, the first and / or second electrodialysis step are operated continuously. By using a feed with constant concentration, resistance is kept low and current efficiency is high compared to batch processes.

[0034] The first aqueous solution comprising formate ions and one or more metal ions may be the result of electrochemically reducing carbon dioxide. Advantageously, the process of the present invention can be applied to the product stream from an electrolyzer. Therefore, in embodiments, the process further comprises a step of electrochemically reducing carbon dioxide, thereby producing an aqueous solution comprising formate ions and one or more metal ions as electrolysis product. Such electrochemical reduction of carbon dioxide is for instance described in WO2017014635, which is incorporated herein in its entirety. This electrolysis product can be used directly as first aqueous solution for the first electrodialysis step. Alternatively or additionally, the electrolysis product can be concentrated before subjecting it to the first electrodialysis step. Concentration can for instance be done using reverse osmosis.

[0035] Because the electrodialysis steps do not rely on addition of chemicals in order to acidify formate ions and concentrate formic acid, there is no need for extensive waste stream treatments, and streams can be readily recycled without needing extensive purification steps. In embodiments, the diluate obtained in the first electrodialysis step is recycled, preferably as feed in the first electrodialysis step.

[0036] In embodiments, the diluate obtained in the second electrodialysis step is recycled, preferably in the acidic compartment of the first electrodialysis step.

[0037] In embodiments, the first alkaline concentrate of the electrodialysis step is recycled, for instance as electrolyte in an optional step of electrochemically reducing carbon dioxide.

[0038] When streams are recycled, they may be subjected to purification and / or concentration steps (e.g. selected from reverse osmosis, electrodialysis and freeze concentration) prior to being recycled.

[0039] According to the invention, there is also provided a system suitable for the production of a concentrated aqueous formic acid solution, preferably using the process as described herein.

[0040] The system may comprise an electrolysis unit configured for electrochemical reduction of carbon dioxide.

[0041] The system comprises a first electrodialysis unit comprising bipolar membranes (BPM-ED unit), suitable for performing bipolar membrane electrodialysis. The electrodialysis unit preferably comprises an inlet, which inlet is preferably configured to receive a product stream from the electrolysis unit, if present. The electrodialysis unit preferably also comprises at least an outlet for acidic concentrate.

[0042] The system also comprises a second electrodialysis unit configured for receiving the acidic concentrate of the first electrodialysis unit.

[0043] Optionally, the system comprises a concentration unit for further concentrating the concentrated formic acid stream from the second electrodialysis unit.

[0044] The system may also comprise means for recycling one or more streams, such as a diluate or the alkaline concentrate from the first electrodialysis unit, a diluate from the second electrodialysis unit, and / or a diluate from the concentration unit. The system may optionally also comprise one or more purification and / or concentration units, such as a reverse osmosis unit, for purifying and / or concentrating one or more recycle streams.

[0045] Figure 1 is a schematic representation of a process and / or system for the production of a concentrated aqueous formic acid solution. First aqueous solution 10 comprising formate ions and one or more metal ions is subjected to bipolar membrane electrodialysis in first electrodialysis unit 1. Second aqueous solution 11 comprising formic acid is subjected to a second electrodialysis step in second electrodialysis unit 2, thereby obtaining a concentrated aqueous formic acid solution 12. First aqueous solution 10 may be obtained from electrolyzer 5, in which carbon dioxide may electrochemically be reduced into formate. In addition to second aqueous solution 11, the electrodialysis step may also produce alkaline concentrate 13 which may be recycled as electrolyte to electrolyzer 5, and / or diluate 14, which may be recycled as feed for bipolar membrane electrodialysis. If diluate 14 is recycled for bipolar membrane electrodialysis, it may be concentrated in reverse osmosis unit 4, to make the concentration of formate ions in stream 15 suitable for being subjected to bipolar membrane electrodialysis again. Diluate 18 obtained in the second electrolysis step may be recycled, for instance to the acidic compartment of the first electrodialysis unit (not shown). Concentrated aqueous formic acid solution 12 is optionally further concentrated in concentration unit 3, obtaining further concentrated formic acid solution 20 and a further diluate 19, which may also be recycled (not shown). EXAMPLES

[0046] Comparative example 1

[0047] Electrodialysis with bipolar membranes (ED-BPM) was performed using a stack with five repeating units of a bipolar membrane, an anion- exchange membrane and a cation-exchange membrane. Each membrane had an area of 10 x 10 cm, resulting in an effective surface area of the membranes of 0.15 m2.

[0048] The different compartments were provided with inlets / outlets for, respectively, the feed / diluate, the acidic concentrate, the alkaline concentrate, and the electrolyte. A flow rate of 600 mL / min was used.

[0049] In this example, a 1.0 M potassium formate solution was used as feed for the bipolar membrane electrodialysis, and a 0.5 M Na2SC>4 solution was used as electrolyte. Experiments were conducted at current densities of 300 A / m2and 500 A / m2, respectively.

[0050] During the experiment, pH and conductivity of the different streams were measured using a pH / conductivity meter. In addition, the concentration of the different streams was measured using titration, and changes in volume were recorded. The voltage over the membrane stack (5 repeating units) was also measured.

[0051] Results for both formic acid and potassium hydroxide at both current densities are shown in figures 2-5. It can be observed that the concentration of formic acid in the acidic concentrate and the concentration of KOH in the alkaline concentrate increased over time. However, the current efficiency dropped considerably with increasing concentrations. At a formic acid concentration of 2.5 M, the current efficiency had already dropped to below 70 % for the experiment at 500 A / m2and even to below 50% for the experiment at 300 A / m2.

[0052] It was found that the decrease of current efficiency was mainly caused by diffusion of formic acid to the feed / diluate compartment and the alkaline concentrate compartment, and leakage of OH- to the feed / diluate compartment and the acidic concentrate compartment, leading to neutralization of the species. Another factor contributing to decreasing current efficiency was water transport over the membranes.

[0053] Example 1

[0054] The same experiment as in comparative example 1 was performed, except that an electrodialysis (ED) stack without bipolar membranes was used for electrodialysis of a 1.0 M formic acid solution. As electrolyte, a 1.0 M potassium formate solution was used. The experiment was conducted at a current density of 300 A / m2.

[0055] Because the ED stack does not have bipolar membranes, there is no compartment for alkaline concentrate, and the different compartments were provided with inlets / outlets for, respectively, the feed / diluate, the acidic concentrate, and the electrolyte. The total effective surface area of the membranes (5 repeating units) was 0.10 m2.

[0056] The results for the concentration step using ED are also shown in figures 2-5.

[0057] When comparing the current efficiency of the electrodialysis of formic acid to the bipolar membrane electrodialysis of potassium formate, a remarkable difference is observed. When using ED to concentrate formic acid, the current efficiencies were much higher than for BPM-ED. Even at high formic acid concentrations above 5.0 M, current efficiency for ED was still above 80%.

[0058] This is also reflected in the specific energy consumption (SEC) of ED vs. ED-BPM, as shown in figure 5. For ED-BPM, the costs per kg of formic acid are 0.9- 1.5 kWh / kg at 300 A / m2, and 1.5-2.6 kWh / kg at 500 A / m2, whereas the costs per kg of formic acid are only 0.44-0.58 kWh / kg at 300 A / m2.

Claims

Claims1. Process for the production of a concentrated aqueous formic acid solution, comprising the steps of:- providing a first aqueous solution comprising formate ions and one or more metal ions,- subjecting the first solution to a first electrodialysis step, wherein the first electrodialysis step comprises bipolar membrane electrodialysis, thereby obtaining an acidic second aqueous solution comprising formic acid, an alkaline third aqueous solution, and a diluate,- subjecting the second aqueous solution comprising formic acid to a second electrodialysis step, thereby obtaining a concentrated aqueous formic acid solution.

2. Process according to claim 1, further comprising a step of electrochemically reducing carbon dioxide, thereby producing the first aqueous solution comprising formate ions and one or more metal ions.

3. Process according to any one of the preceding claims, further comprising the step of concentrating the first aqueous solution before subjecting it to the first electrodialysis step, preferably using reverse osmosis.

4. Process according to any one of the preceding claims, wherein the concentrated formic acid solution has a formic acid concentration of 15 % or more by weight relative to the weight of the concentrated formic acid solution, preferably 20 wt.% or more.

5. Process according to any one of the preceding claims, further comprising the step of recycling the diluate obtained in the secondelectrodialysis step in the first electrodialysis step.

6. Process according to any one of claims 2-5, further comprising the step of recycling the alkaline third solution obtained in the first electrodialysis step as electrolyte for the first electrochemical reduction of carbon dioxide.

7. Process according claim 5 or 6, wherein at least one of the recycling steps comprises concentrating the recycling stream, preferably using one or more selected from the group of reverse osmosis, electrodialysis and freeze concentration.

8. Process according to any one of the preceding claims, wherein the concentrated aqueous formic acid solution is further concentrated by freeze crystallization.

9. Process according to claim 8, wherein freeze concentration is driven by a refrigeration cycle comprising one or more stages.

10. Process according to any one of the preceding claims, wherein the steps are performed continuously.

11. System suitable for the production of a concentrated aqueous formic acid solution, preferably using the process according to any one of claims 1-10, the system comprising:- optionally, an electrolyzer;- a first electrodialysis unit comprising one or more bipolar membranes,- a second electrodialysis unit configured to receive an acidic aqueous solution comprising formic acid from the first electrodialysis unit.

12. System according to claim 11, further comprising a concentration unit, preferably selected from a freeze concentration unit or a distillation unit, configured to receive a concentrated formic acid solution from the second electrodialysis unit.

13. System according to claim 11 or 12, further comprising one or more recycling loops for recycling one or more streams from the first and / or second electrodialysis unit.

14. System according to claims 13, wherein at least one of the one or more recycling loops comprises a concentration unit, preferably selected from the group consisting of reverse osmosis units, electrodialysis units or freeze concentration units.

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