Integrated Process for Co-Production of Carboxylic Acids and Halogen Products from Carbon Dioxide
The present disclosure is a method and system for production of carboxylic based chemicals. A method for producing at oxalic acid may include receiving an anolyte feed at an anolyte region of an electrochemical cell including an anode and receiving a catholyte feed including carbon dioxide and an alkali metal bicarbonate at a catholyte region of the electrochemical cell including a cathode. Method may include applying an electrical potential between the anode and cathode sufficient to reduce the carbon dioxide to at least one reduction product and converting the at least one reduction product and an alkali metal hydroxide to an alkali metal oxalate via a thermal reactor. The method may further include converting the alkali metal oxalate to oxalic acid at the electrochemical acidification electrolyzer. The method may further include the co-production of halogen products from the anolyte region of the electrochemical cell, by the oxidation of hydrogen halides used as an anolyte feed. The halogen may then be further reacted with other chemicals to produce chlorinated organics as well as inorganic compounds such as sodium hypochlorite.
The present disclosure generally relates to the field of electrochemical reactions, and more particularly to methods and/or systems for producing carboxylic acids from carbon dioxide and co-producing halogen products.
BACKGROUNDThe combustion of fossil fuels in activities such as the electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for altering the Earth's climate, changing the pH of the ocean, and other potentially damaging effects. Countries around the world, including the United States, may be seeking ways to mitigate emissions of carbon dioxide.
One implementation may be to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals. If the carbon dioxide may be converted using energy from renewable sources, it will be possible to both mitigate carbon dioxide emissions and to convert renewable energy into a chemical form that may be stored for later use. Electrochemical and photochemical pathways may be likely mechanisms for carbon dioxide conversion.
SUMMARY OF THE PREFERRED EMBODIMENTSThe present disclosure is a method and system for the production of carboxylic acid based chemicals, including carboxylic acids and salts, and producing halogen co-products. A method for producing at oxalic acid may include receiving an anolyte feed at an anolyte region of an electrochemical cell including an anode and receiving a catholyte feed including carbon dioxide and an alkali metal bicarbonate at a catholyte region of the electrochemical cell including a cathode. The method may include applying an electrical potential between the anode and cathode sufficient to reduce the carbon dioxide to at least one reduction product and converting the at least one reduction product and an alkali metal hydroxide to an alkali metal oxalate via a thermal reactor. The method may further include receiving the alkali metal oxalate at an electrochemical acidification electrolyzer and converting the alkali metal oxalate to oxalic acid at the electrochemical acidification electrolyzer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain the principles of the disclosure.
The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:
Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings.
The present disclosure describes a method and system for production of carboxylic based chemicals, including carboxylic acids and salts. The method may employ an electrochemical cell reaction to produce carbon monoxide, CO, or alkali metal formate from a carbon dioxide feedstock. A thermal reaction with an alkali metal hydroxide, may be used to combine, for example, two alkali metal formate molecules, into a alkali metal oxalate product.
The alkali metal oxalate may be then converted to oxalic acid by a membrane based electrochemical acidification process, where protons (H+ ions) formed at the anode may be used to replace the alkali metal ions, and the alkali metal ions (M+) may be captured as alkali metal hydroxide (MOH) at the cathode, and may be recycled to be used as the alkali metal hydroxide used in the intermolecular condensation process unit operation.
Alternatively the alkali metal oxalate may be converted to oxalic acid through treatment with mineral acid, such as HCl, HBr, HI, H2SO4, H3PO4, or the like. For example, treating sodium oxalate with aqueous HCl may result in an oxalic acid solution comprising NaCl. The oxalic acid may be extracted from the solution via extraction with an organic solvent such as alcohol, ether, halo-organic, ketone, amide, or ester. Useful solvents include, but are not limited to, methanol, ethanol, propanol, diethyl ether, methyl ethyl ether, methyl tert-butyl ether, tetrahydrofuran, dioxane, methylene chloride, chloroform, carbon tetrachloride, chlorobenzene, di-chlorobenzene, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, acetone, butanone, dimethylformamide, N-methyl pyrrolidone, and the like. The oxalic acid may also be recovered from the solution through crystallization from the aqueous solution. Crystallization may require concentrating the solution and/or cooling the solution.
After removal of oxalic acid, the aqueous solution comprising salt, NaCl for example, may be recycled by sending it to an anolyte compartment of an electrochemical cell. Halide ions, for example chloride, may be oxidized to form halogen (for example chlorine). The halogen may be isolated from an anolyte stream after exiting an anolyte compartment of an electrochemical cell. The halogen may be reacted with hydrogen, for example hydrogen produced during a thermal alkali metal formate to alkali metal oxalate calcination reaction. Hydrogen may also be obtained from another source. The mineral acid (HCl for example) formed by the reaction of hydrogen with halogen may be used to acidify alkali metal oxalate, completing the cycle. The energy produced (heat or electrical energy) by reacting halogen with hydrogen may be captured and used in the process of the invention (in the thermal calcination reaction for example) or may be used elsewhere.
Before any embodiments of the disclosure are explained in detail, it is to be understood that the embodiments may not be limited in application per the details of the structure or the function as set forth in the following descriptions or illustrated in the figures. Different embodiments may be capable of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of terms such as “including,” “comprising,” or “having” and variations thereof herein are generally meant to encompass the item listed thereafter and equivalents thereof as well as additional items. Further, unless otherwise noted, technical terms may be used according to conventional usage. It is further contemplated that like reference numbers may describe similar components and the equivalents thereof.
Referring to
Catholyte region may include a cathode. Anolyte region may include an anode. An energy source (not shown) may generate an electrical potential between the anode and the cathode of electrochemical cell 110. The electrical potential may be a DC voltage. Energy source may be configured to supply a variable voltage or constant current to electrochemical cell 110. A separator may selectively control a flow of ions between the anolyte region and the catholyte region. Separator may include an ion conducting membrane or diaphragm material.
Electrochemical cell 110 may operate to perform an electrochemical reduction of carbon dioxide in an electrochemical cell producing carbon monoxide (CO) and hydrogen as cathode products and oxygen as an anode product when using an anolyte comprising sulfuric acid (H2SO4).
The CO generated from electrochemical cell 110 may be separated from the hydrogen and then passed to a thermal reactor 120. Thermal reactor may react the carbon monoxide with an alkali metal hydroxide, such as KOH via a thermal intermolecular condensation reaction to form alkali metal formate. Thermal reactor 120 may operate to perform a thermal decomposition reaction or a carbonylation reaction, which may be reactions which incorporate CO into organic and inorganic chemical structures.
Alkali metal formate formed from thermal reactor 120 may be passed to another thermal reactor 130. Thermal reactor 130 may perform a second thermal intermolecular condensation reaction employing an alkali metal hydroxide (e.g. KOH) that may promote the reaction to produce alkali metal oxalate. While system 100 of
Alkali metal oxalate from thermal reactor 130 may be dissolved in water and may be passed to an electrochemical acidification electrolyzer 140. Electrochemical acidification electrolyzer 140 may produce a dicarboxylic acid, such as oxalic acid, and KOH along with oxygen and hydrogen byproducts. Electrochemical acidification electrolyzer 140 may be a membrane based unit including of at least three regions, including an anode region, one or more central ion exchange regions, and a cathode region. It is contemplated that an energy source (not shown) may generate an electrical potential between the anode and the cathode of electrochemical acidification electrolyzer 140 sufficient to produce oxalic acid. Alkali metal oxalate may be passed through the central ion exchange region where alkali metal ions may be replaced with protons, and the displaced alkali metal ions pass through the adjoining membrane into the cathode region to form MOH. The anode reaction may utilize an acid, such as sulfuric acid, producing oxygen and hydrogen ions.
The hydrogen byproduct resulting from electrochemical acidification electrolyzer 140, as an alternative embodiment, may be used as a fuel to produce steam or used in another chemical process that may utilize hydrogen, such as a hydrogenation process.
The dicarboxylic acid, such as an oxalic acid product may be purified to produce a final purified product, or may be further processed as a chemical intermediate to produce another product, such as monoethylene glycol, using a reduction process such as an electrochemical reduction or a catalytic hydrogenation.
Aqueous KOH from electrochemical acidification electrolyzer 140 may be passed to an evaporator 150. Evaporator 150 may evaporate the water from aqueous KOH product using steam or another heat source, converting it into a concentrated aqueous solution and/or solid with 5% or less water content as needed in electrochemical cell 110 and thermal reactor 120.
Referring to
Referring to
System 200 may include an electrochemical cell 110. Electrochemical cell 110 may operate to perform an electrochemical reduction of carbon dioxide with a alkali metal carbonate cathode feed, which may be formed from the reaction of CO2 with MOH, to produce alkali metal formate along with oxygen as an anode product when using an anolyte comprising sulfuric acid (H2SO4). The alkali metal formate product solution concentration from the catholyte compartment of electrochemical cell 110 may range from 1 wt % to 30 wt % or more based on the formate ion, and preferably range from 5 wt % to 20 wt % as formate. The corresponding % weight as the alkali metal formate, for example alkali metal formate may be based on the molecular weight of the alkali metal compound.
Alkali metal formate may be passed to a thermal reactor 120. Thermal reactor 120 may perform a thermal intermolecular condensation reaction with an alkali metal hydroxide (e.g., KOH) to produce alkali metal oxalate.
Alkali metal oxalate from thermal reactor 120 may be dissolved in water and may be passed to an electrochemical acidification electrolyzer 140. Electrochemical acidification electrolyzer 140 may produce dicarboxylic acid, such as oxalic acid, and KOH along with oxygen and hydrogen byproducts. Electrochemical acidification electrolyzer 140 may be a membrane based unit including of at least three regions, including an anode region, one or more central ion exchange regions, and a cathode region. Alkali metal oxalate may be passed through the central ion exchange region where alkali metal ions may be replaced with protons, and the displaced alkali metal ions pass through the adjoining membrane into the cathode region to form KOH. The anode reaction may utilize an acid, such as sulfuric acid, producing oxygen and hydrogen ions.
The hydrogen byproduct resulting from electrochemical acidification electrolyzer 140, as an alternative embodiment, may be used as a fuel to produce steam or used in a side process that may utilize hydrogen, such as in a chemical hydrogenation process.
The dicarboxylic acid, such as oxalic acid product may be purified to produce a final purified product, or may be further processed as a chemical intermediate to produce another product, such as monoethylene glycol, using an electrochemical reduction or thermochemical process.
Aqueous KOH from electrochemical acidification electrolyzer 140 may be passed to an evaporator 150. Evaporator 150 may evaporate the water from aqueous KOH product using steam or another heat source, converting it into a concentrated aqueous solution and/or solid with 5% or less water content as needed in the electrochemical cell 110 or thermal reactor 120.
Referring to
Referring to
Catholyte region 340 may have a mounted cathode 342, which may be a metal electrode with an active electrocatalyst layer on the front side facing membrane 330. High surface area cathode structure 344 may be mounted with direct contact pressure between the face of cathode 342 and cation membrane 330.
As shown in
Electrochemical cell 110 may include a catholyte region 340 which includes cathode 342 having an electrocatalyst surface facing membrane 330. High surface area cathode structure 344 may be mounted between membrane 330 and cathode 342, relying on contact pressure with cathode 342 for conducting electrical current into the structure. The interface between high surface area structure 344 and membrane 330 may utilize a thin expanded plastic mesh insulator screen (not shown) to minimize direct contact with the high surface area cathode material with the membrane 330.
Feed stream 312 may feed into catholyte region 340, flowing through the high surface area structure 344 and across the face of cathode 342 where cathode reduction reactions between carbon dioxide, electrolyte, and cathode material at the applied current and voltage potential produce exit stream 314, the exit stream including a formate.
Stream 314 may be the exit solution and gas mixture product from the cathode reaction which flows by pH monitoring sensor 374 and temperature sensor 352 and then into catholyte gas/liquid disengager 380 where the gas exits as stream 382 and formate/electrolyte overflow exits as stream 384, and the gas-depleted stream leaves the disengager as stream 386. Stream 386 may then enter an input of catholyte recirculation pump 390, which then passes through heat exchanger 392 which uses cooling water 372, then passes by temperature sensor 352. A fresh catholyte electrolyte feed 394 may be metered into stream 386 which may be used to adjust the catholyte flow stream pH into the catholyte region 340 and control a product overflow rate and sets the formate product concentration, with the pH monitored by pH sensor 374. Carbon dioxide flow stream 396 may be metered into the flow stream which enters the catholyte region 340 as stream 312.
In an alternative embodiment, as shown in
Referring to
Electrochemical acidification electrolyzer 140 may include input feeds 430 and 432 and may produce a solution of dicarboxylic acid (e.g. oxalic acid) 456, oxygen 420 from the anolyte region 402, and KOH 442 from the catholyte region 410. Anode region 402 may include a titanium anode 404 with an anode electrode catalyst coating facing cation exchange membrane 406a. The central ion exchange region 408 may contain a plastic mesh spacer to maintain the space in the central ion exchange region between cation exchange membranes 406a and 406b. Optionally, a preferred material may be the use of a cation ion exchange material between the membranes, so that there may be increased electrolyte conductivity in the ion exchange region solution. Catholyte region 410 may include a cathode 412.
Anolyte region 402 may have feed stream input 430 including sulfuric acid, which may flow through the anolyte region 402 and exit as stream 414 including a gas and liquid, passing by temperature sensor 416 into anolyte disengager 418, where the gas exits as stream 420 and liquid overflow as stream 422. Gas-depleted stream 424 may exit the anolyte disengager 418 and deionized water stream 426 may be metered into the stream 424 as well as sulfuric acid make-up stream 428 to maintain acid electrolyte strength in the anolyte region 402. Stream 424 may pass through optional heat exchanger 426 which may have cooling water supply 428 to cool or maintain the stream 424 temperature, and the stream 424 enters the anolyte region 402 as stream 430.
Catholyte region 410 may include feed stream 432 which may be the recirculating alkali metal hydroxide (e.g. KOH) in the catholyte loop, which enters catholyte region 410 and flows by cathode 412, which may generate hydrogen gas and hydroxide (OH−) ions, and forms a alkali metal hydroxide from the combination of alkali metal ions crossing the membrane 406b with the hydroxide ions formed at the cathode 412 from the reduction of water. Exit stream 434 from the cathode region 410 may contain alkali metal hydroxide and hydrogen gas from the cathode reactions, and passes by temperature sensor 436 and then into catholyte disengager 438, where hydrogen gas 440 may be separated from the catholyte solution, which exits catholyte disengager 438 as recycle stream 444 and alkali metal hydroxide product overflow stream 442. Recycle stream 444 may pass through optional recirculation pump 446 and then through optional heat exchanger 448, which uses cooling water supply 450. The stream then passes by temperature sensor 452, and then may have a deionized water addition stream 454 added to the stream to control the alkali metal hydroxide concentration in the catholyte recirculation loop, and then reenters the catholyte region 410 as stream 432.
In an alternative embodiment, the anolyte comprising sulfuric acid may be replaced with an anolyte comprising HBr, producing bromine and hydrogen ions at a much lower voltage potential than required for the generation of oxygen at the anode.
Electrochemical formate cell 502 may be similarly configured to the cell as shown and described in
The anolyte product stream 508 from catholyte compartment 504 comprises a mixture of chlorine gas and NaCl depleted brine solution. The chlorine gas may then be separated or disengaged as stream 510 from stream 508 as a co-product, and the separated depleted brine solution stream 512 may then be processed in a series of steps typically used in chlor alkali processes comprising dechlorination of the depleted brine, resaturation of the brine solution with NaCl using a bed of solid NaCl salt, followed by a brine purification step to remove impurities, such as metals and hardness (such as Ca+, Mg+, and Ba+), from the brine solution to impurity levels typically used to achieve long life operation of separator 503, to produce a purified saturated NaCl brine solution stream 522 which is electrolyzed in the anolyte compartment 504 of formate cell 502.
Chlorine gas 510 may then be processed in various ways, such as removal of water from the gas by condensation, and then the chlorine gas may then be used for producing various useful co-products from the system, for example, the generation of sodium hypochlorite by a reaction with NaOH, the generation of HCl through reaction with hydrogen, as well as reactions with organics, such as to produce EDC (ethylene dichloride) by reaction with an external supply of ethylene. Many other reaction co-products made with the chlorine gas 510 may be envisioned.
Brine dechlorination unit 514 may be used to remove residual chlorine from depleted brine solution 512 using a selected reducing agent, which may be chosen from those typically used such as sodium sulfite, sodium hydrosulfite, and hydrogen peroxide among others. The dechlorinated brine then may be passed to brine saturator unit 516, where the depleted brine NaCl concentration may be increased from a typical 150-240 gm/L as NaCl to a concentration of 300-320 gm/L as NaCl using a brine saturator, which may consist of a bed of solid salt crystals in an apparatus typically called a briner. The saturated brine may then be passed through brine purification system 518, which may consist of chemical precipitation steps for the removal of most of the hardness in the solution, typically by the addition of NaOH and sodium carbonate under alkaline conditions, followed by filtration to remove the precipitated hardness containing solids, then followed by an ion exchange purification step utilizing chelating ion exchange resin beds to reduce the hardness levels in the brine to typically 20-50 ppb or less. The sulfate component in the brine may be reduced by the chemical precipitation, or by the use of commercial system that utilizes nanofiltration to preferentially remove sulfate from brine, for example the SRS system—sold by Aker Chemetics. The purification chemicals also may include HCl and NaOH used for regenerating the chelating ion exchange columns. Stream 520 may be an effluent stream containing the precipitated carbonates, sulfates, and metals effluent from the purification of the saturated brine solution, which may be processed and recycled back to the process with a minimum amount of material requiring disposal. Purified brine solution 522 may then pass into anolyte compartment 504 of electrochemical formate cell 502. The recirculation of the anolyte loop is not shown, but the brine flow rate may be metered so as to maintain the desired brine concentration in the electrochemical cell anolyte loop and overflow stream 508, with the brine concentration typically in the range of 150-240 gm/L as NaCl. In an embodiment, the anolyte brine concentration may be operated lower NaCl concentrations, to as low as about 100-140 gm/L, which may result in a decrease in chlorine efficiency and the generation of more byproduct oxygen in the chlorine gas stream, but which may be useful in reducing the brine flow rate through the brine purification system with a reduction in brine processing costs.
Solution feed stream 548, which may be an aqueous mixture of sodium formate, sodium bicarbonate, and dissolved carbon dioxide which may include a gaseous carbon dioxide component which may be in the form of gaseous micro-bubbles, may be passed into catholyte compartment of electrochemical cell 502. In catholyte compartment 506, which preferably incorporates a high surface area cathode structure, carbon dioxide may be electrochemically reduced to formate, and the formate may combine with the sodium ions (Na+) passing through the adjacent separator 503 to form sodium formate. In addition, any cathode inefficiency side reactions forming hydrogen (H2) at the cathode may produce hydroxide ions (OH−), and these hydroxide ions may react with carbon dioxide to form sodium carbonate in the catholyte solution. The sodium carbonate may then further react with excess carbon dioxide to form sodium bicarbonate. In addition, it is believed that the other sodium ions may combine with carbonic acid and the other potential carbon dioxide equilibrium species at the operating catholyte pH to further form additional sodium carbonate and sodium bicarbonate.
The reduction reaction products may exit as stream 524, where they may be separated or disengaged into gas stream 526 and solution stream 530. Gas stream 526 may be passed into separator 528, which may separate carbon dioxide from any byproduct hydrogen so that they may be reused or recycled in the other system 500 unit operations. Gas separator 528 may be any suitable membrane-based or molecular sieve pressure swing gas separation unit technology that may be capable of the separation of carbon dioxide and hydrogen. The separated gases may then be further purified and compressed as needed for recycle or reuse to the process.
Solution stream 530 comprising mainly sodium formate and sodium bicarbonate may then be split into recycle stream 532, which may be recycled back to electrochemical formate cell 502 catholyte compartment, and product stream 531 which may go to evaporator-crystallizer 550. Recycle stream 532 may have several input streams, including the introduction-of-carbon-dioxide stream 534, optionally a sodium bicarbonate stream 536 from reactor-dissolver unit 560, a side stream 538 leaving stream 532 which may go into an optional electrochemical acidification cell A 540 and may have an acidified product stream 546 back into stream 532, and may have the addition of water to the stream as needed to prevent precipitation in stream 532 and catholyte compartment 506, and having all of the inputs/outputs into stream 532 ending up as solution stream 548 which may be sent into catholyte compartment 506.
Electrochemical acidification cell 540 may be used to acidify a small portion taken from catholyte loop stream 532, and which may reenter anolyte recycle stream 532 as stream 546.
Electrochemical acidification cell 540 may be the same design as the acidification cell as shown in
Catholyte product stream 531, which may contain high concentrations of alkali metal formate and alkali metal bicarbonate may then be passed to evaporator-crystallizer unit 550, which may evaporate sufficient water from the solution and continuously precipitate a alkali metal bicarbonate crystal product as stream 556, a liquid concentrated alkali metal formate stream 554, and a water product stream 552 which may be condensed and used elsewhere as needed in the process, such as in oxalate solution dissolver 572. Evaporator-crystallizer 550 may utilize steam for providing the energy requirements for evaporating the water from the stream 531 input stream to the unit. Evaporator-crystallizer 550 may be a multiple evaporator effect unit, consisting of multiple units to efficiently utilize the energy of the input steam, or any other suitable types of units may be utilized.
In addition, Evaporator-crystallizer 550 may use steam as well as mechanical means for producing a vacuum to further reduce the energy requirements for the evaporation of the water from the solution. Any suitable evaporator-crystallizer unit or system may comprise suitable metallurgy for the operating conditions of the system. Alkali metal formate may have a solubility in water that may be about 8 to 10 times more than that of alkali metal bicarbonate, so the solubility difference allows the easy separation of alkali metal formate from alkali metal bicarbonate using solution temperature differences to enhance the separation. Other methods for the separation of alkali metal formate from alkali metal bicarbonate may be employed including fractional crystallization, falling film crystallization, and the like. A continuous process for the separation may be preferred, although batch processing may also be used.
The alkali metal bicarbonate crystal stream 556 from unit 550 may be in the form of an aqueous slurry, which may then be separated, washed, and dried by any suitable means to produce a dried alkali metal bicarbonate product 558. Equipment such as centrifuges and vacuum belt filters may be used for the separation of the alkali metal bicarbonate crystals from the 556 stream slurry, and the mother liquor from any water rinses may be recycled back to unit 550. The alkali metal bicarbonate product 558 may also be recrystallized or further purified by any suitable means to obtain a final product with purity suitable for specialty uses, such as food grade quality product. A portion of the stream 556 slurry or stream 558 alkali metal bicarbonate product as stream 560 may be utilized in reactor-dissolver 561, which may be used to convert alkali metal carbonate to alkali metal bicarbonate using an additional carbon dioxide gas stream 563. Reactor-dissolver 561 may also have an NaOH input stream 562, which may then be converted to alkali metal bicarbonate. The NaOH may be supplied from one or both of the electrochemical acidification units 540 and 576 if required.
Alkali metal formate stream 554 may be a concentrated alkali metal formate solution that contains 50 wt % or less water, and preferably 40 wt % or less water, and more preferably 30 wt % or less water. The formate solution stream 554 may be viscous and may contain from 0.1 wt % to 30 wt % alkali metal bicarbonate depending on the water solubility of alkali metal bicarbonate in the alkali metal formate solution. The solution concentrations of the alkali metal formate and residual alkali metal carbonate may be varied as needed to achieve the desired final residual alkali metal bicarbonate concentration in the alkali metal formate solution. Alkali metal formate stream 554 may then passed to alkali metal formate liquid dryer where the residual water may be removed by any suitable means such as by vacuum evaporation and the like. The alkali metal formate may be an alkali metal formate melt, consisting of a small percentage of water, in the range of 0.01 wt % to 5 wt % as water, and may have between 0.1 wt % to 20 wt % alkali metal bicarbonate. The alkali metal formate melt stream 566 may then be passed into alkali metal formate thermal reactor 568 for high temperature conversion of the alkali metal formate to alkali metal oxalate (calcination). A suitable catalyst 567, such as NaOH, sodium hydride, sodium borohydride, sodium ethoxide, sodium methoxide, KOH, KH, KOEt, KOMe, KOtBu and the like may be added into the sodium formate before it enters thermal reactor 568. The introduction of catalyst 567 may help to reduce the calcination temperature and improve the conversion yield of alkali metal formate to alkali metal oxalate to a range of 50% to 99% or more, and preferably 70% to 99% or more. The reaction may also provide suitable yields without the need for the addition of catalyst 567. Hydrogen may be a major byproduct reaction from thermal reactor 568 and may be recovered for use in the process. Thermal reactor 568 may be operated in different configurations, such as under a partial vacuum, under an inert atmosphere such as nitrogen, or with the use of any suitable gas that may improve the efficiency of the chemical conversion of the formate to oxalate. The addition of other chemicals to thermal reactor 568 may also be useful, so as to obtain a clean flowing purified product. Thermal reactor 568 may be any suitable type equipment that may heat the alkali metal formate to suitable temperatures and control the thermal or calcination atmosphere. Thermal reactor 568 may include tunnel furnaces, rotary kilns, high temperature spray dryers, high temperature rotating drum/flaker units, fluid bed reactors, and other commercial calcining equipment and designs that may be commercially available.
The alkali metal oxalate product stream 570 leaving thermal reactor 568 may be cooled, and passed to oxalate solution dissolver 572, where alkali metal oxalate solids are dissolved in water, and may be filtered by various available methods to remove any insoluble materials and obtain a clear, filtered product solution, free of suspended solids. The alkali metal oxalate product may contain alkali metal carbonate and/or alkali metal bicarbonate as byproduct(s) of the calcination. The solution may be concentrated sufficiently so that the alkali metal oxalate/alkali metal bicarbonate solution may not require a larger amount of energy or steam for water evaporation in evaporator-crystallizer 576.
Alkali metal oxalate solution stream 574 may then be passed to electrochemical acidification cell 576, where the alkali metal oxalate solution passes through the ion exchange compartment of the cell and may be converted to oxalic acid stream 580 and carbon dioxide stream 579 which may produced from the acidification of any alkali metal carbonate present in alkali metal oxalate stream 574. Electrochemical acidification cell 576 may utilize the same chemistry and configuration as electrochemical acidification cell 540, producing oxygen and hydrogen a co-products, as well as NaOH as stream 578.
Referring to
System 600 may include an electrochemical cell 610. Electrochemical cell 610 may operate to perform an electrochemical reduction of carbon dioxide with a alkali metal bicarbonate cathode feed, which may be formed from the reaction of CO2 with NaOH, producing alkali metal formate along with chlorine gas as an anode product when utilizing hydrochloric acid (HCl) as an anolyte, which may be produced in electrochemical unit 670 which may use a purified NaCl solution input feed stock. It is contemplated that the alkali metal bicarbonate cathode feed, also referred as a catholyte feed, may be a solution containing alkali metal bicarbonate and alkali metal carbonate. Also, the catholyte feed may be specified as an alkali metal bicarbonate containing solution, such that it may contain alkali metal carbonate as well as smaller amounts of other salts that may be added such as alkali metal sulfates, alkali metal chlorides, alkali metal phosphates, and the like where these added components may enhance the electrochemical reduction of carbon dioxide to formate at the cathode. In addition, the alkali metal containing solution may also contain one or more organic homogeneous catalysts to also enhance and lower the potential at the cathode for the reduction.
Alkali metal formate may be passed to a thermal reactor 620. Alkali metal formate may be separated from bicarbonate present in the catholyte by various means as described in
Alkali metal oxalate from thermal reactor 620 may then be dissolved in water and may then be passed to an electrochemical acidification electrolyzer 630. Electrochemical acidification electrolyzer 630 may produce a dicarboxylic acid, such as oxalic acid, and NaOH along with oxygen and hydrogen byproducts. Electrochemical acidification electrolyzer 630 may be a membrane based unit including of at least three regions, an anode region, one or more central ion exchange regions, and a cathode region. Alkali metal oxalate may be passed through the central ion exchange region, where alkali metal ions may be replaced with protons, and displaced alkali metal ions pass through the adjoining membrane into the cathode region to form NaOH. The anode reaction may produce chlorine gas when utilizing an HCl feed from electrochemical unit 670. Alternative, the anode reaction may utilize a different acid, such as sulfuric acid, producing oxygen and hydrogen ions. Alternatively, electrochemical acidification electrolyzer 630 may be an electrochemical electrodialysis unit, utilizing bipolar membranes, producing oxalic acid as well as smaller amounts of hydrogen and NaOH.
The hydrogen byproduct resulting from electrochemical acidification electrolyzer 630, as an alternative embodiment, may be used as a fuel to produce steam or used in a side process that may utilize hydrogen, such as in a chemical hydrogenation process. The chemical hydrogenation process may be, for example, the hydrogenation of an oxalic acid solution or the hydrogenation of an ester of oxalic acid, such as dimethylcarboxyalate (DMO) and diethylcarboxalate (EDO), that may form high purity monoethylene glycol (MEG).
Aqueous NaOH from electrochemical acidification electrolyzer 630 may be passed to an evaporator 640. Evaporator 640 may evaporate the water from aqueous NaOH product using steam or another heat source, converting it into a concentrated aqueous solution and/or a solid with 5% or less water content. The NaOH may be reacted in reactor 680 with CO2 to form an alkali metal bicarbonate solution, which may be passed to the catholyte compartment in electrochemical cell 610. NaOH may also be converted to a solid for use as a catalyst in thermal reactor 620.
Electrochemical unit 670 may be an electrochemical acidification electrolyzer, a type such as electrochemical acidification electrolyzer 630, where a purified NaCl brine solution is passed into the ion exchange compartment and may be acidified, producing an HCl product stream as well as co-producing NaOH and hydrogen in the cathode compartment. The anolyte may utilize sulfuric acid and generate oxygen from the oxidation of water. The purified brine may be produced by brine purification and recycle unit 660, utilizing an NaCl solid feed and using various purification chemicals as needed to produce the purified brine, suitable for use in electrochemical unit 670. Electrochemical unit 670 may comprise other types of electrochemical units, such as electrodialysis units which may utilize bipolar membranes, as well as any other suitable type of electrolyzer that may produce HCl.
System 600 in another embodiment, may also produce alkali metal hypochlorite (for example NaOCl), as a co-product from the system, utilizing chlorine and NaOH produced from electrochemical unit 670 and electrochemical acidification electrolyzer 630. Alternatively, chlorine may be reacted with organics to produce various chlorinated chemical products, such as ethylene dichloride (EDC). MOH may be a separate product of the process, or may be converted to alkali metal carbonate or alkali metal bicarbonate, thus converting more carbon dioxide to useful chemicals.
In another embodiment, the alkali metal formate, produced in electrolyzer 610 may be passed directly to electrochemical acidification electrolyzer 630, skipping thermal reactor 620, producing formic acid. The formic acid may then be converted to other suitable chemicals, such as methyl formate, or reacted with various salts to produce alkali metal formates, such as calcium formate. Methyl formate may also be converted to produce amides such as formamide or dimethylformamide via reactions with amines.
In another embodiment, electrochemical unit 670 may comprise a two compartment cell having an anode compartment and a cathode compartment separated by a separator or membrane. In this embodiment, NaCl may be fed to the anolyte compartment producing chlorine, and sodium hydroxide and hydrogen would be produced in the cathode compartment.
Referring to
Referring to
The HCl product from electrochemical unit 755 may be operated to produce a product stream containing HCl or a solution mixture containing HCl and NaCl. The HCl—NaCl solution mixture composition is such that the NaCl is soluble for the selected HCl concentration in the solution mixture. The HCl solution product concentration may range from 1 wt % to 35 wt %, and more preferably in the range of 5 wt % to 30 wt %, and more preferably in the range of 10 wt % to 25 wt %. The NaCl concentration, depending on it's solubility in the specific HCl concentration, may range from ppm amounts to 15 wt %, and more preferably from 0.01 wt % to 10 wt % or less. The additional alkali metal in the HCl solution product will produce additional alkali metal bicarbonate co-product with the alkali metal formate in the first electrochemical cell, and is a means of producing additional co-product. The HCl:NaCl ratio product as a feed to electrochemical cell 705 anolyte compartment may be used to control the pH in the catholyte of electrochemical cell 705, since the hydrogen ions (H+) or protons may pass in proportion to the sodium ions (Na+) present in the HCl solution composition in the electrochemical cell anolyte. The additional sodium ions crossing the membrane to the catholyte compartment may also produce additional sodium bicarbonate as a co-product from the entire process.
Sodium formate from the electrochemical cell 705 catholyte compartment may then be passed through evaporator crystallizer 710, where unreacted sodium bicarbonate may be separated from the sodium formate product stream. Nanofiltration or other separation methods may also be used in place of evaporator-crystallizer 710 or may be used in conjunction with the 710 unit. The separated sodium bicarbonate may be recycled to reaction unit 760, with any excess sodium bicarbonate that may be passed to carbonate reactor 740.
The purified sodium formate stream from evaporator-crystallizer 710 may then be passed onto electrochemical acidification electrolyzer 715 where sodium formate may be converted to formic acid.
Electrochemical acidification electrolyzer 715 may produce formic acid in addition to preferably co-producing NaOH along with chlorine and hydrogen. Electrochemical acidification electrolyzer 715 may be a membrane based unit comprising of at least three regions, including an anode region, one or more central ion exchange regions, and a cathode region. Sodium formate may be passed through the central ion exchange region, where sodium ions may be replaced with protons, and the displaced sodium ions may pass through the adjoining membrane into the cathode region to form NaOH. The anode reaction may produce chlorine gas when utilizing an HCl feed from electrochemical unit 755. Alternatively, the anode reaction may utilize a different acid, such as sulfuric acid, producing oxygen and hydrogen ions. Alternatively, electrochemical acidification electrolyzer 715 may be an electrochemical electrodialysis unit, utilizing bipolar membranes, producing formic acid as well as smaller amounts of hydrogen and NaOH.
The hydrogen byproduct resulting from electrochemical acidification electrolyzer 715, as an alternative embodiment, may be used as a fuel to produce steam or used in a side process that may utilize hydrogen, such as in a chemical hydrogenation process to produce downstream chemical products from formic acid.
Aqueous NaOH from electrochemical acidification electrolyzer 715 may be passed to an evaporator 735. Evaporator 735 may evaporate the water from aqueous NaOH product using steam or another heat source, converting it into a concentrated aqueous NaOH solution ranging from 5 wt % to 70 wt %, and more preferably from 20 wt % to 50 wt %. The NaOH solution from evaporator 735 may then be sold as the NaOH solution, preferably as a 50 wt % NaOH solution, or converted to several co-products, including sodium carbonate, sodium bicarbonate, and sodium hypochlorite. NaOH may also be used, if required, in producing any of the downstream formic acid based products.
A portion of the NaOH solution from evaporator 735 may then be passed and reacted in reactor 740 with CO2 to form a sodium bicarbonate and/or sodium carbonate solution, which may then be precipitated, crystallized, and dried to produce solid products for commercial sale.
The NaOH from evaporator 735 may also be converted to sodium hypochlorite with any chlorine that may be produced from electrochemical cell 705 and electrochemical acidification reactor 715. The NaOCl concentration from hypochlorite reactor may depend on the concentration of the NaOH used in the reaction and any dilution of the product with deionized water to achieve a specific specification product, and the concentration may typically range from 3% to 20 wt % as NaOCl.
Electrochemical unit 775 may an electrochemical acidification electrolyzer, a type such as electrochemical acidification electrolyzer 715, where a purified NaCl brine solution is passed into one or more ion exchange compartments and may be acidified, producing an HCl product stream as well as co-producing NaOH and hydrogen in the cathode compartment. The HCl product from electrochemical unit 755 may be operated to produce a product stream containing HCl or a solution mixture containing HCl and NaCl. The HCl—NaCl solution mixture composition is such that the NaCl is soluble for the selected HCl concentration in the solution mixture. The HCl solution product concentration may range from 1 wt % to 35 wt %, and more preferably in the range of 5 wt % to 30 wt %, and more preferably in the range of 10 wt % to 25 wt %. The HCl product may then be passed onto the anolyte compartment of electrochemical unit 705.
The anolyte of electrochemical unit 755 may alternatively utilize sulfuric acid and generate oxygen from the oxidation of water, and not produce chlorine or another halogen. The preference may be to produce a co-product anolyte product, such as chlorine.
The purified brine may be produced by brine purification and recycle unit 750, utilizing an NaCl solid feed and using various purification chemicals as needed to produce the purified brine, suitable for use in electrochemical unit 755. Electrochemical unit 775 in an alternative embodiment may comprise other types of electrochemical units utilizing an NaCl feed stock and converting it into an acid and a base, such as electrodialysis units which may utilize bipolar membranes, as well as any other suitable type of electrolyzer that may split salts, such as NaCl, and produce HCl and NaOH.
System 700 in another embodiment, may also produce sodium hypochlorite (NaOCl), as a co-product from the system, utilizing chlorine and NaOH that may be produced from electrochemical unit 705 and electrochemical acidification electrolyzer 715. Alternatively, the chlorine may be reacted with organics to produce various chlorinated chemical products from the process, such as ethylene dichloride (EDC). The NaOH may also be a separate product from the process, or may be converted to sodium carbonate or sodium bicarbonate, thus converting more carbon dioxide to useful chemicals.
Formate CO2 Reduction ChemistryThe postulated chemistry of the reduction of CO2 at the cathode may proceed as follows.
Hydrogen atoms may be adsorbed at the electrode from the reduction of water as shown in equation (1).
H++e−→Had (1)
Carbon dioxide may be reduced at the cathode surface with an adsorbed hydrogen atom to form formate, which may be adsorbed on the surface as shown in equation (2) as follows:
CO2+Had→HCOOad (2)
The formate adsorbed on the surface then reacts with another adsorbed hydrogen atom to form formic acid that may be released into the solution as shown in equation (3):
HCOOad+Had→HCOOH (3)
A competing reaction at the cathode may be the reduction of water where hydrogen gas may be formed as well as hydroxide ions as shown in equation (4):
2H2O+2e−→H2+2OH− (4)
Operating the electrochemical cell at higher pressures (above atmospheric), may increase the current efficiency and allow operation of the cells at higher current densities.
Anode ReactionsThe anode reaction may be the oxidation of water into oxygen and hydrogen ions as shown in equation (5) as follows:
2H2O→4H++4e−+O2 (5)
Below may be the various preferred and alternative embodiments for the process, arranged in different categories.
Formate Formation from CO
The thermal intermolecular reaction of alkali metal formate CO with KOH may be as shown in equation (6) follows:
CO+KOH→HCOOK (6)
The KOH may be consumed in the reaction. Under the right conditions, both formate and oxalate may both be produced, and which may decrease the number of process steps. The production of both would require the separation of these carboxylic acids from each other.
Carbon monoxide may also be selectively absorbed in a alkali metal carbonate and bicarbonate aqueous solutions to produce formate, where M may be an alkali metal which may be shown as in equations (7) and (8) as follows:
CO+MHCO3→MOOCH+CO2 (7)
2CO+M2CO3+H2O→2MCOCH+CO2 (8)
These reactions may not require MOH, such as NaOH or KOH, in the reaction for the formation of M-formate as catalysts.
Oxalate from Formate
The thermal intermolecular reaction of alkali metal formate with KOH may be as shown in equation (9) as follows:
2HCOOK+KOH→K2C2O4+H2 (9)
Optionally, sodium or potassium carbonate may also be used for converting formate to oxalate, but the conversion yields have been shown to be significantly lower. Under the right operating conditions and temperatures, the yields may be significantly improved.
Anode Oxidation ReactionsThe anode reaction when utilizing sulfuric acid in the anolyte, is the oxidation of water generating hydrogen ions and oxygen as shown in equation (10) as follows:
2H2O→O2+4H++4e− (10)
If hydrobromic acid, HBr, is used in the anolyte, the reaction is the oxidation of the bromide to bromine as follows:
2HBr→Br2+2H++2e− (11)
If sodium chloride, NaCl, may be used in the anolyte, the anode reaction, such as in the formate cell in
2NaCl→Cl2+2Na++2e− (12)
Sodium ions may pass through the ion permeable separator from the anolyte compartment to the catholyte compartment and combine with any formate from the reduction of carbon dioxide to form sodium formate and any by-product hydroxide ions formed from the reduction of water at the cathode may form NaOH.
If hydrochloric acid, HCl, may be used in the anolyte, the reaction may be the oxidation of the chloride to chlorine with the co-production of hydrogen ions as shown in equation (13) as follows:
2HCl→Cl2+2H++2e− (13)
Sodium carbonate, Na2CO3, dissolved in solution may be converted to sodium bicarbonate, Na2HCO3, with reaction with CO2 as shown in equation (14) as follows:
Na2CO3+CO2+H2O→2NaHCO3 (14)
Sodium hydroxide, NaOH, reaction with CO2 in solution may be converted to sodium carbonate, Na2CO3, as shown in equation (15) as follows:
2NaOH+CO2+H2O→2Na2CO3+H2O (15)
Chlorine Reaction with NaOH
Sodium hydroxide, NaOH, may be reacted with chlorine to produce sodium hypochlorite, NaOCl, as shown in equation (16) as follows:
2NaOH+Cl2→NaOCl+NaCl+H2O (16)
The following present various exemplary combinations of cell configurations, electrode structures, and anolyte/catholyte compositions that may be used in the electrochemical CO and/or formate, and electrochemical acidification (EA) electrolyzers in the above described processes.
The cathode of the electrochemical cell 110 and electrochemical acidification electrolyzer 140 may be a high surface area electrode. The void volume for the cathode may be from about 30% to 98%. The surface area of the cathode may be from 2 cm2/cm3 to 2,000 cm2/cm3 or higher. The surface areas may be further defined as a total area in comparison to the current distributor/conductor back plate area with a preferred range of from 2 to 1000 times the current distributor/conductor back plate area.
The cathode of the electrochemical cell 110 may be electrolessly plated indium or tin on a copper woven mesh, screen or fiber structure. Indium-copper intermetallics may be formed on the copper woven mesh, screen or fiber structure. The intermetallics may be harder than the soft indium metal, and allow better mechanical properties in addition to usable catalytic properties. The indium electrocatalyst coating may also be prepared by electroplating indium onto a suitable corrosion resistant metal substrate, such as a tin plated copper or tin plated stainless steel metal high surface area substrate. Alternatively, the indium can be plated onto a high surface area carbon substrate, such as a carbon felt or cloth composed of fibers, which may already have an applied tin-based coating. Also, the indium electrocatalyst may be applied by co-electroplating a mixture of indium with another metal, such as Sn, Zn, Bi, or Pb, onto a suitably prepared coated metal or carbon substrate, such that the coating is physically and chemically compatible with the applied indium/co-metal electroplated external layer. Suitable indium-based electrocatalytic coatings are corrosion resistant to the catholyte solutions, salts, and operating pH of the catholyte.
In the electrochemical reduction of carbon dioxide metals including Pb, Sn, Hg, Tl, In, Bi, and Cd among others may produce formic acid (or formate) as a major C1 product in aqueous solutions. Alloy combinations of these metals such as Hg/Cu, Sn—Cd, Sn—Zn, Cu—Sn, may form at various performance efficiencies. One of the issues may be that a number of these metals, such as Sn and Cu, may be that the surface changes and deactivates or loses the Faradaic conversion activity in producing formate. The surface then may have to be reactivated by a reverse current or polarity. In the production for formation of C2+ chemicals, such as oxalic acid and glycolic acid, metals such as Ti, Nb, Cr, Mo, Ag, Cd, Hg, Tl, As, and Pb as well as Cr—Ni—Mo steel alloys among many others may result in the formation of these higher C2+ products.
In another embodiment, the cathode surfaces may be renewed by the periodic addition of indium salts or a mix of indium/tin salts in situ during the electrochemical cell operation. Electrochemical cell 110 may be operated at full rate during operation, or temporarily operated at a lower current density with or without any carbon dioxide addition during the injection of the metal salts.
In another exemplary embodiment, in preparing cathode materials for the production of C2+ chemicals, the addition of metal salts that may reduce on the surfaces of the cathode structure may be also used, such as the addition of Ag, Au, Mo, Cd, Sn, etc. to provide a catalytic surface that may be difficult to prepare directly during cathode fabrication or for renewal of the catalytic surfaces.
In another exemplary embodiment, Magneli phase titanium oxides, in particular Ti4O7 may be used as a cathode base material, which may also be impregnated or coated with one or more of the aforementioned suitable cathode metals, which may include Ag, Au, In, Mo, Cd, Sn, Cu, Hg, Tl, Bi, Ti, Nb, Zr, As, Cr, Co, Zn, Pb, and their alloys and combinations. These magneli phase materials may be in the form of three dimensional porous materials in the shape of plates, foams, pellets, powders, and the like. One manufacturer, Atraverda, Ltd., manufactures these under the tradename of Ebonex.
Cathode 412 for the electrochemical acidification electrolyzer 140 may include stainless steels and nickel electrodes. Cathode 412 may include coatings on the cathode to reduce the hydrogen overpotential.
An alkali metal hydroxide range for the electrochemical acidification electrolyzer 140 may be 5% to 50% by weight, and more preferably 10% to 45% by weight. The alkali metal hydroxide examples may be NaOH, KOH, CsOH and the like.
Cathode materials for the cathode of electrochemical cell 110 for carbon monoxide production from CO2 may include precious and noble metals, Cu, Ag, Au, and their oxides, specifically the oxides of copper. Other d-block metals, such as Zn and Ni, may be selective for CO reduction in aqueous media. Regardless of specificity for CO as a CO2 reduction product, a cathode for electrochemical cell 110 for an aqueous system for CO2 reduction to CO may have a high hydrogen overpotential to prevent competing H2 formation.
Anions used for CO production at the cathode may be any species stable at working potentials such as sulfate, chloride or bicarbonate. CO2 reduction to CO may favor high pH due to limited competing H2 formation; however there may be a practical pH maximum at around 8.5 for a saturated CO2 solution due to the formation of carbonic acid on dissolution. There may be no strict lower limit that may have been observed. Depending on the chemistry of the system, the pH of the catholyte region of electrochemical cell 110 may range from 3 to 12. The pH may be a function of the catalysts used, such that there may be no corrosion at the electrochemical cell 110 and catholyte operating conditions.
Electrolytes for the electrochemical cell 110 for forming CO and formates may include alkali metal bicarbonates, carbonates, sulfates, and phosphates, borates, ammonium, hydroxides, chlorides, bromides, and other organic and inorganic salts. The electrolytes may also include non-aqueous electrolytes, such as propylene carbonate, methanesulfonic acid, methanol, and other ionic conducting liquids, which may be in an aqueous mixture, or as a non-aqueous mixture in the catholyte. The introduction of micro bubbles of carbon dioxide into the catholyte stream may improve carbon dioxide transfer to the cathode surfaces.
The electrochemical cell catholyte solution may also comprise an aqueous or non-aqueous based solution. The solution may contain an organic solvent, for example methanol or ethanol, that may help provide a higher solubility of carbon dioxide in the solution over that of aqueous solutions. The organic solvent may be fully soluble in the aqueous catholyte solution or may also be present as an emulsion in the catholyte solution. The solvent may be a polar or aprotic type solvent. Solvents may include carbonates, such as propylene carbonate, acetone, alcohols—including primary, secondary, and tertiary types, dimethyl sulfoxide, dioxane, aromatic hydrocarbons such as toluene and cyclohexane, chlorinated as well as fluorinated solvents such as chloroform, aprotic solvents such as acetonitrile, and the like. An aqueous solvent in this disclosure is defined as a solution containing less than 50 wt % of organics in comparison to the water content. The organics may be chosen in regards to improving carbon dioxide solubility in the solution, as a moderator of the catholyte salt solubility such as formate, and in improving or enabling the cathode chemistry in the formation of C2+ compounds, which may be promoted by the use of the organics which exclude water in the cathode reaction. Examples of a C2+ carbon dioxide reduction products are oxalate, glycolate, and glyoxylates. Other carbon dioxide products may include acetic acid, ethanol, methanol, as well as others.
Electrolytes for the anolyte region of the electrochemical cell 110 may include: alkali metal hydroxides, (e.g. as KOH, NaOH, LiOH) in addition to ammonium hydroxide; inorganic acids such as sulfuric, phosphoric, and the like; organic acids such as methanesulfonic acid in both non-aqueous and aqueous solutions; and alkali halide salts, such as the chlorides, bromides, and iodine salts such as NaF, NaCl, NaBr, LiBr, KF, KCl, KBr, KI, and NaI, as well as their hydrogen halide forms, such as HCl, HF, HI, and HBr. The alkali halide salts may produce, for example, fluorine, chlorine, bromine, or iodine as halide gas or dissolved aqueous products from the anolyte region. Methanol or other hydrocarbon non-aqueous liquids may also be used, and they would form some oxidized organic products from the anolyte. Selection of the anolyte would be determined by the process chemistry product and requirements for lowering the overall operating cell voltage. For example, using HBr as the anolyte, with the formation of bromine at the anode, which require a significantly lower anode voltage potential than chlorine formation. Hydriodic acid, HI, may form iodine at anode potential voltages even lower than that of bromine.
Preferred anolytes for the system include alkali metal hydroxides, such as KOH, NaOH, LiOH; ammonium hydroxide; inorganic acids such as sulfuric, phosphoric, and the like; organic acids such as methanesulfonic acid; non-aqueous and aqueous solutions; alkali halide salts, such as the chlorides, bromides, and iodine types such as NaCl, NaBr, LiBr, and NaI; and hydrogen halides such as HCl, HBr and HI. The hydrogen halides and alkali halide salts will produce for example chlorine, bromine, or iodine as a halide gas or as dissolved aqueous products from the anolyte compartment. Methanol or other hydrocarbon non-aqueous liquids can also be used, and would form some oxidized organic products from the anolyte. Selection of the anolyte would be determined by the process chemistry product and requirements for lowering the overall operating cell voltage. For example, the formation of bromine at the anode requires a significantly lower anode voltage potential than chlorine formation, and iodine is even lower than that of bromine. This allows for a significant power cost savings in the operation of both of the electrochemical units when bromine is generated in the anolyte. The formation of a halogen, such as bromine, in the anolyte may then be used in an external reaction to produce other compounds, such as reactions with alkanes to form bromoethane, which may then be converted to an alcohol, such as ethanol, or an alkene, such as ethylene, and the halogen hydrogen halide byproduct from the reaction can be recycled back to the electrochemical cell anolyte.
Catholyte cross sectional area flow rates may range from 2 to 3,000 gpm/ft2 or more (0.0076-11.36 m3/m2). Flow velocities may range from 0.002 to 20 ft/sec (0.0006 to 6.1 m/sec).
Catholyte region of the electrochemical cell 110 may include at least one catalyst. The catalyst may be a homogeneous heterocyclic catalyst which may be utilized in the catholyte region to improve the Faradaic yield to formate. Homogeneous heterocyclic catalysts may include, for example, one or more of pyridine, tin 2-picoline, 4-hydroxy pyridine, adenine, a heterocyclic amine containing sulfur, a heterocyclic amine containing oxygen, an azole, a benzimidazole, a bipyridine, a furan, an imidazole, an imidazole related species with at least one five-member ring, an indole, a lutidine, methylimidazole, an oxazole, a phenanthroline, a pterin, a pteridine, pyridine, a pyridine related species with at least one six-member ring, a pyrrole, a quinoline, or a thiazole, and mixtures thereof.
Operating electrochemical cell 110 at a higher operating pressure in the catholyte region may allow more dissolved CO2 to dissolve in the aqueous electrolyte. Typically, electrochemical cells may operate at pressures up to about 20 to 30 psig in multi-cell stack designs, although with modifications, they could operate at up to 100 psig. The electrochemical cell 110 anolyte may also be operated in the same pressure range to minimize the pressure differential on the membrane separating the two electrode regions. Special electrochemical designs may be required to operate electrochemical units at higher operating pressures up to about 60 to 100 atmospheres or greater, which may be in the liquid CO2 and supercritical CO2 operating range.
In another embodiment, a portion of the catholyte recycle stream may be separately pressurized using a flow restriction with back pressure or using a pump 390 with CO2 injection such that the pressurized stream may be then injected into the catholyte region of the electrochemical cell 110, and potentially increasing the amount of dissolved CO2 in the aqueous solution to improve the conversion yield.
Catholyte region and anolyte region of electrochemical cell 110 may have operating temperatures that may range from −10 to 95° C., more preferably 5-60° C. The lower temperature may be limited by the electrolytes used and their freezing points. In general, the lower the temperature, the higher the solubility of CO2 in the aqueous solution phase of the electrolyte which may result in obtaining higher conversion and current efficiencies. However, operating electrochemical cell voltages may be higher, such that an optimization may be required to produce the chemicals at the lowest operating cost. In addition, the operating temperatures of the anolyte and catholyte may be different, whereby the anolyte is operated at a higher temperature and the catholyte is operated at a lower temperature.
The electrochemical cell 110 and the electrochemical acidification electrolyzer 140 may be zero gap, flow-through electrolyzers with a recirculating catholyte electrolyte with various high surface area cathode materials. For example, flooded co-current packed and trickle bed designs with various high surface area cathode materials may be employed. The stack cell design may be bipolar and/or monopolar.
The anode of the electrochemical cell 110 and the electrochemical acidification electrolyzer 140 may include one or more anode coatings. For example, for acid anolytes and oxidizing water under acid conditions, electrocatalytic coatings may include: precious metal and precious metal oxides such as ruthenium and iridium oxides, as well as platinum and gold and their combinations as metals and oxides on valve metal conductive substrates such as titanium, tantalum, or niobium as typically used in the chlor alkali industry or other electrochemical processes where they may be stable as anodes. Magneli phase titanium oxides, in particular Ti4O7 may be used as an anode material, which may also be impregnated or coated with the aforementioned precious metals and precious metal oxides. These magneli phase materials may be in the form of three dimensional porous materials in the shape of plates, foams, pellets, powders, and the like. One manufacturer, Atraverda, Ltd., manufactures these under the tradename of Ebonex. For other anolytes such as alkaline or hydroxide electrolytes, the electrocatalytic coatings may include carbon, graphite, cobalt oxides, nickel, stainless steels, and their alloys and combinations which may be stable as anodes under these alkaline conditions.
Membrane 330, 406a, 406b may be cation ion exchange type membranes such as those having a high rejection efficiency to anions. For example perfluorinated sulfonic acid based ion exchange membranes such as DuPont Nafion® brand unreinforced types N117 and N120 series, more preferred PTFE fiber reinforced N324 and N424 types, and similar related membranes manufactured by Japanese companies under the supplier trade names such as Flemion®. Other multi-layer perfluorinated ion exchange membranes used in the chlor alkali industry and having a bilayer construction of a sulfonic acid based membrane layer bonded to a carboxylic acid based membrane layer may be employed to efficiently operate with an anolyte and catholyte above a pH of about 2 or higher. These membranes may have a higher anion rejection efficiency. These may be sold by DuPont under their Nafion® trademark as the N900 series, such as the N90209, N966, N982, and the 2000 series, such as the N2010, N2020, and N2030 and all of their types and subtypes. Hydrocarbon based membranes, which may be made from of various cation ion exchange materials may also be used if the anion rejection may be not as critical, such as those sold by Sybron under their trade name Ionac®, AGC Engineering (Asahi Glass) under their Selemion® trade name, and Tokuyama Soda among others.
Electrochemical Formate Cell ExamplesAn electrochemical bench scale cell with an electrode projected area of about 108 cm2 was used for much of the bench scale test examples. The electrochemical cell was constructed consisting of two electrode compartments machined from 1.0 inch (2.54 cm) thick natural polypropylene. The outside dimensions of the anode and cathode compartments were 8 inches (20.32 cm) by 5 inches (12.70 cm) with an internal machined recess of 0.375 inches (0.9525 cm) deep and 3.0 inches (7.62 cm) wide by 6 inches (15.24 cm) tall with a flat gasket sealing area face being 1.0 inches (2.52 cm) wide. Two holes were drilled equispaced in the recess area to accept two electrode conductor posts that pass though the compartment thickness, and having two 0.25 inch (0.635 cm) drilled and tapped holes to accept a plastic fitting that passes through 0.25 inch (0.635 cm) conductor posts and seals around it to not allow liquids from the electrode compartment to escape to the outside. The electrode frames were drilled with an upper and lower flow distribution hole with 0.25 inch pipe threaded holes with plastic fittings installed to the outside of the cell frames at the top and bottom of the cells to provide flow into and out of the cell frame, and twelve 0.125 inch (0.3175 cm) holes were drilled through a 45 degree bevel at the edge of the recess area to the upper and lower flow distribution holes to provide an equal flow distribution across the surface of the flat electrodes and through the thickness of the high surface area electrodes of the compartments.
For the anode compartment cell frames, an anode with a thickness of 0.060 inch (0.1524 cm) and 2.875 inch (7.3025 cm) width and 5.875 inch (14.9225 cm) length with two 0.25 inch (0.635 cm) titanium diameter conductor posts welded on the backside were fitted through the two holes drilled in the electrode compartment recess area. The positioning depth of the anode in the recess depth was adjusted by adding plastic spacers behind the anode, and the edges of the anode to the cell frame recess were sealed using a medical grade epoxy. The electrocatalyst coating on the anode was a Water Star WS-32, an iridium oxide based coating on a 0.060 inch (0.1524 cm) thick titanium substrate, suitable for oxygen evolution in acids. In addition, the anode compartment also employed an anode folded screen (folded three times) that was placed between the anode and the membrane, which was a 0.010 inch (0.0254 cm) thick titanium expanded metal material from DeNora North America (EC626), with an iridium oxide based oxygen evolution coating, and used to provide a zero gap anode configuration (anode in contact with membrane), and to provide pressure against the membrane from the anode side which also had contact pressure from the cathode side.
For the cathode compartment cell frames, 316L stainless steel cathodes with a thickness of 0.080 inch (0.2032 cm) and 2.875 inch (7.3025 cm) width and 5.875 inch (14.9225 cm) length with two 0.25 inch (0.635 cm) diameter 316L SS conductor posts welded on the backside were fitted through the two holes drilled in the electrode compartment recess area. The positioning depth of the cathode in the recess depth was adjusted by adding plastic spacers behind the cathode, and the edges of the cathode to the cell frame recess were sealed using a fast cure medical grade epoxy.
A copper bar was connected between the two anode posts and the cathode posts to distribute the current to the electrode back plate. The cell was assembled and compressed using 0.25 inch (0.635 cm) bolts and nuts with a compression force of about 60 in-lbs force. Neoprene elastomer gaskets (0.0625 inch (0.159 cm) thick) were used as the sealing gaskets between the cell frames, frame spacers, and the membranes.
Example 1The above cell was assembled with a 0.010 inch (0.0254 cm) thickness indium foil mounted on the 316L SS back conductor plate using a conductive silver epoxy. A multi-layered high surface area cathode, comprising an electrolessly applied indium layer of about 1 micron thickness that was deposited on a previously applied layer of electroless tin with a thickness of about 25 micron thickness onto a woven copper fiber substrate. The base copper fiber structure was a copper woven mesh obtained from an on-line internet supplier, PestMall.com (Anteater Pest Control Inc.). The copper fiber dimensions in the woven mesh had a thickness of 0.0025 inches (0.00635 cm) and width of 0.010 inches (0.0254 cm). The prepared high surface area cathode material was folded into a pad that was 1.25 inches (3.175 cm) thick and 6 inches (15.24 cm) high and 3 inches (7.62 cm) wide, which filled the cathode compartment dimensions and exceeded the adjusted compartment thickness (adding spacer) which was 0.875 inches (2.225 cm) by about 0.25 inches (0.635 cm). The prepared cathode had a calculated surface area of about 3,171 cm2, for an area about 31 times the flat cathode plate area, with a 91% void volume, and specific surface area of 12.3 cm2/cm3. The cathode pad was compressible, and provided the spring force to make contact with the cathode plate and the membrane. Two layers of a very thin (0.002 inches thick) plastic screen with large 0.125 inch (0.3175 cm) holes were installed between the cathode mesh and the Nafion® 324 membrane. Neoprene gaskets (0.0625 inch (0.159 cm) thick) were used as the sealing gaskets between the cell frames and the membranes. The electrocatalyst coating on the anode in the anolyte compartment was a Water Star WS-32, an iridium oxide based coating, suitable for oxygen evolution in acids. In addition, the anode compartment also employed a three-folded screen that was placed between the anode and the membrane, which was a 0.010 inch (0.0254 cm) thick titanium expanded metal material from DeNora North America (EC626), with an iridium oxide based oxygen evolution coating, and used to provide a zero gap anode configuration (anode in contact with membrane), and to provide pressure against the membrane from the anode side which also had contact pressure from the cathode side.
The cell assembly was tightened down with stainless steel bolts, and mounted into the cell station, which has the same configuration as shown in
The anolyte used was a dilute 5% by volume sulfuric acid solution, made from reagent grade 98% sulfuric acid and deionized water.
In this test run, the system was operated with a catholyte composition containing 0.4 molar potassium sulfate aqueous with 2 gm/L of potassium bicarbonate added, which was sparged with carbon dioxide to an ending pH of 6.60.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 0.4 M K2SO4, 0.14 mM KHCO3
Catholyte flow rate: 2.5 LPM
Catholyte flow velocity: 0.08 ft/sec
Applied cell current: 6 amps (6,000 mA)
Catholyte pH range: 5.5-6.6, controlled by periodic additions of potassium bicarbonate to the catholyte solution recirculation loop. Catholyte pH declined with time, and was controlled by the addition of potassium bicarbonate.
Results:
Cell voltage range: 3.39-3.55 volts (slightly lower voltage when the catholyte pH drops)
Run time: 6 hours
Formate Faradaic yield: Steady between 32-35%, calculated taking samples periodically. The final formate concentration: 9,845 ppm
The same cell as in Example 1 was used with the same cathode, which was only rinsed with water while in the electrochemical cell after the run was completed and then used for this run.
In this test run, the system was operated with a catholyte composition containing 0.375 molar potassium sulfate aqueous with 40 gm/L of potassium bicarbonate added, which was sparged with carbon dioxide to an ending pH of 7.05.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 0.4 M K2SO4, 0.4 M KHCO3
Catholyte flow rate: 2.5 LPM
Catholyte flow velocity: 0.08 ft/sec
Applied cell current: 6 amps (6,000 mA)
Catholyte pH range: Dropping from 7.5 to 6.75 linearly with time during the run.
Results:
Cell voltage range: 3.40-3.45 volts
Run time: 5.5 hours
Formate Faradaic yield: Steady at 52% and slowly declining with time to 44% as the catholyte pH dropped.
Final formate concentration: 13,078 ppm
The same cell as in Examples 1 and 2 was used with the same cathode, which was only rinsed with water while in the electrochemical cell after the run was completed and then used for this run.
In this test run, the system was operated with a catholyte composition containing 0.200 molar potassium sulfate aqueous with 40 gm/L of potassium bicarbonate added, which was sparged with carbon dioxide to an ending pH of 7.10.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 0.2 M K2SO4, 0.4 M KHCO3
Catholyte flow rate: 2.5 LPM
Catholyte flow velocity: 0.08 ft/sec
Applied cell current: 9 amps (9,000 mA)
Catholyte pH range: Dropping from 7.5 to 6.65 linearly with time during the run, and then additional solid KHCO3 was added to the catholyte loop in 10 gm increments at the 210, 252, and 290 minute time marks which brought the pH back up to about a pH of 7 for the last part of the run.
Results:
Cell voltage range: 3.98-3.80 volts
Run time: 6.2 hours
Formate Faradaic yield: 75% declining to 60% at a pH of 6.65, and then increasing to 75% upon the addition of solid potassium bicarbonate to the catholyte to the catholyte loop in 10 gm increments at the 210, 252, and 290 minute time marks and slowly declining down with time 68% as the catholyte pH dropped to 6.90.
Final formate concentration: 31,809 ppm.
The same cell as in Examples 1, 2, and 3 was used with the same cathode, which was only rinsed with water while in the electrochemical cell after the run was completed and then used for this run.
In this test run, the system was operated with a catholyte composition containing 1.40 molar potassium bicarbonate (120 gm/L KHCO3), which was sparged with carbon dioxide to an ending pH of 7.8.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 1.4 M KHCO3Catholyte flow rate: 2.6 LPM
Catholyte flow velocity: 0.09 ft/sec
Applied cell current: 11 amps (11,000 mA)
Catholyte pH range: Dropping from around 7.8 linearly with time during the run to a final pH of 7.48
Results:
Cell voltage range: 3.98-3.82 volts
Run time: 6 hours
Formate Faradaic yield: 63% and settling down to about 54-55%.
Final formate concentration: 29,987 ppm.
The same cell as in Examples 1, 2, and 3 was used, except for using 701 gm of tin shot (0.3-0.6 mm diameter) media with an electroless plated indium coating as the cathode. The cathode compartment thickness was 0.875 inches.
In this test run, the system was operated with a catholyte composition containing 1.40 molar potassium bicarbonate (120 gm/L KHCO3), which was sparged with carbon dioxide to an ending pH of 8.0
The cell was operated in a batch condition with no overflow for the first 7.3 hrs, and then a 1.40 molar potassium bicarbonate feed was introduced into the catholyte at a rate of about 1.4 mL/min, with the overflow collected and measured, and a sample of the loop was collected for formate concentration analysis.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 1.4 M KHCO3Catholyte flow rate: 3.2 LPM
Applied cell current: 6 amps (6,000 mA)
Catholyte pH range: Dropping slowly from around a pH of 8 linearly with time during the run to a final pH of 7.50
Results:
Cell voltage range: 3.98-3.82 volts
Run time: Batch mode: 7.3 hours
Feed and product overflow: 7.3 hours to end of run at 47 hours.
The formate Faradaic efficiency was between 42% and 52% during the batch run period where the formate concentration went up to 10,490 ppm. During the feed and overflow period, the periodic calculated efficiencies varied between 32% and 49%. The average conversion efficiency was about 44%. The formate concentration varied between 10,490 and 48,000 ppm during the feed and overflow period. The cell voltage began at around 4.05 volts, ending up at 3.80 volts.
Example 6The same cell as in Examples 1, 2, and 3 was used, except for using 890.5 gm of tin shot (3 mm diameter) media and with a tin foil coating as the cathode. The cathode compartment thickness was 1.25 inches and the system was operated in a batch mode with no feed input. Carbon dioxide was sparged to saturate the solution in the catholyte disengager.
Packed Tin Bed Cathode Detail:
Weight: 890.5 gm tin shot
Tin shot: 3 mm average size
Total compartment volume: 369 cm3
Calculated tin bead surface area: 4,498 cm2
Calculated packed bed cathode specific surface area: 12.2 cm2/cm3
Calculated packed bed void volume: 34.6%
In this test run, the system was operated with a catholyte composition containing 1.40 molar potassium bicarbonate (120 gm/L KHCO3), which was sparged with CO2 to an ending pH of about 8.0
The cell was operated in a batch condition with no overflow and a sample of the catholyte loop was collected for formate concentration analysis periodically.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 1.4 M KHCO3Catholyte flow rate: 3.0 LPM (upflow)
Catholyte flow velocity: 0.068 ft/sec
Applied cell current: 6 amps (6,000 mA)
Catholyte pH range: Increasing slowly from around a pH of 7.62 linearly with time during the run to a final pH of 7.73
Results:
Cell voltage range: Started at 3.84 volts, and slowly declined to 3.42 volts
Run time: Batch mode, 19 hours
The formate Faradaic efficiency started at about 65% and declined after 10 hours to 36% and to about 18.3% after 19 hours. The final formate concentration ended up at 20,500 ppm at the end of the 19 hour run.
Example 7The same cell as in Examples 1, 2, and 3 was used, except for using 805 gm of indium coated tin shot (3 mm diameter) media and with a 0.010 inch (0.0254 cm) thickness indium foil mounted on the 316L SS back conductor plate using a conductive silver epoxy as the cathode. The cathode compartment thickness was 1.25 inches and the system was operated in a batch mode with no feed input. Carbon dioxide was sparged to saturate the solution in the catholyte disengager. The tin shot was electrolessly plated with indium in the same method as used in Examples 1-4 on the tin-coated copper mesh. The indium coating was estimated to be about 0.5-1.0 microns in thickness.
Indium-Coated Tin Shot Packed Bed Cathode Detail:
Weight: 890.5 gm, indium coating on tin shot
Indium coated tin shot: 3 mm average size
Total compartment volume: 369 cm3
Calculated tin bead surface area: 4498 cm2
Packed bed cathode specific surface area: 12.2 cm2/cm3
Packed bed void volume: 34.6%
In this test run, the system was operated with a catholyte composition containing 1.40 molar potassium bicarbonate (120 gm/L KHCO3), which was sparged with CO2 to an ending pH of about 8.0
The cell was operated in a batch condition with no overflow and a sample of the catholyte loop was collected for formate concentration analysis periodically.
Operating Conditions:
Batch Catholyte Recirculation RunAnolyte Solution: 0.92 M H2SO4
Catholyte Solution: 1.4 M KHCO3Catholyte flow rate: 3.0 LPM (upflow)
Catholyte flow velocity: 0.068 ft/sec
Applied cell current: 6 amps (6,000 mA)
Catholyte pH range: Decreased slowly from around a pH of 7.86 linearly with time during the run to a final pH of 5.51
Results:
Cell voltage range: Started at 3.68 volts, and slowly declined to 3.18 volts
Run time: Batch mode, 24 hours
The formate Faradaic efficiency started at about 100% and varied between 60% to 85%, ending at about 60% after 24 hours. The final formate concentration ended up at about 60,000 ppm at the end of the 24 hour run. Dilution error of the samples at the high formate concentrations may have provided the variability seen in the yield numbers.
Example 7The same cell as in Examples 1, 2, and 3 was used with a newly prepared indium on tin electrocatalyst coating on a copper mesh cathode. The prepared cathode had calculated surface areas of about 3,171 cm2, for an area about 31 times the flat cathode plate area, with a 91% void volume, and specific surface area of 12.3 cm2/cm3.
In this test run, the system was operated with a catholyte composition containing 1.40 M potassium bicarbonate (120 gm/L KHCO3), which was sparged with CO2 to an ending pH of 7.8 before being used.
The cells were operated in a recirculating batch mode for the first 8 hours of operation to get the catholyte formate ion concentration up to about 20,000 ppm, and then a fresh feed of 1.4 M potassium bicarbonate was metered into the catholyte at a feed rate of about 1.2 mL/min. The overflow volume was collected and volume measured, and the overflow and catholyte loop sample were sampled and analyzed for formate by ion chromatography.
Operating Conditions:
Cathode: Electroless indium on tin on a copper mesh substrate
Continuous Feed with Catholyte Recirculation Run—11.5 days
Anolyte Solution: 0.92 M H2SO4
Catholyte flow rate: 3.2 LPM
Catholyte flow velocity: 0.09 ft/sec
Applied cell current: 6 amps (6,000 mA)
Results:
Cell voltage versus time, displaying a stable operating voltage of about 3.45 volts over the 11.5 days after the initial start-up.
Continuous Run time: 11.5 days
Formate Concentration Versus Time: The formate concentration varied between 17,000 ppm and 28,000 ppm based on samples taken daily.
Formate Faradaic yield: The calculated formate current efficiency versus time measuring the formate yield from the collected samples. The formate yield varied between about 30% to 60% based on the daily interval samples taken.
Final formate concentration: About 28,000 ppm.
Catholyte pH: The catholyte pH change over the 11.5 days, slowly declined from a pH of 7.8 to a pH value of 7.5 at the end of the run. The feed rate was not changed during the run, but could have been slowly increased or decreased to maintain a constant catholyte pH in any optimum operating pH range.
The same cell as in Examples 1, 2, and 3 was used with a newly prepared indium on tin electrocatalyst coating on a copper mesh cathode. The prepared cathode had calculated surface areas of about 3,171 cm2, for an area about 31 times the flat cathode plate area, with a 91% void volume, and specific surface area of 12.3 cm2/cm3.
In this test run, the system was operated with a catholyte composition containing 1.40 M potassium bicarbonate (120 gm/L KHCO3), which was sparged with CO2 to an ending pH of 7.8 before being used.
The cells were operated in a recirculating batch mode for the first 8 hours of operation to get the catholyte formate ion concentration up to about 20,000 ppm, and then a fresh feed of 1.4 M potassium bicarbonate was metered into the catholyte at a feed rate of about 1.2 mL/min. The overflow volume was collected and volume measured, and the overflow and catholyte loop sample were sampled and analyzed for formate by ion chromatography.
Operating Conditions:
Cathode: Electroless indium on tin on a copper mesh substrate
Continuous Feed with Catholyte Recirculation Run—21 days
Anolyte Solution: 0.92 M H2SO4
Catholyte flow rate: 3.2 LPM
Catholyte flow velocity: 0.09 ft/sec
Applied cell current: 6 amps (6,000 mA)
Results:
Cell voltage versus time: The cell showed a higher operating voltage of about 4.40 volts, higher than all of our other cells, because of an inadequate electrical contact pressure of the cathode against the indium foil conductor back plate. The cell maintained operation for an extended run.
Continuous Run time: 21 days
Formate Faradaic yield: The calculated formate current efficiency were measured versus time from the collected samples. The formate Faradaic current efficiency declined from about 50%-60% from the first 4 days, averaged at about 4%-45% in days 5 through 14, and slowly declined into the 20%-25% range in days 17 through 20.
Formate Concentration Versus Time: The formate concentration averaged between about 20,000-30,000 ppm in days 1 through 16, and then declined to about 10,000 to 14,000 ppm in days 17 through 20. On day 21, 0.5 gm of indium (III) carbonate was added to the catholyte while the cell was still operating at the 6 ampere operating rate. The formate concentration in the catholyte operating loop was 11,330 ppm before the indium addition, which increased to 13,400 ppm after 8 hours, and increased to 14,100 ppm after 16 hours when the unit was shut down after 21 days of operation.
Catholyte pH: The catholyte pH change over the continuous operation period, which stabilized at about 7.6 after 2 days, and then operated in the 7.6 to 7.7 pH range until shutdown at about 20 days. The feed rate was not changed during the run, but could have been increased or decreased to maintain a constant pH operation in an optimum range.
Example 9The same electrochemical cell as used in examples 1, 2, 3, 8 and 9 was assembled for some further test runs. The only difference was that a previously used electroless indium on tin on copper mesh cathode operated during the runs was rinsed and was then electroplated with an additional indium surface coating layer using an indium sulfamate electroplating solution (Indium Corporation). The estimated applied indium coating thickness on the cathode was about 1 micron. Experiments conducted with the electroplated cathode provided formate Faradaic yield efficiencies of 60-65% with cell operating at 10 to 11 amperes, a current density of 90-100 ma/cm2. The anolyte solution was 1 M sulfuric acid, and the catholyte solution was a 1.0 M KHCO3 solution. Cell runs were conducted in multiple 3 hour batch runs, using a catholyte recirculation flowrate of 3 liters/min with carbon dioxide bubbled into the catholyte disengager.
Example 10The same cell as in Example 9 with the same indium electroplated cathode (previously used in several cell runs) was then used in an electrochemical system to demonstrate the reduction of carbon dioxide at the cathode to produce an alkali metal formate and co-producing chlorine at the anode from an alkali metal halide.
A 300 gm/L solution of sodium chloride was made up from reagent grade sodium chloride for use as an anolyte. The catholyte solution was a 0.25 M Na2CO3 solution made from reagent grade potassium bicarbonate. The chlorine evolved from the anolyte disengager was absorbed in a sodium hydroxide solution containing 80 grams of NaOH in 250 mL of solution volume to produce sodium hypochlorite. Carbon dioxide was sparged into the catholyte solution disengager and the catholyte solution recirculation flowrate was 9 liters/min.
The electrochemical cell was operated for 2 hours at an applied current of 12 amperes (110 ma/cm2 current density). The cell voltage started at about 5.16 volts and slowly declined to about 4.15 volts at the end of the run as the catholyte solution temperature increased from 24.2° C. to about 32.9° C. The pH of the catholyte increased from a starting pH of about 7.75, and the catholyte pH was controlled to a maximum pH of about 8.10 by the addition of dilute 10 wt % HCl in 10 mL aliquots. A total of 80 mL of the 10 wt % acid was added to the catholyte during the run. At the end of the two hour run, the anolyte and catholyte were drained from the cell and collected for analysis. The sodium hydroxide solution absorber solution was analyzed for sodium hypochlorite.
The anolyte and sodium hypochlorite solutions were analyzed for chlorine content using a standard iodometric analysis method using potassium iodide and dilute sulfuric acid and using 0.1 N sodium thiosulfate for the titration. The final anolyte solution volume (280 mL) as found to contain 3.19 gm/L chlorine, for a total of 0.89 gm of chlorine. The NaOH absorber (260 mL final volume) contained a concentration of about 113.34 gm/L NaOCl, which is equivalent to 31.74 gm of NaOCl, or equivalent to 30.23 gm of chlorine. Thus, the total amount of chlorine collected from the cell anolyte and sodium hypochlorite absorber was determined to be 31.12 gm. The calculated theoretical amount of chlorine was determined to be 31.74 gm, so the calculated anolyte Faradaic efficiency of NaCl to chlorine was 98.0%.
The catholyte formate solution concentration was analyzed by ion chromatography, and was found to contain 2,221 ppm of formate, for a total of 2.24 gm as the formate ion. The calculated theoretical as formate ion was 20.61 gm, for a calculated formate Faradaic yield of 10.87%.
Example 11The same cell as in example 10 was used with the same cathode and anode (previously used in several cell runs) was then used in an electrochemical system to demonstrate the reduction of carbon dioxide at the cathode to produce an alkali metal formate and co-producing chlorine at the anode from a hydrogen halide.
A 10 wt % solution of hydrochloric acid was made up from reagent grade 37 wt % hydrochloric acid for use as an anolyte. The catholyte solution was a 1.0 M NaHCO3 solution made from reagent grade potassium bicarbonate. The chlorine evolved from the anolyte disengager was absorbed in a sodium hydroxide solution absorber containing 41 grams of NaOH in 210 mL of solution volume to produce sodium hypochlorite. Carbon dioxide was sparged into the catholyte solution disengager and the catholyte solution recirculation flowrate was 9 liters/min.
The electrochemical cell was operated for 2 hours at an applied current of 12 amperes (110 ma/cm2 current density). The cell voltage started at about 3.38 volts and slowly increased to about 3.53 volts at the end of the run as the catholyte solution temperature increased from 25.9° C. to about 30.2° C. The pH of the catholyte increased from a starting pH of about 7.82, and the catholyte pH slowly increased to about a pH of 8.00 after 30 minutes, and then slowly declined to a pH of 7.80. At the end of the two hour run, the anolyte and catholyte were drained from the cell and collected for analysis. The sodium hydroxide solution absorber solution was analyzed for sodium hypochlorite.
The final anolyte solution volume (430 mL) as found to contain 3.01 gm/L chlorine, for a total of 1.30 gm of chlorine. The NaOH absorber (220 mL final volume) contained a concentration of about 130.27 gm/L NaOCl, which is equivalent to 28.66 gm of NaOCl, or equivalent to 27.30 gm of chlorine. Thus, the total amount of chlorine collected from the cell anolyte and sodium hypochlorite absorber was determined to be 28.60 gm. The calculated theoretical amount of chlorine was determined to be 31.74 gm, so the calculated anolyte Faradaic efficiency of NaCl to chlorine was 90.1%.
The final catholyte formate solution concentration was analyzed by ion chromatography, and was found to contain 4,811 ppm formate, for a total of 3.71 gm present as the formate ion. The calculated theoretical as formate ion was 20.61 gm, for a calculated formate Faradaic yield of 18.0%.
The use of a higher HCl concentration would improve the yield of HCl to chlorine. The final HCl concentration determined by acid-base titration was found to be 34.9 gm/L, which is about 3.5 wt % as HCl. In a typical cell operation, the concentration would be kept at a constant HCl concentration. The formate yield was lower than expected, and may have been due to some cathode coating degradation from the previous runs or during storage between runs.
Thermal Conversion of Alkali Metal Formate to Oxalate ExperimentsExperiments were conducted to determine some process conditions in the thermal conversion of alkali metal formate. Temperature, calcination time, and the addition of various catalysts that may improve the yields to oxalate were evaluated. Carbonate was determined by a standard two step titration method using standardized 0.1 N HCl as the titrant and phenolphthalein and bromocresol green as the pH indicators.
Example 12Table 1 shows the results of a set of experiments that were conducted in a thermal furnace using a nitrogen atmosphere. Experiments were conducted to evaluate the conditions and yields in the thermal conversion of alkali metal formate. Temperature was varied as well as calcination time, and the use of various catalysts were evaluated. These samples were prepared using reagent grade sodium formate crystal and the addition of reagent grade potassium hydroxide pellets. The chemical reagents were mixed together, and placed in a 100 mL nickel crucible. The crucible was calcined at the times and temperatures as given in Table 1. At 420° C., for time periods of 0.5 to 1.0 hrs, the percent yield of the potassium formate to potassium oxalate using the potassium hydroxide catalyst ranged from 73.71% to 78.53%. The oxalate content was analyzed by both permanganate titration and by Ion chromatography. At 440° C., the conversion yield to oxalate was about 77%.
Table 2 shows the results of the same procedure as in Example 12, except that potassium bicarbonate was added to potassium formate as a catalyst. The calcination temperature was 420° C. for 30 minutes in a nitrogen atmosphere in the thermal oven.
Table 3 shows the results of the same procedure as in Example 12, KOH was added to potassium formate as a catalyst. The calcination temperature was 440° C. for 30 minutes in a nitrogen atmosphere in the thermal oven. Table 4 shows the results using no KOH catalyst.
Table 5 shows the results of the same procedure as in Example 12, KOH was added to potassium formate as a catalyst. The calcination temperature was 480° C. for 30 minutes in a nitrogen atmosphere in the thermal oven. Table 6 shows additional results using a KOH catalyst.
Table 7 shows the results of the same procedure as in Example 12, magnesium oxide powder was added to potassium formate as a catalyst. The calcination temperature was 420° C. in a nitrogen atmosphere in the thermal oven.
Table 8 shows the results of the same procedure as in Example 12, sodium borohydride (NaBH4) powder was added to potassium formate as a catalyst. The calcination temperature was 440° C. in a nitrogen atmosphere in the thermal oven. Table 9 shows the results using NaBH4 and KOH as co-catalysts at the same temperature.
Table 10 shows the results of the same procedure as in Example 12, except that sodium hydride (NaH) powder was added to sodium formate as a catalyst. The calcination temperature was 440° C. in a nitrogen atmosphere in the thermal oven.
Table 11 shows the results using NaH added as a catalyst to potassium formate at various time and temperatures.
Alternative anolyte solutions may be employed to generate chemical products such as bromine at the anode region of electrochemical cell 110, which may be used to brominate organics as intermediates in making ethanol, ethylene, and other chemicals based on bromine chemistry. The oxidation of sulfur compounds in the anolyte region, such as sodium sulfide or SO2, or the direct or indirect oxidation of organics, and conducting the partial oxidation of organics, such as methanol to formaldehyde, are also contemplated.
Various alkali metal hydroxides may be employed at the electrochemical cell 110 and/or a thermal reactor 120, 130. For example, hydroxides of lithium, sodium, potassium, and rubidium, and cesium may be used. Further, alkaline earth metal hydroxides may also be used.
Thermal reactors 120, 130 may perform thermal intermolecular condensation reactions using alkali metal hydroxides. Such condensation reactions may include chemical reactions in which two molecules or moieties (functional groups) combine to form one single molecule, together with the loss of a small molecule. When two separate molecules may be reacted, the condensation may be termed intermolecular. Since the reaction occurs at elevated temperatures, the reactions may be characterized as “thermal intermolecular condensation step”. If water may be lost, the reactions may be characterized as “thermal intermolecular dehydration step”. These reactions may occur in an aqueous solution phase, such as with the reaction of CO with the alkali metal hydroxide, or as a melt of the alkali metal carboxylic acid and the alkali metal hydroxide in the thermal reaction.
Thermal reactors 120, 130 may operate at about 40 to 500° C., and more preferably at about 50-450° C. The operating temperatures may depend on the decomposition temperatures of the carboxylic acid and the optimum temperature to get the highest yields of the carboxylic product. A residence time of the reaction at optimum reaction temperatures may range from 5 seconds to hours, and the equipment chosen to conduct the reaction may be designed to provide the rate of heating and cooling required to obtain optimal conversion yields. This may include the use of cold rotating metal that may rapidly chill the hot thermal product after the thermal reaction period may be completed.
Thermal reactors 120, 130 may operate in air or an enriched oxygen atmospheres, as well as inert gas atmospheres, such as nitrogen, argon, and helium. Carbon dioxide and hydrogen atmospheres may also be employed to obtain the highest yield in the reaction, as well as partial CO atmospheres. Thermal reactors 120, 130 may be operated under a full or partial vacuum.
The use of CO from other sources, such as from the production of syngas from methane or natural gas reforming may be employed. CO may also come from other sources, such as process waste streams, where may be it separated from carbon dioxide.
Alkali metal hydroxide concentration ranges may be 2% to 99%, more preferably 5 to 98% by weight. The alkali hydroxide may run in molar excess of the alkali metal carboxylic acid being thermally processed in the initial reaction mix or in a continuous process where they may be mixed together. The anticipated molar ratios of the alkali metal carboxylic acid to alkali metal hydroxide may range from 0.005 to 100, and more preferably 0.01 to 50. It may be preferable to use the least amount of alkali metal hydroxide as possible for the reaction to reduce the consumption of the hydroxide in the process.
The process operating equipment that may be employed for thermal reactors 120, 130 may include various commercially available types. For the CO reaction with alkali metal hydroxide, the equipment that may be used may be batch operation equipment, where gas may be injected into a solution mix of the alkali hydroxide. This may also be done in a continuous manner where there may be a feed input of fresh alkali metal hydroxide into a continuous stirred tank reactor (CSTR) with a CO feed into the solution through a gas diffuser into the solution. Alternatively, counter-current packed towers may be used where CO may be injected into the tower counter-current to the flow of alkali metal hydroxide.
For a alkali metal oxalate operation, thermal reactors 120, 130 may include equipment such as rotary kilns, and single pass plug flow reactors that may be used if the process required the thermal processing of a mixture of alkali metal formate and alkali hydroxide as a solid or hot melt mix. Preferably, the equipment would be operated in a continuous fashion, providing the required residence time for the reaction to go to completion at the selected temperatures, which may be followed by a cooling section.
A thermal intermolecular condensation process may also be conducted to produce higher carbon content carboxylic acids as well as converting the carboxylic acids into esters, amides, acid chlorides, and alcohols. In addition, the carboxylic acid products may be converted to the corresponding halide compounds using bromine, chlorine, and iodine.
Catalysts for the thermal conversion of the alkali metal formate may consist of various bases, including alkali metal hydroxide as well as other compounds that are bases. In addition, alkali metal and other hydrides may be used, since they also act as bases. Any other suitable catalysts that may be compatible with the formates in the calcination and provide high conversion yields are suitable for the process.
It is contemplated that method for production of dicarboxylic acid, such as oxalic acid, may include various steps performed by systems 100, 105, 200 and 205. It may be believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described may be merely explanatory.
Claims
1. A method for producing oxalic acid and co-products, comprising:
- receiving an anolyte feed including a hydrogen halide at an anolyte region of a first electrochemical cell including an anode;
- receiving a catholyte feed including carbon dioxide and an alkali metal bicarbonate at a catholyte region of the first electrochemical cell including a cathode;
- applying an electrical potential between the anode and the cathode of the first electrochemical cell sufficient to reduce the carbon dioxide to formate and to oxidize the hydrogen halide to a halogen;
- converting the formate to an alkali metal oxalate via a thermal reaction;
- receiving the alkali metal oxalate at a first electrochemical acidification electrolyzer;
- converting the alkali metal oxalate to oxalic acid and co-producing alkali metal hydroxide, hydrogen, and a halogen at the first electrochemical acidification electrolyzer;
- converting an alkali metal halide in a second electrochemical acidification electrolyzer to produce a hydrogen halide containing solution, an alkali metal hydroxide, and hydrogen; and
- feeding at least a portion of the hydrogen halide containing solution to the anolyte region of the first electrochemical cell;
- feeding at least a portion of the hydrogen halide containing solution to the first electrochemical acidification electrolyzer.
2. The method of claim 1, wherein the anolyte feed to the first electrochemical cell includes water and a hydrogen halide containing solution generated from the second electrochemical acidification cell, wherein the hydrogen halide solution comprises a soluble mixture of hydrogen halide and alkali metal halide with the hydrogen halide concentration ranging from about 5 wt % to 40 wt %.
3. The method of claim 1, wherein the hydrogen halide includes at least one of hydrogen bromide or hydrogen chloride.
4. The method of claim 2, wherein an alkali metal halide content in the hydrogen halide containing solution ranges from 0.01 wt % to a solubility limit of the alkali metal halide in the hydrogen halide solution.
5. The method of claim 2, wherein the alkali metal halide is at least one of sodium chloride, potassium chloride, sodium bromide, or potassium bromide.
6. The method of claim 1, wherein the alkali metal hydroxide from the first and second electrochemical acidification cells includes potassium or sodium hydroxide.
7. The method of claim 1, wherein the converting the formate to the alkali metal oxalate via the thermal reaction includes receiving a catalyst.
8. The method of claim 7, wherein the catalyst is selected from the group consisting of alkali metal hydroxides, alkali metal ethoxides, alkali metal methoxides, and alkali metal hydrides.
9. The method of claim 6, wherein converting the alkali metal oxalate to oxalic acid at the first electrochemical acidification electrolyzer comprises:
- passing the alkali metal oxalate through an ion exchange region of the first electrochemical acidification electrolyzer bounded by one or more cation ion exchange membranes;
- producing an alkali metal hydroxide and hydrogen in the catholyte compartment, and
- oxidizing a hydrogen halide containing solution in the anode compartment to produce a halogen.
10. The method of claim 9, further comprising:
- converting the oxalic acid to an oxalic acid ester with an alcohol.
11. The method of claim 10, further comprising:
- converting the oxalic acid ester to ethylene glycol by hydrogenation.
12. The method of claim 1, further comprising:
- reacting the halogen with an organic compound to produce a halogenated organic compound.
13. The method of claim 1, further comprising:
- reacting the halogen with an alkali metal hydroxide to produce an alkali metal hypochlorite product.
14. The method of claim 12, wherein the organic compound is at least one of ethylene or propylene.
15. The method of claim 14, wherein the halogenated organic is at least one of ethylene chloride, propylene chloride, ethylene bromide or propylene bromide.
16. A method for producing formic acid and co-products comprising:
- receiving an anolyte feed including a hydrogen halide at an anolyte region of a first electrochemical cell including an anode;
- receiving a catholyte feed including carbon dioxide and an alkali metal bicarbonate at a catholyte region of the first electrochemical cell including a cathode;
- applying an electrical potential between the anode and the cathode sufficient to reduce the carbon dioxide to at least an alkali metal formate and to oxidize the hydrogen halide to a halogen;
- converting the alkali metal formate to formic acid and co-producing alkali metal hydroxide, hydrogen, and halogen at a first electrochemical acidification electrolyzer;
- converting an alkali metal halide in a second electrochemical acidification electrolyzer to produce a hydrogen halide containing solution, an alkali metal hydroxide, and hydrogen;
- feeding at least a portion of the hydrogen halide containing solution to the anolyte compartment of the first electrochemical cell; and
- feeding at least a portion of the hydrogen halide containing solution to the first electrochemical acidification electrolyzer.
17. The method of claim 16, wherein the anolyte feed to the first electrochemical cell includes water, hydrogen halide, and alkali metal halide with the hydrogen halide concentration ranging from about 5 wt % to 40 wt %.
18. The method of claim 16, wherein the hydrogen halide includes at least one of hydrogen bromide or hydrogen chloride.
19. The method of claim 16, wherein the alkali metal halide content in the hydrogen halide containing solution ranges from 0.01 wt % to a solubility limit of the alkali metal halide in the hydrogen halide solution.
20. The method of claim 16, wherein the alkali metal halide is at least one of sodium chloride, potassium chloride, sodium bromide or potassium bromide.
21. The method of claim 16, wherein the alkali metal hydroxide from the first and second electrochemical acidification cells is at least one of potassium hydroxide or sodium hydroxide.
22. The method of claim 16, wherein the alkali metal formate is at least one of potassium formate or sodium formate.
23. The method of claim 16, wherein converting the alkali metal formate to formic acid at the first electrochemical acidification electrolyzer comprises:
- passing the alkali metal formate through an ion exchange region of the first electrochemical acidification electrolyzer bounded by one or more cation ion exchange membranes;
- producing an alkali metal hydroxide and hydrogen in the catholyte compartment; and
- oxidizing a hydrogen halide containing solution in the anode compartment to produce a halogen.
24. The method of claim 23, further comprising:
- converting the formic acid to a formate ester with an alcohol.
25. The method of claim 23, further comprising:
- converting the formic acid to a formamide.
26. The method of claim 16, further comprising:
- reacting the halogen with an organic compound to produce a halogenated organic compound.
27. The method of claim 16, further comprising:
- reacting the halogen with an alkali metal hydroxide to produce an alkali metal hypochlorite product.
28. The method of claim 16, wherein the halogen is at least one of bromine or chlorine.
29. The method of claim 26, wherein the organic compound is at least one of ethylene or propylene.
Type: Application
Filed: Jul 14, 2014
Publication Date: May 4, 2017
Inventors: Jerry J. Kaczur (North Miami Beach, FL), Prasad Lakkaraju (East Brunswick, NJ)
Application Number: 15/320,218
