US20120247970A1

US20120247970A1 - Bubbling air through an electrochemical cell to increase efficiency

Info

Publication number: US20120247970A1
Application number: US13/076,920
Authority: US
Inventors: Erik C. Olson; Kim R. Smith
Original assignee: Ecolab USA Inc
Current assignee: Ecolab USA Inc
Priority date: 2011-03-31
Filing date: 2011-03-31
Publication date: 2012-10-04

Abstract

Methods and systems for electrochemical cell optimization, namely increasing cell efficiency and rate of chemical product, are disclosed. In particular, a device wherein air or an oxygen-containing gas is bubbled through an electrochemical cell and methods of use are disclosed to reduce the voltage demand and power usage of an electrochemical cell. The optimized electrochemical cell also achieves increased chemical generation from electrolysis for using in various settings, including for example, housekeeping applications.

Description

FIELD OF THE INVENTION

The invention relates to methods and systems for electrochemical cell optimization. In particular, an electrochemical cell with improved efficiency and processes for obtaining such improved efficiency bubble an oxygen-containing gas, such as air, through a cell without the use of an oxygen-consuming electrode. The use of an oxygen-containing gas in an electrochemical cell without an oxygen-consuming electrode, namely a cathode, demonstrates the mechanical usage of the oxygen-containing gas rather than a chemical reaction or consumption of the oxygen-containing gas. The enhanced efficiency of the electrochemical cell requires less voltage while at least maintaining the rate of chemical conversion for use in various settings, including for example, housekeeping applications.

BACKGROUND OF THE INVENTION

Electrochemical cells are used for a variety of purposes, such as a means for water treatment and generation of chemicals, including hypochlorite and/or caustic solutions for use in various sanitizing, cleaning and/or disinfecting purposes. In general, electrolysis uses an electric current to split water into its two constituent elements: hydrogen and oxygen. Electricity enters the water at a cathode, a negatively charged terminal, passes through the water and exits through an anode, a positively charged terminal. Hydrogen is collected at the cathode (negatively charged electrical current) and oxygen is collected at the anode (positively charged electrical current). The reaction of water in an electrolytic cell is a redox process, as an oxidation reaction occurs at the anode while a reduction reaction occurs at the cathode.
The electrolysis of aqueous sodium chloride produces chlorine gas at the anode and hydrogen gas at the cathode as a result of the reduction of water at the cathode to form hydroxyl ions and hydrogen gas and the oxidation of chloride ions from sodium chloride solution at the anode to produce chlorine gas. A basic solution of sodium hydroxide (or “caustic” or “alkali”) as well as an acidic solution of hypochlorous acid (formed from the reaction of chlorine with water) is formed. Depending upon the structure of an electrochemical cell, various effluents may be produced. For example, a cell divided by a membrane produces both hypochlorous acid and sodium hydroxide (caustic). Alternatively, if the electrochemical cell is not divided by a membrane, the chlorine gas or hypochlorous acid and caustic react to form hypochlorite (sodium hypochlorite, commonly referred to as bleach). As one skilled in the art appreciates a variety of electrolyte solutions and sources may be utilized and impact the products generated from an electrochemical cell.
Conventional electrochemical cells are equipped with at least an anode and a cathode in the interior and often have a dual structure in which the anode and cathode are separated by a membrane to divide the cells into an anode chamber and a cathode chamber. The barrier membrane provides the advantage of preventing the products at the anode chamber from mixing with the products from the cathode chamber. A variety of cell structure designs may be utilized, including variations in the number of cell chambers, type of membranes, etc., which impact the products generated from a particular electrochemical cell. Various, non-limiting examples of electrochemical cell structures are disclosed, for example in U.S. Pat. No. 3,616,355, U.S. Pat. No. 4,062,754, U.S. Pat. No. 4,100,052, U.S. Pat. No. 4,761,208, U.S. Pat. No. 5,313,589, and U.S. Pat. No. 5,954,939.
Despite the variations in electrochemical cell structures and design, there remains a need for optimized electrochemical cells. Efforts to enhance electrochemical cell efficiency have included, for example, pressurizing a cell, using of oxygen-consuming cathodes (also referred to as oxygen depolarized cathodes) and using oxygen-consuming cathodes concomitantly with an oxygen-containing gas circulated through a cell. See e.g., U.S. Pat. No. 7,670,472, U.S. Pat. No. 7,025,868, and U.S. Pat. No. 6,712,949. However, there are various difficulties in the construction of electrochemical cells and the safety of such cells if used with significantly elevated pressure levels. In addition, there are design detriments of using oxygen-consuming electrodes, such as added expense and mechanical and chemical fragility of the cathodes. As a result, there remains a need for electrochemical cells with enhanced efficiency and longevity. Accordingly, there is a significant market for electrochemical cells able to utilize a variety of electrolyte sources to generate chemical products in situ with improved efficiency and reduced energy consumption.
Accordingly, it is an objective of the claimed invention to optimize electrochemical cells and processes for on-site chemical generation.
A further object of the invention is to reduce voltage requirements of an electrochemical cell without decreasing the rate of chemical conversion.
A further object of the invention is an improved electrochemical cell structure compatible with bubbling an oxygen-containing gas, such as air, through one or more of the chambers to reduce voltage requirements and enhance cell efficiency.

BRIEF SUMMARY OF THE INVENTION

The invention pertains to the field of electrochemistry and more particularly, a device and methods of use for enhanced electrochemical efficiency through improved cell design. In particular, the invention provides the advantage of an overall increase in efficiency of an electrochemical cell, resulting in the lowering of volts required while at least maintaining and preferably increasing the rate of chemical conversion without requiring the use of an oxygen-consuming electrode. The advantageous results of the invention are obtained by bubbling an oxygen-containing gas, such as air, through at least one of the chambers in an electrochemical cell.
According to an embodiment of the invention a method for increasing electrochemical cell efficiency includes obtaining an electrochemical cell configured with at least one anode and cathode separated by a membrane to form at least two chambers, wherein said cathode is not an oxygen-consuming electrode, providing an electrolyte source, and bubbling an oxygen-containing gas through at least one chamber of the cell causing a decreased voltage demand without decreasing the rate of chemical conversion and without the electrolytically reducing the oxygen-containing gas. Preferably, the oxygen-containing gas is air that is bubbled through the cathode chamber of said cell. According to embodiments, the methods of the invention result in at least a thirty percent reduction in voltage, more preferably a fifty percent reduction in voltage.
According to a further embodiment of the invention a method of increasing caustic production from an electrochemical cell with decreased voltage demands includes obtaining an electrochemical cell configured with at least one anode and cathode separated by a membrane to form at least two chambers for the production of caustic, wherein the cathode is not an oxygen-consuming electrode, providing an electrolyte source to the cell, wherein a catholyte source includes a source of an oxygen-containing gas, and reducing voltage demand of said cell as a result of bubbling an oxygen-containing gas through the cathode chamber of said cell, wherein the cell has a reduction in voltage of at least thirty percent.
A further embodiment of the invention is an electrochemical cell designed for increased efficiency for producing chemical effluents such as caustic and comprises at least one anode and cathode electrode, wherein said electrodes are corrosion-resistant and said cathode is not an oxygen-consuming electrode and is not iron, and wherein said electrodes are separated by at least one membrane to form at least one anode and cathode chamber, an electrolyte source for providing an anolyte and catholyte source, a source of an oxygen-containing gas bubbled through at least one of said chambers, and a source of electric current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B show diagrams of a micro flow electrochemical cell according to an embodiment of the invention (FIG. 1A) and the flow directionality of the air and catholyte in the micro flow electrochemical cell (FIG. 1B).

FIG. 2 shows the measurement of sodium hydroxide production over time based on change in pH of an improved electrochemical cell according to the invention compared to a control cell.

FIG. 3 shows the measurement of electrochemical cell efficiency over time based upon sodium hydroxide production and voltage demand of the improved electrochemical cell according to the invention compared to a control cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of this invention are not limited to particular methods and systems for electrochemical cell optimization, which can vary and are understood by skilled artisans. It is further to be understood that all terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting in any manner or scope. For example, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” can include plural referents unless the content clearly indicates otherwise. Further, all units, prefixes, and symbols may be denoted in its SI accepted form. Numeric ranges recited within the specification are inclusive of the numbers defining the range and include each integer within the defined range.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention pertain. Many methods and materials similar, modified, or equivalent to those described herein can be used in the practice of the embodiments of the present invention without undue experimentation, the preferred materials and methods are described herein. In describing and claiming the embodiments of the present invention, the following terminology will be used in accordance with the definitions set out below.
The term “about,” as used herein, refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients used to make the compositions or carry out the methods; and the like. The term “about” also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term “about”, the claims include equivalents to the quantities refers to variation in the numerical quantity that can occur.
As used herein, the term “cleaning” means to perform or aid in soil removal, bleaching, microbial population reduction, or combinations thereof.
As used herein, the terms “feed water” and “water” refer to any source of water that can be used with the methods, systems and apparatus of the present invention. Exemplary water sources suitable for use in the present invention include, but are not limited to, water from a municipal water source, or private water system, e.g., a public water supply or a well. The water can be city water, well water, water supplied by a municipal water system, water supplied by a private water system, and/or water directly from the system or well. The feed water can also include water from a used water reservoir, such as a recycle reservoir used for storage of recycled water, a storage tank, or any combination thereof. In some embodiments, the water source is from the sump of a mechanical washing device such as a dishwasher. In some embodiments, the water source is from a dispenser for a solid block of feedstock. In some embodiments, the water source is not industrial process water, e.g., water produced from a bitumen recovery operation. In other embodiments, the water source is not a waste water stream.
The term “hard water,” as used herein, refers to water having a level of calcium and magnesium ions in excess of about 100 ppm. Often, the molar ratio of calcium to magnesium in hard water is about 2:1 or about 3:1. Although most locations have hard water, water hardness tends to vary from one location to another. Further, as used herein, the term “solubilized water hardness” refers to hardness minerals dissolved in ionic form in an aqueous system or source, i.e., Ca⁺⁺ and Mg⁺⁺. Solubilized water hardness does not refer to hardness ions when they are in a precipitated state, i.e., when the solubility limit of the various compounds of calcium and magnesium in water is exceeded and those compounds precipitate as various salts such as, for example, calcium carbonate and magnesium carbonate. Salts formed from water hardness ions have low solubility in water as they are formed by metal cations interacting with inorganic anions. As concentration in a solution increases and/or temperature or pH of a water source increases, the salts will precipitate from solution, crystallize and form hard deposits or scales on surfaces, causing the undesirable effects on equipment such as electrochemical cells. A threshold inhibitor or threshold agent (as used synonymously) inhibits the crystallization of water hardness ions from solution, without necessarily forming a specific complex with the water hardness ion, thereby inhibiting the scaling, film and/or residue traditionally observed in cells. Not wishing to be limited by theory, it is believed that the threshold inhibitors work by interfering with the growth of the scale crystals.
The term “hypochlorite,” as used herein, refers to a salt of hypochlorous acid. A hypochlorite ion is ClO⁻and therefore a hypochlorite compound is a chemical compound containing this group having a chlorine in the oxidation state (+1). The oxidative state results in very low stability, causing hypochlorites to be very strong oxidizing agents. One skilled in the art may recognize that other chlorine-containing bleaches such chlorate ions or even chlorine dioxide can be formed by modifying the pH or starting materials for an electrochemical cell. A common example of a hypochlorite is the bleaching agent sodium hypochlorite. As used herein, sodium hypochlorite (NaOCl) may be used interchangeably with hypochlorite. Hypochlorous acid is a more effective sanitizer than hypochlorite and is chemically preferred when the pH of a bleach solution is decreased. For purposes of describing the present invention, the description of the use of threshold agents for electrochemical cells producing hypochlorite shall also be understood to incorporate cells producing hypochlorous acid. For further purposes of the present invention, hypochlorite and hypochlorous acid shall also refer to a chlorine-containing oxidant as described herein.
The terms “threshold agent” and “threshold inhibiting agent,” as used herein, refer to a compound that inhibits crystallization of water hardness ions from solution, but that need not form a specific complex with the water hardness ion. Threshold agents are capable of maintaining hardness ions in solution beyond its normal precipitation concentration. See e.g., U.S. Pat. No. 5,547,612. This distinguishes a threshold agent from a chelating agent or sequestrant; however, according to the invention the threshold agent may be either a chelating agent and/or sequestrant. Threshold agents may include, for example and without limitation, polycarboxylates, such as polyacrylates, polymethacrylates, olefin/maleic copolymers, and the like. The threshold agent according to the invention must survive the electrochemical cell's conditions to ensure it is not deactivated and prevented from inhibiting scaling, and further must not cause any decrease in chlorine generation. As used herein, the terms “chelating agent” and “sequestrant” refer to a compound that forms a complex (soluble or not) with water hardness ions (from the wash water, soil and substrates being washed) in a specific molar ratio. According to the invention, the threshold agent is preferably characterized as substoichiometric, such that the threshold agent is effective at concentration levels that are lower than would be expected based on a stoichiometric equivalence of the threshold agent and the scale-causing component present in the electrochemical cell or treated water source.
The term “water soluble,” as used herein, refers to a compound that can be dissolved in water at a concentration of more than 1 wt-%. The term “water insoluble,” as used herein, refers to a compound that can be dissolved in water only to a concentration of less than 0.1 wt-%. The terms “slightly soluble” or “slightly water soluble,” as used herein, refer to a compound that can be dissolved in water only to a concentration of 0.1 to 1.0 wt-%.
The term “weight percent,” “wt-%,” “percent by weight,” “% by weight,” and variations thereof, as used herein, refer to the concentration of a substance as the weight of that substance divided by the total weight of the composition and multiplied by 100. It is understood that, as used here, “percent,” “%,” and the like are intended to be synonymous with “weight percent,” “wt-%,” etc.
The methods and systems according to the invention provide for electrochemical cell optimization, namely the reduction in energy consumption and demand of an electrochemical cell. The claimed methods and electrochemical cells according to the invention significantly reduce the voltage requirements of an electrochemical process without the use of mechanically and chemically fragile gas permeable electrodes, such as oxygen-consuming cathodes. Rather, the present invention uses an oxygen-containing gas without an oxygen-consuming electrode, namely a cathode, in order to take advantage of the mechanical benefits of the oxygen-containing gas with an electrochemical cell. Unlike the electrochemical cell designs of the prior art, the present invention does not utilize the oxygen-containing gas for any chemical reaction or consumption by the system.
The claimed cell optimization discovery was unexpectedly made while attempting to electrochemically-generate hydrogen peroxide using sodium carbonate in place of sodium sulfate. An electrochemical cell was assembled in a manner to allow air to be bubbled through the cathode chamber and yielded an increase in the rate of sodium hydroxide production, rather than hydrogen peroxide. According to the invention, the bubbling of the air (an oxygen-containing gas) through the cathode chamber yielded a significant decrease in voltage demands of the cell.
As a result, the invention provides an improvement over those in the art using various forms of gas diffusion electrodes to allow air to pass for production of hydrogen peroxide, without achieving enhanced electrochemical cell efficiency. See for example, U.S. Pat. No. 4,350,575. As used herein, “gas diffusion electrode,” “oxygen-consuming electrode” and “oxygen depolarized electrode” (used herein synonymously) refer to either a porous or non-porous electrode having high gas permeability and low liquid permeability in order to maintain a stable interface between liquid and gas. One skilled in the art will ascertain that gas diffusion cathodes are recognized as useful for eliminating hydrogen production from a chlor-alkali cell, along with reducing electrochemical cell voltage as a result of eliminating hydrogen gas from surfaces of the electrochemical cell which may interfere with electrical conductivity and therefore efficiency.
According to a preferred embodiment of the invention, an electrochemical cell has overall increased efficiency. In particular, an electrochemical cell according to the invention requires decreased voltage without any decrease in rate of chemical conversion. According to a preferred embodiment, an electrochemical cell requires decreased voltage while increasing the rate of chemical conversion as a result of bubbling air through one of the chambers in the electrochemical cell, wherein no gas diffusion or oxygen-consuming electrodes are employed in the electrochemical cell.
Methods of Increasing Electrochemical Cell Efficiency
The methods according to the invention are used to increase the efficiency of an electrochemical cell. According to an embodiment of the invention, the methods of the invention maintain and/or increase the rate of generating chemicals while decreasing voltage demand if the electrochemical cell. According to an embodiment, an oxygen-containing gas is added to at least one chamber of the electrochemical cell during the electrochemical production. According to a further embodiment, an oxygen-containing gas is added to the cathode chamber of an electrochemical cell having two or more chambers in order to pass through the cathode chamber and increase cell efficiency.
According to the invention, an oxygen-containing gas is understood to include pure oxygen, a mixture of oxygen and inert gases, carbon dioxide, oxygen-enriched air, compressed air, air or the like. Pure oxygen is understood to mean a source having at least 99% purity by volume. According to a preferred embodiment, the oxygen-containing gas is air which is preferably used rather than elemental oxygen due to the decreased cost of adding air to an electrochemical cell. According to a further embodiment of the invention, air is added with a catholyte solution provided to the electrochemical cell. For example, air is streamed into the electrochemical cell with the liquid electrolyte. As recognized by the present invention, the mechanical reaction of the oxygen-containing gas, rather than the chemical reaction obtained in cells having an oxygen-consuming cathode, does not rely on the source of the oxygen-containing gas. As a result, beneficially according to the invention any source of oxygen may be employed.
According to an embodiment of the invention, the oxygen-containing gas is added to an electrochemical cell via a steady stream for the dispersion of gas bubbles into the electrochemical cell. One skilled in the art will ascertain various mechanisms for the dispersion of the oxygen-containing gas into an electrochemical cell. For example, a pressurized pump may be utilized to add air via an inlet directly into the electrolyte solution that is added to an electrochemical cell. According to an embodiment, the addition of the air directly to the liquid flow into the electrochemical cell is preferred. Pressurized pumps are preferred to provide optimum flow rates. According to an embodiment of the invention a flow rate of the oxygen-containing gas is adjusted to the mechanical action of the electrochemical cell. According to an embodiment of the invention a flow rate (L/h) of approximately 10-200 may be utilized, preferably 10-100. In addition pressure of the pump may vary from approximately 0.1-5 psi, preferably less than 2 psi. One skilled in the art will ascertain the various amounts, rates and pressurized conditions for the administration of the oxygen-containing gas suitable for use according to the invention to provide ideal circulation of the electrolyte solution through the electrochemical cell.
According to an embodiment of the invention, the oxygen contained in an oxygen-containing gas that is passed through the cathode chamber does not react with the cathode. As a result, the increase in electrochemical cell efficiency does not result from a “cleaning” effect of the electrode contacted with the oxygen or oxygen-containing gas passed through the electrochemical cell according to the invention. Rather, according to the invention, the oxygen-containing gas is believed to both enhance the crossing of the ion exchange membrane, such that the oxygen-containing gas induces a microscopic vacuum across the membrane, enabling enhanced transport. In addition, it is believed that the bubbling of the oxygen-containing gas increased turbulence within the electrochemical cell to enhance the mixing of the electrolyte solutions throughout the cell and minimize any ‘dead zones.’
According to a further embodiment of the invention, the oxygen in the oxygen-containing gas that is passed through the electrochemical cell according to the invention is not reduced by the cathode to generate water by products. Preferably, the oxygen passed through the cathode chamber is not reduced to promote the formation of water by products. In addition, the electrochemical cell and methods according to the invention do not generate hydrogen peroxide. As demonstrated by the present invention, there is no formation of peroxide intermediates from chemical reactions of oxygen and hydrogen.
As one skilled in the art may recognize, an oxidized anolyte produces H⁺ protons and oxygen, and the H⁺ protons move from an anolyte chamber to the catholyte chamber, wherein the catholyte feed stream is reduced to form peroxide ions, which react with the H⁺ protons to form hydrogen peroxide in the catholyte chamber. However, according to an embodiment of the invention, the oxygen-containing gas that is preferably provided to the catholyte chamber of an electrochemical cell and contacts a catholyte feed stream and cathode does not result in the reduction to generate water byproducts for the formation of hydrogen peroxide in the catholyte chamber.
Voltage Reduction
The methods according to the invention demonstrate a significant decrease in energy consumption by an electrolytic cell. Specifically, according to the invention the methods generate a decrease in an electrochemical cell's voltage demand. One skilled in the art shall ascertain, voltage demands are at least in part based upon factors such as anolyte and catholyte solutions, membrane thickness, pressure, conductivity and other factors. However, determination of these and other optimum parameters for a particular electrochemical cell can be readily ascertained by those skilled in the art through routine experimentation based upon the beneficial description provided herein.
According to preferred embodiments of the invention, at least a 20% voltage reduction is obtained. Preferably, at least a 30% voltage reduction is obtained based upon methods and use of the present invention. Still more preferably, at least a 40% voltage reduction is achieved, with most preferred embodiments demonstrating voltage reductions of at least 50%, at least 60% or greater.
According to a further embodiment of the invention, the methods of increasing electrochemical cell efficiency results in either a maintained or decreased voltage demand of the cell while increasing the rate of chemical conversion of said cell. Preferably, the methods according to the invention result in both a decreased voltage demand of the cell and increased chemical conversion as a result of bubbling air through at least one chamber of the electrochemical cell.
Additional modifications to the methods according to the invention may further increase the efficiency of an electrochemical cell. For example, modifications of the cell pressure may yield further power reduction. According to a preferred embodiment of the invention, at least ambient cell pressure is used. According to a further embodiment, cell pressure is increased above ambient pressure up to about 50 psi to provide cumulative enhancements of electrochemical cell efficiency. However, according to an embodiment of the invention, the efficiency of the electrochemical cell is significantly enhanced without the requirement of increasing the pressure of the cell. As a result, the invention provides an added benefit over art requiring electrolysis to be carried out under pressurized conditions—pressurizing at a critical partial gas pressure or higher in order to obtain a decrease in voltage of the electrochemical cell.
In addition to increasing the efficiency of an electrochemical cell, the methods according to the invention can be used to increase the production of an electrolysis product. For example, an electrochemical process for the electrolysis of inorganic materials, such as the aqueous inorganic metal salt solution of sodium chloride brine, according to the invention has an increased chemical production rate. Chemicals generated from an electrochemical cell vary according to the anolyte and catholyte used for a particular electrochemical cell. Exemplary chemicals generated can include, but are not limited to, hypochlorite and sodium hydroxide.
One skilled in the art to which the invention pertains shall appreciate that electrochemical cell modifications can include varying catholyte and anolyte solutions to yield specific chemicals. For example, according to an exemplary embodiment of the invention, a catholyte comprises deionized water and an anolyte comprises sodium carbonate solution. According to this preferred embodiment of the invention, generated chemicals include sodium hydroxide (i.e., caustic soda). Alternative embodiments may employ sodium carbonate or sodium sulfate as an anolyte, in addition to alternative catholyte solutions.
Electrochemical Cell
Electrochemical cells suitable for use according to the invention can be configured in a variety of manners. According to an embodiment of the invention an electrochemical cell configuration, such as shown in the micro flow configuration of FIG. 1, is used for high efficiency production of a variety of chemicals for cleaning applications. The methods and uses according to the invention maybe utilized with these and additional electrochemical cell structures. For example, exemplary electrochemical cell structures include, but are not limited to those described in U.S. Pat. Nos. 6,790,339, 6,773,575, 6,767,447, 6,761,815 and 6,712,949.
According to an embodiment, the methods and uses according to the invention may be adapted according to the type and structure of a particular electrochemical cell. For purposes of convenience, the disclosure is primarily directed to an electrochemical cell comprising, consisting of and/or consisting essentially of a cell configured with at least one anode and cathode separated by a membrane to form at least two chambers, an electrolyte source and a source for bubble air through at least one chamber of the cell. One skilled in the art can appreciate that additional modifications to the electrochemical cell may be suitable for use according to the invention.
According to an embodiment of the invention, a non-oxygen-consuming electrochemical cell utilized for a chlor-alkali process involves at least a two chambered cell. Most often, brine (sodium chloride) is circulated through an anode chamber, from which chlorine gas evolves. In addition, water is converted to hydrogen and forms sodium hydroxide (caustic) from the cathode chamber. However, according to the invention the oxygen that is bubbled through the cathode chamber is not converted into water by products. The methods and mechanisms of the present invention differ from the prior art in that an oxygen-consuming cathode is not utilized. Although the prior art recognizes the benefit of oxygen-consuming cathodes in reducing hydrogen generation from the cathode and still generating sodium hydroxide, the methods and electrochemical cells according to the invention do not experience the detriment of hydrogen generation within the electrochemical cell. Notably, the mechanical circulation of the oxygen-containing gas prevents the build-up of hydrogen on the electrochemical cell surfaces and does not experience any increase in voltage requirements. Rather, the claimed invention experiences a decrease in electrochemical cell voltage demand as a result of the bubbling of the oxygen-containing gas through the cell.
According to an embodiment of the invention, the electrochemical cell suitable for carrying out the methods of the invention is a cell divided by a cation exchange membrane into a cathode chamber and an anode chamber. According to this embodiment, the cathode chamber houses a cathode electrode that is not an oxygen-consuming cathode and the anode chamber houses an anode electrode. The electrochemical cell is further supplied with an electrical current and may house a pressure-maintaining means. According to this embodiment, an aqueous solution is passed into the anode chamber, referred to as the anolyte, and an aqueous solution, referred to as the catholyte, is passed into the cathode chamber. In addition an oxygen-containing gas is passed into the cathode chamber of the electrochemical cell via an inlet.
As set forth, in a non-limiting embodiment of the invention, the electrochemical cell for use according to the invention adds air directly to one chamber of the cell via an inlet. Preferably, air is added directly to the cathode chamber of the electrochemical cell via an inlet. However, according to the invention the structure of the cell may be modified or vary, for example the cell may have more than two chambers.
According to non-limiting embodiments of the invention, a variety of types of electrodes known by those of ordinary skill in the art may be utilized for the electrochemical cell. The electrode materials useful according to the invention are electrical conductors that are stable in the media to which they are exposed. According to the electrochemical cell of the invention, the electrodes are connected to negative (cathode) and positive (anode) current sources and remain stable in the media of the cell. For example, corrosion-resistant electrodes may be utilized according to the methods and cell designs of the invention. These preferably include stainless steel and/or titanium and alloys of the same. According to a preferred embodiment, the cathode electrode is not iron and is not an oxygen-consuming cathode.
One skilled in the art will ascertain that additional materials may be selected for use as electrodes according to the invention, including for example, aluminum, niobium, chromium, manganese, molybdenum, ruthenium, tin, tantalum, vanadium, zirconium, nickel, cobalt, copper, iridium, alloys of the same and combinations of the same known to one of ordinary skill in the art. The metal selected for use of the electrodes according to the invention, particularly cathodes, are non-oxygen consuming cathodes. According to a further embodiment, the metal selected for use of the electrodes according to the invention, particularly cathodes, are non-porous, solid surfaces.
Various electrochemical cells known to skilled artisans use porous electrodes constructed of a variety of metals. The electrochemical cell of the present invention provides a significant advantage over prior art cells using gas diffusion electrodes. According to an embodiment of the invention, the electrodes utilized for the electrochemical cell are not gas diffusion electrodes. According to a further embodiment of the invention, the cathode is not a gas diffusion electrode, such that the solid surface of the cathode electrode does not permit the flow of catholyte and air through its surface. As a result, preferred embodiments of the invention, use steel and/or titanium electrodes that do not permit the electrolyte solutions and oxygen-containing gas of the invention to flow through any porous electrodes. According to a preferred embodiment of the invention, the non-porous cathode is titanium and does not permit the reaction of the oxygen-containing gas to form hydrogen peroxide. More preferably, the non-porous cathode is selected from a material that does not include iron so as to eliminate any reaction of oxygen and iron on the electrode.
The use of non gaseous diffusion electrodes (e.g. non-oxygen-consuming cathodes) according to the invention is divergent from the teachings of the prior art. For example, U.S. patent application Ser. No. 12/086,374 titled “Oxygen-Consuming Zero-Gap Electrolysis Cells with Porous/Solid Plates” discloses that energy savings are achieved through the replacement of hydrogen-evolving cathodes with oxygen-consuming cathodes. Such references disclose energy savings of as much as 30% through the use of oxygen-consuming cathodes (also referred to as oxygen-depolarized cathodes or oxygen-depolarized cells). The present invention achieves at least the same energy savings through the use of the methods and electrochemical cell designs without the use of costly and mechanically and chemically fragile cathodes, providing a significant and unexpected benefit.
According to non-limiting embodiments of the invention, a variety of membranes may be used to separate the at least two chambers of the electrochemical cell. The membranes suitable for use according to the invention are generally flat diaphragms which separate the anolyte from the catholyte. According to the invention, more than one membrane or diaphragm can be utilized to create an electrochemical cell having at least two chambers, namely a cathode and anode chamber. Preferably, the membrane is a cation exchange membrane or a semi-permeable micro porous diaphragm. One skilled in the art will appreciate the various cation exchange membrane and semi-permeable micro porous diaphragms suitable for use in an electrochemical cell. For example, a commercially-available cation exchange membrane is a NAFION membrane (available from DuPont®).
According to further non-limiting embodiments of the invention, an electrochemical cell may be pressurized and/or have controlled temperatures. See, e.g., U.S. Pat. No. 4,105,515, which is incorporated by reference in its entirety. For example, for pressurization of a cell, a cell may have pressure controlling valves equipped on the catholyte tank or anolyte tank, outlet, etc. Alternatively, the pressure of an electrochemical cell may be controlled as a result of controlling the amount of electrolyte circulated to the electrochemical cell as either the amount of anolyte and/or catholyte. One skilled in the art may ascertain additional modifications for the control and/or modification of the pressure of an electrochemical cell according to the invention and based upon the disclosure of the invention.
According to an embodiment of the invention, in a process for increasing electrochemical cell efficiency or in an electrochemical cell designed for increased efficiency for producing chemical effluents such as hydrogen peroxide and/or caustic, the improvement comprises circulating a source of air bubbled through at least one of said chambers, in addition to optionally maintaining the pressure in the electrochemical cell at least above ambient pressure up to about 50 psi.
According to an embodiment of the invention, at least one electrolyte feed stream is provided to the electrochemical cell. According to the invention, more than one electrolyte feed stream may be provided to the electrochemical cell, such that at least one anolyte feed stream is provided to the anolyte chamber and at least one catholyte feed stream is provided to the catholyte chamber. According to a further embodiment, the source of an oxygen-containing gas is provided with the at least one electrolyte feed stream. Preferably, the oxygen-containing gas is provided directly to the cathode with the electrolyte feed stream.
Additional Electrochemical Cell Modifications
According to a further embodiment, a “specialty salt” or sodium carbonate block may further comprise performance enhancing additives and can be used to generate the anolyte solution.
According to a further embodiment, a threshold agent may be added to electrochemical cell according to the invention to enhance cell longevity in addition to enhancing cell efficiency. The threshold agent utilized according to the invention prevents the scaling of the electrodes and membranes in electrochemical cells, namely the cathode of a cell. According to a preferred embodiment, the threshold agents according to the invention are water soluble polymeric systems capable of preventing hard water scale formation caused by hard water deposits, particularly in systems supplied with water having high levels of carbonate, hydroxide and/or phosphate ions along with water hardness ions traditionally leading to buildup in cells causing the unsightly residue, film and scaling that is detrimental to cells. Use of the threshold agent of the present invention obviates the need to “soften” the water source used in an electrochemical cell. Threshold agents according to the invention are substantially stable in the presence of chlorine or are substantially chemically-resistant to chlorine and the corrosive conditions of the electrochemical cell. According to a preferred embodiment, the threshold inhibiting agents may be a polycarboxylate or related copolymer. Exemplary polycarboxylates that may be utilized as threshold inhibiting agents according to the invention include for example: homopolymers and copolymers of polyacrylates; polyacrylates; polymethacrylates; noncarboxylated materials such as polyolefinic and polymaleic copolymers, such as olefinic and maleic hydride copolymers; and derivatives and salts of all of the same. Additional description of suitable threshold agents for use according to the present invention are described in U.S. patent application Ser. No. 12/986,312, titled “Control of Hard Water Scaling in Electrochemical Cells.”
All publications and patent applications in this specification are indicative of the level of ordinary skill in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated by reference.

EXAMPLES

Embodiments of the present invention are further defined in the following non-limiting Examples. It should be understood that these Examples, while indicating certain embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the embodiments of the invention to adapt it to various usages and conditions. Thus, various modifications of the embodiments of the invention, in addition to those shown and described herein, will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

Example 1

A micro flow through electrochemical cell was modified to allow air to be bubbled through the cathode chamber according to the cell design and assembly shown in FIGS. 1A-B. The exemplary cell configuration of FIGS. 1A-B, or similar configurations, can be used to produce a variety of different chemicals, which include but are not limited to hypochlorite, sodium hydroxide, hydrogen peroxide, etc., for on site cleaning applications at a high rate of efficiency.
The micro-flow cell design provides a generic method for evaluating different electrochemical processes, reactions, feed sources and concentrations, cell design, rates, efficiency, etc. as outlined according to the following procedures. An electrochemical cell was configured with a NAFION F424 membrane creating a two chambered cell having a cathode and anode chamber housing a 316 SS anode and a titanium cathode. Peristaltic pumps were attached and circulated deionized water through the cell to clean tubes, chambers, membrane and electrodes. Catholyte and anolyte solutions were prepared, using a 20% sodium carbonate anolyte solution and deionized water as catholyte solution.

Example 2

An electrochemical cell was configured with a 316 stainless steel anode and 20% sodium carbonate anolyte solution. A titanium cathode and deionized water catholyte were also used. Leads of the power source were connected to the proper electrode (i.e., + anode attaches to 316 SS and − cathode attaches to Ti electrode in this instance). A pH electrode was placed into the catholyte solution and recorded initial pH. The catholyte and anolyte solutions are circulated through the cell. A power source is supplied to the cell and amps are adjusted to a desired level (while adjusting volts to maximum level to ensure amps are being controlled). The power supply for the cell was set to 1 amp and voltage was monitored as it fluctuated. The power supply was run for 15 minutes while recording amps, volts, and pH at the 1, 2, 3, 4, 5, 10, and 15 minute marks.
An increase in the rate of sodium hydroxide production (evidenced by increase in pH of catholyte) and a decrease in voltage were achieved and demonstrated in TABLE 1. A control cell was configured to exclude the bubbling of air through the cathode chamber and sodium hydroxide product and voltage are demonstrated in TABLE 2.
Results were evaluated to compare experimental results to control values. An increase in rate of sodium hydroxide production while maintaining equal or lower voltage levels indicates a significant improvement in electrochemical cell performance and efficiency.

TABLE 1A

(air bubbled through cathode chamber)

Time (min.)	AMPS	VOLTS	pH (catholyte)	ppm H ₂0₂

0	1.0	34.5	6.04	0
1	1.0	32.22	10.41	0
2	1.0	31.54	11.11	0
3	1.0	30.67	11.31	0
4	1.0	28.89	11.45	0
5	1.0	27.8	11.6	0
10	1.0	25.2	11.78	trace
15	1.0	22.3	11.93	trace

TABLE 1B

(air bubbled through cathode chamber)

Time (min.)	AMPS	VOLTS	pH (catholyte)	ppm H ₂0₂

0	0.5	63.7	6.11	0
1	1.0	35.1	10.02	0
2	1.0	34.1	10.87	0
3	1.0	32.5	11.13	0
4	1.0	32.3	11.33	0
5	1.0	31.6	11.41	0
10	1.0	26.3	11.7	0
15	1.0	23.6	11.85	trace

TABLE 1C

(air bubbled through cathode chamber)

Time (min.)	AMPS	VOLTS	pH (catholyte)	ppm H ₂0₂

0	1.0	28.3	6.46	0
1	1.0	29.5	10.11	0
2	1.0	30.5	10.9	0
3	1.0	31.9	11.17	0
4	1.0	29.5	11.38	0
5	1.0	29.2	11.46	0
10	1.0	22.2	11.67	0
15	1.0	18.24	11.85	0

TABLE 2

(Control - no air through cell)

Time (min.)	AMPS	VOLTS	pH (catholyte)	ppm H ₂0₂

0	0.0	63.7	6.71	0
1	0.0	63.7	7.3	0
2	0.0	63.7	7.91	0
3	0.0	63.7	8.2	0
4	0.0	63.7	8.31	0
5	0.0	63.7	8.59	0
10	0.1	63.7	9.48	0
15	0.2	63.8	10.41	0

TABLES 1 and 2 are further represented in FIGS. 2 and 3 wherein sodium hydroxide production was measured along with pH and voltage, respectively. As demonstrated in the figures, the control electrochemical cell (without bubbling air through cathode chamber) did not result in any increased efficiency, as measured in both decrease in voltage requirements and increase in pH from sodium hydroxide production (as achieved with the cell design of bubbling air through the cathode chamber of the electrochemical cell.
The inventions being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the inventions and all such modifications are intended to be included within the scope of the following claims.

Claims

1. A method for increasing electrochemical cell efficiency comprising:

(a) obtaining an electrochemical cell configured with at least one anode and cathode separated by a membrane to form at least two chambers, wherein said cathode is not an oxygen-consuming electrode;

(b) providing an electrolyte source to said electrochemical cell; and

(c) adapting said cell to bubble an oxygen-containing gas through at least one chamber of said cell, wherein said cell achieves a decreased voltage demand without decreasing the rate of chemical conversion and wherein said oxygen-containing gas is not electrolytically reduced by said cell.

2. The method of claim 1 wherein said oxygen-containing gas is air that is bubbled through the cathode chamber of said cell.

3. The method of claim 1 wherein said method achieves an increase in rate of chemical conversion.

4. The method of claim 1 wherein said cell achieves at least a thirty percent reduction in voltage.

5. The method of claim 4 wherein said cell achieves at least a fifty percent reduction in voltage.

6. The method of claim 1 wherein said oxygen-containing gas is compressed air.

7. The method of claim 1 wherein said oxygen-containing gas is air.

8. The method of claim 1 wherein said electrochemical cell does not generate hydrogen peroxide.

9. A method of increasing caustic production from an electrochemical cell with decreased voltage demands, comprising:

(a) obtaining an electrochemical cell configured with at least one anode and cathode separated by a membrane to form at least two chambers for the production of caustic, wherein said cathode is not an oxygen-consuming electrode;

(b) providing an electrolyte source to said electrochemical cell, wherein a catholyte source includes a source of an oxygen-containing gas; and

(c) reducing voltage demand of said cell as a result of bubbling an oxygen-containing gas through the cathode chamber of said cell, wherein said reduction in voltage is at least thirty percent.

10. The method of claim 9 wherein oxygen in said air is not electrolytically reduced by said cell.

11. The method of claim 9 wherein said anode and cathode are corrosion-resistant electrodes and said cathode is not iron.

12. The method of claim 9 wherein said electrochemical cell does not generate hydrogen peroxide.

13. The method of claim 9 wherein said oxygen-containing gas is air.

14. The method of claim 9 wherein said reduction in voltage is at least a fifty percent reduction.

15. An electrochemical cell designed for increased efficiency for producing chemical effluents such as caustic comprising:

(a) at least one anode and cathode electrode, wherein said electrodes are corrosion-resistant and said cathode is not a non-porous, non-oxygen-consuming electrode that does not comprise iron, and wherein said electrodes are separated by at least one membrane to form at least one anode and cathode chamber;

(b) an electrolyte source for providing an anolyte and catholyte source;

(c) a source of an oxygen-containing gas bubbled through at least one of said chambers; and

(d) a source of electric current.

16. The electrochemical cell of claim 15 wherein said source of an oxygen-containing gas is air and said air is bubbled through the cathode chamber of said cell.

17. The electrochemical cell of claim 15 wherein said cell achieves at least a fifty percent reduction in voltage and does not decrease the rate of chemical conversion of said cell.

18. The electrochemical cell of claim 15 wherein said cell either maintains or decreases the voltage requirements and increases the rate of chemical conversion of said cell.

19. The electrochemical cell of claim 15 wherein said anode is stainless steel and said cathode is titanium.

20. The electrochemical cell of claim 15 wherein said electrochemical cell does not generate hydrogen peroxide.