Electrolytic Process to Separate Alkali Metal Ions from Alkali Salts of Glycerine

Abstract

Methods and apparatus for separating alkali metal ions from alkali salts of glycerine to thereby form clean glycerine. These methods are enabled by the use of alkali ion conductive membranes in electrolytic cells that are chemically stable in low pH conditions. The alkali ion conductive membrane preferably includes a chemically stable ionic-selective polymer membrane. A layered composite of a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane is disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application No. 60/985,033, filed Nov. 2, 2007, which is incorporated by reference. This application is a continuation-in-part of U.S. patent application Ser. No. 11/622,314, filed Jan. 11, 2007, which is incorporated by reference.

FIELD OF THE INVENTION

The present invention provides method of separating alkali ions from glycerine based byproducts. The present invention further provides a method of converting alkali salts of glycerine into glycerine. These methods are enabled by the use of alkali ion conductive membranes in electrolytic cells that tolerate low pH environments.

BACKGROUND OF THE INVENTION

Biodiesel is an alternative fuel source to diesel, JP-8, and standard gasoline that is growing in popularity and market penetration in the United States and worldwide. Current methods of making biodiesel are outlined in the publication Biodiesel Production Technology, NREL, July 2004 (NREL/SR-510-36244). Biodiesel is defined as a fuel comprised of mono-alkyl esters of long chain fatty acids derived from triglycerides. A “mono-alkyl ester” is the product of the reaction of a straight chain alcohol, such as methanol or ethanol, with a triglyceride to form glycerine (also known as glycerin or glycerol) and the esters of long chain fatty acids. The triglycerides are commonly obtained from vegetable oils and animal fats of various origins. As used throughout this specification the term “triglycerides” may be used interchangeably with fats or fatty acids, vegetable oils and animal fats. Biodiesel has a general formula R′OOCR, where R′ is a straight chain lower alkyl and R is a C₁₄ to C₂₄ hydrocarbon chain.

One method of making biodiesel involves the reaction of triglycerides with methanol and with a hydroxide salt catalyst according to reaction (1) below.

Where R represents R₁, R₂, or R₃ which may be the same or different C₁₄ to C₂₄ hydrocarbon chain, and R′ is an unbranched, straight chain lower alkyl, such as methyl or ethyl.

The process of reaction (1) uses NaOH/KOH as a catalyst for reaction between triglycerides and alcohol to form a two phase soapy glycerol based ester with biodiesel. The two phase mixture is washed with water to separate biodiesel. This process is cumbersome, complex, inefficient, and may elevate the cost of biodiesel.

Biodiesel may also be manufactured by reaction of triglycerides with an alkali alkoxide, such as alkali methoxide or alkali ethoxide, according to reaction (2) below.

Where R represents R₁, R₂, or R₃ which may be the same or different C₁₄ to C₂₄ hydrocarbon chain, and R′ is an unbranched, straight chain lower alkyl, such as methyl or ethyl.

From reaction (2), alkali metal salts of glycerine are commonly formed as a byproduct in the manufacture of biodiesel. It would be an improvement in the art to provide apparatus and methods to separate alkali metal ions from glycerine byproducts such as the alkali metal salts of glycerine. Such apparatus and methods are provided herein.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for separating alkali ions from glycerine byproducts, especially alkali metal salts of glycerine. These methods are enabled by the use of alkali ion conductive membranes in an electrolytic cell. The alkali ion conductive membrane preferably includes a chemically stable ionic-selective polymer membrane. A layered composite of a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane may also be used to take advantage of the chemical stability of the ionic-selective polymer in low pH conditions and the high alkali-ion selectivity of cation-conductive ceramic materials.

The electrolytic cell includes an alkali ion conductive membrane configured to selectively transport alkali ions. The membrane separates the electrolytic cell into an anolyte compartment configured with an electrochemically active anode and a catholyte compartment configured with an electrochemically active cathode.

The glycerine containing alkali salts may be introduced into the anolyte compartment. Additional reaction byproducts may be present in the anolyte compartment, including Oxygen or Chlorine. Hydrogen gas, or ammonium hydroxide, water, or another compound that is electrochemically oxidizable to generate protons is introduced into the anolyte compartment. Water or alkali salt or alkali base solution is introduced into the catholyte compartment.

In the disclosed method, an electric current is applied to the electrolytic cell to produce hydrogen ions at the anode in the anolyte compartment. This necessarily lowers the pH within the anolyte compartment. The hydrogen ions react with the alkali salts of glycerine to form clean glycerine according to the following reaction:

C₃H₅(OM)₃+3H⁺→C₃H₅(OH)₃+3M⁺

Hydrogen ions may be produced at the anode in the anolyte compartment by oxidation of water, hydrogen, or another compound having oxidizable hydrogen. When hydrogen gas is oxidized at the anode to form hydrogen ions, this reaction may be facilitated by the use of suitable electro-catalyst compounds associated with the anode.

The influence of the electric potential causes free alkali ions (M⁺) to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment. Water is decomposed in the presence of alkali ions in the catholyte compartment to form alkali hydroxide solution and hydrogen gas according to the following reaction:

M⁺+H₂O+e⁻→MOH+½H₂

Clean glycerine (C₃H₅(OH)₃) is removed from the anolyte compartment.

To increase the efficiency of the apparatus and method, hydrogen gas produced in the catholyte compartment may be introduced into the anolyte compartment or used to generate power the process. The alkali hydroxide solution produced in the catholyte compartment may be removed for use in other industrial processes, including the manufacture of biodiesel.

Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment, but may refer to every embodiment.

Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In order that the manner in which the above-recited and other features and advantages of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 provides a schematic view of an electrolytic cell in the apparatus and process for separating alkali metal ions from alkali metal salts of glycerine; and

FIG. 2 provides a schematic view of another electrolytic cell in the apparatus and process for separating alkali metal ions from alkali metal salts of glycerine;

FIG. 3 is a schematic representation of the two compartment electrolytic cell used in Examples 1 and 2;

FIG. 4 is a graph of voltage versus time showing the voltage as sodium is removed from the two compartment cell over time in a batch process of Example 1;

FIG. 5 is a graph of voltage versus time showing the voltage as sodium is removed from the two compartment cell over time in a batch process of Example 2;

FIG. 6 is a graph of current versus voltage to drive sodium across a sodium conductive membrane in a two compartment electrolytic cell to separate sodium from a solution containing sodium salts of glycerine;

FIG. 7 is a graph of conductivity and current at different voltages;

FIG. 8 is a graph of sodium removal from glycerine;

FIG. 9 is a graph of current density and voltage versus time for the operation of a two compartment electrolytic cell;

FIG. 10 is a graph of water content in the anolyte solution after 15 hour of testing the two compartment electrolytic cell; and

FIG. 11 is a graph of current density and voltage versus time for the operation of a two compartment electrolytic cell with a polymer membrane and with anolyte containing ammonium hydroxide.

DETAILED DESCRIPTION OF THE INVENTION

The present embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the methods and cells of the present invention, as represented in FIGS. 1 through 10, is not intended to limit the scope of the invention, as claimed, but is merely representative of present embodiments of the invention.

FIG. 1 illustrates a general schematic view for one electrolytic apparatus and process for separating alkali metal ions from alkali salts of glycerine within the scope of the present invention. The apparatus and process for separating alkali metal ions includes an electrolytic cell 10.

The electrolytic cell 10 uses an alkali ion conductive membrane 12 that divides the electrochemical cell 10 into two compartments: an anolyte compartment 14 and a catholyte compartment 16. An electrochemically active anode 18 is housed in the anolyte compartment 14 where oxidation reactions take place, and an electrochemically active cathode 20 is housed in the catholyte compartment 16 where reduction reactions take place. The alkali ion conductive membrane 12 selectively transfers alkali ions (M⁺) 22 from the anolyte compartment 14 to the catholyte compartment 16 under the influence of an electrical potential 24 while preventing water transportation from either compartment to the other side. In one embodiment, the membrane 12 may include an ionic-selective polymer membrane stable in the low-pH environment of the anolyte compartment. In another embodiment, the membrane 12 may include a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

The electrolytic cell 10 is operated by feeding the alkali salts of glycerine 25 or similar byproducts from the manufacture of biodiesel into the anolyte compartment 14. Hydrogen gas 26 may also be introduced into the anolyte compartment 14.

Water 28 is fed into the catholyte compartment 16. During operation, the source of alkali ions in the catholyte compartment 16 may be provided by alkali ions 22 transporting across the alkali ion conductive membrane 12 from the anolyte compartment 14 to the catholyte compartment 16.

The anode 18 may be fabricated of various materials, including those discussed below. In one embodiment, the anode 18 is fabricated of titanium coated with advanced metal oxides. The anode 18 may include a catalyst 30 to facilitate oxidation of hydrogen gas to form hydrogen ions. The cathode 20 may also be fabricated of various materials, including those discussed below. In one embodiment, the cathode 20 is fabricated of nickel/stainless steel.

Under the influence of electric potential 24, electrochemical reactions take place at the anode 18 and cathode 20. Oxidation of hydrogen gas to form hydrogen ions occurs at the anode 18. The hydrogen ions exchange with the alkali salts of glycerine to form glycerine, also known as glycerin and glycerol. Reduction of water to form hydrogen gas 32 and hydroxide ions occurs at the cathode 20. The hydroxide ions react with available alkali ions to form an alkali hydroxide (MOH) solution 33. The alkali hydroxide solution is removed for use in other chemical processes, such as the manufacture of biodiesel. The hydrogen gas 32 produced in the catholyte compartment 16 and any supplemental hydrogen gas necessary for reactions in the anolyte compartment 14 may be combined to provide the hydrogen gas 26 that is introduced into the anolyte compartment 14. Glycerine 36 is removed from the anolyte compartment 14 for further processing or separation.

The chemical reactions in the electrolytic cell 10 are summarized below:

At the anode/anolyte compartment:

6H₂→12e⁻+12H⁺

4C₃H₅(OM)₃+12H⁺→4C₃H₅(OH)₃+12M⁺

At the cathode/catholyte compartment:

12H₂O+12e⁻→12OH⁻+6H₂

12OH⁻+12M⁺→12MOH

Overall reaction of the electrolytic cell 10:

12H₂O+4C₃H₅(OM)₃→12MOH+4C₃H₅(OH)₃

Unreacted or excess hydrogen gas 34 may be collected and removed from the anolyte compartment 14. Hydrogen gas 34 may be recycled and added to the hydrogen gas 26 introduced into the anolyte compartment 14. The hydrogen gas 34 may provide fuel to an alternative energy generating process, such as a PEM fuel cell or other device known to one of ordinary skill in the art for energy generation. This may help offset the energy requirements to operate the electrolytic processes. The hydrogen gas 34 may be used for chemical processing operations known to one of ordinary skill in the art.

FIG. 2 illustrates a general schematic view for another electrolytic apparatus and process for separating alkali metal ions from alkali salts of glycerine within the scope of the present invention. The apparatus and process for separating alkali metal ions includes an electrolytic cell 100.

The electrolytic cell 100 uses an alkali ion conductive membrane 112 that divides the electrochemical cell 100 into two compartments: an anolyte compartment 114 and a catholyte compartment 116. An electrochemically active anode 118 is housed in the anolyte compartment 114 where oxidation reactions take place, and an electrochemically active cathode 120 is housed in the catholyte compartment 116 where reduction reactions take place. The alkali ion conductive membrane 112 selectively transfers alkali ions (M⁺) 122 from the anolyte compartment 114 to the catholyte compartment 116 under the influence of an electrical potential 124 while preventing water transportation from either compartment to the other side. In one embodiment, the membrane 112 may include an ionic-selective polymer membrane stable in the low-pH environment of the anolyte compartment. In another embodiment, the membrane 112 may include a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

The electrolytic cell 100 is operated by feeding the alkali salts of glycerine 125 or similar byproducts from the manufacture of biodiesel into the anolyte compartment 114. Water present in the anolyte compartment oxidizes to form hydrogen ions and oxygen. It will be appreciated that other compounds besides water may be oxidized to form hydrogen ions.

Water 128, or a low concentration alkali salt or alcohol solution, is fed into the catholyte compartment 116. During operation, the source of alkali ions in the catholyte compartment 116 may be provided by alkali ions 122 transporting across the alkali ion conductive membrane 112 from the anolyte compartment 114 to the catholyte compartment 116.

The anode 118 may be fabricated of various materials, including those discussed below. In one embodiment, the anode 118 is fabricated of titanium coated with advanced metal oxides. The cathode 120 may also be fabricated of various materials, including those discussed below. In one embodiment, the cathode 120 is fabricated of nickel/stainless steel.

Under the influence of electric potential 124, electrochemical reactions take place at the anode 118 and cathode 120. Oxidation of water to form hydrogen ions occurs at the anode 118. The hydrogen ions exchange with the alkali salts of glycerine to form glycerine, also known as glycerin and glycerol. Reduction of water to form hydrogen gas 132 and hydroxide ions occurs at the cathode 120. The hydroxide ions react with available alkali ions to form an alkali hydroxide (MOH) solution 133. The alkali hydroxide solution 133 is removed for use in other chemical processes, such as the manufacture of biodiesel. The hydrogen gas 132 produced in the catholyte compartment 116 may be collected and removed from the catholyte compartment 116. The hydrogen gas 132 may provide fuel to an alternative energy generating process, such as a PEM fuel cell or other device known to one of ordinary skill in the art for energy generation. This may help offset the energy requirements to operate the electrolytic processes. The hydrogen gas 132 may be used for chemical processes known to one of ordinary skill in the art. Oxygen gas 134 may be collected from the anolyte compartment 114 and used for chemical processes known to one of ordinary skill in the art. Glycerine 136 is removed from the anolyte compartment 114 for further processing or separation.

The chemical reactions in the electrolytic cell 100 are summarized below:

At the anode/anolyte compartment:

6H₂O→3O₂+12H⁺+12e⁻

4C₃H₅(OM)₃+12H⁺→4C₃H₅(OH)₃+12M⁺

At the cathode/catholyte compartment:

12H₂O+12e⁻→12OH⁻+6H₂

12OH⁻+12M⁺→12MOH

Overall reaction of the electrolytic cell 100:

18H₂O+4C₃H₅(OM)₃→12MOH+4C₃H₅(OH)₃+3O₂+6H₂

The invention includes the process of separating alkali ions from glycerine byproducts of biodiesel production. In this method, the glycerine byproducts, which include alkali salts of glycerine, may be fed into and purified by an electrolytic cell configured similar to the electrolytic cells 10, 100 and illustrated in FIGS. 1 and 2. As discussed above in relation to the electrolytic cells 10 and 100, hydrogen ions are produced in the anolyte compartment which replace the alkali metal ions in the glycerine byproducts resulting in clean glycerine. The free alkali ions are transported from the anolyte compartment across the alkali ion conductive membrane into the catholyte compartment where they combine with hydroxide ions to form an alkali hydroxide solution.

Other oxidizable materials than hydrogen gas and water can be used in the anolyte. For example, chloride ions from alkali metal chloride present in the anolyte can be oxidized to form chlorine gas. Alternatively, ammonium hydroxide can be added to the anolyte that can be oxidized to release protons.

The phrase “substantially impermeable to water,” when used in the instant application to refer to a membrane, means that a small amount of water may pass through the membrane, but that the amount that passes through is not of a quantity to diminish the usefulness of the present invention. The phrase “essentially impermeable to water,” as used herein in reference to a membrane, means that no water passes through the membrane, or that if water passes through the membrane, its passage is so limited so as to be undetectable by conventional means. The words “substantially” and “essentially” are used similarly as intensifiers in other places within this specification.

Electrode materials useful in the methods and apparatus of the present invention are electrical conductors and are generally substantially stable in the media to which they are exposed. Any suitable electrode material, or combination of electrode materials, known to one of ordinary skill in the art may be used within the scope of the present invention. Non-limiting examples of some electrode materials include titanium coated with advanced metal oxides, nickel, Kovar (Ni—Fe—Co), stainless steel, carbon steel, and graphite.

In some specific embodiments, the anode material may include at least one of the following: dimensionally stable anode (DSA, generally comprising ruthenium oxide-coated-titanium or RuO₂/Ti), nickel, cobalt, nickel tungstate, nickel titanate, platinum and other noble metals, noble metals plated on a substrate such as platinum-plated titanium, metal oxides based on titanium, stainless steel, lead, lead dioxides, graphite, tungsten carbide and titanium diboride. In some specific embodiments, the cathode material may include at least one of the following: nickel, cobalt, platinum, silver, alloys such as titanium carbide with small amounts (in some instances up to about 3 weight %) of nickel, FeAl₃, NiAl₃, stainless steel, perovskite ceramics, and graphite. In some embodiments, the electrodes may be chosen to maximize cost effectiveness by balancing the electrical efficiency of the electrodes against their cost.

The electrode material may be in any suitable form within the scope of the present invention, as would be understood by one of ordinary skill in the art. In some specific embodiments, the form of the electrode materials may include at least one of the following: a dense or porous solid-form, a dense or porous layer plated onto a substrate, a perforated form, an expanded form including a mesh, or any combination thereof.

In some embodiments of the present invention, the electrode materials may be composites of electrode materials with non-electrode materials, where non-electrode materials are poor electrical conductors under the conditions of use. A variety of insulative non-electrode materials are also known in the art, as would be understood by one of ordinary skill in the art. In some specific embodiments, the non-electrode materials may include at least one of the following: ceramic materials, polymers, and/or plastics. These non-electrode materials may also be selected to be stable in the media to which they are intended to be exposed.

Other variations, including variations of electrode material, shape, and in some instances, placement could be made within the scope of the invention by one of ordinary skill in the art.

The alkali ion conductive membrane 12, 112 utilized in the processes and apparatus of the present invention are alkali cation-conductive, and physically separate the anolyte solution from the catholyte solution. The membrane 12, 112 preferably includes a chemically stable ionic-selective polymer membrane. Such membranes may be stable in a wide range of pH conditions, and particularly in the low-pH environment of the anolyte compartment. For example, the membrane preferably can tolerate a pH range from about 0.5 to 14. The membrane 12, 112 may include a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

Layered ceramic-polymer composite membranes are also particularly suitable for use as alkali cation-conductive membranes in the present invention, such as, but not limited to, those disclosed in U.S. Pat. No. 5,290,405, which is incorporated herein in its entirety by this reference. Layered ceramic-polymer composite membranes generally comprise ionic-selective polymers layered on alkali cation-conductive ceramic materials. In some specific embodiments, the alkali cation-conductive ceramic materials of the layered ceramic-polymer composite membranes may include at least one of the following: NaSICON-type materials or beta-alumina. Ion-selective polymer materials have the disadvantage of having poor selectively to sodium ions, yet they demonstrate the advantage of high chemical stability. Therefore, layered ceramic-polymer composite membranes of cation-conductive ceramic materials with chemically stable ionic-selective polymer layers may be suitable for use in the present invention. In some specific embodiments, the types of ion-selective polymer materials which may be used in the layered ceramic-polymer composite structure or as a cation conducing polymer membrane may include at least one of the following: polyelectrolyte perfluorinated sulfonic polymers, polyelectrolyte carboxylic acid polymers, Nafion® materials (from DuPont Fluoroproducts, Fayetteville, N.C.) and polyvinyl chloride (PVC), matrix-based polymers, co-polymers or block-copolymers.

In some specific embodiments, the polymers for the layered ceramic-polymer composite membranes may include at least one of the following features and use characteristics, as would be understood by one of ordinary skill in the art: high chemical stability; high ionic conductivity; good adhesion to ceramic materials; and/or insensitivity to impurity contamination.

In one embodiment, the alkali ion conductive membranes conduct lithium ions, sodium ions, or potassium ions. It may be advantageous to employ membranes with low or even negligible electronic conductivity, in order to minimize any galvanic reactions that may occur when an applied potential or current is removed from the cell containing the membrane. In some embodiments of the present invention it may be advantageous to employ membranes that are substantially impermeable to at least the solvent components of both the catholyte and anolyte solutions.

In some specific embodiments, the alkali ion conductive membrane may include at least one of the following features and use characteristics, as would be understood by one of ordinary skill in the art: a solid form; a high alkali ion conductivity at temperatures below about 100° C.; low electronic conductivity; an alkali-ion transfer efficiency (i.e. high transference number) greater than about 90%; a high selectivity for particular alkali cations (e.g. Na⁺) in relation to other alkali or non-alkali cations; stability in solutions of alkali-ion containing salts and chemicals of weak or strong organic or inorganic acids; a density greater than about 95% of theoretical density value; substantially impermeable to water transport; resistant to acid, alkaline, caustic and/or corrosive chemicals.

In some embodiments of the alkali ion conductive membrane of the present invention, the ceramic membrane may not be substantially influenced by scaling, fouling or precipitation of species incorporating divalent cations, trivalent cations, and tetravalent cations; or by dissolved solids present in the solutions.

For those embodiments utilizing an alkali ion conductive ceramic membrane, the alkali ion conductive ceramic materials are preferably configured to selectively transport alkali ions. They may be a specific alkali ion conductor. For example, the alkali ion conductive ceramic membrane may be a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. The alkali ion conductive ceramic membrane may comprise a material having the formula M_1+xM^I₂Si_xP_3−xO₁₂ where 0≦x≦3, where M is selected from the group consisting of Li, Na, K, or mixture thereof, and where M^I is selected from the group consisting of Zr, Ge, Ti, Sn, or Hf, or mixtures thereof; materials of general formula Na_1+xL_zZr_2−zP₃O₁₂ where 0≦z≦2.0, and where L is selected from the group consisting of Cr, Yb, Er, Dy, Sc, Fe, In, or Y, or mixtures thereof; materials of general formula M^II₅RESi₄O₁₂, where M^II may be Li, Na, or any mixture thereof, and where RE is Y or any rare earth element.

In some specific embodiments, the MSICON materials are selective for sodium, referred to herein as NaSICON (Sodium Super Ion CONducting) type materials. The NaSICON materials may include at least one of the following: materials of general formula Na₅RESi₄O₁₂ and non-stoichiometric sodium-deficient materials of general formula (Na₅RESi₄O₁₂)_1−δ(RE₂O₃.2SiO₂)_δ, where RE is Nd, Dy, or Sm, or any mixture thereof and where δ is the measure of deviation from stoichiometry, as disclosed in U.S. Pat. No. 5,580,430, and as explicitly incorporated herein by this reference in its entirety. In some specific embodiments, the NaSICON-type materials may include at least one of the following: non-stoichiometric materials, zirconium-deficient (or sodium rich) materials of general formula Na_1+xZr_2−x/3Si_xP_3−xO_12−2x/3 where 1.55≦x≦3. In some specific embodiments, the NaSICON-type materials may include at least one of the following: non-stoichiometric materials, sodium-deficient materials of general formula Na_1+x(A_yZr_2−y)(Si_zP_3−z)O_12−δ where A is selected from the group consisting of Yb, Er, Dy, Sc, In, or Y, or mixtures thereof, 1.8≦x≦2.6, 0≦y≦0.2, x<z, and 6 is selected to maintain charge neutrality. In some specific embodiments, the NaSICON-type materials may include sodium-deficient materials of formula Na_3.1Zr₂Si_2.3P_0.7O_12−δ.

Other exemplary NaSICON-type materials are described by H. Y—P. Hong in “Crystal structures and crystal chemistry in the system Na_1+xZr₂Si_xP_3−xO₁₂”, Materials Research Bulletin, Vol. 11, pp. 173-182, 1976; J. B. Goodenough et al., in “Fast Na⁺-ion transport skeleton structures”, Materials Research Bulletin, Vol. 11, pp. 203-220, 1976; J. J. Bentzen et al., in “The preparation and characterization of dense, highly conductive Na₅GdSi₄O₁₂ nasicon (NGS)”, Materials Research Bulletin, Vol. 15, pp. 1737-1745, 1980; C. Delmas et al., in “Crystal chemistry of the Na_1+xZr_2−xL_x (PO₄)₃ (L=Cr, In, Yb) solid solutions”, Materials Research Bulletin, Vol. 16, pp. 285-290, 1981; V. von Alpen et al., in “Compositional dependence of the electrochemical and structural parameters in the NASICON system (Na_1+xSi_xZr₂P_3−xO₁₂)”, Solid State Ionics, Vol. 3/4, pp. 215-218, 1981; S. Fujitsu et al., in “Conduction paths in sintered ionic conductive material Na_1+xY_xZr_2−x(PO₄)₃”, Materials Research Bulletin, Vol. 16, pp. 1299-1309, 1981; Y. Saito et al., in “Ionic conductivity of NASICON-type conductors Na_1.5M_0.5Zr_1.5(PO₄)₃ (M: Al³⁺, Ga³⁺, Cr³⁺, Sc³⁺, Fe³⁺, In³⁺, Yb³⁺, Y³⁺)”, Solid State Ionics, Vol. 58, pp. 327-331, 1992; J. Alamo in “Chemistry and properties of solids with the [NZP] skeleton”, Solid State Ionics, Vol. 63-65, pp. 547-561, 1993; K. Shimazu in “Electrical conductivity and Ti⁴⁺ ion substitution range in NASICON system”, Solid State Ionics, Vol. 79, pp. 106-110, 1995; Y. Miyajima in “Ionic conductivity of NASICON-type Na_1+xM_xZr_2−xP₃O₁₂ (M: Yb, Er, Dy)”, Solid State Ionics, Vol. 84, pp. 61-64, 1996. These references are incorporated in their entirety herein by this reference.

The alkali ion conductive ceramic membranes may be used or produced for use in the processes and apparatus of the present invention in any suitable form, as would be understood by one of ordinary skill in the art. In some specific embodiments, the form of the ceramic membranes may include at least one of the following: monolithic flat plate geometries, supported structures in flat plate geometries, monolithic tubular geometries, supported structures in tubular geometries, monolithic honeycomb geometries, or supported structures in honeycomb geometries. Supported structures may comprise dense layers of alkali cation-conductive ceramic materials supported on porous supports.

A variety of forms for the porous supports are known in the art and would be suitable for providing the porous supports for ceramic membranes with supported structures, including: ceramic layers sintered to below full density with resultant continuous open porosity, slotted-form layers, perforated-form layers, expanded-form layers including a mesh, or combinations thereof. In some embodiments, the porosity of the porous supports is substantially continuous open-porosity so that the liquid solutions on either side of the ceramic membrane may be in intimate contact with a large area of the dense-layers of alkali cation-conductive ceramic materials, and in some, the continuous open-porosity ranges from about 30 volume % to about 90 volume %. In some embodiments of the present invention, the porous supports for the supported structures may be present on one side of the dense layer of alkali cation-conductive ceramic material. In some embodiments of the present invention, the porous supports for the supported structures may be present on both sides of the dense layer of alkali cation-conductive ceramic material.

In some specific embodiments, the ceramic membrane may comprise two or more co-joined layers of different alkali ion conductive ceramic materials. Such co-joined ceramic membrane layers could include solid NaSICON materials joined to other ceramics, such as, but not limited to, beta-alumina. Such layers could be joined to each other using a method such as, but not limited to, co-firing, joining following sintering, etc. Other suitable joining methods are known by one of ordinary skill in the art and are included herein.

A variety of materials for the porous supports are known in the art and would be suitable for providing the porous supports for ceramic membranes with supported-structures, including: electrode materials, NASICON-type materials, β^I-alumina, β^II-alumina, other cation-conductive materials, and non-conductive materials such as plastics or ceramic materials, metals, and metal alloys. The thickness of the dense layer of alkali cation-conductive ceramic material in monolithic structures is generally from about 0.3 mm to about 5 mm, and in some instances from about 0.5 mm to about 1.5 mm. The thickness of the dense layer of alkali cation-conductive ceramic material in supported-structures is generally from about 25 μm to about 2 mm, and often from about 0.5 mm to about 1.5 mm. Layers as thin as about 25 μm to about 0.5 mm are readily producible, as would be understood by one of ordinary skill in the art. In some specific embodiments, the ceramic membranes are structurally supported by the cathode and the anode, each of which is porous. This may dictate characteristics of both the form of the membranes, and/or of the cathode and/or anode. In some specific embodiments, the porous substrate must have similar thermal expansion and good bonding with the membrane as well as good mechanical strength. One of ordinary skill in the art would understand that the number and configuration of the layers used to construct the ceramic membrane as supported-structures could be widely varied within the scope of the invention.

The alkali ion conductive ceramic membrane materials may be prepared and processed according to known processes and techniques, including those disclosed in U.S. patent application Ser. No. 11/622,314, which are incorporated herein.

In one embodiment of the processes and apparatus of the present invention, the electrolytic cell may be operated in a continuous mode. In a continuous mode, the cell is initially filled with anolyte and catholyte solutions and then, during operation, additional solutions are fed into the cell and products, by-products, and/or diluted solutions are removed from the cell without ceasing operation of the cell. The feeding of the reactants anolyte and catholyte solutions may be done continuously or it may be done intermittently, meaning that the flow of a given solution is initiated or stopped according to the need for the solution and/or to maintain desired concentrations of solutions in the cell, without emptying one or both compartments. Similarly, the removal of solutions from the anolyte compartment and the catholyte compartment may also be continuous or intermittent.

Control of the addition and/or removal of solutions from the cell may be done by any suitable means. Such means include manual operation, such as by one or more human operators, and automated operation, such as by using sensors, electronic valves, laboratory robots, etc. operating under computer or analog control. In automated operation, a valve or stopcock may be opened or closed according to a signal received from a computer or electronic controller on the basis of a timer, the output of a sensor, or other means. Examples of automated systems are well known in the art. Some combination of manual and automated operation may also be used. Alternatively, the amount of each solution that is to be added or removed per unit time to maintain a steady state may be experimentally determined for a given cell, and the flow of solutions into and out of the system set accordingly to achieve the steady state flow conditions.

In another embodiment, the system is operated in batch mode. In batch mode, the anolyte and catholyte solutions are fed into the cell and the cell is operated until the desired concentration of product is produced at the anolyte and catholyte. The cell is then emptied, the product collected, and the cell refilled to start the process again. Alternatively, combinations of continuous mode and batch mode production may be used. Also, in either mode, the feeding of solutions may be done using a pre-prepared solution or using components that form the solution in situ.

It should be noted that both continuous and batch mode have dynamic flow of solutions. In continuous mode, the anolyte make up solution is added so the alkali ion concentration is maintained at a certain concentration or concentration range. In a batch mode, a certain quantity of alkali metal salt is used and alkali ion loss in the anolyte due to its transfer through the membrane to the catholyte in not replenished. The operation is stopped when the alkali ion concentration in the anolyte reduces to a certain amount or when the appropriate product concentration is reached in the catholyte.

Several examples are provided below which discuss specific embodiments within the scope of the invention. These embodiments are exemplary in nature and should not be construed to limit the scope of the invention in any way.

EXAMPLE 1

A two compartment cell with a cation conduction polymer membrane (Nafion® 324) was evaluated to separate the sodium from glycerine feed. Schematic of the two compartment electrolytic cell used for testing is shown in FIG. 3. The electrolytic cell 200 uses an alkali ion conductive polymeric membrane 212 (Nafion® 324) that divides the electrochemical cell 200 into two compartments: an anolyte compartment 214 and a catholyte compartment 216. An electrochemically active anode 218 is housed in the anolyte compartment 214 where oxidation reactions take place, and an electrochemically active cathode 220 is housed in the catholyte compartment 216 where reduction reactions take place. The alkali ion conductive membrane 212 selectively transfers sodium ions (Na⁺) from the anolyte compartment 214 to the catholyte compartment 216 under the influence of an electrical potential. Gasket 222 may be used to seal the connection between the anolyte compartment and the membrane 212. Gasket 224 may be used to seal the connection between the catholyte compartment and the membrane 212.

The test was conducted at a temperature of 40° C. The anolyte feed to the anode compartment was glycerine with 15,000 ppm Na⁺ and the catholyte feed to the cathode compartment was 15 wt % NaOH. The cell was operated at 0.75 amps of current. The anolyte volume was 1500 ml and the catholyte volume was 1500 ml

FIG. 4 presents the cell operation for a period of over 50 hours and the voltage response as a function of operation time. The anolyte and catholyte samples before and after testing were analyzed by ICP method to complete sodium mass balance analysis. The analysis shows that 50.67% of the Na⁺ was removed from the anolyte and transferred to the catholyte across the polymer cation conductive membrane.

EXAMPLE 2

A two compartment cell with a cation conduction polymer membrane (Nafion® 324) was evaluated to separate the sodium from glycerine feed. Schematic of the two compartment cell used for testing is show in FIG. 3. The test was conducted at a temperature of 40° C. The anolyte feed to the anode compartment was glycerine with 27,000 ppm Na⁺ and the catholyte feed to the cathode compartment was 15 wt % NaOH. The cell was operated at 0.75 amps of current. The anolyte volume was 1500 ml and the catholyte volume was 1500 ml. Table 1 below presents the performance data.

FIG. 5 presents the cell operation for a period of over 60 hours and the voltage response as a function of operation time. The anolyte and catholyte samples before and after testing were analyzed by ICP method to complete sodium mass balance analysis. The analysis shows that 48.15% of Na⁺ was removed from the anolyte and transferred to the catholyte across the polymer cation conductive membranes. Table 1 below presents the performance data.

TABLE 1
Tests performance data
Exam-	Glycerine	Starting	Current	Initial Sodium	% Sodium
ple	Type	Voltage	Density	Concentration	Removed
1	SWF1080-	15.22	50 mA/cm2	15,000 mg/L	50.67%
	47
2	SWF1412-	7.88	50 mA/cm2	27,000 mg/L	48.15%
	90

EXAMPLE 3

A solution of 0.5 wt. % NaOH in glycerol was made and heated to 90° C. in the anolyte. The electrodes were placed at a distance from the ceramic NaSICON membrane in the anolyte and catholyte solutions at a set distance apart. The separating distance was 1.76 mm between electrodes. The anode was DSA and the cathode was Ni. FIG. 6 is a plot which presents the sodium transfer current-versus voltage to drive sodium across the two compartment cell to thereby separate sodium from sodium salt of glycerol. FIG. 7 is a plot which shows conductivity at different voltages. It also shows current at different voltages, similar to FIG. 6.

During the test, the glycerol started turning yellow at the 8 voltage region (FIG. 7). This suggests limiting the operating voltage below 7 volts and its associated current. This information was used to calculate the active membrane area needed to recycle 10 mM lbs/yr of 8 wt. % glycerol. The leveling of the conductivity graph (FIG. 7) may be due to the glycerol polymerizing.

EXAMPLE 4

A two compartment electrolytic cell equipped with a NaSICON ceramic was operated with 5 wt. % sodium hydroxide in glycerine anolyte feed and low concentration sodium hydroxide in the catholyte. The cell was operated at 80° C. for approximately 17 hours. Titration analysis of the anolyte and catholyte mass balance demonstrates removal of sodium from the anolyte stream. The sodium removal from glycerine is illustrated in FIG. 8. From the results of FIG. 8, over 50% sodium removal from glycerine is easily achieved.

EXAMPLE 5

Salt Removal Cell Test with Porous Alumina Yttrium Doped Zirconium Oxide Separator

An electrolytic cell was equipped with a two inch porous alumina membrane in a two compartment device. The anolyte feed was glycerine based sodium salt solution in methanol solvent, the catholyte feed was 9.62 wt % concentration NaOH. The cell was operated at constant voltage of 20 V (FIG. 9) and room temperature. The catholyte current efficiency measured was 97.68%. The sodium concentration of catholyte solution went from an initial 9.62 wt % to 10.58 wt % during the duration of the test providing evidence of sodium removal from the anolyte glycerine based sodium salt solution. The moisture in the anolyte feed stream after 15-hour test duration time was measured by Karl Fisher titration method is show in FIG. 10.

EXAMPLE 6

An electrolytic cell was equipped with a two inch polymeric Nafion membrane in a two compartment device. The anolyte feed was glycerine based sodium salt solution, the catholyte feed was IM concentration NaOH. The cell was operated at constant current of 50 mA per sq.cm. (FIG. 11) and at 40° C. Ammonium hydroxide was added to the anolyte to maintain the pH of the solution between 7 and 10. Ammonium hydroxide is an alternative to water and hydrogen as the source of protons. The sodium removal current efficiency measured was 61.6%. The sodium concentration of catholyte solution increased from an initial 1M to 2.15M during the duration of the test providing evidence of additional amount of sodium removal from the glycerine based sodium salt anolyte solution. Ammonium hydroxide allowed the operation of the cell at high current density allowing for additional amount of sodium removal from glycerine based anolyte stream,

While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.

Claims

1. A method for converting alkali salts of glycerine into glycerine comprising:

obtaining an electrolytic cell comprising an alkali ion conductive membrane configured to selectively transport alkali ions, the membrane separating an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode;

introducing a solution containing an alkali salt of glycerine (C3H5(OM)3) into the anolyte compartment, wherein M is an alkali metal;

applying an electric current to the electrolytic cell thereby: i. producing hydrogen ions at the anode in the anolyte compartment to facilitate the following reaction: C3H5(OM)3+3H+→C3H5(OH)3+3M+; ii. causing alkali ions (M+) to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment; and iii. decomposing water in the presence of alkali ions in the catholyte compartment according to the following reaction: M++H2O+e−→MOH+½H2; and

removing glycerine (C3H5(OH)3) from the anolyte compartment.

2. A method for converting alkali salts of glycerine into glycerine according to claim 1, further comprising the steps of:

introducing hydrogen gas into the anolyte compartment; and

introducing water into the catholyte compartment.

3. A method for converting alkali salts of glycerine into glycerine according to claim 2, wherein the anode comprises a catalyst to facilitate oxidation of hydrogen gas into hydrogen ions.

4. A method for converting alkali salts of glycerine into glycerine according to claim 1, wherein hydrogen ions are produced at the anode in the anolyte compartment by oxidation of water.

5. A method for converting alkali salts of glycerine into glycerine according to claim 1, wherein hydrogen ions are produced at the anode in the anolyte compartment by oxidation of hydrogen.

6. A method for converting alkali salts of glycerine into glycerine according to claim 1, wherein the alkali ion conductive membrane comprises a chemically stable ionic-selective polymer membrane.

7. A method for converting alkali salts of glycerine into glycerine according to claim 6, wherein the chemically stable ionic-selective polymer membrane is selected from polyelectrolyte perfluorinated sulfonic polymers, polyelectrolyte carboxylic acid polymers, Nafion® materials, and polyvinyl chloride (PVC) matrix-based polymers, co-polymers or block-copolymers.

8. A method for converting alkali salts of glycerine into glycerine according to claim 1, wherein the alkali ion conductive membrane comprises a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

9. A method for converting alkali salts of glycerine into glycerine according to claim 8, wherein the chemically stable ionic-selective polymer membrane is selected from polyelectrolyte perfluorinated sulfonic polymers, polyelectrolyte carboxylic acid polymers, Nafion® materials, and polyvinyl chloride (PVC) matrix-based polymers, co-polymers or block-copolymers.

10. A method for converting alkali salts of glycerine into glycerine according to claim 8, wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li.

11. An apparatus for converting alkali salts of glycerine into glycerine comprising: an electrolytic cell comprising:

an anolyte compartment comprising an electrochemically active anode having a source of oxidizable hydrogen and C3H5(OM)3 in which oxidizable hydrogen is oxidized to form hydrogen ions to thereby facilitate the following reaction: C3H5(OM)3+3H+→C3H5(OH)3+3M+;

a catholyte compartment comprising an electrochemically active cathode separated from the anolyte compartment by a alkali ion conductive membrane configured to selectively transport alkali ions (M+) into the catholyte compartment, wherein the catholyte compartment has a source of water, in which the water is decomposed in the presence of alkali ions to form alkali hydroxide and hydrogen gas;

a fluid path for removing alkali hydroxide produced in the catholyte compartment; and

a fluid path for removing glycerine (C3H5(OH)3) from the anolyte compartment.

12. An apparatus for converting alkali salts of glycerine into glycerine according to claim 11, wherein the source of oxidizable hydrogen is hydrogen gas.

13. An apparatus for converting alkali salts of glycerine into glycerine according to claim 12, further comprising a fluid path for transporting hydrogen gas produced in the catholyte compartment to the anolyte compartment.

14. An apparatus for producing biodiesel according to claim 12, wherein the anode further comprising a catalyst to facilitate oxidation of hydrogen gas into hydrogen ions.

15. An apparatus for converting alkali salts of glycerine into glycerine according to claim 11, wherein the source of oxidizable hydrogen is water.

16. An apparatus for converting alkali salts of glycerine into glycerine according to claim 11, wherein the alkali ion conductive membrane comprises a chemically stable ionic-selective polymer membrane.

17. An apparatus for converting alkali salts of glycerine into glycerine according to claim 16, wherein the chemically stable ionic-selective polymer membrane is selected from polyelectrolyte perfluorinated sulfonic polymers, polyelectrolyte carboxylic acid polymers, Nafion® materials, and polyvinyl chloride (PVC) matrix-based polymers, co-polymers or block-copolymers.

18. An apparatus for converting alkali salts of glycerine into glycerine according to claim 11, wherein the alkali ion conductive membrane comprises a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane.

19. An apparatus for converting alkali salts of glycerine into glycerine according to claim 18, wherein the chemically stable ionic-selective polymer membrane is selected from polyelectrolyte perfluorinated sulfonic polymers, polyelectrolyte carboxylic acid polymers, Nafion® materials, and polyvinyl chloride (PVC) matrix-based polymers, co-polymers or block-copolymers.

20. An apparatus for converting alkali salts of glycerine into glycerine according to claim 18, wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li.