Process and apparatus for removing chloride and sodium ions from an aqueous sodium chloride solution

Abstract

The present invention discloses a process and apparatus for removing sodium and chloride ions from an aqueous sodium chloride solution, such as seawater or brine. The process includes electrolyzing aqueous sodium chloride to remove chloride and sodium ions in the form of chlorine gas and sodium metal. Preferably, a photovoltaic device, such as a triple junction amorphous silicon solar cell, provides the electrical energy for the electrolysis. The process utilizes electrode material that facilitates the production of chlorine gas and inhibits the evolution of hydrogen from the aqueous sodium chloride solution. The sodium is deposited onto a metal surface having a high hydrogen overpotential to produce sodium amalgam. The processed solution from the electrolysis has a reduced sodium chloride content and may be further processed to produce fresh water for human consumption or agricultural purposes. The sodium amalgam is removed from the aqueous sodium chloride solution and transported to and coupled against an air depolarizing fuel cell in water to produce electrical power with the sodium air fuel cell, power that may be used to operate the apparatus or other machinery. The product of the reaction between the sodium amalgam and the fuel cell is sodium hydroxide that may be reacted with the chlorine gas to produce sodium hypochlorite.

Description

FIELD OF THE INVENTION

The present invention relates to a process and apparatus for reducing sodium chloride concentration from an aqueous sodium chloride solution, such as sea water or brine, by removing chloride and sodium ions. More particularly, the present invention discloses a process and apparatus, wherein chloride and sodium ions are removed from the aqueous sodium chloride solution and may be used in subsequent processes. The sodium may be collected and used in the production of power from a sodium-air fuel cell and chlorine gas may be neutralized with the sodium hydroxide produced in the sodium —air fuel cell to produce sodium hypochlorite.

BACKGROUND OF THE INVENTION

Water is a precious and life sustaining liquid. Over 97% of the world water supplies are tied up in the ocean. The remaining 3% of the water exists as fresh water supply in natural and man-made lakes and the polar ice caps. With the looming environmental catastrophe brought upon by the global warming there is already a shortage of fresh water supply in the world. Many of the developing countries are suffering from the failure of the monsoon and consequently the supply of fresh water in reservoirs. The global warming is causing the ice caps to recede thereby melting the fresh water supplies at the polar ice caps into the water that runs into the oceans. While fossil fuels themselves are dwindling and wars have been waged to control the remaining residues it is possible to foresee a day when wars will be fought for control of fresh water supplies. The danger is not in any future date but can happen within our generation.

The general approaches for addressing water shortages can be categorized in terms of three basic strategies: conservation, identifying new sources of freshwater and converting brackish water to freshwater. The goal of water conservation is to promote responsible usage of water. Water conservationists seek to obtain the maximum benefit from existing freshwater sources through the encouragement of responsible usage practices. Although water conservation is clearly beneficial, it is unlikely that it alone will meet the growing water needs of the world.

The identification of new sources of freshwater involves searching for new aquifers, typically to supply the growing needs of a particular region as it develops or expands. New technologies permit geologists to search deeper into the earth for water and to search in regions that were previously inaccessible. Other strategies for obtaining freshwater from new sources include extracting water from icebergs or the polar icecap. More controversially, growing regions with few local water resources are seeking a redistribution of the available resources through water transportation. The southwestern states, for example, have inquired into the possibility of shipping water from the Great Lakes to meet their water needs. Although the search for new freshwater resources will continue and new sources will be identified, it is clear that freshwater is a finite resource that is politically contentious and that the most accessible sources have already been located and tapped and are being consumed. Future sources will become increasingly expensive to both locate and utilize.

A third strategy, water conversion, seeks to produce freshwater from brackish water in an attempt to exploit the vast potential of the world's oceans. Water conversion is most commonly practiced today in arid regions, such as the Middle East, where little or no freshwater resources are naturally available. Conversion of brackish water to freshwater involves a desalination process in which freshwater or purified water is separated or extracted from brackish water. There are a variety of methods available already to convert brackish water into potable water. Evaporation and condensation, reverse osmosis, and electrodialysis are some of the very common techniques.

A common desalination process in use today is based on reverse osmosis, a separation method that uses pressure to induce diffusion of pure water from brackish water through a semi-permeable membrane to provide pure water on one side of the membrane and concentrating salts and impurities on the other side of the membrane. Although reverse osmosis is an effective desalination technique, it is slow, expensive and requires high energy consumption. Related techniques, such as electrodialysis, show improved energy efficiency, but nonetheless remain costly, especially for high volume throughputs. Reverse osmosis and electrodialysis work very well with salt content under 60,000 ppm. Anything more than that, such as sea water that may have salt content of 350,000 ppm, would require higher pressures (close to 1000 psi) and the current membranes, in addition to being expensive cannot sustain such high pressures.

Evaporation/distillation is the preferred method. It is somewhat expensive but work is being performed to continuously decrease the cost by efficient designs.

The current invention overcomes deficiencies in the prior art and provides a novel process and apparatus for the reduction of salt content from aqueous sodium chloride, such as brine, which incorporates a system that electrolytically dissociates NaCl into chlorine gas and sodium metal. Preferably, the system is powered by solar energy. Further, the sodium may be collected and used to fuel a sodium-air fuel cell that produces power and produces sodium hydroxide. The sodium hydroxide may then be reacted with the chlorine gas to produce sodium hypochlorite, which is an industrial bleach.

SUMMARY OF THE INVENTION

The present invention discloses a process for removing salt (NaCl) from an aqueous sodium chloride solution, such as seawater or brine. The process includes electrolyzing an aqueous sodium chloride solution to remove chloride and sodium ions in the form of chlorine gas and sodium metal. Typical electrolysis of a sodium chloride solution will produce chlorine at the anode and hydrogen at the cathode. The process of the present invention on the other hand, utilizes electrode materials that facilitate the production of chlorine gas and inhibit the evolution of hydrogen from the aqueous sodium chloride solution. The sodium cations are deposited onto a metal surface having a high hydrogen overpotential. Since the hydrogen overpotential is high at this surface, the cell voltage has to be raised sufficiently high to produce any hydrogen. However at these potentials sodium cations deposit to produce sodium metal. For example, if the chosen substrate for cathode is mercury, the sodium deposits on mercury forming sodium amalgam. The sodium metal is transported to and coupled against an air depolarizing cathode to form a metal-air fuel cell to produce power. Initially the cell is started with some low concentration of sodium hydroxide. However the product of the reaction between the sodium amalgam and the air electrode of the fuel cell being sodium hydroxide, it is possible to react the sodium hydroxide with the chlorine gas to produce sodium hypochlorite.

An embodiment of the present invention discloses a process for lowering the sodium chloride concentration in an aqueous sodium chloride solution, as well as, making sodium hypochlorite and producing electrical power. The process includes electrolyzing a source of aqueous sodium chloride solution, such as brine, in a first container. The electrolysis splits sodium chloride in the aqueous sodium chloride solution, which produces chlorine gas, sodium metal and processed solution. The sodium cations are collected on a surface, such as a metal surface coated with a material having a high hydrogen overpotential, and removed from the first container. The aqueous sodium chloride solution having depleted NaCl content is then collected and may be utilized as is or further processed to remove contaminants. The amalgamated surface may be coupled to an air depolarizing electrode to form a metal-air fuel cell in a second container containing dilute sodium hydroxide to anodically dissolve the sodium metal from the amalgam in the water thereby extracting electrons and producing more concentrated sodium hydroxide and electrical power. The electrons from the anodic dissolution of sodium metal flow through the external circuit to the cathode of the fuel cell, where oxygen reduction occurs. The sodium hydroxide from the second container is flowed to a third container where the chlorine gas from the first electrolysis cell is bubbled thereby producing sodium hypochlorite.

The electrolysis above may comprise applying an electric potential across an anode and a cathode, wherein the anode is in contact with the aqueous sodium chloride solution. Chlorine evolution will take place at the anode. Since chlorine is a reactive gas, graphite, or lead dioxide coated graphite or platinum or DSA type of electrode substrates are preferable. Preferably, the electrodes are RuO₂ coated titanium electrodes. Chlorine is evolved at the anode. The sodium cations are deposited at the mercury-coated surface, and forms sodium amalgam at the surface. Preferably, the process includes restricting the flow of dissolved chlorine gas to the cathode. In a preferred embodiment, a membrane in disposed in the first container between the anode and the cathode. The membrane restricts the flow of dissovled chlorine to the cathode, i.e. amalgamated surface, while permitting sodium ions to permeate through. In the absence of the membrane, the dissolved chlorine will react with the mercury surface producing passivating film of mercurous chloride.

In a preferred embodiment, the electric potential for the process is supplied by a photovoltaic device in electrical communication with the anode and the cathode. The photovoltaic device may comprise at least one triple junction solar cell, which may be at least one triple junction amorphous silicon solar cell.

In a preferred embodiment, the process incorporates a stainless steel belt coated with mercury acting as the cathode surface. Mercury coating of stainless steel may be performed using traditional electroplating process using appropriate pre-treatment steps. The stainless steel belt may be adapted to move from the first container into the second container where the belt is coupled to the fuel cell. Preferably, a recirculating stainless steel belt is adapted to continually flow in a repeating route between the first container and the second container.

An embodiment of the disclosed process produces processed solution having a sodium chloride concentration lower than the sodium chloride concentration of the source of aqueous sodium chloride solution by splitting and removing chloride anions and sodium cations. The processed solution may be discharged from the first container and further processed to produce fresh water for human consumption or agricultural use.

An embodiment of the present invention discloses an apparatus lowering the sodium chloride concentration in an aqueous sodium chloride solution, as well as, making sodium hypochlorite and producing electrical power. The apparatus of the present invention uses a source of electrical energy for electrolyzing a source of aqueous sodium chloride solution, such as brine, contained in a first container, an electrolyzer in electrical communication with the source of electrical energy splits sodium chloride in the aqueous sodium chloride solution resulting in chlorine gas, and sodium amalgam and processed solution. A sodium collector collects sodium on a mercury coated surface and transports the sodium from the first container to a second container containing dilute sodium hydroxide and an air depolarizing fuel cell electrode. The aqueous sodium chloride solution having the NaCl removed is then collected from the first container and may be utilized as is or further processed to remove other contaminants. The fuel cell electrode is coupled to the amalgamated surface in said second container, which dissolves the sodium amalgam in dilute sodium hydroxide to produce concentrated sodium hydroxide and electrical power. This power may be utilized to subsidize the overall power requirement of the electrolysis system, such as operation of the belt or the electrolysis itself. The sodium dissolves in the water producing electrons through the external circuit to the cathode of the fuel cell, where oxygen reduction occurs. The sodium hydroxide from the second container is allowed to flow to a third container where the sodium hydroxide reacts with the chlorine gas producing sodium hypochlorite. A first conduit is used to flow the sodium hydroxide from the second container to a third container. The sodium hydroxide from the second container is flowed to a third container where the sodium hydroxide reacts with the chlorine gas producing sodium hypochlorite.

In a preferred embodiment, the source of electrical energy is a photovoltaic device for converting solar energy to electricity. The photovoltaic device may comprise at least one triple junction solar cell, preferably at least one triple junction amorphous silicon solar cell.

In a preferred embodiment, the electrolyzer comprises an anode in electrical communication with the source of electrical energy and in contact with the source of aqueous sodium chloride solution and a cathode in electrical communication with the source of electrical energy. The source of electrical energy generates an electric potential between the electrodes and the said anode liberates chlorine ions from the aqueous sodium chloride solution and the chlorine ions combine to produce the chlorine gas. The cathode, i.e. amalgamated surface, contacts and collects the sodium cations. In a preferred embodiment, a membrane is disposed in the first container between the anode and the cathode, wherein the membrane inhibits the flow of dissolved chlorine to the cathode.

In a preferred embodiment, the sodium collector is a stainless steel belt coated with a material having a high hydrogen overpotential, such as mercury, tin or lead. The stainless steel belt may comprise a recirculating stainless steel belt adapted to flow from the first container to the second container, wherein the recirculating stainless steel belt collects the sodium metal in the first container, flows to the second container and contacts the dilute sodium hydroxide in the second container to release the sodium as sodium cations and releasing electrons. The electrons flow through the external circuit to the cathode and reduce oxygen from the air to produce hydroxyl ions. The stainless steel belt may comprise a recirculating stainless steel belt adapted to continually flow in a repeating manner between the first container and the second container.

The disclosed apparatus produces processed solution having a sodium chloride concentration lower than the sodium chloride concentration of the source of aqueous sodium chloride solution by splitting and removing chloride ions and sodium cations. The processed solution containing depleted sodium chloride content may be discharged from the first container through an outlet and collected and further processed to produce fresh water for human consumption or agricultural use.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution to reduce sodium chloride content, fueling a sodium-air fuel cell with sodium liberated from the aqueous sodium chloride solution and producing sodium hypochlorite.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution to remove chlorine and sodium ions from the aqueous sodium chloride solution to reduce the salt (NaCl) content of the aqueous sodium chloride solution.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution to produce chlorine gas and sodium metal, substituting the evolution of hydrogen gas at the cathode surface.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is deposited on a moving stainless steel belt, which has been coated with a material having high hydrogen overpotential, such as mercury.

An embodiment of the present invention provides a process and apparatus for electrolyzing, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is deposited on a moving stainless steel belt, wherein the stainless steel acts as a cathode.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is deposited on a moving stainless steel belt, wherein the moving stainless steel belt removes sodium from the cathode vicinity that exposes fresh surface for more sodium to be deposited.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is deposited on a moving stainless steel belt to produce sodium amalgam, wherein the sodium amalgam is coupled against an air depolarizing fuel cell electrode.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is used to produce a sodium-air fuel cell which produces power, wherein the power may be used to move the belt or do other useful work such as processing the processed solution further to produce potable water.

An embodiment of the present invention provides a process and apparatus for electrolyzing aqueous sodium chloride solution to produce sodium hypochlorite, wherein sodium liberated from the electrolysis of the aqueous sodium chloride solution is used to fuel a sodium-air fuel cell that reacts to produce sodium hydroxide and wherein chlorine gas liberated from the electrolysis of the aqueous sodium chloride solution reacts with the sodium hydroxide from the sodium-air fuel cell to produce the sodium hypochlorite.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to assist in the understanding of the various aspects of the present invention and various embodiments thereof, reference is now be made to the appended drawings, in which like reference numerals refer to like elements. The drawings are exemplary only, and should not be construed as limiting the invention.

FIG. 1 is a fragmentary, cross-sectional view of a tandem or cascade photovoltaic device comprising a plurality of p-i-n-type cells, each layer of the cells formed from an amorphous silicon alloy material;

FIG. 2 is an illustration of desalination apparatus for use with an embodiment of the process of the present invention;

FIG. 3 is a block diagram of an embodiment of the process of the present invention, wherein chlorine and sodium ions are removed from aqueous sodium chloride solution to produce processed solution having a sodium chloride concentration less than the aqueous sodium chloride solution;

FIG. 4 is a block diagram of an embodiment of the process of the present invention, wherein the chlorine removed from aqueous sodium chloride solution is used to produce sodium hypochlorite and power; and

FIG. 5 is an illustration of the splitting of the NaCl in the aqueous sodium chloride solution, wherein a membrane inhibits the chlorine ions from migrating to the cathode, wherein sodium metal is collected by a cathodic belt.

DETAILED DESCRIPTION OF THE INVENTION

An illustration of an apparatus, generally referred to as 100, for the production of sodium hypochlorite (NaOCl) power and aqueous hydrogen chloride having a lower concentration of hydrogen chloride than the original source is presented in FIG. 2. A preferred embodiment of the instant invention utilizes a renewable source of solar energy, sunlight, to effect the formation of sodium hypochlorite (NaOCl), power and aqueous hydrogen chloride having a lower concentration of hydrogen chloride than the original source. Although sunlight is the preferred source of energy, the electric potential may be supplied by any source of power, such as a generator powered by coal, natural gas or other fossil fuels. Sunlight is collected by a photovoltaic device 102 and converted to electrical energy that is used to produce an electrical potential across electrodes 104 and 105, which induces the electrolysis of sodium chloride (NaCl). The aqueous sodium chloride solution may be brine. The brine 101 may be a natural source of brine, such as seawater, or the brine may be any NaCl containing water. The brine may be processed at the natural source or the brine may be collected and transported to a facility designed for the processing of the brine. The brine may also include chemically or physically pre-treated forms of brine. The photovoltaic device 102 may be placed into, on the surface of or adjacent to the source of aqueous sodium chloride solution. The photovoltaic device 102 includes one or more materials that are capable of absorbing solar energy and converting it to electrical energy. The electrical energy produced by the photovoltaic device 102 is provided as an electrical potential to electrodes 104 and 105 that are in contact with the aqueous sodium chloride solution 101. Preferably, the electrodes 104 and 105 are RuO₂ coated titanium electrodes. The potential between the electrodes 104 and 105 of the photovoltaic device 102 is sufficient to drive electrolysis of NaCl in the brine that liberates chlorine and sodium ions from the aqueous sodium chloride solution.

The aqueous sodium chloride solution may enter the first container 112 by any acceptable means. For example, the aqueous sodium chloride solution may be pumped through an inlet 108 or poured into the first container 112. The first container 112 may be positioned with respect to the aqueous sodium chloride solution to allow gravity to flow the aqueous sodium chloride solution into the first container. The aqueous sodium chloride solution may enter the first container through an inlet 108 and may exit the first container through an outlet 109. Preferably, the inlet 108 provides a means of access to the first container 112 for the aqueous sodium chloride solution entering the system to be processed and the outlet 109 provides a means of discharging processed solution to exit the system. The aqueous sodium chloride solution may enter the first container 112 continually or intermittently. To control the desired flow of aqueous sodium chloride solution into the first container, the inlet 108 may be equipped with a valve 114. Similarly, to control the flow of processed solution out of the first container 112, the outlet 109 may be equipped with a valve 115.

Preferably, a membrane 111 is set between the anode 104 and the cathode 105. Preferably, the membrane 111 comprises a sodium conducting fluorinated polymer membrane, such as NAFION®, wherein the membrane inhibits the flow of dissolved chlorine gas to the cathode 105 as the chlorine gas is liberated by the anode 104 while the sodium cations are free to flow through the membrane 111, as illustrated in FIG. 5. In a preferred embodiment, the membrane 111 is a perfluorosulfonic acid polymer membrane, such as a NAFION® membrane. Preferably, the membrane 111 has a thickness of between 0.025 mm and 0.13 mm, depending on the cell resistance. Preferably, the inlet 108 and outlet 109 are set on opposite sides of the membrane 111 and sodium ions and processed solution flow through the membrane 111 as the chloride ions combine to produce chlorine gas. For example, referring to FIG. 2, the system may be designed so that the aqueous sodium chloride solution enters the first container 112 at the top portion of the first container 112 and the processed solution exits the first container 112 at the bottom portion of the first container 112. This encourages the flow of sodium and remaining processed solution to flow through the membrane. In an alternative embodiment, the outlet 109 may be connected to a pump to force the flow.

As the electric potential is applied across the electrodes 104 and 105, chlorine gas is liberated at the anode 104 and sodium metal is collected at the cathode 105. The chlorine gas is emitted from the first container 112. Preferably, the emission of the chlorine gas occurs on the side of the first container 112 having the inlet 108 and the collection of sodium occurs on the side of the first container 112 having the outlet 109. The chlorine gas is collected and used at a later point in the process. Preferably, the chlorine is collected and fed to a third container 130 to react with sodium hydroxide, as described below.

The sodium metal collected by the cathode 105 is transferred to a second container 120. The cathode 105 may be an amalgamated surface, such as metal coated with a material having a high hydrogen overpotential to avoid the evolution of hydrogen at the cathode 105. Examples of substrate metal that may be used include but are not limited to stainless steel, titanium and high performance corrosion resistant alloys, such as HASTELLOY®. In a preferred embodiment, a motorized recirculating belt 110 is used to collect and transfer the sodium cations from the first container to the second container, as illustrated in FIGS. 2 and 5. The optimum speed of the belt will be dependent on the current density and will be set to maximize efficiency. To collect the sodium, the recirculating belt 110 preferably comprises a stainless steel belt amalgamated with a material having high hydrogen overpotential, such as mercury, lead or tin. Mercury is the preferred material due to its high hydrogen overpotential. As the sodium collects on the belt 110 in the first container 112, the sodium deposits on the surface of the belt 110 to produce sodium amalgam, then the belt 110 carries the sodium amalgam to the second container 120. To assist in sodium cation deposition, the cell voltage is preferably at least 1.8 V-2.0 V, depending on current density. For example, at 1000 mA/cm² the cell voltage may be about 2.2 V to about 2.4 V. Rollers 124 may be used to maintain the recirculating route of the belt from the first container to the second container. Preferably, the rollers 124 are positioned to not contact the sodium-contacting surface, as illustrated in FIG. 2. Although a recirculating belt 110 is preferred, any collection and transfer system may be used. For example, a cathode having the above noted properties may be dipped into the first container for a time sufficient to attract the sodium, then removed and dipped into the second container to allow the sodium to dissolve in the dilute sodium hydroxide. The electrical contact may be a brush type of contact also.

In the second container 120, the sodium amalagam dissolves in dilute sodium hydroxide and couples against an air-depolarizing cathode 122 located in the second container 120. As the sodium amalagam dissolves in water molecules in the dilute sodium hydroxide, electrons are released. These electrons flow through the external circuit to the cathode, i.e. air-depolarizing fuel cell 122, where oxygen reduction takes place. The combination of sodium metal anode and oxygen cathode will form a metal-air fuel cell. The sodium air fuel cell produces power that may be used to move the belt 110 or other functions. Additionally, this reaction produces concentrated sodium hydroxide, which may be used to neutralize the chlorine gas to produce sodium hypochlorite. The mercury resists dissolving in the second container 120 as the sodium is released and the belt 110 returns to the first container 112 to collect more sodium. Preferably, the dilute sodium hydroxide is about 0.01M and the concentrated sodium hydroxide has a higher molarity than the dilute sodium hydroxide, as sodium cations react with water in the dilute sodium hydroxide to produce more NaOH molecules and reduce the water content. Pure water may be used in place of the sodium hydroxide, however dilute sodium hydroxide is preferred. The needs of the system only require that water molecules are available to remove sodium from the cathode. The dilute sodium hydroxide is preferred because the concentrated sodium hydroxide may be reacted with the chlorine gas as detailed below.

A second conduit 131 transfers the concentrated sodium hydroxide from the second container 120 to a third container 130. A first conduit 132 collects and transfers the chlorine gas emitted from the first container 112 to the third container 130. A pump may be used to feed the chlorine gas through the first conduit 132 to the third container 130. In the third container 130, the chlorine gas is bubbled through the concentrated sodium hydroxide. The chlorine gas reacts with the concentrated sodium hydroxide to produce sodium hypochlorite. The sodium hypochlorite may be collected and processed for commercial purposes.

Unlike traditional water electrolysis, where oxygen gas and hydrogen gas are produced, aqueous sodium chloride electrolysis typically produces hydrogen gas and chlorine gas. In a batch process, if the electrolysis continuously removes chlorides as chlorine gas and sodium ions as sodium metal, the residual aqueous sodium chloride will become weak in salt content. However, a preferred embodiment of the present invention does not evolve hydrogen but removes sodium cations to reduce the salt (NaCl) content. The inventors of the present invention have found the evolution of hydrogen at the cathode may be substituted with sodium metal deposition by the proper choice of cathode material.

Hydrogen overpotential on certain metals is very low and in certain metals is very high. For example on platinum surfaces hydrogen overpotential is very low which indicates that hydrogen evolution is very favorable on this substrate. In contrast, hydrogen overpotential on lead, tin and mercury is high and hence hydrogen evolution is unfavorable. Therefore, to avoid hydrogen evolution during aqueous sodium chloride electrolysis, the cathode material should be a material with high overpotential, such as lead, tin or mercury. In a preferred embodiment, the cathode material is mercury, the material with a highest overpotential. Since the hydrogen overpotential will be high, the cell voltage will be high (about 1.8 to 2.0 V depending on current density). At the high cathodic potentials, the sodium cations from aqueous sodium chloride will begin to deposit on the mercury cathode. In effect, aqueous sodium chloride is electrolyzed to produce chlorine and sodium. If the aqueous sodium chloride solution from the electrolysis bath is not changed, gradually there will be a decrease in the NaCl content and a gradual rise in the electrolytic cell resistance since ionic conductivity will be gradually going down with the removal of sodium and chloride ions from the water. At some point the processed solution can be removed and fresh aqueous sodium chloride solution may be added to continue the process. The electrolysis reaction is given by
2NaCl(l)→2Na(l)+Cl₂(g)
where the half cell reactions are
2Cl⁻(aq)—Cl₂(g)+2e⁻ (anode; oxidation)
and
Na⁺(aq)+e⁻→Na (l) (cathode; reduction)

The decrease in salinity as indicated by decrease in conductivity is an indication to remove the spent electrolyte and replace it with fresh aqueous sodium chloride solution. This conductivity change can be continuously monitored and automatic sensing systems can dump the spent electrolyte and re-supply with fresh electrolyte. The sodium (Na) is deposited onto a coated belt, which is moving continuously in a recirculating fashion from the first container to the second container, as illustrated in FIGS. 2 and 5. When the belt containing sodium (Na) is moving into the second container it works as an anode against the air electrode. The sodium amalgam-air fuel cell also provides additional power. Sodium dissolves in water producing electrons, which flow through the external circuit to the cathode where the oxygen reduction takes place. The product is sodium hydroxide (NaOH). The reaction is given by
Na⁺(aq)+OH⁻(aq)→NaOH(l)
where the half cell reactions are
Na(l)→Na⁺(aq)+e⁻(anode; oxidation)
and
2H₂O(l)+2 e⁻→H₂(g)+2OH⁻(aq) (cathode; reduction)
Chlorine reacts with the sodium hydroxide (NaOH) to form sodium hypochlorite (NaOCl). The reaction is given by
2NaOH(aq)+Cl₂(g)⇄NaOCl(aq)+NaCl(aq)+H₂O(l)

FIG. 3 is a block diagram of aspects of a preferred embodiment of the present invention, wherein a photovoltaic device is a source of electrical energy to initiate the process to produce processed solution having a lower sodium chloride concentration than the source of aqueous sodium chloride solution. Electrodes are placed into contact with a source of aqueous sodium chloride solution 200 and 201, such as seawater or brine. A source of solar energy 202, e.g. sun, provides solar energy to the photovoltaic device 203. The photovoltaic device provides an electric potential across the electrodes 204, which induces electrolysis of the sodium chloride 205. The electrolysis produces chlorine gas and sodium 206. The chlorine gas bubbles out from the aqueous sodium chloride 208 and the sodium deposit onto a cathode 207. By removing chlorine and sodium ions, weak aqueous sodium chloride is produced 209. Continuation of the electrolysis further reduces the concentration of sodium chloride in the water.

FIG. 4 is a block diagram of aspects of a preferred embodiment of the present invention, wherein a photovoltaic device is a source of electrical energy to initiate the process to produce processed solution having a lower sodium chloride concentration than the source of aqueous sodium chloride solution, to produce power via a sodium air fuel cell and to produce sodium hypochlorite. Electrodes are placed into contact with a source of aqueous sodium chloride solution 300 and 301, such as seawater or brine. A source of solar energy 302, e.g. sun, provides solar energy to the photovoltaic device 303. The photovoltaic device provides an electric potential across the electrodes 304, which induces electrolysis of the sodium chloride 305. The electrolysis produces chlorine ions that combine to produce chlorine gas and sodium cations 306. Preferably, the chlorine gas bubbles from the aqueous sodium chloride solution 311 and is collected for later use and the sodium is deposited onto an amalgamated surface 307. The sodium amalgam couples against an air-depolarizing electrode 308. The sodium air fuel cell produces power 310 and a byproduct of the coupling is sodium hydroxide 309. The sodium hydroxide combines with the chlorine gas to produce sodium hypochlorite 312.

The photovoltaic device preferably utilizes a semiconductor as the active material. A preferred photovoltaic device structure for the instant invention is the n-i-p structure. This structure consists of an i-type (intrinsic) semiconductor layer interposed between an n-type semiconductor layer and a p-type semiconductor layer. In a typical simple device, a transparent conducting electrode layer is contacted to the p-type layer and a metal electrode is contacted to the n-type layer. In such a device, incident sunlight passes through the transparent electrode and p-type layer and is absorbed by the i-type layer. Absorption by the i-type layer leads to promotion of electrons from the valence band to the conduction band and to the formation of electron-hole pairs in the i-type layer. The electrons and holes are the charge carriers needed to produce electricity. The adjacent p-type and n-type layers establish a potential in the i-type layer that separates the electrons and holes. The electrons and holes are subsequently conducted to oppositely charged collection electrodes that provide the potential needed to induce the water splitting reaction.

Representative semiconductor materials suitable for the absorption of solar energy include crystalline silicon, polycrystalline silicon, amorphous silicon, microcrystalline silicon or related materials, including alloys of silicon with germanium. Other materials such as GaAs, CdS and CuInSe₂ may also be used. These materials can be prepared as n-type, i-type or p-type and have bandgap energies that permit absorption of at least a portion of the solar spectrum. Amorphous silicon is a preferred i-type material in a n-i-p structure because it possesses a direct bandgap and exhibits a high absorption strength over a wide wavelength range of the solar spectrum. The high absorbance of amorphous silicon is desirable because it leads to efficient absorption of sunlight in thinner devices. Thinner devices require less material and are correspondingly more cost effective.

A variety of n-i-p photovoltaic structures are operable to provide the electrical potential required to effect the water splitting reaction of the instant invention. A basic objective is to use a photovoltaic device structure that maximizes the absorption of solar energy and the conversion efficiency of solar energy to electrical energy. Several strategies for improving the performance of amorphous silicon based photovoltaic devices are possible. These strategies include the use of microcrystalline silicon to form the p-type layer, integration of two or more n-i-p structures to form tandem devices, and inclusion of a back reflector layer in the structure. U.S. Pat. No. 4,609,771, the disclosure of which is hereby incorporated herein by reference, discloses the use of microcrystalline silicon p-type layers in solar cells. The inventors therein demonstrate that microcrystalline silicon has a higher transparency to sunlight than amorphous silicon. As a result, use of a microcrystalline silicon p-type layer allows more incident sunlight to reach the i-type layer and a higher concentration of charge carriers is produced as a result. U.S. Pat. Nos. 4,600,801 and 5,977,476, the disclosures of which are hereby incorporated herein by reference. The strategy associated with tandem devices is to couple multiple n-i-p structures in series in an attempt to harvest as much incident sunlight as possible. Although high, the absorption efficiency of i-type amorphous silicon layers is substantially less than 100%. Placement of a second n-i-p structure directly below the n-i-p structure that is directly incident to the sunlight provides an opportunity to capture light not absorbed by the first n-i-p structure. Tandem structures that include the stacking of three n-i-p structures to form triple cells have also been described. Additional strategies such as bandgap tailoring of the i-layer from one n-i-p structure to the next have also been demonstrated to improve the light harvesting efficiency of tandem structures. In these strategies, separate n-i-p structures in a tandem structure include i-type layers that are optimized to selectively absorb different portions of the solar spectrum. Amorphous silicon, for example, may be used to absorb the short wavelength portions of the solar spectrum, while SiGe alloys having various amounts of Ge may be used to absorb longer wavelengths since alloying with Ge reduces the bandgap. Representative triple cell structures are described in the following journal publications, the disclosures of which are hereby incorporated herein by reference: “Roll to roll manufacturing of amorphous silicon alloy solar cells with in situ cell performance diagnostics” by M. Izu and T. Ellison (Solar Energy Materials & Solar Cells vol. 78, p. 613-626 (2003)) and “Triple-junction amorphous silicon alloy solar cell with 14.6% initial and 13.0% stable conversion efficiencies” by J. Yang, A. Banerjee and S. Guha (Applied Physics Letters, vol. 70, p. 2975-2977 (1997)).

Back reflecting layers are reflective layers that are typically deposited directly on the substrate. The role of a back reflecting layer is to reflect any light passing through all of the n-i-p cells stacked in a tandem device. Through this reflection process, light that is initially not absorbed is redirected to the stacked n-i-p devices for a second pass and improved absorption efficiency results. Representative back reflecting layer materials include Al, ZnO, Ag, Ag/ZnO, and Al/ZnO.

An important advantage associated with amorphous silicon is the ability to manufacture it in a large scale continuous manufacturing process. Crystalline silicon, on the other hand, can only be prepared in a slow, smaller scale process because of the slow crystallization processes associated with its formation. Consequently, great efforts have been directed at the large scale production of amorphous silicon. Modern web rolling processes permit the high speed production of single and multilayer thin films amorphous silicon based devices. The production of amorphous silicon on a continuous web has been previously described in, for example, U.S. Pat. Nos. 4,485,125; 4,492,181; and 4,423,701, and U.S. application Ser. No. 10/228,542 the disclosures of which are hereby incorporated herein by reference.

A p-i-n multiple cell device may be obtained by reversing the order of the n-plus and p-plus layers or regions of the n-i-p multiple cell device described above. Referring to FIG. 1, a tandem or cascade-type photovoltaic cell, formed of successive p-i-n layers each including an amorphous silicon alloy material, is shown generally by numeral 10. The photovoltaic device illustrated in FIG. 1 may be used with the present invention. FIG. 1 shows a p-i-n type photovoltaic device such as a solar cell made up of individual p-i-n type cells 12a, 12b and 12c. Below the lowermost cell 12a is a substrate 11 which may be transparent or formed from a metallic surfaced foil. The substrate material 11 may be stainless steel, aluminum, tantalum, molybdenum or chrome, as well as substrates formed of synthetic polymers, glass or glass-like material on which an electrically conductive electrode is applied.

Each of the cells 12a, 12b and 12c include an amorphous semiconductor body containing at least a silicon alloy. Each of the semiconductor bodies include an n-type conductivity region or layer 20a, 20b and 20c; an intrinsic region or layer 18a, 18b and 18c; and a p-type conductivity region or layer 16a, 16b and 16c. The term “amorphous” as used herein includes all materials exhibiting long-range disorder, regardless of their short or intermediate range order and regardless of whether those materials are otherwise labeled polycrystalline or crystalline. As illustrated, cell 12b is an intermediate cell and, as indicated in FIG. 1, additional intermediate cells may be stacked atop the illustrated cells.

For each of the cells 12a, 12b and 12c, the p-type and n-type layers of semiconductor material are characteristically light transmissive and highly conductive. The intrinsic layers of semiconductor material are characterized by an adjusted wavelength threshold for solar photoresponse, high light absorption, low dark conductivity and high photoconductivity, including sufficient amounts of a band gap adjusting element or elements to optimize the band gap for the particular cell application. Preferably, the intrinsic layers of semiconductor material are band gap adjusted to provide cell 12a with the lowest band gap, cell 12c with the highest band gap and cell 12b with a band gap between the other two, as light enters the semiconductor material from the top. However, the intrinsic layers of semiconductor material are band gap adjusted to provide cell 12a with the highest band gap, cell 12c with the lowest band gap and cell 12b with a band gap between the other two, if light enters the semiconductor material from the bottom. The n-type layers of semiconductor material are characterized by low light absorption and high conductivity. The thickness of the band gap adjusted layers of intrinsic material may be in the range of 800 to 5,000 angstroms. The thickness of the n-type and p-type layers may be in the range of 25 to 400 angstroms.

The foregoing is provided for purposes of explaining and disclosing preferred embodiments of the present invention. It will be apparent to those skilled in the art that variations and modifications of the present invention can be made without departing from the scope or spirit of the invention. For example, variations in the transfer of the sodium cations after collection, variations in the route of the belt, different sources of electrical energy, different sources of aqueous sodium chloride solution and variations in the conduit system for the chlorine gas and sodium hydroxide will be apparent to those skilled in the art. These changes and others may be made without departing from the scope or spirit of the invention in the following claims.

Claims

1. A process for reducing sodium chloride concentration in an aqueous sodium chloride solution, comprising

electrolyzing said aqueous sodium chloride solution in a first container, said electrolysis splitting sodium chloride in said aqueous sodium chloride solution producing chloride anions, sodium metal and processed solution, said chloride anions combining producing chlorine gas, said processed solution having a sodium chloride concentration lower than said aqueous sodium chloride solution sodium chloride concentration;

bubbling said chlorine gas from said first container;

collecting said sodium metal on a surface producing sodium amalgam;

removing said sodium metal from said first container; and

removing said processed solution from said first container.

2. The process of claim 1, further comprising coupling said surface to an air depolarizing fuel cell electrode in a second container containing dilute sodium hydroxide, said coupling anodically dissolving said sodium metal in said dilute sodium hydroxide producing concentrated sodium hydroxide and electrical power.

3. The process of claim 2, further comprising reacting said concentrated sodium hydroxide with said chlorine gas in a third container producing sodium hypochlorite.

4. The process of claim 1, said electrolyzing comprising:

applying an electric potential across an anode and a cathode;

contacting said aqueous sodium chloride solution with said anode, said contacting with said anode liberating said chloride anions; and

contacting said sodium metal discharging with said cathode, said cathode comprising said surface.

5. The process of claim 4, said electric potential supplied by a photovoltaic device in electrical communication with said anode and said cathode.

6. The process of claim 2, said collecting comprising collecting said sodium metal on a stainless steel belt coated with a material having high hydrogen overpotential producing sodium amalgam.

7. The process of claim 6, said material having high hydrogen overpotential selected from the group consisting of mercury, tin and lead.

8. The process of claim 3, further comprising restricting the flow of chloride anions and chlorine gas to said cathode.

9. The process of claim 8, said restricting comprising disposing a membrane between said anode and said cathode, said membrane inhibiting the flow of said chloride anions and said chlorine gas to said cathode.

10. The process of claim 9, said selective membrane comprising perfluorosulfonic acid polymer.

11. The process of claim 8, further comprising moving said stainless steel belt from said first container containing said aqueous sodium chloride solution into said second container containing dilute sodium hydroxide and said fuel cell, said stainless steel belt collecting said sodium metal in said first container, said sodium metal dissolving in said dilute sodium hydroxide in said second container.

12. The process of claim 11, said moving comprising a recirculating said stainless steel belt from said first container to said second container

13. The process of claim 12, said recirculating stainless steel belt adapted to continually flow in a repeating route between said first container and said second container.

14. The process of claim 1, said aqueous sodium chloride solution comprising brine.

15. The process of claim 5, said photovoltaic device comprising at least one triple junction solar cell.

16. The apparatus of claim 15, said at least one triple junction solar cell comprising at least one triple junction amorphous silicon solar cell.

17. The process of claim 4, said anode comprising a RuO2 coated titanium anode and said cathode comprising a mercury coated stainless steel cathode.

18. The process of claim 1, said removing said processed solution comprising discharging said processed solution from said first container through an outlet.

19. The process of claim 16, said processed solution having a sodium chloride concentration of about zero.

20. An apparatus for reducing sodium chloride concentration in an aqueous sodium chloride solution, comprising:

a source of electrical energy;

a first container containing said aqueous sodium chloride solution;

an electrolyzer for electrolyzing said aqueous sodium chloride solution, said electrolyzer in electrical communication with said source of electrical energy, said electrolyzer splitting sodium chloride in said aqueous sodium chloride solution to produce chlorine anions, sodium metal and processed solution, said chlorine anions combining producing chlorine gas, said processed solution having a sodium chloride concentration lower than said aqueous sodium chloride solution sodium chloride concentration; and

a sodium collector for collecting said sodium metal on an surface producing sodium amalgam.

21. The apparatus of claim 20, further comprising:

a second container containing dilute sodium hydroxide; and

an air depolarizing fuel cell electrode in said second container, said air depolarizing fuel cell electrode coupling to said surface, said coupling dissolving said sodium metal in said dilute sodium hydroxide to produce concentrated sodium hydroxide and electrical power.

22. The apparatus of claim 21, further comprising:

a third container;

a first conduit for flowing said concentrated sodium hydroxide from said second to said third container; and

a second conduit for feeding said chlorine gas from said first container to said third container, said concentrated sodium hydroxide reacting with said chlorine gas to produce said sodium hypochlorite in said third container.

23. The apparatus of claim 20, said source of electrical energy comprising a photovoltaic device, said photovoltaic device converting solar energy to electricity.

24. The apparatus of claim 23, said photovoltaic device comprising at least one triple junction solar cell.

25. The apparatus of claim 24, said at least one triple junction solar cell comprising at least one triple junction amorphous silicon solar cell.

26. The apparatus of claim 20, said electrolyzer comprising:

an anode in electrical communication with said source of electrical energy and said anode in contact with said aqueous sodium chloride solution,

a cathode in electrical communication with said source of electrical energy, said source of electrical energy generating an electric potential between said anode and said cathode, said anode liberating chlorine anions, said cathode contacting and collecting said sodium cations, said cathode comprising said surface.

27. The apparatus of claim 26, said anode comprising a RuO2 coated titanium anode and said cathode comprising mercury coated stainless steel.

28. The apparatus of claim 20, said aqueous sodium chloride solution comprising brine.

29. The apparatus of claim 20, said sodium collector comprising a stainless steel belt amalgamated with a material having a high hydrogen overpotential.

30. The apparatus of claim 29, said material selected from the group consisting of lead, tin and mercury.

31. The apparatus of claim 29, said stainless steel belt comprising a recirculating stainless steel belt adapted to flow from said first container to said second container, said recirculating stainless steel belt collecting said sodium metal in said first container, flowing to said second container and contacting said dilute sodium hydroxide in said second container to release said sodium metal.

32. The apparatus of claim 31, said stainless steel belt comprising a recirculating stainless steel belt adapted to continually flow in a repeating manner between said first container and said second container.

33. The apparatus of claim 26, further comprising a membrane disposed in said first container between said anode and said cathode, said membrane inhibiting the flow of said chlorine anions and said chlorine gas to said cathode.

34. The apparatus of claim 33, said selective membrane comprising perfluorosulfonic acid polymer.

35. The apparatus of claim 20, further comprising an inlet and an outlet, said inlet transferring said aqueous sodium chloride solution into said first container, said outlet discharging said processed solution out of said first container.

36. The apparatus of claim 20, said processed solution having a sodium chloride concentration of zero.

37. The process of claim 2, said coated surface comprising a stainless steel belt coated with mercury.