Abstract:
A device as described for the generation of high purity carbon dioxide (CO2) and hydrogen (H2) by electrochemical decomposition of aqueous solutions of liquid and solid organic acids. A d.c. power source is used to apply a preselected current to an electrochemical cell, consisting of an ion permeable membrane and two electrodes. The generation rate of CO2 and H2 is continuous and proportional to the applied current; it can be stopped instantaneously by interrupting the current. Small Battery operated generators can produce propellant CO2 and H2 to deliver fluids from containers. Other uses include the creation of anaerobic environments in incubation chambers.

Description:
BACKGROUND OF THE INVENTION 
     The use of “in-situ” carbon dioxide generation, to deliver fluids is described in U.S. Pat. No. 5,398,851. In this instance, carbon dioxide is produced by chemical reaction of a sodium carbonate with an aqueous citric acid solution. These fluid delivery devices are rather inaccurate, mainly due to the difficulty to achieve and maintain a constant gas generation rate. Furthermore, their use is limited, since gas generation can not be controlled, varied or stopped. 
     Improvements over such devices are described in the Maget U.S. Pat. No. 5,971,722. In this instance the gas generator is electrochemically controlled and thereby achieves higher accuracy levels, without adding complexities or cost. 
     Commercial carbon dioxide gas generation systems, such as Becton-Dickinson&#39;s GasPak (tm), which are used in microbiology, are also based on the chemical reaction between sodium bicarbonate and citric acid. 
     In all instances, when carbon dioxide is generated by chemically reacting a metal (bi)carbonate with an acid, the reaction, once started is difficult to control, can not be conveniently stopped and at reaction completion residues containing chemicals, binders and additives are present, dissolved or in suspension in solution. 
     Hydrogen-driven fluid delivery is exemplified by the Disetronic Infuser disposable syringe pump, which uses a galvanic cell as a hydrogen source. 
     Commercial hydrogen gas generation systems, such as GasPak used in microbiology, are based on the chemical reaction of sodium borohydride with citric acid. 
     Again, these hydrogen generators are difficult to stop or to control to achieve a constant pre-set gas generation rate. 
     The present invention, based on selecting CO2 and H2 containing organic compounds, and an electrochemical decomposition process, results in the controlled generation of CO2 and H2, the rate being variable at will, which includes stops and restarts, and without formation of by-products. 
     The electrochemical cell, suitable for the practice of the present invention, is described in the Maget U.S. Pat. No. 6,010,317 and is hereby included by reference. 
     The organic acids suitable as sources of carbon dioxide, are of the family of mono- and polycarboxylated hydrocarbons, exemplified by formic acid, oxalic acid, tricarballylic acid, succinic acid, and the likes. 
     By applying energy, provided from an external d.c. power source, an anodic process strips hydrogen from the organic acid, releases hydrogen which is ionized and transported to the counterelectrode, while releasing carbon dioxide as the anodic product. In the instances when the organic acid only contains hydrogen and carbon dioxide, such as formic acid and oxalic acid, the anodic gas released during the process is mainly carbon dioxide, and the cathodic gas is mainly hydrogen. In this manner the CO2 and H2 generation rates are controlled by the applied current, and can thus be readily started, stopped or regulated. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is the primary object of this invention to provide for a carbon dioxide generator which can be controllably operated. 
     It is another object of this invention to provide for a small, compact, self-contained, battery operated carbon dioxide generation apparatus. 
     It is the third object of this invention to provide carbon dioxide on demand, at a rate predictable from an applied current. 
     It is the fourth object of this invention to generate a high purity carbon dioxide. 
     It is a fifth object OF this invention to generate carbon dioxide without production of insoluble reaction by products. 
     The sixth to the tenth objects of this invention are similar to objects one to five, except that they are related to the generation of hydrogen. 
     These and other objects of the instant invention will become more apparent from the claims, specification, drawings and experiments. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the schematic drawing of a CO2 and H2 generator, using an aqueous solution of an organic acid (formic acid) as a CO2 ad H2 source; 
     FIG. 2 is a schematic illustration of the use of a solid organic acid (oxalic acid) contained in a porous, or water permeable, bag or pouch; 
     FIG. 3 is a schematic of the control circuitry of the generator; 
     FIG. 4 is similar to FIG. 2 but shows the use of a grid/basket used to hold the solid acid; 
     FIG. 5 is a schematic illustration of a continuous generator; and 
     FIG. 6 is a sketch of the electrochemical cell module, used to decompose the organic acid. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment consists of a container  1  enclosed by closure  2  with two outlets,  3  and  4 . Within container  1  is located an electrochemical cell module  5  sealed by means of seal  6  to prevent fluid from leaking past module  5 . The aqueous organic acid solution  7  is stored in an upper chamber in container  1  and is in contact with the anode  9  of cell module  5 . Cathode  9  and anode  10  are connected to the power supply  11  and current controller  12 . The dc power supply can be AC/DC converter or a battery. 
     The electronic circuit providing a constant current to electrochemical cell is illustrated in FIG. 3. A battery or AC/DC converter  11  supplies electric power to a constant current controller chip  40  such as Micronics Inc. precision current controller MX 963 or a TPS 7101 manufactured by Texas Instruments Corp. Controller chip  40  is connected to a load resistor  41  via sense leads  42 . These leads provide feedback for the controlling chip  40  to sense the current passing through electrochemical cell  5 . The output of current controlling chip  40  is thereafter connected to electrochemical cell  5  and returns to the power source via ground lead  50  thereby completing the circuit. 
     The other preferred embodiment illustrated in FIG. 2 is similar to the previous description, except that the organic acid  14  is a solid contained in a water permeable bag or pouch  13  made of natural or synthetic fiber or film. The purpose of the bag  14  is to prevent the solid organic acid  14  from forming a deposit on anode  9  thereby hindering the electrochemical process. 
     Another embodiment similar to that illustrated in FIG. 2 is shown in FIG. 4. A metal, plastic or ceramic perforated basket or grid  18  is used to hold solid organic acid  14  which may be in the form of powder, granule or the like. Perforated basket or grid  18  functions to hold the solid acid to prevent formation of deposits on anode  9 . It therefore prevents “fouling” of the electrode while still allowing the CO2 to escape. Illustrated is a battery operated unit. The battery can be external or an integral part of the system. 
     The embodiment illustrated in FIG. 5 shows a continuous generator that is line power operated. It has an organic acid storage tank  20 . The acid  7  would flow through line  22  that has a valve  24  and on into container  1 . The flow of acid is metered by means of a level sensor  26  that sends a signal through wire  27  to control unit  28  to open/close valve  24 . 
     The electrochemical cell module  5  is illustrated in FIG.  6  and its structure and manner of functioning are more thoroughly described in U.S. Pat. No. 6,010,317 which is incorporated by reference into this specification. The module  5  comprises four basic parts. The module parts are a first collector mounted at one end of an outer shell  50 , a second current collector  52  mounted at one end of an inner shell  54 , an ion exchange electrochemical cell  56  sandwiched between the current collectors, and a ring seal member  58 . The electrochemical cell comprises an electrolytic membrane with electrodes  60  and  62  formed integrally on opposite sides of the membrane. 
     By activating switch  28 , a current, the magnitude of which is set by variable resistor  41  is applied to e-cell module  5 , thereby promoting the following reactions, when the organic acid is formic acid: 
     Anodic Reaction 
     
       
         HCCOOH→CO 2 +2H + +2e −   (1) 
       
     
     Cathodic Reaction 
     
       
         2H + +2e − →H  (2) 
       
     
     The anodic reaction results in the stripping of carbon dioxide from the acid, generation of hydrogen, which is ionized and transported through the ion permeable membrane where it is released as hydrogen gas at the cathode. 
     Similarly, in the case of oxalic acid, the following reactions take place:                           
     Cathodic Reaction 
     
       
         2H + +2e − →H 2   (4) 
       
     
     However, in this instance, the ratio CO 2 /H 2 =2, as compared to a value of 1 for the formic acid decomposition process. Therefore, the current efficiency, i.e. moles of CO 2  produced/mA-hr is favorable for polycarboxylated acids such as oxalic acid. 
     In both instances, CO 2  bubbles  8  evolve from the anode and are evacuated via outlet  4 . Hydrogen gas, formed at the cathode, is evacuated via outlet  3 . 
     In some cases, the generation of hydrogen is beneficially used as an independent gas stream, or mixed with evolving CO 2  to create 50/50 CO 2 /H 2  anaerobic gas mixture in the case of formic acid, or a 66.7/33.3 gas mixture in the case of oxalic acid. 
     Whenever hydrogen is not beneficially used, the cathode reaction can be mitigated by using an air depolarized cathode, i.e. supplying oxygen or air, to the cathode chamber, such that reactions (2) and (4) now become: 
     Air Depolarized Cathode Reaction 
     
       
         2H + +2e − +1/2O 2 →H 2 O  (5) 
       
     
     and the electrochemical decomposition process results solely in the production of carbon dioxide and water. 
     During the electrochemical decarboxylation process, the concentration of the organic acid decreases progressively, resulting in a progressive increase of the e-cell voltage necessary to maintain a fixed current, i.e. a fixed CO2 and H2 generation rate. The cell voltage increase is the result of concentration polarization of the anode. Therefore, whenever a fixed voltage is applied to e-cell module  5 , the rate of CO2 and H2 generation decreases progressively as the reaction proceeds, unless additional formic acid is provided to reinstate the initial concentration. However, in the case of a solid organic acid, the situation is different. 
     The solubility of oxalic acid in water is approximately 9.5 wt % at room temperature. If operating conditions, i.e. current applied to the e-cell module  5  is such that the dissolution rate of oxalic acid is greater than its consumption rate, the acid concentration remains constant at 9.5 wt % through-out the process, until the solid acid is totally consumed. Therefore, for self-contained systems, i.e. without the influx of additional acid, solid organic acids offer the opportunity to generate CO2 and H2 at a constant rate under operating conditions of an applied voltage instead of an applied current. This result is beneficially used whenever simple circuitry, i.e. only a resistor or potentiometer, is required for reasons of economy, while still a constant rate of CO2 and H2 generation is expected. 
     EXAMPLE I 
     A solution consisting of 6 mL of 88% formic acid and 14.4 mL of deionized water is electrically decomposed at 25-30° C., while a constant current of 80 mA is applied to the electrochemical cell. The CO2 and H2 generator is operated continuously for 77 hours. The average rate of gas generated at the anode during this time period is approximately 31.5 cc/hr. The gas composition of both anodic and cathodic streams is: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Anodic gas 
                 Cathode gas 
               
               
                   
                 composition 
                 composition 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Carbon dioxide 
                 99.2  
                 3.5 
               
               
                   
                 Hydrogen 
                 0.3 
                 95.7  
               
               
                   
                 Oxygen 
                 0.3 
                 0.8 
               
               
                   
                 Carbon monoxide 
                 ND 
                 ND 
               
               
                   
                 ND = non detected 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE II 
     6.8 grams of oxalic acid, stored in a porous bag, are placed in 13.2 mL of deionized water. The current applied to the electrochemical cell is constant at 40 mA. The generator is operated continuously over a period of 87 hours. The average rate of gas generated at the anode is 33.8 cc/hr. The average rate of gas generated at the cathode is 16.6 cc/hr. 
     The gas composition of both anodic and cathodic streams is: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Anodic gas 
                 Cathodic gas 
               
               
                   
                 composition 
                 composition 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Carbon dioxide 
                 99.1  
                 4.0 
               
               
                   
                 Hydrogen 
                 0.4 
                 95.4  
               
               
                   
                 Oxygen 
                 0.5 
                 0.6 
               
               
                   
                 Carbon monoxide 
                 ND 
                 ND 
               
               
                   
                 ND-non detected 
               
               
                   
                   
               
             
          
         
       
     
     At the end of the test, the storage bag for the oxalic acid was visibly depleted of all solid acid. 
     EXAMPLE III 
     A saturated solution of oxalic acid in deionized water was electrochemically decomposed by applying a battery voltage supplied by two series-connected alkaline AA batteries. By manually changing the resistance from a variable resistor box, it was possible to change the current and voltage applied to the electrochemical cell. The following results were observed: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Electrochemical 
                 Current flowing 
                 Approximate gas 
               
               
                 cell voltage 
                 through the cell 
                 generation rate 
               
               
                 volts 
                 mA 
                 at the anode, cc/hr 
               
               
                   
               
             
             
               
                 1.11 
                 36.0 
                 26.6 
               
               
                 1.12 
                 38.5 
                 28.0 
               
               
                 1.15 
                 42.8 
                 29.2 
               
               
                 1.16 
                 47.5 
                 36.0 
               
               
                   
               
             
          
         
       
     
     Although the preferred embodiments of this invention have been described by way of examples only, it will be understood that modifications may be made without departing from the scope of the invention, which is defined in the following claims.