Abstract:
A system and method for producing dry gas, such as methane or carbon dioxide, incorporates an electrochemical device that removes water and hydrogen from a mixed gas stream. The electrochemical device uses one electrochemical cell to strip hydrogen and water from the mixed gas stream and a second electrochemical cell, combined with a dry feed stream, to remove any residual water from the mixed stream and produce pure, dry gas.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to methane gas production, and more particularly to a system and method that obtains pure, dry gas by removing hydrogen and water from a mixed gas stream.  
         BACKGROUND OF THE INVENTION  
         [0002]    Sabatier reactors are known in the art for converting carbon dioxide (CO2) and hydrogen (H2) into methane (CH4) and water (H2O). These reactors can be used to obtain pure methane for use in other applications, such as fuel. Known methane production systems include an electrolyzer, which separates water into gaseous hydrogen and oxygen, a Sabatier reactor that receives the carbon dioxide, carbon monoxide, hydrogen and water and converts them into methane and water, forming a “wet” methane stream, where methane gas is intermingled with water vapor. Excess hydrogen left over from the Sabatier reaction may also be mixed into the wet methane stream.  
           [0003]    Although an electrochemical separator can be used to easily remove the excess hydrogen from the wet methane stream, it is difficult to completely remove the residual water from the stream. Currently known water pumps remove less than all of the water from the stream and do not produce pure, dry methane as a final product. If the methane will later be cooled in a liquefaction process, any residual water will form undesirable ice crystals before the methane gas condenses to liquid. Thus, any methane gas to be liquefied in a later process must be absolutely dry, with no residual water. Currently known systems, however, have been able to remove only a portion of the water from the methane gas stream. Other gas production systems (e.g., carbon dioxide gas production systems) encounter similar difficulties in forming a completely pure, dry gas.  
           [0004]    There is a desire for a gas production system that can produce pure, dry gas.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to a system and method that removes water from a mixed gas stream containing a desired gas, hydrogen, and water via an inventive electrochemical device acting as a pump and a separator that removes both excess hydrogen and water from the stream. In one embodiment, the electrochemical device includes two electrochemical cells. Hydrogen and water are removed from the mixed gas stream as the stream passes through the first electrochemical cell. As the mixed gas stream passes the anode of the second electrochemical cell, a dry feed stream, such as a hydrogen stream, is fed to the cathode side of the second electrochemical cell. The dry feed stream creates a partial pressure differential with respect to the mixed gas stream, causing the dry feed stream to absorb any residual water from the mixed gas stream. Further, the potential applied across the second electrochemical cell prevents any portion of the dry feed stream from diffusing into the mixed gas stream. As a result, the gas stream that is output from the second electrochemical cell is pure, dry gas, with virtually no water mixed into the desired gas.  
           [0006]    An alternative embodiment incorporates a third electrochemical cell that act as metering device to send the dry feed gas stream back to the electrolyzer after the stream has absorbed excess water from the mixed gas stream. The third electrochemical cell can be controlled to feed the feed hydrogen in a metered fashion as needed to maintain sufficient reduction of carbon dioxide in a methanation reactor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a block diagram illustrating a gas production system incorporating an electrochemical pump and separator device according to one embodiment of the invention; and  
         [0008]    [0008]FIG. 2 is a representative diagram of the electrochemical device used in the system shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0009]    [0009]FIG. 1 illustrates one embodiment of an overall gas production system  100  that incorporates a novel electrochemical device to obtain pure, dry gas. Note that although the example described below focuses on removing hydrogen and water from a wet methane stream, the invention can be applied to any gas production system requiring removal of water and hydrogen from a mixed gas stream.  
         [0010]    In the example shown in FIG. 1, the system  100  includes an electrolyzer  102 , a Sabatier reactor  104 , and an electrochemical device  106  that acts as a water pump and a hydrogen separator. To produce methane, a stream  108  containing carbon dioxide, hydrogen gas, and water is fed into the electrolyzer  102 . As is known in the art, the electrolyzer  102  separates the water into hydrogen and oxygen gases. The oxygen gas is then output from the electrolyzer  102  as a separate product that may be captured and used in a separate application.  
         [0011]    In addition to outputting oxygen, the electrolyzer  102  sends carbon dioxide, carbon monoxide, hydrogen and water to the Sabatier reactor  104 . The Sabatier reactor  104  reduces the hydrogen and carbon dioxide into methane gas and water vapor. Extra hydrogen may be sent to the Sabatier reactor  104  to ensure that all of the carbon dioxide it receives is converted to methane. Any excess, unconverted water from the electrolyzer  102  is fed to the Sabatier reactor  104  along with the hydrogen and carbon dioxide.  
         [0012]    Once the carbon dioxide has been converted to methane and water, the methane, water and any excess hydrogen gas is sent as a wet methane stream  110  to the electrochemical device  106 . The device  106  acts as both a water pump and a hydrogen separator. As is known in the art, the excess hydrogen can be easily separated from the stream using an electrochemical cell because there is a direct relationship between current and the number of protons pumped through the electrochemical cell.  
         [0013]    To remove excess water from the wet methane stream  110 , the electrochemical device  106  incorporates a second electrochemical cell that acts as a pump to send the water in the stream  110  into a dry hydrogen feed stream  112  via a partial pressure differential between the two streams  110 ,  112 . The dry hydrogen feed stream  112  is supplied from a separate hydrogen gas source (not shown).  
         [0014]    [0014]FIG. 2 illustrates one embodiment of the electrochemical device  106  and the flow path of the wet methane stream  110  in more detail. As noted above, the device  106  receives a wet methane stream  110  containing methane, hydrogen, and water from the Sabatier reactor  104 . The wet methane stream  110  is introduced to an anode of a first electrochemical cell  200 . The cell voltage applied to the first electrochemical cell  200  is kept at a level that is sufficient to move protons through the cell  110  while still low enough to avoid electrolyzing the water in the methane stream  110 . This first electrochemical cell  200  will be able to transfer some of the water and virtually all of the hydrogen in the methane stream  110  to the cathode of the cell  200 , but the methane stream  110  may still contain significant amounts of remaining water that need to be removed to obtain pure, dry methane. The residual water occurs when there is too much water in the methane stream  110  relative to the hydrogen protons needed to carry the water away from the stream  110 . In some cases, the methane stream  110  may still contain as much as 10% hydrogen and 50-100% water saturation.  
         [0015]    After being sent past the first electrochemical cell  200 , the methane stream  110  is diverted to the anode side of a second electrochemical cell  202 . Dry hydrogen gas is sent to the device  106  as a feed stream  112  on the cathode of the second cell  202 . When a potential is applied across the second cell  202 , any remaining hydrogen in the methane stream  110  is pumped to the cathode of the second cell  202  along with more of the water vapor from the methane stream  110 . The dry hydrogen feed stream  112  on the cathode of the second cell  202  helps remove even more water from the methane stream  110  by creating a partial water pressure differential between the hydrogen feed stream  112  and the methane stream  110 . More particularly, water molecules in the wet methane stream  110  will be pulled by protons in the hydrogen feed stream  112 , drawing water molecules away from the methane stream  110 .  
         [0016]    The potential applied across the second cell  202  prevents hydrogen from the hydrogen feed stream  112  from diffusing into the methane stream  110  as the hydrogen feed stream  112  absorbs the excess water from the methane stream  110  by driving protons back to the hydrogen feed stream  112 . Once the excess water has been removed from the methane stream  110  in the second cell  202 , pure, dry methane gas  114  is output from the device  106 .  
         [0017]    The hydrogen feed stream  112 , which has now absorbed water from the methane stream  112 , is routed to a third electrochemical cell  204 . The current through the third cell  204  is controlled via any known manner so that the third cell  204  acts as a metering device, outputting controlled amounts of hydrogen and water to the electrolyzer  102 . This information is used to control the rate at which hydrogen is generated in the electrolyzer  102 . As a result, the moisture in the hydrogen/water feed stream  112  sent to the electrolyzer  102  allows control over the moisture content in the hydrogen electrolyte.  
         [0018]    The inventive system and method therefore provides higher water removal efficiency than currently known systems by incorporating additional at least one additional electrochemical cell and a dry feed gas to extract additional water from a mixed gas stream via partial pressure driving force. By sending the mixed gas stream, such as a wet methane stream, through more than one drying stage, the inventive system can generate a pure, dry desired gas without adding undue complexity to the system. Further, by recycling the dry feed gas after it has absorbed water from the mixed gas stream and sending it back to the electrolyzer in the system, the system can use the output of the electrochemical device to maintain the moisture level in the electrolyzer. Regardless of the type of gas sent through the inventive system, an electrochemical device according to the present invention can reliably remove excess water and hydrogen from a mixed gas stream containing the desired gas.  
         [0019]    It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.