Abstract:
The electrochemical gas purifier system consists of a cell module with accessory components mounted in a single framework. The module consists of a number of single cells each capable of purifying and ultimately producing hydrogen gas at pressures exceeding 2000 psi. The process comprises introducing a contaminated hydrogen stream to a cell comprising an anode and a cathode with an electrolyte membrane disposed therebetween. The hydrogen is oxidized on the anode to protons which electrochemically migrate across the membrane to the cathode where they recombine with electrons which have passed through an external power source. The contaminants exit the anode side of the cell while the purified hydrogen exits the cathode side of the cell.

Description:
TECHNICAL FIELD 
     The present invention relates to a hydrogen purification system and process, and especially relates to an electrochemical hydrogen purification process and system. 
     BACKGROUND OF THE INVENTION 
     Hydrogen manufacturing typically comprises the reformation of hydrocarbon fuels such as methane, natural gas, methanol, or gasoline. In one process the hydrocarbon fuel is injected into a catalyst bed and heated by steam or other means until the hydrocarbon breaks down to form hydrogen gas and carbon containing gases such as carbon dioxide and carbon monoxide. The product hydrogen must then be separated from these gaseous byproducts as well as excess fuel gases or vapors. This separation typically employs a pressure swing absorption, temperature swing absorption, permeoselective membranes or cryogenic separation. 
     Product hydrogen is used in many different applications as a process fluid, reactant, or for its special low molecular weight properties. In many of these applications, such as heating, petroleum refining, or food processing, small quantities of resident carbon containing gases or vapors in the hydrogen stream does not constitute a problem. For other applications, such as reducing microelectronic wafers, acting as a carrier gas for laboratory analyses, and as a fuel for fuel cells, the hydrogen purity is very important since the presence of carbon containing contaminants results in deleterious effects on the process and/or system. For these applications, hydrogen is typically purified by passing the hydrogen through thin palladium membranes. This process, which represents a final clean-up step for the hydrogen stream, is very expensive due to the high capital cost of the equipment and process costs. 
     What is needed in the art is a simple, efficient hydrogen purification process which can be readily employed in various systems. 
     SUMMARY OF THE INVENTION 
     The subject of the current invention is an electrochemical device which makes use of a proton exchange membrane to effect a separation of hydrogen from contaminants such as carbon monoxide and carbon dioxide. The method of the present invention for purifying hydrogen comprises: introducing a contaminated hydrogen stream to an anode of an electrochemical cell; applying a voltage across the membrane assembly (anode/membrane/cathode) wherein the voltage is less than that which will cause water electrolysis; oxidizing the hydrogen in the hydrogen stream on said anode to form protons; moving the protons from the anode, across the membrane to the cathode; and recombining the protons to form substantially pure hydrogen gas. 
     The above description and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings, which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a schematic diagram of the electrochemical hydrogen purifier of the present invention; 
     FIG. 2 is a block diagram of a fuel cell system using the electrochemical hydrogen purifier of the present invention; and 
     FIG. 3 is a cross sectional view of an electrochemical hydrogen purifier of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an electrochemical hydrogen purifier  101  comprises: an anode  103 , a cathode  107 , a proton exchange membrane  108  disposed therebetween, and a power source  106  with the positive terminal connected to the anode  103  and the negative terminal connected to the cathode  107 . This purifier can be arranged similar to a conventional electrochemical cell or a bi-polar electrochemical cell. In operation, a contaminated hydrogen stream  102  is introduced to the anode  103  where a voltage of about 1.5 volts or less causes the hydrogen to oxidize forming protons  105  which migrate across the membrane  108  to the cathode  107 , while electrons pass through the power source  106 . At the cathode  107 , the protons  105  and electrons reform hydrogen  109  which is removed from the purifier  101  for use as a high purity hydrogen gas, while the contaminants  104  vent from the purifier  101  or are directed for further processing. The further processing may include removal of water from the hydrogen stream using conventional means, including molecular sieves, thermal processes, condensers, thermal membranes, desiccants, porous plates, among others. 
     The membrane, which can be a conventional material capable of transporting protons from the anode to the cathode, preferably has a high water concentration, i.e., greater than about 25 volume % water, to enhance ionic mobility of protons. Possible membranes include, but are not limited to, proton exchange membranes such as perfluoroionomers, or other ionomers containing styrene, fluorostyrene, and styrene-divinylbenzene structures, among others, with those having chemically bound sulfonic acid groups preferred due to their high electrochemical pumping efficiency. 
     The membrane should additionally have sufficient thickness to inhibit leakage of contaminants from one side of the membrane to the other under standard operating conditions. Typically the membrane is up to about 200 mils thick, with a thickness of about 7 mils to about 10 mils preferred for operating conditions comprising a pressure differential of about 400 psi or less, with greater pressures preferably having membranes with a thickness exceeding about 7 mils. 
     Disposed on each side of the membrane  108  are electrodes, anode  103  and cathode  107 , capable of oxidizing and recombining hydrogen, respectively, and having a thickness to provide sufficient catalytic activity to achieve desired performance levels. Possible electrodes include, but are not limited to, finely divided noble metal base electrodes, such as platinum, palladium, ruthenium, chromium, iridium, rhodium, and alloys thereof, typically having a thickness of less than about 1 mil. 
     Referring to FIG. 2, a schematic of a fuel cell system  20  which utilizes an electrochemical hydrogen purifier, liquid or gaseous hydrocarbon fuel  22  together with water  25  converts to hydrogen gas and carbon based byproducts in a reformer  24 . The contaminated hydrogen gas stream  102  enters purifier  101  where the carbon contaminants, such as carbon dioxide and carbon monoxide are removed from the hydrogen stream and, if desired, the pressure of the hydrogen stream is increased accordingly. While the carbon contaminant stream  104  vents from the purifier  101 , the purified, pressurized hydrogen gas  109  enters fuel cell stack  28  along with an oxidant  26 , normally air. Within the fuel cell  28 , a controlled recombination of hydrogen and oxygen occurs to produce electric power  29 . The benefit of the purification system  101  is that the purified hydrogen gas  109  used in fuel cell  28 , enhances the performance and reliability, while reducing maintenance requirements. 
     Now with reference to FIG. 3, a schematic representation of fuel processor module  30  is shown having a membrane  108 , an anode  103  and a cathode  107 . Screen packages  32 ,  35 ,  37  are sandwiched between the anode  103  and separator plate  34 , cathode  107  and separator plate  36  and separator plates  36  and  38 , respectively. Screens  32 ,  35  collect current from the anode  103  and cathode  107 , while screen  37  serves as a fluids distributor of the coolant water. 
     Important factors in effectively purifying the hydrogen stream  102 , include the voltage applied across the anode/membrane/cathode assembly, the flow rate of the contaminated hydrogen stream  102 , and the hydrogen residence time on the anode  103 . During operation the power source  106  applies a sufficient current across the anode/membrane/cathode assembly to oxidize the hydrogen, forming protons, while not breaking down water contained within the membrane into hydrogen and oxygen gas which would cause membrane dehydration and subsequent inoperability. Typically a voltage of about 1.5 volts or less under standard operating conditions is applied across the assembly with a voltage of about 0.20 to 0.80 preferred. It will be understood by an artisan that under different conditions the voltage which will cause water electrolysis changes. For example, for a cell operating at about 1 atmosphere and 25° C., a voltage exceeding about 1.48 volts will cause water electrolysis, while at higher pressure, a higher voltage is required to induce water electrolysis. 
     The flow rate of the contaminated hydrogen stream  102  should be sufficiently slow to attain a sufficient residence time on the anode  103  to oxidize the majority of the hydrogen in the hydrogen inlet stream  102 , and preferably to oxidize greater than about 85 percent of the hydrogen, with an oxidation of greater than 95 percent of the hydrogen especially preferred. Typically, a hydrogen inlet flow rate in cubic centimeters per minute (cc/min.) is greater than or equal to 7.52 times the total number of amps passed through a cell. For example, a cell having an active area of 1 square foot (ft 2 ) can remove 7520 cc/min. of hydrogen when passing 1000 amperes of current. With an inlet fuel stream which is 60% hydrogen, the unit can process 12533 cc/min. of total gas flow. Residence time is a function of the total number of amps passed through the cell and the concentration of the hydrogen in the inlet stream and total gas flow rate. For a 1 ft 2  cell passing 1000 amps of current, the residence time should be greater than 0.25 min. for a total hydrogen flow rate of 7520 cc/min. 
     In the case of some proton exchange membrane fuel cells which operate with reformed hydrocarbon fuels, trace carbon-bearing contaminants, such as carbon monoxide, absorb onto the anode, masking the active area of the cell. An analogous mechanism can occur with the electrochemical gas purifier, eventually rendering the purifier ineffective and/or inefficient. Consequently, periodic current reversal for short time periods of less than about a minute, with about 10 seconds to about 40 seconds preferred, effectively desorbs the adhering contaminants, resulting in a clean electrode surface. 
     In the case where substantially complete hydrogen recovery is important, i.e., possibly aerospace applications, and/or where purification of the non-hydrogen components, i.e., carbon monoxide, carbon dioxide, etc., is important, a multi-cell purifier arrangement with the cells operated in series, can be employed. In this arrangement, due to decreased hydrogen content, it is preferred to operate subsequent cells at a lower current density than previous cells in order to facilitate hydrogen ion transport across the membrane of the cell. For example, a two cell stack with each cell having an active area of 1 ft 2 , with a 60% hydrogen stream flowing at a rate of approximately 138000 cc/min., in order to recover about 100% of the hydrogen, the stack can be operated at a 1,000 amp/cm 2  current density across the first cell and a 100 amp/cm 2  current density across the second cell. This arrangement will recover about 91% of the hydrogen in the first cell, and about all of the remaining hydrogen in the second cell, thereby recovering about 100% of the overall hydrogen. 
     The purifier of the present invention is a simplified, efficient, cost effective manner of purifying, and if desired, pressurizing hydrogen for commercial use. This system enables: (1) the production of pure hydrogen (i.e., greater than 99.9999% pure) via the separation of hydrogen from carbon containing gases; (2) a controlled hydrogen output by adjusting the current applied to the purifier enabling consumption of a metered flow of hydrogen (this is typical of an automotive fuel cell application whereby hydrogen rate must be metered based on load demand); (3) the generation of hydrogen gas at the desired pressure; and (4) a commonality of environment and hardware exist with electrochemical systems, particularly with proton exchange membrane cells. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.