Abstract:
A hydrogen generator working by hydrolysis of the metal borohydride is described comprising a reaction chamber ( 7 ) which in its bottom part has a liquid collecting area ( 30 ) and leads by short and non-complex connecting components to a conduit end ( 38 ) through which the exhaust products ( 31 ) of the reaction are discharged into the environment, generally the atmosphere and thereby saving weight and volume. By using given high pressures and temperatures for the reaction, the danger of crystallization of exhaust products is prevented.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a hydrogen generator which can be a hydrogen generator comprising a container for containing an aqueous solution of at least one metal hydride, a reactor chamber containing a catalyst, a pump for pumping the aqueous solution from the container to the reactor chamber, a first liquid collecting area communicating with the reactor chamber for collecting the reaction exhaust products, an exhaust products outlet exiting from the collecting area and a gas outlet for extracting the gaseous products, or a hydrogen generator comprising a container for containing a liquid reaction agent, a reactor chamber containing at least one metal hydride in solid form, a pump for pumping the liquid reaction agent from the container to the reactor chamber, a first liquid collecting area communicating with the reactor chamber for collecting the reaction exhaust products, an exhaust products outlet exiting from the collecting area and a gas outlet for extracting the gaseous products, and to a method of operating it. 
       BACKGROUND OF THE INVENTION 
       [0002]    Hydrogen generators of the kind mentioned above usually serve to supply fuel cells with gaseous hydrogen, e.g. for vehicles such as small aircrafts. Particularly for this purpose low volume and light weight are essential. Hydrogen is chemically generated by a following reaction 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where MBH 4  and MBO 2  respectively represent a metal borohydride and a metal metaborate. While H 2  is the useful product, MBO 2  and residual water, partly in the form of steam, are exhaust products. These exhaust products, dissolved in aqueous solution, have a high tendency to crystallize. 
         [0003]    According to US 2004/0009379 A1 the exhaust product is separated into dry residuals which are collected in a special vessel, and steam which can be vented to the atmosphere. The drying equipment and the mentioned vessel lead to a bulky and heavy construction. 
         [0004]    According to US 2006/0225350 A1, the exhaust is processed in a gas/liquid separator and the exhaust is drained to a collecting tank. Again, the construction is unfavourable with respect to size and weight. This prior art, further, uses a closed loop control of the pressure and the temperature in the reaction chamber, mentioned pressures are from 0 to 41 kPa and mentioned temperatures are 20 to 50° C. 
         [0005]    Due to U.S. Pat. No. 7,083,657 B2, the exhaust product is separated into its gaseous and liquid components, the liquid component being fed back to the reaction chamber. In this prior art, the problem is further discussed that in the reaction exhaust products the salt product tends to crystallize, thereby clogging the reaction chamber or the downstream conduits and apparatus. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the invention to provide for a compact, lightweight hydrogen generator. For such compact generators with small sized element, it is particularly necessary to avoid crystallization of the reaction exhaust products which, in this case hypercritically congests the conduits. For preventing the crystallization, the conduit extending from the exhaust products outlet via the controllable valve to a conduit end carries heating devices at least along part of its length, an elevated temperature counteracts the crystallization. 
         [0007]    To obtain such compact and lightweight generator, according to the invention, in the generator the exhaust products outlet opens to the environment via a controllable valve. By such disposals, the components for separating or drying the exhaust products are no longer necessary. 
         [0008]    A further aspect with reference to the compactness is the fact that the saturation water vapour partial pressure depends on the temperature but keeps materially constant if the gas pressure of the mixture containing the water vapour in the reaction chamber is increased. Upon hydrolyses, it is possible to generate the hydrogen under a high pressure in the reaction chamber by controlling the flow rate of the pump and the power of heating devices of the reaction chamber, where a preferred temperature is at least 70° C. For this reason, the reactor chamber preferably contains temperature and pressure sensors, it is coupled to temperature adjusting devices, and the sensors are connected to a control unit controlling the pump and the temperature adjusting devices. 
         [0009]    For maintaining the desired pressure in the reaction chamber the first liquid collecting area contains first liquid level sensors coupled to a control unit controlling the controllable valve which opens and closes in intervals. 
         [0010]    In the generator the gas outlet is coupled via a cooling device to a gas/liquid separator containing a second liquid collecting area including second liquid level sensors, as is known in the prior art; To said area a discharging conduit is connected which leads via a controllable valve, which opens and closes in intervals, alternatively to the environment or to a point upstream of the pump. The liquid is almost pure water which, if conducted to the environment, is discharged similarly as the reaction exhaust products, and if conducted to a point upstream of the pump, can be used for diluting the fuel, similarly to the prior art U.S. Pat. No. 7,083,657 B2, or for a cleaning cycle 
         [0011]    For avoiding crystallization of the reaction exhaust products in the reaction chamber and its downstream elements, high pressure and high temperature according to mutual relations are preferably used, as characterized in claims  9  to  11 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings. 
           [0013]      FIG. 1  shows an embodiment of the hydrogen generator of the invention 
           [0014]      FIG. 2  shows a section view of an alternative embodiment of the invention 
           [0015]      FIG. 3  shows a section view of another alternative embodiment of the invention 
           [0016]      FIG. 4  shows a graph of the correlation between pressure and temperature 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    A hydrogen generator  1  according to  FIG. 1  comprises a bag shaped fuel container  2  that contains an aqueous solution of a metal hydride, in the present example sodium borohydride, which via a conduit  3  and a check valve  4  is conducted to a pump  5 , here a peristaltic pump, which pumps the solution under pressure via a conduit  6  to a reaction chamber  7 . Contained in the reaction chamber is a fixed bed catalyst device  10 , e.g. consisting of porous ceramic substrates with a supported catalyst of known kind. An alternative catalyst device, not shown, consists of movable catalyst supports to be immersed into the solution in the reaction chamber. The reaction chamber can be heated by a heater  11  and can be cooled by fans  12 . According to alternatives not shown, the heater  11  and the cooling fans  12  can be replaced by heat pipes. In the reaction chamber, the reaction mentioned above takes place, releasing hydrogen and reaction exhaust products. 
         [0018]    The hydrogen escapes the reaction chamber through a gas outlet  13 , flows through a cooling coil  14  and is fed into a gas/liquid separator  19 . In this separator, the hydrogen is withdrawn through filter units  20  and a pressure regulator  21  and fed via a conduit  22  to a fuel cell system  23 . 
         [0019]    The reaction chamber  7  contains a pressure sensor  27  and temperature sensors  28 . A control unit  29  in the hydrogen generator  1  picks up the values of the pressure sensor  27  and the temperature sensors  28  and regulates the flow rate of the pump  5  as well as the power of the heater  11  and of the fans  12  according to a given program. 
         [0020]    In the lower part of the reaction chamber  7  there is a first liquid collecting area  30  wherein the slurry like exhaust products  31  accumulate. An exhaust products outlet  32  at the bottom of area  30  extends to a conduit  36  and further via a controllable valve  37  to a conduit end  38  that is located outside the casing of the hydrogen generator  1 . In area  30  two liquid level sensors  39 ,  40  are arranged at slightly different level heights and are connected to the control unit  29 , which controls the valve  37 . When the higher liquid level sensor  40  gives a signal, the control unit  29  opens the valve  37  between conduit  36  and conduit end  38 , discharging the exhaust products  31  through end  38  into the surrounding environment. As surrounding environment the atmosphere is usually defined; also the surrounding environment can be a collection container open to atmosphere, that is no part of the hydrogen generator and not shown in the figures, wherein the exhaust products are accumulated. When the lower liquid level sensor  39  gives a signal, the control unit  29  closes the valve  37 . To maintain the predefined pressure in the reaction chamber  7  the level difference between the sensors  39 , 40  is small as well as the discharged amount, keeping the gas volume in chamber  7  almost constant. The frequency of opening the valve  37  is rather high and the duty rate can be in the order of 1:10. The path from the outlet  32  to the conduit end  38  is short and does not contain complex elements, thereby almost preventing the danger of crystallization. As a further measure the conduit  36  and the valve  37  are at least partially heated. 
         [0021]    In the lower part of gas/liquid separator  19 , the water condensed from the mixture of hydrogen and steam and cooled in the cooling coil  14  accumulates in a second liquid collecting area  45 . In the bottom of area  45 , a discharging conduit  46  is connected that leads to the valve  37 . Two level sensors  47 ,  48  are placed in the area  45  and connected to the control unit  29 , working similar to level sensors  39 ,  40 . The valve  37 , controlled by control unit  29 , can block the exit of the liquid through the discharging conduit  46 , can connect the discharging conduit  46  to the conduit end  38  so as to discharge the condensed water or can connect the discharging conduit  46  to a conduit  49 , located to a point upstream of the pump  5  either into the fuel container  2  or directly into pump  5 . The latter can be suitable for a cleaning cycle for restarting the generator after an interruption, thus preventing clogging of the system by crystallization; or for diluting the solution of the metal hydride. 
         [0022]    According to different embodiment shown in  FIG. 2 , located directly in the reaction chamber  7  and fixed by fixing devices  51  there is a solid block  52  of a chemical hydride. A reaction agent in the fuel container  2  that is an aqueous acidic solution such as phosphoric acid to which possibly a catalyst is added, is transported via the conduit  3  to the check valve  4  and pump  5  and further to a distributer  53 , that is located at the top of reacting chamber  7  and arranged to spray the reaction agent to the solid block  52  thereby originating the reaction. The gas outlet  13  in this embodiment is situated near the bottom of the chamber  7 , but above the liquid collecting area  30  as in the embodiment of  FIG. 1 . 
         [0023]      FIG. 3  shows a still different constellation of the reaction chamber  7 , which is supplied by the fuel solution of  FIG. 1  and is equipped with the fixed bed catalyst device  10  and comprises cooling rips  54 , and the liquid collecting area  30  is constructed as a separate container  55  connected to the catalyst device  10  by a tube  56 . The gas outlet  13  is arranged in the top part of the container  55 , and the exhaust products outlet at the lower part of the container  55  at a level lower than the lower level sensor  39 . 
         [0024]    In the graph of  FIG. 4 , the pressure in kPa at the ordinate is plotted against the temperature in ° C. at the abscissa. Three curves  60 ,  61 ,  62  show limits of the correlation between pressure and temperature. Curve  60  shows generally the temperature limit dependent on the pressure, in the area left of curve  60  (lower temperatures) the danger of crystallization is too high. Curve  61  and  62 , referring to the embodiment of  FIG. 1 , show limits of the correlation of pressure and temperature for different concentrations of sodium borohydride in water, i.e. curve  61  for a 25% and curve  62  for a 20% solution, wherein for both cases the operating points should be selected between curve  60  and curve  61  or respectively  62  for obtaining optimal conditions to avoid crystallization of the exhaust products. These curves appear in the tables of claims  10  to  12 .