Abstract:
The invention discloses a pressure electrolyzer comprising a pressure vessel ( 2 ) and an electrolytic-cell block ( 3 ) arranged in the pressure vessel ( 2 ) and containing a plurality of electrolytic cells ( 4 ) combined in the form of a stack. The electrolytic cells ( 4 ) in each case contain anodes ( 11 ) and cathodes ( 12 ), and an electrolyte-circuit system is provided for supplying an anolyte to the anodes ( 11 ) and for supplying a catholyte to the cathodes ( 12 ), and wherein the electrolytic-cell block ( 3 ) comprises a housing ( 5 ) which is sealed relative to the interior of the pressure vessel ( 2 ). The invention provides that the housing ( 5 ) of the electrolytic-cell block ( 3 ), together with the pressure vessel ( 2 ), forms at least two chambers ( 7, 8 ) separated from each other, which constitute a part of the electrolyte-circuit system and which are defined relative to the electrolytic cells ( 4 ) by the housing ( 5 ) and relative to the atmosphere by the pressure vessel ( 2 ). In preferred embodiments, one of the chambers ( 7 ) which is separated from another chamber constitutes a part of an anolyte circuit, and another of the chambers ( 8 ) which is separated from said one chamber constitutes a part of a catholyte circuit.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
       [0001]     The present invention relates to a pressure electrolyzer, and to a method of operating said pressure electrolyzer.  
         [0002]     Pressure electrolyzers are known for the eletrolytic splitting-up of water into hydrogen and oxygen. Such known pressure electrolyzers comprise a pressure vessel and an electrolytic-cell block, which is arranged in said pressure vessel and contains a plurality of electrolytic cells which are combined in the form of a stack. The electrolytic cells in each case contain anodes and cathodes, and an electrolyte-circuit system is provided for supplying an anolyte to the anodes and for supplying a catholyte to the cathodes. The electrolytic-cell block comprises a housing which is sealed relative to the interior of the pressure vessel.  
         [0003]     A pressure electrolyzer of this description is known from German Publication DE 25 48 699 C3. In the case of this known pressure electolyzer, means for the purification, cooling and circulation of the electrolyte are also arranged in the pressure vessel in addition to the electrolytic-cell block.  
         [0004]     An object of the invention is to provide an improved pressure electrolyzer. Furthermore, an improved method of operating such a pressure electrolyzer is to be described.  
         [0005]     As far as the apparatus is concerned, this object is met by the claimed pressure electrolyzer.  
         [0006]     Advantageous further developments of the pressure electolyzer according to the invention are set out in the dependent claims.  
         [0007]     As far as the method is concerned, the set object is met by the claimed method. Advantageous further developments of the method according to the invention are set out in the dependent claims.  
         [0008]     The invention provides a pressure electrolyzer which comprises a pressure vessel and an electrolytic-cell block which is arranged in the pressure vessel and contains a plurality of electrolytic cells combined in the form of a stack. The electrolytic cells in each case contain anodes and cathodes, and an electrolyte-circuit system is provided for supplying an anolyte to the anodes and for supplying a catholyte to the cathodes. The electrolytic-cell block comprises a housing which is sealed relative to the interior of the pressure vessel. It is provided, according to the invention, that the housing of the electrolytic-cell block, together with the pressure vessel, forms at least two chambers, which are separated from each other, which constitute a part of the electrolyte-circuit system and which are defined relative to the electrolytic cells by the housing and relative to the atmosphere by the pressure vessel. An advantage of the pressure electrolyzer according to the invention is the simplicity of the design of the electrolyte-circuit system. A further advantage is that relatively little space is required by the pressure electrolyzer according to the invention.  
         [0009]     According to a particularly preferred embodiment of the pressure electrolyzer according to the invention, one of the chambers, being separated from the other chamber, constitutes a part of an anolyte circuit, and the other chamber, being separated from said one chamber, constitutes a part of a catholyte circuit. An advantage of this embodiment is that, by separating the anolyte and catholyte circuits, it is possible to achieve a higher degree of gas purity and, consequently, a wider range of load control.  
         [0010]     A particularly preferred embodiment of the pressure electrolyzer according to the invention provides that the chambers which are separated from each other are separated by separating walls which extend between the housing and the pressure vessel.  
         [0011]     According to a preferred embodiment, the pressure vessel has the shape of a cylinder and the longitudinal axis of the electrolytic-cell block is arranged in the pressure vessel parallel with the axis of the cylinder.  
         [0012]     According to a further preferred embodiment of the pressure electrolyzer according to the invention, the separating walls extend between the housing and the pressure vessel parallel with the axis of the cylinder.  
         [0013]     According to a particularly preferred embodiment of the pressure electrolyzer according to the invention, the housing of the electrolytic-cell block is composed of a plurality of cell frames of the electrolytic cells, which cell frames are stacked side by side.  
         [0014]     According to an advantageous further development in this regard, it is provided that, in each case, each anode has its own anode cell frame and, in each case, each cathode has its own cathode cell frame, and that the anode cell frames comprise passages, which produce a connection between the anode and the chamber which constitutes a part of the anolyte circuit, and that the cathode cell frames comprise passages which produce a connection between the cathode and the chamber which constitutes a part of the catholyte circuit.  
         [0015]     A further advantageous embodiment provides that the electrolytic-cell block is arranged with its longitudinal direction substantially horizontally, and that that chamber, which is defined by the housing of the electrolytic-cell block and the pressure vessel, is vertically subdivided in the longitudinal direction of the electrolytic-cell block into the two chambers which are separated from each other.  
         [0016]     A particularly advantageous further development of the two last-mentioned embodiments provides that the passages producing a connection with the anodes are in communication, in each case on the upper side and on the lower side of the anode cell frames, with the one of the chambers, which is separated from the other chamber and constitutes a part of the anolyte circuit, and that the passages producing a connection with the cathodes are in communication, in each case on the upper side and on the lower side of the cathode cell frames with the other of the chambers, which is separated from said one chamber and constitutes a part of the catholyte circuit. The advantage hereof is that it is possible for the electrolyte to circulate automatically without an additional electrolyte-circuit pump, due to a gas lift effect because of the gases rising in the cells and the lower density of the electrolyte/gas mixture in the cells, relative to the degassed electrolyte columns outside the cells. The product gases oxygen, respectively hydrogen, collect along the crowns of the two chambers which are separated from each other.  
         [0017]     According to a further advantageous embodiment of the pressure electrolyzer according to the invention, essential components of the electrolyte-circuit system, in particular a gas separator and/or an electrolyte cooler, are provided within the pressure vessel. The advantage hereof is a particularly space-saving, compact design of the pressure electrolyzer.  
         [0018]     According to a further advantageous embodiment, one of the separating walls is arranged below the electrolytic-cell block and simultaneously forms a mechanical support for said block.  
         [0019]     A particularly advantageous embodiment of the pressure electrolyzer according to the invention provides that the anodes and cathodes are encircled by a sealing element, which extends about their periphery and which forms a lateral end of the electrolytic cell in the region of the anode, respectively of the cathode, and which is sealingly fitted between the cell frames.  
         [0020]     A particularly preferred embodiment hereof provides that the anode and the cathode of an electrolytic cell are each separately encircled in a sealing element.  
         [0021]     The sealing elements of the anode and the cathode are preferably jointly sealingly fitted between two successive cell frames.  
         [0022]     The sealing elements preferably also form a lateral end of the electrolytic cell on a diaphragm which is arranged between the anode and the cathode.  
         [0023]     The diaphragm is preferably sealingly fitted between the sealing element of the anode and the sealing element of the cathode.  
         [0024]     The sealing elements are preferably arranged about the periphery of the anode and/or of the cathode so as to extend in the manner of a frame.  
         [0025]     According to a preferred embodiment, it is provided that the sealing elements are designed to be led about the edge of the anode and/or of the cathode from the front side to the rear side of said anode and/or said cathode.  
         [0026]     According to a particularly advantageous embodiment hereof, the sealing elements are provided with one or more sealing lips, which extend about the periphery of the anode, respectively of the cathode, and which rest sealingly against a sealing face of the anode cell frame, respectively of the cathode cell frame.  
         [0027]     It is preferably provided, in this regard, that the sealing lips project in a perpendicular direction relative to the flat side of the anode, respectively of the cathode, and that the sealing faces of the anode cell frame, respectively of the cathode cell frame, are designed to extend parallel to the flat side of the anode, respectively of the cathode.  
         [0028]     The sealing elements are preferably produced from an elastomer.  
         [0029]     In this regard, it is particularly advantageous when the sealing elements are produced by extrusion-coating of the edge of the anode, respectively of the cathode, using the elastomer.  
         [0030]     According to an advantageous further development of the pressure electrolyzer according to the invention, two adjacent electrolytic cells are separated from each other by a bipolar metal sheet which is encircled by a sealing element, which extends about the periphery of said bipolar metal sheet and forms a lateral end of the electrolytic cell in the region of the bipolar metal sheet and is sealingly fitted between the cell frames of the adjacent electrolytic cells.  
         [0031]     The sealing element is preferably arranged to extend about the periphery of the bipolar metal sheet in the manner of a frame.  
         [0032]     According to an embodiment, the sealing element is arranged on one side of the bipolar metal sheet.  
         [0033]     According to another embodiment, the sealing element is designed to be led about the edge of the bipolar metal sheet, from the front side to the rear side thereof.  
         [0034]     According to a preferred embodiment, the sealing element is provided with one or more sealing lips which extend about the periphery of the bipolar metal sheet and rest sealingly against a sealing face of the anode cell frame, respectively of the cathode cell frame, or against the bipolar metal sheet itself.  
         [0035]     A preferred embodiment hereof provides that the sealing lips project in a perpendicular direction relative to the flat side of the bipolar metal sheet, and that the sealing face of the anode cell frame, respectively of the cathode cell frame, is designed to extend parallel to the flat side of the bipolar metal sheet.  
         [0036]     The sealing element is preferably produced from an elastomer.  
         [0037]     It is possible for the sealing element to be produced by extrusion-coating of the edge of the bipolar metal sheet using the elastomer.  
         [0038]     The separate chambers in the pressure vessels are, according to an advantageous development, connected to each other at a geodetically low point so as to permit an alignment of the levels between the anolyte and the catholyte. The connection is provided by a connecting pipe which extends outside the pressure vessel. By way of alternative, it is possible for the chambers within the pressure vessel to communicate via an opening or a membrane in the separating means.  
         [0039]     For the purpose of the electrical insulation, the pressure vessel is, according to an embodiment, of an electrically non-conductive material. By way of alternative, each of the chambers defined by the housing of the electrolytic-cell block and the inner side of the pressure vessel are encapsulated by a shell of an electrically insulating material. It is also possible for the housing of the electrolytic-cell block and/or the pressure vessel to be provided with an electrically insulating coating or lining.  
         [0040]     The separation of the gas takes place within the pressure vessel in regions disposed geodetically above the electrolytic-cell block. It is advantageous, in this regard, to arrange gas-coalescing aids, in the form of knit fabrics and/or ultrasonic transmitters and/or flow-deflecting means in the gas-separating regions.  
         [0041]     According to a further development of the pressure electrolyzer according to the invention, a water-supply tank is provided. The water-supply tank is connected to a feed-water source via a feed-water filling pipe comprising a feed-water fill-up valve, to a bottom region of one of the separated chambers via a feed-water inflow pipe comprising a feed-water inflow valve, and to a top region of the one of the separated chambers via a pressurizing pipe comprising a pressurizing valve. The water-supply tank, additionally, can be evacuated relative to the atmosphere via a ventilation pipe comprising a ventilation valve, the top region of the one of the separated chambers being filled, during operation of the pressure electrolyzer, with one of the product gases.  
         [0042]     According to an advantageous embodiment, it is provided that the one of the separated chambers, to which the water-supply tank is connected via the feed-water inflow pipe and via the pressurizing pipe, is the other separated chamber constituting a part of the electrolyte circuit.  
         [0043]     As far as the method is concerned, the set object is met by a method of filling a pressure electrolyzer according to the two last-mentioned embodiments, including the following method steps: 
        a) pressure-free filling of the water-supply tank via the feed-water filling pipe with the feed-water fill-up valve open, while the ventilation valve is open, the pressurizing valve is closed and the feed-water inflow valve is closed,     b) closing the ventilation valve and the feed-water fill-up valve,     c) opening the pressurizing valve, so as to bring the water-supply tank to the pressure prevailing in the pressure vessel via the pressurizing pipe,     d) opening the feed-water inflow valve, so as to admit metered quantities of feed water from the supply tank into the pressure vessel, and     e) closing the feed-water inflow valve.        
 
         [0049]     The metered addition of the feed water from the supply tank into the pressure vessel preferably takes place under gravity, the water-supply tank being arranged spatially above the pressure vessel.  
         [0050]     By way of alternative, it is also possible for the metered addition of the feed water from the water-supply tank into the pressure vessel to be carried out by means of a feed-water pump included in the feed-water inflow pipe, which pump may then advantageously have a low capacity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]     Preferred exemplified embodiments of the invention will be explained hereinafter, with reference to the drawings, in which:  
         [0052]      FIG. 1  shows a diagrammatic cut top view of a pressure electrolyzer according to an exemplified embodiment of the invention;  
         [0053]      FIG. 2   a ) shows a cut side view of the pressure electrolyzer shown in  FIG. 1 , to illustrate the anolyte circuit;  
         [0054]      FIG. 2   b ) shows a sectional view corresponding to  FIG. 2   a ) to illustrate the catholyte circuit;  
         [0055]      FIG. 3  shows a cut plan view of a part of the electrolytic-cell block, to demonstrate the most important components of the electrolytic cells;  
         [0056]      FIG. 4  shows an enlarged view of the detail marked with the reference Z in  FIG. 1 , to explain the manner in which the flow is led in the anolyte circuit and in the catholyte circuit,  
         [0057]      FIGS. 5   a ) to  c ) are detailed illustrations on different scales to demonstrate the manner in which the electrolytic cells are sealed at the housing of the electrolytic-cell block; and  
         [0058]      FIG. 6  is a diagrammatic block illustration which includes additional components for the filling of the pressure electrolyzer with feed water. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0059]     In  FIG. 1 , a pressure electrolyzer, which is designated as a whole by the reference number  1 , comprises a pressure vessel  2 , in which is arranged an electrolytic-cell block  3  which is composed of a plurality of electrolytic cells  4  combined in the form of a stack.  
         [0060]     As is shown more clearly in  FIG. 3 , in which two electrolytic cells  4  and their essential components are illustrated, an electrolytic cell  4  comprises an anode  11  and a cathode  12 , which are separated from each other by a diaphragm  13  which is arranged therebetween. A bipolar metal sheet  14  is disposed between two adjacent electrolytic cells  4 , by means of which bipolar metal sheet the cell cavity on the anode side of the one electrolytic cell  4  is separated from the cell space on the cathode side of the adjacent electrolytic cell  4 , while simultaneously being maintained in electrical contact with each other. For information regarding the further design of the elctrolytic-cell block  3 , reference is made, for example, to German publication DE 197 81 442 C1. This shows, in particular, the manner in which it is possible to feed current into the cell block, what voltage is applied to the end plates and to the pressure vessel, and the manner in which the electrical insulation is ensured.  
         [0061]     Referring again to  FIG. 1 , it will be seen that two chambers  7 ,  8  are formed between a merely roughly indicated housing  5  of the electrolytic-cell block  3  and the pressure vessel  2 , which chambers are separated from each other by separating walls  9 ,  10  which extend between the housing  5  and the pressure vessel  2 . The two chambers  7 ,  8  which are separated from each other constitute a part of the electrolytic-circuit system of the pressure electrolyzer, with one of the chambers  7 , which is separated from the other chamber, constituting a part of an anolyte circuit and the other of the chambers  8 , which is separated from said one chamber, constituting a part of a catholyte circuit. As is diagrammatically shown by respective arrows in  FIG. 1 , the reaction products of the electrolytic process taking place in the pressure electrolyzer, being oxygen from the anolyte circuit in the separated chamber  7  and hydrogen from the catholyte circuit in the separated chamber  8 , are collected and drawn off and are, in each case, replaced by a corresponding quantity of fresh water.  
         [0062]     As is further shown diagrammatically in  FIG. 1 , additional essential components of the electrolytic-circuit system, for instance electrolyte coolers  19 ,  20 , are arranged in the separated chambers  7 ,  8 . The pressure vessel  2  has the shape of a cylinder in which the electrolytic-cell block  3  is arranged with its longitudinal axis parallel with the cylinder axis. The separating walls  9 ,  10  extend between the housing  5  of the electrolytic-cell block  3  and the pressure vessel  2  parallel with the axis of the cylinder.  
         [0063]     As will be seen in  FIG. 4  showing the enlarged Detail Z of  FIG. 1 , together with  FIG. 3 , the housing  5  of the electrolytic-cell block  3  is composed of a plurality of cell frames  15 ,  16  of the electrolytic cells  4 , which cell frames are stacked one on top of the other. In this regard, its own anode-cell frame  15  is provided, in each case, adjacent to each anode  11 , and its own cathode-cell frame  16  is provided, in each case, to each cathode  12 .  
         [0064]     As is shown in  FIGS. 2   a ) and  b ), in which the pressure electrolyzer  1  of  FIG. 1  is, in each case, illustrated in a vertical sectional view, passages  17   a, b  are provided in the anode cell frames  15  to produce a connection between the anode side and the chamber  7  constituting a part of the anolyte circuit, cf.  FIG. 2   a ), and the cathode cell frames  16  comprise passages  18   a, b  which produce a connection between the cathode side and the chamber  8  constituting a part of the catholyte circuit.  
         [0065]     As will also be seen in  FIGS. 2   a ) and  b ), the space defined by the housing  5  of the electrolytic-cell block  3 , the longitudinal direction of which is arranged horizontally, and by the pressure vessel  2 , is subdivided vertically and in the longitudinal direction of the electrolytic-cell block  3  into the two chambers  7 ,  8  which are separated from each other. The passages  17   a, b , which produce a connection between the anode  11  and the anolyte circuit, are in communication, in each case, on the upper side, respectively the bottom side of the anode cell frames  15  with the one of the chambers  7 , which is separated from the other chamber and constitutes a part of the anolyte circuit, and the passages  18   a, b , which produce a connection between the cathode side and the catholyte circuit, are in communication, in each case, on the upper side, respectively the bottom side of the cathode cell frames  16  with the other of the chambers  8 , which is separated from said one chamber and constitutes a part of the catholyte circuit. The space in the pressure vessel  2  above the electrolytic-cell block  3 , which is divided axially by the separating wall  9 , serves chamber  7  on the left-hand side of the separating wall  9 , for the separation of oxygen, and chamber  8  on the right-hand side of the separating wall  9 , for the separation of hydrogen. It is possible to arrange gas-coalescing aids in the form of hydrophobic knit fabrics, ultrasonic transmitters, flow-deflecting means and baffle plates in the gas-separating region above the electrolytic-cell block  3 . The electrolyte circuit on the two sides, i.e. in the anolyte circuit through the chamber  7  and in the catholyte circuit through the chamber  8 , in each case takes place automatically as a result of a gas lift effect, due to the gases rising in the electrolytic cells  4  and due to the lower density of the electrolyte/gas mixture in the cells relative to the degassed electrolyte columns outside in the separated chambers  7 ,  8 . If required, it is possible to provide an electrolyte-circulating pump, but this is not illustrated. One of the separating walls  10  is arranged below the electrolytic-cell block  3  and simultaneously constitutes a mechanical support for said block. The pressure vessel  2  thus serves as a supporting structure for the electrolytic-cell block  3 .  
         [0066]     As is shown by the  FIGS. 5   a ) to c) in illustrations on different scales, the anodes  11 , which are, in each case, electrically connected to an anode electrode, and the cathodes  12 , which are, in each case, electrically connected to a cathode electrode, are surrounded, in each case, by a sealing element  21 ,  22 , which extends about their periphery and forms a lateral end of the electrolytic cell  4  in the region of the anode  11 , respectively of the cathode  12 . The sealing elements  21 ,  22  are sealingly fitted between the cell frames  15 ,  16  of an electrolytic cell  4 , i.e. between the anode cell frame  15  and the cathode cell frame  16 .  
         [0067]     Available in each case to the anode  11  and the cathode  12  of an electrolytic cell  4  is its own separate sealing element  21 , respectively 22, and said sealing elements  21 ,  22  are jointly sealingly fitted between said cell frames  15 ,  16  of a cell  4 , the diaphragm  13 , which separates the anode  11  and the cathode  12  of an electrolytic cell  4  from each other, being sealingly fitted between the sealing elements  21 ,  22 . As a result hereof, the sealing elements  21 ,  22  also form a lateral end of the electrolytic cell  4  on the diaphragm  13 . The sealing elements  21 ,  22  are provided to extend in the form of frames about the periphery of the anode  11 , respectively of the cathode  12 .  
         [0068]     As the enlarged illustration in  FIG. 5   c ) shows, the sealing elements  21 ,  22  are designed to be led about the edge of the anode  11 , respectively the cathode  12 , from the front side to the rear side thereof.  
         [0069]     As is shown in  FIGS. 5   b ) and c), the sealing elements  21 ,  22  are provided with a plurality of sealing lips  41 ,  42 , which extend along, respectively parallel to the periphery of the anode  11 , respectively of the cathode  12 , and which project in a direction perpendicular to the flat side of the anode  11 , respectively of the cathode  12 , and which sealingly rest against a sealing face  45  of the anode cell frame  15 , respectively a sealing face  46  of the cathode cell frame  16 , with said sealing faces  45 ,  46  being designed to extend parallel to the flat side of the anode  11 , respectively of the cathode  12 .  
         [0070]     The sealing elements  21 ,  22  are produced from an elastomer and are formed by extrusion-coating of the edge of the anode  11 , respectively of the cathode  12 .  
         [0071]     As  FIGS. 5   a ) to c) further show, the bipolar metal sheet  14 , which separates two adjacent electrolytic cells  4  from each other, is also sealingly fitted between the cell frames  15 ,  16  of two adjacent electrolytic cells  4 , i.e. between the anode cell frame  15  of the one electrolytic cell  4  and the cathode cell frame  16  of the adjacent electrolytic cell  4 .  
         [0072]     The bipolar metal sheet  14  is encircled by a sealing element  24  which extends in the manner of a frame about its periphery and which constitutes, in the region of the bipolar metal sheet  14 , a lateral end of the electrolytic cell  4  and is sealingly fitted between the cell frames  15 ,  16  of the adjacent electrolytic cells  4 .  
         [0073]     As is shown in the enlarged illustration of  FIG. 5   c ), the sealing element  24  is arranged on one side of the bipolar metal sheet  14  and is provided with a plurality of sealing lips  44 , which extend along, respectively parallel to the periphery of the bipolar metal sheet  14  and which rest sealingly against the bipolar metal sheet  14  itself. The sealing lips  44  project in a direction perpendicular to the flat side of the bipolar metal sheet  14 . The cell frames  15 ,  16 , between which the bipolar metal sheet  14 , together with its sealing element  24 , is fitted each have sealing faces  47 ,  48  which are designed to extend parallel to the flat side of the bipolar sheet  14  and against which the sealing element  24 , respectively the bipolar metal sheet  14 , rest sealingly. The sealing element  24  is produced from an elastomer.  
         [0074]     Referring now to  FIG. 6 , an advantageous arrangement  30  for filling the pressure electrolyzer  1  with feed water and a corresponding method will be described. A water-supply tank  31  is provided and said tank is connected to a feed-water source via a feed-water filling pipe  32  comprising a feed-water fill-up valve  33 , and to a lower region of the separated chamber  8  via a feed-water inflow pipe  34  comprising a feed-water inflow valve  35 , and is further connected to an top region of the separated chamber  8  via a pressurizing pipe  36  comprising a pressurizing valve  37 . In addition, it is possible for the water-supply tank  31  to be evacuated relative to the atmosphere via a ventilation pipe  38  comprising a ventilation valve  39 . The top region of the separated chamber  8 , which is in communication with the pressurizing pipe  36 , is filled during operation of the pressure electrolyzer  1  with pressurized hydrogen as the product gas. The method for filling the pressure electrolyzer  1  includes the following method steps:  
         [0075]     a) A first step involves a pressure-free filling of the water-supply tank  31  via the feed-water filling pipe  32  with the opened feed-water fill-up valve  33 , while the ventilation valve  39  is open and the pressurizing valve  37  is closed and the feed-water inflow valve  35  is closed.  
         [0076]     b) The ventilation valve  39  and the feed-water fill-up valve  33  are then closed.  
         [0077]     c) Then the pressurizing valve  37  is opened, so as to bring the water-supply tank  31  via the pressurizing pipe  36  to the pressure prevailing in the pressure vessel  2 .  
         [0078]     d) Finally, the feed-water inflow valve  35  is opened, so as to admit metered quantities of feed water from the water-supply tank  31  into the pressure vessel  2 .  
         [0079]     e) The filling operation is then concluded with the closing of the feed-water inflow valve  35 .  
         [0080]     It is possible for the metered addition of the feed water from the water-supply tank  31  into the pressure vessel  2  to take place under the effects of gravity, with the water-supply tank  31  being arranged spatially above the pressure vessel  2 . By way of alternative, it is also possible for the metered addition of the feed water from the water-supply tank  31  into the pressure vessel  2  to be carried out by means of a feed-water pump included in the feed-water inflow pipe  34 .  
         [0081]     During operations, different electrolyte concentrations set in on the anolyte side and on the catholyte side, as a result of which there is an osmotic pressure buildup with a rising electrolyte level on the cathode side. The difference in the concentrations is caused by a surplus of hydroxyl ions on the cathode side, as a result of which water flows from the anode side to the cathode side. Although an equalization of the concentrations between the catholyte and the anolyte is, in principle, possible via the cell diaphragms, a complete equalization is, as a rule, not possible due to the limited porosity of the cell diaphragms. It is therefore possible to provide for the equalization of the concentrations to be brought about by an additional charge of feed water to the catholyte. It is possible for the equalization to be carried out through a bore or a membrane in the separating wall  9 ,  10  at the foot of the electrolytic-cell block, or through a connecting pipe disposed on the outer side of the pressure vessel  2  and connecting the anolyte and catholyte chambers. For adjusting the flow, it is possible to provide an adjustable valve in the connecting pipe. The connecting pipe is preferably provided at a geodetically low point, so as to prevent gas-charged feed water from being exchanged.  
         [0082]     In the event of an inadequate insulation of the current-carrying parts of the electrolyzer, it is possible that undesirable shunt currents occur, which must be prevented. For the electrical insulation, it is possible for the interior of the pressure vessel  2  to be provided with a non-conductive lining, e.g. a coating, or it is possible for the pressure vessel  2  itself to be manufactured from a non-conductive material. It is also possible for the anode cell frames and the cathode cell frames to be electrically insulated relative to the interior of the pressure vessel  2 . In this regard, it is possible for the insulation to be composed of a material which, simultaneously, protects the cell frames against the corrosive action of the electrolyte. Furthermore, it is possible for the insulation to be composed of insulating shells, which constitute the chambers for the anolyte and the catholyte and, accordingly, encapsulate the chambers defined by the outer side of the electrolytic-cell block and the inner side of the pressure vessel  2 .