Abstract:
An electrolyte cell includes a membrane ( 114 ) supporting frame ( 104 ) which having an aperture ( 180 ) having a stepped sidewall, including a peripheral sealing ledge ( 182 ) in which is set a seal ( 184 ), a membrane ( 114 ) whose periphery is urged against the seal ( 114 ) by a subframe ( 202 ) mounted in the aperture ( 180 ), the sub-frame ( 202 ) being provided with vertically extending stand-offs ( 218 ) at each corner so as to define a cavity partially bounded by the frame ( 104 ) and sub-frame ( 202 ) at the top of bottom of the aperture ( 180 ), the top and bottom edges of the sub-frame ( 202 ) being provided with a plurality of through-holes ( 216 ).

Description:
This invention relates to an electrolytic cell and in particular, but not exclusively, an electrolytic cell for the production of chlorine gas by electrolysis of hydrochloric acid. 
     BACKGROUND OF THE INVENTION 
     A known design of such a cell is a series of planar electrodes suspended in a circulating electrolyte across which a voltage is applied. A membrane is supported to cover each electrode to provide separation of the hydrogen and chlorine gas produced by the electrolysis of the electrolyte, which gases are then separately extracted from the cell. 
     The heat produced by the electrolysis process is removed from the cell by the circulation of the electrolyte but will still subject the cell components to a range of operating temperatures in a given work cycle. 
     Such a stack of electrode/membrane components has been formed by stacking a series of frames interposed between the electrodes and membranes to form sealed interfaces with them, and to form common manifolds for transporting the electrolyte to and from the electrodes and membranes of the cell sealing being obtained by applying pressure to the stack by clamping them together. A disadvantage of this approach is that all the seals are, in effect, fully formed at the same time as the pressure is applied to the stack and failure of one seal can mean having to reassemble a large part or all of the structure. Particular difficulty is associated with the formation of the manifold seals a construction requires the components to be manufactured to close dimensional tolerances. Thermal cycling also introduces physical stresses that can prejudice seal security during use of the cell. 
     Such cells include one or more large-area, thin membranes with no ability to support themselves which must be supported in the cell so as to allow flow through the membrane but not to by-pass it. Provision must also be made to provide flow paths for electrolytes to both sides of the membrane which ensure that the flow of the electrolytes is evenly spread across the area of the membranes and so the area of the electrodes of the cell. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a membrane-supporting frame assembly for an electrolytic cell which is readily assembled as part of an electrolytic cell, more easily and reliably sealable in the cell and with simplified electrolyte flow distribution arrangement. Accordingly, the present invention provides a membrane-supporting frame assembly including a frame which has an aperture having a stepped sidewall, including a peripheral sealing ledge in which is set a seal, a membrane whose periphery is urged against the seal by a sub-frame mounted in the aperture, the sub-frame being provided with vertically extending stand-offs at each corner so as to define a cavity partially bounded by the frame and sub-frame at the top of bottom of the aperture, the top and bottom edges of the sub-frame being provided with a plurality of through-holes, and at least one through-hole through the frame to provide fluid communication through the frame to the cavities. 
     On assembly of the membrane-supporting frame assembly in an electrolytic cell, electrode plates sandwich the frame assembly pressing the sub-frame onto the sealing ledge to seal the periphery of the membrane to the sealing ledge. At the same time the sub-frame defines cavities top and bottom for the collection and distribution of electrolyte with an even flow pattern with simple drillings in the sub-frame with the flow path through the frame being reduced to a single entry and exit port thereby providing savings in both material and machining costs compared to designs requiring multiple through-holes through the frame. 
     Conveniently, the sub-frame is provided with a plurality of membrane supports which are engagable with the membrane which suspend the membrane on the frame prior to mounting of the sub-frame in the aperture. Preferably, the sub-frame is engagable with the membrane supports to positively locate the sub-frame in the aperture. 
     The frame preferably includes a continuous seal circumscribing the outside of the aperture at the front and back of the frame, most preferably these continuous seals are aligned. 
     The sub-frame may include a vertical cross-beam to provide support to the membrane in the assembled cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: 
     FIGS. 1A and 1B are vertical, cross-sectional part views of an embodiment of the electrolytic cell including a frame assembly according to the present invention; 
     FIGS. 2A and 2B are vertical, cross-sectional, exploded part views of part of the cell of FIG. 1; 
     FIG. 3 is an end view of the membrane-supporting frame of the frame assembly of the present invention viewed in the direction A of FIG. 2A; 
     FIG. 4 is an end view of the membrane-supporting frame of FIG. 3 viewed in the direction B of FIG. 2A; 
     FIG. 5 is an isometric view of an upper connector of the cell of FIG. 1; 
     FIG. 6 is an end view of an electrode of the cell of FIG. 1; 
     FIG. 7 is an end view of a sub-frame of the frame assembly of FIG. 1; 
     FIG. 8 is an end view of a membrane of the cell of FIG. 1; 
     FIG. 9 is a top view of the sub-frame of FIGS. 3 and 4; 
     FIG. 10 is a cross-sectional view of the sub-frame coupling frame taken in the direction X—X of FIG. 7; and 
     FIG. 11 is a cross-sectional view of the membrane-supporting frame taken in the direction XI—XI of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, an exemplary embodiment of an electrolytic cell  100  according to the present invention includes a series of three membrane-supporting frames  102 , 104 , 106  each associated with a respective electrode assembly commonly designated  108  and a membrane commonly designated  114 . Embodiments may be constructed with only two such frames or many more such frames and certainly cells with up to  25  frames are considered practicable with the present invention. 
     Each frame  102 ,  104 ,  106  has four through-holes with common designations  120 ,  122 ,  124 ,  126  two of which are shown in FIGS. 1 and 2, the upper two through-holes  120 ,  122  being of larger diameter than the lower two through-holes  124 ,  126 . Each of through-holes  120 ,  122 ,  124 ,  126  is surrounded by a respective annular recess  128 ,  130 ,  132 ,  134  in the frame  102 ,  104 ,  106 , with eight through-holes,  136 , equally spaced round the base of each recess. The through-holes  120 ,  122 ,  124 ,  126  and respective surrounding annular recesses  128 ,  130 ,  132 ,  134  together define a respective circular wall  138 ,  140 ,  142 ,  144  which is formed to stand proud of the adjacent planar surface of the frame  102 ,  104 ,  106 . 
     Two larger diameter annular coupling members  146  (as shown in FIG. 5) are attached to each frame  102 ,  104 ,  106  by bolts  150  which are undersized in holes  136 , the coupling members  146  being generally aligned with the two larger through-holes  120 ,  122 , as shown in FIG.  1 . Similarly, two smaller diameter coupling members  148  are attached to each frame  102 ,  104 ,  106  by bolts  150  which are undersized in holes  136 , the coupling member  148  being generally aligned with the two smaller through-holes  124 ,  126 , also as shown in FIG.  1 . O-ring seals  152 ,  154  set in retaining grooves in the larger and smaller coupling members  146 ,  148  seal the interface between the frames  102 ,  104 ,  106  and the coupling members  146 ,  148 . Because the through-holes  136  are oversized relative to the bolts  150 , the coupling members  146 ,  148  can, to some degree, move laterally relative to the frames  102 ,  104 ,  106  after attachment while continuing to be securely sealed together. 
     O-ring seals  156 ,  158  are set into the cylindrical inner surfaces  160 ,  162  of the larger and smaller coupling members  146 ,  148 , which surfaces are of diameters which are a push fit on the outer cylindrical surfaces  164 ,  166  of the walls  138 ,  142  of the next adjacent frame, the interface so formed being sealed by a respective seal  156 ,  158 . 
     An annular recess  168 ,  170  in each of the larger and smaller coupling members  146 ,  148 , respectively, accommodates the head of the bolts  150  of the adjacent frame with sufficient clearance to allow the above described lateral movement of the coupling members  146 ,  148  on the frames  102 ,  104 ,  106  during assembly. 
     Each frame  102 ,  104 ,  106  has a generally rectangular aperture  180  having a stepped sidewall including a peripheral sealing ledge  182  in which is set a rectangular seal  184 . The aperture  180  is circumscribed on each side of the frame  102 ,  104 ,  106  by a respective seal  186 ,  187 . 
     The top edges of the apertures  180  are both slightly arched upwards to encourage flow of the electrolyte to the respective exit through-holes from the apertures  180 . 
     A number of membrane support pegs  188  extend outwardly from the sealing ledge  182  above the seal  184  on which the membranes  114  (see FIG. 8) are temporarily supported during assembly of the cell by inserting them through matching holes  192  in the membrane  114 . 
     Electrode assemblies  108  include an electrode back plate  196  dimensioned so as to seal to the frame seals  186  and  187  in the assembled cell and which supports an expanded metal electrode mesh  198  on supports  200  so it is positioned adjacent a membrane  114  of the assembled cell. 
     A generally rectangular, open sub-frame  202  with cross-member  204  is dimensioned to fit within the aperture  180  and so as to sit on the sealing ledge  182  of each frame  102 ,  104 ,  106  and urge the membrane  114  into sealed relationship with the seal  184  set in the seal ledge  182  when pressed by an electrode plate  196 . 
     An electrolytic cell sub-unit is defined between the consecutive pairs of electrode plates  196  sealed to each side of a given frame  102 ,  104 ,  106 , the aperture  180  of the frame of each such cell being divided into catholytic and anolytic cell sections by the respective membrane  114  supported by within a frame  102 ,  104 ,  106 . 
     The catholyte and anolyte are circulated to the electrolytic cell subunits by respective common manifolds  124  and  126  and from the electrolytic cell by respective common manifolds  120  and  122 , which are of larger diameter than the manifolds  124  and  126  to handle the additional volume due to the gases generated by the cell during its operation. The electrolytes are passed to the aperture  180  of a given frame by pipes  206 ,  208 , and from the aperture by pairs pipes  210  and  212  all coupled to a respective conduit passing through the frame to the respective manifold  120 ,  122 ,  124 ,  126 . Two exit pipes being provided, in view of the additional volume to be removed from the frame compared to what is input into the frame. 
     The pipes  210  and  212  are coupled to the through-holes in the frame which enter the manifolds  120  and  122  towards their tops so as to electrically isolate the acid entering a manifold from liquid already present. 
     Referring to FIG. 11, a catholyte input conduit  214  passes generally vertically from the lower edge of each frame  102 ,  104 ,  106  to the catholyte cell side of each membrane  114  at the lower inner edge of the frame aperture  180  and is coupled to input pipe  206 . A pair of output conduits (not show) in the upper edge of the frame are coupled to one of the output pipes  210 . 
     Each sub-frame  202  has a series of through-holes  216  through the upper and lower edges of the sub-frame  202 , as shown in FIG. 10, the sub-frame  202  being provided with stand-offs  218  so when the sub-frame is mounted in the aperture  180  of a frame  102 ,  104 ,  106 , a cavity is formed for the distribution and collection of the catholyte to or from the pipes  200  and  210  respectively. Each sub-frame  202  is provided with a number of drillings (not shown) which engage with the membrane locating pins  188  of the frame. On pushing home the sub-frame  202  the membrane  114  is pushed against the seal  184  and when the electrolytic cell stack is closed up the seal is held together by an adjacent frame. The centre bar  204  of the sub-frame  202  is sufficient to hold the membrane  114  against the mesh electrode  198 . 
     Referring now to FIGS. 4 and 11, covered recesses  220 , formed by capping grooves previously milled into each frame  102 ,  104 ,  106 , are coupled via through-holes (not shown) in the frame  102 ,  104 ,  106  to pipes  208  and  212 . The recesses  220  are in fluid communication with the interior of the aperture  180  of the frame  102 ,  104 ,  106 , via a number of through-holes  214 . The covered recesses  220  distribute and collect the anolyte from and to the pipes  208  and  212  from the aperture  180  of the frames  102 ,  104 ,  106 . 
     The provisions of the many through-holes to feed the electrolytes to the membrane ensures the flow of the electrolytes are evenly spread across the area of the electrodes. 
     The seals  184  and  186  have, in this embodiment, a Shore hardness of 60 and 80, respectively so the outer seal determines the degree of sealing. The inner seal  184  is not fully clamped up but this is not important as small leaks across this seal  184  are not important. 
     All the seals of the cell may be covered with a suitable grease, for example a fluorocarbon grease. 
     Referring to FIG. 1, the electrolytic cell includes an end plate  240  which presses four manifold capping members  242  and an electrode plate  196  (but with no mounted electrode mesh), the latter by means of an interposed insulating plate  243 , against the end frame  102  of the stacked frames. The capping members  242  are as the coupling members  146 ,  148  on one side so they can seal similarly to the adjacent frame  102  but each has a cylindrical recess rather than a through-hole thereby sealing the end of the manifolds. 
     The other end of the cell assembly includes a plate  248  which is as the frames  102 ,  104 ,  106  at the manifold region but with a flat central section which serves to press an electrode plate  196  against the frame  106  to seal with it when itself pressed by an endplate  249  abutting the central portion of the plate  248 . 
     The manifolds are completed by end plates  244 ,  246  of appropriate diameter fastened to the plate  248  in the same manner the coupling members  146  are attached to the frames  102 ,  104 ,  106 , which end plates include similar parts  248  and  250  for the flow of the electrolytes to and from the various manifolds. 
     In this embodiment the frames are of PVDF and are about 990 mm wide, 1220 mm high and 35 mm thick. 
     The electrode assembly  108  may be constructed of any suitable materials. In the illustrated embodiment it is constructed as a sandwich of materials. The cathode side of plate  196  is of Hastelloy, the centre supports  200  are aluminium and the anode  198  is coated titanium mesh supported on a titanium plate side of plate  196 . 
     Referring to FIGS. 3,  4  and  6 , the frames  102 ,  104 ,  106  and the electrode  194  have laterally extending shoulders  230 ,  232  which can rest on suitably distance support bars to facilitate assembly, each new component being slid up to the already assembled components. 
     As already described, the manifold seals are fully formed during assembly. The electrode frame seals  186 ,  187  and membrane/frame seals  184  are fully formed by clamping the assembly together by pressing laterally extending pressure beams  234  (see FIG.  1 ), generally aligned with the transverse portions of the electrode/frame seals  186 ,  188 . 
     The electrolytic cell operates as follows. 
     A catholyte and anolyte, each being hydrochloric acid, are pumped into the common manifolds  124  and  126 , respectively, passed upwards either side of the membrane  114  within each frame  102 ,  104  and  106 , to exit via pipes  210  and  212  to the upper common manifolds  120  and  122 , respectively. 
     A current of between 50 and 1500 Amps is passed through the cell generating between 5 and 140 kg of chlorine gas per day for the illustrated three-frame cell and an estimated 40 to 1100 kg of chlorine gas per day for a 25-frame cell. The chlorine produced is cooled and then washed to remove as many contaminants as possible. 
     The cell is operated under vacuum to minimise leakage, hold the minimum inventory of chlorine in the system and also to allow conventional vacuum dosing into water for disinfection, the rate of production being controlled such that the chlorine is produced as required obviating the need for on-site storage of chlorine.