Electrolysis process using liquid electrolytes and porous electrodes

This process can be carried out in an electrolytic cell which is non-partitioned or partitioned by at least one separator and has at least one porous electrode. The electrolyte enters parallel to the surface of the electrode and is forced by at least one restriction point to flow through the porous electrode parallel to the flow of charge.

The invention relates to a process for the electrolysis of liquid 
electrolytes by means of porous electrodes in partitioned or 
non-partitioned electrolytic cells. The process is suitable for reactions 
which evolve gas and for reactions which do not evolve gas. It can be used 
with electrolytes of high conductivity and low conductivity, for example 
in the electrolysis of alkali metal chlorides and in the removal of toxic 
metal salts in the ppm range. 
As is known to anyone skilled in the art, the overvoltage at an electrode 
is a function of the current density. Attempts are therefore made to 
employ electrodes having a large surface area, for example perforated or 
porous electrodes. 
If the electrolyte has a high conductivity, the electrodes are frequently 
immersed in the electrolyte and are wetted on both sides or they are 
subjected to the electrolyte only on one side. However, the large surface 
area of porous electrodes can thereby only be utilized in part, because 
differences in concentration build up as a result of the reaction, and the 
exchange of material is limited. 
If the electrolyte has a low conductivity, solid bed electrolyzers, such as 
are described, for example, in Chem.-Ing.-Techn. 55 (1983) No. 1, pages 
23-30, are frequently employed. In partitioned cells the electrolyte is 
allowed to flow through a thick electrode of coarse pores perpendicularly 
to the flow of charge. When the electrode material is utilized in an 
optimum manner, this results, under conditions of limiting flow, in an 
electrode which becomes thicker in the direction of flow. Adjusting or 
indeed optimizing the space-time yield by controlled variation of the flow 
rate within the electrode can only be effected by altering the width of 
the electrode. This is technically unsatisfactory. A further disadvantage 
is the obligatory coupling of electrolyte flow and conversion, which 
requires a very exact design. As a result of the long path of flow, the 
solid bed cell described is limited to relatively coarse particles. 
The object of the invention therefore consists in making better use of the 
internal surface area of porous electrodes and avoiding the disadvantages 
described. 
An electrolysis process for partitioned and non-partitioned cells having at 
least one porous electrode is therefore proposed, wherein the electrolyte 
enters parallel to the electrode surface and is forced by at least one 
restriction point to flow through the porous electrode at least partially 
parallel to the flow of charge. 
In accordance with a further embodiment of the invention, the electrolyte 
can also be forced by several restriction points to flow at least 
partially parallel to the flow of charge several times. The distances 
between the restriction points in this embodiment can be so chosen that 
the electrolyte flows through electrodes of ever-narrowing cross-section 
or ever-widening cross-section. The electrolyte can also be forced, on its 
flow path, to flow through sections of electrode having different 
properties, for example through thicker sections of electrode or electrode 
sections having a different pore size. 
A further advantageous embodiment of the invention, which is also suitable 
for reactions evolving gas, consists in the electrolyte, after flowing 
through the porous electrode, flowing down at the surface in a thin layer 
under the action of gravity and forming a phase boundary to a gas space. 
As it flows down, the electrolyte can at the same time wet a perforated 
counter-electrode. In this way, gas bubbles formed at this 
counter-electrode can give up their gas content by a short route to the 
immediately adjacent gas space. In order that the flow through the porous 
electrode should be as uniform as possible, the pressure drop should be 
many times the hydrostatic pressure. 
Porous electrodes are to be understood as meaning electrodes having a large 
internal surface area, for example piled particles, loose or compressed 
fiber fleeces or grids which can lie one on top of another in several 
layers. Porous electrodes are also known by the name "gas diffusion 
electrodes". Electrodes of this type can also assume the function of a 
diaphragm as a result of the special layer construction. The electrodes 
can be flat or curved. The electrolytic cells can be non-partitioned or 
can be partitioned by separators, such as, for example, ion exchange 
membranes or diaphragms. The process can be operated with various 
counter-electrodes, specifically with perforated, solid and porous 
electrodes. The arrangement of the electrodes within the space and the 
direction of flow of the entering electrolyte can be of any desired type. 
For reactions which evolve gas, it is preferable to have only one 
restriction point, so that the gas and the electrolyte pass into the rear 
space behind the electrode. It is particularly advantageous in this case 
if the electrolyte can be withdrawn at the lower end of the electrode. A 
falling film which contains bubbles and which flows down in a thin layer 
under the action of gravity is then formed on the rear side of the porous 
electrode. The electrolyte can, however, also be allowed to flow from the 
rear side to the front side. If, in this case, a perforated electrode is 
arranged as the counter-electrode at a short distance from the front side 
of the porous electrode, here too the electrolyte can flow freely 
downwards while wetting both electrodes. A vertical arrangement of 
electrodes is to be preferred in layouts having a free falling film, but 
any layout having an angle to the horizontal of between 1 and 179 degrees 
is also possible. The space behind the porous electrode or the perforated 
electrode is to receive not only the electrolyte flowing down in a thin 
layer, but also the gas formed. This gas space forms a common phase 
boundary with the downward flowing electrolyte. In large electrolytic 
cells, the gas space itself need only be a few millimeters deep. This 
arrangement can also be used for reactions in which there is no evolution 
of gas, by introducing an extraneous gas into the electrolytic cell. 
Compact bipolar arrangements without bipolar separating walls can be 
constructed in this manner, since the narrow gas space acts as an 
insulator. 
In many cases partitioned cells are used in order to prevent the products 
formed from reaching the counter-electrode. If the process according to 
the invention is used, however, it is possible to dispense with this 
expensive mode of construction. It is possible in this case to employ 
non-partitioned cells in which at least one porous electrode is employed, 
the flow through which is in a direction parallel to the transport of 
charge. The flow through the porous electrode takes place substantially 
free from back-mixing. The dwell time of the electrolyte in the active 
layer of the porous electrode can be made extremely small by suitably 
choosing the layer thickness and the flow rate. This enables undesirable 
side reactions to be suppressed effectively. 
Electrolyzers according to the process proposed only differ slightly from 
the state of the art in their fundamental design. Many electrolyzers can, 
therefore, be converted in a simple manner by adding restriction strips. 
The conversion from solid electrodes to porous electrodes is also 
certainly advantageous in many cases. Advantage can be taken of this to 
save energy and to increase the space-time yield. The process also results 
in a saving of electrode material, since a greater surface area becomes 
usable per unit of mass. This also applies particularly to dilute 
solutions of electrolyte operating under conditions of limiting current. 
In this case the space-time yield of the electrode can be increased 
considerably by decreasing the diameter of the pores or of the particles. 
The electrolyte throughput and the conversion can be matched to one another 
in a simple way through the number of restriction points. Subsequent 
adjustment to changed requirements is also readily possible. Optimization 
of the flow rate in order to save further energy can be accomplished 
easily by selecting different distances between the restriction points at 
a constant electrode surface area. 
The invention is illustrated in greater detail using, as examples, FIGS. 1 
to 9. The diagrams are greatly simplified and in some cases only show 
sections of cell stacks; in some cases only a half-cell layout is 
outlined. The flow of electrolyte is indicated by arrows.

FIG. 1 shows a porous electrode 3 which is enclosed by two side walls 2 and 
11. These side walls are intended to represent a further electrode or a 
separator or a casing wall, such as, for example, a so-called bipolar 
separator. In FIG. 1 the electrolyte 1 enters from below between the side 
wall 11 and the porous electrode 3. As a result of the transversely 
arranged strip-shaped restriction points 5, the electrolyte is forced to 
flow transversely through the porous electrode 3, i.e. parallel to the 
transport of charge. In order to absorb the compressive forces acting on 
the porous electrode, it is expedient to mount distance pieces on both 
sides of the porous electrode; these distance pieces are known per se in 
electrolysis processes and are therefore not illustrated here. The 
restriction points 5 can restrict the stream of electrolyte partly or 
wholly. 
FIG. 2 shows a half-cell having a separator 6 and a so-called bipolar 
separator 8 between which a porous electrode 3 of varying thickness is 
located. The stream of electrolyte 1 entering from below is deflected 
several times by restriction points 5. In this case the restriction points 
5 are so constructed that they also screen off the individual sections of 
electrode. The arrangement is preferentially suitable for very dilute 
electrolytes. 
FIG. 3 shows a half-cell layout having a separator 6 and a bipolar 
separator 8 between which a porous electrode 3 is located. The electrolyte 
1 flowing in from below meets only one restriction point 5, so that the 
electrolyte 1 flows through the porous electrode 3 in a single pass. The 
restriction points shown are to be understood as symbols. The structure 
can, for example, be designed in such a way that, although the space 
between the separator 6 and the porous electrode 3 has an inflow, it does 
not have an outflow, and the space between the porous electrode 3 and the 
bipolar separator 8 only has an outflow and no inflow. 
FIG. 4 shows the same arrangement as FIG. 3, but turned upside down. The 
electrolyte 1 now flows from above into the space between the separator 6 
and the porous electrode 3. As the result of a decreasing amount of 
electrolyte, the distance between the porous electrode 3 and the separator 
6 can decrease toward the bottom. The lower restriction point 5 forces the 
electrolyte to flow through the porous electrode 3 parallel to the 
transport of charge. If it is then arranged that the electrolyte 1 can 
flow out freely from the space between the porous electrode 3 and the 
bipolar separator 8, a falling film which can flow out downwards in a thin 
layer through the action of gravity is formed on the rear side of the 
porous electrode 3. This arrangement is preferentially suitable for 
reactions which evolve gas. The gas bubbles formed pass, from the falling 
film as it flows down, by a short path to the phase boundary at the 
immediately adjacent gas space 10 and there liberate their gas content by 
bursting. By means of a suitable arrangement of the layers in its porous 
structure, the porous electrode 3 should preferably release the resulting 
gases on its rear side. 
FIG. 5 shows a section of FIG. 4, but with an additional diaphragm 7. This 
shows the detail of the falling film of electrolyte, with the gas bubbles 
9, flowing downwards at the rear side of the porous electrode 3. As stated 
above, the diaphragm 7 shown can be an integral part of the porous 
electrode 3. 
FIG. 6 shows a section of a non-partitioned single-pole stack of cells. The 
arrangement comprises porous electrodes 3 and perforated 
counter-electrodes 4. The electrolyte 1 entering from above is forced by 
the lower restriction point 5 to flow through the porous electrodes 3. It 
thus comes into contact with the perforated counter-electrode 4 which is 
located a short distance away. If--as already described similarly in FIG. 
4--arrangements are made for the electrolyte to flow out freely, it can 
flow down under the action of gravity with the formation of a phase 
boundary at an immediately adjacent gas space 10. The gas space 10 is 
closed at the top by the upper restriction point 5. The arrangement is 
preferentially suitable for reactions in which a gas is formed at the 
perforated electrode 4 or at the porous electrode 3. The gas can be 
removed together with the electrolyte. 
FIG. 7 shows a section of a non-partitioned stack of cells with single-pole 
connections, a porous electrode 3 and an electrode 4 which has a solid 
structure. Both sides of the electrode 4 are used as a working surface. 
The electrolyte 1 enters from below between two porous electrodes 3 and is 
deflected several times on its way by the restriction points 5. 
FIG. 8 shows a section of a non-partitioned stack of cells which has 
bipolar connections and operates only with porous electrodes 3 and 4. In 
order to prevent a short circuit on the electrolyte side, bipolar 
separators 8 are located between each cell unit. The restriction points 5 
located in this region can be electron conductors. The restriction points 
5 between two operating electrodes must, of course, be insulators. On its 
way, the electrolyte 1 entering from below between the operating 
electrodes is deflected several times by restriction points 5 in the 
manner identified. 
FIG. 9 shows a section of a non-partitioned stack of cells with single-pole 
connections, a porous electrode 3 and a porous counter-electrode 4. To 
make them more readily distinguishable, the electrodes have been given 
voltage symbols. The electrolyte 1 entering from below is forced by the 
upper restriction point 5 to flow through the two porous electrodes 3 and 
4 parallel to the transport of charge. The products formed at the anode 
and at the cathode, labeled 1a and 1b, can be removed separately, 
substantially unmixed, in order to subject them to suitable working up.