NO742331L - - Google Patents

Description

Bipolar electrolysis cell.

In the past, electrolysis of brine^.to provide-

bringing chlorine took place in electrolytic cells with graphite anodes

and the work at low current density. In such previously used

cells,, erosion of graphite made it now dy end to use an inventive thickness of 2.5 cm or, more, which made

an electrode distance (the distance from center line to center line between electrodes of: equal polarity) of 8.5 and up to 10 cm is necessary. Previously used diaphragm electrolytic cells that worked with low current density with thick electrodes and a large electrode distance, had a low ratio of current per volume unit cell volume, often as low as 18 or even 9 amps per litres

cell volume.

Newer electrolytic cells use metal anodes. Such anodes have a smaller distance between electrodes of the same polarity and they can be used at higher current densities. To take advantage of the greater economy of such diaphragm electrolytics. cells, the electrolysis should take place at a high current density at

the anode, e.g. over approx. 800 amps per m 2 anode surface and

.. • 2 ■• ' preferably over 1000 amperes per m anode surface. The electrodes should also be high, usually approx. 90 cm and preferably

above. 1.20 m or more.

During electrolysis at high current density (e.g. current densities of around 1000 amperes per m anode surface) with . high electrodes (e.g. above about 1.20 m) and small distance between the electrodes (e.g. with a distance of from 0.3 to 0.6 cm between an anode and the diaphragm of the corresponding cathode) occurs more problems. Large volumes of gas are formed per unit cell volume,

something that causes the anolyte to foam. The cattle amount of gas

in the anolyte causes the IR drop in the anolyte to increase. The concentration of chloride ions in the anolyte gets a non-uniform distribution. |according to the present invention, the effect of such can

problems in electrolysis operations are reduced.

It has been shown that the problems associated with foam-forming ice in the anolyte, high electrical resistance in the anolyte, and . non-uniform chloride ion concentration can be reduced by a quick 'removal of the foam containing evolved chlorine gas from the an&lytt chamber* separating the evolved chlorine gas from the anolyte and returning the anolyte which has had the chlorine gas removed to the anolyte chamber 1 the cell.

In the electrolytic cells where this invention is particularly applicable, the electrolysis usually takes place at a high current density, e.g. over 800 amps per m and, often over 1000 amperes per m2 and between high electrodes, 90 cm, 1.20 m high and even over 1.80 m high. The electrodes in these electrolytic cells

stands closely samiSib and often has a distance between the electrodes (of- .

stand meliom an anode and the adjacent cathode) of 0.6 cm or even 0.5 or 0.3 cm. Such electrodes will have a small distance between the center lines of electrodes with the same polarity, often 7.5 cm, or less. Such electrolytic cells have large current loads

nings per volume unit total internal cell volume, often 22 amps per liter cell volume and large current loads per m internal horizontal cell surface, often 25,000 amperes or more per m<2>horizontal cell surface. Such ceifers develop large amounts of carbon dioxide per surface unit horizontal cell surface. The rapid development of this chlorine gas causes a chlorine gas foam to form in the anolyte.

According to the present invention, anolytic foam containing the gas developed at the anodes is fed to a chamber for gas separation which can be a reservoir for brine or a feed tank for brine. The foam consisting of chlorine and anolyte is separated in the separation chamber. The separation occurs above the level of the anolyte in the separation tank and is caused by simultaneously changing the momentum of the foam and leading the foam into the separation chamber. Then, the gas-free anolyte, either mixed with brine or alone, is returned from the gas silica chamber to the cell. The gas-free anolyte is introduced into the cell through

a descending tube that opens below the surface of the electrolyte in the cell. In this way, one has a effective prevention of. that gas can enter the returned anolyte and the

■ iste^e^mgjenhom a riser is forced into the chamber for gas separation. The return pipe for liquid can be placed away from the gas riser 1 in a horizontal direction thereby giving a rotating movement to the anolyte.

The apparatus which is to secrete chlorine and the method, according to the present invention, can be used both on monopolar and bipolar diaphragm cells. But since bipola^e cells

to a greater extent it has. combination, of high current densities, assume distances between the electrodes and high electrodes resulting in . high current loads per unit area of the cell's horizontal cross-section (eg more than 25,000 amperes per m 2 of the cell's horizontal cross-section perpendicular to the cell) shows the examples of this type of cells. However, the invention can also be used

on monopolar electrolytic cells operating under conditions described above where foaming and foaming are serious problems. The apparatus for chlorine excretion and the method in the present invention are also applicable with diaphragm cells

which work at high current densities, small distance between the electrodes, small distances between the center lines of electrodes of equal polarity and high graphite electrodes.

In the attached drawings you can see the following:

Fig, 1 shows the general arrangement of the interior

in an electrolyzer where the individual parts are separated for the sake of overview.

Fig. 2 is a perspective sketch which is partially cut through an electrolysis apparatus, according to the present invention. Fig. 3 is a section schematically showing the flow of electrolyte and gas 1 in the anolyte chamber. Fig. 4 shows the individual parts in a single bipolar unit. •'. Figs. 5, 6 and 7 are partially cut-away perspective* sketches of an electrolyser according to the present invention showing ..different alternative placements of the chlorine riser. -

An assembly of bipolar units forming an electrical series of bipolar cells in an electrolyser is shown in fig.; in and partially cut through in fig. 2,. ^.; and 4. Bipolar units 11, 12, 13 and 14 form bipolar cells 16, 17 and 18.

The end unit 11 constitutes a cathodic half-cell, while the end unit 14 constitutes an anodic half-cell. The intermediate bipolar units 12 and 13 are bipolar units which provide both anodic and cathodic half-cells.

In addition to the haly units at the ends 11 and 14, an electrolyzer will normally comprise at least one bipolar unit 12 and may comprise several (up to 10 or 15 or more) bipolar units 12 and 13.

In a type of bipolar electrolysis apparatus to which the present invention is applicable, each of the bipolar units is placed as bipolar unit 12, in a channel frame 110. Channel frame

110 is shown in detail in fig. 4. The channel frame 110 consists of the side walls 122 and 123 in the top 121 and the bottom 124. The channel frame 110 has different openings. Opening 116 in the side wall 123 is used to empty the catholyte chamber and thereby recover the liquid catholyte products. The opening 112 in the cylinder head 121 is used to remove gaseous cathode products.

The openings 117, 118, 119 and 219 in the side walls 122 and 123 and in the top 12l lead to the anolyte chamber. Opening 119 together with the riser for chlorine 220 is used to remove the foam in the gaseous anolyte products such as chlorine and electrolyte. Opening 117 and opening 219 together with the return pipe for anolyte .221 are used to supply brine to the anolyte chamber.

A brine reservoir 151 is located above each individual cell, such as cell 12, and is hydraulically connected to the ariolite volume within the cell by a gas riser 220 and an anolyte return pipe 221. While a single brine reservoir 151 is shown for h<y >is a single cell, a single brine reservoir 151 can serve 2 or more cells or the entire electrolysis apparatus.

.The brine reservoir, placed above the cell, and with devices

for removing the foam from the anolyte chamber and for separating the foam into liquid and gaseous components, and devices for

lead the liquid back to the cell, there is no separation tank. When the separation tank is also referred to as a brine reservoir, it is understood that the separation tank does not need to be filled with brine. When the electrolyser is in operation, the brine containing from approximately 310 to approximately 325 g/l is supplied. sodium chloride from a collecting tank which is not -<§r shown into pipe 155.

The brine feed can be divided into two streams, one of which enters the cell through pipe 153 and the other stream enters the feed tank for

brine 151. Optionally, all the brine can enter the cell through pipe 153.

The flow passing through pipe 153 enters the cell through opening 117. Opening 117, in the side wall 123«...is at a horizontal level between the cell bottom 124 and the cell top 121. The opening for the brine 117 is usually at a level between a quarter and three quarters of the vertical distance between the cell top 121 and the cell bottom 124 and preferably between one quarter and one half of the vertical distance from the cell bottom 124 to the cell top 121.

When part of the brine enters the reservoir 151, the brine entering the reservoir 151 is saturated by the chlorine gas entering the reservoir through the brine riser 220. for

return of anolyte 219.

The anolyte return downpipe 219 exits the upper surface of the anolyte in the cell sufficiently far down to prevent chlorine in the gas bubble formed between the cell top 121 and the top of the anolyte and foam from entering

in the downcomer 219. The downcomer 219 need only open slightly below the surface of the anolyte, but it will usually open more than 15 cm below the surface of the anolyte and preferably

go as far down into the anolyte as possible without washing the diaphragm 101 of the cathodes 41. In an example of the present invention

several electrode pairs, i.e. several anodes and cathodes, can be removed

and the downpipe for the return of anolyte 219 is led into the space thereby created. In this way, the downpipe can go further. than . halfway down the cell, i.e. mouth out further down the cell than half the vertical distance between the cell top 121 and the cell bottom 124. , To achieve the best results are displaced; the downpipe for the return of anolyte 219 horizontally in relation to the riser pipe for chlorine gas 220. In this way, the horizontal displacement of the supply pipe for brine 153 and opening 117" and the downpipe 219 in relation to each other and in relation to the riser pipe for chlorine gas 220 and the location of the brine feed under the upper surface of the brine cause a rotating movement in the brine.

It. the following reaction takes place at the anodes in the ano-listening tube:

The carbon dioxide released at the anode 31 bubbles up along the surface of the anode. If a perforated anode is used with two opposing surfaces separated from each other to provide one hollow channel for gas flow and flow of anolyte, the chlorine gas released at the surface of the anode as against the cathode 41 will pass through the perforation in the anode dg bubble up between the two opposite surfaces of the aaod.

The chlorine gas that bubbles up through the anolyte follows

the path shown by the dashed arrows in fig. 3bg forms a foam of chlorine and anolyte. The lifting effect which is to blame?^the liberated chlorine gas,. carries the foam up through the cell',' through opening

119 At the top of the cell and in through the riser for chlorine 220 to the reservoir for brine 151. When the riser for chlorine 220 does not extend above the surface of brine and foam in the reservoir 151, additional foam will form in the reservoir for brine

151 with recycling of the gas-filled foam back to the cell

and you will get very little gas release. The riser for chlorine 220 therefore protrudes from the bottom of the reservoir for brine. 151

at a distance corresponding to the depth of the electrolyte and foam in the reservoir 151. For best results, the chlorine gas riser 220 does not stop until 5 15 cm from the top of the brine feed tank 151.

When the riser for chlorine 220 protrudes above the electrolyte' and the foam in the feed tank for brine which. described before, will

moderate gas production. A better gas release is obtained when the foam is subjected to a change in momentum before it leaves the riser, e.g. when the flow direction of gas and

fluid changes. With a change in torque comes a change in the product of the mass and the velocity vector. With a change

by the velocity vector one means either a change in the direction of the flow or a change in flow speed. This can e.g. take place when the opening 22å on the riser ..for chlorine 220 is cut diagonally as shown in fig. 3, which causes the direction

on the foam drum is changed before it is led out of the riser 220. When the opening 222 is at an angle of from 30 - 75° in relation to the horizontal, good gas separation is obtained. Particularly good gas separation is obtained when the opening 222 is at an angle of more than 45° in relation to the horizontal.

Even more efficient gas separation is provided, especially at high flow rates, when the riser for chlorine 220 and the opening 222 are made in the form of a T-tube, a Y-tube or an elbow tube and thereby provide an even greater change in direction and moment of the foam before this leaves the riser 220. Such "fittings" on the outlet 222 of the riser 220 provide a change in direction and

torque on the foam before it leaves the riser220. An appliance

for chlorine excretion using T-tubes, Y-tubes and elbow tubes are

shown in fig. 5, 6, and 7.

You can also use other devices for the excretion of gases together with the apparatus for excreting chlorine

described according to the present invention. The foam can e.g. after it has passed through the riser 220 and the opening 222 is made to abut against plates or to pass through various forms of screens. The riser for chlorine 220 should have a cross-section of less - 2 p than 0.01 m per 1000 amperes and preferably less than 0.005 m per 1000 amps.

The chlorine cell in the present invention, which has current loads of over 22 amperes per liter cell volume and often exceed 50 amperes per liter cell volume, will have less than cav 1.6 cm cross-section on the riser for chlorine per liter cell volume and for-. incrementally less than 0.2 cm 2 in cross-section on the riser for chlorine per liter cell volume.

In this way, a satisfactory . speed of the foam in the riser. If the cross-section of the riser per 1000 amps is smaller than 370 cm 2 , the foam will percolate or bubble into the separator tank 151 rather than flow into the tank 151 and the gas separation will not be as efficient.

Furthermore, when constructing and operating cells according to the present invention, the brine reservoir or separator tank 151 should have a horizontal cross-section greater than

2 2

about. 100 cm per 1000 amperes and preferably over 130 cm per 1000 amps. Particularly preferred are reservoirs for brine or separator tanks for chlorine 151 which have a horizontal cross-

at 2

average over 190 cm per 1000 amperes, i.e. up to 370 cm per

2

1000 amperes or possibly up to 500 cm per ..1000 amperes or more.

The following reaction takes place on the electrically active surfaces in the catholytic chamber;

where Me is a metal and Me<+>is a metal ion.

The hydrogen gas bubbles back through the catholyte chamber and then into the chamber behind the catholyte and into the chamber 111 in fig. 4 and finally out through the hydrogen outlet 112 into the tube 113 shown in fig. 4 on top of the bipolar unit. The cell fluid, which contains from 120 - 160 g/l sodium hydroxide and approximately 160 - 210 g/l sodium chloride, is led out of the catholyte chamber through the opening 116 in the wall 122 and is then separated.

It is emphasized that although the invention is described with reference to specific embodiments, it is not limited

of such changes as can be carried out in accordance with the scope of the invention according to the subsequent claims.

Claims (1)

1. Electrolytic cell for electrolysis of brine at . current density of over 2.5 amps per cm horizontal surface where a foam of chlorine and electrolyte forms, grade*., is e r t in that the said cell contains: devices with a smaller cross-section than 1.6 cm per liters of cell volume to pass foam from the cell to a separator tank; devices for changing the momentum of the foam before it is fed into the separation tank; devices to then feed the foam into the 'separation' . the tank above the liquid level in it; and devices for returning the liquid from the separation tank to the electrolytic cell. 2. Electrolytic cell according to claim 1, characterized in that the said separation tank has a horizontal cross-section that is greater than 93 cm <2> per 1000 amps.;3. Electrolytic cell for electrolysis of brine at current densities of over 2.5 aiaper per cm horizontal cross-section where there. a foam of chlorine and electrolyte is formed, character i s e r t in that the said cell is equipped with: devices with a cross-section of less than 1.6 cm2 per liters of cell volume to convey foam from the cell to a separator tank; ■ devices to change the direction of the flow of foam before the foam is fed into the separation tank; devices to feed the foam into the separation tank above the level of the liquid in it; and devices to lead the liquid from the separation tank to the electrolytic cell,;4. Electrolytic cell according to claim 3, characterized in that said separation tank has a horizontal cross section that is greater than 93 cm p per 1000 amps.;5. Method for electrolysing brine~in an electrolytic cell at a current above 2.5 amperes per cm p of horizontal cell surface where a foam of chlorine and electrolyte is formed, characterized by leading the foam from the electrolytic cell through a device for removing foam with a cross-section of less than 370 ein per, 1000 ampere to eh separation tank; changes the direction of the flow of the foam before it enters the separation tank; feed the foam into the separation tank and above the liquid level therein, thereby separating the foam into a gas* and a liquid fraction and finally returning the liquid separated in this way to the electrolytic cell below the surface of the electrolyte i . formed .6. Method according to claim 5, characterized in that the said separation tank has a horizontal cross-section that is greater than 93 cm 2 per 1000 amps. f 7. Method of electrolysing a brine in an electrolytic cell at a current of more than 2.5 amperes per cm2 horizontal cell surface where a foam of chlorine and electrolyte forms; characterized by passing the foam through a device to remove foam • which has a cross-section of less than 360 cm 2pr. 1000 amperes from the electrolytic cell to a separator tank;. changes the momentum of the foam before it is fed into the separation tank; leads the foam into the separator tank above the level of the electrolyte therein and thereby separates the foam into a liquid and gas fraction; and finally leads the liquid thus separated to the electrolytic pellet below the surface of the electrolyte therein. .8. Procedure according to claim 7, character! see r t in that the said separation tank has a horizontal cross-section 2 which is greater than 93 cm per 1000 amps.