Electrolyzers with Bipolar Electrodes

With electrolyzers of this type the tank must be designed in such a way, so that it will not conduct the current. The electrodes must divide the entire space into a series of cells (compartments) which are carefully insulated. The feeding water inlet to the separate compartments must be as small as possible to reduce the current losses caused by short circuits across the electrolyte, to a bare minimum. 

This problem of an electrically non-conducting tank was solved by the firm of Elektrizitdts A.G., formerly Schuckert, by assembling it from several metallic parts joined together by non-conducting materials. 

The middle iron plates e1, e2 etc. act as bipolar electrodes, and have on both sides extended perforated sheets n. The terminal monopolar electrodes have only one sheet n each. The diaphragm frame holds the asbestos cloth. The top edges of the electrodes and diaphragm frames have angle irons a, b, which are riveted and form a flat surface beneath the electrolyte level. This surface iB 

This electrolyzer usually has 15 electrodes and 14 diaphragms and the total voltage is in the range of 28 to 30 V. 

The Schmidt type electrolyzer consists of a series of frames e, with a nickel plated iron plate inside which acts as electrode (see Fig. 74). Each frame has two holes w, at the lower comers for the inlet of the electrolyte, and two 

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Fig. 45. Schematic diagram of an open type electrolyzer with bipolar electrodes inserted into grooves in nonconductivo walls of the vessel.

When it is necessary to collect the gases liberated during the electrochemical process because they are either the product required or a troublesome byproduct, the vessel is closed by a suitable cover which usually is used to support the electrodes. In the case of the filter-press electrolyzers with bipolar electrodes the top of the frames usually serves as a cover. Eleotrolyzers with monopolar electrodes are closed by covers made of steel, concrete, cement, stoneware, etc. Seals between covers and vessels or electrodes are made of asbestos cord, bitumen, asphalt, mixture of kaolin and tar, cement, synthetic resins, etc. 

Typical pilot process conditions are Reactor 4001 steel enamel tank. Electrolyzer -10 bipolar electrodes with a surface of 0.9 m2/300 A. A suspension of 1 kmol MnS04 H20 in 535 kg H2S04 [55-60%] at 85 °C is circulated from the tank to the electrolyzer. 

Fig. 2.2 Schematics of hydrogen production using an alkaline electrolyzer unit with bipolar electrode geometry.

A method was proposed for electrolytic cadmium refining in molten CdCl2 using electrolyzer with bipolar cadmium electrode [235]. The results demonstrated that high-purity Cd could be obtained in a single electrolysis cycle. 

Fig. 79. Bamag type of bipolar electrolyzer with extended electrodes.

Filter-pross electrolyzers, which are similar to a plate and frame filter press, are sucossfully used when gases are evolved in course of the electrolysis. Some designs of these types of electrolyzers will be described in detail in the chapter dealing with the electrolysis of water. The electrolyzer consists of number of frames with bipolar plate electrodes c which are separated by diaphragms d. Framos Fig. 18. Schematic diagram of a filter-press... 

The Krebscosmo Bipolar Cell Electrolyzer BMZ 7.5 which is basically made from steel and titanium. The partition wall of the bipolar electrode is a PTFE foil. Anodes are expanded titanium sheet with noble metal coatings and the cathode structure is a perforated steel sheet. The cell units can be arranged in parallel groups of bipolar elements. Each cell element has 2.5 m of membrane area that operate at a nominal amperage of 7.5 KA. The cell block of the BMZ 7.5/64 electrolyzer consists of 64 elements with 16 series elements with 4 current paths with a nominal block amperage of 30 KA. Electrolytic performance is not disclosed (74). 

Bipolar electrolysis systems are characterized by the type of electrolyte. The proton exchange membrane (PEM) system, developed by the General Electric Compare (GE), uses as the electrolyte a thin membrane of sulfonated fiuorocaibon (Nation ) that conducts electricity when saturated with water. Electrodes are formed by depositing a thin platinum film on opposite sides of the merrtbrane to form a bipolar cell. An electrolyzer is made by stacking 50-200 cells in series, with srritably formed separators to direct the exhaust gases into charmels at the sides. Since the membrane is the electrolyte, only pine water needs to be supphed to the cell. When the cell oper-... 

Electrolyzers for the production of chlorine and sodium hydroxide, including both diaphragm and membrane cells, are classified as either monopolar or bipolar. The designation does not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyzer construction or assembly. There are many more chlor-alkali production facilities with monopolar cells than with bipolar cells. 

CEC BITAC 800 Electrolyzer. The Chemical Engineers Corporation (CEC) bipolar BITAC electrolyzer was jointly developed with Tosoh Corporation. The design follows the filter-press principle. Up to 80 bipolar electrode frames are clamped together by end plates and spring-loaded tie rods. The frames are made of special titanium alloy for the anode and nickel for the cathode. The electric current flows along the nickel pans, since the electrical conductivity of nickel is six times higher than that of titanium. Gas and electrolytes leave the cell compartment in overflow mode with little pressure fluctuation. Transparent PTFE tubes are attached at the electrolyte inlet and outlet nozzles of each element. Anolyte recirculation takes place through an external loop. 

Principles. Hydrochloric acid (22wt% HCl) is fed into the cells in two separate circuits, a catholyte circuit and an anolyte circuit. During electrolysis the concentration is reduced to ca. 17 %, and the temperature increases from 65 to 80 °C. A part of the depleted acid is separated from the catholyte stream, concentrated in the absorption plant to ca. 30 %, and fed back into the main stream. The electrolyzer is bipolar, with pairs of electrodes arranged like the leaves of a filter press. A diaphragm or membrane (e.g., Nafion 430) separates the anode compartment from the cathode compartment to prevent mixing of the gaseous products. 

Water electrolyzer units typically consist of several cells or electrodes arranged in two basic configurations, tank type operated in unipolar configuration, or filter press type operated in bipolar configuration. Tbe most common configuration, see Fig. 2.1, is the unipolar tank type where each electrode has only one polarity and all the electrodes of the same polarity are connected in parallel. The anodes and cathodes are alternately connected, with the... 

The stoneware vessel is provided with side grooves, into which bipolar graphite electrode plates between glass partitions are inserted. Each compartment thus formed is provided with bottom and side openings for the circulation of the electrolyte. The electrolyzer is placed on insulators in an outer larger stoneware vessel with a lead coil, through which the cooling water is pumped. 

Conventional hydrochloric acid electrolyzers consist of 30-36 individual cells connected in series (bipolar arrangement). The cells are formed from vertical electrode plates manufactured from graphite, between which there are diaphragms (for instance made from PVC fabric, distance to the plates ca. 6 mm). The feed with hydrochloric acid (22 wt%, identical for anode and cathode compartment) and the removal of the gases produced take place according to the filter press principle (see Chapter 2). Chlorine leaves the cell with the anolyte, hydrogen with the catholyte. 

In a bipolar electrolyzer, each electrode has both a positive and a negative face, with the positive face in one cell and the negative face in the adjacent cell. Proponents say that bipolar electrolyzers take up less floor space than unipolar ones and that they are better suited to high-pressure and high-temperature operation (which is more efficient). Their drawbacks are that they require much more precise tolerances in construction and that they are more difficult to maintain. (If one cell fails, the entire assembly has to be shut down.)... 

The technological development of electrolyzers started with a mono polar cell consisting of a cathode part and an anode part separated by a diaphragm, hi multi-cell systems, bipolar plates are used carrying the cathode material for one cell and on its backside the anode material for the neighbor cell. The functions of the bipolar plate are the continuous supply of the membrane electrode with H2 on one side and with O2 or air on the other side and the regulation of the water balance by providing moisture for the membrane on the H2 side and remove the product water on the O2 side. 

These systems were mostly non-pressurized bipolar electrolyzers produced by Bamag, Norsk Hydro, BBC/DEMAG and DeNora, whose outputs amounted to approximately 300 Nm h hydrogen. Rectangular (Fig. 11.5) and circular electrodes and cells with an active electrode area of up to approximately 3 m were used. 

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