Bipolar arrangement

If bipolar membrane electrolysers are installed (Fig. 15.10), such as the ICI BiChlor, then even less floor area is required for the same production capacity. In both membrane cases the space available also depends on the chosen operating current density. Utilising the 150 kA available, nine bipolar electrolysers can operate up to 16.7 kA each. This would require 112 anodes per electrolyser to manufacture the 200 000 tonnes per year of caustic soda, utilising about 360 V of the 450 V available. With bipolar electrolyser centres of 6.5 m, including operator walkways, an area of around 60 m by 14 m or 840 m2 will be required. However, more extensive pipework modifications are required with bipolar arrangements. 

The electrodes were arranged as a stack of bipolar electrodes as shown in Figure 23. Aluminum formed at the lower electrodes in each individual cell flows concurrently together with the chlorine gas to the central vertical shaft, where the metal sinks to the bottom and the gas rises to the top. The gas movement promotes the necessary circulation of the electrolyte. Due to the compact bipolar arrangement and the short interpolar distance (10-20 mm) the electrical energy consumption was as low as 9.5 kWh/kg Al. 

In a bipolar arrangement, the sacrificial electrodes are placed between the two parallel electrodes without any electrical connection. The two monopolar electrodes are connected to the electric power source with no interconnections between the sacrificial electrodes. This cell arrangement provides a simple setup, which facilitates easy maintenance. When an electric current is passed through the two electrodes, the neutral sides of the conductive plate will be transformed to charged sides, which have opposite charge compared with the parallel side beside it. The sacrificial electrodes are known as bipolar electrodes. It has been reported that EC cell with monopolar electrodes in series connection was more effective where aluminum electrodes were used as sacrificial and iron was used as anode and cathode. And, electrocoagulation with Fe/Al (anode/cathode) was more effective for the treatment process than Fe/Fe electrode pair (Modirshahla et al. 2007). 

In a related study, Lin et al. [42] used a bipolar arrangement of the same reactor and electrode system mentioned above to compare the performance of the two configurations. The bipolar design was shown to yield higher current efficiencies for cyanide destruction at the expense of a higher power consumption. Nonuniform copper deposition on various cathodes showed that the reactions were not occurring uniformly with the bipolar arrangement. 

Figure 6 presents a sketch of a modern monopolar diaphragm cell (there are bipolar arrangements as well). 

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. 

KIFl) or four heavy chains (e.g., BlmC). Tetrameric BlmC has an unusual bipolar arrangement in which pairs of motor domains He at opposite ends of a central rod segment. 

Figure 6. Bipolar arrangement of electrodes in a channel electrolyser.

In the bipolar arrangement, the bipolar plate becomes the main constmction element, of the fuel cell stack. Besides the performance of the electrochemical components, the characteristics of the bipolar plate determine the performance of the fuel cell stack with respect to specific power and volume. As the bipolar plate is also a significant cost contributor in the fuel cell stack, an optimized bipolar plate has to fulfil a series of requirements. 

In the bipolar arrangement of elementary cells the following overall reaction occurs in the separation wall between the depolarized anode and the cathode ... 

Textures correspond to various arrangements of defects. When the isotropic liquid is cooled, the nematic phase may appear at the deisotropization point in the form of separate small, round objects called droplets (Fig. 12). These can show extinction crosses, spiral structures, bipolar arrangements, or some other topology depending on boundary conditions. Theoretical studies based on a simple model confirm the stability of radial or bipolar orientation (Fig. 5) [22]. Considerations based on improved theoretical models yield stable twisted... 

Fig. 4 Repeating unit in a fuel cell stack with bipolar arrangement...

B. Bipolar arrangement (filter-press type electrolyzer) FIGURE 5.2. Schematic of cell assemblies. 

In the monopolar orientation, up to 30 membranes, with an effective area of about 1.5 m each, can be put together to achieve a production capacity of 9.3 tons of NaOH per day (1(X)% basis) at a current density of 6kAm . The ExL (Fig. 5.28) is the bipolar version of the ExL. The bipolar arrangement is realized by pressing the nickel cathode pan onto the nickel plated back of the anode pan. Up to 60 elements can be combined in one electrolyzer, to realize a production capacity of 22 tons of NaOH per day at a current density of 7kAm . The ExL electrolyzer (dense pak) represents... 

The evaluation of the integral depends on the mode of operation of the cell. Most often the mode is one of constant current supplied to a bipolar arrangement. This results in a current density which is nominally constant and for which the electrode potential, will be a complex function of c. Direct integration is therefore not possible a numerical technique must be employed using the expression for i to evaluate appropriate values of . This is why the reaction model is an essential element of the reactor model. 

Figure 12.24. Diagram of a simple capacitor. In practice a bipolar arrangement is used.

Engineering issues of fuel cell stack and systems design will be dealt with in the following chapters. Hence, issues of, e.g., coflow or counterflow within one cell, stoichiometry and utilization of fuel and oxidant, temperature and current distribution in a fuel cell of technical scale, and, certainly, issues of stacking cells into a bipolar arrangement will not be discussed here. 

Hence, to accumulate the necessary voltage for technical applications, e.g., 200 00 V, for an electrical power train in a car, cells must be crmnected in series. Dedicated bipolar arrangements of cells have been designed and put into operation for serial connection, taking into consideration also the necessary parallel mass flow of fuel and oxidant from a manifold into each individual cell and the respective removal of the product. Such an arrangement of cells is called a fuel cell stack, combining the electrical serial connection of individual cells with a parallel connection for mass flow. 

The most common fuel cell stack design is the so-called planar-bipolar arrangement (Figure 1-2 depicts a PAFC). Individual unit cells are electrically connected with interconnects. Because of the configuration of a flat plate cell, the interconnect becomes a separator plate with two functions ... 

Use of bipolar electrodes to form an ES stack is shown in Figure 5.9. The bipolar arrangement can effectively minimize the volume of the stack and circumvent the use of additional materials and external connections. In addition, the intimate surface level connection can help overcome macroscopic resistances generated from solder joints, long interconnects, and tabs that contact only part of the collector foil. The reduction in packing material (grid weight) for a module also improves cell performance. 

In a bipolar arrangement the elements are connected in series with resultant low current and high voltage. The cathode of a cell is connected directly to the anode of the adjacent cell, as shown in Figure 55. The operation of a bipolar electrolyzer can be easily monitored by measurement of element voltages. If element upsets occur, a safety interlock system actuates the breakers (short-circuiting switches) and isolates the electrolyzer from the electric circuit. As the influx and efflux of electrolytes for the cells with different electric potential are gathered in common headers, problems of stray current may arise. 

Fig. 1 Classic bipolar arrangement in fuel cell stacks. Cells are arranged electrically in series and in parallel for the reactant gas and coolant streams...

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