Cathode catholyte

Cell Cathode Catholyte (Electrolyte) Conversion benzene (%) Selec- tivity (%) Current efficiency (%) Ref. 

The electrolysis was carried out in stacks of 24 cells in parallel in a filter press (Fig. 6.3). Each cell (Fig. 6.4) consisted of a cathode spacer, lead cathode, catholyte distribution block, membrane, anolyte distribution block and lead dioxide anode and will have its own catholyte and anolyte streams with compositions shown in Fig. 6.2. The catholyte distribution block includes turbulence promoters and the cell was designed to minimize the cathode—membrane gap (0.8—3.2 mm) because of the poor conductivity of the catholyte required to obtain the high selectivity for adiponitrile. The electrical connection was bipolar with 300 V appUed across the stack, i.e. about 12 V per cell, which gives a cathode current density in the range 0.4-0.6 A cm . The total cell current was 2870 A. 

As mentioned in Section 17.1, the anodic and cathodic compartments of an electrochemical cell can be separated by an ion-exchange membrane or a porous diaphragm. The division of a cell is often practiced in industrial processes, despite the additional costs, the need for additional seals and possible maintenance problems. A separator may indeed allow a more independent choice of anode/anolyte or cathode/catholyte, enable current eftkiency to be maintained due to the exclusion of redox shuttles and help to isolate electrode products or prevent the formation of explosive or toxic mixtures, for example H2-O2. However, if possible, undivided cells are preferred, as they lead to lower ohmic drops and to much simpler technologies. 

To achieve this pH, a diaphragm is needed to separate the electrolyte near the cathode (catholyte) and the electrolyte near the anode (anolyte). The anolyte is acidified during operation as water decomposes to oxygen and protons as the anodic reaction [2]. The need for a diaphragm adds to the operational complexity and difficulty of conventional electro winning cells. 

The electrolysis was performed in stacks of 24 cells mounted in a filterprcss (Fig. 6 J) and the anolyte and catholyte flows were each in parallel from single reservoirs. Each cell is a relatively complex structure with lead cathode, catholyte chamber, turbulence promoter, membrane, anolyte chamber and lead alloy anode. The celt must be gasketed and each electrolyte chamber must have pipework attached at inlet and outlet and a flow distributor to give an even electrolyte flow, Even so, the cell had to be designed to minimize the interelectrode gap since energy consumption was a consideration and the catholyte and membrane had relatively high resistances. Moreover, rapid dismantling and replacement of membranes was essential... 

Small amounts of propionitrile and bis(cyanoethyl) ether are formed as by-products. The hydrogen ions are formed from water at the anode and pass to the cathode through a membrane. The catholyte that is continuously recirculated in the cell consists of a mixture of acrylonitrile, water, and a tetraalkylammonium salt the anolyte is recirculated aqueous sulfuric acid. A quantity of catholyte is continuously removed for recovery of adiponitrile and unreacted acrylonitrile the latter is fed back to the catholyte with fresh acrylonitrile. Oxygen that is produced at the anodes is vented and water is added to the circulating anolyte to replace the water that is lost through electrolysis. The operating temperature of the cell is ca 50—60°C. Current densities are 0.25-1.5 A/cm (see Electrochemical processing). 

Separation of the anode and cathode products in diaphragm cells is achieved by using asbestos [1332-21 -4] or polymer-modified asbestos composite, or Polyramix deposited on a foraminous cathode. In membrane cells, on the other hand, an ion-exchange membrane is used as a separator. Anolyte—catholyte separation is realized in the diaphragm and membrane cells using separators and ion-exchange membranes, respectively. The mercury cells contain no diaphragm the mercury [7439-97-6] itself acts as a separator. 

Fig. 25. OxyTech MGC electroly2er a, membrane b, anode assembly c, manifold spacer d, anolyte outlet e, catholyte outlet f, bulkhead g, brine inlet h, NaOH inlet i, insulating channel j, bulkhead insulator k, interface material 1, cathode assembly m, interceU bus n, tie rod o, current distributor p,...

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

A.sahi Chemical EHD Processes. In the late 1960s, Asahi Chemical Industries in Japan developed an alternative electrolyte system for the electroreductive coupling of acrylonitrile. The catholyte in the Asahi divided cell process consisted of an emulsion of acrylonitrile and electrolysis products in a 10% aqueous solution of tetraethyl ammonium sulfate. The concentration of acrylonitrile in the aqueous phase for the original Monsanto process was 15—20 wt %, but the Asahi process uses only about 2 wt %. Asahi claims simpler separation and purification of the adiponitrile from the catholyte. A cation-exchange membrane is employed with dilute sulfuric acid in the anode compartment. The cathode is lead containing 6% antimony, and the anode is the same alloy but also contains 0.7% silver (45). The current efficiency is of 88—89%, with an adiponitrile selectivity of 91%. This process, started by Asahi in 1971, at Nobeoka City, Japan, is also operated by the RhcJ)ne Poulenc subsidiary, Rhodia, in Bra2il under Hcense from Asahi. 

A simple electrochemical flow-through cell with powder carbon as cathodic material was used and optimized. The influence of the generation current, concentration of the catholyte, carrier stream, flow rate of the sample and interferences by other metals on the generation of hydrogen arsenide were studied. This system requires only a small sample volume and is very easily automatized. The electrochemical HG technique combined with AAS is a well-established method for achieving the required high sensitivity and low detection limits. 

A similar situation arises when a vertical metal plate is partly immersed in an electrolyte solution (Fig. 1.48c), and owing to differential aeration the upper area of the plate will become cathodic and the lower area anodic. With time the anodic area extends upwards owing to the mixing of the anolyte and catholyte by convection and by the neutralisation of the alkali by absorption of atmospheric carbon dioxide. 

Thus if the cathodic and anodic sites are separated from one another by the geometry of the system and if the solution is relatively stagnant the pH of the anolyte will decrease whereas that of the catholyte will increase. 

In neutral solutions the application of cathodic polarisation prevents crack initiation and this could be taken to indicate that hydrogen embrittlement is not the operative mechanism, since the discharge and entry of hydrogen might be expected to fracture the specimen more readily. The beneficial effect of cathodic polarisation has been interpreted , however, to result from more rapid film repair in the alkaline catholyte generated by the cathode reaction. The film serves as a barrier to rapid hydrogen entry. Consistent with this is the observation that in an environment of low pH (e.g. 10 N HCl) where film formation would not be expected, cathodic polarisation has no effect upon crack propagation. 

Half-cell one half of an electrochemical cell, comprising one electrode (anode or cathode) and its immediate electrolyte (anolyte or catholyte). 

Catholyte. The electrolyte in the isolated cathode compartment may be either the same supporting electrolyte as in the cell or 0.1 M sulphuric acid the formation of mercury(I) sulphate causes no difficulty. 

In the diaphragm-cell process, a solid cathode (iron) is used where hydrogen is evolved [reaction (15.4)]. Porous asbestos diaphragms are used to prevent mixing of the catholyte and anolyte, but owing to the finite permeability of these diaphragms, the alkaline solution that is produced near the cathode stiU contains important levels of chloride ions as an impurity. 

Finally, nickel is electrolytically produced from the purified nickel-bearing solution. In electrowinning of nickel, the hydrogen discharge reaction competes with nickel deposition at the cathode. This counterproductive effect is minimized by keeping the acidic anolyte solution separate from the catholyte by means of a diaphragm cloth. 

To conclude this section, reference may be drawn to what is called the Placid process for recycling lead from batteries. Placid denotes the leaching of lead in warm, slightly acidic, hydrochloric acid brine to form soluble lead chloride. Lead is won from the lead chloride on the cathode of an electro winning cell and is collected. Chloride anions are released simultaneously, but then react immediately with hydrogen ions that have been produced stoichio-metrically from electrolysis of water in the anolyte and passed into the catholyte through a membrane. The hydrochloric acid that is formed is returned as a make-up content to the leaching bath. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

The hulk solution may become depleted. In free-convection experiments, the cathode and anode compartments are often separated by a diaphragm to prevent interaction of the convection patterns. Under these conditions, replenishment of the catholyte does not take place. Examples of sagging limiting-current plateaus caused by bulk depletion can be found in... 

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