Anolyte circulation

Tests with these inclined louvers [9] (Fig. 16.4) showed that the cell voltage dropped, this drop being more prominent (see Fig. 16.5) with an increasing current density, as was expected. This suggests that this innovation of the anolyte circulation acting upon the anode not only lowers the energy consumption but also lowers the material costs for the cell because no specific internals are required and manu-... 

Plotting the oxygen content measured at a current density of 4 kA m-2 versus the measured anolyte pH, the relationship as shown in Fig. 16.11 is obtained as expected. This proves again the high internal anolyte circulation of the KU Single Element that makes possible an oxygen content of 0.6% in chlorine at an anolyte pH of only 2.5. 

Anolyte mixer operation —Anolyte circulating pump flow rate —Total organic level in anolyte circuit... 

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). 

Nickel. Most nickel is also refined by electrolysis. Both copper and nickel dissolve at the potential required for anodic dissolution. To prevent plating of the dissolved copper at the cathode, a diaphragm cell is used, and the anolyte is circulated through a purification circuit before entering the cathodic compartment (see Nickel and nickel alloys). 

In the case of well brine, the anolyte is not circulated as the iodide content is too high. In this situation, a portion of the anolyte containing chlorine is added to the well brine and after oxidising the iodide to iodate the brine is sent to the RNDS . 

Sodium orthoarsenate is also obtained electrolytically by the method described under calcium arsenate (p. 198). Yields up to 100 per cent, may be obtained 9 by employing a cell with a diaphragm between iron electrodes. The anolyte should contain sodium arsenite, or sodium hydroxide and arsenious oxide (equivalent to 150 g. As2Os per litre), and the catholyte sodium hydroxide (150 g. per litre). With a current density of 3 amps, per sq. dm. the current efficiency is 100 per cent. A solid crust of sodium arsenate forms around the anode. The process may be rendered continuous by circulating the anolyte and removing the precipitated arsenate. Iron or nickel electrodes are... 

An apparatus similar to that used in the popular disc electrophoretic technique can be used for isoelectric focusing. One modification concerns the insertion of the tubes in a jacket through which water of 2°C can circulate only the tips of the tubes will be in contact with the electrolytes. Platinum electrodes should be used and arranged at the extremities of the tubes. The anolyte and catholyte are 0.02 Jlf phosphoric acid (pH 2.2) and 0.01 M sodium hydroxide (pH 12.0). Excess persulfate should be eliminated by maintaining a current of 1 mA per tube for 10 minutes. 

The plate-and-frame cell design is usually applied to divided cells. Anode and cathode chambers are separated by, for example, a cation-exchange membrane. Anolyte and catholyte have separate circulation systems with separate feeds and outlets and separate waste-gas purification (Fig. 4). 

In practice, solutions are first pumped into the test cell, and then circulated for a period of several hours to condition the membrane. Next, an electrolysis is performed to further condition the material. These solutions are then discarded, fresh catholyte and radioactive anolyte are added, and electrolysis is conducted at a given membrane current density. A sample of anolyte and the entire catholyte solution are then pumped to a sample collector for weighing and determination of radioactivity. Other experiments may then be repeated at other current densities, or the sequence repeated with new solution concentrations. Thus twelve different combinations of anolyte and catholyte concentrations are used. 

The system has two circulating loops, one for the anolyte solution and one for the catholyte solution. In the anolyte loops, Ce(III) is oxidized to Ce (IV) in the T-cell and passed through the reaction chamber where the organic wastes are introduced gradually. Carbon is converted to carbon dioxide chlorine compounds are converted to elemental chlorine, which is scrubbed and converted to hypochlorite sulfur and other elements are converted to salts, such as sulfates. These salts remain in anolyte solution, which must be periodically replaced as the concentration of the salts increases. 

Cathodes and anodes have been integrated into separate electrolyte circulation systems. Electrolyte management consists of preventing alkafinization at the cathodes and acidification at the anodes by mixing anolyte and catholyte and thus neutralizing both electrolytes to pH 7. An additional advantage of mixing anolyte and catholyte is that anionic nutrients captured in the anolyte end up in the catholyte, and likewise, cationic nutrients end up in the anolyte. Thus, cathodes and... 

Pollutants present in the soil are recovered from the electrode solutions and also, the chemicals to enhance the separation and transportation of pollutants are supplied to the electrode solutions. Therefore, a circulation system is important for the management of the electrode solution and to maintain the parameters like temperature and pH of the anolyte as well as the catholyte. A proper circulation control system enables complete mixing of electrode solutions and sample withdrawal also if necessary. It might also include a process piping to distribute any chemical amendments to electrode wells and to extract the contaminants from the electrode solutions (e.g. precipitation and ionic exchange). 

There is essentially no consumption of acid except the small amount needed at the start of each batch to neutralize a dilute solution of sodium silicate (0.5% Si02) to pH 9 at 60-90°C to form silica nuclei to start the process. Narrow uniform spacing between the membranes is required to minimize power cost and avoid silica deposition. Water is added to the anode compartment since it is slowly transported to the cathode compartment, from which sodium hydroxide solution is constantly withdrawn. Anolyte and catholyte are circulated from the corresponding electrode... 

In a typical run operation started by circulating the sodium sulfate electrolyte solution, the sulfuric acid anolyte solution and the sodium hydroxide catholyte solution through their respective compartments in the cell. The electrolyte solution was then heated to the temperamre specified for the run. Sixty milliliters of the sodium silicate solution were fed into the electrolyte and the electrolyte-sodium silicate mixture circulated to obtain a homogeneous solution. Silica concentration of the homogeneous solution was 0.21 g Si02 per 100 ml. Silica concentration at this point of the operation is referred to as sodium silicate concentration at the time of nuclei formation or SSn. The pH of the solution was 10.35. 

Anolyte and Catholyte Circulating System 3/8" polyethylene tubing with fast and tight polypropylene fittings. 

When all the cell compartments are full and water is running down the drain the circulation rate is increased to 0.1-0.2 gal/min in the anolyte and catholyte compartments and 2-2.25 gal/min in the electrolyte compartment. 

These circulation rates give about a one psi difference between the electrolyte compartment and the anolyte and catholyte compartments. The difference is required to keep the membranes apart and against the membrane screens, thus maintaining a constant distance between membranes. 

Circulation rates and psi adjusted to same values as in Step 6 water cleanup, that is 0.1-0.2 gal/ min anolyte, catholyte and 2-2.5 gal/min electrolyte with a one psi pressure difference between the membranes. 

The 3-compartment cell is satisfactory for acids that do not decompose at the anode, but it would be unsatisfactory for splitting NaCl, because CI2 would form at the anode. To alleviate this problem a cation-exchange membrane could be added next to the anode, and H2SO4 could be circulated as the anolyte. The additional membrane could be a perfluorinated membrane that resists oxidation at the anode, and that addition would improve the chemical stability of the cell. 

The calculation of the distance between the anode and the separator is slightly complex as the asbestos diaphragm is deposited on the cathode screen, and the anode blade is located 4-5 mm from the diaphragm with the anolyte in between, and contains dispersed chlorine gas bubbles. The gas-solution mixture is circulated effectively by the convection of the two-phase flow resulting in a relatively low gas void fraction in the electrolysis zone. The superficial resistivity, Pmix. based on Eq. (163), would be about 1.2 times the resistivity of the solution free of gas bubbles, which is 1.2/0.59 = 2.03 cm. The ohmic drop between the anode and the diaphragm can then be calculated from Eq. (165) and the current as ... 

Both anolyte and catholyte are circulated in the cell chambers. Uniform concentration is achieved by forced or natural circulation as shown in Fig. 5.25. 

From a physical standpoint, the role of the WTC in the anolyte and catholyte water balances explains its effects in process control. Electrolysis tends to increase the strength of the catholyte, which then requires dilution in order to maintain the proper concentration. The water added to the circulating caustic and the water flux through the membranes provide this dilution. When one source diminishes, the other must increase. Therefore, a reduction in the WTC requires an increase in the flow of dilution water. 

---