Current efficiency membrane cell

Influence of Electrolysis Conditions. Among the various electrolysis conditions, brine purity has the most significant effect on the life of the membranes. The presence of a small amount of multivalent cations leads to formation of metal hydroxide deposits in the membrane, and thus causes a decrease in current efficiency, an increase in cell voltage, and damage to the polymer structure of the membrane. With perfluorocarboxylic acid membrane, the presence of more than 1 ppm of calcium ion will begin to cause these problems in a very short period (1 - 8). To obtain stable current efficiency and cell voltage, it is therefore essential to establish effective brine purification methods. 

Perfluorinated membranes used in chlor-alkali cells normally have a thin layer of carboxylate on the cathode-facing surface of a sulfonate membrane. Nafion 901 was introduced as such a membrane [38]. It achieved 33% NaOH concentration with 95% current efficiency in cells operating at 3 kA/m and 3.3 to 3.9 V. The carboxylate layer can be prepared by lamination, but the layer can be... 

In conclusion, the alkaline earth metal ions, precipitated in the membrane, affect both current efficiency and cell voltage. At sufficiently high concentrations, they physically distort the membrane structure. The effects of the anions, such as sulfate, and nonionic species, such as silica, are relatively minor, but these impurities often form insoluble compounds with other chemical species entering the membrane, thereby seriously affecting the membrane structure. 

Note that the current efficiency used for the calculations is 94%. At startup, it usually will be 96% or more. With time and the gradual deterioration of the membranes, the current efficiency will decline. Membranes are often removed when the current efficiency reaches an arbitrary value of about 93%. Cell room auxiliary equipment often is designed for this condition. However, all cells usually do not reach low current efficiency simultaneously. Cell renewal should begin before all membranes have reached... 

Membrane and electrode damage effect cell performance, i.e., cause lower current efficiency, increased cell voltage, and, as a result, increased power consumption [143]. Some impurities affect the anode or cathode coating and cause an increase in overvoltage or simply deposit in the membrane, increasing its resistance and thus the cell voltage. The increase in voltage may in some cases be partially reversible when the impurity concentration drops to the recommended limits. 

Current Efficiency. Current efficiency for caustic production in diaphragm and membrane cells can be estimated from collection of a known amount of caustic over a period of time and from a knowledge of the number of coulombs of electricity passed during that time period. An alternative method involves analysis of the gases evolved during electrolysis and determining the anolyte composition. Material balance considerations (7) show the expression for the caustic efficiency for membrane cells to be... 

Data, for a 32% caustic concentration at 90°C and a current efficiency of 96.0%, obtained in laboratory cells using a DSA anode and an activated cathode, where the membrane is against the anode at a 3-mm gap. 

The anode and cathode chambers are separated by a cation-permeable fluoropolymer-based membrane (see Membrane technology). Platinum-electroplated high surface area electrodes sold under the trade name of TySAR (Olin) (85,86) were used as the anode the cathode was formed from a two-layer HasteUoy (Cabot Corp.) C-22-mesh stmcture having a fine outer 60-mesh stmcture supported on a coarse inner mesh layer welded to a backplate. The cell voltage was 3.3 V at 8 kA/m, resulting ia a 40% current efficiency. The steady-state perchloric acid concentration was about 21% by weight. 

For a profitable electrochemical process some general factors for success might be Hsted as high product yield and selectivity current efficiency >50%, electrolysis energy <8 kWh/kg product electrode, and membrane ia divided cells, lifetime >1000 hours simple recycle of electrolyte having >10% concentration of product simple isolation of end product and the product should be a key material and/or the company should be comfortable with the electroorganic method. 

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. 

PSB electrolytes are brought close together in the battery cells where they are separated by a polymer membrane that only allows Na ions to go through, producing about 1.5 V across the membrane. Cells are electrically connected in series and parallel to obtain the desired voltage and current levels. The net efficiency of this battery working at room temperature is about 75%. It has been verified in the laboratory and demonstrated at multi-kW scale in the UK [92]. 

Electrolysis of water contained within a perfluorinated sulphonic acid membrane (Membrel water electrolysis) developed by ABB [133,201-205] and Sasakura [200]. Current efficiencies reach 14-15% at current densities of 10000 A m2 and more. The cells are generally immersed directly in the water so that ozone is introduced directly into the water to be treated, Fig. 18. 

The current efficiency for pure Cr(III)-sulfuric acid is in the range of 90%. Organics, present in the industrial liquors, especially higher dicarboxylic acids, interfere, thus necessitating the use of divided cells. The scheme of the membrane cell used is shown in Fig. 29. 

This type of cell is limited by the performance of the anion-exchange membrane. The membrane can tolerate only a limited concentration of acid in the anolyte before backdiffusion of protons through the anion membrane becomes rather significant, causing a decrease in the cell s current efficiency, and acid gets into the salt stream. Unconverted salt in the product solutions can be eliminated by the use of the three compartment device, but this option substantially adds to the capital costs and operating complexity of the plant. 

The limiting current density may be defined as the current density at which the depression of the Na+ concentration at the interface of the membrane s sulphonic and carboxylic layers results in an abrupt rise in cell voltage and drop in current efficiency. 

The circulation conditions obtained in the small laboratory cell cannot be attained in a full-scale cell. The effects of Na+ diffusion to the membrane and non-uniformity in its intracell concentration cannot be entirely eliminated, and a greater decrease in current efficiency will tend to occur at high current densities. 

The ML32NCH electrolyser equipped with the Aciplex F-4401 membrane has been in commercial operation at 6 kA m-2 for approximately one year at Asahi Chemical s chlor-alkali plant. As shown in Figs 17.16 and 17.17, the electrolyser has achieved a cell voltage of 3.17 V and a current efficiency of 96%, while operating at 6kA m-2. This operation is continuing the present plan is to investigate the performance of the ML32NCH at a current density of 8 kA m-2. 

Several test runs have been carried out using 1.5 dm2 laboratory cells with the F-8934 at 8kA m-2. The current efficiency at 8kA m-2 was about 1% lower than that at 5kAnT2, and no further decline was observed. F-8934 has also had a similar evaluation result in full-scale pilot cells at 8 kA m-2. AGC has been obtaining slightly lower current efficiencies than its desired target of 97% at the beginning of the membrane lifetime. This has led AGC to ensure that the other stepped-up design concept should be applied to the membrane for 8 kA m-2 operation. 

Fig. 19.13 Plot of current efficiency stability at 8 kA nrf2 operation in a laboratory cell using the F-8934 membrane.

Mauritz and Gray analyzed the IR continuous absorption of hydrated Na OH - and K OH -imbibed Nafion sulfonate membranes for the purpose of correlating this phenomenon to the current efficiency (cation transference number) of chlor-alkali electrochemical cells.In this case, the similar issue of OH ( defect proton ) conductivity is important. A distinct continuous absorption appeared in the spec-... 

In laboratory experiments, selective membranes were already applied years ago for the pH-control during the electro-dialysis of pH-sensitive colloids. In the three-compartment cells used, the electrode chambers were rinsed with distilled water. On account of the high mobility of the H+ ions, the desalting cell was inclined to become acid. To oppose this effect, anode- and cathode membranes with different polarity were sought for. At the beginning the influence of these membranes on the current efficiency — i.e. the amount of salt removed per unit of charge flown through — was mentioned only sporadically (5,23). 

Also is claimed the electrolytic conversion of sodiumchloride in sodiumhydroxide and chlorine (18). In all these instances, the selective membrane is applied in order to increase the current efficiency by either impeding the disappearance of OH- ions from the cathode cell to the anode cell, or that of H+ ions in the reverse direction. 

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