Catholyte efficiency

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. 

The current efficiency (Fig. 19) (100% = 1 mol/2 F) also shows a maximum with current density. At high currents, insufficient C02 is available to neutralize the hydroxide at low currents, the catholyte pH drops, lowering the rate of C02 absorption. This maximum is seen to shift with C02 pressure, as expected [28] the behavior is strikingly similar to that seen in H2 separation (Fig. 3). 

The working cathode also generates a boundary layer. The water that is reduced at the cathode is supplied by the bulk catholyte. This stripping effect forms a layer of approximately 37 wt.% caustic on the surface of the cathode. Again, the thickness of this layer is determined by the efficiency of the internal mixing within the cathode compartment. 

The tests demonstrated that diffusion is the most likely mechanism for transfer of energetic organic species across the membrane from the anolyte to the catholyte chamber. Both the parent species in Composition B (TNT and RDX) as well as a TNT breakdown product, trinitrobenzene (TNB), move to the catholyte. Under operational conditions that provided a processing efficiency of approximately 80 percent for the energetics mixture, the rates of transfer were TNT = 2.6 g/hr/m2, TNB = 0.6 g/hr/m2, and RDX =1.4 g/hr/m2. 

Reduction of substituted nitrobenzenes under alkaline conditions, usually with aqueous sodium acetate as electrolyte and a nickel cathode, is the classical method due to Elbs [45] for the formation of azo- and azoxy-compounds. Protons are used in the electrochemical reaction so that the catholyte becomes alkaline and under these conditions, phenylhydroxylamine reacts rapidly with nitrosobenzene to form azoxybenzene. Finely divided copper has long been known to catalyse the reduction of nitrobenzene to aniline in alkaline solution at the expense of azoxybenzene production [46]. Modem work confirms that whereas reduction of nitrobenzene at polycrystalline copper in alkaline solution gives mainly azoxybenzene, if the electrode is pre-oxidised in alkaline solution and then reduced just prior to the addition of nitrobenzene, high yields of aniline are obtained with good current efficiency... 

In the search for greater energy efficiency and longer life more complex laminated fluorinated membranes have been developed for modern chloroalkali cells having sulfonate groups on the side in contact with the anolyte and carboxylate groups on the side in contact with the catholyte. Ultimately, all chlorine and sodium hydroxide may be manufactured in membrane cells. 

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... 

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

The preliminary conceptual process design and the corresponding flow sheet have shown an efficiency of 39-41% (LHV). These values depend on assumptions regarding the operability of the electrolyser and the crystalliser, which separates components in the spent anolyte and catholyte. 

It will be demonstrated that current efficiency also decreases with an increased concentration of the caustic. Should the transport of current be performed by hydroxide ions alone (the transference number of OH- is t ), the formation of 1 gram-equivalent of hydroxide would be accompanied by the simultaneous emigration of t gram-equivalents of OH- from the cathode compartment. Actually, however, only an a -th part of the current is transported by the caustic, thus merely (l. x) gram-equivalent of OH are transferred from the catholyte. The current efficiency 7)i in per cent is, therefore, given by the equation ... 

The concentration of caustic solution produced was very low and also current efficiency was very small, between 75 and 80 per cent. This was due to the migration of hydroxyl ions from the catholyte to the anolyte which gave rise to hypochlorite ions. These were then anodically oxidized to chlorate ions under... 

When using diaphragms there is no risk of the anolyte being mixed by thermal convection so unlike the bell-jar electrolyzer higher temperatures may be used which lower the specific resistance of the electrolyte. The increased temperature has also a positive effect upon the current efficiencies as both, migration of hydroxyl ions from catholyte to anolyte and solubility of chlorine in brine, are reduced. By this, formation of hypochlorite is limited and caustic and chlorine losses are reduced. 

The Hargreaves-Bird electrolyzer was loaded with 2600 A, current density on the surface of the diaphragm 2 A/sq. dm and voltage 3.6 to 4.0 V. 150 to 170 grams of sodium carbonate per one litre catholyte was obtained while currents efficiency was between 80 to 85 per cent. The temperature was maintained at about 80 °C. 

It can be clearly seen from the last equation that in the course of electrolysis the catholyte becomes alkaline and the ammonia which has been set free can escape into the ambient atmosphere. Further losses of ammonia are caused by its anodic oxidation to nitrogen or nitrate. Another difficulty is caused by the hydroxyl ions, which in an alkaline solution also take part in the conduction of current and are then discharged at the anode, which reduces the current efficiency. 

Alvarez-Gallegos and Pletcher (1998) used the flow divided three-electrode cell fed with 02 and flow circuit depicted in Fig. 19.2 to generate H202 at a three-dimensional RVC cathode. Maximum current efficiencies of 56-68% were obtained for 10mM HC1 and 10 mM H2S04 (pH sa 2) as catholytes at Ecat values... 

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