Chlorine production, electrochemical cells

Electrochemical processes require feedstock preparation for the electrolytic cells. Additionally, the electrolysis product usually requires further processing. This often involves additional equipment, as is demonstrated by the flow diagram shown in Figure 1 for a membrane chlor-alkali cell process (see Alkali AND chlorine products). Only the electrolytic cells and components ate discussed herein. 

Electrochemical reduction in aqueous acid is useful in the treatment of waste liquors obtained from the formation of chloroacetic acid by chlorination of acetic acid. The liquors contain further chlorination products. These are reduced in an undivided cell at a magnetite cathode and a carbon anode to give excellent conversion to monochloroacetic acid [73]. 

Own experiments in divided cells using Nation membrane separators and hypochlorite solutions in the ppm range of concentration resulted in current efficiency values for active chlorine reduction of a few percent. Shifting the pH to higher values complicated the experiments. A buffer stabilised the pH but the relatively high concentration of buffer ions hindered the electrochemical reaction. Thus, quantification is difficult. Kuhn et al. (1980) showed reduction inhibition when calcareous deposits were precipitated on the cathode, but practical experiments showed the decrease of chlorine production in this case. 

Pillai, K.C., Kwon, T.O., Park, B.B. and Moon, I.S. (2009) Studies on process parameters for chlorine dioxide production using Ir02 anode in an un-divided electrochemical cell. J. Haz. Mat. 164, 812-819. 

The Production of Sodium and Chlorine. Many electrochemical processes depend for their success on ingenious devices for securing the purity of the product. As illustration we may consider a cell used for making metallic sodium and elementary chlorine from sodium chloride. 

The rate of production of chlorine gas by electrochemical cells is sometimes determined by addition of oxygen to the chlorine stream at a known rate, and accurate determination of the oxygen concentration at a well mixed point downstream of the point of addition. To the chlorine produced by diaphragm cells, which already contains 0.8% by volume oxygen, additional oxygen is added at the rate of... 

Salt was first electrochemically decomposed by Cruickshank in 1800, and in 1808 Davy confirmed chlorine to be an element. In the 1830s Michael Faraday, Davy s laboratory assistant, produced definitive work on both the electrolytic generation of chlorine and its ease of liquefaction. And in 1851 Watt obtained the first English patent for an electrolytic chlorine production cell (11). 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

Chlorine gas is liberated at the positive electrode and caustic soda and hydrogen are formed at the negative electrode. The hydrogen, although pure, is usually seen as a by-product of low value that may be pumped to a nearby oil refinery or burnt to raise steam. (The terminology used for electrochemical cells is outlined in Box 4.1.)... 

By definition, power expressed in watts is equal to amperes x volts, and energy expressed in watt-hours is equal to amperes x volts x time (in hours). Therefore, the calculation of energy consumption requires a knowledge of the overall reaction and the number of Faradays required to produce the desired product, the operating cell voltage, and the cell current efficiency, which is illustrated here for the case of electrolytic chlorine production. The main anodic electrochemical reaction during the electrolysis of brine is the discharge of the chloride ions to produce chlorine, as described by reaction (4). When the chlorine current efficiency, ci2> is 100%, one Faraday of electricity will produce... 

G. van der Heiden, Diaphragm Cells for Chlorine Production, Proc. Symp. held at the City University, London (1976), Society of Chemical Industry, London (1977), p. 33 J. Appl. Electrochem. 19, 571 (1989). 

A typical chlorine production plant using membrane cells is pictured in Fig. 9.8. Electrolysers are operating at atmospheric pressure and 85°C.The main electrochemical characteristics of brine electrolysis cells using membranes are (i) operating current density 300-500 mA.cm (ii) cell voltage 3.0-3.6 V (iii) NaOH concentration 33-35 wt% (iv) energy consumption 2600-2800 kWh/ton Clj at 500 mA.cm (v) efficiency 50% and (vi) steam consumption for concentrating NaOH to 50% 180 kWh/ton CI2. The production of one ton of chlorine requires -1.7 tons of NaCl and less than 1 ton of water vapour. 

Organic Pollutants in Water Using DSA Electrodes, In-Cell Mediated (via Active Chlorine) Electrochemical Oxidation, Fig, 5 Dependence of the electrochemical free chlorine production efficiency on the chloride content of the electrolyzed water under standard conditions using four different anode materials (iridium oxide, mixed iridium/ruthenium oxides, platinum, doped diamond) (Adapted from [26]. Reprinted with permission from Johnson Matthey Pic)... 

In 1800, Cruickshank was the first to prepare chlorine electrochemically [38] however, the process was of little significance until the development of a suitable generator by Siemens and of synthetic graphite for anodes by Acheson and Castner in 1892. These two developments made possible the electrolytic production of chlorine, the chlor-alkali process, on an industrial scale. About the same time, both the diaphragm cell process (1885) and the mercury cell process (1892) were introduced. The membrane cell process was developed much mpre recently (1970). Currently, more than 95 % of world chlorine production is obtained by the chlor-alkali process. Since 1970 graphite anodes have been superseded by activated titanium anodes in the diaphragm and mercury cell processes. The newer membrane cell process uses only activated titanium anodes. 

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