Brine electrolysis cell

In the last twenty-five years a new process has been developed in the chlor-alkali industry that uses a membrane to separate the anode and cathode compartments in brine electrolysis cells. The membrane is superior to the diaphragm used in diaphragm cells because the membrane is impermeable to anions. Only cations can flow through the membrane. Because neither Cl- nor OH- ions can... 

The most important commercial application of perfluorinated ionomer membranes is currently in the chlor-alkali industry. These materials are used as permselective separators in brine electrolysis cells for the production of chlorine and sodium hydroxide. This... 

A laboratory membrane brine electrolysis cell, designed for automated operation, was constructed ( 1,2). This system enables the measurement of the sodium ion transport number of a membrane under specific sets of conditions using a radiotracer method. In such an experiment, the sodium chloride anolyte solution is doped with 22Na radio-tracer, a timed electrolysis is performed, and the fraction of current carried by sodium ion through the membrane is determined by the amount of radioactivity that has transferred to the sodium hydroxide catholyte solution. The voltage drop across the membrane during electrolysis is simultaneously measured, so that the overall performance of the material can be evaluated. 

Perfluorinated ionomers such as Nafion are of significant commercial importance as cation exchange membranes in brine electrolysis cells ( 1). Outstanding chemical and thermal stability make this class of polymers uniquely suited for use in such harsh oxidizing environments. The Nafion polymer consists of a perfluorinated backbone and perfluoroalkylether sidechains which are terminated with sulfonic acid and/or carboxylic acid functionality. 

In a brine electrolysis cell, electricity is used to produce chlorine at the anode (the reaction is pH independent) and reduce water into hydrogen and caustic soda (the reaction is pH dependent) at the cathode according to the following electrode reactions ... 

Schematic diagram of a membrane brine electrolysis cell. 

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. 

At present about 77% of the industrial hydrogen produced is from petrochemicals, 18% from coal, 4% by electrolysis of aqueous solutions and at most 1% from other sources. Thus, hydrogen is produced as a byproduct of the brine electrolysis process for the manufacture of chlorine and sodium hydroxide (p. 798). The ratio of H2 Cl2 NaOH is, of course, fixed by stoichiometry and this is an economic determinant since bulk transport of the byproduct hydrogen is expensive. To illustrate the scde of the problem the total world chlorine production capacity is about 38 million tonnes per year which corresponds to 105000 toimes of hydrogen (1.3 x I0 m ). Plants designed specifically for the electrolytic manufacture of hydrogen as the main product, use steel cells and aqueous potassium hydroxide as electrolyte. The cells may be operated at atmospheric pressure (Knowles cells) or at 30 atm (Lonza cells). 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

The feed brine of the DEP contains a large quantity of carbonates. Therefore, at pH5 carbon dioxide is degassed. When hypochlorite is added, chlorate and bromate are formed in the feed of the electrolysis cells. These reactions have a slow velocity. The result of this is that conversion is only partial ... 

It appears that all of the bromide that is converted to bromine and bromate in the electrolysis process is eventually recycled to the feed brine. At the cathode of the electrolysis cell bromide is formed again. 

After extensive research and several tests, the option selected was recycling hypochlorite to the feed brine of the electrolysis cells. For this purpose, hypochlorite feed pipes were manufactured and the hydrochloric acid feed capacity to the brine degassing tanks was enlarged. 

Viton hoses were instead selected for the feed brine to the electrolysis cells. These are chemically resistant to chlorine-containing brine. There are several specifications of Viton hose available. For working with brine in an electrolysis environment, special attention had to be given to rupture resistance of the hoses with respect to operator safety. 

There is some increase of sodium chlorate (100 mg kg-1) and sodium bromide (40 mg kg-1) in the cell-liquor. This is a result of the chlorate that is formed in brine degassing. Therefore, more chlorate is fed to the electrolysis cells. Since not all the chlorate is reduced at the cathode, an increase in chlorate in the cell-liquor is observed (see Fig. 14.4). 

An additional advantage of the hypochlorite recycling process is the chlorination of the feed brine in the brine-degassing unit. Organic and nitrogen-containing components are oxidised. The reaction products are removed via the vent-gas to the chlorine destruction unit. Less NCI3 is formed in the electrolysis cells because part of the... 

An additional advantage is the oxidation of all organic and nitrogen-containing components of the brine in the brine degassing tanks. These impurities are not fed to the electrolysis cells, but the products removed to the chlorine destruction unit and incinerator. Control of NCI3 concentrations in chlorine liquefaction has become easier. 

Another brine species of high interest with respect to the cell voltage and membrane life is aluminium. In the electrolysis cells aluminium forms an aluminosilica complex [1] that can damage the electrolyser membrane. This has a negative effect similar to that of iron migration in terms of power consumption. The necessity then of iron and aluminium removal (to mention only the most important elements) from the brine to their lowest possible levels is obvious. 

Wolff, J.J. (1985) Ion exchange purification of feed brine for chlor-alkali electrolysis cells the role of Duolite C-467. Rohm c Haas Bulletin IE-D-285, March. 

Figure 19.16. Basic designs of electrolytic cells, (a) Basic type of two-compartment cell used when mixing of anolyte and catholyte is to be minimized the partition may be a porous diaphragm or an ion exchange membrane that allows only selected ions to pass, (b) Mercury cell for brine electrolysis. The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they are in parallel at low voltage, (d) Bipolar electrical connections 50 or more cells may be series and may require supply at several hundred volts, (e) Bipolar-connected cells for the Monsanto adiponitrile process. Spacings between electrodes and membrane are 0.8-3.2 mm. (f) New type of cell for the Monsanto adiponitrile process, without partitions the stack consists of 50-200 steel plates with 0.0-0.2 ram coating of Cd. Electrolyte velocity of l-2 m/sec sweeps out generated Oz.

Chlorine is produced almost entirely by the electrolysis of aqueous solutions of alkali metal chlorides (Fig. 1), or from fused chlorides. Brine electrolysis produces chlorine at the anode and hydrogen along with the alkali hydroxide at the cathode. At present, three types dominate the industry the diaphragm cell, the membrane cell, and the mercury cell, and there are many variations of each type. 

Currently the chlorohydrin process is only used for the epoxidation of propylene, where it still accounts for some 48% of world installed capacity. The yields are 88-89%. In most cases, the plant is integrated with a chloro-alkali facility that supplies both the required chlorine and sodium hydroxide. The recycle to the electrolysis cells of the brine solution produced in the dehydrochlorination step has been considered but not applied, most probably, for technical and economic reasons. In general the aqueous solution of calcium or sodium chloride is disposed of. 

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