Membrane brine electrolysis

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. 

Schematic diagram of a membrane brine electrolysis cell. 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

In the brine electrolysis system, silica is also contained in raw salt. Silica will precipitate on to membranes in the presence of calcium, strontium, aluminium and iodine resulting in the loss of current efficiency [8-10]. Silica can also be removed in a column filled with ion-exchange resin containing zirconium hydroxide, just like the iodide ion. 

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 The released Na dissolves in the Hg and is withdrawn to another zone where it forms salt-free NaOH with water, (c) Monopolar electrical connections each cell is connected separately to the power supply so they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. 

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... 

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. 

Nafion-315 is currently used in the SPE cell for brine electrolysis. The SPE electrolyzer exhibits a 15-20% energy savings when compared to conventional brine electrolyzers, primarily due to the decrease in ohmic and cathode overvoltages. Figure 5 shows the schematic of the SPE electrolyzer along with a typical membrane electrolyzer. The current distribution across the membrane of an SPE electrolyzer is more uniform than that of a typical brine electrolyzer. 

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

Originally the monomer in Fig. 9 was prepared by DuPont by the synthetic scheme shown in Fig. 12 [33]. Thermolysis of the acyl fluoride in Fig. 12 did not give a desired monomer but gave a cyclo compound. In order to prevent the cyclization, a new synthetic route was developed as shown in Fig. 13, which was applied to the synthesis of Dow membranes [34]. A chlorine atom was introduced to the acyl fluoride to improve the selectivity of vinyl ether formation. The Dow membrane was also developed for brine electrolysis, but was not commercialized probably because of its high cost. Difficulty in the preparation of the acyl fluoride in Fig. 13 is one of the causes. Recently, new synthetic processes for the short side chain monomer were developed, as represented in Fig. 14. 

Perfluorosulphonic Nation membrane separators are used in direct contact with electrodes as solid polymer electrolytes (SPE) in fuel cells . In this case, the membrane is both the electrolyte and the separator. The use of perfluorosulphonic membranes as SPE started 30 years ago with the US space program Gemini and the realization of low temperature H2/O2 SPE fuel cells. Since then, the feasibility of operating the SPE fiiel cells on hydrogen/halogen couples has been demonstrated. In addition, the introduction of perfluorinated membranes for use in water and brine electrolysis and more recently in organic synthesis has taken place . 

Nafion materials, and more generally perfluorinated ionomers, are particularly suitable for water and brine electrolysis and, to date, no viable alternative has been found for SPE applications. The dissolution of Nafion membranes allows the preparation of material with high porosity and high electroactive area. Such structures are required for the development of high power density SPE fuel cells. In recent work, Aldebert et al. have presented different methods for the preparation of SPE... 

Finally, as mentioned above, it is imperative that the brine quality is excellent so that no impurities are deposited in the membrane, disrupting its structure and its chemical integrity. Thus, all efforts, be they operations-related or design-related, should preserve the physical, mechanical, and chemical properties of the membrane during electrolysis to realize sustained maximal performance from the membrane. 

Nearly all applications of caustic soda require separation of the chloride from the hydroxide. In sodium-brine electrolysis, fortunately, the phase equilibrium allows a rather effective separation by evaporation of the liquor. If water is removed until the concentration of NaOH approaches 50%, nearly all of the NaCl falls out of solution. After cooling, the residual concentration is about 1.0-1.1%. This removal of salt causes the concentration of NaOH to increase. The solution produced by evaporation therefore can contain somewhat less than 50% NaOH. This is discussed in some detail in Section 9.S.3.3. Dissolved salt is not acceptable in some uses of NaOH, and so there has always been a split market. Part has been reserved to a purified version of the diaphragm-cell product and to mercury-cell, and now membrane-cell, NaOH. 

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. 

Caustic soda synthesis via brine electrolysis was described in Eqn (15.3). The main alternate chloralkali manufacturing technologies are based on diaphragm, mercury, and membrane cells (Burney, 1993 Venkatesh Tilak, 1983). Of course, the NaCl and NaOH in Eqn (15.3) are fuUy dissociated in the aqueous solution, i.e., NaCl(aq) Na" - -Cl and NaOH(aq) Na - -OH , so that an alternate, and... 

It was the development of ion-exchange membranes which opened a new era in brine electrolysis. These membranes have the extraordinary property of allowing the passage of electrolytic current and yet almost completely preventing the mixing of the hydroxide formed in one compartment of the electrolytic cell with brine introduced as raw material into the other compartment. Hence, a "membrane cell" could be developed for large scale industrial electrolysis, giving satisfactory quality of products without environmental hazard. This is likely to make the "mercury cell" vanish as the enormous investment into new installations pays off. 

Fig. 5. Brine-electrolysis installations (a) with mercury cells and (b) with membrane cells.

Hence, the air-breathing membrane cell is the future of brine electrolysis and is likely to promote further significant increase in the volume of production around the world. 

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