Chlor-alkali current efficiency

T.D. Gierke, Ionic Clustering in Nation Perfluorosulfonic Acid Membranes and Its Relationship to Hydroxyl Rejection and Chlor-Alkali Current Efficiency, Paper presented at the Electrochemical Society Fall Meeting, Atlanta, GA (1977). 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

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. 

The analysis of ICIETB (described in Section 18.2) showed that stable voltage was the dominant factor to be considered. The many years of experience that ICI has in designing, operating and providing technical support for chlor-alkali plant has provided the knowledge that stable voltage (and current efficiency) is achieved by ... 

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-... 

The first electrochemical application of the D -statistic deals with the lack- of-association (i.e. independence) hypothesis concerning current efficiency and current load in diaphragm-type industrial scale chlor-alkali cells [18], Table 5 demonstrates that the two factors are independent with the understanding that the current efficiency/current load relationship may indirectly be influenced by other technical variables, e g. cell potential, and impurities. 

Table 5. Testing the independence of current efficiency and current load via eight different diaphragm-type chlor-alkali cells [18]...

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

Perfluorinated ionomer membranes have been developed for use as separators in chlor-alkali electrolysis cells. Using an automated test apparatus, the current efficiency and voltage drop of such a high performance membrane were evaluated as a function of several cell parameters. Results are plotted as three dimensional surfaces, and are discussed in terms of current theories of membrane permselectivity. 

The variation of current efficiency with solution concentration in the chlor-alkali environment is an added complicating feature of these membranes behavior. Kruissink (9) has performed elaborate calculations to yield the effect of electro-osmotic water transport on permselectivity, in classical terms. Results suggest that the minimum seen in t a+ (at lower NaOH concentrations than used here)... 

Figure 9. Current efficiency vs. NaOH catholyte concentration for Nafion 227 membrane in a chlor-alkali cell (34). Conditions current density, 31 A/dm2 temperature, 85° C anolyte concentration, 4.4 N NaCl cell voltage, 4.6 V.

Figure 6. Schematic potential seen by a hydroxyl ion as it moves across a Nafion perfluorinated membrane in a chlor-alkali cell. This potential consists of two parts a constant sloping portion that arises from the voltage drop across the membrane and an oscillating part that arises from electrostatic restriction of the hydroxyl ions. Physically, the hills and troughs correspond to the channel and cluster regions, respectively. For simplicity, a one-dimensional, periodic, model potential is used to evaluate the membrane current efficiency although the real potential is three-dimensional and aperiodic.

The measurement and control of transport properties for ion exchange membranes is the key element in optimizing the operating conditions for modern chlor-alkali membrane cells. Ideally, a membrane should allow a large anolyte-catholyte sodium ion flux under load, while at the same time the hydroxide ion and water fluxes are kept minimal. Under these conditions, high current efficiency and low membrane resistance can be realized simultaneously in a cell producing concentrated caustic and chlorine gas. 

DuPont has recently announced the development of a new high performance chlor-alkali membrane Nafion 901X. Caustic soda is produced at 33 wt% with over 94% current efficiency. The Nafion 901X is capable of operating at minimum voltage and high current efficiency for extended periods estimated to be in excess of two years (76). 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

The terminology employed in the chlor-alkali industry for comparing the performance characteristics of various cells is the energy consumption expressed in kilowatt hours per metric ton (kW h/M.T) of Cl2 or NaOH. This is related to the cell voltage ( ) and the current efficiency (17) as... 

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... 

The major use of perfluorinated membranes, at present, is as separators in chlor-alkali ceiis. The combination of low resistances, high current efficiencies at high solution concentrations, and high temperatures that can be achieved results in 20-30% lower energy requirements than those achieved with diaphragm or mercuiy cells. The long membrane lifetime (typically 2 years) results in low cost for membrane replacement. (Asbestos diaphragms usually last for only a year or less.)... 

Generalized Caustic and Chlorine Current Efficiency Expressions. The caustic current efficiency can be directly determined by the caustic collection technique, using the amount of caustic produced over a certain period of time, a knowledge of the number of coulombs passed during the same period, and Faraday s law. However, this is a tedious and time-consuming method. On the other hand, direct measurement of the chlorine produced is much more difficult for practical reasons. Hence, the chlor-alkali industry relies on indirect methods that provide rapid and accurate current efficiency values. The theory underlying the indirect methods is discussed in this section. 

The chlor-alkali producers sometimes prefer this version of the caustic current efficiency equation since it avoids the need for measuring the applied load. 

Ch/ NaOH Appendix 4.4.1). Note that the hydrogen inefficiency arises from the BCLs due to the discharge of the available chlorine at the cathode. However, the chlor-alkali industry routinely measures only the chlorine current efficiency of diaphragm cells using the relationship in Eq. (92) detailed in [1,7,13]. 

Product Purity. In chlor-alkali cells, there are two parasitic cathode reactions, which lead to blind current efficiency losses ... 

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