Membranes chlor-alkali

Bissot, T.C. (1985) Sulfate ion transport through perfluorinated chlor-alkali membranes an example of co-ion fractionation in membrane. Paper presented at the meeting of the Electrochemical Society, Toronto, Canada. 

Several years ago, ICI ETB looked at the total market for chlor-alkali membrane technology and considered how to develop an electrolyser that would extend its product and market range. A detailed analysis was carried out and many chlor-alkali producers were consulted. As a result of this work ICI ETB has developed an electrolyser called BiChlor. 

Ion-exchange membrane is designed to allow the transport of primarily sodium ions and water from the anolyte to the catholyte compartment, whereas the diaphragm allows the percolation of all the anolyte through the separator. The cation-conducting, ion-exchange membrane is structured to reject anions, as indicated in Fig. 26.9. The chlor-alkali membranes in use today consist of one or more perfluorinated ion-exchange polymeric mate-... 

A chlor-alkali membrane designed to produce 30-35 percent NaOH consists of at least two distinctly different polymer layers, as shown in Fig. 26.9. The anode side of the membrane is about a 0.1-mm film of sulfonic acid polymer, whereas the cathode side is... 

Figure 8.3 The Chlor-Alkali Membrane Electrolysis cell...

If there is a gap situated between electrode and membrane (or in other cases diaphragm), this is called finite gap principle. A minimum gap of roughly 1 mm is needed to ensure sufficient mass transport. Some tenth of a volt in cell voltage can be saved, if the zero gap principle is applied to chlor-alkali membrane cells. According to this principle, electrodes, which can be transmitted by the feed, are arranged directly on the surface of the membrane on both sides with adjacent transport and contact elements. This principle is used... 

Membrane electrolyzers for water decomposition use cation exchange membranes of the type used in the chlor-alkali membrane process, in this case made H+-ion conducting by different pretreatment. So, the ions generated at the anode (H2O -> 2H++ I/2O2 + 2e ) pass through the membrane and form at the cathode hydrogen (2H+ + 2e -> H2). 

Three types of perfluorinated chlor-alkali membranes are noteworthy. The first of these, the homogeneous carboxylate films. 

Thus all successful chlor-alkali membranes currently employ a perfluorocarboxylate polymer to lower the rate of hydroxide ion transport. The sulfonate portion of some of these membranes is present mainly to add strength to the thinner carboxylate barrier layer. Fabric backing is also used in some cases to improve physical strength. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

In this investigation, a sample of Nafion NX-90209 chlor-alkali membrane was used (E.I. du Pont de Nemours and Co., Polymer Products Department, Wilmington, DE). This membrane has sulfonate and carboxylate polymer layers and is reinforced with an open weave fabric. 

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). 

Parameter Correlations for A Multicomponent Transport Model for Chlor-Alkali Membrane Cells," Presention at the 157th Meeting of the Electrochemical Society, St. Louis, Mo. 5-11, 16, 1980, Olin. 

When chlor-alkali membrane cells are operated on an industrial scale it is desirable that the membrane be cross-linked so that the activation energy barrier to microstructure transition becomes extremely high. This will ensure that the transport properties of the membrane are invariant with time, even at high current densities. 

A newer chlor-alkali membrane-cell process, in which the diaphragm is replaced by a polymeric membrane to separate the cell compartments, has been adopted in much of the industrialized world. The membrane allows only cations to move through it and only from anode to cathode compartments. Thus, as CF ions are removed at the anode through oxidation to CL, Na ions in the anode compartment move through the membrane to the cathode compartment and form an NaOH solution. In addition to forming purer NaOH than the older diaphragmcell method, the membrane-cell process uses less electricity. 

Cation Exchange Membranes for the Chlor-Alkali Membrane Process... 

The effect of the cost of the ion exchange membrane on the total cost of electrodialysis or electrolysis is large because the membrane is relatively expensive. The lifetime of the membrane depends on the purpose and conditions of electrodialysis or electrolysis. A membrane for the electrodialytic concentration of seawater to produce sodium chloride has a lifetime of over 10 years, and that in the chlor-alkali membrane process, which is operated at ten times or more higher current density than that of seawater concentration, is over 5 years. However, in applications for food industries, the lifetime of the membrane is relatively short due to periodical sanitary cleaning of the electrodialyzer by acid or alkali solution, and sometimes oxidizing agents. 

Y. Nishiki, S. Nakamatsu, K. Aoki and K. Okuda, Estimation of optimum anode geometry in chlor-alkali membrane cells, J. Appl. Electrochem., 1984, 19, 90-94. 

The principle of the chlor-alkali membrane cell has been known for a considerable time, and patents for this approach to chlor-alkali manufacture were granted in the early 1960s. Membrane cells combine the purity of mercury cell caustic with the power efficiency of diaphragm cells, while... 

A. Nidola and R. Schira. Deactivation of Low Hydrogen Overvoltage Cathodes in Chlor-Alkali Membrane Cell Technology by Metallic Impurities, In M.M. Silver and E.M. Spore (eds), Advatutes in the Chlor-Alkali and Chlorate Industry, The Electrochemical Society Inc., Princeton, NJ (1984), p. 206. 

H.L. Yeager and A.A. Gronowski, Factors Which Influence the Permselectivity of High Performance Chlor-Alkali Membranes, In T.C. Welligton (ed.). Modem Chlor-Alkali Technology, Vol. 5, Society of Chemical Industry, London (1992), p. 81. 

T.C. Bissot, Sulfate Ion Transport Through Perfluorinated Chlor-Alkali Membranes. In U. Landau, R.E. White, and R.D. Vaijian (eds). Engineering of Industrial Electrolytic Processes, PV 86-8, The Electrochemical Society, Pennington, NJ (1986), p. 194. 

---