Membrane cells/processes

The membrane cell concept has been known for many years, but early work failed as a result of non availability of suitable ion exchange membranes which would resist the very demanding conditions within the chlor-alkali cell. It was also realized that the graphite anodes, which wore away in use, would present major difficulties from the engineering side, so it was not until metal anodes became available that interest revived. However, much of the early work and patents of the 1950s set the scene for the way in which this technology was to develop. 

Other manufacturers such as Asahi Glass, Tokuyama Soda, De Nora, Oxytech, Chlorine Engineers and Uhde offer a variety of either monopolar or bipolar cells and most of these technologies now have representative plants. Membranes are almost all Nafion based. 

Reversible anode voltage Reversible cathode voltage Total 

The actual time when a membrane is changed depends very much on local power costs, the type of membrane and cell systems used. In the economic assessments for membrane installation high priority must be placed on obtaining the right system overall, including the costs of all the replacement operations and other maintenance activities and also the correct choice of current density. As current densities increase so do power consumptions, while capital costs reduce. However, higher current densities will tend to lead to shorter operating lives, so optimization studies are essential in any project assessment. 

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Fig. 1. Flow diagram for chlor-alkali production by a membrane cell process.

In the membrane process, the chlorine (at the anode) and the hydrogen (at the cathode) are kept apart by a selective polymer membrane that allows the sodium ions to pass into the cathodic compartment and react with the hydroxyl ions to form caustic soda. The depleted brine is dechlorinated and recycled to the input stage. As noted already, the membrane cell process is the preferred process for new plants. Diaphragm processes may be acceptable, in some circumstances, but only if nonasbestos diaphragms are used. The energy consumption in a membrane cell process is of the order of 2,200 to 2,500 kilowatt-hours per... 

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

Membrane-based separation, lactic acid production and, 14 120 Membrane biocompatibility, in hemodialysis, 26 823—824 Membrane bioreactors, 16 26 Membrane-bound enzymes, 10 338 Membrane cell process, 9 620 Membrane cells... 

Other typical uses of titanium in a membrane cell process are in the pipelines for hot anolyte and in the anolyte collecting tanks as well as dechlorination systems that normally work under vacuum and sometimes limit the use of loose linings. 

Of the chlorine production capacity installed in Germany, which totalled 4.4 million tonnes in 2003, 50% were from the membrane cell process, 27% from the mercury cell process and 23% from the diaphragm cell process. The mercury cell process has been the subject of environmental policy criticism for years because of its use of mercury cathodes and resulting pollutant emissions. Hence, no new mercury plants will be... 

Membrane cell process -> alkali chloride electrolysis... 

Alercury has a high vapor pressure at the normal cell operating conditions hence it is always found in the reaction products. Although the mercury is almost completely recovered and returned to the process, environmental problems associated with mercury, combined with the less efficient eneigy utilization compared to the modem membrane cell process, has effectively stopped the building of new mercury cell plants. Furthermore, in the 1990s, membrane cells will most likely replace most of the present mercury cells. For details related to mercury cells, see references 8 and 16 and general references. 

F. Hine and A.J. Acioli Maciel,... the Amalgam Cell and the Membrane Cell Processes for Chloralkali Production,/. Appl. Electrochem. 22, 699-704 (1992). 

Summary. Membrane cell processes have become important to modem technology to a great extent because of the development and utilization of perfluorinated membranes. The combination of metal anodes and the perfluorinated membranes has provided a modem revolution in the area of chlor-alkali production. 

Chlor-alkaU membrane cell process (for the production of chlorine gas and concentrated NaOH)... 

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. 

Brine and soft water of ultrapure quality is essential for smooth and efficient operation of membrane cell process since Ca2+ and Mg2+ ions can harm the performance of ion-exchange membranes in the following ways. The precipitated Ca(OH)2 and Mg(OH)2 offer increased electrical resistance across the membrane thereby increasing the cell voltage. Furthermore, the anolyte diffusion layer characteristics are affected which would alter the optimum current density. More seriously, the membrane performance is affected... 

Since the first membrane cell installation at the Nobeoka plant by Asahi Chemical Industry in 1975, several membrane cell plants have been constructed, especially in Japan, as a pollution-free chlor-alkali process. By the end of 1982, the total capacity of the membrane cell process in the world was estimated to be about 600,000 tons of NaOH per year.112... 

Currently, major research and developmental emphasis is toward achieving an energy consumption of less than 2000 kWh/M.TNaOH. The membrane technology is so advanced that a reduction of —100 kW h/M.T NaOH will reach the practical minimum value. Thus, the membrane cell process promises to be the main technology for chlor-alkali production in the near future. 

Membrane cell process for manufacture of caustic soda has mostly replaced the mercury cell process due to better efiftciency. The mercury cell process also had environmental pollution issues that have been removed in the membrane cell process. 

In the membrane-cell process, sodium or potassium chloride brine is fed to the cell and distributed equally among the anode compartments, while water fed into a second header flows into the cathode compartments or into an externally recirculating stream... 

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