Membrane cell installation

Next, ways to augment liquefaction and bring chlorine recovery to more than 99% are considered. To eliminate some of the many variables, only the off-gas from a two-stage liquefier, as in Fig. 7.2, condensing 97.7% of the chlorine from a membrane-cell installation is utilised. The gas is available at 800 kPa. 

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... 

Still, a small relative difference in performance between two types of cell or two different proposals raises the question of its sustainability. The discussion of membrane performance in Section 4.8.5 makes it clear that performance deteriorates with time. The rate or extent of deterioration becomes just as important as the initial performance. It is essential for anyone studying the justification for a new membrane-cell installation or evaluating the differences between offers to understand this and to quantify it to some degree. The following are important factors ... 

Generally, the brine is free of mercury in new membrane-cell installations. However, if a mercury plant is converted to membrane technology, it is very likely that the brine will be contaminated with mercury. Laboratory tests have shown that the concentration of mercury in the catholyte will be about 1,000-fold lower than the concentration in the anolyte, and it is usually considered that lOppb Hg in the catholyte has no short-term effect on voltage. 

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. 

A bipolar pilot membrane cell with six cell elements each of 1.8 m2 has been installed and run with industrial electrolyte. 

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... 

Commercial application of membrane cell technology began in 1975 with the installation of the Nobeoka No 1 (Japan) using Asahi Chemical Co. electrolyzers, Reed Paper (Canada) using Hooker MX electrolyzers and American Can of Canada (Canada) using Ionics Chloromate electrolyzers. By the end of 1982 world capa-... 

The gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

The process is used on a large scale to recover mineral acids from salt solutions obtained in pickling and etching processes. In this application only anion exchange membranes are installed in a stack as indicated in Figure 19. By feeding in alternating cells a mixture salt and acid and pure water in counter current flow more than 95% of the acids can be removed from the feed solution. 

For a pervaporation cell used, the hydrodynamics can be controlled by respecting the proportions of a perfectly stirred reactor. The diameter of the cell with baffled double-envelope is of 0.07 m. The membrane is installed at the bottom of the cell. This assembly also allows the temperature control of the membrane. Thus, it is possible to suppose that the experiments are carried out without polarization of temperature. 

Membrane blistering can also occur during shutdowns, when reverse currents flow and water is transported from the cathode side to the anode side. A similar situation arises when a membrane is installed backward. The accumulation of water in the membranes leads to void formation. It is, therefore, essential that the reverse currents be suppressed by flushing the cell to remove the hypochlorite, by lowering the temperature to lower the diffusion rates, and by maintaining the same electrolyte concentrations in both the compartments. Dilution of the catholyte, or continuous flow of the anolyte is necessary to reduce diffusion/anolyte dehydration with salt deposits on the anode face of the membrane. 

The treated brine handled in the clarifier is alkaline and not highly corrosive. At least in diaphragm plants, carbon steel often can be used as the major material of constmction. Since brine is more corrosive in the presence of oxygen, those parts near or above the water line are sometimes coated. With mercury or membrane cells, there is also the possibility of pickup of harmful elements to be considiered. It is common practice to use coated walls and rubber-covered internals in these cases. When clarifiers are installed at grade level, the bottoms are frequently of concrete. This too is more characteristic of diaphragm-cell plants. Membrane cells can be damaged by pickup of such compounds as sulfates and silica. 

The filtration membranes are sensitive to fouling as well as to free chlorine. This situation is least troublesome in a membrane-cell plant, where the problem components already have been removed from the brine. In mercury-cell plant applications, an installation in the brine recycle loop should include some means of dechlorination. The usual choice is treatment with activated carbon, which is covered in Section 7.5.9.3B. The membranes are in spiral-wound modules placed in cylindrical housings and assembled as on the skid shown in Fig. 7.81. Figure 7.82 shows the construction of a modular element. The low-sulfate permeate flows through the membranes into spacer channels... 

Two disadvantages of this approach are the constraint on membrane-cell capacity and the fact that all the caustic can be regarded as the less marketable mercury-plant product. A larger membrane capacity can of course be installed to eliminate the first disadvantage and alleviate the second, but only at the expense of installing and operating an evaporator. 

Besides the choice of evaporator type, evaporation process design includes selection of the number of effects to be installed. This choice is primarily a matter of a classical economic balance between the cost of supplying more effects and the benefits they offer in energy consumption. As the preceding section made clear, the number of effects in a caustic evaporator is never very great. For technical and economic reasons, diaphragmcell evaporators usually have three or four effects and membrane-cell evaporators have two or three. 

Several mercury/diaphragm hybrid plants have operated for many years. We can expect no more to be built. New mercury cells will not be installed. Any expansion at a mercury-cell site would first of all probably be based on membrane cells and second probably be an occasion for retirement of at least some of the mercury cells. 

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.

---