Chlor-alkali diaphragm process

Figure 3.3 Separation system structure flowsheet for the chlor-alkali diaphragm process. Energy flows, electricity and steam, are marked with dashed arrows.

Figure 13. Flow diagram of the chlor-alkali diaphragm process...

It is essential for all technical chlor-alkali electrolysis processes - as subsequently discussed in Section 6.19.2.2 - that the transport of hydroxide ions formed at the cathode into the anode compartment is excluded (membrane process) or at least largely suppressed (diaphragm process). In the mercury cell process, OH ions are not formed in the entire process. 

The wastewater generated in the membrane cell and other process wastewaters in the cell are generally treated by neutralization.28 Other pollutants similar to those in mercury and diaphragm cells are treated in the same process stated above. Ion exchange and xanthate precipitation methods can be applied in this process to remove the metal pollutants, while incineration can be applied to eliminate some of the hydrocarbons. The use of modified diaphragms that resist corrosion and degradation will help in reducing the amount of lead, asbestos, and chlorinated hydrocarbon in the wastewater stream from the chlor-alkali industry.28... 

Summary of Raw Waste Loadings Found in Verification Sampling of Unit Product of Chlor-Alkali (Mercury Cell and Diaphragm Cell Processes)... 

FIGURE 22.6 General wastewater treatment process flow diagram at a diaphragm cell plant for production of chlor-alkali. 

Diaphragm cell A family of electrochemical chlor-alkali processes using cells with semi-permiable membranes which minimize diffusion between the electrodes. The overall reaction is 2NaCl + 2H20 = 2NaOH + H2 +C12... 

Glanor A Chlor-Alkali process using a bipolar diaphragm cell. Developed by PPG Industries and Oronzio de Nora Impianti Elettrochimic in the early 1970s. 

For over a hundred years the chlor-alkali industry has used the mercury cell as one of the three main technologies for the production of chlorine and caustic soda. For historical reasons, this process came to dominate the European industry - while in the United States the asbestos diaphragm cell took the premier position. Over the last two decades developments in membrane cells have brought these to the forefront, and membrane cells of one kind or another now represent the technology of choice worldwide. 

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... 

Chlorine gas, CI2, is prepared industrially by the electrolysis of molten NaCl (see Section 19.8) or by the chlor-alkali process, the electrolysis of a concentrated aqueous NaCl solution (called brine). Chlor denotes chlorine and alkali denotes an alkali metal, snch as sodium.) Two of the common cells nsed in the chlor-alkali process are the mercnry cell and the diaphragm cell. In both cells the overall reaction is... 

Fig. 28. Flow diagrams of the Mercury and Diaphragm chlor—alkali processes.

The choice of technology, the associated capital, and operating costs for a chlor—alkali plant are strongly dependent on local factors. Especially important are local energy and transportation costs, as are environmental constraints. The primary difference in operating costs between diaphragm, mercury, and membrane cell plants results from variations in electricity requirements for the three processes (Table 25) so that local eneigy and steam costs are most important. 

Figure 21.26 A diaphragm cell for the chlor-alkali process. This process uses concentrated aqueous NaCI to make NaOH, CI2, and H2 in an eiectrolytic celi. The difference in iiquid ievei between compartments keeps a net movement of solution into the cathode compartment, which prevents reaction between OH" and Ci2. The cathode eiectroiyte is concentrated and fractionaiiy crystallized to give industrial-grade NaOH.

As Figure 21.26 shows, the sodium salts in the cathode compartment exist as an aqueous mixture of NaCl and NaOH the NaCl is removed by fractional crystallization, which separates the compounds by differences in solubility. Thus, in this version of the chlor-alkali process, which uses an asbestos diaphragm to separate the anode and cathode compartments, the products are CI2, H2, and industrial-grade NaOH, an important base. 

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. 

The unit operations in a commercial chlor-alkali plant can be generally classified as follows (1) brine purification, (2) electrolytic cells, (3) H2 and Cl2 collection, and (4) caustic concentration and salt removal. In this section, the general process flowsheets for diaphragm, membrane, and mercury cell technologies are discussed with emphasis on the need for brine purification and the manner in which it is carried out. 

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