Causticizing efficiency

Current Efficiency. Current efficiency for caustic production in diaphragm and membrane cells can be estimated from collection of a known amount of caustic over a period of time and from a knowledge of the number of coulombs of electricity passed during that time period. An alternative method involves analysis of the gases evolved during electrolysis and determining the anolyte composition. Material balance considerations (7) show the expression for the caustic efficiency for membrane cells to be... 

For estimating CI2 efficiency (Tlcb) term Cl2(a) in equation 14 should be dropped. The corresponding expression for caustic efficiency for diaphragm cells is... 

It may be noted that the oxygen evolution reaction (14) or (15) is thermodynamically favored over the chlorine evolution reaction. It results in the generation of electrons at the anode. Hence, the chlorine efficiency will be lower than the caustic efficiency. It is interesting to mention here that the caustic efficiency will never be 100% because of the oxygen evolution reaction, as the H+ generated via Eq. (14) must react with the OH ions generated at the cathode to preserve the electroneutrality of the system. 

Caustic efficiency losses from the formation of HOCl, OCl, and... 

F. Current Efficiency Equation for Membrane Cells. The chlorine and caustic efficiency expressions derived from the material balance for the various pertinent species across the electrolyzer are given by Eqs. (49) to (53). These equations need further simplification to eliminate the F/I term and the brine flow rate terms, especially the depleted brine flow rate q, because of the practical difficulties associated with its measurement. 

It is important to note that the process chlorine efficiency is essentially the same as the cell caustic efficiency since, from Eq. (60), it can be seen that ... 

Thus, Eqs. (81) and (88) represent the chlorine and caustic efficiency expressions derived rigorously from the appropriate material balances while accounting for the BCLs. Equations (81) and (88) can be simplified, by expressing the depleted brine flow rate in terms of caustic current efficiency as ... 

All the sodium in the sodium amalgam formed in the cell is converted to caustic in the decomposer. However, all of the caustic generated in the decomposer is not collected, as some slips back to the cell. Hence, the measured caustic efficiency NaOH equal to ... 

These protons will be transported to the cathode side, lowering the caustic efficiency. Thus, as long as O2 evolution is occurring at the anode, it appears impossible to achieve 100% caustic efficiency, even though one may approach it. 

Thus, the perfluorosulfonate membranes provide low caustic efficiencies at high caustic strengths, while the perfluorocarboxylate membranes offer high current... 

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