Diaphragm cell chlor-alkali

The chlor-alkali cell in this diagram electrolyzes an aqueous solution of sodium chloride to produce chlorine gas, hydrogen gas, and aqueous sodium hydroxide. The asbestos diaphragm stops the chlorine gas produced at the anode from mixing with the hydrogen gas produced at the cathode. Sodium hydroxide solution is removed from the cell periodically, and fresh brine is added to the cell. 

The first electrochemical application of the D -statistic deals with the lack- of-association (i.e. independence) hypothesis concerning current efficiency and current load in diaphragm-type industrial scale chlor-alkali cells [18], Table 5 demonstrates that the two factors are independent with the understanding that the current efficiency/current load relationship may indirectly be influenced by other technical variables, e g. cell potential, and impurities. 

Table 5. Testing the independence of current efficiency and current load via eight different diaphragm-type chlor-alkali cells [18]...

The Spearman-Hotelling-Pabst test may be used for determining if a Z)-type statistic in Eq.(15) is significant, by comparing the computed D, D, etc. to lower quantiles of the statistic, tabulated e g. in [31], The diaphragm-type chlor-alkali cells in Section II.3 with rs =... 

The rapidly growing use of C102 in the pulp and paper industry has led to the rapid growth of sodium chlorate, NaC103, production in recent years. Sodium chlorate is produced by the electrolysis of NaCl brine in a cell that is very similar to a diaphragm chlor-alkali cell, except that it has no diaphragm. The overall reaction is as follows ... 

Varjian, R. D. 1981. Energy Analysis of the Diaphragm Chlor-Alkali Cell. Lectures in Electrochemical Engineering, AIChE Symposium Series, 219 - 226. 

Detailed investigations by MacMullin et al.72 show that the value of Kx/r for many porous beds is 3.666 0.098. Since the tortuosity of a porous asbestos diaphragm is —1.5 and Kx varies in the range of 5-5.5, the model describing the porous beds is applicable for the separators used in diaphragm chlor-alkali cells. 

Mass transfer through separators in chlor-alkali cells has been studied by several authors and the general one-dimensional model (see Fig. 16) proposed by Mukaibo82 is discussed in this section. Anolyte flows from the anode compartment through the diaphragm toward the cathode side under a hydrostatic head with a velocity of v. Hydroxyl ions generated in the cathode compartment move... 

Figure 16. Mass transfer through the diaphragm and the concentration profile of H+, OH, and Cl2 in a diaphragm-type chlor-alkali cell.

FIGURE 3.1. Schematics of mercury, diaphragm, and membrane chlor-alkali cells. 

Galvanostatic and Potentiostatic Polarization Measurements. Electrode processes may be classified into two types reaction or charge-transfer controlled, and diffusion or mass-transfer controlled. The electrode processes in diaphragm and membrane chlor-alkali cells are charge-transfer controlled. On the other hand, the formation of sodium amalgam on a mercury cathode, is diffusion-controlled. 

The E° and E° values shown in parentheses are the equilibrium electrode potentials for reactions (142) and (143) at 25°C and with reactants and products at unit activity. In practice, chlor-alkali cells operate at different temperatures and concentrations, hence, the El and ° terms should be properly corrected using the Nemst equation. Typical conditions encountered in diaphragm-type chlor-alkali cells are as follows ... 

In the case of a diaphragm or membrane chlor-alkali cell, there are three ohmic drops to consider anode to separator, cathode to separator, and across the separator. The anode to cathode distance in ELTECH s H-4 cell is 7.74 imrt, the thickness of the diaphragm is 3.05 mm, and the cathode to diaphragm distance is negligible. The anode to diaphragm ohmic drop, // a/s. niay now be estimated from the conductivity data of the electrolytes, which are presented in the appendix to this section. 

The ohmic drop of the hardware depends on the nature of the materials used, and the intercell design configuration. These values have been found to be about 0.25 V for H-4 type diaphragm chlor-alkali cells, at a load of 150kA or 2.32 kA m . ... 

Ru02 losses can occur via the dissolution of the intermediates in the above reactions. However, the published corrosion data for Ru from O2 evolution are not reliable as the data forecast Ru losses as high as 40gm hr [80], which are inconsistent with the observations in diaphragm chlor-alkali cell operations. 

The principles involved in the brine flow across a vacuum-deposited asbestos diaphragm in a chlor-alkali cell are similar to those for filtration operations. When the flow is laminar... 

A major differentiator among the various chlor-alkali cells is the quality of their product caustic solutions. Diaphragm cells produce a liquor containing about 11% NaOH and... 

The chlorine evolution reaction and the hydrogen evolution reaction at the anode and the cathode, respectively, in a chlor-alkali cell are controlled by the electrochemical and/or chemical steps rather than by mass transfer. However, the transport phenomena across the separator, either a porous diaphragm or an ion-exchange membrane, are governed by the solution flow near the surface. The disproportionation reaction of hypochlorites in a chlorate cell is diffusion-controlled process. Consequently, knowledge... 

Industrial chlorate electrolysis takes place in undivided cells, where sodium chlorate and hydrogen gas are formed as described by reaction 1. More detailed, reactions 2 and 3 show the main anode and cathode reactions of chloride oxidation and hydrogen evolution, respectively. Note that these electrode reactions are similar to those in a chlor-alkali cell, though while a chlor-alkali cell has a membrane or diaphragm separating an acidic anolyte from an alkaline catholyte, the chlorate cell is undivided with an electrolyte at close to neutral pH. Chlorine formed therefore dissolves as in reactions 4 and 5 and, chlorate is formed in a disproportionation reaction, number 6 below [3]. 

The electrode reactions in a membrane cell are the same as those in a diaphragm cell but the separator is now a cation-permeable membrane. The development of modern membrane cell technology dates only from about 1970 when it was recognized that the perfluorinated membranes (section 3.2.2) had the properties essential to the chemistry of a chlor-alkali cell. 

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