Membranes carboxylate layer

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

Asahi s investigations showed that a Na+ concentration of 1.1 N was necessary in compartment II to maintain a current efficiency of 96% in the carboxylic membrane during operation with 3.5 N brine in compartment I and 32% caustic soda in compartment III. The Na+ concentration in compartment II was generally far lower than that in compartment I, and clearly indicates the tendency for depression of the Na+ concentration at the interface of the sulphonic and carboxylic layers in the normal... 

In summary, the results indicate that the Na+ concentration at the interface between the sulphonic and carboxylic layers in the normal membrane is substantially depressed from that of the anolyte compartment, and that the concentration at this interface strongly affects the current density. 

The limiting current density may be defined as the current density at which the depression of the Na+ concentration at the interface of the membrane s sulphonic and carboxylic layers results in an abrupt rise in cell voltage and drop in current efficiency. 

The membrane is exposed to chlorine and brine on one side and strong caustic solution on the other side at temperatures of around 90 °C. Only ion-exchange membranes made of perfluoropolymer can withstand such severe conditions. The ion-exchange groups are sulfonate (SOf) or carboxylate (CHCOO-). Modern membranes of this kind consist of three layers, a carboxylic layer on the cathode side, a sulfonate layer on the anode side and a reinforcement layer of fabric in between. In addition, both sides are provided with a hydrophilic coating [3, p. 81 15]. The thickness of the membrane is only one- to two-tenth of a millimeter. It must be mentioned that the hydrate water of the cations is taken along through the membrane. 

Salt rejections by the NS-300 membrane toward synthetic seawater improved as the isophthalamide content of the barrier layer increased. Surprisingly, membrane flux peaked rather than simply declining as a function of increasing isophthalamide content. This is illustrated by the data in Table II. Maximum water permeability characteristics were found at an approximate copolymer ratio of 67 percent isophthalic and 33 percent trimesic groups. The differences in the magnesium sulfate versus sodium chloride rejection appear to be due to the anionically charged nature of the membrane barrier layer, which is rich in carboxylate groups. 

These performance goals have now largely been attained by continued improvements through several generations of materials. Currently, commercial perfluorinated ionomer materials for this application consist of membranes with carboxylate or mixed carboxylate-sulfonate functionality the latter membranes often have layered structures with the carboxylate layer exposed to the caustic catholyte solution. Fabric reinforcement is used in some instances to improve strength. 

Bilayer carboxylate membranes can be produced by surface modification of Nafion-type membranes. In a process used by Asahi Chemical the sulfonate on a Nafion-type surface is reduced to sulfinic and sulfenic acids, then oxidized to a carboxylate layer of 2-10 pm thickness ... 

Perfluorinated membranes used in chlor-alkali cells normally have a thin layer of carboxylate on the cathode-facing surface of a sulfonate membrane. Nafion 901 was introduced as such a membrane [38]. It achieved 33% NaOH concentration with 95% current efficiency in cells operating at 3 kA/m and 3.3 to 3.9 V. The carboxylate layer can be prepared by lamination, but the layer can be... 

With the carboxylate layer on the membrane, the concentration of NaOH in the catholyte can be maintained at about 32%, which compares favorably with the 12% product from diaphragm cells. Consequently, considerably less thermal energy is needed for evaporation to the commercial 50% product. 

These composite membranes are prepared either by lamination of the perfluoro-carboxylate and perfluorosulfonate films [75-77] or by the chemical conversion [78-80] of one surface of the perfluorosulfonic acid to perfluorocarboxylic acid to realize a carboxylate layer thickness of 5-10 xm. 

The membrane conductivity data used in the present calculation are those for a carboxylic acid membrane [97-99], as the overall conductivity of a composite membrane is dictated by the carboxylic layer. Diffusion coefficient data for chloride and chlorate ions across the commercial composite membranes are not available. Hence, an average of the diffusion coefficient data for the chloride ion for carboxylic acid membranes [100,101] was used, assuming the same temperature dependence as that of membrane conductivity. The diffusion coefficient of the chlorate ion was assumed to be the same as that of the chloride ion. Variations in Dq- and k with anolyte concentration, under commercial operating conditions, were reported [102] to be weak, and hence, these dependencies were not considered here. 

Impurities such as Ni and Mg, whose hydroxides have low solubilities, tend to precipitate near the membrane surface on the anode side, causing an increase in the ohmic drop across the membrane. On the other hand, impurities such as Ca, Sr, and Ba, with higher solubilities, are prone to deposit on the cathode side of the membrane in the carboxylate layer, leading to the formation of large voids and therefore, to decreased current efficiencies. The actual situation may be more complex because of mutual interactions of impurities to form complex species. 

Calcium in the brine is particularly harmful because of the moderate solubility product of its hydroxide ( 10 ). Calcium ions penetrate the membrane and precipitate as hydroxide near the membrane-catholyte interface or at the interface of the sulfonate and carboxylate layers. The voltage drop increases greatly when Ca(OH>2 accumulates to a concentration of about 1 mg cm [18]. 

There are three types of blisters, water, salt, and others. Water blisters are formed when there is high water transport across the layers. Salt blisters result finm localized heating of the membrane because of high local current density or nonuniform current distribution. Proper cell design can alleviate this problem (Chapter 5). The other type of blister arises when the acidity of the anolyte is high. When the anolyte pH is below 2, the carboxylate layer protonates to a nonconductive carboxylic acid. This will increase the voltage and the internal vapor pressure, and finally result in the formation of voids in the carboxylate layer. 

IE. Effects of Sulfate. Excessive amounts of sulfate in the brine can lead to the precipitation of certain sulfate compounds in the carboxylic layer of the membrane. This reduces the current efficiency. Therefore, a maximum sulfate level of 8gplNa2S04 in the feed brine is recommended. The sulfate concentration can accumulate to this level unless it is controlled by purging or by some removal process. Section 7.5.7 discusses a number of candidate processes. 

Preparation of the Membranes. Modern cation exchange membranes consist of three layers, the carboxylic layer on the cathode side, the sulfonate layer on the anode side and a reinforcement layer of fabric in between. 

As early as 1848, it had been suggested that sensory receptors transduce only one sensation, independent of the manner of stimulation. Behavioral experiments tend to support this theory. In 1919, Renqvist proposed that the initial reaction of taste stimulation takes place on the surface of the taste-cell membrane. The taste surfaces were regarded as colloidal dispersions in which the protoplasmic, sensory particles and their components were suspended in the liquor or solution to be tested. The taste sensation would then be due to adsorption of the substances in the solution, and equal degrees of sensation would correspond to adsorption of equal amounts. Therefore, the rate of adsorption of taste stimulants would be proportional to the total substances adsorbed. The phenomenon of taste differences between isomers was partly explained by the assumption that the mechanism of taste involves a three-dimensional arrangement for example, a layer of fatty acid floating on water would have its carboxylic groups anchored in the water whereas the long, hydrocarbon ends would project upwards. 

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