Membrane chlorine alkaline

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

Many of today s available membranes meet most of these requirements. In particular, the Nafion-type cation-exchange membrane has quite satisfactory properties for applications in the chlorine-alkaline electrolyses as well as in electrodialysis [6], Anion-exchange membranes often show lower stability in strong alkaline solutions than cation-exchange membranes. 

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

In addition to the monopolar membrane described above a large number of special property membranes are used in various applications such as low-fouling anion-exchange membranes used in certain wastewater treatment applications or composite membranes with a thin layer of weakly dissociated carboxylic acid groups on the surface used in the chlorine-alkaline production, and bipolar membranes composed of a laminate of an anion- and a cation-exchange layer used in the production of protons and hydroxide ions to convert a salt in the corresponding acids and bases. The preparation techniques are described in detail in numerous publications [13-15]. 

The fundamental principle of SPE reactors is the coupling of the transport of electrical charges, i.e. an electrical current with a transport of ions (cations or anions), through a SPE membrane due to an externally applied (e.g. electrolysis) or internally generated (e.g. fuel cells) electrical potential gradient. For example, in a chlorine/alkaline SPE reactor (Fig. 13.3), the anode and cathode were separated by a cation-SPE membrane (e.g. Nafion 117) forming two compartments, containing the anolyte (e.g. 25 wt% NaCl solution) and the catholyte (e.g. dilute sodium hydroxide), respectively. 

The antimicrobial activity of iodine is less dependent than chlorine on temperature and pH, though alkaline pH should be avoided. Iodine is also less susceptible to inactivation by organic matter. Disadvantages in the use of iodine in skin antisepsis are staining of skin and fabrics coupled with possible sensitizing of skin and mucous membranes. 

This thin-film-composite membrane has been found to have appreciable resistance to degradation by chlorine in the feed-water. Figure 2 illustrates the effect of chlorine in tap water at different pH values. Chlorine (100 ppm) was added to the tap water in the form of sodium hypochlorite (two equivalents of hypochlorite ion per stated equivalent of chlorine). Membrane exposure to chlorine was by the so-called "static" method, in which membrane specimens were immersed in the aqueous media inside closed, dark glass jars for known periods. Specimens were then removed and tested in a reverse osmosis loop under seawater test conditions. At alkaline pH values, the FT-30 membrane showed effects of chlorine attack within four to five days. In acidic solutions (pH 1 and 5), chlorine attack was far slower. Only a one to two percent decline in salt rejection was noted, for example, after 20 days exposure to 100 ppm chlorine in water at pH 5. The FT-30 tests at pH 1 were necessarily terminated after the fourth day of exposure because the microporous polysul-fone substrate had itself become totally embrittled by chlorine attack. 

Unlike CA membranes, polyamide membranes cannot tolerate free chlorine or any other oxidizers. Some manufacturers quote 200 - 1,000 ppm-hrs of exposure until the membrane rejection is lost.21 This means after 200 - 1,000 hours of exposure to 1 ppm free chlorine, the membrane rejection will be unacceptably low. Chlorine attack is faster at alkaline pH than at neutral or acidic pH. 

Chemical stability - Ceramic membranes are not degraded by organic solvents and can withstand exposure to chlorine. Many crystalline oxides are relatively insoluble in acidic and alkaline media hence cercunic membranes coitposed of such oxides should be relatively inert under extreme pH conditions. 

The toxicity of hypochlorite arises from its corrosive activity on skin and mucous membranes. Corrosive burns may occur immediately upon exposure to concentrated bleach products. Most of this corrosiveness stems from the oxidizing potency of the hypochlorite itself, a capacity that is measured in terms of available chlorine . The alkalinity of some preparations may contribute substantially to the tissue injury and mucosal erosion. Sodium hypochlorite when combined with an acid or ammonia may produce chlorine or chloramine gas, respectively. An inhalation exposure to these gases may result in irritation to mucous membranes and the respiratory tract, which may manifest itself as a chemically induced pneumonitis. 

An important component of cuticle is 18 - methyl - eicosanoic acid [40]. Fatty acid is bound to a protein matrix, forming a layer in the epicuticle [41,42], and this layer is referred to as F - layer [43]. The F - layer can be removed by treatment with alcoholic alkaline chlorine solution in order to enhance wettability. The cuticle and epicuticle control the rate of diffusion of dyes and other molecules onto the fibre [44]. The cortex, however, controls the bulk properties of wool and has a bilateral structure composed of two types of cells referred to as ortho and para [45,46]. The cortical cells of both are enclosed by membranes of at least three distinct layers within which the microfibrils fit. Cells of intermediate appearance and reactivity designated meso - cortical have also been reported [47]. Cortical cells on the ortho side are denti-cuticle and thin, those on the para side are polygonal and thick [47]. Fig. 1-7 illustrates the bilateral structure which is responsible for the crimp of the... 

Particular attention was devoted to the control of membrane fouling and membrane cleaning. Acid-alkaline washing was tested and low concentration chlorine solutions were also used. The recovery of initial fluxes was generally 50% with the new modulus, and higher then 95% with a used modulus. These results indicate the existence of a certain irreversible fouling of the new membranes, which come to steady state values, and does not increase with membrane reuse. 

Ammonium is determined in many laboratories in a CF system in which the Berthelot reaction is implemented. In the Berthelot reaction, ammonium reacts with chlorine and phenol in the presence of sodium nitroprusside as catalyst in alkaline medium. EDTA is added to prevent interference of calcium and magnesium. Modern systems have been developed that use macroporous polytetrafluoroethylene (PTFE) membranes. In these systems a sample is introduced into a stream to which sodium hydroxide solution is added. Ammonia diffuses through the PTFE membrane into a stream of de-ionized water and the stream is fed through the flow-through cell of a conductivity meter. In this system a minimum of reagents is required and the only interference is from volatile amines. 

A wide variety of materials and coatings have been evaluated for use as cathodes in diaphragm and membrane cells, most of them nickel-based. Some of the compositions disclosed in the patent literature are presented in Table 4.6.3, and Trasatti [13] and Conway and Tilak [4] have reviewed kinetic factors involved during the course of the HER on various materials in alkaline media. Various activation procedures have been employed to modify the siuface morphology of nickel to increase the active surface area. Although many promising compositions and methods were reported [25-186], most were not tested in conunercial chlorine cells. This section discusses selected compositions and techniques of particular interest. Section 4.6.7 addresses the coatings that have been used in commercial operations. 

Major impurities (calcium, magnesium, and other metals) are removed from solution by precipitation (Section 7.5.2). The solids are separated from the treated brine by settling (Section 7.5.3) and one or two stages of filtration (Section 7.5.4). The precipitated solids are removed from the settler for disposal, and the residual brine contains a few ppm of hardness. This is not acceptable in membrane cells. An ion-exchange process therefore follows in that case (Section 7.5.5). The fully treated brine then is ready for use in the cells but is alkaline and contains carbonate. Most plants add acid to the brine in order to improve cell operation and chlorine quality, and this is the subject of Section 7.5.6. 

Acidification of brine before sending it to the electrolyzers is the standard but not universal practice. Purified brine is alkaline and contains carbonate added in excess during chemical treatment. Both OH and COi reduce the effective anode current efficiency by consuming chlorine in the cells. The addition of HCl to the brine neutralizes the contained OH and decomposes the CO to CO2 and water. More chlorine is produced with no increase in electrical load. Addition of excess acid can even neutralize some of the OH" that enters the anolyte by leakage back through diaphragms or membranes. Section 4.4.3 gives a more fundamental explanation of the effects of acid addition. 

Integrated units are available that combine the outputs from the anode and the cathode chambers of conventional membrane cells to make 3-7% sodium hypochlorite. Higher concentrations are possible, but the units do not handle precipitated salt. The brine may be similar to that used by conventional chlorine cells (300 g salt) or it may be more dilute. The entire anode effluent may be mixed with the cathode effluent without separating chlorine or caustic. In nonconventional cells with alkaline tolerant anodes, cathode effluent may be recycled to the anode, or a mixture of sodium chloride and sodium hydroxide may be fed into the anode. The concentration of sodium hypochlorite is determined by the concentration of brine, the relative volumes of effluent from the anode and the cathode, the flow rate, and the current. The units are usually sized for point-of-use operation. Conventional cells may also be used to separate chlorine gas from the anode effluent for point of use. °... 

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]. 

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