Chlor-alkali process advantages

Table 3 summarizes the advantages and disadvantages of the three chlor-alkali processes. 

Chlorine and Alkali from Sodium Chloride etc. 283 Tab. 3 Advantages and disadvantages of the three chlor-alkali processes [3, p. 117]... 

The advantages and disadvantages of the three chlor-alkali processes are summarized in Table 18. The three chlor-alkali processes can be compared in respect to the quality of the chlorine and caustic produced, and the equipment and operating costs. 

The ideal chlor-alkali process is one that is energy-efficient and does not use mercury. A type of cell offering these advantages is the membrane cell, in which the porous diaphragm of Figme 19-24 is replaced by a cation-exchange membrane, normally made of a fluorocarbon polymer. The membrane permits hydrated cations (Na" " and H3O ) to pass between the anode and cathode compartments but severely restricts the backflow of Cl and OH ions. As a result, the sodium hydroxide solution produced contains less than 50 ppm chloride ion contaminant. 

Among the various desulphation systems available in the chlor-alkali industry, the RNDS offers excellent advantages in terms of its low initial and running costs and environmental issues. In addition, it is a simple system and the process design results... 

Mixed-flow dissolvers are an attempt to combine the advantages of upflow and downflow. Upflow-downflow dissolvers were conceived in order to add a filtration zone for the brine produced in an upflow mode. Problems arise when suspended solids accumulate on the filter salt and impede the flow of brine. This mode is not often found in the chlor-alkali industry. Downflow-upflow dissolvers, on the other hand, are rather commonly used. A familiar example in the industry is the highly successful Lixator . The term, devised from lixiviate, is a trademark of Cargill Salt Company. The Oxford Universal Dictionary defines lixiviation as the action or process of separating a soluble from an insoluble substance by the percolation of water. The basic apparatus (Fig. 7.19) consists of a cylindrical body above a conical section with a false bottom. Water or depleted brine is fed through a spray or distributor into the bed and flows to the bottom of the vessel. At the bottom is an opening in the false bottom that allows the brine to flow into the annulus between the two shells and then up to an overflow point. 

Chlor-Alkali Industry. ) The replacement of the mercury process in the chlor-alkali industry by a nonmercury process is considered to be urgent, for the elimination of mercury pollution and for energy conservation. The membrane process has been proven as an effective alternative to the mercury process because of its important advantages, such as 1) Freedom from mercury pollution. 2) Lower electric power consumption. 3) Small steam consumption. 

The carboxylated ion exchange membrane has been advantageously used to obtain up to 35 wt% of caustic soda with the current efficiency of more than 96% in the industrial ion exchange membrane process for chlor-alkali production by electrolysis of sodium chloride. (lO)... 

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