Caustic production

Faraday s law states that 96,487 coulombs (1 C = 1 A-s) are required to produce one gram equivalent weight of the electrochemical reaction product. This relationship determines the minimum energy requirement for chlorine and caustic production in terms of kiloampere hours per ton of CI2 or NaOH... 

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... 

Fig. 38. Caustic purification system a, 50% caustic feed tank b, 50% caustic feed pumps c, caustic feed preheater d, amonia feed pumps e, ammonia feed preheater f, extractor g, trim heater h, ammonia subcooler i, stripper condenser j, anhydrous ammonia storage tank k, primary flash tank 1, evaporator reboiler m, evaporator n, caustic product transfer pumps o, purified caustic product cooler p, purified caustic storage tank q, ammonia stripper r, purified caustic transfer pumps t, overheads condenser u, evaporator v, evaporator vacuum pump w, aqueous storage ammonia tank x, ammonia scmbber y, scmbber condenser 2, ammonia recirculating pump aa, ammonia recycle pump. CW stands for chilled water.

Mild steel cathodes are used extensively in chlor-alkah and chlorate cells. Newer activated cathode materials have been developed that decrease cell voltages about 0.2 V below that for cells having mild steel cathodes. Some activated cathodes have operated in production membrane cells for three years with only minor increases in voltage (17). Activated cathodes can decrease the energy consumption for chlorine—caustic production by 5 to 6.5%. 

The changing balances which do occur have to be resolved . This can be by trade changes such as increasing exports of either EDC or caustic to balance the plant. Some markets are not easy to enter. For example, while Australia imports over one million tons of caustic soda it does so in 25 000 DMT shipments. This is 50 000 liquid tons of material. Relatively few plants have access to such facilities or can indeed store such a quantity. With some Asian countries becoming more self-sufficient in caustic production Japan had to increase its port handling facilities, raising the load size as well as increasing the number of ports to three. 

Local manufacture of caustic is a direct function of local chlorine production and accounts for 126 000 DMT per annum, with the balance of the demand being imported. No local caustic production is used in the alumina industry. Table 10.1 summarises the caustic markets. 

Another aspect that may be taken into account is that of membrane electrolysers having a lower power consumption (Fig. 15.4). Not only does the new technology save power but it also requires less steam to evaporate the cell caustic product to 50%. Additionally, salt removal equipment required in diaphragm plants uses power. This benefit can also be turned around so that for the same power consumed by a diaphragm cell room extra volumes of rayon-grade caustic soda can be produced from the membrane electrolysers. 

The use of polyperfluorosulfonic acid membranes as the cell separator was first demonstrated about three decades ago. Yet it was not until the mid-1980s when the economic advantages of membrane cells over the traditional mercury- and diaphragm-cell technology were fully demonstrated—consequent to better membrane performance, higher caustic product concentrations, and lower power consumption. Retrofitting chlor-alkali facilities with membrane cells accounted for much of the growth and sustenance of this industry over the past two decades. 

C and caustic products of 20-40 weight percent. The cell voltage increases dramatically with caustic strength. The ohmic drop of the membrane or its electrical resistance increases with increasing caustic concentration and also with brine concentration but to a lesser extent with brine than caustic strength. 

In April 1975, Asahi Chemical started operation of a membrane chlor-alkali plant with a capacity of 40,000 MT/Y of caustic soda using Nafion perfluorosulfonic acid membrane. In 1976, this membrane was replaced by perfluorocarboxylic acid membrane developed by Asahi Chemical. The total caustic production capacity of plants based on Asahi Chemical s membrane chlor-alkali technology using perfluorocarboxylic acid membrane will reach 520,000 MT/Y in 1982, at seven locations in various countries. 

Figure 6. Relationship between NaCl in caustic product and concentration of NaOH in catholyte. Key Q, NEOSEPTA-F C-1000 fl), NEOSEPTA-F C-2000.

Figure 36-10, p. 427, shows the arrangement for cationic substitution. Notice that this is ACCACCACCA rather than ACACACAC. In this particular case, a caustic product feed is acidified by replacing cations M with hydrogen ions (H ) from an acidic makeup stream. Other applications of this principle are replacing with ammonium ions (NH/) or X with hydroxide (OH ) or acetate (OAc ) ions. 

Ion-exchange membranes for chlorine-caustic production should satisfy the following requirements,92 some of which tend to be conflicting with each other as will be elaborated in Section 6.2 ... 

The theoretical energy requirement for chlorine and caustic production was shown in Section 4.4.2 to be ... 

Thus, during a cell shutdown, the transport of anions is solely governed by diffusion (Fig. 4.8.27). The flow of anions into the cathode compartment can be minimized by lowering the cell operating temperature before shutdown, as is evident from Eq. (33), via the variations in D with cell temperature, T. Because of this effect of maximal carryover of anions at i -> 0, it is important that the cells be designed so that the ratio of active membrane area to the peripheral inactive area is high in order to achieve low anionic impurity content in the caustic product. 

FIGURE 4.8.28. Effect of current density on the NaCl concentration in the caustic product at 90°C and 200 gpl NaCl in the anolyte, calculated using Eq. (30) [95]. 

C. Effect of Operating Temperature. Increase in temperature lowers the electric field-driven migration rate and increases the diffusion rate. Hence, the chloride and chlorate concentrations in the caustic product will increase at elevated temperatures as shown in Figs. 4.8.31 and 4.8.32. Note that the activation energy for the diffusion of Cl ions is about 10 kcal mol , implying that the Cl levels decrease by about 30% for every 10°C decrease in the temperature. 

These are usually 48-50% NaOH and 45-50% KOH. This is a unique advantage of the mercury cell. Another advantage of merciuy cells is the purity of the caustic product. High purity requires a high grade of water, and demineralized water is the standard material here. 

Allows hardness to break through ion-exchange beds and attack membranes damage to evaporators and contamination of caustic product Equipment damage deactivation of anode coating Lower-quality product accumulation in compressor suction duller, higher bromaie concentration in bleach Accumulation of very finely divided solids in membrane s... 

The caustic recycle flow to membrane cells is metered and distributed in essentially the same way as the brine flow. Manipulation of the extent of dilution of the caustic recycle also helps to control the concentration in the cells. With mercury cells, caustic production depends on the feed of water to the decomposers. Pure water is metered to each decomposer at a rate proportional to the operating load. 

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