Caustic soda concentration

Caustic soda concentrations of 50% are produced directly from equation 11. This advantage is offset by higher operating cell voltages and some mercury contamination of the environment. This latter problem has been diminished or solved to an acceptable extent (31) however, it continues to influence the choice of cells for new plants. No new mercury cells have been installed in the United States since 1970 (32). 

As indicated in Fig. 17.2, the membrane process has long been characterised by substantial reductions in electric power consumption, through constant advances in membrane, electrolyser and electrode technologies. In the early years of its commercial establishment, some 25 years ago, it yielded a caustic soda concentration of 20% or lower, with less than 90% current efficiency. Today, the caustic soda concentration is 33%, the current efficiency is 97%, and the ohmic drop of the membrane has been lowered by approximately 1.0 V. During the same period, advances in electrolyser design have improved the uniformity of intracell electrolyte concentra-... 

The main by-products are 1-chlorobutadiene, produced from the residual dichloro 2-butenes or formed during the reaction, polymers, sodium chloride and monochloro-butenes (l-chloro 1-butene, 2-diloro 2-butenes, 2-chloro 1-butene, etc.) To control the undesirable polymerizations, the reaction takes place in an oxygen-free environment, at the lowest possible temperature, and with an inhibitor. Also effective is the presence of a solvent (methanol, ethanol) or a catalyst In this case, however, it is necessary to raise the caustic soda concentrations (30 per cent) or to employ other bases (liquid ammonia, ion exchange resins, etc.). In the absence of catalyst, the residence time is 3 to 5 h. and selectivity exceeds 95 molar percent for a once-through conversion of nearly 95 per cent... 

The Nafion membranes utilized in the early 1970 s produced caustic soda concentrations of 10-15wt% at electrolytic power consumptions of approximately 3450 KWH/MT NaOH. Advancements in the technology of membranes by duPont, Asahi Glass Co., and Asahi Chemical Co., Tokuyama Soda Co., have achieved membranes that today can produce caustic soda concentrations of 28-40wt% with caustic current efficiency well over 90% for long term operations. 

A boundary layer with a gradient in caustic soda concentration also forms at the surface of the membrane facing the catholyte based on a similar principle, resulting in a caustic soda concentration on the membrane surface which is higher than that in the bulk phase. Since this tends to reduce the current efficiency and electric conductivity of the membrane, it is necessary to minimize the boundary layer thickness or reduce the caustic soda concentration in the bulk phase. It is also essential to purify the brine with ion-exchange resin of high selectivity, in order to prevent precipitation of metal ions as hydroxides in the membrane and the boundary layer (74). 

Caustic Soda Concentration. The maximum electric conductivity of caustic soda solution occurs at a concentration of about 20% at the ordinary electrolysis temperature, and the membrane conductivity tends to decline sharply with caustic soda concentration in the catholyte exceeding 20% (26). The boundary layer effect described in the previous section also makes relatively low concentrations preferable. With increasing concentration of caustic soda, moreover, the allowable concentration of multivalent cation in the brine must be decreased exponentially because the solubility products of multivalent cation hydroxides are constant, and operational difficulties... 

Although as previously described it is preferable to operate the cell at around 20% - 30% of caustic soda concentration to minimize electrolysis power consumption, higher concentrations of catholyte are generally preferable to minimize steam consumption in the evaporation process. However, Asahi Chemical has developed a heat recovery evaporator which greatly reduces the need for external stream supply and therefore permits a significant reduction in the total energy consumption of electricity and steam (74,76). 

Caustic soda concentration. Other conditions being the same, the cathodic current efficiency decreases as the caustic soda concentration increases to more than 15-18%. 

Internal precipitation is less likely when the caustic soda contained in the cathodic compartment is diluted. With caustic soda concentrations in the range of 10-15%, the total concentration of impurities may reach a maximum value in the order of 1 - 5 pg/g, depending on the type of membrane [228]. If the membrane is operated in the acid state [232], [239], this limit of concentration may be increased to 20-30 pg/g [232], [241]. Obviously, this is a compromise, because the advantage of operating under safe conditions for the membrane (even in the presence of relatively high impurity concentrations) is counterbalanced by the lower cathodic current efficiency typical of the acid state of the cationexchange membranes. 

The limitation of the caustic soda concentration to 10% means that an additional step must be introduced into the process. To increase the concentration to the 50% solution normally traded, water must be evaporated and this requires additional energy and plant. The evaporation stage does, however, reduce the problem from chloride contamination since on cooling much of the sodium chloride crystallizes out from the 50% sodium hydroxide solution. Even so, the chloride level remains about 1% and this is not an acceptable level for all the applications of sodium hydroxide. 

B. Exchange Capacity. The degree of ionic permselectivity of the membrane is determined by the number of exchange groups and hence, the fixed-ion concentration in the membrane. During electrolysis, the water content of the membrane decreases with increasing caustic soda concentration. The fixed-ion concentration thus increases. 

Modern materials (e.g. Nafion 901) make use of the best features of both materials, see Figure 7. Most of the membrane is fabricated from the sulphonate resin to obtain the low resistance but the catholyte surface is coated with a thin film of carboxylate resin which permits the high caustic soda concentration to be produced. The plastic net gives the membrane additional mechanical and physical strength. 

Higher caustic soda concentrations of up to 50 wt% are still intended. However, the membranes, which until now have been developed for this purpose, showed too high voltage drop and insufficient stability. 

Iron or stainless steel is used as cathode material for reaction 4 at low caustic soda concentrations (diaphragm process), pure nickel is necessary at high concentrations (membrane process). Hydrogen evolution needs at these materials a relative high overpotential of up to 300 mV. It can be significantly decreased if coatings of, e.g., ruthenium oxides, are applied on the cathode surface (see also essay - Hydrogen Evolution Reaction ). 

The salt precipitated in the liquor flash tank is isolated from the rest of the salt precipitated in the evaporator and used as seed crystals in the cooling system to help diminish coil scaling and supersaturation of the product liquor with sodium chloride. The sodium chloride and triple salt (NaOH -NaCl-Na2S04) precipitated in the liquor flash tank and cooling system is removed from the cooled product liquor with centrifuges. The salt precipitated in the three effects flows countercurrent to the liquor flow so that all of the salt is discharged from the last effect, the effect that has the coldest liquor and the lowest caustic soda concentration. 

From the hydrocarbon phase distributed in the aqueous phase as droplets, COS will be partitioned into the aqueous phase, and then a diffiision limited fast reaction will take place in the aqueous boundary layer. Since the rate of hydrolysis of COS in the aqueous phase by caustic soda depends also on the aqueous phase COS concentration, Ccas,w> increasing Cqh, u by increasing caustic soda concentration can lead to a reduction in ficos by reducing Ccos,i . An increase in the electrolyte (NaOH) concentration will reduce the COS solubility according to the Setch-enow (1892) relation. Further, COS diffiisivity will also be reduced as the caustic soda concentration is incretised. Dewitt (1980) has thereby demonstrated experimentally and theoretically that the COS extraction rate is maximum at an intermediate NaOH concentration. 

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