Caustic chlorine electrolysis

The most common source of pure hydrogen in small amounts is as a byproduct from caustic-chlorine electrolysis plants. A typical 100-ton/day chlorine plant would produce around one millon scfd of pure hydrogen, that is far in excess of the needs of a typical small consumer of hydrogen such as a fats and oils hydrogenation unit. 

Since 1960, about 95% of the synthetic ammonia made in the United States has been made from natural gas worldwide the proportion is about 85%. Most of the balance is made from naphtha and other petroleum Hquids. Relatively small amounts of ammonia are made from hydrogen recovered from coke oven and refinery gases, from electrolysis of salt solutions, eg, caustic chlorine production, and by electrolysis of water. In addition there are about 20 ammonia plants worldwide that use coal as a hydrogen source. 

Caustic soda electrolysis (better known as the chloralkali process) uses salt water (brine) to produce chlorine (CI2), caustic soda (NaOH) and hydrogen (H2). The anodic and cathodic reactions are respectively Cl —> %Cl2 + e and Na + H2O + e NaOH + V2H2. 

Paleologou, M., P-Y. Wong and R. M. Berry, A Solution to Caustic/Chlorine Imbalance Bipolar Membrane Electrolysis , J. Pulp Paper Sci. 18(4), J138-J145 (1992). 

Because chlorine and caustic soda are electrolysis coproducts and chlorine cannot be stored economically, caustic soda production has been very... 

Chlorine and caustic soda are coproducts of electrolysis of aqueous solutions of sodium chloride [7647-14-5] NaCl, (commonly called brine) following the overall chemical reaction... 

Aqueous Hydrochloric Acid. Muriatic acid consumption in 1993 was about 1.57 million metric tons (100% basis). The largest captive use of aqueous HCl is for brine acidification prior to electrolysis in chlorine/caustic cells and the largest merchant markets for HCl are steel pickling and oil-well acidizing, which accounted for 25 and 16% of merchant production, respectively, during 1989. 

The electrolysis of NaCl brine for the production of chlorine and caustic soda is one of the oldest and certainly one of the most important industrial electrochemical processes (22—26). The overall reaction is... 

There have been a number of cell designs tested for this reaction. Undivided cells using sodium bromide electrolyte have been tried (see, for example. Ref. 29). These have had electrode shapes for in-ceU propylene absorption into the electrolyte. The chief advantages of the electrochemical route to propylene oxide are elimination of the need for chlorine and lime, as well as avoidance of calcium chloride disposal (see Calcium compounds, calcium CHLORIDE Lime and limestone). An indirect electrochemical approach meeting these same objectives employs the chlorine produced at the anode of a membrane cell for preparing the propylene chlorohydrin external to the electrolysis system. The caustic made at the cathode is used to convert the chlorohydrin to propylene oxide, reforming a NaCl solution which is recycled. Attractive economics are claimed for this combined chlor-alkali electrolysis and propylene oxide manufacture (135). 

Many related processes use charged membranes and/or EMF. Electrodialytic water dissociation (water splitting), diffusion dialysis, Donnan dialysis, and electrolysis are related processes. Electrowsis (chlorine-caustic) is a process of enormous importance much of which is processed through very special membranes. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction... 

Chlorine, hydrogen, caustic soda, and sometimes caustic potash are coproducts of the electrolysis of saturated aqueous solutions of sodium chloride called brine. The overall chemical reaction is given as... 

Electrolysis of brine to produce chlorine and caustic soda by-product . 

Production of hypochlorite takes place in a two-step absorption unit in which 23% caustic solution is fed counter-currently to the chlorine feed-stream. In the first step -the liquid jet-loop reactor - about 90% of the chlorine is converted to hypochlorite. In step two - a packed column - a very efficient absorption [1-3] is carried out in which the chlorine concentration in the off-gas is reduced to <1 ppm. The operating window of this apparatus with respect to chlorine load is quite large and varies from 100 to 6000 kg h-1 of chlorine. This high capacity is necessary for the consumption of peak loads from the electrolysis plant during short time periods. During start-up or shutdown of one electrolyser the total chlorine peak load can vary from 100 to 300 kg in just a few minutes. 

The electrolysis reaction and types of cells were described adequately under caustic soda. The chlorine gas, contaminated with water from the electrolytic cell, is cooled to 12-14°C to liquefy most of the water, then dried in a tower of sulfuric acid. The pure chlorine gas is compressed to 40 psi and condensed by cooling at -20 to -40 °C to liquefy the gas. 

In contrast to this direct chlorination there is the oxychlorination of ethylene using hydrogen chloride and oxygen, the other major method now used. Since the chlorine supply is sometimes short and it is difficult to balance the caustic soda and chlorine demand (both are made by the electrolysis of brine), hydrogen chloride provides a cheap alternate source for the chlorine atom. Most of the ethylene dichloride manufactured is converted into vinyl chloride by eliminating a mole of HCl, which can then be recycled and used to make more EDC by oxychlorination. EDC and vinyl chloride plants usually are physically linked. Most plants are 50 50 direct chlorinationroxychlorination to balance the output of HCl. 

In applications such as the electroreduction of Figure 15.21(c) and electrolysis of brine to caustic and chlorine, single membranes serve as diaphragms between electrodes, permitting passage of ions but retarding diffusion and mixing of feed and product solutions. 

Chlorine is the most abundant of the halogens, especially in sea water, a ton of which contains in grams chlorine 15,000, bromine 97, iodine 0-17 (ratio 106 6,000 1). Its preparation depends on the discharge of its ion, either directly (i.e., electrolytically) or by oxidation. The older methods of oxidation (by manganese dioxide or by air in presence of certain catalysts) have now been replaced for technical purposes by the electrolysis of sodium chloride, which is primarily for the production of caustic soda, the chlorine being a by-product the chloroparaffins which are now so much used as solvents were developed to utilize this chlorine. 

Industries producing chlorine, sodium hydroxide (also known as caustic soda), sodium carbonate (or soda ash) and its derivatives and compounds based on calcium oxide (or lime) are usually included under this category. As both sodium hydroxide and chlorine have a common raw material, sodium chloride, they are produced in quantities that reflect their equal molar ratio, irrespective of the market for either product. Since they are produced by electrolysis, they require a cheap source of brine and electricity... 

The electrolysis of brine is carried out on a huge scale for the industrial production of chlorine and caustic soda (sodium hydroxide). Because the reduction potential of Na+ is much higher than that of water, the latter substance undergoes decomposition at the cathode, yielding hydrogen gas and OH-. 

This is one of the largest electrochemical industries in the world. It consists in the electrolysis of sodium chloride as brine to give chlorine and caustic soda. Chlorine is used in the preparation of vinyl chloride for PVC, as a bleaching agent for paper and paper pulp, as a disinfectant, besides other chloration applications. Caustic soda is important in mineral processing, and in the paper, textile, and glass industries. Table 15.2 shows recent data for industrial consumption of chlorine and caustic soda in the USA. 

Other steps in these processes are purification of the sodium chloride brine before electrolysis, evaporation of water to further concentrate the caustic soda, removal of oxygen from the chlorine. The hydrogen obtained in partitioned cells is very pure and can be used, for example, in the food industry. There is now a general changeover to membrane cells worldwide. 

From this it can be seen that there is a clear correlation betweon the average efficiencies experimentally determined by analysis of samples drawn at certain intervals during electrolysis and the momentary efficiencies at the end of each period which have been calculated from equation (XI-24). The efficiencies calculated are somewhat lower than the experimental data, which agrees with the fact that efficiencies are calculated at the ends of the periods when the concentration ca is higher than the average concentration during the period under observation. It is evident from what has been said before, that satisfactory efficiency can only be attained if a permanent excess of chloride is maintained in the electrolyte. Therefore, the diaphragm method is not suitable for the production of concentrated caustic solutions because efficiency may become uneconomical ly low and also the total conversion of chlorine into hydroxide is not possible. This process only yields diluted solutions of caustics which are rich in chloride after electrolysis they are concentrated by evaporation and the salt (NaCl or KC1) separated from them by crystallisation is returned to the process. 

Alkali chloride electrolysis — Chlorine with its coproduct sodium hydroxide (also called caustic soda or simply caustic) has been produced on an industrial scale for more than a century by the electrolysis of a saturated solution of sodium chloride (brine). The process, which is often referred to as brine electrolysis or chlorine-... 

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