Feed Brine

Eig. 20. Cut view of Chlorine Engineers membrane bag cell a, manifold b, frame c, partition plate d, sealing plug e, recirculated NaOH inlet f, cathode g, anode h, cathode can i, membrane bag j, base k, butterfly valve 1, feed brine m, depleted brine n, caustic outlet. 

The brine feed to the electroly2ers of all the processes is usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte. Table 14 gives the specifications of the feed brines requited for the membrane and diaphragm cell process to reali2e optimal performance. 

Table 14. Typical Specifications for Feed Brine to Electrolyzers...

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

The feed brine is also a source of impurities. It can contain dissolved and entrained air and so can contribute oxygen and nitrogen. The brine may also contain carbonate that was added during chemical treatment in order to remove dissolved calcium. The carbonate will be converted to carbon dioxide in the cell environment. 

Carbon dioxide permeates the membranes at least as readily as does chlorine. This fact will produce a significant increase in CO2 concentration in a recycle system. For best results at highest chlorine recovery, it will pay to keep the CO2 concentration in the membrane feed-gas low. This can be accomplished in most plants by acidification of cell-feed brine. Acidification is highly recommended in any case when a very high degree of chlorine recovery is required, whether by... 

Elsewhere in this book, White and Sandel [7] discuss the integration of chlorine and ethylene dichloride (EDC) processes. The oxygen content of the chlorine fed to an EDC unit must be kept within the process specification. This can be achieved by liquefying at least part of the chlorine in order to reject non-condensables or by acidifying the brine fed to the cells. Oxygen results from the anodic oxidation of hydroxide ions free acid in the feed brine will neutralise those ions and so reduce the amount of oxygen formed. 

To overcome membrane scaling, the operating pH of the feed brine to the unit was lowered to a range between 4 and 7. A simple modification was made to the plant to control the pH of the plant feed brine by mixing acidic dechlorinated brine with alkaline dechlorinated brine. This modification has proven to be effective and no further membrane fouling has occurred over the last two years. 

As was mentioned previously, an effective system, RNDS , has been developed to remove particular impurities from brine used in membrane electrolysis procedures. The basic concept of RNDS is to bring the feed brine into contact with an ion-exchange resin containing zirconium hydroxide for the adsorptive removal of impurities. For the removal of the sulphate ion from brine, commercial plants utilising RNDS are already in service. For the elimination of iodide and silica, pilot-scale testing is being planned. 

Recycle and cathodic reduction. The most elegant solution for the Diaphragm Electrolysis Plant (DEP) appears to be recycling of the hypochlorite solution and reduction of the chlorate and bromate on the cathode of the electrolysis cell - the hypochlorite solution is added to the feed brine of the cells and the chlorate and bromate are converted to chloride and bromide at the cathode. 

The feed brine of the DEP contains a large quantity of carbonates. Therefore, at pH5 carbon dioxide is degassed. When hypochlorite is added, chlorate and bromate are formed in the feed of the electrolysis cells. These reactions have a slow velocity. The result of this is that conversion is only partial ... 

It appears that all of the bromide that is converted to bromine and bromate in the electrolysis process is eventually recycled to the feed brine. At the cathode of the electrolysis cell bromide is formed again. 

Now that the circle is closed, there is no longer any bromine-containing waste stream flowing from the destruction units. All bromide in the feed brine is returned to the cell-liquor. This is an excellent achievement for the environmental aspects of the DEP ... 

After extensive research and several tests, the option selected was recycling hypochlorite to the feed brine of the electrolysis cells. For this purpose, hypochlorite feed pipes were manufactured and the hydrochloric acid feed capacity to the brine degassing tanks was enlarged. 

Viton hoses were instead selected for the feed brine to the electrolysis cells. These are chemically resistant to chlorine-containing brine. There are several specifications of Viton hose available. For working with brine in an electrolysis environment, special attention had to be given to rupture resistance of the hoses with respect to operator safety. 

There are no longer any chlorate and bromate emissions from the chlorine and hypochlorite destruction units. All hypochlorite is recycled into the feed brine. This process has been operating efficiently since July 1999. 

An additional advantage of the hypochlorite recycling process is the chlorination of the feed brine in the brine-degassing unit. Organic and nitrogen-containing components are oxidised. The reaction products are removed via the vent-gas to the chlorine destruction unit. Less NCI3 is formed in the electrolysis cells because part of the... 

Recycling the hypochlorite to the feed brine has provided an excellent possibility of eliminating completely the chlorate and bromate emissions of the chlorine destruction unit of a diaphragm electrolysis plant. The main advantage of the hypochlorite recycling and cathodic reduction procedure is the reduction of bromate to bromide. 

To maintain the acidity of the brine at pH5, more hydrochloric acid is required during hypochlorite recycling to the feed brine. This extra acid demand is the cause of the largest increase of variable production costs - approximately 100 000 Dutch guilders per year. Alternative solutions showed variable costs up to one million Dutch guilders per year. The investment for this project proved to be the best economical alternative to solve the chlorate and bromate emissions problem. 

Yet more research and development effort concentrates on the diaphragm cell caustic evaporator, finding ways to aid the evaporation of the 10-12% caustic soda in brine to make it into a saleable product. Work is directed into methods of removing the salt products and impurities and preventing corrosion of the equipment. Recovery of the salt from the evaporated caustic soda is an important part of a diaphragm cell plant as the recovered salt is used in the strengthening of the feed brine. 

Owing to limitations on the water balance of the plant, a 40-50% conversion can be undertaken before a salt evaporator needs to be installed to remove excessive water. The feed brine for the membrane cellroom is taken from brine made from fresh imported salt and the weak brine is returned, combined with the diaphragm brine, restrengthened with recovered salt from the evaporators and fed to the diaphragm cells. The concentration of brine to the diaphragm cells could be weaker than normal, at around 270-280 g l-1 rather than 300-310 g l-1 to assist with the water balance. 

After conducting trials on various prototypes and testing a large number of components over many years, ICIETB installed a BiChlor demonstration electrolyser at Id s Lostock Plant in the UK. The BiChlor electrolyser achieved an oxygen content of approximately 1.5% in chlorine with alkaline feed brine. 

In the Improved B-1 at the Kashima factory, low-oxygen content chlorine gas can be obtained by adding hydrochloric acid to the feed brine. Figure 19.8 shows the dependence of the oxygen content in the chlorine gas upon the content of the hydrochloric acid in the feed brine at 6 kA m 2 current density operation. 

There are two steps in the basic process the upstroke and the downstroke (see Fig. 24.1). During the upstroke, feed brine solution bearing impure salt is pumped into the bottom of the ion-exchange resin bed. The impurity (MX) is sorbed by the resin particles according to Equation 24.1 and a purified brine solution is collected from the top of the bed. Next, during the downstroke, water is pumped into the top of the bed, desorbing the brine impurity from the resin according to Equation 24.2 so that a solution of the brine impurity is collected from the bottom of the bed. The total cycle typically takes about 2-10 min to complete and repeats successively. 

Wolff, J.J. (1985) Ion exchange purification of feed brine for chlor-alkali electrolysis cells the role of Duolite C-467. Rohm c Haas Bulletin IE-D-285, March. 

Fabricator acetyla- feed brine Hydraulic, P meation constant, A times total rejection... 

As with the modified S S membrane, the L-S membrane was found to be asymmetric. The side of the membrane away from the casting surface had to be in contact with the feed brine during se rvi ce,... 

When feeding brine shrimp to the zebrafish make sure that enough is available so that all fish may feed. 

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