Conversion from diaphragm process

Transducers The ciirrent-to-pressiire transducer (I/P transducer) is a conversion interface that accepts a standard 4-20 rnA input current from the process controller and converts it to a pneumatic output in a standard pneumatic pressure range (normally (),2-L0 bar [3-15 psig] or, less frequently, 0,4-2,0 bar [6-30 p.sig]). The output pressure generated by the transducer is connected directly to the pressure connection on a spring-opposed diaphragm actuator or to the input of a pneumatic valve positioner. 

The conditions for a conversion from the mercury and the diaphragm process to the membrane process are discussed below. 

Lime Soda. Process. Lime (CaO) reacts with a dilute (10—14%), hot (100°C) soda ash solution in a series of agitated tanks producing caustic and calcium carbonate. Although dilute alkaH solutions increase the conversion, the reaction does not go to completion and, in practice, only about 90% of the stoichiometric amount of lime is added. In this manner the lime is all converted to calcium carbonate and about 10% of the feed alkaH remains. The resulting slurry is sent to a clarifier where the calcium carbonate is removed, then washed to recover the residual alkaH. The clean calcium carbonate is then calcined to lime and recycled while the dilute caustic—soda ash solution is sent to evaporators and concentrated. The concentration process forces precipitation of the residual sodium carbonate from the caustic solution the ash is then removed by centrifugation and recycled. Caustic soda made by this process is comparable to the current electrolytic diaphragm ceU product. 

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. 

I. Primary Filtration. Classically, sand has been used as the medium in bed filters. While it is acceptable in most diaphragm and mercury brine processes, the possibility of dissolving small quantities of silica usually disqualifies sand from use in membrane plants. Other media such as garnet and anthracite are used instead. These have little effect on filter design, and plant conversions usually continue with the same vessels on line but the new fill substituted for sand, lypical particle sizes for both sand and anthracite are about 1 mm, but sand is, on the average, a bit smaller and contains more fines. Regardless of the fill, the apparatus still is frequently referred to as a sand filter. 

About 30 % of electrical energy can be saved by application of oxygen depolarized cathodes (ODC) in the membrane process, as mentioned above. The energy saving will be increased to 50 % if the conversion to this technology starts from the diaphragm or amalgam process. 

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