Caustic soda regeneration system

This takes place at 170°C and between 1 to 1.5.106 Pa absolute, in the presence of a palladium catalyst, in a series of agitated reactors. Unconverted hydrogen is recycled after scrubbing with caustic soda and water. The catalyst system in suspension in the effluent is- recovered by centrifuging and hexahydrobenzoic acid evaporation before regeneration and recycle. 

Lime solution is employed by some operators since caustic soda is costly. However, the pipelines and spray nozzles tend to get choked due to formation of insoluble calcium salts. A variant of this method is the use of the dual alkali process wherein NaOH solution is circulated in the system and lime is used to regenerate NaOH from the spent scrubbing liquor. Other systems of proprietary design are also offered which are low in capital costs and utility consumption. 

When the reaction between solvent B and solute C is irreversible, the value of Xe approaches zero. Such would be the case if H2S were extracted from LPG using an aqueous solution of caustic soda as the solvent. However, if the chemical reaction is reversible, so that the solute can be recovered, then Xe will have a low value that is dependent on the concentrations of the reactants and the temperature. Such a system might be the extraction of H2S from LPG using an aqueous ethanolamine solvent. The extract phase subsequently would be regenerated to permit reuse of the amine solvent. As shown in Figure 11-6, the equilibrium concentration in the extract approaches a constant value limited by the concentration of the reactant in the solvent and the stoichiometry. 

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