B. Choosing the acid

HCl costs less in North America and is sometimes considered in its 35% commercial form. There is no heating effect bur corrosivity is more severe. 

If the choice of the acid arises, it means considering its thermal impact and also the modification in the density of the acidified caustic which becomes a solution of H2SO4 + Na2S04 or HCl + NaCl. 

Accordingly 40 to 180 kg of pure H2SO4 is required to neutralize a cubic meter of spent caustic. The added sulfuric acid, 10 or 18 N, will therefore dilute the spent caustic by 80-44 liters to 360-200 liters of water. 

Acidified spent caustic specific gravity varies from 1.01 to 1.06 whereas sprung acid specific gravity depends on the dominant make up in phenolic acids (sp.gr. from 1.02 to 1.05), in HC (sp.gr. from 0.75 to 0.9) or even in disulfides (sp.gr. from 0.95 to 1.05). 

Although this risk exists, it is not common. It has occurred during neutralization of very concentrated spent caustic by 20 N acid. 

---