Optimal Design of Neutralization Process in 1,5-Dihydroxynaphthalene Production with Solubility Difference of Naphthalene-1,5-disulfonic Acid and Naphthalene-1,5-disulfonic Acid Disodium in Aqueous Sulfuric Acid Solutions

An optimal design of the neutralization process in 1,5-dihydroxynaphthalene production was proposed and discussed. In the new strategy, instead of sodium chloride, sodium sulfate was used in the salting-out process to overcome the drawbacks of the traditional strategy. Then the precipitated mixtures, mainly naphthalene-1,5-disulfonic acid (1,5-H2NDS) and sodium sulfate, were neutralized to obtain naphthalene-1,5-disulfonic acid disodium (1,5-Na2NDS) and sulfuric acid. By adjusting the concentrations of sulfuric acid solutions to 0.3 and 0.5 mass fractions, 1,5-Na2NDS and 1,5-H2NDS were precipitated out, respectively. The yields of both 1,5-H2NDS and 1,5-Na2NDS reached 0.98. 1,5-Na2NDS was sent to the alkali fusion process, while 1,5-H2NDS was reused in the neutralization. In neutralization, the mole ratio (1,5-H2NDS to sodium sulfate) was chosen as 2.5 and the reaction temperature was 353 K. In order to determine the separation conditions, the solubilities of 1,5-H2NDS·2H2O and 1,5-Na2NDS·H2O in aqueous sulfuric acid solutions were measured over the temperature range from 277 to 337 K at atmospheric pressure by a dynamic method. The new strategy is particularly instructive for the industrial production of 1,5-dihydroxynaphthalene.