Pure naphthalene isolation by liquid extraction

Soviet coking plants produce technical naphthalene by crystallization followed by hot pressing (90% of output) and a small proportion by distilling or rectifying a naphthalene fraction after removing phenols and pyridine bases and washing with concentrated sulfuric acid. Pure naphthalene is mostly made by subjecting pressed naphthalene to sulfuric acid washing and then rectifying the washed product. The present commercial processes are complicated, labor-intensive and associated with the formation of highly toxic wastes. A substantial proportion of the naphthalene is lost during processing. Moreover, the processes give rise to dangerous working conditions and cannot be fully automated. In addition the methods preclude the isolation of the impurity benzothiophene (thionaphthene), which could be used as an intermediate for the production of certain plant growth stimulants and polymer additives. The coking industry is probably the only commercial source of this substance, since proposed synthetic methods for producing benzothiophene have received no commercial backing either in the Soviet Union or anywhere else. It seems that the most rational commercial approach to the isolation of both pure naphthalene and a benzothiophene concentrate is to adopt a physico-chemical method such as counterflow liquid-phase extraction or extractive or azeotropic rectification. A mathematical model has been derivedmore » for optimizing the counterflow liquid-phase extraction of the naphthalene fraction with two mutually immiscible solvents. The raffinate from the experimental apparatus yielded a naphthalene product within the GOST specifications. Laboratory tests have shown that benzothiophene concentrates can be made with a yield of 91 to 94% of theoretical.« less