Removal of tar base from coal tar aromatics employing solid acid adsorbents

Abstract

Aromatic compounds from coal tar generally contain a small amount of tar bases, such as quinoline and isoquinoline. These nitrogen-containing compounds can poison the acid-type catalysts and downgrade the aromatic products because of stinking odor. Four solid acid catalysts, silica-alumina, HY, NH4-mordenite, and γ-alumina are used to remove tar bases by adsorption. Wash oil (WO), refined naphthalene (RN), and an intermediate distillate from the China Steel coke plant (Taiwan) contain quinoline ranged from 0.03 to 8.9%. Quinoline and isoquinoline can be selectively removed from a mixture due to their strong chemisorption on acidic sites, thus the remaining compounds are not disturbed following adsorption. Naphthalene, a neutral compound, is physically adsorbed on solid acids, and is desorbed near its boiling point. Silica–alumina gives the best adsorption results because its wide-ranging pore sizes are accessible to the bulky quinoline molecule. The adsorption of HY and mordenite are significantly decreased because of the extremely diffusion limitation of quinoline in pore channels. Solid acids can be completely regenerated in air at 500–600°C. The adsorption ability of silica–alumina can be completely restored even after three cycles of regeneration. The adsorption rate increases with temperature although the capacity decreases. This work demonstrates that the adsorption through the use of solid acids is an effective method that can be used to reduce the amount of tar bases in coal tar aromatics.